WO1992019349A1

WO1992019349A1 - Oil and water separation system

Info

Publication number: WO1992019349A1
Application number: PCT/US1991/002887
Authority: WO
Inventors: David A. Flanigan; Mark F. Schubert
Original assignee: Conoco Specialty Products Inc.
Priority date: 1991-05-02
Filing date: 1991-05-02
Publication date: 1992-11-12

Abstract

An oil-water separation system wherein an emulsion layer (31) comprised of oil and water components in the form of a dispersed phase of droplets of a relatively small size in a liquid continuous phase is formed at the interface of an oil and water layer in a separation vessel (11). A stream of the emulsion layer (31) is taken from the vessel (11) into an oil-water hydrocyclone separator (41) which outlets an underflow stream (47) wherein in the stream a substantial portion of the dispersed droplets are coalesced so that when the coalesced droplets are returned with the underflow stream to the emulsion layer (31) in the separator vessel, such coalesced droplets are separated by gravity or the like into the layered components for removal from the separation system.

Description

Description Oil and Water Separation System

Backα wpd φf The IPYβrrøpη This invention relates to an oil/water separation system and more particularly to a multiphase 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 will 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 wet 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 oil 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 very low quantities which can cause desalting and corrosion complications. These components could include solid particulates, oil field producing chemicals and production stimulants. Part erf this problem ste s from an increase in the use of secondary and tertiary recovery methods which lead to the production of tightly θmulsifiθd 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 oil industry pι___ ems 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 structured 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 increase in buoyancy and increase 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 forms a layer in a separation vessel.

Summary of the Invention 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 on a multiphase separation system. The emulsion layer is comprised of finely dispersed oil or water droplets in a continuous phase. An outlet is provided on the vessel containing the mixture to discharge a stream from the emulsion layer into the inlet of a hydrocydone. The hydrocydone is arranged to coalesce the droplets and discharge the at least partially separated coalesced droplets components and continuous phase component back into the vessel containing the emulsion layer for further separation of the components. Oil and water outlets communicating with the respective oil and water layers in the vessel carry the separated components from the vessel for further processing or disposal.

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;

Figure 2 is a schematic drawing of a separation system in accordance with the present invention for separating components of an emulsion layer in a desalting vessel; and

Figure 3 is a graph showing the effect on droplet size distribution after multiple passes of a mixture through a coalescing hydrocydone.

Description of the Preferred Embodiment

Referring to Figure 1 of the drawing, an enclosed vessel 11 , such as a two or three phase separator, desalter or the like, is shown for receiving a fluid mixture into an inlet 13. The inlet mixture may be from a variety of sources such as a production stream from an oil and gas well, or a process stream of crude oil feed in a petroleum refining operation. The inlet stream will likely contain a fluid mixture comprised of oil and water or oil, water and gas, and may even indude solids. It is recognized that the presence of solids in such a mixture as is treated herein is not unlikely in that solids are a possible source of emulsion formation in the mixtures treated by the separation system disdosed herein. In a typical mixture for separation, oil, water and gas components enter the vessel 11 through inlet 13 where they will tend to at least partially gravity separate. The vessel 11 is shown having a first compartment 15 for receiving the mixture with a baffle 17 at one end dividing the vessel to thereby form a second compartment 19. An outlet line 21 is provided on the second compartment to provide an outlet for separated components such as the oil component 22 shown in the second compartment. An outlet line 23 communicates with the bottom portion of chamber 15 in the vessel. Another series of outlet lines 25 are also shown disposed on the bottom side of the vessel 11 , and indude standpipes 27, 28 and 30 which extend an inlet end 29 of each standpipe upwardly into the compartment 15 for communicating the outlet 25 with an emulsion layer 31 which forms in the compartment 15. Due to variations in the composition and characteristics of the emulsion layer, the liquid level and size of the emulsion layer will vary. Therefore, the series of standpipes 27, 28 and 30, by menas of associated valves 32 in flowpath 39, are arranged to take off a portion of the emulsion layer 31 as the position of the emulsion layer varies. An oil layer 33 and water layer 35 are shown positioned respectively above and below the emulsion or suspension layer 31. A vent line or passageway 37 is positioned on top of the vessel 11 and communicates with the upper interior portion of the vessel to vent or convey any gas separated from the mixture away from the vessel 11.

