WO1987001402A1

WO1987001402A1 - Method for producing of cellulosic pulp

Info

Publication number: WO1987001402A1
Application number: PCT/SE1986/000389
Authority: WO
Inventors: Punya B. Chaudhuri
Original assignee: Chaudhuri Punya B
Priority date: 1985-09-03
Filing date: 1986-09-02
Publication date: 1987-03-12
Also published as: SE455314B; SE8504083D0; SE8504083L

Abstract

A method for the production of cellulosic pulp comprises defibration of a fibrous raw material consisting of wood chips, straw, bagasse, reed or the like. The fibrous raw material is impregnated by an aqueous solution of sodium carbonate. The fibrous raw material impregnated with sodium carbonate is fed to a pressure vessel (5). The material in the pressure vessel (5) is acidified by supplying sulphur dioxide to the pressure vessel (5), said sulphur dioxide reacting with said sodium carbonate to generate sodium sulphite and carbon dioxide, said reaction at least partly taking place in situ in the fibrous raw material. The material in the pressure vessel (5) thereafter for a short period of time is heated by saturated steam at a pressure of 20-100 bar, which further promotes said reaction. Thereafter the material is rapidly discharged and caused to expand in a second vessel (22) under a substantially lower and substantially constant pressure.

Description

METHOD FOR PRODUCING OF CELLULOSIC PULP

TECHNICAL FIELD The invention relates to a method of producing cellulosic pulp for paper making comprising defibration of a fibrous raw material consisting of wood chips, straw, bagasse, reed or the like.

BACKGROUND ART The usual method of manufacture of CTMP (Chemi-Thermo Mechanical Pulp) consists of preimpregnation of the wood chips by sodium sulphite and/or caustic soda, heating to about 130°C and defibration in a disc refiner. A major disadvantage of this process is the high electrical energy consumption, about 1.5-2 M h/t. A considerable portion of the electric energy supplied to the system is not utilized but can be recovered in the form of steam. In modern plants this steam is used as a heat source for drying paper and for other purposes.

There is also a well-known method for defibration of chips, the so called "Masonite process", in which steam is used to heat the chips, which thereafter are caused to "explode" for the production of fibres. However, in this case the pulp becomes brown in colour by the treatment and the resultant fibre obtains a hydrofobic character, and therefore this fibre pulp (Masonite pulp) is unsuitable for paper making. The method, however, is used for manufacture of fibre board. A variation of this method by preimpregnation of chips is described in US-A-1872996.

DISCLOSURE OF THE INVENTION The purpose of the invention is to offer a new method which is particularly designed for using in the first place steam for defibration of chips or other fibrous raw material. Particularly there is an object to achieve a high yield and low energy consumption. Further there is an object to hinder discolouration of the fibrous raw material and at the same time to impart favourable properties to the pulp. These and other objects can be obtained according to the invention therein that the fibrous raw material is impregnated by an aqueous solution of sodium carbonate, that the fibrous raw material impregnated by sodium carbonate is charged in a pressure vessel, that the material in the pressure vessel is acidified by supplying sulphur dioxide to the pressure vessel, said sulphur dioxide reacting with said sodium carbonate to form sodium sulphite and carbon dioxide, which reaction at least partly taking place in situ in the fibrous raw material, that the material in the pressure vessel thereafter for a short duration is heated by saturated steam at a pressure of 20-100 bar, which further stimulates said reaction, and that the material thereafter suddenly is discharged and caused to expand in a vessel at an essentially lower and essentially constant pressure.

As the reaction between sodium carbonate and sulphur dioxide takes place in a closed pressure vessel, the produced carbon dioxide is partly formed in situ in the chips and partly dissolved and filling up the vessel. The high pressure steam which is introduced in the pressure vessel after sulphur dioxide having been introduced accelerates the following reaction to the right Na₂C0₃ + S0₂ * Na₂S0₃ + C0₂

In the pressure vessel the temperature as well as the pressure rise due to the high partial pressure of carbon dioxide. Tests have confirmed that when spruce chips (picea abies) is reacted with sulphur dioxide in stoichiometric quantities in relation to the sodium carbonate quantity in a closed vessel (106 parts of sodium carbonate reacting with 64 parts of sulphur dioxide) the pressure inside the pressure vessel is increased from an overpressure of 1 bar to an overpressure of 6 bar when the temperature is increased to 95°C from room temperature, the total pressure of 6 bar being the sum of the partial pressure of 5 bar for carbon dioxide and 1 bar for water vapour. However, the process according to the invention operates at considerably higher temperature and pressure levels than according to this theoretical example. When the chips is subjected to a temperature of between 235° and 300°C by injection of saturated steam having a pressure of between 35 and 80 bar, the resultant pressure in the pressure vessel becomes between 40 and 100 bar. Therefore, an extra pressure of 5 to 20 bar can be obtained in the vessel by the chemical reaction.

