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Preparation of composite ceramic membrane from titania powders
Powder Technology 81 (1994) 249-257
ELSEVIER
Preparation of composite ceramic membrane from titania powders Robert B. Hutchison *, H. Edward Curry-Hyde, Judy A. Raper School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington, N S W 2052, Australia Received 14 March 1994; revised 24 August 1994
Abstract
A titania-based composite membrane has been developed. The membrane is prepared from titania powder and stainless steel mesh by an aqueous slurry technique. The effects of dispersion technique and heat-treatment temperature on the nature of imperfections and titania agglomerates were investigated. The membranes were characterised by determining their pore size distributions, the pressure drop to air flow and the membranes' flexibility. Keywords: Dispersions; Ceramics; Membranes; Titania
1. Introduction Since the first development of composite ceramic membranes using sols and powder suspensions on porous inorganic supports [1], papers have been published on a zirconia ceramic membrane supported on an Inconel 600 mesh [2,3]. These membranes were prepared using aqueous powder suspensions to coat the metallic substrates and are currently used for liquid filtration. The substrate supports the membranes and also maintains the membrane under compressive loading which decreases the likelihood of cracks forming due to pressure difference or mechanical stress. The recently published work [2,3] is on developing applications for this composite zirconia membrane. A potential commercial application of these composite ceramic membranes is in the area of high temperature gas cleaning, such as simultaneous flyash removal and selective catalytic reduction (SCR) of NOx from coalfired power station exhaust streams [4]. Ideally, a composite membrane could act as a support for the catalyst, and could also act as a barrier filter for the flyash in the gas stream. The system, operated at elevated temperatures, would filter the fiyash, remove the NO~ and leave a clean hot gas stream well suited to waste heat recovery systems. Since the preferred catalyst for the SCR reaction is V2Os supported on titania [5], the best ceramic membrane for this purpose would be a metal mesh-supported titania membrane. Previously published * Corresponding author.
0032-5910/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved
SSD! 0032-5910(94)02890-Z
studies [1-3] on metal mesh-supported membranes have not used titania as the membrane material. Simultaneous removal of NOx and flyash requires consideration of both temperature and pressure drop. Current filtration practice involves operation at low temperature (below 200 °C) and low face velocities using high surface area bags which produce low pressure drop (less than 1 kPa). The cost savings in fan capacity afforded by the low pressure drop are somewhat offset by the energy required to cool the exhaust gas to a temperature at which the fabrics making up the filter can survive. For this reason the use of ceramic membranes for gas filtration potentially provides an economic solution provided that the pressure drop can be kept to a minimum. Methods to achieve this would include reduction of the membrane thickness and modification of the pore structure. The work described here represents an investigation into the preparation of stainless steel mesh-supported titania membranes from titania powder. The stainless steel support could enable temperatures of up to 800 °C to be used.
2. Experimental
2.1. Preparation Due to its particle size (0.2 /xm) and availability, a common pigment grade of titania (anatase, Degussa Kronos 1001) was used to prepare the ceramic mem-
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R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
branes in this study. A suspension of the titania was prepared using various compositions such as those given in Tables 1 and 2. The composition given in Table 1 Was that which, with other preparation techniques detailed later, produced crack-free membranes. The additives included in the suspension were used to ensure the formation of an even film without bubbles which does not crack during drying. A dispersant (tetra sodium pyrophosphate, TSPP) was used to aid formation of a homogeneous suspension; a film-forming agent (poly(vinyl alcohol), PVA) was used to prevent failure of the films during coating and drying; and a plasticiser (glycerol) was used to prevent cracking of the membrane during drying and handling before heat treatment [6]. The metal mesh (316 stainless steel, 100 mesh) was cut to size and treated in a sulfuric acid and potassium dichromate bath [1] to clean and etch the surface to ensure adequate adhesion of the titania powder to the metal surface. Stainless steel was used as it was readily available and resistant to corrosion. It can be used at the temperatures required for the SCR of nitric oxides and would be relatively unaffected by the typical environment within flue ducts. After acid treatment the mesh samples were washed in water and maintained wet until coated later on the same day. To make the required suspension, the additives were first mixed together. The PVA was dissolved in hot distilled water to form a 10 wt.% solution and allowed to cool. The PVA solution, glycerol and TSPP were added and agitated until the TSPP had fully dissolved. The titania was added to the solution and dispersed by one o f three techniques: bead mill, ball mill or ultrasonic dispersion. Ultrasonic dispersion was the most successful and required the titania to be added to the solution, along with 50% of the distilled water and wet Table 1 Composition of suspension for making ceramic membranes Material
Proportion (by weight)
Water Titania PVA Glycerol Na4P2OT- 10H20 (TSPP)
0.54 0.43 0.016 0.013 0.0032
Table 2 Composition of suspension used in preparation of cracked and holed membrane seen in Fig. 7 Material
Proportion (by weight)
Water Titania PVA Glycerol Na4P2OT- 10H20 (TSPP)
0.63 0.33 0.019 0.013 0.0019
ground with a mortar and pestle. The rest of the distilled water was added and the suspension was dispersed by agitation in an ultrasonic bath for 15 min. The latter was required to break up the smaller agglomerates as uncoated titania can be difficult to disperse fully in aqueous solutions due to the high agglomerate tensile strengths [6]. Once the suspension was adequately mixed, the wet mesh was dipped into the agitated suspension and dried in air oriented horizontally. The membranes were not force dried as fast drying is known to cause cracking due to the capillary tension between adjacent pores of different diameters [7]. At least 16 h was allowed for drying of the membranes. After drying, the meshes were heat-treated in atmospheric air. To standardise the heat-treatment process, the temperature was raised to the treatment temperature at 1 °C min -1, held at that temperature for 1 h and then cooled at 1 °C min-1. The treatment temperatures were: 400, 500, 600, 700, 800, 900 °C and room temperature (i.e. no heat treatment). 2.2. Characterisation Reliable assessment of the dispersion of titania powder in a liquid is known to be difficult [8]. Dispersion was ascertained by analysing the size distributions of the particles making up the suspension to ensure that no agglomerates were present. After diluting the suspension with the PVA/glycerol/TSPP solution, a Malvern Mastersizer, an instrument that uses forward light scattering to measure particle size [9], was used to measure the titania particle size distribution to assess the degree of dispersion. The changes that occurred in the membrane during heat treatment were studied using simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) with the evolved gases analysed by mass spectrometry. This enabled identification of the processes occurring during heat treatment and identification of the additives that left residues in the membrane. The technique measured the sample weight and heat flux as the temperature was raised at a predetermined rate (5 °C min-1). The gaseous components evolved from the membrane were identified by passing the gaseous products through a mass spectrometer. As mentioned above, one of the potential disadvantages of ceramic membranes may be their high pressure drop. In order to quantify this aspect of membrane performance, the pressure drop of each prepared membrane was measured using the simple rig shown in Fig. 1. The specific resistance to flow of air at ambient temperature (Pa s m -a) was determined by measuring the pressure drop as a function of superficial gas velocity.
