Oct-2001

Improved predictions for amine sweetening facilities

A review of the work that has been done to avoid the severe problems that benzene, toluene, ethylbenzene and xylene can impose on sulphur recovery assets

Gavin D McIntyre, Vicente N Hernandez-Valencia, Kevin M Lunsford
Bryan Research & Engineering

Article Summary

Recent data on the solubility of HC and BTEX in amines have resulted in better quantitative models, which have been incorporated into process simulation programs. These data show that the amine concentration and type is significant in affecting BTEX (benzene, toluene, ethylbenzene and xylene) solubility. Increasing amine concentration and pressure increases HC and BTEX solubility, while increasing temperature decreases HC and BTEX solubility (for typical commercial amine absorbers).

Confirmation of a model with plant data for MEA, DEA, MDEA, and DGA suggests that the predictions may be used for engineering analysis and optimisation. For amines such as TEA and DIPA, published operating data are yet to be made available.

With existing amine sweetening units, the quantity of BTEX absorbed in amine solvents affects down-stream processes  – such as emissions, if the stripper is vented – or the sulphur recovery unit.

Amine solutions absorb an amount of hydrocarbons (HC) and BTEX. These dissolved HCs obtained by contacting with feed gas are ultimately released in the regenerator’s overhead. This overhead either vents to the atmosphere or feeds a sulphur recovery unit (SRU). HC content for regenerator vents discharging to the atmosphere must comply with recent stringent regulations. For acid gas feeds to a Claus unit, excessive HCs may result in catalyst fouling, sub-quality sulphur product, or the need for more sophisticated burner design.

To better understand and quantify HC and BTEX solubility in aqueous amines, the Gas Processors Association commissioned research Project 971. Preliminary results from this project have been used to improve models for HC and BTEX solubility predictions. Model predictions are compared with operating facilities and guidelines for minimising HC absorption in amine facilities are presented. If the overhead of the regenerator is fed to a burner such as in a Claus plant, the BTEX components are more difficult to destroy relative to other HCs. These BTEX components tend to deactivate the Claus catalysts, severely limiting the catalyst life.

Hydrogen sulphide (H2S), carbon dioxide (CO2) and some HCs are absorbed by the amine solution in the contactor (Figure 1).

Some of the HCs are removed in the rich flash by pressure reduction. The solvent is regenerated and the acid gas and HCs are rejected to the overhead stream. A small portion of the acid gas remains in the lean amine from the bottom of the stripper. This simple flow sheet becomes more complicated depending on the process application and product specifications. Additional draws can remove semi-lean amine from the regenerator, and side coolers may be attached to the absorber to remove heat of absorption.

The lean rich exchanger can be removed entirely. For this simple amine facility, absorbed HCs can only be rejected in the rich flash or the acid gas exiting the regenerator. If HCs are not removed in the rich flash to an acceptable level before the regenerator, other process modifications must be made. For more complicated cases, the potential locations for HC rejection need to be identified.

Equilibrium data
The literature sources for the solubility of light paraffins in aqueous amine solutions are shown in Table 1 (following page). The streams processed by amine sweetening units contain a high concentration of light HCs. The initial concern with HC absorption was based on product loss. Therefore, most of the solubility data are for methane, ethane, and propane.

Recently, Critchfield et al reported on the solubility of n-pentane in aqueous amines. Overall trends for HC solubility in amines in the operating range of a typical absorber can be summarised:
- HC solubility increases with increasing amine concentration
- Increasing pressure increases HC solubility
- Increasing temperature decreases HC solubility.

The type of amine also influences HC solubility. MEA generally shows the lowest HC solubility followed by DEA, TEA, and then DGA, MDEA and DIPA, with DIPA having the greatest affinity for HCs relative to the other amines. These recent findings are somewhat contradictory to the current industry doctrines.

In contrast to the eight articles listed in Table 1, which report on HC absorption in amines, Table 2 lists only three papers which provide data for BTEX solubility in amines. Hagerty and Hawthorn report data for benzene and toluene in MDEA and DGA solutions. Critchfield et al compares benzene solubility in MEA, DEA, DGA, MDEA and DIPA.

Again, contrary to popular belief, the data show DIPA has a greater affinity for benzene than either MDEA or DGA. DGA showed similar affinity for benzene as MDEA. Table 2 also lists the highly anticipated yet unpublished results from GPA project 971. From the little published data for aromatics in amines, the overall trends of BTEX solubility appear to be similar to that of HC solubility:
- Increasing amine concentration increases BTEX solubility
- Increasing pressure increases BTEX solubility
- Increasing temperature decreases BTEX solubility.