Flow pattern, contacting residence time distribution

However, there are two possible diflSculties in determining A lO by physical absorption. First, the flow patterns and residence-time distributions of the phases may be undetermined (Cao is not a known function of CX), hence the value of ki a cannot be deduced from . Second, in efiicient contacting devices the gas and liquid may approach equilibrium quite closely. The determination oik a depends on the difference between the actual and the equilibrium extent and necessitates extremely accurate measurement of the flow rates. 

This assumes that all chemical species have the same residence time distribution, and is very convenient to compute the reaction paths for different contacting patterns. Matsuyama and Miyauchi [16] have also considered some aspects of this. An important conclusion of Wei [15] is that for a reactor with distribution of residence times, all reactions are slowed down in comparison with those in a plug flow reactor, but the faster reactions are slowed down a great deal more than the slower ones. Consequently, the occurrence of distribution of residence times makes all reaction rates of the characteristic species nearly equal. That is, the differences between the various reaction rates are decreased, thereby decreasing the selectivity. This is similar to the diffusion effects considered in Chapter 3. 

The residence time distribution (RTD) is a probability distribution function used to characterize the time of contact and contacting pattern (such as for plug-flow or complete backmixing) within the reactors. Excessive retention of some elements and shortdrcuiting of others due to backmixing and other dispersive phenomena lead to a broad distribution in the residence times of individual molecules in the reactor. This tends to decrease conversion and exerts a negative influence on product selectivity/yield. The RTD depends on the flow regime and is characterized by Reynolds (Re) and Schmidt (Sc) numbers. 

Macromixing is the process whereby parts of a fluid having different histories come into contact and mix-up on a macroscopic scale. It is thus a consequence of the macroscopic hydrodynamic pattern. The development of powerful computer codes make it possible now to determine the average velocity pattern in any kind of equipment, at least in single phase newtonian fluids. This probably won t eliminate methods based on the characterization of complex flow by internal age distributions and residence time distributions (RTD), which can be determined with tracers. Such methods have proven their efficiency for building up flow models well adapted to scale up and to calculation of chemical conversion -... 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

This distribution, or spread of residence times is an important characteristic of the system and may have a profound effect on its performance as a reactor or contacting device. In many cases the actual flow pattern through the system is conveniently determined by the addition of a tracer at the inlet, the concentration of which is then monitored at the outlet. The analysis of the response curves then provides the desired information. 

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