Ind. E n g . C h e m . Res. 1990,29, 2346-2354
2346
Skogestad, S.; Morari, M. LV-Control of a High-Purity Distillation Column. Chem. Eng. Sci. 1988b, 43 (l),33-48. Skogestad, S.; Lundstrom, P.; Jacobsen, E. W. Selecting the best distillation control configuration. AIChE J. 1990,36 (51,753-764. Takamatsu, T.;Hashimoto, I.; Hashimoto, Y. Selection of manipulated variables to minimize interaction in multivariate control of distillation column. Inst. Chem. Eng. 1987, 27 (4), 669-677.
Waggoner, R. C. Distillation Column Control. AIChE Modular Instruction. Series A: Process Control; AIChE: New York, 1983; Vol. A5, pp 19-29.
Received for review May 5, 1989 Revised manuscript received May 23, 1990 Accepted June 1, 1990
SEPARATIONS Changes in Particle Morphology during Drying of Drops of Carbohydrate Solutions and Food Liquids. 1. Effects of Composition and Drying Conditions Tarric M. El-Sayed,+David A. Wallack,* and C. Judson King* Department of Chemical Engineering, University of California, Berkeley, California 94720
Morphological changes of drops of sucrose and maltodextrin solutions, coffee extract, and skim milk have been monitored during drying. One method involved videotaping drops suspended in a stream of hot air. A second method was t o sample a stream of drops of uniform initial size falling through a column with a controlled temperature profile. Suspended drops show a first period of near-spherical shrinkage, closely following predictions for a voidless sphere. This is followed by a second period of rapid inflate-deflate cycling (boiling) and a third and final period during which the drop grows and/or shrinks to reach a solidified morphology. The effects of composition, air temperature, initial solute concentration, and presence or absence of dissolved gases were determined. Particles of coffee extract from the falling-drop dryer evidence surface blowholes, replicating what is observed in commercial spray-dried coffee. This phenomenon is rationalized in terms of viscous resistance to sealing flows.
Introduction Drops of liquid solutions undergo changes in morphology-size, shape, and appearance-as they dry evaporatively to form solid particles. In most cases, an initially spherical liquid drop does not simply become a solid, spherical particle. Appearances of typical particles from a commercial instant-coffee spray dryer are shown in Figure 1. Particles develop internal voidage and surface irregularities (folds, blowholes, etc.). They also expand, collapse, and/or shrivel. These phenomena are not limited to spray drying. For example, similar morphologies have been found for fly-ash particles (Fisher et al., 1976). In spray drying, changes in particle morphology during drying govern the bulk density of the product and should affect drying rates (see part 2) and ease of redissolution. For spray drying of a food product, certain morphological changes, e.g., surface ruptures, probably cause substantial losses of volatile flavor and aroma components. The goal of the present research was to gain further insight into how the nature of the material being dried and the drying conditions govern the development of particle morphology. Two experimental approaches were used. In
* To whom correspondence should be addressed. +Present address: Clorox Corporation, Technical Center, Pleasanton, CA 94566. *Present address: 3M Corporation, St. Paul, MN 55119. 0888-5885/90/2629-2346$02.50/0
one, drops suspended from a glass fiber or a thermocouple were dried in a stream of hotair. The appearances of the drops during drying were recorded continuously by video photography onto videotape for subsequent playback. In the second method, a stream of drops of initially uniform size fell through a vertical column with a known, predetermined temperature field. Samples obtained at various levels along the fall were frozen in situ, freeze dried, and observed by scanning electron microscopy. Drops were formed from aqueous solutions of sucrose, maltodextrin, coffee extract, and skim milk, chosen so as to display different types of morphological features. Previous Work. Microscopic observation is useful for examining the particle morphology of a dried product but does not give direct information on how the final morphology came about. One technique that has been used for monitoring changes in particle morphology continually during drying has been observation of stationary, suspended macrodrops, typically 1-5 mm in diameter. Charlesworth and Marshall (1960) reported differences in morphological development during drying of suspended drops of various salts and coffee extract. Similar experiments were carried out by Abdul-Rahman et al. (1971) and Crosby and Weyl(l977). Artifacts are introduced into suspended-drop experiments by the suspension device, the lack of free rotation, and the relatively large drop size. Alternative approaches are experimentally more complicated. Toei and co-workers 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2347 VIDEO CAMERA
SUSPENDED DROP
---.Fc.Ic
,,
1..
.
\ % ' , I
,,.-miG
.
