Background: During the chemical treatment of comminuted cellulosic fibrous material, for example softwood chips, the primary goal is to remove as much of the non- cellulosic material as possible so that relatively pure cellulose fibers, dissociated from the non-cellulosic material, are produced. This non-cellulosic material, for example lignin, which is preferably removed, consists essentially of adhesives that bind the cellulose fibers together and give support or structure to the wood chips, or tree. When these binding agents are removed, the liberated cellulosic material loses its structural integ¬ rity and is released as individual fibers or masses of fibers. These fibers, usually as aqueous slurry, typical¬ ly cannot support a load, and they behave like a viscous liquid rather than a rigid solid.

In most chemical treatments of comminuted cellulosic fibrous material, either in a batch mode or continuous treatment, treatment liquids are typically circulated in and around the material to distribute pulping chemicals and heat. This treatment typically takes place in a cylindrical treatment vessel. After treatment with the pulping chemicals, the liquids containing the used or spent chemicals and products of the reaction are typically removed from the vessel by means of a screen assembly. This screen assembly typically consists of a perforated plate or a parallel bar assembly or any assembly that permits the passing of liquid while retain¬ ing the cellulosic material within the vessel. A preferred structure of such a screen is illustrated in a Finnish patent application 950826 of A. Ahlstrom Corporation. The motive force behind the removal of the liquid my be the

superatmospheric pressure within the vessel or a pump located outside the vessel.

As the pulping reaction progresses and more and more of the non-cellulosic material (and some cellulosic material, for example hemicellulose) is dissolved and removed from the remaining cellulose, the original material, for example wood chips, loses its rigid structure. This change in character has an effect on how well a screen can effectively separate spent liquor from the cellulose. For example, in the early stages of the cook, the chips essentially maintain their original structure as chips and can easily be retained on the screen surface as liquid is drawn through the screen. In later stages of the cook, the rigid structure of the chips may be lost and the soft, malleable cellulosic material may easily pass through the screen assembly, which earlier retained the firmer chips. Typically, conventional cooking equipment, for example continuous digesters, are designed to account for this softening of the chips. For example, the screens used to remove liquids later in the cooking process typically have smaller apertures, that is, holes or spacing between bars, than those used earlier in the cooking process.

However, when recently developed cooking methods are used, such as the continuous EMCC® or Lo-Solids processes sold by Ahlstro Machinery, Inc. of Glens Falls, NY, or the SuperBatch or EnerBatch batch processes, the increased amount of non-cellulosic material removed increases the potential for cellulose fibers to pass through or clog a screen assembly used to remove liquor from the later stages of cooking. One accepted measure of the amount of non-cellulosic lignin present in a cooked cellulosic material is the kappa number . For example, in conventional cooking of softwood chips a typical kappa number ranges from 30 to 35. However, when using the new

methods described above, kappa numbers of 10 to 20 are typical. Even lower kappa numbers can be achieved for hardwoods, which typically contain less lignin and are easier to delignify. As a result, the cellulosic material which is treated using these newer methods can produce much more delignified, or softer, cellulosic material in the later stages of cooking. This softer material is more difficult to screen without passing fibers through the screen or plugging the screen with fibers making them less efficient and making the entire pulping process more difficult to control.

Typically, in a continuous pulping procesε, such as the EMCC and Lo-Solids processes, the final treatment stages in the cooking vessel, that is, the digester, are co-current or counter-current cooking, co-current or counter-current cooling, or co-current or counter-current washing of the pulp mass prior to discharge from the vessel. The motive force behind this movement of liquid is typically a pump which draws liquid through a screen assembly located along the internal wall of the vessel. Typically, special attention has to be given to minimizing the potential for drawing fibrous material through these screens, since, as described above, the delignified material is in a softer, more pliable state. The potential for screen pluggage may make these cooking vessels more difficult to operate than vessels operated using more conventional methods which do not result in as low kappa numbers.

One object of the present invention is to provide a pulping process which does not succumb to these short¬ comings, but is easier to operate and control. In one embodiment this is effected by eliminating screen assem- blies from the later stages of the cooking process, when the kappa number is below 50.

