METHOD FOR CONVERTING OSMOTIC ENERGY INTO HYDRAULIC ENERGY AND FOR DESALINATION

Abstract

The method p1 for converting osmotic energy into hydraulic energy and the method p2 for desalination, include pressurisation/depressurisation and isochoric washing of an aqueous solution containing a salt in the presence of a selective hydrophobic nanoporous material of which the nanoporous volume within the material is only accessible to fresh water and which has a nanoporosity volume fraction ranging from 0.2 to 1 so as to convert osmotic energy into hydraulic energy or conversely to desalinate water, preferably sea water or brine.

Claims

1. A process for converting osmotic energy into hydraulic energy, comprising: 1a) contacting (i) a first aqueous solution comprising a salt, and (ii) a selective hydrophobic nanoporous material whose nanopore volume within the material is accessible only to fresh water and which has a volume fraction of nanoporosity in a range of from 0.2 to 1, to obtain a mixture, 1b) pressurizing the mixture to a pressure in a range of from 10 to 1000 bar, to obtain a pressurized mixture wherein water intrudes into the nanoporous material, to obtain a pressurized mixture, 1c) carrying out a first_isochoric washing of the pressurized mixture using a second aqueous solution comprising a salt, to obtained a washed mixture, and 1d) depressurizing the washed mixture, wherein water is expulsed from the nanoporous material, diluting the aqueous solution, and collecting hydraulic energy, wherein a salt concentration of the aqueous solution is greater than a salt concentration of the first aqueous solution, a difference in salt concentration between the first aqueous solution and the second aqueous solution being within a range of from 0.5 to 25 mol/L.

2. The process as claimed in claim 1, wherein the second aqueous solution has a salt concentration of less than or equal to 25 mol/L.

3. The process as claimed in claim 1, wherein the first aqueous solution has a salt concentration in a range of from 0 to 2 mol/L.

4. The process as claimed in claim 1, wherein the nanoporous material is selected from the group consisting of MOFs, zeolites, imogolites, mesoporous silicas, mesoporous organosilicon compounds and aerogels.

5. The process as claimed in claim 1, wherein the nanoporous material has pores whose mean diameter is in a range of from 0.5 to 5 nm.

6. The process as claimed in claim 1, wherein the nanoporous material has constrictions whose mean diameter is in a range of from 0.2 to 1 nm.

7. The process as claimed in claim 1, wherein a volume of nanoporous material relative to a volume of the reaction medium represents a ratio M (v/v) in a range of 0.2 to 1.

8. The process as claimed in claim 1, further comprising 1e) carrying out a second isochoric washing of the mixture obtained at the end of the depressurizing, diluting, and collecting 1d) using a first aqueous solution.

9. The process as claimed in claim 1, further comprising converting the hydraulic energy collected in the collecting 1 d) into mechanical or electrical energy.

10. The process as claimed in claim 8, wherein the pressurizing 1b), the first isochoric washing 1c), the depressurizing, diluting, and collecting 1d), and the second isochoric washing 1e) are repeated a number n of iterations, n being an integer greater than or equal to 2.

11. The process as claimed in claim 1, which is performed at a temperature in a range of from 5 to 150? C.

12. A process for desalinating a solution comprising a salt, comprising: 2a) contacting (i) a first aqueous solution comprising a salt, and (ii) a selective hydrophobic nanoporous material whose nanopore volume within the material is accessible only to fresh water and which has a volume fraction of nanoporosity in a range of from 0.2 to 1, to obtain a mixture, 2b) pressurizing the mixture to a pressure in a range of from 10 to 1200 bar, wherein water intrudes into the nanoporous material, to obtain a pressurized mixture, 2c) carrying out a first isochoric washing of the pressurized mixture using a third aqueous solution, the third aqueous solution being pure water or fresh water, to obtain a washed mixture, and 2d) depressurizing the washed mixture, wherein water is expulsed from the nanoporous material, diluting the third aqueous solution, and collecting salt-depleted water.

13. The process as claimed in claim 12, further comprising 2c) carrying out a second isochoric washing of the washed mixture obtained at the end of the depressurizing, diluting, and collecting 2 d), the second isochoric washing being carried out using a second aqueous solution.

14. The process as claimed in claim 13, wherein the pressurizing 2 b), the first isochoric washing 2 c), the depressurizing, diluting and collecting 2d) and the second isochoric washing 2 c) are repeated a number m of iterations, m being an integer greater than or equal to 2.

