Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 2,1978
2. For a low-velocity separation the maximum degree of separation occurs a t a gas velocity near 1.5 times the U,f of the mean dolomite a t low pressure and near the U,f of the mean dolomite at high pressure. The degree of separation is generally higher at high-pressure rather than low-pressure operation and does not appear to be affected by the size distribution of char (420 to 1410 pm) and dolomite (595 to 2000 pm). The rate of separation is a function of the dolomite feed rate, and these two rates are equal when the rate is low. In order to prevent accumulation of ash particles in the bed, the data indicate for the conditions tested that the cross-sectional area of the char-ash separator should be so sized that its unit cross-sectional area removes not more than 400 kglmin m2 of ash agglomerates. 3. The maximum degree of separation for a high-velocity conical separator occurs at a gas velocity between the terminal velocity of the mean char and that of the mean dolomite. The rate of separation decreases asymptotically with the gas velocity a t the nozzle. 4. The data obtained in this investigation indicate that the low-velocityseparation process is better for process design and control of particle separation. This is based on test results which show that the high-velocity separation process is sensitive to the gas velocity a t the nozzle, the nozzle design, the bed composition, and the bed height. The data indicate that the low-velocity separation process can be controlled by the gas velocity in the char-ash separator. Additional hydrodynamic data and high-temperature operation results may alter this conclusion.
141
Literature Cited Archer, D. H., Vidt, E. J., Keairns, D. L., Morris, J. P., Chen, J. L.-P., "Coal Gasification for Clean Power Production", Proceedings of the Third International Conference on Fluidized-Bed Combustion, Hueston Woods, Ohio, 1972, issued as EPA 650/2-73-053, NTlS No. PD 231977, Dec 1973. Chen, J. L.-P., Keairns. D. L., Can. J. Chem. Eng., 53, 395 (1975). Cockerham, R. G., British Patent 1 047 71 1 (1962). "Bench Scale Research on Continuous Gasification Unit," Consolidation Coal Company R&D Report No. 16 to the Office of Coal Research, Department of the Interior, 1968. Gluckman, M. J., Yerushalmi, J., Squires, A. M., "Defluidization Characteristics of Sticky or Agglomerating Beds", "Fluidization Technology", Vol. 11, D. L. Keairns, Ed., p 395, Hemisphere Publishing Corp., Washington, D.C., 1976. Jequier, L., Longchambon, L., Van de Putte, G., J. lnst. Fuel, 33, 584-591 (1960). Lee, B. S., Tarman, P. B., "Status of the Hygas Program", paper presented at the Sixth Synthetic Pipeline Gas Symposium, Chicago, Ili., Oct 28-30, 1974. Merry, J. M. D., Chen, J. L.-P., Keairns, D. L., "Design Considerations for Development of a Commercial Fluidized-Bed Agglomerating Combustor/Gasifier", "Fluidization Technology", Vol. 11, p 423, Hemisphere Publishing Corp.. Washington, D.C., 1976. Mii, T., Yoshida, K., Kunii, D., J. Chem. Eng. Jpn., 6, (1). 100 (1973). Rowe, P. N., Kienow, A. W., Agbim, A. J., Trans. lnst. Chem. Eng., 50, 310, 324 (1972a). Rowe, P. N., Nienow, A. W., Agbim, A. J., "The Role of Particle Size and Density Difference in Segregation in Gas Fluidized Beds", Proc. PACHEC. Kyoto, Japan, 1972b.
Received for review December 30,1976 Accepted November 17,1977 This work is being performed as part of the Westinghouse Coal Gasification Program and has been funded with federal funds from the Energy Research and Development Administration under Contract E(49-18)-1514.The content of this publication does not necessarily reflect the views or policies of the funding agency.
Intermediate Reactions in the Catalytic Hydrodenitrogenation of Quinoline Charles N. Satterfield,' Michael Modell, Ronald A. Hites, and Claude J. Declerck Depaflment of Chemical Engineering, Massachusetts lnstitute of Technology, Cambridge, Massachusetts 02 739
The intermediate reactions in the hydrodenitrogenation (HDN) of quinoline in the vapor phase on a sulfided commercial NiMo/Alp03 catalyst were studied in a continuous-flow microreactor at 500 psig (3.55 MPa) and 1000 psig (7.0 MPa), temperatures of 230 to 420 OC,and quinoline partial pressures of about 13 to 110 kPa. Under all conditions studied, quinoline is rapidly hydrogenatedto an essentially equilibrium concentration of Py-tetrahydroquinoline (Py-THQ). The dominating initial reaction pathway varies greatly with temperature. At the lower temperatures the concentration of quinoline is much less than that of Py-THQ and the latter is then converted either to o-propylaniline or to decahydroquinoline. At the higher temperatures the equilibrium concentration of Py-THQ is much diminished relative to quinoline, and the conversion rate of the latter to Bz-tetrahydroquinoline and subsequently to decahydroquinoline and its products becomes significant. Under the more extreme conditions, a significant fraction of quinoline was converted to various high molecular weight substances including some nitrogen-containing molecules which may be difficult to react further. The various steps in quinoline HDN are compared and contrasted to the analogous steps in pyridine HDN.
