Ind. Eng. Chem. Res. 1988,27, 1301-1312
1301
Interaction of Supercritical Fluids with Lignocellulosic Materials Lixiong Li and Erdogan Kiran* Department of Chemical Engineering, University of Maine, Orono, Maine 04469
T h e extent of dissolutions and the ease of precipitations of the dissolved fragments from a variety of lignocellulosic model compounds and wood species in single-component and multicomponent supercritical fluids have been studied. The behavior of D-glucose, D-Xylose, xylan, arabinogalactan, a-cellulose, kraft lignin, red spruce, sugar maple, and white pine in supercritical carbon dioxide, ethylene, nitrous oxide, n-butane, ammonia, and methylamine has been examined. T h e extent of dissolution of red spruce has been further investigated in binary mixtures of carbon dioxide-ethanol, carbon dioxide-water, carbon dioxide-sulfur dioxide, nitrous oxide-methylamine, ethylene-ammonia, ammonia-water, and ethanol-water and in a ternary mixture of carbon dioxide-water-ethanol. The residues and dissolved fractions following extractions have been characterized by chemical, spectroscopic, and thermal techniques. Supercritical fluids and their use as process solvents in a wide range of application areas including natural products, biochemicals, food, pharmaceuticals, petroleum, fuel, polymers, and specialty chemicals industries have been discussed in a number of recent review articles (Williams, 1981; Paulaitis et al., 1983a; Randall, 1982) and monographs (Schneider et al., 1980; Paulaitis et al., 1983b; Penninger et al., 1985; McHugh and Krukonis, 1986). These fluids offer unusual possibilities for selective extractions and fractionations, separation and purification, material deposition and impregnation, nucleation and particle size regulation, and chemical reactions and syntheses. Some specific examples are decaffeination of coffee, deodorization of oils and fats, liquefaction of coal and biomass, and fractionation of polymers. Literature on the behavior of lignocellulosic materials in contact with supercritical fluids is not extensive. The majority of the publications (Table I) deal primarily with liquefaction or gasification of wood via pyrolysis under supercritical conditions. For example, supercritical fluid extraction of spruce wood using organic solvents, such as acetone, tetrahydrofuran, dioxane, and toluene have been studied as an alternative to distillation of wood (Calimli and Okay, 1978,1982). I t has been reported that use of supercritical acetone can result in complete cellulose liquefaction with minimal char formation (Koll et al., 1979). Liquefaction studies using various organic fluids including acetone, ethyl acetate, and alcohols with birch wood have shown that both the carbohydrates and lignin undergo degradation (Koll et al., 1979). Supercritical methanol has also been studied in conjunction with liquefaction of aspen and western red cedar wood species (Labrecque et al., 1984). Pyrolysis pathways of lignins under supercritical conditions have also been reported (Lawson and Klein, 1985). Other work on the interaction of supercritical fluids with lignocellulosic materials has focused on extraction of low molecular weight constituents from plant materials (Stahl et al., 1978; Hubert and Vizthum, 1978) or wood (Froment, 1981; Amer, 1983). Extraction of organic substances such as tall oil and turpentine from coniferous woods with carbon dioxide, nitrous oxide, nitrogen, lower alkanes such as methane, ethane, and propane, and lower alkenes such as propylene are reported in the patent literature (Froment, 1981). Another patent describes a process for extraction of lignin from sulfite pulping liquors (Avedesian, 1985). Separation of alcohols and hydrocarbons from tall oil soaps (a byproduct of pulping operations) has also been
* To whom correspondence
should be addressed.
explored (Amer, 1983). Extraction of resin and fatty acids from wood and extraction of wax from wood bark with propane, nitrous oxide, carbon dioxide, and ethylene have been reported (McDonald et al., 1983). Studies with other plant materials include extraction of menthol from peppermint leaves (Stahl et al., 1978) and nicotine from tobacco (Hubert and Vitzthum, 1978) using supercritical carbon dioxide. Among other studies are the hydrolysis of wood cellulose to glucose in water-sulfur dioxide systems near or above the critical region (Vick Roy and Converse, 1985) and the extraction of spruce wood with methylamine-water binary systems for lignin removal (Beer and Peter, 1985, 1986). There is a need to study both the fundamental and applied aspects of interactions of supercritical fluids with lignocellulosic materials, in particular wood and its constituents. Understanding the behavior of compounds of varying complexity ranging from relatively simple model compounds (such as glucose) to complex polymers (such as lignin) or multicomponent polymeric networks (such as wood) or solutions (such as black liquors) is valuable for applications in the pulp and paper and natural products industries. Of particular interest are the solubility, stability, and reactivity of these compounds in various supercritical fluid systems. From a practical perspective, such a data base may eventually lead to the development of novel processes which will permit effective utilization of wood and other lignocellulosic materials as a source of chemicals, pulp, and energy (Kiran, 1986a, 1987a,b). The present paper is focused on the interaction of supercritical fluids and lignocellulosic materials. It describes our general methodology, experimental system, and the results on the behavior of various model compounds and wood species in single-component and multicomponent supercritical fluid systems.
