**2. Process fundamentals**

MD is a thermally driven process, in which water vapour transport occurs through a nonwetted porous hydrophobic membrane. The term MD comes from the similarity between conventional distillation process and its membrane variant as both technologies are based on the vapour-liquid equilibrium for separation and both of them require the latent heat of evaporation for the phase change from liquid to vapour which is achieved by heating the feed solution. The driving force for MD process is given by the vapour pressure gradient which is generated by a temperature difference across the membrane. As the driving force is not a pure thermal driving force, membrane distillation can be held at a much lower temperature than conventional thermal distillation. The hydrophobic nature of the membrane prevents penetration of the pores by aqueous solutions due to surface tensions, unless a transmembrane pressure higher than the membrane liquid entry pressure (LEP) is applied. Therefore, liquid/vapour interfaces are formed at the entrances of each pore. The water transport through the membrane can be summarized in three steps: (1) formation of a vapour gap at the hot feed solution–membrane interface; (2) transport of the vapour phase through the microporous system; (3) condensation of the vapour at the cold side membrane–permeate solution interface (Jiao et al., 2004; Peinemann et al., 2010).

Various MD configurations can be used to drive flux (El-Bourawi et al., 2006; Khayet, 2011; Lawson & Lloyd, 1997; Susanto, 2011; Zhigang et al., 2005). The difference among these configurations is the way in which the vapour is condensed in the permeate side. Figure 1 illustrates the four commonly used configurations of MD described as follows:

1. In direct contact membrane distillation (DCMD), water having lower temperature than liquid in feed side is used as condensing fluid in permeate side. In this configuration, the liquid in both sides of the membrane is in direct contact with the hydrophobic microporous membrane. DCMD is the most commonly used configuration due to its

Today, multistage vacuum evaporation is the predominant method used for liquid concentration in food industry. The main drawbacks of this system are high energy consumption and heat induced deterioration of sensory (color changes, off-flavor formation) and nutritional characteristics (Ibarz et al., 2011; Kadakal et al., 2002; Simsek et al., 2007; Toribio & Lozano, 1986; Varming et al., 2004). Recently, technological advances related to the development of new membrane processes including membrane distillation have been proved to overcome this limitation (Bagger-Jorgensen et al., 2011; Cassano & Drioli, 2007; Hongvaleerat et al., 2008; Kozak et al., 2009; Onsekizoglu et al., 2010b; Valdes et al., 2009). This chapter will cover the process features, theoretical aspects and the relevant mathematics related to water transport mechanism in membrane distillation. The most basic concepts of osmotic distillation, a membrane distillation variant operating at lower temperature will be also discussed. The suggestions for membrane selection taking into account the membrane material and module configuration together with contact angle and membrane wettability will be presented in detail. The process parameters affecting the transmembrane flux and the most promising applications for enhancement of flux will be highlighted. Applications in food industry and long term performance of membrane distillation systems will be evaluated. The possibility of integrating membrane distillation

with other existing processes and suggestions for future work will be presented.

membrane–permeate solution interface (Jiao et al., 2004; Peinemann et al., 2010).

illustrates the four commonly used configurations of MD described as follows:

Various MD configurations can be used to drive flux (El-Bourawi et al., 2006; Khayet, 2011; Lawson & Lloyd, 1997; Susanto, 2011; Zhigang et al., 2005). The difference among these configurations is the way in which the vapour is condensed in the permeate side. Figure 1

1. In direct contact membrane distillation (DCMD), water having lower temperature than liquid in feed side is used as condensing fluid in permeate side. In this configuration, the liquid in both sides of the membrane is in direct contact with the hydrophobic microporous membrane. DCMD is the most commonly used configuration due to its

MD is a thermally driven process, in which water vapour transport occurs through a nonwetted porous hydrophobic membrane. The term MD comes from the similarity between conventional distillation process and its membrane variant as both technologies are based on the vapour-liquid equilibrium for separation and both of them require the latent heat of evaporation for the phase change from liquid to vapour which is achieved by heating the feed solution. The driving force for MD process is given by the vapour pressure gradient which is generated by a temperature difference across the membrane. As the driving force is not a pure thermal driving force, membrane distillation can be held at a much lower temperature than conventional thermal distillation. The hydrophobic nature of the membrane prevents penetration of the pores by aqueous solutions due to surface tensions, unless a transmembrane pressure higher than the membrane liquid entry pressure (LEP) is applied. Therefore, liquid/vapour interfaces are formed at the entrances of each pore. The water transport through the membrane can be summarized in three steps: (1) formation of a vapour gap at the hot feed solution–membrane interface; (2) transport of the vapour phase through the microporous system; (3) condensation of the vapour at the cold side

**2. Process fundamentals** 

Fig. 1. Schematic representation of MD configurations

Membrane Distillation: Principle, Advances,

**4. Membrane characteristics** 

**4.1 Membrane materials** 

et al., 2009)

et al., 2004; Nagaraj et al., 2006a; Shin & Johnson, 2007)

appropriate membrane are summarized in this section.

Limitations and Future Prospects in Food Industry 237

offer several advantages, including low-equivalent weight, high water solubility, steep positive temperature coefficients of solubility and safety in foods and pharmaceuticals (Jiao

The selection of the membrane is the most crucial factor in MD separation performance. As stated earlier, the membrane used for MD process must be hydrophobic and porous. There are various types of membranes meeting these expectations; however the efficiency of a given MD application depends largely on additional factors such as resistance to mass transfer, thermal stability, thermal conductivity, wetting phenomena and module characterization. Membrane and module related characteristics affecting selection of the

A large variety of membranes including both polymeric and inorganic membranes of hydrophobic nature can be used in MD process; however polymeric membranes have attracted much more attention due to their possibility to modulate the intrinsic properties. Polytetrafluoroethylene (PTFE), polypropylene (PP) and polyvinylidenefluoride (PVDF) are the most commonly used polymeric membranes due to their low surface tension values (Table 1). Hydrophobic porous membranes can be prepared by different techniques like sintering, stretching, phase inversion or thermally induced phase separation depending on the properties of the materials to be used. The useful materials should be selected according to criteria that include compatibility with the liquids involved, cost, ease of fabrication and assembly, useful operating temperatures, and thermal conductivity (Li et al., 2008; Liu et al., 2011). Among them, PTFE membranes are the most hydrophobic ones showing outstanding thermal stability and chemical resistance properties (they are low soluble in practically all common solvents). The main disadvantage of PTFE membranes is the difficulty of processing. PTFE membranes are generally prepared by sintering or stretching. PP exhibits

**Polymer Surface tension** 

Table 1. Critical surface tension values of some polymers (Adapted from Oliver, 2004; Pabby

**Polytetrafluoroethylene (PTFE)** 19 **Polyvinylidenefluoride (PVDF)** 25 **Polypropylene (PP)** 29 **Polyethylene (PE)** 31 **Polypropylene (PP)** 34 **Polyvinyl alcohol (PVA)** 37 **Polysulfone (PS)** 41 **Polycarbonate (PC)** 45 **Polyurethane (PU)** 45

**(Dynes/cm)** 

convenience to set up in laboratory. However, direct contact of the membrane with the cooling side and poor conductivity of the polymeric material results heat losses throughout the membrane. Therefore, in DCMD the thermal efficiency which is defined as the fraction of heat energy used only for evaporation, is relatively smaller than the other three configurations.


Each of the MD configurations has its own advantages and disadvantages for a given application.
