The effect of supercritical fluid treatment on superficial gas permeability of Douglas-fir was investigated by using carbon dioxide either alone or amended with methanol. Permeability increased in a majority of samples, although nearly one-third of the specimens declined in permeability. Improvements in permeability appeared to be unrelated to variations in temperature, pressure, or the presence of a cosolvent, suggesting that the conditions were uniformly suitable for extractive solubilization. Decreased permeability may reflect redeposition of solubilized extractives to positions where they again diminish permeability as conditions drop out of the critical range, a possibility that will be studied further.Keywords: permeability, Douglas-fir, SCF-CO2, heartwood, supercritical fluidsKeywords: permeability, Douglas-fir, SCF-CO2, heartwood, supercritical fluid

Demessie, Endalkachew Sahle

Hassan, Ali

Kumar, Satish

Levien, Keith L.

Morrell, Jeffrey J.

Undetermined

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SUPERCRITICAL CARBON DIOXIDE TREATMENT: EFFECT ON PERMEABILITY OF DOUGLAS-FIR HEARTWOOD1 Endalkachew Sahle Demessie Graduate Research Assistant Ali Hassan Graduate Research Assistant Keith L. Levien Associate Professor Depanment of Clem~cal Engineering Oregon State Unlven~tv Satish Kumar Research Associate and Jefrey J. Morrell Associate Professor Depaament of Forest Products Oregon State University Corvallis, OR 97331 (Received August 1994) ABSTRACT The effect of supercritical fluid treatment on superficial gas permeability of Douglas-fir was inves- tigated by using carbon dioxide either alone or amended with methanol. Permeability increased in a majority of samples, although nearly one-third of the specimens declined in permeability. Improve- ments in permeability appeared to be unrelated to variations in temperature, pressure, or the presence of a cosolvent, suggesting that the conditions were uniformly suitable for extractive solubilization. Decreased permeability may reflect redeposition of solubilized extractives to positions where they again diminish permeability as conditions drop out of the mitical range, a possibility that will be studied further. Keywords: Permeability, supercritical fluids, Douglas-fir. INTRODUCTION Supercritical fluids (SCFs) are increasingly assessed as to their abilitv to extract and de- ' This is Paper 3032 of the Forest Research Laboratory, posit chemicalS in semiporous materials, in- Oregon State University, Cowallis, OR. Present addresses cluding wood (paulaits et al, 1982; Eckert et of S. Kumar and E. Sahle Demessie are Forest Research Institute, P.O. Box New Forest, Dehra Dun, India, and, al., 1986; McHugh and Krukonis 1986; John- U.S. EnvironmentalProtection Agency, MS489, RiskRe- Ston et al. 1989). SCFS offer a novel combi- duction Laboratory, Cincinnati, OH 45268, respectively. nation of gas- and liquid-like properties that Woalond Akr Sccmcr. 270). 1995. m. 296300 s 1995 by Ihc Soclcty ofWaad Sctenrr and Technology Drmcsic n a/.-SUPERCRITICAL FLUID TREATMENT OF DOUGLAS-FIR 297 no. I .  Schematic of the permeability measurement apparatus. make them unique as solvents and reaction media. Because of their abilities to penetrate wood readily, SCFs may be especially useful in wood processing, either by cawing mate- rials that can subsequently be deposited when temperatures or pressures are varied or by sol- ubilizing materials naturally present in the wood, such as extractives, resin acids, or fatty acids, so that they can be removed for other uses (Froment 198 1; Puri and Mamers 1983; Ritter and Campbell 1991; McDonald et al. 1983; Williams 198 1). Supercritical fluid ex- traction may also enhance permeability of wood by removing extractives clogging pit membranes or by solubilizing pits to the extent that they deaspirate. Enhancing permeability may be particularly useful in refractory species such as Douglas-fir, in some populations of which the combination ofreduced pit numbers and aspiration has a dramatic effect on overall permeability (Krahmer 1961). Removal