Environ. Sci. Technol. 1996, 30, 3435-3440
Membrane-Based Integrated Absorption-Oxidation Reactor for Destroying VOCs in Air PURUSHOTTAM V. SHANBHAG, ASIM K. GUHA,† AND KAMALESH K. SIRKAR* Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102
This proof-of-concept research describes a novel membrane-based, integrated absorber-reactor operating at ambient temperature and atmospheric pressure. It degrades volatile organic compounds (VOCs) in a gaseous stream by ozonation in an inert stagnant fluorocarbon (FC) phase having a high ozone solubility. This FC phase acts as a reaction medium and a liquid membrane. The reactor has two sets of nonporous silicone capillaries. The VOC-containing gas flows through one set and supplies the VOC to the FC phase. The O3-O2 stream flowing through the other supplies O3 to the FC phase. There is also a set of microporous Teflon tubules through which water flows removing oxidation products partitioning from the FC phase. With trichloroethylene (TCE), 60% conversion was obtained for 50 000 ppmv TCE in N2 flowing at 30 cm3/min and 40% for 18 000 ppmv flowing at 50 cm3/min. For 220 ppmv TCE feed, 90% conversion was obtained at 20 cm3/min flow rate and 60% at 60 cm3/min. A conversion in excess of 97% was achieved for a toluene feed of 205 ppmv present in N2 flowing at 11 cm3/min. No FC phase regeneration is required; it is constantly cleaned by the ozonation reactions. The materials of construction were found to hold up well under repeated experimentation.
Introduction Site remediation frequently involves the removal of toxic organic compounds from soil via vaporization; the volatile organic compounds (VOCs) thus removed into the gas phase, typically air, can be treated by a variety of methods including incineration, catalytic oxidation, carbon adsorption, and more recently biofiltration. These methods have some serious drawbacks, which limit their use in site remediation. Incineration. Since the gas stream is relatively dilute in VOC concentrations and significantly humid, incineration * Author to whom all correspondence should be addressed. Telephone: 201-596-8447; fax: 201-642-4854; e-mail: sirkar@admin. njit.edu. † Present address: Middlesex County Health Dept., Air Pollution Program, 928 Livingston Ave., North Brunswick, NJ 08902.
S0013-936X(95)00916-3 CCC: $12.00
1996 American Chemical Society
will be expensive and unattractive, requiring supplemental fuel firing. The possibility of dioxin formation from chlorinated hydrocarbons is an added concern. The combustion products require secondary control devices. Catalytic Oxidation. Catalytic oxidation is a comparatively new technology that operates at above ambient temperatures principally to “light-off” the catalyst to oxidize the pollutants to CO2 and H2O and also HCl in the case of chlorinated hydrocarbons. Since conventional Pt- and Pdbased catalysts are inhibited by Cl2, catalysts incorporating V2O5 and Cr2O3 are being developed that operate at temperatures above 550 °C (1). Carbon Adsorption. Carbon adsorption requires pretreatment of the VOC-containing gas stream to reduce the moisture content, as relative humidities in excess of 40% significantly reduce the bed capacity (2). Carbon adsorption merely concentrates the VOCs; further treatment involves either incineration of the spent carbon or regeneration of the spent carbon bed, which transfers the VOCs to another medium. The VOCs derived from site remediation are often mixtures, and recovery and reuse as solvents is neither sensible nor cost effective. Biofiltration. As a method of treating VOCs in the gas phase, biofiltration is as yet unable to handle BTX, halogenated hydrocarbons, and other priority pollutants effectively (3); it also lacks the convenience of on-site pollutant handling capabilities of other methods. Biofilters are also not amenable to concentration spikes. There is no simple destructive method available to handle a broad array of dilute VOCs in gases obtained from contaminated soils. We describe here a novel high efficiency integrated membrane-based absorption-reaction device to remove VOCs from a gas phase. The VOCs are absorbed via silicone capillary membranes into a stagnant, inert reaction medium; this medium is simultaneously loaded with ozone via another set of silicone capillary membranes to oxidize the VOCs. The inert reaction medium used in this study is a low vapor pressure fluorocarbon (FC). It has a high solubility for ozone (14 times that in water) and VOCs. It is also not implicated in upper atmospheric depletion of ozone and is being used extensively in the semiconductor industry. Furthermore, the silicone capillary membranes virtually eliminate any loss of FC vapor into the two gas streams. An aqueous stream provided through a set of microporous Teflon tubules removes organic and inorganic oxidative