Limitation on reaction

A characteristic peculiarity of isoxazole derivatives is the relatively facile ring cleavage under suitable conditions, and this is a severe limitation on reactions of substitution in the isoxazole series. The best, though as yet inadequately, studied reactions, are the... 

As an example, consider the use of PVPy as a solid poison in the study of poly(noibomene)-supported Pd-NHC complexes in Suzuki reactions of aryl chlorides and phenylboroiuc acid in DMF (23). This polymeric piecatalyst is soluble under some of the reaction conditions employed and thus it presents a different situation from the work using porous, insoluble oxide catalysts (12-13). Like past studies, addition of PVPy resulted in a reduction in reaction yield. However, the reaction solution was observed to become noticeably more viscous, and the cause of the reduced yield - catalyst poisoning vs. transport limitations on reaction kinetics - was not immediately obvious. The authors thus added a non-functionalized poly(styrene), which should only affect the reaction via non-specific physical means (e.g., increase in solution viscosity, etc.), and also observed a decrease in reaction yield. They thus demonstrated a drawback in the use of the potentially swellable PVPy with soluble (23) or swellable (20) catalysts in certain solvents. 

Bench scale experiments. The reactors used in these experiments are usually designed to operate at constant temperature, under conditions that minimize heat and mass transfer limitations on reaction rates. This facilitates an accurate evaluation of the intrinsic chemical effects. 

Still another advantage of fluidized bed operation is that it leads to more efficient contacting of gas and solid than many competitive reactor designs. Because the catalyst particles employed in fluidized beds have very small dimensions, one is much less likely to encounter mass transfer limitations on reaction rates in these systems than in fixed bed systems. 

Thus, due to limitations on the available computer memory, DNS of homogeneous turbulent reacting flows has been limited to Sc 1 (i.e., gas-phase reactions). Moreover, because explicit ODE solvers (e.g., Runge-Kutta) are usually employed for time stepping, numerical stability puts an upper limit on reaction rate k. Although more complex... 

We have discussed these reactions previously in connection with equilibrium limitations on reactions, and we will discuss them again in Chapter 7, because both use catalysts. These reactions are very important in petrochemicals, because they are used to prepare industrial H2 and CO as well as methanol, formaldehyde, and acetic acid. As noted previously, these processes can be written as... 

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. 

It is apparent from these considerations that mass transport may severely limit reactions of coals. We next proceed to a brief discussion of some examples of mass transport limitations on reactions. 

On the other hand, use of whole cells as the vehicle for immobilization of enzymes is not without problems. These disadvantages include the susceptibility to mass transfer/diffusional limitations on reaction rates and possible losses in the yield of the desired product as a consequence of unwanted side reactions. In addition, there are potential problems associated with maintaining the integrity of the immobilized cells—supplying the nutrients, energy sources, or cofactors necessary to maintain the cells in a sufficiently viable condition to mediate the reaction(s) of interest. 

In Lhe case of slow reacLions for which k,. A di(Y, the overall rate constant Arnet is equal to the rate constant for transformation of the precursor complex, k. However, for very fast reactions, k is greater than A difr, so that the overall rate constant is close to Ardiff- Therefore, it is important to understand that there is an upper limit on reaction rates that can be measured experimentally for diffusion controlled processes. In section 7.5 it is shown that A difr for molecular reactants is given by... 

Fig. 7 Confirmation of mass transfer limitation on reaction rate using Mariotte setup O 825 rpm, X 300 rpm then 825 rpm.

Batch reactor studies with different agitator speeds have shown that there was no evidence for external mass transfer limitation on reaction rates in the examined cases when the agitator speed was beyond 200 rev/min [9,19,21,55]. 

Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective utilization of the catalyst, one usually suspends a powdered or granular form of the catalyst in the liquid phase. This type of reactor is useful when one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid (e.g., in the hydrogenation of various oils). The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Mass transfer limitations on reaction rates can be quite significant in those instances where three phases (the solid catalyst and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

The purpose of the initial inspection shall be to verify information provided concerning the facility, including verification of the limits on reaction vessels set forth in paragraph 9. 

The only difference between this example and the previous discussion is the reversibility of the fast isomerizationreaction. The net rate of formation of i-CgHig is the difference between the rates of the forward and reverse isomerization reactions. This net rate will depend on the concentrations of both n-CeHig and i-CgHig. We will examine the concentration profiles of both isomers and determine qualitatively the effect of a pore diffusion limitation on reaction selectivity. For simplicity, n-C Uu will be designated as N and i-CsHm as I . 

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