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.