US2858213A

US2858213A - Wood chip digestion

Info

Publication number: US2858213A
Application number: US579540A
Authority: US
Inventors: Leonard G Durant; James E Irvine; Rohe V Pennington; Surino Alphonse
Original assignee: Condi Engineering Corp
Current assignee: Condi Engineering Corp
Priority date: 1956-04-20
Filing date: 1956-04-20
Publication date: 1958-10-28
Anticipated expiration: 1975-10-28

Description

Oct. 28, 1958 L. G. DURANT ETAL WOOD CHIP DIGESTION 4 Sheets-Sheet 1 F'led April 20, 1956 :E28 2EME :35:8

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.coNoEEcEw Rohe V. Pennington Alphonse Suf ino Oct. 28, 1958 L. G. DURANT ErAL 2,858,213

WOOD CHIP DIGESTION Filed April 20, 1956 4 Sheets-Sheet 2 FIG.2

W210 le 245 ...unllllemnh Y Leonard G. Durclm James E. Irvine Rohe'V. Pennington Alphonse Surino M lh-LMW l AT'TYv L. G. DURANT ETAL WOOD CHIP DIGESTION Oct. 28, 1958 4 sheets-sheet s Filed April 20, 1956 Oct. 28, 1958 L. G. DURANT ErAL 2,858,213

woon CHIP DIGESTION A Filed April 20, 1956 4 Sheets-Sheet 4 W2 27j/enfers F G-5 Leonard G. Durant domes E. Irvine Rohe V. Pennington Alphonse Surno O Airy g .me .mm l s c.. -I --u.,--: g fkllll l. .m Q l Wm '4| M m H o p 'Il F m g .me )11, I mm .MH --.UTIL ou B mT -1- S I I I I l lll/ /A m l 3 n I w I l I I l l l III 8 e 2,858,213 Patented Oct. 28, 1958 WGOD CHIP DIGESTION Leonard G. Durant, James E. Irvine, Rohe V. Pennington, and Alphonse Surino, Pittsfield, Mass., assignors to Condi Engineering Corporation, Pittsfield, Mass., a corporation of Massachusetts Application April zo, 1956, serial No. 579,540

11 Claims. (C1. 927) This invention relates to the digestion of lignocellulosic material, exemplified by wood chips, to render them suitable for being yielded into semi-chemical or prime chemical pulp. In the digestion stage, the chips are subjected, while under the influence of heat and pressure, to treatment with a chemical that reacts with the ligneous material that binds the cellulosic iiber bundles together, to render it into a water-soluble reaction product that can be removed to free the bers from their bundles.

A major problem is to obtain complete and rapid impregnation and diffusion of the digestant chemical liquor (hereinafter called cooking liquor) into the chips. Much diculty has been encountered in trying to solve this problem, so it is an object of this invention to improve on the eicacy of impregnation of the entire chips with the cooking liquor. Chips include a certain quantity of non-condensable gases that tend to inhibit-complete impregnation, so it is another object of this'invention to devise Ways and means for removing such gases automatically and otherwise to eiect better impregnation and complete diffusion of the cooking liquor throughout the chips. While heat enhances good impregnation, such heat tends to stimulate chemical reaction between the cooking liquor and the chips, so it is an object of this invention to limitfthe degree of such reaction during the impregnation treatment of the chips and then transfer the chips to a subsequent digestion in a non-submerged cooking zone. Heat also tends to promote acid hydrolysis of the chips so it is another object of this invention to inhibit signiiicantly such hydrolysis.

Since to be economically feasible, any such digestion treatment must be on a continuous basis, it is another object of this invention to devise new ways and means for continuously feeding chips to the chip-preparation impregnation and diffusion zone; for controlling the concentration of the reactive chemicals in the cooking liquor with which the chips are impregnated; for evacuating chips therefrom and conducting them to the digester for cooking therein; for evacuating cooked chips from the digester, and for then ejecting them to a place of further treatment such as refining, all on a dependable and cheap continuous basis. Another object is to reduce the time required for the digestion of cellulose bearing materials and reduce the quantity of equipment now required for such digestion so that the cost of producing the pulp will be materially cut down. Another object is to devise a continuously operable feeder of chips to the system that is dependable, with long life, and which loses a minimum of steam from the steam-bearing system to which it feeds the chips. Other objects include saving manpower; effecting steam consumption with no peak loads on the boiler house; and giving better control over the cooking time.

These objects, and perhaps others, can be realized by the practice of this invention by continually carrying out action steps in successive treatment zones so that thechips are continually fed to a steaming zone wherethey are prepared for the ready acceptance of the cooking liquor by appropriate steaming to be penetrated by condensed steam, thus relieving them automatically of their non-condensable gases; next submerging the thus conditioned chips in hot cooking liquor under controlled conditions of time and temperature that assures complete penetration and diiliusion of the liquor into the chips but limit the chemical reaction therebetween so that in this zone of impregnating treatment, there is minimized any fiber-degrading action; the chips' themselves now contain an amount of absorbed and absorbed liquor of controlled chemical concentration sucient to enable them to be cooked rapidly but insuflicient to cause cellulose degradation, so they are transferred while under pressure and in submergence enough to be pumpable4 from the impregnation zone to a drainage zone Where they are relieved, while still under pressure, of their VeX- cess liquor which drains' freely therefrom; the drained chips are then cooked in only their retained liquor and in non-submergence in a gaseous environment enclosed in a digester; after being cooked, the chips are evacuated in a particular manner from the digester into an ejector zone having a receiver for the digested chips, from whence they are ejected while in suspension in a stream of steam and projected or blown to a place of subsequent treatment. The liquor from the drainer goes to an accumulator where it is fortified for re-use and where its temperature, pressure, and concentration is maintained as well as its liquid-level, from whence it is recirculated to the impregnation zone-details of which are hereinafter explained more fully. All of the action steps are carefully controlled as to pressure, temperature, and the rate of downward movement or the time of transit of the chips through each treatment zone and between such zones.

The steaming and the impregnation take place in a single enclosed tank in whose bottom section is maintained a regulated constant depth pond of cooking liquor having an overlying steam atmosphere in a steaming zone, into which latter the chips are containually'fed. Steam is supplied to the steaming zone at a predetermined controlled pressure, and liquor is supplied to the pond to maintain its liquid level constant and at a temperature less than that of both the steaming and the digesting zones, whereby there is a limited cooking of the chips while in the impregnating and diiusing zone. The speed of liquor into the chips is faster than the rate of penetration of the heat 4of the liquor because there is a rapid heat transfer and diiusion, with the result that the temperature of the liquor tends to drop. And much the same happens with the steam. The heat dlfusion is great' enough to cause the steam to condense in the chips, and this in turn forces out the non-condensable gases.

