The role of interphase nitrogen transport in the dynamic measurement of the overall volumetric mass transfer cefficient in air-sparged systems

It has been shown both theoretically and experimentally that interphase nitrogen transport may have a significant influence on the rate of interphase oxygen transport, and thereby also on the value of the volumetric mass transfer coefficient of oxygen, k_la, determined in mechanically agitated bubble fermentors using the variants of dynamic method presented in the literature. The experiments were carried out in 1M KCI solution at five stirrer frequencies and two gas inlet levels. The gas interchanges were performed either without interrupting the aeration and agitation of the charge (A) or with the aeration and agitation of the charge turned on at the same time (B). The applied variants of the interchange were N₂→ O₂→, O₂→ N₂, N₂→ air, air→ N₂, O→ O₂, and O→ air. In the two last variants the oxygen dissolved in the charge was removed by reacting with sulfite ions. The k_la values calculated by allowing for the nitrogen transport for procedure A were approximately equal to the values obtained by disregarding the nitrogen transport, whereas those for procedure B were higher (up to 40%), than the values obtained disregarding the nitrogen transport.