WO2021080436A1

WO2021080436A1 - Separator

Info

Publication number: WO2021080436A1
Application number: PCT/NO2020/050252
Authority: WO
Inventors: Ketil Pettersen; Tor Kindsbekken KJELDBY; Ivar Øystein LARSEN
Original assignee: Equinor Energy As
Priority date: 2019-10-23
Filing date: 2020-10-14
Publication date: 2021-04-29
Also published as: GB201915368D0; GB2589558A

Abstract

A separator for separating oil and water is provided. The separator comprises an inlet for liquid comprising oil and water; outlets comprising an outlet for separated oil and an outlet for separated water; and a separator bed that extends at an angle between the inlet and the outlets. The separator bed is arranged such that, in use when liquid flows from the inlet to the outlets, the liquid flows along the separator bed under the action of gravity and separates into the separated oil and the separated water. A method of separating oil and water using the separator is also provided.

Description

SEPARATOR

The invention relates to a separator for separating oil and water and a method of separating oil and water. A number of separators for separating oil and water are known. These include for example gravity separators (also known as tank separators) and pipe separators.

A gravity separator is typically a large vessel in which separation occurs.

The fluid to be separated (i.e. comprising oil and water) is put into the large vessel and left for a period of time until the oil and water have separated by settling. Once separation has occurred the separated fluids can be removed via separate outlets. Separation using such a gravity separator can be slow and of limited efficiency.

This type of separator also results in a large volume of partly processed fluid being bound up inside the vessel. Pipe separators are long closed flow pipes typically with a circular cross section. The fluid flows through the pipe due to a pressure differential between the inlet and the outlets of the pipe separator. The length of the pipe allows the fluid some time to flow through the pipe. It can thus separate within the pipe under flowing conditions before reaching the outlets. However, pipe separators require a long closed flow pipe that may not be possible or ideal for a given application.

Thus, there is a desire for an alternative and/or improved separator for separating oil and water.

In a first aspect the present invention provides a separator for separating oil and water, the separator comprising: an inlet for liquid (i.e. a liquid mixture) comprising oil and water; outlets comprising an outlet for separated oil and an outlet for separated water; and a separator bed that extends at an angle between the inlet and the outlets, wherein the separator bed is arranged such that, in use when liquid flows from the inlet to the outlets, the liquid flows along the separator bed under the action of gravity and separates into the separated oil and the separated water. In a second aspect the present invention provides a method of separating oil and water, the method comprising: providing a liquid (i.e. a liquid mixture) comprising oil and water at an inlet; flowing the liquid comprising oil and water from the inlet over an angled separator bed under the action of gravity such that the liquid comprising oil and water is separated into separated oil and separated water. The method of the second aspect may be performed using the separator of the first aspect. The following discussion, including optional features, may be applicable to the first and/or the second aspects of the invention.

It has been realised that it is possible to have a separator in which the liquid flows along a separator bed under the action of gravity and that the separation can occur as the liquid flows over the angled bed. Compared to a horizontal separator bed the height of the liquid flowing over the bed is decreased in a separator with an angled separator bed. By reducing the liquid height the distance over which the oil and water needs to travel to separate is reduced. As a result, separation can occur more quickly.

The angled separator bed may be referred to as tilted.

The separator (i.e. volume in which separation occurs) may be for operating at or near ambient pressure. The method may be performed at or near ambient pressure. The fluids being separated may be subjected to ambient pressure or near ambient pressure. The gas above the liquid may be at or near ambient pressure. This may for example be at or near atmospheric pressure.

The separator may be an open channel system. This means that the liquid may flow through the separator with a free surface. The gas above the flow may be at rest and at ambient pressure, e.g. standard atmospheric pressure.

The separator bed may be an open channel. The volume in the channel may be in fluid communication with ambient pressure.

Flow of liquid through the separator may be based on open channel flow. Thus the liquid may flow through the separator with an upper free surface.

The separator bed may extend in a longitudinal direction between the inlet and the outlets. The length of the separator and the separator bed may be its dimension in the longitudinal direction.

The width of the separator and separator bed may be the lateral dimension that is perpendicular to the longitudinal direction. The height of the separator may be the height of the liquid flowing over the separator bed in use, i.e. the dimension in a vertical direction.

The inlet and outlets may be at opposite ends of the separator bed. The inlet may be at or near a first end of the separator bed. The outlets comprise a first outlet for separated oil and a second outlet for separated water. The first and second outlets may be at (including towards) a second end of the separator bed opposite to the first end. The first outlet may be at a location higher than the second outlet.

