GB2329849A

GB2329849A - Oil and water separation system involving recycling of scrubbing water

Info

Publication number: GB2329849A
Application number: GB9720886A
Authority: GB
Inventors: Ali Hussain Al-Kandari; David Brownlee Rochford
Original assignee: Kuwait Oil Co; BP Kuwait Ltd
Current assignee: Kuwait Oil Co; BP Kuwait Ltd
Priority date: 1997-10-01
Filing date: 1997-10-01
Publication date: 1999-04-07
Anticipated expiration: 2017-10-01
Also published as: GB9720886D0; GB2329849B

Abstract

An apparatus and method for separating a mixture of oil and water comprises the steps of a) adding recycled water from a later stage c) and separating the mixture; b) washing the separated oil with scrubbing water and then separating the mixture; c) recovering the washed oil and recirculating some of the used scrubbing water back to stage a). The initial mixture may be crude oil obtained form an oil well, and may be heated to 20-100‹C before entering at least one primary degasser. There may be at least one primary separator 12, which may be of any suitable type, including gravity or hydrocyclone, and may contain baffles or weirs (see figs 5A to 5D). The oil from the primary separator may be heated to 50-90‹C before entering at least one secondary separator, 25A, 25B, which acts as a desalter. Here the oil is mixed with the scrubbing water, which may be fresh, aquifer, river, sea water, or purified formation water, and may contain an oxygen scavenger. At least 10% of the scrubbing water is recycled to the primary separators, and may be treated beforehand, eg by separation 40, with the rest being sent to waste.

Description

SEPARATION PROCESS The present invention relates to a process and apparatus for separating oil and water e.g. from a crude oil well.

When crude oil is produced it is mixed with variable amounts of water andlor gas. With increasing age of a well, the proportion of water to oil tends to increase until the oil production becomes uneconomic.

The oil and water are usually crudely separated, and then the oil dewatered prior to transport and sale, and the water deoiled prior to disposal either by evaporation, injection into the underground reservoir or disposal to estuaries or the sea. Thus both separations have to be efficient both for economic and environmental reasons.

It is known to pass the crude oil, water gas mixture to a primary degasser, to separate gas, and then to a preliminary degasser and primary separator to give a first oil product of small water content and a first water product of low oil content.

The first oil product may be dehydrated e.g. in an electrostatic desalter separator to remove more water and then passed to a second desalter in which both desalter stages involve washing with water to remove salts to give a second oil product of reduced salt and water content for sale. The Aqueous effluent from the desalter (second water product) and the first water product from the primary separator are passed to a waste water purification system to remove their oil and solids.

The waste water purification systems are expensive. Improvements have been described to increase the effectiveness of the separation, e.g. with electrostatic separators and/or hydrocyclones, and/or heating but still the purification systems are needed.

A process and apparatus have now been devised which can provide a substantially purer first water product from the primary separator, and may avoid the need for the waste water purification systems.

The present invention provides a process for separating oil and water from a mixture thereof, which comprises the steps of (a) contacting said mixture with a recycled second water product from step (d) of the process to form a first oil product and a first water product and separating said first products; (b) contacting said first oil product with scrubbing water to produce a second oil product and second water product and separating said second products, (c) recovering said second oil product and (d) passing at least some of said second water product to step (a) The present invention also provides an integrated apparatus for separating oil and water from a mixture thereof which comprises (1) a primary separator having a first entry for said mixture and second entry for recycled water, or a combined first/second entry for both mixture and water, and a first upper exit for first oil product and second lower exit for first water product, (ii) a secondary separator having a third entry for said oil product from a line connecting said first and second separator, a fourth entry for a third water product and a third exit for a cleaned oil product and a fourth exit for second water product.

(iii) a recycle line from said fourth exit to said second entry or combined first/ second entry.

The oil water mixture can contain up to 60% (by volume) water e.g. 1-50% but preferably 5-40% especially 10-25%. The mixture may also comprise natural gas with a gas to oil ratio (GOR) (expressed as standard cu. ft. gas per barrel of oil, scfpb) of up to 100 e.g. 10-50. The mixture is obtained from an oil well, usually directly but may be at least partly degassed in a primary or preliminary degasser before entry to the primary separator. The well may be sub sea, with the separator on the sea bed or preferably on a surface platform or onshore but preferably the well is on land with the separators on the surface. Preferably the well is in a place where space and weight are not critical e.g. as may occur in a desert or scrub land or jungle inland or close to the sea shore. The water in the mixture is formation water and can contain up to 300,000 ppm total dissolved salts e.g. 5000-300,000 ppm total salts such as 100,000-250,000 ppm. The salts are usually soluble halides and/or sulphates of alkali metals and/or alkaline earth metals, e.g. sodium potassium, magnesium and/or calcium.

