Toluene Extraction from Petroleum with Water G. B. ARNOLD AND C. A. COGHLAN The Texas Company, Beacon, N . Y .
-t
Liquid-liquid equilibrium data on the extraction of toluene from a toluene concentrate with water at 274' and 302' C. are presented. The toluene concentrate was prepared by fractionation of hydroformed naphtha which was found to be the most attractive source of toluene. Calculations are made from the equilibrium data on the solvent dosage and number of stages of extraction required to prepare toluene of high purity. Consideration is given to the major factors involved in a process for the recovery of toluene from a toluene concentrate from hydroformed naphtha by extraction with water at high temperatures.
I
T BECAME evident during the early stages of the war that the supply of toluene would not be sufficient for normal demands and for the possible needs of the military. Furthermore, i t was highly doubtful if the desired production, estimated to be four times the 1940 production, could be attained by expansion of the coal tar industry alone. This fact led to the investigation of methods of making and isolating toluene from petroleum. A study of the toluene content of representative naturally occurring and reformed naphthas, summarized in Table I, indicated that although toluene was present in almost all the naphthas studied, the most attractive source appeared t o be hydroformed naphtha. Hydroformed naphtha, prepared by dehydrogenation of straight run naphtha (3), could be fractionated to produce a toluene concentrate of from 50 to 60 volume % toluene. To prepare substantially pure toluene, however, it is necessary t o subject this toluene concentrate to further purification procedures. This paper presents a study of the preparation of high purity toluene by liquid-liquid extraction employing water at elevated temperatures as the solvent.
extract purity is definitely limited unless dual solvent or other special techniques are used. However, employing solvents of the second group and extract recycle, similar to reflux in distillation, it is theoretically possible to prepare an extract of very high purity. Maloney and Schubert (6) have presented an excellent analysis of a system of this type in their treatment of the n-heptane-methylcyclohexane-aniline system. Water is only partially miscible with toluene under the conditions investigated, and by use of liquid-liquid extraction with extract recycle, it is theoretically possible to recover substantially pure toluene from a mixture of toluene and nonaromatic hydrocarbons. Jaeger ( 4 ) determined the solubility of several pure hydrocarbons and hydrocarbon fractions in water at elevated temperatures. Griswold and Kasch ( 2 ) subsequently measured the solubility of several petroleum fractions in water. These data indicated that the solubility of the aromatic hydrocarbons in water increases rapidly in the range of 200' to 300' C. and that aromatic hydrocarbons are more soluble in water than the olefinic or saturated hydrocarbons. MATERIALS
The equilibrium data were obtained on a toluene concentrate prepared from the product of a commercial hydroformer. The full range naphtha was fractionated to obtain a crude fraction boiling from 88" to 121' C. which was then refractionated into small cuts. The fractions containing toluene as the only aromatic hydrocarbon were combined, to form the toluene concentrate. Pertinent tests on the hydroformed naphtha and the toluene concentrate prepared therefrom are given in Table 11. Commercially available nitration grade toluene was used in all experiments requiring toluene. EQUIPMENT AND PROCEDURE
To obtain the equilibrium data necessary for the consideration of a commercial process for the extraction of toluene from a toluene concentrate with water, i t was necessary to carry out batch extractions at elevated temperatures and pressures. The equipment used in the work is shown in part in Figure 1 and dia-
TABLE I. TOLUENE CONTENT OF NAPHTHAS Wt. % Toluenein Wt.% 93-121O C. Toluene in Fraction Naphtha
R
Straight-run naphthas
25.7 2.4 12.9 4.9
8.5 0.8 0.4 0.G 0.8
6.0 20.5 58.0
1.3 4.3 14.0
0.8
Reformed naphthas Representative thermally cracked naphtha Representative catalytically cracked naphtha Representative hydroformed naphtha
TABLE 11. TOLUENE CONCENTRATE EMPLOYED IN EQUILIBRIUM STUDIES
Gravity A.P.I. Aniline boint. O C. Bromine addition No. C.F.R.M. octane No. Benzene. vol. 9% Toluene, vol. % ' Xylenes vol. % Total aiomatics, vol. %
PREVIOUS INVESTIGATIONS
Polar solvents have been proposed in the patent literature and several systems have been investigated which could be conceivably applied in the separation of toluene from other hydrocarbons in the toluene boiling range. I n general, these solvents fall into two groups: those with which toluene is completely miscible and those with which toluene is only partially miscible. The preparation of a high purity toluene is not practical by liquidliquid extraction employing solvents of the first group because the 177
Hydroformed Naphtha 49.8 15.0 4 76.9 1.4
7.8 12.2 43
Toluene Concentrate from Hydroformed Naphtha 49.4
..5 ..
