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
  • D21C1/00—Pretreatment of the finely-divided materials before digesting
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21B—FIBROUS RAW MATERIALS OR THEIR MECHANICAL TREATMENT
  • D21B1/00—Fibrous raw materials or their mechanical treatment
  • D21B1/02—Pretreatment of the raw materials by chemical or physical means

Definitions

• the present invention relates to a method for preheating wood chips prior to steaming said chips, for the purpose of improving heat economy.
• wood chips is meant here and in the following chips which are obtained by chipping wood logs and which are used as a starting material in the manufacture of cellulose pulps using known chemical, semi-chemical, chemi-mechanical, and mechanical processes, such as sulphite, sulphate, refiner and thermomechanical methods.
• the invention is thus suitable for the preparatory treatment of the starting material, i.e. the wood chips, in cellulose pulp manufacturing processes.
• the chips are steamed in order to enable chemicals subsequently charged to the process to penetrate and diffuse into the chips more readily, while at the same time facilitating the release of lignin, resins etc. from said chips.
• the Swedish Patent Specification Serial No 149 053 teaches the simultaneous moistening (moisture equalization) and transportation of wood chips to the top of a digester, or to chip bins or silos, or to a steaming vessel.
• the chips are conveyed in water which has been heated to 30-40oC with fresh steam and which may contain a minor quantity of alkali.
• This method of procedure requires the provision of special apparatus and involves the consumption of expensive fresh steam.
• the chips become totally saturated, which renders subsequent impregnation of the chips with cooking liquor difficult, and introduces an unnecessary, high quantity of water to the process. This negatively affects the concentration of the waste liquor, and therewith the heat economy and chemical recovery.
• the Swedish Patent Publication No 227 648 teaches a method for heating chips stored in outdoor stacks at temperatures around 0°C or lower, in order to produce a more uniform pulp raw material. This is effected by blowing hot air or steam into the bottom of respective stacks so as to initiate a desired enzymatic hydrolysis of the extractive substances of the wood, which hydrolysis, as a result of its exothermic reaction, is able to raise the temperature throughout, the whole of the stack. In this way the bottom layer of chips is heated preferably to between 1 and 5°C, at most to
• the aforementioned problems are resolved by means of the present invention, which relates to a method for preheating wood chips prior to steaming said chips, in which method the chips are heated during the preheating process to progressively higher temperatures in one or more stages, after which the chips are finally heated in the steaming vessel to a temperature of about 120°C.
• the method is characterized in that in the first preheating stage, and in the stage or stages immediately following said first stage, the chips are he.ated directly with moisture-saturated hot air, optionally admixed with an inert or neutral gas, said hot air having been heated to a temperature of between 55-99oC, preferably 70-90°C.
• Preheating of the chips in accordance with theinvention is preferably carried out in a chip bin located in the pulp mill and normally placed in front of and above the steaming vessel.
• the chips are preheated continuously in the bin as they are charged thereinto by means of hot, moisture-saturated air blown into said bin, the temperature of said chips increasing progressively as the chips pass down through the bin.
• the temperature of the chips has normally reached to about 95°C. Residual heating of the chips to about 120°C is effected in the steaming vessel by means of fresh steam blown thereinto.
• the hot chips are fed down from the steaming vessel directly into a digester.
• the air used to preheat the chips is suitably heated in a contact device, in which hot water or hot condensate is caused to meet the air in counter-flow.
• the condensate suitably has a temperature of about 80°C, before delivering its heat to the air passing in counter-flow thereto.
• the cooled condensate is continuously recycled to a direct condenser, where it is reheated to a temperature of about 80°C, for example using vacuum steam drawn off from different effects in an evaporation station.
