Ján Cvengroš

Heat and mass transfer in the evaporating film of a molecular evaporator

Abstract The hydrodynamic conditions in the wiped-film on the evaporating cylinder of a molecular evaporator have a considerable effect on the distillation output and composition of outlet streams from the evaporator. In this study, selected parameters are investigated in the laminar and turbulent regimes in the film, depending on the evaporator’s liquid load and temperature. Results show that in the case of a turbulent film without concentration and temperature gradients at the same temperature of the evaporating cylinder surface, a higher portion of the feed is evaporated at a lower content of the more volatile component in the distillate. The model studied shows that there are only small differences in the relative volatility as a measure of separation efficiency in the turbulent or laminar film on the evaporating cylinder. The qualitative agreement between experimental results and the results from the model proposed shows that the model describes well the phenomena that occur within the distillation space of the molecular evaporator.

Feed temperature influence on the efficiency of a molecular evaporator

Abstract 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.

Film wiping in the molecular evaporator

This study deals with a wiper for a molecular evaporator. Its wiping element has the shape of a cylinder with a screw thread with coarse pitch (lead) on its surface. The wiping element bears on the evaporating cylinder due to centrifugal force and rolls over it, bringing the liquid on the evaporating cylinder into an intensive complex movement. The screw thread on the wiping element surface increases the permeability of the liquid through the element, while restricting the formation of a longitudinal wave which develops in front of the element. An analysis of residence time distribution curves shows that there are three different liquid flow regimes on the evaporating cylinder. In this study, conditions of the formation of these regimes and their consequences are discussed. Optimum conditions for distillation can be reached at peripheral speeds of the wiper at which increased retention of a liquid on the evaporator occurs. This leads to an increased residence time. The increased residence time is a positive factor which makes efficient evaporation under gentle conditions at a lower evaporating temperature possible.

Residence time distribution in a wiped liquid film

Abstract The present work concerns the evaluation of experimental residence time distribution (RTD) curves obtained in a wiped liquid film formed on the surface of the evaporator cylinder of a molecular evaporator. The dependent variables in the study are the peripheral load and liquid viscosity. The wiping process is modelled by a cascade of apparatuses with laminar falling film ideally mixed at the exit from each stage in a mixer having zero thickness. The comparison of experimental and model data shows that the model developed explains well the actual processes taking place in the film wiped by a segmented wiper. By comparison of the shape of the experimental RTD curves with theoretical model curves, a criterion can be derived, indicating the efficiency of wiping.

Model of temperature profiles during condensation in a film in a molecular evaporator

Abstract The characteristics of the falling film of condensate flowing downwards along the condenser surface of a molecular evaporator are presented. They result from solving the mathematical model of glycerol vapour condensation. The significance of the film surface temperature of the condensate and of the effect of the process parameters determining film temperature such as evaporation rate, condenser temperature and condenser height are discussed. Measures for enhancing the vapour condensation efficiency are also treated.

Stable radicals as tracers for residence time distributions

Abstract In a search for stable and safe tracers suitable for investigating systems for residence time distributions, such as those systems for which visual observation is not possible, sensitive high temperature systems etc., stable radicals proved valuable. An example is given of their application to determine residence time distributions in a scraped-film prepolymerization reactor operating at a temperature of 100–160 °C, where analysis of the resulting solid sample is difficult under normal conditions and where there are considerable health hazards when using radioactive tracers in a pilot plant environment. The method is described. Substantial time savings in obtaining response data can be achieved over those obtained from heat-resistant dye tracer injection measurements.