Fluidized countercurrent solvent extraction of oil pollutants from contaminated soil. Part 1: Fluid mechanics

• Fluid mechanics of solvent–solid flow were studied in countercurrent extraction.
• Axial solid holdup gradient intensified with the increase of solvent flow rate.
• Solid holdup gradient was attributed to the existence of small size particles.
• Operating curves were obtained based on flooding and solid holdup gradient.
• Solid holdup could be estimated by the ratio of slip velocity to terminal velocity.

Liquid–solid countercurrent solvent extraction is a potential application way for the removal of oil pollutant from contaminated soil. This study focuses primarily on the fluidization performance of liquid–solid two-phase flow in an extraction column (2.1 m × 50 mm diameter), with the solvent and solid flow rates ranging from 10 to 800 L h−1 and from 8 to 107 kg h−1, respectively. Several key hydrodynamic parameters, including pressure drop, solid holdup, superficial velocity of the solvent and solid, and dimensionless slip velocity, were investigated experimentally. At a given solid flow rate, a uniform axial distribution of solid holdup was observed at low solvent flow rate, whereas the axial solid holdup gradient increased with the solvent flow rate. The appearance of a solid holdup gradient was mainly attributed to the presence of small particles. An operating curve for the soil remediation by countercurrent solvent extraction was obtained according to the critical flow rate of the solvent and solid for predicting the occurrence of flooding and the appearance of the solid holdup gradient.