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CHE655 – Plant Design Project #2 Summer 2014 DESIGN OF A CUMENE PRODUCTION PROCESS (Courtesy of the Department of Chemical Engineering at West Virginia University)

Introduction Cumene (isopropyl benzene) is produced by reacting propylene and benzene over an acid catalyst. Cumene may be used to increase the octane in gasoline, but its primary use is as a feedstock for manufacturing phenol and acetone. The plant where you are employed has been buying cumene to produce phenol. Management is considering manufacturing cumene rather than purchasing it to increase profits. Someone has made a preliminary sketch for such a process and has submitted to the engineering department for consideration. Your group is assigned the problem of evaluating the sketch and recommending improvements in the preliminary design. Note that optimization is NOT required in this design project.

Process Description Figure 1 is a preliminary process flow diagram (PFD) for the cumene production process. The raw materials are benzene and propylene. The propylene feed contains 5 wt% propane as an impurity. It is a saturated liquid at 25°C. The benzene feed, which may be considered pure, is liquid at 1 atm and 25°C. Both feeds are pumped to about 3000 kPa by pumps P-201 and P202, are then vaporized and superheated to 350°C in a fired heater (H-201). The fired heater outlet stream is sent to a packed bed reactor (R-201) in which cumene is formed as a desired product and p-diisopropyl benzene (PDIB) as an undesired product. The reactor effluent is sent to a flash unit (V-201) in which light gases (mostly propane and propylene, some benzene, cumene and PDIB) are separated as vapor in Stream 9. Stream 10, containing mostly cumene and benzene, is sent to a distillation column (T-201) to separate benzene for recycle from cumene product. The desired cumene production rate is 100,000 metric tons/yr.

Process Details Feed Streams Stream 1: benzene, pure liquid, 25°C and 1 atm Stream 2: propylene with 5 wt% propane impurity, saturated liquid at 25°C Effluent Streams Stream 9: fuel gas stream, credit may be taken for LHV of fuel Stream 12: cumene product; must be > 99 wt% purity

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Equipment Pump (P-201): The pump increases pressure of the benzene feed from 1 atm to about 3000 kPa. Pump (P-202): The pump increases the pressure of the propylene feed to about 3000 kPa. Fired Heater (H-201): The fired heater desubcools, vaporizes, and superheats the mixed feed up to 350°C. Air and natural gas must be fed to the fired heater. Natural gas is priced at its lower heating value. The fired heater is 75% efficient. Reactor (R-201): The reactor feed must be between 300°C - 400°C and between 2800 kPa - 3200 kPa. Benzene must be present in at least 50% excess. Conversion of the limiting reactant is 92%. The reactor may be assumed isothermal, and the exothermic heat of reaction is removed by vaporizing boiler feed water to make high-pressure steam. Credit may be taken for the high-pressure steam. Main reaction:

C3 H 6



C6 H 6

propylene benzene

 C9 H 12 cumene

Side reaction:

C3 H 6  C9 H12  C12 H18 propylene cumene

p - diisopropyl benzene

Flash Vessel (V-201): This is actually a combination of a heat exchanger and a flash drum. The temperature and pressure are lowered in order to separate the propane and propylene from the cumene and benzene. Cooling water is used to lower the temperature. Distillation Column (T-201): Here all cumene and PDIB impurity in Stream 10 goes into Stream 12, and is in the liquid phase. All benzene, propylene and propane go to Stream 11, and is also in the liquid phase. A distillation column requires both heat addition and heat removal. Heat removal is accomplished in a condenser (not shown), which requires an amount of cooling water necessary to condense the contents of Stream 11. Heat addition is accomplished in a reboiler (not shown), which requires an amount of high-pressure steam necessary to vaporize the cumene in Stream 12.

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Design of Heat Exchanger in V-201 A detailed design of the heat exchanger in V-201 is required for base-case conditions. It should be assumed that cooling water is available at the conditions specified in the Appendix of this problem statement. For this heat exchanger design, the following information should be provided:            

Diameter of shell Number of tube and shell passes Number of tubes per pass Tube pitch and arrangement (triangular/square/..) Number of shell-side baffles, if any, and their arrangement (spacing, pitch, type) Diameter, tube-wall thickness, shell-wall thickness, and length of tubes Calculation of both shell- and tube-side film heat transfer coefficients Calculation of overall heat transfer coefficient (you may assume that there is no fouling on either side of the exchanger) Heat transfer area of the exchanger Shell-side and tube-side pressure drops (calculated, not estimated) Materials of construction Approximate cost of the exchanger

A detailed sketch of the exchanger should be included along with a set of comprehensive calculations in an appendix for the design of the heat exchanger. You should use ASPEN Exchanger Design & Rating (EDR) in the ASPEN Plus simulator to carry out the detailed design.

Cumene Production Reaction For final design of this process, the kinetics for the reactions given below should be used. For the desired reaction: k1 C3 H 6  C6 H 6  C9 H12

propylene benzene cumene

r1  k1c p cb

mole / g cat sec

 24.90  k1  35 .  104 exp   RT 

For the undesired reaction: C3 H 6



propylene

k

C9 H12 2  C12 H18 p  diisopropyl benzene

cumene

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r2  k2c p cc

mole / g cat sec

 35.08  k2  2.9  106 exp   RT 

where the units of the activation energy are kcal/mol, the units of concentration are mol/L, and the temperature is in Kelvin. For a shell and tube packed bed, the recommended configuration, the following data may be assumed: catalyst particle diameter dp = 3 mm catalyst particle density cat = 1600 kg/m3 void fraction  = 0.50 heat transfer coefficient from packed bed to tube wall h = 60 W/m2°C use standard tube sheet layouts as for a heat exchanger if tube diameter is larger than in tube sheet layouts, assume that tube area is 1/3 of shell area

