Amine Filtration Fundamentals

1. Role of Filtration in Amine Systems

Amine solutions used in gas sweetening service inevitably accumulate contaminants over time. These contaminants originate from the inlet gas, from corrosion of carbon steel equipment, from thermal and oxidative degradation of the amine itself, and from well-treatment chemicals that carry over into the gas processing facility. Without effective filtration, these contaminants degrade amine performance, cause persistent foaming, accelerate equipment corrosion, and reduce overall treating capacity.

Filtration serves as the second line of defense in maintaining amine solution quality. The first line of defense is proper inlet separation—removing free liquids, solids, and entrained hydrocarbons from the sour gas before it contacts the amine. However, even with excellent inlet separation, dissolved and finely dispersed contaminants will accumulate in the circulating amine over weeks and months of operation. Filtration is the primary means of removing these contaminants from the amine loop on a continuous basis.

Two-Stage Filtration Approach

Industry best practice uses a two-stage filtration system:

1. Mechanical Filter (Particulate Removal): Cartridge or bag filters remove suspended solids such as iron sulfide (FeS), iron oxide (Fe₂O₃), pipeline scale, and formation fines. These particulates cause erosion, valve seat damage, and contribute to foam stabilization.
2. Activated Carbon Bed (Dissolved Contaminant Removal): A granular activated carbon (GAC) vessel adsorbs dissolved hydrocarbons, surfactants, amine degradation products (such as heat-stable salts), and other organic compounds that cause foaming. The carbon bed must always be located downstream of the mechanical filter to prevent particulate fouling of the carbon.

GPSA Chapter 21 recommends filtering a minimum of 10–20% of the total amine circulation rate through the filtration system as a slip stream. Many operators filter a higher percentage—or even 100% of circulation—when contamination is severe or when the treating unit is in critical service.

Contaminant Types and Filtration Method

Failure to maintain adequate filtration manifests first as increased foaming tendency in the absorber and regenerator columns. Persistent foaming leads to amine carryover into the treated gas or acid gas streams, reduced treating capacity, off-specification sweet gas, and in severe cases, complete loss of treating capability requiring a unit shutdown for amine replacement.

2. Mechanical Filter Sizing

Mechanical filters provide the first stage of amine filtration by removing suspended particulate matter from the circulating solution. Proper sizing ensures adequate solids removal capacity without excessive pressure drop or frequent filter element changes.

Filter Types

Three primary mechanical filter technologies are used in amine service:

Cartridge Filters: The most common filter type in amine systems. Standard pleated or wound cartridge elements are housed in a pressure vessel with removable head. The 10 μm nominal rating is the industry standard for amine service, capturing iron sulfide and other particulates while allowing dissolved contaminants to pass through to the downstream carbon bed. Cartridge filters operate at a flux rate of approximately 0.5 GPM per square foot of filter area.

Bag Filters: A lower-cost alternative to cartridge filters. Bag elements are typically rated at 25 μm, providing coarser filtration but at a higher flux rate of approximately 1.0 GPM/ft². Bag filters are often used as a prefilter upstream of a finer cartridge filter, or in applications where the solids loading is high and frequent element changes are expected. The larger particle size rating means that finer iron sulfide particles may pass through, so bag filters alone may not provide adequate protection for sensitive downstream equipment.

Automatic Backwash Filters: Self-cleaning filters that use a reverse-flow mechanism to dislodge collected solids from the filter media and discharge them as a concentrated waste stream. These filters offer the lowest operating cost because elements do not require manual replacement, but they have the highest capital cost. Automatic backwash filters are typically justified on large amine systems (circulation rates above 500 GPM) where the cost of frequent manual filter changes becomes significant.

