1.0 INTRODUCTION Design is a creative process whereby an innovative solution to a problem is conceived. In this modern age of industrial competition, a successful chemical engineer needs more than a knowledge and understanding of the fundamental sciences and the related engineering subjects such as thermodynamics, reaction kinetics, and computer technology. The engineer must also have the ability to apply this knowledge to practical situations for the purpose of accomplishing something that will be beneficial to society. However, in making these applications, the chemical engineer must recognize the economic implications which are involved and proceed accordingly. All design starts with a perceived need. In the design of a chemical process, the need is the public need for the product, creating a commercial opportunity, as foreseen by the sales and marketing organization. Within this overall objective, the designer will recognize sub-objectives, the requirements of the various units that make up the overall process. Before starting work, the designer should obtain as complete, and as unambiguous, a statement of the requirements as possible. If the requirement (need) arises from outside the design group, from a customer or from another department, then the designer will have to elucidate the real requirements through discussion. When writing specifications for others, such as for the mechanical design or purchase of a piece of equipment, the design engineer should be aware of the restrictions (constraints) that are being placed on other designers. A well-thought-out, comprehensive specification of the requirements for a piece of equipment defines the external constraints within which the other designers must work.
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2.0 PROBLEM STATEMENT Pursuant to instruction from our lecturer we proceeded to come up with a preliminary design of a process to manufacture 1000kg/h of methyl ethyl ketone from dehydrogenation of 2butanol. The design work included coming up with a block diagram, a detailed mass and energy balance, a flow sheet diagram and a detailed design of a distillation column.
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3.0 LITERATURE REVIEW 3.1 BACKGROUND 3.1.1 Nature of methyl ethyl ketone (product description) Methyl ethyl ketone, also known as 2-butanone, is a colorless organic liquid with an acetonelike odor and a low boiling point. It is partially miscible with water and many conventional organic solvents and forms azeotropes with a number of organic liquids. MEK is distinguished by its exceptional solvency, which enables it to formulate higher-solids protective coatings. The molecular formula of methyl ethyl ketone is CH3COCH2CH3; its molecular structure is represented as:
Fig. 1 2D and 3D dimensional molecular structures of MEK
Some physical and chemical properties of MEK are presented in Table 1 below. Because of MEK’s high reactivity, it is estimated to have a short atmospheric lifetime of approximately eleven hours. Atmospheric lifetime is defined as the time required for the concentration to decay to 1/e (37percent) of its original value. 3.1.2 Overview of production and use Generally, Methyl ethyl ketone production is accomplished by one of two processes: (1) Dehydrogenation of secondary butyl alcohol or (2) As a by-product of butane oxidation. Page | 3
Property
Value
Structural formula: CH3COCH2CH3 Synonyms: 2-butanone, ethyl methyl ketone, MEK, methyl acetone Molecular weight (grams)
72.1
Melting point, °C
-86.3
Boiling point, °C
79.6
Density at 20°C, g/L
804.5
Vapor density (air at 101 kPa, 0°C = 1)
2.41
Critical temperature, °C
260
Critical pressure, MPa
4.4
Surface tension at 20°C, dyne/cm
24.6
Dielectic constant at 20°C
15.45
Heat of combustion at 25°C, kJ/mol
2435
Heat of fusion, kJ/(kg*K)
103.3
Heat of formulation at constant pressure, kJ/mol
279.5
Specific heat: vapor at 137°C, J/(kg*K) liquid at 20°C, J/(kg*K
1732 2084
Latent heat of vaporization at 101.3 kPa, kJ/mol
32.8
Flashpoint (closed cup), °C
-6.6
Ignition temperature, °C
515.5
Explosive limits, volume % MEK in air lower upper Vapor pressure at 20°C, mm Hg
2 12 77.5
Viscosity, MPa*s (=cP) at 0°C at 20°C at 40°C Solubility at 90°C, g/L of water
0.54 0.41 0.34 190
Table 1: Physical and chemical properties of MEK
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Figure 2 illustrates the production and use of MEK. Major end-users of MEK include protective coating solvents (61 percent), adhesives (13 percent), and magnetic tapes (10 percent). Vinyls are the primary resins that employ MEK as a solvent. Methyl ethyl ketone is commonly used as a solvent in rubber cements, as well as in natural and synthetic resins for adhesive use. It is also the preferred extraction solvent for dewaxing lube oil and is used in printing inks. Overall, the projected use of MEK is expected to gradually decline. The growing trend towards water-based, higher-solids, and solvent-less protective coatings, inks and adhesives is reducing the demand for MEK. The installation of solvent recycling facilities will also reduce requirements for fresh solvent production. Although MEK is favored as a solvent due to its low density, low viscosity, and high solvency, its addition on the EPA’s hazardous air pollutants list will likely cause potential users to consider other comparative solvents such as ethyl acetate.
