Gas 2025 Issue

Hence, regeneration was not taking place properly. The low temperature out the top of the regenerator is a result of the loss of steam at the top of the column section. The steam generated in the reboiler has three objectives:  Heat the feed solvent to the reboiler temperature (sen- sible heat load).  Break the reaction bond between the CO 2 /H 2 S and the MEA.  Provide enough energy for a reflux flow between 7-10% of the main circulation flow rate. For the regenerator heat load, the energy required to heat the solvent is increased when the feed temperature is cooler than the recommended 195°F minimum. According to plant data, the rich amine feed temperature was only 160°F (71ºC), which means the steam has to heat up the solvent by 90°F instead of 45-55°F in a typical amine unit. The extra steam wasted in heating up the solvent reduces the amount of steam available to regenerate the CO 2 and H 2 S from the rich amine solvent. This means the regenerator runs out of steam before steam reaches the top of the regenerator, so rather than stripping acid gases out in the top of the regenerator (ideal situation), rich amine travels deeper down the regenerator and even into the reboiler before the amine is finally regen - erated. This is one of the most common causes of regener- ator corrosion. H 2 S is a slightly weaker acid gas than CO 2 (carbonic acid) , so it is more easily regenerated higher up the regenerator. Excessive CO2 in the hottest part of the regenerator and reboiler leads to excessive corrosion all the way through the hot lean piping until the amine solvent is cooled in the lean/ rich (L/R) exchanger. As the simulation graphs show (see Figure 4 ), CO 2 is still being regenerated in the bottom of the regenerator, whereas most H 2 S is removed by the time the amine solvent flows into the reboiler. This is a situation with a very high corrosion potential.

Contactor t emperature pro f ile

Vapour Liquid

Temperature (Fahrenheit)

Figure 3 Predicted absorber temperature bulge

the amine that will cause corrosion of both carbon steel and stainless steel. Regeneration of the amine solvent is achieved by counter-currently reacting the solvent with steam flowing up the regenerator tower. MEA is a very strong primary amine, so it does not want to give up its reacted acid gas easily. It takes significant steam energy to regenerate rich MEA. One can tell that the unit has generated sufficient steam to properly regenerate the rich amine solvent by checking the regenerator overhead temperature. For MEA solvents, this temperature must be 225-235°F (107-113ºC). The actual value was only 192°F (89ºC).

Regenerator : Hydrogen sulphide in vapour phase

Regenerator: Carbon dioxide in vapour phase

0E+0

5E+2

1E+3

2E+3 1.5E+3 Concentration (ppmw) 2.5E+3

3E+3

3.5E+3

4E+3

0E+0

1E+5

2E+5

4E+5 3E+5 Concentration (ppmw) 5E+5

6E+5

7E+5

8E+5

Figure 4 H 2 S and CO 2 desorption curves in the regenerator column. H 2 S (left) CO 2 (right)

Gas 2025