Gas Hold-Up and Mass Transfer in a Vessel with an Unsteady Rotating Concave Blade Impeller

Sebastian Frankiewicz,Szymon Woziwodzki

doi:10.3390/en15010346

Abstract

The steady mixing of gas-liquid systems is used where a large development of the interfacial area is required. However, the presence of gas in the liquid reduces the efficiency of mass transfer by reducing the mixing power, due to the creation of gas formations behind the impeller blades and the reduction in density. The efficiency of mass transfer can be increased by using a concave blade impeller or unsteady mixing. Mass transfer efficiency studies for these impellers and unsteady mixing are limited. This paper presents an analysis of the influence of the impeller construction on the gas hold-up and volumetric mass transfer coefficient kLa. Impellers with a different number of concave blades, and with alternatively arranged concave blades, were analyzed. The obtained results were compared with the standard flat blade turbine. The obtained results indicate that the arrangement of the concave blades has the greatest effect on reducing the gas hold-up and kLa. Higher values were obtained for the four-bladed and six-bladed impellers. A comparison of the gas hold-up rate for the unsteady and steady mixing has shown that for steady mixing greater gas hold-up is achieved. The volumetric mass transfer coefficient for unsteady mixing is also greater compared to steady mixing, indicating greater efficiency in mass transfer.

Highlights

Mechanical mixing is the most frequently used unit operation in many industries including in chemical, food, or pharmaceutical
The power requirements for the BT-33 impeller were examined; in the second, step gas hold-up and mass transfer were studied for all impellers
The purpose of mixing of two-phase systems is to increase the interfacial area and to improve the mass transfer. This is done using impellers with concave blades, which limit the size of the gas caverns

Summary

Introduction

Mechanical mixing is the most frequently used unit operation in many industries including in chemical, food, or pharmaceutical. Mixing is carried out in single-phase and multiphase systems to intensify the mass transfer and is especially used for gases that are poorly soluble in liquids. The intensification of mass transfer is realized by increasing the interfacial area in the liquid-liquid and gas-liquid systems. It can be implemented in several ways, including by using unsteady mixing or eccentrically mounted impellers. Mixing through oscillation is used in the mass exchange processes, among others, in OBR and OBC extraction columns [1]. Ni and Gao [2] show that in the OBC column an approximately five-fold increase in the mass transfer coefficient is observed for water-air systems. The use of oscillation in fermentation contributes to an approximately twofold increase in the mass transfer coefficient [3]

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