Electrostatic coalescer

The present invention is related to electrostatic coalescers, and more precisely to an electrostatic fluid coalescer device comprising a housing having an inlet and an outlet for a fluid, electrodes located within the housing and exposed to the fluids and a power unit associated with the housing for supplying power to the electrodes. In the context of this application the term "exposed to the fluids" means that the electrodes comprise at least one surface which is in contact with the fluid.

Coalescers are devices which perform a process by which two or more droplets or particles merge during contact to form a single daughter droplet. The primary use of coalescers is to separate emulsions in various processes. Electrostatic coalescers comprise electrode plates with different polarities to create electrostatic fields in the emulsion and thus perform separation.

The invention will be illustrated by means of examples used for dehydration of process fluid (crude), but it can be used in any other field where coalescers are relevant solutions.

As a consequence of the coalescence process, water bridges or conductive paths formed by other components of the emulsion can be formed between plates with different polarities. This leads to shutdown and loss of operation time.

To avoid said shutdowns caused by water bridges or similar, some coalescers comprise insulated electrode plates.

An example of this technique is shown in WO01/85297. This publication describes an electrostatic coalescer device comprising a tube member with an electrically insulating layer portion and an interacting pair of a first and a second electrode arranged outside and adjacent to the insulating portion, so that the electrodes are not in contact with the emulsion. Insulated electrodes can only operate with AC or pulsed DC sources. Devices comprising insulated electrodes are thus limited in their range of application.

As mentioned above, coalescence will occur when small droplets meet to form bigger drops. This process requires that droplets move towards one another until they collide. Said movement of the droplets is caused by the electrostatic fields created by the electrode plates. When the orientation of the field changes, the direction of movement of the charged droplets changes too. If this change of direction occurs before the droplets have collided with one another coalescence will not take place. The success of the coalescence process depends thus on the relationship between the time the electrostatic field is applied in a single direction (frequency) and the distance between droplets. Another factor which influences the process is intensity of the electrostatic field and flow velocity of the emulsion

through the coalescer. To achieve an effective coalescence DC fields should be applied to the emulsion. However, the presence of insulation materials on the electrodes' surface facing the emulsion leads to highly reduced electrostatic field intensities and unsatisfactory results. In other words, the insulation materials represent also a limitation regarding the composition of the fluid subject to coalescence (relationship between water and oil fractions).

US 2001/0017264 shows a method for separating the constituents of a dispersion where uncoated electrodes in contact with the dispersion are used for providing low pulsating DC voltage (500V-5kV). Electrodes providing higher voltage are coated or covered with an insulating layer.

US 4,252,631 describes an electrostatic coalescence system where DC electrodes provide a high field gradient. Said DC electrodes are situated in a sone where drop sizes and drop population and thus the chances of water bridges are very small. An object of the invention is to limit the short circuit current caused mainly by water bridges.Another object of the invention is to limit loss of coalescer operation time, caused by shutdown due to water bridges or other conductive paths in the fluid causing short circuit of the electrodes.

Another object of the invention is to provide gentle turbulent flow in order to avoid break-up of formed water droplets.

The invention has also as an object to provide a power source for an electrostatic coalescer which permits use of AC and DC voltage sources.

Another object of the invention is to provide a robust, self adjusting, passive arrangement for the power supply. These and other objects are achieved by an electrostatic coalescer device according to the invention, which is characterised in that at least one electrode comprises several electrode segments. All electrode segments in each electrode have substantially the same polarity.

The term "electrode segments" in the context of the present application refers to bodies of electrically conductive material which are insulated from one another, where all segments or bodies have the same polarity. Electrode segments present thus a limited area defined and surrounded by insulation. In case of a water bridge (that is a path establishing connection between electrodes), limiting the conductive area leads to reduction of the short circuit current ( I = J j . dS where the current I is the integral of the dot product of the current density vector J and the differential

surface element dS, i.e. the net flux of the current density vector field flowing through a surface S).

