Aerosol coagulation

Landgrebe, J. D., and Pratsinis, S. E., Gas Phase Manufacture of Particulates Interplay of Chemical Reaction and Aerosol Coagulation in the Free-Molecular Regime." Ind. Eng. Chem. Res., 28 1474-1481 (1989)... 

The most complete theory for aerosol coagulation is that of Fuchs (1964). Since the attachment of radon progeny to aerosols can be considered as the coagulation of radon progeny (small diameter particle) to aerosols (large diameter particle), it is reasonable to use Fuchs theory to describe this process. The hybrid theory is an approximation to Fuchs theory and thus can be used to describe the attachment of radon progeny to aerosols over the entire aerosol size spectrum. 

A history of various studies and theories of aerosol coagulation is given by Gucker 41). Kivnick and Johnstone 71) treat the subject of coalescence of droplets in a turbulent jet. Aerosol build-up techniques are presented by Fahnoe, Lindroos, and Abelson 31). 

Volk, Jr., M and Moroz, W.J., "Aerosol Coagulation in an Acoustic Field", CAES Technical Report Number 354 -74, The Pennsylvania State University, University Park, Pennsylvania, 80 pages, (1974). 

The second method for aerosol coagulation in turbulent flows arises because of inertial differences between particles of different sizes. The particles accelerate to different velocities by the turbulence depending on their size, and they may then collide with each other. This mechanism is unimportant for a monodisperse aerosol. For a polydisperse aerosol of unspecified size distribution, Levich (1962) has shown that the agglomeration rate is proportional to the basic velocity of the turbulent flow raised to the 9/4 power, indicating that the agglomeration rate increases very rapidly with the turbulent velocity. Since very small particles are rapidly accelerated, this mechanism also decreases in importance as the particle size becomes very small, being most important for particles whose sizes exceed 10-6 to 10"4 cm in diameter. In all cases brownian diffusion predominates when particles are less than 10-6 cm in diameter. 

The aerosol properties discussed in previous chapters relate primarily to individual particles. For the most part, discussions have avoided consideration of interference effects between particles. But one area where interpartide effects cannot be neglected is aerosol coagulation, also known as aggregation or agglomeration. 

Otto, E., et al. (1999). Log-normal size distribution theory of Brownian aerosol coagulation for the entire particle size range Part 11—Analytical solution using Dahneke s coagulation kernel. J. Aerosol Science. 30, 1, 17-34. 

Aerosols are unstable with respect to coagulation. The reduction in surface area that accompanie.s coalescence corresponds to a reduction in the Gibbs free energy under conditions of constant temperature and pressure. The prediction of aerosol coagulation rates is a two-step process. The first is the derivation of a mathematical expression that keeps count of particle collisions as a function of particle size it incorporates a general expression for tlie collision frequency function. An expression for the collision frequency based on a physical model is then introduced into the equation Chat keep.s count of collisions. The collision mechanisms include Brownian motion, laminar shear, and turbulence. There may be interacting force fields between the particles. The processes are basically nonlinear, and this lead.s to formidable difficulties in the mathematical theory. 

Thus there is a lai e discrepancy between the theoretical predictions of the collision efficiency for aerosol coagulation by differential sedimentation (taking into account inter-particie fluid motion) and experimental measurements for coagulation by turbulent shear in aqueous suspensions. We do not know whether this discrepancy is due to the ba.sic difference in the coagulation mechanisms (differential sedimentation vs. turbulent shear), different phenomena operating in the different fluid media, or some other as yet unidentified effect. 

To predict the size distribution of a uniform aerosol coagulating in a chamber without deposition on the walls, the following procedure can be adopted The volumetric concentration of aerosol is assumed constant and equal to its (known) initial value. The change in the number concentration with time is calculated from (7.75). The size distribution at any time can then be determined for each value of u — < r /Woo front the relation n = (JV /0) (tj). using the tabulated values. The calculation is carried out for a range of values of t). 

Marlow (1981) has extended this development to the transition and free-molecule regimes and determined that the effect of van der Waals forces on aerosol coagulation rates can be considerably more pronounced in these size ranges than in the continuum regime. 

Alam, M. K. (1987) The effect of van der Waals and viscous forces on aerosol coagulation. Aerosol Sci. Technol. 6, 41-52. 

Gryn, V. I., and Kerimov, M. K. (1990) Integrodifferential equations for nonspherical aerosol coagulation, USSR Comput. Math. Math. Phys. 30, 221-224. 

Mulholland, G. W., and Baum, H. R. (1980) Effect of initial size distribution on aerosol coagulation, Phys. Rev. Lett. 45, 761-763. 

Pnueli D., Gutfinger C., Fidrman M., A turbulent-brownian Model for Aerosol Coagulation, Aerosol Sd. Technol, 1991, Vol. 14, p. 201-209. 

Landgrebe, J. D. and S. E. Pratsinis, A Discrete Sectional Model for Particulate Production by Gas Phase Chemical Reaction and Aerosol Coagulation in Free Molecular Regime, J. Colloid Inter/ Sci., 139, 63-86 (1990). 

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