Quantitative Treatment of Resistance to Mass Transfer Dispersion

Quantitative Treatment of Resistance to Mass Transfer Dispersion 

If it is considered, in the first instance, that the distribution is energy controlled and not diffusion controlled, a solute molecule will desorb from the stationary phase when it randomly has sufficient kinetic energy to break its association with a molecule of stationary phase, as discussed in chapter 1. Similarly, a molecule will be absorbed under the same conditions. 

if (kd) is the desorption rate constant, then the mean desorption time (td) for 

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Consider a local concentration of solute migrating down a column. During this migration, adsorption and desorption steps will continuously and frequently occur. In addition, each occurrence will be a random event. Now a desorption step will be a random movement forward as it releases a molecule into the mobile phase, where it can move forward. Conversely, an adsorption step is a step backward, as it results in a period of immobility for the molecule while the rest of the zone moves forward. The total number of random steps taken as the solute mean position moves a distance (l) along the column is the number of forward steps plus the number of backward 

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