Effect of channel size and flow modulation on filtration in monolith reactors with cocurrent gas-suspension downflow

Filtration and catalytic reaction studies of gas/(kaolin-kerosene) suspension flows through monolith reactors were conducted in isoflow-rate and in flow modulation modes in cocurrent downflow. Monoliths with small (1-mm) and large (4-mm) square channel openings were investigated. The effect of deposition of fines on the catalytic conversion in monolith reactors was assessed using the catalytic hydrogenation of α-methylstyrene as a model reaction on 1%Pd-alumina washcoated monolith blocks. The catalytic tests revealed that conversion was a decreasing function of monolith bed specific deposit. To reduce the impact of deposition on catalytic conversion, three feed flow rate modulation strategies (ON-OFF liquid-, ON-OFF gas- and gas/liquid alternating modes) for on-line self-cleaning of deposits were tested. The dynamics of liquid pulsations generated via these cyclic operation strategies was monitored using electrical capacitance tomography imaging. The corresponding liquid saturation, pressure drop and pulsation intensity were compared for clean fines-free two-phase flows. Under isoflow filtration, monoliths with large opening channels were found to favor detachment of kaolin deposit layers and to promote sieving filtration. Instabilities induced by the three flow modulation strategies favored detachment and removal of the deposits only for the large-opening channel monoliths. Conversely, detachment followed by re-deposition downstream in the bed in small-channel opening monoliths aggravated the deposition and pressure buildup in flow modulation. Hence, small-opening channels can be recommended for isoflow filtration of suspensions whereas large-diameter channels are preferable for cyclic operations.