Absorption Oil

Absorption oil is a light liquid hydrocarbon, typically a naphtha or kerosene fraction with a boiling range of approximately 120 to 220°C, used in natural gas processing plants to absorb propane, butane, and heavier hydrocarbon components from the raw gas stream, producing natural gas liquids (NGLs) as a separate liquid product. In an absorption oil plant, the raw gas contacts the absorption oil in a vertical vessel (the absorber) where the heavier hydrocarbon components dissolve preferentially into the liquid oil phase while methane and ethane remain in the gas. The enriched (rich) absorption oil flows to a stripping still where it is heated, releasing the absorbed NGLs as vapor that is subsequently condensed and separated into propane, butane, and natural gasoline products. The stripped (lean) absorption oil is cooled and recirculated to the top of the absorber. Absorption oil plants have largely been replaced by cryogenic turboexpander plants in modern NGL recovery facilities, but many older plants in Alberta and Saskatchewan continue to operate on this principle.

Key Takeaways

The equilibrium between the hydrocarbon components in the gas and in the absorption oil is described by K-values (equilibrium ratios), where K = y/x for each component (y = mole fraction in gas, x = mole fraction in oil). Components with low K-values are preferentially absorbed into the oil. At typical absorber conditions (2 to 7 MPa, 5 to 30°C), the K-values for propane are approximately 0.2 to 0.5 (mostly absorbed), while for methane they are approximately 20 to 50 (mostly in gas phase). The molecular weight of the absorption oil and the operating pressure and temperature are selected to achieve the desired NGL recovery while minimizing methane and ethane absorption (which reduces gas shrinkage).
Absorption efficiency (the fraction of each component absorbed from the gas) depends on the oil circulation rate relative to the gas flow. The absorption factor A = L / (K × V), where L is the molar flow of oil, K is the equilibrium ratio, and V is the molar flow of gas. Higher A values give better absorption efficiency but require more oil circulation and more energy for regeneration in the still. An absorption factor of 1.0 to 2.0 for propane is typical for economic operation. Plants designed for high propane recovery (more than 90%) use high oil circulation rates and multiple absorption stages (many trays or a tall packed section in the absorber).
The molecular weight of the absorption oil is a key design parameter. Heavier oils (higher molecular weight, lower vapor pressure) absorb more efficiently and circulate better at low temperatures but are harder to strip cleanly in the regeneration still and can accumulate in the gas stream as mist. Lighter oils are easier to strip but have higher vapor pressure, leading to greater absorption oil losses in the exit gas. Typical absorption oils used in Alberta plants are Stoddard solvent (a refined naphtha, boiling range 150 to 200°C) or a site-specific refinery cut.
In a refrigerated absorption plant (a common configuration in Alberta from the 1950s through the 1980s), the raw gas is pre-cooled by mechanical refrigeration (propane refrigerant cycle) before entering the absorber. Lower temperature significantly reduces K-values for propane and butane, increasing their absorption into the oil. Propane recovery in a refrigerated absorption plant is typically 70 to 85 percent, compared to 50 to 65 percent in an ambient-temperature absorption plant. Cryogenic turboexpander plants (developed in the 1970s and 1980s) can achieve 95 to 99 percent propane recovery at lower capital and operating cost per tonne of NGL, which is why they have largely replaced refrigerated absorption plants for new builds.
Absorption oil losses occur through two mechanisms: entrainment (small oil droplets carried out with the exit gas from the absorber) and dissolution (methane and ethane absorbing into the oil, increasing its vapor pressure and causing it to evaporate into the exit gas). Oil losses reduce the plant's NGL revenue and require periodic makeup of fresh absorption oil. In older plants without modern mist eliminators, oil carryover was a common problem that contaminated the sales gas and caused pipeline metering errors. Modern plants use high-efficiency mist eliminators (wire mesh or vane-type) at the top of the absorber vessel to reduce oil entrainment to less than 1 milligram per standard cubic metre of exit gas.

How Absorption Oil Plants Extract NGLs

A natural gas processing plant using absorption oil works like a sponge and a squeeze cycle. The gas enters the absorber vessel and rises through a series of trays or packing. Lean absorption oil flows in at the top and trickles downward through the packing, picking up propane, butane, and pentane-plus from the rising gas stream. The cleaned gas (now leaner in C₃+ components) exits from the top. The rich absorption oil, loaded with dissolved NGLs, exits from the bottom of the absorber.

The rich oil goes to the still (a vertical vessel with a reboiler at the bottom). Heat from the reboiler raises the temperature of the oil, reducing the solubility of the absorbed NGLs and driving them out of solution as vapor. The vaporized NGLs rise through the still, are condensed in an overhead condenser, and separated into propane, butane, and natural gasoline (C₅+) in a downstream fractionation train. The hot lean oil from the still bottom is cooled in a heat exchanger (which also preheats the incoming rich oil) and pumped back to the top of the absorber for the next cycle.

