We consider the long-time behaviour of a free-molecular gas in a time-dependent vessel with absorbing boundary, in any space dimension. We first show, at the theoretical level, that the convergence towards equilibrium heavily depends on the initial data and on the time evolution law of the vessel. Subsequently, we describe a numerical strategy to simulate the problem, based on a particle method implemented on GPGPU. We observe that the parallelization procedure on GPGPU allows a marked improvement of the performances when compared with the standard approach on CPU.
