CA2093098A1 - A method of recovering and reusing sulphur in a sulphate cellulose plant - Google Patents
A method of recovering and reusing sulphur in a sulphate cellulose plantInfo
- Publication number
- CA2093098A1 CA2093098A1 CA 2093098 CA2093098A CA2093098A1 CA 2093098 A1 CA2093098 A1 CA 2093098A1 CA 2093098 CA2093098 CA 2093098 CA 2093098 A CA2093098 A CA 2093098A CA 2093098 A1 CA2093098 A1 CA 2093098A1
- Authority
- CA
- Canada
- Prior art keywords
- sulphur
- gases
- sulphuric acid
- liquor
- stage
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
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
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/04—Regeneration of pulp liquors or effluent waste waters of alkali 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
- D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green 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
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
Landscapes
- Treating Waste Gases (AREA)
- Polysaccharides And Polysaccharide Derivatives (AREA)
- Paper (AREA)
Abstract
Abstract The invention relates to a method of separating sulphur from a cellulose cooking process of a sul-phate cellulose plant and of recycling it for being reused.
In the method, the sulphur is separated and recovered as odour gases produced at various stages of the pro-cess, which gases are passed to combustion of odour gases and oxidated after that by means of a catalyst into sulphur trioxide, which as an aqueous solution produces sulphuric acid. The sulphuric acid again is used for replacing the outside sulphuric acid needed by the plant.
(Figure 1)
In the method, the sulphur is separated and recovered as odour gases produced at various stages of the pro-cess, which gases are passed to combustion of odour gases and oxidated after that by means of a catalyst into sulphur trioxide, which as an aqueous solution produces sulphuric acid. The sulphuric acid again is used for replacing the outside sulphuric acid needed by the plant.
(Figure 1)
Description
2~3~
A method of recovering and reusing sulphur in a sulphate cellulose plant The invention relates to a method of recovering and reusing sulphur in a sulphate cellulose plant, in which method sulphur existing in liquor cycle is sep-arated from it as gaseous compounds and oxidated into sulphur trioxide, after which sulphuric acid is pre-pared from the sulphur trio~ide and the sulphuric acid obtained in this manner is reused in the cellu-lose manufacturing process.
In a sulphate cellulose plant, cellulose pulp is cooked of wood by means of alkaline liguor, after which the cellulose pulp obtained is separated from the liquor. The pulp is washed before bleaching in order that the alkaline cooking liquor contained in the pulp can be separated from it. The spent cooking liquor contains plenty of soap, which is separated from the liquor and passed to a tall oil plant. The spent liquor, i.e. the black liquor, is concentrated by evaporating and burned after the concentration in a soda recovery boiler in order to provide sodium carbonate and sodium sulph~de as well as to recover the energy contained in the black liquor. Smelt pro-duced in the soda recovery boiler is then dissolvedin water or in dilute white liquor, after which-it is treated in a known manner by causticizing it chemi-cally, in conseguence of which the final result of the whole process is a new alkaline cooking liquor.
This process is fully known per se and it is there-fore not described here in greater detail.
The brown cellulose pulp obtained is then bleached in a known manner by means of various chlo-rine compounds, which is also fully known per se and obvious to one skilled in the art and it is not de-... .. ~ .
.: : . : . , , ......................................... :
.. - ., .. . .. , . . , : .
2~3~98 scribed as such in greater detail.
Among cooking and bleaching chemicals, as well as among chemicals used in a tall oil plant there are also sulphurous substances, such as sulphuric acid, which is used both for preparing chlorine dioxide for bleaching and for splitting tall oil in the tall oil plant. The sulphur passes through the process and ty-pically tends to concentrate in the alkaline cooking liquor, which is rather harmful, and attempts are made to prevent that in different ways. Similarly, so-called odour gases are produced at different stages of the process, which gases contain sulphur as reduced gaseous compounds, which causes deposits of sulphur and odour nuisances in the environment. At-tempts have been made to eliminate the odour gasesfor instance by collecting the gases and by burning them either in existing boilers, in a lime kiln or in a separate combustion plant for odour gases. When using a separate combustion plant, the energy con-~0 tained in the gases is recovered by means of a waste-heat boiler and flue gases are scrubbed by a scrub-ber, which is typically a NaOH scrubber. In the scrubber, SOz gases contained in the flue ~ases pro-duce sulphate and sulphite salts, which pass together with white liquor to a pulp treatment plant and are thus recycled. Numerous methods have been presented for desulphuration of the process. Finnish Patent 67243, for instance, discloses a method in which a part of the fresh liquor is circulated as a parallel desulphuration cycle in the cellulose cooking pro-cess, whereby the liquor is carbonated and the hydro-gen sulphide produced is separated by stripping, after which it is burned and prepared into sulphuric acid by means of an aqueous solution. This equipment is very complicated and an expensive lnvestment.
