close

Вход

Забыли?

вход по аккаунту

?

Variability in Transport Properties for Blackbutt Timber in New South Wales Within and Between-Tree Variability.

код для вставкиСкачать
Dev. Chem. Eng. Mineral Process. 14(1/2), pp. 173-181, 2006.
Variability in Transport Properties for
Blackbutt Timber in New South Wales:
Within and Between-Tree Variability
S.J. Cabardo" and T.A.G. Langrish
Department of Chemical Engineering, University of Sydney, Sydney,
New South Wales 2006, Australia
Variability is a key issue in the processing of biological materials, in this case the
hying of hardwood timber. This paper reports the measurements of variability of
transport properties. which are relevant to the drying of blackbutt, Eucalyptus
pilularis Sm, from northern New South Wales. Specifically, within-tree and betweentree variations are reported for two blackbutt regrowth logs. An analysis of variance
showed that some timber properties were affected by the board positions within-trees
and between-trees. Circumferential and radial efects were significant for the withintree variability of most transport properties. Similarly, radial and circumferential
effects were signlficant for most of the transport parameters between trees, but can be
tentatively stated because only two regrowth logs were assessed. Timber boards with
high initial moisture contents had higher rates of diffirsion and low basic densities
using principal components analysis. A possible reason is that if there is less wood
material per unit volume, these vacant spaces may be occupied by water, and there is
also less resistance for diffusive transport of moisture.
Introduction
Large quantities of timber from hardwood plantations in Australia are now being
harvested, from 1 million cubic metres in 2000-01 to a predicted level of 9.2 million
cubic metres in the latter half of the decade as reported by Fer uson et al. [l]. In
NSW, the consumption of eucalypts sawn woods was 263,000 m in 2000-01 [2]. In
the future, Australian timber drying companies may have to rely on getting most of
their timber supplies from these hardwood plantations. However, the companies
report a growing difficulty in handling this type of timber for a number of reasons,
with the increased amount of variation in timber properties affecting its dried quality
being significant. Anecdotally, plantation timber appears to be more variable with
K.
~~
* Author for correspondence (scabardo@chem.eng.usyd.edxau).
I73
S.J. Cabardo and T.A.G.Langrish
regards to its properties compared with old growth timber. As a result, it is possible
that large variation in intrinsic properties may require better control, hence optimized
drying schedules that account for these variabilities.
It is probable, on the basis of previous studies in the literature about the drying of
hardwoods [3], that the main transport mechanism of moisture movement within
hardwoods is diffusion. This is due to hardwoods consisting mainly of heartwood.
Cell wall pits are aspirated within heartwood, thus, water cannot flow by convection
through these cells. The diffusion model for hardwoods was also confirmed by the
work of Wu (41 because it predicted the same moisture content distributions, i.e.
moisture content as a function of distance within the timber, as those observed for
Tasmanian eucalypts, at various times during drying.
The tree species chosen for this study was blackbutt (E.piZularis Sm.), because
blackbutt is the predominant planted hardwood species in NSW, Australia [S]. It is
most abundant in many coastal areas from south of Bega up to south-eastem
Queensland. In summary, the variation o f the diffusion coefficient (a transport
property), in addition to basic density and green moisture content for blackbutt, was
investigated in this paper.
Experimental Procedure and Apparatus
I Preparation of Samples
Blackbutt boards (28 mm thick x 108 mm wide x 900 mm long) were cut from two
regrowth logs; log 685 that was taken from the Newry Creek region, and log 686 from
the Lower Bucca area. These boards were taken from different locations within each
log, as shown in Figures 1 and 2. Each ‘ A ’ board (the first 900 rnm from the bottom
end) has its corresponding ‘E’ board that was taken from the top end of each log
(approximately 3.6 metres up the log). A 20 mm thick sample was taken from each
end of each green ‘A’ and ‘E’ board to calculate its corresponding initial moisture
content using the oven-dry method. The remaining board length was kiln dried in a
drying tunnel, using the conventional drying schedule published in the Australian
Timber Seasoning Manual [ 6 ] suitable for mixed sawn blackbutt boards (25 mm
thick) as shown in Table 1. Each kiln charge consisted of six boards, resulting in four
kiln charges (four experiments). The boards were left to dry, with the schedule
changed based on the number of days. Each experiment took 21 days to run. The final
moisture contents were calculated by cutting another 20 mm sample from each board
at the end of the drying process, and repeating the oven-dry method. Tirnber density
was based on the oven-dry weighdgreen volume, where the sample dimensions were
measured for green volume.
