Project title
Preparation of acetone via the dehydrogenation of isopropyl alcohol (IPA). Problem Statement
Acetone is typically produced in commercial quantities as a by-product during the formation of phenol. However, acetone manufactured thus generally contains small amounts of the reactant benzene and the desired product phenol. In the past, these impurities were deemed to be within allowable limits. However, recent downward revisions of these limits by the US Food and Drug Administration has made alternative processes (which do not involve benzene) more attractive. We wish to design of one such alternative process to produce 45000 tons of 99.9 mol% pure acetone per year, using isopropyl alcohol as the reactant (via the dehydrogenation of isopropyl alcohol). Design a simplified chemical reactor for the production of acetone.
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1. Acetone Introduction Acetone
belongs to ketone family (dimethyl ketone) with scientific name 2Propanone.
In 2010, the worldwide production capacity
for acetone was estimated at 6.7 million tons per year.
The United States had the highest production capacity capacit y with 1.56 million tons per year (2010)
The second largest production is in Taiwan T aiwan and followed by China (2010).
It is a common building block in organic chemistry.
(Stru (Stru ctural for mula of ace acetone) tone)
1.1 Product Descri Descri ption :
Acetone is a clear, colorless, low-boiling, flammable and volatile liquid characterized by rapid evaporation and a faintly aromatic, sweetish odor. It readily mixes with most organic solvents and mixes completely with water. 1.2 Produ ct Uses Uses:
Roughly 75% of the available acetone is used to produce other chemicals, and 12% is used as a solvent. Applications range from surface coatings, films and adhesives to cleaning fluids and pharmaceutical applications. Some consumer and commercial applications include Lacquers for automotive/furniture finishes. Cellulose acetate films and fibers. Photographic films and plates casting. Coatings and inks. Resin thinners and clean-up operations. General purpose cements. Degreasing and degumming agents. Paint, varnish, lacquer strippers. Nail polish removers.
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1.3 Physical H azard I nfor mation:
Acetone is a highly flammable material in both the liquid and vapor forms, has a relatively high vapor pressure, and should be handled only with adequate ventilation and in areas where ignition sources have been removed (e.g. matches and unprotected light switches). The flash point for acetone is -4ºF /-20ºC. 1.4 Proj ected demand for acetone:
There is about 7% increment in the requirement of the acetone worldwide. With this increasing demand the requirement of the acetone in some upcoming years is: Year
Proj ect D emand (mill ion Tones)
2010
6.7
2011
7.2
2012
7.9
2013
8.4
2014
9.0
2015
9.63
2016
10.3
2017
11.02
1.5 Capacity:
The capacity of the desired plant to produce acetone is 45000 tons per year. The capacity data of some previous years is:
Group # 5
Year
Quantity (Tones)
Year
Quanti ty (Tones)
2004
31000
2008
39000
2005
32000
2009
40000
2006
34000
2010
42000
2007
36000
2011
45000
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2. Methods for the preparation of acetone: Acetone is prepared by following of the three methods: 2.1 Cumene Process for Ph enol and A cetone :
This process converts two relatively cheap starting materials, benzene and propylene, into two more valuable ones, phenol and acetone. Other reactants required are oxygen from air and small amounts of a radical initiator . Cumene process is a process of producing phenol (C6H5-OH) and acetone (CH3-CO-CH3) from benzene (C6H6) and propene (C3H6).
