Phase and reaction equilibria in the modelling of hot water extraction, pulping and bleaching

Susanna Kuitunen

    Research output: ThesisDoctoral ThesisCollection of Articles


    This work is part of a joint effort to develop a pulping and bleaching simulator by utilizingfundamental thermodynamic, reaction kinetic, and mass transfer equations extensively. Carbohydrate and lignin polymers are modelled using monomeric pseudo-compounds havingdifferent properties and reactivity. Otherwise, well-known chemical compounds are used in theconstruction of chemical reaction libraries. The main objective of the present work was to study the importance of modelling reaction andphase equilibria simultaneously with irreversible chemical reactions. Based on the mechanismpresented in the literature, libraries consisting of irreversible reactions were constructed forthe following cases: the hot water extraction of wood, kraft liquor impregnation into wood,lignin oxidation in oxygen delignification conditions, and alkaline extraction of chlorine dioxidedelignified pulp. By studying the above-mentioned cases, it was noted that many chemicals and substrates inlignin and in carbohydrates dissociate to the extent determined by their pKa value and pH.Depending on whether they are non-dissociated or dissociated, compounds react differently orthey do not react at all. The evolution of pH is determined by the chemicals added, thecomposition of the fibres, and the reactions taking place. Furthermore, due to the cationexchange property of the fibres, the pH in the fibre wall liquid is lower than in the externalliquid phase. As a conclusion, simultaneous modelling of all phenomena in a single platformprovides the opportunity to study the reaction mechanisms in depth. Furthermore, the model,based on the real physico-chemical phenomena, is supposed to have more capabilities onpredicting process behaviour than traditional correlation models. The implementation of unit operation models (mixer, retention tower, and washer), fromwhich various process sequences can be built, were also introduced. The laboratoryexperiments were simulated with a batch reactor model, which is capable of reproducing thepressure evolution in addition to chemical composition. Novel method for solving the phaseand reaction equilibria was developed and demonstrated.The simulations produce concentrations of pseudo-units and real chemical compounds thusenabling model validation with sophisticated analytical tools like nuclear magnetic resonancespectroscopy and capillary electrophoresis. In order to use standard every-day analyses (kappanumber, intrinsic viscosity, TOC, etc.) for the model validation, the equations converting thechemical composition data into generally used engineering parameters were gathered andimplemented.
    Translated title of the contributionFaasi- ja reaktiotasapainojen mallinnuksesta kuumavesiuutossa, keitossa, ja valkaisussa
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Alopaeus, Ville, Supervising Professor
    • Alopaeus, Ville, Thesis Advisor
    Print ISBNs978-952-60-5617-3
    Electronic ISBNs978-952-60-5618-0
    Publication statusPublished - 2014
    MoE publication typeG5 Doctoral dissertation (article)


    • modeling
    • reaction kinetics
    • mass transfer
    • phase equilibrium
    • Donnan equilibrium
    • reaction equilibrium
    • hot water extraction
    • pulping
    • bleaching


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