Mass transport system

Darling, E. M., Jr. Computer Modeling of Transportation-Generated Air Pollution. A State-of-the-Art Survey. Final Report. DOT-TSC-OST-72-20. Cambridge, Mass. Transportation Systems Center, 1972. 131 pp. 

The packaging of triacylglycerol into chylomicrons or VLDL provides an effective mass-transport system for fat. On a normal Western diet, approximately 400 g of triacylglycerol is transported through the blood each day. Since these two particles cannot cross the capillaries, their triacylglycerol is hydrolysed by lipoprotein lipase on the luminal surface of the capillaries (see above). Most of the fatty acids released by the lipase are taken up by the cells in which the lipase is catalytically active. Thus the fate of the fatty acid in the triacylglycerol in the blood depends upon which tissue possesses a catalytically active lipoprotein lipase. Three conditions are described (Figure 7.23) ... 

Triacylglycerol in the forms of chylomicrons or very low density lipoproteins constitutes the mass transport system of fat in the blood. Excessive levels, particularly of VLDL, can give rise to various pathological problems which are grouped together under the title lipoproteinaemias and are discussed in Chapter 11 (Appendix 11.9). 

These two conditions (Eqs. (4.97) and (4.98)) are usually sufficient for assuming the medium as quiescent in dilute systems in which both cua.s and cda,oo are small. However, in nondilute or concentrated systems the mass transfer process can give rise to a convection normal to the surface, which is known as the Stefan flow [Taylor and Krishna, 1993]. Consider a chemical species A which is transferred from the solid surface to the bulk with a mass concentration cua.oo- When the surface concentration coa,s is high, and the carrier gas B does not penetrate the surface, then there must be a diffusion-induced Stefan convective outflux, which counterbalances the Fickian influx of species B. In such situations the additional condition for neglecting convection in mass transport systems is [Rosner, 1986]... 

ETFE is used in electrical applications for heat-resistant insulations and jackets of low voltage power wiring for mass transport systems, for wiring in chemical plants,... 

In an internal mass-transport system there is a transport of mass, but no material is added to or removed from the sample during or after the exposure. Such systems can thus be utilized in closed volumes, but there is usually some maximum distance over which material can be transported, so that there will generally be a low-frequency cutoff in the spatial response of such systems. 

In an external mass-transport system, material is added to and/or removed from the sample either during or after the exposure. In such systems the mass transport typically occurs in a separate development step subsequent to the optical exposure. Clearly, external mass-transport systems cannot be used in closed volumes, but it is possible to have a spatial frequency response that extends to zero. 

V. INTERNAL MASS-TRANSPORT SYSTEMS A. General Characteristics... 

One of the most general characteristics of internal mass-transport systems is that the detailed mechanisms responsible for the refractive-index changes are at best incompletely understood. In most cases this is not from lack of effort on the part of researchers, but it is a consequence of the nature of these systems. The mass transport takes place within the volume of the material, over distances typically in the range from a few pm to about 0.1 pm (depending on the particular application). The characteristics of the transport processes appear to depend critically on the transport distance, and thus, in general, it is very difficult to establish the mechanisms for changes in microscopic regions from measurements on macroscopic areas. 

Basic Mechanisms. The mechanisms that have been proposed for the various internal mass-transport systems fall into three categories photopolymerization of a major constituent, photopolymerization of a minor constituent, and photocrosslinking. There is a scarcity of definitive information, and these theories are not firmly established. 

External mass-transport systems require either the addition or removal of material to create the desired pattern of refractive-index variation. The use of the latter method is by far the more common and we discuss two techniques for accomplishing this the use of solvents to dissolve portions of a film, and selective evaporation of a fraction of the material in the film. 

As with internal mass-transport systems, there are limits on the distances over which mass can be transported, but there are some important differences. If the transport takes place by diffusion through a matrix, it is frequently difficult to have significant transport over distances of more than a few micrometers, although in special cases this can be extended to a reasonable fraction of a millimeter. However, for samples in the form of a thin sheet or film, the transport can take place in the direction normal to the film. In the plane of the film the spatial frequency response can then extend to dc (zero spatial frequency). In systems, such as photoresists, involving the complete removal of portions of the sample, it is in principle possible to remove material to substantial depths in the sample, but in most practical situations the requirements for mechanical stability of the remaining portions of the sample limit the depths to which one can remove material when making very fine patterns. Nonetheless, in the directions transverse to the direction of mass transport it is possible for the spatial frequency response to extend to dc. 

Mass transportation is any kind of transportation system in which large numbers of people are carried within a single vehicle or combination of vehicles. Airplanes, railways, buses, trolleys, light rail systems, and subways are examples of mass transportation systems. The term mass transit is commonly used as a synonym for mass transportation. 

The desirable features of mass transit systems are balanced by a number of serious drawbacks. In the first place, such systems are economically feasible only in areas that have relatively large populations. As the number of inhabitants per square mile decreases, the efficiency of a mass transportation system also decreases. 

Finally, mass transportation systems are simply not as convenient as the automobile. A person can step into her or his car and drive virtually anywhere with a minimum of inconvenience. No mass transportation system can approach this level of ease. 

The popularity of mass transportation systems varies inversely with the availability of the private automobile. Over the past century, as cars have become less expensive, consumers have opted for private transportation over subways, buses, trolleys, light rail systems, and other forms of mass transit. Between 1915 and 1980, au-... 

The high permeability of fractures causes them to preferentially focus fluid flow. The effectiveness of fractures as mass transport systems for fluids is evident from a casual examination of mineralisation in fractured rocks and leakage of groundwater at fracture outcrops. Similarly, these fractures act as preferential hydrocarbon pathways, focusing their flow from source beds to surface. 

Alcan Mass Transportation System-P.O. Box 1250 8048 Zurich Switzerland... 

Applications of Flexible Adhesives in Mass Transportation Systems 529... 

Composite plastics and rubbers are being used preferably in other transportation applications (wagons of metros, trains, airplane interiors), where flammability as well as VOC are important. In general, PF or epoxy composites are used, and certainly it is compulsory to use carefully selected flame retarders in such mass-transport systems. In such applications, it should always be remembered that, not only flames are important in these applications, hut, the smoke production is very important and critical too. 

New injection technology, particularly for diesel engines and those for mass transportation systems, is creating an even bigger need for cleaner fuel, and this, in turn, is demanding more of nonwoven filter media. 

Transportation engineers are concerned with the planning and layout of highways, airports, harbors and ports, and mass transportation systems. They plan and design transportation terminals and devise and operate systems for the control of vehicular traffic. 

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