Improved sulfateProcess Technology
Deep Delignification
The concept of deep delignification was first proposed by Professor Hartler of the Royal Institute of Technology in Sweden in the late 1970s. Subsequently, researchers from the Swedish Forest Products Research Institute and the Royal Institute of Technology proposed four basic principles for deep delignification and reducing carbohydrate degradation based on extensive research.
Chemical principles of improved sulfate cooking technology
1.Four principles for improving cooking delignification selectivity
a,Keep the alkali concentration as uniform as possible during the cooking process, that is, the alkali concentration is lower at the beginning of cooking, and the alkali concentration is appropriately higher near the end of cooking;
In the traditional cooking process, the cooking alkali is added at the beginning of cooking, so the alkali concentration is high at the beginning of cooking. High-concentration alkali easily causes carbohydrate degradation. As the cooking proceeds, the alkali concentration will become lower and lower due to the continuous consumption of alkali. At the end of cooking, the lower alkali concentration is not conducive to the dissolution of lignin. Therefore, this principle can make the alkali concentration more uniform during the cooking process, which not only protects carbohydrates at the beginning of cooking, but also provides favorable conditions for the dissolution of lignin at the end of cooking.
b, The HS- concentration in the cooking liquid should be as high as possible,
especially at the beginning of cooking and the beginning of a large amount of delignification, a higher HS- concentration should be maintained, so as to improve the strength and yield of pulp and accelerate delignification.
c, The concentration of dissolved lignin and Na+ in the cooking liquor should be as low as possible, especially in the stage of residual lignin removal;
This principle is conducive to the diffusion of degraded lignin from the fiber.
d, Maintain a low cooking temperature, especially in the early and late stages of cooking;
This principle can improve the selectivity of delignification.
2.Factors affecting delignification selectivity during cooking
1). Hydroxide and hydrogen sulfide concentrations
During the cooking process, the three reactions of lignin sulfidation, degradation and condensation depend on the cooking conditions.
High alkali concentration is conducive to lignin degradation, while high sulfidation degree can reduce lignin condensation. During the kraft cooking process, lignin removal and carbohydrate degradation are two reactions that exist simultaneously. According to their empirical rate formula, we can find that:
In the initial delignification stage, the reaction order of OH- and HS- concentrations is 0, which means that neither OH- nor HS- concentration has any effect on the delignification rate.
In the bulk delignification stage, the reaction order of OH- concentration is 0.7-0.8, and the reaction order of HS- concentration is 0.1-0.4, which means that HS- concentration has a greater effect on the delignification rate.
In the residual delignification stage, the reaction order of OH- concentration is 0.7, and the reaction order of HS- concentration is 0, which means that the delignification rate in this stage is mainly affected by OH- concentration.
As for the degradation of carbohydrates in the whole process, it will not be affected by HS-, but mainly by OH-. Therefore, a lower OH- concentration can avoid the large-scale degradation of carbohydrates, while not affecting the degradation of lignin, and can increase the selectivity of cooking.
2). The “memory effect” of lignin
means that the rate of delignification is related to the process conditions in the early stage of cooking, that is, the cooking conditions in the initial delignification stage will affect the delignification rate in the residual delignification removal stage, and even affect the bleachability of pulp.
3). Dissolved lignin and sodium ions
When the content of dissolved lignin in the cooking liquid increases, it will hinder the removal of lignin. This is mainly because when the content of dissolved lignin increases, it will lead to
a, the condensation reaction of lignin increases;
b, the diffusion rate of lignin degradation products decreases;
c, the concentration reaches a state of equilibrium
Therefore, the concentration of dissolved lignin should be kept low during the cooking process.
4). Cooking temperature
With the increase of cooking temperature, the increase in the rate of carbohydrate molecular chain breakage is greater than the increase in the rate of lignin removal. Therefore, in actual production, under the condition of meeting the needs of delignification, the cooking temperature should be as low as possible to reduce the degradation of carbohydrates.
5). Sulfide
During the kraft cooking process, sodium sulfide hydrolysis can produce HS- and OH-. At the beginning of cooking, the pH of the solution is high, and the concentrations of sulfide ions and hydrogen sulfide ions are basically the same. As the cooking reaction proceeds, the pH value of the cooking liquid gradually decreases, and the concentration of hydrogen sulfide ions continues to increase. After entering the stage of large-scale delignification. Sulfide ions are almost all present in the form of hydrogen sulfide ions, which have a relatively strong nucleophilic ability, 1.56 times that of hydroxide ions. They are easier to attack the positively charged center of lignin methylene quinone intermediates than hydroxide ions, thereby causing degradation reactions of lignin macromolecules.
