1471
Ind. Eng. Chem. Res. 1995,34,1471-1479
Catalytic Ceramic Filters for Flue Gas Cleaning. 1. Preparation and Characterization Guido Saracco* and Laura Montanaro Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24. 10138-Torino, Italy
This paper deals with the preparation and the characterization of ceramic porous filters, whose pores were deposited with a y-Al2O3 layer via the so-called “urea method”, in order to increase their specific surface area. Once activated with a suitable catalytic principle, these filters can find a potential application in flue gas cleaning according to a combined action: rnechanical particulate removal catalytic abatement of chemical pollutants (nitrogen oxides, volatile organic compounds, etc.). Both the obtained filters and the bulk y-Al203 powder synthesized through the above method, were characterized from either a structural (BET surface area measurement, Hg porosimetry, differential thermal analysis-thermal gravimetry analysis, X-ray diffraction, scanning electron miscroscopy (SEM) observation, gas permeation) or a catalytic viewpoint. In this last context, isopropyl alcohol dehydration was chosen as a model reaction since it is directly catalyzed by y-A1203 (thanks to its acidic properties) with no need of further catalytic activation. A reaction mechanism is proposed for the test reaction, based on the existence of two types of active sites (A and B). On A-sites isopropyl alcohol gives a n intermediate adsorbate and decomposes provided vicinal B-sites are available for water adsorption. A lunetic rate expression is worked out on the basis of experimental runs performed on a batch-operated differential reactor. The knowledge of this rate expression and of the structural characterization parameters are of major importance for the interpretation and the modeling of the results of a pilot-plant study on the above filters, performed using the same model reaction and presented in part 2 of this series. The urea method is demonstrated t o be a reliable tool to deposit a y - A l 2 0 3 layer all over the pore walls of the filter, markedly increasing its specific surface area. Drawbacks of the procedure employed are though the occurrence of pore blocking after a few deposition cycles and the occasional presence of cracks in the deposited layer.
+
Introduction
Catalytic
The use of catalytically activated porous barriers for application in chemical processing has been widely studied in the past decade. These activated barriers can be considered as multifunctional reactors, which typically allow coupling of a chemical reaction, catalytically promoted, and a separation (Westerterp, 1992). In this context, most attention has been focused on the so-called catalytic membrane reactors (Saracco and Specchia, 1994; Saracco et al., 1994), which enable molecular separation and consequently allow enhancement of conversion for equilibrium-limited reactions (e.g., hydrogen permeation through the membrane improves the per-pass conversion of dehydrogenation reactions). In a less investigated concept the porous barrier may be used as a filter for particulate removal from flue gases (e.g., from waste incinerators, pressurized fluidized bed coal combustors, diesel engines, boilers, etc.) and simultaneously abate chemical pollutants (e.g., nitrogen oxides, volatile organic compounds, etc.) by catalytic reaction (Figure 1). The use of porous ceramic filters for high-temperature applications is meeting an increasing interest (Zievers et al., 1991; Alvin et al., 1991; Clift and Seville, 1993). High-temperature particulate removal allows performance of any heat recovery on clean flue gases, and keeping the temperature high enough so that chemical pollutants can be catalytically destroyed. Conventional filter bags based on polymer materials cannot withstand temperatures higher than about 200 “C. This implies
* Corresponding author. E-mail:
[email protected].
Dusty flue gas
/
4
Dust cake
\
I Catalytic filter
Figure 1. The catalytic filter concept.
relevant energy consumptions for the cooling down of flue gases and for their subsequent reheating up to temperatures suitable for the catalytic converters (e.g., selective catalytic reduction of NO, with ammonia js typically performed on honeycomb structures at 350400 “C (Bosch and Janssen, 1987)). Considerable space savings can be achieved if filters are catalytically activated, allowing elimination or at least reduction of the catalytic converter section of the plant. Only a few, very recent reports appeared in literature concerning this topic. At the Babcock & Wilcox laboratories catalyst pellets for the SCR reaction of NO,s have been entrapped in a ceramic fiber tissue and baghouse filter modules have been assembled and tested with positive results: up to 95% NO, removal
0888-5885/95/2634-1471$09.00/0 0 1995 American Chemical Society
1472 Ind. Eng. Chem. Res., Vol. 34,No. 4, 1995
efficiencies were achieved with simultaneous separation of fly ashes (Kudlac et al., 1992). At 3M Ceramic Technology Center suitable catalysts for soot combustion have been deposited on ceramic fiber filters for use in the cleaning of Diesel-engine emissions: vanadium impregnated fibers allowed direct catalytic combustion of 45% of the filtered soot at Diesel-exhaust temperatures (Morrison and Federer, 1992). This paper describes the preparation and the characterization (BET surface area measurement, Hg porosimetry, differential thermal analysis-thermal gravimetry (DTA-TG) analysis, X-ray diffraction, scanning electron microscopy (SEM) observation, gas permeation) of ceramic candle filters based on porous sintered a-Al2O3, modified by intrusion into their pores of a y-Al203 phase through the so-called "urea method". This material is perhaps the most widely employed catalyst support material. It can be activated with specific catalytic principles, depending on the particular reaction of interest (NO, reduction, VOC abatement, etch However, y-Al203, thanks to its acidic properties (Knozinger and Ratnasamy, 19781, exhibits an intrinsic catalytic activity toward certain reactions (e.g., dehydrations). Therefore isopropyl alcohol dehydration was chosen as a model reaction to assess the capability of y-Al2O3deposited filters of catalytically converting chemical compounds passing through them. Despite its low percent weight fraction, y-Al2O3 would reasonably control the catalytic activity of the filter toward the chosen model reaction, whereas the transport properties across the filter would be mostly determined by its structure. Hence, the aims of this paper are, beyond introducing and describing the preparation procedure adopted, the structural characterization of the thereby modified filters and the assessment of a kinetics law for isopropyl alcohol dehydration on the deposited y-Al2O3. Based on these pieces of information, in part 2 of this series (Saracco and Specchia, 1995) a numerical modeling of the performance of these filters will be attempted and its predictions will be compared with pilot-plant results, concerning the capability of these activated porous barriers to react away trace compounds passing through them.
