WO1998023351A1

WO1998023351A1 - Separator

Info

Publication number: WO1998023351A1
Application number: PCT/GB1997/003295
Authority: WO
Inventors: Neville Paul Chamberlain; Alison Colvin Stewart
Original assignee: Kvaerner Paladon Limited
Priority date: 1996-11-29
Filing date: 1997-11-28
Publication date: 1998-06-04
Also published as: NO983473D0; EP0888158A1; NO983473L; GB9624867D0

Abstract

A separator, e.g. a three-phase separator, includes an inlet (4) and outlets (8, 9 and 12) for separated materials. Between the inlet (4) and the outlets (8, 9, 12) one or more baffles (13, 14) are provided for controlling fluid flow. The baffles are perforated and have a variable free area in different vertical zones thereof. By selecting the shape and size of the perforations, and percentage of perforated area, flow within the separator may be controlled to provide optimum separation conditions.

Description

SEPARATOR

This invention relates to a separator and, more particularly, to a separator for separating a mixture of relatively high and low density liquids into its separate high and low density constituent parts. The invention is particularly applicable to a 3- phase separator for separating a mixture of high and low density liquids and gas into its separate gaseous, high density and low density constituent parts. It should be appreciated, however, that the invention is of more general applicability and may be used in 2-phase sepaiators intended to separate high and low density liquids and in 2-phase separators intended to separate gas from liquid.

3 -phase separators are commonly used in the oil and gas industry for separating a stream which comprises an aqueous liquid, a hydrocarbon liquid, and a gas into its separate aqueous liquid, hydrocarbon liquid and gas constituent parts. The initial stream may consist of a gas flow in which liquid droplets or foam are entrained, or a substantially liquid flow in which gas bubbles are entrained. The exact design of the separator depends to an extent on the nature of the expected input flow, but in essence the function of the 3 -phase separator is to separate the three constituent parts of the input flow into three separate outputs, one consisting of substantially only aqueous liquid, one consisting of substantially only hydrocarbon liquid, and a third consisting of substantially only gas.

A typical 3-phase separator comprises an elongate tank arranged with its axis substantially horizontal. The inlet flow is delivered to one end of the tank and outlets for gas, aqueous liquid and hydrocarbon liquid are provided at the opposite end of the tank. As the material to be separated flows along the tank the gaseous component migrates to the top of the tank for removal through the gas outlet and the aqueous component settles to the bottom of the tank for removal through an aqueous liquid outlet. The hydrocarbon liquid forms a central zone of the tank between the gaseous component and the aqueous component and, near the outlet of the tank, flows over a weir to a hydrocarbon liquid outlet.

A 3 -phase separator may include at least one perforated baffle along its length for controlling the flow of liquid along the tank to allow separation of the aqueous and hydrocarbon fractions of the liquid, and to allow any gas bubbles in the liquid to migrate to the surface thereof to join the gas flow above the liquid phases. Depending on the expected nature of the input flow, and the degree of separation required, more than one perforated baffle may be provided along the length of the tank. Traditionally, the percentage of the baffle area which is open for fluid flow has been substantially constant over the entire height of each baffle.

Similarly, certain 2-phase separators of the prior art have included perforated baffles, the percentage of baffle area which is open being substantially constant over the entire height of each baffle.

It is believed that optimum separation conditions exist if a uniform plug flow is maintained throughout the separator independent of the depth of the discreet phases. However, this desirable flow pattern is difficult to obtain because of the complexities of the flow pattern within the tank induced by internal fittings within the tank, including the baffle plates, which themselves are intended to control flow.

We have now discovered that the flow patterns within the tank can be improved and brouglit nearer the optimum uniform plug flow characteristics if perforated baffles are used within the tank which have a hole configuration which, rather than being uniform, varies in different zones thereof. By appropriately selecting the hole configuration for each particular baffle the flow patterns in the region of that baffle can be improved without materially reducing the effectiveness of the baffle in terms of inducing phase separation or controlling gross bulk movements of the separator contents. The term "baffle" as used herein means a generally wall-like structure which extends across the fluid flow direction in a tank and which pennits the restricted flow of fluid therethrough. A baffle may be a self-supporting structure which is mounted in a tank or may be fonned by a plurality of separate members which are secured to an appropriate support structure in the tank to form the baffle.

