Amine HSS Buildup

 Heat stable salts form when strong acids react irreversibly with amine molecules, creating salts that the regenerator cannot break apart. In a healthy amine system, the chemistry is reversible: acid gases such as H2S and CO2 are absorbed in the contactor and stripped out in the regenerator. When stronger acids enter the system, however, the resulting salts remain in solution and continue to build up over time.

The acids responsible for HSS formation generally fall into two categories:

• Organic acids. These include formic and acetic acid, with smaller contributions from oxalic, glycolic, and propionic acids. They are typically formed in situ through amine oxidation and degradation.
• Inorganic acids. These include chlorides, sulfates, thiosulfates, thiocyanates, and nitrates. They usually enter the system through external contamination, makeup water, or reactions involving oxygen and acid gases.

Each of these anions ties up free amine, reducing the active concentration available to absorb acid gas and increasing the corrosivity and foaming tendency of the solution.

 HSS rarely results from a single root cause. In most plants, HSS accumulation is the cumulative outcome of several mechanisms working together over months or years.

Oxygen Ingress

Oxygen ingress is one of the largest drivers of HSS formation in most amine units. When oxygen enters the system, it oxidizes the amine and produces organic acids, particularly formic and acetic acid, which then react with active amine to form heat stable salts.

Common oxygen entry points include atmospheric breathing of storage tanks without nitrogen blanketing, leaking pump seals, poorly sealed sample points, and air ingress through low-pressure sections of the loop. Even small amounts of oxygen exposure, sustained over time, can produce significant HSS accumulation. Tightening oxygen control is often the highest-leverage action a plant can take to slow HSS growth.

Acid Gas Side Reactions

CO2 and H2S, while reversibly absorbed under normal conditions, can contribute to degradation pathways when combined with oxygen, elevated temperatures, or specific contaminants. CO2-rich service is especially vulnerable to amine degradation routes that produce bicine and other heat stable compounds, particularly in MDEA and DGA units.

Thermal Degradation

High reboiler skin temperatures and excessive stripper bottom temperatures accelerate amine breakdown. Thermal degradation produces a range of heavier compounds, some of which contribute directly to HSS formation. Reboiler tube fouling is a common but often overlooked driver because it forces operators to push temperatures higher to maintain duty.

External Contamination

Chlorides, sulfates, and other inorganic ions frequently enter through makeup water, upstream chemicals, or carryover from inlet streams. In gas gathering systems with multiple field inlets, contamination events can introduce significant HSS precursors in a short period. Once these ions are in the loop, they persist until the amine is reclaimed or replaced.

The damage caused by elevated HSS levels is cumulative and affects nearly every aspect of unit performance.

Reduced Treating Capacity

Each HSS molecule ties up one molecule of active amine. As HSS climbs, the effective free amine concentration drops, even though total amine concentration measured by titration may appear unchanged. That is why a unit can look acceptable on paper while delivering off-spec sweet gas in the field.

Accelerated Corrosion

HSS increases the corrosivity of the amine solution, especially in hot lean-amine sections such as the regenerator overhead, lean-rich exchanger, and reboiler tubes. The mechanism involves both reduced solution pH and the chelating behavior of certain HSS anions, which mobilize iron from carbon steel surfaces. Plants with chronic HSS issues frequently see iron levels in circulating amine climb above 10–20 ppm, indicating active material loss. For a deeper look at corrosion mechanisms, link to the internal article on Amine Corrosion Causes and Prevention.

Increased Foaming Tendency

HSS contributes to foam stability through changes in surface tension and by promoting corrosion products that act as foam stabilizers. Many foaming events trace back to HSS accumulation as a contributing root cause. For the broader picture, link to the internal article on Amine Foaming Causes and Troubleshooting.

Higher Viscosity and Energy Use

Elevated HSS increases amine viscosity, which raises pump load and reduces heat transfer efficiency in lean-rich exchangers. Reboiler duty climbs to maintain stripping performance, increasing fuel and steam costs.

Shortened Equipment Life

The combination of corrosion, fouling, and accelerated degradation shortens the service life of trays, exchanger tubes, reboilers, and reclaimers. The capital impact of unaddressed HSS often far exceeds the cost of proactive monitoring.

 Because HSS accumulates gradually, the symptoms tend to emerge slowly and can easily be mistaken for other problems. Operators should watch for:

• Gradual decline in treating efficiency at constant inlet conditions.
• Rising reboiler duty without changes in acid gas loading.
• Increasing corrosion coupon weight loss or iron-in-solution trends.
• More frequent foaming events.
• Darkened amine solution color; healthy amine is usually clear to pale yellow, while degraded amine may appear dark amber to nearly black.
• Steady increase in makeup amine consumption.

