Cellular foam regime

Previous work with Oldershaw columns (209-211), however, spells a note of caution to Fair et al. s conclusion. For a fixed system, higher Oldershaw column efficiencies were measured under cellular foam conditions than under froth conditions. For this reason, Gerster (212) warned that when cellular foam can form, scaleup from an Oldershaw column may be dangerous. The conclusions presented by Fair et al. (208) do not extend to Oldershaw columns operating in the Cellular foam regime. Other considerations for scaleup from pilot columns (above) may also be important. The scaleup procedure recommended by Fair et al. (208) is... 

Cellular foam occurs at low vapor velocities in small columns, where the wall provides foam stabilization. It occurs with some systems or tray designs but not with others and is promoted by surface tension effects such as the Marangoni effect (99). Cellular foam is uncommon in industrial columns. The foam that causes problems in industrial installations is mobile foam, where the bubbles are in turbulent motion. Mobile foam is associated with the froth and emulsion regimes. Cellular foam is encountered in bench-scale and pilot-scale columns. If cellular foam occurs in the test unit, caution is required when scaling up the results. 

The compression behavior of PP foams is influenced by the cellular stmcture and by the mechanical properties of the PP matrix polymer. Figure 2(a) shows the compressive stress-strain curve of a PP foam schematically. The au-ve displays linear elastic behavior at low strains followed by a long collapse plateau, tmncated by a regime of densifica-tion in which the stress rises steeply [1, 3]. 

During compression of polymeric foams, three characteristic stages of deformation are commonly observed. At low deformations, the polymer foam is in the linear elastic response regime, i.e., the stress increases linearly with deformation and the strain is recoverable. The second phase is characterized by continued deformation at relatively constant stress, known as the stress collapse plateau. And the final phase of deformation is densification where the foam begins to respond as a compacted solid. At this point the cellular structure within the material is collapsed, and further deformation requires compression of the solid foam material (Ouellet et al. 2006). As mentioned above, a specific technique is required to obtain stress-strain curves of ferroelectrets in thickness direction because the thickness in ferroelectrets is normally very thin, corresponding to very small defiections. Dansachmiiller et al. developed an experimental technique that allows obtaining the stress-strain curves in ferroelectret films (Dansachmiiller et al. 2005). This method may also be used to obtain the stress-stain curve for a polymer foam film without oriented macro-dipoles. The schematic of the experimental setup is shown in Fig. 4. 

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