Humidity equilibrium method

Relative humidity method. Water absorption is defined as the water absorbed by a dried protein powder with equilibration against water vapor at a known relative humidity. This method, also known as the equilibrium moisture content (EMC) method, was described first by Mellon et al. (10). Huffman et al. (11) used... 

While the convective heat flux expressiou is straightforward, the expression for drying rate needs explanation. The drying rate can be calculated from this formula, when drying is controlled by gas-side resistance. The driving force is then the difference between absolute humidity at equilibrium with solid surface and that of bulk gas. When solid surface is saturated with moisture, the expression for F is identical to Equation 3.48 when solid surface coutains bound moisture, F will result from Equation 3.46 and a sorption isotherm. This is in essence the so-called equilibrium method of drying rate calculation. 

Alden et al. extended the method by estimating the equilibrium relative humidity based on the wet-bulb temperature (TJ, the partial pressure of water at the solid s temperature, and the saturated vapor pressure of water at 7 [46]. Thus, the moisture content of a given material can be inferred from the estimated temperature difference (AT) and the equilibrium relative humidity. This method has been successfully implemented to control the endpoint in batch fluid-bed dryer for aqueous and aqueous-alcohol granules. An pirical method was used to determine the desired AT value by frequent sampling of the dried material and moisture content determination. The realtime AT was estimated using a computer program based on the temperature measurements this value was then compared with the desired value and used as the drying-endpoint indicator. 

Henry s linearized analysis, with averaged values for the coefficients tr and environmental change in temperature and humidity. The method is able to follow the response in core temperature and bale moisture to the new equilibrium level. Walker also used the technique to determine the equilibration of overdried fleece wool in an undisturbed bin. After 1 day, the moisture uptake is calculated to penetrate 150 mm into the fibrous mass, reaching 700 mm after 10 days and 1.1m after 100 days. 

If the one-point calibration in ambient air is not sufficient, the next best approach is to use the calibration box method.- The air state is created in a closed box made of nonhygroscopic material, like metal or plastic. A controlled state of humidity is maintained by exposing the air in the box to a liquid surface of a saturated salt solution. In practice, a dish containing the saturated water solution of a salt is placed on supports at the bottom of the box. The air in the box is circulated by means of a small fan. The box should be airtight and positioned in a constant temperature environment. The calibrated instruments are placed in the box. A dewpoint hygrometer can be used as a reference. A wide range of humidity can be created by using solutions of different salts. Table 12.5 shows a few examples of equilibrium humidities achieved with different salt solutions. 

The fact that the equilibrium moisture content may be considerable at low humidities is of especial importance in the oven methods. Under ideal conditions no water vapor should be present in the oven, but this is impossible to attain in practice. It is difficult to maintain a dry atmosphere in an air oven, and most commercial vacuum ovens are not air-tight. Thus, the discrepancies in results of different investigators have frequently been traced to different humidities in their ovens. Any attempt to reduce the relative humidity by increasing the oven temperature introduces the danger of error from thermal decomposition. 

An interesting point arises from this method. The particles of material are exposed to water vapour at a set humidity and hence the results equate to that humidity. The measured equilibrium absorption is substantially the same as the equilibrium absorption that would be obtained by immersion in an aqueous solution which would maintain the test humidity. As this implies, the equilibrium water absorption is reduced if the water is not pure. Hence, exposure to aqueous solutions should be made at the concentration of interest. As the humidity approaches 100% even small amounts of a salt have a significant effect on equilibrium absorption. 

Thermogravimetric analyses were carried out in 10°-30° temperature increments with 200-mg samples using a conventional (Mauer) TGA system. Automatic recording of weight change was used to follow reaction to equilibrium, but actual weighings were recorded only by manual operation. The sample was bathed continuously in air of controlled humidity (Pmo = 7.9 torr) flowing at 180 cc/min. Precautions were taken to minimize drafts and convective currents, and buoyancy correction curve was made to 950°C. Further details on experimental methods are available (12). 

When both temperature and humidity are controlled, the standard conditioning time is a minimum of 16h, and where temperature only controlled at 23°C or 27°C, a minimum of 3h. At the sub-normal and elevated temperatures it is simply specified that the time should be sufficient for the test piece to reach equilibrium with the environment. Tables of approximate times required to reach equilibrium in both air and liquid media have been given6 for a wide range of temperatures and various test piece geometries, and a relevant selection of these is reproduced in an annex to the standard. Also, specific instructions are given in some test method standards. 

Such a method is detailed in ISO 252833 and ASTM E9634 for sheet materials in general. To avoid leaks, the wax seal must be applied very carefully using the templates specified, and the temperature and humidity during exposure of the sealed dishes must be controlled closely. The test must be continued until the increase in weight is substantially linear with time, i.e. equilibrium has been reached. 

ASTM D-1412. Standard Test Method for Equilibrium Moisture of Coal at 96 to 97 Percent Relative Humidity and 30°C. 

Archaeological fragments of bones and teeth take up fluorine from the surrounding soil and accumulate it in their mineral phase when they are exposed to a humid environment. Geological time spans are needed for this process to reach equilibrium and for the fluorine distribution to become uniform. In cortical parts of long bone diaphysis, an initially U-shaped fluorine concentration profile can be observed, which decreases from the outer surface and the marrow cavity towards the inner parts of the bone and carries information on the exposure duration of the buried object in its shape. The time dependence of the profile slope is usually described in a simplified way by a diffusion model. The quantitative mathematical evaluation of these profiles may provide information on the exposure duration and the physical condition of the samples. Therefore, several attempts to use fluorine profiling as a dating method have been undertaken [3,39], The distribution of... 

Originally, water activity could not be measured directly. One method was to measure the weight loss of a product held at a range of controlled relative humidities, which also has the effect of holding the product over a range of water activities. If a product is held at its own water activity it neither gains nor loses weight, and this point is described as its equilibrium water activity. 

Average contact angle values for various highly purified diagnostic liquids on films prepared from purified rat skin collagen are presented in Table II. Contact angles were measured with the films (a) drop spread from distilled water on a platinum sheet and air dried to a specularly smooth continuous film in a greasefree container at a temperature that never exceeded 20°C (b) prepared as in Method a but then reswollen with distilled water for at least 1 hr and measured while still completely water swollen in equilibrium with relative humidities of 50% and >99%... 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

There are several advantages to the dynamic vapor sorption device. First, any humidity value can be dialed in, whereas salt solutions are not available for every humidity value and some are quite toxic. Second, since the weight is monitored as a function of time, it is clear when equilibrium is reached. The dynamic devices also give the sorption/desorption rates, although these can easily be misused (see the drying kinetics section later). The salt solution method, on... 

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