## 3 Simultaneous and Selective H\({}_{2}\)S-CO\({}_{2}\) Removal: A Case Study  
In practice, it is normal that both H\({}_{2}\)S and CO\({}_{2}\) appeared together in gas streams; thus, the removal of both from the stream can be accomplished by two different approaches: (i) selective H\({}_{2}\)S-CO\({}_{2}\) removal and (ii) simultaneous CO\({}_{2}\) and H\({}_{2}\)S removal. Some proposals about the two styles of processing are summarized in Tables 5 and 6. Since both gases have similar affinities and characteristics, the simultaneous removal of both species may have a series of benefits, since this removal can be carried out using known technologies: absorption, adsorption, and membranes. It is said in [69] that the removal of H\({}_{2}\)S has a higher cost than that of CO\({}_{2}\), costs which are increased by the operating and capital expenses in relation with what is used in the removal process. Thus, the removal of H\({}_{2}\)S and CO\({}_{2}\) is possible by the use of known carbon capture and storage technologies; also of consideration is the fact that CO\({}_{2}\) and H\({}_{2}\)S competed in the transport, which on one hand points to the influence of CO\({}_{2}\) with respect to H\({}_{2}\)S removal and, on the contrary, H\({}_{2}\)S towards CO\({}_{2}\) removal [26]. The above implies that environmental laws were not fulfilled in relation to tolerable limits, which at the end resulted in an increase of the operational costs. Some other investigations about the selective H\({}_{2}\)S/CO\({}_{2}\) removal are summarized next. As is mentioned above, ionic liquids are a series of chemicals proposed to remove H\({}_{2}\)S from gas streams. Some of these chemicals are investigated to separate CO\({}_{2}\) and H\({}_{2}\)S from methane and water [70]. Density functional theory investigations demonstrated the selective separation of H\({}_{2}\)S from CO\({}_{2}\). This gas bonds to the nitrogen of the anion of the ionic liquid and forms a new C[sbm]N bond, and H\({}_{2}\)S transfers a proton to one of the nitrogen atoms of the ionic liquid; thus, HS' is formed. Other simulations show that methane has weaker interactions with this type of chemical than CO\({}_{2}\) and H\({}_{2}\)S. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Raw Material** & **Plasticizers** & **Dopants** & **Acronym** \\ \hline Alkali lignocarbon and polyvinyl alcohol & Glycerol and water & nano-CuO and Cu\({}^{2+}\) & CLA/PVA \\  & & & Cu-CLA/PVA-1 \\  & & & Cu-CLA/PVA-2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Different alkali lignocarbon-based materials used in the removal of H\({}_{2}\)S. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technology** & **Characteristics** & **Industrial Use** & **Reference** \\ \hline Physical absorption & Dimethyl ether of polyethylene glycol & Yes & [65] \\  & N-methyl-2-pyrrolidene & Yes & [65] \\ Ionic liquid absorption & Ionic liquids & No & [66] \\ Hybrid membrane & Supported liquid membranes & No & [67] \\ Adsorption on pristine materials & Iron-oxide-based materials & Yes & [26; 68] \\  & Molecular sieves-based materials & Yes & \\ Adsorption on composite materials & Metal oxide/silica & No & [26; 68] \\  & Metal oxide/activated carbon & No & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Technologies for the selective H\({}_{2}\)S-CO\({}_{2}\) removal. The selective separation of H\({}_{2}\)S from an acid gas mixture using an acid gas enrichment unit (AGEU) was investigated in [71]. The operation uses absorption with NaOH solutions; however, the addition of Na\({}_{2}\)CO\({}_{3}\) and NaHCO\({}_{3}\) to the alkali solution avoids CO\({}_{2}\) absorption and thus favored the selective separation of H\({}_{2}\)S. Different operational results are included in the work. H\({}_{2}\)S is selectively separated using a column filled with 2-tertiarybutylamino-2-ethoxyethanol [72], and the results are compared to those derived by the use of methyldiethanolamine as an absorbent.