Valmet Continuous Cooking is Valmet’s system for more cost-effective pulp production. It is the first-hand choice for kraft mills pulping hardwoods (eucalyptus, acacia, birch etc.), softwoods (pine, spruce etc.) or bamboo.
In 1948 Valmet received the first order for a commercial continuous digester. The development of the continuous cooking system has resulted in a number of innovative features. The new digester process builds on the strength of previous technologies and experience that has been built up over the years.
The latest steps of development make use of the most recent progress in research while also focusing on simplifying the cooking system. This lead to the Valmet Continuous Cooking concept.
Simplifying the system has significantly reduced the investment and operational cost. Increased flexibility and a process that is easier to operate are other benefits from the simplification of the system. The development aslo includes a new layout where pipe length, accessibility & maintenance has been improved.
Valmet ImpBin™ chip impregnation system establishes a thorough impregnation of the chips with liquor before cooking. The atmospheric Impbin combines the basic features of the traditional chip pre-steaming bin, impregnation vessel and flash system. Impregnation of chips at low temperature for a longer time has proven to be very effective generating pulps with very low reject content. Effective impregnation in combination with the benefits of Valmet’s continuous cooking process produce pulps with high yield and excellent pulp quality.
Valmet’s continuous cooking process chemistry is a further development of the “four rules of Kraft cooking” originally developed at STFI and the Royal Institute of Technology. The Continuous Cooking system is a very flexible process allowing the liquor to wood ratio to be controlled in a unique way during the various parts of the cooking process.
This added process flexibility allows the alkali profile to be optimized achieving a more selective cook with less carbohydrate degradation resulting in better pulp quality and higher yield. Valmet’s continuous cooking process re-circulates black liquor to the impregnation and cooking stages. As a result the hydrogen sulphide concentration is increased during the impregnation and cooking stages. By optimizing the hydroxide and increasing the hydrogen sulphide concentration it has been possible to increase the selectivity of the Kraft process in the initial cooking phase, the bulk delignification stage, and thereby decreasing the amount of slowly reacting residual phase lignin. The above has led to an increase in the rate of delignification in the bulk phase which is utilized to decrease the cooking temperature. Decreasing the temperature minimizes carbohydrate degradation and strength losses.
The Continuous Cooking system design is based on results from the latest research on delignification chemistry. Benefits of a two-vessel system compared to a single-vessel are many. A two-vessel system has no radial temperature and alkali concentration variations, well defined impregnation zone, simple liquor to wood ratio control of the different cooking zones, recirculation of black liquor to the impregnation phase for process benefits.