Franz Thurner

Progress towards understanding the drying of porous materials wetted with binary mixtures

Abstract Drying of porous materials containing binary mixtures changes the composition of the moisture to a certain extent. Generally the more volatile component is removed preferentially, especially when the drying rate is low. At sufficiently high drying rates the moisture always evaporates with constant composition; the drying is nonselective. This effect is due to the liquid-side mass transfer resistance. Nonselective drying is also obtained when the moisture has a pseudo-azeotropic composition; this depends on the relative volatility, the gas-side mass transfer and the residence time of the gas in the dryer. If such a pseudo-azeotropic composition does exist, then even the less volatile component may be removed preferentially, provided the initial composition is above the pseudo-azeotropic one. The selectivity of the drying process also depends on the size of the drying sample as well as upon whether the drying process is run continuously or intermittently. The complex interactions of phase equilibria, gas- and liquid-side mass transfer and capillary flow for given drying conditions were analyzed first for the case of evaporation of binary mixtures from a free liquid surface and second for the case of drying single porous bodies containing binary mixtures. In both cases experimental results were obtained with mixtures of isopropyl alcohol and water. They confirm the phenomena and tendencies as predicted from theoretical considerations.

Convective Drying of Porous Materials Containing Binary Mixtures

The estimation of the size of a dryer requires the knowledge of the drying rate as a function of the moisture content, called drying curve. In addition to that, for the drying of materials containing mixtures it is important to know the composition of the moisture as a function of the moisture content, called composition curve.

Intermittent drying of porous materials containing binary mixtures

Abstract The selectivity during the drying of porous materials containing binary mixtures depends on the thermodynamic equilibrium, the gas-side mass transfer and the liquid-side mass transfer. Very often the drying process is nonselective owing to the controlling liquid-side mass transfer. However, in this case at the early stages a selectivity might be expected, because the liquid-side mass transfer resistance is negligible at the beginning. This so-called initial selectivity can be extended to the whole drying process by intermittent drying. For this purpose the time required for the development of the steady state concentration profile at the beginning (transient time t T ) and the time required for the degraation of the steady state concentration profile during recovery (relaxation time t R ) had been estimated theoretically and experimentally in this work.

Wet-bulb temperature of binary mixtures

Abstract When binary mixtures are evaporated, a wet-bulb temperature does exist, provided that the evaporation is non-selective. The wet-bulb temperature can be calculated based on the energy balance, the kinetic equations for heat and mass transfer and the thermodynamic equilibrium. The calculated wet-bulb temperatures were found to be in good agreement with the experimental ones, obtained with mixtures of isopropyl alcohol and water.

Drying of polyvinyl alcohol in a closed-circuit dryer

Abstract The sorption isotherm and the drying rate curve represent the basis for the design of a dryer for solvent wetted polyvinyl alcohol (36 wt.% methyl acetate, 62 wt.% methanol, 2 wt.% water). In this study the sorption isotherms of polyvinyl alcohol wetted with a mixture of methyl acetate and methanol were measured at 40, 60 and 80 °C, and the drying rate curves of solvent-wetted polyvinyl alcohol layers 3, 10 and 20 mm in height in a dry air stream (air temperatures 30, 60 and 100 °C, air velocities 0.2 and 1 m s −1 ) were investigated. The hygroscopic equilibrium moisture content can be determined by means of the sorption isotherm, and the belt area of the dryer by means of the drying rate curves.

Filterzentrifugen für die pharmazeutische Industrie

Bei der Aufarbeitung pharmazeutischer Wirkstoffe ist die Abtrennung in Filterzentrifugen ein hoch effektives Verfahren, mit dem hohe Produktreinheiten erreicht werden. Voraussetzung dafur ist, dass der abzutrennende Wirkstoff fur den Trennschritt formstabil z. B. als Kristall vorliegt. In den verschiedenen Typen von diskontinuierlichen Filterzentrifugen, die alle die spezifischen Anforderungen der pharmazeutischen Industrie erfullen konnen, laufen grundsatzlich immer die gleichen sequentiellen physikalischen Prozesse ab. Aufgrund apparatespezifischer Besonderheiten weisen die verschiedenen Zentrifugentypen unterschiedliche Starken auf, die dann abhangig von Rahmenbedingungen wie etwa den Produkteigenschaften und den Anforderungen an den Trennprozess eine Auswahl spezifischer Zentrifugentypen vereinfacht.