US2229886A - Method of digesting fibrous materials - Google Patents

Description

Jan. 28, 1941. T. DUNBAR METHOD OF DIGESTING FIBROUS MATERIALS Filed Dec. 19, 1934 2 SheeLs--Sheel l Jan. 2s, 1941. T DUNBAR 2,229,886

METHOD OF DIGESTING FIBHOUS MATERIALS Filed Dec. 19, 1934 2 Sheets-Sheet 2 s; :S 203147010@ ZDJJMM 2130417.17 103041.97

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METD @F MMGESTING mRlUS MATE i t S Thomas L. hunbar, aterto, lil. Y., assigner tc Chemipulp Process Inc., atertown, hl. W., a corporation oit New Work Application necesitarit, isst, serai no.. 'nant it cli-aims.

This invention relates to an improved method of and apparatus for the digestion of fibrous material.'

lin the usual method of digesting wood chips by the quick cook method the digester is rirst completely lled with chips and then acid is admitted. The mass is brought up to reaction temperatures by admitting direct steam. Ordinarily the steam is admitted slowly 'for it had been thought that this was necessary to permit the liquor to penetrate the chips and thus to insure thorough cooking of the centers ci the chips. Thus, under the. old practice, 'from about two to four hours were allowed for this preliminary saturation period, that is to say the admission of steam was controlled so that a temperature of approximately 1l0l70 C. was reached only after the two to five hour period.

lin the early stages of the cook the chips settled and during subsequent cooking the relief was so manipulated that the chips were completely immersed in the liquor. This was thought to be necessary for it was believed that exposed or uncovered chips would be over cooked or burned.

Recently, as disclosed in the U. S. patent to Dunbar 1,859,384, it has been proposed to rst saturate the chips with hot acid under high hydrostatic pressure prior to the actual commence ment of the cook and in this way to more quickly initiate the boiling and to economize on steam consumption.

1t has now been found that if the chips are "subjected to a thorough saturation with the and with a; quantity less than that necessary to completely submerge the chips. denite statement" of the mechanism of the reaction is advanced, it would appear that at least a portion' of the fibrous material is digested in a vapor or a mixed liquid-vapor phase rather than in a continuous liquid phase as heretofore.

It is therefore an object of the present invention to providev a new methodrof delignifying cellulosic material. V Y

Another object is to provide a novel digestion While n process characterized by a markedly reduced liquor requirement.

Another object is to devise a process of digesting' pulp in which celluloselosses are minimined.

A further object is to provide a process of the character described in which heating costs are materially diminished.

With 'these and other equally important and related objects in view, the invention comprehends the concept of subjecting iibrous material such as wood chips to a special preliminary treatment whereby the chips are thoroughly saturated with a digestion reagent and subsequently cooking such chips under-improved conditions of pressure concentration and quantity ci reagent.

In order more clearly to explain the invention there is shown in the accompanying drawings a typical form of apparatus which may be employed to effectuate the process, in which:

Figure 1 is an elevation of one form of apparatus, certain well known parts being omitted for simplicity.

Fig. 2 is a detail of the digester illustrating the relationship oi chip and liquor levels.

Fig. 3 is a diagram of temperature and pressure conditions obtaining during typical treat ments according to the invention.

In the past it was generally assumed that an essential condition of digestion of chips was a complete submergence of the chips in the digesting liquor, for it was found that when the liquid level was lowered below the level of the chips a considerable degree of burning of the uncovered chips occurred. In the older methods, such burning almost of necessity occurred for the chips `were wetted with acid only on the surface or to a small depth below the surface. -Under the thermal conditions of the older operations such surface liquid was evaporated. Since the liquid medium, in effect, served as the thermal control, such relatively dry chips Were overcooked.

I have found that a digestion process may be carried out in which a high liquid level need not be maintained if certain conditions are fuliilled. Such conditions essentially are a complete saturation of the chips with the digestion agent at temperatures below but preferably close to reaction temperatures and such a control of the conditions of temperature and pressure that the upper portion of the mass of chips is in contact with the reagent in the vapor or mixed liquid and vapor phase.

Thorough impregnation of the chips is rather difcult to achieve. The chips themselves are a heterogeneous substance: the fibrous structure contains substances which are of differential chemical compositions, hardness, porosity and permeability. Thus the structural composition of the middle lamella differs from that of the secondary layers of `the cell wall. It is therefore diicult to force a saturating/solution into the chips so as to insure a substantially uniform penetration coupled with uniform concentration of the saturant throughout the body of the chip.

