The use of Membrane Technology for Vegetable Oil processing with Green Solvents
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusOpen Access
© 2019 Mohamed Hussein Ali Abdellah
Utilisation of n-hexane for the extraction of vegetable oil from oilseeds presents growing concerns for the environment and human health. In spite of its high miscibility with the vegetable oil constituents, low boiling point and low heat of vaporisation, n-hexane has been recently classified as carcinogenic, mutagenic and reprotoxic solvent. As a consequence, the industrial usage of hexane, particularly in food-related industries, is expected to decline. Bioderived solvents such as terpenes, which include d-limonene, p-cymene and alpha-pinene, represent promising alternatives to n-hexane. Although terpenes show high solubility with vegetable oils, the recovery of these solvents from vegetable oil mixtures by conventional evaporation is infeasible due to their high boiling points and latent heat of vaporisation. Membrane separation technology, such as solvent resistant nanofiltration represents a potential alternative to heat intensive conventional evaporation. In this thesis, the sorption kinetics of pure solvents and their corresponding canola oil solution (0-100 wt% oil) in free standing polydimethylsiloxane (PDMS) films was studied. The PDMS films were prepared from 50 wt% PDMS solution in n-heptane with a polymer to cross-linker ratio of 2:1 (w/w). The highest sorption levels were observed with pinene followed by limonene and cymene while sorption was negligible in pure oil. Pure solvent flux curves were not linear with transmembrane pressure, but this was shown to be consistent with Fickian diffusion behaviour for highly swollen polymers. In the case of oil/solvent mixtures, the total sorption level decreased exponentially with increasing the oil content in the mixture which is explained by the decrease in the degree of membrane swelling due to the decrease of solvent activity. However, the partial oil uptake from the mixture was higher than that of the pure oil which is also attributed to the effects of membrane swelling induced by the solvents. A binary component Flory-Huggins model was used to calculate the interaction parameter between each penetrant and PDMS while a multicomponent Flory-Huggins model was used to fit the sorption isotherms of the oil and the solvent from the binary mixtures. The performance of laboratory-made polydimethylsiloxane/polyacrylonitrile (PDMS/PAN) composite membranes was studied for the recovery of terpenes from their binary mixture with canola oil. The membranes were prepared by the solution casting method from 7 wt% PDMS solution in n-heptane with a polymer to cross-linker ratio of 2:1 (w/w). The effects of transmembrane pressure (5–30 bar), feed temperature (25–40 deg. C) and feed concentration (10–30 wt% oil) on the membrane performance were investigated in terms of permeate flux and oil retention. In compliance with the observed sorption levels, pure limonene and its oil solutions showed the highest permeate flux followed by cymene then pinene. For the different oil/solvent solutions, the oil retention was comparable and increased with transmembrane pressure to more than 80 % beyond 20 bar. This increase in oil retention is attributed to the increase in solvent flux with pressure whereas the oil flux almost remained unchanged. Increasing the feed temperature resulted in an improvement in the permeate flux and a slight deterioration in membrane selectivity. The effect of cross-flow velocity on the membrane performance was also investigated and used to develop a mass transfer correlation for the boundary layer. Moreover, a mathematical model originating from the Maxwell-Stefan diffusion model in combination with the multicomponent Flory-Huggins solubility model were used to predict the flux of the solvent and the oil which was in a good agreement with the experimental data. The long-term stability of the PDMS/PAN membrane in the different oil/solvent mixtures was investigated. The membrane showed excellent stability with no observed change in performance over a period of 1 year. Furthermore, the degumming of crude canola oil diluted with terpenes by ultrafiltration was investigated. The performance of polyethersulfone, polysulfone and ceramic ultrafiltration membranes was studied at a pressure of 3 bar, feed temperature of 25–40 deg. C and feed concentration of 10–30 wt% oil. The membrane performance was evaluated in terms of phospholipid retention, oil retention and permeate flux. The polymeric membranes used in this thesis proved inadequate due to irreversible swelling of the polymer structure. The average phospholipid retention of the ceramic membrane (MWCO 5 kDa) was 95+/-2% which corresponds to a residual phosphorus content of less than 30 ppm which is consistent with conventional degumming. Conversely, the average oil retention was 16+/-3% which could be problematic in the industrial application. The highest permeate flux was observed with hexane then cymene, limonene and pinene/oil solutions. Membranes experienced fouling during operation indicated by the gradual flux deterioration with time. However, washing with n-hexane at 40 deg. C was efficient to recover the ceramic membrane performance.
KeywordsSolvent Resistant Nanofiltration; Terpenes; Vegetable Oil; Green Solvents; Maxwell-Stefan; Solution-diffusion; Degumming; Polydimethylsiloxane; Ultrafiltration; Thin-film Composite Membranes
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