US2996421A - Pulp manufacture - Google Patents

Description

Aug. 15, 1961 F. B. K. GREEN 2,996,421

PULP MANUFACTURE Filed Dec. 18, 1957 8 Sheets$heet 1 INVENTOR 1 rank ,B. 5191? en "m J4. 2AM

ATTORNE Aug. 15, 1961 F. B. K. GREEN PULP MANUFACTURE 8 Sheets-Sheet 2 Filed Dec. 18, 1957 91 een W I 6 ATTORNE Aug. 15, 1961 F. B. K. GREEN 2,996,421

PULP MANUFACTURE Filed Dec. 18, 1957 8 Sheets-Sheet 4 INVENTOR rankfll' ATTORNE 1961 F. B. K. GREEN 2,996,421

PULP MANUFACTURE INVENTOR.

Fran/ E. If. reen xim k ATTORNEY Aug. 15, 1961 F. B. K. GREEN PULP MANUFACTURE 8 Sheets-Sheet 6 Filed Dec. 18, 1957 I NTOR Frank B. .jreerz M ATTORNEA Aug. 15, 1961 F. B. K. GREEN 2, 96,

PULP MANUFACTURE Filed Dec. 18, 1957 8 Sheets-Sheet '7 @L/ Li i 7 /80 INVENTOR Frank 5K. green ATTORNE 8 SheetsSheet 8 Aug. 15, 1961 F. B. K. GREEN PULP MANUFACTURE Filed Dec. 18, 1957 2 ILF I e g 6w 3 m QM as x, S W Qww .E o Qwwm MB I {I u iwfiwmwmm ml mm S m x Q 8N F .QN NQN K 8w United States Patent 1 Claim. Cl. 162-17 This invention relates to the manufacture of pulp from wood or other fibrous material for making paper and the like and, more particularly, to methods and apparatus for cooking or digesting or softening the wood or other fibrous material to convert it into a pulp by a continuous operation.

As a result of technological and economic developments and considerations in the manufacture of paper, paper board and the like from wood pulp or pulp of other fibrous materials, most of the operations in a pulp and paper mill other than the pulp cooking and digesting steps are carried on in a continuous fashion instead of in a batch-type manner. Accordingly, it may also be desired that the initial cooking and digesting steps wherein the ligneous and other non-cellulosic encrustan-ts or binding materials in wood are softened or removed to render the wood fibres desirably separable for preparation of a pulp slurry also being carried out in a continuous manner for integration with the other continuous processes of the mill.

According to this invention, then, methods and apparatus are provided for continuously processing chips of wood through cooking and digesting steps wherein the wood is subjected to the softening or cooking or digesting action of chemical liquors at elevated temperatures and substantially super-atmospheric pressure to discharge the cooked wood into subsequent pulp mill operations and apparatus. The apparatus embodying and for practicing this invention also is provided for disposition in a pulp mil-l to occupy a minimum of space and yet provide a plurality of parallel, separately controllable lines of flow for concurrently processing the same or different types of wood or other raw materials through a plurality of softening and cooking and digesting zones with an optimum efiiciency in subjecting the raw material to a plurality of different cooking or digesting liquors and/ or at a plurality of concentrations as the cooking proceeds for achieving optimum cooking efliciency and minimal degradation of the cellulose fibres during the cooking steps for optimum yield of finished product.

One object of this invention is to provide a method of the character described for continuously processing wood or other fibrous materials through a plurality of cooking or digesting steps or zones at elevated temperatures and pressures, and discharging the cooked or digested raw materials under pressure for further preparation into pulp for making paper, paper board, or the like.

Another object of this invention is to provide a method of the character described for continuously subjecting wood or other fibrous materials to the action of cooking or digesting liquors or chemicals at elevated temperatures and pressures and continuously discharging the cooked or digested raw materials under pressure.

Still another object of thi invention is to provide a method of the character described for subjecting chips of wood or other fibrous cellulosic materials sequentially Patented .Aug. 1 5, 1961 to the cooking or digesting action of cooking or digesting liquors having a plurality of different concentrations and as a continuous process.

A further object of this invention is to provide apparatus of the character described for continuously and automatically processing pulp raw material'through feeding, softening, cooking, and blowing steps at elevated temperatures and pressures as a continuous flow operation.

A still further object of this invention is to provide apparatus of the character described for continuously cooking or digesting and processing chips of wood or the like at elevated temperatures and pressures and having provision for subjecting the chips to a plurality of diiierent cooking liquors or to cooking liquors of a plurality of different concentrations continuously.

'Still another object of this invention is to provide apparatus of the character described for continuously cooking or digesting chips of Wood or the like for pulp-manufacture through a plurality of diiferent feeding, softening, cooking and blowing zones or steps, and including means for arranging the apparatus for continuous flow operation to occupy a minimum of space in the pulp mill.

A still further object of this invention is to provide, in continuous digesting apparatus of the character described, means for arranging and-supporting and interconnecting the various components of the apparatus for compensating for varying thermal expansion movement .and strains and stresses-among the various components operating at diiferent temperatures and pressures.

Other objects and advantages will be apparent from the following description, the accompanying drawings, and the appended claim.

