Contacting and Separation Equipment - the Electrical Alternative

Solvent Extraction 1990, T. Sekine (Editor) © 1992 Elsevier Science Publishers B.V. All rights reserved

1313

CONTACTING AND SEPARATION EQUIPMENT - THE ELECTRICAL ALTERNATIVE P. J .

BAILES

Department of Chemical Engineering, University of Bradford, Bradford, West Yorks., BD7 1DP, UK ABSTRACT Present understanding of the way in which electric fields can be used to improve equipment performance is based upon a few fundamental observations. Thus, drops forming in an electric field are subject to additional forces that can result in drop rupture or the formation of smaller than usual drops. Conversely, when in close proximity to each other, or with the bulk interface, drops can be made to coalesce very rapidly in the presence of an electric field. In other circumstances drops may be accelerated through the continuous phase whilst exhibiting vigorous internal circulation and enhanced mass transfer rates. These observations either singly or combined have formed the foundation for several recent investigations which are reviewed. An essential precondition that must be satisfied before electric fields can be properly applied is that the continuous phase liquid must be electrically insulating in character. This ensures that sufficient field may be established to have the desired effect on the dispersed phase, which must be relatively conducting. The technology is appropriate for breaking emulsion liquid membranes, hydrometallurgical solvent extraction of metals and crude oil dehydration and desalting. INTRODUCTION Solvent extraction involves the intimate contact of two immiscible or partially miscible liquids and their subsequent separation.

The first

process, if it is to be efficient, requires large numbers of drops to be created;

the second process requires the recombination of drops to form a

separated bulk liquid. achieve high

Designers of solvent extraction equipment strive to

rates of drop break-up

transfer performance.

and coalescence

to maximise mass

The wide variety of equipment that is available

today bears testimony to the different approaches that have been adopted. Almost without exception these designs rely on improvements brought about by mechanical means.

However, this situation is now beginning to change as

increasing attention is focussed on the direct use of electric fields as a means of securing better performance in certain applications. There are two areas of application where electric fields are uniquely effective. splitting

One concerns crude oil dehydration, the other refers to the of

electrically environmental successful

liquid

membrane

augmented

phase

separation

over

competing

benefits

commercial

emulsions.

exploitation

of

In both offers

cases

definite

technology. electric

fields

the

use

of

commercial and In as

particular, a means

of

1314 promoting the removal of the multitude of stable micro-droplets of brine present in crude oil reaching the well-head, has been in progress for most of this century.

This experience is undoubtedly an important precursor to

wider acceptance of the role that electric fields can play in chemical engineering unit operations, not least solvent extraction. Developments within the oil industry are leading to an increasing need for off-shore production platforms where there are enormous savings to be made by improving the performance and thereby reducing the size and weight of platform separation equipment; there are also great incentives to move towards automated procedures.

Increasingly the industry is having to cope

with emulsions where the crude oil is both exceptionally viscous and has a density close to that of water (e.g. Eocene oils).

This combination of

factors makes gravity separation very slow and as such can greatly increase the size of separator needed for a given flow of oil.

As oil fields go

through their productive life the water content of the emulsion produced at the well-head

increases dramatically.

The outcome may be an emulsion

containing 60% water that must be treated on an off-shore rig to give oil containing at most 2% water and possibly a good deal less, depending upon pipeline requirements. These changing demands are sure to impinge on the design of oil field electric treaters in a way that is of interest to the designers of solvent extraction equipment for reasons that will now be considered. CRUDE OIL DEHYDRATION AND LIQUID MEMBRANE EMULSION SPLITTING Crude oil dehydration is not a solvent extraction operation but both processes have in common, a requirement for liquid phase separation.

The

fact that crude oil operations involve rather stable water-in-oil emulsions at elevated temperatures was considered, in the past, sufficient reason to differentiate

between

the

equipment was concerned. technology,

the

subject

two

processes

as

far

as

phase

separation

However, by embracing liquid membrane emulsion of

solvent

extraction has

at

the

same time,

expanded to include an interest in breaking stable water-in-oil emulsions, this

being

essential

in

order

to

recover

the

internal

phase.

The

stabilising surface active agents are natural in origin in the case of crude oil, and deliberately introduced in the liquid membrane process.

In

either event, the result is that the natural rate of phase disengagement is far too slow and must be accelerated to give equipment of acceptable size. Thus,

the

relative

positions

of

solvent

extraction

and

crude

oil

dehydration with regard to their phase separation requirements has changed. They are now closer together.

