Methane purification by adsorptive processes on MIL-53(Al)

Highlights

• MIL-53(Al) presents an adsorption capacity of 4.3 mol kg⁻¹ at 3.5 bar for CO₂.

• Fixed-bed experiments revealed high selectivity of MIL-53(Al) material for CO₂.

• Lab-scale VSA cycles were designed to produce 96.5% CH₄ from a 40CO₂/60CH₄ mixture.

• Two industrial-scale PSA processes were designed and optimized by simulations.

• The obtained purity values allow the distribution via pipelines of upgraded CH₄.

Abstract

The main polluting compound in natural gas and biogas is CO₂. Therefore, the removal of CO₂ from these fuels is a major process in the industry for upgrading their energy content. The separation by Pressure Swing Adsorption (PSA) is energy-wise efficient and the porous aluminium terephthalate – MIL-53(Al) MOF has been pointed out as a promising adsorbent to carry out this separation. In this work, MIL-53(Al) tablets (Basolite^® A100) provided by BASF are evaluated to carry out CO₂/CH₄ separation by adsorption. The adsorption capacity of CO₂ and CH₄ was assessed by dynamic experiments in a fixed-bed reactor, carried out at 303 K and pressures up to 3.5 bar. The evaluated material presents an adsorption capacity of 4.3 mol kg⁻¹ at 3.5 bar for CO₂. Fixed-bed experiments adsorption and desorption in helium flow revealed high selectivity of MIL-53(Al) material for CO₂, with a separation factor of 4.1 at 303 K and pressures of 0.1–3.5 bar, thus showing to be promising for a PSA process. The measured single and binary breakthrough curves were simulated with a mathematical model for fixed bed column. Two VSA cycles, both with 4-steps but with different pressurization types were designed to produce 96.5% CH₄ from a 40:60 CO₂/CH₄ mixture; experimental validation confirms a good model prediction. Two industrial-scale PSA processes were designed and optimized by simulations, a case similar to natural gas upgrade (but lower inlet pressure) and biogas upgrade. The CH₄ recoveries were determined as 92.8% and 72.9%. The productivities were estimated as 2.09 and 2.78 mol kg_ads⁻¹ h⁻¹ and the power consumptions as 17.0 and 5.1 W h mol⁻¹_CH4. The obtained purity values allow the distribution via pipelines of upgraded CH₄.

Introduction

CO₂ and H₂S are two of the main polluting compounds in natural and biogas. Therefore, the separation of CO₂ from CO₂/CH₄ mixtures is a major process in the industry for upgrading these fuels energy content (Xiang et al., 2011). Nowadays, adsorption, membrane and absorption processes are the three main technologies for CO₂/CH₄ separation (Li et al., 2011). Pressure Swing Adsorption (PSA) process became a key unit operation for gas separation/purification in the 1960s with the patents of Skarstrom (US Patent no. 2,944,627) (Kima et al., 2013, Skarstrom, 1960) and Montgareuil (FR Patent no. 1,223,261 and US Patent no. 3,155,468) (Guerin De Montgareuil and Domine, 1960, Guerin De Montgareuil and Domine, 1964, Kima et al., 2013). The PSA based separation is considered as a feasible technology to be employed in natural gas and biogas upgrade due to its high energy efficiency, its simplicity, low capital investment cost, and ease of control (Xiang et al., 2011). The content of the impurities, such as ethane, propane, CO₂, H₂S, and N₂ that must be stripped off in natural gas depends on its origin, and frequently has important variations even in a single region (Cavenati et al., 2006). Typical composition of natural gas is presented in Table 1.

Methane produced by anaerobic decomposition of organic matter, in digesters or landfills, the so called biogas/biomethane, has an even wider range of compositions, depending on the origin of the organic matter and the used process. It is mainly composed of CH₄ (40–75%) and CO₂ (15–60%) but other compounds can be found in some quite important concentrations as H₂S (5–20,000 ppm), H₂O (0–10%). Volatile organic compounds, NH₃, N₂, O₂, and CO can be found in trace concentrations (Cavenati et al., 2008, Heymans et al., 2012, Ryckebosch et al., 2011, Santos et al., 2013). However, for methane distribution via pipelines or liquefaction for long distance transport, the impurities of the methane (natural gas or biogas) must be removed, since they can cause corrosion in pipelines, compressors, gas storage tanks and engines (Ryckebosch et al., 2011). As a result, the methane for domestic gas must have 97% purity or higher (Kima et al., 2013). Additionally, the presence of CO₂ also lowers the calorific value of the fuel (Ryckebosch et al., 2011). To inhibit the formation of dry ice and corrosion in the liquefaction step, impurities in the upgraded natural gas/biogas, such as CO₂ and H₂O, must be reduced to ppm level (Johansson, 2008). If the upgrading process does not reach these requirements an extra step is needed before liquefaction. Table 2 presents the typical composition of liquefied natural gas and biogas, including the maximum impurities concentration to be able to liquefy biogas (Johansson, 2008).

