Feed temperature influence on the efficiency of a molecular evaporator

The temperature of a liquid entering the evaporating cylinder of a molecular evaporator is one of the important technological parameters that determine an evaporator's operation. In this model study, the effect of the feed temperature on the film surface temperature on the evaporating cylinder was observed for various liquid loads and for various differences between evaporation surface temperatures and feed temperatures. The length of the evaporating cylinder measured from the top, along which the temperature on the film surface reaches a steady-state, constant value (asymptotic temperature) increases with decreasing feed temperature, with rising the liquid load and with rising evaporator cylinder temperature. Along this length, which may reach up to several hundred millimetres, evaporation occurs at a reduced rate 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 asymptotic temperature. (C) 2000 Elsevier Science S.A. All rights reserved.