R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257 air in
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Fig. 1. Schematic of pressure drop testing rig.
The apparatus consisted of a bubble tube (A) to measure the air flowrate through a 25.4 mm diameter circular sample of membrane held in place by a membrane holder (B). The pressure drop was measured with a micro manometer (C) or a mercury manometer (D) according to the appropriate pressure drop range. The internal morphology of the heat-treated membrane was assessed by determining the pore size distribution using mercury porosimetry and the surface area using the single-point BET nitrogen adsorption method. This information gave insight into the effect of heat treatment on the titania powder pore structure. Optical and scanning electron microscopies were used to assess the surface morphology of the membrane and the packing of the titania particles at the surface of the membrane.
3. Results and discussion
To produce ceramic membranes with the desired characteristics, it is first necessary to have some understanding of the effect of preparation parameters on the membrane characteristics and performance. This paper describes an experimental programme aimed at producing membranes with the desired characteristics. These include achieving dispersion of the titania powder and the effect of heat treatment on the final pore size distribution of the membrane. From these experiments, information relating to the factors affecting the physical strength of the membrane was also obtained.
251
milling was performed for periods varying between 5 min and 6 h. The degree of dispersion was assessed by observing drops of the suspension on a microscope slide immediately after stopping the ball mill. As the milling continued there were fewer obvious aggregates within the suspension a n d its form was more even. After 4 h, no obvious aggregates were seen in the suspension so it was assumed to be fully dispersed. It was also noticed that the viscosity of the suspension increased as the milling progressed for up to 4 h after which it remained constant. In the second method, the suspension was placed in a bead mill with zirconia beads (diameter 1.5 ram) for periods between 10 min and 3 h. No differences could be observed between any of the bead-milled samples. It was suspected that the zirconia beads were too small to break up effectively the larger aggregates due to the relatively low kinetic energy of collisions between the beads. No increase in suspension viscosity was noticed during the bead milling. In the third method, when the suspension was agitated in the presence of an ultrasonic field for approximately 10 min, a drop examined on a microscope slide did not appear to contain any aggregates. The effect of the ultrasonic field on dispersion of the titania can be seen clearly in Fig. 2. The mean particle size of the titania suspension was significantly reduced .after a 10 min exposure to the ultrasonic field producing a dispersed suspension. However, the dispersion was clearly unstable since, when the ultrasonics were stopped for more than a few minutes, aggregation occurred rapidly, forming large agglomerates. Membranes were prepared from a suspension with the composition given in Table 1 using each of the dispersion techniques. The membranes prepared using ultrasonic dispersion were free of cracks, whereas the other membranes were cracked in each mesh interstice. It is thought that the bead mill did not significantly break up the aggregates of titania, while both the ball mill and ultrasonic dispersion were successful in break-
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3.1. Effects of various dispersion techniques Three dispersion techniques were evaluated. The first was ball milling with 10 mm ceramic balls to disperse the titania, the second was bead milling with 1.5 mm ceramic beads and the third was agitation in an ultrasonic bath. In the first test, the suspension was placed in a ball mill with 10 mm diameter ceramic balls. Ceramic ball
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252
I~B. Hutchison et aL / Powder Technology 81 (1994) 249-257
ing the titania aggregates. The difference between the ball milling and the ultrasonic dispersion techniques was not the degree of dispersion obtained by the techniques but the time between the end of the dispersion and the coating of the metal mesh. The ballmilled titania is thought to have reagglomerated in the 10 min after stopping the ball mill before the mesh was coated. However, the titania dispersed ultrasonically was kept from reagglomerating by coating within the ultrasonic bath (see Fig. 2). The membrane dipped while in the ultrasonic field was thus formed from fully dispersed titania particles which reagglomerated in a few minutes once the membrane was placed on the drying rack. Thus, the effective particle size increased before evaporation of a significant quantity of the solvent. Furthermore, the aggregates formed are thought to have been interlocked and relatively strain-free with van der Waals bonds extending throughout the suspension. These bonds are thought to have been sufficiently numerous to resist the surface tension forces exerted by the evaporation of the solvent from the pores [7]. In this way a membrane free of cracks was able to be produced.
3.2. Effect of heat treatment Transformations that occurred during heat treatment of the dried samples were studied using simultaneous DSC and TGA with analysis of the evolved gas using a quadrupole mass spectrometer. Two runs on the dried suspension (see Table 1) were performed; the first in a helium atmosphere and the second in an air atmosphere. The first run in helium permitted more detail to be obtained from the mass spectrometer, whereas the second run in air simulated the actual processes occurring during the heat treatment of the membranes. The initial investigation was on a membrane in helium atmosphere. A simple spectrum was obtained with two endothermic peaks coincident with two mass loss peaks. The mass loss peaks were observed at 160 and 285 °C. The magnitudes of the mass losses were 2.1 and 2.2%, respectively. It is thought that the peak at 160 °C was due to the evaporation of most of the glycerol. Decomposition of the PVA is believed to account for the peak at 285 °C. The TGA/DSC spectrum for the dried suspension heated in an air environment shows a much more complex system, as seen in Fig. 3. Mass loss peaks (and loss percentages) occurred at 55 (2.0), 200 (2.4), 240 (1.9) and 400 °C (1.9%), with a further very broad peak centred at approximately 300 °C (1.4%). The total mass loss of 9.6% corresponds well with the sum of the masses of the PVA, glycerol and the water of hydration of the TSPP (9.8%). The initial loss of 2.0% at 55 °C is believed to be adsorbed water from the initial solvent and the TSPP as pure TSPP loses its
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water of hydration at 80 °C. Glycerol boils in the pure state at 290 °C and its proportion in the material was 3.5%. The spectrum suggests that this component comes off in stages, possibly depending on the physical environment of each of the glycerol molecules. The PVA decomposes in air beginning at 200 °C and was essentially completely decomposed by 470 °C. There are a number of peaks in the decomposition of PVA and the interaction with the glycerol molecules is thought to account for some of the wide peaks in the spectrum.
