The models conventionally used in the design of equipment for fluid-fluid contacting, such as film and penetration theories, do not account for factors such as interference between neighbouring concentration fields, curvature of the phase interface, etc. In concentrated dispersions and under conditions of large penetration depth, these factors could become important. It is thus difficult, in such cases, to justify a priori the use of the above theories in design. This work examines such situations with the aid of a particle based description which accounts for interference phenomena. The objective is to study the effect of factors usually neglected in the conventional approaches to design, and to provide quantitative criteria to decide when and by how much these approaches will be in error, and whether simple modifications are possible which will ensure their applicability in such situations. The sparged and stirred contactor, with a first-order reaction in the continuous phase, is chosen as an example. It is shown that, while the physical mass transfer rate and coefficient are substantially influenced by bubble curvature and holdup under the conditions examined, provided a physical mass transfer coefficient measured under similar conditions is used, the traditional models can be used with confidence to predict the effect of chemical reaction.