Industrial Moisture and Humidity Measurement

A Practical Guide

Roland Wernecke, Jan Wernecke

• Publisher: Wiley-VCH
• Year: 2013
• Print ISBN:
  9783527331772
• eBook ISBN:
  9783527652433
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Moisture analysis covers a variety of methods for measuring high levels of moisture, as well as trace amounts, in solids, liquids, or gases. There are many applications where trace moisture measurements are indispensable for manufacturing and for process quality assurance. Trace moisture in solids must be controlled for plastics, pharmaceuticals and heat treatment processes. Measurement applications in gases and liquids include, for example, drying processes, hydrocarbon processing, pure gases in the semiconductor industry, natural gas pipeline transport, the conditioning of food and other products.

Written by experts with over 20 years of experience in the field, this one-stop guide covers all aspects of these measurements, including both the theory and a wealth of practical know-how. As such, it includes guidelines on installation, on the realization of standards for absolute and relative humidity, verification and traceability measurements, equipment calibration methods and the latest research developments.

Backed by numerous case studies, this practical book serves the needs of those working in the industry tasked with performing or developing new techniques and processes for moisture and humidity measurement. As a result, the scientist or engineer has all the information required for accurate, reliable, economically viable and efficient moisture measurement.

1

Water – Substance of Life

1.1 Water as a Natural Resource

Water plays a key role in the formation and evolution of our planet and the life it supports. Water is present, in different aggregate states, throughout all parts of the Earth. In solid and liquid forms, water covers approximately 71% of the Earth's surface, while the large amount of gaseous, liquid, and solid water in the atmosphere governs both the global climate and the local weather. Global climatic balance and heat transfer are determined mainly by the large oceanic circulations of warm and cold water. Water has a high degree of transparency for visible light, one of the key factors that enabled the formation and evolution of life in the primeval oceans. The atmospheric layer of water vapor reflects heat emitted from the surface of the Earth and thus prevents the freezing of the planet. Liquid water forms the landscapes on continents, and its presence or absence determines the degree of biological activity and suitability for agriculture.

Every form of life on Earth requires water vapor in the respiratory air. Water is an essential component of each organism, and needs to be present in sufficient amounts at all times. Humans, for example, may survive without the intake of proteins, carbohydrates, and fiber for several weeks. Without water, however, survival is possible for only a few days. Humans require a daily amount of 2–3 l of water in order to maintain biological functions. Water has thus always been a central element in human consciousness and culture. In the ancient philosophies, for example, water is one of the four basic elements, and it is a symbol of purity and life in every religion.

Apart from its immediate nutritional function, humans make use of the different properties of water in many ways, for example,

• for the generation of energy,
• for the transportation of people and goods,
• as a building material,
• in industrial manufacturing processes,
• for relaxing and recreation,
• for the removal of waste,
• as a cleaning agent, and
• for the interim or final storage of many different materials.

1.2 Physical and Chemical Properties of Water

1.2.1 The Water Molecule

Water is a molecule consisting of two hydrogen atoms and one oxygen atom (Figure 1.1).

Figure 1.1 Binding angles in a water molecule.

The difference in electronegativity between oxygen and hydrogen, and the arrangement of atomic orbitals, results in an angle of 104.5° and thus the formation of an electric dipole. The negative charge of the molecular dipole is located at the free pair of valence electrons of oxygen, while the hydrogen ions form the positively charged pole. Different symmetric and asymmetric oscillation modes (Figure 1.2) can be excited and result in a variation of the dipole moment. The dipole character of water molecules is the reason for the formation of the strong intermolecular hydrogen bonds that cause clustering (Figure 1.3).

Figure 1.2 Vibration states of a water molecule.

Figure 1.3 Clustering of water molecules.

This results in a density that is significantly lower than that of chemically similar substances. The diameter of H₂O molecules of 0.28 nm also differs from that of other components of air, such as

• H₂: 0.23–0.29 nm,
• N₂: 0.32–0.36 nm,
• O₂: 0.29–0.35 nm, and
• CO₂: 0.33 nm,

which makes the physical and chemical properties of water so unique. More than 40 anomalies of water, in terms of chemical, thermodynamic, electrical, or optical properties, to name but a few, have been observed in various experiments. Details can be found in the specialist literature.

1.2.2 Physical Properties

In the terrestrial atmosphere, water can be present in the solid, liquid, and gaseous states. All three phases are colorless and possess a high optical transparency in the visible and ultraviolet range. Infrared and microwave radiation, in contrast, is absorbed by water molecules due to the positions of the molecular energy orbitals. Water molecules are electrically neutral, but possess a dipole moment due to the inhomogeneous charge distribution.

