Introduction to Chemical Engineering

CHE-201

4.5 RECYCLE AND BYPASS

Air Conditioner Unit:

Fresh air containing 4.00 % water vapor is to be cooled and dehumidified to a

water content of 1.70 mole % H2O. A stream of fresh air is combined with a

recycle stream of previously dehumidified air and passed through the cooler. The

blended stream entering the unit contains 2.30 mole % H2O. In the air conditioner,

some of the water in the feed stream is condensed and removed as liquid. A

fraction of the dehumidified air leaving the cooler is recycled and the remainder is

delivered to a room. Taking 100 mol of dehumidified air delivered to the room as a

basis of calculation. Calculate the moles of fresh feed, moles of water condensed,

and moles of dehumidified air recycled.

Air Conditioner Unit:

AIRCOND.

n1 (mol)

0.960 DA0.040 W (v)

n2 (mol) n4 (mol)

0.977 DA0.023 W (v)

0.983 DA0.017 W (v)

0.983 DA0.017 W (v)

0.983 DA0.017 W (v)

n5 (mol)

n3 [mol W(l)]

DA = Dry AirW = Water

100 mol

An Evaporative Crystallization Process:

The flowchart of a steady-state process to recover crystalline chromate (K2CrO4) from an aqueous solution of this salt is shown

below:

Forty-five hundred kilograms per hour of a solution that is one-third K2CrO4 by mass is joined by a recycle stream containing

36.4% K2CrO4, and the combined stream is fed into an evaporator. The concentrated stream leaving the evaporator contains

49.4% K2CrO4; this stream is fed into a crystallizer in which it is cooled (causing crystals of K2CrO4 to come out of solution) and

then filtered. The filter cake consists of K2CrO4 crystals and a solution that contains 36.4 % K2CrO4 by mass; the crystal account

for 95% of the total mass of the filter cake. The solution that passes through the filter, also 36.4% K2CrO4, is the recycle stream.

1. Calculate the rate of evaporation, the rate of production of crystalline K2CrO4, the feed rates that the evaporator and

crystallizer must be designed to handle, and the recycle ratio (mass of recycle)/(mass of fresh feed).

2. Suppose that the filtrate were discarded instead of being recycled. Calculate the production rate of crystals. What are the

benefits and costs of the recycling?

EVAPORATOR CRYSTALIZERAND FILTER

H2O

4500 kg/h

33.3% K2CrO4

49.4% K2CrO4

Filtrate36.4% K2CrO4 solution

Filter cakeK2CrO4 solid

36.4 % K2CrO4 solution(the crystals constitute 95% by mass of the filter cake)

Process Flowchart:

EVAPORATOR CRYSTALIZERAND FILTER

Fresh feed4500 kg/h

0.33 kg K/kg0.667 kg W/kg

0.494 kg K/kg0.506 kg W/kg

Filtrate (recycle)m6 (kg /h)

m5 (kg soln/h)0.364 kg K/kg0.636 kg W/kg

Filter cakem4 (kg K(s)/h)

0.364 kg K/kg0.636 kg W/kg

x1 (kg K/kg)(1-x1) (kg W/kg)

m1 (kg /h)

m2 (kg W(v)/h)

m3 (kg /h)