[I) I
Figure 1. Particles of dried coffee extract from a commercial sprav dryer. Figure 2. Suspended-drop apparatus.
have utilized an upwardly flowing air stream with an ultrasonic field (Toei and Furuta, 1982; Furuta et al., 1982) or a shaped velocity field (Furuta et al., 1983) to hold drops in place for observation of solute crystallization and morphological changes during drying. Greenwald and King (1981,1982) and Alexander and King (1985) have used a device wherein a stream of drops of initially uniform diameter falls through a column with a known temperature profile, with samples being collected a t different levels along the column for examination by scanning-electron microscopy (SEM). In an extensive study, Verhey (1972, 1973a,b) demonstrated that vacuoles (internal voids) in spray-dried milk powders tend to be under partial vacuum and contain the gas present during atomization. He concluded that internal voids can come from entrainment of air bubbles during atomization. The results reported by Greenwald and King (1981, 1982) demonstrate that internal voidage can also result from desorption of dissolved air which is initially present or is absorbed upon atomization. Under typical spray-drying conditions, no bubble nucleation occurs within drying drops if dissolved or entrained inert gases are absent. Particles without internal voids are formed from degassed feeds when the atomizer is blanketed with steam (Verhey, 1972, 19'i3a,b) or sealed (van der Stege and R'alstra, 1987). Greenwald and King (1981,1982) observed that substantial internal voidage and particle expansion tended to occur for aqueous solutions as drop temperatures reached or exceeded 100 "C during drying. This reflects the development of large partial pressures and mole fractions of water vapor, joining the inert gas present in the bubble. Alexander and King (1985) examined tendencies for the formation of surface folds as drops dry and concluded that the higher viscosity associated with solutes of higher molecular weight resists flow sufficiently to preclude smoothing of surface irregularities under the surface-tension driving force. Experimental Apparatus a n d Procedure Suspended-Drop Device. The device used for monitoring morphological characteristics of suspended drops is shown schematically in Figure 2 and is described in more detail by El-Sayed (1987). Hot air was supplied through a hed of 1-mm diameter, beaded 4A molecular sieves (Linde Division, Union Carbide Corp.), which served to provide a flat velocity profile. The bed was 11.4 cm deep within a copper tube, 25 cm long, 2.5 cm inside diameter,
and surrounded by heating tape. The air temperature was measured a t the tube exit with a 0.12-mm Type E thermocouple and was relatively constant across that portion of the flow field that could affect drying of the drop (ElSayed, 1987). Exit-air temperatures between 25 and 250° could be maintained within 2 "C. Air humidity was computed from wet- and dry-bulb temperatures. The suspension filament was either annealed glass, 80-100 pm in diameter with a glass bead 100-300 pm in diameter on the tip, or a Type-E thermocouple with 0.1%" leads. The metal exhibits a thermal conductivity about 2 orders of magnitude greater than that of glass and provides many nucleation sites for bubbles. There was also a tendency for drying drops to climb the metal leads. Therefore the experiments with the glass filaments were felt to have fewer artifacts. However, suspension from the thermocouple allowed continuous measurement of drop temperature during drying. Solutions used to form drops were twice filtered through Whatman No. 50 filter paper. Drops 1.4-2.0 pL in volume were formed with a Hamilton 2.5-pL syringe. The syringe needle was coated with Fisher Mold Release to facilitate transfer of the drop from the syringe to the suspension device. The drop of solution on the suspension filament was quickly placed over the air stream, and the video camera was started. A General Electric VHS video camera was used with lenses giving 5OX magnification. Drop diameter was ascertained by reference to the bead diameter, determined beforehand by microscopy. Time was measured from the start of drying by prorating frames on the videotape over the entire time period. The operating procedures are described in more detail by El-Sayed (1987). Falling-Drop Dryer. The system for generating a single stream of drops of initially uniform size was that used previously by Alexander and King (1985); see also Alexander (1983) and El-Sayed (1987). This device uses a fine orifice pulsed by a piezoelectric system, followed by an electrical charge-deflectiondevice. A jet of feed solution flows through the orifice and breaks up a t resonant frequency. The deflection device reduces the number of drops so that catch-up and coalescence do not occur. For later experiments with coffee extract (Wallack, 1989), the charge-deflection apparatus was disconnected from the pulsed-orifice device, and the entire stream of droplets was allowed to pass down the column. In these instances, an air-aspiration technique, similar to that introduced by Rergland and Liu (1973, was used to impart radial spread
2348 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 MORPHOLOGICAL EVENTS
SUPPLY AIR
-.
THERMOCOUPLES
AIR STRAIGHTENER
, '1 i
1I
3 0 5 Cnl
]In ?I'
Cnl
+
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Figure 4. Typical sequence of morphological events for suspended drops dried at temperatures above the boiling point. FIBERGLASS INSULATION
i
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initial corm. ?OX ( w ~ W ) M ~ T i i r z 2 0 0 9 . Do:lAmm Re i7.2
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Figure 3. Falling-drop dryer.
