Trace Mercury & Metal Contaminant Removal from Cryogenic Natural Gas: Liquid Metal Embrittlement (LME) Prevention, Sulfided Chemisorbents & MRU Bed Sizing Guide

1. Executive Screening Matrix & Rapid Selection Flowchart

Elemental mercury ($Hg^0$) is an invisible, highly toxic trace contaminant naturally present in many gas reservoirs across Southeast Asia (Thailand, Indonesia, Malaysia), the North Sea, Australia, South America, and North Africa ($5\text{ to }>500\ \mu\text{g/Sm}^3$). In ambient processing, trace mercury is benign. However, in cryogenic liquefaction units (LNG baseload facilities, FLNG vessels, and turboexpander NGL extraction plants), mercury condensation triggers catastrophic Liquid Metal Embrittlement (LME) of cryogenic Brazed Aluminum Heat Exchangers (BAHX / Cold Boxes). A dedicated Mercury Removal Unit (MRU) guard bed is a non-negotiable safety critical barrier.

                           Raw Natural Gas Mercury Screening
                                (Inlet Hg: 5 to 500 ug/Sm3)
                                             |
             -----------------------------------------------------------------
             |                                                               |
    Wet / Sour Upstream Gas                                        Dry / Sweet Feed Gas
    (Prior to Acid Gas & Dehydration)                              (Upstream of Cryogenic Cold Box)
             |                                                               |
  Metal Sulfide on Alumina (CuS / MoS2)                          Sulfur-Impregnated Activated Carbon
  Tolerates Liquid Water & Heavy HC                              Highest Dynamic Hg Capacity (12-15 wt%)
  Non-Leaching Chemisorbent Matrix                               Deep Polishing to < 0.01 ug/Sm3 (10 ng/Sm3)
             |                                                               |
             -----------------------------------------------------------------
                                             |
                                 Vessel Configuration Check
                                             |
             -----------------------------------------------------------------
             |                                                               |
   Inlet Hg < 50 ug/Sm3                                            Inlet Hg >= 50 ug/Sm3
             |                                                               |
   Single Operating Bed                                            Lead-Lag Dual Bed Architecture
   (3 to 5 Year Service Life)                                      (On-Line Sorbent Changeout Without Shutdown)

1.1 The Multi-Criteria Mercury Removal Technology Matrix

2. Liquid Metal Embrittlement (LME) Failure Mechanisms in BAHX

Cryogenic Cold Boxes in LNG facilities are constructed from vacuum-brazed 5083 and 3003 aluminum alloys. Aluminum offers outstanding thermal conductivity and remains ductile at $-162^\circ\text{C}$ without brittle transition. However, aluminum is uniquely vulnerable to mercury attack:

2.1 Amalgamation vs. Liquid Metal Embrittlement (LME)

1. Mechanical Oxide Breakdown: Aluminum surfaces are protected by a tenacious passive oxide film ($\text{Al}_2\text{O}_3$). In dry cryogenic gas, micro-abrasion, thermal cycling, or trace moisture breaches this passive layer.
2. Grain Boundary Wetting: Liquid elemental mercury ($Hg^0$, freezing point $-38.8^\circ\text{C}$) condenses directly on cold aluminum fins and parting sheets. Elemental mercury has zero dihedral contact angle on bare aluminum, rapidly wetting and penetrating the grain boundaries via capillary action.
3. Catastrophic Intergranular Fracture: Mercury atoms disrupt interatomic metallic bonds at crack tips. Under normal operating internal pressure, the aluminum experiences instantaneous brittle fracture along grain boundaries without warning or prior plastic deformation.
4. The Moomba Incident (2004): Trace mercury caused massive brittle rupture of a cold box at the Moomba gas plant in Australia, releasing high-pressure hydrocarbon gas that ignited into a massive vapor cloud explosion, shutting down gas supply to major metropolitan centers for months.
5. Design Mandate: All major LNG licensors (Air Products AP-C3MR, ConocoPhillips Optimized Cascade, Shell DMR) mandate an MRU outlet mercury limit of $< 0.01\ \mu\text{g/Sm}^3$ ($10\ \text{ng/Sm}^3$).

3. Chemisorption Sorbent Chemistry & Operating Placement

3.1 Chemisorption Reaction Kinetics

Unlike physical adsorption, mercury removal in fixed beds relies on irreversible chemical binding: * Sulfur-Impregnated Carbon: Activated carbon is impregnated with $10\text{--}15\ \text{wt}\%$ finely dispersed elemental sulfur or polysulfides: $$Hg_{(g)} + S_{(s)} \rightarrow HgS_{(s)} \quad (\text{Cinnabar}, \Delta H_r = -106\ \text{kJ/mol})$$ Mercury sulfide ($HgS$) has an extremely low vapor pressure ($< 10^{-10}\ \text{bar}$ at $25^\circ\text{C}$), ensuring that captured mercury will never desorb back into the treated gas stream even during process upsets. * Copper Sulfide on Alumina ($CuS / \text{Al}_2\text{O}_3$): Used when the gas is wet or located upstream of the amine plant. The reaction replaces copper with mercury: $$Hg_{(g)} + 2 CuS_{(s)} \rightarrow HgS_{(s)} + Cu_2S_{(s)}$$

3.2 Process Location: Upstream vs. Downstream

• Downstream of Molecular Sieve Dehydration (Standard): Placing the MRU immediately after dry gas dust filters ensures the gas is bone-dry (dew point $< -100^\circ\text{C}$) and warm ($20\text{--}35^\circ\text{C}$). This enables using low-cost sulfur-impregnated activated carbon without risk of capillary condensation of water or hydrocarbon liquids into the micropores.
• Upstream of Acid Gas Removal (Sour Gas Treating): If inlet mercury is extreme ($> 200\ \mu\text{g/Sm}^3$), mercury will accumulate in the amine solvent, volatilize into the acid gas offgas, poison the Claus sulfur plant catalyst, and contaminate produced water. In this scenario, an upstream water-tolerant $CuS$ bed is installed at the wellsite or plant inlet.

