EP1062181B1 - Method for drying and producing microporous particles - Google Patents

Abstract

In a process for drying microporous, fluid-containing particles, the heat required for increasing the temperature is supplied by convection by reducing the interfacial tension of the fluid, preferably to 0 to 1/10, in particular to 0 to 1/20, of the interfacial tension of the fluid at room temperature, by appropriately increasing the temperature at from close to the critical pressure to supercritical pressure of the fluid. Furthermore, microporous, three-dimensionally networked particles are prepared by a process in which the drying process is used. In addition, an apparatus is used for carrying out the drying process, the apparatus comprising a pressure container having an inner container and pressure-withstanding outer container and suitable measuring and control apparatuses and pump apparatuses and heat exchangers, the inner container being provided for holding the particles to be dried and a gap being provided between the inner container and the outer container.

Description

The present invention relates to a method for drying microporous, Fluid containing particles and a method of making microporous, spatially crosslinked particles using this drying process.

It is known to use hydrogels e.g. Silicic acid hydrogels caused by gel precipitation can be made from water glass under supercritical conditions too to dry microporous, spatially cross-linked silica particles. In the supercritical drying the interfacial tension in the microporous, spatially crosslinked particles containing fluid completely or largely canceled with the aim of shrinking the microporous, spatially cross-linked particles Avoid to a large extent when drying, as characteristic when shrinking Properties of the microporous, spatially cross-linked particles in whole or in part get lost. Such a product obtained by supercritical drying is used in Called gel airgel. In contrast to conventional drying without special ones Arrangement in which the gels suffer a large volume contraction and xerogels arise, so there is only one when drying near the critical point low (<15%) volume contraction instead.

The state of the art for the production of aerogels by means of supercritical drying is described in detail e.g. in Reviews in Chemical Engineering, Volume 5, No. 1-4, S. 157-198 (1988), which also describes the pioneering work by Kistler.

In the known processes for the production of airgel, the heat required for bypassing the two-phase region of the fluid contained in the pores of the particles to be dried is conductively supplied through the container wall (cf. Reviews in Chemical Engineering, Volume 5, No. 1 to 4 (1988); Ind. Eng. Chem. Res. 1991, 30, 126-129; and Journal of Materials Science 29 (1994), 943-948). As is known, the wall / volume ratio becomes less favorable with increasing container volume, which increases the batch times when the scale is enlarged. Furthermore, the thickness of the pressure vessel wall increases with the vessel diameter. When heat is supplied from the outside into a thick-walled, pressurized container, thermal stresses in the container wall limit the temperature difference between the inside and outside of the pressure container, so that the specific heat input (watt / m ³ ) in the pressure container is also reduced.

WO-A-95 06 617 relates to hydrophobic silica aerogels obtained by Reaction of a water glass solution with an acid at a pH of 7.5 to 11, extensive removal of ionic components from the hydrogel formed Wash with water or dilute aqueous solutions of inorganic bases under Maintaining the pH of the hydrogel in the range of 7.5 to 11, displacing the in the hydrogel contained aqueous phase by an alcohol and subsequent supercritical drying of the alkogel obtained.

One method of producing silica airgel on a pilot scale is by White described in Industrial and Engineering Chemistry, Volume 31 (1939), No. 7, pp. 827-831, and in Trans. A.J. Chem. E. (1942), pp. 435-447. The process includes the following steps: production and aging of silica hydrogel, comminution the hydrogel to granules, separation of salt from the gel formed, exchange of the water in the gel by alcohol, entry of the drained gel into one Pressure vessel, heating up the pressure vessel, lowering the pressure Atmospheric pressure, evacuation of the pressure vessel and subsequent removal of the Airgel. A disadvantage of this method is that all steps are discontinuous be carried out and are therefore very time, personnel and cost intensive. White does not mention continuous processes for granule production and desalination. When exchanging water / alcohol, White prefers one with "overlaying / soaking / drainage" for the liquid phase procedure to be described which is an intermittent Represents the loading of the solid body with liquid. One in time White considers even flow to be less economical.

According to US-A-3,672,833, the known methods for the desalination of gels and to replace water with other solvents extremely lengthy and therefore costly processes. To avoid this, the US patent describes the Gel preparation from lower alkyl orthosilicates proposed. However, these require a lot of energy in their manufacture.

The object of the present invention was to provide an improved More economical process for drying microporous, fluid-containing Particles as well as a improved, more economical process for the production of microporous, spatially to provide crosslinked particles using the drying process, the above-mentioned disadvantages of the prior art can be avoided.

