Impinging stream device

Because of the importance of the residence time distribution in the impingement zone, the basic requirement when designing the experimental equipment is that it should be suitable for understanding the residence time and its distribution of particles in the impingement zone. For this purpose, the rest of the equipment should be as simple as possible. From the point of view of residence time distribution, this means shortening the residence times in the spaces other than the impingement zone as much as possible. 

Wu and Wu [50, 51] studied the residence time distribution of particles in a typical co-axial horizontal gas-solid impinging stream contactor (ISC). The structure of the experimental impinging stream device is shown in Fig. 3.1. The body of the contactor is made of plexiglass. The horizontal cross section of the upper chamber is a square of 0.4x0.4 m, and the bottom is tapered sharply in order to avoid the solid material heaping up. This equipment essentially meets the requirements stated above. 

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In addition to those shown in Fig. 2, there are many other different structures. Different impinging stream devices may have different flow configurations, although all of them contain the same essential elements (1) the streams flow in opposite directions and impinge against each other, and (2) each stream contains at least one continuous phase. 

One of the necessary conditions for carrying out a process with gas-solid impinging streams is to accelerate particles from zero to a velocity close to that of gas streams before the opposing streams impinge against each other. Therefore a piece of tube is normally set up in an impinging stream device for each stream for particle acceleration. 

On one hand, solid materials to be processed with an impinging stream device have various sizes, while, on the other hand, the relative velocity between gas and particles varies from time to time in acceleration and deceleration stages of particle motion. Both factors make the value for Rep vary continuously with considerably large amplitude, which may be across various flow regimes. So, the variation of the drag coefficient, Cd, in various flow regimes has to be taken into account in the solution of the motion equations for the particle in various stages. 

A theoretical analysis is helpful for understanding the basic characteristics of impinging stream processes and the performances of the related devices. In an impinging stream device, where the residence time distribution of particles is most important is in the impingement zone, because this zone is the major active region for heat and mass transfer between phases in such a device. Unfortunately, it is basically... 

In most cases, the major part of the particles movement in this stage is in the turbulent regime because of the high relative velocity between particles and gas flow, and so this space is also an active region for heat and mass transfer in the impinging stream device. 

The residence time distribution of particles is related to the properties of the particles and the gas flow, including the size distribution, and the velocity of gas flow and its profile. In practically applicable impinging stream devices, the particles being processed usually have relatively narrower size distribution the diameter of the tube to particles size ratio, d Jd,h is normally very large ( 15) while the gas velocity is high... 

Using the procedure described above, input a step change of the tracer to the hopper and then measure the response of the screw feeder to the step change as a known function of time, which is used as the input signal to the following impinging stream device for the measurement of RTD. 

A set of typical data calibrated is shown in Fig. 3.7. It demonstrates that there is good regularity and reproducibility of the data, suggesting the calibrated curves can be used as the input signals to the impinging stream device for RTD measurement to yield sufficiently accurate results. 

In the derivation above, the input signal C (t) is an arbitrary function of time, without any restraint condition, but, of course, it should be known CAo is the response of the impinging stream device to CAi(r) that can be measured by sampling at the outlet of the device. 

The mean residence times of the particles in various sub-spaces of the impinging stream device are determined by interpretation of the experimental data of 27 sets in total, and the conditions tested range as follows ... 

Generally it can be considered that power consumption should not be a problem in the application of gas-continuous impinging stream devices. 

In a gas-continuous impinging stream device with liquid as the dispersed phase, the liquid is usually atomized into fine droplets with nozzles of an appropriate type, and ejected into gas flows to form droplets-in-gas suspensions before impingement. This can be called the Primary Atomization, and it defines the primary dispersity of liquids. The mechanism of primary atomization and the methods for predicting size distribution (SD) and mean diameter (MD) of the sprayed droplets have been widely reported and some sources of references may be found, e.g., in Ref. [69]. 

In order to obtain experimental evidence for understanding the influence of the impingement between the opposing droplets-in-gas suspension streams on the dispersity of liquid, and also to get some practically applicable information for designing and operating impinging stream devices, the experimental investigation described below was carried out. 

To make sampling easier, the open horizontal co-axial two-impinging stream device is employed in the experiments, as shown in Fig. 5.1. 

Figure 5.1 Impinging stream device for the measurement of droplet size distribution. 1 critical nozzle 2, 3 rotameter 4 frame 5 guider 6 apron A-A impingement plane.

To solve the problem that the residence time of material in a simple impinging stream device is very short so that drying cannot be carried out individually, in addition to the improvements described above, some technological schemes have been proposed which combine the method of impinging streams with other method some of these are introduced below. 

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