US2474862A

US2474862A - Process and apparatus for continuous digestion of fibrous materials

Info

Publication number: US2474862A
Application number: US512978A
Authority: US
Inventors: Richter John Christoffer Carl
Original assignee: Kamyr AB
Current assignee: Metso Fiber Karlstad AB
Priority date: 1942-10-15
Filing date: 1943-12-04
Publication date: 1949-07-05
Anticipated expiration: 1966-07-05
Also published as: FR905821A; CH243348A

Description

July 5, 1949.

C. F. C. RICHTER PROCESS AND APPARATUS FOR CONTINUOUS DIGESTION 0F FIBROUS MATERIALS Filed D96. 4, 1943 rro c INVENTCR: Johan Chrisfoffer Fredrik 0 rl Richter A fforney Patented July 5, 1949 PROCESS AND APPARATUS FOR CONTINU- OUS DIGESTION OF FIBROUS MATERIALS Johan Christoi'ier Fredrik Carl Richter, Oslo,

Norway, assignor to Aktiebolaget Kamyr, Karlstad, Sweden, a company of Sweden Application December 4, 1943, Serial No. 512,978 In Norway October 15, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires October 15, 1962 2 Claims. 1

Digestion of wood or similar fibrous materials to cellulose, or for converting wood into sugar, is effected at an increased pressure and at an elevated temperature, 130 to 180 C. or higher, according as the digestion is carried out according to the bisulphite process, the sulphate process, the sulphurous acid process, or some other process. In practice, processes of digestion of this kind are exclusively carried out periodically, that is to say, the digestion is effected batch-wise, a digester being filled with the raw material, such as wood chips, after which the filling opening is closed and the digester liquor is admitted, and the temperature and pressure are then run up" in accordance with curves gained from experience, the pressure being determined by the pressure of the steam plus the partial pressure of the gases developed. During the running up and digestion period the cock is usually "gassed oil," either in order to keep the pressure down, by discharge of air and $02, or in order to recover certain by-products, such as turpentine.

Many attempts have been made to solve the problem of continuous digestion which would consist in continuously feeding the raw material, such-as wood chips, into one end of the apparatus used for the digestion, and causing such raw material to move through said apparatus while admitting the digester liquor, steam, and so forth, and finally discharging the material from the other end of the apparatus in the form of pulp, that is to say, a completely digested product. Methods and apparatuses of the most various kinds have been proposed and tested, but so far no fully tried-out and generally accepted solution of the problem has come forth which has been put in practice on a commercial scale.

Among the problems which must be solved in order to eiiect a continuous digestion, the following may be noted:

A. The continuous feeding of the raw material, such as wood chips, into the apparatus against a high pressure.

B. A satisfactory impregnation of the raw material with the digester liquor in conjunction with heating of the raw material as well as of the digester liquor.

C. Feeding of the raw material through the apparatus in such manner that control is had of the movement of the mass and of the digester liquor with relation not only to the apparatus but also with relation one to the other, and in such manner that full control is had of the disother words, of the process of digestion itself.

D. The discharge 0! the digested-product.

Heretofore practically all proposed solutions of the problem were based on the same basic principle as that applied to ordinary periodic digesters, namely that the pressure in the digester is determined by the steam pressure corresponding to the digesting temperature plus the partial pressure of the gases present.

It has been found in practice that if the temperature of the digester liquor is at the vaporizing point, the liquor has a pronounced tendency to "travel" relatively to the mass towards the discharge point. This phenomenon is well known from the blowing" of ordinary digester-s. where there is risk of the liquor "going first, while the mass or pulp remains behind and must be washed out of the digester or pried out with the use of crowbars or the like.

This phenomenon is the reason why practically all attempts hitherto made of performing a continuous digestion have failed.

The abovementioned diiliculty can be avoided by feeding the wood chips into the bottom of the digester and discharging the digested pulp and the liquor over a discharge lip at the top of the digester, so that the digester always remains filled up to its top.

But this solution of the problem suffers from certain disadvantages, and it is desirable, therefore, to solve the problem in such mannerthat the material is charged into the top of a vertical digester and the completed product is discharged from the bottom of the digester. This will be evident from the following considerations upon which the present invention is based:

1. For practical reasons the digester should be vertical with its inlet at the top and the discharge at the bottom. In the first place, the material or mass which is specifically heavier, is most easily fed downwards. In the second place, the feeding apparatus when placed at the top of the digester, is more easily accessible for inspection and repair, than if it is placed at the bottom of the digester. In the third place, the digester is most easily emptied if the discharge is eilected from the bottom of the same.

