Small Scale Fluid Catalytic Cracking Unit T. RICE, J. K. CARPENTER,
AND
Gulf Rereorch 6 D.v.lopnsnf
A SMALL . . to
scale fluid catalytic cracking unit has been developed aid IU solying many problems encountered in the operation of Gulf's five commercial fluid c a t a l q q cracking units which have capacities of 20,wO to 65,000 barrela per day. I n the past, Gulf Research & Development Co. has operated a %barrel-perday tluid pilot unit along with various bench scale fixed-bed units (3)to answer these problems. Although ked-bed data were helpful in screening or preliminary comparisons, operation of the %barrel-per-day pilot unit waa found necessary in order to obtain reliable quantitative data for predicting the behavior of catalysts and charge stocks in commercial fluid catalytic cracking units. Unfortunately, the %barrel-per-day pilot unit is relatively expensive to operate and requires large quantities of catalyst and oil which axe frequently not available in experimental work. The need for a research tool capable of providing reliable data for fluid catalytic crackmg operations, but requiring only smell amounts of feed stock, catalyst, and manpower for operation, has been answered by the development of a small scale fluid unit. The small scale unit was patterned after the 2-barrel-per-day fluid unit in order that the product distribution data obtained from the two units would be in good agreement. The unit is essentially a downflow type of tluid unit and dBers from other small scale fluid units which have been described (I, 8 ) in several important respects. All of the essential features of the 2-barrelper-day fluid unit, such as continnous catalyst circulation between the reactor and the regenerator, oil injection into the regenerated catalyst transfer line, and a u t o m t i c control of catalyst levels and circulation, have been retained in the small scale unit. The size of the unit is such that the total catalyst inventory is 1700 to 3000 grams, and the oil charge rate can be varied between 400 and 1400 grama per hour. The data obtained with the small scale unit agree very closely with those obtained on the %barrel-per-day pilot unit and, &B a result, the s m a l l scale unit can be used to investigate many of the charge stock and cataly?t evaluation problems which were formerly studied in the %barrel-per-day unit. Recycle fluid catalytic cracking data can be accurately predicted from single-pasa data obtained on the small scale unit by means of correlations. In addition, because of its aize the small scale unit can be used t o study problems of the following types:
.'
1. The behavior of experimental catalysts under actual fluid cracking conditions. 2. Evaluation of charge stocks which are ahort in supply., These may be experimental stock8 prepared on bench scale tlnits or stocks pre ared from crudes wluch are not yet in production. 3. The ewect of oila containing hi h percentages of metala or other contaminants on cracking cataqysts. Prior to the development of the mall scale unit, mch problema could not be investigated unless e compromise procedure involving operating conditions somewhat different from actual tluid cracking conditions was adopted.
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C. D. ACKERMAN
Ca, Piflrburgh,
Pa.
Construction of Plostic Model Expediter Development of Miniature Unit
The development of a small scale fluid unit presented various problems, some of which could only be solved by experimental work after the unit was in operation. In order to expedite the development, a plastic model first was built. This model proved invaluable in answering problems on the size limitations of the equipment and the range of particle siae of the catalyst which could be circulated satisfactorily. The results of this work indicated that catalyst could be circulated a t controlled rates in the desired quantities in '/,-inch IPS (International Pipe Standard) transfer lines. The 40f micron fraction of commercial equilibrium catalyst wa8 circulated in the model without difficulty. If waa found that, in order t o obtain satisfactory circulation with fresh catalyst, i t was necessary to eliminate the sharp edges by attrition. A pneumatically controlled slide vJve wa8 designed and used on the plastic model to oontrol catalyst circulation. The same type of valve was installed on the small scale unit to control catalyst levels in the reactor and regenerator a~ well as the catalyst circulation. A series of visual tests in a glass tube was made to determine the range of superficial velocities that would be above the minimum Euidization velocity but would not be so high that slugging would be caused in vessels of small diameter. The diameters of the various vessels in the unit were chosen to give velocities in the desired range. A f t e the unit was'placed in operation, i t was found that the principal operating difficulty was coke formation in the transfer, line from the regenerator to the reactor. The coking dEiculty WBB not eliminated until, aa in the Z-harrel-pepday unit, the oil was injected vertically into the regenerator to reactor catalyst line, 88 shown in Figure 1. Several other schemes of injecting the oil into the regenerated catalyst stream were tried, but none were successful. Small Scale Unit Resembles Pilot Plant i n All Importont Respects
The small scale unit is built a8 a miniature fluid unit and incorporates the snme type of flow as a downflow fluid unit (Figure 1). This type of flow w q chosen in order to pmcess the oil in aa nearly aa possible the same manner a8 the 2-barrel-perday unit and thus obtain comparable results. The reactor consists of a catalyst bed, 80 inches high, l>/,inch IPS diameter, and a disenghging section, 18 inches high and 6 inches in diameter. Catalyst and oil enter the reactor at the bottom. The reactor products flow through a cyclone a t the top of the reactor to remove moat of the catalyst. Any catalyst not removed by the cyclone is trapped in the bottom of the fractionator. The catalyst removed by the cyclone is returned t o the stripper. The catalyst level in the reactor is regulated by a dif-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 8
PILOT PLANTS
REACTOR PRESSURE CONTROLLER
D I F F E R E N T I A L PRESSURE TRANSMITTER SPENT CATALYST FLOW CONTROLLER
Figure 1 .
