Diffusional processes

The two constants kj and k describe exactly the same kind of diffusional processes and differ only in direction. Hence they have the same dependence on molecular size, whatever that might be, and that dependence therefore cancels out. 

Diffusion and Mass Transfer During Leaching. Rates of extraction from individual particles are difficult to assess because it is impossible to define the shapes of the pores or channels through which mass transfer (qv) has to take place. However, the nature of the diffusional process in a porous soHd could be illustrated by considering the diffusion of solute through a pore. This is described mathematically by the diffusion equation, the solutions of which indicate that the concentration in the pore would be expected to decrease according to an exponential decay function. 

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

Williams (W2) has recently modified the analysis of Hart and McClure by considering in more detail the effect of diffusional processes on the gas-phase reaction zone. The results of his study show that the diffusional processes tend to stabilize the gas-phase combustion process, indicating that the postulated solid-phase reactions are probably the underlying cause of the instability. 

As a result of the diffusional process, there is no net overall molecular flux arising from diffusion in a binary mixture, the two components being transferred at equal and opposite rates. In the process of equimolecular counterdiffusion which occurs, for example, in a distillation column when the two components have equal molar latent heats, the diffusional velocities are the same as the velocities of the molecular species relative to the walls of the equipment or the phase boundary. 

Whatever the physical constraints placed on the system, the diffusional process causes the two components to be transferred at equal and opposite rates and the values of the diffusional velocities uDA and uDB given in Section 10.2.5 are always applicable. It is the bulk How velocity uF which changes with imposed conditions and which gives rise to differences in overall mass transfer rates. In equimolecular counterdiffusion. uF is zero. In the absorption of a soluble gas A from a mixture the bulk velocity must be equal and opposite to the diffusional velocity of B as this latter component undergoes no net transfer. 

Thus, the diffusional process does not give rise to equal and opposite mass fluxes. 

Thus, the diffusional process in a liquid gives rise to a situation where the components are being transferred at approximately equal and opposite mass (rather than molar) rates. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

Colburn, A. P. (1939) Trans. Am Inst. Chem. Eng. 35, 211. The simplified calculation of diffusional processes, general considerations of two-film resistances. 

After completion of this diffusional process within a few /us, in other words after the consumption of S04 and formation of Py + or G+, interestingly, a larger amount of Py+ was formed for PyODNn with a shorter distance separation between the G-region and Py moiety in the time scale of 100 /is (Fig. 3a). This secondary formation of Py + was attributed to the hole transfer from G + to Py in DNA (Scheme 3, path b). Since the hole transfer from the G-region to the Py moiety is considered to be much slower for Py-ODN5 compared to the PyODNn (n=l 4), in the case of PyODN5 Py + was formed only from the initial collision process the time profile of Py+ at 470 nm for PyODN5 was subtracted from those for PyODNn (n=l 4). This... 

Transient absorption with a peak at 410 nm, assigned to TIOH+, was observed after the electron pulse during the pulse radiolysis (Fig. 5). Together with the decay of TIOH+, transient absorption with a maximum peak at 520 nm assigned to Ptz +was observed. In the case of PtzODNl, the observed formation rate of Ptz + and decay rate of TIOH+ were almost the same (Fig. 6a). Therefore, the hole transfer from G + to Ptz in DNA is faster than the diffusional process for PtzODNl (kht>kobs=l-4xl05 s 1). 

Upon examination of the columns of Pe and PM values for each drug in Table 16, one discovers that the influence of PF is minimal. Therefore, Pe = PM, i.e., permeation of the drugs across the ABL/cell monolayer/filter barriers is governed by the cell monolayer. The remaining questions are (1) To what extent is the transmonolayer diffusional process of uncharged and cationic species gated by the transcellular and paracellular routes and (2) What are the governing factors ... 

At low temperatures diffusion will be rapid compared to chemical reaction and diffusional limitations on the reaction rate will not be observed. In this temperature regime, one will observe the intrinsic activation energy of the reaction. However, since chemical reaction rates increase much more rapidly with increasing temperature than do diffusional processes, at... 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

The inhibition of amino-acid transport has been regarded as the main toxic effect of mercury compounds [82], The biochemical mechanism underlying the inhibition is unclear. In unfertilized sea-urchin eggs an interaction with the amino-acid carrier was found, whereas in fertilized eggs inhibition of amino-acid transport was indirect and might result from an elevation of the Na + content of the egg caused by the inhibition of the Na+ pump [83]. The action on the diffusional process could be mediated by an effect on membrane phospholipids or on membrane proteins, or by interaction with Ca2+ which stabilizes membrane structure. Mercuric chloride in skate liver cells inhibited amino acid transport, decreased Na + /K + -ATPase (adenosinetriphosphatase) activity, impaired volume regulatory mechanisms and increased the permeability of the plasma membrane to potassium [84]. It has been suggested that... 

A solute molecule moves according to two diffusional processes a viscous process with displacement of solvent molecules (Stokes diffusion) and a process associated... 

C[, corresponding to the Stokes diffusional process, can be written as the product of the Stokes friction coefficient multiplied by a correcting factor fr taking into account the finite thickness of the solvent layers... 

The importance of free volume effects in diffusional processes at a molecular level should be further emphasized. An empirical relationship between viscosity and free volume was proposed by Doolittle ... 

In developing the equations governing the thermal and diffusional processes, Hirschfelder obtained a set of complicated nonlinear equations that could be solved only by numerical methods. In order to solve the set of equations, Hirschfelder had to postulate some heat sink for a boundary condition on the cold side. The need for this sink was dictated by the use of the Arrhenius expressions for the reaction rate. The complexity is that the Arrhenius expression requires a finite reaction rate even at x = —°°, where the temperature is that of the unbumed gas. 

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