1
PLANT DESIGN FOR THE PRODUCTION OF 400,000 METRIC TONNES OF NITRIC ACID PER ANNUM FROM AIR OXIDATION OF AMMONIA GAS BY ANDREW OFOEDU DEPARTMENT OF CHEMICAL ENGINEERING FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI. SEPTEMBER 2013
2
EXECUTIVE SUMMARY This report describes the detailed design of a plant to produce 400000 tonnes of nitric acid per year by Ostwald Process. The single pressure process was selected as the most advantageous, having considered several factors one of which is efficient energy management. The process begins with the vaporization of ammonia at 1000 kPa and 35°C using process heat. Steam is then used to superheat the ammonia up to about 80°C. Filtered air is compressed in an axial compressor to a discharge pressure of about 740kPa and temperature of 155°C. Part of the air is diverted for acid stripping. This preheated air and the ammonia vapour are then mixed and passed through the platinum/rhodium catalyst gauze in a converter for oxidation. The reaction gas flows through a series of heat exchangers for recovery of energy as either high-pressure superheated steam, or as shaft horsepower from the expansion of hot tail gas in the turbine. Considering the proximity to market, sea port and source of raw materials, it was decided to site the plant in Eleme, Rivers State. The plant’s
3
estimated capital investment is ₦5.41 billion. The rate of return on investment is 26.25% and the payback period is estimated to be 3 years and 7 months. Thus, the project is both technically and economically feasible.
TABLE OF CONTENT Title page-----------------------------------------------------------------------i Executive Summary---------------------------------------------------------------------------ii Table of content-------------------------------------------------------------------------------iii CHAPTER ONE 1.0 Introduction-----------------------------------------------------------1 1.3-------------------------------------------------------------------------------Design justification-------------------------------------------------------------------3 1.4-------------------------------------------------------------------------------Design Objectives---------------------------------------------------------------------4 CHAPTER TWO 2.0 Literature review------------------------------------------------------------------------5 2.1 History of Nitric acid production-------------------------------------------------------5 2.2 Ammonia oxidation chemistry----------------------------------------------------------8
4
2.3 Emission and Control-----------------------------------------------------------------------14 2.4 Structure and bonding---------------------------------------------------------------------15 2.5 Reactions------------------------------------------------------------------------------------16 2.6 Uses--------------------------------------------------------------------------------------------19 2.7 Safety------------------------------------------------------------------------------------------21 2.8 Pinch technology in modern plant------------------------------------------------------22 2.9 Plant Location -------------------------------------------------------24 2.9.5 Plant layout-------------------------------------------------29 2.9.6 Process routes for the production of nitric acid-----------------33 CHAPTER THREE 3.0 Material balance ------------------------------------------------------42 3.1 Conservation of mass -----------------------------------------------42 3.2 Methods of material balancing -----------------------------------43 3.3 Materials balance assumptions-----------------------------------44 3.4 Summary of material balance calculations-------------------44 3.5 Material balance for each unit------------------------------------44
5
CHAPTER FOUR 4.0 Energy balance -------------------------------------------------------53 4.1 Conservation of energy---------------------------------------------54 4.2 Energy balance assumptions -------------------------------------56 4.3 Summary for energy balances------------------------------------56 CHAPTER FIVE 5.0 Chemical design--------------------------------------------------------61 5.1 Process units of Production--------------------------------------------61
Engineering Nitric
acid
CHAPTER SIX 6.0 Equipment design and specification ---------------------------66 6.1 Problem specification------------------------------------------------67 6.2 Analyzing the problem solution----------------------------------68 6.3
Preliminary design-----------------------------------------------------------------------68
6.4 Material Selection-----------------------------------------------------------------------69 6.5 Design optimization---------------------------------------------------------------------69 6.6 Summary of calculation---------------70
design
and
equipment
specification
CHAPTER SEVEN 7.0 Process control and instrumentation----------------------------73
6
7.1 Objective----------------------------------------------------------------------------------73 7.2 Plant control instrumentation-------------------------------------74 7.3 Alarms and safety trips --------------------------------------------77 7.4 Lining, piping, valves and pumps -------------------------------78 7.5 Pipe support------------------------------------------------------------81 CHAPTER EIGHT 8.0 Safety and considerations---------------------------------------82
environmental
8.1 Safety-----------------------------------------------------------------------------------82 8.2 Hazard and Operability study-------------------------------------------89 8.3 Environmental assessment-------------------------------------------------97
(HAZOP) impact
CHAPTER NINE 9.1 Overview----------------------------------------------------------------103 9.2 Economic Consideration--------------------------------------------103 9.3 Cost estimation--------------------------------------------------------------------------106
9.6 Economic analyses calculation-----------------------------------108
7
CHAPTER TEN 10.0 Start up and shut down procedure----------------------------113 10.1 Emergency shut down and emergency depressurization ------------------------------------------------------------------------------------ 114 10.2 Notification------------------------------------------------------------114 10.3 Record keeping -----------------------------------------------------115 10.4 Startup operation---------------------------------------------------116
CHAPTER ELEVEN 11.0 Conclusion/ Recommendation----------------------------------------------------118 11.1 Conclusion-----------------------------------------------------------------------------118 11.2 Recommendation -------------------------------------------------------------------119
REFERENCES----------------------------------------------------------------120
APPENDIX I Tables and Charts-------------------------------------------------------------------------------123 APPENDIX II
8
Material Balance Calculation------------------------------------------------------------------126
APPENDIX III Energy Balance Calculation------------------------------------------------------------------132 APPENDIX IV Equipment Design Calculation----------------------------------------------------------------137 APPENDIX V Equipment Costing Calculation---------------------------------------------------------------141
9
CHAPTER ONE INTRODUCTION 1.1 BACKGROUND INFORMATION Nitric acid is a strong acid and a powerful oxidizing agent with enormous possibilities for applications in the chemical processing industry. It has commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent, catalyst and hydrolyzing agent. In relation to world production, approximately 65% of all nitric acid produced is used for the production of ammonium nitrate (specifically for fertilizer manufacture).
10
Nitric acid is now produced commercially using the stepwise, catalytic oxidation of ammonia with air, to obtain nitrogen monoxide and nitrogen dioxide. These nitrogen oxides are subsequently absorbed in water to yield between 50% and 68% strength nitric acid by weight. For applications requiring higher strengths, several methods of concentrating the acid are used. The traditional methods are: (a) Extractive distillation with dehydrating agents such as sulphuric acid or magnesium nitrate; (b) Reaction with additional nitrogen oxides. The latter technique has the greatest application in industry. The chemistry of ammonia oxidation is remarkably simple with only six main reactions that need to be considered. 1.1.1 PROPERTIES AND USES Nitric acid is an oxidizing mineral acid with physical and chemical properties that make it one of the most useful inorganic minerals. It is a colorless liquid at room temperature and atmospheric pressure. It is soluble in water in all proportions and there is a release of heat of solution upon dilution. Its high solubility in water is the basis for the process methods used for commercial nitric acid manufacture. It is a strong acid that almost completely ionizes when in dilute solution. It is also a powerful oxidizing agent with the ability to passivate some metals such as iron and
11
aluminum. A compilation of many of the physical and chemical properties of nitric acid are presented in the Appendix. Arguably the most important physical property of nitric acid is its azeotropic point, this influences the techniques associated with strong acid production. The constant-boiling mixture occurs at 121.9°C, for a concentration of 68.4%(wt) acid at atmospheric pressure. Nitric acid has enormously diverse applications in the chemical industry. It has commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent, catalyst and hydrolyzing agent. The most important use is undoubtedly in the production of ammonium nitrate for the fertilizer and explosives industries, which accounts for approximately 65% of the world production of nitric acid. Nitric acid has a number of other industrial applications. It is used for pickling stainless steels, steel refining, and in the manufacture of dyes, plastics and synthetic fibers. Most of the methods used for the recovery of uranium, such as ion exchange and solvent extraction, use nitric acid. An important point is that for most uses concerned with chemical production, the acid must be concentrated above its azeotropic point to greater than 95%(wt). Conversely, the commercial manufacture of ammonium nitrate uses nitric acid below its azeotropic point in the range 50 -65 %(wt.). If the stronger
12
chemical grade is to be produced, additional process equipment appropriate to super-azeotropic distillation is required. There is a potential health hazard when handling, and operating with, nitric acid. Nitric acid is a corrosive liquid that penetrates and destroys the skin and internal tissues. Contact can cause severe burns. The acid is a potential hazard, the various nitrogen oxides present as product intermediates in the process are also toxic. An assessment of the health risk must be fundamental to the
design
of
any
process.
Further
consideration
and
recommendations for the operating health risk and environmental impact of the plant are presented in the Appendix.
1.2 DESIGN JUSTIFICATION At present, there is no Nitric acid plant in Nigeria. The little Nitric acid produced mainly by fertilizer plants in the country is used up immediately by them to make their fertilizer. This means that most of the all Nitric acid used in the country is imported. A Nitric acid plant sited in the country producing Nitric acid made available to the Nigerian market will not only reduce importation of the acid but also encourage fertilizer production, create job opportunities as well as develop the area in which it is sited.
1.3 DESIGN OBJECTIVES
13
To design a plant that will deliver 400000 metric tonnes of 60%(wt) Nitric Acid per annum. To determine the technical and economic feasibility of the plant.
CHAPTER TWO
14
LITERATURE REVIEW 2.1 HISTORY OF NITRIC ACID PRODUCTION Until the beginning of the 20th century, Nitric acid (HNO 3), also known
as
aqua
fortis
and
spirit
of
niter
was
prepared
commercially by reacting sulphuric acid with either potassium nitrate (saltpetre) or with sodium nitrate (Chile saltpetre or nitre). Up to four tonnes of the two ingredients were placed into large retorts and heated over a furnace (Kirk 1996). The volatile product vapourized and was collected for distillation. An acid of 93-95 %( wt) was produced (Gregory 1999). In 1903 the electric-arc furnace superseded this primitive original technique. In the arc process, nitric acid was produced directly from nitrogen and oxygen by passing air through an electric-arc furnace (Ray 1990). Gregory (1999, p.40) argues that ‘Although the process benefitted from an inexhaustible supply of free feed material (air), the power consumption for the arc furnace was cost prohibitive’ According to Ray (1989, p.8) Researchers returned to the oxidation of ammonia in air, (recorded as early as 1798) in an effort to improve production economics. In 1901 Wilhelm Ostwald had first achieved the catalytic oxidation of ammonia over a platinum catalyst. The gaseous nitrogen oxides produced could be easily cooled and dissolved in water to produce a solution of nitric
15
acid. This achievement began the search for an economic process route. By 1908 the first commercial facility for production of nitric acid, using this new catalytic oxidation process, was commissioned near Bochum in Germany (Ray et al 1989). The Haber-Bosch ammonia synthesis process came into operation in 1913, leading to the continued development and assured future of the ammonia oxidation process for the production of nitric acid. (Ray et al 1989) During World War 1, the intense demand for explosives and synthetic dyestuffs created an expansion of the nitric acid industry. Many new plants were constructed, all of which employed the ammonia oxidation process. This increased demand served as the impetus for several breakthroughs in process technology. These included: (a)
The
development
of
chrome-steel
alloys
for
tower
construction, replacing the heavy stoneware and acid-proof bricks. This enabled process pressures above atmospheric levels to be used. (b) The improved design of feed preheaters enabled higher process temperatures to be attained. Higher temperatures improved the yields and capacities, and also reduced equipment requirements (Ohrue et al 1999).
16
(c) Early developments in automatic process control improved process performance and reduced labor requirements. All of these factors helped to improve the process efficiency. The increasing availability of ammonia reduced processing costs still further. In the late 1920’s the development of stainless steels enabled manufacturers to use higher operating pressures. The increase in yield and lower capital requirements easily justified the use of high pressure operation despite increased ammonia consumption. The introduction of higher pressure processes resulted in a divergence of operating technique within the industry. The United States producers opted for a high-pressure system, using a constant high pressure throughout the process. The European manufacturers opted for a split-pressure system. This latter system entails operating the ammonia oxidation section at atmospheric pressure, while the absorption unit is operated at higher pressures, thus capitalizing on improved absorption rates. (Harvin et al 1979) Recent developments in the ammonia oxidation process have included efforts to reduce catalyst losses in the process. Platinum recovery filters have been installed at various stages in the process. (Ohrue et al 1999) Gold/palladium gauze filter pads have been added on the exit side of the catalyst bed, inside the reactor/converter units. These
17
filters have reportedly ensured a platinum recovery of 80% (Anon 1979). Another trend has been for the use of additional filters in the downstream units. These filters are of alumino-silicate construction. Perhaps the greatest progress in nitric acid production technology has been in the manufacture of strong nitric acid (>90% by weight). Advances in the areas of super-azeotropic distillation and in high pressure absorption are most significant. (Ohkubo et al 1999) Research work is continually being performed in an effort to reduce nitrogen oxide emissions from nitric acid plants. The Humphreys and Glasgow/Bolme nitric acid process is just one example of a new philosophy being applied to the absorption systems of weak nitric acid plants (50-68% by weight). Nitrogen oxide emissions have been reduced from 2000-5000 ppm to less than 1000 ppm (Ray et al 1989). For the production of stronger nitric acid, tail gases are now being treated
by
selective
or
non-selective
catalytic
combustion
systems. These innovative units have reduced the nitrogen oxide emissions to below 400 ppm (Ray et al 1989). 2.2 AMMONIA OXIDATION CHEMISTRY Notably, all commercial nitric acid production methods used today are centered on the oxidation of ammonia. It is therefore appropriate to investigate the chemistry of this process, in the
18
knowledge that it is directly applicable to any of the production processes available. (Chilton 1960) The chemistry of the oxidation of ammonia is surprisingly simple. It begins with a single pure compound, plus air and water, and ends with another pure compound in aqueous solution, with essentially no by-products. The process may be described by just six major reactions as shown as follows: 1.
N H 3(g) +2 O2 → HN O3 (aq )+ H 2 O(l )
2. 4 N H 3(g )+ 5O2 (g) → 4 N O(g )+ 6 H 2 O(l )
3.
2 N O(g) +O2 → 2 N O 2 (g)
4.
2 N O2 (g) ⇌ N 2 O4
5.
3 N 2 O 4 + 2 H 2 O(l) → 4 HN O 3 +2 N O (g)
6.
3 N O2 (g) + H 2 O(l) →2 HN O3 (aq )+ N O(g)
Reaction 1 is the overall reaction for the process. This net result is achieved from three separate, and distinct, chemical steps. The first is the oxidation of ammonia to nitrogen monoxide (Reaction 2). The second is the further oxidation of nitrogen monoxide to nitrogen dioxide (Reaction 3), then nitrogen dioxide to nitrogen tetroxide (Reaction 4). The third and final stage involves the absorption of these nitrogen-based oxides into water to form the nitric acid product (Reactions 5 and 6). In most commercial
19
processes, each of these three stages is conducted in separate process units. (Chilton 1960) The first step in the process is the heterogeneous, highly exothermic, gas-phase catalytic reaction of ammonia with oxygen (Reaction 2). The primary oxidation of ammonia to nitric acid (over a catalyst gauze of 9:l platinum/rhodium alloy) proceeds rapidly at process temperatures between 900-970°C. (Kent 1983) The second step in the process involves two reactions (Reactions 3 and 4). These are the oxidations of nitrogen monoxide to the dioxide and tetroxide forms. The equilibrium mixture is loosely referred to as nitrogen peroxide. Both reactions are homogenous, moderately
exothermic,
gas-phase
catalytic
reactions.
All
reactions shown are highly exothermic. (Chilton 1960) The third step in the process involves cooling the reaction gases below their dew point, so that a liquid phase of weak nitric acid is formed. This step effectively promotes the state of oxidation and dimerization (Reactions 3 and 4), and removes water from the gas phase. This in turn increases the partial pressure of the nitrogen peroxide component. (Chilton 1960) Finally, nitric acid is formed by the reaction of dissolved nitrogen peroxide with water (Reactions 5 and 6). Nitric acid is produced by 2 methods. The first method utilizes oxidation, condensation, and absorption to produce a weak nitric acid. Weak nitric acid can have concentrations ranging from 30 to
20
70 percent nitric acid. The second method combines dehydrating, bleaching, condensing, and absorption to produce a high-strength nitric acid from a weak nitric acid. High-strength nitric acid generally contains more than 90 percent nitric acid. The following text provides more specific details for each of these processes. (Chilton 1960) 2.2.1 WEAK NITRIC ACID PRODUCTION According to Ray(1989, Nearly all the nitric acid produced in the U. S. is manufactured by the high-temperature catalytic oxidation of ammonia.
This process typically consists of 3 steps: (1)
ammonia oxidation, (2) nitric oxide oxidation, and (3) absorption. Each step corresponds to a distinct chemical reaction. 1.
AMMONIA OXIDATION First, a 1:9 ammonia/air mixture is oxidized at a temperature of 1380 to 14700F as it passes through a catalytic convertor, according to the following reaction: 4 N H 3 +5 O2 → 4 NO +6 H 2 O
The most commonly used catalyst is made of 90 percent platinum and 10 percent rhodium gauze constructed from squares of fine wire. Under these conditions, the oxidation of ammonia to nitric oxide (NO) proceeds in an exothermic reaction with a range of 93 to 98 percent yield. Oxidation temperatures can vary from 1380 OF to 16500F. (Chilton 1960) Higher catalyst temperatures increase reaction
selectivity
toward
NO
production.
Lower
catalyst
temperatures tend to be more selective toward less useful products: nitrogen (N2) and nitrous oxide (N2O).
21
Nitric oxide is considered to be a criteria pollutant and nitrous oxide is known to be a global warming gas. The nitrogen dioxide/dimmer mixture then passes through a waste heat boiler and a platinum filter. (Chilton 1960) 2.
NITRIC OXIDE OXIDATION The nitric oxide formed during the ammonia oxidation must be oxidized.
The
process
stream
is
passed
through
a
cooler/condenser and cooled to 100 0F or less at pressures up to 116 pounds per square inch absolute (psia). The nitric oxide reacts non-catalytically with residual oxygen to form nitrogen dioxide (NO2) and its liquid dimmer, nitrogen tetra-oxide: 2 N O2 +O2 → 2 N O2 ⇌ N 2 O4
This slow, homogeneous reaction is highly temperature and pressure dependent. Operating at low temperatures and high pressures promotes maximum production of NO 2 within a minimum reaction time (Kent 1983). 3.
