EP1206671B1 - Drying and/or dry process preservation method using semi-permeable membrane - Google Patents

Abstract

The invention concerns a method for drying moisture-sensitive substances with a solvent which mainly consists in separating said substances from an ambient atmosphere wherein said solvent is present by hermetically sealing said substances in an envelope having advantageously a wall zone non-adherent to said substances wherein said envelope consists essentially of a polymer membrane selectively permeable to the moisturising solvent vapour except to said solvent in liquid state. The inventive method is also useful for preventive treatment of dry substances against moisture.

Description

The present invention relates to the techniques of extractive separation, among which are in especially drying techniques, which consist of bring and / or maintain a product in a dry state determined, in terms of its solvent content humidification determined. The invention aims particularly the provision of industries pharmaceutical, chemical, cosmetic and agro-food of a new membrane separation technique solid / liquid or liquid / liquid as an alternative to concentration, drying and already existing dry storage.

To define and describe the invention in its privileged context, it will often be more convenient, in a for the sake of clarity, to place oneself in the specific case of a drying, i.e. a process involving extraction a solvent responsible for a so-called wet state to avoid. Among the least expensive and most classic of industrial equipment for drying products by ridding of water present, regardless of the water of structure, there are containers to which we bring heat and / or vacuum, such as ovens, possibly agitated, such as drum or pallets. Other more expensive devices, such as freeze dryers that sublimate water or microwaves under vacuum, also allow drying of products. In the case of products with low melting point, manufacturers generally use equipment allowing dry on a fluidized bed.

However, all of these devices can be found chronically overloaded, which involves some time waiting for products to dry outside equipment, during which significant quantities solvent are released to the environment. In addition, these equipment often requires a lot of resources maintenance, cleaning and validation. However, the drying stage, although it is the one that poses the most problems in terms of feasibility, costs and deadlines compared to the above operations, is essential at the end of the manufacturing of a product, because the dry form is the major form of transportation and marketing.

Also, to best meet the needs of industrial, the invention relates to a new process for drying of materials sensitive to humidification by a solvent, which has the particular advantage of being able to be put implemented without requiring any energy input, and in particular of fossil energy, and to be usable both in laboratory than within industrial production sites.

This drying process is characterized in that it mainly consists in separating said materials from a surrounding atmosphere in which said solvent is present by hermetically enclosing said materials in an envelope advantageously having a wall zone not adherent to said materials in which said envelope is essentially constituted by a membrane in selectively vapor permeable polymeric material of the humidification solvent for said materials to be removed presence of said solvent in the liquid state, operating under the effect of differential osmotic pressure between two faces of said membrane, by a migration of gaseous molecules of said solvent within said material polymer which is ensured thanks to the mobility of sites vacant intra-molecular polymer material.

In practice, the method according to the invention therefore puts using a semi-permeable membrane that we can say integrates, because continuous, without perforations and not porous, which is nonetheless designed to pass through by individualized molecules as are molecules in the gaseous state unlike molecules not differentiated from non-vaporized liquids. Such membranes are preferably of the type of those which are described in the international patent application filed on same day as this under the priority of same date covered by French patent application 99 10850 in the names of the same inventors.

The process according to the invention therefore involves preferably, according to the content of the latter, various secondary characteristics which concern the polymer composition used to form the semi-permeable membrane. In particular, the membrane, advantageously continuous, without perforations or porosity, is preferably made of a copolymer formed of rigid blocks ensuring the mechanical strength of said membrane and flexible sequences based on an elastomeric oligomer ensuring membrane permeability selectively for gaseous molecules of said solvent by site mobility vacant and with a preferential affinity for molecules of said solvent.

When we indicate above that the semi-permeable membrane does not adhere to the materials to be dried, it is not not to maintain a large space between their respective surfaces. In general, there is a space sufficient natural between materials in the solid state, blocks or powder, and the membrane, and even materials to the pasty or liquid state can be treated by the method of the invention.

Materials sensitive to humidification by a solvent, targeted by the invention, correspond either to materials containing a solvent which must be eliminated for obtain dry and pure materials, i.e. already dry, but can easily regain moisture if they are not separated from the surrounding atmosphere in which the solvent to which they are sensitive is present. It should be noted that the humidification solvent is not necessarily aqueous in nature and that by therefore, the moisture in question may be due to any solvent other than water.

