**Phase Diagram and Membrane Desalination**

Ayman Taha Abd El-aziem El-gendi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/60419

#### **Abstract**

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Desalination technologies have made a significant impact in seawater and brackish water desalination. Recently, the evolution of membrane development has improved performance to lower operating costs and membranes have become the preferred technology for water desalination. Fortunately, different raw materials can be used for preparing membrane sheets which include either organic or inorganic materials, such as cellulose acetate, polyamide, polyimide, ceramic, natural, or artificial polymers. On the one hand, as a result of the variety of the raw materials which already exist in the entire world, different membrane separation processes might be applied dependent on the nature of the membrane sheet and the requirements of treatment process. On the other hand, there are different types of membranes can be used for membrane desalination by using different technologies such as reverse osmosis (RO), membrane distillation (MD), and forward osmosis (FO). The ternary phase diagram for membrane casting solution has an important role to get the required membranes.

**Keywords:** Desalination technologies, membrane, phase diagram, membrane sepa‐ ration processes

## **1. Introduction**

This chapter intends to focus on using membrane separation processes for desalination based on the selectivity of the membrane, such as the preferential permeation of water (classical selectivity) or the solute (reverse selectivity). Phase inversion process is the most important technique used to prepare asymmetric polymeric membranes. In addition, the morphology

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and performance of membranes depend on their effects, which also depend on the membrane preparation process parameters. From a thermodynamic point of view, study on polymer– solvent– nonsolvent system can be well depicted in a ternary phase diagram. The Flory-Huggins theory was found to be a convenient and useful framework for the thermodynamic analysis of component mixing in a membrane preparation system. As a result of the variety of the raw materials which already exist in the entire world, different membrane separation processes might be applied dependent on the nature of the membrane sheet and the require‐ ments of treatment process. In addition, there are different engineering forms which have been established for membranes that include flat, tubular, spiral wound or hollow fiber. Generally, the membrane preparation techniques are classified as a function of the raw materials used, the engineering forms and the characteristics of the required separation process. Over the past decades, the polymeric membranes have achieved commercial importance in a variety of separations applications in the chemical, food, pharmaceutical, and biotechnology industries. Today, the membrane industry is faced with the challenge of inventing new membrane materials.

In this chapter, we will focus particularly on membrane desalination techniques, such as reverse osmosis (RO), forward osmosis (RO) and membrane distillation for desalination. For these methods, as with any membrane process, the membrane is one of the most important characteristics which determines the usefulness and effectiveness of the entire process. For this reason, several targets of research have shifted in recent years towards developing new and more efficient materials that allow for a compromise between two fundamental properties of the separation, often antagonistic, namely the selectivity and permeability.

However, membrane processes are becoming economically competitive after the development of highly permeable polymer membranes. These membranes are less expensive than inorganic membranes and their implementation is much easier. Several types of polymers can be used such as cellulose acetate, polysulfones, polyamides and polyimides.

This chapter aims at preparing desalination membranes in order to get water selective membranes suitable for the retention of salts water mixtures. Hence two objectives must be reached: first, the selection of water selective materials well resistant in almost pure water and second, the preparation of high flux membranes needed for the recovery of water.

As a matter of fact, the preparation of polymeric membranes usually involves the phase inversion process, in which a homogeneous casting solution induces phase separation into a polymer-rich phase and a polymer-poor phase by the exchange of solvent with nonsolvent in an immersion bath (i.e: as water bath). Phase separation would continue to form the membrane structure until the polymer rich phase is solidified. Solidification during phase inversion could be induced by gelation and/or crystallization of the casted polymer solution. The equilibrium ternary phase diagram system is still a good tool for controlling the morphology and inter‐ preting the membrane structure. Significantly, knowledge of phase equilibria (cloud points, binodals, spinodals, and critical compositions) enables one to change the conditions for the preparation of membranes such as the compositions of the casting solution, the temperature and of the coagulation bath type to obtain an optimum membrane structure.

The phase diagram has an important role to report the agreement between experimental work in order to get the required membranes, and the ternary phase diagram miscibility gaps for the evaluations of membrane-forming system. In the ternary phase diagram (polymer (p)/ solvent (s)/nonsolvent (ns)) a miscibility gap with metastable regions exists. According to the theory of phase separation three modes of phase separation can take place in such ternary system: nucleation and growth of the polymer lean phase, spinodal phase separation and nucleation and growth of the polymer rich phase. Since polymer is one of the components of the ternary system, solidification of a part of the system can take place.