2. In accordance with the experience gained from periodic digestion, the material should pass through zones having diflerent temperatures, substantially in such manner that the material first passes through a comparatively cold zone for the purpose of impregnation, and then through the digestion zone proper at full digestion temperature, and finally through a somewhat colder discharge zone. Gas which is possibly separated a,474,se2

during the heating period in the digestion zone, such as S: in the bisulphite process, will attempt to move upward. and if the mass moves downward, the gas will flow in counter-current to the material, which is thus subjected to a gradual preheating. Simultaneously the upward flowing gas. such as $02, will successively encounter gradually colder material. so that the gas is again dissolved and thus aids to effect the impregnation.

3. The supply of chemicals-boiling liquorshould be eflected exclusively with regard to the predetermined quantity and strength, quite irrespective of the pressure in the digester.

4. The movement of the material and of the digester liquor through the digester should be positively controlled and must not be dependent upon accidents or chances. In other words, and with reference to the phenomenon above mentioned, this means that the flow of the liquor must be brought under absolute control and that its tendency to "travel" relatively to the material must be eliminated. The fiow of the liquor through the digester must be determined exclusively by the quantity of liquor introduced in the digester.

From the considerations above stated it is evident that a primary condition for the process proceeding according to the lines above stated, is that the digester must be completely dead, that is to say. completely filled with liquor, without any steam space at the top. If the liquor is at the vaporization temperature, the steam pressure constitutes the force that causes the tendency of the liquor to travel." This cause must be eliminated in the first place.

The conclusion is that the digester must be maintained under compression, that is to say, the liquor therein must be under a pressure which exceeds the steam pressure of the digester liquor at the maximum digestion temperature reached. This pressure of the liquor may conveniently be identical to a pump pressure which is determined by the aid of a pressure regulator.

It will easily be understood that the digester being maintained under compression by the aid of a pump pressure, the condition above stated under point 2 can also be fulfilled, since in such case there is no obstacle to maintaining different temperatures in diiferent zones, as long as the vaporization temperature at the prevailing pressure is not reached at any point. There is thus also nothing to prevent a cooler impregnating zone being maintained at the top of the digester and a hotter digesting zone at a lower level, in accordance with point 2.

It will be understood from point 3 that the treating liquor itself must not be used as a compression liquid. This is for the reason that the first-named liquor must be controlled in such manner thata predetermined quantity thereof is introduced into the digester, but the last-named liquid must be so controlled that a predetermined pressure is maintained.

It will be understood from point 4 that the compression liquid must not be introduced in the digester at such a point that a variation of the quantity introduced influences the course of the process of digestion. The conclusion is that the compression liquid should be introduced at the bottom of the digester in conjunction with the discharge, where the said liquid has no influence whatever on the course of the digestion itself, whether qualitatively or. quantitatively.

The quantity of compression liquid introduced, which in itself is of subordinate importance, be-

.4 comes automatically equal'to the difference between the quantity of the entire discharge and the sum of the quantities charged into the digester of raw material, liquor, including chemicals, and steam possibly introduced.

By varying the discharge quantity in a suitable manner it is thus possible to introduce a suitable quantity of compression liquid.

As regards the compression liquid itself, the only possibility that presents itself in practice is to use waste liquor, or so-called black liquor, in order not to lower the content of chemicals and dissolved substance in the liquor that is conducted to the liquor recovery. For economical reasons it is not possible to use water as compression liquid.

In accordance with the above considerations, the process according to the present invention mainly consists in maintaining the material and the treating liquor during their movement through the digester at a pressure which is higher than the vapor pressure of the treating liquor at the maximum temperature prevailing in the digester, by pumping a compression liquid into the digester independently of the charging of raw material and treating liquor into said digester. As a compression liquid, waste liquor or so-called black liquor may preferably be used. It is suitable to introduce the compression liquid at the discharge end of the digester, so that said liquid simultaneously serves to dilute and to effect discharge of the digested product.

