- Email: [email protected]
Evaluation of Fractionation of Softwood Pulp in a Cylindrical Hydrocyclone1
Chinese J. Chern. Eng., 14(4) 537-541
(2006)
RESEARCH NOTES
Evaluation of Fractionation of Softwood Pulp in a Cylindrical Hydrocyclone* LU Xiaojiang(p&5)".** and LIU Shijie($l]t&?&)b 'Department of Process Equipment, Tianjin University of Science and Technology, Tianjin 300222, China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton TG6 2G6, Canada
Abstract Pulp fiber length characterization is addressed in this article. It is-suggested that the proposed separation index H(L) is a viable index to the fiber fractionation performance for evaluating hydrocyclones. Fractionation of softwood (coniferous wood) bleached chemithermomechanical pulp (BCTMP) fiber was carried out with a cylindrical hydrocyclone. Pulp fiber length characteristics in different streams were examined using the fiber quality analyzer (FQA), and the cumulative fiber length fraction, the fiber length fraction density function and the separation index H(L) for different streams were obtained. It is found that H(L) is very useful for characterizing the fiber fractionation performance by indicating the separation capacity of hydrocyclone for individual subgroup of fibers in different streams under different operation conditions. Results of H(L) show that there exists a critical fiber length. A higher proportion of fibers longer than the critical fiber length is in the overflow stream, and a higher proportion of fibers shorter than the critical fiber length in the underflow stream. The data obtained from FQA suggest that the split ratio is the most significant parameter for fiber fractionation performance, which is the best when the split ratio is in the range between 0.14 and 0.2. The effect of feed rate on fiber fractionation performance is weak. Keywords evaluation, fiber, fractionation, hydrocyclone
1 INTRODUCTION Softwood (coniferous wood) bleached chemithermomechanical pulp tends to produce a wider fiber length distribution. Fractionation on the basis of fiber length may allow mechanical pulp to be split into several grades, increasing its economic value"]. It may also reduce or eliminate the addition of more expensive chemical pulp to the paper making furnish. Although many studies about the fractionation of pulp into two or more streams on the basis of fiber length have been reported, research is still in progress aiming at developing the new technology for or improvement of the fractionation performance. Some devices are used for pulp fractionation, such as the centrifugal cleaner"], rotating coneL3], flotation and pressure screenL5'. Hydrocyclones have been widely used in pulp and paper industryt6].It owes its popularity to the low manufacturing and maintenance costs for its simple design without moving parts. Combined with moderate pressure drop and reasonable throughput and efficiency, the hydrocyclone turns out to be the most attractive solution to separate or classify materials by particle size or density"].
Gavelin et CZZ.'~] performed a series of fractionation tests using a conventional hydrocyclone. Branion et CZZ.'~] studied the use of conical hydrocyclone to separate pulp into two streams and showed that it had a great potential owing to its high capacity and efficiency. However, the method for evaluation of fractionation ability of hydrocyclones has not been established. The cylindrical hydrocyclone has a cylindrical body and no conic section. It have been used extensively for fractionation of pulp fiber in pulp and paper industry[lO~lll The average length of fibers is now commonly used to evaluate fractionation performance, but it is just one of indexes of the average statistics. The fraCti0~tion performance of the device for individual fiber subgroup is not characterized by the average length. In this study, fractionation of the softwood (coniferous wood) bleached chemithermomechanical pulp fiber was carried out with a cylindrical hydrocyclone. Pulp fiber length characterization in different streams was examined using the fiber quality analyzer (FQA). The effect of feed rate and ratio of overflow to feed rate were examined. A novel evaluation method was used
Received 2005-05-17, accepted 2006-04-07.
* Supported by the Natural Science and Engineering Research Council of Canada. ** To whom correspondence should be addressed. E-mail: [email protected]
Chinese J. Ch. E. (Vol. 14, No.4)
538
for evaluating the fiber fractionation performance, and a new separation index H(L) is used to evaluate fiber fractionation performance for different streams.
