MXPA00006870A - Method of producing aerogels and apparatus for carrying out the method - Google Patents

Abstract

The invention relates to a method of producing aerogels and to an apparatus for carrying out the method. The aerogels are produced semi-continuously by per se known methods in that, following gel formation, the lyogel is transferred to a mobile container for at least two further procedural steps, the container comprising inlets and outlets and in that for these procedural steps, the container is moved to appropriate stations for the supply and discharge of materials, at which stations it is connected to feed and discharge means and in that the procedural steps are carried out there. An apparatus for carrying out the method comprises a container with a container top part and container bottom part, an encircling seal and a bag adapted to be placed into the container, the bag being connected along the opening and at least partially to thickening elements, container top and bottom parts being connected along their periphery in gastight fashion to corresponding parts of a retaining device. The container bottom part with the lower part of the holding device, the seal and thecontainer top part with the upper part of the retaining device can be connected in gastight fashion and at least one of the parts of the retaining devices has an opening to receive the bag and at least one of the parts of the retaining device has a plurality of individual projections or an encircling bead so that the minimum distance between the projections or the bead and the other part of the retaining device is greater than the thickness of the thickening elements of the bag and in that the rim of the bag with the thickening elements is held between the projectional bead and the seal.

Description

METHOD TO PRODUCE AIRBRUSHES AND APPARATUS TO CARRY OUT THE METHOD Description The invention relates to a method for produ aerogels and an apparatus for carrying out the method. The liogels are gels that contain a liquid, the dispersing agent. In the particular case in which the liquid of the gel is water, they are also called hydrogels. In the present Application, the term "lyogel" further includes hydrogels. Aerogels in the broad sense that is in the sense of "gel with air as the dispersing agent", are produced by drying a suitable gel. In this sense the term "aerogeles" includes aerogels in the strict sense of the xerogels and cryogels. In this respect, a dry gel is called airgel in the strict sense when the gel liquid is removed at temperatures above the critical temperature and starts with pressures that are higher than the critical pressure. On the other hand, if the liquid of the gel is removed subcritically, for example, with the formation of a vapor-liquid interface, then often the resulting gel is termed xerogel.

Unless the text mentions an explicit qualification, this Application refers to aerogels in the broad sense. For solid substances, aerogels have a very low density and high porosity. Therefore, because of the small size of the pore, aerogels, particularly those with porosities above 60% and densities below 0.6 g / cu.cm, demonstrate an extremely low thermal conductivity and therefore are used as heat-insulating materials such as those described for example in EP-A-0 171 722. However, by virtue of their low density, aerogels also show low mechanical stability, particularly in relation to total loads and abrasion. For industrial applications, aerogels are predominantly used as a granulate. For the application, what is essential is that the granulated airgel to be used should consist of particles of a suitable shape, preferably a spherical shape, and size distribution. The absolutely necessary steps for the production of aerogels are the production of a suitable gel and drying. In principle, a distinction must be made between supercritical and subcritical drying processes, as described above. As a rule, after the actual production of the gel, additional procedural steps are executed prior to drying. For industrial application, aerogels are used almost exclusively in the form of a granulate that is easier to produce and which is easier to use on the other hand. The process steps that continue to actual gel production are then performed on bulk granulated gel or volumes with typical grain diameters of 0.1 mm to 20 mm and which are typically easily permeable to gases and liquids. In the case of supercritical drying, disclosed for example in WO 95/00617 for the production of Si02 aerogels of Si02 hydrogels obtained from water glass and by supercritical drying, at least one change of solvent is very necessary in order to of changing the original gel liquid, for example water in the aforementioned case, by a liquid, for example methanol or C02 suitable for supercritical drying. The supercritical drying itself is then carried out in autoclaves in consideration of the considerable pressures in the supercritical range. Because of the capillary forces that occur in subcritical drying and the reduction involved, not all lyogels are suitable for subcritical drying to produce an airgel. During drying, the gel is considerably reduced if the meniscus of the liquid leaves the interior of the gel in order to, from a certain point, affect the drying process, to go back more or less completely in its initial form again. Consequently, depending on the qualities of the inner surface of the gel, some minimum stability of the gel network is essential, a modification of the inner surface of the gel often being necessary in order to avoid a reaction of the walls of the adjacent pores in the reduced condition and a collapse of the gel that this causes. The corresponding processes where the inner surface of a lyogel Si02 is modified organically and the resulting gel is dried subcritically to produce an airgel, are disclosed for example in US-A-5 565 142, DE-A-434 254 B and in the Application German Patent Not Published 196 48 798. Gases not suitable for subcritical drying disintegrate during subcritical drying, with a loss of their porous structure and therefore no longer demonstrate the favorable properties of aerogels. Therefore, along with the drying step, typically at least one step for chemical modification is needed.

