Topic: Production of 150 000 metric ton per annum (MTA) of Benzene by Dehydroalk
ID: 2077780 • Letter: T
Question
Topic: Production of 150 000 metric ton per annum (MTA) of Benzene by Dehydroalkylation of Toluene
Process Description:
A chemical plant producing 150 000 metric ton per annum of benzene from the dehydroalkylation of toluene, runs 24 hours per day for 330 days in a year. In the process, the cold feeds to the reactor with a 5:1: 5.6 hydrogen (H2) to toluene (C7H8) to methane (CH4) ratio is preheated in a feed effluent heat exchanger (FEHE) that recovers reaction heat from the hot reactor effluent (stream 13). A furnace brings the preheated reactor feed (stream 9) to a reactor inlet temperature of 621 0C. The toluene dealkylates in a Plug Flow Reactor (PFR) to produce benzene (C6H6) and methane (CH4). A small amount of diphenyl (C12H10) by-product is also formed via coupling of two benzene molecules. The reactions are shown below:
C7H8 + H2 à C6H6 + CH4 (Main Reaction) (1)
2C6H6 à H2 + C12H10 (Side Reaction) (2)
The total amount of toluene fed into the reactor is 318.5 kmol/hr and the selectivity of benzene compared to diphenyl is 44 mol C6H6/mol C12H10. The hot reactor effluent (stream 11) is quenched to 612 0C to prevent further reaction and coking. It is cooled through the FEHE and the cooler to 38 0C. The amount of the quench liquid (stream 12) used is 58 kmol/hr with mole composition of 0.03 CH4, 0.75 C6H6, 0.21 C7H8 and 0.01 C12H10.
The cooled mixture is flashed in a flash drum at 38 0C and 25 bar to release the unreacted hydrogen and methane (steam 15) which is recycled back to the reactor (stream 7) after purging (stream 3) to provide an exit for the methane formed in the main reaction. The recycled hydrogen gas stream is mixed with a fresh hydrogen feed of 446.31 kmol/hr. The amount of methane in the flash overhead stream is 90 % of methane fed into the flash drum. The liquid from the bottom of the flash drum (stream 16) is fed to the separation section consisting of three columns which are the stabilizer, product and recycle distillation columns.
The stabilizer column which operates at 38 0C and 2.5 bar, removes methane as the vapour overhead. The liquid bottoms is fed to the product column operating at 112 0C and 2.5 bar, with 100 mole % pure benzene leaving as the distillate product. The bottoms from this column is fed to the recycle column that separates the diphenyl by-product as the bottoms, with some toluene and takes pure toluene as the overhead distillate which is recycled back to the reactor after mixing with 254.46 kmol/hr of fresh toluene feed.
Instructions
1)Based on the given process descriptions and given piping and instrumentation (P & ID) diagram, develop/draw a process flow diagram (PFD). Label the process units/equipment and streams according to the stream numbers and names (refer process descriptions and Table 1).
2)Solve the mass balance (unknowns) for this process by filling in the blanks in Table 1 (labelled by U/N).
3)provide the calculations for: (i) the overall process (should contain the PFD developed in 1), (ii) each equipment involved in the process.
Note: The table and the diagram is attached as an image with this question.
Purge Sep Recycle gas Quench TC Reactor Cooler Methane Compressor Furnace Fuel FEHE Benzene 18 FC 20 H Feed 1, Toluene fresh feed 2. Hydrogen feed 3. Purge gas 4. Methane (stabilizer gas) Toluene Feed 5. Benzene product 6. Biphenyl product 7. Gas recycle 8. Toluene recycle 9. Furnace inlet 0. Reactor inlet 11. Reactor effluent 12. Quench 13. FEHE hot in 14. FEHE hot out 15. Separator gas out 16. Stabilizer feed 17. Stabilizer bottoms 18, Product bottoms 19. Product column reflux 20. Recycle column re BiphenylExplanation / Answer
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