Fluid purification by forward osmosis, ion exchange and re-concentration

Abstract

An apparatus (100) for purifying fluid, wherein the apparatus (100) comprises an osmosis unit (102) configured for pre-purifying the fluid to be purified by forward osmosis of the fluid, which is to be purified, through an osmosis membrane (104) into a chamber (106) comprising dissolved first ions, in particular first cations and first anions, an ion exchange unit (108) configured for exchanging at least part of the first ions, in particular at least part of at least one of the first cations and the first anions, by second ions, in particular at least one of second cations and second anions, and a re-concentration unit (110) configured for separating the pre-purified fluid after the ion exchange into purified fluid and into a re-concentrate enriched with the respective ions, in particular anions and cations.

Claims

1. An apparatus for purifying fluid, the apparatus comprising: an osmosis unit for pre-purifying the fluid to be purified by forward osmosis of the fluid to be purified through an osmosis membrane into a chamber comprising dissolved first ions; an ion exchange unit for exchanging at least part of the first ions by second ions; a re-concentration unit for separating the pre-purified fluid after the ion exchange into purified fluid and into a re-concentrate enriched with the respective ions, wherein at least part of the first ions have an absolute value of a charge state in solution which is lower than an absolute value of a charge state of at least part of the second ions in solution.

2. The apparatus according to claim 1, wherein the osmosis unit promotes a flow of the fluid to be purified through the osmosis membrane while inhibiting a flow of contaminants from the fluid to be purified towards the chamber and while inhibiting the first ions from flowing through the osmosis membrane towards the fluid to be purified.

3. The apparatus according to claim 1, wherein the first ions comprise first cations and first anions and the second ions comprise second cations and second anions, and wherein the ion exchange unit exchanges at least part of the first anions by the second anions and/or at least part of the first cations by the second cations.

4. The apparatus according to claim 1, wherein the ion exchange unit reversibly exchanges at least part of the ions prior to re-concentration and after re-concentration.

5. The apparatus according to claim 1, wherein the ion exchange unit exchanges back at least part of the ions after the ion exchanging by the respectively other ions by a further reverse ion exchange after re-concentration.

6. The apparatus according to claim 1, wherein the re-concentration unit filters the pre-purified fluid after the ion exchange by a re-concentration membrane so that the purified fluid passes the re-concentration membrane, whereas at least part of the ion exchanged ions are retained by the re-concentration membrane.

7. The apparatus according to claim 6, wherein the re-concentration unit comprises at least one of the group consisting of a nano-filtration membrane, an ultra-filtration membrane, a micro-filtration membrane, and a reverse osmosis membrane.

8. The apparatus according to claim 1, wherein the re-concentration unit re-concentrates by a thermal treatment of the fluid.

9. The apparatus according to claim 1, comprising at least one of the following features: wherein the re-concentration unit re-concentrates by at least one of the group consisting of membrane distillation, vapor compression desalination, freezing treatment, electric dialysis, and lonenkraft processing; comprising a fluid drive unit for providing or enhancing a driving force acting on the pre-purified fluid flowing from the ion exchange unit towards the re-concentration unit; wherein the first cations and the second cations are selected in terms of ion properties so as to increase efficiency of re-concentration in the re-concentration unit.

10. The apparatus according to claim 1, comprising an energy production unit for producing energy during operating the apparatus.

11. The apparatus according to claim 10, wherein the energy production unit supplies the produced energy for operating the apparatus.

12. The apparatus according to claim 1, comprising a pressure exchange unit, comprising at least one of the following features: the pressure exchange unit transmits pressure between fluid propagating from the re-concentration unit to the ion exchange unit and fluid propagating from the ion exchange unit to the re-concentration unit; the pressure exchange unit comprises at least one of the group consisting of a turbine, a booster, a Clark pump, and a Pearson pump.

13. The apparatus according to claim 1, is a closed cycle system in which at least part of the ions are recovered and reused within a closed cycle.

