Determination of termodynamic properties of binary systems at elevated pressures by using non-convetional supercritical fluids

Namen diplomskega dela je bil določiti termodinamske in transportne lastnosti binarnega sistema polietilen glikol (PEG)/propan za dve različni molski masi polietilen glikola: 10 000 g/mol (PEG 10 000) in 35 000 g/mol (PEG 35 000). Določili smo talilno krivuljo s pomočjo modificirane kapilarne metode v visokotlačni optični celici od 0 MPa do 30 MPa in izračunali difuzijske koeficiente z metodo zunanje tehtnice pri 70 °C. Podatke o temperaturah tališč, pridobljenih s kapilarno metodo, smo primerjali s podatki iz literature in dobili podoben trend obnašanja talilne krivulje. Z uporabo različnih superkritičnih fluidov (SCF), lahko spreminjamo potek talilne krivulje. Sistem z ogljikovim dioksidom ima najbolj izrazit temperaturni minimum, medtem ko je temperaturni minimum najmanj izrazit pri uporabi argona, talilna krivulja propana pa se nahaja med talilnima krivuljama za argon in ogljikov dioksid. Difuzijske koeficiente, ki smo jih dobili z metodo zunanje tehtnice, smo prav tako primerjali z literaturo in ugotovili, da molska masa bistveno ne vpliva na vrednost difuzijskega koeficienta, večji vpliv ima tlak. Pri določenem tlaku doseže krivulja difuzijskih koeficientov v odvisnosti od tlaka maksimalno vrednost, razlog za to je, da se pri teh pogojih v vzorcu polimera raztopi maksimalna količina fluida. Primerjali smo tudi difuzijski koeficient v odvisnosti od vrste fluida in ugotovili oz. potrdili, da na vrednost difuzijskega koeficienta vpliva premer kot tudi lastnosti (polarnost, razvejanost itd.) molekule fluida. Ker je molekula ogljikovega dioksida manjša od molekule propana, so bile vrednosti difuzijskih koeficientov sistema z ogljikovim dioksidom večje. Pridobljeni podatki in primerjave so bistvenega pomena za načrtovanje novih, boljših procesov.The aim of the thesis was to determine the thermodynamic and transport properties of the binary system of propane and polyethylene glycol (PEG) of two different molar weight10 000 g/mol and 35 000 g/mol. The melting curves were determined by using the modified capillary method in a high-pressure optical cell in the pressure range from 0 MPa to 30 MPa. In the next step, the diffusion coefficients were determined by the external balance method at 70 °C in the same range of pressures. The data of melting temperatures, obtained by the capillary method, were compared to the data found in literature and a similar trend of the melting curves was observed. By using different supercritical fluids (SCF), the course of the melting curves may vary noticeably. When involving carbon dioxide into the system, a most prominent temperature minimum is deteched, followed by the one of the system with propane, whilst in the case of the system with argon the melting minimum is leastwise and therefore, the melting curve of the system with propane is located between the melting curves for argon and carbon dioxide. When comparing the diffusion coefficients obtained by the external balance method with the literature data it was discovered that molar weight does not significantly affect the value of the diffusion coefficient, the influence of pressure is much more significant. At a certain pressure, the curve reaches its maximum value. A possible explaintaion is that an increase of pressure reflects in a higher gas density and consequently solubility of gas in the polymer increases. Therefore a full saturation of PEG with gas can is expected. Diffusion coefficient varies also depending on the type of fluid involved into the system. It is affected by its properties (polarity, branching, etc.) as well as the diameter of its molecules. Due to the fact that a molecule of carbon dioxide is smaller than the molecule of propane, higher diffusion coefficient are observed for the system with carbon dioxide. Knowledge on behavior of the multicomponent systems at elevated pressures is essential for the design of an economically feasible industrial scale production plant