3.3. Titania powder bonding The possibility of increasing the strength of the bonding between the titania particles was investigated so that membranes of increased strength could potentially be made. Pure titania powder was mixed with water and dried. The titania was divided into three samples and heat-treated in air at 85, 400 and 800 °C for 1 h. Each sample was then placed in a stainless steel mesh basket with 150/zm apertures and suspended in water within an ultrasonic bath. After periods of time, the basket was removed, dried at 85 °C, weighed and then replaced in the ultrasonic bath. The samples lost mass when pieces less than 150 /zm were broken off due to the influence of the ultrasonic field and these pieces passed through the apertures of the mesh. The mass loss of each sample was measured as a function of the ultrasonic field strength. A second test was performed on two samples of the dried titania suspension (containing PVA, glycerol and TSPP). These two samples were heat-treated in air at 400 and 800 °C, respectively. Table 3 shows that the sample heat-treated at 85 °C was broken up completely to particles less than 150 /zm in size at the test conclusion, while the sample of titania heat-treated at 400 °C was slightly broken up and lost 5.6% of its mass. The pure titania sample (heat-treated at 800 °C) and the dried titania suspensions (heat-treated at 400 and 800 °C) did not lose any mass
R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257
253
Table 3 Mass loss of titania heat-treated at different temperatures Ultrasonic field and duration
Mass loss of titania samples (%)
12 W, 2 h 24 W, 2 h 30 W, 1 h
Pure titania heattreated at 85 *C
Pure titania heattreated at 400 *C
Titania suspension heat- Pure titania heattreated at 400 *C treated at 800 °C
Titania suspension heattreated at 800 *C
8 98 100
2.2 3.7 5.6
0 0 0
0 0 0
and were unaltered in appearance after being in the ultrasonic field for a total of 5 h. These results show that aggregates formed from a PVA, glycerol, TSPP and titania suspension heat-treated at 400 °C are stronger than those formed from a titania suspension without any additives heat-treated at 400 °C. Another test on the titania heat-treatment process was to determine whether there was any reduction in surface area as this would be expected if any sintering occurred. Single-point nitrogen BET surface area measurements were made on dried titania suspension heattreated at the same time as the membranes. The total surface area of the particles as a function of heattreatment temperature is given in Fig. 4, which shows that as the treatment temperature was increased the surface area increased to a maximum at 400--600 °C. The low surface area of the room temperature-dried sample compared to the pure titania powder is most likely due to the binder and the plasticiser filling or blocking the mesopores in the material. Both of these compounds start to decompose or vaporise at temperatures below 400 °C. Thus, the pores would start to clear and the surface area would increase as observed. The sample heat-treated at 800 °C shows a significant reduction in surface area compared to the samples heat-treated at temperatures between 400 and 700 °C, and a significant degree of interparticle bonding was believed to be present. 10 •
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The sample treated at 900 °C shows the greatest degree of reduction in surface area and thus a high degree of interparticle bonding would be expected. However, this temperature could not be used for membrane preparation as the stainless steel mesh corroded severely in air at this temperature. Cracks were also evident in every interstice of membranes heat-treated to this temperature. T h e cracking was thought to be due to corrosion products of the wires exerting excessive forces on the membrane and thus causing it to fail. The heat treatment was intended to remove the additional components (other than titania) and to improve the mechanical strength of the membrane through interparticle bonding and imposition of compressive forces by the stainless steel mesh. The TGA/DSC results show that the additional components were removed (except for some of the TSPP) and the ultrasonic test showed that there was a significant increase in the mechanical strength of the membrane. Pore size distributions of membranes prepared using ultrasonic dispersion and heat-treated at different temperatures were determined by mercury porosimetry. A summary of the results is given in Fig. 5. The pore size distribution could be divided into three distinct sections based on the expected membrane morphology: • 300-3 /Lm. These pores are believed to be due to the cracks and holes in the membrane and gaps between touching adjacent mesh in the penetrometer. z
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Fig. 4. Single-point nitrogen BET surface area measurements on dried titania suspensions.
0.04 0 1000
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Fig. 5. Pore size distribution as a function of heat-treatment temperature measured by mercury porosimetry.
254
R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
Unfortunately due to the shape and size of the penetrometers it was not possible to prevent touching between adjacent pieces of membrane. • 3--0.02 /~m. These pores are believed to be due to the spaces between the primary titania particles. 3 /~m is fifteen times the size of the primary particles and 0.02 ~m is one tenth of the particle diameter. • 0.02-0.003 /~m. These pores are believed to be the voids at the junction of the titania particles where small irregularities in the surfaces of the particles will show up as mesopores. Fig. 9 shows that the individual particles are without obvious defects to the definition limit of the electron micrograph (less than 0.010 /zm). Fig. 5 shows that there is little difference in the pore size distribution of membranes treated at the different temperatures up to 700 °C. The membrane treated at 900 °C shows a different structure. There was a larger intrusion volume in the 0.2-1.2/~m range. This suggests that it had larger interstices than the other membranes. This was confirmed ~by electron microscopy of this membrane which also showed corrosion of the stainless steel mesh. The membranes treated at the higher temperatures show lower volumes of very small pores, suggesting that there may have been some sintering between the particles leading to the elimination of these very fine pores. The membranes treated at 400 °C and those not treated at all showed a higher volume of pores in the very small range (less than 50 nm and greater than 3 nm). This can be explained by the presence of the additives in the membrane. These were fully destroyed just below 500 °C and so could not contribute to pore volume above this temperature. While these additives were present on the membrane, their physical structure could have contributed to the very fine pores.
3.4. Membrane morphology After drying at room temperature, the crack-free membranes prepared using ultrasonic dispersion had a glossy surface sheen. This was probably due to glycerol and PVA coating the surfaces. Once this material had been decomposed through heat treatment, the surface took on a matt appearance as the titania particles were exposed. There was no difference in the macroscopic appearance of the membranes treated at temperatures from 400 to 800 °C. Fig. 6 shows titania aggregates with diameters from 5 to 50 /xm on the membrane's surface. This membrane was produced using the suspension recipe given in Table 1, ultrasonic dispersion and heat-treated at 800 °C. These aggregates could have been due to poor dispersion of the titania in the suspension, despite the ultrasonic treatment, but may have formed during the agglomeration of the titania prior to drying of the membrane.
Fig. 6. Surface of titania membrane showing agglomerates of titania and mesh coated with titania.
Fig. 7. Top surface of imperfect titania membrane showing cracks and holes and exposed mesh.
