Water is considered as a vital element for the existence of all living things. Inasmuch as it is the most abundant resource on the planet, only 3% of this water is considered as fresh. In the phase of an increasing shortage of fresh water, the sea has become an alternative solution in filling this insufficiency hence the essence of the desalination process. Since the freezing process of operational temperatures as low as -3C, it greatly reduces the possibility that the problem of corrosion and scale formation will be prevented. Corrosion and scale formation are often problematic in all types of distillation. There are two classifications of the freezing processes. These are the direct and the indirect freezing processes. The direct freezing process includes vacuum freezing processes and refrigerant freezing processes.
Key words: indirect desalination, direct desalination, vacuum freezing, brine, pressure, vapor, ice crystal
Desalination by freezing is a process which is defined by the scientific truth that in chemistry, the ice crystal that are formed are purely made of water particularly when the saline water’s temperature falls to freezing point and additional heat removed (Barduhn, 1975). Furthermore, it is by nature that in all crystals impurities are eliminated from the structure of the crystals. This is different from distillation because the freezing methodology uses the phases change in water from it liquid state to solid. The desalination freezing process often needs the segregation of brine and ice crystals. This is followed by the process of cleaning the ice crystals to facilitate the removal of the salts on the ice crystals. After the cleaning process, melting of the ice is commenced to ensure the production of fresh water (Barduhn, 1975).
Compared to types of desalinations such as distillation, desalination by freezing offers different advantages. This is because this process requires the transfer of relatively less energy. In addition, the process needs limited pretreatments considering that it is characterized by minimal corrosion coupled with limited metallurgical complications (Brian, 1968). This means that to be able to freeze one kilogram of water, it will require heat removal of approximately 80 kcal. The process of evaporating one kilogram of water requires additional 600 kcal. This is an indication of potential reduction of the energy required to facilitate the entire desalination by freezing process (Dickey et al, 1996).
Since the freezing process of operational temperatures as low as -3C, it greatly reduces the possibility that the problem of corrosion and scale formation will be prevented. Corrosion and scale formation are often problematic in all types of distillation (Fleming, 1982). Freezing at low temperatures allows for a broader assortment of materials and construction methodologies. An additional advantage of desalination by freezing methodology is that it is moderately insensible to changes in the deliberation types or the types of substances that are presents in feed water. The attractive features that characterize the freezing process have necessitated the need for more investigations since 1960s (Barduhn, 1975).
Colt industries for instance began the process of developing vacuum-freezing vapor compression desalination process in early 1960s. The capacity of this pilot program increased from about 50,000 to 110, 000 galday-1. In late 1960s, Colt industries were able to show unfailing performance of its units with a capacity of 100,000gpd. The company also had a proposed unit of 500,000-galday capacity (Allo et al, 1979). At the initiation of the VFVC process, there was the development of other vacuum freezing processes coupled with the successful operation of pilot units. These included “vacuum freezing vapor absorption” (VFVA) procedure. Vacuum cleaning “high-pressure ice melting” (VFPIM) methodology (Allo et al, 1979).
During this period other variations of the direct freezing procedure including the subordinate refrigerant freezing were developed. The first proposal of saline water conservation using direct freezing with butane as a secondary refrigerant was in 1960 (Avco Systems Div et al, 1977). The principal problem of the desalination by freezing process lies in the process of extrication of ice from the concentrated brine coupled with the inappropriateness of usual refrigerant compressor (Allo et al, 1979). Despite these challenges, the recent innovation of hydraulic refrigerant compressor can be perceived as part of the elucidation to the compressor problem facing the desalination by freezing process.
Desalination freezing process
Essential components in the freezing process
There are many ways of freezing salt water. All these processes involve three essential steps used in acquiring fresh water from the saline source. The process of forming ice-using heat involves the removal of salty water, separating ice and the brine and then melting the ice to obtain fresh water (Ammerlaan and 1984). This is an indication that all freezing process use functional processes because the use similar mechanisms in the development of ice and in the separation of brine. The four essential components of the freezing desolation processes include the freezing device, washer, melting device, and heat removal system (Barduhn, 1975).
