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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/m3/s. The corresponding productivity
of the monolithic VPSA process in terms of its CO2 production
capacity was estimated to be 0.951 mol CO2/m3/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.