CHE 304 Optional Homework (Due May 27 2009)
A basic simulation of the toluene hydrodealkylation process is required using Provision.
Deliverables: (1) Your own drawing of the PFD and a specification table for the toluene
hydrodealkylation process. (2) PFD from the simulation (PROVISION). Stream Property
Tables are required.
Toluene Hydrodealkylation Process
(Revised from Senior Project of Patricia Quinones, 2002)
The hydrodealkylation of toluene1 (HDA) is a process used to produce benzene. The main
reaction in the process is:
Toluene + H2  Benzene + CH4
Toluene and hydrogen are converted in a reactor packed with catalyst to produce benzene and
methane. Typically the reaction reaches a conversion of 90%. The reaction is highly
exothermic and the typical operating conditions are 500 C to 660 C, and 20 to 60 bar. Figure 1
shows the block diagram of the HDA process.
H2 Recycle
Fuel Gas
Hydrogen
Reactor
Benzene
Separator
Toluene
Distillation
Toluene Recycle
Figure 1: Toluene Hydrodealkylation Block Flow Diagram
1
Analysis, Synthesis, and Design of Chemical Processes Richard Turton, Richard C. Ballie, Wallace B. Whiting,
Joseph A. Shaelwitz. Prentice Hall, PTR. New Jersey, 1998.
15
The HDA process begins with mixing fresh toluene with a stream of recycle unreacted toluene,
the mixing is achieved in a storage tank. The toluene is then pumped to combine it with a
stream of mixed hydrogen and fresh hydrogen gas. The mixture of hydrogen and toluene is
preheated before it is introduced to the heater or furnace. In the furnace the stream is heated to
600 C, the reaction temperature, then introduced into the reactor. The reactor is where the main
reaction happens:
C7H8 + H2 = C6H6 + CH4
This reaction is irreversible, and it requires catalyst. The catalyst consists of chromium or
molybdenum oxides, platinum or platinum oxides, on silica or alumina. Another minor
reversible side reaction2 is often observed:
2 Benzene = Diphenyl + H2
The catalytic process occurs at lower temperatures and offers higher selectivity but requires
frequent regeneration of the catalyst. The products are then cooled and introduced into a pair of
separators that separate the unreacted hydrogen. Portion of the unreacted hydrogen is
compressed and recycle back to the feed and the reactor. The products leaving the separators
are then heated before being introduced into a distillation column, where toluene is separated
from the stream and recycle to the feed. This allow for greater conversion. Then further
fractionation separates methane and toluene from the benzene product. The heating
requirements are achieved with low, high and medium pressure steam. The cooling
requirements are achieve by cooling water at temperature of 30 C and pressure of 1 bar.
The process begins with the first phase of a process: the reactor feed preparation. This begins
with combining 108.7 kmol/hr of a fresh stream of toluene at 25oC, and 1.9 bar, with a recycle
stream of unreacted toluene in a storage tank TK-1 at 112oC and 2.43 bar. Toluene is then
pumped through pump P-1, the discharge pressure of the pump must be 25.5 bar, this is
pumped to a second storage tank TK-2 where toluene is combined with a combined stream of
fresh and recycle of hydrogen. The new two-phase stream is preheated in E-1 to 163.9oC, high
pressure steam at 45 barg is used for this purpose. The stream exists the pre-heater as a onephase stream because the toluene was vaporized. The stream is then introduced into the heater,
H-1, this fired heater is a type of furnace which uses air to combust fuel gas to produce enough
heat to raise the temperature of the stream to 600oC. The reactor feed preparation has now been
completed. The stream is at the desired pressure, 24.81 bar, and temperature, 600oC, necessary
for the reaction to occur.
The stream is then introduced into the reactor R-1, this reactor is a vertical vessel packed with
catalyst. This is the main part of the process where the main reaction, which produces benzene,
happens:
“Benzene”. Kirk-Othmer Encyclopedia of Chemical Technology William Fruscella. John Wiley & Sons, Inc.,
1992.