Another very important consideration for a pulp manufacturer is the strength of the paper that is produced from the pulp. Conventional large, high capacity paper machines require that the paper being formed be strong enough to be able to withstand the high speeds and paper tensions at which these machines operate. The strength of a paper product may be highly dependent upon the pulping process used. Specifically, a process that treats the cellulosic material non-uniformly or excessively damages the cellulose fibers and may result in weaker paper. Also, physical stress of cellulose fibers, caused for example by the mechanical action of an agitator, especially when the fibers are in a hot alkaline state, may also result in a reduction in paper strength.

During the discharge of cooked cellulosic material, for example wood chips, the material typically passes from a pressurized state in the digester (i.e., 5-10 bar) to an unpressurized state (i.e., atmospheric pressure to 1-3 bar) . This digester discharging process (known as blowing ) , when performed when the material is in a hot alkaline state, can also inflict damage to the cellulose and produce strength loss. This damage may be exacerbated if the discharge is aided by a rotating mechanical agitator or discharge device. Therefore, in typical conventional pulp mills, the pulp blown from a digester is typically cooled prior to being discharged into a conduit or pipe referred to as the blowline . Cooling is typically effected by introducing cooling liquor, for example cooler wash filtrate, to the bottom of the digester to produce what is known as a cold blow . Cold blowing ensures that minimal strength loss results from blowing from the digester.

In a preferred method of discharging from a batch or continuous digester, the pulp mass is discharged without the aid of a rotating mechanical agitator. For example,

the digester discharge may have a geometry exhibiting single-convergence and side-relief, such as a DIAMONDBACK discharge sold by Ahlstrom Machinery, Inc. of Glens Falls, NY.

Another object of this invention is to provide a method and apparatus for discharging comminuted cellulosic fibrous material from a cooking vessel by cooling the pulp in a two step process: first, cooling the pulp prior to discharge without the aid of screen assemblies, and second, after discharging the pulp from the vessel, using an indirect heat exchanger to cool the pulp at least 10 °C, preferably at least 20 °C. The cooling medium used in the heat exchanger may be a process fluid that is prefer- ably heated before use.

The Invention

One embodiment of this invention consists of a method of pulping comminuted cellulosic fibrous material to a kappa number below 50, preferably 25 - 15, in a cylindri¬ cal vessel in which the number of required screen assem¬ blies is reduced or eliminated entirely.

Another embodiment of this invention consists of cooling, at least 10 °C, the pulp mass discharged at a consistency of 5 - 20, preferably 6 - 16 %, from a digester by means of an indirect-contact heat exchanger. This heat exchanger supplements or replaces the cooling required in the digester vessel. Thus the screen assembly associated with the cooling circulation is preferably no longer necessary. Furthermore, the cooling medium used in the heat exchanger may be a process fluid, for example, kraft white liquor, that can be heated by the hot pulp passing through the heat exchanger.

Though any suitable heat exchanger could be used, it is a fact that there are not that many heat exchangers

designed for medium consistency pulp. In fact, only a few structures have been suggested in prior art and none of them has proven industrially applicable. A new and preferred device which has lately been designed is that disclosed in a co-pending PCT patent application PCT/FI96/00330 (applicant AHLSTROM PUMPS CORPORATION, title "Menetelma ja laitteisto heikosti lampδa johtavan materiaalin kasittelemiseksi", inventors Henricson, Manninen, Peltonen, Pikka, Vesala and Vikman, priority claimed from FI 953064, FI 954407, and FI 954185) filed simultaneously with this application, the disclosure of which together with the disclosures of the priority applications are included herein by reference. A typical feature of this indirect-contact heat exchanger is that it has heat exchange elements with an interior volume for the heat exchange medium (either in liquid, or gas form e.g. steam) . The interior volume is surrounded by the heat exchange surface which is preferably metallic though other materials will also do as long as it has been ensured that the material is able to withstand both the various chemicals used in the process and also the thermal and pressure stresses. On the first side of the heat exchange surface there is the heat exchange (heating or cooling) medium and on the second side there is a pulp at a consistency of 5 to 20 %, preferably 6 to 16 % flowing at an average speed of less than 5 m/s, preferably between 0.1 - 1.0 m/s.

This invention also includes an apparatus for pulping comminuted cellulosic fibrous material in connection with, preferably continuous, digesting, said apparatus including a digester, means for feeding pulp into said digester, means for discharging pulp from said digester, and means for feeding liquid into said digester, said apparatus having discharge means connected to indirect heat exchange means for changing the temperature of the discharged pulp.

