GB2198054A - Separating gasses from liquids - Google Patents

Abstract

A liquid phase (e.g. hydrocarbons containing associated gasses) is introduced into an initial stage expansion chamber (A) from the top of which the gas and from the bottom of which the liquid separated off are extracted. The liquid from the initial stage (A) is fed to a second stage expansion chamber (B) at a pressure lower than that of the initial expansion chamber, and the gas separated off from the second expansion chamber is compressed in a compressor (K1) and admixed and stabilised, preferably in an ejector (E1), with the liquid phase introduced into the initial stage (A). The gaseous phase which is extracted from the initial stage is preferably fed to a drying device (T) prior to being compressed in a compressor (K3), following which the compressed gas is conveyed through a heat exchanger (V) for heating of the liquid phase introduced into the initial expansion chamber (A). <IMAGE>

Description

A METHOD AND DEVICE FOR STABILISING A LIQUID PHASE The present invention concerns a method for the stabilisation, that is, de-gasification, of a liquid phase such as a carbonaceous oil phase, and a device for implementing the method.

To stabilise oil in stages is known technology which is usually carried out by means of horizontal or vertical containers. The gas outlets from each stage typically are provided with compressors, externally cooled gas coolers and scrubbers for separating the condensate. Known installations require comparatively large amounts of space and considerable cooling capacity, and thus need much energy in order to provide the required cooling. Furthermore, such installations frequently require the liquid phase to be heated, something which entails the need for an external heat source.

It is further known in the art to stabilise oil in a one-stage absorption tower in which the oil to be stabilised is fed counter-current to a stripping gas from a boiler at the bottom of the tower. Such a method requires a minimum of space but needs much heat from an external source.

It is an object of the present invention to provide an apparatus with a reduced need for space and cooling/heating and wherein the number of components is also substantially reduced, especially when compared with step-by-step stabilising, and reduced holding periods are required, leading to smaller equipment dimensions and lower weight which again reduce the investment necessary for the actual processing equipment and any supporting structure.

According to one aspect of the invention, there is provided a method for the stabilisation of a liquid phase by the separation of associated gases, comprising: introducing the liquid phase and associated gas into a first stage expansion chamber and carrying out a first separation; extracting the separated gas from the top of the first chamber and the separated liquid from the bottom of the first chamber; feeding the separated liquid from the first chamber to a second stage expansion chamber and carrying out a second separation at a pressure lower than that in the first chamber; extracting and compressing the gas separated out in the second chamber and mixing it with the liquid phase introduced into the first chamber.

According to a second aspect of the invention, there is provided a device for the stabilisation of a liquid phase by the separation of associated gases, comprising: at least two expansion chambers connected in series, each having an inlet for a liquid phase, an outlet for a gas phase and an outlet for a liquid phase separated off; means for compressing the gas phase separated off in the second and each subsequent expansion chamber; means for mixing the separated and compressed gas from the second and each subsequent chamber with the liquid phase fed into the previous chamber.

The invention is particularly suited for the offshore production of oil and gas where production plants are located in a submerged vessel, where oil is exported via pipelines to the shore or to loading buoys, and where the gas is compressed for export or injected back into the reservoir.

Gas separated off from the first chamber is conveyed from the system, after having been dried. The liquid phase from the second expansion chamber may be led away from the system or into one or several additional expansion chambers as required. The separation of liquid and gas and the stabilising of the liquid fraction will thus be achieved through the reduction in stages of the pressure.The liquid fraction from a previous separation step may conveniently be employed as the pressure-carrying medium in a mixing device, preferably an ejector, in which gas from a subsequent separation step is compressed up to the step in question and in which the liquid and the gas fraction are then mixed in a "static mixer", providing approximate conditions of equilibrium and following which the gaseous and liquid phases are coarsely divided in a tube with enforced rotational movement by means of vanes placed inside the tube. The liquid stays for a certain holding time in the expansion chamber in order to avoid unnecessarily large amounts of gas being carried with the liquid phase to the subsequent pressure reducing stage.

