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Experimental evidence of mass transfer mechanisms during freeze-drying in a capillary porous medium
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 2 1 0 2 e2 1 0 9
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Experimental evidence of mass transfer mechanisms during freeze-drying in a capillary porous medium L.A. Segura*, C.A. Oyarzu´n Food Engineering Department, Universidad del Bı´o-Bı´o, Avda. Andre´s Bello S/N, Casilla 447, Chilla´n, Chile
article info
abstract
Article history:
The morphology of the drying front at pore-scale was studied to gain a better under-
Received 23 December 2010
standing of the mass transport mechanisms that occur at pore-level. Therefore, a glass
Received in revised form
micromodel was built using a photolithographic technique. Micromodels can be defined as
27 June 2012
transparent networks of pores (pore bodies) and constrictions (pore throat) that capture the
Accepted 20 August 2012
complexities of natural porous media. The micromodel was saturated with distilled water
Available online 31 August 2012
and frozen at 85 C and then put into a laboratory lyophilizer maintained at 42 C. Porelevel distribution of ice and vapor as the mass transport process advances were reported.
Keywords:
Ice distribution in the bidimensional porous medium was slightly influenced by pore
Freeze-drying
structure disorder. Two different sublimation regimes were also observed (fast front
Porous medium
advance and slow front advance). Preliminary results show that the drying front for the
Microchannel
slow front regimes scaling over time with an exponent of 0.577. ª 2012 Elsevier Ltd and IIR. All rights reserved.
Mass transfer Sublimation
Preuve expe´rimentale des me´canismes de transfert de masse lors de la lyophilisation dans un milieu poreux capillaire Mots cle´s : Lyophilisation ; Milieu poreux ; Microcanal ; Transfert de masse ; Sublimation
1.
Introduction
Freeze-drying is a dehydration process in which water is removed from frozen materials by ice sublimation (George and Datta, 2002). The three stages of a typical freeze-drying cycle are freeze-drying, primary drying, and secondary drying (May and Louis, 1999). A number of freeze-drying models to predict freeze-drying cycles for product quality control have been published, but there are only a few studies that show experimental evidence of freeze-drying front paths during the process (Zhai et al., 2003, 2005). These authors
studied the sublimation front of pure ice in freeze-drying to investigate basic heat and mass transfer mechanisms in the primary drying stage of vial lyophilization. They concluded that the sublimation front was curved and strongly depended on heat transfer in the vial walls. It is important to note that these experiments were carried out in a non-porous medium. Wang and Chen (2007, 2008, 2009) have also experimentally and numerically studied freeze-drying in porous materials. They have concluded that temperature and ice saturation profiles show sublimation in the pre-interface moving region, which takes place after the interface movement is completed.
* Corresponding author. Tel.: þ56 42 253072; fax: þ56 42 253066. E-mail address: [email protected] (L.A. Segura). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2012.08.014
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Nomenclature a f(x,t) g¼0 g>0 g<0 L p r t
pore to pore distance (mm) function representing point belonging to drying front (e) without gravity effect (e) with positive gravity effect (e) with negative gravity effect (e) Size of the micromodel in the z-direction (cm) pressure (Pa) pore-throat radius (mm) time (min)
Therefore, the sublimation interface can be naturally formed rather than constrainedly inserted (Wang and Chen, 2009). On the other hand, Abdelwahed et al. (2006) point out that pore size influences water vapor flow through dried mass in the freeze-drying process; as a consequence, water vapor diffusion coefficients are influenced by this feature of porous materials (Spieles et al., 1995). It is therefore necessary to study the process at pore-scale to gain a better understanding of freeze-drying and know how pore size influences sublimation front morphology. These results are particularly important in freeze-dried porous materials such as foods, drugs, or rocks. Transparent micromodels are an interesting tool to study freeze-drying in a porous structure. These micromodels can be defined as transparent pore networks (pore bodies) and constrictions (pore throats) that simulate some of the complexities of natural porous media (see Buckley, 1991 for a review). Bead packs and single-pore models are also micromodels. Micromodels help to clarify the fundamentals of transport mechanisms because they visualize complex fluid/fluid/solid interactions and their relationships with geometry and pore-space topology in the displacement process. Micromodels were widely used to study immiscible displacement (Lenormand et al., 1983, 1988; Badillo et al., 2011) and isothermal drying of porous media (Shaw, 1987; Prat, 1993; Laurindo and Prat, 1996, 1998; Tsimpanogiannis et al., 1999; Segura and Toledo, 2005a; Segura, 2007; Segura, 2008; Oyarzun and Segura, 2009). These experimental studies contribute in-depth knowledge about the behavior of these multiphase processes. The physics of moisture transport in a porous microstructure can be visualized with appropriately designed transparent pore networks.
T W x Z
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temperature ( C) width of the micromodel (cm) distance along the width of the micromodel (cm) mean drying front position (cm)
Greek letters a regression exponent (e) Subscripts max maximum distance min minimum distance
Fig. 1 shows classical drying fronts in a bead pack micromodel (Shaw, 1987; Segura, 2008). The micromodel was built by inserting silica spheres with a mean diameter of 0.4 mm into a 15 mm 2.5 cm 4.0 cm gap between two glass slides, thus producing a homogenous, dense, and random packing medium. The micromodel was filled with distilled water after packing and then three edges of the micromodel were sealed by allowing the water to evaporate from the open edge. Watersaturated silica spheres are transparent under an optical microscope, whereas drying zones are dark. The experiments were carried out at room temperature (18 C) and the cell with liquid-saturated spheres was placed horizontally and observed under an optical microscope using transmitted light. The drying front was recorded as drying advanced. Photos (a) and (b) shown in Fig. 1 were taken at a 30-s interval. It can be observed that the drying front is irregular. Shaw (1987) also determined the velocity of the drying front where the front was defined as the distance between the most advanced point and the least advanced point in the two-phase zone. Shaw concluded that the velocity of the drying front scaling over time had an exponent of 0.48. He also observed that the drying front morphologies strongly depended on the system’s capillary pressure, that is, pores with lower capillary pressure dried before those with higher capillary pressure. Laurindo and Prat (1996, 1998) experimentally studied the effect of gravity on the drying front morphologies in classical drying with Plexiglass micromodels. The micromodel used in the experiments corresponds to a network of 140 140 pores connected by 39,000 ducts (throats) with seven randomly distributed widths (from 0.1 to 0.6 mm). The depth of each duct was rectangular with a constant value of 1 mm. Liquid hexane was used in the
Fig. 1 e Drying fronts in a bead pack, dark areas are dried zones (Segura, 2008).
