Absorption: Definition, Gas Dehydration, and Glycol Processing

Absorption in petroleum engineering refers to the process by which a gas or vapor is taken up and dissolved into the bulk phase of a liquid, as distinct from being captured only on the surface of a solid. In oil and gas operations, absorption has two primary commercial applications: gas dehydration, where a liquid glycol solvent absorbs water vapor from a natural gas stream to meet pipeline dew-point specifications; and natural gas liquids (NGL) recovery, where a light hydrocarbon oil (absorption oil) contacts a wet gas stream and absorbs the heavier liquid hydrocarbons such as propane, butane, and natural gasoline, yielding a leaner, drier residue gas and a rich absorption oil that is subsequently stripped to recover the NGL products. A third major industrial application is acid gas treating, where amine solvents chemically absorb hydrogen sulfide (H₂S) and carbon dioxide (CO₂) from sour gas streams in a process that is more accurately described as chemical absorption, rather than the physical absorption that governs glycol dehydration and NGL recovery. Understanding the distinction between physical and chemical absorption, and the process configurations used in each case, is fundamental to the design and operation of surface gas processing facilities worldwide.

Key Takeaways

• Absorption is a bulk-phase phenomenon in which a gas molecule dissolves into the interior of a liquid solvent; it is distinct from adsorption, where molecules accumulate only on a solid surface, and from condensation, which is a phase change driven by temperature or pressure without a solvent.
• Triethylene glycol (TEG) is the most widely used absorption solvent for natural gas dehydration, capable of depressing the water dew point by 40 to 80 degrees Fahrenheit (22 to 44 degrees Celsius) per theoretical stage and achieving lean TEG concentrations of 99.0 to 99.95 wt% in the regenerator.
• The water content of natural gas at saturation follows the McKetta-Wehe chart; at 1,000 psia (6.9 MPa) and 100 degrees Fahrenheit (38 degrees Celsius), saturated gas contains approximately 34 lb H₂O per MMscf (0.54 kg/1,000 m³), and the TEG absorber must reduce this to the pipeline specification, typically 4 to 7 lb/MMscf (0.064 to 0.112 kg/1,000 m³).
• Chemical absorption by amine solvents (MEA, DEA, MDEA) removes H₂S and CO₂ from sour gas through reversible chemical reactions, making the process regenerable and continuous; MDEA is selective for H₂S over CO₂, which is exploited in tail-gas treating and Claus plant feed conditioning.
• Environmental regulations including U.S. EPA 40 CFR Part 63 Subpart HH (NESHAP for natural gas transmission and storage) require emissions controls on glycol dehydration units processing more than 0.9 MMscfd, because TEG regenerators emit benzene, toluene, ethylbenzene, and xylene (BTEX) vapors that are listed hazardous air pollutants.

Absorption vs. Adsorption: A Critical Distinction

The terms absorption and adsorption are frequently confused in both casual usage and technical literature, but they describe fundamentally different physical phenomena. Absorption involves a gas or liquid penetrating into and dissolving within the bulk volume of a second phase, typically a liquid. The absorbed species becomes distributed throughout the entire volume of the absorbent. The driving force is the difference in chemical potential between the gas phase and the dissolved state in the liquid, which is governed by Henry's Law for dilute solutions: the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid, at constant temperature.

Adsorption, by contrast, is a surface phenomenon. Gas or liquid molecules are attracted to and accumulate on the external surface area of a solid adsorbent material, driven by van der Waals forces (physisorption) or by chemical bonds (chemisorption). In oil and gas processing, adsorption is used for gas dehydration with solid desiccants (molecular sieves, silica gel, activated alumina), for mercury removal using sulfur-impregnated carbon, and for gas separation in pressure-swing adsorption (PSA) units. Adsorption beds have a finite capacity determined by surface area and must be periodically regenerated by heating or pressure reduction to desorb the captured species. Absorption processes, by contrast, can be operated continuously by circulating the solvent through a regenerator that reverses the absorption equilibrium by heating or pressure reduction.

In the context of gas dehydration, the operator's choice between absorption (liquid glycol) and adsorption (solid desiccant) depends on the required dew-point depression, gas volume, and the presence of contaminants. Glycol absorption is the dominant choice for most midstream dehydration applications because it is simpler, less capital-intensive, and well-suited to the moderate dew-point depressions (40 to 80 degrees Fahrenheit / 22 to 44 degrees Celsius) required for pipeline gas. Molecular sieve adsorption is preferred for deep dehydration (dew points below -150 degrees Fahrenheit / -101 degrees Celsius) required before cryogenic NGL extraction or LNG liquefaction, and for situations where co-absorption of heavier hydrocarbons by glycol would cause operational problems.

