Ind. Eng. Chem. Res. 1992,31,959-967
959
Formation of Uniform Liquid Drops by Application of Vibration to Laminar Jets? Paul A. Haas Chemical Technology Division, Oak Ridge National Laboratory, Post Office Box 2008, Building 4500N, Oak Ridge, Tennessee 37931 -6268
Gel-sphere processes require the formation of liquid drops of controlled and uniform diameter with production rates of lO3-10' drops/min. In this report, procedures and equipment are described that apply vibration to give a controlled breakup of laminar liquid jet streams into uniform drops. Criteria are also given for the selection of jet diameters, jet velocities, and vibrational frequencies and for the configuration of the apparatus to produce drops of 500-5000-pm diameters. In this work, the standard deviations of product sphere diameters (based on the log-normal size distributions) were 1.01-1.05.
Introduction The preparation of gel spheres by processes developed at the Oak Ridge National Laboratory (ORNL) and elsewhere is based on the conversion of liquid drops into solid gel spheres of hydrated metal oxides. Each liquid drop must contain the amount of metal needed in one product sphere. The feed liquid may contain colloidal oxides, metal salts,organic polymers, metals in complexed or hydrolyzed form, or other chemicals. The diameters of the drops must be controlled, predictable, and uniform in order to produce product gel spheres with uniform diameters and properties. Generally, diameters of the dense product spheres are one-third or less of the initial drop diameters. Depending on the mechanism of gelation, the liquid drops must be dispersed into trichloroethylene (Haas et al., 19801, 2ethyl-1-hexanol(Haas and Clinton, 1966,Haas et al., 19671, silicone oil (Bischoff et al., 1974), NH3 gas or solution (Zimmer et al., 19781, or other fluids. Uniformity of drop diameter is an important factor. Drops of 5W5000-pm diameters, produced at a rate of 103-107 drops/min, are required for the practical production capacities of 1-10 kg/h of gel spheres of 1501500-pm diameters. Equipment and procedures to meet these requirements were developed at ORNL as a part of the gel-sphere process development studies. The methods and apparatus that have given the most useful results at ORNL are described in this paper. An alternative technique using turbulent shear fields has been used to produce smaller drops at practical capacities, but it gives much less uniform product sphere diameters. This method has been reported separately (Haas, 1987). The gel-sphere products formed as discussed in this paper are usually excellent, strong spheres with ideal properties for size distribution measurements. The metal concentrations in the product can also be easily measured, which allows an easy and accurate calculation of the drop diameters to be made from the product diameters. Testing can be done by complete batch or by representative sampling. Therefore, many of the uncertainities involved in the measurement of liquid drop diameters within a second liquid are avoided. However, the possibilities of coalescence or of secondary dispersion before solidification occurs must be considered. The gel-sphere equipment used in this study was operated for hundreds of hours to prepare nuclear fuels; this 'Research sponsored by the Office of Facilities, Fuel Cycle, and Test Programs, U.S. Department of Energy under Contract DE-AC06-84OR21400 with the Martin Marietta Energy Systems, Inc.
operation was an excellent test of the reproducibility and dependability of the equipment and procedures.
Drop Formation from a Laminar Liquid Jet A liquid jet discharged from a small opening at viscous or laminar flow conditions will tend to break into short lengths, which then form spherical drops. This behavior results from surface energy, or interfacial tension. The most probable jet length between break points is about five times the diameter of the jet streams, and this length gives a spherical drop diameter about twice that of the jet. The liquid interfacial tension, viscosity, and density affect the time required for drop formation but do not affect the most probable length between break points or the average drop size. This behavior has been predicted theoretically and observed experimentally in both air and liquid media (Merrington and Richardson, 1947; Haas and Clinton, 1966). The jet mechanism of breakup will not occur for low flow rates of liquid that do not form a jet; instead, the drops will form at the opening according to the dropweight method for measuring surface tension. If the jet flow and breakup are not laminar (as a result of high flow rates or other energy inputs), the jet will break into much smaller drops than would be expected from the laminar mechanism. The laminar breakup of a jet, which results from the growth of small disturbances, is favored at lengths of five times the diameter. If the breakup is allowed to develop from random disturbances of the jet, there is a statistical distribution of breakup position to give a distribution of drop size. The random disturbances become larger and more variable as the jet velocity increases, causing the drops to be less uniform in size. As the liquid viscosity increases, the growth of a disturbance into a break becomes slower; therefore, highly viscous jets may require excessive times or distances before breakup and may not give uniform drops. A controlled and uniform vibration can be applied to liquid jets to produce the disturbances that control jet breakup. If the frequency of the vibration is near the natural frequency for laminar breakup, there will be good control of breakup, and the drops formed will be very uniform in size. This concept has been applied in a number of ways at ORNL and elsewhere and has been very useful and effective for the scaleup of gel-sphere proteases. This paper presenta the procedures, equipment, and results developed from application of the technique at ORNL. The instruments used for the formation of liquid jets and drops in relatively stationary gases (usually air) are called single-fluid nozzles, although they may use multiple
OSSS-5SS5/92/2631-0959$03.~0/0 0 1992 American Chemical Society
960 Ind. Eng. Chem. Res., Vol. 31, No. 3,1992
SEVEN-ORIFICE
NOZZLE)
POLYVINYL CHLORIDE TUBING (PERISTALTIC PUMP O U A L I T Y )
METERED BROTH FLOW- * ,
I
POWER SUPPLY
1
p
r
-
VIBRATOR 6
~EELN:~~
1
(SEE DETAIL)
1
SINGLE - F L U I D NOZZLE
GELATION
~~~~~~N
.:: .
