Two Phase Separators

Abstract

Two-phase separator is also called gas-liquid separator. As its name suggests, it is used for separating gas and liquid in wet gas stream, or more generally the gas/liquid stream, when the complex liquid phase components are not required to be separated from each other.

A pressure vessel is a container designed to hold or process fluid (Liquid, gas or both) at a pressure higher than the ambient pressure. A pressure vessel is defined in ASME VIII Div. 1 Section U-1(a) (2) as a container for the containment of pressure, either internal or external. This pressure may be obtained from an external source or by applying heat from a direct or indirect source or any combination.

As previously stated, the pressure vessels discussed in this article are used for hydrocarbon fluids focusing on two-phase separation.

Oil well fluids (Wellstreams) are produced in two commingled phases (liquid and vapour). The liquid component could be free water and other liquid hydrocarbons, while the vapour component could be a mixture of hydrocarbon gasses, nitrogen, carbon dioxide etc.

Ideally, each component of the well stream should be separated, measured and transported separately. The physical separation into individual phase is the activity that is performed in gas or crude processing facilities. The separation may not be achieved in one operation or involve flowing the fluid through several separate equipment.

A typical gas compression facility has a minimum of one separator installed upstream of the gas compressor. The separator removes the liquid component of the stream before the gas flows into the compressor. If the gas entering into the compressor is wet, there will be severe damage to the compressor.

Processed gas used as equipment fuel gas should be free of liquid to prevent damage to the equipment.

Liquid hydrocarbon pumps require a gas-free liquid to operate efficiently. The presence of gas/vapour may eventually damage the pump or result in inefficient flow.

A separator is a pressure vessel designed and constructed to divide or split a commingled liquid-gas stream into the individual phase free of each other. Note that there might be some minimal entrained liquid in the separated gas.

Also, the separated liquid in two-phase separation may contain a minimal amount of free water. The water may be further removed in subsequent separation processes.

Separators may be classified based on physical appearance or the separation phases. In some cases, the two classifications listed above may be combined. Each type of separator has specific advantages, and the selection is based on a critical analysis of the vessel performing the separation process. Some of the critical selection criteria include:

These vessels are primarily used for low gas-oil ratio fluid separation. Low gas oil Wellstreams mean a larger quantity of oil and less gas. They are suitable for handling fluid with sand and other sediments without fouling compared to horizontal separators. They occupy less plot plan space, which is a significant advantage over horizontal separators when installed on offshore platforms.

For a given capacity of gas, vertical separators are usually larger and more expensive than horizontal separators because the upward flow of gas opposes the falling liquid from the inlet rather than travelling perpendicular as in horizontal separators.

Two-phase separators such as inlet separators and scrubbers are primarily vertical.

Vertical two-phase separators are more popular than vertical three-phase separators. They are very popular in gas compression facilities. The inlet stream flows into the vessel through an inlet nozzle located halfway to the vessel’s height, depending on the maximum liquid level. The inlet stream strikes a deflector installed over the nozzle; this induces separation by causing most liquid to fall to the bottom while the gas flows upward. Some other types of inlet devices will be briefly discussed in this article. Other gases that could not escape as the stream hits the deflector will escape from the liquid surface. The escaping gas flows through a mist extractor located before the outlet nozzle. The mist extractor further removes entrained liquid in the gas by collecting the liquid into larger sizes that will fall to the vessel’s liquid.

Keywords: Two Phase Separators, oil nad gas