The term stems from isopropyl benzene or cumene (C6H5-CH(CH3)2), the intermediate material during the process. 2.2 By the dir ect oxi dation of pr opylene using air :
Acetone can also be produced by the direct oxidation of propylene using air. In this process the catalysis consists of a solution of copper chloride containing small quantities of palladium chloride. The reaction takes place under a moderate pressure and at 100°C. It is exothermic by 61 kcal/mole of acetone produced. The overall reaction is as follows:
Propylene
Oxygen
Acetone
2.3 By th e dehydrogenati on of isopr opyl alcohol :
In this process, an aqueous solution of isopropyl alcohol is fed into the reactor, where the stream is vaporized and reacted over a solid catalyst. The reactions occurring within the reactor are as follows:
→ Isopropyl alcohol
acetone
hydrogen
The primary advantage of this process is that the acetone produced is free from trace aromatic compounds, particularly benzene. For this reason, acetone produced from IPA is favored by the pharmaceutical industry. Group # 5
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3. Comparison between processes and selection of process: Cumene process
Propylene oxidation process
Isopropyl alcohol dehydrogenation process Primary product is phenol Acetone is the primary Acetone is the primary Acetone is by product product product Purity of acetone is low Purity is high Purity is high --Propylene used should be 99% Aqueous solution of the pure isopropyl alcohol can be used Conversion to acetone is low Conversion to acetone is low Conversion to acetone is high Worldwide production method Not used worldwide Worldwide production method Unconverted benzene present Not a dangerous compound Not a dangerous compound along with acetone is present along with acetone present along with acetone dangerous to some process To purify acetone large Less separation process Less separation process number of separation required and production cost is required and production cost is processes are required which low low increase the production cost
Major disadvantage of the production of acetone from cumene process is that some
amount of reactant benzene is present along with desired product which is toxic. The disadvantage of production of acetone from propylene oxidation process is that propylene required for the process should be 99 % pure. So our process for the production of acetone is dehydrogenation of isopropyl alcohol.
4. Introduction of desired process: 4.1 Chemical r eaction:
The reaction occurs in vapor phase at a temperature of 350 °C and a pressure of 1.8 – 2 bars in the presence of catalyst.
→ Isopropyl alcohol
Acetone
Hydrogen
4.2 Side reaction s:
At a temperature lower than 325 °C the following reaction is more expected to occur, and ether (di-isopropyl ether) is obtained as the product instead of acetone.
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Isopropyl alcohol
Di-isopropyl ether
Water
At the higher temperature (>350 °C), dehydration reaction is expected to be significant and propylene is formed.
Isopropyl alcohol
Propylene
Water
So the suitable temperature conditions for the reaction is between 325 °C to 350 °C 4.3 Selection of catalyst:
Trial and error experimentation and scientific analysis provide guidance for the selection of the catalyst. There are some categories of the catalyst which are suitable for some purpose. For example platinum, copper and related metals elements such as palladium are known to catalyze hydrocarbon oxidation and dehydrogenation. Since our desired reaction is the dehydrogenation reaction of hydrocarbon (isopropyl alcohol), so the catalyst used for the process belong to this category. One of the factors considered in the selection of catalyst is their turn over frequency (TOF). 4.3.1 Tur n over f requency: Turn over frequency of the catalyst is defined as the amount of
reactant converted into product by the catalyst in the unit time. For most relevant industrial 2 −1 −2 applications, the turnover frequency is in the range of 10 - 10 s
The turnover frequency of cu chromites catalyst is 0.026 per second
The TOF of carbon supported copper is 0.052 per second double than that of the cu
chromites catalyst. Platinum (with a turnover frequency of 0.66 per second) metal has the high TOF for this
process but it is expensive.
So the suitable catalyst for this process is carbon suppor ted copper .
4.4 Reaction ki netics:
The reaction to form acetone from isopropyl alcohol is endothermic with a standard heat of reaction 62.9 kJ/mol. The reaction is kinetically controlled and occurs in the vapor phase over a catalyst. The reaction kinetics for this reaction is first order with respect to the concentration of alcohol and can be estimated from the following equation:
