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 chemical/biochemical/process industries contain varying concentrations of amphiphilic materials (such as alcohols, surfactants, 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 information 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 development of a systematic and comprehensive understanding of how contaminants impact gas/liquid
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