Experimental and numerical investigation of bubble column reactors

Abstract

Due to various advantages, such as simple geometry, ease of operation, low operating and maintenance costs, excellent heat and mass transfer characteristics, bubble column reactors are frequently used in chemical, petrochemical, biochemical, pharmaceutical, metallurgical industries for a variety of processes, i.e. hydrogenation, oxidation, chlorination, alkylation, chemical gas cleaning, various bio-technological applications, etc. However, complex hydrodynamics and its influence on transport phenomena (i.e. heat and mass transport) make it difficult to achieve reliable design and scale-up of bubble column reactors. Many factors influence the performance of this type of reactors significantly, such as column dimensions, column internals design, gas distributor design, operating conditions, i.e. pressure and temperature, superficial gas velocity, physical and chemical properties of the involved phases. A large variety of scientific studies on bubble column reactors utilizing both experimental and numerical techniques has been carried out during the past decades. In this study a bubble column with a square cross-sectional area has been studied in detail using a combined experimental and computational approach. Chapter 1 introduces bubble column reactors and their variants according to practical requirements. Both advantages and disadvantages of these types of reactors are presented. Key parameters related to the performance of bubble column reactors are also presented. In addition, in Chapter 1 a brief literature review is presented on both experimental and numerical techniques utilized in investigations on the performance of bubble column reactors during the past decades. In Chapter 2, accuracy of a four-point optical fibre probe for measuring bubble properties is investigated in a flat bubble column. Photography is used to validate results obtained from the four-point optical fibre probe. According to the comparison, it is found that the liquid properties have a profound influence on bubble velocity measured by the optical probe. Finally, it is found that the extent of inaccuracy in the determination of bubble velocity can be characterized with the Morton number. The accuracy of the four-point optical fibre probe and its intrusive effect are further studied in a square bubble column operating at higher superficial gas velocity in Chapter 3. Besides bubble velocity, other bubble properties, such as local void fraction, chord length and specific interfacial area, are obtained from measurements with the four-point optical fibre probe. Furthermore, bubble size is determined in different ways. Possible reasons for the discrepancy in the bubble size determination are discussed. The effect of the initial liquid height in the bubble column on the bubble properties is also investigated. Chapter 4 studies the effect of the gas sparger properties on the hydrodynamics in a square bubble column with an Eulerian-Lagrangian model. The performance of the model is first evaluated by comparison with experimental data. Subsequently, the effects of different sparged areas and the sparger location on hydrodynamics, i.e. liquid velocity, turbulent kinetic energy and void fraction are investigated. Furthermore, the residence time distribution of the gas phase is extracted from the numerical simulations. These distributions are used to characterize the gas phase mixing in the bubble column by employing a standard axial dispersion model. The results reveal that the extent of mixing increases when the sparged area decreases. The axial dispersion coefficient increases as the sparged area is shifted towards the side wall. For numerical simulation of bubbly flows, reliable closures are required to represent the interfacial momentum transfer rate (i.e. the effective drag acting on bubbles). Furthermore, the presence of neighboring bubbles in a bubble swarm may result in deviation of the drag force acting on isolated bubbles. Chapter 5 investigates the performance of several drag correlations reported in literature for bubble swarms with the aid of a discrete bubble model. By comparing with experimental data, it is found that Lima Neto’s drag model and Wen & Yu’s model have a better performance at low superficial gas velocity and Rusche’s model can predict the hydrodynamics of the bubbly flows better compared to the other models at high superficial gas velocity. In Chapter 6, breakup models developed in literature are implemented into the Eulerian-Lagrangian model. Moreover, the critical Weber number for bubble breakup studied by many authors in turbulent flows is also incorporated in the model. The performance of different breakup models and the critical Weber number for predicting hydrodynamics and the bubble size distribution are compared with experimental data. Finally, the Eulerian-Lagrangian model is further extended to study the performance of bubble column reactors, i.e. predicting overall gas holdup and phase mixing in Chapter 7. The residence time distribution of the gas phase and tracer particles introduced in the liquid phase are used to study the mixing of both the gas and liquid phase. It is found that the applied model shows very good agreement with empirical correlations reported in literature.