Gas separation: processes – Liquid contacting – And degasification of a liquid
Reexamination Certificate
2000-06-13
2002-07-30
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And degasification of a liquid
C095S179000, C095S193000, C095S196000, C095S209000, C095S231000, C096S234000
Reexamination Certificate
active
06425942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an arrangement for drying a gas, in particular natural gas, using a glycol as well measures to purify contaminated glycol in a gas drying plant.
2. Related Art
Glycols, in particular triethylene glycol, are used in great quantities to dry gases, in particular natural gas. When natural gas is transported at high pressure, the water vapour content must not exceed a certain amount which depends on the pressure. Condensed water may cause pipeline corrosion. Gas hydrates might also form.
In order to reduce the water content of the gas, it is brought into contact with glycol as an absorbent in drying plants. For example, the gas to be dried and glycol act on each other in countercurrent in an absorber. During this process, the glycol not only absorbs the moisture from the gas but also impurities and leaves the absorber laden with water and impurities. It is then regenerated, the water from the glycol-water mixture being eliminated.
The U.S. Pat. No. 5 490 873 teaches a method in which the vapours released in the regeneration are partially condensed and then fed into a separator. Three phases are separated from one another in the separator, a gas phase, a liquid hydrocarbon phase and a water phase. The gas phase is compressed and fed to the process. The liquid hydrocarbon phase is withdrawn for use and the water phase is disposed of.
The dried glycol from the regeneration process is then returned to the absorber. During this cycle the glycol becomes more and more contaminated. The impurities include salts, particulate matter, the decomposition products of the glycol as well as organic impurities which have been absorbed from the gas to be dried. Some of the decomposition products of the glycol and the impurities absorbed from the gas to be dried are organic acids, which may lead to a fall in the pH, which in turn may cause increased plant corrosion. Furthermore, these and other organic impurities and decomposition products can lead to foam formation, which reduces the drying properties of the glycol and can lead to high glycol losses.
Therefore, in conventional plants it is necessary to replace or purify the glycol at regular intervals. The glycol removed from the drying plant is normally purified by distillation.
SUMMARY OF THE INVENTION
An object of the present invention is to develop a gas drying process which permits more economic use of the glycol used, in particular extends the life of the glycol.
According to the present invention, this object is achieved by a method for drying a gas using a glycol, wherein the gas is fed into an absorber, brought into contact with glycol in the absorber, the glycol absorbing at least part of the moisture and impurities from the gas, and then the gas is removed from the absorber. The glycol laden with water and impurities is removed from the absorber and regenerated by heating to drive the water out. At least some of the glycol removed from the absorber is purified before or after it is regenerated by mixing it with at least half the quantity of water, bringing the mixture to a temperature above its cloud point, and keeping the mixture at that temperature for a predetermined time, during which time the impurities flocculate, removing the flocculated impurities, and regenerating the glycol by heating it to drive the water out of the mixture. The regenerated glycol may then returned to the absorber. Surprisingly, it was found that a considerable proportion of the impurities in the glycol are absorbed in this way by the floccules forming above the cloud point so that subsequent separation produces sufficiently purified glycol. In particular, impurities such as oil, black sludge and other oxidation products are removed from the glycol during this process. The inventive method is a very cost-effective drying process as it integrates environmentally-friendly and simple purification of glycol.
In one embodiment, the glycol removed from the absorber is purified before its regeneration. The water added during the purification process is driven out together with the water absorbed in the absorber in one regeneration step by heating. In this mainstream process, the entire amount of glycol to be regenerated, i.e. to be freed of water, is always purified before regeneration. As in this process an appropriate amount of water always has to be added to the entire amount of glycol in circulation, a large amount of energy is required to separate the water and the glycol in one regeneration step.
In a further embodiment, part of the glycol previously regenerated is purified. The glycol is first regenerated in the conventional manner by eliminating the water. Some of the regenerated but still contaminated glycol is then passed into a purification process. The rest of the regenerated glycol is fed into the absorber.
After removal of the flocculated impurities, the glycol-water mixture is preferably mixed again and regenerated with the glycol which has been removed from the absorber and is to be regenerated. This side-stream process makes it possible to purify only part of the glycol in the absorber cycle so that the energy required in the regeneration step to drive out the water absorbed from the gas and the water mixed during the purification process can be reduced.
The glycol-water mixture is preferably filtered and/or centrifuged to remove the flocculated impurities. Apart from these fast separation methods, it is also possible to employ alternatives such as sedimentation or flocculation.
In an advantageous embodiment of the method according to the present invention, the glycol-water mixture is passed over a first anion exchanger prior to heating.
The glycol-water mixture is preferably brought to a temperature of approx. 40° C. to 90° C. before purification and kept at that temperature for between 2 and 30 minutes. The temperature, time required and percentage of water in the mixture depend on each other. A higher percentage of water leads to more rapid clouding or flocculation of the impurities; on the other hand a higher percentage of water increases the time and amount of energy required to drive the water out of the mixture. The contaminated glycol is preferably mixed with water, roughly in a ratio of 1:1.
In a further embodiment of the inventive method triethylene glycol (TEG) is used as a desiccant. The contaminated triethylene glycol is mixed with water in a ratio of 1:1 for the purification process and the triethylene glycol-water mixture is brought to a temperature of approx. 75° C. to 85° C. and kept at that temperature for between 5 and 20 minutes. The temperature is above the so-called cloud point. The mixture of water and TEG is preferably passed over an anion exchanger prior to heating.
The object of the present invention is also achieved by a purification method for cleaning glycol contaminated in a gas drying plant. Contaminated glycol laden with water and impurities is removed from the gas drying plant. The glycol removed is mixed with at least half the amount of water. The glycol-water mixture is then passed over a first anion exchanger, anionic surfactants being removed from the mixture. The cloud point is reduced below the temperature of the mixture causing flocculation of the impurities. The flocculated impurities are then removed. The purified glycol-water mixture is returned to a regeneration stage of the gas drying plant where the water is driven out. The glycol removed is preferably removed from the gas drying plant and passed to the purification step at a temperature which is high enough for the temperature of the glycol-water mixture after the removal of the anionic surfactants to be already above the cloud point without it being necessary to additionally heat the glycol-water mixture before or after it has passed through the ion exchanger. to additionally heat the glycol-water mixture before or after it has passed through the ion exchanger.
The inventive method produces excellent purification results; a considerable amount of the impu
Blakely & Sokoloff, Taylor & Zafman
Ruhrgas Aktiengesellschaft
Smith Duane S.
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