Hindawi Publishing Corporation International Journal of Chemical Engineering Volume 2012, Article ID 569463, 15 pages doi:10.1155/2012/569463 Research Article Effect of Contaminants on the Gas Holdup and Mixing in Internal Airlift Reactors Equipped with Microbubble Generator Surya K. Pallapothu and Adel M. Al Taweel Multiphase Mixing and Separations Research Laboratory, Department of Process Engineering and Applied Sciences, Dalhousie University, Halifax, NS, Canada B3J 2X4 Correspondence should be addressed to Adel M. Al Taweel, al.taweel@dal.ca Received 6 June 2012; Accepted 27 August 2012 Academic Editor: Diego G ´ omez-D´ ıaz Copyright © 2012 S. K. Pallapothu and A. M. Al Taweel. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The impact of contaminants on the gas holdup and mixing characteristics encountered in internal airlift reactors was investigated using a 200 L pilot scale unit equipped with a two-phase transonic sparger capable of generating microbubbles. Small dosages of a cationic surfactant (0–50 ppm of sodium dodecyl sulfonate (SDS)) were used to simulate the coalescence-retarding effect encountered in most industrial streams and resulted in the formation of bubbles that varied in size between 280 and 1,900 μm. Gas holdups as high as 0.14 were achieved in the riser under homogeneous flow regime when slowly coalescent systems were aerated at the relatively low superficial velocity of 0.02 ms −1 , whereas liquid circulation velocities as high as 1.3 ms −1 were achieved in conjunction with rapidly coalescent systems at the same superficial velocity. This excellent hydrodynamic performance represents a 5-fold improvement in the riser gas holdup and up to 8-fold enhancement in the liquid circulation velocity and is expected to yield good mixing and mass transfer performance at low energy dissipation rates. 1. Introduction Virtually all process streams encountered in the chemi- cal/biochemical/process industries contain varying concen- trations of amphiphilic materials (such as alcohols, surfac- tants, organic acids, electrolytes, amines, glycols, proteins, phenols, and finely divided particles) that are introduced as reactants, as impurities in the feed and recycle streams, or formed as products and/or byproducts. Although it is well known that the presence of such materials can significantly impact gas/liquid contacting operations, the manner and magnitude by which these changes take place are still controversial. Consequently, much of the practices prevalent today in gas/liquid contacting are based on infor- mation and observations obtained using relatively clean systems where the equilibrium between bubble breakage and coalescence is quickly approached, a situation that does not truly reflect what is happening in most industrial situations. The fact that there is no definite agreement on even some basic fundamental concepts resulted in gas/liquid contacting operations not achieving their full potential when applied to slowly coalescent industrial streams. For example, whereas some investigators report that the volumetric mass transfer coefficient is positively impacted by the presence of surfactants, others report a negative impact. A significant part of the factors hampering the develop- ment of a systematic and comprehensive understanding of how contaminants impact gas/liquid contacting in industrial situations stems from the strong interaction between the various factors affecting it [1]. Consequently, the influence of the aforementioned contaminants on the specific inter- phase area of contact, a, and the liquid side mass transfer coefficient, k L , could be significantly different. Although there is no clear understanding of the mechanisms by which amphiphilic materials impact k L , there is a general recognition that it is adversely affected in the presence of amphiphilic materials [2–8] Means by which large interfacial area of contact can be generated between the phases are therefore promising avenues by which high k L a values can be achieved in industrial systems. This is usually achieved by increasing the energy input to the system, an approach that can adversely affect the energetic performance of gas