In a further compression stage for export or injection to a reservoir and where gas drying is desirable, the gaseous phase acquired may be fed through a cooler which provides a specific hydrocarbon dew point, and then through an absorption dryer which provides a specified water dew point. After compression, part of the gas stream may be fed back to the adsorption dryer for regeneration of the adsorption medium and thereafter made to join the gas intake of the mixing device or ejector of the first stage. The remaining gas stream may be heat-exchanged with the liquid stream to, preferably, the first expansion stage, following which the gas, having been cooled as necessary, is then exported or injected to the reservoir.

Embodiments of the method and the device of the invention will be described below by way of example, with reference to the attached drawings, in which:- Fig.1 shows a flow chart in outline according to which the method is carried out, and Fig.2 shows an example of an embodiment of a device according to the invention.

Shown in outline in Fig.1 are three separation chambers A, B, C, arranged in a cascade in which a well stream is fed into the expansion chamber A through a pipeline Al, and the gas separated off is let out of the expansion chamber A through the pipeline A2. The liquid separated off in chamber A is let out and conveyed to chamber B via the pipelines B1 and B2. The gas separated off in the expansion chamber B is conveyed through the pipeline B3, inserted into which is a compressor K7 to a mixing device which is preferably an ejector El where the compressed gas which was separated off in the expansion chamber B is introduced and admixed with the well stream which is fed via the pipeline Al into the expansion chamber A.

The liquid separated off in the expansion chamber B is conveyed via the pipeline C1, either away from the cascade or preferably to the expansion chamber C where the gas separated off is conveyed via the pipelines C2 and the compressor K2 in a compressed state into a second mixing device or ejector E2 where it is admixed with the fraction which is let into the expansion chamber B. The gaseous phase which is extracted from the expansion chamber A is conveyed through the pipeline A and preferably through a drying device T and on through a compressor K3.The compressed gaseous phase from the compressor K3 may, if so desired, be split into two streams one of which is used to regenerate the drying medium in the drying device T and the liquid/gaseous phase thereby obtained may be fed by means of ejector El into the well stream which is being let through the pipeline Al to the expansion chamber A.

The remaining part of the stream of the compressed gaseous phase may thereafter be conveyed away from the system, but it is preferably first conveyed through a heat exchanger in order to heat the liquid phase which is fed into one of the expansion chambers, preferably to heat the well stream which is fed into the expansion chamber A. In Fig.1 is shown the compressed gaseous phase fed through a heat exchanger V for heating of the well stream which is being conveyed via the pipeline Al into the expansion chamber A.

Because of the fact that the well stream which is being fed into the expansion chamber A is heated further by means of the heat exchanger V, a greater part of the gaseous phase will be driven off in the expansion chamber A, thus improving the efficiency. If so desired, the compressed gaseous phase may be used to heat the liquid phase which is fed into a subsequent expansion chamber. For example, the heat exchanger V might be inserted into the pipeline B1 or C1.

As will be realised, gas which is separated off will be conveyed up through the cascade while any liquid phase separated off will be conveyed down through it, and in this way the heat energy in the well stream which is let into the cascade will be utilised to its maximum, and this is the reason why the amount of energy required when using the present method will be substantially smaller than in previously known stepby-step stabilisation processes.

A device for implementing the method will be described with reference to Fig.2 in which a well stream and gas are conducted through a spout (2) into an ejector (1) of first stage 19, where the mixture in the ejector nozzle (3) entrains gas through a spout (4) which has been sucked by means of a compressor (5) from lower stage (18) through a cyclone (7).

Further admixing of gas and liquid takes place in a mixing tube (8) in which an approximate equilibrium between gas and liquid is obtained, whereupon the mixture is conducted into the cyclone tube (9) where a coarse separation is achieved by subjecting the mixture to a rotary movement by means of vanes in the tube so that the liquid is separated off in a column (10), deflected downwards in a first stage by means of a baffle (11) and given a holding time in the liquid chamber of about 30 seconds, while gas continues through the tube (12) and is conducted away from the separator unit through a spout (13).

