Bypassing, residence time distribution

In a real stirred tank with bypassing or short-cut flow (Fig. 3.22), highly concentrated tracer comes out early, and the residence time distribution depends on the fraction a of the flow in the bypass (Fig. 3.23). The tailing of the response curve is caused by the perfect mixing in the main part of the tank. 

Figure 3.23. Residence time distribution in a tank with bypassing.

The available models mostly refer to ideal reactors, STR, CSTR, continuous PFR. The extension of these models to real reactors should take into account the hydrodynamics of the vessel, expressed in terms of residence time distribution and mixing state. The deviation of the real behavior from the ideal reactors may strongly affect the performance of the process. Liquid bypass - which is likely to occur in fluidized beds or unevenly packed beds - and reactor dead zones - due to local clogging or non-uniform liquid distribution - may be responsible for the drastic reduction of the expected conversion. The reader may refer to chemical reactor engineering textbooks [51, 57] for additional details. 

The well-mixed drier has fhe disfinct disadvanfage of nof being able to dry particles to very low moisture contents because of fhe wide particle residence time distribution. In other words, some particles leave the bed immediately, having had no opportunity to dry and effectively bypass the bed some circulate within the bed for a long... 

Bypassing the reactor showed that the broadening of the pulse caused by the feed and effluent lines was negligible. Hence, the normalized pulse response is the residence time distribution (RTD) of the recycle reactor. 

Broad residence time distributions of the gas due to dispersion and gas-bypass in the form of bubbles, especially when operated in the bubbling bed regime. 

In conventional fixed-bed reactors, catalyst particles of various sizes are often randomly distributed, which may lead to inhomogeneous flow patterns. Near the reactor walls, the packing density is lower than the mean value, and faster flow of the fluid near the wall is unavoidable. As a result, reactants may bypass the catalyst particles, and the residence time distribution (RTD) will be broadened. Moreover, the nonuniform access of reactants to the catalytic surface diminishes the overall reactor performance and can lead to unexpected hot spots and even to reactor runaway in the case of exothermic reactions. 

Overview In this chapter we learn about nonideal reactors, that is, reactors that do not follow the models we have developed for ideal CSTRs, PFRs, and PBRs. In Pan I we describe how to characterize these nonideal reactors using the residence time distribution function (/), the mean residence time the cumulative distribution function Fit), and the variance a. Next we evaluate E t), F(t), and for idea) reactors, so that we have a reference proint as to how far our real (i.e., nonideal) reactor is off the norm from an ideal reactor. The functions (f) and F(r) will be developed for ideal PPRs. CSTRs and laminar flow reactors, Examples are given for diagnosing problems with real reactors by comparing and E(i) with ideal reactors. We will then use these ideal curves to help diagnose and troubleshoot bypassing and dead volume in real reactors. 

We first encountered in Chapter 3 on mixing in multicomponent systems the problem of bypassing and less than perfect mixing. If we have two or more reactants that must mix in order for reaction to occur, then any deviations from a single-valued residence time distribution will show up as an apparent deviation from the predictions based upon perfect mixing. The spread in the residence time distribution leads to different extents of reaction for the fluid elements with these different times. 

The conventional fluidized beds also possess some serious deficiencies, however. The bubbles that are responsible for many benefits of a fluidized bed represent the fluid bypassing and reduction of fluid-solids contacting. The rapid mixing of solids in the bed leads to nonuniform solids residence time distribution in the bed. The rigorous solids mixing in the bed also leads to attrition of bed material and increases the bed material... 

The residence time distribution of the gas may deviate from plug-flow operation by dispersion and bypassing effects. (We will learn in the Section 4.10.3 why plug-flow operation is mostly advantageous compared to mixed flow.)... 

It was suspected that fluid bypassing was occurring in the plant reactors which would necessitate operation at higher temperatures to effect oxygen siass transfer and reaction. Residence time distribution studies using idealized reactor flow models strongly confirmed this suspicion. 

Radioactive tracers [14] are a useful tool to measure unit parameters such as residence times and distribution of the catalyst and vapors in the reactor, stripper, or regenerator. Bypassing can be detected, slip factors calculated and dilute phase residence times are examples of useful calculations that can point the way to future modifications. This technology is also useful for detecting and analyzing equipment malfunctions. Plugged distributors, erratic standpipes, and main fractionator problems such as salt deposits or flooding can be detected with tracers. 

With the mean value 100 and the deviation 20 time units the distribution has a familiar look see the preceding graph). The function tells us that most of the fluid elements (63%) go through the unit with residence times that are between 60 and 140 time units. There are, however, 18.5% of the fluid elements that bypass with very short residence times and 18.5% that take very long times to emerge due to recirculation cells. Some of these never emerge ... 

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