effect of high temperature drying of recycled paper … of high temperature drying of recycled paper...
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Effect of High Temperature Drying Effect of High Temperature Drying of Recycled Paper on Heat of Recycled Paper on Heat Transfer Rates and Sheet Transfer Rates and Sheet
PropertiesProperties
Dania Dania AlsaidAlsaid, , HussamHussam AlkhasawnehAlkhasawneh and and John CameronJohn Cameron
Objectives:Objectives:
To study the effect of high temperature on paper To study the effect of high temperature on paper drying rates and properties for both virgin and drying rates and properties for both virgin and recycled pulp.recycled pulp.
The process variables studied included the The process variables studied included the effects of temperature, moisture level, effects of temperature, moisture level, recycling, and basis weight on heat transfer recycling, and basis weight on heat transfer and paper properties.and paper properties.
An increase in the paper drying rate will increase the An increase in the paper drying rate will increase the production rate. production rate.
The use of gasThe use of gas--fired dryers (high temperature drying) fired dryers (high temperature drying) is one method to increase the drying rate. is one method to increase the drying rate.
The gasThe gas--fired dryer can provide internal shell fired dryer can provide internal shell temperatures in excess of 200temperatures in excess of 200˚̊C, which significantly C, which significantly increases the drying rate increases the drying rate
With gasWith gas--fired dryers, one or more conventional fired dryers, one or more conventional dryers are replaced with a gasdryers are replaced with a gas--fired dryer. fired dryer.
The Study of High Temperature Drying The Study of High Temperature Drying was Conducted in two Parts:was Conducted in two Parts:
11-- A gasA gas--heated cylinder (heated cylinder (GIT) wasGIT) was installed on the installed on the pilot machine at Western Michigan University. pilot machine at Western Michigan University.
22-- An experimental system was designed to simulate An experimental system was designed to simulate heat transfer into the paper during a typical paper heat transfer into the paper during a typical paper cylinder drying process. cylinder drying process.
Background:Background:
Cylinder DryingCylinder Drying: In conventional paper : In conventional paper drying, iron drums 5 to 6 ft in diameter are drying, iron drums 5 to 6 ft in diameter are heated with steam. heated with steam.
Due to ASTM pressure codes, the steam Due to ASTM pressure codes, the steam pressure is limited to 160 psig or about 190pressure is limited to 160 psig or about 190˚̊ C. C.
Resistance to heat transfer from the Resistance to heat transfer from the condensing steam in the dryer cylinder to the condensing steam in the dryer cylinder to the paper includes the condensate layer, scale, the paper includes the condensate layer, scale, the metal shell, and the contact coefficient metal shell, and the contact coefficient between the shell and the paper.between the shell and the paper.
The contact coefficient accounts for 35 % to The contact coefficient accounts for 35 % to 75% of the total heat transfer resistance. 75% of the total heat transfer resistance.
Due to resistance to heat transfer, the Due to resistance to heat transfer, the maximum surface temperature is about 150maximum surface temperature is about 150˚̊C.C.
The drying rate can be described by the rate of The drying rate can be described by the rate of heat transfer from the dryer shell to the paper.heat transfer from the dryer shell to the paper.
The heat entering the paper heats the water and The heat entering the paper heats the water and solids in the paper, evaporates water or is lost solids in the paper, evaporates water or is lost by radiation and convention. by radiation and convention.
The majority of the heat is used to evaporate The majority of the heat is used to evaporate the water in the paper. the water in the paper.
Equation 1 describes the heat flux from the dryer Equation 1 describes the heat flux from the dryer shell into the paper.shell into the paper.
Q = Q = hchc ((TshellTshell –– TpaperTpaper) (1)) (1)
Q is the heat flux W/mQ is the heat flux W/m22
hchc is the paperis the paper--shell contact coefficient (W/mshell contact coefficient (W/m22
·· ˚̊C) C) T is the temperature T is the temperature ˚̊C of the paper and the C of the paper and the shell.shell.
Commercial Application:Commercial Application:
The trial consisted of replacing one steam The trial consisted of replacing one steam heated dryer on the pilot machine with a gasheated dryer on the pilot machine with a gas--fired dryer and producing 126 lb/1000 ftfired dryer and producing 126 lb/1000 ft22
linear board. linear board.
The surface temperature was increased to 260 The surface temperature was increased to 260 ˚̊C, showed a significant increase in efficiency C, showed a significant increase in efficiency with low with low NOxNOx emissions. A Flynn ribbon emissions. A Flynn ribbon burner model Tburner model T-- 534 was selected and the 534 was selected and the basic configuration is shown in Figure 1.basic configuration is shown in Figure 1.
Figure 1. Gas Fired Paper Dryer, US Figure 1. Gas Fired Paper Dryer, US Patent 6,877,979 (1)Patent 6,877,979 (1)
The paper was heated only on one side through The paper was heated only on one side through the first five steam dryer cans.the first five steam dryer cans.
Several paper samples were collected and the Several paper samples were collected and the curl was measured. curl was measured.