Brief Description of Figures

Figure 1 shows a schematical illustration of an existing digester cooling method,

Figure 2 shows a more detailed illustration of the bottom equipment of a conventional continuous digester,

Figure 3 shows schematically a first preferred embodiment according to the present invention

Figure 4 shows schematically a second preferred embodiment according to the present invention, Figure 5 shows a third preferred embodiment according to the present invention,

Figure 6 shows a fourth preferred embodiment accord¬ ing to the present invention,

Figure 7 shows schematically a fifth preferred embodiment according to the invention,

Figure 8 shows a more detailed illustration of a modern fiberline incorporating the invention, and

Figure 9 shows yet another preferred embodiment according to the invention.

Detailed Description of Figures

Figure 1 illustrates a typical prior art design of a continuous digester 10 having several liquid circulations

12 and 14 with which the liquid surrounding the chips is heated or cooled. The arrangement of Fig. l shows a digester 10 with two circulations 12 and 14 and one extraction screen system 16. In the first circulation 12, near the top of the digester 10, the chips are, after the impregnation usually having a temperature of 110 - 130 degrees, heated to a digesting temperature of 150 - 170 degrees. The heating is performed by extracting liquid through a screenplate 122, heating the liquid in liquid heater 124 and recirculating it into the digester. In the cooking zone, the chips are digested, whereby the chips get softer. At the level of the extraction screens 162 the

kappa number of the pulp is 30 - 50 for softwood and 20 - 30 for hardwood. At this kappa number the chips are still physically hard and it is possible to separate liquid without the risk of clogging the screen plates 162. After this, the cooking continues in the washing zone at a temperature of 140 - 170 degrees to the final kappa number which is so low - typically below 30 often below 20 - that the chips become soft and break down during extraction, clogging the extraction screen plates 142. For this reason the wash circulation 14 is difficult to operate. Here the wash liquid having a temperature of 80 - 100 degrees and flowing upwards in the digester 10 is heated in a liquid heater 164 to 140 - 170 degrees. It is known that this circulation does not work well with pulps cooked to kappa numbers below 30, especially below 25. For this reason, various special arrangements have been proposed, for example the use of manhole screens. These are, however, clumsy and unpractical arrangements and solve the problem only partially.

Fig. 2 shows how cool washer filtrate is pumped by means of a so-called cold blow pump 20 both (1) to the bottom scraper 22 to be introduced via the scraper arms 24 or blades into the pulp, (2) to the counter wash nozzles 26, via which the filtrate is sprayed opposite the pulp flow at the sides of the bottom discharge openings 28, and (3) to the digester dilution header 30 from which the filtrate is sprayed into the pulp slightly above the bottom scraper 22. The cool washer filtrate is used both for cooling the pulp and for diluting it to an appropriate discharge consistency. Fig. 2 also shows the wash circulation screens 142 and header 146 which are located just above the digester dilution header 30. The wash circulation includes, in addition to the screens 142 and header 146, a wash circulation pump 148 which draws the liquor to be circulated from the digester 10 through the screens 142 and pumps it to the wash liquor heater 144

(see Fig. 1) from where the liquor is introduced into the central distribution chamber 32 and from there further into the mass through wash circulation discharge openings 34 in the central distribution chamber 32. In the wash circulation the liquid rising counter-currently upwards in the digester 10 is heated to heat the lower part of the digester 10.

One of the purposes of the present invention is to eliminate or reduce the need for the wash circulation 14 without loosing the possibility to heat pulp in the lower part of the digester 10.

Figure 3 shows a typical embodiment of the present invention as applied to the prior art shown in Figure 1. Fig. 3 illustrates a system 40 consisting of a batch or continuous digester 50, having an inlet for comminuted cellulosic fibrous material (not shown) and an outlet 52, which discharges the pulp to a blowline 54. The blowline 54 passes the pulp at a temperature of 120 to 170 °C to a heat exchanger 56 which discharges the pulp at a tempera¬ ture of 130 to 80, preferably 100 to 90 degrees, to a second conduit 58. The second conduit 58, may pass the pulp to further treatment or to one or more additional heat exchangers similar to heat exchanger 56. The cooling medium enters the heat exchanger through a conduit 60 and exits via a conduit 62. In the embodiment shown the cooling medium is washer filtrate at a temperature of 80 to 100 degrees from a downstream washer (not shown) which is heated in heat exchanger 56 to a temperature of 130 to 170 degrees and used as a washing and cooling medium in the bottom of the digester 50. If additional heating is required, such can be performed with an additional heat exchanger arranged in line 62. Preferably the liquid is heated 5 to 50 °C, more preferably 5 to 20 °C with steam or hot extraction liquid from extraction screens before entering the digester bottom. The hot filtrate enters the

