Background of the Related Art Hydrocarbons are typically found as gases or liquids located in geological formations beneath the earth's surface. These hydrocarbons may be produced by drilling a well into the formations. Gaseous hydrocarbons, typically referred to as natural gas, are frequently found to contain water, carbon dioxide and sulfides, such as hydrogen sulfides and mercaptans.

Natural gas containing sulfides is referred to as being "raw" or "sour," due to its strong malodorous smell. However, the concentration of sulfides in natural gas will vary from formation to formation and perhaps over time. For example, the methane content of natural gas will typically be between 45 and 96 percent and the hydrogen sulfide content will typically be between 0.1 and 150,000 parts per million (ppm).

For many applications, it is desirable to remove some or substantially all of the sulfides from the gas. For example, since hydrogen sulfide in the presence of water is corrosive to steel, it is desirable to removed substantially all of the hydrogen sulfide from natural gas before its use and preferably before transporting the gas or processing the gas in oil field equipment, pipelines and refineries. As a result, many pipeline specifications limit the amount of hydrogen sulfide to less than 0.25 grains (parts) per 100 cubic feet of gas.

Accordingly, removal of hydrogen sulfide from hydrocarbons, such as natural gas, crude oil and refined oil products, is an important concern. Further, because of the large quantities of hydrocarbon materials that contain sulfides, it is important to have a means of removing sulfides that does not harm the environment and is economically efficient.

The removal of sulfides is conventionally accomplished using gas processing plants which are typically located in the field and vary in size from small units located at each wellhead to larger, centrally located plants servicing multiple wells. These plants may be nothing more than a single bubble column or a multi-stage.

These gas processing plants are typically provided with a solution containing a sulfide scavenging component which contacts the natural gas stream and removes the sulfides contained in the gas. Scavenging components, otherwise known as scavengers, that have been used to date have included formaldehyde, sodium nitrite, and triazines. However, some of these scavengers have proven unsatisfactory for a variety of reasons. For example, formaldehyde has been found to be carcinogenic and requires careful handling and expensive disposal.

In recent years, the most widely used scavengers have been the triazines, particularly those formed by the combination of formaldehyde and monoethanolamine. The popularity of this triazine scavenger is primarily due to its good scavenging capacity and moderate cost.

However, the reaction of triazines with the sulfides will lead to the formation of solids if the triazine solution is used to exhaustion. These solids represent a significant disposal problem and must be carefully cleaned from the scavenging process vessels. Despite the predominant use of triazine scavengers, there remains a need for sulfide scavenger solutions and processes that operate more efficiently and effectively. It would be desirable if the scavenger solution and process reduced or eliminated solids formation. It would be further desirable if the scavenger solution could scavenge sulfides without becoming malodorous.

Summary of the Invention The present invention provides a process comprising the steps of removing sulfides from a gas with a solution having at least one scavenger comprising the product of an aldehyde and an amine, adding additional aldehydes, such ethanedial and/or formaldehyde, to a portion of the solution, and removing additional sulfides from the gas with the solution.

The aldehyde may be formaldehyde and the amine may be monoethanolamine, perhaps forming a triazine. More preferably, the aldehyde comprises ethanedial and formaldehyde and the amine is monoethanolamine. The process may further comprise the step of discarding a second portion of the solution that is substantially spent.

An additional aspect of the invention provides a method comprising the step of adding ethanedial and/or formaldehyde to a sulfide scavenger derived from an amine, such as monoethanolamine, for the purpose of regenerating with the amine or derivatives of the amine liberated from the sulfide scavenger upon exposure to sulfides.

The invention also provides a method comprising the step of adding ethanedial and/or formaldehyde to a solution comprising a sulfide scavenger derived from an amine for the purpose of combining with a product formed when the sulfide scavenger engages sulfides.

The invention further provides a method of scavenging sulfides comprising the step of contacting a sulfide-containing gas with a solution comprising ethanedial and/or formaldehyde and a sulfide scavenger derived from monoethanolamine.

Yet another aspect of the invention provides a process comprising the steps of reclaiming or recycling monoethanolamine or a derivative of monoethanolamine from a first scavenger stream, and combining the reclaimed or recycled monoethanolamine or derivative of monoethanolamine, such as a carbonate of monoethanolamine, with an aldehyde to form additional scavenging product. The process may further comprise the step of reacting sulfides from a gas stream with a scavenger containing triazines.

Another aspect of the present invention provides a sulfide scavenging composition comprising the product of a plurality of amines, preferably monoethanolamine, and a plurality of aldehydes, preferably comprising a dialdehyde. The plurality of aldehydes may be aqueous, may comprise formaldehyde or consist essentially of the oxidation products of ethylene glycol. The at least one dialdehyde preferably comprises ethanedial. The scavenging composition may also include excess ethanedial.

Another aspect of the invention provides a composition for scavenging sulfides, comprising a triazine and ethanedial, wherein the molar ratio of ethanedial to triazine is between about 0.5 and about 1.5, preferably about 1 or equal to 1. The triazine may be formed by the combination of formaldehyde and monoethanolamine. It is also preferred that an equivalent of carbonyl groups in the ethanedial be provided for between about 1.5 and about 6 equivalents of hydrogen atoms bonded directly to a nitrogen atom in the liberated amine.

Another aspect of the invention provides a process comprising the steps of combining a plurality of aldehydes with monoethanolamine. Again, the plurality of aldehydes may be aqueous, may further comprise formaldehyde or consist essentially of the oxidation products of ethylene glycol.

Yet another aspect of the invention provides a process comprising the step of combining monoethanolamine with a solution consisting essentially of the reaction product formed by the oxidation of ethylene glycol. The process may further comprise the step of then adding ethanedial to the solution.