In the drawings- FIG. 1 is a side elevation view of apparatusembodying and for practicing this invention;

FIG. 2 is an end elevation view from the left end of .FIG. 1 ofapparatus embodying and for practicing this invention;

FIG. 3 is a partial vertical axial sectionthrough the chip feeding component of FIG. 2 along the line 3-3 thereof;

FIG. 4 is a view on a larger scale partly in section and partly brokenaway of the pre-steaming apparatus of FIG. 2 along the line 44 thereof;

FIG. 5 is a vertical axial section partly broken away of the digesting component of the apparatus of FIG. 2 along the line.55 thereof;

FIG. 6 is an enlarged end view of the rotary valve element of apparatus embodying and for practicing this invention;

FIGS. 7 and 8 are partial vertical axial sections on an enlarged scale of pressure equalizing means of the valve shown in FIG. 6;

FIG. 9 is a vertical transverse section along the line 9-9' of FIG. 5

FIG. 10 is a vertical transverse section on enlarged scale of one of the cooking liquor inlets of the digester component of the apparatus along the line 1010 of FIG. 13;

FIG. 11 is a bottom view of the detail of FIG. 10; FIG. 12 is a horizontal section on the line 12r--12 of FIG. 10;

FIGS. 13 and 14 are top and side elevation views, respectively, of the cooking liquor piping arrangements around the digester component of the apparatus of FIG. 1;

FIG. 15 is a diagrammatic top view indicating cooking liquor spray patterns within the digester component of the apparatus of FIG. 1;

FIG. 16 is a view from above of the inside bottom portion of the digester component of apparatus of FIG. 1 with some of the internal structure eliminated for clarity.

As will be understood from the following description, the novel features and advantages of a continuous digesting method or process according to this invention may be achieved with a variety of apparatus constructions, although the invention will here be described in connection with the preferred embodiment of apparatus here illustrated. Referring to the drawings, in which like characters of reference designate like parts throughout the several views thereof, continuous digesting apparatus embodying and for practicing this invention is indicated generally in side view and end view respectively in FIGS. 1 and 2. as comprising feeding apparatus for introducing chips of wood to be cooked into the digesting system and having driving means 11 operatively connected thereto, pre-steaming apparatus 12 for subjecting the chips of wood or other material to a preliminary steaming treatment or step and having its driving means 13 operatively connected thereto, cooking or digesting apparatus 14 with its associated drive means 15 for subjecting the wood chips or other material being cooked to the progressive cooking steps according to this invention, rotary valve means 16 with its associated drive means 17 for delivering and metering chips from pre-steamer '12 into digester 14, and discharge apparatus 18 with its associated drive 19 for discharging the cooked chips from digester 14 through blow line 20 for further processing and treatment to provide the character and quality of pulp or other fibrous material desired.

As indicated more particularly in. the end view of FIG.

2, each of the aforementioned elements is preferably provided in pairs side by side (although the pair of digesting elements here illustrated are both enclosed within the same outer shell as hereinafter described) to provide two independent parallel vertical processing lines or flows for handling continuously and concurrently two lines of material being processed. This preferred parallel arrangement provides extra operational latitude and throughput control since, as described hereafter, one kind of material or cooking chemicals may be processed through one line of flow while, at the same time, either the same or a different material and cooking chemicals are processed through the parallel line of flow as may be required or desired by the capacity requirements of the mill. Al-

though only one of each of the duplicated general elements of this apparatus may be described in detail, it should be understood that the corresponding member of the second parallel line of flow is substantially a duplicate of the one described with, in some instances, the drive meansbeing mirror images, as indicated in FIG, 2.

The feeding apparatus '10 (see FIG. 3) comprises a generally horizontal elongated barrel or casing 25 having a feed opening or hopper-like inlet 26 in flow communication with an overhead supply of chips to be cooked such as a conventional pulp mill chip bin (not shown) The chips are dropped by gravity into inlet 26 where a screw feeder 27, rotatably driven by a motor 28 through a suitable reduction gear and drive 29-31, urges the chips in a more or less compacted mass to the right of the drawing through barrel 25 so that the chips drop by gravity into and through the vertical outlet 35. Since, as will be explained below, the chips are dropping into a vessel having a superatmospheric steam pressure therein,

means are provided to avoid escape of this steam pressure upwardly or back through the feeding apparatus 10. Such means include a constriction 36 in the inner diameter of barrel 25 'so that, as screw 27 forces chips through barrel 25 and past constriction 36 (toward the" right in in the small diameter portion 37 of the feeding apparatus,

which plug of chips tends to prevent the escape of steam backward through the apparatus. To prevent blow back of steam as at the start-up of the apparatus and/ or at any other time when a plug of chips is not present, a valve plate 40 is provided mounted on a piston 41 operatively connected to a hydraulically or pneumatically actuated cylinder-and-piston arrangement 42 and mounted ,for horizontal movement from the solid line closed position of valve plate 40 in FIG. 3 to the dotted line open position thereof. With valve plate 40 open and to the right in FIG. 3 (the dotted line position thereof) screw 27 forces the plug of chips to the right, thus permitting the chips to break otf from the right hand end of the plug and drop by gravity downwardly through outlet 35. When valve 40 is closed, steam can be controllably vented through vent 43 and/or bypassed to equalized pressure on either side of valve plate 40 through the bypass arrangement, 44, 45.

As indicated in FIG. 1, the entire feeding apparatus and its drive are preferably mounted on an elevated floor 46 of the mill above the remainder of the apparatus, with the chips being dropped by gravity through outlet 35 and into subsequent apparatus through a hole in the floor. The chips drop from outlet 35 through a conduit 50 into the pre-steaming apparatus 12. An expansion joint 51 or other expansion compensating means is preferably interposed at some point between the outlet 35 of feeding apparatus 10 and the inlet 52 of pre-steaming apparatus 12.

Pro-steaming apparatus 12, which is considered a preferred although optional portion of the overall system, comprises essentially a cylindrical tube or casing 60 having therein a conventional screw conveyor or feeder comprising screw flights 61 mounted on a shaft 62 journaled at 63 in the respective ends 64 of pro-steaming apparatus 12 with suitable pressure-resistant packings. Screw conveyor 61, 62 is rotatably driven by a conventional drive 13, and the direction of rotation and pitch of flights 61 is such as to urge chips introduced into the inlet 52 of pro-steamer :12 to the left in the drawing (FIG. 4) to traverse substantially the entire length of the casing 60 and to drop by gravity out of the outlet 65 thereof.