digester 50 through one or more conduits 64. Preferably, the conduits and nozzles are positioned so that uniform upflow is achieved in the washing and post-cooking zone. The cooling medium may also be any other process fluid that is preferably heated prior to use, for example kraft white liquor or black liquor, evaporator condensate or simply cold mill water. Cooking chemicals, such as kraft white liquor or caustic, may be added to conduits 64 for treating the pulp in the bottom of the digester 50.

The indirect heat exchanger 56 has a preferably metallic heat exchange surface, preferably made of so called boiler tubes or finned tubes to ensure the pressure resistance thereof. The heat exchange surface is, in accordance with a preferred embodiment, discontinuous, i.e. preferably formed of a number of continuous surface portions. In other words, the substantially even heat exchange surface is broken at certain intervals by means of ribs arranged against the flow direction on said surface, or by means of changing the shape of the crosssection of the flow channel or by some other means. In any case, said interval, i.e. the length of a continu¬ ous heat exchange surface in the flow direction of the pulp, is less than 2 meters, preferably less than 1 meter and more preferably between 100 and 700 mm's. The size of the heat transfer surface depends on the application but is at least 10 m ² , preferably at least 30 m ² , and even more preferably 50 - 300 m ² .

A heat exchanger described in a co-pending PCT patent application PCT/FI96/NNNNN (applicant AHLSTROM PUMPS CORPORATION, title "Menetelma ja laitteisto heikosti lampδa johtavan materiaalin kasittelemiseksi", inventors Henricson, Manninen, Peltonen, Pikka, Vesala and Vikman, priority claimed from FI 953064, FI 954407, and FI 954185) , filed simultaneously with this application, can be mentioned as a practical example of a heat exchanger

which may be used in the blow line of a digester. A typical feature of said heat exchanger is that it has a metallic heat exchange surface. On one side of this surface, there is the pulp suspension having a kappa number of below 50 and flowing with an average flow speed below 5 m/s. The consistency of the pulp is 5 - 20, preferably 6 - 16 %. On the other side of the heat exchange surface, there is the cooling liquid.

There are various ways to ensure that there is a sufficiently large area of heat exchange surface in the digester blow line. If the requirement for cooling the pulp or the requirement for heating the liquid cannot be met with one heat exchanger arranged in the discharge outlet of a digester, Fig. 4 illustrates a preferred embodiment with a digester 50 having, in this embodiment, four outlets 52, each having a heat exchanger 56' having a diameter of 0.2 to 2 meters and a length of about 1 to 6 meters. After the pulp has been cooled by means of the heat exchangers 56' the four flows are united into one flow by using for instance the apparatus discussed in US patent 4,964,950 of A. Ahlstrom Corporation. In addition to the advantage brought by the smaller size of the heat exchangers 56 another advantage may also be mentioned. Due to the larger open area in the digester bottom, a need for a bottom scraper is smaller. The bottom scraper may be rotated more slowly or the scraper may in some cases be omitted entirely. The discharge flow from the digester 50 may be controlled by the valves arranged preferably downstream of the indirect heat exchangers.

Another preferred embodiment of a heat exchanger in accordance with the present invention is where a heat exchanger, or part of it, has been installed inside the digester. The heat exchange surfaces may be arranged above and/or below the bottom scraper. In accordance with this embodiment the heat exchanger is in the form of preferably

circular elements fastened both to each other and to the digester wall.

