Process and configuration for providing external upflow/internal downflow in a continuous digester

Abstract

A continuous digester configuration and process for continuously cooking and digesting cellulosic material and producing cellulosic pulp is disclosed. The digester configuration comprises a plurality of jump-stage recirculations located along the length of a continuous digester vessel, wherein each of the jump-stage recirculations moves extracted liquor from one elevation of the digester vessel and re-introduces the liquor at a higher elevation in the vessel to maintain an internal liquor downflow substantially throughout the entire digester vessel. The process for continuously cooking and digesting cellulosic material comprises maintaining an internal concurrent liquor downflow substantially throughout the continuous digester vessel, wherein the internal liquor downflow does not exceed 400 gallons per ton of b.d. wood feed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of U.S. patent application Ser. No. 60/240,659, filed Oct. 16, 2000, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to continuous digesters that digest cellulosic fibrous material and produce cellulosic pulp. More particularly, this invention relates to an improved digester configuration and process for use in and with a continuous digester. The configuration and process of this invention produce a heretofore-unseen internal con-current flow or downflow in the digester vessel.

BACKGROUND OF THE INVENTION

[0003] Continuous digesters for production of pulp were developed in the 1940's and commercialized in the 1950's. Original continuous digester designs were completely con-current (downflow of chips, chip-bound-liquor, and liquor). Digesters sold in the 1950's were described as “cold blow” digesters in that they had a set of circumferential extraction screens followed by cold filtrate addition at the very bottom of the vessel, just prior to pulp discharge. The extraction screens extracted hot, spent cooking liquor, which was then replaced with an even larger volume of cold filtrate from a downstream washing process. Filtrate addition was through a circumferential header and a central pipe downcomer whose discharge was at or around the elevation of the extraction screens. Thus, digesters from the late 1950's practiced a simultaneous extraction/dilution sequence at the very end of the process.

[0004] In practice, the “cold blow” digesters were limited in the amount of hot liquor that could be extracted from the screens. These screens were also prone to plugging. As a result, the discharge temperature could not be effectively controlled (i.e., lowered). Also, the lower the extraction flow, the higher the amount of by-products in the discharge stream. This required additional washing requirements from downstream unit operations.

[0005] In response the problems cited above, a high-heat in-digester washing system was developed (circa early 1960's) and continues to be used today. In this system, the bottom of the digester vessel is converted to a continuous counter-current contacting system wherein chips and chip-bound-liquor move vertically down while the interstitial liquor between chips (i.e., free-liquor) moves vertically up. Again, wash filtrate from a downstream washing process is added in excess at the very bottom of the vessel. This filtrate serves as a washing medium within the high-heat wash zone. The high-heat wash zone also served as a heat recovery process in that the hot chip/liquor mixture transferred heat to the net upflowing, cooler filtrate. This upflowing filtrate effectively captures system heat and returns it back into the vessel (verses allowing it to be lost through discharge). Thus, filtrate is heated while pulp is simultaneously cooled for discharge at temperatures below 185° F. In order to affect an upflow, liquor is extracted from a set of circumferential extraction screens located well above the bottom of the vessel (i.e., heights that correspond to chip retention times which range from 30 minutes to 4 hours).

[0006] In the early 1960's, the continuous cooking system could be used to affect a counter-current cooking process wherein cooking chemicals are brought into contact with the chips and bound liquor in a counter-current manner. This process demonstrated advantages vis-a-vis better strength properties, better bleaching properties, and improved cooking uniformity.

[0007] The counter-current process was characterized by having (a) split white liquor (WL) additions, (b) counter-current cooking with a major charge of WL to the very bottom of the vessel, (c) internal recirculation of liquors, and (d) black liquor impregnation. The internal recirculation involved taking a portion of the liquor drawn from the mid-level extraction screens and redirecting it to feed, or front-end, of the process. As such, this represented a jump-stage of upflowing liquor such that the impregnation zone of the vessel (i.e., the top most zone) was an internal-downflow, external-upflow system.

[0008] The counter-current process was further augmented by having split WL additions introduced at various levels (or zones) within the counter-current section of the system. This represents the first attempt to profile alkali concentrations along the height of the digester so as to optimize pulp composition and thus pulp yield and pulp properties. The process contains a recirculation from one elevation of the digester to a second, higher (or upstream) elevation.

[0009] During the 1960's and 1970's, continuous digesters that used high-heat washing became the industry standard. This trend continues to this date. Compared to alternative technologies, namely batch cooking processes and continuous process without counter-current washing, these systems offered superior pulp cleanliness, pulp properties, reliability, and energy efficiency. In spite of this, counter-current cooking technology did not take hold. This can be attributed to numerous factors, perhaps the most important being that there were insufficient economic incentives to mitigate potential risks and development costs. Nonetheless, the continuous cooking systems of the era did not have reliable, sufficiently high wash zone upflows. Limitations on the amount of stable upflow that could be obtained make the use of counter-flowing systems for the purposes of cooking unattractive. In general, the relative amount of upflow is limited by (a) excessive column compaction at the extraction screens, (b) unstable column dynamics in the blow dilution zone, and (c) excessive drag forces in the counter-current zone. In the final analysis, for digesters built in the 1960's and 1970's, the relative amounts of upflow and extraction flow were severely limited by their inherent design and this led to practical operating constraints. These problems were made even worse by increased production rates (and thus increased loading rates), and many commercial systems were (and still are) operated at much higher rates than their original design.

[0010] During the late 1970's and early 1980's, considerable progress was made towards improving the system design so as to allow for relatively higher, more stable upflows. Significant innovations included new screen plate designs, the use of switching valves to minimize header plugging, and optimization of the vessel geometry. These improvements decreased column compaction at the extraction and provided more upflow. At the same time, renewed interest in modified cooking technologies evolved, indirectly, from economic and social pressures to decrease the environmental impact of pulping and bleaching practices. In the late 1970's and early 1980's, researchers performed systematic studies to identify optimal pulping conditions. From this, the general principles of modified cooking evolved. These principles state that pulping selectivity can be optimized by (a) maintaining a lower alkali concentration at the start of bulk delignification, (b) maintaining a higher sulfidity during impregnation and the start of bulk delignification, (c) maintaining lower overall ionic strength, particularly at the end of bulk and throughout residual delignification, and (d) minimizing lignin concentration (while increasing relative alkali concentration) at the end of bulk delignification and throughout residual delignification. It so happened that the counter-current cooking methods described earlier achieved all of these objectives. From the mid 1980's onward, modified continuous cooking methods (MCC) became the industry standard. The combination of better system design, a better fundamental understanding of process optimization requirements, and more relevant driving forces made this technology a commercial reality.

[0011] The “Lo-Solids®” based processes (Marcoccia US Pat. Nos. 5,489,363, 5,536,366, 5,547,012, 5,620,562, 5,662,775) are unique in that they describe means for achieving the benefits of modified cooking in systems with little or no wash zone upflow. This is of great importance since the vast majority of installed commercial units still have difficulty maintaining sufficiently high, stable upflows (i.e., even those built after the 1980's, but particularly those built before).