The freezer is made up of a container in which vapor and ice crystals are formed concurrently. The different freezing processes often differ in the apparatus used in the removal of heat from brine as a way of producing ice crystals that are easily transferred, detached, alienated cleaned and molten. The heat that emanates in the course of the freezing procedure is often transferred to the melting device where it is used in melting ice (Dickey et al, 1996). The process of designing and operationalizing the freezing process are two essential tools in the production of a large size of separate ice crystals rather than clusters of ice to ensure the minimization of the amount for ice trapped in the brine (Fleming, 1982).
In the freezing process the sizes of ice crystals formed is also very important. This is because fine and very small crystal are relatively difficult to wash (Ammerlaan and 1984). The nuclealization and the rate of growth of crystals is always primary in the desalination of brine through the freezing process. The ice crystals bent in the freezing section are propelled as slurry to the washing device where ice crystals are detached from the brine. The offset current wash column is often used as the washer where a percentage of the product water that flows in one direction opposite to ice is utilized in washing the ice as a way of removing the brine that sticks to the crystal exteriors (Ammerlaan and 1984).
There are two differences of wash separation column referred to as pressurized and gravity. These separation columns have proved to be strictly practicable and making a choice linking the two from a monetary viewpoint while considering the specific process applications (Fleming, 1982). In the melter, the process begins with the ice from the washing device through the process of heat transfer of crystallization. This heat is often removed from the brine to the freezer then to the melter. The process is often facilitated by releasing the refrigerant into the melting device. In this process the ice picks heat and it begins melting. Since all freezing processes operate below ambient temperatures, it is often necessary to engage in the continuous removal of the heat from the system (Ammerlaan and 1984).
Continuous removal of the heat is often realized through evaporating the referigirant that exchange heat form concentrated brine or product fresh water and compression vapor. These methodologies of heat removal often result in diverse desalination freezing procedures. Additional predictable auxiliary processes that can be used to facilitate the freezing process include deaerators and heat exchanges among other techniques (Fleming, 1982). The application of heat exchanges is often between feed water streams and brine. It is also applied between feed water stream and the product. The aim of using the deaerators is to facilitate the removal of the dissolved gases into the field water, which in different occasions would be liberated in the process. These gases when lefts to be part of the fresh water would affect the operational efficacy that should characterize the systems (Ammerlaan and 1984).
There are two classifications of the freezing processes. These are the direct and the indirect freezing processes. The direct freezing process includes vacuum freezing processes and refrigerant freezing procedures.
Indirect freezing procedure
In the procedure, salty water does not direct contact with the refrigerant. The formation of ice is on a surface using medical refrigerator to other methodologies. According to the schematic diagram showing the indirect process, the process begins with pumping feed seawater through a heat exchanger for the reduction of its temperature (Fleming, 1982). This water enters the freezing cavity where it is subjected to further cooling to enable the formation of crystals. The blurry slurry and the ice are then pumped into the wash column to allow for the separation of brine and ice (Dickey, 1996). The resulting ice is elated into the melting device to allow for the melting of the ice using the heat released from the condensation of the compressed refrigerator. A small percentage of the resulting water is circumvented to the wash column to allow for the washing of the ice crystals. The major part of the ice passes through the heat exchanging section to enable the cooling of the feed seawater before it is emancipated for storeroom. The brine derived from the washer is then taken back to the heat exchanging section to facilitate the cooling of the seawater before it is discarded (Dickey, 1996).
Figure 1.0 an indirect freezing process
Despite the straightforward nature of the indirect freezing procedure, it is characterized by different disadvantages. One of the disadvantages is that the process often requires relatively high amounts of energy due to the high level of resistance amid the refrigerant and the saline water (Tleimat, 1980). Furthermore, the process also entails the use of big metallic temperature transmission surfaces, which are necessary for the melting and freeing steps. An additional disadvantage is in the complexity, the high price, and the difficulty in operating and maintaining the system (Tleimat, 1980). This explains why the indirect freezing procedure is seldom utilized in the desalination process. There are advantages associated with this process. This includes the direct exchange between the refrigerating element and the feed seawater (Barduhn, 1975).
