WO2003096826A1

WO2003096826A1 - Method for extracting at least one compound contained in a fluid by diffusion through a porous membrane

Info

Publication number: WO2003096826A1
Application number: PCT/FR2003/001459
Authority: WO
Inventors: Manuel Dornier; Marie-Pierre Belleville; Gilbert Rios; Max Reynes; José Sanchez; Fabrice Vaillant
Original assignee: Centre De Cooperation Internationale En Recherche Agronomique Pour Le Developpement (Cirad); Centre National De La Recherche Scientifique (Cnrs); Universite De Montpellier Ii (Umii); Ecole Nationale Superieure De Chimie De Montpellier
Priority date: 2002-05-15
Filing date: 2003-05-14
Publication date: 2003-11-27
Also published as: FR2839656A1; AU2003249408A1; FR2839656B1

Abstract

The invention concerns a method for extracting at least one compound present in a first phase using a porous membrane whereof one first surface is contacted with said first phase and the second opposite surface is contacted with a second phase, said second phase having a lower concentration of said compound relative to that of said first phase, said compound to be extracted being transferred from said first phase to said second phase by diffusion through the pores of said porous membrane, mainly under the effect of the concentration gradient in said compound(s) to be extracted between said first and second phases. The method is characterized in that said porous membrane is a ceramic material membrane, and it consists in providing a static pressure higher by 0.1 to 1 bar relative to atmospheric pressure, preferably higher by 0.2 bar relative to atmospheric pressure, at least in said liquid first phase, the static pressure difference between the two surfaces of the membrane being less than 1 bar, preferably less than 0.6 bar, and/or in providing a static pressure difference between the two surfaces of the membrane, consisting in an excess pressure of 0.1 to 1 bar on the side of said first phase.

Description

METHOD FOR EXTRACTING AT LEAST ONE COMPOUND CONTAINED IN A FLUID BY DIFFUSION THROUGH A MEMBRANE

POROUS

The present invention relates to a method for extracting at least one compound contained in a first liquid or gas phase by diffusion through porous inorganic membranes.

In the case where the compound extracted mainly is a majority compound of said first phase, the operation generally corresponds to a concentration of said first phase. If, on the contrary, the compound extracted mainly is a compound present in low concentration in said first phase, the operation generally corresponds to a purification of said first phase.

More particularly the present invention relates to a membrane extraction process in which a porous membrane is used, a first face of which is placed in contact with said first liquid or gaseous phase and the second opposite face of which is placed in contact with a second liquid phase or gaseous, at least one of said first or second phase being liquid, the two said first and second phases flowing tangentially against said membrane, the latter making it possible to bring said first and second phases into contact without mixing, and said second phase having a lower concentration of the compound to be extracted, the latter being transferred from said first phase to said second phase by diffusion through the pores of said porous membrane,

Methods and membranes of this type are known to those of skill in the art. In these methods, the difference in concentration of the compound or compounds to be extracted prevailing between said first and second phases constitutes the driving force of the mass transfer. The compounds to be extracted are transferred from the high concentration phase to the low concentration extracting phase. The small thickness of the membrane locally generates a gradient of high force which is favorable for obtaining a high mass transfer speed.

According to a first embodiment, contact is established between the two phases without them mixing by using immiscible phases by establishing a liquid / gas contact or a liquid / liquid contact but with two immiscible liquids. (14).

According to another embodiment, a non-wettable membrane is used with respect to the liquids which are in contact with it so that the air or another gas inside the pores remains enclosed and constitutes a gas interface between said first and second liquid phases.

In particular for the treatment of a polar liquid solution, a hydrophobic porous membrane is used. A process of this type is well known to those skilled in the art under the name of "osmotic distillation" or "osmotic evaporation" (1-7). These processes, which can be carried out at ambient temperature and atmospheric pressure, offer the advantage of being very energy-efficient and therefore inexpensive.

More particularly, the use of osmotic evaporation or osmotic distillation processes has been proposed for the concentration of fruit juices (4-7). Osmotic evaporation is a process for concentrating aqueous solutions at room temperature and at atmospheric pressure, the aqueous solution circulates on one side of a hydrophobic membrane whose pores are filled with air while a highly concentrated saline solution • • circulates on the other side, the difference in concentration thus inducing a transfer of water vapor through the membrane. This process allows aqueous solutions to be concentrated at room temperature and at atmospheric pressure. It is more particularly used for the concentration of fruit juice.

The physical phenomenon governing the concentration of the solution is the natural evaporation of water at the solution to concentrate / pore interface of the membrane, and recondensation of the vapor on the side of the saline solution, at the pore / brine interface. . The energy returned during the condensation of water in the brine is transferred through the membrane, which behaves like a heat exchanger, to achieve the evaporation of water from the phase to be concentrated. The energy balance is zero, the process operating without exogenous energy supply.

