ALAN BEERBOWER Esso Standard Oil Co., Baltimore, Md.
E. 0.FORSTER, J. J. KOLFENBACH, and H. G. VESTERDAL Products Research Divisions, Esso Research and Engineering Co., Linden, N. J.
Evaluation of Mixing Efficiency of Processing Equipment Radioactive iodine is the tracer in a method for evaluating the mixing efficiency of process equipment in a short time, at little cost, and without permanently contaminating the products studied
IN
THE production of blends of oils, asphalts, waxes, or greases, a high degree of uniformity is desirable. Although it is relatively easy to blend components of low viscosity, mixing becomes increasingly difficult with materials of high viscosity, such as greases, gels, or other semisolid products. I n m a k h g such materials, it is essential to disperse the ingredients as uniformly as possible, to ensure homogeneity of the final products. This high degree of mixing is desirable when the ingredients react to form the final product or when the dispersion is the final product itself. To make certain that uniformity is obtained, the mixing efficiency of the blending equipment available must be thoroughly known. I n many cases, very little is known of the mixing efficiency and the sampling techniques used to determine uniformity are often inadequate and time-consuming, and, therefore, interfere with production schedules. Oil-soluble dyes have been used occasionally for this purpose. The major drawback with such a technique was the large number of samples that had to be analyzed, generally by destructive testing methods, and the dyed product could not be marketed. I t appeared desirable, therefore, to find a tracer that would permit rapid, nondestructive testing without permanently contaminating or impairing the product. With radioactive iodine-1 32 becoming readily available, this material can be used as a tracer to
provide detailed information about mixing in various types of blending equipment.
Method A perfect mixture is defined as one in which each component is uniformly distributed throughout the mixture. If small amounts of a radioactive tracer
2.33 hours. A method has now been devised by the Brookhaven National Laboratory to provide this isotope, in spite of its short half life, at considerable distances from a nuclear reactor (2). This method makes use of the parent-daughter relationship that exists between tellurium-132 and iodine-132, as shown below: fission
---+
U235
P-, Y Te132
+ other products
P-, Y
+1132
T I / Z= 77 hr.
material are added to a system composed of several components which are to be mixed, the uniform distribution of the radioactive tracer among the other constituents indicates the completeness of the mixing operation. The time required to reach this state of uniform distribution reflects the efficiency of the mixing equipment. The progress of the distribution of the radioactive material can be followed either by measuring the fluctuations in the radioactivity ofonevolume element inside the blending equipment, or by determining differences in the radio activity of individual samples taken a t random from the mixture. I n either case, complete mixing is indicated by the disappearance of fluctuations or differences in radioactivity. Until recently, iodine-132 was not readily available to industry as a tracing material, because of its short half life of
Te'32
Tl,2
+ Xe132(stable)
= 2.33 hr.
The generator provided by the Brookhaven National Laboratory contains solid radioactive tellurium dioxide, which contains an equilibrium amount of radioactive iodine-132. When tellurium dioxide is dissolved with aqueous sodium hydroxide, the iodine-132 is liberated and remains in solution even after the tellurium is reprecipitated by the addition of acetic acid. T h e solution containing the radioactive iodine can be removed from the generator and used in the tracing equipment. T h e remaining tellurium dioxide will decay and build up within 12 hours a maximum amount of new radioactive iodine-132. The details of this operation and the construction of the generator have been given ( 7 ) . The decay of iodine-132 to the stable isotope of an inert gas (xenon) makes the use of this radioactive material even more attractive. After 24 hours, less than VOL. 49, NO. 7
JULY 1957
1075
l/lOOOth of the original radioactivity remains. Under normal conditions this would not be detectable, and the material studied can, therefore, be further processed or shipped to customers without difficulty. T h e amounts of radioactive iodine used depend on the type of counting equipment and the expected length of the mixing processes. In general, 2 to 3 ~ c per . pound of material provides adequate counting with a Geiger counter in counting individually removed samples. If, instead of a Geiger counter, a scintillation counter is used, 0.5 to 1.5 pc. per pound is adequate for a mixing process requiring as much as 40 minutes for completion. These are obviously average figures, and deviations are desirable if longer periods of monitoring are necessary or if the construction of mixing equipment requires high intensities for adequate counting. T h e radioactive material can be added as either an aqueous solution or
an organic solution. If addition of small amounts of water is not objectionable or addition of water is part of the normal procedure, the aqueous solution obtained when iodine-132 is removed from the generator can be used directly. If it is desirable and necessary to exclude water entirely, the radioactive iodine is extracted by treating the aqueous solution with equal volumes of 307, hydrogen peroxide; 80% of the radioactive material can be extracted by using benzene as the extracting material. I n the absence of hydrogen peroxide, extraction of the aqueous solution with benzene yields only 5yo of the radioactive material. This indicates that the radioactive iodine is essentially present in form of iodide and iodate ions in aqueous solution and only to a minor extent as molecular iodine. T h e extracted iodine in benzene solutions can then be used as water-free solutions of the tracer material. The method can be used successfully
3,000
\
2,500k
I TEST SAMPLES
(FROM SURFACE OF KETTLE)
DECAY RATE
/
-
2
L a 1.500y
I
W Q
-1
i
MIXING
COMPLETE
-
W I
c z
3
8
TIM E-M I N UTES Figure 1.