In indirect desalination process, it is necessary to pass the energy for refrigeration through the walls of a heat exchanger, which allows the heat races to occur through a solid barrier. It is possible to classify the indirect desalination process into that which is internally or externally cooled (Tleimat, 1980).
Fixed coating growth system
In this structure, the fluid from which the crystal mass grows is stagnant and referred to as static layer crystallization. The static layer of solution in this process is highly reliable and requires the incorporation of simple equipment the use of any moving parts (Tleimat, 1980). The residence time needed in this process is relatively high because of the mass transfer process that is largely promoted by free convection. There is need for large equipment volumes because of the batch-wise concentration of the slow rate of crystallization (Fleming, 1982). Inasmuch as the capital outlay of the equipment used in this process is relatively high, more economic means such as the incorporation of relatively rapid crystallization in series is necessary. This process is easier to manage but the complication arises from the crystal growth on a cooled surface, which induces a relatively faster rate of crystallization, which may produce impure crystal films. Despite the possibility of forming impure crystals, the easy management considering that there is no slurry handling and moving parts overrides the disadvantage (Barduhn, 1975).
Layer crystallization units on rotating drum
The process the formation of ice in thin layer on the heat exchange surface is often after an appropriate time that allows for the building of the ice formation. Upon formation, the ice is detached from the exterior and using the press technique from the remaining concentrated liquid, it is separated (Tleimat, 1980). During the separation process, one form of layer crystallizer uses a revolving drum engrossed in a concentrated fluid. When the refrigerant is dispersed within the drum, it causes the development of ice on the exterior of the drum. The ice is then scrapped free of the ice as it rotates past a knife (Barduhn, 1975).
Progressive crystallization unit
The progressive crystallization process of indirect desalination uses the concentration occurrence of a solute and ice edge moving from one end of a container to another. The progressive crystallization progress is characterized by having a single ice crystal within the system to ensure that the division of the ice quartzes from the resulting solution is done in an easier manner compared to the conventional methodology (Dickey, 1996). In this approach, there is a possibility of forming impure ice crystals on the surface of the vessel when supercoiling occurs prior to the initial crystallization process (Weiss, 1973).
Outwardly cooled crystallization process employs the use of a heat transfer device that is external the main crystallization vessel. This approach employs a heat exchanger that supercools the liquid feed to ensure that the cold feeds provide the main vessel with a cooling effect. To enables effective freezing during the desalination process, it is necessary to control the conditions within the heat exchanger to prevent premature crystallization (Tleimat, 1980).
The process of modification of the external cooling process entails the recycling of part or all of the mass in the crystallizers. In one process, the recycling process involves the re-cooling of all the crystal slurry through an external heat exchanger to ensure the provision of efficient cooling (Fleming, 1982). Nucleation process occurs in the heat exchanger while the growth of the crystals occurs in the crystallizer. Ideally, the cooling of ice-free liquid promotes the nucleation process and the generation of small crystals by pumping in a heat exchanger, that operates on high super cooling conditions (Weiss, 1973).
The super-cool feed
In the externally cooled process, there is instant formation of ice crystals through the nucleation process. This is instantaneous process is always aimed at minimizing the possibility of a heterogeneous crystallization process within the unit (Dickey et al, 1996). The inside wall is encrusted with a hydrophobic fake to minimize any alterations in nucleation and crystallization. Using low energy, the resulting concentrated solution is often filtered as a technique of effective ice separation. In the process of indirect desalination, a relatively narrow range of super-cooled brine temperatures. This enables the formation of large-plate like free ice crystals of several inches in the bulk of slowly transferring brine in a simultaneous way with the growth and development ice on the cold surface (Weiss, 1973).
