Physical Transport

1 Once a pollutant has been emitted into the atmosphere its 

The series of processes which it undergoes - transport and dispersion, chemical transformation and deposition etc. - define its pathway through the atmosphere. For sulphur, this pathway averages between one and five days, i.e. between a few hundred and more than a thousand kilometres. 

The following sections deal with the transport and diffusion processes of pollutants. Chemical transformation and deposition processes are discussed in Chapter 4. 

The flow field in the mixing layer (or planetary boundary layer) of the atmosphere is responsible for pollutant transport between a source and the receptor sites. 

This field is almost always in a state of change and is characterised by a spectrum of atmospheric motions ranging from micro-scale eddies, through meso-scale to synoptic-scale circulations. The elements of the field critical to pollutant transport are wind speed and wind direction. 

---

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

The Reactions and Physical Transport tlie chemical and biological transfornuition, and water movement, that result in different levels of water quality at different locations in time in an aquatic ecosystem. 

The final link in the physical transportation of natural gas IS distribution, the delivery of natural gas by local distribution customers (LDCs) to local consumers. LDCs deliver gas to consumers from storage or from wellheads accessed by pipelines. 

Mass transfer This phenomenon manifests itself as the physical transport of a metal from one portion of the system to another, and may occur when there is an alloy compositional difference or a temperature gradient between parts of the unit joined by the flowing liquid phase. An exceedingly small solubility of the metal component or corrosion product in the molten metal or salt appears sufficient to permit mass transfer to proceed at a fairly rapid pace. 

Barnes, R.S. Schell, W.R. Physical Transport of Trace Metals in the Lake Washington Watershed. In Cycling and Control of Metals, Proc. of an... 

The residence time is the time spent in a reservoir by an individual atom or molecule. It is also the age of a molecule when it leaves the reservoir. If the pathway of a tracer from the source to the sink is characterized by a physical transport, the word transit time can also be used. Even for a single chemical substance, different atoms and molecules will have different residence times in a given reservoir. Let the probability density... 

The physical transport of particles in a river occurs by two primary modes bedload and suspended load. Bedload consists of material moved along the bed of the river by the tractive force exerted by flowing water. Bedload may roll or hop along the bottom, and individual particles may remain stationary for long periods of time between episodes of movement. Suspended load consists of material suspended within the flow and that is consequently advected by flowing water. Rivers and streams are naturally turbulent, and if the upward component of turbulence is sufficient to overcome the settling velocity of a particle, then it will tend to remain in suspension because the particles become resuspended before they can settle to the bottom of the flow. Suspended load consists of the finest particles transported by a river, and in general is composed of clay- and silt-sized... 

Although the general circulation patterns are fairly well known, it is difficult to quantify the rates of the various flows. Abyssal circulation is generally quite slow and variable on short time scales. The calculation of the rate of formation of abyssal water is also fraught with uncertainty. Probably the most promising means of assigning the time dimension to oceanic processes is through the study of the distribution of radioactive chemical tracers. Difficulties associated with the interpretation of radioactive tracer distributions lie both in the models used, nonconservative interactions, and the difference between the time scale of the physical transport phenomenon and the mean life of the tracer. 

The aim of the newly introduced mesoporosity is to enhance the utilization of the microporous network by improved accessibihty of the active sites that are mostly present in the micropores. Although numerous papers have reported on the improved catalytic performance of desihcated zeolites in catalysis (details in Section 2.4.5), only few works are available that reaUy tackle the hierarchical nature of the desihcated zeohtes and demonstrate that selective sihcon removal leads to an enhanced physical transport in the zeohte crystals. 

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

The rates at which chemical transformations take place are in some circumstances strongly influenced by mass and heat transfer processes (see Sections 12.3 to 12.5). In the design of heterogeneous catalytic reactors, it is essential to utilize a rate expression that takes into account the influence of physical transport processes on the rate at which reactants are converted to products. Smith (93) has popularized the use of the term global reaction rate to characterize the overall rate of transformation of reactants to... 

In freshwater algae, movement of copper into cells occurs mainly by physical transport the plasmalemma is the initial site of copper binding. Copper on the plasmalemma increases its permeability, as shown by the leakage of potassium and other ions from copper-treated cells and entry of copper into intracellular sites (Stokes 1979). 

Smith, J.M., and E. Stammers Physical Transport Phenomenal [translation]. Delft DUM, 1986. 

The classification of wastewater in terms of size distribution is normally done from a practical point of view. Typically, a distinction is made between soluble, colloidal and suspended components (Figure 3.6). While this definition for determining what solids are is rational as far as physical transport processes in sewers are concerned, when dealing with the microbial processes for sewer conditions, an extension of the solids definition is required. Particles larger than about 10-4 pm cannot be transported through the cell wall and are, therefore, from a microbial point of view, considered particles. 

The physical transport of dissolved organic compounds through the subsurface occurs by three processes advection, hydrodynamic dispersion, and molecular diffusion. Together, these three cause the spread of dissolved chemicals into the familiar plume distribution. Advection is the most important dissolved chemical migration process active in the subsurface, and reflects the migration of dissolved chemicals... 

The physical transport of oil droplets into the water column, called dispersion, is often a result of water surface turbulence but may also result from the application of chemical agents (dispersants). These droplets may remain in the water column or coalesce with other droplets and gain enough buoyancy to resurface. Dispersed oil tends to biodegrade and dissolve more rapidly than floating slicks because of high surface area relative to volume. Most of this process occurs from about half an hour to half a day after the spill. 

Conservative Chemical behavior that is controlled by physical processes. In other words, physical transport is much faster than any chemical processes that can either supply or remove the chemical. All of the major ions exhibit conservative behavior. The concentrations of conservative species are directly proportional to each other and to salinity. 

---