Settling of liquid dispersions

Apparatus for aiding the separation of the components of a liquid dispersion comprises at least one pair of electrodes, arranged so as to be able to apply a unidirectional, varying electric field across at least a portion of a flow path 5a of the dispersion, at least one of the or each pair of electrodes being separated from the dispersion by a layer of electrically insulating material. A method for aiding the separation of the components of a liquid dispersion is also described.

The present invention relates to methods and apparatus for settling liquid 
dispersions or emulsions and particularly separating two immiscible or 
partially miscible liquids. 
There are many processes in which it is necessary to separate out the 
components of a liquid/liquid dispersion, for example, the separation of 
water droplets from petroleum derivatives or oil and the extraction of a 
metal ion from an aqueous phase into an organic phase. Where the densities 
of the liquids are sufficiently different then the separation can be 
effected in settling tanks. The denser liquid or phase simply sinks below 
the less dense phase and given sufficient time the two phases can be 
separated sufficiently to be drawn off. However, such a process requires 
the use of large tanks taking up a correspondingly large amount of space 
and, at least for solvent extraction processes, typing up a large volume 
of expensive solvent. Furthermore, this separation procedure may be the 
slowest stage of a more extensive process and therefore determine the 
throughput of the complete process. 
It is known that the application of an electrostatic field can speed up the 
separation of a dispersion, including an emulsion, into its components, 
where the continuous phase of the mixed liquids is an electrical insulator 
and the dispersed phase is an electrical conductor. The use of a.c. fields 
has been proposed, the frequency of the alternating current being mains 
frequency, i.e. 50 to 60 hertz (Hz), or a frequency greater than mains 
frequency. 
The use of pulsed d.c. fields has also been proposed, the frequency of such 
fields being of the order of 10 kilohertz (kHz). 
For both the a.c. and the d.c. apparatus the field strengths have been of 
the order of several kilovolts per cm. 
The use of electrodes coated with insulating material has also been 
suggested in connection with high voltage a.c. fields. 
The equipment required to produce such high frequency and high strength 
electric fields across the dispersion is expensive and complicated. 
Furthermore, the generation of high voltages is inherently unsafe in the 
inflammable atmosphere often associated with processes within the field of 
the present invention. 
According to the present invention there is provided apparatus 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 applying a unidirectional, varying 
electrostatic field across at least a portion of the flow path, at least 
one of the or each pair of electrodes being arranged so that, in use, it 
is separated from the dispersion by a layer of insulating material. 
The present invention also provides a method for aiding the separation of 
the 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 a unidirectional, varying electrostatic field across at least a 
portion of the flow path, at least one of the or each pair of electrodes 
being arranged so that it is separated from the dispersion by a layer of 
insulating material. 
The invention involves the combination of three factors to give an 
unexpected and highly efficient method of liquid phase separation. The 
factors are firstly that the electric field applied across the dispersion 
must be unidirectional. Secondly, this field must be fluctuating, for 
instance, regular or irregular variation of the voltage level. Preferably 
the variation is periodic and more preferably pulsed between zero voltage 
and some appropriate high voltage at a predetermined constant frequency. 
Finally, the high voltage electrode or both electrodes must be separated 
from the dispersion and thereby prohibited from contacting the dispersion 
by a layer of insulating material, for example, perspex or air. 
Preferably, the liquid dispersion is a dispersion of one or more 
electrically conducting liquids in one or more less electrically 
conducting liquids. More preferably the liquid dispersion is a dispersion 
of an electrically conducting liquid in an electrically insulating liquid. 
The apparatus is particularly well-suited to the treatment of dispersions 
where the volume of conducting liquid as a percentage of the total volume 
of dispersion is high, for example, 50%. 
The use of an electrically insulating material on the electrodes reduces 
the loss of charge which can occur by short-circuiting through the 
dispersion. Accordingly it is possible to operate the process on 
dispersions having a relatively large proportion of conducting material in 
the dispersion, without an excessive leakage current from the electrodes. 
The degree of liquid phase separation within the apparatus is controlled by 
the frequency and magnitude of said variation of the electrostatic field. 
At values of the electric field strength across the flow path of said 
mixture of liquids below 1000 volts per cm the effect of frequency is 
pronounced. 
The frequency at which the electric field is varied can have a specific 
value for which phase separation is optimum. This optimum frequency 
depends on the thickness and electrical properties of the electrode 
insulation and on the properties of the dispersion that is being treated. 
