Gas 2024 Issue

‘On the fly’ vs high-performance H2 S 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 L LC Blake Morell BASF Corporation

S elective removal of hydrogen sulphide (H 2 S) has become an important topic over the last two decades. Selective designs are tailored either on maximum or controlled H 2 S 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 H 2 S, 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 environ- mentally viable. Use of a highly H 2 S selective solvent, such as BASF’s proprietary OASE yellow, can provide benefits by optimising the capital investment or reducing the oper- ating cost. Various parameters during the design phase influence the H 2 S selectivity (and consequently carbon dioxide [CO 2] 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 tem- perature itself. H 2 S selectivity of generic solvents rapidly deteriorates once amine temperature exceeds 45°C. A key benefit of the OASE yellow selective solvent is a maintained H 2 S selectivity even in high ambient temperature environ- ments 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 H2 S removal in the presence of CO 2 . 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 H 2 S selective applications in the industry. However, recent stricter sulphur dioxide (SO 2) emission targets that meet the World Bank standard of 150 mg/ Nm 3 often require additional chemistry to further boost the MDEA performance of other amines to achieve tight treated gas H 2S 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 H 2 S compared to the kinetically hindered reaction of CO 2 : CO 2 first must react with water to form carbonic acid before the solvent can absorb the CO 2 . Thus, tertiary amines such as MDEA are often used for selective applications as they cannot form carbamates (the only fast reaction with CO 2 ). The following reactions of tertiary amines take place in aqueous solutions: Reaction of water and amine (fast) R1R2R3N + H 2 O  R1R2R3NH + + OHˉ 2 H 2 O  H 3 O + + OHˉ

H 2 S reaction (fast) H 2 S + H 2 O  HSˉ + H3 O +

CO 2 reactions (overall reaction: slow): CO 2 + 2H 2 O  HCO 3ˉ + H3 O + (slow) HCO 3ˉ + OHˉ  H 2 O + CO 32 - (fast)

In this reaction system, CO 2 co-absorption and, thus, H 2 S selectivity are heavily influenced by reaction conditions. This means higher pressure and temperature, as well as a higher CO 2 /H 2 S ratio in the feed gas, favour CO 2 co-absorption and lower H 2 S selectivity, especially at lean amine temperatures above 50°C, which are typical for the Middle East region. The CO 2 reaction accelerates and strongly competes with the H 2 S reaction. As a result, a cooling system/chiller is often part of the design in these climates to achieve H 2 S selectiv- ity with an MDEA/acidified MDEA solution.  Mass transfer Besides lean amine temperature and feed gas pressure

Gas 2024