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 2 1 0 2 e2 1 0 9
experiments. As in Shaw’s (1987) experiment, three edges of the micromodel were sealed by allowing the hexane to evaporate from the open edge. Experiments were carried out at room temperature (isothermal condition). Fig. 2 shows morphologies of the drying front for three gravity conditions ( g > 0, g ¼ 0, and g < 0) where the liquid phase is shown in black and the gas phase in white. Fig. 2a is a sketch of the three studied conditions. Fig. 2b shows images of the experimental drying fronts at approximately 44% liquid saturation (see Laurindo and Prat (1996, 1998) for details). It can be observed from the figure that the drying front was stabilized for positive gravity ( g > 0), which means that there is an equilibrium between gravity and capillary forces. On the other hand, for negative gravity ( g < 0), gravity tends to dominate capillary forces resulting in a destabilized drying front. Finally, when there is no gravity ( g ¼ 0), only the capillary force acts on the drying process, that is, pores with lower capillary pressure dried before those with higher capillary pressure. Furthermore, the drying front depends on pore size distribution in the network. The objective of the present study was to build a 2D glass micromodel and study the morphology of the sublimation interface (drying front) at pore-scale to better understand liquidegas distribution on the sublimation front and the mass transport mechanisms that occur at pore-level.
2.
Materials and methods
2.1.
Micromodel construction
Using photofabrication techniques, 2D transparent porous media (micromodels) can be prepared on either resin (Bonnet and Lenormand, 1977; Dı´az et al., 2011) or glass plates (Chatzis, 1982; Campbell, 1983). Resin-based micromodels control pore shape better. Pore sizes of 1 mm deep and at least 0.1 mm wide are typically formed. Etched glass micromodels do not have welldefined pore shapes and can contain pores 10 mm or larger. The photo-etched glass technique has been applied in this study
because pore shapes found in biological porous materials are usually irregular. The electronic industry regularly transfers micro-patterns onto substrates (via optical, electron-beam, and X-ray lithography) to produce integrated circuits (Chiu and Shaw, 1997). Optical lithography, also known as photolithography, was used because it is the common micromodel manufacturing technique. Photolithography involves pattern design (photomask) and a photo-etching process described below. The photolithographic technique was used to construct a micromodel similar to the one proposed by Chatzis (1982), but with some modifications (Zamorano, 2007; Dı´az et al., 2011). The steps of the photolithographic technique are described as follows (Fig. 3): a) Glass surface preparation. Rubber gloves were always used to handle the plates. Glass plates were washed abundantly with soap to eliminate fatty compounds and powders and thoroughly rinsed with distilled water. They were then submerged in a 4:1:1 H2SO4:H2O2:H2O (v/v) solution for approximately 120 min. Plates were rinsed with distilled water and placed in an oven for 30 min at 150 C. b) Impregnation with HMDS. Dried plates were impregnated with Hexamethyl di-silazane (HMDS) to improve photoresist adhesion and create a hydrophobic surface. A single plate was impregnated with 250 ml of HMDS and placed in a vacuum (600 mmHg) at 75 C for 10 min. Glass plates were cooled to room temperature and stored in a hermetically sealed container. Glass plates were generally stored with silica gel to reduce humidity. c) Photopolymer application. A thin, homogenous layer of positive ultraviolet (UV) light-sensitive photoresist (Positiv 20, Kontakt Chemie) protected from light was sprayed onto the glass surface (see Fig. 3). If there is dust on the glass surface, the photoresist could spread heterogeneously and the quality of the transferred pattern could be very low. Therefore, the workspace environment has to be dust-free to get high resolution patterns. Plates with defect-free photoresist were soft-dried for 15 min at 70 C.
Fig. 2 e (a) Sketch of the three evaporation cases studied; (b) experimental phase distribution for isothermal drying with liquid saturation z 44%, and the effect of gravity in liquid distribution (adapted from Laurindo and Prat, 1998).
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More details can be found in Zamorano (2007), Oyarzun and Segura (2009), and Dı´az et al. (2011).
2.2.
Freeze-drying experiments
The following experiments were carried out after the micromodel was constructed to visualize sublimation interface morphology (drying front):
Fig. 3 e Photolithographic technique (adapted from Chatzis, 1982).
d) Exposure to UV light. The pattern transparency (photomask) was placed on the photoresist-coated surface (see Fig. 3). A second glass plate was placed over the photomask to ensure direct contact with the photoresist. The assembly was then exposed to UV light in a dark chamber for 15 min. The laboratory UV light source was located 20 cm from the assembly. e) Developing. After UV exposure, the resist was developed by submerging the coated glass plate into a solution of 7 g NaOH per liter of cold water for approximately 2 min. When the image was completely developed, the plate was rinsed with a soft jet of water. The developing process dissolved the photoresist areas previously exposed to UV light (i.e., portions under the transparent zones of the photomask). Portions of plate that needed to be etched were then exposed to subsequent acid attack, while the rest of the portions were protected with photoresist (see Fig. 3). The glass plate was then baked at 150 C for 30 min. f) Etching. Adhesive tape was used before the etching process to protect exposed glass that was not meant to be etched (edges and back of glass plate). Plates were then immersed in a 3:2 HF:H2O (v/v) solution for 8 min. Etched plates were rinsed with a strong jet of water and vigorously brushed to remove any remaining photoresist and deposits formed during the reaction (see Fig. 3). g) Glass plate fusion. A second identical glass plate (unmodified) was placed on top of the etched glass plate to seal the micromodel. Plates were fused in a furnace at 710 C for 20 min (see Fig. 3).