ABSORPTION
The final step introduces the nitrogen dioxide/dimmer mixture into an absorption process after being cooled. The mixture is pumped into the bottom of the absorption tower, while liquid dinitrogen tetra-oxide is added at a higher point. De-ionized process water enters the top of the column. Both liquids flow countercurrent to the nitrogen dioxide/dimmer gas mixture. Oxidation takes place in the free space between the trays, while absorption occurs on the trays. The absorption trays are usually
22
sieve or bubble cap trays. The exothermic reaction occurs as follows: 3 N O2 + H 2 O→ 2 HN O3+ NO
A secondary air stream is introduced into the column to re-oxidize the NO that is formed in Reaction 3. This secondary air also removes NO2 from the product acid. An aqueous solution of 55 to 65 percent (typically) nitric acid is withdrawn from the bottom of the tower. The acid concentration can vary from 30 to 70 percent nitric acid. The acid concentration depends upon the temperature, pressure, number of absorption stages, and concentration of nitrogen oxides entering the absorber. There are 2 basic types of systems used to produce weak nitric acid:
single-stage pressure process and dual-stage pressure
process (Harvin et al 1979). In the past, nitric acid plants have been operated at a single pressure, ranging from atmospheric pressure to 14.7 to 203 psia. However, since Reaction 1 is favored by low pressures and Reactions 2 and 3 are favored by higher pressures, newer plants tend to operate a dual stage pressure system, incorporating a compressor between the ammonia oxidizer and the condenser. The oxidation reaction is carried out at pressures from slightly negative to about 58 psia, and the absorption reactions are carried out at 116 to 203 psia. (Harvn et al 1979) In the dual-stage pressure system, the nitric acid formed in the absorber (bottoms) is usually sent to an external bleacher where air is used to remove (bleach) any dissolved oxides of nitrogen.
23
The bleacher gases are then compressed and passed through the absorber. The absorber tail gas (distillate) is sent to an entrainment separator for acid mist removal. Next, the tail gas is reheated
in
the
ammonia
oxidation
heat
exchanger
to
approximately 3920F. The final step expands the gas in the powerrecovery turbine. The thermal energy produced in this turbine can be used to drive the compressor. 2.2.2 HIGH STRENGTH NITRIC ACID PRODUCTION A high-strength nitric acid (98 to 99 percent concentration) can be obtained by concentrating the weak nitric acid (30 to 70 percent concentration) using extractive distillation. (Imai et al 1999) The weak nitric acid cannot be concentrated by simple fractional distillation. The distillation must be carried out in the presence of a dehydrating agent. Concentrated sulfuric acid (typically 60 percent sulfuric acid) is most commonly used for this purpose. The nitric acid concentration process consists of feeding strong sulfuric acid and 55 to 65 percent nitric acid to the top of a packed
dehydrating
column
at
approximately
atmospheric
pressure. The acid mixture flow downward, countercurrent to ascending vapors. Concentrated nitric acid leaves the top of the column as 99 percent vapor, containing a small amount of NO 2 and oxygen (O2) resulting from dissociation of nitric acid. The concentrated acid vapor leaves the column and goes to a bleacher and a countercurrent condenser system to effect the condensation of strong nitric acid and the separation of oxygen and oxides of nitrogen (NO2) byproducts. (Ohkubo et al 1999)
24
These byproducts then flow to an absorption column where the nitric oxide mixes with auxiliary air to form NO 2, which is recovered as weak nitric acid. Inert and un-reacted gases are vented to the atmosphere from the top of the absorption column. Emissions from this process are relatively minor. A small absorber can be used to recover NO2. (Kirk et al 1981)
2.3 EMISSIONS AND CONTROL Emissions from nitric acid manufacture consist primarily of NO, NO2 (which account for visible emissions), trace amounts of HNO 3 mist, and ammonia (NH3). By far, the major source of nitrogen oxides (NO2) is the tail-gas from the acid absorption tower. In general, the quantity of NO2 emissions is directly related to the kinetics of the nitric acid formation reaction and absorption tower design. NO2 emissions can increase when there is (1) insufficient air supply to the oxidizer and absorber, (2) low pressure, especially in the absorber, (3) high temperatures in the coolercondenser and absorber, (4) production of an excessively highstrength product acid, (5) operation at high throughput rates, and (6) faulty equipment such as compressors or pumps that lead to lower pressures and leaks, and decrease plant efficiency. (Leray et al 1979) Roudier (1979) states that the two most common techniques used to control absorption tower tail gas emissions are extended absorption and catalytic reduction. Extended absorption reduces
25
NO2 emissions by increasing the efficiency of the existing process absorption tower or incorporating an additional absorption tower. An efficiency increase is achieved by increasing the number of absorber trays, operating the absorber at higher pressures, or cooling the weak acid liquid in the absorber. The existing tower can also be replaced with a single tower of a larger diameter and/or additional trays. In
the
catalytic
reduction
process
(often
termed
catalytic
oxidation or incineration), tail gases from the absorption tower are heated to ignition temperature, mixed with fuel (natural gas, hydrogen, propane, butane, naphtha, carbon monoxide, or ammonia) and passed over a catalyst bed. In the presence of the catalyst, the fuels are oxidized and the NO2 are reduced to N2. The extent of reduction of NO2 and NO to N2 is a function of plant design, fuel type, operating temperature and pressure. Spacevelocity through the comparatively small amounts of nitrogen oxides is also lost from acid concentrating plants. These losses (mostly NO2) are from the condenser system, but the emissions are
small enough to
be controlled easily by
inexpensive
absorbers. Acid mist emissions do not occur from the tail-gas of a properly operated plant. The small amounts that may be present in the absorber exit gas streams are removed by a separator or collector prior to entering the catalytic reduction unit or expander. (Kent 1983)
26
The acid production system and storage tanks are the only significant sources of visible emissions at most nitric acid plants. Emissions from acid storage tanks may occur during tank filling. 2.4 STRUCTURE AND BONDING
Fig 2: Two major resonance representations of HNO 3. The molecule is planar. Two of the N-O bonds are equivalent and relatively short (this can be explained by theories of resonance. The canonical forms show double bond character in these two bonds, causing them to be shorter than typical N-O bonds.), and the third N-O bond is elongated because the O is also attached to a proton.
2.5 REACTIONS 2.5.1 ACID-BASE PROPERTIES Nitric acid is normally considered to be a strong acid at ambient temperatures. The pKa value is usually reported as less than −1. This means that the nitric acid in solution is fully dissociated
27
except in extremely acidic solutions. The pK a value rises to 1 at a temperature of 250 °C. Nitric acid can act as a base with respect to an acid such as sulfuric acid. HNO3 + 2H2SO4
NO2+ + H3O+ + 2HSO4–
The nitronium ion, NO2+, is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties it can undergo an autoprotolysis reaction, similar to the selfionization of water 2HNO3
NO2+ + NO3– + H2O
2.5.2 REACTIONS WITH METALS Nitric acid reacts with most metals but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese and zinc liberate H2. Others give the nitrogen oxides. (Ababio 2007) Nitric acid can oxidize non-active metals such as copper and silver. With these non-active or less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry to produce nitric oxide
28
which may react with atmospheric oxygen to give nitrogen dioxide. 3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometries. Cu + 4 H+ + 2 NO3− → Cu2+ + 2 NO2 + 2 H2O Upon reaction with nitric acid, most metals give the corresponding nitrates. Some metalloids and metals give the oxides, for instance, Sn, As, Sb, Ti are oxidized into SnO2, As2O5, Sb2O5 and TiO2 respectively. Some precious metals, such as pure gold and platinum group metals do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to color changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (< 14 carats) and to rapidly assess the gold purity. Being a powerful oxidizing agent, nitric acid reacts violently with many
non-metallic
compounds and the reactions
may
be
explosive. Reaction takes place with all metals except the noble metals series and certain alloys. As a general rule, oxidizing
29
reactions occur primarily with the concentrated acid, favoring the formation of nitrogen dioxide (NO2). (Ababio 2007) However, the powerful oxidizing properties of nitric acid are thermodynamic in nature,
but
sometimes
its
oxidation
reactions
are
rather
kinetically non-favored. The presence of small amounts of nitrous acid (HNO2) greatly enhances the rate of reaction. Although chromium (Cr), iron (Fe) and aluminum (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal oxide layer that protects the bulk of the metal from further oxidation. The formation of this protective layer is called passivation. Typical passivation concentrations range from 20– 50% by volume (ASTM A967-05 2000). Metals which are passivated by concentrated nitric acid are Iron, Cobalt, Chromium, Nickel, and Aluminum.
2.5.3 REACTIONS WITH NON-METALS Being a powerful oxidizing acid, nitric acid reacts violently with many organic materials and the reactions may be explosive. (Kent 1983) Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen, noble gases, silicon and halogens, usually oxidizes them to their highest oxidation states as acids with the
30
formation of nitrogen dioxide for concentrated acid and nitric oxide for dilute acid. (Ababio 2007) C + 4 HNO3 → CO2 + 4 NO2 + 2 H2O OR 3 C + 4 HNO3 → 3 CO2 + 4 NO + 2 H2O Concentrated nitric acid oxidizes I2, P4 and S8 into HIO3, H3PO4 and H2SO4 respectively. 2.5.4 XANTHOPROTEIC TEST Nitric acid reacts with proteins to form yellow nitrated products. This reaction is known as the xanthoproteic reaction (Gregory 1999). This test is carried out by adding concentrated nitric acid to the substance being tested, and then heating the mixture. If proteins that contain amino acids with aromatic rings are present, the mixture turns yellow. Upon adding a strong base such as liquid ammonia, the color turns orange. These color changes are caused by nitrated aromatic rings in the protein. Xanthoproteic acid is formed when the acid contacts epithelial cells and is indicative of inadequate safety precautions when handling nitric acid 2.6 USES 2.6.1 NITRIC ACID IN A LABORATORY. The main use of nitric acid is for the production of fertilizers. Nitric acid is neutralized with ammonia to give ammonium nitrate.
31
According to Gregory (1999, p.408) this application consumes 7580% of the 26M tons produced annually. The other main applications
are
for
the
production
of
explosives,
nylon
precursors, and specialty organic compounds. 2.6.2
PRECURSOR TO ORGANIC NITROGEN COMPOUNDS
In organic synthesis, industrial and otherwise, the nitro group is a versatile functionality. Most derivatives of aniline are prepared via nitration of aromatic compounds followed by reduction. Nitrations entail combining nitric and sulfuric acids to generate the nitronium
ion,
which
electrophilically
reacts
with
aromatic
compounds such as benzene. (Gregory 1999) Many explosives, e.g. TNT, are prepared in this way. The precursor to nylon, adipic acid, is produced on a large scale by oxidation of cyclohexanone and cyclohexanol with nitric acid. 1.6.3
ROCKET FUEL
Nitric acid has been used in various forms as the oxidizer in liquidfueled rockets. These forms include red fuming nitric acid, white fuming nitric acid, mixtures with sulfuric acid, and these forms with HF inhibitor. IRFNA (inhibited red fuming nitric acid) was one of 3 liquid fuel components for the BOMARC missile. (Gregory 1999) 2.6.4
ANALYTICAL REAGENT
32
In elemental analysis dilute nitric acid (0.5 to 5.0%) is used as a matrix compound for determining metal traces in solutions. Ultrapure
trace
metal
grade
acid
is
required
for
such
determination, because small amounts of metal ions could affect the result of the analysis. (Kirk 1981) It is also typically used in the digestion process of turbid water samples, sludge samples, solid samples as well as other types of unique samples which require elemental analysis via flame atomic absorption spectroscopy. Typically these digestions use a 50% solution of the purchased HNO3 mixed with deionized water. In electrochemistry, nitric acid is used as a chemical doping agent for organic semiconductors, and in purification processes for raw carbon nanotubes. 2.6.5
WOODWORKING
In a low concentration (approximately 10%), nitric acid is often used to artificially age pine and maple. The color produced is a grey-gold very much like very old wax or oil finished wood (wood finishing). 2.6.6
ETCHANT AND CLEANING AGENT
The corrosive effects of nitric acid are exploited for a number of specialty applications, such as pickling stainless steel. A solution of nitric acid, water and alcohol, Nital, is used for etching of metals to reveal the microstructure (Gregory 1999). Commercially
33
available aqueous blends of 5–30% nitric acid and 15–40% phosphoric acid are commonly used for cleaning food and dairy equipment
primarily
to
remove
precipitated
calcium
and
magnesium compounds (either deposited from the process stream or resulting from the use of hard water during production and cleaning). The phosphoric acid content helps to passivate ferrous alloys against corrosion by the dilute nitric acid.(Anon 1979) Nitric acid can be used as a spot test for alkaloids, giving a variety of colors depending on the alkaloid.
2.7 SAFETY Nitric acid is a strong acid and a powerful oxidizing agent. The major hazard posed by it is chemical burns as it carries out acid hydrolysis
with
proteins
(amide)
and
fats
(ester)
which
consequently decomposes living tissue (e.g. skin and flesh). Concentrated nitric acid stains human skin yellow due to its reaction with the keratin. These yellow stains turn orange when neutralized. Systemic effects are unlikely, however, and the substance is not considered a carcinogen or mutagen. The standard first aid treatment for acid spills on the skin is, as for other corrosive agents, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent secondary damage. Contaminated clothing is removed immediately and the underlying skin washed thoroughly. (Othmer et al 1981)
34
Being a strong oxidizing agent, reactions of nitric acid with compounds such as cyanides, carbides, metallic powders can be explosive and those with many organic compounds, such as turpentine, are violent and hypergolic (i.e. self-igniting). Hence, it should be stored away from bases and organics.
2.8 PINCH TECHNOLOGY IN MODERN PLANTS One of the most successful and generally useful techniques is that developed by Bodo Linnhoff and other workers: pinch technology. The term derives from the fact that in a plot of the system temperatures versus the heat transferred, a pinch usually occurs between the hot stream and cold stream curves. (Sinnot 2005) Pinch
technology
is
a
relatively
modern
engineering
tool
developed in the late 1970s and early 1980s. This new approach to evaluating the energy requirements of a site quickly identified ways of improving the overall energy use. The name “pinch technology” was applied because the technique identified the point or points in the energy flow where restrictions applied and hence limited one’s ability to reuse low grade energy. The major difference between this new technology and the previous
engineering
approaches
was
the
formalized
methodology involving the rigorous application of thermodynamic principles. Pinch technology was initially adopted by major chemical companies and petrochemical energy. Beet sugar was quite quick to adopt it because of the industry’s energy profile and it is now being adopted by the cane industry too. It has also been shown that the pinch represents a distinct thermodynamic
35
break in the system and that, for minimum energy requirements, heat should not be transferred across the pinch, (Linnhoff et al 1983) 2.8.1
APPLICATIONS
Pinch technology is equally applicable to Greenfield project and refurbishments. In either case, their objectives are to achieve: 1. Minimum energy consumption 2. Optimization of utilities 3. Minimum capital expenditure to achieve these Minimizing energy consumption implies minimizing cooling water requirements too because all of the energy used ultimately has to be rejected again in some low grade form. ( Sinnot 2005) The technology strength are its overall approach to process integration (rather than optimizing a single station) and its blend of thermodynamics with commercial requirements. It also takes into account the operational requirements of the site and does reduce flexibility or availability.
2.9 PLANT LOCATION Plant location refers to the choice of a region or the selection of a particular site for settling up the business or a factory. However, the choice is made only after considering alternative sites. It is a strategic decision that cannot be changed once it is taken.
36
Therefore, careful care must be taken before a decision is made on the location of the plant site (Ray et al 1989). 2.9.1
IDEAL PLANT LOCATION
An ideal plant location is one where the cost of the production is minimal, with a large market availability, least risk involved and maximum gain obtainable. It is a place of maximum net advantage or with lowest unit cost of production and distribution. For achieving this objective, small and large scale entrepreneur can make use of local analysis. 2.9.2
LOCAL ANALYSIS
Local analysis is a dynamic process where the entrepreneur analyses and compares the feasibility of different sites with the aim of selecting the best site for a given enterprise. It considers the following: a. Demographic analysis: it involves the study of the population in the area in terms of total number of people in the area, age composition, per capital income, educational level and occupational structures etc. b. Trade area analysis: it is an analysis of the geographic area that provides continued clientele to the industry. It is advisable to also see the feasibility of accessing the trade area from alternative sites. (Ray et al 1989) c. Competitive analysis: it helps to judge the nature, location, size and quality of competition in a given trade area.
37
d. Traffic analysis: this is done to have a rough idea about the number of potential customers passing by the proposed site during the working hours of the industry. The traffic analysis aims at judging the alternative sites in terms of pedestrian and vehicular traffic passing by the site. e. Site economics: alternative sites are evaluated in terms of establishments, costs and operational costs under this. Cost of establishment of a plant is basically cost incurred for permanent physical facilities but operation costs are incurred for running the plant.
2.9.3 SELECTION CRITERIA According to Ray (1989, p. 76) the important considerations for selecting a suitable location are as follows: I. II. III.
Nature or climate conditions Availability and nearness to the sources of raw materials Transport costs: this should be considered both for obtaining raw material and also distribution or marketing
IV.
finished products to the ultimate users. Close proximity to the anticipated market: the industry’s warehouse should be located within the vicinity of densely
V.
populated areas. Availability of infrastructural facilities such as developed industrial shed or site, link roads, nearness to railway stations, airports or seaports, availability of electricity,
38
water, public utilities, civil amenities and means of VI.
communication are important. Availability of skilled and non-skilled labor and technically
VII.
qualified and trained managers. Banking and financial institutions should be located
VIII. IX.
nearby. Safety and security should be given due consideration Government influences: tax relief, subsidies, liberation and other positive policies of the government to support the start off of any industry should be duly considered before any industry is set up. Also, negative government influences like restrictions for setting up industries in an area for reason of pollution control and decentralization of industries should be considered. Utility costs and availability.
X. 2.9.4
SELECTION OF PLANT LOCATION FOR THE NITRIC
ACID PLANT There were three plant locations proposed. Each was evaluated and the final decision based on maximum net advantage was made. 2.9.4.1
LOCATION
ONE:
AGBARA
INDUSTRIAL
ESTATE
(OGUN STATE) Advantages 1. Relatively cheap available land and labor cost. 2. Relatively close to market (Lagos Nylon and plastic market). 3. Relatively close to sea (Lagos Apapa) for import of raw material and export of product if need be.
