582
INDUSTRIAL AND ENGINEERING CHEMISTRY
cooling bath was used to cool the ammonia to the adsorption temperature. The gas then passed through the adsorbent which was placed in an &inch U-tube dipping into the cooling bath. Approximately 10 grams of adsorbent was used. A calcium chloride-ice mixture was used in the agitated bath. Additional cooling and temperature control was attained through the use of dry ice. During a run, ammonia gas was passed through the system at a rate sufficient to ensure a positive pressure of about 10 mm. of mercury above atmospheric pressure in the system. Adsorption a t the desired temperature was continued for 1 hour, which was considered sufficient for equilibrium since the maximum temperature rise of the adsorbent occurred in 10 minutes. Desorption was carried out by transferring the U-tube containing the adsorbent to a water bath and heating to 100” C. for 1 hour. The desorbed ammonia was absorbed in 2 N sulfuric acid. Back titration with standard sodium hydroxide solution was used to determine the amount of ammonia desorbed. An experimentally determined dead space correction was applied to account for the unadsorbed ammonia initially in the system. The adsorptive capacities thus determined are actually dif-
Vol. 45, No. 3
ferences in adsorptive capacities a t the specified temperature and 100” C., the temperature a t which desorption was carried out. The small amount of ammonia remaining on the adsorbent a t 100” C. is not considered significant for design of commercial adsorbers. RESULTS
The adsorption isobars are shown in Figure 1. Thc points were obtained in a random order. It is apparent that the two materials possess different adsorption characteristics. The fuller’s earth isobar increases sharply as condensation temperature is approached. The isobar for gas-adsorbent carbon indicates that the adsorptive capacity of the carbon approaches a constant value as the condensation temperature is approached. LITERATURE CITED
(1) Deits, V. R . , “Bibliography of Solid Adsorbents,” U. S. Cane Sugar Refiners, Washington, D. C. (1944). (2) Emmett, P. H., Chem. Revs., 43,69-148 (1948). RECEIVED for review June 2 5 , 1952.
ACCEPTED Xovember 3, 1952.
Isomerization of ntanes and Hexanes NATURE AND CONTROL OF SIDE REACTIONS B. L. EVERING, E. L. D’OUVILLE, A. P. LIEN, AKD R. C. WAILTGH‘ Research Department, Standard Oil Co. (Zndiana), Whiting, Znd. NUMBER of publications have described the isomeriztttion of butanes, pentanes, and hexanes (5, 19). Butanes isomerize readily without side reactions under suitably controlled conditions, while pentanes and hexanes disproportionate and crack so readily that the isomerization reaction is secondary. With pentanes the lower boiling products are mainly the result of disproportionation between two molecules:
2CHa(CHz)&K --+ CJLo
+ CaH14
Although pentanes react primarily as shown above, small amounts of higher boiling products are formed because of cracking, and these lead to a hydrogen deficiency and gradual deactivation of the catalyst. The cracking (16)reaction is even more pronounced in the case of hexanes and results in more rapid catalyst deactivation. A number of materials have been investigated as inhibitors of the disproportionation and cracking reactions. Hydrogen ( 6 , 7 , 11, 15, 20, $6, 28), aromatics, naphthenes, and isobutane (2-methyl propane) (6, 16, 18) have been reported to inhibit the undesired side reactions in varying degrees. The present paper is concerned with a more detailed account of the inhibiting effect of hydrogen, aromatics, naphthenes, and isobutane and their effect on catalyst life. All these materials inhibit the disproportionation of pentanes and increase catalyst life during isomerization. Hexanes can be successfully isomerized only in the presence of hydrogen. Olefins nullify the effect of inhibitors and accelerate the disproportionation and cracking reactions. Additional information on the role of hydrogen chloride as a catalyst component is presented, supplementing that previously reported 1
Present address, Arapahoe Chemical Co , Boulder, Colo
(14). Finally a mechanism is proposed based on an intramolecular rearrangement while the hydrocarbon is associated with the ionized aluminum chloride-hgdrogen chloride complex. In this work single experiments are often found to be misleading because of changes in the physical condition of the catalyst during an experiment. Therefore the conclusions d r a m are based for the most part on life studies which are considered more reliable. These data are the outgrowth of the exploratory research which led to the development of the Indiana pentane and hexane isomerization processes. CATALYST AND REAGENTS
The aluminum chloride was obtained from the Hooker Electrochemical Co. In early experiments the aluminum chloride was resublimed under vacuum and stored in sealed glass ampoules. An ampoule was opened just prior to an experiment, weighed, and transferred as rapidly as possible to the reactor without other precautions to avoid contact with moisture in the air. The aluminum chloride as received gave substantially the same results under these circumstances as the resublimed material and therefore was used in all subsequent experiments. The ratio of chlorine t o aluminum corresponded closely to that found by Stevenson and Beeck (26) for their moist aluminum chloride. The authors’ catalyst presumably had the approximate empirical formula of Al~Cls.~(OH)0.6 that they reported. The hydrogen chloride was commercial anhydrous compressed gas from the Harshaw Chemical Go. Hydrogen was obtained from the National Cylinder Gas Co. and used as received. The n-butane, isobutane, n-pentane, and 2,3-dimethylbutane were Phillips Petroleum Co. technical grade hydrocarbons with purity better than 95% by volume; they were used without further purification. Pentanes and hexanes that were used in large quantities were narrow cuts fractionated from virgin midcontinent naphthas. The plant pentanes were 99+% C.;
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1953
one stock contained 12% isopentane (2-methylbutane), the other 42% isopentane. The olefin content was 0.06% by the Francis bromide-bromate method. The hexane-containing charge stock was a light naphtha of 67' C. end point containing 0.009 weight % sulfur and had the composition shown in Table I.
