1. 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 mol% 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 0.870 mole fraction ethanol solution (the balance is water). To aid in the distillation process, a mixture of benzene and water is added to the distillation column. This benzene/water mixture is formed by combining a fresh feed of 0.97 mol fraction benzene in water with a recovered benzene solution. The plant wishes to produce 73.0 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 that produces a 10.70 mol/hr stream of pure water, and a 0.93 mol fraction benzene mixture in water; from this benzene mixture a small amount is purged and the remaining 90.00 mol/hr is recycled back into the fresh benzene stream to form the feed into the column
a. Draw the process flow diagram. On the flow chart, label all variables and unknowns.
b. What is the basis of calculation for the process?
c. Calculate the total degrees of freedom over the entire process. Next, calculate the degrees of freedom around each unit operation. Label all calculations.
d. List the algebraic equations for the material balances and the order for which you would solve for all of the unknowns. If the problem is underdetermined list which unknowns you can solve for.