Plug pulse system

Another system is the plug pulse system seen in Fig. 11. The discharge valve is alternatively activated with a gas pulse. This system works well for free flowing materials. The minipot is a variation of the plug pulse system The operation of the minipot is depicted in Fig. 12. 

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. 

Konrad was the first to address the issue of pulsed piston transport using the properties of the solids as they slide through the pipe in a plug-like motion. The friction generated in such systems often can be likened to bin and hopper flow and design, requiring shear stress measurements such as carried out by the Jenike shear stress unit. The final expression using the Konrad approach can be written for horizontal flow as... 

In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

Fortunately, it is not always necessary to recover the system RTD curve from the impulse response, so the complications alluded to above are often of theoretical rather than practical concern. In addition, the dispersion model is most appropriately used to describe small extents of dispersion, i.e. minor deviations from plug flow. In this case, particularly if the inlet pipe is of small diameter compared with the reactor itself, the vessel can be satisfactorily assumed to possess closed boundaries [62]. An impulse of tracer will enter the system and broaden as it passes along the reactor so that the observed response at the outlet will be an RTD and will be a symmetrical pulse, the width of which is a function of DjuL alone. 

Ideally, a sample is introduced into a chromatograph as a perfect plug. In practice, this is not the case, and diffusion occurs because of the injector. For narrow-bore and microbore applications, injectors capable of introducing the required sample volumes are commercially available and optimized to reduce dispersion. This is not the case for capillary LC, and homemade injection systems include the sample tube technique, in-column injection, stopped-flow injection, pressure pulse-driven stopped-flow injection (PSI), groove injection, split injection, heart-cut injection, and the moving injection technique (MIT). Of the injection techniques, only the split injector, MIT and PSI approaches can introduce subnanoliter sample volumes accu-... 

Next the reactor system in which the CSTR is preceded by the PFR will be treated. If the pulse of tracer is introduced into the entrance of the plug-flow section, then the same pulse will appear at the entrance of the perfectly mixed section Xp seconds later, meaning that the RTD of the reactor system will be... 

Cases a and b demonstrate the effect of the residence time tp in the plug flow reactor, 0.2 and 1. As seen, in case a tp was too short for the pulse introduced in reactor 1 to reach its maximum value of unity. The effect of the recycle, R = 0 and 10 is demonstrated in cases a and c. As observed, the concentration of the tracer in reactors 2 and 3 becomes identical at tp = 0.2 the concentration are different in the absence of a recycle. The effect of Xi is demonstrated in cases a, d and e. In case a and d all ti = QiA i = are equal in case a ii = 10 (i = 1,. .., 4) and in d pi = 100 (i = 1,. .., 4). Note that a large pi indicates a smaller volume of reactor for a constant Qi. Thus, it is clearly demonstrated that the system attains faster its steady state values for p.j = 100. Case e demonstrates the effect of large p. =100 for reactors 2 and 3, by comparison to the behavior of reactors 1 and 4 for which pi = 10. It is observed that the response of reactors 2, 3 is faster than the response of reactors 1 and 4. It should be noted that if the tracer is introduced into reactor 2, the behavior is similar, however the concentration in reactor 2 remains constant and begins to change only after tp time units have passed. 

Fig.4.4-2 demonstrates a plug flow reactor containing a "dead water" element of volume Vd where the active part is of volume Vp. The system contains also two perfectly mixed reactors 1 and 2. A tracer in a form of a pulse is introduced into reactor 1 and is transferred by the flow Qi into reactor 2 where it accumulates. 

The distribution system should put in and withdraw liquid without disturbing the plug flow. As the size increases this becomes more importum. A combination of a distribution systses plus a frii to hold the packing seems to work well. The Teed pulses should be input across the entire column diameter in as cl use to a square wave as possible. Otherwise, the obtained resolution is decreased. The distribution system and all piping and velves thou Id have a minimum volume and distance of travel between the injection device and the downstream valves which isolate the products. This minimizes mixing and makes control easier. 

The initial and boundary conditions that apply to this equation depend on whether one is dealing with a pulse or a step stimulus and the characteristics of the system at the tracer injection and monitoring stations. At each of these points the tubular reactor is characterized as closed or open, depending on whether or not plug flow into or out of the test section is assumed. A closed boundary is one at which there is plug flow outside the test section an open boundary is one at which the same dispersion parameter characterizes the flow conditions within and adjacent to the test section. There are then four different possible sets of boundary conditions on equation (11.1.29), depending on whether a completely open or completely closed vessel, a closed-open vessel, or an open-closed vessel is assumed. Different solutions will be obtained for different boundary conditions. Fortunately, for small values of the dispersion parameter, the numerical differences between the various solutions will be small. 

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