The time of transit of the chips uniformally downwardly through the` steaming zone is regulated whereby to equalize both the temperature and moisture content of the chips and to relieve the chips of their non-condensable gases, meanwhile minimizing acid hydrolysis of the chips. The time of transit of the chips downwardly through the controlled depth pond of impregnating cooking liquor is regulated whereby the chips are impregnated with the liquor until it is thoroughly diffused therein while there is limited'cooking or chemical reaction between the cooking liquor and the chips by controlling the cooking liquor concentration and temperature to that end. Pressure in the tank is maintained to bey high enough toaid in the `mechanical evacuation of the chips` from the pond and in their pumped passage while in submergence to the draining zone while in submergence, preferably by pumping. From the draiuer With the steam pressure therein, the impregnated chips go to a digester tank wherein the chips saturated with their absorbed and 2,858,213 4 .rr .f

t f adsorbed liquor are cooked not in submergence as is usual, but in their retained liquor in an enclosed gaseous envlronment.

So that one concept of this invention may be made clear, without too much involvement with specific apparatus, it might be described in diiferent words las follows: making use of a pair of tall enclosed tanks with the rst connected at its bottom to the top of the second through a chip-draining station, the second has a gaseous atmosphere wherein cooking or digestion o-f the chips takes place and from the bottom of which they are evacuated and ejected, while the tirst contains a pond of cooking liquonoverlain lwith a steam atmosphere. A self-supporting but everchanging column of the chips is established and maintained in the first tank extending from the bottom to the `top thereof so that the top section of the column is in the steam atmosphere while the lower section vof the `column is in submergence in the cooking liquor. As chips are mechanically evacuated from the entire cross sectional area of the bottom of column in the .iirst tank, pumped to the drainer and conveyed therethrough to the .topof the second tank, an equal replacing quantity of new chips is lautomatically fed to the top of the first tank so that while the column of chips remains substantially constanttas a column, the component chips that go to make it up are continually changing by moving downwardly through the entire length of that column, thus .passing first through the steam atmosphere of the tank and .then submergedly through the pond in that tank. There .is a similar self-supporting but ever-changing column of downwardly moving chips in the second tank, extending substantially from bottom to top thereof. As chips are evacuated from the bottom of the second tank, an equal replacing quantity of chips is fed automatically .to the top of that second tank so-that while the column ,of chips remains substantially constant as a column, the component chips that go to make it are continually changing by moving downwardly through the entire length of that column, thus passing through the steam atmoshpere of that second tank, from the entire cross-sectional area of the bottom of which they are mechanically evacuated as digested chips.

As the chips ymove downwardly in the columnar form of the mass thereof, they pass .through the overlying steam atmosphere in the iirst tank while Athe time of their passage downwardly therethrough .as well as the temperature and ,pressure of that atmosphere Ais such Athat they are steamed to relieve them of Ltheir tnon-condensable gases and otherwise to make them receptive to the subsequent treatment in the cooking liquor, as previously-described. As the individual chips move downwardly with and in the column that they go 'toI make up, and pass from the overlying steam atmosphere into submergence in the subnatant pond of cooking liquor that is heated to a temperature less than that of the overlying steam atmosphere, their conditioning therein permits them to be completely impregnated and diffused with the heated cooking liquor of the pond thereof in that first tank. But here again, .the `temperature of the pond of cooking liquor, its concentration, its depth, and the time of transit of the chips therethrough must be regulated since the main purposelof the pond directly beneath the overlying steaming atmosphere is to facilitate complete penetration and diffusion of the cooking liquor into and through the chips whilelimiting any cooking thereof.

While .in submergence and under pressure, the chips are conducted through an enclosed pathway from the pond to the drainer station to be drained of all but the cooking liquor absorbed and adsorbed by the chips, whereupon these chips still under pressure are fed into the second tank to the top of the column of chips therein where they are .digested or cooked in a gaseous atmosphere in the presense only of the steam of that atmosphere and their retained liquor. After cooking for the proper time, namely for the controlled time that it takes lli a chip with its columnar adjacent chips to travel downwardly in the second tank from top to bottom thereof, the resulting digested chips are evacuated from that zone and ejected to a place of subsequent treatment, such as washing, screening, etc. This columnar treatment of chips with the rst column extending downwardly through a steaming zone into a chip-submerging impregnating zone, and then whose chips are to be made into another column in which columnar formation they are digested in non-submergence and only inthe residual liquor in the chips, sets certain definite requirements for particular apparatus to be used to carry out these action steps. Sothe invention includes features relating to the proper and pressure-equalized feeding of the chips into the first tank; and to the control of the liquid level of the pond therein, of the evacuation of the chips from the bottom of the column in that tank while adding an equal quantity of chips to the top thereof, of moving the evacuated chips while under Vequalized pressure through the drainer to the top of the second tank, of the evacuation of the chips from the botom of the `column in the second tank; of ejection of the evacuated chips to another place; and of the temperatures and pressures in the various treatment zones as well as of the rate of movement of thecolumnar formation of Ychips downwardly through each zone; and to details of the accumulator as well as to its environment and control.

In the accompanying drawings, Figure l is a diagrammatic showing with legends, of the main steps of the process of this invention. Fig. 2 is a somewhat diagrammatic side elevational view of the system of this invention, with parts in section. Fig. 3 is a partial isometric view offa detail-'of the manifold used to yinject either steam or liquor into the vessels. Fig. 4 gives a flow-diagram of tne system `of this invention. Fig. 5 is a partially diagrammatic view for giving more details of the chip-level sensing controls. Fig. 6 is for showing the -main control panel.

Since Fig. 1 is self-explanatory, let yus refer lto Figs. 2 and 3, showing the system, there is a chip-bin A for receiving chips, or other cellulosic material to be treated, having an arch-breaker 11, and a suitable rotary chipdischarging mechanism 12, operated :by a variable speed' motor driven mechanism 13,'passes-chips downwardly of the feed pipe 14 at a regulatable rate to a multivane chip feeding mechanism B for delivering Chips through pipe 15, valved as at 16 to and into a primary cylindrical tank or vessel C to form a supported 'column of such chips from bottom to `top thereof. That vessel tis divided by a liquid-level L into an upper steaming zone lD having a steam atmosphere 17 and a lower `impregnating zone E made up of a subnatant pond of cooking liquor i3.

