Atmospheric, dispersion

The way a substance is released or vaporized determines how it is dispersed in the atmosphere. Accordingly the initial and boundary conditions for the dispersion calculation have to be fixed. This constitutes the part of the analytical treatment most affected by uncertainties. We distinguish between releases with large or small initial energy. 

If the kinetic energy on release is small mixing of the released material with the surrounding air is relatively slow. On the other hand, if a jet with high kinetic energy is released mixing is fast (vid. Sect. 10.3). 

Releases can be instantaneous and last for but a short time ( puff release ) or continuous, which then lead to plume formation. 

Both types of release are relevant for safety analyses of process plants. Puff or continuous releases of a flammable gas can form a vapour cloud capable of a vapour cloud explosion (VCE). The same consequence may follow releases with high initial kinetic energy. Toxic gases spread analogously. The consequence may then be the formation of gas clouds with lethal concentrations. 

The dispersion is passive or airborne if the released material is lighter than air. If it is heavier than air, on the other hand, we are dealing with dense gas dispersion. A combination of both types is possible. For example, refrigerated ammonia is dispersed as a dense gas in the first place. After being warmed up by the surrounding air its dispersion behaviour becomes passive. 

The objective must be to reduce atmospheric emissions to a minimum or at least below legislative requirements. However, there is inevitably some residual emission and this must be safely dispersed in the environment. The factors that affect the dispersion of gases to atmosphere are3  

In addition to these components, some NOx will be formed. For fuel oil, this will be of the order of 300 ppm. 

Environmental lapse rate (ELR) Dry adiabatic lapse rate (DALR) 

The third factor affecting dispersion is turbulence. Mechanical turbulence is caused by the roughness of the Earth s surface. Away from the surface, convective turbulence (heated air rising and cooler air falling) becomes increasingly important. The amount of turbulence and the height to which it operates depends on the surface roughness, wind speed and atmospheric stability. 

Calculation of pollution concentration at ground level requires specialized modeling techniques that are outside the scope of this text. 

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When gases that are somewhat soluble in a Hquid concentrate are used, both concentrate and dissolved gas are expeUed. The dissolved gas then tends to escape into the atmosphere, dispersing the Hquid into fine particles. The pressure within the container decreases as the product is dispersed because the volume occupied by the gas increases. Some of the gas then comes out of solution, partially restoring the original pressure. This type of soluble compressed gas system has been used for whipped creams and toppings and is ideal for use with antistick cooking oil sprays. It is also used for household and cosmetic products either where hydrocarbon propeUants cannot be used or where hydrocarbons are undesirable. 

D. B. Turner, Workbook of Atmospheric Dispersion Estimates, US EPA, OAP, Pub. AP26, Research Triangle Park, N.C., revised 1970, U.S. Department Printing Office Stock No. 5503-0015. 

D. B. Turner, A Workbook of Atmospheric Dispersion Estimates, Pubhc Health Service Pubhcation No. 999-AP-26, U.S. Government Printing Office, Washington, D.C., 1967. 

Brummage et al.. The Calculation of Atmospheric Dispersion from a Staek, CONCAWE, The Hague, 1966. 

Holland, Workbook of Atmospheric Dispersion E.stimates, U.S. EPA Publ. AP-26, 1970. 

Maximum Ground-Level Concentrations The effective height of an emission having been determined, the next step is to study its path downward by using the appropriate atmospheric-dispersion formula. Some of the more popular atmospheric-dispersion calculational procedures have been summarized by Buonicore and Theodore (op. cit.) and include ... 

The specific requirements to complete the NSR process will vaiy depending on the source location and characteristics, the federal and state regulations which apply, the compliance status of the facihty if it is existing, and the nature of other sources in the area. Atmospheric dispersion modehng is often necessary to determine the maximum offsite ambient air concentrations of the various pollutants that will be... 

CCPS G-37. Understanding Atmospheric Dispersion of Accidental Releases. American Institute of Chemical Engineers, Center for Chemical Process Safety, New York. 

Many sophisticated models and correlations have been developed for consequence analysis. Millions of dollars have been spent researching the effects of exposure to toxic materials on the health of animals the effects are extrapolated to predict effects on human health. A considerable empirical database exists on the effects of fires and explosions on structures and equipment. And large, sophisticated experiments are sometimes performed to validate computer algorithms for predicting the atmospheric dispersion of toxic materials. All of these resources can be used to help predict the consequences of accidents. But, you should only perform those consequence analysis steps needed to provide the information required for decision making. 

ApSimon, H. M., and WUson, J. J, N., Modeling atmospheric dispersal of the Chernobyl release across Europe. Boundary-Layer Meteorol. 41, 123-133 (1987). 

