In order to throw light on the mechanism of freeze-drying and to obtain fundamental design for that process, experiments with market milk were carried out from the standpoint of chemical engineering research.
The experimental apparatus used are schematically shown in Fig. 1, and the results obtained are summarized as follows:
(1) As illustrated in Fig. 2, the typical freeze-drying process can be divided into four stages, namely, i) the primary stage, ii) the steady stage characterized by constant drying rate, iii) the first stage, and iv) second stage, each characterized by decreasing drying rate.
(2) As the process goes on, the frozen sample comes to be divided into two distinctive layers, i.e., the ice layer and the dried porous one, each having a temperature markedly different from that of the other, as shown in Fig. 6. (See also Fig. 12.)
(3) There is a linear relationship between the drying rate and the heating rate, as shown in Fig. 8 and by Eq. (1). In this, the heat coming from the outsystem must be considered, as it causes the experimentally observed values of the drying rate to be greater than the theoretical ones, when the heat supply is small.
(4) The drying rate per unit area is found to be constant, namely, to be independent of the area of the vessel, when the area is larger than a fixed value, 100cmcm² or so. On the other hand, when it is smaller than 100cmcm² or so, the drying rate is affected by the area of the vessel, as shown in Fig. 10.
(5) In most cases, the heat coming through the side walls of the vessel has influence on the temperature distribution in the frozen sample, and the outer said of the sample is more rapidly dried than the inner side of the sample.
Fig. 12 indicates the presumed profile of the sublimation surface, which is affected not only by the heat coming through the side walls, but by the cracks developed during the process, as well as by the thermocouples inserted in the sample.

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Square slices of soap, 1-3mm thick, were cut off from two kinds of curds for toilet soap and were dried in a hot air current at temperatures between 40°C and 70°C, and their drying mechanism and the optimum drying conditions were investigated.
In the first place, the values of average water diffusivity in soap, Dob, were calculated from the experimental results and by Equation (1). Since the values widely differed with υ/υ0 as shown in Fig. 2, the authors employed the figures of log (1-υ/υ₀) vs. log t/RR², the drying curves as plotted in Figs. 4, 5 etc.
From the study of these figures or D_m in Tablc 2, it may be concluded that:
(1) the proper temperature range for soap drying is from 50°C to 60°C,
(2) the differences in physical or chemical properties of soap seem to have strong influence on the drying curuves or D_m as Lederer reported, and
(3) the drying rate of soap at 50°-60°C is constant over the range of relative humidity from 10% to 70%.

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The mechanism of through-flow drying of cocoons was studied under various drying conditions and the measurement was taken of the actual length of silk thread reeled off a cocoon.
The equipment employed in these experiments was as shown in Fig. 1. The experimental results are summarized in Table 1. The drying curve and the temperature distribution of a cocoon are illustrated in Table 1, as to each chrysalis, silk layer and internal space.
From the temperature distribution study, it was made clear that the moisture of a cocoon is mostly contained in the chrysalis and the moisture in the silk layer is quickly lost in the first stage of drying. The temperature of the chrysalis was found to be higher than that of the wet bulb in a hot air.
The effects of various factors on the drying rate are shown in Figs. 3-6. Observed pressure drops through the cocoon bed and mass transfer capacity coefficients calculated from the experimental data are shown in Figs. 7 & 8, which may be expressed by the following equations.
(1)
ha=30.8G^0.43 (2)

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The mechanism of through-flow drying of viscose staple fiber was studied under various drying conditions and the experimental results were summarized as shown in Table 1. The experimental equipment employed was the same as illustrated in our previous report.1)
Kinds of viscose staple fibers used are as follows:
Crimp staple fiber, 1.5 denier×11/8" cut
Crimp staple fiber, 2.0 denier×2" cut
Crimp staple fiber, 5.0 denier×3" cut
Bright staple fiber, 1.5 denier×11/8" cut
The correlation of drying data, as expressed by a drying-time curve and a material-temperature curve, is illustrated in Fig. 1, and the effects of various factors on the drying rate are summarized in Figs. 2-5. Observcd pressure drops through beds of staple fibers are plotted in Fig. 6, which, again, are expressed by the follpwing equation:
ΔP=30υg^1.77 (1)
where the depth of bed is 10cm.
Mass transfer capacity coefficients calculated from the experimental data are plotted in Figs. 7 & 8, and are expressed as the functions of mass velocitv of air and size of thread.
kGa=9.25G^0.8D^-0.27 (2)

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