TABLE OF CONTENTS This chapter discusses the concepts, theory, and sizing equations for the separation of two immiscible liquid phases (in this case, those liquids are normally crude oil and produced water). The separator design concepts presented in Chapter 4 relate to the two-phase separation of liquid and gas and are applicable to the separation of gas that takes place in three-phase separators, gas scrubbers, and any other device in which gas is separated from a liquid phase.
When oil and water are mixed with some intensity and then allowed to settle, a layer of relatively clean free water will appear at the bottom. The growth of this water layer with time will follow a curve as shown in Figure 5-1. After a period of time, ranging anywhere from 3 minutes to 30 minutes, the change in the water height will be negligible. The water fraction, obtained from gravity settling, is called "free water." It is normally beneficial to separate the free water before attempting to treat the remaining oil and emulsion layers.
"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 (1) a producing well or (2) 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...
