Performance Evaluation of Fly Ash-based Tubular Ceramic Membrane in Liquid Phase Separation Processes
4.3 Separation of glycerol from biodiesel .1 Chemicals
4.3.3 Results and discussions
biodiesel is quite simple, fast and cost effective than the conventional method like Gas Chromatography technique (Nogueira et al., 2019).
A microscope with 20X magnification (Model No.: Zeiss Axio Scope.A1, Make: Carl Zeiss Microscopy GmbH Germany) was used to capture the images of droplets of biodiesel emulsion prepared at 60 oC. The droplet size distribution and average size of the biodiesel emulsion were evaluated by analysing three different microscope images using ImageJ software (Open-source software, https://imagej.nih.gov/ij/).
On completion of each experiment, the setup was initially flushed with methanol for 30 minutes as the emulsion easily solubilises in methanol, which makes the cleaning easy. This was followed by cleaning the setup with 1 g/L surf excel solution for 30 minutes. An aqueous NaOH solution (1 wt.%) was allowed to pass through the setup for another 30 minutes to remove the traces of emulsion that may be present in the setup (Atadashi et al., 2015). Finally, flushing with Millipore water was carried out and the water permeability of the cleaned membrane was measured again. It was found that the hydraulic permeability of the cleaned membrane was within ±4% of its original value, which signifies the reusability of the membrane for further experiments.
also reported an enhanced permeate flux with increasing transmembrane pressure for ceramic membranes (Khemakhem et al., 2009; Jana et al., 2010; Suresh et al., 2016; Kumar et al., 2016).
Fig. 4.18 Effect of applied pressure on the permeate flux (Cross flow velocity: 8.33×10-6 m3/s)
As evident from the flux versus time curves (Fig. 4.18), a relatively rapid flux decline is noticed at the beginning of the filtration, followed by a more gradual decrease, until a pseudo-steady state flux is reached (Rodrigues and Fernandes, 2012). However, it is worth to mention that only at the start of the filtration process, the effect of increasing the applied pressure on permeate flux is significant and as filtration proceeds, the difference between steady state permeate flux for all the investigated pressures decreases (Buetehorn et al., 2010; Gomes et al., 2013). The resistance of the cake layer formed by retained agglomerates on the membrane surface controls the flux at this phase of separation process. A previous study also indicated that thicker cake layer is formed at higher applied pressures due to which the influence of flux at latter phase of cross flow filtration is not significant (Buetehorn et al., 2010; Hong et al., 1997).
Fig. 4.19 (a) Glycerol separation mechanism across the membrane (b) Contents of soap and free glycerol in the permeates obtained under different applied pressures
While looking at the membranes’ performance regarding glycerol retention from biodiesel, it can be said that the membrane successfully reduced the glycerol content of biodiesel from 8.33 wt.% (feed) to 0.0107-0.0231 wt.% (permeate) at various pressures (207 – 483 kPa) through the mechanism of size exclusion (Fig. 4.19). This can be attributed to the fact that in the emulsion of biodiesel, glycerol and acidified water is agglomerated due to their high affinity towards each other and constitutes the dispersed phase. These agglomerated compounds remain
suspended in the continuous phase of the emulsion, primarily consisting of biodiesel. Besides this, some residual soap molecules are retained even after the addition of acidified water combines with glycerol to form micelles. These micelles also contribute to the dispersed phase of emulsion and help in the retention of glycerol by the membrane. Methanol is found to be present in the continuous as well as the dispersed phase, owing to its affinity towards both the phases (Gomes et al., 2010; Gomes et al., 2011). It has been found that the droplets of dispersed phases have an average diameter of 31.11 µm at the experiment temperature, as observed under microscope (Fig. 4.20 (a)). Fig. 4.20 (b) represents the droplet size distribution of the emulsion, where it can be seen that most of the droplets formed are in size range of 11-20 µm and 31-40 µm. The mentioned average size of the dispersed droplets is far bigger than the pore size of the membrane, thus aiding in very high rejection of glycerol across the membrane. The permeate obtained primarily consists of biodiesel, with small quantity of methanol and traces of soaps, glycerol and water in it. The rejected agglomerates of water and glycerol are found in the retentate, along with the micelles formed by soap and glycerol. The schematic of the rejection of glycerol from biodiesel is depicted in Fig. 4.19 (a). These findings are in line with the works reported by Gomes et al. (Gomes et al., 2011). However, the increased pressure across the membrane slightly declined the retention performance of the membrane, leading to little higher free glycerol content in the permeate. At higher pressure, the augmented shear on membrane surface may lead to tearing of emulsion droplets, thus allowing a fraction of it to pass through the separation barrier (Sutrishna et al., 2012).
