The aim of this study is to investigate the mass-transfer characteristics of a large paddle impeller by comparing with those of the down-pumping pitched blade turbine and the Rushton turbine, and also to probe the application of surface aeration to batch hydrogenation in polymerization processes and the ethylene oxide additional reaction. Laboratory and industrial data were used for this purpose. As a result, it has been found that, with the large paddle impeller, large k(L)a values of surface aeration without sparging can be obtained continuously at any liquid level because of the combined effect of surface breakage, bubble entrapment and efficient liquid circulation by axial pumping capacity of the impeller. Using these experimental k(L)av data, the improvement in the operation time for a 12 m(3) alkoxylation reactor was estimated. The reactor was then retrofitted with a large paddle impeller, and the actual operation time of 17 h was found to be strictly identical to the estimation, that is 75% reduction of the usual 70 h. This result shows the advantage of large paddle impellers in industrial processes and the accuracy of the estimation procedure. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier BY. All rights reserved.