A Microcatalytic System for Gas Oil Cracking Nai Y. Chen’ and Stanley J. Lucki Research Department, Paulsboro Laboratory, Mobil Research and Development Corp., Paulsboro, N . J . 08066 A fixed bed microcatalytic system was developed to conduct gas oil cracking tests. The amount of gas oil and catalyst required is low as 0.1 cc with material balance of better than 95% attained. The test includes an in situ regeneration cycle. Accurate coke-on-catalyst can be determined reproducibly to f0.2 mg by converting carbon to CO? and measuring the amount of CO? volumetrically. The u n i t is simple in design and its operation and related analytical work can be handled by one laboratory technician. A test can be completed in less than three hours with analytical work taking an additional two to three hours. Extensive use of gas chromatography, including a temperature-programmed gas chromatograph, provides the basis for product analysis. Octane rating of gasoline range product can be calculated based on mass spectroscopic PONA analysis of samples prepared by trapping the gasoline range product during chromatographic separation
T h e speed of catalyst evaluation has always been influenced by the necessary minimum amount of catalyst needed for testing its performance. The early work on cracking catalyst evaluation used 200 cc (Rescorla et al., 1948) of sample and long reaction times (2 hr) to obtain sufficient product for characterization and octane number determination by the available analytical techniques. I n so doing, the data on conversion and product selectivity always reflected the combined effect of two variables, namely, the time-dependent intrinsic catalytic activity and catalyst fouling. The interplay between these two variables has been shown mathematically (Weekman, 1969) and demonstrated experimentally (Nace, 1969) to be such that catalyst characterization cannot adequately be made by a single set of operating conditions. At constant catalystto-oil ratio, conversion could be independent of or even increasing with decreasing vapor residence time (increasing space velocity); or conversion could decrease with increasing reaction temperature (Nace, 1969). Thus, proper characterization of a new cracking catalyst often necessitates evaluation a t several operating conditions. More recently Ciapetta and Henderson (1967) described a modified testing method for cracking catalysts using only 5 grams of sample. Their method facilitates catalyst evaluation under more realistic reaction conditions, but a t a sacrifice of octane number determination and cycle oil characterization. I t also requires removal of the spent catalyst from the reactor for conventional coke analysis. We have developed a microcatalytic test system which reduces the amount of catalyst and oil required to as low as 0.1 cc by taking advantage of a combination of modern analytical techniques and design details; combines cracking and regeneration and provides information on product quality. The system is, furthermore, specifically designed to be flexible in operating conditions and to provide faster results than were previously possible.
’ Present address, Research Department, Central Research Division, Mobil Research and Development Corp., Princeton, N.J. 08540. T o whom correspondence should be sent.
Apparatus
Characteristics of Microcatalytic Test System. INTEGRAL DESIGN.All the necessary equipment for a complete test including material balance and in situ catalyst regeneration are self-contained in one unit. The entire operation, including product analyses, requires only one technician. MICRO IN SIZE. Amount of gas oil required for an acceptable material balance and a complete product analysis is as low as 0.1 cc. The amount of catalyst needed is equally small. ANALYSES.Temperature-programmed gas chromatographic analysis is being used to determine the product distribution of both gas and liquid products in terms of carbon number distribution. The method is not only faster than the distillation method, but also provides additional information regarding the product distribution of the “unconverted” fraction. Combination of gas chromatography and mass spectroscopy provide a PONA analysis of the product in the gasoline boiling range. An octane rating can be calculated based on previously developed correlation between PONA and octane number. All analyses except mass spectroscopic analysis can be handled by the one technician conducting the test. VISUALOBSERVATION. By constructing the unit in glass, the operator can follow the test with visual observationsLe., changes in coloration of the catalyst, chromatographic separation of wide boiling range charge stock, coke accumulation in the preheater, etc. Description of Test Unit. CRACKING CYCLE.Figure 1 shows the schematic diagram of the unit during the cracking cycle. The charge stock (either wide cut MidContinent gas oil or Light East Texas gas oil) is placed in a heated syringe to avoid wax separation. The syringe pump delivers the oil through an electrically heated (low voltage) capillary stainless steel tubing which pierces through a silicone rubber septum into the preheater section of the “Vycor” glass reactor. The preheater and the reactor are made from a single piece of 11-mm i d . tubing with a concentric 3-mm 0.d. glass thermowell. Heat is supplied via a low heat capacity heater made by winding -10 R of flat (3 mm) nichrome wire (-10 amp) around a Ind. Eng. Chem.
