Method and system for freeze crystallization of feed liquid

Abstract

The invention relates to a method for freeze crystallization purification of feed liquid (F), the feed liquid (F) containing at least one chemical, the method having the followings preliminary steps of: determining the at least one chemical contained in the feed liquid (F), calculating a phase diagram of the at least chemical, and process steps of: adjusting the temperature and pressure of the feed liquid to reach physical circumstances for freeze crystallization of the at least one chemical to form salt of said chemical, ice and brine in high pressure of 5 – 200 MPa, preferably to 15 – 150 MPa, most preferably 60 – 100 MPa, by using a piston (90) extending at least partially outside the tank (40) and being attached to a hydraulic actuator (32) located outside the tank (40), separating the salt from the ice and brine in the tank and collecting the salt, and using a digital control unit (150) to control the hydraulic actuator (32) based on the phase diagram. The invention also relates to a corresponding system.

Description

[0001] METHOD AND SYSTEM FOR FREEZE CRYSTALLIZATION OF FEED LIQUID

[0002] The invention relates to a method for freeze crystalli zation of feed liquid, the feed liquid containing at least one chemical , the method having the followings preliminary steps of : determining the at least one chemical contained in the feed liquid, calculating phase diagram of the at least chemical for separation, and process steps of : adjusting the temperature of the feed liquid in a tank to reach physical circumstances for freeze crystalli zation of the at least one chemical to form salt of the chemical , ice and brine , separating the salt from the ice and brine , and collecting the salt .

[0003] The invention also relates to a system for freeze crystalli zation of feed liquid .

[0004] A Dutch company SoliQz B . V . has created a process called "SoliQz" for freeze crystalli zation (EFC) of chemicals . SoliQz is a very selective melt crystallisation process that uses a hydraulic wash column to purify a broad range of chemicals . In general , the EFC uses an aqueous stream as feed liquid with dissolved salts that is cooled to the eutectic conditions , namely temperature , and both salt and ice crystals are formed . Their yield is controlled via temperature control , namely the cold withdrawn from the crystalli zer . The salt and water crystals can be separated in a settler due to the difference in density . A hydraulic wash column can be used to separate the ice crystals from the mother liquor .

[0005] The problem of the above described SoliQz proces s is that it requires a lot of energy to reach the eutectic conditions of the chemical wherein salts are formed . In addition, a large amount of ice is created in comparison to salts . The process is also difficult to implement as a continuous process and therefore is poorly suited for large scale separation of salts . In some cases it is also desired to form salts that are not as pure as the salt formed in the eutectic conditions in which case the eutectic freeze crystalli zation point need not be reached .

[0006] The objective of the present invent ion is to provide a method and a system for free ze crystalli zation of feed liquid that is more energy efficient than the methods and systems of the prior art and better suited for continuous process . The characteristic features of the method according to the invention are described in Claim 1 and those of the system for implementing the method are stated in Claim 12 .

[0007] The purpose of the method according to the invention is achieved with a method for freeze crystalli zation purification of feed liquid, the feed liquid containing at least one chemical , the method having the followings preliminary steps of determining the at least one chemical contained in the feed liquid and calculating a phase diagram of the at least chemical for separation . The method also includes process steps of pressuri z ing of the feed liquid in a tank to high pressure of 5 - 200 MPa, preferably to 15 - 150 MPa, most preferably 60 - 100 MPa by us ing a piston extending at least partially outside the tank and being attached to a hydraulic actuator located outside the tank for pushing and pulling the piston, adjusting the temperature and pressure of the feed liquid by expanding the feed liquid adiabatically in the tank to reach physical circumstances for freeze crystalli zation of the at least one chemical to form salt of said chemical , ice and brine . In addition, the method includes steps of using a digital control unit to control the hydraulic actuator based on the phase diagram and separating the salt from the ice and brine in the tank and collecting the salt from the tank to a salt tank . The method according to the invention uses the basic principles of known EFC process in an elevated pressure to reduce the amount of energy needed to reach the conditions for freeze crystalli zation of the chemical to form salt . The adiabatic work is very efficient compared to temperature changes in atmospheric pressure . The digital control of the hydraulic actuator that drives the piston used for pressuri zation is very accurate and is used to reach the most efficient areas of the chemical ' s phase diagram to stay in the crystalli zation phase of the chemical with minimal energy usage . The most energy efficient way to implement the crystalli zation phase requires quick adjustment of the piston to achieve the desired rate of expansion while the formation of ice tends to increase pressure . The method according to the invention enables efficient chemical recovery and recycling of many commonly used process chemicals such as Na2SC>4 , MgSCh or Ca2SO4 •

[0008] Preferably freeze crystalli zation, melting of the ice, heat transfer and separation phases of the salt , brine and product liquid all take place in the same tank . Thus , high pressures can be used cost efficiently as there is no transfer pumping of the pressuri zed feed liquid need .

[0009] In this application the term chemical must be interpreted to contain both elements and different chemical compositions of elements . In other words , a chemical is a unique form of matter with constant chemical composition and characteristic properties .

[0010] In this context , the physical circumstances refer to temperature , pressure and concentration of the feed liquid .

[0011] Preferably the process steps are implemented in the order presented above . Preferably the control unit is controlling the phys ical circumstances in the tank so that they are kept within 1 - 10 ° C of the boundary curve of freeze crystalli zation conditions to ensure that the formation of ice is continuous during the adiabatic expansion .

[0012] According to an embodiment the digital control follows the pressure and temperature curve to maintain feed liquid in an area of freeze crystalli zation . Thus , the control of pressure must be quick and precise in order to maintain in the optimal area for energy efficiency and yield .

[0013] Preferably the brine is separated from the ice and collected to a brine tank . As the brine is separated from the ice , the brine is in concentrated state and can be used in further uses .

[0014] Advantageously also the product liquid is collected to a product liquid tank .

[0015] In an embodiment of the invention the physical circumstances of the freeze crystalli zation are the physical circumstances of the eutectic freeze crystalli zation point of the feed l iquid . In the eutectic point the salt can be recovered in pure form .

[0016] Alternatively, the method is used in an area of phase diagram of the feed liquid wherein the water freeze crystalli zation or solid salt formation by freeze crystalli zation is taking place .

[0017] Preferably the brine is recycled to the tank as feed liquid for improving the yield of the salt . The yield of the freeze crystalli zation may by 70 - 90 w-% , meaning that 10 - 30 w-% of the chemical is in the brine and can be recovered by repeating the freeze crystalli zation using the brine as feed liquid . The method may further include a step of recycling the brine at least once , preferably 2 - 10 times , most preferably 4 - 8 times to the tank as feed liquid to increase the concentration of the brine . By recycling the brine more water can be removed from the brine thus increasing the concentration of the brine for further use .

[0018] In an embodiment of the invention the feed liquid contains two or more chemicals which are each separated as salts in separate freeze crystalli zation phases , each phase having physical conditions characteristic of the chemical ' s phase diagram to achieve freeze crystalli zation . For example , the physical circumstances can be adjusted to reach the freeze crystalli zation of the first chemical , after which the formed salt is then separated and recovered . Then the physical circumstances , namely pressure , temperature and the concentration of the feed liquid is adjusted so that the freeze crystalli zation of the second chemical is achieved .

[0019] Preferably the phase diagram is calculated for each chemical and the physical circumstances are adjusted accordingly for separation of each chemical individually .

[0020] Preferably the physical circumstances for freeze crystalli zation for each chemical are kept within 1 - 5 ° C range of the freeze crystalli zation boundary . This means that only minimum amount of energy is used to reach the necessary circumstances for the freeze crystalli zation .

