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Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material.
Brunauer, Emmett and Teller's model of multilayer adsorption is a random distribution of molecules on the material surface. Adsorption is the adhesion [1] of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. [2] This process creates a film of the adsorbate on the surface of the adsorbent.
The adsorption sites (heavy dots) are equivalent and can have unit occupancy. Also, the adsorbates are immobile on the surface. The Langmuir adsorption model explains adsorption by assuming an adsorbate behaves as an ideal gas at isothermal conditions. According to the model, adsorption and desorption are reversible processes.
Vacuum swing adsorption (VSA) uses the same principle as PSA but swings between vacuum pressures and atmospheric pressure. [2] PSA and VSA techniques may be combined and are called "vacuum pressure swing adsorption" (VPSA) in this case.
gas molecules only interact with adjacent layers; and; the Langmuir theory can be applied to each layer. the enthalpy of adsorption for the first layer is constant and greater than the second (and higher). the enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction. The resulting BET equation is
The amount of adsorbed gas depends on pressure, temperature and adsorbent type. Since this adsorption process is exothermic, an increase in pressure or a decrease in temperature enhances the efficiency of the adsorption process. It is possible to mix the ANG and CNG technology to reach an increased capacity of natural gas storage.
The Hertz–Knudsen equation describes the non-dissociative adsorption of a gas molecule on a surface by expressing the variation of the number of molecules impacting on the surfaces per unit of time as a function of the pressure of the gas and other parameters which characterise both the gas phase molecule and the surface: [1] [2]
The proportionality between number flux and pressure is only strictly valid for a given temperature and a given molecular mass of adsorbing gas. However, the dependence is only on the square roots of m and T. Gas adsorption experiments typically operate around ambient temperature with light gases, and so the langmuir remains useful as a ...