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1.0 Introduction
Sulphuric acid (H 2SO4) is one of the most important compounds made by the chemical industry. It is used to make, literally, hundreds of compounds compounds needed by almost every industry. There are two major processes in production of sulphuric acid, they are by contact process and lead chambers and it is available commercially in a number of grades and concentration. The contact process produces a purer, more concentrated acid but require purer raw materials and the use of expensive catalyst. The lead chamber process is used to produce much of the acid used to make fertilizers. It produces a relatively dilute acid (62% - 78% H2SO4). In both processes Sulphur dioxide is oxidized and dissolved in water. Some sulphuric acid is also made from ferrous sulphate waste solutions from pickling iron and steel and from waste acid sludge from oil refineries. refineries . They are many uses of Sulphuric Acid (H2SO4) in industries. By far the largest amount of sulfuric acid is used to make phosphoric acid to make the phosphate fertilizers, fertilizers, calcium dihydrogenphosphate and the ammonium phosphates. It is also used to make ammonium sulfate, which is a particularly important fertilizer in sulfur-deficient. in sulfur-deficient. paints, pigments hydrofluoric acid 3% 3%
pulp paper 1%
fibres 6%
metal processing 13%
phosphates 8%
phosphate fertilizers 66%
Figure 1: Uses of Sulphuric Acid (H 2SO4) 1|Page
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Figure 1 showed the percentage of uses if sulphuric acid in industries. It is widely used in metal processing for example in the manufacture the manufacture of copper and the manufacture the manufacture of zinc and in cleaning the surface of steel sheet, known as 'pickling', prior 'pickling', prior to it being covered in a thin layer of tin, used to make cans for food. It is also used to make caprolactam, which is converted into polyamide into polyamide 6 and in the manufacture the manufacture of titanium dioxide, used, dioxide, used, for example, as a pigment.Amongst its many other uses is in the manufacture of hydrofluoric hydrofluoric acid and phenol and phenol with propanone all of which are used in many industries.
1.1 Manufacture of sulfuric acid
The process for producing sulphuric acid has four stages: a) Extraction of Sulphur. b) Conversion of Sulphur to sulphur dioxide. c) Conversion of sulphur dioxide to sulphur trioxide. d) Conversion of sulphur trioxide to sulphuric acid. 1.1.1 Extraction of Sulphur
Easily the most important source of sulfur is its recovery from natural gas and oil. These contain sulfur contain sulfur compounds, both compounds, both organic and hydrogen sulfide both of which must be removed before they are used as fuels or chemical feedstock. Another important source of sulfur is as sulfur dioxide from metal refining. Many metal ores occur as sulfides and are roasted to form an oxide and sulfur dioxide, for example in the manufacture of lead: of lead:
Other metals manufactured from their sulfide ores include copper, copper, nickel and zinc. Worldwide about 35% of the sulfur is obtained as sulfur dioxide from sulfide ore roasting and this is increasing, as plants which traditionally passed the sulfur dioxide to atmosphere are recovering it as sulfuric acid. In particular, China makes most of its sulfuric acid from pyrites, an iron sulfide ore. Sulfuric acid is also obtained from ammonium sulfate, a by-product in the manufacture of poly (methyl 2-methylpropenoate) and also recovered from 'spent' sulfuric acid. 2|Page
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1.1.2 Conversion of Sulphur to Sulphur dioxide
If sulfur is the feedstock, it must first be converted to sulfur dioxide. Molten sulfur is sprayed into a furnace and burnt in a blast of dry air at about 1300 K. The sulfur burns with a characteristic blue flame:
As excess air is used the emerging gas contains about 10-12% sulfur dioxide and 10% oxygen, by volume. The gases are very hot and so are passed through heat exchangers (waste heat boilers). The gases are cooled to about 700 K and the water in the surrounding boiler pipes is converted into steam. In manufacturing one tonne of sulfuric acid, one tonne of high pressure steam is also produced. 1.1.3 Conversion of sulphur dioxide to sulphur trioxide (The Contact Process)
A typical plant contains one cylindrical vessel which acts as a fixed bed reactor with four separate beds of catalyst, known as a converter, heated to 700 K, through which the sulfur dioxide and air pass:
The catalyst, vanadium(V) oxide on silica, is generally in the form of small pellets, to which caesium sulfate has been added as a promoter (Figure 2). The function of the promoter is to lower the melting point of vanadium(V) oxide so that it is molten at 700 K. 1.1.4 Conversion of sulphur trioxide to sulphuric acid
The sulfur trioxide formed from the third bed (and the small amount from the fourth bed) are now converted tosulfuric acid. Sulfur trioxide reacts with water and the reaction can be expressed as:
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However, water itself cannot be used for absorption as there is a large temperature rise, and a sulfuric acid mist is formed, which is difficult to handle. Instead, sulfuric acid of about 98% concentration is used. This is kept at this concentration by addition of water and removal of acid at that concentration. To keep the temperature at about 400 K, the heat is removed by heat exchangers, Figure 2.
