8 Analysis of Accelerated and Natural Aging of Impregnated Charcoals WILLIAM S. MAGEE, JR.
Downloaded by UNIV OF ROCHESTER on January 17, 2018 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch008
Development and Engineering Directorate, Edgewood Arsenal, Aberdeen Proving Ground, Md. 21010
Abstract This paper presents an analysis of kinetic data for degradation during accelerated and natural aging of activated charcoals impregnated with salts of copper, s i l v e r and chromium. The degradation, as measured by decreased a b i l i t y to remove cyanogen chloride from humidified a i r streams, appears to consist of two simultaneous processes. Although the rate constants for these processes d i f f e r by an order of magnitude, they each have a second order dependence upon the initial moisture content of the charcoals and v i r t u a l l y identical simple Arrhenius activation energies. The analysis suggests an aging mechanism based upon non-degrading physi-sorptive sites and fast and slow degrading chemi-sorptive sites. A mathematical model for the mechanism well approximates both accelerated and natural aging data within the appropriate temperature and humidity ranges. An extension to the model for seasonably variable temperatures gives r i s e to integrals which can be evaluated only by numerical techniques.
Introduction The general adsorbent used in respiratory protective systems is activated charcoal. For removal of toxic vapors from a i r , such systems prove more than adequate as measured by service times. However, for removal of toxic gases, such systems do not match the performance with vapors. Fortunately, impregnation of activated charcoal with various substances markedly improves performance with respect to gases. A typical case, studiedlextensively during World War II, i s the enhancement of service lives for removal of hydrogen cyanide, arsine and cyanogen chloride by the impregnation of charcoal with an ammoniacal solution containing compounds of copper, silver and
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Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
PETROLEUM DERIVED CARBONS
Downloaded by UNIV OF ROCHESTER on January 17, 2018 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0021.ch008
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chromium. Interest i n these materials has again arisen i n connec tion with several aspects of environmental protection. Although freshly impregnated charcoal and that from dry storage exhibit the improved service lives, impregnated sorbent which contains moisture deteriorates during sealed storage eventually down to the level of unimpregnated material. The deterioration i s v i r t u a l l y irreversible, only a fraction of the lost service capability being recoverable. As a basis for an attempted elimination (or at least allevia tion) of the deterioration during storage, this paper presents an analysis of sealed aging data for both natural storage and accelerated aging of experimental charcoals. The analysis suggests a mechanism for deterioration which i s used to formulate a mathematical model. The model yields results which compare favorably with data within the appropriate ranges of temperature and v i r t u a l l y a l l physically realizable moisture contents. An extension of the model for seasonably variable storage tempera tures leads to integrals which can be evaluated only by numerical techniques. The use of the extension i s b r i e f l y discussed i n terms of r e a l i s t i c long term storage. Development of the Model .Tfinag anH rrwnrV»rc^.3.4 modified an equation developed by Wheeler and RobellJL and applied the result to studies of service lives of fixed beds of sorbents exposed to a variety of gases and vapors. The modified equation based on simple mass balance has the form 1^ = D[(W/C Q) - (d/C K)An(C /C)] o
o
o
(1)
where Jt^ i s the service l i f e (or breakthrough time) as determined by a concentration £ of the toxic material appearing i n the effluent a i r stream, (^is the influent concentration of the toxic, D i s the dynamic saturation capacity of the bed (weight of toxic picked up per unit weight of charcoal i n the bed), i s the volu metric flow rate,
