Transfer between phases enhancement

Chemical reaction always enhances the rate of mass transfer between phases. The possible magnitudes of such enhancements are indicated in Tables 23-6 and 23-7. They are no more predictable than are specific rates of chemical reactions and must be found experimentally for each case, or in the relatively sparse literature on the subject. 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

Heat and mass transfer, especially mass transfer, in multiphase systems are problems commonly encountered in processing units in the chemical, petrochemical, and many other process industries. Because transfer rates significantly affect the efficiencies and technical-economic indexes of the processes, the enhancement of transfer has been a continuing topic of interest in chemical engineering since the late 1930s. A vast number of theoretical and experimental investigations have been carried out in the search for new methods of enhancing transfer between phases. 

In comparison, the reduction of specific resistance is an effective way of enhancing transfer between phases and has great potential. 

All the researches, developments, and applications of IS, RPB, and IJ show the extreme importance of increasing relative velocity for enhancing transfer between phases. 

Elperin and Tamir considered that, in impinging streams with gas as the continuous phase, transfer between phases is enhanced by the factors below ... 

Without doubt, the gas-continuous impinging streams (GIS) method has been proved by a number of investigations to significantly enhance transfer between phases [58-60, etc. ]. This feature gives it good application potential. 

In the previous chapters, the materials in the dispersed phases of impinging streams discussed are essentially solid particles, although a few aspects of liquid as dispersed phase were mentioned. Since liquids and solids have densities of the same order of magnitude, quite different from those of gases, the analysis and conclusions described in those chapters, including enhancing transfer between phases, the motion of particles, the residence time and its distribution, and the hydraulic resistance and the related problems, etc., are, in principle, also applicable for the occasions where, instead of solid, liquid is in the dispersed phase without significant deviation. 

As mentioned, like any other technical method, the method of impinging streams (IS) cannot be a universal tool. On one hand, IS has the outstanding advantage of significantly enhancing heat and mass transfer between phases while on the other, it also has its intrinsic faults. From the discussions in the previous chapters, the essential characteristics of gas-continuous impinging streams can be summarized briefly as follows ... 

Its effect on enhancing heat and mass transfer between phases is very significant ... 

It can be seen in Figs 7.18 and 7.19 that in range of the impinging velocity w0 from 5.53 TO 16.62 m s-1 the values for volumetric mass transfer coefficient and the gas-film one ranged from 0.577 to 1.037 s l and 0.00641 to 0.0416 m s1, respectively, showing clearly the effect of impinging streams enhancing transfer between phases. 

It can be seen that in the combined multifunctional impinging stream gas-liquid reactor shown in Figs. 7.21 and 7.23. the working principles and action in enhancing transfer between phases for each sub chamber of absorption are the same as those of the reactor shown in Fig. 7.10. The first difference between the two reactors is that a pair of impinging streams is added in the direction perpendicular to the flow axis of the original pair of impinging streams. As a result, the utilization factor of the space inside the sub chamber is increased. 

In principle, gas-continuous impinging streams (GIS) can be applied for the combustion of gases, powdery solids and sprayed liquids. Since gas-combustion is relatively simple and the process is essentially independent of the major feature of GIS, i.e., that it significantly enhances heat and mass transfer between phases, the discussions in this chapter will focus on the combustion of the latter two kinds of fuels. 

Enhancing transfer between phases As mentioned above, the combustion of both droplets of liquid fuel and particles of solid fuel are multiphase reactions normally... 

As mentioned earlier, in gas-continuous impinging streams heat and mass transfer between phases are enhanced efficiently mainly by the following factors (1) Very high relative velocity between phases round the impingement plane, even higher than in common devices by several tens of times (2) Oscillation movement of particles or... 

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