2354
I n d . E n g . C h e m . Res. 1990, 29, 2354-2357
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
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2355
2
30
6-
25-
?
20-
E
--\+,+-2-+/+
I
3
0 +
Model Experimental Temperatdre
- 60
2
-
3
-
40 20
Figure 3. Percent evaporation versus normalized time, predictions of voidless sphere model, and temperature profile for drops of 30% (w/w) sucrose solution initially 0.191 mm in diameter.
Figure 1. Solvent collector.
bFigure 2. Coolant and solvent feed delivery systems for solvent collector.
relation fitted by Furuta et al. (1984) to their experimental data for that system. Effective binary diffusivities for the dissolved coffee solids-water system (coffee extract) were taken from a correlation (Wallack, 1989) fitted to the results of Luyben et al. (1980), covering weight fractions from 0.09 to 0.67 water and temperatures from 30 to 70 "C. Because of the wet-bulb depression, this temperature range covered the drying conditions reported in this work.
Experimental Apparatus and Procedure The falling-drop dryer described in part 1 of this work was used to dry a stream of drops of initially uniform size. The resultant drops and particles were collected at the eight sample ports along the column height for observation of morphology by scanning electron microscopy (SEM), as described in part 1, and for determination of the moisture content. Solvent Sampler. The water contents of particles collected at the various ports were determined through use of a solvent sampler based on the same principles as the sampler used by Etzel and King (1984). As is shown in Figures 1 and 2, an open mixing cup was continuously fed fresh solvent, dimethyl sulfoxide (DMSO), which dissolved the droplets raining on the open surface. The solution drained continuously through a second line. The cup was jacketed and cooled with chilled 2-propanol. The outer casing of the sampler fit through two collars in series, one fixed and one mobile, in the alignment sleeve. This sleeve fit over the port to position the collector within the column and insulate the column interior from the environment during collection.
The DMSO flow rate used was the highest consistent with there being adequately high concentrations for analysis. The inlet solvent flow rate typically varied from 1.0 mL/min for sampling at the bottom of the column to 2.0 mL/min at the top of the column. The solvent sampler was left in place through a port for two to three sampler residence times, typically 10 min, before 10-mL samples were collected from the exit line into sealed bottles (Wallack, 1989). The temperature of the solvent cup affected the amount of moisture condensed into the solvent from the air in the column, the amount of solvent lost by evaporation, and the rate and capacity for dissolution of the droplets. The most appropriate cup temperature was determined by exploratory experiments to be 40 "C. This temperature is well below the boiling points of DMSO and water, so evaporative losses were not great. It is high enough to enable dissolution of sucrose, maltodextrin, and coffeeextract solids. DMSO fumes at the surface of the cup and provides a counter-flux that limits condensation of moisture from the air. A slight continual overflow from the sampler cup was maintained so as to purge condensed moisture from the surface and to wash the outer walls of the collector. Exploratory measurements showed that the rate of moisture condensation into the solvent collector at low flow rates of methanol solvent were of the order of 1 mg of H,O/g of solvent, for DMSO with a solvent cup temperature of 40 "C or for methanol with a solvent cup temperature of 20 "C (El-Sayed, 1987). Water concentrations in the solvent from the lower sample ports for all runs except that reported in Figure 3 were about 15 mg of H,O/g of solvent. For the run shown in Figure 3, water concentrations in the solvent from the lower ports were about 5 mg of H,O/g of solvent. Analytical Methods. The resulting solution contained DMSO, water, and dissolved solids and required analyses for concentrations of the latter two. In all cases, Karl Fischer analysis, using a Computrac MS-1 Karl Fischer titrator (Qunitel Corp., Tempe, AZ), was used to determine the water content. Methanol was added to the sampled DMSO solution in a volumetric ratio of 101 before analysis so as to avoid interference of DMSO with the measurement. The moisture content of the methanol and a methanol/DMSO blank was also determined. Two different procedures were used to determine the dissolved solids: (1) Sucrose. The solutions were analyzed for sucrose content by means of a Spectra-Physics SP8000B highperformance liquid chromatograph (HPLC) equipped with
2356 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 I v
i
I
I
I
1
I
vi
v)
3!
!OO 180
9
2
160
80
+
0
TemDerature
+
20
0
1
1
20
40
VD',
I
1
,
0
60
(sin")
I
I
I
1220 60
,
vi
241 +,-
+-+
-
.. i - " 3 20S c
16 -
.. i-" 2.. 30-
o Experimental Temperalure EndoiCAP
0
-
m
g 0
20
60
40
VD',
20
1
40
r',
20
80
(s/mm'i
Figure 5. Percent evaporation versus normalized time, predictions of voidless sphere model, and temperature profile for drops of 30% (w/w)coffee extract initially 0.253 mm in diameter.
an Aminex HPX-42A stainless steel column (Bio-Rad Laboratories). The column was jacketed so that it could be heated with water circulated from a bath maintained at 80 "C. The mobile phase was water purified by means of a Milli-Q water purification system (Millipore Corp.) and fed to the column at a flow rate of 0.7 mL/min. The detector was a Model R401 differential refractometer (Waters Associates, Inc.). (2) Maltodextrin. The solutions were analyzed for maltodextrin content by HPLC, as described for sucrose solutions. The largest peak was calibrated and monitored (Wallack, 1989). (3) Coffee Extract. Solutions of coffee extract in water and DMSO were analyzed gravimetrically. Known weights of solution were placed in the prechilled freeze-drying apparatus (part 1) and were dried at 50 "C and 10 Pa for 24 h. Coffee solids remained after this time (Wallack, 1989).
