May-2024

On the fly vs high-performance H2S selective solvent

A focus on highly selective designs in low-pressure tail gas treating units using BASF’s proprietary technology in comparison to generic MDEA solutions.

Ashraf Abufaris, Basf Middle East Chemicals LLC
Blake Morell, Basf Corporation

Article Summary

Selective removal of hydrogen sulphide (H2S) has become an important topic over the last two decades. Selective designs are tailored either on maximum or controlled H2S selectivity, depending on the application. The reaction equilibrium in sulphur recovery units (SRU or Claus section) prevents complete conversion of the sulphur species in feed gas to elemental sulphur. Typically, an SRU with two to three Claus reactors can only achieve 93-98% sulphur recovery efficiency. However, higher recoveries of 99.8% and above are achievable if the remaining sulphur compounds in the SRU tail gas are hydrogenated to H2S, which is removed in a selective amine unit (tail gas treating unit, TGTU).

Selection of the proper amine technology for the TGTU is essential to make these projects economically and environmentally viable. Use of a highly H2S selective solvent, such as BASF’s proprietary OASE yellow, can provide benefits by optimising the capital investment or reducing the operating cost.

Various parameters during the design phase influence the H2S selectivity (and consequently carbon dioxide [CO2] slip) in TGTUs, such as absorber height, amine circulation rate, and absorber internals in the mass transfer zone. However, one of the most effective levers is the amine temperature itself. H2S selectivity of generic solvents rapidly deteriorates once amine temperature exceeds 45°C. A key benefit of the OASE yellow selective solvent is a maintained H2S selectivity even in high ambient temperature environments and subsequent high lean amine temperatures of up to 50°C. This avoids installing/operating costly chillers for solvent cooling and makes the design reliable, robust, and flexible for various operational scenarios.

Against this backdrop, key parameters for these selective designs are discussed, followed by operational start-up data from OASE yellow solvent swaps.

Design options to influence H2S selectivity
A number of factors influence H2S removal in the presence of CO2. Adjusting these parameters plays a critical role in unit optimisation throughout the design, commissioning, start-up, and operation phases:

Amine type
Historically, methyldiethanolamine (MDEA) has been widely used in H2S selective applications in the industry. However, recent stricter sulphur dioxide (SO2) emission targets that meet the World Bank standard of 150 mg/Nm3 often require additional chemistry to further boost the MDEA performance of other amines to achieve tight treated gas H2S specifications. Besides the performance- related characteristics, properties such as volatility, stability, acid gas loading capacity, and of course commercial aspects are important selection criteria.

Lean amine temperature
Selective treatment with amine-based solvents generally takes advantage of the rapid reaction of H2S compared to the kinetically hindered reaction of CO2: CO2 first must react with water to form carbonic acid before the solvent can absorb the CO2. Thus, tertiary amines such as MDEA are often used for selective applications as they cannot form carbamates (the only fast reaction with CO2).
The following reactions of tertiary amines take place in aqueous solutions:

Reaction of water and amine (fast)
R1R2R3N + H2O Δ R1R2R3NH+ + OHˉ
2 H2O Δ H3O+ + OHˉ

H2S reaction (fast)
H2S + H2O Δ HSˉ + H3O+

CO2 reactions (overall reaction: slow):
CO2 + 2H2O Δ HCO3ˉ + H3O+ (slow)
HCO3ˉ + OHˉ ΔH2O + CO32- (fast)

In this reaction system, CO2 co-absorption and, thus, H2S selectivity are heavily influenced by reaction conditions. This means higher pressure and temperature, as well as a higher CO2/H2S ratio in the feed gas, favour CO2 co-absorption and lower H2S selectivity, especially at lean amine temperatures above 50°C, which are typical for the Middle East region. The CO2 reaction accelerates and strongly competes with the H2S reaction. As a result, a cooling system/chiller is often part of the design in these climates to achieve H2S selectivity with an MDEA/acidified MDEA solution.

Mass transfer
Besides lean amine temperature and feed gas pressure (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 H2S is predominantly gas-phase driven, CO2 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 liquid/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 decisions is the type of amine. A typical gas sweetening amine unit with primary or secondary amines such as monoethanolamine (MEA) or diethanolamine (DEA) would require a very high amine circulation rate, as these solvents absorb both H2S and CO2 almost without selectivity towards H2S. For this reason, a more H2S selective amine must be considered to reduce solvent circulation/inventory, the amount of CO2 recycled to the SRU, and reboiler duty.

Figure 1 illustrates a comparison between generic amines such as MDEA and acidified MDEA against proprietary amines offered by BASF Gas Treating (OASE yellow and Flexsorb SE Plus).

OASE yellow was developed to enable selective removal of H2S in both high (natural gas) and low-pressure 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 enables lower achievable treated gas H2S specifications.