Study of the drying kinetics of certain pharmaceutical preparations
Abstract Generated abstract
The study examines the drying kinetics of the pharmaceutical preparations sulfadimethoxine and sulfadimezine under convective heat supply, addressing the need for data to select efficient continuous drying methods and calculate drying equipment. Experiments were performed with a DI-8 moisture meter at several temperatures and material layer thicknesses, and drying curves, generalized curves, and drying-rate curves were constructed to determine critical moisture contents, drying rates, and relative drying coefficients. The calculated drying times agreed well with experimental data using Lykov’s approach, and the materials were characterized as capillary-porous substances with extended first drying periods. The work identifies thinner layers as giving higher drying rates, recommends mixing or fluidized-bed drying for productivity, and supports drying temperatures of 160°C for sulfadimethoxine and 150°C for sulfadimezine.
Full Text
STUDY OF THE DRYING KINETICS OF CERTAIN PHARMACEUTICAL PREPARATIONS
G. D. Speshchi, G. V. Chubykina
(Presented by the scientific seminar of the Department of PAKhP of the Faculty of Chemical Technology)
The absence of data on the kinetics of the drying process of materials makes it difficult to choose the optimal drying method, the operating regime of the process, and to calculate drying apparatuses.
For a number of pharmaceutical preparations these questions are subject to clarification in connection with the ongoing transition of their production to continuous methods and the need to use continuously operating, highly efficient drying installations.
Ensuring, along with the required productivity of dryers, high economic indicators associated with their manufacture and operation is impossible without data on the duration of the process.
The present work is devoted to investigating the dependence of the drying rate of sulfodimethoxine and sulfodimezine on the temperature regime and the thickness of the layer of the material being dried, as well as to testing the possibility of using A. V. Lykov’s method to determine the drying duration of these substances under convective heat supply.
To study the drying process, a DI-8 moisture meter was used, the main parts of which are: a drying cabinet with an electric heater and fan, technical quadrant scales, and a thermoregulating device. The scales provide a weighing accuracy of ±50 mg. To measure and regulate the temperature, a contact thermometer with a scale from 100 to 200°C was used.
A sample of the material under investigation was placed in a Petri dish and set on the scale platform. A sheet of asbestos was placed under the dish, which practically eliminated heating of the material directly from the heater located beneath the platform.
The initial moisture content of the materials was kept constant for all experiments and was determined by the usual method. The material was taken in definite quantities corresponding to three different layer thicknesses \(\delta\), approximately equal to 3, 6, and 9 mm.
The temperatures of the experiments were chosen with allowance for the thermal stability of the substances and the conditions of their drying in existing installations. Drying of sulfodimethoxine was carried out at 140, 150, and 160°C; drying of sulfodimezine at 130, 140, and 150°C.
During the experiments, readings on the quadrant-scale dial were taken every minute. The experiment was ended after the reading had repeated five times. After each experiment the final moisture content of the material was determined.
From the data of each experiment, a drying curve was plotted in the coordinates: moisture content of the material \((w,\ \%)\)—time \((\tau,\ \text{min})\).
The moisture content corresponding to a given reading on the balance scale was calculated from the expression
\[ w_n=\frac{m_{\mathrm{vm}}-a_n}{M_c}, \]
where \(m_{\mathrm{vm}}\) is the mass of moisture in the material taken for the experiment, g;
\(a_n\) is the difference between the given \((n\)-th) and the first (before drying) readings on the balance scale, g;
\(M_c\) is the mass of dry material taken for the experiment, g.
\[ m_{\mathrm{vm}}=m_{\mathrm{v.ost}}+a_k=M_c\cdot w_n; \]
\[ M_c=M_{\mathrm{vm}}-m_{\mathrm{vm}}; \]
\[ m_{\mathrm{v.ost}}=M_c\cdot w_k; \]
\(m_{\mathrm{v.ost}}\) is the residual moisture in the material after the experiment, g;
\(a_k\) is the difference between the last and the first readings on the balance scale, g;
\(w_n,\ w_k\) are the initial and final moisture contents of the material, g/g d.m.;
\(M_{\mathrm{vm}}\) is the mass of wet material taken for the experiment, g.
From the drying curves obtained, generalized drying curves were constructed for each material at all values of \(\delta\) [1, 2].
Figure 1 gives generalized drying curves at \(\delta=3\ \text{mm}\). \(\tau\) is the time from the beginning of the experiment to the moment when the moisture content \(w\) is reached, min.
The results of the generalization show that there is no need to take drying curves for all temperature conditions; one experimental curve obtained under any condition may be used for a given value of \(w_n\).
Fig. 1. Generalized drying curves. A—for sulfadimethoxine; Б—for sulfadimezine
From the generalized drying curves plotted in semilogarithmic coordinates, the drying-rate coefficients for the first period were determined.
More illustrative are the drying-rate curves. For the materials under study, rate curves were constructed by the method of graphical differentiation of the generalized drying curves. Figure 2 gives the drying-rate curves for \(\delta=3\ \text{mm}\). From these curves, the values were determined—
values necessary for calculating the total drying time \(\tau_{\text{tot}}\): \(w_{k1}\), \(w_{k2}\), \(w_p\), \(N\), \(\chi_1\), \(\chi_2\).
Calculations showed that the drying time calculated by formula [3]
\[ \tau_{\text{tot}}=\frac{1}{N}(w_n-w_{k1})+\frac{1}{\chi_1}\lg\frac{w_{k1}-w_p}{w_{k2}-w_p}+\frac{1}{\chi_2}\lg\frac{w_{k2}-w_p}{w_k-w_p}, \]
is in good agreement with the experimental data.
\(w_{k1}\), \(w_{k2}\) are the first and second critical moisture contents, %;
\(w_p\) is the equilibrium moisture content, %;
\(N\) is the drying rate for period I, %/min;
\(\chi_1\) and \(\chi_2\) are the relative drying coefficients for periods II and III.
According to the character of the curvilinear sections of the drying-rate curves, it may be concluded:
1) the substances studied belong to capillary-porous materials of complex structure, with the upper section of the curve, between the first and second critical points, corresponding to removal of capillary moisture, and the lower section to adsorption moisture (Fig. 2);
Fig. 2. Drying-rate curves. A—for sulfadimethoxine, B—for sulfadimezine
2) for both sulfadimethoxine and sulfadimezine, period I is extended. This indicates that the drying rate of these substances depends to the same degree both on the parameters determining the state of the drying agent and on the parameters determining the properties and state of the material being dried.
The best results in terms of drying rate were obtained for \(\delta = 3\) mm. With an increase in \(\delta\), the rate of the process decreases significantly. To ensure high productivity of the drying apparatus, drying should be carried out with mixing or turning over of the material. For substances with a capillary-porous structure, a fluidized bed is recommended.
The data obtained make it possible to confirm as optimal drying temperatures: for sulfadimethoxine, \(160^\circ\)C, and for sulfadimezine, \(150^\circ\)C.
Conclusions
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The influence of the temperature regime and of the material layer thickness on the drying kinetics of sulfadimethoxine and sulfadimezine has been determined.
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Data have been obtained for determining the drying time of these substances.
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Recommendations have been given for choosing the drying method.
References
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A. V. Lykov, Theory of Drying. Moscow, “Energiya,” 1968, p. 133.
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V. V. Krasnikov, V. A. Danilov. IFZh, 11, No. 4, 1966, 623.
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V. N. Kiselnikov, N. M. Talanov. “Khimiya i khimicheskaya tekhnologiya,” XV, No. 7, 1972, 1097.