WO1992019347A1

WO1992019347A1 - Oil and water separation system

Info

Publication number: WO1992019347A1
Application number: PCT/US1991/002885
Authority: WO
Inventors: Bradly D. Nelson; Reimer Z. Hansen; Samuel R. Rials
Original assignee: Conoco Specialty Products Inc.
Priority date: 1991-05-02
Filing date: 1991-05-02
Publication date: 1992-11-12

Abstract

An/water separation system wherein a finely dispersed emulsified fluid layer (32) is formed in a separator vessel (23) at the interface of a water layer (26) and an oil layer (30) in the vessel. A stream (34) of the emulsion layer (32) is passed through a hydrocyclone separator (40) for separating and outletting a more dense (48) and less dense (70) component of the emulsion layer stream (34). The more dense component (48) of the stream is passed back to the emulsified layer (32) in the separator vessel (23). The less dense component (70) of the stream (34) is passed back to the oil layer (26) in the vessel (23). Provisions are included for injecting chemicals into the streams at varying points (54, 68) in the system for enhancing separation.

Description

Description Oil and Water Separation System

Background of the Invention

This invention relates to an oil/water separation system and more particularly to a multi-phase separation process, a common use for which is found in oil field production and refining operations, to enhance the gravity separation of immiscible liquids by promoting coalescence of finely dispersed emulsified interfaced fluids forming at an oil/water interface.

A variety of separation systems, commonly found in petroleum industry applications are concerned with an emulsion layer formed in various types of systems which provides problems as to economical separation of the primarily oil and water components thereof.

During the production of petroleum hydrocarbons there is often a substantial amount of water produced. The amount of water will vary depending on many factors, such as: (1) the type of reservoir and formations from which the fluids are produced; (2) the age of the well producing the fluids; (3) the type of enhanced oil recovery (EOR) system that is used, as for example waterflood and steam flooding, both of which wiH increase the amount of water produced.

As oil is produced it must be separated from the water. The ease of this separation is affected by the fluid properties as well as physical and chemical factors. Some factors which may lead to the formation of suspensions and emulsions and thus adversely affect separation of oil and water are:

1. Tight reservoirs with low porosity and permeability, where oil droplets will be sheared simply by moving through the reservoir as the oil is produced;

2. The addition of chemicals such as may be used in chemical floods or corrosion inhibitors used in the well;

3. Shearing of fluid droplets due to pumps or any number of devices which may cause high turbulence such as a valve; and

4. Solids particles which can serve to stabilize emulsions as in oil we colloids.

Another situation leading to the problems involves world crude supplies getting much poorer in quality. Available crudes are getting heavier, more sour, and dirtier. Heavy, viscous crudes hold more particulate matter, and hold it longer. This adverse change in crude oϋ feedstocks is having significant operating and corrosion implications on refinery units. There are a number of components which may appear in crude oil stocks even in w y low quantities which can cause desalting and corrosion complications. These components could include solid particulates, ou field producing chemicals and production stimulants. Part of this problem ste s from an increase in the use of secondary and tertiary recovery methods which lead to the production of tightly emulsified fluids from water floods, caustic floods, surfactant floods, fire floods and the general use of well stimulant chemicals.

Thus, it is seen that a variety of oϋ industry problems which may be associated with drilling, producing or refining petroleum hydrocarbons, deal with oil and water emulsions and ultimately with the separation of these emulsions into their component parts. It is likely that oil/water separation in other industrial environments has similar problems which may be treated accordingly as described herein. Many devices and methods have been used to enhance the effectiveness of such oil/water separation. Such devices and methods may involve the use of chemicals to facilitate phase separation, the addition of heat to reduce viscosity of the fluids, the use of structure packing, specially designed flow paths, filters and other such mechanical devices to structure flow that produces contact of the components in a mixture to promote coalescence, or the use of electrostatic devices to create electric fields and charges that promote coalescence and separation of mixture components.

