Specific Permeability of Wood to Water Part 1 - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 2144–2154

Specific Permeability of Wood to Water Part 1: Longitudinal 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

Two apparatuses for measuring the specific permeability of wood specimens are presented. The developed method was used to measure specific permeability to water in the longitudinal direction (of the log) for three wood species, pine (Pinus sylVestris), birch (Betula pendula), and eucalyptus (Eucalyptus grandis). The wood species were cooked to a certain degree under kraft cooking conditions. The effect of pretreatments on the specific permeability of uncooked wood was also studied. These pretreatments include boiling in water, steaming, and impregnation with cooking liquor. Wood species were cooked to a certain degree with an H factor of up to 600 for pine, 376 for birch, and 400 for eucalyptus. The average specific permeabilities to water ranged from 1 × 10-13 to 3 × 10-12 m3/m for cooked pine, from 1 × 10-13 to 1 × 10-12 m3/m for cooked birch, and from 3 × 10-11 to 5 × 10-11 m3/m for cooked eucalyptus. Introduction Definitions. 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. Permeability as a convection term together with a diffusion term forms the basis for the modeling of mass transfer in pulping processes. Several assumptions to determine the magnitude of the bulk flow of fluid through wood must be made, states Siau.1 The assumptions according to Siau1 are as follows: (1) The flow is viscous and linear. (2) The fluid is homogeneous and incompressible. (3) The porous medium is homogeneous. (4) There is no interaction between the medium and the substrate. (5) Permeability is independent of the length of the specimen in the flow direction. The steady-state flow of liquid is described by Darcy’s law k)

QL m ˙ L Q/A ) ) ∆p/L A∆p F A∆p

Previous Systems Measured. Previous measurements of specific permeability were made on several economically important wood species. The processes where this wood is used are, for example, drying, preservative treatment, fire-retardant treatment, and pulping. It is important to know how fast fluid, gas or liquid, can flow through the porous wood structure. This phenomenon is related to the production rate and economics of the process. The specific permeability of wood to fluid varies depending on the direction of flow: longitudinal (axial) or perpendicular (radial or tangential). A literature survey indicates that the greatest number of systems are measured in a longitudinal direction, as in the publications of Comstock,2,3 Rice and D’Onofrio,4 Vazquez et al.,5 Bradic and Avramidis,6 and Lin et al.7,8 Perpendicular (radial or transverse) measurements were made by Kininmonth,9 Tesoro et al.,10 and Choong and Fogg,11 where

(1)

where k is the permeability [m3/(m Pa s)], Q is the volumetric flow rate (m3/s), L is the length of specimen in direction of flow (m), A is the cross-sectional area of specimen (m2), ∆p is the pressure differential (Pa), m ˙ is the mass flow (kg/s), and F is the density (kg/m3). Specific permeability is the product of permeability and viscosity. It is not affected by the properties of measuring fluid, and it is a function of only the porous structure of the specimen1 K ) kη

(2)

where K is the specific permeability (m3/m) and η is the viscosity of fluid (Pa s). * 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.

Figure 1. Schematic diagram showing how the specimens were drilled from 50-mm-thick disks of sapwood and heartwood along the fiber direction.

10.1021/ie801901f  2010 American Chemical Society Published on Web 01/20/2010

Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

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Figure 4. Schematic diagram of the vacuum apparatus. Differential pressure sensor (PI) and temperature sensor (TI).

Figure 2. Schematic diagram of the experimental setup for measuring the permeability of liquid water in a wood specimen. TI, temperature indicator; PI, pressure indicator; dashed line, RS232 serial connection.

Figure 3. Schematic diagram of the specimen holder. Outer diameter, OD; inner diameter, ID.

the target of the measurements was drying or preservative treatment. Ratios of longitudinal to transverse specific permeability as high as 106 were observed in the previous studies.1,9,10,12 This is obvious because the liquid must be transported vertically inside living trees. Depending on the application, measurements have been made with either a gas and/or liquid phase. The work of Comstock3 studied the assumption that specific permeability should be independent of the fluid phase and should be a function of the wood only. The measurements for liquid are affected by air blockages that can decrease the flow. The measurements for gas are affected by the expansion of the gas, nonuniformity of the pressure gradient, and molecular effects.5,13 The role of

viscosity decreases when the capillary dimension becomes the same magnitude as or smaller than the mean free path of the molecules and the air flow changes to non-Darcian flow.13 The superficial calculated permeability becomes higher when measured with gas than with liquid.13 The real specific permeability can be calculated from measurements, such as those presented by Comstock,3 Lu and Avramidis,13 and Vazquez et al.5 Gas measurements also allow the capillary and pore size measurements of wood.13 It is also assumed that gases do not modify the structure of wood. Several fluid components have been studied in previous works. For example, in his early studies, Comstock2,3 did research on nitrogen and helium gas permeability, as well as water, n-amyl alcohol, and iso-octane liquid permeability. Hayashi et al.14 studied the permeability of six wood species to five liquids. Later, Charuk et al.15 studied the permeability of softwood with nitrogen, creosote, and an aqueous solution of fluoride. Rice and D’Onofrio,4 Lu and Avramidis,13,16,17 Vazquez et al.,5 and Bradic and Avramidis6 studied the permeability of air through wood. The permeability of electrolytes through wood was studied by Chen et al.18 and Blokhra et al.19 Previous Measurement Techniques. Measurements have been concentrated mostly on longitudinal permeability. Traditionally, specimens are put in to a holder and tightened when measured in the longitudinal direction. Comstock2 used a Plexiglas specimen holder and maintained the pressure gradient for the water flow with nitrogen in a pressurized water tank. Prior to each experiment, the water was freshly distilled and degassed in vacuum for at least 15 min. The temperature was regulated at 25 °C, and the pressure was varied up to 100 psi (0.689 MPa). The cylindrical specimens were used as such or saturated with water. The saturation was performed by the evacuation of air, and after that, the water was allowed to penetrate into the wood. According to Comstock,2 the wooden material remained in its green condition. Charuk et al.15 used an apparatus in which the cylindrical specimen was placed in a pressure vessel. The temperature range was wide, from 20 to 130 °C. Rice and D’Onofrio4 used a flexible tube as a specimen holder and clamped it around the outside. The vacuum pump generated the pressure gradient of the air flow. The air was dried before entering the wood, and three different pressure levels were used. Lu and Avramidis13,16,17 used a similar clamping technique to attach the specimen. The specimen was prepared from fresh cut logs and dried moderately. The side surfaces were coated

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Figure 5. Error bars of the specific permeability measurement of a single specimen of pine sapwood, H ) 300. (]) T ) 21 °C, vacuum apparatus; (0) T ) 11 °C, overpressure apparatus; and (4) T ) 36 °C, overpressure apparatus.

multicomponent aqueous mixture of electrolytes. In this work, the specific permeabilities to pure liquid water of steamed, boiled, impregnated, and kraft-cooked wood species was measured. Experimental Section

Figure 6. Mass flow and pressure drop of a single specimen of pine sapwood. (]) T ) 21 °C, vacuum apparatus; (0) T ) 11 °C, overpressure apparatus; and (4) T ) 36 °C, overpressure apparatus; solid line, pressure difference.

