FIELD OF THE INVENTION

The present invention relates to a composition comprising an aqueous solution of piperazine and an alkanolamine, preferably 3-(dimethylamino)-l,2-propanediol, and a process for using said aqueous composition for removing acid gases including H ₂ S, from gaseous mixtures containing H ₂ S.

BACKGROUND OF THE INVENTION

Fluid streams derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of acid gases, for example carbon dioxide (C0 ₂ ), hydrogen sulfide (H ₂ S), sulfur dioxide (S0 ₂ ), carbon disulfide (CS ₂ ), hydrogen cyanide (HCN), carbonyl sulfide (COS), or mercaptans as impurities. Said fluid streams may be gas, liquid, or mixtures thereof, for example gases such as natural gas, refinery gas, hydrocarbon gasses from shale pyrolysis, synthesis gas, and the like or liquids such as liquefied petroleum gas (LPG) and natural gas liquids (NGL).

Various compositions and processes for removal of acid gasses are known and described in the literature. It is well-known to treat gaseous mixtures with aqueous amine solutions to remove these acidic gases. Typically, the aqueous amine solution contacts the gaseous mixture comprising the acidic gases counter currently at low temperature or high pressure in an absorber tower. The aqueous amine solution commonly contains an alkanolamine such as triethanolamine (TEA), methyldiethanolamine (MDEA), diethanolamine (DEA), monoethanolamine (MEA), diisopropanolamine (DIPA), or 2-(2- aminoethoxy)ethanol (sometimes referred to as diglycolamine or DGA). In some cases, an accelerator, is used in combination with the alkanolamines, for example piperazine and MDEA as disclosed in USP 4,336,233; 4,997,630; and 6,337,059, all of which are incorporated by reference herein in their entirety. Alternatively, EP 0134948 discloses mixing an acid with select alkaline materials such as MDEA, to provide enhanced acid gas removal.

Tertiary amines, such as 3-dimethylamino-l,2-propanediol (DMAPD), have been shown to be effective at removing C0 ₂ from gaseous mixtures, see USP 5,736,116.

Further, in specific processes, e.g., the Girbotol Process, tertiary amines have been shown effective in removal of H ₂ S, but show decreased capacity at elevated temperatures, for examples see "Organic Amines-Girbotol Process", Bottoms, R.R., The Science of Petroleum, volume 3, Oxford University Press, 1938, pp 1810-1815.

While the above compounds are effective, they each have limitations which detract from their universal use. In particular, it would be desirable to have and aqueous composition comprising an alkanolamine for removing acid gases including H ₂ S from a gaseous mixture and/or an aqueous alkanolamine solution which is efficient at removing acid gases at a commercially viable capacity when the aqueous solution is used at an elevated temperature, for example above 140°F.

As such, there is a need for an aqueous absorbent composition and method to use said composition, which is effective at removing acid gases including hydrogen sulfide from gaseous mixtures, preferably at elevated operating temperatures.

SUMMARY OF THE INVENTION

The present invention is an aqueous alkanolamine solution composition and process using said aqueous alkanolamine solution composition for removing acid gases including hydrogen sulfide through contact with gaseous mixtures containing hydrogen sulfide, preferably wherein the temperature of the aqueous alkanolamine solution is equal to or greater than 140°F, said composition comprising (i) an amino compound, preferably in an amount of from 0.1 to 75 weight percent, having the general formula:

R^NCHzCIKOITlCHzOH (1) wherein R ^] and R ² independently represent lower alkyl groups of 1 to 3 carbon atoms, for example, methyl, ethyl, propyl, and isopropyl groups, more preferred R ¹ and R ² groups include methyl and ethyl groups, especially preferred amino compounds include

3-(dimethylamino)-l,2-propanediol in which R ¹ and R ² are both methyl groups, and 3-(diethylamino)-l,2-propanediol in which R ¹ and R ² are both ethyl groups; (ii) piperazine, preferably in an amount of from 0.1 to 15 weight percent; and (iii) optinally a physical solvent, preferably selected from cyclotetramethylenesulfone, dimethyl ethers of polyethylene glycol, l,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone, N-formyl- morpholine, N-acetylmorpholine, triethylene glycol monomethyl ether, or mixtures thereof, wherein weight percents are based on the total weight of the aqueous alkanolamine solution, wherein said aqueous alkanolamine solution does not contain an acid having a pKa of 8 or less or an acid-forming material capable of forming in aqueous medium an acid having a pKa of 8 or less.