TABLEI. COMPOSITION OF MID-CONTINENT LIGHT NAPHTHA Volume % 1.5 35.9 3.0 1.0 1.0 23.3 13.4 17.4 3.0 0.5
Isopentane n-Pentane Cyclopentane !2,2-Dimethylbutane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane n-Hexane Methylcyclopentane Benzene Total
583
molecular reaction law, this afforded a ready means of correcting for the variable contact time and following the activity of the catalyst during a life study. Using octane number as a rapid measure of the degree of isomerization and substituting octane number and time in the unimolecular reaction equation, the rate constant k was obtained. T o avoid confusion this rate constant is called the catalyst activity index, a. The value 01 is valid only for comparative purposes and is given by the following expression:
23
a = A log t
A B-C
where A = equilibrium octane number minus octane number of feed B = equilibrium octane number C = octane number of product t = reaction time in hours
100.0
IN ABSENCE OF INHIBITORS The equilibrium octane number was established experimentally TABLE 11. REACTIONOF PENTANES
Hydrocarbon AlCls wt Yo of hydrocarbon HC1, 'wt. '% of hydrocarbon Temperature, C. Contact time, min.
u
Butanes and lighter wt. % Octane number (CF'RM) Isopentane, vol. % based on C6
Mixed Pentanes& 11.5 12.4 3.3 3.3 100 100 120 5 55.0 81.1 84 b
42.5 80b6 83
n-Pentane 11.4 3.3 66
10
14.5 66.8 18.7
42 vol. % isopentane and 58 vol. % n-pentane. Estimated from octane number.
APPARATUS AND PROCEDURE
The apparatus has been described in detail (7). The reactors had a capacity of 1490 ml. and were stirred mechanically at 1725 r.p.m. They were jacketed so that they could be quickly heated by steam or cooled by water; less than 5 minutes were required t o reach reaction temperature or to quench by cooling. Auxiliary equipment was provided for introducing measured uantities of hydrocarbons, hydrogen chloride, and hydrogen. rovision was made for the withdrawal of the liquid product from the settled aluminum chloride catalyst.
8
( 7 ) . Thus the effect of inhibitors and other added materials was determined by noting the change in catalyst activity during a life study. REACTIONS OF PENTANES
I n Table I1 are shown the results of bringing pentanes into contact with aluminum chloride and hydrogen chloride. Although the pentanes are isomerized at 100' C. to near an equilibrium mixture of isopentane and n-pentane, disproportionation t o products boiling below and above the pentanes was a major reaction. Disproportionation is a rapid reaction, since 5 minutes gave almost as large an amount of disproportionation products as 2 hours. Lowering the temperature t o 66' C. reduced disproportionation but also reduced isomerization proportionally, so t h a t there was no substantial improvement in the ratio of isomerization t o disproportionation.
A series of experiments was carried out as follows:
*
The reactor was charged with aluminum chloride, hydrocarbon, hydrogen chloride, and the inhibitor. Agitation was started and the reaction mixture was quickly raised t o temperature. After a given time, agitation was stopped and the reaction mixture was quickly cooled. The catalyst was allowed t o settle and the bulk of the li uid Lydrocarbon product was withdrawn through a bleed-out t&e extending two thirds of the way to the bottom of the reactor. The reactor was then recharged and the procedure repeated a number of times. As liquid roduct was withdrawn from the reactor into a flask, the voktile products t h a t separated passed successively through a soda-lime tube t o remove hydrogen chloride, a dry iceacetone condenser to condense light hydrocarbons, and ti wet-test meter t o measure the effluent hydrogen and any other noncondensable gases. The products were first distilled in a low-temperature fractionating column packed with glass helices and equivalent t o 20 theoretical plates to separate the butanes and lighter from the liquid roducts. The composition of the liquid products either was &ermined by careful fractionation in a wire gauze packed column testing 60 to 80 theoretical plates or was estimated by the determination of the motor-method octane number (CFRM; ASTM D-357). All the life studies on pentanes were made at 100' C. with 11.5 weight yo aluminum chloride and 3.2 weight % hydrogen chloride based on charge. The contact time was held constant at 3 hours except in the first run, which was 2 hours; thus conversion t o isopentane was a direct measure of catalyst activity. The life studies on light naphtha were made at 165' C. (unless otherwise specified) with 10.7 weight yo aluminum chloride and 3.1 weight % hydrogen chloride based on charge. I n this case it was not feasible t o use constant contact time throughout the life study because of the wide variation in catalyst activity; therefore the isomerization of light naphtha t o near equilibrium composition was obtained by varying the contact time. As the isomerization reaction has been shown to follow closely a uni-
60
80 40
8
60
8
20
W
d
2 W I-
40
=.
I
I?:
W
-I
z 4
z
w 80 n 0
e
60
40
- = * - -
.._
I.
.
MOLES PENTANE / MOLE AICI,
Figure 1. Isomerization of Pentanes A. B.