degradation products from the inert reaction medium [e.g., HCl for chlorinated hydrocarbons like trichloroethylene (TCE)] (Figure 1). The microporous Teflon tubules are wetted by the fluorocarbon (FC) phase. The aqueous organic interface where partitioning occurs is on the tubule internal diameter. The aqueous phase flowing through the Teflon capillaries can also be dosed with secondary oxidizers like H2O2 to destroy any recalcitrant VOCs in synergy with ozone. Moreover, ozonation of organic pollutants or intermediate oxidation products partitioning into the aqueous phase can be enhanced by the presence of reactive free radicals, especially the hydroxyl radical. The hydroxyl radical is an extremely powerful oxidizing agent and is able to handle a fairly broad range of organic pollutants (hydroxyl radical concentrations in aqueous phase have values in the range
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FIGURE 1. Schematic of integrated absorption-oxidation of VOCs in air.
of 10-10 and 10-12 M and rate constants with organic molecules are in the range of 10+8-10+10 M-1 s-1). The presence of OH radicals in conjunction with ozone and H2O2 on organic pollutant degradation in aqueous media has been quite extensively covered in literature under the nomer of advanced oxidation processes (4). The integrated absorption-oxidation system for VOCs in gas phase is novel. It has three flowing streams and uses three fiber sets. It exploits the use of a FC phase as a reaction medium initiated in the aqueous-FC ozonation studies (58). However, unlike aqueous-FC systems systems with the pollutant partitioning between water and FC phase, this system has pollutant being absorbed by FC phase from a gas. Most of the reaction takes place in the FC phase on the gas sides rather than on the FC-aqueous interface as found in earlier studies. Although aqueous-FC ozonation studies (7, 8) showed successful destruction of pollutants like phenol, toluene, nitrobenzene, TCE, and acrylonitrile, the aqueous phase in this study plays a secondary role, i.e., to remove O3-destroyed products. The effective kinetics of degradation of the pollutants in the present system is likely to be quite different from those found in earlier studies where water played a crucial role in the destruction process. The inert FC-based reaction medium concentrates the VOCs and ozone, thereby enhancing the oxidation rates of VOCs. The stagnant inert FC phase is constantly regenerated as the breakdown of VOCs proceed, and the reaction products are continuously stripped away by extraction into the aqueous phase and the gaseous products are removed by the gas phase. Moreover, ozonation of double bonds in an inert reaction medium generates epoxides that degrade instantaneously in the presence of the aqueous phase (9). This solves any secondary disposal problems of any recovered VOC or partially oxidized products, because no products aside from organic acids or mineral acids are formed. The disposal of such species requires just neutralization, and then the aqueous stream can be discharged or recycled (Figure 2). This system is amenable to concentration spikes, since the VOCs are being concentrated in the inert reaction medium. Scaling up of such a process requires additional membrane modules in parallel to provide the required membrane area to treat the VOCs. By employing nonporous silicone capillaries and microporous Teflon hollow fibers (or tubules), the reactor materials have been made inert to the harsh oxidizing environment and have been found to remain unaffected after several hundreds of hours in use. Conventional gas absorption towers or plate columns provide a gas-liquid interfacial area/per unit volume (a)
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around 0.1-4 cm-1. Hollow fiber membrane-based gas absorbers have a values as high as 30-90 cm-1 (10, 11). Therefore, highly compact membrane reactors, no more than 1-3 ft high, are possible. If conventional bubbling were employed, considerable FC phase loss would be incurred by FC volatilization. FC fluid is quite costly, and this loss would be economically unacceptable. One can also argue why not mix the two gas streams together and send it to the reactor and use only one set of silicone capillary membranes for the gas phase. This eliminates the possibility of recycling the O2 stream depleted of ozone back to the ozonator. Further recycling of the O2 stream is desired for economic efficiency. Although air can also be used, the O3 concentration would be reduced by a factor of 2.5-3.0, leading to a much bigger reactor. The process proposed here is general enough to be applicable to almost all VOCs. The device is compact and modular, easily skidmounted, and usable at contaminated sites. Only an ozonator, a metering pump, and an electric power supply are required.