6). Composition of black liquor
Lignin in pretreated black liquor has a selective effect on displacement cooking delignification, mainly because
a. Black liquor lignin has a surfactant effect, which is conducive to the penetration and swelling of alkali in the black liquor to the fiber;
b. HS- in the black liquor is loosely connected to lignin and is easily released during the pretreatment process, thereby increasing the concentration of HS-, which promotes the reaction of HS- with lignin in the fiber raw material ;
c. The pretreatment temperature of black liquor is low, and the lignin in the black liquor is replaced before it condenses again after releasing HS-, which has no effect on the further delignification of displacement cooking.
7). Soluble lignin
The initial delignification stage is controlled in the black liquor pretreatment stage, so that the soluble lignin does not appear or appears very little in the second stage of cooking, thus reducing the subsequent condensation of active lignin, which is conducive to deep delignification.
Deep delignification technology
Deep delignification technology includes intermittent displacement cooking and continuous modified sulfate cooking.
Displacement cooking
1.Types of displacement cooking technology
- RDH(Rapid Displacement Heating)
- Super-Batch
- Enerbatch(Cold Blow)
- DDS(Displacement Digester System
2.Basic process of displacement cooking
- Loading wood chips
- Pre-impregnation with warm black liquor
- Displacement with hot black liquor
- Supplementation of hot black liquor
- Heating cooking
- Insulation
- End point displacement
- Putting pot
- Storing white liquor from alkali recovery system
3.Basic characteristics
a. Use warm black liquor for pre-impregnation under pressure, and then use hot black liquor for displacement, which can make full use of the heat energy in the black liquor and reduce the amount of steam;
b. Through the initial stage of wood chip impregnation, the wood chips can be fully impregnated, making the liquid penetration and delignification more uniform, and the alkali in the warm black liquor can also neutralize some acidic groups inside the wood chips, which helps to reduce the fluctuation of the active alkali effect and improve the uniformity of the pulp;
c. The liquid ratio is large, and the digester is filled with cooking liquid during the whole cooking process.
d. The pulp cooled by warm black liquor is pumped to the spray pot, which is conducive to improving the quality of pulp.
In the displacement cooking process, the rapid displacement heating cooking technology, that is, RDH, is a typical representative. Its operating procedures are basically the same as the displacement cooking process mentioned above.
In addition, the DDS cooking system is also an important displacement cooking technology.
In April 2004, the world’s first DDS displacement cooking system was successfully tested in Guangdong Dingfeng Paper Co., Ltd.
The basic principle of DDS is the same as that of RDH. The difference is that the DDS white liquor is divided into 3 parts, which are added in the sub-charging, warm charging section and hot charging section respectively. When charging, the DDS cooking system uses black cold liquid plus white liquid filling to ensure that the PH is greater than or equal to 12, which can strengthen the pre-impregnation effect of black liquor and increase the effect of warm charging. In addition, DDS cooking adopts an advanced automatic control system to better solve a series of problems such as bias flow, prediction of tank liquid level, and prediction of blockage during the discharge process.
Modified kraft continuous cooking technology
Types
- Modified Continuous Cooking (MCC) system
- Extended Modified Continuous Cooking (EMCC) system
- Iso-Thermal Cooking (ITC) system
- Lo-solids Cooking (Lo-solids Cooking) system
- Compact CookingTM system
- Black Liquor Impregnation (BLITM) cooking technology
Properties and uses of chemical pulp
Properties of chemical pulp
After the plant fiber raw materials are pulped by chemical method, the content and distribution of the components left in the pulp and the physical properties of the pulp are closely related to the fiber raw materials and cooking methods. For the same raw material, the pulp properties obtained after cooking with different methods and different process parameters will also have a certain degree of difference.
The properties of chemical pulp are affected by many factors. For chemical pulp, the most important chemical properties are the hemicellulose content and the degree of polymerization of cellulose in some cases.
When alkali pulping, the degree of polymerization of cellulose decreases and the carboxyl content increases. In addition, the distribution of the degree of polymerization of carbohydrates in the fiber cell wall will also change, which will affect the properties of chemical pulp, and the residual lignin in chemical pulp will also affect the physical properties of chemical pulp, such as the strength and color of pulp, and will also affect its chemical properties, especially in the bleaching process, which will affect the bleachability of pulp.
The properties of chemical pulp depend on:
- Raw material type;
- Cooking method and process conditions;
- Polysaccharide type, content and distribution;
- Lignin content and distribution;
- Cell type and fiber morphology;
- Yield and strength;
- Bleaching performance;
- Pulping performance;
- Water filtration performance;
- etc.
The use of chemical pulp
It needs to be determined according to the type of raw materials, cooking methods and product quality indicators. For example, sulphate pulp has high physical strength and can be used to produce high-strength paper, such as linerboard, sack paper, packaging paper and bleached cardboard.
The overall strength of bisulfite pulp is not as good as that of sulphate pulp, but some of its physical properties make it particularly suitable for making certain paper types, such as newsprint, tissue paper, translucent glass paper, oil-proof paper and many high-grade printing papers.
The strength of grass pulp is relatively low, and its bleached chemical pulp can be used to produce general cultural paper or to make higher-grade cultural paper with wood pulp.
