FI56714C - SAFE VIDEO PROCESSING WITH SULFUR PROCESSING ILLALUKTANDE GASER - Google Patents

Description

Γ1 ANNOUNCEMENT rr71> l jBOrg IBJ (11> UTLÄCGNINGSSKAIFT 00/14 • Jtfxjg c (45) Patent oyönrctty 10 03 1930, ÄA2r, Patent oeddelat ^ '(51) Kv.lk. ^ lnt-Cl. * D 21 C 11 / 08 FINLAND —FINLAND (21) PM.nttlh.k.mu.-Ptnt.n.eknln! 751690 (22) H »k« rnl * pilvl - Ansttknlng «di« 06.06.75 '' (23) Alkuptlvi— Glltlghtttdag 06.06.75 (41) Become Public - Bllvlt offentllf qq p jtr

Patent and Registration Office .... ........ , , . „.

_ '(44) Date of commencement of installation and date of issue. -

Patent · and registration register Antkkan utlajd och uti.ikriftan pubtlcerad 30.11.79

(32) (33) (31) Pyyd «« y atuolkaui— Baglrd prloritat 97.06.7U

Sweden-Sverige (SE) 7 ^ 07539-l (71) Skogsägarnas Industri Aktiebolag, Skogsudden, S-355 90 Växjö,

Sweden si-Sverige (SE) (72) Nils Viktor Mannbro, Asarxun, Sweden-Sverige (SE) (7 ^) Oy Jalo Ant-Wuorinen Ab (5 ^) The method of burning malodorous gases from the sulphate process - Sätt vid förbränning av sulfatprocessens illaluktande gaser

Numerous systems for burning malodorous gases leaving sulfate-thimases have already been described in the literature. These systems usually involve the combustion of odorous gases 1) in recovery boilers 2) in lime kilns 3) in gas kilns using auxiliary fuel, e.g. gas or oil ^) in contact kilns and by catalytic conversion to, for example, elemental sulfur

The gas is introduced into the combustion at a concentration of 3) either above or below the explosive 6).

As a result, systems have been developed in which 7) the gas is diluted with combustion air 8) the undiluted gas is collected in a gas clock, etc.

2 S6714 scrubber systems are used to supplement or reserve combustion systems. It should be noted that all or part of the combustion oxidation can be replaced by washing with oxidizing agents dissolved in a scrubber, usually bleach liquor solutions containing oxidant residues such as hypochlorite or chlorine at different pH values. However, the most commonly used scrubber technique uses temporal, i.e. hydroxide-containing liquids, which can consist of 9) different alkali solutions of different concentrations. .

Malodorous gases are often referred to as non-condensable gases and this, of course, means their condensation, depending on the case, in the condensing system used in a given temperature range. Boilers and black liquor depressurization steps thus provide a mixture of gases and water vapor and, in some cases, condens turpentine and steam first. The turpentine concentration of the gases and the factors affecting the condensation of turpentine and the separation of the different condensate fractions in the gas treatment system are important factors in the design of systems with a low risk of explosion.

With respect to the treatment of gases in the condensation and / or absorption stages, their composition and properties can be classified as follows: 12) ionizable substances such as H 2 S and MeSH (methyl mercaptan) which can be absorbed into temporal lye solutions also at higher temperatures 13) compared to 12) more easily condensable gases, such as dimethyl disulfide (Me S) having a boiling point of 18 ° C and dimethyl sulfide (Me_S) having a boiling point of 38 ° C, and methanol (MeOH) having a boiling point of 65 ° C.

1¾) a combination of the above, e.g. using activated carbon absorption steps for methyl sulfides.

It is known that upon absorption into the lye, which is recycled through the digester, more dimethyl sulfides are continuously formed. However, such absorption acceptably reduces the presence of the most strongly odorous gases.

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The water condensates formed by the lye vapors of the sulphate process become malodorous and are contaminated by the gases dissolved in it. In processes using highly closed systems, other organic substances also accumulate in the condensate, and methanol is of particular interest. To purify condensate taken directly to the recipient or back to the process, steps have been included in the system to remove gases and methanol.

Very particularly suitable for this purpose is the displacement of condensate by steam, the heat content of which is utilized differently in the process. Preferably, such displacement of condensate occurs with steam that is passed between the black liquor evaporation steps (This system originally comes from a sulfite pulp process in which fermented ethanol is recovered.).

