Sampling method aids catalyst selection Helps in determining efficiency of catalysts to oxidize contaminants in atmospheres PETROLEUM Improved selection of catalysts to aid oxidation of contaminants in closed atmospheres results from a novel sampling and analysis method developed by Dr. J. Enoch Johnson and associates at the Naval Research Laboratory. The method involves use of a multiport, axial sampling technique, which can handle multiple contaminants that vary in ease of oxidation. Catalyst selection for this use is critical because various contaminants are present in low concentrations. Typical uses of catalytic oxidation to purify atmospheres are in submarines and hyperbaric chambers. Expected uses may be in purifying air in somewhat tightly closed office buildings, air terminals, and the like. The method, developed by Dr. Johnson and coworkers Dr. Franklin S. Thomas and James K. Musick, can provide answers to such questions as how does the oxidation proceed as the atmosphere passes through the catalyst bed, what is the extent of oxidation as a function of bed depth, and how does any deactivation of the catalyst bed occur? To provide easy correlation between performances of different catalysts, the group relates effective reaction rate constant logarithmically to concentration of contaminants and bed depth. This relatively simple relation is practical because the high temperature, 600° F., highly turbulent gas flow, and large excess of oxygen make reasonable an assumption that the reaction in the cylindrical bed should be of first order, Dr. Johnson says. Combined. Heat and mass transfer effects, normally important (in addition to chemical activity) to the rate of conversion in catalytic reactions, can be combined in the effective rate constant, Dr. Johnson adds. This can be done because of the fixed operating conditions which the group used in their experimental work and which could be expected in typical applications of catalytic oxidations to remove contaminants. Because of the kinds of conditions under which the catalysts are expected to perform, the best test method requires use of different bed depths with constant sample flow rate. For this reason, five sampling ports are in the
bed area, arranged to reach to the axis of the bed. Sampling in this way means certain conditions must be met, Dr. Johnson says. Sample tubes must be noncatalytic and small enough to have minimal effect on the flow pattern. The bed diameter and depth must be large compared to the effective catalyst particle diameter to minimize bed edge and end effects. Operating flow is at a space velocity of 21,000 reciprocal hours to minimize back diffusion effects. Sampling rates for analysis by gas chromatography must be low enough to have negligible effect on conditions in the bed. Hydrogen high. Initial concentrations of all contaminants except hydrogen were held at 100 p.p.m., Dr. Johnson says. This concentration is above what would be expected to be met in most atmospheres. Hydrogen was as high as 1%, because such concentration might be encountered in submarines, and the temperature rise resulting from oxidation of this much
hydrogen could affect the activity of the catalyst. Typical results using commercial catalysts such as unsupported Hopcalite, a mixture of manganese and copper oxides, and supported Hopcalite (on alumina), show some differences in efficiency in removal of carbon monoxide. Even greater differences are found if benzene is present as well: Supported Hopcalite hasn't enough activity to remove all carbon monoxide. With five sampling ports, oxidative activity of a given catalyst toward as many as four simultaneous contaminants has been determined in a single run under a single set of operating conditions, Dr. Johnson points out. In addition to answering other questions about catalyst activity, this experimental approach permits measuring such changes within the bed as the progression of front-end poisoning through the bed with time. However, the group found uniform deactivation of these catalysts across the bed with time, even with severe deactivation.
NRL team checks out sampling apparatus used in measuring catalyst efficiency
