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Specific Permeability of Wood to Water Part 2: Perpendicular Specific Permeability of Steamed, Impregnated, and Kraft-Cooked Wood Juha-Pekka Pokki,*,† Ville V. Laakso,‡ Panu Tikka,‡,§ and Juhani Aittamaa† Department of Biotechnology and Chemical Technology, Faculty of Chemistry and Material Sciences, Helsinki UniVersity of Technology, P.O. Box 6100, FIN-02015 TKK, Finland, Department of Forest Products Technology, Faculty of Chemistry and Material Sciences, Helsinki UniVersity of Technology, P.O. Box 6300, FIN-02015 TKK, Finland, and SciTech SerVice Oy Ltd., Tekniikantie 12, Innopoli 1, FIN-02150 Espoo, Finland
A method for measuring the perpendicular specific permeability of wood specimens to water is presented, which is a modification of the method presented in part 1 of this work. The specific permeability in the perpendicular direction of the log of three wood species, pine (Pinus sylVestris), birch (Betula pendula), and eucalyptus (Eucalyptus grandis), was measured from kraft-cooked wood of different cooking degrees. The effect of various forms of pretreatment on the specific permeability of uncooked wood to water was also studied, including steaming with and without cooking liquor impregnation and boiling in water. Wood pieces were cooked to a certain degree with an H factor of up to 600 for pine, up to 300 for birch, and up to 250 for eucalyptus. The average specific permeability of cooked pine ranged from 2 × 10-15 to 3.5 × 10-14 m3/m, that of cooked birch ranged from 1.7 × 10-15 to 8 × 10-14 m3/m, and that of cooked eucalyptus ranged from 2 × 10-15 to 3.5 × 10-14 m3/m. Introduction According to Siau,1 permeability is a measure of the ease with which fluids are transported through porous media when exposed to a pressure gradient. The basic assumptions and the equations used in this measuring project are presented in part 12 of this work. Permeability is important in mass transfer in the chemical pulping process. The longitudinal direction is typically assumed to be the main direction of mass transfer in the pulping process, but the chip dimension is much longer in the longitudinal direction. A project measuring the perpendicular direction was motivated by the fact that the chip dimension in the perpendicular direction is smaller. The purpose of the two parts of this work was to determine the magnitude of specific permeability in both directions and their ratio as a function of the progress of cooking, as indicated by the H factor. Previous systems for measuring specific permeability can be divided according to the flowing phase and direction of flow. According to Comstock,3 gas permeability measurements are considered to be more reliable than liquid permeability measurements for dry wood. Gas permeability measurements do not suffer from problems of air-bubble blockage that can occur for liquid measurements. In this work, the objective was to measure the magnitude of specific permeability of kraft-cooked wood to cooking liquor as a function of the cooking degree. Although the density and viscosity of cooking liquor is higher than water, liquid water approximates alkaline cooking liquor relatively well. The gas measurements were not considered relevant because the kraft-cooked wood chip is filled with liquid through the industrial cooking process. * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Biotechnology and Chemical Technology, Faculty of Chemistry and Material Sciences, Helsinki University of Technology. ‡ Department of Forest Products Technology, Faculty of Chemistry and Material Sciences, Helsinki University of Technology. § P.T. was affiliated with the Department of Forest Products Technology at Helsinki University of Technology when this work was performed, and is now affiliated with SciTech Service.
