Packed Bed Fluidization Conditions throughout a 4-Step Pressure Swing Adsorption Cycle

One of key problems that arise when designing large, packed beds is the existence of high velocities that may lead into its mechanical disintegration. When gas is fed through a packing material at such a high velocity, the bed can loosen until the particle-fluid mixture expands and behaves as if it were a fluid. When fluidization occurs, the adsorbent grinds against itself and the wall of the column resulting in poor separation. Additionally, dusting, lifting, churning, and coning of beds have all been attributed to fluidization. In this effort, the conditions for fluidization phenomena in a bed of BAX 1100 and the effect of the amount of weight compressing a bed of BAX 1100 were determined. For this purpose, a single bed PSA unit specifically designed to evaluate pressure drop and fluidization conditions was constructed from top to bottom of 5” of 6 mm glass beads, 65” of BAX 1100, and 5” of 3A Zeolite.

To determine the conditions of fluidization, a 4-step pressure swing adsorption cycle was created. First, the bed undergoes a pressurization (Press) step, wherein the bed is pressurized from the top of the column from atmospheric pressure (14.7 psia) to 70 psia. Next, the bed undergoes a feed step, wherein the bed is fed from the bottom of the column at a constant flow rate at 70 psia while simultaneously flowing gas out of the top of the column. Then the bed undergoes a co-current depressurization (CoD) step wherein the bed is depressurized in the same direction of the feed from 70 psia down to 40 psia out of the top of the column. Finally, the bed undergoes a counter-current depressurization (CnD) step wherein the bed is counter-currently depressurized back to atmospheric pressure through the bottom of the column. These steps were individually tested in the cycle to determine for the Press, CoD, and CnD steps the shortest time the step pressure can switch between the targeted values or for the Feed step at what flowrate the step can be run at without fluidization conditions occurring. Keeping the bed height of 75”, runs were tested with 2”, 5”, and 10” of glass beads while adjusting the height of BAX 1100 accordingly. In the end, Feed fluidization velocity did not appear to be strongly dependent on the weight of glass beads while the CoD fluidization step time did.