Flow pattern ideal

Flow Pattern Ideality. A straightforward interpretation of the observed kinetics can only be made if the flow pattern in the reactor used corresponds to an ideal flow pattern. In particular for plug flow reactors, deviations from the ideal reactor behavior can be encountered. For perfectly mixed reactors such as a batch reactor and a continuous stirred tank reactor, the rotation speed of the stirrer is the key parameter that needs to be set sufficiently high to ensure complete mixing. Deviations from the ideal plug flow pattern can, for example, be caused by a less-dense packing of the catalyst pellets near the reactor wall, by a too high dilution of the catalyst bed with inert pellets or by the importance of effective axial diffusion compared to convection (15). 

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

Multiphase Reactors. The overwhelming majority of industrial reactors are multiphase reactors. Some important reactor configurations are illustrated in Figures 3 and 4. The names presented are often employed, but are not the only ones used. The presence of more than one phase, whether or not it is flowing, confounds analyses of reactors and increases the multiplicity of reactor configurations. Gases, Hquids, and soHds each flow in characteristic fashions, either dispersed in other phases or separately. Flow patterns in these reactors are complex and phases rarely exhibit idealized plug-flow or weU-stirred flow behavior. 

In another land of ideal flow reactor, all portions of the feed stream have the same residence time that is, there is no mixing in the axial direction but complete mixing radially. It is called a.plugflow reactor (PFR), or a tubular flow reactor (TFR), because this flow pattern is characteristic of tubes and pipes. As the reaction proceeds, the concentration falls off with distance. 

Side entering mixers are used for blending pui-poses. The side entering propeller type mixer is economical and establishes an effective flow pattern in almost any size tank. Because the shaft seal is below the liquid level, its use in fluids without corrosive and erosive properties is usually ideal. 

Hot water basins are used to distribute water in crossflow towers. Here, water is pumped to an open pan over the wet deck fill. The bottom of the pan has holes through which water is distributed. Manufacturers will fit specially shaped plastic drip orifices into the holes to give the water an umbrella shape for more uniform distribution. Different size orifices are used for different flow rates. Ideally, the basin will be almost full at maximum flow. This way, sufficient depth is retained for good water distribution as turn down occurs. The turn down ratio can be extended by the addition of hot water basin weirs- a pattern of baffles perhaps 2... 

Table 7-4 shows flow patterns and applications of some commercially available impellers. Generally, the axial flow pattern is most suitable for flow sensitive operation such as blending, heat transfer, and solids suspension, while the radial flow pattern is ideal for dispersion operations that require higher shear levels than are provided by axial flow impellers. Myers et al. [5] have described a selection of impellers with applications. Further details on selection are provided by Uhl and Gray [6], Gates et al. [7], Hicks et al. [8] and Dickey [9]. 

Figure 8-22 shows the F(6) eurves for laminar flow in a tubular reaetor and for other idealized flow patterns. 

Fig ure 8-22. Curves for reaotors with idealized flow patterns. 

Displacement flow, ideal Ideal flow pattern in an enclosure, in which uniform air diffusion is provided without mixing. 

Mixed flow, ideal The flow pattern in an eticlosure in which the air is completely mixed and has the same conditions at every point. 

As the flow of a reacting fluid through a reactor is a very complex process, idealized chemical engineering models are useful in simplifying the interaction of the flow pattern with the chemical reaction. These interactions take place on different scales, ranging from the macroscopic scale (macromixing) to the microscopic scale (micromixing). 

Non-ideal reactors are described by RTD functions between these two extremes and can be approximated by a network of ideal plug flow and continuously stirred reactors. In order to determine the RTD of a non-ideal reactor experimentally, a tracer is introduced into the feed stream. The tracer signal at the output then gives information about the RTD of the reactor. It is thus possible to develop a mathematical model of the system that gives information about flow patterns and mixing. 

It is shown in Section 9.9.5 that, with the existence of various bypass and leakage streams in practical heat exchangers, the flow patterns of the shell-side fluid, as shown in Figure 9.79, are complex in the extreme and far removed from the idealised cross-flow situation discussed in Section 9.4.4. One simple way of using the equations for cross-flow presented in Section 9.4.4, however, is to multiply the shell-side coefficient obtained from these equations by the factor 0.6 in order to obtain at least an estimate of the shell-side coefficient in a practical situation. The pioneering work of Kern(28) and DoNOHUE(lll who used correlations based on the total stream flow and empirical methods to allow for the performance of real exchangers compared with that for cross-flow over ideal tube banks, went much further and. 

Establish ideal flow patterns This is usually assumed to be the case for plug-flow and continuously stirred tank reactors, but are all conditions for ideal mixing fulfilled For example, a rule of thumb is that the diameter d of the PFR should be at least lOx the diameter of the catalyst particles to eliminate the influence of the reactor wall. Also, the amount of catalyst should be sufficient to avoid axial gradients. Another rule is that the ratio of the bed length L to the reactor diameter d, i.e. L/d, should be >5-10. Higher values are preferable, but these may cause other problems such as temperature gradients and pressure drops. 

