ISOMERIZ
AN
Effect of Various Additives
Isomerization of n-pentane to isopentanc in the presence of aluminum chloride-hydrogen chloride catalyst is accompanied nornially by con&hrable cleavage and disproportionation. Results of experiments are reported in which this reaction was modified by introducing I arious organic additives with the feed. Among those found to be most effective for repressing side reactions were certain naphthenic and aromatic h~drocarbons. Straight run CG fractions containing small or moderate concentrations of benzene or Cg naphthenee also proled satisfactoq Some aromatic derivatives-chloi.obenzenzene and diphenyl ether~
T IS now general knowledge that whereas the reversible isoiiierization of butane to isobutane proceeds smoothly under the influence of aluminum chloride-hydrogen chloride type catalysts; higher paraffins, broadly speaking, under similar treatment undergo pronounced disproportionation and cracking in addition t o isomerization. However, these side reactions can be repressed effectively in the isomerization of paraffins by the introduction of certain materials added with the feed. The use of hydrocarbon additives (4, 9, 10, 11, 16, 17) as well as hydrogen (4, 8, 12) has hecn discussed recently in considerable detail. This paper is concerned with a n independent study of the effect of various organic additives in controlling side reactions in the isomerization of n-pentane to isopentane. A41thought8hereis Some overlapping of t,he work presented by others ( 4 ) it is hoped that the new information and confirmation of the previous data will aid in arriving a t a clearer understanding of the chemistry of the reactions. EXPERIMENTAL METHODS
Two types of experiments were conducted. Batch tests were carried out in a turbomixer autoclave. Relatively small amounts of materials sufficed in this type of test; this permitted a rather rapid survey of a wide variety of additives. Continuous tests directed toward practical application of the method were made in bench scale units of the aluminum chloride pickup type (5).
BATCH TESTS.The mixture of n-pentane, organic additive, and anhydrous aluminum chloride was sealed in a stainless steel turbomixer autoclave of 250-ml.'capacity. Anhydrous hydrogen chloride was pressed in from a small weighing bomb. The autoclave was surrounded by a preheated oil bath and the mixture kept. a t 75' C. under stirring for 6 hours. The volatile products then were Bashed immediately from the autoclave into a dry-ice trap by opening the exhaust valve. After the vessel was cooled, any higher boiling upper layer from the autoclave was added t,o the distillate and the combined product was analyzed by careful fractionation in a Podbielniak column. CONTINUOUS TESTS.A sketch of t,he equipment used for the continuous tests is shown in Figure 1. The blend of pentane and additive was pumped a t a constant rate from a calibrated pressure charger by means of a high pressure differential plunger pump into the bottom of the saturator consisting of a %foot length of 1-inch inside diameter steel tubing. This was packed a t the start of each run with granular anhydrous aluminum chloride. The effluent from the saturator, consisting of a solution of aluminum chloride in the charging stock, was discharged through a short, well insulated 0.25-inch transfer line to the top of the 1
Present address, Standard Oil Company (Ind.), Whiting, Ind.
2374
showed similar properties. The concentration of il given additive must be controlled carefully to obtain the optim u m effect because an excess was found to repress isomcriAation as well as the side reactions. Thus w h i l e 5 LO 2 0 volume of cyclohexane gave good results, the optimum concentration of benzene was about 0.25 to Q.5 volume 7'. Both batch Lests and continuous run* of Lhe aluminum chloride pickup type were made. In the latter tlpe of operation the additive has the additional function of repressing sludge formation in the aluminum chloride supp l y zone. The mechanism o f the reactions is discussed.