Introduction Improved methods for removal of organonitrogen compounds from fuels, as by hydrodenitrogenation, will become of greater importance in the future as it becomes necessary to rely increasingly on processing of lower grade petroleum and on liquids derived from coal and oil shale, which contain much higher concentrations of nitrogen compounds than most liquid fuels processed today. Nitrogen compounds interfere with catalytic reactions such as catalytic cracking, lead to poor 0019-7882/78/1117-0141$01.00/0
color and instability of petroleum products, and in the concentrations existing in unprocessed fuels may produce unacceptably high levels of NO, in combustion gases. Of the various types of nitrogen compounds present, the heterocyclic structures are most difficult to remove by hydrodenitrogenation (HDN). Postulated HDN mechanisms of representative heterocyclic nitrogen compounds have been recently summarized (Cocchetto and Satterfield, 1976). Pyridine HDN is a good model reaction for the nitrogen compounds in the lighter-boiling fuel fractions and the liter-
0 1978 American Chemical Society
142
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 2, 1978
Scheme I. Reaction Pathways in Quinoline Hydrodenitrogenation I
Q
I1
PY-THQ
111
OPA
PBz
tetrahydroquinoline (Bz-THQ), decahydroquinoline (DHQ), propylcyclohexylamine (PCHA) and then ammonia plus propylcyclohexane (PCH),shown as steps IV, V, VI and VII. (We specifically identified all the species shown in Scheme I except for propylcyclohexylamine, but it is a plausible intermediate to propylcyclohexane.) One of the important conclusions from the present study is that quinoline and Py-THQ are in thermodynamic equilibrium under essentially all reaction conditions of interest. Thus a t increased temperature and lower hydrogen pressure the equilibrium shifts back toward quinoline and the second pathway becomes more important. A complicating factor is that decahydroquinoline can be formed from either Py-THQ or Bz-THQ, but the relative importance of the two alternate steps varies greatly with reaction conditions. PBz could plausibly be hydrogenated to PCH (step X), but our results suggest that this is probably not significant. Shih et al. (1977) hypothesize that step IX is important on the basis of bondstrength arguments.
a a cCJH uc'H 1. I ::] a- [a:ucJHI H
-+
+
NH. -+
NH
p1
x7
+ NH
-+
-+
I
H Bz-THQ
DHQ V
PCHA VI
PCH VI1
ature is summarized by Satterfield and Cocchetto (1975). Dinneen (1962) reports that a gas oil fraction having an average molecular weight of 335, obtained from shale oil, contained 1.8 wt % N, with pyridines comprising 35% of the nitrogen compounds; indoles, quinolines, and tetrahydroquinolines 25%, and larger multiring compounds 40%. Snyder (1970) reported that for a 370-455 "C distillate from a California crude, containing 0.45% N, 42% of the nitrogen was in the form of pyridine, quinoline, and their hydroxy derivatives, 7% as indoles, and 40% as carbazoles. For middle distillate fractions, quinoline, which has a boiling point of 283 "C, is a good model compound for characterization of HDN reactions, but only a few studies have been reported of its catalytic hydrodenitrogenation.
Previous Studies Doelman and Vlugter (1963) report studies of quinoline HDN with a fixed bed of CoMo/A1203 catalyst utilizing recycle of product gas, a t temperatures of 150 to 400 "C and largely a t 8.1 MPa pressure. Flinn e t al. (1963) describe laboratory studies of HDN reactions of model compounds, including quinoline, in solution and in representative petroleum fractions on a NiW/A1203 catalyst under industrial processing conditions. Larson (1967) briefly discusses the relative reaction rates of different steps in quinoline HDN in conjunction with a discussion of poisoning in hydrocarbon processing. Madkour et al. (1969) describe the results of four batch runs on quinoline HDN in a high-pressure autoclave using CoMo/Al203 catalyst a t 200-450 "C, 8.1MPa pressure and 2 h reaction time, three of the runs being with added dichlorethane to provide HCl in situ. Aboul-Gheit and Abdou (1973) describe HDN studies of five individual heterocyclic nitrogen compounds, including quinoline, as dissolved in paraffin oil, and carried out in a stirred batch autoclave using CoMo/AlzO3 catalyst a t 350-400 "C, 20.3 MPa hydrogen pressure and contact time of 1to 6 h. Frost and Jensen (1973) hydrodenitrified a crude shale oil and report rate constants for the quinoline-type compounds present, which include pyridines. After our work was completed, a study was published by Shih et al. (1977) on quinoline HDN dissolved in a white oil, utilizing a sulfided NiMo/AlzO3 catalyst in a stirred batch autoclave at temperatures of 300 to 375 "C, pressures of 2.17 to 13.9 MPa, quinoline concentrations of 0.1 to 1.0 wt % in the liquid, and reaction times up to 13 h. The reaction mechanism is complex, but previous studies and our present results indicate that there are two pathways: one involves the successive formation from quinoline of 1,2,3,4-tetrahydroquinoline(Py-THQ), o-propylaniline (OPA), and then ammonia plus n-propylbenzene. These are shown as steps I, 11, and I11 in Scheme I. A second pathway involves the successive formation from quinoline of 5,6,7,8-
Apparatus and Procedure The experimental apparatus was essentially a continuous flow, fixed bed, catalytic microreactor immersed in a fluidized sand bath furnace. Details are given in the thesis by Wilkens (1977), and an earlier version, limited to studies up to about 1.1 MPa, is described by Satterfield et al. (1975). Quinoline (Baker Analyzed Reagent Grade) was pumped into a preheater by an ISCO Model 314 Precision metering pump capable of providing a continuously variable flow rate and was calibrated for the range from 0.3 to 18 cm3/h. The reactor was a 316 stainless steel tube of 4.6 mm inside diameter, mounted vertically. The catalyst was American Cyanamid Aero HDS-3A NiMo/AlzOa commercial hydrotreating catalyst supplied as extrudates (3.1 wt % NiO, 15.0 wt % Moo3).These were ground and sieved to provide a 20/24 mesh fraction. The catalyst charge was 1.48 g (2.17 cm3) giving a packed bed of 13.2 cm length. A single charge was used for all experiments reported here and it had reached a steady-state level of activity during about 275 h of previous exposure to HDN reactions of pyridine in the presence