NGL Liquid-Solid Treating Fundamentals

1. Overview of Solid-Phase NGL Treating

Solid-phase treating of NGL streams uses fixed beds of adsorbent materials to remove trace contaminants that liquid treating methods—such as caustic washing, amine scrubbing, and water washing—cannot adequately address. While liquid treating is effective for bulk removal of H₂S, CO₂, and mercaptans, certain contaminants require the selectivity and low-concentration polishing capability that only solid adsorbents can provide. These contaminants include carbonyl sulfide (COS), elemental mercury, arsenic compounds, residual water, and trace organic sulfur species.

In a typical NGL treating train, solid adsorbent beds are positioned downstream of the liquid treating steps. The liquid treaters handle the heavy lifting—reducing bulk sulfur levels from hundreds or thousands of ppm down to the low ppm range—while the solid beds perform the final polishing to meet stringent product specifications. This sequential arrangement is deliberate: exposing solid adsorbents directly to high contaminant loadings would result in rapid bed exhaustion and uneconomical change-out frequencies.

Target Contaminants for Solid-Phase Treating

Each contaminant class presents unique challenges that make solid adsorption the preferred or only practical removal method:

Liquid vs. Solid Treating Comparison

Understanding the complementary roles of liquid and solid treating methods is essential for designing an effective overall treating system:

Position in the Treating Train

The placement of solid adsorbent beds within the overall treating sequence follows a logical progression from bulk removal to fine polishing:

• First stage — Liquid treating: Amine contactors remove bulk H₂S and CO₂. Caustic wash towers remove mercaptans and some COS (by hydrolysis). Water wash may follow to remove dissolved caustic or amine from the NGL
• Second stage — Molecular sieve beds: Remove COS by catalytic hydrolysis to H₂S and CO₂, followed by adsorption of the H₂S product. Also provide final dehydration if needed. Positioned after liquid treating to avoid contamination of the sieve with amine or caustic carryover
• Third stage — Activated carbon or specialized beds: Remove mercury, arsenic, or other trace metals. Typically the final treating step before the NGL enters the product pipeline or downstream processing

This staged approach ensures that each adsorbent operates in its optimal concentration range and is not overwhelmed by contaminants that should have been removed upstream. Proper pretreatment of the NGL before it contacts the solid beds is critical to achieving design adsorbent life and reliable treating performance.

2. Molecular Sieve Treatment

Molecular sieves are the primary solid adsorbent used for COS removal and final dehydration of NGL streams. These synthetic zeolite materials have uniform pore structures at the molecular scale, allowing selective adsorption based on molecular size and polarity. In NGL treating service, molecular sieves serve a dual function: they catalytically convert COS to H₂S and CO₂ through hydrolysis, and they adsorb the resulting H₂S along with residual water.

Sieve Types and Selection Criteria

Four standard molecular sieve types are commercially available, each with different pore diameters and adsorption characteristics. Selection depends on the target contaminant and the NGL composition:

For propane COS treating, type 4A is the most common selection because it adsorbs COS and H₂S while minimizing propane co-adsorption. Type 5A is used when broader sulfur species removal is required but comes with higher propane losses during regeneration. Type 13X provides maximum contaminant removal capability but also adsorbs significant quantities of NGL, increasing regeneration energy requirements and product losses.

COS Hydrolysis Mechanism

The primary mechanism for COS removal on molecular sieves is catalytic hydrolysis rather than direct adsorption. The sieve surface catalyzes the reaction of COS with water already adsorbed on the sieve:

The H₂S produced by hydrolysis is then adsorbed on the sieve. This two-step mechanism means that effective COS removal requires both adequate water content on the sieve (not bone-dry feed) and sufficient H₂S adsorption capacity. If the sieve becomes saturated with H₂S, COS breakthrough will occur even if water and COS are still reacting. The optimal moisture content in the feed NGL for COS hydrolysis is typically 50–200 ppm—enough to sustain the reaction without flooding the sieve capacity with water.

