DEHYDROGENATION OF METHYL= CYCLOHEXANE OVER A PLATINUM= ALUMINA CATALYST IN ABSENCE OF ADDED HYDROGEN A. W .
R l T C H l E AND A.
C.
N I X O N
Shell UeLelopment Co.. Emerjiille. Calif.
The feasibility of using hydrocarbons as fuels for high speed aircraft in the range of up to about Mach 10
is being studied. The fuel would have to provide cooling through latent, sensible, and endothermic reaction heat. Complete conversion of methylcyclohexane to toluene and hydrogen would give a total heat sink of about 2000 B.t.u. per pound of fuel, about half coming from the endothermic reaction. At moderate temperatures (700" to 1 100" F.), high conversion (95%) and high selectivity for toluene (99%) were obtained a t 1 0-atm. pressure and space velocities up to 100. Only slight conversion was required to provide the hydrogen necessary for maintaining catalyst stability and activity-e.g., 3% a t 1 0-atm. pressure. The specific first-order rate constant was 1 set.-' a t 820" F. and 10 atm.; the activation energy was about 12 kcal. The reaction rate declined somewhat with increasing pressure above 10 atm.
EMPIRICAL study of the catalytic dehydrogenation of propane, methylcyclohexane,and several higher paraffinic hydrocarbons over a chromia on alumina catalyst has been reported (70). This arose out of work done under an Air Force contract to investigate the possibility of utilizing endothermic reactions for cooling high speed aircraft ( 9 ) . The present paper is based on a continuation of this work and relates a detailed study of the dehydrogenation of methylcyclohexane over a platinum-alumina catalyst. As aircraft speeds increase, the problem of coping with aerodynamic heating becomes more and more critical. T h e stagnation temperature rises from 600" F. a t Mach 3 to 1000° F. a t Mach 4 ; l6OO0 F. a t Mach 5 to 2500" F. a t Mach 6 ; and so forth ( 5 ) . The only heat sink on board the plane is the fuel, and this is already being utilized in a minor way with present aircraft to cool the engine oil. More elaborate schemes for utilizing the heat sink in higher speed aircraft have been suggested, taking advantage of latent, sensible, and endothermic reaction heat (7). Heretofore, only thermal (uncatalyzed) cracking reactions have been seriously examined for this purpose ( 7 7 ) . These have the inherent disadvantages of requiring high temperatures to achieve sufficiently high reaction rates and of suffering decreasing heat sink (due to hydrogen transfer reactions) as the conversion increases above 60% ( 7 7 ) . Accordingly, in the present study we considered catalytic reactions as a possible way of circumventing these limitations, although we realized that some disadvantages, such as catalyst weight and greater heat exchanger complexity, are introduced. Various endothermic reactions can be written which should be susceptible of catalysis. These all involve the breaking of carbon-hydrogen or carbon-carbon bonds. The former type are generally more desirable because of the greater energy of the C-H bond. T h e enthalpies involved in some typical reactions are shown below. N
A
Feed Propane Propane n-Octane n-Octane Methylcyclohexane Decalin
Products
Calculated E q u i l . Con- Heat of RPaction w r s i o n at at 1000" h.. 70 Atm. R.f.u.lLb
C3H6, H Z
40
CzH4, CH4 Ethylbenzene, H D Styrene, H Z Toluene, Hz
95 99 99 99
Naphthalene, H 2
(100)
510 ( 1 2 ' 5 d t 100';) 740 970 1440 940
(1000)
All the feeds listed can undergo many other reactions. These are mainly endothermic cracking reactions but hydrogen transfer. which is exothermic, can also occur, so that the endothermicity of the over-all reaction is reduced. All of the hydrocarbons listed would have a sensible and latent heat capacity of about 1000 B.t.u.,Ib. from 70' F. to 1000' K (1340" F.),so that ifextensive dehydrogenation can be achieved the total heat sink will be increased in the most favorable case by about 150%. The total enthalpy required for cooling a high speed aircraft will be a function of many factors, including aircraft speed. altitude, mission requirements, design, and engine configuration. At present it is considered that most of the heat sink will be required to cool the engines. The leading edges. electronic equipment, and living space, which also require cooling, would impose only a minor load. Any speed greater than about Mach 4 . 5 would probably require some endothermic reaction augrnentation for adequate cooling. In contrast to the previous paper ( 7 0 ) , which covered the use of a chromia-alumina catalyst. the present paper relates the dehydrogenation of methylcyclohexane to toluene using a platinum-alumina catalyst. This catalyst is considerably VOL. 5