39
4. Availability of infrastructural facilities such as link roads, 5. 6. 7. 8. 1.
public utilities etc. Availability of financial institution. Relatively secure. Availability of social amenities and means of communication. Disadvantages No local source of raw material nearby meaning all raw
materials have to be transported to the plant location. 2. The major roads that will be used for transportation (i.e form Apapa to Agbara) are bad and one is prone to experience hold up on it. 3. Transport cost will be very high for both bringing in of raw material and marketing finished product as the target market is Lagos and things are known to be very expensive there. 4. The Nylon and plastic market in Lagos is not large enough to exhaust all nitric acid produced by the plant. 5. Additional cost of providing water and electricity for the plant. 2.9.4.2
LOCATION TWO: ABA (ABIA STATE)
Advantages 1. 2. 3. 4. 5.
Relatively cheap available land and labor cost. Availability of market (plastic and Nylon market) Availability of financial institution. Relatively secure. Availability of social amenities and means of communication.
Disadvantages
40
1. Not close to source of raw material 2. Additional cost of providing water and electricity for the plant. 3. Market available not enough to exhaust all nitric acid produced in the plant. 4. Lack of infrastructural facilities such as sea port, airport and railway stations nearby. 2.9.4.3
LOCATION
THREE:
ELEME,
PORT-HARCOURT
(RIVERS STATE) Advantages 1. Close to source of raw material: National Fertilizer Company of Nigeria (NAFCON), an ammonia and fertilizer plant at Onne, Port-Harcourt, Rivers State bought over by Notore started operation in Jan 2009. Their production of ammonia per day of ammonia was 1,000MT as at 2009 of anhydrous ammonia (more than enough raw material for our nitric acid plant). Eleme Petrochemical located in Eleme, Port-harcourt, Rivers State is also billed to come up with an ammonia plant in 2014 which will make available to the Nigeria market 2300MT. 2. Availability of market in Port-Harcourt, closeness to sea for export of product if necessary. 3. Availability of public utilities such as water, sea port, airport, 4. 5. 6. 7.
etc. Availability of both skilled and unskilled labor. Availability of banking and financial institutions. Availability of social amenities and means of communication. Relatively secure.
41
Disadvantages 2. High cost of land 3. No regular power supply
2.9.5
PLANT LAYOUT
Having selected a suitable site for the chemical plant, it is possible and necessary to make a preliminary decision regarding the layout of the plant equipment. (Ray et al 1989) Although the equipment has not been designed in detail, preliminary estimates of the physical size of each item should be available in the equipment list. Any sizing differences between the initial and final estimates should not be too excessive, and appropriate areas should be allowed around the plant items when determining the layout. A
preliminary
determination
of
the
plant
layout
enables
consideration of pipe runs and pressure drops, access for maintenance and repair and in the event of accidents and spills, and location of the control room and administrative offices. The preliminary plant layout can also help to identify undesirable and unforeseen problems with the preferred site, and may necessitate a revision of the site selection. (Baasel 1989) The proposed plant layout must be considered early in the design work, and in sufficient detail, to ensure economical construction and efficient operation of the completed plant. The plant layout adopted also affects the safe operation of the plant, and acceptance of the
42
plant (and possibly any subsequent modifications or extensions) by the community. There are two schemes that can be adopted for determination of the plant layout. (Buckhurst & Harker 1973) First, the ‘flowthrough’ layout (or ‘flow-line’ pattern) where plant items are arranged (sequentially) in the order in which they appear on the process flow sheet. This type of arrangement usually minimizes pipe runs and pressure drops (and is often adopted for small plants). Second, the equipment is located on site in groupings of similar plant items, e.g. distillation columns, separation stages, reactors and heat exchanger pre-heaters, etc. The grouped pattern is often used for larger plants and has the advantages of easier operation and maintenance, lower labor costs, minimizing transfer lines and hence reducing the energy required to transfer materials. These two schemes represent the extreme situations and in practice some compromise arrangement is usually employed. The plant layout adopted depends upon whether a new (‘grass
roots’)
plant
is
being
designed
or
an
extension/modification to an existing plant. Space restrictions are the most common constraints; however, space limitations are usually imposed even with new sites. Other factors to be considered are: (a) Siting of the control room, offices, etc., away from areas of high accident risk, and upstream of the prevailing winds.
43
(b) Location of reactors, boilers, etc., away from chemical storage tanks. (c) Storage tanks to be located for easy access, and a decision made as to whether all tanks (for raw materials and product) should be located together or dispersed around the site. (d) Labor required for plant operation. (e) Elevation of equipment. (f) Requirements of specific plant items, e.g. pumps. (g) Supply of utilities, e.g. electricity, water, steam, etc. (h) Minimizing plant piping systems. (i) Suitable access to equipment requiring regular maintenance or repair. (j) Plant layout to facilitate easy clean-up operations and dispersion of chemicals in the event of a spillage. (k) Access to the plant in the event of an accident. (1) Siting of equipment requiring cooling water close to rivers, estuaries, etc. (m) Location of plant waste and water drainage systems (separate or combined?) and treatment tanks. (n) Adopting a plant layout that will act to contain any fires or explosions.
44
(o) Spacing between items of equipment (insurance companies specializing in the insurance of chemical plants have specific recommendations for the distances required between particular items of equipment). The layout of plant equipment should aim to minimize: (i) damage to persons and property due to fire or explosion; (ii) Maintenance costs; (iii) Number of plant personnel; (iv) Operating costs; construction costs; (v) Cost of plant expansion or modifications. Some of these aims are conflicting, e.g. (i) and (iv), and compromises are usually required when considering the plant layout to ensure that safety and economic operation are both preserved. The final plant layout will depend upon the measures for energy conservation within the plant and any subsequent modifications, and the associated piping arrangements. The process units and ancillary buildings are laid out in such a way to give the most economical flow of materials and personnel around the site. Hazardous processes are located a safe distance from other buildings. Consideration for future expansion is also put in place. The ancillary buildings and service required on the site include: Administrative block
45
Laboratory Storage for both raw materials and products Maintenance workshop Utilities (generator, steam boiler, transformer station) Store for maintenance and operation supplies Other amenities like car park, restaurant and clinic.
Fire Station
Tank Farm
Emergency Water
Plant Area Workshop
Stores
Roads Canteen Medical Center Waste Incinerator
Utilities
Car Pack
Offices
Auditorium
Roads
Fig 1.1 Expected plant layout.
2.9.6
PROCESS ROUTES FOR THE PRODUCTION
OF NITRIC ACID CHILE SALTPETRE/NITRATE PROCESS
46
Chile saltpetre is material which contains sodium nitrate NaNO 3 with percentage around 35-60%, and remaining percentage compounds with KNO3 and NaCl. This raw material Chile saltpetre is concentrated by crystallization in pre-treatment of ore to attain 95% NaNO3 and remaining KNO3 as feed raw material. (Kent 1983) Sulphuric acid with 93% is mixed with the refined Chile saltpetre as per the ratio required as per stoichiometry and sent into a retort which is made with cast iron and the mixture is heated to 200oC with help of furnace flue gasses and coal fire. Thus at this temperature, the following reaction is carried forward to produce HNO3, nitric acid vapors. NaNO 3 + H2SO4 → NaHSO4 + HNO3 All hot vapors of nitric acid are sent to cool down in water circulated cooled silica pipes, condensed HNO3 are collected in receiver which has material resistance to nitric acid. Uncondensed gas which escapes from the collector is scrubbed with cooled water in packed bed tower to collect nitric acid in dilute format. Liquid sodium bi-sulphate is collected from the bottom outlet of the retort. Advantage: it was one of the first methods used in the manufacture of nitric acid. Disadvantage: source of raw material can be exhausted.
47
Fig 1.2: Manufacture of nitric acid from Chile Saltpetre.
BIRKELAND-EYDE PROCESS/ARC PROCESS This process is based upon the oxidation of atmospheric nitrogen by atmospheric oxygen to nitric oxide at very high temperature. An electric arc is used to provide the high temperatures, and yields of up to 4% nitric oxide were obtained. ( Ohrue 1999) N2 + O2 →2NO The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to nitrogen dioxide 2 NO + O 2 →2NO2 This nitrogen dioxide is then dissolved in water to give dilute nitric acid.
48
3 NO2 + H2O → 2HNO3 + NO Advantage: unlimited source of raw material (air) Disadvantage: The process is very energy intensive and is only feasible when electricity is available and cheap. WINSCONSIN PROCESS/NITROGEN FIXATION PROCESS Atmospheric oxygen and nitrogen are combined in a high temperature regenerative furnace operating at about 2000 oC. Nitric oxide is formed with a yield of nearly 2%. Advantage: it does not use electricity to provide the high temperature and therefore does not have the disadvantage of the Birkeland-Eyed process. Disadvantage: cannot compete favorably with the Ostwald process. Another method of production of nitric acid via nitrogen fixation is the nuclear nitrogen fixation route. This method directly combines oxygen and nitrogen. Yields of nitrogen oxide of 5-15% have been reported by exposing air at 150 and 400 oF to radiation from Uranium 235. Advantage: gives a greater yield of nitrogen oxide than the Winsconsin process Disadvantage: with this method comes all the disadvantages of nuclear reaction (problem of managing the radiation which is harmful to living things)
49
OSTWALD PROCESS In this process, anhydrous ammonia is oxidized to nitric oxide, in the presence of platinum or rhodium gauge catalyst at high temperature of about 500K and a pressure of 9bar. (Ray et al 1989) 4 NH3 (g) + 5 O2 (g) → 4 NO
(g)
+ 6 H2O
(g)
(∆H= -905.2KJ)
Nitric acid is then reacted with oxygen in air to form nitrogen dioxide. 2 NO
(g)
+ O2 (g) → 2NO2 (g)
(∆H= -114KJ/mol)
This is subsequently absorbed in water to form nitric acid and nitric oxide 3 NO2 (g) + H2O
(l)
→ 2 HNO3 (aq) + NO
(g)
(∆H= -117KJ/mol)
The nitric oxide is cycled back for re-oxidation. Alternately, if the last step is carried out in air: 4 NO2 (g) + O2 (g) + 2H2O
(l)
→ 4HNO3 (aq)
The aqueous HNO3 obtained can be concentrated by distillation up to about 68% by mass. There are 2 basic types of systems used to produce weak nitric acid: Both processes follow the basic Ostwald process for the catalytic oxidation of ammonia. In summary, this involves an oxidation stage whereby ammonia is reacted with air in a catalytic
50
converter at temperatures in the range of 850-950°C. Reaction gases pass through a series of energy recovery stages before entering an absorption column. The bottoms from the column are bleached of dissolved nitrogen peroxide using air, and the resulting solution is the weak nitric acid product (Roudier et al 1979). The major difference between the two processes lies in the initial conversion
stage.
The
dual-pressure
process
employs
a
conversion stage operating in the range l00-350kPa, and a reactor temperature of about 865°C. The single-pressure process however operates
the
converter
at
800-1100
kPa,
with
a
reactor
temperature closer to 940°C. ( Harvin et al 1979) 1. Single-stage pressure process: in this case, the plant is operated at a single pressure throughout.
51
Fig 1.3. Process flow diagram for single-stage pressure process. Advantage: Less expensive as less equipment’s are used. The single-pressure process uses a higher ammonia conversion
pressure.
This
higher
pressure
provides
advantages in terms of equipment design, e.g. smaller converter dimensions and a single heat-exchanger-train layout.( Leray et al 1979) The higher temperature and the favorable pressure both increase the energy recovery from the process. Limited space availability may favor the single-pressure process Disadvantage:
52
Less efficient as the overall process is favored by varying pressure. Experimental work indicates that the rate loss of catalyst (without a catalyst recovery system) is approximately three times more rapid at 973°C than at 866°C. This means that more catalyst is lost in the single-stage pressure process ( Harvin et al 1979). Absorber efficiency is reduced prompting the need for larger absorber thereby increasing cost. 2. Dual-stage pressure process: here, the plant is operated at different pressures and different stages. Advantages: The first reaction (catalytic conversion
of
anhydrous
ammonia to nitric oxide) is favored by lower pressure while the remaining reactions are favored by higher pressures. This variation in pressure is achieved in dual-stage pressure process. (Harvin et al 1979) Capacities of 1130-1360 tonnes per day favor the dualpressure process, because of the possibility of absorption up to 1550 KPa. Less catalyst is lost because of lower operating temperature
53
Fig 1.4.
Process flow diagram for dual-stage pressure process.
The process selected for this design of nitric acid is single-stage pressure Ostwald process because of its above mentioned advantages.
54
Fig1.5: Selected Process flow diagram for Nitric acid plant.
CHAPTER THREE
55
MATERIAL BALANCE Material balance is one of the most important components of a process design. Overall raw material of the entire process determines the qualities of raw materials required and the products produced in the process. Balance over individual process units determines the process stream flows and their compositions and also the sizes of the various process equipment used in the process. Material balance on the plant used in the production of 400000 tonnes of Nitric acid per year. Mass flow rate = 400000 x 1000
3.1
kg year
kg = 50000 hr
CONSERVATION OF MASS
For a steady state process, the accumulation term will be zero; but if a chemical reaction takes place, particular chemical specie may be formed or consumed in the process. When there is chemical reaction, the material balance equation is given as, Input + Generation = Output + Consumption If there is no chemical reaction, the steady state balance reduces to; Input = Output
56
A balance equation can be written for any identifiable specie present, elements or compound; and for the total material.
3.2
METHODS OF MATERIAL BALANCE
There are two basic methods of material balance and they are; (a)
Algebraic Method
The algebraic method of material balancing is one of the simplest and most common methods applied in balancing the materials that flow through a system. It involves the systematic and sequential technique in indentifying some variable sets which are related by some sets of linear or non-linear equations whose solution depends on the resulting degree of freedom for the system. This degree of freedom provides us with the limit of freedom for which we can set values for some of the variable which is referred to as the design variables. A choice of values for the design variables result in a corresponding value for the remaining variables. The solutions to the equation set are obtained by the various method of solution for simultaneous equations, most appreciably the methods of substitution and elimination. The algebraic method is most efficient for simple system but it may be inappropriate for complex systems involving large number of units.
The split fraction and method is
recommended for such systems. (b). Split Fraction Method
57
This method is based on the theory of recycle processes published by Magier (1964). The method is based on the realization that the basic function of most chemical processing units (Unit Operation) is to divide the inlet flow of a component between two or more outlet streams. This method is ideal in carrying out material balancing of complex of multi-unit plants.
3.3
MATERIALS BALANCE ASSUMPTIONS
The following assumptions were made during the material balance calculations: 1. The system is operating at steady state i.e. there is no accumulation of any sort in the system. 2. There is negligible amount of inert in the process air. 3. Reasonably high conversion in the reactors. 4. Effect of side reactions is minimal.
3.4
SUMMARY
OF
MATERIAL
BALANCE
CALCULATIONS From the steady state material balance equation, the flow rates of each stream are calculated as follows.
3.5
MATERIAL BALANCE FOR EACH UNIT
Basis: 1hr THE COMPRESSOR 1a 1a Stream 1
Stream 2
58
Stream 3
Components O2 N2 Total
Stream 1( kg/hr) Composition
Stream
49720. 187080 236750
1a( kg/hr) 49720. 187080 236750
O2 N2 Total
THE MIXER Stream 2
Stream 5
Stream 4
59
Components
Stream 2(Kg/hr) Stream
4 Stream 5(Kg/hr)
(Kg/hr) O2
42760
-
42760
N2
160860
-
16086 13500
NH3
-
13500
H2O
-
65
Total
203620
TOTAL
13565 217185
65 217185 217185
THE COVERTER
Stream 5
Stream 7
Stream 6
Composition
Stream 5
Stream 6
Stream 7
60
O2 N2 NH3 H2O NO HNO3 NO2 Total TOTAL
42760 160860 13500 65 217185 217185
270 21060 21330
11660 160860 23320 195840 217170
OXIDISATION VESSEL
Stream 7
Stream 8
Component
Stream 7
Stream
O2 N2 NH3 H2O NO HNO3 NO2 Total
(Kg/hr) 11660 160860 23320 195840
(Kg/hr) 160860 1460 33530 195850
8
61
ABSORBER
Stream 6 + Make -up water
Stream 9
Stream 8
Component
Stream 6 + Stream make-up
8 Stream 9
(Kg/hr)
(Kg/hr)
160860 1460 33530 195850
160860 270 23830 8600 30000 672 224232
water O2 N2 NH3 H2O NO HNO3 NO2 Total
(Kg/hr) 270 28120 28390
62
TOTAL
224240
STRIPPER
224232
Stream 10
Stream 9
Stream 3
Component O2 N2 NH3 H2O NO HNO3 NO2 Total TOTAL
Stream 9 (Kg/hr) 160860 270 23830 8600 30000 672 224232
Stream 11
Stream 3 (Kg/hr) 6960 26170 33130 257362
Stream 10 (Kg/hr) 4030 160860 4260 580 7260 176990
Stream 11 (Kg/hr) 30000 20000 50000 226990
63
COMBUSTION CHAMBER Stream 13 Stream 10
Stream 12
Component O2 N2 NH3 H2 H2O NO HNO3 NO2 CH4 C2H6 CO2 Total TOTAL
Stream 10 (Kg/hr) 4030 160860 4260 580 7260 176990
Stream 12 37370 140580 177950 354940
Stream 13 (Kg/hr) 2440 301440 330 26710 410 7520 860 50 24480 364240 364240
64
PURIFICATION REACTOR
Stream 13
Stream 14
Component
Stream 13
Stream 14
O2 N2 NH3 H2 H2O NO HNO3 NO2 CH4 C2H6 CO2 Total
(Kg/hr) 2440 301440 330 26710 410 7520 860 50 24480 364240
(Kg/hr) 2670 303790 170 30230 20 380 27000 364260
Table 3.1: Summary of Material balance on each stream. STREA MS
COMPONENT FLOW RATE (Kg/hr)
TOTA L
65
1 1a 2
O2
N2
4972 0 4972 0 4427 60
1870 80 1870 80 1608 60 2617 0
NO2
CH C2 H6 4
CO2
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
65
-
-
-
-
-
-
-
-
NH3
H2O
NO
-
-
-
-
-
-
3
6960
4
-
-
5
4276 0
1608 60
135 00 135 00
6
-
-
270
210 60
7
1166 0
-
-
8
-
-
-
9
-
10
4030
11
-
12
3737 0
13
2440
14
2670
1608 60 1608 60 1608 60 1608 60 3000 0 1405 80 3014 40 3037 90
270
-
HNO 3
65
238 30 426 0 200 00 267 10 302 30
-
-
-
-
-
-
-
-
-
-
-
-
335 30
-
-
-
300 00
670
-
-
-
580
-
726 0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
410
-
752 0
86 0
50
20
-
380
-
-
233 20 146 0 860 0
244 80 270 00
Table 3.2: Process matrix of the Nitric acid production process EQUIPMENT
ASSOCIATE
2367 50 2367 50 2036 20 3313 0 1356 5 2171 80 2133 0 1958 40 1958 50 2242 32 1769 90 5000 0 1779 50 3642 40 3642 60
66
NUMBER 1 2 3 4 5 6 7 8 9
NAME COMPRESSOR SPLITTER MIXER CONVERTER OXIDISATION UNIT ABSORBER STRIPPER COMBUSTION CHAMBER PURIFICATION REACTOR
STREAMS 1, -1a 1a, -2, -3 2,4, -5 5, -6, -7 7, -8 6, 8, -9 3, 9, -10, -11 10, 12, -13 13, -14
CHAPTER FOUR ENERGY BALANCE The Energy balance gives the account of all the energy requirement of the process which is based on the principle of conservation of energy. The principle states that energy can either be create nor destroyed but can be transformed from one form to another. Also energy can be transferred from one body to another. If a plant uses more energy than its competitor, its product could be priced out of the market.
