Autothermal operating conditions

In the first part of the chapter, a state-of-the-art review and also a thermodynamic analysis of the autothermal reforming reaction are reported. The former, relevant to both chemical and engineering aspects, refers to the reaction system and the relevant catalysts investigated. The latter discusses the effect of the operating conditions on methane conversion and hydrogen yield. 

One of the advantages of the continuous stirred-tank reactor is the fact that it is ideally suited to autothermal operation. Feed-back of the reaction heat from products to reactants is indeed a feature inherent in the operation of a continuous stirred-tank reactor consisting of a single tank only, because fresh reactants are mixed directly into the products. An important, but less obvious, point about autothermal operation is the existence of two possible stable operating conditions. 

Obviously, in designing and operating a stirred-tank reactor it is necessary to be aware of these different operating conditions. Further discussion of the dynamic response and control of an autothermal continuous stirred-tank reactor is given by Westerterp et al.m. 

Measure the emissions from a partial oxidation/autothermal fuel processor for a proton exchange membrane (PEM) fuel cell system under both cold-start and normal operating conditions. 

Table 7 shows a summary of the operating conditions and the product syngas compositions for several autothermal reforming plant designs [15]. 

Given the similarity between equations (6-76) and (6-77), one may conclude that the analysis of multiplicity for the autothermal system is similar (which is indeed the case), where the parameter is found in place of the heat-transfer coefficient. Note that eg can also be written as a function of the operating conditions C and T. 

The space velocity, often used in the technical literature, is the total volumetric feed rate under normal conditions, F o(Nm /hr) per unit catalyst volume (m X that is, PbF o/W. It is related to the inverse of the space time W/F g used in this text (with W in kg cat. and F q in kmol A/hr). It is seen that, for the nominal space velocity of 13,800 (m /m cat. hr) and inlet temperatures between 224 and 274 C, two top temperatures correspond to one inlet temperature. Below 224 C no autothermal operation is possible. This is the blowout temperature. By the same reasoning used in relation with Fig. 11.5.e-2 it can be seen that points on the left branch of the curve correspond to the unstable, those on the right branch to the upper stable steady state. The optimum top temperature (425°C), leading to a maximum conversion for the given amount of catalyst, is marked with a cross. The difference between the optimum operating top temperature and the blowout temperature is only 5°C, so that severe control of perturbations is required. Baddour et al. also studied the dynamic behavior, starting from the transient continuity and energy equations [26]. The dynamic behavior was shown to be linear for perturbations in the inlet temperature smaller than 5°C, around the conditions of maximum production. Use of approximate transfer functions was very successful in the description of the dynamic behavior. 

In lUusIration 10.2 we saw that when one nses a battery of stirred tanks for carrying out an exothermic reaction under isothermal conditions, there may be occasions when the heat requirements for the various tanks may be of opposite sign. Some tanks will require a net input of theamal energy, while others will need to be cooled. It is often useful in such situations to consider the possibility of adiabatic operation of one or more of the tanks in series, remembering the constraints that one desires to place on the temperatures of the process streams. Another means of achieving autothermal operation is to use a network consisting of a stirred-tank reactor followed by a tubular reactor. This case is considered in Illustration 10.6. 

Jaisinghani and Ray (40) also predicted the existence of three steady states for the free-radical polymerization of methyl methacrylate under autothermal operation. As their analysis could only locate unstable limit cycles, they concluded that stable oscillations for this system were unlikely. However, they speculated that other monomer-initiator combinations could exhibit more interesting dynamic phenomena. Since at that time there had been no evidence of experimental work for this class of problems, their theoretical analysis provided the foundation for future experimental work aimed at validating the predicted phenomena. Later studies include the investigations of Balaraman et al. (43) for the continuous bulk copolymerization of styrene and acrylonitrile, and Kuchanov et al. (44) who demonstrated the existence of sustained oscillations for bulk copolymerization under non-isothermal conditions. Hamer, Akramov and Ray (45) were first to predict stable limit cycles for non-isothermal solution homopolymerization and copolymerization in a CSTR. Parameter space plots and dynamic simulations were presented for methyl methacrylate and vinyl acetate homopolymerization, as well as for their copolymerization. The copolymerization system exhibited a new bifurcation diagram observed for the first time where three Hopf bifurcations were located, leading to stable and unstable periodic branches over a small parameter range. Schmidt, Clinch and Ray (46) provided the first experimental evidence of multiple steady states for non-isothermal solution polymerization. Their... 

In indirectly heated reformers operating conditions may vary considerably, but the requirements to the catalyst are similar to those in the tubular reformer. The catalyst activity must be larger for low-temperature adiabatic prereformers, where catalyst effectiveness factors will be outside the strongly diffusion-controlled regime. This implies that a simplified activity expression as in Figure 3.3 is not valid for a prereformer. In an autothermal reformer the requirements to the catalyst includes high temperature stability properties and a good distribution over the catalyst bed of the partly converted gas from the burner. In micro-scale reactors catalyst stability is a major issue. 

Table 5.11 Operating conditions for autothermal fuel processors running on different fuels as calculated by Semelsberger and Borup.

The autothermal reformer operating conditions were set to a S/C ratio of 2 and O/C ratio 0.73. The residual hydrogen from the anode was combusted in an afterburner. The thermal and compression energy ofthe hot combustion gases was used for two purposes firstly, to evaporate water similar to the design of Cutillo et al. described above and secondly, to drive an expander turbine directly connected to the compressor shaft and recovering energy for compression. In a practical 5-kW system this is unlikely to work, because pressure drops in the components will leave little space for... 

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