Ind. Eng. Chem. Fundam. 1984, 23, 109-113
where the superscript * denotes perfect gas and the subscripts A and B refer to the primary and secondary reference compounds, respectively. As applied to a liquid isotherm at 25 "C c
where
(mb/RTc)i
E [(H/RTc)ilT,=298.15/T,; P,=P/P,]
-
[(H/RTc)iIr,=298.15/r~; P,=P~./P~]
That is, both (mb/RTC),and (AHb/RTc)A are found from the respective reference compound equations but at the same reduced pressures and reduced temperature as the experimental compound. Acknowledgment
The financial support of the Gas Processors Association (Project No. 792), the National Science Foundation (Project CPE8023182) and the Texas Engineering Experiment Station (Project No. 9127) are gratefully acknowledged. This paper was presented at the 32nd Canadian Chemical Engineering Conference at Vancouver in October 1982. Nomenclature
Cp = isobaric heat capacity, J / g K E = voltage drop, V H = enthalpy, J / g AHH,= enthalpy correction due to exit temperature L i H b = enthalpy correction due to pressure I.= electrical current, A M E = mass flow rate of experimental fluid, g/h hfF = mass flow rate of Freon-11, g/h P = pressure, MPa Qc = heat transfer rate due to experimental fluid, J / h Q L = heat transfer rate due to heat leaks, J / h R = gas constant Re = electrical resistance, Q T = temperature, "C (IPTS-68) or K (eq 6-8) Greek Letters
109
@ = isothermal throttling w = acentric factor
coefficient, J / g MPa
Subscripts 1 = denotes inlet to calorimeter 2 = denotes exit of calorimeter
A = primary reference compound B = secondary reference compound c = critical value i = either compound A or B R = reference temperature, 25 OC r = reduced quantity by division by critical value Superscripts u = denotes saturation * = denotes perfect gas state Registry No. Water, 7732-18-5. L i t e r a t u r e Cited Bier, K.; Ernst, G.; Maurer, 0.J . Chem. Thermodyn. 1974, 6 , 1027-1037. Cediel, L. E. Ph.D. Dlssertatlon Texas A8M University, College Station, TX, 1983. Clark, R. G.; McKinley, C. Proc. Ann. Conv. Natural Gas Processors Assoc. 1967, 46, 11-18. Eubank, P. T.; Smith. J. M. J . Chem. Eng. Data 1962, 7 , 75-78. Jenkins, A. C.; Berwaldt, 0. E. Ind. Eng. Chem. Process D e s . b e v . 1963, 2, 193-196. Judd, N. F.; Mayhew, C. J.; McElroy, P. J.; Willlamson, A. G. J . Chem. Thermodyn. 1980, 72, 465-473. Keenan, J. H.; Keyes, F. G.; Hill, P. G.; Moore, J. G. "Steam Tables"; Wiley: New York, 1969. Lenoir, J. M.; Robinson, D. R.; Hipkln, H. G. J . Chem. Eng. Data 1970, 75, 23-26. Mather, A. E., paper presented at the Miami Beach Meeting of the American Institute of Chemlcal Engineers, Nov 1978. McConnell, J. R.; Fleckenstein, R. R.; Kidnay, A. J.; Yesavage, V. F. Ind. Eng. Chem. Process Des. Dev. 1984, in press. Miyazaki, T.; HeJmadi, A. V.; Powers, J. E. J . Chem. Thermodyn. 1960, 72, 105-1 24. Nelson, J. M.; Holcomb, D. E. Chem Eng. hog. Symp. Ser. 1953, 7 , 93-98. Ng, H. J.; Mather, A. E., paper presented at the 169th National Meeting of the American Chemical Soclety, Phlladelphia, April 1975. Sagara, H.; Aral, Y.; Salto, S. J . Chem. Eng. Jpn. 1977, 70, 95-100. Sahgal, P. N.; Gelst, J. M.; Jambhekar, A.; Wilson, G. M. I n "International Advances in Cryogenlc Engineering"; Timmerhaus, K. D., Ed.; Plenum Press: New York, 1965; VoI. 10, pp 224-232. Sood, S. K.; Haselden. G. G. AIChE J . 1972. 78, 999-1004. Thlnh, T. P.; Ramalho, R. S.; Kallaguine, S. Can. J . Chem. Eng. 1973, 51, 86-93. van Kasteren, P. H. G.; Zeldenrust, H. Ind. Eng. Chem. Fundam. 1979, 78, 333-339. Wiener, L. D., paper presented at the Dallas Meeting of the American Institute of Chemical Engineers, March 1966. Wormald, C. J. J . Chem. Thermodyn. 1977, 9 , 901-910.
