GB2588925A

GB2588925A - High efficiency water treatment process

Info

Publication number: GB2588925A
Application number: GB1916571.1A
Authority: GB
Inventors: Drak Alex; Efrat Tomer
Original assignee: IDE Technologies Ltd
Current assignee: IDE Technologies Ltd
Priority date: 2019-11-14
Filing date: 2019-11-14
Publication date: 2021-05-19
Also published as: PE20210967A1; GB201918123D0; CN110845043A; WO2021095018A1; CL2019003771A1; GB201916571D0; GB2588977A; BR102020002545A2; MX2020001510A; CN110902765A

Abstract

A process for treating water comprises delivering feed water to an input chamber (2, Fig. 1), passing pressurised feed water from the chamber through a semi-permeable membrane (8, Fig. 1) to produce product water and brine and removing sparingly soluble salts in the brine by passing it through a salt removal unit (20, Fig. 1), e.g. a fluidised bed crystalliser containing seed particles. The brine is subject to a series of re-circulation steps comprising: (i) returning the brine to the input chamber in a recirculation step to pass the brine through the semi-permeable membrane for re-concentration of the brine; and (ii) carrying out a series of step-by-step recirculation steps for continued re-concentration of the brine and crystallisation of low soluble salts from the recirculated brine for the purpose of increasing the recovery by membrane separation. The saturation indexes of low soluble salts during these steps are maintained within a controlled crystallisation zone: calcium carbonate logarithm of saturation index (Log SI) of 0.5 - 2.0; calcium sulfate saturation index of 100% - 400%; silica saturation index of 100% - 220%; and a barium sulfate saturation index of 100% - 5000%. The process may also include anti-scalant and/or deactivation steps.

Description

HIGH EFFICIENCY WATER TREATMENT PROCESS

This invention relates to an improved water treatment process, in particular an improved method for cleaning feed water, such as industrial effluent, mining impacted effluents, BWRO brine and sea water.

TECHNICAL FIELD.

Water treatment processes are known in the art for providing clean water from sea water, brackish water or other industrial effluents. For example, desalination processes exist to provide desalinated water from sea water. Reverse osmosis (RO) occurs when salt water solution is compressed against semi-permeable membranes at a pressure higher than its osmotic pressure. An example of this process is the "Plug-Flow Desalination" method which involves passing of pressurized feed flow through pressure vessels having semipermeable membranes. The feed then separates into a non-pressurized flow of desalted permeate (product water') and a pressurized flow of brine effluent (waste product').

Filtration methods may also be used to clean water. Nanofiltration (NF) also involves a semi-permeable membrane filtration-based method that uses nanometer sized cylindrical through-pores. Nanofiltration can be used to treat all kinds of water including ground, surface, and wastewater. Nanofiltration membranes have the ability to remove a significant fraction of dissolved salts.

However, the efficiency of these types of process is often poor and is highly dependent upon the quality of the feed water, which may also be highly variable.

The treatment of fluids such as sea water, brackish water, industrial effluents, wastewater etc. normally involves the use of semi-permeable membranes. During the process the fluid introduced into the system is compressed against the semipermeable membrane at pressure higher than the fluid osmotic pressure. As a result, the fluid separates into two parts, a first part comprising a product stream, which is the part of the fluid that passes through the semipermeable membrane, and a second part comprising a brine stream, which is the part of the fluid that doesn't pass through the semipermeable membrane. The separation degree, or the ratio between the volume of the product and the volume of the fluid introduced into the semipermeable membrane, depends on the osmotic pressure and the chemical composition of the fluid. Generally, the fluid chemical composition is the limiting factor of the maximum separation degree that can be achieved in the semipermeable membrane. It is desirable to provide an improved process (or method) that minimizes or overcomes the fluid chemical composition limitation.

It is an aim of the present invention is to provide an improved process for the treatment of fluids, in particular water, that overcomes or at least alleviates the abovementioned drawbacks.

SUMMARY OF THE INVENTION.

The present invention relates to an improved process for treating fluids by semipermeable membranes that overcomes or minimizes the afore-mentioned fluid chemical composition limitation by gradual crystallization of low solubility salts, thereby allowing the fluid to reach the maximum separation degree that is governed by the fluid osmotic pressure. The present invention is based on the combination of a gradual fluid concentration by passing the fluid through the semipermeable membranes, and the removal of low solubility salts by heterogeneous crystallization of the low solubility salts on seeds.