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: -2~3~98 The object of the present invention is to pro-vide such a method of separating sulphur from the process of a sulphate cellulose plant by means of which method the amount of sulphur can be regulated in a simple and easy manner, due to which the sulphur separated can be recycled in order to replace a sepa-rate sulphuric acid needed for the bleaching stage and for tall oil cooking, while the sulphur content of the liquor can be maintained on a desired level.
10 The method of the invention is characterized in that the gaseous sulphur compounds are recovered as odour gases produced at different stages of the cel-lulose manufacturing process and separable under the influence of physical conditions only and the sul-phuric acid is prepared from the odour gases by oxi-dating them and by dissolving thus formed sulphur trioxide in a liquid mainly consisting of water.
The substantial idea of the invention is that the sulphur is recovered as odour gases produced at the various stages of the process, which gases are separated from the process and oxidated in such a way that substantially all sulphur in the odour gases is changed into sulphur trioxide, after which sulphuric acid is prepared from the sulphur trioxide in a man-ner known per se, which sulphuric acid again is re-cycled to the process, to the stage of preparing chlorine dioxide or of cooking tall oil. In this way it is possible to replace a great deal of the sul-phuric acid needed by the plant by the sulphuric acid obtained, while the environmental drawbacks are re-duced substantially. The definition in claim 1, viz.
"as odour gases separable under the influence of phy-sical conditions only", means that the odour gases are separated from the solutions of the process on the basis of the temperature and other physical con-. . .
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ditions without a separate treatm~nt carried out by chemical substances, the main purpose of which treat-ment is only to prepare sulphurous gases and to sepa-rate them from the process solution at some stage of the cellulose process. Moreover, ~he sulphur content of the liquor can be maintained sufficiently low by means of the method of the in~ention, due to which the drawbacks in the use and the service life of the equipment caused by the sulphur can be largely avoid-ed. The method of the invention especially improvesthe economical profitability of the cellulose plant, because distinct savings are achieved by this method in chemical costs.
The invention will be described in greater de-tail by way of example by means of the enclosed draw-ings, in which Figure 1 shows schematically a cellulose cook-ing and recovery process and a method according to the invention, 2~ Figure 2 shows schematically one manner of pre-paring sulphuric acid in the method of the invention and Figure 3 shows schematically another manner of preparing sulphuric acid in the method of the inven-tion.
Figure l shows a cellulose cooking process con-ventional per se, in which process wood 1 to be cook-ed is brought into a digester 2. In the digester it is cooked together with alkaline cooking liquor into cellulose pulp, after which the spent alkaline cook-ing liquor, i.e. the black liquor, is washed off from the cellulose in a washing department 3 and the wash-ed cellulose pulp is brought into a bleaching plant 40 The black liquor is led further to an evaporation plant 5 and 6, where water is separated from it un-, - . . : .