A typical operational threshold for action is 1–2 wt% total HSS on a free-amine basis, depending on amine type and service. Many plants operate well past 4–5 wt% before recognizing the problem, by which point reclamation is often urgent.

 Accurate HSS management requires laboratory analysis. There is no reliable way to estimate HSS from operational data alone.

Total HSS Measurement

This is typically reported as wt% on a free-amine basis and is the primary number used to track HSS trends over time.

Ion Chromatography Speciation

This identifies the specific anions present, such as formate, acetate, chloride, sulfate, and thiocyanate. Speciation helps determine the likely source: high formate and acetate often point to 

oxygen ingress, while high chloride or sulfate points to external contamination.

Amine Strength and Degradation Products

This separates active amine from degraded species and provides a fuller picture of solvent health.

Corrosion Monitoring

Iron, nickel, and chromium levels in circulating amine, along with corrosion coupon data, show whether HSS is actively driving material loss.

A quarterly sampling program is the minimum recommended frequency, with monthly sampling for higher-risk units. Regular trending allows problems to be caught early, before HSS reaches the point where reclamation becomes a major intervention.

 Solvent Reclamation

Reclamation is the primary method for removing accumulated HSS from the system. The main approaches are ion exchange, either continuous or batch, which selectively removes HSS anions while leaving active amine in solution, and electrodialysis or thermal reclaiming for heavier degradation products. The right choice depends on amine type, HSS composition, and economics.

Caustic Neutralization

In some systems, controlled caustic addition is used to neutralize HSS and free up bound amine. This must be done carefully. Over-neutralization introduces sodium salts that can become additional contaminants and may contribute to stress corrosion cracking in carbon steel. Caustic neutralization is a tactical tool, not a long-term strategy.

Partial Amine Replacement

When HSS levels are very high or the amine is otherwise compromised, replacing a portion of the inventory with fresh amine may be the most economical route. This is usually a last resort given the cost of new amine and disposal of the contaminated solution.

Preventing Oxygen Ingress

Because oxygen-driven degradation is the dominant HSS mechanism in most plants, oxygen exclusion delivers the highest return on effort. Nitrogen blanketing on storage tanks, mechanical seal upgrades, sample point improvements, and routine leak surveys all help. Plants that successfully control oxygen ingress can extend amine life by years.

Routine Monitoring and Maintenance

Sustainable HSS control comes from disciplined routine practice: scheduled sampling, lab analysis, mechanical filtration maintenance, activated carbon replacement, and process condition monitoring. A preventive approach almost always costs less than reactive intervention.

Operators may observe:

• Gradual decline in treating efficiency 
• Increased corrosion rates 
• Rising reboiler duty 
• Higher foaming frequency 
• Darkened or degraded solvent appearance 

Because these changes often occur gradually, routine monitoring is essential.

Accurate assessment of HSS levels requires laboratory analysis.

Key diagnostic methods include:

• Total heat stable salt measurement 
• Ion chromatography (to identify specific salts) 
• Solvent composition analysis 
• Corrosion monitoring data 

Regular sampling and trending are critical for early detection.

Solvent Reclamation

Reclamation is the primary method for removing heat stable salts from the amine system.

This process:

• Removes accumulated salts 
• Restores solvent performance 
• Reduces corrosion potential 

Partial Solvent Replacement

In some cases, replacing a portion of the amine solution may be necessary to reduce HSS concentration.

Preventing Oxygen Ingress

Minimizing oxygen entry into the system is one of the most effective ways to slow HSS formation.

Monitoring and Maintenance

Routine testing and system monitoring help ensure HSS levels remain within acceptable limits.

• Ignoring gradual performance decline and assuming it is just “how the unit runs now.”
• Delaying reclamation until HSS reaches crisis levels.
• Treating symptoms, such as adding antifoam or increasing reboiler duty, without addressing the root cause.
• Failing to identify and eliminate the source of contamination.
• Adding caustic without proper monitoring and speciation data.

 Nexo Solutions provides full-spectrum heat stable salt management for amine units across gas processing, LNG, and refinery operations. Services include HSS measurement and trending, ion chromatography speciation, solvent quality evaluation, reclamation strategy development, corrosion risk assessment, and onsite testing and troubleshooting.

If your unit is showing signs of HSS accumulation, or if you want to build a preventive monitoring program before performance starts to decline, the team can help.

Contact Nexo Solutions to discuss your amine system performance and HSS control strategy.