Abstract: In gas separation by adsorption,the gas flow in an adsorber is virtually a variable mass flow.The velocity distribution in an adsorber was investigated by a two-dimensional model with consideration of radial porosity and mass variation caused by sorption.Four conclusions are drawn as follows.(1) Porous media themselves have a role in the gas distribution that they make the gas flow uniformly.But the adsorbent in the feed end is greatly affected by entrance effect,in this region the velocity is in a W-shaped distribution,and some parts of the region are in a fluidization state.(2) During the adsorption step there is a greater change in velocity located at the mass transfer zone,while in the other three steps the change in velocity is approximately linear along the bed.(3) During the depressurization step there is a higher velocity in the bed,which blasts the sorbent and causes the sorbent's attrition and pulverization,so it is better to control the rate of depressurization.(4) Mass variation has important effect on the pressure and velocity,so it can not be easily ignored.