Page 213 - Chemical engineering design

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CHEMICAL ENGINEERING
recycled to the absorption column. A purge is taken from the water recycle stream
to prevent the build-up of impurities.
1. Draw up an information flow diagram for this process.
2. Estimate the split faction coefficients and fresh feeds for each stage.
3. Set up the resulting material balance equations, in matrix form.
4. Solve the equations using a suitable spread-sheet.
5. Adjust the values chosen for the split-fractions and feeds, so the results meet
the constraints,
6. Draw a flow-sheet for the process.
Treat the C 4 ’s, other than isobutene, as one component.
Data:
1. Feedstock composition, mol per cent: n-butane D 2, butene-1 D 31, butene-2 D
18, isobutene D 49.
2. Required production rate of MTBE, 7000 kg/h.
3. Reactor conversion of isobutene, 97 per cent.
4. Recovery of MTBE from the distillation column, 99.5 per cent.
5. Recovery of methanol in the absorption column, 99 per cent.
6. Concentration of methanol in the solution leaving the absorption column, 15
per cent.
7. Purge from the water recycle stream, to waste treatment, 10 per cent of the
flow leaving the methanol recovery column.
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8. The gases leave the top of the absorption column saturated with water at 30 C.
9. Both columns operate at essentially atmospheric pressure.
4.3. Water and ethanol form a low boiling point azeotrope. So, water cannot be
completely separated from ethanol by straight distillation. To produce absolute
(100 per cent) ethanol it is necessary to add an entraining agent to break the
azeotrope. Benzene is an effective entrainer and is used where the product is not
required for food products. Three columns are used in the benzene process.
Column 1. This column separates the ethanol from the water. The bottom product
is essentially pure ethanol. The water in the feed is carried overhead as the ternary
azeotrope of ethanol, benzene and water (24 per cent ethanol, 54 per cent benzene,
22 per cent water). The overhead vapour is condensed and the condensate separated
in a decanter into, a benzene-rich phase (22 per cent ethanol, 74 per cent benzene,
4 per cent water) and a water-rich phase (35 per cent ethanol, 4 per cent benzene,
61 per cent water). The benzene-rich phase is recycled to the column as reflux. A
benzene make-up stream is added to the reflux to make good any loss of benzene
from the process. The water-rich phase is fed to the second column.
Column 2. This column recovers the benzene as the ternary azeotrope and recycles
it as vapour to join the overhead vapour from the first column. The bottom product
from the column is essentially free of benzene (29 per cent ethanol, 51 per cent
water). This stream is fed to the third column.
Column 3. In this column the water is separated and sent to waste treatment. The
overhead product consists of the azeotropic mixture of ethanol and water (89 per
cent ethanol, 11 per cent water). The overheads are condensed and recycled to
join the feed to the first column. The bottom product is essentially free of ethanol.