END USE Protective coating solvent
Adhesive solvent
PRODUCTION Dehydrogenation of secondary butyl alcohol By-product of Butane oxidation
Magnetic tapes
Lube oil dewaxing Chemical intermediate Printing ink Miscellaneous
Fig. 2 production and uses of MEK
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3.2 APPLICATIONS 3.2.1 as a solvent Butanone is an effective and common solvent and is used in processes involving gums, resins, cellulose acetate and nitrocellulose coatings and in vinyl films. For this reason it finds use in the manufacture of plastics, textiles, in the production of paraffin wax, and in household products such as lacquer, varnishes, paint remover, a denaturing agent for denatured alcohol, glues, and as a cleaning agent. It has similar solvent properties to acetone but has a significantly slower evaporation rate. Butanone is also used in dry erase markers as the solvent of the erasable dye. 3.2.2 as a welding agent As butanone dissolves polystyrene, it is sold as "polystyrene cement" for use in connecting together parts of scale model kits. Though often considered an adhesive, it is actually functioning as a welding agent in this context. 3.2.3 Other uses Butanone is the precursor to methyl ethyl ketone peroxide, a catalyst for some polymerization reactions. It can also initiate crosslinking of unsaturated polyester resins. 3.3 SAFETY 3.3.1 Flammability Butanone can react with most oxidizing materials, and can produce fires. It is moderately explosive; it requires only a small flame or spark to cause a vigorous reaction. Butanone fires should be extinguished with carbon dioxide, dry chemicals or alcohol foam. Concentrations in the air high enough to be flammable are also intolerable to humans due to the irritating nature of the vapor.
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3.3.2 Health effects Butanone is an irritant, causing irritation to the eyes and nose of humans, but serious health effects in animals have been seen only at very high levels. When inhaled, these effects included birth defects. Butanone is listed as a Table II precursor under the United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances. On December 19, 2005, the U. S. Environmental Protection Agency removed butanone from the list of hazardous air pollutants (HAPs). After technical review and consideration of public comments, EPA concluded that potential exposures to butanone emitted from industrial processes may not reasonably be anticipated to cause human health or environmental problems. Emissions of butanone will continue to be regulated as a volatile organic compound because of its contribution to the formation of tropospheric (ground-level) ozone.
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4.0 METHYL ETHYL KETONE PRODUCTION This section discusses the methods which are used for production of methyl ethyl ketone. 4.1 SECONDARY-BUTYL ALCOHOL DEHYDROGENATION The majority of MEK manufactured is produced by dehydrogenation of secondary-butyl alcohol. This subsection discusses the 2-butanol dehydrogenation process. 4.1.1 Dehydrogenation Process Description Methyl ethyl ketone manufacture by secondary-butyl alcohol dehydrogenation is a two-step process where the first step involves the hydration of butenes to produce secondary-butyl alcohol. The second step consists of the dehydrogenation of secondary-butyl alcohol yielding MEK and hydrogen gas. These steps are illustrated by the following reactions:
OH
(1)
CH3CH=CHCH3 Butene
(2)
OH CH3CHCH2CH3
Aqueous
H2SO4
Zn or Brass 400-550°C
CH3CH2CH3 Sec-butyl alcohol
CH3CCH2CH3 MEK
+
H2 Hydrogen gas
Sec-butyl alcohol
Since the first reaction (1) does not involve MEK as a product, this discussion will focus on the second step of the reaction. Figure 3 illustrates the process of secondary-butyl alcohol dehydrogenation. Initially, preheated vapours of secondary-butyl alcohol are passed through a reactor (Step 1) containing a catalytic bed of zinc oxide or brass (zinc-copper alloy) which is maintained between 400° and 550°C (750° and 1,025°F). A mean residence time of two to eight
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seconds at normal atmospheric pressures is required for conversion from secondary-butyl alcohol to MEK. Product gases from the reaction vessel are then condensed via a brine-cooled condenser (Step 2) and sent to a distillation column for fractioning (Step 3). The main fraction (methyl ethyl ketone) is typically obtained at an 85 to 90 percent yield based on the mass of secondary butyl alcohol charged. The uncondensed gas may be scrubbed with water or a non-aqueous solvent to remove any entrained ketone or alcohol from the hydrogen-containing gas (Step 4).The hydrogen may then be re-used, burned in a furnace, or flared.