Mixing studies on grease in laboratory kettle
INNER PADDLES
OUTER I PADDLES
PROBE -PROBE
K6 14 OUTER W M : 28 INNER RRM. 15,000 LE.
KI
32.6 R.f?M. 25,000 LE. Figure 2.
1076
Grease plant kettles
INDUSTRIAL AND ENGINEERING CHEMISTRY
a t temperatures as high as 150' C. without concern as to evaporation of iodine-132. Experiments have shoiz-n that little radioactive tracer (less than 5y0jwas lost in mixing a material for 1 hour a t about 150' C. Such small evaporation losses are immaterial, as one is concerned only with fluctuations i n radioactivity during the mixing period rather than absolute intensities. T h e only precaution necessary in working a t high temperatures is that vapors emanating from the mixture must be adequate1)vented. Usually, such ventilation already exists for high temperature work. The extent of mixing can be folioiz-ed by either of two counting techniques. I n one case, small samples are taken a t random from the blending equipment, and the radioactivity is measured in a Geiger counter, proportional counter, or scintillation counter. The counts of the samples corrected for decay loss are compared, and differences in counts beyond the experimental error are attributed to incomplete mixing. In the other case, use is made of the fact that the radiation emitted by iodine-132 is hard and can he detected readily through steel walls 1, 8 to 0.25 inch thick. A scintillation counter can therefore detect radiation derived from iodine-132 outside the mising vessel. By combining the scintillation counter with a rate meter and recorder, a tracing of the fluctuation in radioactivity is obtained as the mixture passes by the volume element monitored by the scintillation counter. 4 s mixing progresses, the counts received b!- the scintillation counter fluctuate less and less and eventually become uniform. In both cases, complete mixing is defined as the time elapsed until differences or fluctuations in radioactivity disappear, and the activity of the samples decreases according to the decay rate of iodine-132. Because tellurium 132 is a by-product of uranium fission and readily available in large amounts, iodine-1 32 is relativelJcheap and readily available. The generator supplied by the Brookhaven Laboratory provides enough iodine-I32 to permit mixing studies for 1 week to 10 days; hence is an extremely attracti1.e tool for evaluation of both laboratory and plant operation. Safety. Because of its short half lifee. health hazards attached with the operation are small, and safety precautions do not interfere seriously with continued plant operation. Precautions and safety equipment are those commonl>used in radiation work as defined by the regulations of the Aromic Energy Cornmission. The immediate vicinity of the mixing vessel must be roped off IU prevent unauthorized people from approaching the contaminated area during the measurements. Health officials present when these experiments were performed found no objection to the use of this isotope in the described manner.
M I X I N G EFFICIENCY Short discussions with the personnel involved with operation of the blending equipment prior to the experiment educate them and assure them that their health is not endangered. Monitoring the area surrounding the mixing equipment indicated that no radiation was detectable 10 feet from the kettle in any of the pilot plant and plant experiments. The radiation intensity 5 feet from the kettles or tanks was less than 2 mr. per hour and hence within safety limits. Monitoring the mixing equipment 24 hours after the termination of experiments indicated no detectable radiation outside the kettle walls and less than 2 mr. per hour from the equipment containing the original iodine-132 solution. The lubricants produced in the plant experiments showed less than 0.05 mr. per hour at the surface (including background) after 48 hours' storage and were shipped as previously agreed upon with the U. S. Atomic Energy Commission.
c
W
$ J W
a
Y
CI RCU LAT I O N
W I-
3
zI a n.