The present invention provides a process for treating water, the method comprising: (a) delivering feed water to a first input chamber; (b) passing pressurized feed water from the first input chamber through at least one semi-permeable membrane to produce a product water output and a brine output; and (c) removing sparingly soluble salts present in the brine output by passing the brine through a crystallizer unit in a series of re-circulation steps, comprising: (i) returning the brine output to the first input chamber in a recirculation step to pass the brine output through the semi-permeable membrane for re-concentration of the brine output; and preparing it for a next circulation step of sparingly soluble salts removal; and (ii) carrying out a series of step by step recirculation steps (i) for continued re-concentration of the brine output and crystallization of low soluble salts from the recirculated brine for the purpose of increasing the recovery by membrane separation, wherein the saturation indexes of low soluble salts during these steps are maintained within the following range: (1) calcium carbonate logarithm of saturation index (Log SI) is in the range of 0.5 -2.0; (2) calcium sulfate saturation index is in the range of 100% -400%; (3) silica saturation index is in the range 100% -220%; and.

(4) barium sulfate saturation index is in the range 100% -5,000%.

The maintenance of the salts at a predetermined supersaturation degree maintains the salt in a controlled crystallization zone.

Preferably., the process includes a step of reducing the pressure of the brine output before step (c).

Preferably, the calcium carbonate logarithm of saturation index (Log SI) is maintained in the range of 1.0 -1.5. Preferably, the calcium sulfate saturation index is in the range of 150% -300%. Preferably, the silica saturation index is in the range 130% -175% and preferably the barium sulfate saturation index is in the range 150% -1,000%.

Optionally, the fluid may be provided with an anti-scalant to prevent or delay crystallization of the salts. In the present invention, the process preferably includes the step of de-activating any anti-scalant in the brine output that may interfere with the salt crystallisation process. The de-activating step may comprise passing the brine output through an anti-scalant deactivation unit.

Preferably, the process includes the step of injecting ferric chloride or ferric sulfate for deactivation of precipitation that may interfere with antiscalant components, preferably being provided at a dosing rate between 0 to 10 mg/L as Fe, more preferably 0.05 to 0.5 mg/L as Fe.

Any suitable apparatus may be provided for carrying out the controlled crystallization step of the salts in the brine output but preferably the brine output is recirculated through a fluidized bed reactor or crystallizer unit wherein the brine output is pumped through a bed of seed particles, the seed particles acting as crystallization sites.

Preferably, the hydraulic load in the fluidized bed crystallizer is maintained in the range between 40 -120 m/hr, preferably 60-80m/hr and/or the residence time in the fluidized bed crystallizer is maintained in the range of 1.0 -12 minutes, more preferably 3.0 to 8.0 minutes.

Periodically, without interrupting the operation of the reactor, a lower portion of the bed is preferably discharged, and fresh seed material is introduced.

The process may also include a step of removing the brine output from the first input chamber when reaching a predetermined osmotic pressure in brine output and delivering fresh feed water to this chamber. For example, re-direction may occur upon detecting a predetermined reduction in the efficiency of the semi-permeable membrane, such as by the detection of a predetermined maximum salt concentration corresponding to the maximum osmotic pressure at which the membrane can operate.

The process may further comprise cleaning the input chamber during removal of the brine.

The process of the present invention preferably combines (1) fluid concentration by the semipermeable membranes, (2) anti-scalant deactivation, (3) low solubility salts removal by crystallization on the seeds surface in a controlled crystallization zone and, where necessary, (4) anti-scalant addition.

The process preferably operates in the semi batch mode, meaning that: * the brine produced in the semipermeable membrane is recirculated back through the anti-scalant deactivation, salt removal and anti-scalant addition units.

* the product is produced continuously in the semipermeable membrane.

* the raw / fresh fluid is added continuously to replace the volume of product that is withdrawn from the system, to keep the constant volume of the fluid in the system.

* the raw / fresh water is mixed with the produced brine before introduction to the semipermeable membrane.

* The entire system volume is replaced by the raw / fresh fluid after reaching the osmotic pressure limitation in the semipermeable membrane.

In the described above mode, the fluid is concentrated gradually every time it passes through the semipermeable membrane, the concentration of high solubility salts increases gradually. The low solubility salts, which concentration reaches the controlled crystallization zone, are crystallized on the seeds surface in the salt removal unit so that their concentration is maintained inside the controlled crystallization zone.