Surprisingly, it was found that this problem can be solved by those for heating to temperatures that are at least close to the critical Temperature of the fluid are, the required heat is supplied by convection. Furthermore was found that this measure was carried out particularly advantageously in one apparatus can be in which a pressure vessel, an inner vessel and a pressure-bearing Has outer container, wherein a gap is provided between the inner and outer containers is, and the device suitable measuring and control devices as well Has pumps and heat exchange devices. Furthermore, it was found that a Particularly economical production of microporous, spatially cross-linked particles is possible if in addition to using the drying process mentioned above any necessary washing and / or desalting or fluid exchange in the pores of the microporous particles and any separation that may be required sorbed gases or substances are carried out in countercurrent in the moving bed become.

The invention thus relates to a method for drying microporous fluid containing particles by reducing the interfacial tension of the fluid, preferably to 0 to 1/10, in particular to 0 to 1/20, at room temperature existing interfacial tension of the fluid by using near to supercritical Pressure of the fluid increases the temperature accordingly. The invention The process is then characterized in that one for increasing the temperature supplies the required heat convectively.

To do this drying process preferably a device is used which with a pressure vessel an inner container and a pressure-bearing outer container as well as suitable measuring and Has control devices as well as pump and heat exchange devices, the inner container is intended to hold the particles to be dried, and a gap or space between the inner container and outer container is provided.

The area in which work is preferably carried out according to the invention can thereby be defined that the microporous particles their properties during drying not lose; this means that e.g. the apparent density of the product is not significantly increased that the thermal conductivity of the product does not increase significantly, that preferably no shrinkage of over 15%, especially no shrinkage of over 10% occurs. This fact can also be described in that the Airgel must not become a xerogel (gel dried at normal pressure)

The interfacial tension mentioned above is determined as described in "The Properties of Gases and Liquids "by Reid, Brausnitz, Sherwood, McGraw Hill, 1977, pp. 601 ff. is described, the interfacial tension at the temperature to be tested (and pressure) with the at room temperature and atmospheric pressure below im other identical conditions are measured and compared.

(a) Herstellen von mikroporösen, Porenflüssigkeit oder Fluid enthaltenden Teilchen,

(b) gegebenenfalls Waschen und/oder Entsalzen der in Stufe (a) erhaltenen, Porenflüssigkeit enthaltenden Teilchen mittels eines Lösungsmittels und/oder Wasser,

(c) gegebenenfalls teilweises oder vollständiges Austauschen der Porenflüssigkeit oder des Lösungsmittels oder des Wassers in den Teilchen durch ein Fluid unter Erhalt von mikroporösen, Fluid enthaltenden Teilchen,

(d) Trocknen der mikroporösen, Fluid enthaltenden Teilchen und

(e) gegebenenfalls Abtrennen sorbierter Gase und/oder Stoffe von den getrockneten Teilchen aus Stufe (d),

In a further embodiment, the invention relates to a method for producing microporous, spatially cross-linked particles by

(a) producing microporous particles containing pore liquid or fluid,

(b) optionally washing and / or desalting the particles containing pore liquid obtained in step (a) using a solvent and / or water,

(c) optionally, partially or completely replacing the pore liquid or the solvent or the water in the particles with a fluid to obtain microporous, fluid-containing particles,

(d) drying the microporous, fluid containing particles and

(e) optionally separating sorbed gases and / or substances from the dried particles from step (d),

The inventive method is then characterized in that the Carry out drying as described above and that steps (b), (c) and (e), provided that they are carried out in countercurrent in the moving bed by in step (b) the particles obtained from step (a) a solvent stream and / or Water flow counteracts, in step (c) the particles countercurrent to the fluid leads and leads in step (e) the dried particles towards an inert gas stream. Preferred embodiments of the invention are as follows Description, the dependent claims, the figure and the example described.

The only figure in the accompanying drawing shows schematically a device that is suitable for carrying out the drying process according to the invention.

