oil and gas separators.docx

2.2.1.1 Primary Separation Section

The initial bulk separation of the liquid and gas takes place in this section; it is usually designed with inlet baffling called diverter so that when the fluid entering the separator hits the inlet diverter, it causes a sudden change in the momentum of the mixture.This sudden change in momentum creates some form of centrifugal force for vertical separators or a sudden change in direction for horizontal separators, and as a result of difference in gravity it results in the bulk separation of the gas from the oil. The gas flows to the top of the separator while the oil flows to the bottom of the separator.

Baffle plate diverters are the most common type of diverter used for horizontal separators, they come in varying shapes: flat plate, spherical dish or a cone. The centrifugal diverter is another type of diverter commonly used for vertical separators. Vertical separators are more efficient in separation, but are also more expensive. Different materials such as stainless steel have been tried for the inlet baffling device but structural channel iron has proved to be of a good choice (Francis & Richard, 1995). After the initial separation the fluid moves to the Gravity Settling section.

2.2.1.2 Gravity settling section:In this section, the gravity settling and separation takes place. Due to the large decrease in the velocity of the fluid and the difference in densities, the oil droplets settle and separate from the gas (Gravis, 1960). The gas then flows to the mist extractor section while the oil droplets flow to the liquid collection section.

2.2.1.3 Mist Extraction section:The mist extractor removes the very fine oil droplets that did not settle out from the gas stream in the gravity settling section before they are removed from the separator through a pressure control valve. The mist extractor contains coalescing elements that provides a large surface area which helps to coalesce the small water droplets as the gas pass through it (Arnold et. al, 2008).

2.2.1.4 Liquid Collection section:This section collects the oil and retains it for some time before being discharged out of the separator in order for it to attain equilibrium with the gas. This section requires sufficient retention time to allow separation of free water and oil and to allow for the removal of gas breaking out from the solution. (API, 1989).

2.3 Types of Separators

2.3.1 Three Phase Horizontal SeparatorsThree- phase horizontal separators are used in production streams of high to medium gas to liquid ratio. They are well suited for production with large volumes of gas and/or liquid where there is a constant flow and small liquid surge characteristics (Abdel-Aal &Mohamed , 2003). A block diagram of a horizontal separator is shown below in figure 2.2.

Gas out Mist Extractor Pressure control Inlet ValveInlet diverter Liquid collection Section Gas-liquid interface Water out Level control valve Oil out Level control valve

Figure 2.2 Schematic of a general horizontal three phase separator, the separation of gas from the liquid is brought about by the inlet diverter and the liquid-liquid separation is achieved by employing the difference in densities between the oil and the water and the use of thermal or electrostatic force to break the oil-water emulsion. The level of the separated water is controlled by an

interface controller which is used to operate the water discharge valve.

The horizontal separator is designed in such a way that the produced fluid from the reservoir enters into the separator from the side and hits an inlet diverter, where as a result of change in momentum and fluid densities difference, the first bulk of separation between the liquid and the gas is achieved. The gas flows horizontally to the gravity settling section which is at the top of the vessel; here the entrained liquid droplets (usually 100µm diameter and above) are

Oil Bucket

separated by gravity. The liquid droplets of smaller diameter entrained in the gas are separated out in the mist extractor before leaving the separator through the pressure relief valve. The liquid which was initially separated from the gas at the inlet diverter flows through the down comer which directs the flow to the bottom of the separator below the oil-water interface. As the liquid flows down-wards it is “water-washed” as it flows through water layers, this helps to coalesce and aid the separation of the droplets of water that are entrained and suspended in the continuous oil phase (Ken et al 1999). The primary aim of the inlet diverter is to ensure that the amount of gas carried with the liquid is reduced to the barest minimum. Its secondary function is to control the level of liquid so that it is not injected above the gas-oil interface as this will mix with the liquid entrained in the vessel causing difficulty in controlling the oil-water interface.To ensure thorough separation of oil and emulsion from the water, the separator should be designed to have a large enough volume to allow adequate time for separation. As the separation takes place in the separator, an oil pad is formed this oil pad is the oil and emulsion layer formed at the top of the water. The level of this oil pad is controlled by a weir and the water level is controlled by an interface controller which is used to operate the water discharge valve. The separated oil and emulsion flows above the weir and are collected in separate compartments, whose level is controlled by a level controller used to operate the oil discharge valve.

Horizontal separators come in two basic designs; their difference lies in the control methods used for the oil and water phases’ level.

2.3.1.1 Spill-over and weir interface controller separatorThe first subset of horizontal separators uses the spill-over and weir interface control as shown below in Figure 2.3(a)

Figure 2.3(a) Horizontal three-phase Separator with interface level control and weir (Arnold et al, 2008)

This method makes use of a vertical plate which is often called a dam plate; this allows the water to collect at the upstream of the weir. As a result of the difference in densities of oil and water and the volume of the liquid flowing in, the oil is forced over the weir into a separate compartment from where it is collected at interval, and the water is discharged through an

interface level controller valve. This method has some advantages in the sense that it is comparatively inexpensive and also has a greater utilization of retention time (retention time is the time required to adequately remove free water from the mixture of oil and water). Its major disadvantage lies in the fact that, the level controller is operated by sensing the density difference between the oil and water; once the liquids are a little emulsified the efficiency of the controller will reduce thereby allowing some oil out with the water (Leon 1977).

