RESOLUTION OF EMULSIONS

Abstract: A method and apparatus are described for separating the components of an emulsion comprising an electrically conductive component dispersed in a continuous phase of a substantially electrically insulating component, for instance, a water-in-oil emulsion. The method includes the steps of introducing bubbles of an electrically insulating gas into the emulsion, whilst simultaneously applying an electric or electrostatic field across the emulsion.

Description: RESOLUTION OF EMULSIONS Field of the Invention This invention relates to a method and apparatus for separating emulsions, in particular water-in-oil emulsions, into their component phases. Background to the Invention Existing techniques whereby a water-in-oil emulsion may be resolved into its constituent phases make use of chemical and physical means for achieving the resolution. The need for an emulsion breaking operation may arise in connection with a wide variety of processes. For example, in the production of crude oil at the well-head, where saline formation water is present as fine stable droplets in the oil coming to the surface, it is necessary to break the emulsion in order to dehydrate the oil. In other cases, it may be essential to recover the components of an emulsion after it has been deliberately created for a specific purpose. This occurs in processes using emulsion liquid membrane technology where, for instance, a very stable water-in-oil emulsion is used to treat an aqueous effluent feed stream to remove an impurity. The species to be removed from the aqueous feed stream transfers through the outer oil phase of the emulsion into the water droplets or internal phase of the emulsion. To encourage this mass transfer to occur it is necessary to have some appropriate chemical or physical driving force. A simple example of such a process is the extraction of ammonia from an effluent water into an emulsion of sulphuric acid in kerosene. A crucial feature of the economics of processes that employ emulsion liquid membranes concerns the ability to re-use the oil phase repeatedly. Economic viability depends on being able to break the used emulsion speedily to remove the spent internal phase in bulk and recycle the oil phase to make fres emulsion for re-use. In the oil industry, crude oil emulsions that are slow t resolve using a purely physical method such as gravit separation, can often be treated with chemical additives whic serve to destabilise the emulsion. However, the latter cours of action is not feasible for liquid membrane emulsions. Her the desire to recreate a stable emulsion using the separate oil phase mitigates against the use of chemical demulsifiers. In the case of liquid membrane emulsions it is therefor necessary to consider the use of physical methods alone to speed up the separation of an emulsion. In general, the resolution of emulsions requires that th small droplets of the dispersed (internal) phase coalesc together, until they become large enough to be removed easil from the continuous phase. Where the densities of the two phases are different, the denser phase simply gravitates fro the emulsion and, given enough time, the two phases can b separated sufficiently for each to be drawn off. The tim required for this separation is reflected by the size of the settling tanks which are typically required. These may b very large and may contain a large inventory of expensiv liquids. In addition, the phase separation step may be th slowest stage of a more extensive process and therefore limit the throughput of the overall process. During the resolution of a water-in-oil emulsion, those droplets that have grown by coalescence must gravitate throug the emulsion to reach the bulk interface between the separate water layer and the unseparated emulsion. The viscosity of the emulsion tends to be high, especially for liquid membran emulsions. This hinders the passage of the water droplets an they can be very slow to reach the bulk interface. One method that has proved useful specifically for enhancin the rate of separation of a water-in-oil emulsion is t subject the emulsion to an applied high voltage gradient. Th electric field assists the process of phase separation b promoting coalescence between the water droplets in th emulsion. Several possible mechanisms for electrically aide coalescence have been identified [Waterman, L.C. (1965) Chem Eng. Progress, vol 61, (10), 51], all of which rely upon th attraction of opposite charges on adjacent droplets to caus an increased incidence of collision followed by coalescence The larger droplets produced by this process gravitate muc more quickly to the bulk interface than their much smalle antecedents could do when under the same gravitational field Many different types of electric field have been shown to b effective for emulsion resolution, including: AC field [Cottrell, F.G. (1911) US Patent No 987 114, March]; pulse AC fields [Wolfe, K.M. (1944) US Patent No 2 364 118]; D fields [Siebert, F.M. and Brady, J.D. (1919) US Patent No 290369, January]; and pulsed DC fields [Stenzel, R.W. (1958 US Patent No 2 855 356, October]. A particular feature of some oil field emulsions and mos liquid membrane emulsions is that the water content of th emulsion may be 20% or more. Under such circumstances it i often impossible to