14. The apparatus according to claim 1 is at least one of the group consisting of a seawater desalinisation plant, a brackwater desalinisation plant, a portable, in particular backpack-based, apparatus for mobile water purification, a domestic water purification apparatus for purifying water for a building, an industrial water purification apparatus for purifying water for a plant, an agricultural water purification apparatus, a mining water purification apparatus, a municipal apparatus for purifying fluid, a naval apparatus for purifying fluid, an aircraft apparatus for purifying fluid, and a spacecraft apparatus for purifying fluid.

15. A method of purifying fluid using an apparatus of claim 1, the method comprising: pre purifying the fluid to be purified by forward osmosis of the fluid to be purified through an osmosis membrane into a chamber comprising dissolved first ions; subsequently exchanging at least part of the first ions by second ions by an ion exchange unit; separating the pre-purified fluid after the ion exchange into purified fluid and into a re-concentrate enriched with the respective ions.

16. The apparatus according to claim 1, wherein at least part of the first ions are monovalent ions and at least part of the second ions are multivalent ions.

17. The apparatus according to claim 1, wherein the first ions comprise first cations and first anions and the second ions comprise second cations and second anions, and wherein the first cations are smaller than the second cations and/or the first anions are smaller than the second anions.

18. The apparatus according to claim 1, wherein the first ions comprise first cations and first anions and the second ions comprise second cations and second anions, and wherein the first anions and the first cations represent dissolved sodium chloride; and/or wherein the second anions and the second cations represent dissolved magnesium sulphate.

19. The apparatus according to claim 1, wherein the ion exchange unit reversibly exchanges at least part of the first ions by second ions.

Description

(1) The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

(2) FIG. 1 illustrates a schematic view of an apparatus for purifying fluid according to an exemplary embodiment of the invention.

(3) FIG. 2 illustrates a pressure exchange unit of an apparatus according to an exemplary embodiment of the invention.

(4) The illustrations in the drawings are schematic. In different drawings, similar or identical elements are provided with the same reference signs.

(5) Before describing the figures in further detail, some basic considerations will be summarized based on which exemplary embodiments have been developed.

(6) Exemplary embodiments of the invention may involve one or more of the following concepts: Forward osmosis can be used as a procedure for water purification (in particular for water treatment or water desalinisation). In a corresponding system, it is possible, but not necessary that the fluid to be purified by forward osmosis is pre-treated or pre-processed. Reversible ion exchanging can be implemented in order to reduce the osmotic pressure of a draw solution for a membrane-based re-concentration. For example, the ion exchange may exchange NaCl into MgSO.sub.4. However, many other combinations of materials are possible which can be exchanged by reversible ion exchange, in order to obtain an advantage for subsequent re-concentration. A re-concentration stage (preferably, but not necessarily, membrane-based and/or thermally based) may be implemented. Optionally, an energy recovery mechanism may be integrated in the fluid purification process (see reference numeral 118 in FIG. 1). Optimally, an energy production mechanism may be integrated in the fluid purification process (see reference numeral 116 in FIG. 1).

(7) According to an exemplary embodiment of the invention, an osmotic water treatment is provided which can be used, for example, for drink water treatment, wastewater treatment, seawater desalinisation (including brackwater desalinisation), etc. with high throughput and low energy consumption. Exemplary embodiments may be applied in communal and industrial water supply and waste water management, in mining, in agriculture, in the military sector, in the navy, and for food processing. Also aircraft and spacecraft applications are possible according to exemplary embodiments of the invention.

(8) Advantages of a system according to an exemplary embodiment of the invention are the high energy efficiency and its high robustness. Operation of such a system is simple and involves only a small effort in terms of required skills of operators (which renders the system particularly appropriate for low developed regions) and maintenance (which renders the system appropriate for difficult applications).

(9) The amount of chemicals required for the process is very small, which reduces costs and pollution, and relaxes the logistic effort. In view of the small sufficient pressure values involved in the process, simple and cost efficient components may be implemented in the system. Small pressure values can be obtained by using ion exchange against multivalent ions, thereby reducing, in turn, the osmotic pressure of the draw solution. As a result, pressure tubes can be made of plastic rather than of steel. It is sufficient to implement simple valves and pumps, etc. As compared to thermal purification systems, the system for purifying fluid according to an exemplary embodiment of the invention can be implemented at any desired location, since it is independent of any economic heat source (such as heat dissipated by a power plant).

