## 2 State of the Art
### Methanol Synthesis  
In the synthesis step, carbon monoxide and carbon dioxide react with hydrogen according to the exothermic reactions (2) and (3). Additionally, both reactants are involved in the water gas shift reaction (4), which strongly influences the reaction mechanism. The generation of methanol is limited by chemical equilibrium and favored at low temperatures and pressures. However, in order to obtain significant reaction rates, the synthesis is conducted in the presence of a Ni/Zn/Al\({}_{2}\)O\({}_{3}\) catalyst at temperatures between 200-300 \({}^{\circ}\)C and pressures between 50-100 bar, resulting in a yield of 5-14 mole-% methanol at the outlet [3,5,23,28]. The crude product can be easily separated from the unreacted syngas through flashing near ambient temperature. The unreacted syngas is recycled to the inlet of the reactor and combined with fresh make-up gas. As the synthesis  
Figure 1: Range of the syngas composition for the commercially applied methane reforming technologies. The composition is expressed by the three main stoichiometric modules. gas contains impurities in the form of inert gases (methane, butane, propane), a small fraction has to be purged from the loop. These features together with the metallurgical restrictions and the catalyst properties determine the methanol synthesis reactor design and the configuration of the synthesis loop. With respect to this study an indirect-cooled pseudo-isothermal reactor [29; 30; 31] and a direct-cooled adiabatic quench reactor are investigated [32]. The synthesis concepts mainly differ in heat integration, make-up gas introduction and removal of impurities. The isothermal reactor is modeled according to the worldwide commercially used Lurgi Steam Raising Reactor, which is based on a tube shell design. On the shell side, the released heat of reaction is raised as medium-pressure steam, which is typically used in the reboilers of the distillation columns or for driving the syngas compressors. Regarding the second concept, a multistage fixed bed reactor was chosen according to the design of Imperial Chemical Industries' (ICI) reactor. The quench design is the dominating technology for methanol production with a market share of 70%. It consists of up to five fixed catalyst beds which are separated in one reactor vessel. A part of the circulating gas is mixed with fresh, cold syngas and fed to the inlet of the reactor. The remainder is injected as a quench gas between the catalyst beds to ensure an efficient reactor cooling. The sensitivity of the methanol fraction at the reactor outlet to the inlet composition is shown in the ternary composition diagram in Figure 2 for equilibrium conditions of 50 bar and 250 \({}^{\circ}\)C. The area of optimal syngas composition for maximum methanol production is identified by the elongated ellipse. The results of the sensitivity analysis correspond to the stoichiometric measures given above. Furthermore, the syngas compositions for three basic reforming technologies (SMR, ATR and DMR) and the three analyzed cases are highlighted. Both reactor models were integrated under three different scenarios of syngas generation. Due to kinetic limitations, the real methanol fraction at the outlet is far below equilibrium yield. Regarding Lurgi's isothermal reactor, the methanol outlet fraction was 6.9 mole-%, 10.5 mole-%, and 9.4 mole-% for Cases I, II, and III, respectively. A four-bed quench reactor based on [32] is integrated in the Cases I, II, and III in Figure 2. The distribution of the inlet gas and quench gas is subject to an optimization to maximize the methanol yield at the reactor outlet. The simulation results show that the methanol mole fraction in the effluent  
Figure 2: Ternary diagram for the methanol yield at an equilibrium pressure of 50 bar and an equilibrium temperature of 250 \({}^{\circ}\)C for a given inlet composition. The syngas composition for the basic reforming technologies SMR, ATR and DMR and the investigated cases are shown. is in a low range of 4-5 mole-% for all cases.