When, however, such thorough and homogeneous saturation has been eiected the material then becomes a potentially reactive mass, presenting an optimum surface of contact of the main reactive materials, i. e. the acid (or equivaient reagent) and the ligno-cellulose aggregates. By'reason of this physical condition a novel type of digestion is possible.

lThe conditions to be observed in impregnating the chips to secure the thorough saturation will vary somewhat depending upon the particular type of raw material and reagent employed. Due to the great complexity, from a chemical and physicchemical standpoint, of the fibrous material treated, it is extremely diicult to positively define the mechanism of the reaction. However, it does appear that such impregnation does involve such phenomena as'selective solubility, osmcsis, the ionic condition of the saturant, and possibly cataphoresis. Of these, selective solubility and osmosis undoubtedly play a very important part. The hot acid, employed in the present process for the saturant, takes into solution certain of the non-cellulosic and non-ligneous material, such for example as some of the polysaccharidesv other than cellulose. The acid also mixes with the water held in the prosenchymatous elements of the wood. As is known, there may be a considerable variation in this water, mechanically held for the most part, and known as the moisture content. -It is quite probable that the mechanism of impregnation is quite analogous to the phenomena involved in the normal vital processes of the plant and presents the same type of selectivity of passage or permeability, permitting relatively pure substances like water to permeate the membranes quite freely, ionic solutions like dissolved salts less readily, and complex organic substances least of all. Hence the `characteristics of the impregnating solution, such as its temperature and hydrogen ion concentration, are important features to be taken into consideration.

In carrying out the present process hot acid (below reaction temperatures) is employed as a saturant and this'is employed under conditions of excess quantity of acid, high hydrostatic or penetrating pressure and optimum recirculaiton. Due to the relatively high temperatures employed the acid tends to vpenetrate the cellular structure more rapidly. The recirculation tends not. only to a uniform temperature throughout the mass., that is to say 'of both the acid and the chips, but also serves to leach or wash -outf some of the acid soluble and mechanicallyheld substances in the wood. VWhen the wood is treated under these vconditions of a hot, high pressure, recirculating ,fibrous material may be obtained.

In these circumstances a, digestive reagent,

namely the acid, at a uniform and relatively high temperature, is practically homogeneously distributed through the mass and held in the interand intra-cellular spaces. This type of preliminary treatment therefore is carefully to be distinguished from other forms of initial treatments in which the woody structure is merely wetted on the surface. A chip which is thoroughly saturated, in the manner described, tends to retain the liquid content under much more drastic conditions of temperature than would a surface wetted chip. It is believed that the novel digestion process of the present invention, involving a low liquid level, is permitted by this fact.

In the second stage of the improved process, that is the digestion stage proper, the conditions are so regulated that a low liquid level is maintained, that is to say a liquid level below the level of the chips even after the chips have settled to their maximum degree. In these circumstances it is probable that a portion of the chips are actually digested in a mixed liquid and vapor phase. lDue to the effective preliminary saturation the digesting liquor is held within the body of the chips and is there raised to reaction temperatures.` Due to the optimum surface contacts, coupled with the initial elevated temperature of the acid, the mass may be quickly brought into reaction. During digestion a relatively high partial pressure of gas is maintained in the digesting zone so as to commensurately inhibit the evolution of free SO2 from the liquid medium; in other words to maintain the free acid concentration of the liquor.

Another important feature of the present invention made possible by the thorough preliminary impregnation is that the mass is quickly brought upl to reaction temperatures. In the preferred method of operation this is achieved by rapidly diminishing the pressure after the saturating period so that a maximum quantity of steam, and hence of heat units, may be admitted at the commencement of the cook. The preliminary saturating period therefore eliminates the necessity heretofore obtaining in the art of bringing up the steam pressure slowly,

The principles of the invention will more readily be appreciated upon a consideration of a specic illustrative process in which they are embodied.

As shown in the drawings, a unit may comprise as the major apparatus elements the acid storage tank l, the hot acid accumulator 2 and any de sired number of digesters 3. Where the digestive reagent is a metal bisulphite this may be admitted to the storage tank from the acid towers by means of the pump 4 and line 5. Such acid is usually at relatively low temperature. If desired this acid may be given a mild degree of preheat either by interposing a heat exchange in line 4 or a closed heating coil in the tank. When the acid in tank I is at atmospheric pressure the temperature cannot be greatly raised without loss of SO2 gas- The liquid in tank I is also preheated by hot overhead accumulator gas vented through line 6 and relatively low pressure digester gases entering through line 1 in a manner more fully to be described.

