BACKGROUND Field of the invention

[0001] The present invention concerns an apparatus for separating hydrocarbons and water, in particular of the kind using a process gas for lifting oil.

Prior and related 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 a 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 or 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 fluid containing mainly water, e.g. the water output from a hydro-cyclone.

[0004] Separators of the kind described herein use a process gas, e.g. N ₂ or C0 ₂ , 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. The pressure at the inlet may be reduced in one or more stages, and may involve pressures below atmospheric pressure to enhance bubble formation and degassing.

[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 1 779 911 Al, EP 2 263 768 Al and EP 2 442 881 B l 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.

[0007] W09965588 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 is 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.

[0008] WO2010080035 and WO2013109345A1 provide examples of a vertical, cylindrical separator tank in which a gas, e.g. N ₂ , 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.

[0009] 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.

[0010] A general objective of the present invention is to solve at least one of the problems above while retaining the benefits of prior art. More particularly, objectives of the present invention include improving the efficacy and separation rate in a prior art separator, reduce the amount of process gas required and/or reduce the cost of operation.

SUMMARY OF THE INVENTION

[0011] This is achieved by an apparatus for separating hydrocarbons from water according to claim 1. [0012] More particularly, the invention concerns an apparatus for separating hydrocarbons from water, comprising a tank segment with a fluid inlet through a cylindrical tank wall, a hydrocarbon outlet at a top end and a water outlet at a bottom end. A helical guide is attached to the inside of the tank wall below the fluid inlet, a deflector changing a radial flow to a tangential flow is mounted at the inlet and an inner cylindrical wall is attached to the helical guide. The inner cylindrical wall extends to the top end of the tank segment and comprises a perforated area along most of its axial length..

[0013] In operation, an initial fluid contains dissolved gas, including process gas supplied upstream from the inlet. The deflector at the inlet, the inner cylindrical wall and the helical guide cause the initial fluid to rotate and rise within an annular space formed by the tank wall and the inner cylindrical wall in a conventional manner. Inside the inner cylindrical wall, the fluid form a vortex. Water relieved from gas and hydrocarbons sinks along the inner surface of the inner wall. The perforated area limits the radial fluid flow from the annular space to the inner cylindrical space, such that the rotational flow in the annular space is effectively separated from the vortex in the inner cylindrical volume all the way to the top of the tank segment. The area and distribution of holes in the perforated area can provide an additional pressure drop over the perforated area, thereby increasing the release of gas in the inner cylinder without affecting the circular flow adversely.

[0014] Preferably, the perforated area comprises a inclined guide at each hole. The inclined guides ensure laminar flow through the perforated area, as opposed to numerous small vortices caused by abrupt changes in speed and/or direction of the flow through the holes.

[0015] In some embodiments, the apparatus further comprises a subsequent tank segment, wherein the fluid inlet of the second tank segment is connected to the water outlet of the previous tank segment through a channel. The subsequent tank segment is preferably similar to the tank segment discussed previously, but may, at least in principle, have a different design.

[0016] The internal pressure in each subsequent tank segment may be less than the pressure in any previous tank segment. This allows a gradual reduction of pressure, for example to or below atmospheric pressure, and may be useful to ensure efficient flow within each tank segment over a large pressure difference from first inlet to last outlet. Additional supplies of process gas at each segment are not necessarily required, as more gas is released from the fluid with every pressure drop.

[0017] In some embodiments, however, the channel has a subsequent gas inlet for process gas. Additional process gas may be particularly desirable in applications where the pressure difference between the first inlet and the last water outlet is relatively small, i.e. where a large fraction of gas is released in the first tank segment. However, additional process gas may also be supplied to subsequent tank segments for different reasons.

[0018] In a preferred embodiment, the channel is outside the cylindrical tank wall. This simplifies manufacturing and maintenance, and improves the internal flow in the segments. In particular, a pipe or channel outside the tank segment is readily available for mounting sensors, a gas inlet, for monitoring, operating valves etc. A radial outlet below the helical guide in a previous tank segment is also ideal for receiving the water in the vortex.

[0019] In some embodiments, the apparatus further comprises an annular expansion chamber covering the inlet and extending along the entire outer circumference of the tank wall. This expansion chamber automatically equalises any pressure differences from several pipes providing initial fluid, and presents the same pressure at several, e.g. 4 or 6, inlets distributed along the circumference of the tank wall.

[0020] Preferably, a mixer for mixing the process gas into the fluid is provided upstream from the inlet. The mixer can be of any kind known in the art, and ensures that the process gas is evenly distributed in the fluid entering through the inlets.

[0021] In a preferred embodiment, the fluid inlet comprises a flow regulator. The flow regulator can be of any conventional design, e.g. a rotatable throttle body as in a butterfly valve or a sliding plate for covering or uncovering a slit in the tank wall. The flow regulator adjusts the fluid flow into the apparatus, e.g. by fully closing a fraction of the inlets or by reducing or increasing the aperture through every inlet. In this manner, the apparatus can be adapted to a range of input volume rates, and of course also to varying volume rates. Sensors, controllers, actuators and control algorithms, e.g. feedback to adapt the inlets to a measured flow or feed forward to adapt the inlets in advance to receive a known change, are generally known, and may be adapted to the apparatus of the invention in a conventional manner.

