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Abstract
Gas-inducing stirred tank reactors (GISTs) are attractive for industrial chemical processes where efficient mixing holds the key to product yield and quality particularly in situations where the reaction has low conversion per pass as is the case for the Fischer-Tropsch synthesis (FTS) of hydrocarbons. However, these benefits can only be properly harnessed if there is a reliable set of quantitative relations between operating variables and the mixing attributes as well as reaction metrics. Therefore, in this project, the hydrodynamics and mass transfer in a multiphase system have been intensively investigated using electrical process tomography (electrical resistance tomography (ERT) and electrical capacitance tomography (ECT)).
Unlike conventional methods, tomography provides a visualisation of the interaction between different phases by generating cross-sectional phase distribution images of the vessel. The technique is non-invasive, using electrical signals corresponding to changes in the component distribution within the vessel with the aid of reconstruction algorithms. Various parameters such as stirring speed, particle size, solid loading, temperature, pressure, and partial pressure have been studied in both reactive and non-reactive systems.
In addition, computational fluid dynamics (CFD) software (FLUENT 6.3) was used to elucidate the hydrodynamic behaviour within the system. Dynamics bubble behaviour over time in GIST (gas-liquid system) was successfully modeled using common Laguerre equation that based on birth, growth and death (bubbles break-up and coalescence), where the onset of gas bubble dispersed in liquid starts about 0.2 s.
The dispersed phase holdup increased with stirring speed and solid loading (0-40 g/L). Global solid phase hold-up profile exhibited a sigmoid-shape with respect to the impeller Reynolds number, indicative of three solid suspension regimes across the stirring range (0 to 1200 rpm) investigated. It is evident that at stirring speeds above 800 rpm, vortex formation sets in and gas is pushed towards the impeller shaft causing a maximum gas hold-up in the immediate shaft vicinity. The magnitude of this maximum increased with agitation rate. Phase hold-up distribution and mass transfer profiles were adequately captured by generalized Chapman-Richards equations. The dependency of the model parameters on particle size was also obtained in all cases. Mass transfer coefficients (gas-liquid and liquid-solid) for GIST were superior to that of the externally gas-sparged gas-liquid system, suggesting that the gas-inducing impeller indeed enhances mixing performance and interphase mass transfer.
In the FT reaction, the steady-state gas phase hold-up dependency on temperature was shown to be due to contributions from both thermal expansion and reaction-induced changes in the liquid phase. The latter model was combined with the standard Arrhenius representation of the rate behaviour at a given feed composition, to yield a new relation that may be employed to evaluate online reaction rate from unobtrusive dispersed phase hold-up measurements if the reactor is endowed with ECT capabilities.