Adsorption processes gravimetric measurement

The most common method used for the determination of surface area and pore size distribution is physical gas adsorption (also see 1.4.1). Nitrogen, krypton, and argon are some of the typically used adsorptives. The amount of gas adsorbed is generally determined by a volumetric technique. A gravimetric technique may be used if changes in the mass of the adsorbent itself need to be measured at the same time. The nature of the adsorption process and the shape of the equilibrium adsorption isotherm depend on the nature of the solid and its internal structure. The Brunauer-Emmett-Teller (BET) method is generally used for the analysis of the surface area based on monolayer coverage, and the Kelvin equation is used for calculation of pore size distribution. 

Until relatively recently, the fact that an experimental isotherm necessarily contained composite information concerning the adsorption of the two components of a binary solution was considered to be a major problem. For a rigorous interpretation it was felt necessary to process the data to obtain the so-called individual adsorption isotherm or separate adsorption isotherm of each component. However, this is not at all straightforward and requires the introduction of a number of assumptions relating to the structure of the adsorbed layer. The main problem is of course to know the composition of the adsorbed layer. One assumption often used in the case of volatile components is that introduced by Williams (1913) the solid will adsorb the same amount of each component from the vapour in equilibrium with the solution as from the solution itself. This of course implies that the adsorbed layer has the same composition at the liquid-solid and gas-solid interfaces and it requires numerous gravimetric measurements from the vapour... 

Most adsorption data were collected by volumetric method until microbalance of high sensitivity appeared few years ago. It can hardly say which method is superior to the other, and both methods need the value of the skeleton volume of sample adsorbent. This volume has to be subtracted from the whole volume of the sample container to obtain the volume of void space, which is used for the calculation of the amount adsorbed. The skeleton volume of sample adsorbent was directly used in the calculation of buoyancy correction in gravimetric method. This volume was usually determined by helium assuming the amount of helium adsorbed was negligible. If, however, helium adsorption cannot be omitted, error would yield in the skeleton volume and, finally, in the calculated amount adsorbed. However, the effect of helium adsorption would be much less for volumetric method if the skeleton volume is considerably less than the volume of void space, but the volume of void space cannot affect buoyancy correction. In this respect, helium adsorption would result in less consequence on volumetric method especially when the skeleton volume was determined at room temperature and pressures less than IS MPa. The skeleton volume (or density) was taken for a parameter in modeling process in some gravimetric measurements. However, the true value of skeleton volume (or density) can hardly be more reliable basing on a fitted parameter than on a measured value. Therefore, one method of measurement cannot expel the other up to now, and the consequence of helium adsorption in the measured amount adsorbed should be estimated appropriately. 

Figure 1.10. Adsorption process of helium (He(5.0)) on AC Norit R1 Extra at T = 298.17 K during 58 hours measured gravimetrically (magnetic suspension balance, Rubotherm). The interruption of measurement data at about 24 h is due to limitations in data storage capacity, i. e. an overflow of data, which made a change of the data storage device necessary.

Gas adsorption processes may last for seconds, hours or - sometimes -even days. Therefore one never can be sure whether mermodynamic equilibrium in a volumetric experiment has been reahzed. Hence the time which should elapse between opening the expansion valve and reading of instruments, especially thermometer and manometer has to be chosen according to experience or accompanying gravimetric measurements which - contrary to volumetry / manometry - also provide information on the kinetics or the sorption process, cp. Chap. 3. 

Figure 3.28. Experimental installation of an instrument for gravimetric measurements of adsorption processes from the liquid phase using a magnetic suspension balance. The dashed region within the adsorption vessel below the balance indicates the volume filled with liquid [3.46, 3.47]. Rubotherm GmbH, Bochum, Germany.

Figure 3.29. Gravimetric measurements of the adsorption processes of pure liquid water (lower curve) and of dye Levafix Brilliant Red E-4BA and water from aqueous solution (upper curve) in activated caibon F 300 (Chemviron Carbon), [3.47], Rubotherm GmbH, Bochum, Germany.

Microbalances with alphanumerical display and electronic data recording systems allow one to observe the approach to equilibrium for gas adsorption processes in porous sorbent samples. Typical relaxation times can be one or several seconds, minutes, hours, and -sometimes - even days, cp. helium adsorption data Sect. 2 of Chap. 1. Hence gravimetric measurements do allow one to check whether an adsorption system actually has reached its equilibrium state, i. e. these measurements deliver in principle also information concerning the kinetics of the adsorption process, represented for example by (phenomenological) diffusion coefficients, cp. Sect. 2.3 and Sect. 4.4 and [3.27, 3.48). 

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. 

Abstract The physical principles and basic experimental techniques of impedance spectroscopy, i. e. static or frequency dependent dielectric permittivity measurements of sorbent/sorbate systems are given. These measurements can be used to characterize the state of a sorbent material in industrial adsorption processes. Combined with either manometric or gravimetric measurements of adsorption equilibria leading to calibration curves, permittivity measurements also allow fairly simple and quick measurements of gas adsorption equilibria. Kinetic processes and catalytic reactions inside a sorbent/sorbate system also can be observed. Pros and cons of dielectric measurements are discussed. List of Symbols. References. 

Dielectric measurements of gas adsorption systems can be performed fairly quickly, typically within a few seconds [6.3]. Hence the kinetics of adsorption processes being slow on this time scale can be observed. Indeed these processes are sometimes invisible to purely manometric or even gravimetric measurements. As examples we mention internal diffusion, reorientation or catalytically induced chemical reaction processes of admolecules within a sorbent material. The mass of the adsorbed phase normally is constant during processes of this type, whereas the dipole moment of the admolecules and hence their polarization changes, cp. Sect. 3.2. 

Combined dielectric gravimetric measurements of adsorption equilibria Nitrogen (Na, 5.0), methane (CH4, 5.5) and carbon monoxide (CO, 3.7) on pellets AC-20 (Engelhard Process Chemicals, Nienburg) (Fig. 6.28). 

X-ray powder diffraction (XRPD), thermo gravimetric (TGA) analysis, solid-state nuclear magnetic resonance (NMR), and measurements of adsorption isotherms are key methods for characterizing zeolite-like behavior. However, a simple proof for observing structural changes during the sorption processes is XRPD. 

Automation of gravimetric sorption instruments is not an easy matter and for precision measurements permanent supervision of the adsorption / desorption process by an experienced co-worker is strongly recommended. 

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