to the droplet stream, thereby preventing catch-up and coalescence. The column itself is shown in Figure 3. It was composed of four identical sections of flanged copper pipe, each 0.61 m long and 7.6 cm inside diameter. Each section had two sampling ports made from 12.7 cm long, 5.1 cm inside diameter copper pipe, silver soldered 5.1 cm and 35.6 cm from the top of the column section, perpendicular to the axis. The sampler ports were covered and insulated during operation. Each section was also equipped with four equally spaced Type J thermocouples, the first and third being opposite the sampling ports, with signals supplied to a multipoint recorder. The temperature profile was regulated by means of 92-cm lengths of Fibrox heating tape, wrapped around each section and the bottom of the column. The outer layer was a double wrapping of 9-cm-thick Owens Corning Fiberglass insulation. The liquid feed system was essentially the same as that used by Alexander and King (1985). An exhaust blower was connected at a 90" angle to the base of the column with a flexible, high-temperature dryer hose. Droplets and particles impinged into a cup sealed in the base. With the blower, the column operated slightly below atmospheric pressure. The sampler was in the form of a flat strip of brass inserted through the sampler ports. The sampler arm could hold a microscope slide or a sampling cup, 4.4 cm in diameter and filled with liquid nitrogen. A copper sleeve held the sampler arm in position and closed off the port during sampling. Sampling was done one port at a time, proceeding upward from the bottom of the column, so as to minimize thermal disturbances upstream. Liquid or semisolid samples collected on a microscope slide in Dow Corning 200 silicone fluid were examined with a Bausch & Lomb DM series optical microscope. Frozen particles captured in the liquid nitrogen pool were transferred rapidly to a specially
0.7
01 Consun1 Rab
0
10
20
2
2
30
40
50
UDO (SeUllWli J
Figure 5. Measured reduced diameters for drying of a suspended drop of initially 20% (w/w)sucrose solution in air a t 200 O C . Also included is the diameter history predicted from the model of a spherical, voidless drop.
constructed (El-Sayed, 1987) laboratory freeze dryer, operating at 0.7-2.7 Pa. The dryer chamber was prechilled with dry ice to minimize distortion of particles due to flow during pump-down to operating pressure. Freeze-dried particles were mounted on colloidal graphite, without sputter coating, and examined with a Stereoscan 600 scanning electron microscope (Cambridge Scientific Instruments).
Results Suspended Drops. A typical sequence of observed events and corresponding drop temperatures during drying is shown in Figure 4. In Figure 5 , the reduced diameter (diameter/initial diameter) of a drop of initially 20% (w/w) sucrose solution is plotted versus a reduced time variable. The air temperature was 200 OC, and the air velocity was 10 cm/s, giving a particle Reynolds number of 7.2, based upon initial drop diameter. Later in drying when the shape of the drop or particle was nonspherical, the diameter reported is that of a sphere estimated to have the same volume. The normalized time variable is t/D:, where t is elapsed time and Dois initial diameter. Such normalization is suggested by solutions to the diffusion equation and should enable direct comparison for drops of different size, if particles have the same shape and the same solute diffusivities (e.g., the same temperature and composition profiles). Mass diffusivities are known generally to be more rate-limiting in such systems than are thermal diffusivities. This normalized time variable could be made fully dimensionless by including the diffusivity; however, diffusivities in these systems are highly dependent upon con-
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2349 I
1.1,
,
I
I
--
0 204°C.Re 7.2 0 150°C. 88°C. Re 0.7 7.7
1 .o
0.9
a) A smooth 'egg-shell ' results from drying a drop of 20% (w/w) skim-milk solution in 150°C air.
c) A chimney-like protrusion and hollow shell result from drying a drop of 20% (w/w) maltodexmn solution saturated with CO2.
b) A 'kidney-bean' shape results from drying a drop of 20% (w/w) coffee solution in 138°C air.
d) A satellite droplet being ejected from a suspended drop during the inflate/ deflate period.
Figure 6. Frames from videotapes made during drying of suspended drops.
centration and temperature. There are three distinct regions shown in Figures 4 and 5 and all such plots for suspended drops a t high air temperatures: 1. Early in the drying, in period 1, the drop shrinks uniformly, retaining a relativelv spherical shape and no internal voidage. (The lack of voidage was confirmed by optical microscopy of falling drops captured in oil.) The temperature is initially a t the wet-bulb value and then rises above it. A "skin" that is not as reflective as the original drop surface begins to appear a t the very top and very bottom of the drop, growing and joining eventuallv just below the equator. Subsequently, the surface exhibits shallow wrinkles and folds, as the surface layer is less able to flow. In Figure 5, over 50% of the drop volume is lost in this initial period. 2. In period 2, the drop nucleates bubbles internallv and boils, with bubbles eventually growing, rising, and bursting outward repeatedlv through the surface. Rubble nucleation occurs on the support filament in some experiments but also occurs in the bulk of the drop. The temperature rises to, and stays near, the boiling point. There are rapid inflate-deflate cycles during which the diameter swings by large factors. There were many more such cycles (about 70) than are actuallv shown in Figure 5, but the extremes of diameter are representative. For coffee extract, bubbles breaching the surface push wet material from the core of the drop to the outside of the drop. Occasionallv, one can
Diameter histories of drops of initially 20% (w/w)maltodextrin solution a t different air temperatures.
