Chemistry

Drying


Physical drying refers to the separation of adhering liquid (moisture) from solids, also the removal of water vapor from gases (e.g. by desiccant or condensation) and of dissolved or emulsified water from liquids (e.g. organic solvents).

Chemical drying or hardening (e.g. of paints), on the other hand, is based on crosslinking reactions that create a polymer network.

Learning units in which the term is dealt with

Lacquers30 min.

ChemistryMacromolecular ChemistryPolymer networks

The film formation of chemically drying paints is a common example of crosslinking reactions of polymers. Solvent- and water-based paints as well as powder paints are presented.

Car painting30 min.

ChemistryMacromolecular ChemistryPolymer networks

As an important and technically demanding painting process, car painting is described in its individual steps. The electrodeposition coating is dealt with in more detail.


Desiccant

Desiccant are substances that remove water or (less often) other solvents. The water can be chemically bound, for example in the case of sulfuric acid, calcium chloride or phosphorus pentoxide, or drying can take place by adsorption, such as in the case of molecular sieves or bentonite. A distinction is made between static and dynamic drying. In static drying, the drying agent is added to the substance to be dried and then removed again; in dynamic drying, the gaseous or liquid substance to be dried flows through the drying agent.

Desiccants are used in industry to dry materials in the course of production, to protect end products from undesired soaking, or as a chemical agent against physically caused water release or condensation. This serves the longevity of a product, since it can also avoid corrosion and undesired surface changes of a material.

The most commonly used desiccant is air. One speaks of air drying. This is used in almost all areas of manual or industrial production.

  • Drying wood in instrument making or the furniture industry.
  • Drying food as a means of preservation.
  • Drying textiles on the clothesline.
  • Fluidized bed drying of sodium chloride (salt).


Chemical compounds with a special internal structure are also often used as drying agents. & # 911 & # 93 They are used to protect parts or high-quality components of a product from corrosion or condensation from water vapor.
Such agents are, for example, silica gel or zeolites. In organic chemistry laboratories, anhydrous sodium sulfate or magnesium sulfate are often used as a drying agent. Due to their chemical and structural nature, these include water molecules and then change their spatial molecular structure through intermolecular forces. Water molecules can no longer escape from the structure and remain bound.
These drying agents can be regenerated by the action of heat, the bound water is expelled and the drying agent can be reused.
Blue gel also consists of silica gel, but contains an indicator dye that shows the degree of water absorption (blue = dry or pink = moist). Orange gel is also based on silica gel as a drying agent, just another indicator is used to indicate further usability.

For particularly high demands on the dryness of solvents (example: diethyl ether), freshly pressed (bare) sodium wire is used for drying. The sodium reacts irreversibly with water residues to form sodium hydroxide and gaseous hydrogen.

  • Keeping optical, precision mechanical and electronic components dry
    • during transport within the packaging
    • for keeping closed volumes dry (housing, optical components, laser)

    There have also been attempts to use silica gel to compensate for buoyancy in airships in order to replace the mass of fuel used by absorbing water from the air.


    Economical and independent of the weather: Innovative drying of energy wood using the tried and tested dome ventilation method

    As a renewable energy source, wood plays a decisive role in Europe. In addition to residual wood from the forest, such as thin and damaged wood, fast-growing trees from special plantations are becoming increasingly important as a source of raw materials. Freshly harvested wood cannot be used without drying because of its water content of up to 60 percent. In addition: Damp wood increases soot formation and does not burn completely. It is therefore necessary to reduce the moisture content of the wood to around 20 percent. So far, this has mostly been done by natural drying outdoors.

    Dr.-Ing. Joachim Brummack and his team on a suitable drying process for energy wood. With the dome ventilation process, the experts use a principle they have developed and tested, which was originally designed for the composting of biowaste and for the treatment of residual waste. With this method, it is now possible to dry wood chips quickly and regardless of the weather. The process manages without the use of external energy and does not require any fixed structural facilities. Once the drying process has ended, the wood can be stored temporarily without any further effort. The energy required for drying & # 8222 & # 8220 is supplied by the easily degradable components of freshly harvested wood. Even with conventional drying and storage, this energy is converted by microorganisms, but in extreme cases this can lead to the dreaded spontaneous combustion.


    • Warm outside air is sucked in and dehumidified in a rotary dehumidifier. This increases the enthalpy (thus the temperature) and the water content decreases.
    • This air is recooled in a cross-flow heat exchanger. The outside air transfers the heat to the exhaust air.
    • In the last step, the air is humidified. This lowers the temperature. The supply air is blown into the room.
    • A heater in the exhaust air stream heats the exhaust air so that it can absorb moisture in the rotary dehumidifier. This regenerates the rotor.

    In order to be able to operate this process sensibly from an ecological and economic point of view, the energy supplied to the exhaust air flow must be as inexpensive as possible. For example, solar energy, district heating or waste heat from a CHP unit are suitable.

    The picture shows the basic course in the h-x diagram (not to scale).

    With the help of the exhaust air, cooling can also be carried out by means of evaporative cooling (specifically by misting water), since the evaporative cooling cools the supply air by means of heat recovery. This has the advantage that the water does not come into contact with the supply air and therefore the heat recovery does not have to meet any hygienic requirements. But it has the disadvantage of water consumption (including wastewater charges).


    Sour gas-based material and energy self-sufficient drying of sewage sludge and fermentation residues

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Correspondence: Carsten Pätzold ([email protected]), Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany. Search for more papers by this author

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Correspondence: Carsten Pätzold ([email protected]), Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany. Search for more papers by this author

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Technical University Bergakademie Freiberg, Institute for Technical Chemistry, Leipziger Strasse 29, 09599 Freiberg, Germany

    Abstract

    When processing biomass and wastewater treatment, residues with a high water content are produced. They must therefore be dried prior to thermal recycling. Classic drying processes often use fossil raw materials to generate the necessary amounts of energy. With the sour gas based drying on the basis of carbon dioxide / air mixtures an alternative is presented, the CO2 Considered a valuable material (second life) and used in process technology for drying sewage sludge and digestate. In doing so, significant differences in the dry matter content (DS) were measured, especially in the drying of digestate, which in isolated cases amounted to up to 35% DS with the same drying time. This is expressed inter alia. with a higher carbon content of 0.42% by weight (wf) in the dry material, accompanied by a calorific value that is more than 1 MJ kg −1 higher.

    Abstract

    During the processing of biomass as well as during waste water treatment, residual materials with a high water content are produced. They must therefore be dried prior to thermal utilization. Classical drying processes often use fossil raw materials to generate the necessary energy. With sour gas-based drying based on carbon dioxide-air mixtures, an alternative is presented that considers CO2 as a valuable raw material (second life) and uses it in the process for drying sewage sludge and fermentation residues. Especially in the drying of fermentation residues, significant differences in dry substance content (DS) were measured, which in individual cases to up to 35% DS with the same drying time. This is expressed, among other things, in a higher carbon content of 0.42 wt% (wf) in the dry material, accompanied by a heating and calorific value that is more than 1 MJ kg −1 higher.


    Video: ДЕКОРАТИВНАЯ штукатурка своими руками. ТРАВЕРТИН. Камень (December 2021).