Ind. Eng. Chem. Res. 1997, 36, 4557-4566
4557
Combustion of Volatile Organic Compounds over Platinum-Based Catalytic Membranes M. P. Pina, S. Irusta, M. Mene´ ndez, and J. Santamarı´a* Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain
R. Hughes and N. Boag Chemical Engineering Unit, Salford University, Salford M5 4WT, England
A study of the deep catalytic oxidation of volatile organic compounds (VOCs) contained in air streams on catalytic membrane tubes is presented. The complete combustion of toluene and methyl ethyl ketone as single components and in binary mixtures, with concentrations between 100 and 7000 ppm, was achieved by permeating the VOC-containing stream through a Pt/γAl2O3 catalytic membrane operating under the Knudsen diffusion regime. The membrane combustor performed efficiently, allowing total VOC destruction at low temperatures. In addition, substantial changes in the gas feedrate were accommodated, with only a small variation in the combustion temperatures required. A discussion is presented regarding the ability of the flow-through membrane combustor to attain a lower mass-transfer resistance than a conventional catalytic reactor. Introduction The efficient removal of volatile organic compounds (VOCs) from sources such as waste streams, tank loading/unloading operations, inerting systems, fugitive emissions, etc., provides some of the most challenging problems to current pollution abatement technology. Many of the difficulties in handling VOC-containing streams stem from two circumstances: the first relates to the extensive variety of compounds falling within the VOCs category and the second to the wide range of conditions in which they are present. The VOCs category includes aliphatic and aromatic hydrocarbons, halogenated hydrocarbons, aldehydes, ketones, alcohols, glycols, ethers, epoxides, phenols, etc., all of which significantly contribute to industrial airborne emissions. The variation in the nature of existing VOC compounds and of their physical and chemical properties has given rise to a correspondingly wide range of VOC-removal methods, including thermal and catalytic incinerators, flares, adsorption and absorption processes, biofiltration, membrane separation, ultraviolet oxidizers, corona destruction, and plasma technology devices (Moretti and Mukhopadhyay, 1993). The second circumstance, the diversity of conditions under which VOCs must be processed, concerns variations of several orders of magnitude in concentration (a few ppm to percentages), release rates (VOCs are present in streams with magnitudes ranging from cm3/h for the case of fugitive emissions to m3/s when dealing with the pollution from a stack), and temperature (ambient temperature to several hundred degrees), as well as the presence of mixtures, often with significant synergistic effects. This diversity partly reflects the fact that VOCs are produced in a variety of industries, including the chemical industry, manufacture of plastics, food processing, and many other industrial operations handling paints, cleaning agents, adhesives, coatings, and solvents. However, also frequently found is a strong variation with time in the conditions of VOC release from the operations of a given plant. Thus, the * Corresponding author. Phone: +34 976 761153. Fax: +34 976 761159. Email:
[email protected]. S0888-5885(97)00087-0 CCC: $14.00
discharge of organic vapors during tank loading operations or during the venting of process vessels is a finiteduration, intrinsically-unsteady process. Given the above constraints, it seems clear that a VOC-removal method of broad applicability should be efficient, nonselective (i.e., maintains its efficiency when applied to different compounds), and flexible (capable of accommodating substantial changes in the feed flowrate and concentration). Among the factors driving the interest of technology providers to improve on existing VOC-removal methods is the introduction of recent legislation in the European Community that requires emission limits in the different Member States to be set based on the best available technology (European Communities, 1996). Stricter regulations covering the emission of VOCs and other pollutants linked to ozone generation will also be in place in the U.S.A. by the turn of the century, when the new proposed EPA standards set the limit for exposure to ground level ozone at 0.08 ppm measured over 8 h, rather than the current 0.12 ppm over 1 h (Saunders, 1997). Catalytic combustion is competitive with other technologies of VOCs abatement from air streams, due to its lower operating costs with respect to thermal combustion (Ruddy and Carroll, 1993) and to its flexibility compared to other means of VOCs removal (e.g., adsorption, absorption, condensation). Notwithstanding these advantages, the requirements placed upon catalytic systems are severe: in spite of the above-discussed variations in concentration, production rate, and nature of the organic compounds, the ideal catalytic VOCremoval system would attain high conversions (usually higher than 95%) on diluted streams, avoid partial oxidation products (which are often more toxic than the original VOC molecule), be resistant to deactivation by a variety of mechanisms (e.g., coking, sintering, and poisoning, including the action of non-VOC compounds such as sulfur, silicon, chlorine, and polymerizing materials), and operate with a sufficiently low pressure drop. One possibility to obtain an efficient contact between a lean, VOC-containing gas stream and a combustion catalyst would be to make the gas stream flow through the porous structure where the active phase is located © 1997 American Chemical Society
4558 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