During passage through pre-steamer 12 the chips are subjected to the action of steam introduced into the interior of casing 60 at one or more steam inlets indicated as 70, 71, and 72 to achieve preliminary steaming, satu ration, and softening of the chips prior to actual contact with cooking or digesting chemicals. As Will be understood, of course, for certain applications it may also be desired to introduce some chemicals in addition to steam for a preliminary softening or preparatoly step to condition the chips better to receive the principal or primary cooking actions in the subsequent digester '14.

Shaft 62 of the screw conveyor 61, 62 also carries adjacent outlet 65 a plurality of breaker bars 75 adapted to act on the steamed chips adjacent the outlet 65 and break up the plug of chips being conveyed toward the outlet by the screw feeder 61, 62 in order to provide a more uniform dropping of chips out of outlet 65 and also in order to break up inordinately large agglomerations of A bafiie arrangement 76 is also provided at one side of outlet 65 to work, in conjunction with breaker bar 75, to provide a more even flow of chips leaving the outlet from the screw conveyor.

As shown in FIGS; 1 and 2, the pre-steaming apparatus 12 is preferably supported above the subsequent apparatus and suspended resiliently from the floor 46 as by hangers 80 including springs 81 and depending tie rods 83 engaging beams 84 on which rest casing 60. In this manner substantially the entire Weight of pre-steaming apparatus 12 is carried from above, rather than having 'to'ibe supported 'on the remainder of the apparatus below pro-steamer 12, yet the height of pro-steamer 12 with respect to the following apparatus is completely and automatically adjustable as required by the differential thermal expansion movements of the various elements of the whole system which may be subjected to varying degrees of temperatures at various times Without unduly straining the conduit connections at inlet 52 and outlet 65 of pre-steamer 12. A hollow link or pipe 86 interconnects the end of casing 60 opposite to outlet 65 with the corresponding end of digester 14, and pipe 86 is heated with steam to cause heat expansion thereof as rotary valve 16 expands with heat to aid in maintaining substantial axial parallelism between 60 and 14 during heat expansion movements therebetween.

From outlet 65 of pre-steainer 12 the treated chips drop by gravity through a conduit 85 into rotary valve apparatus 16 which delivers and meters chips into digester 14 and, since the cooking pressure in digester 14 is maintained at a level substantially above the super-atmospheric pressure in pre-steamer 12, rotary valve apparatus 16, also is for preventing the escape of such higher pressure from digester 14 back into pre-steamer "12.

Although the rotary valve apparatus 16 may satisfactorily be constructed in a number of dilferent ways to provide the desired functioning, a preferred construction is as shown in my copending application Serial No. 703,590, now U. S. Patent No. 2,933,208, executed and filed on even date herewith. As indicated therein and in the drawing here (see FIGS. 6-8), such structure comprises a substantially cylindrical casing 90 in which is a rotor 91 having radially extending blades 92 mounted for rotation on a shaft 93 rotatably driven by drive means 17 comprising a motor 95 and reduction gear drive 96. At the top of rotary valve 16 is a chip inlet 100 in flow communication with conduit 85 for receiving chip dropping by gravity therethrough, and at the bottom of rotary valve 16 is a chip outlet 101 in flow communication with the chip inlet 102 of digester 14.

Each pair of adjacent blades 92 defines therebetween a pocket for receiving chips entering through chip inlet 100 and for carrying the chips around to drop them out of chip outlet 101 as rotor 91 is rotatably driven by drive means 17. The radially outer edges of blades 92 are in sliding contact with the inner surface of cylindrical casing 90 so that, during rotation of the bladed rotor 91, as each pocket between two adjacent blades 92 comes into flow communication with chip inlet 100, chips drop thereinto, and the softened chips cooperating with the radially outer edges of blades 92, which just clear the inner surface of casing 90, form a seal against escape of pressure from digester 14 as the pocket full of chips is carried around and dropped through chip outlet 101 intochip inlet 102 of digester 14.

As a further means of pressure control at rotary valve 16 and to lessen the bearing load on shaft 93 produced by the substantial pressure difference above and below rotary valve 16, a system of pressure bypasses, vents and pressure equalizers is preferably provided as indicated in the aforementioned copending application and, in some detail, in FIGS. 6-8. Thus a bypass conduit 105 interconnects a point inside casing 90 at substantially the 8 oclock position in FIG. 6 with another interior point at substantially the 4 oclock position. A further bypass conduit 106 connects a point on the interior casing 90 at the 9 oclock position with a substantially diametrically opposite point at the 3 oclock position.

As will be seen from the foregoing, a pocket between adjacent rotor blades 92 at the 6 oclock position of FIG. 6 drops its load of chips through chip outlet 101 and, accordingly, the steam pressure in that pocket is then the high pressure of digester 14. With continued rotation of rotor 91, that same pocket passes and comes into flow communication With the 8 oclock end of bypass 105. Simultaneously, another pocket full of chips about to be dropped through chip outlet 101 is in flow communication with the 4 oclock position of bypass 105,

and the high steam pressure in the 8 oclock pocket is automatically bled 011 and equalized with the lower steam pressure in the 4 oclock pocket so that the pressure differential between digester 14 and the 4 oclock pocket is reduced prior to the 4 oclock pocket reaching the 6 oclock position, and the pressure differential between the 8 oclock pocket and pre-steamer 12 is simultaneously reduced. In the same manner, as the 8 oclock pocket reaches the 9 oclock position in FIG. 6, still a different pocket loaded with chips is then in communication with the 3 oclock end of bypass 106 and a further pressure equalization is obtained through bypass 106 with a further reduction in the pressure differential between the 9 oclock pocket and chip inlet A vent conduit 107 is provided at the 10 oclock position in FIG. 6 and a further vent or pressure equalizing conduit 108 is provided at the 2 oclock position. These two conduits communicate directly with the interior of pre-steamer 12 at steam inlet 70 thereof for further pressure equalization or adjustment between an empty pocket passing the 10 oclock position on its Way to pick up a load of chips from chip inlet 100 and a just filled pocket passing the 2 oclock position on its Way down to drop its load of chips through chip outlet 101.

