Diffusion catalyst temperature

During the studies carried out on this process some unusual behavior has been observed. Such results have led some authors to the conclusion that SSP is a diffusion-controlled reaction. Despite this fact, the kinetics of SSP also depend on catalyst, temperature and time. In the later stages of polymerization, and particularly in the case of large particle sizes, diffusion becomes dominant, with the result that the removal of reaction products such as EG, water and acetaldehyde is controlled by the physics of mass transport in the solid state. This transport process is itself dependent on particle size, density, crystal structure, surface conditions and desorption of the reaction products. 

Analytical detection of constituents in a gas mixture is also feasible by the alternating catalyst temperatures in the temperature region of the diffusion-controlled regime. Even under conditions where the total rate of catalytic... 

Chemical reaction (with insignificant pore diffusion resistance) Temperature Amount of catalyst Reactant concentrations Concentration of active components on catalyst Stirring rate Reactor design Catalyst particle size... 

The calculations set out above were based on the assumption that the catalyst surface was always at a temperature of 900°K, however, practical experience during the investigation set out in Part 1, revealed that the catalyst temperature always increased with increase in gas flow-rate. The dotted curves in Figure 4 illustrate the effect of such a variation from 900°K to 1200°K at the highest velocity, for the different values of n. When diffusion controls, the surface temperature has no effect, but when the chemical reaction rate controls (n= 7) the overall rate increases... 

Any chemical conversion requires time. The time-dependent development of a reaction, the reaction rate r, is expressed by its kinetics, which describes the correlation of r and the determining factors, such as concentration, pressure, temperature, diffusion, catalyst, mass and heat transfer, and so forth. 

Deactivation of the catalyst pellet has been measured in a laboratory continuous stirred tank reactor The catalyst pellet has been placed in the reactor in such a way that the reacting gas has been in contact only with the lateral surface of the cylinder. The gaseous phase of volume 100 ml has been extensively mixed in order to enable us to assume utiiforw concentrations through the reactor and to neglect external diffusion. The temperature in the jacket of the reactor has been regulated to AO C. The temperatures of the... 

Dalla Betta et al. first proposed an inert porous layer, or diffusion barrier, to prevent temperature runaway, and loosely interpreted the effect in terms of a reduction in the rate of combustion. A more rigorous interpretation of the effect of an inert porous layer on catalyst temperature was provided by McCarty et al, who also described the desired properties for diffusion layer materials, including a high thermal conductivity and low specific combustion activity. These authors stated that the high washcoat temperatures found in catalytic combustion of natural gas were due to the high diffusivity of methane in air, which causes the diffusion rate to the catalyst surface to match the rate of heat dissipation by conduction to the gas phase. The diffusion barrier decreases the rate of diffusion of methane to the catalyst surface, thus reducing the catalyst temperature. Modeling work by Hayes et al. confirmed those concepts. ... 

The reactor consisted of a fixed bed of x --in. cylindrical pellets. The pressure was 790 mm Hg. The external area of catalyst particles was 5.12 ft /lb, and the platinum did not penetrate into the interior of the alumina particles. Calculate the partial-pressure difference between the bulk-gas phase and the surface of the catalyst for SOj at each mass velocity. What conclusions may be stated with regard to the importance of external diffusion Neglect temperature differences. 

E. Tronconi and P. Forzatti, Experimental criteria for diffusional limitations during temperature-programmed desorption from porous catalysts, J. Catal, 1985, 93, 197. Y.-J. Huang, J. Xue and J.A. Schwarz, Experimental procedures for the analysis of intraparticle diffusion during temperature-programmed desorption from porous catalysts in a flow system, J. Catal, 1988,109, 396. 

At this level of the model, consider to be given the local current density i, the anode and cathode catalyst temperatures (da and 6c), the anode and cathode channel vapor concentrations (c and Cc), the anode channel hydrogen concentration (ch), and the cathode channel oxygen concentration (Co)-To be determined from the MEA model are the local cell voltage v, the diffusive water flux through the membrane / from cathode to anode, and the heat generated due to membrane resistance and cathode overpotential losses. As mentioned above, in this simplified model, the membrane resistivity is taken to be constant. Other resistances (except to in-plane currents in the bipolar plates in the stack model) are neglected. 

The hydrocarbon product spectrum produced by a Fischer-Tropsch (FT) catalyst is highly dependent upon the catalyst temperature and the rate of diffusion of reactants into the catalyst matrix. The reaction is highly exothermic and, when it is carried out in a fixed bed reactor, if rates of heat removal from the catalyst are not high, hotspots will form, resulting in variation in the product spectrum and also catalyst deactivation. Thin catalyst coatings coated to heat transfer surface areas in a CPR have been found to greatly enhance the yield of the desirable products per unit volume as compared to conventional fixed bed reactors. The reduction in reactor volume and reduction in associated equipment and low-pressure drop make the FT-CPR an attractive technology. 

There are many factors influencing internal diffusion, such as the size of particles and pore of catalysts, molecular diffusion coefficient, temperature, pressure and other parameters in reaction kinetics etc. Among these factors, the size of catalyst particles and reaction temperature are the most important and easily adjustable parameters. The estimation and elimination of internal diffusion effect can usually use the ways as follows ... 

Carbon monoxide oxidation on Pt catalysts is an example of a reaction of practical importance that can lead to multiplicity of steady states when the resistance to diffusion leads to significant concentration gradients. Typical for multiple steady states is a sudden jump from a relatively low rate of reaction or conversion to relatively high values upon an increase of the catalyst temperature. Upon decreasing the temperature, the jump back to the low reaction rate occurs... 

Researchers at the Politecnico di Milano [40-44] have demonstrated the interest of this kind of reactor for the kinetic study of methane oxidation. They have shown additional advantages of this reactor configuration the small annular zone leads to short diffusion distances the gas flow is laminar and theoretically based correlations can be used to describe mass transfer and high radiation losses lead to more isothermal conditions. However, its main advantages are the direct measurement of the catalyst temperature, thus eliminating any heat... 

A fuel cell operates at 0.6 V and 1 Acm . Calculate the temperature distribution through a gas diffusion layer-bipolar plate sandwich on the cathode side. Assume a constant heat flux at the gas diffusion-catalyst layer interface equal to all the fuel cell losses, except those due to ionic and electrical resistance. Assume that half of the resistive losses apply to the anode side and half to the cathode. Ionic resistance through the membrane is O.lOhm-cm. At the outer edge of the bipolar plate assume that heat is removed by a cooling fluid at 60°C, with heat transfer coefficient, h = 1600Wm K. Electrical resistivity of the gas diffusion layer and bipolar plate is 0.080hm-cm and 0.060hm-cm, respectively. There is a contact resistance of 0.005 Ohm-cm between the gas diffusion layer and bipolar plate. Effective thermal conductivity of GDL and bipolar plate is 1.7 Wm K and 20 Wm" K , respectively. There is a thermal contact resistance between these two layers equal to 1°C/W Thickness of GDL and bipolar plate is 0.38 mm and 3.3 mm, respectively. 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeotites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeotites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

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