, . ' 21D~3Q9~
til it is finally concentrated to a dry content as high as possible. Also a separation of soap takes place in a tank 16 before the evaporation 5. The soap obtained is passed to a tall oil plant 13. At present it is known that the alkaline liquor is at the ini-tial stage 5 at first concentrated under atmospheric pressure, whereby it can be heated with steam at a low temperature, after which it is further concentra-ted 6 under a pressure higher than atmospheric pres-sure and by heating directly or indirectly with a steam under a high pressure and at a high temperatu-t re, as a conse~uence of which the final result is alkaline liquor with a dry content of up till 75-80 %
under such a pressure and at such a temperature that it can b~ displaced further by means of a pump and fed into a soda recovery boiler 7. After the normal evaporation plant 5 the figure shows a concentration stage 6 of this kind, from which the liquor is fed into a separate separating reactor ~, in which steam and various sulphurous odour gases, such as H2S, CH3HS, ~CH3)2S, (CH3)2S2 etc., are still separated from it when the pressure falls slightly. Sulphurous odour gases are produced both in the digester 2, in the evaporation plant 5 and in the sulphur separating re-actor 8 after the concentration as well as at the washing stage 3 of the liquor. All these are passed to an oxidation stage 9 of odour gases, in which the sulphur compounds are oxidated in such a way that the final result is sulphur trioxide, which is scrubbed free from flue gases into sulphuric acid at a scrub-bing stage 10. Finally, the flue gases are still scrubbed at another scrubbing stage 11 with an alka-line solution, e.g. sodium hydroxide, with which slight residues of sulphur produce a solution con-taining sodium sulphite~ sodium bisulphite and sodium 21D93~98 sulphate, which solution is fed into the desulphura- -tion reactor 8 or a dissolving tank of the soda re-covery boiler, and after this mainly carbon dioxide goes into a chimney. The sulphuric acid produced is concentrated at a concentration stage 12, whereby the impure dilute sulphuric acid can be led to the tall oil cooking plant 13 and the strong sulphuric acid is led to a preparation 14 of bleaching chemicals to re-place the sulphuric acid to be purchased and brought from outside. The waste acid produced at the prepara-tion 14 of bleaching chemicals is also led to the tall oil plant 13, from whicH is as waste obtained sodium sulphate, which is led together with the black liquor to the evaporation plant 5. The washed cellu-lose pulp is led to the cellulose pulp bleachingplant 4, into which the produced bleaching chemicals, i.e. mainly chlorine dioxide, are led. Respectively, the steam obtained from the sulphur separating reac-tor 8 can be led to the last concentration stage 6 for heating the black liquor. From the sulphur sepa-rating reactor the black liquor is led into the soda recovery boiler 7, from which the green liquor pro-duced by smelt and water is passed to a causticizing stage 15 and after the causticizing further to the digester 2 as the alkaline cooking liquor.
The most preferable way of preparing sulphuric acid is to use a catalyst. In the method according to Figure 2, the odour gases are burned in a waste-heat boiler 21, after which the produced gases containing S02 are oxidated into S03 gases in a catalyst reactor 22 by means of a catalyst. It is possible to provide steam in the waste-heat boiler by combustion, which steam can be used suitably at various stages of the cooking process. As a catalyst can be used for in-stance vanadium pentaoxide (V20s) or other catalyst 2~3~9~
suitable for the purpose and known per se. After thecatalyst reactor 22 the gases are led to a scrubber 23, in which a two-stage scrubbing takes place. At the first stage the gases are scrubbed either with water or a dilute H2SO4 solution, whereby the sulphur trioxide reacts and produces sulphuric acid together with water. In order to get the sulphur trioxide en-tirely removed, the remaining gases are scrubbed at the second stage with a NaOH solution, after which the gases can be led out and the produced solution containing sodium sulphide or sodium hydrogen sul-phide is removed e.g. to the dissolving tank of the soda recovery boiler. Instead of the separate waste-heat boiler 21 and the catalyst reactor 22, catalytic combustion can also be used, due to which sulphur trioxide is directly produced in the combustion cham-ber and the gases are scrubbed as described above.
The embodiment of Figure 3 shows how the flue gases of the waste-heat boiler according to the fore-going example are changed into sulphuric acid byusing an active carbon reactor known per se instead of a conventional catalyst reactor. The figure does not show the waste-heat boiler but the combustion gases coming from the waste-heat boiler through a channel 31, which gases are led into separate active carbon reactor chambers 32. The gases are led below an active carbon layer 33, from which they are con-veyed through the active carbon layer 33 and go fur-ther into the chimney through a channel 34. Washing li~uid, mainly water, is brought above the active carbon layer through a channel 35 and fed by pumps 36 above the active carbon layer 33. The gases contain-ing SO2 are oxidated in the reactor 32 under the in-fluence of the active carbon layer 33, due to which the produced S03 iS combined with the washing liquid, . .
.
.
2 ~ 8 i.e. water, which flows through the active carbon bed, and the sulphuric acid goes through a channel 37 into a collecting tank 38, from which it is pumped by a pump 39 to be further concentrated according to the diagram presented in Figure 1 and to be further re-used.
An example of process savings to be achieved according to the method of the invention is described in the following. The volume flow of the odour gases produced in the process is about 5 Nm3/second, the SO2 content being about 3000 ppm. The mass flow of sul-phur dioxide is 42,9 g/sec. and about 90 ~ of the sulphur dioxide can be oxidated into sulphur trioxide by means of a catalyst. 30,9 g sulphur trio~ide is then obtained per second and, washed into sulphuric acid, 33,5 g sulphuric acid is obtained per second.