Further evidence for this is seen in the

trend towards higher water content crude oil emulsions and the fact that

1315 liquid membrane emulsions also contain substantial water fractions. Chemical demulsifiers have a role in crude oil dehydration but their use does represent a cost to the process consequences. water-in-oil

and there are

environmental

In contrast, the addition of a demulsifier

to break a

liquid membrane emulsion is prohibited, because subsequent

extractions depend upon the ability to re-create a stable membrane emulsion using the organic phase recycled from the separator.

The presence of any

demulsifier at this stage would obviously have a destabilising effect.

It

seems certain therefore, if liquid membrane technology is to form the basis of continuous processes designed to clean up aqueous effluent streams, and if crude oil production from off-shore and sub-sea locations is to expand, that there will be a demand breaking Within

technology

these

that

for non-invasive, non-polluting,

can cope with high water

constraints, the use

of electric

emulsion

content emulsions.

fields

offers

a bright

prospect particularly as new methods of avoiding short-circuiting problems between

the

electrodes

when

treating

water-rich

emulsions,

are

now

available. ELECTRICALLY AUGMENTED PHASE SEPARATION Practical evidence obtained with crude oil/water separations has shown that almost any type of high voltage field will promote to some extent the separation of water-in-oil emulsions.

The mechanisms whereby this can

occur are not clearly understood except, that if a large potential gradient can

be

established

and

maintained

in

the

continuous

phase

of

a

liquid-liquid emulsion, it causes very fine drops to grow by coalescence with each other to a point where they fall out of the continuous phase under the action of gravity.

The technique is specific for water-in-oil

emulsions rather than the inverse, since it requires the continous phase to be relatively insulating in character. F. G. Cottrell, who is much more famous for his pioneering work on gas cleaning by electrostatic precipitation, was granted the first patents on the process of electrical coalescence (Cottrell, 1911; Cottrell and Speed, 1911); although the subject had been studied much earlier (Rayleigh, 1879). It was believed by Cottrell and several other early workers that drops in the presence of either DC or AC fields grew in size as a result of chain formation and coalescence.

Thus Cottrell and Speed (1911) observed that

when a high voltage was applied between two fine wire electrodes immersed in a thin layer of emulsion spread on a microscope slide, the water drops formed chains extending from one electrode to the other. adjacent

globules

in

each

chain

followed

and

the

Coalescence of

drops

nearest

electrodes became larger as new drops were acquired by the chain.

the

Cottrell

1316 was of the opinion that under the influence of AC fields, charged water drops would also vibrate and thereby rupture the enveloping oil films to the benefit of the coalescence process.

Certainly

it has become the

preferred practice in crude oil dehydrators to use two electrodes within the emulsion, one being electrically earthed, the other being AC energised. Gradually it became apparent that in addition to chain formation there were several other electrical effects which might prevail under certain circumstances. all

Waterman (1965) used the term "electrofining" to encompass

the known mechanisms

electrophoresis,

DC

for coalescence

induced

dipole

in DC

fields.

coalescence,

random

It involved collision

of

oppositely charged drops moving in opposite directions, and collisions due to differences in velocity of movement of different sized drops in the same direction.

Systems exemplifying electrofining invariably had an oil phase

of very low electrical conductivity such as kerosene or gas condensate, and a very

low hold-up

of dispersed

phase.

The various mechanisms have

provided a basis for the theoretical work of others and each mechanism has been substantiated by experiment as may be seen from Table 1. Factors

such as form and magnitude

of electric

field,

fractional

volumetric hold-up of dispersed phase, electrode geometry, and electrode insulation, determine which mechanism will dominate.

This realisation has

led to an astonishing number of patents, many of which represent simple variations on the basic theme.

In order to illustrate the manner in which

progress has been made, the salient points arising

in a selection of

patents from the past sixty years or so, are given in Table 2.

The driving

force underlying most of this work was the desire to circumvent current leakage so that high potential gradients could be employed even with wet emulsions.

The nature of this difficulty is well described in a paper by

Hsu and Li (1985).

Essentially the problem arises if a single chain of

water drops happens to extend from one electrode to the other or if, for any other reason, a very low resistance path should develop between the electrodes.

As a result of this, current can flow freely between the

electrodes, sparking often occurs, and unless the emulsion is relatively dry (e.g. < 1% water) or the applied voltage is limited, this causes a total loss of the coalescing force between the electrodes.

It is this

castastrophic effect upon coalescence that is of so much concern in the drive to devise current limiting designs, rather than the more obvious benefit of reduced power consumption.