The flowpath 39 is connected to the outlets 25 on each standpipe 27, 28 and 30 to convey any fluids therein to an inlet 41 on a hydrocydone 43 shown schematically in Figure 1. Coalesdng hydrocydones for use in such a system are described in greater detail in U.S. Patent 4,995,989 to Carroll, et al., the details of which are incorporated herein by reference. Also hydrocydones such as those shown and described in U.S. Patents 4,237,006, 4,764,287,

4,722,796, and 4,749,490 could be operated in accordance with the present invention by dosing the reject or overflow outlet. The details of the disdosure in these patents is also incorporated herein by reference. A low shear pump 42 is shown in Figure 1 for providing sufficient pressure if needed to affect separation in the hydrocydone. U.S. Patent 4,844,817 shows a pump system for this purpose and is likewise incorporated by reference herein. An outlet 45 on hydrocydone 43 feeds into a return line 47 for conveying fluids from the hydrocydone outlet back to the vessel 11. An inlet 49 on the vessel 11 communicates with a position in the vessel corresponding with layer 31 in compartment 15.

Referring now to Figure 3 of the drawings, the graphic plot shown therein plots the normalized volume percent of oil in a mixture against the droplet size of the oil in micrometers to show the effect of passing a fluid mixture through a coalesdng hydrocydone such as is used in the present system. The present invention involves enhandng tiie operation of conventional gravity separators by incorporating a re-drculation loop of the emulsion or suspension layer from a gravity separator through a coalesdng hydrocydone to coalesce emulsified droplets and then returning the coalesced components to the separator for enhanced gravity separation. Figure 3 shows the effect of making multiple passes of a mixture containing droplets of oil in a water continuous phase through a coalesdng hydrocydone. The test for generating these results was performed in a 60 mm de-oiling hydrocydone of the type described in U.S. Patents 4,237,006, 4,764,287 and 4,722,796, with a closed overflow outlet. It is seen that the initial distribution of droplets in the mixture had a preponderance of droplets at about the 16 micron size range with the total distribution having the largest percentage of droplets being less than that size. After five passes through the coalescing hydrocydone the distribution is significantly shifted to place a majority of the droplets in a range greater than about 30 microns, it is well recognized that the larger droplets are much more susceptible to separation such as in a gravity system. In the operation of the separation system as shown in Figure 1 , the mixture to be separated is inletted through line 13 into a first compartment 15 of the vessel 11. Assuming the inletting mixture to be an oil, water and gas mixture, the fluids will at least partially gravity separate in the compartment 15 to form a lowermost more dense water layer 35 and an uppermost less dense oil layer 33 with there to be the likely formation of an emulsion or suspension layer 31 at the interface between the oil and water components. Gas in the mixture will, when coming out of solution in the mixture, rise to the uppermost level in the vessel 11 to be outietted through outlet 37 for further appropriate disposal. The oil layer 33 is permitted to spill over the baffle 17 to thereby become a segregated component 22 in the compartment 19 formed by the baffle. This presumably dry oil component would then be conveyed to a tank for storage or sales or for further processing as may be appropriate. The water layer 35 is outietted by means of outlet line 23 for further processing or disposal as may be indicated by the particular application. The emulsion or suspension layer is taken by way of outlets 25 and standpipes 27, 28 and 30 to the inlet of the hydrocydone 43. The hydrocydone serves to bring the dispersed phase droplets into contact in a confined area under high centrifugal force so as to coalesce the droplets and thereby increase the nominal droplet size. The amount of fluid recycled would be dependent on the size of the emulsion layer and difficulty of separation. In other words, the larger the emulsion layer, the greater the amount of fluid to be recycled.