Experimental Details The physical properties of the fluorocarbon solvent FC 43 (Fluorinert, 3M, St. Paul, MN) are as follows: molecular weight, 670; density, 1.8 g/mL; boiling point, 174 °C; vapor pressure, 1.3 mmHg at 25 °C; kinematic viscosity, 2.8 cP; surface tension, 16 dyn/cm; and solubility of 7 ppm (by wt) into water (12). The geometrical details of the membrane reactor are provided in Table 1. The reactor consisted of three sets of membrane tubules. Microporous Teflon tubules (Impra, Tempe, AZ) were used as one set. The other two sets were comprised of nonporous silicone capillaries (Dow Corning, Midland, MI). Module fabrication consisted of preparation of the fiber bundles and putting them into a transparent FEP shell (0.61 cm i.d., 1.03 cm o.d., Cole Parmar, Chicago, IL) with polypropylene barbed hose connecting crosses (Cole Parmar, Chicago, IL). The tubules were potted using Armstrong epoxies (Beacon Chemical Co., Mt. Vernon, NY) C4-D for the internal tube sheet and A2-A for the external tube sheet. After a 7-day cure, the reactor was tested for leaks, dried, and then used for the experiments. Ozone was generated by a Polymetrics (Colorado Springs, CO) ozonator, using a pure O2 gas cylinder (Matheson, East Rutherford, NJ). The ozonator was operated at a voltage setting of 90 V; the pressure within the ozonator was kept at 9 psig, and the gas was passed through the ozonator at a constant flow rate of 0.6 L(STP)/min. A small portion of the ozonated gas was diverted to the silicone capillaries for reaction purposes; the major portion of the ozonated oxygen was vented after passing through two KI (2% concentration) washbottles linked in series to break down any ozone and then a sodium thiosulfate washbottle also connected in series to break down any entrained iodine. A constant steady stream of VOC was supplied to the absorber-reactor in two ways. Lower concentrations (220 ppmv for TCE and 205 ppmv for toluene) were obtained from a certified standard VOC-N2 gas mixture (Matheson, East Rutherford, NJ). Higher concentrations were obtained by bubbling nitrogen through a pure liquid VOC sample. The concentration obtained would be determined by the VOC vapor pressure at the ambient temperature and pressure (the pressure was kept as close to atmospheric as possible) and the contacting efficiency. To obtain different VOC concentrations, the VOC feed stream was mixed with
FIGURE 2. Integrated absorption-oxidation reactor configuration for the oxidative degradation of VOCs in air. TABLE 1
Details of Integrated Absorption-Reaction Membrane Ozonator first fiber seta
a
module no.
active length (cm)
1
20.3
Nonporous silicone tubules.
b
second fiber seta
third fiber setb
total no.
i.d./o.d. (µm)
total no.
i.d./o.d. (µm)
total no.
i.d./o.d. (µm)
25
305/610
25
305/610
5
990/2280
Teflon tubules.
