GB2050857A - Separation method and apparatus - Google Patents

Abstract

A crude oil feed containing produced water, e.g. 5.0 to 50%, is treated to reduce the water content, e.g. to 0.5 to 5.0%, by passing through a coalescer comprising a backwashable coalescer element disposed in a vessel. Separate streams of oil and water can be withdrawn from the vessel or the oil and water can be passed from the coalescer vessel to a gravity separator from which separate streams of oil and water are withdrawn. <IMAGE>

Description

SPECIFICATION Separation method and apparatus This invention relates to a method of treating a crude oil containing produced water to reduce the water content thereof and also to an apparatus therefor.

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 usually 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 well head whether by pipeline or tanker.

For the separation of certain oil and water mixtures 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 kerosene and from marine fuel oil and also for removing minor amounts of oil from aqueous effluents.

Further, our copending UK Patent Application No.49844/78 (Serial No.2038652) describes a method which involves passing the crude oil first through a filter to remove produced solids and then through a coalescer.

This method has now been further developed.

According to the present invention a method of treating a crude oil feed containing produced water to reduce the water content thereof comprises passing the crude oil feed through a coalescer, the coalescer being cleanable by backwashing and controlling the conditions so as to effect coalescence and either (i) withdrawing the water and oil of reduced water content as separate streams from the coalescer or (ii) passing the oil and water from the coalescer to a gravity separator and withdrawing from the gravity separator a crude oil of reduced water content.

The crude oil feed will normally contain produced solids and since these will tend to be collected by the coalescer (which will thus act as a filter) it is essential that the coalescer is cleanable by back-washing.

The produced solids referred to above are those that are normally present in crude oil such as particles of sand, precipitated salts or pipe scale. These tend to block the coalescer and lead to a pressure build up across the coalescer.

The amount of produced solids in the crude oil feed may be up to 2000 ppm and will usually be in the range 5 to 1000 ppm. The amount of free gas in the crude oil feed will usually be less than 10% by volume.

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.

Since the coalescer will inevitably filter out the produced solids to some extent and most, if not all, filters effect coalescence to some degree, there is an overlap between the terms coalescer and filter.

Accordingly the coalescer element can be any of the elements described as the filter element in our copending UK Patent Application No. 49844/78 (Serial No. 2038652) The coalescer can comprise an element made from a wide range of materials, for example, stainless steel, nylon, polypropylene, polyester or glass fibre. The element can be in the form of a screen, mesh or net made, for example, of polyester or polypropylene fibres, and can optionally be pretreated with a resin.

Alternatively, the coalescer may comprises a granular solid material such as anthracite or sand or the like.

The flow through the coalescer can be axial or radial.

Conveniently means are provided for backwashing a portion of the coalescer element whilst forward flow through the remainder is maintained.

The operation of the coalescer can be effected as follows: pressure upstream of the coalescer can be up to 1000 psig and the pressure drop across the coalescer conveniently less than 40 psi, for example, 1 to 20 psi residence time of oil in coalescer vessel 0.1 to 10 minutes, preferably 0.3 to 3 minutes. The coalescer may be backwashed when a high pressure drop is generated or at fixed time intervals chosen to avoid a high pressure drop.

The liquid used for the backwashing of the coalescer can be a portion of the treated oil which has passed through the coalescer. The backwash liquid can be fed to a system of rotatable nozzles or apertures which rotate with the backwash collecting system.

The coalescer can be preceded by heating (to control the viscosity and/or maintain the temperature of the oil above that at which wax, if present would separate out) and/or degasification.

The method of the present invention is particularly suitable for treating a crude oil containing 5 to 50% by wt of water to produce a crude oil whose reduced water content is from 0.5 to 5% by wt.

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

The water stream withdrawn from the coalescer (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 containing not more than 15 ppm of oil and preferably 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 as the oil clean up.

The crude oil feed can be a crude oil from which the gas has been partly removed, particularly a crude oil containing up to 10% by volume of free gas and from 10 to 50% by weight of water. When the crude oil contains more than about 10% by volume of free gas then a gasseparation stage may precede the coalescer.

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

In addition to this gas the crude oil 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 operation pressures for the coalescer are in the range 30 to 1000 psig and desirably the pressure is such as to retain the dissolved gas in solution.

According to another aspect of the present invention an apparatus for treating a crude oil containing produced water to reduce the water content thereof comprises a coalescer comprising a coalescer element disposed in a vessel, the coalescer element being cleanable by back-washing and the vessel being provided with means for effecting said backwashing and having an outlet connected to a gravity separator the latter having an outlet for the separated water stream and an outlet for the separated oil of reduced water content.

According to one embodiment of the invention an apparatus for treating a crude oil containing produced water to reduce the water content thereof comprises a coalescer comprising a coalescer element disposed in a vessel, the coalescer element being cleanable by backwashing and the vessel being provided with means for effecting said backwashing and an outletforthe separated water and an outlet for the oil of reduced water content connected to the backwashing means.

The invention is illustrated with reference to the accompanying drawings in which Figure 1 is a schematic drawing showing a single coalescer vessel system, the feed to the coalescer and the various streams therefrom.

Figure 2 is a schematic drawing showing a two vessel system employing coalescer and gravity separator, and Figure 3 is a vertical section of a coalescervessel for use in the present invention.

