NO345472B1

NO345472B1 - Water and hydrocarbon separator

Info

Publication number: NO345472B1
Application number: NO20190982A
Authority: NO
Inventors: Eivind Singdahlsen
Original assignee: Stauper Offshore As
Priority date: 2019-08-14
Filing date: 2019-08-14
Publication date: 2021-02-22
Also published as: NO20190982A1

Description

WATER AND HYDROCARBON SEPARATOR

Technical Field

[0001] The present invention concerns an apparatus for separating hydrocarbons and water, in particular of the kind using a process gas for lifting oil out of the water phase from produced water associated with oil and gas production or extraction of oil from oil contaminated water.

Background Art

[0002] Separating hydrocarbons, i.e. oil and/or gas, and water is required in several applications. One typical example is separating oil from bilge water aboard a ship. Another example is separating oil and/or natural gas from well fluid produced at an offshore oil or gas field. In both examples, the input fluid typically has a high content of water. Further, the rate of input fluid may be large, and the space available for a separator tank aboard a ship or in an offshore platform may be limited and costly.

[0003] Hydro-cyclones and other fast liquid-liquid separators are known in the art and are not further discussed herein. The invention concerns a separator for an input process fluid containing mainly oil-contaminated water for example from hydro-cyclones, but will usually also contain gas.

[0004] Separators of the kind described herein use a process gas, e.g. air, N2 or CO2, to form bubbles. Oil in the input fluid attaches to the bubbles, and rises to the surface, whereas the water sinks. Gaseous hydrocarbons also form bubbles and are removed from the top of the separator tank together with process gas and oil. As the amount of dissolved gas in a liquid is proportional to the pressure above the liquid, the pressure in the output liquid is typically equal to ambient pressure to ensure that the gas is released within the separator tank. Any particles in the process fluid will also be removed in the separator.

[0005] WO 02/41965 discloses a separator tank wherein a vortex is set up within a vertical, cylindrical tank to enhance separation. More particularly, tank has a helical guide on its inner surface to create a rotational flow. The rotational flow forces the lighter component, such as oil and gas droplets, towards an inner concentric cylindrical wall where they coalesce and rise to the surface of the liquid, whereas the heavier components move radially outward and downward. Water is discharged through a water outlet in the lower part of the tank.

[0006] EP 1779 911 A1, EP 2263 768 A1 and EP 2442 881 B1 describe different varieties of a vertical cylindrical tank in which separation is enhanced by setting up at least one vortex. These varieties have a vortex breaker in the form of a disc near the water outlet in the lower part of the tank. Further needed is artificial preheating of the fluid and packing devices for obtaining maximum surface area for release of oil.

[0007] WO9965588 describes a separator tank for removing water from oil, in which process gas is added to the oil before the mixture is introduced at the bottom of a first section. Pressures are adjusted such that the gas forms bubbles rising through the fluid. The gas in the bubbles is rapidly heated by the ambient oil, so that its relative humidity decreases and water vapour is pulled from the oil. The gas and water vapour are withdrawn from the top of the container, while the oil is removed from the bottom of a second section. The first and second sections of the container are separated by a partition wall, preferably in the form of a tube. Further needed is artificial preheating of the fluid and packing devices for obtaining maximum surface area.

[0008] WO2010080035 and WO2013109345A1 provide examples of a vertical, cylindrical separator tank in which a gas, e.g. N2, is added to the input fluid, and the mixture is entered into the tank through a central pipe within the tank. The central pipe comprises branches and tangentially oriented nozzles to set up a vortex. An outlet for hydrocarbons at the top, a helical guide on its inner surface, a vortex breaker and an outlet for clean water at the bottom are also provided. Guide plates are needed and there are no devices installed for arresting or reducing the swirling fluid action or unwanted fluid oscillation in the location where hydrocarbons and gas are leaving the water phase. This greatly reduces the efficiency at each stage.