Fig. 5 shows yet another preferred embodiment of the invention. The heat exchanger shown in Fig. 5 is used in connection with an arrangement including a counter-current flow to which alkali is added. The liquid introduced into the digester 50 is a mixture of cooking liquor (white liquor, WL) and wash black liquor (WBL) . The liquid is introduced into the bottom scraper 70 on the upper side of which there are nozzles 72 for distributing the counter¬ current flow into the digester 50. Said liquid is heated, while the liquid introduced via nozzles 74 to the bottom of the digester 50 is not heated. In this way, the load on the blow line heat exchanger 56 is reduced. Since the liquid introduced to the bottom of the digester 50 is cooler, the need for cooling the pulp in the heat exchanger/s 56 is smaller.

In connection with the above embodiments it is not always necessary to eliminate the wash circulation. The heat exchanger may also be used merely for lowering the temperature in the blowline as shown in Fig. 6. In Fig. 6 the pulp may be cooled in the heat exchanger 56 only 10 to 50 °C, typically 10 to 30 °C to reach a temperature of 110 - 80 °C. The pulp entering the heat exchanger has typical¬ ly a temperature of 120 to 170 °C.

Figure 7 illustrates a typical embodiment of the present invention combined with a hot alkali extraction treatment. In this system 80, cooked cellulose pulp is discharged from a digester 50 and fed to a first heat exchanger 56 as shown in Figure 3. After being cooled/heated in heat exchanger 56 the pulp passes to a hot alkali treatment stage 82. ?he treatment typically includes the addition of caustic to the pulp and a retention time of 10 to 150 minutes at a temperature

between 100 and 180°C, preferably between 120 and 170 °C. Note that prior to a hot alkali stage 82, heat exchanger 56 may heat the pulp stream instead of cooling it so that optimum conditions exist in stage 82. In other words, the heat exchanger is used for adjusting the temperature of the pulp to the level of the following treatment.

After passing through stage 82 the pulp may be passed into conduit 84 to be introduced into a second heat exchanger 86 where the pulp is cooled and then passed to washing stage 88 or other subsequent treatment. The pulp may also be passed directly to washing 88 after treatment 82 without passing to a second heat exchanger 86. Of course, any of these flow streams may be divided so that part of the stream, for example stream in conduit 84, passes first to treatment 86 and then to treatment 88 and part of stream in conduit 84 is separated and passes directly to washing 88. After washing 88 the pulp may pass to further treatment such as another washing, or another hot alkali extraction treatment.

The hot alkali extraction treatment may also be replaced by a treatment in which the pulp is discharged from the digester and transferred, preferably via a heat exchanger, into a vessel where the cooking is continued for at least 10 minutes, preferably 15 to 100 minutes, in a fiber phase at a substantially high temperature, however below 190 °C, preferably between 140 to 180 °C, whereby the treatment liquor contains effective alkali l to 40 g/l, preferably 5 to 40 g/l, more preferably 15 to 35 g/l expressed as NaOH.

Figure 8 illustrates a typical modern continuous pulping fiberline, incorporating the present invention. First, comminuted cellulosic fibrous material, for example softwood chips, are pretreated in a vessel or bin 90.

This pretreatment typically lasts from 5 to 120 minutes,

preferably 5 to 15 minutes, and consists of exposing the chips to fresh or contaminated steam in order to initiate the heating process, begin the impregnation of the chips with liquid and dispel undesirable air from the chips. (The removal of air not only makes the chips more per¬ meable to cooking chemicals but also reduces their buoyancy so that they tend to sink during subsequent liquid treatments.) This pretreatment may also include treatment with yield or pulp strength enhancing additives such as sulfide-containing compounds, for example hydrogen sulfide gas or polysulfide liquor, or athraquinone and derivatives thereof.

The pretreated chips are then discharged from vessel 90, at a temperature between 70 and 110°C, preferably between 80 and 100°C, and fed to another pretreatment vessel 92. Though any conventional discharge may be used, preferably the chips are discharged from vessel 90 without the aid of any mechanical agitation or vibration. For example, the vessel discharge from vessel 90 preferably exhibits single-convergence and side-relief so that the pretreated chips flow unencumbered from the vessel 90. One such discharge is available from Ahlstrom Machinery, Inc. of Glens Falls, NY and will be sold under the trademark DIAMONDBACK. This device is disclosed in co¬ pending US patent applications 08/189,546 filed on Feb. 1, 1994 and 08/336,581 filed on December 30, 1994, the disclosures of which are incorporated by reference herein.