With Ea = 72.38 MJ/Kmol and Group # 5
[] 5
3
3
k o = 3.51 × 10 m gas/m bulk catalyst. s
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5. Physical properties of reactants and products: Property
Water
Acetone
Isopropyl alcohol
Hydrogen
Molecular Weight(kg/kmol)
18
58
60
2
0
-95
-88.5
- 259.2
Boling Point( C)
100
56.2
82.2
-252.8
Critical Temperature ( C)
647.3
508.1
508.3
33.2
Critical Pressure (bar)
220.5
47
47.6
13
Critical Volume 3 (m ) /mi n
0.056
0.209
0.220
0.065
Liquid 3 Density(kg/m )
998
790
786
71
Heat of Vaporization(J/mol)
40683
29140
39858
904
Standard Enthalpy of Formation at 298K(kJ/kmol)
-242.0
20.43
-272.60
0
Standard Gibbs Energy of Formation at 298K (kJ/kmol)
-228.77
62.76
-173.5
0
°
Freezing Point( C) °
°
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6. Conversion with temperature change The conversion of isopropyl alcohol to acetone with temperature is: Temperature (°C) 300 310 320 330 340 350
Conversion % 46.3 55.4 64.6 73.4 83.5 90.0
7. Flow sheet:
7.1 Descri pti on of t he process: 7.1.1 Feed dru m: Feed drum is a kind of tank used for the mixing of the recycle stream and feed
stream. Recycle stream concentration is assumed to be same with the feed stream. Feed stream is at room temperature (25 °C) and at a pressure of 2 bars, which is assumed to be constant. The temperature of recycle stream calculated as 111.50 °C from the energy balance around the isopropyl alcohol distillation column. The temperature of the leaving stream from the feed drum calculated as 32.890 °C, by the energy balance around feed drum. In the vaporizer molten salt is used for heating. The temperature at the entrance 7.1.2 Vapor izer: of the unit is the temperature of the mixture leaving the feed drum, which is 32.890 °C. And the leaving temperature is the bubble point temperature of the mixture, which is 109.50 °C. The pressure is 2 bars, and assumed to be constant. Group # 5
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: Since the temperature leaving the vaporizer is not enough for the reaction to carry 7.1.3 H eater out. Due to which a pre-heater is used to maintain the temperature of the feed stream to the reaction temperature of 350 °C which is the reaction temperature. The unit is working at 2 bars, and assumed to be constant. The entrance and leaving temperatures are 109.50°C and 350 °C. The reactor is the starting point for the calculations. The temperature values for 7.1.4 Reactor: the entering and leaving streams is 350 °C, (i-e the process is isothermal). The reaction takes place inside is endothermic, for this reason the reactor has to be heated. For heating, molten salt is used from the furnace. The entrance temperature of the cooler is 350 °C and leaving temperature is 94.70 7.1.5 Cooler: °C. (Temperature is calculated by using energy balance). For cooling purpose water is used. By using refrigerant better results may be obtained. But since it costs too much, due to which it isn’t chosen as the cooling material. From the temperature values it’s easily seen that the load is on the cooler not on the condenser, for this process. But in reality the unit cannot cool that much, and the load is mostly on the condenser. In this process, the mixture cooled down to its dew point. The pressure is 1.5 bars, and assumed to be constant. The temperature of the entering stream is the dew point and the leaving 7.1.6 Condenser: temperature is the bubble point of the mixture. In the condenser water is used as cooling material. 7.1.7 F lash u nit: Flash unit is operating isothermally, for this reason temperature is not changed.
It is 81 °C in the entrance and exit. From trial and error method, (V / F) value is found to be 0.2. The entrance temperature of the unit is the bubble point of the mixture, but if it is its dew point the (V/F) value should be much higher. In the flash unit the hydrogen is flashed out from the mixture of isopropyl alcohol, water and acetone. Along with hydrogen some amount of acetone and isopropyl alcohol is also flashed out. In order to recover these, a scrubber unit is used. 7.1.8 Scrubber: Scrubber operated adiabatically. Water entering the unit is at atmospheric
temperature (i-e 25 °C). The temperature of the off gas, including hydrogen and a very little amount of acetone, is between 40-50 °C. The temperature of the leaving stream (containing water, acetone and isopropyl alcohol) is found to be 28.10 °C from the energy balance around the scrubber. The streams leaving the scrubber and flash unit are mixed together before entering the acetone column. The temperature leaving the flash unit and scrubber are 81 °C and 28.10 °C respectively. The temperature of the mixture is found to be 45.0 °C. This result is obtained by using energy balance around the mixing point.
