The shape and size of the cooker for chemical pulping is constantly being developed and improved. The first boilers were intermittent boilers. Initially, the cooker was a rotating spherical cooker. The rotation was to facilitate the contact between the cooking liquid and the wood chips, which facilitated the penetration of the liquid. The sphere rotates 2.5~5 times per minute, its volume is 35~40m3, and each ball produces 0.2~2.0t of pulp. The spherical cooker was soon replaced by the cylindrical cooker with conical bottom, but the rotation of the cooker still continued for some time. The output of the cylindrical cooker was about 3t of air-dried pulp, with a higher output of 5t of air-dried pulp. The boilers typically rotated once every 10 minutes. Rarely, there are a few cylindrical cookers running and producing small amounts of pulp.
Intermittent cookers can be heated with direct steam or the cooking liquid can be continuously circulated between the cooker and the heat exchanger, through which the cooking liquid is heated. The external circulation greatly improves cooking uniformity compared to rotary or direct steam heating. The volume of the intermittent cooker has been increasing all the time and currently, the cooker volume is up to 400m³.
Rotary Ball Cooker (Steam Ball)
Rotary cylindrical cooker
Vertical Intermittent Cooker
Modern intermittent cooker
Continuous cooking systems are generally associated with the Kamyr system. The history of Kamyr started in 1938 when the first pilot (5t/d) continuous digester was built in Karlsborg, Sweden, and between 1948 and 1952 the Kamyr AB experiment with a capacity of 50t/d was carried out in Fengersfors, Sweden.
The size of the autoclave increases with the production rate. The first continuous cooker had a diameter of 2.5 m and a height of 21 m, with a capacity of 100 t/d of pulp. One of the newest continuous digesters is now located in Fray Bentos, Uruguay, with a diameter of 10.6 m, a height of 60 m, and a capacity of 3200 t/d. In June 2010, a 12.5 m diameter, 72 m high continuous digester with a design capacity of 5160 t/d of pulp was started up in Rizhao, China.
Modern twin-tower continuous cooker by Fray Bentos
Over the years, both process and technology have undergone fundamental progress. Sulfite pulping has almost disappeared in favor of sulfate pulping, due to the obvious advantages of sulfate pulping in terms of economic efficiency, environmental protection, and product quality. With regard to the types of digesters, the most newly built environmentally friendly digesters are the continuous type, as they have a smaller investment cost than intermittent digesters, which require a large amount of equipment and a larger land area. While there have been extensive research and development projects for new alternatives to sulfate pulp production, it is likely that most of the world’s chemical pulp will be produced by continuous digesters using the sulfate cooking process, at least in the short term future.