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
  • D21C11/06—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
  • D21C11/08—Deodorisation ; Elimination of malodorous compounds, e.g. sulfur compounds such as hydrogen sulfide or mercaptans, from gas streams
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/48—Sulfur compounds
  • B01D53/52—Hydrogen sulfide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/64—Thiosulfates; Dithionites; Polythionates
  • C01B17/66—Dithionites or hydrosulfites (S2O42-)
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/10—Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working

Definitions

• This invention relates to a method and a apparatus for absorb ⁇ ing hydrogen sulphide, more specifically a method and an apparatus for selectively removing, by liquid absorption, hydrogen sulphide from a gas containing both hydrogen sulphide and carbon dioxide.
• hydrogen sulphide can be removed from hydrogen-sulphide-containing gases by being absorbed in an alkaline aqueous solution (i.e. an aqueous solution having a pH > 7), such as sodium hydroxide, or by using ethanolamine, such as monoethanol- amine and diethanolamine.
• Absorption may, for instance, be used for producing hydrogen sulphide in pure form, which optionally is further processed to sulphur in a Claus process.
• the gas contains carbon dioxide in addition to hydrogen sulphide
• the carbon dioxide will also be absorbed in the alkaline solution.
• Carbon dioxide has approximate ⁇ ly the same solubility in water as hydrogen sulphide, and the carbon dioxide will therefore compete with the hydrogen sulphide for being absorbed in the solution.
• Hydrogen sulphide and carbon dioxide are absorbed in an alkaline aqueous solution of e.g. sodium hydroxide in accordance with the formulae below:
• the selectivity to hydrogen sulphide i.e. the ratio of mole of absorbed hydrogen sulphide to mole of absorbed (hydrogen sulphide + carbon dioxide), is directly proportional to the contents of hydrogen sulphide and carbon dioxide in the gas.
• the competition on the part of carbon dioxide is especially pronounced when the gas contains more carbon dioxide than hydrogen sulphide, as is mostly the case.
• a gas contains, say, 1% by volume of hydrogen sulphide and 10% by volume of carbon dioxide and efforts are made to absorb the hydrogen sul ⁇ phide in a sodium hydroxide solution
• the selectivity to hydrogen sul ⁇ phide is but 10%, i.e. 90% of the gas absorbed consists of carbon phide is but 10%, i.e. 90% of the gas absorbed consists of carbon dioxide, which means that as much as 90% of the sodium hydroxide is spent in absorbing carbon dioxide.
• the selectivity for the absorption of hydrogen sulphide can be augmented to about 30-50%.
• the reactions taking place during such an absorption can generally be rendered as follows:
• the absorption solution is a carbonate solution
• the hydrogen sulphide is absorbed almost instantaneously, whereas the carbon dioxide reacts only slowly with the carbonate ions to form hydrogen carbonate ions.
• the black liquor is combusted in a soda recovery unit, resulting in the generation of steam and the formation of a melt chiefly consisting of sodium car ⁇ bonate and sodium sulphide.
• the melt is then dissolved in water and causticised, so that the sodium carbonate is converted to sodium hydroxide and white liquor is obtained, which may then again be used for digesting wood.
• efforts have in recent years been made to develop new processes for the combustion of black liquor, in which no melt is formed.
• black-liquor evaporation Such processes can be collectively referred to as "black-liquor evaporation", and one instance thereof is the so-called SCA-Billerud process (E. Hornstedt and J. Gomy, Paper Trade Journal 158 (1974): 16, pp 32-34).
• SCA-Billerud process E. Hornstedt and J. Gomy, Paper Trade Journal 158 (1974): 16, pp 32-34.
• dust which chiefly consists of sodium carbonate and carbon
• a combustible gas which inter alia contains sulphur compounds
• this is attained by a method which is of the type stated by way of introduction and in which the gas is contacted in at least one step with a carbonate-containing alkaline solution whose pH is adjusted during the absorption by the addition of a hydroxide.