[0022] In embodiments with two or more tank segments, the hydrocarbon outlets from the different tank segments may lead into a common outlet pipe. In this case, reduction valves from each tank segment to a common outlet pressure within the common pipe are required. The alternative, one outlet pipe from each segment, may be preferable in embodiments with a few tank segments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention will be described in greater detail below be means of an exemplary embodiment with reference to the accompanying drawings, in which: Figure 1 illustrates a preferred embodiment of the invention with two tank segments,

Figure 2 illustrates a perforated area,

Figure 3 illustrates the interior of a tank in the region around an inlet,

Figure 4 is a cross section along plane B-B in figure 1,

Figure 5 is a cross section along plane A-A in figure 1 and

Figure 6 illustrates a flow regulator at an inlet.

DETAILED DESCRIPTION

[0024] Figure 1 illustrates an apparatus 100 according to the invention. The apparatus 100 comprises a cylindrical tank wall 101 closed by a top cap 102 and a bottom cap 103. An input flow of initial fluid 1 contains a mixture of water, hydrocarbons and a process gas, e.g. N ₂ or C0 ₂ . The process gas is thoroughly mixed with hydrocarbons and water in a mixer 109, and fed into an annular expansion chamber 130 attached around the tank wall 101. In general, the annular expansion chamber 130 receives initial fluid 1 from one or more sources, equalizes the pressure and provide fluid with an equal pressure at one or more inlets 111 distributed around the circumference of the tank wall 101. The expansion chamber 130 preferably has a larger cross-sectional are than the feeding lines supplying input fluid. This dampens noise and undesired signals, for example pressure pulses from an upstream pump.

[0025] Fig 4 is a cross section along the plane B-B in figure 1, and illustrates that the initial flow from mixer 109 flows in both directions through the annular distribution chamber 130 and enters a first stage or tank segment through radial slits 111 through the tank wall 101. The counter currents within the expansion chamber 130 may be used for mixing, and maintain a suitable pressure difference, e.g. 0.3 bar, over the tank wall. In other words, the counter currents prevent that a fast unidirectional flow past the inlets 111 causes a pressure drop that would limit the radial flow into the inlets 111.

[0026] A deflector 1110 is mounted inside each inlet 111 to redirect the radial flow to a tangential flow along the inner face of the tank wall 101. The deflectors 1110 are attached to the tank wall 101 to ensure that the input fluid flows clockwise when view from above. This is advantageous on the northern hemisphere due to the Coriolis force and hence the natural direction of a vortex north of the equator. Accordingly, a deflector 1110 designed for the southern hemisphere would preferably guide the input fluid in the opposite direction, i.e. counter-clockwise when viewed from above. [0027] An inner cylindrical wall 113 guides the tangential flow from deflectors 1110 in the circumferential direction. The inner cylindrical wall 113 can be fluid tight in the area near the helical guide 112, and separates an annular space along the tank wall 101 from an inner cylindrical space provided for a vortex.

[0028] As shown by arrows in figure 1, during operation the input fluid rises along the inner cylindrical wall 113 toward a first perforated area 114, which extends axially to the top cap 102, in general to the top of the tank segment. Due to the pressure drop over the inlets 111, process gas and gaseous hydrocarbons start to form bubbles as the fluid rises through the annular space formed by the tank wall 101 and the inner cylindrical wall 113. The perforated area 114 may cause a further pressure drop depending on the size and number of holes in the perforated area 114.

[0029] Regardless of whether the perforation causes a further pressure drop or not, a vortex is formed in the cylindrical space within the inner cylindrical wall 113 and more gas is released from the fluid. Oil droplets attach to the bubbles formed in the annular space and the inner cylindrical space of the first tank segment 110, and leave the first stage through a first hydrocarbon outlet 115 at the top cap 102. Arrow 2 indicates a flow of hydrocarbons and process gas from the first stage or tank segment 110.

[0030] A vortex in the cylindrical volume formed by the inner wall 113 aids separation. In particular, the density of the fluid within the inner cylindrical wall 113 increases as process gas and hydrocarbons are removed. The denser fluid relieved from gas and hydrocarbons move radially outwards as it descends. The perforated area 114 limits the flow of clean water radially outward to the annular space, and thereby enhances the separation. At equilibrium, the densest fluid, i.e. the cleanest water, collects at the bottom of the tank segment 110 at the greatest distance from the rotational axis. Hence, a water outlet 116 is provided through the tank wall 101 axially between the helical guide 112 and a dividing plate 104 forming the bottom of the tank segment 110 and the top of a subsequent tank segment 120.

