The development and characterization of a thermosiphon photobioreactor for the cultivation of photosynthetic bacteria

Anye Cho, Bovinille (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Sedert hul eerste verskyning in die 1940’s, het geslote fotobioreaktors (FBRs) ’n beduidende hoeveelheid aandag ontvang as biotegnologiese stelsels en as lewensvatbare alternatiewe tot oop stelsels. Hierdie verbeteringe streef om die probleem van die baie hoë materiaal-, operasionele, en produksiekostes (circa 80% van totale koste) wat geassosieer word met die energie-inset vir belugting en beroering deur meganiese en pneumatiese toestelle, op te los. Hierdie tesis beskryf ’n FBR wat ontwerp is om die natuurlike vloeistof sirkulasie in geslote lusse te benut, wat bereik word deur die termosifon-effek wat ontstaan vanuit die temperatuur geïnduseerde digtheid differensiaal weens die mikrobiese ligabsorpsie en daaropvolgende afkoeling. Hierdie FBRontwerp voorsien belangrike energie- en kostebesparings deur die eliminering van meganiese en pneumatiese toestelle. Lig, wat ’n verpligte vereiste vir fotosintetiese mikro-organismes binne FBRe is, kan natuurlike vloeistofsirkulasie (die termosifon-effek) in sonwaterverhittingstelsels (SWVS) aandryf. In hierdie werk word die oorspronklike termosifonlus, gepubliseer deur Close in 1962, struktureel aangepas om aan die ontwerpkriteria van ’n fotobioreaktor te voldoen. Dit het ’n Termosifon Fotobioreaktor (TFBR) geometrie tot gevolg, wat bestaan uit vyf hoofdele: (i) adiabatiese vertikale silindriese opgaartenk, (ii) afgeknotte keëlvormige verkoelingseksie, (iii) adiabatiese sakpyp, (iv) verhittingseksie (opvanger/absorbeerder), en (v) adiabatiese stygpyp. Die TFBR se geometrie was geskaal tot 1 L bedryfsvolume deur ’n enkel parameter optimering vir die stygpypdeursnee en gebou uit glas en ander waterstof ondeurdringbare bykomende eenhede vir Eksperimentele Vloeidinamika (EVD) studies. Om waardevolle insig in die reaktor se termiese en hidrodinamiese werkverrigting te kry, is Reken Vloeidinamika (RVD) gebruik om die ontwerp en werking van so ’n reaktorstelsel teoreties te ondersoek. Die RVD model is gebaseer op ’n 2D tydafhanklike model wat die nieuniforme volumetriese waarneembare verhitting as gevolg van mikrobiese ligabsorpsie, in ag neem. Dit bou op vorige studies op termosifon modellering wat gebruik gemaak het van eenvoudige randtoestande soos uniforme konstante muur temperature en oppervlak vloed deur verantwoording te doen vir die nie-uniforme lig- en hitteverspreiding, wat varieer deur die reaktor volgens ’n Beer-Lambert tipe kurwe. Die ligabsorberende model is geïmplementeer deur ’n Gebruiker Gedefinieerde Funksie (GGF) wat spektrale uitstraling en dempingparameters van Rhodopseudomonas palustris wat eksperimenteel verkry is, geïnkorporeer het. Die TFBR se dryfvermoë gedrewe konveksie was gekarakteriseer deur die Boussinesq benadering, sowel as eksperimentele en teoretiese beraamde hitte-oordrag koëffisiënte. Die resulterende RVD simulasies het beperkte nuttigheid sonder eksperimentele validering, gedeeltelik as gevolg van die kompleksiteit van hierdie studie. Daarom is eksperimentele data van termokoppelsensors en merker beeld opspoorders vasgemaak aan die TFBR wat ’n biomassa lading van 0.5 kg/m3 Rhodopseudomonas palustris bevat, gebruik om die RVD model by dieselfde operasionele kondisies te valideer. Die RVD simulasie resultate het duidelik dryfvermoë gedrewe karakteristieke vloei profiele gedemonstreer en sterk werwelinge gewys by die opgaartenk, terwyl vloeisnelhede hoër was aan die voorkant as aan die agterkant van die stygpypdeel. Hierdie simulasie resultate is gestaaf deur validasie-eksperimente met aktiewe Rhodopseudomonas palustris en het voorspel dat die TFBR se termiese en hidrodinamiese werkverrigting vir alle metingspunte met ’n relatiewe klein verskil van minder as 5% (317.7- 307.9 K) en 10% (0.009-0.0085 m/s), soos onderskeidelik waargeneem. Die vloei visualisering op die stygpypdeel van die TFBR het ook gewys dat ligabsorpsie ʼn beduidende invloed op vloeisirkulasie en vermenging het, wat lei tot ’n bevredigende ooreenkoms tussen die eksperimentele waarnemings en die RVD simulasie resultate vanuit ’n kwalitatiewe oogpunt. Addisionele eksperimentasie met aktiewe en onaktiewe Rhodopseudomonas palustris het gewys dat die bakteriële metaboliese hitte opwekking en afvalfluoressensiehitte ’n aansienlike bydrae gemaak het tot die algehele termiese en termosifoniese effek van die TFBR. Daar was ’n 4% en 8% verhoging in die bestendige toestand temperatuur en verhittingstempo onderskeidelik van die aktiewe mikrobiese ligabsorpsie. Dit stem ooreen met ’n 3% verhoging van aktiewe bakteriële selle in die vrye suspensie regdeur die bestendige toestand kondisies. Oor die algemeen, het die TFBR bevredigende passiewe vloeistof vloei verskaf om die bakteriële selle in suspensie te hou, wat tot 88% van die aktiewe bakteriële lading in vrye suspensie gehandhaaf het.

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