General Methodology Chemically,wood consists of cellulose, lignin, and hemicelluloses (which are all polymeric molecules), low molecular weight extractives, and some inorganic matter (Sjostrom, 1981). It may be anticipated that, by changing the solvent properties of supercritical fluids in a controlled manner, sequential extractions of all wood constituents starting with the low molecular weight extractives could in principle be achieved. However, among the polymeric constituents, lignin is a nonlinear aromatic polymeric network which is cross-linked and does not dissolve in any known solvent without first derivatization or breakdown. For applications such as pulping where lignin removal is essential, our methodology is to use a binary or ternary
0 1988 American Chemical Society 0888-588~/88/262~-1301~01.50/0
1302 Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988
supercritical fluid system in which at least one component is capable of reacting with lignin and the others capable of dissolving and carrying the reaction products. Those fluids and operational conditions that will give high selectivity toward lignin without excessive degradation of cellulose and hemicellulose fractions are specially desirable for pulping and bleaching operations. Use of high temperatures to cause fragmentation of lignin as is done in supercritical pyrolysis studies would however not be desirable if the objective is pulping, since at temperatures greater than 250 "C cellulose degradation becomes appreciable (Kiran, 198613). Hemicellulose degradations start even at lower temperatures. As a result, single-component supercritical fluids with critical temperatures higher than 200 "C would not be suitable, if thermal degradations of the carbohydrate fractions are to be minimized in the process. However, the critical temperature may be lowered by using binary solvent mixtures. Our interest in this study is initially directed to exploring fluids and conditions which may permit dissolutions and extractions with minimal chemical and thermal damage to wood constituents. As single-component fluids, we have therefore started our investigations using those fluids with critical temperatures less than 200 "C. Some of these fluids have reactive capabilities. The behavior of model compounds in these fluids is being studied in terms of the extent of dissolutions and degradations, which may then be used to establish the operational conditions for selective removal of desired constituents from complex networks such as wood. Carbon dioxide (T,= 31.1 "C, P, = 73.9 bar, and pc = 0.468 g/cm3), ethylene (T, = 9.3 "C, P, = 50.4 bar, pc = 0.217 g/cm3), nitrous oxide (T,= 36.5 "C, P, = 72.4 bar, pc = 0,457 g/cm3),ammonia (T, = 132.8 OC, P, = 112.8 bar, pc = 0.234 g/cm3), methylamine (T, = 156.9 "C, P, = 74.3 bar, p c = 0.222 g/cm3), and n-butane (T, = 152.1 "C, P, = 38.0 bar, p c = 0.228 g/cm3) were selected for initial studies (the critical data are from Reid et al. (1987)). These fluids were chosen for the following reasons: carbon dioxide, ethylene, and nitroux oxide have low critical temperatures and have already been reported in extraction of low molecular weight extractives from wood (McDonald e t al., 1983; Froment, 1981; Amer, 1983). Ammonia and methylamine have been evaluated in some conventional pulping processes (O'Connor, 1972; Thillaimuthu, 1977; Obst, 1981; DeHaas and Lang, 1974). Methylaminewater binary mixtures have been studied for removal of lignin at high pressures (Beer and Peter, 1985). As a supercritical fluid, n-butane is known to dissolve polymers such as polystyrene (Saraf and Kiran, 1987) and polyethylene (Erlich and Kurpen, 1963). Furthermore, these fluids differ in their molecular size, polarity, reactivity, and critical properties, thereby permitting the study of these factors on the dissolution of various solutes. As for binary and multicomponent fluids, many combinations have been considered. In the present paper, the focus is on carbon dioxide-ethanol, carbon dioxide-water, carbon dioxide-sulfur dioxide, ethylene-ammonia, ammonia-water, ethanol-water, and methylamine-nitrous oxide binary, and carbon dioxide-water-ethanol ternary mixtures. Among the wood species, softwoods such as red spruce and white pine and hardwoods such as sugar maple have been investigated. They differ in their chemical compositions especially with respect to their hemicellulose and extractives contents. As for the model compounds, initial emphasis has been placed on those which are representatives of major wood constituents, namely a-cellulose, D-
SOLVENT ELIVERY SYSTEM IsOSll
SEPARITW TRAP
TCS2
OVEN 2
Figure 1. Experimental system for high-pressure-high-temperature supercritical fluid extraction. LF = line filter; PG = pressure gauge; SV = shut-off valve; CV = check valve; PHC = preheating coil; TC = thermocouple; FR = flow-through reactor; P/T = pressure transducer/thermocouple; TCS1, TCS2 = temperature control for oven 1, 2 (RTD sensor); MV = micrometering valve; TP = trap. Subscripts: r = reactor; 1, 2, 3 = trap 1, 2, 3.
glucose, xylan, D-xylose, arabinogalactan, and kraft lignin. Experimental Section A. Materials. Red spruce (Picea rubens),eastern white pine (Pinus strobus), and sugar maple (Acer saccharum) were obtained locally and used in the form of sawdust collected from l-mm sieve. D-Xylose, D-glucose, and arabinogalactan were obtained from Aldrich Chemicals, xylan and a-cellulose were obtained from Sigma Chemicals, and kraft lignin (Indulin AT) was obtained from Westvaco. They were used as received. The extraction fluids, ethylene (MG Scientific, 99.5 wt % purity), carbon dioxide (MG Scientific, 99.8 wt 70purity), nitrous oxide (Airco, 99.9 w t '70 purity), ammonia (Northeastern Ammonia Co., 99.99 wt % purity), n-butane (Matheson Air Products, 99.5 wt % purity), methylamine (Linde Specialty Gases, 98.0 wt '70 purity), and sulfur dioxide (Airco, 99.9 wt % purity), were used without further purification. B. Extraction System. The flow-through extraction system used in this study is shown in Figure 1. The system can be operated at pressures up to 400 bar and temperatures up to 200 "C. It consists of solvent delivery systems (SDS1, SDS2, SDS3), an extractor/reactor (FR), a set of separator traps (TP1, TP2, TP3) involving sequential pressure reduction stages, and the appropriate temperature and pressure control and readout units. The solvent delivery system involves two Milton Roy duplex pumps capable of handling up to four different fluids. The pump heads have been modified to circulate a coolant which permits efficient pumping of the fluids. The extractor, made of 316 stainless steel, is a high-pressure tubular reactor with 19-cm3internal volume (15.24 cm long, 2.54 cm o.d., and 1.26 cm id.), obtained from High Pressure Equipment Co. It is rated at 1000 bar. The extractor is positioned in a heated oven, the temperature of which is monitored and regulated by a RTD controller. The extraction fluid before entering the extractor passes through a sufficient length of tubing which is inside the extractor oven and acts as a preheater. The temperature of the fluid is monitored by thermocouples before, during, and after the extractor. The pressures before the extractor are measured with gauges obtained from Augoclave Engineers. The pressures after the extractor, however, are measured by using special flush-mount transducers ob-
Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 1303 Table I. Dissolution of Wood Components in Supercritical Fluids solvent extraction supercritical properties conditions wood species solvents T,,O C P,,bar T,O C P,bar 47.6 250 81 spruce acetone 235.5 52.6 290 81 THF 267 314 330 91 52.1 dioxane 340 81 42.2 320.8 toluene 192.6 250 101 birch diethyl ether 36.6 250 101 196.6 n-pentane 34.1 250 101 235 2-propanol 48.2 250 101 235.5 acetone 47.6 250 101 240 80.6 methanol 250 101 243 64.6 ethanol 270 101 250.4 38.8 ethyl acetate 270 101 42.5 263 2-butanol 270 101 263.6 1-propanol 52.4 280 101 2-methyl-1-propanol 277 43.6 31 40 81 73.9 pine carbon dioxide 260 100 47.6 western red cedar acetone 235.5 350 100 350 280 methanol 240 260 100 79.5 100 350 350 100 southern pine propane 96.7 42.5 105 210 12.7 45 210 nitrous oxide 36.5 45 620 15 210 ethylene 9.9 51.2 15 620 73.9 carbon dioxide 31 40 210 40 620 78.8 170 79.2 90% birch sulfur dioxide 100.4 180 87.4 10% maple +water 374.1 221.2 74.6 spruce 80% methyl amine 157 +20% water 374.1 221.2 180 100
tained from Dynisco. The flush diaphram with these transducers eliminates contamination or plugging from process stream which may be a concern with ordinary gauges. The readings from the pressure gauges and transducers are checked against a calibrated Heise gauge. The pressure of the extraction fluid is measurable with an accuracy of rt0.3 bar. The temperature is read with an Omega digicator with an accuracy of rt0.5 O C . The traps are made of 316 stainless steel. Each trap has an internal volume of about 35 cm3. The sealing is achieved by an inner cone and a Teflon washer positioned by a screw cap as shown in the enlarged insert in Figure 1. Inlet and outlet connections are standard High Pressure Equipment Co. type fittings connected onto the inner cone. The temperature and the pressure after each trap are monitored by using thermocouples and Dynisco flushmount pressure gauges. The pressure in each trap is regulated with high-pressure-high-temperature micrometering valves obtained from Autoclave Engineers. All the traps, valves, and gauge connections are housed in a heated oven. C. Operational Procedure. An accurately weighed sample (about 2 g) is placed in the reactor between two glass wool plugs which help position the sample and prevent particulates from getting into the flow lines. The reactor is then connected to the fluid lines. Before pumping the extraction fluid, the temperature of the extractor oven and the trap oven are brought to the desired set values for that particular solvent system. The whole system is then flushed by the solvent flow at low pressures. The opening of the micrometering valve MV3 (Figure 1) is then reduced to its minimal value, and the pumping of the fluid is started. When the pressure reading a t trap 3 becomes greater than a set value, the opening of the micrometering valve MV2is reduced to its minimal value, and pressure is built until the set value for trap 2 is achieved.