These coalesced droplets, now having a larger droplet size, are returned to the layer 31 in vessel 11 where they are now more readily separated by gravity into the respective oil and water layers for removal from the separation system as described above. Referring next to Figure 2 of the drawings, a desalting operation is shown for treating crude oil to remove excess solids, naturally occurring salts, and water therefrom prior to their being further processed as in a refining operation. In refinery desalters, the feed oil is often washed with a water solution making up 5 to 10 percent of the feed volume to dissolve naturally occurring salts from the crude oil. A source of crude oil 12 is shown being passed through a pump 14 having an outlet stream which passes through a mixing valve 20 into an inlet 22 of a two-phase separator vessel 23. 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 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 separation, 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 interface or rag layer "becomes a relatively large part of the fluid mixture in the vessel and substantially decreases the residence time of fluids in the separating vessel due to the increased volume of this emulsion layer. Prior art 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 solid partides which are a constituent part of the fluids being treated, serve to form a nudeus about which oil forms to envelop the solid partide and thereby create a neutrally buoyant partide 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 residence time of course being affective to promote separation of such small droplets. In order to better treat this rag layer, in a more efficient and simplified manner, an outlet line 34 from the separator vessel 23 passes the rag layer to the hydrocydone 40. If necessary, this may be fadlitated by use of a pump 36, preferably low shear, provided in line 34 between the separation vessel 23 and hydrocydone 40. Optionally pumps 36 could be placed in line 48 downstream of said hydrocydone 40. The rag layer is admitted to the hydrocydone by means of an inlet 38. These fluids are admitted tangentially into the hydrocydone wherein they are caused to separate by the centrifugal action imposed upon the fluids as a result of the geometrical design of the hydrocydone. In the case of oil coated solids, the centrifugal forces on the fluids in the hydrocydone are increased to the point that the oil coating the particulate matter becomes dislodged therefrom and the paniculate matter is forced to the outer wall of the hydrocydone while the oil component tends to migrate to the centerline of the hydrocydone. The particulate matter thus joins water in the system at the outer wall of the hydrocydone. Oil droplets in the mixture passing through the hydrocydone will tend to move towards the core of the separation chamber therein and because of this swirling movement in the hydrocydone and the confined space of the separation chamber, they will tend to become coalesced into larger droplets, such larger droplets being more susceptible to gravity separation in the separation vessel. These partially separated components of the mixture are then passed through the hydrocydone outlet 42 into an outlet line 48. The solids have now had the oil coating removed therefrom to provide a suffident 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 solids are then removed with the water from layer 30 through outlet 28. Whether or not solids are present in the outlet stream in line 48, it is likely that the outlet stream will have coalesced larger droplets therein as a result of the coalesdng action of the hydrocydone and thus will tend to readily separate when they are returned to the suspension layer 32 in separator 23. Optionally, a back pressure device 46 can be provided in the outlet line 48. Outlet line 48 is arranged to pass the hydrocydone outletting stream through alternate flow routes so that this stream can be inletted either before or after the inietting 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 recyded stream may be mixed in the valve 20 with the inietting crude. Operation of a valve 66 permits an alternative route 62 for supplying the hydrocydone outlet stream to the inietting fluids downstream of the mixing valve 20.

The redrculation concept of this application can be used in conjunction with various multiphase separation systems where droplet conditioning of the liquid phase can be used to promote coalescence and enhance separation. In the petroleum industry, examples of such multiphase separation systems are conventional three phase production separators, oil field heater treaters, electrostatic predpitation systems, desalting separators, and the like but the applications described are in no way intended to be limiting. By redudng or eliminating the emulsion interface layer as in the present system, the relative size of the separator vessel can be reduced or its effectiveness increased. This in turn can be equated to increasing the residence time of the fluids in the vessel due to the increased volume for the oil/water components when the emulsion layer is reduced. In certain applications this concept can be used to reduce or eliminate the need for de- emulsifying chemicals or to reduce electrical power or heat requirements on related separator designs. it should be kept in mind that while the disdosure 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 daim:

