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The effects of operating conditions on lactose crystallization in a pilot-scale spray dryer
Journal of Food Engineering 100 (2010) 551–556
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
The effects of operating conditions on lactose crystallization in a pilot-scale spray dryer Debolina Das *, Hazalea A. Husni, Timothy A.G. Langrish School of Chemical and Biomolecular Engineering (J01), University of Sydney, NSW 2006, Australia
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Article history: Received 8 March 2010 Received in revised form 10 May 2010 Accepted 12 May 2010 Available online 16 May 2010 Keywords: Spray drying Operating conditions Pilot-scale spray dryer Lactose Crystallization kinetics
a b s t r a c t The effects of operating conditions on the rate of drying and degree of crystallinity of lactose have been explored in a pilot-scale spray dryer. Temperature, moisture content, feed flow rate, atomizing air-flow rate, main air-flow rate and particle size have been varied to estimate the range of crystallinity of lactose obtainable in a pilot-scale spray dryer. Modulated differential scanning calorimetry (MDSC) and sorption tests (water-induced crystallization) have been used to assess the degree of crystallinity for freshly spraydried samples. The degree of crystallinity could be varied from 18% to 72% by varying the operating conditions while allowing reasonable drying of the material. The study suggested that the use of a lower inlet temperature increased the crystallinity of the product from 25% (at 230 °C) to 60% (at 170 °C). A decrease in product crystallinity was also noted when using a lower atomizing air-flow rate. Statistical analysis with t-tests confirmed these differences to be significant with 95% confidence. The results suggest differences between small and pilot-scale spray dryers due to differences in particle sizes and drying rates. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Crystallization in drying is a relatively new concept that combines the process of crystallization and drying in a single step within a spray dryer, which is conventionally done separately, to facilitate the production of new engineered particles with controlled particle properties and reduced stickiness to improve the yield from processing and to improve the properties during storage and use. This new combined approach is evolving due to the need to stabilize powders produced from biological extracts. The operating conditions play a critical role here in this combined process of solid-phase crystallization and drying because the temperature and moisture content of the particles influence the rate of crystallization. Solid-phase crystallization from amorphous material occurs when molecules of an amorphous solid, having a non-aligned molecular structure, rearrange themselves into a more stable orderly structure (Jouppila and Roos, 1994). Parameters affecting the crystallization process include temperature, moisture content and molecular structure (Lloyd et al., 1996). Solid-phase crystallization of lactose in storage has been widely studied before, mainly due to its importance in the dairy and pharmaceutical industry. Jouppila and Roos (1994) found that the crystallization of amorphous lactose resulted in the loss of adsorbed water. Crystallization of pure lactose occurred at lower humidities than did crystalliza* Corresponding author. Tel.: +61 2 93515661; fax: +61 2 93512854. E-mail address: [email protected] (D. Das). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.05.005
tion of lactose in skim milk powder, and hence these authors concluded that milk fat decreased the rate of crystallization. Milk proteins may also decrease the rate of crystallization of amorphous lactose in milk powder (Ibach and Kind, 2007). Haque and Roos (2004) studied the crystallization of spray-dried and freeze-dried amorphous lactose and observed that the rate of crystallization increased with increasing relative vapour pressure and storage time. Chidavaenzi et al. (2001) found that polyethylene glycol (PEG) increases the degree of crystallinity of spray-dried lactose. This can be explained by the fact that PEG forms hydrogen bonds with water, decreasing the rate of drying. Thus the molecules have a longer period of time to rearrange themselves into an orderly crystalline material. Paterson and Bronlund (2004) measured moisture sorption isotherms for amorphous lactose, crystalline a-lactose and a predominantly crystalline lactose powder. They observed that the sorption isotherms are not temperature dependent over the range of 12–40 °C except for amorphous lactose at 12 °C, which absorbs less moisture than at higher temperatures. They also found that the amount of water absorbed by the crystalline powders at high water activities was dependent on the packing density of the powder. Recent studies (Chiou et al., 2008a,b) have shown that the extent of crystallization can be controlled by adjusting the operating conditions during spray drying. The Williams–Landel–Ferry theory (Williams et al., 1955) for solid-phase crystallization suggests that the rate of crystallization varies with the temperature of the particle (T) and its glass-transition temperature (Tg). Since the
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glass-transition temperature is dependent on the moisture content (Gordon and Taylor equation, 1952), the WLF equation relates the rate of crystallization to the temperature and moisture content of the particles. Recent works by Chiou et al. (2008) have shown that varying the inlet air temperature over the range from 134 to 210 °C for spray-dried lactose powder has increased the crystallinity of lactose by 20% in a small-scale laboratory dryer (Buchi B-290). Although crystallization within a small laboratory-scale spray dryer has been studied, the utilization of larger spray dryers for investigating the crystallization of spray-dried products has not been studied before. The larger scale of a pilot-scale spray dryer, compared with the laboratory-scale dryer used so far (Chiou et al., 2007, 2008a,b), both gives a greater residence time for crystallization to occur, and it gives more degrees of freedom to change and control the temperature and humidity in the pilot-scale spray dryer compared with the smaller laboratory-scale one. Furthermore, pilot-scale spray dryers have the capacity to produce larger droplets from the atomizer, which is an important factor in determining the crystallinity of the final product since they allow larger particle residence times in the spray dryer. This work investigates the variation in crystallinity and sorption behaviour of lactose occurring as a consequence of changing the operating conditions in a pilot-scale spray dryer. 2. Experimental methods and equipment Fig. 1. Process flow diagram of pilot-scale spray dryer operating system.
