Flow and mass transfer in structured packings with modified surfaces

Commonly, structured packings are operated in film flow. In this case, the entire surface of the packing is completely covered by the liquid to make maximum
use of the available surface area on the elements. However, perfect wetting of the packing is usually difficult to achieve. Moreover, when film flow is targeted, the interfacial area cannot exceed the geometric surface area provided by the packing. Thus, other flow configurations must be alternatively considered increasing the interfacial area potentially available for mass transfer. Droplet flow is one way to achieve this goal. Considering the extreme case of a spherical droplet in contact with the packing on a single infinitesimally small point. For equal volume of droplet and film, the interfacial area in this case is almost five times higher compared to film flow. The developed work is meant to characterize flow structures alternative to the classical film flow over structured packings to determine potential improvement in mass transfer performances. To achieve changes in the flow structure, the surface of the packings elements was modified. In particular, surface treatments were used to improve the hydrophobic properties of the surface. In this way, the formation of liquid droplets on the surface was promoted. A preliminary literature review allowed the selection of self-assembled monolayers (SAMs) as surface modification methods due to the simplicity of performing the treatment. A characterization campaign of the modified surfaces was carried out. Smooth metal sheets specimens of which the packings are made were used for the investigation of contact angles via optical techniques. Behavior of obtained droplets and rivulets as well as their tendency to coalesce were also analyzed. Subsequently, the mass transfer performance of the modified packings was compared with that of the standard packings by air-led stripping of isobutyl acetate from an aqueous solution. A model was developed in this work for droplet and rivulet flow in order to determine specific surface area from packings characteristic and liquid flow rate. Experimental liquid holdup values were used as reference parameter for model validation. This work showed that the considered packing coated with the selected SAMs has a worse performance compared to the non-coated packing in terms of liquid holdup and mass transfer performance, at least when tested with aqueous solutions. However, the value of the present work mainly consists of the developed mathematical model and procedure for packing characterization and performance comparison. In fact, this will allow characterization of possible other surface treatments applicable to the surfaces of structured packings.