GB2125703A - Device for removing electrolyte from electrochemical cell exhaust gases - Google Patents

Abstract

Phosphoric acid electrolyte in the exhaust gas of a fuel cell (10) is removed in a device comprising a heat exchanger (34) which reduces the temperature of the gas to below the acid condensation point, followed by an accumulator (36) including a plurality of vertically oriented, spaced apart, open porosity plates (52) which pick up the acid as the gas stream flows between the plates. The plate spacing, pore size and porosity are selected such that virtually all the acid is drawn into the plates and passes down through the pores of the plates and drips off the plates under gravitational forces, whereupon it is removed from the device via a drain (62) and can be recirculated to the fuel cell if desired. <IMAGE>

Description

SPECIFICATION Device for removing electrolyte from electrochemical cell exhaust gases This invention relates to fluid removal from gas streams, and more particularly to removing evaporated electrolyte from fuel cell exhaust gases.

It is known that in electrochemical cells which utilize phosphoric acid as the electrolyte there is some evaporation of the electrolyte into the reactant gas streams as they pass therethrough, particularly into the air (i.e. oxidant) stream which flows at a significantly greater rate than the hydrogen (i.e., fuel) stream. Although this evaporation is slight, it becomes significant over a long period of time and may eventually result in failure of the cell due to an insufficient quantity of electrolyte remaining within the cell. The problem becomes more severe as cell operating temperatures increase. For extended periods or operation, it may be required that this evaporated electrolyte either be replenished intermittentlyorcontinuouslyfrom a separate source, orthe lost electrolyte must be recovered and returned to the cell.Even if electrolyte loss is sufficiently slow such that it will not have to be replaced during the design life ofthe cell, the phosphoric acid which leavesthe cell in the reactant gas stream is highly corrosive annneeds to be removed from the cell exhaust before it does damage to components downstream of the cell. The assignee of the present invention has expanded considerable effort in attempting to develop a commercially suitable solution to this electrolyte evaporation problem.

Apparatus for removing liquids from gas streams are well known in the art. Manytypes are described and pictured in Chemical Engineers' Handbook, Fifth Edition (pp. 18-22 through 18-93), by R. H. Perry and C.

H. Chilton, McGraw-Hill Book Company. One type particularly useful for removing acid mist from a gas stream involves passing the moist gas stream hori zontallythrough a vertically disposed racked fiber bed. Mist particles collect on the fiber surfaces and are moved downwardlythrough and eventually drain from the bed by gravity (pp. 18-88, 18-89).

One object of the present invention is apparatus for removing, electrolyte from a gas stream.

Afurther object of the present invention is low pressure drop apparatus for removing phosphoric acid from the cathode exhaust stream of a fuel cell.

According to the present invention, apparatus for removing phosphoric acid from the gas stream comprises a heat exchangerfor reducing the temperature ofthe gas stream to below the condensation point ofthe acid, and an accumulator downstream of the heat exchanger comprising closely spaced apart porous walls which define substantiallyvertical open passagewaystherebetween through which the gas stream is passed, wherein the spacing between the walls is small enough such that substantially all of the acid in the gas stream contacts the walls ofthe passageways and enters the pores ofthe walls and drainsdownwardlytherethroughto an acid outlet by the action of gravity.

In a preferred embodiment the cathode exhaust gas stream from a phosphoric acid fuel cell stack is first passed through a heat exchanger to reduce its temperaturetobelowthecondensation point of the phosphoric acid which has evaporated into the gas stream during fuel cell operation. The temperature of the gas stream is maintained well above the condensation temperature of water which is also present in the gas stream. As it leavesthe heat exchanger substantially all the acid is believed to be in the form of a liquid, probably in the form of a mist which may be considered as consisting offine droplets or particles of acid. The reduced temperature gas stream, with the mist entrained therein, is then passed through narrow, vertical, open passageways defined between walls of open pore, highly porous, high surface area material.

The pore size of the walls and spacing therebetween is such that substantially all the acid in the stream contacts the walls, coalesces and is drawn into the pores of the wall material, and drains vertically through the walls and out of the apparatus by means of gravity.