or absence of thiophene. Before each run it was resulfided (see below). Total exhaust gas flow rate was measured with soap film flowmeters. Sampling and Analysis. Effluent samples flowed through a pressure let-down valve and thence to a heated gas sampling valve (2-cm3sample loop) and were analyzed with an on-line Varian Model 2820/30 dual column gas chromatograph fitted with mass flow controllers and a thermal conductivity detector. A 10-ft glass column packed with 3% SP-2250 on Supelcoport was used with temperature-programmed sample elution in hydrogen carrier gas. One composite sample obtained over the temperature range of 280 to 420 "C in run 8 (total pressure 7.0 MPa, quinoline partial pressure = 40.4 kPa) was collected in an ice trap for further identification by the gas chromatography-mass spectrometry technique and it was also analyzed by a separate gas chromatograph equipped with a flame ionization detector, but likewise using a temperature programmed column with SP-2250 packing. Absolute detector responses were determined by injecting known quantities of compounds of interest and measuring their peak areas under experimental conditions. These were converted to relative molar responses (RMR). The response for NH3 was calculated on the basis of a RMR value of 48, and the quantities reported may not be quite as accurate as those for other species. Good resolution on SP-2250 was obtained of a synthetic mixture of the following compounds, in the following order: propylcyclohexane, propylbenzene, transand cis-decahydroquinoline, quinoline, Py-tetrahydroquinoline, and y-phenylpropylamine. The only organic com-
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1978 100 1
. I
--I----&-
z w J 0
1
I
143
1
'\
80
I
v7 I-
250
300
'C
TEMPERATURE,
= 13.5 kPa; total pressure = 3.55 MPa; LHSV = 0.47 h-l; contact time a t 330 "C = 2.5
s; run no. 1.
DHO
OPA+BzTHO
C
Figure 1. HDN of quinoline: initial partial pressure
0
A
400
350
TEMPERATURE,
1
Figure 3. HDN of quinoline: initial partial pressure = 111 kPa; total pressure = 3.55 MPa; benzene as internal standard; LHSV = 3.79 h-l; contact time a t 330 "C = 2.44 s; run no. 5.
p
0 YH)
"3
,,I
0
T O PYTHO
\ PBZ
\
PCH
'\
0
a
20 0
250
300
350
TEMPERATURE
400
,'C
Figure 2. HDN of quinoline: initial partial pressure = 40.5 kPa; total pressure = 3.55 MPa; LHSV = 1.406 h-*; contact time at 330 "C = 2.4 s; run no. 2.
pounds shown in the results for which known samples were not available were o -propylaniline (OPA) and Bz-tetrahydroquinoline (Bz-THQ). During much of this study, it was judged (incorrectly) that OPA would be eluted slightly after Bz-THQ and chromatograms were initially interpreted on that basis. However, analysis of a product sample from run 8 by the gas chromatography-mass spectrometry method (see later) showed that in fact OPA and Bz-THQ unfortunately were eluted together. Data points, therefore, show the sum of these two products, but the ratio of the two varies with reactor conditions. For runs numbered 4 or greater, benzene was added to the feed in known concentration as an internal standard to check on pump feed rate stability and sample reliability. Benzene was essentially unreactive under test conditions and only trace quantities of cyclohexane were formed. A considerable quantity of high molecular weight compounds was also formed, especially a t high quinoline partial pressures and at 3.55 MPa, the lower of the two hydrogen pressures used. The representative product sample from run 8 was also analyzed for this group of products by the gas chromatography-mass spectrometry technique. The equipment was a Hewlett-Packard Model 5982A chemical/electron ionization G C M S system interfaced with an on-line computer consisting of a H - P 5933A data system equipped with 2.5 X lo6 words of disk storage and a CRT terminal. These GC/MS data were interpreted based on well-developed principles of mass spectral fragmentation and, in some cases, on comparison of the mass spectra and GC retention times with those of authentic standard compounds. The major reaction products were identified as Bz-THQ, Py-THQ, and DHQ by comparison with spectra published for authentic materials while OPA was identified by characteristic peaks at mle 135 (due to the molecular ion), and at m/e 106 (due to an amino substituted tropylium ion). The detailed mass spectra are given in the thesis by Declerck (1976). Experimental Procedure. For start-up the reactor system was purged first with argon, then with hydrogen, and then
250
300
350
400
T E M P E R A T U R E ,'C
Figure 4. HDN of quinoline: initial partial pressure = 13.0 kPa; total pressure = 7.0 MPa; benzene as internal standard; LHSV = 0.47 h-l; contact time a t 330 "C = 2.4 s; run no. 9. heated to 230 "C before liquid flow was commenced, to prevent condensation of quinoline. For each run five reaction temperatures were studied, at about 230, 280, 330, 380, and 420 OC. After equilibration for 1 h at a specified temperature, a series of three samples were taken, at intervals of 1.5 to 4.5 h, after which the sand bath was raised to the next temperature and the equilibration period of 1 h and sampling procedure were repeated. The total length of each run was about 12 to 16 h. For shutdown the reactor was allowed to cool to 350 "C, purged with argon for about 0.5 h, and the catalyst was resulfided by flowing a 10-90% mixture of H2S and H2 through the system as it was allowed to cool to 150 "C, and while it was further held at 150 "C overnight. This procedure was shown to provide reproducible activity. Runs were made a t 3.55 and 7.0 MPa and at quinoline partial pressures of about 13,40, and 112 kPa. The mass flow rate of both hydrogen and quinoline was held constant throughout any single run. Contact times quoted are based on packed length and empty cross section of the reactor and the volumetric flow rate at reaction temperature and pressure. For a specified total pressure, the quinoline partial pressure was varied by changing the pump flow rate, holding total gaseous exit flow rate constant. Liquid hourly space velocity, LHSV, is therefore proportional to quinoline partial pressure. In one run (run 15) the variation of product distribution with contact time was determined over the range of 2 to 15 s, at 420 "C, an inlet quinoline partial pressure of 36.7 kPa, and a total pressure of 7.0 MPa (Declerck, 1976).
