PREPARATION OF HIGH CAPACITY UNSUPPORTED REGENERABLE CO2 SORBENT

Abstract: A technique for preparing an unsupported, high capacity CO₂ sorbent. The sorbent is comprised of silver carbonate, alkali metal silicate and alkaline earth metal salt binders for structural integrity, and alkali metal carbonate for CO₂ sorption promotion. The sorbent disclosed in this invention has a high silver oxide density, consumes minimum volume, exhibits high CO₂ absorption rates, and resists dusting and degradation for at least 50 absorption/desorption cycles.

Description: Technical Field This invention relates to the preparation of a sorbent, and especially to the preparation of an unsupported, high capacity, regenerable carbon dioxide sorbent. Background Art Regenerable solid metal oxide carbon dioxide (CO₂) sorbents can be produced via paste extrusions and pelletization methods. However, the cyclical life of these regenerable CO₂ sorbents is limited. Cyclical absorption and desorption operations of solid metal oxide regenerable CO₂ sorbents cause volume changes which result in particle deterioration and breakage; "dusting". This eventually leads to increased sorbent bed pressure drop; resulting in higher power requirements. It is common knowledge in the art that the cyclical life of many regenerable metal oxide sorbents can be increased by the addition of binders or by depositing the active ingredients of the sorbents on inactive supports, such as porous ceramics or carbons, to impart strength and provide high gas/solid contact areas. However, the use of supports and high binder concentrations is undesirable in applications where high CO₂ loading densities, in addition to weight and power considerations, are crucial factors. What is needed in the art is a regenerable CO₂ sorbent which provides high loading densities, structural integrity, and high rates of CO₂ sorption. Disclosure of Invention This invention discloses a regenerable carbon dioxide (CO₂) sorbent, a process for producing said regenerable CO₂ sorbent. The sorbent consists of pellets comprised of silver carbonate, binders, and a CO₂ sorption promoter, and is prepared by mixing silver carbonate pellets with a alkali metal silicate and alkaline earth metal salt binders, and an alkali metal carbonate CO₂ sorption promoter. The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings. Best Mode for Carrying Out the Invention The preparation of the sorbent in the present invention consists of forming silver carbonate into pellets of varying shapes, which are held together by electrostatic forces, and contacting these pellets with an aqueous solution containing alkali metal silicate and alkali metal carbonate. The pellets are dried to remove excess solvent, and are contacted with a soluble alkaline earth metal salt which forms an additional external coating; further enhancing pellet strength. Note, heat and vacuum can be applied to accelerate drying. An aqueous solution is prepared for contact with silver carbonate by dissolving alkali metal silicate and alkali metal carbonate in a solvent. The solution is intimately contacted with the silver carbonate, causing the alkali metal carbonate to impregnate the pellet, and the alkali metal silicate and silver carbonate to react forming a silicate coating. The intimate contact can be accomplished by any method which provides gentle tumbling of the pellets, such as a rotary tumbler, or other suitable mixing device known in the art. The pellets are then dried; any method which will insure uniform deposition of the alkali metal silicate and alkali metal carbonate can be utilized, such as a rotary evaporator. A solution of alkaline earth metal salt is prepared and intimately contacted with the dried pellets, as described above. The pellets are again dried, and sieved to remove dust and fines. Once this is complete, the pellets are heated to a temperature sufficient to convert the silver carbonate to silver oxide; liberating CO₂. The preferred solvent is water, although it is possible to utilize any solvent which is inert with relation to the components of the sorbent. The silver carbonate pellets can either be purchased or fabricated from powder via techniques conventionally known in the art, such as utilizing pelletizers or tablet presses. The pellet size, limited by system pressure drop considerations, typically ranges from about 0.30mm to about 3.00mm, with a pellet size between about 0.60mm to about 1.50mm preferred for closed system applications. The preferred alkaline metal silicates include sodium and potassium silicate, and mixtures thereof, although it is feasible to utilize other alkali metal