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J. A. RIDGWAY, Jr., and WILLIAM SCHOEN American Oil Co. (Texas), Texas City, Tex.
New D a t a for Estimating
...
Hexane Isomer Equilibrium Measurements of hexane isomer equilibrium are made with improved accuracy. Dimethylbutane content of the measured equilibrium is less than that calculated from available free energy data; it is consistent with the performance of vapor phase isomerization processes
F O R ESTIMATING equilibrium composition in hexane isomerization, free energy values given by the American Petroleum Institute Project 44 are commonly used. However, this is not a favorable application for calorimetric data. The small free energy of isomerization is obtained as the difference between the large free energies of formation. This procedure magnifies any errors; in hexanes, this magnification is 20 or more so that prediction of exact equilibria cannot be expected. Work on the direct determination of equilibrium composition (3, 4 ) in the 100' to 400' F. range has shown that the calculated equilibria d o not correspond to the observed composition. A similar discrepancy in the 600' to 800' F. range was noted in early studies of isomerization over dual functional catalysts ( 7 ) . This is also indicated by the performance of the newer solid catalyst isomerization processes: Predicted dimethylbutane content at 750' F. is about 27%; yet the upper limit claimed for such processes a t this temperature is only about 2070, and this figure is indicated to be near equilibrium as it is not improved by decreasing space velocity. Further evidence shows a need for revising the calculated equilibrium composition (Figure 1). A blend corresponding to the calculated equilibrium a t 650" F. was isomerized a t that temperature; the product obtained differed in composition from the charge, thus indicating that the charge blend was not the true equilibrium. In particular, comparison of the 2,2-dimethylbutane peak with the average methylpentane peak in the two analyses shows
Figure 1 . Charge and product chromatographs show the calculated equilibrium blend changes in isomerization and, hence, cannot b e the true equilibrium
the product to contain less 2,2-dimethylbutane than the charge. These results raised serious questions as to the true equilibria. This study was undertaken to clarify those questions and to develop, by direct experiment, dependable equilibrium data for the entire temperature range of commercial interest.
Experimental Two American Oil Catalysts were 650' to 750' F. for a vapor used-at phase. and at 122' F. for a liquid phase. These catalysts are, respectively, solid dual funcrional and solid Friedel-Craft types. The high temperature studies utilized combined microreactor-gas chromatography assemblies similar to units described by Emmett (2) and by Martchal
and others (5). 'The reactor (Figure 2C) has a 2.ml. capacity; it was charged with 40- to 60-mesh catalyst and operated at pressure of 5 p.s.i.g. The chromatograph column was a ','4 inch X 12 feet tube packed with fiberbrick carrying 127, isoquinoline; it was operated at 75' F . In most runs, these equipment components were arranged for intermittent operation (Figure 2 A ) . Hydrogen carrier gas was set at 40 ml. per minute, reactor temperature was adjusted, then 10 ml. of charge hydrocarbon was injected over a 15-second period. The run was completed as soon as its product chromatograph was recorded. Alternatively, the equipment can be arranged for continuous operation (Figure 2B). Here a second hydrogen stream (also at 40 ml. per minute) VOL. 51, NO. 9
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level found in these runs are expressed as per cent of the component in question.
H2 REACTOR
FUPNACE
F i g u r e 2. This equipment was used for high temperature studies
8mm
GLASSREICTOP
TUBE
A. Intermittent c h a r g e system B. Continuous c h a r g e system C. Reactor
"TO B A L A N C E CHROMATOGRAPP
Fiducial Limits as yc of Component
Componerit
2,2-DMB 2,3-DMB 2-MP 3-MP
52.3
lt4.0 12.7 &2.7 12.1 13.6
C-MP n-Ca
COLUMY PRESSURE DROP
Experimental Results
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separation of the trio 2,3-dimethylbutane (2,3-DMB), 2-methylpentane (2-MP),
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600
650 REACTOR
700
750
800
TEMPERATURE * F .
