Principle of Diffusion

Very often, the transport of the liquid through the rubber is controlled by diffusion with either a constant or concentration-dependent diffusivity. This transport can be performed under stationary conditions when the concentration of the liquid varies with position, and under transient conditions when the liquid concentration varies with position and time. 

The diffusion behaviour of polymers in a rubbery state is described by Pick s laws with constant or concentration-dependent diffusivity. 

As shown in Chapter 2, transfer of heat by conduction is due to random molecular motions, and there is an obvious analogy between the two processes of heat conduction and of mass transport by diffusion. Pick in 1855 recognized this fact, and put diffusion on a quantitative basis by adopting the mathematical equation of heat conduction derived some years earlier by Pourier (1822). 

The rate of transfer of a substance diffusing through a unit area of a section is proportional to the gradient of concentration normal to the section  

the amount of material diffusing, and C, the concaitration, are both expressed in terms of the same unit of quantity, then the diffusivity D is independent of this unit and has the following dimensions (length) - (time) or in the CGS system cm i .  

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As their name suggests, these models are based on the physical principles of diffusion and convection, which govern the mixing process. According to the flow pattern, the reactor is divided into different zones with different flow characteristics. 

Renal replacement by an artificial device providing intermittent clearance of plasma based on the physical principle of diffusion. 

We will give a brief summary of the general principles of diffusion and preassociation in electrophilic substitutions before discussing the values in Table 3-1 (for more comprehensive reviews see North, 1964 and Ridd, 1978). A precursor A (HN02 in... 

The operation of the 3M Personal Monitoring System is based on the principle of diffusion. The monitor is comprised of a velocity barrier, a static air column, and a sorbent layer at the bottom of the air column. 

It is not passive. The driving force, its continued operation, is based on the difference between the contaminant concentration at the velocity barrier (assumed to be equal to the ambient concentration) and the sorbent surface (the principle of diffusion). That difference is maintained by the continuous adsorption of the contaminant vapors by the sorbent. 

Passive Sampling. These techniques provide for the accumulation of a contaminant on a substrate on the basis of the principles of diffusion, sedimentation, adsorption, or absorption. 

This consideration of the principles of diffusive sampling identifies a range of factors which may influence the performance of a diffusive sampler for monitoring VOC concentrations in indoor air. These factors will potentially be a source of error in such measurements and add to the overall uncertainty of the result given by the measurement procedure. In addition the amount of uncertainty will be influenced by other factors including amount and consistency of background contamination of sorbents, repeatability of analytical determination, formation of artifacts, stability of analyte on the sorbent, recovery of analyte during analyses and presence of interferents. 

Fig. 8.1. Schematic illustration of the TG experiment [upper) and the principle of diffusion measurement [lower). Lower The white and black circles indicate the reactant and product molecules. The concentrations of the reactant and the product are spatially modulated by the sinusoidally modulated light intensity of the grating light. The fringe length A is also indicated...

Purple cloud of potassium permanganate diffusing outward as a solid piece dissolves in a beaker of water, seen from above. The solid piece (black) takes up much less space than the solution of potassium permanganate ions. This demonstrates the principle of diffusion, where a liquid or gas will spread out to fill the available space. This in turn demonstrates the fundamental differences between a solid and a gas or liquid. A solid is a rigid, constrained structure, which contrasts with the fluid, chaotic nature of a liquid or gas. The molecules of a solid are bonded, but solvation or melting breaks the bonds between the molecules. (Courtesy of Andrew Lambert Photography/Science Photo Library)... 

For the gaseous contaminants the release mechanism from the soil is generally based in the principles of diffusion across a porous medium. Basic diffusion equations are used for estimating the theoretical values of the gaseous flux from the waste material. The generic diffusion equation can be represented by a one-dimensional steady-state equation ... 

PRINCIPLES OF DIFFUSION AND MASS TRANSFER BETWEEN PHASES... 

Principles of Diffusion and Mass Transfer between Phases 647... 

Diffusion dialysis was initially applied in viscose rayon production to recover caustic soda using parchment paper as a membrane. Today the largest industrial utilization of diffusion dialysis is to recover acids or alkalis from waste acids and alkalis using anion or cation exchange membranes. Figure 6.28 shows the principle of diffusion dialysis for acid recovery from waste acid solution. The process and membrane performance are evaluated on the basis of the flux of acids or alkalis (dialysis coefficient) and the ratio of the flux of metal salt to that of acids or alkalis (separation coefficient). The total dialysis coefficient [molh-1 m 2 (moll-1)], Uo, is defined by... 

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