Figure 2: A line diagram illustrating a heat exchanger used in the manufacture of sulfur trioxide.
The gases not absorbed contain about 95% nitrogen, 5% oxygen, and traces of sulfur dioxide. The gas stream is filtered to remove any traces of sulfuric acid mist and is returned to the atmosphere using a high stack.
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1.2 Significance of Process instrumentation in industry
Industry has entered into a new age. This can be seen in the advancements of instrumentations that have brought a very important changes in industry. For example, today, most products are manufactured using some type of automatically controlled processing equipment. This equipment is often complex, demanding a variety of skilled personnel to keep it in operation. At one time, most industrial equipment could be easily placed into operation with just a few simple tools and some basic common sense. Today the situation differs in that a large part of equipment and processes contain numerous control devices vital to the performance of precise automatic operations.The instrumentation calibration techniques and troubleshooting methods is needed in order to keep things in a good state of repair and reliability. Equipment breakdown is common problem will be faced in industry. Preventative maintenance and operational efficiency of plant instrumentation have become more important with the increased dependency quality and maximum production. The industrial instrumentation is a rather broad area of study that deals with the measurement, evaluation and control of process variables such as temperature, pressure, flow rate, fluid level, force, light intensity and humidity. These variables are usually involved in a manufacturing operation of some type that eventually leads to a finished industrial product. Automation production operations are largely responsible for a major part of all instrumentation applications. The primary areas of concern in instrumentation are pneumatics, electronics, mechanics and hydraulics. Each of these areas is individually unique, but they are all very similar in many respects.
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2.0 PFD and P&ID Diagram 2.1 PFD
PFD is depicting a given system’s flows, temperature, pressure and mass balance for various operating conditions. Such information is typically presented in the form of tables. PFD present functional information about a system or subsystem. The components such as piping, pumps and valves are represented by standard schematic symbols that illustrate their function in the system, as opposed to their relative sizes, locations or physical shapes. The instrumentation and control information is not included. PFD are typically the first drawing developed by mechanical processes, often in the pre-conceptual or conceptual design place.
2.2 P&ID
P&ID are typically developed from PFD. P&ID communicated detailed information on how to operate, troubleshoot and repair or modify the system or subsystem . In addition to the mechanical components, they include instruments, signal modifiers, controllers and their inter-relationship. They typically do not include tabular parameters as PFD do. P&ID is a very common term used in the world of process industries. A process engineer in a manufacturing plant need to create new and/or modify the already existing P&ID to the as-is plant modifications. A P&ID is a detailed graphical representation of a process including the hardware and software (piping, equipment, instrumentation) necessary to design, construct and operate the facility.
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3.0 Process Description 3.1 Sulphuric Acid
A sulphuric acid with a chemical formula of H 2SO4 is a strong acid and it is widely used for many applications. Sulphuric acid has become one of the most important industrial chemicals and causing a high demand in sulphuric acid manufacturing locally and globally. However it is odorless, colorless and extremely corrosive. Sulphuric acid at a high concentration can cause very dangerous on contact and safety precautions should be strictly observed when handling it. In general, there are several process in sulphuric acid production such as contact process, wet sulphuric acid process and others. Among all of the processes, only contact process are being focused in this mini project.
3.2 Contact Process
Now a days, sulphuric acid is prepared by contact process all over the world. Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of SO 2 to SO3.
3.3 Details Of Contact Process
Following steps are involved in the preparation of H 2SO4. i.
Preparation of SO2.
ii.
Purification of SO2.
iii.
Oxidation of SO2.
iv.
Absorption of SO 3.
v.
Dilution of Oleum.