The microporous, fluid-containing particles which are suitable for drying according to the invention are not subject to any particular restrictions per se. All particles, solids, structures or granules are suitable which are at least partially, preferably entirely, microporous and contain a fluid in the pores. Suitable particles are, for example, gels which consist of inorganic or organic materials or of polymer material, for example of inorganic oxides or hydroxides such as boron or silica, oxides or hydroxides of the metals titanium, molybdenum, tungsten, iron or tin, aluminum oxide or organic gels such as agar-agar , Gelatin or albumin. The process according to the invention is particularly suitable for drying silica gels. Gels can be used which contain compounds with a critical temperature lower than 350 ° C. or mixtures or mixtures thereof, preferably water and / or liquid organic compounds, as a fluid. Suitable as fluid include all compounds that will be mentioned later in the description of the drying fluids. Particularly suitable fluids are water, C ₁ -C ₆ -alkanols or mixtures thereof, methanol, ethanol, n- and isopropanol being preferred. Most preferred is isopropanol. Depending on the fluid present in the pores, one speaks, for example, of hydrogels and alkogels. The method according to the invention is used most frequently in the drying of silica gels which contain water, the abovementioned liquid organic compounds or mixtures thereof as a fluid.

In a preferred embodiment of the invention, the microporous, Fluid containing particles 50 to 97 wt .-%, in particular 80 to 90 wt .-% fluid based on the total weight of the particles under standard conditions (pressure of 1 bar, temperature of 25 ° C). The particle diameters range from 1 to 15 mm, in particular 2 to 6 mm. The particles contain macro, meso and / or Micropores. The microporous particles to be dried can have any shape own, e.g. Pearls (balls) or angular shapes. The drying process according to the invention is also suitable for drying microporous, fluid-containing Particles or structures that have a certain regular arrangement of building blocks can have. Suitable particles, for example, are also present structures crystallized from thermally degradable templates, nanostructures, their regular arrangement is self organized or nanocomposites as well their precursors or clathrates. In addition, it can be the microporous particles also around a microporous cover layer provided with a certain doping act on a non-porous support. Catalysts or are also suitable Compounds created by impregnation or modification of chemically reactive centers received or impregnated or modified during drying. Prefers Aerogels are formed after drying. Do not contain the particles to be dried for drying suitable fluid according to the invention, this can be before drying be replaced by a suitable fluid or a more suitable fluid. So According to the invention, some microporous particles can be dried using water as the fluid become. However, if you want the high critical for water as drying fluid Circumventing temperatures and pressures, either a water-miscible one Drying fluid (miscible at least under the drying conditions), e.g. on Alcohol, can be used, or you can change the water contained in the hydrogel against a fluid more suitable for drying, e.g. an alcohol, whole or partially out. Exchange and drying can also be done at the same time become.

The convective heat supply according to the method according to the invention can be seen in different ways and is not subject to any particular restriction. As Convection medium or current are suitable for all substances that are not decomposed in the can be brought to a supercritical state. These are preferably opposite the particles to be dried are inert. In addition, the convection current from one Certain temperature substances are also added to the structure to be dried chemically modify, impregnate or e.g. Remove traces of water. A Modification may be desirable if, e.g. the interfacial tension can be reduced.

Expediently, drying is used for drying as a convection stream or drying medium, the critical data of which are not too high in order to avoid a greater outlay on equipment. Suitable drying fluids are ammonia, sulfur dioxide, nitrogen dioxide, sulfur hexafluoride; Alkanes such as propane, butane, pentane, hexane and cyclohexane; Alkenes such as C ₁ -C ₇ -n-, iso-, neo-, secondary or tertiary alkenes, for example ethene or propene; Alkanols such as methanol, ethanol or n- or isopropanol or butanols; Ethers such as dimethyl, diethyl ether or tetrahydrofuran; Aldehydes such as formaldehyde or acetaldehyde; Ketones such as acetone; Esters such as the methyl, ethyl, n- or i-propyl esters of formic, acetic or propionic acid; Amines such as mono-, di- and tri-methyl- or -ethyl- or n- or i-propylamine or mixed alkylated amines thereof; as well as mixtures of two or more of these fluids. Of the organic compounds mentioned, C ₁ -C ₆ -alkanols, ethers, ketones, aldehydes, alkanes, alkenes, esters or amines are preferred. Most preferred are C ₁ -C ₃ alkanols, especially isopropanol. In principle, halogenated hydrocarbons are also possible, but these will be avoided for reasons of material selection and environmental protection requirements. Media with high critical temperatures or high pressures, such as water, will also be attempted to be avoided. In addition to the drying fluids mentioned, supercritical carbon dioxide is also suitable as the drying fluid. Because of its favorable critical temperature of 31 ° C, this is particularly suitable for thermally sensitive substances.