a) The micromodel was saturated with distilled water. b) The micromodel was frozen at 85 C to ensure that all the water was frozen. c) The frozen micromodel was then put into a laboratory lyophilizer (Christ, model Beta 1-8 LD) maintained at 42 C, which is the minimum recommended temperature for the equipment. The micromodel was not put in a vial but located in the center of the cold source as shown in Fig. 4. d) Equilibrium temperature was reached after a reasonable time, a vacuum applied, and the evolution of the process observed. e) Evolution of the sublimation interface front was recorded over time to quantify the process kinetics. Many photos of the process were taken for this reason. f) A conventional video camera (Sony Handycam 700) was used to digitize patterns; Photoshop CS2 (version 9.0, Adobe) image processing software provided high quality monochromatic pictures. Network ice saturation (Sw) was also estimated through image analysis. It was calculated as Sw ¼ Vi/V where Vi corresponds to the network volume filled with water (ice) and V is the total network volume, including solid and free space (Segura and Toledo, 2005b).
2.3.
Sublimation fronts
To analyze vaporeice distributions during freeze-drying, the freeze-drying front was characterized by the mean front position z(t) defined as:
Fig. 4 e Diagram of experiment setup in the lyophilizer.
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ZW f ðx; tÞdx zðtÞ ¼
0
(1)
ZW dx 0
where f(x,t) represents a point belonging to the drying front measured from the open side (see Fig. 4), W is the width of the micromodel, and x is the coordinate in the same direction. Since the resulting morphologies are only approximate and had a slight roughness, the following approximation was adopted: 1 zðtÞz $½zmax ðtÞ þ zmin ðtÞ 2
(2)
where zmax ðtÞ ¼ max ½f ðx; tÞ and zmin ðtÞ ¼ min ½f ðx; tÞ as shown 0
3.
Fig. 6 e Liquidegas distribution of the glass micromodel built in this study.
Results and discussion
Typical morphology of the constructed glass micromodel is shown in Fig. 5. It corresponds to a sketch of the micromodel to illustrate its geometrical parameters. A glass micromodel was built following the steps described in the Materials and methods section. Fig. 6 shows a portion of the final micromodel constructed in this study. The size of the micromodel is approximately 7.5 cm 9.0 cm with 130 170 pore bodies (nodes) and 44,500 channels (pore throats). The depth of each channel is approximately constant and equal to 35 (mm). The width of each channel is a random variable characterized by its pore-throat radius, which follows a discrete Log-Normal distribution (see Fig. 7) with a mean of 23.13 (mm) and a standard deviation of 6.57 (mm). The centerto-center pore distance is 500 (mm). To distinguish pore distribution and liquid vapor interface in the network, the final micromodel was partially liquid-filled and observed with the camera. A typical liquidegas distribution in the glass micromodel is shown in Fig. 6. Pressure and temperature varied slightly during the freezedrying process as shown by the points (white squares) of the peT diagram in Fig. 8. Pressure and temperature range variations were 10.58 1.78 (Pa) and 41.16 1.58 ( C),
Fig. 5 e Geometrical parameters of the micromodel. a, pore to pore distance; r, pore-throat radius.
respectively. Three points (squares) show pressure and temperature values for the freeze-drying process. The experiment lasted 5 d for approximately 7 h each day and the micromodel was refrozen at the end of each day (the experiment ended on day 5). No change was observed in ice distribution when the experiment was interrupted. External variables ( p, T ) were maintained near saturated vapor conditions (automatically with equipment) and produced a weak gradient of partial pressure, thus producing a low mass transfer rate. Processed images of the micromodel freezedrying experiment results in Fig. 9 show freeze-drying and ice saturation (Sw) network evolution over time (frozen layer in black and dried layer in white); temperature and the pressure system are also recorded. Saturation time frames (pictures) were taken every 300 min, except at the beginning and at the end of the process to obtain pictures at different network saturations. Fig. 9a shows an image of the drying front after 150 min (Sw ¼ 88.13%). It can be observed from the
Fig. 7 e Discrete Log-Normal probability distribution of the constructed micromodel.
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Fig. 8 e Pressureetemperature diagram. Three points (squares) show pressure and temperature values during the freeze-drying process.
figure that sublimation occurs almost simultaneously across the cross-sectional area of the micromodel. This implies that there are no preferential pores involved in the sublimation process. As freeze-drying advances we can observe the same behavior in Fig. 9b (Sw ¼ 76.59%), Fig. 9c (Sw ¼ 68.03%), Fig. 9d (Sw ¼ 56.98%), Fig. 9e (Sw ¼ 50.4%), Fig. 9f (Sw ¼ 4.71%), Fig. 9g (Sw ¼ 42%), Fig. 9h (Sw ¼ 25.59%), and Fig. 9i (Sw ¼ 21.92%). In general, 2D porous network iceevapor distribution is only slightly influenced by pore structure disorder. However, Fig. 7 histograms show that this disorder is not important. It is possible that a more disordered medium could influence iceevapor distribution results. If freeze-drying experimental patterns obtained in the present study (Fig. 9) are compared with classical isothermal drying patterns in a Plexiglass micromodel with similar characteristics to the one used (Fig. 2), it can be seen that freeze-drying patterns differ considerably from conventional drying patterns. There were no ice clusters in the sublimation process. This phenomenon demonstrates that pore-throat sizes marginally influence the freeze-drying process and probably only locally influence drying front morphology, therefore producing a slightly rough front. Classical drying patterns in the case of a stabilized front ( g > 0) where the drying front has liquid clusters on its neighboring sites show morphology similar to freeze-drying. It is important to establish that front drying shapes in classical drying experiments strongly depend on capillary pressure. Since freezedrying experiments have no liquid phase, these forces are obviously not present. The lyophilizer equipment used in this study was not designed to conduct this kind of experiment, so photos taken do not capture in detail the sublimation interface morphology of the freeze-drying experiments. However, iceevapor distributions shown in Fig. 9 are very similar to those observed in the experiment. The mean front position was computed as drying advances by measuring the front position z(t) for each freeze-drying
Fig. 9 e Vaporeice distributions in a porous medium in the lyophilization process.
stage shown in Fig. 9 using equation (2). We plotted a logelog graph in Fig. 10, z(t)/L vs. t where L is the micromodel size in the z-direction (9.0 cm) and d-i is the day when the experiment was conducted (i ¼ 1, 2, 3, 4, 5). Fast front advance and slow front advance are two different regimes that can be observed in the figure. The distance which must be reached by a given vapor quantity from the vaporeice interface to the
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study could allow scaling relationships which consider operational conditions and characteristics of the porous medium.