Similar observation was also noticed in the case of soap content in the permeate (Fig. 4.19).
The soap content of the permeate was reduced to below 70 ppm, which is achieved by the combined action of acidified water as well as membrane filtration. Acid present in water reacts with the soap molecules, converting them to soluble salts (Gomes et al., 2011). A portion of the residual soap present even after addition of acidified water was further retained by
membranes. It has already been mentioned that the residual soap molecules combine with the available glycerol to form micelles, remaining dispersed in the emulsion along with glycerol and water (Wang et al., 2009). As these dispersed molecules have higher diameters as compared to the membrane pores, they are retained on the membrane surface in the process of filtration. However, the breakage of micelles takes place at higher pressures like the tearing of agglomerated droplets of water and glycerol, leading to slightly lesser retention of soap molecules on the membrane surface.
Fig. 4.20 (a) Microscopic image of biodiesel emulsion (b) Droplet size distribution of
In this context, it is noteworthy to mention that Fig. 4.21 corresponds to the photo comparison of biodiesel emulsion before (feed) and after treatment, where a successful separation is clearly visible. However, the permeate obtained in the pressure ranges of 207-345 kPa contains less than 0.02 wt% free glycerol and thus satisfies the norms prescribed by ASTM D6751 and EN14214 (ASTM D6751-20; EN 14214). Though there are no specific norms provided regarding the soap content in biodiesel, a reduced content is always preferable as it minimizes the filter plugging of the engine. Moreover, burning of soap rich biodiesel in engine leads to the formation of sulphated ash, causing damage to the fuel injectors as well as to the combustion chamber (ASTM D6751-20). Considering the glycerol content in permeate sample, the applied pressures ranging from 207 - 276 kPa could be adequate to be used in separation of glycerol from biodiesel emulsion.
Fig. 4.21 Photo comparison of feed and permeate samples
Table 4.7 Performance evaluation of the membrane in the treatment of glycerol enriched biodiesel Membrane composition Pore size Experimental
Free glycerol in permeate (wt%)
Permeate flux# (Steady state flux)
References Commercial α-AlO3/TiO2 membrane
(Shumacher GmbH-Ti 01070)
0.2 µm T: 60 ˚C TMP: 200 kPa Cf: 10 wt%
0.06±0.009 ~50.00 kg/m2h (Gomes et
Commercial α-AlO3/TiO2 membrane (Jiangsu Jiuwu Hi-Tech Co., China)
0.02 µm T: 40 ˚C TMP: 200 kPa Cf: 0.42 wt%
0.007 9.08 kg/m2h (Atadashi et
al., 2012) Commercial polyether sulfone
membranes (GE Osmonics, USA)
10 kDa T: 25 ˚C TMP: 400 kPa Cf: 0.049 wt%
0.020 55.00 kg/m2h (Alves et al.,
2013) Commercial α-AlO3/TiO2 membrane
(Shumacher GmbH-Ti 01070)
20 kDa T: 50 ˚C TMP: 300 kPa Cf: 6.80 wt%
0.014±0.002 70.00 kg/m2h (Gomes et al.,
Polyacrylonitrile membrane 6 kDa - 0.017 - (Bansod and
Rathod, 2018) Fly ash, Quartz, Calcium Carbonate 0.133 µm T: 60 ˚C
TMP: 345 kPa Cf: 8.33 wt%
0.0187 9.45 kg/m2h This work