This osmotic evaporation or osmotic distillation process therefore consists in causing the evaporation of water by bringing an aqueous solution into contact with a hydrophobic porous membrane; the evaporated water is then selectively trapped on the other side of the membrane by a very concentrated saline solution. The high concentration of this saline solution generates a particularly high osmotic pressure, which can go up to more than 5.10 ^s KPa. Due to the great greed of this "brine" for water, and under the effect of the large difference in osmotic pressure, water is transported through the membrane.

In the membrane concentration processes of the prior art, membranes of an organic nature made up of polymers such as PTFE (polytetrafluoroethylene), PNDF (polyvinylidene fluoride), PP (polypropylene), and other derivatives are used, in particular membranes made up of '' a thin layer of PTFE on a polypropylene support. These organic membranes have the advantage of having hydrophobic characteristics.

However, the inventors have observed that these organic porous membranes have several shortcomings and drawbacks, in particular when one passes to the stage of industrial exploitation, these membranes prove to be fragile mechanically and also degradable by numerous chemicals.

Furthermore, these polymer membranes do not withstand a high static working pressure and above all different working pressures on each side of the membrane, unless recourse is had to thick membranes, which in this case results in a reduction in transfer speed. of the compound to be extracted through the pores of the membrane, since the transfer speed is inversely proportional to the thickness of the membrane. This is why Membrane extraction methods of the prior art using organic membranes, are carried out at atmospheric pressure, without establishing a static pressure gradient between the two faces of the membrane to avoid deformation thereof.

The use of porous ceramic membranes is known in filtration processes such as ultrafiltration and microfiltration. In these filtration methods a forced circulation of a generally liquid phase is established through the membrane, by establishing a strong gradient of static pressure between the two faces of the membrane, in particular greater than 1 bar, said membrane retaining upstream the compounds larger in size than pores (8-9). The membrane acts here as a sieve under the effect of a static pressure gradient.

The manufacture of porous ceramic membranes is well known (8-13), this ceramic material is well suited for the production of membranes of planar or tubular geometry. These ceramic membranes have pores whose physical characteristics are particular in the sense that they consist of stacks or aggregates of grains and are prepared by sintering at high temperature, the pores being formed by the interstitial space between the grains. The geometry of these interstitial pores makes it difficult for the species to be extracted to diffuse through the membrane in the absence of a static pressure gradient, which has led those skilled in the art to use these ceramic membranes in industrial filtration processes. and not in osmotic distillation or evaporation processes.

For the filtration of polar solutions, membranes of ceramic materials coated with apolar compounds have been described, in particular based on reagents comprising polyphosphazene or silane groups (10).

It has also been suggested to use porous ceramic membranes coated with hydrophobic compounds in other gas transfer processes in which the driving transfer force is a large pressure gradient: these are pervaporation or ozonation processes (11-13). In the pervaporation process, so-called “dense” membranes are used, produced by depositing a continuous film of organic polymers on a porous ceramic support, the initial porosity of the ceramic support being blocked by the organic polymer and diffusion through the membrane is made by molecular diffusion in the solid material, ie diffusion between the molecules of said solid material and not through pores.

In the ozonation process, the gas, namely ozone, is transferred to a liquid phase by passing it through the pores of the membrane, under the effect of a static pressure gradient.

Ceramic membranes have a much greater mechanical resistance than porous organic membranes, because they are incompressible and non-deformable, and therefore withstand high working pressures and an installation with large membrane surfaces without damage, which considerably facilitates their implementation. industrial work.

Furthermore, the chemical stability of ceramic membranes is very much higher than that of organic membranes, which facilitates the transfer of corrosive products such as ozone in an ozonation process.

In WO 93/22036, there is described a method for extracting alcohol from a hydroalcoholic solution through an organic hydrophobic membrane in which the driving force for the transfer through the membrane is a concentration gradient in the species to be extracted. , namely an alcohol, in the two liquid solutions on either side of the membrane. In WO 93/22036, it is speculatively indicated the possibility of using ceramic membranes, but WO 93/22036 does not provide any teaching as to the characteristics of the ceramic membranes which must be implemented, in particular no teaching as to the conditions of implementation of the process to obtain an industrially exploitable process with a transfer speed greater than 1 kg. h ^"1. m ^{" 2} , preferably more than 2 kg.h m- ² . It has never been shown that porous ceramic membranes can be advantageously used in membrane extraction processes in which the main driving force for the transfer of material through the membrane is not a pressure gradient but a gradient of concentration of said compound to be extracted in the fluids flowing on either side of the membrane, and this under industrially exploitable conditions, that is to say with a relatively high transfer flux, in particular greater than 1 kg. h ^"1. m ^{" 2} , or even more than 2 kg. h ^" .m ^'2. This possibility was not obvious from the state of the art taking into account the granular structure of porous ceramic membranes and the geometry of their interstitial pores as mentioned previously.

