New Predictive Models for COD from All Stages in Full ECF Bleaching Sequences
This paper proposes new mathematical models for the prediction of chemical oxygen demand (COD) from different stages in full ECF bleaching sequences and discusses several important issues related to model development and interpretation. The effects of different process conditions, like pH, chemical charge, initial Kappa number, carryover, temperature etc., on the formation of COD from the different chlorine dioxide stages (Do, D1, D2) and extraction stages (E, EO, EP, EOP) were studied and corresponding models were developed for softwood and hardwood pulps. The models, based on results obtained from the extensive studies of the kinetics, stoichiometry, and effluent properties of the chlorine dioxide stages and extraction stages in the ECF bleaching of pulp, were tested for accuracy on a vast range of data both from literature and laboratory experiments. The kinetics, stoichiometry and effluent properties have not previously been considered together for optimization and would be of considerable use in bleach plant optimization as well as in process control of different stages. This study will also help in understanding the need for modifications of the conventional conditions in both the chlorine dioxide and the extraction stages to obtain the best mix between a low formation of COD and high bleaching efficiency.
Phase and reaction equilibria in the modelling of hot water extraction, pulping and bleaching
Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Susanna Kuitunen Name of the doctoral dissertation Phase and reaction equilibria in the modelling of hot water extraction, pulping and bleaching Publisher School of Chemical Technology Unit Department of Biotechnology and Chemical Technology Series Aalto University publication series DOCTORAL DISSERTATIONS 37/2014 Field of research Chemical Engineering Manuscript submitted 5 December 2013 Date of the defence 9 May 2014 Permission to publish granted (date) 12 March 2014 Language English Monograph Article dissertation (summary + original articles) Abstract This work is part of a joint effort to develop a pulping and bleaching simulator by utilizing fundamental thermodynamic, reaction kinetic, and mass transfer equations extensively. Carbohydrate and lignin polymers are modelled using monomeric pseudo-compounds having different properties and reactivity. Otherwise, well-known chemical compounds are used in the construction of chemical reaction libraries. The main objective of the present work was to study the importance of modelling reaction and phase equilibria simultaneously with irreversible chemical reactions. Based on the mechanism presented in the literature, libraries consisting of irreversible reactions were constructed for the following cases: the hot water extraction of wood, kraft liquor impregnation into wood, lignin oxidation in oxygen delignification conditions, and alkaline extraction of chlorine dioxide delignified pulp. By studying the above-mentioned cases, it was noted that many chemicals and substrates in lignin 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 or they do not react at all. The evolution of pH is determined by the chemicals added, the composition of the fibres, and the reactions taking place. Furthermore, due to the cation exchange property of the fibres, the pH in the fibre wall liquid is lower than in the external liquid phase. As a conclusion, simultaneous modelling of all phenomena in a single platform provides 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 on predicting process behaviour than traditional correlation models. The implementation of unit operation models (mixer, retention tower, and washer), from which various process sequences can be built, were also introduced. The laboratory experiments were simulated with a batch reactor model, which is capable of reproducing the pressure evolution in addition to chemical composition. Novel method for solving the phase and reaction equilibria was developed and demonstrated. The simulations produce concentrations of pseudo-units and real chemical compounds thus enabling model validation with sophisticated analytical tools like nuclear magnetic resonance spectroscopy and capillary electrophoresis. In order to use standard every-day analyses (kappa number, intrinsic viscosity, TOC, etc.) for the model validation, the equations converting the chemical composition data into generally used engineering parameters were gathered and implemented.This work is part of a joint effort to develop a pulping and bleaching simulator by utilizing fundamental thermodynamic, reaction kinetic, and mass transfer equations extensively. Carbohydrate and lignin polymers are modelled using monomeric pseudo-compounds having different properties and reactivity. Otherwise, well-known chemical compounds are used in the construction of chemical reaction libraries. The main objective of the present work was to study the importance of modelling reaction and phase equilibria simultaneously with irreversible chemical reactions. Based on the mechanism presented in the literature, libraries consisting of irreversible reactions were constructed for the following cases: the hot water extraction of wood, kraft liquor impregnation into wood, lignin oxidation in oxygen delignification conditions, and alkaline extraction of chlorine dioxide delignified pulp. By studying the above-mentioned cases, it was noted that many chemicals and substrates in lignin 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 or they do not react at all. The evolution of pH is determined by the chemicals added, the composition of the fibres, and the reactions taking place. Furthermore, due to the cation exchange property of the fibres, the pH in the fibre wall liquid is lower than in the external liquid phase. As a conclusion, simultaneous modelling of all phenomena in a single platform provides 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 on predicting process behaviour than traditional correlation models. The implementation of unit operation models (mixer, retention tower, and washer), from which various process sequences can be built, were also introduced. The laboratory experiments were simulated with a batch reactor model, which is capable of reproducing the pressure evolution in addition to chemical composition. Novel method for solving the phase and reaction equilibria was developed and demonstrated. The simulations produce concentrations of pseudo-units and real chemical compounds thus enabling model validation with sophisticated analytical tools like nuclear magnetic resonance spectroscopy and capillary electrophoresis. In order to use standard every-day analyses (kappa number, intrinsic viscosity, TOC, etc.) for the model validation, the equations converting the chemical composition data into generally used engineering parameters were gathered and implemented.
Rubik's for cryptographers
Hard mathematical problems are at the core of security arguments in cryptography. In this paper, we study mathematical generalizations of the famous Rubik’s cube puzzle, namely the factorization, representation and balance problems in non-Abelian groups. These problems arise naturally when describing the security of Cayley hash functions, a class of cryptographic hash functions with very interesting properties. The factorization problem is also strongly related to a famous long-standing conjecture of Babai, at the intersection of group theory and graph theory. A constructive proof of Babai’s conjecture would make all Cayley hash functions insecure, but on the other hand it would have many positive applications in graph theory and computer science. In this paper, we classify existing attacks against Cayley hash functions and we review known results on Babai’s conjecture. Despite recent cryptanalytic progress on particular instances, we show that the factorization, representation and balance problems presumably remain good sources of cryptographic hard problems. Our study demonstrates that Cayley hash functions deserve further interest by the cryptography community.