| With the increase of people's demand for high quality oils,the production of low temperature pressed rapeseed oil has increased in recent years.As the rapeseed cake remains more than 10%of the oil and rich tocopherols,polyphenols and other bioactive compounds,how to improve the comprehensive utilization of rapeseed cake has become an urgent problem in the field of oil processing.Moreover,subcritical fluid extraction,as a low temperature oil production technology,has the advantages of high extraction efficiency,excellent product quality and low energy costs.However,it is difficult to achieve the synergistic extraction of oil and bioactive compounds from rapeseed cake by using single solvent,and there is also a relative lack of research on the energy transfer of subcritical fluid extraction process.In this study,the different subcritical mixed solvents were used to extract the low temperature pressed rapeseed cake and the migration rules of rapeseed oil and bioactive compounds were compared.The energy-saving effects of subcritical fluid extraction technology were also analyzed based on the heat pump principle.The main research and results were summarized as follows:?1?Subcritical propane,dimethyl ether,propane-ethanol mixed solvents?1%,3%,5%?and propane-dimethyl ether mixed solvents?1:1,3:1,5:1,v/v?were used to extract rapeseed oil,and the quality of rapeseed oils obtained from different solvents were evaluated.As a result,the extraction yields of oil extracted by different solvents were more than 90%,and mainly including oleic acid,linoleic acid and linolenic acid.Propane-dimethyl ether?5:1,v/v?,and propane-3%ethanol mixed solvent extraction significantly improved the content of tocopherols and polyphenols in rapeseed oil to207.95 mg/100g,and 50.36 mg/100g respectively.The acid value and peroxide value of the rapeseed oil extracted from propane-dimethyl ether mixed solvent?5:1,v/v?were 3.44 mg/g and 2.00 mmol/kg,respectively,which were significantly lower than that of rapeseed oil obtained from single subcritical solvent.Consequently,the extraction yields and fatty acid composition of rapeseed oils obtained from different solvents were similar,and the mixed solvent was beneficial to enhance the content of bioactive compounds and improve the oxidation stability of oil.?2?Subcritical propane,dimethyl ether and propane-dimethyl ether mixed solvent?5:1,v/v?were used to extract the rapeseed cake,respectively,and the migration of oil and bioactive compounds in different solvents were analyzed.The results showed that the extraction yields of oil and bioactive compounds not only raised with the extraction pressure,but also increased with the extraction time,and the increase rate was faster in the initial stage.According to the van't Hoff equation,the enthalpy values and entropy values of oil and bioactive compounds became positive,and the Gibbs free energy values became negative at different temperatures.The calculations of Arrhenius equation showed that the extraction activation energies of rapeseed oil in propane,dimethyl ether and mixed solvent were 10.87,20.13 and 8.75 kJ/?mol·K?,respectively,which were lower than the values of tocopherols and polyphenols in the same solvent.The effective diffusion coefficients of rapeseed oil,tocopherols and polyphenols in the mixed solvent were 3.57×10-125.11×10-12,3.08×10-124.95×10-12 and 2.85×10-124.91×10-12 m2/s,respectively,which were higher than the values in the propane and dimethyl ether at the same temperature.In conclusion,the effect of extraction pressure on the solubility of oil and bioactive compounds was dominant,and the extraction kinetics of oil and bioactive compounds could be well described by using Baümler model.The extraction processes of oil and bioactive compounds were endothermic and spontaneous.In different solvents,rapeseed oil was the most easily extracted component,while dimethyl ether-propane mixed solvent was beneficial to the synergistic extraction of oil and bioactive compounds.?3?Based on the principle of heat pump,the energy transfer process of subcritical fluid extraction system was analyzed,and the subcritical butane extraction of 20 tons of rapeseed cake was used as an example to calculate and verify the actual energy consumption in the production process.The results indicated that the dissolving equipment in the extraction system was equivalent to the evaporator in the heat pump system,the cooling equipment was equivalent to the condenser,and the extraction solvent was equivalent to the working medium.The thermodynamic cycle was completed after vaporization,compression and condensation,with an amount of energy released.By reference to the two terms Virial equation,the compression enthalpy of propane,butane and dimethyl ether was 53.99,47.97 and 51.48 kJ/kg,respectively.And by reference to the Carruth model,the evaporation enthalpy of each solvent was 322.57,351.89 and 394.45 kJ/kg,respectively.When 20 tons of rapeseed cake was countercurrent extracted 4 times at the ratio of 1:1?w/v?,the theoretical calculation of the solvent condensation required heat transfer area of 27.6 m2,and the system energy consumption of 378 kW·h.In actual production,the heat transfer area of the condenser was 36.0 m2,and the energy consumption was 450 kW·h,which were close to the theoretical calculation results.Therefore,the solvent cycle process of the subcritical fluid extraction was consistent with the compression and condensation process of the working medium in the heat pump,and it was feasible to analyze and calculate the energy transfer in the subcritical fluid extraction process with reference to the heat pump principle. |
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