GB2123716A - Electrostatic separation of oil/water emulsions - Google Patents

Abstract

Apparatus for treating water in oil emulsions to promote their separation includes, for applying an electric field to the emulsion, one or more directionally solidified eutectic (DSE) electrode(s). The DSE is preferably a refractory metal carbide. The apparatus is particularly suitable for treating water in oil emulsions containing less than 10% by weight of water, e.g. emulsions of water in crude oil or hazy gas oil. Either constant or pulsed DC or AC potential may be used.

Description

SPECIFICATION Separation of oil/water mixtures This invention relates to an apparatus for treating water in oil emulsions to promote separation of the oil and water and to a method for treating said emulsions.

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 produced water from the oil before transporting the oil which may be by either pipeline or tanker from the production facilities.

Usually the mixture of crude oil and produced water is in the form of an emulsion and one previously proposed technique for separating these emulsions has involved passing the emulsion between two electrode plates to which a high voltage is applied. The high electric field between the plates causes the water droplets to coalesce thereby facilitating their subsequent separation from the oil. This technique has been called electrostatic dehydration.

It is the object of the present invention to improve the performance of the above described separators.

According to the present invention an apparatus for treating water in oil emulsions to promote their separation includes means for applying an electric field to the emulsion by means of one or more directionally solidified eutectic electrode(s).

Conveniently the apparatus comprises a vessel for holdling the emulsion and at least two electrodes, held in spaced apart relationship in such a manner that when an electric potential is applied to one of the electrodes an electric field sufficient to promote separation is applied to the emulsion.

Desirably both or all the electrodes are made of the directionally solidified eutectic.

By the term vessel we mean any means for containing the emulsion including a conduit through which the emulsion may flow.

The oil can be a mineral oil such as a petroleum fraction, for example, gas oil or kerosine, or can be crude oil itself.

In the case of gas oil the water may be in the form of very fine droplets, such mixtures are known in the art as gas oil hazes.

The term directionally solidified eutectic (DSE) is known in the art.

The preparation of directionally solidified eutectics has, for example, been described in The Journal of Applied Physics, Vol.41, No.1, pages 76 to 81 which discloses the directional solidification of a nickel tungsten eutectic and the subsequent production of a multi-needle field emission cathode by the selective etching and pointing of the tips of the filaments.

Further, The Journal of Applied Physics, Vol.46, No.4, April 1975, Pages 1841 to 1845 discloses the preparation of multi-filament arrays of tungsten in uranium dioxide or zirconium dioxide and of molybdenum in gadolinium trioxide. Also at page 1 53 of Phys. Stat. Sol., Vol. 11, 1972 there is described the manufacture of a cathode plate with outgrowing spikes from a directionally solidified eutectic system consisting of filamentary metallic inclusions of nickel/antimony or chromium antimony embedded in a semiconductive matrix such as indium/antimony.

Previously described directionally solidified eutectics have possessed either mechanical weakness in the fibres or irregularity in the array and non-uniformity of the diameters of the fibres. It is preferred therefore to use the refractory metal carbide eutectics described in UK Patent No. 1,583,030, which, in general, have a regular array of fibres of uniform diameter and high strength.

The apparatus is particularly suitable for treating water in oil emulsions containing less than 10% by weight of water.

According to another aspect of the present invention a method of treating a water in oil emulsion to promote the separation thereof comprises applying an electric field to the emulsion employing an electrode made of a directionally solidified eutectic.

The electric potential applied to the electrode can be either AC or constant or pulsed DC. A convenient potential is up to 3 KV (3000 volts) or more.

When constant direct current or pulsed direct current without reversal of sign is employed, then at least the cathode should be made of the directionally solidified eutectic. In the case of alternating current or pulsed direct current with sign reversal than either of the electrodes may be made of the directionally solidified eutectic.

The invention is illustrated by reference to the accompanying drawing which is a vertical section of the apparatus employed in a laboratory experiment.

