Ind. Eng. Chem. Fundam. 1985, 2 4 ,
for data sets 5-8. For butanol, the value for A. was taken from Marcus (1977) and a good overall fit was obtained. The established additivity of Ff and Fbcalculated from appropriate data is an indication, though not proof, of the validity of the proposed model. A few additional conclusions concerning the dissociation of the different acids in the alcohol can also be drawn from the model: HC1 is fully dissociated to ions. H,S04 is dissociated to H+ and HSO,, and also possibly slightly to SO:-. However, dissociation to SO4* cannot be significant, in view of the accuracy of the correlation that did not take this effect into consideration. HN03 appears to partly dissociate reversibly to N2OP This dissociation is a function of the amounts of total acids present. is not significantly dissociated to ions below concentrations of about 1 g-mol/kg of alcohol. Acknowledgment We are indebted to IMI Institute for Research and Development for permission to use ita proprietary data in this study. Nomenclature a = activity A , B = van Laar coefficients Ai,Bi = coefficients A. = concentration of water in alcohol phase in equilibrium with pure water E = percent error in correlation fc,f,'., f", = unit correction factors Fb = bound water concentration Ff = free water concentration H = linear coefficient K = distribution coefficient, chemical equilibrium coefficient N = number of experimental points X = mole fraction or molality in aqueous phase Y = mole fraction or molality in alcohol phase Greek Letters y = activity coefficient u = standard deviation
203
203-208
Subscripts aq = aqueous 1 = species NH = unreacted HNO, S = solvent W = water Registry No. Water, 7732-18-5. Literature Cited Bromley, L. A. AIChE J. 1073, 19. 313. Cejtiin, J. D.Sc. Thesis, Technion-Israel Instlute of Technology, 1959. Crittenden, E. D.; Hixon, N. A. Ind. Eng. Chem. 1058. 46, 266. Elmore, K. L.; Hatfield, J. D.; Dunn, R. L. J. Phys. Chem. 1065. 5 9 , 3520. Enderby, J. E.; Soper, S. I n "Ionic Liquids", Inmam, D.; Lovering, D.G., Ed.: Plenum Press: New York, 1961; p 7. Fredenslund, A,; Gmehlln. J.; Rasmussen, P. "Vapor Liquid Equilibrium Using UNIFAC"; Elsevier: Amsterdam, 1977. Gai, J. F. I n "Proceedings of ISEC, 1977",Lucas, B. H.; Ritcef, G. M.; Smith, H. W., Ed.; ISEC. 1977; pp 316-322. Grinbaum, 8. D.Sc. Thesis, Technlon-Israel Institute of Technology, 1983 (in Hebrew). Hiebert, K. L. ACM Trans. Math. Software 1081, 7 , 1 . I M I InstLute for Research and Development, Internal Reports. 1969-1983. IMSL Inc. "IMSL Library Reference Manual", 9th ed.; Houston, TX, 1982. Kusik, C. L.; Meissner, H. P. AIChESyrnp. Ser. 1078, 74, 14. Lietzke, J. W.; Stoughton, W. D. J. W y s . Chem. 1062, 66, 506. Marcus, Y. I n "Ionic LiquMs", Inmam, D.; Lovering, D. G., Ed.; Plenum Press: New York, 1981; p 97. Marcus, Y. "Introduction to Liquid State Chemistry"; Why: New York, 1977; pp 197-254. Meissner, H. P.; Kuslk, C. L. AIChE J. 1072, 18, 661. Meissner, H. P.; Tester, J. W. Ind. Eng. Chem. Process Des. Dev. 1072,
1 1 , 128. Mellor, J. W. "Comprehensive Treatise of Inorganic Chemistry", Vol. 8; Longmans: London, 1958; pp 550-570. Meyer, R. J.; Pietsche, E. "Gmelins Handbuch der Anorganisher Chemie"; Veriag-Chemie, Wenheim, 1936; Vol. N-3, pp 818-838. Pitzer, K. S. J. Phys. Chem. 1073, 77, 268. Pltzer, K. S.; Peiper, J. C. J. Phys. Chem. 1080, 64. 184. Pltzer, K. S.; Sllvester, L. F. J. Soh. Chem. 1078, 5 , 269. Pltzer, K. S.; Ray, R. N.; Silvester, L. F., J. Am. Chem. SOC. 1077, 9 9 ,
4930. Renon, H. I n "Foundation of Computer Aided Chemical Process Design", Mah, R. S. H.; Seider, W. D., Ed.; Englneering Foundatlon: New, York, 1981; p 53. Stephen, H.; Stephen, T. "Solublllty of Inorganic Compounds" Pergamon Press: Oxford, 1964; Vol. 2, Part 1 , Tab. 2320-2374. Szpruch, E. MSc. Thesis, Technion-Israel Institute of Technology, 1976.