Figure 3. k temperature and reaction rate increase, product composition approaches equilibrium irrespective of the hexane isomer charged
picked u p hydrocarbon from a saturator then carried it to the reactor. The saturator was held a t 75" F. and under these conditions gave a hydrocarbon charge rate comparable to a liquid hourly space velocity of about 1.2. Effluent from the reactor was sampled periodically by switching the position of the four-way valve for 15 seconds so as to send the product stream through the chromatograph column. Calibration. The chromatographs were interpreted by determining peak areas and converting them to mole fraction by use of calibration factors. These factors differed from unity by less than 5%; they were determined from analyses of known blends similar in composition to the mixture being studied. The blends were prepared from Phillips pure grade hydrocarbons which in turn had been checked by chromatographic analysis and found to contain no significant impurities.
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High Temperature Equilibrium. As a first step in determining the equilibrium composition at elevated temperatures, the pure isomers were charged individually at a series of reactor temperatures. Typical results are shown in Figure 3. Referring to the bottom curve, which is obtained when charging n-hexane, it is indicated that product neohexane concentration changes rapidly between 600 ' and 650' F. and then more slowly as the temperature, and correspondingly the reaction rate, becomes high enough for equilibrium to be approached. A similar approach to a relatively temperatureindependent concentration is noted
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A CATALYST 376 8 CONTINUOUS RUN
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Figure 4. When only minor changes are necessary to attain equilibrium, these are accomplished easily
H E X A N E ISOMER EQUILIBRIUM
i
.2,2-DNB 51.7%
i
- 2-MP
24.2%
Figure True equilibrium composition is a p proached more closely in succeeding runs
3-MP 9 . 8 % 2,3-DMB 9.0 %
nC6
when the other isomers are charged. n-CB, 3-MP, and 2-MP approach this concentration from below; 2,2-DMB and 2,3-DMB from above; this apparent anomaly in the case of 2,3-DMB is due to its rapid conversion to 2,2-DMB. Equilibrium is defined as the concentration toward which the curves converge. .4 similar analysis of the data for the concentration of the other isomers in the product allows their equilibrium levels to be estimated. Preliminary results obtained in this manner were used in estimating the true equilibrium for a temperature of about 700" F. A blend of this composition was then prepared and its performance compared with that of the pure components. I t was found as shown in Figure 3 for the neohexane case, that product from this blend corresponded to the equilibrium estimated by interpolation between the curves obtained with the pure isomer charges. In further runs to precisely define the equilibrium, this blend was used. The alternate two-sided approach to equilibrium appeared to have no advantage over the blend, was experimentally more cum-
5.3 %
bersome, and introduced a chance for error in the interpolation step. Complete results from the blend runs are shown in Figure 4. The charge blend did not seriously differ from the product except in 2-MP to 3-MP ratio, and this is not significant in view of the similarity in performance of these two isomers (Figure 3). The pure isomer charge runs and the majority of these blend runs were made using the intermittent charge system. To clear up any questions concerning the validity of data obtained under these experimental conditions, continucus runs (the square points of Figure 4) were also made. I n these runs, the blend charged to the saturator was such that its vapor had the same composition as the blend charged intermittently; a relatively large quantity of material was used so that insufficient hydrocarbon was vaporized to materially change the composition during the experiments. Excellent agreement is shown between intermittent and continuous results. I n developing the equilibrium-temperature picture, it was necessary to determine acccurately the true equi-
librium a t one high temperature and at one low temperature. As the high temperature, 700" F. was chosen. In Figure 3, equilibrium is clearly approached at this temperature, and there is not the question of distorted results from cracking that is encountered at higher temperatures. The 700" F. equilibrium composition found from Figure 4 is given in Table I. The range given for the composition values corresponds to the 90% confidence interval found in the calibration work. In view of the extent and precision of the data (Figure 4 ) , it is believed that this is a fair estimate of the dependability of these data. Low Temperature Equilibrium. The equilibrium composition can be defined as that mixture which would not be altered on being exposed to isomerization conditions. This was the basis for the experimental approach in determining the low temperature equilibrium. In defining this composition, a blend corresponding to the API Project 44 calculated equilibrium at isomerization temperature was first charged. The composition of the product from this run was used as the basis for making up the charge to the second run. The composition of the second run product would have provided charge to the third, but it differed little from the second run charge, thus indicating that it closely approximated equilibrium. Rather than adjust the charge composition further, the second run charge was used in a series of three additional runs in order to develop, through multiple experiment, an average equilibrium composition of improved accuracy. This series of low temperature experi-
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Figure 6. The free energy of isomerization of n-hexane to other hexanes is a linear function of temperature VOL. 5 1 , NO. 9
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Table 1. Temy., F.