weight loss, % 35 62 73 68 20.8 24.3 24.6 22.8 25.3 21.8 36.8 31.0 32.5 39.6 3.9 41 15 91 28 12 96 18.8 6.8 14.2 2.3 6.9 2.1 5.6 89.2 95.4 40.0
carbohydrates loss, %
lignin loss, %
30 38.9 23.4 23.7 20.2 16 43 29.3 26.9 35.3
0 0 30 18.6 47.5 46.9 9.4 38.5 56.7 58.8
reference Calimli and Olcay, 1978
Koll et al., 1979
Froment, 1981 McDonald et al., 1983
McDonald et al., 1983
Vick Roy and Converse, 1985 Beer and Peter, 1985
In a similar fashion, the settings of the micrometering valves MV1 and MV, are adjusted to achieve the desired pressures in trap 1and the extractor. The highest pressure in the system is maintained in the extractor. The lowest pressure is maintained in the last trap which is often set at atmospheric pressure. The extraction is continued for a desired period of time under the set pressure and temperature conditions in the extractor and traps. At the end of the experiment the reactor and the traps are opened and the weight loss of the initial solute is determined. The amount and the physical form of the precipitates collected in the traps are noted. The residue and the precipitates are analyzed by chemical, thermal, and spectroscopic procedures. D. Characterization Procedures. Chemical Analysis. Wood species before and after extraction were analyzed for their acid-insoluble lignin content (Klason lignin), using a modified procedure suitable for small sample size (ca. 200 mg) described in the literature (Effland, 1977). Thermal Analysis. Thermogravimetric analyses were carried out with a Du Pont 951 TGA unit, under flowing nitrogen (60 cm3/min) conditions and a heating rate of 20 OC/min. Typically, 10-mg samples were used. Spectroscopic Analysis. A Digilab FTIR spectrophotometer (Model FTS-60) was used to obtain IR spectra of samples before and after extractions. Standard KBr pellets containing 1%by weight sample were used. Results a n d Discussions A. Dissolution. Single-Component Fluids. These results of the extraction studies with single-component supercritical fluids are presented in Tables 11-V, in four categories, namely, monomers (xylose and glucose), hemicelluloses (arabinogalactan and xylan), cellulose and lignin, and wood species (red spruce, sugar maple, and white pine). The tables include extraction and precipitation
1304 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table 11. Dissolution of Xylose and Glucose operational conditions fluid
results observations trap 1 trap time, Gi&iXG, T, "C P, bar T, "C P, bar h 70 (P2) Solute: Xylose 1.10 5.39 34 152 34 1 2 0.12 R: no change in color or physical form. P I , P2: none 1.03 3.73 38 69 38 10 2 0.82 same as above 1.07 2.42 134 152 134 7 2 68.1 R: dark brown solid. P1: very little, dark brown solid soluble in water. P2: substantial, dark brown solid 1.09 4.16 161 145 161 7 2 75.4 R: dark brown solid, no longer powdery. PI: little, dark brown solid. P2: substantial, dark brown solid
extractor T, "C P, bar T," P,"
CZH,
39
276
COZ NH3
40 161
276 276
NH2CH3 196
310
C2H4
42
283
1.11 5.52
34
145
34
COZ COZ NH3
66 154 161
307 304 280
1.11 4.16 1.11 4.11 1.07 2.45
66 161 138
158 138 158
66 161 138
1 3 4
2 2 2
-0.0 -0.0 98.7
NHzCH3
204
297
1.11 3.97
153
145
153
7
2
86.9
Solute: Glucose 1 2 -0.0
R: no change in color or physical form. PI, P2: none same as above R light brown cake. P1, P2: none R: dark brown crust. P1: viscous, dark brown. P2: substantial, viscous, dark brown R: almost black powder. P1: dark brown powder. P2: viscous, dark brown
T,= reduced temperature ( T / T , ) ;P, = reduced pressure (P/P,). Table 111. Dissolution of Arabinogalactan and Xylan operational conditions
41
280
results trap 1 trap time, P. T. "C P. bar T. "C P. bar h % Solute: Arabinogalactan 1.11 5.46 34 152 34 1 2 0.17
COP NH3
64 141
276 276
1.11 3.74 1.02 2.42
68 136
138 152
68 136
NH3
140
290
1.02 2.54
139
158
NH2CH3 197
303
1.09 4.07
158
C2H4
40
276
1.11 5.39
NzO NH3
76 144
306 276
NHzCH3
197
n-C4H,0
159
fluid C2H4
iZ&ZG,
extractor T."C P. bar T.
