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W. H. KING, Jr. Process Research Division, Esso Research and Engineering Co., Linden,
N. J.
Radioisotopes in Petroleum Refining Radioisotopes can do jobs heretofore impracticable or impossible; they are convenient and economical to use, because they are very easily detected. Only small quantities need be incorporated in a process being traced with the radioisotope
R m m s o T o m s are important tools in the progressive operator's kit. Future generations will wonder how a refinery was ever run economically without them. Uses are increasing more than exponentially and oil companies measure savings in millions of dollars per year. Although radioisotopes having many unique properties are available, the gamma emitters are most often used by the refiner. Gamma-emitting radioisotopes of long half life are principally used for density measurements; gamma emitters of short half life are used principally for tracer work. While most process streams contain hydrocarbons, radiocarbon and tritium have not been extensively used for tracer work because they are beta emitters of long life. The weak beta emission of these isotopes cannot penetrate pipe walls, and they are difficult to detect while the tracer test is in progress. Samples can be counted by well established laboratory techniques. T h e radioisotopes most often used by the petroleum refiner are cobalt, cesium, iodine, rubidium, barium. and scandium.
Several examples have been selected from the experience of Esso Standard Oil Go. refineries a t Everett, Mass., Bayway, N. J., Baltimore, Md., and Baton Rouge, La. These are presented in the following text.
in knockout drums and reactor vessels. I n such installations, the source and detector are fixed on either side of the pipe, reactor, or knockout drum and a continuous record of the density is displayed on a recorder in the control room.
Quantitative Gamma Ray Absorption for Density Measurements
0 Accurate gamma ray absorption techniques can be valuable in troubleshooting.
T h e main advantages of the gamma ray density apparatus are: 0 The measuring equipment can be installed on operating units without need for shutdown. 0 T h e measuring apparatus is not in contact with the material under measurement.
Thus, the apparatus cannot become fouled because of entrained matter in the stream or the nature of the stream itself. Commercially available equipment is being used today by refiners to measure density in the standpipes of fluid catalytic cracking units, pitch and residuum streams, and alkylation acid streams. Other uses include liquidlevel indicators and high level alarms
Radioisotopes Most Often Used by Refiners" Isotopes Used Coco, Cs138, Ir192
Process Investigated General-purpose, radiography, mixing, inventory, trouble-shooting, vapor line sediment Catalyst mixing, inventory, stack losses Catalyst flow rates Liquid flow rates, liquid mixing Heat exchanger leaks Gas flow rates Entrainment Batch mixing (grease)
Ba140, Ce144, Cr61, Rb*6, Sc48, Coco, A11198 1182, Bal*', Sc46, ZrQ6 1181, Ila*, Rb*s, Br*2 Sbl**,Na*4, Br*2 H*,Bra2, Kr85 Ba140, (214, Sb124, Cos0 I182
Ref erences
(1, 8,6-10)
This discussion (8) This discussion (8) (4, 6) (8)
(?a
Data assembled from information published by several major oil companies, with experience gained by author. Research applications omitted.