The entire cycle — absorption, transport, stripping, cooling, recirculation — runs continuously, 24 hours a day. The main energy input is the still reboiler, which typically burns field gas at 3 to 10 gigajoules per tonne of NGLs recovered. The main moving parts are the lean oil pump (circulating oil from the still to the absorber) and the refrigeration compressor (in refrigerated plants). Mean time between failures for a well-maintained absorption oil plant is 2 to 5 years for major turnarounds.

Fast Facts

Absorption oil NGL recovery was the dominant NGL extraction technology in Canada from the 1940s through the 1970s. The first gas plant in the Turner Valley field, built in 1914 to recover gasoline from the casing-head gas (the light liquid hydrocarbons condensed from the wellhead gas), used a primitive form of absorption oil treatment. By the 1950s, modern refrigerated absorption plants were the industry standard for new builds in Alberta, processing gas from the Devonian carbonate pools of central Alberta and the foothills. The Alberta NGL industry's infrastructure of gas plants built in this era, including several large plants still operating in the Pembina, Drayton Valley, and Edson areas, relies on absorption oil technology that has been maintained and upgraded rather than replaced. TransCanada PipeLines' Empress complex on the Alberta-Saskatchewan border, which processes all gas entering the TransCanada system, uses absorption-based processing among its suite of recovery technologies.

Comparison With Modern Cryogenic Plants

The shift from absorption oil to cryogenic turboexpander technology in the 1970s and 1980s was driven by the higher NGL recovery and lower operating cost of cryogenic plants for large-throughput facilities. A turboexpander plant cools the gas to -100°C by pressure reduction through an expansion turbine, condensing propane and heavier components as liquids that are separated in a demethanizer column. Propane recovery in a modern turboexpander plant is 95 to 99 percent, compared to 70 to 85 percent in a refrigerated absorption plant.

For smaller-throughput plants (below 100 thousand standard cubic metres per day), the capital cost of the cryogenic heat exchangers and turboexpander machinery is not justified. Refrigerated absorption plants remain economic at these sizes and continue to be used for smaller gas volumes in the Foothills, the Peace River arch, and the Alberta Deep Basin.

Absorption oil is also called lean oil (when circulating through the absorber without absorbed NGLs), rich oil (after it has absorbed NGLs from the gas stream), or scrubbing oil. Related terms include absorption (the mass transfer process by which a component dissolved in a gas phase is transferred into a liquid phase; the physical process underlying absorption oil NGL recovery), natural gas liquids (NGLs, the propane, butane, and pentane-plus components recovered from raw natural gas in processing plants; the products extracted by absorption oil or cryogenic processing), still (the regeneration vessel in an absorption oil plant where the rich oil is heated to drive off absorbed NGLs, producing lean oil for recirculation and NGL vapor for condensation and fractionation), turboexpander (a turbine that expands high-pressure gas to low pressure and low temperature, condensing heavy hydrocarbons as liquids; the core equipment of modern cryogenic NGL recovery plants that replaced absorption oil technology for large-throughput facilities), and gas plant (a facility that processes raw natural gas to meet pipeline quality specifications and recover NGLs; absorption oil plants and cryogenic plants are the two main types of NGL recovery technology used in Canadian gas plants).

How Absorption Oil Carryover Caused a Pipeline Contamination Problem in the WCSB

A refrigerated absorption oil plant in the Edson area of west-central Alberta processed approximately 400 thousand standard cubic metres per day of Viking and Notikewin gas. The absorption oil was a Stoddard solvent fraction supplied from a Edmonton-area refinery and stored in a dedicated makeup tank. The plant had operated for 18 years without significant oil carryover problems.

Following a planned maintenance turnaround, the absorber vessel was returned to service. During the first week back online, the TransCanada pipeline operator noted elevated total petroleum hydrocarbon (TPH) readings in the sales gas stream at the Edson meter station. The TPH readings were 4 to 7 times the normal background level, suggesting absorption oil contamination of the gas.

Investigation found that during the turnaround, the wire mesh mist eliminator at the top of the absorber had been replaced with a new mesh pad. The replacement pad had been installed with the incorrect orientation (the drainage channels running parallel rather than perpendicular to the gas flow), reducing its effectiveness from 99.5% to approximately 85% oil droplet capture. The resulting oil carryover was contaminating the sales gas at approximately 15 milligrams per standard cubic metre, compared to a specification of less than 1 mg/scm.

The mist eliminator was removed, correctly reinstalled, and the absorber returned to service. The pipeline operator cleaned the affected section of the sales line and the meter run, a process that took 3 days and required temporarily rerouting the Edson area gas through a different custody transfer meter. Total cost including lost production time, pipeline cleaning, and meter calibration: CAD 340,000. The turnaround maintenance checklist was subsequently updated to include a specific verification step confirming mist eliminator orientation with photographic documentation before the vessel is closed and pressured up.