A plug-flow reactor operates under steady state conditions. All properties are assumed to be well-mixed on the cross-flow plane, that is, no mass or energy transfer in the transverse direction. Moreover, the gas velocity u is assumed to be large enough so that diffusion and thermophoresis fluxes in the flow direction x are insignificant and can be neglected. Consequently, the transport equation for particles of size j is

(19–122)

The equations for moment and surface species are

(19–123)

(19–124)