Cooking principle
The aqueous solution of chemicals, that is, cooking liquid, reacts with plant materials to remove lignin and separate fibers from each other.
Cooking is a complex multi-phase chemical reaction process involving multiple compounds and a complex physical and chemical change process.
Why do we say this?
Because plant materials contain multiple compounds and are solid, while cooking liquid is liquid, a chemical reaction needs to occur between the two. First, they must come into contact before a chemical reaction can occur.
The entire cooking process is carried out in stages and can be divided into 5 stages.
- The ions (OH-, SH-, etc.) in the cooking liquid penetrate and diffuse into the material pieces;
- Subsequently, the chemical components such as lignin in the material pieces adsorb the OH-, SH- and other ions in the cooking liquid;
- After adsorption, the cooking liquid and the components such as lignin react chemically;
- The reaction products dissolve and diffuse to the outside of the wood chips;
- Finally, the reaction products are transferred to the surrounding liquid;
The entire cooking process is completed.
The penetration of cooking liquid on the sheet
During the cooking process, the penetration of the liquid can be divided into two types according to the driving force:
- Pressure penetration
Pressure push, that is, pressure difference, that is, the effect of capillary action and external pressure.
The effect of pressure penetration depends on the flow rate of the liquid through the capillary. At the same time, the level of external pressure and the viscosity of the liquid will also affect the effect of pressure penetration.
In general, whether it is alkaline or acidic cooking liquid, the capillary action of the fiber axis is always about 50-100 times greater than the transverse capillary action. - Diffusion penetration
The driving force is the concentration difference of the liquid.
The effect of diffusion penetration mainly depends on the effective cross-sectional area of the capillary. The concentration difference of the liquid is also related to the activity of the drug molecules or ions, and is also affected by temperature and raw material moisture.
The overall penetration path of the cooking liquid
During the cooking process, capillary action, diffusion and chemical reaction are almost carried out simultaneously, but there is a primary and secondary distinction.
In the early stage of cooking, especially when the moisture content of the raw materials is low, the penetration of the liquid is mainly based on the capillary action, while in the middle and late stages of cooking, when the moisture content of the raw materials reaches the fiber saturation point, the penetration is mainly diffusion, especially when the temperature exceeds 140 degrees Celsius, diffusion becomes the main form of liquid penetration.
Due to their morphological differences, different fiber raw materials have different overall penetration paths during their cooking.
For example, for coniferous wood, the cooking liquid enters from the end of the open fiber, that is, the end of the wood chip, passes through the pits, and moves sideways through the rays from one cell to another.
For hardwood, the cooking liquid enters the rays and longitudinal thin-walled cells through the pits from the vessels, and then passes through the pits into the fibers.
Diffusion pathway of cooking liquid ions from the cell cavity into the intercellular layer
Cooking liquid ions first pass through the third and second layers of the secondary wall from the cell cavity, then pass through the first layer of the secondary wall, and finally reach the intercellular layer with a high degree of lignification and low porosity.
During the penetration process of the liquid, it will be affected by many factors, such as the composition of the liquid, temperature, pressure, type of fiber raw material and specifications of the sheet.
1.The influence of the composition and pH value of the liquid on the penetration
The pH value of the liquid changes with the composition of the cooking liquid, and the influence of the pH value on the penetration of the liquid is very important.
Because when the pH value of the cooking liquid is greater than 13, the axial diffusion of the fiber is close to the lateral diffusion, about 1:0.8
When the pH value of the cooking liquid is less than 13, the axial diffusion of the fiber is 10-40 times greater than the lateral diffusion.
Mainly because when the pH value of the cooking liquid is greater than 13, the cooking liquid can swell the fiber cells. The higher the pH value, the greater the swelling effect on the fiber cell wall. The swelling will cause “temporary pores” on the fiber cell wall, thereby increasing the diffusion channel, which reduces the difference between the lateral and axial diffusion of the fiber.
When the pH is less than 13, there will be no temporary pores on the fiber cell wall, so the difference between the axial and lateral diffusion of the fiber is relatively large.
It can be seen that different cooking process parameters should be used when cooking with different cooking liquids.
For example, when using a cooking liquid with a pH value higher than 13, a faster rate of heating can be used.