4.
Conclusions
Preliminary experimental results of freeze-drying experiments carried out in a 2D glass micromodel are presented. Pore-level distribution of ice and vapor as transport phenomena advances is reported. Ice distribution in the bidimensional porous medium is slightly influenced by pore structure disorder, and there are no ice clusters in the sublimation process. Two different regimes during sublimation were also observed: one characterized by a fast front advance and another characterized by a slow front advance.
Fig. 10 e Temporal evolution of the sublimation interface position (drying front) in a porous medium. d-i is referred to as day i with i [ 1,2,.,5.
micromodel open side increases over time due to medium tortuosity. Since the iceevapor front is deeper on the micromodel, it is a more complex path to transport water vapor to the micromodel open side, and the partial pressure gradient weakens in the vicinity of the vaporeice front. These results fully agree with Abdelwahed et al. (2006) and Spieles et al. (1995), who proposed that pore size influences water vapor flow through dried mass in the freeze-drying process. The present study proposed that freeze-drying was controlled by the heat transfer process in the fast front advance regime (up to 300 min, Fig. 10), which is the same as in the freeze-drying experiments carried out by Zhai et al. (2003) and Zhai et al. (2005) in vials (non-porous medium). However, freeze-drying was controlled by the mass transfer process with a slower freeze-drying velocity during the slow front advance regime (from 300 min to the end). It is also possible to observe a linear tendency in the second regime (slow front advance) from the logelog plot in Fig. 10. Equation (3) exhibits this tendency: log
zðtÞ ¼ 0:5774t 2:0911 r2 ¼ 0:99 L
(3)
This suggests that the drying front scaling over time is: zðtÞ a wt ; t[1 L
(4)
with an exponent of a z 0.577. On the other hand, Shaw (1987) found that the velocity of the drying front scaling over time had an exponent of 0.48 for classical drying. One of the features of classical drying is the presence of liquid clusters (see Figs. 1 and 2). This liquid cluster acts as a barrier for vapor flow through the porous structure. The present study proposes that the increase of the exponent (0.577) is because in freeze-drying vapor flows through the porous structure without these clusters, which appear in classical drying. More experiments are necessary to test this analysis, which would provide a more reliable result. Furthermore, a more in-depth
Acknowledgments Financial support from CONICYT e Chile through project FONDECYT N 1120342 is greatly appreciated.
references
Abdelwahed, W., Degobert, G., Fessi, H., 2006. Freeze-drying of nanocapsules: impact of annealing on the drying process. Int. J. Pharm. 324 (1), 74e82. Badillo, G.M., Segura, L.A., Laurindo, J.B., 2011. Theoretical and experimental aspects of vacuum impregnation of porous media using transparent etched networks. Int. J. Multiphase Flow 37, 1219e1226. Bonnet, J., Lenormand, R., 1977. Re´alisation de micromode`les pour l’e´tude des e´coulements polyphasiques en milieu poreux. Rev. Inst. Francais Pe´trole 42, 1415e1428. Buckley, J.S., 1991. Multiphase displacement in micromodels. In: Morrow, N.R. (Ed.), Interfacial Phenomena in Petroleum Recovery. Marcel Dekker, New York, pp. 157e189. Campbell, B., 1983. Photofabrication of Porous Networks in TwoDimensional Glass Plate Micromodels. Petroleum Recovery Research Center, New Mexico. Report 83e3. Chatzis, I., 1982. Photofabrication Technique of Two-Dimensional Glass Micromodels. PRRC, New Mexico. Report 82e12. Chiu, G.L., Shaw, J.M., 1997. Optical lithography: introduction. IBM J. Res. Dev. 41 (1/2), 3e6. Dı´az, R.E., Acun˜a, S.M., Segura, L.A., 2011. Construction of 2D transparent micromodels in polyester resin with porosity similar to carrots. Cieˆnc. Tecnol. Aliment. 31 (4), 960e966. George, J.P., Datta, A.K., 2002. Development and validation of heat and mass transfer models for freeze-drying of vegetables slices. J. Food Eng. 52, 89e93. Laurindo, J.B., Prat, M., 1996. Numerical and experimental network study of evaporation in capillary porous media. Phase distributions. Chem. Eng. Sci. 51 (23), 5171e5185. Laurindo, J.B., Prat, M., 1998. Numerical and experimental network study of evaporation in capillary porous media. Drying rate. Chem. Eng. Sci. 53 (12), 2257e2269. Lenormand, R., Zarcone, C., Sarr, A., 1983. Mechanism of the displacement of one fluid by another in a network of capillary ducts. J. Fluid Mech. 135, 337e353. Lenormand, R., Touboul, E., Zarcone, C., 1988. Numerical models and experiments on immiscible displacement in porous media. J. Fluid Mech. 189, 165e187.
