Oct-2011

Continuous removal of contaminants from amine solutions

A continuous purification system overcomes amine degradation and the formation of corrosion-promoting heat stable amine salts

Sunil Dandekar and Jenny Shao
Eco-Tec

Article Summary

The amine-treating unit is of great importance in gas processing and refinery operations for treating acid gases. It is also attracting increasing attention from the pressing needs of environmental compliance and for meeting stringent levels of H2S and CO2 removal.

Amine scrubbing solutions used for gas treatment at refineries, chemical, sulphur and gas processing plants remove hydrogen sulphide (H2S) and carbon dioxide (CO2) from gas streams. Yet, one of the problems with amine solutions is the gradual accumulation of byproduct contaminants called heat stable salts (HSS). An increased HSS concentration in the amine loop leads to corrosion-related maintenance problems, frequent filter replacements, amine losses, excessive foaming, capacity reduction and more.

Ion exchange resins have been employed in the removal of HSS, but because of the different affinities the various anionic HSS contaminants have towards the resin, successful removal is restricted. However, the continuous online removal of HSS rather than periodic or batch treatment helps to ensure a more stable and uniform conditioning operation (see 
Figure 1).

A proprietary system from Eco-Tec has proven successful over the years in achieving efficient online HSS removal. To date, 57 of these systems, known as AmiPur, have been supplied in the US, Mexico, Europe, China, India and Saudi Arabia. This article details the technology and its various applications, including CO2 sequestration. Various examples are presented, with two case studies in China and Aruba.

HSS formation
Lean amine solutions of 20–50 wt% and relatively free of acid gases enter the top of the absorption tower and flow counter-current to the sour gas stream being fed from the bottom of the column. The acid gases in the gas stream are chemically absorbed, resulting in a rich amine solution that exits from the bottom of the tower and the sweet gas from the tower top. Thermal regeneration of the rich amine is then carried out in a steam stripper, where the absorbed acid gases are liberated and this regenerated (lean) amine is recirculated back to the absorption tower top.

In the absorption process, a common problem is the formation of small quantities of HSS, which are not removed during steam stripping. HSS are formed due to the presence of certain other acidic components in the process gas and liquids, which result in an irreversible reaction with the amine to form HSS. These contaminants include chloride, sulphate, formate, acetate, oxalate, thiocynate and thiosulphate. The resultant salts have a relatively strong chemical bonding with essentially no dissociation with heat (in the steam stripper). This results in the gradual build-up of HSS in the amine circulation loop and, when the tolerable limits of HSS are exceeded, several operational and maintenance problems are encountered, some of which include:
• High corrosion rates, leading to stress corrosion cracking
• High maintenance and repair costs and safety concerns
• Frequent filter replacements
• Foaming and plugging in absorber tower
• Decrease in absorption efficiency and productivity
• Heat exchanger and reboiler tube fouling
• Excessive heat requirements
• Overall unit instability.

High corrosion rates, typical for a number of amine plants, as well as stress corrosion cracking of stainless steel, usually attributed to chloride, create serious safety concerns. High corrosion leads to high repair and maintenance costs, potential environmental implications and lost production. Carbon steel corrosion is often attributed to the amine contaminants that cannot be stripped and thereby accumulate in the amine solution. However, preventive measures to reduce corrosion produce good results and improve amine unit reliability at a reasonable cost.

Continuous reclamation to control impurities
To prevent HSS from building up beyond critical limits, the most straightforward approach is periodic amine purging, which is messy and prohibitively expensive given the current situation of increased competition, cost control and restricted waste discharge. Other periodic amine clean-ups, on- or off-site, are also practised to some extent by plants that have to employ large equipment for vacuum distillation, conventional ion exchange or electro-dialysis. However, this approach of periodic reclaiming is cumbersome and expensive and does not effectively overcome the operational and corrosion problems caused by the anions.

It became apparent that the ideal solution to overcome HSS-related problems was to have online amine purification (not periodic), which would ensure a stable and uniform gas conditioning operation, while the contaminant levels in the amines are restricted from building up and the corrosion rate is minimised.

Continuous online amine reclaiming is now recognised as the most effective solution for HSS-related problems (see Figure 2). Furthermore, if an upset occurs upstream of the amine unit, the equipment is on-site to address the problem immediately. Such an online system, known as AmiPur, was introduced to this application in 1998 by Canadian-based Eco-Tec.

Some of the features differentiating this ion exchange technology from conventional systems include:
• Fine mesh resins
• Short and packed resin bed
• Counter-current regeneration
• Low resin loading
• Short cycles
• Compact and skid-mounted system
• Stainless steel construction
• Full automatic operation.