Novel Vapor-Phase Biofiltration and Catalytic Combustion of Volatile

Oct 18, 2001 - For the biodegradation study, enriched solvent-tolerant bacterial cells were immobilized onto porous glass cylinders within a biofilter...
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Novel Vapor-Phase Biofiltration and Catalytic Combustion of Volatile Organic Compounds Malinee Leethochawalit, Mark T. Bustard, and Phillip C. Wright* Biochemical Engineering and Environmental Technologies Group, Department of Mechanical and Chemical Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, U.K.

Vissanu Meeyoo Centre for Advanced Materials and Environmental Research, Mahanakorn University of Technology, 51 Cheum-Sampan Road, Nong Chok, Bangkok 10530, Thailand

The aerobic biodegradation and catalytic oxidation of vapor-phase 2-propanol (IPA) were investigated. The catalytic oxidation of IPA was carried out over zinc, copper, and chromium oxide catalysts prepared via a sol-gel technique in a fixed-bed reactor operated at atmospheric pressure and in the temperature range of 25-165 °C. The activity of the catalysts was measured by means of the light-off temperature (defined as 50% conversion of IPA). The light-off temperatures of zinc oxide, copper oxide, and chromium oxide are 90, 100, and 110 °C, respectively. The results indicate that, at relatively low temperature (40-100 °C), IPA was partially oxidized, which resulted in acetone formation. The maximum acetone selectivity varied between 30 and 97% at ca. 100 °C, depending on the types of catalyst. For the biodegradation study, enriched solvent-tolerant bacterial cells were immobilized onto porous glass cylinders within a biofilter. Successful biofiltration of high solvent vapor concentrations of up to 34 g m-3 was achieved. An average IPA elimination capacity of up to 280 g m-3 h-1 was demonstrated by this biofiltration system. A slip feed experiment, using acetone, was investigated in order to assess the substrate specificity performance. The results show that the biofilter can deal with an alteration in feed composition and display no major reduction in the elimination performance. This paper shows that the concentration and compound distribution from the exit of a catalytic partial oxidation process are consistent with the inlet conditions of a gas-phase biofilter containing a solvent-tolerant microbial consortium. This points the way toward a potential integrated biofiltration-catalytic combustion system for the overall enhanced pollution abatement performance. Introduction As the world faces even more stringent legislation imposed on the release of malodorous and volatile organic compounds (VOCs), scientists and engineers face new challenges to clean up these waste streams, without causing more environmental problems in the process. Conventionally, the easiest way to abate these airborne pollutants was simply to burn them in a furnace. In addition, techniques such as absorption and adsorption and catalytic combustion have been tried. The full range of usually available technologies is summarized in Table 1, along with advantages and disadvantages. The most useful strategies for a longterm environmental impact would seem to be those that convert the pollutant into less harmful compounds rather than simply changing their phase. Thus, as can be seen from Table 1, biofiltration and catalytic oxidation offer the advantages of high destruction efficiency and lower operational cost. In more recent times, biological methods have become an attractive option, albeit with a number of kinetic and reactor-based limitations. These increases in usage of biological methods result from low costs, operational simplicity, and intrinsically clean technology (Table 1). There is also a minimal requirement of energy, raw materials, and waste/byproduct production.1 * Corresponding author. Phone: +44 131 451 8165. Fax: +44 131 451 3129. E-mail: [email protected].

In gas-phase bioreactors, the contaminants are sorbed from the gas phase to an aqueous phase where the microbial populations mineralize the contaminants.2 However, a major limitation is that the contaminants for biological methods must be fully biodegradable. In comparison with other oxidative processes such as catalytic combustion, harmful intermediates may also still leave the biofilter. As a consequence, the most successful removal in gas-phase bioreactors occurs for low molecular weight and highly soluble organic compounds with simple bond structures.3 In this study, we seek to investigate the possibility of taking the advantages of catalytic combustion and biofiltration to form an integrated pollution abatement process. The overall aim is to avoid the disadvantages (see Table 1) of these systems in isolation. This process works by using catalytic combustion to partially degrade these VOC molecules into less recalcitrant (i.e., more biodegradable) compounds. In many cases, catalytic combustion is not suitable alone, because the byproducts are often toxic. This thus opens the opportunity for posttreatment by a biofilter. Typically, conventional total catalytic oxidation system temperatures in excess of 200 °C are usual.4-6 However, transition-metal oxides and noble metals can partially oxidize VOCs at temperatures of ca. 120 °C.7,8 This would lead to a significant decrease in the energy running costs for an integrated catalyst-biocatalyst reactor treatment system.