In general, the selection of drying fluid depends on various points. If you want to set "close" critical conditions, this determines, among other things thermal stability of the particles to be dried or the end product the selection of the drying fluid and thus also limits the critical temperature of the Drying fluid. In addition, a possible fluid recovery, the toxicological harmlessness, the miscibility with the fluid in the to drying particles, product properties and safety data at the Selection of drying fluid play a role. There is also the option of Drying fluid to add a component that contains functional groups to the implemented, absorbed or adsorbed the surface of the particles to be dried become. This means that a uniform coating can be Coating or impregnation of the particles to be dried can be achieved. A modified application of the drying fluid is e.g. the addition of ammonia too Isopropanol as drying fluid, e.g. to be able to dry acidic hydrogels without that isopropanol decomposes. With methanol as the drying fluid, the addition causes of ammonia, that an undesirable amount of ether is not formed. For example, in If methanol is used as the drying fluid for isopropanol or isobutanol Hydrophobization of a silica gel can be added. In general, for suitable chemical or physical modification of the particles to be dried Components added before or when the critical temperature of the fluid is reached become.

It is sufficient if the drying fluid with that in the particles to be dried contained fluid at least under the conditions present during drying is miscible. However, the same is advantageously used as the drying fluid like fluid contained in the microporous particles. Examples of among the Drying conditions are completely miscible fluids / drying fluids Mixture of water with higher alcohols or aromatics.

The convection current can top the bed of the particles to be dried below, from bottom to top or from an axial distributor to the outside or flow through in reverse. The mechanical stability, the elasticity, the grain size distribution and the mean grain diameter of the particles determine the type of Flow through the bed. Any fine-particle material that may form carried or separated in the fluid circuit. The filling can be at a Inflow from below can be fluidized in whole or in part. The convection current can be circulated using a temperature-resistant pump, or in a straight-forward mode, only "fresh" drying fluid is heated to temperature brought.

In a preferred embodiment of the invention, the drying is carried out in this way performed that you first depressurize the convection medium in the drying room and then the particles to be dried, which are preferably heated are flushed in without pressure. Then the pressure in the drying room is on the desired value in the vicinity of the critical point. Then the Convection medium preferably set a graft flow. Then will the temperature rises to near the critical point. After reaching the near critical to supercritical conditions of the fluid is relaxed, causing the Particles are "dried". The convection medium can be circulated become.

White (Industrial and Engineering Chemistry, Vol. 31 (1939) No. 7, pp. 827 to 831; Trans.A.I.Chem.E. (1942), pages 435 to 447) suggests this in a batch process Drain the liquid in the void volume before starting the drying step. This proposal can with the convective heat input according to the invention be combined.

The interfacial tension of the contained in the pores of the particles to be dried Fluids can also be reduced by adding surface-active substances or a previous modification of the microporous, fluid-containing Particles by e.g. Silanization, organic esterification or etherification or at Silica gels by siloxanization of vicinal silane mono / di / triols of the inner and outer surface.

In a further embodiment, the invention relates to a method for manufacturing of microporous, spatially cross-linked particles through those defined above Levels (a) to (e).

The production of microporous particles containing pore liquid can be carried out according to Processes known to the person skilled in the art take place continuously.

A washing step for the particles obtained in step (a) can take place if undesirable components, such as unreacted starting material or impurities in the Educts to be removed. For this purpose, the particles from step (a) as A moving bed, preferably a water-miscible solvent being led. A desalination step (b) of the microporous, pore liquid or Particles containing solvent can be before, after or simultaneously with the Wash or be provided alone (without washing) if the particles contain unwanted salts. If such a step is used, it will carried out continuously by the particles obtained from step (a) or particles obtained after washing as a moving bed a stream of water contrary results. A suitable ratio or a suitable setting of the material flows of particles to be dried and water or solvents for the production and maintenance of the moving bed can be done by a specialist within the scope of the usual practice Attempts to be determined. This setting depends, among other things. from the height of the moving bed, the internal mass transfer in the particles to be dried and the vortex point, i.e. on the density and grain size or grain size distribution of the microporous, too drying particles. The water flow or solvent flow becomes preferably set so that there is no fluidization in the moving bed and thus too no unwanted segregation comes. The backmixing on the water side or solvent side is lowest when you are at a water or solvent flow rate near the loosening point of the moving bed is working. All are suitable as entry and exit devices for the particles to be dried Types of pumps that are suitable for conveying granular material, whereby modified concrete pumps have proven particularly successful.