Results and Discussion The results of drying-rate experiments are presented in Figures 3-7, along with the corresponding column airtemperature profiles and the predictions of the voidlesssphere model, using the experimental temperature profile as input to the model. Percent evaporation is defined as the percent of the total water initially in the drops that was removed from the particles collected at the particular sampling level. Sucrose. Figure 3 reports the results for drops of sucrose solution. The percent evaporation as a function of time up to t / D 2 = 30 s/mm2 follows the predictions of the model well. This is as expected, since sucrose, a low mo-
,
I
+
/ E
t-
W
+
10-
0 0
- 260 -240 -220 -200
Experimental Model
I
20
,
g
2
-180
p
- 1160 -140 20
p3
-100 - 80
20-
> m
8
-
+
n 5 40'
._ Model
+
40
Figure 6. Percent evaporation versus normalized time, predictions of voidless sphere model, and temperature profile for drops of 31% (w/w)coffee extract initially 0.216 mm in diameter.
28
n
I
-
V D ~( s i n " )
Figure 4. Percent evaporation versus normalized time, predictions of voidless sphere model, and temperature profile for drops of 30% (w/w)maltodextrin solution initially 0.191 mm in diameter. I
Model Experimental Temperature
-
60 40
0,
2 0 -
20 I
1
40
60
80
0
t ' D t (s,?")
Figure 7. Percent evaporation versus normalized time, predictions of voidless sphere model, and temperature profile for drops of 30% (w/w)coffee extract initially 0.182 mm in diameter.
lecular weight carbohydrate, does not develop voidage or surface morphological features for drying under these conditions in the falling-drop dryer (Greenwald, 1980; El-Sayed, 1987). The end of the constant-activity drying period (designated as "End of CAP" on Figures 3-7) is defined as the normalized time when the predicted drop temperature exceeds the wet-bulb temperature for the conditions of the run by more than 2 "C. The region of close agreement with the model in Figure 3 falls within the CAP. There is a noticeable drop off in the indicated drying rate at later times in Figure 3. This may be attributable to scatter or to complications due to moisture absorption from the air for highly dilute samples. As already noted, for this run the moisture content of samples taken from the later ports was unusually low. No correction was made for moisture absorbed from the air. Maltodextrin. Figure 4 shows the extent of drying for free-falling maltodextrin drops. Again the experimental data are in rather good agreement with the predictive model but fall somewhat below the predictions of the model. As is shown in Figure 11of part 1,the morphology developed by drops of maltodextrin solution in the falling-drop dryer consisted of deep surface folds, with internal voids but no residual surface blowholes. For the early part of this run, drying is in the constant-activity period, where external resistances to heat and mass transfer dominate the drying rate. However, the run does extend substantially into the predicted falling-rate period. Apparently the developing internal voidage and surface folds do not affect drying rates substantially for the portion of the
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2357 falling-rate period that is covered. It seems unlikely that surface folding and particle expansion would have offsetting effects 9" drying rate, since both factors should serve to increaivurface-to-volume ratios. Coffee Extract. .Figures 5-7 report temperature and moisture profiles for three runs in which streams of drops of coffee extract were dried at different air temperatures. Figure 5 shows results for relatively large drops of coffee extract at comparatively low air temperatures. Because of these conditions, most of the run is in the constantactivity period. The deviation between predicted and experimental moisture contents is small and within the experimental uncertainty. In Figure 6, for smaller drops dried a t a higher temperature, there appears to be an inordinate increase in the amount of moisture lost relatively early in the drying process, between t / D o 2= 10 and 20 s/mm2. This effect becomes even more pronounced for coffee drops dried at still higher column temperatures and with still smaller initial drop diameters (Figure 7). The extra losses occur at or somewhat before the predicted end of the constant-activity period. A t the later stages of drying in Figures 6 and 7, the slopes of the experimental percent evaporation data are somewhat greater than the slopes predicted by the model of a voidless sphere. The predicted slope decreases over the course of drying, as is expected for the falling-rate period. However, the experimental drying rates in these two figures do not show the same extent of decrease. Because smaller initial drop sizes were used with higher air temperatures in the runs reported in Figures 5-7, the effect of drop size cannot be separated from that of air temperature. Further Discussion. As reported in part 1, drops of both maltodextrin solution and coffee extract undergo surface ruptures during the course of drying at sufficiently high temperature. However, the natures of the rupture and the ensuing morphological development differ between the two materials. The rupture of maltodextrin droplets is followed almost immediately by a collapse and closing of the hole created by the rupture. On rupture, coffee drops have a tendency for material to be pushed from inside the drop through the rupture site. Figure 10 of part 1 of this work, which corresponds to the conditions of Figure 7, indicates that residual blowholes appear in the particles of coffee extract for reduced times of 20-30 s/mm2 and greater. Residual blowholes also appeared at about 30 s/mm2 for coffee extract under the conditions of Figure 6 and were not seen at all for the conditions of Figures 4 and 5 for particles of maltodextrin and coffee extract, respectively (Wallack, 1989). Thus, there appears to be a correspondence between the surge in drying rate and the appearance of residual blowholes in particles for the conditions of Figure 6 and 7. When a blowhole is present, the interior of the drop connects through the blowhole with the surroundings in a direct vapor path, which could lead to a greater rate of drying. The surge in drying rate may also correspond to conditions where wet material from the interior of drops is pushed outward through surface ruptures. Either way, wet material in the interior of the drop would be exposed to hot air, bypassing the normal diffusional limits imposed by a surface region that is depleted in moisture. Surface ruptures and consequent exposure of interior liquid through the ruptures should become more of a factor a t higher air temperatures, in line with the observations
of part 1 of this work. Correspondingly, the deviations from the predictions of the model of a voidless sphere are greater for Figure 7 , where the air temperature is highest.
