Method for crystallising clathrates hydrates, and method for purifying an aqueous liquid using the clathrates hydrates thus crystallised

Abstract

A method for forming, or crystallising, clathrates hydrates of a host molecule in a liquid including water includes the following consecutive steps: cooling the liquid to a temperature no higher than the crystallisation temperature of the clathrates hydrates; and placing the cooled liquid in contact with host molecules that are capable of forming clathrates hydrates and are adsorbed on a solid support that has a large specific surface area and is made of a hydrophobic and apolar material, whereby the host molecules are desorbed from the solid support that has a large specific surface area and is made of a hydrophobic and apolar material, and react with the water of the liquid in order to provide a liquid containing clathrates hydrates and the solid support.

Claims

1. A method for forming or crystallizing clathrates hydrates of a host molecule in a liquid comprising water, the method comprising the following successive steps: cooling the liquid to a temperature less than or equal to the crystallization temperature of the clathrates hydrates; contacting the cooled liquid with host molecules capable of forming clathrates hydrates, wherein the host molecules capable of forming clathrates hydrates are adsorbed onto a solid support having a high specific surface area and made of an apolar and hydrophobic material; whereby the host molecules are desorbed from the solid support having a high specific surface area and made of an apolar and hydrophobic material and react with the water of the liquid to give a liquid containing clathrates hydrates and the solid support.

2. The method according to claim 1, wherein the solid support has a specific surface area from 100 to 2000 m.sup.2/g.

3. The method according to claim 1 that is carried out continuously.

4. The method according to claim 1, wherein the solid support is in the form of discrete particles.

5. The method according to claim 1, wherein the solid support is in the form of a woven fabric of fibres or a nonwoven fabric of fibres.

6. The method according to claim 1, wherein the solid support is made of a material chosen from materials capable of adsorbing the host molecule with an adsorption energy lower than the energy of crystallisation of the clathrates hydrates.

7. The method according to claim 1, wherein the host molecule is chosen from linear or branched hydrocarbons comprising from 1 to 6 carbon atoms, halogenated hydrocarbons, cyclic hydrocarbons comprising 3 to 10 carbon atoms, CO.sub.2, oxygen, nitrogen, H.sub.2S, noble gases, tetrahydrofuran, ethylene oxide, sulphur hexafluoride, tetra-n-butylammonium bromide (TBAB), and N.sub.2O.

8. The method according to claim 1, wherein the host molecule is immiscible with water, or insoluble in water.

9. The method according to claim 1, wherein the host molecule is chosen from host molecules that allow crystallising of clathrates hydrates at atmospheric pressure and at temperatures greater than that of the crystallisation of ice.

10. The method according to claim 1, wherein the host molecule is chosen from the molecules that form a clathrate hydrate having a specific gravity lower than the specific gravity of the aqueous solution.

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

12. The method according to claim 1, wherein the liquid comprising water comprises water and at least one impurity.

13. The method according to claim 12, wherein the liquid comprising water is a solution.

14. The method according to claim 12, wherein the impurity is any element, molecule, or ion different than the chemical species constituting pure water, namely H.sub.2O, OH.sup., and H.sup.+.

15. The method according to claim 14, wherein the impurity is chosen from mineral salts, organic salts, organic compounds soluble in water, and their mixtures.

16. The method according to claim 12, wherein the liquid comprising water is chosen from seawater, brackish waters, public landfill leachates, waters from oil production, waters from extraction of 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 metallurgical industry, effluents from the nuclear industry, reverse-osmosis concentrates, scaling solutions, effluents from the paper industry, and saline aquifers.

17. The method according to claim 12, wherein the liquid comprising water has a concentration of impurity or impurities from 1 mg/L to 500 g/L.

18. The method according to claim 12, wherein the liquid is chosen from solutions of NaCl having a concentration of NaCl greater than 150 g/L.

19. A method for purifying a liquid comprising water and at least one impurity, the method comprising the following successive steps: a1) clathrates hydrates of the host molecule are formed in the liquid by the method according to claim 12; b1) the clathrates hydrates and the solid support are separated from the liquid; c1) the separated clathrates hydrates are dissociated in order to obtain, on the one hand, pure water and, on the other hand, the host molecule once again adsorbed onto the solid support.

20. The method according to claim 19, wherein during step c1), the clathrates hydrates are dissociated by melting them.

21. The method according to claim 20, wherein the host molecule adsorbed onto the solid support and obtained after step c1) is sent back to step a1).