When an odour gas plant is used 8000 hours per year, the final result can be 1022 tons of sulphuric acid.
2050 tons of sulphuric acid is consumed in the tall oil plant per year, whereby about half of the sulphuric acid needed is obtained from odour gases.
Then the sulphuric acid to be brought from outside constitutes only 50 ~ of what would be needed normal-ly. Further, the NaOH scrubbing required after the odour gas plant consumes considerably less chemicals, because most of the sulphur has been removed before the scrubbing stage and hardly any reaction products demanding a removal of the scrubber liquid are then produced, but the scrubber liquid can be recycled for a considerably longer time than previously.
The method of the invention can still be inten-sified considerably in such a way that sulphurous gases are separately removed from the finally concen-trated black liquor. This can be carried out by heat-ing it to a temperature higher than the cooking tem-, 2~@~8 perature and by keeping it there for a sufficient time, whereby almost all the sulphur is separated as sulphurous odour gases with water vapour from the alkaline liquor when the liquor is permitted to ex-pand to a lower pressure, and thus only a very insig-nificant amount of sulphur comes into the soda recov-ery boiler. This procedure can be carried out for in-stance in connection with the last concentration stage, when the liquor is kept at a sufficiently high temperature during the last concentration stage and it is then permitted to expand. Above in the specifi-cation a~d the drawings, the invention has been de-scribed by way of example and it is by no means re-stricted to it. By pplying the method of the inven-tion by means of the above procedure, it is possibleto provide an almost closed sulphur circulation, whereby it is hardly necessary to add sulphuric acid to the process at all. Especially by separating sul-phur from concentrated black liquor in the above man-ner, the greatest part of the sulphuric acid neededfor the process can be obtained by preparing it from the sulphur separated, and thus the sulphur emissions and the drawbacks caused by them can be greatly mini-mized.
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A method of recovering and reusing sulphur in a sulphate cellulose plant The invention relates to a method of recovering and reusing sulphur in a sulphate cellulose plant, in which method sulphur existing in liquor cycle is sep-arated from it as gaseous compounds and oxidated into sulphur trioxide, after which sulphuric acid is pre-pared from the sulphur trio~ide and the sulphuric acid obtained in this manner is reused in the cellu-lose manufacturing process.
In a sulphate cellulose plant, cellulose pulp is cooked of wood by means of alkaline liguor, after which the cellulose pulp obtained is separated from the liquor. The pulp is washed before bleaching in order that the alkaline cooking liquor contained in the pulp can be separated from it. The spent cooking liquor contains plenty of soap, which is separated from the liquor and passed to a tall oil plant. The spent liquor, i.e. the black liquor, is concentrated by evaporating and burned after the concentration in a soda recovery boiler in order to provide sodium carbonate and sodium sulph~de as well as to recover the energy contained in the black liquor. Smelt pro-duced in the soda recovery boiler is then dissolvedin water or in dilute white liquor, after which-it is treated in a known manner by causticizing it chemi-cally, in conseguence of which the final result of the whole process is a new alkaline cooking liquor.
This process is fully known per se and it is there-fore not described here in greater detail.
The brown cellulose pulp obtained is then bleached in a known manner by means of various chlo-rine compounds, which is also fully known per se and obvious to one skilled in the art and it is not de-... .. ~ .
.: : . : . , , ......................................... :
.. - ., .. . .. , . . , : .
2~3~98 scribed as such in greater detail.
Among cooking and bleaching chemicals, as well as among chemicals used in a tall oil plant there are also sulphurous substances, such as sulphuric acid, which is used both for preparing chlorine dioxide for bleaching and for splitting tall oil in the tall oil plant. The sulphur passes through the process and ty-pically tends to concentrate in the alkaline cooking liquor, which is rather harmful, and attempts are made to prevent that in different ways. Similarly, so-called odour gases are produced at different stages of the process, which gases contain sulphur as reduced gaseous compounds, which causes deposits of sulphur and odour nuisances in the environment. At-tempts have been made to eliminate the odour gasesfor instance by collecting the gases and by burning them either in existing boilers, in a lime kiln or in a separate combustion plant for odour gases. When using a separate combustion plant, the energy con-~0 tained in the gases is recovered by means of a waste-heat boiler and flue gases are scrubbed by a scrub-ber, which is typically a NaOH scrubber. In the scrubber, SOz gases contained in the flue ~ases pro-duce sulphate and sulphite salts, which pass together with white liquor to a pulp treatment plant and are thus recycled. Numerous methods have been presented for desulphuration of the process. Finnish Patent 67243, for instance, discloses a method in which a part of the fresh liquor is circulated as a parallel desulphuration cycle in the cellulose cooking pro-cess, whereby the liquor is carbonated and the hydro-gen sulphide produced is separated by stripping, after which it is burned and prepared into sulphuric acid by means of an aqueous solution. This equipment is very complicated and an expensive lnvestment.
