The term"chemical pulping"as used herein, should be understood in the same sense as that used in the art, namely, to processes such as the Kraft, soda, soda AQ, Kraft AQ, sulphite, bisulfite, and other similar processes, whereby chemical reagents remove the lignin from the fiber structure of the feed material. For convenience, the present specification will focus on the Kraft process, but it will be understood by those familiar with this field of technology, thatthe concepts and features described and claimed herein, can readily be incorporated into the other types of chemical pulping processes.

Chemical pulping has long been used in connection with the paper making industry. Moreover, perhaps 90% of all better grade papers used throughout the world, are made predominantly of chemical pulp. For this reason, it can well be appreciated that any improvements by which the yield is increased, would have very favorable economic and environmental consequences.

In general, chemical pulping processes rely on the release of wood fibers by dissolution of the lignin which binds the fibers together.

Because lignin and other non-cellulosic portions of the wood chips are removed in the process, chemical pulping processes typically provide yields of only 40-50% based on the dried chips, which is considerably less than so-called thermo-mechanical pulping (TMP), implemented in rotating disc refiners.

U. S. Patent 4,869,783 discloses one technique for improving the yield of the chemical pulp process, whereby wood chips are partially defiberized in a high compression screw device associated with the feed end of the pulping process. The fibers in the chips are substantially separated from one another but sufficient inter fiber bonding is

maintained to preserve chip integrity and thereby provide chips having an open porous fibrous network. The chips are subjected to chemical pulping to remove a majority of the lignin in the chips. Although highly compressing the chips in the feed portion of the pulping process would indeed appear to improve the bleached yield considerably (e. g., from about 45% to 50% on a comparative basis) with the pulp brightness also generally showing improvement, the strength properties of the pulp deteriorate significantly. Moreover, the present inventor believes that the strength deterioration of the fibers is the main reason why the high compression of feed chips as described in the'783 patent has not seen commercial fruition notwithstanding the impressive data on improved yield and brightness relative to conventional processes. Specifically, it is believed that the loss in strength arises from the shattering and fracturing of fibers prior to feeding the chemical pulp digester.

The process described in International App. No. PCT/US98/14710 published 18 February, 1999, and entitled,"Method of Pretreating Lignocellulose Fiber-Containing Material"overcomes the significant strength deterioration associated with the process described in the'783 patent, by preceding the high compression of the chips with a conditioning step, whereby the chips are exposed to elevated temperature and pressure, preferably in an environment of saturated steam with no pressure barrier between the conditioning step and the compression zone. The conditioning of the chips prior to high compression, results in considerably less brittle fracture and therefore a larger percentage of the original fiber retains acceptable size and strength during pulping and bleaching, rather than being washed away as fines for disposal or going forward with the fiber to the digester.

Thus, relative to conventional chemical pulping, the process and apparatus of said published application produces higher yield, comparable if not better brightness, and comparable strength to that of conventional pulping. This indeed represents a noteworthy advance in the state of the art of chemical pulping. The disclosure of International

App. PCT/US98/14710, is hereby incorporated by reference.

Both of these feed end techniques require high mechanical compression (at a ratio of at least 3 or 4 to 1) before the feed material is introduced into the chemical digestion and recovery system of the plant. For convenience this compression will be referred to as "compressive pretreatment". The present invention advances the state of the art even farther, by improving the processes described in the '783 patent and said International application.

Summary of the Invention The main aspect of the present invention is based on introducing process fluid into the compressive pretreatment stage, in an environment of elevated temperature and pressure, thereby initiating significant chemical pulping action on the chips at a point farther upstream than was previously known. In a preferred implementation of the present invention, this process fluid includes recycle process reagents drawn from process lines farther downstream. In another aspect of the invention, some of this process fluid is extracted as pressate during compression of the chips, and the pressate is reintroduced downstream, into the plant's recovery system. Therefore, there is no net increase in the fluid handling or chemistry requirements of the pulping equipment, the bleaching plant, and the black liquor recovery plant. Moreover, even pressate that does not include recycled process fluid, but does contain at least wood extractives, can be introduced into the recovery system.

