Understanding the benefits of solvent swaps in gas-processing units

Given the global challenges of investing in new gas-processing capital projects, unlocking the potential of existing operating facilities through selective investment is increasingly becoming a norm. One way to achieve additional revenue with minimal or no capital expenditure is to evaluate alternative solvents for existing gas-processing trains. Such solvent swaps can bring a range of benefits, including:

  • increased capacity;
  • reduced energy consumption;
  • deeper contaminant removal; and
  • less solvent degradation.

The paper presents three case studies that demonstrate the value of solvent swaps for enabling treating of higher contaminated feed gas (Case study 1), reducing operational expenditure and reducing solvent degradation products (Case study 2) and meeting tighter product specifications (Case study 3) to increase plant profitability.

In addition to the technical elements, effective execution is important for the success of solvent swaps in brownfield assets. Through its owner–operator–licensor experience and the execution of numerous solvent swaps worldwide, Shell has developed best practices for solvent swaps. The due diligence regarding risk assessments, logistics, updating of procedures, etc. is emphasised, and the potential benefits of “offline” and “on-the-run” solvent swaps are discussed.

In the current economic climate, there may be little appetite for investing in complex greenfield gas-processing projects. However, competition remains intense and operators are seeking ways to unlock the potential of existing operating facilities without major investments.

One way to create additional revenue with minimal expenditure is to swap the solvents in existing gas-processing trains. When unit performance is satisfactory, there may not be a strong case for upgrading to newer-technology solvents such as ADIP®-X and Sulfinol®-X . However, when operators seek to increase capacity, reduce energy consumption, achieve deeper contaminant removal or reduce solvent degradation without major investment, a solvent swap can be an attractive option.

Treating higher contaminated feed gas without additional capital investment

This case study presents a scenario where a gas plant operating company needed to process feed gas with higher H2S content (almost double) and 30% more CO2 than it was designed for while meeting the same treated gas specifications (Table 1). The existing process configuration employs Sulfinol-D solvent (Figure 1).

Table 1: Original and new feed gas contaminant levels and treated gas specifications.

Original design  New condition
Feed gas H2S content, mol % 1.0 2.0
Feed gas CO2 content, mol % 2.5 3.3
Relative feed gas flow,% 100 100
Treated gas H2S specification, ppmv 3.5 3.5

The plant operator asked Shell Global Solutions to evaluate the options for upgrading the performance of its gas-treating unit. Two options were considered.

Option 1: New pretreatment acid gas removal unit

A new acid gas removal unit was considered that would include an absorption system, a flash drum, a regeneration system and, depending on the acid gas quality, possibly an acid gas enrichment section (Figure 2). The new pretreatment unit would reduce the feed gas H2S and CO2 content to the design values for the existing Sulfinol-D system. However, such equipment would require considerable capital investment, need space and add operational complexity, which would likely reduce unit reliability.

Option 2: Solvent swap to Sulfinol-X

Swapping the existing Sulfinol-D solvent for Sulfinol-X was considered as an alternative (Figure 3).

Modelling showed the relative unit performance improvement for Sulfinol-X compared with the existing solvent (Table 2). This demonstrated that the treated gas H2S, CO2, COS and mercaptan specifications could be achieved with a similar solvent circulation rate. The lean solvent temperature to the absorber column would also be similar.

Reboiler duty and steam consumption would be approximately 9% higher owing to the much higher acid gas flow. However, the higher reboiler duty was within the equipment design specifications, so there would be no need to modify the reboiler. Indeed, the evaluation work showed that the solvent change would not require any major equipment modification. Sulfinol-X has similar fluid properties to Sulfinol-D, so the swap would have no impact on pump seals or seal material and the solvent circulation would be within the operating range of the pumps.

Table 2: Unit performance modelling results for Sulfinol-X, relative to Sulfinol-D.
Sulfinol-X
Relative feed gas flow, % 100
Relative solvent circulation flow, % 100
Relative acid gas flow, % 133
Relative reboiler duty, % 109*
Relative steam consumption, % 109*

*The increase in reboiler duty and steam consumption is due to much higher acid gas content in feed gas.

Sulfinol-X has a higher loading capacity than Sulfinol-D, which is why higher amounts of H2S and CO2 can be removed from the feed gas at the same solvent rate. MDEA reacts 1:1 with CO2 whereas diisopropanol amine (DIPA) reacts 2:1. Consequently, it is possible to load an accelerated MDEA-based solvent more (up to 1 mol CO2/mol amine) compared with a DIPA-based solvent (up to 0.5 mol CO2/mol amine). This provides operational cost savings, as less solvent pumping and heating duty are required, and helps to debottleneck capacity for existing Sulfinol-D units.

Sulfinol-X offers faster CO2 and COS reaction kinetics than does Sulfinol-D. Its faster CO2 reaction and different reaction heat give Sulfinol-X a different temperature profile in the absorber to that of Sulfinol-D.