Flow Diagram of Fluid Catalytic Cracking Unit
frrential-pressure controller that opcrates a slide valve in the spent catalyst line (l/., inch 11%) to thc stripper. Thc reactor is wound with six separate heaters that are controllrd by a tcmpcrature controller to give the desired catalyst temperature in the reactor. The catalyst is *tripped with steam in a vessel in which there is a catalyst bed, 7 inches high and ll!k inches IPS diameter, arid a disengaging section, 6 inches high arid 3 inches IPS diameter. Distilled water for stripping steam is pumped by a constant-rate pump t,hrougli a vaporizer into the stripper. The spent catalyst from the reactor ent,ers the bottom of t,he stripper and overflows a t the top of the ll/r-inch section int,o a surge hopper located below the stripper. An enlarged liquid-level gage is uscd as the surge hopper in order that the levrl of the catalyst in the hopper can be observed a t all times. The use of the liquid-level gage also permits observation of the flow of catalyst from the stripper. Catalyst from the hopper passes through a 3/8-inch IPS standpipe, 47 inches long, to the slide valve that is used to control the catalyst circulation. After passing through the slide valve, the catalyst is lifted in a transfer line inch IPS) by nitrogen or a portion of the regeneration air t o the regenerator. The rate of flow of catalyst in the transfer line is regulated by a differentialpressure controller that operatcs the &de valve to maintain the designated differential pressure across a section of the transfer line. A four-way valve located near the rcgenerator is used to divert catalyst flow from the transfer line into a bomb for 30 seconds in order to obtain an accurate catalyst rate. The regenerator is made of 2*/?-inch IPS tubing, 24 inches long, with a 6-inckdiameter disengaging section. A cyclone a t the top of the regenerator removes most of the catalyst fines from the August 1954
rcgciieiator flue gas Catalyst not ieiriovcd bj. tlic cyclone is trapped in IL stone filter and periodically returned to the unit. Spent catalyst enters the bottoin of the regenerator. The catalyst level in the regemrator is controlled by a differential-pressure controller that operates a slide valve at the bottom of the regenerator standpipe. The regenerator standpipe is made of 84 inchrs of 1/2-inch IPS tubing. Additional air to burn the carbon depobit off the catalyst is added a t the bottom of the regenerator. The regenerator is wound with four heaters that are controlled by a ttmperature controller to give the desired catalyst temperature 111 the regenerator. The regenerator pressure can be controlled eithcr higher or lower than the reactor pressure by a differentialprpssure controller between the reactor and regenerator that operates a pneumatic valve in the flue gas line. Oil is pumped into the unit through two preheatem that are '/(-inch IPS lines jacketed with lead to give close temperature control. The first preheater is 42 inches long. The higher temperature preheater is 12 inches long. The oil is injected into the regrnerated catalyst transfer line and serves, along with a small amount of steam or nitrogen, t o carry the catalyst to the reactor. .2 continuous fractionating system is provided to give the desirrd product fractions which are a 10 Reid vapor preusure, 410' I". end point gasoline, a catalytic gas oil, and a pentanr-ficc gas. The fractionator is made of l'/l-inch IPS tubing packed with 1/2-inch Berl saddles for a length of 24 inches. 4 partial condenser with a large surface is used on the fractionator to hold the temperature of the overhead product a t the desired value. A condenser, held a t 34" F., is used to condense the gasoline from the fractionator overhead product. Water is separated from the gasoline by means of a continuous overflow-type separator.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT CATALYST C I R C U L A T I O N ZP"
Jr,
REACTOR LEVEL
Figure 2.