Since the saturation degree of the low solubility salts is maintained in the controlled crystallization zone and never exceeds this zone, the anti-scalant, dosed to the circulated fluid, is maintained at a very low level, much lower compared to the standard semipermeable membrane process without gradual removal of low solubility salts. In case of phosphonate based anti-scalant, the anti-scalant concentration is maintained in the range of 0 -1.5 mg/L as phosphate, preferably in the range of 0.5 -1.0 mg/L as phosphate.

Since the fluid is concentrated gradually by the semipermeable membrane, preferably no chemicals are required to initiate the crystallization process, in contrast to the common crystallization processes in which chemicals are used to initiate crystallization. If ions that make up the low solubility salt are in a non-stoichiometric ratio, the ion which concentration is lower may be added to increase the removal extent of the low solubility salt.

Anti-scalant deactivation is done on the seeds surface so no additional deactivation agents are required. For cases in which low saturation degree of the final brine is required, additional deactivation agents may be used such as oxidants (ozone, hydrogen peroxide etc.) or precipitation agents (ferric chloride, ferric sulfate, aluminium chloride, aluminium sulfate etc.), preferably ferric chloride or ferric sulfate at concentration of 0.05 -0.5 mg/L as Fe.

To make the process continuous, some of the units can be provided with redundancy, to enable sufficient time for their drainage when osmotic pressure limit is reached and refilling with raw / fresh fluid.

BRIEF DESCRIPTION OF THE DRAWINGS.

Embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a water treatment system suitable for carrying out the process according to one embodiment of the present invention; Figure 2 is a schematic diagram of a water treatment system suitable for carrying out a process according to another embodiment of the present invention; Figure 3 is a plot of concentration against temperature of a salt solution, illustrating the different crystallization zones; Figure 4 is a schematic diagram illustrating the process steps of an embodiment of the present invention; and Figure 5 is a plot of concentration against process time for a salt solution, illustrating a controlled crystallization zone.

DETAILED DESCRIPTION.

The present invention provides an improved water treatment process for improving the efficiency of the water treatment, particularly for enabling variable quality feed water to be used with different recovery rates.

Referring to Figure 1 of the accompanying drawings, the basic components of a system that may be used for carrying out a process of the invention for treating feed water is illustrated. The figure shows a reverse osmosis process and system but a nanofiltration membrane may be used as an alternative to the reverse osmosis membrane. Feed water or salt water (FW) is introduced into a single feed chamber 2 from which it is directed through a delivery pipe 2i to a high pressure pump 6. The high pressure pump 6 then pressurizes feed water prior to its passage through a reverse osmosis membrane 8 from which product water PW is produced, together with a concentrated brine stream CW. A pre-treatment unit, such as filter unit (not shown), may optionally be provided to pre-treat the feed water prior to its passage through the membrane.

Conventionally, the brine waste stream would then be discarded. In the present invention, repeated recycling of this waste product occurs. The concentrated brine stream CW is delivered back to the feed chamber 2 via an optional deactivation unit 40 and a desaturation unit 20 comprising a fluidized bed reactor. The deactivation unit deactivates components within the CW which might interfere in a salt precipitation process and the desaturation unit reduces the saturation level of sparingly soluble salts contained in the brine stream CW. The operating conditions for carrying out this process are particularly important to provide enhanced recovery and these are discussed in further detail below. Ideally, the chamber is open to atmosphere to provide an open loop system which enables a reduction in the pressure of the concentrated brine to near atmospheric pressure. Alternatively, a pressure exchanger or energy recovery system (not shown) may be provided to reduce the overall pressure of the brine stream.

The concentrated brine stream delivered back to the feed chamber 2 is mixed with additional feed water FW that is still being delivered to the chamber and then recycled back through the system to provide more product water PW and concentrated brine CW. This recycling loop is repeated a number of times. The system may be dosed with antiscalant (not shown) to prevent scaling of the membrane.

