Gravimetric analysis for the determination of equilibrium adsorption isotherm data and model fitting applied to CO2 capture from the pre-salt natural gas

Abstract

CO₂ separation technologies have an important application in the processing of pre-salt associated gas streams, consisting mainly of methane and carbon dioxide, for natural gas monetization. The Brazilian legislation requires the CO₂ concentration in the natural gas stream to be lower than n = 3% mol for commercialization. An alternative, still little explored for offshore CO₂ capture, is adsorption, whose main challenge is the development of new sustainable-based adsorbents, with high processing capacity and selectivity efficiency, besides economic viability. In this scenario, the present study comparatively evaluated three adsorbents through tests in a gravimetric analyzer coupled to a magnetic suspension balance, on a bench scale, generating unprecedented experimental data for the construction of adsorption equilibrium isotherms, which were used to estimate parameters of classical models. The experimental runs were conducted at temperature T of 293, 328 and 363 K, in a pressure range from p of 0 to 4.7 MPa for CO₂, CH₄, and their binary equimolar mixture. The materials used as adsorbents were: activated carbon prepared from yellow mobim seed, zeolite Y (known as a good CO₂ adsorbent), and a hybrid material of activated carbon and zeolite with a mass ratio of 50/50. The Langmuir, Freundlich, Sips and O'Brien Myers models were initially fitted to the CO₂ adsorption isotherm data at T = 293 K. Then, the influence of temperature and adsorbate on the activated carbon was investigated for the adsorption of pure CO₂ and pure CH₄ at T of 293, 328 and 363 K, using the temperature-dependent Langmuir model. The experimental data of the isotherms of the equimolar binary mixture of CO₂/CH₄ were measured for the activated carbon at these three temperatures, and the IAST and RAST models were fitted to each isotherm. Experimental data measured for pure CO₂ and pure CH₄ were used to evaluate the predictive capacity of these models. The zeolite had a larger CO₂ affinity, i.e., higher adsorption capacity at low pressure (4.58 mmol g-1 at 0.143 MPa). In contrast, the activated carbon had a higher adsorptive capacity for the pressure range (7.03 mmol g-1 at 4.693 MPa). The hybrid material had a combined performance of these two behaviors. Adsorption was more favorable at lower temperatures. The Sips and O'Brien Myers models obtained the best results for the pure components and the binary mixture. The best-fitted models were then used to determine selectivity, revealing that CO₂ exhibits higher selectivity than CH₄ for mixture adsorption on activated carbon at all three temperatures. Along with novel experimental data, the results improved the understanding of the adsorption process by distinguishing the performance of three different materials for CO₂ capture. For activated carbon, a new adsorbent from renewable sources was validated for CO₂, CH₄, and their binary equimolar mixture at three temperatures, contributing to a better understanding of CO₂ separation from CH₄ in pre-salt scenarios. Still, fitted models for adsorption equilibrium isotherms serve as a basis for future studies on cyclic adsorption process simulations.