Accountability of the energy
utilization of a process plant is necessary in every design project. The conservation of energy however differs from the mass in that energy can be generated (or consumed) in a chemical process.
67
Material can change form; new molecular specie can be formed in a process unit and must be equal to the one out at steady state. The same is not true for energy. The total enthalpy of the outlet stream will not be equal to that of the inlet stream if energy is generated or consumed in the processes, such as that due to heat of reaction. Energy can exist in various forms: head, mechanical, electrical energy, and it is the total energy that is conserved. In plant operation, an energy balance on the plant will show the patterns of energy usage and suggest area for conservation and saving.
4.1
CONSERVATION OF ENERGY
As for materials balance, a general equation can be written for energy balance; Energy
out – Energy in
+
Generation
– Consumption
=
Accumulation This is a statement of the first law of thermodynamics. An energy balance can be written for any process step. Chemical reactions will
evolve
energy
(exothermic)
or
consume
energy
(endothermic). For steady state processes, the accumulation of both mass and energy will be zero (0).
68
Energy exists in many forms; the basic forms are listed below: Potential Energy: This is due to position or height due to motion Internal Energy: This is the energy associated with molecules and is dependent on temperature. Work: This is achieved when a force gets through a distance. Work done on a system is positive while work done by a system is negative Kinetic Energy: This is the energy due to motion.
W
Q
Z1 For unit mass of material 2
Z2 2
U U U 1 + P1 V 1 + 1 + Z 1 g+Q=U 2+ P2 V 2 + 2 + Z 2 g+W g g
Where,
Q = Heat transferred across the system boundary W = Work done by the system P1P2 = Pressure in Pressure Out V1V2 = Volume in, Volume out
69
U1U2 = Velocity in, Velocity out Z1Z2 = Height in, Height out g = Acceleration due to gravity (9.81m/s 2) In chemical processes the kinetic energy factor
U2 g
( )
and the
Potential energy factor (zg) are small and negligible and the relation between U and PV is correlated in terms of enthalpy (H) H = U + PV H2 – H1 = Q – w Also, the work term can be negligible in many chemical engineering systems. Hence, H 2 - H1 = Q
4.2
ENERGY BALANCE ASSUMPTIONS
1.
The process is at steady state
2.
No heat is lost from the vessel and from the pipe i.e. there is proper lagging.
3.
Effect of pressure on enthalpy is ignored .
4.
Potential and kinetic energy changes are negligible.
70
4.3 SUMMARY OF ENERGY BALANCE THE COMPRESSOR
Tin= 20°C
Tout=155°C
TABLE 4.1: HEAT BALANCE AROUND COMPRESSOR. Component N2 O2
nin (Kg/hr) 187030 49720
Hin (KJ/Kg) 0 87.56
nout (KG/hr) 187030 49720
Hout (KJ/Kg) 140.4 87.56
PROPERTIES QUANTITY/VALUE Inlet Temperature( °C ) 20 Outlet Temperature( °C ) 155 Heat duty( KJ/hr ) 26259012 Power and Actual Shaft work, 399515.49 and repectively.(KJ/hr and KJ) 475613.68 TABLE 4.2 HEAT BALANCE ABOUT THE AIR HEATER For air component that passes through the air heater Component nin (Kg/hr) N2 187030 O2 49720 Inlet Temperature (oC) Outlet Temperature (oC) Heat Duty( KJ/hr )
Hin (KJ/Kg) nout (KG/hr) 0 187030 152.49 49720 155 200 15107946.75
Hout (KJ/Kg) 80.79 152.49
For nitrous gases recycled back to the air heater
71
PROPERTIES Inlet Temperature( °C ) Outlet Temperature( °C ) Component nin (Kg/hr) N2 160860 NO 1460 NO2 33530 Heat Duty( KJ/hr )
QUANTITY/VALUE 350 200 Hin (KJ/Kg) nout (KG/hr) -161.10 16080 0 1460 -196.5 3350 -226460.6
Hout (KJ/Kg) -161.10 -155.11 -196.5
TABLE 4.3 HEAT BALANCE AROUND THE CONVETER Component nin (Kg/hr) NH3 13500 O2 42760 NO2 N2 160860 H2O 65 Heat Duty( KJ/hr )
Hin (KJ/Kg) nout (KG/hr) 0 270 612.58 11660 23320 693.63 160860 1309.44 21060 20579273.83 KJ/hr
Hout (KJ/Kg) 1902.99 612.58 610.38 693.63 1309.44
TABLE 4.4 HEAT BALANCE AROUND THE WASTE HEAT BOILER (Unit 9) Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg) O2 11660 -669.8 11660 -669.8 N2 160860 -719.25 160860 -719.25 NO 23320 0 23320 -685.09 PROPERTIES QUANTITY/VALUE Inlet Temperature( °C ) 890 Outlet Temperature( °C ) 250 Heat Duty( KJ/hr ) -15976252 Outlet Temperature of Steam 410 (°C )
72
TABLE 4.5 HEAT BALANCE AROUND THE OXIDIZING VESSEL Component O2 N2 NO NO2
nin (Kg/hr) 11660 160860 23320 -
Hin (KJ/Kg) 100.27 107.80 0 -
nout (KG/hr) 160860 1460 33530
Hout (KJ/Kg) 107.80 103.96 131.00
PROPERTIES Inlet Temperature( °C ) Outlet Temperature( °C ) Heat Duty( KJ/hr )
QUANTITY/VALUE 250 350 1240891.54
TABLE 4.6 HEATER
AROUND
Component N2 NO NO2 PROPERTIES
HEAT
BALANCE
nin (Kg/hr) 160860 1460 33530
THE
STACK
Hin (KJ/Kg) nout (KG/hr) -52.909 160860 0 1460 -65.50 33530 QUANTITY/VALUE
Inlet Temperature( °C ) Outlet Temperature( °C ) Heat Duty( KJ/hr ) Heat Duty of Steam( KJ/hr ) Temperature of Steam (°C )
GAS
Hout (KJ/Kg) -52.909 -719.25 -65.50
150 50 -149781.4 -74322.76 118.48
TABLE 4.7 HEAT BALANCE AROUND THE ABSORPTION COLUMN
73
Component NH3 H2O N2 NO HNO3 NO2
nin (Kg/hr) 270 28120 160860 1460 33530
PROPERTIES Inlet Temperature( °C ) Outlet Temperature( °C ) Heat Duty( KJ/hr )
Hin (KJ/Kg) 0 117597.84 167616.12 1437.66 43924.39
nout (KG/hr) 270 23830 160860 8600 30000 670
QUANTITY/VALUE 50 54 -53280.03
Hout (KJ/Kg) 576.48 99657.0 167616.12 8586.24 51600.0 880.32
74
HEAT BALANCE AROUND THE SUPERHEATER AND STRIPPER.
AMMONIA
AMMONIA VAPORIZER Heat Duty = 14978 KJ/hr Outlet Temperature = -28.20 °C THE AMMONIA SUPPERHEATER Heat Duty = -1596252 KJ/hr Outlet Temperature = 26.65 °C THE STRIPPER Heat Duty = -25873200 KJ/hr Inlet Temperature = 250 °C Outlet Temperature = 120°C
VAPORIZER,
75
CHAPTER FIVE CHEMICAL ENGINEERING DESIGN The equipment used in chemical process industries can be divided into
two
centrifuge,
classes: etc
proprietary
which
are
equipment
designed
and
such
as
pumps,
manufactured
by
specialist firms; non-proprietary equipment which includes the reactor, heat exchanger, evaporators, still, condensers and bleaching vessels. The proprietary equipment will only be selected and specified while the non-proprietary equipment will be designed as special, one-off, items for the particular processes and purposes they are expected to serve. The chemical Engineer’s part in the design of “non-proprietary” equipment is usually limited to “selecting” and “sizing” the equipment. Same will be done in this design work.
5.1 PROCESS UNITS OF NITRIC ACID PRODUCTION PLANT The nitric acid process plant comprises: 1.
Ion- Exchange Unit
This unit consists of series of packed beds containing various organic polymer resins for the removal of unwanted divalent and monovalent ions. Used for the generation of de-ionized water.
76
2.
De-ionized water Cooler
Consist of finned fan-type cooler for cooling the circulating deionized water.
3.
Air Compressor
Here air is compressed in two stages. The first-stage compression is a low-pressure compression from atmospheric pressure up to 310 kPa. An axial compressor is used which takes its shaft drive from a gas turbine. The second compression utilizes a centrifugaltype compressor. The centrifugal compressor is more efficient for the air flow-rate (36 000 kg/h) and outlet pressure (1090 kPa). The centrifugal compressor takes its shaft drive from the expansion of tail gas. Intermediate to the two compression stages is an intercooler which allows the air temperature to be lowered from 180°C to 45°C, with a pressure loss of 10 kPa. The temperature drop enables a more efficient second compression stage. 4.
Ammonia Vaporizer
This unit consists of a shell and tube-type heat exchanger with two passes per shell on the tube side. Operating pressure is 1240 kPa. The exchanger is made from mild steel. 5.
Ammonia Super-heater
77
It consists of a shell and tube-type heat exchanger of similar mechanical
construction
to
the
ammonia
vaporizer.
It
is
constructed from mild steel. 6.
Reactor
The reactor is a pressure vessel operating in the range 1050 kPa to 1100 kPa. The bottom section of the reactor is jacketed. Air is preheated in this jacket prior to mixing with ammonia. The bottom section of the reactor also contains a shell and tube-type heat exchanger. This exchanger provides the final stage of tail-gas preheating. Tail gas enters at 235°C and the reaction gases leave the exchanger section of the reactor at 645°C. 7.
Steam Super-heater
This unit superheats saturated steam from 250°C (and 4000kPa) to 380°C. The product steam is of medium pressure and suitable quality for ‘in-house’ application and also for export. The superheater cools the reaction gases from the reactor exit temperature of 645°C to 595°C. 8.
Waste-heat Boiler
A shell and tube-type exchanger required to heat pressurized (4000 kPa) hot water from 117°C to a saturated vapour at 250°C. The waste-heat boiler cools reaction gases from 595°C to 280°C. 9.
Tail-gas Pre-heater
78
Also comprises of shell and tube-type exchanger. It takes reaction gases leaving the platinum filter at about 315°C and 1020 kPa, and subsequently reduces their temperature to 185°C. The cooling medium is tail gas. It enters at about 50°C and leaves the tail-gas pre-heater at 235°C. 10. Cooler/Condenser This unit condenses weak nitric acid from the gaseous mixture and cools the remaining gases from an inlet temperature of 185°C to 60°C. The shell and tube-type heat exchanger uses de-ionized water as its cooling medium.
11. Oxidation Unit The oxidation unit is an empty pressure vessel that takes input reaction gases and blends in additional air from the bleaching column. The extra oxygen provided enables further oxidation to occur and raises the gas mixture temperature to 140°C. At the top of the oxidation unit is a mist eliminator to prevent carry-over of acid vapor by entrainment. At the bottom of the vessel is the weak-acid drain. 12. Secondary Cooler The secondary cooler takes the exit gases from the oxidation unit at 140°C and cools them down to 65°C, a suitable temperature for entry into the absorption column. The cooling medium is
79
circulating warm water from the warm-water loop. The inlet temperature is 50°C and the exit temperature is about 80°C. 13. Absorber The absorber is usually a sieve tray-type column. It has an operating pressure around 990 kPa. A bursting disc is used for pressure relief. Each tray is provided with cooling coils to allow the cooling of the absorption liquor. There are two independent cooling circuits, each uses de-ionized water. The top section has an inlet temperature of 7°C and an outlet temperature of 20°C. The bottom section cooling loop has an inlet temperature of 20°C and an exit of 40°C. The use of two cooling circuits provides greater flexibility in manipulating absorption conditions in the column. The tail gas leaves the column at about 10°C. Weak acid from the cooler/condenser is added to an appropriate tray midway up the column, and make-up water at 7°C is added to the top tray. The acid drained from the bottom of the column contains some dissolved nitrogen oxides. 14. Stripping Column The bleaching column is a smaller sieve tray-type column. Impure acid runs down the column from the top tray and air is bubbled up through the liquor to remove dissolved nitrogen oxides. The acid from the base of the column is the final desired 60% (wt.) product. 15. Storage Tank
80
Stores the supply of nitric acid produced from the process plant.
CHAPTER SIX EQUIPMENT DESIGN The need to design process equipment may arise as a result of the desire to: i. ii.
Modify an existing process equipment or Develop new equipment.
81
Modification of existing equipment may be required as a result of poor performance or the need of scale up (or down). For example, increased market success of a product may lead to increased production. It may be more economical to increase the capacity of the
existing
equipment
rather
than
add
another
line
of
equipment. This is usually the case when operational cost costs (man power, energy etc.) are high. New equipment, on the other hand may be desired as a result of successful laboratory research and pilot plant studies or as a result of satisfactory process simulation using the computer. In either situation (new or existing equipment), the actual design commences with the assessment of the characteristics of the feed materials, the products and the physical and chemical processes required to convert the raw material to products. The overall satisfactory performance and reliability of the equipment would depend on the following factors. I. II. III. IV. V. VI. VII. VIII. IX.
Optimum processing conditions Appropriate materials of construction Strength and rigidity of components Satisfactory performance of mechanical part Reliable methods of fabrication Ease of maintenance and repairs Ease of operation and control. Safety requirements Environmental impact
The typical process equipment design procedure will involve: 1. Specifying the problem
82
2. Analyzing the probably solution 3. Preliminary design, applying chemical engineering process, principles and theories of mechanics relevant to the problem. 4. Selecting appropriate materials of construction. 5. Evaluating and optimizing the design, the
possible
application of computer aided design (CAD) system like HYSIS, Aspen Plus etc 6. Preparing the drawings and specifications
6.1. PROBLEM SPECIFICATION The specification of the problem is the key stone in the quest to design an equipment to meet the needs of the customer. Specification of a problem may include: 1. The quantity of material to be processed in a given time such as the proposed capacity of the equipment. 2. The physical and chemical properties of the product.
Constraints such as: a. b. c. d.
Availability and cost of materials of construction Availability and cost energy, water, oil etc. Budget for production Availability and cost of manpower with relevant skill for
e. f. g. h. i.
fabrication Space to be occupied by the equipment Environmental issues Safety issues Number of working days in the year Ergonomics
83
6.2. ANALYZING THE PROBLEM SOLUTION A thorough analysis will reduce the list for example if the equipment is to be used for small scale processing. All the constraint listed above will need to be considered.
6.3. PRELIMINARY
DESIGN:
ENGINEERING PROCESS PRINCIPLE
APPLYING AND
CHEMICAL
THEORIES
OF
MECHANICS. Probably the most important expression in the design of process equipment is that of mass and energy balance which may be expressed in general term as; Input + generation – output – consumption = accumulation This expression is found in various forms in thermodynamics, fluid mechanics, transport phenomena,
heat transfer, separation
process and other subject areas. It is simply an expression of indestructibility of matter and energy. This expression applies to all raw materials, intermediate and product.
6.4 MATERIAL SELECTION Materials are critical in the design of process equipment. Materials must be selected to take care of possible corrosion problems. Materials
of
construction
should
also
possess
adequate
mechanical properties to withstand tensile, compressive, shear and impact stresses.
84
Stainless steel of various grades finds wide application in process equipment design especially for parts in contact with raw materials and product. Glass, plastic and rubber lined vessels are also used are also used when materials tend to react with steel. Steel of various carbon contents are used for compounds such as shaft, springs and gears and for support structure.
6.5 DESIGN OPTIMIZATION The calculation process in the design of equipment may require simple arithmetic, algebraic, differential calculus or integral calculus. In many cases an exact solution may not be feasible thus necessitating the use of various approximation techniques such as graphical or numerical methods. In many cases also, only some parts of the equipment are designed
on
the
basis
of
analytical
calculations.
Practical
conditions are used to determine the specifications of the remaining part. It is thus not unusual to have several feasible solutions. There is thus the need to select the best solution. The ultimate goal is to minimize cost or maximize profit. In chemical process industries, equipment used are classified into two; Proprietary equipment such as pumps, centrifuges which are designed and manufactured by a specialist firm. Non- proprietary equipment such as reactors, heat exchangers, condenser, bleaching vessels etc are designed as specially requested.