In one embodiment of the present invention, the amino compound (i) preferably is 3-(dimethylamino)- 1 ,2-propanediol or 3-(diethylamino)- 1 ,2-propanediol.

In one embodiment of the present invention, the process further comprises the step of steam stripping the aqueous alkanolamine solution such that an acid gas-lean aqueous alkanolamine solution is formed which may be used in said contacting step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process flow diagram of an absorption process according the present invention.

FIG. 2 is a plot of H ₂ S concentration in a cleaned gas mixture versus the absorbent circulation rate.

DETAILED DESCRIPTION OF THE INVENTION

The aqueous alkanolamine solution of the present invention comprises an amino compound and piperazine. The amino compounds useful in the aqueous alkanolamine solutions of the present invention have the general formula:

R^NCHzCHCOH^HzOH (1) wherein R ^] and R ² independently represent lower alkyl groups of 1 to 3 carbon atoms, for example, methyl, ethyl, propyl, and isopropyl groups. More preferred R ¹ and R ² groups include methyl and ethyl groups. Especially preferred amino compounds include 3-(dimethylamino)-l,2-propanediol in which R ¹ and R ² are both methyl groups, and 3-(diethylamino)-l,2-propanediol in which R ¹ and R ² are both ethyl groups.

The aqueous alkanolamine solution of the present invention contains the alkanolamine in an amount equal to or greater than 0.1 weight percent, preferably equal to or greater than 5 weight percent, more preferably equal to or greater than 10 weight percent and even more preferably equal to or greater than 20 weight percent wherein weight percent is based on the total weight of the solution. The aqueous alkanolamine solution of the present invention contains the alkanolamine in an amount equal to or less than 75 weight percent, preferably equal to or less than 65 weight percent, more preferably equal to or less than 55 weight percent and even more preferably equal to or less than 50 weight percent wherein weight percent is based on the total weight of the solution.

The aqueous alkanolamine solution of the present invention contains piperazine in an amount equal to or greater than 0.1 weight percent, preferably equal to or greater than 1 weight percent, more preferably equal to or greater than 2 weight percent wherein weight percent is based on the total weight of the aqueous solution. The aqueous alkanolamine solution of the present invention contains piperazine in an amount equal to or less than 20 weight percent, preferably equal to or less than 15 weight percent, more preferably equal to or less than 10 weight percent and even more preferably equal to or less than 8 weight percent wherein weight percent is based on the total weight of the solution.

The aqueous absorbent composition of the present invention may optionally contain one or more additional amino compound. Preferably, the additional amino compound is a different or second alkanolamine not described by formula (1) herein above, such as tris(2- hydroxyethyl) amine (triethanolamine, TEA); tris(2-hydroxypropyl)amine (triisopropanol); tributanolamine; bis(2-hydroxyethyl)methylamine (methyldiethanolamine, MDEA);

2-diethylaminoethanol (diethylethanolamine, DEEA); 2-dimethylaminoethanol

(dimethylethanolamine, DMEA); 3-dimethylamino-l-propanol; 3-diethylamino-l-propanol; 2-diisopropylaminoethanol (DIEA); N,N-bis(2-hydroxypropyl)methylamine

(methyldiisopropanolamine, MDIPA); Ν,Ν' -bis(2-hydroxyethyl)piperazine

(dihydroxyethylpiperazine, DiHEP); diethanolamine (DEA); 2-(tert-butylamino)ethanol; 2-(tert-butylaminoethoxy)ethanol; or 2-amino-2-methylpropanol (AMP), 2-(2-amino- ethoxy)ethanol.

Preferred additional amino compounds comprise one or more tertiary amino group. Preferably the additional amino compound has one or more sterically hindered amino group. An aqueous absorption composition comprising a 1 -hydroxy ethyl-4- pyridnlypiperazine compound and an amine having one or more sterically hindered amino group is particularly suitable for the removal of ¾S.

If present, the amount of optional amino compound in the aqueous alkanolamine solution may range from equal to or greater than 0.1 weight percent, preferably equal to or greater than 1 weight percent, more preferably equal to or greater than 5 weight percent based the total weight of the solution. If present, the amount of optional amino compound in aqueous alkanolamine solution may range from equal to or less than 75 weight percent, preferably equal to or less than 50 weight percent, more preferably equal to or less than 25 weight percent based the total weight of the solution.

