Performance of Nanofiltration Membranes for Solvent Purification in the Oil Industry

The extraction stage of edible oil in the oil industry is commonly performed by using toxic solvents (e.g. hexane) and processes with high energy consumption (e.g. distillation, evaporation) to recover the solvent, which represents around 70-75 wt% in the oil-solvent mixture. In this paper, a membrane-based extraction method using nanofiltration (NF) membranes is presented. Commercial nanofiltration membranes made of different polymers (Desal-DK-polyamide NF from GE-osmonics (R), NF30 polyethersulfone NF from Nadir (R), STARMEM (TM) 122 polyimide from MET (R) and SOLSEP NF030306 silicone base polymer SOLESP (R)) were selected and tested to recover the solvent from soybean oil/solvent (10-20-30% w/w oil) mixtures at various separation pressures and constant temperature in a dead-end filtration set up. The selection of the solvent was made in order to compare solvents obtainable from renewable resources, such as ethanol, iso-propanol and acetone, with solvents traditionally used in the industry (i.e. cyclohexane and n-hexane). The structural stability of the membranes towards the different solvents used in this work was verified visually, by the variation of the membrane area and by means of permeate flux assessments. Desal-DK and NF30 showed poor filtration performance and even visible defects after exposure to acetone but a good performance was obtained for the nanofiltration membranes STARMEM (TM) 122 and SOLSEP NF030306 with ethanol, iso-propanol and acetone. For example, considering a mixture with 30% edible oil in acetone, STARMEM (TM) 122 shows a flux and oil rejection of 16.8 L m(-2) h and 70%, respectively. For the same conditions, SOLSEP NF030306 exhibited a flux of 4.8 L m(-2) h with 78% rejection, which shows the potential application of nanofiltration membranes in the oil industry.