a makeup stream of nitrogen (extra dry) (Matheson, East Rutherford, NJ). The gas flow rates were controlled by a pair (one for the VOC feed and the other for the N2 makeup) of diaphragm flow controllers (J&W Scientific, Baxter, Edison, NJ). To calibrate the gas chromatograph (GC) for higher concentrations, two Matheson mass flow controllers (Matheson, East Rutherford, NJ) were used, one to generate a vapor stream and the other to control a diluent N2 stream. The vapor stream was diluted such that the peak areas thus obtained were similar to those obtained when the certified standard gas mixture was injected into the GC and the concentration of the pure vapor stream was determined by back calculation, knowing the individual gas stream flow rates. Ozonation studies were carried out in a reaction loop shown in Figure 3. The reactor-based setup consisted of four major sections all connected to the membrane reactor: (1) an ozonator to supply ozone, (2) a FC-liquid reservoir, (3) a VOC source, and (4) an aqueous phase recirculation unit. The flow rate of the O3/O2 gas stream, supplied to one set of silicone capillaries, was controlled by means of a Teflon needle valve (Cole Parmar, Chicago,
IL) placed at the outlet of the module while its pressure was kept as close to atmospheric as possible. The spent gas stream was passed through a KI wash and then through a soap bubble flowmeter and finally to the exhaust hood. The VOC-laden stream was admitted into the second set of silicone capillaries; its concentration and flow rate were adjusted by the diaphragm flow controllers. The spent VOC stream was sent to the GC to measure the exit concentration. The pressure on the VOC side was maintained close to atmospheric. The aqueous stream was recirculated through the Teflon tubules by means of a Masterflex Pump (Curtin Matheson Scientific, Morris Plains, NJ). The FC phase was admitted into the shell side of the module. VOC-N2 flow rates were measured at 1 atm and 27-30 °C ambient temperature by means of a bubble flowmeter. The aqueous phase was analyzed for the pollutant by a high-performance liquid chromatograph (HPLC, HP 1090A, Hewlett Packard, Paramus, NJ) equipped with a Micromeritics autosampler connected to a Valco valve having a 10µL sample loop. A 10 cm long and 3 mm diameter Hypersil ODS glass column (Chrompack, Bridgewater, NJ) and a UV
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FIGURE 3. Experimental loop of the integrated absorption-oxidation reactor to remove VOCs from air. TABLE 2
Performance of the Integrated Absorption-Reaction Membrane Ozonatora run no.
aqueous flow rate (mL/min)
O3/O2 flow rate (mL/min)b
VOC-N2 flow rate (mL/min)
aqueous TCE concn (ppm)
VOC feed concn (ppmv)c
VOC exit concn (ppmv)
pH of aqueous phase initial final
1 2 3
4.2 3.8 6.6
27 54 29
32 34 50
97 70 81
51 350 50 320 31 860
20 625 17 425 18 210
6.13 6.13 4.99
2.71 2.74 2.95
a Pollutant: trichloroethylene (TCE). FC43 used as shell side liquid. b Gas flow rates measured at 1 atm pressure and 27-30 °C ambient temperature. For the VOC feed, nitrogen (extra dry) was bubbled through pure TCE and blended with a second stream of nitrogen (extra dry) to give the desired feed concentration.
c
filter photometric detector set at 210 nm wavelength were used to analyze the pollutants. Operating HPLC flow conditions set to detect the VOCs in the aqueous phase involved a mobile phase containing 60% acetonitrile and 40% water flowing at 0.4 mL/min. VOCs in the gas phase were analyzed using a GC (Varian 3400, Varian Associates, Sugarland, TX) equipped with a flame ionization detector (FID). A Carbopack C 80/100 column (0.3% Carbowax 20M), (AllTech, Deerfield IL) was used to analyze the pollutants. Gas phase was sampled by an automatic sampling valve. The column temperature was set at 150 °C, while the injector and detector were set at 220 and 250 °C, respectively. Aqueous phase samples were injected into the HPLC to
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observe any degradation products. The pH of the aqueous phase before and after the experiment was measured by a Corning Model pH meter 140 (Corning, NY).
Results and Discussion The experimental results of the membrane reactor degradation studies for high and low inlet concentrations of TCE are shown in Table 2 and Figure 4, respectively. Each result shown in Table 2 represents the steady-state performance recorded at the end of an 8-h period. The removal of TCE from the gas phase by reaction in the FC phase is subject to five major transport resistances: the two boundary layer resistances for each of the gas phases, the two
FIGURE 4. Degradation of TCE in the membrane reactor at different ozone flow rates. FC used: FC 43; average aqueous flow rate ) 3.1 mL/min. (4) VOC-N2 flow rate ) 22 cm3/min; feed TCE level ) 220 ppm. (0, 9) VOC-N2 flow rate ) 34 cm3/min; feed TCE level ) 220 ppm. (O) VOC-N2 flow rate ) 58 cm3/min; feed TCE level ) 193 ppm.
