I n d . Eng. Chem. Res. 1989,28, 161-165
161
Hydrolysis in Supercritical Water: Identification and Implications of a Polar Transition State Gilbert L. Huppert, Benjamin C. Wu, Susan H. Townsend, Michael T. Klein,* and Stephen C. Paspek? Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Reaction of guaiacol (0-methoxyphenol) in supercritical water a t densities of 0 Ipw I0.7 g/cm3 and with added salts a t pw = 0.50 g/cm3 showed its hydrolysis to catechol (0-hydroxyphenol) and methanol to be through a polar transition state. Correlation of the reaction kinetics following a modified Herbrandson analysis provided a quantitative summary of the greater polarity of the hydrolysis transition state relative to the hydrolysis reactants as Vzm6zm- Vwm8wm- V G “ ~ G=. ~190 f 20 ( ~ m ~ . c a l / m o l ~Salt ) ~ / addition ~. a t pw = 0.5 g/cm3 further shifted the transition-state equlibrium toward the transition-state species. A sodium chloride loading of 0.71 mmol/cm3 led to an “effective water density’’ of pW,eff= 1.1g/cm3. Supercritical fluid (SCF) solvents have attracted considerable interest in separation processes because, near T, and P,, their physical and chemical properties can be varied between liquidlike and gaslike extremes with modest changes in pressure (Paulaitis et al., 1983). The same factors motivate interest in SCF solvents as media for chemical reactions (McHugh and Subramaniam, 1986; Manyya et al., 1987; Johnston and Haynes, 1987; Helling and Tester, 1987); examples of participation of the SCF solvent in the reaction are also known (Lawson and Klein, 1985; Townsend and Klein, 1985; Townsend et al., 1988). Townsend et al. (1988) have shown the reactions of a set of heteroatom-containing diaryls in supercritical water to include parallel pyrolysis and hydrolysis pathways, the selectivity to the latter increasing with increasing solvent density. The controlling reaction mechanism or mechanisms were equivocal, however, since the reaction conditions varied from gaslike to liquidlike as the solvent density increased; in the former limit, it would be reasonable to suspect free-radical or pericyclic chemistry, whereas in the liquidlike limit it would be reasonable to suspect ionic pathways. Moreover, to the extent that polar or ionic chemistry is relevant, reaction kinetics and selectivities could be strongly dependent on the highly density-dependent solvent properties. The foregoing motivated the present study of the reactions of guaiacol (0-methoxyphenol) in supercritical water and salt/water solutions. Guaiacol pyrolyzes to methane, catechol, phenol, o-cresol, and char and hydrolyzes to methanol and catechol (0-hydroxyphenol). The goal of this work was to probe the transition-state polarity of the hydrolysis species. This was accomplished in two manners. First, experiments at constant guaiacol concentrations and variable densities provided preliminary evidence of the ionic character of the transition state because the solvent properties, e.g., dielectric constant e (Franck, 1961) or solubility parameter 6 (Giddings, 1961), of supercritical water are highly dependent upon its density. However, the reaction pressure also varied with changes in water density in this first set of experiments. This motivated a second set of experiments with added salts at a constant water loading, where the solvent properties (Quint and Wood, 1985) could be varied dramatically without substantial changes in reaction pressure. In short, the combination of these experiments allowed preliminary discrimination between electrostatic (e, 6) and hydrostatic ‘Present address: BP Americas Research, 9101 E. Pleasant Valley Rd, Independence, OH 44131. 0888-5885/89/2628-0161$01.50/0
( P )solvent effects on hydrolysis reactions in supercritical water.
