METHOD FOR TREATING AN AQUEOUS SOLUTION CONTAINING DISSOLVED MATERIALS BY CRYSTALLIZATION OF CLATHRATES HYDRATES

Abstract

A method is disclosed for treating an aqueous solution containing dissolved materials that are crystallisable by crystallization of clathrates hydrates of a host molecule which crystallize at atmospheric pressure at temperatures higher than the temperature of ice crystallization. This method allows purified water and solid materials or solutions which are highly concentrated in dissolved materials to be produced simultaneously. The disclosure also relates to the implementation of this method.

Claims

1. A method for treating an aqueous solution containing dissolved materials that are able to crystallize or precipitate, by crystallization of clathrates hydrates of a host molecule which crystallize at atmospheric pressure at temperatures higher than the temperature of ice crystallization, said clathrates hydrates being less dense than said aqueous solution containing dissolved materials, wherein the following steps are carried out: a) the aqueous solution is cooled, in first cooling means, to a temperature T1 that is higher than the temperature of ice crystallization and lower than the crystallization temperature Teq of the clathrates hydrates, and this cooled aqueous solution is introduced into a thermally insulated reactor; b) a quantity of the host molecule is added into the reactor containing the cooled aqueous solution, such that the temperature T2 of the aqueous solution remains, following this addition and following the exothermic release due to the crystallization of the clathrates hydrates, lower than the temperature Teq, whereby the clathrates hydrates of the host molecule crystallize homogeneously in all of the volume of the aqueous solution by forming a suspension of clathrates hydrates crystallized in an aqueous solution concentrated in dissolved materials that may further contain crystallized or precipitated dissolved materials; c) the suspension formed during step b) is sampled in the reactor, and said suspension is sent to a decanter wherein it is separated into an aqueous solution concentrated in dissolved materials and free of clathrates hydrates, into a suspension of clathrates hydrates crystallized in the form of a sorbet in an aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials, and optionally into a supersaturated suspension containing crystallized or precipitated dissolved materials and free of clathrates hydrates in an aqueous solution concentrated in dissolved materials; d) from the decanter is withdrawn on the one hand the aqueous solution concentrated in dissolved materials and free of clathrates hydrates, and optionally on the other hand, the supersaturated suspension containing crystallized or precipitated dissolved materials and free of clathrates hydrates; e) the aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in step d), is sent back in whole into the reactor after having been cooled in second cooling means that are different from the first cooling means; or else, if in the decanter there is no supersaturated suspension then a portion of the aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in step d), is sent back into the reactor after having been cooled in second cooling means, and another portion of an aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in step d), is recovered and/or discharged through a purge; f) the suspension of clathrates hydrates crystallized in the form of a sorbet in an aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials obtained in step c), is sent into a liquid/solid separation apparatus wherein solid clathrates hydrates are separated from the aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials which is recycled in order to form at least a portion of the aqueous solution cooled in step a); g) the solid clathrates hydrates separated in step f) are sent into a reactor for dissociating clathrates hydrates in which the clathrates hydrates are dissociated into purified water and into host molecules; h) the purified water and the host molecules obtained in step g) are separated, the purified water is recovered, and the host molecules are recycled in step b).

2. A method according to claim 1, which is carried out continuously.

3. A method according to claim 1, wherein the dissolved materials that are able to crystallize or precipitate are chosen from mineral salts selected from the group consisting of NaCl, organic salts, and water soluble compounds that have a crystallization eutectic in aqueous solution, and optionally having a density that is greater than that of the treated aqueous solution containing dissolved materials.

4. A method according to claim 1, wherein the aqueous solution containing dissolved materials is chosen from seawater; brackish waters; public landfills leachates; waters from oil production; waters for extracting shale gas by the hydraulic fracturing technique; liquids from the agri-food industry; liquids from the pharmaceutical industry; liquids from the chemical industry; mining effluents; effluents from the metallurgy industry; effluents from the nuclear industry; reverse osmosis concentrates; scaling solutions; effluents from the paper industry; and saline aquifers.

5. A method according to claim 1, wherein the treated aqueous solution has a concentration in dissolved materials from 5 g/L to the limit of saturation of these dissolved materials in water.

6. A method according to claim 1, wherein the host molecule is non-miscible in water.

7. A method according to claim 1, wherein the host molecule is chosen from the molecules that form a clathrate hydrate that has a density lower than 1.3, preferably lower than 1.2.