Needless to say, all conclusions drawn in Sections 1.1.1-1.1.7 are ideal-case considerations of an abstract nature aimed at showing the maximum potential of chemical micro processing, and the ideas behind. In reality, a performance less than ideal (but often better than conventional) may be found, at least initially, e.g. for reasons of imperfect exhibition of flow patterns or due to limits of micro flow compared with existing technology. This reality description is given in Chapters 3-5. 

Figure 4.23 Near-ideal multi-lamination flow patterns in the second-generation caterpillar mini mixer as a result of introducing a splitting plate and improving micro structure geometry [50],...

This plate cuts the flow into pieces which are better defined than the poorly defined ones obtained by the first-generation caterpillar mini mixer. In addition, the micro structure geometry was improved by means of simulation. As a result, near-ideal multi-lamination flow patterns were yielded (Figure 4.23), which showed excellent correspondence with simulation [50]. 

Steady-state reactors with ideal flow pattern. In an ideal isothermal tubular pZi/g-yZovv reactor (PFR) there is no axial mixing and there are no radial concentration or velocity gradients (see also Section 5.4.3). The tubular PFR can be operated as an integral reactor or as a differential reactor. The terms integral and differential concern the observed conversions and yields. The differential mode of reactor operation can be achieved by using a shallow bed of catalyst particles. The mass-balance equation (see Table 5.4-3) can then be replaced with finite differences ... 

The flow pattern of fluids in gas-liquid-solid (catalyst) reactors is often far from ideal. Special care must be taken to avoid by-passing of the catalyst particles near the reactor walls, where the packing density of the catalyst pellets is lower than in the centre of the bed. By-passing becomes negligible if the ratio of reactor to particles diameter is larger than 10 a ratio of 20 is recommended. Flow maldistributions might be serious in the case of shallow beds. Special devices must be used to equalize the velocity over the cross-section of the reactor before reactants are introduced onto the catalyst bed. 

Steady-state reactors with non-ideal flow pattern. In fact, all reactors presented as reactors with ideal flow patterns show some non-idealities as already mentioned above. The deviation from the ideal state for multiphase reactors arises from the presence of phases with very different physical properties. 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

Figure 10.11 Idealized flow patterns in membrane separation.

The ideal tubular reactor is one in which elements of the homogeneous fluid reactant move through a tube as plugs moving parallel to the tube axis. This flow pattern is referred to as... 

For a few highly idealized systems, the residence time distribution function can be determined a priori without the need for experimental work. These systems include our two idealized flow reactors—the plug flow reactor and the continuous stirred tank reactor—and the tubular laminar flow reactor. The F(t) and response curves for each of these three types of well-characterized flow patterns will be developed in turn. 

The responses of this system to ideal step and pulse inputs are shown in Figure 11.3. Because the flow patterns in real tubular reactors will always involve some axial mixing and boundary layer flow near the walls of the vessels, they will distort the response curves for the ideal plug flow reactor. Consequently, the responses of a real tubular reactor to these inputs may look like those shown in Figure 11.3. 

The F(t) curve for a laminar flow tubular reactor with no diffusion is shown in Figure 11.6. Curves for the two other types of idealized flow patterns are shown for comparison. 

In the previous section we indicated how various mathematical models may be used to simulate the performance of a reactor in which the flow patterns do not fit the ideal CSTR or PFR conditions. The models treated represent only a small fraction of the large number that have been proposed by various authors. However, they are among the simplest and most widely used models, and they permit one to bracket the expected performance of an isothermal reactor. However, small variations in temperature can lead to much more significant changes in the reactor performance than do reasonably large deviations inflow patterns from idealized conditions. Because the rate constant depends exponentially on temperature, uncertainties in this parameter can lead to design uncertainties that will make any quantitative analysis of performance in terms of the residence time distribution function little more than an academic exercise. Nonetheless, there are many situations where such analyses are useful. 

In this chapter, we describe several ideal types of reactors based on two modes of operation (batch and continuous), and ideal flow patterns (backmix and tubular) for the continuous mode. From a kinetics point of view, these reactor types illustrate different ways in which rate of reaction can be measured experimentally and interpreted operationally. From a reactor point of view, the treatment also serves to introduce important concepts and terminology of CRE (developed further in Chapters 12 to 18). Such ideal reactor models serve as points of departure or first approximations for actual reactors. For illustration at this stage, we use only simple systems. 

As discussed in Section 17.2.3.1, reactor performance in general depends on (1) the kinetics of reaction, (2) the flow pattern as represented by the RTD, and (3) mixing characteristics within the vessel. The performance predicted by ideal reactor models (CSTR, PFR, and LFR) is determined entirely by (1) and (2), and they do not take (3)... 

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