Patent Application
20150031124
The present invention generally relates to a separator device and method for separating gases and liquids and, more particularly, to a separator and method for separating the gas and liquid phases of a foam. The invention finds particular application in fuel cells, but may also be useful in other applications in which it is necessary or desirable to separate the gas and liquid phases of a foam.
The separation of the liquid and gas phases of foams is a well studied and reported area of research. Foams occur in industrial processes when gas becomes dissolved in a processing liquid. Normally the dissolved gas would behave as part of the processing liquid, however, the gas can come out of solution as small bubbles, for example, when there is a release of pressure. These bubbles rise to the surface of the liquid and collect as a foam.
The presence of foams in processing liquids can lead to a number of efficiency and performance problems in industrial processes and other applications. For example, foams result in reduced efficiency of equipment such as pumps, reduced capacity of pumps and storage tanks, reduced effectiveness of the fluid and drainage problems to name but a few.
There is often a need to remove foams from industrial processes to avoid the problems described above. This usually involves separating the gas phase from the liquid phase of the foam and, ideally, this needs to be done rapidly, efficiently and with minimum power consumption.
One method is to use mechanical separation devices such as hydro-cyclones and centrifuges. These techniques consume significant amounts of power which can be a problem in certain industries or applications, for example, portable or automotive applications. Another possibility is to use a helical separator technology, such as that described in the applicant's co-pending patent application GB1201246.4, which utilises enhanced gravitational force to effect mechanical separation at lower power. However, there is an ever present requirement to improve separation efficiency at reduced power and in a smaller physical volume. To achieve this, it is not possible to rely solely on mechanical separation.
An energy efficient alternative to mechanical separation is to use antifoam. The action of antifoam in the disruption of liquid films and bubbles is well known and exploited. The mechanisms of antifoam in the disruption of a foam interface, i.e. the interface between the gas and liquid phases, are varied, depending on the formulation and form of the antifoam. However, it can generally be described by the interaction of a low surface energy or hydrophobic surface with the liquid film.
Generally antifoam agents take the form of particles having a low surface energy or hydrophobic surface. Such low surface energy surfaces can be created by forming the particles from or coating the particles with a low surface energy or hydrophobic material. Examples of such materials from which the particles may be formed or coated include polytetrafluoroethylene (PTFE), halogenated organic polymers, silicone polymers and hydrocarbon polymers such as polythene and polypropylene. PTFE, having a surface energy of approximately 20 mJ/m2, is a very effective hydrophobic material, although any suitable hydrophobic material may be used.
A low surface energy or hydrophobic particle at the interface of an aqueous foam film will rest such that the contact angle will satisfy Young's equation, as seen in
γSL+γLG cos(θc)=γSG Formula (1)
Where γSL, γLG and γSG are the interfacial tensions between the solid and the liquid, the liquid and the gas and the solid and the gas respectively and θc is the contact angle.
Foams consist of a large number of bubbles. Each bubble has a liquid film having two surfaces each of which is a gas-liquid interface as shown in
It is generally not possible to use antifoam as described above in industrial processes where foams are desirable or a necessary product of a particular process, for example, in aeration or oxidation processes. Foams are useful in such processes because they create a large surface area of liquid in contact with a gas which aids the absorption or solution of the gas in the liquid. The presence of an antifoam agent in such processes would be undesirable because it would prevent the formation of the foam. However, once the particular process involving the foam has completed, it may still be desirable to remove the foam prior to the next processing step. Therefore an alternative method of removal is required.
Devices for separating liquids in droplet or aerosol form from gas streams using hydrophobic separation surfaces are known. DE10323155A1 discloses a separator for the removal of liquid in droplet or aerosol form, from a gas stream, comprising a separation element located in a separation region. The element consists of a material with hydrophobic and nanostructure characteristics.
However, devices for separating the gas and liquid phases of a foam using a hydrophobic surface are not known.
Thus according to a first aspect of the present invention, there is provided a separator for separating the gas and liquid phases of a foam, the separator comprising a first side and a second side and having through-flow means provided therein for permitting a foam or a foam phase to pass from the first side to the second side, the separator further comprising at least one foam contacting surface having a low surface energy, and means for recovering at least one separated foam phase from the foam.
