Ind. Eng. Chem. Process Des. Dev. 1981, 20, 49-53
can be calculated by using the equilibrium constant of the L/mol at ionic reaction HS03- = H+ + SO:- (6.2 X 25 "C), and they are shown on the fifth and sixth columns, respectively. The value of pH at the interface decreases from 8.8 to 7.35 with an increase in coexisting SO2 concentration. The experimental result that the rate of absorption of NO during the simultaneous absorption is increased with the concentration of SO2 in the influent stream corresponds to a change in the value of pH at the interface toward favorable value to the complexing reaction of NO with Fe"'-EDTA chelate. In practical situations, however, the rate of absorption of NO may be significantly decreased when the concentration of coexisting SO2 becomes high and the value of pH at the interface of the absorbent is decreased to less than 7. Conclusion The forward step of the complexing reaction: NO + Fe"I-EDTA = FelI1(NO)(EDTA) was found to be first order in NO and one and one-half order in Fe"'-EDTA at pH 7.0, and the forward reaction rate constant was / s 25 "C. The determined to be 1.51 X lo8 ( L / m ~ l ) ' . ~at rate of absorption of NO during the simultaneous absorption of NO and SOz into an aqueous slurry of MgS03 with the addition of FeIII-EDTA can be predicted from the rate of the single absorption of NO into the aqueous solution of Fe"'-EDTA chelate with the same pH. Nomenclature A , = surface area of solid particles, cm2/cm3of slurry C = concentration in liquid phase, mol/L D = diffusivity in liquid phase, cm2/s k = forward rate constant of complexing reaction k c = gas-side mass transfer coefficient, mol/s cm2 atm kL = liquid-side mass transfer coefficient, cm/s k , = mass transfer coefficient for solid dissolution, cm/s
49
N
= k,AgL2/DE NI = mass transfer rate of component I, mol/s cm2 w = concentration of solid in slurry, g/cm3 of slurry or wt % x = contribution of gas to salting-out parameter, L/g-ion kB = gas-phase concentration, ppm zL = thickness of liquid film for gas absorption, cm Greek Letter a = Bunsen absorption coefficient, cm3of gas(STP)/cm3of solution Subscripts A = NO B = Fe"'-EDTA chelate E = MgS03 or SO3*f = in feed stream i = at gas-liquid interface w = water Superscript 0 = without chemical reaction Literature Cited Hattori, H.; Kawai, M.; Miyajima, K.; Sakano, T.; Kan, F.; Saito, A.; Ishikawa, T.; Kanno, K. Kogal 12, 1977, 27. Hikita, H.;Asai, S.; Ishikawa, H.; Okamoto, T.; Sakamoto, S.; Kkagawa, M. J. Chem. Eng. Jpn. 1970, 1 1 , 360. Onda, K.; Sada, E.; Kobayashi, T.; Kito, S.; Ito, K. J. Chem. Eng. Jpn. 1970. 3 , 18. Sada, E.; Kumazawa, H.; Tsuboi, N.; Kudo, I.; Kondo, T. I d . Eng. Chem. Process D e s . D e v . 1870, 17, 321. Sada, E.; Kumazawa, H.; Butt, M. A. J. Chem. Eng. Jpn. 1979a 12, 111. Sada, E.; Matsuda, A.; Yasunishi, A. Preprints for 44th Annual Meeting of the Society of Chemical Engineering Japan, Tokyo, 1979b; p 463. Sada, E.; Kumazawa, H.;Kudo, I.; Kondo, T. Ind. Eng. Chem. Process Des. D e v . 1979c, 18, 275. Sada. E.; Kumazawa, H.; Kudo, I.; Kondo. T. Ind. fng. Chem. Process Des. D e v . 1900, 19, 377. Seidel, A,; Linke, W. F. "SoiubilRies of Inorganic and Metal Organic Compounds", American Chemical Society: Washington, D.C., 1965; p 524.