1 4
2 2
1.8 64.8
139
7
1.17
31.2
152
158
7
2
80.3
34
159
34
1.13 4.27 1.03 2.42
53 137
116 159
53 137
1 4
2 2
4.3 15.9
287
1.09 3.88
159
152
159
7
2
34.9
345
1.02 9.07
159
138
159
34
2
0.6
Solute: Xylan 1 2 -0.0
conditions, extraction time, total dissolution (weight loss based on initial material), and qualitative observations on the nature of the residues after extraction and the precipitates collected in the traps. No attempt was made to monitor the noncondensables. The majority of these initial tests were of a screening nature, therefore extractions were conducted for a fixed time period, and only qualitative assessment of the precipitates was carried out. For some selected system as will be discussed later in the paper, the effect of extraction time, temperature, and pressure was further studied, and residues and precipitates were quantitatively evaluated. Table I1 summarizes the results for the monomeric small compounds D-xylose and Dglucose which are characteristic building blocks of xylan and cellulose. In order to take advantage of the largest density changes that occur near the critical temperature, the extraction temperatures have been maintained at about 1.1times the critical temperature of the respective fluids. The extraction pressures have been maintained around 300 bar. The pressures in the first and second traps have been maintained at around 150 bar
observations
< (P2)
~
~~
R, no change in color or physical form; P1, P2, none same as above R, dark brown solid; P1, little, dark brown solid; P2, Substantial, dark brown solid R, brown solid; P1, little, brown solid; P2, some, dark brown solid R, dark brown, appears to have been sintered; P1, little, dark brown solid; P2, substantial, dark brown solid
R, no change in physical form or color; P1, P2, none same as above R, light brown solid, no longer in powder form; P1, P2, small amount solids R, yellowish brown solid; P1, trace amount; P2: none R, light yellowish powder; P1, P2, none
and near atmospheric pressures, respectively. Under these operational conditions, the results show that these sugars undergo no appreciable dissolution in ethylene or carbon dioxide. However, in ammonia and methylamine, the dissolutions are very high. Glucose undergoes almost complete dissolution. Significant changes in color are observed which suggest chemical transformations. It is to be noted that, in the case of ammonia or methylamine, the extraction temperatures were higher than the melting temperatures of D-glucose (153-156 "c)and D-xylose (156-158 "C) which may be a factor in the observed high degree of dissolutions. We examined the temperature effect by conducting extractions at high temperatures (at 154 "C) with carbon dioxide. There were no precipitates in the traps, indicating lack of dissolution of glucose in carbon dioxide. Additional data with other solutes described below also show that the different behavior in ammonia and methylamine is not merely due to the temperature of extraction. Table I11 shows the results with arabinogalactan and xylan which are hemicelluloses. Xylan is the most abun-
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1305 Table IV. Dissolution of a-Cellulose and Kraft Lignin operational conditions results observations extractor trap 1 trap time, weightloss, fluid T, OC P, bar T, P, T, "C P, bar T, "C P, bar h % (P2) Solute: a-Cellulose CZH4 41 276 1.11 5.39 34 145 34 1 2 -0.0 R, no change in color or physical form; P1, P2, none same as above 124 1 2 0.13 276 1.12 3.74 67 67 same as above COP 68 303 1.13 4.17 53 158 1 2 3.6 53 same as above NZO 78 152 7 15 16.7 138 R, light brown solid; P1, especially; P2, dark, 185 241 1.13 2.12 138 NH3 viscous, soluble in 0.1 M NaOH soln 122 465 7 2 15.7 R, light brown solid; P1, little dark solid; P2, NHzCHB 193 303 1.08 4.06 165 substantial, dark brown solid; both P1 and P2 soluble in water n-C4Hi, 157 345 1.01 9.07 157 138 157 41 2 4.5 R, no change in physical form or color; P1, P2, none ~
41 37 76 188
276 276 303 276
1.11 1.02 1.13 1.14
5.39 3.73 4.17 2.42
33 37 53 138
159 83 83 152
NH2CH3 197 TZ-C~H~,160
310 208
1.09 4.16 1.02 5.44
165 160
145 104
CZH4
COP NZO NH3
Solute: Kraft Lignin, Indulin AT 1 2 -0.0 33 14 2 5.3 37 1.8 1 2 53 7 1.5 14.7 138 165 160
7 34
2 2
16.7 8.8
same as above same as above same as above R, black powder, P1, viscous, dark brown; P2, brown solid R, black solid cake; P1, P2, dark brown powder R, no change in physical form or color; P I , P2, none
Table V. Dissolution of Wood Species, Red Spruce, Sugar Maple, and White Pine operational conditions results observations extractor trap 1 trap 2 weight residue (R); precipitate, 1st trap (Pl), 2nd trap fluid T, "C P, bar T, P, T , "C P, bar T , OC P, bar time, h loss, % (P2) Solute: Red Spruce 159 34 1 2 -0.0 R: no change in color or physical form; P1, P2; 40 276 1.11 5.39 34 CZH, none 148 7 1.5 6.2 118 R, orange brown; P1, P2, none CzH4 182 207 1.61 4.04 148 158 66 66 1 2 -0.0 R, no change in physical form or color; P1, P2, COZ 66 314 1.11 4.25 none 3.7 151 2 2 159 R, orange brown; P1, P2, none COZ 170 290 1.46 3.92 151 54 1 2 1.5 54 103 R, no change in physical form or color; P I , P2, NZO 77 269 1.13 3.69 none 142 7 1.5 15.9 152 R, black; P, viscous, dark brown material; P2, 190 276 1.14 2.42 142 NH3 dark brown solid 10.4 158 7 2 NH2CH3 145 297 0.97 3.97 158 159 R, brown; P1, P2, brown solid 161 7 2 15.6 159 R, brown; P1, trace; P2, dark brown solid 290 1.01 3.88 161 NH2CH3 163 24.3 165 7 2 145 R, brown; P1, P2, dark brown solid 317 1.09 4.25 165 NH2CH3 197 159 5 2 6.5 R, no change in physical form or color; P I , P2, 159 n-C4H," 170 297 1.04 7.80 159 none 145 164 3 2 R, light brown; P1, P2, small amount, light 310 0.845 4.86 164 CzHbOH 163 brown solid 152 157 3 2 11.0 190 297 0.897 4.64 157 152 170 297 0.685 1.34 150 3 2 30.0 R, light brown; P1, P2, beige s o h 150 H20 ~~
CZH4 COZ NH3 NH,CH,
co2 NHZCHS
39
276
1.10
5.:39
34
152
34
66 150 195
317 293 310
1.11 1.04 1.09
4.30 2.62 4.16
66 145 164
165 157 152
66 145 164
68
310
1.12
4.21
66
138
66
169
290
1.02
3.88
162
159
162
Solute: Sugar Maple 1 2 -0.0 1 8 7
2 2 2
-0.0 12.2 16.7
Solute: White Pine :3 2 0.5 14
dant hemicellulose associated with hardwoods (Sjostrom, 1981). In a way similar to the behavior of xylose and glucose, these compounds do not show appreciable dissolutions in ethylene or carbon dioxide. The dissolution of xylan in nitrous oxide is also small. Dissolutions in ammonia or methylamine are significant, especially for arabinogalactan. The extracted portion of xylan with methylamine appears to be volatiles which are noncondensable a t the temperatures of the traps (at about 159 OC). The color changes of these solutes in ammonia or methylamine extraction also indicate chemical transformations.
2
21.1
R, no change in physical form or color: P1, P2. none same as above R, dark brown; P1, P2, brown solid R, dark brown, P1. viscous, dark bruun. P2, dark brown solid
H.no change in physical form
or iulor; P1, P2,
none R, black; P1, trace; P2, dark brown
The n-butane extraction data are interesting in that, even though the temperature of the extraction is much higher than that of ethylene or ammonia extraction, the extent of dissolution is small, indicating that the high degree of dissolution that is observed with ammonia is not a simple consequence of temperature (i.e., increasing volatility) and most likely involves chemical reactions. Table IV summarizes the dissolution data for a-cellulose and kraft lignin (Indulin AT). The extent of dissolution of a-cellulose in ethylene, carbon dioxide, nitrous oxide, and n-butane is not appreciable. The behavior of kraft
1306 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 THERMOGRRVIMETRIC RNRLYSIS
THERMOGRRVIMETRIC RNRLYSIS
?
\ ,
10.3 U
>
g
20;
4...