Prior art devices for solving oil-water separation problems, particularly in petroleum production and refinery operations, generally utilize conventional horizontal or vertical gravity separation vessels. Several methods have been used to promote coalescence in these vessels, however, these methods usually involve treating the entire fluid stream rather than a side stream of the suspension or emulsified layer. The use of chemicals is the most common practice to break the interfacial tension between droplets and to promote separation. Structured packings are also used to allow the fluids to move along corrugated parallel plates or through narrow openings and contact other droplets which coalesce into larger droplets. These droplets can then be more easily separated by gravity forces due to an increase in buoyancy and in surface area.

Addition of heat is often used to reduce the viscosity of the fluids which will significantly increase the droplets ability to migrate through the continuous phase liquid. Increasing temperature may also increase the density difference between the fluids. The use of an electrostatic potential across the fluids can be used to create a polarity field to charge the liquid droplets much like magnetic poles and thus promote separation. The use of heat or electrostatic potential is typically very energy intensive and costly. Most of the prior art separation schemes for dealing with the problems described above have one common major disadvantage in that they are space intensive because of their reliance to a great extent on tankage and time to eventually permit gravity separation of the components of the mixture. The operational efficiency of prior devices and systems for adequately separating the components has been hampered by the presence of these emulsion layers, with such prior art systems either not adequately addressing the problem or addressing the problem at an undesirable economic level. Typically, the addition of heat, chemicals, greater residence time, etc. have been the solution to these problems, with the inherent undesirable characteristics described above.

It is therefore an object of the present invention to provide a simpler, more efficient and less costly method and apparatus for the problem of separating oil and water components of a fluid mixture, wherein an emulsion comprised of finely dispersed emulsified fluids form a layer in a separation vessel.

Summary of the nvention With this and other objects in view the present invention provides a separation technique for treating an emulsion layer that has formed between oil and water layers in a vessel in a multiphase separation system. The emulsion layer is comprised of finely dispersed oil or water droplets in a continuous phase. An outiet is provided on the vessel containing the mixture to discharge a stream from the emulsion layer into the inlet of a hydrocyclone.

The hydrocyclone is arranged to separate the inletted stream into more dense and less dense components which are outietted from the hydrocyclone through underflow and overflow outlets respectively. The more dense component of the stream typically contains water and the smaller droplets of the dispersed phase which have not coalesced in the hydrocyclone separator. This more dense component is returned to the emulsion layer in the separator vessel. The less dense component outlets through an overflow outlet and is returned to the oil layer in the separator vessel. Oϋ and water outlets communicating with the respective oϋ and water layers in the vessel carry the separated components from the vessel for further processing or disposal.

Alternative arrangements provide for injection of separation enhancing materials such as demulsifiers into fluid flow streams at various points in the system.

Brief Description of the Drawings

Figure 1 is a schematic drawing of a separation system in accordance with the present invention for separating components of an emulsion layer formed in a vessel in a separation system; and

Figure 2 is an alternative arrangement of the separation system of Figure 1.

Description of the Preferred Embodiment Referring first to Figure 1 of the drawings, a desalting operation is shown for treating crude oil to remove excess solids therefrom prior to their being further processed as in a refining operation. A source of crude oil 12 is shown being passed through a pump 14 having an outiet passing through a mixing valve 20 into an inlet 22 of a two-phase separation vessel 23 in the case of a desalting system. This separator could be of another type such as a three phase separator, which would of course include a vent line (not shown) for outlet of a gaseous phase. In the desalting system, water is provided by an inlet line 16 from a pump 18 for mixing water into the crude to thereby wash the salts or other dissolved materials from the crude. Mixing valve 20 provides a means for mixing the water with the crude to ensure that the washing process takes place. The mixture emerging from the mixing valve 20 is then passed by means of inlet 22 into the separating vessel 23 wherein by gravity, the more dense water phase migrates towards the bottom of the vessel into a layer 30 with the less dense oil phase migrating to the top of the vessel into a layer 26. A mid-layer or interface layer 32 is formed in the vessel and is comprised of a suspension or emulsion of oil and water which is sometimes referred to as a "rag" layer. This may be an oil in water or water in oil suspension or emulsion and even have changing characteristics in this respect This interlace or "rag" layer becomes a relatively large part of the fluid mixture in the vessel and substantially increases the residence time of fluids in the separating vessel due to the increased volume of this emulsion layer. Typical separation systems often treat this layer by the use of chemicals in addition to increased residence time in order to separate the suspension or emulsion and recover the constituent fluids. In addition to chemical treatment of these fluids, mechanical devices, as well as the use of heat and electrical potential are used for breaking the emulsion and promoting coalescence of the constituent fluids. In the process of trying to find a solution to the problems involved in the separation process described, it has been found that in some instances solids particles which are a constituent part of the fluids being treated, serve to form a nucleus about which oil forms to envelop the solid particle and thereby create a neutrally buoyant particle which is a combination of the more dense solid and the less dense oil coating. This neutrally buoyant component thus becomes an integral part of the rag layer which typifies this process and generates the problems of separation associated therewith. Also, the rag layer may be further brought about by the presence of small droplets of a dispersed phase in a continuous phase of the liquid components. Such small droplets, say less than 20 micron in size are less likely to separate in a gravity system than larger droplets, with a longer residency time of course being effective to promote separation of such small droplets.

