Chapter 4 - Three-Phase Oil and Water Separators

Abstract

This chapter discusses the concepts, theory, and sizing equations for the separation of two immiscible liquid phases, normally crude oil and produced water. Three-phase separator and free-water knockout are terms used to describe pressure vessels that are designed to separate and remove the free water from a mixture of crude oil and water. Because flow normally enters these vessels directly from either a producing well or a separator operating at a higher pressure, the vessel must be designed to separate the gas that flashes from the liquid, as well as separate the oil and water.The term three-phase separator is normally used when there is a large amount of gas to be separated from the liquid, and the dimensions of the vessel are determined by the gas capacity equations. Three-phase separators are designed as either horizontal or vertical pressure vessels. In most designs, the inlet diverter contains a down-comer that directs the liquid flow below the oil–water interface. In vertical separator, flow enters the vessel through the side as in the horizontal separator. The inlet diverter separates the bulk of the gas. Horizontal separators have greater interface areas, which enhances phase equilibrium. This is especially true if foam or emulsion collect at the gas–oil interface. Thus, from a process perspective, horizontal vessels are preferred. However, they do have several drawbacks, which could lead to a preference for a vertical vessel in certain situations. In three-phase vertical separators, a minimum diameter must be maintained to allow liquid droplets to separate from the vertically moving gas.