LL
60
0
!20
180
240
300
360
420
600
540
480
TEMPERATURE ( " C 1
Figure 3. TG behavior of the model compounds and red spruce. Initial weights have been normalized to 100% at 105 "C (the weight losses below 105 "C are not shown in this plot but can be seen in Figures 2 and 4). THERMOGRAVIMETRIC ANALYSIS I
.U
I - >
A n I
cl
\
X I
i Red spruce
Rrobinogalocton
, L -
O
60
120
180
240 300 380 420 TEMPERRTURE ( * C 1
480
540
6OU
Figure 4. Derivative T G plots for the model compounds and red spruce of Figure 3. The plots have been vertically displaced for clarity. The small peaks below 100 OC represent moisture removal.
of 4 h, the total weight loss is only about 11.0%. In contrast, extraction with water at T , = 0.685 leads to much greater (ca. 30%) dissolution. Table V includes some data for white pine samples. Similar to red spruce and sugar maple, white pine shows substantial weight loss when subjected to methylamine extraction and no measurable weight loss in carbon dioxide. It should be noted that the present data on red spruce and white pine conform with the literature values shown in Table I. Even though different species are involved, and different experimental setups and procedures are used, it can be stated that the dissolutions of wood in carbon dioxide, nitrous oxide, and ethylene are not significant. The present data involving other fluids have been obtained at temperatures lower than those indicated in Table I, and thus are not compared. As was mentioned earlier, previous literature deals primarily with supercritical liquefaction of wood, and this is why most of the fluids in Table I are fluids with high critical temperatures, and extraction conditions are above the thermal stability of wood. In the present work, temperatures that may lead to thermal decompositions have been avoided.
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1307 Table VI. Dissolution of Red Spruce in Multicomponent Supercritical and Near Critical Solvents" ouerational conditions extractor trap 1 trap 2 results observations fluid mixture T o r T,, Por T, P, T , P, time, weight residue (R); precipitate, 1st trap P,, bar T, P, "C bar OC bar h loss, 5% ( P l ) , 2nd trap (P2) (mole fraction) "C Binarv Fluids 4.8 R, light brown; P1, P2, brown solid C02(0.76)/CzH50H 167 (107) 325 (108) 1.16 2.98 163 i s 5 163 61 2 partially soluble in ethanol 35.9 R, dark brown; P1, brown water CO,(O.39)/HzO 190, 268 290, 1705 0.856 0.168 155 152 155 5 2 suspension, some black deposit 17.5 R, dark brown; P1, dark brown solid, COz(0.88)/H,0 193 (57) 290 (62) 1.41 4.61 157 152 157 7 2 soluble in NaOH solution 2.3 R, no change in physical form or COz(0.87)/S02 76 (51) 283 (85) 1.08 3.30 61 159 61 1 2 color; P1, P2, none 9.9 R, brown; P1, P2, none 155 (51) 276 (85) 1.32 3.22 150 1 2 COz(O.87)/SOz 9.4 185, 68 275, 85 1.34 3.25 185 1 2 R, brown; P1, dark brown solid Nz0(0.74)/NHzCH3 12.1 R, dark brown; PI, some, viscous, 182 (34) 259 (50) 1.48 5.08 148 145 148 7 1.5 CZH, (0.69)/NH3 black; P2, none 15.6 R, brown; P1, viscous, dark brown; NH,(0.93)/HzO 185 (146) 269 (112) 1.09 2.38 142 152 142 7 1.5 P2, dark brown solid coating 185 (343) 290 (158) 0.74 1.81 150 152 150 7 41.4 R, light brown; P1, P2, brown H20(0.91)/CzH,OH 2 suspension H,0(0.52)/CzH50H 193, 275 290, 99 0.85 2.90 152 152 152 7 32.4 R, light brown; P1, P2, brown 2 suspension ~~
C0~(0.958)/Hz0(0.022)/ 190 (43) CZHSOH
290 (76)
1.47
3.76
Ternary Fluids 157 152 157
7
2
19.3
R, dark brown; P1, black, viscous, soluble in NaOH soln; P2, none
"Critical constants of mixed solvents are either from literature (Hicks and Young, 1975) or have been estimated (values shown in parentheses). The values for nitrous oxide-methylamine mixture were measured in this study. THERMOGRflVIMETRIC R N f l L Y S I S
(0
(0
0
t-
I Y
w
3
w
> Y
b(r
>
ternaries involving carbon dioxide, nitrous oxide, ethylene, water, ammonia, ethanol, and methylamine. There is no measurable dissolution of red spruce in the carbon dioxide-ethanol binary mixture under the specified conditions. Two different compositions for the carbon dioxide-water binary mixture have been examined. One of these is high in water content, and extractions have been carried out at subcritical conditions. Significant dissolution of red spruce is observed in this composition, but solvent-free precipitation of the dissolved constituents is not achieved. For the other composition,extraction conditions are above the critical, and precipitates are in the solid form. It is interesting to note that compared to pure water (see Table V) incorporation of some carbon dioxide increases the extent of dissolutions which however decreases if the carbon dioxide content is further increased. The extent of dissolutions in ethylene-ammonia, nitrous oxide-methylamine, and ammonia-water appears to be intermediate to the extent of dissolutions in the individual solvents. Two different compositions, one composition being subcritical, of ethanol-water mixtures have been studied. The extent of dissolutions is greater than expected from the individual solvents as can be noted from a comparison with Table V. In either of these extractions, the precipitates in the traps have been observed to be present as suspensions in the extraction solvents. Extractions with carbon dioxide-sulfur dioxide mixtures were conducted a t two different temperatures. The extent of dissolution from the higher temperature run is similar to that obtained from ethylene-ammonia extraction, but the nature of the residue and the precipitate is different. The low-temperature extraction resulted in no appreciable dissolution. Table VI1 shows some preliminary results of the behavior of a-cellulose and kraft lignin in carbon dioxidewater binary systems. The data are compared with the behavior of red spruce. It is to be noted that, for all these solutes, the dissolutions are much greater than those observed in carbon dioxide alone. Only one ternary system has so far been evaluated. This ternary mixture of carbon dioxide-water-ethanol (Table
d--Sugar Maple
Y
cc w
0
0
60
120
180
240
300
360
420
480
540
600
TEMPERATURE C ' C I
Figure 5. Derivative T G plots for different wood species: red spruce, sugar maple, and white pine.