In order to better teat this rag layer, in a more efficient and simplified manner, an outiet line 34 from the separator vessel 23 feeds the rag layer to a hydrocyclone 40. If necessary this may be facilitated by use of a pump 36 provided in the line 34 between the separation vessel 23 and hydrocyclone 40. The rag layer is admitted to the hydrocyclone by means of an inlet 38. These fluids are admitted tangentiaϋy into the hydrocyclone wherein they are caused to separate by the centrifugal action imposed upon the fluids as a result of the geometrical design of the hydrocyclone. In the case of oil coated solids, the centrifugal forces on the fluids in the hydrocyclone are increased to the point that the oϋ coating the particuiate matter becomes dislodged therefrom and the particuiate matter is forced to the oiAer wall of the hydrocyclone while the oϋ component migrates to the ceπteriine of the hydrocyclone for discharge from an overflow outiet 44. The particuiate matter thus joins water in the system at the outer wall of the hydrocyclone for discharge at an underflow outlet 42. This more dense component of the mixture which may be comprised of the soKds and water is passed through a control valve 46 into an outlet tine 48. The soϋds have now had the oϋ coating removed therefrom to provide a sufficient density differential with respect to the liquids accompanying them to effect their separation therefrom in the separation vessel 23 to which they are returned. Any such sotids are then removed with the water from layer 30 through outiet 28. Whether or not solids are present in the underflow outlet stream in outiet line 48, it is likely that this more dense component from the hydrocyclone will contain water with smaller droplets of oil dispersed therein. Thus this underflow outlet stream is passed through line 48 to alternate flowpaths 62, 64 for return to the emulsion or suspension layer 32 in separator 23.

Such alternate routes are provided so that this more dense component can be inletted either before or after the inletting crude stream 12 passes through the mixing valve 20. Line 64 by operation of valve 58 provides a flow path into the inlet stream ahead of the mixing valve so that this predominately water stream may be mixed in the valve 20 with the inletting crude. Operation of a valve 66 permits an alternative route 62 for supplying the underflow outlet stream to the inletting fluids downstream of the mixing valve 20. In some situations the mixing valve may be creating more of an emulsion problem for the mixture than is solved by the mixing of tiie underflow stream and crude. In that situation, this alternative flow path 62 could be used to introduce the underflow stream downstream of the mixing valve. In certain situations it may be desirable to introduce the underflow stream into the separator near the tower level of the rag layer to thereby provide for its entry into the vessel 23 separate from the oϋ and or solid components of the mixture. This might be necessary in a situation where it is desirable to continuously remove water form the system such as when the inletting fluid has a substantially large water component in the beginning. In such a situation, it may tx)t be necessary to acid wash water to the inletting mixture. In any event, these various alternative schemes for dealing with the underflow stream being discharged from the hydrocyclone is provided to show that there are any number of separation schemes which may be treated by the system described herein when the basic problem being attacked is that of enhancing separation of the emulsion layer in the separator vessel.