From the spout (13) in the first stage (19) of the separator unit the gas is conveyed through a cooler (14) which takes care of the lowering of the hydrocarbon dew point, a cyclone (22) which separates off free liquid, and on to an adsorption unit (15) which takes care of the lowering of the water dew point, after which the gas is compressed in the compressor (16). Part of the hot outlet stream from the compressor (16) is returned to the adsorption unit (15) in order to regenerate the adsorbent, after which it is returned to the first stage (19) of the separator so that the regeneration heat is returned to the separation process. The remaining gas is then heatexchanged in the tube exchanger (17) with the liquid stream to the first stage (19) of the separator so that the compressor heat from the compressor (16) is returned to the separator process.

Liquid from the first stage 19 is conducted to ejector 11 where it is mixed with gas fed to the ejector 11 by compressor 51 which sucks gas from a lower stage 20 below stage 18. Further admixing of gas and liquid takes place in mixing tube 81 and the mixture is then conducted to cyclone tube in stage 18 which is similar to cyclone tube 9 in stage 19. As above described, gas separated off in stage 18 is fed to ejector 1 via cyclone 7 and compressor 5. The liquid separated off in stage 18 is then transferred to the final stage 20 via loop 30, where it is maintained at a pressure corresponding to the specification for the boiling point for exported oil.

As previously mentioned, heating the well stream which is being let into the cascade with the gas which leaves the compressor (16), in the way shown in the device in Fig.2, is considered the best alternative. The heat from the gas which is given out by the compressor (16) may, however, be used for heating the liquid phase further down the cascade.

Claims (14)

CLAIMS:

1. Method for the stabilisation of a liquid phase by the separation of associated gases, comprising: introducing the liquid phase and associated gas into a first stage expansion chamber and carrying out a first separation; extracting the separated gas from the top of the first chamber and the separated liquid from the bottom of the first chamber; feeding the separated liquid from the first chamber to a second stage expansion chamber and carrying out a second separation at a pressure lower than that in the first chamber; extracting and compressing the gas separated out in the second chamber and mixing it with the liquid phase introduced into the first chamber.

2. A method according to claim 1 and further comprising: feeding the liquid separated in the second chamber to at least one further expansion chamber arranged in cascade with the first and second chambers; extracting and compressing the gas separated in each further chamber and mixing it with the liquid phase introduced into the previous chamber.

3. A method according to claims 1 or 2 further comprising: drying the gaseous phase in a drying device after it leaves the first stage and before it is compressed.

4. A method according to claim 3, wherein a first part of the compressed gaseous phase is mixed with the liquid phase introduced into the first chamber, and a second part is conducted away from the system.

5. A method according to claim 4, wherein the drying device includes an adsorption medium and the first part of the compressed gaseous phase is fed back through the drying device before mixing it with the liquid phase, in order to regenerate the adsorption medium.

6. A method according to claim 4 or 5, comprising passing the second part of the compressed and dried gas through a heat exchanger before it is conducted away from the system.

7. A method according to claim 6, wherein the heat exchanger is used to heat the liquid phase introduced into the first chamber.

8. Device for the stabilisation of a liquid phase by the separation of associated gases, comprising: at least two expansion chambers connected in series, each having an inlet for a liquid phase, an outlet for a gas phase and an outlet for a liquid phase separated off; means for compressing the gas phase separated off in the second and each subsequent expansion chamber; means for mixing the separated and compressed gas from the second and each subsequent chamber with the liquid phase fed into the previous chamber.

9. Device according to claim 8, wherein each mixing means is an ejector.

10. Device according to claim 8 or 9, also comprising means for compressing the gas separated off in the first chamber, and a heat exchanger for extracting heat from the compressed gas and transferring the heat to the liquid phase supplied to the first chamber.

11. Device according to claim 10, further comprising means for separating liquid from the gas separated off in the first chamber before it is fed through the compressing means.

12. Device according to claim 11, in which the liquid-separating means includes a drying device containing an adsorption medium, and further comprising means for supplying part of the output of the compressing means into the drying device in order to regenerate the adsorption medium.

13. Device according to any of claims 10 to 12, in which the heat exchanger is arranged upstream of the mixing means for the liquid phase input to the first chamber.

14. Device substantially as described in the description with reference to Figure 2 of the accompanying drawings.