2.1. Materials Samples of lactose were produced using a pilot-scale spray dryer (diameter 0.8 m, length 2 m), with the operating conditions given in Table 1. In all cases, a 9.09% w/w solution of lactose was spray dried. 2.2. Drying equipment and method The pilot-scale spray dryer (Fig. 1) that has been used here is a short-form dryer with an internal diameter of 0.8 m and a height of 2 m, consisting of an upper cylindrical part, 1.3 m in height and a lower conical bottom, 0.63 m in height (Langrish, 2007). Fig. 1 shows a schematic diagram of the flow process in a typical pilotscale spray dryer. A plenum chamber located in the lid of the dryer holds the nozzle and has an inlet for hot air entry and inlet swirl vanes. A pipe was connected to the spray dryer on top of the cyclone, which was connected to the outlet box of the spray dryer, in order to extract the air from the spray dryer using a 1 m diameter centrifugal fan with a 2 kW electric motor and a frequency inverter speed con-
troller. Compressed air was used to externally mix with the liquid feed within an ultrasonic atomizer (MAD 0331 B1BDG by PNR), producing a full-cone spray pattern of the material used. A negative gauge pressure of at least 0.5 cm water gauge (Pa) was constantly controlled in the spray dryer to prevent internal pressure build-up in the dryer leading to particles leaking out of the dryer by altering the speed of the outlet fan. A peristaltic pump (Ismatec MP-GE, Zurich) was utilized to pump and control the process fluid flow into the spray dryer through the atomizer. The dryer was also equipped with a blower and three 5 kW heaters to control the temperature of the air flowing into the spray dryer. A software program called CEDRIER, developed by Southwell (2000), was used to measure and control the temperature of the air going into the spray dryer according to the temperature set point for each experiment, which was either 170 or 230 °C. The temperature sensing element was a T-type thermocouple (±1 °C). In addition, the inlet air temperature was monitored by taking frequent readings using a K-type thermocouple (±1 °C). An air velocity meter (VelociCheck, model 8330 by TSI Incorporated) was used
Table 1 Summary of operating conditions used for each experiment. Experiment No.
Inlet gas temperature (°C)
Peristaltic pump setting
Feed rate (g min 1)
Atomizing air-flow rate (L min 1)
Particle size (lm)
No. of replications for each set of operating conditions
1 (base case) 2 3 4 5 6 7 8 9 10 11 12 13
170 230 170 230 170 170 170 170 230 230 230 230 230
2.3 2.3 0 0 2.3 0 2.3 0 0 2.3 0 2.3 0
28 28 14.5 14.5 28 14.5 28 14.5 14.5 28 14.5 28 14.5
20 20 20 20 20 20 10 10 10 20 10 20 20
10 10 10 10 10 10 15 15 15 10 15 10 10
3 2 2 2 2 2 1 2 2 1 1 1 1
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2.3. Analytical techniques Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to analyse the extent of crystallinity of freshly spray-dried samples of lactose. 2.3.1. Water-induced crystallization (WIC) A sample was placed on a 150 mm Petri dish, and mass measurements were taken with an analytical balance (Mettler-Toledo AB204S, four decimal figure analytical) to study the extent of amorphous lactose content in the dried powder. A controlled humidity environment with saturated sodium chloride solution (75.3% relative humidity, Winston and Bates (1960)) and temperature of 25 °C was used. To keep the air conditions uniform within the sorption box, a small fan was used for air circulation. The moisture content of the particles were characterized specifically in terms of an initial moisture content, which refers to the moisture content of the particles when freshly spray dried and the moisture sorption percentage, which is essentially the percentage of moisture in the particles at equilibrium after absorption and desorption from its surroundings. The peak height indicates the degree of crystallinity in terms of the change in the moisture content on a dry basis during crystallization, relative to the final moisture content. All three moisture content characteristics were calculated as a percentage of the dry mass of the material (100 kg/kg dry basis). 2.3.2. Modulated differential scanning calorimetry (MDSC) The degree of crystallinity of the products was analysed using modulated differential scanning calorimetry (MDSC). Samples were heated in hermetically sealed pans using a ramp rate of 10 °C/min with a 1 °C modulated signal every 60 s in a modulated differential scanning calorimeter (TA Instruments Q1000) with a refrigerated cooling system. The scans have been normalized using the sample mass. The peak energies for the various samples have then been integrated to compare the different degrees of crystallinity. For each spray-dried product, an average value of the heats of crystallization as well as the glass-transition and crystallization temperatures was taken for each set of samples (three samples in each set). The exothermic peak for the heat of crystallization in the thermogram (Fig. 2) represents the energy absorbed by the sample when transforming from the amorphous state to the crystalline one. A higher peak heat of crystallization indicates a lower crystalline content.
5
Heat Flow (W/g)
to measure the air velocity at the top of the cyclone to give an estimated velocity for the air flow through the spray dryer and consequently, an estimated flow rate. The experimental design consisted of varying the operating conditions, including the inlet gas temperature, the feed flow rate, the atomizing air-flow rate, the main air-flow rate and the particle size in order to assess the effects on the degree of crystallinity and sorption behaviour of the spray-dried products. By following previous studies that had employed pilot-scale spray dryers (Langrish, 2007; Ozmen and Langrish, 2002, 2003), the inlet gas temperatures range chosen for this purpose were 170 °C (lower limit for adequate evaporation of water) and 230 °C (limited by the equipment), which correspond to the lower and upper limit temperatures of the experiments. The atomizing air-flow rate was set to 10 and 20 L/min, with 20 L/min giving average particle sizes of around 10 lm diameter (particle size measurements were done with a Malvern Mastersizer S), and 10 L/min producing particles of 15 lm mean diameter. Total feed flow rates were set to 28 and 14.5 g/min, corresponding to peristaltic pump settings of 2.3 and 0.0, respectively. Table 1 summarizes the operating conditions used for each experiment. The samples were tested for the degree of crystallinity immediately after spray drying using different analytical techniques.
78 J/g Higher exothermic peak corresponding to greater heat of crystallization
3 42 J/g
1 -1
Lowexothermic peak indicating lower heat of crystallization for 170°C inlet temperature
-3 -5 230°C inlet temperature 170°C inlet temperature
-7 -9 0
100
200
300
400
Temperature°C Fig. 2. MDSC graph for lactose from two different operating conditions: (1) 230 °C, 28 g min 1, 20 L min 1 and (2) 170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, liquid flow rates and atomizing air-flow rates, respectively.
