Oscillometric-gravimetric measurement

Figure 5.9. Experimental setup for oscillometric-gravimetric measurements.

The solubility of carbon dioxide in a special polycarbonate (PC) (Goodfellow, UK) has been investigated experimentally by oscillometric-gravimetric measurements at T = 293 K for pressures up to 6 MPa, [5.7, 5.8]. 

The impedance measurements of this system have been combined with oscillometric-gravimetric measurements leading to both the volume of the swelling polymer and the mass of CO2 sorbed at a given gas pressure, cp. Chap. 5. These combined measurements show that there are nearly linear relations between the dielectric permittivity and the gas pressure as well as the volume of the polymer in the sorption state and the gas pressure, cp. data and Figures given in Sect. 3.2. 

H. Rave, R. Staudt, J.U. Keller Measurement of sorption and swelling behaviour of polymer/C02 systems by a combined oscillometric-gravimetric method. Proeedings. of P3AC2, Brisbane, May 14-18, 2000, World Scientific, Singapore 2000. 

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. 

Eqs. (2.9, 2.10). They show a monotonous increase with increasing sorptive gas density and coincide remarkably well thus proving - in this example -that oscillometric and gravimetric measurements lead (within experimental uncertainties) to identical results. 

Combined oscillometric and gravimetric measurements provide a basis to determine simultaneously the mass and the volume of a swelling sorbent material like polymers or resins in a sorptive gas atmosphere. However measurements seem in practice to be restricted to determinations of sorption equilibria of these materials as the kinetics of mass uptake often is very slow. To give an example we mention that in case of sorption of CH4 on pellets of Makrolon 2400 at 35 °C, p = 2 MPa, it took more than 4 days till equilibrium was reached. 

Rotational Pendulum for combined oscillometric-gravimetric sorption measurements of gases in swelling (polymeric) materials. IFT University of Siegen, 1999. 

In this section we will provide the reader with the basic equations allowing one to calculate from combined oscillometric and gravimetric measurements both the mass of gas (m ) adsorbed or absorbed in a swelling sorbent material of mass (m ) and its volume (V ) in the sorbate state at given pressure (p) and temperature (T) of the sorptive gas and sorbent material. We start by mentioning the result of oscillometric measurements, cp. Sect. 2.2, namely the relation (5.25)... 

We also would like to draw reader s attention to the possibility to combine oscillometric, gravimetric, and manometric measurements in a single experiment. The schematic diagram of such an instrument is given in Figure 5.16. It basically consists of a rotational pendulum, a microbalance (two beams as indicated in Fig. 5.16 or single beam as sketched in Fig. 5.15) and a storage vessel for the sorptive gas. This instrument would allow cosorption... 

For swelling sorbents like polymers and resins, combined osciUometric-gravimetric or oscillometric-manometric measurements allow one to determine simultaneously the total mass (m + m ) and the volume (V ) of the sorbent / sorbate system. In this way, for example, a shift in the glass transition temperature of a polymer due to gas absorption can be observed. 

For rigid sorbent materials, combined oscillometric-gravimetric or oscillometric-manometric measurements in a saturation state of an adsorption system also allow one to determine both the total mass (m m ) and the Volume (V ) of the system without using the so-called helium volume hypothesis, cp. Fig. 5.8 and Chap. 1. 

It should be emphasized that in principle impedance measurements also can be added to other experimental methods aiming at an analysis of gas sorption systems. Examples for these are volumetric-gravimetric measurements. Chap. 4, oscillometric measurements. Chap. 5, or ZLC-... 

Figure 6.31. Specific volume of polycarbonate (GoodfeUow, UK) swelling due to the sorption of methane (CH4, 5.5) at T = 308 K for pressures up to 4 MPa and static dielectric permittivity (8,) of the (methane loaded) polymer [52, 6.3]. The volume has been determined by combined osciUometiic-gravimetric measurements, cp. Chap. 5. Figures 6.30, 6.31 refer to the same set of combined oscillometric-gravimetric-dielectric experiments [6.3, 6.41].

Spring balances for gravimetric and/or oscillometric measurements are not considered. Uncertainties of measurements often are to large and, in case of oscillations, the flow field of the surrounding gas becomes turbulent, i. e. the friction forces exerted by the gas on the sorbent sample cannot reliably be calculated from the Navier-Stockes-equations, cp. Chap. 5. 

Figure 5.7. Reduced masses (Hose, f grav) resulting from oscillometric and gravimetric adsorption measurements of He on activated carbon (Norit R1 Extra) at 293 K. Gibbs excess masses adsorbed (= 0, ) are calculated from (f2o c>...

In Figure 5.12 results of measurements are presented. The oscillometric data ( ) show a monotonic increase with the gas pressure (p), while the gravimetric data ( ) are strongly curved, clearly indicating the influence of buoyancy. At pressures above 5 MPa the microbalance reading becomes even negative, i. e. the effect of buoyancy becomes larger than the increase in mass due to absorption of CO2. 

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. 

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. 

Combined dielectric gravimetric oscillometric measurements of sorption equilibria of methane (CH4, 5.5) in polycarbonate (Goodfellow, UK), cp. Chap. 5, Sect. 3.4, (Figs. 6.30, 6.31). 

Ad 4. Dielectric-gravimetric-oscillometric measurements of sorption equilibria of methane in (swelling) polycarbonate. 

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