Full Text Available
Note: Clicking the button above will open the full text document at the original institutional repository in a new window.
Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production
Microalgae are recognised as a source of lipids for bioenergy, nutrients and pharmaceuticals. Photobioreactors, closed vessels for microalgal cultivation, are known to have high energy consumption due to mixing and aeration. Sparging is commonly used for mixing and gas-liquid mass transfer in photob...
Saved in:
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Language: | English |
| Published: |
Centre for Bioprocess Engineering Research
2016
|
| Subjects: | |
| Tags: |
No Tags, Be the first to tag this record!
|
| _version_ | 1867614406357024768 |
|---|---|
| access_status_str | Open Access |
| author | Jones, Sarah Melissa Jane |
| author2 | Harrison, STL |
| author_browse | Harrison, STL Jones, Sarah Melissa Jane |
| author_facet | Harrison, STL Jones, Sarah Melissa Jane |
| author_sort | Jones, Sarah Melissa Jane |
| collection | Thesis |
| description | Microalgae are recognised as a source of lipids for bioenergy, nutrients and pharmaceuticals. Photobioreactors, closed vessels for microalgal cultivation, are known to have high energy consumption due to mixing and aeration. Sparging is commonly used for mixing and gas-liquid mass transfer in photobioreactors, but is energy intensive. The aim of this work was to reduce these energy requirements by optimising conventional sparging and considering surface aeration coupled with mechanical agitation as an alternative. An airlift photobioreactor was used as a base for comparison with two novel, surface aerated reactors: oscillatory baffled and wave photobioreactors. The three bioreactors were compared in terms of power input, mixing, CO2 mass transfer, algal growth and lipid production. Prior to comparison, each photobioreactor was optimised based on these parameters. To calculate power input, isothermal gas expansion equations were used for sparged systems and calorimetry was used for mechanically agitation systems. Mixing was investigated using a salt tracer and phenolphthalein indicator and mass transfer was measured using the gassing-in method. Scenedesmus sp., a high lipid-producer, was cultivated in low nitrate media across a range of mixing rates in each photobioreactor.In the airlift photobioreactor a critical minimum CO2 supply rate (of 2.7×10-5 m s-1) was found, below which carbon was limiting and above which energy was spent on sparging without increased productivity (0.20 g L-1 d-1 biomass; 0.03 g L-1 d-1 lipid). In the oscillatory baffled reactor, insufficient mass transfer limited algal productivity (0.11 g L-1 d-1 biomass; 0.02 g L-1 d-1 lipid). The wave reactor had high CO2 mass transfer coefficients (10 – 140 h-1) in comparison to the airlift (2.7 – 40 h-1) and oscillatory baffled reactors (6.3 – 37 h-1). Sufficient biomass productivity (0.18 g L- -1 d-1) and higher lipid productivity (0.045 g L-1 d-1) at lower power input in the wave reactor resulted in higher energy efficiency compared to the airlift reactor. Life cycle analysis of simulated algal biodiesel production showed that bioreactor energy contributed 99% of total energy consumption. Therefore, the global warming potential was reduced by 73% when the airlift reactor was operated at the critical minimum CO2 supply (with gas compression to 2 bar) and a further 19% when the wave reactor was
used. This work offers an energy efficient alternative to sparging, through the generation of a well-mixed wave in a surface aerated bioreactor. It also offers methods for optimisation of energy usage with respect to mixing and aeration. Reducing bioreactor energy consumption is key to feasibility, and was demonstrated here to reduce energy-related environmental burdens. |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/20064 |
| institution | University of Cape Town (South Africa) |
| language | eng |
| last_indexed | 2026-06-10T12:51:32.293Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2016 |
| publishDateRange | 2016 |
| publishDateSort | 2016 |
| publisher | Centre for Bioprocess Engineering Research |
| publisherStr | Centre for Bioprocess Engineering Research |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/20064 Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production Jones, Sarah Melissa Jane Harrison, STL Bioprocess Engineering Microalgae are recognised as a source of lipids for bioenergy, nutrients and pharmaceuticals. Photobioreactors, closed vessels for microalgal cultivation, are known to have high energy consumption due to mixing and aeration. Sparging is commonly used for mixing and gas-liquid mass transfer in photobioreactors, but is energy intensive. The aim of this work was to reduce these energy requirements by optimising conventional sparging and considering surface aeration coupled with mechanical agitation as an alternative. An airlift photobioreactor was used as a base for comparison with two novel, surface aerated reactors: oscillatory baffled and wave photobioreactors. The three bioreactors were compared in terms of power input, mixing, CO2 mass transfer, algal growth and lipid production. Prior to comparison, each photobioreactor was optimised based on these parameters. To calculate power input, isothermal gas expansion equations were used for sparged systems and calorimetry was used for mechanically agitation systems. Mixing was investigated using a salt tracer and phenolphthalein indicator and mass transfer was measured using the gassing-in method. Scenedesmus sp., a high lipid-producer, was cultivated in low nitrate media across a range of mixing rates in each photobioreactor.In the airlift photobioreactor a critical minimum CO2 supply rate (of 2.7×10-5 m s-1) was found, below which carbon was limiting and above which energy was spent on sparging without increased productivity (0.20 g L-1 d-1 biomass; 0.03 g L-1 d-1 lipid). In the oscillatory baffled reactor, insufficient mass transfer limited algal productivity (0.11 g L-1 d-1 biomass; 0.02 g L-1 d-1 lipid). The wave reactor had high CO2 mass transfer coefficients (10 – 140 h-1) in comparison to the airlift (2.7 – 40 h-1) and oscillatory baffled reactors (6.3 – 37 h-1). Sufficient biomass productivity (0.18 g L- -1 d-1) and higher lipid productivity (0.045 g L-1 d-1) at lower power input in the wave reactor resulted in higher energy efficiency compared to the airlift reactor. Life cycle analysis of simulated algal biodiesel production showed that bioreactor energy contributed 99% of total energy consumption. Therefore, the global warming potential was reduced by 73% when the airlift reactor was operated at the critical minimum CO2 supply (with gas compression to 2 bar) and a further 19% when the wave reactor was used. This work offers an energy efficient alternative to sparging, through the generation of a well-mixed wave in a surface aerated bioreactor. It also offers methods for optimisation of energy usage with respect to mixing and aeration. Reducing bioreactor energy consumption is key to feasibility, and was demonstrated here to reduce energy-related environmental burdens. 2016-06-21T09:26:17Z 2016-06-21T09:26:17Z 2015 Doctoral Thesis Doctoral PhD http://hdl.handle.net/11427/20064 eng application/pdf Centre for Bioprocess Engineering Research Faculty of Engineering and the Built Environment University of Cape Town |
| spellingShingle | Bioprocess Engineering Jones, Sarah Melissa Jane Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| thesis_degree_str | Doctoral |
| title | Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| title_full | Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| title_fullStr | Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| title_full_unstemmed | Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| title_short | Mixing, mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| title_sort | mixing mass transfer and energy analysis across bioreactor types in microalgal cultivation and lipid production |
| topic | Bioprocess Engineering |
| url | http://hdl.handle.net/11427/20064 |
| work_keys_str_mv | AT jonessarahmelissajane mixingmasstransferandenergyanalysisacrossbioreactortypesinmicroalgalcultivationandlipidproduction |