• the inventive method can be implemented with the aid of an apparatus, which is characterised in that it comprises: a container having a gas inlet and a gas outlet and containing a packing arranged in a number of successive steps; means for supplying a carbonate- containing solution to the last step, as seen in the feed direction of the gas, each step having means for supplying the carbonate-containing solution through the step countercurrently to the gas and for recycl ⁇ ing the solution over the step; conduits arranged between the steps for supplying a partial flow of the solution from one step to a preced ⁇ ing step, as seen in the feed direction of the gas; means for supplying a hydroxide to the carbonate-containing solution in at least one of the steps; and an outlet conduit from the first step, as seen in the feed direction of the gas, for discharging liquid containing hydrogen sul- phide.
• carbonate-containing alkaline solution is meant an aqueous solution containing carbonate ions (CO3 2-).
• this solution is an alkali metal carbonate solution, such as a solution of sodium carbonate, potassium carbonate or lithium carbonate.
• Sodium carbonate is especially preferred, being easily accessible as well as fairly inexpensive.
• the carbonate concentration of the solution is not critical, but conveniently is about 0.1-3 M with respect to carbonate, preferably about 1-2.5 M, and most preferred about 1.7 M.
• the carbonate-containing alkaline solution has a pH of at least about 9. pH values below about 9 result in unsa ⁇ tisfactory absorption of hydrogen sulphide, and there is even a risk that hydrogen sulphide already absorbed be released from the solu ⁇ tion.
• the pH of the solution should not be too high, since this would have an adverse effect on the absorption of hydrogen sul- phide, as compared with that of carbon dioxide.
• the pH of the solution should preferably not exceed about 12 in order that the absorption of carbon dioxide should not be too considerable.
• the pH of the solution is in the range of about 10.0-11.0, and most preferred in the range of about 10.2-10.8. If the pH of the solu- tion is adjusted within this last narrow range, optimum separation of hydrogen sulphide is obtained.
• the carbonate solution can absorb more hydrogen sulphide according to the reaction (5) above. Since the pH of the solution is adjusted by the addition of a hydroxide and maintained within the given range of about 9-12, preferably about 10.0-11.0, and most preferred about 10.2-10.8, the absorption of carbon dioxide is kept on a comparatively low level.
• the carbonate-containing alkaline solution is regenerated by the addition of a hydroxide.
• a hydroxide that does not have an adverse effect on the absorption of hydrogen sulphide and is capable of increasing the pH of the solution from the given lower limit of about 9 to the desired value, such as a value not exceeding about 12.0, preferably not exceeding about 11.0, and most preferred not exceeding about 10.8.
• hydroxides of alkali metals or alkaline-earth metals such as sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), cal ⁇ cium hydroxide (Ca(OH) 2 ), and magnesium hydroxide (Mg(OH) 2 ).
• sodium hydroxide is most preferred.
• the temperature of the liquid used in the absorption is not par- ticularly critical and may vary within a wide range, but preferably is below about 80°C, since there is a risk that the absorption of hydro ⁇ gen sulphide decreases at temperatures of about 80°C and above. It is preferred that the temperature is in the range of from room tem ⁇ perature, i.e. about 20°C, to about 80°C, more preferably about 40-70°C, and most preferred about 60-70°C.
• the absorption should be carried out in such a manner that the flow of gas and the flow of absorbing liquid are coun- tercurrent, and the flow of gas should be turbulent and the flow of liquid laminar. Moreover, the separation of hydrogen sulphide is pro ⁇ moted if the volume of absorbing liquid is large compared with the volume of gas from which hydrogen sulphide is absorbed. Such a high ratio of liquid to gas is achieved by recycling the absorbing liquid that is contacted with the hydrogen- sulphide-containing gas.
• the contact between the sulphur-containing gas and the absorbing liquid (the carbonate-containing solution) may involve one or more steps, preferably two or three steps, and most preferred three steps.
• Such multistep contact has the advantage of shortening the length of each individual step, such that the pH of the carbonate-containing solution does not have time to fall below about 9 in the individual step, while at the same time the sulphide content can be kept low in the uppermost step.
