Evaporation, liquid pools, model

Unfortunately, the available data were not sufficiently accurate for the application of mathematical models governing liquid pool evaporation and spreading. An evaporating liquid pool of ammonia does not produce a heavier-than-air gas cloud, as ammonia vapor at its boiling point is lighter than air at commonly occurring ambient temperatures (0 to 20°C). Therefore, a heavy gas cloud could only be formed if there was significant aerosol formation, which is unlikely in the reported conditions. 

CASRAM predicts discharge fractions, flash-entrainment quantities, and liquid pool evaporation rates used as input to the model s dispersion algorithm to estimate chemical hazard population exposure zones. The output of CASRAM is a deterministic estimate of the hazard zone (to estimate an associated population health risk value) or the probability distributions of hazard-zones (which is used to estimate an associated distribution population health risk). 

This is a subroutine that calculates an evaporation rate from a pool of spilled liquid in presence of wind (ORG-40), or in still air (TP-10). It was developed by the U.S. Array for downwind hazard prediction following release from smoke munitions and chemical agents. The code calculates the evaporation rate of a liquid pool, given the physical stale variables, wind speed, and diameter of pool. ORG-40 and TP-10 models are coded as a Fortran 77 subroutine, EVAP4.FOR, in D2PC. The user s manual is Whiiacre (1987). 

SO3 or oleums are usually stored and transported in their liquid form. Therefore, almost all of the accidents that have occurred involved the generation of a liquid pool (with the exception of the Richmond accident [Basket et al., 1994]). Although there are numerous pool evolution models in the literature, most of them deal with nonreactive liquids, with boiling points either much lower or much higher than typical ambient temperatures. The regime of behavior is then clearly either that of a boiling pool or the evaporation of a liquid of low volatility. 

Source models are used to quantitatively define the release scenario by estimating discharge rates (Section 2.1), total quantity released (or total release duration), extent of flash and evaporation from a liquid pool (Section 2.2), and aerosol formation (Section 2.2). Dispersion models convert the source term outputs to concentration fields downwind from the source (Section 2.3). The relationship between source and dispersion models, and the various model types, is shown schematically in Figure 2.1. As shown in Figure 2.1, source and dispersion models are highly coupled, with the results of the source model being used to select the appropriate dispersion model. 

The released ammonia forms a pool of refrigerated liquid which evaporates by heat transfer from the soil. A constant mass value was assumed for the evaporation rate and a heavier-than-air gas dispersion model was used. 

The devolatilization of a component in an internal mixer can be described by a model based on the penetration theory [27,28]. The main characteristic of this model is the separation of the bulk of material into two parts A layer periodically wiped onto the wall of the mixing chamber, and a pool of material rotating in front of the rotor flights, as shown in Figure 29.15. This flow pattern results in a constant exposure time of the interface between the material and the vapor phase in the void space of the internal mixer. Devolatilization occurs according to two different mechanisms Molecular diffusion between the fluid elements in the surface layer of the wall film and the pool, and mass transport between the rubber phase and the vapor phase due to evaporation of the volatile component. As the diffusion rate of a liquid or a gas in a polymeric matrix is rather low, the main contribution to devolatilization is based on the mass transport between the surface layer of the polymeric material and the vapor phase. 

The decrease of the liquid level above the tube by a quarter of diameter promotes the increase of the average heat transfer coefficient at low and moderate heat fluxes. The measuring of the temperature heads showed that it goes on due to surface superheat decrease in the unflooded part of a tube. It ean be explained with help of following point model. There is not boiling but evaporative meehanism of heat transfer in a sintered powder porous media with the capillary transfer of the liquid from the pool to the zone of heat release. 

A case study is performed assuming an instantaneous release of the toxic liquid acrylonitrile from a rail tankwagon. After the release of the toxic liquid a pool of 600m is formed from which evaporation occurs, leading to a vapour cloud. This vapour cloud travels with the wind and disperses. The degree of dispersion is determined by the wind speed, the stability of the atmosphere and the surface roughness. The stability of the atmosphere is indicated by the pasquill-stabUity class. By day, the most common atmospheric stability class is class D and a wind speed of 5 m/s is assumed, this weather condition is abbreviated with D5. At night class F is the most common atmospheric stability, associated with a wind speed of 1.5 m/s this weather condition is abbreviated as FI.5. The evaporation and dispersion calculations are performed with EFFECTS 7.6. A neutral gas model is used for the dispersion calculations. 

The method described above accounts for the fact that the temperature drop due to evaporation of spray droplets may reduce the saturation vapour pressure sufficiently to avoid the production of flammable vapour. This means that in some cases a substance that is flammable at room temperature, such as toluene, may not produce flammable vapour in the cascade from a tank overfilling release. In reality, in such cases, the liquid from the tank overfill will accumulate within the bund and may eventually rise to ambient temperatures and start to produce flammable vapour. This hazard could be modelled using standard pool-evaporation models. 

If the liquid released is not superheated, but relatively volatile, then the vapor loading is due to evaporation. The evaporation rate is proportional to the surface area of the pool and the vapor pressure of the liquid, and can be significant for large pools. These models are primarily dominated by mass transfer effects. Wind and solar radiation can also affect the evaporation rate. 

Kawamura andMacKay (1987) developed two models to estimate evaporation rates from ground pools of volatile and nonvolatile liquids—the direa evaporation and surface temperature models. Both models are based on steady-state heat balances around the pool and include solar radiation, evaporative cooling, and heat transfer from the ground. Both models agree well with experimental data, typically within 20%, with some differences being as high a 40%. The direct evaporation model is the simpler model, whereas the surface temperature model requires an iterative solution to determine the surface temperature of the evaporating pool. 

An important parameter in all of the evaporation models is the area of the pool. If the liquid is contained within a diked or other physically bounded area, then the area of the pool is determined from these physical bounds if the spill has a large enough volume to fill the area. If the pool is imbounded, then the pool can be expected to spread out and grow in area as a fimction of time. The size of the pool and its spread is highly dependent on the level and roughness of the terrain surface—most models assume a level and smooth surface. 

A more complex model for pool spread has been developed by Webber (1991). This model is presented as a set of two coupled diiSerential equations which models liquid spread on a flat horizontal and solid surface. The model includes gravity spread terms and flow resistance terms for both laminar and turbulent flow. Solution of this model shows that the pool diameter radius is proportional to t in the limit where gravity balances inertia, and as in the limit where gravity and laminar resistance balance. This model assumes isothermal behavior and docs not include evaporation or boiling effects. 

Equilibritun flash models for superheated liquids are based on thermodynamic theory. However, estimates of the aerosol fraction entrained in the resultant cloud are mostly empirical or semiempirical. Most evaporation models are based on the solution of time dependent heat and mass balances. Momentum transfer is typically ignored. Pool spreading models are based primarily on the opposing forces of gravity and flow resistance and typically assume a smooth, horizontal surface. 

Evaporation models for nonboiling liquids require the leak rate and pool area (for spills onto land), wind velocity, ambient temperature, pool temperature, saturation vapor pressure of the evaporating material, and a mass transfer coefficient. 

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