Heat transfers between catalyst beds

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

Converters with indirect cooling have been known since the early days of ammonia production, for example, the Fauser-Montecatini reactor [843], [844], [848], [867], [891]-[893], In this converter, tube coils between catalyst beds transfer the reaction heat to a closed hot water cycle under pressure, operating by natural draft. The hot water releases the absorbed heat in an external steam boiler generating about 0.8 t of steam per tonne of ammonia at about 45 bar (ca. 250 °C). 

Figure 17.37. Some measured and predicted values of heat transfer coefficients in fluidized beds. 1 Btu/hr(sgft)(°F) = 4.88 kcal/(hr)(m )(°C) = 5.678 W/(m )(°C). (a) C o mp arisen of correlations for heat transfer from silica sand with particle size 0.15 mm dia nuiaized in air. Conmtions are identified in Table 17.19 Leva, 1959). (b) Wall heat transfer coefficients as function of the superficial fluid velocity, data of Varygin and Martyushin. Particle sizes in microns (1) ferrosilicon, i 82.5 (2) hematite, d = 173 (3) Carborundum, d = 137 (4) quartz sand, d = 140 (5) quartz sand, d = 198 (6) quartz sand, d = 216 (7) quartz sand, d = 428 (8) quartz sand, d = 51.5 (9) quartz sand, d = 650 (10) quartz sand, d = 1110 (11) glass spheres, d= 1160. Zabrqdskystal, 1976,Fig. 10.17). (c) Effect of air velocity and particle physical properties on heat transfer between a fluidized bed and a submerged coil. Mean particle diameter 0.38 mm (I) BAV catalyst (II) iron-chromium catalyst (III) silica gel (IV) quartz (V) marble Zabrodsky et at, 1976, Fig. 10.20).

Monolithic catalysts for two-phase processes are characterized by (1) poor heat and mass transfer between the gas and the outer surface of the catalyst, and (2) no mass exchange between adjacent channels and consequently zero mass transport in the direction perpendicular to flow. The latter, being the predominant contribution to the overall mechanism of radial heat transfer inside the catalyst bed, results in rather poor heat transfer between the monolith and the surroundings. If more intensive heat and mass transfer within the catalyst bed is needed, arranged catalysts are one of the most effective solutions. 

The reactants enter in the annular space between an outer insulated tube and an inner tube containing the catalyst. No reaction takes place in the annular region. Heat transfer between the gas in this packed-bed reactor and the gas flowing counter currently in the annular space occurs along the length of the reactor. The overall heat-transfer coefficient is 5 W/ K. Plot the conversion and temperature as a function of reactor length for the data given in... 

The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heat-ing/cooling media. 

Integration of the above steps into the formulation of the overall reactor model and the inclusion of heat transfer between the catalyst bed and external cooling or heating media. 

The bubble flow reactor (or flooded bed reactor) does not seem to have gained wide acceptance. In this reactor liquid and gas flow rates are limited by the necessity of avoiding to carry the catalyst away. Moreover it is claimed to be unstable reactor runaway can be initiated by hot spots which form near the gas distributor, in the case of exothermic reactions. Nevertheless flooded bed reactors have been used in a few processes where heat transfer between the cold liquid feed and the hot liquid of the reactor is realized by direct contacting and so ensures easily the initiation of the catalytic reaction (Figure 3). 

The model presented here is a significant step forward in the simulation of fixed bed catalytic reactors. It is an early computational fluid dynamics (CFD) model of the continuum type. In recent years supercomputers have led to an increased application of CFD to studies of heat transfer in packed beds. In modeling the fluid flow in the voids confined by the catalyst particles, Nijemeisland and Dixon [2004] investigated the possibility of deriving values for the heat transfer coefficient between the bed and the wall in terms of the local properties of the flow field, but found no statistically valid correlation. They... 

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