The implementation of the drying process according to the invention is explained below by placing itself in the case of so-called wet materials because moistened with a solvent which they must be rid of. This solvent is assumed be water, although the process is applied without significant modification and using the same examples of membranes with any solvent, even non-aqueous but polar. he It suffices for such a solvent to benefit from the polymer material with the same type of physico-chemical affinity as molecules of water vapor. Now such an affinity is produced by polar connections using Van der Waals forces, as is well known to hydrophilic compounds.

The drying method of the invention is based on a phenomenon approaching osmosis. He exploits the fact inside the envelope containing the materials humid, an osmotic type overpressure is created due to an increase in the partial pressure of the solvent humidification to be eliminated. Under such conditions, thermodynamic laws provide that vapor pressures balance on either side of the wall of the envelope. According to the invention, and thanks to this balancing of pressures, between the interior environment and the exterior environment of the envelope, drying is gradually established by solvent evaporation above the material to be dried at inside the envelope, then by diffusion and transfer vapor of the humidifying solvent through the envelope wall. For products in solutions liquids, drying by the hermetically closed membrane implies that partial evaporation of the solvent takes place also in the immediate vicinity of the internal face of the membrane in contact with said solvent, and as the solvent is present in large quantities, the global kinetics of drying is improved considerably.

It is clear that the gaseous molecules of the solvent humidification diffuses by migration through the thickness of the membrane in a unidirectional direction, only from the inside to the outside of the membrane, until thermodynamic equilibrium is established. he is also clear that the evaporation of the solvent contained in the material contained in the envelope assumes that said solvent to be removed from the material has a voltage of non-negligible saturation vapor at the conditions of outdoor temperature, pressure and relative humidity.

As the solvent evaporates and the gaseous molecules of said solvent diffuse through the envelope, the material becomes depleted in liquid solvent and therefore dry. The osmotic type overpressure is maintained inside the envelope as long as the material is wet. The drying stops when the thermodynamic equilibrium, namely the equality of the partial pressures of the solvent to be removed on either side of the membrane, is reached. Thermodynamically, the activity of the solvent to be eliminated, defined as being equal to the ratio of the partial pressure of the vapor of the solvent considered above the product to be dried to the partial pressure of the vapor of the pure solvent at the same temperature decreases so _that there remains an excess pressure, then it remains constant after the pressure equalization is established.

From what has been said above, it appears therefore that the drying of the hermetically sealed moist material in the envelope is governed by two phenomena thermodynamics which combine their effects, namely of a share the evaporation of the solvent thus extracting from the product to dry, preferably in the vicinity of a wall area away from the wet material, at least when it is the solid state, and on the other hand the diffusion of the vapor of the solvent within the membrane causing its migration to through its thickness. Given the phenomena involved, drying takes place as soon as the kinetics solvent evaporation in the envelope is faster that the kinetics of diffusion across the thickness of the membrane. However, it is desirable that the kinetics diffusion is not too slow for drying to be effective. It goes without saying that the gaseous molecules diffuse the faster the thickness of the membrane cross is weaker. So here we see a way of control the drying rate of a wet material, by modification of the thickness of the membrane layer of the envelope.

As is also apparent from the patent application mentioned above, the semi-permeable membrane used for the drying is preferably transparent, or at least translucent, and transparency to visible radiation, or more generally to solar radiation, is particularly advantageous in most practical applications of the invention, in accordance with secondary features of the invention which involve that the wall comprising the semi-permeable membrane is externally exposed to such radiation. In a way general from this point of view, the invention provides for operating the drying through a semi-permeable membrane which, at least in its active part, is transparent to waves electromagnetic whose wavelengths can in particular vary from 10 nm to 0.1 m, in other words radiation ranging from ultraviolet to microwave.

This will cause heating of the atmosphere containing the materials to be dried, on the side internal membrane, thanks to ray confinement having crossed it.

When the active membrane is tightly closed and exposed to a source of radiation, the temperature inside the container will increase by establishing a confinement phenomenon radiation similar to what is commonly called the greenhouse effect. Electromagnetic radiation incidents lose some of their energy while crossing the membrane, so that the spectrum is changed. Of internal side of the film, there are radiations capable to cross the membrane in the other direction, which are therefore re-emitted to the outside, and radiation that cannot that reflect on the membrane and remain confined to the side inside.

