Rich-Oil Demethanizer Fundamentals

1. Purpose in the Lean Oil Absorption Process

The rich-oil demethanizer is a stripping column positioned between the absorber and the oil purification still in a lean oil absorption plant. Its primary function is to remove dissolved methane (C₁) and ethane (C₂) from the rich oil before the oil enters the still column for full NGL recovery. Without this intermediate stripping step, excessive light gases would carry through to the still overhead, contaminating the NGL product and increasing condenser and compression loads on the still.

In a typical lean oil absorption plant, raw inlet gas contacts a heavy absorption oil (typically C₁₀–C₁₄ kerosene-range or heavier) in the absorber at high pressure (600–1,200 psig). The rich oil leaving the absorber bottom contains the target NGL components (C₃+) along with significant quantities of dissolved C₁ and C₂. The demethanizer strips these light components before the rich oil proceeds to the still for C₃+ recovery.

Lean Oil Plant Flow Sequence

Why Demethanizing Is Necessary

The rich oil leaving the absorber typically contains 10–25 mol% dissolved gas, of which 60–80% is methane and ethane. If this gas were carried into the still column without prior removal, several problems would arise:

• Still overhead overload: Excessive C₁/C₂ in the still feed increases the vapor load, requiring a larger column diameter, larger condenser, and higher compression horsepower for overhead gas handling
• NGL product contamination: Methane and ethane dilute the C₃+ product, making it more difficult to meet Reid Vapor Pressure (RVP) specifications for natural gasoline or LPG
• Energy waste: Heating methane and ethane to still reboiler temperatures consumes energy that could be avoided by stripping these components at lower temperatures upstream
• Pressure control: Light gas accumulation in the still system creates pressure control difficulties and can cause relief valve lifting

The demethanizer overhead gas is typically routed to fuel gas, recompressed for sales gas injection, or recycled to the absorber inlet. The degree of C₂ removal is a design variable that depends on whether ethane has commercial value at the plant location. In many lean oil plants, both C₁ and C₂ are rejected overhead as fuel gas since the primary recovery targets are propane and heavier components.

2. Column Design

The rich-oil demethanizer is a stripping column that uses heat input (reboiler) and sometimes steam to drive dissolved light gases out of the rich oil. Unlike conventional distillation columns in the NGL fractionation train, the demethanizer handles a fundamentally different separation: removing light dissolved gases from a heavy oil matrix rather than separating two adjacent hydrocarbon species.

Stage Requirements

The demethanizer typically requires 6–14 theoretical stages, with 8–10 being the most common design range. The number of stages depends on the desired degree of C₁/C₂ removal and the operating pressure. At lower pressures, the relative volatility between dissolved gases and the oil increases, reducing the stage requirement. Conversely, higher operating pressures (which may be desirable for overhead gas handling) reduce relative volatility and require more stages.

Actual tray count typically ranges from 10–24 trays, assuming 55–70% tray efficiency. The lower efficiency compared to conventional hydrocarbon fractionation columns (70–85%) reflects the challenges of contacting a viscous heavy oil with gas on trays.

Pressure Selection

Operating pressure for the rich-oil demethanizer is typically selected in the range of 150–300 psig. The pressure is influenced by several competing factors:

Tray vs. Packing

Tray columns are the traditional choice for rich-oil demethanizer service. The primary consideration is the nature of the liquid phase: lean absorption oils are heavy, viscous hydrocarbons (viscosity 1–5 cP at operating temperatures) that present challenges for both trays and packing.

