banglore 2010, ippta
TRANSCRIPT
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Desilication of Bamboo and Straw
Black Liquors
A.K.Dixit, B.P.Thapliyal,
R.K.Jain and R.M.Mathur
IPPTA Conference
16TH- 17THJULY, 2010
Banglore
Central Pulp & Paper Research Institute, Saharanpur, U.P.
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Structure of Presentation
Why Desilication ? Desilication Technology Developed by CPPRI
-Concept of Submerse Bubble Reactor
-Process Details
Mechanism of Silica Precipitation by lowering of pH Desilication of Bamboo Black Liquor
Desilication of Rice Straw Black Liquor
Commercial Scale Desilication of Rice Straw
Impact of Desilication on Black Liquor Properties Conclusion
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Why Desilication ?Presence of silica in black liquor causes:
Scaling in Evaporator tubes
Hard deposits on wall tubes- CaAl Silicates
Beehives on furnace walls
Possibly responsible for high viscosity of Black
Liquors in agro residue black liquors Slow settling of lime sludge due to calcium- Silicates
formed during causticization
Low dryness of silica mud from causticizers duringmud washing leads to high oil consumption in lime kiln
Difficulty in lime mud re-burning
Thermal analysis studies have shown that silica forms a sheathover lime particles so the CO2 can not escape from CaCO3molecules during calcination in lime kiln.
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Overview of Desilication Technologies
No. of researchers had tried various processes fordesilication of black liquor.
Additions of Cations ( Ca++ , Mg++, Al+++ )
Lowering the pH of black liquor with CO2 or acid
Combined method by lowering pH & addition of
cations
None of these methods could succeed on commercial scale.
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CPPRI Desilication Technology Lowering of pH with CO2 under controlled conditions
Only silica gets precipitated and no lignin precipitation
occurs.
Concept of Submerse Bubble Reactor
Liquor pumped in a tubular system, is treated with gas,which on its downward path sucks the gas in the form
of discreet bubbles.
The shearing action due to flow of the liquor, presses
the gas bubbles in a kneading fashion. This action continuously exposes new gas liquid
interfaces and thus ensures slow reduction of pH
leading to selective precipitation of silica.
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Schematic of CPPRI Desilication Technology
18
1 Flue Gas from
Boiler2,4,6 Circulation Pum
3,5,7 Submerse
Reactor Tank
8 Hot Retention
Tank
9 Feed Pump to
Filter
10 Clarifier
11 Belt Filter12 Filtrate Tank
13 Feed Pump to Mi
Tank
14 Caustic Mix Tank
15 High pH Tank
16 Feed Pump to
Storage
17 Foam Tank
18 Foam TankLiquor Pump
Desilicated BLto Storage
1
9
10
2 3 4 5 6 7
8
11
1213
14
15
16
17
Weak
L From
torage
Caustic
Sod
a
Precipitated
Silica
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Flow Sheet of Mill Scale Desilication Plant in Rice Straw Based Mill
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CHEMISTRY OF DESILICATION
NaOH + SiO2 Na2SiO3 + H2O
60 122
CO2
+ H2
O H2
CO362
2NaOH + H2CO3 Na2CO3 + 2 H20
80 106
Na2SiO3 + H2CO3 Na2CO3 + SiO2 + H2O
122 + 62 106 + 60 + 18
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Mechanism of Silica Precipitation by lowering of pH
Lowering of the pH of black liquor converts silicates into
insoluble polymeric anhydrides of silicic acid.
Lowering the pH of alkali silicate solutions results in;
First in formation of monosilicic acid.
This monomer undergoes polymerization, at a concentration greater
than the solubility of amorphous silica (i.e. 100-200 ppm), forming
higher molecular weight species.
Above pH7 stabilized particles (Sols) grow to a size of about
100 nm.
When salts are present, to neutralize the charge on growingparticles, aggregation of particles occur with the formation of
chains and ultimately, three dimensional gel net works.