2.3.1.2 Oil bucket and weir plate separator

The second design uses an oil bucket and weir plate and it rules out the need for the liquid interface control as shown below in figure 2.3(b)

Figure 2.3(b) Bucket and weir type horizontal three-phase Separator (Abdel et al., 2003)

This method also makes use of a vertical weir plate with a “U” shaped oil bucket fixed in a vertical position creating a hydrostatic “U” tube effect (although both water and oil have different density and height, the pressure is made equal at this point). It uses the difference in densities of the oil and water with the “head” of the liquid. The idea is that, the head of the liquid pushes the oil over the weir into an oil bucket; the level of this bucket is controlled by a controller used to operate the oil dump valve. The separated water flows under the oil bucket and then over the water weir, the level of this weir is controlled by a level controller used to operate the water dump valve (Maurice & Arnold , 2008). This method has an advantage in that the level controls sense the difference between the liquid and the gas but its design uses too much internal baffling and gives a less effective retention time.

2.3.2 Three Phase Vertical separatorsVertical separators are used in production streams having low to intermediate gas to liquid ratio, they are best suited for productions with sand and other sediments as false cones are usually built in them to handle these sands; but in productions where excessive sand production is expected a cone is fitted at the bottom of the separator to properly handle the sand. A block diagram of a vertical separator is shown below in figure 2.4

extractor pressure control valve Gas out Gravity Settling Section

Inlet Liquid-Gas Interface Inlet diverter

Oil outlet Liquid Collection Section

Water outlet

Figure 2.4 schematic of a general vertical separator, showing how crude oil is separated from the water using a flow spreader located at the oil-water interface, the separated oil flows over a weir to the oil chamber whose level is controlled by a level controller

that operates the oil outlet valve: the separated water flows out of the separator through an outlet valve operated by an interface controller which also controls the level of water in the separator.

The vertical separator is designed in such a way that produced fluid flows into the separator from the side and hits the inlet diverter to achieve the initial separation of the gas from the liquid. The separated gas flows to the top of the separator through the gravity settling section designed to allow separation of liquid droplets of diameter greater than/equal to (100µm) from the gas, which then goes through the mist extractor to further remove the liquid droplets

Oil

of smaller sizes before leaving the separator through a pressure control valve which controls and maintains the separator pressure at a constant value (Ken et al 1999). The pressure and level controller of this section is operated as in the case of a horizontal separator.

The liquid which was separated out flows through a down comer downward and to a flow spreader situated at the oil-water interface. The oil leaving the spreader rises up to the oil pad and the water droplet entrained in the oil settles down and flows counter-current to the oil rising. The water is then collected at the collection section located at the separator’s bottom, the rising oil phase flows over a weir to the oil chamber whose level is controlled by a controller that operates the oil outlet valve, where the oil leaves the separator.

In the same way as the oil, the water leaving the spreader flows downward into the water collection section and the oil droplet entrapped in the water rises counter-currently to the water and flows into the oil pad. The water then leaves the separator through an outlet valve which is operated by an interface controller that also controls the level of the water. The gas liberated from the oil rises through the chimney provided in the separator to join the rest of the gas separated; this helps the liquid section of the separator from being over-pressurized.

Vertical separators come in three basic designs, their difference lies in the method of level control used.

2.3.2.1 Oil weir and chamberThe first design contains an oil weir and chamber as shown below in Figure 2.5a:

Figure 2.5(a) Vertical three phase Separator with oil weir and Chamber (Maurice & Arnold , 2008)

The oil weir and chamber gives a good separation between the oil and water phases as the oil leaving the separator needs to rise to the full height of the weir, but this present some problems in that; it takes up space and reduces the volume of the separator which is needed for the retention time of oil and water. This also provides space for solids and sediments to collect. (Abdel et al, 2003).

2.3.2.2 Level control separatorThe second design is on level control as shown below in figure 2.5b. This uses an oil-water-interface float controller to regulate the water outlet valve and control the level of the water in the water section, and a gas-oil displacer float controller to control the level of oil and also regulates the oil dumping valve. This type of design handles sand and solid production best, and it is the easiest to fabricate because it does not have internal baffling and weirs (Maurice& Arnold , 2008).

Figure 2.5(b) Vertical three phase Separator with interface level control (Maurice &Arnold, 2008)

2.3.2.3 External water column separatorThe third design uses an external water column fitted with adjustable weir which is connected to the water section of the separator. The water column is also connected to the gas section to maintain a balance of pressure between the separator and the water column as shown below in figure 2.5c

Water Out

Figure 2.5(c) Vertical Separator with Water leg with or without oil chamber (Maurice & Arnold , 2008)

A simple level controller which controls the water level in the separator is also used to control the height of the water in the column (Abdel-Aal & Mohamed , 2003). It removes the need to install a controller in the oil-water interface and prevents the possible problems associated with it, but the shortcomings of this type of design is that it takes additional space and extra cost.

2.3.3 Three Phase Spherical SeparatorsSpherical separators are a special type of vertical separator without a cylindrical shell between the two heads they are used for small production platforms operating at moderate pressure and are suitable for use in crowded processing areas because of their compact size and ease of transportation. These separators were initially fabricated to take the advantage theoretically, of the best features of both the vertical and horizontal separators, but in actual practice they have proven not to be efficient in separation, they are difficult to use for three phase separation due to having very limited liquid settling section and are very difficult to size and operate (Ken & Maurice, 1999). They are not often used in oilfield facilities as they are very difficult to fabricate and the liquid-level control is very critical, they also have limited separation space and liquid surge capacity (Francis et al, 1995).