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Aug-2017

Unlocking the potential of existing gas processing assets through solvent swaps

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.

Gary Bowerbank, Pavan Chilukuri and Alireza Zahedimanesh
Shell Global Solutions International B.V.
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Article Summary
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 1), reducing operational expenditure and reducing solvent degradation products (Case 2) and meeting tighter product specifications (Case 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.

Introduction
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.

Case study 1: 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).

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.

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 of options
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.

Case study 2: 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-oxazolidone 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.
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