The issue of thermal stability can be better appreciated from Figures 2-5 which show the thermogravimetric (TG) weight loss and derivative weight loss behaviors of the various wood species and constituent solutes. It is clear from the weight loss curves of Figure 3 or the derivative plots of Figure 4 that above 200 "C cellulose decomposition becomes appreciable. In Figure 4 the plots have been shifted along the y axis for clarity. Hemicellulose degradations start a t even lower temperatures. It is for these reasons that temperatures below 200 OC were maintained in the present extraction studies. Binary and Ternary Supercritical Fluids. As has been discussed earlier in the section of methodology, investigation of the dissolution behavior of lignocellulosic materials in binary or ternary supercritical fluids, in which a t least one of the components have reactive capabilities, is of particular interest. Table VI summarizes the preliminary dissolution data for red spruce with binaries and
1308 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 Table VII. Dissolution of Red Spruce, a-Cellulose, and Kraft Lignin in Supercritical COz-HzO Binary Solvents" oDerationa1 conditions extractor results observations trap 1 P or trap time, weight loss, residue (R); precipitate, 1st trap (Pl), fluid mixture T or T,, (mole fraction) "C P,, bar T, P, T,"C P, bar T,"C P, bar h 7 0 2nd trap (P2) Solute: Red Spruce C02(0.88)/H20 193 (57) 290 (62) 1.41 4.68 157 152 157 7 2 17.5 R, dark brown; P1, dark brown solid, soluble in NaOH s o h Solute: a-Cellulose 152 157 7 2
CO2(0.89)/HzO 190 (55) 290 (63) 1.41 4.60
157
C02(0.88)/H20 195 (57) 290 (62) 1.43 4.68
Solute: Kraft Lignin, Indulin AT 155 158 155 7 2
16.7
R, light brown, part of residue can be dissolved in water
12.3
R, dark brown; P1, P2, dark brown solid, soluble in NaOH s o h
Critical constants of mixed solvents have been estimated (values in parentheses).
VI) is rich in carbon dioxide and results in substantial dissolution as well as solvent-free precipitation. This observation is remarkable in that with very small amounts of water and ethanol the levels of the dissolution achievable with water or ethanol as individual solvents have been made possible. This result is supporting the premise outlined in our methodology that certain combinations of reactive solvents with nonreactive carrier fluids could lead to large dissolutions. B. Further Characterization. The dissolution data discussed above do not provide information on the nature of substances that are extracted. Therefore, further characterizations were carried out. Wood Residues after Extraction. Measurement of the lignin content of the wood after extraction provides information on the compositional changes. The lignin content can be determined by either chemical (such as Klason lignin determination) or spectroscopic procedure (such as IR). Thermogravimetric analyses also provide discriminative information. Variations of lignin content, evaluated by these techniques, after various extractions are discussed below. Table VI11 shows the acid-insoluble lignin (Klason lignin) content of wood residues for some extractions selected from Tables V and VI, identified with the extraction temperatures indicated in parentheses after each solvent. The data are listed in increasing order of the total dissolution. The extraction fluids are subdivided into four general categories, based on both the total dissolution and the Klason lignin levels of the residues. In the first category, extractions with carbon dioxide and ethylene at low temperatures lead to no measurable dissolution. In the second category, even though dissolution is measurable, the lignin content of wood residue after extraction shows only small changes, indicating that both cellulose and lignin are removed in proportional amounts. In the third category, there is substantial dissolution which, however, results in increased lignin contents in the residues, indicating that extractions by these fluids lead to greater removal of carbohydrates. Greater carbohydrates removal by the mixtures of water-carbon dioxide and water-carbon dioxide-ethanol may be interpreted to be a cosequence of acid hydrolysis due to carbonic acid that forms in the system. In the final (fourth) category, high degree of dissolution is accompanied with substantial decrease in lignin content, indicating selective lignin removal. A comparison of the weight loss values (see Tables IV and V) on kraft lignin and a-cellulose indicates nearly equal dissolutions (about 16%) in methylamine. However, the dissolution of red spruce in methylamine is in the range from 10% to 24% (see Table V), depending upon the extraction conditions (from below critical to above critical).
Table VIII. Analysis of Red Spruce Residues after Extractions lignin extraction fluid w t loss, content, (T,"C) 70 % comments" 26.5 initial wood sample -0 28.8 I -0 25.1 I 4.4 29.2 I1 6.2 31.0 I1 9.4 26.3 I1 9.9 28.6 I1 11.0 25.8 I1 12.1 28.8 I1 15.6 23.9 I1 15.9 36.0 I11 17.5 37.4 I11 19.3 38.4 I11 N H F H , (197) H20/C2H,0H (193)
24.3 32.4
13.4 17.4
IV IV
"Type I = no dissolution; type I1 = low degree of dissolution without selectivity; type I11 = intermediate degree of dissolution with preferential removal of carbohydrates; type IV = high degree of dissolution with preferential removal of lignin.
The Klason lignin level of the residue after extraction at 197 "C (13.4%) is much lower than that of the initial wood. This shows that methylamine is particularly selective toward lignin moieties in wood. A reverse situation is observed with ammonia extractions. Dissolutions of a-cellulose, kraft lignin, and red spruce are all about 16%,yet the Klason lignin content of red spruce residue after extraction by ammonia is found to be 36% which is higher than the initial lignin content of wood, thus suggesting more selective removal of the carbohydrates. These results are further substantiated by IR analysis. Figure 6 shows the FTIR spectra of red spruce, a-cellulose, kraft lignin, xylan, and arabinogalactan. The characteristic IR bands for lignin are observed between 1510 and 1600 cm-l (aromatic ring vibrations) and between 1470 and 1460 cm-' (C-H deformation and aromatic ring vibrations). Usually 1510- and 1600-cm-' regions are examined to varify the presence or absence of lignin in a given lignocellulosic material (Fengel and Wegener, 1983). As can be seen in Figure 6, the intensity of absorption at 1510 cm-l decreases in going from kraft lignin to red spruce (lower lignin content) and is absent in a-cellulose and hemicelluloses. Thus, 1510-cm-l absorption band has been used to assess relative lignin content of the samples. The FTIR spectra of red spruce and its residues after extractions with ethylene (the high-temperature run at 182 "C), ammonia, and methylamine corresponding to Table VI11 are shown in Figure 7. Examination of the 1510-cm-I
Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 1309
3
-
a, C
a,
m
-
+
C
t
+
E
m E
C
Ln
L
m C
L
L +
4000
2000
1500
450
Wavenumber (cm-') 4088
2888
IS88
450
Wavenumber (cm-')
Figure 6. Infrared (IR) spectra of red spruce (A), kraft lignin (Inand xylan (E). dulin AT) (B),a-cellulose (C), arabinogalactan (D), The spectra have been vertically displaced for clarity.
Figure 8. IR spectra of red spruce (A) and its residues after extractions with ammonia-water (B),carbon dioxide-ethanol (C), ammonia-ethylene (D), and carbon dioxide-sulfur dioxide (E) binary mixtures.
-5
,-. y
a, C
a,
c t
C
4
E
t m
in
f
C
E
L
c
C m L
+
4000 4880
2888
1588
450
Wavenumber (cm-')
Figure 7. IR spectra of red spruce (A) and its residues after extraction with ethylene (B),ammonia (C), and methylamine (D).
lignin absorption band shows that methylamine is indeed effective in removing lignin. In contrast, the spectrum of the residue after ethylene extraction (even though conducted at high temperature) is essentially unchanged from that of initial wood. Figure 8 shows the FTIR spectra of red spruce residues after extractions with the binary fluids corresponding to Table VIII. As shown in Table VIII, Klason lignin values after these extractions remain basically the same as that of initial wood. This is also suggested by the intensity of 1510-cm-l absorption band in the IR spectra, which remains essentially unchanged. Figure 9 compares the IR spectra of residues after extraction with ethanol-water and ethanol-water-carbon dioxide with those of a-cellulose and red spruce. In the binary mixture, the mole fraction of ethanol was 0.48. In the ternary extraction fluid, the
2080
1580
450
Wavenumber (cm.')