When the chips thus treated for a period of 3-70 seconds at above mentioned pressure and temperature is suddenly allowed to escape through the bottom valve to another pressure vessel, where steam and solid products are separated, the sudden expansion will difibrate the material to such an extent that the supply of electrical power in the following refining operation can be substantially reduced. This second pressure vessel is maintained at a pressure of between 5 and 6 bar. The pressure drop from between 40 and 100 bar to between 5 and 6 bar is considerable. Due to the fact that the pressure in this subsequent pressure vessel is maintained at a considerably low pressure it will also be possible to use a reboiler for generating fresh steam at such a pressure (about 5 bar) that it can effectively be used in the subsequent paper making. The steam thus is used twice, once for exploding the chips and thereafter for paper drying. Additionally, a fracture of the condensate can be used for impregnation of the chips.

Further features of the invention are apparent from the claims and from the following description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

In the following description of a preferred embodiment reference^'will be made to the drawing which in the form of a diagram schematically illustrates the preferred embodiment and the apparatus used for carrying out the process.

DESCRIPTION OF PREFERRED EMBODIMENT

The fibrous raw material, usually chips, is fed to a steaming vessel 1, where surplus steam heats the chips (or corresponding fibrous raw material) and fills the fiberlumen with condensated steam which expels the entrapped air from the chips in a manner known per se. The chips thus steamed is fed at uniform rate by a screw conveyor 2 and a rotary feeder 3 into a first pressure vessel or impregnating vessel 4. The pressure in this impregnation vessel 4 is maintained at about 1-8 bar and at a temperature below the boiling point at the 5 prevailing pressure by means of steam from a subsequent second pressure vessel 5 via a first steam conduit 6. Sodium carbonate in water solution is pumped into the impregnating vessel through a conduit 7. The chips is continuously transported downwards in the impregnation vessel at such a rate that the duration of the chips in M^ contact with the sodium carbonate solution will be maximized to 2 hours. Finally the impregnated chips is discharged through the valve 8 in the bottom cone. A stirrer has been designated 9.

The impregnated material is transported through a conduit 10 to a 15 dewatering screen 11, where surplus liquor is removed. The drained liquor is recirculated to the impregnation vessel through a conduit 14. The chips impregnated with sodium carbonate is stored in a buffer bin 12, which is equipped with a discharge screw 13. The impregnated chips is charged via said discharge screw and via a valve 15 into said 20 second pressure vessel 5, which is a high pressure vessel. The filling of the pressure vessel 5 is performed at a very high rate. Instead of batchwise treatment of the impregnated chips in the pressure vessel 5 also continuous treatment under pressure can be conceived. When a proper amount of chips according to the embodiment is supplied to the 25 high pressure vessel 5, the valve 15 is closed and a bottom valve 16 remains closed under the treatment which follows hereafter.

The purpose of the injection of sulphur dioxide is multiple. One purpose is to reduce the pH to a desired pH-value of between 8-12.

30 Another important object is that the sulphur dioxide shall react with the sodium carbonate to form sodium sulphite according to the following reaction Na₂C0- + S0_ * ^Nap^S03 ^{+ C0}2* ^{The sul}P^hur dioxide therefore is supplied in a stoichiometric amount in relation to the sodium carbonate. Further, there is an object to form carbon dioxide

35 through the said reaction, said carbon dioxide contributing to the increased pressure in the treating vessel. Still another object is to form the carbon dioxide in situ in the fibrous raw material in order further to enhance the explosion effect at the subsequent pressure reduction. 5

After injection of sulphur dioxide also high pressure steam is injected into the vessel 5 from a steam accumulator 19 via a conduit 20 and a valve 21. The high pressure steam has a pressure of between 35 and 80 bar. This pressure in combination with the pressure achieved 0 through the chemical reaction between sulphur dioxide and sodium * carbonate will bring about a resultant pressure in the vessel 5 of between 40 and 100 bar at the same time as the temperature is increased to those temperatures which prevail for saturated water vapour at these pressures, i.e. a temperature of between 235° and 5 300°C.