The objective of the present invention is to provide a membrane extraction process in which the transfer of material is mainly carried out by a simple difference in concentration of compound (s) to be extracted between the phases located on either side of said membrane, so as to obtain a transfer flow in said compound (s) through the membrane at a transfer rate as high as possible for industrial exploitation, and in particular a transfer rate of at least 1 kg. h ^"1. m ^{" 2} , preferably 2 kg. h ^"1. m ^{" 2} , more preferably 5 kg. h ^"1. ^{" 2} .

The inventors have demonstrated that it is possible and advantageous, under certain conditions, to use porous ceramic membranes in membrane processes in which the driving force of material transfer is achieved by a concentration gradient of the compound to be extracted contained in fluids on either side of the membrane, reaching material transfer speeds through the membrane greater than 1 kg. h ^" .m ^" ² , or even more than 2 kg. h ^"1. ^{" 2} .

More precisely, the present invention provides a membrane process for extracting at least one compound present in a first liquid or gas phase using a porous membrane, a first face of which is placed in contact with said first phase and of which the second opposite face is placed in contact with a second liquid or gas phase, at least one of the two phases being liquid, the two said first and second phases flowing tangentially against said membrane, the latter making it possible to bring said first and second phases into contact without them mixing, which second phase has a lower concentration of said compound to be extracted, said compound to be extracted being transferred from said first phase towards said second phase by diffusion through the pores of said membrane mainly under the effect of the concentration gradient in said compound (s) to be extracted between said first and second phases.

According to the present invention, said porous membrane is a membrane made of ceramic material, and a static pressure greater than 0.1 to 1 bar relative to atmospheric pressure is established, preferably greater than 0.2 bar relative to atmospheric pressure. , at least in said first liquid phase, the difference in static pressure between the two faces of the membrane being less than 1 bar, preferably less than 0.6 bar, and / or a difference in static pressure is established between the two faces of the membrane, consisting of an overpressure of 0.1 to 1 bar on the side of. said first phase.

In a first embodiment, the same static pressure greater than atmospheric pressure is applied to each side of the ceramic membrane 1.

In another embodiment, a static pressure greater than atmospheric pressure is applied to the two sides of the ceramic membrane but not identical on the two sides, provided that the pressure difference remains less than 1 bar, preferably less than 0, 6 bar to avoid the appearance of a liquid bridge in the membrane.

In another embodiment, a static pressure greater than atmospheric pressure is applied only to said first phase, this overpressure remaining less than 1 bar, preferably less than 0.6 bar.

In another embodiment, a static pressure greater than atmospheric pressure is not applied in said first phase but a partial vacuum is created by vacuum in the compartment of said second phase, which is then a gaseous phase, the difference of pressure static between the two phases located on either side of the membrane corresponding to an overpressure in said first phase of 0.1 to 1 bar, preferably 0.2 to 0.6 bar.

It is possible to apply a static pressure greater than atmospheric pressure in said first and second liquid phases or to create a partial vacuum by vacuum in said second gas phase by regulating the flow rate of the fluids circulating on each side of the membrane using pumps and valves in a known manner.

The inventors have discovered that it is possible to increase the flow of material transfer through a ceramic membrane compared to organic membranes of the prior art, by establishing the concentration gradient in the presence of a static pressure greater than atmospheric pressure and / or by establishing a small difference in static pressure on either side of the membrane, while retaining a small thickness of membrane. The mechanical properties of a ceramic membrane make it possible to withstand static pressures higher than atmospheric pressure and to establish a difference in static pressure on either side of the membrane on thin membranes, namely in particular of 10 to 100 μm without risk of deformation of the membrane, while the membrane extraction processes using organic membranes must be carried out at atmospheric pressure and without .difference in static pressure, in particular to avoid these deformations.

Preferably, a static pressure difference is established between the two faces of the membrane, consisting of an overpressure of 0.1 to 1 bar (10 to 100 kPa), in particular 0.2 to 0.6 bar on the side of said first phase.

The advantageous effect of this pressure difference is surprising. The results observed are probably linked to the greater penetration of liquids into the pores in the case where said second phase is liquid: when the pressure difference increases on either side of the membrane the liquid located in the high pressure compartment penetrates more in the pores which decreases the thickness of the gaseous layer contained in the membrane and therefore likely increases gas flows through the membrane. In the case where said second extracting phase is a gas, a reduction in the pressure on the gas side decreases the partial pressure of the compound to be extracted, in particular the partial pressure of water, and therefore the concentration of the compound in the gas phase which helps to further increase the concentration gradient between the two compartments.

