Gas adsorption equilibria

Ming L., Anzhong G., et al. Determination of adsorbate density from supercriticl gas adsorption equilibria data. 2003 Carbon 41(3) 585-588. 

Keller and Staudt, Gas Adsorption Equilibria Experimental Methods and Adsorption Isotherms, Springer, New York, 2005. 

J.U. Keller Theory of measurement of gas-adsorption equilibria by rotational oscillations. Adsorption 1 (1995) 283-290. 

M. Tomalla, R. Staudt, J.U. Keller Determination of gas adsorption equilibria by volume-gravimetric measurements. In J.U. Keller, E. Robens (eds.) Microba/ance Pecbniques. Multi-Science Publishing, Brentwood 1994, S. 193-204, ISBN 0 906 522 10 2. 

Staudt R., Sailer G., Tomalla M. and Keller J. U. A note on gravimetric measurements of gas-adsorption equilibria. Ber. Bunsenges. Phys. Chem. 97 (1993) pp.98-105. 

The prediction of mixed-gas adsorption equilibria by ideal-adsorbed-solution theorf is based onEqs. (14.124) and (14.128). The following is a brief outline of the procedure. Since there are N "r 1 degrees of freedom, both T and P, as well as the gas-phase composition, must be specified. Solution is for the adsorbate compositionand the specific amount adsorbed. Adsorption isotherms for eachpure species must be known over the pressure range from zero to the value that produces the spreading pressure of the mixed-gas adsorbate. For purposes of illustration we assume Eq. (14.107), the Langmuir isotherm, to apply for each pure species, writing it ... 

A common practice is to develop a specific model for the adsorptive process of interest and use simplistic descriptions (models or empirical) of pure and multicomponent gas adsorption equilibria and kinetics in order to describe the effects of various operating variables to obtain an optimum design. The effort is always closely tied to experimental verification and empirical fine-tuning using actual process data from pilot plants. A comprehensive set of data on pure and multicomponent adsorption equilibria of the components of SMROG on an activated carbon and a 5A zeolite is available in published literature.73... 

Cerofolini and Rudzihski [43] have reviewed the theoretical principles of single gas and mixture adsorption on heterogeneous surfaces. Their review is chronologically arranged from the earliest to the latest approaches. In the same book, Tovbin [44] reported the application of lattice-gas models to explain mixed-gas adsorption equilibria on heterogeneous surfaces he also discussed [45] the kinetic aspects of adsorption-desorption on flat heterogeneous surfaces. The book [46] also contains other papers on different aspects of adsorption for the reader interested in surface diffusion processes. 

Cerofolini, G.F. and Rudzihski, W. (1997). Theoretical principles of single- and mixed-gas adsorption equilibria on heterogeneous sohd surfaces. In Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces, Studies in Surface Science and Catalysis, Elsevier, Vol. 104 (W. Rudziliski, W.A. Steele and G. Zgrablich, eds). pp. 1-103. 

The state of the art procedure for design of cyclic PSA or TSA processes using activated carbon adsorbents is to simultaneously solve the partial differential equations describing the mass, the heat, and the momentum balance equations for each step of the process using the appropriate initial and boundary conditions. These numerical calculations are carried out over many cycles for the process until a cyclic steady-state performance solution is achieved. Many different numerical integration algorithms are available for this purpose. The core input variables for the solution are multicomponent gas adsorption equilibria, heats, and kinetics for the system of interest [37]. 

Calculation procedures for adsorption and ion exchange differ only in detail from liquid-liquid extraction since an ion-exchange resin or adsorbent is analogous to the solvent in extraction. All coordinate systems used to represent solvent-solute or liquid-vapor equilibria may be used to display three-component solid-liquid, or solid-gas phase equilibria states. For the case of gas adsorption, equilibria are usually a function of pressure and temperature, and so isobaric and isothermal displays such as Fig. 3.21, which represents the propane-propylene-silica gel system, are convenient. 

Among the theories of predicting mixed-gas adsorption equilibria from pure component adsorption isotherms, the ideal adsorbed solution theory (IAST) [14] has become the standard and often serves as a benchmark for the purpose of comparison by other models. IA ST is a thermodynamically rigorous theory based on the mixing of individual components at constant spreading pressure to form an ideal solution. It has the advantages that (1) no mixture data are required and (2) the theory is independent of the actual model of physical adsorption. 

