Recovery of kraft black liquor with direct causticization using titanates (2002)

The feasibility of direct causticization of kraft black liquor using titanates has been evaluated. This has been done by studying the kinetics of the reaction between sodium tri-titanate and sodium carbonate and describing the kinetics by appropriate reaction models. Furthermore, three different chemical recovery processes have been studied from an energy point of view: the conventional kraft recovery cycle using lime, the borate causticization process (as an add-on to conventional kraft recovery process) and the titanate causticization process combined with black liquor gasification has been made. Finally the carbon and carbon species transition during black liquor pyrolysis and gasification in the presence of sodium titanium compounds was studied. The experimental studies have been carried out in three different reactors; a small scale fixed bed reactor, a fluidized bed reactor and a pressurized entrained flow reactor. In the small scale fixed bed reactor the kinetics of the direct causticization reaction between sodium tri-titanate and sodium carbonate was studied at various temperatures between 800°C and 880°C, and in pure nitrogen or in nitrogen with 0.5-5% CO2. By fitting different kinetic models to the experimental data the overall reaction rate was found to be diffusion controlled at the lower temperature and chemical reaction controlled at higher temperatures. Furthermore, this change in mechanism was seen to be influenced by the carbon dioxide concentration in the reaction atmosphere in the way that the mechanism change occurred at higher temperatures. In the semi-batch fluidized bed the direct causticization reaction was studies at temperatures between 800-825°C. Conversions of the sodium carbonate up to 88% were obtained. The fitting of different kinetic models to the experimental data showed that the overall reaction rate in this equipment was controlled by the time taken for the reactants to achieve physical contact. The studies in the pressurized entrained-flow reactor were done using both pure sodium carbonate and using black liquor solids. The experiments were carried out at 900°C, 950°C and 1000°C; at 0.5 MPa and 1 MPa; in pure nitrogen and in 2% carbon dioxide. The experimental results showed that sodium penta-titanate formation is faster with black liquor than with sodium carbonate and that the rate increases with temperature. Furthermore, carbon dioxide in the reaction gases slows down the reaction rate and no clear influence of pressure was seen on the reaction rate. Finally, the reaction rates for the direct causticization reaction between sodium tri-titanate and sodium carbonate was found to be fast enough to be carried out in an entrained flow reactor system, e.g. a Chemrec gasification system. Moreover, the char carbon was gasified both in the presence and absence of carbon dioxide. For the black liquor used in this work, the char carbon was gasified from two to four times faster in carbon dioxide than in nitrogen. Carbon was gasified faster at higher temperature in both the presence and absence of carbon dioxide. The rate of gasification was slightly faster at 0.5 MPa than at 1 MPa total pressure, both in nitrogen and in 2% carbon dioxide. CO was found to be the main carbon gas formed form the black liquor in all experiments. The comparison between the lime, borate and titanate based chemical recovery processes were made by mass and energy balances with the system defined from melted sodium carbonate to white liquor ready to be used in the digester. The lowest energy demand was found in the direct causticization process with titanates; it is only 27% of the energy demand for the other two processes.