LANTS Fluid-Bed Catalytic Cracking Unit J. A. MARSHALL'
AND
J. W. ASKINS
Shell Oil Co., P.O. Box 2527, Houston, rex.
square inch gage and reactor temperatures as high as'1000" F. can be used. Thus, the bench scale downflow unit covers the operating range normally desired of a large scale pilot plant for the study of operating variables but utilizes only a fraction of the amount of feed stock required for a larger unit. Catalyst from the addition hopper is transferred with air into the regenerator (Figure 1). After the catalyst has been regenerated to the desired degree and heat'ed t o the desired temperature, catalyst flow is started into the reaator. The catalyst rate is controlled with a valve which is actuated by the pressure drop created by the flowing catalyst in the transfer line from the regenerator t o the reactor. Sufficient and constant pressure drop is required across the valve t o assure steady flow; this is accomplished by maintaining a constant differential pressure between the reactor and regenerator. The oil rate is measured gravimetrically and is controlled by a gear pump, which forces the oil through two preheaters in series and then into the reactor. The vapors pass upward through the
N CONNECTION with catalytic cracking research programs, a need was felt several years ago for a bench scale, continuous, downflow fluid-bed catalytic cracking unit. This need was motivated principally by the lack of satisfactory correlation between laboratory batch units and continuous commercial plants. The principal requirement of such a unit was that it require a minimum of feed stock but still have a reactor that would operate as a true moving fluid bed in order t o give results that could be correlated with commercial fluid-bed operation. With such a unit, many evaluations normally carried out on large scale pilot plants could instead be performed a t a considerable savings in time and expense on the bench scale unit. Furthermore, representative data obtained on bench scale equipment could be correlated with commercial fluid-bed operation. An apparatus having these features has been developed and placed in operation. This bench scale unit has a 2-inch diameter reactor, 6 feet long, and has a feed rate of from 0.25 t o 0.65 gallon per hour. Satisfactory runs can be made with a feed stock requirement of only one third to one half gallon per run.
Unit Covers Operating Range of larger Plant but Uses Only Small Quantity of Feed Stock Figure 1 shows a simplified flow diagram indicating the principal equipment of the bench scale, continuous, downflow fluid-bed catalytic cracking unit. The unit consists essentially of a loading hopper, regenerator, reactor and stripper, spent catalyst receiver and scales, charge tank and scales, oil charge pump and heaters, water and brine condensers, total liquid product receiver, gas sampler, and wet test meter. This equipment is contained in a steel framework occupying an area 2l/1 feet wide by 10 feet long by 13'/2 feet tall as shown in Figure 2. The instrument board, shown in Figure 3, consists of a 10 x 8 foot panel. Figure 4 shows the range of operating variables of the bench scale unit, except for reactor preasure and temperature, The range of inlet oil mole fraction is shown for typical operation. Weight hourly space velocity can be varied from 0.5 to 24 and catalyst-to-oil ratio can be varied from 2.7 to 22. Reactor pressure can be varied from 5 to 25 pounds per 1 Present address, Shell Oil Co., Wood River Refinery, W o o d River, Ill.
6
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8 . AIR
Figure 1.
Simplified Flow Diagram
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The regenerator (%inch, Schedule 10, stainless steel pipe, 12 feet long) which also acts as a storage vessel has a capacity of approximately 4.5 cubic feet of catalyst and is wrapped with electrical resistance heaters sufficient to heat this catalyst from room temperature to 1000" F. in approximately 4 hours. In order to determine the effcots of carbon level on regenerated catalyst on the conversion-severity relationship and t,he yield structure, the regenerator and regenerator instrument lines can be aerated with carbon dioxide. In this way, partially regenerated cata,lyst can bc heated to the usual temperature without the carbon tlcposit burning off. Catalyst holdup and bed density in the regenerat,or are measured by a manometer. From t,hese mexsuremcrit,s bed height can be calculated.
Catalyst Is Transferred to Reactor by Nitrogen Stream The flow of catalyst from the regenerator to the reactor is controlled by a stainless steel needle valve which has a built-in stainless steel screen t,o prevent foreign particle6 such as vessel scale from plugging it. After the catalyst passes through the valve, it is picked u p by a nitrogen stream and is transferred to the reactor. The stem of the valve is aerated so that catalyst cannot accuniulat,e between thc stem anti bonnet and possibly "freeze" the stem. Cooling fins are located ahead of thr packing gland which ut,ilizes high temperature steam parking. In order t80 maintain catalyst temFigure 2. Steel Framework Containing Equipment of Bench Scale Unit perature the catal>-st valve is electrically compensated down t o the cooling fins. The catalyst transfer line connecting the regenerator ant1 rcaccatalyst bed into the disengaging zone and then go overhead through a porous stainless steel filter. The spent catalyst passes downward into a countercurrent internal stripping section. Reactor bed level is controlled automatically by a catalyst throttling valve which discharges spent catalyst from the stripper. After passing through the porous stainless steel filter, the cracked products pass down through a water-cooled condenser into a concentric disengaging zone where the condensed hydrocarbons drop into a high pressure gage glass, and the uncondensed vapors continue to a second refrigerated condenser where some of the lighter hydrocarbons are condensed. The remaining gases pass through the reactor pressure-control valve and on to the gas sampler and wet test meter. The liquid is drained intermittently from the two high pressure accumulators into a tank where the total liquid product is periodically measured. This tank is manifolded so that gas from it is combined with the regular hydrocarbon gas stream.
