Encyclopedia of Renewable Energy. James G. Speight
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acid gas removal from gas streams. In the process, carbon dioxide (CO2) and hydrgoen sulfide (H2S) are absorbed by treating the gas stream (invariably by passing the gas stream though the liquid absorbent) with an aqueous solution of monoethanolamine (MEA), diethanolamine (DEA), diethylene glycol (DEG) or triethylene glycol (TEG), where a reversible chemical reaction takes place in the liquid phase. Chemical reactions can increase the rate of absorption, increase the absorption capacity of the solvent, increase selectivity to preferentially dissolve only certain components of the gas, and convert a hazardous chemical to a safe compound.
See also: Absorption, Gas Cleaning, Gas Processing, Gas Treating, Scrubbing, Stripping.
Absorption Dehydration
An example of absorption dehydration is known as glycol dehydration and diethylene glycol, the principal agent in this process, has a chemical affinity for water and removes water from the gas stream. In this process, a liquid desiccant dehydrator serves to absorb water vapor from the gas stream.
Essentially, glycol dehydration involves using a glycol solution, usually either diethylene glycol (DEG) or triethylene glycol (TEG), which is brought into contact with the wet gas stream in a contactor. The glycol solution will absorb water from the wet gas and once absorbed, the glycol particles become heavier and sink to the bottom of the contactor where they are removed. The gas stream, having been stripped of most of its water content, is then transported out of the dehydrator. The glycol solution, bearing all of the water stripped from the gas stream, is put through a specialized boiler designed to vaporize only the water out of the solution. The boiling point differential between water (100°C, 212°F) and glycol (204°C, 400°F) makes it relatively easy to remove water from the glycol solution, allowing it to be reused in the dehydration process.
As well as absorbing water from the wet gas stream, the glycol solution occasionally carries with it small amounts of methane and other compounds found in the wet gas. In the past, this methane was simply vented out of the boiler. In addition to losing a portion of the gas stream that was extracted, this venting contributes to air pollution and the greenhouse effect. In order to decrease the amount of methane and other compounds that are lost, flash tank separator-condensers work to remove these compounds before the glycol solution reaches the boiler. Essentially, a flash tank separator consists of a device that reduces the pressure of the glycol solution stream, allowing the methane and other hydrocarbons to vaporize (flash).
The glycol solution then travels to the boiler, which may also be fitted with air or water cooled condensers, which serve to capture any remaining organic compounds that may remain in the glycol solution. The regeneration (stripping) of the glycol is limited by temperature: diethylene glycol and triethylene glycol decompose at or before their respective boiling points. Such techniques as stripping of hot triethylene glycol with dry gas (e.g., heavy hydrocarbon vapors, the Drizo process) or vacuum distillation are recommended.
In practice, absorption systems recover 90 to 99% by volume of methane that would otherwise be flared into the atmosphere.
See also: Absorption, Gas Cleaning, Gas Processing, Gas Treating.
Absorption Oil
Absorption oil (also called absorber oil, scrubbing oil, wash oil) is low-boiling liquid hydrocarbon used to absorb or remove the higher-boiling liquid hydrocarbons from a gas stream. Typically, absorption oil is a hydrocarbon-based liquid that is contacted with a gas stream to remove higher boiling components, such as occurs in the recovery of natural gasoline from the gas stream (often referred to as the wet gas stream).
An absorption plant is a facility used to recover the condensable portion of the gas stream to remove acid gas stream. Typically, the absorption takes place in a counter-flow (countercurrent) vessel, where the gas stream enters the bottom of the vessel and flows upward through the absorption oil. The plant also includes the necessary ancillary thermal units to distill the absorbed hydrocarbons from the higher boiling absorption oil.
See also: Absorption, Absorption Processes, Gas Cleaning, Gas Processing, Gas Treating, Olamine Processes.
Absorption Process
The absorption method of extraction is similar to using absorption for dehydration. The main difference is that in the absorption of liquids from the gas stream, absorbing oil is used as opposed to glycol. This absorbing oil has an affinity for gas liquids in much the same manner as glycol has an affinity for water. Before the oil has picked up any gas stream liquids, it is termed lean absorption oil.
The oil absorption process involves the countercurrent contact of the lean (or stripped) oil with the incoming wet gas with the temperature and pressure conditions programmed to maximize the dissolution of the liquefiable components in the oil. The rich absorption oil (sometimes referred to as fat oil), containing gas liquids, exits the absorption tower through the bottom. It is now a mixture of absorption oil, propane, butanes, pentanes, and other higher-boiling hydrocarbons. The rich oil is fed into lean oil stills, where the mixture is heated to a temperature above the boiling point of the gas liquids but below that of the oil. This process allows for the recovery of around 75% by volume of the butane derivatives, and 85 to 90% by volume of the pentane derivatives, and higher-boiling constituents from the gas stream.
The basic absorption process above can be modified to improve its effectiveness, or to target the extraction of specific gas stream liquids. In the refrigerated oil absorption method, where the lean oil is cooled through refrigeration, propane recovery can be upwards of 90% by volume, and approximately 40% by volume of the ethane can be extracted from the gas stream. Extraction of the other, higher-boiling liquids can be close to 100% by volume using this process.
The absorption method, on the other hand, uses an absorbing oil to separate the methane from the gas stream liquids. While the gas stream is passed through an absorption tower, the absorption oil (lean oil) soaks up a large amount of the gas liquids. The absorption oil (enriched oil), now containing gas liquids, exits the base of the tower after which the enriched oil is fed into distillers where the blend is heated to above the boiling point of the gas liquids, while the oil remains fluid. The absorption oil is recycled while the gas liquids are cooled and directed to a fractionator tower. Another absorption method that is often used is the refrigerated oil absorption method where the lean oil is chilled rather than heated, a feature that has the potential to enhance recovery rates.
More specifically, to allow the process to operate at low temperatures, the feed gas must be injected with ethylene glycol solution to avoid hydrate formation in the heat exchangers. The feedstock (the gas stream) is cooled by propane refrigeration and separated in a cold separator, typically at approximately -18°C (0°F). The separator liquid is sent to the ethane recovery unit (the deethanizer) while the separator vapor is routed to the absorber operating at 400 psig. Refrigerated lean oil is used to absorb the higher molecular weight hydrocarbons (the C3+ constituents) from the gas stream thereby producing a lean gas and a propane rich bottom which is sent to the deethanizer. The deethanizer operates at a lower pressure, typically at 200 psig, producing an ethane rich gas and a rich oil bottom containing the C3+ components.
The overhead stream from the deethanizer is compressed to the sales gas pipeline or used as fuel gas. The bottom product is further processed in a rich oil still which regenerates a lean oil to be recycled back to the absorber and an overhead distillate containing the C3+ constituents. The C3+ stream can be fractionated in a propane recovery unit (the depropanizer) which produces the propane and butane product. Because of the high-boiling material of the lean oil, a heater is used in the rich oil still. If necessary, the lean oil composition can be controlled using a lean oil still (not shown) to remove the heavy tails of the lean oil from the process.
See also: Absorption, Gas Cleaning, Gas Processing, Gas Treating.
Absorption Tower
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