Process for removing contaminants from dialkyl ethers of polyalkylene glycols

A process is described for removing contaminants from a contaminant-laden solution of dialkyl ethers of polyethylene glycols (polyether solvent) by mixing the solution with an aqueous base solution. The mixture produces at least two liquid phases of different densities, which are then separated from each other. The lighter liquid phase is predominantly the polyether solvent while the heavier liquid phase is predominantly the aqueous base solution and the contaminants. The process is particularly useful in removing triethylene glycol and other glycol-based dehydration solvents.

FIELD OF THE INVENTION 
This invention relates to a process for removing contaminants from dialkyl 
ethers of polyalkylene glycols. More specifically, it relates to an 
extraction process using an aqueous base solution to extract contaminants 
from a solution containing contaminants and dialkyl ethers of polyalkylene 
glycols. 
BACKGROUND OF THE INVENTION 
Natural gas that is produced from subterranean reservoirs usually contains 
mixtures of hydrocarbon gases (principally methane and ethane) and it may 
contain appreciable quantities of nonhydrocarbon gases (nitrogen, helium, 
water vapor, carbon dioxide and hydrogen sulfide). For efficient 
transportation and processing of the natural gas, it is frequently 
necessary to remove one or more of the nonhydrocarbon components. 
Water vapor contained in natural gas is often removed from the gas in the 
vicinity of where the gas is produced so that the gas can be more 
economically and efficiently transported, stored and processed. If the 
water is not removed, if may accelerate pipeline and equipment corrosion 
and it may freeze or form hydrates and thereby plug pipelines, valves and 
orifices. 
In a well known and commonly used process for dehydrating natural gas, the 
wet gas is contacted with a glycol such as triethylene glycol (TEG) in a 
countercurrent absorption column. Water vapor is absorbed by TEG as the 
gas flows up the absorption column countercurrent to the glycol flowing 
down the column. The TEG loaded with water is then heated to remove the 
water from it, and the lean TEG is recycled for mixing with wet gas in a 
continuous process. Although TEG has a low vapor pressure at temperatures 
typically used in this dehydration process, a small amount of TEG 
vaporizes or is entrained, thereby becoming a trace component of the 
natural gas mixture. 
Acid gases (principally CO.sub.2 and H.sub.2 S) present in the natural gas 
are removed to produce a "sweet" gas which will not interfere with further 
processing of the gas and will meet customer specifications. One process 
to remove acid gases from natural gas involves contacting the natural gas 
with a liquid solvent containing dialkyl ethers of polyalkylene glycol. 
One such polyether is the dimethyl ether of triethylene glycol. Mixtures 
of such polyether solvents are sold by Norton Company under the trademark 
SELEXOL.RTM.. 
In a process for removing acid gases from natural gas, the natural gas is 
mixed with a polyether solvent in a conventional countercurrent absorption 
column under superatmospheric conditions. The solvent absorbs most of the 
H.sub.2 S and CO.sub.2, and it absorbs essentially all of the triethylene 
glycol and other contaminants previously introduced into the gas. The 
solvent containing dissolved H.sub.2 S and CO.sub.2 may then be 
regenerated by flashing in a series of flashing steps, followed by 
stripping to remove H.sub.2 S, and then recycled to the top of the 
absorption column for reuse. For economic reasons, it is desirable to 
recycle as much of the solvent as possible, thereby minimizing the need to 
add fresh solvent. 
One problem with recycling the polyether solvent is that contaminants such 
as triethylene glycol (from upstream dehydration) and corrosion inhibitors 
(used in the transportation of the gas through pipelines) accumulate 
during extensive recycling of the solvent. The accumulation of these 
contaminants may significantly reduce the efficiency of the solvent to 
remove acid gases. In addition, some of the contaminants may precipitate 
out of the solvent and deposit on process equipment such as pumps, piping 
and heat exchangers. This loss of efficiency can increase the operating 
expense of acid gas removal and it may decrease the gas capacity of the 
gas processing facility. 