It should be readily understood that the term ‘foam’ encompasses froth and can be fine (having small gas bubbles) or coarse (having larger gas bubbles) or a combination of foam having different-sized gas bubbles
Preferably, the separator comprises means for recovering at least a liquid phase from the foam, more preferably at least a liquid phase and a gas phase from the foam.
Generally, the separator of the invention will be provided as part of or in cooperative relationship with an industrial unit generating a foam comprising at least one useful and reusable component, there being provided means for supplying foam from the industrial unit to the separator, and means for recovering the at least one useful and reusable component and supplying the same to the or an alternative industrial unit.
In this document, in the context of a material or a surface, reference to low surface energy shall be taken to mean hydrophobic and vice versa. By low surface energy we preferably mean an energy of less than about 40 mJ/m2, more preferably less than about 30 mJ/m2 and even more preferably less than about 25 mJ/m2.
The at least one foam contacting surface is preferably formed of one or more hydrophobic polymer materials. Such materials may for example be selected from polyolefins, polystyrenes, polyvinyls, polyvinylhalides, polyvinylidenehalides, polyhaloolefins, poly(meth)acrylates, polyesters, polyamides, polycarbonates, polyolefinoxides, polyesters and PEEKs.
More preferably, the one or more hydrophobic polymer materials may be selected from linear or branched polyethylene-linear, isotactic polypropylene, polyisobutylene, polystyrene, polymethylstyrene, polyvinyltoluene, polyvinyl fluoride, polyvinylidene fluoride, polytrifluoroethylene, polytetrafluoroethylene, polyvinylchloride, polyvinylidene chloride, polychlorotrifluoroethylene, polyvinylacetate, polymethylacrylate, polyethylacrylate, polymethylmethacrylate, polyethyl methacrylate, polybutylmethacrylate, polyisobutylmethacrylate, poly(t-butyl methacrylate), polyhexylmethacrylate, polyethyleneoxide, polytetramethylene oxide, Polytetrahydrofurane, polyethyleneterephthalate, polyamide-6,6, polyamide-12, polydimethylsiloxane, polycarbonate and polyetheretherketone. In particular, the one or more hydrophobic polymer materials may be selected from Table 1 which also lists the hydrophobic polymer materials' respective surface energies.
Rather than use an antifoam agent such as hydrophobic particles, A separator unit according to the present invention comprises a low surface energy or hydrophobic foam contacting surface which exploits the same mechanism of foam disruption described above, rupturing the liquid/gas interfaces of the foam as it passes from the first side of the separator to the second side. The foam contacting surface may be formed from or coated with a low surface energy material, for example, PTFE, although any low surface energy or hydrophobic material may be used, as described above. The foam contacting surface may be a substrate or a plate over which or past which the foam or a liquid containing the foam flows. More than one foam contacting surface may be provided to increase the surface area in contact with the foam thereby enhancing foam disruption. The foam contacting surface or surfaces may be orientated parallel to or at an angle to the flow of the foam or the liquid containing the foam. The foam contacting surface or surfaces may be located in the region of the through-flow means.
In one preferred embodiment of a separator unit according to the present invention, the foam contacting surface and the through-flow means may be one and the same thing. In such an embodiment the foam contacting surface has holes therethrough which permit the foam or a phase thereof to pass from the first side of the separator to the second side. The holes increase the specific surface area of disruptive interface. As described above, the foam contacting surface may be formed from or coated with a low surface energy or hydrophobic material and more than one foam contacting surface may be provided.
A preferred foam contacting surface having holes therethrough could be a mesh or a perforated plate. Such a mesh or plate may be formed from or coated with a low surface energy or hydrophobic substrate or medium. Preferably, such a mesh would be formed of low surface energy or hydrophobic polymer filaments or the mesh may be formed of filaments of another material and coated with the low surface energy or hydrophobic material. The mesh or perforated plate hole size can vary in diameter from 0.1 mm to 10 mm, with the polymer filaments when present ranging from 50 μm to 1 mm in thickness. The disruptive interface phenomenon is enhanced when surface material has a low surface energy and a microfilament or rough finish at the length scale of the foam liquid film thickness.