The accumulator 2, which is preferably spherical inshape, serves a number of important purposes in the present method. It is of relatively great capacity and functions as a heat regenerative plant in which heat units ofthe -hot fluids discharged from the digester are imparted to a body of cooler acid ladmitted from the storage and eductor' I2 to the accumulator.

till* till aaeacse tank; coincidentally such cooler acid absorbs free gas evolved during the cooking. The accumulator is so constructed and designed that the temperature, pressure and concentration of the acid may carefully be controlled within narrow ranges. This is of especial importance in the present method which, as noted above, involves an optimum temperature of the acid duringvthe penetration period.

For these several purposes therefore the accumulator is in communication with the acid storage tank I throughthe pump 8, line 9, drop leg lll, second eductor I2 and discharge branch i3. The eductor I is connectedto the relief header it associated with the digester. The line 9 is preferably provided with the check valve 9. The eductors are each provided with suitable control valves I0' and I2' respectively whereby the ow of fluids therethrough may be regulated.

It Will be seen thatrelatively cool (or slightly preheated) digester liquor may be forced under any desired velocity by pump 8 through line 9 and eductor I0, thence through the drop leg II Eductor I0, being in communication with line I4, aspirates fluids in the line and thus induces a suction or diminished pressure, depending in degree upon the particular construction of the eductor and the velocity of the acid flowing through line 9. The condensable or absorbable components of this fluid are condensed by and/or absorbed in the flowing stream of cooler acid. Such double action is accentuated and supplemented bythe second eductor I2 in the same fluid line.

rl'he second eductor I2, as shown, is connected in a separate accumulator recirculation circuit. This comprises the drawoif or' inlet branch I5, controlled by valve I', recirculating pump I 6 and dischargeline I'I. When pump I6 is operated liquor is withdrawn from the accumulator and forced, at any desired velocity, through line II to intimately contact with fluids owing into the eduztor I2 through the drop leg II. In order more accurately to control the temperature of the acid within the accumulator either a direct or indirect heater may he interposed at any desired point in the recirculating line.

Where the exigencies of a particular situation so recommend, the eductor I2 may be omitted and the drop leg I I may be connected to the suc.. tion side of pump I6, the line I'I then being directly connected to the accumulator. While the lines I3 and I5, for simplicity, are shown closely spaced near the bottom of the accumulator, it will of course be understood that they may be located at any desired position. Thus the drawoff line may be positioned near the bottom and the discharge line I3 at any desired elevation. An advantage of placing the line I5 near the bottom is that the induced circulation in the accumulator is then contra to the natural (convective) flow and thus tends to a more uniform temperature throughout the mass of liquid.

As noted above, the pressure conditions within the accumulator may be controlled within any desired range. '.Io accomplish this the vent line l is provided with a pressure regulating valve E' so that whenever'the pressure in the accumulator rises above a determined point, say of the order of from twenty-five to one hundred pounds more or less, gases are automatically vented to the storage tank I. Similarly abypass line I8 provided with a pressure regulating valve I8' leads from the header I4 to the top of the accumulator. This valve may be set at any desired pressure diiferential, say of the order of from one to three or four pounds. When the pressure in relief line i4 exceeds by this amount the pressure in the accumulator, the valve I3 operates and uids are bypassed around the eductor and passed directly to the accumulator.

The accumulator is provided with the gauge glass I9 to indicate the liquid level and this enables the operator to control the ow of fluids thereto. A liquid level indicating and/or recording instrument may also be utilized.

One or, as shown, a battery of digesters may be employed. These are each provided with a removable manhole cover to permit charging of the chips. The relief branch 2l, provided with control valve 2 I leads from the upper portion of the digester to the main header I 4. A second relief opened the liquid level in the digester may be brought to a low point which is materially below the normal level of the chips. The gauge glass 25 is lengthened to extend a suitable distance below the liquid level which obtains during the cooking operation.

The digesters may be charged with acid from the accumulator through the line 26,- valve 2E', pump 21, header 28 and inlet branches 29. These branches may selectively be connected to the header by properly manipulating the valves 29 in the manner well known.

