Ind. Eng. Chem. Res. 1999, 38, 3535-3542
3535
Vacuum Drying of Sintered Spheres of Glass Beads Hiromichi Shibata* and Yumiko Iwao Department of Chemical Engineering, Fukuoka University, 8-19-1, Nanakuma, Jonan-ku, Fukuoka, 814-0180 Japan
Air drying of sintered spheres of glass beads was investigated under vacuum as well as at atmospheric pressure for comparison. The critical moisture contents at pressures of 0.97-26.7 kPa were close to that at atmospheric pressure. Nevertheless, their mechanisms were different. The drying rate curves for the falling rate period were predicted from a receding evaporation front model with either a two-step moisture distribution in the pendular state or a flat moisture distribution in the pendular state. The predicted as well as the observed drying rate curves for the falling rate period at pressures of 0.97-1.47 kPa decreased either by two stages or monotonically, depending on the shape of the respective moisture distributions. At subatmospheric pressures such as 13.0 and 26.7 kPa, it was, however, difficult to determine the configuration of the water. Introduction Vacuum drying in air has widely been used for a long time in the chemical, food, and pharmaceutical industries and recently in the wood industry. The advantages of vacuum drying in these industries are shorter drying times, lower final moisture contents, and lower drying temperatures for heat-sensitive materials than those in air drying at atmospheric pressure. These advantages of vacuum drying were, however, expected from the properties of vacuum itself as well as the industrial experience even in early times despite a lack of knowledge of drying mechanisms. Although there have been numerous papers concerning drying characteristics under vacuum, they were not enough to elucidate the drying mechanism leading to the prediction of drying rate curves, especially in the vicinity of the critical moisture content as well as the comparison between the drying rate curves under vacuum including the critical moisture contents and those at atmospheric pressure, both of which are important in the selection of drying methods.1-6 Therefore, in this work, the drying rate curves of sintered spheres of glass beads are investigated under vacuum as well as at atmospheric pressure, the drying mechanism, especially in the vicinity of the critical moisture content, is revealed, and a model is proposed. The sintered glass beads were selected as a model sample because their physical properties and important drying data accumulated by many investigators were available, and sintered spheres of glass beads with a homogeneous structure as well as a wide porosity range (depending on the extent of sintering) could be produced.7 Thus, the purpose of this work is to elucidate a part of the fundamental aspect of vacuum drying kinetics by use of the model sample. Experimental Section As shown in Figure 1, the same drying chamber under vacuum as described in a previous paper8 was used to measure the drying rate curves of sintered spheres of glass beads saturated with water. The sample sphere * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (92) 865-6031.
Figure 1. Flow diagram of the experimental apparatus: A, blower; B, gas heater; C, stop valve; D, needle valve for drying medium; E, removable glass window for sample inlet/outlet; F, sample; G, electronic balance; H, condenser; I, cold trap; J, oil rotary vacuum pump; K, silicone oil manometer; L, mercury manometer; M, needle valve for air purge; N, temperature controllers; O, personal computer; P, heater and insulation; Q, wire; R, thermocouples for gas and wall temperature; S, thermocouple for measurement of sample temperature during drying; and T, cocks.
was degassed in a vacuum container for 1 h and then saturated gradually for 1 h (to sufficiently degas the air in the sample as well as the air dissolved in the water) with water flowing in through a needle valve equipped with the top part of the vacuum container. It took in total more than 2 h for the sample to be saturated. The experiments were carried out at wall temperatures of 70-110 °C. The temperature distributions at the wall of the drying chamber were within (3 °C. As the temperature of the sample during drying could not simultaneously be measured while the sample was being weighed, another sample, in the center of which a thermocouple was inserted, was placed in the drying
10.1021/ie990013p CCC: $18.00 © 1999 American Chemical Society Published on Web 08/05/1999
3536 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
chamber to measure its temperature during drying. The flow rate of air was only regulated by use of a needle valve located below the drying chamber and was not measured. Instead, the temperature of the sample was measured and adjusted as it was heavily dependent on the flow rate of air even under vacuum, especially during the constant rate period. The temperature of the sample for the constant rate period could be adjusted within a small range of 0.4-17.5 °C over a wide experimental pressure range of 0.97-101.3 kPa. The pressure in the drying chamber was maintained by regulating another needle valve located at the compartment of an electronic balance. It was regulated in the accuracy of (0.1 mmHg in the pressure range of less than 20 mmHg (2.67kPa) when the oil manometer was used and (1.0 mmHg in the pressure range of more than 20 mmHg (2.67 kPa) when the mercury manometer was used. Concerning the usage of the oil manometer, the air in the oil manometer was evacuated by a vacuum pump until the pressure in the oil manometer reduced to a pressure of less than 0.01 mmHg, and then the measurement of pressure in the drying chamber was carried out by operation of the cocks T, as shown in Figure 1. The drying experiments were conducted at pressures of 0.97-26.7 kPa as well as at atmospheric pressure for comparison by use of a wind tunnel for drying as described previously.9 The weight data measured by use of an electronic balance during drying was transmitted to a personal computer for analysis. The glass beads used as the sample material were sieved by 35 and 60 mesh sieves. Table 1 shows the physical properties of sintered spheres of glass beads (0.25-0.42 × 10-3 m in diameter) used as samples in this work. In addition, some of the data presented in this work are referred to data from a previous paper.8
Table 1. Properties of the Sample sample
2R, m
CI
θs, deg
No. 14 No. 19 No. 22 No. 45 No. 53
0.0245 0.0245 0.0235 0.0233 0.0230
0.344 0.367 0.285 0.289 0.271
0.208 0.232 0.162 0.155 0.150
18.0 7.5 27.0 27.0 27.0
evaporation front at y point in the observed drying rate curve for the second falling rate period as shown in Figure 2. For eq 1, Ix < 1, which indicates that the water transfer continues and the funicular state is still present in the sample at moisture contents Cca to Cx. If Ix ) 1, the water transfer ceases. Iy has the same meaning as Ix. If Ix
