Ind. Eng. Chem. Process Des. Dev. 1084, 23, 705-710 Rossini, F. D.; Pitzer, K. S.; Taylor, W. J.; Ebert, J. P.; Kilpatrick, J. E.; Beckett, C. W.; Williams, M. G.; Werner, H. G. A.P.I. Research Project 44. Satterfild, C . N.; Huff, G. A., Jr. Can. J. Chem. Eng. 1982, 6 0 , 159. SatterfieM, C. N.; Huff, G. A., Jr. J. Catal. 1982. 73, 187. htterfkld, c. N.; Way, P. AICh E J. 1972, 18, 305. Thomson, W. J.; A r d t , J. M., Wright, K. L. Prepr. Fuel Chem. Dlv. Am. Chem. SOC. 1980, 25, 101. Vannice, M. A. J . C 8 f d . 1975, 37,482.
705
Vannice, M. A. Catal. Rev. Sci. Eng. 1978, 1 4 , 153.
Received for reuiew November 29, 1982 Revised manuscript received September 1, 1983 Accepted December 12, 1983 This study was supported by the Office of Fossil Energy, U.S. Department of Energy under Grant DE-FG22-81PC40771,
Loss of Volatile Trace Organics during Spray Drying Mark R. Etrei+and C. Judson King' Department of Chemlcal Englneerlng, Unlversiiy of Callfornia, Berkeley, Callfornia 94720
Retentions were measured for volatile acetates in sucrose solutions during spray drying at high nozzle pressures and high air temperatures. The use of a local sampler of novel design served to demonstrate the course of the loss of volatile acetates from the nozzle region onward to the asymptotic retention predicted by the selectivediffusion theory. Results for a fan-spray pressure atomizer are compared wkh those for a two-fluid atomizer and with predictions based upon mass-transfer theory.
Introduction Most of the compounds which give foods their characteristic aroma and flavor are highly volatile with respect to water. Therefore they are easily lost during spray drying. Previous work on the retention of volatile compounds and other food-quality factors during spray drying has been reviewed by Bomben et al. (1973), Bruin and Luyben (1980), and King et al. (1984). Thijssen and Rulkens (1968) found that the retention of volatile compounds in spray-dried foods is much higher than would be expected from equilibrium considerations alone. They attributed this to the rate-limiting effect of liquid-phase diffusion of the volatile compounds to the drop surface. As the water concentration decreases, the diffusion coefficients of these compounds decrease more rapidly than that of water. Below a certain water content the outer layers of the drying drop become essentially impermeable to the volatile compounds, while still selectively transmitting water. This is known as the selective-diffusionconcept. Most investigations of the losses of volatile compounds during spray drying have measured the effects of operating variables (air temperature, feed temperature, feed composition) on the retention of volatile compounds in the final dry product. The stages of spray drying where most of the volatile compounds are lost remain unidentified by these experiments. Rulkens and Thijssen (1972) studied the retention of volatile compounds in the dry product as a function of the feed dissolved-solids concentration, feed temperature, and air inlet temperature. The most striking effect they observed was an increase in the retention of volatile compounds with increasing dissolved-solids content of the feed. The effect of dissolved-solids content can be interpreted through the selective-diffusion theory. Kieckbusch and King (1980) developed a spray-sampling device to measure the losses of volatile compounds from sucrose solutions of various concentrations in the vicinity of a pressure atomizer during spray drying. They noticed Polaroid Corp., Waltham, MA 02254. 0196-4305/84/1123-0705$01.50/0
very high losses of volatile compounds occurring within 20 cm of the atomizer, even for sucrose solutions as concentrated as 40 wt % . Their mass-transfer analysis indicated that the primary additional mechanisms, other than diffusion within stagnant drops, were probably (1)diffusion from the very thin liquid film leaving the atomizer before disintegration, (2) drop formation itself, and (3) internal circulation of drops in the period just after formation. Kieckbusch and King (1980) found that the losses of volatile components in the vicinity of their pressure atomizer increased with increasing temperature of the liquid feed. They also found that higher drying-air temperatures and higher sucrose concentrations increased the retention of volatile Components at a given percent water evaporation. Their experiments used operating conditions relatively mild compared to those typically used in industry, and all of those experiments resulted in total loss of the volatile compounds in