~ , ' ' . . - .
.: , .
.. . . .
' - ' ' : ' " ~ :
:
: -2~3~98 The object of the present invention is to pro-vide such a method of separating sulphur from the process of a sulphate cellulose plant by means of which method the amount of sulphur can be regulated in a simple and easy manner, due to which the sulphur separated can be recycled in order to replace a sepa-rate sulphuric acid needed for the bleaching stage and for tall oil cooking, while the sulphur content of the liquor can be maintained on a desired level.
10 The method of the invention is characterized in that the gaseous sulphur compounds are recovered as odour gases produced at different stages of the cel-lulose manufacturing process and separable under the influence of physical conditions only and the sul-phuric acid is prepared from the odour gases by oxi-dating them and by dissolving thus formed sulphur trioxide in a liquid mainly consisting of water.
The substantial idea of the invention is that the sulphur is recovered as odour gases produced at the various stages of the process, which gases are separated from the process and oxidated in such a way that substantially all sulphur in the odour gases is changed into sulphur trioxide, after which sulphuric acid is prepared from the sulphur trioxide in a man-ner known per se, which sulphuric acid again is re-cycled to the process, to the stage of preparing chlorine dioxide or of cooking tall oil. In this way it is possible to replace a great deal of the sul-phuric acid needed by the plant by the sulphuric acid obtained, while the environmental drawbacks are re-duced substantially. The definition in claim 1, viz.
"as odour gases separable under the influence of phy-sical conditions only", means that the odour gases are separated from the solutions of the process on the basis of the temperature and other physical con-. . .
- . : . . . - - . :
: , : .
:. . . . . ..
. :: ~ .- , : - . . ..
-'~ ' ~,:' '' ', -'' ' :' - : ' .: ' :'' ,. , '' - : - :, , ,-... . : - . :
, ~: . . .: . ..
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ditions without a separate treatm~nt carried out by chemical substances, the main purpose of which treat-ment is only to prepare sulphurous gases and to sepa-rate them from the process solution at some stage of the cellulose process. Moreover, ~he sulphur content of the liquor can be maintained sufficiently low by means of the method of the in~ention, due to which the drawbacks in the use and the service life of the equipment caused by the sulphur can be largely avoid-ed. The method of the invention especially improvesthe economical profitability of the cellulose plant, because distinct savings are achieved by this method in chemical costs.
The invention will be described in greater de-tail by way of example by means of the enclosed draw-ings, in which Figure 1 shows schematically a cellulose cook-ing and recovery process and a method according to the invention, 2~ Figure 2 shows schematically one manner of pre-paring sulphuric acid in the method of the invention and Figure 3 shows schematically another manner of preparing sulphuric acid in the method of the inven-tion.
Figure l shows a cellulose cooking process con-ventional per se, in which process wood 1 to be cook-ed is brought into a digester 2. In the digester it is cooked together with alkaline cooking liquor into cellulose pulp, after which the spent alkaline cook-ing liquor, i.e. the black liquor, is washed off from the cellulose in a washing department 3 and the wash-ed cellulose pulp is brought into a bleaching plant 40 The black liquor is led further to an evaporation plant 5 and 6, where water is separated from it un-, - . . : .
