Dense gases

Fig. 36. Mechanisms for permeation of gases through porous and dense gas-separation membranes.

Initial plume volume flux for dense gas dispersion, voliime/time Continuous release rate of material, mass/time Instantaneous release of material, mass Release duration, time T Absolute temperature, K... 

Initial volume of released dense gas material, length x,q,z Distance in dimensional space, length... 

Dense Gas Dispersion A dense gas is defined as any gas whose density is greater than the density of the ambient air through which it is being dispersed. This result can be due to a gas with a molecular weight greater than that of air, or a gas with a low temperature due to autorefrigeration during release, or other processes. 

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). 

The first step is to determine if the dense gas model is apphcable. If an initial buoyancy is defined as... 

If the group has a value greater than or equal to 2.5, then the dense gas release is considered continuous. If the group value is less than or equal to 0.6, then the release is considered instantaneous. If the value... 

FIG. 26-61 Nomograph to estimate downwind concentrations due to an instantaneous dense gas release based on the Britter-McQnaid correlation. 

Puttock, J. S. (ed.), "Stably Stratified Flow and Dense Gas Dispersion." Oxford University-Press, New York, 1988. 

The model contains a surface energy method for parameterizing winds and turbulence near the ground. Its chemical database library has physical properties (seven types, three temperature dependent) for 190 chemical compounds obtained from the DIPPR" database. Physical property data for any of the over 900 chemicals in DIPPR can be incorporated into the model, as needed. The model computes hazard zones and related health consequences. An option is provided to account for the accident frequency and chemical release probability from transportation of hazardous material containers. When coupled with preprocessed historical meteorology and population den.sitie.s, it provides quantitative risk estimates. The model is not capable of simulating dense-gas behavior. 

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. 

SLAB is one of the most widely-used dense gas models in the public domain. The model accepts evaporating pool sources, jet releases at any elevation, and instantaneous volume sources. 

SLAB calculates chemical concentrations at positions downwind and heights above the ground. Tlic plume may be denser-than-air, neutrally-buoyant, or less dense than air. Thermodynamics effeci.s are accounted for, including latent heat exchanges due to the condensation or evaporation ot liquids, Time averaged results may be calculated. SLAB is the easiest of the publicly-available dense gas models to set up and mn. It has been extensively validated against large-scale field data. 

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. 

The assumption that the probability of simultaneous occurrence of two particles, of velocities vt and v2 in a differential space volume around r, is equal to the product of the probabilities of their occurrence individually in this volume, is known as the assumption of molecular chaos. In a dense gas, there would be collisions in rapid succession among particles in any small region of the gas the velocity of any one particle would be expected to become closely related to the velocity of its neighboring particles. These effects of correlation are assumed to be absent in the derivation of the Boltzmann equation since mean free paths in a rarefied gas are of the order of 10 5 cm, particles that interact in a collision have come from quite different regions of gas, and would be expected not to interact again with each other over a time involving many collisions. 

As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After unfreezing , rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory [1] is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions [2], In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational sites libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic. 

The orientation of linear rotators in space is defined by a single vector directed along a molecular axis. The orientation of this vector and the angular momentum may be specified within the limits set by the uncertainty relation. In a rarefied gas angular momentum is well conserved at least during the free path. In a dense liquid it is a molecule s orientation that is kept fixed to a first approximation. Since collisions in dense gas and liquid change the direction and rate of rotation too often, the rotation turns into a process of small random walks of the molecular axis. Consequently, reorientation of molecules in a liquid may be considered as diffusion of the symmetry axis in angular space, as was first done by Debye [1],... 

FIGURE C.1 Elemental sulfur burns in air with a blue flame and produces the dense gas sulfur dioxide, a compound of sulfur and oxygen. 

A vertical CVD reactor (cf. Figure lb) consists of an axlsymmetrlc enclosure with the deposition surface perpendicular to the Incoming gas stream. The reactant gases are typically Introduced at the top and fiow down towards the heated susceptor. Thus, the least dense gas Is closest to the growth Interface which destabilizes the fiow. The result Is recirculation cells which Introduce not only film thickness and composition variations but also broaden Junctions between layers. This Is particularly of... 

Molecular bromine is highly toxic, as is methyl bromide (CH3 Br), a dense gas used as an insecticide. Methyl bromide is produced by bromination of methane CH4 + Br2 CH3 Br + I Br Molecular bromine is also used in the synthesis of dyes and pharmaceuticals. 

The efficiency of drilling operations can be increased using a drilling fluid material that exists as supercritical fluid or a dense gas at temperature and pressure conditions occurring in the drill site, such as carbon dioxide. 

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