How It Works: Triethylene Glycol Gas Dehydration

The triethylene glycol (TEG) absorption process for natural gas dehydration is one of the most common unit operations in the natural gas midstream industry. A typical TEG dehydration unit consists of an absorber (also called a contactor column), a glycol heat exchanger, a flash separator, a filter system, a still column (glycol regenerator), a reboiler, and a condenser. The process operates on a closed-loop solvent circulation circuit: lean (water-poor) TEG enters the top of the absorber and flows downward by gravity, countercurrent to the wet gas stream that enters at the bottom and flows upward. As the gas and glycol contact one another on the absorber trays or packing, water vapor preferentially partitions into the liquid glycol phase because of TEG's extremely high affinity for water at absorber conditions, typically 80 to 110 degrees Fahrenheit (27 to 43 degrees Celsius) and 200 to 1,200 psia (1.4 to 8.3 MPa).

The now-rich TEG (loaded with absorbed water) exits the bottom of the absorber and passes through a flash separator, where dissolved light hydrocarbon gases that co-absorbed with the water are released and can be used as fuel gas or vented under controlled conditions. The rich TEG then flows through a heat exchanger, where it picks up heat from the outgoing lean TEG, and enters the still column at the top of the reboiler. In the regenerator (still), the rich glycol is heated to approximately 350 to 400 degrees Fahrenheit (177 to 204 degrees Celsius) at near-atmospheric pressure. At these conditions, the absorbed water is driven off as steam and exits through the still column overhead condenser. The lean TEG, now restored to approximately 99.0 to 99.5 wt% concentration (or up to 99.95 wt% with stripping gas or vacuum enhancement), is cooled and pumped back to the top of the absorber to repeat the cycle.

The achievable water dew-point depression is a function of TEG concentration, glycol circulation rate (gallons of TEG per pound of water absorbed), number of theoretical equilibrium stages in the absorber, absorber temperature, and operating pressure. The Kremers correlation and commercial simulators such as HYSYS, Pro/II, and GPSA data book charts are used to optimize these parameters. Increasing the lean TEG concentration from 99.0 wt% to 99.9 wt% can improve dew-point depression by 15 to 25 degrees Fahrenheit (8 to 14 degrees Celsius), which is why stripping gas injection into the reboiler -- which dilutes the water vapor partial pressure in the regenerator overhead and allows a higher lean TEG concentration -- is widely used in applications requiring deep dehydration. Stripping gas consumption is typically 1 to 3 standard cubic feet per gallon of TEG circulated.

The glycol-to-water ratio (GWR), expressed in U.S. practice as U.S. gallons of TEG circulated per pound of water removed, directly affects both the achievable dew-point depression and the operating cost. Typical GWR values range from 2 to 6 gal TEG/lb water, with higher ratios improving performance at the expense of higher heat and pump energy consumption. The GPSA Engineering Data Book provides design charts relating GWR, lean TEG concentration, number of theoretical trays, and achievable dew-point depression, which form the basis for initial dehydrator sizing in new gas plant designs.

NGL Recovery by Absorption Oil

Before the widespread adoption of cryogenic processing for NGL extraction, absorption oil (also called lean oil) was the primary method for recovering propane, butane, and heavier hydrocarbons from wet natural gas streams. In this process, a light petroleum oil (typically a naphtha or kerosene-range product) is contacted with the wet gas in an absorber column. The heavy hydrocarbon components of the gas dissolve preferentially into the absorption oil -- following Henry's Law solubility relationships -- while methane and ethane, which are much less soluble, pass through as lean residue gas.

The rich absorption oil, loaded with propane-plus components, is then stripped in a distillation column (the still or absorber oil still) to recover the NGL as a mixed stream, which is subsequently fractionated into propane, butane, and natural gasoline products. The lean absorption oil, now stripped of the absorbed hydrocarbons, is cooled and recycled back to the absorber to repeat the cycle. The efficiency of NGL recovery depends on the molecular weight and viscosity of the absorption oil, the operating temperature of the absorber (lower temperatures favor absorption by increasing Henry's Law solubility), and the number of theoretical stages. While lean oil absorption has been largely replaced by more efficient turboexpander cryogenic plants in new construction, existing lean oil plants continue to operate at many older gas processing facilities, particularly in mature North American gas fields. See also: absorption oil.