The present invention relates to a process, preferably operated continuously, and a device for working up different solvents such as are used for result in the preparation and extraction of a very wide variety of products, such as, for example, cellulose ethers. In the following text, "working up" is to be understood as meaning the treatment of materials in which different solvents are used or employed. In the process and the device according to the invention, the various solvents used are separated from one another and are kept separate. In the following text the process and the device are described primarily with reference to the particularly preferred application to cellulose ethers. It is to be understood, however, that the invention has broader application.
The preparation of cellulose ethers is known, such as, for example, carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC) or methylcellulose (MC) and mixed ethers thereof, in the presence of a moistening or suspending agent during the etherification reaction. Aqueous, aqeuous-organic or mixed organic moistening or suspending agents whose properties are particularly advantageous for the etherification reaction are employed in this process. It is also possible to use etherifying agents themselves, in excess, as a moistening or suspending agent.
The byproducts of the reaction (for example, salts) formed in the course of the etherification must subsequently be removed from the reaction product in order to prepare purified products. It is known in this connection that the moistening or suspending agent used in the reaction as a rule has only a small capacity for dissolving salts and does not contribute substantially to purifying the cellulose ether. The extraction of the impurities from the reaction product is therefore carried out by means of an extracting agent which is better suited to this purpose.
After the reaction, the moistening or suspending agent must be removed from the cellulose ether. Various separation processes, operating continuously or discontinuously, are employed for this removal. Typical for this use are mechanical separation equipment (for example, decanters, filters or centrifuges) or dryers (for example, rotary vacuum dryers or pneumatic dryers which have been rendered inert). In the case of cellulose ethers which are insoluble in hot water, the moistening or suspending agent can be expelled in hot water.
In the case of the mechanical separation processes the moist, crude cellulose ether still contains a certain content of the moistening or suspending agent. This content becomes mixed with the second solvent or solvent mixture which has been specially selected for the extraction. Thus, subsequent to the extraction, an involved separation by distillation of the solvents becomes necessary to enable them to be recycled to the etherification reaction or extraction.
In the course of the removal of the moistening or suspending agent from the crude reaction product by drying, the particles of cellulose ether become horny as a result of the heat treatment. The hornification closes the surface of the particles, so that penetration by the extraction agent in the course of the subsequent extraction with the solvent particularly selected for this purpose is impaired. This causes a prolonged extraction time or a greater usage of solvent. A further disadvantage of intermediate drying of the crude cellulose ether lies in the fact that the dissolving quality of the purified cellulose ether is impaired as a result of the hornification.
In the case of cellulose ethers which are soluble in hot water, when the reaction medium is expelled by means of hot water, the content of reaction medium which has entered the water must subsequently be removed by expensive methods.
A process and a device of the type described above are known, for example, from the literature reference "Proceedings of the Filtration Society, Filtration and Separation", 1979, pages 176-180 and from DE-B 2,038,310, EP-A 0,052,337 (=U.S. Pat. No. 4,398,024) and EP-A 0,053,741 (=U.S. Pat. 4,404,370). When using the processes and devices of the state of the art, however, the different solvents are obtained in the course of the separation as a common mixture of solvents which is only separated at the end of the process.