The process fluid introduced upstream of the high compression device can be one or more of (a) steam (b) recirculated black liquor of any concentration, (c) recirculated white liquor of any concentration, (d) a vapor phase extract from the digester, (e) an externally supplied liquor including sulphite, NaOH, Na2S, etc., or (f) a combination thereof, e. g., vapor phase from the digester plus steam. The combination of high temperature, high pressure, and high compression achieves chemical

impregnation into destructured chips upstream of the digester, to a far superior degree than was previously possible.

In a particularly effective implementation of the invention, the feed material is delivered under pressure to a compression device which has a variable speed motor such that the duration of exposure of the chips can be controlled therein. In the compression device, the chips experience an initial processing of consolidation and extraction of fluids through the walls of the compression device into a pressurized collar or flash tank. Thereafter the material is further consolidated at a high compression in the ratio of at least 3 to 1 and up to 8 to 1 or more, before discharge. The relative duration of the exposure of the material as between the consolidation phase and the high compression phase, can be pre-established by, for example, the relative screw shaft lengths and wall characteristics defining these regions. In general, the total travel time of the material in the compression device may range from a few seconds to more than a minute, with generally 50%-80% of the travel time associated with consolidation, and 20%-50% of the time associated with compression above a ratio of about 3 to 1. In this manner, steam and chemicals can penetrate into the chips during partial defibration, during the initial consolidation phase, even prior to the continuing defibration in the high compression, plug portion of the device.

The various aspects of the present invention can thus be defined according to the following implementations.

In a chemical pulping process wherein feed stock containing lignocellulosic fibrous material is mechanically compressed to a ratio of at least about 3/1 before introduction into a chemical digestion system with optional recovery system where process fluids optionally including recycle process reagents remove lignin from the fibrous material, the improvement comprises introducing a portion of a process fluid into the fibrous material, before the material is mechanically compressed.

In a chemical pulping process wherein feed stock containing

lignocellulosic fibrous material is mechanically compressed at a ratio of at least about 3/1 before introduction into a chemical digestion system with optional recovery system, the improvement comprises exposing the material to an environment of pressure above 30 psi and a temperature above 120°C before and during the mechanical compression, and in this environment, contacting the material with a chemical liquor or vapor which initiates dissolution of the lignin from the fiber in the material.

In a chemical pulping process wherein feed stock containing lignocellulosic fibrous material is mechanically compressed before introduction into a chemical digestion and recovery system, the improvement comprises exposing the material to an environment of saturated steam at a pressure above 30 psi before the mechanical compression. During the compression of the material, a pressate containing wood extractives is produced, and the pressate is introduced into the recovery system downstream of the mechanical compression.

In another aspect, the invention is directed to a modification of the feed end of a system for chemical pulping of lignocellulosic feed material. A conditioning chamber has an inlet for receiving feed material, and means for exposing the feed material to elevated temperature and pressure for a target time period of exposure. A compression device maintained at elevated temperature and pressure, receives the conditioned feed material, consolidates the conditioned feed material including removal of extractives, compresses the conditioned material at a ratio of at least 3/1, and discharges the compressed material into a feeder for the chemical digester. The exposure time period in the conditioning chamber, and the travel time period in the compression device, are both adjustable. The conditioning chamber is preferably a variable speed transfer conveyor pressurized at a pressure above 30 psi with saturated steam, and the compression device is preferably a variable speed high compression screw pressurized with saturated steam at a pressure above 30 psi.

