A METHOD OF PREPARING DIGESTING LIQUOR

Abstract: A method of preparing digesting liquor having high sulphidity from a spent liquor obtained from digesting cellulosic fiber material, preferably without a conventional causticizing step is described, said method comprising thermal decomposition of the spent liquor under reducing conditions in a reactor at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500 to 1600 DEG C so that a combustible gas phase containing hydrogen sulphide is formed in the reactor and extracted therefrom, and a phase of solid or molten material comprising substantially sodium sulphide which is dissolved in aqueous liquid to produce said digesting liquor. According to the invention hydrogen sulphide is recovered from the gas phase and is returned to the reactor to be present during the thermal decomposition of the spent liquor.

Description: A method of preparing digesting liquor The present invention relates to a method of preparing digesting liquor having high sulphidity from a spent liquor obtained from digesting cellulosic fiber material, preferably without a conventional causticizing step, said method comprising thermal decomposition of the spent liquor in at least one reactor at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500° to 1600°C, said thermal decomposition being carried out under reducing conditions without addition of oxygen-containing gas or by the controlled addition of an oxygen-containing gas in a quantity corresponding to from nearly 0 up to 80% of the quantity of oxygen required stoichiometrically for complete oxidation of substances produced during the thermal decomposition, whereby a combustible gas phase is formed in the reactor and extracted therefrom, and a phase of solid or molten material comprising substantially sodium sulphide, said solid or molten phase being dissolved in aqueous liquid to produce said digesting liquor, said gas phase containing hydrogen sulphide. During combustion of spent liquor from the sulphate pulp industry, for instance, two partial goals are aimed at, the first being that the organic wood substance released is combusted so that its chemical energy is converted to useful thermal and electric energy and the second being that the inorganic chemicals used shall be recovered and converted to active form. This means, among other things, that the sulphur shall be recovered in sulphide form, which requires under-stoichiometric combustion, while the energy recovery requires over-stoichiometric conditions. This means that two opposing processes must be performed simultaneously in one and the same combustion chamber which, with conventional soda recovery unit technology, results in optimizing problems. The inorganic part recovered from the soda recovery unit, the-melt, is dissolved to green liquor and consists of a plurality of chemical compounds. Most of these compounds consist of sodium carbonate and also sodium sulphide, the latter constituting the first one of the recovered active digesting chemicals in the white liquor. However, the cycle is not a hundred per cent efficient and some of the digesting chemicals are obtained in inactive form, thereby constituting ballast in the white liquor. Preparation of the white liquor is the last step in the recovery of chemicals. Here the sodium carbonate in the green liquor is converted to sodium hydroxide by the reaction with calcium oxide (CaO), i.e. caustic lime. The second one of the two active digesting chemicals in the white liquor has thus been recovered. The process is generally termed causticizing and an insoluble sludge consisting of calcium carbonate lime sludge is obtained as a by-product. Calcium oxide is recovered from the lime sludge by combusting it in a long, rotating furnace. The chemical cycle requires considerable energy and is not particularly harmless to the environment. The chemical composition of a normal melt from a soda recovery unit is typically as follows: Compound of melt (approx. ) Sodium carbonate Na2C03 70 Sodium sulphide Na2S 27 Sodium sulphate a2S0 3 Sodium sulphite Na2S<_3 Sodium thiosulphate Na2S2θ3 small quantities Sodium chloride NaCl After causticizing, the chemical composition of a normal white liquor is typically as follows: Compound Content in σ/1 (as NaOH) NaOH 100 Na2S 40 Na 2 C0 3 • 20 Na2S0 2.8 The white liquor recovered through a conventional soda recovery unit process has a sulphidity of 25-30%. (Sulphidity = N a-S x 1( > Q %) NaOH + Na_S Swedish patent SE-C-8502731-6 describes and alternative process to the soda recovery unit technique, which is based on a gasification technique in which the organic substances in the black liquor are gasified in a first process in a pressurized reactor by means of "flash- pyrolysis" to give CO, CO2, H2 and H2S, the residue obtained consisting primarily of the inorganic components of the spent liquor in solid or molten form having a composition corresponding to the melt from the soda recovery unit, i.e. mainly consisting of sodium carbonate and a small proportion of sodium sulphide. As with the soda recovery unit technique, this melt is dissolved and a green liquor is obtained which is treated in the same known manner as when the soda recovery unit technique is used. The chemical composition of spent liquor from the sulphate pulp industry is typically as follows: Sodium + Potassium 8.7 mol/kg DS black liquor Sulphur 1.8 "- Chlorine 0.05 "- Carbon 29.6 "- Hydrogen 39.2 "- Oxygen 21.0 "- The mole ratio Na:S is 4.88:1. Under certain operating conditions, such as at a pressure of 1.5 bar abs. and a temperature of 950°C in the gasification reactor, a melt is obtained consisting of 59 per cent by