The detail of the baffle design will depend on the expected nature of the input flow and, in particular, on the expected ratio of the component parts thereof, the viscosity and surface tension thereof, and on other operating conditions such as temperature, flow rate, and required degree of separation. Also, the detail of the design of the baffle will depend on whether the in-flow of fluid to be separated is at the top or at the bottom of the inlet zone of the separator. However, it would appear to be generally true that operation of a separator can be improved if the variation in baffle free area in accordance with the invention is observed.

Accordingly, one aspect of the present invention provides a separator for separating a mixture of high and low density components into its constituent high and low density parts, the separator comprising: an elongate tank with an inlet for receiving an inlet mixture of high and low density components and outlets, spaced from the inlet, for separated high and low density components respectively; and at least one baffle in the tank between the inlet and the outlets thereof, the baffle having a different proportion of free area in a zone adjacent the bottom of the tank than it has in a zone further up the tank. hi one embodiment of the invention in which the inlet for fluid to be separated is close to the top of the tank the baffle has several zones of progressively decreasing free area from the bottom thereof towards the upper edge thereof. In the alternative, the baffle may have a continuously decreasing free area from the bottom thereof upwardly.

In another embodiment of the invention in which the inlet is at or close to the bottom of the tank the baffle has several zones of progressively decreasing free area from the upper edge thereof towards the bottom thereof. In the alternative, the baffle may have a continuously decreasing free area from the top thereof downwardly.

In a particularly preferred embodiment of the invention the upper edge of the baffle is located spaced from the top of the tank to provide a completely open area above the upper edge of the baffle. Preferably, the upper edge zone of the baffle itself is imperforate. Alternatively, the baffle may include a perforated zone at the top of the tank to assist in gas distribution and foam retention, and an imperforate region immediately below the upper perforate region followed, in the downward direction, by one or more perforate regions for controlling liquid flow.

In a particularly preferred embodiment of the invention the upper edge of the baffle is spaced from the top of the tank and is imperforate, and the region between the imperforate upper edge of the baffle and the lower edge thereof includes four horizontal bands of progressively increasing free area from the uppermost of said bands to the lowermost thereof. In one particularly preferred embodiment of the invention the free area of the uppeπnost band may be about 7% of the total surface area and the free area of the lowermost band may be about 30% of the total surface area.

Preferably, the free area of the baffle is provided by rectangular perforations. The desired increase in free area towards the bottom of the baffle may be obtained by increasing the density and/or the size of the perforations. In one embodiment of the invention the perforations adjacent the imperforate upper edge of the baffle are rectangular openings measuring 10mm x 25mm and the perforations adjacent the bottom of the baffle are square holes 25mm x 25mm. It should be appreciated, however, that the present invention is not limited to the use of rectangular perforations and, indeed, perforations of other shapes may prove desirable in certain circumstances. For example, the holes may be fonned by perforating and defonning regions of the plate to produce funnel-like openings. Alternatively, the baffle may be formed with a multiplicity of long thin holes extending either vertically or horizontally. Such an anangement can be produced by perforating a solid plate or by fabricating a baffle from a series of strips, bars, angles or other profiles which are spaced apart to provide the required holes. The baffle may, in fact, be fabricated from any combination of plate, strips, bars, angles or other profiles to have particular flow characteristics. If the baffle extends into the gas flow region of the separator the openings may be designed to improve coalescence of droplets, e.g. by selection of an appropriate aspect ratio for the holes. hi the case of a separator having a plurality of baffles each baffle may exhibit the prefened increasing free area towards the bottom thereof in accordance with the present invention. Alternatively, one or more of the baffles may have other specific designs to improve separation or through-put which do not exhibit the prefened variation in free area in accordance with the invention.