The lyogels for the xerogels that are used in the catalysis area, before drying, are typically subjected to the steps to modify the surface, which means additional procedural steps. The execution of this method on an industrial scale presents problems, also because of the low mechanical stability of the gels. In principle, the simplest are batch processes. In this regard, there are two substantial alternatives: On the one hand, the gel, for different procedural stages, can be transferred to various suitable tools, for example, exchange devices, reaction vessels, autoclaves, etc. The transport of the gels from one tool to the next must be carried out in a safe manner and therefore slow and / or costly since otherwise grain fracture can very easily occur. In addition, large utensils which are favorable from the investment point of view, give rise to relatively long loading times and, consequently, also considerable differences in drying time which can have a substantial effect on the quality of the gel. Another alternative is to conduct all the reaction stages in a utensil that will then certainly need to incorporate quite complicated tubing so that the various liquids and gases can be fed and eliminated. The general drawbacks of the described alternatives lie in the fact that in areas of cost, few and large utensils must be used and yet many utensils have a small maximum size in relation to the quantity that is going to be produced, which results in production costs very tall. On the other hand, large utensils can also cause bad distribution effects and drying times of great variation. For supercritical drying, it is very difficult to imagine anything other than batch processing. In the case of subcritical drying, continuous processes are indeed desirable but considerable problems arise from the typically necessary use of too corrosive chemicals. The materials of many moving parts of for example the bands for surface modification and the mechanism that moves the bands must be resistant to corrosive materials. The construction and cost problems that result from and are caused by the typically long extended drying times are substantial.

Therefore, the aim of the present invention was to find methods to produce aerogels but which are simple and at minimum cost, which allow the production of larger quantities of aerogels. Another object of the invention was to provide a useful apparatus for carrying out the method. The problem is solved by a method for producing aerogels and characterized in that, after gel formation, the lyogel is transferred to a mobile container during at least two additional stages of the process, said container having inputs and outputs and , for these procedural steps, the container is transferred to the corresponding stations for the admission and discharge of substances, stations where they are connected to feed and eliminate elements and in which the steps of the procedure are executed there. It is also possible for a plurality of process steps to be executed in a station. By using a plurality of containers that are connected to the stations one after the other, it is possible, while simultaneously using all the stations, to produce airgel semicontinuously. The method according to the invention offers several advantages over a batch process: because, on the one hand, the gel has to be transferred less frequently between various utensils, the gel receives moderate treatment during the stages of the process in a difficult manner. there is a grain fracture. On the other hand, the complicated pipe mentioned above is avoided. In addition, as the exchange between several stations is carried out relatively fast, smaller containers can be used resulting in more precise drying time distributions and fewer problems in relation to the uniform distribution of solvents and / or liquid reagents. In addition, the method within certain limits can be more easily graduated, since only the size of the containers has to be altered. In comparison with the behavior of a continuous process that requires considerable operating expenses in the design and construction of bands, the method used here has the advantage that the gel and all the reagents are separated from the moving parts, giving as result in substantially simpler utensils and thus lower investment costs. In addition, the containers are sealed in a gas-tight manner so that air waste problems do not arise and leakage difficulties occur with the utensils.

