US1808830A - Method of and apparatus for digesting wood - Google Patents

Description

June 9, 1931.

J. E. BQND 1,808,830

METHOD OF AND APPARATUS FOR DIGESTING WOOD Fil'ed May 27, 1929 1 \i 22 252% ll 8 25 26 I 0 I9 20 I v 5 J8 J7 I l q M 01-0! I Y 215202172257 Wcfizess l Patented June 9, 1931 UNITED STATES PATENT OFFICE JOSEPHE. BOND, OF APPLETOIN', WISCONSIN METHOD OF AND APPARATUS FOR DIGESTIN'G WOOD Application filed May 27, 1929. Serial No. 366,295.

paratus are commonly employed. This process admittedly has many shortcomings in the quantity of cellulose produced and in the efficiency of the process as a whole but is used in preference'to other more eflicient methods because of the relatively quicker timewithin which pulp may be produced. The process of digestion consists in subjecting the chips to the action of an acid,- and depends for its success on the difference in solubility of the important constituents of wood: that is, the lignin, sugar and resin of wood are relatively more soluble in the particular acid employed than cellulose. However, cellulose under extreme conditions of strength of acid, temperature, pressure andtime is soluble in the same acid and, therefore, the process must be carried on in such manner that the more soluble constituents may be attacked and dissolved without impairing the strength and hydration capacity of the cellulose.

Inasmuch as the wood must for practical purposes be treated in the form of chips or other divisions of substantially uniform size the difiiculty encountered is that of securing penetration of the acid to the interior parts of thersmall body of wood before the exterior portions have been treated to such an extent that the uality of the cellulose fibers is harined. xperience has shown that during the first hour or two in the digester, the temperature should be 110 C. in order to prevent the formation of an insoluble lignin compound around the chips which renders the chips impervious to subsequent treatment. However, it is desirable to raise the temperature of the mass within the digester as quickly as possible. This involves the admittance of steam at high pressure and usually there is a percentage ofburned or useless chips that have beenrendered useless by proximity to the steam inlet. 50 It is a well known physical fact also that solubility isgreatly increased where there is eflicient agitation; this for the reason that if the solvent and the body to be dissolved remain quiescent the solvent immediately around the body becomes saturated and its efliciency is lost. In the processes heretofore employed, circulation of the chips has been effected solely by means of steam jets, although in some cases the acid without the chips has been circulated, as in the Mitscherlich process.

My invention contemplates, in effect, a' combination of the advantages of both the RittenKellner and the Mitsch'erlich processes, and, in addition, a radical departure from the practice commonly employed. For example, I accomplish the quick saturation and cooking of the direct process with the ad vantages of the indirect process.

The invention will be more readily understood by reference to the accompanymg drawings, and will be'more specifically pointed out in the subsequent description of the apparatus employed. i In the accompanying drawings, Fig. l is a vertical sectional view through a digester constructed in accordance with my invention, the illustration being somewhat diagrammatic as'to certain details; and Fig. 2 'is a sectional view on the line 22' of F 1.

' The apparatus which I prefer, to employ consists of a lined shell 10 having a covered inlet opening 11 at the top. through-which the chips are admitted. The casing has a conical bottom 12 and is supported'in the usual manner.

' As a means for carrying out the process of my invention, I make a departure from the usual -digester construction by providing a large centrally disposed pipe 13 having an open top that terminates short of the upper end of the digester. The lower end is in communication with the discharge outlet of a centrifugal or screw pump 14, the impeller of which is indicated at 15. The im p-eller is mounted on a relatively long shaft 116 having outboard bearings 1718 and actuated by a motor 19. The inlet portion 20 of the pump casing is extended to receive a plurality of ship propellers 21 mounted on the shaft 16, having the function of disintegrating the chips, breaking up any lumps or clumpsof material and generally assisting in the agitation. The inlet pipe 22 is in I communication with the lower end of the conical portion of the shell.

Arranged exteriorly of the digester in a,

suitable casing 23 is an acid heating tank containing steam coils 24. A- pipe 25 connects with the inlet pipe 22 and a centrifugal pump 26' in the pipe line 25 withdraws acid from the digester, passes it through the tank and returns it through the pipe 27 to the inlet casing 22 adjacent to the zone of operation of the propellers 21. Fresh acid may be admitted through the pipe 25a, as may be necessary at the start of operations.

Also connected to the casing 21 is a pipe 28 adapted to admit steam if and when required.

