Amine Fundamentals
The removal of sour or acid gas components such as hydrogen sulfide (H2S), carbon dioxide (CO2), carbonyl sulfide (COS) and mercaptans (RSH) from hydrocarbon streams is a process requirement in many parts of the hydrocarbon processing industry. This is especially true with the increasingly stringent environmental considerations coupled with the need to process natural gas and crude oil with increasingly higher sulfur levels.
Alkanolamines are chemical solvents that are the most widely employed for gas sweetening purpose.
Why DGA?
There are several advantages of DGA over other amines that site it as the preferred and most suitable amine for gas processing. Major advantages are:
a. removal of COS and CS2
b. High reactivity, a 4 ppm H2S specification can generally be obtained for applications with low operating pressure and high operating temperature which is the case for all southern area gas plants
c. Enhanced mercaptan removal in comparison to other amines
d. Easily reclaimed to reverse the BHEEU formed by the reaction of DGA with CO2 and COS
Even though it might be the best amine for Sothern’s area climates it does have some disadvantages.
Major disadvantages are:
a. DGA absorbs aromatic compounds (Benzene, Toluene, xylene), BTX, from the inlet gas which potentially complicates the sulfur recovery unit operation. Aromatics suspend on the catalyst preventing H2S and SO2 from reacting in the sulfur plant.
b. Higher solvent cost relative to MEA and DEA
Alkanolamines have three functional groups: an amino nitrogen, an alcohol group (hydroxyl) group and alkane (hydrocarbon) arm. In general, the hydroxyl group serves to reduce vapor pressure, modify base strength and increase water miscibility, while the amino group provides the necessary alkalinity in water solutions to promote the reaction with acid gases. Also, all the alkanolamines have at least one alkane (hydrocarbon) arm that separates the hydroxyl and amino group and provides a degree of chemical stability.
HOC2H4OC2H4-NH2
DGA is a primary amine which means that it only contains one hydrocarbon arm
The overall equilibrium reactions applicable for H2S and CO2 and primary and secondary amines are shown below with a primary amine. A qualitative estimation of the velocity of the reaction is given.
For hydrogen sulfide removal
RNH2 + H2S —— RNH3+ + HS-
RNH2 + HS- —— RNH3+ + S—
The overall reactions between H2S and amines are simple since H2S reacts directly and rapidly with all amines to form the disulfide and sulfide.
For hydrogen sulfide removal
2RNH2 + CO2 —— RNH3+ + RNHCOO-
RNH2 + CO2+ H2O —— RNH3+ + HCO3-
RNH2 + HCO3 —— RNH3+ + CO3–
DGA Lab Analysis:
Several lab analyses are accomplished in lab center. Beside gas chromatograph and water & steam analysis, precise DGA and TEG analysis are also carried out. These analyses are listed below; however for more sophisticated analysis, such as dissolved HC, TDS, or heat stable salts (HSS), special vendors such as MPR or amine experts are contacted.
The DGA analyzed is lean DGA sample. The sample is collected from the cool DGA stream after FCV-106/108. Normally the required DGA analysis includes the following:
???? Appearance
???? Solution strength and composition
???? Acid gas loading
???? Heat Stable Salts (HSS) (Not carried out in UGP lab)
???? Ion Content (iron, chloride, sodium…etc)
???? Total Suspended Solids (TSS) (Not carried out in UGP lab)
???? Total Dissolved Solids (TDS)
???? Foaming (Not carried out in UGP lab)
???? pH
Amine Loss Mechanism
Amine Losses are due to vaporization, entrainment, solubility, and degradation. The difference between estimated losses and actual current plant losses is attributed to mechanical losses such pipe and flange leak, filter change, and pump seal leaks. The ranking of DGA losses from highest to lowest is as follows:
1- Mechanical
2. Entrainment
3. Vaporization
4. Degradation