Regardless of the size or shape of a separator, each gas-liquid separator contains four major sections.
Figures 2-7 and 2-8 illustrate the four major sections of a horizontal and vertical two-phase separator, respectively.
Inlet Diverter Section
The inlet stream to the separator is typically a high-velocity turbulent mixture of gas and liquid. Due to the high velocity, the fluids enter the separator with a high momentum. Collision or abruptly changes the direction of flow by absorbing the momentum of the liquid and allowing the liquid and gas to separate.
This results in the initial “gross” separation of liquid and gas. The inlet diverter, sometimes referred to as the primary separation section. Therefor this section is used to reduce the momentum of the inlet flow stream, perform an initial bulk separate ion of the gas and liquid phases, and enhance gas flow distribution. There are varieties of inlet devices available and these will be discussed in more detail in a later section.
Liquid Collection Section
The liquid collection section, located at the bottom of the vessel, it acts as a receiver for all liquid removed from the gas in the inlet, gas gravity, and mist extraction sections. The liquid collection section provides the required retention time necessary for any entrained gas in the liquid to escape to the gravity settling section. In addition, it provides a surge volume to handle intermittent slugs.
In three-phase separation applications, the liquid gravity section also provides residence time to allow for separation of water droplets from a lighter hydrocarbon liquid phase and vice-versa. Due to the smaller difference in gravity between crude oil and water, compared to gas and liquid in two-phase separation, Liquid-liquid separation requires longer retention times than gas-liquid separation.
Also in in three phase separators, a coalescing packs are sometimes used to promote hydrocarbon liquid – water separation, though they should not be used in applications that are prone to plugging, e.g. wax, sand, etc.
Gravity Settling Section
As the gas stream enters the gravity settling section, its velocity drops and small liquid droplets that were entrained in the gas and not separated by the inlet diverter are separated out by gravity and fall to the gas liquid interface, preconditioning the gas for final polishing by the mist extractor.
The gravity settling section is sized so that liquid droplets greater than 100 to 140 microns fall to the gas-liquid interface while smaller liquid droplets remain with the gas. Liquid droplets greater than 100 to 140 microns are undesirable as they can overload the mist extractor at the separator outlet.
In some horizontal designs, straightening vanes are used to reduce turbulence. The vanes also act as droplet coalescers, which reduces the horizontal length required for droplet removal from the gas stream.
Mist Extractor Section
Gas leaving the gravity settling section contains small liquid droplets, generally less than 100 to 140 microns. Before the gas leaves the vessel, it passes through a coalescing section or mist extractor. This section uses coalescing elements that provide a large amount of surface area used to coalesce and remove the small droplets of liquid. As the gas flows through the coalescing elements, it must make numerous directional changes. Due to their greater mass, the liquid droplets cannot follow the rapid changes in direction of flow. These droplets impinge and collect on the coalescing elements, where they fall to the liquid collection section. Quoted liquid carryover from the various types of mist extraction devices are usually in the range of 0.1 – 1 gal/MMscf.