Fig. 7 shows holes formed during membrane preparation with diameters greater than 100 /~m. This membrane was produced using an early recipe (given in Table 2), dispersed ultrasonically and heat4reated at 400 °C. The defects are most likely formed by incomplete coverage of the mesh due t o the lower proportion of titania in the suspension. The membranes produced with this recipe also had two distinct sides with titania concentrated on one of them. This was due to the horizontal orientation of the membrane while being dried. Undulations in the mesh plane also caused the titania to concentrate at valleys in the mesh. Concentration of titania on the lower side of the mesh was also apparent from the fact that the wires on top of the mesh were exposed (see Fig. 7), whereas the wires on the lower side were covered with a coating of titania. For comparison, the top of a membrane prepared using the suspension recipe given in Table 1, ultrasonic dispersion and heat-treated at 800 °C, can be seen in Fig. 6. Drying cracks of less than 10 /zm wide can also be seen in Fig. 7. Imperfections in other membranes were caused by bubbles or foam in the
R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
suspension during dipping whilst in the ultrasonic bath. These bubbles produced very thin sections of membrane that cracked or fell away during drying. The thickness of the crack-free membranes was 20-50 /~m in the centre of the mesh interstice. When the membrane was thinner than this, drying cracks and holes from incomplete coverage were more prevalent. Thicker membranes also cracked during the drying process, probably due to uneven rates of drying. The surface would have fully dried before the centre of the membrane leading to uneven shrinkage of the membrane. After preparation, some membranes were set in resin to enable cross sections to be examined by electron microscopy. A surface examination was also performed to analyse the shape of individual mesh interstices, the powder aggregates and the membrane pores at higher magnification than possible with optical microscopy. Examination of cross sections of an imperfect membrane produced with the recipe given in Table 3, using ultrasonic dispersion of the titania and heat-treated at 400 °C, showed that the cracks passed all the way through the membrane (see Fig. 8). These cracks were not concentrated around the wires, indicating that the titania powder adheres to the stainless steel mesh and does not fall off. The general shape of all the membranes was as would be expected from a liquid suspension process (see Fig. 8). That is, the membrane was thickest at the wires and thinnest in the centre where the two menisci were closest. Bubbles formed during the preparation process are evident in imperfect membranes and these are often the source of cracks. Fig. 7 shows some interstices cracked in a cross formation. These cracks are believed to have been formed by stresses occurring during the drying process due to the use of the suspension with a lower titania concentration. Examination of the appearance of one interstice of the mesh (250 /~m in diameter) showed that there were
Fig. 8. Cross section of imperfect titania m e m b r a n e showing cracking and the asymmetric distribution of titania between the m e s h interstices resulting from drying the m e s h in a horizontal orientation.
255
two main patterns of cracking. The thicker membranes showed cracks that followed the wires, most likely due to shrinkage of the ceramic towards the centre of the mesh interstice. The thinner membranes had cracks that travelled diagonally across the cell with the widest part of the cracks in the interstice centre. These cracks were most likely caused by shrinkage away from the centre and towards the wires in the mesh. Examination of the agglomerates of titania particles (see Fig. 9) showed the shape of the individual particles but did not show how the particles were attached to each other. No direct evidence of interparticle bonding could be observed. The particles were formed in aggregates with large pores surrounding compact agglomerate structures. The range in pore sizes measured from Fig. 9 was from less than 0.05 to 0.3 /zm. This correlates well with the mercury porosimetry results (Fig. 5) which indicated that the pores within the membrane were in the range 0.02--0.25 ~m. The optimum parameters for preparation of the membranes with the techniques used in this study were: • Ultrasonically dispersed titania with the slurry recipe shown in Table 1. • Dip-coating of the mesh followed by drying in a horizontal orientation. • Heat-treating at a temperature of 800 °C. These conditions produced membranes which were crack-free and with the highest degree of bonding between the titania particles.
3.5. Pressure drop of membranes Pressure drops across crack-free membranes were measured as a function of air velocity and heat-treatmenl temperatures. These measurements were compared witt the specific resistances for imperfect titania/stainles~ steel mesh membranes and a commercial Zirconia Inconel mesh membrane [1]. The results are summariset
Fig. 9. Titania agglomerates forming surface of m e m b r a n e heat. treated at 800 °C.
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R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257
in Table 4. It is apparent that no difference was observed for membranes heat-treated at different temperatures. The cracked or holed membranes had a lower pressure drop than the perfect membranes, which in turn had a lower pressure drop than the commercial zirconia on Inconel membrane.
3.6. Flexibility The flexibility of the optimum membranes was assessed by bending the membranes around a cylindrical former. Once a membrane had been bent around the former and then straightened, the resistance to air flow at room temperature was measured. This was repeated with formers of successively reduced radius until the membrane failed. Fig. 10 shows the results of this test. The membrane was bent around a former of radius 11 mm without failure. The failure of the membrane after bending around a former of radius 8 mm was not catastrophic but was caused by two adjoining interstices of the metal mesh having the titania membrane crack and fall out (see Fig. 11). No other cracks or holes were observed. This demonstrates the ability of filters made of this ceramic/metal mesh composite to withstand flexing and strains which could be imposed during installation. In addition, the vibrations that occur with the movement of large quantities of air are not expected to destroy the membrane. The metal mesh limits the extent of Table 4 Specific pressure drops of membranes Sample
Specific resistance (Pa s m -1)
Perfect titania/mesh membranes Cracked titania/mesh membranes Commercial zirconiafInconel membrane Commercial coated fabric filter
9.5 x 105 (2.0-8.2) × 105 15.0 x 105 1.7 × 105
12
Fig. 11. Surface of titania membrane following bending test showing failure in two adjacent interstices.
any membrane damage to the immediate vicinity of the failure. This would thus prevent complete failure of the filter unit.
4. Conclusions The work of Davidson et al. [1] in producing zirconia and alumina membranes on metal mesh has been extended to enable the preparation of a flexible composite membrane of titania and stainless steel. The prepared membranes have no cracks evident through optical microscopy and porosimetry. The unstable nature of the colloidal suspension causes agglomeration of the titania before evaporation of the solvent which prevents crack formation. Avenues for further work include proving the effectiveness of the membranes as filters, development of a technique to enable drying in orientations other than horizontal and development of methods of preparation that will reduce the pressure drop. This membrane could be used at temperatures up to 800 °C and, when activated with a NO× catalyst, has potential to be used for the simultaneous filtration of flyash and reduction of nitric oxides from coal-fired power station exhausts.
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Acknowledgements The financial assistance for this study was provided by NERDDC/ACARP. R.B.H. gratefully acknowledges the support of the Australian Postgraduate Research Scholarship and the Malcolm Chaikin Scholarship.
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Fig. 10. Flexibility of membrane assessed by wrapping around cylinders of decreasing size and measuring pressure drop as a function of air flow.
[11
A.P. Davidson, M.P. Thomas and S.W. Summers (Alcan International Ltd.), Composite membranes, E P Patent No. 348 041
(1989).
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A.P. Davidson, M.P. Thomas, D.C. Azubike and P.M. Gallagher, Proc. Vth World Filtration Congr., 1990, pp. 235-241. Anon, Filtr. Sep., 29 (1992) 381-384. H.E. Curry-Hyde and M. Andrews, paper in preparation, 1994. H. Bosch and FJ.J.G. Janssen, Catal. Today, 2 (1987) 369-532. T. Entwistle, in A.D. Wilson, J.W. Nicholson and H J . Prosser (eds.), Surface Coatings 2, Elsevier, Amsterdam, 1988, p. 212.
[7] [8] [9]
257
L.L. Hench, in L L Hench and D.R. Ulrich (eds.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, pp. 52-63. J.E. Hall, R. Benoit, R. Bordeleau and R. Rowland, Z Coat. TechnoL, 60 (1988) 49-61. T. Allen, Particle Size Measurement, Chapman and Hall, London, 1981, p. 251.
