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2.1.1 The Removal of Acidic Gas from Natural Gas
The common and most widely used gas purification processes can be classified into the following five categories [13, 16]:
1. Absorption using organic solvents.
2. Adsorption onto solid surfaces.
3. Application of membranes.
4. Chemical conversion to convert into another compound.
5. Cryogenic condensation.
Among the five common methods, the most important method for gas purification is the absorption using organic solvent. Alkanolamines such as monoethanolamine
MEA, diethanolamine
DEA, methyldiethanolamine
MDEA, and
di- isopropanolamine DIPA are the most commonly used solvents for the absorption of
acidic gases such as CO
2
and H
2
S [13]. The capability of alkanolamine to absorb acidic gas depends on the functional groups of alkanolamine i.e hydroxyl group and
amino group. The hydroxyl groups are capable of reducing the vapor pressure and increase the water solubility, whilst the amino group is capable to provide the
alkalinity in water solution, that cause the acidic gas absorption. Methyldiethanolamine MDEA is one of the common alkanolamine which is
widely used in petrochemical industries. MDEA is used as an intermediate material in the synthesis of pharmaceutical products e.g. analgesic and antispasmodic agent,
personal care products e.g. fabric softener and foaming agent on the shampoo, and the most importantly in gas processing plants [17]. The structural formula of MDEA
is presented in Figure 2.1. MDEA has two ethanol functional groups and one methyl group. Those groups are attached to a nitrogen atom. Due to the existence of nitrogen
atom with a pair of free electrons, MDEA forms weak base in aqueous solution, hence MDEA is often used for scrubbingsweetening of acidic gases CO
2
and H
2
S from raw natural gas. MDEA chemically binds with the acidic gases and when heated it
releases the absorbed gases [13, 18].
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Figure 2. 1 Chemical Structure of MDEA
The acidic gas purification using MDEA solution occurs according to Reactions 2.1
– 2.6 [13, 19 − 20]: Ionization of water:
H
2
O ↔ H
+
+ OH
-
2.1 Ionization of dissolved H
2
S: H
2
S ↔ H
+
+ HS
-
2.2 Hydrolysis and ionization of dissolved CO
2
: CO
2
+ H
2
O ↔ HCO
3 -
+ H
+
2.3 Protonation of MDEA:
R
2
NCH
3
+ H
+
→ R
2
NCH
4 +
2.4 Acid-basic reaction with the amine:
R
2
NCH
4 +
+ HCO
3 -
↔ R
2
NCH
3
+ H
2
O + CO
2
2.5 R
2
NCH
4 +
+ HS
-
↔ R
2
NCH
3
+ H
2
S 2.6
The basic flow diagram for an acid gas absorption process is shown in Figure 2.2. The gas treating process includes an absorber unit and a regenerator unit. The typical
operating range of temperature and pressure are 35 ⁰C to 50 ⁰C and 5 atm to 205 atm
respectively in the absorber while in the regenerator at 115 ⁰C to 126 ⁰C and 1.5 atm
to 1.7 atm, respectively.
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Figure 2. 2 Flow diagram of a typical amine treating process [13].
Acidic gases such as CO
2
and H
2
S are absorbed in an absorber to produce a purified gas as the product and a rich amine solution i.e. an amine solution + dissolved CO
2
and H
2
S. The rich amine is then routed into the regeneration unit a stripper with a reboiler to produce a lean amine that is to be recycled. Further, the H
2
S-rich stripped gas stream is then commonly routed into a Claus process to convert it into elemental
sulfur, and the CO
2
generated during desorption can be used for enhanced oil recovery EOR [13].
2.1.2 Process Wastewater from Natural Gas Sweetening Operation
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