26.4 Technical Questions

1. Why is blow down valve introduced in the flow process ?

Ans: It reduces the pressure of pulp slurry from 80atm to 1atm before entering the blow tank.

2. Why is hot water added to the second filter and then the spent water is circulated to the first filter ?

Ans: Hot water has maximum efficiency to remove dissolved chemicals from leached pulp. Therefore, hot water is used in the second filter which eventually becomes spent water. The spent water can remove maximum solids from the pulp. The operation is counter-current in effect.

3. What is the technical difference between Sulfate(Kraft) Process and Sulfite Process ?

Ans:

i) Sulfate process is an alkaline process whereas the Sulfite process is an acidic process.

ii) The pulp has to be bleached more in the Sulfate Process as compared to the Sulfite process.

iii) For Sulfate process, digestion time is 2-5hrs at temperature 170-176°C and pressure 660- 925kPa whereas for Sulfite process, time required is 6-12hrs at temperature 125-160°C and pressure 620-755kPa.

iv) Raw materials are less significant and fibers have better strength properties in Sulfate process.

4. Why does the black liquor enters at various sections of the digester ?

Ans:

• Black liquor is recycled to the base of digester tower for cooling digested chips so as to avoid mechanical weakening of fibres.

• It is withdrawn as side stream from digester and circulated through heat exchanger to reheat and control the digester temperature.

• It is also used along with white liquor to adjust solid - liquid ratio in the digester entry section. Thereby, desired operating conditions of the digester are met.

5. How is turpentine recovered in the pulp manufacturing process ?

Ans: Turpentine is obtained by the distillation of the resin obtained in the pulp manufacturing process.