Economic Analysis When evaluating alternative cases, you should carry out an economic evaluation and profitability analysis based on a number of economic criteria such as payback period, internal rate of return, and cash flow analysis. In addition, the following objective function should be used. It is the equivalent annual operating cost (EAOC), and is defined as EAOC = -(product value - feed cost - other operating costs - capital cost annuity) A negative EAOC means there is a profit. It is desirable to minimize the EAOC; i.e., a large negative EAOC is very desirable, although you are not being asked to carry out optimization. The costs for cumene (the product) and benzene (the feed) should be obtained from the Chemical Marketing Reporter, which is in the Evansdale Library. The “impure” propylene feed is $0.095/lb. The capital cost annuity is an annual cost (like a car payment) associated with the one-time, fixed cost of plant construction. The capital cost annuity is defined as follows:

capital cost annuity  FCI

i (1  i ) n (1  i ) n  1

where FCI is the installed cost of all equipment; i is the interest rate, i = 0.15; and n is the plant life for accounting purposes, n = 10. For detailed sizing, costing, and economic evaluation including profitability analysis, you may use the Aspen Process Economic Analyzer (formerly Aspen Icarus Process Evaluator) in Aspen Plus. However, it is also a good idea to independently verify the final numbers based on other sources such as cost data given below.

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Other Information You should assume that a year equals 8000 hours. This is about 330 days, which allows for periodic shut-down and maintenance.

Final Comments As with any open-ended problem; i.e., a problem with no single correct answer, the problem statement above is deliberately vague. You may need to fill in some missing data by doing a literature search, Internets search, or making assumptions. The possibility exists that as you work on this problem, your questions will require revisions and/or clarifications of the problem statement. You should be aware that these revisions/clarifications may be forthcoming. Moreover, in some areas (e.g. sizing/costing) you are given more data and information than what is needed. You must exercise engineering judgment and decide what data to use. Also you should also seek additional data from the literature or Internet to verify some of the data, e.g. the prices of products and raw materials.

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Cost Data Raw Materials Benzene (> 99.9 wt% purity)

see Chemical Marketing Reporter

Propylene ( 5 wt% propane impurity)

$0.095/lb

Product Cumene (> 99 wt% purity)

see Chemical Marketing Reporter

Utility Costs Low Pressure Steam (446 kPa saturated)

$3.00/1000 kg

Medium Pressure Steam (1135 kPa saturated)

$6.50/1000 kg

High Pressure Steam (4237 kPa saturated)

$8.00/1000 kg

Natural Gas (446 kPa, 25C)

$3.00/106 kJ

Fuel Gas (446 kPa, 25°C)

$2.75/106 kJ

Electricity

$0.08/kWh

Boiler Feed Water (at 549 kPa, 90C)

$300.00/1000 m3

Cooling Water $20.00/1000 m3 available at 516 kPa and 30C return pressure  308 kPa return temperature is no more than 15C above the inlet temperature Refrigerated Water available at 516 kPa and 10C return pressure  308 kPa return temperature is no higher than 20C

$200.00/1000 m3

Waste Treatment

$1/kg organic waste

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Equipment Costs (Purchased) Pumps

$630 (power, kW)0.4

Heat Exchangers $1030 (area, m2)0.6 Compressors

$770 (power, kW)0.96 + 400 (power, kW)0.6

Turbine

$2.18105 (power output, MW)0.6 assume 65% efficiency

Fired Heater

$635 (duty, kW)0.8 assume 80% thermal efficiency assume can be designed to use any organic compound as a fuel

Vessels

$[1.67(0.959 + 0.041P - 8.310-6P2)]10z z = (3.17 + 0.2D + 0.5 log10L + 0.21 log10L2) D = diameter, m 0.3 m < D < 4.0 m L = height, m L/D < 20 P = absolute pressure, bar

Catalyst

$2.25/kg

Reactor

Cost as vessel with appropriate additional volume for cooling coil (fluidized bed) or tubes (shell and tube packed bed)

Packed Tower

Cost as vessel plus cost of packing

Packing

$(-110 + 675D + 338D2)H0.97 D = vessel diameter, m; H = vessel height, m

Tray Tower

Cost as vessel plus cost of trays

Trays

$(187 + 20D + 61.5D2) D = vessel diameter, m

Storage Tank

$1000V0.6 V = volume, m3

It may be assumed that pipes and valves are included in the equipment cost factors. Location of key valves should be specified on the PFD.

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Equipment Cost Factors Pressure (absolute)

< 10 atm, 0.0 10 - 20 atm, 0.6 20 - 40 atm, 3.0 40 - 50 atm, 5.0 50 - 100 atm, 10

Carbon Steel Stainless Steel

0.0 4.0

does not apply to turbines, compressors, vessels, packing, trays, or catalyst, since their cost equations include pressure effects

Total Installed Cost = Purchased Cost (4 + material factor + pressure factor)

Heat Exchangers For heat exchangers, use the following approximations for heat transfer coefficients to allow you to determine the heat transfer area: situation

h (W/m2°C)

condensing steam

6000

condensing organic

1000

boiling water

7500

boiling organic

1000

flowing liquid

600

flowing gas

60

References 1. Felder, R.M. and R.W. Rousseau, Elementary Principles of Chemical Processes (2nd ed.), Wiley, New York, 1986. 2. Perry, R.H. and D. Green, eds., Perry’s Chemical Engineering Handbook (6th ed.), McGraw-Hill, New York, 1984, p. 9-74.

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