A further aspect of the invention provides a process comprising the step of adding ethanedial and/or formaldehyde to a partially spent sulfide scavenging solution containing a carbonate of monoethanolamine for the purpose of forming a sulfide scavenger.

The invention also provides a process comprising the steps of scavenging sulfides with a solution having at least one scavenger comprising a triazine, then adding ethanedial and/or formaldehyde to the solution; and then scavenging additional sulfides. The process may further comprise the step of adding monoethanolamine to the solution.

Brief Description of the Drawings So that the manner in which the above recited features and advantages of the present invention are attained can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are, therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Figure 1 is a schematic flow diagram of a single vessel system for scavenging sulfides from natural gas; Figure 2 is a schematic flow diagram of a dual vessel system for scavenging sulfides from natural gas; Figure 3 is a schematic flow diagram of the system of Figure 2 further providing for recycle of the scavenging solution; Figure 4 is a schematic flow diagram of the system of Figure 2 further providing a mixing vessel for regenerating a portion of the scavenging solution; Figure 5 is a schematic flow diagram of the system of Figure 4 further providing for the on-site generation of the scavenging solution; Figure 6 is a schematic flow diagram of the system of Figure 5 providing for on-site generation of a fresh scavenging solution and separate regeneration of a spent solution; and Figure 7 is a schematic flow diagram of a single vessel system for on-site generation, regeneration and in-line generation.

Detailed Description of the Preferred Embodiment The present invention generally provides sulfide scavenger solutions and processes that have high sulfide scavenging capacity and provide a reduction or elimination of solids formation. The invention utilizes an aldehyde preferably a dialdehyde, such as ethanedial, for the purpose of regeneration with amines, amine carbonates, or other derivatives or forms of amines that are liberated when certain scavenger solutions remove sulfides, including hydrogen sulfide and mercaptans. The scavenger solutions that have been discovered to liberate amines upon reaction with sulfides are those formed by combining an amine and an aldehyde, such as scavengers formed by the combination of monoethanolamine and formaldehyde, the combination of monoethanolamine and ethanedial and the combination of monoethanolamine and a mixture containing both formaldehyde and ethanedial.

For example, sulfide scavenger solutions containing triazines are commonly formed by the 1:1 molar reaction of monoethanolamine and formaldehyde. More specifically, three moles of monoethanolamine and three moles of formaldehyde are necessary to form one mole of triazine. One mole of triazine has now been found to scavenge about two moles of sulfides and liberate between about one and about two moles of monoethanolamine, a derivative of monoethanolamine or its functional equivalent. Therefore, the present invention provides additional aldehydes, such as ethanedial and/or formaldehyde, to interact with the liberated amine and form a product that is further capable of scavenging sulfides. In this manner, the scavenging solution is described herein as being partially regenerable.

The scavengers of the present invention may be prepared by combining the amine and aldehyde in an active-hydrogen:carbonyl group ratio of between about 0.5 and about 3.

However, it is preferred that the components be present in the ratio of two equivalents of active hydrogen in the amine for each carbonyl group equivalent in the aldehyde. An "active hydrogen," as used herein, is a hydrogen atom bonded directly to a nitrogen atom. Thus, one mole of a primary amino group (NH2) has two active hydrogens and is therefore capable of interacting with one carbonyl group equivalent, wherein the carbonyl group equivalent may be provided either by one mole of a monoaldehyde or by one-half mole of a dialdehyde. One mole of a secondary amino group (NH) has one active hydrogen and is therefore capable of interacting with one-half carbonyl group equivalent (CHO group). A tertiary amino group will not combine with an aldehyde to form a stable compound when the manufacturing procedures of this invention are followed.

One aspect of the invention provides a composition comprising the product of a plurality of aldehydes and an amine, preferably monoethanolamine. Preferably, the plurality of aldehydes include formaldehyde and at least one dialdehyde, most preferably ethanedial.

A convenient and preferred source of the plurality of aldehydes is the reaction product produced by the catalytic oxidation of ethylene glycol. After combining the plurality of aldehydes with monoethanolamine, it may be desirable to add excess ethanedial and/or formaldehyde in order to allow for subsequent in-situ regeneration of liberated monoethanolamine and derivatives of monoethanolamine, such as carbonates of monoethanolamine.

Another aspect of the invention provides a composition comprising the product of an amine carbonate, such as monoethanolamine carbonate, and an aldehyde or mixture of aldehydes, preferably ethanedial and formaldehyde. It is believed that an amine carbonate is formed in the scavenger solution when the liberated amine comes into contact with carbon dioxide in the natural gas stream. This may be the primary form of the liberated amine, particularly where the scavenging is performed in a multiple stage process before the liberated amine is regenerated with an aldehyde. Although the exact nature of the liberated amine is not known, it is preferred that about one-half carbonyl equivalent of aldehyde be combined with each mole of amine carbonate.

Yet another aspect of the invention provides a sulfide scavenger solution comprising a triazine and excess aldehyde, such as ethanedial and/or formaldehyde. Because the amount of amine being liberated from a triazine is now known, it is possible to provide a sulfide scavenger solution that contains a sufficient amount of ethanedial and/or formaldehyde to interact with the amine (or the amine carbonate) as it is being liberated. Just as the original triazines liberate amines that are regenerable with formaldehyde and/or ethanedial to form secondary scavengers, these secondary scavengers may also liberate amines that are themselves regenerable. Therefore, the amount of aldehyde provided may account for the full or partial extent of the regeneration desired, preferably making consideration for the minimum tolerable strength of the scavenging solution before it must be discarded. While adding any amount of excess aldehyde is deemed beneficial in accordance with the invention, the molar ratio of carbonyl groups in the aldehyde to triazine is preferably greater than about 2.

