Catalytic heat transport

To proceed with the topic of this section. Refs. 250 and 251 provide oversights of the application of contemporary surface science and bonding theory to catalytic situations. The development of bimetallic catalysts is discussed in Ref. 252. Finally, Weisz [253] discusses windows on reality the acceptable range of rates for a given type of catalyzed reaction is relatively narrow. The reaction becomes impractical if it is too slow, and if it is too fast, mass and heat transport problems become limiting. 

For a more detailed analysis of measured transport restrictions and reaction kinetics, a more complex reactor simulation tool developed at Haldor Topsoe was used. The model used for sulphuric acid catalyst assumes plug flow and integrates differential mass and heat balances through the reactor length [16], The bulk effectiveness factor for the catalyst pellets is determined by solution of differential equations for catalytic reaction coupled with mass and heat transport through the porous catalyst pellet and with a film model for external transport restrictions. The model was used both for optimization of particle size and development of intrinsic rate expressions. Even more complex models including radial profiles or dynamic terms may also be used when appropriate. 

Much research is done in the field of catalyst-design and reactor engineering in order to improve mass- and heat-transport properties [7], Another purpose is to improve the selectivity. This is, in some cases, strongly related to the transport properties, but in other cases the choice of catalytic material and design is essential. 

The mass transfer effects cause, in general, a decrease of the measured reaction rate. The heat transfer effects may lead in the case of endothermic reactions also to a decrease of the equilibrium value and the resulting negative effect may be more pronounced. With exothermic reactions, an insufficient heat removal causes an increase of the reaction rate. In such a case, if both the heat and mass transfer effects are operating, they can either compensate each other or one of them prevails. In the case of internal transfer, mass transport effects are usually more important than heat transport, but in the case of external transfer the opposite prevails. Heat transport effects frequently play a more important role, especially in catalytic reactions of gases. The influence of heat and mass transfer effects should be evaluated before the determination of kinetics. These effects should preferably be completely eliminated. 

In addition to these mass transport steps, heat conduction can also be important in heterogeneously catalyzed processes. For exothermic reactions the heat generated at the catalytic site must be dissipated away from the catalyst and into the reaction medium while heat must be supplied to the active sites for endothermic reactions. In liquid phase processes heat transport is generally not a significant factor since the liquid tends to equalize the temperature throughout the reaction medium and, thus, facilitate temperature control. In vapor phase processes, however, heat transport can be a significant problem. 

In catalytic reactions mass transfer from the fluid phase to the active phase inside the porous catalyst particle takes place via transport through a fictitious stagnant fluid film surrounding the particle and via diffusion inside the particle. Heat transport to or from the catalyst takes the same route. These phenomena are summarized in Fig. 8.15. 

Similar constitutive equations are used to approximate the integrals representing the interfacial heat transfer rates by convection and conduction through the stagnant films in the vicinity of a catalytic solid surface. Hence, the film model can be used to approximate the interfacial heat transport (3.167) by ... 

The simulation starts with a temperature level of 20 C over the entire length of the exhaust pipe. Five seconds after the engine start the heated brick reaches a temperature of approx. 400 C. At this time, the reduction of the hydrocarbon emmisions is still low because of the very small catalytic surface of the heated brick. As a result of the good convective heat transport, the second brick of the EHC reaches the ignition temperature level fast. Thus the conversion of the pollutants can be increased to 80-90 % within 9 s. The electrical heating of the first brick plus the reaction heat set free at the EHC help to warm-up the main catalyst and total combustion of the hydrocarbons is completed after only 25 s. 

Adsorption, and chemisorption in particnlar, is closely allied to heterogeneons catalytic reactions both involve similar mass and heat transport constraints, in addition to bond formation at the solid snrface. In fact, adsorption is viewed as a precnrsor to catalytic reaction, and desorption is viewed as the step snbseqnent to the reaction itself. Adsorption of the reactant(s) and prodnct(s) must be strong enongh to deflect the original bonds, bnt not so strong as to poison the catalyst. This phenomenon has been related to the adsorption potential snggested by Polanyi (see Section 14.3.2). 