The oil water mixture is usually at 20-100 C such as 30-60"C or 35-55"C when it enters the first primary degasser or separator and may be at a pressure of 1-20 bar such as 5-10 bar.

The mixture can meet the recycled water before entry to the primary separator e.g. in a feed line thereto or in a degasser before the separator or can meet the recycled water in the separator itself, or more than one of these meetings can be used.

The primary separator may be any vessel for separating 2 substantially immiscible liquids and in particular one having means for separating gas as well.

Examples of such separators are gravity separators (which are preferred) or a enhanced gravity separators e.g. induced or dissolved gas flotation units, and hydrocyclones. The separator may have an initial mixing zone and then a separation zone, or the mixing zone may be in a separate vessel from the separator.

Preferably however there is no discrete mixing zone; the oil/water mixture and recycled water are passed with the separator and separation commences.

In a separator functioning by gravity alone or assisted, the liquids are mixed thoroughly in the mixing zone if present, and this zone (or part of the separator) may contain moving parts e.g. agitation means such as paddles or stirrers and/or pumps, but preferably the zone has no moving parts, the mixing being effected by special distribution of the inlets for the mixture and second water product. Thus the mixture and recycled second water product may enter together into the mixing zone from one inlet, or may enter separately, in particular with the mixture inlet below the level of the water inlet; preferably entry is by both means.

The entry of oil gas mixture, alone or with recycled second water may be directly into a primary gravity separator e.g. as ajet, especially under the oil/water boundary level, but preferably to aid separation the entry is via at least one distributor tube under the boundary, the distributor tube(s) being preferably at least partly perforated and/or slotted to direct the flow evenly. The tube may simply traverse the separator but preferably has at least one especially 2-14 arms therefrom, each arm being preferably at least partly perforated. The perforations are usually below the oil/water level; advantageously the tube and arms extend horizontally. Preferably the tube and arms are disposed within the gravity separator to distribute the oil water mixture substantially uniformly into it.

After the mixing zone (if present) the mixture and recycled water pass to a separation zone, which may be in a hydrocyclone, but is preferably part of a combined mixer and separator in a gravity separator. In the gravity separator, the oil and water layers separate, and any gas separates above both. A flow divider may be used to separate the layers of oil and water, such as a weir or baffle of particular adjustable height, while an outlet pipe or pipes from the separator may be located at an appropriate height above the separator floor to remove the oil layer; the first water product may be removed via an outlet (or outlets) low in the separator wall, or less preferred in the floor. The first water product is preferably collected from the water layer in the separator via one or more collector tubes, which may be perforated and may traverse the separator, optionally with at least one side arm extending therefrom and also optionally perforated. If desired the tube and/or arm may be open at its far end. The separator may have vertical walls or have walls tapering in or outwards or waisted or convex. The separator may be cylindrical as in a horizontal cylinder, or be a tank with a rectangular side view, and a rectangular plan view, or preferably a circular plan view like a drum.

If desired the separator may comprise electrostatic means to help separate the layers in particular to help break any emulsions; preferably however, such electrostatic means is absent. Thus a preferred gravity separator has one or more inlets for the mixture and second water into a mixing zone without moving parts and a separation zone with at least two outlets for oil and water, but again no moving parts or electrostatic or other moving emulsion breaking means. The separator and/or mixing zone may contain baffles or weirs or other non moving devices to encourage phase separation and/or to elongate the flow path of the separating liquids therein, e.g. with reversal of the horizontal flow direction at least once, such as 1-4 times; baffles disposed to create S or Z or sinusoidal shaped paths in plan view are useful. When the entry of oil, water mixture is via a distribution tube, the path elongation may be used but is preferably absent.

The feeds to the primary separator pass continuously into the separator in a volume ratio of oil/water mixture to second product water of 100:2-10, in particular for mixtures containing 5-50% water and 95-50% oil/by volume especially 5-25% water and 95-75% oil. The volume ratio in the separator of oil to total water may be 10:1 to 1:10,preferably7:1 to 1:3, in particular 7:1 to 1:1 especially 7:1 to 3:1. The percentage of total water to total oil/water mixture and second water product is usually 5-50% e.g. 10-30% such as 12-25% (by volume).

