Quasi-kinetic models deal with processes that are controlled by mass transfer rates rather than by chemical reaction rates. These models assume nearly instantaneous attainment of equilibrium within the region of interest, so changes in the species distribution are controlled by the rate of transfer of substances into or out of that region. These models are constrained by continuity equations making them similar to the chemical reactors models in Chapter 4.

Local equilibrium assumption

Most of the models considered in this chapter rely on the local equilibrium assumption. This assumption requires that the rates of chemical reaction and local mass transfer within the model’s spatial domain are fast relative to the residence time of a slug of solution within that domain. Knapp (1989) and Bahr and Rubin (1987) have evaluated conditions where the local equilibrium assumption is valid and the Knapp treatment is summarized by Zhu and Anderson (2002).

For the local equilibrium assumption to be valid, both the mineral and solution reaction rates and the transport rate to and from the minerals’ surfaces must be fast. These constraints are best tested using the i rst Damköhler number , DaI, and the Péclet number , Pe. DaI compares the rate of consumption (or production) of a species by chemical reaction to the rate of delivery (or removal) of that species by advection.