Occasionally in the operation of catalytic cracking units optimum catalyst transfer conditions are not realized. Conventionally such problems have been tackled on the basis of differential pressure surveys. I n one case control of reactor and regenerator temperatures was not So smooth as usual and some catalyst transfer line erosion was observed. These difficulties were attributed to poor catalyst circulation. Differential pressure surveys indicated that catalyst was surging through the trafisfer line rather than flowing smoothly but could not specifically locate the source of the trouble. Gamma ray absorption techniques, a t several places on the transfer line, called a U-bend, pinpointed the difficulties. Figure 1 shows the U-bend and special frame used to support the scintillation detector and a cobalt-60 source of gamma rays. T o obtain accurate and reproducible readings, the apparatus was designed to secure the detector exactly opposite the cobalt source on the diameter of the pipe and yet be easily movable. As the major point of interest in these tests was density fluctuation, a strong source of about 100 mc. was used. This produced, a t the scintillation detector, a counting rate of about 100,000 counts per minute, giving statistical accuracy of the density fluctuations. By VOL. 50, NO. 2
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Figure 1. This framework was used to position a shielded source and a scintillatioq detector accurately on a U-bend qf q,catalytic cracking unit. Accurate measurement of fluid catalyst density and fluctuations helped pinpoint the cause of poor catalyst transfer rates
comparing data taken at several points on the transfer line, the point of poor aeration was easily determined. Additional aeration taps were placed a t this point. The density fluctuations were reduced to a great extent and the catalyst flow became smooth and steady. As a result, catalyst circulation rate was increased 22yo and the unit was brought under much better control. KO serious transfer line erosion has been observed since these improvements were made. The knowledge gained from this test has been applied to other operating units and to the design of new cracking units. Similar techniques have been used on fluid cokers and other fluidized catalyst processes. Figure 2 shows a recording obtained under poor fluidization conditions during study of the behavior of fluid coke in standpipes. During this test, large bubbles of gas interrupted the coke flow; the large spikes, representing almost 1007, voids, show the frequency of those giant bubbles. When the standpipe is properly fluidized, the curve in the lower field is typical. This recording is
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Figure 2. pipe
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Bubble flow in coker stand-
The upper curve i s typical of a poorly operating fluidized solids standpipe or bed. The large spikes show fhe frequency of giant gas bubbles passing up the pipe, a condition known as slugging
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essentially a wiggly line not deviating more than 3 or 4y0from the 10 to 12% voids level. A special application of the gamma ray technique was to determine the fluid level on distillation trays. The first step was to calibrate the trays with gamma rays a t a very small angle to the tray. The source was placed 6 inches below the tray and the detector 6 inches above the tray on the opposite side of the tower. For the tower tested, 12.5 feet in diameter, the angle of the gamma ray beam was about 5" to the surface of the liquid. This permitted each '/I6 inch of liquid on the tray to be represented by a 3/i-inch path through the liquid, which is easily detected by the decrease in radiation. Rapid Qualitative Density Measurement. The need for rapid density measurements is often of an emergency nature, when time does not permit setup of an elaborate arrangement such as shown in Figure 1. Refiners keep available small sealed gamma ray sources and a scintillation detector. The source can be placed quickly on one side of a pipe and the detector on the other. Readings taken in this manner can be quickly calculated in terms of density within the pipe. when the approximate thickness of the walls of the pipe is known. The value of this highly portable equipment was demonstrated recently on a catalytic cracking plant startup, During the startup, trouble was experienced in establishing circulation through the regenerator standpipe 120 feet high. Although pressure gages were installed every 10 to 12 feet along the standpipe, the point of difficulty was not easily determined; the gamma ray technique with the portable equipment was called into action. A three-man team made density measurements and observed density fluctuations on each of 10 floors of the unit. Approximately 3 minutes were required per point, Within a n hour, rough density figures and fluctuations were calculated for all points. The trouble was localized as caused by too much aeration a t the top of the standpipe. This excess aeration was decreased, proper flow was established in the standpipe, and oil was turned into the unit, all within an hour! The cost of the equipment is approximately $800; the value of products produced in the time saved by employing this troubleshooting technique is several orders of magnitude higher. Similar equipment has been used in other cat plant startups and processes employing fluidized solids. The radioactive sources are mounted in lead cylinders 3 inches long and 3 inches in outside diameter, with a I/?inch hole 11/2inches deep causing the gamma rays from the source to be collimated into a narrow beam. The