Received for review December 22, 1983 Accepted September 10, 1984
Influence of Water on Guaiacol Pyrolysis J. R. Lawson+ and M. T. Kleln" Department of Chemlcal Engineering, Unlverslty of Delaware, Newark, Delaware 19716
The influence of water on guaiacol pyrolysis was examlned through a series of pyrolyses spanning reduced water densities from 0.00 to 1.6 at 383 'C. Neat guaiacol pyrolysis yielded catechol and char as major products and minor products including phenol and o-cresol; methanol was not observed. Water provided a parallel reaction pathway that was formally equivalent to guaiacol hydrolysisto catechol and methand. The overall reaction selectivity to hydrolysis was a continuous function of water density at the condiiins examined. The observed product spectra and qualitative kinetics were summarized in terms of free-radical steps for neat pyrolysis, to which guaiacol soivatlon and hydrolysis steps were added to account for the influence of water.
Introduction The appeal of using supercritical fluid (SCF) solvents in separation processes (Paulaitis et al., 1983) has encouraged their use in the extraction of volatile products from macromolecules, such as biomass (Koll and Metzger,
'E.I. du Pont de Nemours & Co., Inc., Wilmington, DE. 0196-4313/85/1024-0203$01.50/0
1978; Olcay et al., 1983) and coal (Whitehead and Williams, 1975; ROSSand Nguyen, 1983; Squires et al., 1983; Blessing and ROSS,1978;Jezko et al., 1982; Barton, 1983; Vasilakos et al., 1983; Fang et al., 1983; Maddocks et al., 1979; Smith et ale,1983; Amestica and Wolf, 19W. Product yields are generally dependent on both the resources and the SCF solvent extractant, and Squires et al. (1983) have recently demonstrated the importance of thermal decomposition 0 1985 American Chemical Society
204
Ind. Eng. Chern. Fundarn., Vol. 24, No. 2, 1985
Table I. Summary of Conditions for Guaiacol Pyrolysis in Water
T, "C 383 383 383 383 383 383 383
f f f f
1.5 1.5
1.5
1.5 f 1.5 f 1.5 f 1.5
cG,ox io4, mol/cm3 3.62 3.67 3.62 3.62 3.62 1.80 3.62
mol/cm3
cBpx 104, PWa
mol/cm3
reaction time, min
0
0 0.04
1.44-2.24 0.97-1.85 1.02-1.90 1.30-2.25 0.57-2.80 1.40-2.22 1.22-1.71
5-90 5-90 5-90
cw,o x 104, 7.52 70.9 137.0 282.0 282.0 368.0
0.40
0.77 1.59 1.59 2.08
5-90 5-180 5-90 15-135
OBased on TC,W= 374.1 " C ; P c ,=~3206 psia; Vc,w = 56.5 cm3/mol; reactor volume = 0.65 cm3.