silicone membrane resistances, and finally the FC membrane resistance. The residence time of the VOC stream was found to have a major role in determining the TCE conversion defined as follows:
conversion, XA )
C Ain - C Aout C Ain
(1)
Between runs 1 and 2 and run 3, the TCE-N2 flow rate was almost doubled (the residence time consequently was almost halved) causing the TCE conversion to fall from a value of 60% to a value of 40%. However, for a given flow rate of a VOC-containing gas, higher conversions would require larger membrane surface areas or longer residence times or both. The substantial change in the pH of the aqueous phase over the duration of each run indicated that some of the TCE had been mineralized. Also it was found that at the end of the experiment, some TCE had broken through the FC-phase-based liquid membrane and dissolved into the aqueous phase. The conversion of TCE for runs 1 and 2 was in the range of 60-65%, which clearly shows that, in the flow rate range under consideration, the stripping of TCE into the ozone-containing gas phase did not play any major role in the removal of TCE from the TCE-containing feed gas phase. Since TCE had broken into the aqueous phase, it is clear that the reactor was being operated in the ozone-limited reaction regime. The runs shown in Table 2 represent the upper limit of the operational capability of this particular device. Continuous operation for reasonable lengths of time at these high feed concentrations also demonstrates, unequivocally, that this device can handle high transient concentrations. The results of studies carried out at lower TCE feed concentrations are shown in Figure 4. At the lowest feed flow rate of about 22 mL/min, the steady-state conversions were observed to be of the order 90%. No observable evidence of TCE was found in the aqueous phase. The TCE-N2 residence times ranged from 1 s for the lowest flow rate of 22 mL/min to 0.38 s for the highest flow rate of 58 mL/min. The aqueous phase recirculation rate was set at an average value of 3.1 mL/min for all experiments. From Figure 4, it is clear that the O3/O2 gas phase boundary layer
FIGURE 5. Degradation of toluene in the membrane reactor at different ozone flow rates. FC used: FC 43; average aqueous flow rate ) 3.2 mL/min. (O) VOC-N2 flow rate ) 11 cm3/min; feed toluene level ) 205 ppm. (0) VOC-N2 flow rate ) 25 cm3/min; feed toluene level ) 205 ppm. (9) VOC-N2 flow rate ) 23 cm3/min; feed toluene level ) 205 ppm, no ozone.
does not control the removal of TCE; there is virtually no observable stripping of TCE into the O3/O2 phase since there was no change in conversion despite a 4-fold increase in the ozone flow rate. The pH change in these experiments was much lower since the TCE loading levels in these experiments were much lower. The experimental results for toluene as a model VOC are shown in Figure 5. Here again it is observed that at low VOC flow rates (high residence time) the removal of toluene is in excess of 97%. At a higher VOC flow rate, however, it is found that as the O3/O2 flow rate falls, the conversion starts to fall. This may be explained in one of two ways: the observed fall in conversion of toluene is a result of lowering of the ozone dosage rate or a result of reduced stripping of toluene from the FC phase into the O3/O2 gas phase. In order to ascertain that the toluene is being removed by reaction with ozone and not by stripping, the system was run in an identical manner with one major difference, the ozonator was not switched on, so no ozone was supplied to the reactor. Under these circumstances, it is found that the observed toluene conversion is around 25%, less than half of that observed when ozone is supplied to the reactor. This indicates that the toluene is indeed being removed by reaction and not because it is being stripped into the O3/O2 gas phase. Also it must be noted here that during the actual ozonation process, due to depletion of the VOC in the FC phase by reaction, there will be re-absorption of some of the VOC that had been stripped into the O3/O2 gas phase. Furthermore, by providing an adequate membrane area, there is ample opportunity to dose an adequate amount of O3, cause some stripping of the VOC into the O3/O2 gas phase, and yet have substantial area to effectively re-absorb any VOC from the O3/O2 gas stream leaving the reactor. During these experiments, no trace of the inert fluorocarbon was detected in the GC. This indicated that the silicone capillary wall was an effective barrier to the permeation and stripping of the high molecular weight fluorocarbon into the gas phase. The reactor was exposed to the harsh oxidizing conditions for a total period of 90 h without any visible degradation in the materials of construction. Earlier studies (7, 8) have indicated that such membrane devices have