Moisture content change
points (%)
Dry bulb temperature PC)
Wet bulb depression PC)
I74
Green
60
40
35 30 25
55
3
55
3
60
4
60
4
65
5
65
8
15tofinal
(usually 12)
70
70
10
15
20
Variability in Transport Properties for Blackbutt Timber in New South Wales
Figure 1. Cross-section of the
bottom end, showing where
each ' A ' board was taken
within log 685.
Figure 2. Cross-section of the
bottom end, showing where
each 'A' board was taken
within log 686.
II Drying Tunnel
Figure 3 shows a schematic of the overall design of the pilot-scale batch dryer used
for drying 800 mm timber boards. This conventional kiln was used to produce
controlled drying conditions, with initial dry-bulb and wet-bulb temperatures of 55°C
and 52"C, respectively, and an air velocity of 1.30 m/s through and around the boards.
Manipulating the steam flowrate to a steam-injection system enabled control of the
wet-bulb temperature to its desired set-point. The steam system consisted of a dryer
and six-point steam injection pipe over a 300 mm duct. The control mechanism for
the dry-bulb temperature involved the flowrate adjustment, using a control valve, of
100 kPa (gauge) steam to a finned heat exchanger. The total mass of the timber stack
was measured using a platform balance. It has a capacity of 200 kg and a resolution of
5 g. This information was transmitted through an RS 232 interface to a computer to
estimate the average moisture content of the timber stack. This value was used to
determine if the set-points of the dry-bulb and wet-bulb temperatures had to be altered
according to the drying schedule shown in Table 1.
I 75
S.J. Cabardo and T.A. G. Langrish
W a l a r Mains
Pump
Figure 3. Schematic diagram of timber drying tunnel.
111 Fining Procedure for Diffusion Coefficients
The predicted overall drying curve is dependent on the diffusion coefficients and
operating temperatures for each experiment (Langrish et al. [8]). The model is based
on solving Fick’s second law of diffusion for mass transfer and the conduction
equation for heat transfer. Moisture is assumed to diffuse through the timber and
evaporate near the surface. It is often difficult to measure the diffusion coefficient
directly, but it can be fitted to experimental data, as here. Schaffner [7] and Wu [4]
both successfully fitted diffusion coefficients to observed data for eucalypt timbers.
Therefore, least-squares parameter fitting was used to adjust the values of the
reference diffusion coefficient (Or) and the activation energy (DE) (with T as the
average temperature of the timber boards) in an Arrhenius-type equation:
D = D, ex& D,/T)
...(1)
The predicted parameters, D, and DE,are then used to calculate the diffusion
coefficient, D.
176
Variability in Transport Properties for Blackbutt Timber in New South Wales
The parameter fitting minimizes the sum of squares of the difference between the
predicted and actual average moisture contents for each board. Typical initial values
of D,and DEwere 1.13 x los5m2s-' and 3750 K, respectively. The recorded moisture
contents that were measured throughout the drying process, and the dry and wet bulb
temperatures, were dependent on each board and each experiment. The fitted initial
moisture content was set equal to the actual initial moisture content. Fick's second
law of diffusion for mass transfer and the conduction equation for heat transfer were
then solved simultaneously, with convective boundary conditions at the board
surfaces and symmetry at the centerline of each board. Overall, the final adjusted
values of D, and DEcharacterized the drymg behaviour of the corresponding timber
board.
