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. 

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. 

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. 

Coadsorption equilibria of CH4 / N2 gas mixtures of AC Norit R1 Extra at 298 K, 0.3 MPa. The data indicate partial molar Gibbs excess amounts of CH4 (upper figure) and Nj (lower figure) and their dependence on the sorptive gas concentration (ycH4)-... 

Coadsorption equilibria of gas mixtures of known initial concentrations on porous sorbent materials can be measured fairly easily using a microbalance combined with a gas chromatograph or mass spectrometer to analyze the sorptive s gas concentration at adsorption equilibrium. A schematic of such an instrument is given in Figure 3.25. It basically consists of a microbalance, gas supply system, a sample loop including a gas circulator to maintain equal and constant sorptive gas concentrations within... 

Coadsorption Equilibria for Gas Separation and Purification Processes, PH-D Thesis, IFT, University of Siegen, in preparation, 2003. 

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. 

Figure 4.2. Instrument for volumetric-gravimetric-chromatographic measurements of coadsorption equilibria of gas mixtures on porous materials. The gas storage vessel can be recognized on the left hand, the microbalance in the middle above the thermostat and the gas chromatograph on the right hand side of the picture. IFT, University of Siegen, 1990.

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. 

Table 4. I. Data for coadsorption equilibria of CH4/N2 gas mixtures at T=298K on activated carbon NORIT Rl, [4.11]. p...sorptive gas pressure, ycH4 - molar concentration of CH4 in the gas phase, p...density of the sorptive gas, ncH4, nN2-..number of moles ofCH4, N2 adsorbed per unit mass of sorbent.

Figure 4.3. Coadsorption equilibria ofCH4 / N2 gas mixtures at T = 298 K on AC Norit Rl for sorptive gas concentrations ranging from ycH4 = 18.1 % at p = 0.11 MPa to ycH4 = 23.7 at p = 7.93 MPa. Uncertainties of the data are approximately three times of the diameter of the graphical symbols used [4.11].

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 4.6 a. Coadsorption equilibria of a binary gasmixture CH4 / CO2 with... 

Data are given in [mmol/gAC] The lines show predicted coadsorption equilibria based on pure component data and the lAST formalism [4.10],... 

Figure 4.6 b. Coadsorption equilibria of a binary gas mixture CH4 / N2 with ycH4 = 9 % on AC Norit R 1 at 298 K. Data were taken by volumetric - gravimetric measurements [4.4],... 

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 4.17. Coadsorption equilibria data for a binary gasmixture CO2 / CH4 with yco2 = 20.4 %mol, Ych4 = 79.6 %mol on AC D47/3 at 293 K. The quantity In (p/n) is correlated to the absolute amount adsorbed (n) for n = nco2 " > d n = n,o, = nco2 + ncH4. .. . Lines refer to adsorption isotherms of the generalized Langmuir type for adsorbents with fractal dimensions cp. Chap. 7, [4.17].

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.

Densimetric-volumetric measurements have been performed recently at our Institute determining again coadsorption equilibria of (CO2, CH4) on AC D47/3 at T = 293 K for pressures up to 1.4 MPa, cp. Sect. 3.4. Results are identical within experimental uncertainties with those received by densimetric-gravimetric measurements. Sect. 3.4. Hence discussion of this method can be postponed to Sect. 4 where all experimental methods outlined in this chapter will be evaluated from both the experimental and the theoretical point of view. 

Commercially available instruments for VGMs usually need several grams of sorbent material to perform reasonably accurate measurements. For coadsorption equilibria of binary mixtures of unsaturated gases (with or without a supercritical carrier gas), wall adsorption in instrument s tubes, valves, and vessel may cause a serious problem. Calibration experiments in the empty instrument, i. e. without a sorbent, may solve the problem but not always. In these cases additional measurements of sorptive gas concentrations are indispensable. 

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