The mesh or perforated plate, as appropriate, may be arranged in various configurations and at various orientations to the flow of foam. One or more layers may be provided perpendicular to, parallel to or at an angle to the flow. Alternatively a randomly packed configuration may be provided. In addition, a pleated pan fold arrangement may be provided either parallel to or at an angle to the flow. Furthermore, an annular fan fold configuration with combined flow flowing inside or outside the annular space may be provided.
When a foam is passed over the low surface energy surface or through the low surface energy mesh, the gas liquid interfaces of the foam are ruptured and the gas and liquid separate into a dense liquid phase and a gaseous phase. This phenomenon can also enhance separation in mechanical separation devices, such as hydro-cyclones, centrifuges or in the applicant's helical separator technology referred to above. To achieve this, the surfaces in contact with the foam in such devices are formed from materials having a low surface energy and/or a microfilament or rough finish at the length scale of the foam liquid film thickness. In addition, including a low surface energy mesh upstream or downstream of the mechanical separation devices enhances the separation of the gas and liquid phases.
According to another aspect of the present invention, there is also provided a method for separating the gas and liquid phases of a foam comprising: providing a separator unit in accordance with the invention; supplying a foam to the first side of the separator; passing the foam through the through-flow means towards the second side of the separator; and at or downstream from the second side of the separator, recovering at least one of a separated liquid phase of the foam and a separated gas phase of the foam.
The foam may be generated by an industrial unit and the at least one separated phase of the foam may be recovered and supplied to the or an alternative industrial unit.
The industrial unit may be a fuel or electrolysis cell.
Alternatively, the industrial unit may be a biochemical fermentation unit or froth floatation unit.
According to another aspect of the invention there is provided a foam reduction apparatus comprising a low surface energy material and means for contacting foam, when said foam is input to the foam reduction apparatus, along a surface of said low surface energy material.
At least a portion of the surface of said low surface energy material may be convex or pointed so that it is projecting away from other portions of the surface.
Such a portion of the surface of said low surface energy material may be formed by plural convex regions on the surface.
The portion may be formed by elongate strands of a mesh structure.
The surface, or surfaces, may be oriented at least partly parallel to a direction of flow of fluid past the surface(s).
The, or each, surface may comprise flexible material and may be held at or proximal to its/their upstream end(s), so as to inhibit movement of its upstream end whilst permitting lateral movement of a portion of the surface distal from its upstream end.
The surface may comprise a plurality of surfaces which are held in position proximal to one another so that they are at least partly parallel to one another and to the primary direction of fluid flow at their respective upstream ends.
The plurality of surfaces may be held in position so that they are spaced apart from one another in a direction transversal to the primary direction of fluid flow.
The plurality of surfaces may be attached to one another along an axis at least partly parallel to the primary direction of fluid flow and held in position so that they each extend from said axis radially outward from said axis.
A gas-liquid separating apparatus may be provided, comprising a separator according to the first aspect of the invention, and a foam reduction apparatus according to the other aspect of the invention.
A fuel cell system comprising a separator and/or a foam reducing apparatus as described herein may be used for the combined generation of heat and power, to provide motive power to a vehicle, or to generate power in an electronic apparatus, or any combination of two or more of such uses can be provided.
While the separator for separating the gas and liquid phases of a foam described hereinbefore has various applications, a particularly effective application is use of the separator in a catholyte regeneration system for a redox fuel cell, for example.
In an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it, and this oxidised species is fed to the cathode.
There are a number of constraints on this step of oxidising the redox couple. Oxidation of the redox couple should occur as rapidly as possible as a reduction in flow rate of the catholyte through the cathode will reduce the rate of energy production. The rate of energy production will also be reduced if oxidation of the redox couple is not as complete as possible, i.e. if a significant proportion of the redox couple remains unoxidised. The provision of apparatus which rapidly and completely oxidises redox couples present in catholyte solutions is made challenging by the need to ensure that the energy consumed when the oxidation step is taken is relatively low, otherwise the overall power generation performance of the fuel cell will be reduced. Additionally, the apparatus used to oxidise the redox couple should be as compact as possible, especially when the fuel cell is intended for use in portable or automotive applications.