Adsorption processes for the CO₂ depletion from natural gas and biogas streams are based on CO₂ selective materials, either equilibrium based or kinetic based selectivity (Cavenati et al., 2006). Carbon molecular sieves, zeolites, aluminosilicates, and titanosilicates are among the most used adsorbents for this separation (Ryckebosch et al., 2011). Besides these classical porous materials, the relatively new class of crystalline porous materials Metal-Organic Frameworks (MOFs) are attracting a great attention in large-scale carbon dioxide separations. To that purpose, Keskin et al. summarized the majority of the existing studies considering the application of MOFs for CO₂-related gas separation (Keskin et al., 2010). The MOFs with the highest CO₂ capacity are Mg/DOBDC/MOF-74(Mg)/CPO-27-Mg>HKUST-1/Cu-BTC>Co/DOBDC/MOF-74 (Co)/CPO-27-Co>Ni/DOBDC/MOF-74(Ni)/CPO-27-Ni>bio-MOF-11 >Zn/DOBDC/MOF-74 (Zn)/CPO-27-Zn>CUK-1>YO-MOF>SNU-M10>H₃[(Cu4Cl)₃-(BTTri)₈] (Keskin et al., 2010). Yet, many of these high CO₂ adsorption capacity materials also present high heat of adsorption, which makes them not an ideal adsorbent. The most studied MOFs for CO₂ and CH₄ separation are Cu-BTC, MIL-53(Al) and its amino form. Although, Cu-BTC presents a high selectivity and moderate/low heats of adsorption, making it a very good candidate to this separation, its moderate steam stability reduces its applicability in natural gas and biogas upgrade, where water is an important contaminant (Li et al., 2014). On the other hand, MIL-53(Al) high steam stability, its moderate/low CO₂ and CH₄ heats of adsorption, and its relatively high selectivity of CO₂ towards CH₄, makes it an ideal candidate (Keskin et al., 2010, Li et al., 2014). Several authors already studied the adsorption of CO₂ and CH₄ in MIL-53(Al) and its amino form, in powder form or in small granulates (Bourrelly et al., 2005, Boutin et al., 2010, Boutin et al., 2011, Couck et al., 2009, Heymans et al., 2012, Peter et al., 2013, Rallapalli et al., 2010, Rallapalli et al., 2011, Ramsahye et al., 2007, Stavitski et al., 2011). Mil-53(Al) and amino-MIL-53(Al) are both flexible materials, this flexibility leads to two main features on the adsorption isotherms. The first interesting feature is a step on the isotherm, leading to two plateaus due to the opening of the structure from the narrow pore (np) to the large pore form (lp) (Bourrelly et al., 2005, Couck et al., 2009, Rallapalli et al., 2011, Trung et al., 2008). This “opening” pressure depends on the guest molecule and temperature. The second main feature is that the adsorption branch of the isotherm and the desorption branch of the isotherm do not close, presenting a hysteresis (Couck et al., 2009). The flexible behaviour of both structures has been deeply studied by Boutin et al., 2010, Boutin et al., 2011. They presented generic temperature loading phase diagrams for both structures, and predicted that the breathing effect in MIL-53(Al) and in its amino form, is a very general phenomenon, which should be observed in a limited temperature range unrelatedly to the type of adsorbates because it is expected that the affinity of any adsorbate for the np form of the framework is always higher than for the open lp structure (Boutin et al., 2010, Boutin et al., 2011). The presence of amino groups leads to a decrease of the pore volume (Boutin et al., 2011). Therefore, the adsorbed amount of CO₂ decreases 1.5 molecule per unit cell when the amino group is introduced in the MIL-53(Al) structure (Bourrelly et al., 2005, Boutin et al., 2011). Peter et al. (2013) reported almost infinite selectivities from binary breakthrough experiments for MIL-53(Al)_NH₂ at low pressures, significantly higher than for 13X zeolite. However, the selectivity values decrease drastically (even below the ideal selectivity obtained from the adsorption equilibrium isotherms) when performing the same type of experiments at high pressures. Nevertheless, the desorption behaviour of CO₂ from MIL-53(Al) type structures underlined the easy regeneration of those materials when compared to 13X zeolites, which requires very low pressures to fully regenerate. Finsy et al. (2009) identified two distinct selectivity mechanisms in the separation of CO₂ and CH₄ with MIL-53(Al) granulates. At low pressures (<5 bar) they reported selectivities of about 7, in this pressure range the adsorption of CO₂ is due to strong specific interactions of the molecules with the framework hydroxyl groups, while CH₄ is adsorbed in an unselective way. At high pressures, the increasing adsorption of CO₂ reopens the framework leading to a sudden increase in adsorbed amount. Consequently, the average separation factor decreased from 7 to 4 (Finsy et al., 2009). Pellet and powder forms of commercial MIL-53(AL) (Basolite^® A100) were compared by Heymans and co-workers, they obtained lower adsorption capacities for pellets than for powder, however similar separation selectivity was observed for both forms (Heymans et al., 2012). Although, the authors measured the isotherms of both components above 40 bar, no step is observable on the reported adsorption equilibrium isotherms. Indeed, the authors used the Toth model to fit the obtained equilibrium data, taking in consideration only one adsorption site (Heymans et al., 2012). One may conclude that the commercial form of this material has not the typical reported breathing behaviour of the MIL-53(Al). Indeed, the absence of breathing behaviour in MIL-53(Al) material electrochemically synthesized at lab scale has also been reported recently by Martinez Joaristi et al. (2012).

In this work, a MIL-53(Al) (Basolite^® A100) shaped sample is evaluated for CO₂/CH₄ separation by PSA. First, the adsorption equilibrium of both components was studied by means of dynamic method (single component breakthrough experiments). Second, binary fixed-bed adsorption experiments were carried out, and these were used to validate the fixed-bed mathematical model. Afterwards, two 4-step PSA experiments were carried out at lab-scale to validate the separation efficiency and the global PSA model. Finally, industrial-scale column sizing was performed and industrial PSA cycles were designed and simulated, having in consideration two different scenarios: biogas upgrade and a scenario with composition and flow rates similar to natural gas, but lower feed pressure.