rather than diffuse into it. Porous catalytic membranes are well suited for this purpose, as they have often been developed to allow penetration of reactants into a porous matrix containing the catalyst (e.g., Veldsink et al., 1992; Saracco et al., 1995; Herguido et al., 1995). The usual configuration has involved the use of a catalytic membrane where reaction takes place with reactants fed from opposite sides, while there are fewer examples of the use of a flow-through membrane where a premixed gas stream reacts. This was the approach employed in a series of works by Saracco et al. (Saracco and Specchia, 1995; Saracco et al., 1996), who used catalytically modified fly-ash filters for alcohol dehydration and for the reduction of nitrogen oxides with NH3. The concept was taken a step further in a previous work by Pina et al. (1996), where toluene combustion was attempted using a Pt/Al2O3 catalytic membrane operating in the Knudsen diffusion regime. Since in the Knudsen diffusion regime the probability of collisions between the molecules and the wall of the pores is maximized, this type of membrane was expected to give a considerably higher efficiency in the reaction of diluted streams such as those commonly encountered in VOC removal. Indeed, the preliminary results presented in the previous work showed that the membrane could perform efficiently in the combustion of VOCs at low temperatures. In this work, a broader study of this concept has been undertaken, using different membrane preparation methods and testing the membrane combustor with two different types of VOCs (representing aromatics and oxygenates, respectively), alone or in mixtures. A comparison of the same catalytic reactor operating as a flow-through membrane and as a conventional (monolith) reactor is also presented. Experimental Section Membrane Preparation and Characterization. The starting materials used in this work for membrane development were commercial tubular microfiltration membranes (SCT) with internal and external diameters of 7 and 10 mm, respectively. The membranes used had separation layers of a different nature on top of the R-Al2O3 support: γ-Al2O3 (200 nm pore diameter), γ-Al2O3 (5 nm pore diameter), or ZrO2 (5 nm pore diameter). A γ-Al2O3 phase was deposited inside the porous structure of this membrane by means of successive cycles, each involving vacuum filtration of a 1 M bohemite sol, followed by drying and calcination at 600 °C. A similar procedure was used in previous works (Coronas et al., 1994; Santos et al., l995), to deposit silica inside the R-Al2O3 structure. The tubes were then subjected to enamelling at both ends, in order to facilitate sealing and to obtain a well-defined permeation length. The total length of the membrane tubes ranged from 110 to 375 mm, depending on the particular membrane used, while the permeation length (which was coincident with the catalytically active length of the membrane) ranged from 35 to 290 mm. Two different methods were used to deposit catalytic material (Pt) on the membranes. The first involved wet impregnation with chloroplatinic acid, followed by drying and calcination. Impregnation was carried out after deposition of γ-Al2O3, by immersing the membranes in a chloroplatinic acid solution containing 0.5 g of Pt/L. The amount of Pt incorporated onto the membrane was routinely determined by UV-visible spectrometry of the solution before and after impregnation. The accuracy of this procedure to determine the Pt loading was tested
by elementary analysis of the membranes using plasma analysis (IPC) and found to be satisfactory. The Pt loading on the membranes used in this work was between 0.016 and 0.45 wt %, referred to the total weight of the active length of the membrane. It must be noted, however, that Pt is almost exclusively deposited on the γ-Al2O3 phase introduced rather than on the original R-Al2O3, which accounts for most of the membrane mass. The second method employed chemical vapor deposition (CVD) to load Pt on the membrane. This was achieved by evaporating an organic precursor (cyclopentadienylallylplatinum) under mild vacuum (rotary pump) and then permeating the vapors through the membrane, while maintaining its outer side under high vacuum (diffusion pump). The Pt-containing organic vapors diffused through the membrane, becoming adsorbed on the previously deposited γ-Al2O3 phase, where they reacted and decomposed. Although the CVD system could be heated to any desired deposition temperature, both the organic evaporation and the deposition sections where kept at room temperature to prepare the membranes used in this work. In this case the total amount of Pt deposited was calculated from the weight increase of the membrane after calcination to remove any remaining organic material. Both methods of membrane preparation employed were aimed at obtaining a Pt phase dispersed on a γ-Al2O3 support, with a pore structure capable of permeation fluxes in the Knudsen regime. It was therefore important to know the permeation characteristics of the membrane (permeation regime, permeation flux for a given pressure drop), the texture and surface area of the membranes before and after introduction of Pt, and the dispersion of the Pt deposited onto the membranes. To this end, the membranes were characterized by BET surface measurements, H2 chemisorption, mercury intrusion porosimetry, thermogravimetry (TG), scanning electron microscopy (SEM), and permeation measurements. The homogeneity of the Pt distribution along the membrane radius was studied using X-ray photoelectron spectroscopy (XPS) analysis. The measurements were performed on a SSI 301 spectrometer using focused monochromatic Al KR radiation. The analysis was carried out across several points of thin rings cut from different membrane samples. The procedure has been described in a previous work (Herguido et al., 1995). The Pt/Al atomic ratio was determined at radial positions 300 µm apart, which yielded five internal measurement points plus the inner and outer membrane surfaces. Each of the analysis points corresponds to a window with a diameter of irradiated area between 150 and 300 µm. Characterization of Pt/γ-Al2O3 Catalysts. In order to clarify the results obtained in the combustion of mixtures of VOCs, some additional catalyst characterization data were needed. To obtain a greater sensitivity, the characterization measurements were carried out using Pt/γ-Al2O3 catalyst particles (rather than a catalytic membrane, where the sample would be diluted by the large proportion of R-Al2O3 phase present), with a high (3%) Pt loading. On these samples, temperatureprogrammed reduction (TPR) and diffuse reflectance infrared Fourier transform (DRIFT) measurements were undertaken. TPR experiments were carried out on a quartz reactor loaded with 100 mg of catalyst, under a mass-flow-
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4559
Figure 1. Experimental setup.
