Processes transport

A transport process, as used herein, is one that moves chemicals and other properties of the fluid through the environment. Diffusion of chemicals is one transport process, which is always present. It is a spreading process, which cannot be reversed 

Interfacial transfer is the transport of a chemical across an interface. The most studied form of interfacial transfer is absorption and volatilization, or condensation and evaporation, which is the transport of a chemical across the air-water interface. Another form of interfacial transfer would be adsorption and desorption, generally from water or air to the surface of a particle of soil, sediment, or dust. Illustration of both of these forms of interfacial transfer will be given in Section l.D. 

Mass transport problems are solved with the diffusion equation, often represented as 

Transport processes are involved when a current is passed through a galvanic cell. Ions and neutral species that participate in the electrochemical reactions at the anode or cathode have to be transported to the respective electrode surfaces. The basic concepts of transport processes are briefly outlined in this chapter. The reader is referred for a comprehensive treatment to the book on physicochemical hydrodynamics by Levich [1]. Phenomena resulting from transport processes in porous electrodes of fuel cells are covered in chapter XVI. 

So far we have not gone in-depth into the nature of the transport processes responsible for fluxes of material between and within reservoirs. This section includes a very brief discussion of some of the processes that are important in the context of global biogeochemical cycles. More comprehensive treatments can be found in textbooks on geology, oceanography and meteorology and in reviews such as Lerman (1979) and Liss and Slinn (1983). 

If variations in fluid density and diffusivity can be neglected we have 

In most situations a fluid would be turbulent implying that the velocity vector, as well as the concentration c exhibits considerable variability on time scales smaller than those of prime interest. This situation can be described by writing these quantities as the sum of an average quantity (normally a time average) and a perturbation 

From Equation (34), the transport flux F,i, then becomes 

Note that the averages of V and c are equal to zero. The continuity equation can now be written as 

The macroscopic description of nonequilibrium states of fluid systems requires independent variables to specify the extent to which the system deviates from equilibrium and dependent variables to express the rates of processes. 

The three principal transport processes are heat conduction, diffusion, and viscous flow. 

Each transport process is described macroscopically by an empirical linear law. 

Molecular theories of transport processes in dilute gases are based on gas kinetic theory. 

Transport processes in liquids are visualized as the motion of molecules from one cage to another with the cages being made up of neighboring molecules. 

The polarity of a molecule is also important. Apolar substances, such as benzene, ethanol, diethyl ether, and many narcotic agents are able to enter biological membranes easily. By contrast, membranes are impermeable to strongly polar compounds, particularly those that are electrically charged. To be able to take up or release molecules of this type, cells have specialized channels and transporters in their membranes (see below). 

Free diffusion is the simplest form of membrane transport. When it is supported by integral membrane proteins, it is known as facilitated diffusion (or facilitated transport). 

Channel proteins have a polar pore through which ions and other hydrophilic compounds can pass. For example, there are channels that allow selected ions to pass (ion channels see p. 222) and porins that allow molecules below a specific size to pass in a more or less nonspecific fashion (see p. 212). 

Transporters recognize and bind the molecules to be transported and help them to pass through the membrane as a result of a conformational change. These proteins (permeases) are thus comparable with enzymes—although with the difference that they catalyze vectorial transport rather than an enzymatic reaction. Like enzymes, they show a certain affinity for each molecule transported (expressed as the dissociation constant, in mol L ) and a maximum transport capacity (V). 

Free diffusion and transport processes facilitated by ion channels and transport proteins always follow a concentration gradient— i. e., the direction of transport is from the site of higher concentration to the site of lower concentration, in ions, the membrane 

FIGURE 1.4 Illustration of basic fuel ceU processes and their relation to the thermodynamic properties of a cell. The electrical work performed by the cell, corresponds to the reaction enthalpy, — A//, minus the reversible heat due to entropy production, —TAS, and minus the sum of irreversible heat losses at finite load, Qi. These losses are caused by kinetic processes at electrochemical interfaces as well as by transport processes in diffusion and conduction media. 

Qi or T]i are complex functions of cell design, specific properties of cell materials, cell composition, thermodynamic conditions, and current density of operation. All efforts in fundamental fuel cell science are geared toward unraveling these functional dependencies. Applied research and development focuses on finding ways to minimize contributions of Qi and whereby optimal values of Cceii and PceU could be obtained. The target values and constraints on performance, cost, durability, and lifetime of this optimization problem are set by end-user requirements. 

FIGURE 1.5 Polarization and power density curves of a typical PEFC. 

