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.

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Article Summary

The most widely applied tail gas unit (TGU) is the SCOT-type with a hydrogenation reactor to reduce all sulphur species to H2S and subsequently recover and recycle H2S to the Claus thermal reactor. World Bank standards 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 performance, enhanced solvent selectivity, and the development of increasingly sophisticated process simulation tools.

Included in a rigorous model for reaction kinetics and catalyst deactivation mechanisms capable of predicting SRU and TGU performance are molecular reaction pathways and reaction rates for chemical species that are encountered in tail gas hydrogenation reactors as functions of 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 performance 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 catalyst 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 poisoning 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 Hâ‚‚S, 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 recovered as elemental sulphur or emitted to atmosphere.

Fundamental Claus process chemistry converts Hâ‚‚S to sulphur in two to three stages with about 95-97% overall recovery efficiency:

H2S + 3 O2 ⇌ SO2 + H2O     (Thermal stage)
2H2S + SO2 ⇌ 3 Sx + 2H2O     (Thermal and catalytic)
The TGU recovers unconverted sulphur from the Claus unit as Hâ‚‚S, 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-Hâ‚‚S sulphur species to Hâ‚‚S by hydrolysis and hydrogenation, continuation of Claus, and conversion of carbon monoxide to hydrogen and CO2. Hydrogen aids hydrogenation reactions and, in addition, CO emissions are reduced. The main reactions in the catalyst bed are:

COS, CSâ‚‚        - hydrolysis on alumina
SOâ‚‚, Sx, COS, CSâ‚‚     - hydrogenation on Co/Mo
CO         - water gas shift on Co/Mo

The process chemistry is more complex, with several parallel reactions as well as reactions between SOâ‚‚ 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 Hâ‚‚S. After quenching to remove water and heat of reaction, Hâ‚‚S is recovered using an amine selective for Hâ‚‚S. The off-gas is incinerated and vented to atmosphere. Amine regeneration recycles Hâ‚‚S to the SRU reaction furnace. A high degree of sulphur recovery is achieved by substantial conversion of all species to Hâ‚‚S. The TGU admits a low sulphur slip with an overall SRU/TGU recovery performance of 99.9% or better.

The primary performance characteristic of a TGU catalyst is that SOâ‚‚ should be fully converted. If SOâ‚‚ enters the quench circuit, it will foul, corrode, potentially deactivate,  and subsequently degrade the amine. Secondly, a high degree of conversion is required for COS, CS2, and mercaptan; otherwise, these components pass through the amine system, are incinerated and discharged to atmosphere as SOâ‚‚. Finally, any elemental sulphur not converted will plug and corrode the quench circuit.

Historically, TGU design is based on fresh catalyst. The designer selects the temperature and catalyst quantity needed to achieve high conversion of non-H2S sulphur compounds and meet environmental performance requirements. The importance of compliance with permitted environmental emissions from SOR to EOR means sufficient catalyst inventory must be provided such that even in an aged condition, the needed sulphur recovery is achieved.

A temperature profile of the TGU reactor is commonly used to provide insight into catalyst performance and health. The adiabatic reactor experiences a temperature rise from the exothermic reactions associated with the conversion of various sulphur species to Hâ‚‚S and shift of carbon monoxide to hydrogen. Figure 2 shows three approximately equal segments of the bed, with the percentage contribution of each bed to the overall temperature rise across the entire bed: top zone is green and shows 70% of DT, middle zone is blue with 30% DT, and bottom zone is red with negligible DT. The fresh catalyst is achieving almost complete conversion in the first two zones.

The overall temperature rise across the bed is virtually constant across time (not shown), reflecting that even with sulphur slip as outlet concentrations of non-Hâ‚‚S species increase, there is still a high percentage conversion. The magnitude of temperature rise is related primarily to the concentration of sulphur dioxide, carbon monoxide, and elemental sulphur in the feed. The relative amount of temperature rise in each zone reflects the degree of conversion in each zone.

Mid-life activity distribution shifts to a 20% rise in the top zone, 70% in the middle zone, and 5% in the bottom, eventually moving lower into the middle and bottom zones as the top and middle zones deactivate. Incidentally, the temperature profile chart reflects declining activity, which results from deactivation by ageing and poisoning. The significance of shifting reactor bed temperature profile and its implications for ageing, poisoning and potential bed life and the new modelling tool is the subject of the commentary here and in Part 2 of this article.

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