Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Abufalgha, Ayman A."

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    Behaviour of oxygen transfer in a simulated multiphase hydrocarbon-based bioprocess in a bubble column reactor
    (Stellenbosch : Stellenbosch University, 2018-03) Abufalgha, Ayman A.; Clarke, Kim Gail; Pott, Robert William M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Linear alkane hydrocarbons can be converted into higher value products and product-precursors through the addition of an oxygen moiety. One method of upgrading these hydrocarbons is through biologically-mediated oxidation, whereby an oxygen group is added to the alkane backbone by a microorganism to produce a more reactive, and more valuable molecule. To achieve this, operating conditions conducive to this biological process need to be reached; one key is liquid-phase oxygen concentration, which is governed by the oxygen transfer rate into the reaction fluid. This study focuses on the behaviour of the overall volumetric oxygen transfer coefficient (KLa) and, the interfacial area (a), in a model four-phase (air-water-hydrocarbon-deactivated yeast) hydrocarbon-based bioprocess in a bubble column reactor (BCR). It sought to give an understanding on the impact of the alkane concentration, superficial gas velocity and yeast loading on the KLa and the interfacial area of the system. The experiments were conducted at different alkane concentrations of (2.5 to 20% v/v n-C14-20), superficial gas velocities of (1 to 3 cm/sec), and yeast loading of (0.5 to 6 g/l Saccharomyces cerevisiae). The yeast cells were deactivated by heating, to suppress oxygen utilisation, while maintaining cell integrity. The KLa was measured using the gassing-out method (GOP) using a dissolved oxygen (DO) probe which recorded the rate of DO in the system every 5 seconds until oxygen saturation was achieved. The probe constant (Kp) was evaluated at all experimental conditions to ensure the accuracy of the KLa determination. The interfacial area was calculated from the gas hold-up (Ɛ𝐺), and Sauter mean diameter (𝐷32) data. 𝐷32 was evaluated by high-speed photography and image analysis on the acquired images, performed using MATLAB software, while the Ɛ𝐺 was calculated as the difference in the liquid level before and after sparging the reactor. For three phase systems (air-water-hydrocarbons), it was found that at low superficial gas velocity the system was non-homogeneous, with an increased hydrocarbon concentration at the top of the reactor. The spatial homogeneity of the system has not been previously investigated in the literature, particularly in the the bubble column reactor when the process contains hydrocarbons. The KLa measurements were thus affected by the spatial variations in concentrations and therefore could not be reliable. To overcome this, the system was analysed for spatial liquid-phase homogeneity using a physical sampling methodology, which established the optimal superficial gas velocity (2 cm/sec) to achieve homogenous flow in the three phase system. For the four-phase system, the addition of yeast to the BCR resulted in a change in the flow regime and therefore increased the homogeneity in the system at low superficial gas velocity (1 cm/sec). This result suggets that the addition of yeast cells to the reactor likely changed the fluid properties such as viscosity and the surface tension. Investigations on the impact of yeast loading, alkane concentration and superfical gas velocity on KLa and interfacial area in a multiphase hydrocarbon-based bioprocess in a BCR were undertaken for this project. It was found that an increase in the yeast loading resulted in a decrease in KLa at constant superficial gas velocity in the three phase (air-water-deactivated yeast) system. This decrease might be attributed to the existence of diffusion blocking effects as well as the increasing fluid viscosity in the system. A similar effect was found in a four phase (air-water-hydrocarbon-deactivated yeast) system, when the values of KLa were depressed with increasing yeast concentration at high superficial gas velocities and constant alkane concentration (2.5% v/v). However, increasing the constant alkane concentration at the same conditions caused a decrease in KLa for all superficial gas velocities. At low yeast loading (0.5 g/l), the interfacial area increased to an average of 190 m2/m3 when the superficial gas velocity increased from 1 to 3 cm/sec. However, increasing yeast loading from 0.5 g/l to 6 g/l saw a decrease in the interfacial area to approximately 70 m2/m3 at the maximum levels of superficial gas velocities. The observed decrease in the interfacial area at high yeast loadings was a result of the increasing 𝐷32, likely due to the effect the yeast has on fluid properties. Increasing the hydrocarbon concentration from 2.5 to 10% v/v decreased KLa, possibly due to the increase in fluid viscosity and the surface tension, dampening turbulence in the system. However, further increases in alkane concentration above 10% v/v caused a significant increase in KLa for the lowest superficial gas velocities and constant yeast loading. An increase in the superficial gas velocity also decreased the values of KLa with an average of 0.3 s-1 at highest alkane concentration (20% v/v) for all constant yeast loading. The interfacial area had marginal increases of about 20 m2/m3 when the alkane concentration was increased from 2.5 to 20% v/v. This increase in interfacial area with increasing superficial gas velocity was due to the increase in the gas hold up, since 𝐷32 was not influenced by the alkane concentration. It can be concluded that these trends of KLa values were not entirely as expected, since the strongest effect came from alkane concentration, rather than superficial gas velocity, which is usually identified as the strongest factor in oxygen transfer studies. Further, the interfacial area was significantly influenced by the variations in superficial gas velocity (1 to 3 cm/sec). The oppositional effects between the interfacial area and KLa values suggest that the interaction between the yeast cells and the hydrocarbons may have changed the system hydrodynamics such that the oxygen transfer coefficient KL was effected, since interfacial area is seen to be fairly constant over the variable ranges. This work significantly furthers our understanding of the fluid mixing properties and KLa in a bubble column reactor, as well as the behaviour of the overall volumetric oxygen transfer coefficient in simulated hydrocarbon-based processes in this reactor configuration, which has the potential to impact and inform the operation of an industrially relevant hydrocarbon based bioprocess.