An additional aspect of the invention provides a process comprising the steps of combining a plurality of aldehydes with an amine, preferably monoethanolamine. The plurality of aldehydes may be aqueous and preferably include at least one dialdehyde, especially ethanedial. In the case of a ready-made scavenger solution, the most preferred plurality of aldehydes will be those formed by the catalytic oxidation of ethylene glycol, typically comprising about 18 percent formaldehyde and about 22 percent ethanedial.

Additional aldehydes may be supplied to the product as desired for purposes of regeneration of liberated amines. It is important to avoid the simultaneous use of a polyamine with a dialdehyde, because this combination will form an insoluble, polymer that precipitates and is unusable.

A further aspect of the invention provides a process comprising the step of combining monoethanolamine or a carbonate of monoethanolamine with an aldehyde for the purpose of forming a sulfide scavenger. One embodiment of the invention, comprises the steps of: (a) scavenging sulfides with a solution having at least one scavenger comprising the product of an amine and an aldehyde; then (b) adding aldehyde to the solution; and then (c) scavenging additional sulfides, Optionally, the process may further comprise the step of (d) adding additional amine to the solution along with the ethanedial. Such processes may be performed in either a batch mode, a semi-batch mode or, most preferably, a continuous mode. In the continuous mode, a first portion of the scavenger solution is discarded following exposure to sulfides and a second portion is regenerated by addition of aldehydes, such as formaldehyde and/or ethanedial, and recycled for addition contact with sulfides. Most preferably, the amine is monoethanolamine and the aldehyde is ethanedial, formaldehyde or a mixture of ethanedial and formaldehyde. It should be recognized that the present invention encompasses both the on-site generation of the scavenger and the manufacturing of the scavenger in a centralized plant for distribution to a large number of scavenging plants.

The invention also includes a method comprising the step of adding ethanedial and/or formaldehyde to a sulfide scavenger derived from monoethanolamine for the purpose of combining with monoethanolamine or derivatives of monoethanolamine liberated from the sulfide scavenger upon exposure to sulfides.

A process is also provided comprising the step of combining reclaimed monoethanolamine with ethanedial and/or formaldehyde. Similarly, the process may comprise the steps of reclaiming monoethanolamine or a derivative of monoethanolamine from a first scavenger stream and combining the reclaimed monoethanolamine or derivative of monoethanolamine with an aldehyde to form a second scavenger stream. The process may further comprise the step of reacting sulfides from a ggNs stream with a scavenger containing triazines.

Finally, the invention provides a method of scavenging sulfides comprising the step of contacting a sulfide-containing gas with a solution comprising ethanedial and/or formaldehyde and a sulfide scavenger including a product of an amine and an aldehyde. The preferred amine is monoethanolamine and the preferred aldehydes are ethanedial, formaldehyde or a combination thereof.

It should be recognized that the molar ratios of the components discussed herein are the theoretical amounts required to produce the desired product. However, kinetics and equilibrium may dictate that the actual amounts of one component or another vary slightly from the theoretical requirement. Furthermore, it may be desirable in some embodiments of the invention, such as in the on-site generation of a scavenger solution, that the component solutions be diluted for processing convenience and provision of sufficient solvent, such as water. For convenience and/or process simplicity, it may be preferred that the amine solution and the ethanedial solution be sufficiently diluted so that a 1:1 volumetric ratio of the solutions provides a 2:1 molar ratio of active hydrogens in the amine to carbonyl groups in the ethanedial.

In general, the scavenging compounds of the present invention are prepared by the exothermic combination of an amine, or blends of amines, with any C1 to C4 aldehyde. The preferred monoaldehydes include, but are not limited to, formaldehyde, acetaldehyde, propionaldehyde (propanal), and n-butyraldehyde (l-butanal), most preferably formaldehyde.

The preferred dialdehyde is ethanedial.

Following the ratio of one carbonyl group equivalent per two equivalents of active hydrogen, the product of the present invention allows for a stoichiometrically balanced hydrogen sulfide scavenger having substantially no free aldehyde. Because of the low toxicity of ethanedial, it is less critical to maintain a stoichiometric balance and, in fact, may be desirable to provide excess ethanedial in the scavenger solution to allow regeneration to occur simultaneously with the liberation of amine. It is also environmentally safer to maintain a storage tank of free ethanedial on-site for use in the regeneration.

The aldehydes of the present invention may or may not contain water as their only solvent or may contain water with various amounts of methanol, such as between about 4% and about 25%. Methanol and/or water can be utilized in the scavenger solution to reduce the viscosity for use in natural gas applications. In addition, methanol, isopropanol and/or ethylene glycol can be added to the solution to act as an antifreeze, which may be desirable for use in colder climates.

In accordance with the methods and apparatus disclosed herein, sulfide scavenger solutions may be prepared by combining an amine and aldehyde in any effective ratio.

However, it is preferred that the scavengers be prepared by combining the amine and aldehyde in an active-hydrogen:carbonyl-group ratio of between about 0.5 and about 6. It is most preferred that the active-hydrogen:carbonyl-group ratio be about 2, regardless of the specific amine or aldehyde, so that the product is stoichiometrically balanced.

Ethanedial is the preferred dialdehyde for use in preparing the dialdehyde-based scavengers of the present invention or for regeneration of liberated amine(s). Ethanedial may be obtained and used in any concentration, including anhydrous solutions, but the preferred solution contains about 40% ethanedial in water.

Monoethanolamine (MEA) is the preferred alkanolamine for use in preparing aldehyde or dialdehyde based scavengers and may be used in any concentration, including anhydrous solutions. For use in cold climates, a solution having about 85% MEA may be preferred because it has a freezing point that is lower than more concentrated MEA solutions, such as a solution having greater than about 99% MEA. However, when the freezing point is not of concern, such as in a production facility for production of scavenging solutions, a more concentrated MEA solution is preferred, for example a solution having about 99% MEA or greater. The more concentrated MEA will produce a more concentrated scavenging solution and reduce the costs of shipping the solution from the manufacturing plant to various scavenging plants.