1
ORGANIC
Figure 1. Diagram of a single-fluid pulsed nozzle for drop formation.
openings and jets. For these single-fluid nozzles, a distinction will not be made between the jet diameter and the i.d. of the capillary or orifice used to form the jet, although the jet diameter may be slightly smaller (from venturi effects) or slightly larger (if the jet is slowed by energy losses from wetting of the tip). These effects are not significant for the preferred ranges of drop formation conditions. Using a two-fluid nozzle, the jet and drop formation takes place within a second fluid stream. The velocity of this second fluid is usually a more important variable than the capillary or orifice diameter of a twcduid nozzle.
Vibration Control Apparatus For control of drop formation, a vibration is applied to produce a disturbance of controlled frequency in a liquid jet. Some regulation of the size of the disturbance (Le., the power input to the vibration) is also needed. Control of the frequency and power for an ac electrical power supply is an easy, convenient, and dependable technique. A variable-frequency oscillator and amplifier can be used to drive small electromechanical vibrators as a source of mechanical vibration-similar to an audio system without the diaphragm to produce sound. The arrangementa most useful for gel-sphere work at ORNL will be described; other arrangements will be mentioned and referenced. The power inputs needed for control of drop formation are small, so only the smallest of commercial vibration systems are needed. The system at ORNL consisted of the following units (from Alpha-M Corp. Dallas,TX): (1) 25-W solid-state amplifers for frequencies of 2-20 OOO Hz, Model OC-25, and (2) Model AV-6 vibrators (effective moving mass of 0.04 lb, 1 - 2 4 dc resistance, 2 4 impedance at 1 kHz, 2.5-A maximum current, 82-g acceleration maximum, 2-20 000-Hz useful range of frequencies). This type of vibration apparatus supplies a sine-wave ac potential, but the movement applied to the nozzle or broth will be out of phase and may have a less regular shape. The ac potential generates a force by the vibrator coil, but the movement is a compIex function of the mass
and the spring constants of the components that move. Electronic equipment can also be used to measure the vibration amplitude and shape and to supply more complex ac power patterns, such as square waves. Limited tasting of such electronics did not show any improvement in drop formation results, and use of the more complex equipment is not justified. A convenient and dependable procedure for applying the vibration to the liquid jet is to pulse the feed line using the arrangement shown in Figure 1. Results are no better than the best results with other arrangements, but the advantages of the pulsed feed tube are as follows: (1)The vibrator is mounted in a fixed position away from the column top and nozzle. The nozzle can be adjusted in position, removed for clean out, or replaced without moving or damaging the vibrator. (2) The vibration is much “cleaner” as the pulse on the jet is completely linear. With the nozzle attached to the moving element of the vibrator, complex nonlinear motions and satellite droplets are much more likely. (3) The flow channels are simple and easily cleaned. The arrangement shown in Figure 1is effective at low power inputs, and the tubing line connecting the vibrator to the nozzle can be as much as 6o-cm long without causing difficulty. However, gas bubbles in the line must be avoided because they can damp out the vibrationsparticularly those of high frequency used in forming small drops. A thin-walled, highly elastic (rubber) tubing gives poor performance, but the heavy-walled poly(viny1 chloride) tubing supplied for peristaltic pumps has given good results and no tube fatigue problems have been encountered. The optimum amount of pulse volume at the capillary tip is only a fraction of the drop volume. This gives a small reduction in the jet cross section which develops into a break between drops in a short distance. Large pulses to give drops at the capillary tip are likely to give satellite drops and poorer uniformity of drop size. Lightweight nozzles for formation of the liquid jets can be attached directly to the moving element of the vibrator so that the nozzle moves in-line with the jet. The two