122
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Liquld Phase,
Component 2,Z-DMB 2,3-DMB 2-MP 3-MP n-Cs Total
Equilibrium Composition
Mole
%
51.7 9.0 24.2 9.8 5.3 100.0
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300
200
Yapor Pressure,
P.S.I.;I. 14.83 11.35 10.49 9.49 7.85
_ _ _ _ _ _ ~ 6 1 . 0 =t1 . 8 8 . 1 ?c 0 . 4 20.2 + 0 . 7 7.4 i 0.2 3.3 i 0.2 __ 100.0
ments is shown in Figure 5. A solid Friedel-Craft-type catalyst was used in a stirred reactor. All experiments were performed at 122” F. with a reaction period of 4 hours. This temperature and these conditions have given effective isomerization. From these results, the average liquid phase equilibrium composition was determined. These values along with API Project 44 vapor pressure data and the corresponding calculated vapor phase compositions are given in Table I. The indicated 90% confidence interval of the composition data was derived by assuming the measurements were of the same quality as the calibration work and correcting for the smaller number of determinations.
49.1 9.2 25.3 10.8 5.6
400
500
Vapor Phase, IIole
%
36.5 9.5 30.1 14.7 9.2
27.9 9.2 32.1 17.9 12.9
Calculation of Final EquilibriumTemperature Relation. I n determining the equilibrium composirion at intermediate temperatures, advantage was taken of the nearly linear relation between temperature and free energy of isomerization. The free energy of isomerization from n-hexane to the individual isomer was calculated for 122 O F. and for 700’ F. These are shown in Figure 6 together with the curves for free energy of isomerization derived from the API Project 44 study. Except for the high temperature region of the n-C6 2-MP curve, all are found to be straight lines and provided authority for our straight-line interpolation with all except the 2-MP isomer. In that case, a straight line to 600” F. and a
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21.7 8.9 32.9 20.2 16.3
600
700
17.5 8.4 33.0 21.9 19.2
14.5 i 0 . 3 8 . 0 =k 0 . 3 33.0 =t0 . 9 23.0 i 0 . 5 21.5 i 0 . 8 __ 100.0
slight curve at higher temperatures, essentially following the API data, were used. Comparison of the two sets of data shows relatively good agreement. The maximum difference is less than 400 calories, and this is less than 1% of thc free energy of formation of the hexanes a t 600” F. The thermodynamic approach involves small differences between large numbers and suffers in accuracy for this reason. From Figure 6, the free energy of isomerization of the individual isomers can be determined at intermediate trmperatures. Using these values, the intermediate temperature compositions of Table I were calculated and the complete composition-temperature picture of Figure 7 was developed. Conclusion
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The data of Figure 7 provide the temperature equilibrium relation sought ; they are relatively dependable as indicated by an average 90% confidence interval estimated to be &3% of the component in question. Comparison of neohexane concentrations found in this study with those reported by other investigators (3: 4 ) shows fair agreement with low temperature values obtained by direct experiment. Equilibrium neohexane concentrations calculated from the best thermodynamic data are consistently high.
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literature Cited
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( 1 ) Ciapetta, F. G . , Hunter, J. B., IND. EXG.CHEM. 45, 147-59 (1953).
(2) Emmett, P. H., “.4dvances in Catalysis,” Vol. E,pp. 645-8 Academic Press New York, 1957. 1. 7 1 ~. Evering. B. L.. d’Ouville. E. L.. J . Am. Che;: Sac. 7 i , 440-5 (1949). ( 4 ) Koch, H., Richter, H., Ber. 7 7 , 127 (1 944). ( 5 I Markchal, J., Convent, L., van Rysselberge, J., Ret.. Znst. franc. t i t t o l e et ann. combustibles 12, 1067-74 (1957).
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RECEIVED for review January 12, !959 800
ACCEPTEDMay 18, 1959
Figure 7. The complete vapor phase hexane isomer equilibrium is calculated from the free energy-temperature relation
Divieion of Petroleum Chemistry, Symposium on Isomerization and Related Processes, 135th Meeting, ACS, Boston, Mass., April 1959.
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