CA1097465A

CA1097465A - Black liquor energy recovery

Info

Publication number: CA1097465A
Application number: CA272,239A
Authority: CA
Inventors: James R. Prough
Original assignee: Kamyr Inc
Current assignee: Kamyr Inc
Priority date: 1976-11-30
Filing date: 1977-02-21
Publication date: 1981-03-17

Abstract

ABSTRACT OF THE DISCLOSURE

The net energy used for digestion of wood chips compared to the prior art is reduced, and in fact a net energy gain from the digestion process can be provided by proper black liquor heat recovery. Black liquor is with-drawn from the digester and the solids concentration of the withdrawn black liquor is increased to at least about 60%
while steam is generated having a minimum temperature at or higher than the temperature necessary to treat chips in the digester system. Essentially all of the generated steam is recovered and utilized for treatment of chips in the digester system so that the chips are the final condenser for the steam and so that no condensing of the steam with water is necessary. The waste heat that normally would be disposed of in the condenser water is utilized to preheat chips before or during digestion. The concentrated black liquor is then burned to generate heat, and the heat generated by the burning is utilized further in the digester system for chip treat-ment, resulting in a net energy gain from digestion.

Description

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BAC~GROUI\'D ~D SU~ RY OF TH:E: INVENTION

Conventionally duxing the digestion of wood chips to produce pulp, a fair amount of energy is consumed by the burning of fossil fuels for the genexation of steam for heating of the chips, white li~uox, wash ~ater, etc.
~Yhile there have been proposals to minimize the amount of ~nergy required for digestion (see U. S. Patent No. 3,414,03g) by treatment of the chips with some of the heat extracted from the black liquor, at least a vexy large portion of the heat of the b]ack liquor has been wasted by the heating o~ condenser water. The disposal of the condenser water is of course a water pollution problem since discharge thereof into a body ; of water can result in thermal pollution thereof. With conventional black li~uor heat recovery systems such as shown in Pulp and Paper Manufacture, ~olume I, McGraw Hill/Unit Operations, McCabe and Smith, McGraw Hill, and US Patent 3,414,038 heat recovery cannot be entirely efficient since the heat from the last evaporator effect is not suficient for use in the digesting process.
According to the present invention a method and appaxatus are provided whereby the amount of energy that is re~uired for digestion is greatly reduced (i.e., a 44 1/2%
reduction in steam use for the pulping process r with an overall reduction in steam use for pulping and evaporation of 6~%), and considering heat recovered from burning of the 7~6~

concentrated black liquor a net energy gain can be provided from the digesting process. The resulting decrease in energy cost can have a fantastic influence on the economic viability of a mill, especially when worldwide increases in energy prices at a greater rate than labor, capital and material axe considered.
According to the method and apparatus of the present invention, the energy used during digestion is greatly decreased over the prior art by generating steam having a minimum temperature at or higher than the tempera-ture necessary to treat chips in the digester system during concentration of the withdrawn black liquor. Chips in the digester system are then used as the final condenser for the steam, so that no condensation of steam with cooling lS water is necessary, thereby achieving the reduced energy requirements and reduced thermal pollution problems accord-in~ to the invention.
` According to the present invention a method ior recovering heat from black liquor from a digester system for digesting chips is provided comprising the steps of ; Withdrawing black liquor from the digester; incrPasing the solids concentration of the withdrawn black liquor to at least about 60%, while generating steam having a minimum temperature at or higher than the temperature necessary to treat chips in the di~ester system; recovering essentially all of the generated steam and utilizing essentially all of ~7~

