Apr-2023

Kinetic model for TGU hydrogenation reactors: Part 2 Catalyst model validation

A rigorous high-fidelity kinetic model can help designers and operators forecast the life expectancy of reactor catalyst beds.

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

Article Summary

Part 1 of this article, published in PTQ’s Q1 2023 issue, involved the development of a model for the reactions occurring in the hydrogenation reactor of a sulphur recovery unit (SRU). The model addressed 11 different reactions and alluded to the catalyst ageing and poisoning that inevitably occurs over the life of the catalyst. Part 2 takes a deeper look at catalyst deactivation from a modelling standpoint and quantifies how deactivation can be included in the reaction kinetics model. Part 3 will detail catalyst deactivation and poisoning mechanisms. The model is validated with a case study.

Catalyst ageing and poisoning model
Ultimately, catalyst activity sets the sulphur recovery performance of the tail gas unit (TGU). Deactivation of heterogeneous catalysts, such as the ones used in the TGU hydrogenation reactor, occurs by ageing and poisoning; this is a ubiquitous problem that causes gradual loss of catalytic rate. For a comprehensive TGU design, catalyst deactivation over the life of the catalyst charges and its effect on meeting sulphur emission requirements must be addressed.

When fresh catalyst is loaded into the TGU reactor and activated, it has maximum surface area and activity. On start-up, the catalyst is immediately exposed to several possible deactivation stresses, most causing irreversible damage. Mechanisms that alter catalyst activity do so by affecting the dispersed, active, metal-sulphide phases of cobalt and molybdenum and the high surface area alumina support. Alumina (and titania) are often used in the process industry as supports for many heterogeneous catalysts, as well as for the Claus process, so one can draw on this larger body of knowledge and on sulphur recovery industry experience.
The activity of these catalysts is strongly related to the γ-alumina (or mixed phase alumina) surface area of the base, alumina crystallites and their microporous structure that facilitates accessibility to the reactants. The alumina matrix has hydroxyl ions on the catalyst surface that serve as weak Brønsted-Lowry acid sites, promoting hydrolysis and Claus reaction. Their extensive surface area both supports and interacts with the active cobalt and molybdenum metals.

Activity declines as a function of time, and exposure to normal process conditions is treated as ageing and related to loss of surface area and active sites. The remaining fraction active surface area can be represented by an ageing factor, AF. Ageing tends to occur uniformly throughout the catalyst bed, with catalytic activity or conversion of reactive species declining rapidly at first and then slowly over the catalyst’s life. Spent catalyst activity approaches about 50% of fresh activity, and the model is fitted to this operating data for ageing, as observed in the measurements of the sulphur-plant data shown in Figure 1.¹

Assays of used TGU catalysts report surface area, crush strength, carbon, and sulphate.² Typically, when the surface area reaches 120 m2/g, they are considered spent. Other ‘spent’ criteria are carbon-on-catalyst as seen at levels approaching 1% and crush strength declining to half the fresh value. Sulphate is not always observed, but about 1% is not unusual (although spent catalyst may have substantial sulphate). Activity testing reported on abused catalyst (due to exposure to extreme temperatures) in spent conditions expresses about 50% of fresh activity, with carbon typically 0.1 or 0.2%.

Catalyst deactivation and poisoning is a vast and fascinating subject in its own right, and it deserves much more than a cursory treatment. Part 3 will take a deeper plunge into the subject so, for now, the discussion of mechanisms and causes of deactivation is deferred. Suffice it to mention that hydrothermal ageing, sooting, chemisorption of poisons (especially of oxygen) and, by sulphation, coking and sintering are all causes of deactivation. Ultimately, the loss of specific surface area directly affects catalyst performance as the number of active Al-OH surface sites falls.

Quantification of poisoning
Generally, fresh alumina catalyst has a specific surface area of 300-350 m2/g; with initial ageing, surface area declines to 240-260 m2/g and then stays relatively stable, declining only slowly over several years until ‘spent’, at approximately 120 m2/g. Hydrothermal ageing tends to occur uniformly throughout the catalyst bed, approaching about 50% of fresh activity when spent.

Poisoning is treated as activity loss related to any of several contaminants in the feed. Certain streams that wind up at the TGU are known to contribute to poisoning. Although not ideal for an SRU, this method of disposal is sometimes taken as the one with least consequences. SRUs that process BTEX-containing acid gas can pass those species on to the TGU, especially in lean acid gas situations. The effect of BTEX is thought to be reduced at temperatures below 240°C, as discussed elsewhere.³

Deactivation is connected with catalyst pore and active site distribution.4 Catalytic activity is conveniently defined in terms of the observed external rate constant kobs, which is equal to the product of the catalyst active site-based intrinsic rate constant kintr, the effectiveness factor η, and the active site surface number density, σ (number of sites per area of surface), and an ageing factor, AF:

kobs = kintr η σ AF

Poisoning corresponds to a loss of active sites, i.e., σ = σ0 (1 - α), where α is the fraction of sites poisoned. The effect on activity is a combination of site number density, poison selectivity, mass transfer resistance, and loss of surface area. Deactivation directly affects:
•    Selectivity: how quickly the poison interacts with the catalyst active sites; selective poisoning preferentially affects sites near the pore mouth and slowly progresses along the pore, vs non-selective, which progresses more or less uniformly along the entire length of the pore.
•    η: effectiveness factor, i.e., reaction rate with mass transfer resistance/intrinsic reaction rates without mass transfer resistance.
•    hT: Thiele modulus, i.e., the ratio of kinetic rate to mass transfer (diffusion) rate.

The activity response to poisoning depends on the combination of selectivity and Thiele modulus. The approximate order is as follows:
•    Half-order for non-selective, large hT
•    First-order for small hT (<2)
•    Reciprocal function (1/(1+σ hT)) (selective with large hT).