Patent Application
20180029963
This invention relates to processes designed to treat mono ethylene glycol (“MEG”) used in the oil and gas industry to control hydrates formation. More particularly, the invention relates to MEG reclamation processes which are designed to remove salts from a wet MEG feed stream.
In the oil and gas industry, dry (lean) MEG is used to control the formation of hydrates within a produced stream. Once used, the now wet (rich) MEG is, in turn, dried and cleaned by way of a MEG regeneration and reclamation process so the MEG can be used again in hydrate control. The systems and methods used to recover MEG usually include three sections: pre-treatment, flash separation, and MEG regeneration. These sections can be followed by salt management and followed or preceded by a calcium removal section.
The MEG used for hydrate inhibition in natural gas pipelines has a similar volatility to organic acids such as acetic acid. Therefore, if water from a well contains organic acids, the organic acid tends to stay with the MEG solution. Removing this organic acid can require large, costly and wasteful blowdown or purges of MEG.
Other options for the removal of organic acid from rich MEG include boiling off the organic acids, distillation of the rich MEG and organic acid, and precipitation using divalent cations. The boiling off method requires a relatively large heating duty to heat the whole stream and/or vacuum conditions. Because the relative volatilities of MEG and organic acid are similar this method also produces large MEG losses. The distillation method, similar to the boiling off method, requires a large heating (reboiling) and cooling (condensing) duty. Although the MEG losses are reduced by distillation, more equipment is required (e.g., column, reboiler, condenser). The precipitation method requires the addition of chemicals to increase the pH and a solid separation step such as centrifuge or filtration (see e.g. US 2015/0119609 A1).
A preferred embodiment of a system and method for removing acetic acid and other short chain fatty acids described as organic acids from a rich mono-ethylene glycol (“MEG”) solution does so by:
The stripping steps take place in respective stripping columns. A portion of the gas exiting the gas/organic acid stripping column can be recycled to the MEG/organic acid stripping column to reduce total gas usage. A portion of the waste stream exiting the gas/organic acid stripping column can be recycled back to the gas/organic acid stripping column to reduce the amount of caustic solution used as well as the amount of waste.
This method can be used as part of a MEG regeneration and reclamation process to remove acetic acid and other short chain fatty acids described as organic acids from rich MEG. The method reduces the amount of blowdown required, reduces the overall MEG loss, and does not require boiling off, distillation or precipitation steps.
Unlike the boiling off method, this method requires no heat input, no vacuum conditions, and has relatively low MEG losses. Unlike the distillation method, this method requires a low energy input, no heat input, much less cooling duty, operates at a much lower and therefore safer temperature, and does not require additional large equipment items like a reboiler or condenser. Unlike the precipitation method, this method operates at a low pH (as normally seen in rich MEG) so pH adjustment is minimized and does not require a solid separation step which can be expensive, complicated and have a low reliability.
For the purpose of this disclosure, an “organic acid” is a carboxylic acid having a general formula R—C(O)OH, where R is H (formic acid), CH3 (acetic acid), CH3CH2 (propionic acid), or CH3(CH2)2 (butyric acid)) and where the total number of carbon atoms C is no greater than 4.
Referring to
The gas 23, 33 that has stripped the organic acid from the stream 21 exits a top end of the MEG/organic acid stripper column 25 as a gas stream 27 containing the organic acid. A rich MEG stream 29 without organic acid (or substantially free of organic acid) exits a bottom end of the column 25. “Substantially free of organic acid” means at least 90% of the acetic acid present in the rich MEG stream 21 is removed. If other organic acids are present in stream 21, preferably at least 50% of those organic acids are also stripped from the rich MEG stream 21.
The gas stream 27 containing the organic acids stripped from stream 21 is routed to a gas/organic acid stripper column 30 where the stream 27 is contacted by the solution 41. Solution 41 can be a dilute sodium hydroxide solution (e.g., 1.0 wt % NaOH and water) or its equivalent. A gas stream 31 without organic acid (or substantially free of organic acid) exits at the top end of the column 30. “Substantially free of organic acid” means at least 90% of the acetic acid is stripped from the gas stream 27 containing organic acids. Preferably at least 95% is removed.
A waste water stream 35 containing organic acid exits the bottom end of the gas stripper column 25. This stream 35 can be recycled to the gas/organic acid stripper column 30. The gas steam 31 can be recycled to the MEG/organic acid stripper column 25 as a recycle gas stream 33.
Preferably, the temperature range operated in is about 40° C. to 60° C., with 60° C. for the rich MEG stream 21 and gas stream 27 containing organic acids, 40° C. for the gas make-up and recycle streams 23, 33, and 40° for solution 41 and water streams. Those temperatures can vary depending on the application-specific requirements.
Referring to
The use of a caustic solution such as a dilute sodium hydroxide solution in the gas/organic acid stripper removes >99% of the acetic acid from the stripping gas. About 90% of the now substantially organic acid-free stripping gas is then recycled in the MEG/organic acid stripper column. This minimizes the amount of stripping gas make-up. A portion of the caustic solution can also be recycled to minimize the amount of caustic and water make-up as well as reduce the waste stream exiting the gas/organic acid stripper column.
The preferred embodiments described above and illustrated in the drawing figures provide examples of the system and method. The following claims define the inventive system and method and cover the full range of equivalents to which the recited elements of the claims are entitled.
This application is a continuation application of U.S. patent application Ser. No. 15/089,076, filed Apr. 1, 2016, the contents of which are hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15089076 | Apr 2016 | US |
| Child | 15675260 | US |
