Experimental Methodology for Heterogeneous Studies in Pulping of Wood

Abstract

The heterogeneous nature of the kraft pulping of wood is experimentally demonstrated in this paper, and a new methodology for kraft pulping investigation is presented. The strategy proposed here enables the measurement of alkali and of lignin concentrations in the pulping liquor, both inside and outside of the wood chips. With this procedure, it is possible to independently determine, in both entrapped and free liquors, the time histories of the concentrations of alkali and lignin as well as total dissolved solids, which are a direct result of the mass transfer and of the chemical reactions that take place during this heterogeneous process. The influence of the chip thickness and of temperature on the relative rates of these two phenomena is also highlighted. This experimental methodology establishes the foundations for the development of a macroscopic heterogeneous kraft pulping model that can be experimentally validated in pulping conditions, even for modified digester processes.

Improved impregnation efficiency and pulp yield of softwood kraft pulp by high effective alkali charge in the impregnation stage

“…Thinner chips can also contribute to this goal via homogeneous impregnation (Hartler and Onisko 1962;Jimenez et al 1990;Gullichsen et al 1992;Egas et al 2002;Malkov et al 2003). However, this approach has limitations not only because of the higher energy demand for chip production but also for higher amounts of pin chips and fines, which result in yield losses.…”

Heterogeneous studies in pulping of wood: Modelling mass transfer of alkali

“…One assumption refers to not accounting for the mass transfer limitations of alkali in thin chips. This implies that the concentration profiles of alkali in both the free and the entrapped liquors are the same along the cooking experiment, which is not confirmed experimentally, as shown by Egas et al [10]. Moreover, the reactions between alkali and the wood components other than lignin and carbohydrates are neglected, thus leading to unrealistic alkali profiles during cooking.…”

“…This paper proposes a model for the mass transfer of alkali from the bulk liquor to the inside of the wood chips that is based on data on the composition of both free and entrapped liquors. The pulping experiments followed the methodology proposed by Egas et al [10] and the relevance of the information gathered on the entrapped liquor has been confirmed by Simão et al [14,15]. This mass transfer model relies on a lumped parameter approximation, in which the concentration profile of alkali inside the chip is represented by its spatial average concentration.…”

“…The experimental procedure adopted here has already been described in detail by Egas et al [10]. Handmade Eucalyptus globulus chips (size: 30 mm × 30 mm × 6 mm) were cooked in a six-vessel computer controlled batch-digester system described by Romanenko and Castro [16].…”

“…However, in the model proposed for impregnation, it is assumed that no chemical reactions occur between alkali and wood components and that sodium does not adsorb on the solid matrix. These phenomena may not be always negligible [10,12,13] and this may influence the diffusion parameters calculated. Moreover, the low range of temperatures used in the experimental studies (25-100 • C) does not enable using their diffusion model in normal pulping conditions.…”