Uninhibited Inhibited with 0.5% of benzene
0 9% isopentane 0 Q
$' 6 butanes and lighter
No benzene added
A life study was made at 100' C. The results are shown in Figure l A , where isomerization and disproportionation are plotted against the age of the catalyst. The hydrocarbon was mixed pentanes containing 42 volume yo of isopentane. Initially the main reactions are disproportionation and isomeriza-
INDUSTRIAL AND ENGINEERING CHEMISTRY
584
0
Vol. 45, No. 3
0
0.5% BENZENE
209* BENZENE 1.0 % XYLENE
-I
n
z 40
IO
UJ
w z
0 100
50
I50
200
tion, both of which decline as the activity of the catalyst, decreases on-ing to the formation of complex resulting from side reactions. Because the isopentane content is known with somewhat less certainty in the early stages of the life test, owing to the disproportionation products, that portion of the curve is shown as a dotted line. The isomerization activity of the cat,alyst declines very rapidly and virtually no isomerization occurs after a catalyst age of 55 moles of pentane per mole of aluminum chloride. However, a t this point the catalyst is still active for disproportionation, as is shoxn by the product,ioii of 32y0 butanes and lighter; apparently the disproportionation reaction requires a less active catalyst than the isomerization reaction. During this life test 1300 grams of butanes and lighter were formed by disproportionation, Jv-ith only 44 grams entering into a complex with the aluminurn chloride. Products that formed out to 78 moles of pentanes per mole of aluminum chloride were combined and fractionated on a 60theoretical-plate column. As shown in Table 111, large quantities of hydrocarbons both lighter and heavier t'han pentanes are formed, confirming the observation that the primary butaneproducing reaction is disproportionation. The fraction boiling below pentane was mainly isobutane v i t h but a small amount of n-butane and a trace of propane. The hexanes are a mixture of the five isomers in approximately equilibrium proportions.
TABLE 111. COVPOSITION OF PE:\TAAE DISPROPORTIO\ ATIO\ PRODUCT T o 1 o/c
EFFECT OF ARoaraTIcs.
1 0
5 0 28 0 15 5 21 6 18 3 10 6
ioo.0
A life test was made with 0.5 volume
% benzene added t o the mixed pentane feed containing 42 volume % isopentane. The effect in controlling the disproportionation of pentanes is shown in Figure 1B. The benzene inhibited the disproportionation reaction to such an extent that an average of only 4% butane was produced, whereas the isomerization reaction proceeded readily. There was a marked increase in catalyst life; the catalyst was still producing a prodisopentane a t a catalyst age of uct containing 66 volume 212 moles of pentane per mole of aluminuni chloride. The rise in the isomerization curve to a maximum is attributed to the better contact between catalyst and hydrocarbon; the better
r0
I00
150
02
250 3
200
m
MOLES P E N T A N E / M O L E AlCl,
Figure 3.
Effect of Benzene Concentration and Xylene on Pentane Isomerization
Propane n-Butane Isobutane Isopentane n-Pentane Hexanes Heptanes and hearisi Total
50
250
MOLES PENTANE / MOLE AICI,
Figure 2.
a
Effect of Cyclohexane on Pentane Isomerization
5 % cyclohexane. €3 % isopentane, cb % butanes and lighter 2.5% osclohexane. 0 o/p isopentane, c) % butanes and lighter
contact results IT hen the solid aluminum chloride is converted to the highly active liquid aluminum chloride-hydrocarbon complex by the accumulation of unsaturated hvdrocarbons on the catalyst. A subsequent decline in activity is the result of the further accumulation of hydrocarbon, which tends to dilute the catalyst without any further improvement in contact between reactants.
TABLEIv. DISPOSITION O F BENZENE DURING OF PENTANES 55
169 29
159
2.0 2.0 0.5 0.5
1.30
1.94
0.27 0 41
ISOMERIZATION
0 07 0 15 0 02
0.06
At a catalyst age of 185 moles of pentanes per mole of aluminum chloride, the benzene was omitted. No immediate effect on the disproportionation reaction appeared, probably because sufficient benzene remained dissolved in the catalyst complex to inhibit the reaction. However, in the subsequent run, also u ithout added benzene, a large amount of disproportionation was obtained. The inclusion of 0.5 volume % benzene in the following run again inhibited the disproportionation. d life study was made using 2.0 volume % benzene. Figure 2 compares these results with those obtained using 0.5 volume % benzene. Increasing the benzene concentration from 0.5 to 2.0 volume yo results in an average decrease of 10% in isopentane production; these results are in approximate agreement with 5lavity ( 1 6 ) . The increased benzene concentration reduces the dispropoi tionation from 4.2 to 1.5m-eight %. Other aromatics are not so effective as benzene. -4 minimum of 1 volume % of xylene is necessary t o inhibit the disproportionation of pentanes and the catalyst life is much shorter than is obtained with benzene, as shown in Figure 2. The aromatic inhibitor is partly extracted by the catalyst and is partly converted during isomerization to toluene and higher alkyl aromatics, as shown in Table IV for the case of benzene. The evtraction of benzene is greatest in the early stages of the life test. Toluene appears in the product in ainountc that increase with the age of the catalyst. EFFECT OF A-APHTHENES. Naphthenes are similar to benzene in the inhibition of pentane disproportionation. Methylcgclopentane and cyclohexane are effective as disproportionation inhibitors, as also is cyclopentane, which is discussed later. I t is immaterial whether one starts with methylcyclopentane or
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1953
G I
40
0
0.590 BENZENE
50
100
Is0
I
11
HYDROGEN
t 9
\o
200
250
MOLES PENTANE / M O L E AlCl3
rr
Figure 4.