According to the present invention, the transfer flow of said compound (s) through the membrane can reach at least 1 kg. h ^"1. m ^{" 2} , preferably at least 2 kg. h ^"1. m ^{" 2} , more preferably at least 5 kg.h ^_1 .m ^"2 .

In a preferred embodiment, said porous ceramic membrane has the following characteristics:

- a thickness of 10 to 1000 μm, preferably from 10 to 200 μm.

- an average pore diameter of between 0.01 and 5 μm, preferably 0.1 to 1 μm,

- A volume porosity of said membrane is at least 20%, preferably 30 to 80%.

The term “volume porosity” is understood here to mean the proportion of the vacuum of the membrane.

These membrane characteristics make it possible to obtain optimum transfer flows of said compound to be extracted through the membrane, without risk of penetration of liquid into the pores with undesired passage of liquid through the membrane, and while retaining good properties of mechanical strength of the membrane.

In a particular embodiment, said membrane comprises a macroporous ceramic support coated with a porous ceramic layer of pores with diameters of 0.01 to 5 μm, preferably 0.1 to 1 μm and with a volume porosity of at least 20%. preferably 30 to 80%. Said support macroporous then has the function of ensuring the mechanical strength of the assembly.

Said ceramic materials more particularly consist of one or more metal oxides chosen in particular from alumina A1 ₂ 0 ₃ , zirconia ZrO _a , titanium dioxide Ti0 ₂ and silica Si0 ₂

According to an advantageous alternative embodiment of the invention, said membrane is coated on at least one of its faces, preferably on its two faces, with a coating which makes said membrane non-wettable with respect to said first and / or respectively second liquid phase flowing in contact with it. Said coating is produced on the surface of the grains forming the membrane and does not slightly modify the initial porosity thereof.

Thus, in the case of the implementation of two miscible liquid phases, it is possible to avoid the penetration of liquid into the porous support and to avoid the mixing of the two phases respectively placed in contact with the two faces of the membrane by the constitution of a film. gaseous corresponding to the air contained in the pores. And, the diffusion of the compound to be extracted through the membrane is done by gas diffusion as mentioned previously. The notion of wettability of a solid like the membrane, involves physicochemical interactions of the hydrophobic-hydrophilic type between the membrane and the liquid phase in contact with it and obviously also depends on the size and the shape of the pores. This is why, according to the present invention, the pores must preferably be of diameter less than 5 μm to preserve the non-wettability of the membrane even with a hydrophobic coating.

The surface properties of the membrane must be adapted according to the polarity of the liquid (s) in contact with the membrane so that the membrane is not wetted by at least one of the liquids in contact with it.

For the treatment of polar liquid, it is therefore preferable for said membrane to be coated with a hydrophobic coating, in particular in a process according to the invention of osmotic evaporation or osmotic distillation type in which the compound to be extracted is water, and we realize a dehydration of said first liquid phase which is an aqueous solution, using said second phase having a low concentration of water

In one embodiment of this dehydration process by osmotic evaporation, said second liquid phase is brine at saturation. The term “brine” is understood here to mean a solution of salt, for example of CaCl ₂ or of MgCl ₂ , preferably concentration greater than 5 mol.l ^"1 .

As mentioned previously, the use of a ceramic membrane also makes it possible to implement membrane extraction processes in which said second phase is a gas phase with establishment of a small pressure difference with respect to the other face of the membrane by creating a partial vacuum in the compartment of said second gas phase.

In this embodiment, said second phase can be a gaseous phase with low partial pressure of water vapor. Dry air or an inert or neutral gas at reduced pressure, such as CO ₂ or N _2, can be used as the gas phase.

Similarly, when the method according to the invention applies to the extraction of a compound from a first liquid phase to a second liquid phase by gas diffusion through a porous non-wettable membrane, the pores of the membrane can be filled with an inert or neutral gas such as

C0 ₂ or N ₂ rather than by air.

However, in the case of the use of two immiscible fluids, namely two immiscible liquids or a gas and a liquid, the pores of the membrane can be filled with one of the two fluids.

The present invention applies more particularly to the concentration of aqueous solution such as fruit juices. Said first liquid phase is a fruit juice and the concentration of said fruit juice is carried out by extracting water from said fruit juice. For the concentration or the purification of solution in general, the process according to the invention makes it possible to treat the product without heating insofar as the transfer of material between the two compartments on either side of the membrane is linked to a difference concentration between the phase to be treated and the extracting phase. Thus this process is particularly interesting for the treatment of heat-sensitive solutions. The temperatures used are most often below 35 ° C. the treated product does not undergo thermal degradation.