Results Figures 1 through 3 show the effect of temperature on product composition at a total pressure of 3.55 MPa, and Figures 4 and 5 at 7.0 MPa total pressure. For each set of conditions the concentration in the products of each of the following species were determined: quinoline (Q), Py-tetrahydroquinoline (Py-THQ), decahydroquinoline (DHQ),
144
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2,1978 100
I
8 w
-
7-N
=\
I
I
80
Table I. List of Heavy Molecular Weight Compounds Tentatively Identified by GC/MS GC reten-
tion time, min
250
300
3 50
400
Compound
Mol wt
2.3
126
2.9
124
3.3
120
13.0
133
16.3
250
17.2
244
18.6
238
T E M P E R A T U R E , 'C
Figure 5. HDN of quinoline: initial partial pressure = 40.4 kPa; total pressure = 7.0 MPa; benzene as internal standard; LHSV = 1.406 h-l; contact time at 330 O C = 2.4 s: run no. 8. o-propylaniline and Bz-tetrahydroquinoline as a mixture (OPA Bz-THQ), propylbenzene (PBz), propylcyclohexane (PCH), and "3. For all runs numbered 4 or following (Figures 3-5) some benzene was mixed with the quinoline fed (volume ratio of benzene to quinoline = 0.27) to serve as an internal standard to check on possible fluctuations of the quinoline feed pump. Benzene has been reported not to affect the HDN of quinoline (Doelman and Vlugter, 1963) and comparison of similar runs here with and without benzene showed no significant effect of its presence. Essentially no cyclohexane was formed and the maximum amount (at 430 OC) was estimated to be less than 0.1% of the benzene fed. Steady-state activity of the catalyst was shown to be achieved by reproducing a run at 12.7 kPa quinoline and 3.55 MPa total pressure after a series of runs with quinoline at various other conditions amounting to a total of about 130 h on stream. In addition to the compounds identified (which add up to 100% in Figures 1 to 5), a large variety of highel' molecular weight compounds were also formed. These comprised a total of less than a few percent at, e.g., 3.55 MPa and 40 kPa quinoline, 2.4 s residence time, but increased substantially in quantity as quinoline partial pressure was increased from 40 to 111 kPa. The conversion of quinoline to NH3 was very low below 400 "C with 2 s contact time, increasing to 5 4 %at 420 "C and 2 s contact time. The effect of increased contact times from 2 to 15 s a t 36.7 kPa quinoline, 420 "C, and 7.0 MPa pressure was studied, but at the longer contact times material balances showed a considerable loss of carbon and nitrogen in terms of the molecular species of Scheme I. At least in part this was caused by the increased formation of complex higher molecular weight compounds. At the longest contact times where the feed rate was the lowest, complete steady state also may possibly not have been achieved. With increased contact time Q, THQ, OPA, and DHQ completely disappeared, and NH3 and PCH and PBz appeared, the latter two in the ratio of about 2:l. According to Scheme I, the moles of ammonia formed should be equal to the sum of the moles of PCH and PBz formed. Experimentally the formation of PCH and PBz was about twice that of the ammonia, suggesting that some of the intermediates may disappear by mechanisms yet undetermined to form propylcyclohexane and propylbenzene and nitrogen compounds other than NH3 which are not identified. Gas Chromatographic Mass Spectrometry (GC/MS). The GC/MS method was used to analyze the averaged reaction products from run 8 (Figure 5) obtained over the temperature range of 230-420 "C. This set of reaction conditions yields an intermediate amount of high molecular weight material in contrast to high-pressure, low-concentration quinoline studies on the one hand and low-pressure, high quinoline concentration runs on the other.