4i:2 4i:2
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1.
High Pressure Batch Equilibrium Bomb
grammatically in Figure 2. This equipment consisted essentially of a 10,000-ml. stainless steel bomb (maximum working pressure in excess of 5000 pounds per square inch) and facilities for charging and removing the contents of the bomb by positive displacement. The bomb, in the center background of Figure 1 behind the charge burets, was supported by a shaft attached to the center of the bomb. Agitation was obtained by rotating the bomb about its axis. The current for the electric heaters on the bomb was supplied through brushes mounted on the disk behind the bomb. The temperature of the contents of the bomb was determined by a thermocouple inserted through the shaft into the center of the bomb. The use of mercury as a positive displacing medium in high pressure work is well known. At the time this work was carried
Figure 2.
Batch Extraction Equipment
Vol. 42, No. 1
out the general availability of mercury made it imperative t o use some other medium for positive displacement of the conterits of the bomb and Wood's metal, which melts a t about 74" C., was employed. However, it was necessary to provide equipment which maintained the Wood's metal well above the melting point to keep it in a liquid state. The batch equilibrium extractions were carried out by first connecting the union on the bottom of the bomb to the high pressure pump lines. The bomb was then evacuated. The desired amounts of water and toluene concentrate were charged to the bomb by means of the high pressure pumps. This procedure permitted the bomb to be charged a t temperatures above the true boiling point of the charge materials without loss. I n all cases the amounts of the toluene concentrate and water were adjusted so that the total charge t o the bomb was approximately 6000 ml. After charging, the union at the bottom of the bomb was broken, and the contents of the bomb were agitated by rotating the bomb while heating. The bomb was heated until the temperature of the contents of the bomb was slightly above the temperature desired for the extraction. By approaching the temperature of extraction from the high side, the equilibrium between the two phases within the bomb was quickly obtained, whereas had the temperature of extraction been approached from the low side equilibrium would have been obtained only after prolonged agitation. During this time the pressure within the bomb increased to approximately 1800 and 2700 pounds per square inch gage for extractions a t 274"and 302" C., respectively. After agitation of the contcnts of the bomb a t the desired temperature for 60 minutes, the bomb was stopped in a vertical position and allowed to settle 45 minutes while the temperature was maintained constant. These times were demonstrated as being more than adequate for the contents of the bomb to come to equilibrium and to settle. During the settling time the line from the Wood's metal reservoir and the line to the water-cooled condenser were connected to the bottom and top of the bomb, respectively. After the settling period was completed the contents of the bomb were displaced by forcing Wood's metal into the bottom of the bomb. Water, preheated to about 90 C., was pumped into the Wood's metal r e s e r v o i r . The Wood's metal flowed from the reservoir through a VOLUME PER CENT TOLUENE, SOLVENT-FREE BASIS preheater where it was heated to the same temperature as the extraction bomb. From the preheater the Wood's metal, a t extraction temperaEQUILIBRIUM CURVES ture, flowed to the bottomof thebomb. When the Wood's metal was started into the bomb the 0 20 40 60 80 100 bomb pressure was VOLUME PER C E N T TOLUENE IN HYDROCARBONRICH PHASE, SOLVENT-FREE BASIS noted. The valve on the top of the Figure 3. System Toluene-Hydrobomb was then formed Naphtha-Water at 274" opened slowly t o and 302" C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1950
179
EQUILIBRIUM DATA FOR SYSTEM TOLUENE-HYDROFORMED NAPHTHA-WATER TABLE111. LIQUID-LIQUID Hydrocarbon-Rich Phase Val. % a t 15.6' C. Toluene, Toluene Water water-free
Naphtha 71.4 63.6
17.1 24.0
57.4 44.9
28 6 40 0 76.9
..
