stability of powder islands, EPJ Web of Conferences 140 EDP Sciences
Powder Properties and Liquid Flow, Chemical Engineering Transactions. ISBN 978-88-95608- 48-8 57 pp. 1939-1944 The Italian Association of Chemical Engineering
dehydrated food powders. We investigate how the physico-chemical properties of the powder grains affect the
dispersion process. We used insoluble and non-cohesive grains to form islands of powders at a static water
surface. Liquid ?wicking? can avoid the formation of lumps, lead to complete sinking of powder grains when the
grain contact angle is below a critical value in the range 51°-77°. The effect of grain size on powder sinking, or
on the depth of island formed, was experimentally studied to understand the impact of particle size
enlargement on powder dispersion. The interplay of grain contact angle and size was also quantitatively
demonstrated. We also report the conditions leading to the detachment of a powder island from the interface,
forming a powder lump that is wet outside and dry inside. Importantly, introducing flow in the liquid by agitation
does not necessarily improve the dispersion process.
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.
by the use of surfactants. 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.