Membrane clarification of juices has been studied using membrane modules of different configurations, such as plate-and-frame (Sheu et al., 1987), hollow fiber fiber wall thickness around 0.2 mm. These modules are generally operated with the feed flow into the fiber, with the permeate moving radially outward through the module while the retentate is collected from the end of the tube. The permeate flows through the membrane into the permeate channel and spirals toward the Flat plate 300 Moderate 0.03–0.25 Moderate Good Not self-
supporting Low
Tubular 60 High 1.0–2.5 Low Excellent Not self-
supporting High
Hollow fiber
1200 Low 0.02–0.25 High Fair
Self-supporting Low
Spiral wound
600 Low Very high Moderate
Self-supporting Low
Jiraratananon et al. (1997) investigated the UF of pineapple juice using a tubu-lar membrane. The module housing was stainless steel with an equivalent diameter of 350 mm. There were 19 tubular membrane channels, each channel was 4 mm in diameter, 500 nm long, and the effective area was 0.12 m2. They used the MF and UF membranes of pore sizes 0.1 and 0.01 μm, respectively. The obtained perme-ate flux and rejection of macromolecules were 6.37 × 103 m3/m2 h and 84%–87%, respectively, at a pressure of 300 kPa and a cross-flow velocity of 2.0 m/s. de Barros et al. (2003) studied the cross-flow UF of depectinized pineapple juice tubular ceramic membrane (0.01 μm) and polysulfone hollow fiber membrane (100 ka). They observed that the permeated flux obtained in tubular ceramic membrane is higher than that obtained in polysulfone hollow fiber membrane. Higher permeate flux in tubular ceramic membrane can be explained by the fact that the turbulent regime flow increases the rate of solute diffusion from the membrane surface to the bulk.
This leads to less compact cake, causing a higher permeate flux compared with the flow obtained due to cake formation and the laminar flow in hollow fiber membrane.
Sarkar et al. (2008a) studied the UF of enzyme-treated mosambi juice in a flat sheet rectangular channel 6.5 mm in height under a laminar flow regime using 50,000 (MWCO) polyethersulfone membrane. The permeate flux was about 10.5 L/m2 h at a pressure of 500 kPa and a cross-flow velocity of 0.12 m/s.
4.2.5 eleCtriC Field–assisted ClariFiCation oF Fruit JuiCe 4.2.5.1 Basic Principle
To overcome problems with membrane fouling and concentration polarization, an external electric field can be applied across the membrane during cross-flow mem- brane filtration of a solution containing a charged solute. In this process, the electri-cal field acts as an additional driving force to the TMP. External electric field from a regulated DC power supply is applied across the membrane surface in the form of continuous or pulse mode. This process utilizes an electrophoretic force that drags the charged solutes away from the membrane surface, thus limiting the solute accu-mulation on the membrane surface (Figure 4.6). The concentration polarization layer and deposited layer over the membrane surface are thereby reduced, resulting in an increase in permeate flux.
4.2.5.2 Apparatus Design
Two different modules are widely used for electric field–assisted UF such as flat sheet (Sarkar et al., 2008c) and tubular (Yukawa et al., 1983). An electric field can be applied across the membrane with one electrode on either side of the membrane (Figure 4.7a), or the electric field may be applied between the membrane and another electrode. Tubular module consists of one electrode that is centrally located within the tubular membrane (Figure 4.7b).
4.2.5.3 Use of Continuous Electric Field
The application of continuous electric field can be found in several areas of indus- trial applications, for example, separation and fractionation of protein solution, separa-tion of biopolymer, in the treatment of mineral and biological slurry, gelatin solution,
Anode
Bulk flow
Cross flow Membrane
δg and permeate are Cb, Cg, and Cp, respectively. δc and δg are the thickness of concentration boundary layer and gel layer, respectively. (Reprinted from Sarkar, B. et al., J. Membr. Sci., 311, 112, 2008c. With permission.)