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: The acetone column is used to separate the acetone from the 7.1.9 Acetone distil lati on column mixture. The entrance temperature is 45 °C. The leaving temperatures for the top and bottom product are 102.3 °C and 105 °C, respectively, which are the bubble and dew points. Top product of the unit includes acetone (99wt% of acetone which is desired). From the bottom isopropyl alcohol, water and a very little amount of, 0.1 %, acetone is discharged. 7.1.10 I sopropyl alcohol colu mn: In this distillation column, isopropyl alcohol and water are
separated. The entrance temperature is 105 °C. The leaving temperatures of the top and bottom products are both111.50 °C. The top product is recycled to the feed drum. For this reason it’s assumed to have the same concentration with the feed stream. The bottom product is pure water and it’s thrown away. Since its temperature is very high it cannot be recycled to the scrubber. But if a cooler is used, a recycle can be used. 7.2 Descri pti on f the process in th e reactor
The reaction occurring in the reactor is in vapor phase. So the isopropyl alcohol is first vaporized and then passed from the reactor. The process is continuous. Since the dehydrogenation of the isopropyl alcohol is the endothermic reaction, so heat has o be supplied to the reactor to maintain the temperature at 350 °C. For heating purpose the molten salt is used. The molten salt is circulated through the small scale furnace where it is heated and its temperature is raised above the 350 °C and this heated molten salt is used to provide the heat to the reaction during the process. In order to heat the molten salt in the furnace, natural gas is burned in the furnace in the limited amount and this amount of heat is used to heat the molten salt which in turn provides the heat to the reaction.
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8. Selection of the type of reactor used in the process: 8.1 Choi ce of r eactor (Batch, CSTR or PF R)
Selection of the reactor type for a given process is subjected to a number of considerations a) Temperature and pressure of the reaction. b) Need for removal or addition of the reactants and products. c) Required pattern of the product delivery. d) Catalyst use consideration such as requirement for solid catalyst particle and contact with fluid reactants and products. e) Relative cost of the reactor. Some guideline for the reactor selection is: For conversion up to 90%, the performance of five or more CSTRs connected in series
approaches to that of PFR. Batch reactors are best suited for small scale production, very slow reactions or those
requiring intensive monitoring and control. For large operations CSTR or PER is used CSTRs are used for slow liquid phase and slurry reactions. For gas phase reactions PER
is more preferable. For endothermic reactions the plug flow reactor is used. For exothermic reaction that has
a large temperature raise during the reaction, recycle reactors are the best choice. For small
mixed flow reactor is used. For large
best choice.
plug flow reactor is
Since our reaction is the gaseous phase and endothermic reaction so the choice is the plug flow reactor. 8.2 Choice of t he bed (F ix ed, fl ui dized or movin g)
Use of the catalyst requires modifications to basic reactor design to fixed bed reactors, moving bed reactors or fluidized bed reactors. 8.2.1 Fixed bed reactors: These are used in the heterogeneous catalyst reactions and pressure
drop across the bed is small. The design of the fixed bed reactor is very easy as compared to the moving bed and fluidized bed reactors. Their size is also compact as compared to other fluidized Group # 5
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bed and moving bed reactors. The energy requirement is also small because no amount of energy is needed as in fluidized bed and moving bed required to fluidize or move the bed. 8.2.2 F lui dized bed reactor s: These are the reactors with a gas phase working fluid that requires
gas flow around and across the fine particles at a rate sufficient to fluidize the particles suspended within the reactor. Since the catalyst bed has to be fluidized so the energy requirement in these reactors is large. Pressure drop is also large as compared to the fixed bed reactors because the pressure is dissipated to fluidize the bed. The volume of the reactor required is also large as compared to fixed bed reactor, because the void spaces between the fluidized beds occupy the more volume. 8.2.3 M oving bed reactor : These units are fluid reactors used where the fluid contain solid
particles that can be separated from the suspension fluid. Mostly suitable for liquid phase reactions or where the slurry travels through the reactor. Moving bed reactors are not preferred for the gas phase reactions. In these reactors the pressure drop is the greater among all. From above points we see that the suitable reactor for our process is fixed bed plug flow reactor, with the reaction occurring in the tubes and the heat exchanging material flowing outside the tubes.
9. Design steps for the reactor: Standard design steps for the reactor are given below: a) Collect together all the kinetic and thermodynamic data on the desired reaction and the side reactions (if present). The kinetic data required for reactor design will normally be obtained from laboratory or pilot plant studies. Values will be needed for the rate of reaction over a range of operating conditions: pressure, temperature, and flow-rate and catalyst concentration. b) Collect the physical property data required for the design. c) Identify the predominant rate-controlling mechanism: kinetic, mass or heat transfer. d) Choose a suitable reactor type, based on experience with similar reactions, or from the laboratory and pilot plant work. e) Make an initial selection of the reactor conditions to give the desired conversion and yield. f) Size the reactor and estimate its performance. Exact analytical solutions of the design relationships are rarely possible; semi empirical methods based on the analysis of idealized reactors will normally have to be used. g) Select suitable materials of construction. h) Make a preliminary mechanical design for the reactor: the vessel design, heat-transfer surfaces, internals and general arrangement. i) Cost the proposed design, capital and operating, and repeat steps 4 to 8, as necessary, to optimize the design. Group # 5
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The design of the reactor must satisfy the following requirements: i.