the product. This work develops further fundamental understanding of how volatile components are lost from different atomizers, with operating conditions more similar to those used industrially. Experimental Equipment and Procedure The spray drying tower shown in Figure 1 was used to atomize liquid feed solutions of various viscosities and concentrations at pressures up to 7.0 MPa and is described in detail by Etzel (1983). The liquid feed consisted of sucrose solutions with individual acetates present at levels of about 650 ppm w/w each. Water in the spray evaporated and the volatile acetates were lost as the drops traveled downward with concurrently flowing air. The collection system allowed for continuous sampling of the spray at different axial positions. The drying air was heated by an open-flame burner to a maximum temperature of 243 "C and had a constant flow rate of 0.00755 m3/5, expressed at 0 "C and 0.1012 MPa. Spraying Systems Co. two-fluid and fan-spray atomizers were used (spray setup No. 2A and spray tip No. 950017-TC). Spray Sampling. The sampler and sampling system are depicted in Figures 2 and 3. A miscible solvent (methanol) was pumped to a small bowl where it would @ 1984 American Chemical Society
708
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 I+
122
57
i F
-
ReLocking Pinhi
u
Polk-Positionin:
Cooling F l u i d Return Cooling F l u i d I n l e t
Compressor
Chilled Solvent
) E d 2
-
Sample Bottle Dry I c e / l s o propanol Both
I
0 0, ~
P T I
2 3 4
5 6
(Dimensions in c m )
Pressure Sensor Temperature Sensor Gas Shut-Off V a l v e Safety Control Relay Aluminum Plates Glass Door ( W 1 3 c m , H 1 5 c m ) Round W i n d o w ( I O c m d i a ) Accumulator
d
Y
‘1(
COOllnQFluid Pump
Methanol (Solvent)
Air Pump
Figure 3. Sampler flow system.
Figure 1. Spray-drying apparatus.
‘AI
uminum Bar
----CoollnQ Jacket
\
\\
Cooling Fluid
Outlet Solvent Inlet
Ice-Water Both
w
Exhaust Fan
Piston Pump
CoolinqFluid Reservoir
the atomizer. The collector of Kieckbusch and King works on the prinicple of lowering the fugacity of the volatile acetates by cooling. The collector used in the present work lowers the fugacity of the organic volatile components both by cooling and by using an organic solvent. This collector and the collector designed by Kieckbusch and King (1980) were used to sample sprays produced under identical conditions in order to verify the accuracy of the new sampler. The samplers were found to yield identical results, within their individual experimental errors (Etzel, 1983). The collector used in this work also performed well with viscous fluids and at large sampling distances. Analytical Procedures. The acetate concentration, specific gravity, weight fraction sucrose, and weight fraction water were determined for each sample. Analyses for both sucrose and water were necessary because of the presence of large amounts of methanol. The concentrations of the volatile acetates were obtained by direct sample injection into a gas chromatograph (Perkin-Elmer Model No. 3920) with a flame-ionization detector. The sucrose concentration was determined with a refractometer, using a reconstituted aqueous solution after the methanol was removed by evaporation. The specific gravity was determined with glass pycnometer bottles. The weight fraction water was determined by Karl Fischer titration. The retention of volatile compounds (RV) was determined by
Sampling Outlet
Figure 2. Sample-collection device.
mix with the impinging spray and flow out a sample collection tube. The sampler designed by Kieckbusch and King (1980),and also utilized by Zakarian and King (1982), was not used for this work because it did not perform well with viscous feeds and at large sampling distances from
Terms are defined in the Nomenclature section. The ratio of specific gravities is included in this calculation so as to convert the constant volume of sample injected into the gas chromatograph to weight.
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 707
Table I. Propyl Acetate Retentions at the End of the Liquid Sheet, as Predicted by the Simpeon-Lynn Model sheet angle, sheet acetate sheet veloc, uncor sheet atomization atomizer flow wt% rad retention, % m/s length, cmb press., MPa sucrose rate x lo6, ms/s 1.53 88 5.05 82.3 1.0 T 40 7.00 1.1 T 1.61 88 5.05 82.3 50 7.00 1.50 80 5.05 82.3 2.1 L 60 7.00 1.3 T 1.44 81 3.79 61.7 40 3.55 1.43 61 3.79 61.7 2.4 L 50 3.55 2.8 L 1.37 68 3.79 61.7 60 3.55 1.59 85 5.05 82.3 1.3 T 50" 7.00 3.8 L 1.24 49 2.84 46.3 60 1.83 "Column air temperature = 243 OC. bL= laminar; T = turbulent.