The system is provided with a detector (not shown) for checking the efficiency of the reverse osmosis process. In this respect, it is to be appreciated that repeated recycling of the brine stream will reduce the efficiency of the process over time as the saline concentration of the feed water increases. To address this issue, the system is provided with a series of gate valves 12, 14, 16 and 18. During normal recycling of the concentrated water (CW) through the system, valves 12, 14 and 18 are open and valve 16 shut. Once a predetermined reduction in the efficiency of the process is detected, these valves are shut and valve 16 opened. This temporarily shuts down the system/process to empty the chamber 2. Once empty, valve 16 is shut and the other valves opened to allow fresh water to enter the chamber 2 and a new recycling process is commenced.

Figure 2 of the accompanying drawings illustrates an alternative example of a system for carrying out the process of the present invention. Identical features already discussed in relation to Figure 1 are given the same reference numerals and only the differences will be discussed in detail. The system includes both a first feed chamber 2 and a second feed chamber 4 with the process changing to the second feed chamber during emptying and re-filling of the first feed chamber. When the concentration of the feed water in the first chamber 2 reaches a predetermined level, the delivery pipe 2i is shut and feed water is introduced into the system from a second chamber 4 via delivery pipe 4i. This feed water is then passed through the desaturation unit 20, pumped through the reverse osmosis membrane 8 to provide concentrated brine CW and product water PW. The concentrated brine is recycled back to the second chamber 4 via the deactivation unit 40 and desaturation unit 20 and a return pipe 4R for recycling through the system with further feed water.

While feed water is being introduced from the second chamber, the highly concentrated brine water CW in the first chamber is removed via outlet pipe 2o. The chamber is cleaned and fresh feed water is introduced into the chamber 2.

Once the system detects a predetermined reduction in the efficiency of the reverse osmosis process, the system reverts back to the use of the first chamber 2. In this respect, as the first feed chamber over time, the feed water from the second chamber reaches a predetermined concentration, preferably being around the maximum osmotic pressure at which the reverse osmosis membrane can operate, at which point the inlet 4i of the second chamber is closed and feed water is again delivered through the system from the first chamber 2 back to the first chamber via the pressure exchanger 40 and return pipe 2R. The concentrated brine in the second chamber is removed via outlet 4o and fresh water is delivered into the second chamber 4.

Any appropriate number and arrangement of feed chambers and associated delivery and return pipes may be provided in the system. Groups of chambers may work simultaneously.

Various pre-and post-treatments may be provided. In the case of a desaturation unit (20) this may only be operational when a predetermined salt concentration is reached.

An example of a desaturation unit is a fluidised-bed type crystalliser, such as that sold under the name Crystalactor®.

The controller for the system automatically redirects delivery of concentrated water from the first to the second chamber or vice versa, preferably upon detection of predetermined reduction in the efficiency of the overall process. Alternatively, the controller may automatically shut down the system as described in Figure 1.

The basic component parts of the system shown in Figure 1 and 2 carry out the process of the invention under predefined conditions to provide optimized results. The fluid concentration is done gradually by passing the fluid through the semipermeable membrane. The semipermeable membrane is a type of biological or synthetic membrane that will allow certain molecules or ions to pass through it by diffusion or occasionally by more specialized processes of facilitated diffusion, passive transport or active transport. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry.

When fluid is introduced into the semipermeable membrane and is compressed against the semipermeable membrane at pressure higher than the fluid osmotic pressure, the fluid is separated into two parts. The first part is the part of the fluid that passes through the semipermeable membrane, that part is called product. The second part is the part of the fluid that doesn't pass through the semipermeable membrane, that part is called brine.

Since generally the salts are rejected by the semipermeable membrane, the brine part contains more salts than the product part. The salts in the brine may be divided into the two parts -high solubility salts such as sodium chloride, potassium chloride etc., and low solubility salts such as calcium carbonate, calcium sulfate, barium sulfate, silica etc. During the fluid concentration in the semipermeable membrane, the low solubility salts may pass the solubility limit of these salts and as a result may start to crystallize. The undersaturated salt' is salt that has concentration lower than the solubility limit, the supersaturated salt' is salt that has concentration higher than solubility limit and the 'saturated salt' is salt that has concentration equal to the solubility limit.

The crystallization process, among other things, depends on the degree of salt supersaturation. Below saturation (below solubility limit), there is not enough energy to start any type of crystallization (stable zone). At high degree of supersaturation, there is enough energy to start crystallization process everywhere (unstable zone) -on all available surfaces as well as in the solution. At low degree of supersaturation, there is enough energy only for crystallization on the available surfaces and not for crystallization in the solution (metastable zone). At very low supersaturation, the crystallization will only begin on some of the available surfaces with the lowest surface energy (controlled crystallization zone). These zones are illustrated in Figure 3 of the accompanying drawings.

