Abstract
Hydrogen sulfide is removed from natural gas via adsorption on zeolites. The process operates very effectively, but there is still potential for improvement. Therefore, in this article, the adsorption of hydrogen sulfide was investigated on eight LTA zeolites with different cation compositions. Starting with the zeolite NaA (4 A), which contains only Na+ cations, the Ca2+ cation content was gradually increased by ion exchange. Equilibrium isotherms from cumulative breakthrough curve experiments in a fixed-bed adsorber at 25°C and 85°C at 1.3 bar (abs.) were determined in the trace range up to a concentration of 2000 ppmmol. From a comparison of the isotherms of the different materials, a mechanistic proposal for the adsorption is developed, taking into account the specific positions of the cations in the zeolite lattice when the degree of exchange is increased. The shape of the isotherms indicates two energetically different types of adsorption sites. It is assumed that two mechanisms are superimposed: a chemisorptive mechanism with dissociation of hydrogen sulfide and covalent bonding of the proton and the hydrogen sulfide ion to the zeolite lattice and a physisorptive mechanism by electrostatic interaction with the cations in the lattice. As the degree of exchange increases, the proportion of chemisorption sites seems to decrease. Above an exchange degree of 50%, only evidence of physisorption can be found. It is shown that this finding points to the involvement of weakly bound sodium cations at cation position III in the chemisorption of hydrogen sulfide.
Highlights
The amount of natural gas used in 2019 was approx. 24% of the global energy consumption [1]
Since sulfur components in natural gases are present in trace concentrations, industrial separation takes place by temperature swing adsorption in cyclic fixed-bed processes
The adsorption of hydrogen sulfide on Linde-Type-A structure (LTA) zeolites with calcium exchange rates of 0%, 19%, 35%, 50%, 70%, 82%, 89%, and 92% was measured at temperatures of 25°C and 85°C
Summary
The amount of natural gas used in 2019 was approx. 24% of the global energy consumption [1]. When used and transported via pipelines or as LNG, natural gas is subject to strict purity requirements that make it necessary to purify the raw gas Sulfur components such as hydrogen sulfide (H2S), mercaptans (CxHx+2SH), and carbonyl sulfide (COS) cause problems during transport and use because of their toxic and corrosive properties. Since sulfur components in natural gases are present in trace concentrations, industrial separation takes place by temperature swing adsorption in cyclic fixed-bed processes. Polar adsorbents such as zeolites, silica gels, and silica-alumina gels are often used because they are usually easy to regenerate [4, 5]. A large number of zeolite structures are known [9]
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