Effect of mixing behaviour on gas‐liquid mass transfer in highly viscous, stirred non‐newtonian liquids

When declaring and using process engineering data such as, for exmple, mixing times or k L a values, it is assumed that these apply to the entire reactor contents. The condition of uniform mixing, necessary for this, is generally met in the regime of turbulent flow. When highly viscous and, above all, strongly non‐Newtonian liquids are stirred in small reactors, there are frequently also regions of laminar flow and completely stagnant zones, which are only partially mixed or not at all. The present paper pursues the question to what extent is the gas‐liquid mass transfer in stirred, highly non‐Newtonian liquids influenced by the mixing behaviour of the reactor. The results show that, below certain Reynolds numbers, three relatively distinct regions exist, with different mixing intensities. Between an almost ideally mixed region in the vicinity of the stirrer and a completely stagnant and dead one, there is a zone of very slight motion. This finding demonstrates that the usual determination of integral or volume‐based data in the literature is neither logical nor adequate for the case under consideration. Thus, the gas‐liquid mass transfer takes place mainly in the relatively well mixed region. Therefore, use of a k L a value also requires the volume of this region to be declared. Only the knowledge of this volume enables us to correlate the k L a values in such a way that the sorption characteristics thus obtained appear suitable for scale‐up. This procedure, which is illustrated by numerous examples of measured results for different stirrers, is important whenever design data for highly viscous liquids are to be worked out on the basis of laboratory measurements. Only when larger equipment is used, in which turbulent flow can be achieved, this problem does not arise because of the relatively uniform mixing of reactor contents.