(10) A functional principle involved in an exemplary embodiment of the invention is the physical phenomenon of forward osmosis according to which the solutions being separated from one another by a semipermeable membrane equilibrate their concentrations.

(11) Without a separating membrane, the dissolved particles would be equally distributed in the entire volume of both sides of the chamber (under the influence of mixing entropy). By intentionally preventing this thanks to the provision of an osmosis membrane dimensioned to enable only the fluid to be purified (in particular water) to pass the membrane, but to disable this for both the contaminants and the ions, the only possibility of an equilibration is a dilution of the higher concentrated draw solution until the concentration difference is equilibrated or the hydrostatic pressure of the fluid column on the side of the diluted solution balances out the osmotic pressure. For this purpose, a draw solution is provided in the chamber on the side of the osmosis membrane opposing the fluid to be purified. This draw solution shall have an ion concentration being higher than the solution of the fluid to be filtered or purified (i.e. the raw fluid). Highly advantageously, the draw solution may be re-concentrated after each cycle, thereby separating the obtained pure fluid or product fluid. This allows to obtain synergistically purified fluid and a re-concentration of the osmotic agent (i.e. the osmotically active dissolved matter in the draw solution). By such a substantially loss-free recycling of the draw solution, a closed cycle process can be obtained.

(12) A corresponding system will be described in the following in further detail referring to FIG. 1:

(13) FIG. 1 illustrates a schematic view of an apparatus 100 for purifying fluid according to an exemplary embodiment of the invention. The fluid to be purified flows from a fluid source 120 into an accommodation space 122 on the left-hand side of an osmotic membrane 104.

(14) The apparatus 100 comprises an osmosis unit 102 configured for pre-purifying the fluid to be purified by forward osmosis of the fluid to be purified through the osmosis membrane 104 into a chamber 106 comprising, as draw solution, dissolved first cations (in the shown embodiment Na.sup.+) and first anions (in the shown embodiment Cl.sup.) of a first dissolved salt (sodium chloride, NaCl, in the shown embodiment). The osmosis membrane 104 is a semipermeable membrane which is configured (in particular in terms of pore size or zeta potential) so that it can be passed by water as the fluid to be purified, whereas the semipermeable membrane is configured so that it cannot be passed by contaminants in raw fluid to be purified and cannot be passed by the first cations and the first anions of the draw solution. The first cations and the first anions which are dissolved in a liquid carrier such as water are located in the chamber 106 prior to the start of a fluid purification process. For instance in the scenario of seawater desalinisation, the raw fluid may be seawater with contaminants and may also include dissolved sodium chloride, i.e. may also comprise a certain concentration of the first cations (Na.sup.+) and the first anions (Cl.sup.). However, the concentration of the ions (i.e. cations and anions) shall be larger in the chamber 106 (for instance 5% or more) as compared to their concentrations in the raw fluid (for instance 3% or less). The forward osmosis unit 102 is configured for promoting, driven by the phenomenon of forward osmosis, a flow of the fluid to be purified through the osmosis membrane 104 while inhibiting a flow of contaminants from the fluid to be purified towards the chamber 106 and while inhibiting the first cations and the first anions from flowing through the osmosis membrane 104 towards the fluid to be purified. This draws pure water through the osmosis membrane 104, as indicated by arrows 124, while forcing the contaminants of the raw fluid (which may also be denoted as feed solution, feed water or feed fluid) to remain within the accommodation space 122.

(15) As indicated by reference numeral 126, the so processed pre-purified fluid together with dissolved sodium chloride (Na.sup.+, Cl.sup.) flows into an ion exchange unit 108. The ion exchange unit 108 is configured for exchanging the first cations (Na.sup.+) by second cations (Mg.sup.2) and for exchanging the first anions (Cl.sup.) by second anions (SO.sub.4.sup.2). Hence, this ion exchange exchanges monovalent small ions by bivalent larger ions which has a pronounced positive impact on the osmotic pressure (more precisely, advantageously reduces the latter), therefore improving the below described re-concentration efficiency. A skilled person will understand that other ions and/or other valent ratios are possible.

