Apparatus for continuously producing water-soluble hydroxyalkyl cellulose and related ethers

#The invention relates to a continuous process for the manufacture of water-soluble hydroxyalkyl cellulose or water-soluble mixed ethers thereof, based on hydroxyalkyl cellulose, and to equipment for carrying out the process.

#The family of the cellulose ethers, used for a long time in many industrial fields, include water-soluble hydroxyalkyl celluloses (HAlkC) and related ethers, such as hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethyl-hydroxypropyl cellulose (HEHPC), hydroxyethyl-methyl cellulose (HEMC), and hydroxyethyl-carboxymethyl cellulose (HECMC). The properties, manufacturing processes and fields of application of these cellulose ethers are described, for example, in Ullmanns Encyklopaedie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), Verlag Chemie-Weinheim, 4th Edition, Volume 9, 1975, keyword "Celluloseather" ("cellulose ethers"), pages 192 to 212.

#A continuous process for the manufacture of HEC is known (Ullmann, page 206) in which 2 to 8% of ethylene oxide is absorbed, but this leads to water-insoluble products. Higher degrees of etherification are said to lead to a product having an undesirable pasty consistency, resulting in difficulties in the subsequent use of such product in manufacturing processes. Therefore, prior art processes for the manufacture of water-soluble hydroxyalkyl cellulose and mixed ethers thereof are normally carried out discontinuously as suspension ("slurry") processes in the presence of inert organic solvents, such as isopropanol, tert.-butanol or acetone.

#FIG. 1 is a diagram of the process and of an apparatus for performing the process.

#Examples of water-soluble hydroxyalkyl celluloses and the mixed ethers based on hydroxyalkyl cellulose contemplated by the invention include in particular hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethyl-hydroxypropyl cellulose (HEHPC), hydroxyethyl-methyl cellulose (HEMC), hydroxyethyl-ethyl cellulose (HEEC), hydroxyethyl-hydroxypropyl-methyl cellulose (HEHPMC) and hydroxyethyl-carboxymethyl cellulose (HECMC), and, if a substituent is present which does not carry an hydroxyalkyl group or is such a group (for example, carboxymethyl, methyl or ethyl), the degree of substitution by this substituent is at most about 1.0. Preferably, HEC or HECMC are manufactured by the process according to the invention.

#The cellulose used is either of natural origin, for example, cotton linters or wood pulp, or it is present in the regenerated form, such as cellulose hydrate; the particle size of the cellulose should, as far as possible, be smaller than about 1 mm, in particular smaller than about 0.75 mm, before the start of the alkalization.

#The alkali metal hydroxide--customarily NaOH-- is used in the solid form or, more advantageously, in the dissolved form as an aqueous alkali metal hydroxide solution with a concentration of from about 2 to about 60% by weight. Preferred organic solvents used are isopropanol, acetone, hexane, tert.-butanol or mixtures of these solvents, and these can also already have been mixed with water. The indicated composition of the initial slurry merely indicates the sum of the constituents, which are detectable therein, at the start of the process. Of course, for example, the cellulose has already been partially transformed into alkali cellulose after a certain period. For the manufacture of certain hydroxyalkyl celluloses or mixed ethers based on hydroxyalkyl cellulose, solid or liquid etherifying agents, for example, Na monochloroacetate, monochloroacetic acid, propylene oxide or solutions of these can already be added under atmospheric pressure and at room temperature (about 20.degree. to 25.degree. C.) to this first process step (alkalization) which can be carried out under atmospheric pressure. The etherifying agents which are gaseous under atmospheric pressure and at room temperature, such as ethylene oxide, are added in liquefied form under pressure only after the slurry has been transferred into an apparatus which is suitable for elevated pressure. The reaction conditions are here selected such that the cellulose is initially only partially etherified (incipient etherification), i.e., a range from about 0.2 to about 1.0 is attained for the degree of substitution. The term "degree of substitution" is to be understood as the MS (molar degree of substitution) in the case of hydroxyalkyl substituents or other etherifying agents which are capable of multiple substitution. It is to be understood as the DS (average degree of substitution) in the case of etherifying agents which are not capable of multiple substitution, and, in the case of several different substituents (for example, HE and CM) it is also to be understood as the sum of the MS or DS values of the different substituents.