Page 201 - Chemical engineering design

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CHEMICAL ENGINEERING
Component 1, isopropyl alcohol (k D 1)
Unit 1, Reactor. The conversion per pass is given as 90 per cent, so for each mol entering
only 10 per cent leave, hence ˛ 211 is fixed at 0.1. For this example it is assumed that
the conversion is independent of the feed stream composition.
Ž
Unit 2, Condenser. Most of the alcohol will condense as its boiling point is 82 C. Assume
90 per cent condensed, ˛ 421 D 0.9 (liquid out) and ˛ 321 D 0.1 (vapour out). The
actual amounts will depend on the condenser design.
Unit 3, Scrubber. To give a high plant yield, the scrubber would be designed to recover
most of the alcohol in the vent stream. Assume 99 per cent recovery, allowing for
the small loss that must theoretically occur, ˛ 431 D 0.99.
Unit 4, First column. The fraction of alcohol in the overheads would be fixed by the
amount allowed in the acetone product specification. Assume 1 per cent loss to the
acetone is acceptable, which will give less than 1 per cent alcohol in the product;
fraction in the bottoms 99 per cent, ˛ 541 D 0.99.
Unit 5, Second column. No distillation column can be designed to give complete separation
of the components. However, the volatilities for this system are such that a high
recovery of alcohol should be practicable. Assume 99 per cent recovery, alcohol
recycled, ˛ 151 D 0.99.

Component 2, Acetone (k D 2)
Unit 1. Assume that any acetone in the feed passes through the reactor unchanged,
˛ 212 D 1.
Ž
Unit 2. Most of the acetone will condense (b.p. 56 C) say 80 per cent, ˛ 322 D 0.2,
˛ 422 D 0.8.
Unit 3. As for alcohol, assume 99 per cent absorbed, allows for a small loss, ˛ 432 D 0.99.
Unit 4. Assume 99 per cent recovery of acetone as product, ˛ 542 D 0.01.
Unit 5. Because of its high volatility in water all but a few ppm of the acetone will go
overhead, put ˛ 152 D 0.01.
Component 3, Hydrogen (k D 3)
Unit 1. Passes through unreacted, ˛ 213 D 1.
Unit 2. Non-condensable, ˛ 323 D 1, ˛ 423 D 0.
Unit 3. None absorbed, ˛ 433 D 0.
Unit 4. Any present in the feed would go out with the overheads, ˛ 543 D 1.
Unit 5. As for unit 4, ˛ 153 D 1.
Component 4, Water (k D 4)
Unit 1. Passes through unreacted, ˛ 214 D 1.
Unit 2. A greater fraction of the water will condense than the alcohol or acetone
Ž
(b.p. 100 C) assume 95 per cent condensed, ˛ 324 D 0.05, ˛ 423 D 0.95.
Unit 3. There will be a small loss of water in the vent gas stream, assume 1 per cent lost,
˛ 434 D 0.99.
Unit 4. Some water will appear in the acetone product; as for the alcohol this will be
fixed by the acetone product specification. Putting ˛ 544 D 0.99 will give less than 1
per cent water in the product.
Unit 5. The overhead composition will be close to the azeotropic composition, approxi-
mately 9 per cent water. The value of ˛ 154 (recycle to the reactor) must be selected