Modeling and Optimization of CO2 Fixation using Microalgae Cultivated in Oil Sands Process Water

• Author / Creator

  Kasiri, Sepideh

• Biological fixation of CO2 using microalgae is a potential CO2 reduction strategy which can be applied to the oil sands operations in Alberta. These operations also produce large amounts of oil sands process water (OSPW), which can act as a growth medium for the microalgae. In this thesis, three different microalgae: Botryococcus braunii, Chlorella pyrenoidosa and Chlorella kessleri were investigated for their ability to grow in OSPW, and to uptake CO2. Also, the effect of phosphate, nitrate, CO2 concentrations and light intensity were studied using a fractional two-level and a full two-level factorial designs. These investigations eliminated Botryococcus braunii as it cannot grow in OSPW, and demonstrated that Chlorella kessleri has a higher CO2 uptake rate than Chlorella pyrenoidosa. Moreover, CO2 concentration, light intensity and phosphate concentration proved to have the strongest effects (in that order) on the growth and CO2 uptake rate of Chlorella kessleri. To make biological fixation of CO2 economically competitive compared to other CO2 capture techniques, it is necessary to optimize CO2 fixation rate and operate the algal culture at the optimal process conditions. Response surface methodology (RSM) was also used to develop statistical models for the CO2 uptake rate and specific growth rate of Chlorella kessleri. The quadratic models developed were used to determine the optimal sets of CO2 concentration, phosphate concentration and light intensity for CO2 uptake rate and specific growth rate in batch operation. A multi-objective optimization technique was then used to maximize the CO2 uptake rate and specific growth rate simultaneously. Moreover, to have a better understanding of microalgal dynamic behaviour, data generated using a central composite experimental design (CCD) with varying light intensity, CO2 and phosphate concentration was used to develop a mathematical model that describes the kinetics of algal growth and CO2, phosphate, nitrate and ammonium uptake rate of Chlorella kessleri cultivated in OSPW. This nonlinear dynamic model was used to determine the optimal CO2 concentration, phosphate concentration and light intensity for CO2 uptake and algal growth over a period of time in a batch culture using a multi-objective optimization technique to maximize CO2 fixation and algal growth simultaneously. Finally, a lab-scale closed raceway photobioreactor was designed and assembled for cultivation of Chlorella kessleri in OSPW with the aim of combining CO2 capture and algal production. A fed-batch model describing the dynamics of microalgae growth and CO2, phosphate and ammonium uptake rate was developed based on the modification of a previously developed model for batch cultures, and was successfully validated against experimental data. The CO2 uptake rate and algal growth can be maximized by properly manipulating the CO2 and phosphate concentrations and availability and intensity of light. A model-based optimization method was used to calculate the optimal feeding strategies for CO2, phosphate and light intensity.

• Subjects / Keywords

  • Microalgae
  • Optimization
  • Modelling
  • Kinetics

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3ZS2KN18

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.