METHOD FOR WATER PURIFICATION BY DIRECT OSMOSIS AND CRYSTALLISATION OF CLATHRATES HYDRATES

Abstract

A method is disclosed for purifying, by direct osmosis, a first liquid including water and at least one impurity, in which the method comprises the consecutive steps of: contacting the first liquid with a first side of a semi-permeable membrane, a second aqueous liquid containing an osmotic agent being in contact with the second side of the semi-permeable membrane, whereby water is extracted by direct osmosis from the first liquid through the semi-permeable membrane and passes into the second liquid containing the osmotic agent; forming clathrates hydrates of a host molecule in the second liquid containing the osmotic agent into which the water has passed; separating the clathrates hydrates from the second liquid containing the osmotic agent; and dissociating the separated clathrates hydrates to obtain pure water and the host molecule.

Claims

1. A method for purifying a first liquid comprising water and at least one impurity, by direct osmosis, in which the following successive steps are performed: a) contacting the first liquid with a first side of a semi-permeable membrane, a second aqueous liquid containing an osmotic agent being in contact with the second side of the semi-permeable membrane, whereby water is extracted by direct osmosis from the first liquid through the semi-permeable membrane and passes into the second liquid containing the osmotic agent; b) forming clathrates hydrates of a host molecule in the second liquid containing the osmotic agent into which the water has passed; c) separating the clathrates hydrates from the second liquid containing the osmotic agent; and d) dissociating the separated clathrates hydrates to obtain pure water and the host molecule.

2. The method according to claim 1, which is continuously performed.

3. The method according to claim 1, wherein the first liquid and the second liquid are aqueous solutions.

4. The method according to claim 1, wherein the impurity is any element, molecule, ion, or other, different from the elements constituting pure water, that is H.sub.2O, OH.sup.?, and H.sup.+.

5. The method according to claim 1, wherein the impurity is selected from mineral salts selected from the group consisting of NaCl, organic salts, water soluble organic compounds, and mixtures thereof.

6. The method according to claim 1, wherein the first liquid is selected from the group consisting of sea water; brackish waters; landfills leachates; oil production waters; waters from shale gas extraction by the hydraulic fracturing technique; agro-food industry liquids; pharmaceutical industry liquids; chemical industry liquids; mining effluents; metallurgical industry effluents; nuclear industry effluents; reverse osmosis concentrates; scale-forming solutions; paper industry effluents; and saline aquifers.

7. The method according to claim 1, wherein the first liquid has a concentration of impurity(ies) from 1 mg/L to 500 g/L.

8. The method according to claim 1, wherein the first liquid is selected from NaCl aqueous solutions the NaCl concentration of which is higher than 150 g/L.

9. The method according to claim 1, wherein the osmotic agent is chosen by taking as a criterion the osmotic efficiency and optionally the economic profitability of the method.

10. The method according to claim 1, wherein the osmotic agent is chosen from salts, wherein the salts are selected from the group consisting of mineral salts, ionic compounds, proteins and carbon hydrates.

11. The method according to claim 10, wherein the osmotic agent is sodium chloride.

12. The method according to claim 1, wherein the second aqueous liquid is a synthetic aqueous liquid, with a regulated, controlled, composition and concentration.

13. The method according to claim 12, wherein the second aqueous liquid only consists of water and the osmotic agent.

14. The method according to claim 1, wherein the host molecule is water immiscible.

15. The method according to claim 1, wherein the host molecule is chosen from host molecules which enable clathrates hydrates to be crystallised at atmospheric pressure and at temperatures higher than that of ice crystallisation.

16. The method according to claim 1, wherein the host molecule is chosen from molecules which form a clathrate hydrate having a specific gravity lower than the specific gravity of the second liquid, optionally wherein the specific gravity is lower than 1.3.

17. The method according to claim 1, wherein the host molecule is cyclopentane or cyclohexane.

18. The method according to claim 1, wherein the second liquid containing the osmotic agent separated in step c), is sent back to the second side of the semi-permeable membrane.

19. The method according to claim 1, wherein the host molecule obtained in step d) is sent back to step b).

20. A system facility for implementing the method according to claim 1, comprising: an osmotic enclosure separated into a first chamber and a second chamber by a semi-permeable membrane; means for sending a first liquid comprising water and at least one impurity into the first chamber to contact a first side of the semi-permeable membrane; means for sending a second aqueous liquid containing an osmotic agent into the second chamber to contact a second side of the semi-permeable membrane; a reactor for crystallising and growing clathrates hydrates in which clathrates hydrates of a host molecule are formed in the second liquid containing the osmotic agent; means for removing the second liquid from the second chamber of the osmotic enclosure and sending it into the reactor for crystallising and growing the clathrates hydrates; means for separating the clathrates hydrates from the second liquid containing the osmotic agent; and means for dissociating the clathrates hydrates separated in the means for separating the clathrates hydrates from the second liquid, to obtain pure water and the host molecule.

