The synthesis of multiple-effect evaporator systems using minimum utility insights—II. liquid flowpattern selection

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The concept of a “heat shunt” provides the insight needed to predict the extra utility consumption that is caused by the liquid flowpattern of a multieffect evaporator system. When no liquid bypassing of the effects is permitted, the liquid flowpattern requiring the minimum use of utilities follows one of two possible shortest “temperature paths” through the process, i.e. either a “down first” path that starts at the feed temperature, proceeds all the way to the lowest effect temperature, reverses, proceeds to the highest effect temperature and finally returns to the final concentrated product temperature; or an “up first” path that proceeds first to the highest effect temperature, etc. A simple calculation permits one to choose between these two paths. The liquid enters each effect the first time the selected path passes the effect temperature. When liquid bypassing is allowed, the paper shows that parallel feed to the effects is best. Major assumptions are constant boiling point elevation and negligible heat of mixing. An example illustrates all ideas.