The presence of small diameter droplets of the dispersed phase in the emulsion generates a problem that is solved particularly well by the present system in that as this emulsion layer passes through the hydrocyclone, these smaller sized droplets are brought together with the swirling force of the mixture moving through the hydrocyclone. The confined space of the cyclone separator together with the constant movement of the fluids increases the probability that these dispersed phase droplets will merge and form larger droplets. These larger droplets then are more buoyant and less dense and thus more likely to migrate to the core of the hydrocyclone for discharge through the overflow outlet. Alternatively, if these larger droplets are returned by flowpath 48 to the vessel 23, they are more likely to gravity separate into their respective layer in the vessel for discharge with a separated oil or water component.

The oil or less dense component emerging at the overflow outlet 44 of the hydrocyclone is passed by means of fiowline 60 back to the separation vessel 23 through inlet 70 at or near the upper level of the rag layer to thereby promote its further separation from the rag layer 30. in this case, the oil would flow upwardly into layer 26 to be discharged by means of outlet 24 for further processing or disposal as appropriate. Hydrocyctones which can be used in the operation of the system described herein are particularly described in U.S. Patents 4,237,006, 4,764,287, 4,772,796, 4,749,490 and 4,810,382, the details of which are incorporated herein by reference.

Inlet lines 68, 54 and 35 are shown for providing a means of injecting chemicals into the various fluid streams. Chemical injection line 54 is provided for inletting a chemical into the underflow stream or water leg 48 exiting from the hydrocyclone 40. This would provide further treatment of the underflow stream to aid separation of any remaining oϋ components therefrom. Injection of chemicals at this point would have the advantage of providing more intense treatment of these most difficult to separate fluids in the line 48 prior to the combination of the underflow stream with the inletted fluids to the separation vessel 23. Then, the chemicals would also be available to work on the inletting crude stream 12 as weH as mix into the emulsion layer 32 in the vessel 23.

In addition, a chemical injection line 68 is shown for injecting chemicals into the oil component shown exiting the hydrocyclone at outlet 44 prior to the readmission of the oϋ stream into the inlet 70 of the separation vessel 23. Again, introduction of chemicals at this point in the system will provide for more concentrated treatment of that component by the chemical prior to its being remixed with the other fluids in the separation vessel 23. Another chemical injection line 35 is shown feeding into the outlet line 34 from separator 23 to provide a means -for injecting chemicals into the mixture passing to the hydrocyclone 40, upstream of the pump 38.

In the operation of this system just described with respect to Figure 1, as for example, in a desalting operation, the crude oil being treated likely contains a concentration of solids particles in the form of salts or heavy metals which provide downstream problems as to either corrosion of the refining and process systems or in the product derived from the crude. To remove these solids, such fluids are inletted by means of inlet line 12 and pump 14 to tiie separation vessel 23. In the case of a classical desalting operation, water would be added by means of inlet lines 16 and pump 18 to mix with the crude and by means of mixing valve 20 to wash the salts from the crude for subsequent separation in the tank or separating vessel 23. In some operations, there may be sufficient water in the incoming oil line, or as in a production separation situation wherein washing is not taking place, so that it would be undesirable to utilize the mixing valve 20 or to add additional water to the system. In any event, the fluids to be separated are inletted by means of inlet 22 into the separation vessel 23. Vessel 23 serves as a residence vessel for permitting fluid components of the mixture to separate by density into more dense and less dense layers. The more dense layer, which in this typical desalting system is water and perhaps solids will migrate to the bottom of the separation vessel 23 for removal therefrom by means of line 28. The lighter phase to the system will migrate to the upper level 26 for removal therefrom by means of the exit line 24. Typically the fluids being treated by the system of the present invention have the common problem of developing a suspension or emulsion layer. Such suspension or emulsion layer may be increased by many factors such as the effect of oil coated solids or small diameter droplets, etc. A portion of this emulsion layer 32 is taken by line 34 into the inlet 38 of the hydrocyclone 40. An inlet line 35 provides means to inject treating materials into the line 34 prior to the mixture entering the hydrocyclone. Such materials might be demulsifying chemicals or other such chemicals to aid in the separation process by enhancing separation in the hydrocyclone or with the chemicals effectiveness being enhanced by its mixing with the components in the hydrocyclone to facilitate further separation downstream of the hydrocyclone.