silicates, which are soluble in the selected solvent. The silicate forms a thin exterior coating which imparts strength to the pellet structure. The desirable amount of silicate is determined via a balance between the optimum structural integrity and maximum reaction rates. Large amounts of silicate impart strength, but also clog the pores reducing sorption rates. Approximately 3.0 wt% to about 8.0 wt% silicate is preferred, with about 5.0 wt% especially preferred. Alkali metal carbonates, such as carbonates of cesium, potassium, and sodium, and mixtures thereof, distribute throughout the pellet's interior structure and serve as a CO₂ sorption promoters. As is well known in the art, the high pH associated with alkali metal carbonates enhances the CO₂ sorption rate. Since silver oxide has only a moderately alkaline pH, approximately 10.2, the addition of the alkali metal carbonate significantly increases the pellet's alkalinity; enhancing the rate of CO₂ sorption. An alkali metal carbonate wt% of between about 8.0 wt% to about 20 wt% is preferred, with a range between about 8.0 wt% to about 15.0 wt% more preferred, and about 10.0 wt% especially preferred. The alkaline earth metal salt binder concentration is preferably between about 2.0 wt% to about 5.0 wt%, with about 3.0 wt% especially preferred. As with the alkali metal silicate factors such as structural integrity and sorption rates must be taken into consideration when determining the amount of alkaline earth metal salt binder to be utilized. Various alkaline earth metal salt binders can be utilized, with nitrates and chlorides of calcium, magnesium, and barium, and mixtures thereof preferred; such as calcium chloride, magnesium chloride, barium nitrate, calcium nitrate, among others. The wt% of silver carbonate, alkali metal silicate, alkali metal carbonate, and alkaline earth metal salt to be utilized are determined by the desired wt% of each in the final product. Any solvent used in the production process is evaporated. The prepared sorbent, once having been screened to remove any dust and fines, can be loaded into a reactor for the removal of CO₂ from a gaseous stream, typically air. The sorbent is then activated for CO₂ sorption by heating it to temperatures sufficient of convert the silver carbonate to silver oxide. Typically these temperatures will be about 160°C to about 220°C. Note, temperatures above about 250°C can irreversibly damage the silver oxide sorption abilities. After cooling to near ambient temperatures, the sorbent is capable of removing CO₂. For example, air in a closed environment is passed through the reactor where the alkali metal carbonate reacts with the CO₂ and water in the air to form the bicarbonate ion. The silver oxide then reacts with the bicarbonate ion to form silver carbonate, alkali metal carbonate, and water; leaving the alkali metal uninhibited from continuing to remove carbon dioxide from the air stream. Once the silver oxide content is converted to the carbonate form, the reactor bed loses its CO₂ sorption capabilities and is ready for regeneration. Regeneration consists of heating the sorbent bed to a temperature sufficient to cause CO₂ liberation; converting the silver carbonate to silver oxide. Typically temperatures between about 160°C to about 220°C are sufficient for CO₂ liberation. Since the sorbent is unsupported, no inert support is utilized, it contains a higher silver oxide density than the prior art. Also, due to the binder coating, the sorbent resists dusting and degradation for at least 50 absorption/desorption cycles; while other regenerable metal oxide sorbents, known in the art, begin dusting in the very early cycles, if not immediately. The following is a generic method which can be utilized to produce a carbon dioxide sorbent comprised of: 2.0 wt% to 5.0 wt% calcium nitrate, 3.0 wt% to 5.0 wt% sodium silicate, 10.0 wt% to 20.0 wt% potassium carbonate, and 70.0 wt% to 85.0 wt% silver carbonate. 1. The silver carbonate powder pelletized in a rotating disk agglomerator, and sieved to provide between 0.60mm to 1.40mm diameter pellets. 2. A sodium silicate/potassium carbonate solution is mixed with the pellets. To insure uniform coating, a rotating flask is utilized. The aqueous solution is added at a level that provides 3.0 to 5.0 wt% sodium silicate and 10.0 to 20.0 wt% of the potassium carbonate to the pellet. The silicate uniformly concentrates on the exterior surface of the gently tumbling pellets and serves to enhance the structural integrity of the pellet. The potassium carbonate is uniformly distributed within the pellet and serves to enhance the rate of CO₂ sorption. 