Preferential oxidation PROX

Depending on the reason for converting the produced gas from biomass gasification into synthesis gas, for applications requiring different H2/CO ratios, the reformed gas may be ducted to the water-gas shift (WGS, Reaction 4) and preferential oxidation (PROX, Reaction 5) unit to obtain the H2 purity required for fuel cells, or directly to applications requiring a H2/CO ratio close to 2, i.e., the production of dimethyl ether (DME), methanol, Fischer-Tropsch (F-T) Diesel (Reaction 6) (Fig. 7.6). 

The main unit is the catalytic primaiy process reactor for gross production, based on the ATR of biodiesel. After the primary step, secondary units for both the CO clean-up process and the simultaneous increase of the concentration are employed the content from the reformated gas can be increased through the water-gas shift (WGS) reaction by converting the CO with steam to CO and H. The high thermal shift (HTS) reactor is operating at 575-625 K followed by a low thermal shift (LTS) reactor operating at 475-535 K (Ruettinger et al., 2003). A preferential oxidation (PROX) step is required to completely remove the CO by oxidation to COj on a noble metal catalyst. The PROX reaction is assumed to take place in an isothermal bed reactor at 425 K after the last shift step (Rosso et al., 2004). 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

Figure 15.24 Pictorial representation of preferential oxidation (PROX) of CO in an H2 rich atmosphere with a limited amount of O2 to supply pure FI2 by removal of toxic CO in less than 10 ppm range for PEM fuel cells.

Preferential Oxidation (PROX) of CO in Excess H2 on Novel Metal Catalysts... 

Table 3.1 shows the catalytic performance of supported Au catalysts for the preferential oxidation (PROX) of CO in H2 together with the actual reaction conditions and targeted performances. Au/A1203 [61-63], Au/Mn203 [58], Au/ Fe203 [54, 60, 61, 64—66] and Au/Ce02 [54, 60-62, 67-70] have been reported to... 

Schuessler et al. [85] of XCELLSiS (later BALLARD) presented an integrated methanol fuel processor system based on autothermal reforming, which coupled fuel/water evaporation with exothermic preferential oxidation (PrOx) of carbon monoxide. The reactor technology was based, in contrast to most other approaches, on a sintering technique. 

Alternative technologies to the PSA process for H2 purification include, after the HTS reaction, a low-temperature shift (LTS) reaction followed by C02 scrubbing (e.g., monoethanolamine or hot potash).11 The LTS reaction can increase the H2 yield slightly. However, the product stream, after the HTS, needs to be cooled to about 220 °C. Preferential oxidation (Prox) and/or methanation reaction as shown in Equations 2.6 and 2.7, respectively, removes the traces of CO and C02. The product H2 has a purity of over 97%. 

Removal of Trace Contaminants from Fuel Processing Reformate Preferential Oxidation (Prox)... 

For the long-term durability of PEMFC, the acceptable CO concentration appears to be 10-100 ppm. To meet the requirement, three possible reactions can be considered preferential (or selective) oxidation, methanation, and Pd (or Pd alloy) membrane processes. Preferential oxidation (PrOx) of CO can convert CO to CO2, without excessive hydrogen oxidation (to water), to acceptable levels of CO using multi-stage reactors... 

Produced required amount of Catalytica Energy System s autothermal reformer (ATR) catalyst. Produced required amount of NexTech platinum (Pt)/ceria medium-temperature-shift (MTS) catalyst. Obtained Eos Alamos National Eaboratory preferential oxidation (PROX) reactor. 

The processor can be divided into four zones based on the operating temperature reformer, scmbber-high temperature shift (S-HTS), low temperature shift (LTS), and preferential oxidation (PROX). Operating temperatures are nominally 700°C in the reformer, 400°C in the S-HTS, 200°C in the LTS, and 100°C in the PROX. In actual operation, temperatures are not discrete rather, there is a gradual decrease between each zone. The middle zones, S-HTS and LTS, comprise 70% of the total heat required during start-up. The fuel requirements and heat flow for a 50-kilowatt (kW) processor are shown in Figure 1. 

Under normal operating conditions, in which the combustor is sufficiently warm and operated under fuel rich conditions, virtually no NOx is formed, although the formation of ammonia is possible. Most hydrocarbons are converted to carbon dioxide (or methane if the reaction is incomplete) however, trace levels of hydrocarbons can pass through the fuel processor and fuel cell. The shift reactors and the preferential oxidation (PrOx) reactor reduce CO in the product gas, with further reduction in the fuel cell. Thus, of the criteria pollutants (NOx, CO, and non-methane hydrocarbons [NMHC]), NOx CO levels are generally well below the most aggressive standards. NMOG concentrations, however, can exceed emission goals if these are not efficiently eliminated in the catalytic burner. 

Decrease of 20% to 90% observed in autothermal reforming (ATR) and preferential oxidation (PrOx) catalysts, including loss in activity and CO conversion in PrOx... 

A conventional FPS, shown in Fig. 14.2, includes a reformer, two WGS reactors, and two Preferential Oxidation (PrOx) reactors, located downstream of the WGS. For PEM fuel cells, it is a necessity to assure < 10 ppm of CO in the cell stack. These reactors form a considerable fraction of the FPS weight, volume, and cost. Replacing this train by an integrated hydrogen permeation selective membrane on the water gas shift reactor, shown in Fig. 14.3, results in a considerable reduction in the number of components, cost, and volume of the FPS. This will make fuel cell power plants practical and affordable for power generation in a wide range of applications, especially for residential and transportation. Numerous published works [8, 9] in the area of catalytic membrane reactors can be quoted in the experimental [10] and numerical [11, 12] domains. 

Ernst et al. (1999) suggest that the fuel processing system be comprised of a steam-reforming reactcn, water-gas shift (WGS) reactors (at high and low temperatures), and a preferential oxidation (PROX) reactor to oxidize carbon monoxide, as shown in Figure 19.17. Kinetic data are provided for the reformer and the two shift reactors. 

The most common process for the chemical purification of the hydrogen rich gas is the preferential oxidation (PROX) of carbon monoxide. The preferential oxidation is promoted by precious metal based catalysts. Precious metal catalyst promotes the reaction of hydrogen and oxygen as well. So the main disadvantage of PROX is the side reaction of hydrogen with oxygen to water and heat. Furthermore precious metal based catalysts are expensive. 

The CO dean-up section typically consists of different units, usually a one- or two-stage WGS reactor, followed by aunit to remove the final traces of CO such as a selective oxidation (SELOX) - also called preferential oxidation (PrOx) unit, a methanation unit or a physical separation method (Pd-Ag-based membrane, PSA). It would often be desirable to eliminate the LT-WGS unit, as it constitutes a rather large-sized and heavy unit However, heat management restrictions and the still rather low efficiency of the PrOx unit require low GO concentrations in the feed. On the other hand, methanation would become more attractive once an enhanced selectivity permits conversion of CO without conversion of CO2, as this then no longer requires upstream CO2 separation. 

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