Co 2 transfer at high pH is not passive

1. Oxygen Transfer ln (C -C) vs. Time( min) s (Transfer Coefficient/K A = -0.05147) L 4 3 2 1 0 0 20 40 60 80 100 120 140 -1 -2 -3 -4 -5 ln (Cs-C) vs. Time Linear (ln (Cs-C) vs. Time) CO2 Transfer in Algal Bioreactors Miranda A. Spang, Nigel W.T. Quinn, & David Brune Background Algal grow at high pH is much faster than predicted by passive CO2 transfer O2 and CO2 transfer rates and predicted algal growth rate Possible Implications for the Biofuels and CO2 Sequestration • Field algae ponds and open lab bioreactors at high pH (10 -11 ) have been shown to grow algae at rates up to ten times faster than can be explained by standard passive CO2 transfer. kinetics • Standard CO2 transfer is described by dc/dt = KLA (Cs-C) which reduces to  KLA KH pCO2  or (KLA)[CO2]aq.   --   (EQUATION 1) • dc/dt is the rate of change of oxygen concentration • KL is the mass transfer coefficient oxygen • A- interfacial area for mass transfer • Cs is the saturation concentration of oxygen, mg/l • C is the concentration of dissolved oxygen in the liquid • KH is Henry’s Law Constant • pCO2 is the partial pressure of CO2 • However Zeman (1) has shown that transfer at high OH concentration is controlled by the equation,  JCO2 = CO2 transfer rate as mole/m2- sec =(((Df)x(Kd)x (OH))^^ (-0.5)) x (CO2aq)--(EQUATION 2) • JCO2 = CO2 absorption rate per unit surface area [mol s−1 m−2]  • Df  is diffusivisty of CO2 • kd is kinetic constant for hydroxide reaction • OH is hydroxide concentration The Kla or mass transfer rate of O2 was determined to be 0.05 per minute by the slope of the line of ln [Cs- C1]/ [Cs – C1] vs time. The transfer rate of CO2 was calculated using the data for oxygen transfer and multiplying it by the corresponding molecular weight ratio of O2 to CO2 = 31.9988g/mol divided by 44.0096g/mol to get 0.023 per min and then multiplying by the carbon concentration at atmospheric equilibrium for CO2 in water (0.15 mg C/l) to get 0.0034mg mg C/min or approximately 5 mg C/liter per day. This translates into 10 mg volatile solids per liter per day as algal biomass. Thus to increase the algae biomass ten fold from 100 mg/l to 1,000 mg/l this rate suggests a 90 day residence time. Observation: Algal growth in high pH environments such as algal ponds and open bioreactors has been found to be much greater than can be explained by passive CO2 transfer kinetics. Co2 transfer at high pH is not passive Hypotheses: CO2 transfer kinetics is not diffusion-dominated in high pH environments such as algal ponds and open laboratory bioreactors. Instead, CO2 transfer assimilation appears to be a function of hydroxyl ion concentration. Kinetic expressions can be derived for algal biomass and are expected to independent of species If final data supports the hypothesis, then this new understanding of CO2 transfer at high pH could potentially help increase algal productivity in large scale projects such as biofuel production and CO2 sequestration from CO2 emitting industries. • Zehman (1) suggests that there is a point at which CO2 transfer changes from passive diffusion to a OH dominated reaction.  However, this transition has not been quantified. Thus our objectives are to : • Determine the Kla or mass transfer rate of O2 in bioreactors and use this rate to find the mass transfer rate of CO2 for passive diffusion. Use this rate to predict the amount of algal biomass that should be expected based on passive diffusion alone. • 2. Compare the predicted rate of algal biomass to the actual rate • 3. Determine and Compare the actual Kla or mass transfer rate of CO2 for bioreactors with and without algal biomass at pH 10 – pH 11 where the hydroxyl ion concentration is high • 4. Determine the transition point between diffusion dominated and hydroxyl ion dominated C02 transfer for a number of candidate algal species for biofuel production Objectives Bioreactor Setup Scenedesmus sp was grown in a series of open bioreactors with atmospheric CO2 as the only carbon source and with modified BG-11 growth medium containing a minimum amount of phosphorous to prevent buffering at high pH. A bioreactor without algae hydroxide solution containing the same BG-11 growth medium started at a pH around 11.5 was used as a control and then natural dropped as it was exposed to atmospheric CO2. The bioreactor containing Scenedesmus sp started around pH 8 and eventually maintained a pH of approximately 11 after equilibrium has been reached. Alkalinity was equivalent in both the algae reactors and the control at the start of the experiment. The BG 11 growth media for the algae bioreactors started with an alkalinity of 2 milliequivalents Na2CO3 and the control with an alkalinity of 2 milliequivalents of NaOH. All reactors were kept at a fixed mixing rate of 300 rpm using magnetic stirrers. The bioreactors were evenly spaced between two sets of florescent lighting on either side of the reactors. In order to prevent algal growth in control, the control was be shielded from light.. Preliminary data confirms that algae are growing faster than the predicted rate by passive diffusion This rate of biomass production 20mg/l/day is double the amount predicted by passive CO2 transfer. The next steps will include calculating the CO2 transfer in the control bioreactor without algae, gathering more data, and determine the transition point between diffusion dominated and hydroxyl ion dominated C02 transfer for a number of candidate algal species for biofuel production. . Acknowledgements I would like to give a very special thank you to Rolf Melhorn for the use of his lab, his patience, and guidance throughout this project. I would also like to thank Tryg Lundquist. "A special thank you to the S.D.Bechtel, Jr. Foundation for support of the Industry Initiatives for Science and Mathematics Education (IISME) Fellows" Scenedesmus Subspicatus Scenedesmus Subspicatus Filter caps to prevent bacterial contamination Contact information David Edward Brune, PhD, PE Prof of Bioprocess and Bioenergy University of Missouri Miranda Spang, MA ASCEND School 3709 E. 12 Street Oakland, CA 94601 mirandaspang@gmail.com Nigel W.T. Quinn, PhD, P.E. , Group LeaderHydroEcological Engineering Advanced Decision SupportOffice: (510) 486-7056 (LBL) nwquinn@lbl.gov