Exhaust gases cannot be released into the atmosphere, but if they are returned to the process, there will be a certain increase in indeterminate degassing at various points in the process and the gradual formation of methyl sulphides from other sulphide sulfur compounds. Thus, it is important to burn as by-products worthless gases that are formed from different sources in the process. The combustion of sulfur compounds mainly produces SO, which in the atmosphere is gradually converted to SO 2, corresponding to the acidic products formed therefrom. Thus, in the sulfate process, it is important to encapsulate the sulfur compounds so that they bind to the alkaline liquor solutions and pass through the digester as active sulfide compounds. The amount of such sulfur compounds in the process is controlled by the composition of the alkali replacement chemicals (make-up) in the process. The basis for this adjustment is obtained by analyzing the molar ratio of S / 2 Na in process liquor solutions and make-up solutions. In practice, the usual way to control the sulfur flow involves first absorbing some of the gases into a lye solution from the white liquor side and burning the rest of the gases. If the combustion takes place in a recovery boiler, alternatively in a honeycomb, the possible recovery of the burned sulfur depends on whether or not the system comprises a scrubber system for the flue gases. Gas firing in a lime kiln is often considered cheaper, but e.g. afterwards, in the next wash, sulphate ions, respectively gypsum, are formed in the system.

Due to the known drawbacks associated with direct gas combustion in special furnaces, compromise solutions for absorption and combustion systems have been proposed. The gases are then, in some cases, washed with the lye of the white liquor system. The dilute 4,56714 lye of such a white liquor system again degasses when the lye is heated in a soda melt solvent. White liquor components evaporate from digesters and evaporate from the plant.

For this reason, systems have been proposed in which the black liquor intended for the evaporation plant is alternatively oxidized with air with oxygen already during the blowing of the soup. Another system tested involves oxidizing the gases formed together with the black liquor, but all the black liquor must be oxidized to retain the sulfur compounds until the lye is combusted. Smelly oxidizing air must also be burned. Unless all of the black liquor is oxidized, sulfur compounds are again removed from evaporation to gases and condensates and accumulated in the process until a new equilibrium is reached involving various vague removals.

To avoid vague removals, it is possible to transfer the absorptide Liquor from the white liquor side to the black liquor side and lead the evaporated black liquor (concentrated liquor) to a recovery boiler. In order not only to absorb the ionizable substances in the gas mixture but also to condense the other substances in the gas mixture, a cooled alkaline liquor is preferred, but white liquor is difficult to cool without crystallization causing clogging. The consequent dilution of the concentrated liquor is thermally disadvantageous. If the white liquor itself is used, the active cooking chemicals are also lost.

Certain systems operate at a temperature of 50-60 ° C in an alkaline absorption liquor, while other systems in which mainly only ionizable and easily condensable compounds are absorbed can use the normal lye temperature for the digester's white liquor. Thus, there are particular problems with the use of white liquor to be passed to the cooking stages, and with the prevention of crystallization of white liquor on cooling surfaces, etc. Low temperature systems therefore prefer to use dilute lye into which molten soda can be dissolved - but odor already begins in soda solvent.

The present invention provides a solution to the following problems: a) selection of an absorption liquid b) removal of sulfur dioxide from gas combustion c) gypsum formation in a lime kiln d) re-evaporation of absorbed gases e) utilization of absorbed sulfur compounds as cooking chemicals 5 56714 f) back to absorption liquid

According to the invention, the sulfur compounds are absorbed in a small part of the black liquor, preferably in a partially evaporated so-called pre-evaporated black liquor, and the black liquor with the sulfur compounds absorbed therein is continuously removed from the black liquor recycled from the absorption step. The volatile sulfur compounds are thus passed past the remaining evaporation steps and mixed with the resulting concentrated liquor. Pre-evaporation removes volatile substances, including sulfur compounds, from the total amount of black liquor. The substances are mainly ionizable in nature, acidic and can therefore be collected in a small part of the pre-evaporated black liquor, which has a higher hydroxide content than before evaporation. The sulfur compounds in the absorption oil are burned together with the concentrated lye in a recovery boiler. In the soda boiler and its flue gas chemical recovery system, the absorbed sulfur compounds are collected and regenerated into sodium sulfide useful for cooking as efficiently as other sulfur compounds in black liquor.

