A Review of Water Desalination Technologies (Coursework Sample)

A Review of Water Desalination Technologies
Introduction
Water is one of the most abundant and precious natural resources on Earth, covering over 70 percent of the planet. However, it is available in a state not readily consumable or usable to humans. For instance, most of the freely available water in lakes, oceans, and underground aquifers is saline (salty) or filled with solid impurities and other pollutants. Consequently, there is scarcity of fresh water, a situation that affects more than 40 percent of the world population, according to a UN report (Water and Waste Digest, 2018). To address this problem, several water desalination technologies have been developed and applied to purify water for human consumption. While not all of these technologies have been fully implemented, and while most of them are not 100 percent efficient, they play an important role in solving the water scarcity problem. This research paper examines the major desalination technologies in use today, highlighting the concept behind their application and demonstrating their mechanism through diagrams.
Concept Definition: Desalination is the process through which dissolved minerals are eliminated water to make fresh/soft water. This technology is currently utilized to purify sea water for domestic use and agriculture.
Classification of Desalination Technologies
There are three broad categories of desalination technologies, which utilize different mechanisms to separate dissolves minerals from water. These technologies are: Evaporation and condensation, Filtration, and Crystallization.
1 Evaporation and Condensation
Evaporation and condensation was the first technology to be utilized to purify saline water at a commercial scale. This technology is based on the concept of injecting thermal energy (heat) to sea water to produce vapor, which is then condensed to produce fresh water (Chua & Rahimi, 2017). The heat is generated via external combustion processes such as burning fuel.
There are different subcategories of the evaporation and condensation desalination method, which include multi-effect distillation (MED), Multi-stages flash (MSF), and Vapor compression distillation (VCD).
1 Multi-Effect Distillation
The multi-effect distillation method is typically used to desalinate water. As the name suggests, it is based on the concept of distilling sea water through multiple stages. In each stage, sea water is spayed on heated pipes to produce steam (Chua & Rahimi, 2017). The process is repeated in up to 14 stages, and the evaporating steam is condensed to produce distilled water. In this process, each stage is cooler than the previous one, which causes the vapor to gradually cool down until it comes out as distillate water.
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Figure 1: Multi-Effect Distillation (Source: Curto, D., et al. A Review of the Water Desalination Technologies)
In the above scheme, the last cell is for condensing the steam in a heat exchanger (distillate condenser) which is cooled by sea water.
2 Multi-Stages Flash (MSF)
The multi-stage flash distillation (MSF) method involves heating incoming sea water to near boiling point, and then decreasing the water’s pressure in a series of stages to generate vapor (Sitton-Bragg, 2015). As the name suggests, a portion of the incoming sea water is flashed into steam in several stages.
Figure 2: Multi-Stages Flash (Source: Curto, D., et al. A Review of the Water Desalination Technologies
In the above scheme, the operating concept involves injecting sea water via the inlet into the heat exchange chambers, which raises the water’s temperature. At the brine stage, the water has reached near boiling point. The water is then passes through valves into stages with decreasing pressure and temperature. As it flows along the tubes, the feed sear water carries the latent heat off the steam, causing it to condense into clean water.
3 Vapor-Compression Distillation
The vapor compression distillation process is a low-pressure desalination technique that involves passing sea water through a series of heat exchangers, where it is heated by spraying it on the tubes. The difference with the MED is that the VCD involves compressing the vapor to change the water’s boiling point. Since pressure and temperature are directly proportional (PV = nRT), an increase in pressure at a constant volume causes a rise in temperature. Accordingly, VCD plants utilize the compression of vapor to produce heat, which in turn is used to evaporate the feed water.
Figure 3: Vapor-Compression Distillation (Source: Curto, D., et al. A Review of the Water Desalination Technologies)
As the feed water circulates through the chambers, it is compressed alternately, causing it to expand and change from liquid to vapor. As the feed water changes its state from liquid to vapor, heat is absorbed and ejected by from the system, thereby lowering the temperature in the chambers, which eventually cools the vapor into saline-free water.
1 Filtration Technologies
Filtration technologies utilize osmosis mechanisms to purify salty water. This process involves using two liquids with varying concentrations separated by a semi-permeable membrane. Osmotic pressure causes the liquid with a lower concentration to pass through the membrane towards the liquid with a higher concentration. The filtration process utilizes the osmosis concept in two ways, giving rise to four different desalination methods: Reverse osmosis, forward osmosis, electrodialysis, and nano-filtration.
4 Reverse Osmosis
Reverse osmosis involves inverting the normal mechanism of osmosis, whereby saline sea water is forced by pressure through a membrane. As a result, there is movement of molecules from a state of high concentration to a state of lower concentration, producing salt-free water on the other side of the membrane (Salinas-Rodriguez et al., 2021). The side with the feed water (saline water) remains with salt residues, which is prevented from passing through the membrane.
Figure 4: Reverse Osmosis (Source: Curto, D., et al. A Review of the Water Desalination Technologies).
In this regard, the operating principle in reverse osmosis is that it only allows the passage of selected molecules through the semipermeable membrane (Curto et al., 2021). This process is not only the most advanced system for desalinating water, it is also the most efficient since it produces up to 4 and half times less greenhouse emissions.
5 Forward Osmosis
Forward osmosis utilizes the natural mechanism of osmosis to separate concentrated salts from sea water. In this technique, molecules move from a diluted solvent to a concentrated one via a semipermeable membrane. The process utilizes osmotic pressure to force water through the membrane while the dissolved salts are retained behind.
Figure 5: Forward Osmosis (Source: Curto, D., et al. A Review of the Water Desalination Technologies).