Water Vapor L40/WV
Water vapor measurements in thermal analysis
Difference between water vapor and relative humidity in thermal analysis
Water vapor and relative humidity are distinct concepts in thermal analysis. Water vapor refers to the gaseous form of water created when liquid water is heated to its boiling point or above. When this steam is introduced into a controlled environment or instrument, it is referred to as water vapor application.
On the other hand, relative humidity pertains to the amount of water vapor present in the air compared to the maximum capacity of the air to hold moisture at a specific temperature. This measurement is expressed as a percentage. Relative humidity, also known as RH, is a concept relevant in atmospheres below the boiling point of water, representing the moisture content in the air.
It’s important to understand that above the pressure-dependent boiling point of water (e.g., 100°C at sea level), water exists solely in its gaseous state as water vapor. To create controlled gaseous environments for experiments, water vapor generators are used to produce steam, which can then be mixed with carrier gases like air, nitrogen, or helium. This allows for the adjustment of water vapor concentration within the sample gas, typically measured in units like volume percentage (Vol.-%), weight percentage (wt.%), or parts per million (ppm).
In various thermal analysis applications, water vapor generators are employed, especially in combination with instruments like thermogravimetric analyzers (TGA), simultaneous thermal analyzers (STA) within an elevated pressure range, as well as dilatometers. These generators are essential for creating specific gas atmospheres for controlled experiments involving reactions such as adsorption, desorption, reduction, oxidation, or transformation measurements.
Pressure dependent measurements under water vapor atmospheres
Furthermore, aside from regulating concentration and temperature, the atmospheric pressure can be managed to exert a significant impact on experiments. The manipulation of pressure is particularly relevant for altering equilibrium conditions in reactions. For instance, it plays a crucial role in investigations involving processes like coal or biomass gasification. These experiments can be effectively studied using our High Pressure STA Analyzers.
However, it’s important to note that increasing the pressure level introduces new factors to consider. As an illustration, elevating the pressure level leads to a shift in the boiling point of substances, with temperatures increasing until they reach a critical point.
Conversely, the upper limit for the pressure level of gaseous water is determined by the saturation vapor pressure curve. When the pressure exceeds a certain threshold, the water vapor will undergo condensation. At elevated temperatures or pressures, beyond the critical point, the density of liquid water matches that of gaseous water, preventing further condensation. This state is referred to as supercritical, such as in the case of superheated water vapor.
To create a H2O atmosphere inside the furnace at temperatures exceeding 100°C, it is essential to employ a water vapor generator alongside our systems. This generator evaporates water independently, with the option to introduce the resulting water vapor into the sample chamber without blending it with additional purge gases. This setup facilitates the creation of a pure 100% H2O atmosphere around the sample. However, if needed, the concentration can be adjusted by incorporating dry gases using Mass Flow Controllers (MFCs). The mixture is specified as a variable concentration in terms of volume percentage (Vol.-%), weight percentage (wt.%), or parts per million (ppm) of water vapor in a dry carrier gas.
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Applications
The typical example for water vapor applications at elevated temperatures and pressure levels are coal and biomass gasification experiments.
The provided example illustrates a typical gasification experiment involving charcoal. In this experiment, a coal sample was subjected to an isothermal plateau under a nitrogen atmosphere at a pressure of 50 bar, utilizing a High Pressure TGA-Thermo balance. The mass signal displayed a decline corresponding to the release of volatile components, occurring between 20 and 40 minutes. Subsequently, water vapor was introduced into the chamber, initiating the gasification of coal. Over a span of 150 minutes, the coal was nearly entirely consumed, leading to the production of reactive gases such as H2, CO, CH3OH, and others, as indicated by the prominent red mass loss curve. The entire process can be summarized as follows: Carbon reacts with water vapor to yield a mixture of carbon monoxide and hydrogen. The resulting carbon monoxide can further react with a second water molecule to produce carbon dioxide and additional hydrogen, and ultimately, the resulting hydrogen can combine with carbon monoxide to generate methane and other hydrocarbons.
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