3. Results and discussion The results are presented and discussed in terms of the crystallinity of the spray-dried particles. The spray-dried products were collected from two outlets, one directly below the dryer and the other from the bottom of the cyclone separator, and they were analysed separately. Based on the results, the operating conditions have been assessed to investigate which conditions affected the crystallization of the particles most greatly and why. 3.1. Degree of crystallinity The heats of crystallization obtained from MDSC for the various operating conditions have been summarized in Table 3. The values for the degree of crystallinity in Table 3 have been obtained from the heat of crystallization values from MDSC using the data given by Gombás et al. (2002) for lactose. Table 2 shows the percentage degrees of crystallinity for lactose corresponding to the transition energy values shown by Gombás et al. (2002). Table 3 shows two different population classes in the degree of crystallinity for the material from the outlet box. While case 1 gave a degree of crystallinity of 60 ± 12% with an inlet temperature of 170 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1, all the other cases (cases 2–7, Table 3), combining the variations of all three operating parameters, produced materials with degrees of crystallinity between 22 ± 4% and 31 ± 15%. The results from these cases (cases 2–7) can be grouped together as one population class. According to the WLF equation, the effects of increasing the inlet gas temperature on the drying rates and the drying behaviour are likely to increase drying rates, making the products drier. This situation is contrast with the effects of increasing the gas humidity, which are likely to decrease drying rates, making the products wetter. Comparison of the results between cases 1 and 2 shows that increasing the inlet temperature from 170 to 230 °C decreased
Table 2 Transition energy values with different extents of crystallinity obtained by Gombás et al. (2002). Transition energy (J g 112 99 96 78 56 39 34 29 14
1
)
Crystallinity (%) 0 5 10 20 30 50 60 70 80
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Table 3 Variation in the degree of crystallinity for different operating conditions. Operating condition
Case 1: base case:170 °C, 28 g min 1, 20 L min 1 (runs 1 and 5) Case 2: higher inlet temperature – 230 °C, 28 g min 1, 20 L min 1 (runs 2 and 12) Case 3: higher inlet temperature and lower feed flow rate – 230 °C, 14.5 g min 1, 20 L min 1 (runs 4, 10 and 13) Case 4: base-case inlet temperature and lower atomizing air-flow rate – 170 °C, 28 g min 1, 10 L min 1 (run 7) Case 5: base-case inlet temperature and lower feed flow rate – 170 °C, 14.5 g min 1, 20 L min 1 (runs 3 and 6) Case 6: base-case inlet temperature, lower feed flow rate and lower atomizing air-flow rate – 170 °C, 14.5 g min 1, 10 L min 1 (run 8) Case 7: higher inlet temperature lower feed flow rate and lower atomizing airflow rate – 230 °C, 14.5 g min 1, 10 L min 1 (runs 9 and 11)
Difference between material and glass-transition temperature (TTg) is higher
Final (outlet) moisture content (kg/kg DA)
Heat of crystallization (J/g)
Degree of crystallinity (%)
Outlet box
Cyclone separator
Outlet box (at bottom of the dryer)
Cyclone separator
0.25, 0.08 0.13, 0.02
34 ± 12 64 ± 5
41 ± 8 57 ± 6
60 ± 12 25 ± 2
48 ± 10 29 ± 4
0.04
66 ± 3
50 ± 5
24 ± 4
36 ± 6
0.11
64 ± 8
26 ± 1
26 ± 4
72 ± 1
0.13, 0.08
54 ± 15
47 ± 10
31 ± 15
38 ± 10
0.23
62 ± 9
46 ± 13
26 ± 4
40 ± 13
0.02, 0.07
75 ± 6
40 ± 0.5
22 ± 4
48 ± 1
Higher rate of crystallization
More crystalline product
Higher inlet gas temperature
Rapid decrease in moisture content
Increase in glasstransition temperature
Smaller difference between material and glass-transition temperature (TTg)
Low rate of crystallization leading to less crystalline product
Fig. 3. Qualitative diagram summarizing the possible arguments regarding the effects of using higher inlet gas temperature in the pilot-scale spray dryer.
the crystallinity by 35%, whereas other parameters, like the feed flow rate and the atomizing air-flow rate, did not have any significant effect on the extent of crystallization. Low heats of crystallization, with average values of 34 ± 12 J g 1 corresponding to 60 ± 12% crystallinity (Table 2, Gombás et al., 2002) were obtained for the particles collected at the outlet using the base-case operating conditions (170 °C, 28 g min 1, 20 L min 1). In addition, the particles obtained from the outlet at the average values for the cyclones of 41 ± 8 J g 1, corresponding to 48 ± 10% crystallinity, which are close to the values for the particles from the outlet at the bottom of the dryer. In contrast, when the inlet gas temperature was increased to 230 °C (28 g min 1, 20 L min 1), the MDSC analysis showed that the average values for the heats of crystallization were 64 ± 5 J g 1 (25 ± 2% crystallinity) for the particles from the bottom outlet and 57 ± 6 J g 1 (29 ± 4% crystallinity) for the particles from the cyclone. The average heats of crystallization gave an almost twofold difference for the particles from the outlet box when the inlet gas temperature was increased, while the degree of crystallization decreased by 35% from 60 ± 12% to 25 ± 2%. Table 3 illustrates the degree of crystallinity for the two operating conditions. Statistical analysis with t-tests was carried out using the method set out by Finlayson et al. (2007) and Prins et al. (2003) to determine if there were any significant differences between the mean heats of crystallization. As seen in Table 1, the number of repeated runs var-
ied for each operating condition, so the variance for each average mean was assumed to be unequal. By choosing a significance level of 95%, the t-tests were carried out by stating the null hypothesis as ‘‘no significant difference between the average means” and the alternative hypothesis as ‘‘there is significant difference between average means”. The difference was found to be significant with 95% confidence between the average heats of crystallization for the particles that were spray dried at 170 °C, 28 g min 1 and 20 L min 1 and at 230 °C, 28 g min 1 and 20 L min 1, for inlet air temperatures, liquid flow rates and atomizing air-flow rates, respectively. This indicates that the particles that were spray dried at the base-case conditions of 170 °C, 28 g min 1 and 20 L min 1 have significantly higher degrees of crystallinity compared with the particles that were spray dried at the higher temperature but with the same feed flow rate and atomizing air-flow rate. Fig. 3 summarizes the possible arguments regarding the effects of using higher inlet gas temperatures in the pilot-scale spray dryer. The MDSC results from Fig. 2 suggest that the lower argument in Fig. 3 is more significant for the pilot-scale dryer since the degree of crystallinity decreased on using a higher inlet gas temperature. This is affected by the rate of drying, which plays a critical role here. While an increase in inlet temperature increased the material temperature, it also decreased the moisture content rapidly, leading to a rise in the glass-transition temperature. It is therefore possible that this rapid drying of the material due to
D. Das et al. / Journal of Food Engineering 100 (2010) 551–556
Moisture Content Change (%)
10
Greaterpeak heights for 230°C inlet temperature condition Lower peak for 170°C indicatingmore crystalline content
8 6 4
Operating Condition 1 Operating Condition 2
2
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calculated using the Gordon–Taylor equation (1952). The glasstransition temperature for pure water is 137 °C (Johari, Hallbrucker, & Mayer, 1987) and for pure lactose it is 101 °C (Roos and Karel, 1991), together with the curvature constant (k) of 7.42 (Roos, 1993). The estimated glass-transition temperatures at the outlet were predicted to vary between 92 and 101 °C for the different operating conditions. However, the outlet air temperatures were measured to be in the range of 35–85 °C, which is significantly below the glass-transition temperature, and hence it is unlikely that the crystallization rates after spray drying were very rapid.