Referring to the drawing the apparatus comprises a pair of electrodes 1 and 2 supplied with electrical power from cables 3 and 4 which are connected to the electrodes 1 and 2 by stainless steel sleeves 5 and 6. The electrodes 1 and 2 and sleeves 5 and 6 are located in a cavity 7 in a block of polytetrafluoroethylene 8 the block 8 being located in the neck of a funnel 9. The block 8 has an aperture 10 extending axially therethrough in which is held a 1 mm bore capillary glass tube 11 the lower end of which extends into a beaker 12. The block 8 is sealed against the surface of the funnel 9 by an inert compressible material 13 such as plasticine. Two sets of screws 14 and 1 5 screwed into block 8 establish contact with the sleeves 5 and 6 and electrodes 1 and 2 respectively.Cable 3 is connected to an ammeter (not shown) and cable 4 is connected to a high tension supply the common being earth.

The electrical supply was either AC, DC or pulsed DC.

A water in oil emulsion consisting of a stablised Forties crude oil containing 2% by weight of water dispersed as droplets (of water) up to a few micron in diameter was poured into the funnel and allowed to flow between the electrodes under gravity with the electrical supply was connected to the electrodes.

In Examples 1 to 3, the electrodes were prepared from a 10 cm long piece of material of 1 mm square cross section machined from a directionally solidified eutectic composed of TaC, Ni and Cr prepared as described in Example 2 of UK Patent No. 1,583,030, and which had been cut in half by a slitting wheel and each half etched to form an array of points. The fibres consisted of tantalum carbide in a nickel-chromium matrix and occupied 1 0% of the volume of the solidified ingot. The fibre diameter was between 1 and 2 micron. In the comparative experiments the electrodes were made of stainless steel. Results are recorded in the following table.

Table 1

Co#ected Electrical sample in Resid#e Example Electrodes supply beaker in funnel Example 1 Eutectic A.C. Increase in droplet size Several large areas of 3 KV (up to 20 micron) of free water plus many several droplets droplets up to 70 micron Comparative Stainless A.C. Increase in droplet size Similar to collected sample Test 1 steel 3 KV (up to 50 micron) for large no. of droplets Example 2 Eutectic Pulsed Increase in size (up Large areas of free D.C. to 20 micron) for water up to several mm 3 KV large no. of droplets across plus large droplets (up to 70 mm)# Comparative Stainless Pulsed Increase in size (up Several areas of free Test 2 steel D.C. to 20 micron) for water plus large no. of 3 KV several droplets droplets up to 30 micron Example 3 Eutectic D.C. Increase in size of large Several large areas of 3 3 KV no. of droplets (up to free water plus many 50 micron) droplets up to 50 microns Comparative Stainless D.C. Increase in size for Several clusters of Test 3 steel 3 KV several droplets droplets up to 100 micron up to 20 micron The above described examples and comparative tests show that the eutectic electrodes are more effective than the stainless steel electrodes in increasing the droplet size in both the collected emulsions and the residues.Stainless steel electrodes are used in conventional electrostatic dehydrators.

Example 4 Experiments were performed using gas oil hazes containing 500 ppm of water. These were mixed in 100 ml batches using ultrasonic vibration. The gas oil haze was run through a viewing cell at a rate of 6 ml mien~' with a DC voltage of 3 kV applied. The droplets in the haze were micron and submicron in size.

Initially upon application of the voltage no large droplet formation was visible although during the course of the experiment, many droplets up to 50 jum in diameter began to build up on the surface of the test cell.

Samples of the gas oil haze were run through the cell at voltages of zero, 1 kV and 3 kV and the resulting liquid was subsequently examined in a nephelometer to establish the degree of turbidity. This was also done with stainless steel electrodes for zero volts and 3 kV.

The results obtained show the ability of the eutectic electrodes to clarify the gas oil hazes.

The nephelometer was calibrated by setting the zero using a sample of pure gas oil and the 100% setting was obtained using an untreated 500 ppm mixture of water in gas oil. Table 2 shows the spread in values of turbidity on this scale, obtained for four different samples all treated at 3 kV with the same electrodes.

Table 2 Values of relative turbidity for four samples of gassil haze treated with DSE electrodes at 3 kV Relative Sample turbidity ( /0) 1 11 2 21 3 34 4 10 Table 3 shows the difference in relative turbidity obtained for samples subjected to different voltages. They are given in the order in which they were performed. About 10 ml of the gas oil haze was run through in each case before collection of the sample so as to avoid any effects from the previous test.