Regenerator Stores 4.5 Cubic Feet of Catalyst In the following discussion of the principal features of the bench scale unit, reference is made to the detailed flow diagram shown in Figure 5 . Catalyst is charged manually into the catalyst addition hopper (1 cubic foot capacity) through a funnel containing a standard 20-mesh Tyler screen, which removes any foreign matter. It is then pressured into the regenerator with air through one of the regular regenerator aeration lines, through which air is continually flowing.
Figure 3.
Instrument Board for Bench Scale Unit
July 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
tor consists of 3/1e-inch stainless steel tubing having an inside diameter of approximately 1/8 inch. This internal diameter determines the quantity of transfer medium necessary t o maintain smooth transfer of catalyst. Since the amount of transfer medium has a direct effect on the inlet oil mole fraction, it should be maintained as low as practical. The transfer line is about 3 feet in length, and differential pressure taps for measuring the catalyst rate are about 30 inches apart. The catalyst transfer line is electrically heated with a beaded resistance wire and can be maintained a t temperatures up to 1100' F.
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Oil Charge System Includes Two Preheaters That Raise Temperature of Charge to 700' to 1000° F. The charge tank consists of a steam-traced 1-gallon steel container. The charge rate is gravimetrically measured on a Toledo indicating scale which is accurate within 1 gram. A 8/le-inch stainless steel pump suction line, suspended in the charge tank, has the bottom Lfz inch enlarged t o contain a small screen (42-mesh) to prevent foreign material from getting into the pump or heater coils. The pump discharge line contains a rupture disk which will burst a t 230 pounds per square inch gage if a heater becomes plugged. The charge oil rate is controlled by a pump which consists of a motor, a Graham variable gear reduction box, and a a/,gallonper-hour Zenith gear pump element. Two of the advantages of this type of pump over the reciprocating positive-displacement type are: (1) the pump rate can be set accurately and quickly and ( 2 ) flow is steady, resulting in a nonpulsating feed stream t o the reactor. The charge pump and lines are steam traced. There are two charge oil heaters connected in series. The first heater is a coil of 20 feet of 3/1B-inch stainless steel tubing cast in an aluminum block (12 inches long by G inches in diameter); this is maintained at GOOo F. by electrically heating. The second heater consists of a coil of 20 feet of I/*-inch stainless steel tubing cast in another aluminum block of the above dimensions. This cylinder is also electrically heated and isammaintained at 700' to 1000' F. By the use of this system of preheat, which permits high heat flux, sufficient heat ia supplied t o the feed without long heater residence time, thus essentially eliminating any thermal cracking in the preheat section. Three coils are cast in each cylinder Bnd coils are changed if the one in use plugs. Between the two heaters an oil replacement purge line ties into the oil line. Nitrogen enters the oil line a t this point and keeps the high temperature block heater free of catalyst when oil is not flowing. This bleed can be switched t o air for burning out the heater if it cokes. After a run is completed and the oil flow is stopped, the oil replacement bleed purges the high temperature heater of oil. The low temperature heater is back purged through the drop-out valve.
Filter at Top of Reactor Separates Hydrocarbon Vapors and Catalyst d.
The reactor consists of a 2-inch, Schedule 80, stainless steel pipe 6 feet in length; it has a maximum catalyst holdup of about 3.3 pounds, leaving a minimum disengaging zone of 1 foot below the filter. Catalyst holdup is measured by the differential pressure across the bed and maintained a t the desired level by a pneumatic recorder-controller which operates a catalyst valve a t the bottom of the internal stripper. Catalyst holdup, bed density, and bed height are obtained in the same manner as for the regenerator. A porous stainless steel filter, which separates the hydrocarbon vapors and catalyst, is located in the top of the reactor. The filter is a bayonet type (Micro Metallic Corp., designated B-149) and has an average pore diameter of 35 microns (Grade E). The function of the internal stripping section is to remove the adsorbed and occluded hydrocarbons associated with the catalyst.