In recent years there has been an

upsurge of interest in electrode insulation as a means of preventing this collapse of the electric field over the entire electrode and in different forms its use is now being proposed for crude oil desalting applications

1317 TABLE 1 Charged drop coalescence - mechanisms and models Mechanism

Model/Experimental Verification

Random collision

Sjoblem and Goren, 1966; Prestridge, 1973; Sadek and Hendricks, 1974

Induced-dipole coalescence

Waterman, 1965; Williams and Bailey, 1983; Yamaguchi et alt 1985, 1987

Electrophoresis

Seibert and Brady, 1919; Waterman, 1965; Turner, 1967

Pearl chaining

Pearce, 1954; Taylor, 1986; Ise et al, 1989

TABLE 2 A selection of electrical coalescence patents in chronological order Year

Inventor(s)

1926

de Brey

Pulsating DC fields of high voltage, bare metal electrodes

1936

Dillon

Pulsating AC fields with a capacitor in the external circuit, bare metal electrodes

1936

Heinrich

Unidirectional voltage pulses of 10 ^s or less with a maximum mark:space ratio of 1:10; bare metal electrodes

1944

Wolfe

Pulsating AC fields or very high frequency AC with a dielectric layer interposed between the two electrodes and a number of capacitors in the external circuit

1945

Deutsch

Pulsating DC with a bare metal energised electrode and an insulation coating on the earthed electrode

1958

Stenzel

Emulsion treating by uniform AC fields followed by pulsed DC (half wave rectified AC) to give two electric treating zones; bare metal electrodes

1973

Prestridge

Emulsion treated first by AC then by fluctuating unidirectional field using only one power source to generate both fields

1977

Richards and Clark

AC field applied by insulation coated high voltage electrode

1986

Bailes and Larkai

Pulsed DC applied by insulation coated high voltage electrode

Patented innovation

1318 (Bailes and Larkai, 1984; Prestridge and Johnson, 1987, 1989). The

great

advantage

of

using

a high

voltage

electrode

that

is

completely coated in a layer of insulating material, is that the insulation layer localises the ill-effects of any short-circuit caused by the mass of conducting drops.

Thus the field is automatically almost fully sustained

in the event that a local conducting path occurs.

The disadvantage is that

the insulation coating serves to raise the level of the applied voltage required to produce the same effective field in the emulsion.

The essence

of the problem may be seen by representing the system as a two layer condenser as shown in Figure 1.

I Material 1 - Electrode coating ^

Solid-liquid interface

Material 2 - Continuous phase liquid

Figure 1

Two layer condenser

In such a system the field distribution that immediately follows the sudden application of a DC field is governed by the electrostatic requirement of equal dielectric displacement (surface charge density) in the two layers, thus: Dx - D2

or

E 1 /E 2 - ^ A l

(1)

Thereafter there appears a surface charge at the interface and hence a discontinuity relation

in the dielectric

between

the

fields

displacement

in

the

two

such

layers

that ultimately

the

is

the

dictated

by

requirement for conduction current continuity, that is: El/E2 = K 2 A l Typical

values

for

(2) the

relative

permittivities

and

electrical

conductivities would be as follows: 6! - 3.5, e 2 = 1.32 and K X = 3.16 x 10" 1 3 , K 2 = 4.00 x 10~ 9 S nT 1 thus, initially, E;|/E2 = 0.38 and finally, E 1 /E 2 = 1.27 x 10 4 . It

is

immediately

apparent

that

resetting

to

the

initial

condition

1319 maximises the field in the oil layer.

This is the reason why pulsed DC is

so effective when used with an insulation coating on the high voltage electrode.

It also explains why continuous DC does not promote very much

coalescence when used with insulation coated electrodes, a fact that has been reported by others (Richards and Clark, 1977). The benefit of using high voltage pulsed DC in conjunction with a thin coating of insulating material on the high voltage electrode, may be seen in Figure 2.

The ability of the system to continue operating even though

the gap between the electrodes may be bridged by water is clearly evident from Figure 2b where both electrodes dip into the same water.

Figure 2a

depicts the dispersion with no electric field applied; Figure 2b shows the effect of applying a pulsed high voltage field between the high voltage electrode,

which

comprises

a

thin-walled

plastic

tube

filled

with

electrolyte in contact with the curved HV supply wire, and an earthed metal rod.

It is apparent that the field causes ripples at the bulk liquid

interface and drop size increases through coalescence as the dispersion flows down between the electrodes.