Chemical factors:
The design must provide sufficient residence time for the desired reaction to proceed to the required degree of conversion. ii.
Mass transfer factors:
For example with heterogeneous reactions the reaction rate may be controlled by the rates of diffusion of the reacting species; rather than the c hemical kinetics. iii.
Heat transfer factors:
Removal or addition of the heat of the reaction. iv.
Safety factors
The confinement of hazardous reactants and products, and the control of the reaction and the process conditions.
10. Feed to the reactor: Feed used for the preparation of the acetone is 87% weight solution of the isopropyl alcohol. Here we are using the 87 weight percent solution of the isopropyl alcohol because commercially isopropyl alcohol is available in two grades. i. ii.
Anhydrous 87% by weight.
Due to which we are using the 87 weight percent solution of isopropyl alcohol.
11. Material balance across the reactor: Start calculation from the reactor. Basis: 100 kgmol/hr of the isopropyl alcohol are entering in the reactor. Since the solution used for the preparation of acetone is 87%. So the number of moles of water entering in the reactor is calculated as:
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Acetone
Hydrogen Water Isopropyl alcohol
→
At a temperature of 350 °C the conversion is 90%. So
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From material balance
Mass in
=
Mass of water in + Mass of IPA in =
Mass out Mass of water out + Mass of IPA out + Mass of acetone out + Mass of hydrogen out
6000 + 896.5
=
5220 + 180 + 600 + 896.5
6896.5 kg/hr
=
6896.5 kg/hr
So the material balance is justified.
Summary of material balance:
Component
Acetone Hydrogen IPA Water
Nu mber of moles enter in g in reactor
100 kgmol/hr 49.8 kgmol/hr
M ass of component enteri ng in r eactor
6000 kg/hr 896.5 kg/hr
Number of mol es leavin g f rom r eactor
90 kgmol/hr 90 kgmol/hr 10 kgmol/hr 49.8 kgmol/hr
M ass of component leaving fr om reactor
5220 kg/hr 180 kg/hr 600 kg/hr 896.5 kg/hr
From the 100 kmol/hr of the isopropyl alcohol entering in the reactor the amount of the acetone produced per year is 45700 tons, which is very close to the desired amount of the product. So we are not applying the material balance again, but using these calculations in the process. So, By using 100 moles of the isopropyl alcohol per hour the amount of acetone produced is 45700 ton per year.
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12. Energy Balance across the reactor: The reaction occurring in the reactor is isothermal at the temperature of 350 °C
Temperature at inlet = 350 °C
Temperature 350 °C
Component
Nu mber of moles enteri ng in r eactor
Acetone Hydrogen IPA Water
100 kgmol/hr 49.8 kgmol/hr
Nu mber of mol es leavin g fr om reactor
90 kgmol/hr 90 kgmol/hr 10 kgmol/hr 49.8 kgmol/hr
at
outlet
=
H eat of f ormation of components
-216.685 KJ/gmol 0 -272.290 KJ/gmol -241.826 KJ/gmol
12.1 Calcul ations at the in let of r eactor
Reference temperature = 25 °C Inlet temperature = 350 °C Sensibl e heat at in let
∫
∫ Group # 5
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( )
Components In
IPA Water
Moles In ni
kgmol 100 49.8
Standard heat of formation
KJ/gmol -272.290 -241.826
Heat of phase change
KJ/gmol 39.858 40.65
Sensible heats
KJ/gmol 20.014 10.476
KJ
12.2 Calcul ations at the outl et of r eactor
Reference temperature = 25 °C Outlet temperature = 350 °C Sensibl e heat at ou tl et
∫ ∫ ∫ ∫ Group # 5
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Components Out
