In chemical pulping, using cooking chemicals and high temperature, lignin is dissolved from wood tissue in order to separate fibres from the wood matrix. It is very important that cooking chemicals are presented homogenously in all parts of the wood tissue before cooking temperature is reached, otherwise the quality of pulp will suffer. Poor impregnation of cooking chemicals is reflected as decreased uniformity of pulping (Gustafsson et al. 1989; Gullichsen et al. 1992; Malkov et al. 2003), resulting in steep delignification gradients inside the chips (Malkov et al. 2003) and higher amounts of rejects in the final pulp. Non-uniform pulping may also lead to complications in the performance of downstream operations such as bleaching (Stone, Green 1959). Cooking chemicals are transferred into wood by penetration and diffusion. Penetration is defined as the flow of liquor into the air-filled voids of the chips under the pressure gradient, and diffusion as the movement of chemical ions through liquid under the influence of concentration gradients. In fresh wood both mechanisms occur at same time, but most of the chemicals are transferred into wood by diffusion. The role of penetration is to fill fiber cavities with liquid, which enables faster and more uniform diffusion of chemicals into wood.

In conventional digesters, all of the required cooking chemical, in the form of white liquor, is added at the beginning of the process. In liquid phase cooking, as temperature rises, the distribution of chemicals within wood occurs mainly by diffusion. If the rate of temperature rise is slow enough and the penetration of wood chips is complete, a sufficient amount of chemicals is able to enter the chips before cooking temperature is reached. In steam phase cooking, the role of impregnation is emphasized because all the chemicals have to be transferred into the chips during the impregnation stage (Kleinert, Marraccini 1965; Kleinert 1968). In some modified cooking processes, for example in the WLI process (White Liquor Impregnation process) and its commercial version, Enerbatch, most of the alkali is transferred into the chips at low (90ºC) temperature during white liquor impregnation, resulting in homogeneous cooking with a low reject content and improved yield (Jokela 1987; Wizani et al. 1992; 1996). In today’s modified cooking systems impregnation is typically carried out with black liquor enhanced with white liquor having a high HS-/OH ratio to improve the selectivity of cooking (Gullichsen 1999, Sixta et al. 2006). The role of the impregnation stage is only to secure penetration of chemicals into the chips. Most of the chemicals are transferred into the chips by diffusion during the cooking stage. In order to maximize pulp yield and to avoid non-uniform cooking, cooking temperatures have been reduced from 170ºC to 150ºC with softwoods. Cooking time has been extended from 1 h to 4-6 h (Marcoccia et al. 1999, Sixta et al. 2006). At the same time as cooking times have been extended, digester capacities have been increased. Before the 1980s, digester capacities were typically under 1000 ADt/d, whereas the capacities of modern digesters can be over three times higher (Toivonen 2004). Digester capacity is mainly increased by increasing the digester’s diameter. As the digester diameter has grown, the risks related to channeling and non-uniform temperature and alkali profiles in cooking have increased, causing variations in pulp quality (Haas 1990; Toivonen 2004). Proper and uniform impregnation of chips prior cooking is one possibility to minimize drawbacks of high capacity digesters.