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In different types of reactors, gas and solid particles are brought into contact and gas-solid mass and heat transfer is to be optimized. This is for example the case with heterogeneous catalytic reactions, the porous solid particle providing the catalytic sites and the reactants having to transfer from the bulk flow to the solid surface from where they can diffuse into the pores of the catalyst [Froment et al., 2010]. Gas-solid heat transfer can, for example, be required to provide the heat for endothermic reactions taking place inside the solid catalyst. Intra-particle mass transfer limitations can be encountered as well, but this chapter will focus on interfacial mass and heat transfer. The overall rate of reaction is on the one hand determined by the intrinsic reaction rate, which depends on the catalyst used, and on the other hand by the rates of mass and heat transfer, which depends on the reactor configuration and operating conditions used. Hence, the optimal use of a catalyst requires a reactor in which conditions can be generated allowing sufficiently fast mass and heat transfer. This is not always possible and usually an optimization is carried out accounting for pressure drop and stability limitations. This chapter focuses on fluidized bed type reactors and the limitations of conventional fluidized beds will be explained in more detail in the next section. To gain some insight in where gas-solid mass and heat transfer limitations come from, consider the flux expression for one-dimensional diffusion of a component A over a film around the solid particle in which the resistance for fluid-to-particle interfacial mass and heat transfer is localized:

Gas-Solid Heat and Mass Transfer Intensification in Rotating Fluidized Beds in a Static Geometry