Worked Example: Baseload LNG Feed Gas Pre-Treatment Screening, Cold Box Freezeout & BAHX Mercury LME Hazard Evaluation

1. Problem Statement & Industrial Context

A 5.0 MTPA (Million Metric Tonnes per Annum) baseload LNG liquefaction facility is being designed to process $350.0\text{ MMSCFD}$ ($413,000\text{ Sm}^3/\text{h}$) of raw associated natural gas at $62.0\text{ barg}$ ($63.01\text{ bar abs}$) and $28.0\text{ }^\circ\text{C}$.

The liquefaction train utilizes a propane pre-cooled mixed refrigerant (C3MR) cycle. In the Main Cryogenic Heat Exchanger (MCHE), gas is chilled down to $-161.0\text{ }^\circ\text{C}$ inside Brazed Aluminum Heat Exchangers (BAHX) with plate-fin headers fabricated from aluminum-manganese alloy AA 3003.

 [Raw Feed: 350 MMSCFD, 62 barg] ──> [Pre-Treatment Train] ──> [BAHX Cryogenic Cold Box: -161 °C]
  CO2 = 1.8 mol% (18,000 ppmv)        AGRU (aMDEA)              Must satisfy strict cold box limits:
  H2O = 65 ppmv                       Mol Sieve 3A              - CO2 < 50 ppmv (Dry ice freezeout)
  Hg  = 12.0 ug/Nm3                   Carbon Bed MRU            - H2O < 0.1 ppmv (Hydrate/ice plugging)
  Benzene = 280 ppmv                  Scrub Column              - Hg < 0.01 ug/Nm3 (BAHX LME rupture)
  C6+ = 0.42 mol%                     NRU                       - Benzene < 1 ppmv (Crystallization)

Feed Gas Contaminant Assay vs. Baseload LNG Cold Box Specifications

Engineering Objectives

1. Evaluate solid freeze-out precipitation temperatures for $\text{CO}_2$ dry ice and benzene.
2. Quantify the metallurgical hazard of Mercury Liquid Metal Embrittlement (LME) on Brazed Aluminum Heat Exchangers.
3. Assess the LNG storage tank nitrogen rollover risk.
4. Define the complete sequential architecture of the upstream midstream pre-treatment battery required before cryogenic chilling.

2. Cryogenic Thermodynamic Modeling & Freezeout Formulations

2.1 Carbon Dioxide ($\text{CO}_2$) Solidification Boundary in Cryogenic Methane

Solid $\text{CO}_2$ precipitates directly from liquid natural gas when the chemical potential of dissolved $\text{CO}_2$ equals that of pure solid dry ice. Based on Kurata & Kohn solubility data:

$$T_{\text{freeze}} \approx \frac{-3200}{\ln(y_{\text{CO2}}) - 15.2} - 273.15 \qquad (^\circ\text{C})$$

• At the feed concentration of $18,000\text{ ppmv}$ ($y = 0.018$): $$\ln(0.018) = -4.017 \implies T_{\text{freeze}} = \frac{-3200}{-4.017 - 15.2} - 273.15 = \frac{-3200}{-19.217} - 273.15 = 166.5\text{ K} = \mathbf{-106.6\text{ }^\circ\text{C}}$$
• Because the MCHE operates down to $-161.0\text{ }^\circ\text{C}$, feed gas containing $18,000\text{ ppmv } \text{CO}_2$ would freeze solid at $-106.6\text{ }^\circ\text{C}$, completely plugging all plate-fin passages over $54\text{ }^\circ\text{C}$ before reaching the bottom of the column!
• To prevent freeze-out down to $-161.0\text{ }^\circ\text{C}$, $\text{CO}_2$ must be depleted below $50\text{ ppmv}$: $$y = 5.0 \times 10^{-5} \implies \ln(y) = -9.903 \implies T_{\text{freeze}} = \frac{-3200}{-9.903 - 15.2} - 273.15 = 127.5\text{ K} = \mathbf{-145.7\text{ }^\circ\text{C}}$$ At $50\text{ ppmv}$, liquid methane solubility provides safe operating margin against dry ice deposition.