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Mechanism of Silica Precipitation by lowering of pH
Central Pulp & Paper Research Institute, Saharanpur, U.P., INDIA
(Mono silicic acid)
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Desilication of Bamboo Black Liquor
Desilication of Bamboo Black liquor was carried out on
Pilot Plant in a bamboo based mill.
70 litres of weak black liquor was desilicated in each
trial. Flue gas was collected from chemical recovery Boiler
stack.
CO2 content in flue gas was 12%.
Desilicated black liquor was filtered under vacuum. Filtered desilicated black liquor was subjected to re-
alkalisation.
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Desilication of Bamboo Black Liquor
Black Liquor Characteristics Desilication
(%)Before Desilication After Desilication
pH SiO2, g/l PH SiO2, g/l
12.8 4.2 10.5 2.1 48.810.4 1.5 64.3
10.3 1.2 71.4
10.2 0.8 80.9
10.1 0.5 88.9
10.0 0.4 90.5
9.8 0.4 90.5
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Desilication of Bamboo Black Liquor
Silica precipitation starts at a pH around 10.4.
90% desilication is achieved at a pH of 10.1.
Results show that residual silica content is almost
same after pH level of 10.0.
Carbonation of black liquor beyond pH 10.1 does notfacilitate silica precipitation.
Optimum pH range for precipitation of silica is 10.1
Residual silica content in black liquor is 0.4 g/l in this
pH range. Residual silica is difficult to remove bycarbonation.
6 gpl NaOH is required for regaining the pH and
viscosity of black liquor.
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PHYSIO-CHEMICAL ANALYSIS OF BAMBOO BLACK LIQUOR
BEFORE AND AFTER DESILICATION
PARAMETERSORIGINAL
BLACK LIQUORDESILICATED
BLACK LIQUOR
1. pH at 3
0o
C12.
8 12.3
2. Total Solids, %w/w 19.0 19.2
3. RAA,gpl as NaOH 7.0 4.0
4. Silica, gpl 4.2 0.4
5. Sulfur, %w/w 2.8 2.85
6. Organics, % w/w 63.2 62.3
7. In organics,% w/w 36.8 37.7
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Viscosity of Desilicated and Original
Bamboo Black liquor
8
22
55
128
240
8
20
40
80
190
50
55
60
65
70
%
V
Desilicated Blackiquor
Original Black iquor
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Swelling Volume Ratio of Black Liquor
Carbonated Black Liquor
Realkalised Black Liquor
Original Black Liquor
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Desilication of Rice Straw Liquor
Black Liquor Characteristics Desilication
(%)Before Desilication After Desilication
pH SiO2, g/l pH SiO2, g/l
12.0 13.3 10.5 6.9 48.1
10.4 5.8 56.3
10.3 1.5 88.7
10.2 1.0 92.5
10.1 0.7 94.7
10.0 0.4 97.0
9.9 0.3 97.7
9.7 0.3 97.7
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Desilication of Rice Straw Liquor
Silica precipitation in rice straw black liquor starts atpH around 10.3.
Around 97% desilication is achieved at pH 10.0.
Below pH 10.0 there is only marginal improvement
in desilication efficiency.
Residual silica content is 0.4 gpl at this pH and 0.3
gpl at pH levels 9.9 to 9.7.
Optimum pH range for silica precipitation in ricestraw black liquor is 10.0 9.9. Around 98% silica is
precipitated in this pH range.
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Commercial Scale Desilication of Rice Straw
A full commercial scale desilication plant
was installed in 2006 in Coastal papers
Limited.
Capacity of theplant is 20M3 black liquor / hr
Flue gas is tapped from Power boiler.
CO2 content of flue gas is 9-11%.
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INPUT CONDITIONS FOR B.L. DE-SILICATION.
