Fundamental and applied aspects of powder dispersion in liquids.

Abstract

The dispersion of powders in water is a key step in many process industries. During dispersion, the powder grains often form heterogeneous lumps that are wet outside and dry inside, the presence of which reduces the efficiency of the overall dispersion process. Often, it is difficult to predict and control the conditions under which lump formation occurs. This study considers the consequences of adding insoluble grains to a static air–liquid interface from a funnel. By continuing to add grains, the stacks grow until either the lower grains disperse in the liquid, or the complete stack breaks free from the surface and sinks as a lump. Herein, the effects of grain contact angle, density, size and mass flow rate on these processes are studied experimentally and a theoretical analysis given. The maximum number of grains scales with the Bond Number (Bo) as Bo-1.82 when stack detachment is observed and with an exponent -2.0 when grains disperse into the liquid. As a result of these different scaling exponents, a critical Bond number above which grains wet and disperse can be identified. Moreover, the formation of a jet, entraining air into the liquid occurs when the kinetic energy of the grains are sufficiently high. In the case of a moving liquid, stack formation is also assessed in a purpose-built 2-D flow cell. The understanding gained from the foregoing analysis of dispersion or lump formation of insoluble grains is used to study the rehydration performance of soluble food powders. The subsequent discussion also considered the factors influencing the rehydration process of food powders, focussing on powder properties, including moisture content, molecular weight, grain size and density, as well as the agitation speed, liquid temperature and the mass flow rate of powders added to the liquid. The present findings have identified conditions under which lump formation occurs, and hence how these undesired phenomena can be avoided in applications requiring the efficient dispersion of grains across a liquid interface such as in the reconstitution of dehydrated food powders.