see a satellite droplet ejected from the surface during an inflate-deflate cycle (Figure 6d). 3. In period 3, once the drop has lost most of i t s moisture, the inflate-deflate cycles cease and the particle becomes a balloon encapsulating vapor. The temperature rises rapidly from the boiling point toward the air temperature. 'I'he particle grows and/or shrinks to reach a final, solidified morphology. For sucrose (Figure 5 ) , the particle collapses with a near-spherical shape until little or no voidage remains. Successive inflation and deflation were also reported by Charlesworth and Marshall (1960) for a suspended drop of 26% (w/w) coffee extract dried in 132 "C air. In Figure 5, predictions are also shown for the model of Frey and King (1986), used in part 2 of the present work, postulating a sphere with no internal voidage. During period 1, the experimental data agree well with the predictions of the model. As bubbles appear, the drop volume and diameter exceed the predictions of the model, as would be expected. Bubble nucleation occurs late in period 1 and during period 2 as the temperature rise and increase in solute content cause the drop to become supersaturated in dissolved air. Substantial drop expansion and initiation of inflate-deflate cycling do not occur until the drop temperature rises close to the boiling point. This is a manifestation of the phenomenon, pointed out by Greenwald and King (1982), whereby internal bubbles develop high partial pressures and hence high mole fractions and high volumes of water vapor as the temperature rises and the vapor pressure of water approaches atmospheric. As the drop temperature reaches the boiling point, bubble volumes become still larger, stretching the surface layers of the drop. The vapor pressure of water and hence the total pressure in the cavities within the bubbles exceed atmospheric pressure, and there is a driving force for the bubble to burst through the outer surface of the drop. Bursting is followed by collapse of the bubble as the air and water vapor flow out. Repeated bubble nucleation, growth, bursting, and collapse produce the inflate-deflate cycle. (1) Effect of Air Temperature. Results for drying of drops of 20% (w/w) solutions of maltodextrin (Grain Processing Corp., Muscatine, Iowa) are displayed in Figure 7, for three different air temperatures. In line with the higher driving forces, steeper initial slopes occur a t the higher temperatures. Initiation of the inflate-deflate sequence and final drying occur sooner at 204 "C than at 150
2350 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 1 . 2, 1,1[
I
,
i
,
,
Initial Conc 20% (wiw) Tair=lSO"C R e = ? ?
1
,
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sucrose
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initial Conc 20% (wiw) Tair=150"C.Do=1.4mm, Re.77
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,
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10
20
30
40
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Figure 8. Diameter histories of drops of initially 20% (w/w) solutions of different substances at an air temperature of 150 "C.
Figure 9. Diameter histories of drops of initially 20% (w/w) maltodextrin solution with different initial contents of dissolved gas.
"C, again in line with the higher driving force. No bubble growth or inflate-deflate cycling occurs for air at 88 "C, which is below the boiling point. Finally, the degree of expansion of the ultimate dried particle is greater at higher temperature, perhaps relating to higher internal pressures and/or lower viscosity of surface material. (2) Effect of Composition. Observed reduced diameter is reported in Figure 8 as a function of normalized time for drying of 20% (w/w) drops of sucrose, maltodextrin, coffee extract, and skim milk, all under the same conditions. Initial drying rates are nearly the same for all four substances because of the dominance of external resistances to heat and mass transfer early in the drying process. Drops of sucrose solution remain in period 1longer than do drops of maltodextrin solution and coffee extract, which initiate expansion sooner. Sucrose has the lower molecular weight, and hence, water diffusivities at a given dissolved solids concentration (w/w) are greater for sucrose solutions than for solutions of maltodextrin and coffee extract (see sources of data cited in part 2 ) . Hence, drops of maltodextrin and coffee extract enter the falling-rate period sooner. Concomitantly, the drop temperature would rise sooner, leading to earlier bubble nucleation, growth, and expansion. Dissolved solids in skim milk consist of approximately 53% lactose and 37% caseinate (Webb and Johnson, 1965). One might expect the water diffusivity to be similar to that for sucrose solutions, which would explain why skim milk, like sucrose, enters the falling rate period and develops expansion tendencies later. During and after drying, skim milk exhibits a smooth surface (Figure 6a), which appears to be set in place early in drying. This probably results from the protein, possibly denatured. During period 2, skim milk does not exhibit inflate-deflate cycles, probably becuase of the high surface rigidity. During period 2 , especially late in that period, drops of maltodextrin solution exhibit more and smaller bubbles than do drops of sucrose solution. This may reflect the lower diffusivity of dissolved air and water in concentrated maltodextrin solutions, implying limitations on bubble growth rate. Coffee extract exhibits properties more similar to maltodextrin solutions than to sucrose solutions or skim milk in period 2 . However, as already noted, drying drops of coffee extract tend to push material out through breaks in the drop surface, whereas drops of maltodextrin solution do not. Also, the surfaces of drops of coffee extract appear to stretch upon inflation and contract upon deflation. The surface stretching and extrusion of material may reflect the presence of surfactants and/or viscoelastic behavior
from some solutes of very high molecular weight. The different substances behave quite differently in period 3. For skim milk, the rigid surface keeps the final product expanded, despite internal voids that must be under vacuum (Verhey, l972,1973a,b). There is no final expansion or collapse, as occur for the other substances. In period 3, drops of sucrose solution slowly collapse to relatively smooth, solid particles. Diffusivities are probably still high enough so that water vapor can diffuse out through the surface, and the viscosity is probably stili low enough to permit flow that enables full collapse and smoothes the surface (Alexander and King, 1985). Unlike sucrose, drops of maltodextrin remain expanded upon final drying, although surface bubbles do collapse. The lesser collapse tendency may reflect higher viscosity and/or a lower diffusivity of water than for sucrose. In period 3, drops of coffee extract expand but then collapse upon cooling (Figure 6b), whereas drops of maltodextrin do not. The expansion would correspond to viscosity and diffusivities being more similar to those for maltodextrin than to those for sucrose. The ultimate shrinkage upon cooling would result from the lower internal pressure associated with lower temperature and may reflect more pliability or elasticity than for dried maltodextrin. Drops of maltodextrin solution dry sooner than do drops of sucrose solution (Figure 8), probably because they are expanded throughout most of the drying. Drops composed of solutions of mixtures of sucrose and maltodextrin showed a gradual transition from the properties of drops of sucrose solution to those of drops of maltodextrin solution (El-Sayed, 1987). (3) Effect of Initial Solute Concentration. The results for drops of 20%, 30%, and 40% (w/w) maltodextrin solution of equal volume show longer drying times for lower initial solute concentrations (El-Sayed, 19871, as would be expected from the greater amount of water to be removed. Higher initial solute content also leads to larger final particle volume, with the volume of the expanded particle being approximately proportional to the initial solute content. (4) Effect of Dissolved Gas. Figure 9 shows observed particle diameters versus reduced time for drops of 20% (w/w) maltodextrin solution dried a t 150 "C. The drops differ in initial content of dissolved gas. One drop was saturated with air, as was the usual case. Another was formed of solution saturated by prolonged sparging with COP. The third drop was formed from solution degassed by boiling. Although the degassed drop would absorb some air and the C02-saturated drop would lose some C 0 2 by desorption during the transfer process to the filament tip,
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2351
a) 0.0 scc/mm?
