Autothermal performance

There are reaetions where the heat of reaetion ean be employed to preheat the feed when an exothermie reaetion is operated at a high temperature (e.g., ammonia N2 + SHj o 2NH3 or methanol CO + 2H2 o CH3OH synthesis, water-gas shift reaetion CO -1- H2O o H2 -1-CO2). These proeesses may be performed in fixed-bed reaetors with an external heat exehanger. The exehanger is primarily used to transfer the heat of reaetion from the effluent to the feed stream. The eombina-tion of the heat transfer-reaetion system is elassified as autothermal. These reaetors are self-suffieient in energy however, a high temperature is required for the reaetion to proeeed at a reasonable rate. 

Flow reversal performance is controlled weakly by the period r. Flow reversal is an autothermal operation and as such exhibits parametric sensitivity. Greater stability can be ensured by the conventional expedient of providing cooling in the catalyst bed. It can also be done through bypassing part of the reactor effluent gas around the recuperator section. 

To demonstrate the accurate approximation approach for parameter estimation, we summarize a case study presented in Vasantharajan and Biegler (1990). Ammonia synthesis performed in a Haber-Bosch reactor is operated at high pressures in an autothermal manner, and produces ammonia from the following catalyzed reaction N2 -t- 3H202NH3. 

The generation of hydrogen from ethanol via catalytic autothermal partial oxidation (Equation 6.18) has been performed at temperatures of 430-730 °C using catalytic systems based on noble metals [202, 203]. Ethanol oxidation follows a very complex pathway, including several reaction intermediates formed and decomposed on both the supports and active metals that integrate the catalytic systems [204, 205]. 

Heinzel et al. [77] compared the performance of a natural gas autothermal reformer with that of a steam reformer. The ATR reactor was loaded with a Pt catalyst on a metallic substrate followed by a fixed bed of Pt catalyst. In the start-up phase, the metallic substrate was electrically heated until the catalytic combustion of a stoichiometric methane-air mixture occurred. The reactor temperature was increased by the heat of the combustion reaction and later water was added to limit the temperature rise in the catalyst, while the air flow was reduced to sub-stoichiometric settings. With respect to the steam reformer, the behavior of the ATR reactor was more flexible regarding the start-up time and the load change, thus being more suitable for small-scale stationary applications. 

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. 

Beil, A. and Seume, J. (2006) Unsteady performance of a PEMFC system including autothermal methane reforming, in Proceedings of the 4th International ASME Conference on Fuel Cell Science, Engineering and Technology, June 19-21, Irvine, CA. 

The reactor impregnated with nickel showed inferior performance again. Deactivation was observed, which was assumed to originate from coking, sintering, oxidation of the nickel or even losses of volatile nickel species. With increasing temperature, enhanced formation of by-products, namely methane and ethane, was observed in the reformate both under partial oxidation conditions and in the autothermal mode, which was attributed to thermal cracking. 

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. 

Schuessler et al. [85] applied sintering of copper and aluminum powder to form micro structured plates for an integrated autothermal methanol reformer (see Section 2.7.2). The powders were compressed before sintering at a pressure of 1000 bar. Sintering of copper is performed at temperatures between 500 and 700 °C, which allows for the bonding of the plates in a second sintering step (see the next section). 

The isolation and concentration of petroleum products can be performed in several ways. The most efficient method is passive adsorption. In this method, the sample along with a tube filled with Tenax TA adsorbent is placed in a thermostated (60-70 °C) tightly closed container, such as a glass jar, for over 10 h. Under these conditions, a balance between compounds present in the headspace of the sample and the sample adsorbed on the polymer adsorbent is established. Adsorbed compounds are subjected to thermodesorbtion then, the desorbed compounds together with the carrier gas are injected onto a GC column, where they are separated and then identified. This approach has enabled easy detection and identification of trace amounts of petroleum products. Headspace analysis with passive adsorption on Tenax TA is normally used for separation and concentration of analytes. Gas chromatography coupled with an autothermal desorber and a mass spectrometer (ATD-GC-MS) is the best technique for separation of multicomponent mixtures... 

In the extreme case the whole reforming reaction could be performed without a tubular reformer by autothermal catalytic reforming in a design similar to a secondary reformer. In this case it would be necessary to use oxygen or oxygen-enriched air instead of air [402], [489]-[491], [1485]. 

To further illustrate typical Prox performance, results are shown from tests done with an 02/C0 ratio of 1 1 and a reactor space velocity of 440,000 h using a representative shifted reformate from an autothermal methane reformer (CO 500-10,OOOppm 02 1000-20,000ppm H20 = 32% H2 = 32% C02 = 14% ... 

Rh-Ce catalyst exhibited a high ethanol conversion over 95% and high selectivity to H2 and CO in a very short residence time of <10ms under autothermal condition. Increasing steam/carbon ratio increased the H2 yield due to the participation of WGS reaction. The POE and WGS reactions were also performed in a two-stage reactor. 

Ru impregnated y-Al203 catalyst was washcoated on cordierite monolith/ceramic forms and pellets. Among them, the Ru supported on ceramic foams exhibited better performance in the autothermal reforming of ethanol probably due to smaller pore size and higher tortuosity of the support. 

Rh was loaded by impregnation. Addition of a small amount of 02 in the autothermal reforming reaction increases the overall H2 yield, decreases the operating temperature, and reduces the coke and CH4 formation. Detailed studies on the effect of H20/Et0H and 02/Et0H on the catalytic performance were evaluated. 

Another example is monolithic-type reactors, which have found their main application in the field of combustion. A monolith bed allows better autothermic operations with a minimal pressure-drop. This concept was used to improve performances in commercial methanol into formaldehyde conversion by adding a... 

If the water quantity added as feed increases up to a value corresponding to neutral energetic balance between exothermic and endothermic reaction steps, the overall process is denominated autothermal reformer (ATR). This approach combines both SR and POX catalytic processes and it has been recently proposed to optimize the performance in terms of compactness and efficiency of small-medium production plants. This technology could permit a compromise between the good efficiency of SR and the fast start up of POX. However, it needs a careful control of in going mass stream [6, 7]. 

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