GB2499820A - Improved fuel cell electrolyte regenerator and separator - Google Patents

Abstract

The invention concerns a separator for a liquid electrolyte regenerator of a fuel cell system. A helical fluid channel (100) formed on a helix (150) is arranged to conduct liquid and gas of a gas-liquid mixture and separate the liquid from the gas-liquid mixture. Preferably the helical channel (100) is an enclosed channel or pipe along which the gas-liquid mixture is constrained to travel, and the overall diameter (DHelix) of the helical channel is close to twice the pipe diameter (Dpje) of the helical channel along at least a portion of the helical channel. The helical channel can form part of a bulk gas-liquid separator or a gas-liquid contactor and separator or a condensing heat exchanger

Description

IMPROVED FUEL CELL ELECTROLYTE REGENERATOR AND

SEPARATOR

The present invention reiates to an indirect or redox fuel cell system, and in 5 particular to a liquid electrolyte regenerator and separator for such an indirect or redox fuel cell system.

Fuel cells have applications in stationary, back-up and combined heat and power (CHP) contexts, as well as in fuel cells for the automotive industry and in micro fuel cells for electronic and portable electronic devices.

10 Fuel cells are devices that produce electrical energy using the chemical properties of a fuel (often hydrogen) and oxygen to directly create electrical current. They are technically similar to a battery although, unlike a battery, they do not store energy but produce electrical energy from an external fuel source as required,

15 Fuel cells were initially demonstrated in 1839, by Sir William Grove, however, a truly workable fuel cell was not demonstrated until 1959. After use in NASA's space programme, interest in fuel cells decreased until the 1990s when they were considered as a replacement for combustion engines because of their potential to be a more efficient and clean way to create

20 power. Fuel Cells now find use in a range of applications such as transport, stationary power and even laptop computers.

In its simplest form, a fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction product(s), producing electrical energy and heat energy in the process. When hydrogen is used as fuel and

air or oxygen as oxidant, the products of the reaction are water and heat. The hydrogen and air/oxygen gases are fed respectively into catalysing, diffusion-type anode and cathode electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes.

5 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 fue!) of a redox couple to oxidise it, and this oxidised species is fed to the cathode.

There are a number of types of fuel cell which are normally distinguished by io the electrolyte they contain. The best-known types are alkaline, molten carbonate, phosphoric acid, solid oxide and Proton Exchange Membranes (REM). PEM membranes include Polymer Electrolyte Membranes. Direct methanol and regenerative fuel cells are the subject of extensive research. Fuel cells utilising alkali electrolyte have an inherent disadvantage in that the 15 electrolyte dissolves C02 and therefore needs to be replaced periodically. Polymer electrolyte or PEM~type cells with proton-conducting solid cell membranes are acidic and avoid this problem.

PEM fuel cells are used in automobiles. Most fuel cells used in vehicles produce less than 1.18 volts of electricity which is not enough to power a 20 vehicle. Therefore, multiple cells are assembled into a fuel cell stack. The potential power generated by a fuel cell stack depends on the surface area of the membrane in each cell and the total number of the individual fuel cells that comprise the stack.

A PEM fuel cell comprises a polymer electrolyte membrane (PEM) sandwiched between an anode and a cathode. Anode and cathode flow plates are attached to the anode and cathode respectively via respective backing layers. The anode flow plate acts to distribute hydrogen across the 5 anode. The cathode flow plate 110 distributes oxygen/air across the cathode and channels water as a by-product away from the cathode and provides heat as another by-product. An electrical current flows between the cathode and anode flow plates.

The anode typically comprises platinum particles uniformly supported on io carbon particles. The platinum acts as a catalyst by Increasing the rate of the oxidation process. The anode is porous so that the hydrogen fuel can pass through it. Similarly, the cathode too typically comprises platinum particles uniformly supported on carbon particles. The platinum of the cathode acts as a catalyst by increasing the rate of the reduction process. The cathode is is porous so that oxygen can pass through it,

A problem exists in that it has proved difficult in practice to attain power outputs from such PEM-type fuel cells approaching the theoretical maximum level, due to the relatively poor electrocatalysis of the oxygen reduction reaction. A further problem is that expensive noble metal electrocatalysts 20 such as platinum are often used, causing a significant cost impact,

A recently-developed technology addresses these problems and promises to make PEM fuel cells competitive with conventional electricity generators, such as diesei generators, by replacing the fixed platinum catalysts on the cathode with a liquid regenerating catalyst system.

Such a liquid regenerating catalyst system is described in international published patent application W02010128333, the contents of which are incorporated herein by reference.

in a known liquid regenerating catalyst system, liquid electrolyte ('catholyte') is 5 continuously pumped through the fuel ceil, by a pump into a regenerator and then back to the fuel cell. Air is forced by a blower into the regenerator at an input port and air (depleted of oxygen), wafer vapour and heat are output from the regenerator at an output port. As well as providing gas-liquid contacting, the regenerator also includes a gas-liquid separator which allows the 10 regenerator to remove air/oxygen from the catholyte and return the catholyte, substantially free of air/oxygen, to the stack.

This liquid electrolyte regenerating technology reduces platinum content by up to 80% and simplifies the overall fuel cell system. As a consequence the technology not only radically reduces cost, it also improves durability and 15 robustness of the system. This technology overcomes the three major limitations associated with conventional PEM fuel cell operation, namely catalyst loading, catalyst agglomeration and heat management. Additionally, a peak performance power density of nearly 900 mW/cm2 has been achieved, which is a substantial improvement over a previously announced peak power 20 record of around 600 mW/cm2.

A known redox reaction occurs within a fuel cell of the liquid regenerating catalyst systems described above. The composition of a redox mediator couple and/or a redox catalyst of the redox reaction has been described in international patent applications having publication numbers 25 WQ/2007/110863, WO/2009/040577, WO/2008/009993, WO/2009/093080,

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WO/2009/093082, WQ/2008/009992 and WO/2009/093081, the contents of which are incorporated herein by reference.