In the bottom of tank C (sometimes called-the impregnator vessel) there is an evacuator mechanism F for accomplishing continuous discharge of chips from theI bottom of the column thereof in the tank by means of a'variable speed motor mechanism 19 driving a shaft 2%, that has Vmoving in closed paths chip-impelling angled, blades depending from :arms 21 extending radially from a shroud 2.2 comprising a coned top onta cylinder rotating with the arms and having an yopening 23 therein through which chips in submergence are passed downwardly through pipe 24 into pump 25 that forces the chips while submerged in cooking liquor and under pressure `up through delivery conduit 26 to deliver them under pressure tangentially to a drainage station G that comprises the main or outer steam-tight inclined tank 27 having .a steam dome 28 with an annular bottom 29 from which ruprising a stack.30 down which fall chips and liquor fed tangentialto thesteam dome 28 by the delivery conduit 26. The chips and liquor swirl upwardly to spill over the upper Weir-like edge of the stack 30, but meanwhile tramp metal or other detritus gravitates to the annular bottom 29 from which it can be removed through hand-hole cover such as 31. Above the bottom of the main inclined tank 27, there is fixed an inner tank or cylindrical drainer 32 having perforations 33, and rotatable therein is a screw conveyor 34. Thus chips and liquor descending through stack 30 enter drainer 32 up which the screw conveyor 34 moves them, meanwhile their liquor drains from them through the perforations 33 and ows from the tank through outlet 35 and pipe 36 for recovery treatment such as in accumulator tank J. The chips that are so drained of the cooking liquor but While retaining their absorbed and adsorbed liquor pass downwardly through pipe 37, valved as at 38, into digestion tank or zone H, where the chips are digested columnuarly in non-submergence but in the presence of steam and their own adsorbed and absorbed liquor. After being properly digested, they are removed from the bottom of the column thereof in the digester vessel, tank, or zone H by another evacuator mechanism F by means of a variable speed motor mechanism 19' driving a shaft 20', that rotates blade-bearing arms 21' extending radially from a shroud 22' having an opening 23 therein through which chips are passed downwardly through pipe 24'. The, speed of the motor 19 is controlled and regulated by a speed controller 119 through its circuit W3. These primed numerals represent apparatus similar to that to which their unprimed corresponding numerals have been applied. But here pipe 24 leads tangentially into an axial flow ejector station I, from whence through

pipes

39 and 40 digested chips are blown by a steam stream to further treatment such as into a blow tank from which they go to refining or any other desired treatment. Since it usually becomes important to reclaim and otherwise control the cooking liquor used, I indicates an accumulator tank or station. Cooking liquor of controlled strength is pumped by pump 47 from that tank I through suitable valved liquor inlet line 41 to a conventional automatic throttle valve 42 controlled and regulated by a differential-pressure type recording liquid-level controller 43 for regulating the elevation of the liquid-level L of the pond 18 of cooking liquor so that it is maintained constant, or at least as nearly so as possible. S the automatic valve 42 regulates the flow of liquor through pipe 44 and its branches, each of which leads to an annular. manifold 45 having injection nozzles such as 46 extending therefrom into the tank C, whereby liquor can be sprayed into the tank in a fairly well dispersed or distributed manner. The manifolds 45 are distributed along the tank C as may seem desirable. Another pipe 49 also leads from the accumulator vessel or tank I and goes to and into the pipe 24 for giving control of the quantity of liquor mixed with the chips discharged from tank C by evacuator F to make them pumpable (at a consistency of say by pump 25 up through pipe 26 to the drainage station G. Still another pipe 48 is used to lead from the top of the accumulator tank I upwardly to the steam dome 28 of the drainage station G. Other

valved pipes

70 and 71 leading into the accumulator tank J are for conducting certain incoming chemical-bearing liquids thereinto, and

valved pipes

72, 73 and 48 are outlet pipes. Y

From any suitable source, steam under pressure is supplied through steam inlet line 50, valved at 50a, to the steaming zone D in the primary tank C by means of an annular manifold 145 in all respects like manifold 45 and having the same kind of injection nozzles 46 to supply steam into the tank, in a well distributed manner. The inlet of steam into the manifold 145 is controlled and regulated by a conventional pressure-temperature recording controller 51, that controls and regulates throttle-valve 50a in steam line 50 through circuit W4 to maintain substantially constant predetermined pressure in the impregnator and also throttle-valve 52 in vapor-outow line 53 for venting non-condensable gases that have been released from the chips during pre-steaming thereof. Chip-level sensing control means maintains a constant level of chips in the top portion of the impregnator. This controller comprises a transmitter 157 containing a source' of radio-active energy such as cesium-137 broadcasting gamma rays across the tank to a receiver 158 mounted diametrically opposite. Both transmitter and receiver are mounted externally and can be adjusted vertically by means of cables suspended from a motor-driven winch 56. The actual height of the transmitter and the receiver relative to the bottom of the tank, and thus its set poin is recorded from the signal given by potentiometer 156. The gamma rays are transmitted through a vertical band of say 12" in height. The strength of the signal at the receiver depends upon the height of the chips in the tank within this band. The received signal is converted to an electrical current, amplied, and registered on recording controller 55, and through circuit W1 used to regulate the speed of the motor 13 on-the bin discharger 12. The radiation measuring and monitoring devices containing the radio-active material and the receiver, are made by The Ohmart Corporation of Cincinnati, Ohio, A similar chip-level sensing control is applied to the digester zone or tank H, to which similar but primed reference numerals have been applied, The difference is that the control 55' is connected by circuit W2 to the motor drive 19 for regulating the speed of that motor and thus the effective chip-discharge rate of the evacuator F on the primary vessel or tank C.

Digester zone or station H is provided with a steaminlet line 58, for delivering steam under pressure to the

branch lines

59 and 60, each respectively feeding steam to an annular manifold 245 having injection nozzles, patterned after manifold 45 and its nozzles 46. A further branch steam line 69 is for conducting chip-blowing steam to the ejector station I. Inlet of steam through line 58 is controlled and regulated by a conventional pressure-temperature recording controller 61 (similar to controller 51) operating through circuit W5 an automatic throttle-valve 62 and also an automatic throttle valve 63 included in vapor outllowv or gas-0E line 64. Valved vapor outlet or gas-off pipe'65 from the steam dome 28 of the drainage station G, and also vfrom valved vapor outlet or gas-off pipe 66 from the high point of the inclined tank 27 of that station, can all join into a common exhaust or gas-off line 67 leading to further treatment of the vapors, if desired, 'such as for turpentine recovery.