The radiation thus confined causes a significant increase in temperature prevailing in the atmosphere closed by the membrane. In addition, the membrane itself heats up. It is advantageous according to the invention, to add to the composition of the film constituents capable of improve such a confining effect during its implementation film work, by adding different molecules. Thus, according to a preferred embodiment of the membrane object of the invention, the incorporation of particles of silica of small particle size (of the order of a nanometer by example) advantageously between 2% to 20% by weight in the composition of polymers constituting it, allows significantly improve the temperature rise to the interior of the membrane, and thereby the speed of diffusion drying through the membrane increasing the partial pressure of the solvent. (Addition of silica in the polymer is carried out according to conventional methods known to those skilled in the art.)

Under these conditions, the invention provides a means economical to operate drying, using energy natural solar radiation, readily available and not polluting, excluding all energy, and in particular fossil fuel. An additional benefit of the effect of confinement of radiation is linked to the increase in internal membrane temperature, as far as this promotes the mobility of macromolecular chains, and therefore the diffusion of gas molecules through the membrane.

In all cases, the evaporation of the solvent and the migration of solvent molecules in the vapor phase continue as long as the quantity of solvent, by mass per unit of volume, which is present in the atmosphere at inside the membrane, above the product to be dried, remains greater than the amount of solvent material per volume unit present outside the membrane, thus causing the thermodynamic rebalancing which makes migrate molecules across the membrane. We understand well here that the higher the temperature inside the membrane is important, the more the solvent tends to vaporize above the product to dry, and therefore more molecules will migrate quickly through the membrane.

Without wishing to be limiting of the reality of phenomena involved, we can explain how the membrane and its selective vapor permeability, by a exclusive transfer of these vapors through the membrane, directly linked to the inherent structure of the chains macromolecular of the copolymerized material. This transfer of vapors operate by migration of molecules throughout the thickness of the membrane according to an operating mode which can be analyzed in more detail as explained below.

The transfer by migration of gas molecules from solvent is made possible by the existence within of polymeric material and at the microscopic scale of spaces or intra-molecular vacant sites that constitute globally what is called free volume. In one polymer material, in fact, the total volume occupied by the polymer includes a first volume corresponding to the volume what would the polymer occupy if it was condensed and a second volume corresponding to spaces not occupied by the polymer and which is known as free volume.

In accordance with the invention, the capacity is exploited vacant sites to selectively ensure the migration of gas molecules through the thickness of the material polymer, excluding the same solvent in the liquid state, constituting the polymer of the membrane so that from preferably by flexible sequences, it presents a selective chemical affinity with said gas molecules solvent, which are advantageously compact and can thus freely migrate to vacant sites in the material.

Optimal implementation of the process according to the invention therefore requires that the semi-permeable membrane can establish physico-chemical interactions with said molecules, and that it has a quantity of sufficient free volume available, vacant sites being for this purpose sufficiently large and sufficiently mobile to welcome the steam molecules and train them to across the entire thickness of the membrane.

It should be noted that the size of the sites vacancies is advantageously of the order of angstroms (10-10 m), making it only possible to transfer molecules dissociated by migration across the thickness of the membrane which is on the order of a few tens of micrometers. The liquid molecules being non-individualized cannot cross the membrane by through vacant sites.

In the preferred embodiments of the invention, the polymer material is a copolymer formed of rather crystalline rigid sequences, ensuring the mechanical resistance of the material, and flexible sequences rather amorphous elastomer, ensuring the permeability of the solvent vapor material by site mobility vacant. The chemical nature of the flexible sequences is chosen so that said sequences have a selective affinity for gaseous molecules of solvent, without possibility of interaction with others constituents of matter.

In the production of a film with a semi-permeable membrane suitable for enclosing material to be dried by the process according to the invention, the membrane can be combined polymer with a mechanical reinforcement support layer which it is advantageously made integral by adhesion, with or without the use of a layer adhesive intermediary in the film thus formed. This layer support may in particular be made of a textile material or the like made in particular of nonwoven fibers, the nature is chosen so that it does not disturb the capabilities of the membrane. Such a nonwoven is by example based on polypropylene resin fibers or polyester resins.

