Fabrication and Characterization of Low-cost Ceramic Support Membrane
2.2 Results and discussions
2.2.1 Characterization of sawdust
2.2.1.1 Effect of particle size of sawdust on the porosity and pore size of the membrane
Particle size distribution of both raw sawdust (screened through B.S.S. 30, 44, 60, 72 and 100 mesh) and the sample mixtures for membrane fabrication are shown in Figure 2.3 (a- f), respectively. It is observed that particle size of sawdust sieved through 30 B.S.S. (Figure 2.3b) and 44 B.S.S. mesh screen (Figure 2.3c) leads to wide range of distribution. This is due to the presence of wide variety of particle size of sawdust (combination of both fine and coarse particles) whereas the combinations which are prepared using sawdust screened through 60 and 72 B.S.S. mesh shows single mode distribution (Figures 2.3d and e). This nature of particle distribution attributes to a narrow range of particles which provides narrow range of pores.
Figure 2.3f shows bimodal nature of particle size distribution of six different compositions using sawdust separated through 100 B.S.S. mesh which infers the presence of wide range of particles causes wide range of pores. Particle size of sawdust involving statistical parameters such as mode and average size is summarized in Table 2.2. Interestingly, it is found that the median particle sizes of sawdust screened through 60 and 72 B.S.S. are small due to the presence of fine particles of sawdust only (refer Figure 2.3d and e – narrow particle size distribution), provides completely dense membranes, and is not suitable for fabrication of membrane support of a catalytic membrane reactor. Hence, the other sawdust samples (30, 44 and 100 B.S.S.) are considered for further study.
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0.1 1 10 100 1000
0 5 10 15 20 25 30 35 40
Volume (%)
Particle size (m) Raw sawdust screened through (B.S.S.)
30 44 60 72 100
(a)
0.1 1 10 100 1000
0 2 4 6 8 10 12 14 16
Volume (%)
Particle size (µm) Sawdust screened through 30 B.S.S.
SM1 SM2 SM3 SM4 SM5 SM6
(b)
0.01 0.1 1 10 100 1000
0 1 2 3 4 5 6
Volume (%)
Particle size (µm)
Sawdust screened through 44 B.S.S.
SM1 SM2 SM3 SM4 SM5 SM6
(c)
0.1 1 10 100 1000
0 5 10 15 20 25 30 35
Volume (%)
Particle size (µm) Sawdust screened through 60 B.S.S.
SM1 SM2 SM3 SM4 SM5 SM6
(d)
0.1 1 10 100 1000
0 10 20 30 40 50
Volume (%)
Particle size (µm)
Sawdust screened through 72 B.S.S.
SM1 SM2 SM3 SM4 SM5 SM6
(e)
0.1 1 10 100 1000
0 5 10 15 20
Volume (%)
Particle size (µm) Sawdust screened through 100 B.S.S.
SM1 SM2 SM3 SM4 SM5 SM6
(f)
Figure 2.3 Volume weighed particle size distribution of (a) raw sawdust particles and sample mixtures sieved through mesh sizes of (b) 30 B.S.S., (c) 44 B.S.S., (d) 60 B.S.S., (e) 72 B.S.S.
and (f) 100 B.S.S.
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The average pore diameter of the fabricated membrane from different sample mixtures having different weight ratios of raw materials with three different particle size ranges of sawdust of 500, 355 and 150 µm, sieved through 30, 44 and 100 B.S.S. screen, respectively, are also shown in Table 2.3. Table 2.3 shows median particle size and average pore size of the membrane.
