I Engineering Approaches I Gas Residence Time in Fluidized Beds
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A. R. HUNTLEY,' W. GLASS, and J. J. HEIGL Esso Research & Engineering Co., Linden, N.
A new method measures distribution of
N e w Krs5Technique Developed
residence times using a radioactive tracer
The new technique developed for measuring gas RTD's uses radioactive krypton-85 -as the tracer, in order to study air-fluidized pilot unit systems with average gas residence times of 1 to 5 seconds. Such residence times are too short for accurate study by the older methods. This krypton-85 technique has been applied thus far in a small 4inch diameter vessel and in a large 36inch diameter vessel. What Kind of Tracer Was Needed? A radioactive gas was desired to permit measuring tracer concentration in situ, thus avoiding the sampling lag problem. I t had to have some hard gamma radiation to penetrate the l / %inch thick steel walls of the large pilot unit vessel. Its adsorption of the catalyst had to be negligible. Its half-life had to be at least a few weeks. Finally, it had to be readily obtainable at moderate cost.
A represents a very complex flow situation. To determine FLUIDIZED
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Tr
J.
BED
just what the capability of a given fluid bed reactor is, the actual flow patterns as well as the chemical kinetics involved should be known. The complexity of the flow patterns usually forces the adoption of a simplified model where relatively few flow pattern parameters define the bed ( 4 ) . Experiments to determine the flow patterns directly are difficult to conceive and even more difficult to carry out. One experiment that sheds considerable, although indirect, light on the gas flow patterns-and an experiment that moreover can usually be carried out-is the determination of the distribution of gas residence times. Even under steady-state conditions, where the average residence time of the gas passing through the bed is fixed, some of the gas flows very rapidly through the bed while some of the gas remains in the bed for a much longer time. I n fluidized systems where the gas is the reactant, the gas that stays in the bed a long time may be overconverted; some of the product may be degraded to less valuable material. The gas that passes through very rapidly may well be under-converted. It is possible to determine quantitatively the fraction of the gas that remains in the bed for any set period of time. Such a gas residence time distribution (RTD), together with other physical measurements if necessary, can usually be related to the parameters in any idealized flow pattern model of a fluidized bed. The KrS5 technique developed is eminently suitable for determining gas residence time distributions even in systems with an average residence time around 1 second. H o w A r e Gas RTD's Determined?
Experimental determinations of RTD's in fluid beds have been reported by many investigators (7, 3, 5). One common technique that has been employed is to inject a quick, instantaneous pulse of a tracer gas into a feed gas Present address, The Lurnrnus Co., Newark, N. J.
stream. The curve of tracer content of the exit gas stream us. time is then determined. This directly represents the RTD. Thus, if the gas passed through the bed in true piston flow (or plug flow), then the output pulse would look just like the input pulse, but would lag by the gas residence time in the bed, as in Figure 1. I n this example, each portion of the gas has the same residence time. The standard deviation of the residence times, u,which is a common characterizing factor for an RTD, would be zerci Piston flow of the fluidizing gas never occurs in reality, however. A range of gas residence time is always found, as shown in the more typical R T D in Figure 2. Another method commonly used to measure residence time distributions involves a step input function of a gas tracer. The output curve resulting from a step increase in input tracer concentration is equivalent to the integral of the output curve that would have resulted from a pulse input ( 7 , 3 ) . This is illustrated in Figure 3. The R T D can thus be deduced by differentiating the observed output curve from a step change input. The step function can, of course, be either a buildup, as illustrated, or a decay-Le., a sudden decrease in inlet tracer concentration. A decay is simply the inverse of a buildup. The crucial analysis of the exit gas stream has been done in the past either by pulling a sample stream through an appropriate continuous analyzer or by taking numerous closely-timed samples. Such sampling techniques usually involve time lags of the order of 1 to 3 seconds. They can thus be used efficiently only when the average gas residence time to be measured is of the order of 10 seconds or so, in order that the necessary sample lag corrections can be held to a reasonable minimum. Fluid beds with average residence times well below 10 seconds are also of practical interest. Techniques were developed to permit measuring RTD's without incurring sample lag corrections and which were applicable to systems of very low average residence time. These techniques are detailed below.