In freezing desalination this type of freezing and vapor compression techniques has been used in the desalination of brine. Water can play a significant role as a refrigerant in vacuum cleaning (Consie, 1969). When water is used as a refrigerant in freezing desalination, a vacuum is used in the vaporization of a fraction of water, which provides the refrigeration effect essential in lowering the temperature of the product hence reducing product heat and causing the occurrence of ice crystallization (Tleimat, 1980). In this process, the washed ice is melted using direct contact compression of water steam into the melting condensation unit. All the vacuum freezing processes are composed of a crystalizing unit in which ice quartzes and vapor are fashioned concurrently. This process is successful by the maintenance of the vessel closer to the triple point. The methodologies used in the removal of vapor can be grouped as vapor compression systems and vacuum freeze, vapor compression systems and absorption freeze, and ejector absorption system and vacuum freeze (El-Nashar, 1984).
In vacuum freeze and vapor compression systems, the process employs a mechanical compressor in the removal of the vapor phase. The compression of this vapor is to permit its condensation as either a heat transfer surface or pure crystals (Dickey et al, 1996). In absorption freeze and vapor compensation, the absorption of water vapor occurs in a material with lower vapor pressure below that of the triple point while the absorbent must be regenerated (Tleimat, 1980). Alternatively, a conventional refrigeration cycle can be used in the provision of the heat that is necessary in driving off absorbed air (Fleming, 1982). The ejector absorption and vacuum freeze methodology uses different mechanisms in the removal of vapor through the absorption cycle or in some instances, the approach uses low-pressure steam-ejector. The steam used in driving the ejector is also used in the regeneration of the absorbent. The ejector in this situation plays the role of a thermal compressor that raises the quality of the removed vapor to enable its condensation (Fleming, 1982).
Vacuum compression system
The use of the vacuum compression in freezing desalination entails the incorporation of a large multi-blade compressor-to-compressor vapor from the freezer to the melting unit. There two major components of a vacuum freezer which include the vapor removal unit that keeps the slurry at the freezing point as a way of keeping ice particles suspended with a vapor interface (El-Nashar,1984). An economical system will have the freezer and the vapor removal unit on the same facility (Tleimat, 1980). The process of calculating the sizes of the evaporation and condensation units can be calculated from existing correlations especially when the standard thermodynamics are known. The success of the vapor compression system occurs when the primary compressor compresses the water vapor from a pressure relatively lower that of the vapor pressure but in equilibrium with the brine at its freezing point. This in most cases is often up to above the vapor pressure of pure water, which is at 0°C (El-Nashar, 1984).
There is need for an auxiliary refrigerating cycle to facilitate the removal of excess energy from the system. Through the standard of ammonia cycle the process of heat removal through the heat of condensation of excess water vapor, will require the rejection of heat to ambient cooling water (Cheng and Hopkins, 1982). There are different modules that increase the efficient of the compressor. These include evaporative feed pre-coolers, and multi compressor modules (Tleimat, 1980). This is based on the consideration that in order to realize maximum low energy consumption, it is important to seek different solutions (Consie, 1969)
Direct contact freezing desalination
This is an essential large-scale desalination of seawater through the circulation of a cold refrigerant in a heat exchanger. This is a technique of removing the heat form the seawater. The ice formed on the surface of the heat exchanger is then removed, cleaned by a washing process and melted as a way of producing fresh water (Tleimat, 1980). In this form of desalination, the heat that is removed from the seawater is in direct contact with the refrigerant. It is also possible to conduct the direst freezing desalination process using a secondary refrigerant methodology. In this mode, there is the compression of a refrigerant with relatively lower solubility compared to water. This refrigerant is often cooled to temperatures closer to that of the freezing point of salt water before it is mixed with the seawater. In the heat exchange as the refrigerant evaporates, heat is absurd from the mixture and this allows the water to freeze into ice (Dickey et al, 1996).
In this process, the crystallizers provide a platform for intimate mixing between butane and Freon, which are the refrigerants and the product that is to be subjected to freezing. During this process the referigirant, which in its liquid state is subjected to intense pressure, allows for its expansion through a product nozzle into the product liquid where it is allowed to vaporize at relatively lower pressure levels (Tleimat, 1980). The vaporization process allows for a freezing effect, which facilitates the formation of ice, which at times acquires the physical appearance of solute crystals within the product. From an ideal perspective the direct freezing-melting system is characterized by compression of a crystallizer, and an ice nucleation, this allows for the growth of the nuclei to a size that is suitable for separation. In addition, the direct freezing melting system is also composed of an ice crystals separator-melting unit and a washing unit (Tleimat, 1980).