In accordance with principles hereinbefore eX- plained, the digester is provided with means to recirculate the liquor during the saturating stage and when desired' during any desired period of the cooking stage. For this purpose the recirculating line 39 is provided with the inlet branches 3l and 32 which extend within the digester and are suitably perforated. The branches are provided with the control valves 3|' and 32 respectively so that one or both can be connected to line 30. The other end of line 30 is connected to the suction side of pump 33 and this discharges through line 34 into the lower portion of the digester.

In one preferred modication a heater 35 is provided in the recirculation circuit, the purpose of which is to maintain the recirculating stream of acid at the optimum temperature during the soaking period. Consonant with this purpose the ow of heating medium through the heater may be controlled by a thermostatic element so positioned as Ato be responsive to the temperature of the liquor in the circuit and preferably to that within the digester. 35 may be of any desired type, such as an indirect heat exchange, the heating medium of which may be steam, hot spent liquor, waste gases or the like.

Since the present invention permits the use of a relatively strong acid during the soaking period, and since such soaking is carried out at elevated temperatures, means are provided to maintain the acid concentration and conserve heat units by returning any evolved gas to the soaking 1iq uor. As shown, a gas line 3l leads from the upper portion of the digester and discharges into the liquid recirculation line preferably through The heater the eductor 38. It will be understood that any free gas which is liberated from the liquor during the soaking period may thus be returned to maintain the free acid concentration of the soaking liquor as well as to conserve the heat; value in the gas.

If desired such gas line 31 may also be employed as a means of increasing the free acid concentration. Thus, if desired, the line 31 may be connected by a valved branch to header I4 so that the aspirating effect of the eductor 38 will be applied to the uid in header I4. Hence, Whenever desirable, hot relief fluids discharged from a mass of chips undergoing digesting may be admitted to another mass of chips undergoing soaking, preferably in the manner indicated, that is into the recirculating stream by way of an eductor. Heat is thus directly imparted to the soaking liquor and the free acid content of such liquor raised. It will thus be seen that the temperature of the soaking liquor may be controlled by direct heat exchange with hot iiuids and/or by indirect methods, as by the heater 35. By proper regulation of these two the temperature and concentration of the acid may be maintained at any desired value.

The concentration of the liquor in the digester, both during the soaking period and the subsequent digestion period, may, if desired, automatically be controlled by utilizing a tank of liquid or gaseous SO2 and controlling the ow of this gas into the digester by means of an electric controller, such for example as shown in prior Patent 1,888,632. Such control is well known in the art and may comprise essentially a pair of plates or electrodes, the current through which is a function of the concentration of the liquor (electrolyte). This typeof electrical control may also be employed to govern the ow of relief fluids from the header into the digester recirculating `line during the soaking period.

The temperature required to be developed for the actual cooking of the chips may be derived from any sutiable source. In the preferred method the temperature is brought up by means of direct steam admitted from a steam header through a plurality of steam inlets 40.

The digesters are provided with suitable temperature and pressure recordining instruments by means of which a continuous record of these two factors, plotted vagainst time, as indicated in Fig. 3, may be obtained. The digesters are also provided with valved blow oif lines (not shown) through which pulp is discharged at the end of the cook.

The character of the process of digestion will have been appreciated from the foregoing description of the apparatus. It may be assumed that material within say the second digester is undergoing digestion and relief uids are flowing through header I4 and valves 2I, 22 and 24 of number one digester are closed. In starting the treatment in the rst digester the manhole cover 20 is removed and the digester completely lled contained therein is displaced and escapes through open valve 23. When SO2 gas or liquor begins to discharge from line 23 the valve 23' is closed and the pump 21 maintained in operation until a high hydrostatic pressure is built up on the acid.

When the acid has been pumped in so as to obtain a high pressure, of the order of seventy to ninety or more pounds, the valves 26 and 29 are closed and pump 21 stopped. Valves 2|', 22 and 24' are maintained closed and valve 3|' (and if desired valve 32') is opened. The pump 33 is then operated and the high pressure acid is withdrawn from the upper and forced into the lower part of the digester.

