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

Abstract

Summary 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 produced-water 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)/ft2 (or gpm/ft2) for freshwater operation and 13.0 gpm/ft2 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. Oil-droplet 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 48-in.-deep bed.