with epoxy resin. The specimen holder was vacuum rubber tubing that was tightened with clamps. Nine pressure levels and different sample lengths were used. According to Kininmonth,9 the experimental setup is more demanding for measurements in the transverse direction than for those in the longitudinal direction because the sealing of the side grain is essential to avoid leaking. He used green-wood test specimens, air-dried or saturated with water prior to measurements. The measurements were made in the transverse (perpendicular to fiber) direction. The end grain of the specimens was coated with polyurethane coating. The surface to be coated was dried momentarily on a hot plate. After being coated, the specimen was placed in Pyrex glass cells where two neoprene washers clamped the specimen. The motivation for this research was to study the specific permeability of wood chips when they are treated as in the kraft pulping process. The wooden chips were heated and degassed (air-removal) with water vapor and impregnated with cooking liquor prior to cooking. The entrapped air prevents proper impregnation, and during cooking, it can decrease the density of the wood chips and thus prevent the proper flow of the chips inside the digester. Cooking liquid impregnation is therefore an important step to obtain uniformly cooked chips. The air removal is dependent on the air permeability, but impregnation is dependent on liquid permeability. The cooking liquor is a

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 pine and birch logs were fresh, whereas the eucalyptus logs had dried during sea transport to Finland. 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 that were stored in sealed plastic bags in a freezer at -20 °C. Freezing of fresh wood material before kraft cooking is a normal procedure in a pulping laboratory environment. Freezing in plastic bags preserves the moisture content after a large number of samples are divided into more convenient portions. Freezing also prevents darkening of the wood, as well as the development of mold and other micro-organisms. However, freezing does not have a significant effect on the wood fiber characteristics, and it is considered to be a safe way to store wooden samples for pulping. None of the wood disks cracked during storage in the freezer, unlike all of the rejected samples, which were left in another storage room to dry at approximately 20 °C. Also, Comstock2 found that freezing caused no obvious changes in permeability. In fact, both pine and birch from Finland were sawn into disks outdoors during winter when the temperature was around -10 °C and the logs were already frozen. For South American eucalyptus, a temperature of less than 0 °C is not typical, but it was decided to store all of the samples in the same way. Wood species differ from each other in terms of structure and many other characteristics that can influence the permeability. Pine is categorized as softwood, whereas birch and eucalyptus are hardwoods. The fundamental difference is that softwood does not have the relatively large-diameter vessels that can be seen with the naked eye in hardwood. These vessels are aligned in the longitudinal direction of the log and are mostly responsible for the water transportation in the wood. In softwoods, the fluids flow from tracheid to tracheid through bordered pit pairs. Also, the rays might provide an important flow path.


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a

Table 1. Average Specific Permeability of Pine Sapwood, Overpressure Apparatus

avg dev K

avg L

NoS (d, c)b

symbol

ID ) 6, T ) 12 ( 2 °C 3.0 × 10-12 1.3 × 10-12 3.5 × 10-12 1.0 × 10-12 4.1 × 10-12 1.6 × 10-12 6.4 × 10-12 1.2 × 10-12 1.6 × 10-12 8.0 × 10-13 5.1 × 10-12 2.5 × 10-12 1.5 × 10-11 2.5 × 10-12 3.9 × 10-12 1.8 × 10-12 5.3 × 10-13 4.7 × 10-14

3.1 × 10-13 2.7 × 10-13 3.6 × 10-13 3.5 × 10-13 2.2 × 10-13 6.6 × 10-13 2.7 × 10-12 6.7 × 10-13 1.0 × 10-13

0.043 0.043 0.043 0.041 0.019 0.023 0.026 0.045 0.045

4 (0, 4) 4 (1, 3) 4 (0, 4) 4 (1, 3) 2 (1, 1) 4 (2, 2) 3 (1, 3) 5 (4, 1) 4 (1, 3)

] ] ] ] ] ] ] ] ]

NA NA NA NA NA NA NA NA NA

Cook ID ) 6, T ) 33 ( 2 °C 1.4 × 10-12 2.2 × 10-12 7.4 × 10-13 1.0 × 10-12 1.7 × 10-12 4.0 × 10-13 1.8 × 10-12 2.9 × 10-12 1.3 × 10-12 9.3 × 10-13 9.9 × 10-13 6.6 × 10-13 5.0 × 10-13 6.2 × 10-13 3.9 × 10-13 1.6 × 10-12 1.6 × 10-12 1.6 × 10-12 1.4 × 10-12 1.4 × 10-12 1.4 × 10-12 1.5 × 10-12 1.8 × 10-12 1.0 × 10-12 1.3 × 10-13 2.1 × 10-13 7.2 × 10-14

2.6 × 10-13 3.4 × 10-13 4.7 × 10-13 1.7 × 10-13 2.8 × 10-13 9.0 × 10-13 2.8 × 10-13 2.9 × 10-13 5.8 × 10-14

0.043 0.043 0.043 0.041 0.019 0.023 0.026 0.045 0.045

4 (3, 1) 4 (2, 2) 4 (3, 1) 3 (2, 1) 2 (1, 1) 1 (1, 0) 1 (1, 0) 6 (3, 3) 4 (1, 3)

0 0 0 0 0 0 0 0 0

90.8 68.2 64.4 62.9 61.6 58.9 56.9

30.7 24.9 23.7 21.8 20.1 16.4 15.8

Cook ID ) 12, T ) 11 ( 2 °C 1.0 × 10-12 1.9 × 10-12 1.5 × 10-13 5.5 × 10-12 8.3 × 10-13 2.5 × 10-12 2.4 × 10-12 2.6 × 10-12 2.2 × 10-12 1.5 × 10-12 1.9 × 10-12 1.1 × 10-12 2.3 × 10-12 3.1 × 10-12 1.2 × 10-12 2.0 × 10-12 2.9 × 10-12 1.4 × 10-12 2.7 × 10-12 3.0 × 10-12 1.9 × 10-12

5.1 × 10-13 4.0 × 10-13 3.6 × 10-13 4.7 × 10-13 3.8 × 10-13 3.0 × 10-13 4.4 × 10-13

0.043 0.045 0.046 0.040 0.043 0.042 0.041

2 (0, 2) 3 (0, 3) 2 (0, 2) 2 (0, 2) 3 (0, 3) 3 (0, 3) 3 (0, 3)

4 4 4 4 4 4 4

90.8 68.2 64.4 62.9 61.6 58.9 56.9

30.7 24.9 23.7 21.8 20.1 16.4 15.8

Cook ID ) 12, T ) 36 ( 2 °C 1.3 × 10-12 1.6 × 10-12 1.1 × 10-12 1.1 × 10-12 2.3 × 10-12 4.1 × 10-13 1.4 × 10-12 1.7 × 10-12 9.6 × 10-13 1.2 × 10-12 1.6 × 10-12 7.5 × 10-13 1.5 × 10-12 1.8 × 10-12 8.5 × 10-13 1.1 × 10-12 1.4 × 10-12 7.3 × 10-13 1.6 × 10-12 1.9 × 10-12 1.2 × 10-12

6.4 × 10-13 1.0 × 10-12 3.4 × 10-13 2.4 × 10-13 4.3 × 10-13 3.2 × 10-13 4.1 × 10-13

0.043 0.045 0.046 0.040 0.043 0.042 0.041

2 (0, 2) 3 (1, 2) 2 (0, 2) 2 (0, 2) 3 (0, 3) 3 (0, 3) 3 (0, 3)

× × × × × × ×

H factor

yield (%)

Cl number

avg K

max K

0 100 200 300 400 500 600 boiled 30 min steamed 15 min

87.2 66.4 63.1 57.5 56.9 56.0 53.4 NA NA

NAc NA NA NA NA NA NA NA NA

Cook 2.1 × 10-12 2.2 × 10-12 2.8 × 10-12 2.7 × 10-12 1.2 × 10-12 3.5 × 10-12 7.1 × 10-12 2.9 × 10-12 2.3 × 10-13


87.2 66.4 63.1 57.5 56.9 56.0 53.4 NA NA

0 100 200 300 400 500 600 0 100 200 300 400 500 600

min K

Specimens were drilled from the same year ring (i.e., distance from the log center). b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time. c NA ) not analyzed. a

