Patent GB-742902-A (GB)
Title: Improvements in or relating to the continuous production of cellulose esters
IPC: C08B3/06
Filed: 1953-08-27
Abstract: Cellulose esters are continuously produced by progressively eliminating the water from a dilute dispersion of cellulose, the last stages of this elimination being effected by displacement of the water in stages with a carboxylic acid solvent for the ester to be produced, and esterifying continuously the cellulose thus impregnated with acid by means of an organic acid anhydride in the presence of an esterification catalyst and of a solvent for the cellulose ester, an apparatus being used which is provided with a cylindrical worm member having interrupted worm threads in the form of blades inside a coaxial cylindrical casing having spiral ribs on its inner surface coacting with the blades of the worm member, the worm member and casing being rotatable and reciprocable relative to each other, and the apparatus being provided with tubes for leading in the reactants, an outlet duct and temperature control means.  The carboxylic acid solvent used for eliminating the water may first amount to 3-10 times the weight of the cellulose and may then be gradually reduced to from 2-6 times the weight of the cellulose.  The cellulose may be defibred in the presence of 15 to 30 times as much water as cellulose, by weight and the resulting dispersion then further diluted with water. The dispersion may be produced in a defibring apparatus such as a paper mill beater.  The apparatus in which the water is eliminated from the dispersion may be a perforated conveyer belt, a vacuum filter, a perforated basket centrifuge, or a perforated horizontal disc which slowly rotates in such a manner that various portions thereof pass successively under conduits fed with the carboxylic acid, the concentration of which increases from one conduit to the next.  The passage of liquid through the disc may be aided by vacuum means.  The preferred apparatus is analogous to the pulp press of a paper mill.  The solvent used during esterification may be the carboxylic acid used for displacing the water from the cellulose dispersion, or may be another solvent such as methylene chloride. In an example, sheets of wood pulp are defibred with an excess of water at ordinary temperature and the suspension thus obtained is diluted with more water in a tub provided with a mechanical stirrer to bring the cellulose content of the suspension to 2 per cent.  This suspension is discharged on to the wire gauze horizontal endless belt of a pulp press through which the water drains away and on which the residual sheet of cellulose pulp is subjected to the action of a series of press rolls with corresponding suction boxes under the upper run of the belt.  The water remaining in the pulp sheet is then replaced by acetic acid by the action of sprays and corresponding suction boxes.  The last spray (with respect to the direction of travel of the pulp sheet) is supplied with glacial acetic acid and each of the remaining sprays is supplied with diluted acetic acid from the suction box immediately following the spray concerned. After passage underneath the last spray the cellulose sheet emerges impregntaed with almost anhydrous acetic acid.  It is pressed without heating between two rolls which may be rotary vacuum filters so that the proportion of acetic acid in the sheet is reduced to about twice that of the cellulose, and the sheet is then passed through a comminuting device.  The comminuted sheet is passed together with 300 litres per hour of glacial acetic acid into the feed hopper of a horizontal cylindrical esterifying apparatus provided with four temperature control water jackets and two inlet tubes for reactants. A mixture of glacial acetic acid, sulphuric acid and acetic anhydride is also introduced continuously into the esterifier.  The temperature of the water circulated through the four jackets is 12, 20, 40 and 55 DEG  C. respectively, so that the temperature of the reaction mixture is raised progressively during its passage through the mixer.  The cellulose triacetate solution formed in the mixer is discharged through an outlet duct at the far end of the mixer and the triacetate is precipitated, washed and dried by known means.  Cellulose propionate, butyrate and acetopropionate may be prepared by similar means.  Specification 740,171 is referred to.