An energy cost minimizing strategy for fermentation dissolved oxygen control |
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| Authors: | R. W. Shields E. I. Kao |
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| Affiliation: | (1) Advanced Process Control Technologies Group Fermentation Process Research and Development Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, USA |
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| Abstract: | A cost-minimizing mathematical model for on-line control of dissolved oxygen using agitation speed and aeration rate was developed. In pilot scale monensin fermentation using Streptomyces cinnamonensis, this algortihm provided stable control of dissolved oxygen at 40%, reducing energy usage 27.8%. The agitation and aeration profiles provided by the algorithm respresent the pathway of least energy cost for control at the desired dissolved oxygen level. Other observed advantages of bivariable control were reduction of foaming, evaporation, and gas holdup. Reduced maintenance of compressors and agitator motors could also be expected due to decreased load. Monensin productivity equivalent to fermentation with constant agitation and aeration was not obtained, however, with potency reduced 14.8% with the dissolved oxygen control strategy.List of Symbols A m2 cross sectional area of fermentor - A1, A2, A3, A4 constants of polynomial fit to Calderbank's equations - BP N/m2 gauge back pressure - Cag $/W/s cost of electrical power - CQ $/m3 cost of compressed air - CE mol/m3/s carbon dioxide evolution rate - D m impeller diameter - DO, DOmeas, DOsp % dissolved oxyen saturation at any time, measured, and setpoint respectively - h m height of liquid in fermentor - H N/m2/mmol Henry's constant for oxygen in water - Hav average gas holdup in fermentor - kLa, kLameas, kLksp s–1 oxygen mass transfer coefficient at any time, measured, and setpoint respectively - N, Nsp s–1 agitation speed at any time and setpoint respectively - Na, Na, sp aeration number at any time and setpoint respectively - Ni total number of impellers - Np impeller power number - Ns number of impellers into which air is directly sparged - OU, OUmeas mol/m3/s Oxygen uptake rate at any time and measured respectively - P W ungassed agitation power - Pg, Pg,meas, Pg,sp W gassed agitation power at any time, measured, and set point respectively - Q, Qmeas, Qsp m3/s aeration rate at any time, measured, and setpoint respectively - T K fermentation temperature - ug m/s linear gas velocity - V m3 fermentation liquid volume -  mole fraction of oxygen in fermentation off-gas -  calculation constant -  motor efficiency -  $/s sum of agitation and aeration costs -  kg/m3 liquid density |
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