In use there may be no pressure drop along the longitudinal axis of the separator. This may be because the separator is an open channel. As a result there may be no closed volume in which a pressure gradient can form. The separator may be arranged so that pressure has no influence on fluid flow. The pressure gradient in the longitudinal direction may be negligible, e.g. zero.

The flow of liquid through the separator from the inlet to the outlets is free flow based on gravity. The flow of liquid may be flow caused by gravity alone. This is achieved by the separator bed being angled.

In use, the gas velocity of gas above the liquid in the separator may be zero (i.e. the gas is stationary) or near zero. The gas velocity may be negligible. This may mean that the gas does not have any or only limited effect on the movement of liquid. In this case the angle of the separator bed may be required so that the liquid flows from the inlet to outlets.

The separator bed is angled relative to the horizontal. The separator bed may be angled downwards from the inlet towards the outlets. Thus the inlet may be at a vertical height that is greater than one or both of the outlets.

The separator may comprise a separation region (this may also be referred to as a separation volume). This may be the region that contains the liquid passing between the inlet and the outlets. The separating region may be the region in which the liquid containing a mixture of oil and water separates into the separated oil and water. The bottom of the separation region may be provided/bounded by the separator bed.

The separator, separation region and/or open channel may have a non circular cross section. This may be possible because the separator is not a pressurised system.

The separator, separation region and/or open channel may have a rectangular cross section.

The separation region may be formed from a bottom plate (i.e. the separator bed) and side plates. The side plates may be extend upwards vertically from the bottom plate. The side plates may be perpendicular to the bottom plate. The bottom plate and side plates may form an open channel.

The separating region and/or separator bed may be a large and/or flat bed over which the liquid flows from the inlet to the outlets. The separator bed may be a planar surface. The separator bed may provide a large flat area that is tilted over which the liquid flows. The separator may be referred to as a flat bed separator.

The angle of the separator bed may affect the liquid height and/or the liquid velocity. At a greater angle (i.e. further from horizontal) the liquid height of the liquid flowing over the separator bed may be lower compared to if the separator bed is at a lower angle for a given volumetric flow rate.

At a greater angle the liquid velocity of the liquid flowing over the separator bed may be higher compared to if the separator bed is at a lower angle for a given volumetric flow rate.

The angle of the separator bed may be selected to optimise the separation.

The method may comprise selecting the angle of the separator bed to optimise the separation.

When the liquid height is reduced, the maximum distance that any liquid phase in a dispersion needs to travel to separate into the distinct phases is reduced. As a result, the time required for separation may reduce when the liquid height is reduced. Consequently, separation may occur more quickly when the angle of the separator bed is increased.

When the liquid velocity is increased, the amount of time the liquid takes to travel from the inlet to the outlets is decreased. This results in the liquid being into the separator for less time which means that there is less time for the dispersion to separate into the separate phases.

This means that as the angle increases the time required for separation decreases but concurrently the length of time available decreases. Thus an optimum angle (or range of angles) at which the liquid height is reduced whilst the velocity is not increased too much such that separation can occur into the separated oil and water phases over the length of the separator bed.

The angle of the separator bed may be between 1 and 10 degrees, e.g. about 3 degrees.

The angle of the separator bed may be adjustable. The angle of the bed may be adjustable in use. This may be beneficial as the optimum angle may change if the incoming liquid at the inlet changes.

The angle of the separator bed may be adjustable to control the liquid height and/or the liquid velocity of the liquid flowing through the separator. The angle of the separator bed may be chosen to optimise the liquid velocity and/or the liquid height to optimise the separation. The angle may be increased to decrease the height of the liquid (to reduce the time required for separation) and decreased to decrease the fluid velocity (to increase the time the liquid is in the separator, i.e. in the separating region.

The inlet may be higher than the outlets. The inlet may be higher than at least the lowermost outlet, e.g. the water outlet.

The height of the liquid in the separator, i.e. in the separating region/flowing over the separator bed, may be small. The liquid may flow in a thin layer over the separator bed. For example, the height of the liquid may be less than 1m, less than 0.5m or between 10 to 40cm.

By the height of the liquid being low the separation distance may be low and thus the separation time may be short. Also, by only having a small depth of liquid in the separator at any one time, the liquid may be less prone to sloshing.

Therefore, the separator may be suitable for use in an offshore location such as on a floating vessel.

The separator may be up to 50m long. For example the separator may have a length between 1 and 40m, 1 and 20m, 1 and 10m or about 5m. The length may be the distance the liquid travels from the inlet to the outlets. This may be the length of the separator bed. The length of the separator may affect the length of time the liquid is in the separator, e.g. the length may affect the time it takes for liquid to travel from the inlet to the outlets.