reactor. This line mas provided with a U-bend to pruvcnt ariy backflow of hydrogen chloride into the sat>urator. The I .%-inch inside diameter reactor was packed in some instances with 0.25inch semiporcelain Berl saddles, in others with 3- to 8-mesh quartz rock chips. Effluent from the bottom of the reactor passed into a settling trap I'rom which accumulated sludge-t,hat is, hydrocarboncatalyst comples--was periodically discharged. The hydrocarbon-hydrogen chloride stream vias released through an automatic pressure control valve and was cont>inuouslywashed in two glass towers containing Tvatcr and caustic, respectively; the hydrocarbons xere collected in receivers refrigerat,ed by dry ice. Anhydrous hydrogen chloride vr-as admitted at a definite rate at the top of the reactor. The rate was controlled through the use of a quartz or Pyrex capillary (1) sealed in a steol nipple wit,h Wood's metal. A constant differential pressure was maintained between the two ends of the capillary. Each capillary was calibrated in a separate apparatus by measuring the flow of gas into a mercury buret; this maintained the desired pressure in bhe downstream side by a manually controlled needle valve. A rapid check on flon- also was readily obtainable at any time by noting the rate of pressure drop on the upstream side with the valve closed between the capillary and the hydrogen chloride source. Constant upstream pressure was maintained by keeping the supply tank of liquid anhydrous hydrogen chloride in a constant temperature bath a t about 67" F. t o prevent any condensation of hydrogen chloride in the capillary or feed lines. Heating of the tubes was accomplished by means of oil baths. Temperature was controlled manually by Variacs which operated electric heaters on t.he side arms of the baths. Product analyses are reported only after sufficient time on stream a t the conditions in question t)ogive a reasonable approach to operating equilibrium. MATERIALS
CATALYST.Anhydrous aluminum chloride from GivaudanDelawanna, Inc., was used throughout this work without further purification. Pure anhydrous hydrogen chloride was supplied as a iiquified compressed gas by Marbon Corporation, PENTANE.Phillips Petroleum Company's pure n-pentane having a specified purity of over 99.5% was used in all batch tests. Technical n-pentane samples, of compositions indicated in Table 11, n-ere used for the continuous runs. Sample B was obtained from Shamrock Oil arid Gas Corporation, all others from Phillips Petroleum Company. NAPHTHENES.Pure methylcyclopentane was prepared by isomerizing cyclohexane in the presence of aluminum chloridehydrogen chloride and fractionating the reaction product. Cyclohexane additive for the continuous runs was from Eastman Kodak Company; samples which showed traces of i~enzoric:were
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 1948
INSULATED
PRESSURE
WATER CIRCULATING
i II
ii
PUMP
TRANSFER
2375
LINE
PRESSURE
i
GAGES
I THERMoCouPLE
OIL
---+.
i
REACTOR
BATH
y/
TO
Figure 1.
GAGE
WASHING
TOWER
\
Continuous Isomerization Unit
treated with oleum followed by caustic washing. All other naphthene samples were prepared by hydrogenating the corresponding aromatics in the presence of Universal Oil Products Company (U.O.P.) hydrogenation catalyst. AROMATIC HYDROCARBONS. The benzene, toluene, ethylbenzene, and isopropylbenzene were C.P. samples. Secondary butylbenzene and the mono- and di-kit-butylbenzenes were prepared by alkylating benzene with the corresponding olefins (6). n-Propyl-, n-butyl-, and n-amylbenzene were prepared by hydrogenating the corresponding ketones in the presence of copper-alumina catalyst (6). The other aromatic hydrocarbons were obtained from Gesellschaft fiir Teerverwertung m.b.H., Duisburg-Meiderich, Germany. AROMATIC DERIVATIVES. All of the aromatic hydrocarbon derivatives were from Eastman Kodak Company. PARAFFINS. Phillips pure isobutane was used. Pure nhexane was obtained by treating selected narrow boiling fractions of Skellysolve B with chlorosulfonic acid and fractionating to give a product boiling at 68.7" to 68.9 O C., n%' 1.3750. STRAIGHTRUN FRACTIONS. Skellysolves B and C, commercial Cs and C7fractions, respectively, were purchased from Skelly Oil Corporation. The acid-treated materials were prepared by agitating samples of the Cg and C7 fractions under cooling with successive small portions of 105% sulfuric acid, until the last traces of aromatics were removed; the samples then were washed with caustic and water and used without redistillation, The Trinidad Ca fraction was obtained from Trinidad Leaseholds, Ltd.; mid-continent kerosene was from Deep Rock Oil Corporation; its boiling range was 198" to 260" C. DISCUSSION
Cyclohexane and the lower molecular weight NAPHTHENES. mono- and di-substituted cyclohexanes used in concentrations of about 5 t o 10% in the feed blends were effective in repressing
cracking in favor of isomerization, both in batch tests (Table I) and continuous operation (Table 11). The higher molecular weight members of the series containing substituents with a total of four or more carbon atoms were less effective for inhibiting cracking. It was found, however, in the continuous tests that certain hi- and dicyclics repressed both cracking and isomerization. An outstanding fact in connectio~i AROMATIC HYDROCARBONS. with the use of aromatic hydrocarbons as cracking inhibitors is that the concentration required is much lower then in the case of' naphthenes. The criticality of aromatic concentration for t h r case of benzene is strikingly demonstrated in Figure 2. Here, both the degree of isomerization as indicated by the isopentane concentration in the pentane fraction and the extent of cracking as shown by the concentration of butanes in the product htqve been plotted against the percentage of benzene in the feed. The abscissa has been expanded to show better the effect of variations in benzene concentration in the low range by using a scale in l), which the distance from the origin is proportional t o log (v where v is the volume per cent of benzene in the feed. Optimum isomerization was obtained with about 0.25 t o 0.5 volume % benzene. Thus, as increasing proportions of the additive are employed cracking becomes less and less. The isomerization reaction simultaneously becomes more selective and reaches a peak a t LB point where cracking has become quite small. Further increase in additive concentration beyond this point now gradually inhibits isomerization just as it inhibited cracking in the lower concentration range. It was found that inhibition of isomerization by the higher benzene concentrations could be counteracted to some extent by a moderate increase in temperature but the optimum benzene concentration remained the same. A variety of different aromatic hydrocarbons was investigated.
