## 2 The Technologies
### Absorption Processes to Remove Hazardous H\({}_{2}\)S from Gas Streams
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Solvent** & **Gas Stream** & **Contactor** & **Temperature** & **\% Capture** & **Reference** \\ \hline NaCl & H\({}_{2}\)S and NH\({}_{3}\) in N\({}_{2}\) & Column & 25 \({}^{\circ}\)C & Near 99 & [13] \\ Fe-EDTA and others & H\({}_{2}\)S and CO\({}_{2}\) in CH\({}_{4}\) & Column & Ambient & 100 & [14] \\ ChCl and others & H\({}_{2}\)S in N\({}_{2}\) & Bubbled stirrer reactor & 30–70 \({}^{\circ}\)C & 100 & [10] \\ MDEA & H\({}_{2}\)S and CO\({}_{2}\) in N\({}_{2}\) & Rotating packed bed & 30–45 \({}^{\circ}\)C & Near 100 & [15] \\ Modified lye & H\({}_{2}\)S in N\({}_{2}\) & reactor & No data & 98 & [16] \\ Yellow phosphorous & H\({}_{2}\)S in N\({}_{2}\) & Bubble reactor & 55–80 \({}^{\circ}\)C & 88 & [17] \\ \hline \end{tabular}
\end{table} 
Table 1: Removal by absorption technologies of H\({}_{2}\)S and other gases. The improvement in the absorption properties of diethanolamine with respect to the removal of H\({}_{2}\)S from sour industrial off gas was investigated in [22]. The influence of lean amine H\({}_{2}\)S impurity (LAHI), lean amine temperature (LAT), and column pressure (CP) on H\({}_{2}\)S removal was studied. The increase of LAHI and LAT is detrimental for H\({}_{2}\)S removal from the gas stream and LAT, LAHI (83%) being the key factor on H\({}_{2}\)S removal, whereas LAT (15%) and CP (2%) have a minor impact on this efficiency. The next investigation used machine-learning operations to study the solubility of H\({}_{2}\)S in fifteen ionic liquids [23]. No less than six machine-learning operations were used, and the respective results were compared. The conclusions showed that the least-squares support vector machine predicted H\({}_{2}\)S solubility into the ionic liquids well with R\({}^{2}\) (0.99798), RMSE (0.01079), MSE (0.00012), RRSE (6.35%), RAE (4.35%), MAE (0.0060), and AARD (4.03). It was found that H\({}_{2}\)S solubility decreased with temperature and has a direct dependence with the pressure. Ionic liquids such as [OMIM][Tf2N] are the best choice for H\({}_{2}\)S capture. The next investigation [24] explored various aspects of the processing and technologies used in acid gas removal (AGR). The work summarized processing by chemical absorption and mechanisms involved in the removal process; it also showed the main amine-based solvents currently used in such tasks. Absorption by physical methods is also discussed, summarizing pros and cons of the most used absorbents. Industrial applications of AGR processes were considered. The removal of H\({}_{2}\)S from industrial gas streams using an iron/copper bimetallic catalytic oxidation desulfurization system was investigated in [25]. The absorbent is formed by adding N-methyl pyrrolidone (NMP) and CuCl\({}_{2}\) aqueous solution to an iron-based ionic liquid (Fe-II). The acidity and viscosity of the system are greatly reduced by addition of NMP and water, improving gas-liquid mass transfer efficiency. The presence of the copper(II) salt increased the oxidative properties of the solution, allowing for the improvement of the Fe\({}^{3+}\) catalytic influence on the oxidation of H\({}_{2}\)S to the monomer form of sulfur.