• Sieve trays: Most common for new designs; provide good vapor-liquid contact, are relatively insensitive to fouling, and handle the high liquid loading characteristic of oil service
• Valve trays: Preferred when turndown flexibility is needed; the variable orifice area maintains tray efficiency across a wider range of vapor rates than fixed-orifice sieve trays
• Bubble cap trays: Used historically and still found in older installations; provide excellent low-vapor-rate performance and seal against liquid weeping, but are more expensive and harder to maintain
• Structured packing: Generally avoided for heavy oil service due to the risk of channeling, poor wetting at low liquid rates, and difficulty cleaning fouled packing beds

Liquid Loading Considerations

The rich-oil demethanizer operates at very high liquid-to-vapor ratios compared to conventional fractionation columns. The liquid rate is dominated by the heavy oil flow, while the vapor rate is relatively modest (only the dissolved gases being stripped). This high L/V ratio requires careful tray hydraulic design:

• Larger downcomers (15–20% of tray area) to handle the high liquid loading without backup flooding
• Adequate weir height (2–3 inches) to maintain liquid seal on the tray while avoiding excessive pressure drop
• Consideration of foaming tendency, which is aggravated by the surface-active nature of some absorption oils and by trace contaminants (corrosion inhibitors, amine carryover)

Typical Design Parameters

3. Reboiler Design

The reboiler is the primary energy input to the demethanizer, providing the heat necessary to vaporize dissolved C₁ and C₂ from the rich oil. Reboiler design for this service requires careful attention to oil thermal stability, because excessive temperatures cause the absorption oil to degrade, forming heavy ends that increase viscosity, reduce absorption capacity, and foul heat transfer surfaces.

Temperature Limits for Oil Stability

The maximum allowable reboiler tube-wall temperature is the most critical design constraint. Absorption oils begin to thermally degrade (crack) at temperatures above their decomposition point, which depends on the oil type:

The film temperature is the temperature at the heat transfer surface (tube wall), which is always higher than the bulk liquid temperature. A general design rule is to limit film temperature to no more than 450°F for kerosene-range oils. Exceeding this limit leads to coking on tube surfaces, reduced heat transfer coefficients, and accelerated oil degradation.

Heating Medium Selection

The choice of heating medium is driven by the temperature requirements (300–400°F bottoms) and the need to limit film temperatures:

Hot oil heating is the most common choice for rich-oil demethanizer service because it provides fine temperature control and avoids the extreme film temperatures associated with direct firing. The hot oil supply temperature can be precisely set to maintain tube-wall temperatures below the oil degradation threshold.

Reboiler Type: Thermosiphon vs. Kettle

Both thermosiphon and kettle reboilers are used in demethanizer service. The selection depends on the specific operating requirements:

• Thermosiphon reboiler: Preferred when minimizing oil residence time at elevated temperatures is important. Natural circulation driven by the density difference between the reboiler inlet (liquid) and outlet (two-phase) provides continuous flow, reducing the risk of localized overheating. Requires adequate static head from the column bottoms sump.
• Kettle reboiler: Provides a large liquid inventory and vapor disengagement space within the shell. Simpler piping and no dependence on static head, but the larger holdup volume means longer residence time at reboiler temperature, which can accelerate oil degradation. Most common for smaller installations.

For either type, the reboiler duty is determined by the mass of C₁/C₂ to be stripped and the sensible heat required to raise the rich oil from the feed temperature to the bottoms temperature:

Where m_oil is the oil mass flow rate, C_p is the oil heat capacity (typically 0.45–0.55 BTU/lb-°F), ΔT is the temperature rise across the column, m_gas is the mass rate of stripped gas, and λ_strip is the effective heat of desorption.

4. Overhead System

The demethanizer overhead vapor is predominantly methane and ethane with minor amounts of C₃+ and trace oil carryover. The overhead system must handle this gas efficiently while minimizing NGL losses and recovering any entrained oil.

Partial Condenser and Reflux

Many rich-oil demethanizers operate with a partial condenser to provide liquid reflux, which improves C₃+ recovery by washing heavier components back down the column. The partial condenser cools the overhead vapor sufficiently to condense a portion of the C₂+ content while allowing C₁ (and most C₂) to exit as vapor.

The reflux drum (accumulator) serves as a three-phase separator when water is present in the system. The liquid hydrocarbon phase is returned to the column top tray as reflux, the water phase is drained, and the vapor exits for downstream handling. In some designs, particularly where C₂ rejection is complete, the column operates without reflux (simple stripping column), and the overhead is sent directly to fuel gas or compression.