The volume ratio of water in the oil/water mixture to recycled second product water is usually 100: 1-50 e.g. 100:10-50 such as 20-40.

The inlet temperature of the mixture may be 20-100 C in particular 25-90"C, while the inlet temperature ofthe second water product may be 30-80"C e.g. 40 75"C in particular 45-65"C. The temperature of the recycled second water product is usually above that of the inlet temperature of the mixture when they meet e.g. 5 30"C above especially 12-250C above. The temperature of the liquids in the primary separator is usually 30-90"C, in particular 35-70"C such as 40-55"C. If required or desired one or both of the mixture and second water product may be heated or cooled before entry to the separator.

The primary separator may be operated under pressure, e.g. under 0-3 bar g when there is no preliminary degasser, but after such a degasser the separator may be at a pressure of 2-40 inches water gauge (5-100cm water gauge).

The soluble salt content ofthe water in the separator may be 10,000350,000 ppm, e.g. 50,000-300,000 ppm such as 200,000 to 250,000ppm.

If desired chemicals may be present in the water in the primary separator to aid phase separation e.g. demulsifiers, and/or to aid wetting of insoluble solids present e.g. sand and/or corrosion products, by oil or water, in particular to aid oil wetting so the/solids leave the separator in suspension in the oil phase, in preference to leaving in suspension in the first water or remaining at the interface reducing the rate of phase separation.

The residence time for the liquids in the separator is usually 1-60 hours, in particular 2-15 hr preferably 4-8hr, or may be 15-50 hrs.

There may be one primary separator with one inlet for recycled second product water and one outlet for first water product, or there may be more than one primary separator, arranged in parallel or series or both with respect to the oil/water mixture. When entering in parallel separate streams of oil/water mixture pass to each, but in series a stream of mixture passes to the first primary separator and some first water is removed, and the residual partly separated upper layer enriched in oil is passed to a further primary separator to remove more water; 1-4 separators in series or 1-10 separators in parallel may be used. The recycled second water stream may be passed to each separator in parallel to give a number of streams of first water product or may be passed to one or more e.g. each separator of a number of separators in series, optionally with passage of the first water product from that separator to another primary separator e.g. from the first one to the next primary separator. The subsequent separators in series after the first may be provided with means to break emulsions and/or increase separation e.g. electrostatic separators but preferably such electrostatic means are absent.

Each primary separator has contact of an oil containing layer with a separately added water layer, the water being directly recycled second water or coming from another primary separator; hence each separator has an oil feed and a water feed.

Preferably with primary separators in series the recycled second water may be passed to the last of the primary separators with recycle of the aqueous phase produced to the penultimate separator etc; the overall effect in this case is feed of the mixture to the first of the series of the separators and feed of the recycled water to the last of the series to give a first oil product leaving from the last primary separator and a first water product leaving from the first primary separator. The conditions e.g. of oil to total water ratio in each separator may be the same or different.

Thus the contact of oil water mixture and recycled water may be in a single stage or more than one co-current or preferably counter current stage, each stage representing a primary separator.

In order to aid separation of gas in the primary separator(s), the oil/water/gas mixture is preferably passed through a degasser at a lower pressure than well pressure to remove the majority of the gas from the liquids as a gas stream and leave a liquid stream. More than one degasser may be used, in series or parallel, in particular 2-4 especially in series. The, or each degasser may be a horizontal or vertical one; in particular a combination of both is preferred. The gas stream may be separated or at least some preferably all may be passed to the primary separator above the total liquid level therein to mix with gas separated there, prior to its removal from the separator; by this means all the gas is recovered in one line.

Prior separation of at least some gas from the oil/water/gas mixture also aids separation of oil and water in the primary separator by reducing emulsion formation and making the phase separation occur more cleanly. In order to aid this separation a prior liquid phase separation step may be performed on the oil/water/gas mixture to remove a primarily liquid phase which can be passed to the lower part of the primary separator, and leave an oil/water/gas mixture of reduced liquid content and higher gas content, which is easier to degas. This prior liquid separation is preferably in a gravity separator with an internal weir to separate the water stream.