NO. 1
MARCH
1966
59
more reactive than the chromia catalyst, and thus permits a much higher space velocity in carrying out the reaction. Since the catalyst must be carried aboard an aircraft: its weight constitutes a penalty as far as the payload of the aircraft is concerned! and therefore must be minimized to the greatest extent possible. A considerable amount of work has been done on the dehydrogenation of naphthenes over platinum-alumina catalysts in connection with reforming studies for the improvement of octane number of gasoline (2)-e.g., Platforming. However, these studies differ considerably from those reported here. They generally involve relatively low space velocities and a high ratio of hydrogen to hydrocarbon in the feed. Long catalyst life is a primary goal. In our case, although a long catalyst life would not be unwelcome, our primary goal is high space velocity in order to minimize the size of the reactorheat exchanger. The necessity of providing hydrogen in the feed, by either conveyance or recycle, would constitute a complication preferably avoided. Method
The dehydrogenations were carried out a t 1- to 30-atm. pressure. 842' to 1293' F.. a t LHSV's of 5 to 150 in the absence of added hydrogen. Catalytic reactions were studied using both laboratory-prepared and commercial platinum-alumina catalysts. Thermal reactions were studied in the same equipment, with quartz chips replacing the catalyst. Tests were for 20 to 90 minutes' duration. Product material was analyzed by mass spectrometr) (gas products) and by gas-liquid chromatography, from which conversions and selectivities to desired products were calculated. Coke and polymer formed appeared to be negligible and were not included in the conversion computations. Heat sinks produced were not measured directly but \I. ere calculated from thermodvnamic heats of reaction based on the experimental conversions and product distributions and from the sensible and latent heats (calculated from the freezing point to the block temperature, or to 1340' F.). First-order rate constants \here calculated from the experimentally obtained conversions. Equilibrium conversions for 1- to 30-atm. pressure are shown in Figure 1 These values, together 1% ith the thermodynamic heats of reaction and latent and sensible heats. were calculated from the data of American Petroleum Institute Research Project 44 Experimental
The system for carrying out the reaction studies consisted of a hot tube reactor with conventional devices for measuring feed flow rates and collecting and measuring reaction products.
A I , 4- or '/*-inch 0.d. thermowell entered the reactor from the bottom end and contained the thermocouples used to measure the catalyst bed temperatures. A single thermocouple was placed between the metal furnace liner and the reactor tube in the region of the catalyst bed. The temperature given by this thermocouple was taken as the reactor wall temperature. A detailed description of this reactor system has been given (70). The catalyst was contained in the annular space between the thermowell and the reactor wall. The tube dimensions were such that the annular thickness of the catalyst bed was "16 inch (20-ml. volume of catalyst), using the '/l-inch 0.d. thermowell, and l t ' l B inch (7-ml. volume) using the '/*-inch 0.d. thermowell. The catalyst size was 10 to 20 mesh. The laboratory catalyst was prepared by impregnating alumina granules (Harshaw 01 04) with platinum tetramine hydroxide solution to give a 1% platinum content. The commercial catalyst was a UOP-R8 Platforming catalyst which contained 0.76y0 platinum. Prior to carrying out the experiments, the catalyst was reduced in situ with hydrogen for 30 minutes a t 300' C. and then for 1 hour a t the reaction temperature. GLC analyses were carried out with a 20-foot stainless steel column ('/'*-inch 0 . d . ; 10% C;rbowax on Chromasorb M.') at a column temperature of 70 C. Carrier gas was helium a t a flo\v rate of 60 cc. per minute. The feed was Phillips methylcyclohexane (MCH, 99+%), pure grade, passed through a silica gel column before use. Sulfur content was reduced from 8 to 2 p.p.m. by passing the feed over silica gel. Unless specifically stated otherwise, the temperatures given are the furnace block temperatures. The bulk of the experiments were carried out a t 10-atm. pressure.