Traditionally, a variety of heats of adsorption and desorption for pure and multicomponent gas-solid systems have been defined by using thermodynamic models [3-6]. Experimental techniques have also been developed to measure these heats [4,7]. These models generally use the actual amounts adsorbed as the primary variables for representing the extents of adsorption of the adsorbates. Unfortunately, the Gibbsian surface excesses (GSE), and not the actual amounts adsorbed, are the only true experimental variables for measuring the extent of adsorption [8-10]. In view of this fact, a detailed thermodynamic model for multi-component gas adsorption equilibria using GSE as base variables has already been developed [9]. 

J. U. Keller and R. Staudt, Gas Adsorption Equilibria, Experimental Methods and Adsorptive Isotherms, Springer Science, Business Media, Inc., Boston, MA, 2005. 

Al-Ameeri, R.S., and Danner, R.P., Improved tracer-pulse method for measurement of gas adsorption equilibria, Chem. Eng. Commun., 26( 1), 11 -24 (1984). 

Sircar, S., Data representation for binary and multicomponent gas adsorption equilibria. Adsorption, 2(4). 327-335 (1996). 

Maurer, R.T., Multimodel approach to mixed-gas adsorption equilibria prediction, AlChE J., 43(2), 388-397... 

Buss, E., Gravimetric measurement of binary gas adsorption equilibria of methane-carbon dioxide mixtures on activated carbon. Gas Sep. Purif, 9(3), 189-198 (1995). 

In Sect. 3 the measurement methods for gas adsorption equilibria which are presented in this book are outlined. Several other phenomena in gas adsorption systems like the kinetics of the mass exchange process, which could not be considered here are mentioned in brief in Section 4. There also some general information on gas adsorption systems will be given and references for the various fields mentioned will be provided. 

Gas adsorption equilibria can be measured by several basically different methods. In this section we are going to outline the classical ones, namely volumetry/manometry and gravimetry as well as some newer ones, oscillometry and impedance spectroscopy. Emphasis is given to the underlying physical principles. Complementary remarks deal with possibilities to measure binary coadsorption equilibria with and without gas phase analysis. Technical details of all the measurement methods are given in the subsequent chapters, Chaps. (2-6). Prior to considering the measurement methods some general remarks on experimental work with gas adsorption systems are in order. 

The standard method to measure pure gas adsorption equilibria most often used today is the volumetric or manometric method. Chap. 2. Basically it is the mass balance of a certain amount of gas partly adsorbed on the sorbent material. This method can be realized in either open or closed systems, the former ones often using a carrier gas, the adsorption of which normally being neglected. Complemented by a gas analyzer (chromatograph, mass spectrometer) this method also can be used to measure multicomponent or coadsorption equilibria. 

As mentioned above multicomponent gas adsorption equilibria can be measured by... 

Table 0.2. Measurement methods for single and multicomponent gas adsorption equilibria.

In view of limitations in time and number of printed pages not all of the experimental methods to measure gas adsorption equilibria, which are discussed in today s literature, could be taken into account. To give reason for this the following remarks should be helpful. 

Calorimetry as a method to determine gas adsorption equilibria is not considered here in view of excellent presentations of this field in the literature [0.6, 0.37]. 

Once V is known, m = m (p, T, m ) can be calculated from Eq. (1.5). Such a situation is very common for gravimetric measurements of helium gas adsorption equilibria. An example is sketched in Figure 1.8 showing gravimetric adsorption data of activated carbon Norit R1 Extra exerted to He (5.0). As can be seen, for high pressures the reduced mass data ( ) easily can be linearly correlated, i. e. adsorption of helium has reached a state of saturation. Hence, Vhc can be determined via Eq. (1.6) and also the mass of helium adsorbed initially at low gas pressures can be calculated from Eq. (1.5) as ... 

In view of the fact that the most often used measurement methods for gas adsorption equilibria, namely the volumetric/manometric method, Chap. 2 and the gravimetric method, Chap. 3, only allow one to measure instead of (m ) or (mQE) the so-called reduced mass (f2), cp. Eq. (1.5),... 

Impedance Spectroscopic Measurements of Pure Gas Adsorption Equilibria on Zeolites,... 

Figure 2.1. Experimental setup for (static) volumetric measurements of pure gas adsorption equilibria.

Figure 2.6. Experimental set-up for (static) volumetric / manometric measurements of multicomponent gas adsorption equilibria.

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