- Pressure drops in a pebble bed reactor type were investigated. Measurements of porosity and pressure drop in the bed were carried out in a cylindrical packed bed facility. Air and water were used as working fluids. The inhomogeneous porosity distribution in the bed and the additional wetted surface introduced by the wall caused variations in pressure drop. The importance of the wall effects and porosity were investigated by using different bed-to-particle diameter ratios. Four different bed-to-particle ratios were used in these experiments (D/d p = 19, 9.5, 6.33 and 3.65). A comparison between the predictions was made using several empirical correlations including the and KTA () in the literature. Analysis of the data indicated the importance of the bed-to-particle size ratios on the pressure drop. The comparison between the present and the existing correlations showed that the pressure drop of large bed-to-particle diameter ratios (D/d p = 19, 9.5 and 6.33) agreed with the original KTA correlation. However the published correlations cannot be expected to predict accurate pressure drop for certain conditions, especially for pebble bed with D/d p (bed-to-particle diameter ratio) 5. An improved correlation was obtained for a small bed-to-particle diameter ratio by fitting the coefficients of that equation to the obtained experimental database. 2012 Elsevier B.V. All rights reserved.