The most preferred product is prepared by combining a stoichiometrically balanced amount of monoethanolamine with ethanedial. This stoichiometrical balance is achieved at a ratio of 2 active hydrogens per carbonyl group. It is preferred that the temperature be maintained below about 200°F. It is most preferred that the temperature be maintained between about 120 and about 150OF.

The color of the product is effected both by the temperature and the ratio of components. At or near a stoichiometrically balanced combination, occurring below about 120OF, the product will take on a transparent reddish yellow tint. When the combination takes place at temperatures between about 120 and.atbout l50°F, the product will have a medium reddish brown coloration. It has been found that as the amine/aldehyde reaction temperature increases above this range, the product will have an increasingly darker and less transparent appearance. When the product reacts with sulfides, the darkness and opaqueness of the product will continue to increase until the product is substantially spent and has a very dark or black, non-transparent coloration.

Additionally, regardless of the exact active hydrogen:carbonyl ratio, a monoethanolamine/ethanedial product, even using liberated monoethanolamine, shows less tendency to form solids in a system when reacted with hydrogen sulfide and/or mercaptans than other products presently commercially available. It is also a considerable advantage that scavenger solutions made with at least in part ethanedial have absolutely no odor when fresh or when spent. The non-odorous nature of the product is particularly desirable for applications in close proximity to residential areas or in production environments where personnel are present.

The removal of hydrogen sulfide and/or mercaptans from hydrocarbon gases, hydrocarbon liquids and aqueous systems is accomplished by scavenging agents that are the products of combining an amine, such as alkanolamine, with a monoaldehyde and/or a dialdehyde. The following chemical mechanisms describe formation and regeneration of scavenging products by combining monoethanolamine with ethanedial.

The combination of two moles of monoethanolamine with one mole of ethanedial are believed to produce one mole of dialdimine and two moles of water as described by Equation 1.

EQUATION 1 HOCH2CH2NH2 + CHOCHO + NH2CH2CH2OH + H2O (MEA) (Ethanedial) (MEA) (Water) o HOCH2CH2N=CHCH=NCH2CH2OH + 2 H2O (Dialdimine) (Water) It has been observed that the scavenging activity of the scavenger formed by MEA and ethanedial will decline over time if the scavenger is not utilized. Two moles of dialdimine are believed to further interact with one mole of an ethanedial hydrate as described by equation 2.

EQUATION 2 HOCH2CH2N=CHCH=NCH2CH20H + (HO)2CHCH(OH)2 + HOCH2CH2N=CHCH=NCH2CH2 (Dialdimine) (Ethanedial Hydrate) (Dialdimine) -3 HoCH2CH2N=CHCH=NCH2CH2o-CH2(olI)CH2(OII)-OCH2CH2N=CHCH=NCH2C H2OH The polymerization process illustrated by Equation 2 is believed to continue over a period of several days or weeks until a large straight chain copolymer is formed. The formation of a large copolymer chain drastically reduces the scavenging properties of a ethanedial and monoethanolamine mixture. Thus it is important to control the initial combination of monoethanolamine and ethanedial, so that dialdimine is formed as close as possible to the time of its use as a scavenging agent. Hence the importance of in-situ formation of dialdimine on site where the scavenging activity is to take place.

The dialdimine scavengers, whether alone or in combination with other scavengers such as triazines, are further believed to scavenge three moles of hydrogen sulfide in accordance with equation 3 (two moles of hydrogen sulfide) and equation 4 (a third mole of hydrogen sulfide).

EQUATION 3 HOCH2CH2N=CHCH=NCH2CH2OH + 2 H2S (Dialdimine) (Hydrogen Sulfide) + HOCH2CH2NHCH(SH)CH(SH)NHCH>CH20H (Spent Product I) EQUATION 4 HOCH2CH2NHCH(SH)CH(SH)NHCH2CH2OH + H2S (Spent Product I) (Hydrogen Sulfide) HOCH2CH2NHCH(SH)CH(SH)2 + NH2CH2CH2OH (Spent Product II) (Liberated MEA) After absorbing the third mole of hydrogen sulfide, one mole of MEA is liberated.

This MEA is then free to absorb other components of the gas, i.e., carbon dioxide or hydrogen sulfide, or interact with other components of the scavenger chemical. As shown in equation 5, if the sour gas contains carbon dioxide, then it will be absorbed by the MEA to form MEA carbonate.

EQUATION 5 NH2CH2CH2OH + H20 + CO2 o HOCH2CH2NH2*H2CO3 (Liberated MEA) (Water) (Carbon Dioxide) (MEA-Carbonate Salt) Ethanedial can also be used to regenerate MEA-Carbonate Salts back to a useful dialdimine. Equation 6 describes regeneration of dialdimine by combining one mole of monoethanolamine carbonate salt, recovered after desulfurization of natural gas, with one mole of fresh monoethanolamine and one mole of fresh ethanedial. The regeneration of dialdimine also releases one mole of carbon dioxide and three moles of water.

EQUATION 6 HOCH2CH2NH2*H2CO3 + CHOCHO + NH2CH2CH2OH (MEA-Carbonate Salt) (Fresh Ethanedial) (Fresh MEA) o HOCHCH2N=CHCH=NCH2CH2OH + 3 H2O + CO2 (Dialdimine) (Water) (Carbon Dioxide) Similar scavenging agents are formed by combination of an alkanolamine or a polyamine with a monoaldehyde, such as formaldehyde. Furthermore, the regeneration of MEA and/or an MEA-Carbonate Salt, as set out in equation 6, may also be accomplished with other aldehydes, such as formaldehyde, or a mixture of aldehydes.