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3.3.1 Step I - Preparation of SO 2
SO2 is obtained by burning sulphur or by heating iron pyrite (FeS 2) in pyrite burner. S + O2 4FeS2 + 11O2
SO2 2Fe2O3 + 8SO2
3.3.2 Step II - Purification of SO 2
SO2 contains a number of impurities such as dust particles, Arsenous oxide, vapours, sulphur etc. These impurities must be removed otherwise catalyst loses its efficiency (catalyst poisoning). 3.3.2.1 Dust Chamber
SO2 is first passed through the dust chamber where steam is spread over the gas to remove dust particles, which settle down. Fe(OH) 3 also sprayed over to remove oxides of Arsenic. 3.3.2.2 Washing Tower
SO2 is then passed through a washing tower after cooling. Here it is sprayed by water to remove any other soluble impurities. 3.3.2.3 Drying Tower
The gas is now dried by passing through drying tower where concentration H 2SO4 (dehydrating agent) is sprayed. H2SO4 removes moisture from SO2.
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3.3.3 Step III - Oxidation of SO 2 To SO3 3.3.3.1 Contact Tower
Oxidation of SO 2 is carried out in contact tower where V 2O5 is filled in different pipes. SO2 here reacts with air (O2) to produce SO3. Under above conditions 98% SO 2 is converted into SO3. 2SO2 + O2
2SO3
3.3.3.2 Conditions Necessary For Maximum Yield Of SO 3
Oxidation of SO2 is a reversible and exothermic process in which volume of product is less than the volumes of reactants. In order to obtain maximum amount of SO 3, according to LeChatelier’s Principle following conditions are necessary. 3.3.3.2.1 Temperature
A decrease in temperature favours reaction in forward direction. Optimum temperature for this process is
450oC to 500 oC.
3.3.3.2.2 Pressure
Since volumes of reactants are greater than the product (3:2), therefore, according to LeChatelier’s Principle a high pressure is favourable. Optimum pressure is about 1.5 to 1.7 atmosphere. 3.3.3.2.3 Use of Catalyst
At low temperature, rate of reaction was decreases. To increase rate of reaction a catalyst vanadium pentaoxide (V2O5) is used.
3.3.4 Step IV - Absorption of SO 3 in H2SO4
SO3 is not directly passed in water, because a dense fog of minute particles of H 2SO4 is produced. It is therefore, dissolved in concentrated H2SO4 to form pyrosulphuric acid (oleum). SO3 + H2SO4
H2S2O7 (OLEUM)
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3.3.5 Step V - Dilution of Oleum
Oleum is now diluted with water to form H 2SO4 of required concentration. H2S2O7 + H2O
2H2SO4
3.4 Outline of Contact System Sulphuric Acid Plant
In process drawing (Figure 3) sulphur dioxide (SO 2) gas having a temperature of 300 to 500 oC generated from roasting furnace is relieved of large-grained dust particles and impurities by dust Cottrell. Fine dust and impurities that are still not removed are washed off by the next washing tower and cooled down to 30 to 40 oC through cooling tower. In the cooled gas a large quantity of mist is present, removed by first mist Cottrell, intermediate tower and second mist Cottrell. In the gas passed through mist Cottrell, 20 to 40 g/m 3 moisture is contained ; if this is passed on to the next blower heat exchanger, corrosion will be increased ; so in order to reduce moisture to about 0.1 g/m3, it is dried by a drying tower. Gas that is dried is passed through a filter by a blower and sent to heat exchanger and converter. Gas sent into theconverter produces catalytic reaction through catalysis and is oxidized into sulphur trioxide (SO 3). Temperature necessary for catalytic reaction is constantly maintained by the heat exchanger. Gas coming out from the converter by way of the heat exchanger is cooled, but is still too high in temperature (200 oC) for the absorption tower, so it is futher passed through a cooler to bring the temperture down to 100 oC. Sulphuric acid concentration most suitable for sulphur trioxide absorption is that point where total vapour tension is minimum; and below this, concentration component pressure of water will be great, making sulphur trioxide form a mist with water and difficult to absorb. Sulphur trioxide which has become about 100 oC will be absorbed by 93.8% sulphuric acid through the absorption tower, and sulphuric acid of 98% concentration will be produced. In the case of two absorption towers, by circulating fuming sulphuric acid in the first tower, it will be possible to produce fuming sulphuric acid.
