Pharmaceutical granulation drying methods

The stability of pharmaceutical granulations dried by microwaves is comparable with that provided by alternative methods. Microwaves are nonionizing and do not possess the amount of energy required for the formation of free radicals or the liberation of bound water conditions that foster product instability (Fig. 6). 

The first direct compression excipient, spray-dried lactose, was introduced in the early 1960s as a filler specifically designed for direct compression processes. Over many years, more direct compression API and excipients, especially diluents and binders, were developed. Since these are now commercially available, design of direct compression formulations is readily possible. However, despite the simplicity of the direct compression process, the pharmaceutical industry still produces most tablets by wet granulation methods.1... 

Slugs or preforms are usually 1 to 2 in. diameter in pharmaceutical manufacture. Suited to granulation of moisture-sensitive materials and those unable to withstand high temperature of drying in wet granulation methods. 

Historically, the most common method of drying of pharmaceutical powders has been tray-drying. With this method, wet powder or granulation is placed on paper-lined trays, usually solid or perforated metal, which are then placed directly onto racks in a drying chamber (oven) or onto movable racks, or trucks, that are wheeled into an oven. The heat and low relative vapor pressure of solvent provided by the flow of heated, dry air throughout the chamber provide a driving force for solvent transfer to and subsequent removal from the particle surfaces of the powder. This results in the gradual overall loss of solvent from the bulk powder. 

A number of methods are routinely used within the pharmaceutical industry to produce granulations. These are traditionally classified as dry granulation and wet granulation, depending on whether a liquid is used to aid the agglomeration. 

Dry granulation involves the aggregation of particles by high pressure to form bonds between particles by virtue of their close proximity. Two approaches to dry granulation are used in the pharmaceutical industry slugging and roller compaction. In either method, the material can be compacted with a binder to improve the bonding strength. 

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