Removal of Oil, Grease, and Suspended Solids From Produced Water With Ceramic Crossflow Microfiltration

Abstract

Summary Results of studies of two onshore and two offshore pilot plants that useceramic crossflow microfiltration (CCFM) to separate oil, grease, and suspendedsolids from produced water are discussed. The method is capable of producingpermeate quality with less than 5 mg/L (detection limit) of dispersed oil andgrease and less than 1 mg/L of suspended solids. Introduction The produced water after separation from oil during the primary separationprocess contains free and emulsified oil, particulates, and various chemicaladditives. The oil and grease and/or suspended solids must be reduced toacceptable levels before the water can be discharged overboard or reinjectedfor waterflood. The current U.S. Environmental Protection Agency (EPA) standards foroverboard discharge are 72 mg/L maximum for any 24- hour period and 45 mg/Lmaximum average for any 30-day period. More stringent limits are beingconsidered and probably will period. More stringent limits are being consideredand probably will be recommended and implemented. The water quality necessaryfor injection water is based primarily on the reservoir characteristics. Forexample, some sources relate limits to no more than 3 mg/L suspended solidsover 5 um in size. Other sources indicate that a criterion must be establishedfor each formation based on its history and individual characteristics so thatplugging will not occur. The best practicable control technologies currently available for theseparation of oil, grease, and suspended solids from produced water includeparallel-plate coalescers, gas flotation, granular media filtration, gravityseparation. and chemical treatment (water clarifiers/emulsion breakers). It isdesirable to develop netechnologies with higher operational reliability, loweroperating cost, and less sensitivity to operational upsets and that produce alower waste-stream volume for disposal. New technologies will be required tomeet current limits and reinjection requirements. as well as future, morestringent discharge regulations. CCFM has emerged as a state-of-the-art technology for oil/water separationand suspended-solids removal. The ceramic membranes have inherently superiorphysical integrity. chemical resistance, and thermal stability over polymericmembranes. The performance of CCFM can be maintained and optimized through theuse of several operational enhancement techniques, such as chemicalpretreatment, backpulsing, fast flushing, and chemical cleaning. This paper reports the results of studies of two Louisiana onshore and two Gulf of Mexico offshore pilot plants where an explosion-proof pilot unit (XP-1)was used to evaluate the performance of ceramic membranes for the separation ofoil, grease, and suspended solids from produced water. A data base ofoperational parameters was also developed, and the factors that affect the CCFMparameters was also developed, and the factors that affect the CCFM operationwere examined. Theoretical Membrane Description. The asymmetric microporous -alumina membranes(commercially known as Membralox and manufactured by Societe des Ceramiques Techniques in Tarbes, France) are 2.8-ft-long monolithic elements containing 19channels (also called lP-l9, see Fig. i) arranged in a honeycomb configuration. The membrane pore diameters used for the pilot-plant studies ranged from 0.2 to0.5 um; the average membrane layer thickness was 30 to SO um; and the surfacearea normal to the crossflow velocity per element was 2.18 ft2. per element was2.18 ft2. Factors Affecting Permeate Flux. In crossflow filtration, water to befiltered flows tangentially inside the surface of each channel, permeatesthrough the membrane layer and the supporting permeates through the membranelayer and the supporting structures, and exits on the lateral external surface. The permeate flux is an important performance criterion and may be influencedby such factors as crossflow velocity, transmembrane pressure differential, temperature, and feed characteristics. Crossflow Velocity. The crossflow velocity is the average rate at which thewater flows tangentially to the membrane surf ace. A range of 3 to 15 ft/sec isrecommended, depending on feed characteristics of viscosity, particulateloading, and other constraints imposed by pressure-drop limitations. Anincrease in crossflow velocity generally results in a flux increase. Acrossflow velocity that is too high, however, sweeps away the dynamic layerformed from pretreatment and wastes energy. Transmembrane Pressure Differential. The transmembrane pressuredifferential, is defined as pressure differential, is defined as where Pi=tube-side (feed) inlet pressure, psi; p2=tube-side (retentate) outletpressure, psi; and permeate outlet pressure, psi. In general, the flux increases with Al, up to the mass-flow-controlledregion. Beyond this point, the flux becomes independent of Al, as a result ofconcentration polarization. Temperature. The effect of temperature on permeate flux is generallysignificant, primarily because liquid viscosity decreases with increasingtemperature. Therefore, an enhanced flux can be expected for produced water astemperature increases. Permeate Flux and Permeability. The permeate flux can be determinedaccording to where u=permeate flux, gal/(ft2-D); qp--permeate flow rate, gal/D; and Am=membrane surface area, ft2. The permeability of the membrane to the permeating liquid can then becalculated by dividing u by . Operational Chemical Pretreatment. The produced water usually is chemically pretreatedbefore being processed through the membrane. The pretreated before beingprocessed through the membrane. The purpose of the pretreatment is to producediscrete solids that purpose of the pretreatment is to produce discrete solidsthat flocculate a portion of the emulsified oil and suspended solids in theproduced water. Through the flocculation process, the discrete solids producedwater. Through the flocculation process, the discrete solids deposit on themembrane surface to form a hydrophilic dynamic layer, thus preventing the oilysubstances and fine particles from penetrating the membrane to cause fouling. The chemicals used for penetrating the membrane to cause fouling. The chemicalsused for the pretreatment should be chemically compatible with the producedwater and should not add corrosiveness or scale-forming potential to thewater. Backpulsiing and Fast Flushing. During CCFM, the permeate flux is limited bythe progressive accumulation of the dynamic layer on the membrane surface. Asthe thickness of the dynamic layer increases, the permeability of the membranegradually decreases, causing either a rise in Al, or a reduction in flux.