Gas 2024 Issue

applications (acid gas enrichment or tail gas treatment). The proprietary combination of several amines and a promoter system provides higher acid gas capture capacity and ena- bles lower achievable treated gas H 2S specifications. Flexsorb SE Plus is a proprietary gas treating agent developed by ExxonMobil Research and Engineering Company specifically for selective H2 S removal. The tech- nology is well known in the industry for its high perfor- mance, even at high ambient temperatures. The selectivity advantage allows H 2 S removal at lower solvent circulation rates, resulting in lower energy consumption compared to conventional processes. The reliable and simple-to-operate process is characterised by low corrosion and lower foam- ing compared to conventional gas treating solvents. The technology has a commercially proven track record since 1983, with more than 140 reference plants. Both OASE yellow and Flexsorb SE Plus can achieve high treated gas purity to meet the stricter SO 2 emission targets of the World Bank Standard (150 mg/Nm 3 ). In addi- tion, both solvents can maintain H 2 S selectivity at high lean amine temperature (+50°C), which makes designs without an expensive chiller unit possible. On the contrary, generic amines would need high energy demand and lower lean amine temperature to achieve the required H 2 S selectivity, which increases the unit’s costs significantly. Case study 1: Switching from MDEA to OASE yellow In this case study, a refinery located in Germany operated a TGTU with generic MDEA solvent, which often limited the refinery’s operations. The feed gas to this unit includes 1.2 mol% H 2 S and 30 mol% CO 2 . The constraint encountered at the TGTU is the environmental permit limit for SO 2 emis- sions from the thermal oxidiser. This environmental con- straint typically occurred in the summer months due to an increase in the lean amine temperature in the TGTU. As the lean amine temperature approached 37.8°C, the amount of H 2 S that slipped to the thermal oxidiser increased signif- icantly, resulting in increased SO 2 emissions. To mitigate this increased emission during the summer, the overall SRU capacity needed to be limited, even with the use of rented chillers. These factors warranted a review of alternative technol- ogies and a solvent changeover. However, the refinery had already passed the turnaround period, and the operation team had the challenge of changing over the solvent while the unit operated ‘on the fly’. As part of this evaluation, BASF was requested to ana- lyse the possibility of utilising OASE yellow in this unit to reduce the treated gas H 2 S content during the summertime from 90 ppmv to 35 ppmv. OASE connect, an in-house rate-based model simulator, was used to assess the possi- bility of meeting this target without any mechanical modifi - cations to the existing equipment. Based on the simulation results, it was concluded that with the addition of the OASE yellow enhancer system to the existing MDEA inventory, meeting the H 2 S target was not only possible but also the unit could reduce the solvent circulation rate to approximately 65% of the current oper- ating level.

Generic amine

Proprietary amine

F lexsorb SE Plus

O ASE yellow

Promoted MDEA

MDEA

Energy demand

High Low

Low High

Lean amine temperature range

Figure 1 Selective solvent performance comparison

(partial pressure), absorber internals, column height, and liquid/gas (solvent/feed gas) ratio all strongly affect the total mass transfer of individual components from the gas into the liquid phase. While the mass transfer of H 2 S is predominantly gas-phase driven, CO 2 reaction kinetics are mostly related to resistance in the liquid phase. Absorber height and mass transfer surface determine vapour/liquid contact, which directly impacts reaction selectivity. The liq- uid/gas (solvent/feed gas) ratio itself not only impacts mass transfer but also influences the temperature profile within the absorber, impacting reaction kinetics. Selecting the right solvent While designing a TGTU, one of the most important deci- sions is the type of amine. A typical gas sweetening amine unit with primary or secondary amines such as monoetha- nolamine (MEA) or diethanolamine (DEA) would require a very high amine circulation rate, as these solvents absorb both H 2 S and CO 2 almost without selectivity towards H 2 S. For this reason, a more H 2 S selective amine must be con- sidered to reduce solvent circulation/inventory, the amount of CO 2 recycled to the SRU, and reboiler duty. Figure 1 illustrates a comparison between generic amines such as MDEA and acidified MDEA against propri - etary amines offered by BASF Gas Treating (OASE yellow and Flexsorb SE Plus). OASE yellow was developed to enable selective removal of H 2 S in both high (natural gas) and low-pressure

90 80 100

0 30 20 10 40 50 60 70

30 20 40 50 60 70

HS spec Amine circulation rate (%)

TGTU feed gas: HS 1.2%v, CO 30%v

Conversion within 72 hours

Figure 2 Case study 1 OASE yellow conversion results

Gas 2024