Suitable connections for the aforementioned bypass and pressure equalizing conduits are indicated generally in FIG. 7 and 8 as comprising pipe connections 110 penetrating the end wall 111 of valve casing 90 and in flow communication with passages 112 leading to the interior of casing 90 where they communicate sequentially with pockets between adjacent rotor blades 92 as such pockets coincide with the inner open ends of passages 112.

As each pocket full of chips drops through chip outlet 101 of rotary valve 16, it enters digester 14 through chip inlet 102 thereof to be subjected to chemical cooking or digesting liquor and high pressure and temperature as it passes through the several cooking or digesting zones in digester 14.

A preferred satisfactory construction for digester 14 is indicated more particularly in my copending application Serial No. 703,588, now US. Patent No. 2,969,113, executed and filed of even dates herewith, and in some detail here in FIGS. 5 and 9-16. Digester 14 comprises an outer substantially cylindrical shell within which are mounted a plurality of troughs 121-124. In these troughs are screw conveyors 125-128, respectively, rotatably mounted on shafts 129-132, respectively, journaled in the end plates 133 and 134 of shell 120 and rotatably driven from drive means 15 mounted at one end of digester 14 and comprising electric motors and 141 with corresponding reduction gear and chain-and-sprocket drives 142-145. The screw conveyor flights and direction of rotation are arranged to provide for urging chips dropped into the upper troughs 121 and 122 from the chip inlets 102 axially toward the right in FIGS. 1 and 5, while the lower screw conveyors 127 and 128 urge chips in the lower troughs 123 and 124 axially to the left in FIGS. 1 and 5.

As will be understood in the structure here illustrated, four troughs and screw conveyors are shown as a part of the system having the two independent parallel lines of flow mentioned above. Thus, troughs 121 and 123 are in one line of flow whereas troughs 122 and 124 are in the separate parallel line'of flow and are operated independently of troughs 121 and 123 regardless of the fact that all four are contained within the single cylindrical casing 120 of digester 14. As with the foregoing elements of the apparatus embodying and for practicing this invention, only one set of troughs and screw conveyors, 121, 123, 125, 127 will be discussed in detail,

the corresponding set of troughs and screw conveyors being understood to be substantial duplicates.

As chips enter chip inlet 102 from rotary valve 16, they fall upon trough 121 and are conveyed axially therealong (to the right in FIGS. 1 and 5) by screw conveyor 125,which is preferably provided with a circular baffle or flange member 148 at the left or entrance end thereof. In progressing axially along trough 121 the chips are passed beneath a plurality of cooking or digesting liquor inlets 150-154 from which cooking or digesting liquor is sprayedonto' the chips. Each of these liquor inlets comprises a plurality of sprayheads 155 mounted on a circular cover 156 carried by inlet collars or flanges 157 in the top of shell 120 of digester 14.

As indicated more particularly in FIGS. -12, each cover 156 has a depending flange 166 and a bottom plate 161 through which are mounted sprayheads 155. A transverse partition 162 divides the interior of cover 156 into two separate chambers whereby the liquor introduced through pipe 163 may be separately controlled and, indeed, of completely diiferent composition than the liquor introduced through pipe 164. A portion of the sprayheads (indicated as three in FIGS. 11 and 12) are arranged with their outlet orifices 165 directed to spray liquor from pipe 163 toward one side of digester 14 to accommodate, for example, trough 121 while the remaining sprayheads 155 are directed to spray liquor received from pipe 164 to the opposite side of the digester to accommodate trough 122. That is, covers 156 are oriented in collars 157 in casing 120 so that the partitions 162 run axially of casing 121).

In this fashion, a liquor spray pattern is achieved as indicated somewhat diagrammatically in FIG. 15 with liquor being introduced to the chips in the upper digester trough at a plurality of points spaced axially of the digester and with, at each point, a separately controllable spray pattern directed independently to each of the two parallel lines of flow. As pointed out hereafter, the provision of a plurality of separate liquor inlets 150-154 has the advantage, according to this invention, of subjecting the chips being cooked or digested to cooking or digesting liquor of varying concentrations in various Zones of the digester as the chips progress therethrough so that substantially raw chips entering the digester at chip inlet 102 may first be subjected to highly concentrated cooking liquor at inlet 150, thereafter somewhat less concentrated cooking liquor through inlets 151 and 152, and, after the chips have progressed substantially through the upper trough of the digester, to substantially less concentrated cooking liquor through inlets 153 and 154 whereby an optimum efliciency and use of cooking chemicals is achieved as well as a minimum of degradation of the cellulose as might be experienced if chips in an almost cooked condition were subjected to the harsh action of strong cooking liquor. The internal pressure within casing 120 is maintained at the desired high level as herein described by the admission of steam through steam inlet 166.

Since it is desired, according to this invention, to saturate the chips with cooking liquor and thereafter subject them to high steam pressure-rather than to maintain the chips immersed in liquor phase cooking liquor-the bottoms of upper troughs 121 and 122 are perforated so that liquor sprayed on top of the chips as they are continuously being agitated by screw conveyors 125 and 126, will saturate the chips as completely as may be, and then any excess will drain off through the perforated trough so that, at various points along the travel of chips through the digester, the chips are being subjected to cooking liquor which they may soak up, but are not being agitated or maintained in a liquid pool of liquor.

The excess liquor draining from trough 121 is ultimately collected, as described hereafter, at the bottom of digester 14 and removed through liquor outlets 170 and 171. Because of the arrangement of dam 172 and overflow arrangement hereafter described in the bottom section of casing 120 of digester 14, liquor draining from the left half of trough 121 is collected through outlet 170 separately from liquor draining from the right half of trough 121, which is collected through outlet 171. These 8 separately collected batches of'liquor, then, being of different concentrations since they are collected at different points in the cooking cycle, may be recirculated as indicated in FIG. 14 for re-application to chips moving through the digester.

