New developments in acid gas removal

One of the challenges when designing a natural gas treatment plant is to optimize the combination of the acid gas removal (AGR) unit and the sulphur recovery unit (SRU). With some feed gases, a simple AGR will supply a lean acid gas that is costly to treat in the SRU. Below are three examples where Jacobs found the perfect fit of AGR and SRU.

Case studies to optimise the h2s concentration for subsequent sulphur recovery

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Article Summary

Hot flash technology and Sulfinol-M
for a gas plant that processes 3 million Nm3/hr of natural gas, a standard AGR with DGA would supply typically 35% H2S to the downstream SRU. Jacobs together with Shell developed an alternative scheme with an integrated hot flash enrichment and employing Sulfinol-M. Based on this approach the H2S concentration could be increased from 35 mol% to 62 mol%.

The sour gas contains considerable quantities of CO2 and heavy hydrocarbons.
By oversizing the main absorber, the concentration of CO2 in the treated gas can be tailored to the specification.
Sulfinol-M is a water based solvent containing MDEA and sulfolane and can be used for both selective and non-selective removal of H2S in the presence of CO2.

How does the scheme in Figure 1 work?
Sour feed gas is processed in a Sulfinol-M AGR system containing an enrichment step. The enrichment occurs in an additional enrichment flash vessel and an enrichment absorber.

In the non-selective AGR absorber, Sulfinol-M absorbs the H2S, CO2 and trace sulphur components, the treated gas leaves the top of the absorber.

Rich solvent from the bottom of the absorber passes a hydrocarbon flash vessel, is heated and then flashed at reduced pressure in the enrichment flash vessel. The CO2 rich flash vapour flows to an enrichment absorber where the H2S and some trace sulphur components are absorbed preferentially by Sulfinol-M. The gas leaving this absorber is routed to the incinerator, whereas the rich solvent from the enrichment, together with the flashed solvent from the hot flash vessel, is treated in the regenerator. Finally the enriched acid gas from the regenerator goes to the SRU for sulphur recovery.

By applying Sulfinol-M and hot flash enrichment, instead of a standard DGA absorber, a considerable increase in H2S concentration could be achieved. As a consequence, costs could be saved in the downstream SRU. It was estimated that by applying this scheme, the overall cost for AGR and SRU was reduced by 25%.

Acid gas removal for coal gasification
In coal gasification plants the acid gas removal can be a challenge. Especially when biomass is mixed into the feed, the current designs are often not adequate. With a higher biomass content, the resulting CO2 concentration in the acid gas goes up and H2S goes down (and in the case considered can be as low as 13% H2S). This lean gas still has to be treated in the SRU. Jacobs proposed two modifications to an existing AGR system, to achieve an acceptable acid gas for the SRU. To better understand what is involved, first a short description of the acid gas removal (AGR) system will be given (Figure 2).

Acid gas passes a hydrolysis unit (not shown) where hydrogen cyanide/carbonyl sulphide (HCN/COS) are removed. The gas than passes an AGR absorber using Sulfinol M, which decreases sulphur content in the syngas to less than 20 ppm by volume. The loaded Sulfinol is flashed in a hot flash vessel with recontactor, and is then regenerated. A common regenerator is used for both the AGR and the SCOT absorber.

The first modification is to operate the hot flash vessel at a higher temperature and route the gas from the recontactor directly to the SCOT burner (line A), bypassing the Claus unit. In this way a considerable part of the H2S is absorbed in the SCOT absorber.

The second modification is to use the SCOT absorber/regenerator as an enrichment system. Therefore part of the acid gas from the regenerator is routed directly to the SCOT absorber (line B). This additional enrichment comes at a cost, because more solvent has to be circulated and more steam has to be provided for the regenerator. But by the modifications that Jacobs proposed, the existing plant could handle more biomass and still process a Claus feed gas of 40% H2S.

Tuned enrichment

In the gas plant shown in Figure 3, 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 and the low H2S case.

How does it work?
The sour gas is routed at 60 barg to the AGR absorber where it is contacted with an activated MDEA solvent. Here virtually all 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 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 8.0 barg.

To remove any H2S from the flash gas, 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 by 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 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 used in both the AGE and TGT absorbers.

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

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