reactions during extractions at temperatures in excess of 350 "C. It thus seems likely that the high yields of coal volatiles (Scarrah, 1983) and low yields of cellulosic char (Modell, 1977) observed in supercritical water (T, = 373 "C) extractions are a result of both pyrolytic fragmentation and mass transfer steps. This motivated our use of the model compound guaiacol as a probe into the pyrolysis pathways and kinetics contributing to the extraction of volatiles from lignin with supercritical water. The details of a macromolecule's pyrolysis pathways, kinetics, and mechanisms are usually obscured by the complexity of both ita structure and conversion product spectra. However, the relative simplicity of a model compound's structure and reaction product spectra permits quantitative analysis of reaction pathways, which can be used to model the related reactions of the macromolecule's pyrolysis. Lignin is a polymer of single-ring phenolic monomers (Freudenberg and Neish, 1968; Harkin, 1967; Glasser and Glasser, 1974),most of which contain at least one methoxy substituent in a ring position ortho to the hydroxyl. Guaiacol (o-methoxyphenol) thus models key aspects of lignin structure. Previous guaiacol pyrolyses (Ceylan and Bredenberg, 1982; Bredenberg and Ceylan, 1983; Vuori and Bredenberg, 1984; Shaposhinikov and Kosyukova, 1965; Kravchenko et al., 1970; Connors et al., 1980; Klein and Virk, 1981) indicate that ita major low molecular weight products are catechol, phenol, cresols, methane, and carbon monoxide, along with trace amounts of saligenol and other phenolics; methanol yields are low. It is important to note that the formation of ill-defined high molecular weight material, hereafter referred to as char, is important in guaiacol pyrolysis (Klein and Virk, 1981) as well as in 2,&dimethoxyphenol,veratrole (Klein and Virk, 1981),and saligenol (Klein and Virk, 1981; Gardner et al., 1959; Cavitt et al., 1962) pyrolyses. It is clear that the selectivity to single-ring phenolics could be increased by a reduction in the formation of char. We begin our discussion of the influence of supercritical water on guaiacol and, by inference, lignin pyrolysis by detailing our experimental methods. We then delineate results by describing the pyrolysis products observed, their temporal variations, and product interrelationships. Our discussion examines the importance of operating in the P,, pr,w 1)and the implications supercritical region (Tr, of the present results in lignin processing. Finally, a set of candidate elementary reactions is presented that qualitatively explain the observed pyrolysis products and kinetics.
-
Experimental Section Pyrolysis of guaiacol was examined neat and in water. The reaction conditions and the concentrations of the initial reactants and the internal standard are summarized in Table I. All pyrolyses were conducted at 383 "C, which corresponds to a reduced temperature of 1.014 relative to the critical temperature of 374 "C for water. These ex-
periments allowed comparison with previous neat pyrolyses and determination of the influence of water on guaiacol pyrolysis. Guaiacol was commercially available (Aldrich) in a purity of 98% and was used as received. Closed, constant volume and essentially isothermal batch reactors were fashioned from */4-in. nominal Swagelok fittings. The heat-up period required less than 1 min and was small relative to typical reaction times. A representative experimental procedure was as follows. Clean reactors were tared. A nonreactive internal standard (biphenyl) was added, the precise amount of which was determined by reweighing the reactor. The reactant guaiacol was added volumetrically with a 1-pL syringe, and tap water was then added with a 1-mL syringe. In each case, the amount of material added was confirmed by reweighing the reactor. The reactor was sealed and placed in a sand bath preheated to 383 "C, and, after a predetermined reaction time it was subsequently removed from the sand bath and quenched by running water. The reactor was allowed to air-dry at room temperature and was reweighed to check for leakage. The reactor was opened and its contents dissolved in approximately 4 mL of acetone. The reaction products were separated by GC and identified by comparison with the retention times collected from standard samples. Three columns were used. A 50 m X 0.25 mm i.d. borosilicate glass WCOT capillary column with SE-54 as the stationary phase was used for identification of all products. A 6 f t X 2 mm i.d. borosilicate glass packed column with 10% SP-2100 on 100-120 mesh Supelcoport was used for identification of all compounds with the exception of the strongly absorbing catechol. An 8 f t X 2 mm i.d. stainless steel packed column with 3% Dexsil400 on 100-200mesh Supelcoport was used for analysis of catechol after silylation of the sample with n,o-bis(trimethylsily1)trifluoroacetamide.No analyses of permanent gases or water were made. These analyses allowed calculation of an aromatic ring balance that was used as illustrated in equation 1. n
Nchar
= NGU,O -
cNi
(1)
1=1
Since aromatic rings were expected to be stable at 383 "C, the difference between the moles of single aromatic rings and recovered (Cni=,Ni) after reaction was charged (NGU,o) a measure of the extent to which polymerization and condensation reactions to char occurred (Nchar). Results Neat guaiacol pyrolysis yielded catechol, phenol, cresol, and char as major products, along with trace amounts of two other compounds that eluted in the range of methyl catechols. The changes in the neat pyrolysis product spectrum with reaction time are shown in Figure 1. The continuous decrease in reactant concentration with reaction time corresponds to a least-squares estimate of a
Ind. Eng. Chem. Fundam., Vol. 24,
No. 2, 1985 205
ll----l
1
0.0
NEAT PYROLYSIS
0.8 0.7 0.8
0.8
- 0.6
- 0.6
%.* 0.4
0.4
0.3
0.3
02
0.2
0.1
0.1
0 18
20 30 40 W
0
60 70 80 00 100
0
10
20
30 40 60 80 70 SO BO 100
Ruction h. min.