It plays the role of reinforcement or support layer reinforcing the mechanical strength of the envelope, especially when it is essentially made up of a very mechanically weak membrane due to the mass importance of flexible sequences. He is not excluded from combining with the active membrane in semi-permeability several layers of reinforcement, which for example surround.

A similar backing layer, based on a felt permeable non-woven fibers, can be made with other materials, such as paper. We choose in particular those intended for the packaging of foodstuffs, advantageously having a permeability to water included between 50,000 and 100,000 g / m2 per 24 hours and a grammage between 26 g / m2 and 100 g / m2. Their transparency to the visible light is sufficient to preserve the effect of greenhouse favoring the drying speed.

Depending on the circumstances of each particular application, we may also prefer to choose a sufficiently thick and resistant polymer membrane to represent the only constituent of a monolayer film, in which case we can plan to support the membrane, on the place of use, on a micro-perforated plate by example. It is obviously advisable, each time, to choose a support layer which does not alter the semi-permeable properties of the membrane. For this, it is advantageous that the material constituting said support layer has a openwork structure. It is also advisable to make sure, before to fix the membrane to the support layer, that the composition chemical of the material of said support layer either compatible with that of the membrane so that the adhesion between the membrane and the support layer is facilitated. A multi-layer film suitable for carrying out the process drying according to the invention is advantageously carried out by hot calendering of the appropriate polymer, under control the thickness of the membrane.

Being a membrane based on a copolymer formed of rigid sequences and flexible sequences, the respective proportion of each of the links in the polymer material determines the degree of permeability of the membrane. Given what has been said previously on the feasibility of the migration of gas molecules to across the membrane, we understand that a polymer material intrinsically hydrophilic in nature, i.e. in which the flexible sequences are those of an oligomer hydrophilic and are proportionally functionally majority compared to rigid sequences, authorizes essentially the transfer of water vapor. However, hydrophilicity being a direct consequence of the presence of polar groups, said hydrophilic oligomer can establish also polar-type interactions with a solvent polar organic and therefore the process according to the invention makes it possible to dry materials moistened with water or any other polar organic solvent to analogous behavior, such as methanol, ethanol, acetone which are frequently used in synthesis organic.

Under these conditions, it has been found that polymers of the poly (ether-block-amide) type are particularly suitable for carrying out the process carrying out the drying in accordance with the invention. A poly (ether-block-amide) is a block copolymer, the macromolecular chains of which consist of successive sequences of polyether blocks representing the flexible sequences and of polyamide blocks representing the rigid sequences. Such a copolymer results from the polycondensation of polyamide oligomer chains with ends of dicarboxylic chains with polyether-diol oligomer chains. A particular poly (ether-block-amide) comprises a single type of polyamide block and a single type of polyether block. In particular, in the context of the present invention, polyamide blocks formed solely of polyamide 12, denoted PA-12 (- (CH ₂ ) ₁₁ -CO-NH-), and polyether-diol sequences formed either of polyethylene glycol, denoted PEG (-O- (CH ₂ ) ₂ -), or polytetramethylene glycol, noted PTMG (-O- (CH ₂ ) ₄ -).

By placing itself in the particular case where the membrane is thus based on a poly (ether-blocks-amide), we present below a non-limiting functional explanation of selective permeability properties of the membrane to with respect to water, an aqueous solvent or a solvent organic with similar polar behavior.

The PA-12 blocks of poly (ether-block-amide) are crystalline and rather hydrophobic. Inherently, they have therefore tend to have a rather repulsive effect with regard to polar solvents. On the other hand, they bring the properties mechanical elements necessary for the membrane to resist certain industrial uses for which it is intended. The polyether blocks are rather amorphous rather hydrophilic. They therefore present, within the membrane selectively permeable to molecules in the gas phase, a selective affinity for molecules which lend themselves, by polar attraction, to physico-chemical interactions specific with macromolecular chains. Those are then the polyether blocks which, due to their affinity with polar diffusing molecules, ensure the transfer of water vapor and solvent vapors polar through vacant sites available.

