A feed containing 15% by weight of solids is to be concentrated to 40 % solids using a double effect backward feed evaporator. The first effect is heated by steam at 413 K. Vapour from the second effect is condensed at a pressure corresponding to 325 K. The heat transfer area is 10 m2 and the overall heat transfer coefficients are 3 and 2 kW/m2 in the first and second effects respectively. The boiling point rise is 3 K in the first effect and zero in the second effect. If the feed enters at 310 K what is the maximum feed rate that can be accommodated?
It may be assumed that the heat loads on the two effects are equal and that the latent heats of evaporation and specific heats of the solutions are constant at 2330 kJ/kg and 4.18 kJ/kg respectively.