Analytical comparison of the multi-stage flash and long-tube vertical distillation processes

Abstract

Heat transfer area is probably the most significant cost component of distillation plant, usually accounting for some 35–40% of the overall capital cost. Approximate equations are developed for both multi-stage flash and long-tube vertical multiple effect processes expressing the required heat transfer area in terms of the performance ratio, operating temperatures and heat transfer coefficients. The expressions are used to compare the two processes and to give an indication of the impact of the high heat transfer coefficients reported for the fluted tube multiple effect process upon heat transfer requirements. It appears that a multiple effect long-tube vertical falling film plant requires a smaller heat transfer area than does a multi-stage flash process. Improvements in plant layout such as the incorporation of feed heaters into the evaporator bundles, the use of flash tanks for product cooling and a stricter design of vapour disengagement vessels could reduce the costs of the multiple effect process below those of multi-stage flash distillation.