The percent evaporation (%EV) was calculated from the ( 5 expression %EV =
[
1
-(-')(&)I wwf
wsd
X
100
4
(2)
where qwd wws - (WWb/sw) -- (3) wsd wss In the definitions of these terms, "sample" refers to the total of the collected fluid and the water added to the sample bottle before sample collection. The water-to-sugar ratios in the spray and in the sample bottle were not equal because water was added to the sample bottle to prevent sugar crystallization. It is important to note that this definition of the percent water evaporation is not the same as that used by Kieckb w h and King (1980) and zakarian and King (1982). The present definition results in 100% evaporation at total dryness; the definitions used in the previous studies result in percent evaporation at total dryness equal to the weight percent water in the feed. The results were generally reproducible to within 2 or 3% for both acetate retention and percent evaporation. Sheets Losses for Fan-Spray Atomization A fan-spray atomizer forms a thin liquid sheet, usually in the shape of a circular sector. The sheet thins as it travels away from the orifice, and eventually it breaks into ligaments of liquid. These ligaments then collapse into drops. Volatile compounds can be lost from the sheet, the ligaments, and the drops. Simpson and Lynn (1977) presented a model for desorption from a planar, laminar, liquid sheet of constant and uniform velocity. This model can be used to predict the losses of propyl acetate from sheets. Photographs of the sprays were used to estimate the spray angles and sheet lengths needed for this model. Pseudo-binary diffusivities for the acetates in sucrose solutions were taken from the work of Frey and King (1982). A value of 1.25 times the measured sheet length was used in the model as an estimate of the effect of the waviness of the sheet ( E b l , 1983). Table I summarizes the information used in, and the predictions of, the Simpson-Lynn Model. Retentions of Volatile Acetates General Trends. Figure 4 shows the retentions of the acetates vs. axial distance for atomization of a 50 wt % sucrose solution in 200 "C (inlet temperature) column air, using the fan-spray nozzle a t an atomization pressure of 7.00 MPa and a liquid feed temperature of 49.5 OC. As noted by Kieckbusch and King (1980) and Zakarian and King (1982),the effects caused by differences in drop sizes can be largely eliminated by plotting measured retentions vs. the percent water evaporation. A similarity analysis shows that, under certain assumptions, both the percent evaporation and the loss of volatile compounds are functions of 7/Ro2 once drops are formed (Kerkhof and
-
35 c
0
c
a
4"
-
% 25E
.--
c
z
15-
-
-
Ethyl Acetate
A Propyl Acetate
m
Butyl Acetate
0
40% sucrose
A
-
50% sucrose 60% sucrose
-
60 - m
-3
I
101
5
I
I
I
I
I
IO
15
20
25
30
Distance From Atomizer ( c m )
Figure 5. Retention of propyl acetate w. axial distance for different feed concentrationsof sucrose; other conditionsthe same as in Figure 4.
Schoeber, 1974; Zakarian, 1979). Here T is residence time and Ro is initial drop radius. Initially, in Figure 4, the rate of acetate loss is greater than the rate of water loss, but this difference decreases as the percent evaporation increases. This change in slope is due to a change in the ratio of the rate of removal of acetates to the rate of removal of water in the nozzle region. In the liquid sheet, acetates are lost at a much greater rate than water. In sufficiently dry drops, the selective diffusion concept predicts that water is lost more rapidly than acetates. Therefore the (negative) slope should greatly decrease as the percent evaporation increases. In line with this analysis, the order of the retentions for the three acetates in Figure 4 corresponds to that for liquid-phase controlled resistance to mass transfer (Kieckbusch and King, 1980). However, the precision of the data is not sufficient to allow determination of the exponent which
60 -
I
I
40% sucrose A 5 0 X sucrose m 60% sucrose
1
1
I
-
m
m
\ 55L
-
I
\
I
1 I
1
1 25 -
101
IO
'
I
20
I
I
I
I
30 40 % Evaporation
I
7.00 MPo A3.55 MPo m 1.83 MPa
I
50
60
Figure 6. Results of Figure 5, replotted vs. percent evaporation.
