54) Note: This problem is worth double credit.It is desired to concentrate a caustic soda solution from 12.5 to 40.0% in a double-effect evaporator. Feed solution at a flow rate of 50,000 lb/h enters the evaporator at 120?F, and steam at 15 psig is available to concentrate this solution. Pressure in the vapor space of the second effect is kept at 5.92 in Hg absolute (24 in Hg vacuum), and water at 85?F from a cooling tower is available to condense the vapor from the second effect. For backward feed operation, calculate the
(a) steam flow rate required.
(b) steam economy.
(c) area per evaporator effect.
(d) cooling water flow rate required, assuming the water should be returned to the cooling tower no hotter than 125?F.
The overall heat transfer coefficients for the two effects are 350 and 450 Btu/h•ft2•?F, respectively.Answers:(a)21,400 lb/h(b)1.61(c)1960 ft2(d) 421,000 lb/h