3. A distillation column is separating methanol, ethanol, and butanol at a press
ID: 702267 • Letter: 3
Question
3. A distillation column is separating methanol, ethanol, and butanol at a pressure of 2 atm. The column has a total condenser and a partial re-boiler. The feed is saturated liquid with the flow rate of 100 kmol/h comprising 45 mol% methanol, 30% ethanol, and 25% butanol. The external reflux ratio is 2.0, and the reflux is a saturated liquid. We desire a 98% recovery of ethanol in the distillate and a 99.1% recovery of butanol in the bottom product. Find D and B, and compositions of distillate and bottom products.Explanation / Answer
Distillate product = ethanol = LK
Bottom product = BUTANOL = HK
Methanol = LNK = distillate
Fractional recovery of ethanol in distillate FReD = 0.98
Fractional recovery of ethanol in bottom FReW = 0.02
Fractional recovery of butanol in distillate FRbD = 1-0.991 = 0.009
Fractional recovery of butanol in bottom FRbW = 0.991
Ethanol balance at distillate
DXD, E = FReD x F x zF, E
= 0.98 x 100 x 0.30
= 29.4 kmol/h
At bottom
BXB, E = FReB x F x zF, E
= 0.02 x 100 x 0.30
= 0.6 kmol/h
Similarly for butanol
At bottom
BXB, B = FRbB x F x zF, B
= 0.991 x 100 x 0.25
= 24.78 kmol/h
At distillate
DXD, B = FRbD x F x zF, B
= 0.009 x 100 x 0.25
= 0.225 kmol/h
Methanol balance at distillate
DXD, M = F x zF, M = 100 x 0.45 = 45 kmol/h
At bottom
XB, M = 0
BXB, M = 0
Total distillate
D = 45 + 29.4 + 0.225 = 74.6 kmol/h
Total bottom
B = 0 + 0.6 + 24.78 = 25.4 kmol/h
Composition of bottom
XB, M = (BXB, M) /B = 0/25.4 = 0
XB, E = (BXB, E) /B = 0.6/25.4 = 0.024
XB, B = (BXB, B) /B = 24.78/25.4 = 0.976
Composition of distillate
XD, M = (DXD, M) /D = 45/74.6 = 0.603
XD, E = (DXD, E) /D = 29.4/74.6 = 0.394
XD, B = (DXD, B) /D = 0.225/74.6 = 0.003
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