Question 1 of 6: Process Design

Question 1: Process Design (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

This is the classic hydrodealkylation (HDA) flowsheet-synthesis problem of Douglas. The answer is built in Douglas’s hierarchical order: fix the input–output structure from the stoichiometry, then the recycle structure, then the separation train — the whole separation being dictated by the very wide spread of boiling points the question supplies.

(a) Input–output structure and stoichiometry

The desired reaction consumes one H₂ per toluene and makes one benzene and one methane; the reversible side reaction dimerises benzene to diphenyl and releases H₂. Because methane is produced (and enters with the makeup hydrogen), and hydrogen is fed in large excess, the gas phase accumulates light components (H₂, CH₄) that must leave the process — this forces a gas purge. The two feeds are pure toluene and a 95/5 H₂/CH₄ gas; the products are benzene (99.97 %), a fuel-gas purge, and diphenyl.

(b) Recycle structure

Toluene conversion per pass is deliberately kept low (to hold selectivity to benzene and suppress diphenyl), so unreacted toluene is recycled. Hydrogen is fed at a 5:1 ratio to suppress coking, far in excess of stoichiometry, so unreacted hydrogen gas is recycled with a slip-stream purge to reject the methane made in the reactor and brought in with the feed. Two recycles (a gas loop and a liquid toluene loop) and one purge therefore fall directly out of the stoichiometry and the coking constraint.

(c) Separation train — driven by the boiling-point spread

The boiling points span an enormous range: H₂ (−253) and CH₄ (−162) are permanent gases at any reasonable separation temperature, whereas benzene (80), toluene (111) and diphenyl (255) are readily condensable liquids. That single fact sets the whole train:

• Quench and phase split. The 620–705 °C effluent is quenched to 620 °C (coking control) and then cooled; a flash separator makes the first, easy cut — gas (H₂ + CH₄) overhead for recycle and purge, aromatic liquid (benzene + toluene + diphenyl) as bottoms.
• Stabiliser column. The aromatic liquid still holds dissolved light ends; a stabiliser strips residual H₂/CH₄ to fuel gas so they do not spoil the benzene product.
• Benzene (product) column. Benzene (80 °C) is taken overhead at 99.97 % purity; toluene and diphenyl leave as bottoms.
• Toluene (recycle) column. Toluene (111 °C) is recovered overhead and recycled to the reactor; diphenyl (255 °C) leaves as bottoms.

(d) Process alternatives to discuss

Several genuine design choices arise, and a complete answer weighs them:

• Diphenyl — recycle or burn. Because the dimerisation $\mathrm{2C_6H_6\rightleftharpoons C_{12}H_{10}+H_2}$ is reversible, recycling diphenyl to the reactor drives it back toward benzene and raises overall yield; alternatively diphenyl can simply be rejected and burned as fuel. Recycle improves yield at the cost of an extra stream and column duty.
• Purge rate vs. hydrogen loss. A larger purge keeps methane (and hydrogen partial pressure) in check but wastes valuable H₂; a membrane or PSA unit on the purge can recover hydrogen for recycle instead of a simple bleed.
• Reactor conversion vs. selectivity. Higher per-pass conversion shrinks the recycle but increases diphenyl formation and coking; the low-conversion/high-recycle design trades reactor size against selectivity.
• Heat integration. The hot quenched effluent is an obvious source to preheat the cold toluene/hydrogen feed (a feed–effluent exchanger), which is standard on HDA plants and greatly reduces fired-heater duty.