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Final Report: Novel Approach to Detoxification of Polychlorinated Solvents A Waste-to-Useful Fuel Conversion
EPA Grant Number: R823179Title: Novel Approach to Detoxification of Polychlorinated Solvents A Waste-to-Useful Fuel Conversion
Investigators: Timmons, Richard B.
Institution: University of Texas at Arlington
EPA Project Officer: Krishnan, Bala S.
Project Period: October 1, 1995 through September 1, 1998 (Extended to May 31, 2000)
Project Amount: $303,082
RFA: Exploratory Research - Engineering (1995)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:This project involved a detailed evaluation of a catalytic hydrodehalogenation process as an alternate route to disposal polychlorinated waste solvents. Current oxidative disposal methods (e.g., incinerators) are accompanied by emission of undesirable by- products representing only partially oxidized chlorocarbons. Examples of these atmospheric by-product emission compounds include highly undesirable molecules such as furans and dioxins. In contrast, a hydrogen reduction conversion route offers the potential of a disposal process free from unwanted by-products. In fact, the reduction route can theoretically provide simply pure hydrocarbons and hydrogen chloride, both useful product compounds. The reductive hydrodehalogenation process is favorable in terms of thermodynamic considerations. For example, as illustrated below for the compound 1,1,1- trichloroethane (TCA), reductive processes leading to simple hydrocarbons, such as ethylene and/or ethane, are accompanied by large negative free energy changes:
CH3CCl3 + 2H2 CH2=CH2 + 3HCl G = -113 kJ/mol (1)
CH3CCl3 + 3H2 CH3=CH3 + 3HCl G = -214 kJ/mol (2)
Reactions (1) and (2) are used simply as illustrations. In fact, favorable thermodynamics apply in these hydrogen reduction processes, even when larger hydrocarbons, including aromatics, are obtained as reaction products. The favorable energetics are driven by the formation of HCl product molecules. Despite favorable reaction energetics, prior attempts to carry out the direct hydrodechlorination processes have not been successful. Although the direct thermal reaction can be carried out, it requires unreasonably high reaction temperatures and excessively long reaction times. Additionally, carbon (actually in the form of soot) is produced as a major product, requiring further disposal needs as it contains chlorinated residues. To accelerate the rates of the reduction reactions, numerous prior attempts have been made using catalysts to accelerate the conversion rates.
Despite studies with a wide-range of catalytic compositions, the previous catalytic investigations have been unsuccessful. In each case, rapid catalyst deactivation has been encountered, as a result of coke formation and deposition on the catalytic surfaces. Thus, unlike projected dechlorination routes, such as those illustrated in reactions (1) and (2), the actual catalytic processes also can lead to some elemental carbon in lieu of hydrocarbon formation. Although small amounts of elemental carbon may be formed, the buildup of this material on the active catalytic sites leads to catalyst inactivation. In fact, once initiated the rate of catalyst inactivation increases rapidly with continued reaction time. With the rapid coke buildup on the catalyst, the percent conversion of the polychlorinated reactant decreases rapidly and partially dechlorinated chlorocarbon product molecules appear in the effluent stream. The goal of the present study was to devise a catalytic system that would eliminate (or at least severely retard) coke formation thus providing continuous long-term catalyst stability. Additionally, the research objectives included successful 100 percent conversion of the polychlorinated reactants to pure hydrocarbons and HCl. It was felt that only partial dechlorination was of little utility in that this would simply convert one waste material into a different waste product.
Summary/Accomplishments (Outputs/Outcomes):The strategy involved in our studies was to construct the catalyst using support material known to resist coking in other processes. For this reason, we selected small pore synthetic zeolites, in particular the form known as Z5M-5. In these materials, the pore sizes are sufficiently small to prevent formation of the large cyclic carbon intermediates believed to serve as the precursors to rapid coke formation. The catalyst composition also included dispersed metal atoms known to have good hydrogenation ability. A large number of metals were evaluated for this purpose. From these studies, it was determined that the activity and long-term stability of platinum was, by far, the most effective catalyst for our purpose. Because of the relatively small particle-size ZSM-5 support material, several compounds were evaluated as binders to help provide more mechanically stable catalysts. Again, experimental studies indicated that the preferable material for this purpose was alumina. Therefore, small amounts of alumina were incorporated to improve the mechanical and handling properties of the catalysts. Combined Pt/ZSM-5/Al2O3 catalysts, developed in this work, were successful in promoting complete hydrodechlorination of 1,1,1- trichloroethane for prolonged periods, with only ethane and HCl as detectable conversion products. In fact, the catalytic activity expressed in terms of complete 1,1,1-trichloroethane hydrodechlorination per unit weight of catalyst was a factor of 1,000 higher than achieved before with any other catalytic material. Despite the relative success of the aforementioned catalyst, slow catalyst deactivation could be observed with catalyst use. For example, although complete 100 percent conversion of the TCA could be maintained for more than 240 hours, small amounts of 1,2-dichloroethylene were detectable in the product effluent after approximately 120 hours of on-stream time. Additionally, the relative yield of the 1,2-dichloroethylene increased slowly with continued reaction, again indicating a slow decrease in catalyst activity.
