ACTIVATED CARBON ADSORBENT FOR PSA DRIERS

A Type I water adsorption isotherm by the Brunauer classification with a moderate strength of water adsorption is the preferred adsorption characteristic for applications in pressure swing adsorption (PSA) driers. Zeolitic adsorbents provide the Type I isotherm shape, but they hold water too tightly. Aluminas exhibit the Type IV isotherm shapes and they do not provide good water adsorption capacities. It is possible to oxidize the surface of a normally hydrophobic activated carbon in order to produce a hydrophilic carbon with the desired Type I water adsorption characteristics for PSA drier applications. Nitric acid can be used as the oxidizing agent. Fuming nitric acid treatment can cause physical disintegration of the carbon. A moderate concentration of nitric acid prevents this problem, but the rate of oxidation is slow. Addition of oxidation catalysts like copper (II) acetate during the oxidation reaction hastens the reaction rate and a variety of water isotherm shapes can be produced by controlling the reaction time, temperature, catalyst type, and concentration. The effects of these variables on the shape of the water adsorption isotherm of the treated carbon are studied and a procedure is described for production of a preferred Type I water isotherm for PSA drier applications. © 1990.