apply the desired electric field by simpl passing the emulsion between metal electrodes because th presence of a large number of aqueous phase droplets tends t cause an electrical short-circuit between the electrodes This problem can be overcome by coating the high voltag electrode in a layer of insulation, so that those conductio paths that do develop only cause a localised diminution of th electric field rather than a complete collapse of the fiel throughout the inter-electrode region. The potential difference that must be sacrificed whe introducing a layer of insulation between the high voltag electrode and the emulsion may be minimised by operating unde pulsed voltage conditions. At optimum frequency, pulsed D fields have been shown to be particularly good for promotin coalescence when the high voltage electrode is coated wit insulation [Bailes, P.J. and Larkai, S.K.L. (1986) US Paten No 4601834, July] . Experimental evidence suggests that best performance is achieved when the emulsion to be treated flows in close proximity to the insulation coated electrode [Bailes, P.J. and Larkai, S.K.L. , Proceedings of the International Solvent Extraction Conference 1990 (Kyoto) (Process Metallurgy, Vol 7b, pl411, publd. Elsevier Science 1992)]. It follows from this that the larger the electrode area in contact with the emulsion the better. The present invention aims to provide a method and apparatus for separating the components of an emulsion, which can allow improved separation rates to be achieved. The invention also provides a novel method of effectively using insulated electrode surfaces. In practice, the new method works in an unexpected way causing another beneficial effect which acts synergistically to bring about a substantial improvement in the rate of emulsion resolution. Statement of the Invention According to a first aspect of the present invention there is provided a method for separating the components of an emulsion comprising an electrically conductive component dispersed in a continuous phase of a substantially electrically insulating component, the method comprising the step of introducing bubbles of an electrically insulating gas into the emulsion, whilst simultaneously applying an electric or electrostatic field across the emulsion. The types of emulsion which can be separated using this method include water-in-oil emulsions, in which an electrically conductive aqueous phase component is dispersed in a continuous organic phase component which is electrically insulating. The continuous phase must be substantially non- conductive, so as to allow an electric field to be established in it. The method thus depends upon the use of electrically insulating gas bubbles, which are injected into an emulsion whilst it is under the influence of an applied electric or electrostatic field. It is considered that these bubbl essentially create a large gas:continuous phase dielectr interface in the emulsion, at which electric charge wi accumulate. This surface charge then promotes coalescen between adjacent droplets of the dispersed phase in t continuous phase. In this way, the gas bubbles effective serve as electrodes, and their presence ensures that more the emulsion is exposed to the applied electric field. T result is that introduction of gas bubbles considerab enhances the rate of emulsion resolution, compared to the ra which would be observed in the absence of gas introduction The second benefit of introducing gas into the emulsion ste from the fact that gas bubbles moving through the emulsi appear to form channels through it. It has been discover that these channels help to provide pathways down whi coalesced droplets of the dispersed phase can move relative quickly, experiencing much reduced hindrance as they pa through the emulsion towards the bulk interface. The presen of the gas bubbles thus improves not only the rate of inte droplet coalescence but also the rate at which the coalesc droplets gravitate from the emulsion. The electrically insulating gas is preferably air or so other similarly inert gas. The electric or electrostatic field is preferably applied means of one or more pairs of electrodes. More preferably t field is a unidirectional electrostatic field applied betwe a relatively high voltage electrode and a relatively l voltage electrode. The high voltage electrode is preferab separated from the emulsion by a layer or electrical insulating material. Preferably the voltage applied to the relatively high volta electrode is between 10 and 20 kV. The voltage across t electrodes is preferably pulsed at a frequency of from 1.5 50 Hz, a typical pulse rate being around 15 Hz. As indicated above, where the field is applied using an electrode coated with an electrically insulated layer, the applied electric field is preferably a pulsed DC field, allowing the treatment of an emulsion having a relatively high dispersed phase content. However if uninsulated electrodes are to be used to apply the electric field, the field is preferably an AC field. It is preferred that the emulsion is caused to flow through the electric field and the gas is introduced into the emulsion in such a way that the gas flow rate to emulsion flow rate ratio is preferably in the range 100:1 to 600:1, more preferably in the range 160:1 to 490:1 and most preferably in the range 300:1 to 