(327e) H2 Purification By Pressure Swing Adsorption Using CuBTC

H₂ purification by Pressure Swing Adsorption using CuBTC

Bruna Silva (bnogueira@fe.up.pt), Ana M. Ribeiro (apeixoto@fe.up.pt), Jong-San Chang (jschang@krict.re.kr), José M. Loureiro (loureiro@fe.up.pt), Alírio E. Rodrigues (arodrig@fe.up.pt)

Abstract

Hydrogen is one of the most important industrial and potential chemicals, used in many applications, such as hydrocracking, hydrogenation of oils and in the production of methanol and ammonia. Moreover, due to environmental concerns hydrogen has increasingly received attention as it is a promising energy source for electrical power generation and transportation fuel (Ribeiro et al., 2009). Therefore, the demand of hydrogen is continuously increasing with the resulting worldwide attention and research motivation for advances in the field of hydrogen production and purification.

Nowadays, the catalytic reforming of natural gas combined with a water gas shift reaction step is the main and the most cost-effective process for hydrogen production at a commercial scale (Sircar et al., 1999). The hydrogen production by steam methane reforming (SMR) originates a hydrogen stream containing several impurities, such as water vapor, CH₄, CO₂, N₂ and CO. The use of hydrogen in fuel cell applications requires a high purity (99.99+%), which is attained by removing these impurities (Ribeiro et al., 2008). The commercial technology that is widely used for hydrogen purification and separation from steam methane reforming is pressure swing adsorption (PSA) (Ruthven et al., 1994). Several studies have been done focusing on PSA processes for hydrogen separation and purification (Yang and Lee, 1998; Park et al., 2000; Huang et al., 2008; Lopes et al., 2011; You et al., 2012).

A new class of adsorbents named Metal Organic Frameworks (MOFs) represents an excellent alternative to the conventional materials used so far (Silva et al., 2012). The diversity in the configurations of these materials results from co-ordination between inorganic metal atoms and organic ligands or linkers, to form highly porous network structures (Chowdhury et al., 2012; Dasgupta et al., 2012). The use of MOFs is advantageous compared to the utilization of zeolites, since their skeleton accepts almost all the cations of the periodic table (Ferey et al., 2011). Among the different MOFs, CuBTC is one of the few materials that have already commercial availability. This material is composed of copper dimers coordinated to the oxygen atoms of benzene-1,3,5-tricarboxylate (BTC) linkers, forming a regular porous network with a large surface area (BET surface area up to 1600 m²/g), high pore volume, high chemical stability and Lewis acidity (Plaza et al., 2012). Because of these features, this material has a high potential for applications in the field of adsorption, such as gas separation and gas storage (Min Wang et al., 2002; Dathe et al., 2005; Millward and Yaghi, 2005).

In this work, hydrogen purification from mixtures that also contain CO_2, CO, CH₄, and N₂ was performed by PSA with CuBTC as adsorbent (supplied by KRICT). First, the equilibrium adsorption of each pure gas was measured in a magnetic suspension microbalance (Rubotherm, Germany). Second, single component, binary and ternary fixed-bed adsorption experiments were carried out. At the beginning of a breakthrough experiment, a stainless steel column filled with CuBTC and equipped with thermocouples, started to be fed with the gas mixture. During the adsorption and desorption steps, the temperature histories and the molar flow rate were recorded. Additionally, samples were collected in the loops of a multi-port valve for subsequent determination of the molar composition of the exit gas in the gas chromatograph. The same column, employed for the fixed-bed breakthrough curves described above, was used for PSA experiments. 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.

The mathematical modelling of a multicomponent adsorption in a fixed bed involves the material, momentum and energy balances that govern the process, taking into account axial dispersion and mass transfer resistances. The mathematical model developed was applied for the simulation of the breakthrough experiments. This fixed-bed model establishes the basic procedure to simulate adsorption-based cyclic processes. The modeling of a PSA unit involves the same conservation equations used for the fixed-bed simulations coupled with the appropriate boundary conditions for each step. The previously developed model was validated against the results obtained in breakthrough experiments and PSA tests.

The results of the breakthrough experiments showed that the mathematical model describes well the dynamics of the adsorption processes that take place in the fixed bed. The adsorption and desorption showed a good fitting with the simulation results, since the molar fraction, molar flow rate and temperature histories were well predicted by the model. The typical roll-up that appears in multicomponent fixed bed adsorption is also observed and well predicted by the model. The increase of the molar flow rate of the adsorbed gases observed when the desorption step begins was well described by the model. The experimental pressure and temperature histories for all PSA experiments were well predicted by the model. The composition and the molar flow rate of each gas in the different experiments were generally well fitted by the model. In terms of PSA experiments with ternary mixtures, the best performance was achieved for a mixture with 78 % H₂, 18 % CO₂ and 3 % CH₄ and a total cycle time of 516 s: 262 s, 62 s and 192 s for pressurization + feed, blowdown and purge steps, respectively. For this non-optimized cycle, a hydrogen purity of 99.97 % was attained, with a recovery of 45.76 % and a productivity of 6.86 mol.kg^-1.h^-1 (simulation results).

In conclusion, this work demonstrates that the mathematical model developed is a powerful tool for the prediction of PSA experiments, which can save time and money, reducing the number of required experiments.

Acknowledgements

The authors acknowledge the financial support given by FCH JU, through FP7 EU Project Hy2Seps2 - "Hybrid Membrane - Pressure Swing Adsorption (PSA) Hydrogen.

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