Preheater
1
2
Reactor
Condenser
Hydrogen
Scrubber
Solvent
4
Column
Product storage and loading 3
Fig. 3 methyl ethyl ketone from secondary butyl alcohol by dehydrogenation Alcohol to recovery
A liquid-phase process for converting secondary-butyl alcohol to methyl ethyl ketone has been developed and is used sometimes. In this process, secondary-butyl alcohol is mixed with a highboiling solvent containing suspended finely divided Raney or copper chromite catalyst. The reaction occurs at a temperature of 150°C (300°F) and at atmospheric pressure allowing MEK and hydrogen to be driven off in vapour form and separated as soon as each is formed. The
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advantages of this process include a better yield (typically 3 percent better), longer catalyst life, simpler product separation, and lower energy consumption. 4.2 N-BUTANE OXIDATION Another method of manufacturing Methyl ethyl ketone is by liquid-phase oxidation of nbutane. However, MEK has occasionally been commercially available in significant quantities from the liquid-phase oxidation of butane to acetic acid. Depending on the demand for acetic acid, this by-product methyl ethyl ketone can be marketed or recycled. This subsection discusses MEK production via n-butane oxidation. 4.2.1 N-butane oxidation description process Figure 4 illustrates the liquid-phase oxidation of n-butane. Initially, n-butane and compressed air or oxygen are fed into a reactor (Step 1) along with a catalyst, typically cobalt, manganese or chromium acetate to produce acetic acid, MEK and other by-products such as ethanol, ethyl acetate, formic acid, and propionic acid. This process produces the following chemical reaction: O CH3CH2CH2CH3 + n-butane
O2 Oxygen or air
O
CH3COH + CH3CCH2CH3 + Acetic acid
MEK
Other byproducts
+
H2O Water
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Fig. 4 Methyl ethyl ketone from n-butane by liquid phase oxidation
Air is bubbled through the reactant solution at 150° to 225°C (300° to 440°F) with pressures of about 5.5 MPa (800 psi). Conditions must be carefully controlled to facilitate MEK production and prevent competing reactions that form acetic acid and other by-products. Process conditions can be varied producing different ratios of product components through the choice of raw material, reaction conditions, and recovery methods. Vapors containing crude acetic acid and the various by-products including MEK are separated from unreacted n-butane and inert gases (Step 2), then stripped or flashed to remove dissolved butane and inert gases (Step 3), and sent to the purification section (Step 4). Unreacted nitrogen leaving the reactor carries various oxidation products (formic, acetic, and propionic acids; acetone, MEK, methanol, etc.) and some unreacted butane and is sent to a separator (condenser) for removal/recycling of unreacted hydrocarbons (Step 5). The purification section of the plant is complex and highly specialized utilizing three phase distillation in conjunction with straight extraction. The low-boiling organics such as MEK are separated from the crude acetic acid by conventional distillation. Azeotropic distillation is used Page | 11
to dry and purify the crude acetic acid. Recovery and purification of the various by-products require several distillation columns and involve extractive distillation or azeotrope breakers or both. Liquid organic wastes are typically burned in boilers to recover their heat value. 4.3 N-BUTENE OXIDATION A new one-step process that converts olefins to ketones called OK technology was developed. Specifically, MEK is produced via direct oxidation of n-butenes at about 85°C (185°F) and 690 kPa (100 psi), using a proprietary, and homogenous non-chloride catalyst. Advantages of this process are that it is noncorrosive, environmentally clean, and economical because of low capital investment and low energy needs. The process is currently in lab-scale operation; however, plans are underway to design a facility for large scale production. 4.4 JUSTIFICATION OF THE PROCESS USED The justification of the method used was based on the problem statement given to the group by the supervisor.
