Nozzle Atomization

Spray production by methods involving high-speed ejection of a liquid through an orifice (nozzle atomization) and ejection from a spinning disk by centrifugal force (rotary atomization) are the simplest and most important situations because they require knowledge of only one material velocity—that of the liquid. Spray production by the action of an incident air stream on a jet of liquid involves, of course, the velocity of both the liquid and the air. 

If a liquid is forced through an orifice (nozzle) under a pressure, the velocity of the liquid in the channel of the orifice becomes so high that turbulent flow 

The surface tension forces keeping the jet together will be air. The second radius of curvature for the jet being infinitely large. The critical radius at which a continuous jet of liquid becomes unstable and breaks up to form 

It is difficult, of course, to determine the value of u in a flowing system thus experimental verification of such an analysis is not a trivial matter. However, if one assumes that (is proportional to the injection pressure, the product of the pressure and should be constant. In practice, the agreement is not quite exact. If one were to use an excess pressure—that is, the pressure in excess of that at which chaotic flow begins—the agreement might logically be expected to improve. 

Since theories for predicting the drop size of a spray based on the characteristics of the liquid and the apparatus are complex and sometimes unsatisfactory, it is usually necessary to measure sizes for each given situation. In general, however, the following rules hold for most fluid ejection systems  

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A spray-dryer eonsists of a feed tank, a rotary or nozzle atomizer, an air heater, a drying ehamber, and a eyelone to separate the powder from the air. A rotary atomizer uses eentrifugal energy to form the droplet. Pressure-nozzle atomizers feed solution to a nozzle under pressure, whieh forms the droplet. Two-fluid nozzles feed solutions separately into a nozzle head, whieh produces high-speed atomizing air that breaks the solution into tiny droplets. Both the feed solution and the drying air are fed into the drying ehamber in a standard eoeurrent flow [27]. 

Decrease feed nozzle atomizing steam and increase delta coke... 

Nozzle (atomizing) air can contribute significantly to product movement and can also be a source of a significant increase in product attrition. 

In order then to determine what influences flavor retention during drying, one must focus attention on the very early stages of dehydration. In fact, it has been shown that the major fraction of total volatiles lost during nozzle-atomized spray drying occurs within ten centimeters of the pressure nozzle (17, 33, 35). 

Fig. 4. Tail-form chamber employing nozzle atomization. System is particularly suited for dense particles requiring high-pressure atomization. (Stork-Bowen)...

A multistory, tail-form dryer chamber using nozzle atomization is shown in Fig, 4. 

A) Narrow Distribution - Single Fluid Nozzle Atomization... 

Spray. Direct type, continuous operation Suited for large capacities. Product is usually powdery, spheric, and free-flowing. High temperatures can be used with heat-sensitive materials. Product may have low bulk density See comments under Liquids. Pressure-nozzle atomizers subject to erosion Requires special pumping equipment to feed the atomizer. See comments under Liquids Not applicable Not apphcable Not applicable Not applicable Not applicable... 

The results reported in this chapter are confined to two fuels propane gas supphed tangentially at station 3 and hquid kerosene supplied with an ultrasonic whistle nozzle atomizer on the central axis at the base of the combustor. 

Fig. 7 Buchi/Brinkmann Lab Scale Spray-Dryer. A stock solution or suspension is pumped from the beaker through a nozzle which sits above the large glass particle formation vessel on the left-hand side. The nozzle atomizes the feedstock at temperatures ranging from ambient to 250° C into the particle formation vessel, using air or an inert gas such as nitrogen to dry and move particles into the cyclone and collection chamber on the right-hand side. Product temperature is monitored by a temperature probe mounted between the particle formation vessel and the cyclone. Solvent or water are exhausted through a fine particle filter bag in series with the cyclone, which also collects fines.

If the binder solution is added continuously, then the method of addition (pneumatic or binary nozzle, atomization by pressure nozzle) should be considered in any end-point determination and scale-up. 

Viscosity CSt 3-15 80"C 2-4 20 C Preheat system, fuel nozzle atomization... 

Spray Dryers A pumpable feed is atomized into droplets by a rotary or nozzle atomizer, as described under Entrainment Dryers. An integral fluid bed or belt may be added below the dryer to give longer residence time and some agglomeration. Semibatch and continuous operation is possible. 

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

Two-fluid nozzle atomization In two-fluid nozzle atomizers, the liquid feed is fed to the nozzle under marginal or no pressure conditions. An additional flow of gas, normally air, is fed to the nozzle under pressure. Near the nozzle orifice, internally or externally, the two fluids (feed and pressurized gas) are mixed and the pressure energy is converted to Kinetic energy. The flow of feed disintegrates into droplets during the interaction with the high-speed gas flow which may have sonic velocity. 

The spray angle obtained with two-fluid nozzles is normally on the order of 10° to 20°, a very narrow spray pattern that is related to the spread of a free jet of gas. Spray drying chamber designs for two-fluid nozzle atomization are very specialized according to the application. 

The droplet size produced by a two-fluid nozzle atomizer varies inversely with the ratio of gas to liquid and with the pressure of the atomization gas. The capacity of a two-fluid nozzle is not linked to its atomization performance. Therefore two-fluid nozzles can be attributed with some turndown capability. 

Two-fluid nozzles share with pressure nozzles the lack of high feed capacity combined with fine atomization in one single unit. Many spray dryer applications with two-fluid nozzle atomization have a very high number of individual nozzles. The main advantage of two-fluid nozzles is the capability to achieve very fine atomization. 

For spray dryers with pressure nozzle atomization, the mean particle size of the dried product varies in the range from 50 to 250 pm. 

Figure 12-94a shows a cocurrent cone-based tall form chamber with roof gas disperser. This chamber design is used primarily with pressure nozzle atomization to produce powders of large particle sizes with a minimum of agglomeration. The chamber can be equipped with an oversize cone section to maximize powder discharge from the chamber bottom. This type of dryer is used for dyestuffs, baby foods, detergents, and instant coffee powder. 

Figure 12-94 > shows a countercurrent flow chamber with pressure nozzle atomization. This design is in limited use because it cannot produce heat-sensitive products. Detergent powder is the main application. 

Liquid droplet size (decreases with increasing atomization air or nozzle atomization ratio NAR) Increases size and spread of granule size distribution... 

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