Two-fluid atomization

Atomization of melts has, in principle, some similarity to the atomization of normal liquids. The atomization processes originally developed for normal liquids, such as swirl jet atomization, two-fluid atomization, centrifugal atomization, effervescent atomization, ultrasonic piezoelectric vibratory atomization, and Hartmann-whistle acoustic atomization, have been deployed, modified, and/or further developed for the atomization of melts. However, water atomization used for melts is not a viable technique for normal liquids. Nevertheless, useful information and insights derived from the atomization of normal liquids, such as the fundamental knowledge of design and performance of atomizers, can be applied to the atomization of melts. 

Numerous atomization techniques have evolved for the production of metal/alloy powders or as a step in spray forming processes. Atomization of melts may be achieved by a variety of means such as aerodynamic, hydrodynamic, mechanical, ultrasonic, electrostatic, electromagnetic, or pressure effect, or a combination of some of these effects. Some of the atomization techniques have been extensively developed and applied to commercial productions, including (a) two-fluid atomization using gas, water, or oil (i.e., gas atomization, water atomization, oil atomization), (b) vacuum atomization, and (c) rotating electrode atomization. Two-fluid atomization... 

Atomization Rotary atomization Pressure nozzle atomization Two-fluid nozzle atomization... 

If the initial feasibility evaluation is successful, it is reasonable to commit additional materials for a spray-drying trial. A laboratory dryer at least 500 mm in diameter is recommended for such tests. Bench-scale spray dryers are available but are limited in their ability to provide adequate atomization or sufficient process air flow for the successful production of dried particles. The laboratory unit, however, eom-bined with very fine atomization (two-fluid or rotary) will often produce aceeptable product for further testing. A series of tests can be performed at different inlet-outlet temperature combinations using small quantities of material and these samples ean be tested for chemieal stability to evaluate thermal effects from process air contact. The relationship between outlet temperature and final product moisture can also be established for this seale. While samples produced in a laboratory dryer are suitable for evaluating the effeet of spray drying on the product, they are not suitable for use in downstream proeessing because the fine particle distribution produced as a result of the small drying chamber dimensions may not be representative of the final spray-dried produet. 

The Bechtel confined zone dispersion (BCZ) process involves the injection of a fine slurry mist of pressure hydrated dolomitic lime or calcitic lime, using two-fluid atomizing nozzles. A demonstration at the 70 MWe Seward Station of the Pennsylvania Electric Co., performed in 15.2 m of ductwork with a 2.4-m by 3.4-m cross section, achieved a 50% removal of SO2 at a Ca S ratio around 1.1. 

Two-fluid nozzles do not operate efficiently at high capacities and consequently are not used widely on plant-size spray diyers. Their chief advantage is that they operate at relatively low pressure, the hq-uid being 0 to 400 kPa/m" pressure, while the atomizing fluid is usually no more than 700 kPa/m" pressure. The atomizing fluid may be steam or air. Two-fluid nozzles nave been employed for the dispersion of thick pastes and filter cakes not previously capable of being handled in ordinaiy atomizers [Baran, Ind. Eng. Chem., 56(10), 34-36 (1964) andTurba, Brit. Chem. Eng., 9(7), 457-460 (1964)]. 

For proper use of the equations, the chamber shape must conform to the spray pattern. With cocurrent gas-spray flow, the angle of spread of single-fluid pressure nozzles and two-fluid pneumatic nozzles is such that wall impingement wiU occur at a distance approximately four chamber diameters below the nozzle therefore, chambers employing these atomizers should have vertical height-to-diameter ratios of at least 4 and, more usually, 5. The discharge cone below the vertical portion should have a slope of at least 60°, to minimize settling accumulations, and is used entirely to accelerate gas and solids for entty into the exit duct. 

Two-Fluid (Pneumatic) Atomizers This general category includes such diverse apphcations as venturi atomizers and reac tor-effluent quench systems in addition to two-fluid spray nozzles. Depending on the manner in which the two fluids meet, several of the breakup mechanisms may be apphcable, but the final one is high-level turbulent rupture. 

As shown by Table 14-12, empirical correlations for two-fluid atomization show dependence on high gas velocity to supply atomizing energy, usually to a power dependence close to that for turbulent breakup. In addition, the correlations show a dependence on the ratio of gas to liquid and system dimension. 

TABLE 14-12 Exponential Dependence of Drop Size on Different Parameters in Two-Fluid Atomization... 

This is remarkably similar to the empirical two-fluid atomizer relationships of El-Shanawany and Lefebvre [J. Pnergy, 4, 184 (1980)] and Jasuja [Trans. Am. Soc. Mech. Tngr, 10.3, 514 (1981)]. For example, El-Shanawany and Lefebvre give a relationship for a prefilming atomizer ... 

Improve spray distrihiition. Improve atomization hy lowering hinder fluid viscosity. Increase wetted area of the bed per unit mass per unit time hy increasing the number of spray nozzles, lowering spray rate, increase air pressure or flow rate of two-fluid nozzles. 

Two-Fluid Atomization Air-Blast Plain-Jet 15-130 [79]-[82] Industrial gas turbines Simple, cheap, good atomization Narrow spray angle. Atomizing performance inferior to prefilming air-blast type... 

Two-fluid atomization, sometimes also termed twin-fluid atomization, two-phase atomization or aerodynamic atomization, is one of the commonly used techniques in many areas. Two-fluid... 

Figure 2.6. Schematic of internal-mixing two-fluid atomizers.

For low melting-point metals (for example, solder materials), the liquid metal and the atomization gas may be mixed internally inside the atomizer for both low and high melting point materials, the two fluids can be mixed externally outside the atomizer in the nearnozzle region. 

In internal mixing atomization (for example centrifugal-pneumatic atomization), 159] the liquid metal and gas enter the swirl jet atomizer tangentially under pressure (Fig. 2.13)J159] The two fluids rotate, form a mixture, and accelerate in the confuser. Due to the strong centrifugal force, the liquid metal forms a film at the nozzle exit even without the presence of the gas. With the applied gas, the liquid film is atomized into a fine dispersion of droplets outside the nozzle. 

Most commercial and near-commercial atomization processes for liquid metals/alloys involve two-fluid atomization or centrifugal atomization. As suggested by many experimental observations, two-fluid atomization of liquid metals is typically a three-stage process, 3IX 3 yl whereas centrifugal atomization may occur in three different regimes.[5][320] Many atomization modes and mechanisms for normal liquids may be adopted or directly employed to account... 

Figure 3.12. Schematic showing Liquid Jet-Ligament Breakup mode (left) and Liquid Film Sheet Breakup mode (right) in two-fluid atomization of melts.

---