Effect of Hydrogen on Pentane Isomerization P e n t a n e feed c o n t a i n i n g 1270isopentane
cyclohexane, as they readily isomerize t o an equilibrium mixture under these conditions. However, higher concentrations of naphthenes are required. A mixed pentane feed containing 5.0 volume yo added cyclohexane was isomerized under the same conditions as in the previous life studies in which aromatics were used as inhibitors. Comparison of the curves in Figure 3 shows that the conversion to isopentane is slightly lower than with 0.5 volume % benzene, but that the catalyst life is somewhat longer. Although an exact quantitative relationship has not been established, Figure 3 shows that 5.0 volume yo cyclohexane is about as effective as 0.5 volume yo benzene. A decrease in cyclohexane content to 2.5y0a t a catalyst age of 195 moles results in a doubling of the disproportionation reaction. EFFECTOF HYDROQEN. Hydrogen is another effective inhibitor for the disproportionation of pentanes. Table V, showing the effect of various hydrogen pressures on isomerization a t 100' C. indicates t h a t the minimum effective hydrogen pressure lies between 130 and 200 pounds per square inch. At temperatures as high as 150' C. a partial hydrogen pressure of 900 pounds per square inch is necessary. The critical pressure is dependent not only on temperature, but also on catalyst activity Thus the pressures quoted above for fresh catalyst will be lower with less active catalyst. The effect of increasing the hydrogen pressure above the critical value is to retard isomerization, as well as to effect a slight additional decrease in the disproportionation reaction.
TABLEV.
CONVERS~ON OF PENTANES I N , T H E PRESENCE OF
HYDROGEN
(Mixed pentanes containing 12% isopentane treated a t 100' C. for 120 minutes with 11.5% AlCla and 3.3% HC1 by weight) Hydrogen pressure, Ib./sq. inch 100 150 200 250 At room temperature At reaction temperature 130 200 260 330 Butanes and lighter, wt. % of charge 69.5 2 3 2.2 1.8 Octane No. (CFRM) . . . 8 2 . 2 82.2 80 4 Isopentane, vol. % of pentanes 71 71 71 64
Parallel life studies, as shown in Figure 4,were carried out on a mixed pentane feed (containing 12 volume % isopentane) a t 100" C. with 0.5 volume % benzene and with 260 pounds per square inch partial pressure of hydrogen. (A comparison between Figures l B , and 4 shows that despite the 30% difference in isopentane content of the feeds, the isopentane contents of the products from the benzene-inhibited runs differ by only 4 to 6%.) Under the same conditions the rate of pentane isom-
585
erization is 35 to 40% lower with hydrogen than with benzene. Disproportionation as measured by butane formation was 1.1 weight % for hydrogen as compared t o 2.1 weight % for benzene. Although hydrogen is effective in maintaining catalyst life, it suppresses the isomerization of pentanes. Hydrogen is consumed in moderate amounts. I n experiments at 100' C., the hydrogen consumption amounts t o about 0.0043 mole of hydrogen per mole of isopentane produced.
TABLE VI.
CONVERSION OF PENTANES I N PRESENCE OF BUTANES Isobutane
n-Butane Butane moles Mixed bentanes4, moles AlCla grams HC1, 'prams Temperature, C. Reaction time, min. Butanes and lighter wt '3' of charge Composition of liquid pkodouot, vol. 3 '% Isopentane n-Pentane Hexanes Heptanes and heavier a 12% isopentane, 88% n-pentane.
5.0 4.4 72 20.5 100 120 42
51 16 29 4
4.0
..3.. ...5 100 180
... ... ...
4.9 4.4
... ...
100 180 4.5
51 23 17 9
100
1 80
H
5 60 0
T z W
r\\
F 40
4 20
"
IUU
13u
MOLES PENTANE /'MOLE AICI,
Figure 5.
Effect of Olefins on Pentane Isornerization
0.38% a m y l e n e added to p e n t a n e feed c o n t a i n i n g 12% isopentane 0.5% benzene. 0 70 i s o p e n t a n e 0 % b u t a n e s a n d lighter 2 . 0 % benzene. Yo i s o p e n t a n e 70 butanes a n d lighter
EFFECT OF BUTANES. Table VI shows the effect of butanes on the conversion of pentanes. A study was made using equal volumes of n-butane and pentane in each of the first two runs, and isobutane in like amount in the third run.' n-Butane was not effective as a disproportionation inhibitor; the results were approximately the same as in the runs in -which no inhibitor was added. When isobutane was substituted for n-butane in the third run of the study, the net production of butanes fell from an average of 42 weight % based on pentanes in the first two runs t o 4.5 weight %. Although isobutane inhibits the formation of light products, it leads to the formation of higher boiling products. These are attributed to alkylation of hydrocarbon from the aluminum chloride-hydrocarbon sludge produced during the n-butane step. EFFECT OF OLEFINS. Pines and Wackher ($1)have reported a
586
Vol. 45, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
combined inhibition of added hydrogen and cyclic inhibitors. A minimum effective hydrogen pressure is observed for the hexanes as in the case of pentanes. At 100" C. a hydrogen pressure of 1000 pounds per square inch is effective in inhibiting side reactions; lowering the hydrogen pressure to 500 pounds per square inch and increasing the reaction temperature to 156" C. result in a gross amount of butane and lighter. Under these same conditions the addition of 1.0% cyclopentane does not alter the results, while the addition of 10% cyclopentane brings the side reactions under control.