10.1021/ie001108q CCC: $20.00 © 2001 American Chemical Society Published on Web 10/18/2001

Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5335 Table 1. Summary of VOC Abatement Technologies, Adapted from References 17 and 18a

techniques

average capital cost (%)

average operating cost (%)

thermal oxidation

100

100

high destruction efficiency wide applicability heat recovery up to 95% good for varying concentrations and types of VOCs

possibly more harmful secondary waste produced unsuitable for batch operation halogenated compounds possibly requiring additional control equipment downstream

catalytic oxidation

120

50

lower temperature than conventional thermal oxidation high destruction efficiency

high capital cost possible poisoning of the catalyst by some compounds halogenated compounds possibly requiring additional control equipment downstream

adsorption

120

40

recoverable compounds

high capital cost selective adaptability often requirement of large units moisture and temperature limitation prior removal of dusts and mists

absorption

80

25

low capital cost reasonably high efficiency recoverable compounds

lack of equilibrium data

20-600

13

low operating cost good performance at low concentration of pollutant less secondary waste

unflexible to change in the pollutant concentration and loading slow poor performance at high loading or with complex compounds

biofiltration

a

advantages

disadvantages

Average capital and operating costs are compared to thermal oxidation as a reference point at 100%.

As part of assessing the potential for an integrated catalytic oxidation-biofiltration process, the partial oxidation of 2-propanol (IPA) on three different oxide catalysts (Cu, Zn, and Cr) prepared by a sol-gel technique was studied. The effects of the catalyst choice on the light-off temperature and acetone selectivity are investigated. After the catalytic combustion study, we investigate the possibility of using an enriched microbial consortium in vapor-phase bioreactors to treat both similar concentrations and types of compounds found in the exit gases of the catalytic combustion process described above. Concentrations tested here are up to 1 order of magnitude higher than those that have previously been shown for IPA- and acetone-degrading microorganisms. Normally such concentrations would be prohibitorily high for conventional biofilter systems. Thus, this integrated system offers a dual guarding process, i.e., upstream protection of the biofilter by a catalytic combustion process and downstream posttreatment of the combustor by the biofilter. This may offer an effective system for near-zero pollution breakthrough. IPA is chosen as a model system here because more than 1 million tons are produced annually.9 It is used in many industries ranging from cosmetics, rubber, pharmaceuticals, textiles, and other fine chemicals, resulting in a high level of organic-solvent-contaminated wastes. For example, over 52 ktons of airborne IPA solvent emissions occurred within the U.K. in 1995, and it was found to be the largest single contributor to the total U.K. solvent VOC emissions.10 Experimental Section Two different experimental programs were carried out, catalytic combustion and biofiltration. These are described below. Catalytic Oxidation of IPA. (a) Catalyst Preparation. Three metal oxides were used as the oxidation catalysts in this study. The catalysts were prepared by urea hydrolysis of metal salt solutions using an ap-