A = difference XL = latent heat of vaporization, J / g pjT = Joule-Thomson coefficient, OC/MPa p = density, g/m3
Received for review September 17, 1982 Revised manuscript received July 25, 1983 Accepted August 10, 1983
An OpticaVElectronic Technique for Measuring Bubble Coalescence Times K. Tre-tang Chuang,' Andrew J. Stlrllng, and James C. Baker Atomic Energy of Canada Limited, Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario KOJ 7JO
Bubble coalescence time can be used to characterize the degree of foaminess in a gas-liquid contactor. A new method for measuring the coalescence time for bubbles in transparent liquids is described. Since it employs an optical source and electronic detectors, the technique eliminates the tedious analysis of moving pictures tradRionaliy used for coalescence studies.
Introduction
Many industrial gas-liquid contactors rely on the generation of bubbles to facilitate mass and heat transfer.
Tray columns used in distillation are a typical example. Thermodynamic principles indicate that in a pure liquid bubbles coalesce rapidly after coming in contact (Ross,
0196-4313/84/1023-0109$01.50/00 1984 American Chemical Society
Ind. Eng, Chem. Fundam., Vol. 23, No. 1, 1984
110
Q A
(a!
L
SEQUENCE-OF-EVENTS I N PdRE L I O ' J ~ C
-
( 3 )
SECbENCE-3f-ErENTS P I L 33 C ' l Y T E ' h l N G SUDrGCTCNT
6 (:I
tRO'nTH
(0)
CON'GC1
I
Figure 1. Definition of coalescence time.
c3
U 1967). In other liquids, bubbles in contact can remain in juxtaposition to generate foam. As a result, the coalescence time can be a parameter affecting foaming tendency. Unfortunately, there are no reliable methods available for the prediction of foaming properties of gas-liquid mixture. When a new process is being implemented, it is important to measure the system foaminess before the contactor design is accepted. In the Girdler-Sulfide (GS) process for heavy water production, hydrogen sulfide gas is bubbled through water on sieve trays to separate the hydrogen isotopes. When heavy water plants were operated a t lower pressures, the trays were stable. At pressures above 1850 kPa, excessive foaming was observed. This caused flooding of the trays and anti-foaming agents were required to get tray stability. Sagert and Quinn (1976a) have reported that the bubble coalescence time of H2S in water increased with H2S pressure. At the GS process temperature, 30 OC, the coalescence time at 1850 kPa was 140 ms, which was more than 50 times longer than that a t atmospheric pressure. Since plant experience indicated that H,S and water purities also influenced the degree of foaminess, bubble coalescence time measurements could give a quick guide to the plant operators for the control of column stability. Marrucci and his colleagues (1969a,b) reported their studies on bubble coalescence times for bubbles grown on adjacent nozzles. Subsequently, Sagert and Quinn (1976a,b, 1978a,b) published similar studies for various gas-liquid mixtures. In those studies, coalescence time was determined with high-speed moving pictures. This method is not satisfactory for extensive laboratory studies because (a) results are not available for several days after an experimental run, (b) hundreds of frames must be examined to determine coalescence time, and (c) the instant at which bubbles touch must be decided subjectively. This paper describes a simple and direct technique for measuring coalescence time that is suitable for use in a research laboratory or in a production plant.
Definition of Coalescence Time In discussing bubble coalescence it is necessary to define a "bubble" as including both the gas volume and the liquid surface layer surrounding it. Consider two spherical bubbles of equal volume, approaching at constant velocity. In a pure liquid, the separating film thins until the bubble separation is equal to the sum of the bubble radii. Thereupon, the bubbles coalesce. This process is shown in Figure la.
rfl I f 1 C3GLES:iNCE
DETECTION PRINCIPLE
Figure 2. Principle of bubble coalescence detection.