2.2 Benzene ($\text{C}_6\text{H}_6$) Solid Crystallization

Pure benzene freezes at $+5.5\text{ }^\circ\text{C}$. In light cryogenic hydrocarbons, benzene solubility drops drastically:

$$T_{\text{freeze}} \approx \frac{-2600}{\ln(y_{\text{benzene}}) - 12.8} - 273.15 \qquad (^\circ\text{C})$$

• At feed concentration of $280\text{ ppmv}$ ($y = 2.8 \times 10^{-4}$): $$\ln(y) = -8.181 \implies T_{\text{freeze}} = \frac{-2600}{-8.181 - 12.8} - 273.15 = \frac{-2600}{-20.981} - 273.15 = 123.9\text{ K} = \mathbf{-149.2\text{ }^\circ\text{C}}$$
• At the spec limit of $\le 1.0\text{ ppmv}$ ($y = 1.0 \times 10^{-6}$): $$\ln(y) = -13.816 \implies T_{\text{freeze}} = \frac{-2600}{-13.816 - 12.8} - 273.15 = 97.7\text{ K} = \mathbf{-175.5\text{ }^\circ\text{C}}$$ A benzene concentration of $\le 1.0\text{ ppmv}$ guarantees that benzene will remain completely in solution throughout the entire $-161\text{ }^\circ\text{C}$ liquefaction circuit!

3. Metallurgical & Safety Hazard Evaluations

3.1 Mercury Liquid Metal Embrittlement (LME)

• The Physics of Amalgamation: Elemental mercury ($\text{Hg}$) is liquid at ambient conditions. When trace mercury contacts bare aluminum alloy (AA 3003) inside Brazed Aluminum Heat Exchangers, it penetrates the grain boundaries via capillary wetting, amalgamating with the aluminum matrix.
• Catastrophic Rupture Mechanics: Once amalgamation initiates, mechanical tensile stress triggers rapid intergranular cracking. Because BAHX core walls are only $0.8\text{ mm}$ to $1.2\text{ mm}$ thick, complete wall perforation and catastrophic structural failure occurs within hours to days.
• Moisture Reaction: During plant turnarounds, when equipment warms up in the presence of ambient humidity, the mercury-aluminum amalgam undergoes rapid catalytic galvanic oxidation: $$4\text{Al(Hg)} + 3\text{O}_2 + 6\text{H}_2\text{O} \longrightarrow 2\text{Al}_2\text{O}_3 \cdot 3\text{H}_2\text{O} + 4\text{Hg}$$ Mercury is regenerated continuously, destroying entire cold box assemblies.
• Evaluation: At $12.0\ \mu\text{g/Nm}^3$, the feed exceeds the $0.01\ \mu\text{g/Nm}^3$ limit by $1,200\times$. This constitutes an immediate, catastrophic LME safety hazard requiring 100% fail-safe mercury removal before cold box commissioning.

3.2 Nitrogen Rollover & Tank Stratification

• The Rollover Mechanism: Nitrogen ($M_w = 28.01$) is heavier than methane ($M_w = 16.04$). When LNG containing $> 1.0\text{ mol\% } \text{N}_2$ is sent to storage tanks, density differences cause stratification into distinct layers.
• Nitrogen preferentially boils off from the top layer, increasing its density relative to the warming bottom layer. When densities equilibrate, massive convective overturning (tank rollover) occurs.
• Consequence: Spontaneous vapor generation increases by up to $50\times$ the normal boil-off rate (BOG), overwhelming tank pressure relief valves and creating an explosive vapor cloud or severe emergency flaring.
• Evaluation: At $1.75\text{ mol\% } \text{N}_2$, an upstream Nitrogen Rejection Unit (NRU) or end-flash degassing column is mandatory to ensure final LNG product contains $< 1.0\text{ mol\% } \text{N}_2$.