[0012] During the 1960's and 1970's, digesters were built with multiple filtrate additions points (i.e., in addition to the bottom, provisions were made for filtrate addition to the feed and to the extraction screen elevation via the so-called “quench” circulation). The simultaneous extraction dilution sequences at the quench (and at the bottom of cold blow digesters) is fundamentally different than the simultaneous extraction/dilution sequence prescribed by Lo-Solids® cooking due to the fact that these operations do not purge and dilute throughout bulk delignification as prescribed by Lo-Solids®. Similarly, in the 1970's and 1980's, several mills modified their digester configurations so as to be able to extract at both the extraction screens and wash (or bottom zone screens). In some cases, filtrate was introduced at the quench and in others a jump stage recirculation from the wash screen elevation up to the extraction screen elevation was used. The Lo-Solids® process was determined to be unique form these earlier processes (even though bulk delignification was known to occur downstream of the extraction screens) owing to the fact that these earlier processes did not purge and dilute at the start of middle of bulk delignification. Furthermore, jump-staging via internal recirculation emulates a counter-current process wherein dissolved organic material (DOM) is pushed upstream (versus the cross-flow Lo-Solids® process where DOM is purged and diluted).

[0013] In view of the above, however, there is still a continuing need for alternative continuous digester configurations and processes to improve digester performance and efficiency.

SUMMARY OF THE INVENTION

[0014] The present invention relates to continuous digester configurations and processes designed to optimize heat recovery, optimize washing efficiency, optimize operations stability, and obtain the benefits of modified cooking in a continuous digester.

[0015] In accordance with the purposes of this invention, as embodied and broadly described herein, this invention, in one aspect, relates to a digester for continuously cooking and digesting cellulosic material and producing cellulosic pulp comprising a plurality of jump-stage recirculations located along the length of a continuous digester vessel, wherein each of the jump-stage recirculations moves extracted liquor from a first elevation of the digester vessel and re-introduces the extracted liquor into the vessel at a second higher elevation to maintain an internal liquor downflow substantially throughout the entire digester vessel.

[0016] In another aspect, this invention relates to a digester for continuously cooking and digesting cellulosic material and producing cellulosic pulp comprising a jump-stage recirculation located along the length of a continuous digester vessel, wherein the jump-stage recirculation moves extracted liquor from a first elevation of the digester vessel and re-introduces the liquor into the vessel at a second higher elevation to maintain an internal liquor downflow substantially throughout the entire vessel that does not exceed 400 gallons per ton of b.d. wood feed.

[0017] In yet another aspect, this invention relates to a process for continuously cooking and digesting cellulosic material and producing cellulosic pulp in a continuous digester comprising (a) maintaining an internal, con-current liquor downflow substantially throughout a continuous digester vessel, wherein the internal liquor downflow does not exceed 400 gallons per ton of b.d. wood feed.

[0018] Additional advantages of the invention will be set forth in part in the figures and detailed description, which follow, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory of preferred embodiments of the invention, and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic view of an exemplary continuous digester system according to one embodiment of the present invention, shown interconnected to other components of a mill for producing cellulose pulp.

[0020] FIG. 2 is a schematic view of one embodiment of a continuous digester configuration of the present invention.

[0021] FIG. 3 is a schematic view of another embodiment of a continuous digester configuration of the present invention.

[0022] FIG. 4 is a schematic view of one embodiment of jump-stage recirculations of the present invention shown associated with circumferential digester screens.

[0023] FIG. 5 is a schematic view of yet another embodiment of a continuous digester configuration of the present invention.

[0024] FIG. 6 is a schematic view of a known continuous digester and its internal liquor flow.

[0025] FIG. 7 is a schematic view of two alternative liquor downflow configurations of the present invention.

[0026] FIG. 8 is a schematic view of a side-by-side comparison of a known liquor flow configuration and a liquor flow configuration according to the present invention.

[0027] FIG. 9 is a graph of the effect of downflow cooking according to the present invention on digester discharge.

[0028] FIG. 10 is a graph of the effect of downflow cooking according to the present invention on filtrate-cooler duty.

[0029] FIG. 11 is a graph of the effect of downflow cooking according to the present invention on thermal DR.

[0030] FIG. 12 is a graph of the effect of downflow cooking according to the present invention on extraction-filtrate conductivity.

[0031] FIG. 13 is a graph of the effect of downflow cooking according to the present invention on oxygen delignification efficiency.

[0032] FIG. 14 is a graph of the effect of downflow cooking according to the present invention on brownstock yield.

[0033] FIG. 15 is a graph of the effect of downflow cooking according to the present invention on pulping selectivity.

[0034] FIG. 16 is a graph of the effect of downflow cooking according to the present invention on cooking uniformity.

[0035] FIG. 17 is a graph of the effect of downflow cooking according to the present invention on EA profiles.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention may be understood more readily by reference to the figures, detailed description, and the examples, which follow, where like reference numerals represent like elements throughout, unless the context clearly dictates otherwise.

[0037] It is to be understood that this invention is not limited to the specific methods, conditions and/or parameters described, as specific methods and/or method conditions and parameters may, of course, vary. It is also understood that the terminology used herein is used for the purpose of describing particular embodiments only and is not intended to be limiting. It must also be noted that, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include plural references, unless the context clearly dictates otherwise.

[0038] Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment.

[0039] The present invention (based upon functioning commercial systems) demonstrates that the ability to extract from a circumferential digester screen section is dramatically enhanced if: (a) the free liquor flow is con-current (i.e., liquor flows down) to the chip flow at any/all screen locations, and (b) the free liquor downflow does not exceed 400 (±300) gallons per ton of b.d. wood feed, preferably anywhere in the vessel, as measured. The enhanced extraction ability refers herein to (a) ability to extract more per unit area of screen section before plugging or chip column consolidation occurs, and (b) the ability to extract uniformly across the circumference.

[0040] As stated above, the process and configuration according to this invention creates free liquor downflow. Also, excessive downflow is avoided. Free liquor downflow cannot be measured directly and can only be estimated by calculation. The calculations require that several assumptions be made and are based upon measured inputs that will have significant errors. As a result, calculated values for free liquor downflow will have significant margins of error. For a 1000 ton pulp per day system, errors of 200 gpm (or 100% of targeted values) are not uncommon.

[0041] In order to estimate the potential magnitude of calculation error, a real-time, overall mass balance must be performed. The sum of all measured or inferred inflows is subtracted from the sum of all measured or inferred outflows.

Mass of Inflows=Mass of wood as wood chips+Mass of wood moisture+Mass of condensed steam+Mass of total cooking chemical added+Mass of total wash filtrate added+Mass of seal water leakage into the system,  Equation (1).

Mass of Outflows=Mass of total extraction liquor+Mass of filtrate in outflowing pulp slurry+Mass of pulp fiber in out flowing pulp slurry+Mass of all vented steam,  Equation (2).

Error in Mass Balance=Mass of Inflows−Mass of Outflows,  Equation (3).