Segmenting of the electrodes or electrode plates in one or both of two electrode plates (pair) with different polarity in a multitude of local electrode segments leads to an electrostatic coalescer with higher efficiency, i.e. higher percentage accumulated time between shutdowns due to short circuiting water bridges between electrode plates.

The reduction in short circuit current achieved by the invention permits to apply relatively high voltage electrostatic fields ( 2-1OkV HV) to the fluid, leading to high efficiency. The exposed electrodes are among other things able to provide DC fields of the above mentioned magnitude.

In one embodiment of the invention, each electrode segment is individually connected to the power supply. This arrangement gives the possibility of controlling the behaviour of the single segments in an effective way. It facilitates also to adapt the electrostatic fields to the fluids composition, which varies in the vertical direction (if the fluid flows in the horizontal direction) because the different fluid components have different densities and also in the horizontal direction as a consequence of the performed coalescence.

In one embodiment the device according to the invention comprises current limiting elements connected to the electrodes. The presence of current limiting elements (e.g. high resistance resistors) connected to each local electrode (or segment, see below) leads to a robust self adjusting passive arrangement part of the power supply system. The high resistance resistors limit the short circuit current (between plates of opposite polarity) to a level which allows simple control and management of randomly occurring short circuits.

In a variant of this embodiment each electrode segment is equipped with a current limiting element. In a variant of these embodiments the current limiting elements are resistors. Use of a single current limiting element for each segments leads to more reliable operation since failure in a single elements does not result in failure of the whole system. This is especially important in a marine environment where the equipment is not readily accessible for changing out defective parts. This will also have the benefit of easy adaptation of the device to different fluids and different electric field intensities as opposed to the case where a conductive or semi conductive layer of material is deposited on the electrode surface. In one embodiment the device according to the invention comprises non- insulated electrode segments (the segments' surface in contact with the fluid is non- insulated). This embodiment permits the use of DC voltage sources. As mentioned

before in one embodiment of the invention each electrode is insulated from the other electrode segments.

The segments can be formed as pins. In a variant of this embodiments the segments are formed as truncated cones. In another embodiment the segments are formed as discs.

In one embodiment of the invention the electrode segments are adapted to create a DC field in the fluid, that is they are preferably non-insulated to avoid high insulation losses and they are situated in the coalescer arrangement and connected to a power source so as to create DC fields. In one embodiment the device is adapted to create DC ₅ DC pulsed and AC fields. In a variant of this embodiment the device is adapted for generation of all these fields simultaneously. This can e.g. be implemented by the process fluid being subjected to a sequence of fields when flowing through the electrostatic coalescer as e.g. first an AC field, thereafter a DC-pulsed field and in the end a DC field. The invention comprises also a method for separation of two or more fluid components by coalescence, characterised in that it comprises creating a DC and/or a pulsed DC and/or an AC field in the fluid by means of segmented electrodes. As mentioned before this can be achieved by a combination of electrode design and placement of the electrodes in the arrangement. In an embodiment said method comprises simultaneously creating pulsed DC fields and AC fields or DC fields and AC fields or pulsed DC fields and DC fields. This will result in a fluid which is subjected to different electrostatic fields along the housing.

In a variant of the method according to the invention it comprises creating the fields by means of segmented electrodes. In this way the negative effects of water bridges are avoided.

The invention will now be described in detail by means of examples illustrated in the drawings, where:

Figure l is a diagram of the power unit in one embodiment of the invention. Figures 2 shows a detail of the electrode arrangement in the embodiment shown in Figure 1.

Figures 3 and 4 show the electrostatic fields created by means of the device according to the invention.

Figures 5- 10 show a first embodiment of the invention.

Figures 11-13 show a second embodiment of the invention. Figures 14-16 show a third embodiment of the invention. Figures 17-19 show a fourth embodiment of the invention.