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
2376
TABLE I. B.vrcH T L ~ T S ~ Added Compound
Grams
?;one
t . .
Cycloliexane AIethyIcyclohexane Ethylcyclohexane Dimethylcyclohexanes Isopropylcyclohexane 1,3- a n d l,4-ethylmethylcyclohexane n-Butylcyclohexane iert-Butylcvclohexane 1-Isopropyi-4-methylcyclohexane Diethylcyclohexanes 1-Ethyl-4-isopropylcyclohexane
8.6
8,5 8.7 8.6
Mole % in P i o d u c t ~L-CIHLOn-CaHlo 2-CaI-11% n-CfiH12 C G-
54.1 8.1 KAFHTHENES 1.6 2.5 0.0 0.4
11.8
6.8
45.5 55.4 57.3 57.2 51.2 45.5
46.5 35.6 40.2 39.7 37.2 49.3
6.4
6.5 2.5 2.7 6.3 2.4
0,s 2 ,6
36.3 25.0 32.9
43.0 43.7 20.6
8.7 10.9 8.9
0.2 2.8
40,9 24.9
39.0 42.0
8.0
48.7 50.7 7.2
49.7 48.0 91.9 96.9 84.3 93.9
0.7 0.6 0.9 0.5 5.7 0.0
47.0 43.5 71.9 35.0
48.2 50.1 25.4 60.9
1.5 2.8 2.7 0.9
44.7
1 .9
5.0 2.8
8.6
8.7 8.8 8 ,1 8.1
19.5 35.0
8,s
14,4 22.3
8.8
19.2
12 0
5.5
BEXZENE A N D ?l-.\LKILBESZEXES
Brnaene Toluene Ethylbenzene n-Propylbenzene n-Butslbenzene
1. o 1.0 1.7
Isopropylhenzene sec-Butylbenzene teit-Butylbenzene
1.7 1.6 1.7 1.7
p.Di-te,.t-butylbeiizene
0.9 0.7 0.0
2.0 3.0
2.3 0.0
1.6 0,2
Biphenyl 2.0 %..i 0.1 31.5 65.1 0.8 Diphenylinethane 2.0 2.2 2.4 35.3 GO.] Kaphthalene 2.0 8.2 3.7 1.0 56.1 30.7 22.3 2-Methylnaphthalene 2,0 ii , 7 4.: 0.4 37.1 DERIVATIVES O F .kHOVAl'IC H I D R O C A R B G S J Chlorobenzene 3.3 0.6 62.4 32.1 1.6 1. o p-Chlorotoluene 23.3 0.9 58 0 9.0 2.1 6 8 o-Dichlorobenzene 50.7 7.7 I .7 10.6 11.8 10.2 p-Dichlorobenzene 26.3 9.4 2.0 49.1 5.4 9.8 Diphenyl ether 53.6 2.1 44.3 0.7 1.4 Diphenyl ketone 43.2 2.2 23,2 7.4 2.0 24.0 Aniline 26.7 13.5 13.8 43.7 2 3 2.4 Diphenylamine 25.3 2.3 49.4 4.7 2.0 18.3 Materials charged in caoh test were 85 grams of n-pentane, 13 grams of anhydrous aluminum chloride, 2.7 * 0.4 gram of anhydrous hydrogen chloride, a n d t h e indicated additive. Conditions were 6 h o p a t 75" C. Maximum pressure developed ranged f r o m 48 t o 120 lb.,'aq. In.gage. Q
The results obtained with each of these, classified according to type, are included in Table 1. The data are limitcd to a single concentration with each additive. Of the n-alkylbenzenes the effect of toluene equaled that, of benzene. The higher alkylbenzenes inhibited both cracking and isomerization. The branched alkylbenzenes exhibited considerable variation in behavior. With tert-butplbenzene the effluent contained 71.9 mole yo of isopent,ane, the highest concentration produced v i t h any of the additives tested. Results with 1,4-dimethylbenzene equaled those with benzene or toluene. Several polycyclic aromatics exhibited positive inhibiting action. A 4 ~ o aDERIVATIVES. r ~ ~ ~ ~ Aromatic hydrocarbon derivatives which ivere effective as inhibitors are chlorobenzene and diphenyl e t her, I n the presence of isobutane the extent of isomPARAFFIXS. erization was somewhat greater than when no