Oil Purification Still Fundamentals

1. Purpose and Process Overview

The oil purification still (also called the stripping still or lean oil still) is the column responsible for removing absorbed NGL components (C₃+) from the rich absorption oil, regenerating it to lean oil quality for recirculation to the absorber. This is the final and most energy-intensive step in the lean oil absorption cycle, where the NGL product is liberated from the oil carrier and sent to fractionation or product storage.

In the standard lean oil plant flow sequence, rich oil from the absorber first passes through the rich-oil demethanizer to remove dissolved methane and ethane. The demethanizer bottoms—stripped of C₁/C₂ but still laden with C₃+ NGL—feed the oil purification still. Heat input to the still drives the absorbed NGL out of the oil as overhead vapor, while the regenerated lean oil exits the column bottoms and is cooled before returning to the absorber.

Position in the Lean Oil Plant

Key Design Objectives

The oil purification still must achieve several competing objectives simultaneously:

• Maximum NGL recovery: Strip essentially all C₃+ components from the oil to maximize product recovery and revenue. Typical lean oil retention targets are 1–3 mol% NGL remaining in the regenerated lean oil
• Oil quality preservation: Regenerate the oil without thermal degradation, coking, or excessive oxidation that would impair its absorption capacity and physical properties
• Energy efficiency: Minimize reboiler duty and steam consumption while maintaining required NGL removal. Heat integration with the lean/rich oil exchanger is essential
• Product quality: Produce an overhead NGL stream suitable for fractionation or direct sales, meeting vapor pressure and composition specifications

The NGL product from the still overhead typically routes to a fractionation train (depropanizer, debutanizer) for separation into individual products, or it may be sold as a mixed NGL stream (Y-grade) to a downstream fractionation facility. The composition of the overhead product depends on the absorption oil type, operating conditions, and the degree of stripping achieved.

2. Still Column Design

The oil purification still is a trayed stripping column operating at substantially lower pressure than the upstream demethanizer. The reduced pressure increases the relative volatility between the NGL components and the heavy absorption oil, improving stripping efficiency and reducing the number of theoretical stages required.

Stage Requirements

The still typically requires 4–12 theoretical stages for adequate NGL removal, with 6–8 stages being the most common design range. The relatively modest stage requirement reflects the large volatility difference between the light NGL components (C₃–C₅) and the heavy absorption oil (C₁₀–C₁₄ or heavier). Actual tray count ranges from 8–20 trays, depending on tray efficiency (typically 50–65% for heavy oil stripping service).

Tray efficiency in the still is generally lower than in the demethanizer because the bottoms section operates at higher temperatures with more viscous oil and greater foaming tendency. The combination of high liquid loading, elevated viscosity, and surface-active degradation products in aged oil can reduce tray efficiency to 45–55% in older installations.

Operating Pressure

The still operates at significantly lower pressure than the demethanizer, typically in the range of 30–75 psig. This low pressure is essential for effective NGL stripping:

Column Sizing

Column diameter is determined using the Souders-Brown correlation for maximum allowable vapor velocity. For oil purification still service, the Souders-Brown capacity factor (C_SB) is typically 0.20–0.25 ft/s, which is lower than conventional fractionation service (0.30–0.35 ft/s) to account for foaming tendency with heavy absorption oil.

Where V_max is the maximum allowable superficial vapor velocity (ft/s), C_SB is the Souders-Brown capacity factor (0.20–0.25 for still service), ρ_L is the liquid density, and ρ_V is the vapor density. Design vapor velocity is typically set at 70–80% of V_max to provide margin for foaming and operational upsets.

Foaming Considerations

The oil purification still is particularly susceptible to foaming because the oil has been exposed to process contaminants throughout the absorption cycle. Iron sulfide particulates, corrosion inhibitors, amine carryover from upstream treating, and thermal degradation products all contribute to stable foam formation. Design measures to mitigate foaming include:

• Reduced C_SB factor (0.20 vs. 0.30 for clean service) to lower vapor velocities
• Larger downcomers (18–22% of tray area) to allow foam drainage
• Anti-foam injection capability (silicone-based, 5–15 ppm) at the feed tray
• Mesh pad or vane-type demister in the column top section

Typical Design Parameters

3. Steam Stripping vs. Reboiler-Only Operation

The oil purification still can operate with reboiler heat input alone or with a combination of reboiler heat and direct steam injection. The choice between these two modes has significant implications for NGL recovery, oil quality, energy consumption, and overhead system complexity.

Reboiler-Only Operation

In reboiler-only operation, all heat input comes from the column reboiler (thermosiphon or kettle type). The reboiler vaporizes NGL from the oil at the column bottoms, and the resulting vapor rises through the trays, stripping additional NGL from the descending rich oil. This is the simpler configuration, requiring no steam supply, no water handling in the overhead system, and no three-phase reflux drum.