Acetone IPA Hydrogen Water
Moles Out nout
kgmol 90 10 90 49.8
Standard heat of formation
KJ/gmol -216.69 -272.290 0 -241.826
( )
Heat of phase change
Sensible heats
KJ/gmol 30.2 39.858 40.65
KJ/gmol 33.940 22.6 9.466 11.388
KJ
Energy balance equation is:
Here kinetic and potential energies are neglect able with no work done and no accumulation of heat. So the general energy balance equation becomes:
∑ ∑
6
This shows that in order to carry out the reaction we have to supply the 6.3111 ×10 KJ of heat on the basis of 100 kg-mol of isopropyl alcohol entering in the reactor. 12.3 Supply of the heat:
From the energy balance calculations, it is also seen that the reaction is endothermic and we have to supply the heat to the process. This heat is supplied by the use of the molten salt:
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From energy balance Energy supplied by the molten salt
= Energy absorbed by the reacting fluid
The mass flow rate and the temperature are selected and optimized and that values of the flow rate and temperature are selected which makes the process most efficient at minimum cost. From this relation we see that the inverse relation exist between the mass flow rate of the salt and the inlet temperature of the salt. By increasing the inlet temperature of the salt, mass flow rate has to be 6 decreased to exchange 6.3111 × 10 KJ of heat. Similarly if we decrease the inlet temperature of the salt then mass flow rate has to be increased to exchange this amount of heat. So a suitable value of both temperature and mass flow rate has to be selected to make the process optimum.
13. Design of reactor The reactor used in the process is fixed bed plug flow reactor. 13.1 Perf orman ce equation for the reactor:
The performance equation for the fixed bed plug flow reactor is:
∫
Where W is the weight of the catalyst. is the flow rate of the isopropyl alcohol. rate of the reaction. The weight of the catalyst is found from this performance equation. Since the rate equation of reaction is
is the
In the form of conversion the rate equation becomes Group # 5
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Where
⁄
By putting the values of conversion from 0 to 1 in rate equation we obtain the values of . From this we find the values of 1/ . Plot the graph between and 1/ , the weight of catalyst can be calculated. 13.2 Table of and
From the rate equation:
Group # 5
and
0
0.0061
163.9344
0.1
0.004990909
200.3643
0.2
0.004066667
245.9016
0.3
0.003284615
304.4496
0.4
0.002614286
382.5137
0.5
0.002033333
491.8033
0.6
0.001525
655.7377
0.7
0.001076471
928.9617
0.8
0.000677778
1475.41
0.9
0.000321053
3114.754
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⁄ ⁄
13.3 Gr aph between
Graph between
and
and
3500
3000
2500
2000
) A r / 1 (
1500
1000
500
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Conversion (XA)
From this the area under the curve = 730 So
∫ 13.4 Weight of catalyst
Weight of catalyst = W =
Group # 5
× 730 = 100 × 730 = 73000 kg
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13.5 Volu me of catalyst
From the general gas equation the concentration of IPA = 13.6 Space ti me
The time needed to treat the one reactor volume is called the space time.
13.7 Catal yst par ti cles size
The following shapes of catalyst are frequently used in applications:
20-100 µm diameter spheres for fluidized-bed reactors.
0.3-0.7 mm diameter spheres for fixed-bed reactors.
0.3-1.3 cm diameter cylinders with a length-to-diameter ratio of 3-4.
Up to 2.5 cm diameter hollow cylinders or rings.