The need to balance these conflicting requirements gives rise to inefficiencies in cell performance, particularly in automotive and electronic applications and in combined heat and power.
In operation of a redox fuel cell, the catholyte may be provided flowing in fluid communication with the cathode through the cathode region of the cell. The redox mediator couple is at least partially reduced at the cathode in operation of the cell, and at least partially re-generated (re-oxidised) by reaction with the oxidant after such reduction at the cathode. The at least partial regeneration of the redox mediator couple is effected in a regeneration zone.
The re-generation process typically requires the contacting of liquid catholyte with large volumes of air, greater than 4:1 on a volume basis (standard temperature and pressure) and up to 20:1 or greater. The interfacial area generated for mass transfer of oxygen from the air into the catholyte is in the form of high internal phase volume foam of small bubble radius. The rate of regeneration of the catholyte is proportional to the interfacial area for mass transfer. High rates of regeneration are required.
Once the catholyte has been regenerated the residual gases, mainly nitrogen, need to be removed from the catholyte before the catholyte is supplied to the fuel cell cathode. The catholyte must not contain gas bubbles or foam. Disengaging the “spent” gases must be done rapidly, efficiently and with minimal power consumption. As described above, mechanical separation, such as hydro-cyclones and centrifuges, may be used but these use an unacceptable amount of power especially when used in portable or automotive applications.
In redox fuel cell systems an antifoam agent cannot generally be added to the catholyte as there is a need to generate gas-liquid interfaces to promote oxidation. However, as described above, the mechanism of foam disruption can be exploited by passing the foam past a low surface energy or hydrophobic surface, such as PTFE, in order to rupture the foam interfaces.
Thus, according to another aspect, the present invention also contemplates a fuel cell comprising the separator unit of the invention located in a region of the cell adapted to regenerate by oxidation a redox couple useful in the cathode region of the cell.
Such a fuel cell may comprise: at least one membrane electrode assembly, the membrane electrode assembly comprising an anode and a cathode separated by an ion selective polymer electrolyte membrane; an anode chamber adjacent the anode of the membrane electrode assembly; a cathode chamber adjacent the cathode of the membrane electrode assembly; means for supplying a fuel to the anode chamber of the cell; means for supplying an oxidant to the cell; means for providing an electrical circuit between respective anodes and cathodes of the cell; a catholyte solution comprising at least one non-volatile catholyte component, the catholyte solution comprising a redox catalyst and/or mediator couple; means for contacting the redox catalyst and/or mediator couple with the oxidant to generate a foam comprising oxidised catalyst and/or mediator couple; and means for supplying the foam to the separator unit of the invention and recovering for further use at least the liquid phase of the foam.
The invention also contemplates the use of the separator according to any of the foregoing in separating the gas and liquid phases of a foam. Such a use may include supplying at least one useful separated phase from the foam to an industrial process, for example in a catholyte regeneration system of a redox fuel cell, or in separating hydrolysis gases from an electrolyte liquid.
By “cathode chamber” is meant that part of the cell bounded on one side by the cathode side of the membrane electrode assembly. Alternatively, or as well, the “cathode chamber” may be thought of as that part of the cell in which at least a part of the catholyte flowing therethrough in operation of the cell contacts the cathode side of the membrane electrode assembly.
Likewise, by “anode chamber” is meant that part of the cell bounded on one side by the anode side of the membrane electrode assembly.
Various embodiments of the present invention will now be more particularly described by way of example only, with reference to, and as shown in the accompanying drawings, in which:
With reference to
The separation section 2 may comprise multiple low surface energy surfaces or meshes or mesh packing 10 so to increase the specific surface area of disruptive interface.
Although the separator is described as comprising three sections, such sections may not be distinct standalone sections. The sections may be integrated into a single continuous separator in which foam is received at one end and separated gas and liquid phases exit at the other. For example, the collection section 5 may be integral to the separation section 2 because as the gas and liquid phases are separated in the separation section, the denser liquid phase will tend to drop under gravity and collect at the bottom of the separation section, where it can be removed via outlet 8, and the lighter gas phase will tend to rise above the liquid phase and collect in the upper part of the separation section, where it can be removed via outlet 9. This characteristic of the gas and liquid phases is illustrated by the arrows in the separation section 2 in
In operation foam is supplied to the feed section 3 of the separator via inlet 4 as indicated by the leftmost arrow in
The separator may be a standalone separator or module. Alternatively, it may be integrated into another device to enhance its performance, for example, a mechanical separation device as described hereinbefore.