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4.0 Instrumentation In Contact System Sulphuric Acid Plant
Figure 3: Process flow diagram (PFD) Contact System Sulphuric Acid Plant
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4.1 Selective Process
The process that is selected for the study is involving the reaction in the absorption tower. The drying acid absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant. Drying of the gas is necessa ry:
To avoid corrosion caused by wet SO 2 gas before the converter and by wet SO3 after conversion
To avoid loss of production due to the formation of acid mist in the absorption tower
To keep a clear stack,
To avoid acid condensation during shut-downs and thus protect the catalyst from degradation The absorber acid absorbs the SO 3 formed in the converter. Absorption of SO 3 from the
gas is necessary to:
Remove all SO3 and acid mist from the gas stream before it exits the tower.
Produce 98.5% acid in the absorption tower
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4.1 Selective Process
The process that is selected for the study is involving the reaction in the absorption tower. The drying acid absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant. Drying of the gas is necessa ry:
To avoid corrosion caused by wet SO 2 gas before the converter and by wet SO3 after conversion
To avoid loss of production due to the formation of acid mist in the absorption tower
To keep a clear stack,
To avoid acid condensation during shut-downs and thus protect the catalyst from degradation The absorber acid absorbs the SO 3 formed in the converter. Absorption of SO 3 from the
gas is necessary to:
Remove all SO3 and acid mist from the gas stream before it exits the tower.
Produce 98.5% acid in the absorption tower The Absorption Towers equipped with a ceramic packing to increase the contact surface
between the SO3 gas and sprayed acid. It is also equipped with mist eliminator to prevent scabbing of any mist drops from the tower.
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Figure 4: Process Equipment and Instrumentation layout of the process
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Table 1: Abbreviation of various instrument and physical variables List of Abbreviations & Specifications Equipments
Instruments
HP : Hoper for sulfur feed SMT : Sulfur Melting Tank
Level All having 0 to 10 ft level
DST : Dirty Sulfur Tank
L1 : SMT Tank Level
SF : Sulfur Filter
L2 : DST Tank Level
CST : Clean Sulfur Tank
L3 : CST Tank Level
AB : Air Burner with Ash Filter inside
L4 : Boiler Water Level
DR : Dryer
L5 : ACT Tank Level
AF : Air Filter
L6 : AST Tank Level
WHB : West Heat Boiler 4SCR : Four Stage Convertor/Reactor
Temperature
HE : Heat Exchanger
T0 : SMT tank temp. 120 to 160 oC
2SAR : Two Stage Absorber
T2 : AB temp. 800 to 900 oC
ACT Acid Circulation Tank
T3 SO line to boiler temp. 800 to 900 oC
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Table 1: Abbreviation of various instrument and physical variables List of Abbreviations & Specifications Equipments
Instruments
HP : Hoper for sulfur feed SMT : Sulfur Melting Tank
Level All having 0 to 10 ft level
DST : Dirty Sulfur Tank
L1 : SMT Tank Level
SF : Sulfur Filter
L2 : DST Tank Level
CST : Clean Sulfur Tank
L3 : CST Tank Level
AB : Air Burner with Ash Filter inside
L4 : Boiler Water Level
DR : Dryer
L5 : ACT Tank Level
AF : Air Filter
L6 : AST Tank Level
WHB : West Heat Boiler 4SCR : Four Stage Convertor/Reactor
Temperature
HE : Heat Exchanger
T0 : SMT tank temp. 120 to 160 oC
2SAR : Two Stage Absorber
T2 : AB temp. 800 to 900 oC
ACT : Acid Circulation Tank
T3 : SO 2 line to boiler temp. 800 to 900 oC
AST : Acid Storage Tank
T4 : Boiler temp. 700 to 900 oC T5 : SO2 line to 4SCR temp. 600 to 700 oC
Pumps All Having 0 to 500 rpm speed
T6 : 1st stage temp. of 4SCR , 825 to 945 oC
P1 : MST pump to DST
T7 : 2 nd stage temp. of 4SCR, 625 to 725 oC
P2 : DST pump to CST
T8 : 3 rd stage temp. of 4SCR, 450 to 550 oC
P3 : CST pump to AB
T9 : 4 th stage temp. of 4SCR, 400 to 450 oC
B1 : Air Suction Draft for dryer P4 : ACT to 2SAR pump
Switches
P5 : ACT to AST pump
S1 : SF pressure switch
P6 : AST pump for final product out
S2 : DR humidity switch S3 : DR pressure switch S4 : AF pressure switch CS1 : Venting SO 3 concentration switch 30% = low & 60% = high