, . ' 21D~3Q9~
til it is finally concentrated to a dry content as high as possible. Also a separation of soap takes place in a tank 16 before the evaporation 5. The soap obtained is passed to a tall oil plant 13. At present it is known that the alkaline liquor is at the ini-tial stage 5 at first concentrated under atmospheric pressure, whereby it can be heated with steam at a low temperature, after which it is further concentra-ted 6 under a pressure higher than atmospheric pres-sure and by heating directly or indirectly with a steam under a high pressure and at a high temperatu-t re, as a conse~uence of which the final result is alkaline liquor with a dry content of up till 75-80 %
under such a pressure and at such a temperature that it can b~ displaced further by means of a pump and fed into a soda recovery boiler 7. After the normal evaporation plant 5 the figure shows a concentration stage 6 of this kind, from which the liquor is fed into a separate separating reactor ~, in which steam and various sulphurous odour gases, such as H2S, CH3HS, ~CH3)2S, (CH3)2S2 etc., are still separated from it when the pressure falls slightly. Sulphurous odour gases are produced both in the digester 2, in the evaporation plant 5 and in the sulphur separating re-actor 8 after the concentration as well as at the washing stage 3 of the liquor. All these are passed to an oxidation stage 9 of odour gases, in which the sulphur compounds are oxidated in such a way that the final result is sulphur trioxide, which is scrubbed free from flue gases into sulphuric acid at a scrub-bing stage 10. Finally, the flue gases are still scrubbed at another scrubbing stage 11 with an alka-line solution, e.g. sodium hydroxide, with which slight residues of sulphur produce a solution con-taining sodium sulphite~ sodium bisulphite and sodium 21D93~98 sulphate, which solution is fed into the desulphura- -tion reactor 8 or a dissolving tank of the soda re-covery boiler, and after this mainly carbon dioxide goes into a chimney. The sulphuric acid produced is concentrated at a concentration stage 12, whereby the impure dilute sulphuric acid can be led to the tall oil cooking plant 13 and the strong sulphuric acid is led to a preparation 14 of bleaching chemicals to re-place the sulphuric acid to be purchased and brought from outside. The waste acid produced at the prepara-tion 14 of bleaching chemicals is also led to the tall oil plant 13, from whicH is as waste obtained sodium sulphate, which is led together with the black liquor to the evaporation plant 5. The washed cellu-lose pulp is led to the cellulose pulp bleachingplant 4, into which the produced bleaching chemicals, i.e. mainly chlorine dioxide, are led. Respectively, the steam obtained from the sulphur separating reac-tor 8 can be led to the last concentration stage 6 for heating the black liquor. From the sulphur sepa-rating reactor the black liquor is led into the soda recovery boiler 7, from which the green liquor pro-duced by smelt and water is passed to a causticizing stage 15 and after the causticizing further to the digester 2 as the alkaline cooking liquor.
The most preferable way of preparing sulphuric acid is to use a catalyst. In the method according to Figure 2, the odour gases are burned in a waste-heat boiler 21, after which the produced gases containing S02 are oxidated into S03 gases in a catalyst reactor 22 by means of a catalyst. It is possible to provide steam in the waste-heat boiler by combustion, which steam can be used suitably at various stages of the cooking process. As a catalyst can be used for in-stance vanadium pentaoxide (V20s) or other catalyst 2~3~9~
suitable for the purpose and known per se. After thecatalyst reactor 22 the gases are led to a scrubber 23, in which a two-stage scrubbing takes place. At the first stage the gases are scrubbed either with water or a dilute H2SO4 solution, whereby the sulphur trioxide reacts and produces sulphuric acid together with water. In order to get the sulphur trioxide en-tirely removed, the remaining gases are scrubbed at the second stage with a NaOH solution, after which the gases can be led out and the produced solution containing sodium sulphide or sodium hydrogen sul-phide is removed e.g. to the dissolving tank of the soda recovery boiler. Instead of the separate waste-heat boiler 21 and the catalyst reactor 22, catalytic combustion can also be used, due to which sulphur trioxide is directly produced in the combustion cham-ber and the gases are scrubbed as described above.
The embodiment of Figure 3 shows how the flue gases of the waste-heat boiler according to the fore-going example are changed into sulphuric acid byusing an active carbon reactor known per se instead of a conventional catalyst reactor. The figure does not show the waste-heat boiler but the combustion gases coming from the waste-heat boiler through a channel 31, which gases are led into separate active carbon reactor chambers 32. The gases are led below an active carbon layer 33, from which they are con-veyed through the active carbon layer 33 and go fur-ther into the chimney through a channel 34. Washing li~uid, mainly water, is brought above the active carbon layer through a channel 35 and fed by pumps 36 above the active carbon layer 33. The gases contain-ing SO2 are oxidated in the reactor 32 under the in-fluence of the active carbon layer 33, due to which the produced S03 iS combined with the washing liquid, . .
.
.
2 ~ 8 i.e. water, which flows through the active carbon bed, and the sulphuric acid goes through a channel 37 into a collecting tank 38, from which it is pumped by a pump 39 to be further concentrated according to the diagram presented in Figure 1 and to be further re-used.