weight of a2Cθ3 and 31 per cent by weight of Na2S. Through Swedish patent SE-C-465 039 it is known to continuously supply sulphur compounds to the black liquor in order to alter the composition of the melt. By altering the mole ratio Na:S to 1.5:1 by the addition of 4 mol sulphur/kg DS thick liquor a melt having high sulphidity consisting substantially of 14 per cent by weight of Na2C*3 and 80 per cent by weight of Na2≤ is obtained under otherwise identical operating conditions. Due to its large content of sodium sulphide the melt thus consists for the most part of active digesting chemicals, i.e. white liquor. However, the difficulty is to maintain the low mole ratio Na:S, and if possible to reduce it even further, in order to obtain a white liquor of nearly 100% sulphidity. The amount of available sulphur compounds in a sulphate factory, for instance, is limited and only a small portion of the total amount of black liquor can therefore be treated in a gasification process of the type described, unless sulphur is supplied externally. In practice this is impossible to maintain continuously for sulphur balance reasons. It is therefore difficult to maintain a low mole ratio under certain operating conditions - as is accentuated by the following operating example. Increased pressure displaces the reaction towards the formation of carbonate in the following equilibrium reaction: Na2Cθ3 + H2S ^→ Na2S + CO2 + H2O. When the gas produced is supplied to a gas turbine a higher pressure is desirable in the gasification reactor. In this operating example, therefore, the melt obtained contains an even larger proportion of sodium carbonate and an even larger addition of sulphur compounds is necessary. If, for instance, the previously specified chemical composition of the black liquor and the amount of sulphur added remain the same but the pressure is increased from 1.5 bar abs. to 25 bar abs., i.e. the mole ratio Na:S of 1.5:1 is maintained, a melt is obtained consisting substantially of 52 per cent by weight of Na2Cθ3 and 39 per cent by weight of Na2S, i.e. a melt in which the carbonate content has increased from 14% to 52% by weight. This means that causticizing is still required and also that the sulphidity in the melt decreases, which may be a disadvantage. It is clear from the above that if increased pressures in the gasification reactor are used, mole ratios are required which are lower the higher the pressure is, and increasing amounts of sulphur compounds must be added in order to displace the above reaction to the right. It will be recognized that the quantities of sulphur required would be unmanageably large, in particular in the upper pressure levels. The object of the present invention is to provide a method of preparing digesting liquor having high sulphidity from spent liquor which enables an essential part of, or even all the spent liquor produced in a digesting process to be dealt with so that conventional recovery technique such as causticizing and lime sludge burning are reduced and eliminated, respectively. The method according to the invention is characterized in that hydrogen sulphide is recovered from the gas phase extracted and is returned to the reactor to be present during the thermal decomposition of the spent liquor. According to a preferred embodiment of the invention the quantity of hydrogen sulphide returned is adjusted so that the mole ratio between sodium supplied by the spent liquor and sulphur supplied partly by the spent liquor • and partly by said return of hydrogen sulphide amounts to 4:1 and below, preferably 2.8:1 and below, most preferably 1.5:1 and below. Recovery and return of hydrogen sulphide preferably take place continuously. The invention enables gasification of black liquor under the required low mole ratio Na:S in order to produce a melt in the gasification step consisting substantially of sodium sulphide (Na2S) which, after being dissolved in liquid (in a quenching step, for instance, or in a liquid film cyclone), immediately produces a white liquor having very high sulphidity, preferably 100% sulphidity, i.e. N 2S + H 2 0—*>NaHS + NaOH. The invention enables the complicated and expensive causticizing step, also including the lime kiln, to be eliminated from the recovery cycle. White liquor having high sulphidity also enables alteration of the digesting process. High sulphidity is particularly favourable in the initial stages of pulp digestion. Recirculation of hydrogen sulphide establishes such a high partial pressure of hydrogen sulphide in the gasification step that the equilibrium reaction Na2C03 + H2S k Wa.'i--' + co 2 + H 2° is displaced so far to the right that the formation of Na2C03 is suppressed. According to thermo-chemical equilibrium calculations performed, this is achieved at a gasi f ication pressure of 1.5 bar abs., for instance, and when the mole ratio Na:S in the black liquor supply to the gasification reactor, including the sulphur in the recirculated hydrogen sulphide gas, is 1.0 or lower. At the higher gasification of 25 bar abs. mentioned earlier, the above mole ratio must be reduced to 0.5 or lower. In the two operating cases exemplified the actual partial pressure of H2S can be seen in the following table: Operating pressure Mole ratio H2S partial reactor bar abs. Na:S pressure bar abs. 1.5 ≤l.O 0.07 25.0 ≤0.5 2.0 The partial pressure 0.07 bar abs. (0.0492x1.5=0.0738) in the first case corresponds to 4.7% of the total pressure 1.5 bar abs. and the partial pressure 2.0 bar abs. in the second case corresponds to 8% of the total pressure 25.0 bar abs. This means, therefore, that the concentration of hydrogen sulphide must be increased