controlled stream containing 5% H2 in N2, with a heating rate of 5 °C/min, from room temperature to 400 °C. DRIFT data were obtained using a Mattson Research Series spectrometer, equipped with a SpectraTech high-pressure/temperature environmental chamber and a nitrogen-cooled MCT detector. Approximately 30 mg of Pt/γ-Al2O3 catalyst was loaded into the DRIFT cell, to which the VOC-containing gas stream was fed using a system analogous to that described below for the reaction experiments. Catalyst reduction/oxidation treatments as well as reaction experiments were carried out in situ. DRIFT spectra of the catalyst samples under different atmospheres (inert, air, reaction) were recorded by co-adding 300 scans at a resolution of 4 cm-1. Positions, number of bands, and intensities were established from the second-derivative spectra. Reaction System. The reaction system employed is sketched in Figure 1, consisting of the following: (i) A feed section in which a mass-flow-controlled stream (air) was saturated with the selected organic compound and mixed with a second mass-flow-controlled air stream to give the desired VOC concentration (from 100 to 7000 ppm). Saturation was achieved by bubbling the air stream through a series of three flasks containing the VOC, the first at room temperature and the other two immersed in an ice bath. A sintered-glass frit was used to disperse the air in very small bubbles throughout the liquid VOC. Both toluene and methyl ethyl ketone (MEK) were used as representatives of aromatic and oxygenated VOCs, respectively. The volatile organic compounds could be fed to the reactor alone or in a binary mixture, in which case a duplicate saturation system was used, as shown in Figure 1. The feed system also allowed in situ reduction pretreatment of the catalytic membrane, which was carried out in H2 for 2 h at 400 °C. (ii) A reaction section, with a membrane reactor unit containing the catalytic mem-
brane within an outer steel shell. The arrangement was similar to that described in recent works (e.g., Coronas et al., 1995). Unlike previous works, however, in this case the feed mixture entered the membrane tube side and then permeated outward across the membrane wall. Up to seven temperature measuring points were used in the membrane unit: three located axially on the tube side and four in contact with the external surface of the membrane. The maximum temperature difference observed between any two positions during the experiments reported in this work was 20 °C, although temperature differences were usually much smaller, around 10 °C. The temperatures reported below are calculated as the average of the readings of the four thermocouples in contact with the membrane wall. (iii) An analysis section, comprising an on-line gas chromatograph with a FID detector, which was used to analyze the reactor feed and exit streams, and on-line CO and CO2 analyzers for the exit stream. Carbon balances were usually in the 90-110% range, depending on the concentrations used. Partial oxidation (oxygenates) products were not observed with the VOCs used in this work. Results and Discussion Membrane Characterization. The nomenclature and the main characteristics (γ-Al2O3 loading, Pt loading and dispersion, preparation method) of the membranes used in this work are given in Table 1. It can be seen that the highest values of Pt dispersion attained with the impregnation method are around 30% (values for CVD membranes are not available). This relatively low dispersion is due to the fact that, although the overall Pt loadings on the membrane are low, always below 0.45% and usually under 0.2%, the impregnated Pt
4560 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Nomenclature and the Main Characteristics of the Membranes Used in This Work membrane
type of sepn. layer
pore diameter (sepn. layer) (nm)
wt % γ-Al2O3 loading
wt % Pt
preparation method
CVD1 CVD2 CVD3 AB1 AB2 AB3 AA0 AA1 AA2 AA3 AA4 ZA1 ZA4 ZA2 ZA5 ZA3 ZA6
γ-Al2O3 γ-Al2O3 γ-Al2O3 γ-Al2O3 γ-Al2O3 γ-Al2O3 R-Al2O3 R-Al2O3 R-Al2O3 R-Al2O3 R-Al2O3 ZrO2 ZrO2 ZrO2 ZrO2 ZrO2 ZrO2
5 5 5 5 5 5 200 200 200 200 200 20 20 20 20 20 20
0 2.4 3.2 0 1.7 2.5 0 2.15 2.3 4.7 4.9 2.2 2.5 4.3 4.64 5.7 6
0.05 0.13 0.19 0.02 0.09 0.17 0 0 0 0.13 0.13 0 0.25 0 0.25 0 0.45
CVD CVD CVD impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation impregnation
Pt dispersion (%)
32
27 15 35 14
Table 2. Permeation Characteristics of Some Membranes Used in This Work wt % γ-Al2O3 R (Ncm3 N2/ β (Ncm3 N2/ % Knudsen membrane loading cm2‚min‚bar) cm2‚min‚bar2) contribution AB0 AB1 AB2 AB3 AA0 AA1 AA2 AA3 AA4 ZA1 ZA2 ZA3 Figure 2. Distribution of Pt across the membrane radius, from XPS measurements.