In the specific case of a poly (ether-block-amide), the ethers functions of polyether blocks allow to give the macromolecular chains of the membrane additional rotations around the oxygen atom. This movement of the chains brought by the oxygen atoms of polyether blocks generates a greater amount of volume free available and therefore better permeability of the membrane.

The number-average molecular masses of polyether sequences and PA-12 sequences are chosen from so that they are not too high for the membrane permeability capacities are optimal. Indeed, the amount of free volume available increases when the number-average molecular mass of the blocks polyethers decreases. Number average molecular weight PA-12 sequences is advantageously between 1500 and 5000 and that of the polyether sequences between 650 and 2000.

The proportions of each of the polyamide blocks and polyether are chosen so as to obtain a material inherently hydrophilic in nature. Advantageously, the poly (ether-blocks-amide) used for the implementation of the process comprises from 30 to 60% by weight of polyether blocks and from 70 to 40% by weight of polyamide blocks.

Speaking more generally, we can say here that according to the invention it is preferable that the polymer composition of the membrane material has a majority of flexible oligomer sequences characterized by the hydrophilic affinity sites which they comprise, and for a minority of rigid oligomer sequences, the proper behavior is rather hydrophobic.

It is worth remembering here that the drying kinetics are all the faster the polyether sequences are in large quantity. Furthermore, PTMG, because of these four CH ₂ groups, is less polar than PEG and therefore the drying kinetics of a material moistened with water or a polar solvent is slower when the polyether sequences of the poly (ether-blocks-amide) are based on PTMG. For products that must retain a residual amount of solvent, it is therefore preferable to use PTMG as polyether blocks. However, this is in no way limiting on the various forms of implementation of the method which is the subject of the invention.

Although the membrane which has just been described in example is particularly suitable for drying materials containing water or any polar solvent with similar behavior, it should be emphasized that would not go beyond the scope of the process of the invention if the materials contained a nature wetting solvent apolar, in which case it would obviously be necessary to provide a rather hydrophobic in nature, for example a membrane made of polyolefins, so that it exhibits selective affinity for molecules gaseous of said non-polar solvent.

Whatever the chemical composition of the membrane used for implementing the process according to the invention, said membrane is chosen to be continuous, that is to say devoid of any micro-perforations, therefore impermeable to bacteria and microorganisms. She is possibly sterilized. Sterilization can be performed by usual sterilization techniques such as by autoclaving (120 ° C), by β or γ rays or by ethylene oxide. The use of a membrane beforehand sterilized keeps materials in the middle sterile. The membrane is obviously chemically inert vis-à-vis the solvent to be removed. Its properties also make it possible to resist, for several hours, the heat, at least up to 80 ° C, making it possible the possible introduction of the envelope containing the materials in an oven, the supply of heat by the oven accelerating drying. In this case, cleaning and validation of the ovens is not necessary between two drying.

The thickness of the membrane is chosen so that that the vapor migration is not too slow. She is in practice greater than 2 µm but less than 1000 µm in order to be in the orders of magnitude of a film generally flexible which can be coiled in rolls. Of preferably the thickness of the semi-permeable membrane is between 10 and 100 μm, and advantageously between between 10 and 60 µm.

The membranes used advantageously have a permeability to water vapor of between 7,000 and 20,000 g / m2 / 24 h (according to standard ASTM E 96 B, at 38 ° C and 50% RH), a permeability to oxygen from 6,500 to 18,500 cm ³ / m2 / 24 h / bar and a permeability to carbon dioxide of between 72,000 and 177,000 cm3 / m2 / 24 h / bar. Said membranes have a density of 1.02 to 1.07 g / cm3 and a Shore hardness of 25 to 75 D. The elongation at break is close to 365% and the load at break close to 3.9 daN / mm2.

According to an advantageous characteristic of the process of the invention, the material, once dry, does not resume moisture if kept inside the envelope hermetically sealed. Indeed, as we have exposed above, once the thermodynamic equilibrium is reached, no more vapor transfer can occur through the thickness of the polymer membrane. This one being more impermeable to liquids, the recovery moisture from the product is no longer possible. Through therefore, the dry storage of the material is assured. If you place a dry material, but sensitive to humidity and in particular air humidity, in a hermetically sealed envelope consisting of a membrane having the characteristics described above, said material remains dry. It is therefore clear that the introduction dry matter, but sensitive to humidification, in said envelope constitutes a preventive treatment against humidification.

