Syngas Purification Using MIL-125(Ti)_NH2

Parallel to the removal of a specific component (H2S or CO2) from a gas mixture; the purification of H2 or tuning of (H2+CO) composition by adsorbing all its impurities except the target gas(es) is the main objectives in syngas purification. Depending on their origin, the feed gases can contain a variety of impurities: N2, CH4, C2H6, C2H4, CO, CO2, H2S, NH3, H2O. In commercial applications for hydrogen purification, PSA cycles use layered beds with activated carbons and 5A zeolites. Gas purity can reach 99.999% at 70-80% recovery. Syngas itself can also be processed into high value products, such as methanol or synthetic fuel via Fischer-Tropsch (FT). However, H2/CO ratio in syngas is about ~0.7 while the ideal H2/CO ratio of the incoming syngas for Fischer-Tropsch (FT) synthesis is 2 and the required (H2–CO2)/(CO+CO2) ratio for methanol production is 2.1. Therefore, syngas must be conditioned and PSA processes are alternatives to amine absorption processes. Another aim in syngas treatment can be to purify CO2 to purities above 95% required for Carbon Capture and Storage (CCS). This is a relatively new research field without much experimental data and would support the new EU directive on CO2 quality required for geological storage. The EU project “Dynamis‟, taking into account health and safety considerations as well as technical and storage issues, recommended the following target values: H2S < 200ppm, CO < 200ppm, SOx < 100ppm, NOx < 100ppm, H2O < 500ppm, O2+CH4+N2+Ar+H2 < 4%vol (all non-condensable gases). Thus, the challenge is to find an adsorbent that simultaneously removes all impurities, eliminating the need for a multi-layered separation column.

In this work, MIL-125(Ti)_NH2 supplied by KRICT - Korea Research Institute of Chemical Technology as granulates is proposed as potential adsorbent for this task, aiming an adsorption based process. The titanium(IV) 1,4-benzenedicarboxylate (or terephthalate) MIL-125(Ti)_NH2, with the chemical formula Ti8O8(OH)4-(O2C-C6H3NH2-CO2)6 has a three-dimensional pseudo-cubic structure with highly interconnected porous. This possesses two types of cages corresponding to the octahedral (12.5 Å) and tetrahedral (6 Å) vacancies of a cc packing accessible through triangular narrow windows of ca. 6 Å. The carbon monoxide, carbon dioxide, methane, nitrogen, and hydrogen adsorption equilibrium data and adsorption dynamics (single, binary, and ternary adsorption and desorption breakthrough curves) on this adsorbent were assessed at 4.5 bar and 323 K. MIL-125(Ti)_NH2 granulates present an high selectivity for CO2 and exhibited easy regeneration. The single component, binary, and ternary fixed-bed adsorption experiments were simulated in order to validate the mathematical model. The same column, employed for the fixed-bed breakthrough curves described above, was used for PSA experiments, previously designed by simulation using the validated model. A PSA cycle of four elementary steps was performed. The PSA cycle started with the co-current pressurization with feed, followed by feed, blowdown and purge steps. In the purge step, a stream of pure hydrogen was used countercurrently at low pressure to regenerate the column.