When sweetening gas, the addition of selected surfactants to a hydrogen sulfide scavenger will typically enhance the performance of the scavenger by promoting intimate contact between the scavenger and the hydrogen sulfide in the gas. Surfactants are especially useful in systems where aqueous and/or hydrocarbon fluids are present and tend to interfere with contact. Applications where gas is sweetened through use of in-line injection, scrubbing towers, or a combination of the two benefit the most from surfactants. When surfactant- containing scavengers are used in contact towers (scrubbers), the foam height within the tower may be controlled by type and amount of surfactant employed as well as the adjustment of the fluid level within the tower. In this way, carryover problems may be avoided.

All surfactants may be used in selected hydrogen scavenger applications. The surfactants may be nonionic, anionic, cationic, amphoteric, or combinations thereof. An example of a nonionic surfactant useful in combination with the alkanolamine/ethanedial scavenger is the product of nonylphenol and between about 10 and about 12 moles of ethylene oxide.

Surfactants can be used to fine tune a formulation for application in specific circumstances, such as tower height, in-line injection, hydrogen sulfide concentration, and the like. The amount of surfactant that is added to the scavenger of the present invention may be any amount, but will typically vary from about 0.05% to about 5% by volume of the total scavenger solution depending on the particular application. The preferred surfactant concentrations are less than one percent (1.0%).

The sulfide scavengers of the present invention may be manufactured in centralized manufacturing plants, on-site or a combination of the two locations. For on-site generation of scavengers, ethanedial is the most preferred aldehyde due to its low toxicity. For centralized manufacture of scavengers, it is cost effective to use the formaldehyde/ethanedial stream produced by the catalytic oxidation of ethylene glycol. In a typical aldehyde manufacturing plant, this formaldehyde/ethanedial stream is separated to provide a formaldehyde stream and an ethanedial stream. However, in accordance with the present invention, this mixed aldehyde stream may be used effectively to produce scavengers, either alone or in combination with different aldehydes or a separated stream of ethanedial.

The sulfide scavengers of the present invention can be applied in various ways. When sweetening natural gas, the scavenger can be placed into various gas/liquid contacting devices, such as bubble columns, spray columns, columns or towers having packing or trays, well-head injectors, in-line injectors, in-line static mixers (with or without atomization). The gas/liquid contacting devices may be used alone or in various combinations (including one or more similar units), in series or parallel, and in any useful order depending upon the particular application, as will be recognized in the art. Multiple injection points or contacting towers may be utilized as necessary to accommodate the volume of gas and/or the hydrogen sulfide levels specified. Furthermore, the scavengers may be applied in a manner consistent with new or existing equipment, such as a batch process, a semi-batch process or, most preferably, a continuous process.

The scavengers and processes of making and using the scavengers of the present invention may include in-situ, ex-situ or a combination of in-situ and ex-situ regeneration of liberated amines. In-situ regeneration of amines, as used herein, refers to applications having excess aldehyde present in the scavenging solution so that liberated amines may be combined with an aldehyde without requiring a separate vessel. Ex-situ regeneration of liberated amines, as used herein, refers to application having vessels that are separate from the gas/liquid contacting devices for combining aldehyde with the liberated amines.

Furthermore, the sulfide scavengers of the present invention may also be used in conjunction with crude oil, oil field waters, various hydrocarbon streams in refineries, sewage treatment plants (i.e., sewage gas for odor control and removing sulfides from anaerobic digesters), animal feedlot wastes, paper mills, cogeneration, land fills, mining, manufacturing such as coke plants, geothermal, and any other application where H2S or mercaptans are a problem. The scavengers may also be useful for removing sulfides in other applications, streams, gases and liquids, although not specifically mentioned, as will be apparent to one of skill in the art.

The rapid combination or reaction that occurs between alkanolamines and ethanedial and the lower toxicity of ethanedial, as compared to formaldehyde, are important characteristics for on-site and on-demand manufacturing of a organic sulfide scavenger formed by the combination of an alkanolamine and ethanedial.

Figure 1 is a schematic flow diagram of a single vessel system 10 for scavenging sulfides from natural gas. Natural gas is produced through a well bore 12 extending into a gas producing formation 14. The gas is transported out of the formation under its own pressure or with a pump (not shown) and into a bubble column 16, a spray column 18, a packed column, other liquid/gas contacting devices or combinations of devices. In the case of the bubble column 16 and spray column 18 of Figure 1, the gas is released into bubble column 16 at a point under the scavenger solution 20, which is preferably maintained at a level between about 45 and about 85 percent full. The gas rises through the liquid scavenger solution 20, disengages from the liquid, flows upward through the column 16 and column 18, and then passes out of the system 10 to some other device, such as a gas transportation pipeline. The scavenger solution 20 is passed into the spray column 18 and bubble column 16 from a scavenger source or vessel 28. The scavenger solution contacts the gas and reacts with the sulfides in the gas to form various water-soluble compounds. The gas passing out of the columns 16, 18 will preferably contain little or no sulfides and is said to have been "sweetened." Over time, the amount of sulfides removed from the gas will approach the reactive capacity of the scavenger solution 20 and the solution 20 will begin to pass sulfides, in which case the scavenger solution is said to be "spent." When the concentration of sulfides in the gas reaches an undesirable level, the spent scavenger solution may be withdrawn from the column 16 and replaced from the scavenger source 28. More preferably, a spent scavenger stream is continuously or semicontinuously withdrawn from the column, while a substantially continuous stream of unreacted or "fresh" scavenger solution is provided into the column 18 or column 16, alone or in combination.

Level control in the bubble column 16 may be achieved in any conventional manner known in the art, such as with the use of high and low level sensor switches 22, 24. The switches are preferably in electronic communication with a controller (not shown) which controls the rate at which the pump 26 withdraws substantially spent scavenger solution from the bottom of the column 16 for disposal.