In Fig. 3 there are shown graphically two typical runs utilizing the present process; the full lines indicating the temperature and pressure condition in one and the broken lines indicating these conditions of the other. As shown in Fig. 3, the hot acid admitted from the accumulator soon reaches a practically constant value of about '15 C. or 80 C. in the illustrative methods. During its passage through the recirculation circuit heat may be imparted to the acid by means of the heat exchange 35 so as to compensate for that absorbed by the chips. As noted above, the degree of heat imparted to the recirculating acid may automatically be controlled so as to establish and maintain the temperature of the acid at any predetermined value. 'Ihe action of the heater 35 may be supplemented by means of the introduction of hot relief fluids conveyed from header I4 to line 31 by a branch line (not shown) or by the introduction of direct steam from a line (not shown).

During this recirculation any gases evolved from the liquor in the digester are withdrawn through line 31, mixed with and absorbed in the hot recycle liquor in eductor 38 and returned to the digester. The insertion of the eductor 38 in the recycle line serves to impose a reduced pressure in line 31 and thus facilitates the withdrawal of gases. When the line 31 is connected to header I4 the eductor functions similarly.

The -time of recirculation may vary considerably depending upon such factors as the character of the chips, temperature of the acid, etc. In the illustrated procedures, starting with relatively warm acid, the recirculation may be continued for approximately onev and one-half hours (as shown in the full lines), or about four hours (as shown in dotted lines). During this time, using apparatus of typical size, the equivalent of approximately three or more digester charges of acid is recirculated through the chips. l

Such recirculation, as noted, displaces air from the digester, preheats and saturates the chips with hot acid close to reaction temperature, and

leaches and removes the extractives from the chips. Optimum penetration and area of Contact is thus achieved.

It is particularly to be observed `that While the preliminary impregnation is by recirculation to and from the digester, the process is not limited shown on the chart, a decided and abrupt drop in pressure from the order of eighty-live or ninety pounds down to thirty pounds more or less. This, as will be appreciated, enables the employment of low pressure direct steam for bringing the mass up to reaction temperature. Such relief flows 'through header Hl, eductor l0 and drop leg Il into the accumulator.

While at rst blush it would appear that such a drastic drop in pressure would seriously dilute the acid, due to evaporation of SO2, such loss does not in fact obtain to any material degree.

y This would appear to be due to the fact that a large volume of the acid is actually held in the interand intra-cellular spaces of the chips and hence the free surface of the acid is much less than would be expectable from that quantity of acid. Furthermore, during the latter stages of such withdrawal, i. e. when the pressure is lower and a commensurate increase in the evolution of SO2 would be expected, a large quantity of the gas is constrained to pass through a mass of wetted chips which tend to reabsorb and probably to adsorb the gas.

As indicated above, the preferred heating medium is -direct steam introduced through the steam inlets 40. As shown on the chart, admission of the steam soon builds up the pressure 40 within the digester and the temperature lthen increases at a relatively rapid rate. After the pressure has been raised to the operating level,` this may then be maintained by opening valve 2|.

In order to insure uniform thermal conditions 45 during the beginning, or any later stage, of the reaction, the liquid may be recycled to and from the digester by closing the branch 3 I, opening the branch 32 and pumping the liquor through the p recirculating line. By operating the heater 35 50 during this recycling period the quantity bf direct steam and extent of condensation and consequent dilution of acid can correspondingly be controlled.

During the progress of the cook the liquid level 55 tends to raise due to the increased volume of condensate. As shown in Fig. 3, the top relief 2l may be closed and side relief 24 opened and liquor withdrawn some time after reaction temperatures have been reached, and in the illuse@ trated methods after the fourth and sixth hours respectively. By proper manipulation of valves 2l and 24 the desired low liquid level may be maintained.

During this period a relatively large quantity 65 of chips is above the liquid level and is undergoing digestion. Since these exposed chips are thoroughly saturated into the core the reaction proceeds when reaction temperatures are reached. Such chips therefore may be considered as being 7u contacted vwith liquid acid on the interior and on the external surfaces with a mixed liquid and vapor phase of the reagent. In any event, whatever may be the mechanism of the reaction. it is found that with the described low liquid m level treatment the uncovered chips are uniformly and thoroughly digested and produce the same character of pulp as that immersed in the'liquor.

As is known in the art, as the reaction proceeds a considerable quantity of free gas is evolved. This is probably due to the fact that the reaction produces organic acids which tend to displace and set free an equivalent amount of sulphurous acid. This tends to build up an excess pressure. In the present operation such pressure is relieved by opening valve 2l and recovering the chemical and heat values by absorbing such gases in the eductor l0, drop leg ll and eductor l2. In typical operations according to the present method such gas relief may be operated a short time after the fifth hour in one case and after the sixth hour in the other. This relief is controlled, as shown in Fig. 3, so as to maintain the pressure substantially constant down to the blow down period. In the present method pressures during the cooking are maintained at approximately seventy-rive pounds and optimum temperature of about 135 C.

