Towards complete impregnation of wood chips with aqueous solutions. Part I. A retrospective and critical evaluation of the penetration process.


This paper evaluates the process of liquid penetration into wood chips from the retrospective viewpoint of industrial development and research and provides a summary of currently available knowledge in this area. During the past few decades, researchers have been dealing mainly with pulping chemistry and the back-end of pulping processes, while less attention has been paid to the front-end of cooking. As a result, the latest developments in chemical pulping have been driven by pulping chemistry, and the importance of front-end phenomena has been ignored. In spite of the existence of several concepts and theories, several aspects still remain unclear and more research is needed to achieve a complete understanding of the process of liquid penetration into wood chips. Special emphasis is given to the need for model of liquid penetration and for new reliable and accurate methods that provide direct continuous data on the process. The paper also discusses the effect of different factors on the penetration process as well as techniques used to improve the penetration efficiency. More effort has to be directed to research concerning techniques designed to promote liquid penetration, including chip presteaming. Industrial applications of chip presteaming allow some scope for improvement, especially in batch cooking systems. To achieve the ultimate target of effective heating and fast gas removal from the chips, special attention has to be paid to optimisation of steaming parameters such as retention time and the pressure-temperature relationship. Against this background, developing a model of chip presteaming is particularly important.