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4.5 DISTILLATION Distillation as a separation process is indispensable in the production of methyl ethyl ketone from dehydrogenation of 2-butanol. The separation of liquid mixtures by distillation depends on differences in volatility between the components. In distillation, the greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation consists of column, a re-boiler and a condenser system. Vapor flows up the column and liquid counter-currently down the column. The vapor and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporized in the re-boiler and returned to provide the vapor flow. In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. If the process requirement is to strip a volatile component from a relatively non-volatile solvent, the rectifying section may be omitted, and the column would then be called a stripping column. In some operations, where the top product is required as a vapor, only sufficient liquid is condensed to provide the reflux flow to the column, and the condenser is referred to as a partial condenser. When the liquid is totally condensed, the liquid returned to the column will have the same composition as the top product. In a partial condenser the reflux will be in equilibrium with the vapor leaving the condenser. Virtually pure top and bottom products can be obtained in a single column from a binary feed, but where the feed contains more than two
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components; only a single “pure” product can be produced, either from the top or bottom of the column. In engineering terms, distillation columns have to be designed with a larger range in capacity than any other types of processing equipment, with single columns 0.3–10 m in diameter and 3–75 m in height. Designers are required to achieve the desired product quality at minimum cost and also to provide constant purity of product even though there may be variations in feed composition. A distillation unit should be considered together with its associated control system, and it is often operated in association with several other separate units. The vertical cylindrical column provides, in a compact form and with the minimum of ground requirements, a large number of separate stages of vaporization and condensation. A complete unit will normally consist of a feed tank, a feed heater, a column with boiler, a condenser, an arrangement for returning part of the condensed liquid as reflux, and coolers to cool the two products before passing them to storage. The reflux liquor may be allowed to flow back by gravity to the top plate of the column or, as in larger units, it is run back to a drum from which it is pumped to the top of the column. The control of the reflux on very small units is conveniently effected by hand-operated valves and with the larger units by adjusting the delivery from a pump. In many cases the reflux is divided by means of an electromagnetically operated device which diverts the top product either to the product line or to the reflux line for controlled time intervals.
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5.0 PROCESS DESCRIPTION 5.1 DEHYDROGENATION OF 2-BUTANOL Methyl ethyl ketone (MEK) is manufactured by the dehydrogenation of 2-butanol. A description of the processes listing the various units used is given below: 5.1.1 Reactor A reactor in which the butanol is dehydrated to produce MEK and hydrogen, according to the reaction:
CH3CH2CH3CHOH 2-butanol
CH3CH2CH3CO MEK
+
H2 Hydrogen
The conversion of alcohol to MEK is 88 per cent and the yield is taken as 100 per cent. Initially, preheated vapours of secondary-butyl alcohol are passed through a reactor (Step 1) containing a catalytic bed of zinc oxide or brass (zinc-copper alloy) which is maintained between 400°C and 550°C (750°F and 1,025°F). A mean residence time of two to eight seconds at normal atmospheric pressures is required for conversion from secondary-butyl alcohol to MEK. 5.1.2 Cooler-condenser In the cooler-condenser the reactor off-gases (i.e. product gases) are cooled and most of the MEK and unreacted alcohol are condensed. Two exchangers are used but they are modeled as one unit. Of the MEK entering the unit 84 per cent is condensed, together with 92 per cent of the alcohol. The hydrogen is non-condensable. The condensate is fed forward to the second distillation column which is the final purification stage. The MEK is cooled to a temperature of 32 °C. The water is fed to the cooler at a temperature of 24 °C.
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5.1.3 Absorption column In the absorption column the uncondensed MEK and alcohol are absorbed in water. Around 98 per cent of the MEK and alcohol can be considered to be absorbed in this unit, giving a 10 per cent w/w solution of MEK. The water feed to the absorber is recycled from the next unit, the extractor. The vent stream from the absorber, containing mainly hydrogen, is sent to a flare stack. 5.1.4 Extraction column In the extraction column the MEK and alcohol in the solution from the absorber are extracted into trichloroethylane (TCE). The raffinate, water containing around 0.5 per cent w/w MEK, is recycled to the absorption column. The extract, which contains around 20 per cent w/w MEK, and a small amount of butanol and water, is fed to the first distillation column. 5.1.5 Distillation column I In the distillation column the unit separates the MEK and alcohol from the solvent TCE. The solvent containing a trace of MEK and water is recycled to the extraction column. The recovery is 99.99%. 5.1.6 Distillation column II In the second distillation column, also known as the final the purification stage which produces a 99.9% pure MEK product from the crude product from the first column. The residue from this column, which contains the bulk of the unreacted 2-butanol, is recycled to the reactor. The steam generated by the re-boiler in this unit is at a temperature of 140 °C. The following is the block diagram for the production process of methyl ethyl ketone.