Concentration
Control Valves
C1 : concentration of SO 3 from 4SCR 60% to
V1 : Steam for melting sulfer tank 14 | P a g e
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90% C2 : Concentration of sulfuric acid in 1 st stage
V2 : Air to bumer control valve
of 2SAR 97% to 99% C3 : Concentration of sulfuric acid in 2 nd stage
V3 : SO2 flow control to boiler
of 2SAR 92% to 94% V4 : Boiler feed water control Pressure
V5 : SO2 flow control to 4SCR
PT1 : AB pressure 0 to 20 kgpcme
V6 : Coolant flow control to HE
PT2 : Boiler Pressure 30 to 50 kgpcm 2
V7 : Air control for 3rd stage 4SCR
PT3 : Vapor pressure in ACT
V8 : Air control for 4 th stage 4SCR
PT4 : Vapor pressure in 2SAR
V9 : SO 3 flow control from 4SCR V10 : Recycle SO3 flow control for 2SAR
Flow
TV1 : Three way valve for vent or recycle of SO3
F2 : CST to AB flow of sulfur 0 to 10 CFPM
V11 : Dilution water control to ACT
F3 : SO2 gas flow to boiler 0 to 10 CFPM
V12 : Air control for ACT
F4 : SO2 gas flow to 4SCR 0 to 10 CFPM
V13 : Vent control of sulfuric acid vapor
F6 : Sulfuric acid flow from 4SCR to ACT 0 to 10 CFPM
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5.0 Description of instrument used in Contact Process
The Absorbing Acid System absorbs the sulphur trioxide (SO 3) that is formed in the converter to produce sulphuric acid. The reason for absorbing the SO3 is to achieve the desired production of sulphuric acid and to avoid environmental problems that would result if the SO3 were allowed to escape to the environment. Efficiently absorbing the SO 3 will also help to maintain a clear stack by avoiding the formation of sub-micron mist particles that would form the moment the SO 3 containing gas leaves the stack and reacts with the moisture in the air.
5.1 Equipment of Process
A typical Absorbing Acid System consists of the following items of equipment: a) Absorbing Tower b) Acid Pump Tank c) Acid Pump d) Acid Cooler e) Piping f) Instrumentation and Control 5.1.1 Absorbing Tower
A typical Absorbing Tower is a vertical cylindrical vessel designed to contact process gas and strong sulphuric acid (98.5% H 2SO4) for the purpose of absorbing SO 3. The tower may be constructed of specialty alloys or the more traditional brick lined carbon steel.
The
tower will be equipped with a packing support, packing, acid distributor, and mist eliminator. 5.1.2 Acid Pump Tank
An acid pump is required to circulate the acid from the pump tank up to the distributor inside the Absorbing Tower. The acid flows by gravity over the packing and drain out the bottom or side of the Absorbing Tower back to the pump tank. The pump can be a vertical submerged centrifugal pump in which case it would be mounted in the pump tank. External vertical centrifugal pumps are also available as well as horizontal centrifugal pump types, although the latter is uncommon. 16 | P a g e
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5.1.3 Acid Pumps
An acid pump is required to circulate the acid from the pump tank up to the distributor inside the Absorbing Tower. The acid flows by gravity over the packing and drain out the bottom or side of the Absorbing Tower back to the pump tank. The pump can be a vertical submerged centrifugal pump in which case it would be mounted in the pump tank. External vertical centrifugal pumps are also available as well as horizontal centrifugal pump types, although the latter is uncommon. 5.1.4 Acid Cooler
The absorption of water into sulphuric acid is an exothermic reaction and will cause the temperature of the acid to rise unless the heat of absorption is removed. The heat is typically removed in either a plate heat exchanger or anodically protected shell and tube heat exchanger. 5.1.5 Piping
Acid piping is required to carry the acid from one piece of equipment to the next. Materials vary considerably depending on the acid concentration, temperature and cost. 5.1.6 Instrumentation and Controls
Instrumentation and controls are required to monitor the operation of the system and control its operating parameters. Acid concentration, flow and temperature are the most important factors in ensuring the system is performing its function of gas drying as efficiently as possible.