In order to regenerate the liquid electrolyte (catholyte) in the liquid regenerating catalyst system, if is necessary to create a large gas-liquid 5 interfacial area to enable the reaction together of sufficient electrons, protons and oxygen molecules to form the oxidized catholyte and the water byproduct. This can be achieved by the creation of gas bubbles in the liquid stream or liquid droplets in a gas stream (both these methods being known generally as gas-liquid contacting). The total surface area of the gas bubbles 10 is maintained for sufficient time to achieve sufficient mass transfer, after which separation of the gas and liquid streams is performed as rapidly as possible with minimal energy input. This separation is done prior to the input of the liquid electrolyte into the fuel cell so as to provide good operation of the fuel cell.

15 Therefore, a bubble generator for a liquid electrolyte fuel cell system is arranged to input liquid electrolyte and gas, to generate gas bubbles in the liquid electrolyte and to output the liquid and gas in bubble form.

Preferably most of the electrolyte liquid output from the cathode region is converted into a foam form by the formation of bubbles within it. The bubbles 20 greatly speed the re-oxidation of the electrolyte liquid during the regeneration process, prior to the electrolyte being input once again to the PEM fuel cell.

The fuel cell uses cathode electrolyte (catholyte) in liquid form, and best performance of the fuel cell is obtained when the electrolyte at the cathode is free of gas. However, as explained above, the electrolyte output from the

regenerator Is mixed with air and then contains a significant proportion of gas and is preferably in a bubbled or foamed form.

Cyclonic separation is a known method of separating fine particles from a gaseous (or liquid) stream without the use of filters, through vortex separation. 5 The combination of centrifugal effects and gravity are used to separate mixtures of solids and gas, and/or solids and liquid, and/or liquid and gas.

International patent application W02009006672 describes a gas-liquid separator used in the petroleum industry, in which an input mixture of fluids flows downward in an outer pipe along a spiral guide vane such that a gas 10 and a liquid are separated centrifugally,

In cyclonic separation, a high speed rotating (gas) flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone upwards in a straight stream through 15 the centre of the cyclone and out of the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and strike the outside wall, then falling to the bottom of the cyclone where they can be removed. In a conical cyclone, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus 20 separating smaller and smaller particles. Such conical cyclones find application in sawmills, vacuum cleaners, and in the separation of gas and liquid in a gas-liquid mixture.

However, tests with conical cyclones have shown poor performance when attempting to separate gas-liquid foams when the gas-liquid ratio is

approximately 4:1, as is required In the liquid electrolyte regeneration system for best performance of the regenerator. High values of g (acceleration) are required to break down such foams, requiring a large amount of energy to accelerate the two phase mixture, giving rise to large operational cost due to 5 parasitic power loss. Also very high carry-over of liquid into the gas stream and carry-under of gas into the liquid have been observed as a result of inadequate downward momentum of the liquid stream. These problems can be partly reduced by utilising a Gas Liquid Cylindrical Cyclone (GLCC).

A known gas liquid cylindrical cyclone was developed by Chevron and the 10 University of Tulsa for the purpose of separation of oil and gas (e.g. Rosa, E, The cyclone gas-liquid separator: operation and mechanistic modelling, Journal of Petroleum Science and Engineering 32, 87-101(2001)). In one design a gas-liquid mixture enters a cyclone at an input port, gas exits at an upper output port and the liquid is extracted tangentially from the cyclone at a is lower output port thereby increasing the diameter of the gas vortex and improving separation.

A problem arises with such a cyclone separator: the time ('residence time') during which the liquid is under high g conditions is limited. This is partly because the fluid velocity is slowed by wall drag and by separation of the gas. 20 However, the predominant mechanism for limited residence time is the effect of gravity dragging the fluid out of the cylindrical section of the cyclone. These two mechanisms for fluid slowing result in a downward motion, instead of radially outward motion (as desired). To some extent this can be offset by increasing the tangential inlet velocity but doing this requires extra pumping

energy which adds increased pressure drop which in turn costs more (both energetically and financially).

It is an object of the invention to provide an improved fuel eel! electrolyte regenerator that addresses the above-described problems and limitations and, 5 in particular, to provide a regenerator that is capable of separating gas and liquid contained in a gas-liquid mixture as rapidly as possible with minimal energy Input. This is particularly desirable In the case of a gas-liquid mixture where a large gas-to-liquid ratio is employed (for example, ten times as much air as liquid, which results in a "dry foam"). Such a case conventionally 10 requires a more energy or time intensive separation process due to the domination of surface tension effects on coalescence.

In attempting to address the above problems and limitations, it has been found that better performance can be obtained by inducing spiral flow of the two phase gas-liquid mixture and additionally constraining the flow of the 15 mixture within an enclosed helical channel (i.e. a pipe rather than an open cylinder). By doing this, a high gravitational separating force (represented by effective acceleration gSff) can be sustained for a longer time interval, achieving more effective bubble collapse and therefore faster separation of gas and liquid.

20 This technique is particularly effective in breaking down a gas-liquid mixture in the form of foam (comprising bubbles), said foam containing liquid (electrolyte) and air/oxygen. However, it should be understood that the technique is also capable of providing improved separation of gas and liquid of a gas-liquid mixture when the gas-liquid mixture comprises little or no foam

or bubbles, or indeed a two phase mixture of immiscible liquids which display a difference in density (i.e. liquid-liquid separation).

According to an aspect of the invention, therefore, there is provided a separator for a liquid electrolyte regenerator of a fuel eel! system, the 5 separator comprising a helical channel in the form of a fluid channel formed on a helix and arranged to conduct a gas-liquid mixture and separate liquid from the gas-liquid mixture.