(16) After this primary ion exchange, the ion exchanged pre-purified fluid is forwarded, via a pressure exchange unit 118, to a re-concentration unit 110, as indicated by reference numerals 128, 132. The pressure exchange unit 118, shown in detail in FIG. 2, is configured for transmitting pressure between two opposing fluid flows between the ion exchange unit 108 and the re-concentration unit 110 and functions as an isobaric energy recovery component.

(17) The apparatus 100 furthermore comprises a fluid drive unit 114 such as a pump for increasing pressure of the pre-purified fluid flowing from the ion exchange unit 108 towards the re-concentration unit 110.

(18) According to an exemplary embodiment of the invention, the pressure exchange unit 118 and the fluid drive unit 114 can be integrally formed as one common entity, i.e. a pump with integrated energy recovery function fulfilling both tasks of pressure exchange between the fluid flowing upstream and downstream, as well as driving fluid in the upstream direction (such as a Clark pump or a Pearson pump, as manufactured by Spectra Watermakers).

(19) The re-concentration unit 110 is configured for separating the pre-purified fluid after the ion exchange into purified fluid (which may also be denoted as permeate or product water or product fluid) and into a re-concentrate (which may also be denoted as retentate) enriched with the respective second cations (Mg.sup.2+) and second anions (SO.sub.4.sup.2). The re-concentration is accomplished by filtering the pre-purified fluid after the ion exchange by a re-concentration membrane 112 (and/or thermally), such as a nanofiltration membrane, of the re-concentration unit 110 so that the purified fluid passes the re-concentration membrane 112 and can be conveyed to a destination 130, such as an end user consuming the purified water.

(20) Another part of the fluid including a high concentration of the second cations (Mg.sup.2+) and second anions (SO.sub.4.sup.2) is retained by the re-concentration membrane 112. This other part of the fluid including the high concentration of the second cations and second anions retained by the re-concentration membrane 112 is then conducted back via the pressure exchange unit 118 towards the ion exchange unit 108, the reference numerals 134, 136. Subsequently, the same ion exchange unit 108 as mentioned above exchanges back the second anions by the first anions (SO.sub.4.sup.2.fwdarw.2 Cl.sup.) and the second cations by the first cations (Mg.sup.2+.fwdarw.2 Na.sup.+) in a further ion exchange procedure after the described re-concentration. Hence, the ion exchange unit 108 is highly advantageously configured for reversibly exchanging the ions prior to re-concentration and after re-concentration. Thus, the apparatus 100 operates as a closed cycle system in which the anions and the cations are continuously and repeatedly recovered and reused within a closed cycle without the need to supply new draw solution to the system for each batch of water to be purified.

(21) As indicated by reference numeral 138, the recovered draw solution is then conducted back into the chamber 106, where it can be used for purifying new water to be purified and being delivered from the fluid source 120.

(22) Contaminant enriched fluid which has been supplied into the accommodation volume 122 and which has been unable to pass the osmosis membrane 104 is forwarded to a drain 140 such as a wastewater disposal. It is also possible that the contaminant enriched fluid is introduced into a new purification cycle.

(23) Optionally, an energy generation unit 116 may be provided which is configured for generating energy from an osmosis-based rising pressure level in the osmosis unit 102 (more specifically on the draw solution side). Simplified, one might say that a corresponding pressure increase in the draw solution contains energy which can be used. Reference is made to U.S. Pat. No. 3,906,250. As indicated schematically by various arrows in FIG. 1, the energy generation unit 116 is configured for supplying the generated energy for operating one or more of the various components of the apparatus 100.

(24) In the following, the operation of the apparatus 100 be described in further detail:

(25) Purified water is drawn, in the osmosis unit 102, from the contaminated raw water side (left hand side of the osmosis membrane 104 according to FIG. 1) onto the draw solution side (right hand side of the osmosis membrane 104 according to FIG. 1), and thereby dilutes the draw solution in the chamber 106.

(26) Optionally, this procedure can be simultaneously combined with a pressure retarded osmosis in order to produce energy. For example, this energy may be used in order to supply operation energy to one or more components of the apparatus 100.

