Mass transfer limitation on reaction

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. 

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. 

GP 11] [R 5] A judgement on mass-transfer limitations on the reaction rate according to the Mears criterion was made [121], This inequality predicts no such limitations in the boundary layer when the Mears criterion is smaller than 0.15. Using process parameter data applied in a number of experiments, the highest... 

The flow terms represent the convective and diffusive transport of reactant into and out of the volume element. The third term is the product of the size of the volume element and the reaction rate per unit volume evaluated using the properties appropriate for this element. Note that the reaction rate per unit volume is equal to the intrinsic rate of the chemical reaction only if the volume element is uniform in temperature and concentration (i.e., there are no heat or mass transfer limitations on the rate of conversion of reactants to products). The final term represents the rate of change in inventory resulting from the effects of the other three terms. 

In this case the reaction rate will depend not only on the system temperature and pressure but also on the properties of the catalyst. It should be noted that the reaction rate term must include the effects of external and intraparticle heat and mass transfer limitations on the rate. Chapter 12 treats these subjects and indicates how equation 8.2.12 can be used in the analysis of packed bed reactors. 

Before terminating the discussion of external mass transfer limitations on catalytic reaction rates, we should note that in the regime where external mass transfer processes limit the reaction rate, the apparent activation energy of the reaction will be quite different from the intrinsic activation energy of the catalytic reaction. In the limit of complete external mass transfer control, the apparent activation energy of the reaction becomes equal to that of the mass transfer coefficient, typically a kilocalorie or so per gram mole. This decrease in activation energy is obviously... 

At steady state, the rates of each of the individual steps will be the same, and this equality is used to develop an expression for the global reaction rate in terms of bulk-fluid properties. Actually, we have already employed a relation of this sort in the development of equation 12.4.28 where we examined the influence of external mass transfer limitations on observed reaction rates. Generally, we must worry not only about concentration differences between the bulk fluid and the external surface of the catalyst, but also about temperature differences between these points and intraparticle gradients in temperature and composition. 

It is known that high mass velocities are to be employed within the reactor tubes to minimize heat and mass transfer limitations on the catalytic reaction rates. It is also known that the effectiveness factors for the catalysts commonly employed often differ appreciably from unity. 

In this section the effect of mass transfer limitation on the drop conversion rate and the order of drop conversion will be treated, and it will be shown that a process for which the real chemical reaction is of first order in the reactant A (which is dissolved in the dispersed phase) can still be influenced by the effect of segregation when the chemical conversion rate is limited by mass transfer of the reactants. 

The first two conditions seem to be fulfilled only for (a) liquid-liquid systems (b) liquid-gas systems spray towers and bubble columns and (c) gas fluidized systems in an aggregative fluidization state. To what extent the third condition is fulfilled depends, in most cases, on mass transfer limitation of reactions between two or more components. 

Substituting 1 for xs in the preceding equation yields tj = 1, which indicates that there is no mass-transfer limitation. On the other hand, if xs approaches zero, 7] also approaches zero, which is the case when the rate of mass transfer is very slow compared to the reaction rate. 

In this example, we examine the effects of mixing and mass transfer limitations on the yields of competitive-consecutive reactions of the type... 

Fio. 19. Influence of micromixing and mass transfer limitations on the yield of competitive-consecutive reactions (of which one reaction is homogeneous and the other is wall-catalyzed) in a tubular reactor. 

Equation (6.2.26) shows that decreasing the catalyst particle size and increasing the fluid velocity can significantly increase the mass-transfer coefficient. These simple variables may be used as process handles to decrease the influence of external mass-transfer limitations on the observed reaction rate. 

Pan states that CH4 is more reactive than ammonia so that there is likely to be some mass transfer limitation on methane as well as ammonia. Making an assumption that the surface mole fractions of reactants will be of the order of half the bulk gas-phase levels, approximate reaction probabilities for NH3 and CH4 can be calculated. Collision rates are about 2 x 10 molecules cm" s" so that the reaction probability for ammonia and methane is about 10" and for oxygen about 2.5 x 10". These are sufficiently close to the values for independent oxidation of CH4 and NH3 to make it likely that the same surface reactions are also involved in the co-oxidation. 

In surfactant-based reaction media mass transfer limitations on the reaction rate are much suppressed in comparison to stirred two-phase systems without surfactants. The reason for this pronounced difference is the different characteristic length scales of the systems. This may be illustrated by the following example. Twenty grams per litre of a... 

It is interesting to consider the effect of internal mass transfer limitations on the observed kinetics. Taking into account eq. (9.199) the reaction rate for n-th order kinetics at high values of Thiele modulus is given by r - k,C / Thiele modulus for n-th order kinetics is... 

In applications where the temperature range of operation is between 1000 and 1400 °C, there is still a lack of heat-resistant materials. For these applications, a ceramic catalyst system, extruded and completed with support and active phase in one piece, would be the ultimate solution. A surface area-enhancing washcoat is probably not needed at these temperatures, since both mass transfer limitations and reaction rates are high. Probably, only a surface area around 1-10 m g would be sufficient, which could be achieved with fine-tuned extrusion techniques. Hence, complicated washcoat-support interactions can be avoided. Among the several materials that are reported suitable for support extrusion in this review, there is a possibility for some of them to be used as the active component. For example, promising support materials like NZP may be active, depending on the specific ionic substitution. On the other hand, metal structures probably have too low a surface area to be used without washcoat. 

Ishikawa H, Tanaka T, Kuro K et al. (1987) Evaluation of tme kinetic parameters for reversible immobihzed enzyme reactions. Biotechnol Bioeng 29 924-933 Jeison D, Ruiz G, Acevedo F et al. (2003) Simulation of the effect of intrinsic reaction kinetics and particle size on the behavior of immobihzed enzymes under internal diffusional restrictions and steady state operation. Proc Biochem 39(3) 393-399 Katchalski-Katzir E, Kraemer DM (2000) Eupergit C, a carrier for immobDization of enzymes of industrial potential. J Mol Catal B Enzym 10 157-176 Kheirolomoom A, Khorasheh F, Fazehnia H (2002) Influence of external mass transfer limitation on apparent kinetic parameters of peniciUin G acylase immobihzed on nonporous ultrafine silica particles. J Biosci Bioeng 93 125-129... 

Operation in a perfusion mode differs from fed batch operation in that the former not only involves a continuous supply of fresh substrate in the feed stream, but also continuous removal of soluble metabolic products and waste components present in the process fluid. Such removal minimizes the potential for inhibition of the biochemical reaction by soluble products. Perfusion systems are most frequently employed in the cultivation of mammalian cells (see Section 13.3.2.2). However, readers should note that diffusional or mass transfer limitations on biochemical reactions may be imposed by the use of permselective membranes. 

The effect of mass transfer limitation on the overall process can also be expressed in terms of the effectiveness factor, relating the actual reaction rate to that observed if the catalyst surface were exposed to the same... 

The inlet conditions for the numerical simulations are based on the experimental conditions. The simulations are performed with the three different models for internal diffusion as given in Section 2.3 to analyze the effect of internal mass transfer limitations on the system. The thickness (100 pm), mean pore diameter, tortuosity (t = 3), and porosity ( = 60%) of the washcoat are the parameters that are used in the effectiveness factor approach and the reaction-diffusion equations. The values for these parameters are derived from the characterization of the catalyst. The mean pore diameter, which is assumed to be 10 nm, hes in the mesapore range given in the ht-erature (Hayes et al., 2000 Zapf et al., 2003). CO is chosen as the rate-limiting species for the rj-approach simulations, rj-approach simulations are also performed with considering O2 as the rate-hmiting species. 

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