An example of process savings to be achieved according to the method of the invention is described in the following. The volume flow of the odour gases produced in the process is about 5 Nm3/second, the SO2 content being about 3000 ppm. The mass flow of sul-phur dioxide is 42,9 g/sec. and about 90 ~ of the sulphur dioxide can be oxidated into sulphur trioxide by means of a catalyst. 30,9 g sulphur trio~ide is then obtained per second and, washed into sulphuric acid, 33,5 g sulphuric acid is obtained per second.
When an odour gas plant is used 8000 hours per year, the final result can be 1022 tons of sulphuric acid.
2050 tons of sulphuric acid is consumed in the tall oil plant per year, whereby about half of the sulphuric acid needed is obtained from odour gases.
Then the sulphuric acid to be brought from outside constitutes only 50 ~ of what would be needed normal-ly. Further, the NaOH scrubbing required after the odour gas plant consumes considerably less chemicals, because most of the sulphur has been removed before the scrubbing stage and hardly any reaction products demanding a removal of the scrubber liquid are then produced, but the scrubber liquid can be recycled for a considerably longer time than previously.
The method of the invention can still be inten-sified considerably in such a way that sulphurous gases are separately removed from the finally concen-trated black liquor. This can be carried out by heat-ing it to a temperature higher than the cooking tem-, 2~@~8 perature and by keeping it there for a sufficient time, whereby almost all the sulphur is separated as sulphurous odour gases with water vapour from the alkaline liquor when the liquor is permitted to ex-pand to a lower pressure, and thus only a very insig-nificant amount of sulphur comes into the soda recov-ery boiler. This procedure can be carried out for in-stance in connection with the last concentration stage, when the liquor is kept at a sufficiently high temperature during the last concentration stage and it is then permitted to expand. Above in the specifi-cation a~d the drawings, the invention has been de-scribed by way of example and it is by no means re-stricted to it. By pplying the method of the inven-tion by means of the above procedure, it is possibleto provide an almost closed sulphur circulation, whereby it is hardly necessary to add sulphuric acid to the process at all. Especially by separating sul-phur from concentrated black liquor in the above man-ner, the greatest part of the sulphuric acid neededfor the process can be obtained by preparing it from the sulphur separated, and thus the sulphur emissions and the drawbacks caused by them can be greatly mini-mized.
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Claims (13)
1. A method of recovering and reusing sulphur in a sulphate cellulose plant, in which method sul-phur existing in liquor cycle is separated from it as gaseous compounds and oxidated into sulphur trioxide, after which sulphuric acid is prepared from the sul-phur trioxide and the sulphuric acid obtained is re-used in the cellulose manufacturing process, wherein the gaseous sulphur compounds are recovered as odour gases produced at different stages of the cellulose manufacturing process and separable under the influ-ence of physical conditions only and the sulphuric acid is prepared from the odour gases by oxidating them and by dissolving thus formed sulphur trioxide in a liquid mainly consisting of water.
2. A method according to claim 1, wherein the liquid used for dissolving is dilute sulphuric acid.
3. A method according to claim 1, wherein, for intensifying the separability of the odour gases, the black liquor is heat treated during its concentration process by keeping the black liquor at a temperature higher than the cooking temperature for a predeter-mined delay time and by allowing the liquor expand after this in such a way that the sulphurous gases separated from it are released to be recovered.
A. A method according to claim 3, wherein the black liquor is heat treated in the last concentra-tion stage.
5. A method according to claim 3, wherein the black liquor is heat treated after the last concen-tration stage.
6. A method according to claim 1, wherein flue gas containing sulphur dioxide is prepared from the odour gases by burning the odour gases in a waste-heat boiler and by changing the sulphur dioxide ob-tained in this way through further oxidation into sulphur trioxide.
7. A method according to claim 6, wherein the sulphur dioxide is changed into sulphur trioxide by leading it through an active carbon layer and the sulphur trioxide is changed into sulphuric acid by leading water through said active carbon layer in a direction opposite to that of the gas flow.
8. A method according to claim 6, wherein the gases containing sulphur trioxide are led to a two-stage wash, at the first stage of which water or sul-phuric acid is fed to the gases in order to change the sulphur trioxide into sulphuric acid and at the second scrubbing stage of which the gases are scrub-bed with sodium hydroxide in order to bind the re-maining sulphur.
