Dec-2014

Tail gas catalyst performance: part 2

The second part of a two-part account of time and temperature effects on tail gas catalyst performance gives a background to reaction modelling and pilot plant 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 TGU 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, presented in two parts. The first part of the article (PTQ, Q3 2014) provided an introduction to reactor modelling, process and catalyst development history, and chemical equilibrium. The second part develops reactor modelling with a kinetic reaction model, the effects of temperature and space velocity, catalyst activation, catalyst deactivation, and determining TGU catalyst health from a commercial unit temperature profile.

Reactor modelling
In order to predict the performance of TGU reactor systems, a basic framework of chemical equilibrium, reaction chemistry and catalyst activity is used. This, in turn, provides a tool to evaluate the effects of space velocity and temperature on reactor performance.

Requisites for good reactor design and operation must be met to achieve good performance. The assumption in modelling and predicting performance is that all the other things are done correctly. With good gas distribution across the catalyst bed and acceptable pressure drop for process line-up and inlet concentrations within the design boundary, one can move forward to the impact of gas rate and time and temperature on performance.

The influence of temperature on performance has competing effects in kinetics and equilibrium, impacting conversion. The kinetics for reactions of importance are favourably influenced by higher temperature, proceeding to higher conversion at a given space velocity. Equilibrium effects from higher temperatures usually result in higher equilibrium concentrations for the species, which the system is designed to destroy, limiting lower values for outlet concentration. Equilibrium considerations were addressed in the first part of the article and this part deals with kinetics.

Conversion is a term of frequent reference in this article. Conversion for CO (or COS) is expressed as disappearance across the reactor and is adjusted for the equilibrium ‘back pressure’ of the reacting component:
                                                                               COout – COequilibrium
Conversion  = 1  -   -------------------------                
                                                                               COin – COequilibrium

Tail gas reactor temperature has historically ranged between 200°C and 325°C. This fits within the region of active catalyst functions and meets the required minimum temperature for catalytic activity function, about 200°C for low temperature catalysts and 240-300°C for conventional tail gas catalysts. Maximum temperature is generally limited to 345°C (650°F).

Kinetics: first order reaction model
Reaction kinetics describes how fast a reaction proceeds and allows prediction of how far a reaction proceeds toward equilibrium in a reactor system. The reactor system is defined by gas composition, gas flow rate, and catalyst type and volume. A first order reaction kinetic model with equilibrium is useful to describe the conversion of reactants in the tail gas reactor system and is helpful to understand performance effects of operating variables.

Tail gas reaction kinetics can be represented reasonably with first order reaction models as a useful representation of tail gas reaction kinetics. Experimental data for a Criterion C-534 tail gas catalyst is fitted into the reaction modelling equations; those models are used to evaluate design parameters and test operating considerations or troubleshooting to improve performance and assess catalyst activity. 

Sulphur species conversion:
catalyst activity, temperature and space velocity
Modelling the reactor as a continuous plug flow reactor, first order reactions kinetics provides a means for quantifying the gas loading effect. First order reaction kinetics models are reasonable models for both COS hydrolysis, COS sour shift and CO water gas shift conversion in the tail gas system. Although the complete expression is complex when considering mass transfer, diffusion, adsorption and surface reactions, it is possible to simplify the expression to kinetic activity. At extremes, other resistances exert influence.

Reactor loading is represented in the kinetic expression as gas hourly space velocity (GHSV), a ratio of the gas rate per unit of catalyst. The basis used for these expressions is actual volumetric gas flow rate per hour divided by catalyst volumetric inventory.

The relationship between conversion, actual gas hourly space velocity, GHSV and catalyst activity, k, is defined in first order reaction model as:

rate of disappearance of component a = dCa / dt = k * Ca

and overall for the reactor, the equilibrium conversion is given by:
ln (1-Conversion eq) = -k * 3600 / GHSV,

Equilibrium conversion (Conver-sion eq) represents the amount of component a, which can be actually converted, adjusting for equilibrium residual:
Conversion_eq = 1 – (Ca_out –Ca_eq)/(Ca_in – Ca_eq)
and 
Conversioneq = 1 – (Ca_out – Ca_eq)/( Cain – Ca_eq) = 1- e-k * 3600 / GHSV
where:    k = first order activity catalyst, mol/sec
Ca_in = inlet concentration of component A
Ca_out = outlet concentration of component A
Ca_eq = equilibrium concentration of component A, set by WABT and composition of outlet stream
GHSV = actual gas hourly space velocity.