The separator, i.e. the separating region, may have a large cross section compared to the height of the liquid. In other words, the liquid in the separator may have a large surface area compared to its volume.

By having a small depth of liquid and/or a large cross section in the separator, it may be possible to achieve stabilisation of the oil. This may be because gases can easily escape from the oil during separation when it is in a thin layer (due to the small distances any entrained gases have to travel, due to the large surface area of the oil to facilitate release of gases and/or due to the fact that separation may be at ambient pressure). This may help facilitate future handling and/or transport of the oil.

The length and/or width of the separator may be significantly larger than the height. For example, the length and/or width may be an order of magnitude larger than the height of the liquid in the separator, i.e. the width and/or length may be at least 10 times the height. The width may be greater than the height. This may be possible because the system is not operating under pressure.

The width and/or length may each be between 10 and 50m, e.g. about 20m.

The separator may comprise a top plate. The top plate may cover the top of the separating region. This may be connected to the top of each of the side plates.

The top plate may be for collecting gases that leave the liquid as it passes through the separator/separating region. For example, the top plate may be for collecting hydrocarbon gases from the oil.

The top plate may protect the liquids in the separator. For example, it may protect hydrocarbons in the separator from oxidising.

The separator may still operate at ambient pressure and/or be based on open channel flow even when a top plate is present.

The separator bed/ separating region may be divided along its width into parallel sections (i.e. parallel channels). The width of each of the individual sections and/or the total width of the sections through which fluid flows may be greater than the height of the liquid in the separator.

By dividing the separator bed/separating region greater control of the liquid in the separator may be possible. This may for example having the liquid in narrower channels may allow sloshing to be minimised. This may for example be particularly useful if the separator is on a moving vessel.

This may allow the width of the separating region through which liquid flows to be adjustable. This is because the separator may be arranged so that it can be controlled through how many of the parallel channels the liquid flows.

Each parallel section may have a respective outlet for oil and/or an outlet for water. Alternatively, after the separation has occurred the separated oil and/or separated water from each section may be recombined before reaching a common outlet for separated oil and/or a common outlet for separated water.

The inlet may comprise a distribution system. This may be arranged to distribute the liquid across the upstream side of the separator bed. This may be useful when the width of the separator bed is greater than the width of the inlet/pipe feeding the inlet.

The width of the separator bed may affect the process volume that can be handled by the separator.

If the separator bed is sectioned into parallel channels, the distribution system may be used to control how many of the parallel channels the liquid flows through. This may mean that different process volumes can be accommodated by the separator.

The separator needs to have a sufficient width to handle the incoming volume of liquid, especially if the height of the liquid through the separator is small.

When the process volume is low (i.e. the volumetric flow rate is low) the width may be decreased. This may be to ensure that the liquid has sufficient height to avoid turbulence affecting the separation of the oil and water.

When the process volume increases (i.e. the volumetric flow rate increases) the width may be increased. This may be achieved by using more of the parallel channels. The width may be adjusted to keep the height of the liquid within an optimum range, such as within 10cm to 1m. For example the width may be increased to decrease the height for a given volume of liquid and may be decreased to increase the height for a given volume of liquid.

The liquid may have a short residual time in the separator, i.e. it may take liquid a short time to travel from the inlet to the outlets. The residual time may be less than 1hour, less than 30minutes or about 15 minutes, for example.

The liquid flowing through the separator may have a velocity that is in the range typically found in free surface channel flow. To ensure optimal oil-water separation there may exist an optimum bed angle. This angle gives the optimum balance between fluid residence time (time for gravity separation to occur) while at the same time being sufficiently high to take advantage of the separation-promoting effects that can be achieved by fluid flow and gentle oil-water interfacial shear forces.

The flow velocity along the separator bed may be greater or smaller than the flow velocity of the liquid at the inlet.

The volumetric flow rate of liquid into the inlet and liquid out of the outlets may be equal.

The oil and water, once separation has begun, may have different flow velocities. This may result in shear movements at the oil/water interface. These shear movements may help separation of an emulsion layer, e.g. at or near the interface.

The liquid may be degassed before it enters the separator. This is because the separator may not be suitable for collecting or handling any significant volume of gas. The separator may comprise a degasser. The degasser may be arranged to degas the liquid before it reaches the separator bed. The degasser may be located upstream of the inlet.

The separator may comprise a weir plate. The weir plate may be positioned so that water does not pass over the weir plate but oil does. The outlet for separated water may be upstream of the weir plate and the outlet for separated oil may be downstream of the weir plate.

The height of the weir plate may be adjustable. The weir plate and/or the oil outlet may be adjusted based on the height of the interface of the separated water and oil. This may allow control of the outlet of water and oil from the separator.