Brief Description of the Drawinqs These and other objects and advantages of the invention will become even more apparent from the following description of the preferred embodiments, made with reference to the appended drawings, in which: Figure 1 is a schematic of the front end of a conventional Kraft chemical pulping plant with continuous vertical digester, with the feed portion backfit with a first embodiment of the invention including inclined impregnator; Figure 2 is a schematic flow diagram of a Kraft chemical pulping plant including compressive pretreatment according to the invention, and downstream processes including digesting, washing, optional oxygen delignification, bleaching, and liquor recovery; Figure 3 is a schematic of a variation of the invention as shown in Figure 1, as would be implemented in a new plant with continuous vertical digester; Figure 4 is a schematic of a second embodiment of the invention including an inclined impregnator, in conjunction with a conventional continuous inclined digester; Figure 5 is a schematic of a variation of the embodiment shown in Figure 4, wherein the high compression device discharges directly into the vapor phase region of an inclined continuous digester; Figure 6 is a schematic of a third embodiment, whereby the invention is used in conjunction with a batch digester; Figure 7 is a schematic of a third embodiment, whereby the invention including impregnator is used in conjunction with a batch digester; Figure 8 is a schematic of a fourth embodiment, in which the compression device discharges directly into a vertical impregnator situated within the inlet to a vertical digester; Figure 9 is a longitudinal section view of a first embodiment of a high compression device for implementing the present invention;

Figure 10 is a longitudinal section view of a second embodiment of a high compression device adapted for use with the present invention.

Figure 11 is an electron micrograph illustrating the internal cross section of partially defibrated softwood chips using pressurized inlet conditions followed by high mechanical compression; and Figure 12 is an electron micrograph illustrating the internal cross section of partially defibrated softwood chips using atmospheric inlet conditions followed by high mechanical compression.

Description of the Preferred Embodiments Figure 1 is schematic of one possible hardware implementation of the present invention. Figure 1 shows how the compressive pre- treatment disclosed in International App. PCT/US98/14710, can be implemented in a commonly known vertical continuous digester system 10 such as the Kamyr type digester supplie by Ahistrom Machinery Inc. The compressive pretreatment portion 12 according to the invention is shown in phantom.

Wood chips are deposited in an atmospheric chip bin 14 and delivered via a chip metering valve 16 with pressurized discharge, to a horizontal presteaming conveyor 18. The pressure therein is below 30 psi (gauge) when operated in a conventional manner. However, in accordance with the invention, a pressurized conveying screw 20 having an inlet intersecting the vertical drop leg 22, exposes the chips to an environment of saturated steam, at a higher pressure, in the range of 30-150 psi or more, depending on the optimization of process conditions. This pressurized conveyor 20 is driven by a variable speed motor 24 such that the travel time through the conveyor can be controlled as an independent variable.

The conditioned chips are then introduced via feed column 26 into a specially modified MSD Pressafiner or RT Pressafiner (available from Andritz Inc., Muncy, PA) with a pressurized inlet or similar

compression device 28 which destructures the fibers and at the same time presses out liquid 30 and drives out air that may be present naturally or otherwise introduced, as will be described below.

Therefore, the chips upon emerging from the discharge 32 of the compression device are receptive to the rapid and deep take-up of fluids (liquid or vapor) that may be present at the discharge. For this reason, an inclined wet impregnator/conveyor 34 (such as is available from Andritz Inc. as Model 60/30) is provided extending from the discharge of the compression device, back to the drop leg 22. All or some of the impregnator preferably contains process liquid, most commonly recirculated black liquor in the case of Kraft pulping.

One suitable compression device 28 shown in Figure 9 has a variable speed motor 36, an inlet section 38 in which the screw core or shaft 46 has a substantially uniform diameter, a consolidation or extraction section 42, through which the screw core gradually increases in diameter, forming an annulus of decreasing flow cross-section along the substantially cylindrical, perforated wall 44, followed by a plug section 46 which typically has a substantially cylindrical core without screw flights, and preferably includes adjustability 48 of the flow cross- section and/or flow restrictor elements. In this particular embodiment, the screw flight outer diameter 50 remains substantially constant and the flights are not interrupted, but other variations having the desire functionality according to the present invention, are within the ordinary skill of practitioners in this field of technology.

In Figure 9 the pressurized conveyor 20 is shown in section, feeding into a Topwinder type forced feeder 52 (available from Andritz Inc.) which is particularly effective for feeding non-chip material such as bamboo, bagasse, sawdust, and similar materials, which cannot be introduced at a sufficient volummetric rate by relying on gravity alone.