INDUSTRIAL A N D ENGINEERING CHEMISTRY
opposite end of the lead cylinder is fastened to a 3-foot pipe, ' / 2 inch in diameter. Sources u p to 20 mc. of cobalt can be safely carried about from point to point. Those most often used are 1 and 10 mc. cobalt-60 or of iridium-192. The source collimator described is not a safe storage container for sealed sources, as radiation escapes from the defining hole. The collimator protects the operator and reduces scatter of the gamma rays during measurements. For safe storage the collimator is detached from its %foot handle and locked in a lead storage container 2 inches thick. The portable scintillation detector consists of a battery-operated rate meter with a variable time constant and an external scintillation detector unit containing a sodium iodide crystal and photomultiplier tube. The scintillation detector is preferred to the Geiger counter because of the large sensitivity range. Scintillators are good up to 15,000 counts per second; GeigerMiiller tubes do not perform well above 1000. As scintillators are much more sensitive than Geiger counters: smaller and safer sources can be used with them. A three-man team can operate this equipment rapidly and efficiently. Oneor two-man teams have been used at a sacrifice of speed. One man carries the source and holds it in position on one side of the pipe under measurement. A second man holds the scintillation detector on the opposite side of the pipe, while the third man views both the source and detector to be sure that they are in line. The third man also records data. Sediment in Vapor Lines. The portable equipment is also useful in measurement of the amount of sediment in vapor lines. Over a period of rime, sediment forms in the overhead vapor lines from distillation towers. In the past, the sediment has been detected by pressure drop in the line, although this is not entirely satisfactory. By making two density measurements with the portable equipment on a horizontal vapor line, the amount of sediment can be determined. One density measurement is made on a vertical diameter and a second on a horizontal diameter of the pipe. When these measurements are equal, the sediment in the vapor line is essentially zero. When the vertical measurement shows a higher density than the horizontal, the difference in density can be calculated to reveal the extent of the sediment within the pipe. By making several measurements along level or inclined vapor lines, the extent' of the: sediment deposit can be determined prior to shutdown of the equipment for cleanout. For example, using a 5-mc. source of iridium-192, several %inch vapor lines were surveyed
NUCLEAR TECHNOLOGY prior to cleaning. O n lines where sediment was detected, the maintenance personnel found exactly the amount of sediment predicted by the measurement of gamma ray density. Unnecessary work was saved by not opening other vapor lines, which ordinarily would have been suspected of containing sediment. The portable gamma ray equipment has also been used to measure level in knockout drums, catalyst hoppers, and similar places where the normal level gages failed to operate and answers were needed quickly. Future Possibilities. Petroleum refiners of today consider gamma ray absorption equipment one of their most valuable tools. For the first time, refiners see into operating pipes and vessels. Where will this technique lead in the future? I t should be possible for a refiner to employ a technique such as telefluoroscopy. Today, radiographic inspection produces on photographic film a still picture of the object between the radioactive source and the film. This technique can become rapid enough to provide motion radiographic pictures. The operator in the control room of a refining process will be able to look a t his television screen and see the conditions a t the top of the reactor bed, observe certain key trays on distillation towers, and observe other essential points in processing equipment where erosion, corrosion, or stoppages may be interfering with operation.
Tracer Applications in Reflning I n general, use of radioisotopes as tracers involves incorporating the isotope directly into the process fluid. Many complex problems can be analyzed through this technique. The simplest problem is measuring the fluid flow through a pipeline; the more complex problems are typified by studies of mixing kinetics and material balance. For tracer experiments, extreme care is exercised in selecting the proper radioisotope. Gamma emitters are usually preferred in refining work, because they can be readily measured and detected. Secondary considerations are the chemical properties. Sometimes it is necessary to use a specific element, and therefore the choice is highly restricted. This is further complicated because all elements d o not have gammaemitting isotopes. Sometimes a suitable element can be selected from the same chemical family-for example, in tracing sodium sulfate in sulfuric acid substitution of rubidium for sodium may be perfectly legitimate. If the sulfate tracing test could be completed in a short time, 15-hour sodium-24 would be preferred. For longer test
