Assessment of CO2/CH4 Separation Performance of 3D-Printed Carbon Monoliths in Pressure Swing Adsorption

In recent years, the benefits of high material loading, digital tuning, and flexibility in geometric design have made three-dimensional (3D) printing an especially promising way of formulating adsorbent monoliths. However, most studies in this area have focused on material development and have failed to evaluate the performances of 3D-printed adsorbent monoliths in cyclic gas separation processes. Thus, this study evaluates the influence of adsorption pressure (3-10 bar), adsorbate superficial velocity (1.40-2.30 cm/s of 60% CH4/40% CO2), and adsorption time (2.5-10 min) on the pressure swing adsorption (PSA) separation performance of 3D-printed activated carbon monoliths for CO2/CH4 separation. The CH4 recovery and productivity were found to both increase with the adsorbate superficial velocity and adsorption time; however, increasing these process variables yielded greater CO2 bypass into the effluent CH4 stream thus reducing the CH4 purity. Meanwhile, the CH4 purity was found to increase with pressure; however, elevating the adsorption pressure gave rise to a greater amount of CH4 co-adsorption and longer cycle times which reduced the CH4 recovery and productivity. As such, the PSA experiments revealed that monoliths exhibited their best performance at 3 bar pressure, 1.40 cm/s superficial velocity, and 5 min adsorption time. Under these conditions, the packed bed generated 100% CH4 purity, 38% CH4 recovery, and 2.3 mmol CH4/h.g(monolith) productivity, which is comparable to other benchmark materials. As another benefit, probe gas experiments from 0.44 to 8.55 cm/s also revealed that printed monoliths with 200 cells per square inch cell density can reduce pressure losses by similar to 60% from 1.5 mm beads. As such, it was concluded that 3D-printed adsorbent monoliths can both reduce the energy consumptions of PSA processes compared to commercial beads while also achieving comparable light species purity, recovery, and productivity. Overall, this study further cements the promise of 3D printing as a pathway for adsorbent structuring and provides a fundamental understanding of 3D-printed adsorbent monolith process performance.