Designing of spiral wound nanofiltration multistage process for oil concentration and solvent recovery from soybean oil/n-hexane miscella

Abstract

The present work demonstrates the potential of oil/n-hexane miscella separation from a local factory via a hybrid nanofiltration-evaporation process. In the membrane separation, solvent resistance nanofiltration (SRNF) membranes lab-made of polyvinylidene fluoride (PVDF) as support, poly-dimethylsiloxane (SI) or cellulose acetate (CA) as coating materials and a commercial composite membrane were used. To perform this study, a representative spiral-wound (SW) module made-up with the membranes previously mentioned was employed. For miscella mass transfer through the SW module, a plug-cross-mixing flow (PCMF) model was used. From experimental miscella permselectivity data at T = 30 °C and Δp = 20 bar, an analytical final expression of mass balance was obtained which correlated the retentate oil concentration with the membrane area. The multistage process of such membranes was integrated by a number of single-stage in series. In the multistage membrane performance evaluation, several restrictions were imposed on the model considering the operational conditions of the local factory that uses an evaporation and steam stripping process. Seven stage (total membrane area 702 m2) using PVDF-10% SI membrane gave the best oil/n-hexane separation effectiveness. From this, a hybrid SRNF-evaporation process was proposed in which the selected membrane multistage process replaced the first and the second evaporators. The permeate stream rich in n-hexane (90.5 w%) was fed to the section of the expeller in which the oil was depleted, and the retentate stream enriched in oil (86.7 w%) was pumped to the third-stage evaporator and strippers, where the oil was concentrated to >99 w%. The overall energy demand was significantly reduced (≈50%) and there was around 60% less consumption of cooling water and steam by the SRNF-assisted processes in comparison with the conventional factory evaporation process.