Experimental And Theoretical Study On Phase Equilibrium Of Supercritical System

As a new-type, high-efficiency and green chemical technology,supercritical fluid technology has shown great prospects of application inmany significant industry fields. The study on phase equilibrium ofsupercritical system is an important constituent part of supercritical fluidscience and technology. In this thesis, the experimental and theoretical studyon phase equilibrium of pure and mixed solid solutes in supercritical fluidwith cosolvent was carried out in-depth. The main works in this thesis are asfollows:(1) The experimental apparatus of the study on phase equilibrium ofsupercritical system was improved. The equilibrium solubility of solidsolutes in supercritical fluid with cosolvent was measured using aflow-type apparatus and the equilibrium solubility of pure and mixedsolid solutes in supercritical fluid was analyzed by ultravioletspectrophotometer successfully.(2) In this experiment, CO₂was used as a supercritical fluid; the solubilityof4kinds of pure solid solutes were measured, included3,5-dinitrobenzoic acid (3,5-DNBA),3-nitrobenzoic acid (3-NBA), 2-nitrobenzoic acid (2-NBA) and3-aminobenzoic acid (3-ABA); thesolubility of3kinds of mixed solid solutes (mass ratio as1:1) weremeasured, included3,5-DNBA+3-NBA,3,5-DNBA+2-NBA and3-NBA+3-ABA. Ethanol, ethyl acetate, n-propanol, and ethyleneglycol were used as a cosolvent, respectively. The experiment wasinvestigated at (308,318, and328) K from (8.0to21.0) MPa. The molefraction of cosolvent was0.015,0.035, and0.055, respectively. Thenumber of experimental data was amount to273.(3) On the basis of experimental data, the effects of temperature, pressure,the type and concentration of cosolvent on the solubility of pure andmixed solid solutes in supercritical CO₂were investigated; the solubilityenhancement effect factor e was defined in order to study theenhancement effect of cosolvents on the solubility of solutes; moreover,the effects of temperature, pressure and the concentration of cosolventon the enhancement effect factor were analysed; the effects of thedifferent molecular structures and the type, quantity and position offunctional groups on the solubility of solutes were compared; thedifference of the solubility of identical solute in pure or mixed systemwas discussed, and the solubility enhancement coefficient SE wasdefined to study the tendency; the appropriate condition for separatingmixed solids in supercritical CO₂was confirmed by defining the mixtureseparation factor μ and the separation efficiency HE. (4) The solubility data of solid solutes in pure supercritical CO₂werecorrelated by7common used semi-empirical models; the solubility dataof solid solutes in supercritical CO₂with cosolvents were correlated by3common used semi-empirical models; the correlative abilities of thesesemi-empirical models were compared and the influencing factor of theprecision of correlation was discussed.(5) Solubility data from49different compounds of different functionalgroups were collected from literature to compare and evaluate7common used semi-empirical models. The results indicated that thecorrelative and predictive capability of models depend on the accuracyof the expression of density's function and the fitting-parameters. Themore precisely the expression of density's function describes therelationship between the solubility of solute and the density of solvent,or the more exactly the fitting-parameters reflect the effects oftemperature and pressure on the solubility of solute, the more accuratelythe models correlate. The predictive capability and applicability of thesemodels for different types of solutes at different conditions aredemonstrated in this work. The results indicated that with uppertemperature and pressure, all of the accuracy of the models decreasesobviously; with a higher mole fraction solubility of solute, most of theaccuracy of the models increases a little; with more experimental datapoints, most of the accuracy of the models decreases. (6) Based on the Kumar-Johnston (KJ) model, the modified KJ model wasproposed by the theory of solution. The modified KJ model was verifiedby901groups of solubility data containing153groups of experimentaldata and748groups of data in literature. The results indicated thatcomparing with KJ model, the modified KJ model proposed in thisthesis is more suitable to describe and predict the solubility of pure andmixed solids in supercritical CO₂.(7) Based on the Sovova model, considering the effects of temperature onthe solubility, the modified Sovova model was proposed by introducinga function of temperature. The modified Sovova model was verified by1431groups of solubility data containing120groups of experimentaldata and1311groups of data in literature. The results indicated thatcomparing with the Sovova model, the modified Sovova modelproposed in this thesis is more suitable to describe and predict thesolubility of solid solutes in supercritical CO₂with cosolvent.(8) Based on the Bartle model, a new semi-empirical model consists of anew density function was proposed by the theory of solution. Theprecision of new proposed model in this thesis was tested by1214groups of solubility data containing153groups of experimental data and1061groups of reliable data in literature. The results indicated that thenew proposed model can describe the solubility of solid solutes insupercritical CO₂better. All the research in our work would be used for the development, design andindustrial application of the supercritical fluid technology.This project research was financially supported by the funds awarded byNational Natural Science Foundation of China (No.20776006) and theCentral University Basic Scientific Research Projects of the Ministry ofEducation (No. ZZ1103).