Referring more particularly now to Fig. 4, 'and the accumulator I that enclosed vessel has a liquid-level LL with a subnatant pond of cooking liquor 74 and an overlying gas space 75. The liquid-level is maintained constant by means of a liquid-level controller 81, of known type, that operates through wire circuit W, lautomatic throttle-valve 77 in liquor supply line 71, that leads from a liquor heater 78 to which steam is supplied through steam line 79 valved as at 80 by an automatic throttlevalve controlled by a temperature recording controller 76, for maintaining constant the temperature of the liquor in the accumulator I. Fresh fortifying component chemicals that go to make up the cooking liquor come into the heater 78 through pipe 83 by means of a pump 84 from a mixing tank 85 to which is supplied through pipe 86 with black liquor, and through pipe 87 with a constant rate of flow of white liquor. LLL represents the liquid-level in the mixing tank that is maintained constant by means of a diierential pressure liquid-level controller 88 of known type, operating automatic throttle-valve 89. Automatic throttle-valve 90 on the white liquor line 86 is operated automatically through circuit W7 by a usualconductivity cell and recording concentration controller' 91 located in the liquor withdrawal pipe 72 from the accumulator going to the pump 47 on its way through pipe 41 to the mpregnator vessel C. The function of 91 for regulating the concentration of the strength of the cooking liquor. Fortifying liquor from pipe 41 can bypass back to the heater 78, by being shunted through pipe 93 to join again liquor in pipe 83 going to the heater 78. 94 represents means iby which-condensate .can be drained .from the heater 78. The digested-stock blown rom1the ejector station I s.though blow-line .'39 (ox-'40) goes toa usualblow-tank 85 having a discharger-95in its bottom.

'Operation of this system is carried .out by chips from the chip-bin A passing therefrom :at a controlled rate by variable :speed motor '1-3 .and its gdrive, to :pass down .feed pipe 14 to .and through -multi-vane feeder B into vessel C :to ll that vessel .with a .column of chips -extending from lbottom to Ltop, thus extending :upwardly from the bottom through the pond d8 of impregnating cooking liquor .through the overlying :gaseous atmosphere 18 :of pressured steam. While .the .column fis maintained -substantially constant .in height, its :chips -gradually move downwardly in the column to be removed by the bottom of the .column by the :evacuator Blades .depending from arms 21, driven by motor !1i9-plow .or rake chips from the entire cross-sectional :area of .the bottom of the impregnating vessel to discharge-.through door.23 ,of the shroud 22 into -pipe 24 wherein they are diluted with more cooking yliquor :pumped .through pipe 24 by pump 2S from accumulator I. This dilution :is .to assure :that the impregnated chips are suiiiciently in submergence to be -pumpable by .pump 25 into ywhoseeye .they go to be pumped through pipe 26 to and linto the draincr station G (tank 27). '.'Ihe -drainer .=is under .the same pressure and temperatureas is the .impregnator vessel C.

Here they :are delivered -tangentially into .the ,steam dome 28 where they swirl upwardly to spill .over and to fall down stack .30 whereuponscrew conveyor 34 moves them .uphill over the perforated casing 32. Liquor ,drains from 'the :chips through perforations 3?: tothe bottom of the drainer and to ow therefrom throughpipe 36 to the accumulator I. Chips so drained, with only their residual absorbed and adsorbed liquor fall through pipe 37 into the digester vessel H where .they are digested in non-submergence, namely in a gaseous atmosphere. In the digester, the chips again form a columnthereof from the bottom of the digester to the top while individual chips gradually travel downwardly to be discharged from the bottom lof the column in the digester by evacuator F that is similar to .evacuator F in the impregnator vessel. The digested chips are thus evacuated from the entire cross-sectional area of the digestcr whereupon they are supplied through pipe 24' tangentially to the axial-ow ejector station l. This latter forms the subject matter of a copending patent application of ours.

However that station may `be described brieily as a horizontal closed cylinder vwith a chip -exit at one end and a steam inlet'at the other. There is a rotatable shaft passing axially through the cylinder having blades thereon for stirring digested chips received tangentially into the cylinder. Steam from pipe 69 at the same pressure as steam 'supplied through pipe "58 to the digester enters the steam linlet 'in the cylinder for blowing chips being stirred in the cylinder -by the rotating blades from the exit end of the cylinder through the bore of a sleeve supportedin =that end and thus into and along the

blow pipes

39 and 40 to the blow tank 95 from whence the chips can -got to further treatment. Thus digested chips received tangentially by the ejector cylinder and mechanically stirred therein are blown substantially axially 'from the cylinder without change of their forward moving direction through a constantly open bore by astream of chip-blowing steam, meanwhile maximizing thequantity of chips so-blown -while minimizing the quantity of steam so blown.

vSince it is import-ant that the concentration of active chemicals in the cooking liquor used be carefully regulated, 'aswell as its temperature, the accumulator I has been provided with the controller '76 for regulating and maintaining constant lto'a predetermined degree the temperature of the incoming fortifying lliquors through pipe 71; with controller 81 formaintainng constant the `liquidlevel of :the .pond :oflh'quor tin the zaccumulator; Aand the conductivity-sensingicontroller 491 for regulating as desired [the quantity of black liquor fed to the mixing tank in proportion .to the constant quantity of incoming white-liquor through pipe 87, whereby the proportioning :.of these two liquors is controlled as they leave the tank :85 .on .'their way .to .the heater 78.and the accumulatori. Thistcoutrollerl is placed .as .a sentinel @on .the liquor outlet .pipe 72 leading to .the .impregnation vessel C because through that pipe comes a mixture *that must be regulated to beconstant at apredetermined-concentration, of the `fortiiying lliquor delivered tothe accumulator by the pipe 71, and alsonthe more :or less spent'cooking liquor drained from the 'chips :in the drainer 27 and returned through pipes 36.and 70 itothe accumulator. So that the proportioning of the fliquors in the mixing tank shall 'be fairly exact, its `liquid-level is maintained Aconstant by controller 88.

