Cellular foam

The work of Calderbank and Rennie (C4) has been criticized by Sargent and Macmillan (S2) on the basis that the liquid flow conditions used by Calderbank and Rennie (C4) are not found in distillation columns. They (S2) consider that cellular foams are formed for dilute aqueous solutions only when low gas flow rates are employed. By using an n-pentane-isopentane system, Macmillan (Ml) found that for all gas flow rates, froths with densities less than 0.15 were formed and the froth densities were independent of the factor vs(pg)112 but dependent on tray geometry. The associated problem of foam stability has also attracted considerable attention (Al, D3, Zl). 

Data from L. J. Gibson and M. F. Ashby. Cellular Foams (Cambridge, U.K. Cambridge Univ. Press, 1999). 

Doroudiani S, Park CB, Kortschot MT (1998) Processing and characterization of micro-cellular foamed high-density polyethylene/isotactic polypropylene blends. Polym Eng Sci 38 1205-1215... 

Zhang, X., Guan, R. F., Wu, D. Q. and Chan, K. Y. Enzyme immobilization on amino-functionalized mesostmetured cellular foam surfaces, characterization and catalytic properties, J. Mol. Catal., B, 2005, 33, 43-50. 

Cellular foam (Figs. 6.25b, 6.26a) is a wall-stabilized foam. Bubbles grow to an extent which limits the liquid to thin films between the... 

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. 

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 Foams North America VITA GROUP Central Pacific (Thailand) Corp., Ltd. THAI CENTRAL CHEMICAL PCL... 

Another technique of solid foam preparation is based on gas formation in a melted polymerising bulk or in concentrated water suspension of binding materials (cement, gypsum, lime), occurring after physical or chemical processes. It is also possible to incorporate air in a polymerising or solidifying substance bulk. For example, cellular-concrete represents a material in which gas bubbles are uniformly distributed in the bulk. The material produced when suspensions of binding substances are mixed with a foam is called cellular (foam) concrete. If the gas is formed in the concrete bulk as a result of a chemical reaction, for instance, in the reaction of aluminium powder with the liquid phase of the concrete solution, a gas-concrete is produced. 

Foams usually possess a finite low-frequency elastic modulus, along with static and dynamic yield stresses. These and other aspects of foam flow and rheology can be captured qualitatively and even semiquantitatively by cellular foam models. 

Emulsions with a high volume fraction of droplets (0 > 0.64) and foams show solidlike properties such as a yield stress and a low-frequency plateau value of G. The magnitudes of the yield stress and elastic modulus can be estimated using simple cellular foam models. These and related models show that at low shear rates where the shear stress is close to the yield value, the flow occurs by way of intermittent bubble-reorganization events. The dissipative processes that occur during foam and emulsion flows are still under active investigation. 

Vice versa, starting from the specific surface of a foamed polymer one can find the geometrical size of the constituent cells. Indeed, in those rare cases where the real cellular foam structure may be represented by sphere of the same size, the diameter of the spheres can be easily calculated from Eq. (58) . In the case of more irregular structures, the diameter given by this formular is a mean surface-volume diameter . 

This is a general handbook covering all aspects of plastics technology. Chapter 7 — Cellular (Foamed) Plastics covers 16 pages and is useful, although not up to date. This handbook is undergoing revision. 

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