Conventionally, in order to be placed in conditions favorable to the transfer of material through the membrane, the two said first and second phases of different concentration in said compound (s) to be extracted circulate tangentially on either side of the membrane either co-current or counter-current.

Preferably, the tangential circulation speed of said first and second phase (liquid or gas) between 0.1 and 10m. s ^"1 .

To increase the transfer of compounds across the membrane, the extracted compound can be reacted immediately after passing through said second phase so as to maintain the force gradient and therefore the transfer speed at maximum value, the transformation of said extracted compound can take place do this by homogeneous reaction in said second phase or by heterogeneous reaction if the membrane is used as a support for a catalyst in particular.

When said membrane is coated with hydrophobic compounds, these can be in liquid or solid form and fixed to said membrane either by covalent bond or by weak physicochemical interactions.

More particularly, said porous ceramic membrane is coated with a hydrophobic compound of silane type grafted to the ceramic material by a siloxane bond or the hydrophobic compound is a polyphosphazene compound. The silanes more particularly have a chemical structure of the Y-Si (0-R) _{3 type} , the groups R being in particular H or alkyl, in particular CH ₃ or CH ₂ -CH ₃ , and Y being a hydrocarbon chain. These compounds are grafted by siloxane SO-Cer covalent bonds. (Cer. ^ Ceramic) which is established between an OH group of the ceramic support and the siloxane group of the silane compound. After grafting, it is the nature of the Y chain and of the R groups which determines the surface properties of the grafted ceramic. To obtain a hydrophobic surface Y must be apolar and hydrophobic, in particular as regards the substituents present on the hydrocarbon chain Y.

Polyphosphazene compounds have a linear or cyclic polymer structure of type - (P (Y) ₂ = N-) _n -) these polyphosphazene compounds can be grafted by covalent bonds by reaction with the hydroxyl groups OH present on the surface of the ceramic material . The nature of the group Y also determines the hydrophobic surface properties of the grafted ceramic.

In another embodiment, the hydrophobic coating is produced by depositing a lipid film on said membrane.

By way of illustration, the lipid film can be obtained from vegetable oil or any other lipid fatty material comprising triacylglycerols, fatty acids, phospholipids, glycolipids or sterols. The application of the lipid film requires the prior dissolution of the lipid component in an organic solvent and then deposited by drying and removal of the organic solvent such as dichloromethane or hexane.

In known manner, the geometry of the membrane can be planar or tubular.

The unit useful surfaces are most often of the order of 0.1 m ² and in industrial units, several membranes are installed in series or in parallel to obtain total surfaces of several m ² , or even several tens of m ² . For tubular membranes, the length is close to 1 m in the majority of cases. Finally, the present invention relates to an industrial installation useful for the implementation of a method according to the invention characterized in that it comprises a porous ceramic membrane as defined above.

Other characteristics and advantages of the present invention will become apparent in the light of the examples which follow.

Example 1

Commercial tubular alumina membranes (Exekia® membrane, length 25 cm, diameters 7/10 mm, pore diameter 0.8 μm) have been chemically treated so as to modify their surface properties and in particular their hydrophobicity. In this case, the free surface of the grains making up each ceramic layer has been modified on the surface by the supplier of membranes by grafting silanes to ensure the hydrophobicity of the material.

These membranes are then placed in a tubular casing adapted to the size of the membranes where two solutions are circulated in parallel: a brine (CaCl ₂ , 5 mol.l ^"1 ) and a dilute aqueous solution to be concentrated. To simplify, pure water is used. The brine circulates inside the membrane tube and the water outside but the opposite configuration is possible. The membrane therefore serves as an interface for the 2 solutions and defines two compartments respectively in the inside and outside of the membrane tubes.

Due to its treatment, the membrane is not wetted by liquids and neither of the 2 fluids penetrates inside the pores, this area is the seat of a film of gas, namely air. However, following the difference in concentration between the 2 solutions present, a transfer of water is observed from the diluted compartment to the concentrated compartment. This material flow results from a vaporization of the water molecules on the dilute solution side, then the gas phase transfer of these molecules inside the pores and finally their condensation on the saμmure side. Under the operating conditions tested (pressure close to atmospheric pressure in the two compartments, constant fluid temperatures equal to 30 ° C, speed variable brine circulation between 0.5 and 3 m. s ^"1 ), the values of the measured water flows are of the order of 0.8 kg.h ^_1 m ^{" 2} . For high circulation speeds, the pressure drops generated can in certain cases cause liquid bridges to appear in the membrane pores. Solutes can then be exchanged by diffusion in liquid phase between the 2 compartments which is not desirable.

