Experimental Techniques for kLa determination http://www.youtube.com/watch?v=8441Bz07qQ0 Theory: Narasimha dC AL dt = k L a C * − C AL AL ( ) integrating…. C AL 2 C AL1 ∫ (C 1 * AL − C AL ) dC AL = k L a ∫ dt 0 t C * − C AL1 = k at ln AL L C* − C AL2 AL Therefore a plot of slope kLa. C * − C AL1 ln AL C* − C vs AL 2 AL t should result in a straight line of Static Gassing Out Method (Unsteady State) • • In the absence of respiring organism (no O2 consumption) Sparge vessel contents with N2, displacing O2 • Monitor variation in dissolved oxygen concentration (DO) using a (polarographic) DO probe • plot of kLa C * − C AL1 ln AL C* − C vs AL 2 AL t should result in a straight line of slope Dynamic Gassing Out Method • In the presence of respiring organsims • • • • At time t0. then turn off N2 flow • Sparge vessel contents with air at a known flowrate • Monitor and record variation of DO concentration with respect to time. turn the air supply on again Monitor and record rise in DO Theory: For respiring systems dC AL dt = k L a C * − C AL − q o 2 X AL ( ) Where X is the biomass concentration and qo2 is the specific oxygen consumption rate (mmols O2 / g biomass s) .• Allow DO to fall to 0% saturation. turn off air supply to vessel Monitor and record reduction in DO between t0 and t1 At t1. In this case. we can assume that a quasi-steady state exists at the time of the experiment between oxygen transfer and oxygen consumption. suitable for use with respiring systems • Assumes rapid disengagement of air bubbles when air supply is turned off -inappropriate for highly viscous broths • If headspace aeration is significant. − k L a C * − C AL = q o 2 X AL ( ) is the quasi steady state oxygen concentration at the time of the experiment Substituting for dC AL dt − C AL qo2 X in the original equation − = k L a C * − C AL − k L a C * − C AL AL AL ( ) ( ) Rearranging results in the following equation dC AL dt = k L a C − − C AL AL ( ) Integrating.e. results in…. use N2 blanket . that the timescale of the experiment is several orders of magnitude lower than that of the fermentation. C − − C AL1 = k a(t 2 − t1) ln AL L C− − C AL2 AL Notes of the Dynamic Gassing Out Method • Non-invasive method. i.We assume that when we conduct dynamic kla determinations. time is dC AL dt ! .• Requires DO probe with fast response • CL(1) determined by critical DO concentration for organisms for bacteria and yeast mould cultures plant cultures Ccritical ~ 5-10% saturation Ccritical ~ 10-50% saturation Ccritical ~ 10-30% saturation Important: On the degassing stage of the experiment. the specific oxygen consumption rate can be calculated once the biomass concentration is known (easy to evaluate). For the mathematically challenged. the slope of the line in a plot of CAL vs. dC AL dt = −q o 2 X Therefore from the slope of the line in this region (if both axes of the graph are in the correct units). At this point.Doran 9.5 130 73.5 50 70.5 .5 20 53. Time (s) % Saturation 10 43.5% saturation. calculate the gas-liquid mass transfer coefficient for the reactor.5 – Dynamic kLa measurement An experiment was performed on an exponential phase microbial culture. From the following data of the reoxygenation stage.5 30 60 40 67.5 60 72 70 73 100 73. aeration was resumed and the increase in DO concentration was monitored with respect to time. where the oxygen supply was disconnected and the DO concentration was allowed to fall to 43. 80 70 60 % Saturation 50 40 30 20 10 0 0 50 Time (s) 100 150 Figure 1. Plot of % saturation vs. time Solution: Whats the quasi-steady state O2 concentration? 73. Let 10s = t1 Plot C − − C AL1 vs(t 2 − t1) ln AL C− − C AL2 AL .5% sat. 0611 s-1 Note: due to the form of the equation the line has to intercept at the origin.5 2 1.5 4 3.9502 2 Slope is equal to kLa – 0.4.0611x R = 0.5 3 ln. ...5 0 0 20 40 (t2-t1) (s) 60 80 y = 0.5 1 0. 2.