Figure 1. Temporal variations of the yields of major products from neat guaiacol pyrolysis at 383 O C .
Reaction Time. mm
Figure 3. The influence of water density on the temporal variations of catechol yield from pyrolysis of guaiacol at 383 "C.
1
0.8
O6
0.8
0.6
r-----1
0.7
1
0.8
N, 0.6
N,,
NQU.o
0.4 0.3
I +/+
NQU.o
0.4
0.3 0.2
I
0.2 0.1
0.1
+/
/
-
Pr,w = 0 . 0 7
0 0
10
20
30 40 60 60 70 SO 80 100
.
00 0
10
20
30 40 60 60
0
"
,
I
70
80
90 100
Reaction Time, min.
Figure 2. Temporal variations of the yields of major products from guaiacol pyrolysis in water at 383 O C and reduced density of 1.6.
pseudo-first-order rate constant, k = 0.0198 min-'. The principal primary product was catechol, the molar yield (mol of ilinitial mol of guaiacol) of which increased with reaction time to an asymptotic level of about 0.30 at a residence time of 90 min. Yields of the lesser of the major products, o-cresol and phenol, also increased with pyrolysis time. Phenol showed no signs of an asymptote at 90 min. Methanol was observed in only trace amounts. The influence of water on guaiacol pyrolysis is illustrated in Figure 2, where the changes of the product spectrum with reaction time are shown for guaiacol pyrolysis in water at a reduced density, P ~ ,of~ 1.6. , The continuous decrease in guaiacol concentration corresponds to a least-squares estimate of a new pseudo-first-order rate constant of k = 0.0242 m i d . Both catechol and methanol were major and primary products, with the asymptotic molar yields of catechol in Figure 2 being increased over those in Figure 1by approximately 0.25. The yield of methanol in Figure 2 is approximately 0.4. It is interesting that while the asymptotic yields of methanol and catechol increased greatly with the addition of water, the yields of o-cresol and char decreased and the yields of guaiacol and phenol were essentially unchanged. The total yield of single-ring phenolics in Figure 2 is approximately 0.70 for a reaction time of 90 min. This is a substantial increase from the
Reaction Time, mm.
Figure 4. The influence of water density on the temporal variations of methanol yield from pyrolysis of guaiacol at 383 OC. 0.08,
, , , , , , ,
, ,
,
0.02
0.01
0.00 0
H)
20
30 40 bo 80 70 80 80 100 R m t l o n Tlnn. mhl
Figure 5. The influence of water density on the temporal variations of o-cresol yield from pyrolysis of guaiacol at 383 "C.
yield of 0.35 illustrated in Figure 1at the same reaction time.
206
Ind. Eng. Chem. Fundam., Vol. 24, No. 2, 1985
,
060 r
040
1 ,i
o 36 C
80 min reaction time
r
\
020
,1
016
C
NME 0 2 6 NGUo
010
006
0 00
9 '0 ,,'
/
,
'L
1, . '\
"&
$
0
I
/
-
/" -
I
i
04
'
\
-c,
16 min reaction time
?
08
+
~~
12
16
2
24
pr w
Figure 6. The dependence of methanol yield on reduced density for pyrolysis of guaiacol for 15 and 90 min at 383 "C.
