(227c) Methane and Nitrogen Separation By Gas?SMB in Activated Carbon Monoliths Manufactured Via 3D-Printing

With the rising demand of natural gas, the exploration of unconventional methane-rich sources as increased over the years. These sources commonly contain multiple impurities, with nitrogen posing the highest challenge for removal due to its similar properties when compared to methane. To comply with pipeline standards, nitrogen is extracted if its concentration exceeds 3%. The primary method employed for this separation involves cryogenic distillation, known for its substantial energy requirements that contribute to elevated operational expenses. In addition, this technology is not suitable for medium and small-scale purification of methane streams. In contrast to this technology, adsorption-based processes offer advantages such as reduced energy requirements and easy scalability. However, the performance of the solid materials for CH₄/N₂ separation becomes an issue due to their low selectivity. Hence, simulated moving bed (SMB) technology presents itself as a viable option due to its effectiveness in separating similar compounds, exhibiting selectivities that approach unity.

Activated carbons have shown to be promising adsorbents for selective methane/nitrogen separations. However, to be utilized in the industry, the materials need to be shaped. To date, there is a lack of research regarding nearly isobaric processes for this separation. Thus, this research investigated the performance of 3D-printed Maxsorb activated carbon monoliths with a square shape, in the separation of methane from nitrogen. Fixed-bed tests were performed to assess the single and multicomponent adsorption dynamics and three SMB cycles were conducted. To do so, 48 monoliths with 2.7 x 2.7 x 1.0 cm³ dimensions were packed in 6 columns.

The pure component isotherms of N₂, CH4, Ar, and CO₂ were measured at temperatures of 303, 323, and 373 K and a pressure up to 5 bar, using a gravimetric method. The data was successfully regressed with the Dual-Site Langmuir (DSL) model. The results show that CO₂ has the highest affinity to the stationary phase and Ar the lowest, over the entire range of temperature and pressure studied. Also, to verify if the adsorptive capabilities of each of the printed monoliths were similar, one of the monoliths was fragmented and the adsorption equilibrium of each adsorbate was measured using a volumetric method. Pure component CH₄, N₂, CO₂ and Ar isotherms were collected at 303 and 323 K at a pressure up to 2.5 bar. For all the gases a similarity of adsorption capacities between the two different monoliths was observed (see Figure 1).

Single component, binary and ternary breakthrough tests were performed at 303 K and 1.5 bar, at a total flow rate of 0.200 SLPM. A mathematical model that encompasses mass, energy, and momentum balances was implemented in the gPROMS^® software and utilized to predict the adsorption dynamics of the system. The ternary fixed bed experiments (Figure 2) show that the desorbent is able to be displaced by the mixture and to displace the adsorbed phase, which is an important condition for SMB operation. The simulation results show good agreement with the collected data.

Three simulated moving bed (SMB) cycles were employed to separate an equimolar CH₄/N₂ mixture using each of the desorbent gases to evaluate the impact of the desorbent strength in the process. The experiments were carried out in open loop, with no desorbent recycle. A 1-2-2-1 column configuration was considered, with a switching time of 110 s. The SMB unit was operated at 303 K and pressure up to 2 bar. A feed stream of 0.080 SLPM was used for the argon experiment and 0.040 SLPM for the CO₂ experiment, while operating in open loop mode. The internal profiles for each of these cases are presented in Figure 3. An additional experiment was performed with a desorbent recycle stream (closed-loop), using argon as the desorbent, at a feed stream of also 0.080 SLPM. The open-loop cycles were capable of producing a high purity methane stream (98.5% and 99.2% for the CO₂ and Ar experiments, respectively) with a high recovery (> 96.5%). By operating in closed-loop, the desorbent consumption was reduced in 41%. When argon is used as the desorbent gas, the extract product stream is obtained with a productivity of 6.5 kg·m^-3_ads·h^-1. Although promising results were obtained, the cycles could be further optimized to achieve higher productivity values.