PTQ Q1 2023 Issue

Kinetic model for TGU hydrogenation reactors: Part 1 – model development

Development of rigorous model for reaction kinetics and catalyst deactivation mechanisms capable of predicting SRU and TGU performance is presented

Michael A Huffmaster Independent Consultant Prashanth Chandran, Nathan A Hatcher, Daryl R Jensen and Ralph H Weiland Optimized Gas Treating, Inc.

T he most widely applied tail gas unit (TGU) is the SCOT-type with a hydrogenation reactor to reduce all sulphur species to H 2 S and subsequently recover and recycle H 2 S to the Claus thermal reactor. World Bank stan- dards now require a minimal TGU level of performance for all but the smallest sulphur recovery units (SRUs). Across 50+ years of TGU application, many improvements have been made, increasing process efficiency, raising sulphur recovery, and reducing capital cost. Contributions include more effective process design, improved catalyst perfor- mance, enhanced solvent selectivity, and the development of increasingly sophisticated process simulation tools. Included in a rigorous model for reaction kinetics and cat- alyst deactivation mechanisms capable of predicting SRU and TGU performance are molecular reaction pathways and reaction rates for chemical species that are encoun- tered in tail gas hydrogenation reactors as functions of Since catalyst activity declines, provisions must be made to include sufficient catalyst to achieve environmental performance at EOR when catalyst can be replaced temperature and residence time. The resulting model has been implemented as a fixed bed hydrogenation reactor in OGT | SulphurPro, a rate-based process simulator widely used in modelling sulphur recovery and tail gas treating units (TGTUs). * Engineering analysis must ensure environmental per- formance standards are achieved from start-of-run (SOR) conditions right through to end-of-run (EOR). Quantifying expected performance across time is necessary, too. Catalysts age with exposure to process conditions and are poisoned by process contaminants. Because cata- lyst activity declines, provisions must be made to include sufficient catalyst (or operational flexibility) to achieve environmental performance at EOR when catalyst can be replaced. The model, whose development is discussed in more detail, provides the means to address ageing and poi- soning effects, which will help engineers optimise designs,

forecast performance, and troubleshoot operations using the analysis of operating data against model predictions. Process background The sulphur recovery complex in a refinery or gas plant can be viewed as part of the overall system for extracting H₂S, other acid gases, and organic sulphur compounds from the process. The acid gas removal system is regenerated and produces an acid gas (and often sour water acid gas, SWAG) which is processed in the SRU, comprised of Claus, TGU, and thermal oxidiser. Sulphur compounds are recov - ered as elemental sulphur or emitted to atmosphere. Fundamental Claus process chemistry converts H₂S to sulphur in two to three stages with about 95-97% overall recovery efficiency: H 2 S + 3 O 2 ⇌ SO 2 + H 2 O (Thermal stage) 2 2H 2 S + SO 2 ⇌ 3 S x + 2H 2 O (Thermal and catalytic) x The TGU recovers unconverted sulphur from the Claus unit as H₂S, achieving the very high sulphur recovery required by today’s environmental standards. Although an independent process, a TGU is an integral part of the SRU. Process chemistry involves the catalytic conversion of non-H₂S sulphur species to H₂S by hydrolysis and hydro - genation, continuation of Claus, and conversion of carbon monoxide to hydrogen and CO₂. Hydrogen aids hydrogena - tion reactions and, in addition, CO emissions are reduced. The main reactions in the catalyst bed are:

COS, CS₂ - hydrolysis on alumina SO₂, S x , COS, CS₂ - hydrogenation on Co/Mo CO - water gas shift on Co/Mo

The process chemistry is more complex, with several par - allel reactions as well as reactions between SO₂ and other reduced sulphur species. A more comprehensive discussion of reaction pathways is deferred until later. Figure 1 shows the overall TGU process. Claus unit off-gas is preheated and charged to the hydrogenation reactor, where a cobalt- molybdenum catalyst converts sulphur compounds to H₂S. After quenching to remove water and heat of reaction, H₂S is recovered using an amine selective for H₂S. The off-gas is


PTQ Q1 2023

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