Different alternatives to the conventional recovery of spent pulping liquors in the kraft pulping process have been studied by equilibrium calculations. These calculations were performed using FactSage. Three different black liquor gasification systems with addition of TiO 2 for direct causticization were studied: (i) MTCI conditions (600 °C and 0.3 MPa), (ii) Chemrec atmospheric booster conditions (950 °C and 0.1 MPa), and (iii) Chemrec pressurized system conditions (950 °C and 3 MPa). The studies of the influence of the molar ratio TiO 2 /Na 2 O showed that a TiO 2 / Na 2 O molar ratio of 0.5 is needed to capture all of the sodium in the condensed phase as sodium titanates (i.e., achieve complete direct causticization). Furthermore, at these conditions all of the sulfur is released to the gas phase. Different forms of sodium titanates are formed depending on the TiO 2 /Na 2 O molar ratio, and conceptually all of them could be used to produce NaOH in the consecutive leaching step.
The solid state reaction between sodium tri‐titanate and sodium carbonate, forming mainly sodium pentatitanate, was investigated. Experiments were carried out in a micro‐differential reactor made of quartz glass at various temperatures between 800°C and 880°C and in a pilot fluidized bed reactor operated in a semi‐batch mode. In the former reactor, basic kinetic data was obtained by measuring the release of carbon dioxide. Different kinetic models were considered to describe the conversion, such as the Valensi‐Carter model for diffusion controlled reaction rates and the phase‐boundary model for first‐order reaction kinetics. Furthermore, a model that included both diffusion in the solid material and the chemical kinetics was derived. This model described the experimental data obtained in the micro‐differential reactor very well. However, for the fluidized bed experiments, these different kinetic models did not accurately describe the experimental data. Therefore, an improved model was developed, which also took into account the time taken for the reactants to achieve physical contact. This model gave good agreement with the experimental data.
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