proach similar to that described elsewhere.11 The metal salts [CuCl2, Cr(NO3)3‚9H2O, Zn(NO3)2‚6H2O] and urea were supplied from Fisher Scientific U.K. Ltd. Typically, 0.1 M metal salt and 0.4 M urea were mixed at a 1:4 volume ratio. The solution was then maintained and stirred continuously at 100 °C for 24 h. The resulting gel was then filtered, washed repeatedly with distilled water for complete removal of impurity ions, then dewatered by ethyl alcohol, and dried at 100 °C for 14 h. Then, the catalysts were calcined in air for 6 h at 500 °C. (b) Activity Test. Initial experiments focused on the determination of the “light-off” temperature (Figure 1). The catalytic oxidation of IPA on different oxide catalysts was carried out in a differential packed-bed reactor (6 mm i.d.) mounted in the constant temperature ((1 K) zone of an electric furnace. Typically, 0.2 g of the catalyst sample was packed between two layers of glass wool. Small-diameter thermocouples were located before, in, and after the catalyst bed. Exit gases were analyzed chromatographically, using a Shimadzu 17A fitted with a Carbowax BP22 column (15 m in length and 1 µm film thickness). Unless otherwise stated, regulated gas mixtures (40 cm3/min) containing 1% IPA and 20% excess O2 (balance He) were passed through the catalyst bed at ambient temperature. After stabilization, the temperature was raised at 50 °C/min to the next condition. The light-off temperature was monitored as the temperature at which the conversion of IPA reached 50%. High-Concentration Solvent Vapor Biofiltration. The aerobic biodegradation of high-concentration (15-34 g m-3) solvent vapor in a fixed-bed biofilter was investigated. To build a strong biofilm onto support matrixes, the biofilter was first operated in submerged liquid mode for 7 weeks before it was altered to a gasphase operation. (a) Inoculation of the Bioreactor (Liquid Phase). IPA-utilizing cultures were grown in 4 × 250 cm3 shake flasks to a cell density of 9 × 108 cells cm-3. This was subsequently added to the glass fixed-bed reactor col-

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Figure 1. Schematic of the experimental setup of the catalytic oxidation system.

Figure 2. Schematic of the experimental setup of the three-phase fixed-bed bioreactor.

umn with 1000 cm3 of a minimal salts medium (MSM)12 containing 20 cm3 IPA, thereby giving inoculum and IPA concentrations of 3 × 108 cells cm-3 and 7.9 × 103 g m-3, respectively. This liquid mixture was then recirculated through the bed at 0.006 m3 h-1 using a peristaltic pump (Figure 2) for mixing, biofilm formation, and IPA biodegradation in the liquid phase. The column was constructed of a glass tube with an inner diameter of 0.088 m and a height of 0.7 m. The immobilization matrix was 797 g of Siporax sintered glass pall rings (15 mm o.d., 0.01 m i.d., 0.015 m length, and 1.7 × 10-6 m3 volume). The packed-bed volume was 1.91 × 10-3 m3, and the total surface area was 797 m2, with a voidage of 0.45. Air was supplied to the bioreactor base via a plenum chamber and a perforated-plate distributor at a rate of 0.06 m3 h-1. In each batch, after no IPA concentration was detected from the biofilter, 20 cm3 of pure IPA was added to the column. To allow for comparison of substrate specificity (for later slip-stream experiments), a second liquid-phase bioreactor with identical geometric parameters was operated in parallel for the batch treatment of 7.9 × 103 g m-3 of acetone. Liquid samples were withdrawn from the effluent recycle tube for gas chromatographic analysis (as described above for the catalytic activity tests).

The second-stage gas-phase experiments were then carried out to investigate the long-term efficiency of the biofilter. Experimental conditions were similar to those described above. (b) Gas-Phase Biofilter Operation. After liquidphase biofilm/biodegradation stabilization, gas-phase experiments were performed in the same column by draining the liquid, as shown in Figure 2. Saturated IPA vapor was then created by bubbling air through an IPA solution that was then diluted with air using balancing valves and rotameters, before being supplied to the bioreactor base via a plenum chamber and a perforatedplate distributor at a rate of 0.036 m-3 h-1. Every 7 days, a batch of 2000 cm3 of MSM was washed slowly through the bed to prevent drying out of the biofilm and spent medium was drained off in 5 min. Samples were withdrawn by syringe from the gas inlet and outlet for gas chromatographic analysis. Results and Discussion Oxidations of IPA by catalytic combustion and biofiltration were examined separately under similar conditions, as a lead to a future integrated process. As described earlier, the biological process consists of an

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Figure 3. Catalytic oxidation of IPA on three different catalysts at 40 cm3 min-1. IPA conversion and acetone selectivity of copper oxide (2, 4), zinc oxide (b, O), and chromium oxide (9, 0).