2 CHARACTERIZATION OF PULP FIBER LENGTH AND EVALUATION Fiber length is an important parameter for the paper properties. For each fiber group, one has two reliable properties: the longest dimension and the number of the fibers. Usually, individual fiber lengths are defined according to the longest dimensions measured. The average fiber length can be expressed as[121
2
n,hm+l
'm
(1)
=
&I;. I=l
where i indexes the fiber subgroup; n, is the total number of the fibers within this subgroup, L, is the mean fiber length in this subgroup, and N is the total number of fiber species in the pulp. The averaged fiber length defined by Eq.(l), 1, is the m-th order length average length. Fiber length classifiers often report the fiber length based on Eq.(l). For example, FQA reports three fiber lengths, namely: number averaged length ( i e . , lo), length-averaged-length (ZI) and weight-averaged-length (12). For pulp fibers, the presence of fines will significantly affect the number averaged length 10. The length-averaged-length Zl is the most often used to compare different between samples, so that the fines have only a minor effect on the average length. The longer fibers show more significant impact on the weight-averaged-length Z2. Because of the lack of measurements of individual fiber wall thickness, transversal dimension and fiber mass, other means have to be resorted for defining fiber contents in individual samples. The cumulative &,-fiber length fraction up to the j-th fiber subgroup (in the order from short fiber subgroup to long one) is usually defined as
Xm,j
r=l ~
(2)
fdrn 1=l Eq.(2) can be useful for the representation of the fiber length distribution. For example, xo is similar to the commonly used number fractions in other engineering August, 2006
-
f
. = -Xm,j+~
m ,J
';+I-
Xm,j-l
(3) Lj-1
The fiber length fraction distribution density f expresses the rate of change of cumulative fiber length fraction x, and the difference in f between different streams indicates the separation capacity of fractionation device for individual fiber subgroups In order to show clearly the separation capacity of hydrocyclone for fibers in different streams, it is necessary to consider the difference between the fiber fraction density function in different outlet streams and that in the feed stream. Therefore, a dimensionless quantity can be introduced, which is called the relative separation precision function or H(L). H(L) is defined as the ratio of fiber fraction density function in different streams to that in the feed stream, namely, for the overflow,
(4) and for the underflow,
Jfeed.1
where fovedow, funderflow andhe& are the fiber fraction density function of the overflow stream, the underflow stream and the feed stream respectively. H(L) is thought suitable for indicating the separation capacity of hydrocyclone for individual species fibers in different streams. According to Eqs.(4) and (5), H(L)=l for the feed stream. If H(L)>1, the capacity for capturing fiber at the length corresponding H(L) is better; on the contrary, H(L)c1 means the capturing capacity is weaker.
5n14"
-
sciences. If the fibers are of similar transversal dimensions, then x1 (m=l)is particularly useful. Plots of X I and xo as a function of fiber length are able to indicate the size distribution for individual subgroup of fibers in different streams. Corresponding to the Z,-fiber fractions, a fiber Z,-fi-action density function is given by
3 EXPERIMENTAL A cylindrical hydrocyclone was used in this study, as shown in Fig. 1 . It consisted of a cylindrical body about 400mm in length and 45mm in diameter with upper and lower nozzles. This allowed the overflow stream and underflow stream to be taken from the
Evaluation of Fractionation of Softwood Pulp in a Cylindrical Hydrocyclone
axial nozzle from the normal tangential feed. Fig.2 illustrates the experimental setup. The pulp is fed tangentially to the top of the cylindrical body. The feed rate is controlled by valve 1 and measured with an electromagnetic flowmeter.
I.,=I
Do=37.5mrn
539
was mixed with tap water to make the pulp slurry having a consistency of 0.095% for being used as the feed solution. Feed rate were varied in the range of 30 and 83L-min-', making R in the range between 0.054 and 0.712. Pulp fiber length characterization (the number averaged length lo, the length-averaged-length ZI.) in different stseams are examined using the fiber quality analyzer (Code LDA96, Op Test Equipment). 4 RESULTS A N D DISCUSSION 4.1 The fiber length characterizationof feed stmm Shown in Figs.3 and 4 are feed sample fiber length distributions based on the raw data by FQA. For the softwood BCTh4P pulp, the short fiber content is very high as depicted by the fiber fraction density, although the cumulative fiber fraction did not show quick increase in distribution at short fiber lengths. Results obtained using FQA show that the number averaged length lo, the length-averaged-length 21 for the feed sample are 0.8335mm and 2.5595mm, respectively.