[0042] To a first approximation, it can be assumed that the error in calculated liquor flows will be of the same order of magnitude as the Error in Mass Balance. A suitable (initial) control target for each zone in the digester is a free liquor flow of 100 gpm+the absolute value of the error. Final target flows will be adjusted based on system response. the Total Liquor Flow, the Chip-Bound Liquor Flow, and the Free Liquor Flow. By definition:

Free Liquor Flow=Total Liquor Flow−Bound Liquor Flow  Equation (4).

[0043] The liquor flows have a vector component and so a convention for direction is necessary. For example, liquor flowing vertically downward can be assigned a positive value in which case upflowing liquor would have a negative value. Also, calculations are based on a discrete control volume and so the physical location of the calculated flow must be specified: e.g., at the bottom of the control volume.

[0044] To solve Equation (4), the Bound Liquor is estimated and the Total Liquor flow is determined based on a mass balance around the zone in question. The bound liquor is calculated based upon an assumption of the yield, the original basic wood density, and an assumption for the average degree of chip volume compression in the zone of interest.

[0045] The hydraulic condition of the digester is determined by calculating Total, Free, and Bound liquor flows on a zone-by-zone basis using an iterative calculation. This iterative calculation can be performed from the “top-down” or from the “bottom-up”. For illustrative purposes, the bottom-up calculations will be summarized below.

[0046] The bottom most zone of the digester is the blow dilution zone. Here, wash filtrate is added into the zone and pulp slurry leaves the zone.

[0047] By definition, based on the conventions specified above:

Total Liquor flow=Blowline Flow,  Equation (5).

[0048] Furthermore,

Blowline Flow=Total Flow from Preceding Zone+Filtrate Added in Blow Dilution Zone  Equation (6).

[0049] From Equation (6), the Total flow from the preceding zone (which is the wash screen section) can be calculated. The hydraulics within this preceding zone are then calculated using a similar method:

Total Flow from Wash Screens=Total flow from preceding Zone+filtrate added to wash screen zone+cooking chemical added to wash screen zone−extraction from wash screen zone,  Equation (7).

[0050] Thus, Equation (7) can be used to calculate the preceding zones Total Liquor Flow.

[0051] Using this iterative approach, the total flow for each zone of the digester can be calculated. Once total flows are determined then Equation (4) is used to calculate the free liquor flow in each zone.

[0052] As a result of uniform and high capacity extraction, very efficient radial displacement of downflowing liquor can be achieved (i.e., displacement with, for example, a liquor stream introduced at the same elevation through a central pipe downcomer). This efficient displacement can be used at the very bottom of a digester to affect excellent washing and heat recovery. It has also been found that the enhanced extraction capacity is aided by substantially eliminating any upflowing free liquor anywhere in the digester vessel, although this constraint is not as important as maintaining downflow in the screen sections. Most every digester (including the downflow cold blow digesters from the 1950's) affected an upward component of the free liquor velocity at one or more extraction screen locations as a result of their design balance. Furthermore, the vast majority of operating systems in the world will have such an upward velocity component. Notable exceptions here are severely overloaded systems with excessively high free liquor downflows.

[0053] Accordingly, the present invention is directed to a process and digester design configuration, which take advantage of this observation while utilizing other design configurations that are known to enhance overall process performance.

[0054] In on embodiment, the process of the present invention preferably maintains a downflow substantially throughout the entire reaction vessel, particularly at each screen section, and more particularly at the screen walls. In order to achieve this while avoiding excessive downflow liquor velocities, multiple extractions are employed. Multiple extraction sites further enhance the overall extraction ability of the system. The enhanced extraction ability, and the absence of upflowing liquor both allow for significant reductions in the vessel diameter for any given production rate.

[0055] Multiple extractions dictate the use of multiple WL additions. Preferably, WL additions are made simultaneously with (or downstream of) the individual extractions so as not to have excessive residual chemical in the extraction liquor flows.

[0056] In order to minimize the well known, negative effects of high lignin concentration at the end of bulk delignification, the process and digester of this invention is configured to affect an external upflow. This is accomplished by using jump-stage recirculations, wherein extracted liquor from one elevation is reintroduced at a higher (or upstream) elevation.

[0057] While singular jump-stage recirculations have been used elsewhere (e.g., from the wash to the extraction in overloaded commercial systems, or from the secondary to the primary heating circulation), the present invention is unique, in one embodiment, in that it uses a plurality of such stages throughout the entire bulk and residual delignification stages.

[0058] As such, the resultant internal-downflow, external-upflow process maintains a downflow substantially throughout the entire vessel while affecting a process flow scheme that is essentially counter-current (and multi-staged) throughout the whole of bulk and residual delignification.

[0059] The internal-downflow, external-upflow of the present invention is analogous to the process configuration of a counter-current brownstock or counter-current bleach plant washing trains in that the solid and liquid phases move con-currently within the unit operation itself, but the process involves a solid/liquid separation wherein the liquid is externally transferred to an upstream stage. The practice of extracting some portion of counter-flowing liquor has also been applied in bleach plant systems. The type of a multi-stage, internally con-current (downflow)/externally counter-current (upflow) system of the present invention has never, however, been practiced in or proposed for a continuous digester.

[0060] Multi-stage extraction and/or washing ideally should have an efficient displacement between stages. Another aspect of the present invention is that the screen sections, which typically consist of a set of 2 circumferential screen assemblies, are physically separated by at least 3 linear feet of blank plates. This rest area between screens is meant to provide a sharp interface between the end of one zone (stage) and the beginning of the next. It is also felt that this relaxation between screens will further enhance extraction capacity and extraction uniformity by reversing the effects of chip column compaction at the 1st extraction screen assembly of any set of screens.

[0061] This invention can be practiced in 2, 3, 4 or 5 screen elevation systems or in either a single or twin vessel system.

[0062] Another aspect of the present invention is that every row of screens is preferably divided into no less than 6 individual screen plates, and that liquor is withdrawn from behind each screen plate (out of the reactor) through an individual nozzle followed by individual flow indicator-control instruments. This practice provides maximum diagnostic and control capabilities thus insuring optimal uniformity and capacity from each screen assembly or row.

[0063] Another aspect of the present invention is that the process and configuration preferably includes a computer software program that uses process data from process instrumentation to calculate free liquor velocity and net direction at every elevation of the reactor. These calculations outlined in detail above, are performed in “real time” (i.e., no less than once every hour) and their results are reported to operating personnel. Another aspect of this invention is that the associated software will allow operators to simulate the effect of process operating set point changes on liquor profiles, and will also recommend changes required to insure the free-liquor flow requirements are met.

[0064] The Pulping System

[0065] Referring particularly to FIG. 1, and to provide an example and overview of a pulping system and process, a two-vessel hydraulic 1200 admt/d system operated in an Extended Modified Continuous Cooking (EMCC™) mode, shown generally at 155. The digester comprises a top and a bottom, an inlet at the top for receiving cellulosic fibrous material and an outlet at the bottom for discharging digested pulp.

[0066] In particular, the digester system 155 is fed wood chips from wood yard area 171. The wood chip stream 201 is fed to the digester feed system 301. A chip-liquor slurry 401 is fed to the top of digester vessel 70 having a chip-liquor separator 50. A liquor return stream 601 is fed back to the digester feed system 301.

