I
J. A.
RABO, P.
E. PICKERT, and R. L.
MAYS
Molecular Sieve Products, Linde Co., Division of Union Carbide Corp., Tonawanda, N. Y.
Pentane and Hexane Isomerization Catalyst MB 5390 is an intermediate temperature isomerization catalyst developed for the isomerization of pentane and hexane. New zeolite support provides high activity and selectivity isomers better than 95% to the equilibrium
Tk
INTEREST of the petroleum industry in producing high octane number, low-sensitivity blending stocks has given rise to a variety of catalysts and processes for the isomerization of the Cd to C6 paraffins. Several commercial installations are already in operation and, according to predictions, new capacity will be added in the near future. For the isomerization of pentane and hexane, refiners have been weighing the advantages and disadvantages of low temperature compared with high temperature conversion. This article describes a catalyst which operates in an intermediate temperature range-it thereby combines the advantages of higher isomer yield than is currently obtained with commercial high temperature catalysts and lower investment in pretreating equipment than is required with the low temperature catalysts. Linde Isomerization Catalyst MB 5390 is a new catalyst designed specifically for the isomerization of the C5 and (26 paraffins. The catalyst comprises a noble metal (palladium) on a modified zeolite support (6). I t does not contain activators of any type and is insensitive to the presence of water in the feed. The catalyst isomerizes npentane and the hexanes with excellent selectivity below a temperature of 660’ F. with better than a 95% approach to equilibrium. The temperature ‘required by this catalyst is intermediate between that used in the “high temperature” processes (700’ to 850’ F.) (4, 9) and that used in the “low temperature” processes (200’ to 400’ F.) (7, 5 ) . The refiner can thereby take full advantage of the isomer yields thermodynamically favored by intermediate operating temperature. Ca and CB distillates can be isomerized separately or together with almost identical efficiency, since the optimum operating temperatures for the n-pentane and the hexanes differ by only 30’ F.
Life studies show no change in the performance of the catalyst after over 2000 hours of continuous operation. The feed can be prepared by conventional distillation techniques, since heavier boiling components in concentrations of several per cent can be tolerated. The sulfur tolerance of the catalyst is also good, with an equilibrium relationship existing between the sulfur content of the feeds and catalytic activity. Since the low operating temperature and the absence of activators are conducive to a high degree of stability, pilot plant data can be used, with reasonable safety, to predict performance in a commercial scale unit.
tive to produce near equilibrium isomer yields. Linde Catalyst MB 5390 and one other Linde catalyst were used to define the equilibrium isomer distribution for the pentanes and hexanes. The equilibrium was approached from both directions using Cs and c6 normal and isoparaffin feeds. Close approach to equilibrium was effected by gradually increasing the superficial contact time. Experimental equilibrium points were adapted only for runs in which cracking of the feed did not exceed 5 mole %. As shown in Figure 1, a t a temperature of 626’ F., with n-hexane feed the 2,2dimethylbutane content of the CS product was 16.4 mole %. With 2,2dimethylbutane feed the concentration of the same isomer in the product was 16.7 mole yo. The equilibrium is taken as 16.6 f 0.3 mole Yo at 626’ F. At 680’ F., the equilibrium 2,2-dimethylbutane yield was 15.3 i. 0.3 mole %. As shown in Figure 2, at a temperature of 662’ F., the isopentane (2-methylbutane) content of the C5 product, using both normal and isopentane feeds, was 66.0 =t 0.3 mole yo. Equilibrium compositions for both the pentanes and hexanes are given in Table I. The experimental equilibrium distribution values are very close to the values suggested by Ridgway (7)) however, they disagree with the calculated values and the values given by other authors.
