Drying and gas or vapor contact with solids – Apparatus – For diverse operations on treated material
Reexamination Certificate
2002-02-28
2002-12-03
Walberg, Teresa (Department: 3742)
Drying and gas or vapor contact with solids
Apparatus
For diverse operations on treated material
C034S558000, C034S362000, C034S373000
Reexamination Certificate
active
06487788
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a continuous process for the drying and gel formation of solvent-containing gel-forming polymers, in particular polysaccharide derivatives, by flash cooling of suspensions of the polymers and to an apparatus for the drying of solvent-containing products.
The process comprises the steps: metering the solvent-containing polymer having a solids content of 1 to 65% by weight and a pressure of in particular ambient pressure to 6000 hPa, at a polymer temperature of 20 to 100° C., into an evaporation zone, the evaporation zone having a pressure of 0.1 hPa to 800 hPa, cooling the polymer, freeing the polymer of 2 to 5% by weight of solvent with mechanical stirring, mixing and conveying of the mixture to the lower end of the evaporation zone with formation of a dense gelled product layer, discharging the polymer by means of a mechanical discharge apparatus, optionally with further densification of the polymer, and subsequently drying the gelled polymer in a drying apparatus to a solvent content of not more than 5% by weight.
The apparatus described comprises at least an evaporation container having an evaporation zone, a stirrer and a conveyor means, a product inlet, a vapour take-off with connected vacuum generator, a discharge apparatus connected to the lower end of the evaporation container, one or more conveyor paddles having thrust blades which pass across the entrance of the discharge apparatus at a height of not more than 5 mm, preferably 1 to 3 mm, being provided in the lowermost plane of the conveyor means.
German Offenlegungsschrift 196 02 227 A1 discloses a storage container (paste bunker) having a discharge apparatus. In particular, the described stirrer of the storage container has a sleeve design with variable stirring arm geometry for adaptation to the product consistency.
German Patent Specification 952 264 discloses a three-stage process for converting moist, fibrous methylcellulose into powder form of high dissolution rate. Here, the product containing 50 to 70% by weight of water is first homogenized to give a plastic material and is cooled to 10 to 20° C., a cooled screw press being used; the material is then milled using a hammer mill and is dried in a forced-circulation dryer. It is the object of the invention to provide a process and an apparatus which avoid the disadvantages of the known processes and apparatuses and permit comparably safer continuous operation.
Prior gelling of the cellulose ethers is necessary for the drying and for the millability of the cellulose ethers in a miller-dryer.
In particular, it is therefore the object of the invention to develop a drying process which is suitable for solvent-containing gel-forming polysaccharide derivatives, in particular for a cellulose ether suspension, it being necessary to cool the cellulose ether suspension in order to effect gelling and to meter the gel continuously into a downstream apparatus, in particular into a miller-dryer.
In particular, a cellulose ether suspension must be capable of being converted into a homogeneous, gel-like consistency in order to be able to meter the material into a superheated steam miller-dryer.
The object is achieved, according to the invention, if the preheated polymer suspension is introduced into an evaporation zone and the evaporation zone, for example a cylindrical container, is present under a reduced system pressure and the polymer suspension experiences a cooling effect down to below the flocculation point as a result of flashing into the cylindrical container.
The invention relates to a continuous process for the drying and gel formation of solvent-containing gel-forming polymers, in particular polysaccharide derivatives, particularly preferably of hot, aqueous cellulose ether suspensions, by flash cooling (abrupt let-down with cooling) of suspensions of the polymers, comprising the steps: metering the solvent-containing polymer having a solids content of 1 to 65% by weight, preferably of 5 to 50% by weight, particularly preferably of 10 to 40% by weight, at a pressure of in particular ambient pressure to 6000 hPa, at a polymer temperature of 20 to 100° C., preferably of 30 to 95° C., into an evaporation zone, preferably a paste bunker, the evaporation zone having a pressure of 0.1 hPa to 800 hPa, preferably of 20 hPa to 700 hPa, particularly preferably of 40 hPa to 200 hPa, cooling the polymer, in particular to a temperature of <93° C., in particular <60° C., especially of 60° C. to 29° C., freeing the polymer of 2 to 5% by weight of solvent with mechanical stirring, mixing and conveying of the mixture to the lower end of the evaporation zone with formation of a dense gelled product layer, discharging the polymer by means of a mechanical discharge apparatus, in particular with the aid of a single-screw or multiscrew conveyor, optionally with further densification of the polymer, and subsequently drying the gelled polymer in a drying apparatus to a solvent content of not more than 5% by weight.
The residence time of the polymer in the evaporation zone is preferably from 30 sec to 2 h, particularly preferably from 5 to 30 min.
Preferably, the drying of the gelled polymer is effected at 100 to 500° C., particularly preferably from 100 to 230° C., in a miller-dryer.
Advantageously, the drying of the gelled polymer is carried out in particular at a pressure of about 1000 hPa (i.e. ambient pressure) to 5000 hPa, preferably at 1000 hPa.
Particular advantages are obtained if the drying of the gelled polymer is effected in a superheated steam miller-dryer.
Preferably, the drying of the gelled polymer is effected to a solvent content of not more than 4% by weight.
The polymer particularly preferably to be used in the process is a cellulose ether, in particular methylcellulose.
The solvent used in the process is preferably essentially water.
The invention is distinguished from the prior art, in particular that described in German Patent Specification 952 264, in that the cooling of the polymer suspension takes place not by contact cooling of the suspension but by the flash effect, i.e. during the transition from the higher pressure level in the suspension line to the reduced system pressure in the paste bunker (evaporation zone). In contrast to the screw press, this process takes place spontaneously with simultaneous concentration of the polymer suspension. As described in the present patent, the further gelling is achieved by the available residence time in the paste bunker. In contrast to the screw press, the paste bunker additionally has the desirable function of a buffer. The gelling of the polymer (methylcellulose) takes place at a temperature of 10 to 20° C. according to the teaching of German Patent Specification 952 264 but in the present process it takes place at a substantially higher temperature, but typically at <60° C.
The invention furthermore relates to an apparatus for the drying of solvent-containing products, in particular of solvent-containing gel-forming polymers, comprising at least one evaporation container having an evaporation zone, a stirrer and a conveyor means, a product inlet, a vapour take-off with connected vacuum generator, and a discharge apparatus connected to the lower end of the evaporation container, one or more conveyor paddles having thrust blades which pass across the entrance of the discharge apparatus at a height of not more than 5 mm, preferably 1 to 3 mm, being provided in the lowermost plane of the conveyor means.
The discharge apparatus is preferably a single-screw or multiscrew conveyor.
The evaporation container has an additional jacket for passing through a heat-transfer medium in order additionally to promote the devolatilization of the polymer.
In a particularly preferred variant of the apparatus, the thrust blades pass across the entrance of the discharge apparatus at a height of not more than 5 mm, preferably 1 to 3 mm, in order to ensure complete filling of the screw flights. Uniform product conveying and at the same time a tight connection between paste bunker a
Gehrmann Dietrich
Kempkes Hartwig
Bayer Aktiengesellschaft
Dahbour Fadi H.
Gil Joseph C.
Preis Aron
Walberg Teresa
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