Experimental Section The reaction of guaiacol, neat, in water, and in supercritical salt/water solutions, was studied at 383 O C and at reaction times 0 < t < 30 min. Reagent-grade chemicals (99+%) purchased from Aldrich were used as received except for the wash solvent, tetrahydrofuran (THF), which was high-purity grade to assure low methanol content. The stainless steel reactors consisted of two lI4-in. Swagelok caps that sealed a Il4-in. port connector. The reactor volume (VR) was approximately 0.59 cm3. The reactors were purged with argon, loaded with 0.005 g of biphenyl, a demonstrably inert internal standard (Townsend and Klein, 1985),charged with 30 f 1/.LL(26.6 mg) of guaiacol, and filled with the desired loading of either deionized water or salt solution by syringe (fl /.LL),as appropriate. The reactors were then sealed and placed in an isothermal, fluidized sand bath at 383 “C. The heat-up time was approximately 2 min for all runs. After the desired reaction time had passed, the reactions were quenched in room-temperature water, and the contents were removed by washing with THF. Products were analyzed with a Hewlett-Packard 5880 gas chromatograph equipped with a DB-5 fused silica capillary column and flame ionization detector. Area ratios were converted to mass ratios using response factors from a standard solution. Reduced data are reported below as molar yield normalized by the initial moles of guaiacol charged (yi = moli/molh).
-
Results and Discussion Neat pyrolysis of guaiacol yielded catechol, phenol, gases (CHI and CO), and char as primary and major products; the gases and char were not quantified. Other products included cresol and trace amounts (less than a selectivity, si = y J x , of 0.01) of methanol. The first-order rate constant for the disappearance of guaiacol was (8.03 f 0.8) x lo4 s-l. Selectivities to catechol, phenol, and cresol were about 0.22,0.18, and 0.08, respectively. These results are consistent with previous studies of guaiacol pyrolysis that have been reported in detail elsewhere (Lawson and Klein, 1985; Shaposhnikov and Kosyukova, 1965; Vuori and Bredenberg, 1984). Reaction of guaiacol in the presence of water led to the primary hydrolysis products, catechol and methanol, as well as the neat pyrolysis products. Figure 1summarizes the effect of water density on the selectivity to hydrolysis products. The selectivity to catechol rose from 0.2 under 0 1989 American Chemical Society
162 Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 0.0002
0 800
,
0 600
0
0.0001
4
El
a
4 s,=Y,'X
0400
4 00000f
a
E '
a
'
1
0 1
I
02
.
.
I
a
'
.
3
05
04
03
.
I
06
07
0 200
P, /g ~ r n - ~
Figure 2. Dependence of apparent second-order guaiacol hydrolysis rate constant on water loading at 383 "C. 0 000
2
00
0 2
I
06
0 4
,P ig cm.3
Figure 1. Increase in selectivity to guaiacol hydrolysis with water addition at 383 "C and 30 min.
neat pyrolysis conditions to almost 0.8 at pw = 0.67 g/cm3. The selectivity to methanol increased from near 0.0 at pw = 0.0 to about 0.6 at pw = 0.67 g/cm3. Thus, the reaction of guaiacol in water is via the parallel pyrolysis and solvolysis pathways represented by eq 1and 2, respectively, wherein vl, v2, and v3 are as yet undetermined stoichiometric coefficients. A
,OH
0
08aoH +
v,CO
+
v,CH,
+
v3char
(1)
For first-order pyrolysis and second-order hydrolysis, first order in both water and guaiacol concentrations, the pseudo-first-order reaction rate constant for disappearance of guaiacol is kl k2CWo, at low conversions of water. The selectivity to methanol is therefore k2CWo/(kl+ k2Cwo). Thus, following overall guaiacol conversion and the selectivity to methanol allowed determination of k l and k2. The dependence of k2 on water concentration is shown in Figure 2. The positive slope in Figure 2 suggests that either the reaction order in water concentration is in fact greater than one or the truly second-order hydrolysis rate constant is dependent on water loading. The prejudice