## 2 State of the Art  
The commercial routes for the production of methanol via indirect processes include a cleaning unit for the pretreatment of natural gas (sub-system I), before the conversion to syngas takes place in a reforming unit (sub-system II). Various processes for the production of syngas from natural gas exist, whereby the main technologies are covered within this work. Depending on the reforming technology, the syngas has a typical composition deviating from the stoichiometric composition required by the synthesis unit (Table 1). Therefore, for an adjustment of the composition, several syngas conditioning and reforming technologies are used and combined. The syngas in methanol synthesis should have a balanced composition of CO, CO\({}_{2}\), and H\({}_{2}\), which is expressed by several stoichiometric measures. There is still a disagreement about the ideal composition of a syngas in methanol applications, since the reaction mechanism over CO and CO\({}_{2}\) in reactions (2) and (3) has not yet definitely been clarified. Several studies characterize the composition simply by using the H\({}_{2}\)/CO ratio, which equals 2 in the ideal case. In this approach, the hydrogenation of CO\({}_{2}\) via reaction (3) in the stoichiometric consideration is neglected. Although the methanol synthesis is assumed to proceed mainly over CO, the reaction is 100 times faster in the presence of CO\({}_{2}\)[21]. Taking into account the role of CO\({}_{2}\), a second parameter referred to as stoichiometric module (\(S\) module)  
\[S=(\mathrm{H}_{2}-\mathrm{CO}_{2})/(\mathrm{CO}+\mathrm{CO}_{2}) \tag{1}\]  
\[\mathrm{CO}_{2}+3\mathrm{H}_{2}\rightarrow\mathrm{H}_{2}\mathrm{O}+\mathrm{ CH}_{3}\mathrm{OH}\left(-49.7\mathrm{\ kJ}/\mathrm{mol}\right) \tag{2}\]  
\[\mathrm{CO}+2\mathrm{H}_{2}\rightarrow\mathrm{CH}_{3}\mathrm{OH}\left(-90.7 \mathrm{\ kJ}/\mathrm{mol}\right) \tag{3}\]  
\[\mathrm{CO}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{CO}_{2}+\mathrm{H}_{2} \left(41.0\mathrm{\ kJ}/\mathrm{mol}\right) \tag{4}\]  
is proposed to describe the stoichiometry of both reactions. With respect to commercially used copper-based catalysts, the module ideally has a value of slightly above 2 [22]. Typically, the feed gas composition for methanol synthesis is adjusted to contain 4-8% CO\({}_{2}\) for maximum activity and selectivity [23]. Commercially used catalysts show a high reactivity in reaction (4), whereby CO is involved in the water-gas shift reaction (WGS). Therefore [PERSON] et al. [24] counter, that the stoichiometric module is not adequate to reflect the reactivity of a syngas, since the measure is independent of the WGS reaction. Instead they suggest using the CO/CO\({}_{2}\) ratio for this purpose, whereby the conversion rate increases with the ratio. The effluent from the syngas conditioning unit is compressed in a multistage reactor system to overcome the difference in operating pressure between the reforming unit and the methanol synthesis (sub-system III). The synthesis reactors are based on two major technologies: Direct cooled reactors which are designed as multi-tube reactors and adiabatic fixed-bed reactors with quench gas cooling. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reforming Type** & **Main Reactions** & \(\Delta h_{0}\) \\ \hline \multirow{2}{*}{SMR [25]} & CH\({}_{4}\) + H\({}_{2}\)O \(\rightarrow\) CO + 3H\({}_{2}\) & 206 kJ/mol \\  & CO + H\({}_{2}\)O \(\rightarrow\) CO\({}_{2}\) + H\({}_{2}\) & \(-\)41 kJ/mol \\ \hline \multirow{2}{*}{ATR [10]} & CH\({}_{4}\) + 1.5 O\({}_{2}\) \(\rightarrow\) CO + 2H\({}_{2}\)O & \(-\)520 kJ/mol \\  & CO + H\({}_{2}\)O \(\rightarrow\) CO\({}_{2}\) + H\({}_{2}\) & \(-\)41 kJ/mol \\  & CH\({}_{4}\) + H\({}_{2}\)O \(\rightarrow\) CO + 3H\({}_{2}\) & 206 kJ/mol \\ \hline \hline DMR [10; 26] & CH\({}_{4}\) + CO\({}_{2}\) \(\rightarrow\) 2 CO + 2H\({}_{2}\) & 247 kJ/mol \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the main reforming reactions.