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 Charlesworth, D. H.; Marshall, W. R., Jr. Evaporation from Drops Containing Dissolved Solids. AIChE J . 1960, 6, 9-23. Daemen, A. L. H. The Estimation of the Mean Particle Density, the Vacuole Volume and the Porosity of Spray-Dried Porous Powders. Neth. Milk Dairy J . 1982, 36, 53-64. El-Sayed, T. M. Development of Particle Morphology of Drying Drops. Ph.D. Dissertation, University of California, Berkeley, 1987. English, A. C.; Dole, M. Diffusion of Sucrose in Supersaturated Solutions. J . Am. Chem. SOC.1950, 72, 3261-3267. Etzel, M. R. Loss of Volatile Trace Organics during Spray Drying. Ph.D. Dissertation, University of California, Berkeley, 1982. Etzel, M. R.; King, C. J. Loss of Volatile Trace Organics during Spray Drying. Ind. Eng. Chem. Process Des. Deu. 1984, 23, 705-710. Frey, D. D. Experimental and Theoretical Investigation of FoamSpray Drying. Ph.D. Dissertation, University of California, Berkeley, 1985. 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.; Tsujimoto, S.; Okazaki, M.; Toei, R. Effect of Drying on Retention of Ethanol in Maltodextrin Solution during Drying of a Single Droplet. Drying Technol. 1983-1984,2, 311-327. Furuta, T.; Tsujimoto, S.; Makino, H.; Okazaki, M.; Toei, R. Measurement of Diffusion Coefficient of Water and Ethanol in Aqueous Maltodextrin Solution. J . Food Eng. 1984,3, 169-186. Furuta, T.; Okazaki, M.; Toei, R. Flavor Retention on Drying of a Single Droplet under Various Drying Conditions. Proc. 4th International Drying Symposium, Kyoto, Japan; 1984; pp 338-344. Greenwald, G. C. Particle Morphology in Spray Drying of Foods. Ph.D. Dissertation, University of California, Berkely, 1980. Henrion, P. N. Diffusion in the Sucrose and Water System. Trans. Faraday SOC.1964, 60, 72-82. Keey, R. B. University of Canterbury, Christchurch, New Zealand, personal communication, 1983. Luyben, K. Ch. A. M.; Olieman, J. J.; Bruin, S.Concentration Dependent Diffusion Coefficients Derived from Experimental Drying Curves. In Drying '80; Mujumdar, A. S., Ed.; HemisphereMcGraw-Hill: New York, 1980; pp 233-243. Ranz, W. E.; Marshall, W. R., Jr. Evaporation from Drops. Chem. Eng. Prog. 1952, 48, 141-146, 173-180. Sano, Y.; Keey, R. B. The Drying of a Spherical Particle Containing Colloidal Material into a Hollow Sphere. Chem. Eng. Sci. 1982, 37, 881-889. Toei, R.; Furuta, T. Drying Procedure of a Droplet in Non-Supported State. AIChE Symp. Ser. 1982, 78 (218), 111-117. Trommelen, A. M.; Crosby, E. J. Evaporation and Drying of Drops in Superheated Vapors. AIChE J . 1970, 16, 857-867. van der Lijn, J. Simulation of Heat and Mass Transfer in Spray Drying. Research Report 845; Center for Agric. Pub. & Doc.: Wageningen, Netherlands, 1976a. van der Lijn, J. Ph.D. Dissertation, Agricultural University, Wageningen, Netherlands, 197613. Wallack, D. A. Stickiness and Morphological Development during Drying of Drops. Ph.D. Dissertation, University of California, Berkeley, 1989.
Received for reuiew December 13, 1989 Revised manuscript received June 15, 1990 Accepted June 24, 1990
