Concentration patterns, dispersion modeling

The exploratory study Is not designed to obtain Information on temporal patterns In the chemical concentrations since these studies are expected to be completed In a short period of time. If It Is possible to select the time for the exploratory study. It should be conducted at a time when the concentrations would be expected to be at a maximum. It may be necessary to use the hypothesized dispersion model In order to make this decision. For example, the sampling normally should not be done Immediately following a heavy rain, when the ground Is frozen solid, or when a wind Is blowing at 20 to 30 knots. Temporal trends will have to be addressed In the final study. 

In many risk analyses standard dispersion models, available from the EPA for regulatory compliance purposes, are used to compute concentration patterns for prototypes of a class of sources, and the patterns are convolved with population patterns that are characteristic of the source sites (5, 6). A similar level of analysis detail that relies on measured pollutant (ozone) concentration in each county of the Northeast Corridor rather than on modeled concentrations was used by Johnson and Capel ( 7). 

The mechanism by which solids are distributed throughout a vessel once they are suspended is different from that leading to suspension. It might be expected that the solids distribution would again be affected by the bulk flow pattern, i.e. the mean velocities throughout the vessel as well as the turbulence structure. These flows oppose the gravitational downwards force. As will be shown later, the measured vertical concentration profiles are very complicated, much more so than in solids transport in pipe flow, for example, where a steady decrease in concentration occurs from top to bottom. This concentration decay in pipes can be modelled rather easily by a one dimensional sedimentation-dispersion model. A similar model has been proposed for stirred vessels for the region above the impeller. 

The axial dispersion model can further be compared with the tanks-in-series model using the concept of equal variances. A typical concentration pattern obtained for a complex... 

In most adsorption processes the adsorbent is contacted with fluid in a packed bed. An understanding of the dynamic behavior of such systems is therefore needed for rational process design and optimization. What is required is a mathematical model which allows the effluent concentration to be predicted for any defined change in the feed concentration or flow rate to the bed. The flow pattern can generally be represented adequately by the axial dispersed plug-flow model, according to which a mass balance for an element of the column yields, for the basic differential equation governing llie dynamic behavior,... 

In this study we have employed the simultaneous collection of atmospheric particles and gases followed by multielement analysis as an approach for the determination of source-receptor relationships. A number of particulate tracer elements have previously been linked to sources (e.g., V to identify oil-fired power plant emissions, Na for marine aerosols, and Pb for motor vehicle contribution). Receptor methods commonly used to assess the interregional impact of such emissions include chemical mass balances (CMBs) and factor analysis (FA), the latter often including wind trajectories. With CMBs, source-strengths are determined (1) from the relative concentrations of marker elements measured at emission sources. When enough sample analyses are available, correlation calculations from FA and knowledge of source-emission compositions may identify groups of species from a common source type and identify potential marker elements. The source composition patterns are not necessary as the elemental concentrations in each sample are normalized to the mean value of the element. Recently a hybrid receptor model was proposed by Lewis and Stevens (2) in which the dispersion, deposition, and conversion characteristics of sulfur species in power-plant emissions... 

In solid-liquid mixing design problems, the main features to be determined are the flow patterns in the vessel, the impeller power draw, and the solid concentration profile versus the solid concentration. In principle, they could be readily obtained by resorting to the CFD (computational fluid dynamics) resolution of the appropriate multiphase fluid mechanics equations. Historically, simplified methods have first been proposed in the literature, which do not use numerical intensive computation. The most common approach is the dispersion-sedimentation phenomenological model. It postulates equilibrium between the particle flux due to sedimentation and the particle flux resuspended by the turbulent diffusion created by the rotating impeller. 

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