Hydrocarbon co-absorption by the solvent would be lower with Sulfinol-X compared with Sulfinol-D because of its different composition. Consequently, there would be less hydrocarbon content in the acid gas, which would mean that the Claus unit would need less air and have more room for capacity increase.

Comparison between new pretreatment acid gas removal unit and solvent swap to Sulfinol-X

The proposed solvent swap would require very little capital expenditure, as there would be no major equipment modification. The operating costs would also be considerably lower than for a new pretreatment unit. Following a solvent swap, the unit would have the same operational complexity. In contrast, the installation of a new pretreatment unit would increase operational complexity and most likely reduce plant reliability.

The evaluation concluded that a solvent swap to Sulfinol-X would be the most attractive option for meeting the new feed gas contaminant levels. It would provide the existing unit with the means to treat more highly contaminated feed gas at considerably lower capital investment and operating expenditure than the alternative while maintaining the same level of operability and reliability.

Reducing operating and capital expenditure by reducing solvent losses and solvent degradation

A Middle East operator needed to process contaminated gas in its gas-sweetening unit. The design used a DIPA-based solvent. The DIPA reacts with CO2 to form carbamate, which reacts irreversibly to form oxazolidone. In this case, the high partial pressure of CO2 would have led to an accelerated build-up of DIPA-oxazolidine and other related degradation products.

Even though the DIPA-based Sulfinol solvent had been performing well, continuing to use this solvent with the feed gas containing a high CO2 volume would have meant higher operational expenditure through frequent solvent replenishment or the additional capital cost of a new solvent reclamation unit due to DIPA contribution to the solvent degradation.

The operator worked with Shell Global Solutions to find an alternative solution. A solvent swap to Sulfinol-X was proposed. The solvent swap was made without needing additional capital costs. With Sulfinol-X, the gas-sweetening unit met the required specifications for CO2 and sulphur (H2S, COS and mercaptans) removal using a similar solvent circulation rate.

Compared to Sulfinol-D, for the same sulfolane content, Sulfinol-X requires a lower reboiler duty to remove the same amount of CO2 in the regenerator, as the overall heat of reaction for the accelerated MDEA is lower than that of DIPA. In this example, the reboiler duty was approximately 10% lower compared to the Sulfinol-D case.

The solvent change did not require any equipment modification to the gas-sweetening unit or the downstream units. The solvent circulation requirement was within the operating range of the pumps and had no effect on pump seals or seal materials. The composition chosen for Sulfinol-X was within the material selection guidelines for sulfolane-based solvents, so there was no impact on material selection from the solvent change.

In addition to avoiding frequent solvent replenishment or a new solvent reclamation unit, the performance of the unit was checked for a higher gas throughput. The unit can handle 10% additional gas throughput with approximately 12% higher solvent circulation rate. The solvent composition for Sulfinol-X has lower sulfolane content compared to Sulfinol-D.

This gives a lower solvent temperature in the reboiler and hence increased reboiler capacity. In this example, reboiler duty was limiting capacity, so, instead of reducing reboiler duty (steam requirement), the reboiler was kept close to full capacity to be able to increase the gas processing capacity.

The unit has been running successfully with Sulfinol-X for a significant number of consecutive years.

Improving treating performance to meet tighter gas specifications

A gas processing operating company wanted to reduce the total sulphur content of its final product to meet tighter gas specifications. In collaboration with Shell Global Solutions, a solvent swap from diethanolamine (DEA) to Sulfinol-X was proposed.

Modelling as part of a feasibility study showed that using Sulfinol-X in the absorber (Figure 4) would reduce the total sulphur content in the treated gas to the target levels.

Swapping to Sulfinol-X would reduce the mercaptan content in the treated gas from the absorber. The regeneration gas could be used as fuel and its lower sulphur content would help to ease boiler corrosion issues.

Modelling demonstrated that Sulfinol-X would remove H2S, CO2, mercaptans, COS and organic sulphides to the required treated gas product specifications in a single process. This would cut the equipment count significantly compared with alternative multi-process line-ups and thus reduce capital costs. With fewer equipment items to operate and maintain, unit availability should increase and operating costs should decrease. Operating costs could also be reduced, as Sulfinol-X has a lower steam requirement for stripping of the absorbed acid gases compared with alternative aqueous amine solvents.

Effective solvent swap execution

In addition to getting the best technical solution for each opportunity, achieving effective project execution is important for the success of solvent swaps in brownfield assets. Through its owner–operator–licensor experience and the execution of numerous solvent swaps worldwide, Shell has developed best practices for solvent swaps. These are built on continuous improvements during 50 years of design and operating experience. Shell has designed and commercialised more than 250 operating units to date.

