Evaporation in Double-Effect Reverse-Feed Evaporators. A teed containing 2 wt % dissolved organic solids in water is fed to a double-effect evaporator with reverse feed. The feed enters at 100oF and is concentrated to 25% solids.
The boiling-point rise can be considered negligible, as can the heat of solution. Each evaporator has a 1000-ft2 surface area, and the heat-transfer coefficients are U1 = 500 and U2= 700 btu/h ft2 . The feed enters evaporator number 2 and steam at 100 psia is fed to number 1.
The pressure in the vapor space of evaporator number 2 is 0.98 psia. Assume that the heat capacity of all liquid solutions is that of liquid water.
Calculate the feed rate F and the product rate L1 of a solution containing 25% solids. (Hint: Assume a feed rate of, say, F = 1000 lbm/h and a value of T1
Calculate the area and T1. Then calculate the actual feed rate by multiplying 1000 by 1000/calculated area.)