that hydrolysis might be SN2nucleophilic attack, occurring at low as well as high pw, and the remarkable pressure dependence of the density and thus solvent properties of a supercritical fluid favored the latter explanation. These possibilities were probed via experiments with added salts wherein the global solvent properties could be varied without changing the water density. To affect the hydrolysis transition-state equilibrium significantly, the added salt must dissolve in the supercritical water, dissociation would increase the polarity further. The maximum critical solution pressure for a binary system of water and NaCl has been experimentally determined to be -240
+
e @ k z C m l '5 ' )
3-
A
A
hh
a
'
8
-i
I
'3
no@ A
0
* 0
A
I
1
C
Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 163 was. via changes in the hydrolysis rate constant, which increased with increases in solvent polarity. This indicates that the hydrolysis transition state is more polar than the hydrolysis reactants, which implies that supercritical water is able to support polar, ionic-like chemistry in addition to the free-radical chemistry expected in a gaslike phase. A supercritical fluid is thus an extremely interesting reaction medium since its solvent properties can be easily controlled and manipulated through modest changes in pressure. The Herbrandson and Neufeld (1966) analysis allows illustration of the implications of these findings. Herbrandson and Neufeld applied Hildebrand's regular solution theory to the analysis of kinetics. It is important to emphasize that the supercriticalfguaiacol mixture is not likely a regular solution. We follow the Herbrandson and Neufeld analysis here because it provides a convenient correlation of the waterfguaiacol kinetics data which can be used to assess, quantitatively, the effect of added salts. In its simplest terms, regular solution theory can be viewed as a one-parameter empirical constitutive equation for the excess Gibbs free energy of the water/guaiacol solution. For the bimolecular reaction of eq 3, G+W s Z products (3) The transition-state theory rate constant,
-
can be coupled with the activity coefficient model from regular solution theory, R T In (ri)= Vi ( S i (5) to yield the basic Herbrandson and Neufeld analysis R T In (cTk/k,) = 2BB + (Sw2Vw+ SG'VG- Sz2Vz) (6) where B = v& - VGSG- VwSw (7) Equation 6 follows from eq 4 and 5 by allowing the molar volume of the transition state to be approximately equal to the sum of the molar volumes of the reactants. The solvolysis rate constant will therefore increase with the solubility parameter of the solution, 8, if the transition state is more polar than the reactants, Le., if B > 0. Since the solubility parameter increases with increasing water density (Giddings et al., 1961), the observed increase of k2 with p w of Figure 2 is a qualitative indication that SzV, > SwVw + SGVG. More quantitative analysis requires accounting for the dependence of the parameters of eq 6 on p w , the primary variable in the present experiments. The extreme compressibility of supercritical water over the reaction conditions studied here required modification of the regular solution theory formalism and the concept of solubility parameter. This is because transition-state theory is phrased in molecular terms and regular solution theory is phrased in continuum terms. In the liquid phase, where molecular and molar volumes differ by only the small (-10%) free volume, and the liquid is essentially incompressible, no distinction need be made. However, in the gas or fluid phase, the free volume is large and the volumes occupied by a molecule and the molar volume are quite different. The modification of the regular solution theory formalism to follow therefore sought mixing rules that would assign contributions (Si, Vi)to molecules and not continuous fluids. The present interpretation uses the liquid phase as a reference to define molecule-specific values of Si and Vi.
