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Direct autothermal coupling of endothermie and exothermic reaction has been applied a long time ago in industrial processes, such as secondary steam reforming within ammonia synthesis process (Ridler and Twigg 1989) or hydrogen cyanide synthesis (Agar 1999), where hydrogen combustion is used to produce in-situ heat for the endothermie reactions.

Blanks et al. (1990) present a reactor for production o f synthesis gas from catalytic partial oxidation o f natural gas. The reactions that take place in the reacting stream are methane combustion, combined steam and CO2 reforming, and water gas shift reaction. The reactor consists of three different packed beds, stacked vertically on top of each other. The middle bed is the reaction zone containing a commercial nickel reforming catalyst, while the zones above and below are heat exchange zones and contain inert packing. The reactor operates in unsteady state mode and the flow directions of the feed (natural gas and air) and product (synthesis gas) is reversed periodically, so that the waste heat is efficiently recovered. It was possible to reach a pseudo-state where the overall average bed temperature remained constant and the upper and the lower bed temperature profiles mirror each other as the gas flow direction reverses. In order to avoid hot spots in the catalyst bed, better control can be achieved by pre-burning part of the feed in air to form CO2 and H2O to moderate the heat generated in the reactor.

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Kulkami and Dudukovic (1997, 1998) have discussed coupling of solid catalysed endothermie reaction with an exothermic reaction in a bi-directional fixed bed reactor operated in periodic steady state. The exothermic reaction, usually methane combustion takes place in the bed during an exothermic semicycle when a mixture o f fuel and air is fed into the hot bed. The heat generated is then utilised to drive the endothermie reaction, which occurs during the next endothermie semicycle, the reactants being fed into the bed from the other end o f the reactor. For a successful operation of the reactor, two conditions need to be satisfied. The heat liberated during the exothermic semicycle should be equal to or greater than the energy consumed during the endothermie semicycle, and the product of the front velocity and the semicycle period for the endothermie semicycle must be equal to or greater than that for the exothermic semicycle. The energy efficiency and conversion increase with increasing bed length and decreasing semicycle periods.

Recent studies are dedicated to in-situ hydrogen combustion during dehydrogenation reactions. Kolios and Eigenberger (1999) present two alternative operation modes for styrene production. Thus, in asymmetric operation mode, the feed changes with each semicycle between an ethylbenzene/steam and hydrogen/air mixture. While in the symmetric operation model the feed is the same in both semicycles and the necessary heat is supplied from a catalytic burner installed in the centre o f the reactor. The discontinuous heat supply in the asymmetric operation mode causes strong periodic fluctuations in conversion and a poor efficiency o f heat recovery. The symmetric operation mode allows for a high conversion, high styrene selectivity and high efficiency of heat recovery.

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Choudhary et al. (2000) studied experimentally the coupling o f the exothermic catalytic oxidative combustion and endothermie thermal cracking reactions o f propane to propylene and ethylene in the temperature range o f 750 -850 °C. They show that the process can be guided in an efficient manner requiring no external energy and without coke formation. The exothermicity can be controlled by manipulating the temperature and concentration of O2 related to propane in the feed which also strongly influence the propylene/ethylene product ratio.

A major interest existing currently is the production o f hydrogen or synthesis gas from steam reforming of methane coupled with an exothermic reaction, usually methane combustion, due to increasing demand for hydrogen ranging from hydrogen plants for refineries to small units providing hydrogen for fuel cells (Rostrup-Nielsen 2000). De Groote and Froment (1996) modelled and simulated the catalytic partial oxidation of methane to synthesis gas on Ni base catalyst in an adiabatic fixed-bed reactor. Depending on the degree o f reduction of Ni catalyst, reforming can be either consecutive to the total combustion or it can run in parallel. A temperature peak is observed for consecutive operation, which can be moderated if steam or carbon dioxide is added to the feed, leading also to carbon deposition reduction. High selectivities for CO (90 %) and H2 (95 %) can be obtained if the same system is operated in reverse flow mode (De Groote et al. 1996), while coke formation is moderate.

W olf et al. (1997) theoretically investigated, for atmospheric and elevated pressures, the partial oxidation o f methane to synthesis gas over a highly active platinum catalyst. High hot-spot temperature at the catalyst surface together with large temperature gradients between the gas and catalyst surface were calculated, while the heat

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conductivity of the catalytic material proved to be an important factor for the temperature control.

Ma and Trim (1996) studied experimentally the autothermal conversion of methane to hydrogen as a function o f an oxidation and a steam reforming catalyst configuration. Preheating the catalyst bed at ca. 590 K initiates the methane oxidation over a platinum catalyst, the heat and steam produced facilitate further steam reforming over a nickel catalyst. A two-bed system was found to be inferior in performance to one bed containing two mixed catalysts located on the same support. The theoretical investigation of Avci et al. (2001) also confirmed these results. In addition, intraparticle diffusion limitations are found to be significant.

Gosiewski (2001) investigated the carbon deposition for several reactor types for partial oxidation of methane, finding that for the reversal-flow reactors the accumulation of the heat wave results in higher hot spots, and consequently higher carbon deposition, compared to reactors without flow reversal.

Vesser and Frauhammer (2000) investigated the oxidation o f methane to synthesis gas over platinum in a monolith reactor. They found that a limited supply o f oxygen is favourable to high hydrogen selectivity and that the importance o f very high reaction temperature requires good heat integration for the reactor. Three solutions are suggested for an improved process: internal heat exchange between the hot reaction products and the cold reactor feed gases through a counter-current heat exchange reactor, a distributed oxygen feed along the reactor axis using a membrane shell around the catalyst bed, or a reverse-flow reactor where the gases are fed periodically from opposite ends to the catalytic bed. Simulation and experimental studies (Vesser et al.

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2 0 0 0), proving that high reaction temperature and selectivities can be reached have

demonstrated the effectiveness o f the first alternative. As for a membrane reactor configuration with distributed oxygen feed, this can avoid the high oxygen concentration in the reactor entrance, but no effective increase in reaction selectivity was reported.