Though any conventional feeding devices may be used to feed the chips to vessel 92, such as a High Pressure Feeder sold by Ahlstrom Machinery, Inc. , the pretreated chips are preferably fed to the digester by a slurry pump system 94. These systems are disclosed in co-pending US patent applications 08/267,171 filed on June 16, 1994, 08/354,005 filed on December 5, 1994 and 08/428,302 filed

on April 25, 1995. The disclosures of these applications are included in this application by reference herein. These feeding systems are marketed in the U.S.A. under the trademark LO-LEVEL by Ahlstrom Machinery, Inc..

Prior to, during, or after the feeding of the chips into vessel 92 via conduit 96 treatment liquors may be added to the chips. These liquors may include kraft white, green, or black liquor or sulfite liquor. The added liquor may contain yield or strength enhancing additives as discussed above or a treatment to minimize metal ion concentration. These liquors are typically added via conduits 98 or 100.

At the top of pretreatment vessel 92, for example an Impregnation Vessel sold in the U.S.A. by Ahlstrom Machinery, Inc., the chips are at a temperature between 70 and 110°C and at a pressure of 1 to 20 bar, preferably 1 to 5 bar absolute pressure. The introduction of the slurry to vessel 92 is typically aided by the use of a conventional Top Separator, as sold by Ahlstrom Machinery Inc., but any other conventional device may be used. When a Top Separator is used, typically some of the liquor used to transfer the slurry through conduit 96 is removed and returned via a separate conduit (not shown) to the transfer device 94. This return flow may also be heated or cooled to maintain the desired temperature at the top of the vessel 92.

The chips are typically treated in the vessel 92 for 10 minutes to 4 hours, preferably 0.5 to 1.5 hours. In vessel 92 the chips may typically be treated with black liquor extracted from the formal cooking procesε. This black liquor, designated BL2 in Figure 8, is passed through conduit 102, after having been removed from digester 50 via screen assembly 104, and introduced to conduit 96 prior to vesεel 92. Black liqur BL2 haε

preferably an effective alkali concentration of 10-50 g/l expresεed as NaOH, preferably 10 to 30 g/l and a sodium sulfide (Na ₂ S) concentration of at least about 10 g/l expressed as Na ₂ S.

The treatment temperature in the upper part of vessel 92, that is, above screens 106, should be kept below 120°C, preferably between 80 and 110°C. This long cold impregnation treatment is disclosed in co-pending US patent application 08/460,723 filed on June 2, 1995, the disclosure of which is included by reference herein. This treatment may last from 10 minutes to 4 hours, but preferably lasts from 0.5 to 1.5 hours. The chemicals added to this treatment zone, for example the black liquor added via conduit 102 or the white liquor added via conduit 98, may be cooled if necessary by passing them through heat exchangers or flashing to maintain the desired temperature in vessel 92.

After this treatment with black liquor BL2, and white liquor, in the upper section of vesεel 92, the spent cooking chemicals and reaction products, for example, dissolved lignin, cellulose and hemicelluloseε are removed, or extracted , from the veεεel via εcreen aεsembly 106. This liquor, referred to as BLl, iε typically low in alkalinity (i.e. 5 - 15 g/l effective alkali as NaOH) . BLl may be directed to the chemical/heat recovery syεtem, for example the flaεh tankε, a heat exchanger, or evaporator, or it may be used to pretreat the chips in veεεel 90, for example. One preferred method of recovering heat via a heat exchanger iε diεclosed in co-pending US patent application 08/420,730 filed on April 10, 1995.

In the lower part of vesεel 92, that is in zone 108, the down-flowing, pretreated chips are with white liquor in a co-current or counter-current treatment. For

example, white or green liquor may be added via one or more conduits 110 and be drawn counter-currently through zone 108 to be removed via screenε 106. The liquor added via conduitε 110 may contain yield or strength enhancing additives or chemical complexing agents as described above. The temperature in zone 108 is typically between 90 and 150°C, and is preferably below 140°C. The time in zone 108 is 5 to 60 minutes, preferably 10 to 30 minutes. The slurry is then discharged from vessel 92 by any conventional discharge means, for example with the aid of an outlet device, but is preferably performed without the aid of mechanical agitation. For example, the discharge may be effected using an outlet geometry that permits free-flowing discharge without the aid of mechanical agitation such as a geometry exhibiting single-convergence and side relief. Such an outlet is deεcribed in co¬ pending US patent applicationε 08/410,503 filed on March 10, 1995 and 08/529,411 filed on Sept. 18, 1995.

