GB2038652A - Oil-water separator - Google Patents

Abstract

A mixture containing crude oil, water, solids and not more than 20% by volume of gas is separated into an oil stream containing less than 1% by wt of water and a water stream containing less than 100 ppm of oil. The mixture is passed through a filter 4 to remove solids and then through at least one coalescer vessel 6 containing a coalescer element. The mixture has gas dissolved therein to reduce its viscosity and the separation is carried out under pressure to prevent the gas in solution forming bubbles. The oil and water streams can each be cleaned up by passing through further coalescers 8, 10. A portion of filter 4 can be backwashed while another portion is still filtering, the backwashing being effected by using the oil stream or the feed mixture. Filter 4 can be a screen of polypropylene or polyester fibres, polyamide, stainless steel, or sand, with axial or radial flow. Coalescer element is an annular, radial flow element of pleated paper, glass fibres or other fibrous material. The oil and water settles out under gravity in vessel 6. A separator to reduce the gas content may precede filter 4. <IMAGE>

Description

SPECIFICATION Separation method and apparatus This invention relates to a method for separating a mixture containing a hydrocarbon oil and water, more particularly to separating a mixture of crude oil and produced water and also to an apparatus there for.

In the production of crude oil from an oilfield it is usual for the oil initially produced to be substantially water-free. However, during the life of the oilfield the proportion of water produced with the crude oil usu ally increases and a point is reached when it may be desirable to separate the oil from the produced water before transporting the oil from the wellhead either by pipeline or tanker.

For the separation of certain oil and water mix tures it has been previously proposed to employ a separator known as a coalescer. For example, coalescers have been previously proposed for removing minor amounts of water from aviation kerosine and from marine fuel oil and also for removing minor amounts of oil from aqueous effluents.

It is an object of the present invention to provide a method and apparatus for separating a feed mixture containing crude oil and produced water and pro duced solids to obtain an oil from which the water has been substantially removed. Usually the feed mixture contains gas in solution and also gas in the form of bubbles so that the feed mixture is often in the form of foam.

According to the present invention a method of separating a mixture containing a crude oil, produced water, produced solids and not more than about 20% by volume of gas into an oil stream from which the water has been substantially removed and a water stream containing not more than 100 ppm of oil involving the use of a coalescer comprising a coalescer element located in a vessel comprises:: (a) passing the mixture through a filter to remove from the mixture a substantial proportion of solids that would blockthe coalescer, and (b) passing the filtrate through the coalescer to produce an oil phase and a water phase and separating the two phases to form an oil stream from which the water has been substantially removed and a water stream containing not more than 100 ppm of oil 7 ne method can conveniently be effected by either (i) controlling the conditions at which the coalescer is operated so that the water content of the oil stream is less than 1% by weight, or (ii) allowing the water content of the oil stream to exceed 1% by weight and passing the oil stream through another coalescer to reduce the water content below 1% by wt.

The amount of produced solids in the mixture before filtering may be up to 1000 ppm and will usually be in the range 5 to 1000 ppm. The filtrate may contain less than 5 ppm preferably less than 1 ppm.

The amount of gas will usually be less than 10% by volume.

The oil stream can be treated in a further stage (herein referred to as the oil clean up) to give an oil product containing not more than 0.1% by weight of water and a water containing not more than 1000 ppm of oil.

The water stream (to which may be added the water from the oil clean up mentioned above) can also be treated in a further stage (herein referred to as the water clean up) to give a water product containing not more than 15 ppm of oil and an oil containing less than 5% by weight of water. This oil will normally be a relatively small volume and can be added to the feed to the oil clean up.

The mixture separated by the method can be a crude oil from which the gas has been partly removed, particularly a crude oil containing up to 10% by volume of gas and from 10 to 50% by weight of water. When the crude oil contains more than about 10% by volume of gas then a gas separation stage may precede the filtration for solids removal.

It might be expected that the presence of gas in the mixture would have an adverse effect on the separation in the coalescer but it has been found, surprisingly, that a crude that has not been fully degassed can be treated, for example one containing from 2 to 10% by volume of gas.