-
-
PSI
PSI
PS I
--
Figure 6. Isomerization of Light Naphtha 0.05
promotional effect of olefins during isomerization studies with carefully purified materials. When pentane was isomerized with 0.38% added amylene in the presence of 0.5 and 2.0% benzene, respectively, the results shown in Figure 5 were obtained. Catalyst activity increased slightly, but butanes and lighter increased rapidly from the usual 4 to IO%, then declined as the catalyst lost activity for both isomerization and disproportionation. Increasing the inhibitor to 2.0% benzene produced the same effect t o a lesser degree and led to more rapid deactivation of the catalyst despite a decrease in disproportionation. Under these circumstances the olefin acts as a disproportionation promoter; addition of more benzene to control the increased disproportionation formed larger amounts of alkylated benzene with resulting decrease in catalyst life. Further effects of olefins on disproportionation inhibitors are shown in Table VII. Increasing the olefin content to equal that of the benzene inhibitor (0.5%) leads to even greater disproportionation than shown in Figure 5. When pentanes are inhibited with an equal volume of isobutane, the addition of only 0.5 % amylene immediately destroys the effectiveness of the inhibitor. This is presumably associated with the instability of the hydrocarbons of higher niolecular weight resulting from alkylation. This assumption appears to be verified by the results obtained on adding n-hexane and n-octane t o benzene-inhibited pentanes, as shown in Table VII. The addition of 3% n-hexane had no effect, whereas 3% n-octane caused immediate disproportionation. REACTIONS OF HEXANES
The hexanes behave in many respects in a manner similar to the pentanes, but because of the increased chain length side reactions are more pronounced. For instance, butanes derived from hexanes result more from a cracking reaction than disproportionation, thus creating a greater hydrogen deficiency and more rapid deactivation of the catalyst. Ipatieff (IO),Grummitt (8), Bishop (9), and their coworkers have shown that the hexanes decompose mainly to pentanes and butanes in the presence of aluminum halides and hydrogen halides at elevated temperatures. EFFECTOF INHIBITORS. The results summarized in Table VI11 show that these side reactions can be controlled by the
PS I
-
NO HYDROGEN
-'
I
0
II
,
40
I
I
80
I20
1
MOLES FEED / MOLE A I C 1 3
Figure 7 .
Effect of Hydrogen Pressure on Light Naphtha Isomerization
OF OLEFINSAKD HIGHER PARAFFINS ON TABLEVII. EFFECT PENTANE DISPROPORTIONATION n-
Mixed pentanesa, ml. Inhibitor Inhibitor, vol. % based on Cs AICh, grams HC1, grams Temperature, C. Contact time, min. Catalyst Age, Moles Cs/MoIe AlCl 16 29 42 45 68
Hexane, Amylene, 0.5% 3% 500 1000 1000 Benzene Isobutane Benzene 0.5 100 0.5 72 72 72 20.5 20.5 20.5 100 100 100 60 180 180
n.
Octane, 3% 1000 Benzene 0.5 72 20.5 100 60
Butanes, Wt. %
14 4
12% isopentane. Butanes derived from pentane disproportionation.
EFFECTOF HYDROGEK AND NAPHTHENES ON ISOMERIZATION O F z,%DIMETHYLBUTASE
TABLEVIII. Experiment
1
2
None
None
3
4
Cyclopentane vol. % AlCh, wt. % Lased on hydrocarbon HCI, wt. % ' based on hydrocarbon Temperature, C. Contact time mm. Hydrogen, Ib:/sq. inch
10.7
3.2 100 20 1000
Butanes and lighter, wt. % of charge
0.5
32.6
30.0
2.2
0.2
41.5 8.9
... ... ...
5.1 21.6 6.3
Composition of liquid uroduct, vol. 7 0 Pentanes 2,2-Dimethylbutane 2,3-DimethyIbutane 2-Methylpentane 3-Methylpentane n-Hexane and heavier Cyclopentane
4.4
46.9 31.6 16.6 0.3
...
1.0
10.0
11.0
10.5
11.6
3.1 156
3.1 157
10 600
4.5 14.4
7.3
22.0
...
10
500
... .
.
I
... ...
3.5 160
10 500
37.8 11.9
11.3
6.0
March 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
OCTANE NUMBER -MOTOR
--
no promotional effect t o hydrogen chloride, Beeck (g6) believes that hydrogen chloride retards the isomerization reaction, while others (18, 14) assign a promotional effect to hydrogen chloride. The present data show that hydrogen chloride definitely has a promotional effect and plays a vital role in the catalysis, as shown in Figure 10. When light naphtha is isomerized with aluminum chloride in the absence of hydrogen chloride, there is a rapid decline in catalyst activity even though hydrogen is present. The initial activity of the catalyst is attributed to occluded hydrogen chloride contained in the catalyst. After a catalyst life of only 50 moles of light naphtha per mole of aluminum chloride, the catalyst was essentially inactive. When 2.7 weight % hydrogen chloride based on light naphtha was introduced, the activity immediately increased and compared favorably with the life study curve obtained with hydrogen and hydrogen chloride.
