1. CENTRAL PULP AND PAPER RESEARCH INSTITUTE SAHARANPUR
Innovative Approach for Improved Energy & Chemical Recovery Efficiency in Straw Based Paper Mill
Dr. R.K. Jain, Dr. A.K. Dixit, Dr. Tarun
CENTRAL PULP AND PAPER RESEARCH INSTITUTE SAHARANPUR

2. Structure of Presentation
Problems Associated with Agro Residue Black Liquors
Characterization Agro residue and Wood Black liquor
Viscosity of Agro and Wood Black Liquor
Impact of Higher Viscosity on Chemical Recovery System
CPPRI Effort in Improving Thermal Efficiency of Agro Residue based Chemical Recovery
Process Flow-sheet of Desilication followed by Liquor Heat Treatment
Optimization of Desilication Process Conditions
Studies on Liquor Heat Treatment of Desilicated Black Liquor
Analysis of the Control, Desilicated and Heat Treated Black Liquor samples
Observation
Conclusion

3. Problems Associated with Agro Residue Black Liquors
Chemical recovery operation in the agro based paper mills particularly the mill employing wheat straw as a major raw material suffers several operational problems leading to poor energy & chemical recovery efficiency
Presence of high content of silica and LCC cause higher viscosity of black liquor restricting to achieve thermal efficiency of chemical recovery units in wheat straw / bagasse based mills

4. Characterization Agro residue and Wood Black liquor
Parameters
Agro residue Black Liquor
Wood Black Liquor
Total Solids, %w/w. 16 pH 12
12-13
RAA, as Na2O, gpl
5 - 6
SVR, ml/g
8 - 9
Silica as SiO2, %w/w.
Inorganic as NaOH, %w/w 34 35
Organics, %w/w. 66 65
Lignin Carbohydrate Complex, % w/w
14-20%
3-5%
Carbon as C, %w/w
35.05
34.3
Hydrogen as H, %w/w
5.47
4.5
Sulphur as S, %w/w
1.0
3.0

5. Viscosity of Agro and Wood Black Liquor
Total solids, %w/w.
Viscosity, cps at 1000C
Bagasse Black Liquor
Wood Black Liquor 45
50.2 - 50
138 20 55
384 79 60
1073
165 65
3016
369
High viscosity of Agro Residue black liquor shows poor burning and low thermal efficiency in chemical recovery boiler

6. Impact of Silica on Chemical Recovery Cycle
Following are impact of silica on Chemical Recovery Cycle:-
Scaling in Evaporator
Poor burning behavior of Black Liquor
Slow settling of White Liquor
Make lime mud reburning difficult

7. Impact of Higher Viscosity on Chemical Recovery System
Viscosity is an important parameter for black liquor handling. It has impact on following unit operations:-
- Evaporator Capacity and energy requirement
- Heat Transfer
- Mixing operation
- Droplet size

8. CPPRI Effort in Improving Thermal Efficiency of Agro Residue based Chemical Recovery
To combat the problems of high silica content and higher viscosity of agro residue black liquor, a process consisting of Desilication followed by Liquor Heat Treatment was developed for efficient operation of chemical recovery system in the paper mills employing wheat straw and /or Sarkanda as major raw material.
Desilication process is based on controlled carbonation of the black liquor using flue gas in a submerged bubble reactor resulting in selective precipitation of silica at optimized pH level.
Liquor Heat Treatment Process involves heating of the black liquor at temperature more than pulping temperature ( ° for mins. maintaining optimum level of residual active alkali)

9. Process Flow-sheet of Desilication followed by LHT

10. Optimization of Desilication Process Conditions
Desilication Efficiency at different pH
Black Liquor Characteristics
Desilication
(%)
Before Desilication
After Desilication pH
SiO2, %w/w PH
12.5
3.03
10.5
2.8
17.5
10.3
1.6
47.2
10.2
0.8
73.6
10.1
0.5
83.5
10.0
0.40
86.8
9.9
0.36
88.2

