Densimetric-gravimetric measurements

In practice combined volumetric-gravimetric measurements have been fairly successful [0.31]. Also densimetric-volumetric and densimetric gravimetric measurements using magnetic suspension balances (2 positions and 3 positions types respectively) can be recommended. If swelling sorbent materials are considered (slow) oscillometric measurements are recommended. Chap. 5. In case of multicomponent sorption systems (N > 2) a gas analyzing system has to be used in any case. 

Figure 4.14. Schematic diagram of an instrument for densimetric - gravimetric measurements of binary coadsorption equilibria without using a gas chromatograph. The sorptive gas supplied to the system is assumed to be a binary mixture with well known initial molar concentrations ( yJ.yj ) ...

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... 

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. 

Table 4.3. Comparison of experimental pros and cons of densimetric-gravimetric measurements (DGMs) and densimetric-volumetric measurements (DVMs) of binary gas adsorption equilibria without analyzing the sorptive phase.

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

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. 

In this chapter we will present experimental information (Sect. 2.1), the theory of measurement (Sect 2.2), and uncertainties (Sect 2.3), and several examples (Sect. 2.4) of this method. Two modified versions of the measurement procedure which may be called densimetric-gravimetric and densimetric-volumetric / manometric methods (which especially seems to be suited for online industrial coadsorption measurements) are also outlined (Sect. 3). These methods also may be used to measure adsorption of gases and / or vapors on surfaces of arbitrary sohd materials as for example the inner walls of vessels, tubes, valves etc. of the experimental device(s) used (Sect.3.6). Advantages and disadvantages of the methods proposed are discussed in Sect. 4. A list of symbols used is given in Sect. 5, followed by references to journal articles and books cited. 

In this section we wiU outline the algebraic procedure by which data for a binary coadsorption equiUbrium state, namely the masses (mf,i = l,2) adsorbed on a known mass (m ) of sorbent material and the concentrations of the sorptive gas (yf.y ) in equilibrium with the sorbed phase can be calculated from measured data (p, T, Q, p ) of a densimetric - gravimetric experiment described in Sect 3.1. Here we assume that the adsorption chamber initially has been evacuated. (Step up experiments can be handled in a fairly similar manner and do not require additional information or algebraic procedures.)... 

Comparison of Densimetric-Gravimetric and Densimetric-Volumetric Binary Coadsorption Measurements... 

Densimetric-Gravimetric and Densimetric-Manometric Measurements ofBinary Coadsorption EquiUbria, Adsorption, Science Technology, submitted, 2004. 

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. 

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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