Reformers monolithic

Docter et al. [61] developed an ATR for gasoline with an electrical power equivalent of 10 kW . It was composed of a mixing zone with fuel, air and water injection, and a metallic monolith of 0.51 volume coated with catalyst. The monolith was heated by electricity at the inlet section and operated at a very high 0/C ratio of 1, which is the stoichiometry of partial oxidation. Steam was added to the feed at S/C ratio of 1.5. These operating conditions resulted a low hydrogen content of about 27 vol.%, which was determined for the reformate. The reactor could be turned down by a ratio of 1 10 within 2 s while operating temperatures decreased from 800° C to about 660° C. The efficiency of the reactor was still in the range of 80% at more than 2 kW power output. 

Fichtner et al. [66] used a monolithic microchannel reactor for partial oxidation of methane. The reaction was carried out at 1000°C temperature, 25 bar pressure and residence times in the order of few milliseconds. The adiabatic hot spot formation was calculated to be 2320° C. This excessive hot spot was expected to be reduced in the metallic honeycomb by axial heat transfer from the oxidation to the steam reforming reaction zones. 

Rhodium was chosen as construction material for the reactor, which served as active catalyst species at the same time. Rhodium has a high thermal conductivity of 120 W/(m K). Twenty three foils carrying 28 channels each of which was sealed by electron beam and laser welding. The stack of foils formed a honeycomb which was pressure resistant up to 30 bar. The maximum operating temperature of the reactor was 1200°C. The feed was preheated to 300° C and then fed to the reactor. The experiments were carried out between ambient pressure and 25 bar at 0/C ratio 1.0. After ignition of the reaction between 550 and 700°C, 1000°C reaction temperature was then achieved within 1 min, and mainly carbon monoxide and hydrogen were formed. Only 62% conversion of methane but 98% conversion of oxygen was achieved at 1190°C. The performance of the reactor deteriorated when the system pressure was increased. By-product and even soot formation then occurred downstream the reactor. 

Jung et al. [67] investigated the partial oxidation of methane over noble metal catalysts coated onto metallic monoliths, which 

Catillon et al. [70] investigated the performance of copper/ zinc oxide catalyst coated onto copper foams for methanol steam reforming. Significant improvement of the heat transfer by the copper and consequently higher catalyst activity was achieved compared to fixed catalyst beds. 

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Karatzas et al. [34] performed autothermal reforming of tet-radecane, low sulfur, and Fischer-Tropsch diesel in a monolithic reformer over rhodium/ceria/lanthana catalyst. The reformer had a thermal power output of 14 kW. It was composed of an inert zirconia-coated alumina foam for feed distribution at the reactor inlet and two 400 cpsi cordierite monoliths coated with the catalyst switched in series. At an O/C ratio of 0.45, a S/C ratio of 2.5 and temperatures exceeding 740°C, full conversion of the low sulfur feed was achieved, while the formation of the byproduct ethylene was between 100 and 200 ppm. As shown in Figure 14.7, an increasing S/C ratio suppresses ethylene formation. The catalyst showed stable performance for 40 h duration. Karatzas et al. [44] determined experimentally as shown in Figure 14.8 that the efficiency of their ATR increased with increasing fuel inlet temperature and O/C ratio. 

Figure 10.5 View into metallic monoliths produced by EMITEC (photograph courtesy of Emitec). the monolith and the reactor shell. It holds the monohth and prevents by-pass of gases through the gap [57]. A popular ceramic mat used in automotive exhaust systems is Interam produced by 3M [57j. It degrades at temperatures above 800 °C, and therefore a high temperature ceramic fibre material such as CC-Max from Unifrax must be used for monolithic reformer reactors, which are operated at higher temperature [57].

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

Lenz and Aicher reported the experimental results obtained with an autothermal reformer fed with desulfurized kerosene employing a metallic monolith coated with alumina washcoat supporting precious metal catalysts (Pt and Rh) [78]. The experiments were performed at steam-to-carbon ratios S/C = 1.5-2.5 and... 

In this paper, we summarize results from a small scale methane direct oxidation reactor for residence times between lO and lO seconds. For this work, methane oxidation (using air or oxygen) was studied over Pt-10% Rh gauze catalysts and Pt- and Rh-coated foam and extruded monoliths at atmospheric pressure, and the reactor was operated autothermally rather than at thermostatically controlled catalyst temperatures. By comparing the steady-state performance of these different catalysts at such short contact times, tiie direct oxidation of methane to synthesis gas can be examined independent of the slower reforming reactions. 

