Infusion of fluid into powder beds

When it is desirable that a product contain the maximum volume percentage of selected solids within a fluid matrix, which in turn gels or solidifies, it may be desirable to consider an infusion process. An infusion process permits achievement of a higher solids loading than can be obtained utilizing a pourable premixed slurry. However, infusing a viscous fluid into a bed of fine powder is a daunting task. The wide size distributions and non-uniform shapes frequently encountered with many powders make the classical equations for fluid flow through packed beds difficult or impossible to use. An alternate approach is proposed whereby the mechanism is characterized as flow through parallel capillary channels, rather than just as flow over the particle surfaces. A simple test system can be used to define the effective capillary size, from which the pressure and time required to infuse a fluid into powder mixtures can be predicted as a function of the viscosity of the fluid and the geometry of the vessel into which it is being pumped. The predictability is particularly important when the fluid is a polymerizable monomer with only a narrow operating window before it sets up, such as some potting compounds and energetic applications. Examples are given with various fluid/powder systems.