additive was used but there also was a net production of disproportionation products (Table 11). The effectiveness of isobutane in no way compares with t h a t of cyclic hydrocarbons. The straight chain paraffin, n-hexane, had no apparent effect on the reaction. This might, he anticipakd from the fact that its homolog, ,n-psntane, does not suppress its own decomposition. STRAIGHT RUNFRACTIONS. A cheap source of hydrocarbons of the types which Iwre shoir-n to be good additives is found in straight run or natural gasoline. Fractions in the Cg range even
Vol. 40, No. 12
though predominantly paraffinic can be used without the r w of separating the paraffins from the cyclies (Table I1 and 3). JThen Skellysolve €3 was used as an additive, optimum isonierization was obtained a t an additive concentration of almut 8 volume yc. The Slrellysolve B contained on a volumc: basis 2.3YG benzene, 15TG naphthenes (methylcyclopentanc: and cyclohexane), and 83:5 hexanes. Assuming the hexanes t o bv inert with respect t o inhibiting side reactions, the isomerization is dependent on the presence in the feed blend of 0.18 volurnc~ Sc of benzene and 1.2 volume yoof CSnaphthenes. 3 considerably higher concentration of the same acldit~ivc~ was required after benzene v-asremoved by careful treatment with oleum. Optimum isomerization TYas obtained with 25 t o 359: of the a,cid-t,reated fraction in The feed; this corresponds to about 4 or 59; of C S napht,henes. Excellent results Iwre obtained with a Trinidad straight run CS fraction which cont,ained f1.97~benzene, 247; naphtlicnes, and 69Yc hexanes (volume basis). Only 2.6 volumc: 7;; of ifhis , additive was required. Fractions above the C Grange ivet'e riot as efYrutive as tiic fraction. Thc Skellysolve C wan a C6 fraction containing about 4104 naphthenes and 2.7Yc toluenc. When the p this material, either acid treated (tolucme-free) or un sufficient t o inhibit cracking, isomerizat,ion was poor. The same situation prerailcd Tvith small concentrat ioni: of 21. rnittcontinent kerosene, DUAL FU\CTIQY O F ADDITIVES
111 continuous operation of the type employed 111 thi5 14 o r k the saturator serves as a \upply chamhcr for thv aluniinum chloiidp, fiom nhich the late of input of the latter into th(, reactor can be controlled by controlling the saturator temperature. The input rate a t a saturator temperature of 77" C. \\as of the order of 1.1 to 1.4 pounds per barrel of teed. In the ahspnce of additives, ho??ever, total consumption of aluminum chloride from the saturator approached 2.0 pounds per barrel of pentane charged; the additional consumption arose from the formation of a n insoluble liquid sludge in the saturator which could be
a
I
I
I
SATURATOR REACTOR
1
T E M P E R A T U R E : 77 OC.
CONDITIONS:
IOO°C,
-
150-508 PSIC,
0.10 LHSV,
I O MOLEOLHCI
O N HCBNS. CHARGED.