21. The system according to claim 20, further comprising means for sending back the second liquid containing the osmotic agent separated in the means for separating the clathrates hydrates from the second liquid, into the second chamber of the osmotic enclosure.

22. The system according to claim 20, further comprising means for sending back the host molecule obtained in the means for dissociating the clathrates hydrates, into the reactor for crystallising and growing clathrates hydrates.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0112] FIG. 1 is a simplified flow-sheet, which illustrates an embodiment of the method according to the invention.

[0113] FIG. 2 is a scheme which illustrates a facility for carrying out, implementing, the method according to the invention, as well as the implementation of the method according to the invention in this facility.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0114] FIG. 1 is a simplified flow-sheet, which illustrates an embodiment of the method according to the invention to purify an impurity loaded water, such as an impurity loaded aqueous solution, for example a saline aqueous solution mainly containing NaCl and other impurities, such as sea water.

[0115] Obviously, this saline aqueous solution is only given by way of example, and the method illustrated in FIG. 1 can be used to purify any impurity loaded water.

[0116] In this method, water containing impurities 101 forming for example a first solution is provided, and in a first step 102, this water containing impurities is treated by direct osmosis, and the water is extracted from this first solution under the effect of a direct osmosis phenomenon through a semi-permeable membrane up to a second aqueous solution containing an osmotic agent.

[0117] This second aqueous solution is generally called a draining, draw, solution.

[0118] This second aqueous solution is generally a synthetic solution which only consists of water and of the osmotic agent at a determined concentration. Therefore, this solution only contains one single type of compound in solution, unlike the aqueous solution to be purified.

[0119] In the facility represented in FIG. 2, this step 102 is carried out in the reactor 201.

[0120] During a second step of the method 103, clathrates hydrates are formed in the second solution, by introducing host molecules in the second solution.

[0121] In the facility represented in FIG. 2, the clathrates hydrates are formed in the crystallizing and growing reactor 212.

[0122] The clathrates which have been thus formed in the second solution are then separated from this solution during the following step 104.

[0123] In the facility represented in FIG. 2, the separation of the clathrates hydrates takes place in the separator 215.

[0124] The separated clathrates hydrates are then dissociated in a dissociation step 105 to give a mixture of host molecules and purified water.

[0125] The host molecules are separated from the mixture during a step 106, and purified water 107, which can be recycled, is thus obtained.

[0126] In the facility represented in FIG. 2, the dissociation takes place in the reactor for dissociating the clathrates hydrates 218.

[0127] In the method described in FIG. 1, the direct osmosis method 102 extracts water from the first solution up to the second solution, and the formation of clathrates hydrates 103 in the second solution and then the dissociation 105 thereof, associated with the direct osmosis method thus enables from the first solution, at the end of the method, a purified water 107 to be obtained, as a final product which is recovered and which can be used.

[0128] The second solution containing the osmotic agent which is obtained during step 104 after separating the clathrates hydrates from said second solution, is recycled to the direct osmosis step 102 to be used again therein. The recycling of the second solution to the osmosis step 102 is illustrated by the arrow 108 in FIG. 1.

[0129] In the same way, the host molecules which are collected during the separation step 106 may be recycled in the step of forming the clathrates hydrates 103. The recycling of the host molecules to the step of forming the clathrates hydrates is illustrated by the arrow 109 in FIG. 1.

[0130] FIG. 2 is a schematic view which shows an embodiment of a facility for implementing the method according to the invention, continuously implemented, to purify an impurity loaded water, such as an impurity loaded aqueous solution, for example a saline aqueous solution mainly containing NaCl and other impurities.

[0131] Of course, this saline aqueous solution is only given by way of example and the method illustrated in FIG. 2 may be used to purify any impurities loaded water.

[0132] The facility of FIG. 2, which can be called a purification facility 200 first comprises an osmotic reactor or enclosure 201 which is separated by a semi-permeable membrane 202 into a first compartment or chamber 203 and a second compartment or chamber 204.

[0133] The impurity loaded water to be purified forms a first stream which is introduced into the facility by the pipe 205, and then is sent through a pump 206 and a pipe 207 into the first compartment or chamber 203 of the osmotic reactor 201.

[0134] A so-called osmotic agent solution, comprising water and an osmotic agent, such as NaCl, forms a second stream, which is conveyed through a pipe 208 in the second compartment 204 of the osmotic reactor 201.

[0135] The osmotic agent solution is generally a solution that can be referred to as a controlled, regulated synthetic solution. This means that the composition and concentration of this solution are perfectly controlled and that it does contain little or no, besides the osmotic agent, impurities and undesirable substances. In particular, the choice of the osmotic agent as well as its concentration are perfectly controlled. The osmotic power, potential of the osmotic agent solution can thus be in particular accurately controlled, set, adjusted.