Ind. Eng. Chem. Res., Vol. 31, No. 3,1992 961 Table I. Drop Formation with Single-Fluid Nozzles at High Jet Velocities (Equipment, Stainless-Steel Nozzles Attached to Moving Element of Vibrator; Feed, Ethylene Glycol from an Air-Pressured Pot) drop appearance (stroboscopic total flow calcd jet light), (drops/mimjet) calcd resultsb rate, velocity, cma/min nozzle tvDe cm/s oDtimum eood raneeo optimum D, pm Dld one-orifice 52 460 75 OOO 65000-120 006 1100 2.24 d = 490pm 95OOO 100000->130OOO 69 610 1110 2.27 710 area = 1.88 x cm2 90 135OOO 120000->135 OOO 1040 2.12 105 930 170000 140000->180OOO 1060 2.16 133 D > 1.55d). However, better results will be obtained by changing the orifice or jet diameter in order to achieve conditions nearer to those for natural breakup; the jet diameter should be about half the intended drop diameter. Both the observations of drop formation by use of a stroboscopic light and the measurements of gel-sphere size distributions indicate that the best values for D l d range from -2.05 (for 600-pmdrops) to 2.3 (for 5000-pm drops). At the jet velocities required for drop formation, the gravitational acceleration is not significant for short (1or 2-cm) jet lengths but is easily observed for longer jet lengths. The jets are nearly always directad at a downward or horizontal angle, so the drops are accelerated by gravity to give a wider spacing between drops. The deceleration from air resistance has lesa effect than gravity. Jets formed in a liquid medium are very rapidly accelerated to the liquid velocity, as described later. A series of tests were performed to observed drop formation in air at very high jet velocities and high vibration frequencies. In these tests, ethylene glycol was fed to orifice-type nozzles mounted directly on the moving element of the vibrator. When observed by stroboscopic light, the stream of drops appeared motionless and clean in outline at the optimum frequencies and showed flickering and blurring at frequencies higher or lower than optimum. While no procedure for gelling the drops from these high jet velocities was found, it is believed that the drop sizes were controlled and uniform at these optimum conditions. The results showed good drop formation (judged by visual observation) for jet velocities as high as 1180 cm/s (Table I). The calculated ratio of drop diameter/orifice diameter (Dld) for the best frequency at each jet velocity reproduced well for each nozzle. The smaller nozzles had orifices of noncircular shapes and/or rounded or burred edges so that the values of d were difficult to measure. This uncertainty has probably biased the calculated D l d values in Table I. The orifice of 490-pm diameter gave
180 160
5
1
700 1000
2000
4 '
' 4i
4 DROP DIAMETERS ( p m )
k '
c
3500
%,
REGION OF COALESCENCE AND/OR ORGANIC OCCLUSIONS I N DROPS
140
EXCESSIVE DROP BREAKUP W J
N N
0
60
2
40
20
I
0' 0
J E T FORMATION
I
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I
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I
I
0.02 0.04 0.06 0.08 0.10 0.12
I
I
0.14
ORIFICE OR CAPILLARY DIAMETER
0.16 0.18
(cm)
Figure 2. Preferred jet velocities for single-fluid nozzles.
values very near the expected values of -2.2.
Introduction of Drops into a Liquid Medium In order to produced gel spheres of uniform size, the drops must be carried into the liquid medium without any loss of uniformity. This uniformity may be lost when a drop splits into smaller droplets, when two or more drops touch and merge into a larger drop, or when the broth drop encapsulates a small drop or organic and produces a spherical void inside the solid metal oxide gel. As a result of these problems, highly uniform drops in air can be degraded at the air-liquid interface to give less uniform gel spheres. Uniformity is thus affected by the jet or drop velocity, the viscosity of the organic medium, and the configuration of the nozzle tip with respect to the liquid surface. A right-angle impact on the liquid surface increases all three problems, as compared to anglea of 30-60°. Splitting or splattering are the controlling problems for large drops (larger than 2000 pm). Coalescence is more troublesome with smaller drops and/or higher viscosities for the organic medium. Occlusions of organic to leave voids in the gel are more common with intermediate drop sizes. Empirical equipment adjustments are generally the best way to obtain good size uniformity for a specific gel-sphere preparation. Examples from experimental operations are described in the final portion of this paper. The optimum jet velocities for drop formation will depend on the jet diameter and the liquid medium. High jet velocities tend to increase the difficulties described. Low velocities will give large drops without formation of a jet. Low velocities also result in lower capacities per jet. The optimum jet velocities for nozzles in air as a function of nozzle diameter are shown in Figure 2. It is generally best to operate near the optimum velocity for the nozzle (or drop) diameter needed, using multiple jets to achieve the desired capacity. The drop diameters at these optimum velocities would be about twice the nozzle diameters. Low jet velocities cause dripping problems and no jet formation and tend to allow plugging of some orifices in a multiorifice unit. For large orifices or drops, high jet velocities tend to cause breakup into smaller drops at the organic surface. After the drops are in the liquid medium,
