  We investigate a kinetic model for interacting particles whose masses are
integer multiples of an elementary mass. These particles undergo binary
collisions which preserve momentum and energy but during which some number of
elementary masses can be exchanged between the particles. We derive a Boltzmann
collision operator for such collisions and study its conservation properties.
Under some adequate assumptions on the collision rates, we show that it
satisfies a H-theorem and exhibit its equilibria. We formally derive the system
of fluid equations that arises from the hydrodynamic limit of this Boltzmann
equation. We compute the viscous corrections to the leading order hydrodynamic
equations on a simplified collision operator of BGK type. We show that this
diffusive system can be put in the formalism of nonequilibrium thermodynamics.
In particular, it satisfies Onsager's reciprocity relation and entropy decay.
