Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2001-06-21
2002-10-22
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S216000
Reexamination Certificate
active
06469203
ABSTRACT:
DESCRIPTION
The present invention relates to a process of recovering monomeric esters of substituted or unsubstituted acrylic acids from polymer material having corresponding structural units.
Acrylate polymers, which include acrylic glasses chiefly consisting of polymethyl methacrylate (PMMA), are used for instance for the production of long-lived consumer goods. For this purpose, there are frequently used molding processes in the course of which considerable amounts of waste polymer may be obtained. For expediently reprocessing these production wastes and for utilizing waste materials recirculated from the process of utilization quite a number of proposals have now been made.
It is a well-known fact that acrylate polymers, above all PMMA, belong to the few plastic materials which are excellently suited for direct chemical recycling. This means that at certain temperatures and pressures these polymers can completely be decomposed again into the corresponding monomer units (depolymerization) when heat is supplied in an appropriate way (Grassie, N., Melville, H. W., Bull. Soc. Chim. Belges 1948, p. 142).
In the reports on the “19. Kunststofftechnisches Kolloquium des Eurogress Aachen” (Mar. 11-13, 1998) there is described a continuously operating process of depolymerizing PMMA. The comminuted plastic material is charged into a hot extruder (ZSK 30), in which two tightly meshing screws are rotating with a self-cleaning effect. By means of these screws, nondepolymerized PMMA and other residues are discharged from the extruder. The PMMA will depolymerize in the extruder due to the thermal and the mechanical shearing effect. The resulting MMA is withdrawn as vapor phase through the degassing bell and is condensed. In this process, the MMA content of the condensate varies between 89% and 97%, the yield of MMA is about <97%. In the above-described process, heating the PMMA is effected in the extruder via the shell walls. The ratio of wall surface to reactor volume deteriorates, however, with increasing plant. For large plants on an industrial scale the available shell surface is so small that the extruder must either be made extremely hot, in order to sufficiently decompose the PMMA, or only very much worse yields of MMA are obtained. The necessary increased heating of the extruder shell, however, leads to a local overheating, which contributes to the formation of byproducts and impairs the monomer purity.
Furthermore, it is known to depolymerize PMMA by means of a fluidized-bed pyrolysis. As fluidized material there is used quartz sand having a grain size of 0.3 to 0.7 mm. It is a disadvantage of this process that in the course of time the fluidized material is graphitized with soot. When the soot chips off the sand grain, it can be entrained with the gas stream. To obtain an appealingly clean monomer, this plant therefore requires the implementation of many special filter systems (cooler, cyclone, electrostatic precipitator). In this method, a nitrogen stream is used for fluidizing the sand. It is likewise disadvantageous that after the depolymerization the nitrogen and the MMA gas must again be separated by cooling. The nitrogen stream, which upon separation from the product gas is recirculated to the reactor, must therefore be cooled and heated in constant alternation, where the temperature difference is at least 400 K. For a large-scale process, this is disadvantageous from an economical and ecological point of view (J. Franck, thesis 1993,Hamburg University).
It is the object of the invention to provide a process of recovering monomeric esters of substituted or unsubstituted acrylic acids from polymer material having corresponding structural units, which allows a continuous depolymerization free from residues, and thus provides for the production of high-quality recycled monomeric esters in a high yield. In accordance with the invention, free from residues is understood to be a process which avoids the formation of deposits in the reactor space and thus makes it superfluous to shut down the plant for removing the deposits, so that a continuous operation is ensured.
Furthermore, it is the object of the invention to provide a process as mentioned above, which can be operated on an industrial scale and helps to eliminate the disadvantages such as a poor transfer of heat during the depolymerization, a high amount of apparatus required as well as energetically unfavorable process flows.
The subject-matter of the invention is a process of depolymerizing PMMA, which is characterized in that in a reactor the polymer material is brought in contact with a hot mechanically fluidized solid (heat-transfer medium), and the resulting vapors are withdrawn and condensed, where the hot heat-transfer medium is continuously supplied at one end of the reactor, and cooled heat-transfer medium is discharged at the other end.
By means of the inventive process the reactor volume can be kept small as a result of the very good heat transfer of the fine-grained solid and the related relatively short dwell time of the polymer material. Hence, the dwell time of the resulting monomer vapors in the reactor is less than 6 seconds. The desired esters of the acrylic acids are obtained in very good yields and with a high purity. Thus, the hot, fine-grained heat-transfer medium also ensures that in large-scale plants a sufficient transfer of heat is ensured during the depolymerization.
In accordance with the invention, the mechanically fluidized fine-grained heat-transfer medium produces a rubbing effect in the reactor, which helps to completely prevent an accretion of byproducts resulting from the depolymerization at the inner walls and installations of the reactor. These depolymerization byproducts are continuously discharged from the reactor together with the fine-grained heat-transfer medium, so that an agglomeration of the byproducts in the reactor is prevented. Thus, it is possible to continuously perform the advantageous process, as no depolymerization residues, which otherwise must be removed from time to time from corresponding plants of the prior art, are left in the reactor. The monomer gas stream, which leaves the reactor, has a sufficient purity and need only be liberated from entrained dust particles by means of a cyclone. A separation from a carrier gas stream, which would even entrain more dust particles, is by no means necessary.
The mechanical fluidization and the transport of the fine-grained heat-transfer medium can be achieved by all possibilities well-known to the man skilled in the art, such as by moving or rotating walls, possibly under the influence of gravitation. There is preferred the embodiment where the substances supplied to the reactor are mechanically fluidized, mixed with each other and conveyed in a mixer by means of one or more intermeshing shafts rotating in the same direction, which are provided with coils or other mixing tools. Approximately the same dwell time of all solid particles (plug flow) in the range from 5 to 60 seconds can be adjusted by changing the rotational speed of the screws.
The polymer material is heated within a short period and depolymerized by means of the hot heat-transfer medium fluidized by the coils or mixing tools. The volatile components are discharged, whereas the solid byproducts remaining after the depolymerization are discharged from the reactor together with the heat-transfer medium, so that a contamination of the withdrawn monomer vapors with components originating from the solid byproducts is likewise very advantageously prevented. The mass balance of the heat-transfer medium in the reactor is preferably maintained by supplementing at the top end from a heated receiver.
As indicated above, the transport of the heat-transfer medium in the reactor can preferably be effected by one or more rotating shafts, which are equipped with coils or other mixing tools, from the inlet opening to the outlet opening.
Having been discharged from the reactor, the heat-transfer medium can be supplied to the bottom end of a pneumatic conveying line via a seco
Fuss Werner
Goedecke Ralf
Gropp Udo
Groschang Tobias
Schmalfeld Jörg
Dubno Herbert
Forohar Farhad
Metallgesellschaft Aktiengesellschaft
Myers Jonathan
Padmanabhan Sreeni
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