Reaction rates catalysts effect

Figure 1. Effect of P/Ni ratio on reaction rate. Catalyst Nil2, Ph3P, iyio(CO)g, T=200°C, solvent AcOH...

Figure 2. Effect of Methanol concentration on reaction rate. Catalyst Nil2 Bu3P,LiI. Pressure 1000 psig. T=180°, Solvents AcOH and AcOMe as indicated.

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Roles of Coke and Metals on Catalyst Deactivation. The model compound activity test (Figure 4) and the diffusivity test (Figure 6) clearly show that both coke and metal sulfides have a responsibility for decreasing intrinsic reaction rate and effective diffusivity. However, the former test suggests that coke and metal sulfides do not independently affect the active sites. As shown in Figure 4, in the third bed, coke rapidly covered more than 70% of the original active sites at the start of the run. However, after the run, the ratio of the coke-covered active sites to the original ones dropped drastically to less than 10%, while that of the metal-poisoned active sites became around 70%. This indicates that the metal sulfides deposit on part of the active sites which coke initially covers and permanently poison them. 

Using a developed plug-flow membrane reactor model with the catalyst packed on the tube side, Mohan and Govind [1986] studied cyclohexane dehydrogenation. They concluded that, for a fixed length of the membrane reactor, the maximum conversion occurs at an optimum ratio of the permeation rate to the reaction rate. This effect will be discussed in more detail in Chapter 11. They also found that, as expected, a membrane with a highly permselective membrane for the product(s) over the reactant(s) results in a high conversion. 

In terms of catalysis, important equilibrium processes include low-temperature gas adsorption (capillary condensation) and nonwetting fluid invasion, both of which are routinely used to characterize pore size distribution. Static diffusion in a Wicke-Kallenbach cell characterizes effective diffusivity. The simultaneous rate processes of diffusion and reaction determine catalyst effectiveness, which is the single most significant measure of practical catalytic reactor performance. 

Use of experimental data and graphical analysis to determine reactant order, rate constants, and reaction rate laws Effect of temperature change on rates Energy of activation the role of catalysts... 

When it is necessary to include these effects - slow reaction rates, catalysts, heat transfer, and mass transfer - it can make an engineering problem extremely difficult to solve. Numerical methods are a must, but even numerical methods may stumble at times. This chapter considers only relatively simple chemical reactors, but to work with these you must leam to solve ordinary differential equations as initial value problems. 

The rate constant A is a composite parameter, k = ELk, where E is the effectiveness factor, L the concentration of active sites on the surface of the catalyst, and k the actual rate constant of the transformation of the adsorbed species. The effectiveness factor which can attain values from zero to one is a measure of retardation of the reaction by diffusion of reactants or products into or out ofthe pores of the catalyst. For our purpose it should have a value of one or near to one and with careful experimentation this can be achieved. According to Thiele (14) the effectiveness factor is a function of reaction rate and effective diffusion coefficient. Both these parameters depend on the structure of the reacting compound and therefore the effectiveness factor will tend to change with the nature of the substituents. The effect of structure on reaction rate is more critical than on diffusion coefficient and if the reactivity within the series of investigated compounds will vary over some orders there is always danger of diffusional retardation in the case of the most reactive members of the series. This may cause curvature of the log kva a plot. 

In this Chapter, the progress recently made in the field of electrochemical promotion (EP) of catalytic gas reactions is reviewed. The phenomenon consists of electrochemical polarization of metal or metal oxide electrodes interfaced with solid electrolytes which result in a pronounced increase in the catalytic reaction rate. The effect is also termed non-Faradaic electrochemical modification of catalytic activity (NEMCA effect), since the rate increase may exceed the ionic current by several orders of magnitude. The promotion is not limited to the electrochemically polarized interface between catalyst and solid electrolyte, but extends to the entire catalyst surface exposed to the reactive gas. In fact, one of the major challenges in the field of electrochemical promotion is to elucidate the exact mechanism by which the promoting effect propagates from one interface to the other. 

Non-chloroaluminate ILs, which are in general poor nucleophiles, have proven to be attractive alternative media for Lewis acid catalyzed reactions. ILs may have a reaction rate accelerating effect, and they may improve selectivity and facilitate catalyst recovery. This is the case for scandium triflate catalyzed Diels-Alder cycloaddition [8,9], three-component (aldehyde, aniline, triethylphosphite) synthesis of a-aminophosphonates [10], Claisen rearrangement and cyclization reactions [11], or Friedel-Crafts reactions [12, 13]. 

