Conversions in downer reactors are lower than what is expected from a pseudo homogeneous plug flow reactor model. This was attributed to particles moving as clusters, axial gas mixing and gas to particle mass transfer. In the present paper, a two phase tanks in series model is developed for the downer reactors considering gas to particle mass transfer. The model is compared with recent experimental observations on a downer reactor reported in the literature to estimate mass transfer coefficients. Estimated Sherwood Numbers are observed to increase as a function of the ratio of gas to particle velocities. This result is in line with the recently reported direct observations on gas to particle mass transfer in downers.