Bolts DC power supply
Platinum-coated Titanium sheet (positive electrode)
Stainless steel support (negative electrode)
Feed
Retentate Grids in lower flange
+ –
oil-in water emulsion, etc. Flux improvements in the range of 2–10 times have been reported for BSA (Radovich and Sparks, 1980; Radovich et al., 1985; Wakeman, 1998;
Zumbusch et al., 1998; Oussedik et al., 2000), mineral and biological slurry (Weigert et al., 1999), bentonite and algal cells (Moulic et al., 1976), biopolymer (Hofmann, 2003; Park, 2006), and gelatin (Yukawa et al., 1983; Guizard et al., 1989). The appli-cation of electric field was studied in detail during the UF of citrus fruit juice (Sarkar et al., 2008a,b,c, 2009, 2010). Fruit juice contains low-molecular-weight solutes like sugar, acid, salt, flavor, and aroma compounds and high-molecular-weight solutes, mainly pectin, protein, microorganisms, etc. During membrane clarification, the smaller components permeate through membrane while the larger species, mainly pectin, are retained. In fruit juice, the typical concentration of pectin substances is up to 1.0% (Thakur et al., 1997). Pectin, a complex polysaccharide, has a tendency to form gel in the presence of sugar and acid. Pectin gel is a cross-linked polymer molecule net-work in a liquid medium (Thakur et al., 1997; Lee et al., 2007). During clarification of fruit juice, pectin forms a gel-type layer over the membrane surface, which offers extra resistance to the permeate flow apart from membrane resistance (Rai et al., 2006a).
Pectin being negatively charged at its natural pH (about 3.0), the application of external electric field during cross-flow membrane clarification of fruit juice offers great prom-ise in enhancing permeate flux. Thus, the application of an appropriate external electric field may significantly reduce the gel-type layer formation over the membrane surface caused by charged pectin molecules and consequently may increase the permeate flux (throughput) of the membrane system during clarification. Sarkar et al. (2008a) found flux improvements of up to 34% at 400 V/m for mosambi juice.
4.2.5.4 Use of Pulsed Electric Field
Despite the good filtration performance in electric field–assisted membrane fil-tration, the process is not yet used in large-scale production. The major problems associated with the use of a continuous electric field during UF include changes in feed properties due to electrode reaction (Bowen and Subuni, 1991), requirement of high energy, rise of temperature, etc. The method cannot be effectively used for feeds of high electrical conductivity and heat sensitivity. The conductivity of the feed solution is critical. A low conductivity is preferred in this process, since the energy consumption increases with increasing conductivity (Weigert et al., 1999; Bargeman et al., 2002). A high conductivity also increases the amount of heat and electrolyte gases produced at the electrodes. Weigert et al. (1999) showed that if the conductiv-ity of a cristobalit solution exceeded 2 mS/cm, the advantage of using electro- MF vanished, since the energy requirements of the electric field became too high. These problems can be circumvented by the use of pulsed electric field instead of a continu- ous one. Some references of the use of pulsed electric fields are available in the lit-erature (Wakeman and Tarleton, 1987; Bowen et al., 1989; Bowen and Subuni, 1992;
Robinson et al., 1993). Sarkar et al. (2008b) investigated the effect of pulsed electric field during clarification of mosambi (Citrus Sinensis (L.) Osbeck) juice, using a 30 kDa molecular weight cut-off membrane in cross-flow UF mode under laminar flow regime (Figure 4.8). They observed that at a pulse ratio of 3:1 (3 s on and 1 s off), the flux increased up to 39% at 500 V/m for mosambi juice compared with zero elec-tric field. Moreover, pulsed electric field required 22% less energy per unit volume
0 1 2 3 On time/off time ratio
4 Always on
8 Experimental error: 3%
FIGURE 4.8 Variation in steady-state average permeate flux with pulse ratio for various electric fields during clarification of mosambi juice. The solid lines are only guides for the reader. (Reprinted from Sarkar, B. et al., Sep. Purif. Technol., 63, 582, 2008b. With permission.)
TABLE 4.3
Analytical Measurements on Mosambi (Citrus sinensis (L.) Osbeck) Fruit Juice
of permeate compared with continuous mode to produce almost the same magnitude of permeate flux. As shown in Table 4.3, pectin content in fresh mosambi juice was completely removed by both UF and electro-UF (continuous and pulse mode) with-out change in total soluble solid (TSS) content and the resulting clarified juice had lower viscosity and negligible turbidity.
4.2.6 Models For PrediCtion oF PerMeate Flux