balance transfer 0

Mass Transfer Coefficient Mass transfer coefficients - simplified method to describe complex boundary condition involving flow and diffusion. Mass transfer from a surface to a fluid Mass Transfer Coefficient Flux from surface into the fluid:  Ci  1  Niy y  0   Dij   dS S A  y y 0  -Brackets denote a spatial average. -S is the surface area in the direction of the flux (plane  of constant y = 0 in this case). Now, we define an empirical equation describing the flux: N iy  y  0   k f (C s  C0 ) kf unit = m/s. kf is not an intrinsic property of the system.  Ci  1  Dij   dS Equating these two relations: kf  S  S  Co  S C  y y 0  Mass Transfer Coefficient Non-dimensionalization: Ci C  C s  C0 * y y  L * Results: kf L 1  C *    *  dS  y  Dij S s   y* 0 A new parameter: Sherwood number kf L vL   Sh  f (Re, Sc)  f ( , ) Dij  Dij Mass Transfer Coefficient • Determine Sh experimentally for different system. (Table 7.5) • Estimate flux using the Sh number. – Flux at wall for flat plate= Dij Cs  C0  Sh L Estimation of Flux in the Presence of Convection In many mass transfer problems with convection, the concentration gradient is confined to a thin “boundary layer” of thickness M (M <