Example 2

Similar experiments were carried out using membranes similar to those described in Example 1 but with a pore diameter of 0.2 μm. The results obtained are equivalent in terms of water flow. However, for circulation speeds between 0.5 and 3 m. s ^"1 , the formation of liquid bridges has never been observed.

Example 3

Membranes identical to those mentioned in Example 2 were used to study the incidence of temperature on the evaporative flows. The procedure is carried out under the same conditions as in Examples 1 and 2 except with regard to the temperature, the value of which, kept constant throughout the duration of a test, is successively fixed at 25, 30, 35 and 40 ° C. The water flows increase exponentially with temperature and are respectively equal to 0.5; 0.8; 1 and 1.7 kg.h ^ m ^"2 .

Example 4

Membranes identical to those mentioned in Example 2 were used to study the impact of the pressure on the evaporative flows. Three different types of tests were carried out. The first type (Protocol 1) is carried out under conditions similar to those of Examples 1 and 2 (namely a pressure close to atmospheric pressure in the two compartments). For the second (Protocol 2), an overpressure of 0.2 and 0.3 bar identical is imposed in each of the compartments. Finally for the third (Protocol 3), an overpressure of 0.3 bar relative to the pressure of the brine compartment is imposed in the diluted solution compartment. If we compare the performance of the tests carried out according to protocols 1 and 2, we see that the operating flows are multiplied by 4 or 6 when the pressure of the compartments is respectively equal to 0.2 or 0.3 bar. The conditions of protocol 3 lead to even more interesting results (evaporative flow multiplied by 7 compared to protocol 1). However, under these conditions, the risks of formation of liquid bridges are higher.

Example 5

The impact of the chemical nature of the ceramic support was evaluated through the following experiment. Exekia® zirconium oxide membranes of the same shape and dimensions as in Example 1 but having pore diameters of 0.1 μm were made hydrophilic by means of a treatment identical to that described in the examples above. The performance of these membranes is close to that of the membranes prepared from ceramic supports based on alumina when the pressures of the 2 compartments are identical. But if an overpressure is applied on the diluted compartment side, the flows observed with the zirconia and alumina membranes are respectively of the order of 4 and 3 kg.h-'m ^'2 .

Example 6

A second way of treating ceramic membranes in order to give them a hydrophobic character has been explored. The silanes used in the first route have been replaced by polyphosphazenes. Ceramic membranes identical to those used to carry out the experiments described in examples 1 and 2 were thus treated and made it possible to obtain, under the operating conditions described in example 1, less interesting results: weaker evaporative fluxes (< 0.5 kg.h ^_I m ^"2 ) and greater risk of liquid bridging.

Example 7 A third treatment route for rendering the ceramic membranes hydrophobic was considered. Ceramic supports identical to those of Example 1 were coated with a corn oil / dichloromethane mixture in variable proportions and then put to dry in an oven at 70 ° C. for 1 h. The membranes thus treated were then characterized under the following operating conditions: constant temperature of the fluids and equal to 30 ° C., circulation speed of the brine 1 m. s ^"1 , overpressure in the diluted compartment between 0.2 and 0.6 bar.

The results obtained show that when the oil fraction increases in the mixture, the overpressure to be applied to observe a flow of material between the compartments increases. Thus, for an oil / dichloromethane mixture 25/75 (w / w), the evaporative flux is zero when the overpressure applied is 0.3 and goes to 7 kg.h ^"α m ^{" 2} when it is 0, 6 bar.

Example 8

In all of the above examples, the transfer of material results from the existence of a difference in the concentration of water between the 2 compartments, this difference being generated by the use of a concentrated solution (brine). One can however envisage other means to generate it. In the example described below, brine is no longer used, but a partial vacuum is created in the compartment where the brine circulated. Three membranes were tested in this configuration: an alumina membrane with 0.8 μm pores treated with silanes (cf. example 1), a 0.2 μm alumina membrane prepared according to the same treatment and an alumina membrane treated with the oil of example 7. For the 3 cases, the vacuum generated varies from —0.1 to —0.6 bar relative to atmospheric pressure in said first phase. The appearance of liquid bridges is observed as soon as the depression is equal to —0.4 bar in the case of membranes prepared from 0.8 μm supports, regardless of the treatment of the membrane while for the membrane. prepared from a 0.2 μm pore support, these bridges are only observed when the pressure is less than -0.6 bar.

