COST EFFICIENT KRAFT COOKING METHOD USING POLYSULFIDE COOKING LIQUOR

Abstract

The invention is related to a method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor. In order to obtain a cost efficient system both in aspects of investment costs but also in aspects of heat economy of operating the process is most of the total charge of alkali charged as heated polysulfide liquor to an first atmospheric vessel, wherein the hot polysulfide liquor flashes off steam providing most if not all of the necessary steaming effect for the cellulose material. The polysulfide liquor is then allowed to impregnate the cellulose material at a temperature closer to cooking temperature but still so low that essentially no delignification occurs in impregnation vessel, as the H-factor in impregnation vessel is kept within 1-20.

Claims

1. A method for the preparation of kraft pulp with increased pulping yield from lignin-containing cellulosic material using polysulfide cooking liquor, comprising: feeding the lignin containing cellulosic material which has not been previously steamed to the top of a first vessel operating at an applied pressure in the top of the first vessel of at most 0.2 bar, and establishing an upper level of lignin containing cellulosic material in the first vessel; determining a total charge of alkaline cooking liquor in the form of polysulfide liquor for the first vessel; heating the polysulfide liquor to a temperature above its boiling point, allowing water to evaporate off from the polysulfide liquor; charging at least 80% of the total charge of the alkaline cooking liquor, in the form of the polysulfide liquor, to the first vessel and establishing a lower level of liquor below said upper level, thus keeping steam and the lignin containing cellulosic material in a volume above the lower level of liquor; keeping the suspended lignin containing cellulosic material in the first vessel for a sufficient time to reach an H-factor of at least 1; feeding the suspended lignin containing material from the bottom of the first vessel to the top of a second vessel and cooling the lignin containing cellulosic material at full cooking temperature in the range of from 130- to 160° C. to a final kappa number below 40, while adding any of the remaining charge of the alkaline cooking liquor, preferably in form of polysulfide liquor, during feeding to or cooking in the second vessel.

2. The method according to claim 1, including adding additional steam to the volume of the lignin containing cellulosic material kept above the lower level of liquor.

3. The method according to claim 2, including withdrawing a part of the liquor volume in the first vessel from the wall of the first vessel and circulating said part of the liquor volume back to the volume of the lignin containing cellulosic material in a first circulation.

4. The method according to claim 3, including heating the first circulation from a heat source.

5. The method according to claim 4, including adding the polysulfide liquor to the first circulation.

6. The method according to claim 3, including heating the polysulfide liquor added to the first vessel from a heat source.

7. The method according to claim 6, including adding the heated polysulfide liquor to the first circulation.

8. The method according to any of claims 4, wherein the heat source is hot spent cooking liquor withdrawn from the second vessel.

9. The method according to any of claims 4, wherein the heat source is steam.

10. The method according to claim 1, wherein the liquor in the first vessel has an alkali concentration above 60 g/l and a polysulfide concentration above 3 g/l, when charging the polysulfide cooking liquor, establishing a liquor-to-wood ratio in the range 2.0 to 3.2 in said first vessel.

11. The method according to claim 1, including keeping the suspended liquor containing cellulosic material in the first vessel for a sufficient time to reach an H-factor between 1 and 20.

12. The method according to claim 9, wherein the steam is from the low pressure steam out of a pulp mill.

13. The method according to claim 10, wherein the liquor in the first vessel has a polysulfide concentration above 0.09 mol/l.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a schematic cooking system capable of implementing the inventive method.

DETAILED DESCRIPTION OF THE INVENTION

[0036] In FIG. 1 is shown a 2-vessel kraft cooking system, having a first atmospheric impregnation vessel A and a second steam/liquid phase digester B, wherein the inventive method could be implemented. The function of the system is described in following parts.

[0037] Feeding.

[0038] In this type of system is first the lignin containing cellulosic material, Chips, fed with a conveyor belt CB to the top of the atmospheric impregnation vessel A and sluiced into the top using a conventional sluice feeder SF. A first upper level of chips, LE.sub.1, is established in the vessel. Simultaneously is impregnation liquid added to the vessel establishing a second lower level of liquid, LE.sub.2. In this process is the new treatment liquid added as polysulfide liquor, identified as Orange Liquor in drawing, and between 80-100% of the total charge of alkali to the entire cooking process is charged in this position. In the embodiment shown in FIG. 1 is the polysulfide liquor added to a circulation established in the vessel A, comprising a withdrawal screen SC, in the vessel wall, piping and pumps leading the withdrawn treatment liquor back to center of vessel using a central pipe CP. The new polysulfide liquor could thus be distributed to the entire cross section of the vessel while being subjected to the circulation flow.

[0039] The second lower level of liquid LE.sub.2 is established some 5-15 meter below the upper level of chips LE.sub.1, and thus provides for a volume of cellulose material above the liquid level. This dense packed volume of cellulose material provides for a dead weight that drives a plug of cellulose material down and into the pool of liquor contained in the bottom of the vessel. The dense plug of cellulose material also provides for a condensation volume cooling and finally condensing any steam that may evaporate upwardly against the wood material that have been fed to top of vessel and is kept at lower temperature, preferably at ambient temperature.