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b) 0.4 scc/mm?
4 1 tim
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c) 6.4 sec/mm2
e
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I
t
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e , 52 scc/mm:
Figure 10. Developing morphology of particles of coffee extract from the falling-dropdryer.
the results are still revealing. Gas bubbles nucleate and grow earlier in the C02-saturated drop, leading to departure from the simple-sphere behavior a t a shorter elapsed time than occurs for the air-saturated drop. This results from the solubility of C02 being much higher than that of air. Conversely, nucleation and departure from the simple-sphere case are substantially delayed for the degassed drop. These results provide additional confirmation of the importance of bubble nucleation through desorption of gases other than water vapor. The initially degassed drop boiled more violently than the air-saturated drop during period 2, ruptured more frequently, and ejected small satellite drops (Figure 6d) in the process. This behavior may result from the higher drop temperature achieved before bubble nucleation. The final morphology is, however, typical of air-saturated drops. For the C02-saturated drop, bubbles nucleate and the drop expands early in the process, before the surface of the drop has dried much. Wet material is pushed out through ruptures in the drop. The drop continues to dry by forming a new, wet surface as moisture continues to vaporize into the original C 0 2 bubble and force more material out through the rupture. The final, dried particle is highly expanded, with an appearance similar to that reported by Crosby and Weyl (1977) for drying initially foamed drops.
.lo pm
Figure 11. Developing morphology of particles of maltodextrin solution from the falling-drop dryer.
Falling-Drop Dryer. In Figure 10, a succession of SEM photographs is shown for particles of drying coffee extract (initially 30% w/w), collected in liquid nitrogen a t various levels in the falling-drop dryer and then freeze-dried. The parameter is t/D,', defined as in the suspended-drop experiments, with elapsed time derived from the integral of the inverse of the local drop velocity over the fall distance to the sampler location. The local drop velocity is derived from the terminal velocity with allowances for slowing from the initial jet velocity and for the acceleration imparted by the air drawn into the top of the column (El-Sayed, 1987). The column temperature profile for the run depicted in Figure 10 is shown in Figure 7 of part 2 of this work. The particles sampled early in the fall have relatively smooth surfaces. For longer drying times, the particles display signs of expansion, exhibit blowholes (dark circles) and develop crinkled surfaces. Similar results are shown in Figure 11for drying of drops of maltodextrin solution a t a lower temperature (180 "C). Here there are fewer signs of expansion, and the surface develops isolated folds, rather than crinkles. Optical microscopy of sampled drops revealed that internal voidage occurred for t / D o 2 = 6.4 s/mm2 and greater, but not for shorter times. Surface blowholes were characteristic of coffee extract dried at higher temperatures in the falling-drop dryer and were not seen for drying of solutions of maltodextrin or
2352 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990
'
1
'
1
'
1
Maltodextrin Solution Initial Conc 20°/0 Solids T air = 200°C. Re 5.0 7
Do = 1.4 mm
=
200 pm 0 Falling Drops Prediction
200 pm
Loo
0
0
I-
V D (s/mm2) ~
Figure 14. Comparison of diameter histories of drops of initially 2070 (w/w) maltodextrin solution in the suspended-drop and falling-drop dryers, along with prediction of a voidless-sphere model for the falling-drop drver. Figure 12. Particle of commercial spray-dried coffee extract (Alexander and King:, 1985).