Notwithstanding that the digester is filledwith chips at the startof operation, the bulk will be reduced as the operation proceeds. I have therefore extended the pipe 13 near to the top of the casing to insure the flow of acid over the top of the chips. -However, as

digestion proceeds, the bulk will be lessened and there is no necessity for raising the material materially above the level in the digester. I have,therefore,provided means such as the sleeve 29 within the pipe for opening tube 13. The chips in the tube beingloosely and closing ports 30 when it is unnecessary.

to raise the material to the top of the pipe. The sleeve may be operated by means of a screw 31 having an exterior handwheel and. connected to a shaft'32 rigid with the sleeve.

The operation of the apparatus in carrying out my improved process is as follows:

The digester and the pipe 13 are filled with chips in the usualmanner. After the filling opening is closed, acid is pumped through the pipe 25a and 27 and caused to rise in the circulation of both the chips and the acid will begin. If the chips are dry, steam'will be admitted through the pipe 28 along with the acid. The rapid movement of the chips and acid and the complete agitation atthe .point' of admission of'the steam will prevent burning, as the steam will be condensed very quickly.

In order to quickly raise the temperature I of the mass within the digester without diluting the acid with an excessive amount of steam, I withdraw acid through the pipe 25,

and pass it through the heater returning it to the pump inlet in properly heated condi- "tion.

By indirectly heating the acid and by thorough agitation of the digester contents,

the time of cooking is materially reduced and the yield of cellulose is greatly increased.

The cost of operation is also materially reexpense in. the recovery of S0 By eflt'ectivev circulation and the accurate control of temperature during the penetration or satura. tion period, I am able to reduce the per centage of burned chips, slivers, and tailings and the mechanical device employed for circulation assists materially in the disintegration of the wood chips.

The apparatus employed is typical only and other forms may be devised and utilized to advantage without departure from the spirit of my invention.

I claim: v 1

1. The method of operating a digester having a vertical open top tube therein which consists in charging chips into the digester and tube then delivering hot acid to the bottom of the tube and causing it to rise and overflow over the top of the chips in the space around the tube, then continuously pumping acid and chips from the bottom of the space around the tube and discharging said acid and chips upwardly through said tube, withdrawing cooled acid from the digester, indirectly heating the same at a point exterior of the digester and returning the heated acid to the moving body of material.

2.. The method of operating a digester having a vertical open top tube therein which consists in charging chips into the digester and tube then delivering hot acid to the bottom of the tube and causing it to rise and overflow over the top of the chips in the space around the tube, then continuously pumping acid and chips from-the bottom of the space around the tube and discharging said acid and chips upwardly through said tube, Withdrawing "cooled acid from the bottom of the digester and supplying hot acid to the body of material that is being discharged into said tube.

' 3. The method of operating a digester which consists in supplying chips thereto,

then supplying hot acid and steam while con-- tinuously and mechanically circulating the the contents. of the digester until the temperature of the contents is raised to approximately C., then continuing the circulation ofthe contents and maintaining the temperature substantially constant until saturation is complete, then raising the temper? ature to a proper coking temperature and continuing the circulation of the chips and acid. I

4. The combination of a digester having .the inlet to said pump, and means in the intake for disintegrating the chips.

5. The combination of a digester having a vertical open top tube therein, a pump having its inlet connected to the bottom of the space surrounding the tube and its outlet connected to "said tube, means for supplying hot acid to the digester at a point adjacent to the inlet to said pump, and a ship propeller in the intake for disintegrating said chips before they enter the pump. v

6. The combination of a digester having a vertical open top tube therein, a pump having its inlet connected to the bottom of the space surrounding the tube and its outlet connected to said tube, and means for withdrawing cooled acid from the bottom of the digester, heating the acid exteriorly of the digester and returning the heated acid to the pump.

7. The combination of a digester having a. vertical open top tube therein, a pump having its inlet connected to the bottom of the space surrounding the tube and its outlet connected to said tube, means for continuously. withdrawing cooled acid from the lower portion of the digester, means for indirectly heating the withdrawn acid, and means-for returning the heated acid to the moving body of chips and .acid passing through the pump.

8. Ina digester the combination with the casing of a vertical open top pipe therein, a pump having its outlet connected to the pipe and its inlet connected to the bottom of the casing, and means for varying the effective length of the pipe.

9. In a digester the combination with the casing of a vertical open top pipe therein, a pump having its outlet connected to the pipe and its inlet connected to the bottom of the casing, means providing a port in the pipe at a point below its upper end, andmeans operable from the exterior of the digester for opening said port when the level of material in the digester is adjacent to said port.

In testimony whereof I have aflixed my signature.

' JOSEPH E. BOND.

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