8. A method according to claim 1, wherein the host molecule is cyclopentane or cyclohexane.

9. A method according to claim 1, wherein during step b), the host molecule is introduced into the reactor in the form of an emulsion or solution prepared by mixing the aqueous solution cooled to the temperature T1 and the host molecules separated in step h).

10. A method according to claim 1, wherein, during step c), the suspension of clathrates hydrates crystallized is separated in the form of a sorbet in an aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials is separated, optionally continuously, on the surface of the concentrated aqueous solution.

11. A method according to claim 1, wherein, during step h) the purified water and the host molecules are separated by decantation or degassing.

12. A method according to claim 1, wherein the solid clathrates hydrates separated in step f) are washed prior to step g).

13. A method according to claim 1, wherein the temperature of the aqueous solution in the reactor and/or the concentration in host molecules in the reactor and/or the decanter and/or the quantity of aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in step d), and sent back into the reactor during step e) are adjusted in order to obtain in the decanter during step c), in addition to a suspension of clathrates hydrates crystallized in the form of a sorbet, either only an aqueous solution concentrated in dissolved materials having a determined concentration, or an aqueous solution concentrated in dissolved materials having a determined concentration and a supersaturated suspension containing crystallized or precipitated dissolved materials in an aqueous solution concentrated in dissolved materials.

14. A method according to claim 13, wherein the temperature of the aqueous solution in the reactor, and/or the concentration in host molecules in the reactor and/or the decanter, and/or the quantity of aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in step d), and sent back into the reactor during step e) are adjusted in such a way that eutectic conditions are present in the reactor and/or in the decanter, and that is then obtained in the decanter during step c), in addition to a suspension of clathrates hydrates crystallized in the form of a sorbet, an aqueous solution concentrated in dissolved materials at the eutectic concentration and a supersaturated suspension containing crystallized or precipitated dissolved materials in an aqueous solution concentrated in dissolved materials at the eutectic concentration.

15. A method according to claim 1, wherein the supersaturated suspension containing crystallized or precipitated dissolved materials in an aqueous solution is withdrawn in step d), and is treated in a step of solid-liquid separation whereby on the one hand crystallized or precipitated dissolved materials are obtained, which are recovered, and on the other hand an aqueous solution concentrated in dissolved materials is obtained, which is recycled in the reactor after having been cooled in the second cooling means, and/or is recovered, and/or is fully or partially discharged.

16. A method according to claim 1, wherein seeds of solid dissolved materials are further introduced into the decanter.

17. An installation for the implementation of the method according to claim 1, in order to treat an aqueous solution containing dissolved materials that are able to crystallize or precipitate by crystallization of clathrates hydrates of a host molecule which crystallize at atmospheric pressure at temperatures higher than the temperature of ice crystallization, said clathrates hydrates being less dense than said aqueous solution containing dissolved materials, said installation comprising: a) first cooling means in order to cool the aqueous solution to a temperature T1 higher than the temperature of ice crystallization and lower than the crystallization temperature Teq of the clathrates hydrates; b) a thermally insulated reactor; c) means for conveying the aqueous solution cooled to the temperature T1 from the first cooling means to the thermally insulated reactor; d) means for adding a host molecule in the reactor, whereby the clathrates hydrates of the host molecule crystallize homogeneously in all of the volume of the aqueous solution by forming a suspension of clathrates hydrates crystallized in an aqueous solution concentrated in dissolved materials that may further optionally contain crystallized or precipitated dissolved materials; e) a decanter; f) means for sampling in the reactor a suspension of crystallized clathrates hydrates in an aqueous solution concentrated in dissolved materials that may further contain crystallized or precipitated dissolved materials and means to send said suspension in the decanter wherein said suspension is separated into an aqueous solution concentrated in dissolved materials and free of clathrates hydrates, into a suspension of clathrates hydrates crystallized in the form of a sorbet in an aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials, and optionally into a supersaturated suspension containing crystallized or precipitated dissolved materials and free of clathrates hydrates in an aqueous solution concentrated in dissolved materials; g) means for withdrawing from the decanter the aqueous solution concentrated in dissolved materials and free of clathrates hydrates, and optionally the supersaturated suspension containing crystallized or precipitated dissolved materials and free of clathrates hydrates; h) means for sending back in whole or in part into the reactor the aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in the decanter; i) second cooling means different from the first cooling means for cooling before the reactor the aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in the decanter provided in the means for sending back in whole or in part into the reactor the aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in the decanter; j) optionally, purging means provided in the decanter for recovering and/or discharging another portion of the an aqueous solution concentrated in dissolved materials and free of clathrates hydrates withdrawn in the decanter; k) first liquid/solid separation means; l) means for sending the suspension of clathrates hydrates crystallized in the form of a sorbet in an aqueous solution concentrated in dissolved materials and free of crystallized or precipitated dissolved materials separated in the decanter in the first liquid/solid separation means where solid clathrates hydrates are separated from the concentrated aqueous solution of dissolved materials and free of crystallized or precipitated dissolved materials forming a filtrate; m) means for recycling said filtrate in said first cooling means; n) a reactor for dissociating solid clathrates hydrates separated in said first liquid/solid separation means, in which the solid clathrates hydrates are dissociated into purified water and into host molecules; o) means for separating the purified water and the host molecules obtained in the dissociation reactor, means for recovering the purified water, and means for sending the host molecules back into the reactor.

