Preparation of Wood Pulp by Sulfate (kraft) Process | Chemical Technology - Chemical Engineering PDF Download

26.1 What is pulp?

• Pulp is a commercial fibrous material obtained from bamboo, wood, bagasse (waste material) etc. by mechanical and chemical means.
• Pulping means disintegration of bulky fibrous material to small fibres.
• There are mainly three modes of production of pulp:
  • (a) Mechanical
  • (b) Chemical
  • (c) Semi chemical

26.2 Sulfate (Kraft) Pulping Process (Figure 26.1) 

• Most popularly used process.
• This is an alkaline process.
• Na₂SO₄ is added to the cooking liquor. So its common name is sulfate process.
• The presence of sodium sulfide makes bleaching of pulp easier and the paper produced has better strength.

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

(a) Smelting

2NaR + air → Na₂CO₃ + CO₂

(lignin)

Na₂SO₄ + 2C  → Na₂S + 2CO₂

(from R)    (white liquor)

(b) Causticizing

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:-

• Chips are fed in this device.
• Cut logs are conveyed to the chipper where rotary disks with heavy knives reduce the wood to size 2-5cm flat chips.
• Size reduction is done to maximize penetration of process chemicals.

(b) Digester tower:-

• Continuous digester tower is 25 – 30m tall.
• Chips are preheated with  volatilizing turpentine and non-condensable gases.
• For controlling digestion temperature, cooking liquor is withdrawn as side streams and circulated through heat exchanger.
• Digestion is done to free lignin and other non-cellulosic content.
• Cooking time is about one and a half hours at 170°C.
• To avoid mechanical weakening of fibres, digested chips are cooled with recycled black liquor.
• Temperature is maintained at 140-180°C and pressure at about 10 atm.
• Bottom temperature is maintained at 65°C

(c) Blow down valve:-

• This valve reduces the pressure of the stream from 80atm to 1atm before entering blow tank.

(d) Blow tank:-

• When hot pulp slurry is passed to the blow tank, heat is recovered in the form of steam.
• The chips are preheated with this recovered steam.
• The blow tank has high concentration of pulp and low concentration of water.

(e) Screens:-

• Pulp is screened so as to remove wood knots and undigested residues.

(f) Series of filters

• Pulp is filtered to separate black liquor for chemical recovery plant.
• Black liquor is also recycled back to digester for cooling the digested chips.
• Hot water is added to second filter for better filtration.

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 ?

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.

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)).

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.

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.