Enhanced mass transfer and reduced pressure drop in a compound rotating zigzag bed

Abstract

A rotating zigzag bed (RZB) had achieved the commercial application of Higee in continuous distillation processes, and then a compound rotating zigzag bed (CRZB) was developed from the RZB. The CRZB rotor was composed of a disk layer including several perforated concentric circular ripple disks fixed on a perforated rotating ring at the inner part of the rotor and several coupled rotational-stationary baffles at the outer part of the rotor. And a stationary four-arm liquid distributor was located at the eye of the CRZB rotor. The effective interfacial area (ae) and local liquid-side mass transfer coefficient (kL) of the CRZB rotor were measured by experiments of a chemical absorption of CO2 in NaOH solution, and the local gas-side mass transfer coefficient (kG) of the CRZB rotor was measured by experiments of a stripping of ethanol from an ethanol-water mixture by air. The gas pressure drop across the CRZB was measured with air-water system. Effects of gas flow rate, liquid flow rate and rotational speed on these mass transfer parameters and gas pressure drop were investigated. The stationary four-arm liquid distributor led to stronger end effect of the CRZB rotor than that of the RZB rotor. The end effect of the CRZB rotor had significant effect on the ae and kL of the rotor. The ae of the CRZB rotor was much higher than that of the RZB rotor because the total space in the disk layer of the CRZB rotor served as mass transfer zones and the CRZB rotor had stronger end effect than the RZB rotor. The local volumetric liquid- and gas-side mass transfer coefficients (kLae and kGae) of the CRZB rotor reached 2.3 times higher than those of the RZB rotor at a higher liquid flow rate and a rotational speed of 1200 rpm. The gas pressure drop across the CRZB was lower than that across the RZB, because the coupled rotational-stationary baffles were abandoned at the inner part of the CRZB rotor, which reduced the gas flow resistance and led to a lower pressure drop compared to the RZB. Thus, mass transfer was enhanced and gas pressure drop was reduced in the CRZB compared to the RZB, which showed the CRZB had excellent potential as a representative of the second-generation RZB for commercial applications.