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 an emulsion layer, forms at an oil and water interface within the multiphase separator vessel, the emulsion having oil or water droplets finely dispersed therein, means for enhandng separation of the emulsion layer, which means comprises: a hydrocydone designed, constructed and arranged for coalesdng oil and water components of the fluid mixture induded in said emulsion layer, said hydrocydone having a separating chamber with inlet means at a first inlet end thereof for inlet of the fluid mixture to be separated; emulsion layer outlet means on said vessel communicating with the emulsion layer in said vessel for outietting at least a portion of said emulsion layer from said vessel to the inlet means on said hydrocydone; at least one outlet on said hydrocydone at a second end of said separating chamber opposite said first end for outietting coalesced components of the fluid mixture; and conduit means for directing separated components, outietting said at least one outlet on said hydrocydone, into fluid communication with the fluid layers in said vessel.

2. The separator system of Claim 1 wherein said at least one outlet is arranged for conveying substantially all of the separated oil and water components of the mixture into said conduit to thereby return coalesced components of the emulsion to said vessel.

3. The separator system of Claim 1 wherein said hydrocydone separator is arranged to coalesce dispersed droplets in the continuous phase and to outlet said dispersed droplets in the continuous phase through said at least one outlet into said conduit means for return to said vessel.

4. The separator system of Claim 1 wherein the fluid mixture received by said vessel contains a gas phase and further including gas outlet means on said vessel for outietting a substantial portion of said gas phase.

5. The separator system of Claim 1 and further including oil and water outlet means on said vessel for providing separate flowpaths in fluid communication with each of said respective oil and water layers in said vessel to outlet separate oil and water streams from said vessel.

6. The separator system of Claim 1 and further induding pump means arranged to minimize shearing of the dispersed droplets for applying additional pressure to fluids which have exited from the emulsion layer outlet means.

7. A method for separating oil and water components from a fluid mixture wherein a multiphase separator vessel receives the mixture and at least partially by gravity effects separation of the oil and water components into oil and water layers and wherein an emulsion layer forms at an oil 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 stream of at least a portion of the emulsion layer from the vessel into the inlet of a hydrocydone separator designed, constructed and arranged for separating oil and water components of a fluid mixture by coalescing droplets which are dispersed in the continuous phase of the emulsion; outietting a stream of the oil and water components, including the coalesced droplets in the continuous phase, from the underflow outlet of the hydrocydone; and passing the outietting stream from the hydrocydone into fluid communication with said vessel to thereby return the oil and water components including the coalesced droplets to the vessel.

8. The method of Claim 7 and further induding passing a gas phase from the mixture from said vessel.

9. The method of Claim 7 and further induding passing separate streams of oil and water from said vessel in addition to passing the stream of at least a portion of the emulsion layer from said vessel to said hydrocydone.

10. The method of Claim 7 and further induding permitting the returned oil and water components induding the coalesced droplets to gravity separate in said vessel; and passing such gravity separated oil and water components through separate flowpaths from respective oil and water layers from said vessel.

11. The method of Claim 7 and further induding pumping said portion of said emulsion layer stream after it leaves the vessel to increase the pressure of the stream.

12. A separation system for enhandng tiie separation of components of a fluid mixture in a conventional separation vessel wherein a suspension layer is formed at the interface of two liquid components of the fluid mixture, such suspension layer being comprised of a dispersed phase of droplets of a relatively small size in a liquid continuous phase, comprising means for outietting at least a portion of the suspension layer from the vessel; a coalescing hydrocydone having an inlet coupled with said outietting means and an outlet wherein all the fluids coalesced in said hydrocydone pass through said outlet; and means for returning said coalesced fluids outietting said hydrocydone to said vessel.

13. The separator system of Claim 12 wherein said hydrocydone is effective to coalesce smaller droplets of the dispersed phase into larger droplets and further induding means for returning said coalesced fluids into said vessel for further separation.

14. The separator system of Claim 12 wherein the interface between the liquid components varies to change the level of the suspension layer in the vessel and further induding means for outietting the at least a portion of the suspension layers from varying levels in the vessel.