Effect of flow maldistribution

Delsman et al. [9] proposed a method to predict the influence of flow maldistribution and manufacturing tolerances on the performance of a microreactor with a large number of parallel channels. In this method, a variable reactor parameter (e.g., diameter) is considered as a random parameter with a mean value and a standard deviation, and the microreactor is considered as a plug-flow reactor. Then, a number of relationships between the variable parameter and efficiency as compared to the ideal case are presented. The pressure drop as a function of the mean channel diameter is given by Equation 9.5  

The residence time of the fluid in a microchannel varies also between the channels. The residence time in the single channel is a function of both the fluid flow rate and the channel volume. It can be expressed as 

Since the residence time varies between the channels, a tracer pulse at the inlet of the microreactor will be broadened similar to the case for a tubular reactor with axial dispersion. As a first approximation, the relative standard deviation in the residence time is twice the relative standard deviation in the channel diameter [9]  

This equation shows that a variation of the channel diameter results in a decrease in the overall pressure drop over the reactor, when the total flow rate is kept constant. A small deviation in channel diameter does not result in a large difference in the pressure drop a standard deviation of 10% gives a pressure drop of 6% lower as compared with the ideal case. 

The presence of internal mass transfer limitations depends on the reaction rate and the thickness of the porous catalytic layer and is usually expressed via the effectiveness factor, which is defined as the ratio of the observed reaction rate and the rate that would be observed in the absence of concentration gradient throughout the catalytic layer. For an isothermal layer, the maximum thickness of catalytic coating should not exceed Scat to ensure an effectiveness factor of 0.95 [10]  

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Miscellaneous Effects. Depending on individual design characteristics, there are other miscellaneous effects to consider in the determination of the final sizing of a heat exchanger. These include effects of flow maldistribution of both the sheUside and tubeside fluids, stagnant or inactive regions in the tube bundle, and inactive length of the tube in tubesheets. These effects should be individuaUy assessed and appropriate additional areas should be provided. 

As computers become faster, the complexity of problem that can be usefully simulated increases. Areas of interest include combining computational fluid dynamics (CFD) modelling with chemical kinetics to investigate (and hence reduce) the effect of flow maldistributions on aftertreatment system efficiency, and simulating catalyst deactivation over the lifetime of the catalyst. 

Annular fixed beds with radial cross flow have been described in a number of papers [5-10], where, among others, a design criterion [9] and the effect of flow maldistribution on conversion and selectivity [10] have been dealt with. 

P.R. Ponzi and L.A. Kay. Effect of flow maldistribution on conversion and selectivity in radial flow fixed-bed reactors. AlChE J. 25 100 (1979). 

Radial dispersion tends to reduoe the redial concentration gredieats caused by flow maldistribution, thaieby producing axial dispersion. Heace, in oenain cases the effects of flow maldistribution can he characterized in terms of reductions in Pe. If sharp redial concentration differences persist in spite of redial... 

FSL solutions have been developed for continuons countercurrent extractions in which the effects of flow maldistribution and anial dispetaion are negligibly small.1 4 These solutions involve the following infinite series ... 

Two-Dimensional Flow Nonuniforniity. The two-dimensional flow maldistribution has been analyzed only for a crossflow exchanger. In a series of publications as summarized in Refs. 131 and 137, Chiou has studied the effects of flow maldistribution on an unmixed-... 

K. Chowdhury and S. Sarangi, The Effect of Flow Maldistribution on Multipassage Heat Exchanger Performance, Heat Transfer Eng, Vol. 6, No. 4, pp. 45-54,1985. 

Z. H. Ayub, Effect of Flow Maldistribution on Partial Condenser Performance, Chemical Processing, No. 8, pp. 30-34,37,1990. 

In this section, steady-state two-dimensional velocity profiles in the PBMR will be analyzed, considering the relevance of boundary conditions (BC) and the effect of flow maldistribution. 

Ponzi, P,R, and L.A, Kaye, Effect of Flow Maldistribution on Conversion and Selectivity in Radial Flow Fixed-Bed Reactors, AIChE J. 25 (1979) 100-108. 

Ding, Z., liu, L., andMa, R. (2003). Study on the effect of flow maldistribution on the performance of the hoUow fiber modules used in membrane distillation. J. Membr. Sci. 215, 11. 

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