After discharge from vessel 92, the slurry is preεsurized and heated in preparation for formal cooking in vessel 50. The presεurization and tranεfer to veεsel 50 is effected by a conventional High Pressure Feeder 112, as sold in the U.S.A. by Ahlstrom Machinery, Inc., or by the pumping syεtem deεcribed in previously referenced co¬ pending US patent applications 08/267,171, 08/354,005 and 08/428,302. Prior to, during or after the presεurization by tranεfer device 112 additional treatment chemicalε may be added to the slurry to aid in transfer through conduit 114 and to begin the formal cooking procesε. These chemicals may be white liquor added via conduit 116 or black liquor, BL3, added through conduit 118.

At the top of vessel 50 the slurry may be introduced to the vessel by means of a conventional Top Separator or stilling well aεεembly in which εome of the tranεfer liquid may be drawn off and paεεed back to the transfer

device 112 to aid in the tranεfer to vessel 50. Aε before, the liquid returned through this conduit (not shown) may be heated or cooled as desired. Treatment chemicals may also be added to this return conduit in lieu of adding them to conduit 114.

In the upper zone of vesεel 50, that is, above screen assembly 104, the slurry temperature iε typically between 145° and 180°C and iε preferably between 150° and 170°C. The preεεure at the top of the veεεel typically rangeε between 5 to 12 bar, and is preferably between 7 and 10 bar, absolute. White liquor or similar may be added to cooking zone 130 to control alkalinity during cooking by adding a circulation anywhere in zone 130. The slurry iε cooked at thiε temperature and preεεure for between 0.5 to 4 hourε, preferably between 1 to 2 hours. After this initial cooking stage, spent cooking chemicalε and diεεolved reaction productε are removed from the veεsel by εcreens 104. This black liquor, i.e., BL2, is preferably used to treat the chips in vesεel 92 by for example introducing it via conduit 102 to conduit 96. As dis- cuεεed above, BL2 iε typically high in alkalinity.

After paεεing εcreens 104, the εlurry iε further treated between εcreenε 104 and 132. Thiε treatment may be co-current or counter-current depending upon the liquor volumeε extracted via screens 104 and 132. The treatment time between these screens may be from 1 to 60 minutes and is preferably 5 to 30 minutes long.

The liquor removed via screens 132, referred to as BL3, is lower in alkalinity than BL2. BL3 typically has an effective alkali concentration of leεs than 20 g/l as NaOH, typically lesε than 10 g/l. BL3 is lower in alkalinity because 1) alkali was consumed during the treatment between screens 104 and 132 when the kappa number decreased from 1 to 30 units, typically 1 to 15

units; and 2) because typically BL3 has been diluted by weak black liquor (WBL) from conduit 134 that is preferab¬ ly introduced to the bottom of vessel 50.

In the bottom section of the vesεel 50, that is, zone 136, the cooked pulp is treated with WBL from a downstream treatment, typically a waεhing εtage. This liquor may be relatively cool in temperature, that is, cooler than 120°C, but is preferably hot enough that the temperature in zone 136 is between 100 and 170C, preferably 130 to 160°C. White liquor via conduit 138 may also be added to zone 136 so that a co-current or counter-current cooking occurs. The retention time in zone 136 may be between 1 and 280 minutes, and is preferably between 10 and 90 minutes. The kappa number of the pulp may drop in this zone from 1 to 30 units, typically from 1 to 15 units.

After treatment in zone 136 the slurry iε diεcharged from veεεel 50 to conduit 54. This discharge may be effected using a mechanical agitator but preferably does not include any mechanical agitation as described above for the discharge from vesεel 92. Conduit 54 pasεeε the treated εlurry to heat exchanger 56 in which the temperate of the εlurry iε reduced to between 90 and 150°C, prefer- ably to between 120 to 150°C. The cooling medium may be any available proceεε εtream that needs to be heated, for example white, black liquor or even a medium used in a downstream bleach plant, but it is preferably washer filtrate WBL from a downstream washing εtage. This filtrate may be the same as that introduced to the bottom of vessel 50 via conduit 134. One preferred heat exchanger is that disclosed in a co-pending PCT patent application PCT/FI96/NNNNN (applicant AHLSTROM PUMPS CORPORATION, title "Menetelma ja laitteisto heikoεti lampδa johtavan materiaalin kaεittelemiεekεi", inventorε Henricεon, Manninen, Peltonen, Pikka, Veεala and Vikman,

priority claimed from FI 953064, FI 954407, and FI 954185) , filed simultaneouεly with thiε application.

