Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2000-02-28
2001-12-25
Simmons, David A. (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C055S522000, C055SDIG005, C095S097000, C095S104000, C096S153000, C264S029700, C264SDIG004, C502S416000, C502S427000
Reexamination Certificate
active
06332916
ABSTRACT:
The present invention relates to filtration processes. It is particularly concerned with the use of activated carbon filters to remove toxic chemicals from a breathable atmosphere.
According to the present invention a method of filtering an atmosphere containing a gaseous contaminant comprising passing said atmosphere through an activated carbon filter containing at least one transition metal, the filter material having been prepared in a process including, inter alia, the exchange of the metal ion with a cellulose ion exchange resin.
Preferably the transition metal is copper, cobalt, chromium or silver. A suitable process for the production of the activated carbon filter containing one or more of these metals is described by P A Barnes and E A Dawson in “A New Method for the Production of Metal-Carbon Catalysts” published in the Proceedings of the 6th International Symposium on Catalyst Preparation, University of Louvain-la-Neuve, September 1994. In general terms in this process the starting material is typically an ion exchange material in the form of carboxymethyl cellulose (Whatman CM32) as an alkali metal salt such as a sodium salt. Hydroxyl groups on the cellulose chain are modified to form an ether group which carries a metal carboxylic substituent, for example of formula O(CH
2
)
2
COOM where n is an integer of from 1 to 6 and M is an exchangeable cation. A particular group is OCH
2
COONa, with sodium as an exchangeable cation. An ion exchange reaction is set up with a suitable metal salt, preferably a nitrate or sulfate of the metal, for example copper sulfate, and the resulting residue is dried and then charred in an inert gas flow. It is then cooled under an inert gas and this is followed by activation in a nitrogen stream containing steam, to result in a carbon matrix holding the metal relatively uniformly dispersed throughout. This primary activation may be followed by a secondary oxidation by heating in a flow of oxygen in helium to chemisorb oxygen on the carbon surface. The resulting pore widening improves access to the metal by the gases being filtered.
The amount of metal present is preferably arranged to be between 3% and 18% by weight.
Preferably the metal is copper, but cobalt and silver are also effective, singly or in combinations with one another or copper. When such combinations are contemplated the ion exchange process for each may take place simultaneously. The percentage ion exchange and the carbon activation time both have significant effect on the property of the resultant filter to adsorb hydrogen cyanide. Low ion exchange, less than 50% and preferably about 25% or less has been found to favor dispersion of the metal and to increase the capacity of the fitter to adsorb HCN. Long activation times, for example 6 to 12 hours or more increase the capacity to adsorb HCN very considerably. The activation is catalyzed by the presence of copper, and the metal then becomes a center for evolved gases and in the final product the nucleus for transport passages through which reacting gases may diffuse. Cobalt and silver are also both capable of this catalytic effect.
The reaction of hydrogen cyanide with copper and copper salts on activated carbon results in the release of cyanogen (CN)
2
as a volatile reaction product. Due to the toxic nature of cyanogen additional measures may need to be taken for its removal, particularly when copper is the primary metal present. This is much less the case when cobalt is the primary metal.
The filter material may be arranged to contain chromium, in addition to the copper, cobalt or silver. Where copper and chromium are employed, the presence also of silver is particularly useful. There is no lower limit for the secondary metal. The upper limit may be of the order of 11% by weight.
Particularly good results in terms of the removal of both hydrogen cyanide and the product cyanogen are achieved in activated carbon filters containing copper, chromium and silver. Various examples of filtration and the construction of filters suitable for use in processes according to the invention will now be described, by way of example, with reference to the accompanying drawings.
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Barnes et al Preparation of Catalysts VI, 1995, vol. 91, pp. 361-370 “A new method for the preparation of metal-carbon catalysts”.
Barnes Philip A
Chinn Matthew J
Dawson Elizabeth A
Norman Paul R
Nixon & Vanderhye
Pham Minh-Chau T.
Simmons David A.
The Secretary of State for Defence in Her Brittanic Majesty&apos
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