+
21.7 or
259
I
22.0
CH=
23.8
CH-C
6 - c ~ ~ -CH,-
CH,
257
I
-CHZ- CH,-CH,
251
2"
26.3
374
Two GC/MS identification runs were made. One was to identify specifically the principal compounds present for comparison with the results from the on-line gas chromatograph. This confirmed that the principal species were: decahydroquinoline (cis and trans), Bz-tetrahydroquinoline, quinoline, o-propylaniline, and Py-tetrahydroquinoline. For identification of minor compounds without overloading the mass spectrometer with the main compounds, in a second run the valve to the mass spectrometer was opened at the moment of sample injection, closed while the peaks of the major products were eluted, and then reopened. More than 300 mass spectra were taken and recorded. Table I shows the tentative compound assignments of the more significant GC peaks in order of increasing retention times. The identification of compounds is time-consuming and the structures given are those most probable based on the
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1978 TEMPERATURE ,'C
other compounds identified in this study. Isomeric structures of those shown are also quite possible. 100
Discussion Thermodynamic Equilibrium between Quinoline and Py-Tetrahydroquinoline (Py-THQ).From Figures 1 to 5 it is seen that quinoline is converted rapidly and completely to Py-THQ at the lowest temperatures studied. The increasing amounts of quinoline found a t higher temperatures suggest that a thermodynamic equilibrium between quinoline and Py-THQ is approached, which shifts in favor of quinoline a t higher temperatures. The function K'7.0 = (Py-THQ)/(Q)as determined from the data of run 15 at 7.0 MPa varied from 1.9 at 2.1 s to 2.15 to 2.25 at 2.9 to 8.1 s, indicating that at least a t 420 "C equilibrium is nearly established between these two compounds even a t the shortest contact time (K' = K,.PH?). The functions K'3.55 and K'7.0 (where the subscript refers to the pressure) as calculated for all runs are shown in Figure 6 against the reciprocal of the absolute temperature on a semilog plot. (The total number of runs was somewhat greater than those shown in Figures 1 through 5 . ) The data fall on two different straight lines with good agreement. Linear regression yields the following, both with correlation coefficients of 0.99
430
300
I
I
Partial Pressure, k Pa
F
I
-
2,
Y
-;
1 .o
Run1 8 Run2
A0.5
V Run5
111
8.27 X 10-l1 exp(30 830/RT)
(1)
K'7.0 =
7.00 X
exp(33 115/RT)
(2)
The ratio K'7.01K~3.55varies from 5.7 a t 330 "C to 4.5 a t 420 "C. If complete equilibrium were established, the ratio should be 3.9, suggesting that possibly a slightly closer approach to equilibrium is reached ai. 7.0 than a t 3.55 MPa. Cocchetto and Satterj'ield (1976) estimated the thermodynamic equilibrium between quinoline and Py-tetrahydroquinoline in conjunction with theoretical estimations of the equilibria of various steps in HDN reactions of a variety of heterocyclic nitrogen compounds (see also Cocchetto, 1974). A plot of log K , vs. 1/T yielded a straight line from which the heat of this reaction was calculated to be 31.3 kcal/mol by the Van't Hoff relationship. This is remarkably close to an average value of 32 f 1.2 kcal/mol from eq 1 and 2. If equilibrium is indeed established, by dividing the function K' here by the square of the hydrogen pressure in atmospheres (essentially the same as total Pressure), one obtains the conventional equilibrium constant K,. The absolute values of K , thus calculated from Figure 6 are higher than those previously estimated by a factor of about 32 at 3.55 MPa and 41 a t 7.0 MPa. The previous estimates of free energies of formation were based on a modified Van Krevelen method which we stated could be in error by several kcallmol for saturated and unsaturated heterocyclic: nitrogen compounds. The present experimental studies give more reliable values of K , and suggest that the previously published correlation of log K , vs. 1/T should be shifted upward by a value of log K , of about 1.5, the slope remaining the same. These conclusions are supported by Shih et al. (1977). In studies at 342 "C and 3.55 MPa hydrogen pressure the equilibrium was approached starting with either quinoline or Py-THQ. The equilibrium ratio of THQ to Q was 8.0, which agrees exactly with the curve in Figure 6 for 3.55 MPa. It is concluded that the equilibrium between quinoline and tetrahydroquinoline is more favorable toward Py-tetrahydroquinoline than previously estimated and that the assumption of quasi-equilibrium between these two species is well justified for kinetic analysis and modeling of a NiMo catalyzed reaction over essentially all of the range of conditions of current industrial interest. Thermodynamic Equilibrium between Propylbenzene
/-
13.5
Run 10 12.8 O R u n 7 113
/
L c
€
0.7
+
t
L
:Doelman
and Vlugter ( 1 9 6 3 ) 8.1OMPo Total Pressure, 4 8 0 k P a Ouinol I ne
1.5
1.4 -
K'3.55 =
145
1000 T
1.6
1.7
O K - '
2
Figure 6. Experimentally determined concentration ratio of Pytetrahydroquinoline to quinoline as a function of the reciprocal of the temperature. (PBz) and Propylcyclohexane (PCH). The exit gas concentrations of PBz and PCH ranged from 0.1 to 10 mol % in all runs at 3.55 MPa and up to 380 "C at 7.0 MPa. At 420 "C and 7.0 MPa, the concentrations were considerably higher, reaching 37% and 18%for PCH and PBz, respectively, at 15 s contact time. Using the measured outlet concentrations, the ratio (PCH)/(PBz) (H2)3(in units of atmospheres) was calculated for all runs in which the concentrations exceeded 0.4%. The results are shown in Figure 7, together with the equilibrium constant calculated from published values for the free energies of formation (Stull et al., 1969). The scatter in the data, which may be caused in large part by inaccuracies in measuring low concentrations, makes it difficult to conclude whether or not step X proceeds rapidly to equilibrium. Some kinetic arguments (see later) suggest that step X may be relatively unimportant. Reaction Pathways. The evidence from the present study supports the general reaction scheme suggested by Larson (1967) based on his analysis of the data of Doelman and Vlugter on a CoMo catalyst, but with several differences in emphasis, notably that quinoline and Py-THQ are in equilibrium at all times and the probable importance here of a new reaction step, the hydrogenation of Py-THQ to DHQ. Doelman and Vlugter identified no Bz-THQ below 350 "C, but its content increased rapidly with temperature to about 15% at 450 "C. The sum of OPA and Bz-THQ as a function of temperature that we found (e.g., Figure 5 ) is consistent with their observation and both studies suggest that step IV is unimportant a t low temperatures. In the study by Shih et al. (1977) the analytical system allowed the determination of the principal separate organonitrogen compounds involved in the HDN reactions but not, in general, of NH3 and of the individual hydrocarbon species formed. They interpreted their results in terms of the reaction network of Scheme I up to the formation of OPA or DHQ, likewise concluding that step I was in equilibrium. In one run using n-cetane as a solvent they reported that the major hydrocarbon product was PCH and PBz was present in small
146
-
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1978 T EM PE RAT U R E
e
TEMPERATURE ('C)
C
430
t 24
10
1.4
1000 (
1.5
1.6
)
T
Figure 7. Comparison of experimentally observed ratio of propylcyclohexane to propylbenzene with that for equilibrium conditions; units are atm-3; see Figure 6 for legend.