Sizing Methodology

The required filter area is determined by dividing the filtered flow rate by the design flux rate for the chosen filter type:

For cartridge filters, each standard 30-inch-long cartridge element provides approximately 4.5 ft² of effective filter area. The number of cartridges required is:

Standard cartridge filter housings are available in 7-round and 19-round configurations. The 7-round housing accommodates 7 cartridges (31.5 ft² total area, approximately 16 GPM capacity at 0.5 GPM/ft²), while the 19-round housing accommodates 19 cartridges (85.5 ft² total area, approximately 43 GPM capacity).

Filter Type Comparison

Regardless of filter type, the mechanical filter should be equipped with a differential pressure gauge or transmitter. Rising differential pressure indicates solids accumulation on the filter elements. Cartridge and bag elements should be changed when the differential pressure reaches 15–20 psi to prevent element collapse or bypass.

3. Activated Carbon Bed Design

The activated carbon bed is the second stage of amine filtration, targeting dissolved contaminants that pass through the upstream mechanical filter. Its primary purpose is to adsorb surfactants, dissolved hydrocarbons, amine degradation products, and other organic compounds that cause foaming in the absorber and regenerator columns.

It is critical to understand that the carbon bed is not designed for particulate removal. Iron sulfide and other suspended solids will plug the carbon bed, create channeling, and dramatically reduce its effective life. A properly sized and maintained mechanical filter upstream of the carbon bed is essential.

Sizing by Empty Bed Contact Time (EBCT)

The primary design parameter for activated carbon beds in amine service is the Empty Bed Contact Time (EBCT), defined as the ratio of the carbon bed volume to the volumetric flow rate through the bed:

The recommended EBCT for amine service is 10–20 minutes. A shorter contact time reduces adsorption efficiency and allows breakthrough of surfactants, while an excessively long contact time increases vessel size and carbon inventory without proportional improvement in performance.

Carbon Type and Bed Geometry

Granular activated carbon (GAC) in 12×40 mesh size is the standard media for amine filtration. This particle size provides a good balance between adsorptive capacity (high surface area per unit volume) and acceptable pressure drop through the bed. Finer mesh carbons offer higher capacity but create excessive pressure drop, while coarser carbons have lower adsorptive capacity.

The carbon bed vessel should be designed with a length-to-diameter (L/D) ratio of 2:1 to 3:1. This geometry promotes uniform flow distribution through the bed and minimizes channeling, which would allow contaminants to bypass the carbon and reduce effective contact time. The vessel is typically oriented vertically with downward flow through the carbon bed.

Carbon Bed Design Parameters

Carbon life varies significantly depending on the contamination level in the amine solution. In systems with heavy hydrocarbon carryover or severe degradation, carbon may require replacement every 6 months. In cleaner systems with good inlet separation, carbon life can extend to 18 months or longer. Operators should monitor amine foaming tendency and dissolved hydrocarbon content to determine optimal carbon changeout intervals rather than relying solely on a time-based schedule.

4. Slip-Stream vs Full-Stream Filtration

A fundamental design decision for amine filtration systems is whether to filter a slip stream (a fraction of the total circulation) or the full stream (100% of circulation). This choice has significant implications for equipment size, capital cost, operating cost, and the level of protection provided to the amine system.

Slip-Stream Filtration (10–20% of Circulation)

Slip-stream filtration diverts 10–20% of the total amine circulation through the filtration system, with the remainder bypassing directly back to the absorber. This is the most common configuration in the industry because it provides adequate contaminant control for the majority of amine treating applications at a fraction of the cost of full-stream filtration.

The slip stream is typically taken from the lean amine side of the circuit (downstream of the regenerator and lean/rich exchanger) because the lean amine is at a lower temperature and lower viscosity than the rich amine, improving filtration efficiency. The filtered lean amine is returned to the lean amine piping upstream of the absorber.

At a 10% slip-stream rate, the entire amine inventory passes through the filtration system approximately once every 10 circulation cycles. For a typical amine system with a 15–30 minute circulation time, this means the full inventory is filtered roughly every 2.5–5 hours, which is adequate for steady-state contaminant removal in most applications.

