Chemistry

Adsorbent


Area of ​​Expertise - Physical chemistry, interface chemistry

Also known as adsorbent, mostly solid, pore-rich or finely divided substance, which, due to its large inner surface, is capable of selectively enriching certain substances from gaseous or liquid mixtures at its interface (adsorption).

The most common adsorbents in industry and laboratories are various forms of activated carbon, aluminum oxide,Al2O3, Silica gel, SiO2, Zeolites (aluminosilicates) or synthetic adsorbent resins. In industry, these compounds are of particular importance for the treatment (cleaning) of waste water, exhaust air and exhaust gases, from which both inorganic and organic substances can be removed. Corresponding adsorbers are also used for solvent recovery, for water softening or as catalysts. In the laboratory, adsorbents represent the stationary phase in thin-layer and adsorption chromatography; in addition, due to their ability to adsorb water, they often serve as desiccants for organic solvents. In the medical field, adsorbents are used for detoxification (e.g. medical charcoal).

In addition to the qualitative and quantitative adsorption capacity, its thermal stability also plays an important role in the selection of a certain adsorbent, especially for its ability to be regenerated (desorption).

See also: adsorption, catalyst, desorption

Learning units in which the term is discussed

Applications of adsorption45 min.

Chemistrytechnical chemistryBasic operations

This learning unit gives the student an insight into the technical applications of adsorption. Using the example of nitrogen-oxygen separation, a basic solution to a process-related issue is discussed.

Adsorbents45 min.

Chemistrytechnical chemistryBasic operations

In this learning unit the student gets to know different adsorbents. It teaches what types of adsorbents there are, how the surface of such a substance can be determined and which parameters have an influence on the separation of a substance mixture.

Practical course adsorption (methane at 1200 hPa)60 min.

Chemistrytechnical chemistryBasic operations

The basic terms and processes of adsorption are dealt with in the material. It is used for the preparation and follow-up of the Adsorption internship as well as for carrying out adsorption experiments on a remote-controlled trainer.


Dehumidifiers are used to dry new buildings, after the occurrence of water damage and for rooms with high amounts of water vapor, such as & # 160B. Swimming pools. The focus here is on preventing condensation (and thus the formation of mold) on thermal bridges in the structure (outer walls). In addition, they are used in combination with air humidifiers to keep the humidity in a room constant.

The common dehumidifiers work according to three fundamentally different physical methods:

  • Air cooling with water separation (condensation)
  • Absorption in hygroscopic liquids
  • Adsorption of the water vapor on an adsorbent.

When water condenses, an energy of approx. 0.62 & # 160Wh / g is released. This energy must be dissipated immediately by cooling the air, or subsequently by regenerating saturated absorbers. Typically, the performance in Condensate per day specified. Converted, 10 & # 160kg condensate per day corresponds to an output of 300 & # 160W.

The drying performance depends on the temperature. Air with a temperature of 20 & # 160 ° C can absorb a maximum of 17 & # 160g / m³ water. A room with a floor area of ​​20 & # 160m² therefore contains less than 0.8 liters of water in the air. At 10 & # 160 ° C the value falls by half. In addition, the lower temperature reduces the rate of evaporation. Damp basement walls are therefore very difficult to dry with air dehumidifiers, but rather with additional heating.

Air cooling with water elimination

In the case of air cooling with water separation (condensation drying), the air to be dried is passed through a heat exchanger by means of a fan. The coolant used here is, for example, & # 160B. Tap water, well water or brine for use. The water runs off the cooled surfaces of the heat exchanger and is collected in a condensate container. It is of crucial importance in this process that the surface temperature of the heat exchanger is lower than the dew point temperature of the air. In dehumidifiers for household use or in building drying devices, the dew point is not reached by a closed cooling circuit: A compressor is built into the drying device, which ensures the circulation of refrigerant in a cooling circuit. The warm, moist air is sucked in by a fan and suddenly cooled on the evaporator surface. The air humidity condenses and the water is collected in a container and emptied, or drained away by means of a hose.

Since this process works with temperatures close to freezing point, the evaporator can easily freeze up. Therefore, only devices that have an "automatic defrosting process" are practical.

As a rule, the dehumidifiers that work in this way are equipped with an electronic hygrometer and switch off (or to lower power) as soon as a certain value of the relative humidity is reached. In devices with a condensate container, the water level is monitored and the device is switched off when the condensate container is full.

Absorption in hygroscopic liquids

When absorbing in hygroscopic liquids, the air to be dried is passed over a hygroscopic liquid by means of a fan. This usually consists of an aqueous salt solution of lithium chloride, lithium bromide or calcium chloride. The water vapor passes into the hygroscopic solution and dilutes it. The absorption capacity of the solution increases with increasing pressure, decreasing temperature and increasing water vapor concentration in the air. Due to the heat of absorption that is released, it may be necessary to cool the liquid or the dried air.