Results and Discussion
I Overview
The diffusion parameters are listed alongside the initial and final moisture content and
basic density for each board in Tables 2 to 5. The standard error is a measure of the
difference between the predicted drying rate and the experimental drying rate. All the
resulting standard error values were small, and the fitted curves followed the
experimental values closely. Since D was dependent on temperature, the average
temperature value, T, used was 6OoC, because it reflects the average temperature for
each board during the drying process. The parameters D, and DE should be
characteristic of the timber and should be applicable to boards of different thickness if
diffusion is the dominant moisture transport mechanism for moisture movement. The
resulting values of activation energies for all boards were expected not to vary
significantly because the activation energy should be a property of the major
constituents of the timber matrix. Our values ranged from 3405 K to 3792 K, close to
the values of 3800 K and 3773 K reported by Wu [ 4 ] and Langrish et al. [8],
respectively.
Table 2. Initial and final moisture contents, basic densities, and fitted diffusion
parameters for experiment one.
177
S.J . Cabardo and T.A . G. Langrish
Table 3. Initial and final moisture contents, basic densities, and fitted diffusion
parameters for experiment two.
I
I
1
I
1
I
Table 4. Initial and final moisture contents, basic densities, and fitted diffusion
parameters for experiment three.
Table 5. Initial and final moisture contents, basic densities, and fitted d@usion
parameters for experiment four.
Board
ID
~
5A
5E
12A
12 E
18A
18 E
I 78
D,
K]
[x 1@"
rn's-']
Basic
density
fkg/m3]
3.56
3.64
3.73
3.73
3.68
3.71
2.65
2.06
1.56
1.55
1.79
1.65
613
824
816
822
7 84
819
I@'
DE
[x l d
m2s-']
1.14
1.13
1.12
1.12
1.11
1.12
[x
D
Initial
moisture
content
fkghg]
0.94
0.72
0.57
0.52
0.79
0.61
,
[kg/kg]
0.05
0.06
0.06
0.07
0.035
0.028
0.022
0.018
0.029
0.022
Variabiliy in Transport Properties for Blackbutt Timber in New South Wales
II Between-trees and Within-tree variations
Based on the data in Tables 2 to 5 , it can be seen that the initial moisture contents
decreased from pith to bark and basic density increased from pith to bark. The small
dispersion in final moisture contents at the end of the drying process is a reflection of
the lack of variability in equilibrium behaviour, because the drying behaviour at the
end of drying is dominated by equilibrium. The conventional kiln schedule used for
these drying experiments minimized the dispersion of the final moisture contents, thus
satisfying the allowed amount of variation for final moisture contents associated with
high quality timber according to Australian Standards [9]. Moreover, an analysis of
variance (ANOVA) showed that radial and circumferential effects were significant
sources of the within-tree variation of the diffision coefficients, initial moisture
contents and basic densities. A similar result was found for the variability of the same
properties between-trees. However, the ANOVA for between-trees can only be
tentatively stated because only two regrowth logs were dried, resulting in only one
degree of freedom. Nevertheless, these results support the previous statement that the
behaviour of these three parameters changes across the radius of the log, i.e. from pith
to bark. The observation that the variability of these parameters is caused by the same
effects suggests that there may be a correlation between all three parameters. In
addition, the within-tree variability of the initial moisture content was affected
by the height, the radial x circumferential interaction, and the height x circumferential
interaction.
III Principal Components Analysis
Boards from experiments one to four showed an apparent correlation between high
initial moisture contents, higher diffusion coefficients, and low basic densities.
Timber boards with high densities indicates that there is more wood material per unit
volume. This may either be due to large cell volumes and/or more cell-wall material,
leaving less space for the water to occupy, which explains the low initial moisture
contents for timber boards with high densities [3, 101. In addition, more wood
material means a higher resistance to diffusive transport of moisture [3]. Therefore the
diffusion coefficient is expected to be lower in high density wood. A principal
component analysis (PCA) [ l I] was performed on the three parameters. If these
parameters are closely correlated, then the PCA should result in only one large
principal component, and one large eigenvalue and eigenvector. The results of the
PCA showed that the first and largest eigenvalue accounted for 92% of the total
amount of variation within these parameters.