Thus, fresh and highly concentrated cooking liquor is introduced initially through the first liquor inlet and drains down to the left side of dam 172 still substantially concentrated. This liquor is collected through outlet and is recirculated by pump 175 driven by motor 176 up through conduit 177 to be introduced at liquor inlets 151 and 152. through pipe 163. After the chips have progressed approximately one-half the length of trough 121, however, a substantial cooking or digesting effect has been obtained. Accordingly, the chips in the right half of trough 121 may be subjected to a substantially less concentrated cooking liquor, and this less concentrated liquor is withdrawn from a pool on the right side of dam 172 through outlet 171 and recirculated by pump 180 driven by motor 181 through conduit 182 to be sprayed on the chips on the right half of trough 121 through liquor inlets 153 and 154 in substantially the same manner as above described.

The right-hand end 185 of trough 121 is spaced from the inside 186' of head or end plate 133 of shell 120 of digester 14 so that, as the chips in trough 121 reach the end 185- thereof, they drop by gravity onto lower trough 123 along which they are conveyed back toward the left of the drawing by screw conveyors 127 until they reach the left end 190 of trough 123 which is spaced from the inside surface of end plate 134 of casing 120 of digester 14, at which point the chips, now completely cooked, drop by gravity through chip outlet H5 into the inlet 196 of discharger apparatus 18, described below.

The bottom of trough 123 is also perforated as with trough 121 so that liquor continues to drain through the chips and out of the troughs. It will also be noted that trough 123 is positioned within casing 120' of digester 14- spaced above the bottom thereof to provide beneath trough 123 a collecting pool or sump for collecting the liquor draining through the chips for recirculation, as above described. The aforementioned dam 172 extends across this sump beneath trough 123 approximately at the axial midpoint of the digester 14 whereby, as noted in FIG. 16, the collecting sump beneath the lower trough is divided into approximately two equal size pools for collecting the liquor for recirculation.

A partition 200 extends axially along the centerline of casing 120 to divide the liquor collecting pool beneath the lower trough axially so that there is a pool or sump for each of the parallel lines of flow mentioned above. An additional dam 201 is provided immediately adjacent chip outlet to prevent the uncontrolled flow of liquor in the pool or sump out of chip outlet 195. The foregoing arrangements are diagrammed in FIG. 16 where the elements designated as 202204 represent the liquor and chip outlets for the other one of the parallel lines of flow mentioned above.

The heights of dams 172 and 2191 are correlated with dam 201 being higher than dam 172. Thus, as liquor collects in the left sump and the level therein rises, the cooking liquor will overflow dam 172 into the right-hand sump-before the level of liquor behind dam 201 reaches a height to overflow and escape out chip outlet 195. In this manner, the more highly concentrated liquor in the left half of the digester automatically and constantly replenishes the less concentrated liquor in the right half of the digester, but diluting of the more highly concentrated liquor at the left with spent or weak liquor at the right is avoided.

An overflow pipe 210 is also provided leading from an open overflow end 211 at the right of dam 172 directly through dam 201 to discharge liquor out of the end 212 of pipe 210 directly into chip outlet 195. The height of overflow 211 is lower than the dam 172 so that the level '9 of spent liquor to the right ofdarn l172 cannot build up sufliciently to overflow that dam anddilute the more concentrated liquor. The level of spent liquor to the right of dam 172 will build up, and such liquor is automatically drawn ofi through overfiow 211 and pipe 210 anddischarged directly out of inlet 195.

It will be noted that the arrangement described above as illustrative of apparatus embodying and for practicing this invention results in a continuous digester arrange 'ment whereby the chips are subjected to three distinct cooking zones in the digester 14. First, the raw chips in the left half of trough 121 are subjected to theaction of concentrated cooking liquor for the period of time it takes them to travel about halfway along trough 121. Secondly, a longer cooking zone wherein the chips are subjected to more dilute liquor is provided in the right half of trough 121 and the right half of trough 123. A third and final zone whereinthe chips are subjected only to vapor phase or steam pressure without the addition thereto of any cooking liquor may be provided in the 'left portion of trough 123 immediately prior to the discharge of the chips through outlet 195. To this end, a transverse solid horizontal partition 215 is provided beneath the left half of trough 121 and overlying the entire width of trough 123 so that liquor draining'out the perforated-bottom of the left half of trough 121 flows across partition 215 and down around the outside of trough 123 tothe sump in casing 120 therebelow but does not come in contact with the chips in the left half of trough 123. This partition 215, however, extends for only about half the length of the digester so that the more dilute liquor draining from the perforated bottom of'trough 121 at the right half thereof will drain directly onto chips in the righthalf of trough 123.

It will be understood, of course, that, although the aforementioned three distinct cooking zones are described here, suitable arrangement of a plurality of liquoroutlets and liquor pool dams and recirculation piping and pumps may be made to provide any number of distinct cooking zones and treatment effects as maybe desired in practicing this invention, or, if desired, liquors of the same, instead of different, strength or concentration may beintroduced and/ or recirculatedthroughout. It should also be noted, as diagrammed in FIG. 16, that axial'baflies 220 are provided on the bottom of casing 120 and extend throughout a substantial portion of the length of each half thereof and spaced from liquor outlets 170 and 1-71. The purpose of thesebaflles is to force liquor in the separate pools beneath the lower troughs to flow or circulate around the baffles as it is withdrawn through liquor outlets v17} and 171 to maintain solid matter in suspension in theliquor and avoid deposition of suspended solids-in the sump portion at the bottom of casing 120 of digester '14.

