Dense gas dispersion model

EMGRESP is overly conservative for passive gas dispersion applications. No time-varying releases may be modeled. Dense gas dispersion may be computed for only "instantaneous" releases conditions. 

The concentrations predicted by several popular gas dispersion models at the same distances downwind are presented in Table 17.3. The popular gas dispersion models selected were (1) neutrally buoyant Gaussian dispersion, (2) dense gas model SLAB, and (3) ALOHA version 5.2. When the model called for a concentration-averaging time, a one-minute average... 

Dense-gas dispersion The dense-gas regime is described by the thermodynamic model and a box-type dispersion model (Raj et al., 1987 Mohan et al., 1994). 

A complete analysis of dense gas dispersion is much beyond the scope of this treatise. More detailed references are available (Britter and McQuaid, Workbook on the Dispersion of Dense Gases, Health and Safety Executive Report No. 17/1988, England, 1988 Lees, 1986, pp. 455 61 Hanna and Drivas, 1987 Workbook of Test Cases for Vapor Cloud Source Dispersion Models, AlChE, 1989 Guidelines for Chemical Process Quantitative Risk Analysis, 1989, pp. 96-103). 

Colenbrander, G. W. and J. S. Puttock, 1983, Dense Gas Dispersion Behavior Experimental Observations and Model Developments, International Symposium on Loss Prevention and Safety Promotion in the Process Industries, Harrogate, England, September. 

Use the Britter-McQuaid dense gas dispersion model to determine the distance to the 1% concentration for a release of chlorine gas. Assume that the release occurs over a duration of 500 s with a volumetric release rate of 1 m3/s. The wind speed at 10 m height is 10 m/s. The boiling point for the chlorine is —34°C, and the density of the liquid at the boiling point is 1470 kg/m3. Assume ambient conditions of 298 K and 1 atm. 

When a solute is transferred from a solid into a high-pressure gas, it is then taken downstream in the bulk fluid by convective transport. Depending on turbulence, the solute may travel further by other mass-transport mechanisms such as dispersion. Dispersion spreads the solute axially and radially in a cylindrical stet. Eaton and Akgerman [30] considered both axial and radial effects in a model for the desorption of heavy organics, from carbon, by a dense gas. 

LNG Vapor Dispersion Prediction with the DEGADIS Dense Gas Dispersion Model, Report GRI 0242, Gas Research Institute, Chicago, 111. 

Hanna, S.R., and Chang, J.C. (2001) Use of the Kit Fox field data to analyze dense gas dispersion modeling issues, Atmospheric Environment 35,2231-2242. 

Meroney, R.N. (1988) Guidelines for Fluid Modeling of Dense Gas Cloud Dispersion, Journal of Hazardous Materials Vol. 17, 23 16. 

HAVENS, J.A., SPICER, T., LNG Vapor Dispersion Prediction with the DEGADIS Dense Gas Dispersion Model, Topical Report (April 1988 - July 1990), University of Arkansas, Fayetteville, USA (1990). 

In its initial phase a dense gas cloud spreads less in the vertical direction than a cloud of neutral density. Yet, the belief that a dense gas cloud therefore migrates further than one of neutral density is not correct. The different mechanism of mixing with air leads to faster spreading especially under stable weather conditions. In the long run the density of a dense gas cloud becomes practically neutral due to mixing with air. A phase of passive dispersion, whose modelling was explained in the preceding section, ensues. 

The model for dense gas dispersion in [27] is based on experimental results and similitude relations. It is to be preferred to the simple model presented in the next paragraph. 

In general one can state that the modelling of both airborne and dense gas dispersion comprises numerous problems which are stiU to be solved. This is particularly true for near field and if obstacles like buddings or industrial structures must be accounted for, which is the usual situation for releases from process plants. 

In what follows the simple model of Van Ulden [2, 30] is described. It gives an impression of the mechanisms of dense gas dispersion. In any case the model according to [27] should be preferred for practical apphcations. 

The differences of the results underline the modelling uncertainties still existent in dense gas dispersion. ... 

Mohan M, Panwar TS, Singh MP (1995) Development of dense gas dispersion model for emergency preparedness. Atmos Environ 29(16) 2075-2087... 

Dispersion of gases VDI models for airborne and dense gas dispersion (vid. Sect. 10.5)... 

M. van Sint Aimaland, G. A. Bokkers, M. J. V. Goldschmidt, O. O. Olaofe, M. A. van der Hoef and J. A. M. Kuipers, Development of a multi-fluid model for poly-disperse dense gas-solid fluidised beds, Part II Segregation in binary particle mixtures, Chem. Eng. Sci, 2009, 64, 4237 246. 

In summary, a single model has not been developed that can fully characterize riser gas phase hydrodynamics. The studies indicate that under dense phase conditions, typical of commercial FCC riser operation, a simple axial dispersion model may be adequate to characterize gas mixing. Under dilute conditions, a two-phase core-annular model is a good first approximation to the flow structure. However, both radial dispersion and radial gas velocity profiles must be accounted for to provide a realistic and reliable interpretation. The model suggested by Martin et al. should be further developed and applied to risers of different geometry operating with different powders [83]. However, contact efficiency may provide the simplest means from which scale-up criteria can be developed. 

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