Now for the '.various important controls: 'automatic throttle-valve 42 regulated by liquid-level controller 43 maintains constant the liquid-levelL of the pond of cooking liquor in the impregnator vessel C. Pressure-temperature controller :51 regulates .automatic throttle-valve 50a-on the steam line lf50, and automatic throttle-'valve 52 on -the gas-olf line 53, to the end that the temperature and pressure .in that `vessel are regulatably Amaintained constant .at a predetermined'value. Chip-level sensing gauge and-.controller 55 'based .upon its set point, maintains .constantnthe llevel, or the top vof the column of chips inthe-'vessel'C AThis =isldone since that=con trolleris connected-through wiring 'W1 tothe motor 1B that -drives the chip-bin discharger '12. If the chip level rises that motor isl slowed down, while fall of the chip level speeds up that motor.

Similar chip-levelsensing gauge and controller -55on the digester vessel H, based upon its set point, maintains constant thechip-leveL'or the top of the column :of chips in that vessel, since Vthat controller is connected to motor 19 of the chip I'evacuation F in the impregnator vessel C by .means of lcircuit'vviring W2 whereby iif the chip level rises it will cause lslowing down of jthat motorand vice versa.

One vof the principal advantages of any continuous process over va vbatch-wise operation is accurate, continuous control of the quality and quantity of the end product. This is particularly true in cooking lchemical pulp because thereaction between the cooking chemical and the wood Vdoes not proceed at a constant rate. -Cooking in batches of 10 to 30 tons makes it almost impossible to control the process accurately or continuously during the various stages of the cook. It would, therefore, seem desirable to perform this operation continuously by stages so as to gain -accurate control all lthe way through. In the process of this invention, the cook has been broken down vinto three general phases as follows:

(1) Preparing the chips for ready acceptance of the cooking liquor;

(2) Impregnating Athe 'chips with cooking liquor land permitting the Vchemical to diluse uniformly throughout each individual-chip; and

(3) Permitting the vcooking chemical absorbed and adsorbed by the chips to rea'ctwith the lignin so as to remove it from the cellulosic or ibrous portion o'f the wood.

By controlling over abroad range the conditions under which each vof these three phasestakes place, it is possible to produce pulps with 'a wide range of characteristics. It is certainly conceivable that by properly controlling this three-phase cook, pulps tailor-.made for a specific end product can be delivered from the digester. 'Ihis could eliminate much of the blending of different pulps which is practiced widely in the industry today.

In the practice of this invention, chips arefed continuously from a chip bin by a pocket-type bin discharger driven by a variable speed motor. These chips pass through the multi-vane feeder B, which seals against pressure, into the impregnator C. A chip-level sensing controller 55 maintains a constant level of chips in the top portion fo the impregnator C.

jA constant level of preheated cooking liquor is maintained in the impregnator below the chip level by a conventional differential-pressure level controller 43, which actuates a throttle-valve 42 in the liquor inlet line 41. The "control point of the liquid level controller can be changed by a manual adjustment on the front of the instrument case. A conventional temperature-pressure controller 51 actuating a throttle-valve 50 in the steam inlet line 50 maintains a constant predetermined pressure in the impregnator. The temperature controller part of 51 maintains the impregnator temperature by positioning a throttle-valve 52 in the relief line 53, thus venting non-condensable gases which have been released from the chips during pre-steaming.

Since the chips are being evacuated continuously from the bottom of the impregnator and fed continuously into the top of the impregnator yat a constant rate, presteaming time is regulated by the dilerence between the liquorand the chips-levels and impregnation time is controlled by the liquor level. Thus, in phase 1, the chip preparation, or pre-steaming phase, the two controllable variables, time and temperature, are regulated accurately and continuously at a predetermined point. The control point of either or both of these variables can be pre-set to any desired value. In phase 2, the impregnation phase, there are three controllable variables namely, time, temperature, and liquor concentration. Time, of course, is maintained by controlling liquor level in the impregnator as described above. A temperature controller 76 in the liquor accumulator J actuating a throttle-valve S in the steam line to the liquor heater regulates liquor temperature. Acc umulator liquor level is maintained inthe same manner as in the impregnator but by controller 81. A conductivity cell and transmitter 91 in the liquor line 72 to the impregnator positions a throttle-valve 90 in the black liquor line to the mixing tank, thus controlling liquor concentration. Again, the control points of these instruments can be set at any predetermined value. f Thus, each of the three controllable variables in the impregnation phase is maintained at a pre-set point.

In phase 3, the cooking phase, since the liquor concentration yhas already been set, the controllable variables are time and temperature. A digester chip-level sensing controller 55 identical to the one on the impregnator regulates the speed of the impregnator evacuator motor 19 through circuit W2. Temperature and pressure controller 61'in the digester are identical to those in the impregnator. yThus, once the desired control points of the three phases are set, these values are continuously and automatically maintained for any given through-put rate. v

Through-put is pre-set by adjusting thespeed of the digester evacuator motor. If this speed is changed, then it is, of course, necessary to change the control points of the three

level controllers

55, 43 and 55', to maintain the original times in each of the three zones. To avoid the necessity of calculating the control points for each combination of time and through-put, the operator is provided with three indicating instruments (see Fig. 6) calibrated to read directly (1) steaming time, (2) impregnating time, and (3) cooking time. Since cooking time is a function of through-put and digester level, signals from the digester chip level controller potentiometer 1.56', and the digester evacuator motor controller 119', are fed to the cooking time indicator. impregnating time indicator receives its signals from .the digester evacuator motor controller 119' and the Similarly, thev liquor level controller 43. 'Ihe signals to the steaming time indicator come from the digester evacuator motor controller 119 and the liquor level controller 43. The signals tothe steaming time indicator come from the digester evacuator motor controller 119', and the liquor level controller 43, and the impregnator chip level controller potentiometer 156.

To demonstrate the sequence of operations, let us assume that the operator wants to increase through-put. The steps are as follows:

( l) Operator increases digester evacuator speed (adjust (2) Digester chip-level starts to drop; impregnator evacuator motor speeds up; cooking time indicator reads low.

(3) Operator adjusts digester chip-level controller 55 set point by rotating winch 56' with its transmitter 157' and receiver 158 to operate their controller 55', until cooking time indicator indicates proper value.

(4) Impregnating time and steaming time indicators read low; chip-level starts to drop in impregnator; chip bin discharger motor 13 speeds up.

(5) Operator adjusts set point of impregnator chiplevel by rotating motor-driven winch 56 with its transmitter 157 and receiver 158 to operate their controller 55 and liquor level controller 43 until irnpregnating time and steaming time instruments indicate the proper value.

Thus, by making these simple manual adjustments, the operator has changed throughput without changing the conditions in any of the three stages of the cook. He can also, of course, change any single control point without disturbing conditions in any other phase of the operation.

