Methanol process design

Methanol can be synthesised by hydrogenation of CO2 (equation 2.1) or CO (equation 2.2). The two reactions are linked via the reverse water gas shift (RWGS) reaction (equation 2.3), so that both hydrogenations can always take place, even if only CO2 or CO is added as the reactant (OTT et al., 2012, p. 3f).

CO2 + 3H2 ⇌ CH3OH + H2O Δ𝐻_300𝐾= −49.16 𝑘𝐽/𝑚𝑜𝑙 (2.1) CO + 2H2 ⇌ CH3OH Δ𝐻_300𝐾= −90.77 𝑘𝐽/𝑚𝑜𝑙 (2.2) CO2 + H2 ⇌ CO + H2O Δ𝐻_300𝐾= +41.21 𝑘𝐽/𝑚𝑜𝑙 (2.3) All three reactions are equilibrium reactions and therefore no complete conversion of the educts is achieved. Since both hydrogenations are exothermic, as can be seen from the negative reaction enthalpies ΔH in equation 2.1 and equation 2.2, accompanied by a reduction of quantity of material, high pressures and low temperatures shift the equilibrium to the product side (OTT et al., 2012, p. 4). A

compromise must be found for both parameters, because low pressures are associated with lower investment and production costs and high temperatures improve the kinetics of the process. Therefore, both parameters must be adjusted to ensure the best overall conditions for the process.

The methanol synthesis is decisively influenced by the activity, selectivity and long-term stability of the catalyst. ZnO/Cr2O3 catalysts are used at the beginning of the industrial production of methanol. In the 1960s, these catalysts were replaced by Cu-based catalysts, as Cu is a very active metal for methanol synthesis. Aside from Cu itself, these catalyst systems consist of ZnO and aluminium oxide (Al2O3).

The active areas lie within the Cu centres. The presence of ZnO has a stabilising effect on the Cu. The Al2O3 also stabilises and prevents the sintering of active particles, which would lead to catalyst deactivation. Another possibility of deactivation of this catalyst is by sulphur poisoning, therefore the synthesis gas must be sulphur-free. Today, Cu/ZnO/Al2O3 catalyst systems are almost exclusively used, although improvements are still being discussed and researched.

There are big differences in the composition of these catalyst systems. ICI uses catalysts containing 61% Cu, 30% ZnO and 9% Al2O3. BASF's catalysts are also in similar ranges with 65% to 75% Cu, 20% to 30% ZnO and 5% to 10% Al2O3. However, there are also manufacturers where the Cu has a significantly lower fraction at a minimum of only 25%. (OTT et al., 2012, p. 6)

The reactor is the most important component in all chemical processes. Its design is decisive for an optimisation of methanol production with regard to kinetics, thermodynamics, selectivity and catalyst lifetime. In all cases, a compromise must be found between a sufficient reaction rate and sufficient heat removal. High temperatures have a positive effect on kinetics, but low temperatures have a positive effect on thermodynamics. Therefore, the reactors for methanol synthesis must be equipped with an effective temperature control. It is important to remove the heat generated during the exothermic hydrogenation of CO and CO2 and hence maintain the reactor at the desired reaction temperature. To achieve this, adiabatic and quasi-isothermal reactors are primarily used (OTT et al., 2012, p. 10). In addition, research is performed on other reactor concepts, such as membrane, liquid phase and fluidised bed reactors. These are not yet in industrial use, but offer potentials for optimising methanol synthesis in the future (BOZZANO et al., 2016).

In adiabatic reactors, no active cooling of the reaction zone takes place. Instead, cooling is performed by injecting the educt mixture at various points in the reactor,

as shown in Figure 2.1, a). The injected gas is characterised by a lower temperature than the mixture already in the reactor that is the result of an exothermic hydration.

This type of reactor provides a simple way of controlling the temperature. However, the temperature is not constant over the whole reactor, but shows a saw tooth-shaped pattern. The temperature is lowered with each injection, followed by a continuous increase in temperature before cooling is again caused by the next injection. Additionally, parts of the reaction mixture are injected time-delayed and therefore have a shorter retention time to react and do not pass through the entire catalyst bed. Consequently, the conversion is lower than in reactors with an active cooling system. (HANSEN et al., 2008, p. 2939f)

Figure 2.1: Possible reactor types for methanol synthesis, a) adiabatic reactor, b) quasi-isothermal reactor.

Caption: own presentation based on HANSEN et al. (2008, p. 2940).

Quasi-isothermal reactors are used to achieve better temperature control. In these reactors, the entire reaction zone is kept at an almost constant temperature. This is achieved by active cooling, usually with water vapour. The advantage of cooling with steam is that a simple temperature control by changing the steam pressure is possible. This reactor is usually designed as a standing tube bundle reactor, whereby the reaction medium flows from below through the tubes filled with the catalyst. The cooling medium is passed through the reactor in counter-current on the housing side. The steam generated in this way can be used as an energy source for a compressor or a turbine, or can be used as a heat source for distillation in the

later course of the process. The design of this reactor is shown in Figure 2.1 b). As a modification, it is also possible to pass the reaction mixture through the catalyst on the housing side and to flow the cooling medium through the tubes. In this configuration, gas is often used as cooling medium instead of steam. A further modification is to use double tubes through which the reaction mixture first flows on the inside for heating and then flows back through the outer tube filled with the catalyst, whereby steam is used on the housing side (OTT et al., 2012, p. 10f). The advantages of the isothermal reactor are optimal temperature control, the generation of a small amount of by-products, lower operating costs compared to the adiabatic reactor, and a longer catalyst service life. Disadvantages are that, compared to the adiabatic reactor, the maximum production capacity is lower due to the large total reactor volume through tubes and casings and these reactors are also more expensive to invest due to their more elaborate design (HANSEN et al., 2008, p.

2939).

2.1.3 Projects about renewable methanol synthesis in research and industry