Accordingly, the enhanced feeder 52 for the inlet to the compression device is optional. The material after entering the inlet 38, flows to the right, is consolidated as it passes through the reducing cross-section.

Because the material is at the same elevated pressure conditions as in the pressurized conveyor 20, a pressurized jacket 54 surrounds the perforated wall 44 in the consolidation section 42. Fluid is extracted, either by flashing or simply by draining and is ultimately withdrawn through drain pipes 30 or valve controlled flashing. The material undergoes a particular travel time in a consolidation along distance D1, where consolidation and extraction is occurring at an increasing rate, in distinction from the travel time along distance D2, where a plug is formed and the material experiences very high compression i. e., a ratio above 3/1, preferably in the range 4/1 to 8/1, and even more especially 5/1 or more. As noted above, during both the time of travel along D1 (between the inlet and formation of the chip seal or plug) and travel time along D2 (when the material is in compressed plug form), a significant degree of fiber destructuring occurs in an environment of e. g., high pressure saturated steam. Defibering is most intense along D2. Moreover, if as described below, addition reagents are introduced anywhere upstream of the device discharge 32, for example, atXl in the pressurized conveyor, orX3 in the Topwinder feeder or at the inlet to the compression device, the initial stages of lignin dissolution can begin to a significantly greater extent than is possible in a compression screw device that has a substantially atmospheric environment, because the fibers are more open and reactive. Reagents added upstream can also be used to initiate milder reactions prior to cooking in the digesters, such as subjecting the chips to black liquor with low hydroxyl content and high hydrosulfide content in the case of kraft pulping.

Another compression screw device 28'sable with the present invention, is shown in Figure 10. The structural aspects of this device are disclosed and described in international application PCT/US92/00939 (published as W092/13710). The addition modifications include a variable speed drive, pressurized inlet and high compression ratio. The respective retention times in region D1, where the material is consolidated under steam pressure, and the retention

time for D2, where the material is in the form of a compressed chip plug, are closely analogous to those described with respect to Figure 9.

The device of Figure 10 shows an interrupted flight section 56 where the compressed material can be"worked"to some extent thereby increasing the defibering action as well as producing further pressate, before passing through the discharge section.

With further reference to Figure 1, a high pressure feeder column 58 with return pump 70 following the drop leg 22 delivers a slurry of chips in black liquor or an alternative liquor, at a pressure of 5 to 10 bar, along line 60 to the inlet 62 at the top of the continuous vertical digester 64. It is common in the conventional portion of the system represented in Figure 1, for a black liquor liquid level 66 to be present in the drop leg immediately above the inlet 68 to the high pressure feeder column 58.

Typically, the inlet 62 at the top of the digester includes a pressurized drainer 72 or top separator which extracts much of the liquid in the slurry for return via pump 70 to the high pressure feeder 58. The separated but wet chips drop into the digester proper for chemical pulping action with liquor of increasing intensity as the chips move downwardly through the cooking zone of the digester column.

Due to improved and more selective pulping available with the present invention, the cooking conditions of temperature, pressure and possibly reagent concentration in digester 64 can be reduced to achieve a predetermined degree of lignin dissolution. This in turn will result in an increase in yield due to less degradation of the cellulose and hemicellulose components. Conversely, the cooking conditions can be maintained, however, with a reduction in cook time and concomitant increase in production capacity.

By introducing process fluid in the compressive pretreatment equipment (as shown in phantom) in accordance with the invention, particularly into the pressurized conditioning chamber 20 atX, and/or via X2 at the discharge 32 of the compressive destructuring device 28,

significant benefits can be achieved.

