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Oct-2015

Combining acid gas treating and amine tail gas treating (TIA)

One of the challenges in designing a natural gas treatment plant is to optimise the combination of the acid gas removal (AGR) unit and the sulphur recovery unit (SRU).

Frank Scheel
Jacobs Engineering Group
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Article Summary
With some feed gases, a simple AGR would supply a lean acid gas that is costly to treat in the SRU. The following case study shows how Jacobs found the perfect fit of AGR and SRU.

In the gas plant shown in Figure 1, 50 000 Nm3/h is to be treated to remove H2S, CO2 and mercaptans. Jacobs designed the plant to operate with two different feeds, one with a high H2S content and another with half the amount of H2S. The sour gas flow rate and CO2 content are the same for both cases. By employing a recycle, a good quality stream of acid gas for the SRU could be produced, in both the high H2S and the low H2S case.

The sour gas is routed at 60 bar to the AGR absorber where it is contacted with an activated MDEA solvent. Here, virtually all of the H2S and a large part of the CO2 are absorbed into the solvent. Because this plant produces LPG, the majority of mercaptans in the feed gas can be slipped into the treated gas and later removed in the downstream LPG extraction unit.

The rich solvent is flashed in a rich solvent flash vessel. In this flash stage, entrained and dissolved hydrocarbons as well as some of the H2S and CO2 present in the solvent are flashed off at an intermediate pressure of 8 bar.

To remove any H2S, the flash gas is contacted in the recontactor with a side stream of the lean MDEA supply to the main absorber. The treated flash gas (H2S <100 ppmv) is sent from the recontactor to the fuel gas system. After flashing, the rich solvent is sent to the AGR regenerator where it is stripped with steam. The regenerator overhead gas is sent to the acid gas enrichment (AGE) section.

The AGE absorber is designed for selective H2S absorption (using a promoted selective MDEA), therefore virtually all the H2S is absorbed while only a minor part of the CO2 is absorbed.

The acid gas from the AGR regenerator is routed to the AGE absorber where it is contacted with semi-lean selective MDEA solvent from the tail gas treatment (TGT) absorber. In this section, the bulk of the H2S is absorbed. Due to the low H2S content in the TGT section, this solvent is only partially loaded. H2S removal is completed in the top of the AGE absorber against lean amine from the AGE regenerator.

The treated gas leaves the top of the AGE absorber; it is mainly CO2 with 100-150 ppm H2S and is routed to the incinerator of the SRU.

The rich solvent from the AGE absorber is pumped to the AGE regeneration section. The AGE regenerator is a common regenerator for the selective MDEA that is used in both the AGE and TGT absorbers.

Finally, the AGE regenerator overhead gas is routed to the Claus burner of the SRU.

In the low H2S case, the H2S to CO2 ratio is lower, which would result in a lower quality acid gas to the SRU. Since in this case the hydraulic load of the AGE absorber is smaller, part of the acid gas stream from the AGE regenerator can be routed back to the AGE absorber. In this way, the H2S concentration in the 
acid gas feed to the SRU can be increased. The gas compositions for both cases are shown in Table 1.

By using this recycle, the quality of the acid gas to the SRU can be maintained in both operating cases. The SRU can easily handle the acid gas since it has at least 44% H2S.

This short case study originally appeared in PTQ's Technology In Action feature - Q4 2015 issue.
For more information: frank.scheel@jacobs.com

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