Figure 9. IR spectra of red spruce (A) and its residues after extraction with ethanol-water-carbon dioxide (B)and ethanol-water (C) mixtures and a-cellulose (D).
ethanol-to-water ratio was maintained to be the same as in the binary ethanol-water extraction. As seen from the spectra, selectivity for lignin removal is hindered when carbon dioxide is introduced to the extraction fluid. (In the presence of carbon dioxide, carbonic acid formation leads to hydrolysis of carbohydrates). Again IR spectra confirm the results based on Klason lignin determination. Additional information on the compositional changes after extraction were gathered from thermal analyses. Thermogravimetric behavior of lignocellulosic materials can be used as an analytical tool for a t least qualitative characterization of these compounds (Kiran, 198613). This is particularly evident from the derivative plots such as Figures 4 and 5. There are major differences in the derivative weight loss behavior of lignin as represented by Indulin AT versus cellulose and hemicellulose with respect to the onset of decompositions and the rates of weight loss.
1310 Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 THERMOGRAVIMETRIC A N A L Y S I S
THERMOGRRVIMETRIC A N A L Y S I S
I
U
' ~
I
I
I
0 T
\
/Ji I
/I
?ed
C"
_.
r,
_ I
Ethylene e x t r a c t i o n Rminoy:raction
I
-
Yethyl anine e x t ' n
,
1
I-------
0
60
120
180
240 300 360 420 TEMPERATURE ( " C )
480
540
6W
Figure 10. Derivative T G plots for red spruce and its residues after ethylene, ammonia, and methylamine extractions corresponding to Figure 7 .
60
0
180
\
1
a - c e ~~ o s e
240 300 360 420 TEMPERATURE ("C)
480
540
600
Figure 12. Derivative T G plots for the red spruce residues from ethanol-water and carbon dioxide-ethanol-water extractions and that for pure a-cellulose.
THERMOGRAVIHETRIC A N A L Y S I S I
120
/
THERMOGRAVIMETRIC A N A L Y S I S
u
I? N
Methyl m i n e e x t ' n ( 1 n d u i i n A T )
::
44
tI
E ,
(3
R m n i a e x t r a c t i o n ( Red spruce)
z Y
I
n-
1
+
21 H
C a r W n dioxideethO'lo1 extraction
-1
\------
1
i
fur eioxldo o x t ' n
+ - I U
Methyl amine ext'n(Red spruce)
I
D w
Indul In A; / ,
0
60
120
180
360 420 TEMPERATURE ( " C ) 240
300
480
540
600
Figure 11. Derivative T G plots for red spruce and its residues from ammonia-water, carbon dioxide-ethanol, ethylene-ammonia, and carbon dioxide-sulfur dioxide extractions.
TG analyses can therefore be used to gather additional information on the nature of the residue after extraction. Figure 10 shows the derivative TG plots for red spruce and its residues after extraction with ethylene (at 182 "C), ammonia, and methylamine. The general feature for the residue from ethylene extraction is very similar to that of initial wood; both T G traces show the characteristic shoulder peak that is associated with the hemicellulose content (Kiran, 1986b). This is in accord with Table VI11 and Figure 7. Ammonia and methylamine extraction residues do not display the shoulder peak with as much intensity, suggesting that most of the hemicelluloses are removed with these solvents. This observation is consistent with the dissolution data of Tables I1 and V. Figure 11 shows the derivative T G plots of red spruce and its residues after the indicated binary fluid extractions. This figure shows that the general features of wood are retained after the extractions except for carbon dioxidesulfur dioxide extraction (at 155 "C) for which the shoulder peak representative of hemicellulose fractions is not observed, and the location of the primary decomposition peak is shifted to lower temperatures which can be associated with decreased molecular weight of the carbohydrates. Figure 12 is a similar plot for red spruce residues from the
0
60
120
180
240 3W 360 420 TEMPERATURE ( " C )
480
540
6W
Figure 13. Derivative TG plots for kraft lignin and the precipitates collected in the first trap after ammonia and methylamine extraction of red spruce and for the precipitate from methylamine extraction of kraft lignin.
other binary and ternary extractions of Table VIII. These derivative weight loss peaks are sharper and resemble that of a-cellulose, especially in the case of ethanol-water extraction. Precipitation Fractions. The nature of the precipitates in the traps was also analyzed by thermal and spectroscopic techniques. Figure 13 shows the derivative TG plots for the precipitates collected in the first trap after extraction of red spruce with ammonia and methylamine. The figure also includes the derivative TG plots for kraft lignin (Indulin AT) and its precipitates collected from methylamine extraction. These derivative plots show that precipitates are similar to that of lignin fragments if extraction is carried out with methylamine. Figure 14 is a similar plot for the precipitates collected in the second trap. It includes derivative TG plots for the extracted substances from red spruce, kraft lignin, and a-cellulose, using methylamine and ammonia. In this figure, it is clear that the TG behavior of the precipitates from red spruce extraction shows greater similarities, especially in the case of methylamine extraction, to that of the precipitates from kraft lignin extractions. The behavior of the precipitates from extraction of a-cellulose is significantly different.
Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 1311 THERMOGRRVIMETRIC R N A L Y S I S
u) u)
0 _I
I-
I
a
Indul
I
w
in
AT
3
w
Indul In AT
> Y
+ e
> I
U
w 0
Red
0
80
I20
180
240
300
360
420
480
wnms
540
600
TEMPERRTURE ("C) Figure 14. Derivative TG plots for precipitates collected in the second trap after ammonia and methylamine extraction of red spruce, a-cellulose, and kraft lignin.
4000
28EB
1588
450
Wavenumber (cm" 1
Figure 16. IR spectra of precipitates collected in the second trap after methylamine extraction of red spruce (A), kraft lignin (C), and a-cellulose (E) and ammonia extraction of red spruce (B), kraft lignin (D), and a-cellulose (F) corresponding to Figure 14.
1
4800
2066
15EE
KLASON LIGNIN
450
-'
Wavenumber (cm 1
Figure 15. IR spectra of the precipitates collected in the first trap after ammonia (A) and methylamine (B)extraction of red spruce and after methylamine extraction of kraft lignin (C) corresponding to Figure 13 and kraft lignin (D).
