## 1 Introduction  
Climate change due to the rapid accumulation of greenhouse gases in the atmosphere since the industrial revolution represents a major challenge for our society. The main anthropogenic greenhouse gas is CO2, which is predominantly produced from the massive burning of fossil fuels and, to lesser extent, biomass. CO2 is also naturally present, for example, in natural gas. Indeed, a significant part of the world's natural gas reserves are sour, which means they contain CO2 and/or H2S. The CO2 associated with un- or underdeveloped sour gas fields worldwide has been estimated to total 15 Gt CO2, which is quite substantial [1]. Even before climate change became a major societal issue, scientists had developed technologies to remove CO2 and H2S from high-pressure natural gas so that the gas would meet international commercial and safety standards. Thus, several separation techniques exist or are under development to remove acid gases, such as absorption, adsorption, membranes, cryogenic separation, etc. [2, 3, 4, 5, 6, 7]. The same technologies can be applied to remove CO2 from low-pressure flue gas, remove acid gases from biogas, purify syngas, etc. The concept developed in this paper will be applied to natural gas treatment as an example. For acid gas removal from natural gas, the most mature and widespread technology today is absorption by aqueous amine solutions [6, 7]. Several types of amines have been studied. This work focuses on primary or secondary amines that react with CO2 via electrophphilic addition (Lewis base) followed by deprotonation to form carbamate AmCOO\({}^{-}\). The reaction mechanism is as follows [8]:Electrophilic addition (formation of zwitterion):  
\[RR^{\prime}NH+CO_{2}\leftrightarrow RR^{\prime}NH^{+}COO^{-} \tag{1}\]  
Followed by deprotonation:  
\[RR^{\prime}NH^{+}COO^{-}+RR^{\prime}NH\leftrightarrow RR^{\prime}NCOO^{-}+RR^{ \prime}NH_{2}^{+} \tag{2}\]  
The reaction with H\({}_{2}\)S is a classical acid-base reaction:  
\[RR^{\prime}NH+H_{2}S\leftrightarrow RR^{\prime}NH^{+}+HS^{-} \tag{3}\]  
For natural gas, the removal of CO\({}_{2}\) and H\({}_{2}\)S with aqueous alkanolamines takes place in two stages in an acid gas removal unit (AGRU). First, the acid gases are chemically absorbed by the solvent in an absorption column. This process is spontaneous and exothermic. The sweet gas, depleted in CO\({}_{2}\) and H\({}_{2}\)S, can be sent to a pipeline or to a liquefaction unit. The solvent enriched in CO\({}_{2}\) and H\({}_{2}\)S is subsequently sent to a solvent regeneration column, also called a stripper, in which the acid gases are desorbed and separated from the amine solution. This process is endothermic, which means that energy needs to be supplied. Therefore, the amine solution is heated up to \(\sim\)400 K in a reboiler. In addition, water vapor is generated, which helps the acid gases migrate from liquid to gas phase. The regeneration column is operated near atmospheric pressure [7]. Once the acid gases are separated from the natural gas, the H\({}_{2}\)S must be further separated because it is extremely toxic. The acid gases are therefore sent to a sulfur removal unit (SRU) to convert H\({}_{2}\)S into solid sulfur. There exist many H\({}_{2}\)S conversion processes. In this paper, as an example, the Claus process will be described, which is a standard process for large gas treatment plants [9; 10]. This unit is divided into two stages: a thermal oxidation stage, followed by a catalytic stage.