periods, 19-day rubidium would be convenient. Safety considerations are also important. The product reaching the consumer must be safe from the standpoint of radioactivity. Where appreciable amounts could appear in the finished product, the refiner must use only the short-life isotopes. I n general, when the abnount of radioisotope employed in tracer tests is stored 10 times its half life, the activity level has been reduced to practically zero. With many available isotopes this storage time is reasonable. When longer lived isotopes 'are employed, the activity level of products should be diluted to an extremely low figure. Batch Mixing. Iodine-I32 (half life 2.4 hours) has been used to determine mixing efficiency of different types of kettles in the grease plant (Figure 3). Kettles designated as K-1 and K-6 were selected as representing the two types of agitation. K-1 was an old style kettle with a single-motion paddle running a t a fairly high speed of 32 r.p.m. K-6 was a modern kettle, with counterrotating paddles a t 28 and 14 r.p.m. T h e iodine132 tracer was injected a t the top of the kettle and the amount of radiation a t the bottom was measured with a colimated scintillation detector rate meter and recorder. When the radiation dropped off at a constant slow rate, indicating decay of iodine-132, the experiment was considered concluded and that point was taken as the complete mixing time. K-6, the kettle initially studied, had an indicated mixing time of approximately 40 minutes when agitated with the paddles only. I n actual operation, however, paddle action alone is not always depended on for mixing. I n tests employing paddles and a filling pump circulating the grease a t a rate
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Figure 3. Mixing times of two types of grease kettles. K-1, although of older style, is superior. Curves were obtained from a recording scintillation detector focused at the bottom of the kettle when tracer was injected at top at time zero
of 300 pounds per minute the mixing time was reduced to about 15 minutes. In comparison, K-1 kettle with the filling pump circulating required only 4 minutes to mix the grease. As a result of this study, K-6 kettle is used only for cooling and storage of grease, as K-I can handle the batch mixing requirements. Kettle K-1 is approximately 30 years old, while kettle K-6 is of fairly recent design (see Figure 3). Iodine-132 is available as a daughter product of tellurium-132, which has a 77-hour half life. Tellurium-132 dioxide is conveniently packaged by the Braokhaven National Laboratory in unfits called iodine generators. To obtain the iodine a t any time free of the tellurium, the tellurium is dissolved in caustic and the tellurium dioxide reprecipitated with acid. The tellurium dioxide is filtered and the iodine-132 appears in the filtrate as potassium iodide, potassium iodate, and a small amount of free iodine. These products are water-soluble and can be used for tracing water-base reactions. If a hydrocarbon-soluble iodine is desired, the iodide and iodate are converted to iodine by reaction with peroxide in acid solution. The free iodine will readily be absorbed in hydrocarbon solutions. The chemical versatility and short half life of iodine-132 make it well suited for many tracer experiments. Continuous Mixing. The above mixing experiment is typical of what can be done in a batch process. I n the case of a continuous mixer, the problem is somewhat more complicated and it is necessary to take or monitor samples continuously to determine when mixing is progressing satisfactorily. This can be illustrated by a test made to determine whether an old style jet injection device provided better back-mixing in a n alkylation process than a jet tray of more recent and cheaper design. In the alkylation plant, isobutane, alkylate, and acid emulsion were circulated past jets which injected olefin concurrent to the emulsion stream. Rubidium-86 was use$ in this test. Rubidium is a gamma emitter with 19-day half life and very soluble in sulfuric acid. During a turn around, a jet tray of the older type was replaced with the new type and sample taps were placed in the emulsion stream above the new tray. A similar series of sample taps was provided above a n old style tray. Figure 4 shows the tracer mixing test layout. A measured quantity of rubidium-86 dissolved in sulfuric acid was injected downstream of the sample tap feeding the scintillation detector. The only way the radioisotope could be detected by the scintillation counter was through the backmixing from the injection VOL. 50, NO. 2
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The activity level of the tagged heating oil was about four disintegrations per gram per minute, only a little above background. A long counting time on low level gas counting equipment was required to detect the tritium. This test showed that the quarry was not leaking oil to the surrounding area. The tagged heating oil came in handy in locating a leak in the pipeline leading to the quarry.
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Conclusions and Summation
Figure 4.
Radioactive tracer mixing test layout
Used to measure back-mixing efficiency of jet trays in an alkylation plant. RbS6 was injected downstream from continuous sample-withdrawing lines. A jet tray of new and cheaper design was equivalent to more expensive jet trays of old style