REL TRACER CoNCENTRAT'oN
AVE. RESIDENCE
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Figure 1. With piston flow, output pulse would be identical to input pulse TYPICAL RTD
REL TRACER CONCENTRATION
-TIME
Figure 2. In real systems, some pulse distortion always occurs
Figure 3. Step function equals integral of pulse function VOL. 53, NO. 5
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MAY 1961
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SOLENOID VALVES A, E, a C ARE SIMULTANEOUSLY ACTUATED BY A FIRING SWITCH, WHICH ALSO SENDS A TIMING SIGNAL TO T H E DATA RECORDER.
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a FLUID
S.V.
MONITOR TO
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FLUIDIZING AIR
HYPODERMIC
N2
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Kre5
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TAPE RECORDER RECORDING AT HIGH SPEED
N2
/ / /
/ / / / / / / / / / '
A Krypton pulse injection for large units
TAPE RECORDER PLAYBACK AT LOW SPEED
RECORDER
PRINTER
Krypton detection and data handling system
Kr85 Filled the Bill. Krypton-85 fortunately met all of these requirements. Its radiation is chiefly 0.7 mev. beta (99+%), but some 0.54 mev. gamma (0.65%) is also given off. I t has a 9.4 year half-life (2). The krypton-85 is fairly cheap in 50-curie batches ($15 per curie), so that it is practical to use counting methods which see only the gamma radiation. General Description of KrS5 Technique. The krypton-85 was injected into the fluidizing air just upstream of the fluid bed, either as a decay (step function) or as a pulse. In the small unit (4-inch diameter, up to 0.02 pound of air per second), step functions were used. In the large unit (3-foot diameter, up to 15 pounds of air per second), pulses were used, primarily to conserve krypton. The krypton was detected by suitably located scintillation detectors whose output, appropriately modified, was recorded on a high speed tape recorder. The tape was later played back at a low speed, and the pulses automatically counted for uniformly spaced short time intervals. The counts per interval were printed on a paper tape, which was a direct record of tracer concentration us. time. This tape could easily be used to plot the results, or to serve as input data for a computer program.
382
Major Details of Technique
and Equipment
Tracer. The krypton-85 was obtained from Oak Ridge in 50-curie batches, in 1000-ml. lead-shielded containers at sub-atmospheric pressure. It was essentially pure krypton, a mixture of Krs5and the stable Krp6. The specific activity was 80 mc. per STP cc. Extra containers were purchased to permit diluting portions of the batch by pressuring up with nitrogen. This made the tracer gas flow easier to control. The tracer delivery pressure was controlled by an all-metallic regulator which w a s resistant to radiation damage. A transfer hose was built for minimum internal volume, using 15 gage 0.054 inch I.D. stainless hypodermic tubing within a flexible armor. Tracer Injection. For the step functions in the small 4-inch diameter unit, tracer injection was a simple matter. The KrS5transfer hose was connected to a solenoid valve which discharged through a filter into the fluidizing air line. The switch controlling the solenoid also controlled a timing signal which was tape-recorded along with the tracer concentration data. The large 36-inch diameter unit required pulse functions in order to keep
INDUSTRIAL AND ENGINEERING CHEMISTRY
tracer consumption within reasonable limits. A more complicated injection device was consequently needed, shown schematically (left). Diluted Krs5 was pressured into the Kr chamber at about 25 p.s.i.g., and a 400 p.s.i.g. charge of nitrogen was accumulated in the N2 chamber (separated from the K r chamber by large-ported solenoid valve A ) . Purge air was flowed through the injection line to keep it clear of catalyst. SVhen the firing switch was briefly closed and opened, valves A , B, and C were momentarily actuated. This interrupted the purge air, and allowed the h-2 to sweep the krypton-85 into the fluidizing air stream, giving a pulse of about 0.075-second duration. At the same instant, a timing signal was sent to the tape recorder, indicating the start of the experiment. The Geiger tube mounted on the Kr chamber monitored the injection sequence, and showed whether the tracer flow corresponded to positions of the remote control switches. Consumption of Tracer. The tracer consumption required per test for a given unit depended on the mass flow rate of the fluidizing gas, detector arrangement, tracer input function, and ratio of peak count-to-background count. I n the 4-inch diameter unit, decays consumed about 30 curies per pound
GAS RESIDENCE T I M E
Figure 4. Typical Krs6 decay test in 4-inch diameter unit
