Gas-continuous impinging streams

The idea of impinging streams (IS) was originally presented for enhancing transfer processes in gas-solid systems. In the 30 years from 1961 when the concept of IS was presented by Elperin to the mid-1990s, investigations on IS were mainly concentrated on systems with a gas as the continuous phase, while, to an extent, the dispersed phase was extended to include liquid. 

Part I of this book focuses on problems relating to gas-continuous impinging streams, including basic regulations, properties, and some of its applications. 

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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. 

Gas-continuous impinging streams involve flows at high velocity, and so power consumption naturally becomes an important concern [62]. As is well known, the theoretical or minimum work per unit time for fluid transportation is equal to the product of the pressure drop and the volumetric flow rate of the fluid ... 

The most important operating parameter for gas-continuous impinging streams is the velocity of the gas flow at the outlet of the accelerating tube, also called impinging velocity, u0. Therefore, the most convenient approach is to relate all the individual hydraulic resistances to u0. 

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

On the other hand, the assembly condition and the inter-molecule force of liquids are quite different from those of solids. In gas-continuous impinging streams with solid and liquid as the dispersed phases, respectively, some different phenomena would occur, which may affect the performance of impinging streams and thus are of concern. This chapter discusses the problem of liquid dispersion in gas-continuous impinging streams, which is related to the topic mentioned above, and introduces the related results of investigations. 

Gas-continuous impinging streams with a liquid as the dispersed phase has wide application, such as in the combustion of liquid fuel droplets, absorption, water-spray cooling of air, etc. [9]. In such systems the dispersity of liquids plays a very important role affecting heat and mass transfer rates, because it influences both the interface area and the mean transfer coefficient. Wu et al. [68] investigated the influence of impinging streams on the dispersity of liquid. 

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 the previous chapters the essential principles and characteristics of gas-continuous impinging streams (GIS) were discussed while this and subsequent chapters in Part I will focus on the research and development of applied technologies. The contents have been chosen to be as valuable as possible for practical application, while successful or unsuccessful experiences included can be used for reference. 

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 ... 

On the basis of a thorough understanding of the properties of gas-continuous impinging streams, including its advantages and disadvantages, the circulative impinging stream dryer (CISD) was developed and patented by the author of the present book [11, 103]. Primary tests on a quasi industrial scale have yielded satisfactory results, and it can be expected to be applied industrially in the near future. 

From the discussions above, the following general principle for selection of target systems for IS application can be concluded the gas-continuous impinging streams method is especially applicable to gas-liquid reaction or chemical absorption systems involving fast-irreversible reaction(s) in liquid. 

As stated above, the key elements of a gas-continuous impinging stream absorber are the absorption chamber and the atomizers. 

The gas-continuous impinging stream gas-liquid reactor for the experiments of wet desulfurization of flue gas employs the flow configuration of horizontal coaxial two impinging streams, as shown in Fig. 7.9. 

With the impinging velocity u0 ranging from 5.53 to 16.62 m s 1, the measured volumetric mass transfer coefficient kGa is in the range 0.577 to 1.037 s 1 and kG from 0.00641 to 0.0416 m-s 1, showing clearly the effect of gas-continuous impinging streams enhancing mass transfer ... 

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. 

From the basic properties of gas-continuous impinging streams and the burning models derived in the last section, impinging streams intensify the combustion processes of atomized liquid and powdery solid fuels by the following mechanisms ... 

In addition to the two combustors introduced briefly above, researches and development on the application of gas-continuous impinging streams to combustion has been carried out since the 1970s, e.g., the investigations made by Goldberg and Essenhigh [ 146], Ziv etal. [12], Goldman [147-149] and Liu et al. [150]. Mostly, these works involve experimental studies and model analyses, and mainly aimed at the improvements of combustor structure and the arrangement of burners in order to increase combustion efficiency. Most of the experimental combustors employed were very small and their application seems far from practical. 

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... 

Wu, Yuan, Sun, Qin, Cheng, Rong and Xu, Jingnian. Dispersity of liquid in gas-continuous impinging streams. A paper to be published. 

Wu, Yuan, Li, Qin and Li, Fang. Desulfurization in the gas-continuous impinging streams gas-liquid reactor. Chemical Engineering Science (in press). 

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