Absorption (chemistry)

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Chemical absorption describes the process of taking up or “dissolving” an atom, molecule or ion in another phase. This is not a matter of accumulation on the surface ( adsorption ), but of absorption into the free volume of the absorbing phase .

Chemical absorption is used today, among other things, for cleaning industrial exhaust air, for example for exhaust air from electroplating plants or for removing odors. Due to the legal requirements of the limit values ​​( e.g. TA Luft ) for certain substances, which have been tightened repeatedly, the absorption technology is becoming more and more important.

Absorption can in packed columns columns with, structured packing , tray columns , bubble column reactors , via membrane technology or with the technology falling film regenerative effected.

The process of absorption is always exothermic.

A distinction is made between physioabsorption and chemisorption.

Physioabsorption

In physical absorption, the gas is dissolved in the solvent. Mixing takes place without a chemical reaction. The enthalpy of absorption corresponds to the enthalpy of evaporation

An example of physical absorption is the uptake of carbon dioxide in hexane.

Chemisorption

In chemical absorption, the gas reacts with the solvent, so that a product substance is formed. In this case, the absorption enthalpy corresponds to the reaction enthalpy.

Examples are the reaction of sulfur dioxide with calcium hydroxide or of hydrogen chloride with water .

Absorption of substances

The process of absorption is caused by an affinity of one substance for the other - there is a certain solubility of the substances in one another. The absorption of gases can take place not only in a liquid , but also in a solid substance . The gas is dissolved in the absorbent at a given temperature, producing heat ( heat of absorption or heat of solution). The gas absorbed is called absorbate .

If it is, as a closed system, for example a liquid and a gas flow ( diffusion ), the gaseous atoms or molecules in the liquid until the first solubility is reached the gas in the liquid. A dynamic equilibrium is established , which means that the particle flow into the liquid is just as large as the particle flow into the gas phase.

If no chemical reactions take place when the gases are dissolved, then Henry's law applies approximately to liquids at low pressure . At a given temperature, the concentration of a gas is proportional to its pressure above the liquid:

Here is the absorption coefficient , which depends on the temperature and the substances involved. If several gases are involved in the absorption, they are dissolved in the gas phase independently of one another according to their respective partial pressures :

( Henry-Dalton's distribution theorem )

If several immiscible liquids are involved as absorbents in a gas, the ratio of the concentrations is independent of the amount of the dissolved substance and the liquids and depends only on the temperature and the substances; see Nernst's distribution theorem .

Filtration - Adsorption - Absorption

Filtration
Particulate matter or suspended matter can be separated from a liquid or gas stream by filtration. With these mechanical filters, the retention capacity increases with occupancy, as the suspended matter slowly clogs the fine pores of the filter. The air or liquid resistance of the filter increases and its pore size decreases.
adsorption
If, on the other hand, liquid or gas components are firmly bound to a solid surface by activated carbon or zeolites , this is called adsorption. The following applies to adsorption filters: the more this type of filter is occupied, the lower the retention capacity. The physical bond is also known as physisorption . Adsorption is used for cleaning, drying ( desiccant ) and in refrigeration machines ( adsorption refrigeration machine ).
absorption
If a gas or air flow is passed through a scrubbing liquid, one speaks of gas scrubbing or also of absorption . The gas components to be absorbed (absorptive - unbound, absorbed - bound) are molecularly bound in the scrubbing liquid (absorbent - unloaded, absorbate - loaded). The now bound gas components can be driven out by reducing the pressure and / or by increasing the temperature.

See also

Individual evidence

  1. a b c d e f Baerns, Manfred, Behr, Arno, Brehm, Axel, Gmehling, Jürgen, Hinrichsen, Kai-Olaf, Hofmann, Hanns, Palkovits, Regina, Onken, Ulfert, Renken, Albert: Technische Chemie . 2., ext. Wiley-VCH, Weinheim, Bergstr 2013, ISBN 3-527-33072-0 .
  2. a b Freund, Hans-Joachim .: Textbook of Physical Chemistry . 6., completely revised u. update Wiley-VCH, Weinheim 2012, ISBN 978-3-527-32909-0 .