2
16
C7H8 + H2 = C6H6 + CH4
Toluene + Hydrogen = Benzene + Methane
This is a catalytic exothermic reaction, and the temperature is controlled by injecting hydrogen
into the reactor. Hydrogen is injected at 40.4 kmol/hr 44.96oC and 25.5 bar. The exiting stream
exits at 24.81 bar and 671oC, it contains methane, benzene, toluene, and hydrogen. The one
pass conversion is typically around 75%. The process then continues to the separator feed
preparation phase. This process phase begins with introducing the stream into a cooling water
heat exchanger E-2. This heat exchanger cools the product stream to 38oC, which condenses
most of the toluene and benzene in the stream.
The stream then proceeds to the final phase of the process: the separation. In this phase, the
desired product is separated from the by products and the unreacted components. That is,
benzene is separated from unreacted toluene, unreacted hydrogen and the by-product methane.
This is achieved by introducing the two-phase stream containing benzene, methane, hydrogen
and toluene into a high-pressure phase separator, F-1. In this flash drum the vapor and liquid
are separated. In the overhead mainly hydrogen and methane exits, and in the bottom some
hydrogen, methane, and mainly all the toluene and benzene exit. The overhead stream is split
into two streams; one stream is compressed to 25.5 bar in compressor C-1 and is recycle back
to the feed and to the reactor, the other stream is a portion of the fuel gas stream. The
separation happens at a temperature of 38oC and 23.9 bar.
The partially separated stream is then introduced in a second separator, F-2, where mainly all
the methane and hydrogen is separated in the overhead stream. This separator is a low-pressure
separator. The liquid exiting the low-pressure flash drum consist of mainly toluene and benzene
and traces of dissolved hydrogen and methane. The separation is then completed by heating the
stream in heat exchanger E-3, and then introducing it to a distillation column, T-1. The stream
is heated to a temperature of 90oC in E-3, low pressure steam is used for this purpose. The
distillation column is used to purify the benzene product by separating all the unreacted
components and the by products. This tower consists of 42 sieve trays, a reboiler E-6, a
condenser E-5, a reflux drum F-3, and a reflux pump P-2. Toluene exits as a liquid in the
bottom at a temperature of 112oC and 2.43 bar. The overhead containing benzene, traces of
hydrogen and methane, is condensed in E-5 at a temperature of 112oC and a pressure 2.5 bar.
Cooling water is used to condense the vapor exiting the column. A remaining hydrogen and
methane are then separated in the reflux drum F-3; this vapor stream is combined with the other
gases streams: the overhead of the first separator F-1, and the overhead of the second separator
F-2, which are combined to form the fuel gas. The liquid stream exiting in the bottoms of the
reflux drum is pumped to a discharge pressure of 3.3 bar. The pumped stream is separated in
two streams. One stream is feed to tray one of the column and the other stream is cooled down
to 38oC in heat exchanger E-4. The cooled product stream is then sent to storage.
Simulation Description
Figure 2 shows the PFD of the simulation; the following description refers to this PFD. The
simulation requires certain inputs for each type of equipment. Table 1 is the desired inputs in
stream S1 and S17, the toluene and hydrogen streams, and Table 2 is the desired output streams
S22 and S27.
17
Figure 3: Simulation Process Flow Diagram
Hydrodealkytion of Toluene
C1
SP2
Recycle Hydrogen
S 14
S 12
S 13
S 20
S 15
S 22
S 11
M5
Fuel Gas
M6
S 16
S 25
SP1
S 17
M2
S7
Hydrogen
M4
S9
S 18
R1
S6
S8
M3
S4
S3
SC2
S 28
S5
E2
E1
E5
S2
T ol uene
S 26
F1
S 23
S1
S 10
M1
S 27
P1
Benzene
F2
E4
S 19
S 21
E3
SC1
S 24
Toluene Recycle
Figure 2: Simulation PFD for the toluene hydrodealkylation process.