15. The process as claimed in claim 12, wherein, in the contacting 2a), the volume fraction of nanoporosity of the selective hydrophobic nanoporous material has a in a range of from 0.3 to 0.6.

16. The process as claimed in claim 12, wherein, in the pressurizing 2b), the mixture is pressurized to a pressure in a range of from 200 to 800 bar.

17. The process as claimed in claim 1, wherein, in the contacting 1a), the volume fraction of nanoporosity of the selective hydrophobic nanoporous material has a in a range of from 0.3 to 0.6.

18. The process as claimed in claim 1, wherein, in the pressurizing 1b), the mixture is pressurized to a pressure in a range of from 10 to 500 bar.

19. The process as claimed in claim 1, wherein, in the first isochoric washing 1c), the second aqueous solution comprises the salt selected from alkali metal and/or alkaline earth metal salts.

20. The process as claimed in claim 1, wherein the nanoporous material is selected from the group consisting of ZIF-8, Cu2(tebpz), silicalite, chabazite and SSZ-24.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0054] FIG. 1 represents a filling/emptying cycle with pure water in ZIF-8 in the presence of a salt solution of NaCl at 2 mol/L at 70? C.

EXAMPLES

Example 1: Performance of the Process P1 According to the Invention

[0055] The process according to the invention is illustrated using a selective hydrophobic nanoporous material whose nanopore volume within the material is accessible only to fresh water: ZIF-8 (8).

[0056] This is a material of metal organic framework (MOF) type, consisting of zinc atoms bonded by organic ligands, and has a nanoporous structure in the form of spherical cages organized in a cubic structure. The pores of the ZIF-8 have a diameter of 1.2 nm. The cages (or pores) are interconnected by constrictions with a diameter of 0.34 nm. The nanoporous volume of this material represents a volume fraction of 0.4.

[0057] ZIF-8 is a hydrophobic material with low affinity for aqueous solutions. Filling and emptying of the available volume inside the nanoporous material take place under pressure (intrusion and expulsion of liquid into and out of the nanoporous material). At ambient temperature, the material fills with pure water at a quasi-constant pressure of the order of 250 bar, whereas emptying takes place for a pressure of the order of 180 bar. These pressures are dependent on the temperature, on the nature of the aqueous solution and, to a smaller extent, the duration of filling/emptying (see FIG. 1 and table 2).

[0058] In the presence of salt solution, since the salts are excluded from the available volume inside the nanoporous material, they do not penetrate into the hydrophobic nanoporous material. By osmotic effect, an increase is observed in the intrusion and expulsion pressure, as a function of the salt concentration (see FIG. 1) (9-10). It is this pressure gap which gives rise to the energy recoverable by the process according to the invention. For a solution of sodium chloride NaCl at saturation, i.e., 5.2 mol/L, at ambient temperature, a gap of 405 bar is observed toward high pressures: liquid intrusion occurs at 655 bar, and extrusion of the liquid at 585 bar.

[0059] The process is performed in a high-pressure cell suitable for studying phenomena of filling/emptying of hydrophobic nanoporous materials under high pressure. The cell consists of a rigid cylindrical stainless steel base and a closure which comprises an elastomeric membrane. When the cell has been filled, the variation in volume is provided by a piston which deforms the membrane, the latter allowing simultaneously the pressure integrity, the deformability of the system, and its imperviosity in dynamic regime, for piston velocities of 0 to 500 mm/s. The system as a whole is mounted on a traction machine, which is used for controlling the displacement of the piston. This hydraulic machine is able to deliver/dissipate the energy stored/recovered during filling/emptying phenomena for the material. In the context of an industrial application, this traction machine is replaced by a system for converting mechanical energy to electricity, for example.

[0060] The volume of the cell allows a cylindrical tank to be accommodated. One of the bases of the tank is closed by a microporous membrane which is permeable to the liquid but not to the ZIF-8 powder; the second base is closed by a deformable membrane. This tank comprises two hydraulic connections, which allow the liquid to be circulated and to be changed while retaining the powder inside the tank via the frit. When the tank is filled, it is placed within the cell and immersed in water which is intended for transmitting the pressure.

[0061] The examples below were carried out in a laboratory model comprising this tank, which has to be withdrawn from the cell for changes of liquid.