concentrates on the γ-Al2O3 phase, a small fraction of the total membrane weight, which tends to lower Pt dispersion. The distribution of Pt (Pt/Al ratio) across the membrane radius was obtained for five membranes, AB1, AB2, ZA4, ZA5, and ZA6, using XPS analysis. Radial measurements at different axial positions and also at the same axial position with a different angular direction were undertaken to assess the homogeneity of Pt deposition in the membrane. These measurements showed a good homogeneity; i.e., the variation of the Pt concentration on a membrane prepared as described above depends only on the radial coordinate. The results of XPS analysis in the radial direction are shown in Figure 2. It can be seen that, for a given membrane, the superficial concentration of Pt is relatively homogeneous at the inner membrane positions, while higher values are obtained for both membrane surfaces. Since impregnation was carried out by immersing the whole membrane into the Pt-containing solution, the higher Pt concentration at the inner and outer surfaces can be explained as a result of the evaporation of the liquid film that forms on both sides of the membrane when drawn out of the Pt-containing solution. Given the type of work undertaken, it was also important to know not only the load and distribution of Pt deposited on the membrane but also the membrane permeation characteristics, which were obtained in separate permeation experiments. A linear relationship exists between the normalized permeation flux and the average pressure in the membrane (Lin and Burggraaf,
0 0 1.7 2.5 0 2.15 2.3 4.7 4.9 2.2 4.3 5.7
47 131.11 81.29 63.70 205.67 60.14 95.36 34.11 42.38 88.07 35.19 20.81
0 2.33 2.65 0.49 706.73 128.63 61.97 7.2 0.94 58.67 6.14 5.48
100 98.2 96.8 99.3 22.5 31.9 60.6 82.6 97.8 60.0 85.1 79.4
1991):
F ) 1.06
r2 r + 0.125 P ) R + βPav LτµRT av LτxMRT
where F is the permeation flux normalized per unit of time, area, and pressure difference (mol/m2‚s‚bar), Pav is the average pressure across the membrane (bar), and L, , τ, and r are respectively the membrane thickness, porosity, tortuosity, and pore radius. R and β indicate the Knudsen and laminar contributions to the permeation flux. A good linear fit of F vs Pav was obtained with all the membranes tested in this work. The results are given in Table 2 for some selected membranes. It can be seen that the percentage of Knudsen contribution to the total permeation flux strongly depends on the amount of γ-Al2O3 incorporated into the membrane pore structure. A minimum loading of 2 wt % is required to obtain a preponderance of Knudsen flow with 200 nm separation layer membranes, although this also depends on the thermal treatment of the membranes during preparation and on the distribution of the γ-Al2O3 filling. Thus, membrane AA1 in Table 1 gives a predominantly laminar flow, in spite of having a γ-Al2O3 loading over 2%. An exception to this rule is the microfiltration membrane with a 5 nm γ-Al2O3 separation layer, where Knudsen flow is achieved from the start. However, the thin separation layer does not withstand well the treatment at high temperatures, which is necessary for enamelling the membranes. The comparison between membranes AB0 and AB1 in Table 2 (the only difference is that membrane AB1 has been subjected to 800 °C for a total time of approximately 45
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4561
Figure 3. Permeation flux as a function of pressure drop for different types of membrane.
min) shows that some cracks have developed in the separation layer (the total permeation flux is about 65% higher), even though the cracks are of a sufficiently small size, so that Knudsen flow still prevails. The original level of Knudsen flow contribution can be regained by deposition of γ-Al2O3 (membrane AB3 in Table 2). The amount of γ-Al2O3 introduced has a direct influence on the pressure drop required to attain a given throughput. Figure 3 shows the variation of the space velocity in the membrane reactor as a function of pressure drop for three membranes with different amounts of γ-Al2O3. It seems clear that, for a given membrane type and assuming a homogeneous deposition of γ-Al2O3, the pressure drop increases as the γ-Al2O3 loading is increased. However, it is also clear that a certain amount of γ-Al2O3 deposited into the membrane structure is needed to attain sufficient surface area to support the Pt deposited, and it has already been shown that Knudsen flow becomes preponderant as the γ-Al2O3 loading increases on a given membrane. As a consequence, a tradeoff between pressure drop and combustion efficiency exists, which can be made more favorable by optimizing the amount and location of the γ-Al2O3 deposits within the membrane. This aspect is currently under study in our laboratory. In regards to the stability of the membranes under operating conditions, both SEM micrographs (not shown) and permeation measurements showed that, as could be expected, after the initial treatments at 600 °C needed for the calcination of the bohemite sol introduced in the membrane, and briefly at 800 °C, required for enamelling, the membranes are rather stable at the reaction temperatures involved (below 300 °C); i.e., the SEM micrographs indicate that no cracks or fissures are formed during membrane use, and constant permeation fluxes are obtained at a given pressure drop. Furthermore, the catalytic performance of the membrane also appeared stable on prolonged experiments. In spite of this, a decrease was measured in the BET area of the membranes after 130 h of operation. On an unused membrane the measured BET areas increased from 4.9 to 9.3 m2/g for membranes with γ-Al2O3 loadings of 2.3 and 4.9 wt %, respectively. This corresponds to specific surface areas of 190-200 m2/g for the γ-Al2O3 phase introduced, after allowing for the small contribution of the R-Al2O3 support. However, after use the BET area of the γ-Al2O3 phase in the membranes was around 150 m2/g, a significant decrease from the value measured for the fresh membrane.
Figure 4. Toluene conversion versus average membrane temperature for different total concentrations and feedrates: (a) membrane AA3; (b) membrane AB1.