ELSEVIER
Preparation of composite ceramic membrane from titania powders Robert B. Hutchison *, H. Edward Curry-Hyde, Judy A. Raper School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington, N S W 2052, Australia Received 14 March 1994; revised 24 August 1994
Abstract
A titania-based composite membrane has been developed. The membrane is prepared from titania powder and stainless steel mesh by an aqueous slurry technique. The effects of dispersion technique and heat-treatment temperature on the nature of imperfections and titania agglomerates were investigated. The membranes were characterised by determining their pore size distributions, the pressure drop to air flow and the membranes' flexibility. Keywords: Dispersions; Ceramics; Membranes; Titania
1. Introduction Since the first development of composite ceramic membranes using sols and powder suspensions on porous inorganic supports [1], papers have been published on a zirconia ceramic membrane supported on an Inconel 600 mesh [2,3]. These membranes were prepared using aqueous powder suspensions to coat the metallic substrates and are currently used for liquid filtration. The substrate supports the membranes and also maintains the membrane under compressive loading which decreases the likelihood of cracks forming due to pressure difference or mechanical stress. The recently published work [2,3] is on developing applications for this composite zirconia membrane. A potential commercial application of these composite ceramic membranes is in the area of high temperature gas cleaning, such as simultaneous flyash removal and selective catalytic reduction (SCR) of NOx from coalfired power station exhaust streams [4]. Ideally, a composite membrane could act as a support for the catalyst, and could also act as a barrier filter for the flyash in the gas stream. The system, operated at elevated temperatures, would filter the fiyash, remove the NO~ and leave a clean hot gas stream well suited to waste heat recovery systems. Since the preferred catalyst for the SCR reaction is V2Os supported on titania [5], the best ceramic membrane for this purpose would be a metal mesh-supported titania membrane. Previously published * Corresponding author.
0032-5910/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved
SSD! 0032-5910(94)02890-Z
studies [1-3] on metal mesh-supported membranes have not used titania as the membrane material. Simultaneous removal of NOx and flyash requires consideration of both temperature and pressure drop. Current filtration practice involves operation at low temperature (below 200 °C) and low face velocities using high surface area bags which produce low pressure drop (less than 1 kPa). The cost savings in fan capacity afforded by the low pressure drop are somewhat offset by the energy required to cool the exhaust gas to a temperature at which the fabrics making up the filter can survive. For this reason the use of ceramic membranes for gas filtration potentially provides an economic solution provided that the pressure drop can be kept to a minimum. Methods to achieve this would include reduction of the membrane thickness and modification of the pore structure. The work described here represents an investigation into the preparation of stainless steel mesh-supported titania membranes from titania powder. The stainless steel support could enable temperatures of up to 800 °C to be used.
2. Experimental
2.1. Preparation Due to its particle size (0.2 /xm) and availability, a common pigment grade of titania (anatase, Degussa Kronos 1001) was used to prepare the ceramic mem-
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R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
branes in this study. A suspension of the titania was prepared using various compositions such as those given in Tables 1 and 2. The composition given in Table 1 Was that which, with other preparation techniques detailed later, produced crack-free membranes. The additives included in the suspension were used to ensure the formation of an even film without bubbles which does not crack during drying. A dispersant (tetra sodium pyrophosphate, TSPP) was used to aid formation of a homogeneous suspension; a film-forming agent (poly(vinyl alcohol), PVA) was used to prevent failure of the films during coating and drying; and a plasticiser (glycerol) was used to prevent cracking of the membrane during drying and handling before heat treatment [6]. The metal mesh (316 stainless steel, 100 mesh) was cut to size and treated in a sulfuric acid and potassium dichromate bath [1] to clean and etch the surface to ensure adequate adhesion of the titania powder to the metal surface. Stainless steel was used as it was readily available and resistant to corrosion. It can be used at the temperatures required for the SCR of nitric oxides and would be relatively unaffected by the typical environment within flue ducts. After acid treatment the mesh samples were washed in water and maintained wet until coated later on the same day. To make the required suspension, the additives were first mixed together. The PVA was dissolved in hot distilled water to form a 10 wt.% solution and allowed to cool. The PVA solution, glycerol and TSPP were added and agitated until the TSPP had fully dissolved. The titania was added to the solution and dispersed by one o f three techniques: bead mill, ball mill or ultrasonic dispersion. Ultrasonic dispersion was the most successful and required the titania to be added to the solution, along with 50% of the distilled water and wet Table 1 Composition of suspension for making ceramic membranes Material
Proportion (by weight)
Water Titania PVA Glycerol Na4P2OT- 10H20 (TSPP)
0.54 0.43 0.016 0.013 0.0032
Table 2 Composition of suspension used in preparation of cracked and holed membrane seen in Fig. 7 Material
Proportion (by weight)
Water Titania PVA Glycerol Na4P2OT- 10H20 (TSPP)
0.63 0.33 0.019 0.013 0.0019
ground with a mortar and pestle. The rest of the distilled water was added and the suspension was dispersed by agitation in an ultrasonic bath for 15 min. The latter was required to break up the smaller agglomerates as uncoated titania can be difficult to disperse fully in aqueous solutions due to the high agglomerate tensile strengths [6]. Once the suspension was adequately mixed, the wet mesh was dipped into the agitated suspension and dried in air oriented horizontally. The membranes were not force dried as fast drying is known to cause cracking due to the capillary tension between adjacent pores of different diameters [7]. At least 16 h was allowed for drying of the membranes. After drying, the meshes were heat-treated in atmospheric air. To standardise the heat-treatment process, the temperature was raised to the treatment temperature at 1 °C min -1, held at that temperature for 1 h and then cooled at 1 °C min-1. The treatment temperatures were: 400, 500, 600, 700, 800, 900 °C and room temperature (i.e. no heat treatment). 2.2. Characterisation Reliable assessment of the dispersion of titania powder in a liquid is known to be difficult [8]. Dispersion was ascertained by analysing the size distributions of the particles making up the suspension to ensure that no agglomerates were present. After diluting the suspension with the PVA/glycerol/TSPP solution, a Malvern Mastersizer, an instrument that uses forward light scattering to measure particle size [9], was used to measure the titania particle size distribution to assess the degree of dispersion. The changes that occurred in the membrane during heat treatment were studied using simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) with the evolved gases analysed by mass spectrometry. This enabled identification of the processes occurring during heat treatment and identification of the additives that left residues in the membrane. The technique measured the sample weight and heat flux as the temperature was raised at a predetermined rate (5 °C min-1). The gaseous components evolved from the membrane were identified by passing the gaseous products through a mass spectrometer. As mentioned above, one of the potential disadvantages of ceramic membranes may be their high pressure drop. In order to quantify this aspect of membrane performance, the pressure drop of each prepared membrane was measured using the simple rig shown in Fig. 1. The specific resistance to flow of air at ambient temperature (Pa s m -a) was determined by measuring the pressure drop as a function of superficial gas velocity.
R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257 air in
] i
[ mercury ](D) [man°meter I ~
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I micromanometer
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Fig. 1. Schematic of pressure drop testing rig.