Thermal Reaction
Methylcyclohexane reacted thermally in the metal reactor to give mainly cracked and dehydrogenated products which, by GLC analysis, were identified as benzene, toluene, methylcyclohexenes, and methylcyclohexadienes (Table I). (In the GLC chromatogram of the product material, a number of components emerged after M C H and before benzene. In the chromatogram of a sample of hydrogenated product material, essentially no components emerged between M C H and benzene; further, the area of the M C H peak was greater than for nonhydrogenated product material. O n this basis, the components in the product material that emerged between M C H and benzene were presumed to be methylcyclohexenes and methylcyclohexadienes.) At 10-atm. pressure and 1293' F. extensive reaction occurred, over 60y0of the M C H being converted. Under these reaction conditions a difference of 102' F. was observed between the block and catalyst temperatures. However, the calculated heat of reaction was only 118 B.t.u./lb., because of concurrent exothermic reactions between the reaction products. Lower conversions were observed a t lower temperatures and pressures (Table I). Typical product distributions of the gas products are shown in Table 11. First-order rate constants 'based on the rate of disappearance of M C H were calculated using the equation :
k =
LHSV ~
3600
p
X 22,412 MW X P
T 1 X - X 2.3 log __ 273 1-f
where P
= liquid density
LHSV
=
P
= reactor pressure, atmospheres = reaction temperature, O K. (block temperature)
T
0
Jf
MW
liquid hourly space velocity
= fraction reacted-i.e., = molecular weight
% conversion/100
The rate appeared proportional to the total pressure (Table 60
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
Table I .
Thermal Reaction of Methylcyclohexane (Product distribution in the liquid phase) Catalyst. Quartz chips 20 ml. Catalyst volume. 10-20 mesh Catalyst size. Catalyst bed thickness. 3 / 1 6 inch 20 min. Reaction time. Run 25 27 57
24
Temp..
F.
1112 1112 Catalyst bed 1 Pressure. atm. 5 I.HSV Product components. wt. yc MCH 99 0 Toluene Benzene Methylcyclohexenes plus 0 methylcyclohexadienes 0 Cracked. liquidu Cracked. light gas 0 MCH conversion: 7; First-order rate constant, sec. Heat sink, B.t.u./lb., calcd Reactionc 920 Total at block temp. 1120 Total at 1340' F. ( I Emerging before .MCH in CLC analyses. A H f o r reuction to iiquld crackedproducts =
Table II.
1293 1270 1 5
1202 1202 1 5
Block
1
96 8 0 2 0 1
67.0 1.9 1.2
2 1
2 4 0 4
4
3.2 0.027
4.6 3.1 22.2 33.0 0.34
6
10 996 1121 Based on block temprrature. 350 B.t.u./lb., to ltght g a s
Thermal Reaction of Methylcyclohexene
(Product distribution in the gas phase) Catalyst. Quartz chips Catalyst volume. 20 ml. Catalyst size. 10-20 mesh Catalyst bed thickness. 3 / 1 6 inch 20 minutes Reaction time. 1293" F. Block temp. 1 Pressure. atm. LHSV 5 33 0 MCH con\erGon Gas products Hydrogen Methane Ethylene 5.7 Ethane 13 0 Propylene Propane 8 9 Butadiene 5 2 Butenes n-Butane 0 2 Benzene
10 20 51 0
11 . o 9 5 2 1 1. o 2.6 0 2 0 1
I ) in the range of 1 to 10 atm., since for a given temperature the rate constants were little different a t the two pressures. The apparent activation energy was 39.3 kcal. per mole. Catalytic Reaction