Advantageously, the envelope containing the materials to be dried is either in the form of a flat sealing film a closed system at the usual places of storage or production, either in the form of hermetically sealed bags particularly suitable for drying products in laboratory. The bags are hermetically sealed by means known in themselves, such as by welding hot or ultrasonic or by adhesiveness. In other case, the envelope as a flat film deployable from of rollers is arranged in a "tunnel" covering plants in the open field. Materials to be dried can be stored there deposited on a waterproof film or not representing the base of the enclosed space inside the membrane. They can also be placed on racks or in crates vegetables which themselves are laid on said film. We cover everything, with or without poles, using the semi-permeable membrane, possibly complexed with a support layer. Sufficient sealing can be achieved on the edges of the membrane that is placed on the ground.

In order to promote the speed and the degree of drying, wet materials to be dried can be placed on a grid, which increases the active surface of the membrane. For the same reasons, the material to be dried is advantageously in the divided state, in the form of crystals or small particles. For solid materials in blocks larger than a few millimeters it's advantageous to grind them before packaging in the envelope.

The process according to the invention is suitable for drying solid products, pasty or powdery products as well as of liquid products. Pasty or powdery products particularly targeted by the invention are the powders pharmaceuticals, solid chemicals and specific such as thixotropic products, material organic from agricultural, animal and vegetable origin. As solid products that can be dried and kept dry by the process of the invention, mention may, for example, be made of mechanical and electronic objects, exhibits legal proceedings, textiles such as clothes or household linen. About the products made up of liquid solutions, drying results in an increase in the concentration of the solution, same way as the products in which the material moist remains mostly solid. Liquids such as blood, fruit juices, wines, milk, or waste liquids such as sewage liquids, can be concentrated using the process of the invention.

According to a variant of the invention, the method is no longer used for solid / liquid separation, but for liquid / liquid separation. It is then about to selectively separate a liquid contained in a liquid mixture formed from at least two liquids in nature different chemical. This separation is based on the same operating principle as solid / liquid separation, namely evaporation of a liquid solvent then diffusion of corresponding vapors through a membrane having a selective affinity with respect to said solvent to be eliminated. One of the liquids in the mixture is compatible with the chemical composition of the membrane, therefore in particular of hydrophilic nature in the case of the preferred example considered here while the other is relatively inconsistent with it, of which hydrophobic in nature under the same conditions. The membrane is selectively permeable to vapor from compatible solvent, with which it establishes chemical interactions facilitating the migration of molecules vaporized through the thickness of the membrane. The solvent said to be incompatible, which does not exhibit interactions with the membrane polymer, is gradually freed from compatible solvent, and it thus concentrates until it becomes pure.

A very first advantage of the invention resides in the fact that the polymer membrane protects the product it contains aggressions from the outside environment. In particular, it prevents the risks of cross-contamination from produce and microorganisms to enter the envelope, which allows very little aseptic drying expensive.

Another advantage of the present invention lies in the fact that the quality of drying of fragile products or difficult to dry is optimal. Indeed, for products sensitive to high temperatures, such as chemicals with a melting point below 40 ° C, proteins or natural products and extracts, the process drying prevents any denaturation.

Another advantage is that the process according to the invention makes it possible to perfect the drying of products, partially dried with another technique, by simple packaging of products in a hermetically sealed bag, left at room temperature and pressure.

Another advantage is that the products, even so-called hygroscopic, dried according to the process of the invention do not absorb moisture, at temperature and ambient pressure, when locked up hermetically in an envelope made of a material inherently hydrophilic in nature.

Another advantage of the invention resides in a elimination of risks of toxicity for operators. In effect, the products to be dried being introduced into a hermetically sealed envelope, operators are no longer in contact with toxic or potentially toxic products toxic.

Another advantage finally of the invention is the possibility of cold drying in a room closed to air purification avoiding pollution of the environment by solvents in exhaust gases to the atmosphere.

The invention will now be described in the context specific examples of implementation and application of the process.

