Measurement of Pure Gas Adsorption Equilibria

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

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

Instrument for gravimetric measurements of pure gas adsorption equilibria using a two beam balance (Sartorius Th. Gast, 4104 S). 

One may argue that combined volumetric - gravimetric measurements of pure gas adsorption equilibria could provide a means to measure the mass adsorbed (m ) and the void volume of the sorbent and the sorbate phase (V ) simultaneously. Unfortunately this does not hold true. Instead, specializing equations (4.1. 6) to the case of pure gases (mi—>m, m2 = 0) and combining them one gets... 

MEASUREMENT OF PURE GAS ADSORPTION EQUILIBRIA (N=l) BY SLOW OSCILLATIONS OF A ROTATIONAL PENDULUM... 

DIELECTRIC-GRAVIMETRIC MEASUREMENTS OF PURE GAS ADSORPTION EQUILIBRIA... 

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. 

Figure 6.24. Experimental setup for combined dielectric-manometiic measurements of pure gas and gas mixture adsorption equilibria. IFT University of Siegen, 1990.

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]. 

This chapter is organized as follows In Sect. 2 we consider pure gas adsorption measurements by both two beam and single beam balances. Section 3 is devoted to thermogravimetry. In Section 4 multicomponent gas adsorption equilibria are discussed. Finally in Sect. 5 pros and cons of gravimetry especially compared to volumetry/manometry are elucidated. A list of symbols and abbreviations used is given followed by references dted. 

Combining the dielectric measurements with either manometric, gravimetric or oscillometric measurements of gas adsorption equilibria states, one gets calibration curves allowing one the determination of Gibbs excess adsorbed masses by purely electric measurements which normally can be performed fairly quickly and on site in industrial situations. 

This book is the result of a fruitful collaboration of a theoretician (JUK) and an experimentalist (RS) over more than twelve years in the field of gas adsorption systems at the Institute of Fluid- and Thermodynamics (IFT) at the University of Siegen, Siegen, Germany. This collaboration resulted in the development of several new methods to measure not only pure gas adsorption, but gas mixture or coadsorption equilibria on inert porous solids. Also several new theoretical results could be achieved leading to new types of so-called adsorption isotherms based on the concepts of molecular association and -phenomenologically speaking - on that of thermodynamic phases of fractal dimension. Naturally, results of international collaboration of the authors over the years (1980-2000) also are included. 

There are several reports in the literature that measure binary adsorption equilibria using gas chromatography [4,S,6]. In GC techniques the adsorbent is equilibrated with a continuous flow of carrier gas (gas 1). Then a pulse of gas 2 is injected at the column inlet. A peak of the gas 2 is eluted at the exit of the column after some time. Net retention time (or volume) is calculated from the first moment of the peak after correcting for void volume (by measuring the retention time of a non-adsorbing species). If the carrier gas is inert (i.e. helium) the net retention time is related to the pure component Henry s constant. Typical binary measurements reported so r use a mixture of the two gases as carrier and introduce a small perturbation in composition. The net retention volume is related to the thermodynamic properties by [4]... 

Adsorption measurement for multicomponent systems is a function of the composition, temperature, pressure, and properties of adsorbate and adsorbent. As the number of components increases, the number of measurements needed to define the adsorption equilibrium increases rapidly and eventually becomes infeasible. Adsorption equilibrium models are therefore needed to correlate and predict the multicomponent adsorption equilibria. These models should be able to predict the mixture equilibria using the information available on pure component equilibria, as the latter are relatively easy to measure and furthermore there is an abundance of pure component isotherm data available in the literature. As a result, predictive models for gas mixture adsorption are necessary in the design and modeling of adsorption processes. 

Abstract This introductory chapter provides some background information of the material to be presented experimental methods to measure adsorption equilibria of pure and mixed gases on inert porous solids. Applications of gas adsorption processes in science and technology are outlined. An overview of the contents of the book is given. Remarks on subjects, measurement methods and other fields of adsorption science which could not be considered within this monography are mentioned. Hints to respective literature and references are given. 

Figure 3.5. Magnetic suspension balance (Rubotherm GmbH) for automated gravimetric measurements of adsorption equilibria of pure corrosive gases. On the r. h. side of the balance 6 electro-pneumatic valves (Hoke, Frankfurt) for controlling the gas supply and evacuation of the installation clearly can be recognized.

This book is intended to present for the first time experimental methods to measure equilibria states of pure and mixed gases being adsorbed on the surface of solid materials. It has been written for engineers and scientists from industry and academia who are interested in adsorption based gas separation processes and/or in using gas adsorption for characterization of the porosity of solid materials. 

Table 1. Measurement methods for adsorption equilibria of pure gases and gas mixtures on porous solids [1], Explanations of the various symbols are given in the text of this article.

Today there are several experimental methods available to measure pure gas and gas mixture adsorption equilibria on porous rigid or swelling sorbent materials. All these methods have their specific advantages and disadvantages [1]. Choice of any of them depends mainly on the purpose of measurement and/or accuracy and reliability of data needed. For quick measurements of restricted accuracy gas expansion experiments or volumetric measurements are recommended. If high accuracy data are needed, weighing procedures, i. e. gravimetry should be used... 

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