Typically, delignification requires several hours (1.5 hours) at 170 to 176В вЊ (338 to 349В вЏ). In a continuous digester, the materials are fed at a rate that allows the pulping reaction to be completed by the time the materials exit the reactor. Digesters producing 1,000 tonnes or more of pulp per day are common, with the largest producing more than 3,500 tonnes per day. There are several variations of the cooking processes both for the batch and the continuous digesters. Some digesters operate in a batch manner and some in a continuous process. The wood chips are then cooked in pressurized vessels called digesters. About 40вЂ“60% of all alkali consumption, in the continuous process, occurs in the impregnation zone. A good impregnation is important to get a homogeneous cook and low rejects. In the impregnation, cooking liquor penetrates into the capillary structure of the chips and low temperature chemical reactions with the wood begin. The cooking liquors consist of a mixture of white liquor, water in chips, condensed steam and weak black liquor. The impregnation can be done before or after the chips enter the digester and is normally done below 100В вЊ (212В вЏ). Air remaining in chips at the beginning of liquor impregnation is trapped within the chips. The next step is to saturate the chips with black and white liquor. The steam treatment causes the air to expand and about 25% of the air to be expelled from the chips. Cavities inside fresh wood chips are partly filled with liquid and partly with air. The chips normally first enter the presteaming where they are wetted and preheated with steam. The process Impregnation Д¬ommon wood chips used in pulp production are 12вЂ“25 millimetres (0.47вЂ“0.98В in) long and 2вЂ“10 millimetres (0.079вЂ“0.394В in) thick. For this reason, in the 1940s, the kraft process superseded the sulfite process as the dominant method for producing wood pulp. It enabled the recovery and reuse of the inorganic pulping chemicals such that a kraft mill is a nearly closed-cycle process with respect to inorganic chemicals, apart from those used in the bleaching process. Tomlinson in the early 1930s was a milestone in the advancement of the kraft process. The invention of the recovery boiler by G. Patent 296,935 was issued in 1884, and a pulp mill using this technology began in Sweden in 1890. Dahl in 1879 in Danzig, Prussia, Germany. The kraft process (so called because of the superior strength of the resulting paper, from the German word Kraft for 'strength') was invented by Carl F. A precursor of the Kraft process was used during the Napoleonic Wars in England.

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