[0021] Preferably the salt is separated from brine and ice based on different densities , the salt having a larger density than brine and sinking at the bottom of the tank . Thus , the salt can be removed from the bottom of the tank . Preferably the method further includes the following steps for separation of brine and ice after the removal of salt , in which steps the feed liquid is pressuri zed in the tank to a high pressure , the feed liquid at high pressure is cooled using heat transfer liquid to temperature near freez ing point under high pressure and the feed liquid adiabatically in the tank is expanded to essentially normal pressure causing at least a part of the feed liquid to freeze forming a mixture of ice and liquid brine . In addition, the liquid brine is removed from the tank and the brine is collected to a brine tank, the product liquid is fed into the tank forming a second mixture of ice and product liquid, and the second mixture is pressuri zed adiabatically to high pressure causing the ice to melt . Preferably the method also includes steps of using the second mixture to cool another portion of pressuri zed feed liquid under high pressure thus expanding the second mixture adiabatically to essentially normal pressure and removing the second mixture from the tank as the product liquid . This enables brine to be concentrated and separated from ice very efficiently using adiabatic work .

[0022] Preferably the method according to the invention ut ili zes the fact that the energy needed for cooling the feed liquid near the point of freez ing at high pressure i s significantly less than the energy needed for the same temperature change in normal pressure . In other words the work of adiabatic expansion is more energy-efficient than the use of energy via thermodynamic work . In addition, the use of adiabatic expansion for causing the freez ing of the feed liquid is much more energy eff icient than using external cooling past the freez ing point . By using the hydraulic cylinder for the volume change of the tank , the method can be implemented reliably and cost efficiently . The brine is recovered and collected as a valuable product along with the product liquid which might be pure water . Thus , the invention is usable for a wide variety of different implementations from recovering valuable minerals or chemicals or both from industrial effluents to purification of salty water for human consumption .

[0023] Preferably the method further includes a step of collecting the pressure energy during expansion in the tank into a pressure accumulator using the piston connected to the pressure accumulator . This improves the energy efficiency of the method as approximately half of the expansion pressure energy can be reused for pressuri zation of feed liquid or second mixture along with a half of primary energy . Assuming constant compressibility of water, the work W required to pressuri ze water to pressure p is W = p2, where is the compressibility of water (bar-1) and p is pressure in bar . Actually, the compressibility of water decreases nearly linearly with pressure ; at 0 ° C and normal pressure = 52 • I Ch6 / bar . At 2000 bar = 29 • 10~6 / bar .

[0024] Advantageously the method further includes a step of detecting the formation of ice by a sensor based on pressure change in the tank during expansion and using the control unit to drive the piston to compensate the change of pressure caused by the forming of ice in the tank . By compensating the change of pressure in the tank caused by the formation of ice the pressure levels can be maintained at an optimal phase diagram curve for formation of salts .

[0025] The adiabatic expansion of the feed liquid in the tank is preferably controlled using a control algorithm to control the rate of formation of salt and the mixture of ice and liquid brine while maintaining formation of salt so that the formed salt is in as pure as possible .

[0026] Preferably during expansion, the control algorithm is arranged to control the freeze crystalli zation of the chemical using sensor information including one or more of the following : piston stroke length, tank volume , fluid conductivity, fluid density, activity coefficient ; for control .

[0027] According to an embodiment the control algorithm uses a sensor to monitor information including one or more of the following : piston stroke length, volume , fluid conductivity, fluid density, activity coefficient ; and based on the information controls the movement of the piston during expansion to maintain the crystalli zation of the chemical within ±30 % , preferably within ±10 % range of the temperature and pressure of the point of the feed liquid . By maintaining the expansion and the freez ing of feed liquid at the point of the chemical or at least close to it , the purity of the salt can be improved and thus the efficiency of the process . It is known that salt formed in the eutect ic point has a pure structure .

[0028] According to an embodiment the control algorithm maintains the feed liquid in liquid crystalli zation phase of freez ing while the piston is moving until the movement of the piston stops when the salt and ice is formed . The duration of freez ing improves the purity of both salt and ice . However, if the duration of the expansion is long, the capacity of the process may suffer . It is possible to achieve better purity by either increasing the duration of the expansion or by recycling the brine back to the tank, whichever is the most effective when considering the process at an economical standpoint .

[0029] According to an embodiment the feed liquid can be l ime slurry used, for example , in mines for minerals used in batteries , gold mines or such . In these types of industrial processes lime is used to adjust the pH of the liquids . For example , the lime slurry of gold mines may include low concentrations of titanium, which could be recovered using the method according to the invention . Preferably the method further includes a step of heating the pressuri zed second mixture using the heat of the heat transfer liquid used for cooling of the pressuri zed feed liquid . This improves the energy efficiency of the method .

[0030] According to an embodiment the method further includes a step of utili z ing two tanks with common heat exchange system . By utili z ing two tanks side by side heat can be transferred between the two tanks , which are preferably at different phases of the purification process .

[0031] Preferably the feed liquid is cooled within the tank by bringing cooling liquid inside the tank .

[0032] According to an embodiment the method further includes a step of introducing a freez ing initiation impulse of at the beginning of expansion of the cooled and pressuri zed feed liquid in brine recovery phase to initiate the freez ing of the feed liquid . As with effluents the initiation of freez ing may be hard to predict , as in some cases the feed liquid may become subcooled before freez ing, the freez ing may be timely initiated by a separate impulse . The precise beginning of freez ing facilitates the automation of the method and system .

[0033] Preferably the ice formed during adiabatic expansion is pure ice and the product is then pure water . The definition of pure in this case refers to essentially pure in the sense that it does not contain any significant amounts of impurities . However, the meaning of pure water does not refer to water that is necessarily pure enough for human consumption . This may need further purification steps of other sorts . The freez ing initiation impulse may be created by mechanical input , pressure impact or by energy impact . Most preferably cold is fed inside the tank . The feeding of cold may include an insertion of a cold probe into tank .

[0034] Alternatively, an ultrasonic probe may be used for creating the freez ing initiation impulse . The ultrasonic probe may be located outside the tank, which means that there is no need for any sealing between the probe and the tank .

[0035] The duration of the expansion can be I s - 2h, preferably 2 s - 20 min . The purity of both salt and ice tends to increase by using a longer time for the expansion for simple salts , for example NaCl , that are feed liquids in the process .

[0036] Preferably phase diagram of the feed liquid is estimated by simulation of the expansion, for example by using Ansys Fluent -software . This software is a general-purpose computational fluid dynamics (CFD ) software used to model fluid f low, heat and mass transfer, and chemical reactions . Scientific examples of implementation of EFC-process for multi-component feed liquid can be found on publication " Impurity separation efficiency of multi -component wastewater in a pilot -scale freeze crystallizer" by John et al , published in Separation and Purification Technology 236 ( 2020 ) or on "Salt recovery from wastewater by air-cooled eutectic freeze crystalli zation" by Hasan et al , published in Chemical Engineering Journal 326 ( 2017 ) .

[0037] The purpose of the system according to the invention is achieved with a system for freeze crystalli zation purification of feed liquid, the feed liquid containing at least one chemical , the system comprising a tank for the feed liquid, a feed pump for feeding the feed liquid into the tank and a heat exchange system with heat transfer liquid arranged in connection with the tank for alternatively cooling the feed liquid in the tank and heating a second mixture of ice and product liquid in the tank using the heat transfer liquid to reach physical circumstances for freeze crystalli zation of the at least one chemical to form salt of the chemical , ice and brine . In addition, the system includes an outlet valve for removing salt , the liquid brine and product liquid alternatively from the tank, a product liquid tank for recovering the product liquid, a brine tank for recovering the brine and a salt tank for recovering the salt . The system further includes a piston in connection with the tank for affecting volume of the tank to pressuri ze the feed liquid to high pressure of 5 - 200 MPa, preferably to 15 - 150 MPa, most preferably 60 - 100 MP and adiabatically depressuri ze the tank alternatively for performing the freeze crystalli zation of the chemical to form a mixture of salt , ice and product component liquid brine , the piston extending at least partially outside the tank and being attached to an actuator located outside the tank for pushing and pulling the piston, and a digital control unit with a memory having a phase diagram of the feed liquid, the digital control unit being arranged to control the piston based on the phase diagram of the chemical contained in the feed liquid .