Thermodynamic Equilibria Temperature has a pronounced effect on paraffin isomer equilibria. Consequently, the equilibrium isomer distribution must be determined as a function of temperature to define the conversion limit. Inaccuracies in the published basic thermodynamic data have resulted in gross discrepancies between calculated and experimental equilibrium values, particularly for the hexane isomer distribution. These discrepancies persist even in the recently published literature (2, 3, 7). The experimental equilibrium values are usually given for the narrow temperature range over which the catalyst is sufficiently ac-
Table I.
%
Equilibrium Distribution of Pentane and Hexane Isomers, Mole Temp., Calcd. O F. (8)
n-Pentane Isopentane n-Hexane 3-Methylpentane 2-Methylpentane 2,3-Dimethylbutane 2,2-Dimethylbutane
662 626
66 34 19 17 32 10 22
Exptl. 66 34 20.3 21.5 32.8 8.8 16.7
Temp., Calcd. O F. (8)
Exptl.
=t0 . 3 % zk 0 . 3 %
f 0.3% f 0.3% zt 0 . 3 % rk 0 . 3 % & 0.3Yo
VOL. 53,
680
21 17
21.4 i 0 . 3 % 22.3 0.3%
*
41.0
20
NO. 9
15.3
* 0.3%
* 0.3%
SEPTEMBER 1961
733
Experimental
40 -
30 -
20-
30r
;
X
--------%
2 - METHYLPENVANE
)t----x
X
T
I
I
I
I
20
4Q
60
80
I
100
All isomerization reactions were run using 100 ml. of catalyst in reactors of conventional design. The reactors were surrounded by an aluminum block, heated by a Glas Col heating mantle. The temperature variation in the catalyst bed was not more than S 3 " F. T h r units were equipped with automatic temperature and pressure controls which permitted unattended operation. The liquid product was collected in pressure receivers at dry ice temperatiire. Samples were withdrawn at the same temperature and immediately analyzed by gas chromatography. The hydrogen off-gas and the gas released on depressurizing the liquid product contained most of the C1 and Cz and some of the C S product. These samples were analyzed separately. Table I1 gives the complete product distribution in typical pentane and hexane isomerization runs. The data show that the formation of C1 and C2 was very low. Over 80% of the total C3 was recovered in the liquid product. Thus the data in Table I1 can be used in conjunction with the liquid recovery data in Tables 111, IV: and V to estimate the total gasification for any particular set of process conditions.
CDNTACT TIME, SEC. Figure 1.
Equilibrium
Cg
paraffin isomer distribution at
626" F. i s established b y increasing the contact time of
Process Variables
either n-hexane or 2,2-dimethylbutane feeds on Catalyst
Catalyst MB 5390 isomerizes pentane and the hexanes with a close approach to equilibrium a t temperatures of between 608' and 662" F. At a weight hourly space velocity (W.H.S.V.) of 2, over 90% of equilibrium isomer yields were obtained with both the C 6 and Cti feeds. At a space velocity of 5 and a temperarure of 698' F., the isomrr yield with n-pentane was 95% of equilibrium. I n the temperature range employed, a total pressure of 450 p.s.i. and a hydrogen-to-hydrocarbon ratio of 3 was sufficient to assure long catalyst life. Pressure alone in the range 350 to 600 p.s.i. did not affect performance, provided contact time was held constant; however: pressures below 350 p.s.i. tended to increase cracking, and above 600 p.s.i. caused a decrease in activity.
MB 5390
---z
:
V Q
cc L
70
FEED =NORMAL PENTANE FEED=ISOPENTANE
! I
Pentanes
w
I CONTACT TIME, SEC.
Figure 2.
