Mass transfer characteristics of rotating spiral gas-liquid contacting

Abstract

The first substantial experimental measurements of mass transfer in a rotating spiral channel are reported for counter-current physical desorption of a range of organic solutes from water into air. General relations in terms of bulk properties are developed that allow analysis and comparison across different solute properties, operating conditions and contacting equipment.The phase flow rate ratio and cleaned-phase throughput per passage volume emerge as parameters of principal importance, the former measuring sufficiency of solvent phase flow and the later mass transfer effectiveness and, consequently, required device size. The analytical solution for an infinitely wide channel is used to probe the finite-width experimental results and an apparently universal pattern of differences involving a peak in mass transfer coefficient emerges. As liquid flow rate decreases, the thickness of the liquid layer decreases and the mass transfer coefficient rises. But with further decrease in liquid flow rate and liquid layer thickness, an increasing fraction of the liquid flows in the corner regions under the end-wall menisci and the poor contact in these regions leads to a falling mass transfer coefficient. The peak is found to occur at a similar liquid layer thickness regardless of gas flow rate or solute equilibrium characteristics. Comparison is made with packed columns and rotating packed beds using available data in the literature. The rotating spiral performance suggests device sizes will be many times smaller than those for the two packed devices considered. Dependence of rotating spiral device volume on the square of channel size is demonstrated, showing that further reduction in device volume is possible.

2-D parametric analysis of Gas-Liquid Contacting in a Spinning Spiral Contactor

“…The first extensive mass transfer data of a gas-liquid contacting in a spiral channel have been reported in MacInnes and Ayash [3] and Ayash [9]. The authors investigated desorption of four different organic solutes from water into air at 24, 30 and 49 °C in a spiral channel with 1.5 mm height (parallel to y-axis), 4 mm width (parallel to x-axis) and  sin R [8].…”

“…Further, the value of the surface tension is taken to be that of pure water at 25 °C (0.07 N/m). The other physical properties used in the interface calculations and the 2-D model predictions are phase densities, viscosities, molar densities and diffusion coefficients and those are given in details in [3,9]. Finally, the contact angles are needed and they are taken as those for the air and water contacting in the same experimental channel [1].…”

“…To simplify the task, desorption of acetone from water into air is taken as a reference case ( = 1.15). The properties of this system used in the computations are given in [3] and [9]. Further, the purification factor ( n q f   ) is fixed at 1.2 which is not expected to be unreasonable value for desorption process.…”

“…The experiments carried out in MacInnes and Ayash [3] and Ayash [9] are based on fixing the flow rate of one of the phases, usually the gas phase, and change the other. However, it is important to examine how varying these parameters together affects the process of contacting.…”

“…The latter, on the other hand, dictates the separation rate. Therefore, with the ability to control these parameters independently, optimum contacting of systems having different equilibrium and transferring properties can be achieved , in principle, using the rotating spiral technique [2,3].…”