Received for review January 17, 1980 Accepted July 28, 1980
Chemical Equilibria among Quinoline and Its Reaction Products in Hydrodenitrogenat ion Joseph F. Cocchetto' and Charles N. Satterfleld' Department of Chemical Engineering, Massachusetts Institute of Technobgy, CambrMge, Massachusetts 02 139
Equilibrium constants have been calculated for the various steps believed to be of significance in the quinoline hydrodenitrogenation (HDN) reaction network. Results for the reversible initial reaction steps are compared to more reliable experimental values obtained from studies with quinoline and the three hydrogenated heterocyclic compounds formed from quinoline, each studied individually. The HDN reaction pathways of minimum hydrogen consumption are not thermodynamically favored under representatiie conditions of industrial interest, so the burden of selectively hydrogenating only the heteroring is placed solely on the catalyst.
Quinoline is a model compound representative of the heterocyclic nitrogen compounds found in substantial concentrations in the middle distillate fraction of fuels derived from oil shale, coal, and low-grade petroleum. Since heterocyclic compounds are the most difficult to convert to hydrocarbons and ammonia in processing by School of Chemical Engineering, Cornel1 University, Ithaca,
N.Y. 14853.
hydrodenitrogenation (HDN), a knowledge of the HDN reaction networks of model compounds on representative catalysts assumes some importance to guide studies of the HDN of representative feedstocks. Quinoline has received considerable study since it contains both a heterocyclic ring and a benzene ring, so its reaction pattern is representative of the benzenoid derivatives of pyridine. The steps currently believed to be significant in its overall HDN reaction network are shown in Figure 1. This also lists the acronyms used in this paper for the various compounds dis-
0196-4305/81/1120-0049$01.00/00 1980 American Chemical Society
50
Ind. Eng. Chem. Process Des. Dev., Vol. 20,No. 1, 1981 (PB1
l10 2
z
1
4t (PCH)
Q
quinoline
PyTHO
Py ( o r l , 2 , 3 , 4 1 - t e t r a h y d r o q u i n o l i n e
BzTHQ DHP
Bz (or5,6,7,81 - tetrahydroquinoilne
OPA
o-propylani line
decahydroquinoline ( c i s and trans isomers1
PCHA
propylcyclohexylamine
PE
propylbenzene
PCHE
propylcyclohexene
PCH
propylcyclohexane
Figure 1. Quinoline HDN reaction network.
cussed. Of particular importance is the fact that with present commercial catalysts hydrogenation of either or both of the rings in quinoline is an intermediate step in the overall series of reactions. Our studies in a continuous flow microreactor with quinoline and the three hydrogenated heterocyclic derivatives formed from it in the initial reaction steps demonstrated that all the reactions among these species are reversible over a wide range of industrial processing conditions. A knowledge of the equilibria among them is therefore necessary for the proper interpretation of reaction data, for comparison of different catalysts, and for accurate modeling of this reaction. In addition, it is desirable to have estimates of the equilibria for other steps in this reaction. We have previously estimated the equilibrium constants for some of the HDN reactions of selected heterocyclic nitrogen compounds, including quinoline (Cocchetto and Satterfield, 1976). Standard free energies of formation, estimated by group contribution methods, were employed. These methods are relatively unreliable for heterocyclic nitrogen compounds, so experimental data are especially needed for the reversible reactions involving these substances. The equilibrium between quinoline (&) and Pytetrahydroquinoline (PyTHQ) has been well established from experimental studies (Satterfield et al., 1978),but no experimental measurements appear to have been reported for the other three reversible reactions among the two-ring compounds. Theoretical Values Standard free energies of formation are available only for ammonia, propylbenzene (PB), and propylcyclohexane (PCH) (Stull et al., 1969). The standard free energies of formation of o-propylaniline (OPA), propylcyclohexylamine (PCHA), and propylcyclohexene (PCHE) were estimated by Benson's group contribution technique (Benson et al., 1969), while those for the heterocyclic nitrogen compounds (for which Benson's method is inapplicable) were estimated by a less accurate modified van Krevelen group contribution technique (van Krevelen and Chermin, 1951; Cocchetto, 1974). The estimated standard free energies of formation (for the ideal gas standard state at 1
I O O O i 1, K - '
Figure 2. Theoretical equilibrium constants for hydrogenolysis reactions in quinoline HDN. T
e,
c
1
:I I
L
1 O C
0 0
0 _1
-*t'
/
\
( 1 ) @p7+3H2
= a S H 7
-6 (3)
()'C3H7
e @c3H7.2~,
1 4 ) O/C,H7*H,===
-1
c y C 3 H 7
0 10
20
30
40
1000lT K - '
Figure 3. Theoretical equilibrium constants for additional reactions in quinoline HDN.
