Impregnation, vapor phase and methanol as means of intensifying the softwood kraft pulping process

Abstract

The objective of the research was to find ways to shorten the cooking time, i.e. intensify the kraft pulping process. The reason for undertaking such a study lies in the long standing trend of ever increasing reactor size in the kraft pulping industry. The huge digester size in use presently has lead to severe problems in understanding the behavior of the chip column inside the digester. An intensified process with a drastically shorter pulping time would give a more manageable process and greater freedom in reactor design.

The study was performed using a new experimental digester giving a much greater control over temperatures than what can be achieved with other types of digesters. This enabled experiments that clarify the impact of impregnation, heat-up time, cooking temperature and cooking time to a greater degree than what has been possible earlier. The research on actual intensification centered on understanding the impact of impregnation and the impact of alcohols (methanol) on the overall rate of pulping.

This research supports earlier research that shows how the cooking time can be shortened using alcohols as additives in pulping. It also supports results showing that a fast process can be achieved by using impregnation with high concentrations of cooking chemicals followed by a cooking stage performed with direct steam heating. The fact that the effects work in synergy so that the fastest pulping process identified was one that employed high concentration impregnation followed by heating using methanol steam is a new finding. The decrease in cooking time compared to a conventional liquid phase batch process without proper impregnation is close to 70%.

The present research was aimed only at shortening the cooking time and does not address questions related to actual digester and process design and economical feasibility of the process. Especially the regeneration of cooking chemicals and methanol are an area that will need further study before such question can be addressed.