(a) Figure 2

(a) Normal coalescence

(b) (b)

Electrostatic coalescence

1320 Experimental work (Bailes and Larkai, 1982) has shown in practice that the use of pulsed DC and insulation coated electrodes does not always cause an increase in coalescence as the pulsation frequency is increased. is

invariably

maximised.

an optimum

pulsation

frequency

at which

There

coalescence

A variety of causes for this has been identified, thus:

is the

electrostatic coalescer is an element in the RC of an electrical circuit and when a pulsed voltage is applied it responds accordingly

(Joos and

Snaddon,

molecules

1985);

the

achievable

polarisation

of

the

organic

present in the continuous phase has an influence and makes it desirable to break different emulsions with differing electric fields (Hautermann et

al,

1989) ; and the relative permittivity of the emulsion is a function of its water

content

and

it

affects

the

frequency

response

of

the

system

(Sherman, 1968). For ease of photography, Figure 2 refers to a solvent extraction system in which water is dispersed in an organic phase of Acorga P5100 diluted with kerosene.

This is not an emulsion and is not very challenging

as far as phase separation is concerned.

However, it should be noted that

experience has shown pulsed DC to be very effective with liquid membrane emulsions

(Kataoka

and

Nishiki,

1986;

Yan

et

al,

1987).

Several

investigators have reported the use of AC fields with insulation coating (Hsu et al, Goto et

1983; Fujinawa et al, 1984; Hano et al,

al,

1989; Hauertmann et al,

1989).

1988; Feng et al,

1988;

Not surprisingly, in view of

equation (1) it is found that the use of AC fields also affords a means of developing a field in the emulsion when there is an intervening layer of insulation.

Interestingly, these workers report that there is an advantage

in stirring the emulsion during the electrical treatment. An alternative to fixing the nature of the electrode insulation and finding the pulsation frequency at which the effective field is a maximum, is to keep the applied voltage constant and change the electrode coating. Hsu, Li and Hucal (1983), test a number of different insulation coatings on this basis and conclude that, in order to get best results with liquid membrane emulsions, the coating should be hydrophobic and have a relative permittivity of at least three and preferably greater.

This finding is

again in keeping with expectations based upon equation (1). Phase

separation

applications

for

electric

fields

in

solvent

extraction are, of course, not limited to liquid membrane processes. is ample

evidence

that

electric

fields

can be

conventional hydrometallurgical solvent extractions. true where stagewise equipment is used.

used

to

There

advantage

in

This is particularly

Here it is necessary to repeatedly

separate the two liquid phases and the gravity settlers must be designed to

1321 give a holding Consequently,

time in keeping with the rate of phase disengagement.

for

systems

that

are

naturally

slow

to

separate, large

settlers are required and liquid inventories are proportionately increased. By

comparison

with

liquid

membrane

emulsions,

solvent

extraction

dispersions are unstable and separate easily, nevertheless phase separation rates can be greatly improved by the use of electric fields.

For example,

field tests at Ranchers* Bluebird copper mine in Arizona have shown that electrostatic coalescence vessels can be used at ten times the liquid loading rates of conventional separators (Warren et

al,

1978).

Similar

tests at the United Nuclear Church Rock Mill in the US have shown for amine extraction of uranium, electrostatic coalescer operation at six times the conventional loading rate (Prestridge et

al,

1983).

The speed with which

electrostatic coalescence can occur in solvent extraction dispersions is indicated in Figure 3.

(a) Figure 3

(b)

The effect of an electric field on a flow of dispersion

1322 A dispersion of water drops in LIX 64N diluted with kerosene is shown passing between parallel electrodes 3.3 cm apart.

Figure 3a depicts the

situation with no electric field applied to the jet of dispersion.

Figure

3b shows what happens when a DC voltage of 4 kV pulsed at 10 Hz, is applied to the PTFE coated right-hand electrode of the pair, with the bare netal left-hand electrode earthed. ELECTRICALLY AUGMENTED LIQUID CONTACTING It is well recognised

that inter-drop

coalescence

is a factor in

liquid-liquid contacting operations, insofar as it occurs in the mixing process as part of the drop coalescence-redispersion cycle.

This leads to

the idea that electrically enhanced coalescence can be used to improve mixing performance.

