Process of Separation:
Momentum. Fluid phases with different densities will have different momentum. If a two phase stream changes direction sharply, greater momentum will not allow the particles of the heavier phase to turn as rapidly as the lighter fluid, so separation occurs. Momentum is usually employed for bulk separation of the two phases in a stream.
Gravity Settling. Liquid droplets will settle out of a gas phase if the gravitational force acting on the droplet is greater than the drag force of the gas flowing around the droplet. These forces can be described mathematically using the terminal or free settling velocity.
Very small droplets such as fog or mist cannot be separated practically by gravity. These droplets can be coalesced to form larger droplets that will settle by gravity. Coalescing. Coalescing devices in separators force gas to follow a tortuous path. The momentum of the droplets causes them to collide with other droplets or the coalescing device, forming larger droplets. These larger droplets can then settle out of the gas phase by gravity. Wire mesh screens, vane elements, and filter cartridges are typical examples of coalescing devices.
Filter Separators: A filter separator usually has two compartments.The first compartment contains filter-coalescing elements. As the gas flows through the elements, the liquid particles coalesce into larger droplets and when the droplets reach sufficient size, the gas flow causes them to flow out of the filter elements into the center core. The particles are then carried into the second compartment of the vessel (containing a vane-type or knitted wire mesh mist extractor) where the larger droplets are removed. A lower barrel or boot may be used for surge or storage of the removed liquid.
Flash Tank: A vessel used to separate the gas evolved from liquid flashed from a higher pressure to a lower pressure.
Line Drip: Typically used in pipelines with very high gas to liquid ratios to remove only free liquid from a gas stream, and not necessarily all the liquid. Line drips provide a place for free liquids to separate and accumulate.
Liquid-Liquid Separators: Two immiscible liquid phases can be separated using the same principles as for gas and liquid separators. Liquid-liquid separators are fundamentally the same as gas-liquid separators except that they must be designed for much lower velocities. Because the difference in density between two liquids is less than between gas and liquid, separation is more difficult.
Scrubber or Knockout: A vessel designed to handle streams with high gas-to-liquid ratios. The liquid is generally entrained as mist in the gas or is free-flowing along the pipe wall. These vessels usually have a small liquid collection section. The terms are often used interchangeably.
Separator: A vessel used to separate a mixed-phase stream into gas and liquid phases that are "relatively" free of each other. Other terms used are scrubbers, knockouts, line-drips, and decanters.
Slug Catcher: A particular separator design able to absorb sustained in-flow of large liquid volumes at irregular intervals. Usually found on gas gathering systems or other two phase pipeline systems. A slug catcher may be a single large vessel or a manifolded system of pipes.
Three Phase Separator: A vessel used to separate gas and two immiscible liquids of different densities (e.g. gas, water, and oil).
Parts of a Separator: Regardless of shape, separation vessels usually contain four major sections, plus the necessary controls. These sections are shown for horizontal and vertical vessels in Fig. The primary separation section, A, is used to separate the main portion of free liquid in the inlet stream. It contains the inlet nozzle which may be tangential, or a diverter baffle to take
advantage of the inertial effects of centrifugal force or an abrupt change of direction to separate the major portion of the liquid from the gas stream.The secondary or gravity section, B, is designed to utilizethe force of gravity to enhance separation of entrained droplets.It consists of a portion of the vessel through which the gasmoves at a relatively low velocity with little turbulence. In some designs, straightening vanes are used to reduce turbulence. The vanes also act as droplet collectors, and reduce the distance a droplet must fall to be removed from the gas stream. The coalescing section, C, utilizes a coalescer or mist extractor which can consist of a series of vanes, a knitted wire mesh pad, or cyclonic passages. This section removes the very small droplets of liquid from the gas by impingement on a surface where they coalesce. A typical liquid carryover from the mist
extractor is less than 0.1 gallon per MMscf. The sump or liquid collection section, D, acts as receiver for all liquid removed from the gas in the primary, secondary, and coalescing sections. Depending on requirements, the liquid section should have a certain amount of surge volume, for degassing or slug catching, over a minimum liquid level necessary for controls to function properly. Degassing may require a horizontal separator with a shallow liquid level while emulsion separation may also require higher temperature, higher liquid level, and/or the addition of a surfactant.
extractor is less than 0.1 gallon per MMscf. The sump or liquid collection section, D, acts as receiver for all liquid removed from the gas in the primary, secondary, and coalescing sections. Depending on requirements, the liquid section should have a certain amount of surge volume, for degassing or slug catching, over a minimum liquid level necessary for controls to function properly. Degassing may require a horizontal separator with a shallow liquid level while emulsion separation may also require higher temperature, higher liquid level, and/or the addition of a surfactant.
Separator Configurations :
Factors to be considered for separator configuration selection include:
· How well will extraneous material (e.g. sand, mud, corrosion products) be handled?
· How much plot space will be required?
· Will the separator be too tall for transport if skidded?
· Is there enough interface surface for three-phase separation (e.g. gas/hydrocarbon/glycol liquid)?