'14: Du=31.25mm
Figure 1 A schematic of cylindrical hydrocyclone
hydrocyclone
2.0
0
flow meter
4.0
6.0
8.0
fiber length (L), mm
Figure 3 Cumulative lengtb fraction of feed pulp +XO;Axi
bypass
"." Figure 2 Schematic of experimental setup
0.5
1
If the pulp feed rate to the hydrocyclone QF, the overflow stream rate QOand the underflow stream rate QUare specified, a split ratio, R, can be defined as
R=-QQF
e, Qu+&
(6)
By adjusting the opening of value 2, the flow rate of the underflow stream, and thus R, is controlled. Softwood BCTMP (coniferous wood haft) samples are obtained from Millar Western Forest Product Company, Whitecourt Pulp Mill. The BCTMP pulp
0
2.0
4.0 6.0 fiber length (L), mm
Figure 4 Fiber length fraction density of feed pulp .fo;
O h
Chinese J. Ch. E. 14(4) 537 (2006)
Chinese J. Ch. E. (Vol. 14, No.4)
540
4.2 Evaluation of fractionation of pulp fiber in cylindrical hydrocyclone Variations of the length-averaged-length ll and number averaged length lo of the overflow and underflow streams with split ratio R and feed rate Q p are shown in Figs.5 and 6. Fig.5 shows that the effect of R on the fiber fractionation performance is significant in a particular range for a given feed rate. When the split ratio R is low, the length-averaged-length Z1 and number averaged length lo of the overflow stream increases with increasing the split ratio R. When the split ratio R is high, the length-averaged-length Zl of the overflow stream decreases as the split ratio R is further increased. For the underflow stream, the length-averaged-length I I and number averaged length ZO decreases with increasing R. A good fractionation performance is obtained with R in the range from 0.14 to 0.2. The length-averaged-length I , and number av-
eraged length lo versus the feed rate are shown in Fig.6, showing slight decrease of feed rate on the fiber length in the overflow stream. In most studies on pulp fiber fractionation, the number averaged length or length-averaged-length of pulp fiber were commonly used to evaluate fiber fractionation performance. The difference of the number averaged length or length-averaged-length of pulp fiber in different streams were used to show fiber fractionation performance. But this is only an overall evaluation index. H(L) based on ZI can be used to indicate the separation capacity of hydrocyclone for individual subgroup of fibers in different streams. Figures 7-10 show that there exists a critical fiber length at which (H=l).A high proportion of fibers longer than the critical fiber length appears in the overflow stream (H>l), and a high proportion of fibers shorter than the critical fiber length enter the un-
3.501
4.0 -
3.5
3.0 2.5
~
h
2
0.50
'
0
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
I
I
0.7
0.8
I
0.6
2.0
~
.1
0
split ratio ( R )
1
Figure 5 The effect of split ratio on fractionation
+
(&=41L&-')
2 3 fiber length (L). mm
4
F
Figure 7 Separation index H(L) of overflow and underflow streams with R=0.14 and QF=41L.rnin-'
1,: overflow; 0 underflow; ----feed lo: W overflow; 0 underflow; feed
0 ovefflow; 0 underflow; ----feed
t."
