Transportation engineers defined

Transportation systems can be categorized into modes defined by the type of inliastructure needed to run them. Buses, for example, need a fleet of vehicles and a network of roads and streets. Systems that need rails include trains, subways, light rail and monorail (considered separate categories from standard train systems such as commuter rail), and streetcars and trams. Ferries are used in many cities with extensive waterfront areas. Airports also fall within the domain of transportation engineering. While most transportation systems are designed with the needs of a geographically focused population in mind, airports are unique in that they serve passengers from all over the country—or the world. 

These operations are characterized by different reaction engineering properties. The transport of momentum, heat, and mass take place by different rates in the different operations, and the yield and selectivity obtained for a given chemical reaction will depend upon the type of operation employed. The operations also differ with respect to more loosely defined characteristics, such as ease of operation, and it can be noted in particular that some operations have been studied with considerably more thoroughness than others, and may consequently be designed with greater accuracy and reliability. 

It should be mentioned here that a different definition of the diffusion coefficient is often used in chemical engineering problems, which is more appropriate for the description of reactant or tracer transport. It takes into account the fact that the total fluid contained in a porous substance of porosity e is reduced by this factor relative to the bulk, so that an effective diffusion coefficient D of the reactants is defined such that... 

The work of Crank [38] provides a review of the mathematical analysis of well defined component transport in homogeneous systems. These mathematical models and measured concentration profile data may be used to estimate diffu-sivities in homogenized samples. The use of MRI measurements in this way will generate diffusivities applicable to models of large-scale transport processes and will thereby be of value in engineering analysis of these processes and equipment. 

Altogether, the identification of the coordinating residues in the endogenous hDAT Zn2+ binding site followed by the engineering artificial sites have defined an important series of structural constraints in this transporter. This includes not only a series of proximity relationships in the tertiary structure, but also secondary structure relationships. The data also provided information about the orientation of TM7 relative to TM8. A model of the TM7/8 microdomain that incorporates all these structural constraints is shown in Fig. 4 (36). The model is an important example of how structural inferences derived from a series of Zn2+ binding sites can provide sufficient information for at least an initial structural mapping of a selected protein domain. 

From an engineering perspective, deep-fat frying can be defined as a unit operation where heat and mass transport phenomena occur simultaneously. Convective heat is transferred from the frying media to the surface of the product, which is thereafter conducted within the food. Mass transfer is characterized by the loss of water from the food as water vapor and the movement of oil into the food (Singh, 1995). 

The discussion above provides a brief qualitative introduction to the transport and fate of chemicals in the environment. The goal of most fate chemists and engineers is to translate this qualitative picture into a conceptual model and ultimately into a quantitative description that can be used to predict or reconstruct the fate of a chemical in the environment (Figure 27.1). This quantitative description usually takes the form of a mass balance model. The idea is to compartmentalize the environment into defined units (control volumes) and to write a mathematical expression for the mass balance within the compartment. As with pharmacokinetic models, transfer between compartments can be included as the complexity of the model increases. There is a great deal of subjectivity to assembling a mass balance model. However, each decision to include or exclude a process or compartment is based on one or more assumptions—most of which can be tested at some level. Over time the applicability of various assumptions for particular chemicals and environmental conditions become known and model standardization becomes possible. 

In engineering applications it is often convenient to obtain integral representations which directly involve the field and its fluxes, rather than equations for single- or double-layer densities. This methodology is commonly called the direct method. For Poisson s equation this can be done using the Green s identities for scalar fields. As we already know, Poisson s equation is widely used in transport phenomena and polymer processing, and it is defined as,... 

As is evident from the examples we have cited, most environmental problems are highly complex and often ill-defined. At a minimum, they usually require a synthesis of virtually all elements of the chemical engineer s arsenal—thermodynamics, chemical kinetics, and transport phenomena at the most, an interdisciplinary team is required. An insufficient understanding of molecular-scale processes is frequently the key obstacle in developing innovative approaches to environmental protection. 

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