Effects of the particle moving on the radial liquid distribution
The liquid maldistribution directly affects the reactor performance due to it stands for the improper contacting of gas-liquid phases over catalyst surface and channelling of the flow. It is vitally important to investigate the liquid distribution in the novel three-phase moving bed. Since the movement of particles will destroy the liquid pockets between the particles, it will inevitably affect the radial liquid distribution. The liquid distribution within the bed is rather difficult to quantify, so it is usually expressed by the exit liquid distribution. The exit liquid distribution is measured by a concentric cylinder collector at the outlet, which has been proved as a suitable method in publications47. To investigate the effects of the particle moving on the radial liquid distribution, the exit liquid distribution was measured at a given gas and liquid mass flow rates, while the solid flow rate was adjusted to various values.
For the graphical display, the exit liquid distribution was expressed as the mass flow rate measured in each section. Illustrative results are shown in Figures 10, where the liquid mass flow rate in each section is plotted versus the solid flow rate. The dotted lines, representing the uniform flow, are equal to the mean mass flow rate through all zones of the bed. In the trickle bed operation (us = 0 mm/s), the liquid preferentially flows near the wall, as the bed porosity is higher near the reactor wall48. As a result, the mean liquid mass flow rate in the outermost annulus (section 4) was higher than that of the entire cross-section. The liquid mass flow rate of the remaining three annular sections gradually increased from the inside to the outside, but it was always smaller than the average mass flow rate of the entire cross-section. This was mainly due to the uneven arrangement of particles. In the moving bed operation, the radial liquid distribution would be more even to some extent due to the movement of the particles. Taking Figure 10(a) as an example, the liquid mass flow rate in section 1 increased with the solid flow rate, indicating that the movement of the particles carrying more liquid into section 1. As the solid flow rate increased, the liquid mass flow rate in section 2 increased firstly and then decreased. When the solid flow rate was lower than 1 mm/s, the movement of the particle carried more liquid into section 2 with the increase of solid flow rate. But further increasing the solid flow rate, the movement of the particles carried liquid from section 2 into section 1. Thereby the liquid mass flow rate in section 2 was reduced. The liquid mass flow rate in section 3 and section 4 decreased with the increasing solid flow rate, indicating that the percentage of wall flow reduced with the increasing solid flow rate. When the feed rate was less than 1.5 mm/s, the liquid distribution in the three-phase moving bed became more uniform than that in the trickle bed. This was because the movement of particles promoted the radial dispersion of the liquid and improved the liquid distribution. However, when the solid flow rate exceeded 1.5 mm/s, the liquid mass flow rate in section 1 would far exceed the average liquid mass flow rate of the entire cross-section with the increase of the solid flow rate. This was mainly because the solid flow rate was so large that a large amount of liquid was carried by the moving particles and flowed into the center of the bed. As a result, the liquid distribution became uneven.