18. The installation according to claim 17, which comprises means to prepare an emulsion or solution by mixing the aqueous solution cooled to the temperature T1 in the first cooling means and the host molecules, and means for introducing said emulsion or solution into the reactor.

19. The installation according to claim 17, which further comprises means, for washing the solid clathrates hydrates separated in the first liquid/solid separation means.

20. The installation according to claim 17, which further comprises second solid/liquid separation means for treating the supersaturated suspension containing crystallized or precipitated dissolved materials in an aqueous solution withdrawn in the decanter and obtaining on the one hand crystallized or precipitated dissolved materials, and on the other hand an aqueous solution concentrated in dissolved materials.

21. The installation according to claim 20, which further comprises means for recycling in the reactor the aqueous solution concentrated in dissolved materials obtained in the second solid/liquid separation means.

22. The installation according to claim 21, which further comprises third cooling means, optionally confounded with the second cooling means for cooling before the reactor the aqueous solution concentrated in dissolved materials.

23. The installation according to claim 17, which further comprises means for introducing seeds of solid dissolved materials into the decanter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0165] FIG. 1 is a phase diagram for an aqueous solution of a salt such as NaCl.

[0166] This diagram shows that the formation temperature of pure ice is 0 C. and that the eutectic point for a solution of NaCl is located at a temperature of 21 C. and at a concentration in NaCl of 23% by weight.

[0167] On the abscissa is plotted the concentration in salt (as a % by weight) and on the ordinates is plotted the temperature of the solution (in C.).

[0168] FIG. 2 is a phase diagram for an aqueous solution of a salt such as NaCl containing host molecules, in particular gaseous host molecules that form clathrates hydrates.

[0169] This diagram shows that the freezing temperature of the clathrates hydrates is 7 C., and that the eutectic point for a solution of NaCl is located at a temperature higher than 21 C., namely 13 C.

[0170] On the abscissa is plotted the concentration in salt (as a % by weight) and on the ordinates is plotted the temperature of the solution (in C.).

[0171] FIG. 3 is a flow-sheet of a preferred embodiment of the method according to the invention, implemented continuously.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0172] FIG. 3 shows a preferred embodiment of the method according to the invention, implemented continuously, in order to purify for example a saline solution of NaCl, for example at 35 g/l, which is a content similar to that of seawater.

[0173] It is obvious that this saline solution is provided solely as an example and that the method shown in FIG. 3 can be used to purify any aqueous solution containing dissolved materials that are able to crystallize (crystallisable).

[0174] The saline solution to be purified (1) forms a flow (2) that is joined by a flow of a concentrated salt solution (flow 23) in order to form thus a flow (3) which is sent into a heat exchanger (4) where it is cooled to a temperature of about 15 C. without generating ice, due to its high salt content.

[0175] The cooled flow (5), passes through an emulsifier (6) where it is finely mixed with a host molecule, for example 4% cyclopentane in liquid form at 0 C. (flow 7).

[0176] The emulsion coming from the emulsifier (flow 8) joins a reactor (9) in which the cyclopentane hydrate crystallizes causing a supersaturation in salt of the solution.

[0177] It is obvious that cyclopentane is given solely as an example and that the method shown in FIG. 3 can be implemented with any host molecule.