3.5 3.0 -
- 2.5
2,
0.50 I , 30.0 40.0
I
50.0 60.0
I
70.0
,
I
feed rate (QF), L-min-'
Figure 6 The effect of feed rate on fractionation (R=O. 14) 11: overflow; 0 underflow; ----feed lo: W overflow; 0 underflow;
August, 2006
-.-
feed
~
I
80.0 90.0 100.0
+
2.0
~
0
1
2 3 fiber length ( L ) , mrn
4
5
Figure 8 Separation index H(L) of overflow and underflow streams with R=0.074 and QF=41L.min-' 0 O V G ~ ~ ~0Ounderflow; W; ----feed
Evaluation of Fractionation of Sofhvood Pulp in a Cylindrical Hydrocyclone H(L) L, I
3: i
lln
3.0
4 N n
a 2.51 3 2.0
QF
Qo 0.5
1
QU
R X
0
1
2
3
4
5
fiber length (L), mm
Figure 9 Separation index H(L) of overflow and underflow streams with Rd.71 and QpHLmin-’ overflow; 0 underflow; ----feed
i, j
2
3.0 2.5
3
2.0
1.5
4
1.o
0.5
0
1
2 3 fiber length (L), mm
4
5
Figure 10 Separation index H(L) of overflow and underflow streams with Rd.077 and QF=98L-min-‘ overflow; 0 underflow; ----feed
derflow stream (H
5
6
7
8
9
10
11
12
NOMENCLATURE
f
fiber subgroup
REFERENCES
4.0
$
relative separation precision function fiber length, mm m-th order length average length length-averaged-length total number of fiber species in pulp total number of fiber in individual fiber subgroup feed rate, Lmin-’ overflow stream rate, Lmin-’ underflow stream rate, Lmin-’ split ratio cumulative fiber length fraction
Subscripts
1
3.5
541
Eckert, W.F., Masliyah, J.H., Afacan, A., “Fractionation of softwood TMP by flotation”, TAPPI J, 80, 210216( 1997). Bliss, T.L., “A study of fiber fractionation using centrifugal cleaners”, M.S. Thesis, Miami Univ., Oxford, Ohio (1983). Rewatkar, V.B., “Hardwood fiber fractionation using rotating cone”, Can. J. Chm. Eng., 75, 196204(1997). Eckert, W.F., Afacan, A., Masliyah, J.H., “Fractionation of softwood TMP by continuous flotation”, TAPPI J, 83, 1-8(2OOO). Olson, J.A., “Fiber length fractionation caused by pulp screening, slotted screen plates”, J. Pulp Paper Sci., 27, 255-261 (2001). Smook, GA., Handbook for pulp & Paper Technologists, 2nd edition, Angus Wilde Publication,Vancouver (1999). Slack, M.D., h a d , R.O., Bakker, A., Boysan, F., “Advances in cyclone modeling using unstructured grids”, Trans. Inst. Chem. Engrs, 78A, 1098--1104(2OOO). Gavelin, G, Backman, J., “Fractionation with hydrocyclone”, In: Proceedings of TAPPI Pulping Conference, Atlanta, USA (1991). Branion, R., Ho, S.L., Rehmat, T., Sevilla, E., “Fiber fractionation in hydrocyclone”, In: Roc eedings of 6th World Congr. Chem. Eng., Melbourne, Australia (2001). Liu, S.J., Zhu, Z.X., Li, W., Afacan, A., Masligah, J.H., “Kraft wood-pulp fiber fractionation with air-sparged hydrocyclone”, In: Proceedings of PAFTAC Annual Meeting, Montreal, Canada, January (2002). Ho, S.L., “Synthetic fiber fractionation in hydrocyclones”, Ph.D. Thesis, University of British Columbia, Canada (2001). Liu, S.J., “Pulp fiber size characterization”, J. Tianjin Univ. Sci. Technal., 19,234-241(2004).
