The effect of mixing on Rhodopseudomonas palustris growth and hydrogen production
Date
2024-03
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Biological hydrogen production methods, such as photofermentation, offer a potentially environmentally friendly alternative to the non-renewable hydrogen production methods currently used in industry. However, a significant drawback of photofermentation is the mutual shading effect, as not all the cells are exposed to sufficient light intensities, which decreases light utilisation efficiency and, consequently, hydrogen production. Mitigating this effect is important to make photofermentation more economically viable. Various solutions have been proposed in literature, including designing photobioreactors with an increased surface area to volume ratio; however, this results in increased land and material costs. This study proposes controlled mixing as a solution, as it might increase light/dark (L/D) cycling frequencies. The effects of mixing on purple non-sulphur bacteria (PNSB) cells are still poorly understood, especially when a homogeneous substrate is used. This study aims to determine the effect of mixing on the PNSB, Rhodopseudomonas palustris, using glycerol as the substrate. Mixing would be facilitated via static mixers and varying the flow rate since static mixers have not yet been investigated as a mixing method in PNSB systems. The light distribution through the photobioreactor was modelled while considering light scattering effects, revealing that at a cell concentration of 0.7 g/L, 81% of the cells were exposed to insufficient light intensities for hydrogen production in this study. Therefore, increasing the L/D cycling frequency via mixing was expected to increase hydrogen production. Objectives two and three evaluated the impact of three static mixer designs and three flow rates on the growth and hydrogen production of R. palustris, revealing that R. palustris is sensitive to shear stress from the peristaltic pump, with the static mixers causing significantly less stress. The half-moon and spiral static mixers significantly increased the growth and hydrogen production at a flow rate of 0.15 L/min. Due to the static mixers alleviating the mutual shading effect and their corresponding lower degree of shear stress, they resulted in a higher specific hydrogen production than when the cells were circulated via the peristaltic pump. Mixing via the static mixers, therefore, warrants further investigation. Both the spiral and half-moon mixers resulted in a significant increase in growth and hydrogen production. However, the concentric mixer did not, potentially due to an ineffective flow path. The spiral mixer requires further investigation, as it resulted in the maximum specific hydrogen production, although a large degree of error persists with this static mixer. Interestingly, the static mixers only resulted in a significant increase in hydrogen production at 0.15 L/min, with no significant impact as the flow rate increased. R. palustris may already have been photosaturated when circulated through the peristaltic pump every 3.18 minutes (0.31 L/min). This highlights the importance of considering both the light distribution and the L/D cycling frequency when determining the required photobioreactor depth. Compared to microalgae, R. palustris could thus be recirculated much less frequently with no adverse effects on hydrogen production. This is advantageous as less pumping power would be required for optimal performance in PNSB systems.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MEng)--Stellenbosch University, 2024.