The alkali-washed gas is passed to the oxidation step. Of the known oxidation steps, combustion in a furnace is suitable for the purposes of the invention, in which case combustion in existing furnaces, i.e. lime kilns and / or recovery boilers, is most advantageous. The gases are preferably burned in lime kilns, as these have burners for natural gas, generator gas, e.g. from wood waste, alternatively for oil. Odor gas also brings a small addition of fuel to the lime recovery combustion.

Black liquor oxidation is effective in preventing sulfur compounds from entering gases and condensates, but it does not affect other volatile compounds. One of the disadvantages of black liquor evaporation is the increased formation of scale on the thermal surfaces of the evaporator and the development of oxidative heat before incineration. Therefore, according to the invention, it is of particular interest to oxidize only the black liquor used in the absorption tower. Since Na ^ S forms e.g. NaOH through oxidation, the supply of black liquor to the lye recycling system of the absorption tower can be reduced. In this case, it is also possible to absorb the sulfur compounds in the scrubber tower and in the parallel tower to oxidize the circulating absorption liquor. Depending on the gas concentration, the gas and the oxidation air are alternatively fed separately, mixed with the final oxidation and combustion stage, respectively.

The pre-oxidation, which increases the alkalinity of the absorption liquor, makes it possible to reduce the amount of pre-evaporated black liquor transported past the final evaporation. It follows that the dilution of the concentrated liquor with the pre-evaporated black liquor is reduced. The supply point of the absorption liquor to the system is otherwise a critical part in known gas treatment systems. The reason for this is that if the absorption liquor is introduced into one of the liquors before causticization, boiling, respectively, evaporation, odor formation and condensation contamination start again.

The temperature of the black liquor in the absorption step affects the composition of the exhaust gas, because non-ionizable sulfur compounds can be condensed in the black liquor to a certain extent at a sufficiently low temperature. An important novelty according to the invention is to transport the sulfur compounds included in the black liquor to the recovery boiler without the liquor being heated or evaporated in between. Otherwise, the sulfur compounds would evaporate along the way.

In order to reduce the need for absorption liquor and thus the dilution of concentrated liquor in the application of the invention, a gas having an initial concentration is introduced into the absorption step. The gas jets for blowing intermittent soups can be smoothed by means of a Vaporsphere known in the art or a gas clock which collects the gas without the air mixing with the poppy. The smoothed gas stream, like other relatively steady gas streams, for example from a depressurization or turpentine condensing system in kitchens, is passed to the absorption stage. As far as turpentine compaction is concerned, it is known in practice that even after absorption at low temperature, a condensate containing turpentine can form in the gas line to the combustion. N, k, demisterie and separation devices are thus needed in such a system.

Gases which are not condensed in the lye vapor systems of evaporation plants are often absorbed in the Venemark scrubber, which preferably operates with dilute lye, alternatively with sodium lye, in plants using such make-up *. Another cut-off for the treatment of gases from malodorous surface capacitors is the Delary system, in which the sealing fluid of the vacuum system either consists of dilute lye or is made alkaline with white liquor, alternatively sodium hydroxide. The sealing liquid can then be conveyed to the causticizing plant, respectively, in a white liquor tank instead of being discharged as a malodorous condensate.

According to a third system, Rosenlew / Billerud, the fragrances are enriched as a sealing liquid in the recirculating condensate. The overflow of condensate is led to a stripper, while the gases are removed through a white liquor scrubber. These three systems are described in the article by Rönnholm * (Papper och trä, 1973, n ° · 11, page 715).

It has been found that pre-evaporated, preferably also oxidized, iron liquor can be used according to the invention in such absorption systems and in this way malodorous and contaminated substances are transferred from the evaporation of the absorption liquor directly to the combustion of the concentrated liquor. The condensation of water in black liquor solutions can be limited so that the resulting dilution of the concentrated liquor becomes tolerable, alternatively it can be completely avoided.

