Process for dehydrating natural gas

A process for dehydrating natural gas involving the use of a water absorbing composition containing a glycol and an aromatic solubility depressant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improvement in the drying of natural gas. More 
particularly, it relates to reducing the absorption of aromatics in the 
dehydrating solvent and, correspondingly, reducing the emission of 
absorbed aromatics when the solvent is regenerated. 
2. Description of Related Art 
Natural gas, as it is obtained from underground sources, contains water in 
varying amounts. This water can lead to corrosion and obstruction of 
valves and fittings in transmission lines due to condensation, etc., 
especially when the gas is compressed or cooled. A variety of processes 
have been developed to reduce the water content. 
A reasonably detailed summary of the water vapor problem in natural gas and 
various means which have been proposed for removing water from natural gas 
is disclosed in chapter 11, beginning at page 582 of the publication GAS 
PURIFICATION, Fourth Edition, 1985, Gulf Publishing Company, Kohl & 
Riesenfeld. 
One of the more prominent methods for dehydrating natural gas involves the 
use of hygroscopic liquids, whereby the gas is passed in contact with 
liquid glycols, for example, such as ethylene glycol, and polyglycols, 
namely diethylene glycol and triethylene glycol and blends of such 
compositions at temperatures of approximately 85.degree.-105.degree. F. As 
indicated in the foregoing publication, these glycols have unusual 
hygroscopicity and reasonably good stability to heat and chemical 
decomposition, together with relatively low vapor pressure. After 
dehydration the solvents are regenerated by removing absorbed water and 
then reused. 
In our copending application entitled Gas Dehydration Process, Ser. No. 
08/215,757 filed Mar. 21, 1994, we have disclosed and claimed a process 
wherein a composition consisting essentially of a soluble salt from the 
class consisting of potassium acetate or potassium formate and a glycol 
can be used to enhance the absorption of water from natural gas. In 
particular, it has been found that with the addition of one of the 
aforementioned salts to a glycol, such as diethylene glycol, and in the 
presence of a relatively small amount of water in the solution the 
absorption of water from natural gas is improved by from approximately a 
third to three times. With other glycols the amount of potassium acetate 
or formate solubility will vary, however, with corresponding improvements 
in water absorption. For example, triethylene glycol will only absorb 
about 4% by weight of the potassium salt whereas ethylene glycol will 
absorb about 40% by weight, with corresponding differences in water 
absorbency. 
Notwithstanding the improved dehydration of natural gas with the use of 
glycols and potassium acetate or formate, a problem is always present with 
respect to the concurrent absorption from natural gas in the dehydration 
solution of aromatics, such as benzene, toluene, ethylbenzene and xylene 
and to some extent aliphatic hydrocarbons. In regeneration of the dilute 
dehydration solvent after absorbing water from the gas, the extraction of 
the concurrently absorbed aromatics poses a serious environmental problem. 
In the generation process the solvent is concentrated and the aromatics 
are discharged into the atmosphere. With millions of cubic feet of gas 
being dehydrated annually, thousands of tons of aromatics are discharged, 
exceeding environmental regulations. The preferred process would permit 
absorption of water from the gas while allowing the aromatics to remain 
with the gas for subsequent combustion with the gas. With the use of 
potassium acetate or formate in a glycol the absorption of the aromatics 
is reduced. However, further reduction of aromatics is essential to avoid 
as much as possible the need to dispose of the aromatics during 
regeneration of the solvent. 
SUMMARY OF THE INVENTION 
It has been found that by the use of relatively small amounts of either 
diphenylmethane or 1,1-diphenylethane in either a glycol alone or a glycol 
with potassium acetate or potassium formate the reduction of absorption of 
aromatics in a water absorption composition for natural gas can be 
appreciably reduced. These particular compositions are especially suitable 
due to their high boiling points and relatively low melting points as 
regards the operating temperatures for the water absorption process of 
natural gas and the solvent regeneration process.

DESCRIPTION OF PREFERRED EMBODIMENTS 
It has been found that benzene is soluble to the extent of about 31% by 
weight in pure diethylene glycol (up to 2% water). With the addition of 
approximately 15% by weight of diphenylmethane to pure diethylene glycol 
benzene solubility is reduced to about 4% by weight. Correspondingly, 
diethylene glycol containing 30% by weight of potassium acetate reduces 
the benzene solubility to about 14% by weight and with the addition of 
approximately 6% of diphenylmethane to this glycol--30% potassium acetate 
solution the solubility of benzene in the solution is reduced to about 
2.5% by weight. In the presence of between about 5-15% by weight of water 
the solubility of benzene is likewise further reduced. Following is a 
table, containing examples illustrating the results of benzene solubility 
in various compositions employed for absorption of water from natural gas. 
______________________________________ 
DEPRESSION OF SOLUBILITY OF 
BENZENE (Bz) IN DIETHYLENE GLYCOL 
ALONE -- WITH WATER -- WITH POTASSIUM 
ACETATE (KOAc) AND WITH DIPHENYLMETHANE 
(DPM) AT ROOM TEMPERATURE (30.degree. C.) 
KOAc H.sub.2 O DPM BzSol 
Ex (wt %) (wt %) (wt %) 
(wt %) 
______________________________________ 
1 -- -- -- 31 
2 -- 5.0 -- 18 
3 -- 5.0 6.0 3 
4 30 -- -- 14 
5 30 -- 5.7 2.5 
6 30 5.0 -- 9.6 
7 30 5.0 3.0 0.7 
______________________________________ 
Although natural gas can be dehydrated by passing it through one or more 
stages of dehydrating composition, the normal method is to pass the gas 
through a counter current zone or system with the dehydrating composition. 
Such a process is shown and described on page 589 of the aforementioned 
publication GAS PURIFICATION. The use of such a counter current system 
with a glycol dehydrating composition containing up to about 33% potassium 
acetate or formate and between about 5%-15% by water is likewise disclosed 
in our aforementioned copending application. 
In the preferred process a water dehydrating composition, consisting 
essentially of a glycol, such as diethylene glycol with 25-33% by weight 
of potassium acetate and about 0.5 to 5% by weight of water, is introduced 
into the top of a counter current system. Natural gas containing up to 
about 5% by weight of water is introduced into the counter current system 
at the bottom. To achieve maximum dehydration the process should be 
controlled so as to limit the effluent of the water dehydration 
composition to approximately 15% by weight of water, again as disclosed in 
our aforementioned application. 
In order to obtain the desired depression of aromatic solubility while 
maximizing water removal from the gas, the water dehydrating composition 
or solvent initially introduced at the top of a counter current system 
should contain generally between about 1%-6% by weight of either 
diphenylmethane or 1,1-diphenylethane as an aromatic depressant. It will 
be appreciated that the aromatic depressants proposed herein will be 
adjusted for each particular gas source and dehydration process involving 
any of the various glycols cited with or without the potassium salts. 
The use of the aromatic solubility depressants disclosed not only reduces 
the cost of regeneration of water solvent but also reduces the cost and 
difficulty of disposal of otherwise absorbed aromatics during or 
subsequent to the regeneration process. When dealing with millions of 
cubic feet per day the savings can be formidable. 
Having described the invention and certain embodiments thereof the same is 
only intended to be limited by the scope of the following claims.