[0062] In an alternative version, the cell enables change of liquid in situ by means of high-pressure tapping ports, without demounting of the tank.

[0063] Solution A is pure water. Solution B is a briny water, having an NaCl concentration of 5.2 mol/L. The difference in NaCl concentration between solution A and solution B is therefore 5.2 mol/L.

[0064] The process is performed at 25? C.

[0065] Step 1a) is performed by placing 400 mg of ZIF-8 powder in a tank with a volume of 2.5 cm.sup.3, and making up the volume with pure water (solution A). The tank thus filled is placed in the high-pressure cell, which is closed by the high-pressure cap (ratio M (v/v)=0.18).

[0066] Step 1b): The closed cell is pressurized by vertical displacement of the piston at an imposed velocity of 1 mm/s at ambient temperature. In the course of this displacement, the pressure, which increases within the cell, is transmitted to the liquid/powder collective contained in the tank, via the flexible membrane which closes the tank. The pressure increases initially by compression of the liquid and elastic deformation of the cell, to reach a pressure of 240 bar, which marks the start of the filling of the nanoporous material with liquid. This filling phenomenon is accomplished at a pressure of 260 bar. The filling lasts for 0.5 s, represents a change in volume of 160 mm.sup.3, and operates at an average pressure of 250 bar. This pressure is independent of the quantity of powder: for twice the quantity placed in the tank, for a given piston velocity, the duration of filling is doubled, but the filling pressure range (240-260 bar) remains identical. The energy associated with the phenomenon of transfer of 160 mm.sup.3 at 250 bar is 4 J; the corresponding mechanical power for filling in 0.5 s is 8 W.

[0067] Step 1c) requires the liquid to be changed under pressure so as to keep the fresh water in the nanoporosity. This step was not carried out as such in the laboratory model. The washing step 1c) was performed by withdrawing the tank from the cell, thus imposing a return to ambient pressure, and thus emergence of the liquid from the nanoporosity and hence breakage of the cycle. This operation does not allow the osmotic energy to be converted into hydraulic energy, but makes it possible to evaluate the energy recoverable in an operation without breakage of the cycle, by isochoric washing under pressure.

[0068] For the performance of step 1c), without breakage of the cycle, the isochoric washing under pressure is carried out using a double-acting solenoid actuator. The double-acting actuator comprises a central piston which separates two chambers, one containing brine (solution B) and the second the liquid from the device (solution A). On displacement of this piston, the volume of solution B injected into the cell is identical to the volume of solution A exiting the cell. Moreover, the two fluids are at quasi-identical pressure (except for the slight difference in pressure needed for displacement of the piston), since they are in contact within the cell: they both go from 250 to 650 bar, the pressure level being set by the difference in concentration between the fresh water held in the nanoporous material and the liquid contained in the cell, with a concentration which increases on displacement of the piston.

[0069] The washing procedure, when implemented under pressure, therefore represents only minimal consumption of energy. Throughout the duration of step 1c), the piston controlling the volume of the cell is held in a fixed position, corresponding to a compressed state of the system.

[0070] Step 1d) is carried out by depressurization of the system. The piston is released and moves out of the cell. The movement takes place at a pressure imposed by the phenomenon of the emptying of the water from the nanoporous material at the pressure of 580 bar at ambient temperature.

[0071] In experiments carried out for this example, the velocity of the piston is imposed by the traction machine for example, 1 mm/s in the opposite direction to that of step 1b). The energy associated with the expulsion of the 160 mm.sup.3 of liquid from the nanoporosity (for 400 mg of powder) at a pressure of 580 bar is 9.3 J and corresponds to a motive power of 18 W. The net energy extracted over the course of the cycle corresponds to the difference between the energy stored in step 1b) and the energy recovered in step 1d), i.e., 10 J.

[0072] The energy extracted over the course of the cycle is set by the difference in pressure between steps 1d and 1b multiplied by the change in volume associated with filling and emptying, in other words the available volume within the nanoporous material.

[0073] Accordingly, for 1 kg of ZIF-8, the nanopore volume is of the order of 0.4 L. Taking account of the pressure difference of 330 bar, an energy of 13 kJ is obtained. The difference in pressure recovered of 330 bar is less than the 400 bar of osmotic pressure, owing to the hysteresis between filling and emptying, corresponding to an 18% loss of energy.