The entire digester 14-is preferably supported from below on supports 225 and 226 adequately abovedischarger 18. As will be noted, digester supports 225' and 226 then carry substantially the entire weight of digester14 and its associated drive 15 as well. as rotary valve 16 and its drive 17 as indicated in FIGS. 1 and 2. Since pro-steamer 12 is preferably supported from above, as before described, from resilient hangers atfixedto theifloor above, ithisarrangement makes for satisfactory compensation for'vertical thermally-induced heat expansion movements ofthe various component parts without undue strainon the digester 14 or its supports 225, 226. The eifect, in this connection, of expansion joints 51 in conduit 50 and of tie rod 86 has already been noted. Also expansion joints 133 and .184 are provided in liquor conduits and outlets 170, -171, 177, .182 etc., as desired or necessary.

Referring again to FIGS. 1 and .2,.in the system illustrated, the chips leave digester Li -through chip outlet i195 thereof after the cooking or digesting is completed to the degree desired, and drop into discharger 18 through inlet 196 thereof. "-Discharger '18 discharges the cooked woo-d into blow line 20 for washing, refining, and/ or other subsequent operations tobe performed on:the pulp. As will be understood, a numberv of different blowing, discharging, or refining means may follow digester'14in the line of flow and receive the cooked chips therefrom, .as is well known. A preferred form of discharger for the system as illustrated is shown and described in more detail in my copending application Serial No. 703,589, executed and filed on even dates herewith, as comprising. aniouter casing 260 within which is rot-atably mounted a horizontal screw conveyor (not shown) driven by motor .261 through a suitable reduction gear and drive means 262, 263. As the cooked chips drop into casing 260 through inlet 196, the screw conveyor urges them to the left in FIG. 1, and a series of blades on'the conveyor (described in more detail in my said copending application) sweeps the mass of chips and whatever cooking or digestingliquor may be present past a discharge orifice adjacent blow line 20 and controlled by blow valve 265. As the chips pass the orifice and entrance to blow line 20, the pressure in digester 14 (with which, of course, casing 260 isin flow communication) blows the chips through the orifice and out blow line 29.

Among the advantages of the foregoing apparatus will be noted enhanced operational flexibility. For example, as noted, different kinds of wood (e.g., soft wood such as pine and hard wood such as gum) may be simultaneously processed through the apparatus if it isdesired that the pulp being made into paper should consist of an admixture of different woods. Also'it may be found that, for certain type or quality of paper, part of the pulp furnished to the paper machine should consist of pulp cooked to one particular yield and another part to a different yield. As Will-be apparent from the foregoing description, such a situation is readily handled by the system embodying and 'for practicing this invention by controlling or adjusting the concentration of cooking liquor and/or the length of time required for the chips to pass through the cooking or digesting apparatus. Similarly, it may be desired for a certain type or quality of paper-that part of the-pulp be sulfate or kraft pulp and another portion be manufactured by the well-known sulfite or neutral sulfite processes, and both kinds of pulp can simultaneously be processed through the foregoing apparatus. Also, in such situations, the various kinds or'degrees of pulp are readily mixed and metered by controlling relative speed through the digester, and, if desired, may all be dischaged therefrom through a single common discharge apparatus.

Another important feature of apparatus embodying and for practicing this invention relates to the enhanced uniformity of cooking which can be achieved. Uniform cooking conditions for making pulp from wood depend upon a number of Variable factors, oneof the most important of which is uniform heat distribution. Obviously the desired uniform heat distribution is quite diflicult to achieve in conventional batch-type pulp digesters because of their huge size, etc. Truly uniform conditions are also difficult to attain even in continuous digester apparatus where the arrangement consists of merely a cylindrical tube through which the chipsare urged by a screwmuch like the structure of pre-steamer 12 described above since, with such a tube substantially filled with chips, there is only a relatively narrow surface or area near the top of the tube where the chips are directly exposed to steam, and a rather large and preponderarte area around the sides of the tube for almost direct heat transfer from the chips through the tube walls.

In cooking or digesting apparatus according to this invention, by contrast, steam can circulate freely around and among troughs 121-124 for much more uniform heating of the chips within casing 120, and only a very minor portion of the chips in the digester is in contact with or directly exposed to the outer walls of casing 120. Also the surface of each batch of chips directly exposed to either a vapor area or cooking liquor spray, in' this apparatus, is much greater than a continuous digester comprising merely one or more individual tubes as'in pre-steamer 12i.e., no matter how full of chips are troughs 121-124, still the top surface of the mass of chips directly exposed to a vapor area or to liquor spray is'as wide as the greatest diameter of the trough or screw conveyor. Thus much more uniformcooking is achieved, both from the standpoint of uniform heat distribution as well as increased chip area directly exposed to the cooking liquor spray.

This advantage, as will be understood, is obtained whether the apparatus is constructed as illustrated with a separate trough for each screw conveyor or whether, as may be desired, the apparatus is constructed with a wider trough common to two or more screw conveyors, so long as there is a plurality of troughs and screw conveyors within a common outer casing 120 so that only a very minor portion, if any, of the chips directly contacts the outer casing, so that there are passages for vapor circulation between and among the troughs, and so that as large as possible a surface area of the mass of chips is directly subjected to vapor and cooking liquor spray.

It should also be noted that uniform cooking of wood chips to a uniform yield is rarely, ever, obtained with pulping processes and apparatus heretofore used. As will be understood, in conventional pulping processes and apparatus the actual yield of the pulp produced is a composite of different yields even within the same batch. Such variations stem from a variety of factors. Thus, since the yield varies with the cooking temperature, nonuniform heat distribution within the digester produces variations in yield as between chips in the center of the chip mass, other chips in contact with the outer shell of the digester, and other chips directly subjected to the admitted steam. Non-uniform liquor circulation is another factor, as well as localized dilution of the cooking liquor by the admitted steam and/or non-uniform liquor concentrations at various points in the digester. Variations in the moisture content of the chips entering the digester and variations in the density of the Wood also have a non-uniform effect on the rate at which the cooking liquor will penetrate into the chips and, consequently, the rate at which the chips will be cooked. Another variation, and one of the most important, stems from variations in the size of the chips themselves since smaller chips will be penetrated more quickly by the cooking liquor than larger chips with, of course, variations in the rate of cooking and the final yield achieved.

