Water vapor adsorption temperature

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

Dogadkin, Skorodumova, and Kovaleva (127) studied the reaction of carbon black with sulfur at low temperatures. A solution in toluene was used at 145° in the presence of an accelerator. The sulfur sorption was negatively influenced by surface oxides. The oxygen-containing groups were not affected by the reaction, since no change in the water vapor adsorption was detected. No hydrogen sulfide was evolved under the reaction conditions. 

The effect of temperature on the activity of the fresh catalysts was first studied. For the Hopcalite catalyst with feed A there was complete conversion above 300 °C. However, significant loss of activity was observed at lower temperatures, though activity could be restored by heating the catalyst above 300 °C. This is probably due to water vapor adsorption, given the known sensitivity of other Hopcalite formulations to water. There appears to be almost an on off switch for this material at 300 °C, but the long-term... 

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... 

TPD experiments were performed using an NEVA NAG 110 mass spectrometer with a glass-made vacuum system. Prior to every TPD measurement the sample was heated up at 1173K for Ih under vacuum (10 torr) to remove the adsorbed impurities. The typical water vapor adsorption pressure was around 20 torr and the heating rate was 10 l min. The temperature programmer used in our experiments was Chino, model KP. 

Figure 3. Water vapor adsorption isotherms at room temperature of the basic hydrophilic silica (top) and the hydrophobic grade produced with this material...

The thermal transformation of Type 4A zeolite begins at 550°C, as indicated by the decrease of water vapor adsorption capacity. This capacity is nonexistent after a thermal treatment at 670°C. We have demonstrated that, after grinding the samples which have been heated previously at this temperature, half of the zeolitic phase, characterized by its water retention capacity, remained. The residual zeolite is thermally unstable. It has the same x-ray diffraction pattern as the initial zeolite, but should not have the same chemical composition. We have shown that the solid-solid transformation is accompanied by a closed macroporosity which disappears gradually with sintering. There is every reason to believe that the solid-solid transformation begins at the periphery of the particles and progresses towards the center. 

By more intense oxidation of the carbon, the amount of water vapor adsorbed at low relative pressures (<4 Torr at 25 °C) can be drastically increased. For example. Walker and co-workers showed a 100-fold increase in water vapor adsorption by activated carbon after strong surface oxidation by HNO3 (Mahajan et al 1982). Exchange of the surface H-ions by cations (Li, Na, K, Ca) on the oxidized carbon further increased the moisture capacity at low vapor pressures to amounts comparable with that on zeolites. The ion-exchanged carbon was fully regenerated at 140 °C, in contrast to temperatures >350 °C that are required for zeolite regeneration (Mahajan et al., 1982). 

Evolution of the external surface area and the two types of microporosity of atiapulgite (structural and inter-fiber) were examined as a function of a vacuum thermal treatment upt to 500°C. The methods used include controlled transformation rate thermal analysis, N2 and Ar low temperature adsorption calorimetry, water vapor adsorption gravimetry and quasi equilibrium gas adsorption procedure of N2 at 77K and CO2 at 273 and 293K. Depending on the outgassing conditions,i.e. the residual pressure, the structure folds 150 to 70 C. For lower temperature, only a part (18%) of the structural microp< osity is available to N2,13% to argon and 100% to CC>2.With water, the structure can rehydrate after the structure is folded up to an outgassing temperature of 225°C. 

Tables 2 and 3 give the main results obtained from CO2 and water vapor adsorption respectively. The folding of the structure under different outgassing condition (0.1 Pa) is associated with a decrease of the micropore volume accessible to CO2 observed between 70 and 130°C. The volume of gas, which once adsorbed is able to completely fdl the micropores, was calculated using Dubinin s equation (ref. 12) and conveitied into liquid volume using 1.08 and l.OS g/cm for liquid CO2 at 273K and 293K respectively. The values obtained for temperature lower than that...

Sohd sorbent materials have the abiUty to adsorb water vapor until an equiUbrium condition is attained. The total weight of water that can be adsorbed in a particular material is a function of the temperature of the material and of the relative humidity of the air (see Adsorption). To regenerate the sorbent, its temperature must be raised or the relative humidity lowered. The sohd sorbents most commonly used are siUca (qv), alumina (see Aluminum compounds), and molecular sieves (qv). 

Loading was accomplished by exposing the activated zeolites to benzene-de vapors. The extent of vapor adsorption was determined by the increase in weight of the zeolite. The samples were found to be extremely hydroscopic and thus kept in a vacuum desiccator until their use. After two or three temperature runs the samples began to adsorb water vapor. Therefore, only the data obtained for the first two variable temperature cycles are presented here. The samples studied are listed in Table I. 

The application of open sorption systems can provide dehumidification by the adsorption of water vapor and sensible cooling by adiabatic humidification (after a cold recovery for the dried air) at temperatures between 16 °C and 18 °C. Conventional systems have to reach temperature as low as 6 °C or lower in order to start dehumidification by condensation. For comfort reasons this cold air has to be heated up to about 18 °C before released into the building. This shows that open sorption systems can provide in general an energetically preferable solution. 

The hydration behavior of DPPS (Tc = 55 °C) LB films is shown in Figure 13. The DPPS LB film having phosphoserine head groups little hydrated (Am = 600 10 ng, 20 mol of water per lipid) even near the phase transition temperature. Hydration ability has been reported from adsorption experiments of water vapor to lipid powder to be in the order of PC > PE > PS lipids [43], It has been determined from calorimetry that the amount of non-frozen water around lipid molecules is 10 mol, 7-8 mol, and 0 mol for 1 mol of PC, PE, and PS lipids, respectively [44]. This tendency is consistent with our results that PC molecules are easily hydrated and flaked from the substrate, and PE and... 

Hollabaugh and Chessick (301) concluded from adsorption studies with water, m-propanol, and w-butyl chloride that the surface of rutile is covered with hydroxyl groups. After evacuation at 450°, a definite chemisorption of water vapor was observed as well as of n-propanol. The adsorption of -butyl chloride was very little influenced by the outgassing temperature of the rutile sample (90 and 450°). A type I adsorption isotherm was observed after outgassing at 450°. Apparently surface esters had formed, forming a hydrocarbonlike surface. No further vapor was physically adsorbed up to high relative pressures. 

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