Jul-2014

Tail gas catalyst performance: part 1

The first part of a two-part account of time and temperature effects on tail gas catalyst performance provides a background to reaction modelling and pilot studies

Michael Huffmaster, Consultant
Fernando Maldonado, Criterion Catalysts

Article Summary

The tail gas unit (TGU) process has been developed to remove sulphur compounds from Claus tail gas in order to comply with stringent emission regulations. From the early 1970s to today, TGUs have been improved to meet higher levels of performance for ever tighter environmental requirements and to reduce capital or operating cost. Reactor performance is a critical parameter in achieving a TGU’s environmental performance. Conversion of sulphur species to H2S is a function of catalyst activity, reactor space velocity and temperature. Assessment of the impact of these principal variables on both catalyst bed design and performance is the subject of this article which is presented in two parts. In the first part, an introduction to the Claus–TGU sulphur recovery complex provides a framework for examining the impact of operating and design parameters, process development history, equipment line-up evolution, catalyst developments, and reactor chemistry. The first part also provides an introduction to reactor modelling, describes the reactor pilot plant system, and examines chemical equilibria which affect TGU performance. The second part develops reactor modelling and examines the effects of space velocity and temperature.

Reducing sulphur emissions
TGUs are built for a specific purpose – increasing the overall sulphur recovery of the Claus-TGU sulphur recovery complex to about 99.9% from about 96% achieved on a Claus plant alone (see Figure 1). Their sole purpose and economic justification is reducing sulphur emissions, which improves overall environmental quality. In the reductive tail gas process addressed herein, achieving good performance requires high conversion of sulphur compounds to H2S in the reactor. Achieving good performance therefore requires setting reactor operating conditions based upon understanding the influence of key operating variables affecting the catalyst bed.

The key parameters affecting performance of the catalyst bed are catalyst kinetic properties, temperature, and tail gas loading/space velocity:
• A catalyst’s kinetic properties are determined by its manufacture, activation and aging
• Temperature affects catalytic activity and thermodynamic equilibrium, limiting conversion
• Reactor loading directly impacts space velocity, which controls conversion. 

Tail gas reactor loading and temperature effects are represented in first order reaction mechanics and thermodynamic equilibrium. These relationships provide a good model for understanding the influence of these operating variables and show higher gas loading results in lower conversion. Data from catalyst testing in Criterion’s pilot unit are presented to illustrate these kinetic effects.

Application and interpretation of information for assessment of reactor performance and catalyst activity will be discussed, including:
• Temperature profile observed in the reactor bed
• Changes in conversion, indicated by increased incinerator emissions (SO2 and CO)
• Activity evaluation from reactor inlet and outlet stream composition and  determination of aging, usually by unit testing and analytical evaluation results
• Measuring physical properties and/or activity testing for actual catalyst sample.
This article is intended to help TGU operators and designers improve environmental performance by understanding these effects and applying principles to designs or improving performance of existing units.

The Claus process
The Claus process is the most significant gas desulphurising process, recovering elemental sulphur from gaseous hydrogen sulphide. The multi-step Claus process recovers sulphur from the gaseous hydrogen sulphide found in raw natural gas and from the by-product gases containing hydrogen sulphide derived from refining crude oil and other industrial processes. The Claus process utilised in a sulphur recovery unit (SRU) recovers 94-98% of the sulphur in the feed. Used in conjunction with a reductive tail gas process, the Claus process further increases the recovery achieved in the sulphur recovery complex to 99.9%.

In the Claus process, a concentrated stream of H2S is partially burned to form SO2. The SO2 reacts, first thermally and then in subsequent steps, catalytically, with H2S to form elemental sulphur. The produced sulphur is transported as a melt or as a solid.

H2S + 3/2 O2 → SO2 + H2O + Heat

2H2S + SO2 → 3S + 2H2O + Heat

These reactions to form sulphur are in equilibrium; therefore, conversion to sulphur is increased by condensing the product sulphur from each stage, reheating the mixture and taking subsequent steps to lower reaction temperatures. Higher temperatures limit sulphur recovery by shifting the equilibrium towards the reactants. Increased pressure in the Claus process requires higher reaction temperatures in the second and third stages to maintain margin above the capillary condensation temperature of elemental sulphur. The increased temperatures also then limit sulphur recovery via this process.

When the Claus effluent gases exit the final condenser, the majority of the incoming H2S gas stream has been recovered as elemental sulphur and only residual amounts of unreacted sulphur dioxide and hydrogen sulphide and uncondensed elemental sulphur remain. The gas exiting the Claus unit also contains large volumes of water vapour, a co-product of H2S conversion; hydrogen from H2S cracking and sub-stoichiometric combustion; and large volumes of nitrogen, if air is used in the combustion of the hydrogen sulphide. Additionally, carbon compounds that enter the process or form through combustion (CO, CO2, hydrocarbons) react with sulphur species in the reaction furnace to form carbonyl sulphide (COS) and carbon disulphide 
(CS2) some of which remain unconverted.

The COS and CS2 in tail gas result from the quality of acid gas feed to the Claus unit and are converted in the first Claus reactor. In this reactor, it is possible to utilise alumina catalyst, which can have fairly rapidly aging or more robust titania catalyst. Conditions of temperature, residence time and concentration influences how much of these compounds reach the TGU and, in concert with environmental performance criteria, the degree of conversion required.

The concentration of these compounds is governed by complex equilibrium. This mixture of dilute sulphur and carbon compounds in the steam and nitrogen stream is labelled as ‘tail gas’. The sulphur species included in the tail gas comprise 2-6% of the total sulphur entering the Claus process. Initially, Claus process operators were allowed to incinerate the tail gas mixture to sulphur dioxide, a less odoriferous compound, and discharge it to the atmosphere. Under this scenario, the recovered sulphur comprised only about 94% to 98% of the sulphur entering the process. The balance of the sulphur, 2% to 6%, polluted the environment as SO2.

Reductive tail gas processes
Several types of TGUs were developed to further mitigate the pollution released into the environment by Claus units. The best performing TGUs utilise a cobalt molybdenum (CoMo) on alumina catalyst. These processes, when properly operated, convert nearly all of the sulphur species to 
H2S. The H2S is then captured in 
an amine circuit and recycled to 
the Claus unit. Overall, this achieves a recovery of 99.9% or more of the sulphur fed to the Claus unit, achieving the environmental performance required of virtually all regulatory settings. Less than 0.1% of the sulphur in the Claus feed is released into the environment.