Reaction Studies. Both single components (toluene, MEK) and mixtures were incinerated on the catalytic membranes prepared. Blank experiments carried out on Pt-free membranes demonstrated that the contribution of the membrane material to the reaction could be neglected at temperatures below 300 °C. Three types of Pt-containing membranes have been used in the reaction experiments reported in this work, with or without added γ-Al2O3: membranes with 5 and 200 nm separation layers, in which Pt was deposited by impregnation, and membranes with a 5 nm separation layer, in which Pt CVD was performed. Combustion of a Single VOC. Figure 4a shows the ignition curves obtained in the combustion of toluene in membrane AA3 (200 nm separation layer) using different flowrates at two concentrations. It can be seen that, in agreement with the preliminary results reported (Pina et al., 1996), for a given feed concentration the contact time does not have a strong influence in the toluene conversion achieved within the range of flowrates investigated: all flowrates used can be approximately fit by the same curve. This is so in spite of a more than 2-fold increase in the feed flowrate for the results in Figure 4. On the other hand, an increase in the toluene concentration in the feed clearly shifts the ignition curve toward higher temperatures: the lightoff (50% conversion) and total combustion (99% conversion) temperatures at a toluene concentration of 600 ppm are 147 and 174 °C, respectively, while the corresponding values at 1700 ppm are 174 and 185 °C. By way of comparison, Figure 4b shows the results obtained for toluene combustion on a 5 nm membrane, without added γ-Al2O3 (membrane AB1). Even though
4562 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 5. Temperatures required for 50% and 99% conversion as a function of the inlet toluene concentration. Membranes AB1 to AB3.
Figure 6. MEK conversion versus average membrane temperature for different total concentrations and feedrates. Membrane AB3.
in this case a weak trend with contact time can be discerned, again a single curve can be drawn to approximately represent all the data gathered at the different flowrates, and also in this case when the toluene concentration was increased, the curve was shifted toward higher temperatures. The differences with the previous case are in the light-off and total combustion temperatures obtained, which are higher in this case. Thus, for 600 ppm these are 180 and 207 °C, respectively, while 198 and 247 °C were measured for a higher concentration (1600 ppm). In both cases, the temperatures are significantly higher than those shown in Figure 4a. This shows the importance of having sufficient support material (γ-Al2O3 in this case) for the active phase introduced. Thus, while membrane AB1 contained no supplementary γ-Al2O3, the loading for membrane AA3 was 4.7%, which gave a higher Pt loading (0.13%) after the standard impregnation treatment, and probably also a higher dispersion, although this could not be confirmed due to the inaccuracy in determining the dispersion at the low Pt loading of membrane AB1. When the Pt loading on a membrane without supplementary γ-Al2O3 was increased to 0.34% using repeated impregnation treatments, the performance observed (not shown) improved considerably, being close to that of membrane AB3 (2.5% γ-Al2O3 and 0.17% Pt). A second reason that justifies the introduction of supplementary γ-Al2O3 in the membrane is the possibility of defects in the thin membrane separation layer. A defect such as a crack or pinhole provides a preferential channel that would effectively bypass contact with the active phase. Conversely, if γ-Al2O3 is deposited on the macropores of the membrane, at least two significant advantages are obtained: First, a more robust contactor is obtained, which is less likely to develop a crack running across the whole active thickness of the membrane. Second, the preparation method used tends to self-repair defects in the deposition of γ-Al2O3; i.e., as the bohemite sol is filtered through the separation layer a hypothetical defective region (with less γ-Al2O3 in it) would present a lower pressure drop, and therefore filtration would take place preferentially there. The increase in combustion efficiency as the amount of γ-Al2O3 (and Pt) increases is evident in Figure 5, where the light-off and total combustion temperatures for toluene on different membranes are presented as a function of the inlet toluene concentration. As can be observed, an approximately linear relationship exists between the required temperature and toluene concen-
tration for all the membranes. The temperatures required for 50 and 99% conversion on membranes consistently decrease as the γ-Al2O3 loading increases, i.e., from membrane AB1 (no added γ-Al2O3) to AB3 (2.5 wt %). On the other hand, the pressure drop required to obtain a certain permeation flux on membrane AB1 was considerably lower, as shown in Figure 1. Membrane AB3 was also used for the combustion of MEK. Figure 6 shows the conversion curves obtained for two different flowrates and MEK concentrations. Contrary to what was observed with toluene, the flowrate (contact time) now appears to be a more significant parameter than the feed concentration. Thus, in Figure 6 the conversion-temperature curves for different concentrations are approximately overlapping, while a noticeable shift of the curve takes place when the feed flowrate is increased from 485 to 770 Ncm3/min. It may also be noticed that the light-off and total conversion temperatures for a given molar concentration are lower than those obtained for toluene on the same membrane, which would indicate a higher reactivity of MEK. Tichenor and Palazzolo (1987) also found a higher destruction efficiency for MEK with respect to benzene in mixtures for most of the experimental range investigated, even though the destructibility of aromatics as a group was higher than that of ketones. The fact that different MEK concentrations at a given flowrate follow approximately the same conversiontemperature curve while different flowrates at the same concentration do not would be consistent with a pseudofirst-order reaction rate for the combustion of MEK on Pt. The behavior of toluene seems to be more complex. Thus, an apparent zeroth-order reaction rate has been reported over a wide range of concentrations (Barresi and Baldi, 1994), which would shift into a negative order at low toluene concentrations (Barresi et al., 1995). These results could explain the increase in the light-off and total combustion temperatures observed in this work for increasing toluene concentrations but not the similarity of the combustion curves for different total flowrates. Concerning the membranes prepared by the CVD method, the results obtained displayed trends analogous to those observed for impregnated membranes, although the temperatures required to attain a given conversion were somewhat higher. Parts a and b of Figure 7 compare the performance of membranes AB3 and CVD3 in the combustion of toluene and MEK, using comparable concentration levels. In view of the different Pt
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4563
Figure 7. Performance of catalytic membranes prepared by the CVD method. Comparison of Pt/γ-Al2O3 membranes prepared by impregnation (membrane AB3) and by chemical vapor deposition (CVD3): (a) toluene combustion; (b) MEK combustion. (c) Comparison of two CVD membranes with different γ-Al2O3 and Pt loadings. Space velocity ) 6900 h-1.