In the case of membranes prepared from 0.8 μm pore supports, the evaporative fluxes are practically independent of the applied vacuum and are of the order of 0.7 kg. h ^"1. m ^{" 2} for membranes treated with silanes and 1.4 kg. h ^"1. m ^{" 2} for membranes treated with oil. On the other hand, in the case of membranes prepared from 0.2 μm pore supports, the evaporative fluxes increase with the applied vacuum and the possibility of working with a higher vacuum without risk of liquid bridges, allows fluxes to be obtained. transfer order 2 kg.h - '. m ^"2 .

Example 9

The process is applied to the concentration of fruit juice at temperatures between 20 and 35 ° C by following the procedures described in Examples 1 to 8. The transfer of the water contained in the product to the extracting phase leads to the concentration of the soluble dry extract of fruit juice without the need to heat the product. On various fruit juices with an initial soluble dry extract of between 5 and 25% (orange, passion fruit, pineapple, mango, camu-camu, banana, etc.) clarified or containing a variable pulp content, the process allows to concentrate the product until a final soluble dry extract which can reach, as the case may be, 65%. Since the operation does not require heating the treated product, the sensory (color, flavor) and nutritional (vitamin content, in particular C) qualities of the concentrate produced are superior to those obtained using conventional concentration techniques by thermal evaporation. The water flows measured during the operation vary from 0.1 to at least 4 kg. h ^"1. m ^{" 2} depending on the product treated (soluble dry extract, viscosity, - pulp content), the type of membrane used (type of ceramic, average pore diameter, surface treatment used) and according to the conditions chosen operating procedures (circulation speed, temperature, static pressures on either side of the membrane, nature and characteristics of the extracting phase, position of the extracting phase relative to the membrane).

Fluxes greater than 1 kg. h ^"1. m ^{" 2} are obtained in the presence of static pressure greater than atmospheric pressure and / or difference in static pressure between the two phases of the membrane as explained above. Example 10

All the above examples relate to operations for concentrating aqueous solutions following the transfer of water molecules in the gaseous state through the membrane; this operation is possible thanks to the use of a hydrophobic membrane and an extracting phase having a lower concentration of water than that of the solution to be treated. With the same type of installation, it is possible to envisage the extraction of solutes or solvents contained in any other solution, by adapting the treatment of the membrane so as to make it non-wetting by the solution or solutions which are in contact with it and by generating on either side of the membrane a difference in transfer potential linked to a concentration gradient, in the presence of a small difference in static pressure and / or a static pressure greater than atmospheric pressure at least side of the first phase.

BIBLIOGRAPHICAL REFERENCES

1- COUREL, M., TRONEL-PEYROZ, E., RIOS, G.M, DORNIER, M. and REYNES, M 2001.

The problem of membrane characterization for the process of osmotic évaporation Desalination, 140, 15-25

2- COUREL, M., DORNIER, M., HERRY, J.M., RIOS, G.M, DORNIER, M. and REYNES, M 2000

Effect of operating conditions on ater transport during the concentration of sucrose solutions by osmotic distillation.J. of Membrane Science, 170 (2), 281 - 289 ^'

3- COUREL, M., DORNIER, M. RIOS, G.M and REYNES M.2000

Modeling of water transport in osmotic distillation using asymmetric membrane J. of Membrane Science, 173 (1), 107-122

4- COUREL, M., RIOS, G.M., DORNIER, M., REYNES, M. REYNES, M., and DEBLAY, P. 1966

Concentration of fruit juice by osmotic evaporation Xlème University-Industry Colloquium: Membranes and Separative Techniques, Toulouse, June 13, C / 61-67

5- DEBLAY, P. 1991 Process for at least partial dehydration of an aqueous composition and device for implementing the process. Patent FR 91 13013

6- SHAW, P. LEBRUN, M., DORNIER, M., DUCAMP, M.N., COUREL, M. and REYNES, M. 2001

Evaluation of concentrated orange and passionfruit juices prepared by osmotic évaporation. Lebensmittel. Wissenschaft und- Technologie, 34 (2), 60-65

7- VAILLANT, F., JEANTON, E., DORNIER, M., O'BRIEN, GM, REYNES, M. and DECLOUX, M., 2001 Concentration of passionfruit juice on an industrial pilot scale using osmotic évaporation J. of Food Engineering, 47 (3), 195-202

8- MULDER M. 1991

Basic principles of membrane technology. Kluwer Académie Pub., Dordrecht (NL)

9- CHERYAN M.1998

Ultrafiltration and microfiltration handbook. Tchnomic Pub. Lancaster (PA-

USA)

10- US patent 5,238,569 11- CA patent 2,023,691

12- SAAD ALAMI - YOUNSSI et al.

Grafting δ alumina micoroporous membrane by organosilane: characterization by pervaporation.

Journal or Membrane Science 145 (1998) 27-36

13- PICARD C. et al.