Process Des. Develop., Vol. 10, No. 1, 1971
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4
the end of the regeneration cycle, the receiver is disconnected from the reactors and capped, and the exit end is opened to the evacuated constant volume system connected to a vacuum pump which pumps out condensed oxygen and nitrogen without affecting the condensed carbon dioxide (vapor pressure < l o - ” mm Hg a t -196OC). After pumping, and with the vacuum pump blocked off, the microreceiver is warmed to room temperature. The pressure change of the system is noted and can be converted to milligrams of carbon.
2
Analyses
Figure 1 . Schematic diagram of u n i t (cracking cycle) 1.
Heated syringe
2.
3.
Charge stock Heated SS capillary
4.
Septum
5.
Thermowell 6. Preheater 7. Catalyst bed
8. 9. 10. 11,
Product receiver
12.
To vacuum pump Mercury manometer To vent
13. 14.
Gas collection bulb Sample loop Carrier gas to chromatograph
glass tubing, 16-mm i.d. The heater tubing is insulated by a glass shield -20-mm i.d. Temperature is controlled by a thermocouple placed under the nichrome winding separated by a thin sheet of mica, through a temperature controller. The reactor is essentially isothermal with axial A T about 1 2 ”F. The product from the reactor is collected a t liquid nitrogen temperature in a microreceiver. Total product collection, except hydrogen, is achieved by placing a Tesla coil (high voltage discharge) on the glass wall of the exit end of the receiver above the liquid nitrogen level. Volume of incondensables is registered by a soap film flowmeter and is usually negligible. After proper stripping with helium the receiver maintained a t liquid nitrogen temperature is disconnected from the receiver. The inlet end of the receiver is then connected to a helium line and the exit end opened to an evacuated constant volume system consisting of a manometer, a sample loop, and a gas collection bulb. The liquid receiver is then warmed t o slightly above room temperature to drive off gaseous products. After noting the pressure change which can be converted into the volume of gaseous product produced, helium is admitted into the receiver t o sweep residual gaseous product into the gas collection bulb. The liquid product receiver is disconnected from the unit, capped, and weighed on a Mettler analytical balance. The gross weight minus the tare weight of the receiver gives the weight of the liquid product recovered. REGENERATION CYCLE.Figure 2 shows the schematic diagram of the unit during the regeneration cycle. Air, or a mixture of oxygen and helium is admitted through a stainless steel needle which pierces through the silicone rubber septum into the preheater and the catalyst bed. Combustion products from the reactor are mixed with additional air introduced through a rubber septum and passed through a second microreactor packed with 1 cc, 30160-mesh oxidation catalyst [manganese nodules (Weisz, 1968) in this case] maintained a t 1000”F. Carbon monoxide in the combustion gas is converted to CO?. The product from the second microreactor passes through a Mg(ClOJ)? dryer, which removes water from the stream, into a microreceiver, maintained a t liquid nitrogen temperature. At 72
Ind. Eng. Chem.
Process Des. Develop., Vol. 10, No. 1, 1971
Material Balance. The following data are used in the calculation of the material balance: Rate of charge and duration of cracking cycle; weight of weathered liquid product; weight of gaseous product; weight of carbon as coke. Because of slight fluctuation in the delivery rate of the syringe pump it is desirable to obtain blank rates before and after the run. Both the weight of the gaseous product and the weight of carbon (coke) are obtained by converting their respective volumes using the average molecular weight of the gaseous product (based on composition analysis) and the molecular weight of CO,. respectively. Composition Analyses. GASEOUS PRODUCT. Gas samples from the gas collection bulb are analyzed on a F&M Model 720 temperature-programmed chromatograph over a 2-ft SE 30 or UCW 98 silicone gum rubber column. The amount of light gases (C, to C1 including air. CO,, if any), C4’s, and gasoline range products is then determined. The gas product is also analyzed over a 6-ft x V4-in. Poropak Q column for the separation of individual components in the C1 t o C i fraction, including air and C o n , if any. LIQUIDPRODUCT.A rubber septum is placed on the exit end of the microreceiver containing the degassed liquid product with the receiver chilled in an ice bath. The receiver is then warmed in a hot box maintained at -120°F to avoid possible phase separation. A 2-pl liquid sample is then taken from the receiver through the septum and analyzed in the same temperature-programmed gas chromatograph to obtain the separation of fractions of Cj’s, gasoline range, and C,,- range products. A carbon number breakdown of the products in the gasoline and C k range
Figure 2. Schematic diagram of u n i t (regeneration cycle) 3. CO burner 1.