Comstock3 collected early measurements on the specific permeability of various wood species to gas in the longitudinal, tangential, and radial directions. He observed ratios of longitudinal to transverse permeability as high as 106. Later, Tesoro et al.4 measured sapwood and heartwood specific permeabilities to air in the transverse direction. However, as far as we know, there have been no published specific permeability measurements in the perpendicular direction to liquid. Experimental Section Wood Material. The wood species used in this study were pine (Pinus silVestris), birch (Betula pendula), and eucalyptus (Eucalyptus grandis). The eucalyptus was from Uruguay, South America, and the pine and birch were from Finland, Europe. The average diameter of the logs was 26 cm for pine, 17 cm for birch, and 18 cm for eucalyptus. The logs were sawn into 50-mm-thick disks outside when the temperature was below 0 °C, and the disks were stored in sealed plastic bags in a freezer at -20 °C. Freezing is a common practice to store samples to prevent darkening of the wood, as well as the development of mold and other microorganisms. None of the wood disks cracked during storage in the freezer. The rejected samples that were left in another storage room to dry at approximately 20 °C were cracked. Comstock3 found that freezing caused no obvious changes in permeability. Storing eucalyptus at temperatures below 0 °C is untypical, but it was decided to store all samples in the same way. Preparation of Test Specimens. Drilling of Specimens. Cylindrical-shaped specimens were drilled from the wood disks in the radial direction of the log (plug perpendicular to the fiber orientation) using a special hollow drill. The sapwood and heartwood were separated after the long specimen had been drilled, as illustrated in Figure 1. The cylindrical test specimens were 50 mm long, on average, and 18 mm in diameter. The drilled specimens were stored in sealed plastic bags in a freezer below -20 °C until treatment. Steaming, Impregnation, and Cooking. The procedure was similar to that for the cooks presented in part 1.2 A number of
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Figure 1. Specimens drilled along log radius, perpendicular to the fiber direction.
randomly chosen specimens of each kind were selected for drymatter content analyses, and the average results were as follows: pine sapwood, 43%; pine heartwood, 71%; birch, 59%; and eucalyptus, 62%. Typically, all types of specimens (different species, sapwood, heartwood) were handled separately. The test specimens were treated in four different ways. Some of the wooden specimens were treated with hot water at 80 °C for 60 min; this is referred to below as boiling. Some were steamed for 15 min at 100 °C, some were impregnated for 24 h with white liquor after steaming, and some were further cooked in alkaline conditions using the same white liquor as in impregnation. White liquor is an aqueous mixture of sodium hydroxide and sodium sulfide. The white liquor used was typical industrial kraft pulp liquor and was obtained from a Finnish pulp mill. It had an effective alkalinity of 112 g/L as NaOH and a sulfidity of 29%. To enable more uniform delignification throughout the specimens, the following mild cooking conditions were chosen: liquor-to-wood ratio of 17:1, cooking temperature of 153 °C, alkali (EA) charge of 23% on wood for birch and eucalyptus and 28% for pine. The residual alkali concentrations were not measured. Before being cooked, the specimens were steamed and impregnated as described above. Impregnation was performed with pure white liquor in order to get as much alkali into the wood as possible and thereby improve the homogeneity of the specimens. Despite the attempt to perform uniform delignification, the outer part of the wood sample was always somewhat more affected than the core. An air bath digester with six cooking bombs was used for these cooks. The heating time from 80 °C to the cooking temperature was 60 min. As a result of experience with earlier cooks, the kraft cooking was stopped earlier in the H-factor scale. The number of uncracked specimens was larger this way. Some of the specimens, namely, cook ID ) 12 of pine sapwood, cook ID ) 13 of pine heartwood, cook ID ) 14 of birch, and cook ID ) 15 of eucalyptus, were cooked in the same batch. Consequently, the H factor was the same, but the specimens were not necessarily taken from the same log. Thus, the cooking of the specimens of the aforementioned cooks was similar to that used for the measurements of specific permeability presented in part 12 of this work.
