Masters Degrees (Chemical Engineering)

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Browsing Masters Degrees (Chemical Engineering) by Author "Anye Cho, Bovinille"

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    The development and characterization of a thermosiphon photobioreactor for the cultivation of photosynthetic bacteria
    (Stellenbosch : Stellenbosch University, 2018-12) Anye Cho, Bovinille; Pott, Robert William M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Since their 1st appearance in 1940s, closed photobioreactors (PBRs) have received significant attention as biotechnological systems, as viable alternatives to open systems. These improvements were aimed at addressing the very high material, operational, and production costs (circa 80% of total cost) associated with the energy input for aeration or agitation by mechanical and pneumatic devices. This thesis describes a PBR designed to exploit natural fluid circulation in closed loops, achieved by the thermosiphon effect arising from the temperature-induced density differential due to microbial light absorption and subsequent cooling. This PBR design provides important energy and cost savings through elimination of the mechanical and pneumatic devices. Light, being an obligate requirement for photosynthetic microorganisms within PBRs, can additionally power natural fluid circulation (the thermosiphon effect) in solar water heating systems (SWHS). Therefore, this work begins by structurally adapting the original thermosiphon loop published by Close in 1962 to meet the design criteria of a photobioreactor, resulting in a Thermosiphon Photobioreactor (TPBR) geometry comprising of five main sections: (i) adiabatic vertical cylindrical storage tank, (ii) truncated cone-shaped cooling section, and (iii) adiabatic downcomer, (iv) heating section (collector /absorber) and (v) adiabatic upriser. The TPBR’s geometry was sized to 1-L working volume through a single parameter optimization for the riser diameter, and constructed from glass and other hydrogen impermeable auxiliary units for Experimental Fluid Dynamic (EFD) studies. To get valuable insights into the reactor’s thermal and hydrodynamic performance, Computational Fluid Dynamics (CFD) was used to theoretically investigate the design, and operation of such a reactor systems. The CFD model was based on a 2D transient model which accounted for the non-uniform volumetric sensible heating due to microbial light absorption. This extends on previous studies on thermosiphon modeling which made use of simplified boundary conditions such as uniform constant wall temperatures and surface flux by accounting for the non-uniform light and heat distribution, which varies throughout the reactor as per a Beer- Lambert type curve. The light absorptive model was implemented via a User-Defined Function (UDF) which incorporated experimentally obtained spectral irradiance and attenuation parameters of Rhodopseudomonas palustris. The TPBR’s buoyancy driven convection was characterized by the boussinesq approximation as well as experimental and theoretical estimated heat transfer coefficients. The resulting CFD simulations have limited usefulness without experimental validation, in part due to the complexity of this study. Therefore, experimental data from thermocouple sensors, and marker image trackers fitted to the TPBR containing a biomass loading of 0.5kg/m3 Rhodopseudomonas palustris were used to validate the CFD model at the same operating conditions. The CFD simulation results clearly demonstrated buoyancy driven characteristic flow profiles with strong eddies showed at the storage tank, while flow velocities were tilted more to the front than to the rear riser section. These simulation results were ascertained through validation experiments with active Rhodopseudomonas palustris and predicted the TPBR’s thermal and hydrodynamic performance for all measuring points with a relatively small difference of less than 5% (317.7-307.9 K) and 10% (0.009-0.0085 m/s) as observed respectively. The flow visualization on the riser section of the TBPR also found that light absorption significantly influences fluid flow circulation and mixing which leads to a satisfactory agreement between the experimental observations and the CFD simulations results from a qualitative view point. Additional experimentation with active and inactive Rhodopseudomonas palustris revealed that the bacterial metabolic heat generation and waste fluorescence heat significantly contributed to the overall thermal and thermosiphoning effect of the TPBR. There was a 4% and 8% increase in the steady state temperature and heating rate respectively from the active microbial light absorption. This corresponded to a 3% increase of active bacterial cells in free suspension throughout the steady state conditions. In general, the TPBR provided satisfactory passive fluid flow to keep bacterial cells in suspension, maintaining up to 88% of the active bacterial loading in free suspension.