FIGURE 1.6 Schematic of a typical PEFC with the meandering flow field. Note that the figure is strongly not to scale the MEA thickness is 10 -10 times less, than the in-plane size of the current collector plate. The color indicates an elementary cell fragment, which is often considered in cell modeling. 

Generally, owing to the presence of liquid water produced in the ORR, the flow in the cathode channel is two-phased. However, at low and moderate current densities, the mass fraction of liquid water is small and does not strongly affect the flow s velocity. To a good approximation, it can be considered as a plug flow (well-mixed flow with constant velocity).  

When biochemical systems are studied in vitro, it is typically under well mixed conditions. Yet the contents of living cells are not necessarily well mixed and the biochemical workings within cells are inseparably coupled to the processes that transport material into, out of, and within cells. The three processes responsible for mass transport in living systems are advection, diffusion, and drift. Characterizing which, if any, of these processes is active in a given system is an important component of building differential equation-based models of living biochemical systems. 

In general, the equations governing various transport processes, like equations for chemical kinetics in well mixed systems, are built upon the foundation of mass conservation [23]. 

To derive equations for mass transport we introduce the quantity f, which we define as the mass flux density of species i, expressed in units of mass per unit area per unit time. Given fj, we express the rate of change of total mass inside volume V as equal to the rate of mass flux into the volume  

Since within a continuum system the volume V is arbitrary in Equation (3.44), the 

Equation (3.45) is the general continuum statement of mass conservation of species i written in terms of the mass flux density f, and the concentration field c,. 

According to the free energy change associated with the pertinent reaction, nickel will form nickel tetracarbonyl at low temperatures, and this carbonyl will become unstable and revert back to nickel and carbon monoxide at moderate temperatures. The Mond process for refining nickel is based on these features. In this process, impure nickel is exposed to carbon monoxide gas at 50 °C, whereby volatile nickel tetracarbonyl (Ni(CO)4) forms. No impurity present in the crude nickel reacts with carbon monoxide. Since formation of the 

In this process use is made of an endothermic reaction between the aluminum (in the vapor form) in the crude feed material and gaseous aluminum trichloride 

This reaction is carried out at about 1200 °C. The gaseous aluminum subchloride (A1C1) product is cooled in a separate zone to about 700 °C, to bring about decomposition of the gas into aluminum and aluminum trichloride. The aluminum is obtained in a molten, substantially pure state. The aluminum trichloride is recirculated for the production of additional amounts of the subchloride. 

This process has been tried out on a pilot plant scale mainly as a means of producing pure aluminum from an impure aluminum-iron (20-45%)-silicon (2-20%) alloy, obtained by the carbothermic reduction of bauxite in an electric furnace. 

Purification occurs in this process mainly due to three reasons (i) some of the impurities (such as oxygen, nitrogen, and carbon) present as oxide, nitride, and carbide in the feed 

In addition to thermodynamic and kinetic information, the reactor designer needs other physical property data. For example, when considering 

The topics of heat, mass and momentum transfer, known collectively as transport processes, are fully examined in the books by Welty et al. [21] and Bird et al. [22]. There is a useful introduction to fluid mechanics and heat transfer by Kay and Nedderman [23], while mass transfer is fully discussed by Treybal [24] and Sherwood et al. [25]. Coulson and Richardson [26] also give clear introductions to these subjects. 

Many of the results and correlations in heat and mass transfer are expressed in terms of dimensionless groups such as the Nusselt, Reynolds and Prandtl numbers. The definitions of those dimensionless groups referred to in this chapter are given in Appendix 2. 

The book by Reid et al. [9] is an excellent source of information on properties such as thermal conductivities, diffusion coefficients and viscosities of gases and liquids. Not only are there extensive tables of data, but many estimation methods and correlations are critically reviewed. 

1 Transfer of the solute (adsorbate) from the bulk fluid phase to the surface film (boundary layer) which surrounds the adsorbent particle. This step is controlled by convective flow and turbulent mixing. 

3 Transfer of the adsorbate from the particle surface to the interior of the adsorbent via the pore network. This step can be accomplished in two ways pore diffusion (diffusion through the fluid in the pore) and surface diffusion (the particle travels along the pore surface). 

4 Physical or chemical binding of the adsorbate to the internal surface of the adsorbent. This step is controlled by the molecular interactions described previously for adsorption. Steps 1 and 4 are usually the fastest steps, and therefore are not considered to contribute 

PDab Pi = fluid density p, = fluid viscosity Vs = fluid superficial velocity dp = particle diameter k = mass transfer coefficient Dab = diffusion coefficient of sorbate in fluid. 