The weir plate may be used to control the total amount of liquid in the separator.

A level profiler may be used to determine the height of the interface. This information may be used to control the separator, e.g. the weir plate and/or outlets.

The outlets may be provided at a region of the separator with a greater depth. For example, the separator may comprise a sump for collecting the separated water and oil. This may provide a control volume so as to allow regulation of water and oil take-out from the separator.

The separator may comprise a baffle system. The baffle system may prevent direct flow of fluid from the inlet to the outlet along the separator bed. The baffle system may provide a restriction to fluid flow in the longitudinal direction.

The baffle system may comprise an interface baffle at the height of the water oil interface. This interface baffle may allow oil to pass over the top and water to pass underneath whilst holding back the dispersion near the interface.

The baffle system may be arranged to allow the cleanest oil to pass over the top and the cleanest water to pass underneath whilst restricting the flow of the liquid that is still a dispersed mixture of oil and water.

The separator may comprise and/or be used with a coalescer. This may be used to encourage or speed up separation of the oil and water. The coalescer may comprise plate-shaped electrodes placed horizontally on top of the oil phase, directing the electrostatic field downwards to the separator bed. The electrodes may be uninsulated or insulated such as in VI EC LW (Sulzer) or ePAC (NOV). Alternatively, the coalescer may be an in-line electrostatic coalescer that is provided at, or upstream of, the inlet.

The liquid at the inlet may be a mixture of oil and water. For example, the liquid may be a dispersion (i.e. an emulsion) e.g. a water oil or oil water dispersion. The separator may be for separating at least oil and water. The separator may allow for the separation of other liquids or gas and/or solids in addition to oil and water.

The separator may be formed from a portion of an existing storage tank.

For example, an angled separator bed may be provided to form the separator in the existing storage tank. The separator may be retrofit into an existing system by providing an angled separator bed.

Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic of a separator for separating oil and water with an angled separator bed,

Figure 2a is a schematic of another separator for separating oil and water with an angled separator bed,

Figure 2b is a plan view of the separator shown in figure 2a,

Figure 3 shows a baffle system that may be used in a separator for separating oil and water; and

Figure 4 is a schematic of an open channel system illustrating some relevant dimensions.

Figure 1 shows a separator 1 for separating oil, water and gas. The separator comprises an inlet 2 for liquid comprising a dispersion of oil and water with entrained gas, an outlet 4 for gas and an outlet 6 for liquid. By the time that the liquid reaches the liquid outlet 6 the liquid should have separated into separate oil and water phases (not shown in figure 1). The separator 1 comprises a separator bed 8.

The separator bed 8 extends between the inlet 2 and outlets 4, 6. The separator bed is angled by angle Q relative to the horizontal. The separator bed 8 is arranged so that, in use when liquid flows from the inlet 2 to the outlet 6, the liquid flows along the separator bed under the action of gravity and separates into the separated oil and separated water.

The separator 1 is based on an open channel system and operates at ambient pressure. The liquid flows from the inlet 2 to the outlet 6 under the action of gravity alone. The separator 1 has a length and/or width that are at least 10 times the height of the liquid in the separator. This means that the height of the liquid may be small compared to the volume of liquid in the separator 1.

By having a separator 1 with a small height of liquid to be separated, the separation time may be reduced. Thus the separator 1 may provide a separator that has a relatively short residual time for the liquid in the separator.

The angle Q of the separator bed 8 relative to the horizontal may be adjustable. The flow velocity and height of the liquid in the separator are affected by the angle of the separator. This means that the flow velocity and height of the liquid in the separator may be adjustable. The flow velocity may be increased and height of the liquid decreased by increasing the angle Q and the flow velocity may be decreased and the height of the liquid increased by decreasing the angle Q

The angle Q may be selected to optimise the separation of oil and water in the separator.

The box shown in figure 1 indicates a region in the separator in which steady state flow occurs.

It is known that steady state momentum equations for gas-liquid two-phase pipe flow in a closed channel at an angle Q relative to the horizontal may be described as follows:

Where phase K= g for gas and I for liquid, a_k is the volume fraction of phase k, p is the pressure, x is the abscissa along the channel longitudinal axis, r_k is the shear stress between phase k and the wall, S_k is the length of the periphery of phase k in contact with the channel, A is the channel cross sectional area, r_fc-n is the interfacial shear stress between phase k and neighbouring phase n, S_k-n is the length of the interface between phase k and n, p_k is the density of phase k and g is the gravitational acceleration.