In previous sections, we have considered some physical phenomena, which can complicate the processes occurring in a catalyst particle and influence the global (observable) reaction rate or effectiveness factor. Meanwhile, the presence of liquid condensate in some pores can also affect the intrinsic kinetics of the reaction due to the features discussed in Section II. 

It can be seen from equation (7.15) that the temperature difference is proportional to reaction rate, heat effect and the square of diameter of the reactor, and is inversely proportional to the effective conductivity factor. The temperature difference increases with a decrease in the particle diameter of catalysts because the effective conductivity factor A reduces with the decrease in the size of catalyst particles. When decreasing the particle diameter of catalyst in order to eliminate the effect of inside diffusion on reaction, it also enhances the factor of temperature difference. Therefore, it need to weigh the pros and cons of these factors in order to determine the most appropriate particle size of catalyst and the diameter of reactor. 

Catalytic membranes brought new and attractive applications of metal-incorporated mesoporous materials. Mesoporous nickel-silicate membranes were used as efficient catalysts in the selective oxidation of styrene to epoxy ethyl benzene and benzene to phenol. The use of membranes also offered a very good possibility to control the hydrogen peroxide feed and the selectivity in oxidation of styrene to styrene oxide and to increase the reaction rate. The effect of the H2O2 permeance on the conversion of styrene and benzene was also evidenced [83]. The conversion of styrene with membrane reactor has been compared with that realized in a conventional batch reactor with powdery catalyst indicating superior results. 

The term r ej is called the effectiveness factor. Expression 5.14 should be inserted into the balance equations instead of R. If diffusion does not affect the reaction rates, all effectiveness factors, riej, become equal to unity (=1). In the following, the logical and theoretical appearance of the effectiveness factor is discussed by studying the (molar) mass and energy balances of porous catalyst particles in detail. 

The reaction penetration depths. Id or la, are highly insightful parameters to evaluate catalyst layer designs in view of transport limitations, uniformity of reaction rate distributions, and the corresponding effectiveness factor of Pt utilization, as discussed in the sections Catalyst Layer Designs in Chapter 1 and Nonuniform Reaction Rate Distributions Effectiveness Factor in Chapter 3. Albeit, these parameters are not measurable. The differential resistances, Rd or Ra, can be determined experimentally either as the slope of the polarization curve or from electrochemical impedance spectra (Nyquist plots) as the low-frequency intercept of the CCL semicircle with the real axis. The expressions in Equation 4.33 thus relate the reaction penetration depths to parameters that can be measured. 

This reaction occurs at hi pressure (810 MPa) in the presence of a catalyst, such as sodium methoxide, at low temperature (80°C) [87]. The effects of various alkali metal alkoxides has been investigated, and the activity of the catalyst has been shown to increase with increasing ionization potential of the metal [94]. From kinetic studies it has also been shown that both C02 and H2Q react with the catalyst, resulting in a reduced reaction rate. The effect of C02 is twice as severe as that of water [95],... 

The Effect of Reactant Concentration on Reaction Rates The Effect of Temperature on Reaction Rates The Effect of Catalysts on Reaction Rates... 

The enhancement of the toluene steam dealkylation reaction is feasible by microwave irradiation (Litvishkov et al., 2012). It has been found that the most likely cause of the positive effect of microwave radiation on the reaction rate is an increase in the preexponential factor of the Arrhenius equation for the temperature dependence of the reaction rate. This effect is presumably due to an increase in the active surface area of the catalyst formed by the microwave-assisted thermal treatment. 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Generally speaking, temperature control in fixed beds is difficult because heat loads vary through the bed. Also, in exothermic reactors, the temperature in the catalyst can become locally excessive. Such hot spots can cause the onset of undesired reactions or catalyst degradation. In tubular devices such as shown in Fig. 2.6a and b, the smaller the diameter of tube, the better is the temperature control. Temperature-control problems also can be overcome by using a mixture of catalyst and inert solid to effectively dilute the catalyst. Varying this mixture allows the rate of reaction in different parts of the bed to be controlled more easily. 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

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