Interphase transport effects

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

A similar situation occurs when interphase transport effects are considered. Table 3 gives a survey of experimental criteria for the estimation of interphase transport effects. The most general relationship here is criterion 4. However, again it may be suggested that the separate isothermicity criterion 5 be used first, and... 

Table 3. Experimental diagnostic criteria for the absence of interphase, and combined intraparticle and interphase transport effects in twipip irreversible reactions (power law kinetics only). ...

Table 4 summarizes a number of well-known theoretical diagnostic criteria for the estimation of intraparticle transport effects on the observable reaction rate. Tabic 5 gives a survey of the respective criteria for interphase transport effects. It is quite obvious that these are more difficult to use than the simple experimental criteria given in Tables 2 and 3. In general, the intrinsic rate expression has to be specified and, additionally, either the first derivative of the intrinsic rate with respect to concentration (and temperature) at surface... 

Valent, I., Adamcikova, L., and Sevcik, P. Simulations of the iodine interphase transport effect on the oscillatory Bray-Liebhafsky reaction, J. Phys. Chem. A., 102, 7576-7579, 1998. 

CuCl, PEG6000-(TEMPO)2, and oxygen are essential for the oxidation of benzyl alcohol into benzaldehyde. The presence of C02 improves the reaction, presumably being ascribed to high miscibility of 02 into compressed C02, thus eliminating interphase transport limitation, and expandable effect of PEG in compressed C02 [63, 64],... 

In the most general case, i.e. when intraparticlc and interphase transport processes have to be included in the analysis, the effectiveness factor depends on five dimensionless numbers, namely the Thiele modulus the Biot numbers for heat and mass transport Bih and Bim, the Prater number / , and the Arrhenius number y. Once external transport effects can be neglected, the number of parameters reduces to three, because the Biot numbers then approach infinity and can thus be discarded. 

The overall effectiveness factor is actually comprised of the individual effectiveness factors for intraphase and interphase transport ... 

For industrial reactors, the effectiveness factor (i]) is used to provide a measure of the actual reaction rate, as affected by operating conditions, in comparison to the intrinsic reaction kinetics. Assuming that the trickle bed reactor shown in Fig. 4 is operated so that interphase transport of one of the reactants, steps 2 or 8 above, is controlling. 

Interphase Transport. Transport phenomena between the catalyst surface and the bulk gas may control the reaction rate for fast—extremely exothermic or endothermic—reactions. To study chemical events on the catalyst surface, these transport effects must be minimized. It is simple to check for the influence of transport effects in a CSTCR. For a catalyst mounted in a stationary basket, the stirrer speed is varied while all other variables are held constant. Changes in conversion with varying stirrer speeds indicate the presence of transport effects. A temperature gradient between the gas and the catalyst will exist if the reaction is heat-transfer... 

The significance of the zero value in equation (4-160) is that of an approximation to the equilibrium curve in Figure 4.36, where we state in effect that the equilibrium C 0 for the entire range of q values, i.e., q q o for even very small values of C. Obviously the validity of this assumption will depend very much on the particular fluid-solid system considered and how sharp the C — q curve is. The mass-transfer coefficient kj S is a volume-based quantity, where S is the specific surface area per volume for interphase transport. Now, let us make a simple change of variables... 

The definition of equation (7-1) does not envision differences between bulk and external surface concentrations, a point that will be discussed later. We will first treat the problem of transport limitations within the porous matrix (intraphase), then the combination of boundary-layer (interphase) transport with the intraphase effects. ... 

To this point we have dealt only with transport effects within the porous catalyst matrix (intraphase), and the mathematics have been worked out for boundary conditions that specify concentration and temperature at the catalyst surface. In actual fact, external boundaries often exist that offer resistance to heat and mass transport, as shown in Figure 7.1, and the surface conditions of temperature and concentration may differ substantially from those measured in the bulk fluid. Indeed, if internal gradients of temperature exist, interphase gradients in the boundary layer must also exist because of the relative values of the pertinent thermal conductivities [J.J. Carberry, Ind. Eng. Chem., 55(10), 40 (1966)]. 

Additional discussions of parameter values involved in transport effects on chemical reactor behavior are given by Carberry [J.J. Carberry, Ind. Eng. Chem. Fundls., 14, 123 (1975)] Carberry and White [J.J. Carberry and D. White, Ind. Eng. Chem., 61, 27 (1969)] Cresewell and Patterson [D.L. Cresswell and W.R. Patterson, Chem. Eng. Sci., 25, 1405 (1970)] and Dumez and Froment [F.J. Dumez and G.F. Froment, Ind. Eng. Chem. Proc. Design Devel., 15, 291 (1976)]. We will have more to say about interphase transport in fixed beds later in this chapter. 

Dynamic analysis of a trickle bed reactor is carried out with a soluble tracer. The impulse response of the tracer is given at the inlet of the column to the gas phase and the tracer concentration distributions are obtained at the effluent both from the gas phase and the liquid phase simultaneously. The overall rate process consists the rates of mass transfer between the phases, the rate of diffusion through the catalyst pores and the rate of adsorption on the solid surface. The theoretical expressions of the zero reduced and first absolute moments which are obtained for plug flow model are compared with the expressions obtained for two different liquid phase hydrodynamic models such as cross flow model and axially dispersed plug flow model. The effect of liquid phase hydrodynamic model parameters on the estimation of intraparticle and interphase transport rates by moment analysis technique are discussed. 

The presence or significance of interphase transport resistances is confirmed either by conducting diagnostic experiments or by utilizing a well-established criterion. A preliminary set of experiments that must be conducted in laboratory PBRs in order detect the effect of external mass transport is to measure the dependence of the exit conversion Xa on the linear velocity of the fluid through the catalyst bed, u (m/s), which is in fact the velocity of the fluid relative to the catalyst particles [7, 23]. 

Our analysis indicates that kinetic retardation of the interphase transport is essential for a well-balanced theory away from the critical point. The available simulations of the motion of a diffuse interface near a three-phase contact line, taking into account viscous retardation only and, in effect, assuming evaporation or condensation to be as easy as plain advection, may grossly overestimate the rate of interphase transport, but the latter remains essential even when its order of magnitude is reduced due to kinetic retardation. 

The choice between batchwise and continuous operation, or between a tubular and a mixed reactor, may also be determined by Ae reactor configuration. In many two-phase processes a continuous operation is preferred because only Aen it is possible to have constant interphase transport rates. For an effective mass transfer between a suspended and a continuous phase a strong relative motion between Ae phases is required. This usually calls for vigorous agitation, which at Ae same time will increase Ae residence time distribution. Wien Aat is no serious drawback Ae stirred tank reactor is often preferred (sections 4.5,1,3, 4,6,1,3 and 7.2.3). For gas/solid or liquid/solid contact Ae fluidized bed reactor may be attractive (sections 45,1,4, 5.4.4 and 7,2.2,4),... 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

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