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Table I. Comparison of Bench Scale Unit and Commercial Unit Yields and Product Properties a t Constant Conversion Operating Conditions Commercial Unit Reactor temp. F. 902 Reactor p r e s d r e l b /SCJ inch gage 15 0 Catalyst/oil weiLht ratio 5.2 Space velocity, wt. oil/hour/wt. catalyst 0.95 Inlet oil mole fraction 1.00 Yields % wt basis fresh feed 6.8 Ca and lighter Total Ca 9.3 4 Totai cs Total C; (6.7) Debutanized gasoline 37.0 23.8 Light gas oil 17.9 Heavy gas oil 5.2 Coke ... Conversion, lo&% wt. 450$O F. Carbon 58.3 Conversion, lo&% wt. 450$O F. Product properties Gasoline Sulfur, % wt. Olefins, % wt. Aromatics % wt. Naphthenks and paraffins, % wt. Light gas oil Aniline oint OF. Sulfur, wt: Kinematic visc., os. a t 100' F. A P I gravity Molecular weight Correlated cetane no.
Bench Scale Unit 900 15 0 5 0 0.70 0 40 8.1 10.3 (7 0) 35.0 24.2 17.5 4.9s 4 5 58.3
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21.8 0.80 13.5 27 1 a
21.9 0.82 13.2 274
Assuming 8% hydrogen in coke.
This is accomplished with a nitrogen purge flowing countercurrent t o the catalyst. The internal stripper, which has an inside diameter of 0.75 inch and a catalyst holdup of about 0.28 pound, is connected t o the reactor below the oil and catalyst inlets. At a catalyst rate of 40 pounds per hour, the stripper holding time is 24 seconds and the maximum nitrogen t o catalyst ratio is equivalent to 4.8 pounds of steam per 1000 pounds of catalyst. (Steam may be used for stripping, but it is not as convenient to use as nitrogen.) The stripped catalyst then passes through the catalyst valve and is weighed for determination of the catalyst rate. Periodic samples are combined, and the composite sample is used for carbon determination.
Condensed Hydrocarbons Are Collected and Mewured in liquid Product Receiver After passing through the filter, the hydrocarbon vapors and nitrogen diluent flow into a water-cooled condenser, consisting of a &foot length of 1-inch stainless steel pipe jacketed by a 3-inch pipe. Condensed hydrocarbons drop into a small high pressure gage glass. The uncondensed hydrocarbon vapors and nitrogen then flow into a similar '/*-inch refrigerated condenser. The refrigerant circulated through the jacket consists of an ethylene glycol-water mixture maintained a t about 30' F. The condensed hydrocarbons drop into another high pressure receiver similar t o the one mentioned above. The liquid products collected in both high pressure receivers are intermittently withdrawn t o the same large receiver. From the refrigerated condenser the remaining uncondensed gases, which contain less than 1% Ce+, pass through the reactor pressure control valve and are expanded to atmospheric pressure. These gases combine with any vapors that are flashed off the condensed hydrocarbons after they are drained from the high pressure receivers and constitute the gas make for the run. For runs charging waxy stocks, a wax trap is inserted between the reactor and the condensers t o remove the heavy waxy portion of the product before the condenser, where it would plug the tube. The condensed hydrocarbons are collected and volumetrically measured in the total liquid product receiver. The vessel is
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the flow rate desired and the rotameter capacities are of the same magnitude; therefore, settings can be made accurately. Regenerator-reactor differential pressure, catalyst flow to the reactor, and reactor catalyst holdup are all controlled by pressure recorder-controllers which receive their impulses from differential pressure transmitters. The regenerator pressure control valve is located in the vent line and maintains the vessel 1 t o 3 pounds per square inch above reactor pressure. Pressure drop in the catalyst transfer line operates the regenerated catalyst valve while the pre.ssure drop across the reactor controls the spent catalyst valve. This pressure drop is checked and reactor instrument bleeds zeroed before a run with a manometer containing a 1.97 specific gravity fluid, The pressure in the reactor vessel is controlled by a motor valve located downstream of the product condensers. In this position the valve operates in a cool zone and throttles only the light gases and the nitrogen diluent. Also the condensers are held under pressure which helps in the condensing of the hydrocarbon vapors. For control and measurement of temperatures, a 6-point temperature controller, a 24-point temperature indicator, and an % point temperature recorder are connected in parallel to the thermocouples. Autotransformers are used on all control circuits to minimize temperature cycling and are also used on all circuits not on automatic temperature control. Reactor temperatures are measured by four thermocouples located a t different levels in an axial thermowell which extends from the top flange down the center of the vessel t o the oil inlet. These thermocouples give a true reading of the reactor bed temperature. Temperatures from these couples control four separate electrically heated sections on the reactor. This affords good control of the temperature profile when runs are made a t different bed levels. In order to supply sufficient heat to the catalyst for the low-bed runs and not overheat the section of the reactor covered by the lowest element that is above the bed level, the temperature control point in the bed a t the oil inlet is switched t o control the heat input to the regenerated catalyst transfer line, while the heat input to the lowest reactor element is set by an autotransformer.