[0067] The digester system 155 has a primary heating circulation system 90 and a secondary heating circulation system 100, both with an indirect liquor heater. A circumferential screen assembly for cook liquor withdrawal is located at 80a-c along the length of vessel 70. Liquor is withdrawn from locations 80a, 80b and 80c and circulated to heating systems 90 and 100, and re-circulated back to the vessel 70 through a plurality of jump stages, wherein the “upflow” of liquor is accomplished externally.

[0068] The circumferential screen assembly 80b for cook liquor withdrawal is approximately mid-vessel and extracts a spent cooking liquor stream 110 and directs that stream to a liquor evaporation area 120. After evaporation, concentrated, spent (or “Black”) liquor 130 is directed to chemical recovery and preparation areas 140. A “white” liquor stream 150 (cooking chemical) is fed to digester feed system 301 for forming a chip-liquor slurry for introducing into the top of vessel 70.

[0069] The blow dilution zone of vessel 70 is shown generally at 160. Blow dilution zone 160, located around the bottom area of digester vessel 70, has a side dilution header/nozzle assembly 170 and a bottom head dilution header/nozzle assembly 180. At the very bottom of the vessel 70, a blowline 190 containing a pulp/water slurry stream directs the slurry to a “brownstock” washing and screening area 200. Wash water 210 is added to the brownstock washing and screening area 200. From area 200, filtrate 220 (also called cold blow filtrate or wash filtrate) is directed from the wash to the digester's blow dilution zone 160. Washed brown stock pulp/water slurry 230 is directed from the washing and screening area 200 to bleaching and/or drying and/or paper making areas 240.

[0070] Cooking chemical is added at both the wash and MCC™ circulations. Simultaneous counter-current cooking and washing take place in the zones between the extraction and wash screens. Cooking total retention time is between 4.5 and 5.5 hours, with approximately 3.5 to 4 hours retention in the counter current zones.

[0071] Not shown in FIG. 1, but following digester 15 typically is a 2-stage atmospheric diffuser, pressurized screen room and vacuum washer decker. A 2-stage medium consistency oxygen delignification system followed by atmospheric diffuser and vacuum washer complete the brownstock fiberline system. The fiberline system produces several grades of softwood market pulp from a variety of coastal and interior wood furnishes.

[0072] FIG. 2 shows a simplified schematic view of a continuous digester and process of this invention including a digester reactor 2 with three (3) screen sections (1a, 2a and 3a from the top-down). The digester configuration and process can be extended to include systems with 4, 5 or more screen sections, if desired.

[0073] Wash water or filtrate 6 is added to the center of the digester 7 in addition to being added in the blow dilution zone 8. This wash water 6 is added slightly above screen 3a elevation through a central pipe assembly (not shown).

[0074] Liquor withdrawn from the screen 3a is pumped through jump-stage recirculation line 9 to the central pipe discharge at or slightly above the screen 2a elevation. Likewise, liquor withdrawn from the screen 2a is pumped through jump-stage recirculation line 10 to the central pipe discharge at or slightly above the screen 1a elevation.

[0075] For the embodiment shown, spent liquor is extracted from the system 11 and to recovery operations through the screen 1a.

[0076] For existing systems, screens 1a and 2a typically have two rows of screen plate sections. FIG. 3 illustrates an extension of the basic concept described herein wherein an additional jump-stage recirculation flow 12 is taken from screen 1a and sent to the feed system 15 where incoming wood chips and feed liquor 16 are introduced.

[0077] A preferred method for extending the process as such would be to selectively extract liquor to recovery from the top row 13 of screen 1a and recirculate or jump from the bottom row 14. To achieve this additional recirculation from screen 1a to the feed, the recirculated liquor would have to be cooled in a cooling unit 17 such as a heat exchanger or a flash cooling device.

[0078] The process and digester configuration described and shown by FIG. 3 illustrates that multiple jump recirculation flows are used to affect an external upflow, internal downflow throughout the entire system.

[0079] In application, there will be circulation systems in place at each of the screen elevations. These circulations are used to introduce heat and cooking chemical to the system at the given elevation. The jump stages will be incorporated into the circulations as shown schematically in FIG. 4, where circumferential digester screens are shown at 24.

[0080] Note that it in the preferred embodiment of the invention, the jump-stage recirculation lines 18 are taken downstream of the circulation pump 19, but upstream of the WL and/or filtrate addition point 20 and upstream of the circulation's heater 21. Furthermore, the entry point 22 for any given re-circulation (i.e., into the preceding circulation system) would have to be downstream of any extraction 23 or re-circulation 18 flow drawn from the receiving circulation.

[0081] The external upflow process can be operated with multiple extractions. FIG. 5 is identical to FIG. 2 except that three extraction lines are utilized (i.e., spent liquor is extracted off of screen sections 2a and 3a in addition to screen section 1a). Extraction flows off of screens 2a and 3a are denoted as 26 and 27, respectively.

[0082] Extraction flows 26 and 27 are not desirable in that they will indirectly result in decreased energy and washing efficiencies. The greater the flow of either 26 or 27, the greater the loss in both washing and energy efficiency. These flows would only be utilized if extraction flow 11 were capacity limited (i.e., if the screen extraction capacity was not large enough to induce full uptake of available wash filtrate). In general, extraction flow 11 is to be maximized and extraction flows 26 and 27 would be minimized (i.e., set to the minimal flow required to achieve an overall extraction flow which results in uptake of all available wash filtrate). In general, use of extraction flow from 26 would result in smaller energy and washing penalties than use of extraction flow from 27.

EXAMPLES

[0083] The following examples and experimental results are included to provide those of ordinary skill in the art with a complete disclosure and description of particular manners in which the present invention can be practiced and evaluated, and are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.); however, some errors and deviations may have occurred. Unless indicated otherwise, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric.

[0084] An extended digester trial was performed, wherein the digester process configuration was changed from upflow to downflow modified cooking. The primary objective of the trial was to quantify the effect of downflow cooking on overall mill operations and then use this information to perform a cost-benefit-risk analysis required to convert to permanent downflow operations. The secondary objectives were to identify potential operating constraints and to obtain operating data and results for use in final process design.

[0085] Operating data, technical data from pulp and liquor testing, and process modeling were used to characterize process performance before, during, and after the trial. The results presented at least show that downflow operations gave: (1) improved K# and level control; (2) higher, more stable blowline consistency; (3) improved digester washing and energy recovery efficiencies; (4) improved brownstock diffuser washing; and (5) improved oxygen delignification efficiency. The validated process model predicts no effect on viscosity or yield, but predicts further improvements in washing efficiency for a permanent configuration. Independent sources of cost savings were identified and quantified, and these results were used to predict the effectiveness for the permanent conversion project.

[0086] In particular, a two-vessel hydraulic digester that cooks aspen hardwood and white spruce softwood in campaigns was used. Present production rates for hardwood are substantially over design, 1680+ admt/d digester production verses the original design of 1250 admt/d.

[0087] Increased production rates have resulted in decreased overall brownstock washing performance. In particular, as rates increased, the sustainable upflow in the digester's counter-current wash zone has decreased. This has resulted in less digester washing capacity and lower washing efficiency. The efficiency of other washers in the brownstock system has also decreased.