the generated steam recovered or treatment of chips in the digester system so that the chips are the final condenser for the steam and so that condensing of the steam with water is avoided. The method also comprises the further steps of burning the concentrated black liquor to generate neat and provide chemically recoverable ash, and utilizing the heat from the black liguor burning in the digester system for chips treatment. Preferably the digestion of the chips is countercurrent, and the generated steam is utilized for presteaming chips, treat-ment of chips in the chips bin, or for heating white liquorintroduced into the digester. For the presteaming utiliza-tion of recovered steam, the minimum temperature is about 213F, while for the heating of white liquid introduced into the digester system the minimum temperature is about 255F. The concentrating and steam genexating is accom-plished by passing the withdrawn hlack liquor through a recompression evaporator as a first phase, and a double-effect concentration as a second phase. No more than two 2G effects may be provided after the recompression evaporator otherwise the concentrated li~uid (i.e., 65~ concentrationof solids) discharged from the last effect for transport to the ~urnace would have too low a temperature/ and visco-city problems would arise ~i.e., for a 65~ concentration the temperature must be at 238F or above).

According to the apparatus of the presentinvention, an assembly for recovering heat from black liquor from a digester system is provided comprising a digester system including a chips bin, steaming vessel, continuous digester, and heater for heating white liquor introduced into the continuous digester; means for with-drawing black liquor from the digester; means for increas-ing the solids concentration of the withdrawn black liquor :; to at least about 60% while generating steam having a minimum temperature at or higher than the temperature necessary to treat chips in the digester system; and means : for recovering essentially all of the steam from the solids concentration increasing and steam generating means and for :: utilizing essen*ially all o~ the generated steam ~or the : 15 treatment of chips in the digester system so that condensing steam with water i5 avoided. The concentrating and steam `` genexating means preferably comprises a recompression evaporator directly connected to the black liquor withdrawal means and having a concentrated liquor outlet connected to at least ~0 one evaporator. Preferably first and second evaporators are pxovided, and at one of the evaporators has a gas vent, that gas vent being connected with a gas vent ~rom the pre-steaming vessel and/or the chips bin to a turpentine decanter.
Clean condensate recovered from the recompression evaporator 2S and the second effect may be utilized for wash watex, or fox xeconstitutivn of white liquor from the furnace ash, i5 It is the prima.ry object of the present invention to decrease the energy consumption and thermal pollution associated with chips dicsestion. This and other objects of the invention will become apparent from an inspection of the detailed description of the invention and from the appended claims.

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BRIEF DESCRIPTION OF THE DR~T,~INGS

FIGURE 1 is a schematic view showing exempiary apparatus according to the present invention; and FIGURE 2 is a flow chart showing exemplary temperature, flow, and energy use criteria for the apparatus of FIGURE 1.

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DETAILED DESCRIPTION OF THI~ INVENTION

.Exemplary apparatus according to the present invention is shown schematically in FIGURE 1 and includes a digester system 10, black liquor withdrawal means 26, means 28 for increasing the solids concentration of the withdrawing black liquor to at least about 60~, while generating steam having a minimum temperature at or higher than the temperature necessary to treat chips in the digester system, means 30 for recovering essentially all of ~he steam from the solids concentration increasing and steam generat-ing means, and means 32 for utilizing essentially all of the generated steam recovered by the recovering means 30 for the treatment of chips in the digester system 10 so that the con-densing of steam with water is avoided.
The di.gester sys~em 10 is conventional, such as shown in U. S. Patents N~.3,445,328 and 3,811,994, and includes a chips bin 12, steaming vessel 14, continuous digester 16, further treatment station 17 connected to the pulp outlet from the digester 16, heaters 18 for heating white liquid introduced into the digester 16, white li~uox introduction lines 19, and preferably a countercurrent impregnation system 20 including a conventional Kamyr high-pressure feeder valve. Wash liquid is introduced into the digester 16 by line 22 from a wash liquid source 23, and a wash liquid heating system 24 may be provided. The black --8~