Example 1:

Hygroscopic granules containing 79% water are reduced to powder form by simple grinding. 100 g of this powder are then introduced into two separate envelopes, the total area of which is 962 cm ² . The envelopes used consist of a semi-permeable membrane formed of a polymer material based on a poly (ether-block-amide) composed of 40% by weight of PTMG sequences and 60% by weight of PA- sequences 12. The thickness of the wall of the envelope is 25 µm. Each envelope is heat sealed so as to obtain a bag. As a control, a flat container containing the powder left in the open air is used.

The samples in the bags were placed on shelves at room temperature, on the one hand, and in an oven at 50 ° C, on the other hand.

We studied the drying kinetics, for 70 hours, control product and granules placed in the envelopes at room temperature and 50 ° C. We observe that the weight of the control sample hardly varies not. In contrast, weight loss is significant for the products contained in each of the two envelopes. The evolution of weight loss over time is substantially identical for each of the two envelopes, but the degree of drying is better when the envelope is introduced into the oven.

For each of the two envelopes, the weight of the product hardly changes after 50 hours. At after 70 hours, the weight of the product introduced into the envelope placed in the oven is 20 g while that introduced into the envelope left at room temperature is 35 g.

Example 2:

Low woody plants are harvested in good time cool and rainy. 175 g of the sample of plant, which has been roughly cut, in an envelope hermetically closed. The total surface of the envelope is 1500 cm2. The envelopes used consist of a semi-permeable membrane formed from a polymeric material based a poly (ether-block-amide) composed of 40% by weight of PTMG sequences and 60% by weight of PA-12 sequences. The thickness of the envelope wall is 25 µm. A bag freezer is used as a control.

The envelope and the freezer bag are hanging in a room without mechanical ventilation. Temperature is maintained around 19 ° C and relative humidity at 70%.

We see that fogging forms very quickly on the inside of the sample bag. Brown spots develop from the fourth day on the sample for then invade it. Fermentation odors emanate from the opening of the bag at the end of the test. We regularly weigh the control bag for the duration of the test and we notice that there is no variation in weight.

The sample introduced into the envelope behaves differently. The plant retains its green color, and a characteristic odor of dry plant is released during test. When the envelope is opened, the plant crumbles under the fingers.

The envelope is weighed regularly to assess the drying kinetics. The loss of humidity is noticeable from that the plant is introduced into the envelope. At the end of fourth day, the plant sample weighs 90 g. From from the sixth day the weight loss is much less important. On the tenth day, the sample weighs only 55 g and is completely rid of the moisture it contained.

Example 3:

A chemical with a melting point close to 40 ° C and containing 20% water is introduced into a hermetically sealed envelope. The envelope is consisting of a semi-permeable membrane formed of a polymer material based on poly (ether-block-amide) composed 40% by weight of PTMG sequences and 60% by weight of PA-12 sequences. The thickness of the envelope wall is 25 µm and the total envelope surface is 2184 cm2. The chemical does not establish interactions physico-chemical with poly (ether-blocks-amide). We also introduces the product in a flat container open, for comparison of drying efficiency, in an oven at 40 ° C.

Under these conditions, the product sublimates and the loss observed, at the end of drying, is 15% of the mass total.

In the case of drying of the product placed in the envelope, the product sublimates inside the envelope but its vapors do not pass through the wall in due to chemical incompatibility of the product to be dried with poly (ether-block-amide). Only the molecules of water vapor diffuses through the wall. The yield calculated for this drying phase is close to 99.5%.

Example 4 :

Freshly cut medicinal plants and laid on the ground are covered by a semi-permeable membrane combined with a polyester nonwoven forming a movie. The membrane is based on a poly (ether-blocks-amide) consisting of 50% by weight of PA-12 blocks and 50% by weight of PEG blocks. The thickness of the active membrane is 18 µm. The grammage of the nonwoven is 30 g / m2 and its thickness of 0.13 mm. The water vapor permeability of this film, measured according to the method described in the ASTM standard E 96 B (38 ° C, 50% RH) is 24,000 g / m2 per 24 hours.

Sealing is done on the sides and at ends of the film with earth so that the "tunnel" thus constructed in the open air is hermetically sealed. The covered plants dry in 48 hours without adding energy, and in particular without fossil fuel consumption. Plant quality is comparable to that of samples hot air dried control panels.

