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 0.840 mole fraction ethanol (0.160 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 88.0 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 11.2 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.

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.

mol/hr mol b /mol mol w /mol mol b/hr mol b/mol mol w/mol mol/hr mol b /mol mol w /mol mol/hr /mol mol /mol mol b /mol mol w /mol istillatio olumn mol/hr mol et/mol mol w/hr mol w/mol mol/hr mol et/mol mol b/mol 88.0unknown 0.950 11.2 0.07000.001 0.160 0.93000.999 0.840 0.050 What flow rate of the ethanol and water feed stream is needed to provide 88.0 mol/hr of the 0.999 mole fraction ethanol stream? Number mol/ hr What flow rate of fresh benzene is required? Number mol benzene/ hr What is the flow rate of the recycle line? Number mol/ hr

Explanation / Answer

Mole fraction ethanol from the bottom of distillation colummn= 88 mole/hr. It contains 0.999 mole fraction Benzene. Moles of Benzene= 88*0,999=87.912 mole/hr

Feed contains 0.84 mole fraction ethanol, let F= feed of ethanol

F* 0.84= 87.912

F= 87.912/0.84=104.65 moles/hr

Let B= Fresh Benzene and R= Recycled Benzene

Writing a Benene balance at Benzene enteing the distillation column

B+R*0.93= (B+R)*0.95

B*0.05= R*0.02 , B =0.4R

Water enteing the column is removed from bottom of distillation column, from the separator and some of it not recyled.

Water entering = 84*0.16=13.44Moles/hr

Water removed from bottom of distillation column= 88*0.001=0.088

Water stream flow rate= 11.2 moles/hr

Water not recycled= 13.44- 0.088-11.2=2.152 moles/hr

This correspond to 0.07 water in non recycled stream . Let it be P

P*0.07= 2.152, P= 2.152/0.07=30.74 moles/hr

Benzene in this =30.74*0.93=28.6 moles/ hr

Under steady state conditions, this is the benzene leaving, this much amount of benzene will have to be supplied as fresh stream. Benzene (B) in the fresh Stream =28.6 moles/hr

But B=0.4R

R= B/0.4= 28.6/0.4=71.5 moles/hr

Related Questions

Navigate

• Browse (All)
• Browse E
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.