[0038] With the use of the hydraulic driven piston to cause the adiabatic expansion and pressuri zation of the tank the system can be implement in such a manner that it can be easily adapted to different types of feed liquids , such a salty sea water, waste waters of mines and quarries , and other type of industrial effluent containing valuable minerals . Now even the high-pressure freeze crystalli zation can be precisely controlled using the digital control unit which enables the process to be kept in optimum conditions for formation of salt with minimi zed use of energy . The system according to the invention can be used to recover valuable chemicals that can be recycled back to use without using expensive and large evaporation columns . The investment and running costs of chemical recovery can be decreased drastically as the system according to the invention is very energy efficient compared to evaporation-based processes .

[0039] Preferably the melting of the formed ice takes place in the tank using adiabatic work done by the piston . Thus , there is no need for separate tanks for melting the ice and no additional heat or pumping is needed which saves energy .

[0040] Preferably the freeze crystalli zation, melting of the formed ice and separation of salts , brine and product liquid takes place in the same tank using adiabatic work done by the piston .

[0041] Preferably the high pressure used in melting of the ice in the tank is considerably lower than the high pressure used during cooling of the feed liquid prior expansion . Preferably the pressure is increased to such a minimum level that phase transition from ice to liquid is achieved without introducing any heating to the mixture of brine and ice . This results in lower pressuri z ing costs and energy savings .

[0042] Preferably the actuator is a hydraulic cylinder fixedly attached to the piston for both pushing and pulling the piston . With the aid of the hydraulic cylinder, the piston affecting the volume of the tank can be moved in both direct ions , i . e . pulled and pushed for compression and expansion .

[0043] Alternatively, the actuator can be an electric motor . However, the electric motor does not allow the piston to move backwards to adjust to the expansion caused by the formation of ice in the tank . According to an embodiment the system includes a transfer pump for recycling brine or removing brine or both .

[0044] The system preferably includes two wear rings located at a distance from each other placed in between the piston and the tank for supporting the piston, and at least one piston ring around the piston for sealing the piston . With these wear rings and the piston ring, the piston can be reliably sealed in the tank which is important when handling feed liquid in high pressures . The feed liquids may also contain abrasive solids .

[0045] The piston ring preferably has a metal reinforcement . The metal reinforcement makes the piston ring rigid and locks the seal in place . The piston ring can also be called as the piston seal .

[0046] According to first embodiment , the system includes a piston sleeve and an outer sleeve placed concentrically relative to each other between the piston and the tank, and each of the piston sleeve and the outer sleeve having one of the wear rings .

[0047] According to a second embodiment the system includes a hydraulic connection line between the piston and the tank, wherein the hydraulic connection line forms part of the volume of the tank and the piston is located at least partially inside the hydraulic connection line and the piston is attached to the actuator, both piston and actuator located at the side of the tank . This kind of implementation enables the combination of the actuator, piston and tank to be smaller in height compared to the first embodiment wherein the actuator, piston and the tank are al l concentric, one after the other .

[0048] According to an embodiment the piston extends partially inside the tank . Thus , the tank provides support for the piston . Preferably the piston has a uniform head surface for pressuri z ing the tank . In other words , the piston is not perforated so that it can cause the pressure change in the tank .

[0049] Preferably the system further includes a pressure accumulator for collecting pressure energy during expansion in the tank using the piston . Thus , a part of the pressure energy during expansion can be recovered by pressuri z ing the pressure accumulator using the opposite side chamber of the piston .

[0050] Preferably the system further includes a sensor for detecting the formation of ice based on pressure change in the tank during expansion, and software for driving the piston to compensate the change of pressure caused by the forming of ice . It has been noticed during experiments that industrial effluent such as lime , tends to go beyond the theoretical freez ing point during expansion before forming ice . This makes it more difficult to maintain pressure at desired curve during expansion . For industrial scale implementation the system must be able to be automi zed to great extent which would be very difficult unless the actual formation of time could not be observed reliably . Now with the aid of the sensor the point of formation of ice can be detected and used for control of the piston for compensation of the pressure increase .

[0051] According to an embodiment the system includes two tanks with common heat exchange system . By using two tanks preferably in opposite phases of the process the heat recovered from cooling the feed liquid at high pressure in one tank can be utili zed for heating the second mixture of ice and product liquid of another tank . A scaleup of capacity of the system can be implemented using multiple sets of two tanks , for example , 8 or 12 tanks in total . Advantageously the heat exchange system includes a part fitted inside the tank submerged in the feed liquid, the part being preferably a spiral pipe . Thus , the pressuri zed feed liquid can be cooled within the tank which makes the system more robust and reliable as there is no need to pump the pressuri zed feed liquid outside the tank . The spiral shape of the pipe optimi zes the surface area for heat transfer efficiency .

[0052] In the system according to the invention preferably only the tank or tanks , the piping and the connecting valves to tank ( s ) are at high pressure , the remainder of the system is at essentially normal pressure which reduces the implementation costs of the system .

[0053] Preferably the tank has a single uniform volume for the feed liquid that is used for the steps . In other words , there is no diaphragms or membrane or other partitioning of the interior volume of the tank which makes the tank robust and easy to manufacture .

[0054] Preferably the system includes a sensor for monitoring information including one or more of the following : piston stroke length, volume , fluid conductivity, fluid density, activity coefficient ; and a control unit with software including a control algorithm arranged to control the movement of the piston during expansion based on the measured sensor information to maintain the freeze crystalli zation of the feed liquid within the physical circumstances of phase diagram of the feed liquid wherein salt is formed .

[0055] Advantageously the system includes a vacuum pump for removal of salt from the tank after the rinsing . This makes it easier to remove the salt , ensuring that the salt is removed and does not get stuck in the tank . According to an embodiment the piston is connected to the hydraulic actuator via a piston rod . This enables the hydraulic actuator to be placed outside the tank .

[0056] With the method and system according to the invention it is possible to recover salts and desalinate liquids very energy-ef f icient ly as a substitute for an evaporation process , which in prior art has been used for chemical recovery, desalination and other feed liquid purification . The use of the method and system according to the invention saves both energy and investment costs as the system is very affordable to implement and does not require expensive auxiliary equipment such a steam plant or steam regeneration facilities .

[0057] In the following, the invention is described in detail with reference to the accompanying drawings showing some applications of the invention, in which

[0058] Figure la shows the theoretical separation energy of desalination processes as a function of the recovery rate ,

[0059] Figure lb shows the phase diagram of sulfaric acid in constant pressure ,

[0060] Figure 2 shows basic view of an embodiment of the system according to the invention,

[0061] Figure 3a shows a block diagram of the method according to the invention,

[0062] Figure 3b shows a simplified block diagram of an embodiment of the method according to the invention having two chemical components ,

[0063] Figure 4 shows the temperature-pressure curve of water during experimental process , Figure 5 shows basic view of a heat exchange system of a second embodiment of the system according to the invention, which utili zes two tanks ,

[0064] Figure 6 shows a phase diagram of NaOH during adiabatic pressure reduction,

[0065] Figure 7a shows a side view of the first embodiment of tank and the actuator,

[0066] Figure 7b shows a cross-section view of Figure 7a,

[0067] Figure 7c shows the enlargement of the detail A shown in

[0068] Figure 7b,

[0069] Figure 8 shows the profile of the piston ring of the piston,

[0070] Figures 9a - 9c show the piston sleeve used for the sealing of the piston,

[0071] Figures 10a and 10b show the outer sleeve used for sealing of the piston,

[0072] Figures I la shows the cross section of the second embodiment of the tank and the actuator side by side ,

[0073] Figures 11b shows the side view of the second embodiment of the tank and the actuator side by side ,

[0074] Figures 12a - 12e show the water pi ston and its part s used in the second embodiment .

