Principles and Modes of Distillation in Desalination Process

*Abubakar Sadiq Isah, Husna Takaijudin and Balbir Singh Mahinder Singh*

## **Abstract**

Distillation has been a very important separation technique used over many centuries. This technique is diverse and applicable in different fields and for different substances. Distillation is important in the desalination section. Various principles are used in desalting seawater and brackish water to fulfill the demands of freshwater. This work explains the modes and principles of distillation in desalination, their types, present improvement, challenges, and limitations as well as possible future improvements. The first and primary mode of distillation is the passive type. As times went by and the demand for freshwater kept increasing, other modes were introduced and these modes fall under the active distillation type. However, each mode has its own advantages, disadvantages, and limitations over each other. The principles and modes of distillation are as significant as understanding the energy sources needed for distillation. Hence, they are the basic knowledge needed for future innovation in the desalination industries.

**Keywords:** history of distillation, desalination, renewable and nonrenewable energy sources, modes of distillation, principles of distillation

#### **1. Introduction**

Over the ages, the world has been evolving in development and resources use, and this has led to enormous waste generation of different states (solid, liquid, and gas). The waste needs to be either treated or recycled, paving ways for different techniques for different wastes to be treated or recycled. One of the important resources on earth is water. It is used for everyday activities such as domestic, industrial, and commercial purposes. This has caused reduction in freshwater quantity globally and shortage in clean water supply because of pollution of the existing sources. Hence, different techniques and approaches are still being investigated that can provide adequate and sustainable freshwater. Distillation has been a promising process of separating components by heating/boiling, which causes evaporation, and cooling, which causes condensation. Distillation is a simple technique of converting liquid to vapor by heating and subsequently condensing it back to liquid after the vapor comes in contact with a cooler surface. Simple distillation may not be efficient for certain modes of treatment; therefore, some other advanced distillations were found like the fractional distillation for petroleum

refining and multi-effect distillation (MED) for desalination. Generally, distillation is meant to separate a homogenous fluid mixture using the differences in the volatility or boiling point of the mixture's components [1].

There are three definitions of distillation relevant to desalination. (a) Distillation is a process in which a liquid sample is volatilized into vapor that is later condensed into liquid with richer volatile components of the original sample. This can be achieved by heating, reducing pressure, or both. (b) Distillation is the process of separating a mixture of fluids using the differences in their boiling point or relative volatility. (c) Distillation is the application of heat to a liquid to cause its partial vaporization, and then, a separate vessel is used to collect the condensed vapor [2].

The cost for all distillation methods varies, but they have a similar process or working principle. The temperature difference allows water to evaporate even at 40°C leaving the dissolved solids behind, which require about 300°C to volatilize [3].

Distillation has various advantages such as (i) the capacity to take care of a wide range of feed flow rate range, meaning they can handle high and low flow rates contrary to some alternative techniques. For example, facultative, stabilization, oxidation, and maturation ponds all require a high flow rate of feed; (ii) it can remove various and lots of substances from feed concentrations. Numerous alternative treatments have different stages or include varied chemicals for a particular impurity removal. For example, alum is used mainly to reduce solids through coagulation and chlorine is used only for the elimination of pathogens; so, it cannot remove suspended solids or other impurities; (iii) it can produce water of very high quality (pure); this is contrary to other techniques that partially treat or only reduce the impurity level of the feed. Distillation is a very well-known technique for purification because of its robustness and versatility [1]. One of the major issues with distillation in desalination is the high energy demand for the process. **Figure 1** shows a representation of the distillation process in desalination. After feedwater is transferred to the basin, the first step is the use of energy, mostly solar energy, to heat the basin water to cause it to evaporate to produce freshwater; the byproduct remains in the basin as brine solution, which can also be extracted.

The aim of this chapter is to elaborate the principles and modes of distillation in desalination and analyze their types, improvements, features, challenges, limitation, cost, gap, and future improvements needed.

**Figure 1.** *Distillation process in desalination.*
