Ethanol is often used as a fuel additive, but to function in most engines, the e

Question

Ethanol is often used as a fuel additive, but to function in most engines, the ethanol must be very pure and contain little water. It is difficult to distill ethanol and water above 90 mole % ethanol because at this concentration the vapor and the liquid phases of this mixture have the same composition. To get around this effect, benzene is often added to the mixture. The distillation column then separates the mixture into a very nearly pure ethanol stream, and a mixture of benzene and water. The benzene may be separated from the water, and then recycled to use again in the ethanol-water distillation. A company wishes to distill a 0.800 mole fraction ethanol (0.200 mole fraction water) solution. To aid in the distillation process, a 0.950 mole fraction benzene (0.050 mole fraction water) mixture containing fresh benzene and a recovered benzene solution is added. The plant wishes to produce 83.0 moles/hr of a 0.999 mole fraction ethanol (0.001 mole fraction benzene) mixture from a distillation column. The other product from the distillation is a water and benzene mixture. This mixture is sent to a separator which produces a 13.0 mol/hr stream of water, and a 0.9300 mole fraction benzene (0.0700 mole fraction water) mixture, some of which is recycled back to the fresh benzene stream. 0 Label the process flow diagram below. Label any quantites as "unknown" if you have to perform a calculation to obtain the numeric value. DO NOT LEAVE ANY ANSWER SPACE BLANK. "et is short for ethanol, "b" is short for benzene, and "w" is short for water.

Explanation / Answer

Material balance of water at mixing point

n9 x 0.07 = n3 x 0.05

n3 = 1.4 n9

Material balance of benzene at mixing point

n9 x 0.93 + 100 = n3 x 0.95

Solve the above equations simultaneously

n9 x 0.93 + 100 = 1.4 x n9 x 0.95

100 = 1.4 x n9 x 0.95 - 0.93 n9

0.4 n9 = 100

Flow rate of recycle line, n9 = 250 mol/hr

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