Surprisingly, it was found that even with an unstable density layer on the fluid side, the moving bed method without problems when washing and / or Desalting can be applied, i.e. that a procedure can be applied can, in which the microporous particles without problems from above hike down. To maintain the unstable density stratification, the Density difference smeared to a sufficient walking bed length and one Minimum relative speed set. Furthermore, it was surprising that then in comparison to a fixed bed exchange operated in batches acceptable specific displacement component requirement is met. Farther it was surprising that when desalting in a moving bed in countercurrent very cheap Requirement relationships (i.e. required fresh water volume to a certain Volume of desalted hydrogel to be obtained). This was all the more More surprising than the literature, as mentioned above, the desalting step as very was shown complex and tedious, which is why in US-A-3,672,833 Hydrolysis of lower alkyl orthosilicates proposed for the production of silica aerogels has been.

All desired degrees of washout and desalination can be set. The washing step and / or desalination step are accelerated by increasing the temperature, i.e. the higher the temperature, the faster they run off. Preferably they are therefore carried out at elevated temperature, the upper limit for the Temperature etc. through the decomposition of the particles to be washed or desalinated, whose clumping / tendency to stick, dissolve in the fluid, etc. is specified. For example, you can desalt some silica gels at around 80 ° C. For improvement the cross-mixing can also pulsate the solvent or water flow be provided. Furthermore, by bubbling in gas, e.g. Air that Hiking layer to be loosened. Silica gel is preferably added in step (b) Desalination of aging.

In step (c), the pore liquid contained in the particles is partially or completely, especially 97 to 99%, replaced by a fluid. suitable Fluids are those described above in describing the microporous, fluid-containing Particles described fluids. Analogous to desalination favor increased Temperatures the exchange. With regard to the suitable temperature, this therefore applies What was said above under stage (b). This also applies to the setting of the moving bed What was said above under stage (b). Everyone can also do the exchange step desired degree of exchange can be set. Such an exchange of the pore fluid can of course be omitted if the particles obtained in step (a) or (b) already contain a suitable fluid. There is also the possibility that in step (c) the pore fluid in the particles first through one with the pore fluid miscible liquid, but not exchanged fluid suitable for drying becomes. In this case the liquid becomes miscible with the pore liquid then replaced by a fluid suitable for drying. In stage (c) there is also the possibility of material flows of different purities at different heights feed. Furthermore, a combination of the exchange step with a Separation of fines or e.g. of adhering oil from the gelation possible and can save a separate classification step if necessary. It can also Summary of desalination in stage (b) and exchange in stage (c) in one Apparatus can be advantageous with appropriate kinetic conditions. Disturb in the exchanged particles traces of the original pore fluid, you can find them in a separate walking bed under special conditions, e.g. through an implementation, remove. By adding suitable components to the foot of the replacement moving bed this is also possible, a combination with an impregnation is also possible of the microporous particles possible.

In step (d) the microporous, fluid-containing particles are dried. The Drying is carried out by means of convective heat, as described above for the drying process according to the invention is described.

In step (e), which may be carried out, the dried particles of absorptively and / or adsorptively bound gases and / or substances separated or exempted. This step is carried out continuously in the moving bed in countercurrent, whereby the dried particles, preferably at reduced pressure, are directed towards an inert gas stream become. Suitable inert gases are nitrogen, carbon dioxide or noble gases. Air or flue gas can also be used under certain circumstances. Regarding the The setting of the moving bed applies analogously to what was said above under stage (b). It there is also the possibility of adding a component to the inert gas phase which with the dried particles react or is absorbed or adsorbed. The separation step can possibly by a displacement adsorption with a stronger adsorbent material can be improved. In some cases, the removal of the absorptive and / or adsorptively bound substances and / or gases by application alone done by vacuum.

Stage (e) can be followed by a continuous final assembly step, in which the microporous, spatially cross-linked particles into the desired shape brought, e.g. by grinding, sieving or mixing with for use suitable additives. There is also the possibility of using the particles obtained to provide a hard shell, e.g. by means of sintering to increase their mechanical strength increase.

The microporous, spatially cross-linked particles obtained are same particles as those above in the invention Drying processes have been described, these particles compared to the above-mentioned are additionally freed from undesirable by-substances.