380
/ /
Y ,: /
1.4
L 3 . 35 . 5 5MMPPo o
1ooO 1.5 T * 1ooO *
(
OK-)
) 1.6
(
OK-)
)
3
T
Figure 8. Calculated equilibrium ratio of cyclohexane to benzene as a function of the reciprocal of the temperature.
Table I1 Reaction
k , min-'
E , kcal/mol
I1 I11 IV V VI VI11
0.12
35
0.07
24 18
2.60 1.30 3.20 0.30
17
31 16
quantity. Reactions I11 and VI were described in terms of an overall disappearance of OPA or DHQ to form a hydrocarbon and NHs but it was hypothesized that the further reaction of OPA probably would require hydrogenation of the benzene ring (e.g., to PCHA by step IX) rather than proceeding by step 111. For a standard set of conditions (342 "C, 3.55 MPa total pressure) they reported the values listed in Table I1 for firstorder reaction rate constants, as well as activation energies based on studies over the temperature range of about 300 to 375 O C . Since the reactions are not truly first order, the rate constants vary with hydrogen pressure, initial quinoline concentration, and other species present. Although their studies were in the liquid phase and not strictly comparable with ours on a quantitative basis, nevertheless, our results are consistent with the general change in product distribution with degree of conversion, temperature, and pressure that their model would imply. The fact that no Bz-THQ was found here at low temperatures is consistent with a negligibly small equilibrium concentration of quinoline, even though the reaction rate constant for step IV is substantial. DHQ appeared here in considerable concentrations a t low temperatures where no Bz-THQ was found, which is consistent with the dominating importance of steps I and VI11 at low temperatures, shifting to steps IV and V at high temperatures where the equilibrium concentration of Py-THQ has greatly dropped.
Larson postulated that decahydroquinoline could form a cyclohexylamine, although a cyclohexylamine was not detected by Doelman and Vlugter. We likewise did not identify a cyclohexylamine, although traces may have been present, and it is a plausible intermediate that is probably more reactive than its analogue, o-propylaniline, in the primary reaction scheme. At a few seconds contact time, the hydrogenolysis of o propylaniline to propylbenzene and NH3 begins to be significant above about 300 "C. The formation of propylcyclohexane at these temperatures could be plausibly attributed to hydrogenation of propylbenzene (step X), but some evidence suggests that reaction X is unimportant. Benzene, used as an internal standard in many of the runs, did not react appreciably; little cyclohexane was detected under all but the most severe reaction conditions, even though equilibrium favors its formation. This may be seen from Figure 8, which displays the equilibrium ratios of cyclohexane to benzene as calculated from standard free energies of formation (Stull et al., 1969), for the two pressures used herein. Under the experimental conditions, equilibrium conversions of benzene to cyclohexane range from 60 to 100%.The lack of appreciable quantities of cyclohexane in the product thus indicates negligible catalytic activity for benzene hydrogenation, at least in the presence of basic nitrogen compounds. The work of Stengler et al. (1964) is also relevant but not conclusive. They studied the HDN of pyridine, aniline, and several other nitrogen compounds on a NiWMo/A1203 catalyst at 50.7 MPa and temperatures of 190 to 360 "C. The products from HDN of aniline were cyclohexane plus a little benzene, but the concentration of benzene was higher than that calculated from thermodynamic equilibrium and benzene by itself under the same conditions was essentially not hydrogenated. The authors interpreted their results to suggest that some type of activated phenyl residue susceptible to hydro-
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1978
genation was left as an intermediate on the surface of the catalyst after the NH2 group was split off. Effect of Quinoline Partial Pressure and Hydrogen Pressure. For a fixed hydrogen pressure and temperature, an increase in quinoline partial pressure (which is equivalent here to a corresponding increase in LHSV) results in lower percent conversions to ammonia, propylbenzene, and propylcyclohexane; i.e., the overall reaction is less than first order, with respect to quinoline. This implies strong adsorption of reactants, which is consistent with the fact that quinoline is a strong poison for various hydrogenation and hydrodesulfurization reactions. Py-tetrahydroquinoline is more basic than quinoline and is probably even more strongly adsorbed so both compounds could inhibit the rate of formation of other intermediates and products by competitive adsorption. An increase in hydrogen pressure a t a fixed intermediate temperature increases the formation of decahydroquinoline (compare, for example, Figures 1 and 4). This is caused primarily by an increase in the equilibrium amount of Py-THQ present since the first-order rate constants for the various reactions are less markedly affected by hydrogen pressure (Shih et al., 1977). Product Distribution and HDN Conversion. Additional products tentatively identified by GC/MS are shown in Table I and give some further clues about other characteristics of the overall set of reactions. Several significant observations are as follows: (a) Some propyl cyclohexene is formed, as well as the more dominating products propylbenzene and propylcyclohexane. (b) 2Methylindoline (mol wt 133) is also formed. This was suggested to occur from quinoline a t reaction temperatures above 400 "C in the reaction scheme of Madkour et al. (1969). (c) The high molecular weight compounds are apparently the result of randomly linking together fragments of intermediates such as