Membrane-based oil-water separation: Membranes, concepts and case studies

1. IntroductionThe demand for efficient oil-water separation is driven by the tightening of discharge limits for e.g. produced water in the oil and gas industry, process water in the petrochemical industry and bilge water in the marine industry. The global produced water production alone is over 71 billion bbl/a – 8.4 billion m3/a [1]. Among the different technologies, particularly ultrafiltration with polymeric membranes can be used to reduce oil in water down to less than 1 ppm leading to an energy-efficient and compact approach for the industry. The first part of the presentation will introduce the ETNA membrane for oil-water separation. The second part will review different oil-water separation technologies with focus on ultrafiltration-based synergy processes, while the final part will present two case studies on industrial oil-water separation processes. 2. Oil-water separation membranesThe membranes chosen for oil-water separation are the ETNA01PP and ETNA10PP membranes (Alfa Laval Nakskov A/S, Denmark) with a molecular weight cut-off of 1,000 and 10,000 Dalton, respectively. Both membranes are surface-modified PVDF membranes on a polypropylene support and are permanently hydrophilic. The hydrophilic surface of these membranes increases the retention of hydrophobic compounds such as oil and thus reduces membrane fouling [2].3. Membrane-based oil-water separation conceptsThe most common technologies in the industry for oil-water separation are skimmers (gravity/density), coalescers (coalescence) and centrifugal separators (centrifugation/density). Skimmers, centrifugal separators and coalescers are very efficient separation processes for initial oil-water separation but their final oil-in-water levels are commonly higher than the level of less than 1 ppm which is achievable by ultrafiltration. Since the industry often requires high oil recovery combined with low oil content in the discharge water, the optimal solution is often a process combination consisting of a conventional separation technology followed by ultrafiltration and thus different ultrafiltration-based oil-water separation concepts will be reviewed.4. Case studies: Industrial scale oil-water separation The potential of ultrafiltration-based concepts will be highlighted in two case studies. The first plant has a capacity of 40,000 tons/a treating bilge water and industrial oil waste with low suspended solids concentration. The process set-up consists of a skimmer, high speed centrifugal separators and paper band filter plus ultrafiltration. The second plant has a capacity of 80,000 tons/a handling oily waste from ships, petrochemical facilities or offshore industry with higher levels of suspended solids. In the process set-up, decanter centrifuges, high speed separators and evaporation are used in combination with ultrafiltration. Both process set-ups result in an oil-rich phase with more than 98% oil and a water rich-phase with less than 2 ppm oil. 5. Conclusion and outlookOverall, this paper shows that ultrafiltration-based oil-water separation concepts can achieve both high oil concentrations combined with very low residual oil in the water phase and offer therefore an excellent alternative for current and future oil-water separation challenges.