The temperature of the aqueous alkanolamine solution which is brought into contact with the gas to be treated is equal to or greater than 120°F, preferably equal to or greater than 130°F, more preferably equal to or greater than 140°F, and even more preferably equal to or greater than 150°F.

In addition to the amino compound and piperazine, the aqueous alkanolamine solution may comprise one or more other compounds used in fluid treatment following well known practices. Illustrative compounds which may optionally be provided include, but are not limited to, one or more of the following: antifoaming agents; physical solvents including glycols and the mono-and di-ethers or esters thereof, aliphatic acid amides, N-alkylated pyrrolidones, sulfones, sulfoxides and the like; antioxidants; corrosion inhibitors; film formers; chelating agents such as metals; pH adjusters such as alkali compounds; and the like. The amount of these optional components is not critical but may be provided in an effective amount following known practices.

In addition to the amino compound, the piperazine, and the one or more optional other compounds used in fluid treatment the aqueous alkanolamine solution may comprise a physical solvent. Preferably a solvent such as cyclotetramethylenesulfone (available under the tradename SULFOLANE), dimethyl ethers of polyethylene glycol (available under the tradename SELEXOL from The Dow Chemical Company), and triethylene glycol monomethyl ether (TGME or METHOXYTRIGLYCOL available from The Dow Chemical Company), l,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone, N-formylmorpholine, N-acetylmorpholine, or mixtures thereof. If present, the amount of physical solvent in the aqueous alkanolamine solution may be present in an amount from equal to or greater than 1 weight percent, preferably equal to or greater than 5 weight percent, more preferably equal to or greater than 10 weight percent based the total weight of the solution. If present, the amount of physical solvent in the aqueous alkanolamine solution may be present in an amount equal to or less than 75 weight percent, preferably equal to or less than 65 weight percent, more preferably equal to or less than 50 weight percent based the total weight of the solution.

The aqueous alkanolamine solutions of the present invention do not contain an acid or acid-forming material, preferably excluded acids or acid forming materials are ones characterized as strong acids which include any organic or inorganic acid having a pKa of 8 or less, preferably 7 or less, more preferably 6 or less. Examples of acids that are excluded include phosphoric acid, phosphorus acid, hydrochloric acid, sulfuric acid, sulfurous acid, nitrous acid, pyrophosphoric acid, telurous acid, and the like. Also organic acids such as acetic acid, formic acid, adipic acid, benzoic acid, n-butyric acid, chloroacetic acid, citric acid, glutaric acid, lactic acid, malonic acid, oxalic acid, o-phthalic acid, succinic acid, o-toluic acid, and the like are excluded from the aqueous alkanolamine solutions of the present invention. In addition, acid-forming materials that are capable of forming acids upon contact with water cannot be present in the aqueous alkanolamine solutions of the present invention.

The invention set forth herein has great application in the petrochemical and energy industries. For example, the present invention can be used for the treatment of fluid streams, gas, liquid, or mixtures, in an oil refinery, the treatment of sour gas, the treatment of coal steam gas, the treatment of hazardous stack emissions, the treatment of land field gasses, and a new series of devices dealing with hazardous emissions for human safety.

The fluid streams to be treated by the process of the present invention contain an acid gas mixture which includes H ₂ S, and may include other gases such as C0 ₂ , N ₂ , CH ₄ ,

C ₂ H ₆ , C ₃ H ₈ , H ₂ , CO, H ₂ 0, COS, HCN, NH ₃ , 0 ₂ , mercaptans, and the like. Often such gas mixtures are found in combustion gases, refinery gases, town gas, natural gas, syn gas, tail gas, water gas, propane, propylene, heavy hydrocarbon gases, etc. The aqueous alkanolamine solution herein is particularly effective when the fluid stream is a gaseous mixture, obtained, for example, from shale oil retort gas, coal or gasification of heavy oil with air/steam or oxygen/steam thermal conversion of heavy residual oil to lower molecular weight liquids and gases, or in sulfur plant tail gas clean-up operations. The process of the present invention is preferably used to remove H ₂ S and C0 ₂ from a gas stream comprising H ₂ S and C0 ₂ optionally in the presence of one or more other acid gas impurities, for example N ₂ , CH ₄ , C ₂ H ₆ , C ₃ H ₈ , H ₂ , CO, H ₂ 0, COS, HCN, NH ₃ , 0 ₂ , and/or mercaptans. Further, the present invention may be used to remove H ₂ S, C0 ₂ and one or more of N ₂ , CH ₄ , C ₂ H ₆ , C ₃ H ₈ , H ₂ , CO, H ₂ 0, COS, HCN, NH ₃ , 0 ₂ , and/or mercaptans from a gas stream comprising H ₂ S, C0 ₂ and one or more of S0 ₂ , CS ₂ , HCN, COS, and/or mercaptans.

