The continuous cooking and displacement cooking are common pulp cooking processing. What is the difference between them? As a paper pulp making machine manufacturer, CNBM has built a lot of paper pulp solution project and provided digester, blow tank, vacuum washer, twin roll press, single screw press, and other paper pulping machines to clients. Today, we will compare these two paper pulping process from three aspects, including working principle, pulping process, and advantages.
Working principle
Displacement cooking
Displacement cooking is the combination of displacement of black liquor and diffusion washing in a batch digester. The cooking waste liquid is replaced with cooking liquid or washing water and the cooking waste liquid is replaced with heat, and cold blow is achieved. Displacement cooking is the reuse of the black liquor from the last cooking, which can remove some of the dissolved lignin, hemicellulose and extracts from the cooking system at the end of a large amount of delignification stage, and it is possible to add cooking liquid in the stage of supplemental delignification. So that the heat energy and chemicals in the black liquor are reused, the cooking conditions of the whole cooking process are balanced, which is beneficial to improving pulp quality and production efficiency.
Continuous cooking
After the raw materials are pretreated by the chemical solution, they are fed into the horizontally arranged horizontal tube container through the feeding device, and the liquid medicine and steam are simultaneously fed at the same time, and are pushed by the screw conveyor in the horizontal tube to transport the raw materials from one end to the other end, and the raw material is dropped into the second horizontal tube by gravity, and then sent to the other end by the screw conveyor of the second horizontal tube until the third tube. The raw material is fully mixed with the chemical solution under high temperature and high pressure by screw conveying and stirring and achieves a good cooking effect within 20-50 minutes.
Pulping process
Displacement cooking
A cooking cycle consists of 6 steps of the pulping process:
- Loading: Put the material, cold black liquor, and some cold white liquor into the cooking digester to ensure the pH value is 12. The cold liquid is used to fill the tablet and neutralize the acid on the surface of the tablet.
- Warm black liquid filling (warm filling): the cooking digester is filled with warm black liquid and cold white liquid, and the cold black liquid added in the charging process is replaced, simultaneously start the pre-heating and delignification.
- Hot black liquor filling (hot charging): Fill the cooking digester with hot black liquor and hot white liquor, and replace the warm black liquor added in the previous step of the warm charging operation.
- Cooking: Heat the cooking digester with steam to the highest cooking temperature required and maintain it at certain cooking temperature for a suitable period of time. At the moment, determine whether continue to circulate the cooking digester cooking liquid according to the temperature of the upper, middle and lower parts of the digester. The total cooking time can be determined by the H factor.
- Displacement: The hot cooking liquid is replaced by the filtrate of the washing pulp, and the liquid in the digester is cooled. The displaced hot black liquor is stored in the tank and reused in the next cooking step. After the displacement, the contents of the cooking digester were cooled to a flash temperature (100 ° C) below atmospheric pressure.
- Discharge: The contents of the digester cooled to below the flashing temperature of the normal pressure are pumped out of the digester for cold blowing, and the rest of the process is the same as the conventional batch cooking.
Continuous cooking
The material which has been dehydrated by the wet preparation enters the return screw, and the material is fed into the pre-steaming screw through the first discharge port of the feed screw and the drum meter, and the material is preheated to low pressure steam to about 85 ° C, The air in the material is removed, and the impregnated material enters the screw feeder continuously and evenly (the screw feeder is used to continuously and uniformly feed the grass material into a cooking tube having a pressure of about 1.0 MPa and act as a pressure sealing device). After the raw material is passed through the feeding screw, it is pressed into a “plug” and then enters the T-shaped tube. The cooking liquid and steam are added from above the T-shaped tube. After the material is dropped into the cooking tube, the area returns to the original volume due to liquid absorption and heat absorption. And enter the cooking stage of about 170 ° C, the screw conveyor in the cooking tube flips and pushes the materials to advance, after 20 to 50 minutes, material is cooked into pulp, in the middle tube, the unloader is cooled by the thin black liquid sent by washing dilute to below 100 ° C, blow into the blow tank and then send to the washing process
Advantages
Displacement cooking
- Good for alkali recovery
When the solid content of black liquor reaches 30% or above, the viscosity of black liquor after cyclic displacement cooking is lower than the viscosity of black liquor after conventional alkali cooking under various process conditions. As the black liquor circulation reaches equilibrium, the viscosity of extracted liquid become lowered. Compared with conventional cooking, displacement cooking is higher in the black liquor residue after cooking, which is beneficial to the next displacement cooking. The black liquor of displacement cooking has less silica content and lower viscosity, which is good for the Alkali recovery of the waste liquid.
- Technological advancement
Since for DDS is upgraded from RDH, it inherits the advantages of RDH, such as heat recovery system that does not require blowing; reduces the coarse slag rate; reduces the chemicals consumption of the bleaching process; the cooking process is equivalent to a section of pulp washing; obtain pulp with low-calorie and high-strength. DDS enables full automation of the production process. It adopts advanced digital technology (such as the prediction of the liquid level in the trough area and the fuzzy logic prediction of the digester) and enables the advance prediction, the prevention of faults to focus on prevention, which is good for the stability of production and quality.
- Environmental protection, energy saving,
DDS cooking has a fully enclosed waste heat and waste gas recovery system. The crude pulp with low Kappa is produced, the amount of bleaching chemicals is greatly reduced, and the wastewater load of the mid-stage is also greatly reduced. Based on more obvious displacement effect and more recovered heat energy, the energy and chemical utilization efficiency are improved, and the steam consumption per t-pulp can naturally be reduced.
- The pulp has a low Kappa number and high strength
Expanded the role of warm charge, a large amount of delignification has been realized in the warm charge process, so the reduction of alkali amount during hot charging means that the damage to cellulose is reduced, at the same time, a large amount of delignification can be obtained during warm charge. It shows that the pre-impregnation effect is more remarkable, and the cell cavity of the raw material can be fully opened, which is favorable for deep delignification, so the Kappa number can be further reduced. Therefore, the oxygen delignification segment is not required and the three-stage bleaching degree can also reach 88% ISO. If aerobic delignification is used, the cooking hardness can be increased, the holding time can be shortened, and the pulp yield can be further improved.
- Increase the yield of pulp
DDS has achieved a large amount of alkali reduction in the stage of delignification and hot charging in the warm charging process, so the cooking reaction is more complete, greatly reducing the crude slag rate and greatly improve the cooking yield.
- High efficiency
High-temperature washing in the digester, the washing efficiency is high, and the residual alkali in the pulp is reduced after washing, reduce the dilution factor.
Continuous cooking
- The raw material of continuous cooking is pre-impregnated and squeezed, which can increase the amount per unit volume and greatly shorten the entire cooking time.
- There is no sudden release of steam in the traditional batch cooking, the air pollution is small, and it is easy to control.
- The supply of electricity and steam is balanced, with no peak load.
- Labor productivity is high, easy to control automatically, less production staff, low labor intensity.
- High pulp yield, the quality is uniform and stable.
