Aerosols diffusional deposition

The basic operations in dust collection by any device are (1) separation of the gas-borne particles from the gas stream by deposition on a collecting surface (2) retention of the deposit on the surface and (3) removal of the deposit from the surface for recovery or disposal. The separation step requires (1) application of a force that produces a differential motion of a particle relative to the gas and (2) a gas retention time sufficient for the particle to migrate to the coUecting surface. The principal mechanisms of aerosol deposition that are apphed in dust collectors are (1) gravitational deposition, (2) flow-line interception, (3) inertial deposition, (4) diffusional deposition, and (5) electrostatic deposition. Thermal deposition is only a minor factor in practical dust-collectiou equipment because the thermophoretic force is small. Table 17-2 lists these six mechanisms and presents the characteristic... 

In many experiments described and referenced in Chapter 1 an important role was played by diffusional deposition of nonvolatile molecular species (and of particulate matter) on the surface of gas ducts. First of all, in chromatographic columns, the nonvolatile molecules deposit within a short distance from the inlet and contribute to decontamination of the elements under study from interfering radioactive nuclides. Meanwhile, especially in chemical experiments with the aid of aggressive gases, unwanted aerosols occasionally form. The latter diffuse much more slowly, and when they catch and then carry a fraction of the nonvolatile molecular species, they make the depositional decontamination less efficient. 

Because Sc is inversely proportional to D, the values of Schmidt number for aerosols are much larger than unity. In practice one meets the problem of diffusional deposition of polydisperse aerosols. A solution to this problem for rectangular and circular tubes was proposed in Refs. [10,20]. 

The mathematics of diffusional deposition is the same for particles ranging from atoms to aerosol particulates. Because of smaller diffusion coefficients, tiny aerosols deposit more slowly than do the molecular entities. 

Chapter 2 calls some fundamental laws and formulae of physical chemistry that are relevant to the content of the book. It deals with diffusion and reactions in gases, as well as with adsorption upon collisions with surfaces, emphasizing the molecular level. It provides some formulae for estimating molecular properties of uncommon compounds. Formulae describing irreversible diffusional deposition of molecular entities and aerosols from flowing gas on the walls of channels are presented quite extensively. 

Kirsh AA, Fuchs NA. Investigation of fibrous filters diffusional deposition of aerosols in fibrous filters. Colloid D 1968 30 630. 

Deposition by sedimentation and impaction is a function of the inertial aerodynamic size characteristics of the aerosol particles. Deposition by diffusion is a function of the diffusional properties of the aerosol. Deposition by interception occurs when one of the edges of a particle touches the surface of the respiratory tract. Interception is an especially important determinant of deposition of fibers. Deposition of particles in the respiratoiy tract by electrostatic precipitation... 

This process governs the rate of deposition of the molecules of nonvolatile compounds on the surface of gas ducts, and contributes to broadening of the chromatographic zones. Being of the order of 0.1 pm at STP, the mean free path of molecules, which is inversely proportional to pressure, reaches 1 cm only at about 0.01 mmHg. In dense enough gas, in the absence of convective flow, the macroscopic picture of migration of molecules (as well as of aerosol particulates) is described by the equations of diffusion. The mean squared diffusional displacement z2D of molecules, the time of diffusion t and the mutual diffusion coefficient >i 2 are related by ... 

Figure 3 Schematic filter characteristic of the human respiratory tract for aerosol particles. Three domains can be recognized the domain of deposition decreasing with particle size is solely due to diffusional particle transport, the domain of minimum deposition is due to simultaneous diffusional and gravitational particle transport, and the domain of deposition increasing with particle size due to gravitational and inertial particle transport.

Zhang L, Yuh CP. Empirical equations for nasal deposition of inhaled particles in small laboratory animals and humans. Aerosol Sci Teehnol 1993 19 51-56. Gradon L, Yu CP. Diffusional particle deposition in the human nose and mouth. Aerosol Sci Teehnol 1989 11 213-220. 

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