Conventional direct contact freezing
In direct contact freezing, a spray of refrigerant is used by jet impact using a nozzle. The main advantage of this conventional approach to direct contact freezing methodology is that it ensures a high production rate per unit volume. In addition, it also ensures the use of relatively lower amounts of energy (Tleimat, 1980). The success in the design of a direct contact freezing desalination plant is highly dependent on the availability of the most suitable refrigerant. There are thermodynamic, economic, physical and chemical requirements that the refrigerants used in the freezing process must possess in order to be suitable for desalination process. For instance it is important for the refrigerant to have a boiling point of -4°C or below and a vapor pressure of 2.8 ×105 Pa (2.8 atm) at room temperature (Tleimat, 1980). It is also important for the refrigerant to be non-toxic, chemically stable in seawater and non-inflammable. In addition, it is also important for the refrigerant to be immiscible with water and its molecular size factors such as those that do not allow the formation of a hydrate when freezing conditions are employed in the desalination process. An additional quality of the refrigerant is that it should be cheap and readily available for commercial purposes (Tleimat, 1980). The price of the refrigerant is a major determinant of the cost of production and the possibility that more users that are commercial will be involved in the use of the direct contact freezing and melting approach of desalination. Examples of refrigerants that could be used in this process include carbon (IV) oxide, nitrate oxide, butane, and Freon (Dickey, 1996).
Butane is one of the cheaper alternatives. The evaporation of butane entails three stages, which include liquid butane, butane vapor, and liquid brine. The rate of evaporation of butane droplets often increases with a diameter ratio compared to the previous one up to a critical value. This is followed by gradual increase with 1`/6 power (Tleimat, 1980). This gives the implication that the evaporation process was largely controlled by the heat transfer through the transient liquid butane film. The choice of the most suitable refrigerant in the process of direct desalination is essential with regard to its stability and price. A hydrocarbon such as butane can be essential ease of its lower vapor pressure, which is considered relatively lower than the atmospheric pressure (Dickey, 1996).
Ice crystallization unit
In direct desalination, energy recovery is considered as one of the most essential aspects in the process. When used as a refrigerant in direct desalination, butane, being and immiscible refrigerant expands under pressure in its liquid form. This passes through nozzles into the liquid product where it boils at a lower temperature (Tleimat, 1980). When vaporized in the freezer, butane eliminates the heat from the brine and this causes a portion of it to freeze into tiny ice crystals (Buros, 1980). There is an alternative of freezing which entails using a vacuum in vaporing a percentage of the water. This provides the refrigeration effect, which is essential in lowering the temperature of the resultant product facilitating the occurrence of the ice crystallization. This process enables the reduction of the residence time used in the crystallization unit by about half-compared to other forms of freezing (Dickey, 1996).
In the ice nucleation unit, there is the production of small ice crystals which when transferred into the crystallization unit grow larger hence the nucleation process. This growth process often occurs at the expense of the smaller ones. Within the crystallization unit, there is need for the control of the formation and growth of ice quartzes to ensure that there is a uniform distribution of the larger ice crystal facilitates the separation process (Dickey, 1996). Such levels of control will facilitate the freezing process and facilitate the reduction of the amount of product carryover into the separated ice stream (Buros, 1980). In this process, it is important to consider that a highly efficient separation process will ensure very low levels of product carryovers onto the separated ice stream hence a more economical process (Tleimat, 1980).
