Adsorption isotherm measurement

It is therefore of the utmost importance to ensure that the standard isotherm is based on a solid known to be free of pores, and especially of micropores. Unfortunately, it is not easy to establish the complete absence of porosity in the solids used in adsorption isotherm measurement the unsuspected presence of such pores may well account for some, at least, of the discrepancies between different published versions of the standard isotherm for a given adsorptive. 

The development of microporosity during steam activation was examined by Burchell et al [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the Nj adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore sfructure characterization. 

Adsorption Isotherm Measurements and Site-Selective Thermodynamics. For heterogeneous surfaces like CSPs, the adsorption isotherms are usually composite isotherms and often a Bi-Langmuir model (Equation 1.15) describes reasonably well the adsorption behavior [54]. 

The surface of silica turns hydrophobic on treatment with organo-silicon chlorides. Water vapor adsorption isotherms measured by Stober (219) showed a very marked decrease in reversible adsorption. Less than 0.3 primary adsorption centers per 100 A remained in the surface after covering with the organosiloxane layer. Similar effects were observed in the adsorption of ammonia. About 2.2 silanol groups per 100 A had not reacted with the trimethylsilyl chloride. Nevertheless, the greater part of these had become unaccessible for water vapor. Apparently, they were hidden underneath a trimethylsilyl umbrella. ... 

At a given temperature adsorption isotherms measure the number of adsorbed molecules as a function of pressure for the fluid that is in contact with the zeolite. The simplest form is the Langmuir isotherm which treats the zeolite as a collection of equivalent adsorption sites in the absence of adsorbate-adsorbate... 

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based... 

Heats of adsorption are usually determined in two ways either by direct calorimetric determination at a chosen temperature, or by calculating the isosteric heats from adsorption isotherms measured at different temperatures and using the Clausius-Clapeyron equation. Thus, isosteric heats of adsorption are calculated from the... 

Figure 3 shows the water-vapor adsorption isotherms measured on the silane-treated fibers along with those obtained on the untreated fibers. A complete discussion of the water adsorption isotherms of the untreated fibers has already been reported [8]. Here, two new features are immediately evident. First, the presence of the silane overlayer has greatly enhanced the water adsorption capacity of the fibers, and, second, the silane-treated fibers that contain 4% and 6% B,0, adsorb significantly more water than the 0% B20, fibers. It is important to note that these data have been normalized to the specific surface areas of the... 

Hageman et al. [3.13] calculated the absorption isotherms for recombinant bovine somatotropin (rbSt) and found 5-8 g of water in 100 g of protein, which was not only on the surface but also inside the protein molecule. Costantino et al. [3.72] estimated the water monolayer M0 (g/100 g dry protein) for various pharmaceutical proteins and for their combination with 50 wt% trehalose or mannitol as excipient. They compared three methods of calculating MQ (1) theoretical (th) from the strongly water binding residues, (2) from conventional adsorption isotherm measurements (ai) and (3) from gravimetric sorption analysis (gsa) performed with a microbalance in a humidity-controlled atmosphere. Table 3.5 summarizes the results for three proteins. The methods described can be helpful for evaluating RM data in protein formulations. 

Figure 4.37 N2 adsorption isotherm (measured at 77 K) of the catalyst before and after the first run.

Second, there are problems in using concentrations, because the true driving force for the adsorption reaction is chemical potential, which is related to activity. For the dissolved component this means that adsorption isotherms measured in one solution do not necessarily apply to other solutions a point commonly overlooked by geochemists, particularly with regard to organic compounds. Also, variation in solution composition can result in the introduction of other ions or compounds that have an affinity for the surface. These ions may severely alter the adsorption of the component of interest. 

Fig. 7. N2 adsorption isotherms, measured at 25° C, for Ag/LSX (a) after drying at room temperature followed by vacuum dehydration at 450° C, (b) after drying at room temperature followed by vacuum dehydration at 350° C, (c) after drying in air at 100° C followed by vacuum dehydration at 350°C and (d) after drying in air at 100°C in air, followed by heat treatment in air at 450° C and, finally, vacuum dehydration at 450° C (Hutson and Yang, 2000).

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

The extraction plant is equipped with two UV detectors, one after the extractor and the other after separator to provide a continuous concentration measurement of the effluents. Moreover it is possible to take samples of the carbon dioxide at the same places for analysis and calibration of the UV detectors (not shown in figure 6) the same way as described for adsorption isotherms measurement. 

The only information needed to predict the mixture surfactant concentration to attain a specified adsorption level is the pure component adsorption isotherms measured at the same experimental conditions as the mixture isotherms. These isotherms are needed to obtain the pure component standard states. 

In this paper we present a new characterisation method for porous carbonaceous materials. It is based on a theoretical treatment of adsorption isotherms measured in wide temperature (303 to 383 K) and pressure ranges (0 to 10000 kPa) and for different adsorbates (N2, CH4, Ar, C3H8 and n-C4Hio). The theoretical treatment relies on the Integral Adsorption Equation concept. We developed a local adsorption isotherm model based on the extension of the Redlich-Kwong equation of state to surface phenomena and we improved it to take into account the multilayer formation. The pore size distribution fimction is assumed to be a bi-modal gaussian. By a minimisation procedure, it is possible to determine the bi-modal pore size distribution function witch can be used for purely characterisation purposes or to predict adsorption isotherms. 

We used excess adsorption isotherms for N2, CH4, Ar, CgHg and n-C4Hio on an activated carbon (F30-470 from Chemviron Carbon) at 303, 323, 343, 363 and 383 K. These data have been measured using a magnetic suspension balance specially instrumented to perform simultaneous high pressure and high temperature adsorption isotherms measurements. The experimental apparatus and the data are presented elsewhere [11,12,13]. 

Nitrogen adsorption isotherms measurements were performed at 77 K on a Micrometries ASAP 2010M and on a Fisons Sorptomatic 1990. Each isotherm point is acquired when the pressure variation, within a fixed time, is lower than a fixed pressure deviation. 

Fig. 2 highlights the isotopic effect for H2 and D2 by showing the adsorption isotherms measured under both sub- and supercritical conditions (Tcrii(H2>=33.19 K, Tcrit(D2)=38.34 K). The lower part illustrates the evaluation of the experimental data according to the Langmuir equation. It becomes clear that the Langmuir fit works very well for nitrogen, shows an acceptable performance for the hydrogens, but is not applicable to helium. The variation of the results is shown in Table 1. 

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