E
i-CgHI2
IN C5
CUT
--
.I .25 .5
2 5 V O L U M E P E R C E N T B E N Z E N E IN F E E D Figure 2. Benzene as Additive
IO
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
2377
them in the process. Careful fractionations were carried out on samples (1 to 7 liters) of the effluent from t h e Test Mole 70in Product Yol. 7 0 Pentane Period, continuous runs with each naphthene and Feedb HoursC i-CaHio n-CaHio i-C6Hiz T L - C S H I Z Cb + Added Compound in Feed with benzene. 67.9 6.3 17.5 B 72-84 8.3 None In all cases a major portion of t h e ?~APHTHENES original additive was recovered, as such 31.4 13.6 54.7 48-60 0.3 B 10.0 Methylcyclopentane or along with isomers. However, vary30.1 9.2 56.8 3.9 60-72 n 5.0 Cyclohexane 14.2 30.1 54.3 1.4 48-60 B ing amounts of higher boiling products 10.0 Cyclohexane 3 2 . 0 1 2 . 6 53.3 0 . 8 1 . 3 72-84 A 10.0 Cyclohexanea were recovered usually. With the lower 13.2 24.8 60.6 1.7 24-60 B 10.0 Methylcyclohexane 4.9 45.5 49.4 0.3 120-168 C 5.0 Ethylcyclohexaned members of the naphthene series these 51.1 4,: 44.2 0.4 36-132 C 5.0 Dimethylcyclohexanes d 60.0 7.0 32.6 0.0 36-120 5.0 C products were characterized as alkylHvdrindand 67.1 13 7 19.2 0.0 91-107 B 10.0 Dkcahydronaphthalene ated naphthenes because of their physi80.9 14.6 4.5 0.0 48-60 B 10.0 Bicyclohexyl cal constants and the fact that they were BENZENE stable to nitration mixture. 4.8 33.6 57.2 D 79-187 4.4 0.10 Beiisened 3.2 47.3 47.2 D 72-204 2.3 0.25 Benzenea This is demonstrated clearly for the 2.6 48.1 49.0 D 24-68 0.4 0.50 Benzene C 4.5 case of cyclohexane. Effluent from the 6 9 . 5 2 6 . 8 D 24-60 0 . 3 2 . 0 Benzene' 70.1 ' 13.5 15.9 D 48-84 0.5 10.0 Benzene i test at 500 pounds per square inch gage PARAFFINS with 10 volume yo of this additive 6.8 47.0 30.3 C 36-168 14.7 1.2 10.0 Isobutaned (Table 11) was fractionated in a 59 6 . 1 3 1 . 2 3 5 . 3 C 48-108 26.2 1.2 20.0 Isobutened 50.4 20.7 17.3 bubble cap column. The residue abovc B 24-36 11.6 20.0 n-Hexane 40" C. was fractionated further into tt. STRAIGHT R U N FRACTION^ series of cuts; properties of these are in12.4 50.1 31 8 B 48-96 5.7 10.0 28.4 22.0 45.8 B 48-60 3.8 20.0 cluded in Table 111. Similarly, in the 3 1 . 9 4 1 . 7 23.7 B 24-72 2 7 33.3 49.0 28.4 20.5 B 86-122 2 1 case of benzene, i t was evident from the 50.0 Acid-treated Skellssolve B 9.3 52.1 36.4 C 24-144 2.3 5.0 Skellysolve Bd physical constants of the higher boiling 10.6 35.3 52.9 E 60-132 1.2 10.0 Skellysolve Bd 23.0 31.8 44.7 E 144-240 0.5 25.0 Skellvsolve B d fractions that these were predominantly 8.3 31.8 58.1 C 60-98 1.8 2.5 Trincdad Co Cutd 7.0 2 7 . 5 60.9 C 24-72 4 . 6 alkylbenzenes. I n the operation with 1.o Acid-treated Skellysolve C d 11.5 68.2 19.1 C 36-48 1.2 Acid-treated Skellysolve Cd 10.0 0.50 volume 70 of benzene in the feed 7 . 1 7 3 . 6 1 8 . 8 C 24-84 0 . 6 5.0 Skellysolve C d 4.4 69.9 21.7 F 24-132 4.1 0.25 Mid-continent kerosene d some 0.33 volume 7,.was recovered un8 6 . 2 3 . 4 1 0 . 3 F 84-144 0 . 1 2.0 Mid-continent kerosened reacted but not all of the remaining a Reactor conditions unless otherwise indicated in subsequent footnotes were 500 lb./ss. in. gage, o.17y0 was accounted for in the alkyllooo C., 0.10 liquid hourly space velocity (LHSV), and HC1 concentration 10 mole % on t h e hydrocarbons charged. saturator temperature was 77O C. benzenes (part may have gone into 6 Comrnercial'n-pentane feeds had the following compositions: the catalyst phase). A larger volume of Composition, Mole % high boiling material mas available for characterization from the Feed 2'-C1H12 n-C6Hia Cs + run with 2.0 volume 76 benzene. D a t a are presented in 0.1 94.4 A 5.5 Table IV. 4.6 95.4 0.0 B
TABLE 11. CONTINUOUS TEPTS~
.
C D E F
d 0
10.0 7.4 0.8 8.3
I
86.9 91.1 98.9 89.6
3.1 1.5 0.3 2.1
I
Measured from s t a r t of indicated conditions. 260 Ib./sq. in. gage. 150 lb./sq. in. gage. 70
drained from the bulk supply of aluminum chloride. Certain cyclic additives in addition t o inhibiting disproportionation also inhibited saturator sludging t o varying degrees. Methylcyclopentane, cyclohexane, methylcyclohexane, and decahydronaphthalene completely suppressed sludge formation in the saturator; the excess aluminum chloride was granular and undiscolored. Some sludging occurred in the flow tests with the other naphthencs. Benzene in concentrations of 0.5% or higher was a good sludge suppressor although the saturator contents after operation with this additive were invariably darkened at the inlet and the dhrlrened portion was usually somewhat sticky. Paraffins were not sludge suppressors. Although desirable from the standpoint of catalyst consumption, complete suppression of saturator sludge has not been found essential for efficient isomerization. The phenomenon of saturator sludging is to be distinguished from the formation of sludge or catalyst complex in the reactor in the presence of hydrogen chloride; this occurred in all cases, even in the presence of the most effective additives.