Full-Stream Filtration (100% of Circulation)

Full-stream filtration passes all of the amine circulation through the mechanical filter and carbon bed. This configuration provides the highest level of protection and is required in the following situations:

• Critical service: Amine units treating gas for LNG feed or pipeline sales where off-specification gas has severe economic consequences
• Severe contamination: Systems processing gas with high hydrocarbon liquid content, well-treatment chemicals, or formation solids
• Chronic foaming: Units with a history of persistent foaming problems that have not responded to slip-stream filtration
• Selective amine service: MDEA units where foam-induced co-absorption of CO₂ degrades selectivity

Slip-Stream vs Full-Stream Comparison

When designing a new amine unit, many operators install piping and valving to allow future conversion from slip-stream to full-stream filtration if contamination problems arise. This approach minimizes initial capital cost while providing a straightforward upgrade path if operating experience reveals the need for more aggressive filtration.

5. Monitoring and Maintenance

Effective filtration requires ongoing monitoring and a disciplined maintenance program. Without regular attention, filter elements become bypassed or exhausted, carbon beds lose their adsorptive capacity, and contaminants accumulate in the amine solution to levels that cause operational problems.

Filter Differential Pressure Monitoring

The differential pressure across the mechanical filter is the primary indicator of filter loading. A clean cartridge filter typically has a ΔP of 2–5 psi. As solids accumulate on the filter elements, ΔP increases progressively. Cartridge or bag elements should be changed when the ΔP reaches 15–20 psi. Operating beyond this threshold risks element collapse, gasket bypass, or rupture, which allows unfiltered amine to pass directly to the carbon bed.

The carbon bed should also be monitored for ΔP. A clean carbon bed typically has a ΔP of 1–3 psi. Increasing ΔP in the carbon bed indicates either particulate fouling (suggesting the upstream mechanical filter is inadequate or bypassed) or carbon bed compaction. If the carbon bed ΔP exceeds 10 psi, the carbon should be inspected and potentially replaced.

Amine Solution Analysis

Regular amine solution analysis provides the most comprehensive picture of filtration system performance. Key parameters and their target values include:

Monitoring Parameters and Action Limits

Carbon Bed Changeout Indicators

Carbon should be replaced when one or more of the following conditions are observed:

• Foam test deterioration: Increasing foam volume and break time in laboratory foam tests indicate that surfactants are breaking through the exhausted carbon bed
• Hydrocarbon breakthrough: Rising dissolved hydrocarbon content in the lean amine downstream of the carbon bed
• Increasing ΔP: Progressive increase in carbon bed differential pressure not explained by flow rate changes
• Time in service: Even without clear breakthrough indicators, carbon should be replaced at least every 18 months as a preventive measure

Preventive Maintenance Schedule

A structured preventive maintenance program for amine filtration should include the following activities:

• Daily: Record mechanical filter and carbon bed ΔP readings; check for leaks at filter housing closures
• Weekly: Verify slip-stream flow rate is within design range; check ΔP trend for abnormal increases
• Monthly: Sample amine for iron content, suspended solids, and foam test; review ΔP trend data
• Quarterly: Comprehensive amine analysis including HSS, pH, amine concentration, and hydrocarbon content
• As needed: Replace filter elements when ΔP reaches action limit; replace carbon when breakthrough is detected or maximum service life is reached

Maintaining detailed records of filter element change frequency, carbon bed life, amine analysis results, and foaming incidents provides valuable trend data for optimizing the filtration program. A sudden increase in filter element consumption rate, for example, may indicate an upstream corrosion problem or a failure in inlet gas separation that should be investigated and corrected at the source.

References

1. GPSA, Chapter 21 — Hydrocarbon Treating
2. GPSA, Chapter 20 — Dehydration
3. Kohl, A.L. and Nielsen, R.B., Gas Purification, 5th Edition, Gulf Publishing
4. Amine Best Practices Group (ABPG) — Filtration Guidelines for Amine Systems