Further aspects of the invention will now be described in the following detailed description of preferred embodiments of the invention which are illustrated, by 10 way of example only, in FIGs. 1 to 5 of the accompanying drawings, in which:

FIG. 1 is a simplified view of a helix of a helical channel for use in a liquid catalyst fuel cell. FIG. 1a is a side view of the helix and FIG. 1b is a perspective view of the helix.

FIG. 2 is a part-transparent perspective view of a helical separator comprising is a helical channel.

FIG. 3a is a perspective view of a helical air-plate heat exchanger for use as a condenser.

FIG. 3b is a part cut-away view of an end portion of the heat exchanger shown in FIG. 3a.

20 FIG. 4a is a perspective view of a tapered helical channel.

FIG. 4b is a perspective view of a vent and core structure arranged to be placed in the centre of the tapered helical channel of FIG. 4a.

FIG, 5 is an internal perspective view of a tapered helical channel interna! apparatus;

FIG, 8 shows a liquid electrolyte fuel eel! system;

FIG. 7 shows a water droplet sitting on a low-energy solid surface;

5 FIG, 8 shows a hydrophobic particle at a gas-liquid interface of an aqueous foam film;

FIG, 9 shows a zoomed-in view of the hydrophobic particle shown in FIG. 8;

FIG. 10 shows the hydrophobic particle of FIG, 9 penetrating opposite surfaces of the liquid film of FIG. 9; and

10 FIG. 11 shows a fluid and gas conducting apparatus comprising a tubular section or vessel.

The helical channel of the invention will now be described.

FIG. 1 is a simplified view of a helix 150 of a helical channel 100 for use in a liquid catalyst fuel cell to separate liquid and gas of a gas-liquid mixture. FIG. 15 1 a is a side view of the helix 150 and FIG. 1 b is a perspective view of the helix 150.

The helix 150 has a helical axis 102 (z), a helical pitch 104 (Ph^x) and a diameter 108, Gas-liquid mixture is input to the channel at an input end 108 of the helical channel and is constrained to travel along the helical channel in 20 the direction of the helix 150 towards an output end 110 of the helical channel 100.

It is worth noting that the image of the helix 150 shown in the figure represents the centre line of the helical channel and the dimension of PheiiK needs to take

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into account the thickness of a waii of the helical channel between adjacent helical flights in a longitudinal direction (parallel to the helical axis 102). That Is, Pneiix should be measured from the top of one flight of the helical channel to the top of the adjacent flight above it, or from the bottom of one flight of the 5 helical channel to the bottom of the adjacent flight below it. This can be more easily understood by referring briefly to FIG. 2 which will be described further below.

Clearly the helical channel has a hydraulic diameter i.e. a cross-sectional channel width or channel diameter (Dpipe) at any one point along the helical 10 channel as shown in FIG. 2, For example, if the channel is defined by a circular section pipe, the channel diameter is the diameter of the pipe (Dpipe) whereas a square section pipe would have a channel diameter DpjPe that is equivalent to the square section's hydraulic diameter.

Through the use of computational fluid dynamics, results have been obtained is which indicate an optimal value of a dimensionless diameter ratio parameter (A), this parameter being a ratio between the overall transverse-axial diameter or helical diameter (DHeiix) of the helical channel and the hydraulic diameter (Dpipe) of a cross-section of the helical channel at any one point on the channel (A~ DHenx/ Dpipe),

20 The optimal value of A resulted in a maximal modified Dean number (Dm) for a chosen set of operational parameters, as will be explained below,

The Dean number (Dn) is a measure of secondary flow (inertia] to centrifugal forces) and Dm takes into account appropriate helical geometrical factors,

By way of explanation, maximal separation should in theory occur at maximum Dm by virtue of the difference in the densities of each phase.

Modified Dean Number is given by:

Dm=ReV(KDpjpe/2)

5 where Curvature of the helix is given by:

K-(DHeHX/2)/[{DHeHx/2)A2+(PHeiix/2TT)A2]

where P^iix is defined as the vertical distance from the bottom of the previous flight to the bottom of the next flight (i.e. DpjPe + thickness of the helical flight), and Reynolds number (Re) for two phase flow given by

10 Re=(pm|X Vimjx Dpjpe)/fJmix where fjmiX (viscosity of the gas-liquid mixture) and pmsX (density of the gas-liquid mixture) are given by

Mmsx = EMGas"K1"£)MLiq and

15 pmix=EpGas+(1-E)PUq respectively, where e is the gas volume fraction, pmjX is the density of the gas-liquid mixture, Vmix is the velocity of gas-liquid mixture, MmjX is the viscosity of the gas-liquid mixture, and |iGas, Muq, Pg3S and puq are the viscosity and density of the gas and liquid, respectively. The gas volume fraction £ is given 20 by:

£ ~ QGas'(Qcas+Quq)

where Qoas and Quq are the flow rates of the gas and liquid respectively.

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In turbulent flow, centrifugal forces are dominated by inertia! forces and thus secondary effects are reduced. This accounts for the delayed development of turbulent flow in helical pipes (Mandal, S.N. & Das, S.K, Gas-Liquid Fiow through Helical Coils in Vertical Orientation, industrial & Engineering 5 Chemistry Research 42, 3487-3494, 2003).

Initial modelling studies have been conducted by the inventors, in which P^nx was fixed as 1.3 times greater than the channel or pipe diameter:

PHelix=1-3 Dpipe

It should be noted that manufacturing constraints (i.e. thickness of pipe wall)) 10 limited this ratio. The free variables were thus flow rate and diameter ratio A. Fiow rates for the liquid and gas were constrained to practical operational values. That is to say, liquid flow rate (Quq) was chosen to be between 3 and 30 Litres per minute (L/min) and gas fiow rate (Qoas) was chosen to be between 12 and 120 Litres per minute (L/min). The diameter ratio A was 15 varied from 0 to 1000 with Dm as an output. The results of this study suggested an optimal value of A occurs when a dsmensioniess pitch parameter (H) approaches unity, where

H = PHe!ix / (27TRHeiix), where:

RHe!ix™Dt-je!ix^2.