11.5 12.4 19 0 11 7 11 1 14.1 14.0 15.1 23 1
19.3 27.4
MI water ml. hidrocadon Naphtha Temperature, 274' C. 0.13 0.9 0.14 0.7
33.8 33.2b 47.1 100.0
77.5 68.6 57.5 41.7
a b
Water-Rioh Phase Val. % a t 15.6O C.
9.2 13.3 15.6 15.8 23.9 18.6 35.7 22.6 .. 64.1 35.9 Check runs under same conditions. Average of the four duplicate runs.
Toluene 1.7 1.8
maintain the pressure within the bomb either equal to or within 100 pounds per square inch gage above the pressure a t the end of the settling period to prevent mixing of the phases during sampling. Two samples were taken for analysis: the center fraction of the hydrocarbon-rich phase and the center fraction of the water-rich phase. The remainder of the contents of the bomb were collected in order to check the material balance for each equilibrium extraction; in most of the experiments it was better than 98% of the materials charged. The samples of the water-rich phase and of the hydrocarbon-rich phase were cooled t o 15.6 O C., and the volume % water in each sample was determined by direct observation. The hydrocarbon portion of each sample was separated and analyzed for toluene by a procedure involving the determination of the aniline point before and after removal of the toluene. For convenience all measurements were made by volume at 15.6' C. or corrected to that temperature. Volume measurements were all made with a maximum reading error of 1%. At the conclusion of each experiment the following data were available by direct observation or analysis: complete data on the materials charged to the bomb and the volume 7 0 water and
Toluene, water-free
M1. water/ml. hydrocarbon
65.0 70.2
37.5 39.0
..
2.3 3.8 5.6
97.4 97.5 97.0 96.7 97.0 96.7 96.9 95.8 94.4
77.3 75.76 89.9 100.0
31.3 22.7 16.8
..
1.1 1.9 3.5 6.0 12.0
97.2 96.2 94.5 92.0 88.0
38.8 50.2 62 9 75.0 100.0
34.7 25.3 17.2 11 5 7.3
0.16 0.8 0.18 0.4 0.3 Temoereture. 302O C. 0.15 1.7 0.19 1.9 0.23 2.0 0.29 2.0 0.56
10.6 18.5 29.3 46.1 100.0
Water
volume % toluene (water-free) on the water-rich and hydrocarbonrich phases. The volume yonaphtha and total volume of the two phases were then calculated from a material balance for each experiment. The degree of reproducibility of these experiments may be observed from the data in Table 111 where the results of four extractions employing the same charge stock and solvent dosage are given. The other runs in Table I11 are the averages of duplicate extractions. Table I1 shows that the toluene content of the toluene concentrate used in this work was 41.2 volume Yo. I n order to obtain equilibrium data over the entire range of toluene content, equilibrium experiments were made on raffinates from previous extractions (low toluene content) and on samples of the toluene concentrate which had been diluted with nitration grade toluene to the desired concentration (high toluene content). This technique permitted accurate determinations of the equilibrium curves and tie lines for both the stripping and enriching sections. EQUILIBRIUM DATA
Following the procedures described, the equilibrium solubility relations on the system toluene-hydroformed naphtha-water
I\
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CHARGE O I L IS ASSUMED TO BE A TOLUENE CONCENTRATE CONTAINING 5 3 . 5 VOLUME PER CENT TOLUENE
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I
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I
ASSUMING 9 5 % RECOVERY OF AVAILABLE TOLUENE FROM A TOLUENE CONCENTRATE A S 98.0 VOLUME PER CENT TOLUENE EXTRACT
n A V
5 10 I5 20 T O T A L THEORETICAL STAGES OF EXTRACTION
c z C
Figure 4. Relation of Solvent Dosage and Theoretical Stages of Extraction for Extraction of Toluene with Water at 214" C.
w
3 0
Q "I
I O
5 TOTAL
I
IO THEORETICAL
I 15 STAGES
I A 20 OF EXTRACTION
00
Figure 5. Effect of Toluene Content of Feed Naphtha on Extraction of Toluene with Water at 274" C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
purity to be a paraffinic compound of specific gravity of 0.700, a n extract oil of 98.6 weight % or of 98.0 volume % toluene should meet the lower limit of the specific gravity specification. The relat,ion between the solvent dosage and number of stages of extraction for the extraction of toluene from a 63.6 volume % toluene concentrate at, 274" C. a t three recovery levels is shown in Figure 4. The effect of the toluene content of the concent'rate is shown in Figure 5 . Selection of any combination of solvent dosage and number of stages of extraction on these curves fixes the extract recycle ratio. Similar calculations for the recovery of toluene at 302" C. are shown in Figurcs 6 and 7 .