atm) are listed by Cocchetto (19791, and details of the estimation techniques are given by Cocchetto (1974). Equilibrium constants were estimated for each of the reaction steps proposed in quinoline HDN, as a function of temperature. Those for the initial ring saturation reactions are presented and compared with experimental values in the following section. Values for the hydrogenolysis steps in the reaction network are presented in Figure 2 and those for additional steps in Figure 3. Both cis and trans isomers of DHQ were considered in estimating the equilibrium constants for reactions involving DHQ. The theoretical equilibrium constants for quinoline hydrogenation to PyTHQ and for the PyTHQ and OPA hydrogenolysis reactions are identical with those published
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 1, 1981 5 1 I
80 D '1
70
50
c
B""t ":t
Po)
1
s. 4 0
c
\
'4
f
20
2100
5
80
0 E : 6
0 -
G
1 300
( 13.3 k
3 50
4 00
4 50
TEMPERATURE, ' C
Figure 4. Equilibrium between propylbenzene and propylcyclohexane.
previously (Cocchetto and Satterfield, 1976). Estimated equilibrium constants for reactions involving heterocyclic nitrogen compounds could be in error by one to two orders of magnitude, while those for the other reactions are more accurate, perhaps within an order of magnitude. In spite of this limitation some important generalizations can be drawn from these calculations regarding the thermodynamics of various steps in quinoline HDN. The initial saturation reactions are all exothermic, with unfavorable equilibrium constants (log K < 0) at the higher temperatures at which HDN is conducted industrially. Since hydrogen is consumed in each of these saturation reactions, increased hydrogen partial pressure shifts these equilibria toward the saturated species. Thus, under HDN conditions, saturation of the aromatic ring as well as the heterocyclic ring of quinoline is potentially reversible; that is, significant quantities of the saturated and unsaturated species could be present if each of these reactions approached equilibrium, depending on the temperature and hydrogen partial pressure. The hydrogenolysis reactions are less exothermic than the saturation reactions (less hydrogen is consumed), but they are essentially irreversible under HDN conditions since their equilibrium constants are so large (see Figure 2). The estimated equilibrium constants for the other reactions proposed in the quinoline HDN network are shown in Figure 3. Dehydrogenation of PCHA (to OPA) can be neglected under HDN conditions since PCHA denitrogenates 90 rapidly that its concentration is always very low and the OPA e PCHA equilibrium is not approached. In fact, in our studies no PCHA was detected in the HDN reaction products. The other three reactions can be considered irreversible under most conditions of industrial interest. Figure 4 shows the equilibrium between propylbenzene (PB) and propylcyclohexane (PCH), calculated from standard free energies of formation (Stull et al., 1969), for representative conditions of industrial interest. Under the usual HDN processing conditions, PCH is the thermodynamically favored product.
Experimental Studies The reaction of each individual heterocyclic compound was studied in a continuous flow, vapor-phase, fixed-bed catalytic microreactor employing a presulfided commercial NiMo/A1203hydrotreating catalyst, in the presence of a
z0"
-
I
I I 1 DHQ F m d (13 3 k Pa)
Lqulbbrium
I
---------
F(y13.3 T H Qk P Fe a )e d
-. g
FyA '1 I
40-
.-
A 20-
H
H (PylHQ)
0
0
I
*3b=
I
I
I
I
(DHO)
I
I
I
I
j
52
Ind. Eng. Chem. Process Des. Dev., Vol. 20,No. 1, 1981 I
-2
-5
,'c
>420
_I
L
330 v
lO0------7
@+?HI-
c
-61
I
1
-i
I
I
-
1
40
E: -4
r
-51
355MPo Oat04 M P a Data
A70
I
1 140
I ---u 300
150
160
1000 I T
K-l
Figure 6. Comparison of experimental and theoretical estimates of equilibrium constants for hydrogenation of quinoline to tetrahydroquinolines. 4 20 I
-21 -3 r
0
-4
375
330
I
100
t.