0 0.0
-2
0.1
0.2
0.3
0.4
0.5
Time (day)
Fig. 4. Example of peak height estimation from moisture sorption data for particles collected from the bottom trays and spray dried at: (1) 230 °C, 28 g min 1, 20 L min 1 and (2) 170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, feed flow rates and atomizing air-flow rates, respectively. The moisture contents are relative to the final moisture content.
the increase in the inlet temperature did not allow enough time for the material to crystallize. The extent of crystallization is hence affected by the balance between the temperature and moisture content of the particles. The degree of crystallinity of the lactose produced with the seven sets of operating conditions were also analysed using the water-induced crystallization. Fig. 4 shows an example of the moisture sorption data and the peak height estimated as a percentage of the dry mass for two different operating conditions. Operating condition 2 in Fig. 4 (base case), with an inlet temperature of 170 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1, gave the lowest peak heights with an average change in moisture content of 7.0 ± 0.5%, while operating condition 1 with an inlet temperature of 230 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1 gave peak heights with an average change in moisture content of 8.0 ± 0.2%. For other conditions, like case 3 (230 °C, 14.5 g min 1, 20 L min 1) and case 5 (170 °C, 14.5 g min 1, 20 L min 1), the average peak heights were estimated to be 8.3 ± 0.2% and 8.4 ± 0.2%, respectively. Statistical analysis with t-tests was carried out to compare the peak heights for each operating condition with the base-case condition. Significant differences were found with 95% confidence only when the conditions of 230 °C, 28 g min 1, 20 L min 1 (case 2), which gave an average peak height of 8.0 ± 0.2%, were compared with the base-case conditions of 170 °C, 28 g min 1, 20 L min 1, which gave an average peak height of 7.0 ± 0.5%. Water-induced crystallization, using the base-case condition, showed the highest degree of crystallinity in the spray-dried sample, whereas case 2 (170 °C, 28 g min 1, 20 L min 1) showed a lower crystalline content. Similarly analysis with MDSC, using the base-case condition, showed the highest crystalline content and case 2 showed lower crystallinity in the spray-dried lactose. Both the analyses confirm that the highest degree of crystallinity was obtained from the base case with a lower inlet temperature of 170 °C, and lower crystallinity was obtained for samples that were spray dried using the other operating conditions. 3.2. Possibility of crystallization after spray drying The outlet moisture content for the base-case operating condition (170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, liquid flow rates and atomizing air-flow rates, respectively) was 16% with a standard deviation of 8%. The outlet moisture contents for the different operating conditions varied between 2–29%. The glasstransition temperatures (Tg) for the outlet (moist) solids were
3.3. Comparison with the literature The results obtained from the present investigation with the pilot-scale spray dryer are different to those that were found by Chiou et al. (2008b) using a small laboratory-scale spray dryer. These authors found that the crystallinity of spray-dried lactose increased with an increase in the inlet air temperature for this smallscale dryer. The particles in a laboratory-scale dryer, being in the range of 5–7 lm, are about half the diameter of those in the large pilot-scale dryer (15 ± 2 lm) and hence the particles dry more quickly and reach equilibrium much faster in the laboratory-scale dryer. The glass-transition temperature is dependent on the moisture content, as seen in the Gordon-Taylor equation (1952). The glasstransition temperature is also a function of the material and its (non-aqueous) composition, and hence this temperature will reach an upper limit quickly if the moisture content of the particle reaches a lower equilibrium limit quickly, as in the case of a small laboratory-scale dryer. In this case, increasing the glass-transition temperature increases the difference between the material temperature and its glass-transition temperature, causing more crystallization to occur. This situation, where the glass-transition temperature of the spray-dried material has an upper limit while the particle temperature has no limit except as constrained by the equipment, may be a characteristic of this laboratory-scale dryer (Buchi B-290). In the pilot-scale spray dryer, the particle temperature may increase when the inlet gas temperature is raised but is then counterbalanced by the rise in the glass-transition temperature, as the materials get drier at higher inlet gas temperatures. Thus, an increase in the inlet gas temperature may decrease the difference between the material and the glass-transition temperatures, resulting in a lower degree of crystallinity for the product. This behaviour of lower crystallinity at a higher temperature was seen in practice with this pilot-scale dryer, with a heat of crystallization of 34 ± 12 J g 1 at 170 °C and 64 ± 5 J g 1 at 230 °C. 4. Conclusions The results from the analyses using modulated differential scanning calorimetry and water-induced crystallization suggest that there is a significant effect of operating conditions on the extent of crystallization of the spray-dried products in this pilot-scale dryer. The overall range of product crystallinity obtained was between 20% and 72% in the experiments, using all the different operating conditions limited by the equipment. Decreasing the inlet air temperature in the dryer from 230 to 170 °C increased the crystallinity of the product by 35% in the outlet box. Using a lower atomizing air-flow rate increased the average heat of crystallization, suggesting a decrease in the degree of crystallization. Statistical analyses using t-tests confirmed with 95% confidence that there were significant variations in the degree of crystallinity using these operating conditions. T-tests also confirmed with 95% confidence that there was no significant change in the heat of crystallization and sorption test values when varying the feed flow rate and the main air-flow rate.