After passing through heat exchanger 56, the pulp may be pasεed through conduit 150 to veεsel 152 to be optionally treated in a hot alkali extraction stage. Alkali, for example white liquor from conduit 154, may be added to any appropriate conduit, for example conduit 150, prior to said treatment. The hot alkali treatment vessel 152 is dimensioned for a retention time of between 10 and 150 minutes. The temperature in the treatment iε between 100 and 180°C and is preferably between 120 and 170°C. A typical kappa reduction in vessel 152 is between 5 and 20 unitε. After treatment in vessel 152, the pulp may pasε via conduit 156 to an optional εecond heat exchanger 158. The pulp may then be passed to further treatment, for example washing, further delignification or bleaching, or to storage.

If desired the pulp temperature can also be increased by means of heat exchanger 158. For example, εteam can be uεed to heat the pulp. In this way the heat exchanger 158 can act as a condenser to produce a source of clean water. If the temperature desired in the hot alkali stage in vessel 152 is attainable without the use of a heating or cooling heat exchanger, the heat exchanger 158 may be omitted.

When the hot alkali treatment in vessel 152 is not used, one or more heat exchangers may be used to cool the pulp to a temperature between 60 and 140°C, preferably between 80 and 120°C, usually between 90 and 100°C. If the downεtream treatment performed at εuperatmospheric pressure, for example a pressurized wash stage, then cooling to a temperature above 100°C is feasible.

Figure 9 illuεtrateε another embodiment of the invention. In this system the loweεt digeεter εcreen (εcreen 142 in Fig. 1) has been omitted and a washing device 88' has been introduced downstream of the heat exchanger. This washing device is preferably a fractionating washer which allows two or more streams of filtrate of varying cleanlinesε to be removed. One εuch washer is the Drum Displacer® washer marketed by Ahlεtrom Machinery. In the example εhown, two εtreamε of filtrate are removed from waεher 88: one of higher alkali and εolidε content which can be uεed for liquor BLl and a εecond of lower alkali and εolidε content which can be used as liquor WBL is used. By using such an external cooling heat exchanger 56 and washer 88, no internal washing or cooling needs to be done in the digester 50 and the lowest screen (εcreen 142 in Fig. 1) may be eliminated. Some WBL may be directed to the bottom of the digeεter 50 via conduit 64 if deεired.

Furthermore screen 162 in Figure 9 may also be eliminated εo that digester 50 has no screens at all. When no screenε are located in veεεel 50, the fractionating waεher 88 can become a means for separating a stronger liquor, e.g., BL2, from a weaker liquor, e.g., BL3.

The digester and heat exchanger syεtem shown in Figures 7, 8 and 9 illuεtrate a εyεtem for effectively cooking comminuted celluloεic fibrouε material to kappa numberε below 50, preferably 25 - 15, and εtill maintain efficient operation, that is good runnability . With such a syεtem, softwood chips, for example, can be cooked to low kappa numbers without causing screen pluggage or fiber damage. For example, the kappa numbers at the screenε in pretreatment veεεel 92 and in digeεter 50, in which large flow rates are required, are typically above 25 and do not impose an operational problem.

Specifically, at screen 106 the kappa number is typically above 60, even above 90; at screen 104 the kappa is typically above 30. The only screen that is exposed to pulp having a kappa number below 25 is screen 132. However, by employing an external heat exchanger, the temperature of the pulp leaving digeεter 50 does not have to be as low as is conventional. Therefore, the flow rate through zone 136 and through screen 132 is lower than conventionally. The flow required through εcreen 132 iε only 2 to 5 m ³ per ton of pulp. In conventional systems, this flow iε typically above 5 m ³ per ton of pulp. By employing conventional profile-bar screens or screens having large areas or inclined barε, aε disclosed in Finnish application 950826, this system can produce low kappa pulp while providing ease of operation and reduced maintenance.