Binary coadsorption equilibria

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. 

Indeed, procedures (b) open various interesting possibilities to measure binary coadsorption equilibria and to design respective instruments for fully automated measurements, cp. for example Chap. 4, Fig. 4.11b. To get an overview, the various possibilities of coadsorption measurements by combining single component methods are sketched in Table 0.2. The numbers in the upper right portion of this matrix scheme indicate the number of components in the gas mixture which can be determined by the respective method. The numbers in the lower left portion of the matrix give the Chapter and Section where more information on this method can be found. Empty fields indicate that we did not do respective measurements and also are not aware of any institution where such measurements might have been realized. 

Abstract Combined volumetric and gravimetric measurements allow one to determine the coadsorption equilibria of binary gas mixtures without sorptive gas analysis, i. e. without using a gas chromatograph or mass spectrometer. The experimental setup, a basic theory and several examples of this method are presented. Two modifications of it, namely densimetric - gravimetric and densimetric - volumetric measurements are outlined. These especially are suited to do quick but still accurate measurements of binary coadsorption equilibria for industrial process control and / or design. These methods also can be used to measure adsorption of gases and vapors on walls of vessels, tubes or surfaces of any other solid materials. List of symbols. References. 

To measure binary coadsorption equilibria by tbe volumetric-gravimetric method one proceeds as follows A sorbent sample of 1 g - 3 g and appropriate counterweights, typically lead or silver balls, are placed to the buckets of the microbalance. Then the sorbent is activated by exposing it to helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for zeolites and inorganic molecular sieves. After cooling down and evacuation (< 10 Pa) the adsorption chamber is prepared for an adsorption experiment. 

In this section we want to present data for binary coadsorption equilibria of gas mixtures on activated carbon (NORIT R 1) which have been taken by combined volumetric-gravimetric measurements. We start with a set of data which have been measured with the instrument depicted in Figs. 4.1,4.2. Then we describe in brief a new type of volumetric-gravimetric instrument including a magnetic suspension balance allowing also measurements with corrosive sorptive gases. Equilibria as well as kinetic data taken at this instrument will be presented. Finally we hint at a commerciahzed version of this instrument offered by BEE - Japan company. 

Figure 4.4. Schematic diagram of an installation for volumetric-gravimetric measurements of binary coadsorption equilibria without using a gas chromatograph. The installation includes a magnetic suspension balance (RUBOTHERM, Bochum, Germany) allowing also measurements with corrosive gases in a large range of temperature (T < 1500 K) and pressure (p < 100 MPa).

Figure 4.11a. Schematic diagram of an instrument for automated binary coadsorption equilibria measurements using the volumetric-gravimetric method. The installation has been designed by BEL-Japan Corp. and includes a magnetic suspension balance, RUBOTHERM, Bochum. (Reprint by permission of BEL-Japan.)...

Instrument for automated combined volumetric-gravimetric measurements of binary coadsorption equilibria of gas mixtures with non-isometric components. The RUBOTHERM magnetic suspension balance is on the left side. The large white closet includes the volumetric / manometiic part of the instrument and is manufactured by BEL-Japan, Osaka. (Reprint by permission of BEL-Japan.)... 

To measure binary coadsorption equilibria by the densimetric-gravimet-ric method one should proceed as follows After preparing a sorbent sample, i. e. activating it, cp. Sect. 2.1 and weighing it in vacuum to determine its mass (m ), the adsorption chamber (AC) is filled with a gas mixture of known concentrations ( y, ) and the circulation pump is turned on. 

In case the two sorptive gas components (1,2) are mixed with a carrier gas of molecular weight (Mo) which practically is not adsorbed on the sorbent material considered, densimetric - gravimetric measurements still can be used to determine binary coadsorption equilibria of the (non-isomeric) components (1, 2). However, the basic equations (4.41, 4.44, 4.47) have to be modified as follows ... 

Densimetric-gravimetric measurements of binary coadsorption equilibria have been performed at IFT using the instrument shown in Figs. 4.13, 4.14 in 2001-2002, [4.17]. The system chosen was carbon dioxide (CO2), methane (CH4), and activated carbon (AC) D 47/3 at T = 293 K for pressures up to... 

Figure 4.18. Instrument for volumetric-densimetric measurements of binary coadsorption equilibria of gas mixtures on porous solids without using a gas chromatograph. The sorptive gas prepared in the system is assumed to be a binary mixture with known initial molar concentrations ( y, y2 ) IFT University of Siegen, 2002.

DGMs densimetric-gravimetric measurements (of binary coadsorption equilibria)... 

Figure 5.16. Scheme of an instrument for manometric-gravimetric-oscillometric measurements of binary coadsorption equilibria in swelling materials (polymers, resins etc.) without sorptive gas analysis by GC or MS. 

In this article a short overview is given of the measurement methods for adsorption equilibria of pure and mixed gases most often used today. After presenting the traditional volumetric and gravimenic method, modem combinations of it, namely the densimetric-volumetric and the densimetric-gravimetric method to measure binary coadsorption equilibria are presented in brief (Section 2). 

For binary coadsorption equilibria with non-isomeric gas components (Ml 7 M2) gravimetric-chromatographic measurements are not needed. Instead densimetric-volumetric measurements are recommended [6]. The measurement procedure can be grasped from the experimental scheme sketched in Figure 5 below. Basically, a gas expansion experiment is combined with a density measurement of the equilibrium sorptive gas mixture by the buoyancy of a sinker coupled to a magnetic suspension balance. 

Figure 5. Densimetiic-volumetric measurements of a binary coadsorption equilibria of premixed gases with molar concentrations (y. y ).

Finally we would like to mention that binary coadsorption equilibria of non-isomeric gas components also can be measured without gas phase analysis by volumetric-gravimetric or gravimetric-densimetric, i. e. combined weighing and density measurements. Both procedures can be realized in an installation similar to that shown in Figure 4. Details are given in [1, Chapts 3,4]. 

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