The apparatus consisted of a bubble tube (A) to measure the air flowrate through a 25.4 mm diameter circular sample of membrane held in place by a membrane holder (B). The pressure drop was measured with a micro manometer (C) or a mercury manometer (D) according to the appropriate pressure drop range. The internal morphology of the heat-treated membrane was assessed by determining the pore size distribution using mercury porosimetry and the surface area using the single-point BET nitrogen adsorption method. This information gave insight into the effect of heat treatment on the titania powder pore structure. Optical and scanning electron microscopies were used to assess the surface morphology of the membrane and the packing of the titania particles at the surface of the membrane.
3. Results and discussion
To produce ceramic membranes with the desired characteristics, it is first necessary to have some understanding of the effect of preparation parameters on the membrane characteristics and performance. This paper describes an experimental programme aimed at producing membranes with the desired characteristics. These include achieving dispersion of the titania powder and the effect of heat treatment on the final pore size distribution of the membrane. From these experiments, information relating to the factors affecting the physical strength of the membrane was also obtained.
251
milling was performed for periods varying between 5 min and 6 h. The degree of dispersion was assessed by observing drops of the suspension on a microscope slide immediately after stopping the ball mill. As the milling continued there were fewer obvious aggregates within the suspension a n d its form was more even. After 4 h, no obvious aggregates were seen in the suspension so it was assumed to be fully dispersed. It was also noticed that the viscosity of the suspension increased as the milling progressed for up to 4 h after which it remained constant. In the second method, the suspension was placed in a bead mill with zirconia beads (diameter 1.5 ram) for periods between 10 min and 3 h. No differences could be observed between any of the bead-milled samples. It was suspected that the zirconia beads were too small to break up effectively the larger aggregates due to the relatively low kinetic energy of collisions between the beads. No increase in suspension viscosity was noticed during the bead milling. In the third method, when the suspension was agitated in the presence of an ultrasonic field for approximately 10 min, a drop examined on a microscope slide did not appear to contain any aggregates. The effect of the ultrasonic field on dispersion of the titania can be seen clearly in Fig. 2. The mean particle size of the titania suspension was significantly reduced .after a 10 min exposure to the ultrasonic field producing a dispersed suspension. However, the dispersion was clearly unstable since, when the ultrasonics were stopped for more than a few minutes, aggregation occurred rapidly, forming large agglomerates. Membranes were prepared from a suspension with the composition given in Table 1 using each of the dispersion techniques. The membranes prepared using ultrasonic dispersion were free of cracks, whereas the other membranes were cracked in each mesh interstice. It is thought that the bead mill did not significantly break up the aggregates of titania, while both the ball mill and ultrasonic dispersion were successful in break-
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3.1. Effects of various dispersion techniques Three dispersion techniques were evaluated. The first was ball milling with 10 mm ceramic balls to disperse the titania, the second was bead milling with 1.5 mm ceramic beads and the third was agitation in an ultrasonic bath. In the first test, the suspension was placed in a ball mill with 10 mm diameter ceramic balls. Ceramic ball
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Fig. 2. Ultrasonic dispersion of titania with particle size measured by Malvern Mastersizer.
252
I~B. Hutchison et aL / Powder Technology 81 (1994) 249-257
ing the titania aggregates. The difference between the ball milling and the ultrasonic dispersion techniques was not the degree of dispersion obtained by the techniques but the time between the end of the dispersion and the coating of the metal mesh. The ballmilled titania is thought to have reagglomerated in the 10 min after stopping the ball mill before the mesh was coated. However, the titania dispersed ultrasonically was kept from reagglomerating by coating within the ultrasonic bath (see Fig. 2). The membrane dipped while in the ultrasonic field was thus formed from fully dispersed titania particles which reagglomerated in a few minutes once the membrane was placed on the drying rack. Thus, the effective particle size increased before evaporation of a significant quantity of the solvent. Furthermore, the aggregates formed are thought to have been interlocked and relatively strain-free with van der Waals bonds extending throughout the suspension. These bonds are thought to have been sufficiently numerous to resist the surface tension forces exerted by the evaporation of the solvent from the pores [7]. In this way a membrane free of cracks was able to be produced.
3.2. Effect of heat treatment Transformations that occurred during heat treatment of the dried samples were studied using simultaneous DSC and TGA with analysis of the evolved gas using a quadrupole mass spectrometer. Two runs on the dried suspension (see Table 1) were performed; the first in a helium atmosphere and the second in an air atmosphere. The first run in helium permitted more detail to be obtained from the mass spectrometer, whereas the second run in air simulated the actual processes occurring during the heat treatment of the membranes. The initial investigation was on a membrane in helium atmosphere. A simple spectrum was obtained with two endothermic peaks coincident with two mass loss peaks. The mass loss peaks were observed at 160 and 285 °C. The magnitudes of the mass losses were 2.1 and 2.2%, respectively. It is thought that the peak at 160 °C was due to the evaporation of most of the glycerol. Decomposition of the PVA is believed to account for the peak at 285 °C. The TGA/DSC spectrum for the dried suspension heated in an air environment shows a much more complex system, as seen in Fig. 3. Mass loss peaks (and loss percentages) occurred at 55 (2.0), 200 (2.4), 240 (1.9) and 400 °C (1.9%), with a further very broad peak centred at approximately 300 °C (1.4%). The total mass loss of 9.6% corresponds well with the sum of the masses of the PVA, glycerol and the water of hydration of the TSPP (9.8%). The initial loss of 2.0% at 55 °C is believed to be adsorbed water from the initial solvent and the TSPP as pure TSPP loses its
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water of hydration at 80 °C. Glycerol boils in the pure state at 290 °C and its proportion in the material was 3.5%. The spectrum suggests that this component comes off in stages, possibly depending on the physical environment of each of the glycerol molecules. The PVA decomposes in air beginning at 200 °C and was essentially completely decomposed by 470 °C. There are a number of peaks in the decomposition of PVA and the interaction with the glycerol molecules is thought to account for some of the wide peaks in the spectrum.