20 That is, the results suggested an optimal value of A occurs when the helical pitch P^iix is equal to u times the overall helical diameter DhsHx, i.e.

Optimum PHeiix = TrDHeiix

This results in maximum modified Dean number being obtained.

Experiments were performed in which the variation of modified Dean number Dm was noted for different values of the dimensionless pitch parameter, H. it was found that Dm is maximum when H approaches unity, as suggested above. This corresponds to an optimal value of diameter ratio A of 1/u, which 5 is physically impossible.

From these results, values of Dm were calculated for different values of A. The optimum value of A provides a maximum value of Dm. Optimising A for maximal Dm (in addition to optimising the dimensionless pitch parameter, H) provides a fully defined system.

10 As can be seen from the above, a consequence of the mathematical relations governing flow in a helical geometry means that the greatest Dm will occur in a pipe of given diameter when the helical diameter is smaller than the pipe diameter (optimum A is 1/ir). However, clearly, such a theoretical optimum value for A is physically impossible, since the overall helical diameter DHeiix 15 cannot be less than twice the pipe diameter Dpjpe (see for example FIG. 2 described below). This means that A is constrained to be greater than or equal to two. In practice, A must be greater than two because of non-zero wall thickness of the helical channel.

Therefore the best physically-realisable value of A is slightly greater than two, 20 that is, as close to two as possible, to achieve maximum value of Dm,

Preferably therefore, according to a preferred embodiment, Dh8hx and Dpipe at any one point are arranged such that the overall helical channel diameter Dhshx is as close to 2*Dpipe as physically achievable.

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FIG. 2 is a part-transparent perspective view of a helical separator comprising a helical channel that has been used successfully to separate electrolyte liquid and gas (air) at full scale flows. The resulting pressure drop in this configuration is less than 12.5% of the pressure drop that occurs in a known 5 GLCC design of similar size, thus performing bulk separation of the liquid and gas.

After bulk separation is performed by the apparatus of FIG. 2, some mist droplets are seen in the air flow in the output region. Also, if is known that the air exiting the separator is saturated with vapour phase liquid.

10 Even though the separator of FIG. 2 is effective at separating gas and liquid, a relatively large proportion of liquid becomes trapped in its vapour phase even upon separation of the gas and liquid phases. This is due to the operating conditions of the FlowCath™ system, i.e. the operating conditions of gas-liquid contacting and the relatively high temperature at which this gas-liquid 15 contacting operation is carried out. This represents a potential application of this technology for a heat transfer application (see FIGs. 3a, 3b, discussed below).

Separation of vapour phase liquid from gas can be performed by known air-air heat exchanger technology. However, although air-air heat exchangers are 20 relatively efficient at the industrial scale, they do not condense enough liquid from the vapour phase to control concentration in at least one known 1 kW net steady state system with the existing size and power constraints of the FlowCath™ System. This limitation represents a problem to solve.

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Two methods of achieving better vapour phase removal of liquid are: (a) increasing surface area of the heat exchanger; and (b) Increasing cold air flow through the heat exchanger. However these methods are non-optimum because of large packaging volume and large parasitic load, respectively. A 5 problem therefore still exists.

In attempting to address the above problems and limitations, it has been found that a second spiral separator (of the same dimensions of the first) can be very effective in removing any entrained liquid phase droplets, giving exceptionally good separation of electrolyte and also some preliminary 10 condensation of water.

Additionally, in a preferred envisaged configuration of a helical separator, a cold-air stream and a hot-air stream (water rich in the vapour phase) are segregated by a metal (e.g. steel) enclosure in a helical flow path.

A first helical channel (e.g. the helical channel 200 of FIG. 2) separates the is liquid and gas of the gas-liquid mixture to produce (a) bulk liquid that is still in the liquid phase, (b) a gas phase saturated with vapour phase liquid and (c) a liquid phase that is entrained in the gas phase in the form of mist. A second helical channel (e.g. the helical channel 302 of FIG.3a or 302 of FIG. 3b discussed below) then separates the saturated gas phase and liquid phase 20 droplets into gas and liquid phase liquid.

Secondary flow within the curved geometry of a helical channel increases heat transfer coefficients, the effect of which is greater for laminar flow. The flow regime for the current FlowCath™ system (and those in the near future) will be laminar for an air-plate condenser. It has been found that confining a

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plate or fin of a heat exchanger within, or as part of, an enciosed helical channel serves to enhance separation of vapour phase liquid and gas as a result of the (approximately two-fold) increase in heat transfer coefficient. This allows the surface area of a condenser to be halved whilst still affecting 5 the same amount of separation.

If follows that an aspect of the invention is providing a separator for a liquid electrolyte regenerator of a fuel cell system comprising a helical channel in the form of a pipe formed on a helix and arranged to conduct a gas-liquid mixture and separate liquid from the gas-liquid mixture, wherein the helical channel of 10 the separator is an enclosed channel along which the gas-liquid mixture is constrained to travel. The helical channel can be used as a heat exchanger (for example, an air-air plate condenser or counter current shell and tube exchanger for denser fluids) for conducting a fluid to be cooled. Such a heat exchanger is particularly useful for conducting and cooling fluid in vapour is phase so as to perform condensing of the fluid,

FIG. 3a is a perspective view of a proposed configuration of a new helical air-plate heat exchanger 300 employing this principle for use as a condenser. The heat exchanger 300 comprises six enclosed helical channels 302, 304, 306, 308, 310, 312 (five channels for the cooling air and the remaining 20 channel to be used for the hot, vapour rich air.)