The black liquor formed is oxidized in its entirety before and / or after evaporation in many plants, and in particular when the black liquor is evaporated directly by the flue gas west. An interesting embodiment is the heating of the air with a Ljungström preheater, after which the air is used for the simultaneous oxidation and final evaporation of the black liquor. It is obvious that this invention is of particular importance in the context of direct evaporation. The black liquor from the absorption step can then be mixed with the concentrated liquor to be oxidized, but preferably directly evaporated! with lye, which is led to combustion in a recovery boiler.

According to the invention, the sulfur compounds absorbed by the black liquor are introduced into a recovery boiler and SO can be separated from the flue gases in different ways.

A

purposes. These include, for example, delignification chemicals in connection with cross-recovery, the production of bleaching chemicals and the cleavage of tall oil. The collection of the combustion product, sulfur dioxide, in one treatment step, as in the present invention, is thus very useful.

To control the sulfur balance of the sulphate process, cross-recovery can be arranged by absorbing SO 2 from recovery boiler flue gases into a lye from which sodium sulphite cooking liquor is prepared, eg for semi-chemical neutral sulphite cooking (N. Mannbro, Svensk Papperstidning, 1955 »page 573)

The hydrogen sulphide contained in the flue gas leaving the black liquor can be absorbed into a bisulphite-containing scrubber liquid, e.g. sulphite waste liquor, alternatively into a digestion liquid (N. Mannbro, Svensk Papperstidning, 1963, page 3 * 0 ·

By introducing sodium sulphite liquors into the black liquor of sulphite soup, cross-recovery or soap digestion, black liquor oxidation according to the invention can be enhanced with lower air or oxygen consumption than without sulphite. Namely, sodium sulphite leads to an increase in NaOH and NagSgO 2. by reacting with the polysulfide formed as an intermediate during oxidation, increasing the ability of the black liquor to absorb hydrogen sulfide and methyl mercaptan.

Since, according to the invention, only a small part of the black liquor is oxidized, it is possible to oxidize with oxygen gas, and in particular when it is available on site to a greater extent for pulp bleaching and / or waste water treatment. The oxidation takes place in a closed system, for example by injecting oxygen gas into a mixture of black liquor and odorous gas. The foam can be separated from the unreacted gas by passing it below the surface of the concentrated liquor. It has been shown that oxidation under these conditions shifts so much towards dime-style sulfides that relatively advanced odor removal can be achieved. It is possible to ventilate the gaseous waste out of the lye tank by means of a conventional ventilation system as an alternative to sucking the combustion air for the various furnaces that way.

Depending on the distance between the different process steps, it may be advantageous to distribute the black liquor to separate absorption plants, e.g. for cooking and evaporation. The collection of several absorption liquors for combustion with fortified lye must be arranged e.g. according to the gas cleaning system of the recovery boilers and according to whether the differentiation has been performed with a higher or lower melt sulfidity capacity compared to the operating recovery boilers.

For continuous cooking, there are systems in which black liquor removal or pulp washing is performed during cooking, in which case the heat content of the black liquor removed from the digester is utilized for suction-alkali evaporation. According to the method described in our SW patent application 9915/69, vapors and gases are first allowed to leave the black alder in the steam system of a high-power evaporator. According to the invention, the non-condensable gases dissolved therein are absorbed in a part of the partially evaporated black liquor. When the evaporation condensate is displaced by steam, either in a separate system or in a steam stream between the evaporation stages, it is advantageous to feed the displaced gases back to the concentrated liquor. The gases already contained in the condensate are condensed again into black liquor of suitable temperature. In conventional systems, odor gases are transported around the process with the white liquor-side absorptiolipeiden when the other hand, is burned in the recovery boiler lipeäkomponenttina in accordance with the present invention.

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Organic substances in black liquor significantly inhibit the crystallization of sodium salts when low-temperature black liquor, for example after air oxidation, is used to condense odorous gases.

The accompanying drawing shows a schematic diagram of a sulphate plant for intermittent pulp cooking.

It comprises, inter alia, a digester 1, a blow tank 2, a blow steam accumulator 3 for recovering heat from various alkali vapor condensates and blow steam condensers 3a, Vaporsphere 4 as a gas buffer for non-condensable gases in 7 alternatively - or as a reserve - for oxidation with 8 active chlorine-containing bleach circulating water. The black liquor and pulp are separated in the washing plant 9, after which the thin liquor 9a from the washing plant in the black liquor tanks and soap separation plants system 10 is mixed with the already evaporated black liquor to give a mixed liquor 10a with a dry concentration of 20-25% Thus, a middle liquor 12a with a dry content of 30-40% is obtained.