Distillation removal of triethylene glycol from polyether solvents such as 
SELEXOL.RTM. is difficult because triethylene glycol has approximately the 
same vapor pressure as some of the components of SELEXOL.RTM.. 
There is a need for a simple process for removing contaminants from a 
polyether solvent to regenerate the solvent for longer usage. Removal of 
contaminants will reduce the need to periodically replace contaminated 
solvent with fresh solvent. 
SUMMARY OF THE INVENTION 
The present invention provides a process for removing contaminants from a 
solvent in which dialkyl ethers of polyalkylene glycols are the solvent's 
principal components. In this process, the solvent is mixed with an 
aqueous base solution. The mixture produces at least two liquid phases 
having different densities. The lighter phase contains solvent as the 
principal component and the heavier phase contains aqueous base solution 
and the contaminants as principal components. The lighter phase may be 
separated from the heavier phase using any conventional phase separation 
process. The lighter phase, substantially free of contaminants, may be 
used again without further treatment. However, if the lighter phase 
contains unwanted ions or particulates or if its pH is unacceptably high, 
the lighter phase may be further processed. Such further processing may 
include, but not limited to, centrifugation, neutralization, filtration, 
and ion exchange. 
The base constituent of the aqueous base solution used in the process of 
this invention may be any base which when mixed with the solvent solution 
yields at least two phases having different densities. The pH of the 
aqueous solution should be higher than about 11 and preferably be higher 
than about 13. Sodium hydroxide is a preferred base. 
The process of this invention is particularly useful in removing 
triethylene glycol from polyether solvents that are used for absorption of 
acid gases from feedstocks such as natural gas, synthetic natural gas, 
ammonium gas and refining gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides for the removal of contaminants from a 
solution containing the contaminants and dialkyl ethers of polyalkylene 
glycols. An aqueous base is mixed with the solution to induce the 
formation of one or more additional liquid phases having different 
densities. The lighter phase, comprising as a principal component dialkyl 
ethers of polyalkylene glycols, is then separated from the heavier 
phase(s). 
The present invention is applicable to extraction of contaminants from 
polyether solvents represented by the formula: 
EQU R.sub.1 O (R.sub.3).sub.x R.sub.2 formula 1 
where R.sub.1 and R.sub.2 can be identical or different and each is a 
linear or branched-chain alkyl group, R.sub.3 is an alkylene group having 
at least 2 carbon atoms, and x ranges from 1 to 10. Nonlimiting examples 
of suitable polyether solvents include dimethyl, diethyl, dipropyl and 
dibutyl ethers of ethylene, diethylene, triethylene, tetraethylene, 
pentaethylene, hexaethylene and heptaethylene glycols, and mixtures of 
such solvents. Particularly applicable solvents used in the process of 
this invention comprise mixtures of dialkyl ethers of polyalkylene glycols 
of the formula: CH.sub.3 O (C.sub.2 H.sub.4 O).sub.x CH.sub.3 where x is 
in the range from 3 to 9. 
In the practice of this invention the solvent may also contain minor 
amounts of hydrocarbons, carbon dioxide, hydrogen sulfide, and other 
compounds absorbed by the solvent in a gas absorption process. For 
convenience, a solvent solution containing as a principal component 
dialkyl ethers of polyalkylene glycols represented by formula 1 above will 
be designated herein as a "polyether solvent". 
The aqueous base solution used in the process of this invention may 
comprise any water soluble compound which undergoes ionization in an 
aqueous solution to produce hydroxyl ions (OH.sup.-) in considerable 
concentration. Preferred bases are water soluble alkali hydroxide 
compounds. Examples of suitable bases include ammonium hydroxide, alkaline 
metal hydroxides such as sodium hydroxide and potassium hydroxide, and 
alkaline earth hydroxides such as calcium hydroxide. Sodium hydroxide is 
particularly preferred because it tends to be less expensive and is 
generally more readily available than other strong bases. 