The practical embodiment of this invention was tested by placing 10 ml of the Applicant's FlowCath® catholyte liquid in a measuring cylinder and passing a flow of air at a flow rate of 0.5 l/min through the catholyte using a ROBU® sintered glass sparge (product no. 18143) to form a foam. As can be seen from
The test was repeated and a PTFE knitted mesh (Textile Development Associates, Inc., product no. PTFEKM22001), as shown in
There will now be described further aspects and embodiments of the invention relating to recent investigations and experiments by the inventors concerning the use of LEM-assisted foam disruption.
A general principle of foam disruption (destruction or breakdown of froth or foam) using Low Energy Material (LEM), typically comprising a mesh will now be explained. Although the performance of meshes have been demonstrated, structures other than meshes should give similar results (i.e. any other matrix of exposed LEM surface—e.g. multiple stands, fibrous mass, solid foam or sintered mass).
Effective gas-liquid separation, when used as part of a fuel cell system, acts to prevent:
In order to minimise both parasitic load (power consumed by the gas-liquid separation reactor) and gas-liquid separation reactor size, this operation must be accomplished with optimum energy and volume efficiency (i.e. using a small gas-liquid separation reactor which consumes little power). Investigations by the applicant have indicated that PTFE meshes are effective in collapsing V4 POM froth or foam. PTFE is a low surface energy material (LEM) and is therefore highly hydrophobic and thus water repelling (having surface energy of around 18 mJ/m2 at 20° C.).
If exposed to an aqueous frothy mixture, the low surface energy material selectively repels the liquid phase. This has the effect of thinning the liquid boundary between bubbles of the foam (the liquid bubble-to-bubble boundary) at the point of contact of the bubbles with the LEM surface, promoting rupture and thereby coalescence of the bubbles i.e. merging or agglomerating of small bubbles into fewer larger bubbles. In the merging process a plurality of bubbles merges or coalesces to form one single bubble, this occurring for multiple groups of bubbles.
The concept of separation by LEM-assisted bubble coalescence and gas-liquid phase segregation will now be described in more detail.
Gas-liquid separation involving LEM materials can be regarded as a two-stage process. As explained above, LEM materials accelerate or promote foam collapse by enhanced or increased bubble coalescence or merging, effectively causing bubble collapse. However, this process alone does not separate gas from liquid; it merely transforms a fine 2-phase flow (containing small bubbles) into a coarse 2-phase flow (containing larger bubbles). That is to say, the process makes small bubbles into larger bubbles.
A further phase ‘segregation’ stage, using gravity or centrifugal force, can bring about true, or complete, separation (by using a segregator apparatus such as a settling chamber, cyclone, helix, etc.)
LEM-assisted coalescence prior to phase segregation by gravity or centrifugal force has an advantageous technical effect that segregation is achieved, overall, more easily and this allows the use of segregation apparatus or ‘plant’ which is smaller and consumes less energy. Hence, LEM-assisted gas-liquid separation is envisaged by the inventors as a two stage process involving, i) (enhanced) coalescence and, ii) phase segregation.
Also, as mentioned earlier herein, phase segregation apparatus for foam destruction/segregation, such as a cyclone or helix, can be improved by lining the interior surface of the phase segregation apparatus with expanded mesh, as will be explained further below (see Table 2 on the next page for test results).
The concept of primary and secondary bubble coalescence will now be explained.
Investigation by the inventors has led to the development of primary and secondary LEM bubble coalescing devices or coalescers (see
The primary coalescer device or apparatus can be mounted within a pipe downstream from the gas-liquid contactor of a fuel cell system. Examples are shown in
The primary coalescer device comprises multiple (typically mesh) surfaces mounted at least partially parallel to the flow stream. This arrangement has the following advantages:
Coursing, or directing, the fluid flow across the surface, as described above, enables exposure or contact of the incoming finer bubbles with the surface of the LEM.