FIG, 3b is a part cut-away view of an end portion of the heat exchanger 300 shown in FIG, 3a, Helical channels 302, 304, etc. can be seen more clearly. Between adjacent (cold-air) helical channels, a gap 320 exists, in which fluid such as hot, vapour rich air can move past the outer surface 310, 312 of each 25 helical channel. As can be seen, the helical channels in this embodiment are

in the form of hollow fins, in this case the 'helical channel' comprises plural hollow fins. The hollow construction allows separate supply of cooling fluid (e.g. air) which will flow over the outer surface of the helical channel but will not mix with the gas-liquid mixture within the helical channel.

5 A currently employed, off-the-shelf air-air plate condenser has approximately 0.8m2 total surface area (UK Heat Exchangers™). By contrast, the heat exchanger shown in FIG.s 3a and 3b has a total cold surface area of 0.431m2, made possible by the two-fold increase in heat transfer coefficient, which allows a significant reduction in overall size of a condenser used in a given 10 PEM fuel cell system.

Further improvement of the heat exchanger may be achieved by increasing the surface area of the fins or cooling surfaces of the helical channel of the heat exchanger, by providing non-smooth cooling surfaces, for example by means of corrugation and/or dimpling of the cooling surfaces (FIG. 3b, 310, 15 312), without any need for increased packaging volume.

A further improvement in separation may be achieved by arranging the fins or cooling surfaces so that they comprise a surface comprising a low surface energy material (for example PTFE). A highly hydrophobic material can induce coalescence by harnessing the de-wetting force to force plateau 20 borders apart, thereby becoming energetically favourable to form a single bubble, rather than two bubbles separated by a plateau border.

According to an embodiment, there is provided a combination of the helices shown respectively in FIGs. 2 and 3 with the exception that the closed section shown in FIG. 3b is left open. In this embodiment, the gas-Hquid mixture

enters the helix as described above for FIG. 2. However, the fluid passage houses several "fins" with flights running parallel to the main helix flight. These fins, the wall of the helix tube and the main helix flights would be coated with a low surface energy material. The diameter of the fins would be 5 slightly less than the main helix diameter to allow the development of helical flow as described above for the embodiment shown in FIG. 2,

The above-described low surface energy fins would not affect the overall fluid flow within the helix greatly, although there would be additional pressure drop per unit length due to an increased internal surface area/friction effect. io However, it is likely that there would not be an overall increase in pressure drop as these low surface energy fins would increase the rate at which gas-liquid separation is effected and therefore would require a shorter overall length of helix. This embodiment allows the overall size of the device to be reduced whilst maintaining effective separation.

is FIG. 4a is a perspective view of the fluid passage of a tapered helical channel 400 having a greater helical diameter at the inlet and a smaller helical diameter at the exit. The gas-liquid mixture enters at the inlet. The separated gas then exits gradually through gas vents in an inner core of the helix (FIG, 4b) with a minimal amount of air exiting through the fluid exit, thus 20 approaching pipe flow and moving away from free surface flow. The venting of air partway down a helix has been proposed by Rosa et al (Rosa, E, Journal of Petroleum Science and Engineering 32, 87-101, 2001,and OAPI patent application publication no. OA11321 (A)). However, Rosa discloses an arrangement having a constant helix diameter unlike the embodiments

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described above and shown In FIGs. 4 and 5 which have a graduated helical diameter and a graduated pipe diameter.

The embodiment shown in FIGs. 4a and 4b provides an improvement over the design shown in FIG. 2. Using a pipe that decreases In diameter as the fluid 5 travels downward towards the outlet will result In an increase in velocity according to the continuity equation. Concomitantly, the effective gravity force acting on the gas-liquid mixture will Increase.

Even in the case of a high gas to liquid ratio the difference in densities is very large such that the contribution of the gas to the momentum of the gas-liquid 10 mixture is minor. Thus, even when complete separation is achieved, the fluid maintains the majority of the momentum it had when entering the helix as a gas-liquid mixture. The decrease in pipe diameter as the fluid travels towards the outlet is such that the liquid will increase in velocity. As a result of the increase In fluid velocity and the decrease in radius of rotation the effective 15 gravity acting on the gas-liquid mixture increases dramatically compared to a constant-section helical separator.

For example, using the same flow rate and composition of gas-liquid mixture the effective gravity imposed on the constant section (FIG. 2) is 10g (i.e. 10 times gravity, that is, 98.1 m/s2), whereas the effective gravity imposed on the 20 tapered helix shown in FIG. 3a and FIG. 3b starts at 10g and reaches a much higher maximum of 21 g.

FIG. 5 is an internal perspective view of an internal apparatus 500 that may be used to define the tapered helical channel 400 shown in FIG. 4a. The internal apparatus 500 has a spiral vane 501 and an Inner wall 502, attached to the

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vane 501, which may have one or more gas vents 503 in It, allowing gas to exit vertically or part-vertically so as to prevent re-entrainment upon fluid exit. This configuration serves to abrogate pulsating flows which can be caused by "slugs" of liquid-rich material forming in the channel. It also serves to reduce 5 the overall size of the channel, to move away from free surface flow and, most Importantly, to increase separation efficiency. The helical channel 400 (FIG. 4a) comprises an outer wall (not shown) adjacent to and surrounding the spiral vane 501 of the apparatus 500.

The gas vents 503 may comprise a microporous membrane 503, which may io form all or part of the inner wall 502 of the helical channel 400, allowing gas to escape early and preventing liquid escape due to the hydrophobic nature of the membrane. The gas vents 503 are arranged to inhibit the passing of liquid through them due at least in part to their small diameter.

The helical channel 400 (FIG. 4a) may comprise a porous bubble generating is element 504 (not shown in FIG. 4a) incorporated in the outer wall of the helical channel 400. The positioning of the porous bubble generating element 504 utilises the already existing differential density between gas and liquid in addition to the imposed centrifugal gravity as a result of the helical flow path in the helical channel 400 to ensure rapid movement of gas from outer wall to 20 Inner wall 502. This arrangement allows the greatest mass transfer rate (which corresponds to reaction rate in this system) between the gas and liquid as a result of the greatest driving force for mass transfer and reaction (high concentration of reactants).