Tracer studies are extremely important in furthering our knowledge of atmospheric dispersion. These studies consist of release of a known quan-... 

Environmental impact (atmosphere dispersion, settling) ASTM methods... 

Methods of estimating gaseous effluent concentrations have undergone many revisions. For a number of years, estimates of concentrations were calculated from the equations of Sutton, with the atmospheric dispersion parameters C, C, and n, or from the equations of Bosanquet with the dispersion parameters p and Q. More common approaches are based on experimental observation that the vertical distribution of spreading particles from an elevated point is... 

The Gaussian Plume Model is the most well-known and simplest scheme to estimate atmospheric dispersion. This is a mathematical model which has been formulated on the assumption that horizontal advection is balanced by vertical and transverse turbulent diffusion and terms arising from creation of depletion of species i by various internal sources or sinks. In the wind-oriented coordinate system, the conservation of species mass equation takes the following form ... 

Inversion Break-up Fumigation - The inversion break-up screening calculations are based on procedures described in the Workbook of Atmospheric Dispersion Estimates (Turner, 1970). The distance to maximum fumigation is based on an estimate of the time required for the mixing layer to develop from the top of the stack to the top of the plume, using ... 

MESOCHEM Chemical Atmospheric and Hazard Assessment System Impell Corporation Becky Cropper 300 Tristate Internat l Suite 400 Lincolnshire, IL 60069 (312) 940-2090 Software for atmospheric dispersion and chemical exposure assessment. A plume dispersion model. 

As noted earlier, atmospheric dispersion of a pollutant largely depends on ... 

Because of extreme venting conditions assumed, effective stack heights and resultant plumes from both 3- and 5-minute discharge conditions attain heights beyond the micro-meteorological conditions assumed in accepted computation models. It is therefore highly probable there will be considerably further atmospheric dispersion and diffusion of the VCM than predicted in the results shown. That is, the ground level concentration can be expected to be considerably lower than the values shown in Table 6. 

The atmospheric dispersion model for dense ammonia vapor evolves a slice of the plume, from the source to receptor (Kaizer, 1989 ... 

Colenbrander, G. W. and J. S. Puttock, 1980, Maplin Sands experiments 1980 interpretation and modelling of liquified gas spills on the sea, in Atmospheric Dispersion of Heavy Hoses and Small Particles, G. Ooms and H. Tennekes, editors, Springer-Verlag, Berlin. 

Lazaro, M. A. et al, 1997, Model Review and Evaluation for Application in DE Safety Basis Documentation of Chemical Accidents Modeling Guidance for Atmospheric Dispersion and Consequence Assessment, ANL/EAD/TM-75, September. 

Reg. Guide 1.145, 1983, Atmospheric Dispersion Models for Potential Accident Consequence Assessments at Nuclear Power Plants, USNRC, February. 

Sagendorf, J. F., 1974, A Program for Evaluating Atmospheric Dispersion Calculations Considering Spatial and Temporal Meteorological Variations, NO A A Tech. Memo ERL-ARL-44. 

Wheatley, C.J., et al., 1988, Comparison and Test of Model for Atmospheric Dispersion of Continuous Releases of Chlorine, SRD Report R438, UKAEA, July. ... 

Chapter 2 discussed the possible influence of atmospheric dispersion on vapor cloud explosion or flash fire effects. Factors such as flammable cloud size, homogeneity, and location are largely determined by the manner of flammable material released and turbulent dispersion into the atmosphere following release. Several models for calculating release and dispersion effects have been developed. Hanna and Drivas (1987) provide clear guidance on model selection for various accident scenarios. 

The effect of atmospheric dispersion on the structure of a vapor cloud may be summarized as follows. In general, the structure of a vapor cloud in the atmosphere... 

Zeeuwen et al. (1983) observed the atmospheric dispersion and combustion of large spills of propane (1000-4000 kg) in open and level terrain on the Musselbanks, located on the south bank of the Westerscheldt estuary in The Netherlands. Thermal radiation effects were not measured because the main objective of this experimental program was to investigate blast effects from vapor cloud explosions. 

To estimate tlie potential iiupaet on tlie publie or tlie environment of aeeidents of different types, the likely emergeney zone must be studied. For example, a liazardous gas leak, fire, or explosion may eause a toxie cloud to spread over a great distance. The minimum atmospheric dispersion model. Vtirious models can be used tlie more difficult models produce more realistic results, but tlie simpler and faster models may provide adequate data for planning purposes. A more tliorough discussion of atmospheric dispersion is presented in Part 111 - Healtli Risk Assessment. 

D. G. Smith, "Role of Real-Time Atmospheric Dispersion Assessment System," ERT, Inc., Concord, MA, March 1987. 

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