A simple phenomenological numerical model of a binary granular mixture is developed and investigated numerically. We attempt to model a recently reported experimental system where a horizontally vibrated binary monolayer was found to exhibit a transition from a mixed to a segregated state as the filling fraction of the mixture was increased. This numerical model is found to reproduce much of the experimentally observed behavior, most importantly the transition from the mixed to the segregated state. We use the numerical model to investigate granular segregation mechanisms and explain the experimentally observed behavior.