However, typically a preferred frequency of variation of the electrostatic 
field is between 30 Hz and 1 Hz. More preferably it is between 20 Hz and 
1.5 Hz and most preferably it is between 15 Hz and 2 Hz. It should be 
appreciated, however, that the optimum frequency may well lie outside the 
above preferred ranges, and in any case good results may be obtained at 
frequencies other than the optimum frequency. 
Typically for voltage gradients in the dispersion flow path of 300 volts 
per cm and below, the phase separation performance can be expected to be 
very sensitive to the frequency of the electric field fluctuation. For any 
particular operating voltage there is an optimum frequency, where the 
operating voltage is above or below 300 volts per cm. 
In general the higher the voltage the better the separation of the phases, 
but it is preferred to use the lower voltages having regard to factors 
such as safety, capitol costs and operating costs. Having regard to these 
factors the preferred electric field strength across the flow path is 
below 1100 volts per cm. More preferably it is below 500 volts per cm and 
most preferably it is below 100 volts per cm. 
Accordingly apparatus in accordance with the present invention effects 
separation of the components of a dispersion at much lower frequencies 
than had previously been considered. Furthermore separation at much lower 
frequencies has led to the appreciation that dispersions can be 
efficiently separated into their component liquids at field strengths far 
below those previously employed. Furthermore the equipment is safer to 
operate because of the avoidance of very high voltages in an environment 
in which flammable organic materials are likely to be present. 
This invention also provides a process for aiding the separation of the 
components of a liquid dispersion, the process comprising passing the 
dispersion along a flow path, and applying across at least a portion of 
the flow path a varying electrostatic field. 
Apparatus and processes in accordance with the present invention may be 
used in many situations where a mixture of two or more liquids is to be 
separated into its component liquids. Examples are as follows: 
1. A solvent extraction process in which a metal ion is to be extracted 
from aqueous solution. The aqueous medium is intimately mixed with an 
organic solvent in which may be incorporated an agent for extracting the 
metal ion. 
2. The separation of water droplets from petroleum derivatives or oil. 
3. Aromatic and aliphatic hydrocarbons having similar molecular weights may 
be treated using selective solvents which are relatively polar, examples 
being Sulfolane and N-methylpyrrolidone in order to extract high purity 
aromatic hydrocarbons for the petrochemical industry. The hydrocarbon and 
solvent phases are partially miscible and need to be separated after 
extraction. 
4. The separation of the components of liquid membrane systems. 
Apparatus in accordance with the present invention may be used alone or in 
conjunction with a settling tank depending upon the degree of separation 
effected by the application of the electrostatic field. In one arrangement 
a dispersion may be passed along a duct between a pair, or a plurality of 
pairs, of electrodes and then fed from the duct into a settling tank which 
may be provided with one or more baffles positioned close to the point of 
entry of the liquid mixture into the tank in order to reduce the 
turbulence in the tank. Each separated liquid phase can be drawn off at an 
appropriate rate in order to keep the total volume of the liquid in the 
tank substantially constant.

Referring to FIG. 1, apparatus in accordance with the present invention 
comprises mixer container 1 into which the two immiscible liquids are fed. 
The liquids are intimately mixed by agitator 2 driven at high speed by 
mixer motor 3. The resultant dispersion is then fed through a tube 4 into 
an electrostatic coalescer 5. Coalescer 5 includes a shallow perspex 
"duct" 5a which is approximately square in plan and which is located on a 
support 6 so as to be inclined gently upwards in the direction of the 
liquid flow. Metal plates (not shown) are located on the exterior surface 
of the top and the interior surface of the base of the duct 5a, the upper 
metal plate being charged and the lower metal plate being earthed. 
Extending into the duct 5a through a side wall thereof are two probes 7 
which are connected via a resistor bank to a suitable cathode ray 
oscilloscope so that the electric field inside the duct may be measured 
and/or monitored. Below the feed duct 4 is drain valve 8. This may be used 
to drain the contents of the coalescer at shut down and could be used to 
remove coalesced aqueous or conducting phase in order to keep the surface 
of the aqueous phase at a constant level in the duct. At the other end of 
the duct 5a is exit pipe 9 leading to a settler tank 10. 