[0075] In the drawings the following reference numerals are used :

[0076] B brine 35 38 hydraulic connection

[0077] E eutectic point line

[0078] F feed liquid 40 tank

[0079] H heat exchange liquid 50 control unit P product liquid 52 salt tank

[0080] S salt 40 75 hydraulic cylinder

[0081] 10 system 75 . 1 bottom of cylinder

[0082] 32 actuator 75 . 2 piston rod

[0083] 34 wear ring 76 pressure accumulator 77 hydraulic control unit 35 120 piston ring

[0084] 78 position sensor 122 metal edge

[0085] 79 pressure pump 124 seal lip

[0086] 80 pressure control valve 126 outer sleeve

[0087] 81 hydraulic driven piston 127 attachment flange

[0088] 90 piston 40 128 support corner

[0089] 91 feed liquid tank 129 bolt hole

[0090] 92 product liquid tank 130 first groove

[0091] 93 brine tank 131 sleeve part

[0092] 94 transfer pump 132 wear ring groove

[0093] 95 feed pump 45 134 piston sleeve

[0094] 96 conductivity sensor 136 water cylinder

[0095] 97 flow sensor 140 bleeding valve

[0096] 98 2-way valve 142 hollow cavity

[0097] 99 thermometer 144 hydraulic valve unit

[0098] 100 2-way valve 50 146 cooling coil

[0099] 101 outlet valve 148 temperature sensor cover

[0100] 102 4-way valve pipe

[0101] 103 proportional integral 150 water piston control 152 piston body

[0102] 104 safety valve 55 154 piston head

[0103] 105 drain valve 156 interior of the tank

[0104] 108 cylinder integrated heat 160 domed head element 164 thermometer cover

[0105] 109 heat exchange system 165 tank casing

[0106] 110 coolant liquid tank 60 166 support plate

[0107] 111 heat exchanger 168 support pipe

[0108] 112 heating liquid tank, 170 lower flange

[0109] 113 pump 172 upper flange

[0110] 114 pump 174 piston platform

[0111] 115 heating element 65 176 piston guide

[0112] 116 software 178 tank inlet

[0113] 117 memory 180 tank outlet .

[0114] 118 piston ring groove The working principals of the method and system according to the invention are described with respect to Figures la - 12e . All the description below uses water as product liquid in the embodiments , but it should be understood that other liquids besides water can also be used in the method . The expansion of water during freez ing is an anomality specific to water whereas most other liquids tend to lose volume during freez ing . However, substances such as silicon, gallium, germanium, antimony, bismuth and plutonium also expand during freez ing .

[0115] From the preferably symmetry of the system follows that the pressure transfer is theoretically isentropic and reversible ; however, the method according to the invention which involves the separation of pure water from a salt solution requires separation energy .

[0116] In practice there are sources of irreversibility . The compressibility of water decreases with pressure , being at 0 ° C 52 « 10~6 / bar at normal pressure and 39 « 10~6 / bar at 2000 bar . The resulting distortion increases with the maximum pressure to a few per cent at 2000 bar and must be compensated with external energy . Also , friction, fluid viscosity and thermal losses must be compensated .

[0117] In addition to the pressure energy, it is also necessary to transfer the "cold" , i . e . , heat energy, nearly reversibly between different stages .

[0118] The pressure used in the process is a degree of freedom for optimi zation . The pressure energy is proportional to the square of the pressure while the yield of product liquid tends to grow proportionally to the pressure . The expense of the equipment grows rapidly towards the highest pressures , while the theoretical separation energy is lower at lower pressures due to lower salt concentration of the brine produced . The optimum pressure of this process is likely to be in the area of 600 - 1000 bar .

[0119] The total amount of water pressure energy to be transferred in the two pressuri zation-expansion processes is 48 MJ or 27 kWh per m3of product liquid if the product liquid is water . It is assumed that the total reversibility of the four energy transfers is 90 % amounting to an energy consumption of 2 . 7 kWh per m3of water as the product liquid . Adding the theoretical separation energy, 1 kWh / m3, and 0 . 5 kWh / m3for losses elsewhere in the process , the total energy consumption would be 4 . 2 kWh / m3.

[0120] This process is suitable for concentrating salts , radioactive materials , etc . , present in various industrial , mining or waste waters .

[0121] Figure 2 , showing a preferred embodiment of the invention . The system 10 is implemented using a hydraulic system to cause the adiabatic pressure change in the tank 40 . The actuator 32 is operated with oil or water hydraulics , pneumatics , electric motor and mechanical screw / bar . The actuator using hydraulics consist of a hydraulic cylinder 75 and a pressure accumulator 76 . The hydraulic cylinder and accumulator of the system are commercially available by for example Kashon Power .

[0122] The embodiment shown in Figure 2 discloses only one tank 40 which is preferably used for the salt separation and desalination process . This is the simplest way of implementation . However, the system preferably includes at least one pair of tanks 40 , as shown in Figure 5 , which are operated together to achieve the best possible efficiency . The two tanks preferably use adiabatically same integrated heat exchange system 109 including a heat exchanger 111 , coolant liquid tank 110 and heating liquid tank 112 , hydraulic control unit 77 , 78 , 79 , 80 and control unit 50 with software 116 and memory . Also , a common brine tank 93 , a salt tank 52 and product liquid tank 92 are used in common for both tanks 40 .

[0123] The method according to the invention comprises the following steps 182 - 220 disclosed in Figure 3a . Reference is made to Figure 2 - 6 as the method and the system according to the invention is explained in more detail . The method according to the invention includes two main phases , namely a preliminary phase and a process phase . Steps 182 - 186 shown in Figure 3a are included in the preliminary phase whereas the steps 188 - 220 are included in the process phase .

[0124] In the first step 182 of the preliminary phase , the composition of the feed liquid is analyzed . The analysis may, for example , a mass spectroscopy, an NMR spectroscopy or a gas chromatography, which is used to determine the chemical components included in the feed liquid . As these components are determined, known methods such as differential scanning calorimetry (DSC) or thermogravimetric analysis ( TGA) can be used in phase 184 for determining a phase diagram for the feed liquid and saving the phase diagram to memory 117 of the control unit 50 . The phase diagram shows the phases of the chemical in different physical circumstances as well as the eutectic point of the feed liquid . I f the feed liquid includes more than one chemical that is desired to be recovered as a salt , the phase diagram shows the eutectic points of each chemical . Based on the phase diagram the software 116 is used in phase 186 to calculate the most energy-efficient way to separate each chemical in the feed liquid with the minimal energy usage in temperature and pressure changes .

[0125] Figure lb shows , as an example , the phase diagram of sulfuric acid in normal pressure . The area with hatching elevating from left to right indicates phase in which the water is forming crystals , i . e . freeze crystalli zation area . The control unit is controlling the physical circumstances in the tank so that they are kept 1 - 10 ° C below the curve between the white liquid area and left-right-elevating hatch area to ensure that the formation of ice is continuous during the adiabatic expansion . In the physical circumstances of the area indicated with hatching elevating from right to left of the phase diagram salts are formed as well as in the eutectic freeze crystalli zation point E . In this embodiment of sulfuric acid, the circumstances are kept in the area of cross hatched triangle since only water is desired to be removed from the feed liquid thus concentrating the sulfuric acid brine . SAG, SAH, SAT , SATri , SAD and SAM indicates areas of physical conditions wherein different salts containing sulphur are formed .

[0126] In the process phase includes two separate main phases , namely salt recovery phase and brine recovery phase . The salt recovery phase is disclosed in steps 188 - 197 . Both salt recovery and brine recovery phases use an adiabatic work cycle shown in Figure 4 . Figure 4 shows the pressure and temperature changes during the adiabatic process cycle as a diagram . The transition from point A to B is the compression of the feed liquid, during which the temperature of the feed liquid rises . The transition from point B to C is the heat exchange of cooling the feed liquid in high pressure . The transition from point C to A is the adiabatic expansion during which a mixture of pure ice and brine is formed in brine recovery phase . In salt recovery phase the salts are formed during this transition from point C to A . The brine or salt is removed at point A as well as rinsing of the tank with fresh water . The pressuri zation of the second mixture of pure ice and fresh water takes place from point A to C during which the ice melts again . The cold pressuri zed fresh water can be used to cool down the pressuri zed feed liquid of another tank which is the transition from point C to D . The transition D to A is expansion of the fresh water back to normal pressure . Below is table 1 showing the energy needed for each action . It should be noted that heat needed for heating the fresh water is nearly the same as the heat recovered during cooling of the pressuri zed feed liquid . In addition, approximately half of the energy needed for compression can be recovered during expansion . Thus , the need for external energy in the process relatively small .

