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Specialised solvent selection

Published by , Editorial Assistant
Hydrocarbon Engineering,

Selective removal of hydrogen sulfide (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. This article will focus on highly selective designs in low-pressure tail gas treating units (TGTUs) using BASF’s OASE® yellow technology in comparison to generic based methyldiethanolamine (MDEA) solutions.

The reaction equilibrium prevents the complete conversion of the sulfur species in the feed gas to elemental sulfur in sulfur recovery units (SRU or Claus section) to elemental sulfur. Typically, an SRU with two to three Claus reactors is only able to achieve 93 - 98% sulfur recovery efficiency. However, higher recoveries of 99.8% and above are achievable if the remaining sulfur compounds in the SRU tail gas are hydrogenated to H2S, which is then consequently removed in a selective amine unit (TGTU).

The 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 OASE yellow, can provide benefits by optimising the capital investment or reducing the operating cost.

During the design phase there are various parameters to influence the H2S selectivity (and consequently the 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. The H2S selectivity of generic solvents rapidly deteriorates once the 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.

This article will discuss the key parameters for these selective designs followed by real operational start-up data from OASE yellow solvent swaps.

Design options to influence H2S selectivity

There are a number of factors that 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.

Type of amine

Historically, MDEA has been widely used in H2S selective applications in the industry. However, recent stricter SO2 emission targets that meet the World Bank standard of 150 mg/Nm3 often require additional chemistry to further boost the performance of MDEA and other amines to achieve tight treated gas H2S specifications. Besides the performance related characteristics, properties such as volatility, stability, acid gas loading capacity and 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 are not able to form carbamates (the only fast reaction with CO2).

This article was originally published in the May 2024 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Ashraf Abufaris, BASF Middle East Chemicals LLC and Blake Morell, BASF Corp., USA

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