Species Transfer in a Porous Solid

When a fluid flows through a porous solid or a porous bed, the species forming the fluid can present some affinities with the solid particles. The affinity of species with respect to the contacting solid, which here form what is called a stationary phase, can be the result of different phenomena such as adsorption, ion exchange, steric exclusion and absorption. 

If we introduce a discrete feed of A and B, characterized by a reasonable interval of time between two inputs, and a discrete collection of A-carrier and B-carrier outputs into Fig. 4.38, then we will have a chromatographic separator. 

When the carrier is a liquid, the instrumentation includes a pump, an injector, a column, a detector and a recorder or a data acquisition system, connected to a computer. The heart of the system is the column where the separation occurs. Since the stationary phase is composed of micrometric porous particles, a high-pressure pump is required to move the mobile phase through the column. The chromatographic process begins by injecting the solute into the top of the column by an impulse type injection. The separation of the components occurs during the elution of the mobile phase through the column. 

The majority of chromatographic separations as well as the theory assume that each component elutes out of the column as a narrow band or a Gaussian peak. Using the position of the maximum of the peak as a measure of retention time, the peak shape conforms closely to the equation C = Cjjjg, exp[-(t -1] ) The modelling of this process, by traditional descriptive models, has been extensively reported in the literature. 

As has been explained previously in this chapter, the building of a stochastic model starts with the identification of the individual states of the process. 

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Stochastic Mode/sfor Chemical Engineering Processes 305 4.6.2.2 Molecular Species Transfer in a Porous Solid... 

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