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2-butanol
Reactor (dehydrogenation) Unreacted alcohol and MEK To flame stack Unreacted 2-butanol
Gaseous products
Coolercondenser
H2 Uncondensed MEK & alcohol Absorption column
MEK and alcohol
Water 0.5% w/w MEK
Extractor TCE (trichloroethyl ane)
Extract Distillation column 1 Crude product
Distillation column 2
Pure MEK (99.9%)
Fig. 5 Block diagram for the production of methyl ethyl ketone
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5.2 MATERIAL BALANCES Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design; to extend the often limited data available from the plant instrumentation; to check instrument calibrations; and to locate sources of material loss. All mass/material balances are based on the principle of conservation of mass that is massr can neither be created nor destroyed with an exception of nuclear processes according to Einstein’s equation; E=mc2. The general conservation equation for any process system can be written as:
For a steady state process the accumulation term is zero and thus for a continuous steady state process, the general balance equation for any substance involved in the process can be written as:
If no chemical reaction takes place, material balance is computed on the basis of chemical compounds mass basis that are used whereas if a chemical reaction occurs molar units are used. Also it is worthwhile to note that when a reaction occurs an overall balance is not appropriate but a reactant balance (a compound balance) is.
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5.2.1 Choosing a Basis The correct choice of the basis for a calculation will often determine whether the calculation proves to be simple or complex. A time basis was chosen in which the results will be presented. The basis for calculations was chosen as 1 hour and thus results will be presented in kg/h.
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5.2.2 MATERIAL BALANCE FOR THE PRODUCTION METHYLETHYLKETONE (MEK) FROM 2BUTANOL Basis used: 1 hour The material balance was done around the following units: (1) Reactor
2-butanol XF
MEK
Reactor
X (kg)
2-butanol H2
XR
RMM of 2-butanol =74 Moles of 2-butanol = Moles of the2-Butanol that reacted
=
From the equation: CH3CH2CH3CHOH
Yields
CH3CH2CH3CO + H2
Mole ratio for the reaction is 1:1 Hence moles of the MEK reacting is 0.01188X Mass of MEK then is 0.01188
72=0.8554
Mass of 2-butanol is Mass of then H2 is 0.01188 2=0.0276 X (kg)
Reactor
MEK = 0.8554 2-Butanol= H2 =0.02376
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All the components leaving the reactor are discharged directly into the cooler condenser for the next operation. (2) Cooler-condenser Condensate (which is then directly sent to the final purification column) comprises: 84% MEK= 0.8 0.8554 92% 2-Butanol=0.92 Incondensable stream comprises: H2= 2-Butanol MEK
MEK = 0.8554
Coolercondenser
2-Butanol=
(Non-condensable)
MEK 2-Butanol
H2=
H2=
(Condensate) MEK 2-butanol
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MEK
(non-condensable) MEK
H2O
K MEK Absorption column
2-Butanol
2-Butanol
H2=
H2=
J
J MEK 2-Butanol H2O=
(3) MEK balance around the absorption column
Overall balance
Performing a new balance around the absorption column to express the
-value in terms of
in the above equations gives the following values: MEK
K
MEK 2-Butanol
H2O
Absorption column 2
MEK Butanol
H2
H2= MEK H2O
and
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Raffinate: MEK H2O {
Stream J: MEK
}
H2O 2-butanol
(4) Extraction column
MEK 2-butanol = H2O
Raffinate
: MEK
B
H2O
Q
Extractor R
ϑ
MEK 2-butanol TCE
R- Recycle from next operation (TCE)
MEK Balance around the extractor
ϑ
Overall balance
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(Which is approximately = ) (5) Distillation column 1 For this unit operation, the balances were obtained from the previous unit operation i.e. the extraction column and are indicated in the block diagram below. TCE
MEK 2-Butanol
Distillation column 1
MEK 2-Butanol
TCE
(6) Distillation column 2
The material balance for the second distillation column is given as follows;
MEK 2-Butanol
Distillation column 2
1000kg/hr (flow rate as given)
2-Butanol (recycled back to the reactor)
Balancing around this gives: MEK:
2-Butanol: Page | 24
(Returning to the reactor) 4.2.3 CALCULATION OF ACTUAL MASS OF THE COMPONENTS IN ALL THE STREAMS The streams are indicated in the diagrams above. 1) Reactor From the balances carried out in the previous exercise the value of X was obtained as 1172.883 kg based on the 1 hour basis. In = out Entering stream: XF + XR= X where: XF = feed and XR = feed as recycle Leaving streams: MEK = 2-butanol H2 2) Cooler condenser In = out MEK 2-butanol Non-condensable MEK 2-Butanol
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H2 3) Absorption column Entering stream: MEK 2-Butanol H2 Raffinate stream: MEK H2O Leaving stream: MEK H2O 2-butanol 4) Extractor Entering stream: MEK H2O 2-butanol Recycle stream = TCE (Tri chloro ethylane)
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TCE: Leaving stream: MEK: 2-butanol 5) Distillation column 1 Entering stream: MEK: 2-butanol Leaving stream: MEK: 2-butanol TCE:
(This is recycled back into the extractor)
6) Distillation column 2 In = out Entering stream: MEK: 2-butanol:
(this is recycled back to the reactor)
Leaving stream 99.99% pure MEK at 1000kg/hr