of extractives or encrustations markedly enhances both gas and liquid per- meability (Stamm 1932, 1963; Anderson and Fearing 1960; Miller 196 1; C6t6 1963; Thom- as and Nicholas 1966; Comstock 1967). A por- tion of this enhancement may result from in- creased capillary movement through the wood cell walls, but such movement is probably overshadowed by the larger improvements in pit permeability (C6t6 1963). Permeability can also be enhanced by steaming, which appar- ently softens pits to the point that they deas- pirate (Nicholas and Thomas 1968; Nary- anappa and Sharma 1990); but such treat- ments can have negative effects on other prop- erties of some wood species. Supercritical carbon dioxide (SCF-CO,) has been used to explode lignocellulose prior to pulping, thereby enhancing the penetration of pulping chemicals. Although these effects sug- gest that such treatments could markedly open wood structure and improve permeability (Pun and Mamers 1993), earlier studies found no evidence of altered wood structure after SCF- CO, treatment at 100 C and 27.6 MPa (Ritter and Campbell 199 1). As part of a larger study evaluating SCF im- pregnation of wood, the trials described in this paper were performed to better quantify the effects of SCF treatment on wood permeabil- ity. MATERIALS AND METHODS One hundred defect-free dowels (1 3 mm in diam. by 10 mm long) of Douglas-fir heart- wood (Psuedotsuga menziesii (Mirb.) Franco) were cut so that the long axis was oriented radially. The dowels were conditioned to 15% moisture content. Specific superficial gas per- meability of each dowel was determined at 0.138, 0.207, 0.276, and 0.414 MPa with a flow-measurement apparatus (Fig. 1). Each dowel was inserted in a rubber stopper to help direct air flow, and air from a storage cylinder was introduced at the desired pressure at one end of the dowel. Inlet pressure was measured by connecting a differential mercury manom- eter, while outlet pressure was measured with a water manometer. Air flow rate was mea- sured with a soap-film flow meter similar to that described by Siau (1971). Higher flow rates were measured with a digital flow meter (Mc- Millan 3 10-3). Superficial air permeability, Ig, was determined at the selected pressures by using Darcy's Law as modified by the Klin- kenberg equation (Siau 1984). The intercept and slope of the regression line of Ig vs. recip- rocal mean pressure were used to calculate per- 298 WOOD AND FIBER SCIENCE, JULY 1995, V. 27(3) TABLE 1. Variables evaluated for effects of SCF treatment on average wood weight loss andpermeability change. Tnsf- Treatment mmt Prcaswc Tmtment T e m m t w c  Buid density Methanol Avg. wwd A a ,  prmcability no. MPal time (mi") ?0 (Urn') addcd vt. loaa change (%). no 2.58 yes 2.92 no 3.26 yes 3.35 no 3.17 no 3.03 yes 2.73 no 3.70 yes 3.37 10 30 60 80 0.747 no 3.24 V ~ U C I  n k t  means of 10 dowels p r  tnsment. Pcrmcability ch- fo l lod  by an ~ t s i a k  significant at u - 0.05 by a W a t .  meability in the absence of slip and a slip fac- tor. The initial permeability measurements were used to segregate 100 dowels into 20 groups of 5 with approximately similar average per- meabilities. The dowels were then treated, 5 at a time, with SCF-CO, with or without 3 mole % methanol at various combinations of pressures, times and temperatures (Table 1). (Methanol was included because its presence increased the solubility of many nonpolar ma- terials in SCF-CO,.) Extraction was performed with an Isco Series 2000 SCF extraction sys- tem consisting of a dual-chamber extraction module, a 260-ml-capacity CO, pump, a 100- ml-capacity cosolvent pump, and a controller. Carbon dioxide continuously flowed through the extraction vessel at a rate of 0.8 glmin, and the average SCF-wood volume ratio was 3: 1. After treatment, the dowels were recondi- tioned to a stable moisture content at 21 C and 65% relative humidity and weighed; gas per- meabilities were then measured under the same temperature/humidity conditions as described above. Permeability values before and after treatment provided