Typical Control Charts
REACTOR-REGENERATOR DIFFERENTIAL PRESSURE
The gas from the gasoline condenser is dricd and recombined with gasoline as feed to t,he stabilizer, The stabilizer, which is made of 1-inch IPS tubing with a 28inch packed scct,ion, is used to obtain R 10 Reid vapor pressure gasoline. I'odbielniak Hclipak Yo. 2917 i.i u s d a 4 the packing material. X condenser in Tyhich liquid propane is vaporized provides reflux for the column. The gas from the stabilizer is metered in a wet test meter and a finger-type rubber tube pump is used to pump enough gas for a fianiple through a caustic scrubber to remove hydrogen sulfide. Operational and Performance Features of Unit Permit Wide Range of Catalyst and Charge Stock Studies
Although it was patterned aftcr the 2-barrel-per-day fluid pilot plant which requires t'hree operators per shift, the small scale unit requires only one operator per shift, principally because it is a much smaller unit and is operated only on a eingle-pass basis. Ho.rvever, the small scale unit is flexible and, with the exception of recycle operat,ions, it can be operated over nearly the same ranges of variables as are obtainable on the 2-barrel-per-day unit. The ranges of operating conditions that are feasible are summarized in Table I. Good temperature control is obtained with conventional instrumentation because of the presence of fluidized beds of catalyst. Kormally, the reactor operates with no gradient or a barely measurable one. The oil and catalyst temperatures before niixing are normally maintained a t l e d e used in larger units in order to attain comparable conditions in the regenerated cat,alyst transfer line. The regeneration temperature may be held a t any desired level to aid in control of the carbon on regenerated catalyst. Carbon on regenerated catalyst is usually controlled to commercial operating levels. It is possible to maintain the carbon on regenerated catalyst a t a level of 0.6yoby weight within about ~ O . O Z ~byo suitahle adjustment of air rate, catalyst level, and temperat,ure. Charts from the catalyst circulation controller are t,ypified
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by the segment shown in Figure 2, and the individually mcasurcd rates are usually found to be within It27, of tlic a Flow st,oppages usually do not occur unless the differcntialpressure control is abnormally rough or the floiv of feed is intcrrupted. Khen the catalyst circulation rate is measurcd by momentary diversion into a bomb, an equal amount of catalyst is added to the regencrator t,o avoid upsetting thc level. Calculated rates from carbon balances average about the samc as measured rates but are less consistent'. Figure 2 includes typical charts of thc reactor and rcgerierator lcvel controllers. Since control of convcrsion arid the lcvcl of carbon on the regenerated catalyst depend on these levels, it is important that they bc consistent. I t ha? been i'ou~idthat the conversion n-ill remain constant viithin 1yoor less during rcasonably smooth operation for consecutive hourly or bihourly wcighings, and that the carbon-on-catalyst, control is good. continuous fractionation was used to ensure good correlation with the 2-barrel-pcr-day pilot unit. Although it might appear that, continuous fractionation would mat,eriallp lengthcri thc time required to obtain a test period, it, has been found that the fract,ionating towers will reach steady-state conditions about as rapidly as the carbon-on-catalpt and flow rates. Any loss in time is more than balanced by the man-hours t,hat, would he rc-
Table I .
Range of Operating Variables of Small Scale Fluid Cracking Unit
Oil charge Pate, grams/hr. Space velocity, wt./(lir.)(wt.) Catalyst circulation, graIna/hr. Catalyst inventory. grams T o t a l unit Reactor Reeenerator Temperature. O F, Oil preheat Reactor Regenerator Carbon biirnuff, granis/Iir. Carbon on regenerated catalyst,
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
400-1400
0,4-25
3300-12000
1700-3000 GO-ROO
300-1200 450-900 876-1000 1000-1200
7c b y wt.