Table 2. Average Specific Permeability of Pine Sapwood, Vacuum Apparatusa H factor 0 100 200 300 400 500

yield (%) 90.8 68.2 64.4 62.9 61.6 58.9

Cl number 30.7 24.9 23.7 21.8 20.1 16.4

avg K 8.8 × 10-13 1.1 × 10-12 1.3 × 10-12 1.3 × 10-12 9.4 × 10-13 7.4 × 10-13

max K

min K

Cook ID ) 12, T ) 21 ( 2 °C 1.0 × 10-12 7.2 × 10-13 1.4 × 10-12 7.5 × 10-13 1.6 × 10-12 1.0 × 10-12 1.3 × 10-12 1.3 × 10-12 1.1 × 10-12 7.7 × 10-13 8.6 × 10-13 5.4 × 10-13

avg dev K

avg L

NoS (d, c)b

5.8 × 10-13 3.6 × 10-13 2.6 × 10-13 2.0 × 10-13 4.8 × 10-13 1.7 × 10-13

0.043 0.045 0.046 0.040 0.043 0.042

2 (2, 0) 3 (3, 0) 2 (1, 1) 2 (2, 0) 3 (3, 0) 3 (3, 0)

symbol + + + + + +

a Specimens were drilled from the same year ring (i.e., distance from the log center). b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time.

Preparation of Test Specimens. Drilling of Specimens. Using a special hollow drill, cylindrical-shaped specimens were drilled from the icy wood disks in the longitudinal direction of the log (plug parallel to the fiber orientation) separately from the sapwood and heartwood as illustrated in Figure 1. The cylindrical test specimens were 50 mm long with a diameter of 18 mm. The drilled specimens were stored in sealed plastic bags in a freezer at -20 °C before treatment. Steaming, Impregnation, and Cooking. All types of specimens (different species, sapwood, heartwood) were handled separately. A number of randomly chosen specimens of each kind were selected for dry-matter content analyses, and the average results were as follows: pine sapwood, 43%; pine heartwood, 71%; birch, 59%; and eucalyptus, 62%. 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, some were steamed for 15 min at 100 °C, some were

impregnated for 24 h with white liquor after steaming, and some were 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 obtained from a Finnish pulp mill. It had the effective alkalinity of 112 g/L as NaOH and a sulfidity of 29%. To enable a more uniform delignification through 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 cooking, the specimens were steamed and impregnated as described earlier. Impregnation was done 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

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Figure 7. Specific permeability of pine sapwood as a function of H factor; symbols according to Tables 1 and 2.

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. Some specimens were also tested without any treatment. 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. During washing, the liquidto-wood ratio was around 100:1. The washing water was changed every 24 h. For each cooking bomb (i.e., for each H-factor level), the weights of three specimens were measured before cooking and after washing and drying 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 the dry-matter content. This was determined according to standard20 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 standard21 SCAN-C 29:72 using these same specimens after they had been ground to powder with a Wiley mill. Description of the Overpressure Apparatus. The newly developed apparatus presented in Figure 2 consists of a specimen holder, an adjustable overflow vessel to generate the pressure difference, balances to measure the mass of permeated fluid, a pressure meter, and a temperature meter. The specimen holder was a plastic hose (i.d. ) 32 mm) with a length of approximately 70 mm. The diameter of the cooked cylindrical shaped specimen was 19 mm after swelling. Initially, the diameter was 18 mm when drilled. The length of the drilled specimens was approximately 50 mm. The length of each specimen was measured before the permeability experiment. The specimen was attached to the holder with hot melt glue (heater, 3 M Polygun EC; glue temperature, 232 °C with module 5; glue, 3 M Scotch-Weld TM 3747 Q). The glue melt flowed into the space between the specimen and the plastic hose. The glued specimen was placed in water to cool. The glue gave a noncompressive attachment. The attachment was weak because the glue did not stick tightly to the wet wood specimen. Thus, the pressure difference was limited to a maximum of 20 kPa because too high of a pressure would break the attachment. The techniques used by other authors did not apply in this study because cooked wood is soft and fractures easily. The specimen in this work was completely penetrated with water and could not be dried. It was easily noticed during the test that a minor

compression of the specimen altered its shape. Some experiments were performed to confirm the sensitivity of specific permeability to pressing of the sample. The clamp was placed around the specimen holder and tightened; it resulted in a decrease of flow. The specimen holder is presented in Figure 3. In this apparatus, tap water flows through the overflow vessel. The deaeration of water used in permeability studies has been discussed in several articles.2,9,18 The dissolved air in liquid can block the channels in the wood and reduce the liquid flow. The dissolving of air into degassed water is difficult to avoid when experiments are performed using a system open to air. After being cooked, the specimens were washed in buckets and stored in closed plastic bags in air-saturated water. The specimens were exposed to the atmosphere for less than 5 min during gluing. The draining of the water from the porous structure of the wood during gluing was insignificant. Thus, the specimens of our measurements remained filled with water before and during the measurements. In our system, the air dissolved in tap water had enough residence time to be liberated. The water was mixed well and heated, and there was a route for the liberated air to escape from the apparatus. However, the industrial cooking process also has dissolved gas in the cooking liquor, despite chip steaming before the cooking digester. A rope moves the adjustable overflow vessel vertically, giving the pressure difference. The outlet of the specimen is open to atmospheric air, but the inlet is connected to the overflow vessel. A differential pressure sensor measures the pressure difference, which is recorded as a function of time. The differential pressure sensor used was Huba Control DTP 05-420 sensor, with a pressure range from 0 to 0.5 bar and an accuracy of 0.5% full scale. The temperature of the inlet water is measured at a 3-cm distance from the specimen and recorded as a function of time. The temperature sensor used was a Nokeval TRCP-13-3-R1/8 Pt100 sensor. The heating system allows the adjustment of the inlet water by circulating the water through a heat exchanger. The pump type was a magnetic-driven gear pump. The total mass flow of water permeated through the specimen was divided into two streams. Both streams were collected in vessels on the balances, and the masses of the collected water were recorded as a function of time from both balances. The cooked specimens were very soft, not allowing for any compression, and the glue was not completely leak-proof. Some of the water could have leaked between the specimen and the glue. This leak was eliminated by the two concentric pipes that were gently pushed against the specimen. In addition, the very sharp edge of the inner pipe held the specimens against the force generated by the pressure drop. The inner pipe separated the real mass flow through the wood. The cross-sectional area of the inner pipe was taken into account in the calculation of specific permeability. The outer pipe collected the rest of the mass flow through the wood, as well as any potential leak between the specimen and the glue into a separate stream. The concentric pipe setup is presented in Figure 2. The balance for weighing the mass flow of the inner pipe was a Mettler-Toledo PG5002-S, and its resolution was 0.01 g. The balance for weighing the mass flow of the outer pipe was a Kern 572, and its resolution was 0.1 g. The signals from the pressure meter and temperature meter were converted into an RS232 signal with a Nokeval 2021-RS24 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.