liquor withdrawal means 26 may include a simple conduit connected to conventional screens between the digesting and washing stages of the digester 16, althou~h other like apparatus may be pro~ided.
~he black liquor concentrating and steam generating means may comprise a variety of particular structures including a reduced number of e~fects from ~onventional evaporator syst~ms,increased heating surface area of conventional systems, large body evaporators, or forced circulations - any means that results in the desired black liquor concentration and genexated steam temperature.
Pxe~erably, however, the means 28 includes a conventional recompression evaporator (such as those manufactured by Rosenblad Corporation of Princeton, New Jersey) having a black liquor inlet 35, a steam heat inlet 36 from a steam source 37, a condensate outlet 38, a steam vapor outlet 39, and a concentrated liquor outlet 40. A compressor 41 pro-~ides the power for the recompression. Steam heat is introduced at 36 in order to have a multiplier efect in the subsequent evaporators connected to the recompression e~aporator 34.
Connected to the recompression evaporator 34 is a~ least one conventional evaporator 42, although it is preferred that two conventional evaporators 42, 42' be provided (but no more than two evaporators otherwise the ~inal concentrated black liquor has a temperature that is 7~&5 low enough to cause viscocity problems). The conventional evaporators 42, 42' may be of any type,such as long-tube vertical evaporators, each of the evapora~ors hav.ing a feed liquor inlet 44, 44', a steam heat inlet 45, 45', a condensate outlet 46, 46', a steam vapor outlet 47, 47', a concentrated liquor outlet 48, 48l, and a vent 4 9 r 4 9 1-The black liquor inlet 44 for the first evaporator 42 is the concentrated liquor outlet 40 from the recompression evaporator 34, and the steam heat inlet 45 for the first evaporator 42 is the steam vapor outlet line 39 of the recompression evaporator 34. The condensate line 3~ from the xecompression evaporator 34 is connected to a pair of ~lash tanks 50, the steam outlet from the second effect flash tank providing the steam for steam inlet 45' o the second evaporator 42'. The l.ine 52 leading from the second efect flash tank 50 is connected to the condensate line 46' from the second evaporator 42i, and the condensate in line 52 is clean condensate, which may be led to the wash liquid source 23, or be used for white liquor recovery. The con-

2~ centrated liquor outlet 48 rom the first evaporator 42 provides the concentrated liquor inlet 44' for the second eYaporator 42'.
The means 30 for recovering essentially all of the steam from the solids concentration increasing and steam generating means 28 includes the steam vapor recovery por-tions of the evaporators 42, 42', and the means for utiliz-ing essentially all o the generated steam recovered by ~lQ-1~7~`~5 the recovery means 30 for treatment of chips in the digester system 10 includes the lines 47, 47' of the evaporators 42, 42', and ~heir connections to various digester system com- j ponents. Although the lines 47, 47' may be connected to a variety of structures in the system 10, it is preferred that the line 47 be connected to the heater 18 for preheat- `
ing of the chips in the digester 16 hy heating of white liquid being recirculated and introduced to the digester 16.
The steam in line 47 has a temperature of about 255F, which .is suitable for use in heating of the white liquor. The line 47' preferably is connected to the steaming vassel 14 or to the chips bin 12 for heating of the chips before intro-duction thereof to the digester 16 in order to facilitate impregnation of the chips with liquid and to heat them to a lS desired preliminary temperature before digestion. The steam in line 47' is at about 213~F which is suitable for the preheating function.
The concentrated liquor line 48' from the second evaporator 42 is connected to means 54 for burning the con~
centrated black liquor to generate heat and to provide chemically recoverable ash, such as a conventional furnace having heat recovery apparatus associated therewith (i.e., a boiler). Means for utilizing the heat from the black liquor burning in the digester system 10 for chips treatment includes a line 55 leading from the means 5~, and connection of the line 55 to various digester system components, such as ~ .7~$iS