Not all the steps in the process have to be executed in the manner according to the invention. And preferably all the steps of the process are preferably carried out after gel formation and in the manner agreed upon in the invention. Preferably, the stations are arranged one after the other in the same sequence as the process steps, so that the containers only have to travel short distances between the stations. Preferably, the last station is placed directly along the first station and thus the containers do not need to be transported relatively long distances while they are empty. In principle, all methods for producing aerogels are suitable for their execution in the manner according to the invention. Particularly it is useful to apply the method to the production of low density aerogels, that is, a density of less than 0.3 g / cu. Cm since those gels are mechanically sensitive. The preferred methods in this case are those for the production of SiO2 aerogels on a liquid sodium silicate base, as described for supercritical drying, for example in WO95 / 06617 and for subcritical drying for example in US-A -5 565 142, DE-A-43 42 548 and German Patent Application unpublished 192 48 798.

There are particular advantages in methods for subcritical drying because, according to the method relatively many steps of the procedure are necessary in that case. The containers for executing the method according to the invention must be perfectly adapted to the method. For example, in terms of their walls and their entrances and exits, they must be designed to adapt the mechanical loads that arise. This in particular is true for the case of supercritical drying where the containers must in principle be designed as mobile autoclaves. The inner walls of the containers as well as the inlet and outlet connectors must, if necessary, resist the corrosive influences, which can be achieved with conventional methods, for example by means of enamel or a coating of suitable synthetic plastics. As regards the material of the container itself, this can be made of glass, steel or even synthetic plastics. The typical container sizes vary in a range between 0.2 cu.m and 2 cu.m but it is also possible to use larger or smaller containers. Preferably, in the container there is at least one element permeable to gases and liquids but not to gels, said element is provided at least in those places where, during at least one of the stages of the process, the gases or liquids emerge from the gel. Said element prevents the gel particles from leaving the container by means of the flow through the pile. The element can be placed right in the container or connected to it separately or not separately. Preferably, said element are filter trays placed in a convenient place and having a suitable shape. Preferably, the flow through the heat is descending. In this case, the material of the stack or dough is preferably in the filter tray. In a particular embodiment, the element permeable to gases and liquids but not gels consists of a bag in which the gels are filled. Preferably, the bag is in a filter screen or screen, the openings of which are small enough to avoid any strain on the bag through the holes and thereby damaging the bag or gel. Then, on the one hand, the gel can be removed more easily from the container but on the other hand, for cleaning or in the case of blocking, it is not necessary to remove the relatively expensive container from the process. On the other hand, it is sufficient to change the bag, which in comparison is cheaper. The bag should consist of a material that is sufficiently resistant to tearing so that the bag with gel can be handled without any problem and not affected by the solvents and reagents used. The permeability of the bag, at least in the points where the liquid flows in and out, for example in the case of a bag with an opening in the upper part, with flow that goes down to the tray, will be markedly greater than that of the rest of the bag . Suitable materials are synthetic woven or nonwoven plastics of suitable permeability which can be made, for example, of polyethylene, polypropylene, PTFE or PVDF. For the trays, cross-linked metal structures can also be used. The bags can also be used to transport the finished airgel to the customer without the need for it to be transferred, so that it can be delivered from the production to the customer in a particularly safe manner, with a minimum of abrasion. For greater ease of handling, the bag is partially connected along its opening to the thickening elements which are preferably flexible, i.e. metal cables, rods or tubes of synthetic plastic which are sewn or welded. It is particularly preferred that the thickening element extends along the entire periphery of the opening. It is useful that the thickening element also has spring properties since in this way the bag can be kept additional and automatically open in the opening. Preferably, the element is then contained in a tube formed in the upper end of the bag because in this way the mechanical stresses can be evenly distributed and the formation of folds is prevented. Preferably, the element is formed by a metal wire coated and sprung which is formed in the ring at its ends, for example by a tube that is pushed on it. The coating is to resist corrosion and can be made of for example PTFE tubes. The problem of providing a useful apparatus for carrying out the method is solved by an apparatus comprising a container with an upper part of the container and a lower part of the container, a surrounding seal and, adapted to be placed in the container, a bag which is connected at least to the thickening elements along the opening, and which further comprises a holding device for the bag. The upper and lower parts of the bag are connected along their periphery in a gas-tight manner to the corresponding parts of the retaining device, so that the lower part of the container can be connected to the lower part of the retaining device, the seal and the upper part of the container can be connected to the upper part of the retention device in a gas-tight manner At least one of the parts of the retention devices has an opening for receiving the bag and at least one part of the retaining device comprises a plurality of individual projections or a surrounding reinforcing projection such that the minimum distance between the projections or the reinforcing