2.
O? inlet tube Added 0 2 for CO burner
4.
Drier
5.
CO? receiver
DRY C-4
ClGASOLlNE
k4-tC5+C6-t-
c , + q
I
116121 F 0 2 4 6 8 1 RETENTION TIME, M I N .
Figure 3. Temperatureprogrammed chromatogram of gaseous product
I
ATT. 0
RETENTION TIME
can also be obtained from the chromatograms. Analyses of the chemical types in the gasoline range product (PONA analysis) can be done by mass spectroscopy. The sample for such analysis can be conveniently obtained by injecting a 25- to 50-11 liquid sample into the temperatureprogrammed gas chromatograph and trapping out the gasoline range fraction a t the exit of the chromatograph in liquid nitrogen. COKE DETERMINATION. The determination of the amount of coke in grams of carbon is straightforward, knowing the volume of the system, and its pressure change before and after CO? expansion. This method has been calibrated quantitatively with CaC03, checked with established analytical methods, and is both accurate and reproducible to 1 0 . 2 mg. Regeneration can be completed in less than 10 min with 1200°F oxygen. I n cases where reuse of catalyst is anticipated, a slower regeneration with air at 950" F is recommended. Results and Discussion
Chromatograms. GASEOUSPRODUCTS. A typical temperature-programmed chromatogram of the gaseous prod uct is shown in Figure 3. The chromatogram of the lighter end of the gaseous product is shown in Figure 4. From these two chromatograms, the average molecular weight of the gaseous product can be calculated. Calculation of the product distribution is done in the conventional way by dividing the products into three groups: light gases (C,-C J, C i s , and Ci- gasoline. LIQUIDPRODUCTS. A typical temperature-programmed chromatogram of the liquid product is shown in Figure 5. A carbon number scale obtained from the analysis of blend of n-paraffins placed on the bottom of the chromatogram allows the division of the liquid product into three fractions: C, and lighter, C s - gasoline, and C1,cycle stock. Figure 6 shows the chromatogram of the charge stock, wide cut Mid-Continent gas oil. The charge stock covers a carbon number range from Cloto CU with about 0.6'7 in the gasoline range and an average carbon number of C?,?.Figure 7 shows a superimposition of Figure 5 over Figure 6. Note that the C1?+cycle stock is considerably different from the charge stock. Hydrocarbons with carbon number greater than C.o have essentially disappeared and a significant amount of C1:r to CIH products are produced. This information is not produced by the conventional bench scale tests. Typical Results and their Comparison with Conventional Bench Scale Tests. T o compare the performance of the micro gas oil test with the conventional bench scale tests (Alexander and Shimp, 1944; Alexander, 1947; Harriz. 1966) three catalysts were used to crack a standard wide
-
Figure 4. Chromatogram of lighter end of gaseous product (Poropak Q Column - T = 160" F)
:C
/SC4f---
I
I
4
6
GASOLINE
CYCLE
I
1
I
8
IO
I
I
1
1
1
14 16 18 2 0 CARBON NUMBER
12
1
STOCK
1
24
1
1
28
1
1
32
1
1
l
36
1
1
40
44
Figure 5 . Temperature-programmed chromatogram of liquid product
L 5 C 4 G
I I
C c GASOLINE
CYCLE
STOCK ~
I
I
I
I
4
6
8
IO
1
I
I
I
1
1
12 14 16 I 8 2 0 CARBON NUMBER
1
1
24
1
28
1
1
32
1
1
36
1
1
40
l
l
44
Figure 7. Superimposition of Figure 5 on Figure 6
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
73
Table I. Comparison of Micro Gas Oil Test with Conventional Bench Test
S = 4; 1.5; 900" F
Conditions: Conditions:
S
=
3; @ = 2; 875" F
/3
Catalyst A Test Method
Conversion, Wt 5% Cycle Above Gasoline C;' Gasoline Total C, Dry Gas Coke Material Balance
Cat-D
Micro
Cat-D
Micro
Cat-C
28.4 71.6 23.2 3.7 0.7 0.8 96.8
28.5 71.5 22.1 3.8 1.7 0.9
28.3 71.7 19.9 4.2 2.7 1.4 94.5
45.1 54.9 36.5 5.0 2.4 1.2 98.5
42.9 57.1 32.1 5.5 3.3 2.0 93.1
52.2 47.8 41.8 3.4 5.3 1.7 97.7
59.4 40.6 42.2 8.6 6.2 2.3 97.1
...