9. A method according to claim 6, wherein the sulphur dioxide is changed into sulphur trioxide in a separate catalyst chamber.
10. A method according to claim 6, wherein sul-phur trioxide is prepared from odour gases by means of catalytic combustion.
11. A method of recovering and reusing sulphur in a sulphate cellulose plant, in which method sul-phur existing in liquor cycle is separated from it as odour gases produced at different stages of the cel-lulose manufacturing process and separable under the influence of physical conditions only, whereby the black liquor is heat treated during its concentration process by keeping the black liquor at a temperature higher than the cooking temperature for a predeter-mined delay time and by allowing the liquor expand after this in such a way that the sulphurous gases separated from it are released for being recovered, and the sulphuric acid is prepared from the odour gases by oxidating them into sulphur trioxide, after which sulphuric acid is prepared from the sulphur trioxide by dissolving it in a liquid mainly consist-ing of water, and the sulphuric acid obtained is used in the cellulose manufacturing process.
12. A method according to claim 11, wherein the black liquor is heat treated in the last concentra-tion stage.
13. A method according to claim 11, wherein the black liquor is heat treated after the last concen-tration stage.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FI921445A FI94266B (en) | 1992-04-01 | 1992-04-01 | Process for recycling and reusing sulfide in a sulfate cellulose plant |
| FI921445 | 1992-04-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2093098A1 true CA2093098A1 (en) | 1993-10-02 |
Family
ID=8535024
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2093098 Abandoned CA2093098A1 (en) | 1992-04-01 | 1993-03-31 | A method of recovering and reusing sulphur in a sulphate cellulose plant |
Country Status (6)
| Country | Link |
|---|---|
| CA (1) | CA2093098A1 (en) |
| ES (1) | ES2080643B1 (en) |
| FI (1) | FI94266B (en) |
| FR (1) | FR2690934B1 (en) |
| PT (1) | PT101244B (en) |
| SE (1) | SE510171C2 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE9704298L (en) * | 1997-11-24 | 1999-05-25 | Kvaerner Pulping Tech | Procedure for bleaching cellulose in a closed system using Caro's acid |
| FI127393B (en) | 2016-09-07 | 2018-05-15 | Valmet Technologies Oy | A system and a method for producing aqueous sulphuric acid |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4098639A (en) * | 1975-06-17 | 1978-07-04 | Mo Och Domsjo Aktiebolag | Process for reducing the requirement of fresh chemicals without increasing emissions in the pulping of cellulosic material |
| SE411772B (en) * | 1978-04-07 | 1980-02-04 | Sca Development Ab | SET TO REDUCE EMISSIONS TO RECIPIENT AND ATMOSPHERES IN CONNECTION OF CELLULOSIC MATERIAL |
| JPS5513141A (en) * | 1978-07-17 | 1980-01-30 | Toshiba Corp | Treatment of offensive odor gas |
| SE442523B (en) * | 1981-06-22 | 1986-01-13 | Mo Och Domsjoe Ab | PROCEDURE FOR PREVENTING EMISSIONS OF SULFUR INHALING GASES IN THE PREPARATION OF CHEMICAL CELLULOSAMASSA |
-
1992
- 1992-04-01 FI FI921445A patent/FI94266B/en active
-
1993
- 1993-03-31 CA CA 2093098 patent/CA2093098A1/en not_active Abandoned
- 1993-03-31 PT PT10124493A patent/PT101244B/en not_active IP Right Cessation
- 1993-03-31 ES ES9300663A patent/ES2080643B1/en not_active Expired - Fee Related
- 1993-03-31 SE SE9301070A patent/SE510171C2/en not_active IP Right Cessation
- 1993-04-01 FR FR9303831A patent/FR2690934B1/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| SE510171C2 (en) | 1999-04-26 |
| FI921445A0 (en) | 1992-04-01 |
| FR2690934A1 (en) | 1993-11-12 |
| FI94266B (en) | 1995-04-28 |
| FI921445A (en) | 1993-10-02 |
| SE9301070L (en) | 1993-10-02 |
| PT101244A (en) | 1994-03-31 |
| ES2080643B1 (en) | 1996-06-16 |
| PT101244B (en) | 1999-09-30 |
| ES2080643A1 (en) | 1996-02-01 |
| FR2690934B1 (en) | 1995-06-30 |
| SE9301070D0 (en) | 1993-03-31 |
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