The absorption step of this invention generally involves contacting the fluid stream, preferably gaseous mixture, with the aqueous alkanolamine solution in any suitable contacting vessel, for examples of representative absorption processes see USP 5,736,115 and 6,337,059, both of which are incorporated herein by reference in their entirety. In such processes, the fluid stream containing H ₂ S and/or other impurities from which the acid gasses are to be removed may be brought into intimate contact with the aqueous alkanolamine solution using conventional means, such as a tower or vessel packed with, for example, rings or with sieve plates, or a bubble reactor.

In a typical mode of practicing the invention, the absorption step is conducted by feeding the fluid stream into the lower portion of the absorption tower while fresh aqueous alkanolamine solution is fed into the upper region of the tower. The fluid stream, freed largely from the H ₂ S and C0 ₂ if present emerges from the upper portion (sometimes referred to as treated or cleaned gas) of the tower, and the loaded aqueous alkanolamine solution, which contains the absorbed H ₂ S and C0 ₂ , leaves the tower near or at its bottom.

Preferably, the inlet temperature of the absorbent composition during the absorption step is in the range of from 120°F to 210°F, and more preferably from 140°F to 200°F. Pressures may vary widely; acceptable pressures are between 5 and 2,000 pounds per square inch

(psi), preferably 20 to 1,500 psi, and most preferably 25 to 1,000 psi in the absorber. The contacting takes place under conditions such that the H ₂ S is preferably absorbed by the solution. The absorption conditions and apparatus are designed so as to minimize the residence time of the aqueous alkanolamine solution in the absorber to reduce C0 ₂ pickup while at the same time maintaining sufficient residence time of the fluid stream with the aqueous absorbent composition to absorb a maximum amount of the H ₂ S gas. Fluid streams with low partial pressures, such as those encountered in thermal conversion processes, will require less of the aqueous alkanolamine solution under the same absorption conditions than fluid streams with higher partial pressures such as shale oil retort gases. A typical procedure for the H ₂ S removal phase of the process comprises absorbing H ₂ S via countercurrent contact of a gaseous mixture containing H ₂ S and C0 ₂ with the aqueous alkanolamine solution of the amino compound in a column containing a plurality of trays at a temperature, of at least 120°F, and at a gas velocity of at least 0.3 feet per second (ft/sec, based on "active" or aerated tray surface), depending on the operating pressure of the gas, said tray column having fewer than 20 contacting trays, with, e.g., 4 to 16 trays being typically employed.

After contacting the fluid stream with the aqueous alkanolamine solution, which becomes saturated or partially saturated with H ₂ S, the solution may be at least partially regenerated so that it may be recycled back to the absorber. As with absorption, the regeneration may take place in a single liquid phase. Regeneration or desorption of the acid gases from the aqueous alkanolamine solution may be accomplished by conventional means of heating, expansion, stripping with an inert fluid, or combinations thereof, for example pressure reduction of the solution or increase of temperature to a point at which the absorbed H ₂ S flashes off, or by passing the solution into a vessel of similar construction to that used in the absorption step, at the upper portion of the vessel, and passing an inert gas such as air or nitrogen or preferably steam upwardly through the vessel. The temperature of the solution during the regeneration step should be in the range from 120°F to 210°C, and preferably from 140°F to 200°F, and the pressure of the solution on regeneration should range from 0.5 psi to 100 psi, preferably 1 psi to 50 psi. The aqueous alkanolamine solution, after being cleansed of at least a portion of the H ₂ S gas, may be recycled back to the absorbing vessel. Makeup absorbent may be added as needed.