Table 3 Effect of voltage on gas oil turbidity using DSE electrodes Sample Voltage (kV) Turbidity (%) 1 0 96 2 3 34 3 0 92 4 1 54 A comparison test with flat electrodes was then made using stainless steel electrodes with a voltage of 3 kV applied. The resulting liquid had a relative turbidity of 83% compared with the original haze.

The DSE electrodes used for the gas oil haze clarification experiments were subsequent examined in a scanning electron microscope and found to be completely free of damage.

Example 5 The results set out in the following Table 4 were obtained in a revised apparatus in which an oil in water emulsion with same composition as before was caused to flow past the electrodes by means of a nitrogen pressure head. In a series of test runs according to the invention, the cathode was a directionally solidified eutectic similar to those previously described. In a series of comparative tests the cathode was a stainless steel plate. In both series the anode was a stainless steel plate. The residence time of the emulsion in the field between the electrodes was less than 1 second. The emulsions were passed through the system three times, samples being taken after each pass.

Table 4 Current Percentage change (water) Voltage flow rate /, {mAJ 12 {mA) 13 (mA) U-/, /1~/2 /2~/3 Total DC (kv) (mI/min) Istpass 2ndpass 3rdpass Istpass 2ndpass 3rdpass U-13DC Cathode 3 8.4 0.5 0.2 0.18 9.2 14.7 30.1 54.0 DSE 3 8.4 0.6 0.2 0.15 9.1 9.3 33.1 51.5 DSE 3 8.4 0.9 0.6 0.1 9.3 16.3 14.7 40.3 DSE 4 8.4 0.4 0.2 - 5.9 30.9 - *36.8 DSE 2 8.4 0.2 0.0005 0.0002 No real change 0 DSE 2 8.4 1.1 1.1 0.7 4.2 4.9 9.0 18.1 DSE 3 8.4 0.8 0.7 0.6 7.3 7.4 6.7 21.4 Stainless steel 3 8.4 0.9 0.9 0.9 4.2 4.6 +8.8 Stainless steel 3 8.4 0.8 0.8 0.8 0 6.4 +6.4 Stainless steel *change in water content for two passes through the system only.

The results exhibit a scatter in the percentage change of water content and current passing through the cell. The results do, however, show that the DSE cathode can improve the efficiency of water removal from the system by over two and a half times compared with a flat stainless steel cathode.

Claims (9)

Claims

1. An apparatus for treating a water in oil emulsion to promote its separation comprising one or more electrodes made of directionally solidified eutectic for applying an electric field to the emulsion.

2. An apparatus as claimed in claim 1 including at least one anode and at least one cathode wherein the apparatus is suitable for use with either constant direct current or pulsed direct current without reversal of sign and the cathode is made of the directionally solidified eutectic.

3. An apparatus as claimed in claim 1 including at least one anode and at least one cathode wherein the apparatus is suitable for use with alternating current.

4. An apparatus as claimed in claim 2 wherein both cathode and anode are made of a directionally solidified eutectic and are held in spaced apart relationship such that when an electric potential is applied across the electrodes an electric field is applied to the emulsion sufficient to cause an increase in droplet size.

5. An apparatus as claimed in claim 4 wherein the directionally solidified eutectic is a nickel tungsten eutectic or tungsten in uranium oxide or zirconium oxide or molybdenum in gadolinium oxide, or nickel antimony or chromium antimony.

6. An apparatus as claimed in claim 4 wherein the directionally solidified eutectic is a refractory metal carbide eutectic.

7. A method for treating a water in oil emulsion to promote the separation thereof which comprises applying an electric field to the emulsion wherein the electric field is applied by means of one or more electrodes made of a directionally solidified eutectic.

8. A method for treating a water in oil emulsion as claimed in claim 6 wherein the emulsion contains less than 10% by weight of water.

9. A method for treating a water in oil emulsion as claimed in claim 7 wherein the water in oil emulsion is a gas oil haze.