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W
W LL
4
Vol. 42, No. 1
1 \ \
CHARGE O I L I S ASSUMED TO BE A TOLUENE CONCENTRATE CONTAINING 5 3 . 5 VOLUME PER CENT TOLUENE
w
I
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I
I
I
A
were determined at 274' and 302 "C. These data are summarized in Table 111. The equilibrium curves, plotted on rectangular coordinates according to the method of Maloney and Schubert ( 6 ) are shown in Figure 3. The commercial possibilities of a solvent extraction process depend primarily on the relation between the number of stages of evtraction and the solvent dosage required for the desired separation. These factors may be calculated from the equilibrium solubility data graphically following the procedure developed bv Maloney and Schubert (6). The maximum toluene content of the extract oil, when employing counterflow extraction, would be realized when the minimum solvent dosage and a n infinite number of stages of extraction were used. Under these conditions the extract phase is in equilibrium with the charge oil. The results of calculations on the minimum solvent dosage and the toluene content of the extract oil for the recovery of toluene from several concentrates of different toluene content are given in Table IV. A study of the toluene content of the water-free extracts shows that toluene of high purity cannot be prepared by liquid-liquid extraction of a toluene concentrate with water either at 274" or 302' C. without resorting to extract recycle operation.
TABLEIv. LIMITING CONDITIONS
O F EXTRACT PURITY AND MINIMUMSOLVENT DOSAGEIN SIMPLECOUNTERFLOW EXTRACTION OF TOLUENE WITH WATER
Extraction Temperature, C.
Toluene Content of Feed, Vol. yo
Toluene Recovered, Val. 3' %
Solvent Dosage, Vol. Water/Vol. Xaphtiia
274 274 274 274 302 302 302 302
60.0 63.5 50.0 40.0 60.0 53.5 50.0 40.0
96.0 95.6 95.4 95.0 96.0 96.5 96.3 96.2
12.2 11.6 11.4 12.2 7.3 7.0 7.0 7.1
TheoretiToluene cal Content of Stages of Water-Free Extrac- Evtract Oil, tion Val. 70 94.6 02.3 90.8 84.3 82.6 79.2 77.3 71.2
Calculations on extract recycle operation, according to the flow shown in Figure 8, were based on the assumption of an extract oil of 98.6 weight % or 98.0 volume % toluene, Rater-free. The specifications for nitration grade toluene call for a material with a specific gravity of 0.871 * 0.002 at 15.6" C. Assuming the im-
ASSUMING 9 5 % RECOVERY OF AVAILABLE TOLUENE FROM A TOLUENE CONCENTRATE AS 90.0 VOLUME PER CENT TOLUENE: EXTRACT
I 5 TOTAL
1 10 THEORETICAL
I
I
15
20
STAGES
OF
A
v
EXTRACTION
Figure 7. Effect of Toluene Content of Feed Naphtha on Extraction of Toluene w-ith Water at 302' C.
Plotting the relation between the number of stages of cxtmction and solvent dosage to prepare an extract of desired purity and recovery from a given charge oil in the manner shown in Figures 4 to 7, inclusive, furnishes an easy method for locating the most reasonable combination of the lowest number of stages and the lowest solvent dosage required for the desired separation. The optimum conditions are represented by the point of maximum inflection of the curves. The curves in Figures 4 to 7 show that, in general, a combination of lower solvent dosage and number of stages of extraction may be obtained by carrying out the extraction at 302' C. than at 274' C. Furthermore, the recovery of toluene from concentrates of high toluene eontent is much more efficient than from the lower toluene content concentrates. PLANT SIZE UNIT
To obtain some conception of the commercial possibilities of n process for the recovery of toluene by water extraction, consideration was given to the major requirements for a plant scale unit. It is evident that in determining the cost of the equipment required for the extraction, the diameter, height of the extraction column, and the pressure required for the operation would be critical factors. -4side from utility requirements the heat load would be the major operating factor. Figure 8 represents the flow considered for a unit to recover toluene of 98.0 volume % purity from a toluene concentrate (53.5 volume % toluene) by extraction with water a t 302" C. The higher extraction temperature, 302" C., was selected because a lower solvent dosage was required a t that temperature than for
Figure 8.