Bed Sizing Parameters

Molecular sieve beds for NGL liquid treating are sized using liquid hourly space velocity (LHSV) and mass transfer zone (MTZ) concepts:

Regeneration

Molecular sieves are regenerated by temperature swing adsorption (TSA), using hot gas to desorb the accumulated contaminants and restore the sieve capacity. The regeneration cycle consists of three phases:

• Heating phase: Hot regeneration gas (typically natural gas or nitrogen) at 450–600°F is passed through the bed in the reverse direction (counter-current to adsorption flow) to desorb water, H₂S, and other contaminants. Heating continues until the bed outlet temperature reaches 400–500°F, indicating thorough desorption
• Cooling phase: Cool gas (ambient temperature) is passed through the bed to reduce the temperature back to adsorption conditions (typically 80–120°F). Cooling must be thorough because sieve capacity decreases at elevated temperatures
• Standby or pressurization: The regenerated bed is pressurized to operating conditions and placed on standby until the online bed reaches breakthrough

Cycle Configuration: Lead-Lag-Regeneration

Most molecular sieve installations use a minimum of three vessels configured as lead-lag-regeneration. During the adsorption cycle, the feed NGL flows through the lead bed first, then through the lag bed. The lead bed handles the bulk of the contaminant loading while the lag bed serves as a polishing bed and guard against premature breakthrough. When the lead bed approaches saturation, it is taken offline for regeneration, the former lag bed becomes the new lead, and the freshly regenerated bed becomes the new lag.

This three-bed configuration provides continuous treating capability while maintaining a safety margin against specification excursions. In some designs, a two-bed system (one online, one regenerating) may be acceptable if the cycle time is long enough and the consequences of brief breakthrough are tolerable.

Capacity and Temperature Effects

Molecular sieve adsorption capacity is strongly temperature-dependent. Dynamic capacity (the usable capacity in cyclic operation) is typically 60–75% of the static (equilibrium) capacity and decreases further at elevated temperatures. For COS treating of propane, typical dynamic capacities are:

• Water capacity: 15–20 wt% at 80°F; 10–14 wt% at 120°F
• H₂S capacity: 8–12 wt% at 80°F; 5–8 wt% at 120°F
• COS removal: Dependent on hydrolysis kinetics and H₂S capacity rather than direct COS adsorption capacity

Sieve performance degrades over time due to coking from heavy hydrocarbon exposure, mechanical attrition from thermal cycling, and poisoning by contaminants such as glycol or amine carryover. Typical sieve life in NGL treating service is 3–5 years, with gradual capacity decline requiring progressively shorter adsorption cycles toward end of life.

3. Activated Carbon Adsorption

Activated carbon is used extensively in NGL treating for mercury removal, trace organic sulfur compound adsorption, and product color improvement. Unlike molecular sieves, which rely on pore size selectivity, activated carbon operates primarily through van der Waals forces and surface chemistry, making it effective for a broad range of contaminants, particularly nonpolar and weakly polar species.

Carbon Types and Selection

The two primary types of activated carbon used in NGL treating are distinguished by their raw material source, which determines their pore structure and adsorption characteristics:

Mercury Removal with Sulfur-Impregnated Carbon

Mercury removal is one of the most critical applications of activated carbon in NGL processing. Elemental mercury in NGL streams attacks aluminum heat exchangers in cryogenic plants through liquid metal embrittlement, causing catastrophic failures. Even trace levels of mercury (parts per billion) are sufficient to initiate aluminum corrosion over time.

Standard activated carbon has limited capacity for elemental mercury. To achieve the ultra-low mercury levels required for cryogenic plant protection (< 0.01 μg/Nm³), the carbon is impregnated with elemental sulfur (typically 10–15 wt% sulfur loading). The mercury reacts chemically with the sulfur to form mercuric sulfide (HgS), which is highly insoluble and permanently bound to the carbon matrix:

This chemisorption mechanism provides several advantages over physical adsorption: the reaction is essentially irreversible under normal conditions, the capacity is predictable based on sulfur loading, and the spent carbon can be safely disposed of since the mercury is locked in an insoluble sulfide form. Typical mercury loading capacities for sulfur-impregnated carbon are 10–20 wt% (as mercury), depending on the sulfur content and carbon pore structure.