Figure 2 is a schematic flow diagram of a dual stage system 30 for scavenging sulfides from natural gas. The system includes two stages, each stage including a gas/liquid contacting device such as a column. As in Figure 1, the gas enters below the scavenger 20 in bubble column 16 and exits the top of column 18. However, then the gas is transported to a second stage, such as a second bubble column 32, through a gas line 34. The gas enters column 32 below the liquid level of the scavenger solution 36, rises through and disengages from the scavenger solution 36, and exits the spray column 38 for transport to some other device, such as a gas transportation pipeline.

The scavenger source or vessel 28 now delivers scavenger solution into the spray column 38 and bubble column 32 for scavenging sulfides from the gas passing therethrough.

Within the column 32, the scavenger solution 36 is preferably maintained at a level between about 25 and about 85 percent full depending on the particular system engineering for optimum contact without blow-by. The solution 36, which is partially reacted with sulfides, is withdrawn from the bottom of the column 32 (the second stage) and transferred to the top of the column 18 and into column 16 (the first stage). The solution 20 contacts more gas and reacts with more sulfides. When the solution 20 is spent or substantially spent, it is withdrawn from the column and discarded.

As already mentioned, the scavenging processes of the present invention may be carried out in batch, semi-batch or continuous modes. The system 30 provides a two-stage, counter-current flow of the scavenger solution and natural gas to obtain a high scavenging efficiency. The most reactive, fresh scavenger solution is provided to the second stage of the system (columns 32, 38) where the sulfide concentration in the gas is already reduced due to sulfide removal in the first stage (columns 16, 18).

Figure 3 is a schematic flow diagram of the system 30 of Figure 2 further providing for recycle of the scavenging solution in the first stage, second stage or both stages.

Recycling may be beneficial for increasing the degree of contact between the gas and the liquid, for example by increasing the volumetric flow rate of liquid being sprayed in a spray column, such as spray columns 18, 38. Furthermore, when the fresh scavenger solution provided by the source 28 contains excess ethanedial for the purpose of combining with liberated amines, it may be desirable to provide a recycle stream from either or both of the first and second stages to the inlet of the second stage (here, column 38). The liberated amine in the recycle stream 42 will combine with the excess ethanedial in the fresh solution stream 44 to form a product which is itself useful as a scavenger of sulfides.

Figure 4 is a schematic flow diagram of the system 30 of Figure 2 further providing a vessel 46 for regenerating a portion of the scavenging solution. While the scavenger solution may be regenerated at any point (such as the solution withdrawn from column 32 or any intermediate point in either column 16, 32) or several points, it is preferred to regenerate the scavenger solution withdrawn from column 16 through pump 26 to stream 48. The solution in stream 48 will be substantially spent and will contain liberated amines unless the system is otherwise provided with excess ethanedial. A first portion of the stream 48 will be discarded and a second portion of the stream 48 is passed through line 42 into the vessel 46 for regeneration. A source of a regenerate aldehyde solution 50 is also provided to the vessel 46.

The regenerated solution exits the vessel 46 and may be directed to either the first stage (any point in column 18 or column 16) through line 54, the second stage (any point in column 38 or column 32) through line 52, or both stages in the same of different amounts. The relative flow rates of substantially spent solution through line 42 and the aldehyde solution through line 56 are preferably controlled to provide about two moles of active hydrogen in the liberated amine for every mole of carbonyl in the aldehyde. This ratio will theoretically yield a stoichiometrically balanced amine/aldehyde product for use as a sulfide scavenger. Note that even amines in the carbonate form are considered to have two active hydrogens.

Furthermore, because of the low toxicity of ethanedial it is acceptable, and perhaps preferred, to provide an excess of ethanedial and thereby allow some additional regeneration to take place within the contacting columns.

Figure 5 is a schematic flow diagram of the system 30 of Figure 4 further providing for the on-site generation of a fresh scavenging solution. An amine storage vessel 58 and an aldehyde storage vessel 50 are located near the site at which a gas containing sulfides is to be sweetened. While this is typically near the well-site, gas may be sweetened at any point according to the present methods. Control valves 62 and 64 are used to control the quantity of fluid that flows from the amine and aldehyde storage vessels, respectively. The ratio of flow through the valves 62 and 64 is determined by the particular molar ratio desired, typically between about 2 and about 6 equivalents of hydrogen atoms bonded directly to a nitrogen atom in the amine for every equivalent of carbonyl groups in the aldehyde, and the degree of dilution of the amine and aldehyde components. The overall rate of scavenger production is determined by the gas stream flow rate through the system 30, the sulfide concentration within the gas, and the acceptable level of sulfide within the sweetened gas stream 66.

The amine and aldehyde components flow together and enter a vessel or static mixer 46 preferably having sufficient volume to provide the residence time necessary for the reaction to be complete. Since the aldehyde/amine combination is exothermic, cooling may be required at times to keep the temperature in the desirable ranges. Cooling may be achieved, for example, by the use of a cooling device (not shown), such as a fin fan, refrigeration system or water jacket arrangement, in thermal communication with the vessel 46.

Furthermore, the vessel 46 may also receive some substantially spent scavenger solution through line 42 for regeneration. In this case, the amount of aldehyde, preferably formaldehyde and/or ethanedial, supplied to the vessel 46 will preferably be based on the amount of active hydrogens in both the aldehyde stream 60 and the recycle stream 42. Again, it is possible and perhaps desirable to supply an excess of aldehyde for the purpose of facilitating in-situ regeneration of liberated amines.