60
50
40
30
I/ W
U ACID TREATED SKELLYSOLVE A SKELLYSOLVE e REACTOR CONDITIONS:
B
B8 77 SO'C, 250-500 PSIG. ON HCBNS. CHARGED
1-
REACTIONS O F ADDITIVES
Some clue as to the manner in which the cracking inhibitors function should be afforded by determining what happens to
VOLUME
Figure 3.
PER CENT
ADDITIVE
IN
FEED
Straight R u n Fractions as Additives
-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2378
TABLE 111. CHARACTER O F HIGH BOILINGPRODUC'TS FROM ISOMERIZATION IX PRESENCE OF CYCLOHEXAXE (Distillation charge: 375 ml. of product > 40° C.5) Cuto Temp., C. TI';R a n g e Fractions6 a t 760 Mm. Vol %. of Fractions 1-3 58.1 1 5 . 2 1 . 3 7 9 6 - 1 . 3 8 9 3 ) Corresponds t o impuri4-6 2'0.6 3 0 . 7 1.3747-1.3952 1 tiesproved t o be present in the original pentane, nis., cyclopentane a n d isomeria hexanes 7-8 77.8 4 0 . 8 1 4059-1.4039 Methylcyclopentane 9-15 80.8 7 7 . 6 1 4222-1.4264 a n d cyclohexane equivalent t o about of t h e cyclo:%ne charged 18 97.0 82.7 1.41,50 17 100.8 87.8 1.4218 18 100.8 89.9 1.4230
1
I1
'
19 20 21
107 125 150
92.9 95.7 96.6
1,4231 1.4238 1.4281
I
1 Higher naphthenes equivalent to aborit 11 27% of the cvclohexane charged
1
22 200 97.4 1.4480 23 217 99.0 1.4618 223 99.7 1,4660 2P Residur ... 100.0 1.4765 This is tho residue from a 50 bubble cap distillation and represents 14.7 vol. % of the effluent from operation a t 500 lb./sq. in. gage with 10 vol. % of cyclohexane. bhpproximate efficiencies of distillation equipment used were 40 theoretical plates for cuts 1 through 18, seven theoretical plates for cuts 10 through 21, a,nd one theoretica! plate beyond cut 21. 0 Cumulative, based on distillation charge.
TABLE I T T . CHARACTER O F HIGHBOILINGPRODUCTS ISOMERIZATION IN PRESENCE OF BEXZEKE (Distillation charge: 138 ml. of product C u t Temp., C. Fractionsb a t 760 M m . 1'01. yoc
> 67'
FROM
C.a)
ny
Range of Fraction8
This is residue from a fractionation in a 3B-inch glass helis-packed column a n d represents 2.38 vol. yo of the effluent from the run with 2.0 r o l . "0 of benzene. b h fractionating column of about sixty theoretical plates waz used through fraction 17. a five-plate column for the remainder of t h e distillation. c Cumufative, baaed on distillation charge.
M ECHASI S5I
The isomerization of saturated hydrocarbons is affected b y various factors (13, 14, 18) which are inherent in the usual type of laboratory technique and whjch include impurities present in the commercially available reagents. For this reason it is desirable for the study of the mechanism of the isomerization reaction, to deal with highly purified reagents; this is possible through the use of a high vacuum technique. The present investigation, although dealing chiefly with commerical grades of reagents or C . P . grades which were not purified rigorously brings out several factors which seem to throw some light on the mechanism of isomerization. The additives used for inhibiting the cleavage of pentanes during isomerization belong t o either one or both of the two categories: alkylatable inhibit>ors,suc.has aromat,ic hydrocarbons; hydrogen donors such as decahydronaphthalene ( 7 ) ; or molecular hydrogen itself. Compounds that can be alkylated and that are known to act also as hgdrogcn donors under certain conditions include isobutane, methylcycloherane, and cyclohexane. I n order to obtain some eonclusive data a3 to the course of isomerization, several experiments were made using highly purified reagents and a high vacuum t,echnique analogous t'o that described previously ( I S , 18, 18). The experimental data are given in Table V.