Bed Sizing and EBCT

Activated carbon beds are sized using empty bed contact time (EBCT), which is the ratio of bed volume to volumetric flow rate. EBCT provides a measure of the residence time available for adsorption:

Non-Regenerable vs. Regenerable Applications

In NGL treating, activated carbon beds are most commonly operated as non-regenerable (disposable) units, particularly for mercury removal. The decision between regenerable and non-regenerable operation depends on the contaminant type, loading rate, and economics:

• Non-regenerable (disposable): The spent carbon is removed and replaced with fresh material when breakthrough occurs or capacity is exhausted. This is the standard approach for mercury removal (where the mercury-sulfur bond is irreversible), arsenic removal, and applications where contaminant loading is low and bed life is measured in years. Change-out intervals of 2–5 years are typical for mercury service
• Regenerable: The carbon is regenerated in place or at an off-site facility using steam or hot inert gas to desorb physically adsorbed contaminants. This approach is applicable for trace organic removal and color improvement where the adsorption mechanism is physical rather than chemical. Regeneration is not effective for sulfur-impregnated carbon used in mercury service because the chemisorbed HgS cannot be desorbed thermally

Spent Carbon Handling and Disposal

Spent activated carbon from NGL treating service requires careful handling due to the accumulated contaminants, particularly mercury. Key disposal considerations include:

• Mercury-laden carbon: Classified as hazardous waste under RCRA if the TCLP (Toxicity Characteristic Leaching Procedure) exceeds 0.2 mg/L mercury. Most sulfur-impregnated carbons pass TCLP because the HgS is insoluble, but testing is required before disposal. Retort or stabilization may be necessary for high-loading applications
• Hydrocarbon-contaminated carbon: Must be purged of residual NGL before handling in open atmosphere. Typical purging procedures use nitrogen or steam to displace hydrocarbons and reduce explosion risk
• Disposal options: Permitted hazardous waste landfill (most common), mercury retorting and recycling (recovers mercury for reuse), or high-temperature incineration for non-mercury carbon

4. Specialized Adsorbents

Beyond molecular sieves and activated carbon, several specialized adsorbent materials are used in NGL treating for specific contaminant removal applications. These materials are selected when conventional adsorbents lack the capacity, selectivity, or economic viability for the target service.

Metal Oxide Beds for H₂S Polishing

Metal oxide adsorbents react chemically with H₂S to form stable metal sulfides. The two most common metal oxide systems used in NGL treating are iron-based and zinc-based sorbents:

Zinc oxide beds are particularly effective for final polishing of NGL streams where residual H₂S levels of 1–10 ppm must be reduced to sub-ppm levels to meet copper strip test specifications. The ZnO-H₂S reaction is thermodynamically favorable and essentially irreversible at normal operating temperatures, providing a sharp breakthrough front and predictable bed life.

Copper-Based Adsorbents for Arsenic Removal

Arsenic compounds (primarily arsine, AsH₃) in NGL streams are a concern for downstream ethylene plants and petrochemical facilities, where arsenic poisons polymerization and hydrogenation catalysts. Copper-based adsorbents (typically CuO on alumina support) are the standard technology for arsenic removal from NGL liquids:

Copper oxide beds operate at ambient temperature in liquid NGL service with superficial velocities of 1–3 gpm/ft² and EBCT of 10–20 minutes. The beds are non-regenerable; spent material is removed and disposed of as hazardous waste due to the arsenic content. Typical bed life ranges from 1 to 3 years depending on inlet arsenic concentration, which can vary from 5 to 100 ppb in arsenic-bearing gas fields.

Alumina Guard Beds

Activated alumina (Al₂O₃) guard beds are placed upstream of molecular sieve or other high-value adsorbent beds to protect them from contaminants that cause permanent capacity loss. The alumina preferentially adsorbs heavy hydrocarbons, glycol, amine, and other liquid contaminants that would foul or poison the downstream sieve. Guard beds are a cost-effective protective measure because alumina is significantly less expensive than molecular sieve and can be replaced more frequently without impacting overall treating economics.

