PTQ Q4 2025 Issue

Reducing refinery SOx emissions footprint

Technical guide on designing an effective TGTU add-on to a sulphur recovery unit utilising process simulation

Ganank Srivastava Bryan Research & Engineering, LLC

S ulphur recovery units (SRUs) have long been seen as the ‘emission controls’ unit of a refinery or gas plant. Contaminated off-gases and wastewater streams generated from various refinery hydroprocessing units are usually first sent to an amine treatment and sour water stripping unit for scrubbing out hydrogen sulphide ( H₂S). Instead of incinerating all this recovered H₂S directly, most operators first send these acid gases to an SRU. The primary function of an SRU is to recover useful and harmless elemental sulphur (Sx) from H 2S. The Claus pro - cess is the industry standard for sulphur recovery, converting H₂S to elemental Sx through partial combustion (thermal) and catalytic reactions. However, a straight-through Claus process typically achieves only around 95% sulphur recov- ery efficiency (SRE). The remaining sulphur-containing molecules, such as unrecovered H₂S or unreacted SO₂, all exit with the Claus tail gas and eventually still make their way to the backend flare stack, contributing to overall sul - phur oxide (SOx) emissions. As refineries look to expand and process more sour crude, the emissions footprint (especially compounded on an annual basis over time) is only going to rise. SOx emis - sions pose serious environmental threats, contributing to acid rain, air toxicity, and ground-level ozone formation. Therefore, it is imperative to look beyond the 95% SRE and push the target towards 99.9% or higher, not only to ensure social responsibility but also to meet regulatory compliance. Solution In the hydrocarbon processing industry, especially in sul - phur block operations, tail gas treating units (TGTUs) have been widely accepted to bridge this gap and play a critical role in ensuring environmental compliance and enhancing SRE. The need for cleaner technologies and stricter envi - ronmental regulations has made the design and operation of TGTUs not just optional, but essential for sustainable operations. TGTUs are usually installed downstream of the Claus unit to capture and convert residual and unconverted sul- phur compounds in the tail gas to increase the overall SRE towards 99.9%. It ensures the plant remains compliant with stringent emission standards. As environmental regulations become more stringent,

process engineers should be provided with a comprehen- sive guide to designing TGTUs that effectively manage SOx emissions in refineries. Process scheme A standard TGTU configuration typically follows the follow - ing scheme after the acid gases pass through the straight- through SRU: • Hydrogenation reactor : Converts all remaining uncon- verted sulphur-containing species back into H₂S. • Quench or cooling system : Reduces gas temperature. • Amine absorber : Selectively absorbs H₂S using a selec - tive solvent. • Amine regenerator : Strips H₂S from the rich amine solution. • Sulphur recycle stream : Recycles H₂S back to the SRU front-end. A representative process flow diagram of a TGTU is shown in Figure 1 . Evaluations criteria Central to the TGTU is the amine absorber, a column tasked with removing H₂S from the treated tail gas stream while allowing CO₂ (either originally present in small amounts in the SRU feed acid gas or generated from hydrocarbon combustion in the front-end thermal step) to slip through. Recovering and recycling CO₂ in the TGTU is counter - productive, as the molecule has no role to play in sulphur recovery or Claus reactions. Moreover, it adds unnecessary load to the amine regenerator that strips H₂S from the cir - culating solvent. Hence, the main objective of a TGTU will be to absorb, recover, and recycle H₂S from the tail gas back to the Claus unit and allow CO₂ and other inerts to be slipped through unabsorbed. This seemingly straightforward function is a delicate bal- ancing act involving mass transfer, thermodynamics, and chemical reaction kinetics. A misstep in absorber design or operation can result in undesirable CO₂ co-absorption or H₂S slip, thereby leading to SOx emissions non-compli - ance, increased energy costs, and, of course, lower than the target 99.9%+ SRE. Therefore, to design, optimise, or troubleshoot such sys - tems, robust process simulation modelling is essential. The

PTQ Q4 2025