Vaporization, liquid pools, model

DESIGNER also contains different hydrodynamic models (e.g., completely mixed liquid-completely mixed vapor, completely mixed liquid-vapor plug flow, mixed pool model, eddy diffusion model) and a model library of hydrodynamic correlations for the mass transfer coefficients, interfacial area, pressure drop, holdup, weeping, and entrainment that cover a number of different column internals and flow conditions. 

The horizontal liquid flow pattern is very complicated due to the mixing by vapor, dispersion, and the round cross section of the column. On single-pass trays, the latter results in the flow path, which first expands and then contracts. A rigorous modeling of this flow pattern is very difficult, and usually the situation is simplified by assuming that the liquid flow is unidirectional and the major deviation from the plug flow is the turbulent mixing or eddy diffusion. In [80], two different models, the eddy-diffusion model and the mixed pool model were developed and tested in the context of the rate-based approach for RD trays. The details of these models can be found in [81]. 

For spills of liquid SO3 or oleum, the pool model should be run first. The results of the pool model are automatically set as inputs to the dispersion model. The dispersion model is subsequently run. For releases of SO3 vapor, the dispersion model should be run directly. For varying input parameters, the distanee- or time-varying functions should be defined. 

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. 

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. 

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. 

Both empirical and pscudomechanistic models based on heat and mass transfer concepts are available and are based on the thermodynamic properties of the liquid and, for the boiling pool, on the thermal properties of the substrate (c.g., groimd). Vaporization rates may vary greatly with time. The dimensions of the vapor cloud formed over the pool are often required as input to some dense gas dispersion models (Section 2.3.2) this is empirical and is not provided by most models. 

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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