Typical guard bed sizing uses 10–15% of the main sieve bed volume, with the same superficial velocity as the main bed. The alumina is replaced on a fixed schedule (typically annually) or when pressure drop increase indicates significant loading.

Lead-Lag Configuration Design

Most solid adsorbent installations use a lead-lag vessel arrangement to ensure continuous treating while allowing bed change-out or regeneration without process interruption. The design principles for lead-lag systems include:

• Lead vessel: Handles the bulk of the contaminant loading. For non-regenerable beds, this vessel is changed out first when its capacity is exhausted
• Lag vessel: Serves as a polishing bed and guard against premature breakthrough from the lead vessel. The lag bed should have sufficient remaining capacity to protect the product if the lead bed fails prematurely
• Switching valves: Allow reversal of flow direction so the former lag vessel becomes the new lead, and a fresh bed is installed in the lag position. Switching can be manual or automated based on breakthrough detection
• Sizing criterion: Each vessel should contain at least 50% of the total required capacity, so either vessel alone can maintain specification for a limited period during change-out operations

Bed Change-Out Procedures

Changing out spent adsorbent beds requires careful planning and execution to maintain safety and minimize downtime:

• Isolation and depressurization: The vessel is blocked in, depressurized, and purged with nitrogen to remove residual hydrocarbons before opening. Multiple nitrogen purge cycles may be required to reduce hydrocarbon concentration below the LEL (lower explosive limit)
• Spent material removal: Spent adsorbent is typically vacuum-extracted through manways or bottom nozzles. Mercury-laden or arsenic-laden materials require special handling procedures, personal protective equipment, and licensed waste haulers
• Vessel inspection: While the vessel is open, internal components (inlet distributor, bed support, outlet collector, vessel shell) are inspected for corrosion, erosion, and mechanical damage
• Fresh material loading: New adsorbent is loaded in lifts, with care taken to avoid segregation of particle sizes and to achieve uniform bed density. Excessive drop height during loading can fracture particles and generate fines that increase pressure drop

Monitoring Breakthrough

Effective breakthrough monitoring is essential for optimizing bed utilization while preventing off-specification product. Monitoring methods vary by contaminant:

5. Design and Operation Considerations

Proper mechanical design and operational management of solid adsorbent beds determine whether the treating system achieves its design objectives for contaminant removal, bed life, and economic performance. This section addresses the hydraulic, thermal, and operational factors that influence treating system performance.

Bed Hydraulics

Pressure drop through fixed adsorbent beds is governed by the Ergun equation, which accounts for both viscous and inertial flow contributions. For liquid NGL service, the viscous term typically dominates because of the relatively low superficial velocities used:

Where ΔP/L is the pressure drop per unit bed length, μ is the liquid viscosity, V_s is the superficial velocity, ε is the bed void fraction (typically 0.35–0.40), d_p is the particle diameter, and ρ is the liquid density. Clean bed pressure drops for NGL treating service typically range from 0.3 to 1.5 psi per foot of bed depth.

Liquid Distribution

Uniform liquid distribution across the bed cross-section is critical for effective contaminant removal and efficient adsorbent utilization. Poor distribution leads to channeling, where the liquid preferentially flows through paths of least resistance, resulting in uneven loading and premature breakthrough through under-utilized bed regions. Key distribution design elements include:

• Inlet distributor: A properly designed inlet distributor (pipe distributor, spray header, or distributor tray) should provide uniform liquid distribution within ±10% across the bed cross-section. Pipe distributors with multiple drip points are common for NGL service
• Bed support: The outlet end of the bed uses a support grid (Johnson screen, perforated plate with wire mesh, or proprietary screen) to retain the adsorbent while allowing treated liquid to drain freely. The support must be strong enough to bear the weight of the adsorbent and liquid holdup during operation and hydrostatic testing
• Top retention screen: A hold-down screen at the top of the bed prevents adsorbent fluidization during flow surges and maintains bed integrity. The screen is weighted or spring-loaded to accommodate thermal expansion of the adsorbent during regeneration cycles

Temperature Effects on Adsorption Capacity

Adsorption is an exothermic process, and capacity decreases with increasing temperature for all adsorbent types. For NGL treating, the practical implications are significant because NGL stream temperatures vary seasonally and with upstream processing conditions:

For installations where summer NGL temperatures exceed 120°F, feed cooling upstream of the adsorbent beds should be evaluated. The capital cost of a feed cooler is often justified by the extended adsorbent life and improved treating reliability at lower temperatures.