from 4.7% to 8% when the total pressure in the reactor is increased from 1.5 bar abs. to 25.0 bar abs. To maintain the higher partial pressure of H2S required in order to obtain a white liquor having high sulphidity, preferably 100% sulphidity, in a quenching step, for instance, according to the invention the H2S content in the process gas is returned to the gasification reactor. This return is achieved by allowing the gas to pass a gas washing apparatus for selective and regenerative absorption of the H2S content. Examples of such absorption processes are the Purisol process which utilizes N-methylpyrrolidone as an absorption liquid, and the Dow Gas/Spec-process which utilizes methyldiethylamine (MDEA) as an absorption agent. Using these processes, for example, more than 99% of the H2S content of the gas can be washed out, driven off from the absorption liquid via a regeneration step, and returned to the gasification step. Some form of sulphur or sulphur compound is added to the gasification reactor when it is started up in order to build up the required high H2S level in the system/cycle, and thus the necessary Na:S mole ratio at the actual reactor pressure. Such external additions of sulphur may also be carried out during the process in order to compensate any sulphur losses in the circulation system or when the partial pressure for H2S is to be increased due to increased pressure in the reactor. Such supplementary sources of sulphur may consist of a sulphurous secondary fuel, sulphurous gas from the digester house or evaporation plant, which can be supplied via the secondary air for the reactor or via the above-mentioned hydrogen sulphide regeneration step, elementary sulphur, sodium sulphate or sulphurous residue acid from the production of chlorine dioxide. However, if the entire flow of black liquor from the factory is gasified - i.e. no soda recovery unit is used - and without chlorine dioxide bleaching, said sodium sulphate and residue acid are not available. The regenerated H2S gas is thus returned to the gasification reactor to maintain the required H2S partial pressure. According to a suitable embodiment the H2S gas may be utilized entirely or partially as atomization gas for the black liquor addition in the burner nozzle. The H2S gas recovered from the process gas from a reactor can be distributed so that one part is returned to this first reactor and the remainder to a second reactor which is supplied with the same black liquor. Thus, according to this embodiment of the invention, two white liquors can be produced one having high sulphidity and the other somewhat lower sulphidity. It is preferred that all the H2S gas recovered is recirculated to the reactor. If for some reason the entire quantity of the H2S gas recovered is not to be recirculated, the surplus is subjected to a suitable process such as the one generally known as the Claus process, for the recovery of elementary sulphur. A measuring device may be disposed in the pipe for the process gas leading from the reactor, to measure the content of H2S or S in the gas. This measuring device is connected to control means that, via a valve, control the supply of supplementary sulphurous material when the process is started up and during continuous operation if a sulphur loss occurs in the circulation or, alternatively, if not all the H2S recovered is to be circulated and instead elementary sulphur is to be recovered for some reason either temporarily or continuously. The invention is applicable to spent liquors from both sulphate and sulphite pulp industry processes. The thermal decomposition occurs under reducing conditions without the supply of oxygen-containing gas or with controlled supply of an oxygen-containing gas in a quantity corresponding to from nearly 0 up to 80%, preferably up to 60%, of the quantity of oxygen required stoichiometrically for complete oxidation of substances produced during the thermal decomposition. The invention is further illustrated by means of the following examples. Black liquor having an Na:S mole ratio of partly 4.88:1 (normal sulphidity) and partly 2.0:1 (100% sulphidity) was fed continuously into a reactor in which the temperature was maintained at 950°C and the pressure at 1.5 bar abs. The dry solids content of the black liquor was 65%. Air pre-heated to a temperature of 500°C was supplied in controlled manner so that the quantity of oxygen reached 45% of quantity of oxygen required stoichiometrically for complete oxidation. The results from various tests are shown in the following Table 1. Test 1 relates to conventional run, that is with normal sulphidity and without recirculation of H2S. An addition of sulphur was made in the subsequent tests (2-6) to provide the reactor with the amount of supplementary sulphur necessary so that the stated Na:S mole ratio was set. Furthermore, in tests 3-6 H2S gas was recirculated to participate in this increase in added sulphur and subsequently to be responsible for maintaining the stated mole ratio. Increased recirculation of H2S gas resulted in a corresponding increase in its partial pressure so that the amount of sodium carbonate formed was reduced and the amount of sodium sulphide correspondingly increased. The gas extracted continuously from the reactor was caused to pass a gas washing apparatus for the H2S content in the gas to be absorbed. The results show that a forced increase in the H2S partial pressure obtained through recirculation of H2S gas gives a controlled decreased Na:S mole ratio in the reactor and accompanying convertion of the black liquor to white liquor having high sulphidity, without causticizing and lime sludge burning.