Figure 4 shows that boards with high initial moisture contents have low basic
densities, and thus high diffusion coefficients. One board (24E) from log 685 was
considered an outlier; although it had a high basic density, it also had a high diffusion
rate which was possibly due to the crack present throughout the length of the board.
However, the presence of cracks has not increased the apparent diffusion coefficients
by an order of magnitude.
I 79
S.J. Cabardo and T.A. G. Langrish
I
\-
24E
\
Basic density 1
(1000s of kgiin3)
\
liiitiai inoisture content (kglkg, dry basis)
Figure 4. Three-dimensional plot of the parameters, together with the line of
maximum variation.
Conclusions
Within a tree, initial moisture contents decreased from pith to bark,, and basic
densities increased from pith to bark. ANOVA analysis showed that circumferential
and radial effects were likely to be significant for the variability of the diffusion
coefficient, initial moisture content and the basic density, both within a tree and
between trees. However, results from the ANOVA of between-trees c.an only be
tentatively stated because only two regrowth logs were dried, whereas a thorough
analysis was conducted for within-tree effects. In addition, timber boards with high
initial moisture contents had low basic densities and high diffusion coefficients. This
observation may be connected with the link between less wood material, more space
for water in the green timber, and the increased ease with which moisture can diffuse
through the timber board.
180
Variability in Transport Properties for Blackbutt Timber in New South Wales
Acknowledgements
This work is supported by the Australian Research Council, Forests NSW - a special
mention to Dr Ross Dickson and Mr Bill Joe, and J. Notaras and Sons, Grafton.
Nomenclature
D
D,
DE
T
Diffusion coefficient, m2s-'
Reference diffusion coefficient, m2 s.'
Activation energy for diffusion, K
Average temperature of the timber boards, K
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10
II
Ferguson, I.S., Fox, J., Baker, T., Stackpole, D., and Wild, 1. 2002. National and Regional Plantation
Wood Availability 2001-2044. Consultmi S Reporf for National Forest Inventov, Burenu of Rural
Sciences, Canberra.
ABARE. 2001, Australian Forest and Wood Products Statistics, March and June quarters 2001.
ABARE, Canberra, p.13.
Keey, R.B., Langnsh, T.A.G., and Walker, J.C.F. 2000. Kiln Drying of Lumber, pp.65-115, 175-181.
Springer Verlag, Berlin.
Wu, Q. 1989. An Investigation of Some Problems in Drying of Tasmanian Eucalypt Timbers. M. Eng.
Sci. Thesis, University of Tasmania, Hobart, pp.140-141.
Boland, D.J., Brooke;, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kieinig,
D.A., and Turner, J.D. 1989. Forest Trees of Australia, CSIRO Melbourne.
hitp://data.brs.gov.nw'mapsew/pfnnt/species.php?speciesid=14,accessed 1112412003.
Mills, R. 1991. Australian Timber Seasoning Manual. Australian Furniiure Research and
Development Insiiiute Limited. pp.160-I 66.
Schaffner, R.D. 1981. Fundamental Aspects of Timber Seasoning, M. Eng. Sci. Thesis, Faculty of
Engineering Science, Faculty of Engineering, University of Tasmania, Australia, pp. 130-135.
Langrish, T.A.G., Brooke, AS., Davis, C.L., Musch, A,, and Barton, G.W. 1997. An Improved
Drying Schedule for Australian Ironbark Timber: Optimisation and Experimental Validation. Drying
Technology, 15( 1 ), 47-70.
AustralianMew Zealand Standard ASMZS 4787:2001: Timber - Assessment of Drying Quality,
Standards Australia, Standards New Zealand, accessed 12/3/2003.
Walker, J.C.F. 1993. Primary Wood Processing. Chapman and Hall Publishers, London, pp.68-74.
Smith, L.I. 2002. A Tutorial on Principal Components Analysis.
hiip://www.cs.oingo.nc.nz/cosc4S3~student~tu1orials/principal~component~.pd~
accessed 19/3/2004.
Документ
Категория
Без категории
Просмотров
0
Размер файла
444 Кб
Теги
wales, properties, transport, timber, blackbutt, south, within, tree, new, variability
1/--страниц
Пожаловаться на содержимое документа