_oIn a(C), where the pressure Π_o=137 MPa and the activity a(C)=1−0.0256 C−0.0045 C². Step 2c) may be performed using a double-acting actuator with a through-type rod. A double-acting actuator with a through-type rod generally comprises a central piston which separates two chambers, one containing the solution B and the second the solution C. On displacement of this piston, the volume of solution C injected into the cell is identical to the volume of solution B exiting the cell. Moreover, the two fluids are at quasi-identical pressure, since they are in contact within the cell (pressure difference situated within a range from 0 to 10 bar, necessary for the circulation of the liquid). The pressure is equal to the sum of the pressure of emptying the material of pure water and the osmotic pressure of the solution in contact with the material at the temperature under consideration. For example, when solution B is an aqueous solution of NaCl at 5.2 mol/L at 25° C., washing commences at the pressure of emptying the solution B of 655 bar, the sum of the pressure of 250 bar of filling the material ZIF-8 with pure water and the osmotic pressure of 405 bar of the solution B, and finishes at the pressure of filling of solution C of 250 bar, the sum of the pressure of 250 bar of filling the material ZIF-8 with pure water and the osmotic pressure of 0 bar of the solution C. The duration of step 1c) may be situated within a range from 10 ms to 10 s and preferably from 100 ms to 1 s.

Advantageously, step 2d) of the process P2 according to the invention may be performed by means of a high-pressure device such as a hydraulic actuator. This may be, for example, the apparatus described in the text (9). The duration of step 2d) may be situated within a range from 10 ms to 10 s and preferably from 100 ms to 1 s. The depressurization applied in step 2d) depends on the nanoporous material used, on the composition of the solution (here C) and on the temperature at which step 2d) is performed. In step 2d), the depressurization and the associated parameters are imposed by the concentration of the solution C, the nature of the nanoporous material and the temperature of performance of the process. In step 2d) of the process P2 according to the invention, the pure water trapped in the nanoporous material is expelled and mixed into solution C. The process P2 has therefore made it possible to desalinate part of the solution B, by passing it into the material and then diluting it in solution C. In the context of the use of the process P2 for the desalination of a concentrated brine, the performance level of the process according to the invention is greater than that of a conventional membrane approach. More specifically, the extra energy cost relative to the minimal thermodynamic desalination cost remains modest relative to the extra cost inherent in the conventional membrane approaches.

Advantageously, the process P2 according to the invention may further comprise a step 2e) of isochoric washing of the mixture obtained at the end of step 2d), the washing being carried out by means of an aqueous solution B. The performance of step 2e) is similar to that of step 1c), but conducted at the pressure of step 1d). Advantageously, in the performance of step 2e) of the process P2 according to the invention, the pressure increases in the system, owing to the return of the solution C to the solution B, at constant volume. This increase in pressure is due to the difference in salt concentration between the aqueous solutions C and B. The process P2 according to the invention may therefore comprise an increase in the pressure, during the washing step 2e). For example, on passage from a solution C (containing no salt or virtually no salt) to a solution B at 5.2 mol/L, the pressure increase during step 2e′) is 405 bar. For infinitely low concentrations, the increase in pressure Π is given by the Van′t Hoff formula Π=i Δc RT, where Δc is the difference in salt concentration, i is the number of ions contained in the salt, R is the ideal gas constant and Tis the temperature; for the specific case of the salt NaCl at a temperature of 25° C., the following phenomenological law allows the osmotic pressure to be expressed irrespective of the concentration c of 0 to 6 mol/L: Π=Π_oIn a(C), where the pressure Π_o=137 MPa and the activity a(C)=1−0.0256 C−0.0045 C². Step 2e) may be performed using a double-acting actuator with a through-type rod. A double-acting actuator with a through-type rod generally comprises a central piston which separates two chambers, one containing the solution C and the second the solution B. On displacement of this piston, the volume of solution B injected into the cell is identical to the volume of solution C exiting the cell. Moreover, the two fluids are at quasi-identical pressure, since they are in contact within the cell (pressure difference situated within a range from 0 to 10 bar, necessary for the circulation of the liquid, between the cell entry and exit pressures). Washing takes place at a pressure equal to the sum of the pressure of emptying the material of pure water and the osmotic pressure of the solution in contact with the material at the temperature under consideration. For example, when solution B is an aqueous solution of NaCl at 5.2 mol/L at 25° C., washing commences at the pressure of filling the solution C of 250 bar, the sum of the pressure of 250 bar of filling the material ZIF-8 with pure water and the osmotic pressure of 0 bar of the solution C, and finishes at the pressure of filling of solution B of 655 bar, the sum of the pressure of 250 bar of filling the material ZIF-8 with pure water and the osmotic pressure of 405 bar of the solution B. The duration of step 1c) may be situated within a range from 10 ms to 10 s and preferably from 100 ms to 1 s.

Advantageously, the process P2 according to the invention may be performed by a repetition of steps 2b), 2c), 2d) and 2e). The process P2 may therefore be performed such that the successive steps are: 2a), 2b), 2c), 2d), 2e), 2b), 2c), 2d), 2e), etc. Steps 2b), 2c), 2d) and 2e) may be repeated a number m of iterations, m being an integer greater than or equal to 2. For example, in normal operation, for an enclosure (reaction medium) with a volume of 100 mL, a volume of water of 50 mL litres and a volume of ZIF-8 powder of 50 mL, the value of m may be of the order of a million.

Without limitation, the processes P1 and P2 also exhibit the following advantages:

- the processes P1 and P2 according to the invention can be performed in the same device;
- the processes P1 and P2 operate by volume osmosis between the outside and the inside of the selective, hydrophobic nanoporous particles;
- the processes P1 and P2 make use of the exchange surface area corresponding to the external surface area of the collective of nanoporous particles. This exchange surface area, set by the size of the nanoporous particles, is between 0.1 and 100 m²/g. This exchange surface area enables rapid operation, with filling/emptying times of between 10 ms and 10 s and preferably between 100 ms and 1 s;
- the processes P1 and P2 are based on an isotropic mechanical stressing of each nanoporous particle, which allows operation to take place in the presence of concentrated solutions having an osmotic pressure of possibly up to 1000 bar.

BRIEF DESCRIPTION OF THE FIGURES

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

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).

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.

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).

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.

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.

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.

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.

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

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.

The process is performed at 25° C.

Step 1a) is performed by placing 400 mg of ZIF-8 powder in a tank with a volume of 2.5 cm³, 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).

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 mm³, 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 mm³at 250 bar is 4 J; the corresponding mechanical power for filling in 0.5 s is 8 W.

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.

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.

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.

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.

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