Figure 8. Combustion of single-component and binary mixtures. (a) Inhibition of MEK combustion by toluene, membrane AB3. Space velocity ) 2660 h-1. (b) Inhibition of MEK combustion by toluene, membrane CVD3. Space velocity ) 3500 h-1. (c) Inhibition of toluene combustion by MEK, membrane AB3. Space velocity ) 2660 h-1.
load on both membranes, the flowrates were set to give the same space time (g of Pt‚s/g of air). Parts a and b of Figure 7 clearly show that for both compounds the combustion temperatures were lower on the impregnated membrane. It must be noticed, however, that only a few results on reaction experiments involving CVD membranes have been gathered so far. There is ample scope for optimization of the preparation conditions, aimed to attain a highly dispersed Pt phase on the γ-Al2O3 support. Other trends displayed by the membranes prepared by the CVD method were also similar to those discussed already for impregnated membranes. Thus, in Figure 7c the performances of two CVD-prepared membranes
with different γ-Al2O3 and Pt contents in MEK combustion are compared. A higher γ-Al2O3 content results in a higher Pt loading for the same CVD parameters (concentration and total feedrate of the organic precursor, temperature, duration of CVD) and in considerably lower temperatures (about 50 °C difference between the light-off temperatures for membranes CVD3 and CVD1). Combustion of Binary Mixtures. The results obtained in the combustion of mixtures of MEK and toluene are plotted in Figure 8, along with the singlecompound curves, which serve as a reference. The inhibition effect of toluene upon the combustion of MEK is evident from the results in Figure 8a: while there is little difference between the curves for combustion of 1750 and 3100 ppm of MEK as a single compound, the
4564 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 10. In-situ DRIFT spectra of MEK and of MEK-toluene binary mixtures (1500 ppm of each compound) over a 3% Pt/γAl2O3 catalyst in N2. Figure 9. TPR diagram of a 3%Pt/γ-Al2O3 catalyst (a) after oxidation in air at 275 °C for 1 h and (b) after exposure to 1500 ppm of MEK in N2 at 300 °C for 2 h.
addition of a similar amount of toluene shifts the curves to significantly higher temperatures, and the effect seems to be analogous for different MEK concentrations. The same result is shown in Figure 8b for a membrane prepared by the CVD procedure. The shift in the MEK combustion curve was roughly the same as that observed in the impregnated membranes (Figure 8a) when similar toluene concentrations (in the 2000-2300 ppm range) were used. However, a stronger inhibiting effect of toluene can be recognized in Figure 8b in the presence of higher toluene concentrations. On the other hand, the inhibiting effect of MEK on toluene combustion is considerably limited (Figure 8c). While some inhibiting effect of MEK on toluene combustion can be ascertained at the lowest level of toluene concentrations employed (i.e., the combustion of 1400 ppm when mixed with 2200 ppm of MEK occurs at cat. 10 °C higher than the combustion of 1750 ppm of toluene as a single compound), this effect was no longer noticeable at toluene concentrations of 2200 ppm and above. The different behavior of toluene and MEK regarding inhibition appears to be in agreement with the competitive adsorption mechanism often postulated to explain the performance of catalytic combustors handling VOC mixtures (Spivey, 1987; Mazzarino and Baressi, 1993). In order to test this hypothesis and to gain insight on the mechanism governing the combustion of VOCs in mixtures, some additional studies were performed using a 3 wt % Pt/γ-Al2O3 catalyst in a powdered form. Curve a in Figure 9 shows the results of a TPR experiment carried out on the Pt/γ-Al2O3 catalyst after oxidation in air at 275 °C for 1 h. A wide reduction band appears, with the main peak at 196 °C. When the same oxidized sample was treated with 1500 ppm of MEK in N2 at 300 °C for 2 h (curve b), the TPR experiment showed a much lower H2 consumption (the peak at 196 °C practically disappears). This means that, under a N2 atmosphere, even oxygen-containing species such as MEK are able to reduce oxidized Pt and suggests that under reaction conditions oxidized and reduced Pt sites could be present on the catalyst surface. The DRIFT study performed under reaction condi-
tions on Pt/γ-Al2O3 complements the TPR results and helps to understand the behavior of mixtures. On γ-Al2O3 (results not shown), three bands characteristic of CdO stretching were observed at 1712, 1665, and 1632 cm-1 (in order of decreasing intensity), when MEK in air or in nitrogen was passed over the solid. This correlates well with the results of Kiselev and Lygin (1986), who studied the adsorption of acetone on γ-Al2O3 and found three characteristic bands at 1692 cm-1 (interaction of CdO with surface OH groups) and 1625 and 1600 cm-1, (coordination of acetone with nonprotonated sites). When 3 wt % Pt is added to the alumina, the adsorption sites are significantly modified, and the reaction atmosphere does have a significant influence on the position of the bands. The different DRIFT spectra obtained with MEK and MEK-toluene mixtures in N2 are shown in Figure 10. It can be seen that for MEK alone in nitrogen, i.e., with the catalyst mostly reduced according to TPR results, the most intense band (CdO stretching) now appears at 1665 cm-1, with a second peak at 1709 cm-1, which probably corresponds to adsorption on the sites still remaining oxidized. When toluene is introduced in the system, the main band shifts to 1689 cm-1, thus indicating a weakening of MEK adsorption in the presence of toluene. Neither the toluene nor the MEK band positons are modified by increasing the temperature to 200 °C in N2. Figure 11 shows the DRIFT spectra of both single components and binary mixtures on Pt/γ-Al2O3 under reaction conditions, i.e., in the presence of air and at temperatures significantly higher than ambient temperature. The curve corresponding to single-compound MEK shows that the main band is now at 1709 cm-1, while the band corresponding to adsorption on reduced Pt (1665 cm-1) decreases considerably. These results can be interpreted as a preferential adsorption of MEK on oxidized sites, which would be in agreement with a Mars-Van Krevelen combustion mechanism. On the other hand, toluene as a single compound presents a relatively weak band at 1603 cm-1. The DRIFT spectra dramatically change when both components are present under reaction atmosphere: The band corresponding to toluene increased markedly in the mixture with respect to the single-component band, while that of MEK decreased. This trend increases with temperature and
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4565
Figure 12. MEK conversion versus average membrane temperature for flow-through and monolith operation modes. Membrane AB3. Space velocity ) 2760 h-1. 1200 ppm of MEK. Figure 11. In-situ DRIFT spectra of MEK, toluene, and binary mixtures (1500 ppm of each compound) over a 3% Pt/γ-Al2O3 catalyst in air, at different temperatures.