The scavenger solution may then be directly contacted with a sour gas stream. In accordance with the present invention, this contact may involve in-line injection at one or more points (such as at point 68), bubble columns, spray columns or other liquid/gas contacting devices, alone or in combination. While the columns 18, 38 are shown having a number of trays or baffles, the columns may be an open spray tower or contain packing or trays. Furthermore, it should be recognized that the columns 18, 38 may be eliminated entirely leaving only the columns 16, 32.

Figure 6 is a schematic flow diagram of the system 30 of Figure 5 providing for on- site generation of a fresh scavenging solution and separate regeneration of a spent solution.

The on-site generation of a fresh scavenging solution is performed substantially as described with reference to Figure 5. The separate regeneration process is performed in a dedicated vessel 70 which may be variously positioned within the system 30 but is shown receiving the recycle stream 40 and delivering the regenerated solution to column 18 of the first stage. A second stream is supplied to the vessel 70 from the aldehyde source 50 for combining with the liberated amines. The control valves 72, 74 determine the relative flows of substantially spent solution and aldehyde into the vessel 70 and are preferably operated to provide one carbonyl group in the aldehyde for every two active hydrogens in the liberated amine. While this embodiment requires a second vessel, the regenerated solution, which is less reactive with sulfides due to its dilution with fully reacted chemicals and water, is isolated in the first stage to avoid diluting the reactivity of the solution in the second stage.

Figure 7 is a schematic flow diagram of a single vessel system for onsight generation, regeneration and in-line injection. The system of Figure 7 operates in a similar manner to that of Figure 5, except that this system operates with a single vessel. In addition, the system of Figure 7 includes inline injection into an upturn in the metro gas pipe 80 before the gas passes through a static mixer 82 and into the spargers of the vessel 32. Preferably, the inline injection occurs at point 68 to an atomizer (not shown) which atomizers the scavenging chemical into up-flowing natural gas. The scavenger is carried along with the gas through the static mixer to provide even greater contact between the scavenger and the hydrogen sulfide.

This inline injection system has been found to treat a significant amount of hydrogen sulfide.

It is important to note that even the scavenger injected inline upstream of the vessel 32 can be regenerated in accordance with the present invention because the liberated amine passes into the vessel and is recycled along with the other scavenging chemical through pump 26.

Finally, it should be noted that this inline injection scheme may be used in conjunction with any of the systems of Figures 1 through 7.

It should be noted that the invention encompasses several variations in the equipment that can be used in conjunction with the scavenging processes described herein. For example, a portion of the dialdimine product could be injected upstream of one or several contact towers using an atomizer nozzle, perhaps directed downward into an upturned portion of the gas pipe, and an in-line static mixer to start the scavenging reaction early and deliver more liberated monoethanolamine into the contact tower(s). Also, the addition of mist eliminators, trays, or packing spargers is considered within the scope of the invention.

The following examples illustrate the function of this invention and some of its preferred embodiments.

Example 1 The following product formulations for scavenging H2S from natural gas were evaluated on Texas Gulf Coast field gas using laboratory scale testing equipment consisting of sparger-equipped clear plastic towers 1.25 inches in diameter by 16 inch in height. The volume of influent field gas was split evenly between the towers and controlled with a flow control meter at four (4) standard cubic feet per hour (>scfh) for each individual tower. The H2S concentration in the influent and effluent gas was measured with a SENSIDYNE/GASTEC pump and H2S analyzer tubes. (Sensidyne and Gastec are trademarks of Sensidyne, Inc. of Clearwater, Florida). This technique allowed the scavenging performance of the present formulations and available commercial products to be directly compared. The relative performance of the product is seen by comparing the amount of hydrogen sulfide slippage through the tower containing the liquid product.

Products 1, 3, and 4 were prepared by placing the monoethanolamine in a vessel, mixing the ethanedial and methanol together in a separate vessel, then adding the ethanedial/methanol mixture to the monoethanolamine. Product 2 was prepared by adding the monoethanolamine to the ethanedial/methanol mixture. The amounts of ethanedial and monoethanolamine are indicated below.

The field gas that was used in the evaluations had the following composition and properties: Methane 92.9% Ethane 0.4% Carbon Dioxide 2.4% Hydrogen Sulfide 12,200 ppm Gas pressure 185 psig Gas temperature 1300 F Product Formulations (wt%) Ingredient Prod. #1 Prod. #2 Prod. #3 Prod. #4 Monoethanolamine 46.00 46.00 47.89 40.52 Ethanedial (60% H2O) 49.86 49.86 48.55 55.26 Methanol 4.13 4.13 3.56 4.22 Product Test Results (ppm H2S slippage) Time (mill) Blank Prod. #1 Prod. #2 Prod. #3 Prod. #4 Start 11,500 30 11,000 395 360 420 2000 60 9,500 395 400 445 1600 90 10,100 400 ~~ 390 410 600 120 10,200 1 400 400 1 410 600 150 10,100 400 400 400 670 180 10,100 650 650 550 820 210 10,100 1100 1000 900 1020 240 10,500 1750 1550 1600 1300 270 1 10,100 2520 2420 2350 2500 300 10,000 6500 4500 4500 4500 330 10,100 7 7300 8000 6800 6200 360 10,000 8800 9300 8500 8200 390 10,500 9800 9000 8500 8500 *This blank was drawn from the wellhead rather than the tower.

Example 2 A formulation of the present invention, labeled Product #5, was tested according to procedures of Example 1, above, along with the industry standard hydrogen sulfide scavenger, SULFASCRUB HSW 0700F. The HSW 0700F product of the SULFASCRUB (a trademark of the Petrolite Corporation, St. Louis, Missouri) product line is available from the Petrolite Corporation of St. Louis, Missouri.

Product #5 is similar to Product #1 of Example 1. Product #5 was prepared by placing 46.00 wt% of a 85% monoethanolamine solution in a vessel, mixing 50 wt% of a 40% ethanedial solution and 4 wt% of methanol together in a separate vessel, then adding the ethanedial/methanol mixture to the monoethanolamine.