18
Table 1: Input streams for the HDA process.
Inputs
Stream: S1
Stream: S17
Molar Rate: 108.7 kmol/hr
Molar Rate: 301.0 kmol/hr
Temperature: 25 C
Temperature: 25 C
Pressure: 1.9 bar
Pressure: 25.5 bar
Molar Composition: 1.00 Toluene
Molar Composition: 0.95H2
0.05 CH4
Table 2: Output streams for HDA process.
Outputs
Stream: S22
Stream: S27
Molar Rate: 304.1 kmol/hr
Molar Rate: 105.6 kmol/hr
Temperature: 35.7 C
Temperature: 38 C
Pressure: 2.5 bar
Pressure: 2.955 bar
Molar Composition: 0.001 C7H8
Molar Composition: 0.004 C7H8
0.587 H2
0.996 C6H6
0.403 CH4
0.009 C6H6
The inputs required in each type of equipment are shown in Table 3 (Specification
Table).
Table 3: Input data for process simulation.
Simulator
Equipment Simulator
Number
Equipment
M-1
Mixer
M-2
Mixer
M-3
Mixer
Mixer
M-4
Mixer
M-5
M-6
Mixer
Pump
P-1
C-1
Compressor
Input
Output
Streams Streams
S1 S24 S2
S17 S14 S16
S16 S3 S4
S6 S15 S7
S11 S18 S20
S20 S25 S22
S2
S3
S12
S13
19
Required Input
Pressure Drop = 0 bar
Pressure Drop = 0 bar
Pressure Drop = 0 bar
Pressure Drop = 0 bar
Pressure Drop = 0 bar
Pressure Drop = 0 bar
Outlet pressure = 25.5 bar, Efficiency 75%
Outlet pressure = 25.5 bar, Efficiency 75%
E-1
E-2
E-3
E-4
E-5
F-1
F-2
SP-1
SP-2
R-1
SC-1
SC-2
Pre-heater S4
Heater
S5
Heat Exchanger S19
Cooler
S26
Cooler
S8
Flash Drum S28
Flash Drum S10
Splitter
S9
Splitter
S13
Reactor
S7
Stream Calc. S21
Stream Calc. S23
S5
S6
S21
S27
S28
S9
S18
S11
S14
S8
S23
S25
Outlet Liquid Fraction = 0
Outlet temperature = 600 C
Outlet temperature = 90 C
Outlet temperature = 38 C
Outlet temperature = 38 C
S10 Pressure = 23.9 bar, Temp = 38 C
S19 Pressure = 2.8 bar, Temp = 38 C
S12 Recycle stream flowrate = 440 kgmol/hr
S15 Recycle to the reactor = 40 kgmol/hr
Duty = 0, product phases, global temp = -22.2 C
75% conversion
Overhead product = 99% benzene,
S24 Bottoms product = 98.85 % toluene
S26 Overhead product = 100% H2, 100% CH4
The thermodynamic system used for this simulation was the Soave- Redlich-Kwong
(SRK). The simulation starts with combining the fresh toluene stream S1 with the recycle
toluene stream S24 in the mixer M-1 to form stream S2. This mixer simulates the mixing
that happens in TK-1 in the original PFD figure 2. The stream S2 is then pumped (P-1)
to 25.5 bar and is combined in a mixer M-3 with stream S16, this stream is a mixture of
fresh feed hydrogen mixed with unreacted hydrogen, it also contains traces of toluene
benzene and methane. Pump P-1 is simulated with an efficiency of 75%. Mixer M-3
simulates the mixture of the liquid phase and gas phase that happens in TK-2 in the
original process. The stream S16 is the outlet of mixer M-2, this mixer combines the
fresh stream of hydrogen S17 and the unreacted recycle stream of hydrogen S14. To
simulate the mixers only the pressure drop is required, which in this case is 0 bar. The
two phase stream exiting the mixer M-3 is then introduced into heat exchanger E-1 in the
tube side. High-pressure stream is used in this heat exchanger in the shell side to heat up
the tube side process stream S4 to an outlet temperature of 163.9 C. The outlet stream S5
is a one-phase stream of all vapor. The pressure drop for all the heat exchangers are set
to be 0.345 bar to simulate a real heat exchanger.