When using a cooking liquid with a pH value less than 13, the heating rate should be appropriately slower. If it is too fast, it may cause uneven cooking.
2.Temperature
When the temperature rises, the viscosity of the liquid will decrease, the surface tension will decrease, and the diffusion coefficient will increase, thereby accelerating the pressure penetration and diffusion rate of the liquid, which is conducive to penetration. However, the temperature cannot be increased too much, otherwise it will cause uneven cooking.
3.Pressure difference
During the liquid penetration process, the pressure difference mainly includes the pressure difference caused by capillary action, the pressure difference caused by the static pressure of the liquid outside the sheet, and the external pressure generated during the cooking and feeding.
Among these pressure differences, the pressure difference generated by the capillary is the largest.
During the process of liquid impregnation, the increase in pressure difference will accelerate the rate of liquid impregnation
4.Types of fiber raw materials and specifications of sheet
Different raw materials have different composition and structure of fiber cells, and different densities, so the rate of liquid impregnation is also different
Generally speaking, raw materials with low density, large cell cavity and low resin content are easier to impregnate, such as grass raw materials, straw, wheat straw, etc.
At the same time, the specifications of the sheet, such as length and thickness, will also affect the impregnation effect
At the same time, in order to increase the exposure of the fiber cell cavity, when chipping, it is generally cut obliquely, which can improve the penetration efficiency.
In addition, the moisture content of the wood chip will also affect the impregnation. When the moisture content is high, the diffusion effect is the main effect, and when the moisture content is low, the capillary effect is the main effect.
Experiments have shown that when the moisture content of the wood chip is greater than 40%, the impregnation rate is the fastest, and the impregnation at this time is mainly pressure impregnation.
So during production, the moisture content of the wood chip is mostly controlled at around 40%.
Chemical principle of alkaline cooking
Alkaline cooking delignification chemistry
Lignin is a three-dimensional high molecular compound composed of propane structural units.
Lignin exists in large quantities in fiber raw materials. One of the purposes of chemical pulping is to remove lignin from fiber raw materials as much as possible.
How to dissolve lignin from plant raw materials?
- Increase the hydrophilicity of lignin by increasing the number of aliphatic or aromatic hydroxyl or carboxyl groups
- Degrade lignin macromolecules into smaller fragments that can be dissolved in water through chemical reactions
- Link hydrophilic substituents to lignin macromolecules to make its derivatives soluble in water
For example, lignin reacts with sulfites to form lignin sulfonates
Chemical reactions of lignin in alkaline cooking
Alkaline cooking delignification characteristics must break down lignin macromolecules into small molecules, so that lignin can be dissolved from fiber raw materials.
The breaking down of lignin macromolecules into small molecules is actually the breaking reaction of each connecting bond between the structural units of lignin macromolecules, and at the same time, it is necessary to reduce the broken alkaline lignin molecules from condensing into macromolecules.
Different cooking methods have different reactants that react with lignin,
For example, caustic soda method reactant: OH-
Sulfate cooking reactant: OH-, HS-
Through chemical reactions, lignin macromolecules are degraded into alkaline lignin and sulfide lignin with smaller molecular weight, simpler structure, and soluble in water.
In lignin macromolecules, each unit is linked together by various mystery bonds, carbon-carbon bonds, etc., and there are also lipid bonds in grass raw materials.
Different chemical bonds react differently in chemical reactions.
In the alkaline cooking process, there are several main chemical reactions of lignin:
1.Alkaline cleavage of phenolic α-aryl ether or α-alkyl ether bonds
2.Alkaline cleavage and sulfurization cleavage of phenolic β-aryl ether bonds
3.Alkaline cleavage and sulfurization cleavage of non-phenolic β-aryl ether bonds
4.C-C bond cleavage between aryl-alkyl and alkyl-alkyl
5.C-Methyl-aryl ether bond cleavage
6.Condensation reaction during alkaline cooking
7.Formation of lignin chromophores during alkaline cooking
Chromophores: refers to unsaturated groups that produce absorption peaks in visible light
For example: unsaturated hydrocarbons with double bond structures, hydroxyl groups, benzene rings, o-quinones, p-quinones, diaromatic rings, etc.