In addition to this gas the mixture will usually contain gas in solution, the amount of which will depend on the pressure. It is preferred to have a substantial quantity of gas in solution since this reduces the viscosity of the mixture and facilitates separation. Suitable operating pressures for the filter and coalescer are in the range 30 to 1000 psig and desirably the pressure is such as to retain the dissolved gas in solution.

The purpose of the filtration is to remove produced solids that are normally present in crude oils such as particles of sand, precipitated salts or pipe scale which would otherwise rapidly block the coalescer and lead to a pressure build up across the coalescer.

The filtration is carried out in such a manner as to remove a substantial proportion of such solids and thus give the coalescer a reasonable working life, for example when the pore size of the element in the coalescer is about 50 microns the filtration will be effected to remove a substantial proportion of solids that would block pores of that size.

The filter for solids removal may be a screen, e.g.

of polypropylene or polyester fibres or other synthetic material such as polyamide or of stainless steel.

Alternatively, the filter may comprise sand or the like from which the filtered solids are removable by backwashing or a similar procedure which avoids dismantling the filter.

The flow through the filter can be axial or radial.

The operation of the filter can be effected as follows: pressure upstream of the filter can be up to 1000 psig and the pressure drop across the filter conveniently less than 40 psi, for example 1 to 20 psi residence time 0.1 to 10 minutes, preferably 0.3 to 2 minutes. The filter may be backwashed or replaced either when a high pressure drop is generated across the filter by the filtered solids or at fixed time intervals chosen to avoid a high pressure drop across the tilter.

Conveniently the filter is backwashed with the feed mixture or oil product and the washings combined with the separated oil product. The backwashing may be effected periodically or continuously by backwashing a sector of the filter whilst the forward flowthrough the remainder is maintained.

The term coalescer is understood by those skilled in the art and refers to any means by which relatively small droplets of oil or water occurring in dispersions such as emulsions which do not readily separate under gravity are coalesced to facilitate separation.

For the avoidance of doubt the term coalescer does not include plate type separators which are unable to separate the small droplets found in crude oil/water mixtures, and electrostatic separators.

The coalescer element may comprise, for example pleated paper, glass fibre or other fibrous material conveniently in the form of an annulus through which the mixture flows radially.

The pore size of the element will be chosen so that it is fine enough to effect coalescence but not so fine as to become rapidly blocked by solids in the filtered crude oil. Usually the pore size of the element will be in the range 20 to 100 microns.

By pore size is meant that the element will retain 95% of particles of the specified pore size. Thus a 20 micron pore size element will retain 95% of particles of 20 microns.

The operation of the coalescer can be effected as follows: residence time of oil phase 0.1 to 5 mins.

preferably 0.5 to 3 mins. The pressure upstream of the coalescer element can be up to 1000 psig and the pressure drop across the coalescerelement preferably less than 100 psi conveniently in the range 1 to 40 psi.

Preferably the operation of the coalescer is effected so that the major component of the mixture (usually the oil) is the continuous phase on the downstream side of the coalescer element. The effect of this is that the coalesced water droplets on leaving the coalescer element pass into the oil phase and settle under gravity towards the base of the coalescer vessel.

The coalescer element may be replaced in the event of a high pressure drop developing, for example caused by trapped solids.

According to a preferred embodiment of the invention a method of separating a feed mixture containing a crude oil from 1 to 50% by weight of produced water, a minor amount of solids up to 1000 ppm and more than 10% by volume of gas to produce an oil containing less than 1.0% by weight of water comprises:: (a) passing the mixture through a separator to reduce the amount of gas to below 10% by vol ume, (b) passing the mixture through a filter to remove solids therefrom, said filter being regen erable by backwashing and backwashing the filter with the feed mixture and combining the washings with the oil product, (c) passing the filtrate containing gas in solution in amount sufficient to reduce the viscosity through a coalescer operated at superatmospheric pressure to retain the dissolved gas in solution and further so that coalesced droplets of water pass into a continuous oil phase on the down stream side of the coalescer and settle under grav ity to produce an oil phase and a water phase and separating the two phases to form an oil stream and a water stream containing not more than 100 ppm of oil and either (i) controlling the conditions at which the coalescer is operated so that the water content of the oil stream is less than 1% by weight, or (ii) allowing the water content of the oil stream to exceed 1% by weight and passing the oil stream through another coalescer to reduce the water content below 1% by wt, and (d) passing the water stream through a further coalescer to remove oil therefrom to reduce the oil content and give a water product.

