Chemical pulping, from alkaline to acidic, includes many different processes. The pulping process used in industry is primarily the use of aqueous solutions of inorganic chemicals to pulping at high temperature and pressure. The dominant chemical pulping process is the alkaline sulfite pulping process (and its variants), which has a wide range of applications because it can be used for pulping all lignocellulosic raw materials. It is also very energy efficient. The sulfate pulping process is particularly suitable for the production of high strength and resilient pulp fibers from coniferous woods, and it is also suitable for hardwoods because of its ability to handle extractable efficiently. Low yields and difficulty in bleaching the pulp are disadvantages of sulfate pulping. However, the recovery of chemicals allows for a closed-loop and reduces environmental contamination.
The output of acid sulfite pulping has been declining over the past decades. Despite its high yield, good selectivity, and ease of bleaching, acid sulfite pulping has often been replaced by alkaline pulping because of the quality of the pulp fibers, the limitations of raw materials, and the difficulty in adapting to contemporary environmental regulations. A few sulfite pulp mills are still operating in Europe. Most high yield chemical pulping processes use neutral or mild alkaline sulfite semi-chemical cooking liquors. In addition, the chemical reaction between the cooking liquor and the lignin at the impregnation stage is similar to that of sulfite pulping, e.g., chemically pretreated thermomechanical pulp (CTMP).
Chemical pulping separates the fibers from the wood structure by dissolving enough lignin from the intercellular layer that no or very little mechanical action is required and the fibers can be separated from each other without damage. The yield and lignin content of the pulp depends on the species of wood and the method of pulping, but the yield of the pulp after the separation of coniferous wood fibers is about 60% and the lignin content of the pulp is about 10%.
The chemical reaction of wood components is non-homogeneous at the solid-liquid interface. Inwood pulping, the chips are immersed in a cooking liquid at high temperature and pressure. To ensure a uniform reaction, uniform distribution of the liquid, and temperature in the wood chips is essential. In order to ensure uniformity of reaction, it is essential that all fibers in the wood get their respective proper share of chemicals and energy. If not homogeneous in this respect, the cooked pulp will contain a large number of unseparated chips (sieve residue), darkening of the pulp color, reduced fines yield, and reduced bleachability and strength of the paper. The basic properties of wood determine most of the characteristics of pulp fibers. Only process aspects that affect pulp properties are discussed here. Chip size is important for mass transfer within the chip, especially at the impregnation stage. The shorter the chip is, the more fibers are cut during the chipping process. Modern chipping machines, which cut in the direction of the grain, produce chips of corresponding length and thickness. Chip thickness is critical for the transfer of chemicals during the impregnation stage and cooking process, as it determines the mass transfer properties.
The goal of chemical pulping is to remove lignin, not only from within the fiber walls but also from the intercellular layer, so that the fibers can be separated better. Ideally, each fiber should receive the same amount of chemical treatment in the same amount of time and at the same temperature. This means that the chemicals as well as the heat energy must be delivered evenly throughout the reaction parts of the woodchip. The cooking process has two main stages: (1) the impregnation stage, where the wood chips are filled with cooking liquid before the delignification reaction begins, and (2) the cooking stage, where the cooking liquid (chemicals) are constantly moving toward the reaction site. Woodchip size – and especially woodchip thickness – is very important in this case. The thicker the wood chips, the longer the distance that the liquid diffuses into the center of the chips.
The interior of a fresh wood chip is partially filled with liquid and partially filled with air, the ratio of which depends on the moisture content of the wood. Air must be removed from the chips before the cooking liquid can penetrate sufficiently into the interior of the chips. Steam is usually used to remove the air. The wood chips are heated with steam, causing the air inside the chips to expand and some (~25%) of the air to be removed. Increasing the water vapor pressure in the wood will remove more air. The diffusion of steam inward and air outward will further reduce the air content of the wood chips. However, the rate of air and water vapor transfer at this stage is slow and can only be accomplished with sufficient time. The important parameters of steam are temperature, time, and vapor pressure. A certain amount of air can also be removed by vacuuming, but this is not used in practice.
A fully steamed piece of wood is immersed in a cooking liquid at a certain pressure, which causes condensation of water vapor inside the piece, further increasing the pressure gradient between the free liquid and the inside of the piece. Capillary pressure is generated under these conditions. The rate of immersion depends on the vapor evaporation conditions and the amount of pressure applied. The following figure shows an example of the relationship between vapor pressure and temperature of the impregnating liquid and the effect of penetration.
The effect of steam pressure on penetration (steam time is 10 min) (spruce wood chips, steam steamed wood chips with 30 ℃ to 90 ℃ water at 2 kg/cm2 (original 2 KP/cm2) pressure, when the penetration rate of 80% to 98%, the time required).
Note: ATM stands for atmospheric pressure.
The reacting ions must diffuse into the interior of the wood chips for the cooking reaction to take place. If the diffusion distance is too long and the diffusion rate too slow, all of the cooking agents is consumed before it reaches the center of the wood chips, which will result in inhomogeneity of the delignification. Therefore, there is a critical equilibrium between the rate of ion transfer, the thickness of the wood chips, and the rate of a chemical reaction. The rate of diffusion is controlled by the concentration difference between the internal and external solutions of the wood chips. Increasing the temperature increases the diffusion rate, but the chemical reaction rate increases even more. On average, thick chips do not remove as much designing as thin chips under the same cooking conditions. This was confirmed by Hartley et al. by cooking heartwood chips of different thicknesses under standard sulfate cooking conditions, varying only the cooking temperature, and by Gullichsen et al. by the same.
Non-uniformity of coniferous wood sulfate slurry when cooked to an average kappa value of 23.4
Thick wood chips produce more sieve residue than thin wood chips, and cooking at high temperatures is more severe. The higher the cooking temperature, the greater the difference between the delignification rate and the penetration rate of the liquid. Cooking uniformity requires that the chips be sufficiently thin and that the difference in thickness between the chips be as small as possible (i.e., the thickness of the chips be as uniform as possible). Other things being equal, thicker chips require more alkali than thin chips to achieve the same degree of delignification. Cooking inhomogeneity can be reduced or even eliminated by proper chipping and screening of chips, good liquid penetration, and sufficiently low cooking temperatures.