Additionally, this arrangement allows an overall very high gas-liquid ratio to be 25 achieved which would not be possible by using a single air injection point.

The porous bubble-generating element 504 may be used to concurrently perform gas-liquid contacting, whilst the turn (flight) of the helix after the porous element can be used for separation. The porous element 504 is shown in FIG. 5 as plural openings or vents, but could equally be a 5 microporous membrane. The porous element 504 allows the helical channel to operate both as a separator and as a regenerator. It should be understood that the gas vent 503 and porous element 504 can be used with a helical channel having a helical diameter that is not tapered or graduated, i.e. is constant,

10 The use of the porous element or aperture(s) may enable more volumetric efficient helix geometry because of the dual function of gas-liquid contacting and gas-liquid separation.

Helical flow devices are widely used in heat transfer applications. However, helical flow has not been used in a liquid catalyst fuel cell system. Such liquid is catalyst fuel cell systems have not included any capability for the destruction of foams or separation of droplet streams having high gas-liquid ratio. The use of a microporous membrane to vent air/gas from a helical channel to achieve separation of a gas-liquid mixture is novel,

FIG. 6 shows a liquid electrolyte fuel cell system 600. In this system, liquid 20 redox cathode electrolyte (catholyte) is circulated through a fuel cell stack 602 in which it is reduced due to the action of the fuel cells 604 in the fuel cell stack 602, The liquid redox catholyte is then passed through a regenerator 606, in which the catholyte is oxidised.

The oxidisation process requires the contacting of liquid catholyte with large volumes of air, the liquid and air being in a ratio greater than 4:1 air-to-liquid on a volume basis at standard temperature and pressure (STP) and ideally up to 20:1 or greater. Interfaces between the liquid and gas/air are generated in 5 the form of bubble membranes or films. It is desirable to maximise the total area of these gas-liquid interfaces in order to maximise mass transfer of oxygen from the gas/air info the catholyte.

The gas-liquid interface in the regenerator is in the form of high internal phase volume foam comprising bubbles having small bubble radius. The rate of io regeneration (oxidation) of the liquid catholyte is proportional to the total interfacia! area of the gas-liquid interface. High rates of regeneration are required so that the fuel cell stack can generate a useful amount of power for a typical use of the fuel cell system 600.

The regenerated electrolyte and gas, mixed with the electrolyte, are then is conducted together into a gas-liquid separator 608 comprising the helical separator (FIG. 2, 200). The separator 608 provides as outputs (a) bulk liquid electrolyte and (b) a less dense gas-liquid mixture comprising gas (air) mixed with liquid electrolyte in the form of droplets/mist and/or vapour-phase liquid. The more dense liquid is fed (in this example by gravity) to a reservoir 610 for 20 collecting the liquid in the mixture in the form of bulk liquid in liquid phase. The separator outputs gas which is conducted into a condenser 612, in this example the condenser shown in FIGs 3a and 3b (FIGs 3a and 3b, 300). External cooling air is passed across cooling fins of the condenser 812 by means of a cooling fan 614. A fluid pump 616 pumps liquid collected by the 25 reservoir 610 and outputs the liquid to the fuel cell stack 602 for use as

electrolyte in the fuel cells 604 of the fuel cell stack 602. The condenser 612 outputs condensed liquid (condensate) to the reservoir 610 in the form of liquid-phase liquid.

Turning again to the regenerator 606, once the liquid electrolyte (catholyte) 5 has been regenerated, the residual gases (mainly Nitrogen) must be removed from the catholyte which is supplied to the cathodes of respective fuel cells 604 and should not contain gas bubbles because such bubbles would interfere with the operation of the fuel cell 604, It is highly desirable that disengaging or separating the residual or "spent" gases is performed rapidly, io efficiently and with minimal power consumption.

Mechanical separation methods such as hydro-cyclones and centrifuges use an unacceptable amount of power. The helical separator 608 can provide an alternative mechanical separation which requires lower power. However, there is an ever present requirement to improve separation efficiency at is reduced power and in a smaller physical volume. Therefore it is desirable to employ a method of separation other than, or additional to, the mechanical separation methods so far described above.

International patent application publication WO 2010/108227 discloses a method and apparatus for dry separation of hydrophobic particles. However 20 WO 2010/108227 is directed to particle separation, and not gas-liquid interface disruption, and does not relate to fuel cells. German patent publication DE10323155A1 discloses a separator for the removal of liquid in droplet or aerosol form from a gas stream. However DE10323155A1 is not concerned with foam or a fuel cell.

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Japanese patent publication JP3038231, incorporated herein by reference, discloses a separation unit membrane composed of hydrophilic parts and hydrophobic parts. Japanese patent publication JP1297122, incorporated herein by reference, discloses a material consisting of a thin film of liquid 5 containing a carrier held in a laminated form with a film composed of only hydrophobic pores used as a liquid film for gas separation.

Separation of liquid and gas in foams has been previously investigated. For example, see "Defoaming: Theory and Industrial Applications", P.R. Garrett, CRG Press, ISBN 0-8247-8770-8, incorporated herein by reference. See also io "The Physics of Foams" D. Weaire & S. Hutzler, Clarendon Press, ISBN 0-19-851097-7, pages 149-150, Incorporated herein by reference. There is also "The effect of high volume fraction of latex particles on foaming and antifoam action In surfactant solutions", P.R, Garrett, S.P. Wicks, E, Fowler, Colloids and Surfaces A: Physicochem. Eng. Aspects 282-283 (2006) 307-328, 15 incorporated herein by reference.

The action of so-called 'antifoam' in the disruption of liquid films and bubbles is well known. There exist various mechanisms by which antifoam can disrupt the gas-liquid interface of foam, the mechanisms depending on the formulation and form of the antifoam, but such mechanisms can generally be 20 described by the following explanation of the interaction between a liquid film and a low surface energy surface. This action results In so-called "de-wetting1. De-wetting describes the rupture of a thin liquid film on a substrate (either a liquid or a solid) and the formation of droplets. The opposite process (spreading of a liquid on a substrate) is called 'spreading'.

