CORRESPONDENCE SIR: We differ with a statement made in the paper, “Oxidation of Contaminative Methane Traces with Radiofrequency Discharge”, by Daniel L. Flamm and Theodore L. Wydeven [ES&T, 10 (6), 591 (1976)l. In the introductory section, it is implied that microwave discharges accomplish only “partial” removal of toxic vapors, and that “pressures less than 100 torr. . . apparently.. . would make the process unattractive for many practical applications”. This, however, is not the case. In the original paper, “Microwave Decomposition of Toxic Vapor Simulants” [ES&T, 9 (3), 254 (1975)], there were many 99% and higher conversions. We also showed significantly higher efficiencies at higher power levels, 99.9+%, the precision of which was limited only by the multiple mass-balance weighings required in the process. It was also to be expected for lower power levels that lower conversions would result-as indeed they did-which were reported accordingly. With respect to the 100 torr pressure range and its practicality, we wish to refer the authors to unit processes which utilize similar vacuum conditions, such as the manufacture of polyglycols, maleic anhydride, cresols, and others (“How to Find the Lowest Cost Vacuum System”, Chem. Eng., p 87, Feb. 2,1976), as well as operations of distillation, freeze drying, vacuum flashing, etc. In this regard, we wish to confirm an initial scaleup of equipment, the plan for which was mentioned in the 1975 paper, whereby multipounds per hour quantities of hazardous pesticides have now been detoxified in a new microwave plasma system: this was accomplished at pressures of 100 torr and lower. Details of the process, sponsored by the U S . EPA Office of Research & Development, Cincinnati, Ohio, are now in process of compilation for submitting to this Journal as a current research paper.
Palo Alto Research Laboratory
L. J. Bailin Barry L. Hertzler
Lockheed Missiles & Space Co. 325 1 Hanover Street Palo Alto, Calif. 94304
SIR: Bailin and Hertzler have misread our paper; we did not make the general statement that “microwave discharges could accomplish only partial removal of toxic vapors”. We specifically referred to the process outlined by Bailin et al. (1975) and said that they used a microwave discharge “for the partial removal of toxic vapor simu1ants”-the adjective “only” was not ours. This statement is based on data in the cited publication: six out of the 32 points they report indicate an extent of reaction of 99% or better, but 10 of these data show less than 90% reaction. At the maximum power reported in that article, 500 W, only 97.9% of the DMMP feed reacted in air. Furthermore, we do not know how effectively the process would remove toxic vapors since the investigators treated two nontoxic organophosphorus compounds and the principal reaction products include organic and organophosphorus materials derived from the feed. In general, the reaction of organic chemicals in microwave discharges has been studied for more than 20 years, and this literature does include reports of nearly complete feed conversion [for instance, R. L. McCarthy, J. Chem. Phys., 22, 1360 (1954);F. J. Vastola and J. P. Wightman, J. Appl. Chem., 14,69 (1964)l. We are well aware of the use of vacua in chemical processing as well as the associated costs. Obviously, the attractiveness of a process is determined by comparing its economics with those of various alternatives. It is our present opinion that the 96
Environmental Science & Technology
costs of microwave energy and vacuum (the average pressure over Bailin et al.’s 32 points was 23 torr; the highest pressure was 82 torr) would make such a process economically unattractive in many applications when it is compared with other means of waste treatment. If helium is required, it could also contribute a significant process cost. Systematic data on the relationship between power, pressure, conversion, mass throughput, and the identity of reaction products would provide a basis for the quantitative evaluation of economics and feasibility. We have not seen Bailin and Hertzler’s unpublished work on pesticides but are glad to learn that such a study has been carried out. We look forward to a quantitative economic comparison between this process and incineration or other conventional technology. Daniel L. Flamm Texas A&M University College Station, Tex. 77843
Theodore L. Wydeven NASA Ames Research Center Moffett Field, Calif. 94035
SIR: In a recent paper, Calvert ( I ) attempted to estimate the hydroxyl radical concentrations in the Los Angeles atmosphere by using the rate of disappearance of selected hydrocarbons determined during the Los Angeles Reactive Pollutants Program (LARPP). His estimates of [HO] differ by as much as an order of magnitude depending on the hydrocarbon selected for calculation. Calvert speculates incorrectly that the discrepancy occurred because the more reactive olefins had been attenuated inadvertently as a result of sampling and storage procedures employed by the Air Resources Board Laboratories. It is a matter of record that the helicopter sampling missions were performed by the Environmental Protection Agency. Each mission required about an hour and a half. Sample bags were transferred from the helicopter directly to opaque black polyethylene storage bags at the end of each mission. The shielded samples were subsequently delivered to the ARB laboratory. Samples were analyzed in chronological order immediately upon arrival. It is highly improbable that the procedures employed by the ARB resulted in the attenuation of the reactive olefins. The stability of light hydrocarbons in plastic bags is well established (2-4). In repeated analyses performed in our laboratories, light hydrocarbon (including reactive olefin) concentrations in Tedlar bags remained unaltered for at least 24 h. We attribute this stability to the fact that Tedlar exhibits a very low affinity for light hydrocarbons and to the fact that the nitrogen dioxide photolysis rate constant, k l , in our laboratory (