1. RAA IN WBL 4 5 gpl
2. SILICA AS SiO2 6 13 gpl
3. pH MAX. REMOVAL OF SILICA 9.9
4. pH OF WBL BEFORE SILICA REMOVAL 11.8 12.0
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PHYSIO-CHEMICAL ANALYSIS OF RICE STRAW BLACK LIQUOR
BEFORE AND AFTER DESILICATION
PARAMETERSORIGINAL
BLACK LIQUORDESILICATED
BLACK LIQUOR
1. pH at 30o C 12.0 10.3
2. Total Solids, %w/w 14.1 9.6
3. RAA,gpl as NaOH 5.0 0.8
4. Silica, gpl 13.3 0.68
5. Sulfur, %w/w 0.06 0.1
6. R2O3, % w/w 0.09 0.05
7. Organics, % w/w 67.65 71.9
8. In organics,% w/w 32.2 28.1
9. Calorific value cal/gram 2578 2940
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PHYSIO-CHEMICAL ANALYSIS OF RICE STRAW BLACK LIQUOR
BEFORE AND AFTER DESILICATION Contd..
PARAMETERS ORIGINALBLACK LIQUOR
DESILICATEBLACK LIQUOR
10. Carbon, % w/w 34.4 36.611. Hydrogen, %w/w
2.5 2.9
12. Nitrogen, % w/w 0.7 1.013. Sodium, % w/w 8.5 11.514. Calcium,% w/w 0.04 0.0615. Sulfate, %w/w as SO4 0.15 0.15
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Viscosity Of Desilicated and Original Rice
Straw Black liquor
33
56
112
316
730
8
12
24
75
130
35
40
45
50
52
%
tl
li
Vi it , . . .
Desilicated Black iquor
Original Black iquor
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Photographs of Mill scale desilication plant
in Rice straw based mill
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DESILICATION PLANT REACTION TANKS
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GAS NOZZLES ASSEMBLY
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FLUE GAS NOZZLES
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CLARIFIER UNDERFLOW PUMP
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PRESSURE FILTER
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SILICA CAKE BEING SCRAPPED
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DUMPED SILICA CAKE
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Impact of Desilication on Black Liquor Properties
Colloidal stability of B.L.- High
Lignin co-precipitation - Nil
Foam generation during gas liquid interaction-moderate and
easily manageable.
Settling of silica sludge- Encouraging results were obtained on
carbonated black liquor.
After three hours of settling the sludge volume stands at only 20% of total
liquor volume and supernatent black liquor has less than 20 ppm
suspended solids.
iscosity of desilicated and realkalized black liquor is fairly close
to the original black liquor in case of bamboo black liquor.
iscosity of rice straw B.L. is reduced significantly after
desilication.
Contd
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Impact of Desilication on Black Liquor Properties..
Contd
No precipitation observed even at 50% Concentration. Original
black liquor gets precipitated at 28% Conc.
There is marginal reduction in the calorific value of realkalized
bamboo black liquor, due to increase in the concentration of
inorganics during realkalisation. However there is no change incalorific value on the basis of organic content of the black liquor.
Calorific value, however increased in case of rice straw black
liquor.
There is a remarkable improvement in the swelling volume ratio(S R) of carbonated & realkalized black liquor when compared to
the original black liquor, which is a pointer towards better
combustibility of desilicated black liquor.
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Conclusion Black liquor properties are improved & burning behaviour of black liquor
is better after desilication.
Successful operation of full commercial scale plant since last four years
has created confidence on CPPRI Technology.
Pilot plant and full commercial scale plant trials have shown that both
Bamboo and straw black liquors containing 2.0 to 14 gpl silica can be
desilicated to a residual silica of 0.5gpl. Successful demonstration of bamboo and rice straw black liquor
desilication has further confirmed the suitability of CPPRI desilication
technology in wide range of silica content of black liquor ( 2-15 gpl).
Adoption of desilication process in bamboo and straw based chemical
recovery will help to; reduce operational problems caused by silica,
save energy during evaporation and
protect environment by making lime sludge suitable for reburning and reducing
solid waste.
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