Without a shear action as described above, which acts to tear or drag the bubbles away from the surface, the only mechanism for bubble removal is buoyancy of the bubbles relative to the liquid, due to their lower density compared to that of the liquid.
If buoyancy is the only mechanism, the active surface becomes isolated from a significant portion of the bubbles by an established gas layer resulting from many coalesced bubbles forming a single volume of gas. As a result the process of gas-liquid separation or segregation is less effective.
In addition, crossflow shear action has also been observed to encourage bubbles to grow by coalescing, or merging, with one another in a ‘snowball-like’ fashion as they are swept downstream across the surface, which typically comprises a mesh structure.
Surfaces may be mounted across the pipe cross section in parallel to one another, or radially, and/or in pleats. Other cross sectional arrangements may include a lattice or grid (of squares, honeycomb, circles, polygons, etc), concentric circles, a spiral or any other structure forming axial channels (see
The primary coalescer may be designed and arranged as a series of discrete ‘elements’ within a pipe. Each element would contain and support a suitable amount of LEM surface. If one element was found to be insufficient to coalesce a given flow or fineness of foam, then plural elements could be installed as required.
In
The example secondary coalescer device 1605 or ‘Bubble Trap’ is mounted within the reservoir 1606 and receives return (fluid) flow via the primary device 1620. The main purposes of the secondary coalescer device 1605 are
The example secondary coalescer device 1605 is illustrated in both
The rack 1605 of the example arrangement shown in
In the embodiment illustrated in
There are at least three mechanisms for this very effective means of foam destruction.
First according to a first mechanism, as described for the primary coalescer, foam destruction begins during its initial passages into the screen array as it moves or courses across the LEM screen surface beneath the inlet 1640 (the flow direction of foam being downward as shown by arrow 1616 in
Secondly, the screens 1602 of the secondary coalescer device 1605 inhibit any surviving froth from advancing laterally with respect to the main fluid flow direction (indicated by arrow 1616 in
De-gassed liquid is allowed to drain through apertures of the screens 1602. However, bubbles larger than these apertures are prevented from passing through the apertures (typically mesh) and are detained or held up until they are collapsed or burst.
Bubbles smaller than the apertures can pass though the apertures with the liquid. However, most of the bubbles are prevented from reaching the screen surface by the detained larger bubbles. This can be considered as advantageous in that, in effect, the screen or mesh acts like a bubble filter with the retention of smaller bubbles (within the regions between adjacent screens) being assisted by a ‘filter cake’ of larger bubbles in a region between the region containing the smaller bubbles and the surface of the screen facing that region (not illustrated).
A third mechanism may arise due to a flow field existing beneath the fluid-plunging inlet of the secondary coalesce device.
It is envisaged that the combination of these three mechanisms makes the LEM bubble trap very effective, test results indicating this effectiveness.
Without the presence of a secondary coalescing device or ‘Bubble Trap’, gravity separation (by upward floating movement of buoyant bubbles within, and relative to, the liquid) would be the only mechanism available to facilitate (a) the destruction/containment of residual foam and (b) gas re-entrainment in the liquid fluid. If such gravity separation is the only mechanism, the gas-liquid segregation reservoir would therefore need to be much larger than if the secondary coalescing device is used. Clearly, the secondary coalescing device provides for use of a smaller reservoir for a given fluid flow rate and a given.
In addition to primary and secondary coalescer stages as described above, the gas-liquid separator system could also be arranged with primary and secondary gas-liquid segregation stages.
For example, following the primary coalescer, gas-liquid mixed fluid flow could be directed to a cyclone or helix or other bulk separator to facilitate bulk segregation of the gas phase from the liquid phase. Liquid (and any residual foam) discharging from the bulk separator liquid output (e.g. cyclone base or helix output) could then be directed to a secondary coalescer bubble trap. Gravity settling within the gas-liquid reservoir would then facilitate secondary segregation.
The advantages of such an arrangement may become apparent on higher-power systems employing high flow rates and/or high flow velocities. Separation by staged coalescence and segregation would allow flow velocities to be gradually reduced before return of the fluid to the reservoir. This would further promote calming of flow and would avoid secondary entrainment to the gas and liquid outlets respectively.