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5.3 ENERGY BALANCES As with mass, energy can be considered to be separately conserved in all but nuclear processes. The conservation of energy, however, differs from that of mass in that energy can be generated (or consumed) in a chemical process. Material can change form, new molecular species can be formed by chemical reaction, but the total mass flow into a process unit must be equal to the flow out at the steady state. The same is not true of energy. The total enthalpy of the outlet streams will not equal that of the inlet streams if energy is generated or consumed in the processes; such as that due to heat of reaction. Energy can exist in several forms: heat, mechanical energy, electrical energy, and are the total energy that is conserved. In process design, energy balances are made to determine the energy requirements of the process: the heating, cooling and power required. In plant operation, an energy balance (energy audit) on the plant will show the pattern of energy usage, and suggest areas for conservation and savings. A general equation can be written for the conservation of energy:
This is a statement of the first law of thermodynamics. An energy balance can be written for any process step. Chemical reaction will evolve energy (exothermic) or consume energy (endothermic). For steady-state processes the accumulation of both mass and energy will be zero. The energy balance was carried out around cooler condenser and the second distillation column. In chemical processes the kinetic and potential energy terms are usually small compared with heat and work terms, and can normally be neglected.
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If the kinetic and potential energy terms are neglected the energy equation reduces to
For many processes the work term will be zero, or negligibly small, and equation above reduces to the simple heat balance equation:
Where heat is generated in the system; for example in a chemical reactor:
heat generated in the system. If heat is evolved (exothermic processes)
is taken as
positive, and if heat is absorbed (endothermic processes) it is taken as negative. process heat added to the system to maintain required system temperature. Hence:
enthalpy of the exit stream enthalpy of the outlet stream. For a practical reactor, the heat added (or removed) Qp to maintain the design reactor temperature will be given by:
Where is the total enthalpy of the product streams, including unreacted materials and byproducts, evaluated from a datum temperature of 25°C;
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is the is the total enthalpy of the feed streams, including excess reagent and inerts, evaluated from a datum of 25°C; Qr is the total heat generated by the reactions taking place, evaluated from the standard heats of reaction at 25°C (298 K). This equation can be written in the form: ∑∫
∑∫
∑[
]
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5.3.1 ENERGY BALANCE FOR THE PRODUCTION METHYLETHYLKETONE (MEK) FROM 2BUTANOL The energy balance was carried around the cooler condenser and the second distillation column (final purification stage). The balances are as indicated below. 4.3.1.1 Cooler condenser
MEKMEK = 0.8554 2-Butanol= 2-butanol 140.74 kg H2= H2 27.87 kg
Coolercondenser
MEK= (Non-condensable)
2-Butanol H2=
Condensate
QR
MEK = 2-butanol
The temperature at which the products of the reactor leave is 400 °C. The condenser cooler lowers cools the products to a temperature of 32 °C. The energy balance is given as shown in the calculations below. Energy balance for MEK Sensible heat to lower the temperature of the condensate MEK from 400 °C to 79.6 °C,
Sensible heat to lower the temperature of the incondensable MEK from 400 °C to 80 °C,
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Sensible heat to lower the temperature of the condensate MEK from 79.6 °C to 32 °C,
No of moles of MEK condensed
⁄ Latent heat of vaporization of MEK,
Total energy required for MEK cooling and condensation,
Energy balance for 2-butanol Sensible heat to be removed to lower the temperature of 2-butamol from 400 °C to 99 °C is determined as follows,
⁄ To condense the 2-butanol, Page | 32
Total heat to be removed from 2-butanol,
Total heat to be removed from the cooler condenser,
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5.3.1.2 Distillation column 2
QC
F=1140.52kg/h XF=0.88
R
D=1000kg/h XD=0.999
QR
B=140.52kg/h XB=0.0088
Taking reflux ratio (R.R) = 1.94 Total energy balance equation is: HF+QB=QC+HD+HB
QC is obtained by a balance around the condenser
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QC
V HV
R HR
D HD
An energy balance at steady state is: HV = QC + HR + HD Values of enthalpy of product (distillate) and reflux are zero as they are both at the reference temperature. Both are liquid and the reflux will be at the same temperatures as the distillate. Enthalpy of vapour: Hv= latent heat + sensible heat For methyl ethyl ketone, latent heat is given as: Ln =
Latent heat of the vapor stream:
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Sensible heat =∑ ∫ Boiling point of methyl ethyl ketone =79.6 ℃ (352.6 K) Sensible heat of MEK, =0.026362
∫
A balance around the condenser yields:
[
]
The quantity of heat that needs to be extracted from the condenser by the cooling fluid is obtained as follows. Page | 36
QR is obtained by an overall energy balance around the column.