These benefits will be more fully appreciated when understood in the context of the balance of the process plant as described with respect to Figure 2. The process begins with the atmospheric presteaming bin 14 as shown in Figure 1 and is followed by the compressive pretreatment stage 12 which, in the preferred embodiment, comprises pressurized conditioning and compressive destructuring of the chips. The pretreated chips are then introduced into the chemical pulping equipment (i. e., digester 64), and thereafter washed 74. The "cooking"in the digester is performed with increasingly strong concentrations of white liquor via line (s) 76. In some variations of the kraft cooking process, a higher temperature black liquor is first introduced prior to the addition of white liquor. After removal of the lignin to the desired Kappa value, the process liquid has darkened considerably and is thereafter referred to as black liquor. The black liquor is washed from the chips and the diluted black liquor is passed through a drain line 78 to the black liquor seal tank 80. The washed chips are then sent to a bleaching plant 82 where the chips are exposed to successive stages of chlorination, extraction and chlorine dioxide bleaching, thereby producing essentially bleached white pulp as the desired end product of the process. Several variations of the bleaching process are also practiced including oxygen pre-delignification, ozone pre-treatment, extraction with oxygen and peroxide addition (EOP), chlorine substitution with chlorine dioxide, etc. In some applications, no elemental chlorine is used (ECF) and occasionally no chlorine of any type is used (TCF).

The liquor from the black liquor seal tank eventually passes through an evaporator array 84 and the concentrated black liquor, containing approximately 65% solids, is delivered to a recovery boiler 86. For simplification in Figure 2, optional black liquor oxidation treatment is not shown. In the boiler, the lignin, wood extractives, and some hemicellulosic material are burned, producing gases and some

particulates which may require scrubbing (i. e., electrostatic precipitator) before discharge through a stack. The smelt, consisting essentially of inorganic chemicals, passes through a chemical recovery process 88.

Upon exiting the chemical recovery process as regenerated white liquor, the liquid can be returned for introduction into the digester or similar chemical pulping unit.

Various points along the process shown in Figure 2 are labelle with an alphabetic identifier. Points B1, B2, B3 and B4, represent various locations in which the used liquor is processed and recovered in the kraft process. Point W represents a line containing white liquor.

Point V represents a vapor phase of cooking (white) liquor that often is deliberately maintained near the inlet of the digester.

In accordance with the one aspect of the present invention, a flow of one or more black liquor B, white liquor W, or vapor phase V from the digester, can be introduced from a downstream line, into the compressive pretreatment stage 12. Furthermore, steam S or external liquor L with chemical reagent, can be similarly introduced in the compressive pretreatment stage.

With reference again to Figure 1, line X1 represents the introduction of any one or combination of the process fluids B, W, V, S or L, into the pressurized conditioning chamber 20 (e. g., pressurized conveyor) of the compressive pretreatment stage. Fluid line X2 represents a different point of introduction of the same or different one or combination of process fluids B, W, V, S or L, at the discharge 32 of the high compression device (e. g., Pressafiner).

It is evident that to the extent process fluid is introduced at any point upstream of the compression zone of the compression unit 28, a pressate will be extracted. In a preferred embodiment wherein, for example, the process fluid through line X, is black liquor, such as from point B2 of Figure 2, the pressate 30 from the compression unit will be a concentrated low volume stream of black liquor and wood extractives.

In another aspect of the invention, this pressate is fed downstream to

the black liquor seal tank 80 for subsequent combustion in the recovery boiler 86. A large portion of wood extractives are thus selectively removed prior to cooking. There is no liquid effluent treatment costs and therefore the incorporation of the invention as shown in Figure 1 will not necessitate expansion in effluent treatment when backfit to an existing chemical pulping plant.

The trend today in Kraft pulping is toward low alkalinity during the initial phase of the cook to help maintain a higher yield. Defibration of the lignocellulose material under pressurized inlet conditions of device 28 according to the present invention (e. g., as shown in Figure 9) produces structural intact material with high levels of exposed lignin and fiber wall material, permitting improved chemical diffusion and more selective lignin dissolution during chemical pulping, and thus improved yield. The introduction of vapor phase chemicals (V) or recycled black liquor B (or low alkalinity white liquor W) to the pressurized inlet of the compression unit, such as shown with process fluid stream X3 (or X1), will help further reduce the total alkalinity required during cooking due to improved penetration during partial defibration in the compression zone 46. Thus, there will be less carbohydrate degradation and yield loss.

