Effect of material type and particle size distribution on pressure drop in packed beds of large particles: Extending the Ergun equation

The dependence of packed bed pressure drop on variables such as particle size distribution (PSD) and material type are of vital importance in the design of industrial equipment including fixed- and fluidized-bed reactors, blast furnaces and fixed-bed gasifiers. The pressure drop across a packed bed is commonly calculated using the Ergun equation that was developed using experimental data from laboratory-scale beds comprised of small, mono-sized, smooth, non-porous, spherical or nearly spherical particles. In the industrial applications much larger poly-dispersed particles are used, raising the need for a correlation based on more relevant measurements. This study deals with an extension of the Ergun equation to packed beds of large coal, char and ash particles with different average particle diameters and different PSD widths. The research presented here has shown the influence of material type and PSD on both particle properties (sphericity) and packed bed properties (voidage and Sauter mean diameter). In turn these have a significant impact on the subsequent bed pressure drop. It was determined that a bed of ash particles has the highest voidage, followed by the char bed and then coal bed with the lowest voidage. The difference may be attributed to differences in particle sphericity as well as the surface roughness of the particles. In all cases the particle diameter had a lesser effect on bed voidage compared to PSD width, as wider PSD was associated with a lower bed voidage due to smaller particles filling the spaces between the larger particles. New values of the Ergun equation constants were obtained via regression analysis from pressure drop data generated for coal, ash and char particles. The values applicable to coal (77.4 and 2.8), char (160.4 and 2.8) and ash (229.7 and 2.3) particles were found to better approximate bed pressure drop compared to those used in the original form of the Ergun equation (150 and 1.75). The modified Ergun equation can successfully be used to predict pressure drop in a composite packed bed of coal, char and ash particles mimicking the bed structure in an industrial packed-bed gasifier.