Mostpreferablythewallsareporousflat plates made from acidproof carbon and which are spaced apartfrom each other defining narrow parallel passageways therebetween. There is virtually no pressure drop across the accumulator since the gas stream is always passing through open passageways and is not being forced through a packed bed or other material which, as itfills, has a decreasing porosityor flow area; thus, even partial filling ofthe plates with liquid acid has virtually no effect on the pressure drop through the device.

The foregoing and other objects, features and advantages ofthe present invention will become more apparent in the light ofthefollowing detailed description of preferred embodiments thereof as shown in the accompanying drawing.

Fig. 1 is a side elevation view, partially broken away and partly in section, showing an acid removal device in accordance with the present invention.

Fig. 2 is a partly broken away view, partly in section, taken along the line 2-2 of Fig. 1.

Referring to Fig. 1,the device according to the present invention comprises a fuel cell stack 10 shown schematically as a single cell, although in actuality a cell stack may comprise hundreds of adjacent cells connected electrically in series. As is well known in the art, each fuel cell ofthe stack 10 comprises a pair of spaced apart electrodes 12, 14 having an electrolyte 16 disposed therebetween, either as a free liquid or, more likely, disposed within a matrix which holds the electrolyte like a sponge. In this preferred embodimentthe electrolyte is phosphoric acid which is held within a silicon carbide matrix.

Fuel, such as hydrogen, represented bythearrow 17, passes through a fuel compartment 18 on the nonelectrolyte facing side of the electrode 12, which is the anode. Anode exhaust gases, represented by the arrow20, leave the stack 10. An oxidant, such as represented by the arrow 24, and which is usually air, passes th rough an oxidant gas compartment 26 on the nonelectrolyte faci ng side of th e electrode 14 wh ich is the cathode electrode. As the air 24 passes through the gas compartment 26 it is heated by the exothermic fuel cell reaction, and a small but not insignificant amount of electrolyte 16 evaporates into the hot air stream. Water produced by the fuel cell also evaporates into the air stream.The cathode exhaust gases, represented by the arrow 28 and which now include electrolyte and water, leave the stack 10 and are fed into an acid removal device represented generally by the reference numeral 32.

Referring to Figs. 1 and 2, the acid removal device 32 includes a housing 33 which encloses a heat exchanger section 34 and an accumulator section 36 downstream thereof. The cathode exhaust stream 28 enters a plenum chamber 38 of the acid removal device 32, via an inlet35, which directs itthrough the heat exchangersection 34. In the heat exchanger section 34, the temperature of the cathode exhaust stream is reduced to a temperature below the condensation temperature of the acid and above the condensation temperature of the water in the gas stream.Preferably the temperature is selected to assure that the acid vapor pressure is insignificant, such that no harmful amount of acid can leave the heat exchanger in the vapor state, and the acid is in the form of a liquid mist of very fine droplets with the droplet size being large enough to be effectively removed by the following accumulator section. The water will remain in the gas phase.

The reduced temperature cathode exhaust gas stream, now carrying an acid mistorfineacid droplets, is directed through the accumulator section 36 which includes high surface area, porous, open pore material to capture the acid droplets, such that a substantially acid free gas enters an outlet plenum 37 downstream ofthe accumulator 36 and is exhausted from the device 32 via an outlet 39.

In the preferred embodiment ofthe acid removal device 32 shown in Figs. 1 and 2, the heat exchanger section 34 comprises a plurality of spaced apart, parallel, hollow rectangularfins40 disposed within the housing 33 and which are fed coolant water (represented bythe arrow 42) from an inlet header44 via individual feed tubes 46. The inlet header 44 is supported from the housing 33 by clamps 43 and brackets 45. The coolantwater passes through the fins 40 and is discharged into a coolant outlet header48via individual coolant outlettubes 50 (Fig. 2).The outlet header 48 is supported from the housing 33 by clamps 47 and bracket 49.The fins 40 are made from stainless steel and coated with a 254 pom thick layer of perfluoroalkoxyto protectthefinsfrom attackbythe acid. The housing 39 is made from carbon steel which has also been coated internally with a 254 calm thick layer of perfluoroalkoxy.