[0074] This loss of energy can be minimized greatly by increasing the operating temperature (see example 2).

[0075] An additional advantage of the process is that the pressures are quasi-constant for filling/emptying times of greater than 0.1 s, allowing steps 1b) and 1d) to be carried out rapidly without additional loss of energy (see table 2). This aspect constitutes a notable advantage of the process according to the invention relative to conventional use of selective membranes for processes for converting osmotic energy, with which the pressure loss increases linearly with the liquid flow rate (that is, inversely to the passage time).

[0076] Taking into account the distance between powder grains, of the order of a micron, the transfer of fluid during washing phases (steps 1c and 1e) can take place over a duration of 0.5 s (identical to the duration of phases 1b and 1d) and requires a pressure difference of a few bar, which is modest when set against the difference in osmotic pressure.

[0077] The average power delivered with the process can be estimated by dividing the energy collected during a cycle by the duration of a cycle. Taking a value of 0.5 s per step of the cycle, the total duration of a cycle is 2 s. Accordingly, from the 13 KJ obtained for a saturated NaCl brine, at ambient temperature, in the context of this example a power of 6.4 KW is obtained for 1 kg of ZIF-8.

TABLE-US-00001 TABLE 1 [NaCl].sub.A [NaCl].sub.B P.sub.osm P.sub.int P.sub.ext P.sub.ext ? P.sub.int E.sub.ext E.sub.tot mol/L mol/L bar bar bar bar kJ/L kJ/L ? 0 0 0 250 180 ?65 ?6.5 0 0 0.5 22 250 202 ?43 ?4.3 2.2 0 1 46 250 226 ?19 ?1.9 4.6 0 2 105 250 285 40 4 10.5 0.38 0 4 268 250 448 203 20.3 26.8 0.75 0 5.2 405 250 585 340 34 40.5 0.83

[0078] Table 1: Energy extracted at 25? C. as a function of the concentrations of solutions A and B. [NaCl]A NaCl concentration of solution A, [NaCl]B NaCl concentration of solution B, P.sub.osm osmotic pressure induced by the concentration difference, P.sub.int intrusion pressure, P.sub.ext expulsion pressure, P.sub.ext-P.sub.int difference between intrusion pressure and expulsion pressure, E.sub.ext energy extracted per unit volume of liquid expelled, E.sub.tot total energy of osmotic origin per unit volume of liquid expelled, n yield of the conversion procedure.

Example 2: Study of the Effect of Temperature on Process Yield

[0079] Example 1 above is reproduced at different temperatures. The process is performed successively at the temperatures of 5, 15, 30, 50 and 70? C.

TABLE-US-00002 TABLE 2 T P.sub.osm P.sub.int P.sub.ext P.sub.ext ? P.sub.int E.sub.ext E.sub.tot ? C. bar bar bar bar kJ/L kJ/L ? 5 384 230 534 304 30.4 38.4 0.79 15 398 235 568 333 33.3 39.8 0.83 30 418 250 613 363 36.3 41.8 0.86 50 446 255 651 396 39.6 44.6 0.88 70 474 260 694 434 43.4 47.4 0.92

[0080] Table 2: Energy extracted as a function of temperature for a solution A at 0 mol/L and a solution B at 5.2 mol/L. P.sub.osm osmotic pressure induced by the concentration difference, P.sub.int intrusion pressure, P.sub.ext expulsion pressure, P.sub.ext-P.sub.int difference between intrusion pressure and expulsion pressure, E.sub.ext energy extracted per unit volume of liquid expelled, E.sub.tot total energy of osmotic origin per unit volume of liquid expelled, n yield of the conversion procedure.

[0081] A reduction in the difference between filling pressure and emptying pressure was observed when the temperature increases. As indicated in table 2, this temperature dependence impacts the dynamic behavior of the system. For a temperature greater than 50? C., the filling pressure becomes quasi-independent of the filling duration over the time range surveyed.

[0082] Operation at 70? C. leads to an increase in emptying pressure, which goes from 180 to 220 bar for pure water, while the filling pressure goes from 250 bar to 260 bar.

[0083] At 70? C., the difference in osmotic pressure is 474 bar. This gives a filling pressure of 260 bar, an emptying pressure of 694 bar, and thus an energy loss reduced to 8%.

LIST OF REFERENCES

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