According to another aspect of the present invention an apparatus for the separation of a mixture containing a crude oil, from 1 to 50% by weight of produced water, a minor amount of produced solids and not more than 20% by volume of gas into an oil stream containing not more than 1% by weight of water and a water stream containing not more than 100 ppm of oil, said apparatus comprising: (a) a filterforthe removal of said produced sol ids, (b) a vessel containing a coalescer element for the separation of the filtered mixture into an oil stream and a water stream said coalescer vessel being connected to the filter so that filtered mix ture is passed as feed to the coalescer vessel.

The pore size of the coalescer element can be in the range 30 to 70 microns.

The apparatus can include a second coalescer element located in a vessel for the separation of the filtered mixture from the filter, said second coalescer vessel being connected to the filter and to the first mentioned coalescer vessel to permit operation in series or in parallel with said first mentioned coalescer.

The second coalescer vessel can be connected to the first coalescer vessel so that the oil stream is passed as feed thereto, said second coalescer having a pore size of 25 to 50 microns.

The filter can have means for backwashing a portion thereof whilst forward flow through the remainder is maintained and for effecting said backwashing with a portion of the feed mixture or oil stream and combining the backwashed material with the oil stream from the coalescer.

The invention is illustrated by reference to the accompanying drawings in which Fig. 1 is a flow diagram of the method in which gas is removed from a crude oil/water mixture prior to filtering and bulk separating, and Fig. 2 is a more detailed flow diagram showing oil clean up and water clean up stages following bulk separation. Fig. 3 is a more detailed flow diagram showing the backwashing of the filter and the arrangement of coalescers in parallel. The compositions of the various feeds and products are given below.

Referring to Fig. 1, a crude oil is passed to an oil/gas separator 2 as described in our copending UK Patent Application Nos. 27650178 Serial No. 2000054 and 42873/78 Serial No. 2007118 from which the degassed oil is passed to a filter 4 for solids removal.

Alternatively, the separator 2 can be a conventional gravity gas/oil separator. The filter 4 was a screen filter containing a polypropylene filter element and having the facility for back flushing a segment of the filter whilst the flow through the remainder of the filter is continued and is available commercially from The Plenty Group Limited of Newbury, England. The filtered oil is then passed to a vessel 6 containing a coalescer element of glass fibres of pore size about 50 microns to effect bulk separation into an oil stream 5 and a water stream 7.

Referring to Fig. 2, the oil stream 5 is passed to an oil clean up vessel 8 containing a coalescer element of pleated paper or glass fibres from which a cleaned up oil product 16 and a water stream 18 are obtained. Water stream 18 can be passed to water stream 7 or to water stream 12.

The water stream 7 is passed to a water clean up vessel 10 containing a coalescer element of pleated paper or glass fibres from which a cleaned up water stream 12 and an oil stream 14 are obtained. Oil stream 14 is combined with water stream 7 to vessel 10.

Referring to Fig. 3, crude oil feed is passed via line 29 to an oil/gas separator 28 from which degassed crude oil is passed via line 30 to a backwashable filter 26 from which filtrate is passed via line 36 to coalescer vessel 20 or via line 38 to coalescer vessel 22. The two vessels 20 and 22 are arranged to work in parallel or in series via line 44. Separated oil is taken from vessel 20 via line 42 and from vessel 22 via line 48. Separated water is taken from vessel 20 via line 40 and combined with that withdrawn from vessel 22 by line 46 and the combined separated water passed via line 50 to an oil water separator 24 from which clean water is obtained via line 52. Filter 26 is backwashed by feed supplied via line 32 and the backwashed material passed via line 34 to the oil product in line 42.Gas dissolved in the water in line 52 can be removed in a cyclone (not shown) prior to disposal of the water.