The hot cathode exhaust gases 28 from the stack 10 are fed into the plenum 38 which directs them between the fins 40 of the heat exchanger section 34.

Since the cathode exhaust gas flow rate, temperature, and pressure are known, the heat exchanger section 34 may be readily sized and constructed, and the coolantwaterinlettemperature and flow rate selected, to reduce the temperature of the exhaust gases to a preselected temperature. That temperature is selected such that upon leaving the heat exchanger section 34 all but a harmless (and virtually insignifi cant) amountofthe acid is still in aformwhich is not or cannot be removed in the following accumulator section 36.

The accumulator section 36 comprises a plurality of closely spaced apart, flat, rectang u lar, open pore, porous plates 52 disposed within the housing 33 and which define a plurality of vertical, narrow, parallel passageways 54 therebetween. The plates 52 are trapped vertically between opposing pairs of channel members 56 (Fig. 2) and horizontallytrapped between the walls ofthe housing 33. Spacing between adjacent plates is maintained by narrow flat stri ps 58 which extend into the channel members 56, as at 57, as best shown in Fig. 2. The strips 58 also help maintain plate flatness, which assures uniformly sized, straight passageways 54.

It should be appreciated that the plates 54 need not be flat, although flat plates are preferred for ease of manufactu re. The plates may, for example, be wave or zig-zag shaped in horizontal cross-section, such that the passageways 54 will be wave or zig-zag shaped, as the case may be. Whether wavy orflat, it is required thatthe plates, and thus the passageway 54 be substantially vertical.

In operation, the reduced temperature acid moist gas stream is directed downwardlythrough the narrow passageways 54. The liquid acid in the gas stream contacts the plates and is drawn therein by capillaryforces. As the droplets accumulate and coalesce within the pores of the plate 52 they eventually are drawn downwardlythrough and begin dripping from tte plates by the action of gravity. The liquid acid is removed from the device 32 through an outlet 60 atthe lower end thereof. A suitable trap 62 or othertype of device is provided at the outlet 60 to preventgasfrom escaping from this outlet along with the acid.

The plates 52 must be made from a material which is compatible with the phosphoric acid electrolyte.

Preferablythe strips 58 are made from the same type of material as the plates 52, but this is not required. It is especially preferred to make the plates 52 from the same mateerl as the electrodes 12,14 (except without catalyst and withoutwetproofing since the plates must be hydrophilic in orderto be wetted by the acid droplets) ofthe stack 10 since that material is known to be highly resistant to attack by phosphoric acid (i.e., acidproof) and also has desirable strength and porosity characteristics.

Thefollowing methodwasusedtomakeflatplates 52 for an acid removal device according to the present invention: 67% byweightpitch based carbon fibers (averaging about 254 sum long and 10 cm in diameter) were mixed with 33% phenolicresinandthe mixture was compression molded to a density of 0.65 gm/cc.

The molded plate was then heat treated stepwise up to 2800#to to convert the binderto acidproof carbon.

The finished plates had a density of 0.056 gm/cc. The finished plates were about 2.03 mm thick, had an open porosity of about 75%, and a mean pore size of about32,umwith 80% oftheporesfalling between 25 and 45 pm.

In one test, flat plates having almostthesame properties asthe type described intheforegoing example and which were made bythe same method were used in an acid removal device very similar in appearance and operation to that shown in the drawing. The accumulator section 36 of the test apparatus comprised 70 of these plates 52 each having a length of 35.56 cm, a height of 8.89 cm and a thickness of 1.77 mm. The plates were spaced from each other a distance of 1.77 mm by strips ofthe same material. In the test the cathode exhaust gas stream entering the acid removal device 32 was at a temperature of about 1 960C and at atmospheric pressure.The phosphoric acid condensation temper atureforthis gas stream was 193#C.The heat exchanger section 34 was sized and constructed to reduce the temperature of the gas stream to about 141 C, well above the water condensation temperature (dew point) of 60 C. Underthese conditions no significant amount of acid was believed to have left the device in the exhaust gas stream. The pressure drop from the gas inlet 35 to the gas outlet 39 was on the order of 17.78 mm of water. Most of this pressure drop was due to inlet and outlet ducting. The pressure drop across the heat exchanger section 34 was on the order of 0.76 mm of water and across the accumulator section 36 was on the order of 0.50 mm of water.