[0127] Table 1 .

[0128] The difference between salt recovery and brine recovery is the physical circumstances in which the expansion step from C to A is done . In the salt recovery phase , the temperature and pressure are kept during expansion in freeze crystalli zation area of the phase diagram of the feed liquid so that freeze crystalli zation takes place forming salts from the chemical contained in the feed liquid . In brine recovery the physical circumstances are kept such that water of the feed liquid freezes during expansion .

[0129] To be more precise in step 188 , the feed liquid F contained in feed liquid tank 91 is pumped to the process vessel , also known as the tank 40 , using a feed pump 95 . The feed liquid suitable for the method according to the invention is preferably filtered and solid material has been removed from it prior entry into the process . The inlet valve 98 is closed when the tank 40 is being filled with feed liquid F to the desired degree . In phase 189 the feed liquid F inside the tank 40 is pressuri zed using a piston 90 connected to a hydraulic actuator 74 . The piston 90 can be implemented as in Figure 5 and 8 but is also possible to use a piston separate unit from the tank, that utili zes a water piston for transfer of pressure to the tank . This corresponds to work from point A to point B in diagram of Figure 4 . As the pressure inside the tank 40 has reached a desired level , for example , 800 - 1500 bar, or preferably 600 - 1000 bar, the feed liquid F inside the tank 40 is cooled under pressure using heat exchanger 111 and cylinder integrated heat element 108 during phase 190 . This corresponds to work from point B to point C in diagram of Figure 4 . In other words , heat exchange liquid is circulated in the cylinder integrated heat element 108 to absorb heat from the feed liquid F , the temperature of which has increased during pressuri zation .

[0130] The cooling is continued until the pressuri zed feed liquid F has reached a temperature near freez ing . The cooling is not continued pass the point of freez ing as it would not be energy efficient . In phase 191 the pressuri zed feed liquid F is expanded back to normal pressure causing a part of it to freeze . The expansion is controlled by the control unit using an algorithm configured to keep the expansion at optimum physical conditions for salt formation based on the phase diagram of the feed liquid that has been saved in the memory . This condition is in the freeze crystalli zation area of the phase diagram of the feed liquid . Preferably the temperature is kept below the transition area, at a range of 1 - 10 ° C from the transition line . The expansion corresponds to work from point C to point A in diagram of Figure 4 . The expansion is caused by use of the piston 90 to increase the volume of the tank 40 . Pure water of the feed liquid F is frozen during the adiabatic expansion as well as salt and liquid brine B are formed . In phase 192 the salt S is removed from the tank 40 by opening a 3-way outlet valve 101 and pumping the salt to a salt tank 52 by using a trans fer pump 94 . Alternatively the salt can be recovered by a small rinse as the salt is accumulated in the bottom of the tank . In phase 193 the tank 40 is rinsed by pumping fresh water from a product liquid tank 92 using another transfer pump 94 while a 2-way valve 98 is opened and the outlet valve 101 is closed . Now the tank 40 contains a second mixture of fresh water P , ice and brine . In phase 212 the mixture of fresh water P , ice and brine in the tank 40 is pressuri zed using the piston 90 connected to the hydraulic actuator 74 . As the pressure inside the tank 40 increases , the temperature of the second mixture increases causing the ice to melt . This corresponds to work from point A to point D in diagram of Figure 4 . In phase 194 the pressuri zed fresh water in the tank is warmed using the heat exchanger 111 and the cylinder integrated heat element 108 just enough that when in phase 195 the fresh water and brine in the tank is expanded to normal pressure the fresh water and brine does not freeze again . The warming of the mixture corresponds to work from point D to point B in diagram of Figure 4 and expansion corresponds to work from point B to point A . The expansion is caused using the cylinder to increase the volume of the tank . In phase 196 the mixture of fresh water P and brine is removed to the product liquid tank 92 by opening valves the outlet valve 101 and a 2-way valve 100 . In phase 197 the outlet valve 101 is closed which is the end of salt recovery phase .

[0131] Although Figure 4 discloses that the second mixture is pressuri zed to same high pressure as used during the cooling of the feed liquid prior expansion, it must be understood that the level of pressuri zation of the second mixture can be much lower, just barely in the level which is able to cause the melting of the ice to save energy . The brine recovery phase consists of steps 200 - 220 . In phase 200 , feed liquid F that is the mixture of water and brine from the salt recovery phase , that is pumped back to the process vessel , also known as the tank 40 , using a feed pump 95 . The feed liquid suitable for the method according to the invention is preferably filtered and solid material has been removed from it prior entry into the process . The inlet valve 98 is closed when the tank 40 is being filled with feed liquid F to the desired degree . In phase 202 the feed liquid F inside the tank 40 is pressuri zed using a piston 90 connected to a hydraulic actuator 74 . This corresponds to work from point A to point B in diagram of Figure 4 . As the pressure inside the tank 40 has reached a desired level , for example , 800 - 1500 bar, or preferably 600 - 1000 bar, the feed liquid F inside the tank 40 is cooled under pressure using heat exchanger 111 and cylinder integrated heat element 108 during phase 204 . This corresponds to work from point B to point C in diagram of Figure 4 .

[0132] The cooling is continued until the pressuri zed feed liquid F has reached a temperature near freez ing . The cooling is not continued pass the point of freez ing as it would not be energy efficient . In phase 206 the pressuri zed feed liquid F is expanded back to normal pressure causing a part of it to freeze . This corresponds to work from point C to point A in diagram of Figure 4 . The freez ing is preferably controlled by an algorithm that maintains the feed liquid in crystalli zation phase of freez ing, i . e . as icy slush, not allowing it to fully freeze as solid ice . The expansion is caused by use of the piston 90 to increase the volume of the tank 40 . Pure water of the feed liquid F is frozen during the adiabatic expansion and liquid brine B is formed .

[0133] In phase 208 the liquid brine B is removed from the tank 40 by opening a 3-way outlet valve 101 and pumping the brine to a brine tank 93 by using a transfer pump 94 . In phase 210 the tank 40 is rinsed by pumping fresh water from a product liquid tank 92 using another transfer pump 94 while a 2-way valve 98 is opened and the outlet valve 101 is closed . Now the tank 40 contains a second mixture of fresh water P and pure ice . In phase 212 the mixture of fresh water P and pure ice in the tank 40 is pressuri zed using the piston 90 connected to the hydraulic actuator 74 . As the pressure inside the tank 40 increases , the temperature of the second mixture increases causing the ice to melt . This corresponds to work from point A to point D in diagram of Figure 4 . In phase 214 the pressuri zed fresh water in the tank is warmed using the heat exchanger 111 and the cylinder integrated heat element 108 just enough that when in phase 216 the fresh water in tank is expanded to normal pres sure the fresh water does not freeze again . The warming of the mixture corresponds to work from point D to point B in diagram of Figure 4 and expansion corresponds to work from point B to point A . The expansion is caused using the cylinder to increase the volume of the tank . In phase 218 the fresh water P is removed to the product liquid tank 92 by opening valves the outlet valve 101 and a 2-way valve 100 . In phase 220 the outlet valve 101 is closed which is the end of process cycle . Then all the previous phases 200 - 220 can be repeated .

[0134] Figure 3b shows a block diagram of basic salt separation phases of the method according to the invention when the feed liquid F has two chemicals as components to be separated and collected . These chemicals are Na2SC>4 and MgSCh which are separated in consecutive stages . In first salt recovery phase the conditions are set for freeze crystalli zation of Na2SC>4 , namely the temperature is set to - 6 ° C during expansion thus forming Na2 SC>4 salt which can be recovered . The brine B containing MgSCh is recycled back to tank for salt recovery of MgSCh during which the temperature is set to -20 ° C for optimal conditions for freeze crystalli zation of MgSCh during to form MgSCh salt . Figure 5 shows the heat exchange system according to the preferred embodiment having two independent tanks 40 . As can be seen from Figure 5 , the two tanks 40 share the heat exchange system 109 but also the salt tank 52 , product liquid tank 92 as well as the brine tank 93 . As the feed liquid inside one tank 40 is cooled before expansion, the heat accumulated to the heat exchange liquid can be used pumped to the heating liquid tank 112 . As the efficiency of the recovery of heat during cooling is 50 % at best , the remainder of the heating can be done using a separate heating element 115 inside the heating liquid tank 112 . Also, the cooled coolant liquid that is used for heating the second mixture of fresh water and pure ice is being collected to the coolant liquid tank 110 , thus ensuring the thermal efficiency of the method .

