The phenomena occurring during the flow of dilute solid-liquid mixtures in a horizontal concentric and 100% eccentric annulus are presented, analyzed and discussed. The experiments were performed in a 5 in long, 7 cm by 4 cm transparent annulus. A range of superficial liquid velocities between 0 and 2.32 m/s were studied, with water and two aqueous Carboxyl-Methyl-Cellulose (CMC) solutions; one with Newtonian rheology (with a viscosity of 6.79 cP) and one with non-Newtonian rheology (with apparent viscosity of 8.3 cP at a shear rate of 170 s(-1)). The solids used were 2 mm glass spheres with sphericity close to one, while the tested solid loading was 2% and 4% w/v. The flow patterns at various experimental conditions were observed visually by video monitoring equipment and were analyzed to produce flow pattern maps. The main parameters affecting the transitions to the particular flow pattern are presented and the conditions for the transition are discussed. A method is presented for flow pattern detection using pressure drop measurements and the positive implications for real-life applications are discussed.