Abstract

Even bubble column reactors (BCR) and airlift reactors (ALR) have been developed in terms of various related aspects towards mass transfer enhancement, the effective analysis of gas diffuser types on mass transfer and gas–liquid hydrodynamic characteristics is still limited. Therefore, the present study aims to analyze the relative effect of different types of air diffusers on bubble hydrodynamics and mass transfer performance to understand their behaviors and define the best type. The experiments were conducted by varying different diffuser types, reactor types (BCR and ALR), and superficial gas velocity (Vg) (0.12 to 1.00 cm/s). Five air diffusers including commercial fine sand (F-sand) and coarse sand (C-sand) diffusers, and acrylic perforated diffusers with orifice sizes of 0.3 mm (H-0.3), 0.6 mm (H-0.6), and 1.2 mm (H-1.2), were used in this study. For every condition, it was analyzed in terms of bubble hydrodynamics and oxygen mass transfer coefficient (KLa). Lastly, the selected diffusers that provided the highest KLa coefficient were evaluated with a solid media addition case. The results of both reactor classes showed that F-sand, the smallest orifice diffuser, showed the smallest air bubbles (3.14–4.90 mm) compared to other diffusers, followed by C-sand, which larger about 22–28% on average than F-sand. ALR exhibited a better ability to maintain smaller bubbles than BCR. Moreover, F-sand and C-sand diffusers showed a slower rising velocity through their smaller bubbles and the tiny bubble recirculation in ALR. Using F-sand in ALR, the rising velocity is about 1.60–2.58 dm/s, which is slower than that in BCR about 39–54%. F-sand and C-sand were also found as the significant diffusers in terms of interfacial area and gas hold-up. Then, the KLa coefficient was estimated in every diffuser and reactor under the varying of Vg. Up to 270% higher KLa value was achieved from the use of F-sand and C-sand compared to other types due to their smaller bubbles generated/maintained and longer bubble retention time through slower rising velocity. After adding 10% ring shape plastic media into the reactors with F-sand and C-sand diffusers, a better performance was achieved in terms of KLa coefficient (up to 39%) as well as gas hold-up and liquid mixing. Lastly, ALR also had a larger portion of mixed flow pattern than BCR. This eventually promoted mass transfer by enhancing the mixed flow regime.

Highlights

  • Gas–liquid and gas–liquid–solid contactors are commonly used in chemical, biological, and biochemical industries [1]

  • In Bubble column reactor (BCR), it can be observed that the air bubbles distribute between 3.14 and 11.28 mm, while larger bubbles were obtained in the higher gas flow, regardless of diffuser classes due to the bubble coalesce and high-pressure promotion at a higher flow [24,27]

  • The main purpose of this work is to study the effect of different air diffuser types on bubble hydrodynamic characteristics and oxygen mass transfer performance

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Summary

Introduction

Gas–liquid and gas–liquid–solid contactors are commonly used in chemical, biological, and biochemical industries [1]. Bubble column reactor (BCR) and airlift reactor (ALR) are the two types of multi-phase reactors often applied to introduce gas and liquid phases together as mass transfer process due to various benefits including efficient mixing behaviors, high mass transfer rate, compactness, and lower operation and maintenance costs [3]. Even these reactor classes have received much attention from researchers and industrial sectors, various challenges have to be overcome to further enhance mass transfer performance as well as oxygen transfer efficiency from gas to liquid phases due to the high oxygen demand in the liquid aspect. The packing reactor could enhance mass transfer performance due to increasing gas hold-up, decreasing bubble size, and decreasing the liquid circulation

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