Vol. 40, No. 12
These data show that aluminum chloride-hydrogen chloride causes a large amount of decomposition of n-pent,ane. The addition of a small aiiiount of benzene inhibits both isomcriaation and the cleavage of n-pent,ane. The effect of benzene is pronounced even when the concentration of' hydrogen chloride is increased from 1 t o 7%. These experiments indicate that benzene inhibits the cleavage of pentane. Therefore, carbonium ions necessary for the isomerization of n-pentane t o isopentane ( 2 ) are not produced in sufficient amounts for isomerization. The mechanism through rdiich the inhibition of the cleavage reaction proceeds is not entirely clear a t present. Its complete clarification will require further experimental x o r k carried out under strictly controlled conditions, including a study of the kinctics of the reaction. The addition of water t o a mixture composed of pentane, aluminum chloride, hydrogen chloride, and benzene caused the isomerization of n-pentane to occur. This is not surprising inasmuch as it was reported already (18)that hydroxgaluminum dichloride which is formed t>hroughthe &ion of water on aluminun1 chloride is not equivalent to aluminum chloride-hydrogen chloride catslyst. The latter catalyst requires an outside source of carbonium ions t,o catalyze the isomerization of n-butane to isohutane; hydroxyaluminum chloride probably forms the carbonium ions necessary for this reaction through halogen-hydrogen interchange between the catalyst, and the alkane (16, 18). The commercial aluminum chloride may contain enough hydroxyaluniinum chloride t,o cause such a n isomerization nithout the introduction of carbonium ions from an outside source. This is brought out in experiment 7, Table V, where the conirnerical grade of aluminum chloride was used as a cat,alyst. It was shown in Table 11 t)hat with the increase in bcrizene concent,ration from 0.25 t o 10% the amount of isomerization (including decomposition) decreases. I n line with the experimental results obtained with t,he highly purified reagents, it is probable that in the presence of benzene t,he carbonium ions necessary to cause the isomerization of n-pentme t o isopcntane were obtained through the presence of small amounts of extraneous material. By increasing the concentration oE benzene the chance for the carbonium ions to react, with it is greatly increased; for that reason the concentration of carbonium ions available for causing the isomerization is decreased. To obtain the same effect with naphthenes as with benzene, the concentration of the former has to be much greater. The reason for this is that t,he inberaction of a naphthene with a carbonium ion does not proceed as readily as with aromatic hydrocarbons. In the case of polyalkylat,ed aromatics, or those which are not readily alkylated, the degree of decomposition is relativcly high; this is in line with t,he ideas presented here. The mechanism discussed is merely a suggestion based on presently available data as to the function of the decomposition inhibitors in the isomerization of n-pont,ane. It is hoped t h a t a systematic study of the isonierizat,ion of saturated hydrocarbons,
HIGHVACUUM EXPERIVEST~"
TABLE
Expt. No. rL-C0Hi2 AlCls HC1 Ha 0 CGHB
4
6
I
2
100 100 9.4 9.4 0 1.09 0
0
0 0
4 5 Chargeb, hIoles 100 100 100 9.5 9.4 9.5 7.3 1.1 7.4 0 0.97 0 1.17 1.17 0 Analysis, Mole Yo
3
6
7
100 9.5 7.4 0.98 1.16
IO0 9.4c 7.4 0 1.17
Temperature i s o C: time 6 hours. These rnaberikls were'rigorohy purified except as indicated, Commercial 11Cla.
December 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
using high vacuum technique, may furnish additional information as t o thc true mechanism of isomerization. LITERATURE CITED
(1) Bloch, H. S., Hoffman, A. E., Oleszko, T. J., and Chenicek, J. A,,
Dir.. of Petroleum Chemistry, A.C.S., New York, 1944. (2) Bloch, H. S., Pines, H., and Schmerling, L., J . Am. Chem. SOC., 68,153 (1946). (3) Cheniceh, J. A., Dr>er, C. Q., Sutherland, R. R., and Iverson, J O., World Petrolaum, Refinery Issue, 1944,p. 146. ( 4 ) Eveiing, B. J,., d'Ouville, E. L., Lien, A. P., and Waugh, R. C., presented before the Pivision of Petroleum Chemistry at the 11 I t h Meeting of the AMERICAX CHEMICAL SOCIETY, Atlantic Citv. N. J. (5) Ipatieff; V. N., Corson, B. B., and Pines, H., J . Am. Chem. SOC.,
58,919 (1936). (6) Ipatieff, V. N., and Haensel, V., Ibid., 64, 520 (1942). (7) Ipatieff, V. N., and Pines, H., Ibid., 59, 56 (1937).