The baffle according to the invention is believed to be particularly effective when used at an upstream region of the tank immediately downstream of the inlet thereof. Often, in prior art separators it has been necessary to provide a specially reinforced baffle at this point in order to withstand the substantial pressure across the baffle. Typically, this has been achieved by use of a double baffle plate arrangement in which two separate baffle plates are bolted to the opposite sides of a support structure in order to provide the necessary strength. It is believed that the prefened embodiment of the present invention will offer less total resistance to flow than that of typical prior art baffles and accordingly when used at the inlet end of a separator it may be possible to use a single baffle plate according to the present invention where, in the prior art, a double baffle plate anangement has been necessary. The prefened embodiment of the present invention controls channelling of flow and the fonnation of a plug flow along the vessel. Accordingly, the invention is especially useful in the context of heavy oils with high viscosities and high drag characteristics. It is to be understood, however, that the invention is generally applicable to a wide range of separators, and in particular may be applied to separators of the type of which the inlet flow is in the upper part of the separator (in which case the proportion of perforated area will in general be higher adjacent the bottom of the battle) and the separators of the type in which the inlet flow is in the lower part of the separator (in which case the baffle will in general have a larger area of perforation adjacent the upper edge thereof).

The invention will be better understood from the following description of a prefened embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein the single figure shows schematically a prefened embodiment of a 3-phase separator according to the present invention.

Referring to the drawing, the illustrated 3 -phase separator 1 comprises an elongate tank 2 which is mounted, in use, with the longitudinal axis 3 thereof substantially horizontal. An inlet 4 is provided adjacent one end of the tank for receiving a mixture of aqueous liquid, hydrocarbon liquid and gas. The mixture received at the inlet 4 is delivered via a vane pack 5 to an inlet zone 6 located adjacent one end of the tank. It should be appreciated, however, that the use of a vane pack is not essential to the operation to the present invention and that other inlet devices may be used. From the inlet zone 6 the mixture flows along the tank towards the outlet end 7 thereof. During the course of passage along the tank gas contained in the inlet mixture rises to the top of the tank and eventually leaves the tank via a gas outlet 8. The aqueous liquid sinks to the bottom of the tank and eventually leaves the tank via an aqueous liquid outlet 9. The hydrocarbon liquid forms a layer between the aqueous liquid and the gas and eventually flows over a weir 10 into an outlet zone 11 from which it leaves the tank via a hydrocarbon liquid outlet 12. As will be understood by those skilled in the art that by controlling the rate at which aqueous and hydrocarbon liquids are removed from the outlets 9,12 in relation to the rate of inlet flow through the inlet 4 the required operating levels within the tank 2 can be maintained. Appropriate sensors and control mechanisms are provided for this purpose.

In order to control the flow of fluid along the tank and to promote separation thereof into its respective constituent parts two baffles 13,14 are provided within the tank. As will be understood by those skilled in the art the number of baffles used will depend to an extent on the size of the installation, the expected flow rate, and the degree of separation required. In all installations, however, at least one baffle will be provided. hi accordance with the present invention at least one of the baffles has a free area adjacent the lower edge thereof which is greater than the free area in a zone of the baffle further up the tank. In the prefened embodiment of the invention the variation in free area is applied to the upstream baffle 13 adjacent the inlet zone 6. The variation in baffle free area may also be applied to the baffle 14. In the alternative, this baffle may be designed with other operating characteristics in mind which will dictate other free area anangements.

In the prefened embodiment of the invention the baffle 13 is divided vertically into five zones or bands. The uppennost zone adjacent the upper edge 15 of the baffle is imperforate. The exact depth of the uppennost zone will depend on operating conditions and on the size of the installation, but for a typical installation in which the tank 2 has a diameter of approximately 3000mm the uppermost imperforate zone may typically have a vertical extent of 100mm. The portions of the baffle 13 located below the uppermost imperforate zone may be divided into four zones or bands of approximately equal vertical extent. Preferably, the free area of the zones progressively increases from the uppennost zone to the lowennost zone, hi one embodiment of the invention the uppennost zone may have a free area of approximately 7%, the next zone down may have a free area of approximately 12%, the next zone down may have a free area of approximately 25% and the bottom zone may have a free area of approximately 30%. It must be stressed that the actual free areas, the relative size of the different zones of free area and the ratio of the free area of the largest zone to the smallest zone will be selected depending on the particular operating characteristics expected for the particular installation in question. The present invention is not limited to any particular values or combinations of values.