• each step has such an extent or length that the pH of the solution at the end of the step has fallen to about 10.0-10.2, the liquid being then drawn off to be regenerated by means of a hydroxide and then recycled to the step at issue.
• the removal of hydrogen sulphide in a method of the above type can be rendered more efficient, while drastically reducing the chemical consumption, if the method is divided into two stages, namely a first stage, in which the main part of the hydrogen sulphide is removed in accordance with the method described above, and a second stage, in which the remaining hydrogen sulphide is essentially removed by combustion to sulphur dioxide, which is absorbed in an alkaline solution in a wet cleaner (scrubber).
• a first stage in which the main part of the hydrogen sulphide is removed in accordance with the method described above
• a second stage in which the remaining hydrogen sulphide is essentially removed by combustion to sulphur dioxide, which is absorbed in an alkaline solution in a wet cleaner (scrubber).
• the remaining hydrogen sulphide must, how ⁇ ever, be removed. According to the invention, this takes place by combusting the hydrogen-sulphide-containing gas from the first stage, such that the hydrogen sulphide is oxidised to sulphur dioxide, whereupon the sulphur dioxide formed is absorbed in an aqueous solution, whose pH is adjusted by the addition of an alkaline solution or an alkaline substance, in a wet cleaner (scrubber).
• the present invention thus provides a method for selectively removing, by liquid adsorption, hydrogen sulphide from a gas con ⁇ taining both hydrogen sulphide and carbon dioxide, characterised in that the gas, in a first stage for removing the main part of the hydro ⁇ gen-sulphide content of the gas, is contacted in at least one step with a carbonate-containing alkaline solution whose pH is adjusted during the absorption by the addition of a hydroxide, and that the gas is then in a second stage combusted in order to convert the remaining hydro- gen sulphide to sulphur dioxide, which is absorbed in an alkali-con ⁇ taining solution.
• the invention further provides an apparatus for selectively removing, by liquid absorption, hydrogen sulphide from a gas con- taining both hydrogen sulphide and carbon dioxide, characterised in that it comprises a) a container having a gas inlet and a gas outlet and containing a packing arranged in a number of successive steps; means for sup ⁇ plying a carbonate-containing solution to the last step, as seen in the feed direction of the gas, each step having means for supplying the carbonate-containing solution through the step countercurrently to the gas and for recycling the solution over the step; conduits arranged between the steps for supplying a partial flow of the solution from one step to a preceding step, as seen in the feed direction of the gas; means for supplying a hydroxide to the carbonate-containing solution in at least one of the steps; and an outlet conduit from the first step, as seen in the feed direction of the gas, for discharging liquid con ⁇ taining hydrogen sulphide, b) a combustion device having an inlet for hydrogen
• the carbonate-containing alkaline solution used in the first stage for hydrogen- sulphide absorption is an aqueous solution containing carbonate ions, such as a solution of alkali metal carbonate, preferably sodium carbonate.
• carbonate ions such as a solution of alkali metal carbonate, preferably sodium carbonate.
• Green liquor is an aqueous solution that usually contains about 1.3-1.4 mole of Na 2 CO3 per litre, about 0.4-0.6 mole of Na 2 S per litre, and about 0.3-0.5 mole of NaOH per litre. According to the invention, it has also been found that white liquor may likewise be used in the first stage as the hydroxide for regenerating the carbonate-containing alkaline solution, when the invention is employed in the sulphate pulp industry for removing hydrogen sulphide.
• White liquor is an aqueous solution that con ⁇ tains NaOH, Na 2 S and Na 2 CO3, usually in the proportions of about 2.3-2.6 mole of NaOH per litre, about 0.4-0.6 mole of Na 2 S per litre, and about 0.25-0.4 mole of Na 2 CO3 per litre.
• white liquor as the hydroxide for regenerating the carbonate-containing alkaline solution constitutes a particular aspect of the invention. Instead of white liquor, use can be made of weak liquor or oxidised white liquor for regenerating the carbonate-containing alkaline solution.
