LANTS
Midget Fluid Catalytic Cracking
.
Units H. W. GROTE, J. HOEKSTRA, AND G. T. TOBIASSON Universal Oil Products Co., Chicago, I l l .
B
.
.
In evaluating laboratory batches of catalyst and specially laboratory scale research. It ECAUSE of the high prepared feed stocks for catalytic cracking it is desirable to cost of research and deis required first that the operate on the smallest scale possible and yet duplicate the velopment projects carried equipment be operable with a conditione to which the catalyst and feed stock are exposed out on a semicommercial minimum of attention to in commercial operation. scale, it is desirable t o restrict economize on m a n p o w e r ; The bench scale fluid catalytic cracking units described the amount and scope of this secondly, that it be able to permit continuous operation with full catalyst circulation work to projects that can only evaluate small batches of caton approximately 1 liter of catalyst and employ continuous be solved in the larger equipalyst in quantities that can debutanieation and automatic bateh fractionation. ment and to broaden as much conveniently be prepared in Comparative product distribution data are given which as possible the range of work the laboratory; and thirdly, show that the essential operating conditions of the larger done in the laboratory. As that it be able to evaluate scale units have been duplicated in the midget units. applied to catalytic cracking, batches of charging stock of These units permit obtaining more complete data than this plan demands laboratory a size that can conveniently conventional laboratory methods with no increase in manmethods which closely apbe prepared by laboratory power requirements and are capable of taking over a conproach the operating condif r a c t i o n a t i o n o r treating siderable amount of work from the larger scale pilot plants tions used in commercial fluid methods. with a great economy of manpower. catalytic cracking units. These three requirements have been met in the past by Although the development 1 apparatus employing the fixed of the fluid catalytic cracking bed technique which has been the standard laboratory approach process has proceeded rapidly on pilot plant and commercial for continuous operations with solid catalysts ever since solid catscales, laboratory research and testing in catalytic cracking are alysts have been known. Therefore, it was only natural that the still largely carried on by methods employing fixed or fluidized earliest laboratory catalytic cracking methods employed the fixed fixed beds of catalyst. catalyst bed. Routine tests and laboratory research in catalytic Although the term “fluidized fixed bed” as used in (3) is selfcracking are still carried on to a large extent in small fixed bed catcontradictory in the light of more recently proposed definitions alytic cracking units. The standard Uniyersal Oil Products ( I ) , it is retained in the present discussion for convenience in referring to the original reference where the term “fluidized (U.O.P.) cracking catalyst activity test employs a 25-ml. bed of k e d bed” was used to describe a fluidized bed of solid particles catalyst. Laboratory cracking units using automatically conentirely confined within a single vessel. The terms “conked” trolled processing and regeneration cycles with 100-ml. beds of catr or “noncirculating” fluidized bed are suggested for use in dealyst were used for several years to study catalyst degradation, to scribing future work with this type fluidized bed. evaluate the product distribution characteristics of laboratory The desirability of using laboratory scale circulating fluid preparation of new catalysts, and to study the effect of processcatalytic cracking apparatus has been recognized for a long time, ing variables. Information obtained by the use of this type apbut the technical difficulties of operation a t this scale retarded paratus is of limited value and may even be misleading, when the early development of such equipment. Preliminary design applied to fluid catalytic cracking as carried out on a commercial studies using glass apparatus have overcome these difficulties, scale, owing to wide differences in operating conditions as indiand three complete bench scale fluid catalytic cracking units cated in Table I. have recently been built in the research and development laboratories of Universal Oil Products Co. and are now in satisfactory operation charging 100 t o 300 ml. of oil per hour and TABLE I. TYPICAL OPERATINGCONDITIONS having catalyst inventories of approximately 1000 ml. LIMITATIONS OF AVAILABLE TECHNIQUES Space velocity wt. oil/wt. cat./hr. Catalyst to oil’ratio Catalyst time in reactor, minutes Carbon content of spent catalyst, wt. %
Although the term “laboratory scale” as applied to catalytic cracking apparatus is not sharply defined, there are three considerations which place a practical limit on the size of such equipment if it is to retain the advantages usually associated with
545
Fixed Catal s t Bel 3-8 0.7-0.06 30-120 Variable, up to 10%
Fluid Catalyst Technique 0.6-3 3-15 3-15 Constant. about 1%
546
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1.