Oil Gas Separation Feed to oil/gas separator: crude oil containing 60% volume of gas 10 ppm by weight of solids 10% wt of water the temperature of the feeds was about 120 F, the pressure was about 400 psig Product from oil/gas separator: crude oil containing 5% volume of gas 10 ppm by weight of solids 10% wt of water

product A * The solids comprised particles of sand, precipitated salts, pipe scale and other material.

Solids Filtration Feed to filter: as productA the temperature of the feed was about 1 200F and the pressure on the inlet side of the filter was about 400 psig. The residence time was about 0.5 minutes. Product from filter: crude oil containing 5% volume of gas 1 ppm by weight of solids 10% wt of water

product B Coalescer Feed to coalescer: as product B the temperature of the feed was about 120 F and the pressure upstream of the coalescer element was about 380 psig residence time of oil phase in coalescer vessel 1 minute.

Products from Coalescer Oil Stream: crude oil containing 5% volume of gas 1 ppm by weight of solids 1.0% wt of water Water stream: water containing 100 ppm of oil Oil clean up Feed: as oil stream above residence time about 2 minutes Product: crude oil containing 5% volume of gas 1 ppm by weight of solids 0.1% wt of water water containing 15 ppm of oil Water cleanup Feed: as water stream above residence time about 1 minute Products: oil containing 1.0%wtof water water containing 15 ppm of oil The coalescer elements in vessel 6 (Fig. 2) and vessels 20 and 22 (Fig.3) for operation in parallel had a pore size of about 50 microns. Initially the pressure drop was about 3 psi and slowly built up to about 20 psi at which point the elements were replaced.

The coalescer element in oil clean up vessel 8 (Fig.

2) and vessel 22 (Fig.3) when operated in series with vessel 20 had a pore size of 30 to 40 microns, whilst that in water clean up vessel 10 (Fig. 2) and 24 (Fig.3) had a pore size of about 15 microns.

In this example the life of the coalescer elements was about 500 hours at which point they were replaced.

The above example was repeated except the pore size of the coalescer element in vessel 6 was about 1 micron (,a) instead of 50 microns (,a). The coalescer element became blocked leading to a high pressure build up and had to be changed after 51 hours. When this example was repeated without the presence of the filter 4 the coalescer element became blocked in 10 minutes.

Water removal in filter In an alternative procedure water which had formed a separate phase at the base of the filter vessel was drained off and thereby removed from the mixture. This step has the advantage of reducing the load on the coalescer by reducing the amount of water in the feed from 10% to 5% by wt. This water can be added to the feed to the water clean up stage.

Use ofdemulsifier Two examples were carried out under identical conditions: one with no added demulsifier and one with 30 ppm of a commercially available demulsifier sold under the Trade Name Tretolite DS 964 added.

Without the demulsifier the water content of the oil stream from the coalescer was 4.0% by wt, with demulsifierthis content was 0.4% by wt. In these two examples the pore size of the coalscer element was 20 microns (,a), the pressures at the inlet to the filter 41 psig, at the inlet to the coalescer33 psig and at the outlet from the coalescer 30 psig. The operating temperature was 120 F.

Claims (16)

1. A method of separating a feed mixture containing a crude oil, produced water, produced solids and not more than about 20% by volume of gas into an oil stream from which the water has been substantially removed and a water stream containing not more than 100 ppm of oil, involving the use of a coalescer comprising a coalescer element located in a vessel the method comprising: (a) passing the mixture through afilterto remove from the mixture a substantial proportion of the solids that would blockthe coalescer, and (b) passing the filtrate through the coalescer to produce an oil phase and a water phase and separating the two phases to form an oil stream from which the water has been substantially removed and a water stream containing not more than 100 ppm of oil.

2. A method as claimed in claim 1 which com prises either (i) controlling the conditions at which the coalescer is operated so that the water content of the oil stream is less than 1% by weight, or (ii) allowing the water content of the oil stream to exceed 1% by weight and passing the oil stream through another coalescer to reduce the water content below 1% by wt.

