Technology for freeze concentration in the desalination industry
Graphical abstract
Introduction
Water is a vital cornerstone of everything accomplished in modern society and the over-exploitation of existing fresh water supplies along with the increasing demand for water for drinking, agriculture and industry is going to be a major problem in the future. According to ‘The Millennium Development Goals Report 2012’ [1], 783 million people, or 11% of the global population, remain without access to an improved source of drinking water and almost 2.5 billion do not have access to adequate sanitation. The World Water Council estimate that the planet will be around 17% short of the fresh water supply needed to sustain the world population by 2020 [2]. Of all the water in the world, the majority of the Earth's water is contained in the oceans (~ 97%), while another 2% is trapped in icecaps and glaciers, resulting in less than 1% being accessible as fresh water [3], [4], [5]. Taking these figures into account, the oceans represent a virtually unlimited supply of water, however, seawater itself is unsuitable for human consumption and industrial/agricultural uses without treatment. For this reason, desalination has become an important method for the production of fresh water with the daily desalination capacity estimated as 71.9 million m3/day at the end of 2011 [6].
Desalination for water supply has grown steadily since the 1960's. Patents filed in 2010 for desalination technologies are double that of 2005, demonstrating the increasing interest and research activity in this field [7]. Based on the process, desalination plants are usually characterized into two main types, thermal processes (including multi-stage flash (MSF), multi-effect distillation (MED), vapor compression distillation (VC), freezing) and membrane processes (reverse osmosis (RO), forward osmosis (FO), electrodialysis (ED)), although there are other processes such as ion exchange and hybrid processes which may also be used. Details and reviews of these technologies and methods are given elsewhere [3], [4], [8], [9], [10]. RO (often referred to as seawater reverse osmosis — SWRO) has become the most internationally widespread desalination technology [11]. In the period 2005–2008, the annual worldwide contracted capacity of RO increased from 20 to 35 million m3/day [12] which represents over 50% of the total installed desalination capacity on the planet [13], [14]. The world's largest desalination plant at Ras al-Khair in Saudi Arabia became operational in 2014 and has the capacity to produce 1.025 million m3/day, through a hybrid system which implements both the multistage flashing (MSF) and reverse osmosis (RO) technologies [15].
Although desalination technology has progressed rapidly, the technology itself is still imperfect. Despite best efforts, desalination is costly and largely inefficient. The desalination process takes a huge amount of pressure or heat to separate the water from the salt and other impurities, which requires energy and therefore money. Large quantities of concentrated brine need to be disposed of [16], and the desalination process is also considered to be detrimental in terms of environmental impact and cost [17], [18]. Therefore, selection of the process to be used and optimisation are vital for successful desalination operations.
In this paper the freeze-melting process will be considered for the purposes of desalination. The fact that the freeze-melting process can purify and concentrate liquids has been known for many years [19]. The simplest natural example is that sea-ice has a much lower salt content than sea-water, a phenomenon used by the inhabitants of the polar regions as a source of drinking water. From an industrial-separations viewpoint, the freeze-melting process has a number of important advantages [9], [20]:
- a)
A very high separation factor,
- b)
High energy efficiency since the latent heat of freezing is low compared to the latent heat of evaporation (333.5 kJ/kg and 2256.7 kJ/kg, respectively [21]), which leads to a lower energy requirement in comparison to other processes,
- c)
Insensitive to biological fouling, scaling and corrosion problems because of the low operating temperature, which means less use of chemicals and thus lower operating costs.
- d)
Absence of chemical pre-treatment means no discharge of toxic chemicals to the environment.
- e)
Inexpensive materials of construction can be utilized at low temperature, which results in lower capital cost.
Despite these important advantages for freeze-melting processes (low energy and low temperature), this technique has only been used to a very limited extent industrially for desalination. This has been largely due to a very conservative approach to the adoption of new technology, the perception that such a process would be mechanically complex, the lack of appropriate test data and that water is a low value product. However, these caveats have recently become less important due to the pressing need for more effective solutions to water pollution problems and the development of mechanically simpler freezing technology. In the rest of this paper the main methods currently being investigated for freeze separation in desalination will be reviewed and the possible uses of this technology will be discussed.
Section snippets
History of freeze separation processes
Freeze separation is a technique first thought to be used by sailors in cold climates to get fresh water on board ships. Sailors found that, when seawater was frozen, all the impurities were concentrated into the center of the ice block. The majority of the ice block was made up of high purity frozen water [19]. Due to the freezing temperatures of the water, the ice layer was often found to be potable for human consumption. This discovery meant that sailors could simply thaw the outer ice and
Principles and major types of freeze separation processes
Desalination by freezing is based on the fact that ice crystals are essentially made up of pure water. During the process of freezing, dissolved salts are excluded during the formation of ice crystals leading to a separation of ice and brine. However, naturally frozen sea ice contains brine pockets i.e. highly saline water trapped in the ice during the process of freezing [36], which means that sea ice, although purer than seawater, is not pure fresh water. Fig. 1 illustrates this process,
Discussion of recent developments in freeze separation processes
In Section 3 the basic types of freeze separation systems have been presented, in this section recent developments and innovations which have occurred in the freeze separation process will be discussed. The most recent freeze separation technology is widely available for use in many different industrial sectors, although the majority is used in food processing and desalination. Other sectors where major research is currently taking place include the treatment of waste streams and investigation
Current cost of freezing processes versus conventional techniques
Freeze desalination processes are currently under development for full scale industrial applications and reliable process economics have not been evaluated and only an estimation of the economic potential can be made at this point in time. Shone [49] made cost comparisons for the production of chilled desalinated water and desalinated ice for use in the mining industries and found that the cost of product water from the freeze desalination process had little difference to that of an RO process
Conclusions
In this paper the freeze concentration process and its characteristics have been described. Even though the process offers numerous advantages including low energy usage and low temperature, the freeze desalination process is currently not being taken up in the desalination industry. The main factors affecting the use of the freezing process seem to be the capital cost and complexity of the process. The current use of the technology in various industrial sectors, including food, waste
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