Vol. 43, No. 2
Steel Racks Containing Equipment for One Midget Cracking Unit
A closer approach to commercial fluid catalytic cracking on a 1,aboratory scale is the fluidized fixed bed apparatus ( 3 )which was developed specifically t o provide laboratory operation more nearly comparable to the condit,ions existing in a commercial fluid catalytic crackjng unit than was attainable in the laboratory fixed bed equipment. In this apparatus, fluid type catalyst was placed in a tube of such dimensions as to leave a free space above the catalyst bed. Processing, stripping, and regeneration occurred in a repeating cycle just as in the earlier fixed bed apparatus. The flow of oil vapor, steam, and air, respectively, was upward with sufficient velocity to fluidize the catalyst. The agitation of solids which is characteristic of the fluidized cat,alyst bed was thus realized in the laboratory. This agitation was evident particularly during the regeneration period of the operating cycle in that no “hot spot” was formed although undiluted air was used as the regenerating medium. Although the fluidized fixed bed provided a closer approach to commercial fluid operation than did the fixed bed units, there remained some important differences. I n the circulating fluid unit, as used on a commercial scale, the average carbon concentrations on the catalyst remain constant in both the reactor and regenerator as long as the operating conditions are held constant. In the fluidized fixed bed, on the other hand, as in any fixed bed unit, the oil contacts a clean catalyst at the beginning of the processing period after which it contacts catalyst with increasing amounts of carbon. LikeJT-ise, the regenerating period begins
with a maximum amount of carbon and ends with clean catalyst. Certain wall effects of the vessel that are due to alternate oxidation and reduction of the metal surface have also been experienced in the fluidized fixed bed apparatus. PRELIMINARY DEVELOPMENT OF MIDGET FLUID UNIT
Although a great deal of valuable information was obtained from the operation of the fluidized fixed bed units it was realized that they were only a compromise and that work should continue toward the ultimate goal-a laboratory scale circulating fluid catalytic cracking unit. The experience obtained in the design and operation of the fluidized fixed bed equipment afforded a valuable background for starting this work. The fluidized fixed bed apparatus used about 1.3 pounds (600 grams) of catalyst; a catalyst of average density occupied a volume of 0.035 to 0.04 cubic feet ( 1 to 1.1 liters) in a 2-inch (5.2-em.) inside diameter tube. This apparatus had operated successfully with linear gas vclocities through the catalyst bed ranging from 0.05 to 0.4 feet per second (1.5 to 12 cm. per second), and this range was selected as a suitable scale for the design of a circulating fluid unit. The initial experimental work on the design of a complete fluid apparatus was done with apparatus fabricated from glass in which the catalyst was circulated with air. Several different vessel arrangements were tried before selection of the final one shown in the flow diagram, Figure 5 . The advantages of this
February 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
547
arrangement over others will become apparent in the discussion of the catalyst flow. An essentially complete glass unit was then fabricated according to this final design and was tested by circulating catalyst with air both a t room temperature and a t temperatures of 300 ' to 400 F. It was only after completion of these circulation tests in glass that the fabrication of a stainless steel unit was begun. Apparatus fabricated from glass was also used to obtain design data for the dehydrating and debutanizing sections of the unit. The use of glass for this purpose is desirable not only because it is easily fabricated but also because it permits direct visual observation of the operation of the equipment in relation to indirect observations such as temperatures and pressures to which one is limited in metal apparatus. One stainless steel prototype unit was first fabricated :t continue the development work begun in glass to a stage where a battery of three units could be installed. Fabrication of the stainless steel equipment was not without certain difficulties. Some of the smaller parts and more complex assemblies called for considerable skill and ingenuity on the part of the mechanical department, and in some cases original designs had to be changed slightly to eliminate fabrication $fficulties. O
