Hot spot occurrence

Figu re 5.10 Hot-spot occurrence in the phenol hydrogenation reactor. 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented an operation of vacuum residue hydrodesulfiirization in the conventional fixed bed reactor system in 1980. We have also conducted a high conversion operation to produce more middle distillates as well as lower the viscosity of the product fuel oil to save valuable gas oil which is used to adjust the viscosity. Vacuum residue hydrodesulfurization in fixed bed reactors mvolves the characteristic problems such as hot spot occurrence and pressure-drop build-up. There has been very little literature available discussing these problems based on commercial results. JafiFe analyzed hot spot phenomena in a gas phase fixed bed reactor mathematically, assuming an existence of the local flow disturbance region [1]. However, no cause of flow disturbance was discussed. To seek for appropriate solutions, we postulated causes ofhot spot occurrence and pressure-drop build-up by conducting process data analysis, chemical analysis of the used catalysts, and cold flow model tests. This paper describes our solutions to these problems, which have been demonstrated in the commercial operations. 

To prevent hot spot occurrence, we evaluated the methods which improved liquid distribution, in the commercial operations. Figure 6 compares changes in the temperature deviation of the first bed during the different runs. The axial number, which is an average of the standard deviation of the temperatures at each level, is defined as an index of a degree of maldistribution. A small number indicates good liquid distribution. We took the countermeasures successively run after run. 

Effective solutions to the problems of the vacuum residue hydrodesulfurization unit equipped with the fixed bed reactors, such as a hot spot, pressure-drop buildup, and catalyst deactivation by coke fouling, were discussed. Improving liquid distribution can prevent hot spot occurrence. Dispersing inorganic solids throughout the reactors can control a pressure-drop increase in the first bed. For a high conversion operation, controlling the conversion in each bed can minimize the coke deactivation in the fourth bed. 

The situation described above is not acceptable for operation. Therefore, hot-spot occurrence is very likely with high exothermic reactions, and must be avoided by a proper design and selection of operating parameters. Computer simulation is a powerful method that can be used to detect such behaviour. 

Unlike THg, a limited data set about the occurrence of organomercury compounds in biota of the Ebro River has been noticed. MeHg concentrations were only determined in zebra mussel specimens in the Flix hot spot [32] as well as some fish species living in Flix dam (Diez, unpublished results). 

These methods hardly take spatial distributions of velocity field and chemical species or transient phenomena into account, although most chemical reactors are operated in the turbulent regime and/or a multiphase flow mode. As a result, yield and selectivity of commercial chemical reactors often deviate from the values at their laboratory or pilot-scale prototypes. Scale-up of many chemical reactors, in particular the multiphase types, is still surrounded by a fame of mystery indeed. Another problem relates to the occurrence of thermal runaways due to hot spots as a result of poor local mixing effects. 

LES/FDF-approach. An In situ Adaptive Tabulation (ISAT) technique (due to Pope) was used to greatly reduce (by a factor of 5) the CPU time needed to solve the set of stiff differential equations describing the fast LDPE kinetics. Fig. 17 shows some of the results of interest the occurrence of hot spots in the tubular LDPE reactor provided with some feed pipe through which the initiator (peroxide) is supplied. The 2004-simulations were carried out on 34 CPU s (3 GHz) with 34 GB shared memory, but still required 34 h per macroflow time scale they served as a demo of the method. The 2006-simulations then demonstrated the impact of installing mixing promoters and of varying the inlet temperature of the initiator added. 

The above simulations as to the occurrence of hot spots once more illustrate the power and promises of LES over RANS-type simulations. The hot spots can never be found by means of a RANS-type of simulation. The same technique was used by Van Vliet et al. (2006) to study the influence of the injector geometry and inlet temperature on product quality and process efficiency in the LDPE reactor. On the contrary, the RANS-based simulations due to R. A. Bakker and Van den Akker (1994, 1996) were pretty much suited to arrive at yield predictions for a fed batch reactor as a whole. 

It is not practical to stir all reaction systems, for example, bulk polymerizations, postpolymerization reactions, fixed-bed catalytic reactors, and plug-flow reactors. Although multipoint temperature sensing is often used as a key solution to determine a runaway in nonagitated vessels, the occurrence of hot spots may not always be detected. 

Heberer T. and M. Adam (2005). Occurrence, Eate and Removal of Pharmaceutical Residues in the Aquatic Environment An Extended Review of Recent Research Data. In D.R. Dietrich, S.F. Webb, and T. Petry (Eds.). Hot Spot Pollutants Pharmaceutical in the Environment. Elsevier, New York, pp. 11-36. 

The occurrence of hot spots must be avoided during the practical operation of high-temperature fuel cells. This can be done in different ways One possibility is to keep the total cell current and the average current density low. However, this solution is unsatisfactory, because future fuel cell research must aim at intensifying the process of power generation and the power density. Figure 3.8 may point to another, more efficient solution For the same cell current, different, possibly unstable... 

The catalyst widely used is based on copper(II) chloride impregnated on alumina. The highly exothermal reaction is carried out at temperatures around 200 °C and pressures of 1.5-5 bar, in fixed-bed or fluid-bed reactors. The fluid-bed technique offers more intensive heat transfer, prevents the occurrence of hot spots and allows more efficient catalyst regeneration. Ethylene conversion of 93-97% can be achieved with selectivity in EDC of 91-96%. 

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

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. 

The transition from the liquid- to the gas-phase reaction regime is often accompanied by a marked increase in the reaction rate, because the gas phase surrounding the catalyst pellet offers less mass-transfer resistance than the liquid phase. For the case of an exothermic reaction, this may have an undesirable effect, as it gives rise to a rather narrow reaction zone with steep temperature gradients. Thus, the catalyst may be exposed to local overheating, which results in subsequent deactivation of the bed or the occurrence of a number of undesirable side reactions. Furthermore, if the heat removed is insufficient, the hot-spot temperature could occur. 

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