Droplets-in-gas suspensions

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 order to obtain experimental evidence for understanding the influence of the impingement between the opposing droplets-in-gas suspension streams on the dispersity of liquid, and also to get some practically applicable information for designing and operating impinging stream devices, the experimental investigation described below was carried out. 

The larger the value for cr, the greater the scattering of the size distribution, and, correspondingly, the poorer the uniformity of the droplet sizes. Part of the data measured at various air-to-liquid mass flow ratio, m.JmL, are shown in Table 5.1. It can be seen that, normally, the impingement between opposing droplets-in-gas suspension streams makes cr smaller. Only those obtained at in the third column of the... 

Essentially, the impingement between the two opposing droplets-in-gas suspension streams does not change the mean diameter of the droplets. 

An aerosol is a suspension of either a solid or a liquid in a gas. Fog, for example, is a suspension of small liquid water droplets in air, and smoke is a suspension of small solid particulates in combustion gases. In both cases the liquid or solid particulates must be small enough to remain suspended in the gas for an extended time. Solid aerosol particulates, which are the focus of this problem, usually have micrometer or submicrometer diameters. Over time, solid particulates settle out from the gas, falling to the Earth s surface as dry deposition. 

Thus, the isothermal is a straight line of slope [Cw(9 — On) + A] with respect to the humidity axis. At the reference temperature 90, the slope is X at higher temperatures, the slope is greater than X, and at lower temperatures it is less than X. Because the latent heat is normally large compared with the sensible heat, the slope of the isothermals remains positive down to very low temperatures. Since the humidity is plotted as the ordinate, the slope of the isothermal relative to the X-axis decreases with increase in temperature. When 9 > Bq and Jf > Jf0, the saturation humidity, the vapour phase consists of a saturated gas with liquid droplets in suspension. The relation between enthalpy and humidity at constant temperature 9 is ... 

Colloid chemistry investigates substance mixtures. These substance mixtures can be heterogenous, such as emulsions (in which tiny droplets of one liquid are dispersed in another), suspensions (consisting of a fine dispersion of solid particles in a liquid volume phase), and aerosols (in which liquid droplets are dispersed in the gas phase). However, there are also homogenous mixtures in which the solute is present in larger, supermolecular aggregates. These homogenous mixtures include micellar solutions and liquid crystalline... 

The correlations for as discussed above are for homogeneous liquids. Bubbling gas-liquid reactors are sometimes used for suspensions, and bioreactors of this type must often handle suspensions of microorganisms, cells, or immobilized cells or enzymes. Occasionally, suspensions of nonbiological particles, to which organisms are attached, are handled. Consequently, it is often necessary to predict how the values for suspensions will be affected by the system properties and operating conditions. In fermentation with a hydrocarbon substrate, the substrate is usually dispersed as droplets in an aqueous culture medium. Details of... 

Just as with emulsions and foams, suspensions can exist with additional dispersed phases present. They may contain, in addition to solid particles and a continuous liquid phase (and possibly a stabilizing agent), emulsified droplets and/or gas bubbles. Figure 2.4 (in Section 2.2.1) shows photomicrographs of a practical suspension that contains suspended oil droplets in addition to the particles. The terminology used to describe such systems can become confusing. Consider an aqueous dispersion of solid particles and emulsion droplets. If the solid particles are adsorbed on the emulsion droplets then it is an emulsion that also contains solids. If, however, the particles and droplets are not mutually associated then the system is at once a suspension and an emulsion. Which term is used becomes a matter of choosing the most appropriate context frequently one or the other is considered to be the primary dispersion while the other phase is considered to be an additive or a contaminant. 

In gravitational separators, separation of droplets and firm suspensions from a gas flow occurs by the influence of gravity. A high degree of gas and liquid separation can only be achieved at a very slow flow speed. The optimum speed as established in practice is 0.1 m/s at a pressure of 6 MPa. The proportion of petroleum suspension separation from the gas at this flow speed is 75—85%. 

A number of colloids are familiar to us. An aerosol consists of liquid droplets or solid particles dispersed in a gas. Examples are fog and smoke. Mayonnaise, which is made by breaking oil into small droplets in water, is an example of emulsion, which consists of liquid droplets dispersed in another liquid. Milk of magnesia is an example of sol, a suspension of solid particles in a liquid. 

In gas dynamic atomization, a liquid stream is broken-up into small entities which are dried very quickly. Because the formation of droplets occurs in aerodynamic suspension, the material experiences no shear and the liquid temperature does not rise above the local dew point, despite high gas temperatures. Since drying and subsequent cooling are rapid, organic materials do not have time to oxidize, degrade, or experience any other damage. Food powders often exhibit better flavor, texture, and instant characteristics than comparable powders from other spray dryers. Because a low pressure stream of slurry is pumped and dispensed, the system can also handle corrosive and abrasive products easily. Control over particle size is normally better. Fig. 7.82 depicts SEM photographs of some typical products. 

One of the most important issues in modeling the suspension polymerization process is the evaluation of the physical and transport properties of the reacting system as well as the calculation of composition and partitioning of the different species (e.g., monomer(s), polymer, initiator(s), etc.) in the various phases present in the system. In a suspension polymerization process, one can identify, at least, three phases the dispersed phase (e.g., polymerizing monomer droplets), the continuous aqueous phase and the gas phase. The dispersed phase can be either homogeneous (if the polymer is soluble in its monomer) or heterogeneous (if the polymer is insoluble in its monomer). In the powder suspension polymerization, the dispersed phase consists of two different phases the polymer-rich and the monomer-rich phase. The continuous aqueous phase contains only small amoimts of monomer and, finally, the gas phase contains monomer and water vapors. 

Operations that generate aerosols (suspensions of microscopic droplets in air), such as vigorous boOing, highspeed blending, or bubbling gas through a liquid, increase the potential for exposure via inhalation. Consequently, these and other such operations on toxic chemicals should also be carried out in a hood. 

Aerosol Suspension of small solid particles and/or liquid droplets in a gas, usually air. 

Colloidal dispersions are those having particles or drops with at least one dimension greater than about 1 nm bnt less than abont 1 rm. These systems are classified as emnlsions when a hquid phase is dispersed in a second liquid, suspensions when a sohd phase is dispersed in a liqnid medinm, foams when a gas is dispersed in a hquid, or aerosols when hqnid droplets or solid particles are dispersed in gas. Other combinations are less common. 

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