fiber length fraction distributing density
Chinese J. Ch. E. 14(4) 537 (2006)
(2006)
RESEARCH NOTES
Evaluation of Fractionation of Softwood Pulp in a Cylindrical Hydrocyclone* LU Xiaojiang(p&5)".** and LIU Shijie($l]t&?&)b 'Department of Process Equipment, Tianjin University of Science and Technology, Tianjin 300222, China b Department of Chemical and Materials Engineering, University of Alberta, Edmonton TG6 2G6, Canada
Abstract Pulp fiber length characterization is addressed in this article. It is-suggested that the proposed separation index H(L) is a viable index to the fiber fractionation performance for evaluating hydrocyclones. Fractionation of softwood (coniferous wood) bleached chemithermomechanical pulp (BCTMP) fiber was carried out with a cylindrical hydrocyclone. Pulp fiber length characteristics in different streams were examined using the fiber quality analyzer (FQA), and the cumulative fiber length fraction, the fiber length fraction density function and the separation index H(L) for different streams were obtained. It is found that H(L) is very useful for characterizing the fiber fractionation performance by indicating the separation capacity of hydrocyclone for individual subgroup of fibers in different streams under different operation conditions. Results of H(L) show that there exists a critical fiber length. A higher proportion of fibers longer than the critical fiber length is in the overflow stream, and a higher proportion of fibers shorter than the critical fiber length in the underflow stream. The data obtained from FQA suggest that the split ratio is the most significant parameter for fiber fractionation performance, which is the best when the split ratio is in the range between 0.14 and 0.2. The effect of feed rate on fiber fractionation performance is weak. Keywords evaluation, fiber, fractionation, hydrocyclone
1 INTRODUCTION Softwood (coniferous wood) bleached chemithermomechanical pulp tends to produce a wider fiber length distribution. Fractionation on the basis of fiber length may allow mechanical pulp to be split into several grades, increasing its economic value"]. It may also reduce or eliminate the addition of more expensive chemical pulp to the paper making furnish. Although many studies about the fractionation of pulp into two or more streams on the basis of fiber length have been reported, research is still in progress aiming at developing the new technology for or improvement of the fractionation performance. Some devices are used for pulp fractionation, such as the centrifugal cleaner"], rotating coneL3], flotation and pressure screenL5'. Hydrocyclones have been widely used in pulp and paper industryt6].It owes its popularity to the low manufacturing and maintenance costs for its simple design without moving parts. Combined with moderate pressure drop and reasonable throughput and efficiency, the hydrocyclone turns out to be the most attractive solution to separate or classify materials by particle size or density"].
Gavelin et CZZ.'~] performed a series of fractionation tests using a conventional hydrocyclone. Branion et CZZ.'~] studied the use of conical hydrocyclone to separate pulp into two streams and showed that it had a great potential owing to its high capacity and efficiency. However, the method for evaluation of fractionation ability of hydrocyclones has not been established. The cylindrical hydrocyclone has a cylindrical body and no conic section. It have been used extensively for fractionation of pulp fiber in pulp and paper industry[lO~lll The average length of fibers is now commonly used to evaluate fractionation performance, but it is just one of indexes of the average statistics. The fraCti0~tion performance of the device for individual fiber subgroup is not characterized by the average length. In this study, fractionation of the softwood (coniferous wood) bleached chemithermomechanical pulp fiber was carried out with a cylindrical hydrocyclone. Pulp fiber length characterization in different streams was examined using the fiber quality analyzer (FQA). The effect of feed rate and ratio of overflow to feed rate were examined. A novel evaluation method was used
Received 2005-05-17, accepted 2006-04-07.
* Supported by the Natural Science and Engineering Research Council of Canada. ** To whom correspondence should be addressed. E-mail: [email protected]
Chinese J. Ch. E. (Vol. 14, No.4)
538
for evaluating the fiber fractionation performance, and a new separation index H(L) is used to evaluate fiber fractionation performance for different streams.
2 CHARACTERIZATION OF PULP FIBER LENGTH AND EVALUATION Fiber length is an important parameter for the paper properties. For each fiber group, one has two reliable properties: the longest dimension and the number of the fibers. Usually, individual fiber lengths are defined according to the longest dimensions measured. The average fiber length can be expressed as[121
