Monolithic Adsorbent-Based Rapid-Cycle Vacuum Pressure Swing Adsorption Process for Carbon Capture from Small-Scale Steam Methane Reforming

Steam methane reforming is the primary route to convert natural gas into hydrogen, both at small and large scales. Most of the effort from the scientific and industrial community has been directed toward CO2 capture from large-scale reforming plants. This Article, therefore, proposes an intensified 8-bed, 14-step vacuum pressure swing adsorption unit to capture CO2 from a high partial pressure stream in a small-scale steam methane-reforming process. The unit employs a monolithic adsorbent, rather than adsorbent pellets or beads; this allows the cycle time to be much shorter than that for pelleted adsorbent systems. The specific energy penalty for carbon capture has been minimized by regulating the blowdown and evacuation pressure, while also meeting the CO2 purity and recovery constraints. The minimum penalty is realized when the CO2 purity and recovery are fixed at the minimum allowable values of 95% and 90%, respectively. The model predicts a specific electrical energy penalty of 9.03 kJ/mol CO2 for a fixed feed processing capacity of 7.10 mol of feed/m(3)/s. The corresponding productivity of the monolithic VPSA process in terms of its CO2 production capacity was estimated to be 0.951 mol CO2/m(3)/s. The productivity was found to be almost double than the values reported in the literature for state-of-the-art MDEA-based capture plants for large-scale steam methane-reforming applications. A dry feed gas was initially assumed for modeling; the presence of moisture was found to cause a drop in CO2 recovery by approximately five percentage points. The monolithic adsorbent system has also been compared to an equally sized fixed bed, packed with pelleted adsorbent. The pelleted adsorbent system was found to have a lower working capacity between the same blowdown and evacuation pressure. The working capacity could only be improved with reducing the regeneration (evacuation) pressure, resulting in a higher vacuum pump penalty.