air per second, for a peak-to-background ratio of 100. I n the 36-inch diameter unit, however, pulses consumed only about 0.07 curies per pound air per second, for a peak-to-background ratio of 100. This low KrS5 consumption was mainly due to the pulse function, although the greater sensitivity of the large scintillation detectors used on the unit also helped keep the consumption down. Detection. The KrS5 concentration was measured in both the small and large units by scintillation detectors containing thallium-activated NaI crystals. On the 4-inch diameter unit, detectors with 1 X 1 inch crystals were used. O n the 36inch diameter unit, large, very sensitive detectors with crystals 2 X 4.5 inch diameter were used, of the type employed for aerial scintillation surveys. Colli-
mators were generally used which provided about 1.5 inches of lead around the sides of the crystals. Each detector was connected to a scaler which provided a 1200-volt power supply. The scaler was not used in the conventional way; it was used simply to amplify and shape each voltage pulse from the detector, for direct recording on magnetic tape. This was done by connecting the Schmitt discriminator output to the tape recorder input. Data Recording a n d Handling. T h e instrumentation system used to detect, record, and print data is shown schematically (p. 382, right). A dual-channel magnetic tape recorder was employed to store data for subsequent playback. During the data storage step, a high tape speed was selected which permitted individual scintillation pulse recording. The output from the scaler’s Schmitt discriminator was fed directly to one channel of the magnetic tape recording head. O n the second channel, a control signal in the form of a constant frequency was recorded. This was turned on at the instant of tracer injection, and was turned off manually after completion of the run, or at any time selected by the operator. Playback of the stored data was done at a reduced tape speed which allowed the digital printer and analog recorder to “ f ~ l l ~ w . ”These units were turned on and off by a control unii which was activated by the signal on the second channel of the magnetic tape. Tracer Disposal. The air carrying the radioactive tracer leaving the large unit was discharged through an 80-foot high stack. The tagged air leaving the small unit was further diluted about 40fold with additional air and then discharged through a 60-foot high stack. The resulting maximum ground concentration of krypton was calculated and was always well below the permissible AEC limit for unrestricted areas.
Typical Results
Typical R T D data obtained with the Kr86 tracer technique are shown in Figures 4 and 5. The decay in Figure 4 was measured in the 4-inch diameter unit when fluidizing cracking catalyst with 0.01 pound air per second. The average gas residence time was 1.7 second, and the standard deviation (g) was 0.98 sec. Figure 5 shows typical results from the 36-inch diameter unit, operating with cracking catalyst and 13.5 pounds air per second, in which a pulse function was used. The corresponding decay result was calculated for comparison with the pulse. The average residence time in the fluid bed was 3.5 seconds, and the u was 1.4 seconds. I n the figures, only enough data points to define the curves are shown: I n Figure 4 the actual count rate was measured every 0.1 second, but points are shown only every 0.2 to 0.3 second. Other Applications
Gas R T D measurements can also be made in nonfluid-solids systems with this technique. The method is the samc whether the solids are fluidized, stationary, or absent. The void volume of any dry system can be determined with the gas R T D technique. If the system contains liquid (especially hydrocarbons), however, the Krs6 technique should be used with caution. Absorption of the tracer in the liquid may give misleading results (6). Acknowledgment
J. A. Wilson developed the recordingdata handling system used in this work. literature Cited (1) Danckwerts, P. V., Chem. Eng. Sci. 2, 1 (1953). (2) Kinsman, S., et al., “Radiological Health Handbook,” p. 240, U. S.
G.P.O., 1957.
(3) Mason, E. A., Sc.D. thesis, M.I.T.,
1950. (4) May, W. G., Chem. Eng. Progr. 55, 12, 49-56 (1959). (5) May, W. G., Dechema Monografih. 32, NO. 451-76, 261-82 (1959). (6) Steinberg, M., Manowitz, E., IND.ENG. CHEM.51, 47 (1959).
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RECEIVED for review September 20, 1960 ACCEPTED March 3, 1961
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Division of Industrial and Engineering Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960. Material supplementary to this article has been deposited as Document No. 6630 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured by citing the document number and by remitting $1 .25 for photoprints or $1 ,25 for 35-mm. microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, Library of Congress.
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Figure 5. Typical v 2 KrS5 pulse-and 0 V equivalent decayin 3-inch diameter unit W
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