22. The method according to claim 1, wherein the solid support comprises beads made of activated carbon.

23. The method according to claim 22, wherein the beads have a diameter from 0.2 to 15 mm.

24. The method according to claim 4, wherein the discrete particles are beads, fibres, or tubes.

25. The method according to claim 24, wherein the beads have a diameter from 0.2 to 15 mm.

26. The method according to claim 6, wherein the solid support is made of a material chosen from activated carbon, oxides of metals, oxides of metalloids, zeolite, and organic polymers.

27. The method according to claim 26, wherein the oxides of metalloid is silica.

28. The method according to claim 27, wherein the silica is mesoporous silica.

29. The method according to claim 10, wherein the specific gravity of the aqueous solution is lower than 1.3.

30. The method according to claim 29, wherein the specific gravity of the aqueous solution is lower than 1.0.

31. The method according to claim 17, wherein the concentration of impurity or impurities is from 100 mg/L to 250 g/L.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph that gives the Temperature of the liquid (T in C.) as a function of time (t in minutes) during the cooling of the liquid and the crystallisation of the clathrates hydrates in a method for crystallising hydrates of the prior art that does not implement a solid support, for example made of activated carbon.

(2) FIG. 2 is a graph that gives the Temperature of the liquid (T in C.) as a function of time (t in minutes) during the cooling of the liquid and the crystallisation of the clathrates hydrates in a method for crystallising hydrates according to the invention that implements a solid support, for example made of activated carbon.

(3) FIG. 3 is a graph that gives the Temperature of the liquid (T in C.) as a function of time (t in minutes) during the step of melting the clathrates hydrates, in a purification or transportation method of the prior art that does not implement a solid support for example made of activated carbon (curve drawn with a solid line), or in a purification method according to the invention that implements a solid support for example made of activated carbon (curve drawn with a dotted line).

(4) The origin of the times is at t.sub.2.

(5) FIG. 4 is a schematic view that illustrates a facility for implementing the purification method according to the invention that uses clathrates hydrates crystallised by the method for crystallising clathrates according to the invention, as well as the implementation of the purification method according to the invention in this facility.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(6) FIG. 4 is a schematic view that illustrates an embodiment of a facility for implementing the purification method according to the invention, carried out continuously, in order to purify a water rich in impurities, such as an aqueous solution rich in impurities, for example a saline aqueous solution predominantly containing NaCl and other impurities.

(7) It is obvious that this saline aqueous solution is only given as an example, and that the method illustrated in FIG. 4 can be used to purify any water rich in impurities.

(8) The solution to be treated is conveyed via a pipe 209 into a cooling device 210. This solution to be treated, purified, is then sent via a pipe 211, into a reactor for crystallising and growing the clathrates hydrates 212 into which host molecules, for example cyclopentane, adsorbed onto a solid support, for example made of activated carbon, have been introduced.

(9) The cooling device 210 comprises a heat exchanger 213 in which an heat exchange takes place between the solution to be treated and the solution concentrated in impurities that circulates in a pipe 214 and then in a pipe 208.

(10) The suitable heat exchangers are known to the man skilled in the art and will not therefore be described in more detail.

(11) The water contained in the cooled solution to be purified conveyed by pipe 211, and the host molecules adsorbed onto a solid support which can be found in the reactor 212 form clathrates hydrates that grow in this reactor 212 for crystallising and growing the clathrates hydrates.

(12) Cyclopentane can preferably be used as the host molecule, but other host molecules may also be used, for example gaseous molecules such as methane, ethane, butane, propane, hydrogen sulphide, carbon dioxide, or their mixtures.

(13) In general, the host molecule is chosen in such a way as to not affect the rest of the method. Consequently, the host molecule may be chosen according to economic criteria or others.

(14) It is particularly advantageous to use cyclopentane as the host molecule, since it allows the clathrates hydrates to be formed in the reactor 212 for crystallising the clathrates at atmospheric pressure, namely at a pressure of 1 bar and at a temperature generally between 20 C. and 6 C.

(15) The solid support is preferably made of activated carbon.

(16) It should be noted that the temperature at which the clathrates hydrates are formed in the reactor 212 is dependent on the concentration of impurities in the reactor 212. Indeed, in general, the concentration of impurities lowers the temperature of crystallisation of the clathrates.

(17) Since water is used to form the clathrates hydrates in the reactor 212, a suspension of clathrates hydrates and of the solid support in a solution concentrated in impurities is therefore obtained in the reactor 212.