the buoyant effect and the dampening of drop oscillations make splitting less likely. Therefore, 3000-4000-pm drops for an internal gelation process (Haas et al., 1980) were formed with the nozzle slightly above or below the liquid surface, so the jet penetrated the liquid surface before division into drops occurred. For the same internal gelation process with smaller drops, high jet velocities resulted in coalescence of the drops at the medium surface and/or organic occlusions within the drops. For organics with low viscosities (trichloroethylene or perchloroethylene), orientation of the jet at 30-45O angles to the liquid surface allowed good operation at the jet velocities shown in Figure 2. If a liquid jet is below the surface of the organic liquid medium when drop formation takes place, some problems that occur at the liquid-gas interface are eliminated. This procedure is useful for the formation of large drops in an organic medium of low viscosity. As the jet diameter decreases (or the organic medium's viscosity increases), the jet velocity decreases rapidly, so that controlled breakup into drops is not practical. A metered flow of the organic drive fluid can be used in a two-fluid nozzle to give a well-controlled laminar jet for drop formation (Haas, 1975). Since this organic flow must be laminar (turbulent flow will disperse the jet into small drops), higher organic viscosities will allow higher velocities of the drive fluid. Since the jet will be accelerated (or decelerated) to the organic velocity before breakup occurs, the maximum jet velocity (and, hence, nozzle capacity) can be increased as the organic viscosity increases. This two-fluid nozzle technique is illustrated by the examples described later. The twefluid nozzles give useful production capacities using silicone oil or 2-ethyl1-hexanol as organic media, but such nozzles are limited to low jet velocities or capacities for trichloroethylene (TCE) or perchloroethylene. External gelation of drop surfaces can be achieved by reaction with NH3 gas. The spheres are then allowed to drop into an ammonia (",OH) solution where gelation is completed. Gelation of the surface by exposure to NH3 is extremely rapid, and the jet breakup into drops must be completed before exposure to NH,. To accomplish this, the jet is directed through a window, with drop formation taking place in an air-purged zone followed by gelation in a NH3-purged zone. Organic polymers are usually added to the metal salt solutions to strengthen the surface gel and prevent smashing a t the ",OH solution surface (Zimmer et al., 1978).
Fabrication of Multijet Nozzles Since the feed system is complex, the formation of uniform multiple jets from one metered feed flow is very important to the practical scaleup of drop formation. A single vibration source is generally applied to the multiple jets. If the jets differ, however, the vibration conditions cannot be optimum for all jets. The vibration frequency should equal the drop formation frequency; therefore, nonuniform flow rates to the different jets will result in varying drop diameters and poor uniformity. If one of the parallel orifices becomes plugged, the flow in the others will increase and subsequently yield larger drops. Any burrs, marred edges, or lack of circularity will change the jet and may prevent controlled drop formation. In this study, multiple nozzles fabricated by five different techniques were used. These nozzle types and their relative advantages and disadvantages are listed below. (1)Short lengths of capillary tubing from a common plenum produce jets (without dripping) over a wide range of velocities, but the capillaries plug easily, are difficult
Ind. Eng. Chem. Res., Vol. 31, No. 3, 1992 963 to unplug, and require high feed pressures. (2) Holes (orifices) drilled in thin-walled (0.25-in.-o.d.) stainless steel tubing show good resulta with orifice diameters >500 pm, but with smaller holes, any burrs or other irregularities limit the jet and drop uniformity. (3) A stainless steel sheet with holes is fabricated by a commercial supplier of spinnerettes to the textile industry. These holes, fabricated by laser drilling, were not significantly better than those made using small drills. (4) Holes drilled in small, machineable ceramic (MACOR) tubes were very uniform, with cleaner edges than those drilled in stainless steel. The ceramic is also less easily wetted by the aqueous feed, preventing large drips from forming as easily. The ceramic was fragile and seemed to lose strength during continued use. This may have been a result of chemical attack or fatigue from vibration. (5) Holes were drilled in thin stainless steel (as in item 2) and then electropolished lightly to smooth the edges of the holes. The electropolishing produced holes as smooth as those in the ceramic of item 4. Excessive electropolishing resulted in rounded holes and with larger jet diameters (thus larger optimum drop size) than the jet diameters expected from the drill size. This technique gives distinctly superior hole and jet uniformity as compared to the other nozzle types and durability superior to that of the ceramic in item 4. One unit of this type is shown schematically in Figure 1.
