Chemical reactors averaging technique

The averaging technique characteristic of the second approach may apply to the case of a tubular reactor where the ratio of the characteristic catalyst particle size to the diameter of a single tube is close to unity, but it is invalid, as will be shown, in the general case of fixed-bed reactors. This approach keeps out of a researcher s field of vision the problem of the reactor stability to local perturbations. At the same time, the technologist is often faced with hot spots in the catalyst bed of a fixed-bed reactor, which make its operation imperfect and even lead to an emergency situation in a number of cases, Until recently, nonuniformity of the fields of external parameters (e.g., nonuniform packing of the catalyst bed or nonuniformity of reactant stream velocity ) was considered the only cause of these phenomena. The question naturally arises whether the provision for uniformity of external conditions guarantees the uniformity of temperature and concentration profiles at the reactor cross-section. The present paper seeks to answer this question, which, as a matter of fact, has not yet been posed in such a form in the theory of chemical reactors. 

The overall interfacial area for the whole reactor can be determined by chemical techniques. These techniques, however, must be used with restrictions. For example, chemical methods are difficult to use for fast-coalescing systems, since the presence of a chemical compound may reduce coalescence rates. Furthermore, in fast-coalescing systems, the specific area may depend strongly on the position in the reactor, which complicates the interpretation of an average value obtained with chemical methods. Indeed, both physical and chemical techniques should be used together to describe the phenomena that occur within gas-liquid reactors. While chemical methods allow the determination of the much-needed average interfacial area, information on the variations of the interfacial parameters, such as aL and dsv, within the reactor, which is important for scale-up, cannot be obtained by this method. 

At the outset, we recognize that a technique that measures overall values cannot be used without the restrictions that arise from the results observed with physical methods. For example, the chemical method can hardly be used with fast-coalescing systems, since the presence of a chemical compound may well reduce the coalescence rates. In fast-coalescing systems, as observed with physical methods, the wide variation of specific contact area at different locations in the reactor negates the meaning of an average value. In fact, physical and chemical techniques should be used simultaneously to identify more fully the phenomena that occur in gas-liquid reactors. While chemical methods provide overall values of interfacial area that are immediately usable for design, we must also know the variations in the local interfacial parameters (a, dgM) within the reactor in order to deal competently with scale-up. These complementary data, measured by physical methods, should be obtained from local simultaneous measurements of two of the three interfacial parameters as discussed above. 

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