(18) The suspension of clathrates hydrates and of solid support in a solution concentrated in impurities formed in the reactor 212, is withdrawn in the reactor 212, and is sent to a separator 215 via a pipe 216.

(19) In the separator, the clathrates hydrates and the solid support are separated from the solution concentrated in impurities by using a liquid/solid separation technique.

(20) The liquid/solid separation technique used to separate the clathrates hydrates and the solid support from the solution concentrated in impurities is dependent on the type of clathrates hydrates but also on other factors. The man skilled in the art can easily choose the suitable technique as a function of the properties of the clathrates hydrates such as for example their particle size distribution.

(21) This liquid/solid separation technique may be chosen for example from all the conventional techniques for liquid/solid separation, such as filtration for example with a filter press, centrifugation.

(22) The solution concentrated in impurities separated in the separator 215 and which thus no longer contains clathrates hydrates or solid support is withdrawn, from the separator 215, via the pipe 214 that feeds the heat exchanger 213 in order to cool the initial solution to be purified 209 in the cooling device 210.

(23) The solution concentrated in impurities, after having fed the heat exchanger 213 in order to cool the initial solution to be purified 209 in the cooling device 210, exits this cooling device 210 via the pipe 208.

(24) Two alternatives are possible for the treatment of this solution concentrated in impurities that circulates in the pipe 208: According to a first alternative, the concentrated solution is introduced in its entirety into the stream of solution to be treated that circulates in the pipe 209. In this case, the impurities will be concentrated up to the eutectic concentration. In order to make the hydrates crystallise in the reactor 212, a temperature T lower than the temperature of the eutectic needs to be set. The impurities, in particular the salts, will thus be withdrawn in solid form via a drain (not shown) located at the base of the reactor 212. According to a second alternative, the concentrated solution is partially discharged in order to form the liquid drain-off, purge, in the form of a concentrate, and the rest of the concentrated solution, which is therefore not discharged, is introduced into the current of solution to be treated that circulates in the pipe 209 (thus, there is no longer discharge in solid form via a drain, purge, located at the base of the reactor 212). The lower the proportion of discharge, the more concentrated the concentrate, but the more concentrated the crystallisation solution as well, and thus the lower the temperature of crystallisation of the hydrates. The eutectic forms the lower limit of this alternative (no more liquid discharge).

(25) The clathrates hydrates and the solid support separated in the separator 215 are withdrawn in the separator 215 via a pipe 217 and conveyed into a reactor for dissociation of the clathrates hydrates 218, in which the clathrates hydrates are separated into host molecules and purified water.

(26) Any known dissociation technique can be used. In general, the dissociation of the crystallised crystals of hydrate into water and host molecules is carried out by an increase in temperature that leads to their melting. In other words, the crystallised crystals of hydrate are melted; the reactor 218 is then called a melter.

(27) Then, the purified water, on the one hand, and the host molecules and solid substrate on the other hand, the host molecules being adsorbed onto the solid support, are separated by a liquid/solid separation technique such as decantation or filtration.

(28) Thus, in the case in which the host molecule is a liquid immiscible with water, the dissociation by melting of the clathrates hydrates in the reactor 218 forms a suspension that is introduced into a filter (not shown) in order to separate the purified water from the host molecule adsorbed onto the solid support and the solid support.

(29) The host molecule adsorbed onto the solid support is recycled with the solid support from the reactor 218 into the reactor 212 via a pipe 219.

(30) In the facility according to the invention, the host molecules and the solid support are thus recycled and used again which increases the profitability of the facility.

(31) The purified water is discharged from the reactor 218 or from the filter via a pipe 220 and can be used.

(32) It is noted that the facility for implementing the purification method according to the invention, shown in FIG. 4, does not comprise a heat exchanger such as a heat pump that would operate between the reactor for crystallising and growing clathrates hydrates 212 and the reactor for dissociation of the clathrates hydrates 218.

(33) The absence of such a heat exchanger such as a heat pump constitutes one of the advantages of the method according to the invention.

(34) To optimise the method, however, it would be of interest to place a heat exchanger between the reactor for dissociation of the clathrates hydrates such as a melter 218 and the stream of solution to be treated or entering flow that circulates in the pipe 209.

(35) The heat of the entering flow would then help to melt the hydrates, while the cold from the reactor 218 such as a melter would allow the stream of solution to be treated or entering flow to already be cooled before entering the cooling device 210.