**6. Process parameters**

#### *Feed concentration*

248 Distillation – Advances from Modeling to Applications

results in the reduction of driving force for water vapour transport leading a decrease in

*Mass transfer through the membrane pores:* The main mass transfer mechanisms through the membrane in MD are Knudsen diffusion and molecular diffusion (Figure 5). Knudsen diffusion model is responsible for mass transfer through the membrane pore if the mean free path of the water molecules is much greater than the pore size of the membrane and hence, the molecules tend to collide more frequently with the pore wall (Li et al., 2008; Nagaraj et

Fig. 5. Mass transfer mechanism involved in water vapour transport through membrane

*<sup>m</sup>* 1.064 *r M <sup>K</sup>*

where *ε* is the fractional void volume, *δ* is the membrane thickness, *τ* is the tortuosity, *M* is the molecular weight of water, *R* is the gas constant and *T* is the absolute temperature.

On the other hand, when the pore size is relatively large, the molecule–molecule collisions are more frequent and molecular diffusion is responsible for mass transfer through the

> ln 1

Both models were successfully applied for predicting the mass transfer through the membrane in DCMD systems (Babu et al., 2006; Bandini & Sarti, 1999; Chen et al., 2009;

*D M <sup>K</sup> Y RT* 

*m*

where Yln is the log mean of mole fraction of air and D is the diffusion coefficient.

Lawson & Lloyd, 1996b; Nagaraj et al., 2006b; Srisurichan et al., 2006).

0.5

(19)

(20)

*RT*

In this case, the membrane diffusion coefficient is calculated using equation:

transmembrane flux (Babu et al., 2006; Babu et al., 2008; Nagaraj et al., 2006b).

al., 2006b; Pabby et al., 2009; Srisurichan et al., 2006).

pores of MD module.

membrane pores (Khayet & Matsuura, 2011).

Permeate flux decreases with an increase in feed concentration. This phenomenon can be attributed to the reduction of the driving force due to decrease of the vapour pressure of the feed solution and exponential increase of viscosity of the feed with increasing concentration. The contribution of concentration polarization effects is also known, nevertheless, this is very small in comparison with temperature polarization effects (Lagana et al., 2000; Pabby et al., 2009). As it is well known, MD can handle feed solutions at high concentrations without suffering the large drop in permeability observed in other pressure-driven membrane processes and can be preferentially employed whenever elevated permeate recovery factors or high retentate concentrations are requested (i.e. concentration of fruit juices) (Curcio & Drioli, 2005; Li & Sirkar, 2005; Schofield et al., 1990b).

#### *Feed temperature*

Various investigations have been carried out on the effect of the feed temperature on permeate flux in MD. In general, it is agreed upon that there is an exponential increase of the MD flux with the increase of the feed temperature. As the driving force for membrane distillation is the difference in vapour pressure across the membrane, the increase in temperature increases the vapour pressure of the feed solution, thus results an increase in the transmembrane vapour pressure difference.

It is worth quoting that working under high feed temperatures was offered by various MD researches, since the internal evaporation efficiency (the ratio of the heat that contributes to evaporation) and the total heat exchanged from the feed to the permeate side is high. Nevertheless, the increase in quality losses and formation of unfavorable compounds (i.e. hydroxymethyl furfural and furan) in fruit juices due to high operation temperatures restricts the temperature levels (Ciesarova & Vranova, 2009; Crews & Castle, 2007; Onsekizoglu et al., 2010b). Temperature polarization effect also increases with the increase in feed temperature (Moon et al., 2011).

#### *Feed flow rate & stirring*

In MD, the increase in flow and/or stirring rate of feed increases the permeate flux. The shearing forces generated at high flow rate and/or stirring reduces the hydrodynamic boundary layer thickness and thus reduce polarization effects. Therefore, the temperature and concentration at the liquid-vapour interface becomes closer to the corresponding values at the bulk feed solution (Winter et al., 2011). Onsekizoglu et al. (2010a) studied the effects of various operating parameters on permeate flux and soluble solid content of apple juice during concentration through osmotic distillation (OD) and membrane distillation (MD) processes. They observed that the effect of feed flow rate on transmembrane flux was less than half of the influence of temperature difference across the membrane.

The effect of flow rate on MD flux becomes more noticeable at higher temperatures especially associated with higher temperature drop across the membrane (Walton et al., 2004). Consequently, higher productivity can be achieved by operating under a turbulent flow regime. On the other hand, the liquid entry pressure of feed solution (LEP) must be taken into account in order to avoid membrane pore wetting when optimizing feed flow rate (Hwang et al., 2011; Khayet et al., 2006).