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May, J.C., Louis, R., 1999. Freeze-Drying/Lyophilization of Pharmaceutical and Biological Products. Marcel Dekker, New York, Basel. Oyarzun, C.A., Segura, L.A., 2009. Design and construction of glass micromodels for the study of moisture transport in softwoods. Dry. Technol. 27, 14e29. Prat, M., 1993. Percolation model of drying under isothermal conditions in porous media. Int. J. Multiphase Flow 19 (4), 691e704. Segura, L.A., Toledo, P.G., 2005a. Pore-level modeling of isothermal drying pore networks. Evaporation and viscous flow. Lat. Am. Appl. Res. 35, 43e50. Segura, L.A., Toledo, P.G., 2005b. Pore-level modeling of isothermal drying pore networks. Effects of gravity and pore shape and size distribution on saturation and transport parameters. Chem. Eng. J. 111, 237e252. Segura, L.A., 2007. Pore-level modeling of isothermal drying of pore networks. Liquid and vapor diffusivity. Dry. Technol. 25, 1677e1686. Segura, L.A., 2008. Drying of porous materials: experiments and modeling at pore level. In: Gutie´rrez-Lopez, G.F., BarbosaCa´novas, G.V., Welti-Chanes, J., Parada-Arias, E. (Eds.), Food Engineering: Integrated Approaches. Springer ScienceþBusiness Media LLC, New York, pp. 301e306. Shaw, T.M., 1987. Drying as an immiscible displacement process with fluid counterflow. Phys. Rev. Lett. 59 (15), 1671e1674. Spieles, G., marx, T., Heschel, I., Rau, G., 1995. Analysis of desorption and diffusion secondary drying in vacuum
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freeze-drying of hydroxyethyl starch. Chem. Eng. Process. 34 (4), 351e357. Tsimpanogiannis, I.N., Yortsos, Y.C., Poulou, S., Kanellopoulos, N., Stubos, A.K., 1999. Scaling theory of drying in porous media. Phys. Rev. E59 (4), 4353e4365. Wang, W., Chen, G., 2007. Freeze drying with dielectric-materialassisted microwave heating. AIChE J. 53, 3077e3088. Wang, W., Chen, G., 2008. Numerical investigation on freezedrying on initially porous frozen material from aqueous solution. In: Proceedings of 16th International Drying Symposium, Hyderabad, India, November 9e12, 2008, vol. A, pp. 508e513. Wang, W., Chen, G., 2009. Freeze-drying of initially frozen materials from aqueous solution. In: Proceedings of 4th InterAmerican Drying Conference, Montreal, Canada, August 23e27, 2009, pp. 395e403. Zamorano, C., 2007. Protocolo para la elaboracio´n de micromodelos transparentes 2D en Vidrio. Thesis to obtain bachelor Food Engineering, Universidad del Bı´o-Bı´o, Chilla´n, Chile. Zhai, S., Taylor, R., Sanches, R., Slater, N.K.H., 2003. Measurement of lyophilisation primary drying rates by freeze-drying microscopy. Chem. Eng. Sci. 58, 2313e2323. Zhai, S., Su, H., Taylor, R., Slater, N.K.H., 2005. Pure ice sublimation within vials in a laboratory lyophilizer; comparison of theory with experiment. Chem. Eng. Sci. 60, 1167e1176.
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Experimental evidence of mass transfer mechanisms during freeze-drying in a capillary porous medium L.A. Segura*, C.A. Oyarzu´n Food Engineering Department, Universidad del Bı´o-Bı´o, Avda. Andre´s Bello S/N, Casilla 447, Chilla´n, Chile
article info
abstract
Article history:
The morphology of the drying front at pore-scale was studied to gain a better under-
Received 23 December 2010
standing of the mass transport mechanisms that occur at pore-level. Therefore, a glass
Received in revised form
micromodel was built using a photolithographic technique. Micromodels can be defined as
27 June 2012
transparent networks of pores (pore bodies) and constrictions (pore throat) that capture the
Accepted 20 August 2012
complexities of natural porous media. The micromodel was saturated with distilled water
Available online 31 August 2012
and frozen at 85 C and then put into a laboratory lyophilizer maintained at 42 C. Porelevel distribution of ice and vapor as the mass transport process advances were reported.
Keywords:
Ice distribution in the bidimensional porous medium was slightly influenced by pore
Freeze-drying
structure disorder. Two different sublimation regimes were also observed (fast front
Porous medium
advance and slow front advance). Preliminary results show that the drying front for the
Microchannel
slow front regimes scaling over time with an exponent of 0.577. ª 2012 Elsevier Ltd and IIR. All rights reserved.
Mass transfer Sublimation
Preuve expe´rimentale des me´canismes de transfert de masse lors de la lyophilisation dans un milieu poreux capillaire Mots cle´s : Lyophilisation ; Milieu poreux ; Microcanal ; Transfert de masse ; Sublimation
1.
Introduction
Freeze-drying is a dehydration process in which water is removed from frozen materials by ice sublimation (George and Datta, 2002). The three stages of a typical freeze-drying cycle are freeze-drying, primary drying, and secondary drying (May and Louis, 1999). A number of freeze-drying models to predict freeze-drying cycles for product quality control have been published, but there are only a few studies that show experimental evidence of freeze-drying front paths during the process (Zhai et al., 2003, 2005). These authors
studied the sublimation front of pure ice in freeze-drying to investigate basic heat and mass transfer mechanisms in the primary drying stage of vial lyophilization. They concluded that the sublimation front was curved and strongly depended on heat transfer in the vial walls. It is important to note that these experiments were carried out in a non-porous medium. Wang and Chen (2007, 2008, 2009) have also experimentally and numerically studied freeze-drying in porous materials. They have concluded that temperature and ice saturation profiles show sublimation in the pre-interface moving region, which takes place after the interface movement is completed.