The dehydrogenation of methylcyclohexane to toluene and hydrogen proceeded rapidly over the platinum-alumina catalyst. High M C H conversions (95+%) with high selectivity to toluene (99fY0) were observed a t LHSV's as high as 100. Catalyst stabilities were good and conversion declined only a few per cent over the 90-minute process period. The reaction was highly endothermic and temperature differences as great as 200' F. were observed between the furnace block and the catalyst bed. The reaction rate was dependent upon the catalyst bed
44 1081 1131 =
1112 1067 10 20 88.8
0.2 0.1 2.8 8.2 11.2 0.04
51
52
1202 1152 10 20
1293 1191 10 20
79.4 0.4 0.7
52 3 2 2 3 1
7.7 7 3 4 6 20.6 0.08
10 5 13.6 17.7 47 7 0 22
57 1003 1128
40 924 1124
118 1104 1154
-22 B.t.u./lb
dimensions. Higher conversions were obtained by reducing the thickness of the catalyst bed and diluting the catalyst with copper granules. For example, with a '/16-inch annular catalyst bed, 93.3yo conversion was observed a t 1022' F.; with a 3/le-inch thick bed, only 6O.l0/, conversion was observed even a t 1112' F. (10 atm., LHSV = 50, Table 111). Further, with a '/16-inch thick bed diluting the catalyst with a n equal volume of copper granules gave 95.8% conversion compared to about 8270 conversion with no dilution (LHSV 100, 10 atm., 1112' F.). A similar enhancement in conversion was observed with a copper-diluted catalyst using a 3/16-inch thick catalyst bed. The data are tabulated in Table 111. Presumably the variations in reaction rate-i.e., conversion-with catalyst bed dimensions were due to variations of heat-transfer rates within the reactor. Reducing the thickness of the catalyst bed or diluting the catalyst with copper granules resulted in better heat transfer from the reactor wall to the catalyst particles. Hence, the temperature of the catalyst particles was probably higher in the l / 16-inchor in the diluted bed than in the 3/16-inch or in the undiluted bed. The primary aim of this research was to obtain high M C H conversions with high selectivity to toluene a t the highest possible space velocity. Best results were obtained with the '/l,-inch annular catalyst bed when the catalyst was diluted with an equal volume of copper granules. Under these conditions an initial M C H conversion of 95.17, with selectivity for toluene of 99.2% was obtained a t an LHSV of 100, 10atm. pressure, and 1112' F. (Table IV). This corresponded to an endothermic heat of reaction of 894 B.t.u., Ib., or a total heat sink of 1814 B.t.u.jlb. a t 1112' F., or 2014 B.t.u.,'lb. at 1340" F. (In a separate experiment it was shown that a 3H2-toluene mixture was thermally stable a t 1340' F.) Catalyst stability was good and a decline in conversion of only about 2.50/, was observed after 90 minutes' reaction. The temperature difference between the furnace block and the catalyst bed (measured in the thermowell) was over 200' F . (Table IV). This suggests that the rate-determining step in VOL. 5
NO. 1
MARCH
1966
61
Table 111.
Contersion, cc VCU/ Vcetslyat
LHSV 69 53 101 50 1/11 0 50 148 1/16 0 167 100 1111 1 129 100 Corrected t o 11 12" F. using activation energy of 11.7 kcal./mole. Run
a
Catalyst Bed Thickness, In.
Dehydrogenation of Methylcyclohexane 1yo Pt on r\lzOa Catalyst. Catalyst Size. 10-20 mesh Pressure. 10 atm.
