Improving Process Efficiency of Algae Based Biofuel and Bioproduct Production using Metabolism based pH Control

Open Access
Author:
Wang, Jun
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 31, 2015
Committee Members:
  • Wayne Roger Curtis, Dissertation Advisor
  • Andrew Zydney, Committee Member
  • Antonios Armaou, Committee Member
  • Maryann Victoria Bruns, Committee Member
Keywords:
  • algae
  • biofuel
  • pH
  • process control
  • nitrogen
Abstract:
Algae based biofuel and bioproducts offer a sustainable alternative to current fossil fuels as well as a source of various high value products. The large scale production of algae based biofuel requires high density of algae culture and economically feasible cultivation methods. One of the important issues associated with high density algae cultivation is pH instability, which is due to the inherent proton imbalance in algae metabolism. Among different nitrogen sources in typical algae culture media, nitrate and ammonium lead to alkalinization and acidification of the media, respectively, as they are consumed. Compared to other pH control strategies such as buffering or acid-base addition, the metabolism based pH control by feeding different nitrogen sources provides a potential pH control strategy that is technically feasible in large scale outdoor biofuel production. To accomplish this, a fed-batch nutrient feeding strategy would need to be adopted in a model-based pH control system. Due to the current lack of understanding of the magnitude of the stoichiometric pH imbalance under different nitrogen sources, the first part of this study presents a quantitative assessment of the proton imbalance for micro algae (Chlorella vulgaris) grown on different nitrogen sources. Our results give tentative experimental values for the pH imbalance associated with nitrate and ammonium media, which can be used as initial parameters in a pH control system based on stoichiometric utilization of nitrogen during algae growth. The preliminary interpretations of these results are also presented as they relate to the biology of nitrogen uptake and metabolism as well as the chemistry associated with media buffering. The second part of this work focuses on the implementation of such metabolism based pH control strategy in both laboratory scale and commercial outdoor scale algae production using facilities in Arizona Center of Algae Technology and Innovation (AzCATI). The outdoor field test shows that: compared with current on demand CO2 buffering control method, metabolism based pH control method could achieve long term stable pH while using only one order of magnitude lower CO2 supply rate. In both cases, culture pH could be maintained at a desired level, however different scales do show significant differences: the pH in the laboratory scale flask can be managed at a much more precise level due to little fluctuation in a well controlled lab cultivation environment while the outdoor production system has fairly high fluctuations with only metabolic pH control since the whole process of outdoor system is inevitably subject to the change of weather and solar cycle. Besides the main line of pH control, the last part of this dissertation consisted of a compilation of various efforts related to algae technology including the development of an optical density sensor and an attempt to cultivate algae on a coal based surface.