The first oil product from the primary separator usually contains 0.5-10% e.g. 1-5% water such as 2 to 4% water. The first oil product is preferably heated e.g. to 50-90"C prior to being passed to the secondary separator(s) where it is desalted by contact with scrubbing water. The contact is preferably without any prior separation of water from the heated first oil product. This contact may also be in a single stage or in more than one stage of cocurrent or especially counter current contact, in particular 2-4 stages of counter current contact. The or each secondary separator may have a mixing and separation zone as described above for the primary separators and may otherwise be as described above preferably without a separate mixing zone. The or each secondary separator (or separators in total) usually has 1-10% ofthe separation volume ofthat ofthe total primary separator(s). The residence time and temperature may be within the ranges specified above, in particular the temperature being 10-35"C e.g. 10-20"C above the temperature of the primary separator such as at 40-750C. The pressure in the secondary separator(s) is usually at 2-7 barg, in particular 3-6 barg. The scrub water may be fresh, aquifer, river or sea water or a purified formation water, which has its salt content reduced e.g. by dialysis or electrolysis; water from a mixture of sources may be used. Oxygen scavenger may be added if desired to the water used to provide the scrub water. The total soluble salt content of the scrub water may be 1-50,000 ppm, e.g. 10-500 ppm, or 500-15000 or 3000-lOOOOppm, e.g. for brackish water or 10,000-50,000ppm for sea water; scrub water has usually less than a fortieth of the salinity of the water of the oil water mixture. The volume percentage of scrub water (in total) to first oil product is usually 1-15% preferably 2-10% such as about 4-7.5%, smaller amounts being used with multistage countercurrent contacts.

In a Preferred process, the contact of first oil product and scrub water is in 2 countercurrent stages with feed to the first secondary separator of first oil product and recycled used scrub water from the second secondary separators to give purified intermediate oil product and second water product, and then feed of that purified oil product and scrub water to the second secondary separator to give exit flows of second oil product (now doubly purified) and used scrub water for recycling to the first secondary separator.

The second oil product usually contains less than 0. 1% water e.g. 200-1000 ppm water or up to 1000ppm (total base sediments and water (BS & W)) and up to 401bs p 1000 barrels (ptb) total soluble salts e.g. 1-30 ptb such as 5-l0ptb if brackish water is used for scrubbing or 10-25ptb if sea water is used for scrubbing; the second oil product may be passed for sale or transport as crude oil or directly for refining. If desired a combination of brackish water and seawater can be used to reduce brackish water consumption achieving a second oil product less than 0.1% (BS & W) e.g. 7-20ptb. If desired the second oil product may be passed through a heat exchanger to heat the first oil product before entry to the secondary separator.

The second water product, which is the aqueous effluent from the secondary separators usually contains up to 10% oil e.g. lOOppm to 10% oil such as up to 1% oil (by volume) e.g. 100-800 ppm or 300-700 ppm oil, and 1000-250,000ppm e.g.

10,000-200,000ppm, such as 40,000-200,000 or 60,000-180,000ppm total soluble salts and insoluble solids up to 100 mg/l e.g. 5-80 mg/l such as 10-40mg/l. At least part e.g. at least 10% or at least 50% such as 100%, or only a part such as 20-80% of the second water product is recycled to the primary separator, either directly or into a line containing oil/water mixture prior to entry to the separator or into a prior degasser, or any combination of these.

The first water product from the primary separator(s) usually contains less than 100 ppm, e.g. 0.1-20 and especially 1-10 or 2-5 ppm oil even during any temporary malfunctions of the secondary separators when the amount of oil in the second water product may be as high as 10%. The first water product can be passed for disposal e.g. as described above, usually without any further purification to remove oil, e.g. without at least one of a degasser, non gravity separator e.g. tilted plate separator and induced gas flotation unit to separate from oil traces especially without the separator, flotation unit or hydrocyclone unit.

If desired the second water product may be treated between leaving the secondary separator and entering the primary one. The treatment may be a purification to remove insoluble solids and/or oil, or to recover purer water (i.e. of lower oil content) therefrom. Thus the second water product may be treated to separate purified water and to leave a liquid of higher oil content e.g. 0. 1 - 5% oil such as 0.3 - 2% oil; the purified water may be passed as such for disposal etc, while the liquid is recycled to the primary separator (or entry lines therefor). This treatment reduces the extra water being added to the primary separator, as well as reducing the extra salt or contaminant e.g. oxygen or corrosion product or oxygen scavenger being added in that water. The treatment may be performed in a tertiary separator, which may be a gravity separator or a hydrocyclone. Treatment to remove solids may be through a filter, with the eluate passing to the primary separator (or entry line therefor).

The process is usually performed continuously, with flow ratios of oil/water mixture and scrub water and control valves controlled to keep substantially constant the oil/water interface level in the gravity separators.