I
4 50
I
1
t
W.3H2
!
c c
3
9
400 TEMPERATURE, " C
Figure 8. Estimated equilibria among quinoline, tetrahydroquinolines, and decahydroquinoline at 3.53 MPa hydrogen partial pressure.
1,'c 1
35 0
I70
i
80c
'6O0 b
! 501
- 5 c- 6 t-
40
-6 .7
I-
/O 300
I 140
350 400 TEMPERA1 U R E , * C
4 50
I
1.60
150 1000 i 1 ,
170
K-'
Figure 7. Comparison of experimental and theoretical estimates of equilibrium constants for hydrogenation of tetrahydroquinolines to decahydroquinoline.
heterocyclic nitrogen compounds is determined by only the corresponding equilibrium constant (a function of temperature) and the hydrogen partial pressure (assuming ideal gas behavior-a reasonable approximation here). These experimentally estimated equilibrium Constants are shown in Figures 6 and 7; also shown, for comparison, are the corresponding equilibrium constants calculated from estimated standard free energies of formation. The data points signify "best" estimates of the equilibrium constants from experimental data, while the error bars show experimental bounds on these equilibrium constants. Agreement between the equilibrium constanta estimated experimentally and theoretically is quite good, considering the latter could be in error by one to two orders of magnitude. The largest discrepancy is observed for the equilibrium constants for hydrogenation of quinoline to PyTHQ, but the experimental values from present data are in excellent agreement with those previously reported
Figure 9. Estimated equilibria among quinoline, tetrahydroquinolines, and decahydroquinoline at 6.98 MPa hydrogen partial pressure.
(Shih et al., 1977; Satterfield et al., 1978). The experimental log K vs. 1/T lines for the other ring saturation reactions are drawn nearly parallel to the theoretical lines in Figures 6 and 7, within the constraints of passing through all the error bars, and of self-consistency. (The slopes of the theoretical lines generally provide accurate estimates of the standard heats of reaction. This is evidenced by the nearly parallel theoretical and experimental log K vs. 1/T lines in Figure 6 for the Q F* PyTHQ reaction.) Equilibrium constants at 330 "C for two of the reactions could not be estimated accurately from the experimental data because of severe kinetic limitations at this relatively low temperature. The experimental log K vs. 1/T correlations were used to estimate the equilibrium composition of the heterocyclic compounds (Q, PyTHQ, BzTHQ, and DHQ) as a function of temperature at hydrogen partial pressures of 3.53 MPa and 6.98 MPa. These are representative of typical pressures of industrial interest. Ideal gas behavior was assumed in the calculations, and the results are shown in Figures
Ind. Eng. Chem. Process Des. Dev. 1901, 20, 53-62
8 and 9. These figures provide reasonable estimates of the relative quantities of the heterocyclics if each of the initial ring saturation reactions in quinoline HDN were at equilibrium. This can actually occur only if the hydrogenation and dehydrogenation reactions among the heterocyclics are fast relative to the PyTHQ and DHQ hydrogenolysis reactions-a condition approached in our studies. The heterocyclic equilibria favor DHQ at lower temperature and high hydrogen pressure, while quinoline is favored at higher temperature and lower hydrogen pressure. This is consistent with the fact that the ring saturation (hydrogenation) reactions are exothermic and consume hydrogen. The behavior of the tetrahydroquinolines is more complex, since they are partially saturated species subject to either hydrogenation or dehydrogenation. Thus at constant hydrogen partial pressure, the equilibrium concentration of PyTHQ or BzTHQ proceeds through a maximum as temperature increases. The effect of hydrogen pressure on the equilibrium concentration of PyTHQ or BzTHQ depends on the temperature. For example, a t 420 O C an increase in hydrogen pressure from 3.53 MPa to 6.98 MPa increases the equilibrium concentration of PyTHQ or BzTHQ, but the opposite effect occurs a t 330 " C (compare Figures 8 and 9). In commercial hydrotreating processes aimed at nitrogen removal, it is desirable to minimize hydrogen consumption for economic reasons. The equilibrium behavior of the heterocyclics has significant implications in this regard. Saturation of the aromatic ring of quinoline to form BzTHQ is thermodynamically (but not necessarily kinetically) slightly more favorable than saturation of the heteroring, to form PyTHQ (see Figures 8 and 9). (This is consistent with the fact that, at HDN temperatures, saturation of benzene to cyclohexane is thermodynamically
53
more favorable than saturation of pyridine to piperidine for a given temperature and hydrogen partial pressure.) For quinoline HDN, particularly at the higher hydrogen pressures required to achieve satisfactory nitrogen removal rates, the equilibria among the heterocyclics permit substantial oversaturation to DHQ (see Figure 9). These results can most likely be extended, at least qualitatively, to other multiring heterocyclic nitrogen compounds such as acridine and carbazole. Thus the burden of selectively hydrogenating only the heterorings is placed solely on the HDN catalyst. Present commercial hydrotreating catalysts, however, have been shown to exhibit little selectivity for HDN reaction pathways of minimum hydrogen consumption (Shih et al., 1978; Satterfield and Cocchetto, 1980).