The hygroscopic liquid needs to be regenerated after a certain period of time. This is usually done by heating outside of the rooms to be dried or by dissipating the resulting steam.

Adsorption of water vapor

When the water vapor is adsorbed, the air to be dried is passed over an adsorbent by means of a fan. In technical applications it is mostly silica gel, often also a so-called molecular sieve. The water vapor accumulates on the adsorbent and condenses there. The adsorptive capacity of the adsorbent increases with falling temperature and rising water vapor concentration in the air. Due to the heat of adsorption and condensation released, it may be necessary to cool the adsorbent or the dried air.

The adsorbent needs to be regenerated after a certain period of time. This is usually done by drying with hot air. In the case of molecular sieves, re-drying can take place. This mainly happens in systems in which the air to be dried is compressed. The re-drying process usually takes place in two drying containers. In a container, the compressed air is sent through the adsorber, which removes the moisture from this air (by attaching it to the granulate of the molecular sieve). Then part of the compressed and dried air is passed through the second container against the atmosphere. The expansion makes the air considerably more absorbent for moisture and removes the previously accumulated water molecules from the molecular sieve. This process is switched back and forth between the two containers at regular intervals, so that a higher or lower percentage of the dried air is available for further use, depending on the desired degree of dryness of the compressed output air.


The technical adsorbents used in laboratory practice and especially in exhaust gas cleaning technology can be divided as follows:

  • Carbon-containing adsorbents (e.g. activated carbon, activated coke, carbon molecular sieves)
  • Oxidic adsorbents (e.g. active clay, silica gels, zeolites, trass)
  • Polymer adsorbents (often styrene polymers with the use of a crosslinking agent)
  • Mixed sorbents (e.g. mixture of hydrated lime and activated coke) these are not yet used technically to any significant extent

Because of their porosity, adsorbents have a large internal surface. This can sometimes be more than a thousand square meters per gram. [1] Pore diameter and pore size distribution depend on the particular adsorbent. While zeolites, for example, have a clearly defined pore diameter, a distinction can be made between macro, meso and micro pores in activated carbons, depending on the pore diameter. The specific volume of the micropores is usually between 0.25 and 1.2 cubic centimeters per gram of adsorbent. [1]

Adsorbents are impregnated for certain separation tasks. The impregnating agent serves either as a reactant (chemisorption) or as a catalyst for heterogeneous catalysis. [2] The adsorbents can be impregnated either by immersion and spray processes or by adsorption from the gas phase. The mass fraction of the impregnating agent can be up to 30 percent. [2] Examples of the use of impregnated adsorbents are activated carbons treated with sulfuric acid or bromine for the separation of mercury from combustion exhaust gases [3] and activated carbons provided with metal oxides for the reduction of nitrogen dioxide in vehicle interior filtration [4].

Adsorbents are used in exhaust gas cleaning systems, water treatment systems, extractor hoods, insoles, adsorption chillers and cabin air filters, among other things. The selection of the respective adsorbent depends not only on the components to be separated, but also on economic and safety aspects, among other things. The interaction between the substance to be adsorbed (adsorptive) and the adsorbent is important for the separation task. [5] The economic aspects also include the regenerability of the adsorbent used. This assumes that there is no reaction between the adsorbed substance (adsorpt) and the adsorbent. [6] When it comes to safety aspects, the reactivity and flammability of activated carbon play a prominent role. [7] Adsorbed hydrocarbons can also represent a fire load that cannot be neglected.


Litigation

The gas is introduced under increased pressure (usually approx. 6-10 bar) into a fixed bed reactor which is filled with the adsorbent, so that it is flowed through. One or more components of the mixture (the so-called heavy component) are now adsorbed. The so-called "light component" can be removed at the exit of the bed. After a while, the adsorber bed is largely saturated, and part of the heavy component also escapes. At this moment, the process is switched over using valves so that the outlet for the light component is closed and an outlet for the heavy component is opened. This is accompanied by a drop in pressure. At the low pressure, the adsorbed gas is now desorbed again and can be recovered at the outlet. Two alternately loaded and unloaded adsorbers enable continuous operation. In order to drive the supernatant of desorbed heavy components out of the adsorber bed, a portion of the desired product is used to rinse in order to avoid contamination.

The exact adjustment of the switching times is based on the desired purity of the gases. The increase in one component always takes place at the expense of its recoverable amount and the purity of the other component.

If you work at pressures below atmospheric pressure, the method is also known as VSA (Vacuum Swing Adsorption), if a pressure is above atmospheric pressure and one below, then it is referred to as VPSA (Vacuum Pressure Swing Adsorption). Except for the pressure range used and the precautions required as a result, these procedures are all identical.


Video: ADSORPTION # Adsorbate # Adsorbent (December 2021).