The influence of water concentration on guaiacol pyrolysis is further illustrated in Figures 3, 4, and 5, where, respectively, the yields of catechol, methanol, and o-cresol are plotted vs. reaction time for parametric values of reduced water density ranging from pr,w= 0.0 to 1.6. Figures 3 and 4 show not only the general increase in catechol and methanol yields with reaction time and water density, but also that the influence of water is continuous and evident at values of pr,wboth greater and less than unity; similarly, the decrease in the yield of o-cresol shown in Figure 5 is a continuous function of water concentration. The influence of water concentration on guaiacol pyrolysis is further illustrated in Figure 6, where, separately for pyrolysis to 15 and 90 min holding time, the yield of methanol is plotted vs. reduced water density pr,w. Methanol yields in Figure 6 appear directly and linearly dependent upon the initial concentration of water. As indicated in the above, the conversion of guaiacol was rather insensitive to water density over the range 0 < pr,w < 1.6. More careful scrutiny of the dependence of guaiacol conversion on water density, at a given reaction time, suggests that at low values of pr,w(0 < pr,w< 0.10-0.15), the addition of water actually decreased guaiacol conversion relative to neat pyrolysis; guaiacol conversion reached a minimum with continued addition of water and, eventually, increased with water density. Relevant data are shown in Figure 7 , where the yield of guaiacol (NG,/NG",-,) is plotted vs. water content for the parametric values of reaction time of 15,60, and 90 min. Inspection of Figure 7 reveals that, at the highest water density investigated (pr,w= 2.1), the conversion of guaiacol was greater than that observed for neat pyrolysis at the identical reaction time of 90 min. Discussion The present results indicate that at 383 "C water reacts with guaiacol in a manner formally equivalent to hydrolysis. This hydrolysis reaction provides the hydrogen (and oxygen) required to redirect the selectivity of guaiacol pyrolysis away from char and methane (Ceylan and Bredenberg, 1982; Bredenberg and Ceylan, 1983; Vuori and Bredenberg, 1984) to catechol and methanol. It is interesting that the hydrolysis appears rather conventional, That is, methanol and catechol yields were continuous functions of water density with no apparent discontinuity at pr,w= 1. Hence it appears that the primary motivation for operating at supercritical conditions is that water is
both hot and dense, the latter attribute having the usual accelerating effect on a rate that is positive order in water concentration. However, it should be noted that the ability of the SCF solvent to dissolve a normally immiscible coreactant could be a profound motivation for operating in the dense region. At least two implications to lignin processing are apparent. First, just as water provided the hydrogen (and oxygen) required to improve guaiacol pyrolysis selectivity, water could provide the hydrogen (and oxygen) required to process a carbon-rich resource such as lignin to less carbon-rich products. Second, although neat guaiacol pyrolysis yielded little methanol, lignin's aromatic methoxy1 groups are appealing (Allan and Matilla, 1971) but chemically unreasonable origins of the modest yields of methanol that arise from the pyrolysis of actual lignin (Allan and Matilla, 1971; Iatridis and Gavalas, 1979; Jegers, 1982). During neat pyrolysis the reaction of aromatic methoxyl groups would occur predominantly by fission of the weaker phenoxy-methyl bonds to phenol and methane precursors rather than fission of the stronger phenylmethoxyl bond to benzene and methanol precursors (Ceylan and Bredenberg, 1982; Bredenberg and Ceylan, 1983; Vuori and Bredenberg, 1984). However, present results show that the water that evolves during lignin pyrolysis (Allan and Matilla, 1971; Iatridis and Gavalas, 1979; Jegers, 1982)in a typical yield of 20 w t 70might react with the aromatic methoxyl groups. These could thus be sources of methanol even in the light of the bond energetics suggested above. Finally, the present results invite consistent mechanistic speculation. A set of elementary reactions that explain the observed product spectra and also qualitative kinetic observations is presented in Figure 8. Neat guaiacol pyrolysis can be interpreted in terms of the set of elementary reactions shown in Figure 8a. The first of these nonchain steps is fission of the phenoxymethyl bond to catechol and methyl radicals, which can in turn abstract hydrogen from guaiacol to yield catechol, methane, and two guaiacol radicals. The guaiacol radical can undergo either p-scission, to yield a phenol radical that can abstract hydrogen from guaiacol, or disproportionate through steps such as those outlined in Figure 8a to form cresol, catechol, and char products. Pseudo-steady-state solution of the mechanism of Figure 8a yields eq 2, 3, 4, and 5 for the rates of guaiacol (G) decomposition and catechol (CA), o-cresol (OC), and char appearance, respectively. -d(G)/dt = 3k1G
+ k5 (k1G/kG)'/'
(2)
d(CA)/dt = [I + a]klG
(3)
d(OC)/dt = @klG
(4)
d(char)/dt = rk,G
(5)
Note that the mechanism requires a correlation of o-cresol and char yields. Our extension of this free-radical mechanism for neat guaiacol pyrolysis to account for the influence of water is shown in Figure 8b. We model neat pyrolysis to occur in parallel with a hydrolysis pathway that involves solvated guaiacol (G*). This solvated or partially hydrolyzed guaiacol (G*) would exist only at dense (not necessarily supercritical) conditions and is prevented from undergoing the free-radical reactions (Figure 8a) that are accessible to the unsolvated guaiacol (G) by admittedly nebulous "cage effects"; subsequent addition of water to the solvated guaiacol (G*) effects the hydrolysis to catechol and
Ind. Eng. Chem. Fundam., Vol. 24,
No. 2, 1985 207
+ KW) d(char)/dt = r k l G o / ( l + KW) d(OC)/dt = PklGo/(l
o 9s O
N,,
O6
NGU o
04
.
