1. Overview & Applications
Molecular sieves are synthetic crystalline aluminosilicates (zeolites) with uniform pore structures that selectively adsorb molecules based on size and polarity. For natural gas dehydration, molecular sieves achieve outlet dew points of -100°F to -150°F—far beyond glycol dehydration capabilities.
LNG Feedstock
<0.1 ppmv H₂O
Required to prevent ice formation in cryogenic exchangers at -260°F.
NGL Recovery
Prevent hydrate formation in turboexpander and demethanizer.
Pipeline Spec
7 lb H₂O/MMscf
Typical spec; mol sieves easily achieve <1 lb/MMscf.
Glycol vs Mol Sieve
TEG limited to ~7 lb/MMscf; mol sieves required for deeper drying.
Why molecular sieves for LNG: Glycol dehydration achieves ~7 lb H₂O/MMscf (dew point -20 to -40°F), but LNG requires <0.1 ppmv H₂O (dew point -100°F or lower) to prevent ice formation in cryogenic heat exchangers. Only molecular sieves can reach this specification economically.
2. Zeolite Type Selection
Zeolite type selection depends on feed gas composition and what contaminants (beyond water) need to be removed. The pore size determines which molecules can enter and be adsorbed.
| Type | Pore (Å) | Adsorbs | Excludes | Primary Application |
|---|---|---|---|---|
| 3A | 3 | H₂O, NH₃ | CO₂, H₂S, C₂+ | Water-only removal; rich gas with high C₃+ |
| 4A | 4 | H₂O, CO₂, H₂S | C₃+, aromatics | Most common for NG; combined H₂O + acid gas |
| 5A | 5 | H₂O, CO₂, H₂S, mercaptans | iso-paraffins, C₅+ | Sour gas with mercaptans; high water loading |
| 13X | 10 | H₂O, CO₂, H₂S, all HC | Large molecules | Bulk acid gas removal; less selective |
Selection Guidelines
- Type 3A: Use when inlet gas has high C₃+ content and you want water removal ONLY. Prevents hydrocarbon co-adsorption that would contaminate regeneration gas and reduce bed life.
- Type 4A: Standard choice for most natural gas applications. Removes water + CO₂ + H₂S simultaneously. Most cost-effective for sweet to moderately sour gas.
- Type 5A: Required when mercaptans (RSH) must be removed along with water. Higher capacity than 4A but co-adsorbs more hydrocarbons.
- Type 13X: Use for bulk CO₂ removal upstream of a 4A polishing bed. Not recommended as primary dehydration sieve due to low selectivity.
Physical Properties
| Property | 3A | 4A | 5A | 13X |
|---|---|---|---|---|
| Bulk density (lb/ft³) | 43 | 42 | 41 | 40 |
| Water capacity @ 77°F (wt%) | 20 | 22 | 21.5 | 28 |
| Bead size (mesh) | 8×12 | 8×12 | 8×12 | 8×12 |
| Max regen temp (°F) | 600 | 600 | 600 | 600 |
| Heat capacity (Btu/lb·°F) | 0.22 | 0.22 | 0.22 | 0.23 |
3. Adsorption Fundamentals
Molecular sieve adsorption is driven by strong electrostatic interaction between polar water molecules and the ionic framework of the zeolite. The adsorption is highly exothermic, releasing ~1800 Btu/lb water adsorbed.
Dynamic Capacity
Design capacity is less than equilibrium capacity due to temperature effects, relative saturation, and safety factors. GPSA Figure 20-37 provides capacity correction curves.
Dynamic Capacity Calculation:
Capacity_design = Capacity_base × F_T × F_sat × F_util
Where:
• Capacity_base = 20-22 wt% for Type 4A at 77°F, saturated
• F_T = Temperature correction factor
- F_T = 1.0 - 0.004 × (T - 77) for T > 77°F
- At 100°F: F_T = 0.91
- At 150°F: F_T = 0.71
• F_sat = Relative saturation correction
- At 100% RS: F_sat = 1.0
- At 50% RS: F_sat ≈ 0.85
- At 10% RS: F_sat ≈ 0.60
• F_util = Design utilization factor = 0.60-0.80
- Accounts for non-uniform flow and aging
- Conservative design: use 0.70
Example: Type 4A at 100°F, saturated gas, 70% utilization
Capacity_design = 22% × 0.91 × 1.0 × 0.70 = 14.0 wt%
Mass Transfer Zone (MTZ)
The MTZ is the region of the bed where water concentration transitions from saturated (inlet) to fresh (outlet) sieve. Bed height must exceed 2-3× MTZ length to prevent premature breakthrough.