is affected by the reaction atmosphere: Under nitrogen at 150 °C, the relative toluene/MEK ratio of band intensities is 0.23. This increases to 0.95 in air at 100 °C, 1.22 at 150 °C, and 1.33 at 200 °C. Thus, it can be concluded that in the presence of MEK and under reaction atmosphere the sites are modified in such a way that toluene adsorption is favored. This explains the results obtained in the combustion of MEK-toluene mixtures, where the presence of toluene inhibited the combustion of MEK: Under the reaction conditions used for binary mixtures, toluene would successfully compete with MEK for adsorption sites, except at low toluene concentrations. Comparison of Flow-Through Catalytic Membrane Reactors and Conventional Reactors. The above-discussed results in Figures 4-8 demonstrate the usefulness of a VOC combustor based on catalytic membranes operating in the Knudsen diffusion regime. The combustion temperatures found in this work for toluene and MEK are among the lowest reported in the literature, in spite of the relatively low dispersion of Pt on the membrane. Concerning this point, when comparing combustion temperatures, it must be taken into account that in this work the temperatures have been measured directly on the catalytic membrane, while the majority of previous works report gas inlet temperatures rather than the temperature of the catalyst. Given the exothermicity of the combustion reactions involved, the difference can be significant, except for very lean VOC streams (e.g., at a toluene concentration of 150 ppm, the differences between the inlet and exit gas temperatures based on the adiabatic temperature rise would be estimated at about 20 °C). The starting hypothesis for this work was that the membrane combustor could increase the VOC removal by lowering the mass-transfer resistances involved. On a conventional fixed-bed reactor carrying out VOC combustion, the reactant molecules have to diffuse through the external film surrounding the catalyst particles, then into the stagnant gas that fills the pores, before reaching the adsorption/reaction sites. A similar situation arises when a monolith reactor is used. Diffusion limitations are especially significant in VOC combustion since, due to the low concentrations usually encountered, a low driving force (concentration gradient) is available for diffusion. However, in the catalytic
membrane system used in this work, the feed flows perpendicularly to and then across the membrane wall; thus, the external film resistance is greatly reduced. With regards to the internal diffusion, the reduction of mass-transfer resistance stems from forced flow under the Knudsen diffusion regime. In order to obtain a direct assessment of the increase in the combustion efficiency that can be attained by the flow-through membrane reactor used in this work, its performance was compared to that of the same membrane reactor operating as a monolith. To this end, monolith operation was simulated by introducing a 4 mm outside diameter quart cylinder axially in the 7 mm internal diameter membrane tube. In this way, a 1.5 mm wide annular channel was created between the external wall of the quartz cylinder and the internal wall of the membrane. Figure 12 shows the conversion-temperature curve obtained for MEK combustion using the same membrane (AB3) in the flow-through and monolith operation modes. It can be seen that not only are the required operating temperatures much lower in the flow-through mode (about 60 °C for lightoff temperatures and well over 100 °C for total conversion) but also the shape of the curve is different. Thus, while a sharp increase of conversion takes place around the light-off temperature in the flow-through operation mode, as expected from an exponential dependence under the kinetic control regime, the change in the monolith operation mode is much more gradual and the curve levels off, increasing only slowly at high temperatures, a positive indication of diffusion control. Analogous results (not shown) were obtained in the combustion of toluene. Also, a similar leveling-off in the conversion-temperature curves at conversions well below 100% can be observed in other works dealing with VOC combustion on monolith catalysts (e.g., Mazzarino and Barresi, 1993; Barresi and Baldi, 1994). Conclusions Flow-through catalytic membrane reactors operating in the Knudsen diffusion regime can be used efficiently as combustors for the removal of VOCs from air streams. The total combustion of toluene and methyl ethyl ketone used as examples of VOCs has been achieved at low temperatures, over Pt/γ-Al2O3 membranes which were prepared by deposition of γ-Al2O3 followed by Pt impregnation. The membranes in which Pt was deposited by the CVD method showed a comparable performance. The membranes used performed stably over extended
4566 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
