Gas—Droplet Mixing

Gas-droplet contact can be either cocurrent or countercvirrent (and sometimes crosscurrent). Up imtil now we have considered that the gas has a large voliune with constant temperatvire and partial pressure. In a spray dryer the gas in contact with a particle constantly changes due to the gas-droplet contact pattern. These different dryer configu- 

Chapter 8 Other Ceramic Powder Fabrication Processes 

FIGURE 8A2 Spray diyer flow patterns (a) Cocurrent flow, (b) countercurrent flow. 

To account for the effect of particle size distribution in addition to the residence time distribution is difficult, because different size particles can remain in the reactor for different periods of time. To account for these effects completely, a population balance must be performed, where the conversion, Xg, is an internal variable (see Chapter 3). This type of treatment is beyond the scope of this chapter. A simplified method of accounting for ffie effects of a particle size distribution, W(r), on the mean conversion, X, is by [20, p. 381] 

The time, t, spent by a droplet in the spray tower of height, z, is given by vector addition of the gas velocity, Vg, and the terminal settling velocity of the droplet, o, (i.e., v = z/t = + )- With countercurrent 

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Spray drying involves three fundamental steps (1) atomization, (2) droplet drying, and (3) gas-droplet mixing. Each of these steps will be discussed in the following sections. The particles produced by spray drying are sometimes spherical and other times in the shape of a punctured spherical shell. The distribution of internal stnicture and chemistry including binder are not uniform inside a spray dried particle because it dries from the outside, first bringing impurities finm the center of the droplet to the surface where they are crystallized out or left behind when the solvent evaporates. 

For flow in spray dryers, this force may be important in the droplets entrance region, where the velocities of injected spray droplets and drying gas are substantially different and intensive gas-droplet mixing leads to high change rates of these gas-droplet relative velocities. 

The high potential and small radius of curvature at the end of the capillary tube create a strong electric field that causes the emerging liquid to leave the end of the capillary as a mist of fine droplets mixed with vapor. This process is nebulization and occurs at atmospheric pressure. Nebulization can be assisted by use of a gas flow concentric with and past the end of the capillary tube. 

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

Emissions of soot on the other hand represent a smaller fraction of the overall emission, but are probably of greater concern from the standpoint of visibility and health effects. It has been suggested that soot emissions from fuel oil flames result from processes occurring in the vicinity of individual droplets (droplet soot) before macroscale mixing of vaporized material, and from reactions in the bulk gas stream (bulk soot) remote from individual droplets. Droplet soot appears to dominate under local fuel lean conditions (1, 2), while bulk soot formation occurs in fuel rich zones. Factors which are known to affect soot formation from liquid fuel flames include local stoichiometry, droplet size, gas-droplet relative velocity and fuel properties (primarily C H ratio). 

Ethene gas when mixed with HBr gas produces droplets of a colourless liquid, bromoethane (Figure 6.2.24). 

A number of fine reviews have appeared recently which address in part the problems mentioned and the models employed. Rietema (R12) discusses segregation in liquid-liquid dispersion and its effect on chemical reactions. Resnick and Gal-Or (RIO) considered mass transfer and reactions in gas-liquid dispersions. Shah t al. (S16) reviewed droplet mixing phenomena as they applied to growth processes in two liquid-phase fermentations. Patterson (P5) presents a review of simulating turbulent field mixers and reactors in which homogeneous reactions are occurring. In Sections VI, D-F the use of these models to predict conversion and selectivity for reactions which occur in dispersions is discussed. 

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. 

One of the primary reasons that the spray dryer-scrubber is able to achieve excellent sulfur dioxide removal with such low liquid-to-gas ratios is the small size of the droplets produced by the high speed centrifugal atomizer. This type of atomizer also has an easily controlled turndown capability which is a desirable feature that has been demonstrated in the pilot tests. As gas flow decreases, the amount of sodium carbonate solution can be decreased in direct proportion without interfering with sulfur dioxide removal efliciency. The atomizer actually produces finer droplets at the lower liquid flow rates. This appears to compensate for any gas-liquid mixing problems that could impair performance. 

Steam-assist flares use high pressure steam to entrain surrounding air and inject it into the core of the flare gas stream. The rapid mixing of the steam and air with the flare gas helps reduce soot formation that tends to lower the flame radiant fraction. Figure 30.14 shows a steam-assisted flare operating under identical flare gas flow conditions with and without steam-assist. Notice without steam-assist, the flame is more luminous and contains more soot this results in higher radiant fractions. The fraction of heat radiated from a flame can also be greatly increased by the presence of liquid droplets in the gas. Droplets within a hot flame can easily be converted to soot [21]. 

Centrifugal Scrubbers. In a centrifugal scrubber, column design and directed sprays cause droplets and gas to mix in a rising vortex such that centrifugal force increases the momentum of collisions between particles and drops. Thus, smaller particles can be captured, and efficiencies as high as 90 percent can be achieved with particles as small as 5 pm, at pressure drops from 2- to 6-in (50- to 150-mm) water gauge. 

Metal matrix composite (MMC) particles can be fabricated through mixing solid particles and atomized liquid droplets in a spray atomization and co-injection process [4, 5, 30, 31, 52]. The process is characterized by a three-phase spray flow (gas/droplets/ particles). Solid particles (usually at least one order of magnitude smaller than atomized droplets), conveyed by the atomization gas or via a separate gas-assisted delivery system, are injected into the droplet spray and likely to be incorporated into the droplets or captured by the droplets surface during frequent impingements, forming composite droplets which are subsequently solid-ifled as composite particles. 

Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets, and small particulate matter. For ICP/MS, it is then only necessary to sweep the ablated aerosol into the plasma flame using a flow of argon gas this is the basis of an ablation cell. It is usual to include a TV monitor and small camera to view the sample and to help direct the laser beam to where it is needed on the surface of the sample. 

A liquid sample must be vaporized to a gas or, more likely, to a vapor consisting of an aerosol of gas, small droplets, and even small particles of solid matter. To be examined, the aerosol is mixed with argon gas to make up the needed flow of gas into the plasma and is then swept into the flame. 

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