Figure l is a diagram of the power unit in one embodiment of the invention. The power unit is connected to an AC power source 5 with a low input voltage (400V AC and 50 Hz in the illustrated embodiment of the invention). The power unit comprises an AC/AC converter unit 7 and output lines 8. On output lines 8 a voltage of 2-1OkV with a frequency of 0-1OkHz is available. The power unit comprises optional modulators 9 which permit feeding the electrodes with different voltages (individual feeding of the electrodes). By means of the modulators (which can be controlled externally to the device) the applied field can easily be adapted to the current type of fluid and flow velocity. The above mentioned devices are encapsulated in a container 6 which is kept at 1 atm. Dry HV connections 10 provide an interface between container 6 and an electrode chamber. The output lines 8 are connected to AC/+ electrodes 1, AC/-electrodes 2, DC/+ electrodes 3 and DC/-electrodes 4. DC electrodes 3 and 4 are connected to the output lines via a diode bridge 11 and a filter 12. The electrodes are divided in segments 13 where all segments in an electrode have the same polarity. The diagram shows also resistors 15 located between the output lines and the electrode segments. The aim of resistors 15 is to control the maximum operation current, limit the current in case of short circuit, and provide increased robustness. This embodiment of the invention provides 2-1OkV HV on the electrodes.

Figure 2 shows the fields that can be created by the electrode arrangement in the embodiment shown in figure 1. Diagrams representing intensity of the electrostatic field vs time are shown in figures 3 and 4. The resultant field distribution in the coalescer depends not only on the fields impressed on the electrodes but also on the geometric configuration of the electrodes, the shape of the segments, and their relative position in space. Electrodes 1 are fed with AC as illustrated in figures 3a and 4a, these electrodes face one another and are situated on a plane along the flow direction. Electrodes 1 provide an AC field between each electrode 1 and a ground electrode 2. Electrodes 3 and 4 are fed with DC pulsed voltage and are also situated in a plane along the flow direction and spaced from one another. They provide a DC pulsed voltage between each electrode 3 and 4 respectively and ground electrode 2 as illustrated in figures 3b and 4b (positive DC pulsed field between 3 and 2, negative DC pulsed field between 4 and 2). However, if the geometric configuration of the electrodes is such that there is an area where DC electrodes directly face one another (that is, where ground electrode 2 is not situated between electrodes 3 and 4, as illustrated on the far right side of figure 2), a DC field will be created in this area. This field is shown in figures 3c and 4c.

As will be shown in detail later on the invention gives the possibility of creating AC, DC pulsed and DC fields in one and the same device. The above mentioned fields can be applied separately or simultaneously.

Electrodes 3 and 4 are non-insulated electrodes while electrodes 1 are non-insulated electrodes in a preferred embodiment of the invention. As mentioned before non- insulated means that the electrode surface in contact with the fluid is not insulated.

Electrodes 1, 3 and 4 comprise electrode segments (single bodies) 13 attached to insulation plates 14.

Figures 5-10 show a first embodiment of the invention. In this embodiment the electrode arrangement defines three electrostatic field zones as explained in relation to figure 2: one zone with DC electrodes directly in front of each other (DC field), one zone with a ground electrode between the DC electrodes (pulsed DC field) and one zone with a ground electrode between AC electrodes (AC field).

Figure 5 is a cut in the horizontal direction. Figure 5 shows a housing 110 having an inlet 111 and an outlet 112 for the fluids 25. Housing 110 comprises a canister 26, a power connection 27 to the power unit and an electrostatic coalescer assembly situated inside a dielectric chamber 30. The assembly is intended primarily to be oriented horizontally, causing small water droplets to grow into bigger droplets which under the influence of gravity will travel vertically, gather at "the bottom" of the canister and hence contribute to the dehydration of process fluid. Gravity G is shown as an arrow perpendicular to the plane of the figure. The electrostatic coalescer subassembly inside the canister comprises electrodes 1, 3, 4 divided into electrode segments and a central ground electrode 2. Electrodes 1, 3 and 4 are situated in a vertical plane along the direction of flow. All electrodes are located within the housing and exposed to the fluids.

