Effect of fluiddynamic conditions on growth rate and biodesulfurization capacity of Rhodococcus erythropolis IGTS8

• R. erythropolis growth rate under different hydrodynamic conditions is modeled.
• Growth rate is strongly depending on stirrer speed but it is hardly affected by the air flow rat.
• Biodeslfurization capacity is also strongly affected by the stirrer speed.
• OTR, OUR, mixing and hydrodynamic stress are joined to explain the results.
• The best results are reached for stirrer speeds between 250 and 450 rpm.

The growth rate and desulfurization capacity accumulated by the cells during the growth of Rhodococcus erythropolis IGTS8 under different fluid dynamic conditions in a stirred and sparged tank bioreactor have been studied. Hydrodynamic conditions were changed using different stirrer speeds and gas flow rates. It was observed that the growth was strongly dependent on the stirrer speed employed. Oxygen transfer limitation was observed at low stirrer speeds (from 100 to 250 rpm). In contrast, at higher stirrer speeds, cell damage was caused by hydrodynamic stress in the turbulent bulk of the broth, yielding again a decrease in growth for stirrer speeds higher than 450 rpm. Moreover, increasing the agitation from 100 to 450 rpm has a positive influence on the development of the desulfurization capacity of the cells during growth, yet this capacity shows a dramatic decrease for higher stirrer speeds. Nevertheless, the change of the air flow rate hardly has any influence on the growth rate and no hydrodynamic stress effect has been detected between 1 and 10 L min−1. A regime analysis of the characteristic times for oxygen mass transfer, oxygen uptake and mixing under different agitation conditions has been made. It was found that the minimum stirrer speed necessary for a satisfactory performance of the bioreactor from the point of view of the cells oxygen demand was 350 rpm, while at stirrer speeds over 450 rpm, growth rate and desulfurization capacity are both negatively affected.