COMPLETED MIDGET UNITS
-
I n spite of these fabrication difficulties, the prototype unit was built and operated, and then a battery of three complete units was finally assembled. Three steel racks, each occupying an area 18 inches wide and 7.5 feet long and containing the equipment for one cracking unit, were erected in a parallel arrangement, as shown in Figure 1. Each rack contains a furnace with the catalyst section of the cracking unit, the dehydrator for the reactor effluent, a debutaniaer for synthetic crude together with the charging pumps, gas meters, and sample holders (Figure 2). The three gasoline fractionating columns for gasoline removal are not mounted on these racks but are placed together a t another location in the same room as shown in Figure 3. The instrument board, shown in Figure 4, consists of four 2 X 7 foot panels and contains the instruments for all three of the units. Each unit is serviced by one parallel section containing a three-point temperature controller, a six-point temperature recorder, and a recording temperature controller as well as switches, indicating lights, and variable transformers for the various heater circuits. The fourth section of the panel board contains a three-point temperature controller for the gasoline fractionating columns with variable transformers and switches for the associated heater circuits and also a thirty-six point indicating potentiometer serving all the units including the fractionating columns. The control desk placed next t o the instrument panel is equipped with three plywood leaves, fastened with hinges. The log sheets are fastened to these so that the log sheet for any of the three units is instantly available for recording data or for inspection by raising the leaves above it. All this equipment together with a cabinet for supplies, a small work table, and a refrigeration unit is located in a space approximately 16 X 23 feet. OPERATING PROCEDURE
Operation of the units is relatively simple. Nominal rates for air to the regenerator and nitrogen to the reactor, stripper, and catalyst valve are set by means of hand control valves located above individual rotameters. With the catalyst valve closed, catalyst is added t o the hopper (the upper vessel in the furnace assembly shown a t the left of Figure 5 ) and flows in turn through the regenerator, reactor, and stripper to the catalyst valve below the furnace. Addition of catalyst is continued until a level is maintained in the hopper indicating that the other vessels are filled to their normal operating levels. Heat is applied to the unit and catalyst circulation is then established by opening the catalyst valve and allowing catalyst to enter the transfer line where a stream of nitrogen carries it back into the hopper.
Figure 2. Rack Showing Furnace Dehydrator, Debutanizer, Charging Pump, Gas Meter, and Sample
Holder From this point the flow is continuously downward through the regenerator, reactor, and stripper in turn, and finally back to the catalyst valve. The catalyst diverter a t the top of the unit is merely a two-way valve which can be opened to divert the entire catalyst stream in the transfer line through an open tube from which the catalyst may be collected for a short time in a container suitable for weighing. The catalyst circulation rate is obtained by weighing the catalyst collected during a measured period of time. Considerable flexibility of operation is provided by the catalyst vessel arrangement described. Since neither oil vapor nor regeneration air is used to transfer the catalyst, there is no difficulty in varying the flow of oil or air independently of catalyst flow. The use of nitrogen for transferring the catalyst to a reservoir a t the top of the unit has several other advantages, The catalyst valve may be located in a cool zone thus facilitating its adjustment. The cooling of the catalyst in the transfer line causes no difficulty because although some reheating occurs in the hopper, a relatively cool catalyst entering the regenerator is desirable since it tends to eliminate afterburning of carbon monoxide in the dilute phase. Since the hopper contains only nitrogen from the transfer stream in addition to catalyst, it may be opened a t any time to add catalyst without disturbing the operation of the unit beyond momentarily lowering the catalyst temperature in the regenerator if the amount added is fairly large. There is no noticeable effect on reactor temperatures of such catalyst additions. After catalyst circulation is established, the raw oil charge pump and the stripping water pump are started, and the nitrogen streams to the reactor and stripper are shut off; if desired, nitrogen stripping may be used instead of steam stripping. Vaporization and preheating of the raw oil charge and the stripping water are accomplished by passing each of them through sections of