It is to be observed that in one form of the present process the maximum reacting temperature is reached at about the eighth hour in one case and the seventh hour in another, and this may be maintained at a substantially constant pressure, by proper manipulation of the valves, until the usual tests indicate the approximate termination of the cook. Steam is shut off some time prior to completion of the cook, in the manner known to those skilled in the art. The pressure is reduced gradually down to 20 or 30 lbs., after which the material is either discharged into the blow-pit or is washed with water and then discharged.

It will be seen that the present type of process presents many advantages. As will be observed from an inspection of the chart, the preliminary saturation period is carried out under high pres sure and in such circumstances as to insure a substantially uniform and relatively high temperature of the mass prior to the cooking period. The quantity of heat units therefore required are materially diminished inasmuch as there is much less of a temperature rise required than in prio-r processes. As compared, for example, to a cold acid cook, the heat units necessary are only those required to bring the mass from a temperature of 70 or 80 up to cooking temperatures (of the order of 115 C.) as against a rise from 40 to 100 C. This diminishes the steam consumption and the liquor necessary to be Iwithdrawn on side relief.

As noted hereinbefore, the preliminary treatment insures a thorough saturation of the chips with acid at a uniform temperature and a uniform concentration. As a result of this type of treatment marked economies are secured. Thus, according to the older methods of digestion, using small size digesters, about 2500 gallons of liquor were required per ton of pulp. When utilizing the principles of the present invention, with the same size digester, the quantity of liquor can be reduced to between 1800 and 2000 gallons. This results not only in large savings in handling costs but also in sulphur and steam costs, the steam cost being reduced substantially in direct proportion to the reduction in the quantity of liquor required. Furthermore, due to the thorough impregnation, a larger ultimate yield is secured because this type of treatment reduces the quantity of shives or uncooked cores in the final product.

The process is so operated as to insure optimum recovery of the heat units as well as the chemical values of the liquor.

An outstanding feature of the present invention resides in the fact that it permits theuse of a low liquid level without in any way impairing the quality of the pulp produced. The lowering of the liquid level, permitted by the special preliminary treatment, as noted above, eiTects savings in the quantity of liquor required and establishes corresponding economies in chemical and steam costs. It will be understood that while the type of recirculation shown is fromthe top to the bottom of the digester, such recirculation may, by the proper by-pass connections to the pump, be made from the bottom to the top, so that if desired during a single soaking and/or cooking period recirculation may be from top to bottom, bottom to top, or an intermittent use of these two. .I

Therefore, while a preferred modification of the invention has been described, it is to be understood that this is given merely to exemplify the underlying principles involved. It will be readily apparent to those skilled in the art that these salient principles may be eiectuated in other specifically different types of methods and one utilizing different apparatus. Thus, while a process has 'been described with respect to the digestion of wood chips with bi-sulphite liquor, it is apparent that it is applicable to other digestion processes operating upon different raw materials and utilizing different reagents. Similarly, while a process has been described with respect to the quick cooking method, i. e. use of direct steam as the heating medium, it is clear, as has been pointed out, that the mass may be brought to reaction temperatures by using indirect heating methods or by a combination of the two. Therefore the invention is not to be considered limited to the particular procedures described, except as such limitations are clearly imposed by the appended claims.

1 claim:

l. In the digestion of brous material in which the material is submitted to a soaking with hot digestion liquor below reaction temperature prior to cooking-that improvement which comprises pumping the hot liquor continuously and directly to and from the digester during such soaking operation and utilizing such pumping operation for simultaneously withdrawing gases from the digester and returning such gases in admixture with liquor to the digester.

2. In a process of the character described, charging a digester with brous material, storing hot preconditioned acid in an accumulator, charging hot acid from the accumulator to the digester and then circulating the hot liquor to and from the digester while maintaining the liquor below reaction temperature, the utilizing the recirculating stream to withdraw gas from the gas space of and return it to the body of liquid in the digester.