The first water product is usually of sufficiently low oil content to be passed for disposal, but may be further purified to remove oil if desired. This product may however contain solids, such as solids from the well e.g. sand or rock, or scaling products from interaction of the formation water salts and salts in the scrub water, or corrosion products. While addition of oil wetting agents can help entrain the solids into the first oil product, some may remain in the first water product. The solids can be separated from this product via a separation means e.g. hydrocyclone or filter or preferably both e.g. with filter first as a primary screen to remove as filtrate the majority of the water for disposal, and then for the remaining wet solids a secondary solid/liquid separator to remove more water, for disposal or recycle to the primary or secondary separator (or feed lines therefor). The solids still remaining may be dewatered further with a cyclone, the liquid from which can be recycled to the primary separator; this dewatering may be performed one or more times, with recycle of liquid and eventual recovery of solids for disposal.

Preferably the process of the invention comprises at least one of the following process features (a) first oil product contains 1-5% water (by volume), when it is heated prior to contact with the scrub water, (b) the contact of first oil product and scrub water is in more than 1 contercurrent stage e.g. 2 separators, (c) the separation of water and oil in the primary separator(s) is at 25-650C especially 40-60"C, (d) the second water product recycled to meet the oil/water gas mixture contains less than 18% oil e.g. 1-10% oil (by volume), (e) the first oil product is not subjected to electrostatic dehydration after it has been heated and prior to contact with the scrub water and (f) the percentage of total water to the total of oil/water mixture and recycled second water in the primary separator(s) is usually 5-60% especially 10-40%. Especially the process has feature (f) and in particular at least one of the other features, preferably (b).

A benefit of the invention is that the clean water obtained as first water product may be pure enough for direct disposal without the need for an expensive deoiling and filtration waste water systems. The clean water can be reinjected with reduced need for extra pressure and with reduced formation deposition problems and hence reduced frequency of well workovers. The recycling operations of the invention may be retrofitted to existing deoiling and dewatering plants. Savings in capital and operation expenditure can be made.

The invention is illustrated with reference to the accompanying drawings which are flow diagrams in which Fig. 1 relates to a basic form of the present process.

Fig. 2 and 3 relate to a process of Fig. 1 with different extra treatment of a recycle stream.

Fig. 4 relates to a process of Fig. 1 with extra solids removal.

Fig. 5A-5D represent plan views of a number of primary separators for use in the process.

Referring to Fig. 1 an entry line 1 for an oil/water/gas mixture from a well head leads to a preliminary separator 2 having an internal weir 3, a water outlet 4 and second outlet 5 leading via line 6 to a vertical degasser 7 having an upper gas outlet 8 and a lower liquid outlet 9, which meets line 10 from outlet 4. Gas outlet 8 is joined via line 11 to a primary separator 12 at upper gas entry 13, while line 10 leads to a lower liquid entry 14 to separator 12. Separator 12 has a gas exit 15 and a lower water exit 16 for first water product and intermediate oil exit 17 for first oil product, while separator 12 has internal means for distributing liquid from inlet 14 (see Fig. 5) and may have a weir (not shown). Oil exit 17 is joined via line 19 and pump 20 to a heat exchanger 21 provided with entry line 22 for purified oil and exit line 23 for cooled oil and exit line 24 for heated first oil product. Exit line 24 leads to a heater 36 and thence to a secondary separator 25, which has two stages with separators 25A and 25B in series spaced by line 26. A line 27 joins separator 25B to line 24 upstream of separator 25A, which has exit 28 for second water product, line 26 is also linked via line 29 to a tank 30 for scrub water e.g. brackish or sea water, tank 30 having an upper air vent 31. Separator 25B has exit line 27 for a water phase and line 22 for purified oil returning to exchanger 21. Exit 28 from secondary separator 25A leads back to primary separator 12, directly via recycle line 32 and entry 33, while line 32 has a branch line 34 to entry linelO.

Water exit 16 in primary separator 12 is joined to clean water line 35.

In use the crude oil/water/gas mixture enters in line 1, water and/or oil is separated in separator 2, passing via line 10 to separator 12; the mixture of reduced oil or oil and water content and higher gas content passes via line 6 to degasser 7 where it is separated into a gas stream passing to entry 13 in separator 12 and a liquid stream passing via line 10 to separator 12. In separator 12, oil water and gas phases separate with gas leaving via exit 15, first oil produced via exit 17 and clean water via exit 16. The first oil product passes via line 19 to exchanger 21 where it is heated by incoming hotter purified oil from line 22 to give purified oil in line 23 for export and a heated first oil product in line 24 which is reheated in heater 36 and then mixed with used scrub water from line 27 and the mixture is separated in separator 25A into a second water product leaving at exit 28 and an oil product passing in line 26 to meet fresh scrub water from line 29. The mixture of oil and scrub water in line 26 is separated in separator 25B into purified oil for line 22 and used scrub water for line 27. The second water product passes from exit 28 via line 32 to primary separator 12 and via line 34 to line 10.

