Catalysts, activity temperature gradients

Catalyst Effectiveness. Even at steady-state, isothermal conditions, consideration must be given to the possible loss in catalyst activity resulting from gradients. The loss is usually calculated based on the effectiveness factor, which is the diffusion-limited reaction rate within catalyst pores divided by the reaction rate at catalyst surface conditions (50). The effectiveness factor E, in turn, is related to the Thiele modulus,

first-order rate constant, a the internal surface area, and the effective diffusivity. It is desirable for E to be as close as possible to its maximum value of unity. Various formulas have been developed for E, which are particularly usehil for analyzing reactors that are potentially subject to thermal instabilities, such as hot spots and temperature mnaways (1,48,51). 

Some reactions have been found to proceed with better results in the absence of solvent, probably because of the creation of temperature gradients which are eliminated in the presence of a stirred solvent. This was observed for the Diels-Alder reaction of a-amino acid precursors with cyclopentadiene catalyzed by heterogeneous catalysts (Si02-Al, Si02-Ti), when the reaction was performed in toluene or in the absence of solvent [53], Microwave activation increased the rate of reaction without reducing the selectivity of the reaction. 

In the suspended state, however, no temperature gradient arises at the catalyst surface since the active site temperature is the same as the boiling point of the solution. Only small magnitudes of reaction rates and conversions were obtained in the suspended state because of diminished rate constant k and enlarged retardation constant K (Table 13.2). 

Steep temperature gradients inside the catalyst layer will enhance the bubble formation and bring about efficient product desorption and effective regeneration of vacant active sites consequently. There irreversible processes are followed by another irreversible act of bubble detachment from the surface. 

The catalyst pellets were 3.2 by 3.2 mm cylinders, and the Pt was superficially deposited upon the external surface. Compute both external mass and temperature gradients and plot AC o, and AT versus the mass velocity. Can you draw any qualitative conclusions from this plot If the reaction activation energy is 30 kcal/mol, what error in rate measurement attends neglect of an external AT What error prevails if, assuming linear kinetics in SO2, external concentration gradients are ignored ... 

The rate and selectivity of a surface-catalyzed reaction can be affected by the existence of concentration or temperature gradients in the quiescent layer of fluid which surrounds the catalyst particle or is contained within its pores upon whose surface much of the exposed active material is distributed. The reactants in the bulk phase reach the reactive sites by diffusion through these regions of the fluid, and they affect the kinetics when the rates of the surface-catalyzed reactions are fast relative to the rate of transport of reactants to the catalytic sites. In the hydrogenation of unsaturated liquids or compounds in solution, the agitation of the liquid-catalyst mixture must be adequate to assure that the solution remains saturated with hydrogen. 

The catalyst bed temperature increases in the direction of gas flow due to the WGS reaction exotherm. Typical temperature gradients in the bed are about 20-30°C. The lifetime and state of activity of the catalyst is conveniently monitored by the temperature profile through the adiabatic bed. As the reaction front moves through the bed when the catalyst ages, so does the temperature rise from the reaction (Fig. 3). 

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