Electrode segments corresponding to electrodes 1, 3 and 4 are implemented as pins in the present embodiment of the invention, and said pins are shaped as truncated cones. Some of the pins (Figure 5, high pins 20) protrude from insulation plates 14 while other pins (low pins 21) are almost in flush with the insulation plates 14. High and low pins (20, 21 respectively) alternate in a direction transversal to the fluid flow. Ground electrode 2 is divided into two segments 22 and 23 joined by a transverse plate 24 (figures 3 and 5). Electrode pins 20, 21, segments 22 and 23 and plate 24 define a gap. Through this gap, the process fluid 25 is allowed to flow, during which period of travel, coalescence is performed. Depending on the type of fluid, it will be advantageous to create turbulence in the flow, to achieve a laminar flow or to provide a combination of these. Turbulence can in some cases be helpful in creating water bubbles but an exaggerated turbulence can lead to explosion of the bubbles before they reach the lower part of the container.

Figure 5 shows plate 14 with a front part facing the process fluid 25 and a back part facing a dielectric chamber 30. Across isolating plate 14 there is substantially no differential pressure since a pressure balancing system, using typically several piston type pressure compensators has been introduced. Balancing pistons 32 are situated in a chamber which on one side is in contact with the fluid inside 31 of the coalescer arrangement (fluid 25) and on the other side is in fluid connection with dielectric chamber 30 providing a pressure balancing system. Between plate 14 and canister wall 32 there is provided a pressure seal 33.

Figure 5 shows also power connection 27 which is a part of the power unit. Resistors 15 are located inside the dielectric fluid chamber(s) 30 and are electrically connected to the back side of the electrode pins 20, 21. From each group 29 of resistors 15 an electrical cable 28 feeds through to the electrical coalescer power source via a high integrity HV penetrator 10 (figure 1) and a protected dielectric fluid filled metal tube. Figure 6 is a view along line A-A (figure 3) of the electrode plates, and shows groups of electrode pins 20 and 21 on insulating plate 14. DC electrodes 3, 4 are situated on the left hand side, while AC electrodes 1 are situated on the right hand side. Two opposite plates for each type of electrode (AC, DC) are provided in this embodiment of the invention but it is possible to have more than one pair of plates for each type. Although this embodiment of the invention shows plate 14 as a common plate for several electrodes it is possible to provide one plate for each type of electrodes.

Figure 7 is a view along line B-B (figure 3) and clearly the shape of the gap between electrodes. Obstructions in the fluid flowing direction are limited as high pins 20 and low pins 21 are aligned in this direction. This figure shows also the composition of pins 20 and 21 which comprise a bolt part connected to resistor 15 and a head part in contact with fluid 25. This figure shows that electrode segments (pins 20 and 21) are insulated from one another by means of plate 14 while presenting a non-insulated surface to the fluid 25. Electrical connection between electrode segments is performed through resistors 15.

Figure 8 shows the power unit casing 6 which is situated on top of the canister, and power connections 27. Cables 28 are lead through conducts 40 to busbars 29. The figure shows too a detail of resistor 15. Reference 41 denotes a penetrator, which is a sintered glass plug comprising metal conductors 28 for connection to the HV source. Penetrators must be able to withstand pressure because they are situated between dielectric chamber 30 (pressure balanced with fluid 25) and connection 40 to casing 6 (at atmospheric pressure).

Figure 9 shows the flow area or gap 50 for fluid 25. This area has a section in the shape of three Xes. This embodiment of the invention provides a turbulent flow, which in most cases increases the efficiency of the coalescence.