point countercurrent to the emulsion stream. The general conclusion of the test was that both types of trays permitted essentially 100% backmixing in the reactor space between jet trays. All other things being equal, the trays of newer and cheaper design are preferred for simplicity of maintenance and lower cost of installation. T h e real advantage of the radioisotope technique was the simplicity of making a side by side comparison of the two trays; only one experimental tray was manufactured for the test If the performance of the new type of tray were to be measured by performance of the alkylation plant, almost all of the oldstyle trays would have had to be replaced with the new style trays before differences in alkylate quality could have been observed. Fluid Solids Tracing. Occasionally in the operation of fluid units it becomes desirable to measure catalysts inventory by means independent of differential pressure. This can be done by injecting a known amount of tracer in the catalyst and measuring the diluted concentration after equilibrium. Thus, the circulating inventory of fluid catalyst can be determined. Barium-I40 (12day half life) is a convenient isotope for this purpose. Barium can be impregnated on catalyk in the laboratory as barium chloride. Tests show that even the high temperatures in a catalyst regenerator will not strip the barium or its daughter lanthanum from the catalyst. Barium-140 was used to measure the circulating inventory of catalyst in a fluid hydroformer, toward the end of a long run to confirm the suspected condition of the unit, where nonfluid catalyst may have been built up. When the unit was down, a volume of catalyst was found equal to that predicted by the radioactive test, After the unit was repaired, 'the performance of the new parts a confirmed quickly by using this type
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of radioactive tracer test. Rubidium-86 has been used with equal success. Fluid solids flow rates can be conveniently measured with isotopes of very short half life. Barium-I37 has a half life of 2.6 minutes and can be obtained by milking cesium-137: which has a long half life. The Tracerlab Co. makes a n ion exchange extraction injection device which quickly strips barium-137 in a Versene solution from the parent cesium-137. This device has been used to measure the flow rate of a fluidized solid, where normal techniques such as heat or carbon balance could not be used. Detectors were placed a t two points on the solids transfer line. T h e extraction injection device was operated to introduce a solution of barium directly into the flowing solids. As the radioactive solids passed each detector unit, the time of travel over the known distance was measured. The flow rate could be measured about ever! 20 minutes, as the radioactivity of the previous test had died out in this period of time, and a new batch of barium-I37 had been generated. Reservoir Leaks. To meet the problem of summer storage of heating oil, the Esso Standard Oil Co. has acquired some abandoned slate quarries to serve as summer oil reservoirs. Theoretically, oil should not seep out of the quarry because the local water table is above the oil level. Before large amounts of oil could be stored in the quarry, a test was necessary, to be certain that the oil would not seep into the ground through the rock strata and contaminate the area, T o detect possible leaks, a n experimental batch of heating oil was tagged with tritium (half life 12.5 years). Tritium was chosen because it will not separate from the oil if the oil percolates through the rock strata or ground. Some high boiling olefins were hydrogenated with a mixture of tritium and hydrogen. This was then blended into a large quantity of heating oil and shipped to the stone quarry.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Radioisotopes are a valuable and versatile tool in petroleum refining. Miith a small group of people trained in their use, refiners can expect very substantial savings, which depend on the magnitude of the radioisotope program. The simplest program-with a few sealed gamma ray sources a t the millicurie level and a portable Geiger or scintillation detector costing approximately $1000-would be valuable for troubleshooting. Applications include density measurements in catalyst plant startups, finding stuck go-devils used in cleaning lines, measuring sediment in vapor lines, checking solid and liquid levels in hoppers and knockout drums, and in special cases observing erosion of internal insulation. A moderate program, including the use of tracers, density apparatus, and radiographic equipment, would carry an equipment cost between $3000 and $6000. Experience has shown that these expenditures can pay for themselves at least ten times per year. Literature Cited
(1) Bartholomew, R. N., Casogrande, R. hf., IND.ENG.CHEM. 49, 428 (1957'1. Reerbower, -A,, Forster, E. O., Kolfenbach, J. J., Vesterdal, H. G., I6id.,49, 1075 (1957). Fries, B. A , , Hull, D. E., Jones, S. B., "Applications of Radioactivity in Petroleum Technology," 4th World Petroleum Congress, Rome, 1955. ( 4 ) Handlos, A. E., Kunstman, R. W., Schissler, D. O., IND.ENG.CHEY. 49, 25 (1957). ( 5 ) Kinsella, A. J., Jr., Mitchell, J. J., Petroleum Proc. 10, 1718-23 (November 1955). ( 6 ) Kunc, J. F., National Ind. Conf. Bd., 4th ,4nn. Conf. Atomic Energy in Industry, New York, 1955. ( 7 ) McMahon, R. E., IND.EKG. CHEM. 47, 844-5 (1955). ( 8 ) Singer, E., Todd, D. B., Guinn, V. P., Ibid., 49, 11 (1957). ( 9 ) Todd, D. B., Wilson, W. B., Did., 49, 20 (1957). (10) Wilson, W. B., Good, G. M., Deahl, T. J . , Brewer, C. P., Appleby, W. G., Zbid.,48, 1982 (1956). \
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RECEIVED for review April 23, 1957 ACCEPTED July 25, 1957 Division of Petroleum Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, 131st Meeting, ACS, Miami, Fla., April 1957.