3. A method as claimed in either claim 1 or 2 wherein the feed mixture contains from 2 to 10% by volume of gas and also gas in solution in amount sufficient to reduce the viscosity of the feed mixture and which comprises operating at a superatmospheric pressure to retain the dissolved gas in solution.

4. A method as claimed in claim 1,2 or3 which comprises operating the coalescer under the following conditions: pressure drop across coalescer element from 1 to 40 psi, residence time of oil phase in the coalescer vessel from 0.50 to 3.0 minutes.

5. A method as claimed in any one of the preceding claims wherein the coalescer is operated so that coalesced droplets of water pass into a continuous oil phase on the downstream side of the coalescer element and settle under gravity towards the base of the vessel to form the water phase.

6. A method as claimed in any one of the preceding claims wherein the filter is operated so that separation of the water from the oil takes place and which comprises withdrawing water as a separate phase from the filter to reduce the amount of water passing to the coalescer.

7. A method as claimed in any one of the preceding claims wherein the filter to remove solids is regenerable by backwashing and which comprises backwashing the filter with the feed mixture or oil stream and combining the washings with the oil stream.

8. A method as claimed in any one of the preceding claims wherein the pore size of the coalescer is in the range 30 to 70 microns.

9. A method of separating a feed mixture containing a crude oil from 1 to 50% by weight of produced water a minor amount of solids up to 1000 ppm and more than 10% by volume of gas to produce an oil containing less than 1.0% by weight of water which method comprises: (a) passing the mixture through a separatorto reduce the amount of gas to below 10% by vol ume, (b) passing the mixture through a filter to remove solids therefrom, said filter being regen erable by backwashing and backwashing the filter with the feed mixture and combining the washings with the oil product, (c) passing the filtrate containing gas in solution in amount sufficient to reduce the viscosity through a coalescer operated at superatmospheric pressure to retain the dissolved gas in solution and further so that coalesced droplets of water pass into a continuous oil phase on the down stream side of the coalescer and settle under grav its to produce an oil phase and a water phase and separating the two phases to form an oil stream and a water stream containing not more than 100 ppm of oil and either (i) controlling the conditions at which the coalescer is operated so that the water content of the oil stream is less than 1% by weight, or (ii) allowing the water content of the oil stream to exceed 1% by weight and passing the oil stream through another coalescer to reduce the water content below 1% bywt, and (d) passing the water stream through a further coalescer to remove oil therefrom to reduce the oil content and give a water product.

10. An apparatus for separating a mixture containing a crude oil, from 1 to 50% by weight of produced water, a minor amount of produced solids and not more than 20% by volume of gas into an oil stream containing not more than 1% by weight of water and a water stream containing not more than 100 ppm of oil, said apparatus comprising: (a) a filterforthe removal of said produced solids, (b) a vessel containing a coalescer element for the separation of the filtered mixture into an oil stream and a water stream said coalescer vessel being connected to the filter so that filtered mix ture is passed as feed to the coalescer vessel.

11. An apparatus as claimed in claim 10 wherein the pore size of the coalescer element is in the range 30 to 70 microns.

12. An apparatus as claimed in claim 10 including a second coalescer element located in a vessel for the separation of the filtered mixture from the filter, said second coalescervessel being connected to the filter and to the first mentioned coalescer ves sel to permit operation in series or in parallel with said first mentioned coalescer.

13. An apparatus as claimed in claim 12 wherein the second coalescer vessel is connected to the first coalescer vessel so that the oil stream is passed as feed thereto, said second coalescer having a pore size of 20 to 50 microns.

14. An apparatus as claimed in claim 10 or 11 wherein the filter has means of backwashing a portion thereof whilst forward flow through the remainder is maintained and for effecting said backwashing with a portion of the feed mixture or oil stream and ombining the backwashed material with the oil stream from the coalescer.

15. An apparatus as claimed in any one of claims 10 to 12 wherein an oil/gas separator is connected to the filter to reduce the gas content of the feed mixture prior to filtration.

16. An apparatus substantially as hereinbefore described with reference to any one of Figures 1,2 or 3.