3.3. Titania powder bonding The possibility of increasing the strength of the bonding between the titania particles was investigated so that membranes of increased strength could potentially be made. Pure titania powder was mixed with water and dried. The titania was divided into three samples and heat-treated in air at 85, 400 and 800 °C for 1 h. Each sample was then placed in a stainless steel mesh basket with 150/zm apertures and suspended in water within an ultrasonic bath. After periods of time, the basket was removed, dried at 85 °C, weighed and then replaced in the ultrasonic bath. The samples lost mass when pieces less than 150 /zm were broken off due to the influence of the ultrasonic field and these pieces passed through the apertures of the mesh. The mass loss of each sample was measured as a function of the ultrasonic field strength. A second test was performed on two samples of the dried titania suspension (containing PVA, glycerol and TSPP). These two samples were heat-treated in air at 400 and 800 °C, respectively. Table 3 shows that the sample heat-treated at 85 °C was broken up completely to particles less than 150 /zm in size at the test conclusion, while the sample of titania heat-treated at 400 °C was slightly broken up and lost 5.6% of its mass. The pure titania sample (heat-treated at 800 °C) and the dried titania suspensions (heat-treated at 400 and 800 °C) did not lose any mass
R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257
253
Table 3 Mass loss of titania heat-treated at different temperatures Ultrasonic field and duration
Mass loss of titania samples (%)
12 W, 2 h 24 W, 2 h 30 W, 1 h
Pure titania heattreated at 85 *C
Pure titania heattreated at 400 *C
Titania suspension heat- Pure titania heattreated at 400 *C treated at 800 °C
Titania suspension heattreated at 800 *C
8 98 100
2.2 3.7 5.6
0 0 0
0 0 0
and were unaltered in appearance after being in the ultrasonic field for a total of 5 h. These results show that aggregates formed from a PVA, glycerol, TSPP and titania suspension heat-treated at 400 °C are stronger than those formed from a titania suspension without any additives heat-treated at 400 °C. Another test on the titania heat-treatment process was to determine whether there was any reduction in surface area as this would be expected if any sintering occurred. Single-point nitrogen BET surface area measurements were made on dried titania suspension heattreated at the same time as the membranes. The total surface area of the particles as a function of heattreatment temperature is given in Fig. 4, which shows that as the treatment temperature was increased the surface area increased to a maximum at 400--600 °C. The low surface area of the room temperature-dried sample compared to the pure titania powder is most likely due to the binder and the plasticiser filling or blocking the mesopores in the material. Both of these compounds start to decompose or vaporise at temperatures below 400 °C. Thus, the pores would start to clear and the surface area would increase as observed. The sample heat-treated at 800 °C shows a significant reduction in surface area compared to the samples heat-treated at temperatures between 400 and 700 °C, and a significant degree of interparticle bonding was believed to be present. 10 •
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The sample treated at 900 °C shows the greatest degree of reduction in surface area and thus a high degree of interparticle bonding would be expected. However, this temperature could not be used for membrane preparation as the stainless steel mesh corroded severely in air at this temperature. Cracks were also evident in every interstice of membranes heat-treated to this temperature. T h e cracking was thought to be due to corrosion products of the wires exerting excessive forces on the membrane and thus causing it to fail. The heat treatment was intended to remove the additional components (other than titania) and to improve the mechanical strength of the membrane through interparticle bonding and imposition of compressive forces by the stainless steel mesh. The TGA/DSC results show that the additional components were removed (except for some of the TSPP) and the ultrasonic test showed that there was a significant increase in the mechanical strength of the membrane. Pore size distributions of membranes prepared using ultrasonic dispersion and heat-treated at different temperatures were determined by mercury porosimetry. A summary of the results is given in Fig. 5. The pore size distribution could be divided into three distinct sections based on the expected membrane morphology: • 300-3 /Lm. These pores are believed to be due to the cracks and holes in the membrane and gaps between touching adjacent mesh in the penetrometer. z
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200 400 600 800 1000 TEMPERATURE (C) Dried titania Suspensions PureTitania Powder: wet with water then dried
Fig. 4. Single-point nitrogen BET surface area measurements on dried titania suspensions.
0.04 0 1000
~v
900C
10 0.1 PORE SIZE (prn)
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o
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Fig. 5. Pore size distribution as a function of heat-treatment temperature measured by mercury porosimetry.
254
R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
Unfortunately due to the shape and size of the penetrometers it was not possible to prevent touching between adjacent pieces of membrane. • 3--0.02 /~m. These pores are believed to be due to the spaces between the primary titania particles. 3 /~m is fifteen times the size of the primary particles and 0.02 ~m is one tenth of the particle diameter. • 0.02-0.003 /~m. These pores are believed to be the voids at the junction of the titania particles where small irregularities in the surfaces of the particles will show up as mesopores. Fig. 9 shows that the individual particles are without obvious defects to the definition limit of the electron micrograph (less than 0.010 /zm). Fig. 5 shows that there is little difference in the pore size distribution of membranes treated at the different temperatures up to 700 °C. The membrane treated at 900 °C shows a different structure. There was a larger intrusion volume in the 0.2-1.2/~m range. This suggests that it had larger interstices than the other membranes. This was confirmed ~by electron microscopy of this membrane which also showed corrosion of the stainless steel mesh. The membranes treated at the higher temperatures show lower volumes of very small pores, suggesting that there may have been some sintering between the particles leading to the elimination of these very fine pores. The membranes treated at 400 °C and those not treated at all showed a higher volume of pores in the very small range (less than 50 nm and greater than 3 nm). This can be explained by the presence of the additives in the membrane. These were fully destroyed just below 500 °C and so could not contribute to pore volume above this temperature. While these additives were present on the membrane, their physical structure could have contributed to the very fine pores.
3.4. Membrane morphology After drying at room temperature, the crack-free membranes prepared using ultrasonic dispersion had a glossy surface sheen. This was probably due to glycerol and PVA coating the surfaces. Once this material had been decomposed through heat treatment, the surface took on a matt appearance as the titania particles were exposed. There was no difference in the macroscopic appearance of the membranes treated at temperatures from 400 to 800 °C. Fig. 6 shows titania aggregates with diameters from 5 to 50 /xm on the membrane's surface. This membrane was produced using the suspension recipe given in Table 1, ultrasonic dispersion and heat-treated at 800 °C. These aggregates could have been due to poor dispersion of the titania in the suspension, despite the ultrasonic treatment, but may have formed during the agglomeration of the titania prior to drying of the membrane.
Fig. 6. Surface of titania membrane showing agglomerates of titania and mesh coated with titania.
Fig. 7. Top surface of imperfect titania membrane showing cracks and holes and exposed mesh.