The resulting ice crystals are collected in the ice crystal separator where they are they are washed with water as a way of eliminating brine from the crystal surfaces. The most essential technology in this phase is how to ensure the growth of ice crystals in the separation unit large enough to facilitate the process of separating the crystals from the solutes (Tleimat, 1980). Nucleation and supercoiling are the factors that minimize the possibility of forming large ice crystals. When the former is very high it creates a large number of small crystals. Lower rates of cooling are desirable because they minimize the possibility of excessive nucleation. Lower rates of nucleation are also considered important in this phase because they facilitate the production of larger crystals at a desirable residence time (Tleimat, 1980). In this phase, it is also important to adjust the rates of agitation with certain limits as a technique of lowering the rates of agitation considering that relatively high mixing rates have a tendency of promoting the formation of small crystals to the resulting mechanical damage (Dickey, 1996).
An additional factor that has an effect on the quality of product crystals is the circulation patterns of ice slurry in the freezing unit using direct contact vaporization of an immiscible refrigerant. One way of mitigating this effect is by ensuring that the butane used as a freezing gent is introduced at the bottom, as it will give a vertical movement to the ice slurry (Dickey, 1996). An additional technique is in the use of a mechanical agitator and another agitator to allow for relatively small flow of butane vapor. When purged in near the bottom of the crystallizer, it allows for an improvement of the mixing as it also the initiation of a reliable operation approach in all circumstances (Dickey, 1996). The small flow of butane vaporous into the crystallizer, which contains negligible levels of incondensable gas and heat is highly effective in the alloying circulation even in situations where the pressure at the bottom half of the crystallizer surpasses the vapor pressure of butane (Tleimat, 1980).
In the design of a secondary refrigerant freezer, the following elements should be considered to allow for an effective direst freezing desalination process. Sufficient dispersion of the liquid refrigerant into the brine is of great importance (Cheng and Hopkins, 1982). The use of normal butane is highly cost effective and the most efficient because it does not allow for the formation of any hydrate (Dickey, 1996). The application of short residence time is not a necessary condition for the formation of small crystals with poor wash ability. In the design of the refrigerating plant, short contact time should be considered with sealer agitation methodologies incorporated (Weiss, 1973).
In direct desalination freezing, some of the most effective methodologies would include the use of fine spray nozzle in the introduction of liquid refrigerant under brine. Furthermore, the pumping of vapor should be through vapor spurges, which should later be introduced under brine. (Weiss, 1973)
Ice crystals must be separated from the brine before melting. This is because the brine is attached to these crystals through interfacial tension. After the formation of the crystals, they are washed to eliminate impurities such as brine from the crystal surface and produce pure crystals (Brian, 1968). The separation process uses a wash column. Pressurized and gravity are the two types of wash columns that can be used in the separation process. In the pressurized type, the ice crystals raise and the hydraulic pressure forces a wash liquid to flow down (Tleimat, 1980). The existing pressure plays the role of squeezing the concentrate trough the filter at the bottom of the wash column. In the process of its flow, the wash column eliminates impurities from the surface of the ice crystals at the wash front, which is the interface of the washed and the unwashed crystals. The wash liquid is exposed to colder crystals and crystalizes on them. This technique ensures that the wash liquid does not mix with the concentrated liquid (Weiss, 1973).
The gravity was Colum approach serves as a relatively simpler design. Its efficiency is in its ability to create the pressure needed in compressing the ice bed. Its working technique is similar to that of the pressurized wash column but at a relatively lower pressure. In this method, ice crystals are formed and moved by hydraulic means to the top of the column. The performance of the wash column using the gravity approach is highly dependent on the size and the shape of the crystal (Weiss, 1973). In the separation, process there exists some surface tension, which can be eliminated by using clean fresh water as a displacing liquid. This is the gravity drainage separation methodology, which not only drains the brine but also adds pure water and allows the filtering of the water through the interstices of the ice bed to ensure the displacement of the remnant of the brine (Brian, 1968).
The melting unit is often difficult to characterize despite its appearance as a relatively straightforward process. This is because there are two approaches these are direct contact and indirect melting unit. The former approach is relatively easy to achieve (Weiss, 1973). However, the process of scaling up is often difficult considering that it is difficult to characterize heat transfer rate because of the drainage of refrigerant and water for the bed. This is different from an indirect melting process, which requires a great transfer surface hence reducing the attainable efficiency coupled with the direct contact-melting unit. In order to optimize the efficiency of the melting unit, it is important to apply the concept of integration. This process allows for the use of healing platform that allows for high-level efficiency in melting the ice crystals to produce fresh water (Cheng and Hopkins, 1982).
