Recovery of Kraft Black Liquor - Alternative Processes and System Analysis

Besides the conventional recovery process, there are several possible processes whereby the cooking chemicals in kraft black liquor can be recovered. A part of one of the possible processes, direct causticization using titanate, has been studied in detail. Direct causticization have several advantages compared to conventional lime-based recovery: e.g. higher production of electricity due to a gasification process, possibility of tailor-made white liquor, no risk of smelt explosions and a smaller amount of non-active chemicals in the white liquor. This process was then compared to other processes, i.e. the conventional lime-based process and autocausticization using boron, from a thermodynamic point of view. All of the compared processes had at least one high-temperature and one low-temperature reaction. In order to be able to compare the recovery processes, the kinetics and equilibrium had to be established for the direct causticization process (in this study, one of the two main reactions was investigated). It was found that the reaction rate for the causticizing reaction was diffusion controlled, and that the equilibrium concentration of sodium hydroxide was high in the formed white liquor (approximately 200 g NaOH/kg solution at 100\ub0C). Furthermore, the material has a low specific filtration resistance, which means that it is possible to perform the reaction in a filter press. In the thermodynamic (enthalpy and entropy) study, it was found that the direct causticization process had the lowest endothermic reaction enthalpy at high temperatures, which means that as little as possible of the high-quality energy was used for the reaction. The thermodynamic efficiency based on entropy showed that the overall efficiency of the sodium cycle (including the bleaching plant) was high for the three processes (around 65%) and that the greatest entropy increase was found in the combustion unit (e.g. a recovery boiler or a gasification unit with a following gas turbine)