Imagine a water-rich liquid mixture of water, guaiacol, and biphenyl. The solubility parameters and molar volumes are, in principle, well defined in this case, and the traditional mixing rules
B
C@Ji
=
(8)
where = XiVi/CXjVj
(9) incorporate the small and essentially constant liquid-phase free volume of the molecular interstices into the numerical values of Si and Vi. It is also reasonable that Vz = VG + VW (Moore and Pearson, 1981). Now consider heating and expanding the solution to T > Tc,mlution and pw = 0.5 g/cm3, respectively. The fluid is now highly compressible, and the solution polarity will no longer be that of the liquid reference. There will now be an appreciable free volume, but the mole fractions are unchanged from the liquid reference. The mixing rules of eq 8 and 9 can be used only if Si or Vi changes with pw. This would be akin to the Gidding's correlations for Si with pi (Giddings et al., 1961). Because the present experiments were at both variable p w (water-only experiments) and constant p w (salt-addition experiments), it was more convenient to retain the numerical values of Si and Viat their liquid-phase reference and redefine the mixing rules to account for the free-volume of the supercritical fluid. The utility of this approach is that the important approximation VZm= VGm+ VWmcan be retained. In this view of the SCF solvent solution, each species has an effective volume fraction of @i
The mixture solubility parameter is then represented as In the limiting case of high solvent loading, 8 scales linearly with p w , as in the Giddings correlation (Giddings et al., 1961). Substitution of eq 11into eq 6, with the liquid reference values for the pure component molar volumes, yields
where
C = VGm(6Gm)2 + VWm(SWm),2 - Vzm(pzm)'t (
5
)
p
G
+ R T In k,
(13)
and
B' = VZmSZm- VGmSGm- VWmSWm
(14) Plotting R T In k(pwfMw + p G / M G ) versus pw yields a linear plot with a slope 2B'6wmfpwm. Relevant data are shown in Figure 4, regression of which provides B' = 190 f 20 ( ~ a l . c m ~ / m o l ~and ) ' / ~C = -21700 f 400 calfmol. The present correlation allows the numerical values of the solubility parameter (Sim) and molar volume (Vim)at the liquid-phase reference to be used in eq 11 to predict a mixture property. The addition of salt is thus viewed as to affect the properties of the mixture but not the reference properties of the reactants or transition-state and species. This permits use of B = 190 (~al.cm~/mol~)'/~ C = -21 700 cal/mol in the construction of Figure 5, a plot of In k versus pw according to eq 12.
164 Ind. Eng. Chem. Res., Vol. 28, No. 2, 1989 -15000 T
-1 6000
1
i
*
= VzmSzm- VGmSGm- VWmSWm = 1% 20 ( ~ a l . c m ~ / m o l ~ ) ~ ~ ~ as a measure of the polarity difference. 3. The hydrolysis rate and rate constant can be manipulated by changing the density of the supercritical water and through the addition of salts.
Acknowledgment We are grateful to Dr. M. Gorbaty of Exxon CRD for helpful comments during the conception of the experiments. We are also grateful for the support of this work by the Petroleum Research Fund, administered by the American Chemical Society, the DOE (DE-FG2285PC80509), and the NSF (CBT-8451240).
0 0
01
02
03
,P
04
05
06
0 7
/g cm.3
Figure 4. Determination of hydrolysis transition-state polarity through variation of water density.
Nomenclature C = concentration, mol/cm3 h = Plank’s constant k , = reaction rate constant in an ideal reference and dilute solution, L mol-’ min-’ K, = equilibrium constant M = molecular weight P = pressure R = gas constant s = y i / x = selectivity S = salt loading, g/cm3 T = temperature V = pure molar volume, cm3/mol x = conversion Greek Symbols
0 :O
025
0 %
0 75
i
pw g c m 3
b w ell = ”
Figure 5. Increase in the effective solution density upon addition of salts.
Figure 5 includes, at the abscissa of pw = 0.5 g/cm3, the rate constants measured upon the addition of salt. The horizontal transposition of these rate constants onto the extrapolation of the “water-only” regression curve (according to eq 12) specifies an effective density of the salt/water solution. This can be interpreted as a measure of the solvent polarity that pure water would obtain at the specified density. The polarity of the salt solutions is then equivalent to that of pure water at the “effective density”. This was as high as 1.1 g/cm3 for NaCl loading of 0.71 mmol/cm3. The effect of salt addition may therefore be viewed as an increase in the polarity of the solution, which shifts the transition-state equilibrium (eq 3) toward the transition-state species. To summarize, the left-hand side of Figure 5 shows that the stability of the transition state can be varied with changes in water density, while the right-hand side shows the ability to manipulate the kinetics through the addition of salts. This suggests promise for the design, manipulation, and control of optimal SCF solvent systems.