These equations can be simplified to provide a momentum balance for open channel flow. In open channel flow there is no pressure difference along the separator and hence y. j_S zero. Also, the shear stress ^between the gas and the wall is zero (because it is open channel flow) and the interfacial shear stress

and T_L-g can be approximated as zero because the gas above the liquid is close to stationary and only put into weak motion by the gas flashing from the flowing liquid. In other words, due to the close to stationary gas as well as low velocities of the liquid, the interfacial shear stress can also be assumed to be negligible.

As a result the steady state momentum equations for liquid flow in an open channel at an angle Q can be simplified as follows:

1.2

This can be rearranged to:

1.3

For liquid in a rectangular open channel as shown in figure 4 with width b and liquid height h, the area of liquid A_t will be b x h and S_L (the length of the periphery of liquid I in contact with the channel will be b+2h.

The volume fraction of liquid

multiplied by the channel cross sectional area, A is equal to the area of liquid A_L.

Putting this into the above equation gives: i(b + 2 h) = bhpigsinO 1.4 A liquid-wall friction factor can be estimated by a typical wall friction model from the literature. . Once a given bed angle has been specified , Equation 1.4 can be used to estimate film height and velocity of the free falling liquid film.

Figure 2a and 2b show a separator 10 that works on the same principles as separator 1 described above. The separator has an inlet 12 for liquid comprising oil and water, an outlet for oil 14, an outlet for water 16 and an angled separator bed 18.

The separator bed 18 in this case is split into a plurality (in this case six) parallel channels 19 (only one channel is labelled for clarity). These channels 19 each extend between the inlet 12 and outlets 14, 16 of the separator 10. The separation of the separator bed 18 into these channels 19 means that better control over the liquid flowing through the separator 10 can be achieved.

The capacity of the separator 10 may be adjusted by using more of the channels 19 when more capacity is required and using fewer of the channels 19 when less capacity is required (i.e. when the volumetric flow rate is lower).

The inlet 12 may comprise a distributor that can be used to distribute liquid across the separator bed 18. This may be used to control which of the channels 19 the liquid flows through. The separator 10 comprises a sump 20 for collecting the separated water and oil. This may provide a control volume so as to allow regulation of water and oil take-out from the separator 10 through the outlets 14 and 16.

The separators 1, 10 may be used with a baffle system 22 as illustrated schematically in figure 3. The baffle system 22 may prevent direct flow of fluid from the inlet to the outlets along the separator bed and this is achieved by the baffle system 22 providing a restriction to fluid flow in the longitudinal direction.

The baffle system 22 comprises an interface baffle 24 at the height of the water oil interface. This interface baffle 24 allows oil to pass over the top and water to pass underneath whilst holding back the dispersion near the interface. Although not shown the separators 1, 10 may be used with other components such as a degasser and/or a coalescer.

Claims

CLAIMS:

1. A separator for separating oil and water, the separator comprising: an inlet for liquid comprising oil and water; outlets comprising an outlet for separated oil and an outlet for separated water; and a separator bed that extends at an angle between the inlet and the outlets, wherein the separator bed is arranged such that, in use when liquid flows from the inlet to the outlets, the liquid flows along the separator bed under the action of gravity and separates into the separated oil and the separated water.

2. A separator according to claim 1 , wherein the separator is for operating at or near ambient pressure.

3. A separator according to claim 1 or 2, wherein the separator bed is an open channel.

4. A separator according to claim 1 , 2 or 3, wherein the separator is arranged so that in use, the flow of liquid through the separator is flow caused by gravity alone.

5. A separator according to any preceding claim, wherein the angle of the separator bed is adjustable.

6. A separator according to any preceding claim, wherein the length and/or width of the separator is at least 10 times larger than the height of the liquid in the separator in use

7. A separator according to any preceding claim, wherein the separator bed is divided along its width into parallel sections.

8. A method of separating oil and water, the method comprising: providing a liquid comprising oil and water at an inlet; flowing the liquid comprising oil and water from the inlet over an angled separator bed under the action of gravity such that the liquid comprising oil and water is separated into separated oil and separated water.

9. A method according to claim 8, wherein the method is performed at or near ambient pressure.

10. A method according to claim 8 or 9, wherein the flow of liquid over the angled separator bed is flow caused by gravity alone.

11. A method according to claim 8, 9, or 10, wherein the method comprises selecting the angle of the separator bed to optimise the separation.

12. The method according to any of claims 8 to 11, wherein the height of the liquid flowing over the separator bed is less than 1m.

13. The method according to any of claims 8 to 12, wherein the length and/or width of the separator is at least 10 times larger than the height of the liquid in the separator.

14. A method according to any of claims 8 to 13, wherein the method comprises using the separator of any of claims 1 to 7.