The compressor performance is often affected by the contamination of the brine spray carried over from the crystallization unit. This explains why it is important to develop a separation device between the compressor and crystallization unit. The role of the refrigerant is to remove ice contained in an evaporating referigirant. In situations where there are dissolved refrigerants, it would be easy to employ vacuum stripping o effluent stream. This can be polished to ensure that it meets environmental standards by carbon absorption where necessary (Weiss, 1973).
There is a variant to the process of direct freezing desalination. This involves the formation of clathrate hydrate, which appears in gaseous state. The clathrate hydrate falls in the category of solids, which have gaseous molecules occupying cages, which consists of hydrogen bonded molecules (Weiss, 1973). These molecules are often at temperatures higher than the freezing point of water. These molecules are crystalline inclusion compounds that are composed of water and gas which occur spontaneously at defined local conditions of temperatures and pressure (Tleimat, 1980). Clathrate hydrates can possess three common crystalline motifs. These are structure I, structure II and Structure H. Each of these motifs contains cavities formed by water molecules that physically trap the gas hydrates to from species within them (Weiss, 1973).
The trapped gaseous molecules are held in position within the hydrogen bonded molecular water lattice by the Van der Waals forces. After about 75% of the host cavities are occupied, a solid thermodynamic and highly stable crystalline cell unit is formed (Cheng and Hopkins, 1982). The resulting hydrates are known as gas hydrates which contain gas molecules that only exist in the as the gasses state and standard pressure and temperature. Upon the melting of the gas hydrate, the hydrocarbon and fresh water are recovered and recirculated. On this case, the hydrocarbon is recycled to facilitate the process of producing freshwater (Brian, 1968).
Figure 2.0 freezing desalination by hydrate
This process of desalination has been considered as relatively advantageous to that of direct freezing because the operating temperatures is relatively higher hence ensuring the reduction of power needs and the cost of operation in the process of melting clathrate hydrate to facilitate the production of portable fresh water (Weiss, 1973). In addition, it also allows for a process of recycling the resulting hydrocarbon making it a relatively cost effective approach with regard to the reduction of the cost incurred through another desalination process (Cheng and Hopkins, 1982). There are organizations such as the Bureau of Reclamation, which has been involved in the commission of work through the Thermal Energy storage Inc., .to facilitate the exploration of this approach to desalination. The commissioning of two pilot projects in Hawaii and San Diego has enabled this (Weiss, 1973).
The essence of freezing desalination and the processes involved
Water is considered as a vital element for the existence of all living things. Inasmuch as it is the most abundant resource on the surface of the planet, only 3% of this water is considered as fresh. In the phase of an increasing shortage of fresh water, the sea has become an alternative solution in filling this insufficiency hence the essence of the desalination process (Weiss, 1973). The phase change process is an approach to freezing desalination considered as an applied process. Freezing is one of the most common phase change processes, which is a less energy, consume, but is to be used on an industrial scale (Fleming, 1982). In the freezing desalination process, the highly expensive membrane in osmosis is not used. An additional advantage of this approach to desalination is that the brine used is less corrosive compared to that used at higher temperatures. The process also involves the freezing of an aqueous solution, which contains salt. This process allows for the formation of ice, which is composed of pure water, and it ensures that impurities are restituted to concentrate the remaining liquid phase (Fleming, 1982).
There are different approaches of freezing desalination. Crystallization on cold wall is an approach that allows the solution to be in contact with a cold surface. This process allows for a crystallization process, which begins to the development of an increasingly impure crystalline layer. To allow for a good later purification of the layer, it is important to ensure a perfect control of the saturation of the brine (Cheng and Hopkins, 1982). The success of the desalination by freezing process rests on the behavior of the salt water in the presence of a gradient as elaborated in the diagram phase of H2O-NaCl in the figure below. This Binary phase diagram gives a presentation of varieties of possible phases in function of the total fraction temperature and NaCl. The possible phase from the Binary diagram can be explained by a combination of ice, water vapor, solid sodium chloride, and solid hydrated sodium chloride (NaCl2H2O) (Weiss, 1973).