Washing, Yield, and Chlorine Number Determination. Washing of the cooked specimens was carried out by soaking them in water at 22 °C for 72 h. The washing water was changed every 24 h. During washing, the liquid-to-wood ratio was about 100:1. For each cooking bomb, the dry weights of three specimens were measured before cooking and after washing in order to determine the yield. The specimens were marked with iron wire before cooking to enable sample identification. The dry weight of the wood pieces before cooking was calculated using dry-matter content, because drying the specimens before cooking would have affected the result. The dry-matter content was determined according to standard SCAN-CM 39:94, with the exception that the amount of wood used was 60 g in total. After washing, these marked specimens were dried for 24 h at 105 °C. The chlorine number determination was done according to standard SCAN-C 29:72 using these same specimens after they had been ground to powder with a Wiley mill. Description of the Vacuum Apparatus. The newly developed vacuum apparatus presented in part 12 of this work consists of a specimen holder, a vacuum system to generate the pressure difference, an overflow vessel, a balance to measure the mass of permeated fluid, a pressure meter, and a temperature meter. A vacuum pump (Vacuubrand membrane pump, MZ2C, 2.4 m3/ h) generated the vacuum for the system. The differential pressure sensor used was a Huba Control DTP 05-420 sensor, with a pressure range from 0 to 0.5 bar and accuracy of 0.5% full scale. The temperature sensor used was a Nokeval TRCP-13-3-R1/8 Pt100 sensor. The balance for weighing mass flow was a Mettler-Toledo PG5002-S balance with a resolution of 0.01 g. The overpressure method presented in part 12 was not suitable for measurements in the perpendicular direction because the leakage between the specimen and the glue was large compared to the real fluid flow of permeability. However, the specimen holder of the vacuum apparatus was suitable after minor modification. The perpendicular specific permeability was known, and during the experiments, it was found to be decades smaller than the longitudinal specific permeability. The length (i.e., thickness) of the specimen had to be at a minimum. The thickness of the specimen used in the measurements was approximately 5 mm; that is, the final specimen was like a coin. This was the minimum achieved by manual slicing. Because the drilled specimens in the radial direction were not exactly circular after cooking, a sealing material (Bostik, Blu-Tack) was used. Each 5-mm-thick coin-like sliced specimen was wrapped with this sealing material and then pushed gently into the standard laboratory glassware NS19 socket specimen holder. The specimen was successfully attached in a way that was tight and prevented leaking. A schematic diagram of the apparatus is presented in Figure 2. The signals from the pressure and temperature meters were converted into an RS232 signal by a Nokeval 2021-RS-24 VDC panel meter. The balances had a builtin RS232 port. The RS232 ports were read and written to file with an in-house program. The programming language was Visual Basic 6 Professional. Measurement Procedure Using the Vacuum Apparatus. The washed specimens were immersed in water and stored in plastic bags. The specimen to be measured was taken from the plastic bag and set on a table for slicing. When the H factor was high, several attempts were needed to obtain a nonfractured slice. After laboratory cooking, the 5-cm-long cylindrical-shaped specimen was sliced with a sharp and thin-bladed knife. Both surfaces of the slice were cut and used in the experiments as such. Pressing and removal of water was avoided in order to keep the porous structure of the wood filled with water.
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factor in Tables 1-4. Also, the average absolute deviation is reported for each H factor. This provides an indication of the scattering of specific permeability in successive experiments; that is, the smaller the value, the smaller the scattering. The uncertainty in H factor was estimated to be 5 units, which relates to approximately 1 min of cooking time when removing the cooking bomb from the air bath. The bombs were cooled rapidly under a flow of cool tap water. The dynamics of cooling was not included in this error analysis. The uncertainty in yield was estimated to be 1%, and that in Cl number was estimated to be 0.5 units. Results and Discussion
Figure 2. Schematic diagram of the apparatus.
Figure 3. Error bars of the specific permeability measurement of a single specimen of pine heartwood at T ) 23°.