* Corresponding author. Tel.: þ56 42 253072; fax: þ56 42 253066. E-mail address: [email protected] (L.A. Segura). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2012.08.014
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Nomenclature a f(x,t) g¼0 g>0 g<0 L p r t
pore to pore distance (mm) function representing point belonging to drying front (e) without gravity effect (e) with positive gravity effect (e) with negative gravity effect (e) Size of the micromodel in the z-direction (cm) pressure (Pa) pore-throat radius (mm) time (min)
Therefore, the sublimation interface can be naturally formed rather than constrainedly inserted (Wang and Chen, 2009). On the other hand, Abdelwahed et al. (2006) point out that pore size influences water vapor flow through dried mass in the freeze-drying process; as a consequence, water vapor diffusion coefficients are influenced by this feature of porous materials (Spieles et al., 1995). It is therefore necessary to study the process at pore-scale to gain a better understanding of freeze-drying and know how pore size influences sublimation front morphology. These results are particularly important in freeze-dried porous materials such as foods, drugs, or rocks. Transparent micromodels are an interesting tool to study freeze-drying in a porous structure. These micromodels can be defined as transparent pore networks (pore bodies) and constrictions (pore throats) that simulate some of the complexities of natural porous media (see Buckley, 1991 for a review). Bead packs and single-pore models are also micromodels. Micromodels help to clarify the fundamentals of transport mechanisms because they visualize complex fluid/fluid/solid interactions and their relationships with geometry and pore-space topology in the displacement process. Micromodels were widely used to study immiscible displacement (Lenormand et al., 1983, 1988; Badillo et al., 2011) and isothermal drying of porous media (Shaw, 1987; Prat, 1993; Laurindo and Prat, 1996, 1998; Tsimpanogiannis et al., 1999; Segura and Toledo, 2005a; Segura, 2007; Segura, 2008; Oyarzun and Segura, 2009). These experimental studies contribute in-depth knowledge about the behavior of these multiphase processes. The physics of moisture transport in a porous microstructure can be visualized with appropriately designed transparent pore networks.
T W x Z
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temperature ( C) width of the micromodel (cm) distance along the width of the micromodel (cm) mean drying front position (cm)
Greek letters a regression exponent (e) Subscripts max maximum distance min minimum distance
Fig. 1 shows classical drying fronts in a bead pack micromodel (Shaw, 1987; Segura, 2008). The micromodel was built by inserting silica spheres with a mean diameter of 0.4 mm into a 15 mm 2.5 cm 4.0 cm gap between two glass slides, thus producing a homogenous, dense, and random packing medium. The micromodel was filled with distilled water after packing and then three edges of the micromodel were sealed by allowing the water to evaporate from the open edge. Watersaturated silica spheres are transparent under an optical microscope, whereas drying zones are dark. The experiments were carried out at room temperature (18 C) and the cell with liquid-saturated spheres was placed horizontally and observed under an optical microscope using transmitted light. The drying front was recorded as drying advanced. Photos (a) and (b) shown in Fig. 1 were taken at a 30-s interval. It can be observed that the drying front is irregular. Shaw (1987) also determined the velocity of the drying front where the front was defined as the distance between the most advanced point and the least advanced point in the two-phase zone. Shaw concluded that the velocity of the drying front scaling over time had an exponent of 0.48. He also observed that the drying front morphologies strongly depended on the system’s capillary pressure, that is, pores with lower capillary pressure dried before those with higher capillary pressure. Laurindo and Prat (1996, 1998) experimentally studied the effect of gravity on the drying front morphologies in classical drying with Plexiglass micromodels. The micromodel used in the experiments corresponds to a network of 140 140 pores connected by 39,000 ducts (throats) with seven randomly distributed widths (from 0.1 to 0.6 mm). The depth of each duct was rectangular with a constant value of 1 mm. Liquid hexane was used in the
Fig. 1 e Drying fronts in a bead pack, dark areas are dried zones (Segura, 2008).
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experiments. As in Shaw’s (1987) experiment, three edges of the micromodel were sealed by allowing the hexane to evaporate from the open edge. Experiments were carried out at room temperature (isothermal condition). Fig. 2 shows morphologies of the drying front for three gravity conditions ( g > 0, g ¼ 0, and g < 0) where the liquid phase is shown in black and the gas phase in white. Fig. 2a is a sketch of the three studied conditions. Fig. 2b shows images of the experimental drying fronts at approximately 44% liquid saturation (see Laurindo and Prat (1996, 1998) for details). It can be observed from the figure that the drying front was stabilized for positive gravity ( g > 0), which means that there is an equilibrium between gravity and capillary forces. On the other hand, for negative gravity ( g < 0), gravity tends to dominate capillary forces resulting in a destabilized drying front. Finally, when there is no gravity ( g ¼ 0), only the capillary force acts on the drying process, that is, pores with lower capillary pressure dried before those with higher capillary pressure. Furthermore, the drying front depends on pore size distribution in the network. The objective of the present study was to build a 2D glass micromodel and study the morphology of the sublimation interface (drying front) at pore-scale to better understand liquidegas distribution on the sublimation front and the mass transport mechanisms that occur at pore-level.
2.
Materials and methods
2.1.
Micromodel construction
Using photofabrication techniques, 2D transparent porous media (micromodels) can be prepared on either resin (Bonnet and Lenormand, 1977; Dı´az et al., 2011) or glass plates (Chatzis, 1982; Campbell, 1983). Resin-based micromodels control pore shape better. Pore sizes of 1 mm deep and at least 0.1 mm wide are typically formed. Etched glass micromodels do not have welldefined pore shapes and can contain pores 10 mm or larger. The photo-etched glass technique has been applied in this study
because pore shapes found in biological porous materials are usually irregular. The electronic industry regularly transfers micro-patterns onto substrates (via optical, electron-beam, and X-ray lithography) to produce integrated circuits (Chiu and Shaw, 1997). Optical lithography, also known as photolithography, was used because it is the common micromodel manufacturing technique. Photolithography involves pattern design (photomask) and a photo-etching process described below. The photolithographic technique was used to construct a micromodel similar to the one proposed by Chatzis (1982), but with some modifications (Zamorano, 2007; Dı´az et al., 2011). The steps of the photolithographic technique are described as follows (Fig. 3): a) Glass surface preparation. Rubber gloves were always used to handle the plates. Glass plates were washed abundantly with soap to eliminate fatty compounds and powders and thoroughly rinsed with distilled water. They were then submerged in a 4:1:1 H2SO4:H2O2:H2O (v/v) solution for approximately 120 min. Plates were rinsed with distilled water and placed in an oven for 30 min at 150 C. b) Impregnation with HMDS. Dried plates were impregnated with Hexamethyl di-silazane (HMDS) to improve photoresist adhesion and create a hydrophobic surface. A single plate was impregnated with 250 ml of HMDS and placed in a vacuum (600 mmHg) at 75 C for 10 min. Glass plates were cooled to room temperature and stored in a hermetically sealed container. Glass plates were generally stored with silica gel to reduce humidity. c) Photopolymer application. A thin, homogenous layer of positive ultraviolet (UV) light-sensitive photoresist (Positiv 20, Kontakt Chemie) protected from light was sprayed onto the glass surface (see Fig. 3). If there is dust on the glass surface, the photoresist could spread heterogeneously and the quality of the transferred pattern could be very low. Therefore, the workspace environment has to be dust-free to get high resolution patterns. Plates with defect-free photoresist were soft-dried for 15 min at 70 C.