• weak liquor an aqueous solution that contains NaOH, Na 2 S and Na 2 CO3, usually in the proportions of about 0.3-0.4 mole of NaOH per litre, about 0.05-0.08 mole of Na 2 S per litre, and about 0.05-0.07 mole of Na 2 CO3 per litre.
• green liquor, white liquor as well as weak liquor contain both carbonate and hydroxide, they may be used either separately or together, both as the carbonate-containing solution and for regenerat ⁇ ing the carbonate-containing alkaline solution.
• the combustion of the hydrogen- sulphide- containing gas takes place in known manner in a combustion device of known type, such as a lime sludge reburning kiln or a bark burning boiler when the invention is used in the sulphate pulp industry.
• a combustion device of known type, such as a lime sludge reburning kiln or a bark burning boiler when the invention is used in the sulphate pulp industry.
• an auxiliary fuel such as oil or natural gas, may be used in the com ⁇ bustion to produce satisfactory oxidation of the hydrogen sulphide to sulphur dioxide.
• the hydrogen-sulphide-containing gas is burnt, the hydrogen sulphide is, at normal combustion temperatures, completely oxidised to sulphur dioxide, and the gas thus contains only negligible amounts of hydrogen sulphide after the combustion.
• the sulphur dioxide formed should be removed, which according to the invention takes place by it being absorbed in an alkali- containing solution in a wet cleaner (scrubber).
• a wet cleaner to selectively remove, by absorption in an alkali-containing solution in a scrubber, sulphur dioxide from a gas which in addition contains carbon dioxide, is a well-known technique, which need not be described in more detail here.
• a scrubber where the absorbing alkali-containing solution is recycled to a high extent.
• the pH of the alkali-containing solution in the S0 2 scrubber is preferably in the range of about 6-8, and most preferred in the range of about 7-7.5.
• the alkali in the alkali-containing solution conveniently is a hydroxide or a carbonate of an alkali metal or an alkaline-earth metal, such as sodium hydroxide, potassium hydroxide, calcium carbonate, calcium hydroxide or magnesium hydroxide.
• Sodium hydroxide is pre ⁇ ferred, especially if the sulphur is to be recycled directly in the pro- cess, e.g. when the method is employed in the sulphate pulp industry. If so, use is conveniently made of a sodium-hydroxide-based spray scrubber, for instance of the type used by ABB Flakt Industri AB and designated the MoDo scrubber.
• the scrubber may then comprise one or more heat-recovery steps, in which cold water is sprayed towards the hot gas, so that hot water having a temperature of about 45-65°C is obtained.
• the solution leav ⁇ ing the scrubber contains sodium sulphite (Na 2 SO3), sodium hydro ⁇ gen sulphite (NaHS ⁇ 3) and sodium sulphate (Na 2 SO-4).
• Na 2 SO3 sodium sulphite
• NaHS ⁇ 3 sodium hydro ⁇ gen sulphite
• Na 2 SO-4 sodium sulphate
• this solution can be mixed with other combust ⁇ ible process liquids, e.g. the black liquor in a pulp mill.
• a combustion under reducing conditions results in sulphur in the form of hydrogen sulphide
• a combustion under oxidising conditions results in sul ⁇ phur in the form sulphur dioxide.
• the method according to the invention is conveniently so designed that its different stages are integrated in the existing equipment. This means that the combustion of the hydrogen- sulphide-containing gas takes place in an existing bark burning boiler or lime sludge reburning kiln. This reduces the need for other fuels in these process steps.
• the sulphur-dioxide-containing flue gases from the bark burning boiler or the lime sludge reburning kiln can then be conducted to an existing SO 2 scrubber.
• Pulp mills may be equipped with separate SO 2 scrubbers for flue gases from the bark burning boiler and the lime sludge reburning kiln and from the soda recovery boiler, respectively.
• SO 2 scrubbers for flue gases from the bark burning boiler and the lime sludge reburning kiln and from the soda recovery boiler, respectively.