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5.2 Absorption
The absorption of SO 3 from the gas relies on equilibrium factors as described in the section Absorption and Stripping. There are many physical factors that affect the degree of absorption that will occur in a tower. One of the primary factors is the vapour pressure of sulphur trioxide/sulphuric acid above the acid. Absorber towers operate in a relatively narrow range of concentration and temperature for effective absorption of SO3. This is apparent when you look at a plot of the total vapour pressure versus acid concentration curve. In a concentration range of 97.5% to 98.5% the equilibrium vapour pressure above sulphuric acid reaches a minimum. Operation in this acid concentration range will result in the maximum absorption of SO3. The vapour pressure curves (better curve to follow) indicate that the lower the acid temperature, the lower the vapour pressure above the acid. This would imply that operating the absorber system at lower temperatures would result in better absorption, however, this is not the case.
At lower temperatures other physical properties such as density and viscosity
begin to affect the absorption process in a negative manner. The specific operating conditions of a plant’s absorber system will depend on the design of the tower, packing characteristics, distributor design, acid flow, mist eliminator efficiency, etc. in addition to the acid concentration and temperature. The optimum operating conditions can be determined by measuring the amount of SO 3/H2SO4 leaving the tower or in the case of a final absorber, observing the opacity of the stack.
The quality of the stack
emissions are observed as the operating conditions are varied. Operating conditions must reach steady state and be held for a period of time before the affects of the new operating conditions are recorded.
A systematic program of varying operating conditions and
observation will allow the optimum operating conditions to be determined.
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6.0 Conclusion and Recommendation
Sulphuric acid (H2SO4) is one of the most important compounds made by the chemical industry. It is used to make, literally, hundreds of compounds needed by almost every industry. There are two major processes in production of sulphuric acid, they are by contact process and lead chambers and it is available commercially in a number of grades and concentration. The process for producing sulphuric acid has four stages which is extraction of sulphur, conversion of sulphur to sulphur dioxide, conversion of sulphur dioxide to sulphur trioxide and conversion of sulphur trioxide to sulphuric acid. The process that is selected for the study is involving the reaction in the absorption tower. The drying acid absorbs the water vapor remaining in the gas after it leaves the Gas Cleaning Section of the plant. The absorber acid absorbs the SO3 formed in the converter. Absorption of SO 3 from the gas is necessary to remove all SO3 and acid mist from the gas stream before it exits the tower. Besides that, it is to produce 98.5% acid in the absorption tower. The Absorption Towers equipped with a ceramic packing to increase the contact surface between the SO 3 gas and sprayed acid. It is also equipped with mist eliminator to prevent scabbing of any mist drops from the tower. Therefore, it is a recommendation for the future process in production of sulphuric acid by contact process. Firstly, it is about single absorption plant with a tail gas scrubber and bypass gas direct to tail gas scrubber during extended period of low or no SO 2. It is recommended because single absorption plants are better able to handle varying operating conditions than double absorption plants. It has lower auto thermal limit than double absorption plants. Besides that, the tail gas scrubber was able to handle varying inlet SO 2 levels and still provide low SO2 emissions. The tail gas scrubbers are also a backup during startups and upset conditions. Lastly, acid plants can tolerate short periods of no gas and still retain sufficient heat in the catalyst to restart without using the preheater. Bypassing gas direct to tail gas scrubber avoids having to use the preheater to maintain catal yst temperatures.
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7.0 References
Carey, F. A. "Reactions of Arenes. Electrophilic Aromatic Substitution". On-Line Learning Center for Organic Chemistry. University of Calgary. Retrieved 27 January 2008. Pelletreau, K.; Muller-Parker, G. (2002). "Sulfuric acid in the phaeophyte alga Desmarestia munda deters feeding by the sea urchin Strongylocentrotus droebachiensis". Jones, Edward M. (1950). "Chamber Process Manufacture of Sulfuric Acid". Industrial and Engineering Chemistry 42 (11): 2208 – 2210. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 653.
Maltis J.P, Renovance, Bashahr: “ Instrumentation in Producing Sulphuric Acid”, published 2008. Pp:38-pp 67.
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