Diffusion Coefficient Of C18Unsaturated Fatty Acid Methyl Esters In Modified Supercritical CO₂

Diffusion coefficients are important physical properties to evaluate mass transfer rates in designing, scaling up and optimizing reactors and separators for extraction or chromatography with supercritical fluids.To determine the molecular diffusion coefficients of C18unsaturated fatty acid methyl esters in supercritical carbon dioxide (SCCO2) or SCCO2containing10mol%ethanol or tetrahydrofuran as modifier, oleic acid and four methyl esters of C18fatty acids, i.e., methyl oleate, methyl ricinoleate, methyl linoleate and γ-methyl linolenate were selected as the typical solutes. The diffusion coefficients were measured at temperatures from313.15to333.15K and pressures from11to27MPa using the Taylor-Aris chromatographic peak broadening technique.The influences of temperature, pressure, density and viscosity of SCCO2or the solvent mixture on the diffusion coefficients were examined. The diffusion coefficients in pure SCCO2and ethanol-modified SCCO2were well correlated with pressure, temperature, fluid density and viscosity.The results show that methyl oleate always diffuses faster than methyl ricinoleate and oleic acid at the same operating condition with or without modifier. It may be caused by bigger molecular weight or size or hydrogen-bond association due to an additional hydroxyl group of methyl ricinoleate and carboxyl group in oleic acid. Moreover, the D12values in modified SCCO2decreased with increasing the number of C-C double bonds for C18-methyl esters, which is consistent with the trend reported in pure SCCO2. It may be the complex spacial configuration related to C-C double bond that causes the above diffusion behavior27predictive models were tested for predicting diffusion coefficients in pure SCCO2. Scheibel, zeroth and second molecular connectivity models of Wells, He-1997, He-Yu-1997, He-Yu-1998, He-Yu-Su-1998, Catchpole-King and Funazukuri-Kong-Kageishowed the best predictive ability with average absolute deviations (AAD) ranging from1.0%to4.0%. The diffusivity data in ethanol-modified SCCO2are compared with the estimation of fifteen predictive models derived from pure CO2in literature.The modified Wilke-Chang equation and Siddiqi-Lucas equation are the best purely predictive model.The free volume model of DHB with two adjustable parameter gives the least errors with AAD globally lower than4.0%both in pure SCCO2and in ethanol-modified SCCO2.In order to get an insight into solute-cosolvent interactions and to predict the cosolvent effects on diffusions, cosolvent effect parameter E is introduced to quantitatively compare the strength of different types of interaction between solute and cosolvent, especially hydrogen bonding. The relationship of the E values of four C18-methyl esters are methyl oleate> methyl ricinoleate> methyl lino leate>γ-methyl linolenate. Six correlations between E and fluid properties (pressure, temperature, fluid density and viscosity) were established with good correlating results for the first time.The equation dependpent on pressure gains the least AAD values (0.94%).