85
6.6 SUMMARY OF THE DESIGN AND SPECIFICATION OF EQUIPMENT CALCULATION. In designing and specifying of equipment for chemical industries, the
variables
/parameters
involved
namely;
pressure,
temperature, density, volume, area, diameter, height, heat duties, heat capacities etc must be carefully calculated. This gives the designer exact data for fabrication and manufacturing. For the production of Nitric Acid; the following equipment are designed and specified; Nitric Acid storage tank, Ammonia storage tank, Absorber, Converter, Oxidation vessel, heat exchangers. FOR REACTORS The
operating
intensity
is
given
for
the
reactors=11296.324kg/m2/24hrs =11296.324kg/m2/day Equipment
Mass of Area(m2) reactant (kg/h) 13500 28.68 23320 49.55
Converter Oxidation Vessel Absorber 33530 71.24 The stripper column has 10 plates FOR STORAGE TANKS Equipment Type
Nitric tank
Acid
storage
Diameter(m) 6.04 7.94 9.52
86
Shape Nature Material Construction Capacity Volume(m3) Diameter(m) Height(m)
Cylindrical Insulated of Stainless Steel 50000000kg/hr 23.8 4.6 13.9
Equipment Type Shape Nature Material of Construction Capacity Volume(m3) Diameter(m) Height (m)
Ammonia Storage tank Cylindrical Insulated Stainless Steel 13565kg/hr 66.8 6.5 19.6
FOR HEAT EXCHANGERS Using the formulae; Q=AUDTm A= Q/UdTm Where Q= Heat Duty of the heat exchanger(KW) A= area(m2) U= Overall Heat Transfer Coefficient(KW/m2)( This is assumed for all) DTm=Log Mean Temperature Difference(Celsius)
87
Using a countercurrent flow; DTM= DT1 −¿ DT2/ ln(DT1 /DT2) DT1 = Thin-Tcout DT2 = Thout –TCin Equipment Q (KJ/hr) Thin Thout TCout Waste Heat Boiler (1) 15976252890 250 30 Air Heater 350 200 150 250
Stack Gas Heater Waste Heat Boiler(2) NH3 Super Heater NH3 Vaporizer
74323
200
Tcout 410
150
30
118.5
149781.4 208
50
150
32
4437.85
330
26.6
28.2
167.2
28.2
33.4
410
149781.4 208
Table 6.1: Table showing the heat transfer area of some equipment Equipment Stack Gas Heater Waste Heat Boiler(1) Waste Heat Boiler(2) NH3 Super Heater NH3 Vaporizer Air Heater
Q(KW/S) 20.65 4437.85 41.61 4437.85 41.61 4133.7
Area(M2) 2 130.5 11.83 127.6 1.87 588.7
88
CHAPTER SEVEN PROCESS CONTROL AND INSTRUMENTATION Instruments are provided to monitor the key process variables during plant operation. They may be incorporated in automatic control loops, or used for the manual monitoring of the process operation. They may also be part of an automatic computer data logging system. Instruments monitoring critical process variables will be fitted with automatic alarms to alert the operators to critical and hazardous situations. It is desirable that the process variable to be monitored be measured directly; often, however, this is impractical and some dependent variable, that is easier to measure, is monitored in its place.
7.1 OBJECTIVES The
primary
objectives
of
the
designer
when
specifying
instrumentation and control schemes are: 1. Safe plant operation: (a) To keep the process variables within known safe operating limits. (b) To detect dangerous situations as they develop and to provide alarms and automatic shut-down systems. (c) To provide interlocks and alarms to prevent dangerous operating procedures. 2. Production rate: To achieve the design product output.
89
3. Product quality: To maintain the product composition within the specified quality standards. 4. Cost: To operate at the lowest production cost, commensurate with the other objectives. These are not separate objectives and must be considered together. The order in which they are listed is not meant to imply the precedence of any objective over another, other than that of putting safety first. Product quality, production rate and the cost of production will be dependent on sales requirements. For example, it may be a better strategy to produce a better-quality product at a higher cost. In a typical chemical processing plant these objectives are achieved
by
a
combination
of
automatic
control,
manual
monitoring and laboratory analysis.
7.2 PLANT CONTROL CONFIGURATION The plant will be designed for manned operation and will be linked to
the
adjacent
fertilizer
manufacturing
plant.
Certain
configurations will be put in place to monitor some key parameters of the plant. The acid plant process control will be embedded in the plant DCS. The instruments of the individual process units will be terminated in junction boxes located at the unit’s skid limits. From here these instruments will be connected to instrument cabinets in the auxiliary room and integrated in the PAS. The plant safety instrument system (SIS) will be independent of the PAS. There will be a link between the PAS and the SIS for data
90
monitoring/logging and maintenance/operational override control purposes. Fire and gas monitoring will also be a dedicated module integrated in the safeguarding system. The process control schemes of some vital units are discussed as follows: Absorption column The process control scheme for the absorption column is presented in fig It was designed from the recommendations presented in the HAZOP analysis. It features ratio control on the make-up water stream. The signals from flow transmitters on this line and on the gas input line are fed to the ratio controller, whereby the make-up water stream is adjusted. Other control features include a pressure controller on the tail-gas outlet stream so that the column absorption pressure can be maintained at the design operating value of 950 kPa. A temperature transmitter on the tail-gas outlet stream provides the signal for control of the overall cooling-water flow rate. This is the temperature which is most useful in determining good absorption. The cooling circuit itself is fed from a common line (on which the overall flow rate is controlled). Small block valves on each of the tray cooling-coil feed lines enable flow rate regulation to each of the coils. These valves feature a removable top whereby a magnetic flow meter may be inserted to read the flow rate. The valves need only be set initially and then periodically adjusted manually.
91
There is no automatic control on the flow rate of the gas inlet stream or weak-acid condensate stream, since both of these flows are predetermined by feed flow rates earlier in the process. Isolation valves and provision for spectacle blinds are included to enable the column to be isolated during shutdown periods. The product-acid solution is withdrawn from the column using a level control valve on this line. The liquid level in the base of the column must be maintained slightly above the level of the plate downcomer to prevent incoming gas from by-passing the sieve plates. All controllers suggested for the absorption column feature HIGH and LOW alarms for good control. The final safety requirement is a relief line with a relief valve protected by a bursting disc. Air heater The process control scheme suggested for the air heater is shown in Fig. This flow scheme features a control valve on the compressed air inlet line. A temperature controller taking its signal from the heater outlet line ensures the flow is regulated to maintain the heater temperature of 250°C. Air pressure is controlled prior to entry into the unit and is kept constant at 7.3 atm. A pressure indicator on both inlet and outlet steam lines enables this parameter to be adequately monitored.
92
The nitrogen oxide reaction gas stream cannot be directly controlled from the air heater. Instead the flow rate, temperature and pressure are predetermined by the reactor feed conditions. Both inlet and outlet lines possess isolation valves for plant shutdown. These lines would be blanked before any platinum recovery work was attempted on the heater. Inlet and outlet lines also feature temperature indicators, consistent with the policy of constant monitoring of this parameter throughout the process. Ammonia Vaporiser and Superheater Pressure indicator and controller will be installed to maintain ammonia vapor at 7.3 atm. Temperature indicator and controller is required to ensure that the outlet temperature of 250°C is achieved in the superheater. The control scheme is shown in the figure below. Ammonia Converter Temperature control system is needed within the converter to ensure that the temperature in the converter does not drop below the reaction temperature of 890-900°C, to avoid loss of heat.
7.3 ALARMS, SAFETY TRIPS AND INTERLOCKS Alarm systems need to be installed in specific areas to alert operators of serious, and potentially hazardous, deviations in process conditions. Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. Where delay or lack of response, by the operator is likely to lead to the rapid development of a hazardous
93
situation, the instrument would be fitted with a trip system to take action automatically to avert the hazard; such as shutting down pumps, closing valves, operating emergency systems. The basic components of an automatic trip system are: 1. A sensor to monitor the control variable and provide an output signal when a preset value is exceeded (the instrument). 2. A link to transfer the signal to the actuator, usually consisting of a system of pneumatic or electric relays. 3. An actuator to carry out the required action; close or open a valve, switch off a motor. The high-temperature alarm operates a solenoid valve, releasing the air on the pneumatic activator, closing the valve on high temperature. 7.3.1
INTERLOCKS
Where it is necessary to follow a fixed sequence of operations for example, during a plant start-up and shut-down, or in batch operations interlocks are included to prevent operators departing from the required sequence. They may be incorporated in the control system design, as pneumatic or electric relays, or may be mechanical interlocks. Various proprietary special lock and key systems are available. Table 7.1: Property measured Flow – rate Level Pressure
Letter Code for Instruments Symbols First letter F L P
Indicating only
Controlling
FI LI PI
only FC LC PC
94
Temperature Humidity IIndicator L-
Level
F-
Flow rate
H-
Humidity
T H
TI HI C P
-
TC HC Controller
T
-
-
Pressure
Temperature
(Source: Sinnott, R.R 1999).
7.4 LINING, PIPING, VALVES AND PUMPS In Fig.7.1, which is the piping and instrument diagrams, there are various mechanical component introduced in the plant to obtain maximum efficiency some of which includes, flanges, valves, piping lines, blinds, gaskets and so on. 7.4.1 VALVES The valves used for chemical process plant can be divided into two broad classes, depending on their primary function: Shut-off valves (block valves), whose purpose is to close off the flow. Control valves, both manual and automatic, used to regulate flow. The table below shows some of the valves used in the P and I diagram (figure 5), their symbols, and functions. Table 7.2: Types of Valves and Symbol Used In PID
NAME
SYMBOL
FUNCTIONS
95
Control Valves
Automatic Valves
Used to control flow in lines.
Fitted on sensitive lines and are either pneumatically or digitally controlled.
Check Valves Fitted in lines of relatively high pressure or velocity Butterfly Valves
Used for control of gas or vapour flows
7.4.2
JOINTS
There are various joints used in fig 3.0 either as flow reducers, or to aid the carrying property of pipe. And effective transport of fluids in the piping flow. Below is a table of the various elbows and joints used in the P and I diagram:
96
Table 7.3: Joints JOINTS
SYMBOLS
FUNCTIONS
AND ELBOWS EQUAL ‘T”
Used to reduce a flow
REDUCER JOINT
line
into
three
equal
lines
Used 90o
T
–
CONNECTOR
in
joining
a
running line to a flow line.
Used in channeling lines also
ELBOW
reduces
flow
speed.
Used in channeling lines LONG
–
RADIUS
in pipe support.
ELBOW Used in branching lines. 45o LATERAL Used
in
pressure flow.
reducing
CHAPTER 8
97
SAFETY AND ENVIRONMENTAL CONSIDERATIONS REDUCER 8.1 SAFETY Safety is the condition of being protected against any danger. Every organization has a legal and moral obligation to safeguard the health and welfare of its employees and the general public.
7.5
PIPE SUPPORT
The good management practices needed to ensure safe operation
The Design a plant’s P and Ioperation. is not complete use of will also of ensure efficient In a without chemicaltheprocessing supports. Pipe in plant helps incan reducing cost and to industry, thesupports chemicals used piping or produced be hazardous number of or pump required to maintain line flow parameter humans the environment if not properly handled and thisand could safety of personnel through operation zone. and financially. equally lead to a lot of damage structurally Below some major type are of support: The is best organizations those that have come to the realization that provision of safety is not only the right thing to do for their I – BEAM Support to carry pipe lines employees, it is also profitable. H – BEAM Support above 2m Advantages of a safe working environment U – CHANNEL 1. Ultimately, safety leads to more profit as less money is spent PLATES taking TO ALIGN VALVES care of legal bills, hospital bills, and repair of equipment. SHOES TO HOIST PIPE INTO PROPER ORIENTATION 2. It gives the company a good name. 3. Happy employees which increases their job performance. Safety is usually considered in three classes: I. II. III.
Safety of the environment Safety of the personnel Safety of the plant and equipment.
98
The term “engineering safety” covers the provision in the design of control systems alarms, trips, pressure relief devices, automatic shutdown system and duplication of key equipment, firefighting equipment and service; personnel protect equipment and so on.
99
8.1.1
SAFETY OF THE ENVIRONMENT
There are several hazards associated with industrial process. These hazards need to be prevented and kept in check in order to protect the environment. Environment in this context refers to the immediate surroundings around the plant. For the safety of the environment to be ensured, the following points should be noted and applied; 1. Flaring of gases should be done minimally. 2. The level of toxicity of effluent should be monitored regularly and kept in check. 3. Storage tanks should be situated in areas away from vehicle traffic. 4. The control room should be attended to at all times to ensure that there is an immediate response if an alarm is triggered. 5. There should be a way of informing the community around the facility if there is danger that might affect them e.g Fire. An alarm is suggested, and this should be tested regularly. 6. Protect pipe racks and cable trays from fire. 7. Fire-fighting system must be provided within the complex. This consist: Fire water pipe network throughout the facility supported by necessary hydrants. Hoses should be permanently placed near these hydrants. The system should have a suitable water pump. It is advised that there be at least 2 pumps. One big one to fill the lines or pump large volume of water into it (when the water is being depleted very fast like in a case of fire) and a smaller jockey
100
pump to maintain the pressure in the line. It would start more frequently than the big pump. It is suggested that the system should have its own separate standby generator. Fire entry suits and other protective clothing, compressed air breathing apparatus and fire blankets should be made available in every building. 8.1.2
SAFETY OF THE PERSONNEL
The personnel of a company refers to the operators and staff of that company who ensure that the production process move on smoothly. Their safety can be ensured in the following was. 1. Provision of personal protective equipment (PPE) and ensuring that they are properly used. 2. Pipes and equipment that contain very hot liquids for example the heater must be lagged to make sure it does not cause injury to personnel. 3. All chemicals in the plant must be properly label with its chemical hazard identification chart and their Material Safety Data Sheet (MSDS) must be available and updated regularly also. 4. First aid kits must be provided in all buildings. 5. Emergency means of transportation must be provided in case of any accident. 6. Emergency exit doors must be provided and these exits clearly marked for all to see. 7. Cleanliness of the facility must be ensured at all times to avoid unnecessary risk or accident. 8. Smoking should be avoided in process area.
101
9. Fire extinguishers must be made available at strategic points within the facility. 10. All ladders must have hand rails and personnel encouraged to use them whenever climbing. 11. Safety signs and symbols should be placed at hazardous area. 8.1.3
SAFETY OF THE PLANT AND EQUIPMENT
A plant includes any machinery, equipment (including scaffold), appliance, implement or tool and any associated computer or fitting used in the production process of a material or substance. There are different types of risks associated with using different types of equipment. To mitigate this risk, the following must be done: 1. As Nitric acid is known to be highly corrosive, regular pigging of the pipelines with both smart and scraper pigs should be conduct so as to check this. 2. Corrosion protection is achieved by the well proven use of suitable austenitic stainless steel where condensation can occur and by regular monitoring of the conditions. 3. Regular maintenance of equipment should be carried out. 4. Check to ensure that equipment and machineries meet health and safety standards before it is purchased. REACTOR Reactor is a vessel in which chemical transformation takes place. The converter, oxidizing unit and absorber are the reactors in this design. The catalytic reactor is designed to give a uniform distribution of the air/ammonia mixture over the catalyst gauzes.
102
Maintenance of the catalyst operating temperature is very important for the NO yield. This is achieved by adjusting the air/ammonia ratio and ensuring that the lower explosive limit for ammonia in air is not exceeded. The following safety steps should be followed in the design and operation of the reactor. The materials going into the reactor must be purified. This is done to remove impurity that will affect the reactor. The reaction condition i.e. temperature, pressure etc must be monitored closely. The reactor should be cleaned accordingly during periodic maintenance. COMPRESSOR While operating the compressor, the following precautions should be observed. The air must be dried properly to avoid water entering into the compressor which could damage it. The air must be filtered properly to avoid foreign particle from entering into the compressor. The proper operating pressure should be maintained at all times. Coolant must be checked regularly and topped. In case of shut down, the shut down and start up procedures should be strictly adhered to. HEAT EXCHANGERS For heat exchangers to work effectively and safely, the following must be implemented.
103
Ensure that the heat exchanger is pressure tested as designed. Ensure that it is cleaned periodically and faults and leakage rectified if found. Inspect after cleaning before coupling back. The water must be purified especially the one going into the boiler to remove chlorine because ignition occurs when chlorine is passed into ammonia forming nitrogen and hydrochloric acid and if chlorine is present in excess, then a highly explosive nitrogen trichloride (NCl3) is formed. As we cannot guarantee 100% conversion of ammonia in the converted and no leak in the boiler, the water should be purified. PUMPS For the pumps to work effectively and safely, the process operators should ensure that; The right operating temperature and pressure must be used. There should be a minimum flow line that helps to maintain a certain rate of flow in the pump preventing it from going into cavitation. Lube oil to gland bearings is available, bottle filled and ready. The lube oil should be changed at regular intervals. The vents should be properly cleaned and capped There should be no particle or foreign body in both the suction and discharge lines. TANKS Nitric acid is normally stored in flat bottomed, roofed tanks, made from low carbon austenitic stainless steel, installed in areas
104
provided with suitable containment facilities. The acid level in the tank is monitored by means of a level indicator. A vent to the atmosphere allows the escape of gas which comes from liquid movement and thermal effects. It is normal to earth the tanks. For the life span of the tanks to be preserved, the following should be done: The tank should be cleaned regularly Periodically, the tanks should be checked for corrosion and the affected part could be painted. The right operating conditions for the tanks must be maintained. 8.1.4
GENERAL SAFETY PRECAUTION
Generally, precaution is taken to prevent accidents or hazards (a potential danger). Hazards can either be intrinsic or extrinsic. Intrinsic hazards are naturally occurring hazards caused by wind, earthquake, lighting and water. The effects of intrinsic hazards include
leakage
in
pipes,
explosion
of
pipes,
collapse
of
production buildings, rupture of welded joints etc. the following precautions can be taken to mitigate the dangers posed by intrinsic hazards: Lightning rods should be installed at strategic points. Drainages should be wide and deep enough, and should also be channeled properly to prevent flooding with the facility. Explosive substances have to be stored in a cool dry place away from sunlight and ultraviolet rays.