Figure 8. Effect of Hydrogen Pressure on Butane Formation from Light Naphtha
Light naphtha having the composition shown in Table I contains both benzene and naphthenes. However, cyclic inhibitors alone are not sufficient for suppressing side reactions of hexanes and for increasing catalyst life, as shown in Figure 6. In the absence of hydrogen the catalyst activity declines rapidly and the catalyst is essentially inactive a t a catalyst age of 70 moles of light naphtha per mole of aluminum chloride. Carrying out the isomerization of light naphtha under 500 pounds per square inch of hydrogen pressure and a t 165' C. produces a remarkable increase in catalyst life, as shown in Figure 6. The activity declines rapidly a t first, then levels out and is essentially constant a t a catalyst life of 240 moles of light naphtha per mole of aluminum chloride. The significance of the difference in the catalyst activity index, a,a t a catalyst life of 100 moles of light naphtha per mole of aluminum chloride for operation in the presence and absence of hydrogen, may be expressed in terms of octane number improvement. In the absence of hydrogen 1hour contact time produces only a 1.5 CFRM increase as compared to a 14.0 CFRM increase in the presence of hydrogen, This remarkable capacity of hydrogen for inhibiting side reactions during isomerization and for supplying the hydrogen deficiency t o maintain catalyst life is the basis for the neohexane and naphtha isomerization processes operated during the war for aviation gasoline production ( 6 , M ) . Figure 7 shows the effect of hydrogen pressure on the isomerizatiqn of light naphtha a t 100' c. Catalyst life increases as the hydrogen pressure is increased. Figure 8 shows the effect of hydrogen pressure on butane formation for these same experiments. The production of butanes and lighter becomes progressively less as the hydrogen pressure is increased. The suppression of butane formation by hydrogen explains a t least in part the longer catalyst life obtained a t the higher hydrogen pressures. Furthermore, hydrogen has a minor effect on suppressing isomerization. Analysis of the products produced a t various hydrogen pressures showed a small but definite decrease in 2,2-dimethylbutane with increase in hydrogen pressure, as shown in Figure 9. The same effect of hydrogen pressure was observed for the isomerization of cyclohexane to metliylcyclopentane (14). When a cross plot of Figure 8 is prepared a t an 80 CFRM level and placed on Figure 9 for comparison, the effect of hydrogen pressure on the disproportionation and cracking reactions as measured by butane formation varies exponentially while the effect of hydrogen pressure on suppressing isomerization is a linear function. HYDROGEN CHLORIDE AS AN ACTIVATOR.There are considerable differences in opinion concerning the effect of hydrogen chloride when added to aluminum chloride. Pines ( $ 1 ) ascribes
h
ISOMERIZATION
1 \
DISPROPORTIONATION AND
12
I
I
0
CRACK ING
I I 200 400 HYDROGEN PRESSURE
I 600
- P.S.I.
J O
800
Figure 9. Effect of Hydrogen Pressure on Isomerization and Decomposition of Light Naphtha MECHANISM OF REACTION
A great deal has been written on the mechanism of the isomerization reaction (3, 18, 17, 2S, bV). Although there is some difference of opinion, a majority favors a carbonium ion mechanism. The present investigation leads one to believe that there has been some oversimplification. Any mechanism must adequately take into account the following facts: Reaction takes place in the catalyst phase. Hydrogen chloride is a necessary component of the catalyst. Hydrogen, aromatics, naphthenes, and isobutane inhibit disproportionation. Disproportionation is inhibited without appreciably affecting isomerization. Olefins destroy the effect of inhibitors and increase disproportionation and cracking. Minimum hydrogen pressure necessary to inhibit disproportionation and cracking increases with chain length. Suppressing effect of inhibitors on isomerization increases with decrease in chain length. Formation of aluminum chloride-hydrocarbon sludge increases with increase in chain length. Hydrogen is consumed. Brown, Pearsall, and Eddy ( 4 ) have shown that no interaction occurs between pure aluminum chloride and pure hydrogen chloride, but that in the presence of a basic aubstance-i.e., a proton acceptor-a complex is formed in which the aluminum chloride and hydrogen chloride are associated stoichiometrically. On this basis i t is postulated that the proton acceptors, which
INDUSTRIAL AND ENGINEERING CHEMISTRY
888
are present as impurities in the commercial reagents employed in this work, serve to force the following reaction to the right. Proton acceptor (moisture, oxygen, olefins, etc.)
+ HCl +
r AICh
1
1+
[proton y c e p t o r
AIC1,-
Vol. 45, No. 3
an over-all electron displacement, which results in an intramolecular rearrangement while the hydrocarbon is associated with the catalyst. This activated complex-intramolecular rearrangement mechanism has the desirable feature of depicting the role of the catalyst and can account for variations in isomerization due to acid strength.
H I t is further postulated that it is the resulting product, in the form of an ionized complex, which acts as the effective catalyst.
