MEANS AND PROCEDURE FOR REMOVAL OF CARBON DIOXIDE

Abstract: Carbon dioxide in air and/or e.g. fuel gas for hydrogen/air fuel cells is absorbed in porous bodies (15) or elements (16), consisting completely or partly of aluminum oxide or aluminate, the surface of which has been impregnated with a solution of alkali carbonate. Under these conditions the carbon dioxide is taken up rapidly with formation of hydrogen carbonate. The carbonate is regenerated by thermal means. The new method is simple and efficient and is particularly suitable for light applications like removal of carbon dioxide from air for metal air batteries, fuel cells etc.

Description: Means and procedure for removal of carbon dioxide. Air contains a little above 300 ppm carbon dioxide. The metal air batteries, mainly the iron- and zink/air batteries, have a very high energy density but have in spite of this not yet found practical application as batteries for electric cars. One reason for this is that the alkaline electrolyte picks up carbon dioxide from the reaction air. The electrolyte has to be changed after a small number of cycles. One possi- bility to go around this problem is to clean the electrolyte by electro-dialysis in a continuous or discontinuous process. The carbon dioxide of the air is, owever, the cause of other problems in this application which cannot be sol- ved by electrolyte change or electrolyte purification. Carbonates are precipitated in channels and air cham¬ bers which is connected with the "creeping" property of the alkaline electrolyte. Such a "carbonate beard" can in its turn cause secondary effects with serious battery damage as a consequence. The best principle to solve this carbonate problem is therefore to remove the carbon dioxide from the air before introduction into the metal air battery. There are many known processes in the process industry for absorption of carbon dioxide. None of these known processes is however suitable for the metal/air batteries because of too great complicity and large volume and weight requirements. The present invention is an efficient and surprisingly simple solution to this problem which meets all require¬ ments put up, in particular the requirements on simplicity, reliability and small size. OMPI The invention can also be used in an analogous manner for elimination of carbon dioxide from air and fuel gas for fuel cell generators with alkaline electro¬ lyte. Another related application is elimination of carbon dioxide from reaction air for chlor alkali cells with air cathodes, electro-chemical so-called oxygen generators with air cathodes etc. There are also many applications outside the proper electro-chemical energy technology, e.g. removal of carbon dioxide from the atmosphere aboard submarines, space vehicles etc. The invention is concerned with an apparatus and a pro¬ cedure for removal of carbon dioxide from gas mixtures characterized in that it comprises a fixed bed absorp¬ tion reactor containing one body or a multitude of bodies of a supporting material, the surface of which at least consists of porous aluminum oxide or aluminate impregnated with a carbonate of an alkali metal. The invention will now be described by means of the drawing. Figure 1 shows in principal a metal/air battery with an absorption reactor for air purificatio . Figures 2 and 3 shov; two embodiments of the fixed bed reacto The basic process for the invention is the reaction between sodium carbonate and carbon dioxide at a tem¬ perature below - 100 C preferably < 50 C. Na 2 C0 3 + C0 2 + H 2 0 →- 2 NaHC0 3 . The sodium carbonate is regenerated by heat treatment at a temperature above 100°C, preferably 150-200°C. 2 NaHC0 3 →- Na 2 C0 3 + H 2 0 + C0 2 . OMPI These reactions are carried out in an absorption rector with the active carbonate absorbed in porous bodies or elements e.g. monolith structures which are exposed to the gas flow. The carrier material consists of aluminum oxide,Al-O-, > or is a mixture of aluminum oxide or aluminate with other resistant oxides like magnesium oxide, zirconium oxide, heat stabilized silicon dioxide, silicates etc. It has been proposed to use a finely ground mixture of potassium carbonate and aluminum oxide for carbon dioxide removal (the U.S. patent 3,865,924). In this case the particles of carbonate and aluminum oxide were separated from each other ("discrete separate particles"). The strong catalytic action with our invention is apparently caused by the impregnation procedure. The presence of aluminum oxide or aluminate in the carrying material is necessary for the technical effect of the invention. It is here apparently a question of a completely unexpected catalytical effect on the reac- tion between carbonate and carbon dioxide. It is not necessary to add moisture to the gases. The small quantitities water vapor which are normally present in air and fuel gas are quite sufficient for fast reaction. The reaction between solid carbonate without carrier is to the contrary very slow under otherwise comparable conditions. If the air is moistened a compact cake is formed. The absorption reactor is designed in the same way as a catalytic reactor e.g. of the type which is used for exhaust gas cleaning in cars i.e. a pellet bed or mono¬ lith structure. The absorption reactor is dimensioned preferably for one battery cycle. The height should be 0,2 meter or somewhat above. Regeneration can take place by means of electrical heat or in connection with the charging procedure. Regene¬ ration may also take place by means of exhaust gas heat in case of hybride systems. Another application is a fuel cell generator of the alkaline type intended to replace-the mechanically driven alternating current generator for heavy trucks. In this particular application the hydrogen for the fuel cell generator is produced by steam reforming of methanol. Absorption reactors according to the in¬ vention can be used as well for purification of the fuel gas as for the reaction air. The exhaust gas heat from the main engine is used for regeneration of the absorption reactors with a scheme with several parallell reactors in a permutated cycle of operation. The application of the invention for a metal air battery is shown in Figure 1. The description that follows shall adhere to this embodiment which,however, should not be considered a restriction of the scope of the invention. Figure 1 shows completely schematically an iron/air battery with circulating electrolyte according to the U.S. patent 3,801,376 . The battery is air cooled. Cooling air and process air are coming into direkt contact with the electrolyte. Figure 1 shows the battery pile (1) which is enclosed in a surrounding electrolyte vessel (2) . The air comes into contact with the air electrodes in an air chamber which is not shown. The air flow in the air chamber is shown by dotted flow arrows (3) . The electrolyte is circula¬ ting through the electrolyte chamber (4) via over flow (5) to the electrolyte trough (6) in the battery vessel. The electrolyte flow is shown with the flow arrows (7) . Air is supplied by means of two fans, one is governed by the temperature (8) and the other one (9) by the air demand for the discharge current in question. There is also a heat exchanger- (10) in the air system for recovery of water. Pre-heat of the process air is taking place in the battery vessel since the process air is first supplied to the surrounding vessel whereby the air is heated up in contact with the hot electrolyte whi¬ le at the same time the air is taking up moisture. The system contains also an electrical heater (11) which is used for heating up the battery during cold whether and for regeneration of the absorption reactor (12) . Regeneration is taking place by means of the heat element (13). Air with a temperature around 150 - 200°C is* flowing up through the absorption reac¬ tor and is taking with it evaporated carbon dioxide. The air is taken in through the valve (14) which is opened for the regeneration procedure. Figure 2 shows a simple embodiment of such an absorp¬ tion reactor which thus consists of a vessel contai¬ ning a bed of porous pellets of activated aluminum oxide (15) . Examples of suitable commercially available carriers of this kind are BASF D10-10 or Kebo 1.1108. Impregnation is taking place with e.g. saturated sodium carbonate solution around 50 C whereafter the pellets are dried in an oven at around 150 C for two hours. Potassium carbonate can also be used but it is not more efficient than the cheaper sodium carbonate. Addition of sodium arsenite, which is a known catalyst for this reaction, does not give a further improvement of the catalytic action of the carrier. The vessel is furnished with an inlet and an outlet. In this case the electrical element (13) is situated in the bed. Figure 3 shows another embodiment with a carrier in the form of a monolith ' structure (16) like those which are used for exhaust gas cleaning or final combustion of stack gases from wood stoves etc. The porous carrier of aluminum oxide has been impregnated with a sodium carbonate solution. The process is complicated with several steps. The reaction is not studied in the literature and it has not been possible to elucidate which step is rate determining. The absorption is, however, taking place rapidly and with a high efficiency i a.small reactor volume thanks to a not clarified catalytic action devel¬ oped by the carrier. An electric car with an iron/air battery for 20 kW and 25 k h designed in principle according to Figure 1 was supplied with an absorption reactor ("carbonate cartridge") according to Figure 2. The reactor had a 3 volume of 15 dm with a height of 2,5 dm. The bed consisted of pellets with beads of 4 mm diameter of BAST D 10-10 which had been impregnated with sodium carbonate as described above. The treated air flow had a carbon dioxide concentration of 37,5 ppm, which was measured after 1 hour of operation, to be compared with the inlet concentration 360 ppm. In a comparative experiment with an active carrier of active silicon dioxide BASF D11-11 under otherwise identical conditions the carbon dioxide content was reduced only to 310 ppm.