The middle liquor is passed to a final evaporation 13 from which concentrated liquor 14 with a dry content of 60-65% is removed. It is mixed with make-up, after which the concentrated lye is passed via line 15b to a recovery boiler 16 where it - together with the absorbed gas - is burned.

The lump gases 17 are fed to a purification and recovery system 18 where heat, alkali (Na 2 SO 4) and preferably also SO 2, etc. are recovered. The purified flue gas is released to the atmosphere at point 19. The soda melt 20 is dissolved in hot dilute liquor in the white liquor system 21 and, while significant steam removal occurs, gases are removed through the steam pipe 22, which also symbolizes vague odor removal from the entire causticizing plant 21. The white liquor 23 conveys bound and volatile odorous sulfur compounds and turpentine are removed from the digester through the degassing pipe 1a to a condensing and turpentine separation plant 24, after which gas from here and from the blowing gas system 4 is led to 25 alkaline scrubbers / 10 56714 impregnated with sulfur compounds, line 2? was transported back to the white liquor system 21. The odors then spread throughout the white liquor system and a substantial portion penetrated the atmosphere through the vapor tube 22 and some accumulated in equilibrium with the odor distribution by recirculation in line 1a. The liquor in line 28 (not shown) in the evaporator at evaporator 11 contributed to the spread of odor. 20.

Example 1

When carrying out the process according to the invention, the washing liquid of the allca lysrubber 5 was changed to black liquor. Thus, middle liquor 12b (absorption liquor) was added, which was oxidized and cooled in oxidation tower 30 with air 30a, which after use 30b was conveyed to lime kiln 7. Barely 2% of middle liquor 12a was fed to oxidation tower 30. The oxidized liquor 12c was passed to the circulating absorption liquor 5a and the upper stream 5b was mixed with 15 concentrated lye 1¾ and burned in a recovery boiler 16. The same treatment was also performed for different gases 11a from evaporator 11 and its equipment during the final condensation phase.

According to the example, the absorbed odors could be completely removed from the white liquor system. These, on the other hand, were recovered useful in Na 2 Srna soda melt 20 after the gaseous sulfur combustion products were gradually returned from the flue gas cleaning plant 18 before being discharged into the atmosphere 19. needed to be transported as combustion air to boilers and furnaces.

The drawing does not show devices well known to those skilled in the art for drip separation from gas pipelines, control devices or gas analyzers, for example for incoming air, nor special safety devices for preventing gas explosions, such as gas transport by steam ejectors instead of fans, etc.

Depending on the quality of the wood, in addition to methyl sulphides, varying amounts of turpentine, methanol, furfural, acetic acid, formaldehyde, etc. are obtained in the condensation steps. It is known to fractally target

Claims (15)