relates the liquid-phase mass transfer coefficient to the diffusivity (Etzel, 1983). Effect of Feed Sucrose Concentration. The effect of feed sucrose concentration is illustrated by Figures 5 and 6, which show results for propyl acetate. One can notice from these figures the very sharp increase in the acetate retention with increasing feed sucrose concentration. The effect of feed concentration was also investigated for an atomization pressure of 3.55 MPa, resulting in essentially the same trends as those for the higher-pressure experiments (Etzel, 1983). The retention for 40 wt % sucrose solution was up to five times less than that for the 60 wt % solution. Although it appears that the 60 wt% sucrose solution would have a substantial retention at total dryness, the 40 and 50 wt 740 solutions appear to approach very low or zero retentions. Most of the acetates that are lost escape within 10 cm of the atomizer in all cases. The acetate losses can be grouped into losses from the liquid sheet, losses during drop formation, and losses from the drops once they are formed. The predicted sheet retentions are 88, 88, and 80% for the 40, 50, and 60% sucrose solutions, respectively, represented in Figures 5 and 6 (see Table I). The data for the 40 and 50% solutions allow extrapolation back to the sheet edge for an estimated sheet retention above 70%. The predicted and experimental losses compare favorably, considering the limitations of the sheet-loss model, and that additional losses due to the drop formation must also be added to the atomization-region losses. The sheet losses do not account for most of the observed losses for the 40 and 50% solutions. Therefore major additional losses must be occurring from the drops. The aerodynamic model described by Rothe and Block (1977) was adapted to this work (Etzel, 1983). Similar aerodynamic models were also developed by Kieckbusch (1978) and Zakarian (1979). This model can be used to predict drop-fall times for the sprays of Figures 5 and 6. Predicted drop-fall times for the 40 and 50% solutions are very close together, but that for the 60% solution is 15% lower. The correlationsof Hasson and Mizrahi (1961)were used to estimate the effect of sucrose concentration on the drop-size distribution of the sprays. The drop-size distribution was estimated to be essentially the same for the different feed concentrations, with slightly greater sizes for the more concentrated solutions. Therefore differences in drop retentions for the different solutions must be largely due to variations in the acetate diffusivities or the effects of viscosity on circulation, oscillation, etc. Since sheet losses are apparently the same for the 40 and 50% solutions (see Table I), this must be the explanation for
the higher retent,ions observed for the 50% solution in Figures 5 and 6. Menting et al. (1970) found that volatile compounds were lost almost exclusively during the constant activity period (CAP) or constant-rate period, where the drop evaporates as if it were pure water. Then the losses of volatile compounds would be expected to become much less when the CAP ends and evaporation becomes controlled by internal mass transfer of water, with the surface sucrose concentration rising toward the critical concentration for selective diffusion. The sharp effect of feed sucrose concentration on the loss of volatile compounds is partially due to the strong effect of sucrose concentration on the length of time the drop evaporates in the CAP. However, the activity of water in a 60% sucrose solution is 0.9 (Chandrasekaran and King, 1971),and therefore 60% feed solutions should not show a CAP. Consequently, the generalization that volatile compounds are lost primarily during the CAP is not always correct. A 50% sucrose solution would have a surface water activity of 0.9 by the time it reached 33% evaporation (or 60% sucrose), even if no concentration gradients were formed in the drop. From Figure 6 the acetate losses obviously do not stop at 33% evaporation. Therefore the losses of volatile compounds occurring outside the CAP are also important. As the sucrose concentration of the liquid feed is increased, at a constant atomization pressure, the sheet length increases, and sheet losses increase. Drop losses decrease as the sucrose concentration increases, because the diffusivity of the volatile compounds is decreased, the CAP is shortened, and the surface reaches the critical sucrose concentration for selective diffusion sooner. These opposing effects could cause a maximum in the product retention of volatile compounds to occur as the feed concentration is increased. Rulkens and Thijssen (1972) report data for the retention of volatile compounds in maltodextrin solutions which indicate that a maximum in retention occurs with increasing feed concentration. Effect of Atomization Pressure. Figure 7 shows the effect of atomization pressure on propyl acetate retention for a 60% sucrose solution atomized with the fan-spray nozzle at 1.83, 3.55, and 7.00 MPa into 200 "C (inlet) column air. If the assumptions of the similarity analysis discussed earlier are valid, the asymptotic acetate retention should not be a function of drop size. The sheet losses do not adhere to the similarity analysis; i.e., they give a different ratio of acetate loss to water loss at different pressures. If the total drop-diffusional losses are not a function of the drop fall time and the drop size, then the