Anti-scalant is generally used to prevent or delay the crystallization process of the low solubility salts on the surface of the semipermeable membrane. There are different anti-scalants in the market specifically developed to prevent or delay crystallization of different low solubility salts. The anti-scalant is limited to a certain threshold of supersaturation, above this threshold, immediate crystallization will occur, below this threshold the crystallization process will start after a certain period of time, called 'induction time'.

Seed material is generally used to reduce the concentration of low solubility salts in the solution by crystallization of the salts on the seeds surface. To control the crystallization process on the seeds surface, a certain supersaturation degree of the low solubility salts should be maintained. The supersaturation should be high enough to initiate the crystallization process immediately with lowest induction time and should be low enough to prevent crystallization process in the solution and on the undesirable surfaces with high surface energy. Therefore, the supersaturation of the low solubility salts should be in the 'controlled crystallization zone' as shown in Figure 3 (and Figure 5).

Accordingly, the process of the present invention carefully controls the crystallization of the salts present in the brine stream by maintaining certain conditions, in particular to maintain the low solubility salts in the controlled crystallization zone as the brine output is recirculated through the process. To control the crystallization process of calcium carbonate on the seeds surface, the supersaturation degree (logarithm of saturation index) of calcium carbonate is maintained between 0.5 and 2.0, preferably between 1.0 and 1.5. To control the crystallization process of calcium sulfate on the seeds surface, the saturation degree (saturation index) of calcium sulfate is maintained between 100% and 400%, preferably between 150% and 300%. To control the crystallization process of silica on the seeds surface, the supersaturation degree (saturation index) of silica is maintained between 100% and 220%, preferably between 130% and 175%.

If anti-scalant is used to prevent or delay the crystallization process on the surface of the semipermeable membrane, in order to control the crystallization process on the seeds surface, the supersaturation degree of low solubility salts should be maintained in the same range as mentioned above. However, due to the presence of anti-scalant, the anti-scalant activity should be eliminated just before the contact of the fluid that contains the low solubility salts and the anti-scalant, with the seeds surface. The anti-sealant activity can be eliminated by a number of different methods, among them (1) chemical addition to capture the anti-scalant (such as ferric chloride, ferric sulfate, aluminium chloride, aluminium sulfate etc.), (2) chemical addition to destroy the antiscalant (such as ozone, hydrogen peroxide etc.), or (3) providing a surface to adsorb the anti-scalant (such as seeds surface etc.) etc. The process of the present invention may use different types of equipment to crystalize the low solubility salts on the seeds, such as a chemical reactor followed by clarifier, fluidized bed reactor etc. A fluidized bed reactor is preferred and has an advantage over other crystallization methods due to the high flow velocity and low residence time. The principle of fluidized bed reactor operation is as follows: the reactor is partially filled with suitable seed particles; the fluid, (in the present case, the brine produced by the semipermeable membrane) is pumped upward through the bed of particles to maintain this in a state of fluidization. The seed particles are used as crystallization sites; they provide the high surface area that lowers the energy required for precipitation. As the crystals become progressively heavier, they gradually travel towards the bottom of the bed. Periodically, without interrupting the operation of the reactor, the lower portion of the bed is discharged, and fresh seed material is introduced. The upward velocity applied is in the range of 40 -120 m/hr, preferably between 60 -80 m/hr. The residence time of the liquid in the reactor is between 2.0 and 12.0 minutes, preferably between 3.0 and 8.0 minutes.

The process of the present invention combines the following criteria to provide optimized conditions; (1) fluid concentration by the semipermeable membranes, (2) anti-scalant deactivation, (3) low solubility salts removal by crystallization on the seeds surface, (4) anti-scalant addition, as illustrated in Figure 4 of the accompanying drawings.

The process operates in a semi batch mode. A semi batch mode means that: * the brine produced in the semipermeable membrane is recirculated back through the anti-scalant deactivation, salt removal and anti-scalant addition units.

* the product is produced continuously in the semipermeable membrane.