[0009] EP 2263 767 A1 describes a separator tank for separating oil and gas from water comprising a cylindrical vertical tank and inner conical segments where the mixture of liquid and gas are forced into a swirling upward motion along the outer conical walls and into the adjacent area at the cone top outlet. The separator does not provide adequate control of liquid flow in the hydrocarbon gas release zone, and the efficiency not satisfactory.

[0010] EP 1779 911 A1 discloses a separator tank, and the inventor is the same as for present application. The separator tank of EP 1779 911 A1 is compatible to the above EP 2263 767 A1 containing a single conical frustum and the separator tank may be couple to one or more similar separator tanks for improving of the overall efficiency. The separator does not provide adequate control of liquid flow in the hydrocarbon gas release zone, and the efficiency not satisfactory.

[0011] The separators above may comprise several tank segments or stages, such that the water output from one stage is the fluid input to the next stage below. Two to four stages are common, and each stage typically requires process gas. The pressure may be equal in all stages. However, it may be desirable to limit the pressure drop in each stage or tank segment to achieve a relatively slow flow within the segment, thereby increasing the amount of oil adhering to the bubbles within the segment, and hence the efficiency of the segment. A limited pressure drop at each stage may require additional stages to arrive at the desired output pressure. Further, marginal control of the liquid motion in the oil –water – gas separation zone may further reduce the efficiency by preventing optimum release of oil and gas which may require additional stages for approved cleaning. Gas – liquid mixers may also constitute a significant part in enhancing the overall efficiency of the process.

[0012] WO2017/164747, which belongs to the current proprietor and is incorporated herein by reference, solves the problems associated with the above prior art by a separation apparatus having a tank segment with a truncated conical shaped tank segment (frustum) located inside the tank segment comprising a pipe spread, securing tangentially distribution of the fluid along the inner wall of the conical tank segment towards the upper part of the conical tank segment. Ring mounted spaced guide vanes are arranged at the upper part of the conical tank segment, arresting the swirling motion of the fluid and combining the remaining motion into axial and radial motion into the space between the vanes. A hydrocarbon outlet is arranged at a tank cap at the upper part of the tank wall, a pipe for scale and debris removal is connected to the lower part of the cone segment, and a water outlet is coupled at a bottom end of the tank segment.

[0013] Further hydrocarbon/water-separators are known from e.g. US 4406789A and US 5030255 A.

[0014] Despite the fact that this separator solves problems with the previous separators by ensuring very clean water and at the same time improving the efficacy and separation rate of the separator, reducing the amount of process gas required, as well as combining the solution with bio-remediation designated limited space for offshore operations in vulnerable areas whenever overboard flow of clean water is required, there is still room for improvements.

Summary of invention

[0015] A general objective of the present invention is to solve at least one of the problems above while retaining the benefits of the solution of WO2017/164747. More particularly, the objectives of the present invention include ensuring that oil is not remixed with the clean water before the water exits the separator. This is achieved by collecting oil that is entrained by the water and carried to the lower section of the separator and remove this oil before it can re-pollute the water, or more particularly by a water and hydrocarbon separator, comprising a tank having an untreated water and hydrocarbon mixture inlet, a water outlet at a lower part of the tank, a hydrocarbon outlet at an upper part of the tank and an insert arranged to set the hydrocarbon and water mixture into a tangential laminar swirl, said insert comprising a conical side wall that has a narrowing cross section in an upward direction, a bottom wall, a quiescer arranged at the top of the insert and a spreader coupled to the water and hydrocarbon mixture inlet and arranged in a lower part of the insert, wherein the bottom wall having a conical or concave shape with its apex facing into the interior of the insert, said bottom wall forming a convex or conical cavity at its underside, and a hydrocarbon reject pipe extending into said cavity.

[0016] In a preferred embodiment, the separator comprises a debris outlet coupled to the bottom wall at a peripheral portion of said bottom wall. This will ensure that debris is collected and securely extracted from the separator.