The process stream is then introduced into the heater E-2, where it is heated up to 600 C.
This temperature is the required temperature that will allow the reaction to happen in the
reactor. The now heated up stream S6 is combined with recycle compress hydrogen S15
in mixer M-4. Since the reaction is highly exothermic, the temperature is controlled by
injection of quench hydrogen at the reactor, this is simulated by the use of mixer M-4.
The feed is then introduced to the main part of the process the reactor R-1. A conversion
reactor is used to simulate this stoichiometric reactor. The reaction stoichiometry must be
entered in order to appropriately use this reactor. This reactor type is used since only the
reaction is known, no other data equilibrium data is known. In order to simulate an
exothermic reactor the duty of the reactor must be specified to be zero Btu/hr. Also the
toluene conversion of 75% must be entered. This is the typical conversion that is realistic
for this process. The process stream now containing a high amount of benzene exits the
reactor at 671oC, this temperature must be decreased in order to separate the unreacted
components with the products. This is achieved by introducing the product stream S8 to
20
heat exchanger E-5. In this heat exchanger the process stream is cooled to 38oC, by using
cooling water. The outlet stream, S28, is a two-phase stream that is introduced into the
first separator F-1. This knock out drum separates mostly all hydrogen and methane and
it also contains traces of toluene and benzene. The pressure and temperature of this flash
drum must be specified to 23.9 bar and 38oC.
The overhead stream S9 is then split into two streams S11 and S12 in splitter SP1. For
this splitter an estimate of what is recycled back is given in table 3. Table 3 shows the
estimates needed for the recycle streams S12, and S15, for splitters 1 and 2. Stream S12
is compressed in a compressor of efficiency of 75% to a pressure of 25.5 bar. The
compressed gas S13 is then introduced into splitter SP-2, which splits the splitter into
stream S14 and S15. Stream S14 is mixed with the fresh feed of hydrogen, and stream
S15 is used to control the temperature of the reactor. The amount of the splits is given in
Table 3. The stream S11 is part of the fuel gas.
The bottom stream S10 of the separator F1 is introduce into the second separator F2,
which separates more hydrogen and methane from benzene and toluene. The liquid
bottom stream S19 is then heated up in heat exchanger E-3 to 90oC. The overhead stream
S18 is combined with the overhead of the first separator F-1 in mixer M-5 to makeup
portion of the fuel gas.
Stream S19 is then introduced into a distillation column, which is simulated by a stream
calculator, SC1. In this piece of equipment the separation required must be specified. In
this case 99% of the benzene must exit at the overhead and 98.85% of the toluene must
exit at the bottom of the separation unit. The temperature and pressure of the overhead
and the bottom must be specified in order for the simulation to be properly represented.
The temperatures are 112oC and 2.43 bar. The bottoms of this stream calculator S24 is
the recycle toluene that is mixed in M1 with the fresh toluene feed. The overhead of this
separator is then introduced into another stream calculator SC2, which simulates the
condenser and reflux drum and pump of the distillation tower. This stream calculator
separates all the hydrogen and methane in the overhead S25, from the toluene and
benzene in the bottoms S26. The stream S25 is then combined in a mixer M6 with the
rest of the gas that composes the fuel gas. The stream S26 is then introduced into a heat
exchanger E4 where the benzene product is cooled down to 38oC before is sent to
storage. Table 4 lists the results of the material balance of each stream in the simulation.