The microporous particles obtainable by means of the method according to the invention can be used in many technical fields. Among other things, they are suitable for the production of transparent or opaque thermal insulation materials (under certain circumstances as a substitute for fluorine-chlorine-hydrocarbon-containing materials). You will also find use as catalysts and catalyst supports, Adsorbent, obtained by coking microporous polymers Carbon aerogels as electrodes (e.g. soaked in electrolyte in capacitive energy stores), Membranes, Cerenkov detectors, super light sponges Storage / storage or as a gelling / thickening / thixotropic agent liquid fuels for space travel, as insecticides, sinterable precursors for Ceramics or high-purity light guides, piezoceramic transducers in ultrasonic transmitters, in acoustic antireflection layers, as dielectrics, as a carrier for Fluorescent dyes, as matting agents, as additives in lubricants, rubber and sealants, in composite materials and in paints and sheets.

A preferably used device for drying microporous fluid containing particles comprises at least one "two-shell" container Inner container and pressure-bearing outer container as well as suitable measuring and Control devices as well as pump and heat exchange devices. The inner container is for holding the particles to be dried provided or determined, and there is a gap between the inner and outer containers or space provided. The inner container can have any shape a rotationally symmetrical shape is preferred, e.g. a cylinder with conical Spout or a ball so that the gap can be rotationally symmetrical. The inner container can be above and / or below be conical. It can be made from any material that comes with the drying temperature to be set still have the required strength. Stainless steel, boiler plate or glass fiber reinforced plastic Am are preferred stainless steel is most preferred. The inner container is preferably thin-walled, the inner container is preferably designed for pressures of less than 6 bar. The outer container consists of materials that have the compressive strength required for drying have. Fine comb steel or heat-resistant steel is preferred. The gap or The space between the inner and outer container provides thermal insulation. It is conveniently with an inert gas, preferably a bad one thermally conductive gas, such as nitrogen or krypton. To improve the Insulation can also be filled with insulation material (e.g. rock or glass wool) become.

The figure describes a device with inner and outer container and suitable measuring and control devices and pumps and heat exchangers, which is particularly suitable for carrying out the drying process according to the invention. The actual dryer consists of the thin-walled inner container 1 and the pressure-bearing outer container 2. The method according to the invention is carried out as follows. First, the inner container 1 is filled with drying fluid via line 3. The particles to be dried are then flushed in from the storage container 4 via line 5 at the top of the dryer with drying fluid. The dryer is closed and the pressure in it is increased to near to supercritical conditions. The pump 6 then presses the drying fluid heated in the heat exchanger 7 into the particle bed from below. The drying fluid is fed from the head of the dryer back to the pump 6 or partially released via the valve 8 to keep the pressure in the dryer constant until near to supercritical temperatures are set in the entire dryer. The pressure is then released via valve 8. The dried particles are drawn off via line 9. A differential pressure control is preferably used between the inner container 1 and the outer container 2, since the inner container 1 is to be constructed as thinly as possible. This differential pressure control operates as follows: increases the level in the storage vessel 10, because the drying fluid circuit a positive pressure is present and drying fluid flow to the storage vessel 10 via the cooler 11, is a level sensor 12 by means of a N ₂ -Splitrange control 13 the pressure of the N ₂ pads in the gap formed between the inner and outer container increased. If the level in the standing container 10 drops, the N ₂ split-range control 13 will correspondingly reduce the pressure of the N ₂ cushion in the gap. In order to avoid the entry of fines in the standing container 10, a small cleaning fluid flow 14 is fed to the standing container 10 via a flow control. This flow of material may also take on the task, inter alia, of reducing the level of components which are formed and are disruptive by supplying fresh fluid. Should the regulation of the differential pressure between inner container 1 and outer container 2 fail, an overflow valve between the inner and outer container (not shown) preferably protects the inner container 1. To protect the inner container 1, the pressure drop between the foot and head of the particle bed should be limited. If corresponding regulations fail, the inner container 1 is protected from destruction by a further overflow valve in a dryer bypass (not shown).

The invention offers the advantages that considerable amounts of energy can be saved because the outer container in the drying process only a small temperature change subject. Next to it is the temperature change stress on the flanges and other parts of the apparatus largely reduced compared to known methods from the state of the art. To load the dryer, only the Inner container, e.g. by evaporative cooling. By eliminating heating and The cooling time of the outer container significantly shortens the batch time.

