Osmosis: The Molecular Theory. Larry Howlett
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Название: Osmosis: The Molecular Theory
Автор: Larry Howlett
Издательство: Ingram
Жанр: Химия
isbn: 9781628473759
isbn:
Before long-term storage, seeds are dried or humidified to establish specific seed water content. One process used to establish initial values for seed water content is equilibrating seeds in a controlled humidity and temperature environment. Data from these tests has been termed water sorption isotherm curves.
Water sorption isotherm data is reexamined in this paper by introducing a new variable, moisture vapor pressure. At given temperature and relative humidity, moisture vapor pressure is a characteristic of moist air that can be readily calculated from empirical equations. We postulate that at equilibrium conditions, the moisture vapor pressure in the seed is equal to the moisture vapor pressure in the equilibrating moist air environment. Furthermore, we propose that the calculated value of the seed moisture vapor pressure is a characteristic property of the seed at the specified equilibrating moisture content and temperature.
Seed Data
Seeds equilibrated under conditions of constant temperature and relative humidity will approach a certain and repeatable value for temperature and moisture content.
Although many factors may be important in maintaining seed vitality during short and long-term storage, we limit this discussion to seed temperature and moisture content. Furthermore, we consider only constant temperature and constant seed moisture content during short or long-term storage.
At equilibrium, there will be no heat or moisture exchange between a seed and its environment. The temperature of the seed will equal the temperature of the environment and the moisture content of the seed will be in balance with the moisture content of the environment.
Equilibration data for yellow dent corn is shown in figure 2. The graph shows storage conditions that will maintain constant seed moisture content.
From figure 2, for seed at 15% moisture content, equivalent storage atmospheres are:
32 °F, 49% relative humidity;
70 °F, 62% relative humidity;
And 90 °F, 68% relative humidity.
At each of these storage atmosphere conditions the seed moisture content will remain constant at 15%. The seed vitality then becomes only a function of the storage temperature. We note that all of this seed storage information is directly available from the seed equilibration curves.
The equilibration curves were generated with the modified Henderson equation:
Where: Mc = moisture content dry weight %; rh = relative humidity %; T=temperature ºC (ºC = (ºF-32)/1.8)
And, the empirical constants for yellow dent corn are:
K=8.6541x10-5; N = 1.8634; C = 49.810.
The equilibration curve, figure 2, is actually a combination of the properties of the seed and the properties of atmosphere where the seed is stored.
Thermodynamics of Moist Air
The psychrometric chart, figure 3, displays some of the thermodynamic properties of moist air.
Thermodynamics relates to energy and its transformations. When a sufficient number of thermodynamic properties of a substance are known, a unique value for the remaining properties can be determined. Thermodynamic properties are directly related to the energy of a substance or a system. Thermodynamic properties of standard air are presented in tables and on psychrometric charts as shown in Figure 3.
In many fields, thermodynamic properties of temperature, pressure, and relative humidity are considered independent thermodynamic variables because they can be directly measured using common instruments. Some commonly used dependent thermodynamic variables are chemical potential, Gibbs free energy, enthalpy, entropy, and vapor pressure.
For an ideal gas, the thermodynamic properties are related by the perfect gas equation, equation 2. This relationship between the thermodynamic variables has been verified by experiments on many gases and gas mixtures.
PV = RT (2)
Pressure, P, volume, V, and temperature, T are thermodynamic properties of a gas and R is the gas constant for a specific gas or mixture of gases.
If we define two variables (i.e. pressure (P) & temperature (T)), the third thermodynamic variable volume (V) is uniquely determined from the perfect gas equation.
For moist air with the gas constant determined from the mixture of gases, three thermodynamic variables are required to determine any other thermodynamic property. Typical and readily measured independent values are Pressure, Temperature, and relative humidity. Some of the unique dependent thermodynamic variables for moist air are humidity ratio, enthalpy, density, and vapor pressure. Note that vapor pressure refers to the pressure of the moisture vapor in the air.
Moisture Vapor Pressure Model
We select vapor pressure as an important property of moist air since it is useful for many engineering applications. A psychrometric chart for atmospheric air that includes vapor pressure is shown in figure 3. At normal pressure, there is a unique relation between temperature, relative humidity, and moisture vapor pressure of air. The dependent thermodynamic properties of moist air are relatively insensitive to small changes in atmospheric pressure.
If we apply this thermodynamic data for air (figure 3) to the equilibration chart (figure 2), we can develop a unique thermodynamic property graph for the seed separate from the atmosphere (figure 4). Each value for temperature and relative humidity determines a specific vapor pressure value for the atmosphere. We propose that the seed must exhibit this same vapor pressure be in equilibrium with the atmosphere. We propose that the vapor pressure data shown in figure 4 represents a characteristic property of the seed that is independent of the storage atmosphere.
We hypothesize that the seed placed in a small-evacuated container at constant temperature will lose some water to the container to produce a moisture vapor pressure in the container. Since the moisture loss from the seed to the container is small, the seed moisture content will remain approximately constant. Thus, we propose that the moisture vapor pressure in the container will correspond to the seed properties shown in figure 4.
When a seed has equilibrated to its environment, the moisture content in the seed is in equilibrium with the seed's environment.
We postulate that the moisture vapor pressure in the seed must equal the moisture vapor pressure in the environment as a condition of equilibrium. We propose that this equilibrated value of seed moisture content, vapor pressure, and temperature are state thermodynamic variables of the seed. Thus, these conditions СКАЧАТЬ