Mercury Removal Unit Fundamentals

1. Mercury in Natural Gas

Mercury occurs naturally in reservoir fluids from geological formations. Concentrations vary by orders of magnitude between regions—from <0.1 µg/Nm³ in North American dry gas to >500 µg/Nm³ in Southeast Asian fields.

Mercury Species

Mercury in natural gas exists primarily as elemental vapor (Hg⁰), with trace amounts of organomercury compounds and mercuric salts:

Species	Formula	Occurrence	Removal
Elemental mercury	Hg⁰	95-99% of total	Easy (adsorption)
Mercuric chloride	HgCl₂	Trace in sour gas	Moderate
Dimethyl mercury	(CH₃)₂Hg	Rare (biogenic)	Difficult

Regional Mercury Levels

Region	Typical Range (µg/Nm³)	Comments
North America (onshore)	0.01–0.5	Generally low; some exceptions
North Sea	0.1–10	Variable by field
Indonesia/Malaysia	50–500	High mercury province
Middle East	1–50	Moderate levels
Australia (NW Shelf)	0.5–20	Variable

Design consideration: Mercury concentration can vary significantly over field life as different reservoir zones deplete. Design for P95 or P99 concentration, not average.

2. Failure Mechanism

Mercury causes liquid metal embrittlement (LME) of aluminum alloys used in brazed aluminum heat exchangers (BAHX). This is the primary driver for mercury removal in LNG facilities.

Four-stage diagram of liquid metal embrittlement showing mercury adsorption on aluminum oxide layer, penetration forming amalgam zone, diffusion along grain boundaries, and crack propagation under stress — LME progression: Mercury breaches oxide layer, forms amalgam, diffuses along grain boundaries, and initiates intergranular cracking under stress.

LME Process

Adsorption: Mercury vapor condenses on cold aluminum surfaces
Amalgamation: Mercury penetrates oxide layer, forms Al-Hg amalgam
Diffusion: Mercury migrates along grain boundaries
Embrittlement: Amalgam weakens grain boundary cohesion
Cracking: Under normal operating stress, intergranular cracks form
Failure: Catastrophic rupture of heat exchanger passages

Critical fact: LME can initiate at mercury levels as low as 10 ng/Nm³. Once mercury contacts aluminum below -39°C (mercury freezing point), damage accumulates. Failures have occurred years after initial contamination.

Equipment at Risk

Main cryogenic heat exchangers (MCHE): $50-100M+ replacement cost
Cold boxes: Brazed aluminum plate-fin exchangers
Kettle reboilers: Aluminum tubes in LNG/NGL service
Cryogenic piping: Aluminum alloy sections

3D cutaway of brazed aluminum heat exchanger showing parting sheets, corrugated fins, header bars, nozzles, and detail of brazed joint vulnerable to mercury attack — BAHX construction: Brazed joints between fins and parting sheets are vulnerable to mercury-induced LME; replacement cost exceeds $50-100M.

3. Removal Technologies

Mercury removal relies on chemical reaction between mercury vapor and solid adsorbents. The dominant technology is sulfur-impregnated activated carbon.

Sulfur-Impregnated Carbon

Activated carbon impregnated with 10-15% elemental sulfur. Mercury reacts irreversibly with sulfur to form stable mercuric sulfide (HgS, cinnabar):

Mercury Capture Reaction: Hg⁰ (vapor) + S⁰ (surface) → HgS (solid) ΔG° = -50 kJ/mol (highly favorable) The reaction is: • Irreversible at operating temperatures (<120°F) • Fast kinetics (equilibrium in <1 second contact) • Exothermic but negligible heat release at ppm levels

Three-scale view of sulfur-impregnated carbon adsorbent: pellet scale showing 3-4mm extrudate, pore scale showing sulfur deposits in macro/mesopores, and reaction scale showing Hg + S → HgS formation — Adsorbent structure: Sulfur deposits in carbon pore network capture mercury vapor through irreversible HgS formation; capacity 10-20 wt%.

Adsorbent Comparison

Adsorbent	Hg Capacity	Max Temp	Regenerable	Relative Cost
Sulfur-impregnated carbon	10-20 wt%	120°F (50°C)	No	1.0×
Metal sulfide (CuS/ZnS)	5-10 wt%	300°F (150°C)	Some types	1.5-2.0×
Silver-impregnated carbon	8-15 wt%	100°F (38°C)	No	5-10×

Process Configurations

P&ID of lead-lag mercury removal unit showing inlet KO drum, lead bed for bulk removal, lag bed for polishing, switching valves, and outlet mercury analyzer with instrumentation — Lead-lag configuration: Achieves 80-90% adsorbent utilization; outlet analyzer monitors breakthrough for bed rotation timing.

Single guard bed: One vessel, full changeout at breakthrough. Suitable for low Hg (<5 µg/Nm³)
Lead-lag (recommended): Two vessels in series. Lead saturates first; rotate to maximize utilization (80-90%)
Multi-stage: Bulk removal (metal sulfide) + carbon polishing. For high Hg (>100 µg/Nm³)

Best practice: Lead-lag configuration is standard for LNG plants. Provides continuous operation during changeouts and achieves 80-90% adsorbent utilization vs. 50-60% for single-bed operation.

4. Bed Sizing Methods

MRU sizing uses material balance for adsorbent quantity and contact time/velocity limits for vessel dimensions. Both constraints must be satisfied.