periods of time, without apparent loss of mechanical integrity, permeation properties, or catalytic performance. The flow-through catalytic membrane reactor extends the range of the kinetically-controlled regime, although at the expense of a higher pressure drop. Obtaining a favorable pressure drop requires optimization of the amount and location of the γ-Al2O3 deposits within the membrane: a low pressure drop, with preponderance of the Knudsen regime, can be obtained from membranes without added γ-Al2O3, although a certain amount of γ-Al2O3 deposits is desirable in order to obtain a more robust membrane and a larger support area for Pt. The addition of γ-Al2O3 becomes essential in applications where reduction of airborne pollutants to single-figure ppm concentrations is required. Under these conditions, even a minor defect on the separation layer would have very detrimental consequences for the intended membrane use. A combustor based on the type of catalytic membranes studied in this work could be used mainly in low- and medium-capacity, end-of-pipe applications. Also, its operation is well suited to handle discontinuous applications (e.g., tank cleaning, emptying of batch reactors, etc.), where there are important relative variations of flowrate over short periods of time. Acknowledgment Financial support from DGICYT, Spain (Projects PB93-0311 and MAT950499), is gratefully acknowledged. We are also grateful to Dr. C. Guimon, of URA CNRS 474, France, for performing XPS analysis on selected membranes. Literature Cited Barresi, A. A.; Baldi, G. Deep catalytic oxidation of aromatic hydrocarbon mixtures: reciprocal inhibition effects and kinetics. Ind. Eng Chem. Res. 1994, 33, 2964-74. Barresi, A. A.; Pommereul, J.; Pelissero, M.; Baldi, G. Catalytic combustion of aromatic solvents over Pt catalysts. Steady state and dynamic behavior. Proc. 2nd Ital. Conf. Chem. Process Eng. 1995, 324-28. Coronas, J.; Mene´ndez, M.; Santamarı´a, J. Development of ceramic membrane reactors with a non-uniform permeation pattern. Application to methane oxidative coupling. Chem. Eng. Sci. 1994, 49, 4749-57. Coronas, J.; Mene´ndez, M.; Santamarı´a, J. Use of a ceramic membrane reactor for the oxidative dehydrogenation of ethane to ethylene and higher hydrocarbons. Ind. Eng Chem. Res. 1995, 34, 4229-34.
European Communities, Council Directive 96/61/CE of 24/9/96 on Integrated Pollution Prevention and Control, 1996. Herguido, J.; Lafarga, D.; Mene´ndez, M.; Santamarı´a, J.; Guimon, C. Characterization of porous ceramic membranes for its use as catalytic reactors for methane oxidative coupling. Catal. Today 1995, 25, 263-69. Kiselev, A. V.; Lygin, V. I. In Infrared spectra of surface compounds; Slutzkin, D., Ed.; Halsted Press: New York, 1986; p 258. Lin, Y. S.; Burggraaf, A. J. Preparation and characterization of high temperature thermally stable alumina composite membrane. J. Am. Ceram. Soc. 1991, 74, 219-24. Mazzarino, I.; Barresi, A. A. Catalytic combustion of VOC mixtures in a monolithic reactor. Catal. Today 1993, 17, 335-48. Moretti, E. C.; Mukhopadhyay, N. VOC Control: Current Practices and Future Trends. Chem. Eng. Prog. 1993, 89 (7), 20-26. Pina, M. P.; Mene´ndez, M.; Santamarı´a, J. The Knudsen-diffusion Catalytic Membrane Reactor: An efficient contactor for the combustion of Volatile Organic Compounds. Appl. Catal. B 1996, 11, L19-27. Ruddy, E. N.; Carroll, L. A. Select the best VOC control strategy. Chem. Eng. Prog. 1993, 89 (7), 28-35. Santos, A.; Coronas, J.; Mene´ndez, M.; Santamarı´a, J. Catalytic partial oxidation of methane to synthesis gas in a ceramic membrane reactor. Catal. Lett. 1995, 30 (1), 189-99. Saracco, G.; Specchia, V. Catalytic ceramic filters for flue-gas cleaning. 2. Catalytic performance and modeling thereof. Ind. Eng. Chem. Res. 1995, 34, 1480-87. Saracco, G.; Veldsink, J. W.; Versteeg, G. F.; van Swaaij, W. P. M. Catalytic combustion of propane in a membrane reactor with separate feed of reactants. II. Operation in the presence of transmembrane pressure gradients. Chem. Eng. Sci. 1995, 50, 283342. Saracco, G.; Specchia, S.; Specchia, V. Catalytically modified flyash filters for NOX reduction with NH3. Chem. Eng. Sci. 1996, 51, 5289-97. Saunders, B. U.S. refineries must piece together RFG, pollution control puzzle. Oil Gas J. 1997, 95 (1), 18-22. Spivey, J. J. Complete catalytic oxidation of volatile organics. Ind. Eng. Chem. Res. 1987, 26, 2165-80. Tichenor, B. A.; Palazzolo, M. A. Destruction of volatile organic compounds via catalytic incineration. Environ. Prog. 1987, 6, 172-76. Veldsink, J. W.; van Damme, R. M. J.; Versteeg, G. F.; van Swaaij, W. P. M. A catalytically active membrane reactor for fast, exothermic, heterogeneously catalysed reactions. Chem. Eng. Sci. 1992, 47, 2939-44.
Received for review January 30, 1997 Revised manuscript received August 15, 1997 Accepted August 24, 1997X IE9700876
X Abstract published in Advance ACS Abstracts, October 1, 1997.