[0088] To remedy the loss of brownstock washing, the mill converted digester operations to Lo-Solids® cooking and has implemented advanced digester control strategies. Both measures have provided improved washing performance and overall digester control is good to excellent. Nevertheless, limitations in overall brownstock washing have reached the point to where they are limiting the extent of delignification and the selectivity of the mill's medium consistency oxygen delignification system. Future plans are to further increase production, and this will make the negative effects of brownstock washing limitations even more pronounced. Finally, even with the present mode of Lo-Solids cooking in place, several digester limitations exist which will prohibit further production rate increases; namely, extraction flow limitations, flash system limitations, and filtrate/blowline cooling limitations.

[0089] Previous results showed that all of the benefits of modified cooking (as well as excellent washing performance) could be obtained for digesters with little or no upflow by utilizing a downflow Lo-Solids cooking configuration. More recent results showed that conversion to a downflow mode of cooking specifically addressed the limitations of the known systems and resulted in improved overall digester operations and performance. The systems are typically severely overloaded two-vessel hydraulic digesters with similar shell and screen configurations.

[0090] FIG. 6 illustrates the presently known “cross-flow” Lo-Solids configuration and FIG. 7 shows two upgrades that permit permanent operation in a downflow mode.

[0091] The basic purpose of the trial was to develop a more rigorous cost-risk-benefit analysis. More specifically, the trial objectives were to:

[0092] Identify and quantitatively predict effect of downflow operations on digester stability;

[0093] Identify and quantitatively predict effect on digester yield, washing efficiency, and energy efficiency;

[0094] Identify and quantitatively predict effect on other mill unit operations (brownstock washing, O2 delignification, evaporators, etc.);

[0095] Identify operating constraints that may influence final process and equipment design; and

[0096] Obtain estimates for capital effectiveness.

[0097] Trial Methodology

[0098] FIG. 8 compares the base case and trial configurations. The trial configuration is a Lo-Solids-type process, wherein spent cooking liquor is extracted during the bulk delignification stage and is replaced with pre-heated white liquor and filtrate.

[0099] Compared to base case operations, the trial mode in accordance with the present invention involved operating the bottom two cook zones of the digester in a downflow mode (i.e., versus upflow). In this particular case, (the trial mode configuration of the present invention), the top most set of screens (i.e., the “Extraction” screens) were not used during the trial and were taken off line. Instead, extractions were performed at the 2nd and 3rd set of digester screens from the top down (i.e., the “Modified” and “Wash” screens, respectively). Primary heating shifted from the bottom circulation and heaters (HR) to the modified circulation and heater (HR). White liquor (WL) addition to the wash circulation was turned off and replaced with filtrate addition at this location. The wash circulation heater was not used during the trial.

[0100] The major differences between the trial configuration, the base case, and the proposed permanent configurations are seen by comparing FIGS. 6 and 7. Table 1 summarizes the relevant differences. The trial configuration resulted in one less cooking circulation (i.e., one less white liquor addition point with heating) than either the base case or the proposed permanent configurations. Also, the trial mode provided for only 2 extraction sites versus the 3 extraction sites in the proposed permanent configurations. As such, the trial mode was operated with only 2 independent cook zones versus the 3 independent cook zones available for either the base case or the proposed permanent downflow configurations.

[0101] Table 1: Comparison of Base Case, Proposed Permanent Downflow, and Trial Configurations. “WL” Refers to White Liquor. 1 Number of WL Number of Number of addition points Extraction Sites Independent to digester from digester Cook Zones Base Case: MCC 2 2 3 Cross-flow Proposed Down- 2 3 3 flow Configurations Trial Down-flow 1 2 2 Configuration

[0102] The practical effects of having one less cook zone for the trial mode than for either the base case or proposed permanent downflow configurations are: the peak cooking temperature would have to be raised, the residual alkali profile would have higher maximum values and larger gradients, and overall digester washing would be compromised. These conditions would result in decreased cooking uniformity, viscosity, and yield—all other things being constant.

[0103] Thus, it was recognized in advance that the trial downflow configuration would give results that were less favorable than the permanent downflow configuration(s) and, in some cases, even less favorable than the present upflow base case. Nevertheless, the trial still met its objectives and provided valuable insights into the anticipated performance of the proposed modifications.

[0104] In order to predict operating results for the permanent modification(s), the base case and trial operating results were used to validate a predictive process model of the digester. The validated model was then used to predict and compare anticipated results for various permanent configurations. Specifically, the trial program consisted of the following elements:

[0105] (1) All relevant and available process data from before, during and after the trial were downloaded from the mill's DCS system; and

[0106] (2) Pulp and liquor samples from before, during, and after the trial were collected and tested.

[0107] Information from items (1) and (2) were used to characterize the base case and trial modes as well as validate the predictive digester model. The validated model was used to predict the outcome for proposed permanent downflow configurations.

[0108] Results

[0109] Two trials were performed. The first trial consisted of a 72-hour campaign. Conversion to downflow provided:

[0110] Improvements in column movement stability, blowline consistency, blowline consistency control, and digester level control;

[0111] An increase in maximum sustainable extraction capacity—from 125 L/s to 165 L/s;

[0112] A decrease in filtrate cooler duty wherein cooling water valve position went from 100% (out of control) to less than 10% in control; and

[0113] Improved blowline temperature control.

[0114] A second trial lasted for 8 days (inclusive of transition times to go on and off downflow mode). It should be noted that during both trials the ability to optimize process conditions was severely limited by (a) the absence of a second white liquor and heating circulation within the digester, (b) modified-circulation heater capacity limitations, and (c) limitations to wash extraction flow imposed by the size of the line and valve. The heater limitations were particularly severe in that they indirectly resulted in limitations to digester extraction capacity and washing efficiency.

[0115] Effect of Downflow Operations on Process Stability and Control

[0116] During the trial, overall digester stability and control were exceptional. In many respects, the noted improvements in stability are difficult to quantify. For example, the mill had an excellent process control system in place and overall process control for the base case was good to excellent. One of the general observations from operating personnel was that the system responded more predictably and more readily to control action and that less control action was required while on downflow mode. While this is clearly a positive effect, it cannot be easily measured. Nevertheless, quantitative results showing improved control were obtained.

[0117] The long-term blowline K# variance for base case operations was 5.4%. This value is from pooled, filtered data over several months and represents excellent K# control—particularly for an overloaded, high-capacity two-vessel digester. The trial K# variance was 3.6%, or 33% lower than the long-term average. This is a very surprising result, particularly since: (a) throughout the trial process conditions were being changed in an effort to optimize the new cooking configuration, (b) the trial configuration had significant limitations on primary heating capacity and this limited K# control action, and (c) the sample space for data was substantially lower for the trial than for the long term result.