360:1. According to a second aspect of the present invention there is provided apparatus for use in separating the components of an emulsion comprising an electrically conductive component dispersed in a continuous phase of a substantially electrically insulating component, the apparatus comprising means for applying an electric or electrostatic field across the emulsion; and means for introducing bubbles of an electrically insulating gas into the emulsion whilst the electric or electrostatic field is being applied across the emulsion. The means for applying the electric or electrostatic field will typically comprise a pair of electrodes, one at a relatively high, and one at a relatively low, voltage. It is preferably capable of applying a high voltage field which is more preferably a pulsed field. The high voltage electrode may be coated with an electrically insulating layer. In this case, the apparatus is preferably used to apply a pulsed DC field across the emulsion, allowing for the treatment of an emulsion having a relatively high dispersed phase content. However, the electrodes may be uninsulated, in which case they are preferably used to apply an AC field. The means for introducing the gas bubbles into the emulsi may take any convenient form, for instance one or more g tubes capable of introduction into the emulsion and connect to a suitable gas supply such as a compressed air line. The apparatus preferably comprises a suitably shap separation vessel in which to contain the emulsion whilst i components are being separated. This separation vess preferably contains the relatively low voltage electrode a pair of electrodes forming part of the means for applyi the electric or electrostatic field. The low volta electrode may, for instance, take the form of a perforat element (for example, a disc or plate), of an electricall conductive material such as stainless steel, positioned insid the separation vessel and connected to earth. The relatively high voltage electrode of the pair electrodes may, for instance, comprise a secondary vess positioned inside the separation vessel, the secondary vesse containing an electrolyte in contact with a voltage suppl In this way, an electric field may be set up between t electrolyte and another electrode contained within t separation vessel. Where the high voltage electrode is to b insulated, the secondary vessel is formed from an electricall insulating material such as polythene. The apparatus preferably additionally comprises delivery mean for introducing the emulsion to be separated into t separation vessel. The delivery means may, for instance, tak the form of a delivery tube made from an electricall insulating material such as PTFE. The emulsion is preferabl introduced into the separation vessel between the tw electrodes of the means for applying the electric field Where the apparatus includes a secondary vessel as a electrode inside the separation vessel, the emulsion i preferably introduced into the separation vessel at a poi immediately adjacent the secondary vessel, between t secondary vessel and the other electrode. The apparatus preferably additionally comprises means by whic the separated components of the emulsion may be separatel removed from the separation vessel. The method and apparatus of the invention may be used fo treating emulsions on a batch or a continuous basis. Am embodiment of the present invention will now be described by way of example only and with reference to the accompanying illustrative drawing, Figure 1, which shows schematicall emulsion separating apparatus in accordance with th invention. Detailed Description of the Drawing Referring to Figure 1, apparatus in accordance with the present invention includes a cylindrical glass separation vessel 1 with a sidear exit pipe 3. The separation vessel is open at the top and partially closed at the bottom to leave a central aperture connected to a pipe 5. Arranged concentrically within the separation vessel is an electrode assembly 7 consisting of a clear thin-walled polythene bottle half-filled with an electrolyte 9. The electrolyte is in contact with a high voltage supply via a wire 11, so that the electrolyte 9 forms a high voltage electrode, insulated by the polythene bottle. Near the bottom of the separation vessel 1 is a horizontal perforated disc 13 of stainless steel which is supported by, and in electrical contact with, a stainless steel rod 15 which in turn is sealed in pipe 5 leading from the base of the separation vessel. The perforated disc 13 is the relatively low voltage electrode of the apparatus, and is connected to earth. The base of th polythene bottle electrode 7 and the disc 13 thus form a parallel electrode system. In operation of the apparatus, a liquid water-in-oil emulsion, for example, may be passed into the vessel 1 through a delivery tube 17 made from an electrically insulating material, in this case PTFE. The emulsion enters the bul fluid located in the vessel at a position immediately beneath the base of the bottle electrode 7. An electric field is applied between the high voltage bottle electrode and the lower, earthed, electrode 13. The liquid emulsion located within vessel 1 is subjected to this electric field. Simultaneously, gas bubbles are introduced into vessel 1 through pipes 21. As a consequence, droplets of the