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been able to perform consistently over longer periods of time. Stability for times greater than 300-400 h is not known at this time. This study of the integrated absorption-oxidation process demonstrates, however, its efficacy as a device to handle VOCs, viz., TCE and toluene in air. The ability of the device to handle both high and low concentrations places it between carbon adsorption (a final cleanup method) and incineration (a method of effectively handling concentrations capable of sustaining combustion). By employing a compact membrane-based ambient ozonation-destruction, costly secondary incineration requirements of dilute air streams are avoided. The device scaleup is fairly straightforward; its capacity is directly related to the membrane area provided. The presence of an aqueous phase may inhibit the formation of hazardous byproducts, e.g., phosgene, which may be formed during the oxidation of TCE, rapidly hydrolyzes in the presence of water yielding two chloride ions (13). The FC phase used is a benign perfluorinated product, whose utility as a blood substitute is being currently researched (14). Since the FC phase is a perfluorinated product, i.e., all the carbon side chains are occupied by fluorine atoms, the fluorocarbon should have an indefinite shelf life, but this supposition needs to be put to test. During the experiments conducted for the lower concentrations of TCE and toluene as feed, the FC phase was continuously recycled without any visible degradation of FC or any deterioration of reactor performance. The utility of this device becomes obvious when the device and the accessories that accompany it, viz., an ozonator with an air/O2 supply and a water recirculation pump, are put on a skid. A few questions need to be answered, for example, the types of byproducts formed (beside HCl). Also the presence of multiple pollutants that would compete for ozone molecules represents a viable direction for future study. The use of secondary oxidizing
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agents in the aqueous phase, viz., H2O2, to improve existing TCE conversions has also to be investigated.
Acknowledgments Partial funding for this project came from the Hazardous Substance Management Research Center (HSMRC) at the New Jersey Institute of Technology (NJIT). The authors would like to thank John Ruffing of 3M for the FC liquids and Anthony Green of IMPRA/ IPE for the Teflon tubules. The authors thank Sudipto Majumdar for his help in setting up the VOC-N2 gas loop.
Literature Cited (1) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control: Commercial Technology; Van Nostrand Reinhold: New York, 1995. (2) Graham, J. R.; Ramaratnam, M. Chem. Eng. 1993, 2, 6. (3) Bohn, H. Chem. Eng. Prog. 1992, 4, 34. (4) Glaze, W. H.; Kang, J. W. Ind. Eng. Chem. Res. 1989, 28, 1573. (5) Stich, F. A.; Bhattacharyya, D. Environ. Prog. 1986, 6, 224. (6) Bhattacharyya, D.; Van Dierdonck, T. F.; West, S. D.; Freshour, A. R., J. Hazard Mater. 1995, 41, 73. (7) A Novel Membrane Reactor for Oxidative Degradation of Hazardous Organic Wastes. Final Project Report BICM-27 & BICM-34; Hazardous Substance Management Research Center, NJIT: Newark, NJ, 1994. (8) Guha, A. K.; Shanbhag, P. V.; Sirkar, K. K.; Vaccari, D. A.; Trivedi, D. H. AIChE J. 1995, 41, 1998. (9) Bailey, P. S. Ozonation in Organic Chemistry Academic Press: New York, 1982. (10) Qi, Z.; Cussler, E. L. J. Membr. Sci. 1985, 23, 321. (11) Karoor, S.; Sirkar, K. K. Ind. Eng. Chem. Res. 1993, 32, 674. (12) Product Manual 3M Fluorinert Electronic Liquids, 1989. (13) Masten, S. J; Hoigne, J. Ozone Sci. Eng. 1992, 14, 197. (14) Yamanouchi, K.; Heldebrant, K. Chemtech 1992, 6, 354.
Received for review December 5, 1995. Revised manuscript received May 30, 1996. Accepted September 9, 1996.X ES950916J X
Abstract published in Advance ACS Abstracts, October 1, 1996.