Surprisingly, it was found that when flowing through a bed of bottom up, as shown in the figure, a large part of the liquid presented can be cold pushed out of the container. Compared to a continuous one Powder process in which large amounts of solvent co-flow with the drying solids have to be heated, so there is more energy saved. Surprisingly, it was also found that conventional silica-alkogel granules, which was obtained from hydrogel by the convective heat supply neither mechanically nor as a result of temperature stresses or is damaged. This also applies to gel pearls in the lowest layer of the bed still have ambient temperature and flowed with 300 ° C hot fluid become.

The invention is illustrated by the following example, which is a preferred embodiment represents the invention, illustrated in addition.

Example: Step (a): Hydrogel production

According to DE-A-21 03 243, DE-A-44 05 202 and DE-A-16 67 568, silica hydrogels have been used manufactured. At least 95% by volume of it had a pearl diameter from 2 to 12 mm. Coarse material was submerged in water Harp sieve separated. Next, the silica hydrogels were added before Desalination undergoes continuous electricity classification.

Stage (b): desalination Apparatus:

Two desalination moving beds, each 11 m high and 800 mm wide, were included Sampling points equipped at different heights. Fresh water was at the foot supply via distributor and salt water at the head via slotted screen cartridges. The Cell wheel sluice at the foot adjusted the flow of solids. At low Flow velocities and in the case of gels which tend to stick could also be used static mixers the cross-mixing in bed can be improved.

Execution:

In each desalination moving bed there was one moving from top to bottom Flow of approx. 510 l / h classified hydrogel from the previous stage (approx. 150 of the 5101 a gap of approx. 2450 l / h flows from below into the gap volume) sent to meet. After about 30 hours at the latest, the patient had become stationary set in the moving bed. The conductivity of the samples taken at the different Locations taken along the bed showed no changes. in the Overflow was measured a conductivity of more than 1 milli-Siemens / cm. The Water in the gap volume of the desalinated hydrogel had a conductivity of 40 Micro-Siemens / cm on what a sodium content of about 1 wt .-% in the gel equivalent.

Stage (c): water / alcohol exchange Apparatus:

The liquid exchange step was 11m high and 500mm wide Moving bed carried out, which was similar to that for desalination was used. The alcohol was supplied above the rotary valve by means of a distributor. The water / alcohol mixture could pass through slotted screens expire. At low flow speeds and with tendencies to stick Gels were able to improve cross-mixing in bed with static mixers.

Execution:

The desalted hydrogel stream from stage (b) of about 1000 l / h was a Isopropanol flow of about 1400 l / h sent. After 10 hours at the latest had become stationary in the moving bed. The densities of the samples from the various sampling points along the bed showed no change more. The residual water content in the gel, which was derived at the foot of the moving bed, was less than 1% by weight. The specific isopropanol demand volume ratio was so 1.4: 1.

Stage (d): drying Apparatus:

The apparatus used corresponded schematically to the device shown in the figure. The equipment used consisted of a 100 bar pressure-resistant outer container made of heat-resistant steel, stainless steel-plated inside, and a 400 mm wide inner container made of stainless steel. The outer container was 8 m high, cylindrical, and had an outer diameter of 600 mm and a wall thickness of 50 mm. The inner container had a wall thickness of 4 mm and tapered at the top and bottom. The usable volume was 1 m ³ . The nitrogen-filled annular gap between the inner and outer container was 50 mm wide in the cylindrical area. The inner container communicated with the drying fluid circuit, which housed the pressure maintenance, circuit pump and heat exchanger. A trunk protruded into the head of the inner container, which had the Alkogel feed line in the center and the screen surface on the outside of the cylinder for fluid / solid separation.

Execution:

The pressure-bearing part of the dryer was heated to 300 ° C with 100 bar steam. The inner container was boiled by adding isopropanol. Alkogel was flushed in with isopropanol which was circulated. The temperature of the alkogel hardly increased during this charging process. After closing the dryer, the annular gap and inner container were brought to 60 bar pressure. With regard to details of the pressure control, reference is made to the figure. The pump was switched on and drying fluid was first fed in at low speed, for example 1 m ³ per hour at a density above 0.7 kg / l. The bed of the alkogel was flowed from below. Then the heat exchanger was heated. The speed of the pump could be increased with decreasing density of the drying fluid. Instead of the density, the temperature at the top of the dryer could also be used as a reference variable. 70% of the isopropanol could be forced out of the circuit when cold. The supercritical temperature at the top of the bed was reached after 50 minutes. It was relaxed without affecting the two-phase area.

Stage (s): separation of sorbed gases / substances Apparatus:

A 3 m ³ silo was used to remove / separate the sorbed gases / substances.