Membrane Distillation: Principle, Advances,

MD system with almost the same energy.

which were higher than those obtained with single-strength juice.

microfiltration) is involved in order to improve both RO and OD flux.

the operation, thus the coupled process was proposed to be more effective.

Limitations and Future Prospects in Food Industry 251

flocculation step using fining agents such as gelatine and bentonite. Hongvaleerat et al. (2008) obtained flux values of about 7-10 kg/m2h in pineapple juice concentrate production by OD

RO or forward osmosis (FO) processes have been proposed as a pre-concentration step before OD or MD promising reduction of processing costs. High quality fruit juice concentrates can be produced economically in this manner. Therefore, an integrated process involving preconcentration of the feed by RO followed by further concentration by OD or MD should yield a high-solids product concentrate of quality comparable to that achieved by OD alone but at significant reduction in processing cost (Martinetti et al., 2009; Nayak & Rastogi, 2010; Wang et al., 2011). The combination of RO and OD processes was evaluated by Cabra et al. (2011) for concentration of Acerola juice, by Kozak et al. (2009) for concentration of Black currant juice, by Galaverna et al. (2008) for concentration of blood orange juice, by Cassano et al. (2003) for concentration of citrus and carrot juices. It is worth mentioning that in all the previously mentioned studies, a clarification pretreatment step (i.e. ultrafiltration of

Criscuoli & Drioli (1999) presented a detailed energetic and exergetic analysis of both RO– MD and NF–RO–MD integrated systems. They observed an improvement in the performance of the integrated system by introducing NF as water pretreatment for the RO–

The coupled operation of MD and OD processes is another promising approach to improve transmembrane flux. In this case, osmotic solution is cooled and the feed solution is slightly heated in order to provide additional driving force. Belafi-Bako & Koroknai (2006) compared MD, OD and coupled operation of OD and MD in terms of flux and final soluble solid concentration in sucrose model solutions and apple juice. Higher water flux and SSC values were achieved with coupled operation confirming an increase in driving force. More recently, Onsekizoglu (2011), have proposed the use of a coupled membrane process capable of concentrating pomegranate juice under very mild conditions. The pomegranate juice was clarified by ultrafiltration in a cross-flow membrane filtration unit (MWCO: 100 kDa). The clarified juice then concentrated by coupled operation of OD and MD, in which the feed solution is gently heated (30.0±2.0°C) and the osmotic solution (CaCl2.H2O) is slightly cooled (10.0±1.0°C). The final step yielded a concentration of the clarified juice (with an initial total soluble solid content of (TSS) 17°Brix) up to 60-62°Brix. The experiments have proven that the driving forces were added in coupled operation, which resulted in enhanced water flux during

Several strategies for reducing temperature polarization through membrane arrangement in MD have been proposed. Some authors have considered the use of spacer-filled channels (Chernyshov et al., 2003; Cipollina et al., 2011; Phattaranawik et al., 2001; Teoh et al., 2008; Wang, 2011). The spacers can improve the flow characteristics at the membrane surface and by promoting regions of turbulence due to the formation of eddies and wakes. Therefore, the temperature polarization can be reduced by improved boundary layer heat transfer. Various surface modification techniques including coating, grafting and plasma polymerization to reduce temperature polarization effect though improvement of membrane surface characteristics have been employed. For example, a novel hollow fiber membrane was proposed by Li & Sirkar (2005) which were commercial porous PP hollow fibres coated with a variety of ultrathin microporous silicone-fluoropolymer layer on surface

#### *Permeate temperature*

The increase in permeate temperature results in lower MD flux due to the decrease of the transmembrane vapour pressure difference as soon as the feed temperature kept constant. It is generally agreed upon that the temperature of cold water on the permeate side has smaller effect on the flux than that of the feed solution for the same temperature difference. This is because the vapour pressure increases exponentially with feed temperature (Alklaibi & Lior, 2005; El-Bourawi et al., 2006).

#### *Permeate flow rate*

The increase in permeate flow and/or stirring rate reduces the temperature polarization effect. Consequently, the temperature at the gas/liquid interface approaches to the bulk temperature at the permeate side. This will tend to increase driving force across the membrane; resulting an increase in MD flux (Courel et al., 2000; Hongvaleerat et al., 2008). It is important to note that as the permeate used in the MD is distilled water and in the OD is hypertonic salt solution; the extent of the effect of flow rate is more prominent in the latter configuration. This is because of the contribution of concentration polarization effects on permeate side in OD.
