The common goal of all chemical pulping processes is to separate fibers by delignification, but the processes can be classified according to the method used to achieve this goal. The delignification is accompanied by reactions of polysaccharides, which can be considered as side reactions, but these reactions play an important role in pulp yield and pulp properties. The reactions of the extractable are also important. Delignification reactions can be carried out under alkaline, neutral and acidic conditions. The mechanisms and results involved are different.
Of particular importance for alkali, delignification is the alkaline hydrolysis of the phenol ether bonds, whereby the lignin becomes soluble in alkaline solutions. The sulphation of sulfur hydrogen ions in sulfate cooking is not known. It may accelerate the cleavage of phenolic ether bonds, leading to direct cleavage of alkyl ether bonds, but it may also prevent the condensation of alkali-sensitive groups, which decreases the delignification rate.
In acid sulfite pulping, the sulfonation of the α-aryl and alkyl ether groups makes lignin water-soluble, followed by sulfite or acid hydrolysis of the alkyl ether bonds. The sulfonation of the terminal groups will prevent the recondensation of the lignin. Neutral sulfite cooking is generally less dignifying, utilizing sulfonation of certain groups to convert the lignin molecule to a hydrophilic sulfonate, which is then dissolved by hydrolysis or sulfite hydrolysis.
All methods of delignification are associated with undesired side reactions. Lignin condensation is the key reaction, which occurs in both acidic and alkaline cooking. The condensed lignin is dark in color and is difficult to dissolve in bleaching. Improper cooking conditions can lead to significant lignin condensation, especially in acid cooking, resulting in high sieve rates and poor pulp quality. Condensation with other phenolic compounds – for example, tannins in the heartwood and bark of pine – strongly limits the feedstock that can be used in the conventional acid sulfite process.
Sulfite pulping is the primary method of producing bleached and natural color pulp for paper production. The sulfite process, which can be applied over a wide pH range, is primarily used for the production of specialty pulp, such as dissolving pulp. Semi-chemical methods, such as neutral sulphite semi-chemical (NSSC) pulping, are used for the production of packaging products such as corrugated core paper.
The influence of process parameters on cooking results can be divided into two areas: those related to raw material properties and those related to process conditions. Raw material properties are difficult to control but should be taken into account when setting cooking operating conditions. For example, the ratio between the species of wood and the basic properties of the wood can only be controlled by good management of the supply logistics, but the quality (size) and conformity of the chips are controlled by the pulp mill itself. Process parameters, such as the amount of cooking agent (grams of chemical per gram of dried wood), the composition of the cooking liquor, cooking time, and temperature are adjusted by process control measures.
The most important parameters for steaming can be either wood-related or process-related. Some of the wood-related variables are morphological properties (size and distribution of fibers), the chemical composition of the wood, density, moisture content, age, and location on the original tree, and its decay condition. Some of the process-related variables are chip cleanliness and chip size, chemical composition, cooking agent dosage (g/g chips), and cooking liquid concentration (g/L), as well as time and temperature and their relationship.
Good process control requires accurate, fast, and practical analytical methods. In-line continuous measurement is preferable, but not always possible. Pulp properties, such as cooking uniformity, residual lignin content in the pulp, degree of carbohydrate degradation, or fiber length distribution, are not easily measured online and must be measured using fast and common laboratory methods. These methods do not require direct measurement of the basic properties, but should reflect their changes accurately and in a meaningful way, and should be sufficiently relevant to the basic properties in question to allow good process control. Such measurements are called predictive, and they give predictions that can be anticipated in terms of quality. Simulation methods are those in which the application of the pulp is simulated in the laboratory. For example, a paper sheet may be copied under standard conditions and its properties, such as paper strength, brightness, or opacity, measured.
