182
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE VIII.
Vol. 42, No. 1
traction, to improve the toluene cont,ent, is indicated. The solHE.u REQGIREILIENTS FOR RECOVERY O F TOLUENE vent dosage and number of stages of extraction required 011 charge B Y WATER EXTRACTION A T 302' c. oils of greater than 50% toluene require an extraction tower. of (Assuming flow in Figure 7 ) feasible dimensions. The pressure required, however, is well R.t.u./Hour above that usually encountered in extraction processes.
Feed preheater, E-2 Water preheater, E-4 Extract recycle preheater, E-6
1,082,000 10,500,000 1,541.000
LITERATURE CITED
-___
Total 13,123,000 Total heat required, 13,123 B.t.u./eal. toluene recovered
(1) Blanding, F. H., and Elgin, J. C., TTans. Am. I7&. Chem. Engrs., 38, 305 (1942). ESG. CmnI.. 34, 804 (1942). i b l Griswold, J., and Kasch, 3. E., IXD. Petroleum (3) Hill, L. R., Vincent, G. A., and Everett, E . I.'.. .\'atZ. News,38, R-456 (1946). (4) Jaeger, A., Brennstoff-Chem., 4, 259 (1923). In\
COh CLUSIONS
A study of the recovery of toluene from the toluene concentrate from petroleum stocks by liquid-liquid extraction with water based on equilibiium data indicates that such a process is feasible if carried out a t temperatures of the order of 302 C. Charge oils of below 50 % vvouldnot be very attractive; the use of stocks which have been dehydrogenated prior to ex-
( 5 ) Keenan, J. H., and Keyes, F. G., "Thermodynamic Properties of Steam," New York, John Wiley 8: Sons,1940. (B) Maloney, J. O . , and Schubert, A . E., Trans. Am. Znst. Chem. E n y s . . 36, 741 (1940). f 7 , Selson, 1 %'. I,., Oil Gas J . , 36, 184 (1938). \'I
RECEIVEDJune 8, 1949. Presented before the Division of Petroleum Chemistry a t the 115th lIeeting of the AhrE~.chh C B ~ SOCIETY, ~ ~ I San Francisco, calif.
Aging of Crac
Catalysts
LOSS OF SELECTIVITY G. A. MILLS Houdry Process Corporation, Marcus Hook, Pa. Petroleum cracking catalysts undergo certain changes during use which result in loss of activity. In some instances there is also a n unfavorable alteration in the distribution of cracked products-gasoline, coke, and gas. This loss of selectivity w-as investigated by studying the effects of conditions to which catalysts may be subjected. The factors found to be important in catalyst aging were: the chemical nature of the gases to which a catalyst is exposed; the time and temperature of this exposure; the type of catalyst; and contamination of the catalyst by metals entrained i n the charge stock. The conclusion was reached t h a t loss of selectivity is due to heavy metals in active form in the catalyst. Even minute amounts of iron,
nickel, vanadium, and copper were harmful. These metals occur in certain petroleum stocks and may be carried by entrainment to the catalyst where they accumulate. Either clay or synthetic catalyst is poisoned. Corroboration was obtained from analysis of the ash of oils causing this type of aging and of catalysts so aged. More usually, loss of selectivity occurs because commercial clay catalyst, but not synthetic, contains iron brought into a catalytically active state through reaction with sulfur compounds contained in catalytic cracking charge stocks. This was confirmed by testing a clay catalyst from which iron had been selectively removed and determining its stability while cracking charge stocks containing sulfur compounds.
T
which have been found important in catalyst aging are: the chemical nature of the gases to which the catalyst is exposed; the time and temperature of this exposure; the type of catalyst; and contamination of the catalyst by metals entrained in the charge stock.
