The objective of this study was to develop mathematical models that describe the course of delignification and carbohydrate degradation during cooking processes as a function of cooking temperature and cooking chemicals concentrations. The cooking processes investigated in this work were the kraft and the autohydrolysis processes. In the case of kraft cooking different existing delignification models were fitted to experimental data from both soft- and hardwood cooks. The fact that data with different temperatures were present enabled the estimation of the activation energies for the reactions associated with the models. For the available data it turned out that a model containing two lignin components of differing reactivities was sufficient for a satisfactory description of the observed delignification. Furthermore, two modifications aimed at augmenting the authenticity of the model were conducted. First, Donnan theory, accounting for the time development of the chemical concentrations at the actual location of the delignification reactions, was incorporated to the model. Second, the rate constants in the model were calculated according to the real temperature during heating and cooling instead of resorting to the computation simplifying concept of isothermal time. Moreover, the square root relationship between the reciprocal lignin content and the anthraquinone charge earlier established for soda-anthraquinone cooks was confirmed in the kraft-anthraquinone context. For carbohydrate degradation during kraft cooking a model based on the endwise reactions peeling, stopping and alkaline hydrolysis was developed. The model provided satisfactory fits to the yield data of all the three carbohydrates (cellulose, glucomannan and xylan) under scrutiny. The portions of material degraded by the different reactions present in the model were calculated based on the parameter estimates obtained from the fits. Furthermore, the impact of dissolution was compared to that of the endwise reactions; for glucomannan and cellulose the endwise reactions degrade more material than does the dissolution, whereas the opposite is true for xylan. The yield loss model for the carbohydrates was also combined with a model for the decrease in the degree of polymerization. By performing a simultaneous fit of the two models an enhanced estimate of the model parameters was obtained. For the autohydrolysis cooks reaction schemes for the delignification and the degradation of xylan and glucan were developed and the corresponding kinetic models were fitted to the data resulting in estimates for the kinetic parameters. The kinetic models were combined with the diffusion equation resulting in a simulation program for the effect of cooking on the wood components on chip level. Furthermore, the kinetic models together with the Ergun equation for the forced flow of the heated cooking liquor through the packed bed led to a simulation program of the spatial developments of delignification and carbohydrate degradation on reactor level.
|Publication status||Published - 2015|
|MoE publication type||G5 Doctoral dissertation (article)|
- modeling, reaction kinetics, kraft process, autohydrolysis, lignin, cellulose, hemicellulose, yield, degree of polymerization, diffusion, Ergun equation