Experimental Study on Oil Removal in Nutshell Filters for Produced-Water Treatment

Laboratory testing was conducted with a nutshell filter to determine the oil-removal performance on the basis of the flux rate, media type, media size, water salinity, and oil concentration. The goal was to determine the operating parameters that allowed <5 parts per million by volume (ppmv) of oil at the outlet, which is the normal operating point for this technology in producedwater treatment. Nutshell filters composed of black walnut or pecan granular media are an established produced water-treatment technology for tertiary oil removal. Guidelines for the size and operation of nutshell filters have evolved mainly by trial and error, with limited published operating data. This laboratory-research program tested a nutshell filter to determine the operating flux limits (flow rate per unit area) that provide suitable oil-removal performance. The separation-efficiency target was defined as 5 ppmv of oil in the outlet stream. The variables tested included medium type [black walnut shell (BWS) or pecan shell (PS)], medium size, filtration flux, water salinity, and oil concentration. The flux limit for common 12/20 media is 12.0 (gal/min)/ft (or gpm/ft) for freshwater operation and 13.0 gpm/ft for saline water. Decreasing the medium size to 20/30 mesh increased the allowable flux limit but at the expense of a substantial increase in the pressure drop. Oildroplet penetration into the filter bed proceeds by means of a near-plug-flow profile, with the top 18 in. of the bed providing 99% of the oil removal. Full breakthrough is a function of inlet concentration, with 20–30 hours of operation expected for a 48in.-deep bed.