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a

Table 3. Average Specific Permeability of Pine Heartwood, Overpressure Apparatus

avg dev K

avg L

NoS (d, c)b

symbol

Cook ID ) 8, T ) 18 ( 2 °C 1.6 × 10-12 1.3 × 10-13 1.2 × 10-13 4.4 × 10-14 3.4 × 10-12 1.9 × 10-13 2.9 × 10-12 2.0 × 10-12 1.5 × 10-12 1.5 × 10-12

4.1 × 10-13 2.4 × 10-14 2.2 × 10-13 7.2 × 10-13 2.4 × 10-13

0.044 0.046 0.036 0.032 0.048

4 (2, 2) 4 (0, 4) 4 (1, 3) 2 (0, 2) 1 (0, 1)

] ] ] ] ]

6.2 × 10-13 6.6 × 10-14 1.5 × 10-12 3.9 × 10-12 2.0 × 10-12

Cook ID ) 8, T ) 35 ( 2 °C 1.8 × 10-12 2.2 × 10-14 1.1 × 10-13 3.7 × 10-14 4.6 × 10-12 1.4 × 10-13 4.8 × 10-12 3.1 × 10-12 2.0 × 10-12 2.0 × 10-12

9.4 × 10-14 2.4 × 10-14 2.4 × 10-13 7.6 × 10-13 5.5 × 10-13

0.044 0.046 0.036 0.032 0.048

4 (0, 4) 4 (0, 4) 4 (0, 4) 2 (0, 2) 1 (0, 1)

0 0 0 0 0

27.9 26.2 25.3 23.2 21.9 18.6 15.1

1.4 × 10-12 9.8 × 10-13 3.8 × 10-14 1.1 × 10-13 3.3 × 10-13 1.5 × 10-13 1.1 × 10-13

Cook ID ) 13, T ) 11 ( 2 °C 3.3 × 10-12 3.0 × 10-14 1.8 × 10-12 1.2 × 10-13 4.1 × 10-14 3.5 × 10-14 1.3 × 10-13 8.3 × 10-14 6.3 × 10-13 3.9 × 10-14 2.3 × 10-13 6.9 × 10-14 1.1 × 10-13 1.1 × 10-13

2.2 × 10-13 7.5 × 10-13 2.2 × 10-14 7.8 × 10-14 1.4 × 10-13 5.7 × 10-14 4.0 × 10-14

0.044 0.045 0.046 0.047 0.042 0.045 0.045

3 (0, 3) 2 (0, 2) 2 (0, 4) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

4 4 4 4 4 4 4

27.9 26.2 25.3 23.2 21.9 18.6 15.1

4.8 × 10-13 6.0 × 10-14 4.1 × 10-14 4.1 × 10-14 4.6 × 10-14 1.0 × 10-13 1.0 × 10-13

Cook ID ) 13, T ) 36 ( 2 °C 1.1 × 10-12 2.4 × 10-14 6.6 × 10-14 5.4 × 10-14 5.6 × 10-14 2.5 × 10-14 6.3 × 10-14 1.9 × 10-14 5.9 × 10-14 3.2 × 10-14 1.8 × 10-13 2.4 × 10-14 1.0 × 10-13 9.8 × 10-14

1.0 × 10-13 4.2 × 10-14 3.1 × 10-14 2.5 × 10-14 3.3 × 10-14 7.1 × 10-14 3.6 × 10-14

0.044 0.045 0.046 0.047 0.042 0.045 0.045

3 (0, 3) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

× × × × × × ×

H factor

yield (%)

Cl number

avg K

100 200 300 400 500

69.3 64.7 59.7 56.2 55.0

NAc NA NA NA NA

8.9 × 10-13 7.0 × 10-14 1.2 × 10-12 2.5 × 10-12 1.5 × 10-12

100 200 300 400 500

69.3 64.7 59.7 56.2 55.0

NA NA NA NA NA

0 100 200 300 400 500 600

96.4 71.7 67.7 63.3 60.3 58.4 57.1

0 100 200 300 400 500 600

96.4 71.7 67.7 63.3 60.3 58.4 57.1

max K

min K

Specimens were drilled from the same year ring (i.e., distance from the log center). b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time. c NA ) not analyzed. a

Table 4. Average Specific Permeability of Pine Heartwood, Vacuum Apparatusa H factor 0 100 200 300 400 500 600

yield (%) 96.4 71.7 67.7 63.3 60.3 58.4 57.1

Cl number 27.9 26.2 25.3 23.2 21.9 18.6 15.1

avg K 2.6 × 10-14 1.1 × 10-14 2.4 × 10-13 7.9 × 10-14 2.0 × 10-14 3.8 × 10-14 1.5 × 10-13

max K

min K

Cook ID ) 13, T ) 21 ( 2 °C 2.8 × 10-14 2.4 × 10-14 1.2 × 10-14 1.0 × 10-14 9.1 × 10-13 1.3 × 10-14 9.0 × 10-14 6.9 × 10-14 2.3 × 10-14 1.8 × 10-14 3.8 × 10-14 3.7 × 10-14 1.7 × 10-13 1.2 × 10-13

avg dev K

avg L

NoS (d, c)b

symbol

1.5 × 10-14 6.3 × 10-15 4.4 × 10-13 6.0 × 10-14 1.1 × 10-14 2.6 × 10-14 5.9 × 10-14

0.044 0.045 0.046 0.047 0.042 0.045 0.045

3 (0, 3) 2 (0, 2) 4 (0, 4) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

+ + + + + + +

a Specimens were drilled from the same year ring (i.e., distance from the log center). b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time.

Figure 8. Specific permeability of pine heartwood as a function of H factor; symbols according to Tables 3 and 4.

Description of the Vacuum Apparatus. The apparatus was a modification of the previous apparatus in which the overpres-

sure of the static head was replaced with a vacuum. Figure 4 illustrates the apparatus used. A vacuum pump (Vacuubrand membrane pump, MZ2C, 2.4 m3/h) generated the vacuum for the system. The pressure was controlled with a manual globe valve and fine-tuned with a needle valve. The volume of the buffer tank was approximately 1 L, and the volume of the collection tank for water was 2 L. The specimen holder in this modification is a standard laboratory glassware NS19 socket. The specimens drilled in the longitudinal direction of the log were almost circular in their cross-sectional area. The specimens were set gently into the specimen holder to avoid any deformation (i.e., to have a minor effect on the area). Friction was sufficient to keep the specimens stationary. No additional sealing in the direction of water flow was needed. Most of the radial flow of the water into the specimen was reduced by wrapping the specimen in laboratory film (Parafilm M, Pechiney plastik packaging). The laboratory film was found to be better than glue because of the elasticity of the laboratory film. The glue would have prevented the gluing of the specimen into the specimen holder when the combined measurement procedure was applied. A cup of water was placed on the balance. The water was at the surrounding room

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Table 5. Average Specific Permeability of Birch, Overpressure Apparatusa avg dev K

avg L

NoS (d, c)b

symbol

Cook ID ) 9, T ) 18 ( 2 °C 1.3 × 10-12 1.5 × 10-12 7.2 × 10-13 2.3 × 10-13 4.5 × 10-13 1.0 × 10-14 2.6 × 10-13 6.4 × 10-13 7.8 × 10-14 9.0 × 10-13 1.6 × 10-12 5.5 × 10-13 1.0 × 10-12 1.3 × 10-12 4.6 × 10-13 2.1 × 10-12 3.9 × 10-12 1.1 × 10-12 2.1 × 10-12 2.1 × 10-12 2.1 × 10-12 1.3 × 10-12 1.6 × 10-12 6.3 × 10-13 7.9 × 10-13 2.3 × 10-12 2.7 × 10-13

3.5 × 10-13 1.4 × 10-13 1.4 × 10-13 4.1 × 10-13 3.4 × 10-13 4.8 × 10-13 4.9 × 10-13 7.2 × 10-13 4.2 × 10-13

0.042 0.046 0.044 0.044 0.047 0.048 0.014 0.041 0.029

3 (2, 1) 6 (0, 3) 6 (0, 3) 6 (0, 3) 6 (0, 3) 6 (0, 3) 1 (0, 3) 6 (0, 3) 4 (0, 3)

] ] ] ] ] ] ] ] ]