the over-all reaction is the rate of heat transfer from the block to the catalyst particles. Apparently, high M C H conversions a t LHSV greater than 100 should be possible if more effective heat transfer to the catalyst particles could be obtained. Attempts to obtain high conversion above 1112' F. were nut successful. Catalyst stability was poor, presumably because of poisoning by products from a concurrent thermal reaction (Table I). At 1202' F. and a n LHSV of 150, a conversion of 81.9% was observed for the first 30-minute period; it declined to 68.5y0 for the second 30-minute period (Table I V ) . At a given temperature and pressure lower conversions were observed with increased space velocities. This effect was studied in a series of "bracketed" runs (Table V). Some catalyst poisoning occurred, and a decline in activity between the standard runs was observed. Presumably the poisoning \vas due to impurities in the feed or to the formation of coke precursors on the catalyst surface. After poisoning, the catalyst could be regenerated (nearly completely) by treatment with hydrogen a t 1-atm. pressure for 1 hour a t the reaction temperature (Table V). First-order rate constants were calculated for a few runs of moderate conversion. From these values, a n apparent activation energy of 11.7 kcal. per mole was calculated for the temperature region 842' to 1022' F. The data are tabulated in Table V I and Figure 2 is a n Arrhenius plot of the data. In these calculations the temperature of the reactor wall was used, as it was felt to be closer to that of the catalyst particles than either the block temperature or that indicated by the central thermocouple. The products obtained from the dehydrogenation reaction indicate that pressure should have a significant effect on the reaction rate. Under the experimental conditions, increasing the pressure from 10 to 30 atm. decreased the conversion about 15% (Table V I I ) , resulting in a reduction in the first-order rate constant by a factor of 2. This effect of pressure on conversion will be investigated further in future work. Effect of Hydrogen Pressure on Catalyst Stability
Good catalyst stability was observed under our reaction conditions without addition of hydrogen to the feed. Work on reforming reactions shows that with a platinum-alumina catalyst some hydrogen must be present in the reaction in order to maintain catalyst stability (2). Presumably, the hydrogen acts to remove or suppress the formation of coke precursors. Although it appeared that under our usual operating conditions (IO-atm. pressure, 842' to 1112' F., LHSV of 20 to 150) the hydrogen generated by the dehydrogenation reaction was sufficient to maintain a clean catalyst surface, it was of interest to determine the minimum partial 62
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Temp.,
F.
Block
Bed
1112 1112 1022 1022 1112
649 838 693 687
Exptl. 60 1 97 2
93 3 67 0 95 8
889
Corr. t o 7772" F
60 1 97 2 82 0" 95 8
OF.
Catalyst, 1% P t on A I 2 0 3 C a t a l y s t V o l u m e : 7 ml. C a t a l y s t B e d Thickness. in. Temperature' Reactor W a l l , ' K ,
I .4 I/T x IO3
Figure 2.
I .5
Temperature coefficient
pressure of hydrogen necessary to maintain good Catalyst stability. Several experiments were carried out without added hydrogen a t 1-atm. pressure, 842' to 932' F., and various low MCH conversions. (The degrec of conversion determined the hydrogen partial pressure during reaction.) The experiments were run for 20 to 30 minutes and the decline in catalyst activity as indicated by the increase in reactor temperature was noted. The hydrogen partial pressure present during reaction was then calculated from the MCH converted (Table V I I I ) . With a hydrogen partial pressure of 0.74 atm. during reaction, the catalyst appeared stable over a 30-minute reaction period. At lower hydrogen partial pressures the activity declined slowly in one case (0.63 atm. Hy) and the catalyst was completely deactivated in another case (0.47 atm. H d Further. treating with hvdrogen (1 atm.) a t 1112' F. for 30 minutes did not regenerate a deactivated catalyst.
Table IV.
Dehydrogenation of Methylcyclohexane Catalyst. Catalyst volume. Pressure. Catalyst size. Catalyst bed thickness. Catalyst diluted with 7 ml. of copper pellets.
at High Space Velocity 1%;Pt on AlnOs 7 ml. 10 atm. 10-20 mesh '/I6 inch (10-20 mesh)
Run
LHSV Reaction time, min. Temp., F. Catalyst Reactor wall Block Liquid product analysis, wt. MCH Toluene Benzene Methylcyclohexenes Cracked MCH conversion, wt. 7G Selectivity for toluene, yG Heat sink, B.t.u./lb. Reaction Total at block temp. Total at 1340 ' F.