[0135] Cold heat transfer liquid is transferred from coolant liquid tank 110 via the 4-way valve 102 to the tank 40 on the left in Figure 5 using the pump 114 . The heat transfer liquid cool s the feed liquid in the tank 40 and exits the cylinder integrated heat element 108 of the tank 40 at an elevated temperature . Then the heated heat exchange liquid is pumped using the pump 114 via the 4-way valve 102 . 1 to the heating liquid tank 112 . The heat exchange liquid entering the heating liquid tank 112 is at a lower temperature than the heat exchange liquid in the heating liquid tank 112 , thus cooling the content of the heating l iquid tank 112 . This cooling is compensated by using the separate heating element 115 to provide more heat . The heat exchange liquid from the heating liquid tank 112 is pumped the tank 40 on the right in Figure 11 for heating the second mixture of fresh water and pure ice in the tank 40 . As the second mixture is heated, the heat exchange liquid cools and is returned via pump 113 and 4-way valve 102 to the coolant liquid tank 110 . As the heat exchange liquid entering the coolant liquid tank 110 is at a higher temperature than the heat exchange liquid in the coolant liquid tank 110 , the temperature of the content of the coolant mixture tank 110 is elevated slightly . To cool the content of the coolant liquid tank 110 , the heat exchange liquid in the coolant liquid tank is fed to the heat exchanger 111 wherein the heat of the heat exchange liquid is transferred to the heat exchange liquid from the heating liquid tank 112 . The cooling of the coolant liquid in heat exchanger can be implemented, for example a heat pump or even cold temperature outside the facility if the climate is suitable .

[0136] Referring to back to Figure 2 , in addition to heat , it is also preferred to recover at least a part of the pressure that is being released during expansion . For this purpose , the system preferably includes a pressure accumulator 76 , that is connected to the bottom of cylinder 75 . 1 to the opposite side of the hydraulic driven piston 81 in relation to the piston rod 75 . 2 . Contrary to Figure 2 , the piston 90 may also form the piston rod 75 . 2 of the hydraulic cylinder . As the method is expansion phase , the hydraulic driven piston 81 is moved towards the end of the cylinder and hydraulic fluid is being led to the pressure accumulator 76 , wherein a pressure is created . As the piston 90 is again being used for pressuri zation of the tank, the pressuri zed hydraulic fluid within the hydraulic accumulator can be led to the end of the cylinder 75 . 1 behind the hydraulic driven piston 81 . In addition, the pressure pump 79 is used to compensate the pressure losses .

[0137] In the method according to the invention the brine is preferably being collected into the brine tank 93 shown in Figure 2 . Since the brine is a valuable by-product of the method according to the invention, the concentration of the brine is preferably as high as possible . For increasing the concentration, the brine is preferably recycled back to the tank as a separate batch . Most preferably the brine is recycled 2 - 10 times so that the concentration of the brine is high for further processing . The efficiency of recovery of brine in each cycle of desalination is approximately 40 - 50 % . The initial concentration of the brine in the mixture can be , for example , 2 , 5 wt% , whereas the desired concentration may be , for example 12 , 5 wt% . The brine may be recycled using a transfer pump 94 .

[0138] The same principle of recycling applies also to salts , the recovery of salt phase can be repeated by using the product liquid of the previous stage as feed liquid .

[0139] In addition to recycling of brine , the other product , i . e . fresh water is being recycled back to the tank when the tank is being rinsed . The rinsing of the tank is important if the purity of both products , the fresh water and brine is important . However, if fresh water is not used for further purposes , the rinsing is optional and may be left out .

[0140] The method and system according to the invention is used for purification of industrial effluents having minerals and chemicals that are valuable but can also be used for desalination of salty water . The freez ing of salty water is known to follow the pressure drop constant according to known principles . However, industrial effluents , such as lime slurry is more difficult to predict . In some cases , the pressure drop causes the feed liquid to become subcooled before the formation of ice begins . Since the ice has a larger volume than the liquid form of the feed liquid, the pressure inside the tank increased due to freez ing . To compensate the increase of volume caused by the freez ing, the piston is utili zed to increase the volume of the tank simultaneously . However, if the precise point of formation of the ice is not known, the automation of the system is hard to implement . For this reason, the system preferably includes a sensor 78 that is used to detect the formation of ice from the sudden increase of pressure inside the tank . As soon as the formation of ice is detected, the cylinder is used to compensate the volume change to maintain the desired rate of adiabatic expansion in the tank to form the mixture of pure ice and liquid brine . With the aid of the sensor 78 , the system and the control of expansion can be automati zed regardless of the feed liquid . The freez ing takes place in a matter of seconds .

[0141] The system preferably includes a control computer used to control the operations of the valves , pumps , cylinder and other auxiliary devices of the system . The control computer preferably includes software means configured for the control of the system based on various sensors shown in Figure 2 . In the system according to the invention only the tank and the lines leading up to the valves connected to the tank are subjected to high pressure , the other parts of the system are maintained in normal pressure . In this case , the normal pressure refers to atmospheric pressure . The method and system can also be implemented using slightly elevated pressure of 2 - 5 bar as a substitute for normal pressure . The volume of a single tank 40 can be 10 - 1000 1 , for example 50 1 . The tank can be made from high-strength steel or carbon-fiber composites or hybrid of both or other reinforced structure . The tank may be equipped with internal heat insulation to reduce heat transmission between the tank and its content during the desalination temperature cycles .

[0142] The tank is preferably made of two materials placed on top of each other, the inner material , that is in contact with the feed liquid, being preferably duplex . The purpose of the use of duplex is to create an inner surface that is highly corrosion resistant and can withstand the harsh conditions . Instead of duplex, the inner surface of the tank can also be made as a surface coating . However, the difficulty of using a coating is its durability in the difficult conditions . The outer material covering the inner material core can be carbon steel which is much cheaper than the inner material and provides adequate strength properties for the tank . The thickness of the tank can be 100 - 150 mm, of which 50

[0143] - 60 mm is carbon steel . A tank with a volume of 41 liters would then weight approximately 200 kg .

[0144] I f pressures near the lower end of the operation range of the invention, namely pressures below 20 MPa are used, the outer material can also be some sort of composite structure .

[0145] The range of movement and the diameter of the piston define the displacement volume of the piston that needs to be proportional to the si ze of the tank . The piston is arranged to displace a volume of 1 - 20 % , preferably 5 - 15 % of the tanks volume .

[0146] Figure 4 shows the desired bath of expansion when the feed liquid is water . The expansion rate and temperature must be held such that the freez ing takes place in controlled manner so that the method can be automi zed . The automati zation of the method is implemented using sensors to detect changes of pressure , volume , temperature and concentration as well as flow sensors . The sensors itself can be such known in the prior art for the particular purpose . The process control can be implemented using programmable PLC technology . The control algorithm is feed liquid sensitive , so that a separate control algorithm is used for each feed liquid . The process parameters are programmed to correspond to characteristics of the feed liquid in different concentrations which are known from the phase diagram which has been determined in the preliminary phases of the method . The control algorithm is designed to maintain the expansion of the desired bath specifically for each feed liquid based on the phase diagram .

[0147] Figures 7a - 12e show different embodiments for the implementation of the piston that causes the change of volume of the interior of the tank and the actuator that operates the piston . There are two embodiments , namely the first embodiment , wherein the actuator is attached directly to the tank, and the second embodiment wherein the actuator and the tank are placed side by side connected with a hydraulic connection line . With regards to the first embodiment , reference is made to Figures 2 , 7a and 7b wherein the tank 40 , piston 90 and the actuator 32 are all in line connected to each other . The second embodiment is shown in Figures I la and 11b . In the second embodiment , the actuator 32 , which is preferably a hydraulic cyl inder 75 , is located side by s ide with the tank 40 , on the side of the tank 40 .

