Absorption:
Absorption is where a liquid is soaked up into something like a sponge, cloth or filter paper. The liquid is completely absorbed into the absorbent material.
Adsorption:
Adsorption is always exothermic. When gas molecules are adsorbed, the entropy on the surface reduces. Hence, the enthalpy should be negative to have a zero Gibbs.
DeltaG=DeltaH-T(DeltaS)
Adsorption refers to individual molecules, atoms or ions gathering on surfaces. Due to the high surface area and porosity, activated carbon and aluminum are good adsorbents to adsorb contaminants. Aluminium will be used as an aluminium oxid to prevent the burning of the metal by the desorption with high temperatures. If the metal burns, it smells terrible. Activated carbon would not be used for the oxygen services and high temperatures, which can burn.
Langmuir suggested that adsorption takes place through this mechanism:, where A is a gas molecule, and S is an adsorption site. If we define surface coverage,, as the fraction of the adsorption sites occupied, in the equilibrium we have. KP is the partial pressure of the gas or the molar concentration of the solution. For very low pressures Teta=KP, and for high pressures Teta=1.
A(g/lig)+S(solid)=AS
Langmuir model for isothermal adsorption
The removal of the sulfur from the gas can be conveyed physically by the water and henry rule: The
The removal of the oxygen from nitrogen through the copper catalyst bed:
- Reduction: CuO(s)+H2(g)=Cu(s)+H2O(g): exothermic
- Deactivation: 2Cu+O2=2CuO: exothermic
Physical adsorption versus chemical adsorption:
Physical absorption occurs when the adsorbate gas molecules are held by physical forces like Van der Waals forces. Chemical adsorption occurs when the absorbate molecule is held on the adsorbent surface by chemical forces as short covalent chemical bonding occurs by the sharing of electrons.
Once all the bonds on the surface of adsorbent are used for chemical bonding, no further adsorption is possible. Hence in chemical adsorption only one layer of adsorbate is formed in the adsorbent
Dehydration with molecular sieve (zeolite):
A molecular sieve works by adsorbing gas or liquid molecules that are smaller than the effective diameter of its pores, while excluding those molecules that are larger than the openings. One water molecule is ca. 3 Angestrom.
Scrubbers:
A scrubber system is used in the removal of harmful and hazardous particulates from industrial gas streams before they are released into the environment.
How to remove H2S from the flue gas:
NaOH react with H2S dissolved in aqueous solution to form NaHS or Sodium Sulfide. This is an acid-base reaction (neutralization): H2S is an acid, NaOH is a base.
Caustic based methods:
Sodium hydroxide (caustic soda):
Unlike amines, sodium hydroxide is not regenerable. It is possible to design caustic scrubbers to selectively remove the H2S and leave the majority of the CO2 in the gas (based on slower CO2 absorption rates)
H2S+NaOH=Na2S+H2O
Amine gas treating:
Amines like Diethanolamine (DEA) and Monoethanolamine (MEA) are used. A positively charged ammonium group (RNH+3) can be gained:
RNH2 + H2S ⇌ RNH+3 + HS−
RNH2 + H2CO3 ⇌ RNH+3 + HCO−3
The resulting ions are more soluble in solution and can be removed from the gas phase.
Sour gas consists of CO2 and H2S
Iron-Based Adsorbents:
Iron oxide pellets are packed into a scrubbing unit and the biogas is passed through.
Fe2O3+H2S=FeS+H2O
How to remove NOx from the flue gas:
- Using ammonia (NH3) with high temperatures and oxygen: NOx will be converted in N2 and H2O.
- Using ammonia (NH3) with oxygen and catalyst: NOx will be converted in N2 and H2.