The formation of clusters during freezing is the reason for the anomalous density change of water, compared to other molecules with similar structure. Due to clustering, water expands in volume during the phase transition from liquid to solid, which is associated with a reduction in density (Figure 1.4). This process continues as temperatures further decrease, as long as crystallization occurs. The crystal structure of water molecules in ice is a monocrystalline hexagonal lattice. Water has the highest density (e.g., smallest volume) at a temperature of around T ≈ 4 °C. A further increase in temperature results in a decrease in density, similar to any other liquid. This anomaly of density has direct implications on the landscape, and on natural processes: Lakes freeze from top to bottom, which allows fish to survive during winter. Water that penetrates into rock crevices and freezes during the night can cause significant frost wedging due to the volume expansion.

Figure 1.4 Density of water as a function of pressure.

A further anomaly of water is the temperature at which melting and freezing occurs. According to the chemical properties of compounds of hydrogen and other elements from group VI of the periodic table, the phase transition temperatures should be as shown in Figure 1.5.

Figure 1.5 Melting and boiling temperatures of hydrogen compounds in the oxygen group of the periodic table and the expected values for H₂O.

The reason for this significant deviation is the strong hydrogen bonds between the molecules, which need to be overcome by using an increased amount of energy. As a consequence, the melting and evaporation temperatures of water at standard pressure are shifted to T_melt = 0 °C and T_evapo = 100 °C, respectively. The strong hydrogen bonds also cause a strong surface tension at interfaces, which results in a high viscosity and a good wetting behavior on polarized surfaces.

Water possesses a high heat capacity, which dampens the low temperature change upon heating or cooling. Thus, the transition between the aggregate states is associated with a significant release or absorption of thermal energy.

1.2.3 Chemical Properties

Water is formed during the combustion or, more precisely, the oxidation of hydrogen according to

(1.1)

The oxidation is an exothermic process, where hydrogen is oxidized and oxygen is reduced. Water can be used as a solvent for a wide range of chemicals, because

• it is soluble in many substances and forms free ions by dissociation,
• it can be easily absorbed due to the low molar mass of the molecule,
• the dipole of the molecules allows for the formation of stable bonds, and
• the hydrogen bonds cause an interlinking with other polar molecules.

Salts, bases, and acids can be dissolved and diluted in water and are used for many chemical reaction processes. Dissolved oxygen is an important factor for life and biological activity. In the atmosphere, the ratio of oxygen to nitrogen is roughly 1 : 4. This ratio is 1 : 1.8 in water, that is, a much higher oxygen content, which is essential for the respiration processes of underwater life forms.

Metals are only corroded by liquid water and in environments with a relative humidity of U > 70%rh. Dissociation of water molecules yields unbound hydroxide ions, OH⁻, which are highly reactive. In the atmosphere, OH⁻ ions react with many substances and pollutants and thus act as a cleaning agent. An example of this process is the natural decomposition of ozone by a reaction with water according to

(1.2)

Man-made atmospheric substances, for example, industrial sulfur compounds, are also, to a certain extent, decomposed and bound by water. This generates condensation nuclei, which causes cloud formation. The particles are then washed out of the atmosphere by precipitation, and into the ground. Investigations into the distribution of thes...

Table of contents

• Cover
• Related Titles
• Title Page
• Copyright
• Preface
• Chapter 1: Water – Substance of Life
• Chapter 2: Thermodynamic Terms and Definitions
• Chapter 3: Water in Solid, Liquid, and Gaseous Materials
• Chapter 4: Moisture and Humidity Measurement Methods in Solid, Liquid, and Gaseous Substances
• Chapter 5: Selection of a Measurement Method
• Chapter 6: Reliability and Traceability of Measurements
• Chapter 7: Moisture Measurement in Meteorology, Agriculture, and the Environment
• Chapter 8: Applications in the Food and Beverage Industry
• Chapter 9: Moisture and Humidity Measurement in Industrial Plants
• Chapter 10: Applications in the Chemical, Pharmaceutical, and Plastics Industries
• Chapter 11: Applications in the Manufacture and Processing of Paper and Textiles
• Chapter 12: Moisture Measurement in the Building Industry
• Chapter 13: Laboratory-Based Moisture Measurement
• Chapter 14: Moisture and Humidity Measurement in Space
• Appendix A: Relevant Units of Thermodynamics
• Appendix B: Tables and Diagrams of Thermodynamics
• Appendix C: Constants and Parameters
• Appendix D: Material Parameters
• Appendix E: Water Adsorption in Products
• Index