Execution:

After relaxing, the airgel was pneumatically transferred to the silo. The silo was then evacuated and a weak stream of nitrogen was allowed to flow through the bed at about 30 mbar pressure. This stream of nitrogen exchanged the gas atmosphere in the silo ten times per hour. As a result, the partial pressure of desorbed alcohol was kept low, the desorption accelerated and completed. The residence time was more than 30 minutes in order to also remove sorbed gases / substances from the Knudsen pores of the airgel. If it was desired or necessary to cool down, the silo was operated at normal pressure and worked with N ₂ in a circular mode over a washer.

packaging:

The continuous assembly step was carried out by grinding and mixing of dopants in a pin mill.

The airgel granules obtained showed a grain size of up to 12 mm, only 2% by volume of the granules having a grain size of less than 2 mm. The average thermal conductivity λ _{10 of} the 2-3 mm fraction of the granulate was better than 18 mW / (m · K) according to DIN 52616, for the powder it was 16 mW / (mK). The transparency of the 2-3 mm fraction was 60% at 1 cm layer thickness. The bulk density according to ISO 3944 was 70 to 130 g / l. The airgel was water-repellent and floated on water. The headspace (the gas phase above the bed) of the airgel did not become explosive at 100 ° C and only explosive at 160 ° C after one hour.

Surprisingly, it was found that the gel despite the rapid heating did not suffer that the abrasion resistance of the gel was sufficient and that hardly any water accumulation occurred in the fluid. In some cases, even one Water depletion observed what the reuse of the solvent without thermal processing enabled, without an increase in water in the Drying fluid entered.

Claims (11)

1. A process for drying microporous, fluid-containing particles by reducing the interfacial tension of the fluid, preferably to 0 to 1/10, in particular to 0 to 1/20, of the interfacial tension of the fluid at room temperature, by appropriately increasing the temperature at from close to the critical pressure to supercritical pressure of the fluid, which comprises supplying the heat required for the temperature increase by convection.
2. A process as claimed in claim 1, wherein the fluid-containing particles dried are gels which contain water, C₁-C₆-alkanols or mixtures thereof as fluid.
3. A process as claimed in claim 1 or 2, wherein gels which contain isopropanol as fluid are dried.
4. A process as claimed in any of claims 1 to 3, wherein silicic acid gels are dried.
5. A process as claimed in any of claims 1 to 4, wherein a drying fluid is used for the convective heat supply.
6. A process as claimed in claim 5, wherein the drying fluids used are C₁-C₆-alkanols, C₁-C₆-ethers, C₁-C₆-ketones, C₁-C₆-aldehydes, C₁-C₆-alkanes, C₁-C₆-alkenes, C₁-C₆-esters or C₁-C₆-amines or carbon dioxide.
7. A process as claimed in claim 5 or 6, wherein the drying fluid used is the same fluid as that contained in the microporous particles.
8. A process for the preparation of microporous, three-dimensionally networked particles by

   (a) preparing microporous particles containing pore liquid or fluid,

   (b) if required, washing and/or removing salt from the particles obtained in stage (a) and containing pore liquid, by means of a solvent and/or water,

   (c) if required, partially or completely exchanging the pore liquid or the solvent or the water in the particles for a fluid to obtain microporous, fluid-containing particles,

   (d) drying the microporous, fluid-containing particles and

   (e) if required, separating off sorbed gases and/or substances from the dried particles from stage (d),

   wherein the drying in stage (d) is carried out as defined in any of claims 1 to 7 and stages (b), (c) and (e) are carried out in a moving bed by the countercurrent method, by passing, in stage (b), the particles obtained in stage (a) countercurrent to a solvent stream and/or water stream, passing the particles countercurrent to the fluid in stage (c) and passing the dried particles countercurrent to an inert gas stream in stage (e).
9. A process as claimed in any of claims 1 to 7, wherein the process is carried out using an apparatus which comprises a pressure container having an inner container (1) and a pressure-withstanding outer container (2) and suitable measuring and control apparatuses and pump apparatuses and heat exchangers (6, 7), the inner container being provided for holding the particles to be dried and a gap being provided between the inner container and the outer container.
10. A process as claimed in claim 9, wherein the inner container consists of stainless steel and the pressure-withstanding outer container of creep-resistant steel.
11. A process as claimed in claim 9 or 10, wherein differential pressure control is used between the inner container and the outer container.

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