What is cooking?
Cooking is the process of delignification.
Hydrolysis is the process of degrading hemicellulose.
The necessary conditions for cooking and hydrolysis
Cooking and delignification power: The driving force for hydrolysis of hemicellulose:
Alkali concentration; PH value: 3.0~4.0;
Sulfur ion concentration; Temperature;
Temperature; Concentration of dissolved hemicellulose;
Concentration of dissolved lignin; Residual hemicellulose content in wood chips;
Time; Time;
Cooking common terms
Cooking liquid: refers to the alkaline solution used for cooking raw materials. Cooking liquor is usually made by mixing white liquor with a certain amount of black liquor.
White liquid: The solution obtained by causticizing the filtrate with Ca (OH)2. The main components are NaOH and Na₂S, and contain a small amount of uncalcified Na2CO3, unreduced Na₂SO4, as well as Na₂ SO3, Na2CO3,and Na₂Sn.
Black liquor: The waste liquid produced by alkaline cooking is called black liquor.
Green liquid: refers to the solution formed by the molten material flowing out of the combustion furnace after the alkali recovery black liquid is evaporated and concentrated, and then sent to the combustion furnace for combustion in dilute white liquid or water.
Active base (AA): NaOH+NazS, expressed as Na₂O or NaOH.
Effective alkali (EA): NaOH+1/2 Na ₂ S, expressed as Na2O or NaOH.
Sulfurization degree: White liquid vulcanization degree refers to the percentage of NazS to active base.
Total alkalinity (TA): NaOH+NazS+Na₂CO3+Na₂SO4+Na₂SO3+Na₂S2O3,excluding NaCl,usually expressed entirely as Na₂O.
Total titratable alkali (TTA): NaOH+NazS+Na2CO3+Na₂SO3, expressed as Na₂O or NaOH.
Alkali usage: Alkali usage refers to the percentage of active alkali usage (mass) to the mass of dry raw materials during cooking. Liquid ratio: the ratio of the dry raw material mass (kg or t) in the steamer to the total liquid volume (L or M3) during cooking. For example, a liquid ratio of 1:4 means that 1 ton of dry raw material requires a total cooking liquid volume of 4M3.
Pulp hardness: Refers to the relative amount of lignin and other reducing substances remaining in pulp. Tankassium permanganate is usually used as an oxidant for determination, expressed in terms of kappa number.
H-factor: A single variable composed of two parameters, cooking temperature and cooking time, defined as the integral of the relative reaction rate constant of lignin removal at cooking temperature with respect to cooking time.
P-factor: A single variable composed of two parameters, hydrolysis temperature and hydrolysis time, defined as the definite integral of the relative reaction rate constant of hydrolyzed hemicellulose at a hydrolysis temperature with respect to hydrolysis time.
Displacement cooking process
1. Loading& Pre impregnation
After a period of time from the start of wood chip loading, black liquor from the displacement tank is injected from the lower part of the cooking tank to pre impregnation the wood chips. The black liquid from the displacement tank usually comes from the 1st filtrate tank of the washing section.
The residual alkali in the pre impregnation solution can be adjusted by adding small amount of white solution.
By injecting pre impregnation solution, a certain amount of hydraulic pressure can also be provided for the tank.
2. Hot liquid filling
Phase 1:
Hot black liquor is pumped from the No.1 hot black liquor tank into the cooking tank to replace the pre impregnation liquid in the tank.
The pre impregnated solution that has been replaced should be refluxed to the replacement solution tank below 100 ℃;
When the temperature of the pre impregnation solution reaches 100 ℃ or above, the reflux is switched to the No. 2 black liquid tank.
Phase 2:
After preheating the wood chips in the tank with certain amount of hot black liquid, the filled tank black liquid is mixed with hot white liquid to increase the alkaline concentration of the cooking solution.
After the hot liquid is filled, the temperature inside the cooking tank rises to 150-160 ℃.
3. Heating and insulation
After the hot liquid filling is completed, if the cooking temperature does not reach the set temperature, the online circulating heater can be heated by medium pressure steam.
During the insulation period, the temperature and pressure inside the steaming tank will remain constant until the H factor reaches the target value.
4. Cold liquid replacement
When the set cooking endpoint is reached, the black liquid in the replacement black liquid tank is pumped from the bottom of the cooking tank, and the hot black liquid in the tank is recovered to black liquid tanks1 and tank2 respectively;
When the temperature of the recovered liquid from the cooking tank is below 100 ℃, the reflux liquid is transferred to the replacement black liquor tank.
By replacing the heat and residual alkali in the tank with cold liquid, and terminating the cooking reaction, it is also equivalent to a stage of pulp displacement washing.
5. Pump and place the tank
After the replacement is completed, the pulp in the steaming tank is pumped into the atmospheric pressure spray tank for storage using a tank pump.
The pulp placed in the tank is diluted with black liquor from the displacement tank.
Usually, the concentration of the pulp sprayed into the tank is about 5.0%.
Cooking assistant -catalyst
Cooking Additive – Penetrant
Dissolved pulp displacement cooking
Poly disc filter mainly used for paper mills