Equilibrium Cg paraffin isomer distribution at 662" F. is established b y increasing the contact time of either n-pentane or isopentane feeds on Catalyst MB 5390
734
INDUSTRIAL AND ENGINEERING CHEMISTRY
Catalyst MB 5390 isomerizes the pentanes between 653' and 698" F. as shown in Table 111. With a hydrogento-hydrocarbon ratio of 3 and a space velocity of 2, 65% isopentane in the C S fraction was produced; a t a space velocity of 5: 6270 isopentane was produced. At a total pressure of 450 p.s.i., these process conditions gave superficial contact times ranging from 15 to 37 seconds. Contact time was calculated as the residence time in the reaction space of a unit volume of reactants at the
PENTANE A N D H E X A N E I S O M E R I Z A T I O N reaction conditions. Along with temperature, contact time was the most important process parameter; both pressure and space velocity could be varied without affecting performance provided contact time was held constant. A total of 2 to 3 wt. % ' hexanes in equilibrium distribution were produced in the reaction. Under conditions yielding 65 mole 7 0 isopentane in the Ci, fraction, the C1 to C ) gas formed constituted less than 1.5 wt. 7, of the product. Over 75 wt. % of the gas was propane. This distribution indicates a high degree of selectivity for carbonium ion reactions. The low production of methane and ethane, of course, means low hydrogen consumption.
Hexanes Hexanes are isomerized by Catalyst
MB 5390 a t temperatures from 608' to 662' F. (Table IV). At a space velocity of 2 and a hydrogen-to-hydrocarbon ratio of 3, over 157, 2.2dimethylbutane was produced with essentially equilibrium yields of the remaining components. The 2,2-dimethylbutane yield corresponds to over 90% of equilibrium. The once-through isomerizate has an octane number of 91 (RON 3 ml. TEL) or research octane number with 3 ml. tetraethyllead. Catalyst activity remained unchanged for over 1000 hours at a pressure of 450 p.s.i. and hydrogen-to-hydrocarbon ratio of 3. Pressure could be varied between 350 and 600 p s i . with no effect on performance provided the space velocity was adjusted so as to maintain a constant contact time. The C1 and Ca fraction was less than 2.0 wt. % of the product and was composed mostly of propane, with less than 2 wt. % and 8 wt. % C1and CB,respectively.
low operating temperature, several per cent C7 can be tolerated in the feed without affecting the life of the catalyst.
Table 11. Product Distribution in Pentane and Hexane Isomerization with Catalyst MB 5390 Feed' Component, Wt. yo n-Hexane n-Pentane
Catalyst life Catalyst MB 5390 is a water insensitive, coke-resistant catalyst with good mercaptan sulfur tolerance and long life under normal operating conditidns. The catalyst contains no activators of any type and requires no activating additive in the feed. Water concentrations in the feed up to saturation have no effect on performance. Its coke resistance extends even to high temperatures at which a large fraction of the hydrocarbon feed is converted to gas. In Table VI, the stability of the catalyst was tested for periods of over 1250 hours using n-hexane feed and for over 1400 hours using n-pentane feed. At the end of these periods the catalyst was producing over 14 mole % 2,2dimethylbutane and 63 mole % isopentane with the respective feeds. No detectable coke was found on the catalyst after these tests were terminated, in spite of the fact that the catalyst had
Methane Ethane Propane Isobutane n-Butane Isopentane n-Pentane 2,2-Dimethylbutane 2,3-Dimethylbutanel 2-Methylpentane 3-Methylpentane n-Hexane Methylcyclopentane Cyclohexane
0.03 0.11 1.55b 0.90 0.38 0.64 0.35 13.88
0.06 0.23 1.02* 2.21 1.10 57.91 34.73 0.36
39.55
1.07
19.30 21.85 1.24 0.18
0.59 0.72 0 0
a Feed contained 2.3y0 methylcyclopen82 3= 2.0% of the total C3 is actane. counted for in the liquid product.
