Catalyst
ecorder for
ytic Cracking Units A n instrument which continuously indicates and records t h e amount of catalyst carried away with t h e flue gas in fluid catalytic cracking units has been developed. I t consists of an optical device for measuring t h e concentration of catalyst and a flowmeter for measuring t h e flue gas velocity. T h e optical device and t h e flowmeter are coupled by a mechanism which automatically multiplies concentration into rate of flow, thus giving t h e instantaneous loss of catalyst per u n i t t i m e . T h e accumulated loss of catalyst i s obtained by automatic integration.
J. H.RAMSER
AND
J. W. HICKEY
T H E ATLANTIC R E F I N I N G C O M P A N Y , P H I L A D E L P H I A , P A .
HE flue gas of catalytic cracking units of the fluid type contains finely divided catalyst which escapes with the flue gas into the atmosphere and therefore is permanently lost. The loss of catalyst may vary from a fraction of a ton to several tons per day depending on operating conditions. The high cost of cracking catalyst may seem to make complete recovery desirable. However, for practical reasons, a complete recovery is not feasible. An upper limit of catalyst concentration in the flue gas may be fixed through local smoke-abatement ord:nances. I n the absence of such ordinances, the upper limit of catalyst concentration is fixed by the fact that a high selective loss of catalyst fines produces poor fluidization of the catalyst in the unit. Therefore, efficient operation of a cracking unit requires that the catalyst loss be known and controlled within such limits as the above mentioned factors warrant. The catalyst loss is usually determined by a spot test in which flue gas is filtered through a porous thimble for 2 hours. This length of time is necessary in order to accumulate readily weighable amounts of catalyst and to cover a complete time cycle of rapper operations in the Cottrell precipitator. The purpose of the rappcrs is to shake the catalyst loose from the collecting electrodes. Most of the catalyst falls into collecting bins but some is carried away with the flue gas, producing a momentary high catalyst concentration in the flue gas. From the concentration determined by the thimble method and a computed value of flue gas velocity at stack conditions, the catalyst loss per unit time may be calculated. The thimble method is not only time-consuming but also has the additional disadvantage that trends in the catalyst loss are not readily discernible unless frequent and accurate spot tests are made. FurthermoIe, this method is of little use when the catalyst concentration fluctuates widely such as may be the case during startups and other periods of unsteady operation. It seemed therefore desirable to develop a completely automatic instrument which continuously records the instantaneous as well as the accumulated loss of catalyst. The catalyst loss recorder to be described in this paper has bcen especially designed to meet the stringent conditions of stability, accuracy, continuous service, and low maintenance which such an instrument should satisfy. The catalyst loss recorder has been used on a commercial cracking unit with satisfactory results. In addition to recording catalyst loss, the instrument has been found helpful in correlating trends in catalyst loss myith operating conditions. It may be used also for checking the operation of the Cottrell rappers since each rapper produces its own peak on the chart of the recorder.
P R I N C I P L E OF OPERATION
The catalyst loss recorder consists of three different functional parts: 1 . A device for measuring the concentration of catalyst in the flue gas. 2. A device for measuring the flow of flue gas in the stack. 3. A device for multiplying the catalyst concentration into the gas flow rate to obtain automatically the instantaneous catalyst loss per unit time. Measurement of Catalyst Concentration. The device for nieasuring the concentration of catalyst in the flue gas consists of a light source mounted on one side of the stack and a thermopile mounted on the other side. Preliminary laboratory investigations, as well as actual calibrations a t the stack, have shown that thc relation between the voltage output, E , of the thermopile and the catalyst conccntration, c , may be approximately represented by the equation c = IC (log Eo - log E ) in which Eo is the voltage output corresponding to zero concentration and k a n empirical constant. The value of the constant depends mainly on the thickness of the light-dispersing medium; i t was found to be little affected by the variations in particle size and surface reflectivity, which are encountered under practical operating conditions. For a stack diameter of 4.0 feet, the numerical value of k was found to be 190 if the concentration is expressed in pounds per million cubic feet. The value of Eo depends on the intensity of the light source. T o be wii,hin the range of the voltage recording instrument, it may be necessary to reduce Eo and E by a common factor. This may be done by means of a potentiometer which is connected across the thermopile. The voltage output of the potentiometer, Tvhich is EO’at zero concentration (EO’ < EO),is then fed into the voltage recorder. The intensity of the light entering the stack may vary slightly with time; this is partly due to variations in light output and partly due to variable losses of light bctween light source and stack. Therefore, it may be necessary occasionally to reset the voltage which corresponds to zero catalyst concentration to the value chosen for Eo’. This may be done, during operation of the unit, by means of two tubes which are pushed into the stack from either side. These tubes completely enclose the light beam when they meet a t the center of the stack. The flue gas with its suspended catalyst is displaced from the tubes by two streams of air Under this condition, the intensity of the light received by the thermopile corresponds to zero catalyst concentration. The
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INDUSTRIAL AND ENGINEERING CHEMISTRY
voltage output of the thermopile may then be adjusted by the potentiometer to the chosen value for Eo'-for example, 4.5 mv. The output voltage of the thermopile is recorded continuously by a Brown Elektronic strip chart potentiometer. If desired, the recorded voltage output of the thermopile may be converted into catalyst concentration by means of a calibration chart or by a scale mounted near the recording pen. For practical purposes, it is often more desirable to know, the instantaneous or accumulated loss of catalyst. Obviously, the loss of catalyst depends not only on the concentration of catalyst in the flue gas but also on the rate of flow of the flue gas. Measurement of Flow. As a primary element for measuring the rate of flow of the flue gas, a reversed t,ypePitot tube is used. The opening of the Pitot tube is placed at a point a t which the gas velocity has a maximumvalue. This point was determined by making a velocity traverse through the stack. The results of 20 t m m s e s , made Over period Of months, have shown that the maximum gas velocity, at a fixed level in the stack, is proportional to the average gas velocity over the cross section of the stack under steady state conditions' At a level which was 13 feet above the inlet to the stack, the proportionality constant was found to be 0.95 * 0.007 and the point corresponding to maximum gas velocity was 10 inches from the center of the stack. During the test period the average gas velocity under steadystate conditions varied from 5g*9 to 77*2 feet per second* The t,emperature of the flue gas varied from 448" to 462" F. and t.he pressure from 760 to 776 mm. of mercury. The gas velocity calculated for the average temperature Of 4500F*and the average pressure of 765 mm. of mercury deviated from the observed gas velocities a t actual stack conditions by less than 1% in each case. I t was therefore unnecessary to compensate the flow measurements for the variations of teniperat'ure and pressure which are encountered during normal operation. The catalyst fines are prevented from entering the Pitot tube by well regulated air streams which do not affect the pressure differential across the tube, as explained below. The Pitot tube is connected to a ring balance flowmeter ( 1 , 2) which records directly the rate of flow of the,flue gas. Multiplying System. I n order to obtain an indication of the rate of catalyst loss at any time, i t is necessary to have a device which automatically multiplies the instantaneous catalyst concentration into the instantaneous rate of flow of the flue gas. The device designed for this purpose is constructed as follows: By means of a pair of Selsyn motors the bridge-balancing wheel of the Brown oteiitiometer is coupled to a cam which is located in the case houging the ring balance flowmeter. A cam follower which is constrained to move in a fixed radial direction rides 011the rim of the cam. The contour of the cam corresponds to the empirically obtained relation between catalyst concentration and voltage output of the thermopile. This relation may be represented approximately by the equation given above. The diktance of the cam follower frbm the center of the cam is proportional to k log Eo' when the voltage output of the thermopile is En. This position of the follower corresponds therefore to zero concentration. When the output voltage is E, corresponding to the concentration c, the distance of the follower from the center of the cam is proportional to k log E'. Therefore, the radial displacement of the roller from the zero position is proportional to concentration c according to the equation. As mentioned above, the Pitot tube is connected to a ring balance flowmeter which automatically converts the differential pressure indication of the Pitot tube into rate of flow. The flowmeter carries two recording pens: one for recording the rate of flow and the other for recording the product of the rate of flow and any quantity which may be fed into the instrument in form of a linear displacement. If this linear displacement is made proportional to catalyst concentration by the cam arrangement described above, the product pen will record the instantaneous catalyst loss. By an integrating device which is built into the flowmeter, the instantaneous catalyst losses are summed up tar
Figure I.
