Simulation and Optimization of Recovery Systems in the Pulp and Paper Industry

The chemical recovery process in the pulp industry, employing magnesium bisulfite as cooking liquor, has long been established and holds the potential to operate as a near closed-loop system concerning chemical usage. Post-combustion of red liquor, magnesium oxide is reclaimed from the ash and hydrated into magnesium hydroxide. In a multistage absorption process, sulfur dioxide (SO2) is extracted from exhaust gas using the recovered magnesium hydroxide, forming magnesium bisulfite, which is then reused as cooking liquor. However, suboptimal process conditions often result in avoidable additional chemical usage, compromising the closed-loop nature of the process. Hence, there is a significant opportunity for enhancing chemical recovery through process optimization. This thesis delves into optimizing chemical recovery within magnesium bisulfite pulping processes using simulation techniques. Simulation enables virtual experimentation, allowing the identification of primary process inefficiencies and the derivation of optimized process designs. This thesis addresses two main optimization potentials: minimizing unwanted precipitation in the absorption system and reducing soot formation during combustion, thereby reducing the necessity for bleaching chemicals in pulp production. The first objective of this study is to employ process simulation to tackle unwanted precipitation in the system. A comprehensive thermodynamic process model of the chemical recovery process, encompassing the complex electrolyte system components (MgO-CaO-SO2-H2O-O2-CO2), was developed to predict precipitations accurately. By utilizing the validated model in process flowsheet calculations, the pH value, temperature, and Mg(OH)2 to SO2 ratio were identified as key parameters influencing undesired precipitation reactions, and parameter limits at which increased precipitations occur were defined. Finally, an optimized process design is presented that minimizes precipitation by optimally adjusting the Mg(OH)2-SO2 ratio, thereby significantly reducing chemical consumption. The second objective of this thesis is to use Computational Fluid Dynamics (CFD) simulations to analyze the effect of red liquor spraying on combustion inside the furnace. By optimizing the spraying behavior, the combustion can be improved, and soot formation can be reduced. For this purpose, a CFD model of the combustion vessel was developed, which describes the spraying and combustion of red liquor. With the developed model, the influence of different spray characteristics, such as particle size, spray angle, and spray type, on the combustion speed was calculated in virtual experiments. The experiments revealed that a smaller particle size, a wider spray angle, and a hollow cone spray type, compared to a full cone spray type, are beneficial for faster droplet combustion. VII This research contributes to understanding and improving chemical recovery processes in the pulp industry, emphasizing the importance of simulation-based optimization for sustainable and efficient operations. The developed simulation models are tools for evaluating process and system designs that extend beyond the performed investigations. The thermodynamic process model developed in this work can support both plant operations and plant design. It enables plant operators to identify the operational limits of the plant and respond to operational problems with a comprehensive understanding of the mechanistic interplay within the process system. In plant design, the model allows the simulation of various process scenarios and supports finding an optimized design tailored to the needs and infrastructure of the plant operator. The developed CFD model facilitates evaluating how different design changes affect flow and combustion conditions in the furnace and how emission reduction measures can be implemented. The applied simulation approaches can be transferred to various combustion scenarios. By adjusting kinetic parameters in the model, the developed simulation tool is not limited to the combustion of the red liquor but can also simulate furnaces using other fuels.