[0136] According to the invention and this is one of the advantages of the method according to the invention in comparison with reverse osmosis methods of prior art which are not associated with a clathrates hydrates crystallisation technique, there is no limit as regards the choice of the osmotic agent.

[0137] Indeed, this choice is not restricted by problems related to recovering, regeneration, and recycling of some osmotic agents, since water is purified by the clathrates hydrates crystallisation technique and not by separating the osmotic agent from the solution containing it.

[0138] The osmotic agent may be chosen for example from salts, ionic compounds and proteins.

[0139] In a preferred embodiment, the osmotic agent is sodium chloride NaCl.

[0140] Other osmotic agents which may be used are the salts mentioned above.

[0141] Since the choice of the osmotic agent according to the invention is in no way limited, this choice may be made in order to fulfil one or more criteria which can be mostly freely chosen.

[0142] All the criteria set out hereinafter could be actually gathered under one and a single overall criterion which is the osmotic efficiency criterion.

[0143] A first mandatory criterion which governs the choice of the osmotic agent is that the osmotic agent has to be capable of creating a water flow through the semi-permeable membrane from the first compartment to the second compartment of the osmotic reactor. This criterion is imperative, because otherwise there is simply no osmosis.

[0144] Another criterion, being optional and preferable, is that the reverse diffusion of the osmotic agent from the second compartment to the first compartment has to be limited. Indeed, otherwise the efficiency loss is significant.

[0145] According to yet another criterion, the osmotic agent may be chosen in order to obtain a water flow by unit area of the semi-permeable membrane which is stable and as high as possible, in other words, in order to obtain a high yield of the membrane. An osmotic agent which fulfils these criteria is NaCl.

[0146] Once again, to fulfil these criteria, the choice of the osmotic agent is not limited by recovering or regeneration of the osmotic agent in a subsequent phase.

[0147] Another criterion or a further criterion which could possibly govern the choice of the osmotic agent is the semi-permeable membrane used, but this criterion is not crucial. One of the advantages to choose the osmotic agent as a function of the type of membrane used is that the osmotic method can be optimised.

[0148] To all the mandatory or optional criteria listed above which all fall within the overall scope of the osmotic efficiency criterion, the economic criterion, that is the cost of the osmotic agent can be added, even if, generally, said osmotic agent remains permanently in the second solution or draining, draw, solution and thus has not to be constantly replenished.

[0149] The osmotic agent solution forming the second stream, which is conveyed through the pipe 208 into the second compartment or chamber 204 of the osmotic reactor or enclosure 201 is a solution which can be referred to as a draining, draw, solution.

[0150] It is to be noted that the concentration of osmotic agent of this draining, draw, solution may be, but is not necessarily, higher than the concentration of impurities of the solution to be treated.

[0151] Indeed, even if a higher concentration of osmotic agent increases the osmotic pressure, however it is the nature of the osmotic agent that is prevalent. Some osmotic agents can thus generate higher osmotic pressures while having concentrations lower than the content of impurity(ies) of the solution to be purified.

[0152] Pure water, from the impurities loaded water to be purified, passes under the effect of the reverse osmosis through the semi-permeable membrane 202 from the compartment 203 up to the compartment 204 of the osmotic reactor 201.

[0153] Thus, in the compartment 204, an osmotic agent solution is formed, which is diluted with respect to the osmotic agent solution, relatively more concentrated, initially present in the compartment 204 and which had been sent in the same through the pipe 208.

[0154] The diluted osmotic agent solution is discharged from the second compartment 204 through a piping 209, which conveys this solution into a cooling device 210. This solution is then sent through a pipe 211, in a reactor for crystallising and growing the clathrates hydrates 212 in which there are host molecules.

[0155] The refrigerating device 210 comprises a heat exchanger 213 in which a heat exchange takes place between the diluted osmotic agent solution and the osmotic agent solution (see below) which flows in a pipe 214 and then in the pipe 208.

[0156] Suitable heat exchangers are known to the man skilled in the art and thus will not be described in more detail.

[0157] Water contained in the cooled diluted osmotic agent solution conveyed by the piping 211, and the host molecules which are in the reactor 212 form clathrates hydrates which grow in this reactor for crystallising and growing the clathrates hydrates 212.

[0158] As a host molecule, cyclopentane may preferably be used, but also other host molecules may be used, for example gaseous molecules such as methane, ethane, butane, propane, hydrogen sulphide, carbon dioxide, or mixtures thereof.

[0159] Generally, the host molecule is chosen so as to have no influence on the rest of the method. Consequently, the host molecule may be chosen according to economic criteria or other criteria.

