The fluidization quality decreases drastically when the reactions involving a gas volume decrease were carried out in a fluidized bed reactor. The model to describe the defluidization behavior for reactions involving gas-volume reduction is highly required to choose proper operation parameters. Herein a phase condensation model is proposed to describe the fluidization and the transfer behavior of the chemical reactions involving a gas volume decrease when they are carried out in a fluidized bed reactor. The phase condensation model concerns an emulsion phase domain. The mass transfer between bubbles and the modeling domain, the governing equations for the hydrodynamics and mass transport, the Brinkman equation, together with the kinetic sub-model are incorporated in the model. The simulation result captures the spatial distribution of reactant concentration and gas velocity which is onion-shaped for the CO2 hydrogenation reaction. The fluidization quality can be predicted by the phase condensation model. The result indicates that lower temperature and higher inert gas content in the feed gas are favorable for good fluidization. A phase diagram of fluidization quality is obtained using temperature and expansion factor as indices. The phase condensation model renders the precise prediction which compares well with published experimental data. (C) 2012 Elsevier B.V. All rights reserved.