Characteristic isotherm

It was noted earlier (p. 115) that the upward swing in the Type IV isotherm characteristic of capillary condensation not infrequently commences in the region prior to the lower closure point of the hysteresis loop. This feature can be detected by means of an a,-plot or a comparison plot (p. 100). Thus Fig. 3.25(a) shows the nitrogen isotherm and Fig. 3.25(h) the a,-plot for a particular silica gel the isotherm is clearly of Type IV and the closure point is situated around 0 4p° the a,-plot shows an upward swing commencing at a = 0-73, corresponding to relative pressures of 013 and therefore well below the closure point. 

The 2eohte and the resin adsorbents show different adsorption isotherm characteristics, particularly at higher concentration (51). The resin adsorbent isotherm is slightly concave upward, whereas the 2eohte isotherm is linear, or even slightly concave downward. Resins, therefore, have an advantage in a UOP Sarex operation that involves high feed-soHds concentration. 

It is not surprising, therefore, that this dynamic technique was chosen to provide information on chiral interactions in compressed films. Given that these stereochemically differentiated systems may have dramatically different HI A isotherm characteristics, and hence different packing arrangements, it is plausible that their flow properties are stereochemically differentiated as well. 

NaY yields a compietely reversible type I isotherm, characteristic of micropore filling common in many zeolites. However, USY-B and DAY yield an isotherm close to type IV. Similar differences in adsorption isotherms were observed for n-hexane, cyclohexane, n-pentane and benzene. Furthermore, many of the isotherms measured on DAY zeolites showed hysteresis loops (Figure 6). 

Sulfated zirconia is a good example of a structural Lewis acid which has been chemically treated to enhance acidity. It has been extensively studied as a solid acid catalyst for vapour phase reactions and we1112 and others14 have found that a mesoporous version of this material is a particularly effective catalyst for liquid phase Friedel-Crafts alkylation reactions and to a lesser extent Friedel-Crafts benzoylations. The commercial (MEL Chemicals Ltd) material SZ999/1 shows a nitrogen isotherm characteristic of a mesoporous solid (surface area 162 m2g, pore volume 0.22 cm3g )- Whereas microporous and mesoporous materials are capable of rapidly catalysing the alkylation of benzene with various alkenes (Table 1), on reuse only the mesoporous... 

The Nj adsorption-desorption measurements (Fig. 2) showed a type IV isotherm, characteristic of mesoporous materials with BET surface areas of 303 and 608m. g for SBAla and SBAlb respectively, and narrow pore size distributions centered at 3.7 and 5.9nm. 

The isotherms of such a surface approach the Langmuir isotherm characteristic of the weakest binding part of the surface Km/2 at large pressures. At low pressures, (A) < C Xo < Kmj the sorption takes place nearly linearly on the most strongly binding sites Kq, Over a considerable range of pressures of A between Km > (A) > Xo, the behavior of the... 

The samples derived from white gels show an irreversible Type IV isotherms characteristic of MSA-type materials, while, those derived from transparent gels are characterized by a reversible Type I isotherms typical of microporous ERS-8-type materials. Irreversible Type I + IV isotherms are observed for the samples with intermediate values of molar ratios. 

In this study, activated carbon fibers (ACFs) deposited by copper metal were prepared by electroplating technique to remove nitric oxide (NO). The surface properties of ACFs were determined by FT-IR and XPS analyses. N2/77K adsorption isotherm characteristics, including the specific surface area, micropore volume were investigated by BET and t-plot methods respectively. And, NO removal efficiency was confirmed by gas chromatographic technique. From the experimental results, the copper metal supported on ACFs appeared to be an increase of the NO removal and a decrease of the NO adsorption efficiency reduction rate, in spite of decreasing the BET S specific surface area, micropore volume, and micro-porosity of the ACFs. Consequently, the Cu content in ACFs played an important role in improving the NO removal, which was probably due to the catalytic reactions of C-NO-Cu. 

Figure 7.1 Migration and shape evolution of an injection profile as predicted by the ideal model in the case of a convex upward isotherm. Characteristics associated with concentrations and shock formation, velocity associated with a concentration, propagation of a band (i.e., of a concentration signal), and formation of a shock (see text), (a) Injection of a pulse with a Gaussian profile. Reproduced with permission from B. Lin, S. Golshan-Shirazi, Z. Ma and G. Guiochon, Anal. Chem., 60 (1988) 2647 (Fig. 1), 1988 American Chemical Society, (b) Injection of a continuous hnear ramp. Reproduced with permission from T. Ahmad, F. Gritti, B. Lin and G. Guiochon, Anal. Chem., 76 (2004) 977 (Fig. 2). 2004 American Chemical Society.

