Gas Dehydration Process

1. Field of the Invention

This invention relates to a process for removal of water from gases. More particularly, this invention relates to a process for the removal of water from natural gas utilizing a drying agent whereby the rate of aromatic compound absorption by the drying agent is reduced.

2. Description of Prior Art

Many gases, such as air, nitrogen and natural gas, contain water vapor in varying amounts depending upon the pressure and temperature of the gases. This water vapor can result in corrosion of and stoppages in valves and fittings in gas transmission systems, including the formation of ice or hydrates where the gas may be compressed or cooled. In order to prevent these potentially serious consequences, numerous processes have been developed for reducing or substantially eliminating the water vapor from said gases. See for example Kohl et at., "Gas Purification" 4th Edition, 1985, Gulf Publishing Company which provides a detailed summary of the problem of water vapor in natural gas and various means which have been developed for removing said water.

There are currently at least three known commercial methods for removing water vapor from gases: absorption by hygroscopic liquids; adsorption by activated solid desiccants; and condensation by compression and/or cooling.

Of these methods, the most prominent method currently in use is absorption by hygroscopic liquids and, in particular, liquid glycols having between two and eight carbon atoms. Such liquids are exemplified by ethylene glycol and polyglycols such as diethylene glycol and triethylene glycol, and blends of such compositions. These glycols are known to have unusual hygroscopicity, reasonably good stability with respect to heat and chemical decomposition, and relatively low vapor pressure.

In a typical glycol dehydration plant, a glycol stream containing about 1-5% water by weight is contacted with natural gas in a counter-current column. During the counter-current contact, the glycol stream removes water from the natural gas stream and the dilute glycol stream is passed to a reconcentration or regeneration process in which the absorbed water is removed, thereby enabling reuse of the glycol. Due to the high temperatures that may be involved in the reconcentration or regeneration process, triethylene glycol is sometimes preferred as the dehydration agent due to its greater thermal stability.

One problem with known processes for dehydration of natural gas is the undesirable tendency of the drying agent to absorb aromatic compounds which may be present in the natural gas.

Accordingly, it is an object of this invention to provide a process for removal of water from gases having improved efficiency over known processes.

It is another object of this invention to provide a process for removal of water from natural gas.

It is yet another object of this invention to provide a process for removal of water from gases whereby the rate of aromatic compound absorption by the drying agent is substantially reduced over known processes.

These and other objects of this invention are achieved by a process for removal of water from a gaseous fluid comprising contacting the gaseous fluid with a dehydrating composition comprising a glycol, at least one aromatics solubility depressant, and at least one dissolved salt comprising at least one alkali metal carboxylate. Examples of common carboxylates are formates, acetates, succinates and oxalates. The glycol is selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, and mixtures thereof.

The process of this invention improves the capacity of glycols to absorb moisture in a gas drying or dehydration process by use of a dissolved salt preferably selected from the group consisting of alkali metal acetates and alkali metal formates. We have found that the addition of potassium acetate or potassium formate in accordance with one embodiment of this invention to a glycol can increase the capacity of the glycol to absorb water from a gas stream by up to three times or more, depending upon the amount of salt added and the particular glycol utilized.

We have also found that alkali metal acetate and alkali metal formate salts not only increase the basic hygroscopicity of the glycols, but their presence also markedly reduces the solubility of the glycols for aromatic compounds normally found, for example, in natural gas. Such aromatic compounds as benzene, toluene, ethylbenzene, and xylene are frequently present in natural gas and are normally extracted and absorbed by glycols alone, thereby reducing their capacity to remove water from the gas stream. In addition, the presence of aromatic compounds, which may comprise 10-40% of the dehydrating composition, not only contaminates the glycols, but also complicates their reconcentration and introduces costly environmental problems related to disposition of the aromatics. We have found that certain neutral organic additives, or aromatics solubility depressants, added to the dehydrating composition decrease the rate of absorption of aromatics. Indeed, the decrease in absorption of aromatic compounds from the gaseous fluids being dehydrated is unexpectedly greater than would be expected based upon the decrease in aromatic absorption resulting from the use of potassium carboxylates or such neutral organic additives alone.

We have found that the three principal glycols normally contemplated for use in drying gaseous fluids, such as natural gas, even in an anhydrous state, have the ability to dissolve substantial amounts of alkali metal carboxylates, for example potassium carboxylates, which, in turn, increases the capacity of the glycols to absorb water. Table I hereinbelow shows the solubilities in glycols at 30.degree. C. of potassium acetate, potassium formate, sodium acetate, lithium acetate hydrate, and lithium acetate. From this data, it can be seen that the solubility of potassium acetate, potassium formate and sodium acetate in anhydrous ethylene glycol and anhydrous diethylene glycol is comparable. However, potassium formate is substantially more soluble in anhydrous triethylene glycol than potassium acetate and, thus, potassium formate is the preferred dissolved salt, particularly when triethylene glycol, which has been the preferred glycol for drying natural gas due primarily to its thermal stability on reconcentration or regeneration, is used.

