Pulp digesters

Rotary kilns and, to a lesser extent, Fluo-SoHds kilns are used to calcine a wet precipitated calcium carbonate filter cake in the kraft or sulfate paper-pulp process (15). Lime is regenerated for use as a causticization reagent in recovering caustic soda for pulp digestion. Losses in lime recovery are replaced by purchased lime (see Paper Pulp). 

Sulphurous acid occurs in bisulphite manufacture, as well as in various processes such as cellulose pulp digestion and oil refining. Since some oxidation to sulphuric acid, which makes the conditions much more corrosive, is often involved, the use of molybdenum-bearing types is generally preferred. 

Cellulose produds There is little scope for the use of austenitic steels in the alkali processes for the digestion of wood pulp, but both molybdenum-free and molybdenum-bearing steels are extensively used for sulphite pulp digestion, choice depending on concentration, temperature and working experience. 

II There are both successful and unsuccessful pulp digester installations. 

To illustrate how different m(X ) and x may happen to be, let s consider as a specific example (others can be found in Saraiva and Stephanopoulos, 1992c) a Kraft pulp digester. The performance metric y, that one wishes to minimize, is determined by the kappa index of the pulp produced and the cooking yield. Two decision variables are considered H-factor (xj), and alkali charge (X2). Furthermore, we will assume as perfect an available deterministic empirical model (Saraiva and Stephanopoulos, 1992c), /, which expresses y as function of x, i.e., that y =/(xi, X2) is perfectly known. 

In order to verify how close to a known true optimum the final solutions found by our learning methodology happen to be, we will describe here its application to a pulp digester, for which a perfect empirical model /(x) is assumed to be available. Other applications are discussed in Saraiva and Stephanopoulos (1992c). 

F. J. Doyle and F. Kayihan, Reaction profile control of the continuous pulp digester, Chem. Eng. Sci., 54, 2679-2688 (1999). 

Tall oil is the generic name for the oil obtained upon acidification of the black liquor residue from kraft pulp digesters. Kraft processing dissolves the fats, fatty acids, rosin, and rosin acids contained in pinewoods in the form of sodium salts and when the black liquor is concentrated to make it possible to recover some of its chemical and heating value, the soaps become insoluble and can be skimmed off. The brown, frothy curd thus obtained is then made acidic with sulfuric acid, converting the constituents to a dark-brown fluid (tall oil). 

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. 

Thomas, B., Carbon brick linings in alkaline pulp digesters, TAPP 37 174-176 (1954). 

Liquor recycle to the sulfiting tower is continued until the pH of the underflow reaches 8.5, at which time a portion of this stream will be bled off for use as pulping digestion liquor. Under these conditions the proportions of salts present in solution are about 82% Na2S03, 4% NaHSOs, and 14% NaHCOs. For some species of wood or for different pulping objectives liquor of about pH 8 may be used. This is made by more prolonged contact between the sulfur dioxide and sodium carbonate solutions (Eq. 15.7). 

PCA was able to distinguish three sources of PCDD and PCDF in crab hepa-topancreas pentachlorophenol, polychlorophenol condensation during pulp digestion, and chlorine bleaching [68]. Following the introduction of controls on the discharge of PCDD and PCDF by the pulp mills in British Columbia (Canada), the concentrations of the chlorine bleaching-related tetrachloro PCDFs have decreased more rapidly than the concentrations of hexachloro PCDDs. The latter are associated mainly with polychlorophenol condensation products and with pentachlorophenol. 

KRAFT PROCESS - A wood-pulping process in which sodium sulfate is used in the caustic soda pulp-digestion liquor. Also called Kraft pulping or sulfate pulping. 

---