GB2171031A

GB2171031A - Electrostatic separation of liquid dispersions

Info

Publication number: GB2171031A
Application number: GB08603859A
Authority: GB
Inventors: Philip Jensen Bailes; Samuel Kojo Larley Larkai
Original assignee: University of Bradford
Current assignee: University of Bradford
Priority date: 1985-02-20
Filing date: 1986-02-17
Publication date: 1986-08-20
Also published as: ZM1186A1; GB2171031B; AU5349786A; GB8603859D0; GB8504338D0

Abstract

A method for aiding separation of components of a liquid dispersion, for instance a water in oil dispersion, involves passing the dispersion along a flow path and applying, by means of at least one pair of electrodes, a unidirectional electrostatic field which is pulsed at a predetermined frequency less than 1Hz. The method is particularly useful where a large volume of dispersion is being treated in the treatment vessel. At least one of the or each pair of electrodes is insulated from the liquid, and has the higher voltage applied thereto.

Description

SPECIFICATION Settling of liquid dispersions The present invention relates to methods and apparatus for settling liquid dispersions or emulsions and particularly separating two immiscible or partially miscible liquids.

Our copending European Patent Application No. 81305155.4 describes apparatus and methods for aiding the separation of the components of a liquid dispersion, the apparatus comprising means for passing said dispersion along a flow path and at least one pair of electrodes for causing an electrostatic field across at least a portion of a flow path, characterised in that the electrostatic field is unidirectional and varying, and at least one of the or each pair of electrodes being, in use, separated from the dispersion by a layer of electrically insulating material. There is described the use of a voltage pulsed between zero and an appropriate high voltage preferably below 1100 v/cm. Pulse rates up to 60 Hz are used, a preferred range mentioned being between 30 Hz and 1 Hz, a more preferred range being between 20 Hz and 1.5 Hz, and most preferred range being between 15 Hz and 2 Hz.It is stated that good results may be obtained at frequencies outside the preferred ranges. However, it has now been surprisingly discovered that very good results can also be obtained with pulse rates below 1 Hz.

Accordingly the present invention provides a method for aiding the separation of components of a liquid dispersion, the method comprising passing said dispersion along a flow path, and applying, by means of at least one pair of electrodes, an electrostatic field which is pulsed at a predetermined frequency less than 1 Hz across at least a portion of a flow path, at least one of the or each said electrodes being separated from the dispersion by a layer of electrically insulating material, and the means for producing a pulsed voltage being arranged to produce a higher voltage at the or each insulated electrode.

The electrostatic field may be a unidirectional (DC) or an AC electrostatic field but is preferably a DC field.

The present invention also provides apparatus for aiding the separation of components of a liquid dispersion, the apparatus comprising means for passing said dispersion along a flow path, and at least one pair of electrodes for applying a electrostatic field which is pulsed at a predetermined frequency less then 1 Hz across at least a portion of a flow path, at least one of the or each pair of electrodes being separated from the dispersion by a layer of electrically insulating material, and the means for producing apulsed voltage being arranged to produce a higher voltage at the or each insulated electrode.

In our above-mentioned European patent specification, it is mentioned that the frequency at which the electric field is varied can have a specific value for which phase separation is optimum and that this optimum frequency depends on the thickness and electrical properties of the electrode insulation and on the properties of the dispersions as being treated. However, at that time the system was not fully understood by the inventors. Now, however, the inventors have developed a theory which leads to the conclusion that polarisation in an electrostatic coalescer is important for its effect on coalescence.

Experimental investigation with the apparatus and dispersion systems described in our abovementioned European patent application led to the establishment of the influence of various electrical parameters, as follows:~ (1) Coalescence increased with applied voltage at fixed pulsed frequency; (2) There is an optimum frequency of pulsation at which coalescence is maximised for a given applied field; (3) The insulation coating thickness influences the optimum frequency such that the thinner the coating the lower the optimum frequency and the stronger the field experienced by the dispersion; (4) The best pulse form of those nominally described as square, half-wave and triangular is the square form; (5) The mark/space ratio for pulsed voltage should be unity for best coalescence; (6) Continuous DC fields cause comparatively little coalescence.

