AN APPARATUS AND A PROCESS WHEREIN A SOLID-LIQUID SLURRY IS CIRCULATED IN A VERTICAL REACTOR BY MEANS OF GAS LIFT ACTION IN A CENTRALLY LOCATED TUBE INSIDE THE REACTOR. ONE OR MORE HYDROCYCLONES ARE USED TO CLARIFY LIQUID PRODUCT LEAVING THE REACTOR AND TO RETURN SLURRY SOLIDS TO THE REACTOR. THE PROCESS IS PARTICULARLY DESIRABLE FOR CATALYTICALLY DESULFUIZING RESIDUAL OILS IN THE PRESENCE OF HYDROGEN.