Estimating the needed amount of desiccant, water or moistener to adjust the equilibrium water activity of dry powder mixtures

Prior to their mixing, further processing or packaging, industrial pharmaceutical and food powder formulations, or their individual components, are sometimes equilibrated under controlled humidity conditions in order to reach a water activity level deemed desirable and safe. Occasionally, there is a need to lower this water activity by adding a desiccant, or raise it through free moisture adsorption or adding a moistener. If the components do not interact chemically, the system is closed, the moisture in the free space negligible and the temperature practically constant, the amount of water gained or lost by the mixture's components is determined by a simple moisture balance equation, which can be used to translate the exchanged moisture into the mass of a given desiccant or moistener needed. To find this mass, one needs to know the mixture's initial composition, the ingredients' masses and moisture contents (in order to calculate their dry mass), and their moisture sorption isotherm equations. The latter can be of any kind, including polynomial, and there is no limit on the number of their coefficients. Knowledge of the desiccant's moisture content, if not zero, or moistener's initial moisture contents, or water activity, and its moisture sorption isotherm equation is also needed. The calculation of the overall amounts of water exchanged and corresponding amount of desiccator or moistener needed has been automated and can be performed with a Microsoft Excel® spreadsheet especially written for the purpose and posted as freeware on the Internet.

► Water needed to adjust dry mixtures' aw is calculated by a mass balance equation.
► The removed or added water translated to the desiccant or moistener's mass needed.
► The calculation program is posted as a freely downloadable MS Excel® spreadsheet.
► The software applies to mixed ingredients pre-equilibrated at different aw levels.