H CH,
:
H C H
I
I
I
I
This ionized complex does not differ particularly from a c d i bonium ion, especially as more recently carbonium ions have been generally assumed to be associated x i t h the catalyst environment (24). I n the carbonium ion mechanism the caibonium ion is depicted as attracting a hydride ion from a hydrocarbon to form a new carbonium ion, which then undergoes rearrangement. The rearranged (isomerized) carbonium ion in turn attracts a hydride ion from another hydrocarbon molecule, thus propagating the chain. The carbonium ion niechanism theory is believed to be inadequate, as it does not account for the fact that inhibitors have a pronounced effect on the disproportionation reaction but only a relatively small effect 011 isomerization. The present results are believed to be nioie adequately explained by picturing the isomerization reaction as taking place through association of the hydrocarbon with the ionized complex, m-hereby activation occurs followed by rearrangement. This concept does not differ substantially from that advanced by Heldman ( 9 , 13) and later by Powell and Reid ( $ 3 ) and Beeck et al. (1,50). Catalytic action may be pictured as a simultaneous proton-anion attack on the hydrocarbon molecule in a manner similar to the push-pull mechanism conceived by Swain (27) to explain displacement reactions of organic halides. Thus as shown below, the electrophilic proton portion of the catalyst attracts the hydrogen on the second carbon atom in the hydrocarbon chain through distoition of the elections in the C-H bond. At the same time the nucleophilic AIC1,portion of the catalyst attracts the hydrogen on the opposite side of the third carbon atom The qimultaneous attack effects
H C H
:
CH,---
Figure 9 suggests that the isomerization and disproportionation reactions are proceeding by different mechanisms. It is postulated that the disproportionation reaction arises when the activated hydrocarbon moves under certain circumstances farther than some critical distance from the catalyst environment, thus giving rise to a free active fragment which is a carbonium ion or ie similar to a free carbonium ion. This carbonium ion can do several things: It can undergo hydride ion transfer with another hydrocarbon molecule, thus propagating the chain, or lose a proton t o become an olefin which may add to the aluminum chloride to form aluminum chloride-hydrocarbon sludge or reform a carbonium ion. W-hen sufficient carbonium ions are formedfor example, when the temperature is raised or olefins are addedthe chances of reaction of a carbonium ion with an olefin increase. In the case of pentanes, reaction of a carbonium ion with a C, olefin leads to a Clo carbonium ion. Such a large carboniuni ion is extremely unstable and may break into two propylene molecules and a Cq carbonium ion. In other words, from one active fragment three potential carbonium ions are forme 1, which leads to an accelerated zero-order chain reaction. Thia implies the necessity for keeping the free carbonium ion concentration below a critical value to avoid gross disproportionation of the hydrocarbon. This is what aromatics, naphthenes, and hydrogen do as inhibitors; they react with carbonium ions, thucontrolling the carbonium ion concentration. From this it ma\be seen horr the disproportionation reaction can be inhibited without appreciable effect on the isomerization reaction. Besides disproportionation, wherein a hydrogen balance i maintained, there is the additional side reaction of crackin:, n-hich leads to hvdrocarbons of lorver molecular weight and a hydrogen deficiency. This cracking reaction becomes mor e pronounced with increase in the length of the hydrocarbon chain. This leads largely to the formation of aluminum chloride-hydrocarbon sludge, which deactivates the catalyst. Besides inhibiting the disproportionation reaction by adding to free carbonium ions, hydrogen is also capable of reacting with the hydrocarbon attached t o the aluminum chloride to split off saturated hydrocarbons of low molecular weight, thus regenerating the cata I p t . This later reaction leads to the hydrogen consumption observed during isomerization in the presence of hydrogen. ACKYOWLEDGMENT
The authors are indebted to the numerous members of the Standai'd Oil Go. (Indiana) Research Laboratories who contrihuted their advice and services, and especially to E. TTT. Thiele, l3.H. Shoemaker, and J. A. Bolt. LITERATURE CITED
( I ) Beeck, O., Otvos, Y. W., Stevenson, D. P., and Wagner, C. D., J . Chem. Phus., 16, 255 (1948). (2) Bishop, J. W., Burk, R. E., and Lankelma, H. P., J. Am. Chem. Sot., 67, 914 (1945). (3) Bloch, H. S., Pines, H., and Schmerling, L., Ihid., 68, 153 (1946).
March 1953
589
INDUSTRIAL AND ENGINEERING CHEMISTRY
(4) Brown, H. C., Pearsall, H., and Eddy, I . P., Ibid., 72, 5347 (1950). (5) Egloff, G., Hulla, G., and Komarewsky, V. I., “Isomerization
of Pure Hydrocarbons,” New York, Reinhold Publishing Corp., 1942. (6) Evering, B. L., Fragen, N., and Weems, G. S., Chem. Eng. News, 22, 1898 (1944). 71, 440 (7) Evering, B. L., and d’ouville, E. L., J . Am. Chem. SOC., (19491.
(8) Grummitt, O., Sensel, E. E., Smith, W. R., Burk, R. E., and Lankelma, H. P., Ibid., 67, 910 (1945). (9) Heldman, J. D., Ibid., 66, 1786 (1944). (10) Ipatieff, V. N., and Grosse, A. V., IND. ENG.CHEM.,28, 461
Pines, H., in “Advances in Catalysis,” Vol. I, p. 251, edited by Frankenburg, Komarewsky, and Rideal, New York, Academic Press, 1948. Pines, H., Kvetinskas, B., Kassel, L. S., and Ipatieff, V. N., J . Am. Chem. SOC.,67, 631 (1945).
