Activation gravimetric measurements

Conductivity, in water activity measurement, 67-70 Confocal laser scanning microscopy to characterize lipid crystals, 575-579 description of, 575, 577 Conjugated dienes and trienes, determination of, 515-517 Conjugated linoleic acid (CLA), fatty acid analysis, 437-438, 445-446 Convection oven, gravimetric measurement of water, 7-8, 10-11... 

Fig. 18. Self-diffusion coefficients of benzene in NaX at 458 K PFG NMR, O (97) and (92) (JENS, A (13) deduced from NMR lineshape analysis, (10). Comparison with nonequilibrium measurements T, sorption uptake with piezometric control (93) , zero-length column method (96) o, frequency-response and single-step frequency-response technique (98). The region of the results of gravimetric measurements with different specimens (92) is indicated by the hatched areas. Asterisked symbols represent data obtained by extrapolation from lower temperatures with an activation energy confirmed by NMR measurements.

IR spectroscopy was mainly used to characterize the sorbed species. The zeolite powder was pressed into self supporting wafers and analyzed in situ during all treatments (i.e., activation, sorption, reaction) by means of transmission absorption IR spectroscopy using a BRUKER IPS 88 FTIR spectrometer (resolution 4 cm" ). For the sorption experiments, an IR cell equipped with IR transparent windows which could be evacuated to pressures below 10" mbar was used [11]. The activated zeolite wafer was contacted with a constant partial pressure (0.001 mbar) of the adsorbate at 308 K until adsorption-desorption equilibrium was reached (which was monitored by time resolved IR spectroscopy). For the coadsorption experiments, the catalysts were equilibrated with 0.001 mbar of both adsorbates admitted in sequentional order. The spectra were normalized for the sample thickness by comparing the intensities of the absorption bands of the adsorbate with the integral intensity of the lattice vibration bands of the zeolite between 2090 and 1740 cm". The surface coverage was quantified by calibration with gravimetric measurements (under conditions identical to the IR spectroscopic experiments). 

For gravimetric measurements, the powders were pressed (c.a.400mg), activated and sulfated in the same conditions as for the IR study. A McBain thermobalance was used. The temperature was increased from r.t. to 450°C (0.5°C.min ) and then kept at 450°C until a plateau was obtained for the weight. 

In a study of the surface area of an Mn02 catalyst [8], ESR measurement of the surface area based on adsorption of DPPH indicated a surface area of 46 m /g, whereas ESR measurement of active adsorbed oxygen indicated a surface area of 42 m /g. These two values compared favorably with the results from the usual BET method, which gave a surface area of 61 m /g. The BET method consists of volumetric or gravimetric measurements of the quantity of gas that will completely cover the surface of the solid with an adsorbed layer. 

Braeuer, P. Salem, M., and Hatting, P Calculation of single adsorption isotherms from gravimetrically measured binary gas mixture adsorption isotherms on activated carbon at high pressures, Sep. Purif Technol., 12(3), 255-263(1997). 

Buss, E., Gravimetric measurement of binary gas adsorption equilibria of methane-carbon dioxide mixtures on activated carbon. Gas Sep. Purif, 9(3), 189-198 (1995). 

Once V is known, m = m (p, T, m ) can be calculated from Eq. (1.5). Such a situation is very common for gravimetric measurements of helium gas adsorption equilibria. An example is sketched in Figure 1.8 showing gravimetric adsorption data of activated carbon Norit R1 Extra exerted to He (5.0). As can be seen, for high pressures the reduced mass data ( ) easily can be linearly correlated, i. e. adsorption of helium has reached a state of saturation. Hence, Vhc can be determined via Eq. (1.6) and also the mass of helium adsorbed initially at low gas pressures can be calculated from Eq. (1.5) as ... 

Differences in the volumetrically and gravimetrically measured Gibbs excess masses depicted in Fig. 1.24 are partly due to different activation procedures of the zeolite samples used in the experiments. This of course should be avoided, but as a matter of fact happened in our experiments and therefore is brought to reader s attention. 

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.

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. 

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... 

On mild steel, the inhibiting activity and passivation characteristics exerted by carboxy and phosphono groups were determined by gravimetric measurements and electrochemical impedance spectroscopy. The organic chemicals under investigation included nitrilo(triacetic acid) (NTA), phos-phonomethyl-iminodi(acetic acid) (MPIDA), di(phosphonomethyl)glycine (DMPG), and nitrilotri(methyl-phosphonic acid) (ATMP). 

Isotherms and isobars are drawn from thermal gravimetric measurements using a Me Bain balance well suitable to impose water vapor pressures controlled by means of a "cold point" (ref. 16). The curves are constructed in graduated steps by increasing (or decreasing) pressure or temperature in small successive increments. Before each experiment, the zeolite is activated in situ at 400°C at 10 1 Pa. For adsorption the initial state is the activated state at 350 C, either at a pressure p under isobaric conditions, or at a pressure of 101 Pa under isothermal conditions. The final state is a state close to saturation. For desorption measurements the previous defined initial and final states are reverse. 

Permeability is so low that, in sorption experiments, large pressure differences and membranes of small thickness have to be employed. These restraints do not apply to condensable vapours or highly permeant gas. In the last case, relaxation processes in the polymer matrix cause changes in the transport behaviour and make the permeability time-dependent. In contrast, only vapours of sufficiently high solubiUty are suitable for the gravimetric measurements used in the sorption experiments. Consequently, there are only few systems for which both sorption and permeation results have been reported. Low et al. conducted the two types of moisture transport experiments on a polyamide 6/elay nanocomposite. The authors found that the activation energy of moisture permeation obtained from the sorption experiment is lower than that derived from the permeation measurement. They concluded that the interaction and contribution of the diffusion and solubility parameters show complex transport behaviour in these nanocomposite films. 

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