[0160] It is particularly advantageous to use cyclopentane as a host molecule, because it enables clathrates hydrates to be formed in the reactor for crystallising the clathrates 212 at atmospheric pressure, that is under a pressure of 1 bar and at a temperature generally between ?20? C. and 6? C.

[0161] It is to be noted that the temperature at which the clathrates hydrates are formed in the reactor 212 depends on the concentration of the osmotic agent in the reactor 212. Indeed, generally, the concentration of osmotic agent lowers the crystallisation temperature of clathrates.

[0162] As water is used to form the clathrates hydrates in the reactor 212, a suspension of clathrates hydrates in a concentrated solution of osmotic agent is thus obtained in the reactor 212.

[0163] The suspension of clathrates hydrates in a concentrated solution of osmotic agent formed in the reactor 212, is removed from the reactor 212, and is sent into a separator 215 through a pipe 216.

[0164] In the separator, the clathrates hydrates are separated from the concentrated solution of osmotic agent by implementing a liquid/solid separation technique.

[0165] The liquid/solid separation technique used to separate the clathrates hydrates from the concentrated solution will depend on the type of clathrates hydrates but also on other factors. The man skilled in the art will easily choose the suitable technique as a function of the properties of the clathrates hydrates as for example their particle size.

[0166] This liquid/solid separation technique may be chosen for example from conventional liquid/solid separation techniques, such as filtration for example with a filter press, centrifugation.

[0167] The concentrated solution of osmotic agent separated in the separator 215 and which thus does not contain clathrates hydrates any longer is removed, from the separator 215, through the pipe 214 which feeds the heat exchanger 213 in order to cool the diluted solution of osmotic agent 209 in the cooling device 210.

[0168] The concentrated solution of osmotic agent is then conveyed through the pipe 208 into the second compartment 204 of the osmotic reactor 201.

[0169] It is thus noticed that in the facility of FIG. 2, the flow of solution containing the osmotic agent forms a closed loop, and the osmotic agent is recycled into the second compartment 204 of the osmotic reactor 201.

[0170] The consumption of osmotic agent is thus limited to the amount initially necessary to form the concentrated solution of osmotic agent and possibly to limited supplies when the facility 200 is in use.

[0171] As in the facility of FIG. 2, the solution of osmotic agent is recycled, it is not necessary to recover the osmotic agent, and the osmotic agent can thus be chosen independently of its ability to be recovered, regenerated.

[0172] The clathrates hydrates separated in the separator 215 are withdrawn in the separator 215 through a pipe 217 and conveyed in a reactor for dissociating the clathrates hydrates 218, where the clathrates hydrates are separated into host molecules and purified water.

[0173] Any known dissociation technique may be used. Generally, the dissociation of crystallised hydrate crystals into water and into host molecules is made by a rise in temperature resulting in melting them. In other words, the crystallised hydrate crystals are molten.

[0174] Then, the purified water and the host molecules are separated by settling or outgassing.

[0175] The choice of one or the other of the settling and degassing techniques is made depending on the more or less volatile nature of the host molecule.

[0176] Thus, in the case where the host molecule is a liquid non miscible with water and which has a specific gravity lower than that of water such as cyclopentane, the dissociation by melting the clathrates hydrates in the reactor 218 forms an emulsion which is introduced into a settler (not represented) to separate purified water from the host molecule. The host molecule is recycled in the reactor 212 by means of a pump 219 and of pipes 220 and 221.

[0177] In the case where the host molecule is a gas, and as it is particularly represented in FIG. 2, the gaseous host molecules are directly recovered in the dissociation reactor and recycled in the reactor 212 by means of a pump 219 and of pipes 220 and 221.

[0178] In the facility according to the invention, the host molecules are thus recycled and used again which increases the facility economic profitability.

[0179] The facility 200 represented in FIG. 2 comprises a second heat exchanger 222 which operates between the reactor for crystallizing and growing the clathrates hydrates 212 and the reactor for dissociating the clathrates hydrates 218 to recover heat energy released by the formation of clathrates hydrates in the reactor for crystallizing and growing the clathrates hydrates 212.

[0180] This second exchanger is generally a conventional heat pump.

[0181] The heat cycle implemented in the heat exchanger 222 uses very little energy, because the exothermic nature of the crystallisation and the endothermic nature of the dissociation enable a heat loop to be formed.

[0182] The purified water is discharged from the reactor 218 or from the settler through a pipe 223 and can be used.

[0183] The part of the facility 200 delimited by a dotted line in FIG. 2 corresponds to the part of the facility and of the method which involves the clathrates hydrates, whereas the part which is not delimited by a dotted line corresponds to the part of the facility and of the method which involves reverse osmosis.

[0184] Of course, insofar as, in accordance with the basic principle of the method according to the invention, both these parts are combined together, they could be arranged in a different way than that shown in FIG. 2.