Testing of LEM mesh materials, carried out by the inventors, will now be described.
A range of LEM meshes and surfaces have been investigated by the inventors in order to determine their effectiveness. Results are given in Table 3 below. Knitted Mesh PTFEKM22001 and Expanded Meshes ET-8300 and 5PTFE7-100ST were selected for testing and were found to give good performance.
Images of knitted mesh PTFEKM22001 (originally developed for surgical applications) are shown in
As the process of LEM-assisted bubble coalescence is a surface phenomenon (i.e. it occurs at a surface), self supporting meshes could be fabricated with a rigid substructure (e.g. wire mesh coated with PTFE). External supporting structures (e.g. racks, frames, supporting ribs, etc.) take up volume, are intrusive to flow and thus contribute to pressure drop along the fluid flow direction. An internal supporting structure would avoid these problematic issues.
Primary and secondary coalescer devices, as described above, were developed by the inventors though a program of testing and development. Devices were tested downstream of a foam generator. Unless otherwise stated, applied fluid flows were 30 L/min (0.5 litres per second) catholyte and 120 L/min (2 litres per second) air at room temperature (around 20 degrees Celsius). Table 4 summarises each prototype design and performance.
During tests, the above described devices were also found to generate much less catholyte mist carryover to the gas exhaust (i.e. less droplets escaping with exiting or output gas). Results of the tests indicate that it is possible, by use of the invention, to obtain two orders of magnitude improvement over conventional gravity and centrifugal foam separation devices (i.e. settling chamber, cyclones, helix etc.)
The following provides a proposed explanation of why a LEM-based gas-liquid separator, examples of which are described above, releases significantly less catholyte mist to the exhaust stream than by gravity or centrifugal based techniques.
Settling chambers, cyclones and helices all exploit differences in phase density to achieve gas-liquid separation and thereby foam collapse. Gravity or centrifugal force is used to induce liquid drainage via the foam's interconnected network of bubble membranes. As a result, bubbles at the foam surface become liquid starved, leading to membrane thinning and therefore weakening. Eventually the weaken membranes rupture under the influence of surface tension. Liquid surface tension then draws the collapsing membrane films into spherical droplets. These become entrained within the separating gas stream and exit the system via the gas exhaust output as mist. This is undesirable. A new surface layer of bubbles is consequently revealed on the upper surface of the froth or foam and the process repeats.
LEM Based Gas-Liquid Separation, Compared with Gravity and Centrifugal Based Gas-Liquid Separation
Due to the hydrophobic nature of PTFE or other low-surface energy material, each liquid bubble membrane contacts the low-energy surface with a ‘low’ contact angle. That is, the outer surface of the bubble's membrane is repelled by the hydrophobic surface and therefore, when the bubble contacts the surface, its membrane becomes angled away more from the surface outwardly from the central point of contact of the bubble with the surface, than if the surface were made of a higher-surface energy material. In other words, the surface of the bubble membrane, in the region of contact of the membrane with the surface, is more outwardly convex than it would be if the surface were made of a higher-surface energy material.
This ‘low’ contact angle acts to undermine and locally thin the membrane at the point of contact, leading to weak surface adhesion. Inherent membrane surface tensions then become sufficient to tear or drag the liquid film membrane away from the PTFE surface and thereby remove the bubble from the surface. As each membrane retracts, liquid, contained within the membrane, flows into surrounding films of one or more other bubbles and two bubbles are coalesced into one. See
According to the above-described process of destruction or coalescence of bubbles by means of low-surface energy material, in a general sense the bubble membranes are not weakened by drainage of the liquid, the integrity of the membranes is maintained as the membranes retract and as such, each membrane is much less likely to disintegrate into droplets. That said, even if some droplets are created anyway, despite this maintained integrity of the membranes, the formation of such droplets would occur as a double encapsulation (i.e. droplet contained within surviving bubbles) and would occur beneath a blanket or region of foam. This provides opportunity for re-absorption of the droplets thus preventing the droplets from being released to the gas output or exhaust.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1201246.4 | Jan 2012 | GB | national |
| 1203565.5 | Feb 2012 | GB | national |
| 1203567.1 | Feb 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/050174 | 1/25/2013 | WO | 00 |