KETTLE+ EXTERNAL CIRCULATION 300 LB. / MIN. PUMP
W
v)
c z
3
8 IO
0
20
40
30
TIME IN MINUTES Measurement of mixing efficiency in grease plant kettles
Figure 3.
Results rr)
i
i
Laboratory. T h e mixing in a 5gallon steam-heated kettle was evaluated using counteracting paddles revolving at 60 r.p.m. This kettle was charged with 22 pounds of grease (corresponding to three fourths capacity), and 30 ml. of an aqueous solution containing 0.5 mc. of iodine-132 was added. Five-gram samples of grease were removed at random after 1, 2, and 3 minutes' mixing and thereafter a t intervals of 20 seconds. Their activity was measured with a standard Geiger counter; complete mixing occurred after 4.8 minutes. Two subsequent runs with the same material under the same conditions reproduced the mixing time to within 0.1 minute. The mixing efficiency of a second kettle of similar but not identical design and equal capacity was evaluated a t another laboratory. The charge was 25 pounds of the same grease, and 30 ml. of an aqueous solution containing 60 pc, of iodine-132 was added. The counteracting paddies revolved at 35 r.p.m. in both directions. The progress of mixing was followed by sampling the top layer in the kettle and by scanning the kettle bottom with a scintillation counter. Both techniques indicated complete mixing in 3.0 minutes. A tracing of the rate meter recording is shown in Figure 1. The shorter mixing time observed with this kettle was attributed to the slightly different paddle design. The mixing efficiency of a third kettle of markedly different design was also determined. This unit had a capacity of 15 gallons and used single motion paddles. Twelve gallons of cutback asphalt blend were stirred while an oil
I
0 X
ND OF ADDITION OF 1132 "0
I
2
3
4
Figure 4.
5
6 7 8 TIME IN MIN.
9
1 0 1 1 1 2
Mixing efficiency of kettle K-2
9500 pounds of modifled wax; 2 1 r.p.m.; 100' C.; approximately 25 mc. of iodine-I32
120
1
I
I
I
I IO 100 0 I
0 X
DECAY CURVE
90 80
I
70
OF
I"?
60 50 4 Z
TIME Figure 5. 2000 pounds of lime soap base;
IN MI".
Mixing efficiency of pressure cooker 0.5 r.p.m.;
100-150° C.;
approximately 3 0 pc. of i o d i n e 1 3 2
VOL. 49, NO. 7
JULY 1957
1077
Table 1. Paddle
Evaluation of Mixing Efficiency of Grease Kettles Complete Grease Temp., Charge, Mixir g Amount of Speed, O c. Lb. Time, Min. Tracer, Mc. R.P.M.
Test Laboratory
60
25
22
Pilot Plant
43 23 23 23 43
25 25 25 94 94
200 200 200 200 83
solution of radioactive iodine-1 32 was added; 100 pc. were used in 0.2 ml. of naphtha. Complete mixing required nearly 20 minutes. Pilot Plants. A pilot-plant kettle with a total capacity of approximately 100 gallons had double action paddles (similar to those used in the laboratory kettle) operating at lower speeds, ranging from 23 to 43 r.p.m. A scintillation counter rather than a Geiger counter was used for counting with a 30-gram sample, instead of the 5-gram samples used in the laboratory. Thus, the concentration of the tracer could be kept lower because of the larger sample and the higher efficiency of the scintillation counter. The experimental conditions and results of five runs in this pilot kettle are shown in Table I together with those for the laboratory tests. They indicate that a n inverse relationship exists between the time required to reach complete mixing and the speed of the paddles as well as the temperature. Further studies are in progress to determine the nature of this relationship. Plants. Various blending operations in different plant equipment were followed, including a 15,000-pound kettle with double action stirrers. 25,000- and 10,000-pound kettles with single action paddles, a pressure cooker, and a 30,000-gallon tank with agitation provided by air jets. The arrangement of the stirrers in the first two kettles is shown in Figure 2. I n the 15,000pound K-6 kettle with outer and inner paddles operating a t 14 and 28 r.p.m., respectively, the same grease used in the laboratory and pilot plant was mixed. T h e 25,000-pound K-1 kettle was used to study a similar grease a t 33 r.p.m.