Leakage or fracturing of the specimen could be detected at the beginning of a measurement as a jet-like flow pattern of water when the vacuum was generated. These specimens were rejected and a new slice was cut. The vacuum was increased in three or four steps and then decreased back to the starting value. The highest vacuum or pressure difference was 20 kPa. At each pressure level, the duration of the measurement was 5-10 min. Result Evaluation and Error Analysis. The calculation procedure and error analysis was similar to that used in part 1 of this work.2 Because the flow in the perpendicular direction was approximately 100 times smaller than that in the longitudinal direction, the time interval for the data logging was set large, usually 30 s. One factor affecting the mass flow was the evaporation of water from the vessel on the balance. This is dependent on the relative humidity, the free area of the water surface, the temperature, and so on. Maximum evaporation takes place during winter, but its effect was a maximum of 5% for the mass flow during this measuring project. One example of error analysis is shown in Figure 3. The second vertical axis provides information on the pressure drop used. The permeability is practically constant during the measurement. As a part of the error analysis, the maximum and minimum average values of the specific permeability is given for each H
Three wood species were tested in this study: pine (sapwood and heartwood), birch, and eucalyptus. The flow direction was perpendicular to the fiber, that is, in the radial direction of the cylindrical shape of the log. The reported values are the H factor, yield, and chlorine number to provide information on the cooking reported in part 1.2 The reported average specific permeability at each H factor is the average of individual experiments. The number of individual experiments is indicated in the column labeled NoS (i.e., number of specimens). The number of experiments performed was, in point of fact, higher because some of the specimens were fractured when being attached to the specimen holder. However, only the successful experiments are reported. Bramhall5 discussed a decreasing liquid flow, and this was also observed in some of these experiments. Because the number of experiments was small and the scattering of results was high, these were not reported. The columns “max K” and “min K” indicate the maximum and minimum average values, respectively, of the specific permeability in individual experiments. The average absolute deviation is a measure of the absolute deviation of a single specific permeability value from the average value, indicating the “noise” in the specific permeability measurement as a function of time. The length (thickness) of the specimens varied from 0.003 to 0.005 m. This is approximately one-half the thickness of a typical mill chip. It was not possible to study the effect of specimen length with these short specimens. Pine, Sapwood. The number of experimental runs was 2 for 30-min-water-boiled specimens and 5 for 15-min-steamed specimens. The number of runs with cooked specimens was 15, usually 2 for each H factor. Table 1 shows detailed results of the experimental runs. It seems that there is an increasing trend between H factors 200 and 500, even though the number of specimens was small; nevertheless, the magnitude of specific permeability was revealed. Boiled and steamed specimens were found to have nearly equal specific permeabilities. Pine, Heartwood. There were two cooks of pine heartwood. Four steamed specimens and 28 cooked specimens were tested. The number of specimens was higher than in the sapwood experiments, and