After a duration of between 3-70 seconds at the said pressure and temperature the contents in the vessel 5 rapidly are blown via valve 16 out into a third pressure vessel 22, which according to the 0 preferred embodiment is maintained at a constant pressure of about 5-6 bar. This sudden pressure reduction causes an explosion within the chips, which results in an initial defibration of the chips. According to the embodiment the pressure vessel 5 thus operates cyclically, each cycle consisting of filling the vessel 5 with impregnated fibrous raw 5 material, closing the valve 15, injecting sulphur dioxide through conduit 17, injecting steam through conduit 20, opening the valve 16 and discharging the contents to vessel 22, closing valve 16 and opening valve 15. This entire cycle is performed during a duration of time of between 1 and 3 minutes. During each cycle, when the container 0 5 is pressurized, also steam is supplied through conduit 6 to the impregnation vessel 4.

The steam released from the third pressure vessel 22 is fed to and is condensated in a reboiler 23, which generates saturated fresh steam 5 having a pressure of 4-5 bar. The pulp from the third pressure vessel 22 is discharged via the conduit 24 and a valve 25 for continued treatment. Fractions of the steam condensate, which can contain metanol and other higher alcohols can be reused via a conduit 26 for impregnating the chips in the impregnation vessel 24 together with the sodium carbonate.

Claims

1. Method of producing cellulosic pulp for paper making comprising defibration of a fibrous raw material consisting of wood chips, straw, bagasse, reed or the like, c h a r a c t e r i z e d in that the fibrous raw material is impregnated by an aqueous solution of sodium carbonate, that the fibrous raw material impregnated by sodium carbonate is charged in a pressure vessel, that the material in the pressure vessel is acidified by supplying sulphur dioxide to the pressure vessel, said sulphur dioxide reacting with said sodium carbonate to form sodium sulphite and carbon dioxide, which reaction at least partly taking place in situ in the fibrous raw material, that the material in the pressure vessel thereafter for a short duration is heated by saturated steam at a pressure of 20-100 bar, which further stimulates said reaction, and that the material thereafter suddenly is discharged and caused to expand in a vessel (22) at an essentially lower and essentially constant pressure.

2. Method according to claim 1, c h a r a c t e r i z e d in that the sodium carbonate is supplied in an amount corresponding to between 0.05-0.30, preferably 0.1-0.3 times the weight of the fibres in the fibrous raw material.

3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that the material in the pressure vessel is acidified by the injection of gaseous or liquid sulphur dioxide, that the acidified fibrous raw material rapidly is heated for 3 seconds-2 minutes, preferably for 5-70 seconds by saturated steam under a pressure 35-80 bar in a pressure vessel, and that the fibrous material thereafter is evacuated and caused to expand in a subsequent pressure vessel (22), in which an essentially constant pressure of 2-10 bar. preferably about 5-6 bar is maintained.

4. Method according to any of claims 1-3, c h a r a c t e r i z e d in that the sulphur dioxide is supplied in a substantially stoichiometric amount in relation to the amount of sodium carbonate for the generation of said sodium sulphite and carbon dioxide.

5. Method according to any of claims 1-4, c h a r a c t e r i z e d in that the pH-value is reduced to a level of between pH 8 and pH 12 by the injection of sulphur dioxide in said pressure vessel prior to the injection of the saturated steam.

6. Method according to any of claims 1-5, c h a r a c t e r i z e d in that the material in the pressure vessel after heating by means of steam is maintained at the higher pressure and temperature under a duration of time of 30-70 seconds.

7. Method according to claim 6, c h a r a c t e r i z e d in that the temperature is between 235° and 300°C.

8. Method according to any of claims 1-7, c h a r a c t e r i z e d in that fresh low pressure steam is generated by the contaminated steam from said second pressure vessel (22) via a steam reboiler (23).

9. Method according to claim 8, c h a r a c t e r i z e d in that a condensate obtained from steam in said second pressure vessel at least partly is recycled and supplied to the impregnation vessel (4) in order to participate in the impregnation process together with said sodium carbonate.