Causticization of green liquor having a high content of sodium hydrogen carbonate Hans Theliander Chalmers University of Technology, Gothenburg, Sweden Keywor-ds: Sodium hydrogen carbonate, Bicarbonate, Causticization, Green liquor SUMMARY Green liquor from a kraft recovery system based on the gasification of black liquor typically has a different composi- tion compared to "normal" green liquor: it contains a significant amount of sodium hydrogen carbonate. The aim of this work was to make an experimental investiga- tion of the causticization of a liquor containing sodium hydrogen carbonate and to transfer the findings to industrial conditions. A laboratory batch reactor was used in the experimental part and a dynamic simulation programme was used to make the transfer to industrial conditions. It was found that the conversion degree of lime can, for certain process conditions, be very low (40-60% of maximum conver- sion degree) when a sodium hydrogen carbonate solution is caus- ticized: this is due to the formation of an obstacle which hinders the mass transport and/or the reaction itself. Aragonite was found only in lime mud from tests where the utilisation degree of lime was low and it was suggested that it is the aragonite that genera- tes the obstacle. Furthermore, it was found that the reaction bet- ween sodium hydrogen carbonate and sodium hydroxide is exot- hermic: the released heat of reaction is of such a magnitude that it must be considered in the heat balance of the causticization plant. The results from a calculations using the dynamic simula- tion programme indicate that, if all of the lime is added to the first vessel (the slaker) and the residence time is about 30 minu- tes, the problems related to the formation of aragonite can be avoided. ADRESS OF THE AUTHOR: Department of Chemical Engineering Design, Chalmers University of Technology, S-412 96 Gothenburg, Sweden A kraft black liquor recovery system based on the gasifica- tion of black liquor typically produces a green liquor having a different composition compared to "normal" green liquor: it contains a significant amount of sodium hydrogen carbonate. The sodium hydrogen carbonate is formed both in an absorption tower where hydrogen sul- phide is absorbed in green liquor as well as in systems where the smelt is quench-cooled. The sodium hydrogen carbonate influences the conditions in the causticization plant: one important consequence is that the lime charge must be increased in order to convert the hydrogen carbo- nate ions to carbonate ions. The causticization operation may, thus, be described as a two-step reaction: in the first, hydrogen carbonate ions are transformed into carbonate ions (slaked lime or hydroxide ions being the hydroxide source) and in the second step, carbonate ions reacts with slaked lime forming hydroxide iyns and calcium carbona- te. The two ions HC03 and CO; are invokved during the first step and, during the second step, C03 and OH- are involved. Consequently, the second step is the "normal" causticization reaction, implying that the final equilibrium conditions are the same as in "normal" causticization. The overall reaction (steps one and two) is: HCOT (aq) + Ca(OH)2 (S) o OH- (aq) + CaC03 (S) + H2 0 [ 11 Here, it can be seen that the molar ratio between the calci- um hydroxide and the hydroxide ion is 1:l wheras in "nor- mal" causticization the corresponding molar ratio is 1:2. Thus, when a pure hydrogen carbonate solution is caustici- zed twice as much lime is needed compared to that of a carbonate solution. In an industrial plant, green liquor pro- bably will be a mixture of sodium hydrogen carbonate and sodium carbonate ions. The molar ratio between the calci- um hydroxide and the hydroxide ion will, thus, be between 1:l and 1:2. The mechanism and kinetics of hydrogen carbonate causticization may be different compared to "normal" causticization even if the final equilibrium conditions are the same as in "normal" causticization. No papers related to the causticization of hydrogen carbonate ions were found in the literature. The aim of this work was to investigate the reaction mechanism, briefly examine the kinetics of the causticiza- tion of hydrogen carbonate ions, estimate the heat of reac- tion and transfer the findings obtained in a laboratory batch reactor to a continuous-operating causticization plant. In the latter part of the study, a dynamic simulation programme was used. Different Mechanisms There are at least three mechanisms possible for the con- version of hydrogen carbonate ions to carbonate ions, as shown in Fig. l. A B C + l H,O+CO; H,O + OH- OH- znd znd znd OH '+Hco;-+ CO:~H,O OH'+HCO;--+ CO:~H,O Fig. I. Three possible mechanisms for the conversion of hydrogen carbonate to carbonate. In Mechanism A, hydrogen carbonate ions react with the calcium hydroxide forming calcium carbonate, carbonate ions and water: 2 NaHCO, (aq) + CU(OH)~ (S) 3 L21 Nu2 CO3(aq) + CaCO:, (S) + H2 0 Mm = -90 U/ m01 M" is the heat of reaction in dilute solutions, thermody- namic data was taken from Perry and Green (1984). Two steps are involved in Mechanism B: in the first, hydrogen carbonate reacts with calcium hydroxide for- ming hydroxide ions, calcium carbonate and water and, in the second step, the hydroxide ions react with hydrogen carbonate ions forming carbonate ions and water: Nordic Pulp and Paper Research Journal no. 111997 Brought to you by | American University of Beirut Authenticated Download Date | 6/12/19 10:56 AM