Flow for Recovery of ToIuene by Extraction with Water at 302' C.
extraction a t the lower temperature, 274' C. The quantities shown are based on the production of 7266 pounds or 1000 gallons of 98.0 volume yo toluene extract oil per hour. The operating conditions were determined from the curves of Figures 6 and 7 (indicated by X ) and are given in Table V.
TABLEV. OPERATINGCONDITIONSASSUMEDFOR PLANT SIZE UNIT TO RECOVER 98.0 VOLUMEyo TOLUENE BY WATER EXTRACTION
a
Capacity assumed, gal./hr. Operating data assumed Toluene content of feed, vol. % Toluene content of extract oil. vol. 7% Toluene content of raffinate oil, vol. % Toluene recovzry, vol. % Temperature C. Solvent dosabe basis feeda, vol. % Recycle ratio basis extract oil Theoretical stages of extraotiona Stripping section Enriching section Total Operating pressure assumed, lb./sq. inch Based on data from Figures 6 and 7.
inch Raschig ring packing-presented as discussion to a paper by Blanding and Elgin (1). Table VI summarizes the data required to arrive a t a figure of 5.1 feet for required diameter of the extraction tower. Assuming 11.1theoretical stages of extraction and 5 feet per stage of extraction (based on plant experience with a large number of extraction towers), a packed section of 61 feet of 1-inch Raschig rings would be required. The ppessure required t o operate the extraction system may be estimated, assuming water and the oil t o be immiscible, as the sum of the vapor pressure of the water and the oil at 302' C. as shown in Table VII.
1000 (7266 lb./hr.)
5 3 . 5 (58.0 wt. %) 98.0 5.6 95.0 302 1250 2:l 5.4
5.7 11.1 1695
TABLE VI. DIAMETER OF EXTRACTION TOWER Specific gravity assumed a t 302' C. Oil-rich phasea Water-rich phaseb Continuous phase Total throughput assumed, gal./sq. ft./hour Diameter of tower required, feet a Based on the curves presented by Nelson ( 7 ) . b Based on data from Keenan and Keyes (6).
0.44 0.72 Water 1,000 5.1
*
~
TABLE VII. PRESSURE REQUIRED FOR LIQUID-LIQUID EXTRACTION OF TOLUENE WITH WATERAT 302 C. O
Lb./Sq. Inch 1275 420
Although the toluene concentrate used in this work contained 41.2 volume % toluene, it was demonstrated on plant size equipment that the average toluene content of the toluene concentrate from hydroformed naphtha was 53.5 volume %. Therefore, that figure was used as the toluene content of the charge oil in all the calculations. The diameter of the extraction column can be estimated from the quantities given in Figure 8 and assuming a total throughput of approximately 1000 gallons per square foot per hour. This figure is probably conservative in light of Colburn's data on 0.5-
Vapor pressure of water a t 302' C.a Vapor pressure of oil fraction a t 302' C. b Total operating pressure i695 a From Keenan and Keyes' (6) tables. b Calculated from vapor pressure tables assuming toluene as the oil component.
The major operating factor would be the heat required. Assuming no heat recovered from the exchanger, E-5 of Figure 8, in which the extract solution is cooled to 177" C., the heat load may be calculated as shown in Table VI11 to be 13,123 B.t.u. per gallon of toluene recovered.
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
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Total 13,123,000 Total heat required, 13,123 B.t.u./eal. toluene recovered
(1) Blanding, F. H., and Elgin, J. C., TTans. A m . 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). f7, \'I
Selson, 1%'. I,., Oil Gas J . , 36,
184 (1938).
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 to hydrocarbons a t about 800" to 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 to 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 that 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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