Fig. 2 shows apparatus with the same features as that in Fig. 1 apart from the connection between secondary separator 25A and primary separator 12. Line 32 joins separator 25A to a small separator tank 40 having upper liquid exit 41 and lower liquid exit 42. Exit 41 is linked via line 43 and pump 44 to line 10, while exit 42 is linked via line 45 to clean first water product exit line 35, which has a pump 46 in it, though this is optional. The operation of the apparatus in Fig. 2 is the same as that in Fig. 1 apart from treatment of second water product leaving separator 25A. This product is passed via line 32 to tank 40, where clean water is separated for exit via exit 42 and line 45 as clean effluent and meets first product water; if desired (not shown) the clean water may be kept separate from first product water when lines 45 and 35 do not meet. The rest ofthe second water product remaining in tank 40 and exiting via line 41 contains more oil, e.g. 0.5 -1% oil, than in the second water product and is returned to the primary separator 12 via lines 43 and 10.

Fig. 3 represents a modification of the apparatus of Fig. 2 with a hydrocyclone 50 instead of tank 40, but otherwise has the same features and is operated in the same way. Also shown is a side line 51 with valve 52 leading off clean water line 45 with return line 53 to separator 12. In normal use valve 52 is closed but may be opened in the event of a problem with the hydrocyclone 50 with recycle from the hydrocyclone of all of its feed back to the separator.

Fig. 4 relates to flow diagram with the majority of the features and the mode of use substantially as for Fig. 1, apart from absence of heat exchanger 21 and heater 36 (and lines 22 and 23 associated therewith) [though these may be present if desired, not shown] and extra treatment for the first water product leaving in line 35. Line 35 joins separator 12 via pump 60 to a filter 61 from which clean water filtrate leaves via line 62. The retained wet solids side of filter 61 is joined via line 63 to a series of liquid separators 64, 65 and 66 in which more liquid is removed from the wet solids. Expressed liquid from separator 64 passes via line 67 to first oil product line 19 to aid in the desalting operation in separators 25A and 25, while the remaining wet solids pass via line 68 to a better solids separator 65, which is a solid liquid cyclone. Liquid separated in cyclone separator 65 is passed via line 69 to line 10, while the remaining wet solids are passed via line 90 to a slurry vessel 66, where remaining liquid is drained off and returned via line 91 to line 69.

Residual solid from vessel 66 is passed to slurry containment 92.

Fig. 5 shows plan views of four different arrangements, 5A-SD of the entry and exit lines of a primary separator for use in the invention. In Fig. SA the separator 12 has an entry and an exit 70 and 71 respectively and two vertical weirs or baffles 72 offset with respect to each other to define a Z shaped passage between them. Entry 70 for oil/water mixture is below the oil/water phase boundary in the separator while there are two exits 71, one above the boundary 71A for oil and one below for water 71B. During its travel from entry to exit the oil/water mixture separates into layers of oil and water which leave through upper and lower exit 71A and 71B respectively. The direction of flow from Exit 71B is in the same direction as and parallel to, but spaced from that from entry 70.

In Fig. SB, the entry 70 is joined to an array of parallel perforated distributor pipes 73 extending laterally from a central pipe 74 and located under the oil/water phase boundary. Upper exit 71A for oil is joined to a cross shaped laterally extending pipe 75 each of the other ends 76 of which is open to collect oil, the pipe 75 being above the oil water phase boundary. Lower exit 71B for water is under the oil/water phase boundary and the direction of flow from exit 71B is the same as in Fig.SA. The direction of flow of oil from exit 71A is in a direction opposed to and at an obtuse angle to the direction of flow from entry 70.

In Fig. 5C entry 70 leads to a central distributor 77 with six laterally extending perforated tubes 78 symmetrically disposed above it. Distributor 77 is under the oil/water phase boundary. There are two exits 71 one 71A above the boundary for oil and one 71B below it for water. The direction of flow from exit 71 is in the same direction as, but at an acute angle to the direction of flow from entry 70.