According to this invention, however, it is preferred to screen the chips prior to introducing them into hopper 26 of feeding mechanism whereby smaller chips are fed into one of the parallel lines of flow while larger chips are fed into the other. In this manner, a greater uniformity of yield and cooking rate is achieved'by subjecting the smaller chips to less cooking than the larger chips either by varying cooking liquor concentration or by varying the through-put rate at which the chips are fed through digester 14.

A similar advantage is achieved according to this in vention by providing one or more additional rotary valves 16 for introducing into digester 14 chips at one or more additional points spaced along the digester so that the largest chips are introduced through chip inlet 102 and, thereafter, progressively, smaller chips are introduced at later positions along digester 14 so that, although all the chips are mixed together as they leave chip outlet 195, the smaller chips have been cooked for less time or at a less rate than the largest chips by virtue of being introduced to digester 14 at a later point in the travel of the chips through the digester.

Such variations in chip size, as is well-known, inevitably occur in pulp manufacture since wood for cooking into pulp is conventionally cut into chips by apparatus which is essentially a rotating disk with a multiplicity of knives thereon. Even when the length and bite of the 12 knives are adjustable, still variations in chip size occur as well as other variations which effect the rate to which the liquor will penetrate the chip. For example, sulfite liquor, as is well known, penetrates into the wood generally with the grain, and there is a tendency for the chipper knife to close the pores in the wood when it cuts across the grain, thus retarding penetration. A sulfate or kraft liquor on the other hand, penetrates the wood from all sides so that the effect of the chipper knife is less noticeable. If it is desired, also, to decrease the cooking time by attempting to start out with smaller chips, one elfect is to produce a larger amount of very fine chips or sawdust which is generally considered useless for pulping and is more conventionally burned for heat.

The foregoing knowledge is well known to men skilled in this art, but it has not been practical heretofore to use such knowledge because existing digesting apparatus could not make use of it. With apparatus embodying and for practicing this invention, however, variations in cook arising from variations in chip size or conditions can, as heretofore noted, be substantially eliminated by separately handling chips of different sizes and conditions through the apparatus either by introducing them into the digester 14 at separate points therealong or utilizing the two parallel vertical lines of flow as noted to produce separate cooking conditions as required.

When the chips are introduced into digester 14 and kept separate therein in different parallel lines of flow, the cooking time and rate is controlled by the speed of the conveying mechanism. When chips are introduced into the same line of how at different points therealong, the cooking time is controlled by the positioning of the point of introduction, and with this apparatus either of both methods of cooking rate control can satisfactorily be utilized so that, by contrast to pulping processes heretofore known, wood chips, regardless of size can be cooked under controllable conditions to the same yield, or controllably to diiferent yields, by controlling or adjusting or altering the cooking time (i.e., through-put rate and distance of travel), cooking liquor concentration, liquor-to-wood ratios, etc.

The same advantages can also be obtained with apparatus embodying and for practicing this invention when it is desired simultaneously to cook difierent kinds of wood, such as hard wood and soft wood. By altering the type and/or concentration of the cooking liquor in the two sides of the digester and adjusting the speed of the screw conveyors, the cooking or digesting of pulp from hard woods on one side and soft wood on the other can be simultaneously accomplished and controlled for a substantially constant output of the two kinds of pulp in the desired relative ratio or proportions.

Particularly in installations where different sizes of chips are introduced into digester 14 at diiferent points therealong, the size of the perforations in the bottoms of troughs 121-124 may advantageously be adjusted with respect to the chip size at various points. For example, if chips which are retained by a one-half inch screen are introduced through chip inlet 102 with smaller chips being introduced at some subsequent point along digester 14, the perforations in the left end of trough 121 could be as large as diameter thus permitting an increased initial liquor-to-wood ratio, since the larger perforations would promote drainage of excess liquor more readily. The perforations in the right-hand end of trough 121 and/or in trough 123, then, would be somewhat smaller to retain Whatever chip size was present in those portions of the troughs with, for certain conditions, the left half of trough '123 having no perforations at all for the introduction thereinto of sawdust for an extremely short cooking period immediately prior to discharging the cooked wood from outlet 195.

As a further step toward uniform cooking results, the inevitably varying moisture contents of the wood (which vary with the season of the year in which the wood was cut, the season in which it is used, the length of storage time after cutting and before use, etc.), are controlled or compensated forand adverse aifects thereof minimized by pre-steaming the chips in pre-steamer 12, as noted, at above approximately 100 lbs p.s.i. This has the effect of driving entrapped air out of the chips and equalizing the moisture content thereof so that a much more uniform rate of penetration of the cooking liquor into the chips is achieved. Such pre-steaming, in addition to substantially equalizing the moisture content of chips also raises the temperature to that of the saturated steam. If super-heated steam, then, is introduced into digester 14 through steam inlet 166 thereof, the moisture content of chips entering digester 14 will be vaporized substantially immediately, thus, in effect reducing the moisture content of the chips and enhancing the rate of penetration of cooking liquor thereinto.

For example, with wood weighing 22.5 pounds per cubic foot bone-dry and having a moisture content of 50% of the bone-dry wood, one pound of the wood would occupy a volume of 77 cu. in., and a half pound of water therein a volume of 14 cu. in. Of this, the actual woody substances occupy a volume of 18 cu. in, with 45 cu. in. of the total Volume being occupied by air in the cells or porosities in the wood. The specific volume of steam at 130 p.s.i. is 5400 cu. in. Since the space occupied by water and air in the wood is 63 cu. in, it is necessary to vaporize that amount of water in the wood into steam to eliminate air and moisture from the wood i.e., .0116 pound of water must be vaporized to achieve the volume of 63 cu. in. With the latent heat of steam at 130 p.s.i. at 866 B.t.u., B.t.u. would be required to vaporize .0116 pound of water under these conditions.

