(83c) Simulation of Selective Treating Processes

The selective gas treating process will be used to highlight the differences between mass transfer rate based simulation and ideal stage / efficiency simulation of sour gas treating.

Gases containing both H₂S and CO₂ present a unique gas treating challenge. Typical examples include:

1. Feeds to Acid Gas Enrichment units which concentrate H₂S so it can be fed to a Sulphur Recovery Unit (SRU) and

2. Tail gas streams produced from the Claus section of SRUs where H₂S must be removed before the remaining gas is incinerated and discharged into the atmosphere.

Selective treating processes use aqueous alkaline solvents to achieve the goal of absorbing H₂S while slipping CO₂. Even though H₂S and CO₂ are both acid gases with similar acidity, the selective treating goal can be met by exploiting differences in the mass transfer characteristics of H₂S and CO₂. This paper discusses in detail the mass transfer processes involved in acid gas absorption, with emphasis on the different aspects which dominate the absorption rate of H₂S and CO₂.

Simulation calculations for sour gas treating columns depend on the ability to accurately predict Vapor Liquid Equilibrium (VLE) of acid gases over the liquid solvents. However, simulation software must do more than predict equilibrium; it must account for the fact that, in practice, real sour gas removal columns do not attain equilibrium. The most challenging part of simulating sour gas removal is predicting the difference between a real column and a series of idealized equilibrium stages. The departure from equilibrium can be addressed in simulation software by using one of two completely different calculation approaches:

1. The performance of each ideal stage can be reduced by an efficiency factor (with or without an additional residence time modifier) or

2. The performance of each real tray or packed bed can be calculated using mass transfer coefficients and mass transfer rate equations.

A series of case studies is used to demonstrate several counter-intuitive aspects of selective treating processes. It is shown that a mass transfer rate based simulation approach is better suited for the selective treating process because, like the real process, its outcome depends on differences in mass transfer rates which, in turn, depend on underlying physical processes. In contrast, it will be shown that considerable additional challenges must still be faced in order to produce correct results from an equilibrium stage / efficiency model of the process.