Fig. 2 e (a) Sketch of the three evaporation cases studied; (b) experimental phase distribution for isothermal drying with liquid saturation z 44%, and the effect of gravity in liquid distribution (adapted from Laurindo and Prat, 1998).
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More details can be found in Zamorano (2007), Oyarzun and Segura (2009), and Dı´az et al. (2011).
2.2.
Freeze-drying experiments
The following experiments were carried out after the micromodel was constructed to visualize sublimation interface morphology (drying front):
Fig. 3 e Photolithographic technique (adapted from Chatzis, 1982).
d) Exposure to UV light. The pattern transparency (photomask) was placed on the photoresist-coated surface (see Fig. 3). A second glass plate was placed over the photomask to ensure direct contact with the photoresist. The assembly was then exposed to UV light in a dark chamber for 15 min. The laboratory UV light source was located 20 cm from the assembly. e) Developing. After UV exposure, the resist was developed by submerging the coated glass plate into a solution of 7 g NaOH per liter of cold water for approximately 2 min. When the image was completely developed, the plate was rinsed with a soft jet of water. The developing process dissolved the photoresist areas previously exposed to UV light (i.e., portions under the transparent zones of the photomask). Portions of plate that needed to be etched were then exposed to subsequent acid attack, while the rest of the portions were protected with photoresist (see Fig. 3). The glass plate was then baked at 150 C for 30 min. f) Etching. Adhesive tape was used before the etching process to protect exposed glass that was not meant to be etched (edges and back of glass plate). Plates were then immersed in a 3:2 HF:H2O (v/v) solution for 8 min. Etched plates were rinsed with a strong jet of water and vigorously brushed to remove any remaining photoresist and deposits formed during the reaction (see Fig. 3). g) Glass plate fusion. A second identical glass plate (unmodified) was placed on top of the etched glass plate to seal the micromodel. Plates were fused in a furnace at 710 C for 20 min (see Fig. 3).
a) The micromodel was saturated with distilled water. b) The micromodel was frozen at 85 C to ensure that all the water was frozen. c) The frozen micromodel was then put into a laboratory lyophilizer (Christ, model Beta 1-8 LD) maintained at 42 C, which is the minimum recommended temperature for the equipment. The micromodel was not put in a vial but located in the center of the cold source as shown in Fig. 4. d) Equilibrium temperature was reached after a reasonable time, a vacuum applied, and the evolution of the process observed. e) Evolution of the sublimation interface front was recorded over time to quantify the process kinetics. Many photos of the process were taken for this reason. f) A conventional video camera (Sony Handycam 700) was used to digitize patterns; Photoshop CS2 (version 9.0, Adobe) image processing software provided high quality monochromatic pictures. Network ice saturation (Sw) was also estimated through image analysis. It was calculated as Sw ¼ Vi/V where Vi corresponds to the network volume filled with water (ice) and V is the total network volume, including solid and free space (Segura and Toledo, 2005b).
2.3.
Sublimation fronts
To analyze vaporeice distributions during freeze-drying, the freeze-drying front was characterized by the mean front position z(t) defined as:
Fig. 4 e Diagram of experiment setup in the lyophilizer.
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ZW f ðx; tÞdx zðtÞ ¼
0
(1)
ZW dx 0
where f(x,t) represents a point belonging to the drying front measured from the open side (see Fig. 4), W is the width of the micromodel, and x is the coordinate in the same direction. Since the resulting morphologies are only approximate and had a slight roughness, the following approximation was adopted: 1 zðtÞz $½zmax ðtÞ þ zmin ðtÞ 2
(2)
where zmax ðtÞ ¼ max ½f ðx; tÞ and zmin ðtÞ ¼ min ½f ðx; tÞ as shown 0
3.
Fig. 6 e Liquidegas distribution of the glass micromodel built in this study.
Results and discussion
Typical morphology of the constructed glass micromodel is shown in Fig. 5. It corresponds to a sketch of the micromodel to illustrate its geometrical parameters. A glass micromodel was built following the steps described in the Materials and methods section. Fig. 6 shows a portion of the final micromodel constructed in this study. The size of the micromodel is approximately 7.5 cm 9.0 cm with 130 170 pore bodies (nodes) and 44,500 channels (pore throats). The depth of each channel is approximately constant and equal to 35 (mm). The width of each channel is a random variable characterized by its pore-throat radius, which follows a discrete Log-Normal distribution (see Fig. 7) with a mean of 23.13 (mm) and a standard deviation of 6.57 (mm). The centerto-center pore distance is 500 (mm). To distinguish pore distribution and liquid vapor interface in the network, the final micromodel was partially liquid-filled and observed with the camera. A typical liquidegas distribution in the glass micromodel is shown in Fig. 6. Pressure and temperature varied slightly during the freezedrying process as shown by the points (white squares) of the peT diagram in Fig. 8. Pressure and temperature range variations were 10.58 1.78 (Pa) and 41.16 1.58 ( C),
Fig. 5 e Geometrical parameters of the micromodel. a, pore to pore distance; r, pore-throat radius.
