Low temperature WGS catalysts

High- and low-temperature WGS catalysts based on Fe and Cu respectively, require slow and carefully controlled activation procedures. After reduction they are highly reactive toward air and can be a fire hazard to the consumer. 

Power law model also provides good kinetic fit for the low-temperature WGS catalysts. Ovensen et al. [53] proposed microkinetic model based on surface redox mechanism and also evaluated the macroscopic power law kinetic model which was found to be an excellent representation of the kinetic data. Koryabkina et al. [54] determined the kinetic parameters for power law expression using catalysts based on copper over different supports. These authors suggested that there was a strong inhibition on the reaction rate by the products. They also proposed that the kinetics could be explained by a redox mechanism. The kinetic parameters obtained from different works are summarized in Table 9.5. 

TABLE 9.5 Kinetic Parameters Obtained from the Power Law of the Low-Temperature WGS Catalysts... 

Today s low- and high-temperature WGS catalysts are sufficiently active and stable for use in stationary facilities for 2-10 years before requiring replacement. The incentive for companies to develop catalysts that last longer than 10 years is limited. Even so, it is reasonable to expect some advance to occur within the next several years just as it did when Cu was added to the iron-chromia high-temperature catalysts about 20 years ago. 

Figure 4.42. The turnover frequencies for the low-temperature WGS reaction as a function of adsorption energies of oxygen and carbon monoxide. The positions of the step sites on noble and late transition metals are shown. As observed experimentally only copper appears to be a suitable pure metal catalyst for the process. Adapted from [139].

Ayastuy, J.L., Gutierrez-Ortiz, M.A., Gonzalez-Marcos, J.A., Aranzabal, A., and Gonzalez-Velasco, J.R. Kinetics of the low-temperature WGS reaction over a Cu0/Zn0/Al203 catalyst. Industrial Engineering Chemistry Research, 2005, 44, 41. 

Tabakova, T., Boccuzzi, F., Manzoli, M., Sobczak, J.W., Idakiev, V., and Andreeva, A. Effect of synthesis procedure on the low-temperature WGS activity of Au/ceria catalysts. Applied Catalysis. 

The catalyst packed was assumed to be the commercial Cu/ZnO catalyst for lower-temperature WGS reaction. A number of studies on the reaction kinetics of the commercial WGS catalyst, Cu0/Zn0/Al203, have been published.43-48 Based on the experimental data of the commercial catalyst (ICI 52-1), Keiski et al.47 suggested two reaction rates for the low-temperature WGS reaction in the temperature range 160-250 °C. The first was dependent only on CO concentration and gave an activation energy of 46.2kJ/mol. The second reaction rate was dependent on CO and steam concentrations with a lower activation energy of 42.6kJ/mol. Because of the proximity of our operation conditions to theirs and the fact that steam is in excess in most of the membrane reactors, Keiski and coworkers first reaction rate expression was chosen for this work. The reaction rate is given in Equation 9.5,... 

Water gas shift catalysis has been practiced for 90 years in hydrogen and syngas production. High- and low-temperature WGS is run in industry in adiabatic reactors using Fe-Cr- and Cu-Zn-based formulations. The established catalysts have not changed dramatically in the last 40 years, but continue to improve marginally. 

While both base metal and precious metal catalyst formulations performed well in a simulated high temperature WGS feed stream (-15% CO), precious metal catalysts were clearly superior when tested with a low temperature WGS feed stream (-5% CO). Results for a Siid-Chemie copper zinc catalyst (T2650) and a Siid-Chemie precious metal-ceria catalyst (PMS5) are shown in Figure 5, tested in a simulated low temperature WGS feed stream. Equilibrium was achieved for the PMS5 catalyst for... 

We have developed a one-dimensional non-isothermal model for the countercurrent WGS membrane reactor with a C02-selective membrane in the hollow-fiber configuration using air as the sweep gas. Figure 1 shows the schematic of each hollow-fiber membrane with catalyst particles in the reactor. The modeling study of the membrane reactor is based on (1) the CO2 / H2 selectivity and CO2 permeance reported by Ho [1, 2] and (2) low-temperature WGS reaction kinetics for the commercial catalyst copper oxide, zinc oxide, aluminum oxide (CuO/ZnO/ AI2O3) reported by Moe [3] and others [4]. In this modeling study, the model that we have developed has taken into account critical system parameters including temperature, pressure, feed gas flow rate, sweep gas (air) flow rate, CO2 permeance, CO2 /H2 selectivity, CO concentration, CO conversion, H2 purity, H2 recovery, CO2 concentration, membrane area, water (H20)/C0 ratio, and reaction equilibrium. 