Conclusion of The Pilot Machine Trial:Conclusion of The Pilot Machine Trial:
It was found that curl was present and the It was found that curl was present and the sheet curled away from the heated side.sheet curled away from the heated side.
Sheet curl was influenced by the amount of Sheet curl was influenced by the amount of drying on each side of the sheet. The drying on each side of the sheet. The recommended location of a single gasrecommended location of a single gas--fired fired dryer was such that it would counteract the dryer was such that it would counteract the natural curl present in the sheet due to the first natural curl present in the sheet due to the first dryer cans.dryer cans.
Experimental System:Experimental System:
The experimental system was designed to The experimental system was designed to simulate the heat transfer into the paper during simulate the heat transfer into the paper during a typical paper cylinder drying paper process.a typical paper cylinder drying paper process.
Figure 2 shows the experimental system Figure 2 shows the experimental system designed for this study.designed for this study.
Figure 2, Experimental Paper Drying Figure 2, Experimental Paper Drying SystemSystem
Experiment:Experiment:
HandsheetsHandsheets were made using a Noble and were made using a Noble and Woods Woods handsheethandsheet machine at 15 seconds machine at 15 seconds refining level by Mead refiner from lap SW refining level by Mead refiner from lap SW pulp and recycled pulp.pulp and recycled pulp.
HandsheetsHandsheets were made at three basis weights, were made at three basis weights, 60g/m60g/m22, 120g/m, 120g/m22 and 180g/mand 180g/m22. .
Paper samples from each grade were dried at Paper samples from each grade were dried at 60% initial moisture content at three different 60% initial moisture content at three different temperatures. temperatures.
The low temperature, 100The low temperature, 100˚̊C, the average C, the average cylinder temperature used on conventional cylinder temperature used on conventional cylinder dryers producing newsprint. The cylinder dryers producing newsprint. The highest temperature, 200highest temperature, 200˚̊C, is representative C, is representative of the gasof the gas--heated dryer surface temperature. heated dryer surface temperature.
Results:Results:Figure 3. Heat Flux, Shell and Paper Temperatures for Figure 3. Heat Flux, Shell and Paper Temperatures for 120g/m120g/m22 Lap Pulp and 200Lap Pulp and 200˚̊C Shell TemperatureC Shell Temperature
0
50
100
150
200
250
- 10 20 30 40 50Time, Sec.
Tem
pera
ture
, 'C
0
20000
40000
60000
80000
100000
Hea
t Flu
x, W
/m^2Shell Temperature
Paper Temperature
Heat Flux
Figure 4. Drying Curve for Recycled Paper at 40% Solid Figure 4. Drying Curve for Recycled Paper at 40% Solid
Content, 180g/mContent, 180g/m22 and 200and 200˚̊C Shell TemperatureC Shell Temperature
Drying Rate versus Time
05
101520253035
- 5.00 10.00 15.00 20.00 25.00Time, seconds
Gra
ms/
(M^2
s)
w arm up zone
constant rate zone
First falling
Second falling
Figure 5 & 6. Effect of Shell Temperature and Basis Figure 5 & 6. Effect of Shell Temperature and Basis Weight on the Average Drying Rates in the First Weight on the Average Drying Rates in the First Falling Rate PeriodFalling Rate Period
55--Virgin PulpVirgin Pulp 66--Recycled PaperRecycled Paper
0
5
10
15
20
25
50 100 150 200 250
Shell Temperature C
Dry
Rat
e in
The
Firs
t Fal
ling
Perio
d (g
/m2.
Sec)
180 g/m2
60 g/m2
120 g/m20
5
10
15
20
25
50 100 150 200 250
Shell Temperatue C
Dry
ing
Rat
e in
The
Firs
t Fa
lling
Per
iod
(g/m
2.Se
c)
60 g/m2
120 g/m2
180 g/m2
Figures 7 & 8. Effect of Shell Temperature and Figures 7 & 8. Effect of Shell Temperature and Basis Weight on Drying Rates in the Second Falling Basis Weight on Drying Rates in the Second Falling PeriodPeriod
77-- Virgin Pulp 8Virgin Pulp 8--Recycled PaperRecycled Paper
0
1
2
3
4
5
6
50.0 100.0 150.0 200.0 250.0
Shell Temperature CD
ryin
g R
ate
(g/m
2.Se
c)
60 g/m2
120 g/m2
180 g/m2
0
1
2
3
4
5
6
50.0 100.0 150.0 200.0 250.0
Shell Temperature C
Dry
ing
Rat
e (g
/m2.