It will be apparent to those skilled in the art that, for each particular gas stream analysis, an optimum combination of alkali metal salt and glycol can be predetermined and prepared for introduction into a counter-current dehydration or staged system. In accordance with one preferred embodiment of the process of the invention, it is preferred that the amount of dissolved salt in the dehydrating composition is less than about 33% by weight of said dehydrating composition. In accordance with a particularly preferred embodiment of this invention, based upon the data shown in Table I, the preferred dissolved salt is potassium formate in an amount comprising in the range of about 10% to 21% by weight of the dehydrating composition. Potassium formate, in particular, imparts an increased capacity to the substantially anhydrous glycol composition to absorb water and at the same time diminishes the ability of the solution to absorb aromatic compounds, such as benzene, as shown hereinbelow.

A typical counter-current system for dehydration of natural gas is taught by the Kohl et al. reference. However, it will be apparent to those skilled in the art that such a counter-current system could be used for drying other gases as well. Due to losses which may occur, monitoring of the concentrations and corresponding continuous additions may be necessary.

In most counter-current gas dehydration systems, the dehydration solvent composition is introduced at the top of a vertical dehydration column and withdrawn at the bottom of the column. Correspondingly, the water-containing gas is introduced at the bottom of the vertical dehydration column and removed at the top, thus passing upwardly through the descending dehydration solvent composition. Alternately, for specific purposes, a gas can be dehydrated by passing it through one or more stages containing a dehydrating solvent composition, such as disclosed by U.S. Pat. No. 4,979,965.

The temperature and pressure of the gas to be dehydrated can have an effect on the process of this invention. For example, for natural gas containing primarily methane, the temperature of the gas to be dehydrated will usually be within the range of about 85.degree.-105.degree. F., having been reduced from higher temperatures when discharged from its underground source. Pressure during dehydration is usually increased to between about 500-1,000 psi. At this temperature, the gas will contain about 0.5 to 5% by weight of water.

The following examples show the results of laboratory dehydration experiments, and illustrate the principles of the process of this invention, on air and methane, utilizing potassium acetate and potassium formate with diethylene glycol (DEG) compared to the use of triethylene glycol (TEG) alone, as the current industry standard dehydrating agent. In these example, the test gases were initially saturated with water at 21.degree. C. to avoid condensation in the lines when processed at 30.degree. C. (86.degree. F.). After saturation, the gases were passed through the respective solutions at atmospheric pressure, and the effluent analyzed. In selected instances, a second stage was utilized, but was found to have minimal, or insignificant, effect on dehydration. Table II shows the dehydration results obtained with air and Table III shows the dehydration results obtained with methane. In each case, the molar water content of air and methane were the same. The water weight percent difference is due to the difference in the respective molecular weights of air versus methane, that is 29 versus 16.

It is clear from the data shown in Table II that air dehydration was significantly enhanced from a third to three times by the use of a composition comprising diethylene glycol and a salt selected from the group consisting of potassium acetate and potassium formate, over the industrial standard triethylene glycol. Similarly, significantly improved results are shown in Table III for the dehydration of methane using a composition comprising diethylene glycol and potassium acetate over triethylene glycol alone. As can be seen, there is a 50% or greater improvement in the dehydration of methane using the diethylene glycol/potassium acetate solution compared to the industrial standard triethylene glycol.

Table IV provides data showing the superiority of a dehydration composition for dehydration of a gaseous fluid in accordance with the process of this invention comprising triethylene glycol and dissolved potassium formate as compared to pure triethylene glycol alone.

In virtually every instance, the amount of water remaining in the treated gaseous fluid after treatment with the anhydrous dehydration composition utilized in accordance with the process of this invention is substantially less than the water content of air treated with pure triethylene glycol alone.

U.S. Pat. No. 1,866,560 to Gordon et al. teaches the use of a solvent comprising 41.6% by weight calcium chloride, 38.4% by weight water, and 20.0% by weight glycerin for dehydrating methane. For comparison purposes, respective percentages of water shown were added, notwithstanding the large amount of initial water required by the Gordon solvent, with the result shown in Table V.

It is clear from a comparison between the data shown in Table V for a Gordon-type solvent and the data shown in Tables I-IV for a glycol/potassium carboxylate composition in accordance with the process of this invention that the process of this invention is substantially more effective in dehydrating methane than the Gordon-type solvents.