With the preferred embodiment of pressurized conditioning followed by compressive destructuring according to the present invention, the chips are not damaged during pretreatment and can be subjected to conventional cooking conditions without deterioration in the yield, strength, or brightness of the fibers. For example, the chips can be cooked to low Kappa number values, or at least to the same values as conventionally cooked chips. This is of particular significance with mills which use TCF bleaching, in which a low initial Kappa number is necessary. Inspection of chips which have been conditioned under high pressure and then highly compressed, reveals an open chip structure in which the chips are quite pliable when rolled between the figures, without internal fracturing of the fiber structure. This is in

contrast to chips inspected after atmospheric presteaming followed by high compression, in which the fibers tend to crumble when rolled between the fingers. The accompanying electron micrographs illustrate the internal cross section of partially defibrated softwood chips, firstly using pressurized inlet conditions (Figure 11), and secondly using atmospheric inlet conditions (Figure 12). The figures clearly indicate atmospheric preheating was not sufficient to prevent fiber shattering during compression in the high compression screw device. The damaged internal structure of the atmospherically heated wood chips explains why the chips crumble when rolled between the fingers, unlike that of the chips compressed under pressurized inlet conditions.

Thus, with the preferred embodiment of the present invention, conventional yield can be increased in a first increment by the use of pressurized conditioning immediately preceding the compressive destructuring. This results from better penetration of chemicals between the fibers at the discharge of the compression device. A further increment of yield and/or efficiency can be achieved by a reduction in the severity of cooking (via decreased temperature, pressure, alkali concentration) to achieve a given Kappa value.

Conversely, because according to the invention the partially defibrated chip structure is more rigid the material can tolerate extensive cooking to lower Kappa values without loss in pulp strength.

As a result of implementing the preferred embodiment, the plant can be optimized as between utilization of the conventional size and footprint of digesting equipment to achieve a higher than standard yield, or else for a given desired throughput, the size and/or footprint of the equipment can be reduced. In particular, smaller digester units can be utilized, to achieve current levels of throughput.

Further operational techniques and details of a typical Kraft pulping plant may be found in the brochure entitled"Continuous Progress in Cooking", available from the Alhstrom Machinery Inc., t1997, and from said U. S. Patent 4,869,783, the disclosures of which

are hereby incorporated by reference. According to the process described in the'783 patent, the cooking is terminated upon reaching Kappa levels greater than 40 (typically about 45 to 70) and is then followed by other processes, such as oxygen delignification, to reach the desired pre-bleaching Kappa of 15 to 25. With the present invention, similar Kappa levels above 40 can be achieved in the brown stock (i. e., the washed pulp from the digester) but significantly, levels in the range of 15-35 with acceptable strength and brightness can also be achieved, for consistency with the operation of a wide variety of brown stock treatment (with or without oxygen delignification).

Figure 3 illustrates an alternative system configuration 100, to that of Figure 1. In this embodiment, which would be suitable for use in a new plant, the rotary valve 16 with pressurized discharge deposits chips directly into the pressurized conveyor 20. A stream of process fluid X, is introduced near the inlet to the pressurized conveyor. The pressurized conveyor discharges immediately into the inlet of the compression device 28, which in turn discharges into a conventional drop leg 22'associated with feeding into the high pressure feeder column 58. By eliminating conveyor 18 (Figure 1) this configuration uses only one (pressurized) conveyor 20 and is thus more compact.

Process liquid along line X2 can be introduced as a vapor or liquid at the discharge of the compression device 28.

Figure 4 shows another system embodiment 200 of the invention, as used in conjunction with an M&D continuous digester 202 (available from Andritz Inc.), or other digester operating on similar principles. As is well known in the relevant field of technology, these digesters have buckets or the like which move along line 204 whereby the material is introduced into a vapor cooking phase in the buckets in the upper portion 206, the buckets travel downwardly through a liquid phase 208 of the cooking fluid, and rise upwardly through the vapor phase again, for discharge through a rotary valve 210 with pressurized inlet. The atmospheric chip bin 14 delivers chips through a rotary valve

16 with pressurized discharge, into a pressurized conveyor 20, which in turn is followed by a compression device 28. The compression device discharges into an inclined wet conveyor or impregnator 34 at atmospheric conditions. The discharge from the impregnator is horizontal conveyed to a rotary valve 210 with pressurized discharge, which in turn feeds the pressurized vapor phase inlet of the digester in a conventional manner. Process fluid can as in the previous embodiments, be introduced at X"X2, and/or X3.