Tank 10 has located therein vertical baffles 11a and 11b which define 
portions 11c and 11d of the tank. Within portion 11c the turbulence of the 
liquid being fed into the tank through tube 9 is dissipated. In portion 
11d there is, during operation of the apparatus, set up a steady state 
position in which dispersion 12 lies between organic phase 13 and aqueous 
phase 14. The depth of dispersion 12 may be used as a measure of the 
effectiveness of the apparatus in achieving separation of the dispersion 
into its component phases. The smaller the depth of the dispersion the 
more effective is the electrostatic coalescer 5. 
A further baffle 11e allows organic phase 13 to accumulate in portion 11f 
of tank 10 and to be led off therefrom through pipe 16. Aqueous phase may 
be led off from a central portion of the tank between baffles 11b and 11e 
through pipe 15. 
The electrical circuit used to charge up plate 5 of the electrostatic 
coalescer is shown in FIG. 2. In this circuit a variable supply from an 
EHT generator of up to 15 kilovolts is alternately switched on and off by 
means of a shunt stabilised triode 20 as used in colour television EHT 
valves. The grid of the triode is connected to a signal generator 23 with 
a range of frequencies of from 0.5 to 60 hertz, the generator being in 
parallel with a 1 M.OMEGA. resistor 24. The cathode 26 of the valve is 
connected to earth and is heated by a 7.3 volt a.c. heater 27. The anode 
22 of the valve is connected in series with a 100 M.OMEGA. resistor 19 and 
thence to the EHT input. The voltage on the anode is fed via line 21 to 
the top plate of the electrostatic coalescer 5. 
It will be appreciated that the supply to the triode heater 27 can be 
derived by any suitable means. However, in this embodiment of the present 
invention the heater supply is derived from an a.c. main supply of the 
standard 240 V, 50 Hz. In FIG. 2, terminal 34 is the live terminal, 
terminal 35 is neutral and 36 is earth. The lead from the live terminal 36 
is taken to a first terminal of the primary winding 29 of a 7.3 V a.c. 
0.3A transformer, via a main ON/OFF switch 33 and a 1A fuse 32 in series 
with the live supply lead. The second terminal of the primary winding 29 
of the transformer is led via a second pole of the previously mentioned 
ON/OFF switch 33 to the neutral terminal 35. 
In parallel with the primary winding 29 of the transformer, across the live 
and neutral supply leads, are an indicator neon lamp 31 and a current 
limiting 270 K.OMEGA. resistor 30. When switch 33 is closed, connecting 
the primary winding 29 to the mains supply terminals 34 and 35, lamp 31 is 
lit, indicating that switch 33 is in the ON position. 
Comprising the remainder of the transformer assembly is an iron core and 
secondary winding 28. A first terminal of secondary winding 28 is 
connected to earth rail 36. The second terminal of secondary winding 28 is 
connected to a first terminal of triode heater element 27, the second 
terminal of heater element 27 being connected to earth rail 36. 
It will be appreciated that the connection of the mains a.c. supply to 
primary winding 29 induces an a.c. voltage across secondary winding 28 of, 
in this case, 7.3 V. This causes an alternating current to flow through 
heater element 27, thus generating heat energy which may be transmitted to 
electrons on cathode 26. 
The above-described control circuit for the electrostatic coalescer 
operates to connect the top plate electrode alternately to ground, via the 
triode valve, and to the supply from the EHT generator. The result is that 
a pulsed d.c. unidirectional current is delivered to this electrode, the 
pulses being of substantially square wave form (see FIG. 3). 
Examples of dispersions, whose separation into component liquid phases may 
be accelerated by the above-described apparatus are as follows: 
1. A dispersion comprising equal parts of organic and aqueous phases, the 
organic phase being 40% cyclohexanol and 60% ESCAID 100 (a commercial 
kerosene diluent which is a product of Exxon) and the aqueous phase being 
water. The viscosity of the organic phase is 2.852 centipoises at 
25.degree. C. and its specific gravity is 0.853. The viscosity of the 
aqueous phase is 0.896 centipoises at 25.degree. C. and the specific 
gravity 1.000. 
2. A dispersion comprising equal parts of organic and aqueous phases, the 
organic phase being 20% LIX 64N (so- called "liquid ion exchange" 
extractant made by Henkel Corporation) and 80% ESCAID 100 and the aqueous 
phase is a solution of 2 g/l sulphuric acid in water. the viscosity of the 
organic phase is 1.953 centipoises at 25.degree. C. and its specific 
gravity is 0.809. The viscosity of the aqueous phase is 0.888 centipoises 
and its specific gravity is 1.0025. 