MTZ Length Estimation:
L_MTZ = (v_s × t_MTZ) / 60
Where:
• L_MTZ = MTZ length (ft)
• v_s = Superficial velocity (ft/min)
• t_MTZ = MTZ formation time (minutes)
- Typically 30-90 minutes for gas dehydration
- Increases with velocity and water content
Design requirement:
• Bed height H ≥ 3 × L_MTZ for reliable operation
• H ≥ 2 × L_MTZ is marginal (close monitoring needed)
Breakthrough time:
t_breakthrough = Cycle_time × (H/L_MTZ - 1) / (H/L_MTZ)
At H = 3×L_MTZ: Safe time = 67% of cycle time
Co-Adsorption Effects
Type 4A and 5A sieves co-adsorb other components, which reduces water capacity and complicates regeneration.
| Component | 3A | 4A | 5A | Impact |
|---|---|---|---|---|
| CO₂ | No | Yes (~10%) | Yes (~12%) | Reduces H₂O capacity 2-5% |
| H₂S | No | Yes (~6%) | Yes (~8%) | Difficult to regenerate; fouls bed |
| Mercaptans | No | Partial | Yes | Irreversible adsorption; poisons bed |
| Heavy HC (C₆+) | No | No | Partial | Deposits in pores; use 3A if high C₆+ |
Sour gas considerations: For gas with >100 ppmv H₂S, either use Type 3A (excludes H₂S) or install upstream amine treating. H₂S adsorbs strongly on Type 4A and is difficult to fully desorb, leading to gradual capacity loss.
4. Bed Sizing Method
Bed sizing involves calculating required sieve mass from water loading, then determining vessel dimensions from velocity constraints and volume requirements.
Step 1: Water Loading per Cycle
Water Loading Calculation:
W_cycle = Q × (C_in - C_out) × t_ads × F_upset / 24
Where:
• W_cycle = Water to be adsorbed per cycle (lb)
• Q = Gas flow rate (MMscfd)
• C_in = Inlet water content (lb/MMscf)
• C_out = Target outlet water (lb/MMscf)
• t_ads = Adsorption cycle time (hours)
• F_upset = Upset factor (1.15-1.25 for flow variations)
Example:
Q = 100 MMscfd, C_in = 60 lb/MMscf, C_out = 1 lb/MMscf
t_ads = 16 hours, F_upset = 1.15
W_cycle = 100 × (60-1) × 16 × 1.15 / 24
W_cycle = 4,540 lb water/cycle
Step 2: Required Sieve Mass
Sieve Mass Calculation:
M_sieve = W_cycle / Capacity_design
Using example above with Capacity_design = 14% = 0.14:
M_sieve = 4,540 / 0.14 = 32,430 lb per bed
Bed volume:
V_bed = M_sieve / ρ_bulk = 32,430 / 42 = 772 ft³
Step 3: Vessel Diameter from Velocity
Diameter Calculation:
Convert flow to ACFM at operating conditions:
Q_acfm = Q_scfm × (P_std/P_op) × (T_op/T_std) × Z
Cross-sectional area from velocity limit:
A = Q_acfm / v_s
Where v_s = 40-80 ft/min per GPSA Diameter:
D = √(4A / π)
Example: 100 MMscfd at 800 psig, 100°F
Q_scfm = 100×10⁶/(24×60) = 69,444 scfm
Q_acfm = 69,444 × (14.7/814.7) × (560/520) × 0.85
Q_acfm = 1,170 acfm
At v_s = 60 ft/min:
A = 1,170/60 = 19.5 ft²
D = √(4×19.5/3.14) = 5.0 ft
Step 4: Bed Height and H/D Ratio
Height Calculation:
H = V_bed / A = 772 / 19.5 = 39.6 ft
H/D ratio = 39.6 / 5.0 = 7.9:1 ← TOO HIGH!
Target H/D = 2:1 to 4:1 for good flow distribution
Solution: Increase diameter or use parallel beds
If D = 8 ft: A = 50.3 ft², H = 772/50.3 = 15.4 ft
H/D = 15.4/8 = 1.9:1 ← Acceptable
Verify MTZ: If L_MTZ = 3 ft, then H/L_MTZ = 5.1× ✓
Pressure Drop
Pressure drop through packed beds is calculated using the Ergun equation. Design ΔP typically ranges from 5-15 psi.
