Kinetics intrinsic

It is possible to eliminate the mass transfer resistances in Steps 2, 3, 7, and 8 by grinding the catalyst to a fine powder and exposing it to a high-velocity gas stream. The concentrations of reactants immediately adjacent to the catalytic surface are then equal to the concentrations in the bulk gas phase. The resulting kinetics are known as intrinsic kinetics since they are intrinsic to the catalyst surface and not to the design of the pores, or the pellets, or the reactor. 

Most research in heterogeneous catalysis is concerned with the measurement, understanding, and modihcation of intrinsic kinetics. 

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It is the magnitude of the various k values in Eqs. (7.1)-(7.4) that describes the intrinsic kinetic differences between the various modes of addition, and the k s plus the concentrations of the different species determine the rates at which the four kinds of additions occur. It is the proportion of different steps which determines the composition of the copolymer produced. 

Intrinsic Kinetics. Chemisorption may be regarded as a chemical reaction between the sorbate and the soHd surface, and, as such, it is an activated process for which the rate constant (/ ) follows the familiar Arrhenius rate law ... 

Table 5.1 presents the intrinsic kinetic parameters (Km and Vln lx) for the free lipase system and apparent kinetic parameters (K and V ) for the immobilised lipase in the EMR using fixed 2g-l 1 lipase concentration. The immobilised lipase showed higher maximum apparent reaction rate and greater enzyme-substrate (ES) affinity compared with free lipase. 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

Example 10.1 Consider the heterogeneously catalyzed reaction A —> P. Derive a plausible form for the intrinsic kinetics. The goal is to determine a form for the reaction rate that depends only on gas-phase concentrations. 

Solution Under the assumption of intrinsic kinetics, all mass transfer steps are eliminated, and the reaction rate is determined by Steps 4-6. The simplest possible version of Steps 4-6 treats them all as elementary, irreversible reactions ... 

But ai = a for intrinsic kinetics. Making this substitution and solving for ... 

Solution The elementary reaction steps of adsorption, reaction, and desorption are now reversible. From this point on, we will set ai = a, pi = P, and so on, since the intrinsic kinetics are desired. The relationships between ai, a, and a are addressed using an eflectiveness factor in Section 10.4. The various reaction steps are... 

Deduce the functional form for the pseudohomogeneous, intrinsic kinetics. 

Few fixed-bed reactors operate in a region where the intrinsic kinetics are applicable. The particles are usually large to minimize pressure drop, and this means that diffusion within the pores. Steps 3 and 7, can limit the reaction rate. Also, the superficial fluid velocity may be low enough that the external film resistances of Steps 2 and 8 become important. A method is needed to estimate actual reaction rates given the intrinsic kinetics and operating conditions within the reactor. The usual approach is to define the effectiveness factor as... 

Example 10.8 How fine would you have to grind the ethylbenzene catalyst for laboratory kinetic studies to give the intrinsic kinetics Assume the small pore diameter of Example 10.7. 

Solution Take 17 = 0.98 as an adequate approach to the intrinsic kinetics. Setting this value for r] and solving Equation (10.25) for gives... 

Catalyst pellets often operate with internal temperatures that are substantially different from the bulk gas temperature. Large heats of reaction and the low thermal conductivities typical of catalyst supports make temperature gradients likely in all but the hnely ground powders used for intrinsic kinetic studies. There may also be a him resistance to heat transfer at the external surface of the catalyst. 

It is a good idea to run the laboratory reactor without catalyst to check for homogeneous reactions. However, this method does not work when the homogeneous reaction involves reactants that do not occur in the feed but are created by a heterogeneous reaction. It then becomes important to maintain the same ratio of free volume to catalyst volume in the laboratory reactor used for intrinsic kinetic studies as in the pilot or production reactors. 

Determine the form of the pseudohomogeneous, intrinsic kinetics for each of these cases. Assume that the surface reaction step, as shown above, is rate limiting. 

The pseudohomogeneous reaction term in Equation (11.42) is analogous to that in Equation (9.1). We have explicitly included the effectiveness factor rj to emphasis the heterogeneous nature of the catalytic reaction. The discussion in Section 10.5 on the measurement of intrinsic kinetics remains applicable, but it is now necessary to ensure that the liquid phase is saturated with the gas when the measurements are made. The void fraction s is based on relative areas occupied by the liquid and soUd phases. Thus,... 

A glib generalization is that the design equations for noncatalytic fluid-solid reactors can be obtained by combining the intrinsic kinetics with the appropriate... 

If kiAi is known with good accuracy, it may be possible to back out the intrinsic kinetics using the methods of Section 7.1. Knowing the intrinsic kinetics may enable a scaleup where kiAj(af — ai) is dilferent in the large and small units. However, it is better to adjust conditions in the pilot reactor so that they are identical to those expected in the larger reactor. Good pilot plants have this versatility. The new conditions may give suboptimal performance in the pilot unit but achievable performance in the full-scale reactor. 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

The intrinsic kinetics of the reactions taking place in the scrubber, i.e. the reaction of NO with the iron chelate forming an iron nitrosyl complex (eq. 1) and the undesired oxidation reaction of the iron chelate (xanpla (eq. 2) wae deteimined in dedicated stirred cell contactors. Typical process conditions were T = 25-55 °C [Fe"(EDTA) "] = 1-100 mol/m [NO] = 1-1000 ppm pH = 5-8 and an oxygen level ranging between 1 and 20 vol%. 

Minimize the effects of transport phenomena If we are interested in the intrinsic kinetic performance of the catalyst it is important to eliminate transport limitations, as these will lead to erroneous data. We will discuss later in this chapter how diffusion limitations in the pores of the catalyst influence the overall activation energy. Determining the turnover frequency for different gas flow velocities and several catalyst particle sizes is a way to establish whether transport limitations are present. A good starting point for testing catalysts is therefore ... 

For testing and optimizing catalysts, the temperature region just below that where pore diffusion starts to limit the intrinsic kinetics provides a desirable working point (unless equilibrium or selectivity considerations demand working at lower temperatures). In principle, we would like the rate to be as high as possible while also using the entire catalyst efficiently. For fast reactions such as oxidation we may have to accept that only the outside of the particles is used. Consequently, we may decide to use a nonporous or monolithic catalyst, or particles with the catalytic material only on the outside. 

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

By comparison with data from a vigorously stirred mini-batch reactor (10 cm ), it could be shown that this micro-reactor operation gave intrinsic kinetic data [111]. This is demonstrated, e.g., by the lower conversion of the branched iso-alcohols respective to the normal-chain ones. 

OS 63] [R 27] ]P 46] A 43% conversion was achieved by micro-channel processing, while both batch experiments and expectation from intrinsic kinetics indicated conversions close to 90% [117] 80% was achieved by micro-channel processing with an additional micro mixer (see the Section Setting micro mixing prior to reaction, below). 

Main axis experimental conversions ( ) intrinsic kinetic model (solid line) kinetic model + mass transfer kinetics (dashed line). Secondary axis variation of computed k,o with flow rate. 

The first two data sets were in good accordance, whereas a fit to the intrinsic kinetics without correction was only valid for short residence times and large flow rates. This is the consequence of achieving a low degree of liquid/liquid dispersion at low flow rates, i.e. the reaction becomes mass-transfer limited in this regime. 

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