• the air used for heating the chips in the chip bin is heated in an air-heating unit which comprises a plurality of indirect condensors built together, and means for supplying spray water to said air such as to saturate the same with moisture.
• the thermal energy input to the air-heating unit is obtained, for example from an evaporation station, by supplying to the indi vidual condensors low-grade vacuum steam drawn from mutually different effects in said station.
• the saturated hot air is heated to a temperature of about 70°C, before being blown into the chip bin.
• the air- heating unit is divided into two similar units which work in parallel and to each of which there is fed vacuum steam drawn from the evaporator, in a manner such that one unit is supplied with vacuum steam from the higher levels of the evaporator, i.e. the hottest steam, while the other unit is supplied with vacuum steam from the lower levels of said evaporator.
• the secondary steam used in the last stage of the chip preheating process is preferably blown into the bottom of the chip bin, said steam has a temperature of at least 100°C and, according to the present method, is suitably withdrawn from an evaporation unit with suitable pressure, or from a so-called flash tank, i.e. an expansion vessel, for driving off steam from digester waste liquor. Normally there are from 1 to 3 such expansion vessels per digester, and hereinafter these vessels will be designated flash tank I, flash tank II etc. , In the order of sequence from the digester.
• the steam used in the steaming vessel is normally taken from flash tank I after the digester and has a temperature of about 125oC.
• steam used in the steaming vessel is preferably taken from flash tank II or from a pre-evaporation effect coupled to flash tank I.
• Steam charged to the steaming vessel in accordance with this latter alternative is more pure than steam charged In accordance with the former, and also provides the best heat economy.
• the surprisingly good heat economy afforded by the method according to the present invention is related to the fact that it has been found possible to cover a high percentage of the heat required to preheat the chips to about 120°C by utilizing air as a carrier medium for waste heat in the form of, for example, lowgrade vacuum steam.
• air as a carrier medium for waste heat in the form of, for example, lowgrade vacuum steam.
• the air mainly only serves as a carrier medium for the steam, which constitutes the heating component.
• the part played by the air in the transfer of heat is thus relatively small, and is of subordinate significance, except when the hot air has a very low final temperature.
• waste heat When applying the method according to the invention, the energy ccosts involved are very low, because a great deal of the heat required to heat the chips to about 125oC is covered by waste heat, which is thus given a profitable value, said waste heat suitably being in the form of low-grade.
• Another advantage is that, in comparison with direct heating of the chips with hot water, the chips treated in accordance with the invention are not saturated with water during the treatment process, which con tributes in turn to improved impregnation of the chips in a subsequent stage and to a higher quality of the final pulp.
• a contributory factor in this connection is that air and volatile organic components are displaced from the chips during the treatment of said chips in accordance with the invention.
• a further advantage is that the saving in energy afforded by the method according to the invention leads to a reduction In the amount of fresh steam which need be charged to the steaming vessel.
• FIG. 1 illustrates schematically the most common system used today for steaming wood chips.
• flash steam having a temperature of about 125°C and taken from a flash tank I, designated 2 in the Figure, is passed through a pipe 1 to a steaming vessel 4.
• Fresh steam is also passed to the steaming vessel, through a pipe 3.
• the chips are heated in the steaming vessel 4 to a temperature of about 120oC, whereafter the steamed chips are charged to a digester, as shown by reference 5, through a gate feeder 13.
• Hot, thin liquor obtained from the digesters is charged to the flash tank I through a pipe 34.
• Liquor in the flash tank I is transferred to a flash tank II, here designated 11, through a pipe 7.
• Chips are fed to the steaming vessel 4 from a chip bin or silo 6 located above said vessel, cthrough a gate feeder 12. Steam from the flash tank II Is not used to preheat the chips, but is transferred to a condenser 9 through a pipe 8.