Filter Preparation Porous a-AlsO3 tubes (length 250 mm; external diameter 10 mm; thickness 1.5 mm) were purchased from S.C.T. (Societe des Ceramiques Techniques, Bazet, France). A transition alumina was precipitated in the pores of these basic tubes via an adapted version of the urea method (Gordon et al., 1959; Ulhorn et al., 1992). The filter was first impregnated with a Al(N03)3 urea aqueous solution. Urea decomposition was then thermally induced promoting in situ precipitation of Al(OH)3 on the pore walls. The product was then dried and calcined. Urea decomposes at an appreciable rate in aqueous solutions a t temperatures higher than 90 "C (Shaw and Bordeaux, 1955). As long as the decomposition of urea proceeds, Al(N03)3 can be hydrolyzed according to the following overall reaction: 2Al(N03), 3CO(NH2), 12H20 2Al(OH), 6NH,N03 3H2CO3 (1)
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+
+ +
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An Al(N03)3.9H20 aqueous solution was prepared at a concentration (750 gL-') close to the solubility limit at room temperature so as to maximize the amount of aluminum hydroxide deposited after each cycle. Urea
was then dissolved into this solution up to a concentration of about 400 g-L-l, in considerable stoichiometric excess (reaction 1). The solution had to be warmed up to about 50 "C so as to ensure complete dissolution of the above salts. The support tube was ultrasonically washed in acetone and dried a t 80 "C for 30 min. Afterward it was cooled and weighed. The tube was then put in a glass cylindrical vessel under vacuum. The prepared solution was then supplied to the vessel so that the entire tube could be impregnated. After 5 min the vessel was brought t o atmospheric pressure. The tube was pulled out, put in another clean vessel, and placed in an oven overnight at 95 "C. The following thermal cycle was then followed to obtain the formation of a transition alumina phase and, a t the same time, to possibly avoid thermal shocks and massive cracking of the layer deposited on the pore walls of the support: drying at 105 "C for 4 h; heating up to 230 "C at a 2 "C-min-l rate; 1h stay at 230 "C; heating up to 400 "C at a 2 "C-min-l rate; 3 days stay at 400 "C; cooling down to room temperature at a 2 '(2.min-l rate. The entire deposition procedure was performed on different samples for a progressively higher number of times (up to 51, so as to study the effect of repeated deposition on the filter characteristics. The obtained samples were characterized via weight-increase measurements, SEM observations (Hitachi S-2300), Hg porosimetry (Porosimeter 2000, Carlo Erba Instruments), BET measurement of specific surface areas (Sorptomatic Series 1800, Carlo Erba Instruments) and catalytic activity toward isopropyl alcohol dehydration (performedon pellets derived from membrane crushing). The described procedure was also performed, starting directly from the promoting solution, in order to obtain some bulk transition alumina powder. This powder was then characterized via X-ray diffraction (Philips 17101, BET measurements, DTA-TG analysis (STA 409, Netzsch), and temperature-programmed-desorptiontechniques by use of a quadrupole mass spectrometer (Balzers QDP 101). The kinetics of isopropyl alcohol dehydration on y-Al2O3 powder compacts were then assessed by use of a batch-operated differential recycle reactor, working out a suitable reaction rate expression accounting for the influence of the main parameters of interest (i.e., temperature, isopropyl alcohol partial pressure, and water partial pressure).