548
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 2
bons in the reactor effluent are not condensed by the condenser a t the top of the dehydrator since water cooling is employed. The gas leaving the dehydrator condenser is passed through a chamber filled with anhydrous calcium chloride for water removal and is then fed to a debutanizer precooler which is merely a jacketed tube refrigerated to maintain a temperature of -40" F. or lower in order to condense all of the Cg+ fraction along with most of the Ca and some lighter hydrocarbons. The condensate from this precooler is fed into the debutanizing column along with the dehydrator bottoms. The debutanieer produces a synthetic crude bottoms fraction, containing less than 0.5% of Cd hydrocarbons; this is collected in a spherical receiver. The gas overhead stream leaving the top of the column through a refrigerated condenser is combined with the gas issuing from the precooler to form a total process gas stream containing 1% or less of Cs hydrocarbons. This gas after being metered and sampled is discharged into the atmosphere. At the conclusion of a test, the spherical receiver containing the debutanized synthetic crude produced during the test period is transferred to an automatic batch gasoline fractionating column where it is surrounded by a heater and used as the still pot. The gasoline is removed as a distillate and the cycle stock is recovered from the spherical receiver on completion of the distillation. DESIGN O F THE FRACTIONATION SYSTEM
Figure 3.
Gasoline Fractionating Columns for Gasoline Removal
It is obvious that all fractionation equipment could have been eliminated from the units, and the necessary separation could have been accomplished in the service and analytical laboratories. However, three cracking units in continuous operation would require a large amount of manpower to accomplish this n orli by the usual laboratory methods. By incorporating automatic fractionating equipment in the apparatus, the over-all manpower requirement is greatly reduced, much intermediate handling of the cracked products is eliminated, and the analyses of the products and calculation of the yields are greatly simplified. A single process gas sample containing all the Cq and lighter hydrocarbons suitable for direct analysis by the mass spectrometer is obtained from each test along with specification gasoline and cycle stock samples. A relatively small investment in equipment, therefore, is amply repaid by savings in analytical costs and simplification of product handling.
'/g-inch stainless steel pipe about 2 feet long and located within the ieactor and stripper furnaces, respectively. The vapor effluent from the stripper is fed into the top of the reactor where it combines with the cracked hydrocarbon vapors leaving the reactor through the reactor cyclone. At this stage the cracking step is completed and the reactor effluent is fed into a train of fractionating equipment which serves to separate the reactor effluent into various fractions so that the yields of gas, gasoline, and cycle stock can be determined. The dehydrator into which the total reactor effluent passes is a small fractionating column packed with Berl saddles in which hydrocarbon is refluxed continuously so that water is stripped from the liquid phase and is carried as a vapor into the reflux condenser at the top of the column. The condensate falls into a trap where the water is separated and passed into the water receiver while the liquid hydrocarbon returns to the column as reflux. The liquid leaving the bottom of the reboiler is anhydrous and can be charged directly to the debutanizer without causing difficulty by ice or hydrate formation in the refrigerated debutanizer condenser. O b v i o u s l y Figure 4. Instrument Board for Laboratory Scale Catalytic Cracking Unit the hydrogen and lighter hydrocar-
February 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
.