Table 2.2 Particle size characteristics of sawdust materials
Sawdust
Mode (µm) 150 212 250 355 500
Average particle size (µm) 298.95 1.836 1.253 205.143 153.43
For the sample mixtures containing sawdust of different sized particles sieved through 100 B.S.S. mesh, the average pore diameter varies between 0.05 to 0.10 µm. Similarly, for membranes made of using sawdust of different sized particles sieved through 30 and 44 B.S.S.
mesh, the average pore diameter varies between 0.04 to 0.10 µm and 0.06 to 0.13µm, respectively, is in good agreement with literatures [115, 116]. It is observed that the average pore diameter of all the fired membranes are very close to each other unlike the particle size of sample mixtures together with different size of sawdust and its distribution. Even, it is quite interesting to note that SM5 membrane containing sawdust screened through 100 and 30 B.S.S.
mesh become cracked and damaged during sintering owing to large particle size and absence of feldspar. This result enlightened the fact of choosing 44 B.S.S. screened sawdust as pore- former for the fabrication of ceramic membrane throughout the study.
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Table 2.3 Determination of particle size and pore diameter for the membrane sintered at 850°C
Membrane
Median particle size D50(µm) of sample mixture
Average pore diameter (µm) of the fired membrane obtained from gas
permeation analysis Sawdust
sieved through 30 BSS
Sawdust sieved through 44 BSS
Sawdust sieved through 60 BSS
Sawdust sieved through 72 BSS
Sawdust sieved through 100 BSS
Membrane 30 BSS
Membrane 44 BSS
Membrane 100 BSS
SM1 383.64 17.15 0.094 0.152 444.39 0.04 0.06 0.05
SM2 23.84 10.92 0.096 0.137 54.34 0.09 0.10 0.09
SM3 104.39 11.20 0.131 0.097 101.73 0.07 0.08 0.07
SM4 30.34 22.85 0.106 0.242 22.72 0.10 0.11 0.10
SM5 326.09 6.493 0.087 0.090 19.44 - 0.13 -
SM6 42.61 13.248 0.072 0.708 401.84 0.07 0.13 0.07
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Table 2.4 Porosity determined using Archimedes’ principle and gas permeation study for the membrane sintered at 850°C
Membrane
Volumetric porosity in % based on Eq. (2.1)
Porosity (%) from gas permeation study based on Eq. (2.2) Membrane
30 BSS
Membrane 44 BSS
Membrane 100 BSS
Membrane 30 BSS
Membrane 44 BSS
Membrane 100 BSS
SM1 52 48 55 45 43 46
SM2 36 38 36 30 34 29
SM3 46 41 48 40 37 39
SM4 34 38 35 28 34 29
SM5 - 38 - - 36 -
SM6 46 36 47 39 34 40
Table 2.4 shows a comparative result of porosities of the fired samples using volumetric porosity determination technique (Archimedes’ principle) and gas permeation study. Results obtained from gas permeation study is considered to be better than volumetric porosity method as the later technique has a tendency to keep water intact into the pores of ceramic body (called capillarity), thus showing higher porosity [115, 116]. But statistically (t-test), it is stated that the obtained porosities of the membranes (made of sawdust screened through 44 B.S.S. sieve) using volumetric porosity and gas permeation differ marginally as the t-Stat value (1.543) is not < -t Critical two-tail (-2.228) or > t Critical two-tail (2.228). However, the variation between the results is prominent from the standard deviation (dispersion) values which confirm that the gas permeation study provides better result. The standard deviation value of the porosities obtained from the gas permeation study is 3.50 which is less than that of the porosity values found from the volumetric porosity, is 4.30, indicates the data points tend to be very close to the mean or expected value for a low standard deviation whereas a high standard deviation
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indicates that the data points are spread out over a large range of values (refer Tables A1 and A2 in Appendix.1). It is true that porosity decreases with increase in average pore diameter and vice-versa but porosity also depends on raw materials present in the sample mixture. On the contrary, it is observed that the porosity of SM5 made of sawdust screened through 44 B.S.S.
(Tables 2.3 and 2.4) increases with increase in average pore diameter as SM5 contains no feldspar which has the ability to reduce porosity of ceramic by forming glassy matrix during sintering.