Structural Characterization Transition Alumina Powder. DTA-TG analysis clearly showed the presence of a couple of endothermic peaks, occurring at 100-200 "C and at 300-400 "C, each one in connection with about 10% mass loss, attributable to the elimination of physisorbed water and reaction by-products. Thermal desorption analysis, performed heating a powder sample from room temperature to 900 "C at a 10 "Camin-l rate, confirmed that physisorbed water is almost entirely released below 200 "C. At higher temperatures, nitrogen oxides leave the sample in large amount together with water and C02, probably due to the decomposition of by-products (e.g., NH4N03, carbonates, etc.). After 3 days calcination at 400 "C a substantially pure y-Al2O3 could be obtained, as confirmed by XRD analysis. Moreover, BET measurements showed that after such a calcination time the y-Al2O3 exhibits a specific surface area of nearly 250 m2.g-'. This value remained practically unaffected after prolonged stay at 400 "C.
Ind. Eng. Chem. Res., Vol. 34,No. 4,1995 1473
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Deposition cycles
Figure 2. Variation of weight increase and of specific surface area of deposited filters as a function of number of deposition cycles.
0
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0.02
0.03
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Face velocity (mod)
Figure 4. Variation of pressure drop across the filter as a function of the superficial velocity of the permeating gas (nitrogen, temperature 25 "C,outlet pressure lo5 Pa). Number of deposition cycles: ( 0 )0 (virgin filter); (W) 1;(+) 2; (A)3.
Figure 3. SEM picture showing extensive pore plugging in a tube sample which underwent five deposition cycles.
Activated Filter. Figure 2 shows the variation of the weight increase (referred to the virgin support) and of the specific surface area of the product, as a function of the number of deposition cycles. The reported data points are mean values of a series of five measurements performed on different tube samples. Beyond three deposition cycles, there is a slow decrease in the slope of the weight-gain line. Simultaneously the BET surface area dismisses a positive trend and remains in the range 7-12 m2*g-l. In spite of the fact that BET measurements may be affected by a certain error owing to the comparatively low specific surface areas, primarily due to the effect of the a - A l 2 0 3 matrix, still a possible explanation of the observed results might be drawn. It seems likely that, from three deposition cycles on, pore plugging becomes relevant, thus preventing the impregnating solution from further entering some parts of the filter. SEM observations on a sample which underwent five deposition cycles clearly confirmed this hypothesis (Figure 3). The effect of pore plugging appears clearly from Figure 4, which reports the pressure drop increase as function of the superficial velocity of pure N2 flowing through candles which underwent up to three deposition cycles (further deposition cycles were not performed for these tests on the basis of the conclusions hereafter drawn). Beyond two deposition cycles, when the occurrence of pore plugging becomes serious, the pressure drop markedly increases, reaching unacceptable levels for practical application (Clift and Seville, 1993). Further, pore obstruction is also detrimental since it
Figure 5. SEM picture showing the deposited y - A l 2 0 3 layer after two deposition cycles. Cracks are occasionally present (see magnification).
prevents part of the deposited catalyst from being reached by the permeating gases. For these reasons it seems inconvenientto perform more than two deposition cycles, at least using the same deposition procedures adopted in this study. More controlled operating conditions might eventually lead to a higher catalyst load in the filter without extensive pore plugging. SEM observations on a twice-deposited sample showed that the deposited y - A l 2 O 3 is homogeneously distributed throughout the thickness of the support tube, giving a few-microns-thick layer, stuck onto the pore walls. However, adhesion of this layer can be improved. In fact some cracks are prone to form close to the grain boundaries of the a - A 1 2 0 3 support (Figure 5). A milder heat treatment (particularly during the drying step) may be helpful to avoid the formation of those cracks by limiting the stresses in the catalyst support layer being formed. Finally Hg porosimetry was performed on the base support and on samples which underwent one and two activation cycles. Pore-distribution histograms are reported in Figure 6 for a comparison. It appears clearly that the distribution shifts toward lower pore radius values at increasing number of impregnations. Further, the pore size distribution range becomes wider.
Kinetics Characterization As earlier stated, the model reaction chosen to assess the catalytic activity of the transition alumina obtained
1474 Ind. Eng. Chem. Res., Vol. 34, No. 4, 1995
Isopropanol
Water
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A-site
B-site
\\%\\\\\\\R\q Water
4 Isopropanol dehydration
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Propylene
Figure 7. Proposed reaction pathway for isopropyl alcohol catalytic dehydration.
10
100
W Figure 6. Hg porosigrams for bare a-Al203 support (a) and for tube samples which underwent 1 (b) and 2 (c) deposition cycles.
is the isopropyl alcohol dehydration:
CH,CHOHCH,
-
H,O
+ CH,CH=CH,
(2)
One of the interesting advantages of this reaction is the almost total absence of side reactions. Contrary to primary alcohols (Butt et al., 1962; Knozinger and Kohne, 1964), the formation of ether can be kept to a minimum (