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Total liquid Product Fractionation Yields Cuts Corresponding to Gasoline and light and Heavy Gas Oils
OIL CHARGE RATE, GAL./HR. 99' API GAS OIL Figure 4.
Range of Operating Variables
wrapped with '/r-inch tubing carrying the glycol-water coolant such that the liquid in the receiver is maintained a t about 60" F. This prevents excessive flashing of the material condensed by the refrigerated condenser when material is drained from the watercooled condenser. A wet test meter with a stainless steel rotor is employed t o meter the product gas. This gas is then vented t o the outside of the building. A composite gas sample is obtained by withdrawing a continuous slip stream from the main gas line by water displacement.
Instrumentation Provides Accurate Control of Process Conditions The flow of air or nitrogen t o all instrument purges, aeration lines, and transfer lines is metered by rotameters. Flows are manually controlled by needle valve settings in the rotameter headers. This system of control is satisfactory since the inlet pressure to the rotameter is constant. I n general, one rotameter meters the flow into three lines. The groupings are made so that
Gas samples and CSand lighter components in the total liquid product are analyzed in the usual manner, the former by mas8 spectrometer and the latter by a combination of Podbielniak distillation and mass spectrometer analysis of Podbielniak fractions. The total liquid product is fractionated in a 30-plate Oldershaw column normally a t a 4:1 reflux ratio. Cuts are usually made to correspond to gasoline, light gas oil, and heavy gas oil. With the above system, a complete product distribution can be obtained. This procedure also eliminates the lengthy line out periods and weight balance difficulties associated with continuous fractionating equipment in the unit and permits considerably more flexibility and accuracy of product analyses and workup. Commercial Yields Can Be Predicted from Bench Scale Data Data typical of operating results are shown in Figures 6 and 7. These were obtained by cracking a 29" API gravity West Texas gas oil a t space velocities ranging from 6.5 to 0.8 over an equilibrium synthetic silica-alumina catalyst. Other operating variables-Le., temperature, inlet oil partial pressure, catalyst-to-oil ratio-were held constant. These graphs, in which the per cent weight yields are plotted versus the depth of conversion, show the data from the bench scale unit to be very consistent with reference to severity change. Reproducibility of data is good. The two points a t 59% weight conversion, which were obtained a t identical operating conditions, are typical of normal operation.
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July 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Typical Yield-Conversion D a t a
Temperature, 900' F.; pressure, 16 Ib,/sq./inch gage; catalyst/oil ratio, 5.0
The bench scale unit correlates well mit,h a 3-barrel-per-day pilot plant ( 1j which has a larger reactor of similar design. Commercial yields can he predicted accurately by appropriate correction of bench scale data for the effects of t'he different process variables. Comparisons of yields and product properties from a commercial unit with those from the bench scale unit at the same conversion level when cracking a 30" API gravity West Texas gas oil are given in Table I. Although the bench scale data are not corrected for any differences in process variables yield structures and severitj- requirements are similar, and product properties are in close ayrcement vit,h t'liose of the commercial unit. A considerable volume of dat,a has been accumulated to support t,he one set of operating data given in Table I. Over 90% of the runs made on the bench scale unit have satisfactory weight balances (98 to 102% weight recovery). One operator can run the unit. On one-shift, operation, the average output is 8 runs per 5-day week. This compares favorably with a large 3-hnrrel-per-day pilot plant ( 1 ) which requires three operators per shift on a three-shift schedule to average five runs per 7-day week while requiring an average of 4 barrels of feed stock per run.
Figure 7.
40 50 60 CONVERSION, 100-SOW 450-t Typical Yield-Conversion D a t a
70
Temperature, 900' F.; pressure, 16 Ib./sq./inch gage; catalvrt/oil ratio, 5.0
Conclusions The fluid catalyst catalytic cracking technique has been successfully adapted to a bench scale, continuous downflow, fluid bed unit, the data from n-hich can be used for the prediction of commercial yields. Space velocity (weight of oil per hour per weight of catalyst) can be varied from 0.5 to 24 by changing oil charge rate and/or reactor holdup; catalyst-to-oil ratio can be varied from 2.7 t o 22 i+ith accurate control of reactor bed holdup and temperature profile. In many cases, this unit has proved to be a successful substitute for a large scale catalytic cracking pilot plant in the evaluation of feed stocks, catalysts, and operating variables, thus resulting in considerable savings in capital expense, operating cost, and time, in addition to having much lower feed stock and catalyst requirements.
literature Cited (1) Trainer, R. P., Alexander, X. W., and Kunreuther, F., ISD.EKG. CHEX, 40, 175 (1948). RECEIVED for review May 4, 19.53.
ACCEPTED
June 2. 19S3.