To give some idea of the temperatures and pressures used it may be said that in vessel C, namely the steaming and impregnation zone,l temperatures used will be dependent upon the type of wood used, the grade of pulp desired and the cooking medium or chemical used. For example, when cooking a prime grade of kraft, using a southern pine wood, temperatures used will be in a range from about 160 C. to 178 C. While pressures used therein will range from p. s. i. g. to 150 p. s. i. g.

For explanation, ifa temperature is used, say 178 C., the temperature of the steaming area 17 of vessel C will be maintained at this temperature, but the chips entering this zone will be at a lower temperature and, because of the inherent natural insulation qualities of wood, they Will resist rising to the temperature of the surrounding steam. It has been yfound by experimentation that the time of residence of chips in this steam temperature should be about 5 minutes in order to prepare them for the rapid acceptance of liquor. At the end of the chip transit through the pre-steaming zone the chips are still about 12 C.-20 C. below the temperature of the surrounding steam. Experimentation has shown, however, that the chips are ready for the quick and ecient acceptance of cooking liquor and that carrying the steaming period longer would tend to generate wood acids which would in turn `attack thevcellulose in the wood, thereby causing degradation of. the resultant pulp. A wide and costly range of experimentation has shown that dilerent species of wood and/ or wood with Widely dilferent moisture content will require a variation from the 5 minute residence in the pre-steaming zone but this time will not vary more than plus or minus 2 minutes in any woods investigated.

The time of residence in the impregnating pond 18 of vessel C, which is needed to assure complete impregnation and diiusion of the liquor through the chip is from l2 to 15 minutes for this prime grade of kraft pulp.v Other grades of pulp, other woods and other cooking liquors will call for more or less time but should never exceed plus or minus 5 minutes. The temperature of the liquor is held at the same point as the temperature in zone 17, The incoming cooler chips have a cooling eiect the evacuators F and F; and the ejector I.

on lthe liquor. It has been found experimentally that thiscooling `effect amountsfto als1much as 9 vC. to 13 C., thus limiting the tendency toward a cooking reaction while the liquor is penetrating fthe chip. In the digester vessel H, with a temperature-'of "178 C., the time of transit of a chip therethrough for the time of residence of a `chip therein, is from say 25 to 30 minutes for the `grade of pulp mentioned herein. These temperatures, pressures, and duration time of treatment vary somewhatdepending on the grade of pulp desired, the typeof wood used, the cooking-medium or chemical used, but because of the prestearning and impregnation of the chip the time in residence is not nearly so long as in conventional batch systemsnor isthe variation so Wide.

The concentration of liquor used for this ydescribed cook lies in Va `range of `from 38 grams per litre to 5G grams per litre of active alkali calculated as Na2O with about.25% to 30% sulphidity. Other grades will require different amounts of chemical or different chemicals, all controllable and all dependent upon the Wood being used andthe grade and type of pulp required.

To start the system, the temperature of the liquor in the accumulator I `must be brought up to impregnating temperature. rThis can be done by using ythe liquor pump 91 to pass the liquor through by-pass pipe 93 to the heater or heat exchanger 78. Concurrently, the pressure vessels or tanks C and Hare being brought up to temperature by the application of steam thereto. The chip discharger motor 13 is started; the multivane feeder B is started; and the chip level gauges 157, 158, 157 and 158' are lowered down 'to Vthe bottom of the vessels by the

Winches

56 and 56 respectively. In order to make the start-up semi-automatic, the hoisting apparatus is designed so that its hoisting speed is the same as the speed at which chips move through the respective vessels at some nominal production. The operator stops the hoist at an appropriate time, and then starts 'the drainer G; its transfer pump 25;

The liquorlevel in the accumulator J ywill have dropped to or very near yto its operating level LL and the liquor make-up pump 84 will have started operating.

Grades of pulp obtainable by the practice of this invention range all the way from high yield low grade pulps of say 94% yield to low yield high grade pulps of substantially pure alpha cellulose of say 42% yield. It is a recognized fact that in conventional and known cooking procedures, the desirable carbohydrate materials are attacked 'and destroyed to a degree along with the undesirable encrusting materials such as lignin. This is due to the slow rate of impregnation of the chemical into the unprepared chip so that during the necessarily long impregnation time (usually about 11/2 hours) the chemical which acts on the lignin first also has time to act on the carbohydrate. In effect the chemical eats its way into the chip, reacting and losing strength as it penetrates so that the lchip in the outer area is exposed to a richer concentration of chemical for a longer period of time which the inner area is exposed to a leaner chemical concentration for a shorter perior of time. The result is a chip which is over cooked in the outer area to destroy some of the desirable cellulose and undercooked in the inner area to retain some of the underable lignin. The resultant pulp may have a yield which is indicative of good pulp quality but which is in actuality a mixture of bad and good. In order to produce a clean pulp with competitive procedures, one must cook for a sufficient time to reduce this center area of the chip to a point Where the rejected portion will be low, thus progressively destroying the outer area. The other way is to accept a high percentage of poor grade material from the inner area which must subsequently be mechanically removed by expensive equipment such as screens. The grades of pulp produced lby this new practice, due to .the rapidity v of thorough impregnation and diffusion of chemical through the chips, are characterized by uniformity of 12 cooking throughout and by consequently having a higher percentage of carbohydrate kor cellulose content. A better description would 'be a larger and more truly proper proportion of desirable carbohydrates to undesirable lignin -thansimilar pulps made by lcompetitive processes.

As this invention may be embodied lin several forms without departiig from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and'not restrictive, since the scope of the invention is defined by the appended claims iand all changes that fall within the metes and bounds of the claims or that form their functional as Well as conjointly cooperative equivalents, are therefore intended to lbe embraced by those claims. For instance, whereas the treatment of wood chips has been described, other lignocellulosic 4particles could be substituted therefor which are desired to be exposed to Acontinuous digestion.