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 4, 1984
pressure should affect only the sheet-loss component of the asymptotic acetate retention. Pressure may also affect losses due to drop formation, circulation, oscillation, etc. The sheet-losses predictions from Table I qualitatively agree with the results in Figure 7. As the atomization pressure is decreased, the drop size increases and the drop fall time decreases. This is because, except within a few centimeters of the atomizer, the higher terminal velocity of the larger drops is more important than their lower initial velocity. Therefore, the Fourier number ((D7/R:), where D is the diffusivity of the acetate) corresponding to a given axial distance is smaller for the low-pressure experiments and this implies higher acetate retentions at a given axial distance. This is not what is observed. The discrepancy is probably mainly due to the large differences in sheet losses among the different atomization pressures (Table I). The small Fourier number (this time containing the liquid water diffusivity) for the lower-pressure experiments also explains why the largest percent evaporations measured increased in the order 22.5,50.0, and 55.5% for the 1.83,3.55, and 7.00 MPa pressure runs, respectively (Etzel, 1983). Effect of Inlet Column Air Temperature. The effect of column air temperature on the loss of acetates was investigated for 50% sucrose solutions atomized at 7.00 MPa into 200 "C and 243 "C inlet column air. The liquid feed temperature was kept at the wet bulb temperature for 200 "C air (49.5 "C), in order to keep the sheet losses for the two runs similar. The higher-temperature experiment gave lower retentions at a given distance, probably because the sheet losses were larger for this experiment and/or the drop losses were higher because of the higher wet-bulb temperature resulting from the higher temperature air. When the data were plotted against percent evaporation there was essentially no difference in the retention between the two runs (Etzel, 1983). This result differs from the finding of Kieckbusch and King (1980) at much lower atomization pressures that higher air temperature increases retentions at a given % evaporation. Dry-Product Retentions. Although retentions measured near the atomizer appear to be close to asymptotic in several cases, they are not necessarily the same as the dry-product retentions. The remaining fall distance is very large in comparison with the distance over which samples were taken in the present work. Additional losses may occur outside the atomization region as the drops approach the column air temperature. This may include losses due to changes in particle morphology, as suggested by Rulkens and Thijssen (1972). It is difficult to conclude from the present data whether the retentions lower in the column are not at all dependent upon the percent evaporation and distance or are only weak functions of these variables. Frey (1984) has measured dry-product acetate retentions for some of the conditions used in this work. For 60% solutions atomized at 7.00 and 3.55 MPa (see Figure 7) the dry-product propyl acetate retentions were 35.5 and 26.5%, respectively. For 50% sucrose atomized at 7.00 MPa (see Figure 4) the dry-product propyl acetate retention was 6.5%. Thus losses lower in the column are substantial. The difference between the two dry-produd retentions for the 60% solutions corresponds closely to the difference in the retentions at 25 cm axial distance for those two runs, as shown in Figure 7. This suggests that the absolute amounts of additional losses are not strongly dependent upon the atomization pressure or therefore the drop size. Two-Fluid Atomizer. Experimental results were obtained for atomization of 50% sucrose solutions with the two-fluid atomizer at a liquid-feed temperature of ap-
101 5
I
1
10
I
I
1
709
15 20 25 Distance F r o m A t o m i z e r ( c m )
30
Figure 8. Comparison of propyl acetate retentions with different atomizers for 50% sucrose feeds. See text for conditions with twofluid atomizer. Fan-spray atomizer conditions as in Figure 4. 40
I
I
I
I
I
I
1
0 Two-Fluid Atomizer
A Fan-Spray
Atomizer
30
t 101
15
I
I
I
I
I
I
20
25
30
35
40
45
J
50
% Evaporation
Figure 9. Results of Figure 8, replotted vs. percent evaporation.
