Numerical Simulation Of Molecular Distillation And Application In Recycling Of Rapeseed Oil Deodorizer Distillate

Molecular distillation (MD) process is useful in the separation and purification of materials of high boiling point, as well as those that are thermally sensitive. It has been gotten more and more applications in food and chemistry industries. However, there remain unexplored larger and more important fields of molecular distillation. Parameters of practical production still depend upon experience and optimization methods. Application of MD on the separation of active components from rapeseed oil deodorizer distillate (RODD) is not systematically studied.The objective of study is to carry out numerical simulation of molecular distillation and separation of components from RODD, which provides technical support for recycling of RODD.In this study, balanced equations of a binary mixture for MD were derived on the basis of relation of heat and mass transfer in liquid films on both the evaporating and condensing surface, allowing for the feed, evaporating and condensing physical-chemical properties. The separation of vitamin E, sterols and fatty acid methyl ester (FAME) with acid and lipase esterification by MD from RODD and simulation for separation of vitamin E from RODD were carried out. The main work and results were as follows.1. Balanced equations of a binary mixture for MD were derived on the basis of relation of heat and mass transfer in liquid films taking account of the properties change of mixture.(1)The stable implicit finite differences Crank-Nicholson method was used for the transformation of partial differential equation into discrete form. The model values agreed well with the experimental values. The maximum relative error did not exceed 5.1%. The effects of feed temperature, condenser temperature, feed concentration and feed rate on MD process were investigated by the simulation of DBP-DBS mixture distillation. As the feed temperature increased, film thickness and surface concentration decreased sharply. It was more remarkable with an increase of feed temperature. The length of the evaporating cylinder along with the surface temperature of evaporating film reached steady-state was decreasing with increasing feed temperature. Evaporation rate of DBP increased with an increase of feed temperature. Evaporation rate decreased and re-evaporation on condensing film taken place with increasing of condensing temperature. Evaporating film surface temperature decreased while degree of evaporation and separation factor increased with the increase of feed concentration. Evaporating film thickness, surface concentration and composition of distillate increased, while degree of evaporation decreased when feed rate increased. The length of the evaporating cylinder along with the surface temperature of evaporating film reached steady-state decreasing with increasing feed rate.(2) Influences of divided evaporator and condenser on MD process were investigated by the simulation of DBP-DBS mixture distillation. Results showed that influence of splashing on desired substance caused by insufficient removal of gases and re-evaporation were minimized by the divided condenser. Gradient concentration of desired substance and increased degree of evaporation were obtained. Desired product concentration could be obtained by adjusting correspond condensing temperature. Evaporating film surface temperature was gradient increased nearly to the evaporator temperature through the gradient temperature increase of divided evaporator. Degree of evaporation increased with the increase of evaporator temperature. Different components could be collected on correspond condenser. The residence time distribution was carried out for wiped film molecular distillation by impulse response method.2. The separation of vitamin E, sterols and FAME with acid and lipase esterification by molecular distillation from RODD.(1) Optimization of sterols, FAME and vitamin E separation from RODD was studied systematically on basis of acid catalyzed methyl esterification. A mathematical regression model was presented for separation of sterols in methyl esterification of RODD. The effects of MD operation parameters on separation of FAME and vitamin E from RODD were investigated. The results showed that the methyl ester rate was above 98% when methyl esterification temperature 60℃, time 1.65hrs ,catalyst 4.4%, methanol/material ratio 100ml/100g. It indicated that the recovery of FAME was 41g/100mL when pressure 5.2Pa, evaporating temperature 120℃, feed temperature 70℃, wiper rolling speed 150 rpm and feed rate 120mL/h.. when experiments with an evaporating temperature of 200℃, feed flow rate of 90 ml h^-1 and wiper rolling speed of 150 rpm was conducted, the content and recovery of V_E was above 34.7%, 89.5% respectively after molecular distillation. Vitamin E was concentrated by molecular distillation by eliminating free fatty acid (FFA), content and recovery in the residue were 26.32% and 69.23%, respectively. Compared to the FAME separation to concentrate vitamin E, vitamin E concentration and recovery of vitamin E by this method was lower.(2) MD and lipase catalyzed methyl esterification were applied for vitamin E concentration. The model of FAME content produced by enzymatic reaction was presented by RSM and compared with the BP neural network model with algorithm with momentous factor. The results indicated that when RODDTGF 20g, reaction temperature 37℃, the amount of water 11.7ml and lipase 30.78 U for hydrolysis time 16 h followed by amount of methanol added 1.32ml with lipase 15 U per time of three times, and total reaction time 40h, the content of V_E was 25.2% after twice MD. Optimization by RSM was better compared to the BP neural network model. 3. Molecular distillation simulation for separating desired components was carried out. Model value of V_E concentration was compared with experimental value.(1) Simulation for separation of V_E from RODD and separation of FFA from soapstock were carried out. Results showed that the tendency of model value was similar to that of experimental value. V_E concentration increased with the increase of evaporator and feed temperature. Evaporator temperature of model was lower 20K than experimental value and relative error of V_E concentration was below 15% when high concentration was obtained. Degree of evaporation increased with the increase of evaporator temperature when FFA separation took place. The relative error between model and experimental value was below 16%. The FFA content in residue decreased when evaporator temperature increased and was similar to model values.(2) MD process software was developed combining the advantages of Matlab data-processing with object oriented design of Visual Basic 6.0 by Matrix VB. The software had the characteristics of friendly interface and conveniently processing.