also this time, the specific permeability increased up to an H factor of 300. Above this H factor, the specimens increasingly became fractured, making the experiments even more demanding. Steamed and impregnated H ) 0 specimens were found to have almost equal specific permeabilities. Birch. The results for birch are presented in Table 3. There are 7 steamed and boiled specimens and 11 cooked specimens. The birch specimens were very easily fractured during experiments. The tests revealed the magnitude of the specific permeability, but the trend as a function of H factor was not easy to conclude. Obviously, the rapid increase in specific
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Table 1. Average Specific Permeability of Pine Sapwood, Perpendicular Direction, Vacuum Apparatus H factor
yield (%)
Cl number
avg K
boiled 30 min steamed 15 min
NAb NA
NA NA
3.8 × 10-15 2.1 × 10-15
0 100 200 300 400 500 600
87.5 70.5 69.8 63.2 60.9 55.1 51.9
27.7 23.7 22.0 19.2 18.0 15.2 13.6
2.4 × 10-14 3.4 × 10-14 2.7 × 10-15 7.7 × 10-15 1.1 × 10-14 1.6 × 10-14 5.7 × 10-15
max K
min K
avg dev K
avg L
NoSa
1.5 × 10-15 9.5 × 10-16
0.0045 0.0047
3 5
6.9 × 10-15 1.9 × 10-14 1.3 × 10-15 3.0 × 10-15 7.2 × 10-15 1.1 × 10-14 3.2 × 10-15
0.0044 0.0038 0.0046 0.0049 0.0045 0.0043 0.0068
2 2 2 2 3 1 2
[ [ [ [ [ [ [
avg dev K
avg L
NoSa
symbol
9.7 × 10-16 3.4 × 10-15 2.9 × 10-15 1.4 × 10-15 3.2 × 10-15 2.5 × 10-15
0.0046 0.0039 0.0046 0.0041 0.0042 0.0044
4 2 2 2 2 2
2 2 2 2 2
1.6 × 10-15 5.1 × 10-16 3.9 × 10-15 7.8 × 10-15 1.1 × 10-15 1.9 × 10-15 1.8 × 10-15
0.0041 0.0044 0.0047 0.0053 0.0048 0.0062 0.0054
2 3 3 3 3 2 2
4 4 4 4 4 4 4
avg dev K
avg L
NoSa
symbol
1.2 × 10-15 8.7 × 10-16 1.6 × 10-14 7.2 × 10-15 7.9 × 10-16 2.4 × 10-15 3.3 × 10-15 8.1 × 10-14
0.0045 0.0048 0.0050 0.0049 0.0049 0.0049 0.0049 0.0049
3 4 2 2 1 2 2 2
9 9 9 9 9 9
symbol
Cook ID ) 4 T ) 23 ( 2 °C 8.0 × 10-15 2.5 × 10-15
1.4 × 10-15 1.9 × 10-15
Cook ID ) 12, T ) 21 ( 2 °C
a
3.1 × 10-14 6.5 × 10-14 3.3 × 10-15 1.1 × 10-14 2.1 × 10-14 1.6 × 10-14 8.4 × 10-15
1.6 × 10-14 2.6 × 10-15 2.1 × 10-15 4.2 × 10-15 6.3 × 10-15 1.6 × 10-14 3.0 × 10-15
NoS ) number of specimens. b NA ) not analyzed.
Table 2. Average Specific Permeability of Pine Heartwood, Perpendicular Direction, Vacuum Apparatus H factor
yield (%)
Cl number
avg K
steamed 15 min 5 100 200 300 400
NAb 87.9 65.8 60.4 56.7 52.8
NA NA NA NA NA NA
2.1 × 10-15 5.2 × 10-15 4.2 × 10-15 2.4 × 10-15 7.4 × 10-15 9.8 × 10-15
0 100 200 300 400 500 600
94.5 70.1 65.4 62.2 57.8 56.1 54.9
27.7 25.5 23.4 20.9 19.6 16.7 15.1
2.3 × 10-15 9.8 × 10-16 5.1 × 10-15 1.1 × 10-14 2.2 × 10-15 3.4 × 10-15 3.2 × 10-15
max K
min K
Cook ID ) 5, T ) 23 ( 2 °C 2.5 × 10-15 5.7 × 10-15 5.9 × 10-15 2.5 × 10-15 8.8 × 10-15 1.7 × 10-14
1.8 × 10-15 4.6 × 10-15 2.6 × 10-15 2.3 × 10-15 6.0 × 10-15 2.4 × 10-15
Cook ID ) 13, T ) 22 ( 2 °C
a
2.7 × 10-15 1.1 × 10-15 1.1 × 10-14 1.8 × 10-14 2.8 × 10-15 4.4 × 10-15 3.5 × 10-15
1.9 × 10-15 8.7 × 10-16 2.0 × 10-15 2.1 × 10-15 1.7 × 10-15 2.3 × 10-15 2.9 × 10-15
NoS ) number of specimens. b NA ) not analyzed.
Table 3. Average Specific Permeability of Birch, Perpendicular Direction, Vacuum Apparatus H factor
yield (%)
Cl number
avg K
boiled 30 min steamed 15 min 0 50 100 150 204 300
NAb NA 95.3 65.5 60.1 59.9 56.4 51.6
NA NA 20.8 15.1 12.2 9.4 8.1 4.3
4.0 × 10-15 1.9 × 10-15 2.8 × 10-14 8.7 × 10-15 1.7 × 10-15 6.3 × 10-15 1.0 × 10-14 8.4 × 10-14
max K
min K
Cook ID ) 14 T ) 23 ( 2 °C
a
8.3 × 10-15 2.3 × 10-15 5.5 × 10-14 1.5 × 10-14 1.7 × 10-15 9.0 × 10-15 1.0 × 10-14 1.0 × 10-13
1.9 × 10-15 1.6 × 10-15 1.5 × 10-15 2.7 × 10-15 1.7 × 10-15 3.7 × 10-15 9.8 × 10-15 6.4 × 10-14
NoS ) number of specimens. b NA ) not analyzed.
