26.3 Functional role of various processes
(a) Chipper bin:-
- • Chips are feed 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• For controlling digestion temperature, cooking liquor is withdrawn as side streams and circulated through heat exchanger.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.
- • 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, now a days 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.