The pH of the aqueous base solution used in this invention should be high 
enough to form at least two distinct phases when mixed with the polyether 
solvent. The inventors have observed that phase separation becomes less 
distinct as the pH of the aqueous base solution falls below about 11. The 
pH of the aqueous base solution should therefore be greater than about 11 
and preferably greater than 13. An aqueous base solution containing from 
about 30 to 60 weight percent alkali metal hydroxide such as sodium 
hydroxide is suitable in practicing this invention. 
The amount of base solution to mix with the polyether solvent can vary 
widely and will depend upon the amount of polyether solvent to be treated, 
the mixing time, and pH of the base solution. The amount of base solution 
should be sufficient to form at least two separate phases after mixing and 
sufficient to absorb the contaminants from the solvent. For illustration 
purposes only, a base to solvent ratio of about 1:100 to about 1:1, 
preferably about 1:25 to about 1:15, may be used. 
Contaminants removed from the polyether solvent include polar compounds 
that are miscible in the aqueous base solution and compounds that react 
with the base to produce ionized solids or polar liquids. Polar liquids 
will generally be more miscible in the aqueous base solution than in the 
polyether solution. Ionized solids and polar liquids heavier than the 
polyether solution may be separated following mixing by conventional 
gravity separation techniques or filtration. Reaction precipitates that 
remain polyether solvent may be removed by filtration. Nonlimiting 
examples of contaminants that may be removed from a polyether solvent in 
accordance with this invention include alcohols such as methanol and 
ethanol, glycols such as triethylene glycol and diethylene glycol, and 
polar aromatic hydrocarbons such as phenol and aniline. 
The process of this invention may be carried out within a wide range of 
temperatures and pressures. The pressures and temperatures should be 
selected that do not degrade the polyether solvent. Pressure-temperature 
relationships suitable in the practice of this invention are well 
understood by those skilled in the art, and need not be detailed herein. 
Mixing of the solvent with the base solution is performed until intimate 
contact of the solvent and base have been effected. For purposes of 
illustration only, a mixing time of 1 minute to 5 minutes for mixing 
twenty liters of solvent with one liter of a sodium hydroxide solution 
should be sufficient. 
The formation of multiple phases after mixing of a polyether solvent and an 
aqueous base and the preferential miscibility of certain contaminants may 
be due to a combination of phase behavior and chemical reaction phenomena. 
While not wishing to be bound by any particular theory, the following is 
offered as one explanation. Polyether solvents are generally slightly 
polar compounds that are highly miscible in water and most hydrocarbons. 
Polyether solvents are believed to be miscible in water because their 
ether groups hydrogen bond with the protons of water. Polar compounds such 
as triethylene glycol are miscible in water and polar organic solvents. 
When a polyether solvent is mixed with the aqueous base solution, the 
strong base reduces the number of protons in the aqueous solution that the 
solvent's ether groups can bond with, thus reducing the solvent's 
solubility in the aqueous base solution. On the other hand, a polar 
contaminate such as triethylene glycol stays in the aqueous solution since 
it reacts, ionizes, and/or hydrogen bonds with the base. 
The process of the present invention may be better understood by referring 
to the Drawing, which is a block flow diagram illustrating one method of 
the present invention. A polyether solvent solution containing 
contaminants and an aqueous base solution enter a static mixer 10. The 
mixer 10 provides intimate mixing of the aqueous base solution and 
polyether solution. Although only one static mixer is shown in the 
Drawing, more than one may be used. The mixture then flows through line 3 
to a separator 20 suitable for separating liquid phases having different 
densities. The phase separation may be performed in a batch or continuous 
process, and may be performed using a settling tank or a centrifuge. The 
heavier liquid phase(s), consisting predominantly of the 
contaminant-bearing aqueous base solution, is removed from the separator 
for further handling. Optionally, the contaminants may be removed from the 
base solution by processes well known to those skilled in the art so that 
the base can be reused in the process of this invention. 