permeability at an H factor of 300 is an indication of fractures of the specimens. Eucalyptus. The number of specimens was 31. The specific permeability of eucalyptus in the perpendicular direction was found to be comparable to those of pine and birch, as can be seen in Table 4. For eucalyptus, specific permeability is practically constant up to an H factor of 100, but it increases rapidly after that. An H factor of 175 can be seen as the limit where the measurements became impossible because of very fractured specimens. Pine, Birch, and Eucalyptus Compared. No major difference in specific permeability in the perpendicular direction can be found when the three species are compared. The specific permeability remains practically constant as a function of H factor. There are some exceptions at high H factors, but they are partly caused by experimental uncertainties. A graphical comparison between all specimens and cooks of specimens is presented in Figure 4.
The magnitude of specific permeability of all species studied in this work ranged from 3 × 10-14 to 2 × 10-15 m3/m. Longitudinal and Perpendicular Specific Permeabilities of Pine, Birch, and Eucalyptus Compared. The purpose of this measurement project was also to obtain the ratio of the permeability in the longitudinal direction to that in the perpendicular direction. The permeability was measured at two temperatures in the longitudinal direction as presented in part 1 of this work.2 The average of these two temperatures is practically the same as the one temperature used in the perpendicular direction. The value of the experiment-weighted average of the specific permeability in the longitudinal direction was calculated. The H factors were also selected to match as closely as possible. The ratio of the specific permeability in the longitudinal direction to that in the perpendicular direction is presented graphically in Figure 5. It can be seen that, for pine sapwood, the ratio of longitudinal to perpendicular specific permeability increases slightly as the H factor increases. Pine heartwood, birch, and eucalyptus all
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Table 4. Average Specific Permeability of Eucalyptus, Perpendicular Direction, Vacuum Apparatus H factor
yield (%)
Cl number
avg K
boiled 30 min steamed 15 min 0 60 108 175
NAb NA 91.6 66.1 62.7 56.6
NA NA 28.1 19.6 14.3 9.16
1.9 × 10-15 2.1 × 10-15 3.8 × 10-15 2.4 × 10-15 5.3 × 10-15 3.4 × 10-14
0 50 100 175 250
87.4 64.9 59.1 56.9 52.3
24.0 18.8 12.6 8.5 4.4
5.8 × 10-15 7.1 × 10-15 4.4 × 10-15 4.8 × 10-15 1.7 × 10-14
max K
min K
avg dev K
avg L
NoSa
symbol
1.0 × 10-15 9.7 × 10-16 2.3 × 10-15 2.0 × 10-15 1.5 × 10-15 6.7 × 10-15
0.0046 0.0045 0.0040 0.0049 0.0054 0.0058
2 6 5 3 2 2
+ + + +
3.0 × 10-15 3.0 × 10-15 3.7 × 10-15 3.0 × 10-15 6.5 × 10-15
0.0057 0.0052 0.0052 0.0066 0.0073
3 2 2 2 1
× × × × ×
Cook ID ) 11, T ) 21 ( 2 °C 2.1 × 10-15 2.6 × 10-15 8.1 × 10-15 3.2 × 10-15 5.6 × 10-15 5.6 × 10-14
1.7 × 10-15 1.4 × 10-15 1.5 × 10-15 1.6 × 10-15 5.0 × 10-15 1.2 × 10-14
Cook ID ) 15, T ) 23 ( 2 °C
a
1.1 × 10-14 1.1 × 10-14 6.0 × 10-15 4.8 × 10-15 1.7 × 10-14
2.6 × 10-15 3.4 × 10-15 2.8 × 10-15 4.7 × 10-15 1.7 × 10-14
NoS ) number of specimens. b NA ) not analyzed.