proximately 47 "C and an inlet column air temperature of 150 "C. The liquid and gas flow rates in all experiments were approximately 0.49 and 0.36 g/s, respectively. This corresponded to an atomization air pressure of approximately 0.27 MPa. Increasing the atomizing air temperature from 49 to 99 " C resulted in a small increase in retention. This is probably due to the lower-humidity air, in the higherair-temperature experiment, causing more rapid onset of the selective-diffusion mechanism. The acetate retention order (ethyl acetate was retained less than propyl acetate) indicated that volatiles losses from the two-fluid atomizer are predominately liquid-phase controlled, following the analysis of Kieckbusch and King (1980). Fan-Spray vs. Two-Fluid Atomization. The superiority of either the fan-spray or the two-fluid atomizer can be ascertained by comparing their observed retentions. Figures 8 and 9 show the results for a 50% sucrose solution atomized into 200 "C inlet column air with a fan-spray atomizer operating at 7.00 MPa, as well as the results for a two-fluid atomizer experiment with an atomization air temperature of 99 "C. The feed temperature was 2-3 "C higher in the fan-spray atomizer experiment, but this difference should not be significant, based on the results of Kieckbusch and King (1980). The inlet column air temperature was 50 "C lower (150 "C) for the two-fluid atomizer experiment. The higher air temperature for the fan-spray atomizer experiment may or may not have caused a slight increase in retention, in view of the conflicting results of Kieckbusch and King (1980) and in the present work for the effect of air temperature on retention.
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 4, 1984
710
55k
040% sucrose
A50% sucrose 60% sucrose
c
g
35
P
?
;
251
I55
i
//
W
IO
15
5
20
25
30 l
Distance F r o m A t o m i z e r icm)
Figure 10. Percent evaporation vs. axial distance;data from Figures 5 and 6.
The potential advantage for the two-fluid atomizer is that it has only drop-formation and drop-diffusional losses and no sheet losses, as opposed to the fan-spray atomizer which has all three. Using this reasoning, it might be expected that the fan-spray atomizer would always have lower retention then the two fluid atomizer. This reasoning appears not to be correct since the fan-spray atomizer had a slightly higher retention, as seen in Figures 8 and 9. On the other hand, for 7.00 MPa atomization pressure and 50% sucrose feed, the predicted sheet losses are only 12% (Table I). The two-fluid atomizer may be advantageous for feeds not satisfactorily atomized by a pressure atomizer, or for cases where long sheets are formed by a pressure atomizer, with consequent large sheet losses. Water Evaporation An estimate can be made, by considering two extreme situations, of whether the drops in the atomization region are drying mostly in the constant-activity period (CAP) (external mass and heat transfer controlling) or in the falling-rate period (internal mass transfer controlling). During the CAP the water flux from the drops would be essentially constant, but since a given amount of removal constitutes a larger fraction of the total water present in solutions with high sucrose concentration, higher-concentration solutions would be expected to give greater percent evaporation at a given distance. The existence of the CAP is a result of the high molecular weight of sucrose, which requires a large weight percentage of sucrose to alter the equilibrium partial pressure of water significantly. On the other hand, if the drops are evaporating mostly during the falling-rate period, the lower water diffusivity for higher sucrose contents should cause these drops to have a lower percent evaporation at any given axial distance than drops with lower sucrose concentrations. Therefore, in that case, the percent evaporation should be highest for solutions containing the lowest percent sucrose. The measured percent evaporation versus distance for the experiments described in Figures 5 and 6 is plotted in Figure 10. The drop sizes and fall times are similar for these experiments (Etzel, 1983). The order of the three curves shown in Figure 10 indicates that a large part of the evaporation must be occurring in the CAP. This offers a possible explanation for the high acetate losses occurring in these experiments, since the surface sucrose concentration should build up to the value required for selective diffusion only after the drying process enters the fallingrate period. Spray-Air Mixing An important observation which was made during each experiment was the nature of the air-flow pattern existing