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Due to the application of the described image processing steps on image sequences up to 1000 images per second, it is possible to determine and to analyse the transport process of several hard particles concerning their location, velocity and acceleration inside the molten bath. 

The previous investigations of hard particle transport processes during laser beam dispersing have shown, that the high speed microfocus radioscopy system is a usable arrangement to observe and analyse the movements, velocities and accelerations of particles inside the molten bath. That possibility was, until now, not given by conventional techniques of process... 

V. K. LaMer, ed.. Retardation of Evaporation by Monolayers Transport Processes, Academic Press, New York, 1962. 

One of the most usefiil applications of the mean free path concept occurs in the theory of transport processes in systems where there exist gradients of average but local density, local temperature, and/or local velocity. The existence of such gradients causes a transfer of particles, energy or momentum, respectively, from one region of the system to another. 

The kinetic theory of transport processes in gases rests upon three basic assumptions. 

Fertziger J H and Kaper H G 1972 Mathematicai Theory of Transport Processes in Gases (Amsterdam North Holland)... 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

Walton D J, Phull S S, Chyla A, Lorimer J P, Mason T J, Burke L D, Murphy M, Compton R G, Ekiund J C and Page S D 1995 Sonovoltammetry at platinum electrodes surface phenomena and mass transport processes J. Appl. Electrochem. 25 1083... 

Electron transfer reactions are conceptually simple. The coupled stmctural changes may be modest, as in tire case of outer-sphere electron transport processes. Otlier electron transfer processes result in bond fonnation or... 

A very important issue - disregard of which is a big source of bad modeling studies - is the dear distinction of transport processes (toxicokinetics) and interactions with targets such as membranes, enzymes, or DNA (toxicodynamics). Figure 10.1-6 gives a rather simplified model of a fish to illustrate this distinction. 

The tme driving force for any diffusive transport process is the gradient of chemical potential rather than the gradient of concentration. This distinction is not important in dilute systems where thermodynamically ideal behavior is approached. However, it becomes important at higher concentration levels and in micropore and surface diffusion. To a first approximation the expression for the diffusive flux may be written... 

C. J. Geankophs, Transport Process and Unit Operations, 2nd ed., AHyn Bacon, Newton, Mass., pp. 373. 

C.. GeankopHs, Transport Processes and Unit Operations, AHyn and Bacon, Boston, 1978. 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

At very high dopant concentrations, transport occurs direcdy between the dopant molecules. The polymer acts only as a binder in most cases. Taking TPD-doped PVK as an example, at low TPD concentrations the hole mobihty first decreases from 3 x 10 cm /Vs to 10 cm /Vs with increasing TPD concentration, because TPD molecules act as hole traps (48,49). At higher TPD concentrations, new direct transport channels between the TPD molecules open up and the hole mobihty increases to lO " cm /Vs for ca 60% TPD doping (Table 1, entries 9—11) (48,49). In this case, there is no evidence for unusual interaction between TPD and PVK that affects the hole transport process. 

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

In engineering appHcations, the transport processes involving heat and mass transfer usually occur in process equipment involving vapor—gas mixtures where the vapor undergoes a phase transformation, such as condensation to or evaporation from a Hquid phase. In the simplest case, the Hquid phase is pure, consisting of the vapor component alone. 

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

Transport processes describe movement of the pesticide from one location to another or from one phase to another. Transport processes include both downward leaching, surface mnoff, volatilization from the soil to the atmosphere, as weU as upward movement by capillary water to the soil surface. Transport processes do not affect the total amount of pesticide in the environment however, they can move the pesticide to sites that have different potentials for degradation. Transport processes also redistribute the pesticide in the environment, possibly contaminating sites away from the site of apphcation such as surface and groundwater and the atmosphere. Transport of pesticides is a function of both retention and transport processes. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

R. A. Home, ed.. Water andMqueous Solutions Structure, Thermodynamics, and Transport Processes, Wiley-Interscience, New York, 1972. 

Early models used a value for that remained constant throughout the day. However, measurements show that the deposition velocity increases during the day as surface heating increases atmospheric turbulence and hence diffusion, and plant stomatal activity increases (50—52). More recent models take this variation of into account. In one approach, the first step is to estimate the upper limit for in terms of the transport processes alone. This value is then modified to account for surface interaction, because the earth s surface is not a perfect sink for all pollutants. This method has led to what is referred to as the resistance model (52,53) that represents as the analogue of an electrical conductance... 

R. J. Friauf, "Basic Theory of Ionic Transport Processes," in J. Hladik, ed., Phjsics ofPlectroljtes Vol. 1, Academic Press, Inc., New York, 1972. 

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

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