3. Ina process of the character' described, admitting a charge of brous material to be delignied to an enlarged digestion chamber, accumulating hot preconditioned digestion liquor in a separate tank, pumping hot liquor from the tank to the digester until a high hydrostatic pressure is established, then pumping thel liquor directly to and from the digester for a period of time sufficiently prolonger to insure substantially complete penetration of the material, and subsequently lowering the liquid level in the digester to a point below the levelof the material and then raising the liquor to reaction temperatures.

4. A process of digesting brous material which comprises admitting the material to an enlarged digester, introducing digester liquor at elevated but below reaction temperature to the digester, utilizing such liquor to displace and vent air from the digester, sealing the digester from access of air while admitting an additional quantity of hot liquor suicient in amount to establish a high hydrostatic pressure on the acid, then recirculating the acid directly to and from the digester for a relatively protracted period While maintaining it below reaction temperature, then drawing oi liquor from the digester to a degree to expose at least a portion of the material before raising the mass to reaction temperature.

5. A method of digesting brous materials which comprises admitting the material to an enlarged chamber to completely fill the chamber, introducing preheated bisulphite liquor maintained at temperatures above 60 C. but below reaction temperatures, building up a high hydrostatic pressure on the liquor and recirculating it to and from the digester, maintaininglthe optimum temperature of the liquor during such recirculation by applying heat thereto, continuing such circulating until a uniform temperature is attained and the material is subsequently completely impregnated with the hot liquor; then lowering the liquid level well below the level of the material and bringing the mass up to reaction temperatures and during such reaction maintaining a high partial pressure of acid gas in the vapor space of the chamber.

6. A method of digesting fibrous materials which comprises lling a digester with the material, admitting hot digestion liquor to the digester while displacing and venting air therefrom, sealing the digester against access of air and admitting additional quantities of the hot bisulphite liquor to impose a high hydrostatic pressure thereon, then recirculating the hot acid directly to and from the digester while automatically regulating the temperature to an optimum vvalue which accelerates penetration but precludes any substantial reaction; maintaining all the material in contact with the acid for a period of time suciently prolonged 'to insure optimum penetration, thenwithdrawing a sucient quantity of the liquor to establish a large vapor space in the digester and expose a considerable quantity of the chips above the liquid level, and then bringing the mass up to digestion temperatures while maintaining a high partial pressure of sulphur dioxide gasv in the vapor space.

7. A method of digesting fibrous material which comprises charging the material to a digester, admitting hot digester liquor below reaction temperature to the digester and contacting it with the material for a period suiciently long to insure penetration of the liquor into the material while recirculating the liquor to and from the digester, withdrawing relief iluids from a second mass of material undergoing digestion and admitting such fluids to the4 said mass undergoing penetration to control the temperature and concentration of said soaking liquor, withdrawing liquor from the digester and lowering the liquid lil low reaction temperatures prior to cooking, that improvement which comprises forcing a quantity ol hot digestion liquor maintained below reaction temperatures into a digester until a high hydrostatic pressure has been established on the liquor, pumping the liquor directly to and from the digester While maintaining the liquor below a temperature at which any substantial cooking takes place, and utilizing such pumping operation for simultaneously withdrawing gases from the upper portion of the digester and returning such gases in admixture with liquor back into the digester.

9. A process of digesting fibrous material which comprises admitting the material to an enlarged digester, introducing a charge of hot digestion liquor to the digester, such charge being greater than that required to fill the digester at normal pressure, the temperature of the liquor being below reaction temperature; recirculating the liquor directly to and from the digester, heating the liquor during such recirculation to main-` tain lt at an elevated optimum but below reaction temperature, continuing such recirculation until the material has been penetrated with liquor, then materially lowering the liquid level below the level of the material inl the digester and cooking the mass.

10. A method of digesting fibrous material which comprises lling a digester with the material, admitting hot digestion liquor to the di gestel' While displacing and venting air therefrom, sealing the digester against access of air and admitting additional quantities of the hot liquor to impose a high hydrostatic pressure thereon; then recirculating the hot acid to and from the digester while automatically regulating the ternperature and concentration to an optimum value which accelerates penetration but precludes any substantial reaction; maintaining all of the material in contact with the liquor for a period of time sufficiently prolonged to insure optimum penetration, then withdrawing a sufcient quantity of the liquor to establish a vapor space in the digester and expose a considerable quantity of Vthe chips above the liquid level, and then bringing the mass up to digestion temperatures THOMAS L. DUNBAR. y

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