.;'
The FTIR spectra of the precipitates corresponding to Figures 13 and 14 are shown in Figures 15 and 16. The IR spectra in Figure 15 show that the precipitates in the first trap from extraction of red spruce and kraft lignin with methylamine are very similar to initial kraft lignin. Yet the precipitate from ammonia extraction of red spruce is different. The IR spectra in Figure 16 confirm the derivative T G results of Figure 14 in that precipitates in the second trap from wood or kraft lignin are of similar nature. Effect of E x t r a c t i o n Time, T e m p e r a t u r e a n d Pressure. The foregoing results have shown that methylamine is effective in preferential removal of lignin from wood. Therefore, further experiments were conducted with methylamine to study the influence of extraction time, temperature, and pressure on the extent of dissolution and precipitation, and the lignin level in the wood residues after extractions. Multiple runs were carried out at each condition to test the reproducibility of the results as well. Figure 17 shows the extent of dissolution of red spruce in methylamine, the amount of precipitates collected in the first trap, and the Klason lignin content in the wood
0
I
2
3
4
5
6
EXTRRCTION T I M E (HOUR) Figure 17. Dissolution (%) of red spruce in methylamine, Klason lignin content (%) in the residues after extraction, and the amount (%) of precipitates in the first trap, as functions of extraction time. Extraction conditions: 185 "C, 275 bar, and 1 g/min solvent flow rate.
residues after extraction, as a function of extraction time. In the figure, the total dissolution and the total precipitates are normalized with respect to the bone dry weight of initial wood. The extraction conditions were 275 bar, 185 "C, and 1g/min solvent flow rate. The pressure in the first trap was immediately reduced to 1bar. As shown in the figure, dissolution initially increases with time and levels off at about 28% by weight. The precipitates which were all in solid from follow a similar trend, increasing until a plateau value. The Klason lignin content of the wood residues decreases with extraction time, from an initial value of 26.5% down to 10.1% after 5 h of extraction. The experiments on pressure dependence show that, in going from 4 to 275 bar (at 185 "C, 2 h, and 1 g/min
1312 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988
methylamine flow), the dissolution increases from 4.5 % to 23.1% and the lignin content of the residues decreases from 28.4% to 16.5% (Li and Kiran, 1987). The experiments on temperature dependence show that the dissolution sharply increases a t temperatures above the critical temperature of the extraction fluid. These results for methylamine and its mixture with nitrous oxide have been discussed in detail elsewhere (Li and Kiran, 1987). The foregoing results on dissolution, lignin level, and amount of precipitates in the first trap were found to be highly reproducible. The standard deviations of repeat runs at each condition were less than 0.6%. Conclusions This study has provided extensive information on the extent of dissolutions of wood and its constituents in a variety of single-component and multicomponent supercritical fluids. The extractions in carbon dioxide, ethylene, nitrous oxide, and n-butane are not substantial, and the interactions appear to be nonreactive. The dissolutions in ammonia and methylamine result from reactive interactions. The dissolved compounds are in general precipitated in dry solid form in the traps. Binary mixtures result in dissolutions which are strongly dependent on the composition and the reactive nature of the component solvents. Among the various fluids, methylamine is more reactive with lignin, whereas ammonia and carbon dioxide-water mixtures are more reactive with carbohydrates fractions of wood. Acknowledgment This research has in part been supported by the National Science Foundation (Grant CBT-8416875). Registry No. CzH4, 74-85-1; COP, 124-38-9; NH,, 7664-41-7; NHZCH,, 74-89-5; N20, 10024-97-2; C4H10, 106-97-8; CZHbOH, 64-17-5; HzO, 7732-18-5; SOz, 7446-09-5; xylose, 58-86-6; glucose, 50-99-7; arabinogalactan, 9036-66-2; xylan, 9014-63-5; a-cellulose, 9004-34-6; indulin AT, 8068-05-1.
Literature Cited Amer, G. I. U.S.Patent 4422966, Dec 27, 1983. Avedesian, M. M. U.S. Patent 4 493 797, January 1985. Beer, R.; Peter, S. In Supercritical Fluid Technology;Penninger, J. M. L., Radosz, M., McHugh, M. A,, Krukonis, V. J., Eds; Elsevier: New York, 1985. Beer, R.; Peter, S. Chem. Ing.-Technol. 1986,58(1),72. Calimli, A.; Olcay, A. Holzforshung 1978, 32(1), 7.
Calimli, A.; olcay, A. Sep. Sci. Technol. 1982, 17(1), 183. DeHaas, G. G.; Lang, C. J. Tappi 1974,57(5), 127. Effland, M. J. Tappi 1977, 60(10), 143. Erlich, P.; Kurpen, J. J. J. Polym. Sci. 1963, AI, 3217. Fengel, D.; Wegener, G. Wood-Chemistry, Ultrastructure,Reactions; Walter de Gruyter & Co.: London, 1983; p 157. Froment, H. A. U.S. Patent 4 308 200, Dec 28, 1981. Hicks, C. P.; Young, C. L. Chem. Reu. 1975, 75(2), 119. Hubert, P.; Vitzthum, 0. G. Angecu. Chem., Int. Ed. Engl. 1978,17, 710. Kiran, E. Paper presented at the AIChE Summer National Meeting, Boston, 1986a. Kiran, E. In Cellulose, Structure, Modification and Hydrolysis; Young, R. A,, Rowell, R. M., Eds; Wiley-Interscience: New York, 1986b. Kiran, E. Explorations 1987a, 3(2), 24. Kiran, E. Tappi J . 1987b, 70(11), 23. Koll, P.; Bronstrup, A.; Metzger, J. V. Holzforshung 1979, 33, 112. Labrecque, R.; Kallaguine, S.; Grandmaison, J. L. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 177. Lawson, J. R.; Klein, M. T. Ind. Eng. Chem. Fundam. 1985,24,203. Li, L.; Kiran, E. Paper presented at the AIChE Annual Meeting, New York, 1987. McDonald, E. C.; Howard, J.; Bennett, B. Fluid Phase Equilib. 1983, 10, 337. McHugh, M.; Krukonis, V. Supercritical Fluid Extraction, Principles and Practice; Butterworths: Boston, 1986. Obst, J. R. Tappi 1981, 64(3), 171. O'Connor, J. J. Tappi 1972, 55(3), 353. Paulaitis, M. E.; Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Rev. Chem. Eng. 1983a, 1(2), 178. Paulaitis, M. E., Penninger, J. M. L., Grey, R. D., Davidson, P., Eds. Chemical Engineering at Supercritical Fluid Conditions; Ann Arbor Science: Ann Arbor, MI, 1983b. Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds. Supercritical Fluid Technology; Elsevier: New York, 1985. Randall, L. G. Sep. Sci. Technol. 1982, 17(1), 1. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. Saraf, V. P.; Kiran, E. Polym. Prepr. 1987, 28(2). Schneider, G. M., Stahl, E., Wilke, G . , Eds. Extraction with Supercritical Gases; Verlag Chemie: Deerfield Beach, FL, 1980. Sjostrom, E. Wood Chemistry-Fundamentals and Applzcations; Academic: New York, 1981. Stahl, E.; Schilz, W.; Schutz, E.; Willing, E. Angew. Chem., Int. Ed. Engl. 1978, 17, 731. Thillaimuthu, J. Tappi 1977, 60(6), 112. Vick Roy, J. R.; Converse, A. D. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier: New York, 1985. Williams, D. F. Chem. Eng. Sci. 1981, 36(11), 1769.
Received for review May 28, 1987 Revised manuscript receioed December 8, 1987 Accepted March 7, 1988