10-120 0.1-1.5
Vol. 46, No. 8
PILOT PLANTS
L/
quired f o r b a t c h fractionation. With the use of continuous fractionation, the gasoline yield and conversion can be observed with changes in operating conditions, and thus it is easy to determine when the desired conversion is obtained. The fractionator combines the functions of quenching the crude vapors and separating the gasoline from the gas oils. ASTM gasoline distillations are obtained on a control basis to allow the operator to adjust the tower temperatures to obtain the desired separation. The liquid levels in the frac345 50 55 60 65 GAS OIL C O N V E R S I O N . % B Y V O L U M E tionator, t h e s t a bilizer, a n d t h e Figure 3. Small Scale and Pilot water separator are Unit Yield Conversion Data all controlled by a simple overflow deX Small scale unit vice and no atten0 Pilot unit t i o n is r e q u i r e d from the oaerator except to drain the accumulators periodically. The water-free gasoline ,flows by gravity to the stabilizer and from the stabilizer reboiler through an overflow to an accumulator for draining. The usual metering and sampling techniques are employed with all liquids and solids being weighed. Ordinary reducing valves and rotameters are used to regulate the pressure and to meter the flow of the gases used in the operation. Calibrated wet test meters are used to measure the flow of the product gases. Flue gas and hydrocarbon gases are sampled by pumping with a finger-type rubber tube pump into displacement bottles in order that a constant sample flow can be maintained. The hydrocarbon gas sample is passed through caustic solution to absorb hydrogen sulfide in order to permit greater accuracy in analysis, and the used caustic is withdrawn a t intervals to permit analysis for sulfur. The flue gas is sampled before and after i t is passed through a combustion furnace to burn all carbon monoxide to carbon dioxide. The operator regularly analyzes these samples for carbon dioxide and may adjust the operating variables to obtain a specified carbon burnoff. Numerous catalytic cracking charge stocks with a boiling range of 600' to 1050' F., as well as distillate stocks of higher and lower boiling range, have been successfully charged to the unit. Test periods are usually only 2 to 16 hours in duration, but continuous operations for periods as long as 10 days have been attained even when the higher boiling range stocks are cracked. Thus, it is possible to obtain several series of test periods or to complete an entire program in a single operation. Continuous operation for reasonably long periods is also advantageous in the evaluation of experimental oils which are limited in supply. The technique used in this case consists of operating on a similar stock until the unit reaches a steady state and then, without interruption of any of the flows, changing to the stock which is limited in supply.
August 1954
I n addition, continuous operation for long periods had made i t possible to obtain catalyst aging or poisoning data. In this type of operation changes in operating conditions and in unit yields may be used to determine the extent of catalyst aging or poisoning, but it is usually desirable to supplement the data with other tests. The catalyst inventory is critical, and care must be exercised to limit the size of catalyst samples withdrawn for analytical or activity tests. Experimental Data Agree Closely with Results from larger Unit
The unit was originally designed to operate a t atmospheric pressure and a large part of the work done to date has been carried out a t essentially atmospheric pressure. A series of calibration runs was made in order to determine how data obtained a t atmospheric pressure on the small scale unit compare with data obtained at about 15 pounds per square inch gage on the 2-barrelper-day unit. After small adjustments had been made for the difference in operating pressure, it was found that the yields from the small scale unit agreed well with the 2-barrel-per-day data. However, it was found that the maximum conversion on the small scale unit wa3 limited to 54% by operation a t atmospheric pressure. Since the charge stock used in this comparison was a heavy KuFait gas oil which is easy to crack, i t was evident that the maximum conversion obtainable on other stocks would be unsatisfactorily low. In order to avoid this undesirable situation and to eliminate the necessity of making pressure corrections, the unit was modified to operate under pressures as high as those used on the 2-barrel-per-day unit. The modifications consisted of adding a reactor pressure controller and decreasing the reactor diameter to 11/&inch IPS in order to maintain a reasonable superficial gas velocity in this vessel. A second series of calibration runs was made in which the same charge stock, catalyst, and operating conditions, including pressure, were used in the small scale unit that were used in a previous series of runs on the 2-barrel-per-day unit. The product distribution obtained from the units is compared in Figure 3. The small scale data without any adjustments are in excellent agreement with the 2-barrel-per-day data. Single curves satisfy the data from both units, and, a t equal conversions, equal yields of carbon, gasoline, Ca, Cs, and Cz and lighter fractions are obtained from the two units. In addition, conversions as high as 64% were obtained on the small scale unit. Conclusions
A small scale fluid unit has been developed that is similar in design and produces results that are in close agreement with those of a 2-barrel-per-day fluid pilot unit. It is being used to investigate many problems, including studies of catalyst contamination, evaluation of experimental catalysts, and evaluation of charge stocks which are limited in supply. The unit is not designed for recycle operation and, because of the size of samples normally obtained, tests that require unusually large samples are not feasible. However, i t is expected that single-pass data on any stock that is feasible as charge for a fluid unit may be obtained with experimental size samples of catalyst and charge stock with accuracy close to that attained on larger pilot units. I n addition, recycle fluid catalytic cracking data can be accurately predicted from single-pass data obtained on this unit by means of correlations. Literature Cited
H.w., et d., IND.ENG.CHEM., 43, 545 (1951). Marshall, J. A., and Askins, J. W., Ibid., 45, 1603 (1953). (3) Rice, T., and Ivey, F. E., Jr., Proc. Am. Petroleum Inst., 27 (III), 84 (1947). (1) Grote, (2)
RECEIVEDfor review March 9, 1954. $CCDPTED M a y 27, 1954. Presented before the Division of Petroleum Chemistry at the 125th Meeting of t h e AMERICAN CHEMICAL SOCIETY,Kansas City, hlo.
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