Gas adsorption manometry

This method is based on the measurement of the gas pressure in a calibrated, constant volume, at a known temperature. 

The technique of gas adsorption manometry is now probably the most widely used it is simple and effective since the pressure transducer provides all the information required to determine the adsorption isotherm. Thus, the pressure and temperature of each dose of gas are measured and the gas is allowed to enter the adsorption bulb. After adsorption equilibrium has been established, the amount adsorbed is calculated from the change in pressure. The most critical features of adsorption manometry are summarized in the following checklist, with more detailed comments given in Section 3.4. 

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Simple gas adsorption manometry. A simple modem set-up with three valves and a capacitance pressure transducer is shown in Figure 3.2. The dosing volume is within the central cross, i.e. in the connecting tubes between the valves and the pressure transducer. 

Gas adsorption manometry with reservoir and double pressure measurement. The corresponding set-up is shown in Figure 3.3. One pressure transducer is used to determine the amount of adsorptive remaining in the reservoir, while the second is used to determine the adsorption equilibrium pressure and also the amount of unadsorbed gas in the central cross and adsorption bulb. This arrangement gives an integral measure-... 

Figure 3 J. Gas adsorption manometry with reservoir and double pressure measurement.

Differential gas adsorption manometry. In the earliest form of this technique (Schlosser, 1959), represented in Figure 3.4, two carefully matched capillaries feed two bulbs (adsorption and reference) from a common reservoir of adsorptive. The pressure difference between the two sides provides a direct measurement of the amount adsorbed on the sample side, provided the gas flow rate through the two capillaries is the same this is no longer true when the difference between the two downstream pressures (on sample side and reference side) is too great For this reason, this technique is limited to downstream pressures lower than, say, 50 mbar. This allows one to determine surface areas by adsorbing Ar at 77 K, but not N2. 

A simplified form of differential gas adsorption manometry (Haul and Dxlmbgen, 1960, 1963) is represented in Figure 3.5. Nitrogen under atmospheric pressure is introduced into both sides at room temperature. The intermediate valve is closed and the two bulbs are immersed in liquid nitrogen. When equilibrium is reached, the pressure difference gives the amount adsorbed. No vacuum is needed (only flushing with... 

Figure 3.4. Differential gas adsorption manometry with capillaries (valves for initial evacuation of equipment or filling of reservoir not represented) (after Schlosser, 1959).

Figure 3.5. Differential gas adsorption manometry from atmospheric pressure (after Haul and Dflmbgen, 1960).

At the other extreme, a sophisticated form of differential gas adsorption manometry (Camp and Stanley, 1991 Webb, 1992) is shown in Figure 3.6. It can be considered as combining the best features of the equipment in Figures 3.3 and 3.5. The differential assembly allows one to eliminate the dead volume correction, provided that the dead volume of the reference side is properly adjusted by means of the glass beads. In addition, an integral measurement is possible by the use of the two reservoirs and an extra differential manometer. 

Figure 3.6. Differential gas adsorption manometry with double reservoir and triple pressure mea ment (after Camp and Stanley, 1991). 

An advantage of gas flow techniques is that they can be used for two veiy different types of procedures, i.e. either the discontinuous point-by-point procedure with a non-adsorbable carrier gas (cf. Section 3.3.1) or a continuous adsorption procedure (Section 3.3.2). The limitation is that the amount adsorbed is evaluated by integration of the gas flow over a period which may range from five minutes to several hours. Therefore, great stability and accuracy of the flowmeter are essential. The checklist given above for gas adsorption manometry is equally applicable to gas flow techniques. 

Characterising the porous structure of Egyptian mortars using thermoporometry, mercury intrusion porometry and gas adsorption manometry... 

The cryogenic adsorption system was specially developed to measure adsorption isotherms of H2 and D2. This system is equipped with a closed helium cycle two-stage Gifford McMahon refrigerator to operate under cryogenic conditions. The adsorption temperature can be kept constant within 0.03 K at 20 K. Adsorption isotherms are obtained by gas adsorption manometry. This method is based on the measurement of the gas pressure in a calibrated, constant volume, at a known temperature. The dead space volume was calculated from a helium calibration measurement at the temperature of interest. Thermal transpiration effect was calibrated according to the work by Takaishi and Sensui [41]. 

In the case of differential or twin arrangements of adsorption manometry (cf. Figures 3.4-3.6), the dead volume determination is not required, but the volume equalization and the symmetry of the set-up are essential. The volume equalization is usually obtained with glass beads on the reference side and sometimes also with adjustable bellows or a piston. The check or adjustment is normally carried out at ambient temperature the introduction of an identical amount of gas on both sides must result in a zero pressure difference between them. 

Since the data provided by the above type of immersion calorimetry experiment can be directly related to the results obtained by gas adsorption calorimetry the question arises which type of experiment is to be preferred In fact, immersion calorimetry, although time-consuming, has certain advantages because of the difficulty in handling easily condensable vapours in adsorption manometry. 