Figure 4. Biodegradation of high-concentration IPA in a liquid-phase biofilter: IPA outlet ([) and acetone outlet (9).

initial liquid-phase startup, followed by the main gasphase biofiltration process. The results of this study are described below. Catalytic Oxidation of IPA. This section demonstrates how to use the catalytic combustion process as a guard cell for a biofiltration unit. It is apparent that IPA is relatively toxic to living cells at high concentration.12 As shown in Figure 3, the overall removal of IPA can be achieved at a temperature of ca. 120 °C in the presence of metal oxides (Zn, Cu, and Cr). The results show that the light-off temperatures of zinc oxide, copper oxide, and chromium oxide are about 10 °C different: 90, 98, and 108 °C, respectively. It is interest-

ing that at low temperature IPA is converted to acetone, which is more rapidly biodegradable. The other main oxidation product is carbon dioxide. It is found that chromium oxide yields the highest acetone selectivity at a temperature of about 100 °C, while zinc oxide exhibits the lowest acetone selectivity. This suggests that chromium oxide is suitable as a guard catalyst cell. Biofiltration Startup Process (Liquid Phase). The decomposition kinetics of a fed-batch liquid-phase fixed-bed bioreactor for the biodegradation of 7.9 × 103 g m-3 of IPA are shown in Figure 4. Acetone is an intermediate in the aerobic biodegradation of IPA. IPA was fed to the bioreactor as the sole carbon source for growth. However, the bacteria can also utilize the

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Figure 5. Biodegradation of high-concentration acetone in a liquid-phase biofilter: acetone outlet ([).

intermediate acetone. This might suggest the involvement of two biocatalytic mechanisms; IPA is first converted to acetone by alcohol dehydrogenase, followed by further mineralization catalyzed by acetone carboxylase. Alternatively, the process could be catalyzed by the action of a broad specificity oxidase-reductase enzyme. The localization and mode of action is the subject of further study. During 341-884 h, the process was affected from an unavoidable stoppage of air and liquid circulation systems for maintenance. At 1220 h, a thick stable biofilm was also visually observed, and the process was then changed over to a gas phase. Acetone Biodegradation. The decomposition of 7.9 × 103 g m-3 of acetone in a separate bioreactor was investigated, and the results are presented in Figure 5. The aim of this study is to assess how well the biological system responds to acetone, the major product of the partial oxidation of IPA. After an absorption period, the acetone biodegradation rate improved to 63 g m-3 h-1. Figure 5 shows that acetone biodegradation is biphasic. In each batch, after addition of 20 cm3 of acetone, the microbes first show a slow biodegradation rate (a lag phase), followed by a rapidly increasing degradation rate. The slow biodegradation rate might stem from two parameters; adsorption of acetone from the extracellular liquid phase through the biofilm and cell membrane. The appearance of acetone inside the cell itself may also cause induction of acetone carboxylase, so that the acetone can be utilized for generation of metabolic energy.13 This, in combination with the stable acetone concentration gradient, causes a higher biodegradation rate in the second phase. Gas-Phase IPA Biodegradation. After the stabilization of the biofilm in a submerged culture as described above, the gas-phase studies began. The startup and acclimation of the biofilter for the biodegradation of IPA vapor are shown in Figure 6. IPA was removed from the gas phase by adsorption into the biofilm for an initial sorption stage of 16 h; during this phase, no acetone was detected in the exhaust gas. This indicates little or no biological oxidation of the IPA vapor, adsorption of any acetone produced into the immobilization matrix,

