Evaporation in Double-Effect Reverse-Feed Evaporators. A feed containing 2 wt % dissolved organic solids in water is fed to two double-effect evaporators with reverse feed. The feed enters at 100°F 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-ft 2 surface area, and the heat-transfer coefficients are U1 = 500 and U2 = 700 btu/h · ft 2 · °F. The feed enters evaporator number 2 and steam at 100 psia is fed to evaporator 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.)
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