2319
v- N.,and Schmerling, L., IND. ENG. CHEM..40, 2354 (1948). (9) Perry, S.F., Trans. Am. Inst. Chem. Engra., 42, 639 (1946). (10) Pines, H. (to Universal Oil Pioduct,s Co.) U. S. Patent 2,405,616 (Aua. 6, 1946). (11) I h i d . , 2406,967 (September 3. 1946). (12) Pines, H., Kvetinskas, B., Kassel, L. S.,and Ipatieff, V. N., J . Am. Cham. S o c . , 67, 631 (1945). (13) Pines, H., and Wackher, R. C.. I h i d . , 68,595 (1946). (14) I b i d . , p. 599. (15) Ibid., p. 2518. (16) Pines, H. and Wackher, R. C., (to Universal Oil Products Co.) U . S. Patent 2,406,633 (Aug. 27, 1946). (17) Ibicl., 2,406,634. (18) Wackher, R. C. and Pines, H.. J . Am. Chem. Soc., 68, 1842 (1946). ( 8 ) Ipatieff.
RECEIVED December 8, 1947.
Presented before the Divkion of Petroleum Chemistry a t the 112th Meeting of t h e AMERICAN CHEMICAL SOCIETY, New York, N. Y.
Esters of Naturally Occurring Fatty Acids J
PHYSICAL PROPERTIES OF METHYL, PROPYL, AND ISOPROPYL ESTERS OF C, TO C,, SATURATED FATTY ACIDS CARL W. BONHORST, PAUL
M
a
ALTHOUSE, AND HOWARD 0. TRIEBOLD
Pennsylvania State College, State College, Pa.
An apparatus is described and the operating procedure is given for the determination of a complete vapor pressure curve on one or two drops of a pure liquid. The apparatus was calibrated, and the vapor pressure curve for each of the methyl, propyl, and isopropyl esters of the naturally occurring Cn to C18 saturated fatty acids was determined. Decomposition of the esters was found to be progressive above 205 C. and occurred over a wide temperature range rather than at a specific temperature. The relationships of the densities and viscosities of the esters to temperature were studied by determining these constants at 20", 37.8", 60", and 98.9" C. Relationships between the vapor pressures, densities, and viscosities indicate that the forces governing these three properties have some factor or factors in common.
T
HE value of physical data for the identification of pure
esters of the fatty acids has been proved (4,11). Among those physical constants which contribute information for the identification of a pure compound are vapor pressure, density, and viscosity. The study of the effects of changes in structure or molecular weight upon physical properties frequently yields valuable information. Further studies of these effects upon the properties of pure compounds may make possible the formulation of methods for the estimation of the composition of mixture.s, or conversely, for the preparation of mixtures having certain desired properties. The propyl and isopropyl esters should yield information of special value in the study of fats and oils, for they are the simplest esters of 3-carbon alcohols with fatty acids. The value of a thorough study of these properties with regard t o esters is enhanced considerably by the need for densities in the calculation of volumes for synthesis, and by the fact that the viscosity characteristics of esters are such as t o suggest their use as lubricants.
An excellent compilation of the vapor pressures of a great number of compounds has recently been published (18). I-Iowever, the only vapor pressure data t h a t seem t o be available for the esters of the naturally occurring fatty acids are those of the methyl esters (4, I S ) . Boiling points at 30 mm. of mercury pressure and densities have been reported for the isomeric 16carbon esters (16). Densities and boiling points a t 20 mm. pressure have been reported for the esters of caproic and caprylic acids with various alcohols (9). Densities of methyl esters (2) have been reported. Viscosities of numerous low molecular weight esters ( 7 ) ,and a large number of diesters (67, and esters of nonanoic, nonenoic, oleic, and cinnamic acids (1) have appeared in the literature. However, complete studies of the relationship of the density and viscosity of the methyl, propyl, or isopropyl esters to temperature have not been reported. APPARATUS
Densities and viscosities may be determined easily aiid accurately with comparatively simple apparatus. Various methods for the determination of vapor pressures on a small quantity of pure liquid have appeared in the literature (4, 8, 1 7 ) . However, all have certain serious limitations and usually yield erroneous results when used outside those limits of temperature and pressure to which they seem t o be intrinsically adapted. Natelson and Zuckerman ( I d ) observed that when a capillary filledrby suspending a drop of liquid from the lower end is placed in an air bath heated to a certain temperature and the pressure slowly reduced, a point is reached at which the level of the liquid in the capillary will fall rapidly. They found that the pressure at this point corresponds t o the vapor pressure of the liquid at the temperature of the bath. I n an effort t o utilize these observations, a new apparatus has been designed for the determination of vapor pressures over a range of 2 t o 100 mm. and a t temperatures up to 220" C.