The combination of experimental observations and a new theoretical approach to the matter has led to the unexpected conclusion that the optimum pulse frequency could well be below 1 Hz. A particular factor which influences the optimum pulse frequency is the volume of the vessel in which coalescence is taking place, and for very large volume vessels the optimum pulse frequency may well be below 1 Hz and perhaps considerably below 1 Hz. It is a preferred feature of the present invention to provide apparatus for the invention in which the electrodes are located within a vessel having a volume greater than 1,000 litres and more preferably a volume greater than 10,000 litres.

The apparatus used may be similar to that specifically described in our above-mentioned copending European patent application. However, the apparatus may be, for instance, apparatus for effecting liquid-liquid extraction comprising a vessel provided with agitation means located in a first, central common region of the vessel, means for feeding a first, lighter liquid and a second, heavier liquid to a said central region whereby there is provided in said central region a dispersion of said liquids and above said dispersion a layer of said first liquid substantially free of said second liquid and below said dispersion a layer of said second liquid substantially free of said first liquid, and means for applying a pulsed unidirectional electrostatic field across at least a portion of said layer above said dispersion.Such apparatus may be similar to that described in, for instance, US Patent Specification No 4221658.

The dispersion should be one of at least one electrically conducting liquid in one or more liquids of relatively lower electrical conductivity. Preferably the dispersion is a water in oil dispersion.

Preferably the voltage is pulsed between zero and an appropriately high voltage. More preferably the maximum field strength across the flow path is below 1100 v/cm.

The invention is especially applicable to dispersions in which the volume of electrically conducting liquid as a percentage of the total volume of the dispersion is relatively high and the invention works surprisingly well with dispersions where the volume of conducting liquid is greater than 30%, and may be as high as 40 or 50%.

To illustrate the effect of the size of the apparatus on the optimum pulse frequency, the apparatus described in our above-mentioned co-pending European Patent Application No 81305155.4 has been modified so that the depth of the continuous phase liquid in the coalescer is 130mm rather than 10mm. The high voltage electrode coating material was perspex (RTM) having a thickness of 0.17mm and the voltage applied to the coalescer was 500V. The following results were obtained:

Frequency (Hz) | Reduction in Dispersion Band Depth (%) 0.014 45.6 0.016 48.8 0.025 48.8 0.083 35.2 0.28 34.4 0.91 35.2 3.1 36.8 10.0 36.8 25.0 36.8 It can be seen that the optimum pulse frequency is around 0.02Hz, and at this frequency the reduction in dispersion band depth of around 50% is an extremely good figure compared with, for instance, a similar voltage applied at a constant DC voltage, and such results on a commercial scale would be a great benefit in separating liquid components in a dispersion.

Claims

1. A method for aiding the separation of components of a liquid dispersion, the method comprising passing said dispersion along a flow path, and applying, by means of at least one pair of electrodes, an electrostatic field which is pulsed at a predetermined frequency less than 1 Hz across at least a portion of a flow path, at least one of the or each said electrodes being separated from the dispersion by a layer of electrically insulating material, and the means for producing a pulsed voltage being arranged to produce a higher voltage at the or each insulated electrode.

2. A method according to Claim 1 in which the electrostatic field is a unidirectional field.

3. A method according to Claim 1 or Claim 2 wherein the dispersion is a water in oil dispersion.

4. A method according to any of the preceding claims wherein the voltage is passed be tween zero and an appropriately high voltage.

5. A method according to any of the preceding claims in which the maximum field strength across the flow path is below 1,100v/cm.

6. A method according to any one of the preceding claims wherein the volume of electrically conducting liquid is greater than 30% of the total volume of the dispersion.

7. Apparatus for aiding the separation of components of a liquid disperion, the apparatus comprising means for passing said dispersion along a flow path, and at least one pair of electrodes for applying a electrostatic field which is pulsed at a predetermined frequency less thanl Hz across at least a portion of a flow path, at least one of the or each pair of electrodes being separated from the dispersion by a layer of electrically insulating material, and the means for producing a pulsed voltage being arranged to produce a higher voltage at the or each insulated electrode.

8. Apparatus according to Claim 7 wherein the apparatus comprises a vessel provided with agitation means located in a first, central common region of the vessel, means for feeding a first, lighter liquid and a second, heavier liquid to a said central region whereby there is provided in said central region a dispersion of said liquids and above said dispersion a layer of said first liquid substantially free of said second liquid and below said dispersion a layer of said second liquid substantially free of said first liquid, and means for applying a pulsed unidirectional electrostatic field across at least a portion of said layer above said dispersion.