Estabilidade de adsorventes de secagem de gás natural: combinando protocolo de envelhecimento com a dinâmica de adsorção em leito fixo.

Abstract

Natural gas is the second most used fuel in Brazil. It is composed mainly of methane and other hydrocarbons; however, it could present some contaminants. Among them, water may cause several problems like heat capacity reduction, transport impairments and corrosion in pipelines and equipment. Water removal may be performed by the TSA (Temperature Swing Adsorption) process, in which case separation takes place by adsorption at low temperatures and bed regeneration occurs at high temperatures upon water desorption. In this work, two kinds of adsorbents typically employed in natural gas dehydration were assessed: LTA zeolite and activated alumina, which were subject to an accelerated aging protocol that emulated a TSA process. The samples were placed in a Parr reactor, where they were humidified, mixed with n-heptane (0.6 mL g -1 ), and pressurized by CO 2 and methane to mimic the natural gas composition. Then, the samples were heated at 250°C and cooled down to room temperature cyclically, samples being withdrawn in the 14 th , 24 th , and 35 th cycles. The Ray-X Diffraction pattern of the alumina sample shows that it underwent a phase transition process after 35 cycles. In the FTIR analysis, both samples present bands around 1560 and 1380 cm -1 after 35 aging cycles, which were associated with PAHs (polyaromatic hydrocarbons), suggesting that organic species were formed inside the samples. Water adsorption

isotherms present the characteristic type II, which is due to the characteristic of macroporous binder material, with LTA zeolite showing the highest CO 2 and methane adsorption capacity, which could be associated with the higher micropore volume in this sample. Breakthrough experiments of CO 2 and H 2 O as single and binary compound were carried out. In CO 2 /H 2 O mixtures, CO 2 adsorption capacity was strongly affected by the vapor water presence, resulting in a reduction in its adsorption capacity of up to 90 %. The column adsorption model, after being validated by the breakthrough experiments, was run using the equilibrium and kinetic data of the LTA and alumina aged samples. In both cases, the dynamic adsorption capacity was consistent with the isotherm outputs. In the LTA, the adsorption amount decreases progressively for increasing aging cycles. On the other hand, the alumina sample showed a significant dynamic and equilibrium capacity reduction in the first 14 cycles, stabilizing at that level of degradation. Furthermore, the aging process also affects the adsorption kinetic due to pore blockage. In such cases, changes in the breakthrough shape may occur, leading to an earlier breakpoint.