Mears’ criterion

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... 

At this point, it is also important to check the plug-flow assumption, as the models assume plug flow for liquid. The correlation of Michell-Furzer (eq. (3.417)) for the liquid is used and the results are shown in Table 5.17. The minimum values of Z/Jp are evaluated by using the Mears criterion (eq. (3.421)). 

Hydraulic parameters and Mears criterion for plug flow... 

The next parameter of importance is the Peclet number of the liquid and the gas phase. For the specified Reynolds number, the Peclet number for the liquid phase using the Michell-Furzer correlation (eq. (3.417)) is 0.74. The minimum value of Z/dp for ethanol conversion between 0.1 and 0.9, evaluated using the Mears criterion (eq. (3.421)), is 2.84 and 62.11 respectively, much lower than the value used in the example, which is about 2500. Thus, the operation can be assumed to follow the plug-flow model. 

Using 324 measuring points taken at temperatures between 35 and 75 °C, hydrogen concentrations between 1.6 10-3 and 11.0 10-3 mol NdnT3 and oxygen concentrations between 1.7 10 3 and 7.3 10 3 mol Ndm-3, a kinetic expression for the reaction was determined on the basis of a Langmuir-Hinshelwood model (Figure 2.30). The Mears criterion was applied to verify that no mass transfer limitation was to be expected for the system from the gas phase to the non-porous catalyst ... 

The Mears criterion can be applied for evaluating the influence of the external diffusion the. This is given by the expression ... 

The well-known Frossling equation seems applicable, as Sh = 2+ 0.6Rep5 Sc1/3. Replacing the values gives Sh = 10 and ki = 2.37 x 10 5m2/s. The Mears criterion becomes ... 

Table 7.5 Relative errors < u and <5W for several reaction orders n, if Mears criterion is just fulfilled...

According to Mears criterion the catalyst cannot be regarded as being isothermal because = 0.051 > 0.025. This illustrates that Mears criterion is too stringent. 

The percentage desulfurization versus liquid flow rate (and, hence, length of the reactor, since LHSV was kept constant) data obtained with this catalyst are shown in Fig. 4-19. As one explanation for the effect shown in this figure, it has been suggested that the axial dispersion in shorter beds causes their poor performance. Is this a viable explanation Based on Mears criterion, what is the... 

According to Mears criterion, the minimum bed-length required to eliminate the axial dispersion effect can be expressed as... 

Table 4-2 Minimum bed-length from Mears criterion...

The Mears criterion, like the Wiesz-Prater criterion, uses the measmed rate of reaction, — r j, (kmol/kg cat. s) to learn if mass transfer from the bulk gas phase to the catalyst smface can be neglected. Mears proposed that when... 

An indication of the presence of bulk diffusion effects can be obtained by using the Mears criterion ... 

For analysis of dispersion effects see Montagna and Shah [A.A. Montagna and Y.T. Shah, Ind. Eng. Chem. Proc. Design Devel., 14, 479 (1975)]. According to the Mears criterion [D.E. Mears, Chem. Eng. Sci., 26, 1361 (1971)], the minimum (Z/dp) required to hold the isothermal reactor length to within 5% of that of the plug-flow case is... 

Mears Criterion for External Mass Transfer Effects In a number of cases, it may be necessary to rapidly estimate whether transport limitations are present or not, and Mears criterion is... 

The criterion makes use of (,-Ra)p, the measured global rate (kmol/kg-s) n, order of reaction dp, the particle diameter (m) Pb, the bed density (kg/m ) k, mass transfer coefficient (m/s) and 04fc = bulk concentration of A (kmol/m ). The intrinsic kinetics of the reaction must be known in order to use Mears criterion. The mass transfer coefficient, k , can be estimated from existing empirical correlations [26-28]. 

Mears Criterion for External Fleat Transfer Effects There is an analogous Mears criterion for predicting possible heat transfer effects [26-28] ... 

The Mears criterion is used to check for external mass transfer limitation in microchaimels ... 

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