c
-
0
t
4 t A
'L,
Reaction Time
-
90 min
\
i
$,
i
P r II
Figure 7. The dependence of guaiacol conversion on reduced water density for pyrolysis for 15, 60, and 90 min at 383 "C.
methanol. Pseudo-steady-state solution of the network shown in Figure 8b was accomplished by: (1) considering the solvation to be rapid at reaction conditions, and thus allowing guaiacol (G) and solvated guaiacol (G*) to be in virtual equilibrium; and (2) recognizing that gas chromatographic analysis at ambient conditions would yield only the s u m of guaiacol and solvated guaiacol (G + G*) as the observable guaiacol species (GO) in kinetic analyses. Rate equations for observable guaiacol (GO)disappearance, and methanol (ME), catechol, 0-cresol, and char appearance that explicitly account for the effects of water are shown as eq 6-10.
+ kHKw)/(1 + KW) + k, [k,/k,(l + KW)Go]1/2)Go(6) d(CA)/dt = [ ( k H K w + (1 + a)ki)/(KW + l)]Go (7) d(ME)/dt = kHKWGo/(l + KW) (8)
-d(G")/dt
((3k1
EL E M EN TA R Y STEPS
REACTION
aoH - + aoH -[aoH @OH
(51
CO +
Hp
OCHz'
HO
2
(61
OCHp.
( a ) Elementary steps for neat guaiacol thermolysis
1 Neat Guaiacol Thermolysis,
(10)
Note that methanol and catechol rates both increase with water concentration and that o-cresol and char rates both decrease. It is interesting that the rate expression for guaiacol disappearance can be in agreement with the conversion data in Figure 7. Recall that these data show that guaiacol conversion decreased with increasing water concentration at low levels of water concentration, reached a minimum value at intermediate levels of water concentration, and eventually increased at high values of water concentration. Note also that this behavior is like that expected from the formal superposition of a fission-controlled free-radical reaction pathway and a bimolecular hydrolysis pathway. The rate constant for the former pathway would have a positive partial molar volume of activation (Eckert, 1972) and would thus decrease with added water (increasing pressure). The rate of the latter pathway would always increase with water concentration because its partial molar activation volume would be negative and its rate expression would be positive order in water concentration. In brief, the candidate mechanisms of Figure 8 and the implied rate eq 2-10 are in qualitative agreement with the present experimental findings. Conclusions We conclude that two competing reactions occur during guaiacol reaction in water. The first is neat pyrolysis that leads to catechol and high molecular weight material as major products. The second is hydrolysis of guaiacol to catechol and methanol, the selectivity to which is a continuous and increasing function of water density. A superposition of free-radical and hydrolysis reaction mechanisms, the latter invoking an ill-defined solvated and caged guaiacol species, allowed formulation of rate expressions that were qualitatively consistent with the temporal variations of observed product yields. These results suggest that hydrolysis reactions may contribute to reduced yields of char and high yields of single-ring products during the "extraction" of volatiles
Time = BO min
dReaction
(9)
a s in ( a ) , above
( b ) Superposition of neat guaiacol thermolysis with guaiacol solvation and hydrolysis Figure 8. Elementary steps for guaiacol pyrolysis neat and in water at 383 "C.