[0118] The improved K# control is felt to be the direct result of improvements in both digester level control and digester discharge control. Both level and discharge control, in turn, are felt to be determined by column movement stability. Table 2 summarizes results for improved digester level and discharge control. Variability in blowline consistency is used here as an indication of discharge control. 2 TABLE 2 Improvements in Digester Level Control and Digester Discharge Stability. Outlet Digester Device Level % Variance dP % Variance Long term avg. for 43.1 34 332 11.8 base case Trial avg. for down-flow 43.0 29 420 4.2 % Diff., trial vs. base 0% −15% 27% −27% case Level indication is based on interpreted signals, expressed as % of height between strain gauges. Outlet Device dP refers to pressure drop across the digester bottom in inches H2O and is related to blow consistency.

[0119] The results in Table 2 show that the variance in level control and outlet pressure drop decreased by 15 and 27%, respectively, when comparing downflow trial results to long-term averages for base case operations.

[0120] Another important process control issue involves the effect that downflow operations had on filtrate-cooler duty. The filtrate cooler is capacity limited when a river or water body temperature is higher, and so normal operations regularly require the cooling water valve opening to be 100%. In other words, the filtrate temperature cannot be decreased enough to sufficiently cool the blowline and the blowline temperature is consequently out of control. For base case operations, the blowline temperature will often exceed the upper limit permissible for the downstream atmospheric diffuser. When this happens, the diffuser is by-passed and the entire brownstock washing system is severely upset. On downflow mode, the filtrate-cooler duty was decreased dramatically allowing blowline temperature to be brought into control. FIGS. 9 and 10 illustrate the stepwise changes in blow discharge and in filtrate-cooler duty as a result of transition to and from downflow cooking.

[0121] Effect of Downflow Operations on Process Performance: Heat Recovery and Washing Efficiencies

[0122] The dramatic decrease on filtrate-cooler duty described above is due to enhanced internal heat recovery for the downflow mode. The radial displacement of hot, down flowing fiber and liquor with unheated filtrate at the wash screen elevation is extremely efficient at displacing heat prior to blowing. Since this heat flows through the wash extraction line to the flash tanks, the filtrate need not be cooled excessively in order to lower the blow discharge temperature, and so the filtrate-cooler duty is decreased.

[0123] Besides improving blow temperature control, the other practical effects of this enhanced internal heat recovery are to decrease the amount of warm water generated in the filtrate-cooler and to increase the amount of energy present in the extraction liquor. Since extraction liquor is sent to the flash tanks, a portion of this energy is recovered in the form of low-pressure steam that is subsequently used for pre-heating the incoming chip stream. Thus, warm water generation is displaced by low-pressure steam generation.

[0124] A useful and practical method for quantifying internal heat recovery efficiency is by calculating the thermal displacement ratio (DRT) for the digester's blow dilution zone. As is the case for washing displacement ratios, an ideal displacement of the component of interest (in this case heat) by the incoming displacement medium (in this case wash filtrate) corresponds to a DR value of 1.00. The lower the displacement ratio, the lower the efficiency. For long-term, base case operations the average DRT was determined to be 0.82 with a variance of 4.8%. For the downflow trial according to the present invention, the average DRT was determined to be 0.94 with a variance of 2.1%. The difference between upflow and downflow efficiencies was 0.12 points, or 14.6%, with downflow mode having higher efficiency. This is an extremely large difference for this parameter. The observed ratio of 0.94 for down flow operations is extraordinarily high compared to typical results for upflow systems but typical of the author's observations for downflow systems. The difference between upflow and downflow variability in DRT was 2.7 points or 56%, with downflow mode having lower variability. This decreased variability is another confirmation of improved digester discharge stability for the downflow mode.

[0125] FIG. 11 shows the stepwise change in thermal DR as a result of transition to and from downflow cooking.

[0126] In general, the thermal and wash displacement ratios are related to one another because both will depend on the filtrate-to-chip contacting efficiency. Since thermal DR was shown to increase as a result of downflow operations, it was expected that the digester's washing efficiency would also increase. Furthermore, digester extraction capacity increased from 125 Us to an average of 142 L/s (and peaked at 165 L/s) while on the trial mode according to this invention. The difference of 17 L/s as compared to upflow operations corresponds directly to an increase in washer filtrate uptake—or increased digester dilution factor. The increased dilution factor is another reason why the digester washing efficiency increases for the downflow according to this invention.

[0127] Extensive sampling of blowline squeezate and brownstock filtrates was performed during and long after the trial. For the data reported here, over 19 sets of samples were collected.

[0128] Samples were tested for sodium and COD in order to directly measure washing efficiencies. Table 3 summarizes the results. 3 TABLE 3 Summary of COD profiling Upflow Downflow % Avg. % Var. Avg. % Var. Diff. Blowline 109939 10 84723 13 −23 2 Stage Atm. Diff. 38757 19 30200 12 −22 3A/3B Filtrate 32640 20 30843 12 −5 #5 Squeezate 1649 15 1577 15 −4 5 Brownstock washing stages (#1 to #5) 3 pre-O2 washing stages (#1, #2, #3A&B)

[0129] Converting from upflow to downflow operations resulted in a decrease in blowline solids of between 20 and 25%. For example, the average blowline COD concentration for upflow operations was approximately 110,000 mg/L with a variance of 10%, whereas the average concentration for the downflow trial was approximately 85,000 mg/L with a variance of 12%. The larger variance is typical of results for direct testing of filtrate solids.

[0130] Measured values for global digester wash DR were 0.82 (8.5% variance) for upflow operations and 0.86 (8.8% variance) for downflow operations. The difference of 0.04 points, or 5%, is a large value for this parameter. The variance, or imprecision of data, is compounded when converting from direct concentration measurements to the efficiency parameter. The large variability in DR is typical of wash efficiency studies and speaks to the practical challenges of performing direct measurement washing studies.

[0131] During the trial, the total increase in filtrate uptake was limited to 40 L/s, and averaged 17 L/s, due to limitations in the modified circulation heater and in the flow capacity for the wash extraction line. Neither of these limitations would exist on a permanent re-configuration. In the absence of these limitations, it is anticipated that the total digester extraction would increase to the current evaporator capacity of 160 L/s and that the corresponding increase in filtrate uptake would be 35 L/s. For a permanent configuration, evaporator capacity would be the ultimate limitation to digester extraction capacity and filtrate uptake. Once again, any additional increase in filtrate uptake will result in further increases in digester washing efficiency.

[0132] Effect of Downflow Operations on Other Areas

[0133] As shown in Table 2, downflow cooking resulted in an increase in blow consistency of approximately 25% and a simultaneous decrease in consistency variability. It is commonly known that increased stability and consistency to the inlet of a diffusion washer results in improved operations and efficiency. Table 3 shows that diffuser extraction filtrate also decreased by approximately 20%.

[0134] Testing around the diffuser showed that the unit's wash DR increased from 0.89 (variance of 5.6%) for upflow to greater than 0.96 (variance of 9.9%) for downflow. The difference of 0.07 points, or approximately 8%, represents a large increase for this parameter. As with the digester DR calculations, it is seen here that the variance of data increases substantially when converting from direct concentration measurements to the DR efficiency parameter. For both the digester and diffuser, differences between upflow and downflow filtrate concentrations are statistically significant at a relatively high confidence interval whereas differences in wash DR's are not.