Example 5 :

We introduce old clothes in a envelope formed by a semi-permeable membrane complexed with a flexible and micro-perforated polyethylene film having a grammage of 43 g / m2. The semi-permeable membrane is base of a poly (ether-block-amide) consisting of 50% by weight PA-12 blocks and 50% by weight of PEG blocks and its thickness is 25 µm. The water vapor permeability of this complexed membrane, measured according to the method described in ASTM E 96 B (38 ° C, 50% RH), is 200 g / m2 per 24 hours.

Clothes should be maintained and stored with a humidity level of 45 to 55% to avoid the appearance of mold if they are too wet or the alteration of the fibers if they are not sufficiently wet.

We also introduce old clothes into a simple polyethylene cover, presenting a water vapor permeability of 18 g / m2 per 24 hours, to study the influence of the semi-permeable membrane on the preservation of clothing.

You can see when the clothes are protected by the complexed membrane that their appearance remains intact even if the outdoor relative humidity is close to 100%, then that under such conditions, clothing protected by the simple polyethylene cover take up moisture and become moldy, which leads to their gradual deterioration.

Naturally, and as it already results from all that above, the invention is not limited to the conditions or quantities which have been specified in relation to selected examples to illustrate the invention in its modes of implementation preferred.

Claims (11)

1. A method of drying materials sensitive to being humidified bya solvent and/or of keeping said materials in the dry state, characterized in that it consists mainly in separating said materials from a surrounding atmosphere in which said solvent is present, by hermetically sealing said materials in an envelope essentially consisting of a semipermeable membrane made of a polymer material which is selectively permeable to the vapor of the solvent humidifying said materials in the presence of said. solvent in the liquid state, effective under a differential osmotic pressure between the two sides of said membrane, by migration of the gas molecules of said solvent within said polymer material, which migration is due to the mobility of intramolecular vacant sites in the polymer material.
2. The drying method as claimed in claim 1, characterized in that said membrane is continuous, nonporous and made of a copolymer formed from hard blocks providing said membrane with mechanical strength and from soft blocks, based on an elastomer oligomer, providing the membrane with selective permeability towards the gas molecules of said solvent, by the mobility of the vacant sites, and exhibiting preferential affinity for the molecules of said solvent.
3. The drying method as claimed in claim 2, characterized in that said soft blocks have hydrophilic sites which provide said affinity with the gas molecules of said solvent by forming polar bonds and in that said soft blocks are functionally in a predominant proportion compared with the hard blocks, the copolymer being overall hydrophilic in nature so that the membrane is selectively permeable for water vapor or for the vapor of polar solvents exhibiting similar behavior.
4. The drying method as claimed in any one of claims 1 to 3, characterized in that said membrane has, in its active regions, a thickness of 2 to 1,000 µm, preferably between 10 and 100 µm.
5. The drying method as claimed in any one of the preceding claims, characterized in that the polymer membrane is fastened to a mechanically reinforcing support layer advantageously made of nonwoven fibers.
6. The drying method as claimed in any one of the preceding claims, characterized in that said envelope has a wall region which does not adhere to said materials, especially when they are in the solid state.
7. The drying method as claimed in any one of the preceding claims, characterized in that said envelope is transparent, or at least translucent, to radiation to which it is exposed externally, so that the drying rate is improved thereby, especially by establishing a greenhouse effect.
8. The drying method as claimed in any one of the preceding claims, applied to the concentration of solutions in said solvent in the liquid state.
9. The drying method as claimed in any one of the preceding claims, characterized in that the material constituting the polymer membrane is a copolymer of the poly(ether-block-amide) type having polyamide-based hard blocks and polyether-based soft blocks with hydrophilic sites, especially ones based on polyethylene glycol or polytetramethylene glycol.
10. The drying method as claimed in any one of the preceding claims, characterized in that said materials to be dried are either covered with a flat film closing off a hermetically sealed system or hermetically packaged in a bag, the film and the bag essentially being formed from the semipermeable polymer membrane, optionally complexed to at least one support layer.
11. The drying method as claimed in any one of the preceding claims, characterized in that said materials being in the solid state, they are laid on a grid and/or presented in divided form.