4. Fixed-Bed Mass Transfer Zone (MTZ) & Reactor Hydraulics

4.1 Fixed-Bed Adsorption Dynamics

Gas passes through the fixed bed in downward flow to prevent bed fluidization. Sorbent utilization follows a traveling Mass Transfer Zone (MTZ): * Equilibrium Saturated Zone ($L_e$): Sorbent at the bed inlet has reached its ultimate thermodynamic dynamic capacity ($L_{\text{cap}} \approx 12\text{--}15\ \text{wt}\% \text{Hg}$). * Mass Transfer Zone ($L_{\text{MTZ}}$): The active reaction zone where gas-phase mercury concentration drops from $C_{\text{in}}$ to $C_{\text{out}}$. The length of the MTZ is governed by internal pore diffusion and chemisorption reaction rates: $$L_{\text{MTZ}} = v_s \cdot \tau_{\text{reaction}} \cdot \left( \frac{d_p^2}{D_{\text{pore}}} \right)$$ * Unused Sorbent Zone ($L_u$): Fresh sorbent awaiting MTZ migration. Breakthrough occurs when the leading edge of the MTZ reaches the vessel outlet.

4.2 Sizing Equations & Superficial Velocity Limits

1. Superficial Gas Velocity ($v_s$): $$v_s = \frac{Q_{\text{actual}}}{A_{\text{bed}}} = \frac{4 Q_{\text{actual}}}{\pi D_{\text{bed}}^2}$$
2. Recommended design range: $v_s = 0.15\text{ to }0.30\ \text{m/s}$ ($30\text{--}60\ \text{ft/min}$).
3. Too low ($< 0.10\ \text{m/s}$): Causes flow maldistribution and gas channeling.
4. Too high ($> 0.35\ \text{m/s}$): Flattens MTZ, causes excessive Ergun pressure drop, and risks carbon particle attrition/dusting.
5. Minimum Gas Residence Time: $$\tau = \frac{H_{\text{bed}}}{v_s} \ge 10\text{ to }20\ \text{seconds}$$
6. Ergun Pressure Drop Equation: $$\frac{\Delta P}{H} = 150 \frac{(1-\epsilon)^2}{\epsilon^3} \frac{\mu v_s}{d_p^2} + 1.75 \frac{1-\epsilon}{\epsilon^3} \frac{\rho_g v_s^2}{d_p}$$ For a typical $3.5\ \text{m}$ bed with $3.5\ \text{mm}$ carbon pellets, design pressure drop is clean and low ($15\text{--}35\ \text{kPa}\ /\ 2\text{--}5\ \text{psi}$).

5. CAPEX vs. OPEX Bed Configuration: Single vs. Lead-Lag

• Single Vessel Configuration (Low Mercury $< 40\ \mu\text{g/Sm}^3$): Sized for a 3 to 5 year bed life aligned with major plant turnaround cycles (TAR). Sorbent changeout requires plant shutdown.
• Lead-Lag Dual Bed Architecture (High Mercury $> 50\ \mu\text{g/Sm}^3$):
• Two vessels operate in series. The "Lead" bed takes full inlet loading and is allowed to break through to $100\%$ saturation. The "Lag" bed acts as the safety polisher.
• Piping manifolds allow isolating the Lead bed, vacuuming out spent sorbent, loading fresh media, and valving the freshly loaded bed in as the Lag vessel without interrupting LNG production.

6. Spent Sorbent Handling, Decommissioning & Safety

Spent MRU media contains up to $150\ \text{kg}$ of toxic mercury per ton of sorbent: 1. Pyrophoric Iron Sulfide Risk: Sorbents that have encountered trace $H_2S$ contain iron sulfides ($FeS$). When exposed to air during vessel unloading, $FeS$ oxidizes exothermically and can ignite the activated carbon bed. Inert nitrogen purging and water misting are mandatory during unloading. 2. Hazardous Waste Regulations (RCRA / Basel Convention): Spent mercury sorbent cannot be disposed of in standard industrial landfills. It must be manifested under hazardous waste rules and shipped to licensed pyrometallurgical thermal retort facilities (e.g., in Germany or the US) where mercury is vacuum-distilled and reclaimed as triple-distilled virgin elemental mercury.

7. Standards References & Recommended Practice

• ISO 6978-1 / 6978-2: Natural Gas — Determination of Mercury — Part 1: Sampling by Chemisorption on Iodine; Part 2: Sampling by Amalgamation on Gold/Platinum Alloy.
• ASTM D5954 / ASTM D6350: Standard Test Method for Mercury in Natural Gas by Cold Vapor Atomic Absorption / Fluorescence.
• API 521: Pressure-relieving and Depressuring Systems.
• EEMUA Publication 179: A Working Guide for the Prevention of Liquid Metal Embrittlement in Cryogenic Heat Exchangers.

ChemProCal Calculation Engine Cross-Reference

To calculate exact MRU vessel diameter, bed height, mass transfer zone margin, sorbent inventory mass, and Ergun pressure drop, use our interactive Cryogenic Mercury Removal Unit (MRU) Fixed-Bed Adsorber Sizer.