Literature Cited Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, 0. R.; O'Neal, H. E.; Rodgers, A. S.; Shaw, R.; Welsh, R. Chem. Rev. 1969, 69, 279. Cocchetto, J. F. S.M. Thesis, M.I.T., Cambrldw, Mass., 1974. Cocchetto, J. F. Ph.D. Thesls, M.I.T., Cambridge, Mass., 1979. Cocchetto, J. F.; Setterfield, C. N. Ind. Eng. Chem. Process Des. Dev. 1978, 75, 272. Satterfield. C. N.; Cocchetto, J. F. Ind. Eng. Chem. Process Des. Dev. 1981, accompanying paper In this Issue. Satterfield, C. N.; Modell, M.; Hites, R. A.; Declerck, C. J. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 141. Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. 6. Am. Chem. Soc.Div. Pet. Chem. Repr. 1977, 22, 919. Shih, S.; Relff, E.;Zawadrki, R.; Katzer. J. R. Am. Chem. Soc. Prepr.. Fuekr Dlv. 1976, 99. Stuli, D. R., Westrum, E. F.. Jr.; Sinke. 0. C. "The Chemical Thermodynamics of Organic Compounds", Wiley: New York, 1969; pp 231, 361, 370. van Krevelen, D. W.; Chermin, H. A. G. Chem Eng. Sci. lS51, 7 , 66.
Receiued for reuiew February 19,1980 Accepted July 21,1980 The work was supported in part by funds from the U S . Environmental Protection Agency.
Reaction Network and Kinetics of the Vapor-Phase Catalytic Hydrodenitrogenation of Quinoline Charles N. Satterfleld" and Joseph F. Cocchetto' Department of Chemical Engineering, Massachusetts Institute of Technology, CambMge, Massachusetts 02 739
Studies were made in a continuous-flow microreactor at 3.55 and 7.0 MPa, 330 to 420 O C , and over a range of contact times, on a presulfided commercial NIMo/A120, catalyst. Quinoline and various intermediate products were reacted individually. In all cases the predominant hydrocarbon product was propylcyclohexane. Reactions among quinoline and its hydrogenated heterocyclic derivatives were all found to be reversible. The complex kinetic behavior observed can be well interpreted in terms of equilibrium limitations on the initial ring saturation reactions as well as large variations among the relative adsorptivities of the various Ncontaining species present initially and formed in the reaction. The kinetics of the irreversible intermediate steps-denitrogenation of o-propylaniline (OPA), hydrogenolysis of Py-tetrahydroquinoline (PyTHQ) and of decahydroquinoline (DHQpare modeled.
The titled reaction is a good model for hydrodenitrogenation (HDN) of six-membered ring nitrogen heterocyclic compounds in middle distillate fuels having a high organonitrogen content such as those derived from oil shale, coal, and low-grade petroleum. The steps curSchool of Chemical Engineering, Cornell University, Ithaca,
N.Y. 14853. 0196-4305/81/1120-0053$01.00/0
rently believed to be significant in the reaction network for quinoline HDN, based on the results of the present and previous studies, are shown in Figure 1 of the paper accompanying this one (Cocchetto and Satterfield, 1980). This also lists the acronyms used in both papers for the various compounds discussed. Model compound studies of various heterocyclic nitrogen compounds have shown that hydrodenitrogenation proceeds via saturation (hydrogenation) of the heterocyclic 0 1980 American Chemical
Society
