Estudo da separação da mistura CH4/CO2 pelo processo de adsorção com modulação de pressão utilizando carvão ativado comercial de origem vegetal

Abstract

The separation of the mixture formed by the major components of raw biogas (CH4/CO2) is of great interest because it promotes the upgrading of biogas to biomethane, which is a renewable fuel that can be fed into the natural gas grid and used as a substitute for fossil fuels. Biogas upgrading by pressure swing adsorption (PSA) is widely used. However, adsorbent materials can be costly when designing a PSA unit or inefficient in the presence of minority contaminants such as water vapor. The aim of this work was to study the separation of the mixture by the PSA process using coconut shell activated carbon (CACC), which is produced on a commercial scale and is low-cost. To this end, the adsorbent was first characterized, and it was shown by nitrogen and hydrogen physisorption that 94% of the pore volume is concentrated in the micropore region. Equilibrium parameters were estimated by applying the Langmuir and Langmuir Multisite models to the single-component equilibrium isotherms of CH4, CO2 and H2O at 298 and 323 K, which indicated affinities and adsorption capacities in the order H2O>CO2>CH4. Based on these parameters, binary and ternary equilibrium isotherms were simulated, and the selectivity and adsorption capacity of CACC were calculated in the presence and absence of humidity and then compared with those obtained for zeolite 13X, used as a reference material. The simulations using the IAST-Langmuir model showed good agreement with the experimental binary equilibrium data, and high equilibrium selectivity of CACC to CO2, which increased in the presence of moisture, indicating its viability for wet biogas enrichment. On the other hand, the simulations for zeolite 13X suggested that the material is not suitable in the presence of water vapor but shows greater selectivity than CACC for the CH4/CO2 system. Therefore, the integration of both materials can be useful for biogas enrichment. Diluted breakthrough curves were obtained (1 to 5 bar), with 5 vol% of the adsorbate of interest in balance with Helium gas. These curves were used to estimate the mass transfer kinetic parameters in the macro/mesopores and micropores, which did not vary with pressure within the range tested. Kinetic and equilibrium parameters were used to simulate the breakthrough curves for binary and ternary systems. The model was able to describe well the behavior and the breakthrough of both adsorbates in CH4/CO2 binary breakup curves obtained at pressures of 1 to 5 bar and a temperature of 298 K, from which it was evident that CACC has the potential to separate the CH4/CO2 mixture in a fixed-bed column (continuous flow). In the ternary mixture simulation (CH4/CO2/H2O), the presence of humidity caused a 10% reduction in CO2 adsorption capacity but did not prevent the mixture from being separated and the production of enriched methane. Simulations carried out for the PSA process showed that CACC can provide elevated levels of purity (≈ 92%) and recovery (≈ 89%) of methane in the dry condition, which decreased in the presence of humidity to 76 and 52%, respectively, under the simulated conditions. Finally, it was concluded that the CACC performed well in the presence of moisture and could therefore be applied in an integrated manner with another material that has greater selectivity in the dry condition, as a preliminary stage in biogas enrichment or in a hybrid bed (e.g., activated carbon and zeolite).