propylcyclohexane, propylbenzene, and o-propylaniline. Other possible linkages than those detected may exist but escaped detection because of their high boiling point. (d) Nitrogen-containing molecules are detected a t very long retention times suggesting that these compounds have low volatility. These first identifications shed some light on reactions occurring on the catalyst surface other than those shown in Scheme I. During the hydrogenolysis of such species as Pyand Bz-tetrahydroquinoline, decahydroquinoline, and 0 propylaniline, transient intermediates are presumably formed and recombination reactions of these reactive species compete with their normal hydrogenation. This suggests that the rate of these recombination reactions should increase with the concentration of the intermediates on the catalyst surface. This is consistent with the experimental finding that the HDN reaction is "cleaner" a t 7.0 MPa than a t 3.55 MPa and that the amount of higher molecular weight compounds increased sharply when the quinoline partial pressure was increased. Moreover, when acetone was added to the reaction products of a run a t 41.5 kPa quinoline, 7.0 PMa total pressure, a white precipitate was formed. This is indicative of the presence of high molecular weight material in the reaction products which perhaps was not vaporized in the gas chromatograph and therefore escaped detection. The GC/MS data also suggest that the formation of propylcyclohexane in relatively large amounts a t long contact times may occur by hydrogenolysis of some of the high molecular weight products. Other possible routes can, however, be imagined. Comparison with Other Studies. Two other studies of quinoline HDN can be compared in some detail with the present results, that of Doelman and Vlugter (1963) on a CoMo/AlaO3 catalyst (about 4 wt % COOand 10.5 wt % Moo3), and the very recent report by Shih et al. (1977) of a liquid phase study on a NiMo/AlzOs catalyst. Most of the Doelman
147
and Vlugter data were obtained a t 8.1 MPa pressure, LSHV = 1and a gas recirculation rate which is equivalent to 15.8 mol of H2/mol of quinoline. This provides a partial pressure of quinoline of about 480 kPa, considerably higher than the highest here. I t is clear that the CoMo catalyst was much less active for the first step of hydrogenation of quinoline to Pytetrahydroquinoline than the NiMo catalyst used here. At the lowest temperature studied, 230 OC, quinoline was converted here almost 100% to Py-tetrahydroquinoline whereas in the Doelman and Vlugter study, which encompassed temperatures from 150 to 450 "C, the maximum ratio of Py-tetrahydroquinoline to quinoline was reached a t 300 "C. Figure 6 plots the value of this ratio that they reported a t 300,350, and 400 "C, from which it appears that equilibrium for this reaction was not closely approached by them until about 350 to 400 "C was reached. In an analogous study of the HDN of pyridine, the first step of hydrogenation to piperidine was likewise found to be more rapid on a NiMo/Al& catalyst than on a CoMo/Al203 catalyst (Satterfield and Cocchetto, 1975). Doelman and Vlugter analyzed their products for quinoline, Py-THQ, Bz-THQ, primary aromatic amines (anilines), primary aliphatic amines (y-phenylpropylamine), and determined nitrogen removal by difference after analyzing the ammonia-free product by the Kjeldahl method. This was shown to agree closely with the total basic nitrogen content as determined by titration with perchloric acid in glacial acetic acid. They did not determine propylbenzene, propylcyclohexane, or NH3 directly. Aniline content was small, but increased to a maximum of about 10% a t 400 "C before decreasing. y-Phenylpropylamine was present in small amounts in their studies, but we found none (this species was well resolved by gas chromatography in a synthetic mixture and we should have identified it if it had been present). This product would be expected if the benzene carbon-nitrogen bond broke in the reaction of Py-THQ. The fact that o-propylaniline is found instead suggests that the latter C-N bond is much stronger than the bond of N with a CH2 structure. In a run a t 342 "C in which y-phenylpropylamine was added to quinoline, Shih et al. (1977) reported that the former reacted very rapidly to form propylbenzene and NH3 so even if it were formed to some extent as an intermediate, little would be expected to be found in the overall product. We observed substantial quantities of decahydroquinoline which decreased with an increase in quinoline partial pressure, but none was reported by Doelman and Vlugter, except for a brief comment that some was found a t very low space velocities. However, this may reflect the considerably higher partial pressure of quinoline in their study. The results of the present work are reminiscient of the observations of Flinn et al. (1963) on the HDN of indole. In the partially reacted products they detected more than 10%in the form of unidentified substances heavier than indole including a number of quinoline and carbazole derivatives. They concluded that one of the reasons for difficulties encountered in high degrees of nitrogen removal by HDN reactions was the formation by side reactions of nitrogen compounds more difficult t o remove than the original reactant. This may also explain, a t least in part, why larger amounts of ammonia were not obtained here when long contact times were used. In their studies with individual compounds they reported that quinoline was less reactive than indole (likewise reported by Aboul-Gheit and Abdou (1973) in studies with single components), whereas in the study of shale oil HDN Koros et al. (1967) found the opposite-that the indole type compounds (includes pyrroles) were less reactive than the quinoline type (includes pyridines). The same was also found by Frost and Jensen (1973) in a study of HDN of shale oil. The reversal of the order of activity in mixtures as compared to studies with individual compounds was attributed by Koros