The fluids inletted to the hydrocyclone 40 are at least partially separated within the hydrocyclone to form a less dense component exiting the overflow outlet 44 and more dense component which is comprised of water and solids particles which outlet through the underflow outlet 42 into a discharge line 48. A control valve 46 is provided in the line 48 to control the outlet flow from the underflow of the hydrocyclone. The more dense component of the mixture, which is typically comprised of the water, oil droplets and perhaps some solid particles, is recycled into the separation vessel 23 by means of alternate flowpaths 62, 64. These flowpaths 62, 64 return the underflow stream into the emulsion layer 32 by introducing the stream into the inletting mixture either upstream or downstream of the mixing valve 20. Routing through these flowpaths is chosen by use of valves 58 and 66.

Next referring to Figure 2 of the drawings, a system is described which is the same in most respects to that shown in Figure 1 except that instead of the pump 36 being placed in the outiet line 34 from separator vessel

23, separate pumps are placed downstream of the hydrocyclone 40. In Figure 2 a low shear type pump 50 is placed in fiowline 48 after or downstream of the chemical injection point corresponding to injection line 54. This placement provides a mixing action in the pump for any materials introduced through line

54. The primaiy purpose of this puπφ is of course to l ^ underflow outlet stream in line 48 so that this stream is under sufficient pressure to gain entry into the inlet 22 of vessel 23. The dispersed droplets which exit the hydrocyclone through the underflow are relatively small and as such are not as susceptible to shear forces in tiie pump as are larger droplets. Larger droplets are likely to have left the hydrocyclone through the overflow outlet 44.

Thus, this placement of the pump has certain advantages in this respect, particularly if sufficient pressure exists in the system to move the emulsion portion stream through the hydrocyclone. Another low shear pump 45 is placed in the overfiow stream between outiet 44 on the hydrocyclone and inlet

70 to the separation vessel 23, again downstream of injection Kne 68 to permit any mixing to take place. A pumping scheme particularly adapted for use herein is disclosed in U.S. Patent 4,844,817 incorporated herein by reference.

Additionally, Figure 2 shows the return line from overflow outlet 44 to inlet line 70 on separator vessel 23, to enter the vessel 23 above the emulsion layer at the oil layer 26. Any water which may be present in this less dense component will likely be easily separated by gravity when it is reintroduced into the oil layer. Thus, such water will drop out and migrate to the water layer 30 for subsequent removal through outlet Kne 28.

It should be kept in mind that while the disclosure herein has been directed primarily to petroleum industry applications, the problems associated with oil-water separation in other industry applications will involve similar concepts and the system described herein will apply equally well to those problems in other industries. Therefore, while particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

Claims

1. In a separation system for treating a fluid mixture wherein a multiphase separator vessel receives the mixture and at least partially by gravity separates oil and water components within the mixture into oil and water layers and wherein a suspension layer forms at an oil and water interface within the multiphase separator vessel, the suspension layer having oϋ or water droplets dispersed in a continuous phase, means -for enhancing separation of the suspension layer, which means comprises; a hydrocyclone designed, constructed and arranged for separating oil and water components of the fluid mixture included in said suspension layer, said hydrocyclone having a separating chamber with inlet means at a first inlet end thereof for inlet of the fluid mixture to be separated; suspension layer outlet means on said vessel communicating with the suspension layer in said vessel for outletting a portion of said suspension layer from said vessel and fiowpath means for passing said portion to the inlet means on said hydrocyclone; an overflow and underflow outlet on said hydrocyclone separating chamber for outletting separated components of the fluid mixture; and conduit means for directing separated components outletting said underflow outlet on said hydrocyclone into fluid communication with said emulsion layer in said vessel.

2. The separation system of Claim 1 and further including means for directing the outietted component from said overflow outiet to the oil layer in said separator vessel.

3. The separation system of Claim 1 wherein the component outletting said overflow outlet of said hydrocyclone is a less dense component which is recycled into fluid communication with the emulsion layer in said separator vessel.