Fig. 7 shows holes formed during membrane preparation with diameters greater than 100 /~m. This membrane was produced using an early recipe (given in Table 2), dispersed ultrasonically and heat4reated at 400 °C. The defects are most likely formed by incomplete coverage of the mesh due t o the lower proportion of titania in the suspension. The membranes produced with this recipe also had two distinct sides with titania concentrated on one of them. This was due to the horizontal orientation of the membrane while being dried. Undulations in the mesh plane also caused the titania to concentrate at valleys in the mesh. Concentration of titania on the lower side of the mesh was also apparent from the fact that the wires on top of the mesh were exposed (see Fig. 7), whereas the wires on the lower side were covered with a coating of titania. For comparison, the top of a membrane prepared using the suspension recipe given in Table 1, ultrasonic dispersion and heat-treated at 800 °C, can be seen in Fig. 6. Drying cracks of less than 10 /zm wide can also be seen in Fig. 7. Imperfections in other membranes were caused by bubbles or foam in the
R.B. Hutchison et al. / Powder Technology 81 (1994) 249-257
suspension during dipping whilst in the ultrasonic bath. These bubbles produced very thin sections of membrane that cracked or fell away during drying. The thickness of the crack-free membranes was 20-50 /~m in the centre of the mesh interstice. When the membrane was thinner than this, drying cracks and holes from incomplete coverage were more prevalent. Thicker membranes also cracked during the drying process, probably due to uneven rates of drying. The surface would have fully dried before the centre of the membrane leading to uneven shrinkage of the membrane. After preparation, some membranes were set in resin to enable cross sections to be examined by electron microscopy. A surface examination was also performed to analyse the shape of individual mesh interstices, the powder aggregates and the membrane pores at higher magnification than possible with optical microscopy. Examination of cross sections of an imperfect membrane produced with the recipe given in Table 3, using ultrasonic dispersion of the titania and heat-treated at 400 °C, showed that the cracks passed all the way through the membrane (see Fig. 8). These cracks were not concentrated around the wires, indicating that the titania powder adheres to the stainless steel mesh and does not fall off. The general shape of all the membranes was as would be expected from a liquid suspension process (see Fig. 8). That is, the membrane was thickest at the wires and thinnest in the centre where the two menisci were closest. Bubbles formed during the preparation process are evident in imperfect membranes and these are often the source of cracks. Fig. 7 shows some interstices cracked in a cross formation. These cracks are believed to have been formed by stresses occurring during the drying process due to the use of the suspension with a lower titania concentration. Examination of the appearance of one interstice of the mesh (250 /~m in diameter) showed that there were
Fig. 8. Cross section of imperfect titania m e m b r a n e showing cracking and the asymmetric distribution of titania between the m e s h interstices resulting from drying the m e s h in a horizontal orientation.
255
two main patterns of cracking. The thicker membranes showed cracks that followed the wires, most likely due to shrinkage of the ceramic towards the centre of the mesh interstice. The thinner membranes had cracks that travelled diagonally across the cell with the widest part of the cracks in the interstice centre. These cracks were most likely caused by shrinkage away from the centre and towards the wires in the mesh. Examination of the agglomerates of titania particles (see Fig. 9) showed the shape of the individual particles but did not show how the particles were attached to each other. No direct evidence of interparticle bonding could be observed. The particles were formed in aggregates with large pores surrounding compact agglomerate structures. The range in pore sizes measured from Fig. 9 was from less than 0.05 to 0.3 /zm. This correlates well with the mercury porosimetry results (Fig. 5) which indicated that the pores within the membrane were in the range 0.02--0.25 ~m. The optimum parameters for preparation of the membranes with the techniques used in this study were: • Ultrasonically dispersed titania with the slurry recipe shown in Table 1. • Dip-coating of the mesh followed by drying in a horizontal orientation. • Heat-treating at a temperature of 800 °C. These conditions produced membranes which were crack-free and with the highest degree of bonding between the titania particles.
3.5. Pressure drop of membranes Pressure drops across crack-free membranes were measured as a function of air velocity and heat-treatmenl temperatures. These measurements were compared witt the specific resistances for imperfect titania/stainles~ steel mesh membranes and a commercial Zirconia Inconel mesh membrane [1]. The results are summariset
Fig. 9. Titania agglomerates forming surface of m e m b r a n e heat. treated at 800 °C.
256
R.B. Hutchison et aL / Powder Technology 81 (1994) 249-257
in Table 4. It is apparent that no difference was observed for membranes heat-treated at different temperatures. The cracked or holed membranes had a lower pressure drop than the perfect membranes, which in turn had a lower pressure drop than the commercial zirconia on Inconel membrane.
3.6. Flexibility The flexibility of the optimum membranes was assessed by bending the membranes around a cylindrical former. Once a membrane had been bent around the former and then straightened, the resistance to air flow at room temperature was measured. This was repeated with formers of successively reduced radius until the membrane failed. Fig. 10 shows the results of this test. The membrane was bent around a former of radius 11 mm without failure. The failure of the membrane after bending around a former of radius 8 mm was not catastrophic but was caused by two adjoining interstices of the metal mesh having the titania membrane crack and fall out (see Fig. 11). No other cracks or holes were observed. This demonstrates the ability of filters made of this ceramic/metal mesh composite to withstand flexing and strains which could be imposed during installation. In addition, the vibrations that occur with the movement of large quantities of air are not expected to destroy the membrane. The metal mesh limits the extent of Table 4 Specific pressure drops of membranes Sample
Specific resistance (Pa s m -1)
Perfect titania/mesh membranes Cracked titania/mesh membranes Commercial zirconiafInconel membrane Commercial coated fabric filter
9.5 x 105 (2.0-8.2) × 105 15.0 x 105 1.7 × 105
12
Fig. 11. Surface of titania membrane following bending test showing failure in two adjacent interstices.
any membrane damage to the immediate vicinity of the failure. This would thus prevent complete failure of the filter unit.
4. Conclusions The work of Davidson et al. [1] in producing zirconia and alumina membranes on metal mesh has been extended to enable the preparation of a flexible composite membrane of titania and stainless steel. The prepared membranes have no cracks evident through optical microscopy and porosimetry. The unstable nature of the colloidal suspension causes agglomeration of the titania before evaporation of the solvent which prevents crack formation. Avenues for further work include proving the effectiveness of the membranes as filters, development of a technique to enable drying in orientations other than horizontal and development of methods of preparation that will reduce the pressure drop. This membrane could be used at temperatures up to 800 °C and, when activated with a NO× catalyst, has potential to be used for the simultaneous filtration of flyash and reduction of nitric oxides from coal-fired power station exhausts.
10
gs
Acknowledgements The financial assistance for this study was provided by NERDDC/ACARP. R.B.H. gratefully acknowledges the support of the Australian Postgraduate Research Scholarship and the Malcolm Chaikin Scholarship.
0,,~
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0.014
References 0.8cm
Fig. 10. Flexibility of membrane assessed by wrapping around cylinders of decreasing size and measuring pressure drop as a function of air flow.
[11
A.P. Davidson, M.P. Thomas and S.W. Summers (Alcan International Ltd.), Composite membranes, E P Patent No. 348 041
(1989).
KB. Hutchison et al. / Powder Technology 81 (1994) 249-257 [2] [3] [4] [5] [6]
A.P. Davidson, M.P. Thomas, D.C. Azubike and P.M. Gallagher, Proc. Vth World Filtration Congr., 1990, pp. 235-241. Anon, Filtr. Sep., 29 (1992) 381-384. H.E. Curry-Hyde and M. Andrews, paper in preparation, 1994. H. Bosch and FJ.J.G. Janssen, Catal. Today, 2 (1987) 369-532. T. Entwistle, in A.D. Wilson, J.W. Nicholson and H J . Prosser (eds.), Surface Coatings 2, Elsevier, Amsterdam, 1988, p. 212.
[7] [8] [9]
257
L.L. Hench, in L L Hench and D.R. Ulrich (eds.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, pp. 52-63. J.E. Hall, R. Benoit, R. Bordeleau and R. Rowland, Z Coat. TechnoL, 60 (1988) 49-61. T. Allen, Particle Size Measurement, Chapman and Hall, London, 1981, p. 251.