Removal of Sulfur Compounds From Diesel/Crude Oil Using Extractive Membrane Contactor and Supported Liquid Ionic Membrane

The feasibility of using an extractive liquid membrane contactor (ELMC) and supported ionic liquid membranes (SILMs) for the desulfurization of hydrocarbon streams, such as raw diesel and whole crude oil, were investigated separately and for each, separation capability and mass transfer rate were studied using ionic liquid (IL) (n-butyl-3-methyl pyridinium methylsulfate) and other extractive organic solvents, such as furfural and light naphtha. The SILM study using n-butyl-3-methyl pyridinium methylsulfate to remove sulfurous species from hydrocarbons exhibits very low mass transfer coefficients (i.e., value of 8.75x10^-7 to 2.1x10^-4 kg/m².s was obtained for different membrane materials, pore sizes and under different experimental conditions). This is attributed to diffusion of big molecules of sulfurous species, such as Dibenzothiophene (DBT), as indicated from the low value of the estimated diffusion coefficient of DBT in IL (in the order of 10^-11 m²/s) obtained. However, the use of extractive liquids, such as furfural and naphtha, in ELMC boost the transfer rate of sulfurous species as compared to IL as indicated in the high value of mass transfer coefficient (k) (i.e., value of 4.4x10^-4 to 6.7x10^-4 kg/m².s was obtained using different membrane materials, pore sizes and under different experimental conditions). This is as a result of the higher diffusion coefficient (in the order of 10^-9 m²/s) of sulfurous species in furfural and light Naphtha when compared to that of ILs (in the order of 10^-11 m²/s). Furthermore, the mass transfer coefficient (k) shows significant dependence on the flow rate of both feed stock and extractive liquid streams, as the mass transfer coefficient of sulfurous compounds increased with increase in the flow rate of either stream or both. The mass transfer resistance offered by the membrane is very significant as clearly indicated by increase in mass transfer coefficient with increasing membrane pore size. This suggests that the diffusion resistance is contributing significantly to the overall resistance of the transfer of the sulfur species from feedstock to extractive liquid streams. This thus indicates that membrane material is clearly not affecting the transfer rate of the sulfurous compounds. Furthermore, when furfural is used with crude oil as feed, the fouling on the membrane surface from the crude oil side is significant, which limits the transfer rate. However, light naphtha is more promising to act as an extractive liquid for the reason that it transfers under the osmotic pressure difference to the whole crude oil side, which prevents fouling and thus promotes faster diffusion of sulfurous species.