TOPIC: Conceptual Flowsheet for Production of Benzene from Toluene. Proposed Solution:

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Janel Ashlyn Young
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1 Norwegian University of Science and Technology Course: Energy and Process Department of Energy and Process Engineering No.: TEP 4230 Trondheim, , T. Gundersen Part: Production Systems Task: 5 Year: 2004 Plenary: Deadline: none TOPIC: Conceptual Flowsheet for Production of Benzene from Toluene Proposed Solution: 1) To remove Methane (CH 4 ) from Hydrogen ( ) in the gas feed for this process will be quite expensive since condensing Methane (atmospheric boiling point of 112 K or -161 C) requires very low temperatures. Further, Methane is a by-product for the main reaction where Benzene is produced, and the conclusion is thus that we should not remove Methane before the reactor. Due to the second argument, membrane technology has not been considered for the separation of Methane and Hydrogen. 2) The task can be solved numerically by iteration or analytically. The numerical iteration (here performed as a trial and error procedure) gives the following results: Conversion (X) Selectivity (S) Status high X low X low X low X high X Fine! The analytical solution of the expression S = f(x) as given under item (e) of the tast text gives the following: X = 1 - exp { [ ln ln (1 S) ] / 1.544} Using S = 0.95 this expression gives X = or 81.8%. From the table above, we see that more iterations are required to identify maximum yield defined as Y = X S. The following can be established: X = 0.91 gives S = and then Y = 0.775, X = 0.89 gives S = and then Y = 0.793, X = 0.88 gives S = and then Y = 0.796, X = gives S = and then Y = 0.797, and finally X = 0.87 gives S = and then Y = It can be concluded that maximum yield is achieved at a degree of conversion of about 87% with a selectivity of about 92% and a resulting yield of about 80%. Maximum yield can also be found analytically through derivation of the relationship between X and S and then identifying the maximum of Y = X S. 3) Since Methane is an impurity of the gas feed and also is a by-product from the main reaction, there will be Methane in the reactor effluent. Since Methane is highly volatile, this component will follow unconverted Hydrogen (even more volatile) through the subsequent separation system. Since (as argued under item 1) it is very expensive to separate Hydrogen and Methane, the solution is to have a purge stream. This means that the stream with volatile components leaving the separation system should be split into two branches, where one branch is the purge stream, and the other branch is the recycle to the reactor. Page 1 of 6

2 4) A simple block diagram for the process based on the conclusions so far is shown on the next page. 5) Since there is no loss of Toluene in the product streams (Benzene and Diphenyl) as a result of the assumption about an ideal separation system, the conversion of Toluene in the reactor must equal the fresh feed of Toluene to the process. If the expression for selectivity is used (see the text for this task, item e): S Benzene produced / Toluene converted = P B / F FT This gives the following result: F FT = P B / S = / 0.95 = kmoles/hr The converted Toluene that does not end up as Benzene will give the by-product Diphenyl (these are the three aromatic components in the system). The produced amount of Diphenyl then becomes (taking account for the stoichiometric fact that it takes 2 Benzene molecules to produce 1 Diphenyl molecule): P D = ½ (F FT - P B ) = ½ ( ) = 6.75 kmoles/hr Purge P G R G F G F FT Toluene Reactor Separation System Benzene P B R T Toluene Diphenyl P D The amount of purge gas (P G ) and recycled gas (R G ) can be found by solving a system with two equations (molar balance for Hydrogen and Methane) with respect to two variables (P G and R G ). One must also use the information that the purge stream contains 40% (molar basis) of Hydrogen (see the text, item 5). The molar balance for Hydrogen is based on: In + Produced = Converted + Lost in Purge Hydrogen is converted in reaction (1) at the same rate as the conversion of Toluene (which is equal to fresh feed of Toluene) and produced in reaction (2) at the same rate as Diphenyl is produced. This gives the following: 0.95 F G + P D = F FT P G Page 2 of 6