NA NA NA NA NA NA NA NA NA

Cook ID ) 9, T ) 35 ( 2 °C 5.9 × 10-13 7.2 × 10-13 3.3 × 10-13 1.5 × 10-13 2.8 × 10-13 1.6 × 10-14 2.1 × 10-13 5.3 × 10-13 1.1 × 10-13 8.4 × 10-13 1.3 × 10-12 3.4 × 10-13 7.4 × 10-13 1.5 × 10-12 2.7 × 10-13 1.0 × 10-12 1.9 × 10-12 5.7 × 10-13 8.8 × 10-13 8.8 × 10-13 8.8 × 10-13 5.2 × 10-13 7.2 × 10-13 1.9 × 10-13 3.1 × 10-13 5.8 × 10-13 4.0 × 10-14

1.5 × 10-13 1.0 × 10-13 8.7 × 10-14 3.0 × 10-13 2.3 × 10-13 2.7 × 10-13 2.3 × 10-13 2.8 × 10-13 1.8 × 10-13

0.042 0.046 0.044 0.044 0.047 0.048 0.014 0.041 0.029

3 (2, 1) 4 (1, 3) 6 (0, 6) 6 (2, 4) 6 (0, 6) 6 (2, 4) 1 (0, 1) 6 (5, 1) 6 (2, 4)

0 0 0 0 0 0 0 0 0

95.3 65.5 60.1 59.9 56.4 51.6 52.9

20.8 15.1 12.2 9.4 8.1 4.3 3.7

Cook ID ) 14, T ) 15 ( 2 °C 6.3 × 10-13 6.3 × 10-13 6.3 × 10-13 5.0 × 10-14 4.9 × 10-14 4.9 × 10-14 9.4 × 10-14 1.3 × 10-13 6.3 × 10-14 3.2 × 10-13 3.5 × 10-13 3.0 × 10-13 2.8 × 10-13 3.5 × 10-13 2.0 × 10-13 3.3 × 10-13 5.4 × 10-13 1.2 × 10-13 1.9 × 10-12 3.3 × 10-12 3.3 × 10-12

1.3 × 10-13 3.3 × 10-14 5.1 × 10-14 9.9 × 10-14 9.7 × 10-14 1.2 × 10-13 2.1 × 10-12

0.049 0.045 0.047 0.046 0.044 0.047 0.048

1 (0, 1) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

4 4 4 4 4 4 4

95.3 65.5 60.1 59.9 56.4 51.6 52.9

20.8 15.1 12.2 9.4 8.1 4.3 3.7

Cook ID ) 14, T ) 36 ( 2 °C 5.0 × 10-13 5.0 × 10-13 5.0 × 10-13 1.6 × 10-14 1.6 × 10-14 1.6 × 10-14 2.8 × 10-14 2.8 × 10-14 2.8 × 10-14 1.2 × 10-13 1.4 × 10-13 9.8 × 10-14 1.4 × 10-13 1.8 × 10-13 1.0 × 10-13 2.0 × 10-13 3.3 × 10-13 6.8 × 10-14 4.4 × 10-13 6.2 × 10-13 6.2 × 10-13

7.2 × 10-14 9.7 × 10-15 1.7 × 10-14 4.7 × 10-14 3.8 × 10-14 6.4 × 10-14 1.1 × 10-13

0.049 0.045 0.047 0.046 0.044 0.047 0.048

1 (0, 1) 2 (0, 2) 1 (0, 1) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (1, 1)

× × × × × × ×

H factor

yield (%)

Cl number


86.8 53.7 48.7 49.9 47.2 47.0 41.1 NA NA

NAc NA NA NA NA NA NA NA NA


86.8 53.7 48.7 49.9 47.2 47.0 41.1 NA NA

0 50 100 150 204 300 376 0 50 100 150 204 300 376

avg K

max K

min K

Specimens drilled close to the surface of the log. b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time. c NA ) not analyzed. a

Table 6. Average Specific Permeability of Birch, Vacuum Apparatusa H factor

yield (%)

Cl number

avg K

0 50 100 150 204 300

95.3 65.5 60.1 59.9 56.4 51.6

20.8 15.1 12.2 9.4 8.1 4.3

3.3 × 10-13 1.3 × 10-14 1.6 × 10-14 2.9 × 10-14 5.9 × 10-14 1.1 × 10-13

max K

min K

Cook ID ) 14, T ) 21 ( 2 °C 3.3 × 10-13 3.3 × 10-13 1.3 × 10-14 1.2 × 10-14 1.9 × 10-14 1.4 × 10-14 3.0 × 10-14 2.9 × 10-14 7.6 × 10-14 4.3 × 10-14 1.5 × 10-13 6.8 × 10-14

avg dev K

avg L

NoS (d, c)b

symbol

1.1 × 10-13 6.3 × 10-15 9.9 × 10-15 1.2 × 10-14 1.8 × 10-14 2.5 × 10-14

0.049 0.045 0.047 0.046 0.044 0.047

1 (0, 1) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

+ + + + + +

a Specimens drilled close to the surface of the log. b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time.

temperature. The balance, pressure sensor, and temperature sensor were read using the same automation as for the overpressure apparatus. Measurement Procedure for the Overpressure 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 the unbound water at the surface of the specimen was dried with paper towels. The handling of the specimens was done gently, to avoid pressing the specimen. The specimen was placed inside the plastic hose on a steel plate, and the melted glue was poured between the specimen and the hose. Then, the specimen and the holder were immersed into water to cool the melted glue. After cooling, the specimen with the holder was removed from the steel plate. The specimen holder was attached to the body of the apparatus. Then, the valve was opened slowly to let the tap water enter the inlet of the specimen. Once the air bubbles had risen to the overflow vessel, the pressure stabilized, and the valve was opened more.

The pressure difference was set to a maximum (20 kPa) at the beginning, and then it was lowered twice and then raised twice back to its original position. This method gave three pressure levels. Each pressure level lasted 5-10 min. Once the pressure cycle was completed, the water was heated by starting the circulation pump, and the pressure cycle was repeated. Measurement Procedure for the Vacuum Apparatus. The specimen was attached gently to the NS19 socket and wrapped in laboratory film to block radial flow. Friction kept the specimen in its position, allowing it to be immersed completely in the water that was in the cup placed on the balance. The vacuum was increased in three or four steps and then decreased gradually. The maximum pressure difference was 20 kPa. The water level increased slowly as the measurement proceeded, and when the water level was higher than 5 cm above the specimen, the water flowed into the collecting tank. The readings of the balance, pressure, and temperature were recorded continuously as the measurement proceeded. The evaporation


of water from the water cup was also studied, but its effect on error was insignificant. Combined Measurement Procedure. This procedure was used to compare the results of the two separate apparatuses, overpressure and vacuum. The procedure started with the attachment of the specimen to socket NS19, and a measurement was performed. If the specimen was not broken or fractured after the test, it was immersed in water to wait for the next procedure. Within a couple of hours, the overpressure tests were started by gluing the specimen to its holder. The overpressure test was performed at two temperatures, the lower temperature first. Error Analysis. The density and viscosity of liquid water was correlated between 5 and 50 °C. The error in density was less than 0.01%, and that in viscosity was less than 2%. The resolution of the Mettler-Toledo balance was 0.01 g in the static mode, but in the dynamic case, the uncertainty was dependent on the way the liquid flowed into the vessel on the balance, as drops or as a continuous flow. It is estimated to be 0.1 g. The time interval needed to scan of all the RS232 ports of the instruments was 0.5 s, which was taken as the uncertainty of the time measurement. The uncertainty of the temperature measurement was estimated to be 1 K; the accuracy of the temperature sensor was better. The uncertainty was caused by the liquid flow patterns, the liquid heat capacity, the heat transfer from the surface of the specimen holder, and the heat capacity of the specimen itself. The uncertainty of the pressure measurements was estimated to be 0.1 kPa, equaling a water static head of 1 cm. The uncertainty of the density and the viscosity of water came from the uncertainty in temperature. The uncertainty in the length and diameter of the specimen was 1 mm in each dimension. The total difference describes the maximum error of the specific permeability as follows

|

| |

|

|

|

|

1 Lη m ˙ η m ˙ Lη ∆F + ∆m ˙ + ∆L + - 2 F A∆p F A∆p F A∆p m ˙ Lη m ˙ Lη m ˙ L ∆A + ∆(∆p) + ∆η F A2∆p F A(∆p)2 F A∆p

∆K )

|

|

|

|

|

(3)

where ∆m ˙ )

| |

| |

mk+1 - mk 1 ∆m ∆m ) ) ∆(∆m) + ∆(∆t) tk+1 - tk ∆t ∆t (∆t)2 (4)