The limitation of reboiler-only operation is that achieving very low NGL retention in the lean oil (below 2–3 mol%) requires higher reboiler temperatures, which increases the risk of oil thermal degradation. To strip the last fraction of C₃+ from the oil using heat alone, the bottoms temperature must approach the oil's thermal stability limit (380–450°F depending on oil type).

Steam Stripping Benefits

Direct steam injection into the column (typically at or below the bottom tray) provides an additional stripping medium that reduces the partial pressure of NGL components in the vapor phase, enhancing their removal from the oil without requiring higher temperatures. The key benefits include:

• Lower reboiler temperature: Steam provides supplemental stripping vapor, allowing the reboiler to operate 20–40°F below the temperature required for equivalent NGL removal without steam. This significantly reduces oil degradation risk
• Better NGL recovery: Steam dilutes the vapor phase, reducing the NGL partial pressure and increasing the driving force for mass transfer from the liquid oil phase. Lean oil NGL retention below 1 mol% is achievable with steam
• Reduced oil degradation: Lower operating temperatures translate directly to longer oil life, lower makeup rates, and better absorption performance over time
• Operational flexibility: Steam rate can be adjusted independently of reboiler duty to fine-tune stripping performance without changing column temperature profile

Typical Steam Rates

Steam injection rates for oil purification stills typically range from 0.3 to 1.0 lb steam per gallon of oil circulated. The exact rate depends on the desired degree of NGL removal, the oil type, and the operating temperature:

Open vs. Closed Steam Injection

Steam can be injected into the still column in two ways:

• Open (live) steam injection: Steam is injected directly into the column through a sparger or distributor below the bottom tray. This is the most common arrangement. The steam mixes with the oil, provides stripping action, and exits overhead with the NGL product vapor. The condensed water must be separated in a three-phase reflux drum
• Closed steam (steam reboiler): Steam heats the oil indirectly through a heat exchanger without mixing. This avoids water contamination of the overhead product but does not provide the partial-pressure reduction benefit. This arrangement is essentially equivalent to a conventional reboiler using steam as the heating medium

Trade-Offs: Water Handling in Overhead System

The primary disadvantage of open steam stripping is the introduction of water into the overhead system. The still overhead vapor contains NGL hydrocarbons, uncondensed steam, and water vapor that must be condensed and separated. This requires:

• Three-phase reflux drum: The overhead accumulator must separate hydrocarbon liquid (reflux and NGL product), water (for disposal or reuse), and vapor (uncondensed light ends). This adds complexity compared to the two-phase separator used in reboiler-only operation
• Larger condenser: The latent heat of steam condensation adds to the condenser duty. For steam rates of 0.5–1.0 lb/gal, the additional condenser load can be 15–30% above the NGL-only condensation requirement
• Water disposal: The condensed water contains dissolved hydrocarbons and must be treated before discharge. Typical disposal routes include produced water systems, evaporation ponds, or deep-well injection

4. NGL Product Recovery

The overhead product from the oil purification still is a mixed NGL stream containing primarily C₃ through C₅+ hydrocarbons, along with minor amounts of C₂ (if not completely removed in the demethanizer), water vapor (if steam stripping is used), and trace quantities of absorption oil mist. The quality and composition of this product determine its commercial value and downstream processing requirements.

Product Composition

The NGL product composition depends on several factors: the inlet gas composition, the absorber operating conditions, the degree of C₁/C₂ removal in the demethanizer, and the still operating parameters. A typical overhead NGL product from a lean oil plant processing pipeline-quality natural gas contains:

NGL Retention Targets

The lean oil leaving the still bottoms retains a small fraction of NGL that was not completely stripped. Typical retention targets are 1–3 mol% NGL in the lean oil, with the exact target determined by economic optimization. Lower retention (better stripping) requires more energy input but recovers more product. The optimal retention level depends on NGL product value, energy costs, and oil degradation considerations.

NGL retention is monitored by measuring the lean oil flash point, which decreases as residual NGL content increases. A lean oil flash point below the design specification indicates incomplete stripping and should trigger investigation of reboiler performance, steam rate, or column operation.

Condenser and Reflux Drum Design

The overhead system includes an air-cooled or water-cooled condenser followed by a reflux drum. In steam-stripping installations, the reflux drum operates as a three-phase separator to handle hydrocarbon liquid, water, and uncondensed vapor:

• Condenser: Sized to condense substantially all C₃+ components while allowing uncondensed light ends (C₁, C₂) and water vapor to pass through. Outlet temperature typically 100–130°F depending on ambient conditions and cooling medium
• Reflux drum: Provides liquid residence time (5–10 minutes) for phase separation. Hydrocarbon reflux is returned to the column top tray; net NGL product is pumped to storage or fractionation. Water phase is routed to disposal
• Reflux ratio: Typical reflux ratios are 0.5:1 to 2.0:1 (reflux to product), with higher ratios improving C₅+ recovery at the expense of increased condenser duty