Sec)
60 g/m2
120 g/m2
180 g/m2
Figure 9 & 10.Figure 9 & 10. Effect of Shell temperature and Basis Effect of Shell temperature and Basis Weight on the Contact Coefficient in the First Falling PeriodWeight on the Contact Coefficient in the First Falling Period
99--Virgin pulp Virgin pulp 1010--Recycled PaperRecycled Paper
0
200
400
600
800
50 100 150 200 250
Shell Temperature C
Con
tact
Coe
ffic
ient
(w/m
2.C
)
180 g/m2
60 g/m2
120 g/m2
0
200
400
600
800
1000
50 100 150 200 250
Shell Temperatue CCon
tact
Coe
ffic
ient
(W/m
2.C
)
60 g/m2
120 g/m2
180 g/m2
Figure 11 & 12Figure 11 & 12. Effect of Shell Temperature and Basis . Effect of Shell Temperature and Basis Weight on the Contact Coefficient in the Second Falling Weight on the Contact Coefficient in the Second Falling PeriodPeriod
1111--Virgin Pulp 12Virgin Pulp 12--Recycled PaperRecycled Paper
0
100
200
300
400
500
0 50 100 150 200 250
Shell Temperature C
Con
tact
Coe
ffic
ient
( W
/m2.
C)
60 g/m2
120 g/m2
180 g/m2
0.0
200.0
400.0
600.0
800.0
0 50 100 150 200 250
Shell Temperature C
Con
tact
Coe
ffic
ient
(W
/m2.
C)
60 g/m2
120 g/m2
180 g/m2
Effect of High shell Temperature on Paper Effect of High shell Temperature on Paper Properties:Properties:
11-- Curl: Curl: The dried papers are cut into a 15 mm width and 15 The dried papers are cut into a 15 mm width and 15 cm length strips. cm length strips.
Curl is measured by calculating the angle of curvature from Curl is measured by calculating the angle of curvature from the edge rise and the distance between the edge and the the edge rise and the distance between the edge and the midpoint of the strip. midpoint of the strip.
The dried papers tend to curl away from the heated surface The dried papers tend to curl away from the heated surface toward the side last to dry. toward the side last to dry.
The side against the dryer shell dries first and once dried The side against the dryer shell dries first and once dried cannot shrink. This noncannot shrink. This non--uniform drying causes paper to curl. uniform drying causes paper to curl.
Figure 13. Effect of Shell Temperature on Paper Curl. Figure 13. Effect of Shell Temperature on Paper Curl.
Paper Curl for 180 g/mPaper Curl for 180 g/m22 Virgin Pulp and Recycled Paper at Virgin Pulp and Recycled Paper at 50% and 70% Humidity50% and 70% Humidity
0
5
10
15
20
25
30
35
50% 70% 50% 70%
Humidity at 23 deg C
Angl
e O
f Cur
vatu
re (d
eg.)
100 deg. C150 deg. C200 deg. C
Recycled
22--Tensile Strength:Tensile Strength:
Figure 14Figure 14. . Effect of Shell Temperature on Paper Strength for Effect of Shell Temperature on Paper Strength for
180 g/m2 Virgin Pup and recycled Pulp180 g/m2 Virgin Pup and recycled Pulp
0
4
8
12
16
20
24
0 50 100 150 200 250
Shell Temperature (deg. C)
Tens
ile In
dex
(N.m
/g)
Virgin PulpRecycled Paper
Conclusion:Conclusion:1. The paper1. The paper--shell contact coefficient was not affected by shell shell contact coefficient was not affected by shell
temperature in the first following rate zone, but decreased temperature in the first following rate zone, but decreased with increasing temperature in the second following rate with increasing temperature in the second following rate zone. The decrease was present with both lap pulp and zone. The decrease was present with both lap pulp and recycled pulp.recycled pulp.
2. Based on conclusion 1, the high temperature dryer should be 2. Based on conclusion 1, the high temperature dryer should be installed near the middle of the dryer chain and both the installed near the middle of the dryer chain and both the beginning and end of the chain should be avoided.beginning and end of the chain should be avoided.
3. Each gas3. Each gas--fired high temperature dryer should increase the fired high temperature dryer should increase the drying rate by 2 to 4 %.drying rate by 2 to 4 %.
ConclusionConclusion--Continued:Continued:
4. High temperature drying should not affect the strength 4. High temperature drying should not affect the strength properties, but can affect sheet curl. The paper will tend to properties, but can affect sheet curl. The paper will tend to curl toward the side dried last. This effect is less for curl toward the side dried last. This effect is less for recycled paper than for lap pulp.recycled paper than for lap pulp.
5. Based on conclusion 4, if only one high temperature dryer was5. Based on conclusion 4, if only one high temperature dryer wasinstalled, this dryer needs to be located such that it installed, this dryer needs to be located such that it counteracts the curling tendency already present in the sheet.counteracts the curling tendency already present in the sheet.
6. Ideally at least two high temperature dryers 6. Ideally at least two high temperature dryers should be installed. This will not only guard should be installed. This will not only guard against increases in curl, but will allow curl to against increases in curl, but will allow curl to be controlled by adjusting the relative be controlled by adjusting the relative temperatures of the high temperature dryers.temperatures of the high temperature dryers.
Acknowledgements:Acknowledgements:
The authors would like to acknowledge the support of The authors would like to acknowledge the support of The Minnesota Department of Commerce, Gas The Minnesota Department of Commerce, Gas Technology Institute and The US Department of Technology Institute and The US Department of Energy for their support.Energy for their support.
Thank YouThank You
Questions?Questions?