A comparison of some methods of estimating volumetric mass‐transfer coefficients in an external‐loop airlift fermenter

Volumetric mass‐transfer coefficients were measured in an 11‐L external‐loop airlift fermenter with deionized water, a fermentation medium, and during a fermentation. Both a Mackareth oxygen electrode and a novel rapid‐response probe were used. When the conventional step‐change dynamic method was used for water, the long, nonlinear response time of the Mackareth electrode made correction of its readings difficult; this problem did not occur when the rapid‐response probe was used. A comparison was made with a method of mass‐transfer coefficient determination which does not involve any assumptions about the gas residence time distribution. However, this method requires that the liquid phase is well‐mixed and this requirement was not met in the airlift fermenter. Comparison of the present results with other KL a determinations for airlift fermenters showed that KL a in water depends on the active gas holdup, the value of KL a/ϵ at 20°C being ca. 0.37 s−1. Although higher gas holdups were obtained with the fermentation medium than for water, the values of KL a/ϵ were lower, ca. 0.22 s−1 at 20°C.