[0040] Steaming

[0041] The cellulose material must be steamed in order to drive out bound air and enable a thorough impregnation. The air must be expelled to such an extent that the cellulose material loses its buoyancy, as well as enablement of impregnation to such an extent that the entire cellulose volume may be fully cooked and reduce the amount of rejects after the cook. No steaming process in practice is capable of expelling 100% of all air bound in the cellulose material, but most system drive out air to such an extent that the wood material loses its buoyancy as well as keeping the amount of rejects at acceptable levels. Wth the experience from ImpBin concepts it has been proven that the steaming concept used in ImpBin works to such an extent that even large chunks of cellulose material becomes fully impregnated and that the reject volumes in some cases are close to zero. In some implementations of ImpBin system was installation of huge reject bins recommended to mill operators by 3.sup.rd party consultants, but after some weeks of operations it was discovered that not even a toothpick sized reject volume was sent to the reject bin, which proves the perfect impregnation effect from using ImpBin in that installation. This should be compared with some perceptions in the pulping industry in the late 1980-ies that the cellulose material required extensive steaming effects in dedicated apparatuses, first steaming in a chip bin, and then also steaming in a separate steaming vessel at slightly higher pressure before suspending the steamed chips in liquor, which was the standard set up in conventional cooking until the late 1990-ties.

[0042] In the system disclosed is the major part steaming effect, or in some cases the entire steaming effect, obtained by addition of hot liquors having a temperature above 100° C., in this case hot liquors containing the polysulfide liquor, in center of vessel A, and due to the fact that the vessel is atmospheric is steam flashed off into the volume of cellulose material. The steam is released from the outlet end of the central pipe CP located in the lower end of the volume of cellulose material located above the second liquid level LE.sub.2. In some cases could several central pipes be used to distribute the steam and the polysulfide liquor more evenly over the cross section, using the multipipe system as disclosed in EP2467533.

[0043] As disclosed is the liquors added to the vessel heated preferably using heat exchangers HE.sub.1 and HE.sub.2. Direct injection of steam may be used, but has the disadvantage that the polysulfide concentration decreases due to the dilution effect of steam condensate. Also, clean steam condensate is expensive to replace if lost, as even ordinary tap water needs thorough and expensive cleaning before use in the steam cycle, so preferably is the clean steam condensate from indirect heat exchangers sent back to the steam cycle.

[0044] A first heat exchanger HE, may be included in the circulation disclosed, and a second heat exchanger HE.sub.2 may be included in supply pipe of the polysulfide liquor, and at least one of these heat exchanger systems are included if not both depending upon need for heating and the starting temperature of the polysulfide liquor.

[0045] In the most preferred embodiment and as disclosed in FIG. 1 is the first heat exchanger HE, using the heat value of the hot spent cooking liquor withdrawn from digester. The spent cooking liquor typically holds full cooking temperature, i.e. 130-160° C. at withdrawal, said temperatures obtained after using live steam from the medium pressure steam net of the mill. This high heat value is preferably used to heat the polysulfide liquor that conventionally is made on site of the mill and is stored in atmospheric tanks holding a temperature of about 70-80° C. Thus the polysulfide liquor may thus be heated easily to a temperature of about 110-130° C. before addition to the system using heat exchangers.

[0046] In the most preferred embodiment and as disclosed in FIG. 1 is the second heat exchanger HE.sub.2 system using the heat value of low pressure steam using live steam from the low pressure steam net of the mill. The low pressure steam is most often available at abundance at the mill, in contrast to medium pressure steam, but is most suitable for heating purposes in the range 100-130° C. The heating obtained in the circulation by the second heat exchanger, preferably in combination with the heating of the polysulfide liquor, is most often sufficient for effective steaming of the cellulose material in warm climate where chips holds an ambient temperature of about 20-30° C. or even higher.

[0047] In particular demanding applications, for example in cold climate with ambient temperatures well below 0° C. and corresponding temperature of the cellulose material, additional steam may be supplied directly to the vessel A as disclosed, using low pressure steam using live steam from the low pressure steam net of the mill. This steam may be supplied in a distribution chamber in the wall of the digester located above the second liquid level LE.sub.2, and preferably implemented as disclosed in EP2591165 previously used for black liquor impregnation in ImpBin and first implemented in cold climate mills.

[0048] With these alternatives for steaming no risk for emission of malodorous sulfur compounds may be experienced, as all liquors added contains no black liquor. The steaming concept may thus be optionally changed from the cold top control previously used in black liquor impregnation using ImpBin. If instead hot top control is implemented, allowing steam to blow through the entire cellulose volume located above the second liquid level LE.sub.2, then the vented gases from the vessel may be sent to turpentine recovery, obtaining turpentine with less sulfur content.