4.22 0 J M Wp + 87.4 Where A = wt %;o of total aromatics, 0 = wt % of total olefins, MWa = av. molecular weight of aromatics, M W P = av. molecular weight of paraffins. The correlation has a standard error of =t1.17 O.N. Table I1 shows a comparison of the calculated octane number with the measured minimicro octane number of gasoline products from conventional bench scale tests. Also shown are the composition analyses and the calculated octane number of gasoline products from micro gas oil tests. TIMEA N D MANPOWER. The test unit is designed for one man operation. Except for the mass spectroscopic PONA analysis of the gasoline fraction for octane number calculation, all equipment and analytical instruments can be easily handled by one laboratory technician. An estimate of the time (in minutes) required to conduct a test is: Cracking Cycle
Catalyst Loading Air Preheat Helium Purge (prerun check) (including cooling liquid receiver in liquid nitrogen, checking temperature profile of reactor and preheater)
10 30 20
Gas Oil Test Helium Purge Product Collection (including weathering of gaseous products, weighing of liquid receiver)
10 10 30
Subtotal Helium Purge (prerun check) (including cooling liquid receiver in liquid nitrogen, checking temperature of reactor and CO burner)
74
Table II. Minimicro Octane Number Correlation Aromatics
Olefins
Wt
Wt
Calcd
O h
M.W.
Yo
M.W." paraffins
Measured
Catalyst
O.N.
O.N.
B B B B B A
26.6 27.8 26.5 27.7 22.6 21.6
113.2 114.3 114.4 115.6 131.1 113.0
26.9 26.5 29.2 28.6 34.2 38.2
102.8 101.8 107.3 105.1 104.0 133.9
96.3 95.7 95.1 95.7
95.6 95.8 95.6 95.7 95.2 94.9
.. .
.. .
"Av molecular wt of olefins is not given by PONA analysis and is assumed to be equal to that of paraffins.
Sample Analysis. Each chromatographic analysis takes about 30 min to complete. With the time required for the temperature-programmed chromatograph to return to the starting position after each analysis a total of 2 to 3 hr is required for the complete analysis of gas and liquid samples. Data Analysis. Doing a desk calculation requires about 2 hr to complete the calculation, consisting of material balance, conversion, and product distribution. Considerable time can be saved with the aid of digital computing machines. Nomenclature
S
= wt hourly space velocity
d = catalystihydrocarbon charge (ratio by wt)
A = wt 0 = wt MWa = av M W p = av
percent of total percent of total molecular wt of molecular wt of
aromatics olefins aromatics paraffins
Literature Cited
110
Regenerotion Cycle
Subtotal Total Time for Test
Catalyst C
Micro
+ (R + 3) = 21.2 A/MW,4+
Coke Burnoff Helium Purge Evacuation of Liquid Receiver Product Collection
5
Micro
cut Mid-Continent gas oil. The micro gas oil tests were made with 0.75- to 1-cc 20130-mesh particles instead of 4/10 mesh used in the larger units. Total gas oil used in each test was 0.5 cc. Results expressed in weight percent basis are shown in Table I. OCTANE RATING. A generalized octane number (Research Octane Number + 3 cc T E L ) composition correlation as shown below is used in calculating octane number from mass spectroscopic PONA analysis.
Ron
Catalyst
=
20
10 10 5 20 65 175 min
Ind. Eng. Chem. Process Des. Develop., Vol. 10, No. 1, 1971
Alexander, J., Proc. Amer. Petrol. Inst. Sec. 111, 51-6 (1947). Alexander, J., Shimp, H. G., Nut. Petrol. News 36, R5378 (1944). Ciapetta, F. G., Henderson, D. S., Oil Gas J . 65, 88-93 (1967). Harriz, C. G., Hydrocarbon Process. Petrol. Refiner 45 (lo), 183-8 (1966). Nace, D. M., Ind. Eng. Chem. Prod. Res. Develop. 8, 24 (1969). Rescorla, A. R., Ottenweller, J. H., Freeman, R . S., Anal. Chem. 20, 196 (1948). Weekman, V. W., IND. ENG. CHEM. PROCESS DES. 8, 385 (1969). DEVELOP. Weisz, P. B., J . Catal. 10, 407 (1968). RECEIVED for review January 19, 1970 ACCEPTED August 20, 1970