Figure 10 shows a detail of electrode plate 14. Plate 14 comprises through going openings where sintered glass cylinders 60 are "glued" to electrode pins 20, 21. This figure shows also pressure seal 33 in section.

Figures 11-13 show a second embodiment of the invention. In these figures, elements corresponding to those shown in figures 1-8 are given the same reference numbers. This embodiment of the invention is adapted to provide a laminar flow gap for fluid 25. Ground electrode 2 comprises in this embodiment of the invention a central plate 71 and side plates 70 joined by means of rods 72 (figures 11 and 13). Ground electrode 2 is situated between electrodes 1, 3 and 4 and in this embodiment of the invention there is substantially no area where electrodes 1 face one another. As a consequence of this the electrostatic fields created by the electrodes are an AC field and a pulsed DC field (as shown in figures 3a, 4a, 3b, 4b) but no significant DC component is created.

The electrode segments 1, 3 and 4 are flat and implemented as discs and bars. As one can see in figures 9 and 11, the gap 50 for fluid 25 does not present any protuberance in the direction of flow, and this embodiment will provide a substantially laminar flow.

Figure 12 shows insulating plate 14 with disc electrodes 80 and bar electrodes 81. As one can see disc electrodes 80 are provided in the lower part of the electrode plate. This arrangement takes into consideration the fact that in the bottom area of fluid 25 there will be a higher water content, and water bridges will most probably occur in this area. Disc electrodes 80 are sidewise suitable for limiting the effects of water bridges. On the upper part of the plates 14, where oil will be the main component of the fluid, bar electrodes 81 are situated to provide laminar flow.

Figure 12 is a section perpendicular to the flow and shows how laminar flow is provided. Discs 80 have each a circular shape and are sidewise insulated from one another (this is not shown in the figure). From the point of view of fluid flow, they provide together a continuous surface. Each bar electrode 81 is oblong and provides a continuous surface to the fluid flow. Bar electrodes 80 are situated between the ground electrode's central plate 71 and the flat electrodes 80. As one can see in figures 11 and 12 a similar configuration is provided for the DC and AC electrodes (1 and 3, 4 respectively).

Figures 14-16 show a third embodiment of the invention. This embodiment of the invention provides DC ₅ DC pulsed and AC electrostatic fields and at the same time substantially laminar flow. The electrode arrangement comprises DC electrodes 3 and 4, and AC electrodes 1 all segmented in the form of discs 90 and bars 91 (figure 15). Ground electrode 2 is situated between DC electrodes and between AC electrodes but does not cover the entire space between DC electrodes 3 and 4, so DC electrostatic fields are provided in the zone free for ground electrode 2. Ground electrode 2 has a shape similar to the one described in relation to the second embodiment of the invention. As shown in figure 16 which is a section along D-D, bars 91 and discs 90 belonging to a single electrode are spaced from one another and an opposite electrode is situated between these. This arrangement provides DC field with high intensity due to the reduced distance between electrodes 3 and 4.

Figures 17-19 show a fourth embodiment of the invention. This embodiment provides DC and AC fields and substantially no pulsed DC fields. In this embodiment of the invention DC electrodes 3 and 4 are segmented into discs 100 and bars 101 and are situated in a similar manner as in the third embodiment of the invention. Ground electrode 2 extends only in the zone between AC electrodes 1 and has a shape similar to the one described in relation to the second embodiment of the invention.

The invention provides thus a device which combines controlled handling of water bridges with high efficiency. Although a limited number of different embodiments have been described it will be obvious for the skilled man that other variant are possible within the scope of the appended claims. AC and DC electrodes can e.g. switch places in relation to the fluid's flow direction, with AC electrodes "meeting" the fluid before the DC electrodes, several groups of AC electrodes (or DC) electrodes can be put together, distances between electrodes facing each other can be varied and so can distances between electrode groups. Modulators can be adapted to provide signals with different shapes (sinus, rectangular pulses, triangular pulses, etc).