In a preferred regeneration technique, the H ₂ S-rich aqueous alkanolamine solution is sent to the regenerator wherein the absorbed components are stripped by the steam which is generated by boiling the solution. Pressure in the flash drum and stripper is usually 1 psi to 50 psi, preferably 15 psi to 30 psi, and the temperature is typically in the range from 120°F to 340°F, preferably 170°F to 250°F. Stripper and flash temperatures will, of course, depend on stripper pressure; thus at 15 psi to 30 psi stripper pressures, the temperature will be 170°F to 250°F during desorption. Heating of the solution to be regenerated may very suitably be affected by means of indirect heating with low-pressure steam. It is also possible, however, to use direct injection of steam. The resulting hydrogen sulfide-lean aqueous alkanolamine solution may be used to contact a gaseous mixture containing H ₂ S. Preferably the clean gas contains equal to or less than 10 ppm H ₂ S, meeting some environmental regulations, more preferably equal to or less than 4 ppm H ₂ S, meeting typical pipeline specifications.

A preferred embodiment of the present invention involves performing the method of the present invention continuously, or as a continuous process. However, the method may be performed batch wise or semi-continuously. Selection of the type of process used should be determined by the conditions, equipment used, type and amount of gaseous stream, and other factors apparent to one of ordinary skill in the art based on the disclosure herein.

EXAMPLES

Examples 1 to 9 are an aqueous amine absorbent solution comprising an

alkanolamine, deionized water, and optionally a second amine, amounts are in parts by weigh based on the total weight of the absorber composition. A gas stream comprising a synthetic mixture containing 4.2 percent H ₂ S, 16 percent C0 ₂ and 79.8 percent N¾ wherein percent is percent by volume, is treated in a pilot scale absorber to remove the H ₂ S and C0 ₂ . For each aqueous amine absorbent solution, the gas stream is treated at three different flow rates. The compositions, process parameters, and residual H ₂ S and C0 ₂ levels for Examples 1 to 9 are listed in Table 1. In Table 1 :

"DGA" is 98% 2-(2-aminoethoxy) ethanol available from Acros Organics;

"MDEA" is 98% methyldiethanolamine available from The Dow Chemical

Company; and

"DMAPD" is 98% 3-dimethylamino-l,2-propanediol available from AK Scientific;

"Piperazine" is 99% piperazine available from Aldrich Chemical.

An aqueous amine absorbent solution is introduced into the pilot scale absorber FIG. 1 via feed line 5 into the upper portion of a gas-liquid countercurrent packed-bed absorption column 2. The gas stream is introduced through feed line 1 into the lower portion of column 2 at a gas flow rate of 10 liter per minute. The absorber pressure is adjusted to 238 psia. The clean gas (i.e., reduced amounts of H ₂ S and C0 ₂ ) is discharged at the top of the absorber 2 through line 3 and residual H ₂ S and C0 ₂ levels are determined by gas chromatography (GC) analysis. The aqueous amine solution loaded with H ₂ S and C0 ₂ flows toward the lower portion of the absorber, and leaves via line 4. The aqueous amine in line 4 is reduced in pressure by the level control valve 8 and flows through line 7 to heat exchanger 9, which heats the loaded aqueous solution. The hot rich solution enters the upper portion of the regenerator 12 via line 10. The regenerator 12 is equipped with random packing which effects desorption of the H ₂ S and C0 ₂ gases. The pressure of the regenerator is set at 17 psia. The gases are passed through line 13 into condenser 14 wherein cooling and condensation of any residual water and amine occurs. The gases enter a separator 15 wherein the condensed liquid is separated from the vapor phase. The condensed aqueous solution is pumped via pump 22 through line 16 to the upper portion of the regenerator 12. The gases remaining from the condensation are removed through line 17 for final collection and/or disposal. The regenerated aqueous solution flows down through the regenerator 12 and the close-coupled reboiler 18. The reboiler 18, equipped with an electrical heating device, vaporizes a portion of the aqueous solution to drive off any residual gases. The vapors rise from the reboiler and are returned to the regenerator 12 which comingle with falling liquid and then exit through line 13 for entry into the condensation stage of the process. The regenerated aqueous solution from the reboiler 18 leaves through line 19 and is cooled in heat exchanger 20, and then is pumped via pump 21 back into absorber 2 through feed line 5.

The flow rate for the aqueous amine absorbent is determined by slowly adjusting downward until the amount of H ₂ S in the purified gas line 3 shows a dramatic increase.

The results for Examples 1 to 9 are graphically represented in the plot shown in FIG. 2. H ₂ S levels, in parts per million by volume (ppmv), are plotted against the amine flow rate in cubic centimeters per minute (cc/min).

Table 1

Not an example of the present invention