A series of catalysts for the low temperature WGS reaction has been prepared by flame spray pyrolysis. The catalysts consist of a traditional Cu/Zn0/Al203 sample, a ceria promoted Cu/Zn0/Al203 catalyst and a series of ceria and/or zirconia supported Cu or Pt catalysts. Flame spray pyrolysis results in high surface area catalysts with good dispersion. The WGS activity of the catalyst samples has been measured in a plug-flow reactor in the temperature range 180-315 °C. ... 

Kumar and Idem [43] synthesized Cu/Ni/Ce02-Zr02 and compared with commercial low- and high-temperature WGS catalysts. Under the reformate conditions, Cu/Ni/Ce02-Zr02 was found to be more stable with no apparent activity loss compared to both commercial low- and high-temperamre WGS catalysts. They proposed that the superior catalytic performance of... 

I. Ivanov, P. Petrova, V. Georgiev, T. Batakliev, Y. Karakirova, V. Serga, L. Kulikova, A. Eliyas, S. Rakovsky, Comparative study of ceria supported nano-sized platinum catalysts synthesized by extractive-pyrolytic method for low-temperature WGS reaction, Catal. Lett. 

Then they proposed Ni-Mo carbide catalyst [35] for the low temperature WGS reaction. Among the various catalysts Nio.25Moo.75 catalyst carburized at 873 K was more active than the other catalysts. Usually, Mo carbide catalysts deactivate with time-on-stream. However, Nio.25Moo.75 catalyst carburized at 923 K exhibits stable activity for 300 min of time-on-stream. The Ni contents of 15% and 25% increased the catalytic activity but further Ni content led to a drop in activity. They proposed that the promotion of the WGS reaction activity was due to the formation of Ni-Mo oxycarbide. 

Reports are also available on CO2 selective membrane reactors for WGS reaction. Zou et al. [40] first time synthesized polymeric C02-selective membrane by incorporating fixed and mobile carriers in cross-linked poly vinyl alcohol. Micro-porous Teflon was used as support. They used Cu0/Zn0/Al203 catalyst for low temperature WGS reaction. They investigated the effect of water content on the CO2 selectivity and CO2/H2 selectivity. As the water concentration in the sweep gas increased, both CO2 permeability and CO2/H2 selectivity increased significantly. Figure 6.18 shows the influence of temperature on CO2 permeability and CO2/H2 selectivity. Both CO2 permeability and CO2/ H2 selectivity decrease with increasing reactimi temperature. After the catalyst activation, the synthesis gas feed containing 1% CO, 17% CO2, 45% H2 and 37% N2 was pumped into the membrane reactor. They are able to achieve almost 100% CO conversion. They also developed a one-dimensional non-isothermal model to simulate the simultaneous reaction and transport process and verified the model experimentally under an isothermal condition. 

To achieve the complete CO conversion, the WGS reaction after coal gasification is conducted in two adiabatic reactors, i.e., high temperature WGS reactor (310—450 °C) and low temperature WGS reactor (200—250 °C). The two reactors are arranged in series so the raw syngas flowing out of coal gasifier enters the high-temperature WGS reactor packed with Fe-based catalyst first, followed by the low-temperature WGS reactor packed with Cu-based catalyst (Man et al., 2011). 

The low temperature shift catalyst functions 100-200°C below the temperature of the high temperature system and is operated between 210 and 240°C. The catalysts used in this process are copper/zinc oxide/alumina materials. The composition and method of preparation of the copper-based precursors are crucial in determining the final properties of the catalyst. The typical composition of a commercial copper/zinc oxide/alumina catalyst may be 33% CuO, 34% ZnO, and 33% AI2O3. Although the ability of copper/zinc oxides to catalyze the WGS reaction was recognized in the late 1920s, routine usage in commercial plants did not occur until 1963. Until that time, the susceptibility of the catalyst to... 

Ceria-zirconia mixed oxide-supported metal nanoparticles are attractive catalysts in low-temperature WGS, CO preferential oxidation (PROX), or three-way catalysis. It was observed that after redox cycles (reduction at 1173K and then oxidation at 823 K), the reducibility of these catalysts was enhanced substantially, which greatly affected their catalytic performance. HAADF studies showed that the pyrochlore-type cation sublattice in the reduced Ce Zr O was retained in the fully oxidized mixed oxide with a Ce Zr Og stoichiometry as far as the oxidation temperature did not exceed 823 K. However, it is not clear whether compositional heterogeneity occurred at the atomic level in Ce Zr Og. Trasobares and coworkers addressed this issue by aberration-corrected STEM, atomic-resolution EELS mapping, and EELS image simulations [66], They synchronously acquired EELS and HAADE signals in... 

---