• the hydro ⁇ gen-sulphide-containing gas is combusted elsewhere, for example in a separate boiler, the sulphur-dioxide-containing flue gases can be con- ducted to the existing SO 2 scrubber for the flue gases from the soda recovery boiler.
• the present invention enables good process economy, a closed chemical system in which all the sulphur is recycled, as well as very low emissions to the sur- rounding atmosphere.
• FIG. 1 shows a preferred apparatus according to the invention for implementing the first stage of the method according to the inven ⁇ tion
• Fig. 2 is a schematic flow chart illustrating the method accord- ing to the invention.
• the apparatus shown in Fig. 1 comprises a tower or container 1 having an inlet 2 for a gas 3 which contains hydrogen sulphide as well as carbon dioxide. At the opposite end of the apparatus, there is pro ⁇ vided an outlet 4 for gas 5 from which hydrogen sulphide has been removed by liquid absorption.
• the contact between the hydrogen-sul ⁇ phide-containing gas and the carbonate-containing solution involves three steps 6, 7 and 8. Each step contains a packing 9, as hinted at in step 6 in the Figure.
• the packing 9 has such a shape as to generate a laminar flow of liquid through the steps 6, 7 and 8. It has been found that a packing in the form of corrugated plates is espe ⁇ cially suitable for this purpose.
• the packing may, for instance, be made of plastic or metal.
• each step has means for supplying the carbonate-containing solution through the step countercurrently to the gas, as well as for recycling the solution over the step.
• these means are made up of pumps 10, 11 and 12 which, via conduits 13, 14 and 15, feed the carbonate- containing alkaline solution to the respective steps 6, 7 and 8, as well as conduits 16, 17 and 18 which conduct the solution from the respective steps to collecting vessels 19, 20 and 21. From these col ⁇ lecting vessels, the solution is recycled over the steps through con- duits 22, 23 and 24, which are connected to the pumps 10, 11 and 12, respectively.
• Fresh carbonate solution preferably sodium carbo ⁇ nate solution
• Fresh carbonate solution is fed to the last step 6, as seen in the feed direction of the gas, through a conduit 25 from a supply (not shown) of sodium carbonate solution.
• the carbonate solution can be generated in the last step by supplying sodium hydroxide solution to this step and allowing the hydroxide solution to absorb carbon dioxide from the gas, such that a carbo ⁇ nate-containing solution is obtained according to the reactions (3) -(4) above.
• a hydroxide preferably a sodium hydroxide solution
• a hydroxide solution may be supplied to the collecting vessels 19, 20 and 21 through conduits 26, 27 and 28, respectively, conducting alkali from a supply (not shown), which preferably is common to all the conduits.
• the supply of sodium hydroxide solution for adjusting the pH of the absorption solution is regulated on the basis of the measured pH values of the solutions in the collecting vessels 19, 20 and 21 (not shown).
• the different steps are further inter ⁇ connected by conduits 29 and 30 for feeding a partial flow of the absorption solution from one step to the preceding step, i.e. from the step 6 to the step 7 as well as from the step 7 to the step 8.
• an outlet conduit 31 is arranged for discharging hydro ⁇ gen-sulphide-containing liquid from the collecting vessel 21 and the step 8.
• Fig. 2 is a schematic flow chart illustrating the method accord- ing to the invention.
• a hydrogen-sul ⁇ phide-containing and carbon-dioxide-containing gas 3 is, in a first stage, treated in an apparatus which includes a container 1 for the absorption of hydrogen sulphide, as described in the foregoing with reference to Fig. 1.
• the absorption of hydrogen sulphide is not, in the first stage, carried out to 100%, i.e. total absorption of hydrogen sulphide in the first stage, but only so far that at least the main part of the hydrogen sulphide, i.e.
• the hydrogen sulphide is absorbed in the first stage.
• a total of at least about 98% of the hydrogen-sul ⁇ phide content of the gas is absorbed by the first and the second stage.
