Drying water vapor pressure

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeabihty coefficient. The dimensions are quantity times thickness in the numerator and area times a time interval in the denominator. These dimensions do not have a pressure dimension in the denominator as does the permeabihty. Common commercial units for WVTR are (gmil)/(100 in. d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(m-s). The WVTR describes the rate that water molecules move through a film when one side has a humid environment and the other side is dry. The WVTR is a strong function of temperature because both the water content of the air and the permeabihty are direcdy related to temperature. Eor the WVTR to be useful, the water-vapor pressure difference for the value must be reported. Both these facts are recognized by specifying the relative humidity and temperature for the WVTR value. This enables the user to calculate the water-vapor pressure difference. Eor example, the common conditions are 90% relative humidity (rh) at 37.8°C, which means the pressure difference is 5.89 kPa (44 mm Hg). 

Fig. 3. Water-vapor pressure over ice and several drying agents.

I. The wet-bulb or saturation temperature line gives the maximum weight of water vapor that I kg of dry air can cariy at the intersecting dry-bulb temperature shown on the abscissa at saturation humidity. The partial pressure of water in air equals the water-vapor pressure at that temperature. The saturation humidity is defined by... 

The state of equilibrium differs from the equilibrium between water and pure water vapor in that, in a gas phase, there is also inert gas (dry air) present. This means that the water pressure is equal to the total gas pressure, p -- p, + ph, not to the water vapor pressure p, . 

Next we will show that the dependence of water vapor pressure on the partial pressure of dry air is very small, and consequently a good approximation is... 

The ground material (40-mesh) is dried in vacuo at room temperature over a desiccant (magnesium perchlorate) that permits practically no water vapor pressure. An assumption was made that at room temperature and in the absence of air, decomposition and oxidation would be negligible. It was found, however, that a direct application of this reference method was not practicable from a routine standpoint, because the time to reach equilibrium was exceedingly long (6 to 9 months). 

Droplets of various hquids were prepared in several ways. For example, a macroscopic drop was first deposited on the substrate and then absorbed from an edge using filter paper. In other cases a macroscopic drop was blown away with a jet of N2 or air. These processes leave a surface that appears dry to the naked eye but still contains many tiny droplets that can be observed with SPFM. If the droplets are of aqueous solutions, the water vapor pressure in the chamber, with which they readily equilibrate, determines their size. For hquids with low vapor pressure, films and droplets can be formed by condensation from a warmed reservoir. 

The equilibrium water vapor pressure over the materials in hydrate forms is greater than that over the materials in hydroxide forms. Elimination of water vapor and hence drying of the material occurs when the ambient water vapor pressure in the system is lower than the equilibrium water vapor pressures given by the above equations. To effect drying, there are two options. 

The other option available for drying involves subjecting the material to reduced pressures. By decreasing the ambient pressure, for example, by evacuation, to a level lower than the equilibrium water vapor pressure over the material, drying can be implemented. This option is particularly important when heating affects the material, in addition to drying it. 

Point 4 above may be correct, but only if the partial water vapor pressure of the product at the product temperature is larger than the existing partial water vapor pressure in the chamber, otherwise the chamber pressure has to be lowered. Since the energy input during secondary drying is not decisive, it would be safer to use a chamber pressure as small as the condenser temperature allows. 

Figure 2.8.1 shows a typical installation for flasks and other containers in which the product is to be dried. The condenser temperature for this plant is offered either as -55 °C or as -85 °C. For this type of plant, a condenser temperature of -55 °C is sufficient as this temperature corresponds with a water vapor pressure of approx. 2.1 10 2 mbar, allowing a secondary drying down to approx. 3 10-2 mbar. This is acceptable for a laboratory plant, in which the limitations are not the condenser temperature but the variation of heat transfer to the various containers, the rubber tube connections and the end pressure of the vacuum pump (2 stage pump, approx. 2 10 2 mbar). Figure 2.8.2 shows that these units are designed for very different needs. The ice condenser in this plant can take up 7.5 kg of ice at a temperature down to -53 °C. 

Partial gas- or vapor pressures during freeze drying can also be measured by a mass spectrometer, and water vapor pressures by hygrometers, sensitive only for water vapor. Both systems are necessary for development- and analytical work, but in production plants they need only to be used to check or identify process data. 

Merika [3.51] emphasized from his 17 years of experience with the quality control of freeze dried transplants the importance of sterility and residual moisture control as the decisive characteristics. Furthermore, the leak tightness of the storage containers was constantly controlled. Merika did not measure the product temperature during drying, but controled the process by measuring water vapor pressure and temperatures of the shelves and the condenser. The residual moisture content after 2 years of storage must be below 5 %. All products were sterilized by gamma radiation. 

The merit of Malinin s work is the comparative study of water content of bones by reproducible methods. The measurement of water vapor pressure during the drying cannot be used dirctly to determine the RM, as Malinin correctly states. Measurement of the description rates (DR) provide a means to follow quantitatively the course of desorption drying. The method is described in Section 1.2.2, but cannot be applied in an installation used by Malinin because the condenser cannot be separated from the chamber by a valve. By using the data given in the paper of Malinin it is possible to estimate the freeze drying process of bone transplants as follows ... 

Take as an example, a small dry particle of NaCl of a given mass (mu) that is introduced into air at a water vapor pressure corresponding to SA in Fig. 14.38a. Assuming that the RH is above the deliquescence point of NaCl, 75% at 25°C, the particle will take up water, dissolve, and form a stable droplet of radius rA. Similarly, if the air saturation ratio increases to Su, the particle will, under equilibrium conditions, take up water and grow to radius ru. 

Smectite-type materials were synthesized with a hydrothermal method [5]. The aqueous solution of sodium silicate (Si02 / NajO= 3.22) and sodium hydroxide was mixed with the aqueous solution of metal chloride to precipitate Si-M (M divalent metal cation, Si M = 8 6) hydroxides. The precipitation pH of Si-M hydroxide was controlled by changing the molar ratio of sodium hydroxide to sodium silicate. After separating and washing of Si-M hydroxide, slurries were prepared from Si-M hydroxide and water. The Si-M slurries were treated hydrothermally in an autoclave at 473 K under autogaseous water vapor pressure for 2 h. The resultant samples were dried at 353 K then we obtained smectite samples. The smectite-type materials are denoted by the divalent species in octahedral sheets and BET surface area, e.g., Ni-481 for the Ni2+ substituted smectite-type material with a surface area of 481 m2g. ... 

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