In this work, a two-dimensional pore network model is developed to study the heat and mass transfer inside a capillary porous wick with opposite replenishment in the dry-out regime. The mass flow rate in each throat of the pore network is computed according to the Hagen-Poiseuille law, and the heat flux is calculated based on Fourier's law with an effective local thermal conductivity. By coupling the heat and the mass transfer, a numerical method is devised to determine the evolution of the liquid-vapor interface. The model is verified by comparing the effective heat transfer coefficient versus heat load with experimental observations. For increasing heat load, an inflation/deflation of the vapor pocket is observed. The influences of microstructural properties on the vapor pocket pattern and on the effective heat transfer coefficient are discussed: A porous wick with a non-uniform or bimodal pore size distribution results in a larger heat transfer coefficient compared to a porous wick with a uniform pore size distribution. The heat and mass transfer efficiency of a porous wick comprised of two connected regions of small and large pores is also examined. The simulation results indicate that the introduction of a coarse layer with a suitable thickness strongly enhances the heat transfer coefficient.