Overhead Gas Disposition

Gas Compression Requirements

When the overhead gas is reinjected into the sales gas pipeline or recycled to the absorber, compression is required from the demethanizer operating pressure (150–300 psig) to the pipeline or absorber pressure (600–1,200 psig). The compression ratio is typically 2:1 to 4:1, achievable with a single-stage reciprocating compressor or a small screw compressor. Compressor horsepower is estimated from:

Where Q is the gas flow rate (ACFM at suction), P₁ and P₂ are suction and discharge pressures (psia), k is the ratio of specific heats (approximately 1.25 for C₁/C₂ mixture), and E is the compressor efficiency (typically 0.80–0.85 for reciprocating).

Oil Mist Elimination

Absorption oil mist carryover in the overhead vapor is a persistent concern. Even small amounts of oil loss (0.1–0.5 gal/MMSCF) accumulate over time and represent both a direct economic loss and a fouling risk for downstream equipment. Common mitigation approaches include:

• Mesh pad demister: Installed in the column top or reflux drum; effective for droplets larger than 10 microns
• Vane-type separator: Installed in the overhead piping; handles higher liquid loads than mesh pads
• Reduced column vapor velocity: Sizing the top section for 60–70% of flood ensures adequate disengagement time
• Anti-foam injection: Silicone-based anti-foams (5–10 ppm) reduce entrainment caused by foaming at the top trays

5. Troubleshooting

Rich-oil demethanizer problems typically manifest as poor C₁/C₂ removal, excessive oil losses, or column instability. The viscous nature of the absorption oil and the dissolved gas content create unique operational challenges not encountered in conventional NGL fractionation columns.

Oil Foaming

Foaming is the most common operational problem in rich-oil demethanizers. The rich oil contains dissolved gases, trace contaminants (corrosion inhibitors, amine, glycol, iron sulfide particles), and surface-active degradation products that promote stable foam formation on trays.

Poor C₁ Removal

Inadequate methane stripping results in excessive C₁ in the stripped rich oil, which carries through to the still column and causes overhead overloading. Common causes include:

• Insufficient reboiler duty: Verify reboiler heat input against design, check for fouled heat transfer surfaces, confirm heating medium temperature and flow
• Tray damage or bypassing: Inspect column internals for tray corrosion, bolt loosening, or downcomer damage that allows vapor or liquid to bypass active tray area
• Operating pressure too high: Elevated column pressure reduces the driving force for gas desorption; consider reducing pressure if overhead gas handling permits
• Oil degradation: Thermally degraded oil has increased viscosity, which reduces tray efficiency and mass transfer rates; monitor oil quality indicators

High Oil Losses

Oil losses from the demethanizer occur primarily through overhead entrainment and are accelerated by foaming, high vapor velocities, and degraded oil quality. Chronic oil losses increase lean oil makeup costs and can foul downstream equipment.

• Check demister pad condition (mesh pads have a service life of 3–5 years and may collapse or become plugged)
• Verify column is not operating above 75–80% of flood velocity at the top section
• Monitor oil quality: degraded oil with high surface tension tends to form finer mist droplets that pass through demisters
• Install or upgrade to vane-type mist eliminator if mesh pad performance is inadequate

Tray Flooding with Viscous Oil

Heavy absorption oils operating near their lower temperature limit can become sufficiently viscous to impede liquid drainage through downcomers, causing backup flooding even at design liquid rates.

• Increase feed temperature: Preheat the rich oil feed using lean/rich oil heat exchange to reduce viscosity before entering the column
• Reduce liquid loading: If the plant is operating above design oil circulation rate, reduce the lean oil rate to the absorber
• Downcomer modifications: In severe cases, column internals can be retrofitted with larger downcomers or modified tray geometry to accommodate higher-viscosity liquids
• Oil change-out: If the oil has degraded significantly (viscosity increase > 30% above fresh oil), schedule a partial or complete oil replacement

Diagnostic Monitoring Parameters

References

1. GPSA, Chapter 16 — Hydrocarbon Recovery
2. GPA Standard 2140 — Liquefied Petroleum Gas Specifications
3. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
4. Campbell, J.M. — Gas Conditioning and Processing, Campbell Petroleum Series