We claim:

1. The continuation process of treating ligno-cellulosic material such as wood chips to conditionthem by digestion as a result of which they may be yielded into pulp, practicable in a pair -of enclosed tanks with the first connected at its bottom to the top of the second through a chip-draining station while in the first there is maintained a pond of cooking liquor having an overlying pressured steam atmosphere and inthe second there is a pressured steam atmosphere, and which process comprises downwardly moving a column of chips in the rst tank by evacuating chips from Vthe bottom thereof to the draining station en route to thesecond tank and by feeding to the top of the first tank a quantity of chips equal to those so-'evacuated lwhereby as a result of such columnar movement Aindividual chips move downwardly through the 'overlying steam atmosphere and then through the subnatant pond, downwardly moving a column of chips in the second tank by evacuating chips from the bottom thereof to a place of further treatment and by feeding to the top of that tank from the draining station a quantity Vof drained chips equal to kthose so evacuated whereby as a result of such columnar movement individual chips move downwardly through the steam atmosphere of the second tank, adding cooking liquor to the pond while maintaining the depth of the pond constant and the `temperature thereof to be lless than that ofthe overlying steam atmosphere, controllably regulating the time of vtransit of 'chips through the overlying steam atmosphere and also through vthe subnatant pond, draining-the chips in nthe drainage station of all but theirabsorbed and adsorbed liquor'while 'maintaining thereon pressure substantially'equal to that ofthe vfirst tank, controlling the temperature of the pressured steam in the second tank to `be at a temperature at which the chips are cooked in non-submergence in their retained liquor, and controllably regulating the time of transit of chips through the second tank to obtainthe desired degree of cooking.

2. The continuous process of treating ligno-cellulosio material exemplified by wood chips to condition them by digestion vas a result of which they vmay be yielded into pulp, which comprises establishing and maintaining the following zones in sequence; a feeding zone, in a single enclosure an overlying steaming zone and an underlying liquid-impregnating zone having therein a pond of chip-impregnating cooking liquor controlled to have a constant liquid level, feeding chips through the feeding zone to the steaming zone in the enclosure to form a supported column of chips therein, an enclosed drainage zone, a digestion Zone, and an ejection zone; supplying steam `to the digestion zone and to the enclosure while controlling the temperature of the steamingzone to be above that of the pond and the temperature of the pond to be less lthan that of the digester zone while regulating the temperature of the steaming zone substantially to free the chips therein of their non- 13 condensable gases and to ready them for impregnation and diffusion by the hot liquor in the pond, passing the impregnatedchips from the bottom of the column .thereof through the drainage Zone while hot to drain them, while still hot passing the drained chips into the digester zone to rform therein a supported column thereof and there cooking the chips in non-submergence to produce digested and softened chips, discharging the chips from the bottom of the column thereof in the digester zone into the ejection Zone, and from the latter ejecting such discharged chips while in suspension in lsteam to a place of V'further treatment, meanwhile regulating the time of transit o'f-thel chips through each of the zones and particularlyl through the impregnatingpond whereby therein there iseffected substantially complete impregnation and diifusion ofthe previously steamed chips and in the digestion zone wherein theyvare cooked only in their retained liquor.

3. The process acc-Ording to claim 2, wherein the passing from the impregnating pond 4of the chips immersed in the cooking liquor is regulated to be at :a rate of liquor withdrawal correlated with the rate of liquor addition to the pond whereby the temperature of the pond is maintained at `a Adegree below that in the steaming Zone.

4. The continuous process of treating ligno-cellulosic material exemplified by wood-chips to condition them by digestion as a result of which they may be yielded into pulp, which comprises establishing and maintaining two 7 self-supporting columns of chips to -be treated, with each in a separate enclosing container having a pressured steam atmosphere but in the first of which there is also a subnatant pond of heated cooking liquor, maintaining the liquid-level of the pond constant, maintaining the pressure and temperature in each container substantially constant, transferring chips from the bottom of the rst column that are in submergence in the pond to a drainer station wherein they are drained of liquor, feeding drained chips to the top of the second column, and removing chips from the bottom of the second column to the atmosphere meanwhile regulating the rate of passage of an individual chip downwardly through each column of chips, whereby pre-steaming followed by liquor-impregnation of the chips takes place in the iirst and in the second digestion in non-submergence in a gaseous atmosphere.

5. Continuously operable wood-chip digesting apparatus which comprises a system having a combined steaming and impregnating tank enclosed for holding a pond of cooking liquor overlain by a steam atmosphere, means for regulatably supplying steam to the tank, means for automatically removing noncondensable gases from the tank, means fo1 regulatably feeding chips to the steam atmosphere to provide in the tank a supported column of chips, means for feeding such liquor to the pond, means for maintaining a constant liquid-level on the pond, means for evacuating chips from the bottom of the column thereof in the tank, a chip drainer, means for regulating the passing to the drainer of evacuated impregnated chips while immersed in liquor, a chip digester under pressure enclosing a gaseous atmosphere, means for passing chips from the drainer to the digester to provide therein a supported column of chips, means for supplying steam to the digester for controlling its temperature, and means for discharging digester chips from the column thereof in the digester; with the additional features of means for controlling the temperature of the steam atmosphere overlying the pond to be greater than that of the pond; means for controlling the temperature of the pond to be not above that in the digester; means for controlling the pressure on the pond to be substantially equal to that -of the digester; and means for controlling the time of transit Iof the chips through each of the steam atmosphere, the pond, and the digester.

6. Apparatus according to claim 5, with means where- 14 by liquor is supplied to the pond at a temperature below that of the overlying steam atmosphere.

7. Apparatus according to claim 5, wherein the means for controllingthe temperature of the pond include means for regulating that temperature to be below that of the steam atmosphere, and the means for controlling the 4 for regulatably supplying steam to the tank, motordriven means for feeding chips to the steam atmosphere of the tank to provide a supported column thereof from the bottom of the tank to its top, means for'feeding cooking liquor to the pond, means for maintaining'a constant liquid-level on the pond, motor-driven means for evacuating chips from the bottom of the column thereof in the tank, a chip drainer, means for pumping to the drainer evacuatedchips while immersed in liquor, ya chip digester` under pressure enclosing a gaseous atmosphere, means for passing chips from the drainer to the digester to provide a supported column thereof from the bottom of the digester to its top, means for supplying steam to the digester for controlling its temperature, and means for discharging chips from the bottom of the column thereof in the digester; with the additional features of chip-level sensing means associated with the tank, and means for controllably connecting the latter means with the motordriven means for regulatably feeding chips tothe tank whereby rise of the chip-level slows down the motor while fall of the chip-level speeds up the motor.