Material Balance Method

Required Adsorbent Mass: M = (Q × C_in × t × SF) / (Cap × U × 1000) Where: M = Adsorbent mass (kg) Q = Gas flow rate (Nm³/h) C_in = Inlet mercury concentration (µg/Nm³) t = Design bed life (hours) SF = Safety factor (1.2-1.5 for Hg variability) Cap = Adsorbent capacity (wt% × 10,000 µg Hg/g adsorbent) U = Utilization factor (0.7-0.9) Example: Q = 100,000 Nm³/h C_in = 50 µg/Nm³ t = 8,760 h (1 year) SF = 1.25 Cap = 15 wt% = 150,000 µg/g U = 0.80 M = (100,000 × 50 × 8,760 × 1.25) / (150,000 × 0.80 × 1000) M = 456 kg

Contact Time Requirement

Minimum Bed Volume (Contact Time): V_min = (Q_act × t_c) / ε Where: V_min = Minimum bed volume (m³) Q_act = Actual volumetric flow at P, T (m³/s) t_c = Minimum contact time (2-5 seconds) ε = Bed voidage (0.38-0.42) Contact time ensures: • Adequate mass transfer for equilibrium • Complete reaction with sulfur sites • Consistent outlet specification Typical minimum: 2-3 seconds for sulfur-impregnated carbon

Vessel Sizing

Vessel Dimensions: Cross-sectional area (from superficial velocity): A = Q_act / v_s Where v_s = 0.5-1.0 ft/s (0.15-0.30 m/s) design velocity Vessel diameter: D = √(4A / π) Bed height: H = V_bed / A L/D ratio target: 1.5:1 to 3:1 Check for fluidization: v_mf = (d_p² × (ρ_p - ρ_g) × g) / (150 × μ) × (ε³ / (1-ε)) Design velocity must be < 0.5 × v_mf

Design Parameters Summary

Parameter	Typical Range	Design Basis
Superficial velocity	0.5-1.0 ft/s	Prevent fluidization
Contact time	2-5 seconds	Reaction equilibrium
L/D ratio	1.5:1 to 3:1	Flow distribution
Pressure drop (clean)	0.5-2.0 psi	Process constraints
Utilization factor	0.7-0.9	MTZ, channeling allowance

Cross-section of mercury removal vessel showing inlet distributor, freeboard, bed restraint screen, sulfur-impregnated carbon bed, support grid, and outlet collector with design parameters — MRU vessel internals: Target L/D 1.5-3.0, velocity 0.5-1.0 ft/s, contact time 2-5 seconds for complete mercury capture.

5. Industry Specifications

LNG Plant Feed Gas

Project/Region	Mercury Limit	Basis
Typical LNG specification	<10 ng/Nm³	BAHX protection
Australia (Gorgon, Wheatstone)	<1 ng/Nm³	Conservative design
Qatar LNG	<10 ng/Nm³	Industry standard
US Gulf Coast LNG	<10 ng/Nm³	FERC/DOE permits

Pipeline Specifications

Pipeline System	Mercury Limit
US interstate (typical)	30 µg/Nm³ (0.001 gr/100 scf)
European grid	100 ng/Nm³
Offshore gathering	No formal limit (case-by-case)

Mercury Measurement

Accurate measurement is critical for compliance verification and breakthrough detection:

Method	Detection Limit	Application
ASTM D6350 (gold trap + AAS)	~0.5 ng/Nm³	Reference method
ASTM D5954 (AFS)	~0.1 ng/Nm³	Ultra-low detection
Online CVAF analyzer	0.01-100 µg/Nm³	Continuous monitoring
Portable gold film	~1 ng/m³	Spot checks

Monitoring strategy: For LNG service, install online mercury analyzer downstream of MRU. Set alarm at 50% of spec limit to allow response time before contract exceedance.

6. Operations & Changeout

Breakthrough Curve

Log-scale graph of mercury breakthrough showing flat baseline during compliant operation, alarm setpoint at 5 ng/Nm³, specification limit at 10 ng/Nm³, and S-curve rise at MTZ exit around month 18-20 — Breakthrough behavior: Outlet remains low until MTZ exits bed; alarm at 50% of spec provides lead time for changeout planning.

Breakthrough Time Estimation: t_bt = (M × Cap × U) / (Q × C_in) Where: t_bt = Time to breakthrough (hours) M = Adsorbent mass (kg) Cap = Capacity (µg Hg/g adsorbent) U = Utilization at breakthrough (0.7-0.9) Q = Flow rate (Nm³/h) C_in = Inlet mercury (µg/Nm³) Breakthrough defined as outlet reaching: • 10% of inlet, OR • Specification limit (whichever is first)

Lead-Lag Changeout Strategy

Normal operation: Lead vessel online (bulk removal), lag vessel polishing
Approach breakthrough: When lead outlet reaches 50% of spec, monitor closely
Lead breakthrough: Isolate lead vessel when outlet exceeds spec
Rotation: Lag becomes new lead; change out spent lead vessel
Reload: Fresh adsorbent in former lead vessel; becomes new lag

Changeout Safety

Spent mercury adsorbent is hazardous waste (EPA RCRA). Changeout requires:

Vessel isolation: Blind flanges, nitrogen purge to <1% LEL
Personnel protection: Supplied air respirators, Tyvek suits
Mercury monitoring: Continuous vapor monitoring (OSHA PEL: 0.1 mg/m³)
Vacuum transfer: Enclosed transfer to sealed containers
Disposal: Manifested transport to licensed hazardous waste facility

Spent Adsorbent Disposal

Method	Process	Cost ($/tonne)
Stabilization + landfill	Cement/polymer encapsulation	500-1,500
Thermal retort	Heat to recover Hg	2,000-5,000
Chemical treatment	Leach and precipitate	3,000-8,000

Cost consideration: Disposal cost is significant for large beds. At 15 wt% loading, 10 tonnes of spent carbon contains 1.5 tonnes of mercury worth $30-50k at commodity prices, but recovery is often uneconomic vs. stabilization.

Mercury Removal Unit Design

<10 ng/Nm³

10-20 wt%

1-5 years

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