Conclusions 1. The reaction of guaiacol in supercritical water is via parallel pyrolysis and hydrolysis pathways. 2. The transition state for hydrolysis is more polar than its reactants. Quantitative correlation according to a modified Herbrandson and Neufeld analysis provided B’
x = conversion 8= mixture solubility parameter 6 = solubility parameter = volume fraction y = activity coefficient K = Boltzmann’s constant u = stoichiometric coefficient p = density, g/cm3
+
Subscripts c = critical property G = guaiacol i = species i o = initial condition T = total W = water Z = transition state Superscript m = liquid reference state Registry No. 2-HOC6H40CH3,90-05-1; 2-HOC6H,0H, 12080-9; CHSOH, 67-56-1.
Literature Cited Franck, E. U. Angew. Chem. 1961, 73, 309-322. Giddings, J. C.; Myers, M. N.; McLaren, L.; Keller, R. A. High Pressure Gas Chromatography of Nonvolatile Species. Science (Washington, D.C.) 1961, 162, 67-73. Helling, R. K.; Tester, J. W. Oxidation Kinetics of Carbon Monoxide in Supercritical Water. Energy Fuels 1987, I , 417-423. Herbrandson, H. F.; Neufeld, F. R. Organic Reactions and the Critical Energy Density of the Solvent. The Solubility Parameter, 6, as a New Solvent Parameter. J. Org. Chem. 1966, 31, 1140-1143. Johnston, K. P.; Haynes, C. Extreme Solvent Effects on Reaction Rate Constants at Supercritical Fluid Conditions. AZChE J . 1987, 33, 2017-2026. Lawson, J. R.; Klein, M. T. Influence of Water on Guaiacol Pyrolysis. I n d . Eng. Chem. Fundam. 1985,24, 203-208. Manyya, R.; Brittain, A.; Dealmeida, C.; Mok, W.; Antal, M. J., Jr. Acid-catalysed dehydration of alcohols in supercritical water. Fuel 1987, 66, 1364-1371.
Ind. Eng. Chem. Res. 1989,28, 165-173 McHugh, M. A.; Subramaniam, B. Reactions in Supercritical Fluids-A Review. Ind. Eng. Chem. Process. Des. Dev. 1986,25, 1-12. Moore, J. W.; Pearson, R. G. Kinetics and Mechanism; New York, 1981. Paulaitis, M. E.; Penninger, J. M. L.; Gray, R. D., Jr.; Davidson, P. Chemical Engineering at Supercritical Fluid Conditions; Ann Arbor Science: Ann Arbor, MI, 1983. Quint, J. R.; Wood, R. H. Thermodynamics of a Charged HardSphere Ion in a Compressible Dielectric Fluid. 2. Calculation of the Ion-Solvent Pair Correlation Function, the Excess Solvation, the Dielectric Constant near the Ion, and the Partial Molar Volume of the Ion in a Water-like Fluid above the Critical Point. J . Phys. Chem. 1985,89,380-384.
165
Shaposhnikov, Yu. K.; Kosyukova, L. V. Pereabotka Drev., Ref. Inform. 1965,3,6. Sourirajan, S.; Kennedy, G. C. The System H20-NaCl at Elevated Temperature and Pressures. Am. J . Sci. 1962,266,115-141. Townsend, S. H.; Klein, M. T. Dibenzyl ether as a probe into the supercritical fluid solvent extraction of volatiles from coal with water. Fuel 1985,64, 635-638. Townsend, S. H.; Abraham, M. A.; Huppert, G. L.; Klein, M. T.; Paspek, S. C. Solvent Effects during Reactions in and with Supercritical Fluids. Ind. Eng. Chem. Res. 1988,27,143-149. Vuori, A.; Bredenberg, J. B-son. Holzforschung 1984,3,133. Received for review February 18, 1988 Accepted September 26, 1988
Heterogeneous Model of a Moving Bed Reactor. 2. Parametric Analysis of the Steady-State Structure Pedro E. Arce,t Orlando M. Alfano,t Irma M. B. Trigatti,$and Albert0 E. Cassano*l§ INTEC,l Casilla de Correo No. 91, 3000 S a n t a Fe, Argentina
This work describes a parametric study performed with a heterogeneous model of a countercurrent moving bed reactor which, in the general case, includes heat transfer with the surroundings. An adequate dimensionless form of the mass and energy balances of the reactor model allows us to identify the characteristic numbers related to the behavior of the reactor or the solid pellets. The analysis of the structure of steady states is performed through the representation of the multiplicity surfaces in a three-dimensional space of dimensionless characteristic numbers. In general, a structure of 1-3-1 steady states is obtained for the adiabatic and nonadiabatic cases. However, for the latter it is possible to detect a pathology of 1-3-5-3-1 steady states in a narrow range of the involved parameters. Through this analysis, it is possible to diagnose the probable operating points of the reactor and draw useful criteria for its analysis and design.