NGL Product Routing

The NGL product from the still can be handled in several ways depending on plant configuration and market conditions:

• On-site fractionation: The mixed NGL is fed to a fractionation train (depropanizer, debutanizer, deisopentanizer) for separation into specification products (HD-5 propane, commercial butane, natural gasoline)
• Pipeline delivery (Y-grade): The mixed NGL is stabilized and pumped to an NGL pipeline for delivery to a central fractionation facility. Product must meet GPA 2140 specifications for pipeline-quality NGL
• Truck loading: For smaller plants without pipeline access, NGL products may be loaded into transport trucks for delivery to market

5. Oil Degradation and Loss Prevention

Maintaining absorption oil quality is one of the most critical operational challenges in lean oil plant operation. The oil purification still is the location where oil degradation risk is highest because the oil is exposed to the most elevated temperatures in the entire absorption circuit. Managing oil quality directly impacts NGL recovery efficiency, operating costs, and equipment reliability.

Thermal Degradation Mechanisms

Thermal degradation of absorption oil occurs through two primary mechanisms when temperatures exceed the oil's stability threshold:

• Thermal cracking: At temperatures above 380–400°F (depending on oil type), hydrocarbon molecules in the oil begin to break down into smaller fragments. This produces light ends that increase the vapor pressure of the lean oil, reduces the average molecular weight, and generates reactive olefinic species
• Polymerization and coking: The reactive cracking products can recombine into heavier, higher-molecular-weight species. Over time, this produces viscous tars and coke deposits on heat transfer surfaces, particularly on reboiler tubes where film temperatures are highest. Coking reduces heat transfer coefficients and can eventually plug reboiler tubes

Temperature Limits by Oil Type

Each absorption oil type has a characteristic maximum operating temperature above which degradation rates become unacceptable. These limits apply to the bulk oil temperature in the still bottoms; film temperatures on reboiler tubes may be 25–50°F higher:

Steam stripping allows the still to operate at lower bottoms temperatures while still achieving adequate NGL removal, which is one of the primary motivations for selecting steam-assisted operation over reboiler-only design.

Oil Makeup Requirements

Oil losses are inevitable in any lean oil plant and must be replaced with fresh makeup oil to maintain the circulating inventory. Total oil losses typically range from 0.02 to 0.10% of the circulation rate, depending on plant condition and oil quality. The primary loss mechanisms include:

• Mechanical carryover: Oil mist entrained in the absorber residue gas, demethanizer overhead, and still overhead. This is the largest loss mechanism, typically 0.01–0.05% of circulation
• Thermal degradation: Cracked light ends that exit with the NGL product or vent gas. Accelerated degradation increases this loss component
• Leaks and drains: Process leaks, pump seal losses, and maintenance drains. Typically minor in well-maintained plants
• Dissolved in NGL product: A small amount of heavy oil dissolves in the NGL product and is carried out of the still overhead

Monitoring Oil Quality

Regular monitoring of key oil quality parameters is essential to detect degradation early and take corrective action before the oil quality deteriorates to the point of impairing absorption performance:

Oil Reclaiming Techniques

When oil quality has deteriorated but full replacement is not economically justified, several reclaiming techniques can restore oil properties to acceptable levels:

• Clay treatment (filtration): Passing the oil through a bed of activated clay (attapulgite or bentonite) adsorbs polar degradation products, reduces color and acid number, and removes surface-active contaminants that promote foaming. Clay beds are typically sized for 0.5–1.0 gpm/ft² superficial velocity and are replaced when breakthrough occurs
• Vacuum distillation: A side-stream vacuum still can separate light degradation products (which reduce flash point) and heavy polymeric material (which increases viscosity) from the useful oil fraction. This is the most effective reclaiming method but requires additional capital equipment
• Partial oil replacement: Draining a fraction (10–25%) of the circulating oil and replacing it with fresh makeup on a periodic basis dilutes degradation products and maintains average oil quality within acceptable limits
• Activated carbon polishing: Supplementary to clay treatment, activated carbon beds can adsorb specific contaminants (sulfur compounds, amine degradation products) that clay alone may not remove effectively

Preventive Measures

The most effective approach to oil quality management is prevention of degradation rather than remediation:

• Maintain reboiler film temperatures at least 25°F below the oil's thermal stability limit at all times
• Use steam stripping to reduce reboiler temperature requirements when economically justified
• Minimize oil residence time at elevated temperatures by using thermosiphon reboilers rather than kettle types where practical
• Ensure adequate lean/rich oil heat exchange to recover heat and reduce reboiler duty
• Maintain oxygen exclusion from the oil system, as oxygen accelerates both thermal cracking and polymerization reactions
• Monitor reboiler tube-wall temperatures using skin thermocouples and respond promptly to unexplained temperature increases that may indicate fouling

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. API Standard 650 — Welded Tanks for Oil Storage