105
The floor around the industry should be properly cast around with concrete or other weather resistant covering and the road networks properly tarred Extrinsic hazards are man-made hazards caused by man due to carelessness.They may include dropping of an oil filter on the floor, igniting flame close to a flammable substance, not knowing how to operate a machine or equipment, ignoring procedure for starting or using equipment, using wrong tools. They can be prevented by the following measures: Personnel coming into the facility for the first time must be given a proper safety orientation on the do’s and don’ts of the company policy on safety and regular safety talks on safety and maintenance of plant must be conducted. Regular training of personnel on safety issues should also take place. Good housekeeping practice by all employees should be encouraged. 8.2
HAZARD OPERABILITY (HAZOP) STUDY
Hazard and operability study sometimes simply referred to as operability
Studies,
provide
a
systematic
and
critical
examination of the operability of a process. They indicate potential hazards due to deviations from the intended design conditions. 8.2.1 BASIC PRINCIPLES
106
A formal operability study is the systematic study of the design, vessel by vessel, and line by line, using “guide words” to help generate thought about the way deviations from the intended operating conditions can cause hazardous situations. The following words are also used in a special way, and have the precise meanings given below: • Intention: the intention defines how the particular part of the process was intended to operate; the intention of the designer. • Deviations: these are departures from the designer’s intention which are detected by the systematic application of the guide words. • Causes: reasons why, and how, the deviations could occur. Only if a deviation can be shown to have a realistic cause is it treated as meaningful. • Consequences: the results that follow from the occurrence of a meaningful deviation. • Hazards: consequences that can cause damage (loss) or injury.
107
108 Table 8.1
HAZOP STUDY:
Deviation No Flow
Weak-acid condensate stream
Possible consequences 1.Pump Failure
Consequence Deficient quality product and high NOx tail gas emission levels. As for I Valve overheats. As for I and 2.
2. Valve fails shut. 3. Line fracture
More Flow
4. Higher feed air.
More Temperature
5. high feed rate causing larger heat of reaction 6. Isolation valve is closed in error while pump running.
More Pressure
humidity
in
7. Thermal expansion on the isolation. Less Flow
Less Temperature
Action Required a) Install LOW LEVEL ALARM on LIC at the base of the absorption column. Covered by a). b) Install kick-back on pumps Covered by a) and b). c) Regular inspection and patrolling of weak-acid transfer lines and seals. Higher make but weaker d) Install a HIGH LEVEL product acid. ALARM on LIC at the base of the absorption column Possible higher NOx See Table 8.6. emission. Lines subject to full Covered by b) delivery pressure f) Perhaps worthwhile installing a pressure gauge upstream of the delivery pump Possible line fracture or g) Provide thermal expansion flange leakage relief in the Valve section. Decreased absorption. Covered by a), c), and d). Lower product make
8. Flange leakage or valve stub blanked but leaking 9. Reaction gas Increased dissolved NOx See Table 8.6. temperature in oxidation concentrations in
109
Maintenance
Table 8.2
HAZOP study:
Deviation No Flow
unit lower. product acid. 10. Equipment failure, Process stops. flange leak, catalyst changeover in reactor, etc.
Ensure all pipes and fittings are constructed of the right materials and arc stress relieved.
Make-up water feed stream. Possible causes
consequences
Action Required
1.Pump Failure
Deficient quality product and high NOx tail gas emission levels. As for I Valve overheats. As for I and 2.
a) Install LOW LEVEL ALARM on LIC at the base of the absorption column. Covered by a). b) Install kick-back on pumps Covered by a) and b). c) Regular inspection and patrolling of weak-acid transfer lines and seals. d) Install a HIGH LEVEL ALARM on LIC at the base of the absorption column See Table 8.6
2. Valve fails shut. 3. Line fracture
More Flow
4. Control valve fails open
Dilute acid formed
product
More Temperature
5. Higher feed temperature to the Refrigeration unit. 6. Failure in the refrigeration unit.
Possible higher emission due to absorption. As for 5.
is
NOx lower
e) Ensure refrigeration unit is well maintained with adequate
110
More Pressure
8. Thermal expansion in the isolation valve section (fire). 9. Flange leakage or valve stub blanket but leaking
Less flow
Less Temperature
Maintenance
Table 8.3. Deviation
7. Isolation valve is closed in error whilst pump running.
I0. Reaction gas temperature in oxidation unit lower. 11. Equipment failure, flange leak, catalyst changeover in reactor, etc .
Line subject to full delivery pressure.
Possible line fracture or flange leakage
control Covered by b). IJ Perhaps worthwhile installing a pressure gauge upstream of the delivery pump. g) Provide thermal expansion relief in the valve section.
Decreased absorption. Higher operating cost in lost water Increased dissolved NOx concentrations in product acid. Process Stops
Covered by a),c) and d)
consequences
Action Required
See Table 8.6
Ensure all pipes and fittings are constructed of the right materials and are stress relieved.
HAZOP study: Gas-inlet stream Possible causes
111 No Flow
1.Flow stopped upstream
No absorption in column. Entire process stops as tailgas Row stops.
2. Line blockage or the isolation valve shut in error.
As for I. Pressure buildup in pipe and secondary As for I. Gases escape into the surroundings.
3.Line fracture
More Flow
4. Increased feed
Possible reduction absorption Efficiency. May cause flooding.
More Pressure
5.Flooding
Unit subject to high pressure, bursting discs may rupture, tail gas release. As for 2. Line fracture or flange leakage.
6. Isolation valve accidently closed 7. Thermal expansion in isolation.
in
More Temperature
8. Insufficient cooling
Decreased absorption, higher pollution.
Less flow Less Temperature
9. Leaking inlet range 10. Overcooling.
As for 3. Increased dissolved gasses
a) Ensure liquid feeds to absorber and other process unit shut down. b) Install LOW FLOW ALARM on FIC. Covered by a) and b). c) Install kick-back on upstream pumps and ensure pressure relief system is adequate d) Ensure regular patrolling of feed transfer lines. e) Plant emergency shutdown procedures F) Ratio control on the liquid feed streams should be sufficient. g) Install HIGH LEVEL ALARM on the FIC. Covered by c). h) Ensure correct sizing on pressure relief system. Covered by b) and c). i) Provide for thermal expansion relief in the design of the isolation valve section j) Ensure accurate temperature control on the internal cooling circuit. Covered by b), and d) Covered by j)
112
H i g h NOx composition
Table 8.4.
11. Improved yield from reactor.
in acid. Higher tail-gas levels possible.
emission
k) Manually increase makeup water Composition flow rate.
consequences No tail gas for expansion. Pressure build up in column and line As for I.
Action Required See Table 8.1. a) Install LOW LEVEL ALARM on PIC. b) Install pressure relief valve with bursting disc. Covered by b). c) Install HIGH LEVEL ALARM on PIC. Covered by a) d) Institute regular inspection of all transfer lines. Covered by b) and c). Covered by b) and c). d) Look to altering make-up water feed rate in response. e) Install HIGH LEVEL ALARM on TIC Covered by b) and c). f) Install thermal expansion relief in isolation valve section. Covered by b) and c).
HAZOP study: Gas – Outlet stream.
Deviation No flow
Possible causes I. No inlet gas flow.
2. Flooding in column.
3. PCV fails shut, line blockage or isolation valve closed in error.
As for 2.
As for 1.
More flow
More Temperature More pressure.
4. Line fracture or flange failure 5. Increased gas feed at inlet. 6. Decreased NOx absorption 7. Higher feed gas or liquid inlet temperature. 8. All of 5, 6, and/or 7. 9. Thermal expansion in isolation valve section (fire). 10. PCV fails shut or isolation valve shut in error.
Transfer line subject to higher pressures. As for 5. Tail-gas emission levels up. Decreased absorption and higher NOx emission. As for 5, 6, and 7. Line fracture or flange leakage. As for 3
113 Less flow
Liquid Carryover Maintenance
Table 8.5 Deviation No flow.
11. Leaking flange or valve stub not blanked and leaking. 12. Flooding. 13. The entrainment device ineffective. 14. Equipment failure, flange leak, catalyst changeover in reactor, etc .
Less tail gas for expansion and release of NOx to the environment.
Covered by a) and d).
Condensation is down, steam lines (corrosion). Process stops.
Replace entrainment device. Ensure all pipes and fittings are constructed of the right materials and are stress relieved
HAZOP study: Liquid-outlet stream. Possible causes I. No liquid inlet from either make-up water or acid condensate. 2. Flooding in column.
consequences See Tables 9.6 and 9.7
Increase in column pressure. Liquid level in column increases adding to flooding problems.
3. LCV fails shut. 4. Line fracture.4. Line fracture.
More flow
5. LCV fails open.
Discharge of acid into the surroundings. Loss of feed to the stripping column Gas begins to bypass the plates causing higher NOx emissions.
Action Required a) Covered by control and alarms specified in Tables 8.1, 8.2 and 8.3. Covered by a). b) Install HIGH LEVEL ALARM on LIC. c) Regular patrolling and inspection of transfer lines. d) Install suitable alarms to stripping column to indicate loss of flow e) Install LOW LEVEL ALARM on LIC.
114 More temperature
6. Higher inlet temperatures
More pressure
7. LCV fails shut or isolation valve close in error. 8. Leaking flange or valve stub not blanked and leakage. 9. Lower inlet stream temperatures or over capacity from cooling circuit. 10. Lower steam temperature.
Less flow Less temperature
NOx dissolved
maintenance
11. Equipment failure, flange leak, catalyst changeover i n reactor, etc.
Less dissolved NOx in acid but higher NOx tail-gas emissions Line subject to full surge or delivery pressure. Loss to surrounding.
Covered in Tables 8.1, 8.2 and 8.3.
Higher concentrations of dissolved NOx in product acid. Higher downstream operating costs. Same as 9. Process stops
See Tables 8.1, 8.3 and 8.4.
Covered by a) Covered by d) and e)
See Tables 8.1,8.3 and 8.4
Ensure all pipes and fittings are constructed of the right materials and are stress relieved
115 Table 8.6:
HAZOP study: cooling-water circuit.
Deviation No Flow
Possible causes 1.Pump Failure
consequences High emissions of NOx in tail gas.
2. Valve fails shut. 3. Line fracture
More flow
4. Control valve fails open.
More temperature
5. Higher feed temperature to the refrigeration unit.
More pressure
7. Isolation valve is closed in error while pump running. 8. Thermal expansion in the isolation valve section (fire). 9. Flange leakage or valve stub blanked but leaking 10. Higher duty from
Less flow
Less Temperature
As for 1 Valve overheats. As for 1 and 2
Product acid is at lower temperature therefore, higher dissolved NOx. Possible higher NOx emissions due to lower absorption. As for 5. Lines subject to full delivery pressure. Possible line fracture or flange leakage.
Decreased absorption. Low quality product and high emissions. Increased dissolved NOx
Action Required a) Install HIGH LEVEL ALARM on TIC on the tail-gas outlet line to indicate high emissions. Covered by a). b) Install kick-back on pumps. Covered by a) and b). c) Regular inspection and patrolling of cooling-water circuit lines and associated d) Install a LOW LEVEL ALARM on TIC on the tail gas outlet line Covered by a) e) Ensure refrigeration unit is well maintained with adequate control Covered by b). f) Perhaps worthwhile installing a pressure gauge upstream of the delivery pump. g) Provide thermal expansion relief in the valved section Covered by a), c), and d).
Covered by a), c), and d).
116 refrigeration unit. Maintenance
11. Equipment failure, flange leak, catalyst changeover in reactor , etc
concentration acid. Process stops
in
product Ensure all pipes and fittings are constructed of the right materials and are stress relieved
117
8.3 ENVIRONMENTAL IMPACT ASSESSMENT (E.I.A) This is the assessment of the possible positive and negative impact that a proposed project may have in the environment, together consisting of the environmental, social and economic aspect. It is a systematic process of identification, prediction evaluation,
mitigating
and
presentation
of
possible
consequences on the environment of proposed actions at a stage in decision making process so that environmental damage can be minimized or avoided. 8.3.1
WHAT EIA DOES
• Describes the project or operation • Describes the environment that will be affected • Predicts the impact on the environment • Adopts options, techniques and controls to reduce negative impact. • Monitors the project or operation to ensure that identified key impact is minimized. 8.3.2
GOAL
• To ensure that decision makers consider the ensuing environmental impacts when deciding whether to proceed with a project or not.
118
8.3.3
BENEFITS
OF
ENVIRONMENTAL
IMPACT
ASSESSMENT • May be prerequisite for permit approval by government or international agencies. • Required by financiers of the proposed projects • Help to prevent environmental problem, risk or costly-time working liabilities. • Boosts Proponent Company’s image. • Repose confident /assurance in Proponent Company. 8.3.4
ENVIRONMENTAL IMPACT ASSESMENT (EIA) OF A
NITRIC ACID PLANT. Negative impact The major negative impact of a nitric acid plant is NOx emissions of the tail gas from the absorption tower especially during start up and shut down before the plant stabilizes. Others include:
Risk of fire/explosion hazard. Reduced soil and marine water quality. Increase in water and electricity demand. Noise pollution.
Positive impact Provide skilled and unskilled job opportunities. Make available nitric acid in the country, thereby encouraging the production of fertilizer as it is a major chemical used in its production.
119
Bring development to the area where the plant is sited. 8.3.5
NOx EMISSION FROM NITRIC ACID PRODUCTION.
Nitric acid production is one of the larger chemical industry sources of NO. Unlike NOx found in combustion flue gas, NOx from nitric acid production is part of the process stream and is recoverable with some economic value. Vent gas containing NOx is released to the atmosphere when the gas becomes too impure to recycle or too low in concentration for recovery to be economically practical. The chemical reactions for each of the nitric acid production process steps demonstrate that NOx must first be created before nitric acid can be produced. The first reaction, 4NH3 + 5O2 ⇌ 4NO + 6H2O + heat Eq. 1 Shows NO forming from the reaction of NH and air. The NO is then oxidized in the second step, 2NO + O2 ⇌ 2NO2 + heat Eq. 2 Producing NO2. The NO2 is subsequently absorbed in water to produce nitric acid. However, as the absorption reaction, 3NO2(g) + H2O(l) ⇌ 2HNO3 (aq) + NO(g) + heat, Eq. 3
120
Shows, one mole of NO is produced for every three moles of NO2absorbed, making complete absorption of the NOx impossible. The unabsorbed NOx, if not controlled, is emitted in the absorber tail gas.
8.3.6 FACTORS AFFECTING NOx EMISSION LEVELS. Re-oxidation of NO into NO2 is a very slow reaction. As more air is added, the reaction becomes increasingly slower as the reactants become diluted with excess nitrogen. Increased temperatures due to exothermic absorption tend to reverses eq. 3 producing more NO2. Low temperature (less than 38 0C [1000F]) is a key factor forhigh absorption efficiency but is also one that is difficult and expensive to control. Completion of the absorption process which reduces NOx emission is aided by increased pressure (800 to 1,400 kPa) Increasing acid strength beyond design specification typically increases the NOx emission rate. Good maintenance practices and careful control ofoperations play important roles in reducing emissions of NOx. 8.3.7
CONTROL
TECHNIQUES
FOR
NOx
EMISSIONS
FROM NITRIC ACID PRODUCTION. Extended absorption Extended
absorption
reduces
NO
emissions
by
increasing
absorption efficiency and is achieved by either installing a single
121
large tower, extending the height of an existing absorption tower, or by adding a second tower in series with the existing tower. Increasing the volume and the number of trays in the absorber results in moreNOx being recovered as nitric acid (1-1.5% more acid) and reduced emission levels. Non-selective catalytic reduction (NSCR). Nonselective catalytic reduction uses a fuel and a catalystto 1. Consume free oxygen in the absorber tail gas. 2. Convert NO2 to NO for decolorizing the tail gas. 3. Reduce NO to elemental nitrogen. The process is called nonselective because the fuel first depletes all the oxygen present in the tail gas and then removes the NOx. It can be operated at any temperature, heat used to operate it can be recovered and it can achieve higher NOx reduction than extended absorption but it is expensive due to the cost of fuel. Selective catalytic reduction (SCR) Selective catalytic reduction uses a catalyst and ammonia in the presence of oxygen to reduce NOx to elemental nitrogen. The process is called selective because the ammonia preferentially reacts with NOx in the absorber tail gas. The following sections discuss SCR used as a NO control technique for nitric acid plants. Proper operation of the process requires close control of the tail gas temperature. Chilled Absorption.
122
Chilled absorption provides additional cooling to the absorption tower. This process is frequently used in addition to other control techniques such as extended absorption. The principal advantage of chilled absorption is improved absorber efficiency due to lower absorption temperature. However, chilled absorption by itself typically cannot reduce NOx emissions to the level that any of the three primary control techniques can achieve. 8.3.8
ENVIRONMENTAL MANAGEMENT PLANT
The following are actions taken to mitigate the negative impacts of the plant sited above. 1. Use extended expander and chilled absorption to increase absorber efficiency and thus reduce NOx emission. 2. Site the plant far away from residential area so as to reduce noise pollution effect and risk of fire. 3. Update on site emergency response plan. 4. Have a generator to provide the electrical needs of the plant and own water supply system. 5. Monitor stack gas emissions. 6. Test liquid waste to make sure the level of chemical in it is acceptable before discharging to sea.
123
CHAPTER 9 ECONOMIC
ANALYSIS
9.1 OVERVIEW Chemical processes have been harnessed to transform resources, and raw materials into more useful and hence more valuable products to improve the living standards of people. This principle is at the core of chemical engineering, and there industries have matured over the last 100 years, and have been very successful at creating wealth. The means of establishing which products to make and how to optimize the process required for the manufacture
have
been
based
on
economic
principles.
Approaches to accounting for the risks to the economic value of projects are also considered to ensure that they deliver the expected benefits. 9.2 ECONOMIC
CONSIDERATION
The following are considered under economic evaluation; 1. Cost and Assets Accounting: This provides a survey of accounting procedures for the analysis of cost and profits as used for industrial applications. 2. Cost Estimation: This provides information regarding the estimation of fixed capital cost and also recurrent operating expenditure.
124
3. Interest concept
and and
Investment calculation
of
costs:
This discusses the
interest,
i.e
payment
compensation for the use of borrowed capital. 4. Taxes and Insurance: Taxes represent a
as
significant
payment from a company’s earnings and although insurance rates are only a small fraction of annual expenditure cover for a plant is essential. 5. Depreciation: This is the measure of the decrease in value of an item, with respect to time and can be considered as a cost incurred for the use of the equipment. 6. Profitability, Alternatives, Investments
and
Replacements: The profitability of an investment is a measure of the amount of profit generated. It is important to assess the profitability accurately and also the profit that could be obtained from alternative investments. 10.4 TYPES OF COST X.4.1
INDIRECT COST
1. Design and engineering cost; which cover the cost of design and the cost of “engineering” the plant: purchasing, procurement and construction supervision. Typically 20% to 30% of the direct capital cost. 2. Contractor’s fees: if a contractor is employed his fees (profit) would be added to the total capital cost and would range from 5% to 10% of the direct cost 3. Contingency allowance: this is an allowance built into the capital cost estimate to cover for unforeseen circumstances
125
(labor disputes, design errors, adverse weather) typically 5% to 10% of the direct cost.