been periodically exposed to high temperatures to accelerate activity fall-off. The introduction of water into both the pentane and hexane feeds up to the point of saturation had no detectable
+
Pentane-Hexane Fraction Catalyst MB 5390 isomerizes the C 6 and Ce fraction most efficiently between 635" and 662' F. (Table V). Since hexane is isomerized a t the lower teniperature, the composition of the hexane fraction in the isomerizate approached the equilibrium more closcly than did that of the pentane fraction. Using a pentane-hexane feed, over 13 mole % 2,2-dimethylbutane and 60 mole % isopentane was produced in the respective fractions a t 664' F. The ability of the catalyst to isomerize the combined Cs and Ce fraction results in greater flexibility of operation, since feeds ranging from the straight C6 cut to the straight Cs cut can be processed efficiently. Heptane is more severely cracked than hexane under similar process conditions. However, since the over-all gasification is low because of the
Table 111. Temperature, O F. Pressure, p.s.i. W.H.S.V., g./g./hr. Ht/hydrocarbon, m/m Contact time, sec.
Pentane Isomerization 662
662
698 450 2 3 36
698 450 5 3 14
Total liquid product, wt. %
c1-c3 C4 c6 f
0.9 3.8 95.3
0.3 1.2 98.5
1.1 3.6 95.3
0.5 1.8 97.7
66.2 33.8
65 35
65.2 34.8
62.5 37.5
Composition of CS fraction, mole % Isopentane n-Pentane
Table IV. Temperature, F. Pressure, p.s.i. W.H.S.V., g./g./hr. Contact time, sec.
Hexane Isomerization 626 450 1 91
626 450 2 45
626 450 2 45
635 450 5 18
Total liquid product, wt. yo Ci-Cs C4 C6
f
1.5 1.4 97.1
0.8 1.1 98.1
1.2 1.0 97.8
0.3 0.2 99.5
20.7 21.5 32.9 8.6 16.4
21.8 21.8 33.3 8.0 15.2
21.5 21.6 33.2 8.2 15.5
33.6 20.8 30.8 5.8 0.0
Composition of CS fraction, mole % n-Hexane 3-Methylpentane 2-Methylpentane 2,3-Dimethylbutane 2,Z-Dimethylbutane
VOL. 53, NO. 9
e
SEPTEMBER 1961
735
Table
V.
Pentane-Hexane Isomerization 50 Rlole yo
+
+Pentane 50 Mole yo n-Hexane
+
Feed
75 Mole yo n-Pentane 25 Mole 95 %-Hexane
Temperature, ‘ F. Pressure, p.s.i. W.H.S.V.,g./g./hr. Hg/hydrocarbon, m/m
644 450 2 3
644 450
653 450 2 3
2
3
Total liquid product, wt. yo
c1-c3 c 4
c3 + Composition of CSfraction, mole
1.2 1.2 97.6
1.6 1.6 96.8
1.8 1.8 96.4
59.8 40.2
62.0 38.0
58.8 41.2
23.3 23.0
23.3 22.9
23.3 22.8
40.4
40.5
40.7
13.3
13.4
13.2
7‘
Isopentane n-Pentane
Table VII. Effect of Sulfur in Pentane Isomerization with Catalyst MB 5390 Sulfur Content, P.P.M.”
Isopentane in C S Fraction, Mole yo
6 12 18 24 30 55 3000 0
62.5 61.0 60.0 58.5 57.5 55.0 27.0b 62.5
As n-butyl mercaptan. * Average conversion during 16-hour test period.
Composition of C Sfraction, mole Yo n-Hexane 3-Methylpentane 2-Methylpentane 2,3-Dimethylbutane) 2,2-Dimethylbutane
effect on performance during several days of operation. The effect of the presence of sulfur is of great importance in reactions utilizing noble metal catalysts. The magnitude of the effect depends mainly upon the amount of sulfur present and the temperature. Permanent sulfur poisoning is caused by the formation of the noble metal sulfides. Temporary poisoning results from the chemisorption of H2S on the noble metal. The interaction between noble metal and sulfur is usually an equilibrium phenomenon in the presence of hydrogen. Since sulfur compounds of the noble metals are less stable at high temperatures, the sulfur poisoning effect is inversely related to temperature. With isomerization catalysts, higher sensitivity to sulfur is experienced than with reforming catalysts. due to the relatively low tem-
Table VI.
