Adsorption moisture isotherms

Figure 9. Moisture adsorption isotherms of enzyme-treated and nontreated peanut...

The dynamic moisture adsorption isotherm is another useful tool, exemplified by the water sorption/desorption of DMP 728 zwitterion as shown in Figure 16. DSC may not be helpful unless the technique is modified by punching a tiny hole in the pneumatic sample cap, from which the moisture or solvent may escape with a narrow thermal peak. Another method is to place the sample in silicon oil on the sample pan. The thermal effect of solvent boiling out is observed in the thermograph. 

Typical plots of the degradation rate as a function of relative humidity are shown In Figs. 2 and 3. These plots strongly resemble the moisture adsorption isotherm (Fig. 4) for the hand-sheets used In this investigation especially for the 80° and 90°C plots. This suggests a linear relationship between the degradation rate and the moisture content. 

Physical Changes that Occur when Hardwoods Are Treated with Dilute NaOH. Adsorption Isotherms. Because of the inability of very dilute NaOH to bring about phase transformations within cellulose, it would be expected that the moisture adsorption isotherm of a hardwood treated as described would be similar to that of the untreated wood. This was found to be essentially true up to at least 90% relative humidity (RH). Using a previously described technique, the equilibrium moisture content (EMC) of wood substance at essentially 100% RH was measured (11, 38) a very marked effect of the treatment was found. 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

As water swells cellulose in an intercrystalline way (i.e. only within the non-crystalline amorphous regions), a relationship would be expected between accessibility and moisture uptake, and this is indeed found (Figure 5.5). Refining causes cellulosic fibres to swell and it would therefore be expected to cause a change in the water adsorption isotherm. This is indeed observed (Figure 5.6). 

The aim of this paper is to study the behavior of a earbonaceous material in PAH abatement in hot gas cleaning in real conditions, e.g. power stations where moisture and CO2 are present. Therefore, the Phe adsorption isotherms eontaining different steam and CO2 concentrations were obtained and fitted to different theoretical models. 

The BET surface areas of the zeolite samples were determined by N2 adsorption-desorption at -196 C in a Micromeritics ASAP 2010 equipment. Prior to the determination of the adsorption isotherm, the calcined sample (0.5 g) was outgassed at 400 C under a residual pressure of 1 Pa in order to remove moisture. The adsorption data were treated with the full BET equation. The t-plot method using the universal t-curve was applied in order to obtain an estimation of the micropore volume, microporous surface and external surface area [7]. 

Moisture sorption of microcrystalline cellulose has been studied extensively. Fig. 6A includes the sorption and desorption studies for microcrystalline cellulose. The inserts are plots of moisture content versus time, which approximately represent the kinetics of sorption and desorption at each humidity. The equilibrium adsorption isotherm (Type II) has been fit to the BET equation (C = 16.48, = 0.033 g/g solid) and this curve is... 

Fig. 6 (A) Moisture sorption isotherm for microcrystalline cellulose adsorption and desorption. (Adapted from Ref (B) BET and GAB equations describing the adsorption branch.

Figure 29 shows curves of H/M ( = h/m) vs. H = lOOh) calculated from experimental sorption data (also plotted) on wood and bark at 25 °C, for both adsorption and desorption. Figure 30 shows the curves of the total moisture content M, and of Mi and M2, all expressed in percent (M = 100 m). These curves were obtained using values of m, ki, and 2 calculated from the curve in Figure 29 for the adsorption isotherm of wood. The curves labelled Mh and M, are derived from the Hailwood-Horrobin sorption isotherm model. 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

The constant pattern water breakthrough curve for isothermal adsorption of trace moisture (Langmuir isotherm) from an inert carrier gas can be described by [15] ... 

For adsorption isotherms, pear cubes (10 mm X 10 mm) were frozen at — 40°C and freeze-dried (Telstar Lioalfa-6 lyophyliser) at 10 Pa. Afterwards, samples were placed in chambers with controlled relative humidity (RH) up to equilibrium was reached. Desorption isotherms were obtained in pear slices with skin (with approximately 8 cm diameter and 10 mm thickness), partially dehydrated in a conventional dryer at 45°C for different times to ensure different final water contents in the samples. After the drying treatment, samples were kept in plastic bags under refrigeration to ensure the internal moisture equilibration and then their water activity was measured. Experiments were carried out in triplicate for each sample. 

Although many books have described the mechanisms of moisture adsorption and adsorption isotherms for drug substances, few reports have dealt with the kinetics of moisture adsorption. Zografi and co-workers reported that the moisture adsorption rate, IV, for water-soluble substances can be represented by the following equations, based on a heat-transport control model5 -601 ... 

Figure 9 The calorimetric response for vapour sorption of an amorphous drug substance with a glass transition temperature of about 95 °C. The experiment was performed in an isothermal microcalorimeter coupled with a vapour perfusion device at 25°C. A linear RH ramp was imposed at 5% RH h. The resulting signal shows initial moisture adsorption, recrystallisation and subsequent moisture desorption...

Water vapor adsorption isotherms have been obtained on cotton from room temperature up to 150°C [303,304]. Theoretical models for explaining the water vapor sorption isotherms of cellulose have been reviewed [303]. Only adsorption theories will be discussed here at ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer (GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 [305]. The GAB equation, which was not discussed in the chapter in the book Cellulose Chemistry and Its Applications [303], can be applied up to RVPs above 0.9 [306]. Initially as the RVP increases, a monomolecular layer of water forms in the cellulose. By a RVP of 0.19-0.22 the monomo-lecular layer is complete [303], and the moisture regain, when a monomolecular layer has just formed, for cotton and mercerized cotton is 3.27 and 4.56%, respectively [261,303]. By a RVP of 0.83 0.86, about three layers of water molecules are formed, and at higher RVPs it is thought that condensation occurs in the permanent capillary structure of the sample [307]. 

As implied by the proposal of Schuchmann et al. [7], many adsorption isotherms may be normalized by plotting the equilibrium moisture content against the free energy... 

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