Reactor Transport Properties

A number of factors limit the accuracy with which parameters for the design of commercial equipment can be determined. The parameters may depend on transport properties for heat and mass transfer that have been determined under nonreacting conditions. Inevitably, subtle differences exist between large and small scale. Experimental uncertainty is also a factor, so that under good conditions with modern equipment kinetic parameters can never be determined more precisely than 5 to 10 percent (Hofmann, in de Lasa, Chemical Reactor Design and Technology, Martinus Nijhoff, 1986, p. 72). 

Thus, the local (static) quantities represent the stationary properties affecting the static pressure and the local reactivity in a nuclear reactor. The flow (dynamic) quantities represent the transport properties affecting the energy, momentum, and mass balances of a flow. 

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. 

A comprehensive discussion of the most important model parameters covers phase equilibrium, chemical equilibrium, physical properties (e.g., diffusion coefficients and viscosities), hydrodynamic and mass transport properties, and reaction kinetics. The relevant calculation methods for these parameters are explained, and a determination technique for the reaction kinetics parameters is represented. The reaction kinetics of the monoethanolamine carbamate synthesis is obtained via measurements in a stirred-cell reactor. Furthermore, the importance of the reaction kinetics with regard to axial column profiles is demonstrated using a blend of aqueous MEA and MDEA as absorbent. 

Fluids are highly compressible along near-critical isotherms (L01-1.2 Tc) and display properties ranging from gas-like to Liquid-Like with relatively small pressure variations around the critical pressure. The liquid-like densities and better-than-liquid transport properties of supercritical fluids (SCFs) have been exploited for the in situ extraction of coke-forming compounds from porous catalysts [1-6], For i-hexene reaction on a low activity, macroporous a catalyst, Tiltschcr el al. [1] demonstrated that reactor operation at supercritical... 

The catalyst must be as homogeneous as possible to get good spectroscopic data. On the other hand, basic engineering rules such as flow patterns through the reactor, heat- and mass-transport properties, dead volume, and catalytic measurements need to be fulfilled. Therefore, preferentially, a thin layer of a catalyst or a sieved catalyst fraction should be applied, especially if the reactions are rapid [31], Moreover, such studies should be performed under realistic conditions (i.e. in gas phase, liquid phase [including catalyst preparation], or even at high pressure). 

Employing 1-hexene isomerization on a Pt/y-ALOj reforming catalyst as a model reaction system, we showed that isomerization rates are maximized and deactivation rates are minimized when operating with near-critical reaction mixtures [2]. The isomerization was carried out at 281°C, which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are the main reaction products, the critical temperature and pressure of the reaction mixture remain virtually unaffected by conversion. Thus, an optimum combination of gas-like transport properties and liquid-like densities can be achieved with relatively small changes in reactor pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like) reaction media for the in situ extraction of coke-forming compounds. 

It will be supposed that the kinetics of all the reactions that are going on and the thermodynamical and molecular transport properties of all the substances present are known, and that it is desired to find out how the composition of the effluent from a reactor depends on the conditions that are imposed. The conditions that must be fixed are the composition, pressure, temperature, and flow rate of the reactant mixture, the dimensions of the reactor and of the catalyst pellets, and enough properties of the heat-transfer medium to determine a relation between the temperature of the tube wall and the heat flux through it. 

This illustration does not pretend to represent the correspondence to be expected between a real scale-model reactor and its prototype, but to show that if the transport properties can be accurately estimated, a useful scale model can be constructed in simple cases. In the real situation, there may be large uncertainties in the best obtainable estimates of the transport properties, and there may be in addition significant effects of... 

The general approach for modelling catalyst deactivation is schematically organised in Figure 2. The central part are the mass balances of reactants, intermediates, and metal deposits. In these mass balances, coefficients are present to describe reaction kinetics (reaction rate constant), mass transfer (diffusion coefficient), and catalyst porous texture (accessible porosity and effective transport properties). The mass balances together with the initial and boundary conditions define the catalyst deactivation model. The boundary conditions are determined by the axial position in the reactor. Simulations result in metal deposition profiles in catalyst pellets and catalyst life-time predictions. 

If solubility alone determined the optimal cleaning conditions, a few selections of T and P would determine the conditions for running the reactor. The transport properties of the fluid interacting with the part to be cleaned may be just as important as solubility in determining the overall success of the cleaning operation. Transport effects and scaleup issues are of crucial concern in cleaning and are discussed in detail in the chapter by M. R. Phelps et al. 

D. Casanave, A. Giroir-Fendler, J. Sanchez, R. Loutaty, and J.A. Dalmon, Control of transport properties with a microporous membrane reactor to enhance yields in dehydrogenation reactions, Cat. Today 25 309 (1995). 

The efforts and advances during the last 15 years in zeolite membrane and coating research have made it possible to synthesize many zeolitic and related-type materials on a wide variety of supports of different composition, geometry, and structure and also to predict their transport properties. Additionally, the widely exploited adsorption and catalytic properties of zeolites have undoubtedly opened up their scope of application beyond traditional separation and pervaporation processes. As a matter-of-fact, zeolite membranes have already been used in the field of membrane reactors (chemical specialties and commodities) and microchemical systems (microreactors, microseparators, and microsensors). 

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