Radial dispersion of mass and heat in fixed bed gas-solid catalytic reactors is usually expressed by radial Peclet number for mass and heat transport. In many cases radial dispersion is negligible if the reactor is adiabatic because there is then no driving force for long range gradients to exist in the radial direction. For non-adiabatic reactors, the heat transfer coeflScient at the wall between the reaction mixture and the cooling medium needs also to be specified. 

Dense CO2 is an ideal reaction medium for oxidation catalysis because its inertness with respect to oxidation offers safety and avoids side-products from solvent oxidation, and its complete miscibility with molecular oxygen provides high concentrations of the oxidant and eliminates mass transfer limitations. Furthermore, the excellent heat transport capacity of SCCO2 allows effective heat control in exothermic oxidation reactions. Recently, a review of catalytic oxidations in dense CO2 has been published. ... 

Fj is the component molar flowrate i = H2O, CO, COj, H2, Inert) rj is the catalyst effectiveness factor e is the catalyst bed porosity (assumed to be equal to 0.5) is the shell tube radius r,i is the membrane tube radius (fco) is the rate of WGS reaction is the reaction temperature (K) n is the number of chemical species cpt is specific heat [J/mol Kj AHreaz is AH of reaction at temperature T [J/molj and U2 is a global heat transport coefficient from catalytic zone to permeation one [J/m -s-K]. 

A detailed example is included here to evaluate thin-film catalytic microreactors as kinetic tools compared with conventional laboratory reactors. Most kinetic studies carried out in laboratory reactors which utilize small catalyst particles and the intrinsic kinetics for rapid reactions could be well hidden by mass and heat transport limitations. Existing criteria for mass and heat transport were estimated for both a microreactor with a thin-film catalyst thickness of 5 pm and a packed-bed laboratory reactor with radii of 2 and 4mm [22]. Based on the calculation of Weitz-Prater... 

Heat and Mass Transfer Using the film theory, both phenomena mainly depend on the film and gas stream thickness and the type of reaction. Other parameters are the interfacial area, the residence time and the axial dispersion. Good mass and heat transport presume a good fiow equipartition in the channels. In mesh reactors the mesh open area determines the interfacial area. Mass transfer coefficients ki a from 3 to 8 L s and higher values in catalytic systems can be achieved [25]. 

When testing catalytic properties, it is of utmost importance that other phenomena than those occurring at the catalyst s active sites do not become a limiting factor. Only then the observations can be directly related to the catalyst properties. Two types of other phenomena are likely to affect the observations, that is, the mass and heat transport phenomena at the catalyst pellet scale and the reactor flow pattern nonidealities at the reactor scale. 

The volume basis for this simple model is the material of the fins or pillars between the channels or in the slits, respectively. This specific volume is responsible for transport through a stack of several catalytically modified microstructured plates without intermediate cooling or heating. Assumption for this simple solution of the heat transport equation is a temperature constant heat production rate, so that only small predicted gradients fit the experiment. The stack height without intermediate cooling/heating is... 

In the previous chapters we predominantly considered catalysis as a molecular event, in which substrate molecules are activated by the catalyst. In this chapter and the next we will emphasize catalytic features of dimensions in space much larger than that of single catalytic centers and times much longer than those associated with the individual molecular catalytic cycles. Often mass and heat transport cause reaction cycles, which occur at different sites, to interact. Under particular conditions this gives rise to cooperative phenomena with oscillatory kinetics and temporal spatial organization. As such, interesting surface patterns such as spirals or pulsars may form. Such complex cooperative phenomena are known in physics as appearances of excitable systems. Their characteristic features are easily influenced by small variations in external conditions. Hence these systems have also features that are called adaptive. 

This chapter deals with the microkinetics of gas-solid catalytic reaction systems. An applied approach is adopted in the discussion, which starts with the formulation of intrinsic rate equations that account for chemical processes of adsorption and surfece reaction on solid catalysts and then proceeds with the construction of global rate expressions that include the individual and simultaneous effects of physical external and internal mass and heat transport phenomena occurring at the particle scale. 

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