6 Flashcards
(28 cards)
Acid gas removal
We want to reduce the contents of CO2 and H2S gases to meet the quality standards for natural gas distributors
Standards for CO2 and H2S
- CO2 : 2mol%
- H2S : 4ppm
How do we go about removing the acid gases?
Via absorption and stripping columns
And when we have a downstream dehydration process via absorption using glycols or MEA
Which fluid package do we use for absorption?
We use Acid Gas Chemical solvents as these are very high non ideal system
Absorption process
- Gas enters bottom with contaminants and a liquid stream also enters from top
- Separation occurs by gas mixture contact with liquid
- Clean gas stream top and liquid with contaminant stream leaves bottom
Stripping process
- Gas (steam) enters bottom and a liquid stream (amine) with contaminants also enters from top
- Separation occurs by liquid mixture contact with gas
- Gas stream with contaminants top and clean liquid (lean amine) stream leaves bottom
Types of amine solvents
MEA/ DEA
How is a stripper modelled on HYSYS?
With a distillation column
What is the operating pressure and temperature like for absorbers?
Typically low
If operating temperature is too high, the solvent can evaporate. More solvent needed for better separation
What does it mean if the concentration of contaminant in the gas is high?
The separation becomes more difficult to carry out
What type of columns are commonly modelled for absorption columns in HYSYS?
Packed columns
Could also use tray columns
When do we use physical absorption?
Usually in high pressures and when the concentration of the contaminant in the gas is high
When do we use chemical absorption?
When the concentration of contaminant in the gas is low
What is the tray efficiency like in absorbers?
Low (20-30%)
The tray efficiency set is a combination of operating experience, predictive correlations using height of liquids on the tray and residence time of gas in the liquid.
When do we use Acid Gas- Chemcial Solvents?
Removal of CO2 and H2S via amines
When do we use Acid Gas - Physical Solvents?
Removal of CO2 and H2S via DEPG, selexol solvent or , methanol
When do we use Acid Gas - Liquid Treating?
Removal of CO2 and H2S via amines from natural gas liquid or liquified petroleum gas
When do we use Acid Gas - Caustic Wash?
- Caustic NaOH
- treating liquid streams only like the previous one
Acid Gas - Chemical Solvents
Addresses highly non-ideal absorption systems
Ensure to add heat stable salts
What is the number of degrees of freedom for absorption columns?
0 ; no reboiler or condenser
Calculation type for absorption columns
- Efficiency : commonly used (default) for H2S and CO2
- Advanced Modelling : assumes rigorous rate-based model to relate mercaptans, COS, CS
Efficiencies calculated for each tray
CO2 ones tends to be lower than H2S
Easier to remove H2S than CO2
Rich amine stream
Bottom of the absorber out
Lean amine stream
Feed liquid stream into the absorber
