Technical publications

ABSTRACT

Semi-empirical mathematical modeling of chlorine dioxide bleaching has been extensively developed during the 80's by Swedish groups (Teder, Germgård and coworkers). More recently, some equations were reviewed and modified to build an ECF bleaching sequence simulator (Mortha, 2000, 2001). However, some limitations appear in previous models because of their partly empirical basis, and as a consequence, some simulator parameters have to fit experimental data. A first purpose of this work was to improve some equations used in former simulators to improve their predictability for different kinds of pulps. A second purpose was to model sequences where the first D₀E stage is split into several (DE) stages at low charges of ClO₂. A new form for the stoichiometric equation during D₀ was input in the model. It assumes an effect of the initial charge of ClO₂ on the delignification stoichiometry. The results obtained fit much better laboratory bleaching data for a variety of pulps.

1. INTRODUCTION

Pulp bleaching simulation is a general objective that has been followed since the initial works of Swedish groups in the 80's who established semi-empirical single stage bleaching models, based on kinetic and stoichiometric studies for various bleaching agents (Teder, Germgård and coworkers). Using a similar formalism, a simulator for the complete DEDED sequence was presented later by Mortha (2000, 2001). In the latter, some equations were reviewed and modified to fit laboratory bleaching data on Kraft pulps. Parameters were adjusted for the different D stages of the sequence. Additionally, the pH equation was derived from a theoretical basis. However this work was limited to a little number of pulps and for ClO₂ charges in the range 1-3%.

The purpose of this work was to improve further the form and significance of some mathematical equations used in former simulators for the D₀E stages. Modeling sequences in which the first D₀E stages is split into several (DE) stages at low ClO₂ charges was studied as an example. Additionally, the simulator predictability was extended to a greater variety of pulps (softwood and hardwood pulps, Kraft or other alkaline pulps).

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ABSTRACT

A new method for semi-quantitative titration of free phenolic groups in cellulosic pulps was proposed by Eshkiki et al¹. It is based upon the reaction between free phenolic groups and chlorine dioxide at 0°C, in the presence of DMSO, in an acidic medium. One free phenolic group consumes theoretically two molecules of ClO₂.The previous protocol was improved and the effect of ClO₂ excess was studied throughout model compounds behavior. Selectivity of this reaction and choice of pH were discussed.

This in situ method was applied on pulps before and after hexenuronic acids removal in order to determine if ClO₂ was consumed by these groups during the titration. Results were compared to ³¹P NMR results obtained on extracted lignin.

1. INTRODUCTION

The majority of the existing methods for functional groups determination in residual lignin are applied on extracted lignin from pulp^2-8. Lignin extraction is a major drawback, because it is an heavy procedure which usually provides limited yield and may create chemical modifications. Therefore, reliable results are not guaranteed by these methods. Methods applied directly on pulp, such as oxidative periodate demethoxylation⁹ or solid state ¹³C NMR¹⁰, suffer from the interference of carbohydrates, interfering reactions with structures or functional groups other than lignin phenolic groups and poor accessibility inside the lignocellulosic matrix.

Eshkiki et al¹. proposed a new wet chemical method for the titration of free phenolic groups in pulps based on the fact that one free phenolic group should consume two molecules of ClO₂ at 0°C, in acidic medium and in the presence of an excess of DMSO, an HclO inhibitor. Improvement of the protocol, influence of pH, ClO₂ excess and better understanding of the reaction mechanism are the subject of the present paper.

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ABSTRACT
The research in this thesis focussed on the issues related to the modelling of two major process areas of the pulp and paper industry, namely, Kraft cooking and ECF (Elemental Chlorine Free) bleaching, their interpretation and eventual development of the models. The thesis can be divided globally into two general parts.

The first part dealt with the issues related to chemical process and environmental engineering during the ECF bleaching and Kraft cooking processes. The research on Kraft cooking predictively investigated the extent and the nature of pulp non-uniformity studied were temperature gradients and chip mixtures: either mixed-size chips, according to a thickness distribution law, or mixtures of different wood species. The research on ECF bleaching involved development of new improved semi-empirical kinetic and stoichiometric models for all the chlorine dioxide stages (D_o, D₁, D₂) and extraction stages, including oxygen and/or peroxide reinforced, (E, EO, EP, EOP) in a full ECF bleaching sequence, in addition to the mathematical correlations for the COD and resistant COD formed in each bleaching stage. The models predict the variations of kappa number, pH, brightness, bleaching chemicals consumption and effluent load.

The second part of the thesis involved use of software engineering skills through deep programming required for the development of simulators using platforms like Visual Studio 2005/MATLAB/CADSIM/ODBC/EXCEL. Comprehensive and advanced simulators were developed for Kraft cooking and ECF bleaching processes and their multi-purpose applications such as an important engineering tool for prediction, decision, diagnosis, process regulation, process optimization or education were demonstrated.

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ABSTRACT
New mathematical models for the prediction of chemical oxygen demand (COD) from different stages in full ECF bleaching sequences are developed and several important issues related to model development and interpretation are discussed. 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 (D_o, D₁, D₂) and extraction stages (E, E₀, E_P, E_OP) were studied and corresponding models were developed for softwood and hardwood pulps. The percentage of resistant COD in the total COD produced in bleaching stages was also evaluated and modelled. 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.

