Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Pellets, mass transfer transport

One must understand the physical mechanisms by which mass transfer takes place in catalyst pores to comprehend the development of mathematical models that can be used in engineering design calculations to estimate what fraction of the catalyst surface is effective in promoting reaction. There are several factors that complicate efforts to analyze mass transfer within such systems. They include the facts that (1) the pore geometry is extremely complex, and not subject to realistic modeling in terms of a small number of parameters, and that (2) different molecular phenomena are responsible for the mass transfer. Consequently, it is often useful to characterize the mass transfer process in terms of an effective diffusivity, i.e., a transport coefficient that pertains to a porous material in which the calculations are based on total area (void plus solid) normal to the direction of transport. For example, in a spherical catalyst pellet, the appropriate area to use in characterizing diffusion in the radial direction is 47ir2. [Pg.432]

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. [Pg.435]

This study was carried out to simulate the 3D temperature field in and around the large steam reforming catalyst particles at the wall of a reformer tube, under various conditions (Dixon et al., 2003). We wanted to use this study with spherical catalyst particles to find an approach to incorporate thermal effects into the pellets, within reasonable constraints of computational effort and realism. This was our first look at the problem of bringing together CFD and heterogeneously catalyzed reactions. To have included species transport in the particles would have required a 3D diffusion-reaction model for each particle to be included in the flow simulation. The computational burden of this approach would have been very large. For the purposes of this first study, therefore, species transport was not incorporated in the model, and diffusion and mass transfer limitations were not directly represented. [Pg.374]

Micropore mass transfer resistance of zeoUte crystals is quantified in units of time by r /Dc, where is the crystal radius and Dc is the intracrystalline diffusivity. In addition to micropore resistance, zeolitic catalysts may offer another type of resistance to mass transfer, that is resistance related to transport through the surface barrier at the outer layer of the zeoHte crystal. Finally, there is at least one additional resistance due to mass transfer, this time in mesopores and macropores Rp/Dp. Here Rp is the radius of the catalyst pellet and Dp is the effective mesopore and macropore diffusivity in the catalyst pellet [18]. [Pg.416]

If zeolitic diffusion is sufficiently rapid so that the sorbate concentration through any particular crystal is essentially constant and in equilibrium with the macropore fluid just outside the crystal, the rate of mass transfer will be controlled by transport through the macropores of the pellet. Transport through the macropores may be assumed to occur by a diffusional process characterized by a constant pore diffusion coefficient Z)p. The relevant form of the diffusion equation, neglecting accumulation in the fluid phase within the macropores which is generally small in comparison with accumulation within the zeolite crystals, is... [Pg.348]

Heat and mass transfer processes always proceed with finite rates. Thus, even when operating under steady state conditions, more or less pronounced concentration and temperature profiles may exist across the phase boundary and within the porous catalyst pellet as well (Fig. 2). As a consequence, the observable reaction rate may differ substantially from the intrinsic rate of the chemical transformation under bulk fluid phase conditions. Moreover, the transport of heat or mass inside the porous catalyst pellet and across the external boundary layer is governed by mechanisms other than the chemical reaction, a fact that suggests a change in the dependence of the effective rate on the operating conditions (i.e concentration and temperature). [Pg.326]

Figures 13 and 14 refer to the situation where only intraparticlc transport effects influence the observable reaction rate. However, a similar behavior is observed if, besides intraparticle heat and mass transport processes, the heat and mass transfer between the catalyst pellet and the bulk fluid phase is also considered. More information about this situation can be found, for example, in the works of Cresswell [26], McGreavy and... Figures 13 and 14 refer to the situation where only intraparticlc transport effects influence the observable reaction rate. However, a similar behavior is observed if, besides intraparticle heat and mass transport processes, the heat and mass transfer between the catalyst pellet and the bulk fluid phase is also considered. More information about this situation can be found, for example, in the works of Cresswell [26], McGreavy and...
The second necessary condition is isothermal operation. This is apparent from the results of Sections 6.2.3.3 and 6.2.3.4 where it has been shown that heat and mass transport may drive the effective reaction rate in opposite directions. Normally, mass transfer control of a reaction means a drop of the effective reaction rate (for positive reaction order), whereas a limited heat transfer in the case of an exothermal reaction will cause the temperature inside the catalyst pellet to rise, and will thus increase the effective reaction rate. When both effects occur simultaneously, an increase as well as a decrease of the effective rate may be observed, indicating either a lower or a higher apparent activation... [Pg.346]

The nature and arrangement of the pores determine transport within the interior porous structure of the catalyst pellet. To evaluate pore size and pore size distributions providing the maximum activity per unit volume, simple reactions are considered for which the concept of the effectiveness factor is applicable. This means that reaction rates can be presented as a function of the key component. A only, hence RA(CA). Various systems belonging to this category have been discussed in Chapters 6 and 7. The focus is on gaseous systems, assuming the resistance for mass transfer from fluid to outer catalyst surface can be neglected and the effectiveness factor does not exceed unity. The mean reaction rate per unit particle volume can be rewritten as... [Pg.177]

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. [Pg.240]

