Question

A plant for the production of NaOH concentrates a solution of 10%w NaOH, 10% NaCl, an 80% H2O by evaporation. During evaporation some NaCl crystallizes out pure. If the fina solution contains 50% NaOH and 1% NaCl, calculate the following: a. b. The pounds of water evaporated per ton of initial solution. (1404) The pounds of NaCl precipitated per ton of initial solution. (196) The weight of the final solution. (400) C.

Answer 1

let, initial solution, F = 1 ton = 2000 lb. Where, 10wt% NaOH, 10wt% NaCl, 80wt% water.

In the evaporation process, initial solution is divided into three parts. One part is being evaporate (M) which contains 100% water. Another part is being precipitated as crystal (C) which contains 100% NaCl. Rest part is the remained solution (S) which contains 50wt% NaOH, 1 wt% NaCl, 49 wt% water.

Overall mass balance -

F = M + C + S.

or, 2000 lb = M + C + S.

mass balance of NaOH -

F * 10% = 0% * M + 0% * C + 50% * S

or, 2000 lb * 0.1 = 0.5 * S

or, S = 400 lb.

Mass balance of NaCl -

F * 10% = M*0% + C*100% + S * 1%

or, 2000 * 0.1 = 0 + C + 0.01*400

or, C = 200 - 4 = 196 lb

and M = F - C - S = 2000 - 196 - 400 = 1404lb.

a. 1404 pounds of water evaporated per ton of initial solution.

b. 196 pounds of NaCl precipitatted per ton of initial solution.

c. 400 puonds of solution remains after each ton of initial solution evaporation.