projection and the other part of the retaining device is greater than the thickness of the wall of the bag and the edge of the bag, together with the thickening elements, are held between the projections or reinforcement projection and the seal. It is the fact that when the container is in use, the reinforcement between the projection and the seal is maintained so that the bag is placed in the container without holding devices. it can not slide into the container, for example by violent flow conditions or the like. In order to ensure reliable sealing of the apparatus, the seal must, even in the compressed state, be thicker than the height of the reinforcement. Therefore, the minimum clearance between the projections and the other part of the retention device must be less than the thickness of the reinforcement. So that the bag is not fixed in the stirrup by the projection, which could result in mechanical loads of a depression, and in that way the seal can also be pressed with sufficient firmness, that distance must be greater than the thickness of the wall of the bag. The seals used are normal or together adapted to the conditions of the process. Preferably, the lower part of the retaining device is an intermediate ring and the upper part of the retaining device is an intermediate ring with a plurality of projections or a reinforcing projection along the inner opening. If the opening in the container is not circular, then here and hereinafter the term "intermediate ring" should be understood as the obvious analogous term adapted to the shape of the container opening. As a result of this arrangement, the bag can easily be placed on the bottom of the container. If the intention is also to place a screen or grid on the bag, then the simplest way to produce the holding device is that the lower part of the holding device is an intermediate ring while the upper part of the holding device is a ring intermediate with a sieve placed on it, supplying the projection on the screen. As an example, a preferred embodiment of the method according to the invention for producing liquid sodium silicate S102 aerogels by subcritical drying is described below with reference to the drawing. The drawing shows a preferred embodiment of the retention device for a bag with thickening elements according to the invention. Relatively large amounts were produced by the method according to Example 7 disclosed in the previously published German Patent Application 196 48 798. Three stations are constructed. The first station consists of a spray tower in which the liquid sodium silicate is sprayed together with the specific amount of hydrochloric acid. According to the design of the mixing nozzle, to give as a result particles of approx. 1 mm in diameter which have a spherical shape and which precipitate in a vapor atmosphere. The particles are extracted at the lower end of the spray tower as an aqueous suspension and delivered to a filtration band where they are washed by rinsing with electrolytic water to remove the salt. The band carries the airgel particles directly to the reaction vessel according to the invention and placed under it, the container, once full, is closed with a lid that is placed on top. A flexible bag has been previously placed in the container and the airgel remains in it. The bag consists of an ECTFE fabric on the bottom and a felt PTFE sewn less permeable in the walls and forming a tube in the opening where there is a PTFE tube with a center of elastic wire that closes in a ring at its ends by a small tube of tantalum. Placed in a lower part (1) of the enamelled steel vessel with a removal joint is a fiberglass reinforced PTFE screen or grid which is electrically conductive to avoid the risk of explosion and which is supported by a total of four parallel meshes. An enamelled steel lid (2) with a stirring connection serves as the upper part of the container. A lower intermediate ring and an upper intermediate ring (3, 4) of steel with a PTFE coating are gas-tightly connected to the bottom of the container (1) or upper part of the container (2) by screws with round head or screw ordinary. The upper intermediate ring (4) also carries an upper support grid (5). The support grid consisting of PTFE rods is joined by clamps under the intermediate ring by a reinforcing ring (6). Between the reinforcement ring (6) and the support grid (5) an additional filter cloth (7) made of ECTFE fabric or of high permeability (150μm mesh) is fixed, which is fixed by means of screws and PTFE washers . Between the upper and lower intermediate rings is a seal (8). The bars of the upper support grid have projections (9) extending to the sealing space so that the PTFE wall (10) of the bag and thereby the bag rests between the seal (8) and the projection ( 9) by means of a tube formed from the wall of the bag and housing the elastic wire coated with PTFE, thus preventing the bag from breaking. The projections are beveled in such a way that they offer enough space for changes in length due to the expansion of the heat. The reaction vessel is then transported to the second station where it is connected to the appropriate tubes. In principle, the gel is acidulated in this season in which the hydrochloric acid passes through it. Then, a mixture of hexamethyl disiloxane and trimethyl chlorosilane is pumped through the airgel so that on the one hand the surface of the airgel is water repellent and on the other hand the trimethyl chlorosilane and water react to produce disiloxane of hexamethyl (HMDSO) and water. The reaction is carried out in forced circulation, continuously decanting the hydrochloric acid that emanates. In the third and final station to which the container is transported, the gel wet with water and HMDSO, which continues to be very acidic, is dried subcritically. For this, the nitrogen at temperatures of approximately 180 ° C passes down through the material of mass. Nitrogen is driven in a cycle, accompanied by moisture that condenses. Then, the container is opened and the bag is lifted up as a complete unit by means of sewn handles or tubes. This composite structure can then be stored separately or transported to the client for further processing. The mobile container that is enameled on the inside to resist corrosion, does not need to be cleaned, since the bag was placed on it, so that it can be used again immediately for the next batch.