[0017] The reject pipe may extend out of the tank through the side wall of the tank, a bottom section of the tank or an upper section of the tank, depending on the requirements.

[0018] Preferably, the spreader is located with its centre immediately above the apex of the bottom wall. This ensures optimal creation of a swirl of fluid.

[0019] An inlet tube for production fluids may extend from the water and hydrocarbon inlet to the spreader from a bottom section of the tank through the bottom wall or from an upper section of the tank through the quiescer and the interior of the insert to immediately above the bottom wall, depending on the requirements.

Brief description of drawings

[0020] The invention will now be described in more detail, referring to the accompanying single figure 1 showing a schematic cross section through the separator of the invention.

Detailed description of the invention

[0021] Figure 1 shows an apparatus for separating hydrocarbons from water. It comprises a tank 1, which forms the outer boundary of the separator. The tank 1 preferably has a cylindrical middle section 1a and rounded upper and lower end caps 1b and 1c.

[0022] A water outlet 2 is formed through the lower end cap 1c and a hydrocarbon (oil and gas) outlet 3 is formed through the upper end cap 3. Any fines in the fluid may also be entrained in the oil and gas flow out of the separator through the outlet 3, while coarser particles will migrate downwards in the separator, as explained below. An inlet 4 for untreated water/oil/gas extends into a central portion of the tank 1.

[0023] Inside the tank 1 is an insert 5. The insert 5 has a conical sidewall 6, which is widest at the bottom and narrower at the top. The bottom of the insert 5 is closed by a bottom wall 7, which has a conical or convex shape with the apex facing into the interior of the insert 5. At the top of the insert 5 is a quiescer 8, i.e. vanes, a grid, grating or other features that counteracts the swirling of the fluids as they flow out of the insert. A set of vertical vanes 17 attached to the inside of the upper part of the insert 5 also act to stop the swirling and counteract pulsation in the fluid.

[0024] At the periphery of the bottom wall 7 of the insert 5 there is a debris outlet 9 to drain debris, scale or other types of small particles that inevitably will collect at the bottom. Due to the concave or conical shape of the bottom wall 7, the debris will collect along the periphery of the bottom wall 7. To efficiently remove the debris, several debris outlets may be located around the periphery of the bottom wall 7.

[0025] The inlet 4 for an untreated mixture of water and hydrocarbons is coupled to a central inlet tube 10, which has its end close to the bottom wall 7 immediately above the apex of the bottom wall 7. At the end of the tube 10 is a spreader 11, which has at least one but preferably a multiple of arms that are directed tangentially to the conical side wall. The central tube 10 may also extend through the top of the insert to a location immediately above the bottom wall 7.

[0026] As is described in the afore mentioned WO 2017/164747, the water and hydrocarbon mixture is mixed with gas in a mixer (not shown) before entering the tube 10.

[0027] A hydrocarbon (oil and gas plus any fines) reject pipe 12 is arranged with an open end below the bottom wall 7 close to the apex of the bottom wall 7. This pipe 12 may extend out through the side of the tank 1, as shown, downwards through the bottom of the tank 1 or upwards through the insert 5.

[0028] The functioning of the separator, including debris collection and oil reject, will now be explained in detail.

[0029] The hydrocarbon and water mixture is mixed with gas before entering the inlet 4. The mixture exits the spreader 11 which sets the mixture into a tangential swirl within the conical insert 5. This swirl is both tangential and laminar. As the mixture is forced upwards inside the insert 5, as indicated by the arrows 13, the narrowing cross section of the insert 5 will increase the rotational speed of the swirl. The swirl will force particles in the flow outwards towards the conical wall 6. When reaching the wall 6, the particles will fall downwards to the bottom wall 7. Due to the concave shape of the bottom wall 7, the particles and debris will collect along the periphery of the bottom wall 7. From here the debris may be removed through the debris outlet 9. This removal may be done continuously or at intervals. A slight under-pressure may be applied to the debris outlet to facilitate the removal.