• the thus-cleaned gas 5, which still contains hydrogen sulphide and carbon dioxide, is then fed from the first stage to the second stage, in which the gas is first combusted in order to convert hydro- gen sulphide to sulphur dioxide, and then wet-cleaned in a scrubber in view of the absorption of the sulphur dioxide formed.
• the combustion takes place in a combus ⁇ tion device 32, which has combustion means 33, e.g. a burner, for combusting the gas 5 supplied through an inlet 34 and oxidising the hydrogen sulphide to sulphur dioxide.
• combustion device 32 may consist of existing equipment, such as a bark burning boiler or a lime sludge reburning kiln.
• the sulphur- dioxide-containing flue gas is discharged through an outlet 35 and conducted to a wet cleaner 36, which comprises a container 37 having an inlet 38 for the sulphur-dioxide-containing gas, an outlet 39 for the cleaned gas 40, and means 41 for supplying an alkali-containing solution.
• the alkali-containing solution contains a hydroxide of an alkali metal or alkaline-earth metal, preferably sodium hydroxide.
• the solution is circulated by means of a pump 42 to nozzles 43, where it is finely divided and is countercurrently brought into contact with the sulphur-dioxide-containing gas, such that the sulphur dioxide is absorbed by the solution.
• the pH of the solution is adjusted to about 6-8, preferably 7-7.5.
• the solution that has absorbed sulphur dioxide is discharged from the second stage at 44.
• the absorption of hydrogen sul ⁇ phide took place at atmospheric pressure, and the feed gas had a temperature of about 60°C and contained 1.13 mole % of hydrogen sulphide and 16.9 mole % of carbon dioxide.
• the gas was saturated with water vapour at the temperature at issue, which corresponded to about 18.7 mole % of water.
• the feed gas flow was 38280 Nm 3 /h, giving the gas a velocity of about 3.1 m/s in the absorption tower.
• the absorption tower had a height of 6.25 m, and the two first steps each had a height of 1.5 m, whereas the last step, as seen in the feed direction of the gas, had a height of 1 m.
• Each step was provided with a packing of the type Mellapack 500 from Sulzer.
• the diameter of the tower was 2.3 m.
• Fresh absorption solution which consisted of 8.8 m ⁇ /h of 2 M sodium carbonate solution having a temperature of about 60°C, was supplied to the last step in the tower along with recycled absorption solution, such that a total of about 50 m ⁇ /h of absorption solution was supplied to the last step in the tower.
• the pH of the absorption solution supplied was about 11.0, which was reduced to about 10.2 during the passage of the solution through the step owing to the absorption of hydrogen sulphide.
• the solution was supplied to a 1.5 m 3 collecting vessel, where the solution was regenerated by the addition of a 2.5 M sodium hydroxide solution having a temperature of about 60°C, such that the pH of the solution was again increased to about 11.0. Then, the regenerated solution was recycled by means of a pump to the last step in the absorption tower for renewed absorption of hydrogen sulphide.
• the absorption solution was recycled in the scrubber at a total flow rate of 170 m 3 /h. From the scrubber was drawn off 1.8 m 3 /h of the solution, which had a total sulphur con ⁇ centration (SO3 2 - + HSO3- + SO4 2 -) of 1.05 mole/1.
• the total degree of separation of sulphur i.e. the degree of separation of hydrogen sulphide in the first stage plus the degree of separation of sulphur dioxide in the second stage, thus was 99.8%.
• the first stage i.e. the degree of separation of hydrogen sulphide in the first stage plus the degree of separation of sulphur dioxide in the second stage
• the selec ⁇ tivity to hydrogen sulphide in a 90-98% separation of hydrogen sul ⁇ phide is 0.15.
• the total selectivity for the whole stage then is about NaOH consumption more than 70% higher than in the inventive method, despite the fact that the total degree of separation of sulphur was lower.

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• 1993
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• 1994
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  • 1994-09-07 WO PCT/SE1994/000829 patent/WO1995007750A1/en active Application Filing
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