Since the saturation degree of the low solubility salts is maintained in the controlled crystallization zone and never exceeds this zone, the anti-scalant, dosed to the circulated fluid, is maintained at a very low level, much lower comparing to the standard semipermeable membrane process without gradual removal of low solubility salts. In case of phosphonate based anti-scalant, the anti-scalant concentration is maintained in the range of 0 -1.5 mg/L as phosphate, preferably in the range of 0.5 -1.0 mg/L as phosphate.

Since the fluid is concentrated gradually by the semipermeable membrane, no chemicals are required to initiate the crystallization process, in contrast to the common crystallization processes in which chemicals are used to initiate crystallization. If ions that make up the low solubility salt are in a non-stoichiometric ratio, the ion which concentration is lower can be added to the system to increase the removal extent of the low solubility salt.

Claims

CLAIMS.A process for treating water, the method comprising: (a) delivering feed water to a first input chamber; (b) passing pressurized feed water from the first input chamber through at least one semi-permeable membrane to produce a product water output and a brine output; and (c) removing sparingly soluble salts present in the brine output by passing the brine through a salt removal unit, the brine output being subject to a series of re-circulation steps comprising; (i) returning the brine output to the first input chamber in a recirculation step to pass the brine output through the semi-permeable membrane for re-concentration of the brine output; and preparing it for a next circulation step of sparingly soluble salts removal; and (ii) carrying out a series of step by step recirculation steps (i) for continued re-concentration of the brine output and crystallization of low soluble salts from the recirculated brine for the purpose of increasing the recovery by membrane separation, wherein the saturation indexes of low soluble salts during these steps are maintained within the following range: (1) calcium carbonate logarithm of saturation index (Log SI) is in the range of 0.5 -2.0; (2) calcium sulfate saturation index is in the range of 100% -400%; (3) silica saturation index is in the range 100% -220%; and.(4) barium sulfate saturation index is in the range 100% -5,000%.
2. The process according to claim 1 further comprising reducing the pressure of the brine output before step (c).
3. The process according to claim 1 or claim 2 wherein the calcium carbonate logarithm of saturation index (Log SI) is maintained in the range of 1.0 -1.5.
4. The process according to claim 1, 2 or 3 wherein the calcium sulfate saturation index is maintained in the range of 150% -300%.
5. The process according to any one of claims 1 to 4 wherein the silica saturation index is maintained in the range 130% -175%.
6. A process according to any one of claims 1 to 5 wherein the barium sulfate saturation index is maintained in the range 150% -1,000%.
7. A process according to any one of the preceding claims wherein the fluid is treated with an anti-scalant to prevent or delay crystallization of the salts.
8. A process according to claim 7 wherein the process includes the step of adding a phosphonate-based anti-scalant, preferably being provided in the range 0.5 -1.0 mg/L as phosphate.
9. A process according to any preceding claim further comprising the step of deactivating any anti-scalant in the brine output that may interfere with the salt crystallisation process.
10. A process according to claim 9 wherein the de-activating step comprises passing the brine output through an anti-scalant deactivation unit.
11. A process according to claim 9 or claim 10 wherein the process includes the step of injecting ferric chloride or ferric sulfate for deactivation, preferably being provided at a dosing rate between 0 to 10 mg/L as Fe.
12. A process according to any one of the preceding claims wherein the brine output is recirculated through a fluidized bed crystallizer unit wherein the brine output is pumped through a bed of seed particles, the seed particles acting as crystallization sites for carrying out the controlled crystallization step of the salts in the brine output.
13. A process according to claim 12 wherein the crystallizer unit also de-activates any anti-scalant in the fluid.
14. A process according to claim 12, 13 or 14 wherein the hydraulic load in the fluidized bed crystallizer is maintained in the range between 40 -120 m/hr.
15. A process according to claim 12, 13 or 14 wherein the residence time in the fluidized bed crystallizer is maintained in the range of 1.0 -12 minutes.
16. A process according to any one of claims 12 to 15 further comprising periodically, without interrupting the operation of the crystallizer unit, discharging a lower portion of the bed and introducing fresh seed material.
17. A process according to any one of the preceding claims further comprising continuously adding fresh feed fluid to replace the volume of product that is withdrawn from the process to keep a constant volume of fluid in the process.
18. A process according to claim 17 further comprising periodically replacing the entire volume of fluid by fresh feed fluid after reaching the osmotic pressure limitation in the semipermeable membrane.
19. A process according to any one of the preceding claims wherein no chemicals are added to initiate the crystallization process step.