Transport different definitions

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... 

Active transport. The definition of active transport has been a subject of discussion for a number of years. Here, active transport is defined as a membrane transport process with a source of energy other than the electrochemical potential gradient of the transported substance. This source of energy can be either a metabolic reaction (primary active transport) or an electrochemical potential gradient of a substance different from that which is actively transported (secondary active transport). 

Undeniably, the speed vector, by its size and directional character, masks the effect of small displacements of the particle. Another difference comes from the different definition of the diffusion coefficient, which, in the case of the property transport, is attached to a concentration gradient of the property it means that there is a difference in speed between the mobile species of the medium. A second difference comes from the dimensional point of view because the property concentration is dimensional. When both equations are used in the investigation of a process, it is absolutely necessary to transform them into dimensionless forms [4.6, 4.7, 4.37, 4.44]. 

Here, r = ln(JB/ thermai) has a somewhat different definition from the usual one it is zero for neutrons, the energy E of which is thermal, (t t) is the transport cross section which may depend on the energy but does not depend on the position thermal neutrons the average logarithmic energy loss (independent of position) <7, the absorption cross section for thermal neutrons which depends on the position. q(r) is the density of fast neutrons per unit r (it is not Fermi s slowing down density Q), multiplied with the velocity, n the density of thermal neutrons times their velocity, /(r) dr is the number of fission neutrons per slow neutron captured in U, for which r is between r and r + dr. Finally pi is the chance of escaping resonance absorption and p2 the thermal utilization. The multiplication constant here... 

A slightly modified form of expression was obtained by Ash and Barrer who used a somewhat different definition of the transport diffusivity. If the cross coefficient can be neglected (I. wO), Eq. (5.9) reduces to Eq. (5.6) with Dq = S), which is the familiar Darken equation/ originally derived for the interdiffusion of two alloys. While Eq. (5.6), being essentially a definition of Dq, is always valid it is evident that the assumption that Dq- is only true in the limiting case where In general both Z)q and are... 

As pointed out in the introduction to this chapter, there are many different definitions of dense phase transport and of the transition point between dilute phase and dense phase transport. For the purpose of this section dense phase transport is described as the condition in which solids are conveyed such that they are not entirely suspended in the gas. Thus, the transition point between dilute and dense phase transport is saltation for horizontal transport and choking for vertical transport. 

Compared to the lUPAC definition, the literature on transport modeling in porous electrodes employs a different definition of the terms micro- and macropores (cf. Sections 15.2 through 15.6) Here, micropores are any pores inside the porous particles that constitute the electrode, and macropores are the (interparticle) void space between these porous particles. This distinction reflects the large difference in size between the large interparticle and small intraparticles pores. In film electrodes composed of porous carbon powders, the size, number, and magnitude (i.e., pore volume) of such macropores depend on the film preparation and are governed by parameters such as particle size, use of polymeric binder, and film compaction. 

One obvious reason for the different values of Vfmin b the different definitions used by researchers to quantify and model minimum transport ctmditions (e.g. pressure minimum curve, saltation, sliding beds, unstable plugging boundary). However, this alone does not explain the wide range of scatter obtained witii respect to experimental data and other models. 

In many cases, it is necessary to estimate the rate at which a heterogeneous catalytic reaction wfll proceed, if it is controlled by external mass transfer. Alternatively, it may be necessary to estimate the concentration difference (Ca,b — Ca ) and the temperature difference (7b — T ) that are required to sustain a known or measured rate of reaction. Calculations of Ca3 — Ca,s and Tb — Tg are the only way to evaluate the influence of external transport when definitive diagnostic experiments are not feasible. Calculations such as these can be performed using Eqns. (9-38) and (9-40), provided that the transport coefficients kc and h are known, or can be obtained from correlations. 

A fundamental difference exists between the assumptions of the homogeneous and porous membrane models. For the homogeneous models, it is assumed that the membrane is nonporous, that is, transport takes place between the interstitial spaces of the polymer chains or polymer nodules, usually by diffusion. For the porous models, it is assumed that transport takes place through pores that mn the length of the membrane barrier layer. As a result, transport can occur by both diffusion and convection through the pores. Whereas both conceptual models have had some success in predicting RO separations, the question of whether an RO membrane is truly homogeneous, ie, has no pores, or is porous, is still a point of debate. No available technique can definitively answer this question. Two models, one nonporous and diffusion-based, the other pore-based, are discussed herein. 

The lower volatihty of JP-8 is a significant factor in the U.S. Air Force conversion from JP-4, since fires and explosions under both combat and ordinary handling conditions have been attributed to the use of JP-4. In examining the safety aspects of fuel usage in aircraft, a definitive study (15) of the accident record of commercial and military jet transports concluded that kerosene-type fuel is safer than wide-cut fuel with respect to survival in crashes, in-flight fires, and ground fueling accidents. However, the difference in the overall accident record is small because most accidents are not fuel-related. 

This definition of the process shows that it differs essentially from sublimation and distillation. A chemical-transport reaction is necessarily reversible a concentration gradient is induced, e.g., by means of a tem-... 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

MTs serve multiple roles in neurons. Besides acting as the substrate for the transport of membrane-bounded organelles, MTs are necessary for the extension of neurites during development they provide the structural basis for maintaining neurites after extension and they also help maintain the definition and integrity of intracellular compartments. The diversity of these functions is reflected in differences in the biochemistry and metabolic stability of different MTs. 

Integer variables are included in the model to ensure that new capacities can only be installed as integer multiples of a given plant capacity (if, e.g., hydrogen liquefaction plants in the model are only defined for a capacity of 100 MWh2, only liquefaction capacities of 100 MWh2, 200 MWh2, etc., can be installed). This approach allows the definition of different capacity classes of, e.g., production plants or transport... 

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