21
Table 4: Simulation Material Balance
Stream Name
Stream Description
Phase
Temperature
Pressure
Flowrate
Composition
TOLUENE
H2
METHANE
H2O
AIR
BENZENE
Stream Name
Stream Description
Phase
Temperature
Pressure
Flowrate
Composition
TOLUENE
H2
METHANE
H2O
AIR
BENZENE
S1
S2
S3
S4
S5
S6
S7
S8
Toluene
Liquid
Liquid
Liquid
Mixed
Vapor
Vapor
Vapor
Vapor
C
25.000
48.242
49.160
43.219
163.905 600.000 586.293 671.015
BAR
1.900
1.900
25.500
25.500
25.155
24.811
24.811
24.811
KG-MOL/HR 108.700 144.653 144.653 845.265 845.265 845.265 885.653 885.653
1.000
0.000
0.000
0.000
0.000
0.000
S9
C
BAR
KG-MOL/HR
0.993
0.000
0.000
0.000
0.000
0.007
S10
0.993
0.000
0.000
0.000
0.000
0.007
S11
0.170
0.618
0.206
0.000
0.000
0.005
S12
0.170
0.618
0.206
0.000
0.000
0.005
S13
0.170
0.618
0.206
0.000
0.000
0.005
S14
0.163
0.617
0.215
0.000
0.000
0.006
S15
Vapor
Liquid
Vapor
Vapor
Vapor
Vapor
Vapor
38.000
38.000
38.000
38.000
44.958
44.958
44.958
23.900
23.900
23.900
23.900
25.500
25.500
25.500
740.254 145.400 300.254 440.000 440.000 399.612
40.388
0.001
0.591
0.399
0.000
0.000
0.009
0.243
0.004
0.021
0.000
0.000
0.732
Stream Name
Stream Description
Phase
Temperature
Pressure
Flowrate
Composition
TOLUENE
H2
METHANE
H2O
AIR
BENZENE
S17
S18
Hydrogen
Vapor
Vapor
C
25.000
38.000
BAR
25.500
2.800
KG-MOL/HR 301.000
3.117
Stream Name
Stream Description
Phase
Temperature
Pressure
Flowrate
Composition
TOLUENE
H2
METHANE
H2O
AIR
BENZENE
S26
0.000
0.950
0.050
0.000
0.000
0.000
0.007
0.165
0.766
0.000
0.000
0.063
0.001
0.591
0.399
0.000
0.000
0.009
S19
0.001
0.591
0.399
0.000
0.000
0.009
S20
0.004
0.000
0.000
0.000
0.000
0.996
S21
0.001
0.591
0.399
0.000
0.000
0.009
S23
0.001
0.591
0.399
0.000
0.000
0.009
S24
S16
Vapor
36.672
25.500
700.612
0.001
0.745
0.249
0.000
0.000
0.005
S25
Liquid
Vapor
Mixed
Vapor
Liquid
Vapor
38.000
35.487
90.000
112.000 112.000 112.000
2.800
2.800
2.455
2.155
2.425
2.500
142.283 303.371 142.283 106.329
35.954
0.731
0.248
0.000
0.005
0.000
0.000
0.747
0.001
0.587
0.403
0.000
0.000
0.009
S27
S22
S28
Benzene Fuel Gas
Liquid
Liquid
Vapor
Mixed
C
112.000
38.000
35.661
38.000
BAR
3.300
2.955
2.500
24.466
KG-MOL/HR 105.598 105.598 304.102 885.653
0.004
0.000
0.000
0.000
0.000
0.996
0.001
0.591
0.399
0.000
0.000
0.009
0.041
0.495
0.337
0.000
0.000
0.127
0.001
0.585
0.404
0.000
0.000
0.009
22
0.041
0.495
0.337
0.000
0.000
0.127
0.248
0.000
0.005
0.000
0.000
0.747
0.004
0.000
0.007
0.000
0.000
0.989
0.970
0.000
0.000
0.000
0.000
0.030
0.000
0.023
0.977
0.000
0.000
0.000