Tracing particle movement for simulation of light history and algal growth in airlift photobioreactors using Positron Emission Particle Tracking (PEPT)

Brighton, Marc

Tracing particle movement for simulation of light history and algal growth in airlift photobioreactors using Positron Emission Particle Tracking (PEPT)

Doctoral Thesis

2017

Publisher

University of Cape Town

Department

Centre for Bioprocess Engineering Research

Faculty

Faculty of Engineering and the Built Environment

Abstract

Microalgae are recognised for their potential in the production of a wide range of both high value and bulk, low value products. Microalgae greatly surpass the land, nutrient, sunlight, energy and water efficiencies of traditional crops. Whether this occurs in open ponds or closed photobioreactors, the limitations of mixing, mass transfer and light provision remain. This study focuses on airlift photobioreactors, where gas is injected into the base of a column, forcing liquid up an inner ‘draft tube’, termed the riser, before gas-liquid disengagement at the liquid surface and return of the liquid down the outer annulus, known as the downcomer. Gas-liquid mass transfer is not regarded as limiting in airlift reactors, particularly with the use of supplemental CO2. Light provision and utilisation by the algae is the limitation. Increasing light provision is not just a matter of increasing surface light intensity as the algal growth efficiency decreases and photoinhibition increases at high light intensities. Further, light attenuation and shading influence the light received at different points in the reactor. Hence, the position of the algal cell relative to the light source and the trajectories of the cells through the reactor determine their ‘light history’ and associated growth of the algal cells. This study determines the light that an individual algal cell experiences, using a novel application of Positron Emission Particle Tracking (PEPT) to determine particle movement. The radial position from PEPT and the dual asymptotic light penetration model of Suh and Lee (2003b) are used to predict the ‘light history’ of the tracer particle. As a suitably long tracer trajectory was used, this 'light history' is descriptive for all algal cells within the airlift reactor. These trajectories were also used to describe fluid movement and mixing in the airlift, linking fluid flow behaviour at specific operating onditions directly to growth rates and maximum algal concentrations attained. Application of PEPT to the aqueous environment of the airlift reactor to represent movement of unicellular algae required development of tracers with near-neutral buoyancy, that do not settle out onto the reactor base or adhere to the walls, and uptake sufficient Gallium-68 (68Ga), a β+ emitting radioisotope. A 200-1000 μm tracer was developed, composed of a calcium alginate hydrogel shell around a droplet of mineral oil with 150-220 μCi 68Ga uptake. A second, larger tracer (500-1000 μm) was developed by coating commercially available cation exchange resins with higher 68Ga uptake (200-800 μCi) with a layer of calcium alginate containing small CO2 bubbles. Calculation of the Stokes numbers suggested that the particle followed fluid elements in the majority of situations. Good correlation between PEPT trajectories and studies using liquid tracers supported this. Three airlift reactors (84 or 100 mm outer diameter, 347 mm tall and with 40 or 70 mm diameter draft tubes) were studied at gas flowrates from 0.22 - 2.12 L/min. PEPT trajectories of 50 - 190 minutes duration comprising 250 - 2500 circulations were obtained for each operating condition using water as the suspension medium. These datasets were more than sufficient to be representative of the particle movement patterns for most operating conditions. Similar PEPT analysis results and simulated growth when using particle trajectories recorded using water or when using a Scenedesmus culture as a suspension medium confirmed the applicability of the PEPT datasets to the characterisation and algal growth simulation of an algal photobioreactor. The majority of the data used had a positional error of ± 2 mm and >30 Hz frequency. Analysis of particle movement in the airlift reactors showed that reducing the gas flowrate increased the time required to complete a circulation or to move through each section of the airlift. This particularly increased the time to travel through the downcomer for airlifts with a narrow draft tube, and to travel through both the riser and downcomer for the airlift with a wide draft tube. Swirling patterns of reduced mixing occurred on entry to the downcomer if it was larger than the draft tube, and recirculation occurred in the draft tube if it was larger; both behaviours being more prevalent at low gas flowrates. Tracers in all configurations showed deviation from laminar flow with slow but significant radial movement in the downcomer, enough to change the light they would experience. Light histories were developed using the PEPT data on particle radial position in the reactor and the light penetration model of Suh and Lee (2003b). Algal growth simulation was carried out based on this light history, which is unique for each airlift reactor geometry and the gas flowrate at which it is operated. The simulation used the photosynthetic factories model of Eilers and Peeters (1988) to calculate light saturation, photoinhibition and algal growth using empirically determined constants for Porphyridium (Wu and Merchuk, 2001) and Scenedesmus (Fraser, 2011). Doubling external light intensity resulted in more than double the growth at 1.28 L/min gas flowrate for both algal species in all three ALRs, along with significant photoinhibition and an increasingly long lag phase at >200 μmol/m2/s incident light intensity for Porphyridium. There was reduced growth in the airlifts with narrow risers and wider downcomers for the lower gas flowrates under strong light (≤0.32 L/min for an 84 mm diameter and ≤0.64 L/min for a 100 mm diameter). This was only due to the flow patterns and resulting light history, showing lower growth due to slower cycling of light intensities, and matches behaviour seen in experimental algal growth. The narrower airlift reached higher algal concentrations with the highest productivity per mol of photons absorbed. However, it produced a similar mass of algae as the wider airlift with a wide draft tube. This wider airlift also showed the lowest levels of photoinhibition, an increasingly important factor for the use of higher light intensities such as sunlight. This study is the first reported application of PEPT in submerged culture bioreactors, including photobioreactors, thus requiring the development of new tracers. The focus on light experienced by an individual cell as a function of time is unique as all experimental work and most simulations present the average light provision to the whole algal culture as the variable of interest, despite recognition at the fundamental cell biology level of the impact of light cycling. Comparisons are made between different airlift reactor geometries and gas flow rates in terms of particle trajectories, fluid flow patterns, photoinhibition, light utilisation and algal growth. The minimum aeration required to prevent decreased algal growth has been determined in terms of necessary movement of algal cells between differing light intensities in airlifts. Additionally, the quantification of potential photoinhibition is useful in airlift design for growth under strong light, particularly for light sensitive species. This study is a new building block for the improved efficiency of microalgal systems towards bio-based products or CO2 recycling.

Keywords

Bioprocess Engineering

Reference:

Collections

PhD / Doctoral

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