Table 1 illustrates the results obtainable using the above-described 
apparatus on the second of the dispersions described above. 
TABLE 1 
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REDUCTION 
PEAK ELECTRIC IN DISPERSION 
FIELD STRENGTH 
FREQUENCY BAND DEPTH 
v/cm Hz % 
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40 0.5 30.9 
1.0 50.4 
2.0 66.3 
4.0 71.4 
6.0 75.6 
8.0 76.10 
10.0 71.00 
20.0 48.8 
30.0 29.3 
40.0 15.8 
50.0 5.3 
60.0 1.0 
80 0.5 35.8 
1.0 64.8 
2.0 82.3 
4.0 85.7 
6.0 86.1 
8.0 86.5 
10.0 84.1 
20.0 78.0 
30.0 73.4 
40.0 60.9 
50.0 39.2 
60.0 21.8 
40 8.0 76.10 
80 8.0 86.5 
120 8.0 89.8 
200 8.0 91.6 
300 8.0 96.2 
550 8.0 97.6 
700 8.0 97.65 
850 8.0 98.0 
1000 8.0 99.4 
1000 1.0 98.3 
8.0 99.4 
60.0 98.1 
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The peak electric field values given in Table 1 are those obtaining inside 
duct 5a as measured by the probes 7. The reduction in dispersion band 
depth is the ratio, expressed as a percentage, of the depth of dispersion 
12 in tank 10 when the electrostatic field is applied across duct 5a to 
the depth when the field is not so applied. 
It can be seen from Table 1 that for electric fields 40 and 80 V/cm the 
maximum reduction in dispersion band depth is obtained at a frequency of 8 
Hz. Although the reduction in dispersion band depth at a given frequency 
at 8 Hz does increase with increasing electrostatic field, a very large 
reduction in band depth is achieved at electrostatic field strengths as 
low as 40 V/cm. Such a low electrostatic field strength may be achieved in 
the above-described apparatus with a voltage level applied to the charged 
electrode as low as 200 volts and there is therefore no need for a high 
voltage generator in order to reach such voltages. It is also apparent 
from Table 1 that at an electric field strength of 1000 V/cm the results 
for 1 Hz, 8 Hz and 60 Hz are nearly the same and very satisfactory. 
The data given in Table 1 are for a duct 5a for which the perspex sheet 
forming the top of the duct is 6 mm in thickness. The upper electrode is 
effectively separated from the dispersion by an insulating coating 6 mm 
thick. Perspex coatings of 3 mm and 10 mm employed with the same 
liquid-liquid dispersion show lower and higher optimum frequencies, 
respectively. The values obtained are given in Table 2. 
Apparatus in accordance with the present invention may or may not include a 
settling tank of a size appropriate for the process in which the apparatus 
is to be used. Having regard to the extent of settling achieved by the 
application of the electrostatic field, such a settling tank where it is 
included can be of substantially smaller capacity than those which would 
have been necessary without the use of the electrostatic field to effect 
precoalescing of the dispersed phase. 
TABLE 2 
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THICKNESS OF 
ELECTRODE OPTIMUM 
COATING FREQUENCY 
mm Hz 
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3.0 5.0 
6.0 8.0 
10.0 12.5 
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Apparatus in accordance with the present invention may, instead of 
including its own settling tank, be added to existing equipment including 
settling tanks. The addition of such apparatus will enable the settling 
stage of the overall process to be effected very much more rapidly for a 
given throughput, and in practice the throughput of the overall process 
can be greatly increased. 
Apparatus in accordance with the present invention when added to existing 
equipment including settling tanks may take a form where one electrode is 
suspended above said equipment, insulated from the liquid contents by air 
or an inert gaseous atmosphere. 
In an alternative embodiment of the invention the fluctuating 
unidirectional electrostatic field can be applied between suitably 
insulated wires immersed in the flow path of the dispersion, the 
electrical connections and spatial arrangement of said wires being such 
that they form a plurality of pairs of electrodes. 
It is found that the shape of the pulse has little effect on the efficiency 
of the electrostatic coalescer. Triangular, square and sine waves have 
been applied to the electrodes of the coalescer 5 but, because of the time 
delays introduced by the perspex wall of the coalescer duct, the wave form 
of the pulse inside the duct has been found to be in all cases a distorted 
and rounded square wave.