Examples of Experimental Results Selection of drop formation conditions for a specific gel-sphere process depends on compromisesfor minimizing the observed problems. The general criteria discussed in previous sections are used in selecting empirical adjustments. If the visual appearance with a stroboscopic light is clear (i.e., a stationary jet and a row of drops with no blurring or flickering), the drop formation is almost certain to be well controlled and uniform. A distance between drops of 1.5 times the drop diameter is a secondary test of good conditions. A smaller separation distance, with blurring or flickering of the drops, indicates a less regular breakup with conditions on the high-frequency or smalldrop-diameter side of the optimum. A larger distance between drops is likely to result in satellite drops and probably indicates conditions on the low-frequency or large-drop-diameter side of the optimum. Degradation of drop uniformity at the air-liquid medium interface can be observed by microscopic inspection of the gel spheres. If a fraction of the uniform drops coalesce so that two drops of diameter D form a larger drop, the larger drop diameter will be 2O.=D = 1.260. This effect is readily apparent from a product having spheres that are primarily two different sizes, with a few of intermediate sizes. Drop breakup from excessive impact will give a continuous, wide distribution of diameters smaller than the intended diameter. Occlusion of organic drops inside the gel sphere will be visible as small holes in the gel surface; the voids will generally be tangent to the surface rather than centered in the gel spheres. Each of the following examples is selected from a number of gel-sphere sample preparation runs where the results were reproduced repeatedly without difficulty. The remarks on possible difficulties and methods to avoid them are provided for completeness of information and do not indicate that the difficulties are probable or hard to avoid. A. Sample Calculations. The calculations of laminar jet conditions for the experimental samples are similar for all cases. Therefore, some calculations will be given and explained in detail, while only the results of calculations
-
or the exceptions may be noted for some individual examples. The desired product sphere diameter and concentration must be known. These, with the feed liquid concentration, are used for calculating a diameter shrinkage factor and the initial drop diameter. If the product is spheres of uniform size, then the bulk density (as determined by pouring slowly and tapping in a graduated cylinder) will be 0.62 times the particle density. That is, uniform spheres can be packed to leave 38% void volume. Shrinkage factors or the ratio of drop to product diameters, DID,, are calculated as follows:
- -- diameter shrinkage = DP
product concentration drop concentration For U02with a density of 10.96 g/cm3 and a concentration of 1.25 M U in the drop
D
(10.96)(1000)/270
-=[ DP
1.25
1
= 3.19
1
= 2.47
0.333
For Tho2with a density of 10.00 g/cm3 and 2.5 M Th in the drop D
(10.00)(1000)/264
DP -=[
2.50
0’333
For U03gel spheres of 1.40 g/cm3 bulk density and 1.3 M U in the feed
To prepare dense U02 spheres of 1200-pm diameters and with DID = 3.19, the drop diameter would need to be 3830 pm. !?or this large a drop, empirical results show an optimum D l d = 2.3. Therefore, the nozzle diameter should be -383012.3 = 1660 pm. The optimum jet velocity (from Figure 2) is -35 cmls. The liquid feed rate to the jet will be flow = (velocity)(area) = (35)
[
(0.166)21r
=
45 cm3/min (2)
The drop or vibration frequency is frequency =
flow drop volume
--
-
(45)(6) (0.383)3~ 1530 drops/min (3)
The U02 flow rate for one jet would be (45)(60)(1.25)(0.27) = 0.91 kg/h 1000
To form a feed of 1.0 (kg of U)/h as U03 gel spheres of 1.40 g/cm3 and 350-pm diameter, the DID, calculated above is 1.99. Then D =
(t)
= (1.99)(350) = 696 pm
d = D/2.1 = 330 pm
964 Ind. Eng. Chem.
Res., Vol. 31, No. 3, 1992
3600-pm WASHED GEL 2400-prn DRIED GEL 1200-pm UO? Figure 8. ORNL internal-gelation proceea yield8 large, uniform spberea which densify to give a high-quality UO?product.
The optimum jet velocity (from Figure 2) is -130 cm/s. The liquid feed rate to one jet will be flow = (velocity)(area)= (130)
(0.033)%
-
(60)= 6.67 cm3/min
Table 11. Siza Distribution Data for Formation of Intermediate-SiMdUO, Gel Spheres (Conditions: UO, Gel Sohe- Dried to 225 " C )
screen size range, wn >420 40(t420
Smce 1(kg of U)/h = 4.2 (mol of U)/h or 3.23 L/h of 1.3 MU, the number of jets = (3.23)(1000)/(60)(6.67) = 8.0. The drop vibration frequency is ikquency =
flow/jet drop volume
-
(6.67)(6) (0.0696)%
3 8 0
38000 drops/min
250-340 95% product yield, primarily because there is leaa distortion during gelation. Product evaluation for -400-run dried microspheres (-650-pm drops) is again
'000
I
800
pernutage Within stated d i m range 0
49.58 27.87 1.82
36W380 34W360
-
weight, g 0 O.wO2 0.073
0.003 0.009
62.40 35.08 2.29 0.13
0.104
I I I l l , , , I I I I LOW-VISCOSITY ORGANIC MEDIUM (TCE),o II HIGH-VISCOSITY ORGANIC. D = I . 0 2 2
I
=
1.022
600
L
2oo IO00.2
I 2
5
10
20 3 0 4 0 5060 70 80
90
FRACTION OF UNDERSIZE0 SPHERES (wt
95
95
95
99.8
Z)
Figure 4. Size distributions for intmmdiate-size drop diametm in two organic liquids.