The greatest effect on the maximum error in the specific permeability came from the area term. The diameter of the inner pipe was defined accurately, but the axial alignment of the specimen with respect to the holder caused some uncertainty. The second effect was the error in mass flow, where the response of data logging was the limiting factor, along with the data sampling interval. However, writing in pen and on paper would not have provided the same speed. A typical error analysis of one specimen is shown in Figure 5, where the same specimen is measured first using the vacuum apparatus and then using the overpressure apparatus. It is possible to maintain a relatively constant permeability during a period of 20 min. The details of the experiments shown in Figure 5 are presented in Figure 6, where the mass flow and pressure drop are presented. The pressure drop is negative in vacuum experiments; the more negative the pressure drop, the higher the driving force and the higher the mass flow generated. There is a decrease in mass flow as a function of time. The obvious

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Figure 9. Specific permeability of birch as a function of H factor; symbols according to Tables 5 and 6.

explanation for this observation is the liberation of air bubbles that partially block the channels inside the wood. The response of the mass flow to the pressure drop is almost linear during the overpressure tests. Often, as in Figure 5, the specific permeability at higher temperatures was lower than that at cool temperatures. If the wooden specimen is assumed to be temperature-independent, the decrease in the ratio of viscosity to density should be compensated by an increase in mass flow as the temperature increases. The increase in mass flow was not sufficient in most cases. The temperature was measured at the inlet of the liquid flow, not at several locations inside the specimen. The density and viscosity were determined according to the measured inlet temperature. The outer surface of the specimen holder was exposed to the ambient air at around 21 °C. Although the plastic hose and the glue are thermal insulators, there could have been a thermal gradient in the specimen. Because the wooden specimens were cut by sawing and kraftcooked in alkaline conditions before the permeability tests, fibers could be blocking part of the porous structure of the specimen. These fibers might have been released when the circulation pump and heating was started. The preparation of the end surfaces of the specimen was discussed in Rice and D’Onofrio,4 Lu and Avramidis,16 and Bradic and Avramidis,6 who trimmed the end surfaces with a thin sharp blade. In our case, the sawn end surfaces are more industrial-like than razor-blade-finished specimens, because the chipping of the log is a harsh process, producing an unfinished end surface of the chip. Several articles, such as those by Comstock,2,3 Chen et al.,18 and Kininmonth,9 have dealt with the degassing and filtration of the liquid used in permeability tests. This is one important factor in obtaining a constant liquid flow. Some tests with porous glass sinter (Schott Duran, #4) were performed for comparison, where a minor decrease in specific permeability to tap water was noticed when the temperature was increased. This can be explained in terms of the dissolved air in tap water. The dissolved air in the tap water is comparable to that in industrial cooking liquor. The white liquor is exposed to the atmosphere while being held in storage tanks before being fed to cooking, and there is also residual air inside the chips. Thus, the kraft cooking process contains some dissolved gas in the liquid. Also, one source of error is the small dependency of the specific permeability on the pressure difference. The setup with two concentric pipes was designed to eliminate the leaks, but there still might have been leakage between the sharpened edge of the inner pipe and the cut section of the wooden specimen.

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Table 7. Average Specific Permeability of Eucalyptus, Overpressure Apparatusa avg dev K

avg L

NoS (d, c)b

symbol

Cook ID ) 10, T ) 18 ( 2 °C 7.0 × 10-11 2.4 × 10-11 7.5 × 10-11 5.7 × 10-11 7.0 × 10-11 4.0 × 10-11 6.7 × 10-11 2.1 × 10-11 6.4 × 10-11 2.7 × 10-11 5.7 × 10-11 2.5 × 10-11 3.5 × 10-11 1.8 × 10-11

2.6 × 10-12 4.8 × 10-12 3.3 × 10-12 3.5 × 10-12 2.6 × 10-12 2.0 × 10-12 1.3 × 10-12

0.046 0.046 0.049 0.047 0.049 0.046 0.048

6 (0, 6) 6 (0, 6) 6 (0, 6) 6 (0, 6) 6 (0, 6) 6 (1, 5) 6 (1, 5)

] ] ] ] ] ] ]

3.8 × 10-11 5.3 × 10-11 4.4 × 10-11 3.6 × 10-11 3.4 × 10-11 2.5 × 10-11 1.7 × 10-11

Cook ID ) 10, T ) 34 ( 2 °C 5.8 × 10-11 1.8 × 10-11 6.5 × 10-11 4.2 × 10-11 5.3 × 10-11 2.9 × 10-11 5.8 × 10-11 1.4 × 10-11 5.5 × 10-11 1.8 × 10-11 3.6 × 10-11 1.7 × 10-11 2.3 × 10-11 1.2 × 10-11

2.2 × 10-12 4.1 × 10-12 7.5 × 10-12 8.1 × 10-12 6.6 × 10-12 6.9 × 10-12 3.7 × 10-12

0.046 0.046 0.049 0.047 0.049 0.046 0.048

6 (0, 6) 6 (0, 6) 6 (1, 5) 6 (1, 5) 6 (0, 6) 6 (1, 5) 6 (2, 4)

0 0 0 0 0 0 0

22.9 16.9 13.1 10.1 6.0

1.5 × 10-11 1.9 × 10-12 2.8 × 10-11 1.1 × 10-11 2.1 × 10-11

Cook ID ) 15, T ) 18 ( 2 °C 1.6 × 10-11 1.3 × 10-11 2.2 × 10-12 1.7 × 10-12 3.1 × 10-11 2.5 × 10-11 1.6 × 10-11 5.5 × 10-12 2.6 × 10-11 1.6 × 10-11

1.5 × 10-12 2.1 × 10-13 3.3 × 10-12 1.1 × 10-12 1.8 × 10-12

0.052 0.053 0.050 0.051 0.046

2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2)

4 4 4 4 4

22.9 16.9 13.1 10.1 6.0

9.0 × 10-12 1.2 × 10-12 2.0 × 10-11 4.2 × 10-12 1.4 × 10-11

Cook ID ) 15, T ) 36 ( 2 °C 1.0 × 10-11 7.7 × 10-12 1.5 × 10-12 8.5 × 10-13 2.4 × 10-11 1.5 × 10-11 4.3 × 10-12 4.2 × 10-12 1.8 × 10-11 1.0 × 10-11

1.0 × 10-12 1.4 × 10-13 1.7 × 10-12 5.5 × 10-13 1.8 × 10-12

0.052 0.053 0.050 0.051 0.046

2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (0, 2) 2 (1, 1)

× × × × ×

H factor

yield (%)

Cl number

avg K

0 48 100 175 250 325 400

89.1 66.6 60.0 56.6 53.7 53.9 53.4

NAc NA NA NA NA NA NA

4.7 × 10-11 6.5 × 10-11 5.5 × 10-11 4.7 × 10-11 4.0 × 10-11 3.6 × 10-11 2.4 × 10-11

0 48 100 175 250 325 400

89.1 66.6 60.0 56.6 53.7 53.9 53.4

NA NA NA NA NA NA NA

0 50 100 175 250

92.4 72.9 62.3 59.4 52.8

0 50 100 175 250

92.4 72.9 62.3 59.4 52.8

max K

min K

Specimens drilled close to the surface of the log. b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time. c NA ) not analyzed. a

Figure 10. Specific permeability of eucalyptus as a function of H factor; symbols according to Tables 7 and 8.