The void fractions for the spherical particles is between 0.4 to 0.41 The diameter of the copper particles selected (from the literature) = 0.3 mm with the void fractions (Porosity = φ) of 0.4 13.8 Volume of r eactor
13.9 Num ber of tubes
The preferred lengths of the tubes (according to the TEMA standard) should be is 6ft, 8ft, 12ft, 16ft, 20ft and 24ft (7.32 m). So Group # 5
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To calculate tube diameter As we know that to prevent deviation from plug flow assumption
⁄
Assuming Diameter of tube = 70 mm
13.10 H eigh t of r eactor (Shell )
Allowance of the reactor height is 20% - 50% of the shell height. For our system assuming the allowance for shell is 20% of the tube height. So
13.11 Di ameter of r eactor
Diameter of the shell is calculated by using the Harvey equation. Harvey equation is:
Where
The tubes used in triangular form. For triangular form
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By putting all these values in the Harvey and solving, we have
13.12 Veri fi cation of the assumpti ons
All the values assumed can be verified from the length to diameter ratio of the given reactor and the pressure drop calculations. If the value of length to diameter ratio and pressure drop lies within the limit of the fixed bed plug flow reactor then the design is accepted, otherwise we have to perform the calculations again with the different assumptions. 13.12.1 Length t o diameter rati o
For the plug flow reactor the length to diameter ratio lies between 3-5. For the desired reactor Length = 10.248 m Diameter = 2.15 m Length to diameter ratio for desired reactor =
⁄
= 4.766
For the desired reactor the length to diameter ratio is 4.766 lies between the allowable limit. So the design is satisfactory. 13.12.2 Pressur e drop calculati on
For the fixed bed reactor to operate economically the pressure drop along the length of the reactor should be less than the 10% of the operating pressure. The pressure drop along the length of the packed bed is calculated by using the Ergun equation. The Ergun equation is:
Where
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⁄ ⁄ ⁄ By putting all the values in the Ergun equation:
The operating pressure is 2 bars.
The pressure drop along the length of the reactor is less than the 10% of the operating pressure. So the design is accepted. 13.12.3 Volu me of tu bes
In plug flow reactor where the reacting fluid is inside the tubes, the volume of tubes should be greater than the volume of the reactor. So In the desired reactor
This is greater than the reactor volume. So all the conditions are satisfied and design is accept ed.
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14.Mechanical design of the reactor 14.1 Thi ckness of the r eactor:
An estimate of the thickness of the shell is obtained from the diameter of the shell. The wall thickness of any vessel should not be less than the values given below; the values include a corrosion allowance of 2 mm:
Vessel Diameter (m)
Minimum thickness (mm)
1
5
1-2
7
2-2.5
9
2.5-3
10
3-3.5
12
Since the diameter of the vessel is 2.15 m, so from the above table the thickness of the shell is 9mm. Thickness of the reactor shell = 9mm 14.2 H ead selection and design
The ends of a cylindrical vessel are closed by heads of various shapes. The principal types used are: 1. Flat plates and formed flat heads 2. Hemispherical head 3. Ellipsoidal head 4. Torispherical head
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Flat head Applicable to low pressure Cheapest from all types
Torispherical head Used up to the operating pressure of 15 bar Above 10 bars their cost should be compared with that of an equivalent ellipsoidal head
Ellipsoidal head Above 15 bars ellipsoidal head is used Economical within pressure limits
Hemispherical head Used for very high pressures Capital cost is high
So the right choice of head is Torispherical head. Thickness of head is calculated by:
Where
√
So by putting the values of all the variables the value of thickness of head is:
14.3 Vessel Suppor ts:
The method used to support a vessel will depend on the size, shape, and weight of the vessel; the design temperature and pressure and the vessel location and arrangement. Types of supports: Saddle support (for horizontal vessels) Brackets support (for vertical vessels)
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Skirt support (for vertical vessels, particularly where
the length is high and effect of wind is prominent) For the desired reactor “Bracket supports” are used.
Bracket Support 15. Specification Sheet
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Equipment
Reactor
Type of reactor
Multi-tubular fixed bed reactor
Operating temperature
350 °C
Operating pressure
2 bar
Volume of reactor
13.6 m
Volume of catalyst
8.16 m
Weight of catalyst
73000 Kg
Number of tubes
485
Shell height
10.248 m
Diameter of shell
2.15 m
Thickness of shell
9 mm
Head type
Torispherical head
Support type
Bracket support
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16. References www.che.cemr.wvu.edu/publications/projects/acetone/acetone-a.PDF http://www.owlnet.rice.edu/~ceng403/gr1998/acetone.html http://www.scribd.com/doc/30134032/Isopropyl-Alcohol www.annualreviews.org/doi/pdf/.../annurev.matsci.35.100303.12073 www.jbrwww.che.wisc.edu/home/jbraw/chemreacfun/.../slides-masswrxn.p Gael D. Ulrich P. T. Vasudevan A Guide to Chemical Engineering Process Design nd and Economics” 2 ed, Process publishing, 1993 th Plant design and economics for chemical engineers by Timmerhaus 5 edition th Chemical engineering design by Coulson and Richardson’s volume 6, 4 edition “
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