performed using results obtained from a screen analysis. Sintering before screening is not necessary for this size, since they are much stronger than the -2500-pm dried spherea and will Withstand the rigors of screening without cracking or chipping. One RUI of -4-h duration (CGT129) produced 5567 g of dried product. A ceramic nozzle containing six orifices of -320-pm diameter was used to produce nominal 65O-wdroplets. A riffled sample of -60 g was taken from the dried product and then screen analyzed. The resulta of this analysis are prwnted in Table 11. From this tabulation, it is evident that >97% of the product was within the acceptable range of 340-380-pm diameter. Plotting these data as in Figure 4 shows a standard geometric deviation of 1.022. The size distribution data from use of a particle counter for sintered UOz spherea using a different type of single-fluid nozzle (Figure 5) also shows a standard deviation D M / D P= 1.022. C. Application for External Gelation with NBs Gas over ",OH Solution. Gelation of a drop surface by exposure to ammonia gas or aqueous NHIOH solution is
Ind. Eng. Chem. Res., Vol. 31, No.3,1992 965 500
-
450
-
400
-
350
-
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300 -
Y
250
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6
200
-
5z
150
-
6 0
m
400 50
0
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SAMPLE IO: I G T - 7 6 # l - A [ R R S ] MINIMUM DlAM i 2 6 5 p m MAXIMUM DIAM = 3 3 4 p m MEAN DlAM i 307.2pm STANDARD DEV. : 6.Spm NO. PARTICLES i 3 9 2 7
VI
? u
I
-
-
-
I
"
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structure for the hydrous oxide gels and gives a stronger shell that will keep its spherical shape through mechanical and chemical stresses. The apparatus and procedures developed to meet these external gelation requirements by Zimmer et al. (1978) were reproduced at ORNL to prepare Tho2and (Th,U)O. spheres. The drops, formed in air using a horizontal n d e , then entered a window into the NH, gas (Figure 6). The nozzle, together with the vibrator, could be moved into a downward position for startup to make it less likely that a drop on the nozzle tip would disturb the formation of the laminar jet. During startup operation, the startupshutdown funnel is in an upper position, to close the gelation box window and to collect the jet or drops from the nozzle. After good jet operation is established, the vibrating nozzle is brought to a horizonal position, and the funnel is lowered so that droplets enter the gelation box. T w o stainless steel tubes lead through the cover into the box. One provides suction for off-gas so no NH, will leave the box and cause plugging of the nozzle. This tube could be located outside the box, at the window. The second tube supplies the NH?gas needed for droplet gelation. Both tubes have a -tuning fork" configuration at the end. The off-gas tube has a number of holes along each leg of the fork, whereas the NH, gas supply fork has a thin slot in each leg. Typical run conditions for all gel-sphere preparation experiments were as follows: nozzle diameter, 0.6 mm; n o d e frequency, 40(t500 vibrations/s; solution flow, 16 mL/min; NH, gas flow, 2.5 L/min; gelation box off-gas flow, 35 L/min; pressure in solution feed tank, 1-2 psig. Size analysis and photographs of (Th,U)O.spheres show good uniformity of particles (Figures 7 and 8). Although the arrangement for providing fmt an air, then an ammonia, atmosphere seems complex, a simpler arrangement was used with excellent results for small batch tests (Figure 9). The gelation chamber is a 4-L beaker with a 2-cm-diameter hole about three-quarters of the way up the wall. A kitchen-type plastic wrap and a rubber band are used to close the top. The NH, is fed into the beaker using a tube poked through the wrap. The single-fluid nozzle was either rigidly attached to the vibrator or the vibrator was attached to a piston in the feed, as described for the two-fluid nozzle (Figure 10). The ap-
-
Figure 6. Quipment arrangement for external gelation using NH, g=.
so rapid that the drops must be preformed in an a m m e &-free phase with time to assume a spherical shape before
exposure to ammonia gels the drop. Mass transfer inside the drop is slow, and the gel surface layer over a liquid core may be weak. Therefore, organic polymer additives are commonly included in the feed solutions for these gelation processes. The organic polymer provides a support
WASHED GEL (1!20-pm)
F
DRIED GEL (670-pm)
REDUCED AND SINTERED GEL (485-prn)
m 1. (Th,U)02partielea (Th/U= 7.6/1), prepared by external gelation in a NH, gw-NH,OH solution.
