Catalysts heat effect

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

Other cases, neglecting heat effects would cause serious errors. In such cases the mathematical treatment requires the simultaneous solution of the diffusion and heat conductivity equations for the catalyst pores. 

It does not matter that there is no known catalyst that can accomplish the reaction in Equation (7.21) directly. Heats of reaction, including heats of formation, depend on conditions before and after the reaction but not on the specific reaction path. Thus, one might imagine a very complicated chemistry that starts at standard conditions, goes through an arbitrary trajectory of temperature and pressure, returns to standard conditions, and has Equation (7.21) as its overall effect. A77. =-1-147,360 J/mol of styrene formed is the net heat effect associated with this overall reaction. 

Taskin, M.E. et al. (2008) CFD study of the influence of catalyst particle design on steam reforming reaction heat effects in narrow packed tubes. Ind. Eng. Chem. Res., 47 (16), 5966-5975. 

Effective Thermal Conductivities of Porous Catalysts. The effective thermal conductivity of a porous catalyst plays a key role in determining whether or not appreciable temperature gradients will exist within a given catalyst pellet. By the term effective thermal conductivity , we imply that it is a parameter characteristic of the porous solid structure that is based on the gross geometric area of the pellet perpendicular to the direction of heat transfer. For example, if one considers the radial heat flux in a spherical pellet one can say that... 

Partial oxidation of methane to syngas over Ni and Co catalysts was effected by use of microwave irradiation, and compared with conventional heating [73]. Although the same conversion levels and H2/CO ratio (2.0 0.2) were observed, the temperature of the catalyst bed was much lower (200 K) when microwave irradiation was used than with conventional heating. Under both activation modes the Ni-based... 

Steinfeld et al. [133] demonstrated the technical feasibility of solar decomposition of methane using a reactor with a fluidized bed of catalyst particulates. Experimentation was conducted at the Paul Scherrer Institute (PSI, Switzerland) solar furnace delivering up to 15 kW with a peak concentration ratio of 3500 sun. A quartz reactor (diameter 2 cm) with a fluidized bed of Ni (90%)/Al2O3 catalyst and alumina grains was positioned in the focus of the solar furnace. The direct irradiation of the catalyst provided effective heat transfer to the reaction zone. The temperature was maintained below 577°C to prevent rapid deactivation of the catalyst. The outlet gas composition corresponded to 40% conversion of methane to H2 in a single pass. Concentrated solar radiation was used as a source of high-temperature process heat for the production of hydrogen and filamentous... 

Our initial work on reaction thermal effects involved CFD simulations of fluid flow and heat transfer with temperature-dependent heat sinks inside spherical particles. These mimicked the heat effects caused by the endothermic steam reforming reaction. The steep activity profiles in the catalyst particles were approximated by a step change from full to zero activity at a point 5% of the sphere radius into the pellet. 

The recycle reactor with large recycle acts as a mixed flow reactor and shares its advantages. Actually, to minimize heat effects the catalyst need not be all at one location, but can be distributed throughout the recycle loop. 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

Another part of our investigation deals with the effect of heat treatment on the leaching behavior of palladium on activated carbon catalysts. Heat treatment is a known technique to increase the performance of catalysts. (3) Therefore, standard carbon supported palladium catalysts were exposed to different temperatures ranging from 100 to 400 °C under nitrogen. The catalysts were characterized by metal leaching, hydrogenation activity and CO-chemisorption. 

Carbamates have been prepared by heating ethyl carbamate with a higherboiling alcohol in the presence or absence of catalysts [31-33], Aluminum iso-propoxide has been reported [34] to be an excellent catalyst for the interchange reaction between ethyl carbamate and benzyl alcohol. The interchange reaction is also effective for /V-alkyl carbamates as well as unsubstituted carbamates [35]. This catalyst is effective in preparing mono- and dicarbamates in excellent yields from primary and secondary alcohols and diols. Other effective catalysts are dibutyltin dilaurate [36], dibutyltin oxide [37], sulfuric acid or p-toluenesulfonic acid [31], and sodium metal (reacts with alcohols to give the alkoxide catalyst) [33]. 

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

SYNTHESIS A solution of 33.2 g of veratraldehyde in 15.0 g nitroethane was treated with 0.9 g of n-amylamine and placed in a dark place at room temperature. In a day or so, separated H20 was apparent and, after a couple of weeks, the mixture completely solidified. The addition of 50 mL EtOH and heating effected complete solution and, on cooling, this provided 1 -(3,4-dimethoxyphenyl)-2-nitropropene as yellow crystals, 29.0 g, with nip of 70-71 °C. The more conventional reaction scheme, 6 h heating of a solution of the aldehyde and nitroethane in acetic acid with ammonium acetate as catalyst, gave a much inferior yield of product (33.2 g gave... 

Thermal reforming a process using heat (but no catalyst) to effect molecular rearrangement of low-octane naphtha into gasoline of higher antiknock quality. 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

Although there appears to be good evidence that redispersion can occur, Smith et 0/.251,252 observe no redispersion either for model or industrial Pt catalysts heated in 3% 02/N2 at 673-873 K. Instead they suggest that re-dispersion involves the recovery of Pt sites lost to the chemisorption of H2 rather than a particle size effect. Why re-dispersion does not occur in this case is not clear, but may be related to the use of a low partial pressure of 02 (see earlier), so that a Pt-Al2 03 complex does not form. 

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