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Langrish, T.A.G., Chan, W.C., Kota, K., 2007. Comparison of maltodextrin and skim milk wall deposition rates in a pilot-scale spray dryer. Powder Technology 179 (1–2), 84–89. Lloyd, R., Chen, X.D., Hargreaves, J.B., 1996. Glass transition and caking of spraydried lactose. International Journal of Food Science and Technology 31 (4), 305–311. Ozmen, L., Langrish, T.A.G., 2002. Comparison of glass transition temperature and stick point temperature for skim milk powder. Drying Technology 20 (6), 1177– 1192. Ozmen, L., Langrish, T.A.G., 2003. An experimental investigation of the wall deposition of milk powder in a pilot-scale spray dryer. Drying Technology 21 (7), 1253–1272. Paterson, T., Bronlund, J., 2004. Moisture sorption isotherms for crystalline, amorphous and predominantly crystalline lactose powders. International Dairy Journal 14 (3), 247–254. Prins, J., Mccormack, D., Michelson, D., Horrell, K., 2003. 7.3.1. Do two processes have the same mean? NIST/SEMATECH e-Handbook of Statistical Methods.. Roos, Y.H., Karel, M., 1991. Plasticizing effect of water on thermal behaviour and crystallization of amorphous food models. Journal of Food Science 56 (1), 38– 43. Roos, Y.H., 1993. Water activity and physical state effects on amorphous food stability. Journal of Food Processing and Preservation 16 (6), 433–447. Southwell, D., 2000. Operability and performance assessment of spray dryers. Ph.D. Thesis, Department of Chemical Engineering. Sydney, Australia, University of Sydney. Williams, M.L., Landel, R.F., Ferry, J.D., 1955. The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. Journal of the American Chemical Society 77 (14), 3701–3707. Winston, P.W., Bates, D.H., 1960. Saturated solutions for the control of humidity in biological research. Ecology 41 (1), 232–237.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
The effects of operating conditions on lactose crystallization in a pilot-scale spray dryer Debolina Das *, Hazalea A. Husni, Timothy A.G. Langrish School of Chemical and Biomolecular Engineering (J01), University of Sydney, NSW 2006, Australia
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 10 May 2010 Accepted 12 May 2010 Available online 16 May 2010 Keywords: Spray drying Operating conditions Pilot-scale spray dryer Lactose Crystallization kinetics
a b s t r a c t The effects of operating conditions on the rate of drying and degree of crystallinity of lactose have been explored in a pilot-scale spray dryer. Temperature, moisture content, feed flow rate, atomizing air-flow rate, main air-flow rate and particle size have been varied to estimate the range of crystallinity of lactose obtainable in a pilot-scale spray dryer. Modulated differential scanning calorimetry (MDSC) and sorption tests (water-induced crystallization) have been used to assess the degree of crystallinity for freshly spraydried samples. The degree of crystallinity could be varied from 18% to 72% by varying the operating conditions while allowing reasonable drying of the material. The study suggested that the use of a lower inlet temperature increased the crystallinity of the product from 25% (at 230 °C) to 60% (at 170 °C). A decrease in product crystallinity was also noted when using a lower atomizing air-flow rate. Statistical analysis with t-tests confirmed these differences to be significant with 95% confidence. The results suggest differences between small and pilot-scale spray dryers due to differences in particle sizes and drying rates. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Crystallization in drying is a relatively new concept that combines the process of crystallization and drying in a single step within a spray dryer, which is conventionally done separately, to facilitate the production of new engineered particles with controlled particle properties and reduced stickiness to improve the yield from processing and to improve the properties during storage and use. This new combined approach is evolving due to the need to stabilize powders produced from biological extracts. The operating conditions play a critical role here in this combined process of solid-phase crystallization and drying because the temperature and moisture content of the particles influence the rate of crystallization. Solid-phase crystallization from amorphous material occurs when molecules of an amorphous solid, having a non-aligned molecular structure, rearrange themselves into a more stable orderly structure (Jouppila and Roos, 1994). Parameters affecting the crystallization process include temperature, moisture content and molecular structure (Lloyd et al., 1996). Solid-phase crystallization of lactose in storage has been widely studied before, mainly due to its importance in the dairy and pharmaceutical industry. Jouppila and Roos (1994) found that the crystallization of amorphous lactose resulted in the loss of adsorbed water. Crystallization of pure lactose occurred at lower humidities than did crystalliza* Corresponding author. Tel.: +61 2 93515661; fax: +61 2 93512854. E-mail address: [email protected] (D. Das). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.05.005
tion of lactose in skim milk powder, and hence these authors concluded that milk fat decreased the rate of crystallization. Milk proteins may also decrease the rate of crystallization of amorphous lactose in milk powder (Ibach and Kind, 2007). Haque and Roos (2004) studied the crystallization of spray-dried and freeze-dried amorphous lactose and observed that the rate of crystallization increased with increasing relative vapour pressure and storage time. Chidavaenzi et al. (2001) found that polyethylene glycol (PEG) increases the degree of crystallinity of spray-dried lactose. This can be explained by the fact that PEG forms hydrogen bonds with water, decreasing the rate of drying. Thus the molecules have a longer period of time to rearrange themselves into an orderly crystalline material. Paterson and Bronlund (2004) measured moisture sorption isotherms for amorphous lactose, crystalline a-lactose and a predominantly crystalline lactose powder. They observed that the sorption isotherms are not temperature dependent over the range of 12–40 °C except for amorphous lactose at 12 °C, which absorbs less moisture than at higher temperatures. They also found that the amount of water absorbed by the crystalline powders at high water activities was dependent on the packing density of the powder. Recent studies (Chiou et al., 2008a,b) have shown that the extent of crystallization can be controlled by adjusting the operating conditions during spray drying. The Williams–Landel–Ferry theory (Williams et al., 1955) for solid-phase crystallization suggests that the rate of crystallization varies with the temperature of the particle (T) and its glass-transition temperature (Tg). Since the
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glass-transition temperature is dependent on the moisture content (Gordon and Taylor equation, 1952), the WLF equation relates the rate of crystallization to the temperature and moisture content of the particles. Recent works by Chiou et al. (2008) have shown that varying the inlet air temperature over the range from 134 to 210 °C for spray-dried lactose powder has increased the crystallinity of lactose by 20% in a small-scale laboratory dryer (Buchi B-290). Although crystallization within a small laboratory-scale spray dryer has been studied, the utilization of larger spray dryers for investigating the crystallization of spray-dried products has not been studied before. The larger scale of a pilot-scale spray dryer, compared with the laboratory-scale dryer used so far (Chiou et al., 2007, 2008a,b), both gives a greater residence time for crystallization to occur, and it gives more degrees of freedom to change and control the temperature and humidity in the pilot-scale spray dryer compared with the smaller laboratory-scale one. Furthermore, pilot-scale spray dryers have the capacity to produce larger droplets from the atomizer, which is an important factor in determining the crystallinity of the final product since they allow larger particle residence times in the spray dryer. This work investigates the variation in crystallinity and sorption behaviour of lactose occurring as a consequence of changing the operating conditions in a pilot-scale spray dryer. 2. Experimental methods and equipment Fig. 1. Process flow diagram of pilot-scale spray dryer operating system.