To prepare for an effective solvent swap, the following factors should be considered as part of the site’s management of change process:

  • the adequacy/capacity of the existing unit design, including, for example, the relief capacity and tray hydraulics;
  • the new heat and material balance resulting from the changed temperature profile of the unit and new solvent;
  • update of site hazard register and chemical safety datasheets;
  • reconfiguration of flow and level instruments in response to the new solvent properties;
  • turndown limitations, which may be an issue in some cases, for example, the flash gas pressure control valve and the regenerator reflux loop;
  • the requirement for new or additional stainless steel cladding in certain column sections, although temperatures are generally lower;
  • the requirement to clean the unit thoroughly beforehand;
  • assessment of solvent quality with potential to reuse in the new solvent;
  • on-site storage of new and existing solvents, and wash water, and the eventual disposal of the existing solvent inventory; and
  • the prevention of cross-train contamination through common facilities and connections needs in the case of multiple trains and a staggered solvent swap. This might require additional facilities, for example, a drain vessel and solvent storage.

In performing due diligence ahead of a solvent swap, risk assessments, logistics, updating of procedures and shut down planning (if required) need to be considered.

Solvent swap: “on the run” versus “offline”

Depending on the similarity of the new and existing solvent ingredients, the solvent swap can be performed “on-the-run” or “offline”. For example, a solvent swap from methyl diethanolamine (MDEA) to Sulfinol-X may (if the existing solvent is of acceptable quality) be performed on the run while a solvent swap from Sulfinol-D to ADIP-X or Sulfinol-X needs to be offline.

On-the-run solvent swaps are performed while the unit is in operation. In this case, it is recommended to reduce the feed gas flow rate and consequently the solvent rate, change the activated carbon filter and perform hydrocarbon skimming in the unit before the solvent swap.

The new solvent ingredients, for example sulfolane, for a solvent swap from MDEA to Sulfinol-M, can be added to the storage tank (buffer vessel or regenerator sump) and then pumped back gradually to the unit until the desired solvent composition is achieved. Water will also need to be bled from the system via the reflux to reduce the water content and maintain the overall inventory.

Following confirmation of the right solvent composition, the solvent rate and gas rate can be gradually increased to normal operating levels. While performing the solvent swap procedure, the following operating parameters have to be constantly monitored:

  • treated gas composition at the absorber outlet;
  • absorber pressure and temperature profile;
  • acid gas composition and flow rate;
  • regenerator operating parameters; and
  • mechanical filter pressure drop.

An offline solvent swap is performed while the unit is in shut down. In general, the following steps need to be followed:

  • draining of the existing solvent inventory;
  • hot water wash and draining of wash water;
  • second water wash and draining of water wash; and
  • loading of the new solvent.

It is prudent to align an offline solvent swap with a unit turnaround and to complete a thorough cleaning of the unit to ensure best performance on re-start. The number of days required for offline solvent swap depends on the size of the unit and whether it is performed during a turnaround or cleaning event.

The time needed for each step is estimated based on the inventory and the planned activities. At least two complete water washes of the system are typically required. For example, when swapping from Sulfinol-D to ADIP-X, the residual Sulfinol-D level in the final rinse water should be less than 1.0 wt%, and the wash water is typically heated and circulated for 8–12 hours.

The benefits of solvent swaps in gas-processing units: A summary

Solvent swaps can help to increase revenue from existing gas processing trains with no or minimal capital expenditure.

The first of three case studies demonstrated the value of swapping to Sulfinol-X for treating more highly contaminated feed gas at considerably lower capital investment and operating expenditure than an alternative pretreatment option while maintaining the same level of operability and reliability.

In the second case study, a switch to Sulfinol-X to treat gas with a high CO2 content has reduced solvent losses and solvent degradation without any equipment modification requirements. The improved solvent performance has avoided the need for high operational expenditure through frequent solvent replenishment or the additional capital cost of a new solvent reclamation unit. The unit is capable of handling 10% additional gas throughput with an approximately 12% higher solvent circulation rate.

The final case study shows how a gas-processing operating company can meet tighter treated gas-product specifications in a single process using Sulfinol-X. This would cut the equipment count significantly compared with alternative multi-process line-ups and thus reduce capital costs.

In addition to the technical elements, effective execution is important for the success of solvent swaps in brownfield assets. In certain circumstances, an on-the-run solvent swap, which avoids shutting down the unit, can be performed with appropriate procedures and monitoring.

More in Global Solution

Acid Gas Removal

Meet natural gas commercial specifications cost-effectively by using the licensed technologies ADIP and SULFINOL technologies to remove carbon dioxide (CO2), hydrogen sulphide (H2S), mercaptans (RSH), carbonyl sulphide (COS) and other organic sulphur compounds 

Contact Gas Processing

Please contact the Shell Gas Treating and Sulphur Removal technologies team for further information and we will respond to your enquiry via email.