Figure 5 shows alternative configuration 300 for use with an M&D type digester 202, wherein the chips are conveyed from the atmospheric chip bin 14 through a rotary valve 16 with pressurized discharge, into a pressurized conveyor 20 which in turn feeds the compression device 28. The discharge 32 of the compression device is directly exposed to the vapor phase 206 of the cooking fluid in the upper, inlet portion of the digester.

Figure 6 shows a feed system configuration 400 that can be utilized upstream of the feed chute 402 to batch type digester. Chips from the atmospheric bin 14 are fed through the rotary valve 16 into a pressurized conveyor 20 and then through the chip compression device 28, which in turn discharges to a transfer conveyor 404 for charging into the batch digester. Figure 7 shows a further modification 500 wherein the compression device 28 discharges into an inclined impregnator 34 prior to transfer for charging at 402 into the batch digester. It can be seen in both Figures 6 and 7 that process flow lines X, and/orX2 can be effectively utilized, as with the previously-described embodiments.

Figure 8 shows yet another system implementation 600, whereby the chip material from the feed bin 14 is conditioned at high pressure at 20, then introduced to the compression device 28, which provides the addition function of the feeder device for the vertical digester 602.

In particular, the compression device 28 discharges into a vertical impregnator 604 where the compressed chips, upon expansion as they

discharge from the compressing unit at 32, are exposed to the liquid 606 and are thereby impregnated immediately as they are conveyed upwardly by screw 608 before dropping down into the main region 610 of the vertical digester.

It can be appreciated that with all the foregoing systems, the preferred implementation includes a variable speed drive 24 on the pressurized conveyor 20, thereby providing control of the exposure of the chips to the elevated temperature and pressure conditions, for a controlled time period, preferably in the range of 10-1800 seconds.

Furthermore, it would be within the ordinary skill of those in this field to provide pressurized steam to maintain saturated conditions in the range generally of 30-150 psi, preferably 40-100 psi. The compression device 28 in all embodiments preferably includes a variable speed motor 36 to provide an independent control on the time interval during which the chips are processed in the device. For example, with respect to Figure 9, the two time intervals during travel over D1 and D2 from inlet 38 to chip plug and in the plug mode respectively, are regulated by the speed of the screw and screw design itself. A range of 5-15 seconds would be especially beneficial where a process fluid line such as X3 of Figure 1, is connected to the inlet 38 of compression device (or at any point upstream of the discharge end 32). Thus, even if a particularly strong liquor is introduced, the time period of exposure can be controlled, at one or both of the conditioning chamber 20 or compression device 28. Moreover, as shown at 90 in Figure 3 the invention can be optimized by continually measuring and controlling the relative mixing of process fluids or with other fluid such as L or S chemical reagents such as B, W, or V for delivery via any of linesx, X2, orx3.

It should also be appreciated that there is no need for a pressure barrier or differential between the pressurized conveyor 20 and the compression device 28. The physical configuration of these two functional units need not be limited to what is shown herein, i. e., a

horizontal conveyor with discharge into a horizontal compression unit.

The functions of these two units could be combined into a single unit having for example, a conditioning chamber leading into a compression chamber, both of which are maintained at elevated temperature and pressure, preferably a saturated steam environment above 40 psi. As noted above, the pressurized environment, as distinct from the atmospheric pressure in the conveyor and compression device as is typical in conventional atmospheric inlet screw devices, not only opens the chips up for more efficient lignin removal, but importantly maintains the elongation of the chips and minimizes brittle fractures and shattering. This advantage contributes significantly to the ability of the present invention, to permit cooking to desired Kappa number levels without loss in pulp strength properties.