[0178] A flow (10) consisting of the cyclopentane hydrate and of the solution supersaturated in salt is sampled in the reactor (9) and is sent into a decanter (11).

[0179] The flow (10) is stabilised in the decanter (11), wherein the cyclopentane hydrate will float on the surface because its density is lower than the density of the solution, and wherein the saline solution will reach its eutectic equilibrium concentration by crystallization of the salt. The salt crystals with a density greater than that of the solution will settle at the bottom of the decanter.

[0180] Three flows are then withdrawn from the decanter (11), namely: [0181] a flow (12) withdrawn in the upper portion of the decanter (11). This flow (12) is a cyclopentane hydrate sorbet composed of cyclopentane hydrate and of a saline solution at eutectic concentration but free of any salt crystal or precipitated salt, [0182] a flow (13), withdrawn in the middle portion of the decanter. This flow (13) is constituted of saline solution at eutectic concentration, [0183] a flow (14), sampled at the bottom of the decanter (11), is constituted of saline solution at eutectic concentration and of salt crystals or of settled precipitated salts but free of any cyclopentane hydrate crystal.

[0184] The flow (13) then joins a second heat exchanger (15) in order to be cooled to about 19 C. before being reintroduced upstream into the reactor (9) in the form of a flow (16).

[0185] The flow (14) is filtered in a filter (17). This filtration operation makes it possible on the one hand to obtain a flow (18) of crystallized or precipitated salts that are recovered (19), while the filtration juice or filtrate at eutectic concentration forms a flow (20) that is reinjected into the same heat exchanger (15) as the flow (13).

[0186] The flow (12) is also filtered in a filter (21) in order to recover the cyclopentane hydrate crystals of the sorbet in the form of a flow (22). The filtration juice (23), also at eutectic concentration, coming from this filtration operation, is reintroduced upstream of the method in order to be mixed with the flow (2) of solution to be purified before cooling in the exchanger (4).

[0187] The cyclopentane hydrate constituting the flow (22) is introduced into a melter (24) where is will be dissociated in order to give an emulsion of cyclopentane and of purified water (flow 25).

[0188] This emulsion (flow 25) is then introduced into a decanter (26) in order to separate the purified water from the cyclopentane.

[0189] In light of the density of 0.75 of the cyclopentane, in the bottom portion of the decanter (26) a flow (27) of purified water is withdrawn which is recovered (28) while in the upper portion of the decanter (26), a flow of cyclopentane (29) is withdrawn.

[0190] This flow of cyclopentane (29) is recycled upstream of the method in order to form with a possible supply of fresh cyclopentane (30), the flow (7), which feeds the emulsifier (6).

[0191] The invention will now be described in relation with the following example, given for illustrative and non-limiting purposes.

Example

[0192] In this example, the treatment is carried out by the method according to the invention of a solution of NaCl at 30 g/L at a flow rate of 10 m.sup.3/day.

[0193] The method is such as described in FIG. 3, it is implemented in an installation that makes it possible to treat 10 m.sup.3 of solution/day and with cyclopentane as the host molecule.

[0194] Table I hereinbelow gives for the flows identified by numbers corresponding to the reference signs in FIG. 3, the flow rate in 10 Kg/h, the salt content in % by weight, the water content in % by weight, the cyclopentane content in % by weight, and the hydrate content in % by weight.

TABLE-US-00001 TABLE I Flow Salt Water Cyclo- Hydrate Flow rate in content content pentane content Number 10 kg/h % % % % 2 41.67 3.1 96.9 0 0 3 210 21.4 78.6 0 0 5 210 21.4 78.6 0 0 7 8 0 0 100 0 8 218 20 76 4 0 10 753 23.7 70.5 0 5.8 12 216 20 59.9 0 20 13 537 23.5 76.5 0 0 14 2.2 50.7 49.3 0 0 16 538 23.5 76.5 0 0 18 0.8 100 0 0 0 20 1.4 23.5 76.5 0 0 22 47.4 0.1 9 0 90.9 25 47.7 0.1 83 16.8 0 27 39.4 0.1 99.9 0 0 29 8 0 0 100 0

[0195] Recall that: [0196] The flow (2) corresponds to the solution to be treated. [0197] The flow (18) corresponds to the crystallized salts. [0198] The flow (27) corresponds to the purified water.