Table 9.1: typical factors for estimation of project fixed capital cost MAJOR
EQUIPMENT
,
TOTA; PCE
PURCHASE COST (F1 TO F9) Equipment erection Piping Instrumentation Electrical Building process Utilities Storage Site development Ancillary buildings 9.4.2
0.40 0.70 0.20 0.10 0.15 0.50 0.15 0.05 0.15
Total physical plant cost (PPC)
PPC = PCE (1 + ∑factors ) = PCE × 3.4 Ranging from f10 to f12 Design and engineering Contractors fee contingency Fixed capital = PPC (1 + f10 + f11
0.30 0.05 0.10 + f12) = PPC × 1.45
126
9.4.3
OPERATION COSTS
An estimate of the operating costs, the cost of producing the product, is needed to judge the viability of a project and to make choices between possible alternative processing schemes. These costs can be estimated from the flow sheet, which gives the raw material and service requirements, and the capital cost estimate The cost of producing a chemical product will include the items listed below. They are divided into two groups. 1. Fixed capital cost: Costs that do not vary with production rate. These are the bills that have to be paid whatever the quantity produced 2. Variable operating cost: Costs that are dependent on the amount of product produced. 9.4.4 FIXED COSTS 1. Maintenance (labor and materials) 2. Operation labor 3. Laboratory costs 4. Supervision 5. Plant overhead 6. Capital charges
9.5 COST 9.5.1
ESTIMATION
THE RATIO METHOD
127
The ratio method is a simple technique whereby known capital cost data for an existing chemical plant are adjusted to provide a cost estimate for the desired plant capacity. This method is also able to update figures to account for inflationary effects of past years. Finally the capital cost figure is adjusted for exchange rate differences between countries .The method is centered around the use of key cost estimation indices such as the CE plant cost index and the Marshall and Stevens (M&S)index. Ratio method calculations; n capacity of designed Cost of Designed plant ¿ Cost of previous plant ( Capacity of previous plant )
Cost of
1200 tons 1200 0.6 =Costof 280 tons/day ×( ) day 280
cost of 280tons /day =¿ $ 60 million
Therefore; 0.6
cost of 1200 / day=$ 60 million ×(
1200 ) 280
= $143.66 million
= ₦22.7 billion 9.5.2
STEP COUNTING METHOD
Step counting estimating methods provide a way of making a quick order of magnitude estimate, of the capital cost of a proposed project. The technique is based on the premise that the capital cost is determined by a number of significant
128
processing steps in the overall process. Factors are usually included to allow for the capacity, and complexity of the process: material of construction, yield, operating pressure and temperature. step counting method calculations; C=14000 NQ 0.615
Where Q=plant capacity, tonne per year. N=number of functional units C=capital cost. Q=400,000tons/yr N=13 0.615
C=14000× 13 ×( 400,000)
= $ 507 million equivalent ¿ 79.8 billion naira . 9.5.3
FACTORIAL METHOD
Capital cost estimates for chemical process plants, are often based on an estimate of the purchase cost of the major equipment items, required for the process, the other costs being estimated as the factors of the equipment cost .The accuracy of this type of estimate will depend on which stage the design has reached at the time.
129
ECONOMIC ANALYSIS CALCULATIONS ESTIMATION OF FIXED CAPITAL COST Rough Estimate Cf = Fl Ce Ce = ₦ 824.43 million Cf = 4.7 × 828.43 = ₦ 3.89 billion
Detailed Estimate PPC = Ce ( 1 + ∑factors ) ∑factors = 3.3 PPC = 824.43 (1 + 3.3) = ₦3.55 billion Total Fixed Capital = 3.55 ( 1 + 0.45) = ₦ 5.15 billion Working capital = 5% × 5.15 = ₦258 million Total Capital Investment Cost = Total Fixed Capital + Working Capital Total Capital Investment Cost = 5.15 + 0.258 = ₦ 5.41billion OPERATING COST ESTIMATION Variable Cost 1) Raw materials
₦ million 0.25
130
2) Miscellaneous
0.24
3) Utilities Cost Cooling water
Negligible
Steam
Negligible
Power
0.4
4) Shipping $ Packaging
1.15
Total
2.04
FIXED COST ₦ Million 1) Maintenance 2) Operating Labor 3) Laboratory Cost 4) Plant Overhead 5) Insurance 6) Royalties Total
1.21 7.26 2.18 3.63 387.15 Not applicable 401.43
Annual Total Operating Cost = 2.04 + 401.43 = ₦ 403.47 million Annual Operating Cost Rounded = ₦ 403.5 million
ANNUAL PROFIT CALCULATION Total Expenses = operating cost + capital finance cost
131
Total fixed capital investment = ₦ 5.15 billion Working capital = ₦ 258 million Total capital cost = ₦ 5.41 billion (Assuming 5% compound interest rate annually and 2 years investment) Total interest = p (1 + r )n = 5.41 (1 + 0.05)2 = ₦ 5.96 billion Operating cost for 2 years = 2 × 0.403 = ₦ 806 million Total Expenses = ₦ (0.806 + 5.96) billion = ₦ 6.77 billion Cost of Nitric acid per ton = ₦ 10500 Annual total cost of Nitric acid = 7100 × 400,000 = ₦ 4.2 billion Total cost of Product for 2 years = ₦8.4 billion
Annual total cost of steam = 10500 kg hour
8000 hours 1 year
₦ 7.5 454kg
132
Annual total cost of steam = ₦ 1.39 million Total income before tax = ₦ (5.68 billion + 2 × 0.00139 billion) = ₦ 8.4 billion Total income after tax (based on 2.5% tax) = ₦ (8.4 – 0.025 × 8.4) billion = ₦ 8.19 billion Profit after tax = ₦ (8.19 -6.77) billion = ₦1.42 billion Payback period (no interest) = Depreciable FCI / Total profit = 5.15 / 1.42 = 3.63 years Therefore, payback period = 3years 8 months rounded. BREAK EVEN ANALYSIS On the assumption that market price of nitric acid will remain constant for a reasonable length of time. The breakdown period for the plant will simply be the inverse of the rate return on the investment ∴
¿
Break even time (yrs) =
Total capital investment annual net profit
5.41 =3.81 yrs 1.42
Break even time= 3yrs 10 months.
133
RATE OF RETURN ON INVESTMENT R O R = yearly profit / total initial investment × 100% = 1.42/5.41 × 100/1 = 26.25%
CHAPTER TEN
134
STARTUP AND SHUTDOWN PROCEDURES Shutdown is that period of time during which a boiler, gas turbine, process heater or nitric acid production unit is allowed to cool from its normal temperature range to a cold or ambient temperature. The shutdown philosophy is based on the nitric acid plant process control and safeguarding philosophy reference and adapted to suit the developments in the design. High nitric acid supply availability is of paramount importance. The level of safeguarding reflects the need for the plant to operate safely whilst ensuring maximum availability. Shutdowns of the main process will be avoided as much as possible within the constraints of safer operation. Additional time is given to the operator to correct process upsets by intentionally accepting cascading events. This in turn will result in fewer disruptions in the process For
all
separators,
low
low
liquid
level
will
cause
the
corresponding liquid outlet SDV (Shut down Valves) to close rather than generating an OSD (Operational Shut Down) On high liquid level and high pressure in the main nitrous gas stream, gas flow is stopped by closing the inlet shutdown valves. This is to avoid liquid carry over to the absorption column and stripper. Gas compressor unit.
135
In the absorption column, a high liquid level and a high pressure will close the corresponding inlet SOV. Trips in the off air compressor package will stop the compressor and the air flow will be directed to the flare.
10.1
EMERGENCY
SHUTDOWN
(ESD)
AND
EMERGENCY DEPRESSURIZATION (EDP) Emergency Shutdown and Depressurization of pressurized vessels and piping is the acknowledged way to reduce the likelihood of escalation from accidental hydrocarbon release incidents. An
ESD
will
be
automatically
initiated
on
confirmed
low
instrument air pressure and manually initiated on confirmed gas detection via ESD push button. The aim of an ESD is to bring the plant to a safe condition by; 1. Isolate
the
plant
from
the
flow
lines,
stopping
all
hydrocarbons containing streams from coming in and going out of the plant. 2. Depressurization the plant. 3. Starting down the fired heaters
10.2
NOTIFICATION
Prior notification of scheduled shutdowns and scheduled start-ups following scheduled shutdowns shall be made in a timely manner and form. Shutdowns and start-ups must be scheduled in pairs with scheduled dates for each. Notification of scheduled start-ups and shutdowns is required only if an exemption from the
136
emissions limit is required. This notification shall contain the following information: 1. Dates and times of the scheduled start-up and shutdown and its duration, and 2. Any other process
variable
that
is
appropriate
as
determined.
10.3 RECORD KEEPING Records shall be maintained and kept on-site and made available for two years indicating hour-by-hour firing rates, flue gas temperatures, NOx emissions and such process variables that are appropriate. Once all of these equipment checks are performed, the complete unit is disassembled, all parts and bearings are rechecked and oiled, the lubrication system is drained and flushed, and the train is re-assembled. A time-consuming aspect of the drive train checkout involves plotting of the unit’s surge curves. Once the unit is operational, the air compressor can be used to blow out downstream air and stream lines. Other equipment debugging procedures are performed according to individual “punch lists” and are summarized as follows: Liquid piping and coded vessels: Pressure tested with water at maximum working pressure. Gas lines: cannot be checked until plant is operating Relief valves: bench tested with required pressure—if serious problems exist, they are sent out for repairs.
137
Heat exchanger: flushed with water or a cleaning solution. Waste Heat Boiler: undergoes a hydrostatic check followed by pre-treatment with chemicals to prevent corrosion due to oxygen or water prior to plant start-up. A final procedure before production starts consists of filling the boiler with water and warming with steam to prevent shock to the system Absorber column: shipped to the plant as a complete package and can be of either a bubble cap or sieve tray arrangement. The column is prepared by flushing with water to clean and check flow and level indicating instruments. Sieve tray columns are more sensitive to gas versus liquid flow and may require 1hr to seal properly whereas a bubble cap unit may take about 20mins Instrumentation: Cannot
be
installed
until
all
other
equipment is in place. A critical component is ammonia/air ratio control system which must be accurately calibrated to read concentrations of about 9-11 Percent ammonia in air.
10.4
STARTUP OPERATIONS
Once all equipment is installed and thoroughly checked for proper mechanical operation (this may take from 2 to 6 months), the plant is ready to undergo initiation of nitric acid production. Preliminary startup operations consist of the following steps: 1. Startup of air compressor system 2. Initiation of water flow to absorber tower
138
3. Platinum gauze lit by hydrogen torch to initiate burning of ammonia (flame is self-sustaining) 4. Ammonia flow is begun Within 2 to 3 weeks of this initial startup, the plant is ready for a test or demonstration run. Test runs usually last 3, 7, or 14 days depending on the contract. During this time, the plant must achieve its peak efficiency, of maximum design rate, and meet all applicable emission regulations. A violation of any of these conditions or other equipment mal-functions results in a cessation of the test run. The conclusion of a successful test run results in the “legal acceptance” of the plant from the contractor. The best point in time to define plant startup is when the ammonia flows to the converter is initiated. Barring no usual problems, the completion of a successful test run and the achievement of maximum production rate should be about one month or less from this starting point. An important point with respect to nitric acid facilities is that the summer months are the most critical for proper operation due to cooling requirements for the exothermic reaction involved. For this reason, most new plants try to come online during the hotter periods when a successful test run would be most meaningful. Because of the requirement for performance testing within 180 days of startup, it is conceivable that testing could be required during the cooler months when a plant would find it easiest to meet applicable emission limitations. In this instance, regulatory agencies might want to conduct testing as soon after startup as possible, consider
139
postponement of tests until the following summer, or consider winter testing and subsequent summer testing.
CHAPTER 11 CONCLUSION AND RECOMMENDATION 11.1 CONCLUSION From the design procedures followed and results obtained, it can be concluded that a plant can be set up to produce 400,000 tonnes of Nitric acid per annum from ammonia oxidation. The excess steam generated in the process can be gathered and sold to increase the total income to be realized from sale of products. Also, the exhaust gases from the turbine is reduced to the lowest minimum (<1000ppm). This is to reduce the NOx emission from the plant which is in line with the Federal Environmental Protection Agency (FEPA) regulations. The produced acid will be sold mainly to fertilizer manufacturing plants and oil servicing companies in Nigeria, and can be exported as well. Finally, an economic evaluation of the plant showed that the rate of return on investment is about 26.25% and the payback time is about 3years and 7 months. Therefore the project can be said to be economically feasible.
140
11.2 RECOMMENDATION Additional control schemes should be put in place to ensure very low nitrous oxide emission; this will contribute to the global objective in reducing environmental degradation. There should be considerations for a two stage air compression to supplement the fluctuations in air requirement due to the anticipated increase in the demand of nitric acid. There should be provisions for preventive maintenance, as this will help to reduce frequent shutdowns due to repairs. It is also anticipated that this plant will be part of a larger chemical complex. Ammonia will be produced by steam reforming of natural gas. The nitric acid plant will take a portion of the ammonia product, and nitric acid and ammonia will then be used to produce ammonium nitrate.
141
REFERENCES Ababio, O.Y. 2005, New General Chemistry, Africana- Fep Publishers, Sydney.
Aneke, L. E. 2009, Principles of chemical engineering process design, De-adroit innovation, Enugu.
Anon, A. 1979 ‘Nitric Acid rolls on’ Chemical Engineering 29 June, pp. 24-25.
Boland, D. & Linnhoff, B. 1979 ‘The preliminary design of networks for heat exchangers by systematic methods’ Chemical Engineering, London 22 April, pp. 25-27.
Brown, K. J. 1989 Process integration initiative (review of the process integration initiatives funded under the Energy Efficiency R&D Program), Energy Technology Support Unit, Harwell Laboratory, Didcot United Kingdom, pp. 221-236.
142
Canon, B.W 1998 Safety and health in workplace, Nostrand Rein hold, New York, pp.201-203.
Cheremisinoff, N. P. 2000, Chemical process equipment, Butterworth Heineman, New Delhi.
Chilton T.H. ‘The manufacture of nitric acid by oxidation of Ammonia : the Du pont pressure process’ Chemical Engineering Progress, Monograph Series Vol. 56, AIChe, New York.
Coulson J. M. & Richardson J. F. 2004 Coulson & Richardson’s Chemical engineering, 6th Ed. Vol. 1, Elseiver publishers London. Durilla, M. 2009, NOx and NO2 control in nitric acid plants, Queens Publishing House, U.S.A.
Felder R. & Rousseau R. 2000, Elementary principles of chemical processes, 3rd Ed. John Wiley & sons, New York.
Gregory T.C 1999, Uses and Applications of chemicals and related materials, Reinhold Publishing, New York.
Harvin R.L, Leray D.G & Roudier L.R 1979, ‘Single pressure or dual pressure nitric acid: an objective comparison’, Ammonia Plant Safety, Vol. 21, pp.173-183, AIChe, New York.
143
Himmelblau, D. M. 2003, Basic principles and calculations in chemical engineering, 6th Ed. Prentice Hall, India. House, F. F. 1969 ‘Engineers guide to plant layout’ Chemical Engineering, NY 76 July 28 p.120. Kent J.A, 1983, Reigel’s Handbook of Industrial Chemistry, Van Nostrand Ranhold Publishing, New York. Kirk B.E & Othmer D.F (Eds) 1981, Encyclopedia of Chemical Technology 3rd Ed. Vol.15 Wiley-Interscience, New York, pp.853871. Linnhoff, B, Dunford, H & Smith, R 1983, Heat integration of distillation columns into overall processes, Chem. Eng. Sc., 38(8), pp. 1175-1188. Martyn, S.R. & David, W. J. 1989, Chemical engineering design: a case study approach, Bell and Bain Ltd, Glasgow.
Max, S.P, Klus, D. T. & Ronald, E.W 2003, Plant design and economics for chemical engineers; 5th Ed., McGraw-Hill, New York.
Ohrue T., Ohkubo K. & Imai O. 1999, Technological improvements in strong nitric acid process, Vol. 21 pp.164-170, AIChe, New York.
144
Perry R. H., Green D. W. & Maloney J. O 2008, Perry’s Chemical Engineers’ Handbook, 8th Ed. McGraw-Hill, New York.
Sinnot, R.K 2005, Chemical engineering design, 4th Ed., Butterworth-Heinemann, London.
145
APPENDIX I TABLES AND CHARTS Table A.1:
Conversion factors for some common SI units
146
Table A.2: Typical Overall Coefficient
147
Table A.3:
Typical Design stress for Plates
148
Figure A.1: Temperature correction factor: for one Shell; two or more even tube passes Heat exchange
APPENDIX II MATERIAL BALANCE CALCULATION Basis: 1hour
4000000tons HNO3 1 year
1 year 8000 hours =50tonsHNO 3
solutions/hour
149
ABSORBER AND STRIPPER 3NO2+H2O ⇌ 2HNO3+NO 50tons HNO3 solution
0.6 tons HNO3 1 ton HNO3 solution
=30 tons HNO3 Produced 30
tons 3
HNO3
tons 1 ton mole 46 tons NO2 100
moles NO2 HNO3 2 ton moles 63 HNO3
tons
NO2 fed tons 1 ton mole 98 tons NO2
HNO3
NO2
converted
=33.528 tons NO2 fed 1 ton mole NO 2
ton
30 tonsHNO3
1
ton
mole 30 tons NO
HNO3 63 tons HNO3
moles
1 ton mole NO
HNO3 = 7.143 tons NO Produced
OXIDISING UNIT 2NO+O2 ⇌ 2NO2 33.528
tons
1 ton mole O2
1 ton mole NO2 32 tons O2
NO2 fed 2
ton
NO2
moles 46 tons NO2
1 ton mole O2
150
=11.662 tons O2 converted 11.662 tons O2
1 ton mole O2
32 tons O2 =21.866 tons NO converted 11.662 tons O2
2 tons moles NO 1 ton mole O2
30 tons NO 1 ton mole NO
1 ton mole O2
2 tons moles
32 tons O2
NO fed fed 1 ton mole O2 1 ton mole NO fed
30 tons NO
fed
=21.866 tons NO fed CONVERTER 4NH3+5O2 ⇌ 4NO+6H2O
23.197
tons 5 tons moles 1 ton mole NO
NO
O2 converted 4 tons moles
30 tons NO
NO =30.929 tons O2 converted
32
tons
O2
converted 1 ton mole O2 converted
Quantity of O2 fed to the converted= (11.662+30.929) =42.591 tons O2 fed. 23.197 NO
tons 4 tons moles 1 ton mole NO NH3 4 tons moles 30 tons NO
NO =13.145 tons NH3 converted.
