The gas sweetening process is the method of removing Hydrogen Sulfides, Carbon Dioxide, and Mercaptans from natural gas to improve its quality and make it suitable for transport and sale. These elements are corrosive and toxic in nature and should be removed.
Reasons for Gas Sweetening Process
Removal of the contaminants from Gas is required for the reason of:
- Corrosion Control
- Toxicity
- Gas/ Liquid product specifications
- To prevent poisoning of catalysts in downstream facilities
- Control of the overall heating value of the natural gas
- To meet environmental requirements
CO2 & H2S are very common contaminants in Natural Gas system
Reason for CO2 Removal for Gas Sweetening:
- When combined with water forms Carbonic Acid which is corrosive
- CO2 reduces the BTU value of the gas
- If present in 2-3% gas is not saleable.
Reason for H2S Removal for Gas Sweetening:
- Extremely toxic gas
- Highly corrosive
Reason for Mercaptans (R-SH) Removal for Gas Sweetening:
- It has smell
- If comes in contact with bacteria (inside the final product storage) then released H2S
- 2R-SH Bacteria R-SSR + H2S
Gas Sweetening Process Treatment – Process Selection
- Air pollution regulations regarding Sulfur Compound disposal and/or Tail gas clean-up requirements
- Type & concentration of Impurities in sour gas
- Temp/pressure at which sour gas is available and at which sweet gas must be delivered
- Specification for the Acid gas
- Specification for the Residue gas
- Hydrocarbon composition of the gas
- Selectivity required for Acid Gas Removal
- Capital cost and operating cost
Chemical Reaction Process (Continuous) for Gas Sweetening
Amine Process
- MEA (Mono ethanol Amine)
- DGA (Diglycolamine)
- DEA (Diethanol Amine)
- MDEA (Methyl Dethanol Amine)
- DIPA (Diiso Propanol Amine)
- Formulated Amines (Proprietory Amines)
- Mixed Amines
Physical Solvents (Absorption)
- Selexol (DOW process)
- Fluor Solvent
Application of Physical Solvent
Consider when
- The partial pressure of acid gas > 50psi
- Heavy HC concentration is low
- Bulk removal of acid gas required
- Selective removal of H2S is required
Advantages of the Combination Process
- Can achieve 30ppm of CO2 and 1ppm of H2S
- Low Energy requirement
- Low foaming & non-corrosive
- High acid gas loading
Combination of Chemical + Physical Solvents
- SULFINOL-M (Sulfolane + MDEA)
- SULFINOL-D (Sulfolane + DIPA)
- UCARSOL (Formatted MDEA based Solvent)
Other Methods
- Molecular Sieve
- Locat
- Sulferox
Physical Solvent v/s Chemical Solvents
Evaluation of Process
Note:
- Acid gas (H2S+CO2+CS2+COS+RSH)
- Acid gas pick-up – Mole of Acid gas removed in Regenerator
- Acid gas loading – Quantity of Acid gas absorbed in the Amine Solution
Process Capabilities
Sulfinol (SHELL TECHNOLOGY) Process of Gas Sweetening
Solvents:
- Sulfinol-M {Sulfolane (20%)+MDEA (50%)+Water (30%)}
- Sulfinol-D {Sulfolane (20%)+ DIPA (50%)+Water (30%)}
- Sulfinol-X Improved Sulfinol M & Sulfinol D with an Acceleration Agent
Note: Sulfolane – Tetrahydro Thiophene Dioxide (Physical Solvent)
Merits of Sulfinol Process
- Higher Acid Gas loading (typical 1.5 Mole of AG/Mole of Sulfinol)
- Lower corrosion Rates
- Ability to slip up to 50% CO2 (Selective removal of H2S from gas FEED containing CO2)
- Lower solvent rate
- Lower Energy of Regeneration
- Lower capital & Energy / Operating cost per unit of Acid Gas removed
- Complete removal of mercaptans
- The high removal rate of COS
- Lower foaming tendency
- Can be used for a wide range of operating pressure (up to 80 bar)
- Sulfolane increases the physical solubility of the acid gas contaminants in the solvent
- Sulfolane improves the efficiency of the absorption process
- Physical solubility of Mercaptants increases.
De-merits of Sulfinol Process
- Payment of Licensing fee
- The solvent is costlier than chemical solvents
- At low acid gas partial pressure, the advantage of a lower circulation rate for the Sulfinol process diminishes as compared to chemical solvents.
How we can removing Mercury from sweetening unit ?