11. FLOW SHEET OF DESILICATION PROCESSTC

12. Characterization of original and desilicated black liquor
Parameters
Control BL
Desilicated
Re-alkalized BL
Total Solids, %w/w.
11.2
11.3 pH
12.54
12.6
RAA, as NaOH
gpl
3.84
4.35 %
3.44
4.92
SVR, ml/g 8 10
Silica as SiO2, %w/w.
3.03
0.36
Inorganic as NaOH, %w/w
31.59
33.82
Organics, %w/w.
68.41
66.18
Residual Silica – 0.04%
SVR improved to 10 showing better burning behavior

13. Heat Treatment of Desilicated Black Liquor
Process involves heating of black liquor at ° for minutes, maintaining optimum residual active alkali level.
Cleavage of the high molecular weight Lignin carbohydrates bonds (LCC) occurs due to depolymerization leading to generation of lower molecular weight entity.

14. SCHEMATIC OF LIQUOR HEAT TREATMENT

15. Variable affecting Liquor Heat Treatment
Residual Active Alkali
Retention time
Temperature
Studies were conducted on optimization

16. Optimization of RAA
Optimization of Residual Active Alkali (RAA) at 1800C
RAA, %w/w
as NaOH
1800C
(% reduction in Viscosity)
50% solids
55% solids
60% solids
3.5
44.72
45.10
48.24
4.5
50.91
52.83
56.41
5.5
61.72
63.08
66.53
6.5
66.58
68.28
70.07

17. Optimization of Retention Time
Optimization of Retention Time at 1800C
Time, minutes
% reduction in viscosity (RAA 5.5% / 180°)
55% solids
60% solids
65% solids 15
62.8
66.1
66.6 20
65.7
68.5
68.9 25
69.9
70.2
72.4

18. Optimization of Temperature
Optimization of Temperature (RAA 5.5%)
Temperature, 0C
% reduction in viscosity
50% solids
55% solids
60% solids
175
61.5
63.1
66.6
180
62.9
64.3
67.1
185
63.9
65.5
68.1

19. Studies on Liquor Heat Treatment of Desilicated Black Liquor
Liquor Heat Treatment of desilicated re-alkalized black liquor was carried out at optimized process conditions.
Control black liquor, desilicated black liquor and Heat treated black liquors were analyzed for various parameters to observe the impact of Desilication coupled with LHT on black liquor properties

20. Desilicated-Re-alkalized-Heat Treated BL
Analysis of the Control, Desilicated and Heat Treated Black Liquor samples
Parameters
Control BL
Desilicated-Re-alkalized-Heat Treated BL pH
12.54
12.55
RAA, as NaOH
gpl
3.84
5.71 %
3.44
2.37
SVR, ml/g 8 15
Silica as SiO2, %w/w.
3.03
0.38
Inorganic as NaOH, %w/w
31.59
34.74
Organics, %w/w.
68.41
65.26

21. Viscosity of control and treated Black Liquor
Total Solids, %w/w.
Value, cps
Control BL
(Without LHT)
Treated BL
(After LHT)
1000C
1050C 50
350
295
111 93 55
885
718
195
155 60
2215
1735
342
259 65
5545
4200
600
432 70 -
1051
719

22. Observations
The following observations are made from the study conducted:
Liquor Heat Treatment coupled with desilication of wheat straw black liquor conducted at optimized conditions resulted in more than 75% reduction in viscosity.
Treated liquor have shown better combustion behavior reflected by higher Swelling Volume Ratio (SVR) of treated black liquor (15 ml/g) as against untreated black liquor (8 ml/g).
It is possible to achieve 5-6% higher concentration of treated black liquor due to lower viscosity and a concentration of more than 65% can be achieved as against 60% of untreated black liquor.
Firing of black liquor at higher dry solids level will also reduces emissions from chemical recovery boiler.
Reduction in silica will help the agro residue based mills in installing full-fledged chemical recovery system
In addition to above benefits, reduction in silica content will make lime sludge suitable for reburning and will improve dryness of lime sludge. This will reduce fresh lime requirement and less fuel will be required for lime mud reburning.

23. Conclusions
The development of Liquor heat treatment in combination with desilication of silica rich wheat straw black liquor and demonstration of the same should help the Indian paper Industry employing wheat straw as raw material in achieving higher chemical recovery and thermal efficiency of chemical recovery operation.

24. THANKS
Central Pulp & Paper Research Institute