Desalination by Membrane Distillation

*Mustakeem Mustakeem, Sofiane Soukane, Muhammad Saqib Nawaz and Noreddine Ghaffour*

### **Abstract**

At present, around 25% of water desalination processes are based on distillation. Similar to classical distillation, membrane distillation is a phased-change process in which a hydrophobic membrane separates two phases. Membrane distillation is considered an emerging player in the desalination, food processing and water treatment market. Due to its high salt rejection, less fouling propensity, operating at moderate temperature and pressure, membrane distillation is considered as a future sustainable desalination technology. The distillation process is quite well known in desalination. However, membrane distillation emerged a few decades ago, and a thorough understanding is needed to adapt this technique in the near future. This review chapter introduces the classical distillation and membrane distillation as an emerging technology in the desalination arena. Heat and mass transfer and thermodynamics in membrane distillation, characteristics of the performance metrics of membrane distillation are also described. Finally, the performance evaluation of MD is presented. The possibility of using low-grade heat in membrane distillation allows it to integrate directly to solar energy and industrial waste heat.

**Keywords:** membrane distillation, desalination, vapor flux, vapor transport, evaporation

#### **1. Introduction**

Distillation is a thermal process in which a component separates out from a multi-component mixture by a phase-change process. When a membrane is put in between the feed and condensing solution, the feed can be vaporized and condensed at the membrane interface (thickness ≈ 200 *μm*). **Figure 1** shows a schematic of a single-stage distillation and membrane distillation setup. The membrane distillation (MD) process combines the use of conventional distillation and membranes processes. It is a hybrid technology that uses the advantages of membrane separation and thermal distillation processes. In MD, the separation process employs a porous hydrophobic membrane between feed and permeate, allowing only solvent vapor to pass through, retaining the liquid/solid phase. Although the membrane provides a mass transfer resistance to the vapors, its employment allows water to condense within a minimal distance. This gives an advantage in creating a large partial pressure gradient across the membrane.

In a typical MD setup, the trans-membrane temperature difference (temperature difference between two sides of the membrane) creates the vapor pressure

## *Distillation Processes - From Solar and Membrane Distillation to Reactive Distillation…*

**Figure 1.**

*A schematic of a typical single stage distillation system and the membrane distillation system. In MD, the vapors generate at the feed-membrane interface, and move to the permeate side and membrane allows only vapors to pass through.*

difference, which drives mass transport [1–5]. The vapor is generated at the feed side and moves through the membrane pores to condense/get collected at the permeate side of the membrane. The salts, due to their non-volatile nature, remain in the feed solution.

MD has advantages over conventional desalination techniques. These include [6]:


Based on the distillate collection methods, MD can be classified into four broad configurations: (1) air gap membrane distillation (AGMD); (2) sweeping gas membrane distillation (SGMD); (3) vacuum membrane distillation (VMD); and (4) direct contact membrane distillation (DCMD) [7]. In all configurations, the feed solution remains in direct contact with one side of the membrane, while on the other side, distillate collection differs based on the type of the variant.


*Desalination by Membrane Distillation DOI: http://dx.doi.org/10.5772/intechopen.101457*

**Figure 2.**

*A schematic of the four different MD configuration (A–D). The feed water flows tangentially in a cross flow regime at the feed side. At the permeate side, distillate taken off through various mechanisms and membrane allows only vapors to pass through.*

gases at the permeate side. The pore wetting possibility is very less as the flow of gas takes off the distillate without condensing it [11]. The main disadvantage of this configuration is the high gas flow to sweep a unit volume of the permeate product. SGMD can be used in desalination and concentration of non-volatile liquids [7, 8].


flux [12–14]. In DCMD, as the name suggests, the membrane remains in direct contact with both solutions, i.e., feed and permeate. **Figure 2(D)** shows a schematic of a typical DCMD set up. The vapors transfer through the membrane and condense in the circulating coolant at the other side of the membrane. To achieve high thermal efficiency, the feed and coolant stream runs in countercurrent directions. DCMD is best suited for applications where water is a major permeate, such as desalination and concentration of fruit juices. Also, the condensation step is carried out inside the module itself. High conduction losses across the membrane are the main disadvantage of DCMD [8, 15].

**Table 1** summarizes the applications, advantages and disadvantages of different MD configurations [4, 6–9, 11, 15–17].
