6. What are the other by-products of the pulp and paper industries ?
Ans: Main byproducts of the Kraft process are turpentine, rosin and tall oil soap.

7. What is sulfidity ?
Ans: The mass ratio of sodium sulfide to caustic soda plus sodium sulfide, (where all masses are expressed on Na₂O basis) is known as sulfidity. The presence of sodium sulfide makes the bleachibility of pulp easier and paper produced has better strength properties.

8. From literature survey, obtain digester kinetics for lignin removal along with relevant technical information that will be useful for its design as well as operation ?

Delignification Kraft pulping rate for softwoods at 160°C (Kleppe (1970)).

Kinetic expression:

At constant sulfidity and alkali charge, the delignification rate is regarded as a homogeneous first order reaction with respect to Lignin concentration (wt %) in the wood using the expression (Vroom (1957)):
-dL/dt = kL where 'k' is the rate constant.
The temperature dependency of 'k' is provided as
Lnk = (43.2 – 16.113/T).
where T is the temperature of the digester.

9. What is solvent pulping ?

Ans. In solvent pulping, lignocellulosic feedstock is contacted with organic solvents such as acetone, methanol, ethanol, butanol, ethylene glycol, formic acid or acetic acid at 140 – 220°C in an aqueous solution consisting of 20 – 60 % water. Subsequent separation and recovery of th solbvent is carried out using distillation. Ethanol is the preferred solvent for solvent pulping. Solvent pulping provides 4 – 5 % higher yield than cracked pulping.

References :

1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008

2. Kirk R. E., Othmer D. F., Encyclopedia of Chemical Technology, John Wiley and Sons, 1999-2012