Vapour dispersion

A quantitative estimate of hazard distances for a variety of events, e.g. vapour dispersion to 50% LEL from pipework releases, up to second degree burns from a fireball. 

Ruan, T.J., Hale, M. and Howarth, R.J., 1985a. Numerical modelling experiments in vapour geochemistry, II vapour dispersion patterns and exploration implications. J. Geochem. Explor., 23 265-280. 

Fig. 10.1 Metal atom reactor. A - Glass reaction vessel. B - Electron beam furnace, model EBSl, G.V. Planer Ltd. C - Vapour beam of metal atoms. D - Co-condensate of metal and substrate vapours. E - Heat shield. F - Furnace cooling water pipes. G - Electrical lead for substrate solution dispersion device. H - Furnace electrical leads. J - Substrate inlet pipe (vapour). K - Substrate inlet pipes (solution). M - Rotation of reaction vessel. N - To vacuum rotating seal, service vacuum lead troughs and pumping systems. 0-Level of coolant (usually liquid nitrogen). P-Capped joint for product extraction. 0-Substrate vapour dispersion device. R-Substrate vapour beam. (From Green, M.L.H., 1980, /. Organomet. Chem., 300, 119.)...

We discuss classical non-ideal liquids before treating solids. The strongly interacting fluid systems of interest are hard spheres characterized by their harsh repulsions, atoms and molecules with dispersion interactions responsible for the liquid-vapour transitions of the rare gases, ionic systems including strong and weak electrolytes, simple and not quite so simple polar fluids like water. The solid phase systems discussed are ferroniagnets and alloys. 

Surface waves at an interface between two innniscible fluids involve effects due to gravity (g) and surface tension (a) forces. (In this section, o denotes surface tension and a denotes the stress tensor. The two should not be coiifiised with one another.) In a hydrodynamic approach, the interface is treated as a sharp boundary and the two bulk phases as incompressible. The Navier-Stokes equations for the two bulk phases (balance of macroscopic forces is the mgredient) along with the boundary condition at the interface (surface tension o enters here) are solved for possible hamionic oscillations of the interface of the fomi, exp [-(iu + s)t + i V-.r], where m is the frequency, is the damping coefficient, s tlie 2-d wavevector of the periodic oscillation and. ra 2-d vector parallel to the surface. For a liquid-vapour interface which we consider, away from the critical point, the vapour density is negligible compared to the liquid density and one obtains the hydrodynamic dispersion relation for surface waves + s>tf. The temi gq in the dispersion relation arises from... 

We saw in Chapter 5 that there is a driving force tending to make dispersions of precipitates in alloys coarsen and we would expect a dispersion of droplets in water vapour to do the same. Water droplets in clouds, however, carry electrostatic charges and this gives a different result for the driving force. 

A similar logie is applieable to the eontrol of explosions involving gas or vapour, but other measures, e.g. dispersion by steam or eontainment by water eurtains, may be applieable to vapour elouds in the open air. Containment or diversion of a blast (e.g. by blast walls) and redueing its effeet by appropriate spaeing of equipment, buildings ete. are also applieable. 

Analyses of gases and vapours tend to utilize the teehniques deseribed on page 308. Many of these methods were traditionally limited to laboratory analyses but some portable instruments are now available for, e.g., gas ehromatography (Table 10.16) and non-dispersive infra-red speetrometry (Table 10.17). 

Drums eontaining flammable liquids are preferably stored outside, so that any flammable vapour ean readily disperse. Similar eonsiderations may apply to the dispersion of any vapour/fumes from drums of toxie liquids or solids. In some eases weather proteetion is provided by a roof. 

Hanna, S. R. and D. Strimaitis, 1989, Workbook of Test Cases for Vapour Cloud Dispersion Modes, CCPS, New York. 

The Hyperion PCNTs are typically prepared as follows [22[ a metallic catalyst [Fe(N03)3 supported on AI2O3] is dispersed in a ceramic boat which is then placed in an electric furnace. After pre-treatment (flushing with Ar at 5Q0°C) the temperature is raised to 900°C under an H2 fiow, and a vapour consisting of H2 and benzene (9 1 vol %) is introduced into the furnace for 2 h. MWCNTs colleet in the ceramie boat and, after cooling to room temperature under an Ar fiow, the product is harvested. 

Chemical methods. A known volume of the gas is passed over a suitable absorbent, the increase in mass of which is measured. The efficiency of the process can be checked by arranging a number of vessels containing absorbent in series and ascertaining that the increase in mass in the last of these is negligible. The method is very accurate but is laborious. Satisfactory absorbents for water vapour are phosphorus pentoxide dispersed in pumice, and concentrated sulphuric acid. 

This type of catalyst is not limited to nickel other examples are Raney-cobalt, Raney-copper and Raney-ruthenium. When dry, these catalysts are pyrophoric upon contact with air. Usually they are stored under water, which enables their use without risk. The pyrophoric character is due to the fact that the metal is highly dispersed, so in contact with oxygen fast oxidation takes place. Moreover, the metal contains hydrogen atoms and this adds to the pyrophoric nature. Besides the combustion of the metal also ignition of organic vapours present in the atmosphere can occur. Before start of the reaction it is a standard procedure to replace the water by organic solvents but care should be taken to exclude oxygen. Often alcohol is used. The water is decanted and the wet catalyst is washed repeatedly with alcohol. After several washes with absolute alcohol the last traces of water are removed. 

When analysing the standard deviation value, which measures the dispersion of measurements, the effect of heteroscedasticity, already discussed in connection with the measurement of vapour pressure, is noted ie the dependency between standard deviation and average (the higher the average, the greater the dispersion of measurements). One way to make this unfortunate property obvious when it comes to analysing data is to calculate the coefficient of variation for each distribution (C 0- If it is more or less constant, there is heteroscedasticity. 

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