The presence of surfactant impurities allows quasi-state films to be maintained after contact. The instant of contact (to)is defined as the point at which deformation begins. Some time after contact (t,), bubbles coalesce. Coalescence time, as shown in Figure lb, is defined at At, where At = t , - to.
Principle of Measurement A technique for measuring coalescence time requires (a) a means for generating bubbles and bringing them together, and (b) a means for determining the times to and t,. In practice, bubble generation influences the detection technique and vice versa. Since the measurement of toand t, cannot be permitted to influence bubble growth, optical methods are attractive. Absorption in clear water is significant only in the infrared region. Reflection of visible light from a moving bubble surface, therefore, offers a potentially simpler basis for determining to and t,. The method developed is based on the growth of two bubbles a t adjacent orifices, closely resembling the approach described by Kirkpatrick and Lockett (1974). A light source and a detector are positioned so that the growing bubbles intercept the light beam and reflect the peripheral rays away from the detector (see Figures 2a-c). As the bubbles grow, the radiant fluxreaching the detector diminishes from @I (its uninterrupted value) until the bubbles are in contact (see regions a-d in Figure 3). A t the instant of contact (see Figure 2d), the axial1 rays can no longer reach the sensor but off-axis rays generate a radiant flux a0a t the detector. In a pure liquid, coalescence occurs at contact and the flux returns to @T, a value which depends on the reflectivity of the interfaces of the resulting bubble (see broken curve in Figure 3). For a foamy liquid, post-contact bubble growth reduces the flux below @o, possibly to zero (see Figures 2e and 3). After coalescence, the transmitted flux returns to @T (see solid curve in Figure 3).
Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984 (a)
-
( t ) REFER TO E V E N T S SHOWN I N
In principle, coalescence time can be measured in a two-part experiment. Firstly, bubbles would be grown in the pure liquid and a0would be measured. Next, bubbles would be grown in the foamy liquid, and the times, toand t,, at which the flux reached a0and 'PT,respectively, would be measured. Apparatus (a) Bubble Chamber. The diagram of the bubble chamber is shown in Figure 4. To accomplish the idealized sequence of events shown in Figure 2, nitrogen bubbles were grown under constant flow conditions, from adjacent orifices in an aluminum plate. Coalescence time measurements were made before the bubbles floated away from the orifices. The bubble coalescence chamber was a Plexiglas cylinder, 9 cm in diameter and 7 cm high, which permitted visual and photographic observations. Constant and equal flow conditions were maintained by Nupro SS2SGD needle valves and 0.55 m of 0.254-mm capillary tube connected directly to the pressure regulator of a compressed nitrogen cylinder. The volume of the orifice was limited to 17.5 X m3, an important precaution in maintaining constant flow at low flow rates. No means for measuring gas flow rates was provided, but the needle valves maintained repeatable bubble formation a t rates as low as 0.09 bubble pairs per second. Appropriate orifice diameters and spacing were determined by trial and error. Spaced too far apart, bubbles floated away from the growth orifice before making contact with each other. A limit to close spacing, however, was set by the need to locate a light source between the orifices. In the experiments described, 2.8-mm diameter orifices were spaced on 3.45" centers. (b) Detection System. Light from a 60-W tungsten halogen lamp was directed into the 0.65 mm space between the orifices using a 0.4 mm diameter fiber optic light guide (see Figure 4). A Fairchild (Type F P T 101) phototransistor was mounted with its 1.3 mm diameter window normal to and centered along the light axis, 10 mm above the plane of the orifices. This setting represented a compromise in maximizing the light detected without disturbing bubble formation. The phototransistor was connected to a +6 V dc power supply through a load resistor RL as shown in Figure 5. RL was selected as 47 kn,so that the collector-emitter voltage (VcE) was a linear function of light flux. In experiments to measure coalescence time RL was increased to 1Mil to increase the detector sensitivity at low light levels. The collector-emitter voltage, VCE, of the phototransistor was fed to a buffer amplifier which provided current driving capability and adjustable band-pass filtering. The
FIGURE 2
___ SIGNAL . . FOR PURE L I Q U I D
- SIGNUL FOR L l Q U l O C O N T U I N I N G
P
SURFeiCTlNTS
L
PURE
(bl
j CONTACT
At
t tc j
TIME
COALESCENCE
Figure 3. Radiant flux received by detector.