Example 1 Referring to Figure 1, a single vessel of diameter 4 feet and height 8 feet containing a backwashable polypropylene mat coalescer element of 3000 sq inch area (the coalescer element was obtained from the Plenty Group Limited of Newbury, Berkshire) was used to treat 10 to 12,000 barrels/day of crude oil at a gathering centre. The crude oil had been previously treated to separate gas as 50 psig at the gathering centre. The content of free gas was less than 10% by volume. 10 ppm of Tretolite DS 964 or Servo 2856 demulsifierwas added to the crude oil upstream of the vessel. The operating pressure was 45 psig.

The crude oil contained: produced solids 5 to 1000 ppm produced water 20% by volume c free gas content < 10% by volume The coalescer was operated at 45 psig at 40"C the latter being the temperature of the crude oil feed.

Residence time of oil phase in coalescer vessel was 2 minutes.

Separated water was run off from the base of the vessel by means of a float controlled valve whilst crude oil containing about 3.5% by wt of water and less than 5 ppm of solids was withdrawn from the top of the vessel. A portion of oil was employed to backwash the coalescer and the backwashed fluids recombined with the dewatered oil.

Example 2 Example 1 was repeated on a smaller scale. The vessel diameter was 12 inches, area of element 60 square inches, crude oil treated at 400 barrels/day. The crude oil feed was preheated to 550C to disperse wax.

Residence time of oil phase in coalescer vessel was 2 minutes.

Example 3 Referring to Figure 2, a polypropylene mat coalescer element similarto that used above was housed in a smaller vessel, i.e. 27 inches diameter and 5 feet in height A crude oil containing 20% by volume of water and 5 to 1000 ppm of solids and 10 ppm of Tretolite DS 964 was treated. The coalescer was operated at 27 bar (400 psig) and 45 C. The free gas content was less than 10% vol. The coalescer was located at a crude oil gathering centre and the feed supplied from a first stage gas separator. Residence time was 0.4 minutes. The total mixture (oil and water) after passing through the coalescer was passed to a gravity separator in the form of a tank 4 feet diameter and 8 feet in height. Gravity separation was effected at 27 bar pressure. A crude oil whose water content varied from 0.50 to 3.5% by vol was withdrawn from the gravity separator.

The abbreviation ppm means parts per million by volume.

Tretolite is a registered trade mark.

In the above Examples 1 and 2 the coalescer vessel employed was as shown in Figure 3. Wet crude oil feed was supplied through the base of the vessel 1 via inlet 2 and caused to flow radially outwardly through an annular coalescer element.

The coalescer is operated so that the oil water interface is below the filter element. Water droplets on passing through the coalescer enter a continuous oil phase and settle to the bottom to form a water phase which is continuously withdrawn via outlet 4.

The vessel is provided with a continuously rotatable backwash cleaning arm 8 by means of which the coalescer element is continuously backwashed. The backwashings are withdrawn via outlet 6. The oil phase is withdrawn from an outlet 10 near the top of the vessel and a fourth outlet 12 is provided for separated gas.

The separated water withdrawn from outlet 4 can be further treated in a water clean up stage by passing through a further coalescer as described in UK Patent No. 2038652A to obtain water containing not more than 15 ppm of oil (total oil including soluble oil as determined by method API - 733 - 58).

Claims (11)

1. A method of treating a crude oil feed containing produced water to reduce the water content thereof which method comprises passing the crude oil feed through a coalescer comprising an element disposed in a vessel, the coalescer element beng cleanable by backwashing and controlling the conditions so as to effect coalescence and either (i) withdrawing the water and oil of reduced water content as separate streams from the coalescer vessel or (ii) passing the oil and water from the coalescer to a gravity separator and withdrawing from the gravity separator a crude oil of reduced water content.

2. A method as claimed in claim 1 wherein the crude oil feed contains 5 to 50% by vol of water and the oil of reduced water content contains 0.5 to 5.0% by vol of water.

3. A method as claimed in either claim 1 or claim 2 wherein an effective amount of a demulsifier is added to the crude oil feed.

4. A method as claimed in any one of the preceding claims which comprises preheating the crude oil feed to disperse wax.

5. A method as claimed in any one of the preceding claims which comprises operating the coalescer so that droplets of water on passing through the coalescer element enter a continuous oil phase.

6. A method as claimed in any one of the preceding claims which comprises further treating the water stream from the coalescer or the separated water from the gravity separator by passing through a further coalescer comprising a coalescer element disposed in a vessel to obtain water containing not more than 15 ppm of oil.

7. An apparatus for treating a crude oil feed containing produced water to reduce the water content thereof comprising a coalescer comprising a coalescer element disposed in a vessel, the coalescer element being cleanable by backwashing and the vessel being provided with means for effecting said backwashing and having an outlet connected to a gravity separator the latter having an outlet for the separated water stream and an outletforthe separted oil of reduced water content.

8. An apparatus for treating a crude oil containing produced water to reduce the water content thereof comprising a coalescer comprising a coalescer element disposed in a vessel, the coalescer element being cleanable by backwashing and the vessel being provided with means for effecting said backwashing and an outlet for the separated water and an outlet for the oil of reduced water content and an outlet for separated gas.

9. An apparatus as claimed in claim 7 or 8 wherein a further coalescer vessel containing an element is provided for treating the separated water, the vessel being connected so that the separated water is passed as feed thereto.

10. A method of treating a crude oil stream substantially as hereinbefore described with reference to any one of the Examples.

11. An apparatus for treating a crude oil stream substantially as herein before described with reference to Figures 1, 2 or 3.