2.1. Materials Samples of lactose were produced using a pilot-scale spray dryer (diameter 0.8 m, length 2 m), with the operating conditions given in Table 1. In all cases, a 9.09% w/w solution of lactose was spray dried. 2.2. Drying equipment and method The pilot-scale spray dryer (Fig. 1) that has been used here is a short-form dryer with an internal diameter of 0.8 m and a height of 2 m, consisting of an upper cylindrical part, 1.3 m in height and a lower conical bottom, 0.63 m in height (Langrish, 2007). Fig. 1 shows a schematic diagram of the flow process in a typical pilotscale spray dryer. A plenum chamber located in the lid of the dryer holds the nozzle and has an inlet for hot air entry and inlet swirl vanes. A pipe was connected to the spray dryer on top of the cyclone, which was connected to the outlet box of the spray dryer, in order to extract the air from the spray dryer using a 1 m diameter centrifugal fan with a 2 kW electric motor and a frequency inverter speed con-
troller. Compressed air was used to externally mix with the liquid feed within an ultrasonic atomizer (MAD 0331 B1BDG by PNR), producing a full-cone spray pattern of the material used. A negative gauge pressure of at least 0.5 cm water gauge (Pa) was constantly controlled in the spray dryer to prevent internal pressure build-up in the dryer leading to particles leaking out of the dryer by altering the speed of the outlet fan. A peristaltic pump (Ismatec MP-GE, Zurich) was utilized to pump and control the process fluid flow into the spray dryer through the atomizer. The dryer was also equipped with a blower and three 5 kW heaters to control the temperature of the air flowing into the spray dryer. A software program called CEDRIER, developed by Southwell (2000), was used to measure and control the temperature of the air going into the spray dryer according to the temperature set point for each experiment, which was either 170 or 230 °C. The temperature sensing element was a T-type thermocouple (±1 °C). In addition, the inlet air temperature was monitored by taking frequent readings using a K-type thermocouple (±1 °C). An air velocity meter (VelociCheck, model 8330 by TSI Incorporated) was used
Table 1 Summary of operating conditions used for each experiment. Experiment No.
Inlet gas temperature (°C)
Peristaltic pump setting
Feed rate (g min 1)
Atomizing air-flow rate (L min 1)
Particle size (lm)
No. of replications for each set of operating conditions
1 (base case) 2 3 4 5 6 7 8 9 10 11 12 13
170 230 170 230 170 170 170 170 230 230 230 230 230
2.3 2.3 0 0 2.3 0 2.3 0 0 2.3 0 2.3 0
28 28 14.5 14.5 28 14.5 28 14.5 14.5 28 14.5 28 14.5
20 20 20 20 20 20 10 10 10 20 10 20 20
10 10 10 10 10 10 15 15 15 10 15 10 10
3 2 2 2 2 2 1 2 2 1 1 1 1
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2.3. Analytical techniques Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to analyse the extent of crystallinity of freshly spray-dried samples of lactose. 2.3.1. Water-induced crystallization (WIC) A sample was placed on a 150 mm Petri dish, and mass measurements were taken with an analytical balance (Mettler-Toledo AB204S, four decimal figure analytical) to study the extent of amorphous lactose content in the dried powder. A controlled humidity environment with saturated sodium chloride solution (75.3% relative humidity, Winston and Bates (1960)) and temperature of 25 °C was used. To keep the air conditions uniform within the sorption box, a small fan was used for air circulation. The moisture content of the particles were characterized specifically in terms of an initial moisture content, which refers to the moisture content of the particles when freshly spray dried and the moisture sorption percentage, which is essentially the percentage of moisture in the particles at equilibrium after absorption and desorption from its surroundings. The peak height indicates the degree of crystallinity in terms of the change in the moisture content on a dry basis during crystallization, relative to the final moisture content. All three moisture content characteristics were calculated as a percentage of the dry mass of the material (100 kg/kg dry basis). 2.3.2. Modulated differential scanning calorimetry (MDSC) The degree of crystallinity of the products was analysed using modulated differential scanning calorimetry (MDSC). Samples were heated in hermetically sealed pans using a ramp rate of 10 °C/min with a 1 °C modulated signal every 60 s in a modulated differential scanning calorimeter (TA Instruments Q1000) with a refrigerated cooling system. The scans have been normalized using the sample mass. The peak energies for the various samples have then been integrated to compare the different degrees of crystallinity. For each spray-dried product, an average value of the heats of crystallization as well as the glass-transition and crystallization temperatures was taken for each set of samples (three samples in each set). The exothermic peak for the heat of crystallization in the thermogram (Fig. 2) represents the energy absorbed by the sample when transforming from the amorphous state to the crystalline one. A higher peak heat of crystallization indicates a lower crystalline content.
5
Heat Flow (W/g)
to measure the air velocity at the top of the cyclone to give an estimated velocity for the air flow through the spray dryer and consequently, an estimated flow rate. The experimental design consisted of varying the operating conditions, including the inlet gas temperature, the feed flow rate, the atomizing air-flow rate, the main air-flow rate and the particle size in order to assess the effects on the degree of crystallinity and sorption behaviour of the spray-dried products. By following previous studies that had employed pilot-scale spray dryers (Langrish, 2007; Ozmen and Langrish, 2002, 2003), the inlet gas temperatures range chosen for this purpose were 170 °C (lower limit for adequate evaporation of water) and 230 °C (limited by the equipment), which correspond to the lower and upper limit temperatures of the experiments. The atomizing air-flow rate was set to 10 and 20 L/min, with 20 L/min giving average particle sizes of around 10 lm diameter (particle size measurements were done with a Malvern Mastersizer S), and 10 L/min producing particles of 15 lm mean diameter. Total feed flow rates were set to 28 and 14.5 g/min, corresponding to peristaltic pump settings of 2.3 and 0.0, respectively. Table 1 summarizes the operating conditions used for each experiment. The samples were tested for the degree of crystallinity immediately after spray drying using different analytical techniques.
78 J/g Higher exothermic peak corresponding to greater heat of crystallization
3 42 J/g
1 -1
Lowexothermic peak indicating lower heat of crystallization for 170°C inlet temperature
-3 -5 230°C inlet temperature 170°C inlet temperature
-7 -9 0
100
200
300
400
Temperature°C Fig. 2. MDSC graph for lactose from two different operating conditions: (1) 230 °C, 28 g min 1, 20 L min 1 and (2) 170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, liquid flow rates and atomizing air-flow rates, respectively.