the heating means 18 or the steam source 37. While the utilization of the hlack liquor concentrating and steam generating means 28 according to the invention results in overall steam reduction for pulping and evaporation of 64~ compared with the prior art, the additional utilization of the heat from the combustion of the black liquor in the furnace 5~ results in a net energy gain from the digestion process, any extra heat not being utilized in the systems 18, 2~ or 37 being used for the generation of electricity or elsewhere in the pulp plant. The ash from the furnace 54 is passed on to conventional white liquor recovery means 56 which may include dissolving tanks, a slaker, a white liquid clarifier, and mud wash, and the like. Lime and water from line 52 and ~ime kiln 58 are provided to the means 56 for generating the white liquor that may be used in line 19.
Turpentine recovery can also be effected ~rom the black liquor treatment system and the like. Vent lines 49 and 49' from evaporators 42 and 42' are connected to a ~ent line 60 from the~ chips bin 12 and/or the steaming vessel 14, and a~ter they merge together the lines lead to a condenser 62. The condensate from condenser 62 leads through line 64 to a conventional turpentine decanter 66 or the like wherein ~urpentine is recovered and exits by a line 67. Vent lines 68 from the condenser 62 and decanter 66 lead to suitable odor control apparatus, and fouled conclensate ~rom decanter 66 - along with fouled condensate ....

in line 46 of evaporator 42 - lead to a suitable disposal or reclamation station 70.
--~ . FIGURE 2 is a ~low chart showing exemplary feed-in flow rates for steam, chips, water and other energy, that might exist in a system according to the inven-tion. The total steam used in the system illustrated in ~IGURE 2 is about 1,30ipounds per ton of Pul~ Produced, and 2,lO0 kilowatt hours for the compressor 41 of recompres-sion evaporator 34 for 21-l/4 tons of pulp pro~uced (usin~ the con-~ersion factor of 3.413 pounds of steam per kilowatt hour at e~aporator conditions, the 2,100 kilowatt hours per 2l.25 ton ~sed by compressor 41 equals about 337 pounds of ste~m per ton of pu~p produced), for a total steam used of only about pounds per ton of-pulp producecl. When the energy gain from burn-ing black liquor in furnace 54 and recovering the heat t1~ereof .~s considered, there is a net energy gain from the system.
It is also noted that essentially no water is used whatsoever in the black liquor treatment system except a very small amount of water which is used for condenser 62. It is also noted that the solids concentration of liquor in line 48' is prefexably 65~ at a temperature o 238F so that no visco-city problems are encountered in the pumping of the liquor to the urnace 54 for treatment therein. Also, the substan-tial amount of clean condensate water in line 52 may be used anywhere in the mill ~hexe clean water is desired, such as in the wash line 22.

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Accordin~ to the method of the present invention, black liquor is withdrawn through a line 26 from a digester 15 of a digester sy tem 10 for the digestion o~ chips, the liquor is concentrated to a solids concentra-tion of at least about 60% and steam is generated therefrom,the steam having a minimum temperature comparable to the minimum temperature necessary to treat chips in the digester system, all the generated steam is recovered, and essen-tially all of the recovered steam is utilized for the treat-ment of chips in the digester system, such as at white liquorheater 18 (approximately 255F steam), and at chips bin 12 or steaming vessel 14 (approximately 213F steam). The concentrated black liquor is burned in a furnace 54, and the energy thereof is utilized elsewhere in the chips digester s~stem, such as at steam source 37, heater 18, heater 24 or for genexating electricity ~or compressor ~1 or the like.
It will be thus be seen that according to the pxesent invention a method and apparatus have been provided which substantially reduce the steam requirements ~or treat-ment of a given amount of chips compared to prior art struc-tures and methods, and which actually can produce a net energ~ gain when black liquor combustion is considered.
Thus, the objects of the present invention have keen achieved.
While the invention has been herein shown and described in what is presentl~ conceiv2d to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within t~e scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and methods.

Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method for recovering heat from black liquor from a digester system for digesting chips, comprising the steps of (a) withdrawing black liquor from the digester, (b) increasing the solids concentration of the withdrawn black liquor to at least about 60%, while generating steam having a minimum temperature at or higher than the temperature necessary to treat chips in the digester system, by passing the withdrawn black liquor through a recompression evaporator as a first phase, (c) recovering essentially all of the generated steam in step (b), (d) utilizing essentially all of the generated steam recovered in step (c) for treatment of chips in the digester system so that the chips are the final condenser for the steam and so that the condensing of the steam with water is avoided, (e) burning the concentrated black liquor from step (b) to generate heat, and (f) utilizing the heat from the black liquor burning of step (e) in the digester system for chip treatment.

