(34j) Hydrodynamics and Mechanism Study of Foam Column Trays with Various Contact Angles

Foam materials have been widely used as equipment internals, a typical application being the column tray for distillation^1-3. Hydrodynamic phenomena are very important fundamentals for further design, optimization and operation of process equipments, which are extremely affected by surface wettability. In this work, cylindrical foam silicon carbide (SiC) devices, with the diameter of 44-mm, the height of 6-mm and the mean pore size of 3-mm, having various contact angle () were fabricated and investigated as fixed valves. A detailed fabrication process was put forward for preparing uniform hydrophobic and superhydrophobic surfaces with tunable contact angles inside and outside of foam SiC materials considering the specialty of foam structure. Subsequently, systematic study of gas distribution, pressure drop and weeping on foam SiC fixed valves with various contact angles was conducted to evaluate the effects of surface wettability. Hydrophilicity, hydrophobicity and superhydrophobicity present remarkably different influences on hydrodynamic performance of foam SiC fixed valves. Research results prove that with the increase of surface contact angles\, nonuniformity of gas distribution above the valves owing to relatively wide distribution of pore size is weaken greatly, especially in the condition of lower gas velocity, which is extremely important for stable, long-periodic and high-efficient operation in industry.

Except for gas distribution, pressure drop is also an extremely parameter for process equipment, which determines energy consumption to some extent. Pressure drop of foam SiC fixed valves is divided into four parts: net dry pressure drop, pressure drop caused by the clear liquid height above the valve, pressure drop by surface tension in the process of bubble formation and growth and additional pressure drop generated by surface wettability of foam SiC fixed valves. Additional pressure drop are generated by different types for hydrophilic, hydrophobic and superhydrophobic surfaces, that is to say, pore blocking and shrinkage because of the spreading of liquids for hydrophilic surfaces, pore blocking and shrinkage because of liquid adhesion for hydrophobic surfaces and pore blocking because of liquid dropsâ?? entrance for superhydrophbobic surfaces. The experimental results indicate that pressure drop is vastly decreased along with the increase of surface contact angles, about 30% decrease at most for hydrophobic surfaces and about 43% decrease at most for superhydrophobic surfaces compared with hydrophilic surfaces under the operation condition of 20-mm weir height. In addition, weeping measurement indicates that quantity of liquid leakage decreases as surface contact angles increase and follows a regular pattern.

In order to gain deep insights into the underlying physical process, an analytical model and mechanism were proposed, which consists of three sections based on the process of gas flowing through foam materials, solid-liquid upper interface and liquid layer. Critical F-factors and critical contact angle values of were used as characteristic parameters to distinguish flow mechanism and process features that mainly were described by phase slip and non-slip, flow patterns and interfacial forces.

References

1. Zhang LH, Liu XK, Li H, et al. Hydrodynamic and mass transfer performances of a new SiC foam column tray. Chem Eng Technol. 2012;35(12):2075-2083.

2. Gao X, Li X, Liu X, Li H, Yang Z, Zhang J. A novel potential application of SiC ceramic foam material to distillation: foam monolithic tray. Chem Eng Sci. 2015;135:489-500.

3. Li H, Fu L, Li X, Gao X. Mechanism and analytical models for the gas distribution on the SiC foam monolithic tray. AIChE J. 2015;61(12):4509-4516.