Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C7/00—Digesters
  • D21C7/10—Heating devices
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/10—Concentrating spent liquor by evaporation
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/02—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/24—Continuous processes
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C7/00—Digesters
  • D21C7/14—Means for circulating the lye
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
  • D21C11/0092—Substances modifying the evaporation, combustion, or thermal decomposition processes of black liquor
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes

Definitions

• the present invention relates to a method for generating steam in a digester plant of a chemical pulp mill.
• the heating is made using a heating circulation.
• a volume of spent hot black liquor typically at full cooking temperature of 130-180 °C.
• the heat value in the hot black liquor volume is used initially to generate steam for steaming vessel and heat white liquor, or alternative liquid volumes charged to the cooking systems.
• the spent hot black liquor is typically sent first to two or more flash tanks connected in series where steam fractions are produced at successively lower temperature and pressure as the liquor is flashed in stages to temperatures of approximately 95-1 10 °C. Finally the black liquor is sent to the evaporator system in the recovery island.
• ImpBinTM a single first impregnation vessel, ImpBinTM, replacing all previous chip bins, steaming vessel and impregnation vessel.
• ImpBinTM the chips steamed and impregnated with hot black liquor that is supplied from digester, and when added to ImpBin is allowed to flash off steam for steaming and using the residual black liquor as the impregnation liquor.
• Different concepts have been proposed where the heating to full digester temperature is replacing the addition of clean steam into digester to usage of flash steam from black liquor. Clean condensate will thus not be lost in digester, and cooking liquors will not be diluted with clean water that increase evaporation work in the recovery operations.
• Valmet AB ' s SE 453,673 One of the first patents disclosing a reboiler producing dirty steam for heating in digester top is shown in Valmet AB ' s SE 453,673.
• US8512514 is disclosed a complex dual heat recovery system, using a first evaporator to generate foul steam for heating in digester top using a first extraction flow from digester, and a second heat exchanger for producing clean steam from a second extraction flow from digester.
• US8691049 concerns a method using the CrossCircTM concept promoted by Valmet AB.
• the CrossCirc concept is hot black liquor added directly into the transfer system in order to reduce steam consumption in digester top for reaching full cooking temperature.
• a reboiler also used to recover residual heat value in the return liquor withdrawn in the top separator, and this reboiler is producing clean steam for chip bin steaming.
• US8808498 is yet another system disclosed where flash steam from black liquor heats white liquor, and the flashed black liquor is used in a second heat exchanger in order to heat white liquor.
• the inventive method is developed for generating steam in a digester plant of a chemical pulp mill comprising:
• HBL hot black liquor
• the evaporated mixed liquor used as impregnation liquid in early chip treatment systems i.e. black liquor impregnation
• black liquor impregnation which is advantageous from an emission point as the evaporated mixed liquor has a lower content of malodorous gases than the content of malodorous gases in the volume of hot black liquor when extracted from digester.
• the inventive method may also further involve that the evaporation of the mixed liquor takes place in a reboiler (Reb) and the charge of alkaline cooking liquor (WL) is introduced into the hot black liquor in or before the reboiler where adequate mixing takes place during the boiling conditions prevailing in the reboiler.
• Reb reboiler
• WL alkaline cooking liquor
• the inventive method may also further involve that said evaporated mixed liquor obtaining a lower dry matter content than the hot black liquor and a lower concentration of alkali than the fresh alkaline cooking liquor.
• the inventive method may also further involve that a clean steam condensate is obtained from the indirect heating in the reboiler and this clean condensate is used as heating medium in a preheater, preheating the alkaline cooking liquor or the mixed liquor before being fed to the reboiler.
• the inventive method may also further involve that the heated evaporated mixed liquor flow (X) is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to the top of the digester (DTOP) where the heated evaporated mixed liquor generates steam as well as hot cooking liquor.