The lighter fluid phase, consisting predominantly of the polyether solvent, 
may be sent directly to a gas processing facility (not shown in the 
Drawing) for reuse. Depending on the solvent's intended use, it may be 
desirable to further treat the solvent to remove particulates, adjust the 
pH or remove alkali ions. Nonlimiting examples of further treatment 
include any one or more of the following: neutralization, filtration, 
centrifugation, and ion exchange. For example, a portion of the lighter 
phase may be filtered (or centrifuged) to remove solids, it may be 
neutralized to reduce the pH and then filtered, it may be deionized by any 
ion exchanger, or it may be neutralized, filtered and then deionized. The 
Drawing illustrates all of these additional treatments except 
centrifugation. 
Referring again to the Drawing, at least a portion of the polyether solvent 
(the lighter phase) is sent from the separator 20 via line 5 to a 
neutralization vessel 30 to reduce the solvent's pH to a desired level. 
Reducing the solvent's pH to approximately the same pH as the 
contaminant-laden feed stream is preferred. Any convenient neutralizing 
agent may be used, including carbon dioxide and an aqueous acid. 
The neutralized solvent is then passed through line 7 to a filter 40 for 
removal of solids. Particulates may have been present in the feed-solvent 
stream or they may have been formed as reaction byproducts during mixing 
of the base solution and contaminant-laden solvent and during 
neutralization of the solvent following mixing. 
Following neutralization and filtration, if the polyether solvent solution 
still contains an unacceptably high alkali ion concentration, the solvent 
may be passed through line 9 to an ion exchanger 50 for further 
deionization. For example, if NaOH is used as a base, Na.sup.+ ions 
remaining in the solvent may be removed by the ion exchanger 50. The 
solvent leaving the ion exchanger will be essentially free of both 
contaminants and alkali ions. 
All the steps of the present invention (mixing and separation and, 
optionally, neutralization, filtration and ion exchange) may be performed 
using conventional equipment. However, since the aqueous base solution is 
corrosive, the equipment should be made of corrosion resistant materials. 
EXPERIMENTAL TEST 
This invention is further illustrated by the following laboratory 
equipment, which demonstrates the operability of the invention and is not 
intended as limiting the scope of the invention as defined in the appended 
claims. 
In this experiment, 174 grams of a solution containing 87.7 weight percent 
SELEXOL.RTM., 9.5 weight percent triethylene glycol and 2.8 weight percent 
water and other contaminants was thoroughly mixed with 15.4 grams of a 50 
weight percent sodium hydroxide solution at 23.degree. C. and atmospheric 
pressure. After mixing, the mixture split into two distinct liquid phases 
with 154.4 grams in the top (SELEXOL.RTM. rich) phase. The two phases were 
separated by decantation and analyzed. The top phase contained about 96.5% 
of the original amount of the SELEXOL.RTM., and about 95% of the 
triethylene glycol has been removed. Compositions in weight percent of the 
two phases as measured by a gas chromatograph are presented in Table 1 
below. 
TABLE 1 
______________________________________ 
Top Bottom 
Component Phase (wt %) 
Phase (wt %) 
______________________________________ 
SELEXOL .RTM. 98.0 0.4 
Triethylene glycol 
0.5 49.0 
Water & other 1.5 50.6 
contaminants 
______________________________________ 
The principle of the invention and the best mode contemplated for applying 
that principle have been described. It will be apparent to those skilled 
in the art that various changes may be made to the embodiments described 
above without departing from the spirit and scope of this invention as 
defined in the following claims. For example, additional equipment such as 
centrifuges, filters, and holding tanks may be introduced into the process 
for reasons of efficiency, economy, control or safety. It is, therefore, 
to be understood that this invention is not limited to the specific 
details shown and described.