(27) The diluted draw solution obtained in chamber 106 after the forward osmosis then flows through the ion exchange unit 108. During a corresponding ion exchange procedure, the small and monovalent ions (Na.sup.+, Cl.sup.) of the draw solution are substituted by larger and bivalent (or more generally multivalent, for instance trivalent) ions (Mg.sup.2+, SO.sub.4.sup.2). As a result, the osmotic pressure of the draw solution is significantly reduced. In the example of the ions according to FIG. 1, two Nat ions can bind to one SO.sub.4.sup.2 ion, and two Cl.sup. ions may bind to one Mg.sup.2+ ion, which allows to reduce the number of dissolved particles by a factor of two, which may consequently reduce the osmotic pressure by a factor of two.

(28) In a subsequent membrane-based re-concentration this results in a high reduction of the needed hydraulic pressure and therefore the energy consumption. When implementing other re-concentration units 110 (i.e. other than membrane-based re-concentration units 110), the configuration of the ion exchange unit 108 may be adapted to obtain corresponding advantages which do not relate to a reduction of the osmotic pressure. An example, for thermally-based re-concentration, would be an exchange against ions which can be deposited thermally at a lower temperature than the ions used in the osmosis unit 102. A corresponding example would be CO.sub.2NH.sub.3.

(29) An embodiment of the invention, in which the ion exchange is reversible, is highly preferred although not mandatory.

(30) Next, the ion exchanged solution flows through the pressure exchange unit 118. The pressure exchange unit 118 transfers a (preferably large, for example more than 50%) percentage (for instance 97%) of pressure of a fluidic flow of concentrate (flowing from the re-concentration unit 110 back to the ion exchange unit 108) to the diluted draw solution (flowing from the ion exchange unit 108 towards the re-concentration unit 110).

(31) The forwardly flowing pre-purified water then passes the fluid drive unit 114 which may be a boost pump for providing a desired or required remaining pressure (i.e. a difference between a required pressure and a pressure of the forwardly flowing pre-purified water downstream of the pressure exchange unit 118).

(32) Subsequently, the forwardly flowing pre-purified water flows towards the re-concentration membrane 112. A further chamber 150 in which the re-concentration membrane 112 is located and in which the forwardly flowing pre-purified water flows via a fluid inlet 152, has two fluid outlets 154, 156.

(33) The permeate, i.e. the pure water and therefore the final product, flows towards the destination 130 via outlet 154.

(34) The concentrate (or retentate) however flows back via outlet 156 to the pressure exchange unit 118 and transfers still present pressure with low loss to diluted draw solution flowing from the ion exchange unit 108 to the re-concentration unit 110, in order to pre-load the latter, in terms of pressure, for the re-concentration unit 110. It is also possible that pressure is used for powering a turbine for generating electric current, is supplied to a booster, etc.

(35) After having left the pressure exchange unit 118, the concentrate flows, preferably but not mandatory in counter direction with respect to the flowing direction of the diluted draw solution, a second time through the reversibly operating ion exchange unit 108 and exchanges the divalent ions (Mg.sup.2+, SO.sub.4.sup.2) with the original monovalent ions (Na.sup.+, Cl.sup.). This increases (for instance doubles) the osmotic pressure in the draw solution.

(36) Finally, the concentrate flows back into the osmosis unit 102, and the cycle commences again from the beginning.

(37) Next, the individual procedural steps and components of the apparatus 100 will be described in further detail.

(38) Within the osmosis unit 102, any configuration is possible which is based on the principle of forward osmosis, i.e. in which a higher concentrated solution extracts a solvent such as water from a lower concentrated solution. This can be accomplished by an artificially prepared draw solution (as in FIG. 1), whereas it is however also possible to implement osmotic dilution or osmotic energy recovery or the like. However, osmosis shall be combined with a (preferably reversible) ion exchange and a subsequent re-concentration. It is for instance possible to dilute fluid to be purified (such as the water) with auxiliary fluid (such as sewage or wastewater) having a lower osmotic pressure, so that the purification (for instance desalinisation) of the mixture of the actual fluid to be purified and the auxiliary fluid can then be accomplished with lower concentrated fluid.