In Fig. 5D an entry 70 leads to a perforated pipe 79 having two perforated arms 80 extending laterally and normally therefrom. Pipe 79 and arms 80 are under the oil/water phase boundary. Each exit 71A and B is joined to an internal perforated collecting pipe 81 of T shape like pipe 79 and arms 80. The direction of flow from exits 71 is in the same direction as and colinear with the direction of flow from entry 70.

Example A plant trial was performed in an apparatus as shown in Fig. 1 with a drum primary separator in arrangement as in Fig.SB.

A crude oil water gas mixture of 86% oil and 14% water (by volume), at a flow rate of 70,000 barrels per day (bpd) with 4 million standard cu ft of gas per day (mscfpd) flowed at 460C into a preliminary separator 2 to separate some water and pass the oil water gas mixture to degasser 7 and hence primary separator 12.

Recycle second water product in line 32 at 55"C at a flow rate of 3,200 bpd, a total salt content of 55000ppm and an oil content of 500ppm was passed to meet the oil gas water mixture and separation occurred in separator 12, in which the liquids were at 430C. The volume ratio of oil to total water in the separator 12 was 100:21. The total salt concentration in the water in the separator was on average 240,000ppm. The separator gave first oil product containing 1-5% water at a rate of 60000 bpd and a first water product of 2-lOppm oil at a flow rate of 13000bpd.

The first oil product was heated by exiting second oil product in line 22 and then reheated in heater 36 to 65"C prior to mixing with used scrub water containing about 30,000ppm total salts in line 27 at flow ratios of 100:6.7. To brackish water in tank 30 containing 7500ppm total salts was added 9ppm ammonium bisulphite oxygen scavenger (per ppm of oxygen in the water). The water obtained at a flow rate of 2000-5000 average 4000 bpd was passed to line 26. Separators 25A and B were at about 60"C and about 59"C respectively. Separator 25B gave a second oil product of about loooppm (base sediments and water) at a flow rate of 60000bpd and separator 25A gave a second water product of 100-lOOOOppm oil at a flow rate of 3,200bpd. All the second water product was recycled to mix with the oil/water/gas mixture.

During the initial trial which lasted a month, the average oil content of the first water product was below 8ppm, and this month included a period of 6hrs during which the process was intentionally upset by raising the oil content of second water product from about 500ppm to 10%. This upset gave a first water product of maximum 15ppm oil. lf this peak is excluded the average for the rest of the month was about 8ppm oil.

In comparison in the month prior to the trial when no recycle in line 32 was performed the average oil content of water, after purification of "first product water" from separator 12 was still greater than lOOppm. In the 6 month period before the trial without recycling, the average oil content was 35ppm with upsets in excess of 100ppm.

The above oil contents in water are based on analysis for aromatics and unsaturated compounds. Total oil levels might be twice as high, but the benefit of at least a 4 fold reduction in oil content shown above would still apply.

Claims

CLAIMS 1. A process for separating oil and water from a mixture thereof which comprises the steps of a) contacting the mixture with a recycled second water product and separating a first oil product and a first water product; b) contacting the first oil product with scrubbing water and separating a second oil product and second water product;

c) recovering the second oil product, and d) passing at least some of the second water product to step a).
2. A process according to claim 1, wherein at least one of the separating steps is performed by gravity separation.
3. A process according to claim 1 or claim 2, wherein the first oil product and the first water product are separated at a temperature of 25 to 650C.
4. A process according to any of claims 1 to 3, wherein the first water product is passed through at least one filter and / or separator to remove solids.
5. A process according to any of claims 1 to 4, wherein the first oil product is heated prior to being contacted with the scrubbing water.
6. A process according to the claim 5, wherein the first oil product is heated to a temperature of 50 to 90"C.
7. A process according to any of claims 1 to 6, wherein the first oil product is contacted with scrubbing water in more than one stage.
8. A process according to the claim 7, wherein the stages of contact are counter current with respect to the movement of the first oil product.
9. A process according to the claim 8, wherein there are 2 to 4 stages of counter current contact between the first oil product and scrubbing water.
10. A process according to any of claims 1 to 9, wherein the scrubbing water contains an oxygen scavenger.
11. A process according to any of claims 1 to 10, wherein the scrubbing water is a combination of brackish water and seawater.
12. A process according to any of claims 1 to 11, wherein at least 10% of the second water product is recycled for contacting with the oil / water mixture.
13. A process according to the claim 12, wherein 20-80% of the second water product is recycled.
14. A process according to any of claims 1 to 13, wherein the recycled second water product is treated to further purify it before contacting with the oil and water mixture.
15. A process according to the claim 14, wherein the purification involves filtration.
16. A process according to claim 14 or claim 15, wherein the purification involves gravity separation.
17. A process according to any of claims 1 to 16, wherein step a) is carried out under pressure lower than that of the mixture and wherein gas is recovered from the mixture.
18. An apparatus for separating oil and water from a mixture thereof comprising