As a part of the error analysis, the maximum and minimum average values of the specific permeability are given for each H factor in Tables 1-8. Also, the average absolute deviation is reported for each H factor. This gives an indication of the scattering of specific permeability in successive experiments: the smaller the value, the smaller the scattering. The uncertainty in the H factor was estimated to be 5 units; approximately 1 min was needed to remove the cooking bomb from the air bath. The bombs were cooled rapidly in flowing cool tap water. The dynamics of the cooling was not included this error analysis. Uncertainty was estimated to be 1% in the yield and 0.5 units in Cl number. Results and Discussion Three wood species were tested in this study: pine (sapwood and heartwood), birch, and eucalyptus. The flow direction was

along the fiber, that is, parallel to the cylindrical axis of the log. The reported values of H factor, yield, and chlorine number provide information on the cooking in previously reported kraft cooking conditions. The purpose of chemical pulping is to delignify the wood, in particular, the middle lamella, to make it possible to separate the individual fibers. The pulping liquor promotes the degradation and dissolution of the lignin, although at the same time, the polysaccharides are attacked. The chemical interaction of the cell wall components with kraft pulp liquor is well-known. On the other hand, understanding of the effects of chemical processing on the cell wall ultrastructure is still limited. The influence of steaming on the ultrastructure of a bordered pit membrane was studied by Nicholas and Thomas.22 They used loblolly pine and found evidence that components in the pit membrane are hydrolyzed during steam treatment. They reported that the hydrolysis results in a general weakening of the membrane, as well as a reduction of the strength of the bond between the border and torus of aspirated pits. Nicholas and Thomas22 came to the conclusion that these chemical changes are responsible for the improved permeability because they reduce the effectiveness of aspiration. 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 (d, c)” (i.e., number of specimens). The same column contains information on the behavior of specific permeability as a function of time for various samples, where d indicates a decreasing trend in specific permeability as a function of time and c indicates a constant trend. This trend was concluded visually from the plots as a function of time. Bramhall23 reported a large number of studies in which decreasing liquid flow, nonDarcian behavior, was studied. The reason for the observed trend was the blockage of pit pores by extraneous material and air bubbles. The liquid water in our study was practically saturated with air. However, the industrial case during impregnation and


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a

Table 8. Average Specific Permeability of Eucalyptus, Vacuum Apparatus H factor

yield (%)

Cl number

avg K

0 50 100 175 250

92.4 72.9 62.3 59.4 52.8

22.9 16.9 13.1 10.1 6.0

6.6 × 10-12 8.3 × 10-13 7.2 × 10-12 3.6 × 10-12 6.5 × 10-12

max K

min K

Cook ID ) 15, T ) 23 ( 2 °C 9.3 × 10-12 3.9 × 10-12 1.2 × 10-12 4.3 × 10-13 8.6 × 10-12 5.8 × 10-12 4.3 × 10-12 2.8 × 10-12 6.9 × 10-12 6.2 × 10-12

avg dev K

avg L

NoS (d, c)b

symbol

1.8 × 10-12 1.5 × 10-13 4.3 × 10-12 9.2 × 10-13 2.4 × 10-12

0.052 0.053 0.050 0.051 0.046

2 (0, 2) 2 (0, 2) 2 (2, 0) 2 (0, 2) 2 (2, 0)

+ + + + +

a Specimens drilled close to the surface of the log. b NoS ) (total) number of specimens. The values in parentheses indicate the numbers of specimens exhibiting a decreasing trend (d) and a constant trend (c) in specific permeability as a function of time.

Figure 11. Comparison between species. Symbols: (]) pine, sapwood, T ) 22 °C; (0) pine, heartwood, T ) 22 °C; (4) birch, T ) 22 °C; (×) eucalyptus, T ) 22 °C.

cooking contains dissolved gas in the cooking liquid as well. The columns “max K” and “min K” indicate the maximum and minimum average values, respectively, of the specific permeability of individual tests. The average absolute deviation is a measure of the absolute deviation of individual specific permeability values from the average value, indicating the “noise” of the specific permeability measurement as a function of time. The length of the specimens was varied from 0.02 to 0.05 m to study the length effect. It was found that the specimen length had no significant effect on the specific permeability; thus, the average length is reported. Pine, Sapwood. The number of tested specimens was 52, including the runs with the cooked specimens (H factors ranged from 0 to 600), that is, steamed and boiled specimens. Table 1 reports the average specific permeabilities of the experimental runs performed in the overpressure apparatus. The two cooks were performed with the same target H factors. The percent yield was determined for both cooks, but a chlorine number was also determined for the samples of cook ID ) 12. The same specimens of cook ID ) 12 were also measured using the vacuum apparatus. The results of the vacuum measurements are presented in Table 2. A comparison of the overpressure and vacuum methods reveals the equal magnitude of the results. The experiments on specific permeability are plotted in Figure 7 as a function of H factor. It is easily seen that, above an H factor of 400, the scattering of the results increases as a function of H factor. This is logical because the rigid structure of the wood is mostly lost. The trends in percent yield and chlorine number are not as obvious and thus are not presented. The decreasing or constant trend in specific permeability as a function of time is reported for each H factor. The classification between a constant and decreasing trend is subjective and was made visually. Generally, it can be

said that the vacuum system shows a higher number of experiments with decreasing specific permeability. There are two obvious reasons for this observation: The slight vacuum liberates air bubbles from the tap water quite easily, whereas the slight overpressure does not promote the formation of air bubbles. Also, the pressure difference compresses the specimen slightly into the NS19 socket. A comparison of the obtained data for steamed and boiled test specimens to the data reported by Siau1 shows equal magnitudes of the specific permeability. The pine sample mentioned in Siau1 was not specified exactly, but it was not cooked in alkaline conditions. Pine, Heartwood. The number of experiments with pine heartwood was smaller because of difficulties in keeping specimens from cracking during experiments for cook ID ) 8. However, the two cooks made it possible to have 30 tested specimens. Table 3 lists these results for the overpressure apparatus. It is noted that, at an H factor of 200, there is a minimum of specific permeability, and then at an H factor of 400, the scattering of the results is the highest. At H factors higher than 400, the specimens fractured very easily. Specimens for cook ID ) 13 were measured using both techniques, and the results are reported in Table 4. The cracking of wooden specimens for cook ID ) 13 also limited the number of experiments. Some of the specimens appeared to be uncracked, but the experiments revealed a large scattering of results. Figure 8 shows the specific permeability in graphical form. As a conclusion, the experiments indicate that the heartwood of pine is less permeable than sapwood when the specimens were kraft cooked. There are several articles, such as those by Comstock,2 Rice and D’Onofrio,4 Bradic and Avramidis,6 Flynn,24 and Comstock,12 in which different wood species were tested. These species were not kraft cooked but were fresh wood. Contrary to the results for sapwood, the vacuum system showed a higher number of experiments with constant specific permeability and a smaller permeability compared to overpressure measurements. The deviations can be explained in terms of the closing of fractures during compression of the specimen against the NS19 socket. Birch. The wooden species of birch cannot be defined as clearly as sapwood and heartwood in the case of pine. The criteria according to which the specimens were drilled were to follow the same year ring close to the surface of the log. The results are presented in Table 5. Samples for cook ID ) 14 were tested using both apparatuses and exhibited similar trends, although the vacuum measurements gave a slightly smaller permeability, as shown in Table 6. The scattering of results shown in Figure 9 is smaller than in the pine experiments. The birch cracked easily during cooking when the H factor exceeded 300. The second cooking (cook ID ) 14) was interrupted earlier than the first (cook ID ) 9).