Light
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Source Assembly
give the accumulated catalyst loss between any time limits. The accumulated catalyst IOSS may be read on the counter which is coupled with the integrator. DETAILS OF CONSTRUCTION AND OPERATION
Catalyst Concentration Meter. The construction of the light source assembly is schematically shown in Figure 1. Tube 1 (length 33 inches, inside diameter 4.0 inches) is welded t o flange 2. The inner tube, 3, may be pushed into the stack by means of the handles, 4,which are welded to the front end of tube 3. Because the inner tube will be heated to about 450' F. while in the stack, the clearance between the outer and inner tube was made about 0.03 inch, when both tubes are a t room temperature. It has been found necessary to make the area between outerand inner tube small in order to reduce friction caused by catalyst fines which are caked together by the moisture in the flue gas. TJvogrooved bearing surfaces are provided at the frontend of tube 1. T o prevent undue sagging of the tube while in the stack, the distance between these bearing surfaces was made to be 8 inches. The outer and inner tubes as well as the handles are made of stainless steel. The formation of ice between the outer and inner tube during freezing weather is prevented b y a steam coil wound around the outer tube. Catalyst fines and moisture (the flue gas may contain up to 30y0 water vapor by volume) are prevented from entering the tube by uniform forward flow of air.. It was found that a single air inlet will not produce a sufficiently uniform flow. As a result catalyst fines gradually drift back and cling to the window of the light source, thereby reducing the light intensity. A sufficiently uniform flow is produced by a series of air entry holes arranged around the circumference of tube 1, indicated in Figure 1, Channel 5 with air inlet 6 is welded directly to tube 1. The air is filtered to prevent slow accumulation of oil and dirt on the window of the light source. After trying a varietv of light sources, i t was found that certaiii sealed beam spotlights (for example, Type 4535 of the General Electric Company) are most suitable as light sources. By clamping the spotlight around its outer rim, 7, mechanical shift due to vibration is reduced to a minimum. Furthermore, the front surface of this type of light source is illuminated quite uniformly; therefore, small shifts in position do not affect the voltage output registered by the thermopile. Light sources consisting of a bulb and a separate parabolic reflector have been found unsatisfactory because the severe vibrations usually existing a t the stack cause a rapid defocusing of the light source. The light source housing, 8, is separated from the rest of the system by window 9. This allows the cover of the housing to be opened for occasional adjustment of the light source u-ithout disturbing conditions in the tube. When it is necessary to clean the window (about once every 2 months), it may be removed from the housing after a blind flange has been inserted a t 10. The housing, 8, is mounted on an adjustable support so that the position of the housing may be shifted to focus the light beam on the thermopile. Bellows 11 provide the necessary flexibility.
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I n order to hold the intensity of the light source as constant as possible, i t was found necessary to provide good voltage regulation. The volt,agc regulation obtainable with a constant voltage transformer was found to be insufficient. .However, satisfactory results were obtained with an electronic voltage regulator (Sorenson, Model 250) provided the load on the regulator is increased to its rated value by a resistor in parallel with the light source. The construction of the assembly containing the thermopile is the same as the one described above escept that window 9 is replaced by a lens and the light source by a thermopile. Stack Flowmeter. The reversed type Pitot tube used as the primary element in the stack flowmeter consists of two Monel tubes which are 5 feet long and have an inner diameter of 0.375 inch. Catalyst fines and moisture are excluded from the tubes by well regulated air streams.
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--
Figure 2.
Purging System For P i t o t Tube
The air purging system used is shown in Figure 2. Filtered air of about 65 pounds per square inch gage pressure is reduced to a nearly constant pressure of 15 pounds per square inch gage by pressure regulator 1. The flow of air is split into two purging streams; the rates of flow are held constant by means of two Moore constant flow regulators, 2 and 3. The latter are controlled by the inlet pressure to the two needle valves, 4 and 5 . An oilfilled manometer, 6, is provided for determining the pressure differential across the Pitot tube. After reading the pressure differential, with air supply valve 7 closed, purging air is admitted and adjust,ed by needle valves 4 and 5 so that the different.ia1pressure indicated by the manometer is the same as Tvithout purging. The rate of flow through each tube should be approximately 10 cubic feet per hour if the velocity in the st,ack is about 75 per feet second. It was found by laboratory t e s h that the indicated differential pressure is not measurably affected by variations of ~ 5 in % each air stream. Although the purging air will exclude effectively catalyst fines and moistmurefrom the interior of the Pitot tube, a deposit of catalyst may gradually build up a t the openings of the tubes. This deposit, may be removed easily by a blast of air which may be produced by first closing valves 7, 8, and 9 and then opening TTide valves 10 and 11. The differential pressure across the Pitot tube is transmitted from the stack to the control room over a pair of lines which are 200 feet long. Tests have shown that, a pressure transmission line of such length is satisfactory for the present purpose. The ring balance flowmeter, installed in the control room, is adjusted t’o read a different,ial prcssure of 1.5 inches of water a t full scale. The different,ial pressure corresponding to a stack velocity of 75 feet per second is 1.1 inches of water. Clutch and Cam Arrangement. As mentioned above, the bridge-balancing wheel of the Brown potentiometer is coupled to the voltage-concentration conversion cam by a pair of Selsyn motors. Since the chosen type of Selsyn motor was too large to be mount,ed inside the instrument housings, it byas necessary to