129
131
132
100
150
150
30
60
90
30
60
30
60
889 1024 1112
893 1029 1112
896 1038 1112
876 1008 1112
874 1008 1112
943 1123 1202
1047 1123 1202
yG 4.2 95 1 0 1 0 1 0 5 95.8 99.2 894 1814 2014
5.4 94 0 0 1 0 1 0 4 94.6 99.4
6.6 92 8 0 1 0 1 0 4 93.4 99.3 872 1792 1992
884 1804 2004
24.9 74 4
25.7 73 5
0 3 0 4 75,l
0 3 0 4 74.3 98.8
99.0
698 1618 1818
188 .11 1 81 . O 0.1 0.4 0.4 81.9 98.9
690 1610 1810
311 .55 3 67.1 0.2 0.6 0.7 68.5 98.0
762 1757 1886
63 1 1393 1755
Table V. Effect of Space Velocity on Conversion Catalyst. 1yc Pt on X 1 2 0 3 Catalyst bed thickness. ' / 1 6 inch Catalyst volume. 7 ml. Pressure. 10 atm. Catalyst size. 10-20 mesh Reaction time. 20 min. Run 133 148 __
Temp., F. Block 842 842 Wall 779 781 LHSV 30 100 Product analysis, wt. 7c 10.1 5 8 . 2 MCH 8 9 . 3 41.5 Toluene 0.5 0.3 Cracked MCH conversion, 7c 8 9 . 9 41.8 Selectivity for toluene, yG 9 9 . 4 9 9 . 3 H Ptreatment at 842' F. for 1 hour before
-
69
164
O
842 788 30 15.9 83.4 0.4 84.1 99.2
842a 788 30"
842 783 50
11.9" 87.6 0 4 88.1 99.5
39.6 59.9 0.4 60.4 99.2
842 790 30 19.3 80.2 0.3 80.7 99.4
Temo.. F. Bdck Wall Conversion, yG k, set.-' ~
71.3 28 1 0.3 28.7 98.0
842 792 30 19.8 79 7 0.4 80.2 99.4
1022 883 50 6.7 92.7 0.5 93.3 99.3
1022 873 100 34 0 65.2 0 6 66.0 98.8
1022 889 50
1022 867 150
6.9 92.5 0.5 93.1 99.3
49.0 50 4 0.6 51.0 98.8
Run 164- 1
Table VII.
7.2 92.3 0.4 92 8 99.5
932 820 100
572 531 50
45.2 54.4 0 4 54.8 99.3
88.6 11.1 0.3 11 4 97.3
Effect of Pressure
Catalyst. 1% Pt on . U 2 0 3 Catalyst volume. 7 ml. Catalyst size. 10-20 mesh ' / I C inch thick Catalyst bed. Temperature. 1022 F. LHSV. 150 Run 75
164-2 76-2
842 752 40,8 0 62
1022 894 50
this experiment
Table VI. Temperature Coefficient Catalyst. 1% Pt on A1203 Catalyst volume. 7 ml. Catalyst size. 10-20 mesh Catalyst bed thickness. ' / l e inch Pressure. 10 atm. LHSV. 100 163
842 788 150
932 820 54.8 1 00
1022 882 66.6 1 45
From the above experiments, it is evident that with this catalyst and a t process conditions of atmospheric pressure and 842' F., in order to maintain good catalyst stability the hydrogen partial pressure must be a t least about 0.7 atm. Hence, the reaction must be carried out a t high conversion. At 10-atm. pressure, only about 2.5% conversion is needed to give a hydrogen partial pressure of about 0.7 atm. In agreement with this, the reactor was successfully operated a t 11yo conversion without apparent catalyst deactivation. In this case catalyst deactivation was tested by bracketing the
Pressure, atm. Conversion. Yc k. set.-'
10 61 1 1 2
20 58 6 0 9
76- 1
30 51 6 0 6
run a t 11% conversion with runs a t 66Oj, conversion (Table VIII). The dehydrogenation reaction was also tested with a commercial Platforming catalyst (UOP-Rg), which contained 0.76y0 Pt and about 0.7% halogen and had a definite acid character. The laboratory catalyst contained 1% Pt, no halogen,
