Pulping operations digester

By-Products. There are three stages within the pulping operation at which wood-derived chemicals can be recovered as by-products. Turpentine is obtained from the reHef of gases after an initial steaming of chips in the digester. Better yields of turpentine are obtained from batch digesters than from continuous systems. Pines and firs give the best yields. Turpentine is composed principally of unsaturated bicycHc hydrocarbons, of which ca 90% are a- and P-pinenes and 5—12% other terpenes. 

In the Kraft recovery process the green liquor, which is an aqueous solution of sodium carbonate, is heated with lime to produce white liquor or caustic soda, which is then returned to the pulp digestion operations. In the production of alumina, lime and soda are fed to bauxite digesters. The CaCO 3 produced during the course of the reaction is rebumed to lime and is recycled. The main difficulties associated with these processes include the extensive mechanical handling through the use of causticizers, settlers, and repulpers in order to produce caustic that is low in carbonate and the high fuel consumption needed to reconvert CaCO to lime. 

The pulp to be treated with cellulolytic enzymes should first be screened in a laboratory flat screen to remove any shives. The screened pulp is beaten either in a laboratory Hollander beater (TAPPI Standard Method T 200 os-70) or in a PFI mill (TAPPI Standard Method T 248cm-85), depending on the amount of pulp to be treated, to a final Canadian Standard Freeness of 200-300 ml. The beating operation helps convert shives into fibers and improves the digestibility of the pulp towards cellulolytic enzymes. The pulp is dewatered to a consistency of approximately 25% and stored in a polyethylene bag after fluffing. 

Historically chemical pulping was carried out in large batch digesters (200-400 m ). However, the trend is towards the use of continuous digesters, now accounting for about 65% of kraft production. The largest in operation have production capacities of more than 2000 tonnes of pulp a day. 

Kraft pulping may be earried out in either bateh or eontinuous digesters, the use of which has been described earlier. The traditional bateh operation is simpler to describe operationally. 

The long warm up period contributes little to the H factor because reaetion kinetics dictate that the rate of delignification more than doubles for every 10°C rise in temperature. The equation is useful as it allows mill operators to determine when to stop the cook even though the operating temperatures have differed from normal, for example because of fluctuations in steam supply to the digesters. Operators merely have to ensure that they cook to a constant H factor (Figure 13.12b), as pulps eooked to the same H factor have essentially the same properties, i.e. yield, lignin eontent (kappa number). Results can be compared provided the initial alkali and sulphidity eoneentrations are the same. 

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