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mount them on the outside. This introduced a complication in the case of the Brown potentiometer since the bridge-balancing wheel is located on the back side of the instrument door. The problem -was solved by devising a mechanical clutch which is engaged when the door is closed and disengaged when the door is opened. The clutch consists of two disks one of which is rigidly mounted on the bridge-balancing wheel. This disk has a conically-shaped depression in the center and an off-axis conical hole. The other disk is connected to the shaft of one of the Selsyn motors. It is constrained t o move in an axial direction by a slot, pin, and spring arrangement. This disk carries a blunt conicallyshaped pin a t the center and a slightly shorter conical pin off the axis. When the door of the instrument is closed, the blunt pin enters its counterpart, thus establishing perfect alignment of wheel and shaft. By a slight rotation of either wheel or shaft, the off-axis pin will find its hole and snap into position by spring action. The pins must be short enough to avoid wedging when the door is opened. An interlock switch cuts off the power supply to the Selsyn motors when the door is opened. This precaution is necessary to avoid spinning of the Selsyn motors when they are disconnected from the bridge-balancing wheel. The cam for converting the voltage output of the thermopile into concentration was designed for a maximum concentration of 100 pounds per million cubic feet. This corresponds to a catalyst loss of 4.07 tons per day with a stack diameter of 4 feet and a flue gas flow rate of 75 feet per second. The point on the cam which corresponds to zero concentration (4.5 mv.) is 2.46 inches from the center of the cam; the point which corresponds to 100 pounds per million cubic feet (1.18 mv.) is 0.51 inch from the center. A displacement of the roller by 2.46 - 0.51 = 1.95 inches corresponds thwefore to 100 pounds per million cubic feet. The contour of the cam between the points indicating zero and maximum concentration was made to correspond to an empirically determined calibration curve, which was practically- identical with the curve represented by the equation mentioned above. The cam is mounted inside the flowmeter housing on the shaft of the second Selsyn motor; the cam follower is connected to the multiplication system of the flowmeter. CALIBRATION
The catalyst loss recorder may be calibrated through a direct determination of catalyst concentration by means of the thimble method. This is the method commonly used for determining the average concentration of catalyst in the flue gas over a time interval of 2 hours. Since the thimble method may give erratic results, it is advisable to make a sufficient number of determinations, if possible with widely different catalyst concentrations. The observed concentrations then are plotted against the logarithm of the average voltage output of the thermopile over a period of 2 hours. The average voltage output may be obtained by integration of the curve traced by the recording potentiometer. It was found t,hat the average value determined by this method is practically identical with the value obtained by neglecting the short-time peaks due to the operation of the Cottrell rappers. A straight line may be fitted to the data by the method of least squares. INSTALLATION
For reasons of flow measurement and greater uniformity of catalyst distribution in the flue gas, the Pitot tube and the optioal part of the catalyst concentration meter were installed on the stack of the catalytic cracking unit. The optical system was installed a t a level which is 13 feet from the inlet to the stack and 25 feet from its outlet; the Pitot tube is at a point 18 inches lower. The spot determinations of catalyst concentration for calibration of the instrument were made a t the same level.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
A platform for installation and maintenance was built around the stack (page 1098). On this platform the air flow regulator for the Pitot tube and the filter for the purging air are located. The Brown potentiometer and the flowmeter were installed in the control room which is near ground level. MA1NTENANCE
The zero point of the catalyst concentration meter is being checked about once every 2 days. For this purpose the sliding tubes are pushed into the stack and left in this position for about 10 t o 15 minutes to remove finely divided catalyst which may have entered the tubes during the sliding operation. I n order to leave enough space for the air to escape, it is advisable to retract one of the tubes by about 0.5 inch after contact is made. When cleansing of the tubes is complete, the voltage input into the Brown potentiometer should be recorded for about 5 minutes to determine its average value. If the average deviates from the voltage which was chosen to correspond to zero concentration, it may be adjusted by means of ‘a 200-ohm potentiometer which is connected in series with the thermopile. In a check run of the instrument which cqvered a period of 3 weeks, the zero point was adjusted 16 times t o a value of 4.5 mv.