in the dryer. This was accomplished by following visible entrained particles. Large recirculation patterns were observed for the experiments of Figures 5 and 6. These patterns of reversed or recirculating flow started more than 30 cm below the nozzle and disappeared at about the level of the nozzle. Reverse flow must occur when the amount of air entrained into the spray is greater than that entering the column. Since more air is entrained into the higherpressure sprays, and the air feed rate to the column is constant, the amount of flow reversal should increase as the atomization pressure increases. This was observed experimentally. Reverse flow of column air should cause the air in the vicinity of the atomizer to be cooler and more humid than in the absence of reverse flow. This should cause lower initial evaporation rates and therefore longer times spent in the CAP for the spray drops than if there was no reverse flow. Thus the experiments with low atomization pressure would be more likely to give higher percent evaporations than the experiments with higher atomization pressure, which is what was observed (Etzel, 1983). The small changes in drop size produce predictions for the variation in the percent evaporation which are the opposite of the observed trends. Therefore changes in drop size must not play a dominant role in the explanation of these experiments. Acknowledgment This research was sponsored by National Science Foundation Grant No. CPE-8006786, administered through the Division of Chemical and Process Engineering. Nomenclature paf = average gas-chromatograph peak area of the feed pas = average gas-chromatograph peak area of the sample sgf = feed specific gravity sgs = sample specific gravity sw = sample weight, g wsd = weight fraction sugar in the spray or drops wsf = weight fraction sugar in the feed wss = weight fraction sugar in the sample wwb = weight of water added to the sample bottle, g wwd = weight fraction water in the spray or drops wwf = weight fraction fraction water in the feed wws = weight fraction water in the sample Literature Cited Bomben, J. L.; Bruin, S.;Thijssen, H. A. C.; Merson. R. L. Adv. FoodRes. 1973, 20, 1. Bruin S.; Luyben, K. Ch. A. M. "Drying of Food Materials: A Review on Recent Developments" I n "Advances in Drying"; Mujumdar, A. S., Ed.; Hemisphere Publishing Co.: New York, 1980; Vol 1. Chandrasekaran, S.K.; King, C. J. J. Food Sci. 1971, 36,699. Etzel, M. R. Ph.D Dissertation, University of California, Berkeley, 1983. Frey, D. D. Ph.D Dissertation, University of California, Berkeley, 1984. Frey, D. D.; King, C. J. J. Chem. Eng. Data 1982, 27,419. Masson, D.; Mizrahi, J. Trans. Inst. Chem. Eng. 1961, 39,415. Kerkhof. P. J. A. M.; Schoeber, W. J. A. H. "Theroretical Modelling of the Drying Behaviour of Droplets in Spray Drying" I n "Advances in Preconcentration and Dehydration of Foods"; Spicer, A,, Ed.; Applied Science Publishers, Ltd.: London, 1974. Kieckbusch, T. G. Ph.D Dissertation, University of California, Berkeley, 1978. Kieckbusch, T. G.; King, C. J. AIChE J. 1980, 26, 718; Erratum: 1981, 27, 528. Kino. C. J.: Kieckbusch, T. G.: Greenwaki, C. G. "Focd-Quality Factors in Spray Drying" I n "Advances in Drying"; Mujumdar, A. S.,Ed.;-Hemlsphere Publishing Co.; New York, 1984; Voi. 3. Menting, L. C.; Hccgstad, B.; Thijssen, H. A. C. J. Food Techno/. 1970. 5, 127. Rothe, P. H.; Block, J. A. I n t . J. MUM-Phase Fbw 1977, 3, 263 Rulkens. W. H.; Thiissen. H. A. C. J. Food Techno/. 1972, 7, 95. Simpson, S. G.; Lynn, S. AIChE J. 1977, 23, 666. Thijssen, H. A. C.; Rulkens, W. H. Ingenieur (The Hague) 1988, 60, Ch45. Zakarhn. J. A. Ph.D Dissertation. Universitv of California. Berkeiev. 1979. Zakarlan, J A.; King, C J Ind. Eng. Chem Process Des. Dev lS82, 21, 107
Received for review December 20, 1982 Accepted December 1, 1983