or complete biodegradation of acetone. To assess the biofilter performance, IPA feed concentrations were varied, breakthrough was monitored, and the removal efficiency was calculated. A sudden concentration increase to 20.9 g m-3, between 24 and 42 h, caused an IPA breakthrough from the biofilter at a concentration of 10.7 g m-3, indicating that the bacterial metabolism is not optimized at this level. A subsequent reduction in the inlet feed concentration to 18 g m-3, at 48 h, allowed stabilization of the biodegradation rates and hence an increase in the IPA elimination effectiveness. The IPA feed concentration was steadily increased to 25.5 g m-3 following 188 h of operation, and IPA was observed to break through between 121 and 188 h at 3 g m-3. However, the acetone concentration continually increased to a maximum level of 17.4 g m-3, indicating significant conversion of IPA but little cometabolism of acetone. The IPA feed concentration was then reduced to a constant level of ca. 15 g m-3 to assess the overall performance of the biofilter when steady-state conditions were approached. Figure 6 also shows that the outlet acetone concentration is reduced in decreasing cycles with maximum values of 17.4, 11.5, 6.1, and 2.3 g m-3 at times 192, 384, 480, and 552 h observed, respectively, following startup. This suggests that the solvent-tolerant bacterial community is implementing the biochemical processes required for more rapid acetone utilization, such as an increase in the transmembrane acetone transport and up-regulation of acetone-degrading enzymatic systems, for example. The biodegradation of the 15 g m-3 IPA vapor feed was successfully carried out after 200 h of operation, with the exhaust gas containing between 0 and 0.7 g m-3 of residual IPA. However, it may be assumed that steady-state biofiltration conditions occurred following 624 h from startup, because both IPA and acetone levels were reduced to below 0.7 g m-3 in the vent gas. During this period, both IPA and acetone are effectively cometabolized by the solvent-tolerant microbial consortium in the biofilter at concentrations that would otherwise be toxic to environmental microorganisms.12

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Figure 6. Biodegradation of high-concentration IPA vapor in a biofilter utilizing a solvent-tolerant mixed bacterial culture. IPA inlet ([), IPA outlet (9), and acetone outlet (2) concentrations were monitored throughout the experiment.

Figure 7. Biofilter elimination capacity for both IPA ([) and total carbon (9).

A concentration spike was applied to the biofilter to assess the microbial metabolic ability to recover following a high concentration pulse. This was carried out between 1152 and 1230 h, where the IPA inlet feed concentration was doubled rapidly to 34 g m-3 and then returned to ca. 16 g m-3, as depicted in Figure 6. Subsequent IPA breakthrough from the biofilter occurred between 1308 and 1640 h at an average concentration of 7 g m-3. However, acetone breakthrough commenced prior to this at 1254 h to an average concentration of 9 g m-3 in the vent gas, indicating biodegradation of IPA during the high concentration spike, although not rapidly enough to ensure total removal of both pollutants. It can also be seen that this exhaust acetone concentration fluctuates more than concentrations of IPA, indicating a metabolic step change in the bacterial culture as it adapts to the shift in the feed gas concentration. Following 1640 h, no further IPA was detected in the vent gas, although elevated acetone levels persist at 10 g m-3 until 1699 h.

However, a further 189 h were required to return the biofilter to steady-state conditions with complete pollutant removal. Although a flow rate of 0.036 m3 h-1 is a relatively low flow rate in comparison to other systems, the pollutant loading is up to 1 order of magnitude higher than previous reports for either IPA or acetone. A typical elimination capacity for IPA in a conventional biofilter is up to 100 g m-3 h-1, for example.2 The equivalent overall elimination capacity (g m-3 h-1) of this biofilter, containing the solvent-tolerant bacterial consortium, is depicted in Figure 7. Although high elimination capacities of up to 460 g m-3 h-1 are observed within the startup period for IPA, this value for total pollutant conversion is considerably lower, between 320 and 160 g m-3 h-1, for example. During steady-state operation, however, the total carbon removal equals the IPA elimination capacity (280 g m-3 h-1), although between 1260 and 1860 h the elimination of acetone is notably reduced. During conditions where

