An evaporator is a device in which a solution is contracted with steam; the steam heats the solution and boils off a portion of the solvent, leaving the solution more concentrated. A multiple effect evaporator is a connected series of evaporators (called effects) through which a solution passes, becoming more concentrated in each unit.

A triple-effect evaporator is used to produce fresh water from sea water, which contains 3.5 wt-% salt. (The salt may be assumed to be all NaCl in this problem). Thirty thousand pounds per hour of sea water are fed to the first effect of the evaporator. A simplified diagram of the process is given below. The composition of the solution leaving the third effect is measured with an electrical conductivity probe, calibrated to provide a reading of the mole fraction of NaCl in the solution. A reading of 0.01593 is obtained. Approximately the same quantity of water is boiled off in each effect. Calculate the evaporation rate in each effect and the wt-% of NaCl in the solution leaving the second effect.

-y HO HO 1st effect 2 nd effect 3 rd effect 30 000 lb/hr 3.5 wt-% NaCl