Pines, H., and Wackher, R. C., Ibid., 68, 595 (1946). Ibid., p. 699. Powell, T. M., and Reid, E. B., Ibid., 67, 1020 (1945). Schmerling, Louis, paper presented before Division of Petroleum Chemistry, 119th Meeting, AM. CHEM. SOC., Cleveland, Ohio. Schuit, G. C. A., Hogg, H., and Verheus, J., Rec. trau. chim., 59, 793 (1940).
Stevenson, D. P., and Beeck, O., J . Am. Chem. Soc., 70, 2890
(1936). (11) Ipatieff, V. N., and Schmerling, L., Ibid., 40, 2354 (1948). (12) Komarewsky, V. I., and Ulick, S. C., J. Am. Chem. SOC.,69, 492 (1947). (13) Leighton, P. A., and Heldman, J. D., Ibid., 65, 2276 (1943). (14) Lien, A. P., d’Ouville, E. L., Evering, B. L., and Grubb, H. M., IND. ENG.CHEM.,44, 351 (1952). (15) McAllister, S. H., Ross, W. E., Randlett, €I. E., and Carlson, G. J., T r a n s . Am. I n s t . Chem. Engrs., 4 2 , 3 3 (1946). (16) Mavitv. J. M.. Pines. H.. Wackher. R. C.. and Brooks. J. A,. IN=:ENG.CHEM.,40, 2374 (1948). (17) Oblad, A, G., and Gorin, M. H., Ibid., 38, 822 (1946). (18) Perry, S. F., Trans. Am. I n s t . Chem. Engrs., 42, 639 (1946).
(1948).
Swain, C. G . , Ibid., 70, 1119 (1948). Swearingen, J, E., Geckler, R. D., and Nysewander, C. W., T r a n s . Am. I n s t . Chem. Engrs., 42, 573 (1946). Wackher, R. C., and Pines, H., J . Am. Chem. SOC.,6 8 , 1642 (1946).
Wagner, C. D., Beeck, O., Otvos, Y . W., and Stevenson, D. P., J . Phys. Chem., 1 7 , 4 1 9 (1949). RECEIVED for review June 14, 1952. ACCEPTED October 20, 1952. Presented before the Division of Petroleum Chemistry at the 111th Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Correlating Diffusion Coefficients in Liquids DONALD F. OTHMER AND MAHESH S. THAKAR Polytechnic Institute of Brooklyn, Brooklyn 2, N . Y .
D
TFFUSION coefficients in liquids are increasinglv important in many theoretical and engineering calculations involving mass transfer, such as absorption, extraction, distillation, and chemical reactions. Comparatively few such data have been published, however, and methods for their correlation and for prediction of data on other substances are needed. A simple method of correlation is presented which allows easy and accurate extrapolation and interpolation of available data, and the prediction of such data when not available. The method follows the principle used t o correlate many other properties of matter in previous papers of this series-namely, plotting logarithmically the property of one material against the same property or the vapor pressure of a reference material. I n this manner, vapor pressures (a), gas solubilities and partial pressures (1‘7), adsorption pressures ( 1 5 ) , vapor compositions, equilibrium constants, activity coefficients, relative volatilities, electromotive forces (9, 1 1 ) , viscosities (IO),reaction rate constants ( I d ) , surface tensions ( l a ) , densities ( 1 3 ) , and azeotropic compositions ( 1 6 ) have already been correlated. The theoretical background for such a plot may vary, b u t the method usually follows the simple steps for making a vapor pressure plot (8):
law-i.e., directly proportional to the absolute temperature and inversely proportional t o the viscosity of the liquid. Eyring (6) and others (3,4,18) have suggested that diffusion is a rate process which varies as a n exponential function of temperature. Thus:
1. On a sheet of logarithmic paper indicate vapor pressures of a standard substance-e.g., water-on the X axis and of the substance in question on the Y axis; then calibrate the X axis with values of temperatures corresponding t o the vapor pressures of the standard substance. 2 . Erect temperature ordinates. 3. Plot points and connect with a straight line, the slope of which is the ratio of the molal latent heat of the substance to that of the reference substance. This ratio is much more nearly constant a t all temperatures-and the line more nearly straightthan the latent heat itself from a plot of log P us. 1 /T.
If E J L is assumed constant, this can be integrated t o give:
DEVELOPMENT OF LOGARITHMIC PLOT FOR DIFFUSION COEFFICIENTS
The variation of diffusion coefficients with temperature has been assumed by Wilke ( 1 9 ) t o be related to the Stokes-Einstein
D
= KeEd/RT
(1)
Taking logarithms and differentiating,
The Clausius-Clapeyron equation for the vapor pressure of liquids is: dT d l o g P = -L
RT2
(3)
At the same temperature, if Equation 2 is divided b y Equation 3, there results:
(4)
Log D = Ed log P
+C
(5)
Equation 5 indicates that, if diffusion coefficients are plotted on logarithmic paper against vapor pressures of a reference liquid at the same temperatures, a straight line results with slope of
EaIL. Diffusion coefficients of different substances are plotted this way in Figure 1. The X or vapor pressure axis is calibrated t o give a temperature scale by known values from a standard table. The energy of activation of diffusion can be calculated from the slope of the line and the latent heat of vaporization of the reference substance a t t h a t temperature.
