26.1 What is pulp?
26.2 Sulfate (Kraft) Pulping Process (Figure 26.1)
Chemical reactions involved
(i) Digestion (hydrolysis and solubilization of lignin)
R-R’ + NaOH→R"COONa + ROH
R-R’ + Na2S →Mercaptans
(ii) Chemical recovery from black liquor
2NaR + air → Na2CO3 + CO2
Na2SO4 + 2C → Na2S + 2CO2
(from R) (white liquor)
Na2CO3 (aq) + Ca(OH)2 (s)→ 2NaOH (aq) + CaCO3 (s)
(green liquor) (white liquor)
CaCO3→CaO + CO2
CaO + H2O →Ca(OH)2
Figure 26.1 Flow sheet of manufacture of pulp manufacture
26.3 Functional role of various processes
(a) Chipper bin:-
(b) Digester tower:-
(c) Blow down valve:-
(d) Blow tank:-
(f) Series of filters
Bleaching of pulp
To produce white paper, the pulp is bleached. The chemicals used to bleach pulp must be environment friendly. Bleaching with chlorine produces dioxins and other undesirable products. So, nowadays pulp is bleached with hydrogen peroxide, ozone, chlorine dioxide, oxygen etc. The objective of bleaching is to remove small fractions of lignin that remains after digestion.
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 whereasthe Sulfite process is an acidicprocess.
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 ?
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.
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 assulfidity. 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 oC (Kleppe (1970)).
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.
Q. 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 oC 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.