i) at least one primary separator having an entry for the mixture and an entry for recycled water, a first upper exit for the removal of a first oil product and a second lower exit for the removal of a first water product;

ii) at least one secondary separator operatively connected to the primary separator having an entry for the first oil product from the primary separator, an entry for scrubbing water, an exit for the removal of a cleaned oil product and an exit for the removal of a second water product.

iii) a recycle line from the exit for the second water product to the entry for recycled water to the primary separator.
19. An apparatus according to claim 18, wherein the primary separator has a common entry for the mixture and for recycled water.
20. An apparatus according to claims 18 or 19, wherein the primary separator further includes an exit for gas.
21. An apparatus according to claim 19 or 20, wherein the level of the first entry in the primary separator is below the level of the second entry.
22. An apparatus according to claims 19 to 21, wherein the level of the first entry is below the level of the oil / water boundary in the separator.
23. An apparatus according to any of claims 18 to 22, wherein the primary separator and / or secondary separator is a gravity separator.
24. An apparatus according to any of claims 19 to 23, wherein the primary and / or secondary separators contain distinct contacting and separation zones.
25. An apparatus according to claim 24, wherein the contacting zone comprises agitation means.
26. An apparatus according to claim 24 or 25, wherein the separation zone contains at least one flow divider.
27. An apparatus according to any of claims 18 to 23, wherein the primary and / or secondary separator comprises at least one flow divider.
28. An apparatus according to claim 26 or 27, wherein the flow divider is nonmoving.
29. An apparatus according to any claims 18 to 28, wherein the primary separator comprises at least one distributor tube into which the oil / water mixture is fed at the entry.
30. An apparatus according to any of claims 18 to 29, wherein the primary and / or secondary separators contain at least one collector tube for removal of first and /or second water products.
31. An apparatus according to claim 30, wherein the collector tubes traverse the primary and / or secondary separators.
32. An apparatus according to any of claims 18 to 31 comprising at least two primary separators.
33. An apparatus according to claim 32, wherein the primary separators are operatively connected in series.
34. An apparatus according to claim 33, wherein the primary separators are operatively connected in parallel.
35. An apparatus according to any of claims 18 to 34, wherein the primary and / or secondary separators comprise electrostatic means.
36. An apparatus according to any of claims 18 to 35, further comprising a preliminary separator into which the oil and water mixture or oil, water and gas mixture is fed prior to entering the primary separator, said preliminary separator being operatively connected to the primary separator.
37. An apparatus according to claim 36, wherein the preliminary separator includes an internal divider.
38. An apparatus according to claims 36 and 37, wherein the preliminary separator is a gravity separator.
39. An apparatus according to any of claims 18 to 38, further comprising at least one degasser into which the oil, water and gas mixture enters, the degasser removing the majority of the gas to leave a liquid stream that passes into the first entry of the primary separator.
40. An apparatus according to claim 22, wherein the primary separator includes an additional entry above the total liquid level in the primary separator into which is passed a proportion of the gas stream formed in each degasser.
41. An apparatus according to claims 39 or 40, comprising 2 to 4 degassers in series.
42. An apparatus according to any of claims 1 to 24, further including at least one heating device between the primary and secondary separators.
43. An apparatus according to claim 42, wherein the heating device includes one heater and one heat exchanger.
44. An apparatus according to claim 43, wherein the exit for the cleaned oil product in the secondary separator is connected to the heat exchanger to allow the cleaned oil product to exchange heat with the first oil product passing therethrough.
45. An apparatus according to claim 18 to 44, further including at least one separator through which the recycle line passes.
46. An apparatus according to claim 45, wherein at least one separator is a solids liquid cyclone.
47. An apparatus according to any of claims 18 to 46, wherein the recycle line includes at least one filter.
48. An apparatus according to any of claims 18 to 47, wherein the first water product is passed through the exit in the primary separator into a discharge line, the discharge line including at least one filter.
49. An apparatus according to claim 47 or 48, wherein the discharge line comprises at least one separator.
50. An apparatus for separating oil, water and gas from a mixture thereof substantially as described herein with reference to the accompanying drawings:
51. A process for separating oil, water and gas from a mixture thereof substantially as described herein.