Feed
perature a t which isomerization reactions are carried out. The tolerance of Catalyst MB 5330 for sulfur was studied using n-pentane feed in which sulfur concentrations were varied from zero to 3000 p,p.m. Sulfur concentrations u p to 10 p.p,m. were found to have no effect on the performance of the catalyst. A prolonged life study was initially carried out with Phillips commercial grade n-pentane which contained 6 p.p.m. sulfur. No effect on catalyst performance was detected after over 1000 hours. The sulfur level in the feed was then increased stepwise and maintained a t each successive concentration until activity leveled cff, and no further change in performance would be detected. Table VI1 shows the steady-state pentane conversion a t various sulfur levels. Sulfur concentrations u p to
Catalyst Life
n-Pentanea
Time on stream, hr.
28
Composition of CSfraction, mole % Isopentane n-Pentane
61.4 38.6
455
62.2 37.8
1000
62.5 37.5
Composition of Csfraction, mole % n-Hexane 3-Methylpentane 2-Methylpentane 2,3-Dimethylbutane( 2,2-Dimethylbutane a 1T.H.S.V. (space velocity g feed/g. catalyst/hour) Pressure = 450 p.s.i.
......
1
736
...... ...... ......
INDUSTRIAL AND ENGINEERING CHEMISTRY
... ... . . I
...
?&-Hexane” 1400
62.0 38.0
... ... ... ...
193
813
1250
...... ......
... ...
23.8 20.6
23.5 20.7
23.3 20.5
41.6 14.0
41.6 14.2
41.7 14.4
= 2 ; Hz/hydrocarbon,
m/m = 3;
10 p.p.m. had no effect on performance. Above this level, however, sulfur acted as a temporary poison. At a concentration of 55 p.p.m., the isopentane yield leveled off a t 54y0 whereas at 3000 p.p.m., the average conversion during a 16-hour run was 27%. I n every case, the full activity of the catalyst was restored by operating the catalyst with the original low sulfur feed. At the 55 p.p.m. sulfur level, the catalyst operated for over 500 hours producing a constant 5477 isopentane in the product. After 2500 hours exposure to feeds containing a variety of sulfur concentrations, the activity of the catalyst was found to be unchanged on reverting to the original feed. Literature Cited (1) Block, H. S., Donaldson, G. R., Haensel, V.: Division of Petroleum
Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. (2) Ciapetta, F. G., Hunter, J. B., IND. ENG.CHEM.45, 147 (1953). (3) Emmett, P. H., “Catalysis,” Vol. VI, p. 159, Reinhold, New York, N. Y., 1958. (4) Heinemann, H., Bednars, C., Knaus, J. A , , Solomon, E., Erdol u. Kohle 12, 2281 (April 1959). (5) Lanneau, K. P., Arey, W. F., Perry, S. F., Schriesheim, A., Holcomb, H. A,, Petrol. Rtfner 38, No. 6, 199 (1959). (6) Rabo, J. A , , Pickert, P. E., Stamires, D. N., Boyle, J. E., Paper 104, Second International Congress on Catalysis, Paris, France, 1960. (7) Ridgway, J. A , , Schoen, William, IND.END.CHEY.51, 1023 (1959). (8) Rossini, F. D., Pitzer, K. S., Arnett, R. L., Braun, R. M., Pimentel, G. C., ”Selected Values of Physical and Thermodynamical Properties of Hydrocarbons and Related Compounds,” Carnegie Press, Pittsburgh, Pa., 1953. (9) Starnes, W. C., Zabor, R. C., Division of Petroleum Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959. RECEIVED for review iVovember 8, 1960 ACCEPTEDApril 26, 1961 Division of Petroleum Chemistry, Symposium on Catalysts and Catalytic Cracking, 138th Meeting, ACS, New York, N. Y . ,September 1960.