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FIG. 7 shows a solid object 701 which has a surface 702 having low surface energy. A water droplet 706 sits on the low-energy surface 702. The droplet will rest in a position such that a defined contact angle, denoted by 0C in the figure, is subtended between the low-energy surface and the surface of the 5 droplet In contact with the surrounding gas/air. The angle is defined by Young's equation, as set out below:

YSL + YLG ==: YSG

jo FIG. 8 shows a low surface energy, hydrophobic, particle 802 at a gas-liquid interface 804 of an aqueous foam film 806.

FIG. 9 shows a zoomed-sn view of the hydrophobic particle 802 and gas-liquid interface 804 shown in FIG. 8. The particle will rest in a position in or on the film 806 so that a defined contact angle between the film surface and the is particle surface will satisfy Young's equation given above, the angle being denoted by 0ci in FIG. 9.

FIG. 10 shows the hydrophobic particle 802 of FIG. 8 penetrating opposite surfaces 804a, 804b of the liquid film of FIG. 9. The opposite surfaces of the liquid film 806 provide two respective gas-liquid interfaces 804a, 804b. When 20 the particle penetrates the film 808 as shown, the particle protrudes from each opposite surface 804a, 804b of the film 806, and a contact angle is defined at both gas-liquid interfaces, the angle being between the (tangential) surface of the particle and the surface 804a, 804b of the film, the angle being denoted by @c2 for the lower gas-liquid interface 804b in FIG. 10, the angle 8ci not being

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shown in FIG, 10 for ease of reading, As the particle penetrates the film 806, the liquid film 806 is ruptured and the bubble bursts thereby achieving the de-wetting effect outlined above. The hydrophobic, or antifoam, particle then moves (for example, due to gravitational force) to the next gas-liquid interface 5 which is provided by a membrane wall or film of an adjacent bubble, and so on.

As can be deduced from the above, introduction of an antifoam agent in the form of hydrophobic particles into a gas-liquid foam is effective in separating gas and liquid in the foam and thereby converting the foam into separate 10 liquid and gas portions.

However, in the liquid catalyst fuel cell system, presence of antifoam particles in the liquid electrolyte could adversely affect operation of the fuel cells. Also it would be disadvantageous if such an anti-foam agent were present in liquid catholyte entering the regenerator because the regenerator generates a gas-is liquid interface by means of bubbles to promote oxidation and an antifoam agent in the liquid electrolyte would inhibit such bubble generation. It can be seen that there exist two conflicting requirements: for generation of foam it is best if no anti-foam agent Is present; whereas anti-foam agent is effective in the destruction of foam. In the liquid electrolyte fuel cell system, both 20 generation of foam and destruction of foam are required.

Embodiments provide an inventive way to avoid this conflict and seek to provide a further-improved helical separator arranged to separate gas and liquid with further improved efficiency. According to these embodiments the helical channel of the helical separator (e.g. the separator shown m FIG 2

„ 2? ~

(FIG. 2, 200) comprises a surface comprising a low surface energy material and arranged to contact the gas-liquid mixture.

It should now be appreciated that it is possible to cause foam disruption/destruction by contacting the foam with one or more hydrophobic 5 surfaces. Such a surface is not merely hydrophobic particles, but is a surface of a solid structure that comes into contact with the gas-liquid mixture comprised of foam or bubbles, thereby causing gas-liquid interfaces to be ruptured.

Low energy surfaces are found in certain polymers, for example 10 polytetrafluoroethylene (PTFE) which has a surface energy of around 18 mJ/m2. Surfaces of such polymers have been used very effectively to break down foams.

According to embodiments, electrolyte foam and low surface-energy material can be moved adjacent to each other (one and/or the other moving). The 15 foam can simply pass along a plane or curved surface of the low surface-energy material or the low surface-energy material can be in the form of a mesh and the mesh and foam can move relative and adjacent to one another.

For example the foam can be forced through a holed member, for example a mesh, comprising low surface-energy material. Alternatively the holed 20 member can be forced through the foam. The use of a holed member or mesh increases the specific surface area of disruptive interface. The hole size can vary from 0.1 millimetre to 10 millimetres, and the holed member can comprise a mesh having filaments of low surface-energy materia! (e.g. polymer) having diameters between 50 micrometres and 1 millimetre.

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As the foam and holed member pass next to each other, the gas-liquid interface of the foam is ruptured and the gas and liquid separate into a denser iiquid phase and a less dense gaseous phase. This operation, when performed prior to further mechanical separation (the further mechanical 5 separation being performed by a further helical separator for example) enhances the overall separation. Such a further helical separator may be a condenser as exemplified by the condenser (FIG, 0, 812).

Including a holed member e.g. mesh either up-stream or down-stream of the helical separator within the liquid electrolyte fuel cell system enhances the io separation of the gas and liquid phases.

Alternatively or in addition, low surface energy materials can be incorporated within the helical separator, internal surfaces of the separator comprising low surface energy material, as described above in relation to the helical separators of FIGs. 2, 3 and 4. Optionally and advantageously, a holed 35 member or mesh can be present inside the helical channel of such a helical separator.

