Cumene Production
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.
I. Backgrond information for Material Balances
Cumene Production Reactions
The reactions for cumene production from benzene and propylene are as follows:
C3H6 + C6H6 → C6H5-C3H7
propylene benzene cumene
C3H6 + C6H5-C3H7 → C3H7-C6H4-C3H7
propylene cumene diisopropyl benzene (DIPB)
The best technology for cumene production is a catalytic process that is optimized at 350°C and 25 atm.
pressure. The benzene is kept in excess to limit the amount of DIPB product.
Process Description
The reactants are fed as liquids from their respective storage tanks. After being pumped up to the required
pressure dictated by catalyst operating conditions, the reactants are mixed, vaporized and heated up to the
reactor operating temperature. The catalyst converts the reactants to the desired and undesired products
according to the reactions listed above. The molar feed ratio is 2:1 benzene to propylene; propylene
conversion is 99%; the product molar selectivity ratio is 31:1 cumene to DIPB. The product gases are
cooled to 40°C at 25 atm. pressure to condense essentially all of the cumene, DIPB, and unreacted benzene
to a liquid. The unreacted propylene and a propane impurity are separated from the liquid and are used as
fuel gas. The liquid stream is sent to two distillation towers. The first distillation tower separates benzene
from cumene and DIPB. The benzene purity level is 98.1 mole%. We have no chemical market for this
stream and plan to sell it as unleaded gasoline. The second distillation tower separates cumene from DIPB.
The cumene is 99.9 mole% pure. The DIPB stream will be sold as fuel oil. A sketch of the process is
attached. The reaction units and process streams are described in the tables.
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Table 1. Description of Process Units
Symbol Name Comments
V-201 Vaporizer Liquid feeds are vaporized and heated for reactor
R-201 Reactor Vapors are reacted over catalyst; temperature 350°C; pressure
25 atm.; 99% propylene conversion per pass; 31/1
cumene/DIPB molar selectivity
S-201 Separator Vapor is cooled to 40°C at 25 atm. pressure, separating
essentially all of the benzene, cumene and DIPB as a liquid
from propylene and propane gases
T-201 Distillation Tower No. 1 Overhead stream contains 98.1 mole% benzene, balance
cumene; bottoms stream contains cumene and DIPB
T-202 Distillation Tower No. 2 Overhead stream contains 99.9 mole% cumene; bottoms
stream contains pure DIPB
Table 2. Description of Process Streams
Stream Number Comments
1 benzene>99.9 mole% pure; liquid feed
2 95 mole% propylene; 5 mole% propane; liquid feed
3 2/1 benzene/propylene molar feed ratio
4 99% propylene conversion; 31/1 cumene/DIPB molar selectivity
5 propylene + propane only
6 0 mole% propylene + propane
7 98.1 mole% benzene purity, balance cumene, sold as gasoline
8 0 mole% benzene
9 99.9 mole% cumene, balance DIPB; 100,000 tons/year production
10 100 mole% DIPB; sold as fuel oil
Table 3. Prices for Feedstock’s and Process Streams
Chemical or Fuel Price
benzene feed, >99.9% $0.90/gallon
propylene feed, 95 mole% propylene, 5 mole% propane $0.095/lb
cumene, >99.8 mole% $0.21/lb
fuel gas $0.080/lb
gasoline $0.60/gallon
fuel oil $0.50/gallon
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II. Background information for Energy Balances (Take note of the slightly different
assumptions and process conditions)
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 P-202, 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. There are no side-reactions or by-products. The reactor effluent is sent to a
flash unit (V-201) in which light gases (mostly propane and propylene, some benzene and cumene) 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, assumed pure
Equipment
Pump (P-201):
The pump increases pressure of the benzene feed from 1 atm to about 3000 kPa. Pump operation may be
assumed isothermal, and the cost of energy may be neglected. (Both of these assumptions are valid for this
semester’s design only.)
Pump (P-202):
The pump increases the pressure of the propylene feed to about 3000 kPa.. Pump operation may be
assumed isothermal, and the cost of energy may be neglected. (Both of these assumptions are valid for this
semester’s design only.)
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.
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The following reaction occurs:
C3 H 6 + C6 H 6 → C9 H12
propylene benzene cumene
There are no side reactions.
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 in Stream 10 goes into Stream 12 (may be assumed pure cumene), and is in the liquid
phase. All benzene, propylene and propane goes 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.
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
Electricity $0.05/kW hr
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 should be no more than 15°C above the inlet temperature, otherwise there is an
additional cost of $0.35/106 kJ
Refrigerated Water $200.00/1000 m3
available at 516 kPa and 10°C
return pressure ≥ 308 kPa
return temperature is no higher than 20°C
if return temperature is above 20°C, there is an additional cost of $7.00/106 kJ
Data
Use data from References [1] or from any handbook (such as Reference [2]). The following data are not
readily available in these references.
Liquid Heat Capacity (range 25°C - 300°C)
Assume that the liquid heat capacity for benzene given in Reference [1] is valid for all organic
liquids.
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Vapor Pressures: Vapor pressures may be interpolated or extrapolated from the following data:
normal boiling point additional vapor pressure point
T (K) T (K) 10-6 P (kPa)
benzene 353 562 4.87
propylene 225 365 4.59
propane 231 370 4.14
cumene 425 631 3.21
Normal heat of vaporization for cumene: 3.81 × 107 J/kmole
Heat of formation for cumene: 3.933 × 106 J/kmole
Economic Analysis
When evaluating alternative cases, 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.
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.
Other operating costs are utilities, such as steam, cooling water, natural gas, and electricity.
The capital cost annuity is an annual cost (like a car payment) associated with the one-time, fixed cost of
plant construction. A list of capital costs for each piece of equipment will be provided by Spring Break.
You will learn to calculate the annuity value in ChE 38.
Other Information
You should assume that a year equals 8000 hours. This is about 330 days, which allows for periodic shut-
down and maintenance.
You should assume that two streams that mix must be at identical pressures. Pressure reduction may be
accomplished by adding a valve. These valves are not shown on the attached flowsheet, and it may be
assumed that additional valves can be added as needed.
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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