Multi-heating rate method

Multi-stage separation calculations require heat balances involving transition between the phases. The enthalpy balance equations must, therefore, include heats of vaporization and condensation. Usually these heats are not calculated separately but are implied by using appropriate methods for calculating liquid enthalpies and vapor enthalpies. Consider, for instance, a system where a vapor stream and a liquid stream enter with enthalpies and // respectively, and leave as a mixed phase stream with vapor enthalpy and liquid enthalpy h2. These are total stream enthalpy rates with units such as kJ/h. If no heat is added to or removed from the system, an energy balance is written as... 

Another method of handling unequal molal overflow rates for multi-component mixtures is to make the calculations on the constant molal flow rates and then to calculate the heat load required to give these rates for a few positions in the column. If the heat input to the still is made equal to the largest requirements, and the condenser and tower cross section correspondingly corrected, a conservative design will be obtained. The check points most commonly employed are (1) around the still, (2) the entire section below the feed plate, (3) the feed... 

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