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The increasing trend in specific permeability and the average absolute deviation can be seen as a function of H factor based on the 57 tested specimens. Eucalyptus. The number of tested specimens was 52. The stability of the flow was much higher than with pine and birch. This was caused by a much higher specific permeability, as can be seen in Table 7. Measurements were done with the overpressure apparatus. The vacuum method gave a smaller specific permeability, with the compression of the specimens against the NS19 socket as the obvious explanation. A graphical presentation is shown in Figure 10. The specific permeability of liquid water in eucalyptus for cook ID ) 10 shows a decreasing trend as a function of H factor. However, the second cooking (cook ID ) 15) for comparison showed higher scattering of the results. The eucalyptus wooden species is very soft and cracks very easily. Pines, Birch, and Eucalyptus Compared. The specific permeabilities of pine sapwood, pine heartwood, birch, and eucalyptus at the “low” and “high” temperatures of the two cooks were averaged to make the number of specimens larger and to smooth out possible differences between the cooks. The specific permeability of pine sapwood was found to vary between 3 × 10-12 and 1 × 10-12 m3/m below an H factor of 400, and that of pine heartwood was found to vary between 1 × 10-13 and 2 × 10-12 m3/m below an H factor of 400, indicating that cooked sapwood is more permeable than cooked heartwood. The specific permeability of birch showed a minimum at an H factor of approximately 150, but generally, the specific permeability was between 1 × 10-13 and 1 × 10-12 m3/m. Eucalyptus exhibited a slight maximum at an H factor of approximately 100, but this behavior might be dependent on the cook and the specimen. The magnitude of specific permeability of eucalyptus was higher, ranging from 3 × 10-11 to 5 × 10-11 m3/m. A comparison is presented in Figure 11. Conclusions The experimental setup for the measurement of the specific permeability of pine sapwood, pine heartwood, spruce, and eucalyptus to water near room temperature was built. Different treatments and combinations of treatments were used for the wooden specimens: boiling in water, steaming, impregnation, and kraft cooking. The experimental setup of this work measured the specific permeability of uncompressed wood. The sapwood of pine was more permeable than the heartwood. The specific permeability of birch was close to that of the pine heartwood. Eucalyptus was found to be the most permeable species in this study. Scattering of the results was the highest with the pine heartwood, followed by the pine sapwood. The most uniform material from the viewpoint of specific permeability was found to be eucalyptus. Eucalyptus showed a decreasing trend in specific permeability as the H factor increased. The specific permeability of pine was almost constant, and for birch, a small increase as a function of H factor was observed. These experiments showed that the convection of liquid water decreased or increased depending on the wood species and the cooking degree, and there was a large variation of specific permeability between the individual samples.

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) Comstock, G. L. Longitudinal Permeability of Green Eastern Hemlock. For. Prod. J. 1965, 13, 441–449. (3) Comstock, G. L. Longitudinal Permeability of Wood to Gases and Nonswelling Liquids. For. Prod. J. 1967, 17, 41–46. (4) Rice, R. W.; D’Onofrio, M. Longitudinal Gas Permeability Measurements from Eastern White Pine, Red Spruce and Balsam Fit. Wood Fiber Sci. 1996, 28 (3), 301–308. (5) Vazquez, G.; Chenlo, F.; Moreira, R.; Arnaud, G. Longitudinal Permeability of Sapele (Entandrophragma cylindricum) to Air. Holtzforschug 1997, 51, 173–176. (6) Bradic, S.; Avramidis, S. Longitudinal air permeability of pinewood with beetle transmitted blue-stain. Holz Roh Werkst. 2007, 65 (3), 183– 185. (7) Lin, R. T.; Lancaster, E. P.; Krahmer, R. L. Longitudinal water permeability of western hemlock. I. Steady-state permeability. Wood Fiber Sci. 1973, 4 (4), 290–297. (8) Lin, R. T.; Lancaster, E. P. Longitudinal water permeability of western hemlock. II. Unsteady-state permeability. Wood Fiber Sci. 1973, 4 (4), 278–289. (9) Kininmonth, J. A. Permeability and Fine Structure of Certain Hardwoods and Effects on Drying. Holtzforschug 1971, 25 (4), 127–133. (10) Tesoro, F. O.; Choong, E. T.; Skaar, C. Transverse Air Permeability of Wood. For. Prod. J. 1966, 16 (3), 57–59. (11) Choong, E. T.; Fogg, P. J. Moisture Movement in Six Wood Species. For. Prod. J. 1968, 18 (5), 66–70. (12) Comstock, G. L. Directional Permeability of Softwoods. Wood Fiber Sci. 1969, 1 (4), 283–289. (13) Lu, J.; Avramidis, S. Non-Darcian Air Flow in Wood, Part 3. Molecular Slip Flow. Holtzforschug 1997, 51, 85–92. (14) Hayashi, S.; Nishimoto, K.; Kishima, T. Study on the Liquid Permeability of Softwoods. Wood Res. 1966, 33, 47–57. (15) Charuk, E. V.; Vologdin, A. I.; Kovrigin, G. S. Die Permeabilita¨t des Kern- und Reifholzes von Nadelho¨lzer gegenuber Flussigkeiten in Gasen. Holztechnologie 1973, 14 (3), 135–138. (16) Lu, J.; Avramidis, S. Non-Darcian Air Flow in Wood, Part 1. Specimen Length Effect. Holtzforschug 1997, 51, 577–583. (17) Lu, J.; Avramidis, S. Non-Darcian Air Flow in Wood, Part 2. Nonlinear Flow. Holtzforschug 1997, 51, 77–84. (18) Chen, P. Y. S.; Sucoff, E. I.; Hossfeld, R. The Effect of Cations on the Permeability of Wood to Aqueous Solutions. Holtzforschug 1970, 24 (2), 65–67. (19) Blokhra, R. L.; Parmar, M. L.; Sharma, R. K. Flow Through Porous Medium: Part II-Permeability of Water & Solutions of Ammonium Nitrate, Sulphate & Phosphate Through Treated Pine Wood Membrane. Indian J. Chem. 1978, 16, 800–802. (20) Wood chips for pulp production: Dry matter content. Test Method SCAN-CM 39-94; Scandinavian Pulp, Paper and Board Testing Committee: Stockholm, Sweden, 1994. (21) Chlorine consumption of pulp. Test Method SCAN-C 29:72; Scandinavian Pulp, Paper and Board Testing Committee: Stockholm, Sweden, 1972. (22) Nicholas, D. D.; Thomas, R. J. Influence of Steaming on Ultrastructure of Bordered Pit Membrane in Loblolly Pine. For. Prod. J. 1968, 18 (1), 57–59. (23) Bramhall, G. The validity of Darcy’s law in the axial penetration of wood. Wood Sci. Technol. 1971, 5, 121–134. (24) Flynn, K. A. A Review of the Permeability, Fluid Flow, and Anatomy of Spruce (Picea Spp.). Wood Fiber Sci. 1995, 27 (3), 278–284.

ReceiVed for reView December 12, 2008 ReVised manuscript receiVed September 16, 2009 Accepted December 17, 2009 IE801901F

Specific Permeability of Wood to Water Part 1 - ACS Publications

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