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6.0 DESIGN OF DISTILLATION COLUMN 2 6.1 DISTILLATION PRINCIPLES Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapor pressure characteristics of liquid mixtures. 6.2 VAPOUR PRESSURE AND BOILING The vapor pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface. Here are some important points regarding vapor pressure:
energy input raises vapor pressure
vapor pressure is related to boiling
a liquid is said to ‘boil’ when its vapor pressure equals the surrounding pressure
the ease with which a liquid boils depends on its volatility
liquids with high vapor pressures (volatile liquids) will boil at lower temperatures
the vapor pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture
distillation occurs because of the differences in the volatility of the components in the liquid mixture
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6.3 DESIGN OF DISTILLATION COLUMN Distillation columns are designed using the vapor-liquid equilibrium data for the mixtures to be separated. The vapor liquid equilibrium characteristics of the mixture will determine the number of stages and hence the number of trays required for the separation. Most distillation columns are designed by use of the McCabe Thiele method. 6.4 McCabe THIELE DESIGN METHOD The McCabe Thiele approach is a graphical one and use the VLE plot to determine the theoretical number of stages required to effect the separation of the mixture (binary in our case). The method assumes constant molar overflow and this implies that:
Molar heats of vaporization of the components are roughly the same.
Heat effects (heats of solution, heat losses to and from the column etc.) are negligible.
For every mole of vapor condensed one mole of liquid is vaporized.
The design process is simple. Given the VLE data/relationship for the more volatile component, operating lines are drawn first.
Operating lines define the mass balance relationships between the liquid and vapor phases in the column.
There is one operating line for the bottom (stripping) section of the column and one for the top (rectifying) section of the column.
Use of the constant molar overflow assumption also ensures that the operating lines are straight.
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In the design done for the distillation column 2 the following criteria was followed. 1. Specification of degree of separation required 2. Selection of the operating conditions 3. Selection of the type of contacting device e.g. plates , pickings 4. Determining the stage and reflux requirements. 5. Sizing the column e.g diameter and height. Assumptions made in the design of the distillation column:
Equimolar overflow
Total condenser
Partial reboiler
Density does not vary with temperature
Theoretical plates i.e perfect phase equilibrium exists between both phases leaving the plate.
1. Degree of separation required The feed to the distillation column contains 88 mol % of the less volatile component (methyl ethyl ketone) and 12 mol % of the more volatile component (2-butanol). An overhead purity of 99.9 mol percent is desired while a bottoms purity of 0.1 mol % is obtained thus the following mole fraction value relate to the more volatile component:
A reflux ratio of 16 was used as calculated based on the minimum reflux ratio.
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QC
F=1140.52kg/h XF=0.88
D=1000kg/h XD=0.999
R
QR
B=140.52kg/h XB=0.0088
Fig. 6 Presentation of the second distillation column for overall material balance
The following vapour liquid equilibrium data was used to draw the VLE curve.
X
0.088
0.278
0.383
0.467
0.478
0.582
0.702
0.803
0.855
0.900
Y
0.192
0.468
0.583
0.644
0.655
0.737
0.823
0.885
0.905
0.940
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1.2
1 y'
Y
0.8
0.6
0.4
0.2
0 0
0.2
0.4
0.6
0.8
xF
1
1.2
X
The value of y’ is read from the graph as shown above.
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2. Determination of stages and reflux requirements The theoretical number of stages was determined by the McCabe Thiele method. This is a graphical method for the determination of the ideal number of stages. This was procedure was carried as follows. Determining the minimum reflux ratio The minimum reflux rate can be determined mathematically from the endpoints of the rectifying line at minimum reflux – the overhead product composition point (xD, yD) and the point of intersection of the feed line and equilibrium curve(x’, y’).
xD=0.99 y’=0.92 x’=0.8768
The equation for the rectifying section is given as follows,
The above equation is plotted in the curve as shown below, and the McCabe Thiele method is used to determine the number of stages.