[0148] First embodiment according to Figures 7a - 10b

[0149] In the first embodiment , the actuator 32 , which is preferably the hydraulic cylinder 75 , is physically attached to the bottom of the tank 40 as shown in Figures 7a - 7c . The actuator 32 operates the piston 90 , which extends at least partially out side the tank 40 . For this reason, the piston 90 must be supported on the tank 40 . Alternatively, the actuator can also be attached on top of the tank, where it is only taking up space vertical ly, and thus not widening the area needed for the tank . However, it is preferred to attach the piston beneath the tank to facilitate bleeding of the hydraulic piston . A complete bleeding is essentially for the durability of the tank as any air mixed with feed liquid will collapse under the high pressure used in the method .

[0150] According to Figures 7a and 7b the tank 40 is preferably supported on a support pipe 168 inside of which is the actuator 32 that operates the piston 90 . The support pipe 168 may be attached to the floor with using a lower flange 170 and to the tank 40 using an upper flange 172 . In Figures 7a and 7b the actuator 32 is the hydraulic cylinder 75 . The piston 90 enters the interior 156 of the tank 40 as shown in Figure 7b and changes the volume of the interior 156 . Preferably the interior 156 has a cylindrical part 158 with a domed head 160 which eliminates any sharp corners which would be weak spots . A thermometer 99 is preferably located in the domed head 160 . The tank 40 may be inside a cas ing 165 and supported on the casing 165 by a support plate 166 surrounding the tank 40 . The piston 90 may be fixed to a platform 174 which is connected to the hydraulic piston 75 shown better in Figure 2 . The platform 174 may be placed movably on a guide pin 176 that guides the platform 174 and the piston 90 upwards as the hydraulic cylinder 75 pushes the platform 174 up towards the tank 40 .

[0151] Figure 7c shows the enlargement of detail A shown in Figure 7b . The system preferably includes at least two wear rings 34 located at distance from each other placed in between the piston 90 and the tank 40 for supporting the piston 90 , and at least one piston ring 120 between the piston 90 and the tank 40 for sealing the piston 90 . In Figure 7c the piston ring 120 is preferably a ring seal . The cross-section view of the piston ring 120 is shown separately in Figure 8 and the piston ring groove 118 reserved in the piston 90 for the piston ring 120 in Figure 7c . The piston ring is preferably made of polymer that is suitable for the harsh pressure and temperature conditions . For example , a piston ring by American High Performance Seals , known as Duralast 4403 , can be used in this purpose . This piston ring 120 includes a metal edge 122 connected to the polymer piston ring which has two seal lips 124 place apart from each other in the longitudinal direction of the piston . The purpose of the metal edge is to make the piston ring piston ring more rigid and to lock the piston ring in place . The two lips of the piston ring enable perfect sealing even with high pressures used inside the tank .

[0152] The wear rings and the piston ring can be placed in between the piston and the tank in two alternative embodiments . In one embodiment the necessary grooves for the wear rings and piston ring are fabricated directly to the tank . However, this is quite difficult to implement as the machining of the grooves to the tank is difficult . In the other, prefer embodiment , the grooves , wear rings and the piston ring are formed to separate sleeve or sleeves attached in between the tank and the piston or the piston rod, whichever enters the interior of the tank . However, the grooves for wear rings and the piston ring are difficult to machine into a single sleeve , and for that reason , in the preferred embodiment there are two separate sleeves , namely the piston sleeve 134 shown in Figures 7c, 9a - 9c and the outer sleeve 126 shown in Figures 7c, 10a and 10b . It must be noted that the tolerance for machining the grooves for wear rings and the piston ring i s small and therefore it requires high precision machining . According to Figure 7c one of the wear rings 34 is placed on a piston sleeve 134 , which is placed around the piston 90 or the piston rod attached to the piston . The piston 90 or the piston rod moves in relation to the pi ston sleeve 134 which is stationary and fixedly attached to the tank 40 via the outer sleeve 126 , shown in Figure 7c . Both piston sleeve 134 and the outer sleeve 126 are stationary in relation to the tank 40 , whereas the piston 90 moves concentrically in relation to these parts . According to Figure 7c, the piston ring 120 is placed in between the piston 90 and the outer sleeve 126 which is placed partly inside the tank 40 .

[0153] The outer sleeve shown in Figures 10a and 10b includes first groove 130 for a static seal , that seals the outer sleeve 126 to the tank . The outer sleeve 126 includes an attachment flange 127 equipped with bolt holes 129 for attaching the outer sleeve 126 to the tank 40 as shown in Figure 16 . A sleeve part 131 is attached to the attachment flange 127 and extends towards the interior of the tank 40 . The sleeve part 131 has two separate diameters , the larger diameter part being sunk into the outer layer of the tank and the smaller diameter part extending into the interior of the tank . The structure with two diameters enables better sealing . The smaller diameter part of the sleeve part 131 preferably has a wear ring groove 132 for a wear ring 34 shown in Figure 7c which is supporting the piston 70 or the piston shaft in correct alignment . A second wear ring 34 is installed in the wear ring groove 132 of the piston sleeve 134 shown in Figures 9a - 9c . The piston sleeve 134 is inserted inside the outer sleeve 126 as shown in Figure 7c . Preferably the piston sleeve and the outer sleeve are machined to such high level of precision that they fit each other with a tolerance of only 0 , 01 - 0 , 1 mm .

[0154] The second embodiment shown in Figures I la - 12e

[0155] The second embodiment , wherein the actuator 32 and the tank 40 are placed side by side connected with a hydraulic connection line 38 , is shown in Figures I la and 11b . In this embodiment , the actuator 32 is preferably a combination of hydraulic cylinder 75 and a water cylinder 136 . The water cylinder 136 is connected to the hydraulic cylinder 75 and is connected to the hydraulic connection line 38 which is connected to the tank 40 . In other words , the interior of the water cylinder 136 and the hydraulic connection line 38 together with interior 156 of the tank 40 form a uniform volume for the feed liquid . A maj or advantage of this embodiment is the smaller height of the equipment , which is important if the tanks are in a mobile facility, such as a standard freight container wherein the space is for the system is very limited . The standard dimension of the freight container does not allow the first embodiment of the tank to be instal led in vertical position and the bleeding of a hori zontally positioned tank would be difficult . By using the implementation of the second embodiment , it is possible to create achieve production capacity of 700 m3 / d within one freight container having the maximum weight of 23 000 kg . I f production capacity of over 1000 m3 / d is needed, it is preferred to use the implementation of the first embodiment in a separate production facility, since it is easier to control the production using smaller number of tanks rather than a larger number of smaller tanks .

[0156] It is also possible to implement the second embodiment using an electric motor as the actuator combined to the water cylinder . However, the electric motor is used in combination with a gearing to convert the movement speed of the electric motor to increase torque does not allow the piston to move away from the tank as the pressure increases during freez ing of the feed liquid like a hydraulic cylinder does . Therefore , the control of the electric motor must be more precise to compensate the increase in pressure caused by the freez ing of feed liquid .

[0157] A water piston 150 used in the water cylinder is shown separately in Figure 12a, and it consists of a piston body 152 , shown separately in Figures 12b and 12c, and a piston head 154 , shown separately in Figures 12d and 12e . The piston head i s preferably removably attached to the piston body 152 so that a rigid piston ring can be fitted around the pi ston rod . In addition, the implementation of the water piston 150 in two parts enables easier handling of the water piston before installation as the complete water piston can be heavy, even over 200 kg . The hydraulic cylinder is driving the water piston 150 that pressuri zes the feed liquid inside the connection line and tank . The movement of the piston may be 200 - 1000 , preferably 300 - 700 mm . The diameter of the piston may be 30 - 200 mm, preferably 50 - 150 mm . The dimensions of the hydraulic piston inside the hydraulic cylinder must be selected so that the desired pressure levels can be achieved . The piston may be made of duplex with a carbon steel core . The piston can also be hollow to reduce the weight of the piston which would otherwise be very heavy .

