Feed temperature influence on the efficiency of a molecular evaporator

The temperature of a liquid entering the evaporating cylinder of a molecula r evaporator is one of the important technological parameters that determin e an evaporator's operation. In this model study, the effect of the feed te mperature on the film surface temperature on the evaporating cylinder was o bserved for various liquid loads and for various differences between evapor ation surface temperatures and feed temperatures. The length of the evapora ting cylinder measured from the top, along which the temperature on the fil m surface reaches a steady-state, constant value (asymptotic temperature) i ncreases with decreasing feed temperature, with rising the liquid load and with rising evaporator cylinder temperature. Along this length, which may r each up to several hundred millimetres, evaporation occurs at a reduced rat e which corresponds to the real temperature on the film surface. Therefore, it is useful to gently preheat the feed before it enters the evaporator in appropriate front-end heat exchangers to a temperature close to the asympt otic temperature. (C) 2000 Elsevier Science S.A. All rights reserved.