(Fr-crit), byproduct yield is equal to the minimum obtained for a submerged feed.Selectivity for several classes of competitive reactions is maximized when the feed is located in a highly turbulent region. The usual practice is to feed close to the impeller with a submerged feed pipe. With submerged feeds, however, many mechanical, metallurgical and/or operational problems must be addressed in detailed engineering design. Surface feed remains an attractive configuration, provided byproduct formation can be suppressed. This study reports several promising configurations for surface feed, all of which achieve high selectivity for the third Bourne reaction. In particular, an up-pumping pitched blade turbine (PBTU) located in the upper third of the tank gives very good results. The up-pumping configuration is robust, with little effect of feed configuration or variations in geometry in the bottom of the tank on the selectivity. There IS maximum impeller speed (NE) above which air bubbles are entrained into and remain trapped within the tank. A modified form of the Froude number, Fr = (NS)-S-2/g, can be used to identify this critical point (Fr-crit = (NES)-S-2/g). When the PBTU is operated at the critical Froude number