HE ability of catalysts to perform efficiently is essential for successful operation in the keenly competitive refining field. However, in commercial operation petroleum cracking catalysts not only tend to lose activity but also selectivity-the ability to produce a desirable distribution of products. Although the effects of aging cracking catalysts have been apparent readily, the causes have been recognized only slowly. The fact is that the aging process does not occur in a simple way from a single cause. During the cyclic cracking process, catalysts are subjected t o hydrocarbons a t about 800" t o 950" F. and to products of combustion a t temperatures as high as 1150" F., perhaps considerably above on the catalyst surface. I n addition there may be present steam and other compounds, such as those containing nitrogen and sulfur, as well as entrained contaminants. These are the conditions causing alterations in catalysts which ultimately are observed as deactivation, Catalyst aging has been investigated by studying the effects of different conditions GO which catalysts may be subjected. The influence of impurities and the effects of various gases a t high temperatuies were determined. It is the purpose of this paper to present these laboratory data togethm with their interpretation as applied t o aging with loss of selectivity. The factors
CATALYST ACTIVITY A N D SELECTIVITY
The cracking properties of a catalyst are usually evaluated by passing oil over the catalyst under standard test conditions and collecting and measuring the reaction products. These products are conventionally separated according to boiling point into fractions classified as gas, gasoline, gas oil, and catalyst deposit or coke. The activity of a catalyst may be measured broadly by the over-all conversion of charge stock into products boiling in a range other than t h a t of the charge stock. Catalyst selectivity is determined by the distribution of products and here refers to the production of gasoline relative to coke and gas. A loss of activity is caused by deposition of a hydrocarbonaceous product on the catalyst during the cracking reaction. This is temporary since activity is restored by burning off the coke with air in frequent regeneration periods. In addition, however, after many successive cycles a permanent aging also is observed. Xormal catalyst aging is considered to be caused by
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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183
obtained with the sample under the standard cracking test conditions. Of course, cases of severe loss of selectivity are obvious since gasoline yield may be decreased with actual increases of coke and gas. The advantage in using a semilogarithmic plot as in Figure 1 arises from the fact t h a t parallel straight lines are then obtained for different catalyst types. The equation, A log coke = gasoline coke constant, and a similar equation for gas may be used to evaluate a coke and a gas constant which serve to quantitatively define the selectivity for a given catalyst. A is a constant defining the slope and has been found to equal 36.0. The selectivity of a cracking catalyst may also be expressed numerically as a carbon-producing factor and a gas-producing factor ( 1 5 ) . These are the ratios of carbon and of gas yields obtained with the catalyst tested t o the carbon and gas yields, respectively, obtained with a standard catalyst at the same conversion of gas oil. These factors appear in the data and the discussion which follows. In the present case, comparisons were made with results obtained with a series of standard catalysts, steam treated to different activities rather than a single standard catalyst run to give different conversions. The numerical values obtained by either procedure are nearly equal. The errors in the catalytic activity test-A, already mentioned, are such that the coke and gas factors ordinarily can be interpreted as showing loss of selectivity only when they are greater than 1.15 or even 1.20.
+
A
15 Figure 1.
25 35 45 GASOLINE VOL. %
55
Catalyst Selectivity-Catalytic Activity Test-A
Houdry Type M = synthetic SiOrAlrOi TCC Filtrol 3 acid-activated bentonite Houdry Type I = low-iron catalyet derived from clay
d#
changes brought about by steam a t high temperatures, or by very high temperatures alone. Under these conditions loss of total surface area is observed ( 4 , 8, IS). On the other hand, aging may result in loss of selectivity. This has frequently been known as abnormal aging or catalyst poisoning. The activity and selectivity of cracking catalysts are evaluated in this laboratory by the catalytic activity test-A (1). Light East Texas gas oil is passed over a 200-ml. fixed bed of catalyst at 800" F. for a 10-minute period a t 1.5 liquid hourly space velocity. The products-gas, gasoline, gas oJ, and coke-are measured under well defined conditions. The coke figure actually refers t o the amount of carbon in the catalyst deposit. Extensive tests (1) have shown that the standard deviations in this test are, in the units employed, gasoline 1.5, coke 0.3, gas 0.7, and gas gravity 0.09. The distribution of products obtained with three catalyst types using this standard cracking test is shcwn in Figure 1. These catalysts were adjusted to different activities by treatment with steam at temperatures above 1000" F Type M is a synthetic catalyst manufactured by the Houdry Process Corporation. T C C Filtrol is an acid-activated bentonite clay manufactured by the Filtrol Corporation for use in Thermofor catalytic cracking. Houdry Type I catalyst is derived from clay (14). Chemical analyses of these catalysts are given in T a b I These catalysts are in the form of cylindrical pellets of ahout 4 m m . diameter and height. I n order to determine catalyst selectivity from the products distribution of a cracking test, i t is not sufficient merely to consider gasoline-to-coke and gasoline-to-gas ratios. These ratios are not constant for a given catalyst but depend on conversion level obtained in the cracking test. This is evident from Figure 1. This plot is the basis for determining the selectivity of any catalyst sample by comparison of the products distributiop
TABLE I. CATALYST COMPOSITION
Dry (220O F.) basis Ignition loss at 1600O F.
so4
Free Ha604 Ignited basis Si01 Ah08 FezOa
Houdry Type M 1.5 0.1
....
NiO
87.5 12.5 0 1 0.01 0.01 0.15 0.001
VZOb
0.005
2 ii Total alkali as NarO cuo
0.001
Composition, Wt. yo TCC T C C Filtrol Filtrol washed 8.5 4.3
0.83
7.8 0.14
...
Houdry Type 1 6.8
..... ..... 76.7 18.2 0.1 4.4 1.0 0.2