• DTOP top of the digester
• the inventive method may also further involve that the heated evaporated mixed liquor is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to a preceding impregnation vessel (IB), preferably at end of the impregnation vessel and preferably directly ahead of transfer of the cellulosic feed material to the digester (D).
• a preceding impregnation vessel IB
• the impregnation vessel D
• the inventive method may also further involve that the heated evaporated mixed liquor flow (X) is heated to a temperature above the prevailing full cooking temperature, and charged at least in part to a preceding impregnation vessel (IB), preferably at start of the impregnation vessel and more preferably directly into a chip volume established over a liquid level (Liq in said impregnation vessel, such that steam released from the heated evaporated mixed liquor will steam the cellulosic feed material before impregnation.
• IB preceding impregnation vessel
• Fig. 1 shows schematically a state of the art digester system
• Fig. 2 shows a modification of the digester system in Fig 1 where the inventive steam generation system is installed;
• Fig. 3 shows a detail view of the steam generating system, with identified process parameters disclosed.
• Fig. 1 illustrates a state of the art digester system of the Compact Cooking G2TM concept, sold by Valmet.
• the Compact Cooking G2TM concept includes a single first atmospheric impregnation vessel IB, ImpBinTM, replacing all previous chip bins, steaming vessel and impregnation vessel.
• ImpBin vessel is the chips CHIN steamed and impregnated with hot black liquor TSRET that is supplied from digester D, and when added to IB/ ImpBin is allowed to flash off steam for steaming and using the residual black liquor as the impregnation liquor forming a liquor level Liqi. below the steam flash off position.
• the system includes the CrossCircTM concept promoted by Valmet AB, where hot black liquor HBL withdrawn from digester is added directly into the transfer system in order to reduce steam consumption in digester top for elevating the temperature. Final heating is obtained by an additional heating with medium pressure steam ST P reaching full cooking temperature in digester top. A first heating of the chips towards full cooking temperature then takes place during the transfer to the top separator TS, and the excess transport liquor TSRET is withdrawn in the top separator and used as the flashing impregnation liquor. In most installations in warmer climates is the steam flashed off from the hot black liquor in the atmospheric ImpBin vessel enough for full steaming of chips and purging of air bound in the chips.
• the extraction position for the hot black liquor at full cooking temperature may be located anywhere in the digester, i.e. within the indicated zone A.
• a dedicated heater used for the white liquor WL charge and the addition to digester top is conventionally made using a white liquor header distributor (not shown) that sprays the white liquor into the top.
• handling concentrated white liquor at full cooking temperature involves increased corrosion exposure, and the heaters may be subjected to alkali corrosion even if the heater is made in stainless steel needing even better and more expensive material in the heater.
• Fig. 2 illustrates an improvement of the Compact Cooking G2TM concept as shown in Fig. 1 , and includes a steam generation system operating according to the inventive method.
• the inventive concept is at least a part of the hot black liquor HBL, holding full cooking temperature which in this example is 141 °C, instead sent to a reboiler Reb for further heating.
• the hot black liquor HBL be heated in preheater HE before being charged to the reboiler, and this alternative will be more described below with reference to Fig. 3.
• a charge of fresh alkaline cooking liquor typically white liquor but any kind of alkaline cooking liquor may be used, is charged into the hot black liquor and preferably before the preheater HE.
• the alkaline cooking liquor holds a temperature below 1 00 °C and stored in atmospheric tanks, and hence the mixed liquor WL+HBL will reach a lower temperature at about 1 30 °C. If a preheater HE is used this temperature may be increased to about 1 34 °C before charged to the reboiler Reb.
• the reboiler holds of volume of mixed liquor WL+HBL and a coil or piping for heating media is installed in this liquor volume, in this case the heating media is medium pressure steam STMP.
• the heating of the mixed liquor will be done indirectly, and clean steam condensate will not be lost in the mixed liquor and dilute it.
• the coil is supplied with medium pressure steam STMP and a condensate STc is extracted from the heating coil.