.Y
549
Various considerations entered into the choice between batch operation or continuous operation of the fractionating equipment t o be installed on the unit. With the fixed bed or fluidized fixed bed equipment there was no choice because the intermittent product stream of varying rate and composition did not adapt itself to fractionation by continuous methods. The constant rate and composition of the reactor effluent from the circulating fluid unit, however, made continuous debutanization and fractionation seem attractive. Batch operation on automatic equipment is open to some of the same objections which apply to the use of standard laboratory methods. More handling is required and in the case of a batch debutanizer, two gas samples requiring separate analysis are obtained. Time is also lost because this work cannot bo done until after the completion of the test period on the cracking unit. With continuous fractionation, however, more “lining-out” time is required to reach a steady state before beginning each test period. After weighing these advantages and disadvantages the decision was in favor of batch operation for the gasoline fractionator and continuous operation for the debutanizer. I n designing the continuous debutanizer, two alternative modes of operation were possible. It could either be operated under pressure so that temperatures below the freezing point of water were not needed or it could be operated at atmospheric pressure in which case refrigerated condensers were required. The high pressure method would also require the use of a compressor for the reactor outlet gas and a pump for the liquid product. This posed a difficult control problem at this scale because the reactor outlet pressure had to be held essentially constant. The use of atmospheric pressure and low temperatures on the other hand meant the complete elimination of water from the product before it entered the debutanizer. That this was the simpler approach is substantiated by the efficient operation of the dehydrating and debutanizing columns on the cracking units which were constructed. INSTRUMENTATION AND CONTROL
I n order to further economize on manpower and to provide smoother operation, automatic control was applied whenever feasible. The three-point temperature controller a t the top of the instrument panel section for each cracking unit (Figure 4) controls the heat input to the heaters surrounding the regenerator, reactor, and stripper, respectively, of each cracking unit. The control temperature is obtained from a thermocouple fastened to the outer wall of each vessel. The dehydrator is controlled by setting the heat input to the reboiler a t a predetermined value on the variable transformer control. Condenser temperature is controlled by a thermostatic throttling valve on the water supply. Liquid level control in the reboiler is accomplished by a fixed overflow leading t o the debutanizer.
550
INDUSTRIAL AND ENGINEERING CHEMISTRY
of electrical contacts. Connected with a pressure tap into the base of the column it regulates the heat input to maintain a constant superatmospheric pressure a t the base of the column and hence controls the pressure drop across the packing and consequently the rate of vapor flow up the column. A thermocouple a t the top of the packed section of the column operates through one control section of a three-point controller to cut off the current to the heater when the predetermined cut temperature is reached. When the current is cut off by this controller, the distillation is shut down completely without attention from the operator.
TABLE 11. TYPICAL OPERATINGDATA
Conversion, 100-vol. % catalytic gas oil 53 . O Processing condition Weight hourly space velocity 3.0 Weight ratio, catalyst to oil 4.4 Charge rate, gallons per hour 0.09 Observed recovery of products, weight 102.8 % of charge Yields adjusted t o loo'?& recovery 199 Dry'gas (C3 and lighter), cu. ft./ bbl. of rharge 5 4 Dry gas ( C , and lighter), a t . % of charge 13.4 Ci fraction vol. "& of charge 41.2 Debutanizdd gasoline, vol. % of charge 47.0 Catalytic gas oil, vol. yo of charge 2.7 Catalyst deposit, a t . % of charge
Gasoline 57.5
30 50
70
100 116 124 1.52 208 282 360 382 405 98.0
-
90 95
E n d point Per cent over Per cent bottoms Hydrocarbon type analysis Olefines, % Aromatics, % Paraffins naphthenes,
+
1.0
20 34 46
Vol. 43, No. 2
52.9 7.9 3.5 4.9 98.6 128 4.1
TYPICAL OPERATING DATA
13.6 42.5
Considerable operating experience has been accumulated on these units, and typical of the operating results are the data shown in Table 11. These were obtained while processing a 31 A.P.I. gravity mid-continent gas oil, having a 50% distillation temperature of 690' F., over a synthetic silica-alumina catalyst at a reactor temperature of 850" F. The data from the 2-barrel-perday pilot plant (9)given for comparative purposes were obtained during operation on a synthetic silica-alumina catalyst. After the conclusion of the series of tests the catalyst was removed from the larger plant, screened on 200 mesh, and charged to the midget unit for the series of tests from which the data in Table I1 were taken. It appears from these data that the observed product distribution is generally similar to that obtained in large scale fluid catalytic cracking units, with the exception that the dry gas yields seem somewhat higher and the debutanixed gasoline yields correspondingly lower. This shift in product distribution may be influenced by three principal points of difference between the midget and full scale reactors: the midget reactor runs a t atmospheric pressure, instead of 3 to 10 pounds gage; the linear velocity in the reactor is decidedly lower than in the large scale units; and the midget reactor employs countercurrent flow which allows contact of cracked vapors with clean catalyst from the regenerator. Stripping of spent catalyst from the reactor has been satisfactory, for in general, the apparent hydrogen content of the catalyst deposit as calculated by oxygen disappearance in the regenerator ranges between 7 and 10%.