I. Introduction A moving bed operation is convenient for a number of processes in chemical and metallurgical industries. Coal combustion and gasification and the direct reduction of iron ores are two important examples at present. Even though counter- and cocurrent operation may be employed, for the case of liquid-liquid (Luss and Amundson, 1967) or gas-solid (Szekely et al., 1976) heterogeneous systems, the countercurrent system seems to be more widely used. For the mathematical modeling of this type of reactor, we need to take into consideration the two phases in countercurrent movement exchanging heat and mass between each other and, eventually, with the environment. The presence of a chemical reaction in one or both phases is also a possibility to be taken into account. These features, indeed, define a physicochemical system whose modeling is highly complex, and therefore, suitable assumptions must be invoked to reduce it to a level of mathematical tractability. The earliest systematic analysis of this type of reactor goes back to a series of papers published by Amundson et al. (Munro and Amundson, 1950; Amundson, 1956a,b; Siegmund et al., 1956). These authors carefully delineated *To whom correspondence concerning this paper should be addressed. Research Assistant from CONICET and U.N.L. t Supporting Research Staff member of CONICET. SMember of CONICET’s Research Staff and Professor a t U.N.L. Instituto de Desmollo Tecnoldgicopara la Industria Qdmica. Universidad Nacional del Litoral (U.N.L.) and Consejo Nacional de Investigaciones Cientificas y TBcnicas (CONICET), 3000 Santa Fe, Argentina. 0888-5885/89/2628-0165$01.50/0
the problem under study and presented their solutions in terms of the Fourier transform. In their analysis, both lumped and intragradient solid particles were considered. However, they limited the study only to the linear range of the reaction rate function with respect to the solid temperature. In this way, they obtained analytical solutions which are valid only for certain particular systems and within a restricted temperature range. Afterwards, Schaefer et al. (1974) studied the behavior of a moving bed reactor at steady state, but employing an exclusively thermal model. The authors showed the possible existence of multiple steady states, using a step function to represent the heat generation curve. In the area of coal gasification and/or combustion, we may refer to the paper of Rudolph (1976), who dealt with the problem of a Lurgi gasifier qualitatively. Yoon et al. (1978) simulated the same type of reactor by using a single temperature for both phases and a shrinking core model for the carbon pellet, with and without an ash layer. Later on, Amundson and Arri (1978) and Arri and Amundson (1978) presented the description of a Lurgi gasifier using a flame-front model for the pellet. They considered different temperatures for the gas and the solid phases and a temperature gradient in the ash layer. More recently, Cho and Joseph (1981) extended Yoon et al.’s model (1978) by including different temperatures for each phase and Caram and Fuentes (1982) presented a simplified model of a countercurrent gasifier which allows the temperature profiles in both phases to be obtained analytically. Regarding the literature in the metallurgical area, there are models of moving bed reactors applied to the direct reduction of iron ores. Tsay et al. (1976a,b), using a three-interface concept for the pellet, proposed a reactor model for the direct reduction of hematite with a mixture 0 1989 American Chemical Society