[0158] The control of freez ing The control unit with software and digital hydraulic control unit both control the different stages of the process by following the input data provided by sensors and meters .

[0159] The input data provides information about the different stages of the process for use by the software of the control unit and thus for controlling the process . The information obtained from the inputs may include information such as valve position, fluid flow rate , pressure , temperature , piston stroke length, volume , fluid conductivity, fluid density and activity coefficient .

[0160] The digital hydraulic control unit is used in different stages of the process :

[0161] • during feeding of feed liquid to the tank, at least one or more of the following is monitored : flow rate , temperature or water conductivity of the feed liquid; for controll ing and monitoring one or more of the following : amount , quality, process temperatures of the feed liquid .

[0162] • during pressuri zation of the feed liquid in the tank, at least one or more of the following is monitored : pi ston stroke length, volume , temperature , pressure ; for controlling and managing of volume changes or freez ing of the feed liquid or both,

[0163] • during expansion, digital hydraulics are controll ing the formation of salt as well as freez ing and composition at a desired controlled rate while maintaining separation of salt , brine and product so that salt is in as pure a form as possible . In the brine recovery phase the digital hydraulics are also controlling the conditions to optimi ze the formation and purity of separated brine .

[0164] • during expansion, digital hydraulics use sensor information including one or more of the following : piston stroke length, volume , fluid conductivity, fluid density, act ivity coefficient ; for control ,

[0165] • during removal second mixture , one or more of the following is monitored : flow rate , temperature , fluid conductivity, density; for enabling control of one or more of the following : fluid quantity, quality, composition; and thus management of controlled separation by optimal and adjustable way,

[0166] • digital hydraulic unit can also be connected to heat exchange control .

Claims

CLAIMS1. Method for freeze crystallization purification of feed liquid (F) , the feed liquid (F) containing at least one chemical, the method having in the following order the followings preliminary steps of: determining the at least one chemical contained in the feed liquid (F) , calculating a phase diagram of the at least chemical for separation, and in the following order process steps of: pressurizing of the feed liquid (F) in the tank (40) to pressure of 5 - 200 MPa, preferably to 15 - 150 MPa, most preferably 60 - 100 MPa, by using a piston (90) extending at least partially outside the tank (40) , the piston (90) being attached to a hydraulic actuator (32) located outside the tank (40) for pushing and pulling the piston (90) , adjusting the temperature of the feed liquid in a tank (40) to reach physical circumstances for freeze crystallization of the at least one chemical to form salt of said chemical, ice and brine, adjusting also the pressure in the tank (40) during the freeze crystallization by expanding the feed liquid (F) adiabatically in the tank (40) by using the piston (90) , and separating the salt from the ice and brine in the tank (40) , and collecting the salt into a salt tank (52) , in which method the hydraulic actuator (32) is controlled by using a digital control unit (150) based on the phase diagram.

2. The method according to claim 1, characterized in that the brine is separated from the ice and collected to a brine tank (93) .

3. The method according to claim 1 or 2, characterized in that the brine is recycled to the tank as feed liquid for improving yield of the salt.

4. The method according to any of claims 1 to 3, characterized in that the feed liquid contains two or more chemicals which are each separated as salts in separate freeze crystallization phases, each phase having individual physical conditions characteristic of the chemical's phase diagram to achieve freeze crystallization .

5. The method according to any of claims 1 to 4, characterized in that physical circumstances for freeze crystallization for each chemical are kept within 1 - 5 °C range of the freeze crystallization boundary.

6. The method according to any of claims 1 to 5, characterized in that the method further includes the following steps for separation of brine and ice after removal of salts: pressurizing the feed liquid (F) in the tank (40) to a high pressure, cooling the feed liquid (F) at high pressure using heat transfer liquid (H) to temperature near freezing point under said high pressure, expanding the feed liquid (F) adiabatically in the tank (40) to essentially normal pressure causing at least a part of the feed liquid (F) to freeze forming a mixture of ice and liquid brine (B) , removing the liquid brine (B) from the tank (40) and collecting the brine (B) to a brine tank (93) , feeding product liquid (P) into the tank (40) forming a second mixture of ice and product liquid (P) , pressurizing the second mixture adiabatically to high pressure causing the ice to meltusing the second mixture to cool another portion of pressurized feed liquid (F) under high pressure thus expanding the second mixture adiabatically to essentially normal pressure, removing the second mixture from the tank (40) as the product liquid (P) .

7. The method according to any of claims 1 to 6, characterized that the physical circumstances of the freeze crystallization are the physical circumstances of the eutectic freeze crystallization point of the feed liquid.

8. The method according to any of claims 1 - 7, characterized that the method further includes a step of detecting the formation of ice by a sensor (78) based on pressure change in the tank (40) during expansion and using a software (116) to drive the piston (81) to compensate the change of pressure caused by the forming of ice in the tank (40) .

9. The method according to any of claims 1 - 8, characterized that the adiabatic expansion of the feed liquid (F) in the tank (40) is controlled using a control algorithm to control the rate of formation of salt and the mixture of ice and liquid brine (B) while maintaining formation of salt so that the formed salt is in as pure as possible.

10. The method according to claim 9, characterized that during expansion, the control algorithm is arranged to control the freeze crystallization of the chemical using sensor information including one or more of the following: piston (90) stroke length, tank (40) volume, fluid conductivity, fluid density, activity coefficient; for control.

11. The method according to claims 9 or 10, characterized that the control algorithm- uses a sensor to monitor information including one or more of the following: piston (90) stroke length, volume, fluid conductivity, fluid density, activity coefficient; and based on the information controls the movement of the piston (90) during expansion to maintain the crystallization of the chemical within the physical circumstances of phase diagram of the feed liquid (F) wherein salt is formed.

12. System (10) for freeze crystallization purification of feed liquid (F) , the feed liquid (F) containing at least one chemical, the system (10) comprising a tank (40) for the feed liquid (F) , a feed pump (95) for feeding the feed liquid (F) into the tank (40) , a heat exchange system (109) with heat transfer liquid (H) arranged in connection with the tank (40) for alternatively cooling the feed liquid (F) in the tank (40) and heating a second mixture of ice and product liquid (P) in the tank (40) using the heat transfer liquid (H) to reach physical circumstances for freeze crystallization of the at least one chemical to form salt of said chemical, ice and brine, an outlet valve (101) for removing salt, the liquid brine (B) and product liquid (P) alternatively from the tank (40) , a product liquid tank (92) for recovering the product liquid (P) , and a brine tank (93) for recovering the brine, a salt tank (52) for recovering the salt, characterized in that the system further includes a piston (90) in connection with the tank (40) for affecting volume of the tank (40) to pressurize the feed liquid (F) to high pressure of 5 - 200 MPa, preferably to 15 - 150 MPa, most preferably 60 - 100 MP and adiabatically depressurize the tank(40) alternatively for performing the freeze crystallization ofthe chemical to form a mixture of salt, ice and product component liquid brine (B) , the piston (90) extending at least partially outside the tank (40) and being attached to an actuator (32) located outside the tank (40) for pushing and pulling the piston ( 90 ) , and a digital control unit (50) with a memory (117) having a phase diagram of the feed liquid (F) , the digital control unit (50) being arranged to control the piston (90) based on the phase diagram of the chemical contained in the feed liquid (F) .

13. The system according to claim 12, characterized that the system (10) includes- two wear rings (34) located at distance from each other placed in between the piston (90) and the tank (40) for supporting the piston (90) , and- at least one piston ring (120) around the piston (90) for sealing the piston (90) .

14. The system according to claim 13, characterized that the system (10) includes a piston sleeve (134) and an outer sleeve (126) placed concentrically relative to each other between the piston (90) and the tank (40) , and each of the piston sleeve (134) and the outer sleeve (126) having one of the wear rings (34) .

15. The system according to any of claims 12 - 14, characterized that the system (10) further includes a sensor (78) for detecting the formation of ice based on pressure change in the tank (40) during expansion, and a software (116) for driving the piston (81) to compensate the change of pressure caused by the forming of ice.

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