208
Ind. Eng. Chem. Fundam. 1985, 24, 208-214
from lignin with supercritical water. Acknowledgment This work was supported in part by the U.S.Department of Energy under Grant No. DEFG 2282PC50799 and in part by the Ngtional Science Foundation under Grant No. CPE 8204440. The valuable assistance of C. B. Baker and R. J. Phillips in the analysis of the reaction products is gratefully acknowledged. Registry No. Guaiacol, 90-05-1. Literature Cited Allan, G. G.; Matllla, T. I n “Lignins: Occurrence, Formation, Structure and Reactions", Sarkanen, K. V.; Ludwig, C. H.. Ed.; Wlley-Intersclence: New York. 1971. Ameatlca, L. A.; Wolf, E. E. Fuel 1984, 63, 227. Barton, P. Ind. Eng. Chem. RocessDes. Dev. 1983, 22. 589. Bbsslng, J. E.; Ross, D.S. Am. Chem. SOC. Symp. Ser. 1978, 77, 171. Bredenberg. J. Bson; Ceylan, R. Fuel 1983, 62, 342. Cavitt, S. B.; Sarraflzadeh, H. R.; Gardner, P. D. J. Org. Chem. 1962, 2 7 .
1211. Ceylan, R.; Bredenberg, J. Bson Fuel 1982, 61, 377. Connors. W. J.; Johanson, L. N.; Sarkanen. K. V.; Wlnslow, P. Holzforchung 1980, 34, 29. Eckert, C. Ann. Rev. Fhys. Chem. 1972, 23, 239. Fong, W. S.; Chan, P. C. F.; Plchaichanarong. P.; Cwcoran, W. H.; Lawson, D. D. "Experimental Obsecvatlons on a Systematic Approach to Supercritical Extraction of Coal”; I n “Chernlcal Englneerlng at Supercritical Fluid Conditbns”, Paulaltis, M. E.: Penninger, J. M. L.; (;ray. R. D., Jr.; Davldson, P.; Ed.; Ann Arbor Science: Ann Arbor, MI, 1983. Freudenberg, K.; Nelsh, A. C. ”Constitution and Biosynthesis of Llgnin”; Springer-Verlag: New York, 1968. Gardner, P. D.;Sarrafizadeh, H. R.; Brandon, R. L. J. Am. Chem. SOC. 1959. 87, 5515. G l a w , W. G.; Glasser, H. R. Macromolecules 1974, 7 , 17. Harkln, J. M. I n ”Oxidative Coupllng of Phenols”, Taylor, W. L.; Battersby, A. I.; Ed.; Marcel-Dekker: New York, 1967.
Iatridis, B.; Gavalas, G. R . Ind. Eng. Chem. Prod. Res. Dev. 1979, 18,
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Received for review May 23, 1984 Accepted November 5 , 1984
Thermophoretically Enhanced Mass Transport Rates to Solid and Transptration-Cooled Walls across Turbulent (Law-of- t he-Wall) Boundary Layers Siileyman A. Goko$ut and Danlel E. Rosner’ Hlgh Temperature Chemical Reactlon Engineerlng Laboratory, Yale UnlVersi@, Department of Chemical Engineering, New Haven. Connecticut 06520
Convectivedlffusion mass transfer rate predictions are made for both solid wall and transpiration-cooled “lawof-the-wall’’ nonkthermal turbulent boundary layers (TBLs), including the mechanism of thermophoresis, i.e., small particle mass transport “down a temperature gradient”. Our present calculations are confined to low mass-loading situations but span the entire particle size range from vapor molecules to particles near the onset of inertial (“eddy”) impaction. I t is shown that, when Sc >> 1, thermophoresis greatly increases particle deposkion rates to internally cooled solid wails, but only partially offsets the appreciable reduction in deposition rates associated with dust-free gas-transpiration-cooled surfaces. Thus, efficient particle sampling from hot dusty gases can be carried out using transpiration “shielded” probe surfaces.
1. Introduction Accurate engineering predictions of mass transport rates in turbulent, nonisothermal forced convection systems are necessary in many technologies (including the production *Professor and Chairman, Department of Chemical Engineering; Director of HTCRE Laboratory. To whom inquiries concerning this paper should be sent. NASA Lewis Research Center, M.S. 106-1, Cleveland, OH 44135.
of fumed chemicals from combustion or arc-jet sources, gaseous fuels from coal, high temperature gas cleaning) and in research (e.g.,the design of gas sampling systems). Until relatively recently many such calculations have been made assuming that Fick or Brownian diffusion (i.e. concentration diffusion) is the dominant nonconvective contributor to the instantaneous mass flux toward (or away from) the surface. However, a significant augmentation in small particle diffusional transport rates due to thermophoresis (drift of particles down a temperature gradient) in laminar boundary layers (LBLs) has been predicted, for both solid 0 1985 American Chemical Society