· Can heating coils or sand jets be incorporated if required?
· How much surface area is available for degassing of separated liquid?
· Must surges in liquid flow be handled without large changes in level?
· Is large liquid retention volume necessary?
Factors to be considered for separator configuration selection include:
· How well will extraneous material (e.g. sand, mud, corrosion products) be handled?
· How much plot space will be required?
· Will the separator be too tall for transport if skidded?
· Is there enough interface surface for three-phase separation (e.g. gas/hydrocarbon/glycol liquid)?
· Can heating coils or sand jets be incorporated if required?
· How much surface area is available for degassing of separated liquid?
· Must surges in liquid flow be handled without large changes in level?
· Is large liquid retention volume necessary?
Vertical Separators :
Vertical separators, are usually selected when the gas-liquid ratio is high or total gas volumes are low. In the vertical separator, the fluids enter the vessel striking a diverting baffle which initiates primary separation. Liquid removed by the inlet baffle falls to the bottom of the vessel. The gas moves upward, usually passing through a mist extractor to remove suspended mist, and then the "dry" gas flows out. Liquid removed by the mist extractor is coalesced into larger droplets which then fall through the gas to the liquid reservoir in the bottom. The ability to handle liquid slugs is typically obtained by increasing height. Level control is not critical and liquid level can fluctuate several inches without affecting operating efficiency. Mist extractors can significantly reduce the required diameter of vertical separators. As an example of a vertical separator, consider a compressor suction scrubber. In this service the vertical separator: · Does not need significant liquid retention volume.
Vertical separators, are usually selected when the gas-liquid ratio is high or total gas volumes are low. In the vertical separator, the fluids enter the vessel striking a diverting baffle which initiates primary separation. Liquid removed by the inlet baffle falls to the bottom of the vessel. The gas moves upward, usually passing through a mist extractor to remove suspended mist, and then the "dry" gas flows out. Liquid removed by the mist extractor is coalesced into larger droplets which then fall through the gas to the liquid reservoir in the bottom. The ability to handle liquid slugs is typically obtained by increasing height. Level control is not critical and liquid level can fluctuate several inches without affecting operating efficiency. Mist extractors can significantly reduce the required diameter of vertical separators. As an example of a vertical separator, consider a compressor suction scrubber. In this service the vertical separator: · Does not need significant liquid retention volume.
Horizontal Separators :
Horizontal separators are most efficient where large volumes of total fluids and large amounts of dissolved gas are present with the liquid. The greater liquid surface area in this configuration provides optimum conditions for releasing entrapped gas. In the horizontal separator,the liquid which has been separated from the gas moves along the bottom of the vessel to the liquid outlet. The gas and liquid occupy their proportionate shares of shell cross-section. Increased slug capacity is obtained through shortened retention time and increased liquid level.Fig also illustrates the separation of two liquid phases (glycol and hydrocarbon). The denser glycol settles to the bottom and is withdrawn through the "boot." The glycol level is controlled by a conventional level control instrument. In a double barrel separator, the liquids fall through connecting flow pipes into the external liquid reservoir below. Slightly smaller vessels may be possible with the double barrel horizontal separator where surge capacity establishes the size of the lower liquid collection chamber.
Horizontal separators are most efficient where large volumes of total fluids and large amounts of dissolved gas are present with the liquid. The greater liquid surface area in this configuration provides optimum conditions for releasing entrapped gas. In the horizontal separator,the liquid which has been separated from the gas moves along the bottom of the vessel to the liquid outlet. The gas and liquid occupy their proportionate shares of shell cross-section. Increased slug capacity is obtained through shortened retention time and increased liquid level.Fig also illustrates the separation of two liquid phases (glycol and hydrocarbon). The denser glycol settles to the bottom and is withdrawn through the "boot." The glycol level is controlled by a conventional level control instrument. In a double barrel separator, the liquids fall through connecting flow pipes into the external liquid reservoir below. Slightly smaller vessels may be possible with the double barrel horizontal separator where surge capacity establishes the size of the lower liquid collection chamber.
As an example of a horizontal separator consider a rich amine flash tank. In this service:
· There is relatively large liquid surge volume leading to longer retention time (this allows more complete release of the dissolved gas and, if necessary, surge volume for the circulating system).
· There is more surface area per liquid volume to aid in more complete degassing.
· The horizontal configuration would handle a foaming liquid better than a vertical.
· The liquid level responds slowly to changes in liquid inventory.
· There is relatively large liquid surge volume leading to longer retention time (this allows more complete release of the dissolved gas and, if necessary, surge volume for the circulating system).
· There is more surface area per liquid volume to aid in more complete degassing.
· The horizontal configuration would handle a foaming liquid better than a vertical.
· The liquid level responds slowly to changes in liquid inventory.
Spherical Separators : These separators are occasionally used for high pressure service where compact size is desired and liquid volumes are small. Factors considered for a spherical separator are:
· compactness;
· limited liquid surge capacity;
· minimum steel for a given pressure.
· limited liquid surge capacity;
· minimum steel for a given pressure.
