OXYGEN TRANSFER, MIXING TIME AND GAS HOLDUP CHARACTERIZATION IN A HYBRID BIOREACTOR
Integration of life sciences & engineering
Integration of Life Sciences & Engineering - Poster (T5-P)
Keywords: Bioreactor, Airlift reactor, Mechanically agitated.
The global oxygen transfer coefficient (kLa), gas holdup () and mixing time were characterized in a hybrid bioreactor (mechanically agitated airlift). The draft tube of the bioreactor was conformed by a filtration module in stainless steal with a pore size of 20m and an irregular geometry in order to increase the filtration area. The total filtration area was 0.162 m2 and the operation volume was 4.5 L for an area-volume ratio of 0.036 m2 L-1. The reactor was agitated with two Rushton turbines, the 6-bladed turbines, 0.075 m diameter were placed at the centerline of the bioreactor vessel. The vertical distance between the impellers was 0.050 m and the lower impeller was located 0.085 m from the bottom of the tank. The bioreactor vessel was rounded bottom, 0.17 m in diameter and its overall height was 0.32 m. The draft-tube, 0.09 m in internal equivalent diameter and 0.015 m tall, was located 0.05 m above the bottom of the tank. The vessel was sparged in the concentric zone through a perforated pipe ring sparger (4 holes of 0.0015 m in diameter located on one concentric sparger). The riser and downcomer area ratio (Ar/Ad) was 0.83. The static liquid height was 0.22 m in all experiments.
For the measurements water and culture medium were used. The mixing time was determined by the acid tracer technique, as the time needed for the tracer concentration to reach 95% of its final steady-state value from the instance of tracer input. Gas holdup, or the volume fraction of gas in dispersion, was measured by the volume expansion method. The overall gas–liquid volumetric mass transfer coefficient (kLa) was measured with the dynamic gassing-in method. The agitation and aeration conditions for water were: stirrer rates 0, 50, 100, 200, 300 and 450 rpm and superficial gas velocities (referred to the riser area UGr) 0, 0.003, 0.004, 0.006, 0.008, 0.010 and 0.012 m s-1; while for the culture media the stirrer rates were the same but the superficial gas velocities (refer to the riser area UGr) were 0, 0.004, 0.008, 0.010 and 0.012 m s-1. Each rehearsal was made by triplicate.
The mechanical agitation may or may not improve the mixing time and the oxygen transfer on both test fluids, nevertheless was observed a decrease in the oxygen transfer efficiency. The highest oxygen transfer efficiencies were gotten at 100 and 300 rpm with a superficial gas velocity of 0.008 m s-1. Although culture media solids (1.4%P/V) decreased up to 20% in the global oxygen transfer coefficient, they did not have any incidence over the mixing time. For both fluids, bubbly flow regime in which the bubbles rose with relatively few interactions among them, persisted until a gas velocity of <0.008 m s-1. At higher aeration rate, the coalesced bubble flow (churn turbulent flow) occurred.
For the proposed reactor correlations for the gas holdup (E) and kLa as function of the agitation speed (N, s-1), solid concentration (Cs, %P/V), superficial gas velocity (UGr, m s-1) or specific pneumatic power (PG/VL, W m-3) and specific mechanic power (PM/VL, W m-3) were gotten for the bubbly and coalesced bubble flows. They showed a good correlation factor (r2 > 94%).
The theoretical consideration of a kLa plot against the gas holdup ratio ε/(1−ε) is expected to be linear in any sparged bioreactor, irrespective of the fluid used and the prevailing flow regime [Chisty, 1989]. In the mechanically agitated hybrid reactor designed, the dependence between kLa and the holdup ratio for the full range of the aeration rates and the turbine speeds tested showed to be higher than the unit in the range from 1.3 to 2.6, possibly by the geometry of the designed filtration module. As a result a better oxygen transfer is expected than in a conventional airlift bioreactor.
The designed reactor is shown as a novel alternative for aerobic fermentations in order to get high cell density cultures and for operations where is require to get biomass for further assays without an environment exposure. This bioreactor can be considered as a prototype for the design and hydrodynamic characterization of a larger bioreactor.
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Presented Wednesday 19, 13:30 to 15:00, in session Integration of Life Sciences & Engineering - Poster (T5-P).