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1.2
q-line 1 stripping operating line
Y
0.8 VLE curve rectifying operating line
0.6 45° line
0.4
0.2
xB
xD
0 0
0.2
0.4
0.6
0.8
1
1.2
X
From the above analysis using the McCabe Thiele method, the theoretical number of stages was obtained as 12 stages. Ideal number of stages obtained= 12 i.e.
Rectifying section= 3 stages Stripping section = 9 stages Page | 44
3. Sizing of the column The sizing of the column was carried out using Carrillo, Martin and Roselle’s correlation (2000). [
]
Where Fv is defined by the following expression √ is the vapor phase superficial velocity is the liquid phase specific mass is the vapor phase specific mass
⁄
⁄
Where
At 760mmHg, data for MEK is as given below
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To obtain the mass flow rate of the gas and the liquid the following balance is carried out as below. VR
R=LR
F
VS
LS
⁄ ⁄ ⁄ ⁄
(
) (
)
Using the value of x in equation (1),
From equation (3),
(
) Page | 46
(
)
(
⁄
)
√ ⁄
⁄
⁄ ⁄
√
Therefore the diameter of the column is,
Determining the height of the column using the following procedure,
[
] √
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[
]
Stages in the upper section= 3 Stages in the lower section = 9
+
The active part of the distillation column is 2.2 m
4. Selection of the type of contacting device to be used Raschig rings will be used as the contacting device in the distillation column. They are ceramic in nature are 1/3 mm in size.
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Raschig rings are pieces of tube (approximately equal in length and diameter) used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic or metal and provide a large surface area within the volume of the column for interaction between liquid and gas or vapour.
They form what is now known as random packing, and enable distillations of much greater efficiency than the use of fractional distillation columns with trays. In a distillation column, the reflux or condensed vapour runs down the column, covering the surfaces of the rings, while vapour from the re-boiler goes up the column. As the vapour and liquid pass each other counter-currently in a small space, they tend towards equilibrium. Thus less volatile material tends to go downwards, more volatile material upwards. Raschig rings made from borosilicate glass are sometimes employed in the handling of nuclear materials, where they are used inside vessels and tanks containing solutions of fissile material, for example solutions of enriched uranyl nitrate, acting as neutron absorbers and preventing a potential criticality accident.
Fig. 7 Raschig rings used for the operation
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7.0 CONCLUSION Distillation column design requires the selection of the right various packing and tower sizing to meet the process, hydraulic, efficiency, and mechanical requirements of the service. Process considerations include operating conditions, flexibility, and solid handling requirements. Hydraulic and efficiency criteria involve selection of a suitable packing material that allows for cost-effective optimization of vessel height vs. diameter. Determining the number of stages required for the desired degree of separation and the location of the feed tray is merely the first steps in producing an overall distillation column design. Other things that need to be considered are tray spacing; column diameter; internal configurations; heating and cooling duties. All of these can lead to conflicting design parameters. Thus, distillation column design is often an iterative procedure. If the conflicts are not resolved at the design stage, then the column will not perform well in practice. It can be deduced from the previous section on distillation column design that the number of trays will influence the degree of separation. As the feed stage is moved lower down the column, the top composition becomes less rich in the more volatile component while the bottoms contains more of the more volatile component. However, the changes in top composition are not as marked as the bottoms composition.
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8.0 REFERENCES 1. Chemical Engineering Design, 4th Edition by R.K Sinnot. 2. Unit Operations of Chemical Engineering, 5th Edition by McCabe and Smith. 3. Li, Y.L., “Production technology and market analysis of methyl ethyl ketone”, Fine and Specialty Chemicals, 12(18), 22—25(2004). (in Chinese) 4. Zhang, Y.X., “Production technology and application status of methyl ethyl ketone”, Journal of Henan Chemical Industry, 11(1), 51—55(2003). (in Chinese) 5. Distillation: An Introduction by M. T Tham. 6. Qi, J., Gao, N., “Market analysis of methyl ethyl ketone”, Petrochemical Industry Technology, 10(3), 61 — 64(2003). (in Chinese) 7. Ma, Y.S., Su, J., Wang, C.M., “A process of ketone from secondary alcohol by dehydrogenation”, C.N Pat., 1289753(2001). 8. Perry’s Chemical Engineering Handbook. 9. Coulson and Richardson’s Chemical Engineering, Volume 2, Fifth Edition. 10. Lecture notes from CHP 461 (Chemical Engineering Design I) and CHP 372 (Mass Transfer I) 11. www.wikipedia.org. 12. www.basf.com
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