[0031] A channel 117 connects the water outlet 116 from the first tank segment 110 to the fluid inlet 121 of the subsequent tank segment 120. The inlet 121 is similar to the inlet 111, the inner cylindrical wall 123 with a perforated area 124 are similar to the wall 113 and perforated area 114 discussed above, etc. In particular, the second tank segment 120, and in general any subsequent tank segment are preferably designed in a similar manner with a deflector 1110, a helical guide 112, an inner cylindrical wall 123 with a perforated area 124 etc. Thus, any subsequent segment 120 can be connected in series to a previous tank segment 110, 120 by an external channel 117. [0032] The external channel 117 facilitates manufacturing, maintenance and inspection. Further, mounting and/or connecting a gas inlet 118 for an additional supply 3 of process gas, valves and other equipment (not shown) is readily performed on or in an external channel 117.

[0033] Every tank segment 110, 120 has a separate hydrocarbon outlet 125. This can be a separate pipe from each stage, or more conveniently inlets to a common pipe 140 leading through the top cap 102 as shown in figure 1. The outlet from pipe 140 illustrated by arrow 4 is a flow of process gas and hydrocarbons similar to the flow from outlet 115 shown by arrow 2. If desired, the outlet 115 from the first tank segment 110 can also be an inlet to pipe 140.

[0034] The water outlet 126 from the last tank segment is shown at the bottom of stage 120, and the last tank segment 120 is provided with a conventional vortex breaker in the form of a horizontal disc 141 at the lower end 103. The water outlet 126 might alternatively be openings through the outer wall 101 as the water outlet 116 from tank segment 110.

[0035] Tests have shown that the efficacy is greatly improved by limiting the radial flow as described above. As a result, typically more than half the hydrocarbon contained in the initial fluid leaves the separator through the first hydrocarbon outlet 115 together with process gas. In turn, this reduces the need for subsequent segments, e.g. to one subsequent tank segment 120 as shown in figure 1, whereas a typical prior art separator tank would require three or four tank segments to achieve a similar concentration of hydrocarbons in the final water flow 5.

[0036] Figure 2 is a cross section through part of a perforated area 114, 124. Each hole is provided with a inclined guide, i.e. a portion 1140 of the inner cylindrical wall that is directed down and radially out from the cylinder. The direction could alternatively be up and radially into the cylinder. The purpose of the inclined guides 1140 is to avoid abrupt changes in speed or direction of the flow velocity through the perforated area 114, 124. The inclined faces 1140 may be punched out from the inner cylindrical wall, e.g. as shown in figure 3.

[0037] Figure 3 illustrates a region around an inlet with a portion of the tank wall 101 removed. The inlet 111,121 itself is an opening through the removed part of the tank wall 101. However, the inlet 111, 121 corresponds to the radial opening into the deflector 1110. The hatching in figure 3 illustrates that the deflector 1110 and the helical guide 112, 122 below the inlet 111, 121 are normally attached to the inner face of tank wall 101. As best seen in figure 4, the deflectors 1110 do not extend radially inward to the perforated area 114. This allows a circular flow to pass the deflector 1110 with only minor disturbances. The inlets 121 to the subsequent tank segment 120 are designed in a similar manner. [0038] The inner circular wall 113, 123 is shown with a fluid tight portion 113, 123 in the region near the helical guide 112. However, the perforated area 114 may extend from the top of the tank segment 110, 120 to the helical guide 112 in the respective tank segments 110, 120.

[0039] Figure 4 is a horizontal cross section through the separator 100 along plane B-B in figure 1. The parts and reference numerals are discussed above. Four radial inlets 111 with deflectors 1110 are distributed around the circumference. In accordance with common practice, the articles 'a' , 'an' and 'the' when used in the claims mean 'at least one', whereas 'one' means exactly one. Thus, 'an inlet' stated in the claims should be construed as 'at least one inlet'. Similarly, the claims imply at least one input flow 1, at least one mixer 109 etc.

[0040] Figure 5 is a horizontal cross section through the separator 100 along plane A-A in figure 1. The cylindrical tank wall 101, second perforated area 124, pipe 140 and disc 141 are concentric, and discussed above. The channel 117 is shown as a half -pipe attached to the outer surface of the tank wall 101. Other designs, e.g. several channels and/or pipes are anticipated, and within the scope of the invention.

[0041] Figure 6 is a section through the region near an inlet 111, 121. During operation, the fluid flows through openings 111, 121 in the tank wall 101, and is deflected to a rotational flow by deflector 1110, helical guide 112, 122 etc. as described above. An axially movable plate represents a general flow regulator 119, controlling the input to the associated tank segment 110, 120. The flow regulator 119 can be of any conventional design, e.g. a rotatable throttle body such a butterfly valve within an external pipe or a sliding plate for covering or uncovering a slit in the tank wall as in figure 6. Either way, the flow regulator 119 adjusts the fluid flow into the apparatus, e.g. by fully closing a fraction of the inlets or by reducing or increasing the aperture through every inlet. In this manner, the apparatus 100 can be adapted to a range of input volume rates, and of course also to varying volume rates. Sensors, controllers, actuators and control algorithms, e.g. feedback to adapt the inlets to a measured flow or feed forward to adapt the inlets in advance to receive a known change, are generally known, and may be adapted to the apparatus of the invention in a conventional manner.

[0042] While the invention has been described with reference to specific examples and embodiments, the scope of the invention is determined by the accompanying claims.