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

Abstract

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 oil in the outlet stream. The variables tested included medium type (black walnut shell or pecan shell), medium size, filtration flux, water salinity, and oil concentration. The flux limit for common 12/20 media is 12.0 gpm/ft2 for fresh water 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 via a near plug-flow profile, with the top 18 inches of the bed providing 99% of the oil removal. Full breakthrough is a function of inlet oil concentration, with 20-30 hours of operation expected for a 48-inch-deep bed. Testing for removal of fine sand particles showed a average 5 µm separation size for 12/20 media at 13.5 gpm/ft2 flux.

Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review

“…It was found that 95% of the oil was trapped in the first 12 inches of the bed and 99% in the first 18 inches of the bed. Large pools of coalesced oil were detected within the medium interspace [118].…”

“…The second type of medium for deep-bed filtration includes various organic, synthetic and inorganic granules, possessing roughly spherical shapes and sizes on the order of millimeters. All these media adsorb oil to various extents, and if the surface chemistry and topology are favorable, they initiate coalescence of oil droplets at their surfaces and therefore significantly contribute to the separation efficiency [118]. If coalescence is promoted at the filter surface, the droplets, which are attached within a short mutual distance for some time, have favorable conditions to merge with each other.…”

Materials and Technologies for the Tertiary Treatment of Produced Water Contaminated by Oil Impurities through Nonfibrous Deep-Bed Media: A Review

“…On the other hand, straining leads to a decrease in the flux rate (pressure drop). Industrial filtration technologies use well-balanced processing conditions in terms of the height of the bed, size of the medium, and inlet flow, which enable cleaning of the deep-bed just once per 24 h, usually through backwashing [27]. More details about the mechanisms of coalescence, experimental techniques, and modeling can be found, for instance, in the review of Kamp et al [28], and the theoretical aspects of emulsion stability are described in the chapter by Danov [29].…”