Patent GB-740684-A (GB)
Title: Continuous process for digesting pulp
IPC: D21C3/24
Filed: 1951-10-17
Abstract: <PICT:0740684/IV(a)/1> Disintegrated cellulosic material suspended in a body of hot digesting liquor in a digester is subjected to continuous digestion at a temperature which is at least 300 DEG  F. during the greater part of the travel of the material from the top to the bottom of the body of digesting liquor, and which is not allowed to fall below 275 DEG  F., the material in the upper part of the body of liquor being subjected to the action of a co-current stream of the hot liquor, and the material in the lower part of the body of liquor being subjected to the action of a counter-current stream of hot digesting liquor, and the finished pulp mixed with liquor is withdrawn from the bottom of the digester under pressure.  The digesting liquor may be a substantially neutral solution which has been rendered weakly acid by carbon dioxide produced by reaction of the liquor with the cellulosic material.  In a modification of the process for carrying out the treatment of bagasse, the material may be treated in a long digester, at the top of which it is preliminarily treated with digesting liquor at 225 DEG  to 275 DEG F. which serves to disintegrate it.  Additional quantities of the hot digesting liquor at 300 DEG  to 350 DEG  F. are fed to the digester at the top and bottom and used liquor is removed from an intermediate part of the digester at a temperature of 275 DEG  to 300 DEG  F., the conditions in the digester being such that the bagasse during its passage in suspension down the digester is maintained for a period of 2-4 hours in the hot digesting liquor at substantially 300 DEG  F. For the treatment of bamboo, the material may be initially treated with digesting liquor at 250-275 DEG  F. so as to disintegrate the bamboo, and additional amounts of digesting liquor at 330-375 DEG  F. may be fed to the digester at both top and bottom, used liquor at 300-340 DEG  F. being removed from an intermediate part of the digester, the conditions being such that the bamboo is suspended in liquor at 340 DEG  F. for 6-10 hour as it passes down the digester. The process may be used for the production of pulp from wood, straw, and flax, as well as bamboo and bagasse, and the product may be used for the production of paper or rayon.  The digesting liquor may be a solution of a hydrotropic salt such as an alkali metal or alkaline earth metal salt of an organic acid derived from a single benzene ring compound, e.g. a salt of xylenesulphonic, cymene sulphonic, toluenesulphonic, or salicylic acid.  The pulp from the digester may be passed directly to a series of screw presses to remove adherent liquor.  Three screw presses and a rotary filter press may be used and washing liquor may be caused to flow in countercurrent to the cellulose pulp.  If desired, the intensity of the treatment, i.e. the extent to which pentosans and lignin are removed from the cellulose fibres, may be controlled by controlling the gaseous pressure in the digester which in turn governs the concentration of dissolved carbon dioxide in the liquor and hence the acidity.  As shown in Fig. 1, the digester has a perforated inner belt 2, through which liquor is withdrawn, about half-way between the top and bottom, and a perforated cone 3 at the bottom and a perforated blister 4 at its top, through which liquor is introduced.  A pump 20 provides forced circulation of cooking liquor from the space between the belt 2 and the wall of the digester 1 to the heater 7 and back to the digester. One or more feed tanks 10 are provided.  Heated solution is delivered to feed tank 10 by means of pipe 11 which is connected to pipe 6. The heated solution is also delivered to pipe 12 which itself is connected to the top and bottom of the digester 1.  By operation of valves in pipes 12a, 12b, the solution entering the digester at the top and bottom respectively may be at different temperatures. The head 23 of the feed tank may be removed and the tank filled with wood chips or other comminuted cellulosic material.  The head is replaced and then hot cooling liquor from the heater 7 is introduced.  A tank 17 is provided for storing under pressure any excess hot liquor forced out of the main digester 1 when the contents of feed tank 10 are pumped into the digester.  Bleed pipes 16, 161 are provided for controlled escape of entrapped air or steam from tanks 10, 17, and a bleed pipe 1611 is provided for controlled escape of carbon dioxide from digester 1.  A pump 18 removes cellulose pulp and liquor from the digester at a chosen pressure whatever the pressure within the digester.  The pulp containing digesting liquor is fed continuously through a cooling vessel to one or more screw presses, washing vessels, and rotary filters.  Hot liquor removed in the first screw press is returned to the heater and thence again to the digester. Lignin may be recovered from used washing liquors, and these may then be concentrated by evaporation to obtain solutions of hydrotropic salt for re-use in the process. If desired the raw cellulosic material may be pretreated with a heated acidic solution, e.g. a solution of sodium xylene sulphonate acidified with xylene sulphonic acid.  After pressing and neutralizing it is mixed with a substantially neutral solution of hydrotropic salt and fed to the main digester.  Specification 560,492 is referred to.