Fluid steam reforming

Our initial work on reaction thermal effects involved CFD simulations of fluid flow and heat transfer with temperature-dependent heat sinks inside spherical particles. These mimicked the heat effects caused by the endothermic steam reforming reaction. The steep activity profiles in the catalyst particles were approximated by a step change from full to zero activity at a point 5% of the sphere radius into the pellet. 

Abedi, J., Yeboah, Y.D., Howard, J., and Bota, KB. (2001) Development of a catalytic fluid bed steam reformer for production of hydrogen from biomass, 5th Biomass Conference of the Americas, Orlando, FL (Cancelled). Abstracts to be published on CD/ROM. 

Arana et al. have performed extensive modeling and thermal characterization experiments on their reactor design. They modeled their design consisting of two suspended SiN - tubes linked with slabs of silicon using two-dimensional computation fluid dynamics and a heat transfer model (Femlab, Comsol Inc.). The heat of reaction of the steam reforming or... 

Fixed- or packed-bed reactors refer to two-phase systems in which the reacting fluid flows through a tube filled with stationary catalyst particles or pellets (Smith, 1981). As in the case of ion-exchange and adsorption processes, fixed bed is the most frequently used operation for catalysis (Froment and Bischoff, 1990 Schmidt, 2005). Some examples in the chemical industry are steam reforming, the synthesis of sulfuric acid, ammonia, and methanol, and petroleum refining processes such as catalytic reforming, isomerization, and hydrocracking (Froment and Bischoff, 1990). 

Chapter 7 is the climax of the book Here the educated student is asked to apply all that he/she has learned thus far to deal with many common practical industrial units. In Chapter 7 we start with a simple illustrative example in Section 7.1 and introduce five important industrial processes, namely fluid catalytic cracking in FCC units in Section 7.2, the UNIPOL process in Section 7.3, industrial steam reformers and methanators in Section 7.4, the production of styrene in Section 7.5, and the production of bioethanol in Section 7.6. 

It is rewarding that both fundamental and applied aspects are dealt with. The deactivation of catalysts in important industrial processes like fluid bed catalytic cracking, hydrotreatment, hydrodesul furization, catalytic reforming, hydrodenitrogenation, steam reforming,... 

Chapter 10 contains a literature survey of the basic fluidized bed reactor designs, principles of operation and modeling. The classical two- and three phase fluidized bed models for bubbling beds are defined based on heat and species mass balances. The fluid dynamic models are based on kinetic theory of granular flow. A reactive flow simulation of a particular sorption enhanced steam reforming process is assessed. 

Finally, it should be noted that FT should be considered as only one of the three steps in the conversion of natural gas into liquids, the other two being syngas generation and hydroprocessing.44 However, new concepts such as the combination of methane steam reforming and FT synthesis, in order to convert methane directly to hydrocarbons, have been explored.45 The idea seems to be operative at 573 K, using Ru and Co catalysts, but very low conversions are achieved.45 Thus, higher performance catalysts will have to be developed and the combination with alternative reaction conditions (such as supercritical fluids) merits consideration. 

Figure 2.18. Schematic representation of the pilot-plant for methane steam reforming. The R-1 is the reforming reactor, which contains a bundle of tweve palladium tubular membranes and a fluid-ized-bed of catalysts. From Adris et ij/.[2.381], with permission from Elsevier Science.

Figure 4. Catalytic Fluid Bed Steam Reforming Reactor...

A schematic and photograph of the pilot-scale catalytic fluid bed reformer are shown in Figure 4. The 30-cm catalytic steam reforming reactor was successfully operated on peanut pyrolysis vapor at a feed rate of 7 kg/hour of vapors. The results are in agreement with those obtained from the 5-cm bench-scale reactor used for the reforming of the aqueous fraction of pyrolysis oil. Typical gas compositions at the outlet of the reformer are shown in Figure 5. These data show that the yield of hydrogen is approximately 90% of maximum. 

The CSP plant consists of a solar collector field, a receiver, a heat transfer fluid loop and a heat storage system. The mirrors of the solar field concentrate the direct solar radiation on the solar receiver set at the focal line. The heat transfer fluid (e.g., molten salts) removes the high temperature solar heat from the receiver that is afterwards collected into an insulated heat storage tank to be pumped, on demand, to the heat users (steam generators, endothermic reactors, etc.), where its sensible heat is released. Finally, the heat carrier fluid is stored in a lower temperature tank ready to restart the solar heat collection loop. The idea to match the CSP plant with natural gas steam-reforming Pd-based MR derives from the thermal level reached by molten salt stream (550°C), which meets the thermal requirements of MR (preferred operating at around 500°C). 

Two systems of structured catalytically active materials have been shown already in Section 25.2.2 (Figure 25.1). On leached brass wires [34], the conversion of methanol was dependent on the procedure of surface treatment and the leaching fluid. Brass of different composition leached in 3.7% HCl or 10% NaOH followed by subsequent calcination at 600 °C and reduction at 250 °C showed nearly no conversion, whereas the same treatment with aluminum-coated brass wires showed good activity under steam reforming conditions. Basic leached catalyst deactivated rapidly. The number of results for CPO and OSR presented in this study was fairly limited. The hydrogen yield and methanol conversion reached approximately 80% and 60%, respectively, at... 

To evaluate the potential of carbon formation in a steam reformer, it is therefore essential to have a rigorous computer model, which contains kinetic models for the process side (reactor), as well as heat transfer models for the combustion side (furnace). The process and combustion models must be coupled together to accurately calculate the process composition, pressure, and temperature profiles, which result from the complex interaction between reaction kinetics and heat transfer. There may also be a temperature difference between bulk fluid, catalyst surface, and catalyst interior. Lee and Luss (7) have derived formulas for this temperature difference in terms of directly observable quantities The Weisz modulus and the effective Sherwood and Nusselt numbers based on external values (8). 

The sorbent should be regenerable. Regeneration must be performed with a gas stream generated by the system or with air. Possible regeneration fluids include air, steam, reformate, and tail gases from a burner. 

It is difficult to be at all quantitative as to when and to what degree these various possible applications will come to pass. Among the many factors which will determine the future energy scene are technical factors (advances in fuel cells, electric vehicles, electrolyzers, LH2 fuelled aircraft, etc.), environment factors (SO2 emissions, mining of fossil fuels, etc.) and, of course, the ubiquitous economics and politics which control all major human activities. What does seem clear is that, in the early years, synthetic fluid fuels will be manufactured by steam reforming, both for economic reasons and for institutional reasons associated with the expertise of the petroleum and gas industries. Electrolytic hydrogen will enter upon the scene more slowly, as it will be dependent upon the availability of cheap or surplus electricity and will tend to be produced by the chemical industry or electricity utilities rather than by the fuel industries. Moreover, its first use is likely to be for chemical synthesis, rather than as a fuel. 

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