In additional experiments, a second catalytic monolith was added immediately after the first monolith. Although tiie residence time was doubled in these experiments, neither the water-gas shift reaction (2) or the steam reforming reaction (1) was found to significantly improve the reaction conversion and selectivity. From these data, it is apparent that the primal hurdle to achieving the perfect reactor operation involves the selective oxidation of CH4 to H2 and CO only. If CO2 and H2O are formed, the amount of available O2 is obviously reduced accordingly. From stoichiometry, this results in unreacted CH4 in the product gases since the reforming reaction is too slow to consume this metiiane at these short residence times. Thus, the only way to improve Sh2 and Sep at these short residence times is to maximize the partial oxidation reaction selectivity. 

Figure 11 Yields of H2, CO, CO2, and CH4 from reforming of benchmark gasoline fuel catalyzed by Rh- or Pt-CGO supported on a cordierite monolith (Conditions OjC = 0.88, SIC = 1.6, GHSV = 9,000...

Similar results were achieved over a Rh/alumina monolith catalyst " using catalytic POX for the reforming of a simulated JP-8 military feed containing 500 ppm of sulfur (as benzothiophene or dibenzothiophene). Stable performance for over 500 h with complete conversion of the hydrocarbons to syngas at 1,050°C, 0.5 s contact time, and LHSV of about 0.5 h was reported. At this high temperature, carbon formation was not reported and the sulfur exited as hydrogen sulfide. 

The differences in reactions at different reactor positions was studied by Springmann et al. who reported product compositions for ATR of model compounds as a function of reactor length in a metal monolith coated with a proprietary noble metal containing Rh. As expected, the oxidation reactions take place at the reactor inlet, followed by the SR, shift, and methanation reactions. Figure 32 shows the product concentration profiles for a 1-hexene feed, which are typical results for all the fuels tested. These results show that steam, formed from the oxidation reactions, reaches a maximum shortly after the reactor inlet, after which it is consumed in the shift and reforming reactions. H2, CO and CO2 concentrations increase with reactor length and temperature. In this reactor, shift equilibrium is not reached, and the increase in CO with distance from the inlet is the net result of the shift and SR reactions. Methane is... 

Recently, the use of Rh supported on washcoated alumina monoliths has attracted interest for ATR of higher hydrocarbons.Reyes et al carried out ATR of n-C6 in monolithic catalysts containing Rh as an active component. A maximum H2 yield of 170% was obtained from the reforming of n-C6 at an O/C ratio of 1, a S/C of 1, preheat temperature of 700°C, and GHSV of 68,000 h Brandmair et al. also carried out ATR of n-C6 over Rh supported on ceramic monoliths at similar conditions, and reported that the Rh catalyst provided better performance over time. 

Fixed bed reactors still predominate for fuel processing. However, fixed beds are susceptible to vibrational and mechanical attrition. Recently, monolithic reactors, either metallic or ceramic, have attracted interest for reforming processes since they offer higher available active surface areas and better thermal conductivity than conventional fixed beds. Low-pressure drop and robustness of the structure are major advantages of monolithic reactors. 

Water gas shift and steam reforming reactions producing H2 under rich conditions (reactions R6 and R7 in Table III, respectively) start to be significantly active at the temperatures above 300 °C (cf. Fig. 22c. These reactions result in a different actual CO C3H6 H2 concentration ratio inside the monolith in comparison with the raw exhaust gas, or the synthetic rich inlet gas mixture used in the lab experiments (Koci et al., 2007b). The reactions with water are characterized by the evaluated rate constants kj T) as well as by the thermodynamic equilibrium constants Keq(7). 

Hydrocarbon Reforming 1 [HCR 1] Micro Structured Monoliths for Partial Methane Oxidation... 

An early application of a combined steam reformer/catalytic combustor on the meso scale was realized by Polman et al. [101]. They fabricated a reactor similar to an automotive metallic monolith with channel dimensions in the millimeter range (Figure 2.65). The plates were connected by diffusion bonding and the catalyst was introduced by wash coating. The reactor was operated at temperatures between 550 and 700 °C 99.98% conversion was achieved for the combustion reaction and 97% for the steam reforming side. A volume of < 1.5 dm3 per kW electrical power output of the reformer alone was regarded as feasible at that time, but not yet realized. 

Von Hippel et al. [104] patented a special reactor head, which allows for a distribution of the two gas flows through each individual channel. Even at a 1 200 °C monolith temperature the heads did not heat up to more than 200 °C, hence silicone rubber was applied for sealing the heads. This concept was applied for coupling methane combustion and steam reforming in separate flow paths [105],... 

Comparison Between Coated Micro Structures and a Conventional Monolith Applied to Autothermal Methanol Reforming... 

Chen et al. [36] performed a comparison of micro structured steel and aluminum plates with a conventional monolith by varying the GHSV. Full conversion could be maintained for autothermal methanol reforming in the micro structures up to a GHSV of 40 000h 1, whereas conversion dropped to 80% at 20 000h 1 at the monolith. Even at 186 000 h, still 95% conversion could be achieved in the stainless-steel micro reactor. No significant performance differences were observed between the steel and aluminum plates. 

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