Vapor sorption rate curves

The goals of the present study were to reexamine vapor sorption in a lightly crosslinked rubber, both as an unfilled sample and in samples containing two types of carbon black in varying amounts. Complications in the vapor sorption-rate curves motivated a more detailed study of the diffusion problems as the main area of concern. 

Figure 4 illustrates the distribution curve of a compound through the sorbent bed and the capacitive breakthrough curve of the compound in the effluent. At the start of collation the compound is distributed through a sation of the sorbent as shown by curve 1. After continued collection this section becomes saturated , that is an equilibrium is established with incoming concentration Ci where the vapor sorption and desorption rates are equal. This is shown by curve 2. The sorption front has moved through the front section of the collection tube (L 2/3) and is being collected on the backup section. If collation continues, the... 

The pressure increase in a reaction chamber that contains a porous substrate when a monomer vapor is introduced by a given flow rate can be utilized to calculate the sorption capability of the porous substrate. The pressure buildup curves are shown in Figure 34.6 for Millipore filter and porous polysulfone film. The pressure buildup curve with a porous glass tube is too slow to be presented in the same time scale. From the slope of the linear portion of the pressure buildup curve, the ratio of monomer sorbed/monomer fed into the system is estimated as 0.636 for the polysulfone film, 0.926 for Millipore filter, and 0.9987 for the porous glass tube. 

Time Dependence. Time dependence may be observed in the diffusion of organic vapors in polymers below their glass transition temperature (Tg) (5). At these temperatures, the rate of diffusion is comparable with the rate of motion of the polymer segments. As a result, the value of the diffusion coefficient attained at a given concentration in an element of the polymer will depend on the time for which this concentration has existed at the element. D has more time in which to approach its equilibrium value in thicker films. Therefore, sorption proceeds more rapidly the thicker the film, and the reduced sorption curves do not coincide as required by Equation 1 describing Fickian diffusion. 

Figures 10.13 and 10.14 show the kinetic plots for the sorption of n-hexane vapors on Styrosorb 2 at 20 and 40°C [16]. The anomalous character of kinetics shown in the nonlinear dependence of 7 versus y/r is clearly seen at the low and moderate extents of bead saturation. By replacing the curved kinetic plots with a set of short stra ht lines and calculating the effective diffusion coefficients, Dg, according to Eq. [10.2] within each linear interval, one arrives at the important conclusion that the diffusion rate of the sorbate molecules increases by more than 1 order of magnitude (F. 10.15) during the sorption process.

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