If one pound of pulp cooked to a 50% yield requires two pounds of steam and, with the enthalpy of saturated steam at 130 p.s.i. being 1193 and the enthalpy of superheated steam at 420 F. and 130 p.s.i. being 1230, there are, of course, 37 B.t.u. superheat available per pound of steam and 74 B.t.u. available per pound of chips. Since only 10 B.t.u. are required for the desired vaporization of the moisture in the chips and the replacement thereby of entrapped air, the foregoing example shows that there is a substantial excess of heat available for this purpose according to the foregoing process.

After initial contact with the superheated steam treatment, the subsequent addition through liquor inlets 150- 154 of cooking liquor at a temperature lower than the saturated steam temperature causes condensation of the steam inside the chips, creating a vacuum or lower pressure area inside the chips than at the surface thereof further enhancing the rate of penetration of the liquor into the chips. Also the superheating treatment, having vaporized or reduced moisture in the chip, provides a situation in which the cooking liquor penetrating the chip is not substantially diluted by residual moisture in the chip, as in the usual case, thus permitting use in this process of cooking liquor having a lower initial concentration than would normally be required to get the desired cooking liquor concentration into a chip which had not been so treated.

As illustrative of a satisfactory system embodying and for practicing this invention, satisfactory results are achieved with the illustrated apparatus in the manufacture of sulfate or kraft pulp from southern pine woods. For a rated capacity of 42 tons of pulp per day for each of the two parallel lines of flow, the digester 14 may satisfactorily be of a size approximately 8 ft. in diameter by 31 ft. long with each of the four screw conveyors 125428 having a diameter of approximately 34 in. and a pitch and speed of rotation to convey a given batch of chips completely through digester 14 from chip inlet 102 to chip outlet 195 thereof in 40 minutes when the level of chips in the troughs is such that the conveyors are 90% full. As will be understood, the other elements of the illustrated system are constructed and '14 dimensioned to correspond to such size and throughput capacity of digester 14. As'previously noted, the cooking rate of total cooking time may vary according 'to the size of the chips, etc., but is preferably maintained within the range of from approximately 20'minutes to approximately 40 minutes total dwell in the digester 14 for kraft or sulfate pulp adapted for the manufacture of kraft liner board. With such an installation, conventional kraft cooking liquor (comprising, as is well understood, sodium hydroxide and sodium sulfide) is supplied to liquor inlet at a concentration of approximately 320 to 400 pounds active alkali (calculated as sodium oxide) per ton of pulp, supplied at a rate of approximately 34 to 40 gallons per minute, and superheated steam within the range of 350 to 375 F. is supplied through steam inlet 166 to maintain the pressure within digester 14 at approximately 120 to 170 p.s.i.

The cooking liquor collected, as above noted, in the sump at the bottom of casing 12% and withdrawn through liquor outlets 170 and 171 is recirculated by pumps 175 and at approximately 200 to 400 gallons per minute through liquor inlets 151-154, and spent liquor is Withdrawn through overflow pipe 210 and discharged out chip outlet at the rate of approximately 30 to 40 gallons per minute under the satisfactory conditions described. With such conditions, wood chips at the rate of 70 tons per day are fed into each of the feeding mechanisms 10 to produce the aforementioned 85 tons of pulp per day out of each of the parallel lines of flow, and cooked to a yield within the range of 55% to 65% finished pulp product.

While the methods and forms of apparatus herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claim.

What is claimed is:

In a continuous digesting method for the preparation of pulp from wood chips and the like in continuous flow, the steps which comprise subjecting a plurality of different batches of said chips preliminarily to the action of super-atmospheric steam pressure until the temperature of said chips and moisture contained therein is raised substantially to the temperature of saturated steam at said super-atmospheric pressure, then subjecting said chips to super-heated steam to vaporize and drive out therefrom residual moisture and air entrapped therein, separately introducing said thus treated different batches of chips into a plurality of different digesting zones for substantially continuous conveying of said chips therethrough, spraying said chips in said digesting zones with a first application of chemical digesting liquor for impregnation of said liquor into said chips while maintaining said chips at an elevated temperature and pressure in said digesting zones and during said conveying of said chips therethrough, draining off from said chips while being conveyed through said digesting zones excess chemical digesting liquor sprayed thereon but which does not penetrate into said chips, maintaining said chips during said conveying thereof through said plurality of digesting zones in diiferent atmospheres of chemical digesting liquor vapor but substantially free of liquid phase chemical digesting liquor, thereafter conveying said batches of chips separately into different digesting zones subsequent to said first digesting zones, spraying said chips in said subsequent digesting zones with a second application of chemical digesting liquor at a concentration less than liquor sprayed on said chips in said first application of chemical digesting liquor for further impregnation and digesting of said chips by said liquor in said digesting zones while maintaining said chips at said elevated temperature and pressure in said vapor atmospheres of said digesting liquor, draining oif said batches .of chips excess digesting liquor from said second application thereof, all of said plurality of chips and said plurality of zones and said plurality of applications of chemical digesting liquor occurring Within a single digesting vessel in which is maintained said super-atmospheric steam pressure, and thereafter separately blowing said chips into atmospheric pressure for use of said blown and digested chips in the further preparation of said pulp.

1,679,336 Dunbar July 31, 1928 16 De la Roza Aug. 20, 1935 Dreyfus July 13, 1937 Pornilio Oct. 31, 1939 'Merrill; May 7, 1940 -Pomilio Nov. 25, 1941 Becker Sept. 26, 1950 Weiss Aug. 19, 1952 Kehoe Nov. 4, 1952 Becker Nov. 6, 1956 Durant Oct. 28, 1958

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