From Fig. 3, it appears that the specific surface area of the disordered DWM materials remains very high till 800°C. Beyond this tenperature, for sample B for instance, its value decreases from 800 to 20 mVg if calcination ten rature is raised from 800 to 1000°C. Whatever the hydrothermal treatment conditions of the synthesis are, the thermal resistance of the materials is similar. Even at 1000°C, the recovered materials exhibit a type IV isotherm, characteristic of mesoporous compounds. A part of mesoporosity is thus maintained with quite a narrow pore size distribution, but the maximum adsorbed volume is sharply reduced. This is in accordance with the very broken appearance of the particles observed by SEM. Thermal stability of these disordered materials is, however, very superior to MCM-41, whose structure does not resist beyond 600°C. This behaviour can be related to the different preparation method that affords compounds with a different structure and also thicker walls. 

From nitrogen adsorption-desorption measurements (Fig. 6), we can observe that all the samples exhibit a type IV isotherm characteristic of mesoporous compoimds. However, there is an evolution of hysteresis loop from type H2 (lUPAC), usually encountered for disordered structures, towards HI commonly observed for MCM-41 when the weight percentage of template decreases below 30 wt.%. For the lower concentrations, the adsorption branch of the isotherm becomes much steeper and the pore size distributions (PSD) become much narrower (see inserts) and centred on about 4.4 nm. Specific surfece area also increases... 

N2-sorption The MCM-41 starting material and the two Pt/MCM-41 samples washed with ethanol show the Na adsorption desorption isotherm characteristic of the MCM-41 materials with a steep N2 uptake at P/P°=0.35 (lUPAC type IV isotherm), whereas the shape of the N2 sorption isotherm of the sample washed with water solely is characteristic of microporous materials. The determination of the pore diameter using the DFT calculation (the pore diameter of the sample... 

A fast gelation was observed for all the materials prepared at TPAOH/S102 = 0.09 producing opalescent gels. After calcination, they were characterised by irreversible Type IV + (1) isotherms, characteristic of mesoporous MSA-type materials... 

Also, operationally, dead volume issues can make it much more difficult to change from one reaction chemistry to another due to long clean-out times. In addition, it is possible to have particles drop out and ultimately plug the system in areas with low flow relative to the bulk of the stream. Hickman and Sobeck also mention thermal requirements for a system to achieve the isothermal characteristics that they needed. Operationally, reduced heat transfer limits the types of reactions that can be efficienfly controlled and also hot spots can lead to an increase in alternative reaction pathways with the formation of impurities. Finally, another problem that they encountered was that given the high surface to volume ratio of the equipment, the materials of constmction caused a problem for some chemistries that they wanted to study. It is not dear if the material of construction issue would be a problem for routine operation. Again, these are not really process analytical problems but they can... 

In the various lab reactors, some are suitable to get the dynamics data, some are not. This is according to the following three requirements. Firstly, the most important reaction condition is its isothermal characteristic because the effect of temperature on reaction rate is exponential, especially to complicated reaction. For a few simple reactions, the exact temperature distribution sometimes can be used as supplementary information on the dynamics, but that is a special case of the individual secondly, the tangibility and uniformity of the settle-time thirdly, whether the sampling and analysis of products is easy or not. The above three points decide the quality of the reactor, which also further decide the precision of dynamic model. 

Figure 1.8 shows the shape of the sorption isotherm characteristic of many dry food products. If the partial pressure of the external atmosphere Pw is nearly zero, then the equilibrium moisture inside the dry product will also be almost zero. Section A of the curve represents a region in which the monomolecular layers are formed, although there may be multimole-cular layers in some places toward the end of A. 

The second type of the blow-off mechanism occurs when the heated reaction products mix with the unbumed stream so fast that there is no time for ignition before the temperature and the radical concentrations drop lower than some critical value. This blow-off mechanism was proposed in [16]. The balance between the chemical time scale determined by the laminar flame velocity and the physical time scale determined by the isothermal characteristics of the flow was used for the correlation equations in [16]. 

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