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Figure 8. Biofilter performance when initiated with acetone followed by a substrate switch to IPA: acetone elimination capacity (9), IPA elimination capacity (×), acetone inlet concentration ([), and IPA inlet concentration (2).

the IPA feed was pulsed to 34 g m-3, the biofilter responds poorly in the short term but later returns to stable operation. However, the biofilter is unable to totally convert all carbon compounds prior to atmospheric release. A minimum value of 10 g m-3 h-1 was obtained at 1476 h, while steady-state conditions were reestablished 487 h later. These perturbations and lowering of the total carbon conversion efficiency demonstrate the likely utility of pre- or posttreatment (either intermittent or continuous) with a system like a catalytic combustor. This shows that, despite the possibility of high solvent loads to the biofilter, the disadvantage of a low ability to handle concentration fluctuations remains (see Table 1). The identical gas-phase biofilter was investigated in a slip-feed experiment in order to assess substrate specificity characteristics within the gas-phase biofilter; acetone alone was supplied as the carbon source to a similarly prepared biofilter, before switching the substrate to IPA. Acetone feed concentrations were steadily increased to 14 g m-3, and then IPA was used (15 g m-3) as the sole carbon source. Although there were fluctuations in the feed during 640-740 h and 1080-1320 h, because of an unavoidable interruption in operation, the IPA average concentration was about 20 g m-3. Figure 8 shows the elimination capacities for both substrates and demonstrates that the biofilter can deal with an alteration in the feed composition efficiently, albeit after a period of time, and displays no major reduction in the elimination performance following this substrate switch. This result shows that these consortia have a significant adaptive ability to variable- and high-concentration feed streams in a long-term operation. However, these data do not elucidate whether this is a nonspecific enzyme such as an oxidase-reductase or a set of complex enzymes including alcohol dehydrogenase and acetone carboxylase.14,15 This forms the basis of current studies. It is also clear that further enrichment on acetone would be required if the system is going to work effectively on an acetone-rich effluent from a catalytic combustor. However, the orders of magnitude of the concentrations tolerated and metabolized by this system are suitable. These have not been demonstrated as achievable before.

Conclusions High conversion efficiencies of IPA were demonstrated in both catalytic combustion systems and gas-phase biofilters. The high acetone selectivity and relatively low temperature achieved for the IPA light-off temperature (100 °C) in the catalytic system lends itself to further optimization. The aim of this preliminary study was to assess the possibilities of linking catalytic and biocatalytic technologies together in an integrated reactor system for the purposes of gas-phase pollution abatement. The first stage of this process has been achieved with strong indications of selection of relevant enzymatic processes, for both IPA and acetone at concentrations of up to 34 g m-3. In addition, we show in this paper that slip-stream operation is possible, with partial oxidation products being feed to the biofiltration system. These high concentrations, utilizing solvent-tolerant microorganisms, mean that there is greater overlap in the catalyst and biocatalyst feed concentrations than has hitherto been the case (see Table 1 for biofilter disadvantages). As shown recently in a review of the biotechnological application of ultrahigh-temperature bacteria by this group,16 hyperthermophiles with temperature stabilities of over 113 °C (and rising) have been discovered. This means that there is a converging of the temperature optima and concentration range between biological and inorganic catalytic systems, if only partial catalytic oxidation is required. Thus, capital and running costs can be kept to a minimum. It is proposed that there is a likely balance between the partial catalytic combustion and biofiltration in a single system either for balance of the feed spikes or for transfer of low biodegradable compounds into higher biodegradable partial oxidation intermediates. Further sensitivity analysis on this model system is the subject of our future work. Acknowledgment P.C.W. and M.T.B. thank the U.K.’s Biotechnology and Biological Sciences Research Council (BBSRC) for financial support (Grant 97/E11124). Heriot-Watt’s Ph.D. Scholarship’s program is also acknowledged.