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to competitive adsorption effects in mixtures in which it was postulated that the more basic quinoline-type compounds would probably be preferentially adsorbed and converted. Comparison of Quinoline and Pyridine HDN. There are several significant differences between the HDN behavior of these two compounds on a NiMo catalyst. The rate of hydrogenation of the heterocyclic ring is much faster for quinoline than for pyridine. With a contact time of 2.9 s, quinoline conversion to Py-tetrahydroquinoline is nearly 100% a t 230 OC and 3.55 MPa whereas with pyridine the conversion is only about 10%a t the same conditions and on the same catalyst (Wilkens, 1977). Equilibrium conditions correspond to essentially 100% of the hydrogenated species in both cases at this temperature and pressure. There is a considerable difference between the hydrogenolysis of o-propylaniline, an aromatic amine, and its homologue n-pentylamine, an aliphatic amine, in pyridine HDN. In quinoline HDN, reaction of o-propylaniline is one of the principal rate-limiting steps, whereas in pyridine HDN, npentylamine is rapidly denitrified. Doelman and Vlugter found aromatic amines to be highly stable and suggested that the C-N bonds in anilines are stronger than in aliphatic amines by having some double bond character, the lone election pair of nitrogen being attracted by the carbon of the benzene ring. This also suggests that possibly a catalyst with stronger acid sites would increase the hydrogenolysis rate and thus the overall rate of the HDN reaction. Indeed, Madkour et al. (1969) found that the presence of HC1 accelerated HDN reactions on a CoMo/Al203 catalyst, studied in the reduced but not sulfided form. During quinoline HDN, numerous high molecular weight compounds are formed though their concentrations are small a t low conversions and low quinoline partial pressures. Some of them have been identified by GC/MS. Certain compounds are nitrogen-containing molecules; others are simply heavy hydrocarbons. In pyridine HDN, alkyl transfer reactions have been noticed (Sonnemans et al., 1972), mainly the formation of N-pentyl piperidine. Analogous disproportionation reactions do not seem to occur during quinoline HDN; a t least they were not detected in this work, nor in the study of Shih (1977). Sonnemans et al. (1974) also observed the formation of an unexpected product C10H16 which did not contain CHs groups and probably not any ring structure. This is reminiscent of some of the high molecular weight hydrocarbons formed here, especially a t high quinoline partial pressures, although all of those identified here have one or more rings. In the HDN of pyridine an enhancement in the catalyst activity was noticed in the presence of sulfur compounds
(Satterfield et al., 1975). One run here was carried out with a feed containing benzene thiol (which readily decomposes to form benzene and H2S) a t 7.0 MPa and 41.5 kPa quinoline, but no significant difference was noticed in the results compared to a similar run performed by the usual procedure. The ratio of benzene thiol to quinoline was only 0.22 w/w and this may not have been high enough to product a significant increase in catalyst activity. However, here the catalyst was resulfided before each run (with the exception of run 2) which was not done in the pyridine studies, and this procedure may have overshadowed a possible effect of H2S as such during reaction.
Acknowledgments The experimental apparatus was constructed by John A. Wilkens. The work was supported in part with funds from the US.Environmental Protection Agency. The GC/MS determinations were made by R. Laflamme and J. Dillon.
Literature Cited Aboul-Gheit, A. K., Abdou, I. K., J. Inst. Pet. London, 59, 188 (1973). Cocchetto, J. F., S.M. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1974. Cocchetto, J. F.. Satterfield, C. N., Ind. Eng. Chem. Process Des. Dev., 15, 272
(1976). Declerck, C. J., S. M. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1976. Dinneen. G. U., Proc. Am. Petr. Inst., 42 (VIII), 41 (1962). Doelman, J., Vlugter, J. C., "Proceedings, Sixth World Petroleum Congress", Section Ill, pp 247-257,The Hague, Netherlands, 1963. Flinn, R. A,, Larson, 0. A., Beuther, H., Hydrocarbon Proc.,Pet. Refiner, 42, 129
(1963). Frost, C. M.. Jensen, H. B., Am. Chem. Soc. Div. Pet. Chern. Prepr., 18 (l),119
(1973). Koros, R. M., Bank, S., Hoffman, J. E., Kay, M. I., Am. Chem. SOC.,Div. Pet. Chem. Prepr., 12, (4),8-165(1967). Larson, 0. A., Am. Chem. Soc., Div. Pet. Chem. Prepr., 12 (4).8-123(1967). Madkour, M. M., Mahmoud, B. H., AWou, I. K., Vlugter, J. C., J. Indian Chem. Soc., 46, 720 (1969). Satterfield, C. N., Cocchetto, J. F., AIChEJ., 21, 1107 (1975). Satterfield, C. N., Modell, M., Mayer, J. F., AIChEJ., 21, 1100 (1975). Shih, S.S.,Katzer, J. R., Kwart, H., Stiles, A. B., Am. Chem. SOC.Div. Pet. Chern. Prepr., 22, (3)919 (1977). Snyder, L. R., Am. Chem. SOC. Div. Pet. Chem. Prepr., 15 (2), C-43(1970). Sonnemans, J. F., Goudriaan, F., Mars, P.. "Fifth International Congress on Catalysis", Palm Beach, Fla., Paper 76,1972. Sonnemans, J., Neyens, W. J., Mars, P., J. Catal., 34, 230 (1974). Stengler, W., Welker. J., Leibnitz, E., Freiberg. Forschungsh. A, 329, 51
(1964). Stull, D. R., Westrum, E. F., Jr., Sinke, G. C., "The Chemical Thermodynamics of Organic Compounds", Wiley, New York, N.Y.. 1969. Wilkens, J. A.. Sc.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1977.
Received for reuiew January 3, 1977 Accepted November 10,1977
