Freeze-Drying with Structured Sublimation Fronts—Visualization with Neutron Imaging

Abstract

The particular structure of the sublimation front in vacuum freeze-drying of porous media is, in most situations, not accessible at the pore scale. The classical measurement techniques access the process only globally. Knowledge about the structure of the front, however, is necessary for prescriptive analysis of freeze-drying, as it dictates not only drying velocity, drying time, and overall energy consumption, but also the material properties after drying. This is especially relevant in situations in which the freeze-drying process is carried out close to the collapse temperature of the product. We, therefore, study the sublimation of ice with neutron tomography and analyze the spatial formation of the dry space using the example of frozen cylindrical maltodextrin with drying parameters at the limit of material collapse. We show that the sublimation front forms unique fractal structures that differ strongly from the usual form of a flat front. Distinct dry fingers covering the sample, in addition to a fractal peripheral sublimation front, were observed. The findings are important for the understanding of freeze-drying processes and will serve as a basis for the development of microscale models of freeze-drying.

Experimental Study of the Impact of Pore Structure on Drying Kinetics and Sublimation Front Patterns

“…In summary, very different drying behaviors were observed for different solid contents, characterized by different pore structures ( Figure 2 ). While for MD5 and MD5 AN the front remained almost flat, in the sample with high solid content, a completely different shape was formed, with significant front widening, in agreement with previous findings [ 15 ].…”

“…The sample dries more heterogeneously, again identified by dry dark gray spots spread over the ice-saturated region, and seemingly also from the bottom side. This finding is similar to the experiment reported in [ 15 ], where maltodextrin with 20% ( w/w ) solid content was investigated using neutron imaging. There, distinct dry fingers developed in the center of the sample and resulted in faster drying of this region, whereas the ice ring, located around the center, dried more slowly.…”

“…The heterogeneity of transport conditions resulting from distributed pore space can affect the development of the sublimation front, which propagates faster in larger pores. The result is a structured sublimation front, a phenomenon such as described in [ 11 , 12 , 13 , 14 , 15 , 16 ]. Hence, the freezing process, i.e., the development of the solid matrix, was investigated in depth [ 17 , 18 , 19 ] to evaluate and subsequently find pore structures favorable to heat and mass transfer.…”

“…The dynamic temporal evolution of the sublimation front can be assessed using in operando imaging techniques with high contrast between the propagating dry pores, solids and ice [ 12 , 13 , 15 , 16 , 20 , 21 ]. At the same time, pore scale models can assess heat and mass transfer kinetics at the front.…”

Observations on the Changing Shape of the Ice Mass and the Determination of the Sublimation End Point in Freeze-Drying: An Application for Through-Vial Impedance Spectroscopy (TVIS)

“…Neutron imaging and in situ tomography are able to scan the partially dried matrix in a glass vial and visualize the sublimation kinetics and in doing so provide true topographic information on the changing structure of the ice front. However, they require a line of sight to the vial(s) under scrutiny, which then impacts the drying process and results in anomalous ice shapes developing during the process [14].…”