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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Chemical processes > Process modelling and optimization: new tools for the pulping industry | Update: July 20, 2011 |  |
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Scientific report of the LGP2 (2006-2009)
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| Researchers of the LGP2 (May 2011) |
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Our goal is to develop new models for chemical and thermal optimization in the pulping industry. We developed a Kraft cooking simulator and an ECF bleaching simulator (elemental chlorine free sequence), based on laboratory experimentation accompanied by industrial scale testing. The developed models rely on kinetics, thermodynamics and mass transfer, and also partly on semiempirical approaches to allow for wood and pulp variability.
A batch Kraft cooking simulator has been improved from early developments. It allows the description of simultaneous mass transfer and delignification kinetics in three phases (initial, bulk and residual) in a plane wood chip. Successive 1st order kinetic laws and variable alkali diffusivity in wood chip wall are assumed. Rate constants and diffusivities are function of temperature and reaction level. In the external liquor, convective external mass transfer takes place. During batch cooking, several steps of temperature and varying chips, differing by size (thickness) distribution or wood nature (affecting chemical kinetics), can be applied. A Ph.D work has targeted the cooking behaviour of various species and mixtures of them.
A full ECF bleaching sequence model was created by assembling single bleaching stages models (D, EP, EO, EOP), based on semi-empirical kinetic and stoichiometric equations. Results were supported by a wide literature survey and new experimental data. A main upgrade of the model is the prediction of COD and hard COD, which had never been previously modelled.
It was found that replacing a single prebleaching stage by a cascade of (DE) stages with optimized intermediate washing, by dividing the reactant charge, yielded much higher bleaching efficiency; up to 30% of the reactant was saved in the full sequence. This was a consequence of a higher reaction selectivity oriented on the free phenolic groups in lignins, avoiding secondary reactions.
In bleaching processes using towers in series with long retention times, transitional periods from one pulp to another yield mixed pulp qualities. To identify this phenomenon pulp quality indicators were searched and finally, certain fibre morphological properties were selected as valuable parameters, allowing easy in-line process control implementation.
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| Figure 1 - Predicting the effects of temperature gradients in a Kraft digester on the kappa number distribution of the pulp. Blue curve, uniform temperature (170°C) – green curve: -2.7°C wall temperature gradient – red curve: -4.5°C wall temperature gradient |
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