Helium is often used in adsorption manometry for the determination of the dead space volume (see Chapter 3), but this procedure is based on the presupposition that the gas is not adsorbed at ambient temperature and that it does not penetrate into regions of the adsorbent structure which are inaccessible to the adsorptive molecules. In fact, with some microporous adsorbents, significant amounts of helium adsorption can be detected at temperatures well above the normal boiling point (4.2 K). For this reason, the apparent density (or so-called true density ) determined by helium pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational temperature and pressure (Fulconis, 1996). 

Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar... 

We report in Figures 1 and 2, with the same presentation and scale, the experimental results obtained by adsorption manometry and gravimetry, for the three gases and the three temperatures. Incidentally, this is a good example where the formerly used representation in adsorbed volume or adsorbed mass is clearly less convenient than the more universal representation in adsorbed amount (or still more precisely in the present case, in surface excess amount ). The shapes of the adsorption isotherms, with very different curvatures from one gas to the other, are a clear indication of the increased gas/solid interaction as one passes from argon to nitrogen and then to carbon dioxide, for which the isotherms are practically of type I. 

Adsorption manometry allows an easy outgassing in a separate unit and lends itself to multiple sample operation, ie to numerous experiments, but requests a satisfactory knowledge about the non-ideality of the gas used. 

Adsorption gravimetry lends itself to in-situ outgassing (up to 500°C with our equipment) but can only accommodate one sample at a time. A major advantage only obtained with the special assembly making use of a sinker is the permanent measurement of the density of the gas phase it allows not only to make at any time a correct buoyancy correction but it also allows to provide the users of adsorption manometry equipment with the data they need to make a safe void volume correction. 

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. 

As we here are mainly interested in adsorption measurement techniques for industrial purposes, i. e. at elevated pressures (and temperatures), we restrict this chapter to volumetric instruments which on principle can do this for pure sorptive gases (N = 1), Sect. 2. Thermovolumetric measurements, i. e. volumetric/manometric measurements at high temperatures (300 K - 700 K) are considered in Sect. 3. In Section 4 volumetric-chromatographic measurements for multi-component gases (N>1), are considered as mixture gas adsorption is becoming more and more important for a growing number of industrial gas separation processes. In Section 5 we discuss combined volumetric-calorimetric measurements performed in a gas sensor calorimeter (GSC). Finally pros and cons of volumetry/manometry will be discussed in Sect 6, and a hst of symbols. Sect. 7, and references will be given at the end of the chapter. 

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. 

This chapter is organized as follows In Sect. 2 we consider pure gas adsorption measurements by both two beam and single beam balances. Section 3 is devoted to thermogravimetry. In Section 4 multicomponent gas adsorption equilibria are discussed. Finally in Sect. 5 pros and cons of gravimetry especially compared to volumetry/manometry are elucidated. A list of symbols and abbreviations used is given followed by references dted. 

To measure adsorption a certain amount of gas of mass (m ) is prepared in the storage vessel and the adsorption chamber is evacuated. Upon opening the expansion valve, the gas expands to the adsorption chamber where it is partly adsorbed on the (external and internal) surface of the sorbent material. This process may last milliseconds, minutes, hours or even several days - as in case of helium on activated carbon (Norit Rl) [2.8]. After thermod)mainic equilibrium, i. e. constancy of pressure (p) and temperature (T) inside the vessels has been realized, these data can be taken as a basis to calculate the mass of the gas adsorbed on the sorbent (m ). That is, volumetric adsorption experiments mainly result in pressure measurements. Hence the name Manometry for this method should be used [2.2]. 

Contrary to manometry/volumetry, very high and very low pressures of the sorptive gas do not pose a serious problem in gravimetric adsorption measurements. Ibis is a due to the fact that in gravimetry, the adsorbed mass is determined by its weight, i. e. a quantity which in principle is physically independent of the gas pressure. 

Abstract This chapter is devoted to the study of coadsorption of gases in nanoporous solids by using the differential calorimetry. In the first part, the thermodynamic principles of adsorption of gases are recalled. Some of them have already presented in chapter one. However a special attention has been paid here to the determination of the adsorption enthalpies and entropies and we focused on the selective adsorption of binary mixtures. Then the specific experimental technique based on the combination of differential calorimetry with manometry and gas phase chromatography or mass spectrometry is shown in details. In the last part, the thermodynamic concepts on coadsorption are illustrated with experimental results taken from studies on gas separation by selective adsorption in mlcroporous solids. 

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

In the manometry technique coupled with CPG or MS, the determination of the amount of each component i adsorbed (n ) consists to perform a mass balance on the gas phase from the total pressure and the mole fraction of component i in the gas phase, measured before and after adsorption [9]. As the system is closed, we consider that the amount of matter leaving the gas phase is equal to that one adsorbing on the solid. 

Fig. 7.13 Equivalent thermodynamic system for measuring the adsorbed amoimt by manometry. Vo dead volume of reactor (known from cedibration with helium) p and p pressure of gas before and after adsorption (measured) = volume of gas at equilibrium (unknown)...

The calorimetry technique coupled with manometry shown in Fig. 7.12 allows the measure of the heat emanated by the adsorption of a given amount of gas on the solid. In thermodynamics it is essential to know at what function of state corresponds this adsorption heat. This depends on the calorimetric technique and experimental... 

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