YEEOLE VALVE
TUBE
-
0
R t 6 U L A l i O IC SUPOLI (0-4Ol.P.I.5Ll
g-
I l O R A t i OSClllOSCOfi H i u i i i i PhcKino i i i i i OR 0111 L L O S C O P l l l A l R O l l 1 1004
Figure 5. Schematic diagram of detector and instrumentation.
( a ) THROUGH ( f )
111
I D E N T I F Y THE SEQUENCE-OF-EVENTS
SHOWN I N F I G U R E 2
RPD I PNT FLUX
Figure 6. Detector signal for growth and coalescence of consecutive bubble pairs.
112
Ind. Eng. Chem. Fundam.. VoI. 23, No. 1, 1984
INDEPENDENT GROWTH
POST-CONTKT
NOTE:
GROWTH
INDEPENDENT GROUTH
CONTACT
COALESCENCE
ARROWS ON 0SClLLOSCOPETRllCESS"OW
POST-CONTACT GROWTH
POST-COALESCENCE 6ROWTH
DETACHMENT
INSTANTS OF THE BUBBLE PHOTOGRAPHS
Figure 1. Correlation between detector signal and sequence of events during bubble formation and coalescence.
resultant signal was then monitored, either with an oscilloscope, chart recorder, or time-interval measurement unit. Correlation between OpticaljElectronic Measurements a n d Photographic Observations In establishing the measurement technique, gas buhhles were generated at the rate of approximately one pair per second in solutions of 10-50 ppm n-amyl alcohol in distilled water. A typical example of the signal generated by the phototransistor is shown in Figure 6. It is sufficiently similar to the predicted signal (Figure 3) that the regions of growth, contact, and coalescence could be identified. Positive correlation between the opticalleledronic signal and the events in the chamber was obtained photographically as follows. The phototransistor signal was recorded by an oscilloscope, operated in ita single-sweep, signaltriggered mode. A pulse, taken from the delayed-trigger output of the oscilloscope, was used to fire a single-shot gas discharge lamp illuminating the bubble chamber. During the formation of a series of bubbles, the trigger delay was increased 80 that the illumination coincided with different stages of bubble powth, contact, and coalescence. Cameras recorded both the oscilloscope traces and the events in the chamber. The exact time of the photograph was determined diredly from the oscilloscope trace because the light flash saturated the detector at the instant of the photograph. Figure 7 shows a series of photographs and oscilloscope traces to indicate that the detector signal correlated precisely with the predicted sequence of events. Experimental Procedure Following the theory, the procedure for tke coalescence time measurement was as follows. (a) Bubbles were generated in distilled water and eowas measured from the detedor signal, in units of the detedor signal 1voltage (see Figure 8a). (b) Without altering the gas flow rate, the distilled water was replaced with the liquid under test and the detector signal was recorded (see Figure 8b). (c) The
Ibl LIQUID UNDER TEST IN A M Y L ALCOHOL1
+
, * I S OEFlNED AS THE TlUE WHEN+ R E d C H L S 4 0
RADIANT FLUX
,ARB "NITS,
t j 15 lOEUTlFlED Ais THE TIME
AT WHlCil+
a,
.
Ij
-4
INCREASES SHIRPL"
,400 ms IN TH1S EXI(MPLEI
Figure 8. Procedure far coalescence time measurements.
time towas identified as the abscissa value corresponding to Qo,and the time t, was identified by the sharp increase in the light flux (see Figure 8b). (d) The measurements (a) and (c) were repeated during the formation of approximately 30 bubble pairs to determine the average coalescence time. An electronic time interval monitor was ussed to speed up the measurements (see Figure 5). The procedure using electronic time interval measurement was as follows. (a) Bubbles were generated in distilled water, and the "STOP^ trigger level of the time-interval unit was set to correspond with a signal level between Qo and QT (see Figure 8). The "START" trigger level was then set to eoby adjusting the control knob to the position where zero (or nonzero) time intervals were fiist displayed (depending on the direction
Ind. Eng. Chem. Fundam. 1884, 23, 113-1 15 I 500
VI
d’i
i
BUBBLE PAIRS PER SECOND
50
I
+STANDARD DEVIATION
P 1
I
I
I
200
w
I
IO
20
0 50
100
1 200
N - A M Y L ALCOHOL CONCENTRATION (ppm)
Figure 9. Bubble coalescence time of n-amyl alcohol.
