Short communication
Intensification of mass transfer in the pulsed bubble column—Comparison of the efficiency of static and pulsed aerator

https://doi.org/10.1016/j.cep.2020.107919Get rights and content

Highlights

  • Liquid pulsations at resonance frequencies caused an increase of gas hold-up.

  • Liquid pulsations at resonance frequencies caused an increase of mass transfer.

  • Static aerator provides higher values of gas hold-up comparing to pulsed aerator.

  • Volumetric mass transfer coefficient was higher when the static aerator was used.

Abstract

Introduction of pulsations to the classical bubble columns makes it possible to reduce the size and velocity of flowing gas bubbles and to obtain nearly spherical shape. As a result, gas hold-up and interfacial area are increased which obviously has an effect on the intensification of mass transfer.

In the work the values of gas hold-up and volumetric mass transfer coefficient obtained in the pulsed bubble columns with static and pulsed aerator were compared. In the tests two types of aerators were used: the first one was static, while the other one was a pulsed aerator fixed rigidly on the surface of the exciter disk. In both cases pulsations with the resonant frequencies up to 70 Hz and amplitude of 0.25–2 mm were introduced into the system, and the apparent gas velocity was set within 3.1–9.3 mm/s.

Equations were proposed to calculate gas hold-up and volumetric mass transfer coefficient with accuracy of ±25% and ±30% for the aerator pulsating at resonance frequencies.

Using a pulsed aerator in most cases lower gas hold-up was obtained than in the case of a static aerator. The volumetric mass transfer coefficient was higher when the static aerator was used.

Introduction

Recently, in the literature we can observe growing interest in the methods for intensification of technological processes and unit operations in chemical engineering [[1], [2], [3]]. Pulsed bubble columns are a group of reactors equipped with a pulsed element [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]] due to which it is possible to introduce pulsations of determined frequencies and amplitudes into a two- or multiphase mixture. When using resonance frequency of pulsations of such an element, the so-called Bjerknes force acting on gas bubbles in the opposite direction to the buoyancy force is generated [[5], [6], [7], [8], [9], [10], [11], [12]]. This leads to an increase of gas hold-up, growth of the interfacial area and a reduction of the flow rate and size of gas bubbles which affects their residence time in the liquid layer. This design solution enables intensification of mass transfer processes which results in a several time increase of mass transfer coefficient, especially for columns with slenderness ratio Hp/D < 8 [8]. Basic information on the hydrodynamics of gas bubble flow and mass transfer in bubble pulsed columns has been presented in our earlier work [8].

In comparison to study [8] in which a static aerator was used, cf. Fig. 1a, in the present work the authors decided to change the design of the apparatus which consisted in mounting the aerator on the surface of the pulsed exciter disk – see Fig. 1b. This solution caused that the aerator was pulsating with a frequency and amplitude equal to that of the pulsed exciter disk during the process of mass transfer in the liquid-gas system.

Fig. 2 shows photographs of a bubble column with the pulsed aerator during the process of mass transfer. Photograph 2a shows the process of aeration without pulsations. Fig. 2b and c shows photographs of the bubble column operating in the conditions of the first fw* and second f1* resonance frequency of pulsations. As can be seen in Fig. 2b and c, when using the resonance frequency of pulsations, a standing pressure wave is formed along the column height which is characterized by the presence of so-called wave nodes and arrows. The interpretation of photographs 2b and 2c shows that gas bubbles flowing along the column height assume shapes very similar to spherical, their number increases, while the diameter and flow velocity decrease.

The purpose of the work was to investigate gas hold-up and volumetric mass transfer coefficient in the pulsed bubble column with a pulsed and static aerator and to compare the results with earlier obtained operating parameters of an analogous bubble column in which a static aerator was used [8].

Section snippets

Experimental set-up and procedures

The main element of the experimental set-up was a column made of glass with a square cross-section, side D = 0.135 m and total height Hc = 2.25 m, mounted on a steel tripod and connected with it with a set of springs. At the bottom of the column, a round steel pulsed exciter disk of diameter dp = 0.105 m and thickness 10 mm was installed. The plate was sealed on a flexible membrane and connected to the eccentric drive installed on the motor shaft. Pulsation frequency f of the exciter disk was

Results

The gas hold-up and volumetric mass transfer coefficient obtained in the pulsed bubble column are shown below.

Conclusions

In this study the process of aeration in the pulsed bubble column was investigated using the static and the pulsed aerator – both working under the conditions of resonant frequencies of pulsation. Analysis of experimental results leads to the following conclusions:

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    Introduction of liquid pulsations at resonance frequencies to the classical bubble column caused an increase of gas hold-up and nearly spherical shape of gas bubbles. Hence, the interfacial area was enlarged which obviously made it

CRediT authorship contribution statement

A. Gwiazda: Investigation, Visualization. M. Dziubiński: Conceptualization, Writing - original draft. P. Budzyński: Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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