Grafting of ceramic membranes with fluorinated silans Separation and Purification Technology 25 (2001) 65-69

14- Pierre F.X., SOUCHON I., MARIN M. 2001-

Recovery of sulfur aroma compounds using membrane-based solvent extraction, J. of Membrane Science 187: 239-253.

Claims

1 Method for extracting at least one compound present in a first liquid or gas phase using a porous membrane, a first face of which is placed in contact with a said first liquid or gas phase and the second face of which opposite is placed in contact with a second liquid or gaseous phase, at least one of the two said first and second phases being liquid, the two said first and second phases flowing tangentially against said membrane, the latter making it possible to bring said first into contact and second phases without mixing, said second phase having a lower concentration of said compound compared to that of said first phase, said compound to be extracted being transferred from said first phase to said second phase by diffusion through the pores of said porous membrane, mainly under the effect of the concentration gradient in said compound (s) to be extracted between these your first and second phases, process characterized in that said porous membrane is a membrane made of ceramic material, and a static pressure greater than 0.1 to 1 bar compared to atmospheric pressure is established, preferably greater than 0.2 bar relative to atmospheric pressure, at least in said first liquid phase, the difference in static pressure between the two faces of the membrane being less than 1 bar, preferably less than 0.6 bar, and / or a difference of static pressure between the two faces of the membrane consisting of an overpressure of 0.1 to 1 bar on the side of said first phase.

2 Method according to claim 1, characterized in that said membrane is coated on at least one of its faces, preferably on its two faces, with a coating which makes said membrane non-wettable with respect to said first and / or second liquid phase flowing in contact with it.

3 Method according to claim 1 to 2, characterized in that said compound to be extracted is water, and dehydration of said first liquid phase which is an aqueous solution is carried out, using said second phase having a low concentration in water. 4 Method according to claim 3, characterized in that said second liquid phase is a saturated brine solution. ^"

5 Method according to claim 3, characterized in that said second phase is a gaseous phase at low partial pressure of water vapor.

6 Method according to one of claims 1 to 5, characterized in that a static pressure difference is established between the two faces of the membrane, consisting of an overpressure, from 0.1 to 1 bar on the side of said first phase .

7 Method according to one of claims 1 to 6, characterized in that the transfer flow in said compound (s) through the membrane is at least 1 kg. h ^"1. m ^{" 2} preferably at least 2 kg. h ^"1. m ^{" 2} , more preferably at least 5 kg. h ^"1. m ^{" 2}

8 Method according to one of claims 1 to 7, characterized in that a treatment is carried out at ambient temperature, in particular at a temperature below 35 ° C. • and said first liquid phase is a heat-sensitive solution comprising compounds capable of being degraded by heating.

9 Method according to one of claims 1 to 8, characterized in that said first liquid phase is a fruit juice and the concentration of said fruit juice is carried out by extraction of water from said fruit juice

10 Method according to one of claims 1 to 9, characterized in that the tangential circulation speed of said first and second phase is between 0.1 and 10m. s ^"1 .

11 Method according to one of claims 1 to 10, characterized in that it has an average pore diameter of between 0.01 and 5 μm preferably 0.1 to 1 μm

12 Method according to one of claims 1 or 11, characterized in that the volume porosity of said membrane is at least 20%, preferably from 50% to 80%. 13 Method according to one of claims 1 to 12, characterized in that said membrane has a thickness of 10 to 1000 microns, preferably from 10 to 200 microns.

14 Method according to one of claims 1 to 13, characterized in that said membrane comprises a macroporous ceramic support coated with a porous ceramic layer of pores with diameters from 0.01 to 5 μm preferably from 0.1 to 1 μm and of volume porosity of at least 20%, preferably from 30% to 80%.

15 Method according to one of the preceding claims, characterized in that the said ceramic material comprises one or more metal oxide chosen from A1 ₂ 0 ₃ , 2r0 ₂ , TiO ₂ and Si0 ₂

16 Method according to one of claims 1 to 15, characterized in that said membrane is coated with hydrophobic compounds in liquid or solid form and fixed to said membrane either by covalent bond or by weak physico-chemical interactions.

17 The method of claim 16, characterized in that said porous ceramic membrane is coated with a hydrophobic compound of silane type grafted to the ceramic support by a siloxane bond.

18 Method according to claim 16 or 17, characterized in that the compound is a polyphosphazene compound.

19 The method of claim 16, characterized in that the hydrophobic coating is produced by depositing a lipid film on said membrane

20 Method according to one of claims 1 to 19, characterized in that said membrane is in the form of a flat or tubular wall.

21 Industrial installation useful for implementing a method according to one of claims from 1 to 20 characterized in that it comprises a porous ceramic membrane. as defined in one of claims 1, 2 and 11 to 20.