~~
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1
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6 I(.,*.I
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Figure 13. Particle-diameter frequencies for particles of initiallv 2 0 7 ~(w/w) maltodextrin sampled from the falling-drop dryer at an air temperature of 180 "C.
sucrose. Blowholes are also seen in particles of commercial spray-dried coffee extract, e.g., Figure 12 (Alexander and King, 1985). Blowholes mav be residues of inflate-deflate cycles of the type observed for suspended drops and in anv event probably result from release of pressurized vapor in cavities inside the drops. As is visible for some particles in Figures 10 and 11, expanded particles sampled and dried in this wav tended to display a deep depression a t some location on the particle surface. This is probably an artifact of the technique, resulting from inward buckling following the sudden reduction in interior pressure due to the rapid cooling associated with the particle striking the pool of liquid nitrogen. As can be seen in parts c, d, and g of Figure 10, the particles sampled at any level displayed different sizes and shapes, even though the initial drops were of a constant size, and all drops experienced the same temperature profile during fall. This phenomenon has been noted before (Greenwald and King, 1981, 1982; Alexander and King, 1985). The range of sizes reflects different degrees of internal voidage and probably results from bubble nucleation being a statistical event, requiring a suitable heterogeneous nucleation site (Greenwald and King, 1981, 1982). Distributions of diameters of freeze-dried particles measured with the SEM are shown in Figure 13 for the same run for which photographs are shown in Figure 11.
The lengths of the bars shown for each sampling port are proportional to the number of particles of each size observed in the sample. For nonspherical particles, the reported diameter is the average of the maximum and minimum diameters. A t short times, i.e., for early ports, the drops are shrinking uniformly and are spherical with narrow size distributions. Drop sizes vary more a t later times as uneven shrinkage, expansion, and collapse occur. In some instances, bimodal distributions are seen a t the later ports. Examination of drops and particles by optical microscopy confirmed that varying degrees of voidage accounted for the differences in particle sizes.
Discussion Comparison of Suspended-Drop and Falling-Drop Studies. Suspended and free-falling drops demonstrate qualitativelv similar sequences of events. There is an early period of drying wherein drops shrink uniformly, followed by bubble nucleation and growth, which are followed in turn bv gross morphological changes such as formation of blowholes, uneven surface shrinkage, expansion, and collapse. Observed effects of air temperature, initial concentration, and dissolved-gas content for falling drops paralleled those observed for suspended drops (El-Sayed, 1987). Reduced diameters as functions of time for particles from the falling- and suspended-drop experiments can be compared, using the normalized-time variable. Such a comparison is made in Figure 14 for initially 20% maltodextrin solution drying in air a t 200 "C and a t a particle Reynolds number of 5.0. Also included is the prediction for a voidless sphere, from the model described in part 2 of this work. By virtue of some early formation of internal bubbles, the diameters of falling drops do not decrease initiallv as rapidlv as either the voidless-sphere prediction or the diameter of the suspended drop. Suspended drops dry in less normalized time, and the final particles are more expanded than for falling drops, probably because the filament promotes heat transfer and bubble nucleation. Another difference is that the apparent surface skin and other surface features form unevenlv on suspended drops, whereas surface features appear to be evenly distributed over the surfaces of falling drops, presumably due to free rotation and no support filament. Development of Blowholes. It was observed that tendencies for blowholes to appear in particles differed for different substances. Since this phenomenon seems not
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2353 to have been explored heretofore, it was given additional attention. Falling drops of coffee extract began to display residual blowholes at a normalized time somewhat greater than that when suspended drops started the inflate-deflate cycle. From Figure 10, it can be seen that blowholes first appear for falling drops at t/D: between 20 and 30 s/mm2. This is somewhat greater than the reduced time a t which inflation-deflation started for suspended drops of coffee extract under similar conditions of initial concentration, air temperature, and particle Reynolds number. (See, for example, Figure 8, where allowance should be made for different operating conditions.) This difference in times between the falling- and suspended-drop experiments can be rationalized through heat-transfer and bubble-nucleation effects from the filament for suspended drops. It may be possible to rationalize tendencies for sealing or closure of ruptures that occur in droplets during period 2 in terms of a competition between a sealing flow due to surface-tension driving force and viscous resistance to that flow. This concept is similar to those employed to rationalize collapse during freeze drying (Bellows and King, 1973), the extent of smoothing of surface folds in spraydried carbohydrates (Alexander and King, 1985), and stickiness and agglomeration tendencies in amorphous powders (Wallack and King, 1988). Equation 1 (Bellows and King, 1973; Bellows, 1972), a derivation for which is given by Wallack (1989), approximates the relationship between surface tension, viscosity, t, = pD/a (1) rupture diameter, and the time required for closure of a rupture through viscous flow driven by surface tension. Here t, is the time required for total closure, D is the initial rupture diameter, a is the surface tension, and p is the viscosity of the surface material of the drying drop. Measurements for aqueous solutions of sucrose and maltodextrin (Supran et al., 1971) indicate that the surface tension does not change much from the value for pure water. Surface tension measurements for 30% and 50% (w/w) aqueous coffee extract made at 20 "C by the Wilhelmy plate method gave values about one-half the value for pure water (Wallack, 1989), thereby indicating the presence of surfactants. Viscosities have been measured for concentrated aqueous solutions of sucrose (Schneider et al., 1963) and maltodextrin and coffee extract (Wallack and King, 1988; Wallack, 19891, as functions of temperature and moisture content. Comparative viscosities (mPa s) of highly concentrated aqueous solutions at 70 "C and 7.0% (w/w) H,O are sucrose 1.0 X lo4, maltodextrin, 1.3 X lo6, and coffee extract 6.7 X lo7. These conditions, although arbitrary, should be representative of those experienced by a drop in the falling-drop dryer. The viscosity of highly concentrated aqueous coffee extract under these conditions is thereby over 3 orders of magnitude greater than that of sucrose solution and 50 times greater than that of maltodextrin solution. Since the viscosity changes over a much wider range than does surface tension, the viscosity is the dominant factor in determining the time for rupture closure through eq 1. The much higher viscosity of concentrated coffee extract qualitatively rationalizes the incomplete closure of blowholes in particles of coffee extract during drying. Indeed, and 4.0 X loT3kg/s2 taking surface tensions of 7.2 X for maltodextrin solution and coffee extract, respectively, D = 1.0 X m based on the size of the blowholes in Figure 10, and the values of viscosity values listed above, the closure times estimated from eq 1 are 0.1 and 3.0 s for
maltodextrin and coffee extract, respectively. Obviously, these times are highly approximate, since actual conditions cover a wide range of viscosities resulting from variations in concentration and temperature. The typical fall time of a drop through the entire drying column was 2-3 s. Thus, the time scales for rupture closure in sucrose and maltodextrin at 70 OC and 93% (w/w) concentration are short compared to residence times in the column, while the time required for rupture closure in coffee extract is comparable to the residence times and the time for drying.
