Drying

Abstract

Drying is a vital process in pharmaceutical formulation involving removal of moisture from solids by applying heat, thus resulting in both heat and mass transfer. The process when theoretically studied utilizes various terms such as absolute humidity, saturation humidity, dew point, relative humidity, drying capacity, wet bulb temperature and the dry bulb temperature which can be understood with the help of psychometric chart. The process of drying of solids when graphically plotted as drying rate versus moisture content is known as drying rate curve. The drying rate curve can be classified as initial adjustment period, constant rate period, first falling rate period and second falling rate period as per the trends of curve. Various types of equipment are used for drying the pharmaceutical solids. The most basic amongst them is a tray drier where the solids are kept on a tray and are dried with hot air. Whereas in fluidized bed dryer, the solids are dried by suspending in air by a constant stream of hot air. On the other hand, when the fluid content is higher, spray drying can be used where the mixture is pumped on a hot drying environment leading to atomization and further leaving particulate solid residue of the mixture. Freeze drying and vacuum drying are the specialized drying processes where drying is achieved by freezing samples or by reducing the pressure around the samples, thus resulting in drying without heating.