Figure 4. Comparison between species. Symbols: ([) pine, sap, T ) 21 °C, cook ID ) 12; (2) pine, heart, T ) 23 °C, cook ID ) 5; (4) pine, heart, T ) 22 °C, cook ID ) 13; (9) birch, T ) 23 °C, cook ID ) 14; (+) eucalyptus, T ) 21 °C, cook ID ) 11; (×) eucalyptus, T ) 23 °C, cook ID ) 15.
show decreasing trends. The ratio of longitudinal to perpendicular specific permeability as a function of H factor was correlated with a line, with the general form KL /KP ) aH + b
(1)
where KP and KL are the perpendicular and longitudinal specific permeabilities, respectively. The coefficients a and b are presented in Table 5. Comstock3 collected a set of directional measurements on specific permeability for different wood species. The data indicated that the ratio of longitudinal to radial specific permeability was 14400 for pine sapwood and 5200 for pine heartwood. In our case the ratio, for kraft-cooked pine varied from 33 to 570 and was 770 for boiled pine sapwood and 110 for steamed pine sapwood. However, steamed or boiled wood is not necessarily comparable to fresh or dry wood. Additionally, this article presents liquid specific permeabilities, whereas Comstock3 reported gas specific permeability values. Conclusions An experimental setup was built for the measurement of the specific permeability to water of pine, spruce, and eucalyptus
Figure 5. Ratio of longitudinal to perpendicular specific permeability as a function of H factor; axis logarithm-scaled. Symbols: ([, s) pine sapwood, T ≈ 21 °C; (2, · · · ) pine heartwood, T ≈ 21 °C; (2, - - -), birch, T ≈ 21 °C; (+, · - -) eucalyptus, T ≈ 21 °C. Lines added for clarity only. Table 5. Slope a and Intercept b of the Correlation for the Ratio of Longitudinal to Perpendicular Specific Permeability species
a
b
R2
range of H factor
pine, sapwood pine, heartwood birch eucalyptus
0.42 -0.12 -0.10 -22.36
92.3 157.6 42.5 6778.7
0.14 0.12 0.18 0.70
0-600 0-600 0-300 0-250
in the longitudinal and perpendicular directions. The specimens were measured near room temperature and for uncompressed wood. The percent yield and chlorine number were analyzed, and some of the specimens were cooked in the same batch to ensure the comparability of the specific permeabilities measured in the two directions. With all of the tested wood species at high H factors, where the yield is approximately 55%, almost one-half of the wood material was dissolved. Despite this and despite some variation in the measured specific permeabilities, in terms of kraft cooking, the specific permeability of individual wood chips was found to remain relatively low until the very end of the cooking process. In these experiments, the magnitude of the ratio of specific permeability in the longitudinal direction to that in the perpendicular direction was found to be approximately 100 for pine and birch and 4000 for eucalyptus throughout the H-factor scale.
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Acknowledgment This project was funded by TEKES (National Technology Agency of Finland). Literature Cited (1) Siau, J. F. Transport Processes in Wood; Springer-Verlag: Berlin, 1984. (2) Pokki, J.-P.; Laakso, V. V.; Tikka, P.; Aittamaa, J. Specific Permeability of Wood to Water Part 1: Longitudinal Specific Permeability of Steamed, Impregnated, and Kraft-Cooked Wood. Ind. Eng. Chem. Res. 2010, 49, in press (doi: 10.1021/ie801901f).
(3) Comstock, G. L. Directional Permeability of Softwoods. Wood Fiber Sci. 1969, 1 (4), 283–289. (4) Tesoro, F. O.; Choong, E. T.; Skaar, C. Transverse Air Permeability of Wood. For. Prod. J. 1966, 16 (3), 57–59. (5) Bramhall, G. The validity of Darcy’s law in the axial penetration of wood. Wood Sci. Technol. 1971, 5, 121–134.
ReceiVed for reView December 10, 2008 ReVised manuscript receiVed September 17, 2009 Accepted December 18, 2009 IE801902B