In a preferred fuel cell stack system it is contemplated that the acid removed by the plates 52 and which drips offthe plates 52 and passes out of the acid removal device 32 will be recirculated to the cells ofthe stack 10 to replenish the lost acid.

In designing the plates 52, the pore size must not be so small that the acid simply fills up the plates and remains immobile. lfthatwereto happen the acid would begin building up on the outer surfaces of the plates and be more likelyto bridge the gaps between the plates thereby decreasing thecross-sectional flow area and increasing the pressure drop and the overall effectiveness ofthe acid removal device.

Bridging may, of course, be prevented by increasing the spacing between plates; however, the spacing cannot be so large asto permit any significant amount of acid to passthroughthe passageways without contacting the plates. It is theorized that best results will be obtained when the spacing between the plates is smaller than the mean free path ofthe acid droplets or particles such that they have a very high probability of contacting the plates. The mean free path ofthe acid is dependent upon droplet or particle size, which is believed to be a function of the temperatureto which the gas stream is reduced in the heat exchanger. It is believed that the plate spacing should be no less than about 0.50 mm and no more than about 6.35 mm. The pores and porosity must be selected such thatthe acid contacting the plates is drawn into the plates while the acid coalesces and continuously drains vertically through the plate material and drips from the plates bythe action of gravity. It is believed that the plates should have an open porosity of between 50 and 80% and thatthe minimum mean pore size should be no less than about 12.7 pm and no longer than about 127pom.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the artthat other various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims (6)

1. A method for removing phosphoric acid from a fuel cell cathode exhaust gas stream including both evaporated phosphoric acid and evaporated water characterized in comprising the steps of: reducing the temperature of the cathode exhaust gas stream to a preselected temperature below the condensation point of phosphoric acid and above the condensation point of water;; passing the reduced temperature gas stream through vertically oriented open passageways defined between a plurality of high surface area walls spaced apart a distance of between 0.50 and 6.35 mm, wherein said walls are made from a material which is resistant to corrosion by said acid and which is wettable by said acid, said material having an open porosity of between 50 and 80% and a mean pore size of between 12.7 and 127 cm, said combination of preselected temperature, wall spacing, open porosity, and mean pore size being selected such that substantially all of the acid in the gas stream contacts the walls as it passes through the passageways, coalesces and enters the pores of the walls, and drainsdownwardlythrough and subsequentlyfrom the walls bythe action of gravity, whereby a substantially acid free gas stream leaves the open passageways.

2. The method according to claim 1, characterized in that said walls are acidproof carbon.

3. The method according to claims 1 or2, characterized in that said high surface area walls are parallel,flat plates.

4. Apparatus for removing phosphoric acid from within a gas stream in carrying outthe process of claims 1-3, characterized in comprising: housing means having gas stream inlet means for receiving a gas which includes both evaporated phosphoric acid and evaporated water, said housing means also including gas outlet means and liquid acid outlet means; heat exchanger means downstream of said inlet means for reducing the temperature of said gas entering said inlet means to belowthecondensation point of phosphoric acid and abovethecondensation point of water;; accumulator means downstream of said heat exchanger means and comprising a plurality of high surface area walls spaced apart a distance of between 0.50 mm and 6.35 mm defining su bstantia I ly vertica P ly oriented open passageways therebetween, said walls being made from a material which is resistant to corrosion by said acid and which is wettable by said acid, said wall material having an open porosity of between 50 and 80% and a mean pore size of between 12.7 and 127 pom; ; and means for directing the reduced temperature gas stream from said heat exchanger means through said open passageways and to said gas outlet means, whereby substantially all of the acid in the gas stream contactssaidwalls as it passesthrough said passageways, enters the pores of said walls, and drains downwardlytherethrough to said acid outlet means bytheaction of gravity.

5. The apparatus according to claim 4, characterized in that said walls are acidproof carbon.

6. The apparatus according to claim 4 or 5 characterized in that said walls are parallel, spaced apart, flat plates defining said passageways therebetween.