While the above criteria are useful for diagnosing the effects of transport limitations on reaction rates of heterogeneous catalytic reactions, they require knowledge of many physical characteristics of the reacting system. Experimental properties like effective diffusivity in catalyst pores, heat and mass transfer coefficients at the fluid-particle interface, and the thermal conductivity of the catalyst are needed to utilize Equations (6.5.1) through (6.5.5). However, it is difficult to obtain accurate values of those critical parameters. For example, the diffusional characteristics of a catalyst may vary throughout a pellet because of the compression procedures used to form the final catalyst pellets. The accuracy of the heat transfer coefficient obtained from known correlations is also questionable because of the low flow rates and small particle sizes typically used in laboratory packed bed reactors. [Pg.229]

Both interphase and intraphase mass transfer limitations are minimized by decreasing the pellet size of the catalyst. Since a packed bed of very small catalyst particles can cause an unacceptably large pressure drop in a reactor, a compromise between pressure drop and transport limitations is often required in commercial reactors. Fortunately, laboratory reactors that are used to obtain... [Pg.231]

For first-order reactions we can use an overall effectiveness factor to help us analyze diffusion, flow, and reaction in packed beds. We now consider a situation where external and internal resistance to mass transfer to and within the pellet are of the same order of magnitude (Figure 12-9). At steady state, the transport of the reactant(s) from the bulk fluid to the external surface of the catalyst is equal to the net rate of reaction of the reactant within and on the pellet. [Pg.755]

Next we can use Equations (12-45) through (12-48) to eliminate from Equation (12-47), so that the total molar transport of A from the bulk fluid to the external pellet surface can be expressed solely in terms of bulk concentration and other parameters of the system (e.g., the mass transfer coefficient, and the specific reaction rate, i). [Pg.756]

Transport to the Catalyst Pellet The rate of mass transfer of H2 from the bulk solution to the external surface of catalyst particles is... [Pg.772]

Most commercial adsorbents consist of small microporous or nonporous crystals formed into macroporous pellets or particles. The solutes carried along the col-lunn by the fluid mobile phase must first be transported from the bulk fluid phase to the external surface of the adsorbent and then they must diffuse inside the particles. Within a particle there are two distinct kinds of diffusion phenomena that contribute to the resistances to mass transfer, the macropore (or inter-crystaUine) diffusion through the pellet and the micropore (or intra-crystaUine) diffusion resistance. The relative importance of macropore and micropore diffusion resistances depends on the pore size distribution within an adsorbent particle. Micropores have diameters smaller than 2 nm, macropores diameters greater than 50 nm while mesopores are in the range of 2 to 50 nm. [Pg.236]

For an endothermic reaction there is a decrease in temperature and rate into the pellet. Hence 17 is always less than unity. Since the rate decreases with drop in temperature, the effect of heat-transfer resistance is diminished. Therefore the curves for various are closer together for the endothermic case. In fact, the decrease in rate going into the pellet for endothermic reactions means that mass transfer is of little importance. It has been shown that in many endothermic cases it is satisfactory to use a thermal effectiveness factor. Such thermal 17 neglects intrapellet mass transport that is, ri is obtained by solution of Eq. (11-72), taking C = Q. [Pg.448]

A limiting case of intrapellet transport resistances is that of the thermal effectiveness factor In this situation of zero mass-transfer resistance, the resistance to intrapellet heat transfer alone establishes the effectiveness of the pellet. Assume that the temperature effect on the rate can be represented by the Arrhenius function, so that the rate at any location is given by r = A... [Pg.465]

For -in. pellets both external and internal mass transport retard the rate. The internal mass-transfer resistance is sizable, as indicated by r values of the order of 0.5. The external resistance seldom exceeded 10% of the total. [Pg.476]

Problem 4-3. Reaction-Diffusion in a Spherical Catalyst Pellet. Consider a spherical catalyst pellet. Assume that transport of product is by diffusion, with diffusivity Deff. As in the problem discussed in Section F, we assume that the transport of reactant within the pellet is decoupled from the transport of product. Mass transfer in the gas is sufficiently rapid so that the reactant concentration at the catalyst surface is maintained at c,x,. [Pg.285]

Gas-solid (catalytic) reactions. Mass transfer is likely to be more important within the pellet than in the external film, and heat transfer more important in the film than within the pellet. In other words, intraphase mass transfer and interphase heat transfer would normally be the dominant transport processes. Thus the pellet can reasonably be assumed to be isothermal. [Pg.764]


See other pages where Pellets, mass transfer transport is mentioned: [Pg.510]    [Pg.857]    [Pg.225]    [Pg.241]    [Pg.435]    [Pg.439]    [Pg.280]    [Pg.281]    [Pg.156]    [Pg.34]    [Pg.98]    [Pg.857]    [Pg.360]    [Pg.138]    [Pg.92]    [Pg.104]    [Pg.332]    [Pg.282]    [Pg.231]    [Pg.392]    [Pg.699]    [Pg.1250]    [Pg.487]    [Pg.358]    [Pg.420]    [Pg.332]   
See also in sourсe #XX -- [ Pg.610 ]




SEARCH



Mass transfer/transport

Mass transport

Pellets, mass transfer

Transport pellets

© 2024 chempedia.info