a measure of the potential effects of SCF treatment on wood structure. The resulting changes in permeability were subjected to an analysis of variance, and dif- ferences between initial and final permeabili- ties in individual treatments were subjected to t-tests at a < 0.05 (Steel and Tome 1980). RESULTS AND DISCUSSION Supercritical fluid treatment increased spe- cific superficial gas permeability by an average of 133%, with a maximum increase of nearly 600% (Fig. 2). These changes were significant in 8 of 10 treatments (Table 1). Approximately 19% of the dowels had decreased permeability after the treatment. The reasons for this de- crease are unclear, although they may reflect redeposition of solubilized extractives on the pit membranes as the temperature or pressure regimes dropped below the critical point (3 1.1 Cosolvent - + - + - -  + - - + Time(min) 30 30 30 30 60 30 30 30 60 30 Ternp.(DC) 40 40 40 40 80 80 80 80 80 80 Pressure(MPa) 20 20 30 30 20 20 20 30 30 30 0 1 2 3 4 5 6 7 8 9 1 0  Treatment No. FIG. 2. Permeability changes of Douglas-fir dowels treated with SFC-C02 at selected temperatures and pres- sures with or without methanol as a cosolvent. Demersie el a/-SUPERCRITICAL FLUID TREATMENT OF DOUGLAS-FIR 299 TABLE 2. Analysis of variance of permeability changes in Douglas-fir heartwood dowels exposed to SC-C02 under differingpressure and temperature regimes with or without methanol as a cosolvent. Source of variation Svm of quara  df Mean quare F-ratio Sib level Main effects 1.8828-5 3 6.2738-6 0.937 0.427 Permeability ,pressure 9.4798-8 I 9.4798-8 0.014 0.907 Permeability.temperature 6.7338-6 1 6.7338-6 1.005 0.319 Permeability .cosolvent 9.7208-6 1 9.720E-6 1.452 0.232 Inte~actions 2.242E-5 3 7.474E-6 1.116 0.348 Permeability.pressure. temperature 2.61 18-6 1 5.6118-6 0.838 0.373 Permeability .pressure.cosolvent 1.9228-5 1 1.9228-5 2.870 0.095 Pe~meability ~temperature.cosolvent 4.174E-8 1 4.174E-8 0.006 0.938 Residual 4.881E-4 73 6.6968-6 Total (corn.) 5.3018-4 79 C, 7.353 MPa atmospheres for pure CO,). They may also represent reaspiration of pit mem- branes after treatment. On the average, the dowels lost 3.32% of their weight during SCF treatment, a value comparable to that found with SCF extraction of pine or birch (Li and Kiran 1988). Permeability changes appeared unrelated to extraction weight losses, suggest- ing that the former reflected more subtle SCF effects on wood structure (Table I). Changes in temperature, pressure, and treat- ment time had variable effects on permeabil- ity, probably because wood permeability is it- self naturally variable. High temperatures gen- erally reduced any increases in permeability, even though high temperatures resulted in a slight decrease (4.2%) in the percentage of dow- els in which permeability decreased after treat- ment. The ANOVA, however, indicated that changes in neither temperature nor pressure significantly affected permeability (Table 2). Because of the complexity of the extractives in Douglas-fir heartwood, it was not possible to determine either the optimum conditions for extraction or which extractives affected permeability. It is possible that elevated tem- peratures reduced extractive removal, thereby reducing any improvements in permeability. Similar decreases in extractive removal with increased temperatures were observed in pon- derosa pine extraction (Ritter and Campbell 199 I). Extractive levels in Douglas-fir are low in comparison with those in such species as southern pine. Thus, more significant changes under different extraction regimes may be seen with these species. The use of a cosolvent produced some im- provement in permeability, especially at 80 C. Cosolvents can produce significant improve- ments in solubility of a wide array of com- pounds (Dobbs et al. 1986; Van Alsten 1986). While it is possible that methanol was not an appropriate cosolvent for extractive removal, alcohol refluxing is a standard method for re- moving extractives from wood. Perhaps higher concentrations in SCF-CO, would have boost- ed the improvements