heavier aqueous component of the water-in-oil emulsion are caused to coalesce and gravitate towards the bottom of the separation vessel. The aqueous component passes through the perforated disc 13 and forms an aqueous layer beneath it. The separated aqueous component is then drawn off by means of tube 19, and the remaining oil phase is drawn off through sidearm 3. In one particular example, a stable water-in-oil test emulsion was prepared by using a high speed homogeniser to mix the aqueous phase into a kerosene which contained a non-ionic surfactant of HLB value 4.9. This test emulsion contained 65% aqueous phase by volume and had a mean droplet size of approximately 2μ. The emulsion was very stable. When allowed to separate at room temperature under normal gravity conditions, only 1.5% of the water dispersed in the emulsion resolved to form a water layer after one month. The test emulsion was fed continuously to the apparatus of Figure 1, and measurements were taken of the aqueous content of the effluent organic phase (removed via pipe 3) in order to determine percentage resolution of the emulsion. Results were obtained with and without the injection of air and these data are set out in Table 1 below. * of emulsion in separation vessel, under influence of applied electric field. Where air was introduced into the apparatus, it was supplied from a standard laboratory compressed air line, at a fairly steady supply rate. Excessive supply rates were found to cause mixing of the oil phase into the aqueous phase to a certain extent. The air was supplied at just above atmospheric pressure, the separation vessel 1 being open to the atmosphere at its upper end. In both cases a pulsed DC electric field was applied to the emulsion, using a pulse rate of 15 Hz and a nominal voltage of 22 kV. The distance between the electrodes was 15 mm. It is clear from Table 1 that introducing air has a dramatic effect on the extent of emulsion resolution achieved in an electric field. The invention thus allows for relatively high resolution rates to be achieved, even when using insulated electrode surfaces (eg, when treating emulsions with relatively high water contents) . In a second example, a stable water-in-oil emulsion comprising by weight: 25 parts distilled water 11.5 parts kerosene 1 part Brij-52™ (diethylene glycol hexa decyl ether surfactant) was electrically treated in a continuous flow experiment at a temperature of 30°C. In this case an emulsion flow rate of 20 ml/min was treated in the apparatus of Figure 1 so that the residence time of the emulsion in the electric field was 15 minutes. Pulsed DC electric field was applied to the emulsion, using a pulse rate of 15 Hz and a nominal voltage of 22 kV. It was found that the percentage resolution of the emulsion was strongly influenced by the air flow rate and that there was an optimum air flow rate. The optimum condition gave 90% resolution for an air flow rate of 0.11 litre/s. The same experiment conducted with zero air flow gave 32% resolution. In order to achieve a 90% resolution of this emulsion with zero air flow it was necessary to reduce the emulsion flow rate to the apparatus from 20 ml/min to 6 ml/min.

Claims: CLAIMS 1. A method for separating the components of an emulsion comprising an electrically conductive component dispersed in a continuous phase of a substantially electrically insulating component, the method comprising the step of introducing bubbles of an electrically insulating gas into the emulsion whilst simultaneously applying an electric or electrostatic field across the emulsion. 2. A method according to claim 1 in which the emulsion is a water-in-oil emulsion. 3. A method according to claim 1 or claim 2 in which the electrically insulating gas is air. 4. A method according to any of the preceding claims in which the electric or electrostatic field is applied by means of one or more pairs of electrodes. 5. A method according to claim 4 wherein the field is a unidirectional electrostatic field applied between a relatively high voltage electrode and a relatively low voltage electrode. 6. A method according to claim 5 wherein the relatively high voltage electrode is separated from the emulsion by a layer of electrically insulating material. 7. A method according to any of the preceding claims in which the ratio of the air flow rate to the emulsion flow rate is in the range 100:1 to 600:1. 8. A method according to any of the preceding claims in which the ratio of the air flow rate to the emulsion flow rate is from 160:1 to 490:1. 9. A method according to any of the preceding claims in which the ratio of the air flow rate to the emulsion flow rate is in the range 300:1 to 360:1. 10. Apparatus for use in separating the components of an emulsion comprising an electrically conductive component dispersed in a continuous phase of a substantially electrically insulating component, the apparatus comprising means for applying an electric or electrostatic field across the emulsion, and means for introducing bubbles of an electrically insulating gas into the emulsion whilst the electric or electrostatic field is being applied across the emulsion.

Inventors: BAILES PHILIP JENSEN, BAILES, PHILIP, JENSEN
Applicants: UNIV BRADFORD, UNIVERSITY OF BRADFORD
Application year: 1993
Main classification: B01D17
IPC codes: B01D17