Water vapor adsorption relative humidity

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. 

Fig. 1 Water vapor adsorption and deliquescence of a water-soluble solid (a) Atmospheric relative humidity, RHj < RHq (b) RH = RH0 and (c) RHj > RH0.

Sorption equilibria and kinetics are influenced by the nature of the adsorbent and the adsorbate, by the mechanism of adsorption, and by environmental parameters such as temperature, relative humidity, concentration of the adsorbate, and air velocity and turbulence past the adsorbent surface. Air velocity and turbulence only affect sorption kinetics the other parameters also affect equilibria. In general, low adsorbate saturation vapor pressure, low temperature, and high adsorbate concentration in the air increase adsorption. Relative humidity does not always affect adsorption. Colombo et al. (1993) found a 35 % decrease in adsorbed mass when relative humidity was changed from <10 % to 35 %, but only an 8 % decrease when the humidity was increased from 35 % to 70 %. Building materials, which are exposed to indoor air in the normal humidity range of 35-70 %, will typically already be covered by at least one monolayer of adsorbed water, and the formation of multilayers will only have a limited influence on sorption properties for other airborne substances. Kirchner et al. (1997) found that an increase in air velocity increased the rate of desorption of a VOC mixture from painted gypsum, but not from carpet. The air velocity of air above the tuft may be insignificant for the desorption processes of carpet fibers deeper in the tuft. 

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). 

The drying mechanisms of desiccants may be classified as foUows Class 1 chemical reaction, which forms either a new compound or a hydrate Class 2 physical absorption with constant relative humidity or vapor pressure (solid + water + saturated solution) Class 3 physical absorption with variable relative humidity or vapor pressure (soHd or liquid + water + diluted solution) and Class 4 physical adsorption. 

Adsorbent beds used in the PADRE process have been recycled on a test stand more than 2000 times with no measurable loss of adsorption capacity. Also, the PADRE resin has a relatively high tolerance for water vapor, allowing efficient treatment of airstreams with a relative humidity of greater than 90%. These two advantages make the PADRE process a cost-effective on-site treatment technology when compared to traditional activated carbon systems. 

For many experimental purposes, activated carbon can be dried by keeping it in a desiccator for several days. The adsorption of water vapor differs from that of most gases. At low humidities, the adsorption of water is very small but it increases rapidly at a relative humidity of 0.3 to 0.4 (Figure 16 4). McBain29 suggests... 

A common problem in removal of vapors from air is the simultaneous adsorption of water vapor. The capacity of the carbon for organic vapors may be reduced by 50 to 70 percent if the relative humidity is 80 percent but there is little effect if the relative humidity is less than 30 percent. This may make it worthwhile to raise the temperature of the feed by 20 to 30°F, even though this decreases the equilibrium capacity for a single adsorbate. 

Vapor-phase sorption onto the same dry Woodbum soil is illustrated in Figure 3.16 where soil uptake is expressed as a function of the relative pressure-equilibrium partial pressure/saturated vapor pressure. The data indicate a BET adsorption process. Increasing relative humidity decreases the adsorption (Fig. 3.17) of dichlorobenzene ultimately resulting in a linear isotherm. It is concluded that at low humidities adsorption on the mineral surfaces is involved. Increasing availability of water reduces the availability of these sites until sorption is due to partitioning into the SOM. It will be demonstrated that the converse of this relation is important in assessing the potential for evaporative loss from soils. 

Humidity. Atmospheric water vapor can significantly reduce the adsorptive capacity of solid sorbents and dosimeters should be tested at the maximum relative humidity expected in field use if this information is not available from the manufacturer. 

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