Product Comparison Results (ppm H2S slippage) Time Blank HSW HSW Prod. #5 Prod. #5 Elapsed 0700F 0700F (minutes) (ppm H2S (% H2S (ppm H2S (% H2S Slippage) removal) Slip) removal) Start 11,000* - - 15 10,800 225 97.9 250 ] 97.7 45 10,200 270 97.4 330 96.8 60 10,200 245 97.6 250 97.5 90 10,100 340 96.6 250 97.5 105 10,100 390 96.1 310 96.9 120 10,000 420 95.8 340 96.6 135 10,200 490 95.2 410 96.0 150 9,900 550 94.4 470 95.3 165 1 9,900 1 650 93.4 530 94.6 180 10,000 790 92.1 - 710 92.9 195 10,000 1025 89.8 980 90.2 210 10,000 1300 8a0 1250 87.5 225 10,000 2150 78.5 1600 84.0 240 10,000 3200 68.0 2200 78.0 255 10,000 6000 40.0 3100 69.0 270 10,000 7000 30.0 4250 57.5 285 10,000 7900 21.0 5800 42.0 300 10,000 8200 18.0 7500 25.0 In the above example, which was carried out at four (4) standard cubic feet of gas per hour for a period of 300 minutes, the overall absorption efficiency of Product #5 is about 83.2% and the overall absorption efficiency of HSW 0700F is about 77.3%. Based upon the relative hydrogen sulfide slippage given in the table above, the monoethanolamine/ethanedial Product #5 yields an overall absorption efficiency that is significantly more than 6 percent greater than the HSW 0700F. Based upon the continuing absorption capacity of Products #1- 4 between 300 and 390 minutes in Example 1, it is believed that the overall absorption capacity of the monoethanolamine/ethanedial product may be as much as 10 percent greater than that of HSW 0700F.

Prior to the test, Product #5 was obServed to have a medium brown color, primarily due to the approximately 1450 F reaction temperature, and a viscosity just slightly greater than water. Immediately upon initiation of the test, exposing Product #5 to hydrogen sulfide, the color began turning a darker brown until reaching a dark black color within about 30 minutes. Once depleted, the spent Product #5 had a viscosity similar to used automotive oil.

Example 3 Two tests were performed to compare the hydrogen sulfide scavenging capacity of six scavenger solution prepared with different molar ratios of monoethanolamine and ethanedial ranging from 1:1 up to 3:1. An aliquot of 30 ml of each scavenger solution, without dilution, was placed into a glass tower. One standard cubic foot per hour (SCFH) of a gas containing about 1% hydrogen sulfide, about 5% carbon dioxide and about 94% methane was bubbled into the bottom of the tower. In each test, a sample of the gas into the towers was measured to provide a control or blank. The tests were carried out over a period of three hours and fifteen minutes, taking measurements of the parts per million of hydrogen sulfide in the gas out of the top of each tower every fifteen minutes. The results of these tests are contained in the table below. Test #1 Test #2 Time Hydrogen Sulfide in Exit Gas (PPM) Hydrogen Sulfide in Exit Gas (PPM) Elapsed Blank 1.5:1 2:1 2.5:1 Blank 1:1 2:1 3:1 0 12,900 - - - 13,500 - - - 15 - 420 12 7 ~ - 9,000 20 5 45 - 1,000 46 22 - 7,800 84 19 75 - 1,300 150 46 - 8,500 210 62 105 - 1,450 375 117 - 10,600 430 108 135 - 1,400 475 210 - 8,600 940 235 165 - 1,100 725 425 - 10,200 1,100 - 550 195 - 1,700 1,000 800 - 10,400 1,700 1,100 The tests show that scavengers having a monoethanolamine: ethanedial ratio of between about 2:1 and about 3:1 remove significantly more hydrogen sulfide from the gas than do scavengers with ratios of 1.5:1 and below. As the data supports, compositions prepared at ratios of 1.5:1 and below react with hydrogen sulfide much slower than those with ratios above 1.5:1 and, therefore, would require unacceptably large scrubber towers in order to provide sufficient contact time to reduce the hydrogen sulfide concentration in many sources of natural gas down to pipeline specifications. A reaction rate that is sufficiently fast to minimize the size and cost of physical equipment, such as scrubber towers, is an important commercial consideration in choosing a sulfide scavenger. The performance shown in the table for compositions prepared at ratios of 1.5:1 and below would not be sufficient to meet these commercial standards. By contrast, the performance of compositions prepared at ratios above 1.5:1 is dramatically improved. In certain circumstances, the compositions prepared at ratios between about 2:1 and about 3:1 have been found to out perform the top sulfide scavenger in the industry.

Example 4 The effect of regeneration on the scavenging capacity of a given feed was evaluated by monitoring the quantity of sulfides scavenged by feeds with variable aldehyde content. Case Case Case #1 #2 #3 Monoethanolamine (moles) 2 3 3 Ethanedial (moles) 1 0 1 Formaldehyde (moles) 0 3 1 H2S Scavenged (moles) 3 2 3.7 MEA Liberated (moles) 1 1 1.3 Case #1 - ThiosweetTM MEA/ethanedial scavenger Case #2 - SulfascrubTM MEA/formaldehyde scavenger (triazine) Case #3 - MEA/mixed aldehyde (50% formaldehyde/50% ethanedial) scavenger The results show that each of the three scavengers liberate MEA upon scavenging hydrogen sulfide. Note that the use of ethanedial provides for a more efficient utilization of MEA since for every two (2) moles of MEA combined with ethanedial, one (1) mole of MEA is liberated. By contrast, the triazine product requires three (3) moles of MEA combined with formaldehyde, while liberating only one (1) mole of MEA.

While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims which follow.