Claims (14)

1. Claims: 1. A method for producing aerogels, characterized in that, after formation of the gel, the lyogel is transferred, by at least two additional steps of the process, to a mobile container comprising inputs and outputs and in which In these process steps, the container is moved to the appropriate stations for feeding and discarding substances, stations where they are connected to feed and discard elements, and where the process steps are performed there.
2. 2. A method according to claim 1, characterized in that, in the places where the gas or the liquids of the gel emerge during at least one of the stages of the process, there are elements permeable to gas and liquids but not to the gels.
3. 3. A method according to claim 1 or 2, characterized in that the element permeable to gases and liquids but not gels is a filtration tray.
4. 4. A method according to claim 1 or 2, characterized in that the element permeable to gases and liquids but not gels is a bag.
5. 5. A method according to one of claims 1 to 4, characterized in that, the flow through the gel is descending.
6. An apparatus for carrying out the method according to one of the preceding claims, comprising a container with an upper part of the container (2) and a lower part of the container (1), a surrounding seal (8) and a bag (10, 11) that can be placed in the container, characterized in that the bag (10) is at least partially connected to the thickening elements (11) along the opening and in which the upper parts and bottom of the container (2, 1) are, along its periphery, connected in a gas-tight manner to the corresponding parts of the retention device (3, 4, 5, 6, 7, 9) and in which the part bottom of the container (1) can be connected in a gas-tight manner to the bottom of the retention device (3), the seal (8) and the upper part of the container (2) can be hermetically connected to the gas at the top of the holding device (4, 5, 6, 7) and in which at least one of the parts of the retention devices (3, 4, 5, 6, 7) comprises an opening for receiving the bag (10) and at least one of the parts of the retention device (3, 4, 5, 6, 7) it contains a plurality of individual projections (9) or a surrounding reinforcement projection so that the minimum distance between the projections (9) for the reinforcing projection of the other part of the retaining device is greater than the thickness of the thickening elements ( 11) of the bag (10, 11) and the ring of the bag together with the thickening elements (11) is supported between the projection (9) or reinforcement projection and the seal (8).
7. An apparatus according to claim 6, characterized in that the lower part of the retention device is an intermediate ring (3) and the upper part of the retention device is an intermediate ring (4) with a plurality of projections (9) or a reinforcing projection along the interior opening.
8. 8. An apparatus according to claim 6, characterized in that the lower part of the retention device is an intermediate ring (3) and the upper part of the retention device is an intermediate ring (4) with a screen (5, 6). ) supplied in this intermediate ring and in which the projections (9) are supplied in this screen.
9. 9. An apparatus according to one of claims 6 to 8, characterized in that the thickening element (11) is flexible.
10. An apparatus according to one of claims 6 to 9, characterized in that the thickening element (11) extends along the entire periphery of the opening.
11. 11. An apparatus according to one of claims 6 to 10, characterized in that the thickening element (11) has molding properties.
12. 12. An apparatus according to one of claims 6 to 11, characterized in that the thickening element (11) is placed in a tube formed in the upper end of the bag.
13. An apparatus according to one of claims 6 to 12, characterized in that the thickening element (11) contains an elastic metal wire connected to a ring.
14. 14. An apparatus according to claim 13, characterized in that the metal wire is coated with a material that is inert with respect to the chemical substances that are used.