On the basis of force and energy balance, a generalized approach has been developed for the prediction of pressure drop in fixed, expanded and fluidized beds, packed columns and static mixers. It has been shown that the Ergun type equation can be derived from first principles. i) Pressure drop in fixed, fluidized and expanded beds : Spherical particles Delta P/L d(p)epsilon(L)(4.8)/epsilon(S)rho V-L(2) = 18/Re-p + 0.33 The above equation has been tested and proven over a wide range of voidage and similarly between this equation and equation proposed by Gibilaro et al. (1985) have been pointed out. ii) Pressure drop in fixed, fluidized and expanded beds: Non-spherical particles and static mixers Laminar regime Delta P/L = 180 mu(L) V-L epsilon(S)/phi(2)d(p)(2)epsilon(L)(3) Turbulent regime Delta P/L = 13.5 epsilon(S)(3) rho V-L(2)/phi d(p) epsilon(L)(3) + 3 C-DN infinity epsilon(S) rho V-L(2)/phi d(p) These equations have been tested and proven for packed beds with packings, with or without internal voidages and Kenics and Sulzer type of static mixing elements. The force and energy balance method has been extended for the estimation of hindered settling velocity in particulate (solid-liquid and gas-liquid) fluidized beds. The similarity between these equations and those proposed by Richardson and Zaki (1954) have been pointed out. It has been proposed that, all the column internals, such as packings and static mixing elements can be characterised on the basis of phase hold-ups (Es and epsilon(L)), size, shape factor of internals and the fluid velocity, These parameters are shown to be sufficient to correlate/predict the pressure drop data for different type of reactor systems. The understanding of individual roles of these parameters in altering the pressure drop characteristics is expected to improve the prediction of the mixing, heat and mass transfer performance of these reactors.