Ergun Equation:
ΔP/L = [150μ(1-ε)²v / (ε³dp²)] + [1.75ρ(1-ε)v² / (ε³dp)]
↑ Viscous term ↑ Inertial term
Where:
• ΔP/L = Pressure gradient (psi/ft)
• μ = Gas viscosity (~0.0001 lb/ft·s)
• ε = Bed voidage (0.37 for beads)
• v = Superficial velocity (ft/s)
• dp = Particle diameter (0.0066 ft for 8×12 mesh)
• ρ = Gas density (lb/ft³)
Typical result: 0.3-1.0 psi/ft for natural gas
For H = 15 ft bed: ΔP = 5-15 psi
5. TSA Regeneration Cycle
Thermal Swing Adsorption (TSA) regenerates molecular sieves by heating to 450-550°F with dry regeneration gas, desorbing water vapor. The complete cycle includes depressurization, heating, cooling, and repressurization.
Cycle Steps
| Step | Duration | Conditions | Purpose |
|---|---|---|---|
| 1. Adsorption | 8-24 hr | Process P&T; gas flows down | Remove water from feed gas |
| 2. Depressurization | 0.5 hr | Vent to flare/recovery | Reduce P for regen; recover inventory |
| 3. Heating | 3-5 hr | 450-550°F regen gas; flow up | Desorb water from sieve |
| 4. Cooling | 2-4 hr | Ambient dry gas; flow up | Cool bed to <150°F for adsorption |
| 5. Repressurization | 0.5 hr | Dry product gas | Equalize with operating bed |
Regeneration Gas Requirements
Heat Duty Calculation:
Q_total = (Q_sieve + Q_water) × F_loss
Q_sieve = M_sieve × Cp × ΔT
= 32,430 lb × 0.22 Btu/lb·°F × 400°F
= 2.85 MMBtu
Q_water = W_cycle × H_des
= 4,540 lb × 1800 Btu/lb
= 8.17 MMBtu
F_loss = 1.20 (20% heat losses)
Q_total = (2.85 + 8.17) × 1.20 = 13.2 MMBtu/cycle
Regen gas flow (at 400°F rise, 4 hr heating):
m_regen = Q_total / (Cp × ΔT × t)
= 13.2×10⁶ / (0.5 × 400 × 4)
= 16,500 lb/hr = 275 lb/min
Volume flow ≈ 5,500 scfm (10-15% of feed)
Temperature Limits
Critical: Do NOT exceed 600°F (315°C) bed temperature. Higher temperatures cause sintering of the zeolite structure, resulting in permanent capacity loss and bead degradation. Control heater outlet and monitor multiple bed thermocouples.
Multi-Bed Configurations
| Configuration | Beds | Mode | Best For |
|---|---|---|---|
| Two-bed | 2 | 1 ads, 1 regen | Simple, low cost; long cycles (16-24 hr) |
| Three-bed | 3 | 2 ads, 1 regen | Smaller beds; short cycles (8-12 hr) |
| Four-bed | 4 | 2-3 ads, 1 regen | Large plants; redundancy; maintenance flexibility |
6. Operations & Troubleshooting
Monitoring Parameters
- Outlet dew point: Primary indicator of bed performance; alarm if rises above target
- Bed temperatures: Install 6-12 thermocouples at different heights; monitor during regen
- Pressure drop: Track over time; increase indicates fouling or fines migration
- Regeneration gas temperature: Ensure reaching 450-550°F throughout bed
Common Problems
| Problem | Symptoms | Likely Cause | Solution |
|---|---|---|---|
| Premature breakthrough | Water in product before cycle end | Incomplete regen; channeling; fouling | Increase regen temp/time; inspect bed |
| High ΔP | ΔP > 20 psi; compressor issues | Fines migration; liquid carryover | Install upstream filters; knock out liquids |
| Reduced capacity | Shorter cycle times over months | Bed aging; irreversible adsorption | High-temp regen; plan bed replacement |
| Bed dusting | Fines in downstream equipment | Thermal stress; mechanical damage | Limit regen rate; install outlet screen |
Bed Life Expectations
Expected service life depends on operating severity:
- Clean, sweet gas: 5-7 years
- Moderately sour gas: 4-5 years
- Heavy HC or mercaptans: 3-4 years
- High cycling or upsets: 2-4 years
Replace when capacity drops below 80% of design or when mechanical issues (dusting, high ΔP) develop.
Operational tip: Avoid liquid carryover to molecular sieve beds at all costs. Free liquids cause massive local heating during adsorption (due to concentrated heat of adsorption), leading to thermal shock, bead fracture, and rapid bed degradation. Install effective inlet separators and knockout drums.
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