3. The excess aqueous solvent is evaporated from the solution/pellet mixture by using a rotating vacuum flash evaporator and applying heat to the mixture to a temperature level ranging from 50°C to 90°C. Use of the rotating flash evaporator insures that the sodium silicate and potassium carbonate are uniformly deposited. 4. Calcium nitrate, to provide 2.0 wt% to 5.0 wt%, is then added as in step 2 to further enhance the structural integrity of the pellet. The wetted pellets are dried as in step 3. 5. The coated/impregnated pellets are sieved prior to use to remove fines and dust. 6. The pellets are packed within a reactor. The reactor is heated to a temperature level between 160°C to 220°C to convert the silver carbonate to silver oxide. An air purge flow through the bed assists in the removal of the CO₂ and water released from the pellets. After cooling the beds to near ambient conditions, the sorbent is ready to remove CO₂. This procedure is repeated when it is required to regenerate the CO₂ sorption capabilities of the sorbent. Pellets prepared by this example have been shown to have CO₂ loading densities ranging from about 192,2 to 240 Kg/m³ (12.0 to 15.0 lbs/ft³); between about 9,67 to 12,00 moles/m³ (0.274 to 0.340) moles/ft³. This invention relates to the production of a regenerable CO₂ sorbent capable of high CO₂ loading densities, and relatively devoid of the low CO₂ sorption rates and dusting problems which plague the art of regenerable metal oxide CO₂ sorbents. Since this sorbent doesn't require a support, contains a relatively low binder content, and has a high metal oxide density, it consumes minimum volume; making it ideal for closed environmental applications. Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the wording of the appended claims.

Claims: A method for preparation of an unsupported regenerable metal oxide CO₂ sorbent, said method using silver carbonate, an alkali metal silicate, an alkali metal carbonate, and an alkaline earth metal salt, which comprises: a. pelletizing silver carbonate, wherein pellets are formed; b. preparing an aqueous solution of alkali metal silicate and alkali metal carbonate, wherein the amount of alkali metal silicate and alkali metal carbonate are determined via the desired wt% of each in the final product; c. contacting said pellets with said alkali metal silicate/alkali metal carbonate aqueous solution, thereby forming silicate coated, alkali metal carbonate impregnated pellets; d. drying said silicate coated, alkali metal carbonate impregnated pellets; e. preparing an aqueous solution of alkaline earth metal salt, wherein the amount of alkaline earth metal salt utilized is determined by the desired wt% of alkaline earth metal salt in the final product; f. contacting said silicate coated/alkali metal carbonate impregnated pellets with said alkaline earth metal salt aqueous solution, thereby forming an alkaline earth metal coating on said silicate coated, alkali metal carbonate impregnated pellets; g. drying said alkaline earth metal/silicate coated, alkali metal carbonate impregnated pellets. A method as in claim 1 wherein the pellet size ranges from about 0.30mm to about 3.00mm in diameter. A method as in claim 1 or 2, wherein the alkali metal silicate is selected from the group consisting of sodium and potassium silicate, and mixtures thereof. A method as in any one of claims 1 to 3, wherein the alkali metal carbonate is selected from the group consisting of cesium, potassium, and sodium carbonate, and mixtures thereof. A method as in any one of claims 1 to 4, wherein the alkaline earth metal salt is selected from the group consisting of nitrates and chlorides of calcium, magnesium, barium, and mixtures thereof. A method as in any one of claims 1 to 5, wherein contacting and drying of said pellets with said alkali metal silicate/alkali metal carbonate aqueous solution, and alkaline earth metal salt aqueous solution is performed within a rotatory evaporator. A method as in any one of claims 1 to 6, wherein heat is applied to any of the operations to reduce sorbent preparation time. A method as in any one of claims 1 to 7, wherein the desired wt% of alkali metal silicate in the final product is between about 3.0 wt% to about 8.0 wt%. A method as in any one of claims 1 to 8, wherein the desired wt% of alkali metal carbonate in the final product is between about 8.0 wt% to about 20.0 wt%. A method as in any one of claims 1 to 9, wherein the desired wt% of alkaline earth metal salt in the final product is between about 2.0 wt% to about 5.0 wt%. An unsupported regenerable metal oxide CO₂ sorbent, which comprises: a. silver carbonate for