ABSTRACT
Nonpolar solvents have been reported to exhibit a nonlinear flux-pressure behavior in hydrophobic membranes. This study explored the flux-pressure relationship of six nonpolar solvents in a lab-cast hydrophobic poly(dimethylsiloxane) (PDMS) membrane and integrated the permeance behavior in the evaluation of the proposed transport model. The solvents exhibited a nonlinear relationship with the applied pressure, along with the point of permeance transition (1.5-2.5 MPa), identified as the critical pressure corresponding to membrane compaction. Two classical transport models, the pore-flow model and solution-diffusion model, were evaluated for the prediction of permeance. The solution-diffusion model indicated a high correlation with the experimental results before the point of transition (R 2 = 0.97). After the point of transition, the compaction factor (due to membrane compaction after the critical pressure) derived from the permeance characteristics was included, which significantly improved the predictability of the solution-diffusion model (R 2 = 0.91). A nonlinear flux-pressure behavior was also observed in hexane-oil miscella (a two-component system), confirming the existence of a similar phenomenon. The study revealed that a solution-diffusion model with appropriate inclusion of compaction factor could be used as a prediction tool for solvent permeance over a wide range of applied transmembrane pressures (0-4 MPa) in solvent-resistant nanofiltration (SRNF) membranes.
ABSTRACT
Vegetable oil processing has been identified as one of the potential nonaqueous applications of membrane technology. Membrane-based processing has been largely attempted on individual steps of the conventional refining process with reasonable success. With the advent of organic-solvent-nanofiltration, membrane desolventizing of hexane oil miscella has received greater attention, revitalizing the prospects of integrated membrane processing. A practical evaluation of membrane augmented desolventizing revealed that approximately 65% energy savings towards solvent evaporation could be achieved in an industrial environment. Further, a pragmatic appraisal advocated that an integrated membrane process with a focus on pretreatment and desolventizing along with physical refining would be a desirable approach for fortifying the benefits. The present review intends to channelize the efforts to overcome the current limitations and highlights the importance of developing better membranes, process evaluation under appropriate practical conditions, and developing suitable cleaning protocols for stable performance. In the case of alternate solvents to hexane, membrane solvent recovery would be a favorable approach to overcome the limitation of associated higher thermal energy requirements. Nevertheless, solvent selection should be based on a composite evaluation of extraction and membrane desolventizing, specific to the type of oil. Finally, a comprehensive process scheme has been proposed to realize the benefits in extraction-refining plants. In this direction, a few pilot demonstration plants need to be established and operated for 1-2 years to understand and overcome the practical difficulties and limitations of the technology, leading to its industrial adoption.