2. A method as recited in Claim 1, comprising the further step of effecting countercurrent impregnation of chips in the digester.

3. A method as recited in Claim 1, wherein step (d) is accomplished by utilizing generated steam for presteaming chips in the digester system before digesting the chips.

4. A method as recited in Claim 1, wherein step (d) is accomplished by treating chips in the chips bin of the digester system before presteaming thereof.

5. A method as recited in Claim 1, wherein step (d) is accomplished by utilizing generated steam for heating white liquor being introduced into the digester.

6. A method as recited in Claim 1, wherein step (f) is accomplished by heating white liquor being introduced into the digester.

7. A method as recited in Claim 1, comprising the further step of utilizing the clean condensate from evaporation of the black liquor for digester wash liquid.

8. A method as recited in Claim 1, wherein the temperature of steam in step (b) is 213°F or greater, and wherein the final black liquor solids concentration is about 65%.

9. Apparatus for recovering heat from black liquor from a digester system for digesting chips, comprising - a digester system for digesting chips including a chips bin, steaming vessel, continuous digester, and heater for heating white liquor introduced in the continuous digester, - means for withdrawing black liquor from the digester, - means for increasing the solids concentration of the withdrawn black liquor to at least about 60% while generating steam having a minimum temperature at or higher than the temperature necessary to treat chips in the digester system, said means comprising a recompression evaporator directly connected to the black liquor withdrawal means, and having a concentrated liquor outlet, and at least one evaporator connected to the recompression evaporator concentrated liquor outlet, - means for recovering essentially all of the steam from said solids concentration increasing and steam generating means, - means for utilizing essentially all of the generated steam recovered by said recovering means for the treatment of chips in said digester system so that condensing of steam with water is avoided, - means for burning the black liquor to generate heat, and - means for utilizing the heat from the black liquor burning in said digester system for chips treatment.

10. Apparatus as recited in Claim 9, wherein said recompression evaporator has a steam outlet and wherein said at least one evaporator is connected directly to said recompression evaporator steam outlet, steam supplied by the recompression evaporator steam outlet used to heat the liquor to be concentrated by said at least one evaporator.

11. Apparatus as recited in Claim 9, wherein said at least one evaporator comprises two evaporators, a first and second evaporator, said first evaporator being connected to said recompression evaporator concentrated liquor outlet, and said second evaporator being connected to a concentrated liquor outlet from said first evaporator, said liquor being concentrated to about at least 60% after passage through said second evaporator, and further comprising means for leading the concentrated liquor from said second evaporator to a furnace.

12. Apparatus as recited in Claim 9, wherein said at least one evaporator has a gas vent, and further comprising means for directly connecting said gas vent and a gas vent from one of said presteaming vessel and said chips bin to a turpentine decanter.

13. Apparatus as recited in Claim 9, wherein said means for utilizing the heat from the black liquor burning in said digester system comprises means for supplying heat to said heater for heating white liquor introduced into said continuous digester.

14. Apparatus as recited in Claim 9, wherein said means for utilizing essentially all of the generated steam recovered by said recovering means comprises means for transporting said steam to said presteaming vessel for steaming of chips therein.

15. Apparatus as recited in Claim 9, wherein said means for utilizing essentially all of the generated steam recovered by said recovering means comprises means for transporting said steam to said chips bin for pre-heating of chips therein.

16. Apparatus as recited in Claim 9, wherein said means for utilizing essentially all of the generated steam recovered by said recovering means comprises means for transporting steam to said heater for heating white liquor introducted into said continuous digester.

17. Apparatus as recited in Claim 9, further comprising means for effecting countercurrent impregnation of chips in said digester.