Droplet-Settling vs. Retention-Time Theories for Sizing Oil/Water Separator

Abstract

Summary This paper discusses two techniques for sizing oil/water separators. Droplet-settling theory, based on Stokes' law governing water-droplet movementthrough a continuous oil phase, is used to develop sizing equations for bothhorizontal and vertical separators. Retention-time theory, which calculatestheoretical fluid-retention time within the separator by relating liquid flowrates vessel geometry, is also presented. Published separator data comparingvessel sizes to allowable capacities are analyzed using both sizingtechniques. Introduction The separation of oil and water phases is one of the most common and leastunderstood processes in a production facility. As fluids flow into the bottomof the wellbore, up the tubing, and through surface chokes and equipment, theoil and water are mixed thoroughly. The liquid must eventually be routed to avessel where it is separated into a continuous oil phase containing dispersedwater droplets (sometimes referred to as an emulsion) and a continuous waterphase containing dispersed oil droplets. These liquids are then routed to oil-and water-treating systems, respectively. The vessels that perform this separation are usually called three-phaseseparators when a significant amount of gas must be separated from the liquidin the same vessel, or freewater knockouts (FWKO's) when there is little or nogas. In some areas, FWKO refers to a vessel where very little gas must beseparated and the separated gas is recombined with the oil and flows out theoil outlet. Other names that describe equipment performing this initialseparation of the liquid phases are wash tanks, settling tanks, and gunbarrels. This paper discusses the validity of two different techniques(retention-time and droplet-settling theory) for choosing vessel size. Ref. 1provides a detailed description of these theories and the derivation ofappropriate equations review of the pertinent literature by the SPE ReprintSeries Committee while it was developing a volume on surface productionequipment indicated that operators have not published actual data on flowrates, liquid properties, and vessel geometries so that field experience can beused to validate these theories. In this paper, we discuss data published byvendors on the capacities of standard low-flow-rate separators. Operators andengineering companies have used these data for several decades to sizeseparators. If these data can be shown to be consistent with one of the sizingtheories described, that theory may present an appropriate scaling factor tosize separators for modem conditions of much higher flow rates. This paper does not discuss the further complicating factor of chemicaltreatment. Conceptually, some optimum economic balance between vessel size andchemical usage should exist. Research is under way to better our understandingof this phenomenon. Equipment Description Fig. 1 shows a typical horizontal three-phase separator. Fluid enters thevessel and hits an inlet diverter, where the majority of the gas is separated. The liquid falls to below an oil/water interface where the liquid is "waterwashed." The oil and its entrained water droplets flow horizontally to theoil weir, where a level controller regulates the rate at which it leaves thevessel. The water-continuous phase and oil droplets entrained in it flowhorizontally to the water outlet. The discharge rate is regulated by aninterface controller. As the oil- and water-continuous phases flow the lengthof the ves- sel, gravity forces cause the water droplets to settleperpendicular to the bulk flow in the oil-continuous phase and the oil dropletsto rise perpendicular to the bulk flow in the water-continuous phase. Similarly, liquid droplets in the gas fall perpendicular to the bulk flow ofthe gas phase. Fig. 2 shows a horizontal FWKO where very little gas is expected and the gasis recombined with the oil. The oil/water separation mechanics are identical tothose in Fig. 1. Oil/water separation can also occur in a vertical vessel, as shown in Fig.3. The liquid is routed to below the oil/water interface by a downcomer. In avertical vessel, the water droplets entrained in the oil settle countercurrentto the upward oil flow and the oil droplets entrained in the water risecountercurrent to the downward water flow. Ref. 1 describes in more detail thedifferent equipment types and operating problems; Ref. 2 presents thedroplet-settling theory as it applies to the settling of liquid droplets in thegas-continuous phase. Oil Treating vs. Water Treating It is intuitively obvious that a separator designed to treat oil will have adifferent flow pattern and internals than one designed to treat water. Is theoil/water separator essentially an oil-treating or a water-treating device?That is, besides its main function of separating the liquid into two phases, isthe quality of the oil or water outlet of overriding concern? Certainly, if theoil outlet is not treated further, as in a gun barrel and many light-oil andcondensate facilities, then the oil quality would govern. If the water is nottreated further, as in a skim vessel, then the water quality is a primaryconcern. Most oil/water separators, however, have both oil and water treatingdownstream. In such cases, we must consider the physics of the situation beforeconcluding that its function is primarily oil or water treating. The oil viscosity is normally one or two orders of magnitude greater thanthe water viscosity. Thus, an oil droplet can rise through the water much moreeasily than a water droplet can settle through the oil. In addition, experiencehas shown that the natural emulsifiers in the liquid tend to make much morestable water-in-oil emulsions than oil-in-water emulsions. The result is that aseparator properly sized to treat the oil will provide a reasonable waterquality. The water-treating system may contain additional separators designedprimarily to ensure a low oil content in the outlet water. Such devices aredescribed in Ref. 1 and are not considered in this analysis. Inlet Diverter Some form of inlet diverter is required, even in the classic FWKO shown inFig. 2. The flow into this vessel normally comes from a higher-pressure source. Gas is liberated as the liquid reaches its new pressure and temperatureconditions. If the gas is not separated by the inlet diverter and is forced torise through the liquid, it will bring water droplets with it, increasing theoutlet-oil water cut.