The present process and an apparatus for performing the same are illustrated diagrammatically and by way of example-in the accompanying drawing. v

Raw material is continuously supplied to the vertical digester 'I from the chip receptacle 2 which is provided with a vibration device 3 for producing a uniform supply of chips to a measuring apparatus 4, for instance in the form of a conveyer screw having a controllable speed, a blade wheel, a shaking apparatus, or some similar wellknown apparatus. Since the digester is assumed to be under a pressure which is higher than the vapor pressure of the digester liquor at the digestion temperature used, the digester must be completely filled with liquor. The feeding of the raw material, such as wood chips, into this liquorfilled space under pressure is illustrated in the drawing as effected by means of a combination of a rotating cell wheel 5, a sieve 6 in a sieve housing provided at the top of the digester and surrounding a feeding screw I, a circulation pump 8, and conduits connecting the cell wheel, the sieve housing, and the pump.

The feeding is effected in the following manner: An empty cell in the cell wheel is partly filled with chips at ii and continues to rotate in the direction of the arrow, more liquor entering the cell at i2, for instance by leakage from It. At It the chips are emptied from the cell by the circulating liquor which is pumped by the pump 8 from the sieve 6 through the connection 8 to the cell in question, and is returned from said cell to the inner side of the sieve 6 wherethe chips brought along are fed downward into the digester by the screws '1, while the liquor is separated through the sieve and is returned in circulation to the cell wheel. The cell continues to rotate after the for gassing off air, for instance, in a well known manner.

While the chips or other raw material are fed into the digester at the top thereof by means of the screw 1, the impregnation or digester liquor may be supplied, in a manner previously known in itself, either directly to the digester at 28, or at 28a through the receptacle l6 and the pump Ii. Fresh liquor, S: or other chemicals, may also be supplied wholly or partially at a later stage, for instance at 28b or at 28c, suitably in connection with a local circulation of the liquor and heating of the same by means of steam, as indicated at H. In the upper portion of the digester the impregnation of the material takes place at a comparatively low temperature, 60 to 120 C., after which heatis supplied at I] either directly in the'form of steam or indirectly by local circulation of the liquor through a heat exchanger in a manner previously known in itself, for which reason such supply of heat has not been illustrated in the drawing. The digestion temperature may for pressure actuated valve 21, so that the desired temperature of 120 C. is maintained at the bottom of the digester. If, on the other hand, the

temperature at the bottom of the digester tends to rise above the said temperature of 120 C. due.

instance be 140 to 180 C., or even higher.

Gases which may be separated due to the heating, will rise upward in counter-current to the material fed into the digester, and will thus produce a certain preheating in the impregnating zone, and, due to the decreasing temperature, the gas will at the same time be dissolved again and take part in the impregnation.

In a similar manner further heat'may be supplied at "a, or if desired, the temperature may be decreased at this point.

The completely digested mass, that is, the mixture of fibres and waste liquor, is discharged from the bottom of the digester, and steam and gases are then allowed to escape at IS in the usual manner, the liquor being separated at 20. while the ready fibre product is conducted away at 2| to be further treated, and the surplus quantity of liquor is conducted at l9 to the liquor recovery. The discharge may be effected, for instance, by the aid of a scraper device 22 and a sluicing device 23 the capacity of which may be controlled by the aid of an automatic relay 2i.

A portion of the liquor separated at 20 is returned by means of the pump 25 and at a sumcient pressure for being introduced at 26 into the bottom of the digester to serve the purpose of diluting the stock or pulp in connection with the discharge of the same. By means of a valve 21 which is inserted in the pump conduit, and which is actuated by the pressure in the digester, the quantity of the compression liquid introduced at 26 is regulated in such manner that the desired compression is maintained in the digester, and consequently, during continuous operation the quantity of liquid pumped in at 26 becomes equal to the difference between the discharge quantity at 23 and the quantities of raw material, digester liquor, and steam introduced at I3, 28, 28b, 28c, l1 and Na, the discharge quantity at 23 being, of course, greater than the sum of the last mentioned quantities.