3. Results and discussion The results are presented and discussed in terms of the crystallinity of the spray-dried particles. The spray-dried products were collected from two outlets, one directly below the dryer and the other from the bottom of the cyclone separator, and they were analysed separately. Based on the results, the operating conditions have been assessed to investigate which conditions affected the crystallization of the particles most greatly and why. 3.1. Degree of crystallinity The heats of crystallization obtained from MDSC for the various operating conditions have been summarized in Table 3. The values for the degree of crystallinity in Table 3 have been obtained from the heat of crystallization values from MDSC using the data given by Gombás et al. (2002) for lactose. Table 2 shows the percentage degrees of crystallinity for lactose corresponding to the transition energy values shown by Gombás et al. (2002). Table 3 shows two different population classes in the degree of crystallinity for the material from the outlet box. While case 1 gave a degree of crystallinity of 60 ± 12% with an inlet temperature of 170 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1, all the other cases (cases 2–7, Table 3), combining the variations of all three operating parameters, produced materials with degrees of crystallinity between 22 ± 4% and 31 ± 15%. The results from these cases (cases 2–7) can be grouped together as one population class. According to the WLF equation, the effects of increasing the inlet gas temperature on the drying rates and the drying behaviour are likely to increase drying rates, making the products drier. This situation is contrast with the effects of increasing the gas humidity, which are likely to decrease drying rates, making the products wetter. Comparison of the results between cases 1 and 2 shows that increasing the inlet temperature from 170 to 230 °C decreased
Table 2 Transition energy values with different extents of crystallinity obtained by Gombás et al. (2002). Transition energy (J g 112 99 96 78 56 39 34 29 14
1
)
Crystallinity (%) 0 5 10 20 30 50 60 70 80
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Table 3 Variation in the degree of crystallinity for different operating conditions. Operating condition
Case 1: base case:170 °C, 28 g min 1, 20 L min 1 (runs 1 and 5) Case 2: higher inlet temperature – 230 °C, 28 g min 1, 20 L min 1 (runs 2 and 12) Case 3: higher inlet temperature and lower feed flow rate – 230 °C, 14.5 g min 1, 20 L min 1 (runs 4, 10 and 13) Case 4: base-case inlet temperature and lower atomizing air-flow rate – 170 °C, 28 g min 1, 10 L min 1 (run 7) Case 5: base-case inlet temperature and lower feed flow rate – 170 °C, 14.5 g min 1, 20 L min 1 (runs 3 and 6) Case 6: base-case inlet temperature, lower feed flow rate and lower atomizing air-flow rate – 170 °C, 14.5 g min 1, 10 L min 1 (run 8) Case 7: higher inlet temperature lower feed flow rate and lower atomizing airflow rate – 230 °C, 14.5 g min 1, 10 L min 1 (runs 9 and 11)
Difference between material and glass-transition temperature (TTg) is higher
Final (outlet) moisture content (kg/kg DA)
Heat of crystallization (J/g)
Degree of crystallinity (%)
Outlet box
Cyclone separator
Outlet box (at bottom of the dryer)
Cyclone separator
0.25, 0.08 0.13, 0.02
34 ± 12 64 ± 5
41 ± 8 57 ± 6
60 ± 12 25 ± 2
48 ± 10 29 ± 4
0.04
66 ± 3
50 ± 5
24 ± 4
36 ± 6
0.11
64 ± 8
26 ± 1
26 ± 4
72 ± 1
0.13, 0.08
54 ± 15
47 ± 10
31 ± 15
38 ± 10
0.23
62 ± 9
46 ± 13
26 ± 4
40 ± 13
0.02, 0.07
75 ± 6
40 ± 0.5
22 ± 4
48 ± 1
Higher rate of crystallization
More crystalline product
Higher inlet gas temperature
Rapid decrease in moisture content
Increase in glasstransition temperature
Smaller difference between material and glass-transition temperature (TTg)
Low rate of crystallization leading to less crystalline product
Fig. 3. Qualitative diagram summarizing the possible arguments regarding the effects of using higher inlet gas temperature in the pilot-scale spray dryer.
the crystallinity by 35%, whereas other parameters, like the feed flow rate and the atomizing air-flow rate, did not have any significant effect on the extent of crystallization. Low heats of crystallization, with average values of 34 ± 12 J g 1 corresponding to 60 ± 12% crystallinity (Table 2, Gombás et al., 2002) were obtained for the particles collected at the outlet using the base-case operating conditions (170 °C, 28 g min 1, 20 L min 1). In addition, the particles obtained from the outlet at the average values for the cyclones of 41 ± 8 J g 1, corresponding to 48 ± 10% crystallinity, which are close to the values for the particles from the outlet at the bottom of the dryer. In contrast, when the inlet gas temperature was increased to 230 °C (28 g min 1, 20 L min 1), the MDSC analysis showed that the average values for the heats of crystallization were 64 ± 5 J g 1 (25 ± 2% crystallinity) for the particles from the bottom outlet and 57 ± 6 J g 1 (29 ± 4% crystallinity) for the particles from the cyclone. The average heats of crystallization gave an almost twofold difference for the particles from the outlet box when the inlet gas temperature was increased, while the degree of crystallization decreased by 35% from 60 ± 12% to 25 ± 2%. Table 3 illustrates the degree of crystallinity for the two operating conditions. Statistical analysis with t-tests was carried out using the method set out by Finlayson et al. (2007) and Prins et al. (2003) to determine if there were any significant differences between the mean heats of crystallization. As seen in Table 1, the number of repeated runs var-
ied for each operating condition, so the variance for each average mean was assumed to be unequal. By choosing a significance level of 95%, the t-tests were carried out by stating the null hypothesis as ‘‘no significant difference between the average means” and the alternative hypothesis as ‘‘there is significant difference between average means”. The difference was found to be significant with 95% confidence between the average heats of crystallization for the particles that were spray dried at 170 °C, 28 g min 1 and 20 L min 1 and at 230 °C, 28 g min 1 and 20 L min 1, for inlet air temperatures, liquid flow rates and atomizing air-flow rates, respectively. This indicates that the particles that were spray dried at the base-case conditions of 170 °C, 28 g min 1 and 20 L min 1 have significantly higher degrees of crystallinity compared with the particles that were spray dried at the higher temperature but with the same feed flow rate and atomizing air-flow rate. Fig. 3 summarizes the possible arguments regarding the effects of using higher inlet gas temperatures in the pilot-scale spray dryer. The MDSC results from Fig. 2 suggest that the lower argument in Fig. 3 is more significant for the pilot-scale dryer since the degree of crystallinity decreased on using a higher inlet gas temperature. This is affected by the rate of drying, which plays a critical role here. While an increase in inlet temperature increased the material temperature, it also decreased the moisture content rapidly, leading to a rise in the glass-transition temperature. It is therefore possible that this rapid drying of the material due to
D. Das et al. / Journal of Food Engineering 100 (2010) 551–556
Moisture Content Change (%)
10
Greaterpeak heights for 230°C inlet temperature condition Lower peak for 170°C indicatingmore crystalline content
8 6 4
Operating Condition 1 Operating Condition 2
2
555
calculated using the Gordon–Taylor equation (1952). The glasstransition temperature for pure water is 137 °C (Johari, Hallbrucker, & Mayer, 1987) and for pure lactose it is 101 °C (Roos and Karel, 1991), together with the curvature constant (k) of 7.42 (Roos, 1993). The estimated glass-transition temperatures at the outlet were predicted to vary between 92 and 101 °C for the different operating conditions. However, the outlet air temperatures were measured to be in the range of 35–85 °C, which is significantly below the glass-transition temperature, and hence it is unlikely that the crystallization rates after spray drying were very rapid.