4. Upstream Pre-Treatment Battery Architecture

To bring the $350\text{ MMSCFD}$ feed stream into full baseload LNG compliance, five sequential pre-treatment unit operations are specified:

 [Raw Feed: 350 MMSCFD, 62 barg, 28 °C]
                  │
                  ▼
 ┌─────────────────────────────────────────────────────────┐
 │ 1. ACID GAS REMOVAL UNIT (AGRU)                         │
 │    - Solvent: 50 wt% aMDEA + Piperazine Promoter        │  ---> Removes CO2 (1.8% to < 50 ppmv)
 │    - Contactor: 24-tray packed column, 3,200 mm ID      │  ---> Removes H2S (14 to < 4 ppmv)
 └─────────────────────────────────────────────────────────┘
                  │
                  ▼
 ┌─────────────────────────────────────────────────────────┐
 │ 2. MERCURY REMOVAL UNIT (MRU)                           │
 │    - Non-regenerable sulfur-impregnated activated carbon│  ---> Chemisorbs Hg as solid HgS
 │    - Dual lead-lag guard beds (CuS / Sulfur on Carbon)  │  ---> Lowers Hg from 12.0 to < 0.01 ug/Nm3
 └─────────────────────────────────────────────────────────┘
                  │
                  ▼
 ┌─────────────────────────────────────────────────────────┐
 │ 3. CRYOGENIC MOLECULAR SIEVE DEHYDRATION                │
 │    - Desiccant: Type 3A Zeolite (excludes methane)      │  ---> Adsorbs H2O from 65 to < 0.1 ppmv
 │    - 3-bed regenerative TSA system (2 on stream, 1 reg) │  ---> Lowers water dew point to -100 °C
 └─────────────────────────────────────────────────────────┘
                  │
                  ▼
 ┌─────────────────────────────────────────────────────────┐
 │ 4. HEAVY HYDROCARBON SCRUB COLUMN / NGL EXTRACTION      │
 │    - Front-end de-ethanizer / wash column               │  ---> Strips Benzene from 280 to < 1.0 ppmv
 │    - Heavy aromatics and C6+ condensation               │  ---> Cuts C6+ to < 0.10 mol%, BTX < 100 ppmv
 └─────────────────────────────────────────────────────────┘
                  │
                  ▼
 ┌─────────────────────────────────────────────────────────┐
 │ 5. NITROGEN REJECTION / END-FLASH BOG COLUMN            │
 │    - Post-liquefaction atmospheric flash drum           │  ---> Rejects N2 from 1.75 to < 1.0 mol%
 │    - Off-gas routed to gas turbine fuel supply          │  ---> Prevents LNG storage tank rollover
 └─────────────────────────────────────────────────────────┘
                  │
                  ▼
 [Purified Feed Gas Ready for Cold Box Chilling to -161 °C]

5. ChemProCal Engine Verification & Cross-Check

from engineering_core.gas_thermo import screen_lng_feed_gas

res = screen_lng_feed_gas(
    feed_flow_mmscfd=350.0,
    operating_pressure_barg=62.0,
    operating_temp_c=28.0,
    h2o_ppmv=65.0,
    co2_ppmv=18000.0,
    h2s_ppmv=14.0,
    total_sulfur_mg_nm3=28.0,
    mercury_ug_nm3=12.0,
    benzene_ppmv=280.0,
    btx_ppmv=450.0,
    c6_plus_mol_pct=0.42,
    nitrogen_mol_pct=1.75,
    oxygen_ppmv=4.0
)

Verification Matrix

6. Key Takeaways for LNG Facilities & Process Engineers

1. Mercury Guard Bed Location: The Non-Regenerable Mercury Removal Unit (MRU) should always be placed immediately downstream of the Acid Gas Removal Unit (AGRU) and upstream of the Molecular Sieve Dehydration beds. Locating it upstream of the molecular sieve protects the desiccant from mercury accumulation and prevents mercury release during high-temperature thermal regeneration.
2. Type 3A vs 4A Molecular Sieve Selection: Type 3A zeolite ($3\text{ \AA}$ pore diameter) must be specified for LNG dehydration. Type 4A zeolite ($4\text{ \AA}$) co-adsorbs methane, ethane, and propane, leading to severe thermal cracking, heavy carbonaceous coking, and premature bed replacement during $280\text{ }^\circ\text{C}$ regeneration cycles.
3. Continuous Mercury Online Detection: Because even sub-part-per-billion mercury levels trigger catastrophic aluminum embrittlement, dual atomic fluorescence spectroscopy (AFS) online analyzers must sample both inlet and outlet streams of the MRU with an automatic trip interlock to divert feed gas before it contacts the Brazed Aluminum Heat Exchanger cold box.