in permeability. Further trials with other cosolvents and treatment con- ditions will be required to understand these effects more fully. CONCLUSIONS The results indicated that SCF-CO, signifi- cantly improved permeability in a majority of Douglas-fir dowels tested. The improvements appeared to have little relationship to changes in treatment pressure, temperature, or time under the experimental regimes examined. Further trials to understand the nature of the permeability change better will examine pres- sure release rate, initial extractive level, and microdistribution of redeposited extractives in the SCF-CO, treated wood. 300 WOOD AND FIBER SCIENCE, JULY 1995, V. 27(3) REFERENCES ANDERSON, A. B., AND W. B. FEARINO, JR. 1960. Solvent seasoning of tanoak. Forest Rod. J. 10(5):234-238. C o ~ s r o c ~ ,  G. L. 1967. Longitudinal permeability of wood to gases and non-swelling liquids. Forest Prod. J. 17(10):41-46. CB-, W. A,, JR. 1963. Structural factors affecting the permeability of wood. J. Polymer Sci. (Part C) 2231- 242. DOBES, 1. M., J. M. WONO, AND K. P. JOHNSTON. 1986. Nonpolar co-solvents for solubility enhancement in su- percritical fluid carbon dioxide. I. Chem. Eng. Data 31: 303-308. E c w ~ r ,  C. A., 1.0. VAN hm, AND T. Sro~cos. 1986. Supercritical fluid processing. Environ. Sci. Technol. 20(4):319. FROMEKT, H. A. 1981. U. S. Patent43088200. Dec. 28, 1981. JOHNSTON, K. P., D. G. PECK, AND S. KIM. 1989. Mod- eling supercritical mixtures: How predictive is it? Ind. Eng. Chem. Res. 28:1115-1125. KRWMER, R. L. 1961. Anatomical features ofpermeable and refmctoq Douglas-fir. Forest Prod. J. 11:439-441. LI, L., AND E. KIRAN. 1988. Interaction of supercritical fluids with lignocellulosic materials. Ind. Eng. Chem. Res. 27:1301-1312. MCDONALD, E. C., J. HOWARD, AND B. BEN=. 1983. Chemicals from forest products by critical fluids ex- traction. Fluid Phase Equilib. lO(2-3):337-344. McHuon, M. A,, AND V. J. KRUKONIS. 1986. Super- critical fluid extraction: Principles and practice. Butter- worths, Boston, MA. MILLER, D. I. 1961. Permeability of Douglas-fir in Or- egon. Forest Rod. J. ll(l):L4-16. NARYANAFTA, D., AND S. N. SWAR)*IA. 1990. Longitu- dinal gas permeability of refractory Cmunrina equise- fifolia as affected by steaming and ponding treatments. J. Indian Acad. Wood Sci. 2l(1):5-11. NICHOLAS, D. D., AND R. I. T n o m .  1968. Influence of steaming on ultrastructure of bordered pit membranes in Ioblolly pine. Forest Rod. J. 19(1):57-59. PAIJLU~,  M. E., V. I. KRuKoms, R. T. K ~ R ~ K ,  AND R. C. RFlD. 1982. Supercritical fluid extraction. Rev. Chem. Engr. 1(2):179. PURI, V. P., AND H. M-. 1983. Explosive pretreat- ment of lignocellulosic residue with pressure carbon di- oxide for the production of fermentation extracts. Bioeng. Technol. 293149-3161. Rmm, D. C.,ANDA. G. CAMPBELL. 1991. Supercritical carbon dioxide extraction of southern pine and pon- derosa pine. Wood Fiber k i .  23:98-113. Suu,  I. F. 1971. Flow in wood. Syracuse University Press, Syracuse, NY. - 1984. Transport processes in wood. Springer- Verlag, New York, NY. S r m ,  A. J. 1932. Effect of chemical treatment upon permeability of wood. Ind. Eng. Chem. 2451-52. - 1963: Permeability ofwood to fluid. Forest Prod. J. 13:503507. S r w ,  R. G. D., AND J. H. TORUUE. 1980. Principles and procedures of statistics. McGraw-Hill, New York NY. THOMAS, R. I., AND D. D. NICHOLAS. 1966. Pit mem- brane structure on loblolly pine as influenced by solvent exchange drying. Forest Rod. I. l6(3):53-56. VAN&, J. GI 1986. Structuraland functional effects in solutionswitb ~ u r e  and entrained-doped supercritical solvent. Pb.D. dissertation, Illinois Institute of Tech- nology, Chicago, IL. WILLIAMS, D. F. 198 1. Extraction with supercritical gas- es. Chem. Eng. Sci. 36:1769-1788. 

Supercritical carbon dioxide treatment: Effect on permeability of Douglas-fir heartwood

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Supercritical carbon dioxide treatment: Effect on permeability of Douglas-fir heartwood

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