3 The molar balance for Methane is similarly based on: In + Produced = Lost in Purge Methane is produced in reaction (1) at the same rate as the conversion of Toluene (which is equal to fresh feed of Toluene). This gives the following: 0.05 F G + F FT = 0.6 P G When introducing figures for the known variables (F FT = and P D = 6.75), the result is a purge gas rate of P G = kmoles/hr and a feed gas rate of F G = kmoles/hr. The recycled amount of Toluene can be found from the degree of conversion in the reactor (X = 81.8%) and the conclusion that converted Toluene must equal fresh feed of Toluene. Total amount of Toluene in the reactor feed stream (F T ) is equal to fresh feed of Toluene (F FT ) plus the recycled amount of Toluene (R T ). From this amount, 81.8% is converted and must again equal the fresh feed of Toluene: (F FT + R T ) = F FT Introducing known figures for F FT this gives recycled Toluene, R T = kmoles/hr. The recycled amount of gas is found from the required Hydrogen/Aromatics ratio for the reactor inlet stream: ( / Aromatics ) in 5.0 If this ratio is set equal to 5.0 and we realize that Toluene is the only aromatic component at the reactor inlet, the expression above becomes: ( 0.95 F G R G ) / ( F FT + R T ) = 5.0 Introducing known figures (F G = , F FT = and R T = 60.05), this gives the amount of recycled gas, R G = kmoles/hr, in other words a huge recycle! In processes with purge streams, the recycle streams tend to be large to avoid loss of raw material in the purge stream. For this particular process, however, it is primarily the large Hydrogen/Aromatics ratio that pushes the recycle stream to such a large value. 6) The purpose of the purge stream is (in this process) to allow Methane to be released from the process. In order to avoid accumulation of Methane in the recycle stream, the amount of Methane in the purge stream must equal the sum of Methane in the feed gas and the amount of Methane produced in reaction (1). This is included in our calculations through the molar balance for Methane, but can be checked at this stage: In with feed plus produced in reaction (1): 0.05 F G + F FT = kmoles/hr Out with the purge stream: 0.6 P G = kmoles/hr The total amount of purge is calculated to kmoles/hr, which means a loss of the raw material Hydrogen equal to kmoles/hr. If the purge stream is reduced, the loss of Hydrogen will be reduced too, since the amount of Methane must be constant. This can Page 3 of 6

4 only be achieved by increasing the recycle so that the fraction of Methane in the recycle and purge is increasing. Using Y PH to describe the fraction of Hydrogen in the purge, we have: Methane that must be released from the process: (1 - Y PH ) P G = constant = kmoles/hr When the amount of purge is reduced, the fraction of Methane (1 - Y PH ) must increase. This means that the fraction of Hydrogen (Y PH ) will be reduced, and this affects the expression that was used to calculate the amount of recycled gas (R G ): ( 0.95 F G R G ) / ( F FT + R T ) = 5.0 If 0.4 is substituted with Y PH, we realize that a reduced Y PH must give an increased R G. 7) The molar streams at the reactor inlet is: Hydrogen: 0.95 F G R G = kmoles/hr Methane: 0.05 F G R G = Benzene: = 0.00 Toluene: F FT + R T = Diphenyl: = 0.00 The molar streams in the reactor outlet is: Hydrogen: 0.4 (R G + P G ) = kmoles/hr Methane: 0.6 (R G + P G ) = Benzene: P B = Toluene: R T = Diphenyl: P D = ) The total reaction equation can be established by considering the net consumption and production of the various species through the reactor: or ( ) + ( ) Toluene = Benzene + ( ) CH Diphenyl Toluene = Benzene CH Diphenyl This gives the following expression for the standard heat of reaction (, means 1000, and. means decimal): H r 0 H r 0 = ( 83) (Benzene) (-75) (Methane) (182) (Diphenyl) (0) (Hydrogen) (50) (Toluene) = - 11, (kmoles/hr) (kj/mole) Page 4 of 6

5 = - 11, MJ/hr = - 11, (MJ/hr) / 3600 (sec/hr) = MW or H r 0 = (kw) / (kmoles Benzene/hr) = kwh / kmoles Benzene Accordingly, this is an exothermic reactor (where chemical energy is converted to thermal energy). 9) Due to the large step in boiling points between Hydrogen/Methane and the three Aromatic components, it is overwhelmingly likely that a simple flash separator will work well in this case. This means that we avoid condensing Methane, which is very expensive. The three aromatic components can subsequently be separated in two distillation columns. The feed stream to the separation system is characterized by the following: Component Boiling Point Flow Rate Name T B (K) (kmoles/hr) Hydrogen Methane Benzene Toluene Diphenyl Since the amount of Diphenyl is very small, it is preferable to separate Benzene from Toluene (and Diphenyl) first, and thereafter separate Toluene from Diphenyl. The conceptual flowsheet for this process then becomes as indicated in the figure below: 10) The major energy consumption elements in this process are the following: Heating: a) The feed gas must be heated from 40ºC to reactor temperature 600ºC b) Fresh feed of Toluene must be heated similarly from 20ºC to 600ºC c) The recycled gas must be reheated from the compressor exit temperature to 600ºC d) The recycled Toluene must be heated from the pump exit temperature to 600ºC e) Heat must be supplied to the reboilers in the two distillation columns, the largest amount to the Benzene/Toluene column, since this separation is most difficult and has the largest flowrates. Page 5 of 6

6 Purge P G R G F G F FT Toluene Reactor Flash Benzene P B R T Toluene P D Diphenyl Cooling: f) The reactor effluent stream must be cooled from reactor temperature (600ºC) to a temperature where the aromatic components are condensing (depends on the pressure, but Benzene has a normal (atmospheric) boiling point of 80ºC). The pressure in the flash separator should not be set too low, since that would result in large compressor duties for the gas recycle stream. g) Cooling must be supplied to the condensers of the two distillation columns, the largest amount to the Benzene/Toluene column, since this separation is most difficult and has the largest flowrates. Power: h) The major contribution here is the recompression of the gas recycle. The feed gas has sufficient pressure at the reactor inlet. Power needed for pumping is neglectable in most processes, certainly in this one. Trondheim, Truls Gundersen Page 6 of 6

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