• FIG 2 illustrates schematically another known method of procedure, In which chips in the chip bin 6 are neated with secondary steam holding a temperature of about 105oC and passed to said bin through a pipe 14.
• the secondary steam may, for example, originate from an evaporation stage or from a flash tank.
• Final heating of the chips (steaming), to about 120oC, is effected in the steaming vessel 4, similar to the Figure 1 system, using flash steam and fresh steam passed to the tank through respective pipes 1 and 3.
• Figure 3 Illustrates schematically a preferred embodiment of the invention.
• condensate passed to a direct condenser 28 through a pipe 30 is heated in said condenser from a temperature of about 20oC to a temperature of about 20oC.
• the resultant hot condensate is then passed through a pipe 29 to an air-heater 31, in which relatively cool air introduced into the heater through a pipe 33 is permitted to pass the hot condensate entering the heater 31 through pipe 29 in counter-flow. In this way, the air is heated from about 40oC to about 70oC, while being washed at the same time.
• the air heater 31 has the form of a counter-flow contact device from which moisture-saturated hot air, taken out through pipe 32, is passed to a chip bin or silo 6 and blown into the lower part thereof. Through condensation and convection the hot, moisture-saturated air is cooled in the chip bin from its input temperature of about 70°C to about 40°C, at the same time as the chips in the bin 6 are heated to a temperature of about 60°C, which constitutes 30-50 % of the total preheating requirement.
• Residual heating of the chips to a temperature of about 120oC is effected partly in the chip bin or silo 6, by blowing thereinto through a pipe 27 secondary steam having a temperature of about 105°C and taken from the upper part of the evaporator 15, and partly in the steaming vessel 4, with the aid of flash steam introduced through pipe 1 and having a temperature of about 125°C, together with a requisite amount of fresh steam introduced via pipe 3, to impart to the chips a tempera ture of about 120°C.
• the air cooled in the chip bin 6 is taken out through the pipe 33 and recirculated to said bin via the air heater 31. Similar to the systems illustrated in Figures 1 and 2, hot, thin liquor taken from the digesters is passed to the first flash tank 2 through the pipe 34.
• the chips were preheated in two stages , firstly In the chip bin 6 with secondary steam 14 to a temperature of about 95°C, and secondly in the steaming vessel 4 with flash steam 1 and fresh steam 3, to a final temperature of about 120°C. It required 30,250 MJ/hr to heat the chips to 95°C in the chip bin, which corresponds to 13.5 tons/hr of secondary steam at a temperature of 105°C. The amount of flash steam 1 and fresh steam 3 required could, in this case, be corre spondingly decreased. If the value of the secondary steam 14 is calculated as 80 % of that of the fresh steam, a corresponding saving in fresh steam amounts to 2.7 tons/hr.
• the chips were heated in three stages, of which the first two were effected in the chip bin 6 and the third in the steaming vessel 4.
• the chips were heated with moisture-saturated hot air 32 from the air heater 31 to a temperature of about 60°C.
• the heat required herefor corresponded to 8.8 tons/hr of fresh steam.
• Continued heating of the chips to a temperature of about 95oC was effected by blowing secondary steam 27 having a temperature of about 105°C into the bottom of the chip bin.
• the steam required herefor was 4.7 tons/hr.
• the amount of flash steam 1 and fresh steam 3 used in the third heating stage in the steaming vessel 4 could be reduced by 13.5 tons/hr.
• Comparison method A 0 0 Comparison method B 2.7 1.5 Method C according to the invention 9.7 5.4

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Paper (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)
• Commercial Cooking Devices (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20341628

Family Applications (1)

Country Status (6)

Cited By (1)

Families Citing this family (10)

Citations (2)

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• 1980
  • 1980-08-29 SE SE8006046A patent/SE422604B/sv not_active IP Right Cessation
• 1981
  • 1981-07-15 WO PCT/SE1981/000218 patent/WO1982000838A1/en not_active Ceased
  • 1981-07-15 JP JP56502598A patent/JPS6262194B2/ja not_active Expired
  • 1981-08-28 US US06/297,400 patent/US4401510A/en not_active Expired - Fee Related
  • 1981-08-31 CA CA000384881A patent/CA1169285A/en not_active Expired
• 1982
  • 1982-02-18 FI FI820547A patent/FI69325C/fi not_active IP Right Cessation

Patent Citations (2)

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