9. `Continuously operable wood-chip digesting apparatus which comprises a system having a combined steaming and impregnating tank enclosed for holding a pond of cooking liquor overlain by a steam atmosphere, means for regulatably supplying steam to the tank, motordriven means for feeding chips to the steam atmosphere of the tank to provide a supported column thereof from the bottom of the tank to its top, means for feeding cooking liquor to the pond, means for maintaining a constant liquid-level on the pond, motor-driven means for evacuating chips from the bottom lof the column thereof in the tank, a chip drainer, means for pumping to the drainer evacuated chips while immersed in liquor, a chip digester under pressure enclosing a gaseous atmosphere, means for passing chips from the drainer to the digester to provide a supported column thereof from the bottom of the digester to its top, means for supplying steam to the digester for controlling its temperature, and means for discharging chips from the bottom of the column thereof in the digester; with the additional features of chip-level sensing means associated with the digester, and means controllably connecting the latter means with the motor- -driven means for regulatably evacuating chips from the tank whereby rise of the chip-level slows down the motor while fall lof the chip-level speeds up the motor.

l0. Continuously operable Wood-chip digesting apparatus which comprises a system having a combined steaming and impregnating tank enclosed for holding a pond of cooking liquor overlain by a steam atmosphere, means for regulatably supplying steam to the tank, means for feeding chips to the steam atmosphere of the tank to provide a supported column thereof from the bottom of the tank to its top, means for feeding cooking liquor to the pond, means for maintaining a constant liquidlevel on the pond, means for evacuating chips from the tank, a chip drainer, means for pumping to the drainer evacuated chips while immersed in liquor, a chip digester under pressure enclosing a gaseous atmosphere, means forvpassing chips from the drainer to the digester to provide a supported column thereof from the bottom of the digester to its top, means for supplying steam to the digester for controlling its temperature, and means for discharging chips from the bottom of the column thereof in the digester; with the additional features of an accumulator tank receiving pressured steam, pipe means for returning drained liquor from the draiuer to the accumulator, means for maintaining in the accumulator a pond of liquor, means for supplying the latter pond with fortifying chemical components of the cooking liquor, means for transferring such fortified liquor to the pond thereof in the impregnating tank, and control'lable automatic means associated With the accumulator for regulatably maintaining constant to a predetermined degree the temperature and pressure therein as well as the concentration of its fortified liquor.

11. Apparatus according `to claim 10, wherein the means for regulatably maintaining the concentration of the fortied liquor comprises a mixing tank, an independent valved feed line for each of at least two active chemical components of cooking liquor leading into the mixing tank, means for conducting mixed liquor from the mixing tank into the accumulator, and regulatable automatic liquor concentration control means in the 1e means for transferring fortified liquor from the accumulator to the pond thereof in the impregnator tank exercising control over the valve in at least one of said feed lines.

References Cited in the tile of this patent UNlTED STATES PATENTS 1,843,466 Traquair Feb. 2, 1932 1,991,244 De La Roza Feb. 12, 1935 2,091,513 Merrill Aug. 31, 1937 2,152,267 Merrill Mar. 28, 1939 2,225,771 Dunbar Dec. 24, 1940 2,229,886 Dunbar Ian. 28, 1941 2,301,136 Moreland Nov. 3, 1942 2,323,128 Hare June 29, 1943 2,490,533 McAlear Dec. 6, 1949 2,638,248 Alvord May 12, 1953 2,675,311 Natwick Apr. 13, 1954 2,680,298 Obenshain June 8, 1954 2,749,240 Ross June 5, 1956 2,803,540 Durant Aug. 20, 1957

Claims

1. THE CONTINUATION PROCESS OF TREATING LIGNO-CELLULOSIC MATERIAL SUCH AS WOOD CHIPS TO CONDITION THEM BY DIGESTION AS A RESULT OF WHICH THEY MAY BE YIELDED INTO PULP PRACTICABLE IN A PAIR OF ENCLOSED TANKS WITH THE FIRST CONNECTED AT ITS BOTTOM TO THE TOP OF THE SECOND THROUGH A CHIP-DRAINING STATION WHILE IN THE FIRST THERE IS MAINTAINED A POND OF COOKING LIQUOR HAVING AN OVERLYING PRESSURED STEAM ATMOSPHERE AND IN THE SECOND THERE IS A PRESSURED STEAM ATMOSPHERE, AND WHICH PROCESS COMPRISES DOWNWARDLY MOVING A COLUMN OF CHIPS IN THE FIRST TANK BY EVACUATING CHIPS FROM THE BOTTOM THEREOF TO THE DRAINING STATION EN ROUTE TO THE SECOND TANK AND BY FEEDING TO THE TOP OF THE FIRST TANK A QUANTITY OF CHIPS EQUAL TO THOSE SO EVACUATED WHEREBY AS A RESULT OF SUCH COLUMNAR MOVEMENT INDIVIDAUL CHIPS MOVE DOWNWARDLY THROUGH THE OVERLYING STEAM ATMOSPHERE AND THEN THROUGH THE SUBNATANT POND, DOWNWARDLY MOVING A COLUMN OF CHIPS IN THE SECOND TANK BY EVACUATING CHIPS FROM THE BOTTOM THEREOF TO A PLACE OF FURTHER TREATMENT AND BY FEEDING TO THE TOP OF THAT TANK FROM THE DRAINING STATION A QUANTITY OF DRAINED CHIPS QUAL TO THOSE SO EVACUATED WHEREBY AS A RESULT OF SUCH COLUMNAR MOVEMENT INDIVIDUAL CHIPS MOVE DOWNWARDLY THROUGH THE STEAM ATMOSPHERE OF THE SECOND TANK, ADDING COOKING LIQUOR TO THE POND WHILE MAINTAINING THE DEPTH OF THE POND CONSTANT AND THE TEMPERATURE THEREOF TO BE LESS THAN THAT OF THE OVERLYING STEAM ATMOSPHERE, CONTROLLABLY REGULATING THE TIME OF TRANSIT OF CHIPS THROUGH THE OVERLYING STEAM ATMOSPHERE AND ALSO THROUGH THE SUBNATANT POND, DRAINING THE CHIPS IN THE DRAINAGE STATION OF ALL BUT THEIR ABSORBED AND ADSORBED LIQUOR WHILE MAINTINING THEREON PRESSURE SUBSTANTIALLY EQUAL TO THAT OF THE FIRST TANK, CONTROLLING THE TEMPERATURE OF THE PRESSURED STEAM IN THE SECOND TANK TO BE A TEMPERATURE AT WHICH THE CHIPS ARE COOKED IN NON-SUBMERGENCE IN THEIR RETAINED LIQUOR, AND CONTROLLABLY REGULATING THE TIME OF TRANSIT OF CHIPS THROUGH THE SECOND TANK TO OBTAIN THE DESIRED DEGREE OF COOKING.