Within a framework of mechanics of multiphase media a mathematical model of the dynamics of a helium and water vapor mixture flowing through a bed of composite adsorbent, based on microspheres and aluminum oxide, is derived. It is shown how to coincide the flow in porous media of adsorber and gas diffusion in a cylindrical granule of adsorbent taking into account helium adsorption by microspheres and water vapor by a porous surface of an adsorbent.