Vapor cloud stability

FIG. 26-31 Estimated maximum downwind distance to lower flammable limit L, percent by volume at ground level in centerline of vapor cloud, vs. continuous dense vapor release rate at ground level. E atmospheric stability. Level terrain. Momentary concentrations for L. Moles are gram moles u is wind speed. (From Bodmtha, 1980, p. 105, by permission.)... 

A plant handling substantial quantities of flammable toluene is located 1000 ft from a residential area. There is some concern that a sizable leak of flammable vapors will form a flammable cloud with subsequent ignition in the residential area. Determine the minimum mass flow rate of toluene leakage required to produce a vapor cloud in the residential area with a concentration equal to the LFL. Assume a 5 mph wind and D atmospheric stability. 

If the boiling point is attained during runaway, a possible secondary effect of the evaporation of a solvent is the formation of an explosive vapor cloud, which in turn can lead to a severe explosion if ignited. In some cases, there is enough solvent present in the reaction mixture to compensate the energy release, allowing the temperature to stabilize at the boiling point. This is only possible if the solvent can be safely refluxed or distilled off into a catch pot or a scrubber. Moreover, the... 

Thomerson and Billings [87] describe field tests in which chlorine was released at up to 70kg min from three 1-ton containers. Typical wind velocities were about 9 m s . Relative humidity was very low, the test site being located in the Nevada desert. It is noteworthy that the temperature of the spilled liquid stabilized at about -50 C, well below the boiling point of —34°C. The wind subcooled the liquid, which was collected in a well-insulated pan, and approximately 50% of the chlorine vaporized during a test. The use of downwind water sprays in these tests reduced the concentration of chlorine in the air by an average of 31%. This was attributed to the induction of dilution air by transfer of momentum from the spray. As noted above, the spray also forced the vapor cloud lower, so that the concentration of chlorine at 1.5 m elevation was actually higher for a distance of 230 m fi om the point of release. In these tests, portable fire water monitors performed relatively poorly. 

The U.S. EPA provides look-up tables to give distances to the toxic endpoint. To use, the facility must determine a release rate and the toxic endpoint for the chemical under question. For a flammable material, distances are computed to e lower flammable limit. Additional tables and equations are provided for calculating distances for vapor cloud explosions and BLEVEs. Separate tables are provided for dense gases and neutrally buoyant gases, urban or rural conditions, and a D stability at 3 m/s wind speed or F stability at 1.5 m/s wind speed. The U.S. EPA also provides tables listing toxic endpoints for the chemicals regulated by 40 CFR Part 68. 

A total of six tests was performed. Each involved transferring liquid chlorine from one or more 1-ton chlorine containers to the open pan. After the pool of liquid chlorine had stabilized, measurements were made of downwind concentration with no mitigation procedure applied. Then one of several different mitigation procedures was performed on the liquid pool. In the first and last tests, the only mitigation technique used was water spray nozzles to knock down the chlorine vapor cloud. In tests 2, 3, and 5, different vapor suppression foams were applied to the liquid pool surface in combination with water spray from the nozzle system and measurements were made of the downwind concentration to determine the effectiveness of the procedure. Test 4 used no foam but did use the water spray nozzles and portable water nozzles. 

The EPA ride specifies defauit values of wind speed, atmospheric stability class, and other parameters for the development of the offsite consequence analysis of worst-case scenarios. It also specifies the end point for the consequence analysis, based on the calculated concentration of toxic materials, the overpressure (1 psi) from vapor cloud explosions, and the radiant heat exposure for flammable releases (5 kW/m for 40 seconds). 

Figure 6 Cloud footprint to an atmospheric concentration of 20 ppm resulting from the rupture of a 50-mm-diameter chlorine pipe containing either chlorine liquid or chlorine vapor. Release conditions Complete rupture of pipe without shutoff, pipe elevation is 5 m above grade, wind speed is 5 m/sec, atmospheric stability class D, 20 ppm is the Emergency Response Planning Guideline-3 (ERPG-3) concentration for chlorine, the concentration at which life-threatening effects might result from exposure for 1 hour.

The chemical dynamics, reactivity, and stability of carbon-centered radicals play an important role in understanding the formation of polycyclic aromatic hydrocarbons (PAHs), their hydrogen-dehcient precursor molecules, and carbonaceous nanostructures from the bottom up in extreme environments. These range from high-temperature combustion flames (up to a few 1000 K) and chemical vapor deposition of diamonds to more exotic, extraterrestrial settings such as low-temperature (30-200 K), hydrocarbon-rich atmospheres of planets and their moons such as Jupiter, Saturn, Uranus, Neptune, Pluto, and Titan, as well as cold molecular clouds holding temperatures as low as 10... 

Key variables temperature, pressure, relative humidity, water vapor mixing ratio, wind speed, wind direction, cloud fraction, stability, PBL height, photolysis rates, and the emission rates of met-dependent primary species (e.g., dust, sea-salt, biogenic aerosol, marine phytoplankton-produced... 

A brief comment concerning your mechanism which stabilizes the dense layer in your suggestion. Your mechanism would apply only to condensed matter, to particles. However, the sulphuric acid vapor which we have measured is also concentrated in a very thin cloud, and here it should be kept in mind that the vapor is formed in situ probably from SO precursor gas and that the lifetime of the sulphuric acid molecule is much less than one day. In the undisturbed stratosphere it is about one day. Under these conditions, where there is more aerosol, the vapor has an even shorter lifetime. And this would tell us that also the gas, and that means the precursor gas, the SO is concentrated in such a thin layer. So this does not necessarily apply only to the aerosols which are formed from the condensable gas formed in turn from the precursor. 

Venus upper atmosphere is even drier than the lower atmosphere, and the average water-vapor mixing ratio above the clouds is only a few ppmv. The very low H2O mixing ratios were hard to explain until it was realized that Venus clouds are 75% sulfuric acid, which is a powerful drying agent. When dissolved in the acid, most of the water reacts with H2SO4 to form hydronium (HaO ) and bisulfate (HSO4) ions. As a result, the concentrations of free H2O in the acid solution and in the vapor over the acid are extremely low. The partial pressure of water at Venus cloud tops is lower than that over water ice at the same temperature. Thus, the clouds are responsible for the extreme dryness of Venus upper atmosphere, and play an important role in the photochemical stability of Venus atmosphere (see Section 1.19.3.3). 

One may effectively control the stability of atmospheric aerosols by spraying concentrated solutions of hygroscopic substances, such as calcium chloride, or solid substances, such as silver iodide and solid carbon dioxide. These substances cause condensation of water vapor (or the formation of small ice crystals in supercooled clouds), and result in precipitation. Analogous means can be used to dissipate fog. 

The stratosphere is very dry and generally cloudless. The long polar night produces temperatures as low as 183 K (-90°C) at heights of 15 to 20 km, cold enough to condense even the small amount of water vapor present to form polar stratospheric clouds (PSCs). The lowest temperatures are more prevalent in the Antarctic, where the polar vortex is more stable than in the Arctic. The exceptional stability of the vortex at the South Pole may be a result of the almost symmetric distribution of ocean around Antarctica. The less-stable... 

The cloud behavior is affected mostly by the wind speed, the vapor evolution rates from the pool, the atmospheric stability class, and the atmospheric relative humidity (Kapias, 1999 Kapias and Griffiths, 1999a). In the majority of the cases, the cloud will initially be denser... 

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