Simultaneous transport processes

These dimensionless groups of fluid properties play important roles in dimensionless modeling equations of transport processes, and for systems where simultaneous transport processes occur. 

I have included the description of heat transfer because I want to discuss simultaneous heat and mass transfer in the next chapter. This simultaneous transport process is important practically and is interesting intellectually, with implications ranging beyond the particular problems presented. However, to discuss this simultaneous process, we need to assure a background in heat transfer. I expect that many who read this book will not have such a background, for this topic is usually buried well inside the engineering curriculum. Accordingly, this chapter is a synopsis to provide this background. 

In gridpoint models, transport processes such as speed and direction of wind and ocean currents, and turbulent diffusivities (see Section 4.8.1) normally have to be prescribed. Information on these physical quantities may come from observations or from other (dynamic) models, which calculate the flow patterns from basic hydrodynamic equations. Tracer transport models, in which the transport processes are prescribed in this way, are often referred to as off-line models. An on-line model, on the other hand, is one where the tracers have been incorporated directly into a d3mamic model such that the tracer concentrations and the motions are calculated simultaneously. A major advantage of an on-line model is that feedbacks of the tracer on the energy balance can be described... 

This approach is possible only if density gradients are not formed in the solution as a result of transport processes (e.g. in dilute solutions). Otherwise, both differential equations must be solved simultaneously—a very difficult task. 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

In heterogeneous reactions, phase boundaries exist between phases and transport processes the intrinsic rate of reaction should be taken into account simultaneously in reactor design. The combination of mass transfer rates and reaction rates leads to the so-called overall rate. The goal is to express the global rate in terms of the bulk properties of the phases, eliminating the interphase properties. 

One of the most important processes involved in the scale-up of liquid parenteral preparations is mixing [1]. For liquids, mixing can be defined as a transport process that occurs simultaneously in three different scales, during which one substance (solute) achieves a uniform concentration in another substance (solvent). On a large, visible scale, mixing occurs by bulk diffusion, in which the elements are blended by the pumping action of the mixer s impeller. On the microscopic scale, elements that are in proximity are blended by eddy currents, and they 43... 

Mitchell used chemiosmotic to describe enzymatic reactions that involve, simultaneously a chemical reaction and a transport process. The operational definition of coupling is shown in Figure 19-18. When isolated mitochondria are suspended in a buffer containing ADP, Pi, and an oxidizable substrate such as succinate, three easily measured processes occur (1) the substrate is oxidized (succinate yields fumarate), (2) 02 is consumed, and (3) ATP is synthesized. Oxygen consumption and ATP synthesis depend on the presence of an oxidizable substrate (succinate in this case) as well as ADP and Pi. 

However, even if such measurements were possible, would the uncertainty of the result be small enough to establish that production does indeed balance observed loss of ozone The calculation of ozone loss in the Antarctic ozone hole was shown to have an uncertainty of 35 to 50%. The uncertainty for analyzing whether production balances loss in the midlatitude stratosphere is similarly 35 to 50%. About half of the uncertainty is in the measurements of stratospheric abundances, which are typically 5 to 35%, and half is in the kinetic rate constants, which are typically 10 to 20% for the rate constants near room temperature but are even larger for rate constants with temperature dependencies that must be extrapolated for stratospheric conditions below the range of laboratory measurements. In addition to uncertainties in the photochemical rate constants, there are those associated with possible missing chemistry, such as excited-state chemistry, and the effects of transport processes that operate on the same time scales as the photochemistry. Thus, simultaneous measurements, even with relatively large uncertainties, can be useful tests of our basic understanding but perhaps not of the details of photochemical processes. 

Most nonequilibrium systems are characterized by variation of velocity, temperature, composition, or electrical potential with position and the consequent transport of momentum, energy, mass, or electric charge. Naturally, transport of two or more of these may occur simultaneously. Attention is focused here, however, on situations where only one transport process occurs and a transport coefficient can be calculated from its measured rate. For example, thermal conductivity can be calculated if the rate of energy transport and the temperature variation in the system are measured. 

We have already seen in Section 2.2 how the transport of both anions and cations is a vital part of biochemistry. We will examine supramolecular models of biological ion channels in detail in Chapter 12 but here we focus on some simple ion transport systems (ionophores) relevant to simultaneous anion and cation binding. Because of the need to maintain overall and local charge neutrality during any transport process the transport of individual ions across a biological or artificial membrane never occurs in isolation. There are two kinds of primary transport processes. Ion exchange or antiport, occurs when chemically different ions of like charge such as Na+ and K+ are simultaneously transported in... 

Ion pair receptors are useful in mimicking biological symporl processes — the simultaneous transport of oppositely charged ions in the same direction. 

Here one makes an effort to describe simultaneously transport-controlled and chemical kinetics processes (Skopp, 1986). Thus, an attempt is made to describe both the chemistry and physics accurately. For example, outflow curves from miscible displacement experiments on soil columns are matched to solutions of the conservation of mass equation. The matching process introduces a potential ambiquity such that experimental uncertainties are translated into model uncertainties. Often, an error in the description of the physical process is compensated for by an error in the chemical process and vice-versa (i.e., Nkedi-Kizza etal, 1984). 

Figure 7. Outline of the current concepts in cation transport by the Na+-K+-pump. Three Na+ and two K+ are thought to be transported consecutively by protein sites containing two negative carboxylate groups. The ions become occluded during the transport process by the simultaneous closure of cytoplasmic and extracytoplasmic gates. A low conductance access channel connects the sites to the extracytoplasmic surface, resulting in an ion well with voltage sensitivity of extracytoplasmic ion binding and dissociation. Reproduced with permission from Glynn, 1993.

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