Preferably, the baffle 13 is in the form of a steel plate and the free areas are provided by way of substantially rectangular perforations through the plate. In order to provide the increased free area towards the bottom of the plate the size and/or density of perforations may be increased relative to those adjacent the upper end of the plate. For example, in the prefened embodiment of the invention the portion of the plate having a free area of 7% is fonned with rectangular perforations 10mm x 25mm at appropriate spacings. The zone having a free area of 12% is provided with perforations 10mm x 25mm at a greater density than those of the 7% free area zone. The zone having a free area of 25% is provided with holes 25mm x 25mm and the zone having the free area of 30% is provided with holes 25mm x 25mm at a greater density than in the zone having a free area of 25%.

It should be noted that in many installations of the prior art the upstream baffles 13 (generally known as the distribution baffle) has been formed by a particularly robust structure constmcted by securing two baffle plates to appropriate support members. It is believed that in the case of the present invention the need for such a double baffle plate anangement is removed in many installations and a single baffle plate only is required. It should be noted, however, that under certain circumstances it may be desirable for two baffles of variable pitch, in accordance with the present invention, to be located closely adjacent to each other so as to produce a combined effect on the flow pattern through the separator.

Whilst the present invention is primarily concerned with the design of baffles to improve the flow characteristics of material through an essentially static separator, the invention is also applicable to the design of separators for use on floating platforms and barges where the baffle, in addition to controlling the flow of material through the separator, is used to control surging of the liquid within the separator due to platform or barge movement..

Also, whilst the present invention is primarily concerned with separators in which the tank extends substantially horizontally, it should be appreciated that the invention is also applicable to separators in which the tank extends substantially vertically and the baffle separates a lower region of the tank into inlet and outlet zones.

Claims

CLAIMS:

1. A separator for separating a mixture of high and low density components into its constituent high and low density parts, the separator comprising: an elongate tank with an inlet for receiving an inlet mixture of high and low density components and outlets, spaced from the inlet, for separated high and low density components respectively; and at least one baffle in the tank between the inlet and the outlets thereof, the baffle having a different proportion of free area in a zone adjacent the bottom of the tank than it has in a zone further up the tank.

2. A separator according to claim 1 wherein the baffle has several zones of progressively decreasing free area from the upper edge thereof towards the bottom thereof.

3. A separator according to claim 1 wherein the baffle has a continuously decreasing free area from the top thereof towards the bottom thereof.

4. A separator according to claim 1 in which the baffle has several zones of progressively decreasing free area from the bottom thereof towards the upper edge thereof.

5. A separator according to claim 1 wherein the baffle has a continuously decreasing free area from the bottom thereof towards the top therof.

6. A separator according to any preceding claim wherein the upper edge of the baffle is located spaced from the top of the tank to provide an open area above the upper edge of the baffle.

7. A separator according to claim 6 wherein the upper edge zone of the baffle is imperforate.

8. A separator according to any of claims 1 to 5 wherein the baffle extends to or closely adjacent to the top of tl e tank and includes a perforate region adjacent the top of the tank followed, in the downward direction, by an imperforate region, followed by one or more perforate regions of different free area.

9. A separator according to any of claims 1 to 7 wherein the free area of the uppermost band is between 5 and 15% of the total surface area of the uppermost band and tlie free area of the lowennost band is greater than 25% of the total surface area of the lowermost band.

10. A separator according to claim 9 wherein tlie free area of the uppennost band is substantially equal to 7% of the total surface area of the uppennost band and the free end of the lowermost band is substantially equal to 30% of the total surface area of the lowennost band.

11. A separator according to any preceding claim wherein the free area of the baffle is provided by rectangular perforations in the baffle.

12. A separator according to any preceding claim wherein the baffle is a perforated plate.

13. A separator according to any of claims 1-11 wherein the baffle is fabricated at least in part from strip, bar or profile material to provide the required open area by gaps between adjacent strip, bar or profile members.

14. A separator substantially as hereinbefore described with reference to the accompanying drawing.