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Thanks also goes to Sutcliffe Croftshaw Ltd. {Waterlink} (Lancashire, England) for technical advice on biofiltraion. Literature Cited (1) Groenestijn, J. W. V.; Hesselink, P. G. M. Biotechniques for Air Pollution Control. Biodegradation 1993, 4, 283-301. (2) Devinny, J. S.; Deshusses, M. A.; Webster, T. S. Biofiltration for Air Pollution Control; Lewis Publishers: Boca Raton, FL, 1999. (3) Heinsohn, R. J.; Kabel, R. L. Sources and Control of Air Pollution; Prentice Hall, Inc.: Upper Saddle River, NJ, 1999. (4) Busca, G.; Baldi, M.; Pistarino, C.; et al. Evaluation of V2O5-WO3-TiO2 and alternative SCR catalysts in the abatement of VOCs. Catal. Today 1999, 53, 525-533. (5) Parida, K. M.; Samal, A. Catalytic Combustion of Volatile Organic Compounds on Indian Ocean Manganese Nodules. Appl. Catal. A 1999, 182, 249-256. (6) Baldi, M.; Finocchio, E.; Fabio Milella, G. B. Catalytic Combustion of C3 hydrocarbons and Oxygenates over Mn3O4. Appl. Catal. B 1998, 16, 43-51. (7) Al-Shihry, S. S.; Halawy, S. A. Unsupported MoO3-Fe2O3 Catalysts: Characterization and Activity during 2-Propanol Decomposition. J. Mol. Catal. A 1996, 113, 479-487. (8) Gil, A.; Ruiz, P.; Delmon, B. Effect of the Support and Added Oxides on the Bistability Observed in the Oxidative Dehydrogenation of 2-Propanol over Copper-Supported Catalysts. Catal. Today 1998, 32, 185-191. (9) Harris, J. W. Hydrocarbon Production Handbook. Hydrocarbon Process. 1991, 64, 154. (10) Derwent, R. G.; Pearson, J. K. Improving Air Quality in the United Kingdom-the Solvents Contribution. Environ. Technol. 1997, 18, 1029-1039.

(11) Yue-xiang, H.; Cun-ji, G. Synthesis of Nanosized Zirconia Particles via Urea Hydrolysis. Powder Technol. 1992, 72, 101104. (12) Bustard, M. T.; Meeyoo, V.; Wright, P. C. Biodegradation of Isopropanol in a Three Phase Fixed Bed Bioreactor: Start Up and Acclimation Using a Previously-Enriched Microbial Culture. Environ. Technol. 2000, submitted for publication. (13) Bustard, M. T.; Meeyoo, V.; Wright, P. C. Biodegradation of High Concentration Isopropanol Vapour in a Biofilter Inoculated with a Solvent-Tolerant Microbial Consortium. Trans. Inst. Chem. Eng., Part C 2001, 79, 129-135. (14) Clark, D. D.; Ensign, S. A. Evidence for an Inducible Nucleotide-Dependent Acetone Carboxylase in Rhodococcus rhodochrous B276. J. Bacteriol. 1999, 181, 2752. (15) Sluis, M. K.; Ensign, S. A. Purification and Chracterization of Acetone Carboxylase from Xanthobacter Strain Py2. Proc. Natl. Acad. Sci. 1997, 8456-8461. (16) Bustard, M. T.; Burgess, J. G.; Meeyoo, V.; Wright, P. C. Novel Opportunities for Marine Hyperthermophiles in Emerging Biotechnology and Engineering Industries. J. Chem. Technol. Biotechnol. 2000, 75, 1095-1109. (17) Bohn, H. Consider Biofltration for Decontaminating Gases. Chem. Eng. Prog. 1992, 34. (18) Wright, P. C. An Examination of Liquid-Film Three-Phase Fludization under Conditions Typical in Biological Systems. Ph.D. Dissertation, University of New South Wales, New South Wales, Australia, 1997.

Resubmitted for review August 2, 2001 Revised manuscript received August 20, 2001 Accepted August 20, 2001 IE001108Q