Acknowledgment This research was supported by Grant CPE84-1450 from the National Science Foundation and by a Merck Predoctoral Fellowship to one of the authors (T.M.E.) from Merck & Co., Inc. Registry No. Sucrose, 57-50-1; maltodextrin, 9050-36-6.
Literature Cited Abdul-Rahman, Y. A. K.; Crosby, E. J.; Bradley, R. L. Drying of Single Drops of Foamed and Nonfoamed Sodium Caseinate Solutions. J . Dairy Sci. 1971,54, 1111-1118. Alexander, K. Factors Governing Surface Morphology in the Spray Drying of Foods. Ph.D. Dissertation, University of California, Berkeiey, 1983. Alexander. K.: King.. C. J. Factors Governing Surface MorDhologv of Spray-Dried imorphous Substances. b y i n g Techno'l. 19g5, 3, 321-348. Bellows, R. J. Dependence of Collapse Temperature on Composition and Concentration. Ph.D. Dissertation, University of California, Berkeley, 1972. Bellows, R. J.; King, C. J. Product Collapse during Freeze-Drying of Liquid Foods. AZChE Symp. Ser. 1973, 69 (132), 33-41. Bergland, R. N.; Liu, B. Y. H. Generation of Monodispersed Aerosol Standards. Enuiron. Sci. Technol. 1973, 7 , 147-153. Charlesworth, D. H.; Marshall, W. R., Jr. Evaporation from Drops Containing Dissolved Solids. AZChE J . 1960, 6, 9-23. Crosby, E. J.; Weyl, R. W. Foam Spray Drying: General Principles. AZChE Symp. Ser. 1977, 73 (1631, 82-94. El-Sayed, T. M. Development of Particle Morphology of Drying Drops. Ph.D. Dissertation, University of California, Berkeley, 1987. Fisher, G. L.; Chang, D. P. Y.; Brummer, M. Fly Ash Collected from Electrostatic Precipitators: Microcrystalline Structures and the Mystery of the Spheres. Science 1976, 192, 553-555. Frey, D. D.; King, C. J. Experimental and Theoretical Investigation of Foam-Spray Drying. 1. Mathematical Model for the Drying of Foams in the Constant-Rate Period. Ind. Eng. Chem. Fundam. 1986,25, 723-730. Furuta, T.; Okazaki, M.; Toei, R.; Crosby, E. J. Formation of Crystals on the Surface of Non-Supported Droplet in Drying. In Drying '82; Mujumdar, A. S., Ed.; Hemisphere-McGraw-Hill: New York, 1982; pp 157-164. Furuta, T.; Okazaki, M.; Toei, R. Retention of Volatile Component in a Single Droplet during Drying. In 3rd International Congress on Engineering and Food; McKenna, B., Ed.; Institute of Chemical Engineers of Ireland: Dublin, 1983. Greenwald, C. G.; King, C. J. The Effects of Design and Operating Conditions on Particle Morphology for Spray-Dried Foods. J . Food Process. Eng. 1981, 4, 171-187. Greenwald, C. G.; King, C. J. The Mechanism of Particle Expansion in Spray Drying of Foods. AZChE Symp. Ser. 1982, 78 (2181, 101-110.
Schneider, F.; Schliephake, D.; Klinimek, D. Density Tables for Aqueous Sugar Solutions. Zucker 1963, 16,465-474. Supran, M. K.; Acton, J. C.; Howell, A. J.; Saffle, R. L. Surface Tension of Common Aqueous and Organic Phases in Food Emulsions. J. Milk Food Technol. 1971, 34, 584-585. Toei, R.; Furuta, T. Drying Procedure of a Droplet in Non-Supported State. AZChE Symp. Ser. 1982, 78 (218), 111-117. van der Stege, H. J.; Walstra, P. Vacuole Formation during Spray Drying of Milk; The Use of a Sealed Atomizing Disk. Neth. Milk Diary J . 1987, 41, 321-328. Verhey, J. G. P. Vacuole Formation in Spray Powder Particles. Neth. Milk Dairy J. 1972,26,186-202; 1973a, 26,203-224; 1973b, 27, 3-18.
