Because the feed underflow in Example 14-1 was not the same as in the other stages, we had to do the feed stage calculation separately, but connected to, the calculation for the remainder of the cascade. In Example 14-2, the feed contained less entrained liquid than the other raffinate streams, but a separate calculation was not required for the feed stage. Explain the reasons for the different handling of the feed stage.

Example 14-1

Example 14-2

In production of sodium hydroxide by the lime soda process, a slurry of calcium carbonate particles in a dilute sodium hydroxide solution results. A five-stage countercurrent washing system is used to remove sodium hydroxide. The feed contains 1 kg dry calcium carbonate and 6.7 kg water. The underflow from the settlers entrains approximately 3.0 kg liquid/kg dry calcium carbonate solids. Inlet water is pure. If 7.0 kg wash water/kg dry calcium carbonate solids are used, predict the recovery of NaOH in the wash liquor.