Displacement Cooking Overview

Currently, batching displacement cooking is one of the energy-efficient cooking techniques in the paper and pulp industry. What do you know about displacement cooking? As a paper pulper machines manufacturer, CNBM can provide the best solution for batching displacement cooking and displacement digester system, and related pulping machines. Today we will learn displacement cooking together.

Working principle of displacement cooking

According to the basic requirements of the extended delignification theory, batching displacement cooking fully recycles the cooking waste liquid to achieve the purpose of high efficiency and energy saving. First, the pulp in the cooking digester is washed with dilute black liquor and recovers the displaced black liquor. And then the washing process is carried out at different stages of the cooking process, the cooking waste liquid and heat are displaced, and the black liquor displaced at each stage can be added to the lye and then used in the next-stage cooking reaction. Adopt cold-blowing technology, which can fully utilize the residual alkali and heat in the black liquor, effectively reduce the steam consumption, also can greatly reduce the non-condensation and air pollution, protect the environment, completely solve the serious noise problems caused by thermal blowing and improve the quality of the pulp. The cooking process optimizes the temperature gradient operation, from low temperature to high temperature, and the temperature is controlled step by step to control the heat preservation operation, which can effectively ensure the production status of different chips.

The main theoretical basis for batching displacement cooking is the “extended delignification “. The theory can be basically achieved by the following operations:

1. The concentration of alkali liquor at the beginning of cooking is low, the concentration of alkali liquor is high at the end of cooking, and the concentration of alkali in the cooking process is required to be as uniform as possible;
2. Keeping a higher concentration of HS-ion in the cooking liquid, especially when a large amount of delignification starts;
3. Keeping a lower concentration of the dissolved lignin and Na+ in the cooking liquor, especially in the late stage of cooking;
4. The highest temperature of cooking is as low as possible, especially at the beginning and end of cooking.

Advantages of displacement cooking

Batching displacement cooking has the following distinct advantages:

• The displacement of black liquor and the continuous circulation of the cooking liquor facilitate the uniform distribution and temperature of the liquid in the entire displacement digester, which provides a prerequisite for extended delignification. At the same time, the higher initial cooking temperature obtained by the replacement of the hot black liquor reduces the total cooking time, and the higher liquid ratio also facilitates the rapid penetration of the liquid, which provides a good precondition for uniform pulping.
• The displacement of black liquor during pulp cooking is beneficial to the permeation of the cooking liquor, thereby ensuring the homogeneity of the delignification of the wood chips in the cross-sectional direction. The continuous black liquor displacement can neutralize and remove the acidic substances and some lignin from the wood chips, creating conditions for the white liquid to enter and react with the wood chips, preventing the incomplete cooking phenomenon inside the wood chips.
• The cooking process adopts the heating measures in the outer tank area, and the stepwise heating of the potting, immersing and low-temperature cooking operation increases the heating rate of cooking and shortens the cooking time.
• The displacement digester always maintains a certain hydraulic pressure during cooking, and the liquid filling operation has a higher liquid ratio, and the replacement effect of the adding process of chemical liquid makes the cooking in the digester uniform evenly and improves the uniformity of the pulp.

Pulp cooking process

Pulping Process

The batch displacement cooking process consists of six stages of charging operation, impregnation operation, low-temperature cooking operation, high-temperature cooking operation, and displacement washing operation and discharging operation.

Charging operation

The material is sent to the displacement digester by the conveying device, and the air in the digester is discharged by the exhaust fan, and the black liquor collected by the black liquor pump is added according to the procedure, and the appropriate amount of whitening liquid is added by the cold white liquor pump. Add black liquor to the digester during the charging process, until the set maximum level and level setting are reached.

Impregnation operation

Pumping 125-135~C black liquor and part of the cool white liquor into the displacement digester, and replacing the liquid added during the charging process. With the filling of the black liquor, the pressure and temperature in the digester are increased, and the pre-delignification starts, it stops when the set amount of liquid is reached.

Low-temperature cooking operation

The black liquor of 155~170~C and the hot white liquor of 160~170~C are pumped into the cooking digester. As the temperature of the displacement digester increases, the delignification continues. The black liquor displaced from operation and the black liquor added during the immersion operation is separately sent to different tank, known as the immersion black liquor tank and the cooking black liquor tank at different temperatures for the operation of the next digester, and the operation is completed after the liquid target value is reached, so that the material in the digester is heated to near the temperature at which it is cooked. The lignin is optimally removed.

High-temperature cooking operation

The operation process is basically the same as the conventional batch cooking, including two stages of heating and heat preservation, that is, using the heat pump of the external circulation system of the digester to extract the black liquor in the digester, and the steam is heated to the required cooking temperature (enter the heat preservation Ib”t section) by the direct steam heater. During the heat preservation period, the cooking liquid continues to circulate in the digester to ensure uniform temperature throughout the digester, and the operation is stopped when the set H factor is reached.

Displacement washing operation

The filtrate from the vacuum washer is used to displace the cooking liquid in the digester, meanwhile, cooling the liquid in the digester. The displaced black liquor of 150-170 ° C is sent to the cooking black liquor tank, and the black liquor of 130-140 ° C is sent to the impregnation black liquor tank for reuse in the next cooking operation when the temperature of the replacement liquid reaches the set temperature. Then stop the operation.

Discharging operation

After the replacement washing is completed, the pressure in the displacement digester disappears, that is, the discharging pulp pump and the diluted black liquor pump are started. The black liquor is added to the upper and middle parts of the displacement digester to dilute the digester, and in the discharging process, the amount of diluted black liquor in different periods is controlled by the digester formula, for guarantying discharge smoothly. This operation of the displacement digester is under normal pressure.

Digester cycle

The operation cycle of each stage is different according to the raw materials and the pulp, especially the difference in the holding time. The cooking cycles of bamboo and eucalyptus are as follows:

Displacement Equipment

The main supporting equipment for batch displacement cooking technology includes displacement digester, black liquor tank, white liquor tank, blow tank, heat exchanger and cyclone separator.