Elements of Physical Transfer Processes

The driving forces, or driving potentials, for transport phenomena are (i) the temperature difference for heat transfer (ii) the concentration or partial pressure difference for mass transfer and (iii) the difference in momentum for momentum transfer. When the driving force becomes negligible, then the transport phenomenon will ceases to occur, and the system will reach equilibrium. 

It should be mentioned here that, in living systems the transport of mass sometimes takes place apparently against the concentration gradient. This uphill mass transport, which usually occurs in biological membranes with the consumption of biochemical energy, is called active transport, and should be distinguished from passive transport, which is the ordinary downhill mass transport as discussed in this chapter. Active transport in biological systems is beyond the scope of this book. 

Transport phenomena can take place between phases, as well as within one phase. Let us begin with the simpler case of transport phenomena within one phase, in connection with the definitions of transport properties. 

Biochemical Engineering A Textbook for Engineers, Chemists and Biologists Shigeo Katoh and Fumitake Yoshida 

Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-32536-8 

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Except for elements with one or more radiogenic isotopes, the isotopic composition of an element in Nature can be considered constant for most applications of stable isotope techniques. However, they are not invariable (see also Chapter 1). Stable isotopes of an element differ slightly in mass, which may cause changes in the element s isotopic composition if a chemical or physical transfer process is sensitive to the isotopes masses and if the transfer is incomplete. For reasons of mass balance, the isotopic abundances cannot be changed if all matter is transferred from a source to a target compartment. 

Physical transfer processes include evaporation and precipitation, and also membrane diffusion as a basic mechanism of element transfer in the human body. A difference in diffusion rates between isotopes inevitably results in their fractionation over the course of element transport (kinetic isotope fractionation effect). When an element undergoes a chemical reaction, this can also be described as an element transfer process from one element species into another. Again, if reaction rates between isotopologs differ, this gives rise to a difference in isotopic... 

Minimization of construction cost has therefore been a prime objective in the development of solar distillation. Probably the most promising method for its accomplishment is the combining of all three primary elements in a distillation process—i.e., heat supply facility, evaporator, and condenser—into a single piece of very simple equipment. Such a unit is the basin-type solar distillation plant (4). But the simplicity of this equipment ceases with its general form, and over-all operation of so many functions makes the physical processes of energy and mass transfer highly complex. 

The parts or elements of a system can be parts in the physical sense of the word or they can be processes. In the strictly physical sense, the parts of a human body or those of a chair constitute a physical system. In our studies of chemical/biological equipment that performs certain chemico-physical functions, we must also consider the various chemico-physical processes that take place inside the system as elements thereof. These processes interact very often with each other to perform the task of the particular chemical plant, called the system. A simple example of this is a chemical reactor in which processes such as mixing, chemical reactions, heat evolution, heat transfer, etc. take place in a controlled way to achieve the task of the reactor, i.e., the change of the input reactants to the output products. 

As seen from (3.29), zooplankton are considered passive elements of the ecosystem subject to physical processes of transference in space as a result of water movement. However, zooplankton are known to migrate mainly in the vertical direction. In the given model we can use a simple mechanism to simulate the process of vertical migration of zooplankton. For this purpose, we divide the whole water thickness into two layers 0 < z < z0 and z0 < z < H. Let us suppose that zooplank-... 

Firstly, we have to recall the physical situation and, in order to facilitate understanding, we should draw a sketch. At high oven temperatures heat is transferred from the heating elements to the meat surface by both radiation and heat convection. From there it is transferred solely by unsteady-state heat conduction which surely represents the rate limiting step of the heating process. 

A vessel in which the properties are uniform is called a stirred-tank (or well-mixed) reactor. If there is no mixing in the direction of flow in the cylindrical vessel for hydrocarbon processing, another ideal type is realized the ideal tubular-flow, orpi -flow, reactor. Here the reaction mass consists of elements of fluid that are independent of each oth gpach one having a different composition, temperature, etc. This classification is of basic significance in design, because simplified treatments of the physical processes of mass and energy transfer are applicable for each ideal reactor. We noted in Sec. 

Electron-transfer (ET) reactions play a central role in all biological systems ranging from energy conversion processes (e.g., photosynthesis and respiration) to the wide diversity of chemical transformations catalyzed by different enzymes (1). In the former, cascades of electron transport take place in the cells where multicentered macromolecules are found, often residing in membranes. The active centers of these proteins often contain transition metal ions [e.g., iron, molybdenum, manganese, and copper ions] or cofactors as nicotinamide adenine dinucleotide (NAD) and flavins. The question of evolutionary selection of specific structural elements in proteins performing ET processes is still a topic of considerable interest and discussion. Moreover, one key question is whether such stmctural elements are simply of physical nature (e.g., separation distance between redox partners) or of chemical nature (i.e., providing ET pathways that may enhance or reduce reaction rates). 

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