Produced wellhead fluids are complex mixtures of different compounds of hydrogen and carbon, all with different densities, vapor pressures, and other physical characteristics. As a well stream flows from the hot, high-pressure petroleum reservoir, it experiences pressure and temperature reductions. Gases evolve from the liquids and the well streamchanges in character. The velocity of the gas carries liquid droplets, and the liquid carries gas bubbles. The physical separation of these phases is one of the basic operations in the production, processing, and treatment of oil and gas. In oil and gas separator design, we mechanically separate from a hydrocarbon stream the liquid and gas components that exist at a specific temperature and pressure. Proper separator design is important because a separation vessel is normally the initial processing vessel in any facility, and improper design of this process component can “bottleneck” and reduce the capacity of the entire facility.
Separators are classified as “two-phase” if they separate gas from the total liquid stream and “three-phase” if they also separate the liquid stream into its crude oil and water components. This chapter deals with two-phase separators. In addition, it discusses the requirements of good separation design and how various mechanical devices take advantage of the physical forces in the produced stream to achieve good separation. Separators are sometimes called “gas scrubbers” when the ratio of gas rate to liquid rate is very high. Some operators use the term “traps” to designate separators that handle flow directly from wells. In any case, they all have the same configuration and are sized in accordance with the same procedure.
FACTORS AFFECTING SEPARATION
Characteristics of the flow stream will greatly affect the design and operation of a separator. The following factors must be determined before separator design:
• Gas and liquid flow rates (minimum, average, and peak)
• Operating and design pressures and temperatures
• Surging or slugging tendencies of the feed streams
• Physical properties of the fluids such as density and compressibility
•Designed degree of separation (e.g., removing 100% of particles greater than 10 microns)
• Presence of impurities (paraffin, sand, scale, etc.)
• Foaming tendencies of the crude oil
• Corrosive tendencies of the liquids or gas