Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2001-03-28
2003-01-14
Simmons, David A. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S139000, C095S902000, C062S644000
Reexamination Certificate
active
06506236
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a process for reducing the level of carbon dioxide in a gaseous mixture and in particular to the removal of carbon dioxide from a gaseous mixture which is to be subjected to downstream processing in which the presence of carbon dioxide is deleterious. The present invention is especially useful in removing carbon dioxide from air which is to be employed as a feed gas in a process for the cryogenic separation or purification of air.
Carbon dioxide is a relatively high boiling gaseous material and removal of this and other high boiling materials for example water which may be present in a gaseous mixture is necessary where the mixture is to be subsequently treated in a low temperature, for example cryogenic, process. If relatively high boiling materials such as carbon dioxide and water are not removed, they may liquefy or solidify in subsequent processing and lead to pressure drops and flow difficulties in the downstream process. In the purification of air, it may also be necessary or desirable to remove hazardous, for instance explosive materials prior to further processing of the gaseous mixture so as to reduce the risk of build-up in the subsequent process thereby presenting an explosion hazard. Hydrocarbon gases, for example acetylene, may present such a hazard.
Several methods are known for removing carbon dioxide and water from a feed gas by adsorption on to a solid adsorbent including temperature swing adsorption (TSA) and pressure swing adsorption (PSA). In each of these techniques, a bed of adsorbent is exposed to a flow of feed gas for a period to adsorb carbon dioxide and water from the feed gas. Thereafter, the flow of feed gas is shut off from the adsorbent bed and the adsorbent is exposed to a flow of purge gas which strips the adsorbent gas and water from the adsorbent and regenerates it for further use. A pressure swing adsorption or thermal swing adsorption process may suitably be employed in a cryogenic process for the separation of air as a pre-purification step. U.S. Pat. Nos. 4,541,851, 5,137,548 and 5,232,474 describe adsorption processes. In these processes water and carbon dioxide are suitably removed from a gaseous mixture by contacting the mixture with a water adsorbent material for example silica gel or alumina and with a carbon dioxide adsorbent material for example a molecular sieve zeolite. It is conventional to remove water first and then carbon dioxide by passing the gaseous mixture through a single adsorbent layer or separate layers of adsorbent selected for preferential adsorption of water and carbon dioxide in a column. Removal of carbon dioxide and other high boiling components to a very low level is especially desirable for the efficient operation of downstream processes.
In TSA, the heat needed to desorb the carbon dioxide and the water from the adsorbent in the regeneration phase is supplied by heated regenerating gas. In PSA, the pressure of the purge gas is lower than that of the feed gas and the change in pressure is used to remove the carbon dioxide and water from the adsorbent with the heat required for desorption being supplied by the heat of adsorption retained within the bed. Generally, the pressure of the regenerating gas is lower than that of the feed gas in TSA also. However, in a TSA process, the adsorption phase is carried on for a prolonged period and the heat of adsorption of the carbon dioxide and water on the adsorbent liberated during most of the adsorption phase is displaced out of the bed by the flow of gas. It is necessary that the adsorbent bed has a substantial capacity for adsorbing carbon dioxide and water. As disclosed in U.S. Pat. No. 5,137,548, the difference between the temperature of the regenerating gas and the temperature at which adsorption is conducted need not exceed 50° C. and can be substantially less provided that the flow of regeneration gas is sufficient to provide the requisite heat of desorption.
It is known to use zeolites in the selective removal of carbon dioxide from a gaseous mixture both by TSA and PSA, especially prior to a cryogenic air separation process. The use of zeolites 13X and 5A for the selective removal of carbon dioxide is also known, for example as described in U.S. Pat. No. 4,249,915. Zeolites containing a binder are typically produced in the form of a bead or extrudate by mixing small particles of the zeolite together with the binder. Known binders include clays, and also for example alumina, silica and mixtures thereof. The binder is employed so as to improve the mechanical strength of the zeolite particles. Further, a binder provides a wide macropore network in the zeolite particle so one would expect enhanced mass transfer as mass transfer is known to be macropore diffusion controlled.
A zeolite with binder may contain of the order of 20 percent by weight of binder. However, the binder is generally inert and does not contribute to the adsorption capacity of the zeolite. Accordingly, as compared to a binderless zeolite, a given total mass of zeolite with binder has a lower volume or mass of actual zeolite available for adsorption. Put another way, to achieve a given level of adsorption, a higher volume of zeolite with binder is required which requires a larger reactor and hence a consequential increase in capital and variable costs.
U.S. Pat. No. 4,381,255 describes a process for producing a binderless zeolite by extruding a mixture of a zeolite and metakaolin clay and then contacting the extrudate with sodium hydroxide whereby the clay is converted to zeolite. Binderless zeolites have been proposed for use in U.S. Pat. No. 5,810,910 in recovering ozone from gas streams, where they are advantageous because they can contain reduced levels of metal impurities which would catalytically destroy ozone. They have been taught in U.S. Pat. No. 5,868,818 for recovering oxygen from air in a PSA process.
Various attempts have been made to improve upon standard binder containing zeolites for removing carbon dioxide prior to air separation. U.S. Pat. No. 5,531,808 describes a process for the removal of carbon dioxide from gas streams using a type X zeolite which has a silicon to aluminium ratio of not greater than 1.15. By employing a zeolite having these characteristics, this process advantageously reduces or avoids the need to refrigerate the gas stream in an air separation process.
U.S. Pat. No. 3,885,927 describes a process in which carbon dioxide is removed from a gas stream containing carbon dioxide at a level of not more than 1000 ppm using a type X zeolite containing at least 90 percent equivalent barium cations at a temperature of −40° C. to 120° C.
BRIEF SUMMARY OF THE INVENTION
It has now been found that binderless zeolites exhibit higher capacity for adsorption of carbon dioxide than would be expected from a consideration of the adsorption capacity of a zeolite with binder and the expected increase in capacity if the binder were replaced by the same zeolite. Similar results have been obtained in relation to the adsorption of N
2
O. The enhanced capacity is especially beneficial for use in a TSA process.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, a first aspect of the invention provides a process for the reduction of the level of carbon dioxide in a gaseous mixture comprising carbon dioxide and at least one other gaseous component which comprises contacting the gaseous mixture and binderless X-type zeolite wherein the binderless zeolite is obtained by producing a zeolite comprising a binder and converting the binder into zeolite so as to reduce the level of binder whereby a first ratio taken between the adsorption capacity for carbon dioxide of the binderless zeolite to the adsorption capacity of the zeolite comprising a binder prior to the reduction of the level of the binder is greater than a second ratio taken between the level of zeolite in the binderless zeolite by weight per cent to the level of zeolite in the zeolite with binder prior to reducing the level of the binder.
In a second aspect, the invention
Golden Timothy Christopher
Malik Nasim Hassan
Raiswell Christopher James
Salter Elizabeth Helen
Taylor Fred William
Air Products and Chemicals Inc.
Chase Geoffrey L.
Lawrence Frank M.
Simmons David A.
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