4. The separation system of Claim 1 wherein said multiphase separator vessel is a desalter device for separating components of a crude oil mixture, and wherein said suspension is in the form of a finely dispersed emulsion layer at said oil and water interface.

5. The separator system of Claim 1 wherein said multiphase separator vessel is a three phase separator.

6. The separation system of Claim 4 and further including a mixing means upstream of the multiphase separator vessel for mixing water into the crude oil mixture before the mixture is introduced into the multiphase separator vessel; and wherein said conduit means is arranged to direct a substantially water component outletting said underflow outlet into the crude oϋ mixture after the mixture has passed through said mixing means.

7. The separation system of Claim 1 and further including low shear pump means downstream of said underflow outlet for increasing the pressure of fluids being directed by said conduit means to said emulsion layer in said vessel.

8. The separation system of Claim 7 and further including means for mixing a separation enhancement material into the fluids outletting said underflow outlet of said hydrocyclone upstream of said pump means to mix said materials with said component outletting through said underflow outlet to enhance separation of the component outletting the hydrocyclone and of the components in the fluid mixture in said separator vessel.

9. The separation system of Claim 1 and further including low shear pump means positioned in said fiowpath means downstream of the suspension layer outlet means on said vessel for increasing the pressure of said portion of said suspension layer passing through said fiowpath means from said vessel to said inlet means prior to said portion being inletted into said inlet means on said hydrocyclone.

10. The separation system of Claim 9 and further including injection means in said fiowpath means upstream of said pump means for mixing a separation enhancement material into said portion prior to said portion of said suspension layer being passed by said pump means into said inlet means.

11. A method for separating oil and water components from a fluid mixture wherein a multiphase separator vessel receives tiie mixture and at least partially by gravity effects separation of the oil and water components into oϋ and water layers and wherein a finely dispersed emulsion layer forms at an oϋ and water interface within the multiphase separator vessel, the emulsion having oil or water droplets dispersed in a continuous phase, wherein enhanced separation of the emulsion layer is effected by; passing a portion of the emulsion layer from the vessel into the inlet of a hydrocyclone separator designed, constructed and arranged for separating oϋ and water components of a fluid mixture by coalescing droplets which are dispersed in the continuous phase of the emulsion, said hydrocyclone separator having underflow and overflow outlets for outletting more dense and less dense components respectively of said portion; outletting a stream of the more dense component, including smaller droplets dispersed in the continuous phase, from tiie underflow outiet of the hydrocyclone; passing the outletting stream from the hydrocyclone underflow outlet into fluid communication with the emulsion layer in the vessel to thereby return the more dense component including the smaller droplets to the vessel; outletting a stream of the less dense component comprised primarily of coalesced dispersed droplets, from the hydrocyclone overflow outiet; and passing the stream of the less dense components into fluid communication with the oil layer in the vessel.

12. The method of Claim 11 and further including permitting the returned oil and water components including the coalesced droplets to gravity separate in said vessel; and passing such gravity separated oil and water components through separate flowpaths from the respective oil and water layers form said vessel.

13. The method of Claim 11 and further including passing the emulsion layer portion from the vessel into a low shear pump and then pumping the emulsion layer portion into the inlet of the hydrocyclone so as to minimize shearing of the dispersed droplets in the mixture.

14. The method of Claim 13 and further including injecting a separation enhancement material into said emulsion layer portion prior to such portion being passed into said pump.

15. The method of Claim 11 and further including passing the underflow outlet stream from said hydrocyclone into a tow shear pumping arrangement and then pumping the underflow stream of the more dense component into fluid communication with the emulsion layer in said separator vessel.

16. The method of Claim 15 and further including mixing a separation enhancement material into the fluids outletting said underflow outlet of said hydrocyclone before passing the underflow stream into said pumping arrangement.

17. The method of Claim 11 wherein a mixing device is positioned upstream of the separator vessel for mixing the fluid mixture as it passes through said mixing device and before the fluid mixture inlets into the separator vessel, and further including; passing a separation enhancement material into the mixture entering the separator vessel before the fluid mixture passes through the mixing device.