Diffusion washing

As the protein solution moves into and through the rectangular flow channel, a parabolic concentration profile develops (Fig. 5 a). Diffusion washes out the flow profile, eventually equalizing the concentration at C0. 

To summarise, flume and chamber studies are helpful for investigating the relative importance of surface diffusion, wash off and plant uptake/release from aerial shoots, when used in conjunction with direct tidal creek flux measurements. However, the results from these experiments should not be extrapolated to give a flux value for the marsh as a whole as they do not account for sediment drainage, water column and creek bank processes. 

One or all of the steps can be used on a product or occur simultaneously. Several steps can be used often, firstadisplacement wash to remove mother liquors and any associated impurities, followed by a dissolution or diffusion wash. 

Washing curves are used to describe washing efhciency. Frequently washing curves are represented by plots of CJCg as functions of wash ratio j as shown in Figure 22.58. The ratio of in mass to C indicating wash efhciency is constant for a portion of the wash cycle and drops rapidly after the wash ratio j increases beyond 0.6. The drop starts to slow down when the wash ratio j reaches about 1.5 when diffusion washing becomes the dominate factor. Solute is not uniformly distributed... 

Wellinghoff et al. (1995) derived a correlation directly from a pore diffusion model based on Pick s law and obtained an equation which predicts the effective distribution coefficient of diffusion washing as a function of four dimensionless numbers. The apparent differences between the two approaches vanish upon closer examination. Namely, when neglecting the factors that cannot be arbitrarily influenced due to process inherent restrictions the two approaches are essentially alike. They only differ in that Wellinghoff et al. (1995) additionally consider a dimensionless area which corresponds to the ratio of the pore surface to the surface of the crystal layer. This factor can only be determined experimentally yet which is fairly complicated and thus limits the applicability of the approach. 

Figure 7.19 Prediction of result of diffusion washing. (Wangnick 1994.)...

C(t) decays toward a plateau value at short time, and then to zero as isotropic diffusion washes out the anisotropy. 

The number of washings may be reduced to about twenty, if time is allowed for diffusion of the alkali from the surface of the catalyst into the surrounding wash water. Use 750 ml. of water in each washing, allow diffusion to proceed for 3-10 minutes, stir again, and decant the supernatant liquid as soon as the catalyst settles to the bottom. 

Cotton linters or wood pulp are nitrated using mixed acid followed by treatment with hot acidified water, pulping, neutralization, and washing. The finished product is blended for uniformity to a required nitrogen content. The controlling factors in the nitration process are the rates of diffusion of the acid into the fibers and of water out of the fibers, the composition of mixed acid, and the temperature (see Cellulose esters, inorganic esters). 

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

Performance assessments are predictions of radioactivity releases, the rate of transfer of contaminants through various media, and the potential for hazard to the pubHc. These are based on a combination of experimental data obtained in the process called site characterization and detaded computations about radionuchdes and their effects. The progressive attack on the metal or ceramic waste container, the diffusion of water into the waste form, the leaching of the radioactive compounds, diffusion out, and washing away of radionuchdes are all considered. 

Fig. 13. Single-sheet diffusion transfer plate (a) stmcture (b) upon exposure to light (c) development and (d) washing off and finish. In (a) the plate is first coated with a receiver layer of small (<5 nm) catalytic sites. The photographic layer is a spectrally sensitized silver haUde emulsion. In (c) the exposed areas develop as silver metal. Unexposed areas diffuse down to the receiver layer and form the printing image. In (d) the emulsion is washed off, revealing...

Washing equipment typically includes cylinder or dmm washers, diffusion ring washers, pressure diffusers, pressure washers, and belt washers. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, permeable sohd phase with which it is associated. The separation usually involves selective dissolution, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid and it may be incorporated within, chemically combined with, adsorbed upon, or held mechanically in the pore structure of the insoluble material. The insoluble sohd may be massive and porous more often it is particulate, and the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated. 

Thorough displacement washing is possible in a nutsche if the wash solvent is added before the cake begins to be exposed to air displacement of filtrate. If washing needs to oe more effective, an agitator can be provided in the nutsche vessel to reslurry the cake to allow adequate diffusion of solute from the solids. 

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