2
n,hm+l
'm
(1)
=
&I;. I=l
where i indexes the fiber subgroup; n, is the total number of the fibers within this subgroup, L, is the mean fiber length in this subgroup, and N is the total number of fiber species in the pulp. The averaged fiber length defined by Eq.(l), 1, is the m-th order length average length. Fiber length classifiers often report the fiber length based on Eq.(l). For example, FQA reports three fiber lengths, namely: number averaged length ( i e . , lo), length-averaged-length (ZI) and weight-averaged-length (12). For pulp fibers, the presence of fines will significantly affect the number averaged length 10. The length-averaged-length Zl is the most often used to compare different between samples, so that the fines have only a minor effect on the average length. The longer fibers show more significant impact on the weight-averaged-length Z2. Because of the lack of measurements of individual fiber wall thickness, transversal dimension and fiber mass, other means have to be resorted for defining fiber contents in individual samples. The cumulative &,-fiber length fraction up to the j-th fiber subgroup (in the order from short fiber subgroup to long one) is usually defined as
Xm,j
r=l ~
(2)
fdrn 1=l Eq.(2) can be useful for the representation of the fiber length distribution. For example, xo is similar to the commonly used number fractions in other engineering August, 2006
-
f
. = -Xm,j+~
m ,J
';+I-
Xm,j-l
(3) Lj-1
The fiber length fraction distribution density f expresses the rate of change of cumulative fiber length fraction x, and the difference in f between different streams indicates the separation capacity of fractionation device for individual fiber subgroups In order to show clearly the separation capacity of hydrocyclone for fibers in different streams, it is necessary to consider the difference between the fiber fraction density function in different outlet streams and that in the feed stream. Therefore, a dimensionless quantity can be introduced, which is called the relative separation precision function or H(L). H(L) is defined as the ratio of fiber fraction density function in different streams to that in the feed stream, namely, for the overflow,
(4) and for the underflow,
Jfeed.1
where fovedow, funderflow andhe& are the fiber fraction density function of the overflow stream, the underflow stream and the feed stream respectively. H(L) is thought suitable for indicating the separation capacity of hydrocyclone for individual species fibers in different streams. According to Eqs.(4) and (5), H(L)=l for the feed stream. If H(L)>1, the capacity for capturing fiber at the length corresponding H(L) is better; on the contrary, H(L)c1 means the capturing capacity is weaker.
5n14"
-
sciences. If the fibers are of similar transversal dimensions, then x1 (m=l)is particularly useful. Plots of X I and xo as a function of fiber length are able to indicate the size distribution for individual subgroup of fibers in different streams. Corresponding to the Z,-fiber fractions, a fiber Z,-fi-action density function is given by
3 EXPERIMENTAL A cylindrical hydrocyclone was used in this study, as shown in Fig. 1 . It consisted of a cylindrical body about 400mm in length and 45mm in diameter with upper and lower nozzles. This allowed the overflow stream and underflow stream to be taken from the
Evaluation of Fractionation of Softwood Pulp in a Cylindrical Hydrocyclone
axial nozzle from the normal tangential feed. Fig.2 illustrates the experimental setup. The pulp is fed tangentially to the top of the cylindrical body. The feed rate is controlled by valve 1 and measured with an electromagnetic flowmeter.
I.,=I
Do=37.5mrn
539
was mixed with tap water to make the pulp slurry having a consistency of 0.095% for being used as the feed solution. Feed rate were varied in the range of 30 and 83L-min-', making R in the range between 0.054 and 0.712. Pulp fiber length characterization (the number averaged length lo, the length-averaged-length ZI.) in different stseams are examined using the fiber quality analyzer (Code LDA96, Op Test Equipment). 4 RESULTS A N D DISCUSSION 4.1 The fiber length characterizationof feed stmm Shown in Figs.3 and 4 are feed sample fiber length distributions based on the raw data by FQA. For the softwood BCTh4P pulp, the short fiber content is very high as depicted by the fiber fraction density, although the cumulative fiber fraction did not show quick increase in distribution at short fiber lengths. Results obtained using FQA show that the number averaged length lo, the length-averaged-length 21 for the feed sample are 0.8335mm and 2.5595mm, respectively.