[0135] Thus, both digester and diffuser wash efficiencies increased by relatively large amounts as a result of downflow operations. This conclusion is supported by testing results, but to a limited degree of statistical confidence. This conclusion is also supported by the following observations: (a) thermal DR's increased substantially and significantly, and there is a strong relationship between thermal and wash DR's since both are a function of chip-to-filtrate contacting efficiency; (b) digester global dilution factor increased substantially and it is known that washing efficiency increases with increased dilution factor, and (c) diffuser inlet consistency increased and both inlet consistency and inlet temperature control improved—these conditions are expected to improve diffuser efficiency.

[0136] Perhaps the biggest potential effect on downstream washing, however, is related to improved blow temperature control and the anticipated elimination of severe upsets due to diffuser by-pass on high blow temperatures. FIG. 12 shows the diffuser extraction filtrate conductivity during the trial. Filtrate conductivity is only an indirect measure of washing effectiveness since it will also depend on wash water purity, the chemical charge to the oxygen reactor, and the oxygen reactor's extent of delignification. Nevertheless, it is a good qualitative indicator of system response to overall conditions.

[0137] As shown in FIG. 12, the trial commenced July 7th Between July 7th and 8th, the digester was undergoing transition from upflow to downflow and the extra wash water make-up flow was being gradually eliminated so as to bring the brownstock washing net dilution factor back down to normal levels. By the end of July 8th brownstock conditions were at target set points. Between July 9th and July 11th both the digester and diffuser ran stable. Filtrate conductivity steadily decreased from a peak value of 36 to slightly below 28 and appeared to be trending further downward. Normal values for extended stable operations are between 32 and 34. The minimum value of 28 observed during the trial is exceptionally low.

[0138] The diffuser was diverted for less than 3 hours due to a mechanical (switch) failure. Once brought back on-line, the conductivity was found to have increased from 28 to 34. It took fully another 24 hours of stable operation to return to approximately 31, i.e., 24 hours to reverse ½ of the effect of a 3-hour upset.

[0139] The second to last day of the trial, a process change was made to the digester in an effort to further optimize extraction rates. Unfortunately, this change proved to be a misstep that caused a system hydraulic upset. Blowline temperatures increased and the diffuser was by-passed for approximately 2.5 hours. As a result, diffuser conductivity increase from 31 to 38 and it took fully 2 days of normalized operation to return to the 31 level.

[0140] The results in FIG. 12 shows that extended, stable operations in downflow mode result in low filtrate conductivity values. The relative effects on diffuser filtrate conductivity are not as large as on blowline and diffuser filtrate COD concentrations. This is likely due to other variables influencing filtrate conductivity values (e.g., chemical charge in oxygen reactor and wash water purity). These results also illustrate the dramatic impact of relatively short system upsets. Typical response time to a 2 to 3 hour upset is between 2 and 3 days. Even after 3 days of stable operations no apparent minimum was observed. Hot blowing and simultaneous diffuser by-pass is substantially worse than diffuser by-pass alone: this is because hot blowing corresponds to loss of digester heat recovery and washing efficiencies.

[0141] While the hot blow excursion in the middle of the trial was unfortunate, downflow operations will significantly reduce the frequency of such hot blows due to (a) enhanced discharge stability, (b) enhanced system response to control action, and (c) greater control range on filtrate-cooler discharge temperature. For example, during the excursion of July 12th, the filtrate-cooler discharge temperature did not change quickly enough to avoid high blowline temperatures—even though there was ample cooler capacity for this purpose. The feedback temperature control loop was not properly tuned for the new, more rapidly responding conditions. It is felt that, with more run time and appropriate adjustments to control loop tuning, the operators and control system will be able to avoid future hot blows under similar circumstances.

[0142] FIG. 13 shows the response of the oxygen delignification system during the trial. The long-term % delignification average for upflow operations is approximately 27%. Achieving 30+% delignification at normal production rates is unusual.

[0143] During the trial, the extent of delignification increased steadily up to the point where the atmospheric diffuser was taken off line. The improved performance is related to improved brownstock washing. The system response to improved washing confirms that the oxygen delignification system is carry-over limited: i.e., that upstream washing limits the performance of this delignification stage. The oxygen system has a 2 to 3 day response time, which is consistent with the response (or lag) time exhibited by the brownstock filtrate cycle. As with the brownstock filtrate cycle, it appears as though the maximum potential system response was never achieved during the course of the trial: i.e., it appears as though filtrate conductivity did not achieve its apparent minimum and so % delignification did not achieve its apparent maximum. During the trial, the peak value for extent of delignification was approximately 15% higher than the long-term average for upflow operations. Long-term downflow operations will result in more than a 15% increase in the average extent of delignification.

[0144] It is interesting to fully consider the inter-active effects between brownstock washing efficiency and extent of oxygen delignification. Since delignification is washing limited, any improvement in digester and/or diffuser washing will result in increased % delignification within the oxygen reactor. This increase in extent of reaction results in the formation of more dissolved organic solids within the reactor—most likely up to the point where the system becomes limited again. The auto-limited generation of additional solids results in increased COD concentration for post-oxygen washer filtrate. This filtrate, in turn, is re-circulated backwards to the pre-oxygen washer. Thus for a carry-over limited system such as this, it is expected that improved upstream washing (i.e., decreased carry-over from the digester/diffuser) will result in increased delignification up to the point where limiting concentrations are achieved again. Further, it is expected that filtrate concentrations around the oxygen delignification stage will remain relatively constant and at carryover concentrations that correspond to the limiting threshold for further reaction at the given level for % delignification. The results presented in Table 3 suggests that this is precisely what happened during the trial. Digester squeezate and diffuser filtrate concentrations decreased by more than 20% whereas pre- and post-oxygen filtrate concentrations decreased by less than 5% as a result of the trial. The small differences in pre- and post-oxygen filtrate concentrations have an extremely low level of statistical significance.

[0145] Improved digester and diffuser washing result primarily in increased extent of oxygen delignification. Little or no decreases in COD carry-over into the bleach plant where observed or are expected. Lower COD concentrations in the pre-oxygen area of the brownstock washing system may have secondary benefits: e.g., less defoamer usage. Another potential secondary benefit is related to the fact it is often necessary to add make-up wash water to the brownstock washing system in order to minimize COD carry-over and post oxygen K# upsets. Lower average COD concentrations for the oxygen feed stock due to improved Digester and Diffuser washing and less Diffuser by-passes will reduce the need for periodic wash-water make-up. The long-term effect of this would be to increase average % solids to the evaporators, and to decrease variability in % solids.

[0146] The decrease in filtrate-cooler duty has a significant effect on the mill's warm/hot water balance. The net effect is to decrease the generation of warm water and the mill's overall use of low-pressure steam. Modifications to the digester heat recovery will allow this decrease in low-pressure steam demand to displace medium pressure steam. Thus, the proposed modifications according to this invention will result in decreased overall steam usage. The filtrate cooler uses fresh water. Decreased filtrate-cooler duty therefore results in decreased fresh water uptake and decreased water treatment costs.