1. INTRODUCTION

The problems of many of the traditional mill pollutants, such as biological oxygen demand (BOD) and newer pollutants such as dioxin and chlorate, are considered to have been solved, as the remaining emissions todays are very low. However, emissions of COD to receiving waters are still an area where further reductions are needed. Much interest is therefore focused on methods that improve the situation with respect to COD. Some models for overall COD coming out bleaching plant are available in literature [1, 3, 5]. All of them are based on either initial Kappa number or final yield from bleach plant. However, these models are less-predictive, cover a narrow range, and are less accurate and less useful in bleach plant optimization. Moreover, yield is often difficult to measure in mills and hence models based on this are restricted in there capability of application. Tsai et.al [5] found COD to be a function of Kappa number and Kappa factor (KF), based on laboratory work performed on southeast U.S. softwood pulp at Kappa 13, 14 and 31 [5]. the relationship reported by Tsai [5] is as follows:

 

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ABSTRACT

For several years now, numerous studies have been conducted to investigate low alkalinity deinking. True neutral deinking was even proposed. Nevertheless, there is still a need for more complete investigations, for essentially two reasons:

- most of the studies were performed using sodium hydroxide as the alkali source and only few with alternative alkaline sources.

- whatever the alkaline source, dosages used for chemicals were relatively high (both total alkalinity and peroxide charge > 0.5 %).

In this paper we investigated the use of Mg(OH)2 as an alternative alkaline source in the deinking of an ONP rich mixture (70% ONP / 30% OMG), by comparison with conventional sodium hydroxide based chemistry. All the work was focused on using low chemical dosages for both alkalinity source (sodium hydroxide, sodium silicate or magnesium hydroxide) and hydrogen peroxide.

These near neutral conditions were evaluated at the laboratory scale using a Lamort Pulper & Flotation cell. The results for each source of alkali were compared in terms of: brightness, ERIC value, COD and bleaching response to sodium hydrosulfite. The economics in terms of variable cost was also assessed.

Very interesting results were observed and the following main conclusions could be drawn:

- under low total alkali (i.e. 0.1 to 1 %) and low peroxide dosage (i.e. 0.2 to 1 %) condi-tions, the deinking efficiency is not decreased and could even be increased when pH is lowered.

- under these conditions, the deinking efficiency is not directly correlated with the hy-drogen peroxide charge since most of the brightness increase comes from deinking and neither from bleaching nor darkening prevention.

- hydrogen peroxide is nevertheless necessary, since lower efficiency is always ob-served without it, with an optimum charge around 2 to 5 kg/t.

- under these near-neutral conditions, the use of Mg(OH)2 as an alternative alkali source appeared to be well adapted for both higher efficient deinking, lower COD re-lease and cost efficiency.

1. INTRODUCTION

The use of recycled fibers to manufacture different kind of papers (newsprints, magazines, tissues and other higher quality paper grades) has constantly increased over the last years. Such recycled pulps are widely obtained from ONP-OMG mixtures through a deinking proc-ess consisting in converting the wastepaper to pulp in a deinking medium (pulping) and separating the detached ink by flotation. The conventional deinking chemistry at the pulper uses caustic soda to swell the fibers and help ink detachment along with hydrogen peroxide to prevent alkaline darkening, sodium silicate for peroxide stabilization and a fatty acid soap for ink anti-redepositing. This alkaline pulping suffers from several drawbacks: the resulting high pH increases COD in the mill effluent and the multi-product recipe induces high chemical cost.

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ABSTRACT

This paper is focused on evaluation and comparison of the different modelling approaches of the ECF bleaching process. The important parameters and tendencies were determined from the existing models and experimental data. Identification of these key governing parameters led to propose new comprehensive empirical models, which were simpler, more accurate, applicable to broader range of process conditions and easily adjustable (if required) with different kinds of wood species. Mathematical models were developed for the first chlorine dioxide (D_o) and extraction (E_o) stage. The chlorine dioxide consumption with respect to kappa number decrease was found to be dependent on temperature, initial kappa number and initial chlorine dioxide charge. The initial ratio of slow and fast kappa numbers in extraction stage was found to be dependent on kappa factor in D_o stage. It is now possible to separate modelling of D and E stages and also reinforcement of E stage with peroxide and oxygen could be predicted with separate models. Also more accurate correlation between K₄₅₇ of D₁ stage and kappa number of E_o stage was established.

1. INTRODUCTION

With increasing chemical costs, tougher environmental regulations and tighter customer demand, there is a need for an accurate and robust kinetic model that can be used for process optimization and control of bleach plant. This paper is focused on evaluation and comparison of the different modelling approaches of the ECF bleaching process. The important parameters and tendencies were determined from the existing models and experimental data. Identification of these key governing parameters led to propose new comprehensive empirical models, which were simpler, more accurate, applicable to broader range of process conditions and easily adjustable (if required) with different kinds of wood species. Mathematical models were developed for each pulp processing unit and linked together to create a generic bleaching stage in a simulator for ECF process. This paper discusses in detail, particularly, the first chlorine dioxide (D_o) and extraction (E_o) stage.

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