47.1
2.7
O
Catalytic Catalytic Gas Oil Gasoline Gas Oil 28.4 58.1 30.0 482 501 506 538 570 609 686 726
...
96.0 4.0
110 128 136 165 214 286 370 388 416 98.5 1.5
-
488 506
512 538 568 608 672 706
750 98.5 I .5
18 32 50
The heat input and liquid level in the debutanixer reboiler are controlled in a manner similar to that in the dehydrator. The cooling of the debutanizer condenser and precooler by direct evaporation of Freon-12 refrigerant in a closed system simplifies control at these points. The capacity of the Freon compressor is such that with the full refrigeration load the suction line pressure will not rise above 8 to 9 pounds per square inch absolute. This ensures that as long as liquid Freon is supplied to the precooler jacket the temperature will not rise above -40" F. which is sufficient for retention of Cb hydrocarbons. A thermostatic expansion valve controls the supply of liquid Freon and prevents overflow of the liquid from the jacketed portion of the precooler. A similar expansion valve is used on the column condenser. The reflux ratio and hence the cut point is controlled by adjustment of the temperature in the column condenser. This temperature is regulated by an instrument actuated by a thermocouple 12 inches from the top of the &foot packed section of the column. The controlled output air from this recording temperature controller opposes the Freon pressure in the condenser jacket, and these two pressures are applied to opposite sides of a flexible metal bellows which operates a valve controlling the escape of Freon from the condenser to the compressor suct,ion line. When high temperatures in the column call for more reflus,the lowering of t,he controlled air pressure opens the control valve to drop the Freon pressure until it balances the lower air pressure. This in turn lowers the condenser temperature and returns more reflux to the column. The reverse operation occurs when less reflux is called for. The gasoline fractionators are also automatically controlled. The reflus ratio is controlled by a 3-to-1 vapor partition arrangement. The water-cooled reflux condenser is operated with a full stream of cooling water. Light hydrocarbons, not condensed a t the water temperature, are returned to the gasoline receiver by the refrigerated final condenser which operates in the same manner as the debutanizer precooler. The heat input to the reboiler is controlled by a flexible metal bellows actuating a pair
CONCLUSION
The fluid catalyst technique can be successfully applied a t laboratory scale to a circulating catalytic cracking unit capable of reproducing the important operating conditions existent in full scale commercial catalytic cracking units. The versatility of these midget fluid catalytic cracking units is beyond the goal originally set. Microspherical and ground synthetic catalysts have been successfully used; the only limitation on catalysts is that the particle size range must generally be restricted to about 30 to 200 mesh for satisfactory operation. These units have been operated a t weight hourly space velocities of 1 to 3 and catalyst to oil weight ratios of 3 to 10 with no evidence that these ranges are necessarily the extremes that can be handled. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of many individuals in the Universal Oil Products organization who took part in the development of the equipment described. LITERATURE CITED
(1) IXD.EXG.CHEX, 41, 1249-50, definitions 1 a n d 4 (1949). (2) Oil Gas J . , 45, 82 (March 1, 1947). (3) T h o m a s , C. L., and Hoekstra, J., IND.ENG.CHEM.,37, 332-4 (1945). R ~ C E I V EApril D 18, 1950. Presented before the Division of Petroleum SOCIETY, Chemistry a t the 117th Meeting of the AXERICH CHEMICAL Houston, Tex.
