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Introduction
Catalytic Gas-Solid Fluidized Bed Reactors (FBRs) have been studied and used for over six decades. From novel laboratory demonstrations [1] to performing nanoparticle coatings [2], to being at the heart of large petrochemical companies [3,4], these reactors have many uses. From an engineering point of view, advantages include: efficient solids mixing, good gas-solid contacting and low pressure drop. A wealth of understanding of the hydrodynamics of FBRs and their effects on reactor performance has been gained, although there are numerous areas where fundamental understanding is lacking. Many studies focus on either a specific hydrodynamic parameter or purely on the reactor performance. By using the one, deductions are made with regard to the other. For instance; hydrodynamic insight is used to infer the effect on the reactor performance or the reactor performance is used to infer the hydrodynamic cause. Few studies have followed an integrated approach, which creates difficulties in modelling an FBR. Depending on the operating velocity (U0) several regimes exist in FBRs, most commonly used being the bubbling, turbulent or fast fluidization regimes. Each regime is characterized by its own hydrodynamic behaviours. The bubbling and fast fluidization regimes have enjoyed much academic attention due to the distinctness of the bubbles and the core annulus, respectively. The turbulent regime has better gas-solids contacting than the bubbling regime without the high solids circulation of the fast fluidization regime.
These reasons make the turbulent regime a popular choice for industry. Commercial examples of turbulent reactors include FCC regenerators, zinc sulphide roasters and Mobil MTG, acrylonitrile, maleic anhydride, phthalic anhydride and ethylene dichloride reactors.
Despite the turbulent regime being popular in industry, it has not received as much attention as the bubbling or fast fluidization regimes [5]. Based on observations of incipiently fluidized bubbling beds, the need for hydrodynamic descriptions of two-phase behaviour arose. The earliest well-known published works on the matter were those of Rowe and Partridge [6,7] and Davidson and Harrison [8–10]. The concept was developed further and gas exchange between the phases was explored [11–16]. Ultimately, leaders in the field such as Kunii, Levenspiel and Grace proposed reactor models based on the theory [17–22]. Generally, these reactor models and the two-phase theory best describe the hydrodynamic behaviour of bubbling fluidized beds [23–26]. The theory entails that most of the gas reagents are contained in a lean, solids/catalyst-deprived phase thatbubbles though a dense, solids-rich (emulsion) phase.
This closely resembles the physical phenomena in the FBR. Since most of the gas throughput is present in the lean phase, the ovement of gas into and out of the emulsion phase often dictates the performance of an FBR. Therefore the description of the interphase mass transfer becomes one of the crucial modelling variables. Most correlations for this transfer are derived on the basis of lowvelocity/ interaction bubbling regime behaviour (small U0/Uc values of 0.02). Uc is the onset velocity of the turbulent regime. In this low U0/Uc regime the bubbles have near-ideal geometries and low interactions with each other. Despite the success of these models at lower velocities, the transfer correlations are not suited for higher velocity operations [25,27–29]. Few attempts have been made to adapt interphase mass transfer correlations for the higher velocity bubbling regime or turbulent regime [30].
Chapter 1 : Introduction.
1.1 Outline of the thesis
Chapter 2 : Literature Review.
2.1 Fluidized bed fundamentals.
2.2 Measuring methods.
2.3 Solids and voids behaviour.
2.4 Fluidized bed reactor models
2.5 Mass transfer
Chapter 3 : Tomography .
3.1 Experimental.
3.2 Results and discussion
3.3 Conclusions
Chapter 4 : Linking Reactor Performance and Hydrodynamics
4.1 Experimental
4.2 Results and discussion
4.3 Mass transfer correlations.
4.4 Conclusions
Chapter 5 : Reactor Performance
5.1 Experimental
5.2 Estimation of bubble size
5.3 Results and discussion.
5.4 Conclusions
Chapter 6 : Concluding Remarks
References
Appendix A : XRT Calibration
Appendix B : Determination of Hydrodynamic Parameters
Appendix C : Reactor Performance Data
