Amine wash

Greatly simplified flowchart of the amine scrubbing process

The amine scrubbing is a commonly used chemical process for removing carbon dioxide , hydrogen sulfide and other acid gases from gas mixtures. The amine scrubbing is based on the principle of chemisorption and thus achieves high purities even at relatively low pressures in the absorption column. The selectivity is also usually higher than with physisorption .

Procedure

For amine scrubbing, slightly alkaline aqueous solutions of amines (mostly ethanolamine derivatives) are used, which reversibly chemically absorb acidic gas components . In the absorber is (ca. 40 ° C) and slightly elevated pressure fed (about 8 bar) fresh amine solution from the head at low temperature. The gas to be cleaned is fed into the separating column from below . The chemical reaction of the absorbed CO ₂ with the amine solution releases energy in the form of heat. The purified gas leaves the column at the top and the loaded detergent (amine solution) is pumped into the desorber at the bottom of the column.

In the desorber, at high temperature (use of thermal energy) and low pressure, the reaction, the chemical equilibrium is reversed and the bound acidic gas is removed from the detergent. The pure gas (with moist components) can thus be drawn off at the head of the desorber. The proportions of detergent and water are separated from the gas stream by a condenser and fed back into the column.

The subsequent regeneration of the washing agent enables the bound gas to be recovered in a highly concentrated manner. This makes a later material use of z. B. CO ₂ or H ₂ S ( Claus process ) possible. This is not possible with other methods such as pressurized water washing.

Diethanolamine and monoethanolamine are mostly used , but methyldiethanolamine , diisopropylamine , diisopropanolamine and diglycolamine are also commonly used:

Monoethanolamine (MEA) only absorbs CO ₂ , with a reaction enthalpy of approx. 85 kJ / mol
Diethanolamine (DEA)
Methyldiethanolamine (MDEA) absorbs H ₂ S and CO ₂ with a reaction enthalpy of approx. 62 kJ / mol
Diglycolamine (DGA)

Mixtures of these amines mentioned are often used in order to use synergy effects , such as a catalytic effect for the reactions of the detergent.

An important evaluation criterion for the various mixtures is the heat output that is used in the evaporator to regenerate acidic gas from the detergent. This can be expressed in terms of the amount of heat absorbed by the CO ₂ .

Reactions

The net reactions of H ₂ S and of CO ₂ are shown here.

{\ displaystyle \ mathrm {R {-} NH_ {2} \ + \ H_ {2} S \ \ rightleftharpoons \ R {-} NH_ {3} ^ {+} \ + \ HS ^ {-}}}

{\ displaystyle \ mathrm {R {-} NH_ {2} \ + \ CO_ {2} \ \ rightleftharpoons \ R {-} NHCOO ^ {-} \ + \ H ^ {+}}}

Response network

Ionization of the water:

${\ displaystyle {\ ce {H2O <=> H + + OH-}}}$

Ionization of the dissolved H ₂ S

${\ displaystyle {\ ce {H2S <=> H + + HS-}}}$

Hydrolysis and ionization of the dissolved CO ₂

${\ displaystyle {\ ce {CO2 + H2O <=> HCO3- + H +}}}$

Protonation of the amine

${\ displaystyle {\ ce {RNH2 + H + <=> RNH3 +}}}$

Formation of the carbamates

${\ displaystyle {\ ce {RNH2 + CO2 <=> RNHCOO- + H +}}}$

These reactions describe possible paths for binding CO ₂ or H ₂ S. According to Henry's law , the equilibrium concentrations that arise depend on the partial pressures of the gases. The carbamates formed represent a group of stable reaction products that are only dissolved again in the desorber.

When CO ₂ is absorbed in the amine / water mixture, the CO ₂ first dissolves in the water and forms carbonic acid . The resulting carbonic acid first breaks down into H + and HCO3- ions. These react with the amine. Thus, the absorbed CO _{2 is} chemically bound reversibly. The chemical equilibrium is therefore only reached when the detergent load is significantly higher. This mechanism can also be used to explain the selectivity of an amine for a gas component.

use

This process is used in refineries , petrochemical plants , in the direct air capture process, in steel mill processes, in natural gas and biogas processing and in flue gas desulphurisation .

One way of reducing emissions is to separate the carbon dioxide from flue gases in coal and gas power plants using amine scrubbing and then to store it ( CCS ). It can also be used as a starting material for various chemical syntheses ( power-to-chemicals ). At present, this procedural path cannot yet be represented cost-effectively.

Exemplary explanation

The mind game with carbonated mineral water makes the principle of absorption more tangible:

If a mineral water bottle is cold and unopened (high pressure inside), a lot of CO ₂ is bound in the water in the form of carbonic acid. In the case of amine scrubbing, this represents the absorber. If the bottle is now opened (lower pressure) and stored uncooled (higher temperature), the CO _{2 is released} from the bond and is released into the environment. This represents the desorber in amine scrubbing. The biggest difference is that amine scrubbing is a mixture of water and amines that can chemically bind significantly more CO ₂ and less heat has to be added during regeneration in the desorber.

Individual evidence

↑ ^a ^b ^c ^d ^e ^f Arthur B. Kohl, Richard B. Nielsen: Gas Purification . 5th edition. Gulf Publishing Company, 1977, ISBN 0-88415-220-0 , pp. 50 ff .
↑ ^a ^b Klinski, Stefan .: Feeding biogas into the natural gas network: Study; this work was carried out within the framework of the project: "Evaluation of the possibilities for feeding biogas into the natural gas network" (FKZ 22021103) . 2nd edition. Agency for renewable raw materials, Gülzow 2006, ISBN 3-00-018346-9 .
↑ ^a ^b ^c ^d Bernd Overmaat: Carbon2Chem. In: ThyssenKrupp. ThyssenKrupp, accessed on September 13, 2019 .
↑ ^a ^b Stefan Klinski: Feeding biogas into the natural gas network . Ed .: Fachagentur Nachwachsende Rohstoffe e. V. 2nd edition. Leipzig 2006, ISBN 3-00-018346-9 , FKZ 22021103, p. 38-39 .

[:0-1] ↑ ^a ^b ^c ^d ^e ^f Arthur B. Kohl, Richard B. Nielsen: Gas Purification . 5th edition. Gulf Publishing Company, 1977, ISBN 0-88415-220-0 , pp. 50 ff .

[:2-2] Klinski, Stefan .: Feeding biogas into the natural gas network: Study; this work was carried out within the framework of the project: "Evaluation of the possibilities for feeding biogas into the natural gas network" (FKZ 22021103) . 2nd edition. Agency for renewable raw materials, Gülzow 2006, ISBN 3-00-018346-9 .

[:1-3] Bernd Overmaat: Carbon2Chem. In: ThyssenKrupp. ThyssenKrupp, accessed on September 13, 2019 .

[:3-4] Stefan Klinski: Feeding biogas into the natural gas network . Ed .: Fachagentur Nachwachsende Rohstoffe e. V. 2nd edition. Leipzig 2006, ISBN 3-00-018346-9 , FKZ 22021103, p. 38-39 .