'14: Du=31.25mm
Figure 1 A schematic of cylindrical hydrocyclone
hydrocyclone
2.0
0
flow meter
4.0
6.0
8.0
fiber length (L), mm
Figure 3 Cumulative lengtb fraction of feed pulp +XO;Axi
bypass
"." Figure 2 Schematic of experimental setup
0.5
1
If the pulp feed rate to the hydrocyclone QF, the overflow stream rate QOand the underflow stream rate QUare specified, a split ratio, R, can be defined as
R=-QQF
e, Qu+&
(6)
By adjusting the opening of value 2, the flow rate of the underflow stream, and thus R, is controlled. Softwood BCTMP (coniferous wood haft) samples are obtained from Millar Western Forest Product Company, Whitecourt Pulp Mill. The BCTMP pulp
0
2.0
4.0 6.0 fiber length (L), mm
Figure 4 Fiber length fraction density of feed pulp .fo;
O h
Chinese J. Ch. E. 14(4) 537 (2006)
Chinese J. Ch. E. (Vol. 14, No.4)
540
4.2 Evaluation of fractionation of pulp fiber in cylindrical hydrocyclone Variations of the length-averaged-length ll and number averaged length lo of the overflow and underflow streams with split ratio R and feed rate Q p are shown in Figs.5 and 6. Fig.5 shows that the effect of R on the fiber fractionation performance is significant in a particular range for a given feed rate. When the split ratio R is low, the length-averaged-length Z1 and number averaged length lo of the overflow stream increases with increasing the split ratio R. When the split ratio R is high, the length-averaged-length Zl of the overflow stream decreases as the split ratio R is further increased. For the underflow stream, the length-averaged-length I I and number averaged length ZO decreases with increasing R. A good fractionation performance is obtained with R in the range from 0.14 to 0.2. The length-averaged-length I , and number av-
eraged length lo versus the feed rate are shown in Fig.6, showing slight decrease of feed rate on the fiber length in the overflow stream. In most studies on pulp fiber fractionation, the number averaged length or length-averaged-length of pulp fiber were commonly used to evaluate fiber fractionation performance. The difference of the number averaged length or length-averaged-length of pulp fiber in different streams were used to show fiber fractionation performance. But this is only an overall evaluation index. H(L) based on ZI can be used to indicate the separation capacity of hydrocyclone for individual subgroup of fibers in different streams. Figures 7-10 show that there exists a critical fiber length at which (H=l).A high proportion of fibers longer than the critical fiber length appears in the overflow stream (H>l), and a high proportion of fibers shorter than the critical fiber length enter the un-
3.501
4.0 -
3.5
3.0 2.5
~
h
2
0.50
'
0
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
I
I
0.7
0.8
I
0.6
2.0
~
.1
0
split ratio ( R )
1
Figure 5 The effect of split ratio on fractionation
+
(&=41L&-')
2 3 fiber length (L). mm
4
F
Figure 7 Separation index H(L) of overflow and underflow streams with R=0.14 and QF=41L.rnin-'
1,: overflow; 0 underflow; ----feed lo: W overflow; 0 underflow; feed
0 ovefflow; 0 underflow; ----feed
t."
3.5 3.0 -
- 2.5
2,
0.50 I , 30.0 40.0
I
50.0 60.0
I
70.0
,
I
feed rate (QF), L-min-'
Figure 6 The effect of feed rate on fractionation (R=O. 14) 11: overflow; 0 underflow; ----feed lo: W overflow; 0 underflow;
August, 2006
-.-
feed
~
I
80.0 90.0 100.0
+
2.0
~
0
1
2 3 fiber length ( L ) , mrn
4
5
Figure 8 Separation index H(L) of overflow and underflow streams with R=0.074 and QF=41L.min-' 0 O V G ~ ~ ~0Ounderflow; W; ----feed
Evaluation of Fractionation of Sofhvood Pulp in a Cylindrical Hydrocyclone H(L) L, I
3: i
lln
3.0
4 N n
a 2.51 3 2.0
QF
Qo 0.5
1
QU
R X
0
1
2
3
4
5
fiber length (L), mm
Figure 9 Separation index H(L) of overflow and underflow streams with Rd.71 and QpHLmin-’ overflow; 0 underflow; ----feed
i, j
2
3.0 2.5
3
2.0
1.5
4
1.o
0.5
0
1
2 3 fiber length (L), mm
4
5
Figure 10 Separation index H(L) of overflow and underflow streams with Rd.077 and QF=98L-min-‘ overflow; 0 underflow; ----feed
derflow stream (H
5
6
7
8
9
10
11
12
NOMENCLATURE
f
fiber subgroup
REFERENCES
4.0
$
relative separation precision function fiber length, mm m-th order length average length length-averaged-length total number of fiber species in pulp total number of fiber in individual fiber subgroup feed rate, Lmin-’ overflow stream rate, Lmin-’ underflow stream rate, Lmin-’ split ratio cumulative fiber length fraction
Subscripts
1
3.5
541
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fiber length fraction distributing density
Chinese J. Ch. E. 14(4) 537 (2006)