Before revision:  A stochastic model is given as a Markov population process that is, in general, infeasible to analyze due to the massive state space sizes involved. Building on recent results on fluid approximation, such a process admits a limit behavior as a system of ordinary differential equations , whose size is, unfortunately, dependent on the number of points in the lattice . Assuming an unbiased random walk model of nodes mobility, we prove convergence of the stochastic process to the solution of a system of partial differential equations of reaction-diffusion type .
After revision:  By assuming random walk, we prove convergence to a system of partial differential equations (PDEs) whose size depends neither on the lattice size nor on the population of nodes .