Hot Spots in Catalyst Beds

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Special care must be taken in the design and operation of adsorption units handling ketone solvents, such as methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), and cyclohexanone, because of their reactivity in the presence of activated carbon. Traces of metal compounds in the carbon act as catalysts for decomposition reactions, which are further accelerated by the presence of heat and moisture. The decomposition reactions are exothermic and if allowed to continue can cause hot spots in the bed and ultimately fires. According to Collins (1988), ketone adsorption and recovery can be accomplished safely if these guidelines are followed ... 

In practice, granular beds comprising a very large number of catalyst pellets are used. It is well known that the efficiency of a catalytic reactor depends crucially on the liquid phase distribution within the catalyst bed [14]. It is likely that the development of hot spots in a catalyst bed is also related to the character of liquid phase distribution. Therefore, it is very important to map the spatial distribution of the liquid phase in a catalytic reactor for various operation regimes. This eventually should lead to the formulation of the mechanisms responsible for the development of critical phenomena on both a micro- and macroscale. 

In summary, major challenges in the partial oxidation of methane are (1) designs to avoid excessive thermal gradients (hot spots) in the catalyst bed (2) reduction of the cost of O2 separation and (3) elucidation of the reaction pathways as a step toward improved catalyst design. 

Figure 5 shows the axial gas and solid temperature profiles during start-up operation. Notice that the hot spot in the reactor moves down the bed as the heat of reaction increases the temperature of the catalyst particles. Also note the significant temperature difference between the catalyst and gas in the early part of the reactor, where conversion is rapid due to the heat of reaction being generated on the catalyst surface. These differences are even more pronounced (over 20 K) near the center of the bed and near the outer wall.11... 

Automatically controlling the outside of the catalyst tube to 370-400° gives a hot spot in the catalyst bed of 530° at the rate specified in equipment used by the checkers. 

The packed bed reactor is used to contact fluids with solids. It is one of the most widely used industrial reactors and may or may not be catalytic. The bed is usually a column with the actual dimensions influenced by temperature and pressure drop in addition to the reaction kinetics. Heat limitations may require a small diameter tube, in which case total through-put requirements are maintained by the use of multiple tubes. This reduces the effect of hot spots in the reactor. For catalytic packed beds, regeneration is a problem for continuous operation. If a catalyst with a short life is required, then shifting between two columns may be necessary to maintain continuous operation. 

In most radial-flow converters, the upper portion of the bed is sealed with excess, unused catalyst. This design prevents feed gas from by-passing the reaction section when the catalyst settles. The KAAP reactor uses a proprietary sealing system to overcome this problem. This sealing system avoids the catalyst maldistribution that can lead to formation of hot spots in the catalyst bed. The system also allows 100% of the loaded catalyst volume to be utilized for the ammonia conversion reaction203. 

The dilution of the feed can be useful to prevent hot-spots in the catalytic bed or to enable experiments at small conversions, which are necessary for the determination of kinetic parameters. However, for such cases it is important to check if the catalyst is stable with and without dilution of the feed. 

Fluidized and transport bed processes have also been developed for better management of the heat released. The former prevents the occurrence of hot spots in the catalyst bed through a more uniform temperature profile. The concentration of the -butane can also be higher, even within the explosion limits, thanks to the barrier to flame propagation constituted by the fluidized bed of particles. Selectivity is not however dissimilar to that in fixed bed operation due to considerable back-mixing of the products and longer residence times. 

Interactions with the carbon surface can make the recovery of certain solvents, such as ketones and chlorinated hydrocarbons, difficult. Ketones and aldehydes can polymerize, releasing large amounts of heat. When this happens in a part of the bed with poor heat transfer, the temperature can reach the ignition point of the solvent. Fires always start with hot-spots in parts of the bed where the airflow is reduced due to poor design. The susceptibility of the carbon bed to autoignition can be reduced by removing soluble alkali sodium and potassium salts, that may be present as impurities in the activated carbon and which can function as combustion and gasification catalysts [29]. 

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 highly exothermic combustion reaction at the top of the catalytic bed produces hot spots in the supported catalysts that increase the problem of deactivation by sintering, but also produce some results... 

---