Industrial adsorption processes

Mass Transfer Rale Consideralions - As discussed previously, the mass transfer mechanism involved in industrial adsorption processes is complex. Generally, basic physical data on the materials involved are insufficient for design. Experimental mass transfer rate data for the specific adsorbate-adsorbent system are usually required for good design. 

TABLE 7.2 Applications of molecular sieves in industrial adsorption processes... 

Major industrial adsorption processes using zeolite adsorbents may be classified as follows (I) hydrocarbon separation processes, (II) drying gases and liquids, (III) separation and purification of industrial streams, (IV) pollution control applications, and (V) nonregenerative applications. Some important commercial processes in each of these areas are discussed briefly. 

One of the first industrial adsorption processes was the separation of n/iso-paraffins in molecular sieves 5A zeolite developed by Union Carbide in the sixties for the octane improvement of gasoline pools and solvent production (Symoniak [1]). Since the seventies, the development of Hysomer and TIP (Total Isomerisation Process) by Shell and UOP respectively, and the demand of environment protection with the restriction of lead in fuel, makes this separation coupled with an isomerisation reactor one of the most licensed adsorption... 

Industrial adsorption processes normally are cyclic processes in which adsorption and desorption steps of the sorbent material alterate periodically. Often the desorption or regeneration step is cmcial and essentially determine the period and the energetic efficiency of the cycle [1.2, 1.14-1.16]. An important quantity to characterize the desorption process is the (molar) enthalpy (AH ) needed to desorb the leading component either of product or waste - of a gas mixture from the sorbent. In Table 1.2 some examples of desorption processes and their industrial applications together with typical values of the molar desorption enthalpy are given. Summarizing it can be stated that in reversible physidesorption processes molar enthalpies of about (10-50) kJ/mol are needed whereas in irreversible chemisorption processes (70-200) kJ/mol are necessary for desorption. ... 

Table 1.2. Desorption enthalpies of cyclic industrial adsorption processes for gas/vapor...

Abstract The physical principles and basic experimental techniques of impedance spectroscopy, i. e. static or frequency dependent dielectric permittivity measurements of sorbent/sorbate systems are given. These measurements can be used to characterize the state of a sorbent material in industrial adsorption processes. Combined with either manometric or gravimetric measurements of adsorption equilibria leading to calibration curves, permittivity measurements also allow fairly simple and quick measurements of gas adsorption equilibria. Kinetic processes and catalytic reactions inside a sorbent/sorbate system also can be observed. Pros and cons of dielectric measurements are discussed. List of Symbols. References. 

Impedance measurements also can be used to monitor industrial adsorption processes consisting of multi-step cycles like pressure swing adsorption processes (PSA) or vacuum swing adsorption processes (VSA), [3.37, 3.48, 3.42, 3.43]. 

In Sect. 4 we present several adsorption isotherms which are solutions of the Maxwell relations of the Gibbs fundamental equation of the multicomponent adsorbate [7.15]. These isotherms are thermodynamically consistent generalizations of several of the empirical isotherms presented in Sect. 3 to (energetically) heterogenous sorbent materials with surfaces of fractal dimension. In Sect. 5 some general recommendations for use ofAIs in industrial adsorption processes are given. 

As far as industrial adsorption processes are concerned it always should be taken into account that isotherms which are favorable for adsorption normally are unfavorable for desorption processes. Also, for column performance, for example packed bed dynamics, the velocity of the mass break through front is inverse proportional to the steepness of the adsorption isotherm. Hence it can be decisive to have accurate equilibria data at hand to get reasonably accurate values of the respective differential quotients [7.2, 7.4, 7.40]. For mixture adsorption this argument becomes even more important. S. D. G. 

Activated carbon is a very important industrial adsorbent because it exhibits a well developed porosity (micro, meso and macroporosity) and this is coupled with a great thermal and chemical stability, a relatively large hydrophobicity (thus favouring the adsorption of non-polar substances in the presence of humidity), low production cost, etc. Additionally, the surface of activated carbon can be functionalised with different heteroatoms (but mainly oxygen), thus modifying the chemical nature. A large and accessible surface area is a necessary but not sufficient condition for the preparation of activated carbons to be used in industrial adsorption processes (gas and liquid phase purification, separation, environmental control, etc.), since the last few years has shown that the chemical composition of the carbons surface plays a very important role in the process. 

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