Gas 2023 Issue

140.1 F

140.3 F

Hydrogen sulphide (V) 0.8

ppmv

Hydrogen sulphide (V) 3.9

ppmv

Sweet outlet-1

Sweet outlet

Carbon dioxide (V) 30.0 ppmv Carbonyl sulphide (V) 189.1 ppmv Carbon dioxide (V) 0.0092 lbmol/hr Carbonyl sulphide (V) 0.0578 lbmol/hr

Carbon dioxide (V) 1.72 mol% Carbonyl sulphide (V) 405.3 ppmv Carbon dioxide (V) 5.3525 lbmol/hr Carbonyl sulphide (V) 0.1262 lbmol/hr

Lean amine-1

Lean amine

MDEA absorber

DEA absorber

130.0 F

Hydrogen sulphide (V) 2

mol%

Hydrogen sulphide (V) 2

mol%

Carbon dioxide (V) mol% Carbonyl sulphide (V) 496 ppm mol Carbon dioxide (V) 16.3438 lbmol/hr Carbonyl sulphide (V) 0.1634 lbmol/hr 5

Sour inlet

Rich amine-1

Rich amine

Figure 1 Absorber case study comparing COS pick-up in MDEA vs DEA

Any base, B, present in solution deprotonates the zwitter- ion. These reactions are responsible for quite significant COS absorption rates into primary and secondary amines, but they do not occur with tertiary amines. Reaction (4) is known to be equilibrium limited. The rate of the reverse reaction is observed to be practically zero for Reaction (5), thus indicat- ing that, for any amine, COS will completely hydrolyse to CO2 and H 2S in the fullness of time. Thiocarbamate formation is significantly limited by the rate of deprotonation, Reaction (7). In fact, for several amines, the COS absorption rate is almost completely determined by the rate of deprotonation. This is unlike CO2 , where the zwitterion deprotonation rate has much less influence on the overall conversion. As a result of these factors, the COS-amine reaction rate is much slower than amine-CO2 . Nevertheless, COS reaction rates are significant enough for a substantial fraction of the COS in a typical feed gas to be removed by primary and secondary amines. However, such is not the case for mercaptans beyond MeSH because they are very weak acids and easily displaced by co-absorbed CO2 and H 2S. Recently, we finished developing a COS absorption model that treats COS as a rigorous mass transfer rate- controlled component and incorporates it along with its reaction kinetics into the OGT ProTreat simulator. The model results were validated against some 20 proprietary sets of field-performance data for various amine systems. They showed the model accurately simulates COS removal in amine absorbers for the first time. What follows is a case study showing: Comparison of absorber outlet concentrations as predicted by the Legacy vs Kinetic Model for the absorber models shown in Figure 1

• Mass-transfer and reaction-rate control in the COS removal model • A comparison between various amines’ performance for COS removal in a simple absorber • Comparison between simulation and actual plant perfor - mance in RSH removal • Summary of literature renditions of VLE in RSH-amine systems. Because of the role played by reaction kinetics, differ- ent types of amines (primary, secondary, tertiary) have quite different COS removal effectiveness. For mercaptans removal, on the other hand, it is mainly the pKa of the amine that determines RSH removal – kinetics plays no role at all. Therefore, it makes sense to treat COS and mercaptans removal in separate ways. Case studies The following case studies elucidate the absorption mecha- nism of COS and mercaptans in typical absorbers: COS Figure 1 shows the simulation of two simple absorbers, one using a 3M solution of DEA and the other using MDEA at the same molar strength and circulation rate. In both cases, the feed gas is 5 mol% CO2 and 2 mol% H 2S; 500 ppmv of COS was assigned to the inlet gas. Callouts attached to the gas streams show the gas analyses. For MDEA and DEA respectively, the CO2 removal effi - ciency is about 67% and 99% compared with 23% and 65% for COS. As expected, amines do not remove COS as effectively as CO2, although a significant amount of pick- up is seen. In addition, DEA, a secondary amine, reacts much faster with COS by forming thiocarbamate via the zwitterion mechanism. This leads to almost three times greater efficiency than MDEA which, as a tertiary amine, cannot form thiocarbamate. Table 1 compares predictions using our new model for COS absorption (‘Kinetic’ in table) with what has been the only type of simulation commercially available until now (‘Legacy’ in table). The Legacy and Kinetic Models give identical predictions of CO2 and H 2S removal, as one might expect. However, the Legacy Model predicts that

DEA

MDEA

Model

CO 2

H 2 S COS

CO 2 H 2 S COS

ppmv

mol%

ppmv

Legacy (equilibrium) Kinetic (reaction rate)

30 30

0.8 0.8

523† 189

1.7 3.9 508†

1.7 3.9 405 † Removal of CO2 and H 2S concentrates COS above its 500 ppmv inlet value

Table 1

Gas 2023