(39) What concerns the (preferably reversible) ion exchange as carried out in the ion exchange unit 108, it is advantageous to operate the ion exchange without external or separate regeneration solution by conducting the solution produced in the re-concentration unit 110 (if desired after a concentration procedure) a second time through the ion exchange unit 108 (preferably but not mandatory in opposite direction with regard to the initial flow direction). Advantageously, the apparatus 100 may, in each cycle, supply artificially produced clean diluted draw solution with predefined, reproducible and always identical composition to the ion exchange unit 108.

(40) For regenerating or re-concentrating the draw solution, the re-concentration membrane 112 may be implemented which retains the dissolved ions in the re-concentration unit 110 by filtering, so that the ions are available for a regeneration of the reversible ion exchange procedure. For example, one of the following membrane separation procedures may be carried out in this context: nanofiltration, reverse osmosis (for instance adapted as seawater reverse osmosis, or brackwater reverse osmosis), ultrafiltration, microfiltration, etc.

(41) However, it is also possible to implement, for re-concentration, a thermal process which may involve distillation. For instance, multi stage flash evaporation, multi effect distillation and/or solar distillation may be applied. Further alternatively, the re-concentration may be accomplished by membrane distillation, vapor compression desalination, freezing procedures, electro dialysis, or Ionenkraft methods. For the case of such non-membrane-based re-concentration procedures, the ion exchange in the ion exchange unit 108 can then exchange an osmotic agent against ions which have a positive impact on the subsequent re-concentration. For the example of thermal re-concentration, primary ions may be exchanged by secondary ions which can be deposited at lower temperature.

(42) As mentioned above, an energy recovery procedure can be optionally involved in the process of purifying fluid. Examples for such an energy recovery are an isobaric energy recovery (for instance in the form of the pressure exchange unit 118), a turbine for generating electric current, a turbocharger, a pump (which may preferably substitute the fluid drive unit 114) with integrated energy recovery function (such as a Pearson pump or a Clark pump, as manufactured by Spectra Watermakers).

(43) FIG. 2 illustrates a pressure exchange unit 118 of an apparatus 100 according to an exemplary embodiment of the invention.

(44) The functioning principle of the pressure exchange unit 118 as an example for an energy recovery component according to an exemplary embodiment of the invention is as follows: The flow of concentrate flows through the pressure exchange unit 118 and leaves the latter with a certain amount (for instance approximately 95%) of the required inlet pressure of the re-concentration unit 110 (in the previous cycle, the concentrate has left the re-concentration unit 110 with for instance around 98% of the inlet pressure, and the pressure exchange unit 118 may have a degree of efficiency of 97%).

(45) The pressure exchange unit 118 shown in FIG. 1 and FIG. 2 is an isobaric energy transfer component which transfers hydraulic pressure from one fluid stream onto another fluid stream. Such a procedure can be carried out with a significantly larger degree of efficiency as if a conversion of the type of energy was required.

(46) As can be taken from FIG. 2, a pressure exchange is accomplished between primary fluid flowing from the ion exchange unit 108 towards the re-concentration unit 110 (see reference numerals 128, 132) and secondary fluid flowing from the re-concentration unit 110 towards the ion exchange unit 108 (see reference numerals 134, 136). A rotating piston (not shown) is arranged within a casing 202 (such as a cylinder). Arrows 204 indicate the respective fluid under pressure, whereas arrows 206 indicates the respective fluid in a pressureless or low pressure state.

(47) In an embodiment in which no energy recovery shall be implemented, it is advantageous to implement a back pressure valve (optionally in combination with a pressure relief valve), in particular when a membrane based re-concentration unit 110 shall be implemented in the apparatus 100.

(48) As alternative to the configuration according to FIG. 2, it is for instance possible to implement a turbine, a booster, a Pearson pump and/or a Clark pump (for instance as manufactured by Spectra Watermakers).

(49) It should be noted that the term comprising does not exclude other elements or steps and the a or an does not exclude a plurality. Also elements described in association with different embodiments may be combined.

(50) It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

(51) Implementation of the invention is not limited to the preferred embodiments shown in the figures and described above. Instead, a multiplicity of variants are possible which use the solutions shown and the principle according to the invention even in the case of fundamentally different embodiments.