[0147] Effect of Downflow Operations on Pulp Yield

[0148] As described above, the trial configuration resulted in process conditions that were expected to have a negative effect on yield and selectivity. Yield audits were performed in order to assess the short and long term impacts of converting to downflow cooking.

[0149] Results from the audits produced the following calibration curve for the aspen hardwood's brownstock yield:

Y=3.75*(log V)/C2+42

[0150] Where Y is brownstock yield (mass of brown pulp per unit mass of bone dry wood feed), V is the brownstock viscosity, and C is the mass fraction of cellulose in pulp. Measured yield values are felt to be precise to within +/−0.25 yield points. FIG. 14 compares measured yield results for upflow and trial downflow operations.

[0151] Based on the data shown in FIG. 14, yield at any given Kappa number decreased by approximately 0.1 points when comparing long-term upflow to trial downflow operations. While this effect is smaller than the limits of precision for the test, it is felt to be a good indication of the trend and magnitude of actual results.

[0152] FIGS. 15 and 16 show the results for cooking selectivity and cooking uniformity. Brownstock viscosity dropped from 76 to 72 cp during the trial. No effect on bleached viscosity was observed. Reject content appeared to increase marginally, but levels in both cases are extremely low and analysis is complicated by the inherent imprecision in this type of testing. It is concluded that yield and selectivity were similar, or perhaps slightly lower, for the trial as compared to typical upflow operations.

[0153] Validated Process Model Predictions

[0154] Testing data from the yield audits and liquor solids profiling, as well as process data from the mill's DCS were used to validate a predictive, steady state process model of the digester. This process model predicts steady-state conditions for alkali profiles, dissolved solids profiles, temperature requirements, and pulping kinetics. These results are used to infer effects of a permanent downflow process on pulping yield, selectivity, and washing efficiency.

[0155] As an example, FIG. 17 compares the effective alkali (EA) profiles for upflow conditions, trial downflow conditions, and permanent downflow conditions. The upflow and trial-downflow profiles were validated by direct testing and measurement, whereas the permanent downflow profile is strictly a model prediction. Note the increase in alkali concentration for trial conditions throughout the digester. This effect, again, is due to the elimination of a cook zone. Permanent downflow configurations eliminate this effect and, in fact, are expected to result in an alkali profile that is marginally better (i.e., “flatter”) than base case upflow conditions.

[0156] The model predicts that both the selectivity (i.e., viscosity at a given Kappa number) and yield are effectively identical for long-term upflow and permanent downflow configurations. The model also predicts that permanent downflow will result in further improvements in digester washing efficiency. While trial conditions resulted in a 25% reduction in blowline solids concentration, the permanent configuration is expected to affect as much as a 35% reduction in this concentration.

[0157] Modification Results

[0158] Based on the results of this trial, the following beneficial results have been identified:

[0159] Displacement of other capital projects (upgrade of cold blow cooler and brownstock washing).

[0160] Lower bleach costs from better O2 delignification.

[0161] Elimination of diffuser bypasses (hot blowing), leading to evaporator off-loading.

[0162] Low and medium pressure steam savings.

[0163] Lower bleach costs from better K# control.

[0164] Fresh water treatment savings due to lower filtrate-cooler duty.

[0165] Less white liquor losses through digester discharge and filtrate by-pass.

[0166] Based on these results, the trial successfully met its objectives: quantitative information has been obtained for the impact of downflow operations on overall digester performance.

[0167] The impact of downflow operations on digester stability, responsiveness, and runnablility at the mill are now known. The extended trial showed that downflow resulted in:

[0168] Improved K# and level control.

[0169] Higher, more stable blowline consistency.

[0170] Improved digester washing and energy recovery efficiencies.

[0171] Improved atmospheric diffuser washing.

[0172] Improved oxygen delignification efficiency.

[0173] The output from a validated digester model predicts that there will be no effect on viscosity or yield, but further improvements still in washing for a permanent configuration.

[0174] This invention has been clearly described in detail, with particular reference to certain preferred embodiments, in order to enable the reader to practice the invention without undue experimentation. Theories of operation have been offered to better enable the reader to understand the invention, but such theories do not limit the scope of the invention. In addition, a person having ordinary skill in the art will readily recognize that many of the previous components and parameters may be varied or modified to a reasonable extent without departing from the scope and spirit of the invention.

[0175] Furthermore, titles, headings, examples or the like are provided to enhance the reader's comprehension of this document, and should not be read as limiting the scope of the present invention. Accordingly, the invention is defined by the following claims, and reasonable extensions and equivalents thereof.

Claims

1. A digester for continuously cooking and digesting cellulosic material and producing cellulosic pulp comprising:

a plurality of jump-stage recirculations located along the length of a continuous digester vessel, wherein each of the jump-stage recirculations moves extracted liquor from a first elevation of the digester vessel and re-introduces the extracted liquor into the vessel at a second higher elevation to maintain an internal liquor downflow substantially throughout the entire digester vessel.

2. The digester of claim 1, wherein the internal liquor downflow does not exceed 400 gallons per ton of b.d. wood feed substantially throughout the entire digester vessel.

3. The digester of claim 1 further comprising a plurality of liquor extraction and addition points along the length of the digester.

4. The digester of claim 3 further comprising a flow-indicator-control system associated with each jump-stage recirculation and liquor extraction and addition points.

5. The digester of claim 1, wherein the digester vessel comprises a plurality of extraction screens along the length of the vessel, each extraction screen having a jump-stage recirculation.

6. A digester for continuously cooking and digesting cellulosic material and producing cellulosic pulp comprising:

a jump-stage recirculation located along the length of a continuous digester vessel, wherein the jump-stage recirculation moves extracted liquor from a first elevation of the digester vessel and re-introduces the liquor into the vessel at a second higher elevation to maintain an internal liquor downflow substantially throughout the entire vessel that does not exceed 400 gallons per ton of b.d. wood feed.

7. The digester of claim 6 further comprising a plurality of liquor extraction and addition points along the length of the digester.

8. The digester of claim 7 further comprising a flow-indicator-control system associated with the jump-stage recirculation and liquor extraction and addition points.

9. A process for continuously cooking and digesting cellulosic material and producing cellulosic pulp in a continuous digester comprising:

(a) maintaining an internal, con-current liquor downflow substantially throughout a continuous digester vessel, wherein the internal liquor downflow does not exceed 400 gallons per ton of b.d. wood feed.

10. The process of claim 9, wherein the continuous digester vessel comprises a plurality of jump-stage recirculations located along the length of the vessel, and each of the jump-stage recirculations moves extracted liquor from a first elevation of the digester vessel and re-introduces the extracted liquor into the vessel at a second higher elevation to maintain the internal liquor downflow substantially throughout the entire digester vessel.

11. The process of claim 9, wherein the continuous digester vessel comprises a plurality of liquor extraction and addition points along the length of the digester.

12. The process of claim 11, wherein the continuous digester vessel further comprises a flow-indicator-control system associated with each jump-stage recirculation and liquor extraction and addition points.

13. The digester of claim 9, wherein the digester vessel comprises a plurality of extraction screens along the length of the vessel, each extraction screen having a jump-stage recirculation.