Claims: Patent claims 1. Apparatus for removal of carbon dioxide from gas mixtures c h a r a c t e r i z e d in that it com¬ prises a fixed bed absorption reactor (12) contain¬ ing one body or a multitude of bodies (15,16) of a supporting material, the surface of which at least consists of porous aluminum oxide or aluminate impregnated with a carbonate of an alkali metal. 2. Apparatus according'; to claim 1 c h a r a c t e r i¬ z e d in that the absorption reactor (12) contains a multitude of bodies (15) the surface of which at least consists of porous aluminum oxide or aluminate impregnated with a carbonate of an alkali metal. 3. Apparatus according to claim 1 c h a r a c t e r i¬ z e d in that the absorption reactor ' (12) contains a monolith reactor (16) the surface of which at least consists of porous aluminum oxide or aluminate im¬ pregnated with a carbonate of an alkali metal. 4. Procedure for removal of carbon dioxide from gas mixtures c h a r a c t e r i z e d in that the gas mixture is brought into contact with a fixed bed of one or a multitude of bodies of a supporting material (15,16) the surface of which at least consists of porous aluminum oxide or aluminate impregnated with a carbonate of an alkali metal, at a temperature be¬ low about 100°C, whereby alkali metal hydrogen carbo¬ nate is formed, said hydrogen carbonate then being decomposed to carbonate and carbon dioxide by heat treatment at a temperature above 100 C. _OM?I

Inventors: HOEGLUND LARS OLOF, LINDSTROEM OLLE, HOEGLUND, LARS, OLOF, LINDSTROEM, OLLE
Applicants: LINDSTROEM AB OLLE, AB OLLE LINDSTROEM
Application year: 1983
Main classification: B01D53
IPC codes: B01D53, H01M8, H01M12