B49CG - Process Design - Engineering Assignment Help

Assignment Task

Question 1

Benzene (C6H6) can be produced via reacting toluene (C6H5CH3) with hydrogen (H2) as shown by the reaction below:
C6H5CH3 + H2 → C6H6 + CH4
Methane (CH4) is produced as the by-product during the reaction. Given the following information and process description, build a flowsheet using Aspen Plus for the production of benzene and complete the given tasks:
Fresh hydrogen feed
stream
Fresh toluene feed
stream
Temperature
(°C)
40 25
Pressure (bar) 25 1
Flow rate
(kmol/h)
350 140
Mole fraction Hydrogen: 0.95
Methane: 0.05
Toluene: 1.00
Fresh toluene stream is mixed with the bottom product of 2nd distillation column. Prior to mixing, the fresh toluene stream is pressurised so that its pressure is identical to the pressure of the bottom product of 2nd distillation column. The mixed stream is heated to 330 °C and pressurised to 25 bar. On the other hand, fresh hydrogen feed is mixed with the recycled vapour product of phase separator. After mixing, the mixed stream is heated to 330 °C via a heater. This stream is then mixed with the toluene stream and fed to reactor. Conversion of toluene is 80 %.
After the reaction, the product stream is cooled to 40 °C and its pressure reduced to 21 bar. This stream then enters a phase separator. 20 % of the vapour leaving the phase separator is purged to remove methane, while the rest of the vapour is compressed, recycled and mixed with fresh hydrogen feed. The liquid leaving the phase separator enters the 1st distillation column to separate hydrogen and methane from benzene and toluene. The bottom stream of the 1st distillation column goes through a valve to reduce its pressure to 12 bar. It then enters the 2nd distillation column, where high purity (at least 99.7 mol%) benzene exits as top product. Toluene is obtained as bottom product of 2nd distillation column, recycled and mixed with fresh toluene feed.
While building the flowsheet, pressure drop across heaters, coolers and phase separator can be assumed as 0 bar. Use RSTOIC reactor to model the reactor, and DSTWU column to model the two distillation columns. Reflux ratio of 1.00 can be used for 1st distillation column, while reflux ratio of 6.00 can be used for second distillation column. For both distillation columns, distillate is expected to exit as only vapour state.

Tasks:
a) What are the feed stage and total number of stages for both distillation columns?
b) What are the flow rates of hydrogen and toluene recycle streams?
c) What is the flow rate of benzene product stream?
Show the overall flowsheet with its workbook (stream table) together with your answer.

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