[0049] The spent cooking liquor typically holds full cooking temperature, i.e. 130-160° C. at withdrawal, said temperatures obtained after This high heat value is preferably used to heat the polysulfide liquor that conventionally is made on site of the mill and is stored in atmospheric tanks holding a temperature of about 70-80° C.

[0050] A second heat exchanger system HE.sub.2 may be included in the circulation disclosed, and a second heat exchanger system HE.sub.2 may be included in supply pipe of the polysulfide liquor, and at least one of these heat exchanger systems are included if not both depending upon need for heating and the starting temperature of the polysulfide liquor. In the most preferred embodiment and as disclosed in FIG. 1 is the second heat exchanger system using the heat value of the hot spent cooking liquor withdrawn from digester. The spent cooking liquor typically holds full cooking temperature, i.e. 130-160° C. at withdrawal, said temperatures obtained after using live steam from the medium pressure steam net of the mill. This high heat value is preferably used to heat the polysulfide liquor that conventionally is made on site of the mill and is stored in atmospheric tanks holding a temperature of about 70-80° C.

[0051] Each heat exchanger may comprise a number of heat exchangers arranged in a system, not shown, using the hotter heating media in countercurrent mode such that the residual heat value in the heating media heats the coldest flow in a first heat exchanger, and the original heat value heats a flow that has passed at least on preceding heat exchanger in a second heat exchanger.

[0052] Feed from Impregnation to Cooking Vessel

[0053] Thus, the first impregnation stage in vessel is implemented in the vessel B and preferably only charged with the polysulfide cooking liquor and as small amount as possible of additional liquids such as wood moisture, steam condensates, and especially no black liquor nor additional water or filtrates. The resulting liquor-to-wood ratio established should be in the range 2.0 to 3.2 and the temperature should be in the range 100-120° C.

[0054] After the sufficient retention time in vessel A, which should have a retention time resulting in an H-factor in the range 1-20 of the impregnation stage, the impregnated cellulose material will be fed to the steam/liquid phase digester B together with the residual treatment liquor. In FIG. 1 is disclosed a transfer system with parallel centrifugal pumps, corresponding to what is disclosed in EP2268862 and/or EP2268861, but conventional sluice feeders may also be used. As disclosed could optionally additional air be supplied to top of digester, in form of pressurized air CA that could raise the pressure in digester top without excessive heating if higher pressure in top is sought for and using lower cooking temperatures. However, it should be realized that the invention may equally well be implemented with a hydraulic digester, i.e. a digester without a steam phase in top and completely filled with cooking liquor. Due to the low H-factor in impregnation the residual treatment liquor contains most of the original charge of alkali as virtually nothing has been consumed for delignification. Here is shown a conventional transfer system with dilution in bottom of the vessel B using withdrawn treatment liquor from the top separator TS in the top of vessel B sent via return line TRRET. Also, a part of the hot spent cooking liquor withdrawn from a screen SC2 is added to the return line in order to raise the temperature ahead of cooking in vessel B. At the top of the digester vessel B is the cellulose material heated to full cooking temperature, in the range 130-160° C. depending upon type of cellulosic material. The heating to full digester temperature is conventionally done by adding medium pressure steam from the MP steam net of the mill. Additional liquid is added in order to reduce the alkali concentration at this point, which in this embodiment is a part of the withdrawn spent cooking liquors, withdrawn from screens SC.sub.2 and SC.sub.3. Most of the withdrawn spent liquor from screens SC.sub.2 and SC.sub.3 is sent to recovery REC, but the heat value is used first in heat exchanger HE.sub.1 as disclosed, and then preferably is finally flashed in a flash tank FT to ambient pressure. The steam flashed off ST.sub.s is preferably sent to LVHC (Low Volume High Concentration) or HVLC (High Volume Low Concentration) systems, the latter after diluting the gases, for disposal and preferably combustion of malodorous gases. As also disclosed is the flashed spent cooking liquor first sent to a knotter, and the knots screened out from the spent cooking liquor is sent to knot handling system and thereafter reintroduced into bottom of vessel A

[0055] In this embodiment is shown a digester B with 2 concurrent cooking zones, one cooking zone above the first screen section SC.sub.2 and a second cooking zone above the final screen section SC.sub.3 in bottom of digester, but any kind of cooking scheme may be implemented in the digester vessel B. In a conventional manner is preferably a final counter current wash zone implemented in bottom of digester by addition of wash water/Wash. The final pulp with a kappa number below 40 is fed out from bottom in flow P.sub.OUT.

Alternative Embodiments

[0056] The invention could be implemented in a number of different ways besides what is disclosed in FIG. 1. The digester vessel B could be operated according to EAPC, MCC, ITC or Lo-Solids Cooking, with or without additional charges of alkali to some digester circulations. If the impregnation vessel is operated with cold top then also black liquor may be added to impregnation vessel in order to reach the desired liquid-to-wood ratios necessary (if the charge of polysulfide liquor is not enough).