(447g) Organic Solvent Reverse Osmosis Membrane for Cyclic Siloxane Purification

Siloxanes and silicone polymers such as silicone fluids, elastomers, and resins are important chemicals with a wide range of applications in the biomedical, electronic, cosmetic, and space industries, etc. Conventional techniques for the separation of the small siloxane molecules from silicone fluid such as stripping and adsorption are complex and energy intensive. Membrane separations could serve as alternatives to or complement these industrial separation techniques to reduce the overall process energy demand. This work presents the application of polymeric membranes in the purification of siloxane oligomers via the rejection of silicone oil contaminants. First, the fundamental transport properties are studied to understand the driving force and mechanism for the separation. These properties are then incorporated into a Maxwell-Stefan (MS) framework for siloxane permeation and separation predictions. Experimental analysis shows that the diffusivities and solubilities of the siloxanes decrease with increasing Si atoms, and the resulting siloxane and oil fluxes are accurately matched with the solution-diffusion (SD) model using these independently-measured parameters. Unit activity analysis confirms that sorption selectivity dominates the SD separation performance with SD4/SOil = 1250 and DD4/DOil = 122. Finally, an octamethylcyclotetrasiloxane (D4) cleansing operation using polydimethylsiloxane (PDMS) membranes was carried out with a feed containing 90/10 wt/wt% D4/silicone oil. With a transmembrane pressure of 20 bar, a high-purity D4 permeate stream (>99wt% D4) was obtained. The discrepancy in SD flux prediction was found to be due to concentration polarization with a concentration polarization modulus of 4.2. Modification to the MS framework could improve solvent/polymer mixture transport prediction in polymer membranes. These findings provide an industrially scalable low-cost alternative to silicone fluid separation and siloxane purification.