Moisture Effects on Sieve Performance

The interplay between moisture and molecular sieve performance is complex. While water is essential for COS hydrolysis (the primary removal mechanism), excessive moisture reduces the capacity available for H₂S adsorption. The optimal balance depends on the relative concentrations of COS, H₂S, and water in the feed:

• Too little water (< 20 ppm): COS hydrolysis rate drops, leading to COS breakthrough even though H₂S capacity remains available. The sieve can adsorb H₂S directly but cannot convert COS
• Optimal range (50–200 ppm): Sufficient water for COS hydrolysis without excessive consumption of H₂S capacity for water adsorption. This is the target range for most NGL treating applications
• Too much water (> 500 ppm): Water occupies a disproportionate share of the sieve capacity, shortening the H₂S adsorption cycle and requiring more frequent regeneration. For feeds with high water content, a separate dehydration bed upstream of the treating sieve may be required

Common Operational Problems

Several recurring operational issues can degrade treating performance and reduce adsorbent life if not addressed:

• Channeling: Caused by poor liquid distribution, bed settling, or particle migration. Symptoms include premature breakthrough with significant unused capacity remaining in the bed. Detected by comparing actual bed life to design capacity and by radial temperature profiles during regeneration (cold spots indicate bypassed zones)
• Attrition and fines generation: Thermal cycling during molecular sieve regeneration subjects the particles to repeated expansion and contraction, generating fines over time. Excessive fines increase pressure drop and can plug outlet screens. Controlled heating and cooling ramp rates (50–100°F/hr) minimize thermal shock
• Premature breakthrough: Can result from higher-than-design contaminant loading, elevated operating temperature, co-adsorption of unexpected contaminants, or adsorbent aging. Investigation should include feed composition analysis, temperature logging, and comparison of actual vs. design loading rates
• Upstream contamination: Glycol, amine, compressor oil, or corrosion inhibitor carryover from upstream processing permanently degrades molecular sieve capacity. These heavy organic compounds fill the sieve pores and cannot be fully removed by standard regeneration. Guard beds and inlet coalescing filters provide protection
• Liquid maldistribution: Particularly problematic in larger-diameter vessels (> 8 ft diameter) where achieving uniform distribution is more challenging. Symptoms similar to channeling; resolved by distributor modification or installation of redistribution plates within the bed

Economic Comparison of Treating Methods

The economics of solid treating are driven by adsorbent cost, replacement frequency, and operating labor. The following comparison provides general guidance for evaluating treating options:

Integration with Upstream Liquid Treating

The performance and economics of solid adsorbent treating are strongly influenced by the effectiveness of upstream liquid treating. Proper integration requires attention to several interface conditions:

• Upstream treating performance: The solid beds are sized based on a design inlet contaminant concentration that assumes the liquid treaters are functioning properly. If caustic or amine treating underperforms, the solid beds will be overloaded and bed life will be shortened dramatically
• Carryover protection: Coalescing filters or knockout drums between liquid treaters and solid beds prevent entrainment of treating solutions (caustic, amine, glycol) that would poison or foul the adsorbent. A 10-micron coalescing filter with a separate liquid drain is the minimum recommended protection
• Temperature coordination: NGL temperature entering the solid beds affects adsorbent capacity. If upstream processing raises the NGL temperature above design, a trim cooler may be needed to protect adsorbent performance
• Flow stability: Adsorbent beds perform best at steady-state conditions. Flow surges and rapid composition changes from upstream upsets can cause premature breakthrough. Surge drums or flow control upstream of the treating beds improve performance reliability

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Standard 2140 — Liquefied Petroleum Gas Specifications and Test Methods
3. GPA Standard 2145 — Table of Physical Constants for Hydrocarbons and Other Compounds
4. API Standard 618 — Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services
5. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 — Pressure Vessel Design