I
I
I
I
I
I
I
400
v)
-
W
-
i
250
LL
0
a
200
1
ERRITRON
1
I
450 -
I \
MAXIMUM DlAM ~ 7 2 7p m MINIMUM DlAM ~ 6 3 4pm MEAN DlAM ~ 6 8 f . f p m STANDARD DEV. ~ 8 . 8w m NO. PARTICLES = 8 7 4 7
r
POWER SUPPLY 0-25 V-A 50-10,000 c p a
VIBRATOR SUPPORT
VISRATOR TIP (STAINLESS STEEL)
4 RUBBER TUBING (THIN WALL)
50
0 1 I 1 I 1 600 6 2 5 6 5 0 675 7 0 0 1 2 5 7 5 0 1 7 5 DIAMETER
I
SOL
I
800 825
850
(pm)
Figure 8. Size analysis of dried (Th,U)02particles. Particles were not screened or shape separated. n
GLASS NOZZLE WITH CAPILLARY
GAS NHI
AIR
DRIVE FLUID (2-ETHYL-1-HEXANOL)
PLASTIC WRAP
2-cm
K 5-cm HOLE
4-LITER PYREX BEAKER
CAPILLARY TIP (ABOUT 0.02 IN. I D )
NHq OH SOLUTION
MANOMETER
SLEEVE
STAND
Figure 9. External gelation test apparatus.
paratus was operated in a chemical hood and oriented so the hood air flow removed the NH, escaping from the hole in the beaker, without carrying it to the nozzle. External gelation of drops by low concentrations of NH3 in an organic liquid is much slower than the gelation in NH3 gas or aqueous NH40H. Therefore, the drop formation techniques described for internal gelation are usually practical. The drop will return to a spherical shape in the organic liquid before gelation takes place. D. Applications Using Two-Fluid Nozzles. The basic concept for breakup of a laminar jet can be applied with the jet centered in a laminarflow of the organic liquid. The calculations for the vibration frequency (using a volume balance for the drop diameter and the broth flow rate) are unchanged. The jet will be accelerated (or decelerated) to the organic velocity before breakup occurs; therefore, the jet velocity and diameter are determined by the organic velocity instead of by the diameter of the capillary or orifice through which it feeds. This effect has some advantages, since (1)the organic flow rate can be adjusted to vary the jet diameter (thus, a two-fluid nozzle can be used for a wider range of average drop sizes than can a single-fluid nozzle), (2) the jet can be accelerated to give a smaller diameter so that the capillary is larger than about half the drop diameter (This may be helpful to prevent plugging); and (3) good jet formation is possible at lower velocities than those practical for single-fluid nozzles. Two limitations that result from the organic flow must also be considered. The organic flow must be laminar, since a turbulent flow will disperse the aqueous solution into much smaller drops. Promoted turbulence is likely at the jet entrance and is more troublesome for smaller nozzles and drop sizes. Therefore, the Reynolds number calculated for the organic flow in a simple tube should not exceed 2000; 600 as a maximum is a safer limit. Laminar
SUPPORT
I I
DISCHARGE INTO SPHERE -FORMING COLUMN
Figure 10. Two-fluid nozzle with vibration.
flow of the organic also gives a parabolic velocity distribution, so the jet velocity and diameter will vary with the position of the jet in the tube. The two-fluid nozzles are usually designed to center the jet, and calculations for both jet velocity and drop size are based on twice the average organic velocity. A two-fluid nozzle with vibration was used to form Thoz sol drops in 2-ethyl-l-hexanol, using the nozzle apparatus shown by Figure 10 (Haas, 1975). A 6.4L volume of Tho, sol was fed in 538 min, using a vibration frequency of 28 800 cycles/min. The 4.2 kg of Tho, spheres produced had an average diameter of 379 pm, with a standard deviation of 3.7 pm (or u = 1.01 for a log-normal distribution). This indicates an average drop diameter of 933 pm, with a standard deviation of 9.1 pm. The calculated drop diameter from that feed volume and frequency is 925 pm. Data for other batches of calcined spheres with average diameters of 283-695 pm show standard deviations of 2.5-5.4 pm (Haas, 1975). The two-fluid nozzle operation without vibration gave D,/D, = u = 1.10. Testa were made using a nozzle arrangement similar to that in Figure 10 to form uranyl nitrate solution drops of 3950-~mdiameters in TCE. The TCE has a low viscosity and high density, giving high Reynolds numbers. Using a Reynolds number of 1000 to estimate the maximum allowable average velocity yields 5.3 cm/s (or 10.6 cm/s for a center-line velocity). With a jet of 0.19-cm diameter, the calculated values are 18.0 mL/min feed rate to give 550 dropsf min. However, this flow rate did not give a jet; instead, drops of 1000-6000-pm diameters were stripped from the nozzle tip. By doubling the flow rate and the vibration frequency, both good jet formation and drop formation were obtained. This gives a Reynolds number
Ind. Eng. Chem. Res. 1992,31,967-973
of 2000 and was border line with respect to turbulent flow. The jet twisted or flickered at thisflow, and slightly higher flow rates gave turbulence. For smaller nozzles, excessive turbulence at the tip was observed at calculated Reynolds numbers of 1000. The TCE flow to the nozzle was cooled to 20 “C and the nozzle discharged into a column of hot TCE for internal gelation of the drops. Useful tests were made with this two-fluid nozzle arrangement. The size distributions showed D W / D pvalues of 1.03-1.05. The single-fluid nozzles above the TCE surface were found to give simpler operation, higher capacities, and slightly better size uniformities.
