Rectisol
Encyclopedia
Rectisol is the trade name for an acid gas removal
process that uses methanol
as a solvent to separate acid gas
es such as hydrogen sulfide
and carbon dioxide
from valuable feed gas streams. By doing so, the feed gas is made more suitable for combustion
and/or further processing. Rectisol is used most often to treat synthesis gas (primarily hydrogen
and carbon monoxide
) produced by gasification
of coal or heavy hydrocarbons, as the methanol solvent is well able to remove trace contaminants such as ammonia
, mercury
, and hydrogen cyanide usually found in these gases. As an acid gas and large component of valuable feed gas streams, CO2 is separated during the methanol solvent regeneration.
), cold methanol at approximately –40 °F (–40 °C) dissolves (absorbs) the acid gases from the feed gas at relatively high pressure, usually 400 to 1000 psia (2.76 to 6.89 MPa). The rich solvent containing the acid gases is then let down in pressure to release and recover the acid gases. The Rectisol process can operate selectively to recover hydrogen sulfide
and carbon dioxide
as separate streams, so that the hydrogen sulfide can be sent to either a Claus
unit for conversion to elemental sulfur
or a WSA Process unit to recover sulfuric acid
, while at the same time the carbon dioxide can be sequestered or used for enhanced oil recovery.
Rectisol, like Selexol
and Purisol, is a physical solvent, unlike amine based acid gas removal
solvents that rely on a chemical reaction with the acid gases. Methanol as a solvent is inexpensive compared to the proprietary Selexol and Purisol solvents. The Rectisol process requires more electrical energy for refrigeration
to maintain the low temperatures required but it also requires less steam energy for regeneration. Although capital costs for methanol solvent (Rectisol) units are higher than proprietary solvent units, methanol as a cold, physical solvent can remove greater percentages of acid gas components providing a higher purity cleaned gas.
The Rectisol process is very flexible and can be configured to address the separation of synthesis gas into various components, depending on the final products that are desired from the gas. It is very suitable to complex schemes where a combination of products are needed, such as for example hydrogen, carbon monoxide, ammonia and methanol synthesis gases and fuel gas side streams.
Amine gas treating
Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines to remove hydrogen sulfide and carbon dioxide from gases...
process that uses methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
as a solvent to separate acid gas
Acid gas
Acid gas is natural gas or any other gas mixture which contains significant amounts of hydrogen sulfide , carbon dioxide , or similar contaminants. The terms acid gas and sour gas are often incorrectly treated as synonyms...
es such as hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
from valuable feed gas streams. By doing so, the feed gas is made more suitable for combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
and/or further processing. Rectisol is used most often to treat synthesis gas (primarily hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
) produced by gasification
Gasification
Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures , without combustion, with a controlled amount of oxygen and/or steam...
of coal or heavy hydrocarbons, as the methanol solvent is well able to remove trace contaminants such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, and hydrogen cyanide usually found in these gases. As an acid gas and large component of valuable feed gas streams, CO2 is separated during the methanol solvent regeneration.
Process description
In the Rectisol process (licensed by both Linde AG and Lurgi AGLurgi AG
Lurgi GmbH is a German engineering, construction and chemical process licensing company. The head office is located in Frankfurt am Main. Lurgi GmbH has been part of Air Liquide S. A. since 2007.- History :...
), cold methanol at approximately –40 °F (–40 °C) dissolves (absorbs) the acid gases from the feed gas at relatively high pressure, usually 400 to 1000 psia (2.76 to 6.89 MPa). The rich solvent containing the acid gases is then let down in pressure to release and recover the acid gases. The Rectisol process can operate selectively to recover hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
as separate streams, so that the hydrogen sulfide can be sent to either a Claus
Claus process
The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the scientist Carl Friedrich Claus, the Claus process has become the industry standard....
unit for conversion to elemental sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or a WSA Process unit to recover sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
, while at the same time the carbon dioxide can be sequestered or used for enhanced oil recovery.
Rectisol, like Selexol
Selexol
Selexol is the trade name for an acid gas removal solvent that can separate acid gases such as hydrogen sulfide and carbon dioxide from feed gas streams such as synthesis gas produced by gasification of coal, coke, or heavy hydrocarbon oils...
and Purisol, is a physical solvent, unlike amine based acid gas removal
Amine gas treating
Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines to remove hydrogen sulfide and carbon dioxide from gases...
solvents that rely on a chemical reaction with the acid gases. Methanol as a solvent is inexpensive compared to the proprietary Selexol and Purisol solvents. The Rectisol process requires more electrical energy for refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
to maintain the low temperatures required but it also requires less steam energy for regeneration. Although capital costs for methanol solvent (Rectisol) units are higher than proprietary solvent units, methanol as a cold, physical solvent can remove greater percentages of acid gas components providing a higher purity cleaned gas.
The Rectisol process is very flexible and can be configured to address the separation of synthesis gas into various components, depending on the final products that are desired from the gas. It is very suitable to complex schemes where a combination of products are needed, such as for example hydrogen, carbon monoxide, ammonia and methanol synthesis gases and fuel gas side streams.