CO₂ sorption; b. alkali metal silicate and alkaline earth metal salt as binders; c. alkali metal carbonate as a sorption promoter. An unsupported regenerable metal oxide CO₂ sorbent as in claim 11, wherein said sorbent preparation comprises: a. pelletizing silver carbonate, wherein pellets are formed; b. preparing an aqueous solution of alkali metal silicate and alkali metal carbonate, wherein the amount of alkali metal silicate and alkali metal carbonate to utilize is determined via the desired wt% of alkali metal silicate and alkali metal carbonate in the final product; c. contacting said pellets with said alkali metal silicate/alkali metal carbonate aqueous solution, thereby forming silicate coated, alkali metal carbonate impregnated pellets; d. drying said silicate coated, alkali metal carbonate impregnated pellets; e. preparing an aqueous solution of alkaline earth metal salt, wherein the amount of alkaline earth metal salt to utilize is determined by the desired wt% of alkaline earth metal salt in the final product; f. contacting said silicate coated, alkali metal carbonate impregnated pellets with said alkaline earth metal salt aqueous solution, thereby forming an alkaline earth metal coating on said silicate coated, alkali metal carbonate impregnated pellets; g. drying said alkaline earth metal/silicate coated, alkali metal carbonate impregnated pellets. An unsupported regenerable metal oxide CO₂ sorbent as in claim 11 or 12, wherein the silver carbonate is in the form of pellets, the pellet size ranging from about 0.30mm to about 3.00mm in diameter. An unsupported regenerable metal oxide CO₂ sorbent as in claim 12 or 13, wherein contacting and drying of said pellets with said alkali metal silicate/alkali metal carbonate aqueous solution, and said alkaline earth metal salt aqueous solution is performed within a rotatory evaporator. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 14, wherein the desired wt% of alkali metal silicate in the final product is between about 3.0 wt% to about 8.0 wt%. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 15, wherein the desired wt% of alkali metal carbonate in the final product is between about 8.0 wt% to about 20.0 wt%. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 16, wherein the desired wt% of alkaline earth metal salt in the final product is between about 2.0 wt% to about 5.0 wt%. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 17, wherein the alkali metal silicate is selected from the group consisting of sodium and potassium silicate, and mixtures thereof. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 18, wherein the alkali metal carbonate is selected from the group consisting of cesium, potassium, and sodium carbonate, and mixtures thereof. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 11 to 19, wherein the alkaline earth metal salt is selected from the group consisting of nitrates and chlorides of calcium, magnesium, barium, and mixtures thereof. An unsupported regenerable metal oxide CO₂ sorbent as in any one of claims 12 to 20, wherein heat is applied to any of the operations to reduce sorbent preparation time. A method for utilizing an unsupported regenerable metal oxide CO₂ sorbent, said sorbent containing silver carbonate, an alkali metal silicate, an alkali metal carbonate, and an alkaline earth metal salt, which comprises: a. loading said sorbent into a reactor to form a sorbent bed; b. heating said sorbent bed to a temperature sufficient to convert said silver carbonate to silver oxide; c. passing a CO₂ contaminated stream over said sorbent bed, wherein said alkali metal carbonate reacts with said CO₂ forming alkali metal bicarbonate, and said silver oxide reacts with said alkali metal bicarbonate forming silver carbonate and alkali metal carbonate. A method as in claim 22, wherein said sorbent bed is heated to between about 160°C to about 22o°C. A method as in claim 22 or 23, wherein the alkali metal silicate is selected from the group consisting of sodium and potassium silicate, and mixtures thereof. A method as in any one of claims 22 to 24, wherein the alkali metal carbonate is selected from the group consisting of cesium, potassium, and sodium carbonate, and mixtures thereof. A method as in any one of claims 22 to 25, wherein the alkaline earth metal salt is selected from the group consisting of nitrates and chlorides of calcium, magnesium, barium, and mixtures thereof.

Inventors: AYLWARD, JOHN R., BIRBARA, PHILIP JAMES, NALETTE, TIMOTHY ARTHUR
Applicants: UNITED TECHNOLOGIES CORPORATION
Application year: 1991
Main classification: B01J20
IPC codes: B01J20, B01D53, B01J2, C01B31, C01G5