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The average deviation of the voltage output between adjustments was A0.17 mv. or &3.8%. This deviation would produce an error of *lo% in the value of the instantaneous catalyst loss. While the sliding tubes are in the stack, the condition of the light source window and of the lens in front of the thermopile is examined. As a rule, cleaning of these parts is only necessary about once every 2 months. At the same time the air flow rates in the Pitot tube purging system are checked and, if necessary, adjusted. After the sliding tubes have been retracted, the instrument is ready for use. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of P. J. Satinsky in testing and calibrating the catalyst loss recorder during its trial period. LITERATURE CITED
(1) Hagan Corp., Hagan Bldg., Pittsburgh 30, Pa., Bull. 1M 48. ( 2 ) Rhodes, T. J., “Industrial Instruments for Measurement and Control,” 1st ed., pp. 269-70., New York, McGraw-Hill Book Co.. 1941. RECEIVED January 3, 1949.
Fluid-Solid Air Sizer and A u n i t for t h e continuous drying and sizing of dolomite employing t h e fluid-solid technique has been developed and i s operating on a commercial scale for the preparation of dry, dust-free rotary kiln feed. Removal and recovery of marketable fines from the feed give an improved product and effect a fuel saving in t h e kiln operation. T h e unit, 5 feet 8 inches in diameter by 15 feet 6 inches high, has a sizing and drying capacity of 45 tons of -4-mesh feed per hour. For 2.070 water feed entering a t 18’ C., drying air enters a t 400” C., dry sized product leaves a t
110’ C. and 0.0270 water, and moist dust-laden air leaves a t 75” C. Air i s heated by an oil burner using No. 2 fuel oil a t a rate of 1 gallon per ton of feed; 8000 cubic feet of free air compressed t o 1.2 pounds per square inch gage are required per t o n of feed. One t o n of feed a t 15% -65 mesh results in 0.9 t o n of sized product a t 670 -65 and 2% -100 mesh and 0.1 ton of dust a t 1.570 +65 mesh, most of which is recovered f r o m cyclones as a dry marketable product. Separations f r o m 20 t o 100 mesh are indicated as possible with this unit.
CLARENCE J. WALL, THE D O R R COMPANY, WESTPORT, WILLIAM J. ASH, NEW ENGLAND L I M E COMPANY, CANAAN,
I
N T H E customary application of fluidization, dust entrain-
ment is merely a nuisance. This paper describes a commercial application of fluidization wherein entrainment is a desirable feature of the operation: in a unit that dries and sizes the feed for two rotary kilns of the New England Lime Company a t Canaan, Conn. The principles of operation of the sizer-dryer are: the use of hot air t o evaporate the moisture and the sizing of the feed by differential entrainment from a fluidized bed. By discharging the sized product a t the bottom of the bed, short-circuiting is minimized and advantage is taken of the maximum possible scrubbing effect of the fluid bed. Limestone or dolomite for lime or dolime production is generally obtained by a quarrying operation with its attendant variable moisture conditions. Some stones decrepitate on heating to such an extent that it has been found expedient to precrush to - 3 / l e inch. Such a fine feed is bound to result in a high percentage of dust carried out of the rotary or FluoSolids kiln. This dust, which is a mixture of lime and unburned stone, constitutes a waste of fuel and of otherwise useful agricultural stone and presents a disposal problem. Preliminary laboratory test work a t the Dorr Company, Westport, Conn., laboratories established that dry limestone could be
CONN. CONN.
successfully sized in a fluidizing unit. An air sizer unit 14 inches in diameter was then installed a t the New England Lime Company’s plant a t Adams, Mass., to prepare a dust-free feed for the FluoSolids limestone calcining pilot reactor. The air sizer operated satisfactorily up to a capacity of 1 ton of feed per hour. When the lime reactor was operated using feed prepared in this sizer, the dust loss from the lime reactor was reduced 50% and the lime reactor fuel requirement was reduced from 42 to 39 gallons of Bunker C oil per ton of lime produced. The New England Lime Company had another plant a t Canaan, Conn., using rotary kilns for calcining dolomite, where it was felt that a sized feed might prove advantageous. The Dorr Company therefore designed a combined dryer and sizer unit for this plant to handle up to 50 tons per hour of feed of variable moisture content. The unit was installed by the New England Lime Company and is in satisfactory commercial operation. DESCRIPTION OF T H E U N I T
Figure 1 is a diagrammatic sketch of the sizer-dryer unit. The welded steel shell is 5 feet 8 inches in inside diameter and the over-all height ia 15 feet 6 inches. The air heater compartment and the windbox are lined with insulating refractory brick.