Claims: C L A I S 1. A method of preparing digesting liquor having high sulphidity from a spent liquor obtained from digesting cellulosic fiber material, preferably without a conven¬ tional causticizing step, * said method comprising thermal decomposition of the spent liquor in at least one reactor at a pressure of from atmospheric pressure up to about 150 bar and at a temperature of about 500° to 1600°C, said thermal decomposition being carried out under reducing conditions without addition of oxygen-containing gas or by controlled addition of an oxygen-containing gas in a quantity corresponding to from nearly 0 up to 80% of the quantity of oxygen required stoichiometrically for complete oxidation of substances produced during the thermal decomposition, whereby a combustible gas phase is formed in the reactor and extracted therefrom, and a phase of solid or molten material comprising substantially sodium sulphide, said solid or molten phase being dissolved in aqueous liquid to produce said digesting liquor, said gas phase containing hydrogen sulphide, characterized in that hydrogen sulphide is recovered from the gas phase extracted and is returned to the reactor to be present during the thermal decomposition of the spent liquor. 2. A method as claimed in claim 1, characterized in that the quantity of hydrogen sulphide returned is adjusted so that the mole ratio between sodium supplied by the spent liquor and sulphur supplied partly by the spent liquor and partly by said return of hydrogen sulphide amounts to 4:1 and below, preferably 2.8:1 and below, most preferably 1.5:1 and below. 3. A method as claimed in claim 1 or 2, characterized in that said recovery and return of hydrogen sulphide are effected in a closed recirculation loop. 4. A method as claimed in claim 2, characterized in that the pressure in said reactor is higher than or equal to 1.5 bar abs. and that said mole ratio is less than and equal to 1:1, respectively. 5. A method as claimed in any of claims 1-4, characterized in that substantially all hydrogen sulphide is recovered and returned. 6. A method as claimed in any of claims 1-4, characterized in that all or almost all hydrogen sulphide is recovered and returned. 7. A method as claimed in any of claims 1-6, characterized in that all or a part of the returned hydrogen sulphide gas is utilized as atomizing agent for the spent liquor. 8. A method as claimed in any of claims 1-7, characterized in that additional sulphur is .added through a secondary sulphur source during the starting-up stage in order to achieve a predetermined mole ratio between sodium and sulphur and during operation to maintain said predetermined value in the event of sulphur losses, or in order to change the mole ratio during operation to an even lower value, resulting in an equivalent increase in the partial pressure of the hydrogen sulphide. 9. A method as claimed in any of claims 1-8, characterized in that a first part of the hydrogen sulphide gas extracted and recovered from said reactor is returned to said reactor and a second part is conveyed to a second, similar reactor being supplied with preferably the same spent liquor for the recovery of a first digesting liquor having a high sulphidity and a second digesting liquor having a lower sulphidity, different partial pressure of the hydrogen sulphide gas being maintained in the two reactors and a lower and higher NarS mole ratio, respectively, within the specified limits thus being maintained in the first reactor and the second reactor, respectively. 10. A method as claimed in any of claims 1-9, characterized in that said measures including extraction of a combustible gas, recovery therefrom of hydrogen sulphide and return of hydrogen sulphide to the reactor and, when applicable, transfer to a second reactor are carried out continuously.

Inventors: NILSSON BENGT, NILSSON, BENGT
Applicants: CHEMREC AB, CHEMREC AKTIEBOLAG
Application year: 1992
Main classification: D21C11
IPC codes: D21C11