For several years, new processes have been emerging in the field of gas solid fluidization, involving complex gaseous chemical reactions, more often at high temperatures. As an illustration of these new technologies, the particular case of silicon CVD (Chemical Vapor Deposition) from silanes will be described. Traditional models of fluidized beds are intrinsically unable to represent such complex operations. Hence, as a first attempt, a new model has been developed to locally calculate mass transfer inside and around an isolated bubble rising through the fluidized bed. Examples of results in the case of CVD are presented. They confirm the applicability of the approach, which however needs further improvements.