Further advantage can be obtained when the helical separator comprises an internal surface having a rough finish. Preferably the internal surface also has a low surface energy, for example it comprise a coating of low-surface energy 20 material. Preferably the roughness of such a rough finish has a dimension (e.g. average dimension) that is of the same order as the average film thickness of the liquid foam. For example the internal surface may have raised portions (bumps or ridges) that have a width which is similar to the average thickness of the film of the foam,

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This approach of using a surface having low-surface energy can also be applied to other gas-liquid separation functions, such as the separation of hydrolysis gases from the electrolyte liquid of the liquid electrolyte fuel cell system,

5 According to an embodiment, the foam is conducted from the gas-liquid contacting section of the regenerator through conducting apparatus comprising three sections;

a feed section for distribution of foam across a mesh section low surface energy mesh packing section for separation

10 8 a phase separation section with two outlets, one for gas the other from liquid

FIG. 11 shows a conducting apparatus 1100 comprising a tubular section or vessel 1100 which contains such a feed section 1102, mesh packing section 1104 and phase separation section 1106 comprising gas section 1108a and 35 liquid section 1108b. Gas is output from the gas section 1106a and liquid is output from the liquid section 1106b. The flow of fluid through the apparatus 1100 is indicated by arrows.

As an example of the use of mesh to perform separation of gas and liquid in a foam, liquid electrolyte, 10mS volume, was placed in a measuring cylinder and 20 air was passed through the catholyte with a flow rate of 0.5 litre/minute using a sintered glass sparge. The foam thus-formed over-filled the measuring cylinder. A PTFE knitted mesh was placed in the throat of the measuring cylinder and air was sparged again using the same conditions. The effect of

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this was to efficiently rupture the foam and separate the gas and liquid phases.

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Claims (27)

1. A separator for a liquid electrolyte regenerator of a fuel cell system, the separator comprising a helical channel in the form of a fluid channel formed on a helix and arranged to conduct a gas-liquid mixture and separate liquid from the gas-liquid mixture.

2. The separator of claim 1 wherein the overall helical diameter (Dheiix) of the helical channel is close to twice the hydraulic diameter {Dpjpe) of the helicai channel along at least a portion of the helical channel.

3. The separator of claim 1 or 2, comprising a porous element, located at an exterior wall region of the helical channel, through which gas can pass.

4. The separator of any preceding claim, comprising a porous element located at an interior wall region of the helical channel, through which gas can pass.

5. The separator of any preceding claim, wherein the helical channel has a surface comprising a low surface energy material and arranged to contact the gas-liquid mixture.

6. The separator of Claim 5, wherein the surface is provided at least partly on a holed member in the channel, holes in the holed member arranged to allow the gas-liquid mixture to pass through the holes.

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7. The separator of Claim 6, wherein the holed member is in the form of a mesh or perforated plate.

8. The separator of Claim 6 or 7, wherein the holed member has holes having diameters between 0.1 millimetre and 10 millimetres.

9. The separator of any one of Claims 6 to 8, wherein the holed member extends across the internal diameter of the helical channel.

10. The separator of any preceding claim, comprising a further helical channel formed on a smaller-diameter helix and in fluid communication with the helical channel, the further helical channel being arranged to conduct liquid and gas of a portion of the gas-liquid mixture that passes from the helical channel to the further helical channel and to separate liquid from the portion of the gas-liquid mixture.

11. The separator of claim 10, wherein the further helical channel has a surface comprising a low surface energy material.

12. The separator of claim 10 or 11, wherein the further helical channel comprises heat conductive material and is surrounded by the helical channel, the further helical channel being for conducting a fluid colder than said gas-liquid mixture.

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13, The separator of any preceding claim, wherein the helical channel and/or or the further helical channel, as defined by Claims 10-12, comprises a non-smooth outer surface.

14. The separator of any preceding claim, wherein the helical channel comprises plural channels formed on respective plural helices substantially parallel to each other.

15, The separator of claim 14, wherein the plural helices have separate longitudinal axes,

16. The separator of any preceding claim, wherein the overall helical diameter and the hydraulic diameter of the helical channel are graduated along the helical channel between an inlet having larger overall helical diameter and larger hydraulic diameter and an outlet having smaller overall helical diameter and smaller hydraulic diameter.

17, The separator of any preceding claim, comprising a gas vent between the helical channel and an inner core surrounded by the helical channel, allowing separated gas to pass through the gas vent between the helical channel and the inner core,

18. The separator of claim 17, wherein the gas vent comprises a hydrophobic material having pores or micro-pores for inhibiting passage of liquid through the pores and allowing passage of gas through the pores.

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19. The separator of any preceding claim, wherein the helical channel comprises a first helical channel for separating liquid from the gas-liquid mixture to produce bulk liquid-phase liquid and gas, and a second helical channel, coupled to the first helical channel, for separating vapour-phase liquid and entrained liquid phase liquid from the gas.

20. The separator of any preceding claim, wherein the gas-liquid mixture comprises liquid in the vapour phase.

21. The separator of any preceding claim, wherein the separator comprises a bulk separator for performing separation of a gas-liquid mixture comprising liquid in liquid-phase, a demister for performing separation of a gas-liquid mixture comprising liquid in both liquid-phase and vapour-phase, and a condenser comprising an air-air plate heat exchanger for performing separation of a gas-liquid mixture comprising liquid in vapour-phase, wherein at least one of the bulk separator, the demister and the condenser has a helical channel.

22. A separator and regenerator apparatus, comprising the separator of any one of claims 1 to 21 and a regenerator arranged to input liquid electrolyte and gas to generate gas bubbles in the liquid electrolyte and output the liquid and gas in bubble form,

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23, Use of a fuel cell system comprising the separator of any one of claims 1 to 21 or the separator and regenerator apparatus of claim 22, for the combined generation of heat and power,

24. Use of a fuel cell system comprising the separator of any one of claims 1 to 21 or the separator and regenerator apparatus of claim 22 to provide motive power to a vehicle.

25, Use of a fuel cell system comprising the separator of any one of claims 1 to 21 or the separator and regenerator apparatus of claim 22 to generate power in an electronic component.

26. A separator substantially as described herein and as illustrated in one or more of the accompanying drawings.

27. A separator and regenerator apparatus substantially as described herein and as illustrated in one or more of the accompanying drawings.

28, A helical heat exchanger substantially as described herein and as illustrated in one or more of the accompanying drawings.

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