Coulson Evaporation

Question 1

A forward-feed double-effect evaporator, having 10 m2 of heating surface in each effect, is used to concentrate 0.4 kg/s of caustic soda solution from 10 to 50 per cent by mass. During a particular run, when the feed is at 328 K, the pressures in the two calandrias are 375 and 180 kN/m2 respectively, and the condenser operates at 15 kN/m2. For these conditions, calculate:
(a) the load on the condenser;
(b) the steam economy and
(c) the overall heat transfer coefficient in each effect.
Would there be any advantages in using backward feed in this case? Heat losses to the surroundings are negligible. Physical properties of caustic soda solutions:

Answer 1

D2 = 0.15 kg/s
        economy = 1.57
        U1 = 2.53 kW/m2 K
        U2 = 1.84 kW/m2 K
        no advantage in 
        using backward feed

Question 2

A liquid with no appreciable elevation of boiling-point is concentrated in a triple-effect evaporator. If the temperature of the steam to the first effect is 395 K and vacuum is applied to the third effect so that the boiling-point is 325 K, what are the approximate boiling-points in the three effects? The overall transfer coefficients may be taken as 3.1, 2.3, and 1.1 kW/m2 K in the three effects respectively

Answer 2

T1 = 381.5 K
T2 = 363.2 K
T3 = 325 K