1. Methods to reduce the emission of gases or vapors containing odorless sulfur compounds and other volatile substances, which gases or vapors, respectively. vapors are emitted during the sulphate pulping process, including black liquor recovery (9, 10) and evaporation (11) with final evaporation step for thick liquor (14), which is supplied to the liquor incinerator (16), respectively. its smoke treatment system (18), while said gases (11a, 25) are preferably treated at possible sustained concentration in two or more steps (5, 6), whereby they are primarily scrubbed (5) with an alkaline liquor (5a) for absorption of a large portion of the sulfur compounds, after which the unabsorbed gas residue (5c) can be emitted but suitably subjected to a secondary odor eliminating step (6), e.g. by absorption (8), as on active koi (possibly lignin carbon), resp. by oxidation with gaseous or dissolved oxidizing agents and preferably oxidation to sulfur and sulfur, respectively. combustion (7) to sulfur oxides, characterized in that the alkaline absorption liquor (5a) of the primary gas treatment step (5) consists of a smaller part of the black liquor (12b), preferably a portion which is based on the amount of black liquor (9a) the evaporation plant is less than 5% and preferably about 0.5% or, as calculated alternatively, on the 1/1 56714 partially evaporated black liquor (12a) is about 1%, and that absorbed substances are combusted to make them as black liquor components (5b, 15b) is fed to the liquor incinerator (16) without being evaporated in the final evaporation plant (13) for other black liquor (12a) and said that the sulfur from the absorbed gases is substantially directly recovered in the form of sodium compound in the solid combustion residue or soda smelting (20). with smoke (17) is released to the atmosphere (19). 2. A method according to claim 1, wherein the absorption liquor (5b) is mixed (15) with thick liquor (14) and passed via (15b) to the liquor combustion furnace (16), characterized in that the increase in the water content of the thick liquor (15b), calculated as a percentage of its resp. the total weight of the mixture, is limited in that it does not detrimentally reduce the heat release from the thick liquor combustion (16) and preferably to a water increase less than 2%, preferably to 1% or less, and that the thick liquor (15b) absorbed with absorbed sulfur compounds is subjected to combustion. ie total oxidation of organic matter, alternatively divided by oxygen oxidation of the sulfur compounds, followed by combustion (16). 3. A process according to claim 1 or 2, characterized in that the black liquor (15a) used for the absorption is prepared in whole or in part by black liquor, whose alkali concentration is increased by evaporation (12) during the simultaneous evaporation of volatile acidic substances (11a). .ex. hydrogen sulphide, which substances are enriched in the absorption liquor (5b) and incinerated (16) together with it. 4. A process according to any of claims 1-3, characterized in that the absorbent liquor (5a) is prepared from black liquor by oxidizing (30) its contents of sodium sulfide with oxygen or oxygen-containing gas such as air (30a) in order to partly to increase the concentration of sodium hydroxide and partly to cool black liquor to improve its absorption properties. 5. A process according to any one or more of claims 1-3, characterized in that all black liquor is oxidized with oxygen or oxygen-containing gas either before (10a) or after partial evaporation (12). 6. A process according to any of claims 1-3, characterized in that a mixture (15b) of absorption liquor (5b) and thick liquor (14) is oxidized with oxygen or oxygen-containing gas. 7. A method according to any one of claims 1-3, characterized in that the absorption liquids (5a) and 16 56/14 gases absorbed via the overflow (5b) are fed to a closed oxidation system with the supply of oxygen and that the liquids are absorbed from the gases. as an oxidation product, further bonded, that is, non-ether strippable sulfur compounds, are mixed with other black liquor (12a) either for common final evaporation (13) or thick liquor obtained therefrom (14). 8. A method according to any one or more of claims 1-3, characterized in that black liquor (5b) from the absorption stage (5) is oxidized and subjected to separate evaporation. 9. A method according to any one or more of claims 4-8, characterized in that the absorption liquor, before (12b) or after the absorption of the sulfur compounds (5b), is oxidized in a closed system with concentrated oxygen without dilution with air of the gas atom (5c). , which leads to secondary odor elimination (6). 10. A process according to nigot or some of claims 4-9, characterized in that the oxygen consumption (30a) of the black liquor oxidation (30) is reduced by being added with sodium sulfite, preferably depleted from neutral sulfite boiling and / or free sip digestion. -hallen bisulfite- resp. sulphite-containing flushing liquid. 11. A method according to nigot or nigra of claims 4-9, characterized in that, in the case of black liquor contact (30) with foam or air formed, possibly side foam used for gas absorption (5), is eliminated by the foam being lowered below the surface of thick liquor (15). 12. A method according to nigot or nigra of claims 1-3, characterized in that the secondary treatment (6) of the residual gas (5c) with oxygen in the black liquor (5) is secondary to the absorbable sulfur compounds (5c). air or other oxygen-containing gas mixture and possibly together with other fuels, e.g. lignin products * bark, generator gas, oil etc. 13. A method according to claim 1, characterized in that gas combustion takes place in the black liquor (16) and / or mesa incinerators (7) of the process and that sulfur (20, 21), as a solid combustion product, is added to free combustion products. the absorbent closure (5b, 15b). 14. A process according to claim 1, characterized in that the secondary treatment of gas components (5c) which are not absorbable in alkali is done with oxidizing bleaching agent (8b), preferably by scrubbing (8) with bleaching liquor containing residues of oxidizing chlorine compounds, so-called. active chlorine, resp. residues of acid-based bleach.