17 tons NH3 1 ton NH3
mole
151
13.145 tons NH3
1 ton NH3 fed 0.98 ton NH3 converted
=13.413 tons NH3 fed. Quantity of NH3 leaving converter= (13.413-13.145) =0.268 tons NH3. 23.197
tons
NO
6 tons moles 1 ton mole NO H2O 4 tons moles 30 tons NO
18 tons H2O 1 ton mole H2O
NO =20.877 tons H2O Produced. 0.005 tons H2O 0.995 tons NH3 0.0679
13.413 tons NH3
Total Quantity of H2O leaving Converter= (20.877+0.0679) = 20.95 tons. Make up H2O is added to ensure efficient chemosorption. Quantity of makeup water= 7.059 Quantity of H2O fed to Absorber= (7.059+21.582) = 28.641 tons of H2O. 30 tons HNO3
1
ton
mole 1 ton mole H2O 18 tons H2O
HNO3 63 tons HNO3
2 tons moles 1 ton mole H2O HNO3
=4.286 tons H2O Required. =0.64 tons O2
152
O2 left to react=6.32 tons O2 {6.96 – 0.64} Assume 50% conversion of NO Amount of NO reacted = 0.5 × 8.6 =4.3tons NO 0.27 tons NH3
4.3 tons NO
5 mols O2 4 mols NH3
1 mol NH3 17 tons NH3
32 tons O2 1 mol NH3
1 mol O2 2 mol NO
1 mol NO 30 tons NO
32 tons O2 1 mol O2
=2.29tons O2 reacted Amount of leaving stripper: 6.32 – 2.29 = 4.03 tons O2 Amount of NO left unreacted: 8.6 – 4.3 = 4.3 tons
=0.48 tons NO Total amount of NO leaving stripper = 4.3 + 0.48 =4.78 tons NO 0.27ton NH3
6 mols H2O 4 mols NH3
1 mol NH3 17 tons NH3
18 tons H2O 1 mol H2O
153
=0.43 tons H2O Total amount of water vapor leaving the stripper = 23.83 + 0.43 -20 =4.26 tons NB: Amount ofH2O in HNO3 solution =20 tons. 4.3 tons NO
2 mols NO2 2 mols NO
1 mol NO 30 tons NO
46 tons NO2 1 mol NO2
=6.59 tons NO2 Total NO2 leaving stripper =6.59 + 0.672 = 7.262 tons NO2 COMBUSTION CHAMBER AND PURIFICATION REACTOR. COMBUSTION CHAMBER CH4 + 2O2 CO2 + 2H2O 2C2H6 + 7O2 4CO2 + 6H2O 2H2 + O2 2H2O 2NO + O2 2NO2 Assume 10 tonnes of natural gas supplied to combustion chamber with composition in wt % CH4:85.7, C2H6: 4.8, N2: 3.2, H2: 6.3 8.75 tons CH4 2 mols O2 1 mol CH4 =34.28 tons O2 required
1 mol CH4 16 tons CH4
32 tons O2 1 mol O2
8.57 tons CH4 1 mol CO2 1.02 1 mol CH4 =23.10 tons CO2
1 mol CH4 16 tons CH4
44 tons CO2 1 mol CO2
1 mol CH4 16 tons CH4
18 tons H2O 1 mol H2O
8.57 tons CH4 2 mols H2O 1.02 1 mol CH4
154
=18.9 tons H2O 0.48
tons 7 mols O2
1 mol C2H6
32 tons 02
30 tons C2H6
1 mol O2
1 mol C2H6
44 tons CO2
30 tons C2H6
1 mol CO2
1 mol C2H6
18 tons H2O
30 tons C2H6
1 mol H2O
C2H6 2 mols C2H6 =17.92 tons O2 0.48
tons 4 mols CO2
C2H6 1.02 2 mols C2H6 =1.38 tons CO2 0.48
tons 6 mols H2O
C2H6 1.02 2 mols C2H6 =0.847 tons H2O 0.3 tons H2
2 mols H2O 2 mol H2
1 mol H2 2 tons H2
18 tons H2O 1 mol H2O
2.7 tons H2O N/B: Assume 50% conversion of H2, 90% conversion of CH4 and C2H6,30% conversion of NO Amount of CH4 leaving combustion chamber = 8.57(0.1) = 0.857 tons Amount of C2H6 leaving combustion chamber = 0.48(0.1) =0.048 tons
APPENDIX III ENERGY BALANCE CALCULATION Unit 3:
THE COMPRESSOR
Tin= 20°C
Tout=155°C
155 T2
Heat, Q = n∆H =
∫ CpdT T1
Components involved N2 an O2 Specific heat capacities; N2 = 1.04 KJ/KgK and O2 = 0.6486 KJ/KgK Enthalpy, H; H1= 1.04(428 −293 ) = 140.4 KJ/Kg H2 = 0.64886(428− 293) = 87.56KJ/Kg H3 = 0.6486 (428 – 293) = 87. 56KJ/Kg Q= Heat output from the Compressor; Q = n∆H = ∑ nHOut −∑ nHIn 187030(140.4) + 49720 (87.56) −49720 (87.56) Q = 2625901 KJ/hr Let q = Volumetric flowrate of air q = Fair// Dair Fair = Flow rate of air, Dair = Density of air q = 236750 Kg/m3/1.178Kg/m3 = 200976.23 m3/hr Theoretical Power of the Compressor= P1Q1ln (P1/P2) = 1 ×200976.23 ln (7.3/ 1) = 399515.49 KJ/hr Actual Shaft Work required =
Theoretical power Efficiency
= 399515.49/ 0.84 = 475613.68KJ Unit 5: AIR HEATER
156
Tin = 155°C
T 0ut = 250°C
Enthalpy, H; 533
H1 and H3 = =
∫ 0.9454+1.315 ×10
-4
428
152.484KJ/Kg
H2 = 0.973504T + 1.76 ×10 =
T −5897.46 T-2
T2 −1187.2T-1
-4
80.79 KJ/Kg
Heat Duty of the Air Heater, Q = n ∆ H = ∑ nHOut − ∑ HIn Q = 187030(80.7782) + 49720(152.484) – 49720(152.484) = 15107946.75 KJ/hr Unit 8; THE CONVETER
Reactions involved in the Converter 4NH3 + 5O2
4NO + 6H2
∆Hr = −907 KJ/K
Balance with reaction; Q = ∆H = ξ∆Hr + ∑nOut HOut− ∑ninHin ξ = extent of reaction ξ=
(270 − 13500)/ -4 = 3307.50 KJ/hr
Specific Heat capacity of NH3 Cp/R =( 3.578 + 3.020 ×10
T + (-0.186 ×10
-3
T-2))
5
0.488 Calculating the enthalpies H1 and H5 = 1.0142(1163 −523) = 615.58 KJ/Kg H2 and H7 = 1.0838 (1163 – 523) = 693.632 KJ/Kg
×
157 1163
H4 = =
∫ 1.7461+1.47376× 10
-3
523
T – 9076.8T2
3035.09 – 1132.10 = 1902.99 KJ/ Kg 1163
H6 =
∫ 0.9385+0.173 ×10 523
T + 387.94T-2)
-4
= 610.38 KJ /Kg Heat
outlet,
Q
=
3307.5(-907.3)
+
(270
427 (612.58)+160860 (693.632)+65(1305.98) ×1902.99+1160 ( 612.58 ) +21060 ( 1305.98 ) – ¿
Q = 20579273.85 KJ/hr Unit 9; WASTE HEAT BOILER Inlet Temperature = 890 ° C and Outlet Temperature= 250°C Enthalpies of the compositions 0.9454+1.315 ×10 ¿
H1 and H3 =
523
∫¿
-4
T −5897.46 T-2)
1163
= 523.70 – 1193.50 = -669.8 KJ/Kg H2 and H4 = =-719.25 KJ/Kg H5 = -685.088 KJ/ Kg Q= (1160 (-669.8) + 16080( -719.25) + 23320(-685.088) – 1160(-669.8) + 160860(-719.25)
158
Q = -15976252KJ/Kg Temperature of Steam coming out from the Waste Heat Boiler Q = mCp (TST – TCW) m= 1000Kg/hr TCW=30°C, Cp = 4.2 KJ/hr TST -30 = 15976252/1000 × 4.2 TSteam=410°C Unit 13: STACK GAS HEATER
Tin = 200°C
TOut =
150°C H1 and H3 = 0.973504T + 8.8 ×10
T2
-5
−¿
1187.2T
= -52.909 KJ/Kg H2 and H
5
= Cp ∆ T = 1.31(-50) = -65.5KJ/Kg
H4= -50.906KJ/kg Q = 160860(-52.909) + 1460(-50.906) + 33530(-65.5) – 160860(-52.909) + 335030(-65.5) Q= -74322.76 KJ/Kg ¿ ×2.15 ׿ TST -30) -74322.76 = 13565
159
TSteam= 118.48 Unit 12: WASTE HEAT BOILER Inlet Temperature = 150°C = 423K Outlet Temperature = 50°C = 323K Enthalpies, H H1 and H3 = 0.97350T + 8.8 ×10
T – 1187.2T-1
-5
= -102.59 KJ/Kg H2 and H5 = 131 KJ/Kg Q = 160860(-104.734) + 1460(-102.59) + 33530(-131) Q=-149781.4 KJ/hr TSteam=103°C AMMONIA VAPORIZER Q + ƛ =13565 × 2.15(T +33.4) ƛ=Latent heat of Steam= 1910.3 Q=149781.4 T = 15169.14/ (13565×2.15) -33.4= -28.199°C AMMONIA SUPERHEATER Q= -1596252KJ/hr, ƛ=3300 Q + ƛ = 13565×2.15(T + 28.2) T = (1596252+3300)/ (13565×2.15) -28.2 T=26.65°C
160
APPPENDIX IV EQUIPMENT DESIGN
CALCULATION
STACK GAS HEATER Q = 74322.76KJ/h, Th in =2000C, Th =118.480C, U =0.102Kw/m2
out
= 1500C, Tc in = 300C,Tc out
Converting Q = 74322.76KJ/h to KJ/s 74322.76/3600 =20.6452KJ/h Tm=( Th ¿−Tc out )−( Thout −Tc ¿ ) Th −Tcout ln [ ¿ ] Thout −Tc ¿
( 200−118.48 )−(150−30) (200−118.48) ln [ ] (150−30) 81.52−120 81.52 ln 120 38.48 0.387 o
∆ T m=99.43 C A=
Q U ∆Tm
20.6452 0.102× 99.53
A = 2.03m 2
161
AMMONIA VAPORIZER Q=
149781.4 KJ 149781.4 ,∈ KJ / sec= =41.606 KJ /S hr 3600
Tc ¿ =−33. 4 o C out =¿−28.19 9o C Tc ¿ Th¿ =208
Thout =167.2 ∆ T m=
(Th¿ −Tc out )−(Thout −Tc ¿ ) Th¿−Tc out ln [ ] Thout −Tc¿
(208+28.199)−(167.2+ 33.4) 208+28.199 ln [ ] 167.2+33.4 236.199−200.6 236.199 ln [ ] 200.6 35.599 35.599 o = =218. 4 C ln 1.177 0.163
U = 0.102KW/m2 A=
Q 41.606 = =1.868 m2 U ∆ Tm 0.102 ×218.4
WASTE HEAT BOILER 1
162
∆ T m=
(Th¿ −Tcout )−(Thout −Tc¿ ) Th¿ −Tcout ln [ ] Thout −Tc ¿
(890−410)−(250−30) 480−220 = 890−410 480 ln [ ] ln 250−30 220 ∆ Tm=333.76 A=
Q 4437.8 4437.8 = = =130.73 m2 U ∆ Tm 0.102 ×333.76 34.04
WASTE HEAT BOILER 2 ∆ Tm=
A=
(208−150)−(50−32) 58−18 40 = = =34.48 208−150 58 ln3.2 ln [ ] ln 50−32 18
Q U ∆ Tm
Q = 149781.4KJ/hr, converting to KJ/s = 41.61KJ/S 41.61 =11.83 m2 0.102×34.48
NH3 SUPERHEATER.
163
∆ T m=
(Th¿ −Tcout )−(Thout −Tc¿ ) Th¿ −Tcout ln [ ] Thout −Tc ¿
∆ Tm=
( 410−26.65)−(330−28.2) 81.55 = =340.9 5o C 410−26.65 ln 1.2702 ln [ ] 330−28.2
Q = 4437.85KJ/S. U = 0.102KW/m2 A=
4437.85 =127.6 m2 0.102 ×340.95
AIR HEATER Q = 14881486.75KJ/hr = 4133.7KJ/S. ∆ Tm=
A=
(350−250)−(200−155) 100−45 55 55 = = = =68.8 4 o C 350−250 100 ln 2.22 0.799 ln [ ] ln ( ) 200−155 45
Q 4133.7 = =588.7 m2 U ∆ Tm 7.02
164
APPENDIX V EQUIPMENT COSTING CALCULATION IN 1998 Cost ∈ A m2=cost of 500 m2 ×(
A m2 0.6 ) 500 m2
IN 2013 Cost ∈2013=cost∈1998 ×(
2013 index ) 1998 index
Index in 1998 = 390 Index in 2013 = 683.6 AMMONIA VAPORIZER
165
Cost of 500m2 = N1.84 million A = 1.87m2 0.6
Cost ∈1998=N 1.84 million ×(
1.87 ) =N 0.06 million 500
Cost ∈2013=N 0.06 million ×(
683.6 ) 390
N 0.11 million. WASTE HEAT BOILER 1 IN 1998 2
2
Cost ∈ A m =cost of 500 m ×(
A m 2 0.6 ) 500 m 2
IN 2013 Cost ∈2013=cost∈1998 ×(
2013 index ) 1998 index
Index in 1998 = 390 Index in 2013 = 683.6 Cost of 500m2 = N1.84 million WASTE HEATER BOILER 2 In 1998 Cost ∈ A m2=N 1.84 million ×(
11.8 0.6 ) 500
166
N 1.84 million ×(0.0236)0.6 cost of A m2=
N 1.84 million × 0.106 N 0.19 million
In 2013 cost of 2013=N 0.19 million ×
683.6 =N 0.33 million 390
AMMONIA SUPERHEATER A = 128m2 Cost in 1998 128 0.6 Cost of 128 m =N 1.84 million ×( ) =N 0.8214 million 500 2
Cost in 2013 cost of 2013=N 0.8124 million ×
683.6 =N 1.424 million 390
WASTE HEAT BOILER 1 A = 130.5m2 In 1998 Cost ∈ A m2=cost of 500 m2 ×(
2
Cost of 130.5 m2=N 1.84 million ×(
cost of 2013=N 0.8219 million ×
FOR AIR HEATER
0.6
Am ) 2 500 m
0.6
130.5 ) =N 0.8219 mi llion 500
683.6 =N 1.44 million 390
167
IN 1998 A m 2 0.6 Cost ∈ A m =cost of 500 m ×( ) 500 m 2 2
2
IN 2013 Cost ∈2013=cost∈1998 ×(
2013 index ) 1998 index
Index in 1998 = 390 Index in 2013 = 683.6 Cost of 500m2 = N1.84 million Cost of A m2=N 1.84 million ×(
cost of 2013=N 2.1 million ×
588.7 0.6 ) =N 2.1 million 500
683.6 =N 3.68 million 390
STACK GAS HEATER 1998 Cost ∈ A m2=N 1.84 million ×(
2.03 0.6 ) =0.068 million 500
cost of 2013=N 0.068 million ×
683.6 =N 0.119 million 390
CONVERTER Cost index 2013 = 680.1 Cost index 1990 = 390
168
Volume = 600 gallon Cost for 600 gallon in 1990 = 17000 0.6
Cost of 1 million gallon∈1990=17000 ×(
Cost ∈2013=$ 1.457(
1000000 ) =$ 1.457 million 600
680 )=$ 2.541million 390
COMPRESSOR In 1990 Cost of eq1 = cost of eq2 Cost of eq1 = $2100
×(
(
cap 1 ) cap 2
7294 ) 447.8
0.6
0.6
Cost of eq1 = $ 11203.4 cost index 2013 Cost in 2013 = cost in 1990 ×( cost index 1990 ) 928.1 cost in 2013 = 11203.4 ( 756.3 )
=$ 13748.4 = # 2.2 million Cost for nitric acid storage tank Given volume of the tank at 1990 = 12 × 1606 gallons Cost at 1990 = $170000 Cost index at 2013 = 683.6 Cost index at 1990 =395
169
Volume of storage tank =28.3m3 to liters =23800l For 5000 gallons (
Cost at 1990 = 170000
5000 ) 6 12 ×10
0.6
1637.96 = $2830.56 = #447228 Cost in 2013 = 1593.39(
683.6 ¿ 395
= $2830.56 = N447228.00 For year 2006 Cost of equipment 1 = cost of equipment 2 For
(
capacity of equipment 1 ) capacity of equipment 2
year
cost∈2006 cost∈2013
=
2006
CPE ∈2006 CPE ∈2013
RATIO METHOD Cost (
of
designed
capacity of designed plant ) capacity of previous plant
plant
=
cost
of
n
Cost of 1200tons per day =cost of 280 per day
= $60 million
( 1200 280 )
previous
0.6
= $144 million
(
1200 ) 280
0.6
plant
170
=#23 billion STEP COUNTING METHOD C = 14000 N Q0.615 N = 13(number of functional units) Q = 400000 tons/yr (capacity of plant) C = 14000 ×13 ×(400000) = # 79.8 billion
0.615
= $507 million