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Wallack, D. A. Stickiness and Morphological Development during Drying of Drops. Ph.D. Dissertation, University of California, Berkeley, 1989. Wallack, D. A,; King, C. J. Sticking and Agglomeration of Hygroscopic, Amorphous Carbohydrate and Food Powders. Biotechnol. Prog. 1988, 4 (l),31-35.
Webb, B. H., Johnson, A. H., Eds. Fundamentals of Dairy Chemistry; AVI: Co.: Westport, CT, 1965.
Receiued for review December 13, 1989 Revised manuscript received June 15, 1990 Accepted June 24, 1990
Changes in Particle Morphology during Drying of Drops of Carbohydrate Solutions and Food Liquids. 2. Effects on Drying Rate David A. Wallack,+Tarric M. El-Sayed,t and C. Judson King* Department of Chemical Engineering, University of California, Berkeley, California 94720
Drying rates and the development of particle morphology were monitored simultaneously for initially uniform size drops falling through a column having a known temperature profile. A sampler with continuously flowing dimethyl sulfoxide solvent was used to determine the extents of drying of the captured particles. Measured water contents were compared with predictions of a model based upon diffusion within a noncirculating, voidless sphere. Drying rates for drops of aqueous solutions of sucrose and maltodextrin agreed well with this model, as did rates for drops of coffee extract drying a t low (189 " C ) air temperature. At higher air temperatures (219 and 254 "C), coffee drops dried a t rates greater than predicted by the model. These results correspond t o observed tendencies for particles of partially dried coffee extract to exhibit residual blowholes and for suspended drops to push wet material from the interior of the drops outward through surface ruptures during drying.
Introduction Commercial spray dryers tend to be quite large, so as to provide adequate fall height for complete drying of drops. For foods and many other substances, much of the residence time required for spray drying is associated with the falling-rate period, in which drying is rate-limited by moisture transport within particles. Given the complex particle morphologies identified in part 1 of this work and by previous workers, one can expect that the development of particle morphology can have significant effects upon drying rates. The purpose of the present research was to investigate the extent to which, and the way in which, drying rate is affected by developing particle morphology. There have been only a few studies of the effects of morphological development upon drying rates of particles formed from drops. Charlesworth and Marshall (1960) observed morphologies and measured weights of suspended drops of aqueous solutions of various salts and coffee extract during drying. They developed correlations for crust-formation times. Trommelen and Crosby (1970) performed similar experiments, comparing drying in air and drying in superheated steam. Toei and Furuta (1982) inferred drying rates from measurements of drop size and temperature for drops of skim milk and NaCl solution floating in an upflowing air stream. Furuta et al. (1983-1984,1985) measured drying rates and rates of loss of volatile organics from single drops of maltodextrin solution suspended in an air stream. Sano and Keey (1982) and van der Lijn (1976a,b) followed weight losses of suspended drops of skim milk and interpreted the results in terms of a diffusion model allowing for the existence of an internal, spherical cavity. As has been noted in part 1, suspended-drop studies can produce artifacts affecting the rates due to the suspension device. Also, drops used for such studies are typically an order of magnitude larger than those present in a spray dryer.
* To whom correspondence should be addressed. 'Present address: 3M Corporation, St. Paul, M N 55119. Present address: Clorox Corporation, Technical Center, Pleasanton. CA 94566. f
Daemen (1982) correlated the amounts of internal voidage and extents of loss of volatile diacetyl from spray-dried skim milk. He noted that higher drying air temperatures led to both greater internal voidage and greater loss of diacetyl and speculated that the greater particle expansion was the cause of the greater loss. The specific objective of the present work was to use the falling-drop dryer, described in part 1, to determine particle morphology and drying rate under controlled conditions of free-fall drying, for drops with sizes close to those encountered in commercial spray dryers. Predictive Model. Predictions of the average water content of a drop versus time were made through the mathematical model of Frey and King (19861, postulating drying of a sphere with no internal voidage. The model allows diffusion coefficients within the sphere to vary sharply with solute concentration, as well as with temperature. External heat- and mass-transfer coefficients for use in the model were taken from the correlation of Ranz and Marshall (1952). The equations underlying the model are presented by Frey and King (1986). Their variable cy (voidage) was taken to be zero. The model, as implemented, does not allow for internal circulation, internal voidage, nonsphericity, surface ruptures, residual blowholes, or surface folds or wrinkles, all of which are reported in part 1 of this work and related past studies. The model thereby affords a basis for determining when these other factors affect the drying rate significantly. The equations were solved numerically by using a computer program supplied by Keey (1983) and modified by Frey (1985), similar to the method used by Sano and Keey (1982). The model and method of solution are described in detail by Wallack (1989), who also gives a program listing; see also Frey (1985), Frey and King (1986), and El-Sayed (1987). Diffusivities for the sucrose-water system were taken from a correlation fitted by Etzel (1982) to the data of English and Dole (1950) and Henrion (1964). Diffusivities for the maltodextrin-water system, as a function of concentration and temperature, were obtained from the cor-
0888-5885/90/2629-2354$02.50/0 0 1990 American Chemical Society
