Modeling of the Kinetics for Methanol Synthesis using Cu/ZnO/Al₂O₃/ZrO₂ Catalyst: Influence of Carbon Dioxide during Hydrogenation

Abstract

A kinetic model for methanol (MeOH) synthesis over Cu/ZnO/Al2O3/ZrO2 catalyst has been developed and selected to evaluate the effect of carbon dioxide on the reaction rates due to its high activity and stability. Detailed kinetic mechanism, on the basis of different sites on Cu for the adsorption of carbon monoxide and carbon dioxide, is applied, and the water−gas shift (WGS) reaction is included in order to provide the relationship between the hydrogenations of carbon monoxide and carbon dioxide. Parameter estimation results show that, among 48 reaction rates from different combinations of rate determining steps (RDSs) in each reaction, the surface reaction of a methoxy species, the hydrogenation of a formate intermediate HCO2, and the formation of a formate intermediate are the RDS for CO and CO2 hydrogenations and the WGS reaction, respectively. It is shown that the CO2 hydrogenation rate is much lower than the CO hydrogenation rate, and this affects the methanol production rate. However, carbon dioxide decreases the WGS reaction rate, which prevents methanol from converting to dimethyl ether, a byproduct. In such a way, a small fraction of carbon dioxide accelerates the production of methanol indirectly within a limited range, showing a threshold value of the CO2 fraction for the maximum methanol synthesis.

Kinetic Modelling at the Basis of Process Simulation for Heterogeneous Catalytic Process Design

“…In order to enhance the hydrogen yield avoiding the direct ethanol combustion, only water should be considered as an additional feedstock, without oxygen. Dry Reforming (DR) of methane, in turn, is often operated in such conditions as to make methane a relatively stable specie, which is often converted in separated reaction stages: SR and DR followed by methanation originate then two different reforming process [4]. Starting from a first-principle thermodynamic analysis and keeping in mind this difference, the equilibrium mixtures arising from a given feed composition can be calculated at least in three ways: 1) any species possibly originated from SR, DR, water gas shift (WGS) and methane conversion (occurring simultaneously) is quantified according to its Gibbs Free Energy, 2) only chemicals are considered originated from ethanol SR plus WGS (methane not present, as it were fully converted in the same step), 3) from DR, with full conversion of ethanol into CH4 and CO in a pre-reformer, followed by a separate WGS and MET equilibration stage.…”

“…The calculations originally performed with PROII [4] have been replaced with similar ones performed in ASPEN Plus ® -the same reactor type and SRK thermodynamic model were used. Notice that the calculation scheme shown in Figure 2 of the cited work may not treat rigorously the formation of coke, since two equilibration stages are shown, but only one is modeled via a Gibbs reactor, while nothing is specified about the capability of other PROII reactor classes to mix solid and vapor phases.…”

“…Table 1: Equilibrium composition (mol/moltot) of a mixture derived from ethanol reforming under the assumptions specified above; in case 3 Water is fed separately to the second reactor (rather than being treated as inert in the first block). The two basic Ethanol SR and DR flowsheets are reproduced in Figure 3 (adapted from [4]). The Product-to-Feed auto-heat exchange is the usual solution when dealing with and endothermic reaction (that requires a pre-heating) which gives byproducts that are still hot enough and if they are not fully oxidized, then can also be used as an additional fuel to sustain the needed heat input, economising fresh and higher-value fuels.…”

“…The Product-to-Feed auto-heat exchange is the usual solution when dealing with and endothermic reaction (that requires a pre-heating) which gives byproducts that are still hot enough and if they are not fully oxidized, then can also be used as an additional fuel to sustain the needed heat input, economising fresh and higher-value fuels. With these configurations and the typical operating condition specified below, the overall material balances obtained are as follows (Table 2, [4]). These results are obtained on the basis of the thermodynamic "atom equilibrium", only, and are therefore useful to find the maximum hydrogen yield or to represent reactors with an excess catalyst hold-up.…”

“…Zhang et al implemented a different kinetic expression for the gas to methanol process, obtained by the combination of the surface reaction of a methoxy species, the hydrogenation of a formate intermediate HCO2 and the WGS reaction [4,5]. The occurrence of possible internal pore diffusional limitation was determined on the basis of the Weisz-Prater criterion and the effective diffusivity for multicomponents mixture was calculated.…”