0 0.0
-2
0.1
0.2
0.3
0.4
0.5
Time (day)
Fig. 4. Example of peak height estimation from moisture sorption data for particles collected from the bottom trays and spray dried at: (1) 230 °C, 28 g min 1, 20 L min 1 and (2) 170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, feed flow rates and atomizing air-flow rates, respectively. The moisture contents are relative to the final moisture content.
the increase in the inlet temperature did not allow enough time for the material to crystallize. The extent of crystallization is hence affected by the balance between the temperature and moisture content of the particles. The degree of crystallinity of the lactose produced with the seven sets of operating conditions were also analysed using the water-induced crystallization. Fig. 4 shows an example of the moisture sorption data and the peak height estimated as a percentage of the dry mass for two different operating conditions. Operating condition 2 in Fig. 4 (base case), with an inlet temperature of 170 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1, gave the lowest peak heights with an average change in moisture content of 7.0 ± 0.5%, while operating condition 1 with an inlet temperature of 230 °C, a feed flow rate of 28 g min 1 and an atomizing air-flow rate of 20 L min 1 gave peak heights with an average change in moisture content of 8.0 ± 0.2%. For other conditions, like case 3 (230 °C, 14.5 g min 1, 20 L min 1) and case 5 (170 °C, 14.5 g min 1, 20 L min 1), the average peak heights were estimated to be 8.3 ± 0.2% and 8.4 ± 0.2%, respectively. Statistical analysis with t-tests was carried out to compare the peak heights for each operating condition with the base-case condition. Significant differences were found with 95% confidence only when the conditions of 230 °C, 28 g min 1, 20 L min 1 (case 2), which gave an average peak height of 8.0 ± 0.2%, were compared with the base-case conditions of 170 °C, 28 g min 1, 20 L min 1, which gave an average peak height of 7.0 ± 0.5%. Water-induced crystallization, using the base-case condition, showed the highest degree of crystallinity in the spray-dried sample, whereas case 2 (170 °C, 28 g min 1, 20 L min 1) showed a lower crystalline content. Similarly analysis with MDSC, using the base-case condition, showed the highest crystalline content and case 2 showed lower crystallinity in the spray-dried lactose. Both the analyses confirm that the highest degree of crystallinity was obtained from the base case with a lower inlet temperature of 170 °C, and lower crystallinity was obtained for samples that were spray dried using the other operating conditions. 3.2. Possibility of crystallization after spray drying The outlet moisture content for the base-case operating condition (170 °C, 28 g min 1, 20 L min 1 for inlet temperatures, liquid flow rates and atomizing air-flow rates, respectively) was 16% with a standard deviation of 8%. The outlet moisture contents for the different operating conditions varied between 2–29%. The glasstransition temperatures (Tg) for the outlet (moist) solids were
3.3. Comparison with the literature The results obtained from the present investigation with the pilot-scale spray dryer are different to those that were found by Chiou et al. (2008b) using a small laboratory-scale spray dryer. These authors found that the crystallinity of spray-dried lactose increased with an increase in the inlet air temperature for this smallscale dryer. The particles in a laboratory-scale dryer, being in the range of 5–7 lm, are about half the diameter of those in the large pilot-scale dryer (15 ± 2 lm) and hence the particles dry more quickly and reach equilibrium much faster in the laboratory-scale dryer. The glass-transition temperature is dependent on the moisture content, as seen in the Gordon-Taylor equation (1952). The glasstransition temperature is also a function of the material and its (non-aqueous) composition, and hence this temperature will reach an upper limit quickly if the moisture content of the particle reaches a lower equilibrium limit quickly, as in the case of a small laboratory-scale dryer. In this case, increasing the glass-transition temperature increases the difference between the material temperature and its glass-transition temperature, causing more crystallization to occur. This situation, where the glass-transition temperature of the spray-dried material has an upper limit while the particle temperature has no limit except as constrained by the equipment, may be a characteristic of this laboratory-scale dryer (Buchi B-290). In the pilot-scale spray dryer, the particle temperature may increase when the inlet gas temperature is raised but is then counterbalanced by the rise in the glass-transition temperature, as the materials get drier at higher inlet gas temperatures. Thus, an increase in the inlet gas temperature may decrease the difference between the material and the glass-transition temperatures, resulting in a lower degree of crystallinity for the product. This behaviour of lower crystallinity at a higher temperature was seen in practice with this pilot-scale dryer, with a heat of crystallization of 34 ± 12 J g 1 at 170 °C and 64 ± 5 J g 1 at 230 °C. 4. Conclusions The results from the analyses using modulated differential scanning calorimetry and water-induced crystallization suggest that there is a significant effect of operating conditions on the extent of crystallization of the spray-dried products in this pilot-scale dryer. The overall range of product crystallinity obtained was between 20% and 72% in the experiments, using all the different operating conditions limited by the equipment. Decreasing the inlet air temperature in the dryer from 230 to 170 °C increased the crystallinity of the product by 35% in the outlet box. Using a lower atomizing air-flow rate increased the average heat of crystallization, suggesting a decrease in the degree of crystallization. Statistical analyses using t-tests confirmed with 95% confidence that there were significant variations in the degree of crystallinity using these operating conditions. T-tests also confirmed with 95% confidence that there was no significant change in the heat of crystallization and sorption test values when varying the feed flow rate and the main air-flow rate.
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