A

counter-current

liquid-liquid

extraction

column

designed to allow simultaneous use of high voltage fields and high shear mixing,

to promote

coalescence

tested in the laboratory

and

redispersion

respectively

(Bailes and Stitt, 1987).

has been

Measurements, with

cumene transferring from kerosene into a dispersed phase which was composed of a 50/50 mixture of N-methyl pyrrolidone and ethylene glycol, reveal substantial improvements in mass transfer rates are possible by matching increases in impeller speed and applied voltage to create more intense coalescence-redispersion cycles. The

concept

of

liquid-liquid

contacting

by

means

of

vigorous

mechanical agitation, balanced by electrostatic coalescence, is one which can be put into practical form so that high hold-up dispersions are just as readily dealt with as dilute mixtures.

This is not true of many other

ideas for augmenting mass transfer by the use of electric fields.

The

problem so often facing the equipment designer is how to reproduce on a commercial scale with realistic throughputs, the mass transfer enhancement observed in the laboratory with single drops or a dilute dispersion.

For

example, it is known that single drops formed at a metal nozzle, which is raised to a high potential, detach prematurely (Stewart and Thornton, 1967; Takamatsu et

al,

carrying a free charge

1982).

These drops can be

accelerated in the presence of a DC field through an insulating immiscible organic liquid and as a result of the increased turbulence within and around the drops they exhibit enhanced mass transfer performance (Bailes and Thornton, 1971; Chang and Berg, 1985; Vu and Carleson, 1986).

Attempts

to employ this knowledge in a practical contactor have been dogged by the difficulty of reproducing the single drop behaviour in a situation where there are numerous nozzles and a respectable hold-up of charged drops. Inherent problems of space charge between the electrodes, drop screening

1323 effects and electrical tracking down the walls and along aqueous filled pipes, all pose difficulties that remain to be solved. laboratory, results.

work

with

Thus,

single

Weatherley

Meanwhile, in the

charged

drops

continues

describes

the

electrical

to

give

useful

enhancement

of

microencapsulation reactions (Murray and Weatherley, 1989) and reports on the

extraction

of

ethanol

from

charged

drops

of

fermentation

liquor

(Laughland et al, 1987). The formation of single charged drops or indeed a spray of charged drops does not have to be accomplished at a nozzle.

It is feasible to

disperse a conducting liquid directly into an insulating liquid from the plane

interface between

the

two bulk phases by means

perpendicular to the interface.

Yoshida et

al,

of a DC

field

1988, have developed a

novel inclined plate contactor based upon this idea but again there seems to be a problem in getting sufficient hold-up fraction of the drops formed. A number of different electrically augmented liquid-liquid contactors have been designed around the simple gravity spray column.

Yamaguchi et al

(1988) report tests on a spray column with four vertical rod electrodes arranged equidistant from each other on the inner wall of a column made from acrylic resin.

DC energisation

in this case causes coalescence,

redispersion and motion of the drops in such a way that mass transfer is significantly enhanced, although the dispersed phase hold-up fraction is somewhat disappointing.

The authors attribute this to the weak electric

field at the core of the column not producing the requisite redispersion. In contrast, Warren and Prestridge their column design

(1979, 1980) deliberately ensure in

(see Figure 4) that the dispersed phase

is first

exposed to a very high potential gradient, which causes drop break-up, and subsequently to a lower field strength, which promotes drop coalescence.

Electrodes

Figure 4

Extraction column with electrically induced droplet break-up and coalescence

1324 The electrodes are shown as being energised by means by Prestridge's dual polarity

circuit

(Prestridge,

1973,

1974).

Also

feasible

is

the

application of AC to one electrode with the other at earth potential.

The

drawback with the design is that when sufficient voltage is applied to the electrodes to cause satisfactory mixing, the polar fluids accumulate within the mixing zone, ultimately forming a conductive path, which allows arcing or short-circuiting to occur between the electrodes.

One answer to this

problem is to modulate the strength of the electric field so that time is provided for coalesced polar drops to gravitate

from the mixing zone.

Prestridge and Johnson (1986) describe this approach and show the advantage of a modified multistage device in the context of crude oil desalting but it is equally applicable to solvent extraction applications. CONCLUSIONS It is clear from the foregoing, that there is considerable interest in the

application

of

electric

fields

to

enhance

rates

of

liquid

phase

separation in solvent extraction and emulsion liquid membrane processes. In these applications, and in the area of crude oil dehydration, there is a demand

for new electrical

emulsions.

technology

to cope with high water

content

Also foreseen are developments in mixing by means of electric

fields, which could lead to novel forms of liquid-liquid contactor. NOMENCLATURE D

electric displacment

E

electric field intensity

e

relative permittivity

K

electrical conductivity

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