[0030] Due to the tangential laminar flow, water in the mixture will tend to collect in droplets of increasing size. When the swirling fluid reaches the top of the insert 5, the swirling is first slowed down due to the vanes 17, the fluid is forced through a quiescer, which in a preferred embodiment is a fine grating 8. This grating 8 forces the fluid into an outwardly directed radial laminar flow, as indicated by the arrows 14.

[0031] The water, which has collected into larger droplets and is heavier than the gas and oil, flows downward on the outside of the insert 5, as indicated by the arrows 15. The water flows down to the bottom of the tank 1, as indicated by the arrows 16, from where it flows out the water outlet 2.

[0032] The lighter oil and gas collect above the insert 5 and is removed through the hydrocarbon outlet 3.

[0033] Some oil and gas, and possibly fines, will be entrained by the water and forced downward in the tank 1. However, as the water settles at the bottom of the tank 1, most of the oil and gas will flow upwards until it meets the underside of the bottom wall 7. Here the oil and gas will collect due to the concave shape (as seen from below) of the bottom wall 7.

[0034] The hydrocarbon reject pipe 12 extends into the concave space of the bottom wall 7 and hence the zone where the oil collects. A slight under-pressure applied to the reject pipe 12 will suck the oil out through the reject pipe 12. This can be done continuously or at intervals.

[0035] An oil sensor on the underside of the bottom wall or at the pipe outlet 12 can be used to determine the presence of oil and hence the need to remove it.

[0036] By collecting the oil entrained with the water as described above, is prevented that this oil finds its way into the water outlet or flows back up to the top of the tank to get remixed with the water. Hence a greater purity of water exiting the separator is achieved.

Claims

1. A water and hydrocarbon separator, comprising a tank (1) having an untreated water and hydrocarbon mixture inlet (4), a water outlet (2) at a lower part of the tank (1), a hydrocarbon outlet (3) at an upper part of the tank (1) and an insert (5) arranged to set the hydrocarbon and water mixture into a tangential laminar swirl, said insert (5) comprising a conical side wall (6) that has a narrowing cross section in an upward direction, a bottom wall (7), a quiescer (8) arranged at the top of the insert (5) and a spreader (11) coupled to the water and hydrocarbon mixture inlet (4) and arranged in a lower part of the insert (5), characterised in that the bottom wall (7) having a conical or concave shape with its apex facing into the interior of the insert (5), said bottom wall (7) forming a concave or convex cavity at its underside, and a hydrocarbon reject pipe (12) extending into said cavity.

2. The separator of claim 1, characterised in that it comprises a debris outlet (9) coupled to the bottom wall (7) at a peripheral portion of said bottom wall (7).

3. The separator of claim 1 or 2, characterised in that said hydrocarbon reject pipe (12) extends out of the tank (1) through the side wall (6) of the tank (1).

4. The separator of claim 1 or 2, characterised in that said hydrocarbon reject pipe (12) extends downward and out of the tank (1) at a bottom section of the tank (1).

5. The separator of claim 1 or 2, characterised in that said hydrocarbon reject pipe (12) extends upwards through the bottom wall (7) and the insert (5) and out of the tank (1) through at an upper section of the tank (1).

6. The separator of any of the preceding claims, characterised in that the spreader (11) is located with its centre immediately above the apex of the bottom wall (7).

7. The separator of any of the preceding claims, characterised in that an inlet tube (10) extends from the water and hydrocarbon inlet (4) to the spreader (11) from a bottom section of the tank through the bottom wall (7).

8. The separator of any of the preceding claims 1 - 6, characterised in that an inlet tube (10) extends from the water and hydrocarbon inlet (4) to the spreader (11) from an upper section of the tank through the quiescer (8) and the interior of the insert (5) to immediately above the bottom wall (7).