Conclusions The gel-sphere processes require the formation of liquid drops of controlled and uniform sizes at rates of 103-107 drops/min. The product spheres allow good measurements of the drop size distributions. The basic technique reported here uses an applied vibration to give a controlled and uniform breakup of laminar liquid jets into drops. Controlled alternating current powers the small electromechanical vibrators to apply vibrations to liquid jets in several different ways. A highly regular and reproducible breakup occurs for vibration frequencies that promote a breakup near the natural frequency of breakup for the laminar jet. With optimum conditions, the drop formation is so regular that the motion appears to be completely stopped when observed with a stroboscopic light. Criteria are given in this paper for selection of jet diameters, jet velocities, vibration frequencies, and the configurations of the drop-formation apparatus. Selection
967
of these parameters for a specific gel-sphere process involves some compromises to minimize the observed problems by empirical adjustments. Preparation of gelsphere samples required repeated runs in which the drop formation was controlled and reproduced without difficulty. Examples of these results and conditions are given. The values of u = DW/D, (the geometric standard deviations for a log-normal distribution) were 1.01-1.05.
Literature Cited Bischoff, K.; et al. “Sol-Gel Processes for Carbide Preparation”. Proceedings of a Panel: Sol-Gel Processes for Fuel Fabrication, Vienna, May 21-24; International Atomic Energy Agency: Vienna, 1974; IAEA-161; pp 95-128. Ham, P. A. Formation of Liquid Drops with Uniform and Controlled Diameters at Rates of lo3to 106-Dropsper Minute. AIChE J. 1975.21 (2). 282-285. Haas, P. A. Turbulent Dispersion of Aqueous Drops in Organic Liquids. AIChE J. 1987,33 (6), 987-995. Haas, P. A.; Clinton, S. D. Preparation of Thoria and Mixed-Oxide Microspheres. Ind. Eng. Chem. R o d . Res. Dev. 1966,5,236-244. Haas,P. A.; et al. Preparation of Reactor Fuels by Sol-Gel Processes. Chem. Eng. h o g . Symp. Ser. 1967,63 (BO), 16-27. Haas, P. A.; et al. Chemical Flowsheet Conditions for Preparing Urania Spheres by Internal Gelation. I d . Eng. Chem. R o d . Res. Dev. 1980,19,459-467. Merrington, A. C.; Richardson, E. G. The Break-up of Liquid Jets. R O C Phys. . SOC.1947,59, 1-13. Zimmer, E.; et al. Aqueous Chemical Processes for the Preparation of High Temperature Reactor Fuel Kernels. Radiochim. Acta 1978,25, 161-169.
Received for review August 7, 1991 Revised manuscript received November 4, 1991 Accepted November 28, 1991
Calculations of Solubilities of Aromatic Compounds in Supercritical Carbon Dioxide Yu-Jane Sheng, Ping-Chin Chen, and Yan-Ping Chen* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China
David Shan Hill Wong Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China
Solubilities of five aromatic compounds in supercritical carbon dioxide are calculated in this work by using the Patel-Teja equation of state. Unlike the traditional van der Waals mixing rules, the energy parameter of the equation of state is evaluated by equating the excess free energy calculated by the equation of state to that from a UNIFAC group contribution liquid model. New UNIFAC group interaction parameters of carbon dioxide and various hydrocarbon groups are obtained through regression of experimental vapor-liquid equilibrium data. A new mixing rule for the excluded volume parameter of the equation of state is proposed, and a generalized correlation of that parameter is presented. With the new mixing rules, solubilities of aromatic solids in supercritical carbon dioxide can be calculated satisfactorily. The results are comparable to those computed by using the van der Waals mixing rule with multiple best-fitted unlike pair parameters.
Introduction Supercritical fluid extraction (SCF’E) is a new separation technology which has received much interest in the processing of pharmaceuticals, natural products, and many other special applications (McHugh and Krukonis, 1986; Brennecke and Eckert, 1989). Carbon dioxide is a promising solvent since it is inexpensive, nontoxic, inflammable, and environmentally acceptable and has a low critical
* To whom correspondence should be addressed.
temperature and a moderate critical pressure. The design and development of supercritical extraction processes also depend on the ability to model and predict the solubilities of solid solutes in supercritical solvents. The prediction is usually difficult because of the large differences in sizes and molecular interactions between the solute and solvent molecules. Cubic type equations of state (EOS), which have been extensively applied in vapor-liquid equilibrium (VLE) calculations, have also been used in computing the solubilities of solids in supercritical fluids (A review paper on
0888-5885/92/2631-0967$03.00/00 1992 American Chemical Society