• both the addition of alkali and heating have positive effects on the content of dissolved hemicellulose in the black liquor, that need time, temperature and high alkali concentration in order to increase cleavage rate on end groups of the hemicellulose.
• the cleavage effect has reached a critical order could the hemicellulose start to precipitate onto cellulose and increase yield from the cook.
• the additional retention time in the reboiler, the heating and the alkali addition all contribute synergistically in increased cleavage rate.
• the common heating of both black liquor and alkaline cooking liquor in a liquor mixture in one single heating apparatus avoids both lignin precipitation risks in a reboiler fed with black liquor with high solids content and low alkali level close to lignin precipitation level as well as avoids alkali corrosion risks in dedicated heaters for concentrated cooking liquor.
• the heated hot black liquor WL+HBL may reach a temperature well over the full cooking temperature, and this flow X may be used in several alternative routes, either for exclusive use or in combinations.
• First option given priority to reduce need for medium pressure steam addition to digester top involves routing the flow X to digester top at the indicated position XALTL The overheated mixed liquor will then flash off additional steam as well as supply of both alkali and black liquor to cook at full cooking temperature.
• Second option given priority to reduce need for medium pressure steam addition to digester top by heating the cellulosic material already in transfer circulation involves routing the flow X at end of impregnation vessel IB and preferably directly ahead of the transfer system at the indicated position XALT2.
• the overheated mixed liquor will heat the slurry with cellulosic material when mixed into this slurry.
• Third option given priority to increase steaming effect in top of impregnation vessel involves routing the flow X at top of impregnation vessel IB, and preferably into the return flow TSRET from the top separator at the indicated position XALT3.
• the overheated mixed liquor will heat the return liquor when mixed into it and more steam will be flashed off when finally dumped into the volume of cold chips lying above the liquor level Liqi_. This option may be preferred in mills operating in cold climate and fed with deep frozen chips.
• Options 2 and 3 for the X flow are specifically adapted for a Compact CookingTM system with an ImpBin, but option 1 may be used in any digester system operating with conventional pressurized impregnation vessels as well as using other chip pretreatment systems with dedicated chip bins and steaming vessels.
• Option 2 may also be used in conventional 2-vessel digester systems where heating is to be obtained in first vessel.
• Fig. 3 is disclosed an embodiment of the invention in a Compact CookingTM system with an ImpBin implemented according to the first option.
• the basic parameters for the Preheater (HE) are as follows:
• the overheated mixed liquor WL+HBL from the reboiler holds same temperature of 155.1 °C.
• an additional amount of steam released at a rate of 4.4 kg/s which is almost 40 % of the black liquor steam STBL rate from the reboiler, while the cooking liquor WL+HBL is charged at full cooking temperature.
• Black liquor typically has a dry matter content exceeding that of fresh cooking liquor by at least 5-20 %, so the dilution by fresh cooking liquor improves flow characteristics trough the indirect heat exchanger and reduce scaling tendencies, that often are seen in evaporation stages as a tar like deposition in the evaporation stages, where similar increase of dry matter content of undiluted black liquor is obtained.
• a cost reduction alternative may the entire heating of the mixed liquor, i.e. black liquor and white liquor, by obtained in the indirect heater before charging to digester, which mean that the white liquor charge to digester top needs no dedicated white liquor heater, which save investment costs for such heater, and enable downscaling of any additional white liquor heater for other charges to the process.
• Indirect heaters for concentrated white liquor are due to working conditions, heat and alkali concentration, most expensive as they need to be made in high alloy corrosive resistant steel.
• the total charge of alkaline cooking liquor needed for the entire cook may be charged to the reboiler, or at least 20-50 % of the total necessary charge.

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• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Paper (AREA)
• Control Of Turbines (AREA)

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• 2015
  • 2015-11-16 SE SE1551483A patent/SE539449C2/en unknown
• 2016
  • 2016-10-18 WO PCT/SE2016/051013 patent/WO2017086856A1/en not_active Ceased
  • 2016-10-18 EP EP16866744.2A patent/EP3377696A4/de not_active Withdrawn

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