The solidious is a presentation of the migration of the total solid state to a mixture. The liquidious is a presentation of the migration of the total liquid state to a mixture (Cheng and Hopkins, 1982). The process of freezing desalination has the objective of ensuring that throughout freezing process the salinity levels of the resulting fresh water are in agreement with the salinity standards of fresh drinking water. According to recent studies, there are different parameters essential for the success of desalination by freezing. For example, a freezing temperature of -5°C for about 24 hours decreases the initial salt concentration of water by approximately 99% (Weiss, 1973).
Furthermore, during this process it is essential to use cylindrical geometry as a strategy of minimizing the possibility of formed trapping in the brine pockets arising from the effects of gravity coupled by the maximum density of the brine. It is also important that during the desalination by freezing process, a compromise of the freezing temperatures coupled with the respect on the speed of freezing and steps must be considered as a technique of ensuring that the resulting fresh water is in accordance with the existing standards of the world health organization (Cheng and Hopkins, 1982).
Economic and environmental aspects
From theoretical perspective, freezing desalination has certain advantages compared to other forms of desalination. These include lower energy requirement, minimal potential for corrosion and little precipitation. From an economic perspective the choice and the type of desalination plant used is highly dependent on the cost and the environmental implications involved. In freezing desalination, the water separation process involves the incorporation of both thermal and electrical energy because of the crystallization process that define the entire procedure (Bouchekima, 2002).
Despite the perceived economic implications involving the use of freezing desalination process, there are currently reductions in unit water cost. This is largely because of the technological developments, which have necessitated the optimization of the design process hence ensuring an improvement in the efficiency of the thermodynamics. Additional factors necessitating the reduction include an increase in the size of the plants, lower interest rates in energy costs and the use of renewable energy sources to power the freezing desalination plans. Energy, which forms an essential component in freezing desalination, has experienced a reduction in consumption by about 90% (Bouchekima, 2002). This can be attributed it the interruption of additional technological approaches in the market which have facilitated the reduction in energy needs.
Greenhouse gas emissions form an essential part of the environmental aspects that concerns freezing desalination. This is because the use of fossil fuel energy contributes to an increase in the concentration of carbon IV oxide, sulphur dioxide and nitrogen oxide in the atmosphere hence contributing to the devastating climate change effects. This explains why in order to reduce the effects greenhouse gas emissions, freezing desalination plants have been integrated with non-polluting renewable energy sources (Bouchekima, 2002).
Brine and chemicals discharged from the desalination process also form part of the environmental aspects. The brine discharge has negative effects on the environment because the high salt density and content has a high probability of affecting sea life when discharged into water (Bouchekima, 2002). Pretreatment residual turbidity affects the success of the photosynthesis process considering that it lowers transmission of sunlight rays into water at its rejection point. Freezing desalination process also involves operation in high and low temperatures, which have the potential of affecting marine life (Bouchekima, 2002)
There are different approaches of freezing desalination. These are direct and indirect desalination. In direct freezing desalination, a spray of refrigerant is used by jet impact using a nozzle. The main advantage of this conventional approach to direct contact freezing methodology is that it ensures a high production rate per unit volume. In addition, it also ensures the use of relatively lower amounts of energy. In direct desalination, energy recovery is considered as one of the most essential aspects in the process. When used as a refrigerant in direct desalination, butane, being and immiscible refrigerant expands under pressure in its liquid form. This passes through nozzles into the liquid product where it boils at a lower temperature. In freezing desalination, butane is one of the cheaper refrigerants. The evaporation of butane entails three stages, which include liquid butane, butane vapor, and liquid brine. The rate of evaporation of butane droplets often increases with a diameter ratio compared to the previous one up to a critical value. This is followed by gradual increase with 1`/6 power.
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