Tackling elemental mercury removal from the wet-gas phase by enhancing the performance of redox-active copper-based adsorbents utilising an operando pre-heating system

Abstract

A simple and facile technology to capture the elemental mercury selectively from moisture-containing natural gas (wet-gas) streams of petrochemical industries is presented. We aimed at exploiting the established redox chemistry of copper(II) towards mercury(0), by utilising simple copper(II) chloride impregnated adsorbents. Tuning the copper coordination sphere with temperature as a control by taking advantage of moisture in the feed, has been successful in enhancing the mercury adsorption through a viable process design. Mercury removal was achieved through chemisorption when the CuCl₂ (Cu(II) : Hg(II) mol ratio = 1.35 : 1) impregnated on solid supports, such as silica, γ-alumina and activated carbon via the wet incipient method and have been used as adsorbents. Supported CuCl₂-based adsorbents were characterised using UV-visible, scanning electron microscopy, energy dispersive X-ray spectroscopy and powder X-ray diffraction, wherein copper complexation was not observed on freshly prepared adsorbents. An in-house experimental setup (wet-gas rig) was designed to perform mercury breakthrough experiments through mercury online monitoring, wherein an add-on moisture saturator was included to allow water vapour in the feed and mimic a wet-gas scenario. In the wet-gas testing rig, two modes of testing were implemented, i.e., with and without pre-heating the incoming gas feed in front of the adsorbent column. The gas pre-heating method introduced for the test under wet-gas conditions resulted in an enhancement of mercury adsorption performance as compared to the dry conditions for selected CuCl₂-containing adsorbents. This enhancement of mercury removal was particularly predominant for CuCl₂ supported on silica, in which the performance increased by four-fold, accounting for a slight pre-heating by 10 °C for the wet-gas in comparison to the test under dry-gas conditions. The redox centre on copper at the hydration sphere, was influenced particularly on the silica support, which consequently resulted in superior reactivity with elemental mercury in the vapour state. Moreover, the gas pre-heating avoided capillary condensation, thereby contributing to the enhancement on mercury adsorbent's life-time. The mercury capture on the adsorbent has been expected to be due to complexation on the silica support that led to a better fixation.