4.8
0.54
12 18 18 8 5
0.59 0.57 0.56 0.58 0.70
paddle speed. I n the 10,000-pound kettle, K-2, the preparation of a modified wax product was followed a t 21 r.p.m. I n the pressure cooker, mixing and cooking of a lime soap concentrate were measured, and in the air-blown tank the formation of an emulsifier concentrate was studied. These examples represent materials ranging from stiff semisolids to slightly viscous liquids. The details of experimental conditions are shown in Table 11, with the approximate amount of tracer used and the counts per minute measured a t the time of complete mixing. I n all cases, the counting was performed through the walls of the blending equipment with a scintillation counter connected to a rate meter and a Speed-0-Max recorder (Leeds & Northrup). Typical tracers of the progress of mixing obtained from the recorder are shown in Figures 3 through 5. In Figure 3, the progress of mixing is followed, and the curve is smoothed out, because reduction of the time scale was necessary for reproduction purposes. The experiments are also reduced to a common scale of activity. Complete mixing was obtained in 42 minutes in kettle K-6, using the paddles alone for mixing purposes. As blending operations occasionally require an assist by external circulation, in another experiment a n external pump was used to withdraw grease from the bottom of the kettle and pump it back into the top. Under these conditions, the mixing time was reduced to 16 minutes, with the same grease. This indicated that the combination of external pump and paddles was more effective than the paddles alone. Also shown in Figure 3 are the results obtained in kettle K-1. Only 4
minutes were required for complete mixing in this kettle, with a similar pump for external circulation. The slower paddle speed employed in kettle K-6 probably accounts for the poorer mixing efficiency noticed in that kettle. Only 8 minutes were required for mixing a similar amount of the fluid material in kettle K-2 provided with a single paddle action (Figure 4). I n this case, mixing progressed during the addition of the radioactive tracer. As the addition took 4 minutes, the starting point was selected a t the halfway point of this operation. In Figure 5 is shown the mixing progress as observed with the pressure cooker, a closed circulating blending system in which the entire content is recirculated by an external gear pump once every 2 minutes. The tracing is different, because passages of the unmixed material are recorded in rapid succession. Under these conditions, complete mixing is believed to be reached when the recorded fluctuations are of the order of magnitude of the statistical fluctuations of the counting equipment and their average value decreases according to the decay rate of iodine-132; 6 minutes were required in this case to obtain uniform blending. This result confirms the efficiency of recirculation as indicated in kettle K-6 when using external pumping.
Acknowledgment The authors wish to thank the members of the staff of the Brookhaven National Laboratory for their advice and help in developing these test methods.
Literature Cited (1 ) Brookhaven National Laboratory, Upton, N. Y . , “The Hot Laboratory, Iodine-I32 Generator” (May 1955) (revised procedure September 1955). (2) Stang, L. G., Jr., Tucker, W. D., Banks, H. O., Jr., Doering, R. F., Mills, ‘I?. H., iVucleonics 12, 22 (1954). RECEIVED for review October 5 , 1956 ACCEPTEDJanuary 4, 1957 Division of Industrial and Engineerinq Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
Evaluation of Mixing Efficiency of Plant Blending Equipment Penetration Time Amount Unworked for of Mm./lO at Complete Mixing, Tracer, Charge, Temp. Circulation Temp., Mc.~ Indicated Min. Lb. O c. Lb./Min. Product Table II.
Paddle Speed,
Kettle
R.P.M.
K- 1 K-2 K-6 Pressure cooker Tank 333 a
33 21 14-28 14-28 0 Air blown
300 0
0 300 1,000 0
Grease Modified wax Grease Grease Lime soap base Emulsifier concentrate
Approximate.
1078
INDUSTRIAL A N D ENGINEERING CHEMISTRY
75 100 66
70 94-150 72
25,000 9,500 10,000 10,000 2,000
+ 100,000
c,P.M. at Time of Complete Mixing‘L
360 Fluid 340 340
...
4 8 42 16 6
60 25 80 15 30
12,000 15,000 15,000 3,000 60,000
Liquid
10
70
5,000
