drying

After you wet a powder to form granules — like wetting flour to make crumbs of dough — you have a damp mass that cannot be pressed into stable tablets. Drying is the step that drives the water (or solvent) back out, leaving granules with just the right amount of residual moisture: dry enough to be stable and free-flowing, but not bone-dry, since a little moisture often helps tablets bond.

Physically, drying removes liquid by supplying heat so the water evaporates and is carried away by air, by vacuum, or by freezing it out under low pressure. In a wet-granulation line the dominant methods are tray (oven) drying and, far more commonly today, fluid-bed drying, where hot air both heats and fluidises the granules at once. The endpoint is judged by loss-on-drying or by Karl Fischer water content, against a target window set during development.

Getting moisture right is a balancing act with real consequences. Too much residual water leaves the product chemically unstable — water drives hydrolysis and microbial growth — and can make the powder sticky and hard to compress. Too little, or drying too hot, can over-dry or even degrade a heat-sensitive drug and produce brittle granules that crumble into fines. Some drugs also change crystal form (or lose water of hydration) on drying, which can alter dissolution.