Although potassium acetate, potassium formate, sodium acetate, lithium acetate hydrate and lithium acetate enhance the ability of ethylene glycol, diethylene glycol, and triethylene glycol to dehydrate gases, the preferred solvent in accordance with the process of this invention is triethylene glycol. As previously indicated, triethylene glycol has good stability under the conditions of regeneration and superior ability, particularly in combination with potassium formate, within designated ranges, to remove water from gases. In addition, its use can reduce the expensive dehydration by 75% or more over known solvents. For initial introduction into a counter-current system used in accordance with the process of this invention, the triethylene glycol solvent is substantially anhydrous. In accordance with a particularly preferred embodiment, the preferred dehydrating composition comprises in the range of about 15 to 20 weight percent of potassium formate, for maximum efficiency. In addition, it is important that the process be controlled so that the water content of the effluent does not exceed about 15% by weight of water, thereby establishing an overall water range within the system of between about 1 and 15% by weight. When water is present in amounts above about 15% by weight, the ability of the dehydrating composition to dehydrate starts to diminish rapidly.

Also as previously stated, the use of alkali metal carboxylates in combination with a glycol in accordance with the process of this invention, reduces the absorption of the aromatics, including benzene, toluene, ethylbenzene and xylene, which occur in natural gas. For example, benzene is soluble in triethylene glycol in all proportions in diethylene glycol in the amount of about 31% by weight. In the presence of 30% by weight of potassium acetate in diethylene glycol, the solubility of benzene is reduced to approximately 14%, and with the addition of 5% by weight of water to a diethylene glycol/potassium acetate solvent, the solubility of benzene is reduced to about 10% or less. As the solvent composition approaches 15% by weight of water, the solubility of aromatics is reduced to approximately 2%. The rate of absorption is, of course, reduced proportionally. Accordingly, regeneration of the solvent composition in the absence of aromatics is substantially improved and, correspondingly, environmental problems associated with the limitation and disposing of aromatics during the generation process are eliminated.

Diphenylmethane (DPM) and 1,1-diphenylethane (DPE) alone or in conjunction with potassium carboxylates are taught by our recently issued U.S. Pat. No. 5,462,584 as lowering the solubility of aromatics in glycols such as triethylene glycol, thereby decreasing the rate of absorption of aromatics from gas streams treated with these glycols. However, diphenylmethane, for example, tends to be stripped from the water-laden triethylene glycol solution during triethylene glycol regeneration. Because triethylene glycol is used in a closed loop and thus, must be regenerated by distillation, the use of diphenylmethane as an additive would require additional equipment for recovery and reuse.

We have discovered neutral organic additives, or aromatics solubility depressants, which are not stripped from the solution during distillation to remove water from the glycol solution and which, when used in combination with alkali metal carboxylates such as potassium carboxylates, exhibit a synergistic effect in reducing the solubility of aromatics in glycols and, thus, reduce the absorption of aromatics by said glycol solutions. In particular, we have discovered that aromatics solubility depressants, such as biphenylphenyl ether and MARLOTHERM-S, a commercially available heat transfer fluid (dibenzyltoluene), when dissolved in a dehydrating composition comprising a glycol selected from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol and mixtures thereof, and a dissolved salt comprising at least one alkali metal carboxylate results in a reduction of the solubility of aromatics, such as benzene, substantially greater than one would expect based upon the reduction in solubilities resulting from glycol solutions containing either an alkali metal carboxylate or one of these aromatics solubility depressants alone.

Table VI shows the solubility of benzene and toluene in various diethylene glycol-salt solutions. Tables VII and VIII show the depression of benzene solubility in triethylene glycol by various aromatics solubility depressants. A positive additive effect is defined as any reduction in benzene solubility over that observed without the additive. Diphenylmethane and 1,1,-diphenylethane, while effective as aromatics solubility depressants, are generally not suitable due to their relatively high volatility. By comparison, the high molecular weight products, MARLOTHERM-S, DOWTHERM "G", and NOVOLAK are all seen as being effective additives. MARLOTHERM-S, having a boiling point of 390.degree. C., resists stripping during distillation of water from the triethylene glycol solution and is also a neutral hydrocarbon. Study of MARLOTHERM-S has also led to the discovery of a synergistic effect, particularly when used with potassium formate.

We have found that a solution (Solution "A") comprising by weight in the range of about 0-40% diphenyl ether, 5-65% biphenylphenyl ether and 0-35% polyphenylphenols is non-volatile with water at 100.degree. C., has a good solubility, even in the presence of water, and gives good results, even without salt. Table IX shows the solubility depression of benzene, toluene, and xylene in triethylene glycol with potassium formate and Solution "A". It can be seen that Solution "A" is most effective at a level of about 15% by weight water.

Table X shows the depression of benzene solubility in triethylene glycol with potassium formate and MARLOTHERM-S under varying conditions. Lines 1-3 show the effect with no additives; lines 4-9 show the effect of MARLOTHERM-S alone; lines 10-13 show the effect of potassium formate at 20.0 wt. percent; and lines 14-17 show the effect of potassium formate and MARLOTHERM-S in combination. It is clear from the data in lines 14-17 that the combination of additives produces a result substantially greater than the additive effect of the individual components.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.