As noted above, our objective was to achieve and maintain complete hydrodechlorination of starting compounds. Thus, whereas the Pt/ZSM-5/Al2O3 catalyst represented a major advance in catalyst activity, it did not meet completely the goal we wished to attain in developing a reductive disposal route for these chlorinated wastes. An important discovery, during additional investigations with the Pt/ ZSM-5/Al2O3 catalyst, was obtained in experiments with ZSM-5 materials having a higher ratio of Si/Al atoms. Whereas the previous studies had used ZSM-5 having a Si/Al ratio of 30/1, we observed a significantly improved catalyst lifetime using a Si/Al ration of 60/1. In light of the sensitivity of the catalyst lifetime to Si/Al ratios in the zeolite, along with previous observations of poor catalytic activity with Pt/Al2O3 combinations, studies were conducted with a Pt/ZSM-5 (i.e., no added Al2O3) catalyst. Previously, the Al2O3 component was thought to be necessary to provide mechanically strong particles and to help prevent plugging of the reactor. However, it was found that judicious choice of ZSM-5 particle sizes (between 300 to 1,000 microns) permitted preparation of Pt/ ZSM-5 catalysts having good mechanical strength and porosity. The simple Pt ZSM- 5 catalyst proved to have exceptional activity and longevity. Using the ZSM-5 material having a Si/Al ration of 60/1, complete dechlorination of 1,1,1-TCA was maintained for over 250 hours at a high TCA feed rate, at which time the reaction was terminated. No sign of catalyst deactivation was detectable at termination with the reaction products consisting of only C2H6 and HCl. Even more significantly, this same catalyst was effective in promoting complete dechlorination of trichloroethylene and tetrachloroethylene for periods up to 800 hours before the runs were terminated. Again no signs of catalyst deactivation were observed with the product stream consisting of only ethane and HCl. Experiments with a C3 halogenated compound, in this case 1,2,3- trichloropropane, were equally successful supporting the generality of this conversion method.
In light of the exceptional stability of the Pt/ZSM-5 catalyst during hydrodechlorination reactions, additional studies were carried out in which only partial hydrogenation of perhalogenated molecules were attempted. Such partial dechlorinations are of interest in terms of environmental concerns and energy savings in the large-scale industrial production of compounds such as chloroform from carbon tetrachloride. It was discovered that the Pt/ZSM-5 catalyst, when operated at temperatures of around 100 C, exhibited excellent stability and high product selectivity in the conversion of CCl4 to CHCl3. Reactions terminated after 500 hours of continuous catalyst use revealed no sign of catalyst inactivation. This study further demonstrates the excellent stability of the Pt/ZSM-5 catalyst with respect to its ability to promote continuous hydrodechlorination reactions without deactivation.
In summation, the primary objective of this research project, namely development of a new catalyst to provide complete hydrodechlorination of polychlorinated reactants was achieved during these studies. The efficacy of this catalyst was demonstrated using a variety of industrial solvents, including a perchlorinated compound. As an added discovery, it was observed that these same Pt/ZSM-5 catalyst, operated at lower temperatures, will successfully promote controlled partial hydrodechlorination of perchlorinated molecules. It is believed that both the complete and controlled partial hydrodechlorination reactions, demonstrated in this work, represent new technology that merits serious consideration for industrial scale applications.
Supplemental Keywords:Sustainable Industry/Business, Scientific Discipline, Waste, RFA, Hazardous Waste, Environmental Engineering, Environmental Chemistry, Hazardous, New/Innovative technologies, alternatives to incineration, hazardous waste generation, hydrocarbons, waste reduction, detoxification, industrial chlorinated solvents, hazardous waste management, mobility of contaminants, chlorinated waste liquid, innovative technologies
Progress and Final Reports:
1999 Progress Report
Original Abstract