of approach). (b) Without altering the gas flow rate, the distilled water was replaced with the liquid under test and the coalescence time was displayed directly for each bubble pair in turn. (c) Coalescence times were recorded and averaged manually. Parameters Influencing Coalescence Time The procedure described provided a direct and simple method for determining the coalescence time for bubbles generated under the controlled conditions. The value of using the method in practice also depends on understanding and, if necessary, controlling all external param-
113
eters which influence coalescent time. Two parameters which may be expected to influence coalescence time include (1) equality of gas flows in each orifice, and (2) bubble formation rate. It was observed that bubble growth and coalescence were sensitive to the difference between the gas flow rates a t the orifices. Whenever the flows differed, bubbles formed out of phase and failed to coalesce. However, the needle valves provided adequate control over gas flow so that coalescence was maintained over hours or days without readjustment. Figure 9 shows the coalescence time as a function of bubble pair formation rate over the range 0.09 to 2.7 bubbles pairs per second for concentrations of n-amyl alcohol in water up to 200 ppm. Three significant observations were made. (a) At high gas flow rates the bubbles separated from the orifice before coalescence. This limited the range of coalescence times which could be measured. (b) Coalescence time was independent of flow rate over the range from 0.42 to 2.7 bubble pairs per second. (c) The standard deviation [a(AT)] of coalescence times measured a t a fixed flow rate was always less than 5 % of the average value. Literature Cited Kirkpatrick, R.; Lockett. M. Chem. Eng. Sci. 1074, 29,2363-2373. Marrucci, G. Chem. Eng. Sci. 1060a, 2 4 , 975-985. Marrucci, G.; Nicodemo, L.; Acierno, D. “Co-Current Gas-Liquid Flow”; Rhodes, E.; Scott, D.; Ed.; Plenum Press: New York, 1969b; pp 95-108. Ross, S. Chem. Eng. hog. 1067, 63, 41-47. Sagert, N.; Quinn, M. Can. J . Chem. Eng. 1076a, 54, 392-398. Sagert, N.; Quinn, M.; Cribbs, S.; Roslnger, E. “Foams”; Akers, R.; Ed.; Academic Press: London, 1976b; pp 147-162. Sagert, N.; Quinn. M. J . ColhM Interface Sci. 1076a, 65, 415-422. Sagert, N.; Quinn, M. Chem. Eng. Sci. 1078b, 33, 1087-1095. Received for review June 11, 1982 A c c e p t e d August 26, 1983
A Pneumatic Probe To Detect Gas Bubbles in Fluidized Beds. 1. Method of Operation Rory L. C. Flemmer Department of Chemical Englneerlng, University of Natal, Durban, Natal, South Africa
in a fluidized bed to be detected reliably at any temperature. It offers minimal interference to the bed and is cheap and simple to construct. A probe is presented which enables gas voids
Introduction I t is necessary in the characterisation of fluidized bed phenomena to know the extent to which bubbling occurs. For this reason many techniques have been developed to detect bubbles. (For a review, see Fitzgerald, 1979). All these techniques have the limitation that they cannot be used in high-temperature beds and most have the disadvantage that they obstruct the flow field to a significant degree. This paper presents a probe which offers minimal obstruction and which c a n be used in beds a t any temperature. o i 9 ~ - 4 3 1 3 1 a 4 1 i o 2 3 - o i 13$01.5010
Principle of Operation The probe, depicted schematically in Figure 1,uses two fine tubes. One of these, the jet tube, emits a small jet of gas to impinge upon the opening of the other, the sensing tube. If this jet is obstructed by particles then, the change in pressure experienced by the sensing tube is readily recorded. In order to compensate for fluctuating bed pressure a t the point of observation, a third fine tube, the compensating tube, has its opening so placed that the small jet does not stagnate upon it; Le., the plane of its opening is 0 1984 American Chemical Society