By constructing the relay 24 controlling the sluicing device 23 as a temperature relay, the discharge may be caused to take place at a desired predetermined temperature. For instance, if the relay 24 is adjusted for the temperature 120 0., then discharge will take place as long as, owing to the supply at 26 of compression liquid having a temperature of, say, 80 C., the temperature in the digester is decreased from, say, 170 C. to 120 C. at the discharge, and during all this time the supplied quantity of compression liquid is of for instance, to the temperature of the compression liquid increasing, then the relay 24 will increase the discharge rate at 23, and as a result hereof the relay 21 will also admit more compressionliquid at 26, so that the desired temperature of C. is maintained at the bottom of the digester. If another temperature, say C., is desired, then the temperature relay 24 is adjusted for this temperature, and in such case a temperature of 30 C. will be maintained at the bottom of the digester. The capacity of the sluicing device 23 shall of course be so large that said device is always capable of discharging not only the quantities of raw material, digester liquor, and steam introduced at I3, 28, 28b, 280, l1, and I la, but also the compression liquid introduced at", irrespective of the quantity thereof, and the temperature of this compression liquid must, of course, always be lower than the discharge temperature.

By the simultaneous use of a valve actuated by the pressure in the digester which controls the supply of compression liquid, and a sluicing device which is influenced by the temperature, the entire course of digestion may thus be automatically controlled and regulated.

The process and the apparatus above described may be used, in principle, regardless of whether the treatment is efiected with bisulphite, suiphate, SOz-water, or some other combination. The supply of chemicals and the supply of heat may be effected in accordance with well known methods which, therefore, need not be further explained.

I claim:

1. The process of continuous digestion of flbrous materials underpressure, which consists in continuously introducing the raw material into an impregnation zone in a vertical digester at the top thereof, said impregnation zone having a temperature substantially below digestion temperature, continuously introducing digester liquor into said impregnation zone in the digester at the top thereof to continuously maintain the digester completely filled with a mixture of raw material and digester liquor, supplying heat to said mixture in a digestion zone below said impregnation zone in said digester, passing said mixture of raw ma terial and digester liquor as a continuous downward flow through the digester from the top to the bottom thereof, continuously discharging the digested product from the bottom of said digester, maintaining said mixture of material and digester liquor during such flow through the digester under a pressure higher than the vaporization pressure of the digester liquor at the maximum temperature prevailing in the digester, thereby preventing generation of vapour from said digester liquor, controlling such pressure by introducing 7 into thedigester in the vicinity oi the bottomand the sum of the quantities charged into the digester per unit of time, and controlling the rate of discharge from the diaester in dependence upon the temperature at the bottom of the digester, thereby eilecting regulation the quantity oi compmsion' liquid introduced in the digester whereby a predetermined temperature is maintained at the bottom of the digester.

2. Apparatus for continuous digestion of iibrous materials, comprising in combination a vertical digester, means for continuously introducing raw material into said digester at the top end thereof, means for continuously introducing digester liquor into said digester at the top end thereof, means for heating the fibrous material and digester liquor in said digester intermediate the top and bottom ends thereof, means for continuously discharging digested product from the bottom end 01' said digester, a conduit connected to said digester in the vicinity of the bottom end thereof for introducing a compression liquid in the digester at the bottom end, means in said conduit actuated by the pressure in said digester for controlling the flow of compression liquid through said conduit so as to maintain a predetermined pressure in said digester, a discharging apparatus having a variable capacity associated with the bottom end of digeeter ior continuously discharging digested product thereirom,

and an adjustable temperature relay actuated by the temperature at the bottom end or the digester and operatively connected to said discharge apperatus so as to control the operation oi said apparatus in response to the temperature at the bottom end of the digester.

JOHAN CHRIBTOH'ER FREDRIK CARL RICHTER.

summons crrnn The following references are 01' record in the ille of this patent:

UNITED sums PATENTS Number Name Date 1,186,655 Freeman June 13, 1916 1,478,614 Morterud Dec. 25, 1923 1,505,934 Olier Aug. 19, 1924 1,679,336 Dunbar July 31, 1928 1,776,761 Morterud Sept. 23, 1930 1,856,453 Bacchus May 3, 1932 1,903,962 Dreyfus Apr. 18, 1933 1,918,181 Cram July 11, 1933 1,933,017 Jansa Oct. 31, 1933 1,954,012 Lemberger Apr. 10, 1934 1,980,390 Dreyfus Nov. 13, 1934 2,008,635 Brubacher July 16, 1935 2,200,034 Merrill May 7, 1940 FOREIGN PATENTS Number Country Date- 519,793 France June 15, 1921 150,782 Great Britain Sept. 6, 1920 63,460 Norway Apr. 28, 1941