respectively. Three points (squares) show pressure and temperature values for the freeze-drying process. The experiment lasted 5 d for approximately 7 h each day and the micromodel was refrozen at the end of each day (the experiment ended on day 5). No change was observed in ice distribution when the experiment was interrupted. External variables ( p, T ) were maintained near saturated vapor conditions (automatically with equipment) and produced a weak gradient of partial pressure, thus producing a low mass transfer rate. Processed images of the micromodel freezedrying experiment results in Fig. 9 show freeze-drying and ice saturation (Sw) network evolution over time (frozen layer in black and dried layer in white); temperature and the pressure system are also recorded. Saturation time frames (pictures) were taken every 300 min, except at the beginning and at the end of the process to obtain pictures at different network saturations. Fig. 9a shows an image of the drying front after 150 min (Sw ¼ 88.13%). It can be observed from the
Fig. 7 e Discrete Log-Normal probability distribution of the constructed micromodel.
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Fig. 8 e Pressureetemperature diagram. Three points (squares) show pressure and temperature values during the freeze-drying process.
figure that sublimation occurs almost simultaneously across the cross-sectional area of the micromodel. This implies that there are no preferential pores involved in the sublimation process. As freeze-drying advances we can observe the same behavior in Fig. 9b (Sw ¼ 76.59%), Fig. 9c (Sw ¼ 68.03%), Fig. 9d (Sw ¼ 56.98%), Fig. 9e (Sw ¼ 50.4%), Fig. 9f (Sw ¼ 4.71%), Fig. 9g (Sw ¼ 42%), Fig. 9h (Sw ¼ 25.59%), and Fig. 9i (Sw ¼ 21.92%). In general, 2D porous network iceevapor distribution is only slightly influenced by pore structure disorder. However, Fig. 7 histograms show that this disorder is not important. It is possible that a more disordered medium could influence iceevapor distribution results. If freeze-drying experimental patterns obtained in the present study (Fig. 9) are compared with classical isothermal drying patterns in a Plexiglass micromodel with similar characteristics to the one used (Fig. 2), it can be seen that freeze-drying patterns differ considerably from conventional drying patterns. There were no ice clusters in the sublimation process. This phenomenon demonstrates that pore-throat sizes marginally influence the freeze-drying process and probably only locally influence drying front morphology, therefore producing a slightly rough front. Classical drying patterns in the case of a stabilized front ( g > 0) where the drying front has liquid clusters on its neighboring sites show morphology similar to freeze-drying. It is important to establish that front drying shapes in classical drying experiments strongly depend on capillary pressure. Since freezedrying experiments have no liquid phase, these forces are obviously not present. The lyophilizer equipment used in this study was not designed to conduct this kind of experiment, so photos taken do not capture in detail the sublimation interface morphology of the freeze-drying experiments. However, iceevapor distributions shown in Fig. 9 are very similar to those observed in the experiment. The mean front position was computed as drying advances by measuring the front position z(t) for each freeze-drying
Fig. 9 e Vaporeice distributions in a porous medium in the lyophilization process.
stage shown in Fig. 9 using equation (2). We plotted a logelog graph in Fig. 10, z(t)/L vs. t where L is the micromodel size in the z-direction (9.0 cm) and d-i is the day when the experiment was conducted (i ¼ 1, 2, 3, 4, 5). Fast front advance and slow front advance are two different regimes that can be observed in the figure. The distance which must be reached by a given vapor quantity from the vaporeice interface to the
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study could allow scaling relationships which consider operational conditions and characteristics of the porous medium.
4.
Conclusions
Preliminary experimental results of freeze-drying experiments carried out in a 2D glass micromodel are presented. Pore-level distribution of ice and vapor as transport phenomena advances is reported. Ice distribution in the bidimensional porous medium is slightly influenced by pore structure disorder, and there are no ice clusters in the sublimation process. Two different regimes during sublimation were also observed: one characterized by a fast front advance and another characterized by a slow front advance.
Fig. 10 e Temporal evolution of the sublimation interface position (drying front) in a porous medium. d-i is referred to as day i with i [ 1,2,.,5.
micromodel open side increases over time due to medium tortuosity. Since the iceevapor front is deeper on the micromodel, it is a more complex path to transport water vapor to the micromodel open side, and the partial pressure gradient weakens in the vicinity of the vaporeice front. These results fully agree with Abdelwahed et al. (2006) and Spieles et al. (1995), who proposed that pore size influences water vapor flow through dried mass in the freeze-drying process. The present study proposed that freeze-drying was controlled by the heat transfer process in the fast front advance regime (up to 300 min, Fig. 10), which is the same as in the freeze-drying experiments carried out by Zhai et al. (2003) and Zhai et al. (2005) in vials (non-porous medium). However, freeze-drying was controlled by the mass transfer process with a slower freeze-drying velocity during the slow front advance regime (from 300 min to the end). It is also possible to observe a linear tendency in the second regime (slow front advance) from the logelog plot in Fig. 10. Equation (3) exhibits this tendency: log
zðtÞ ¼ 0:5774t 2:0911 r2 ¼ 0:99 L
(3)
This suggests that the drying front scaling over time is: zðtÞ a wt ; t[1 L
(4)
with an exponent of a z 0.577. On the other hand, Shaw (1987) found that the velocity of the drying front scaling over time had an exponent of 0.48 for classical drying. One of the features of classical drying is the presence of liquid clusters (see Figs. 1 and 2). This liquid cluster acts as a barrier for vapor flow through the porous structure. The present study proposes that the increase of the exponent (0.577) is because in freeze-drying vapor flows through the porous structure without these clusters, which appear in classical drying. More experiments are necessary to test this analysis, which would provide a more reliable result. Furthermore, a more in-depth
Acknowledgments Financial support from CONICYT e Chile through project FONDECYT N 1120342 is greatly appreciated.
references
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