WO2019118282A1

WO2019118282A1 - Regeneration process of an anionic exchange resin used for mercaptans removal

Info

Publication number: WO2019118282A1
Application number: PCT/US2018/064385
Authority: WO
Inventors: Runyu TAN; Reba S. GEORGETOWN
Original assignee: Dow Global Technologies Llc
Priority date: 2017-12-13
Filing date: 2018-12-07
Publication date: 2019-06-20

Abstract

A method is disclosed for the removal of mercaptans from a gas feedstream. The method provides for passing a gas feedstream comprising mercaptans though a regenerable adsorbent media which adsorbs mercaptans to provide a mercaptan containing-lean gas product and a mercaptan-rich adsorbent media. The regenerable adsorbent media is a strongly basic ion exchange resin.

Description

REGENERATION PROCESS OF AN ANIONIC EXCHANGE RESIN USED FOR

MERCAPTANS REMOVAL

FIELD

The present invention relates to a novel adsorption method for removal of mercaptans from liquid and gas feed streams, and more particularly, an adsorption method for purification of hydrocarbons, petroleum distillates, natural gas and natural gas liquids, associated and refinery gases.

BACKGROUND

Fluid streams derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of acid gases, for example carbon dioxide (C0₂), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbon disulfide (CS₂), hydrogen cyanide (HCN), carbonyl sulfide (COS), or mercaptans as impurities. These fluid streams may be gas, hydrocarbon gases from shale pyrolysis, synthesis gas, and the like or liquids such as liquefied petroleum gas (LPG) and natural gas liquids (NGL).

In natural gas processing, it is often desirable to remove sulfur compounds from the feedstream. Owing to this problem, the technology-of removing these substances is conventionally termed as "sweetening" or deodorization. These sulfur-contaminated compounds are also corrosive, causing damage to technological equipment and transportation systems. Further, practically all sulfur-contaminated compounds are irreversible poisons for many catalysts used in chemical processes. Therefore, such commercially important processes as natural gas steam reforming, individual hydrocarbons and petroleum distillate isomerization, hydrogenation, etc. require practically complete removal of the many sulfur compounds from the process feed before catalysis. Finally, it should be mentioned, that the full oxidation of the organic sulfur compounds leads to sulfur dioxide and sulfur trioxide, whose formation needs to be minimized for ecological reasons.

Removal of sulfur containing compounds is normally done in two steps. In a first stage, the amine treatment removes hydrogen sulfide from the system. Some mercaptans, part of carbon oxysulfide and of carbon dioxide may also be removed in this step. This process is related to absorption. The second step is an adsorption of organic sulfur compounds, especially mercaptans, sulfides, thiophenes, thiophanes and disulfides.

Adsorption of sulfur-contaminated compounds is the most common method for removal of these sulfur compounds, because of the high performance and relatively low capital and operational costs. Numerous processes and adsorbents have been developed for the removal of organic sulfur compounds and hydrogen sulfide, carbon oxysulfide and carbon disulfide, from gases and liquids.

The most widely used physical adsorbents for these sulfur compounds are synthetic zeolites or molecular sieves. For example, USP 2,882,243 and 2,882,244 disclose an enhanced adsorption capacity of molecular sieves for hydrogen sulfide at ambient temperatures. USP 3,760,029 discloses the use of synthetic faujasites as an adsorbent for dimethyl disulfide removal from n-alkanes. USP 3,816,975; 4,540,842; and 4,795,545 disclose the use of standard molecular sieves as a sulfur adsorbent for the purification of liquid hydrocarbon feedstocks. For removal of carbonyl sulfide, mercaptans, and other sulfur compounds from liquid n-alkanes, USP 4,098,684 discloses the use of combined beds of two or more molecular sieves. EP 0781832 discloses zeolites as adsorbents for hydrogen sulfide and tetrahydrothiophene in natural gas feed streams. Regeneration of these molecular sieves requires elevated

temperatures.

Ion exchange resins have been used to remove mercaptans from mixtures of hydrocarbons, such as gasoline and naphtha. For example, see USP 2,718,489 which discloses the use of alcohol with the caustic to improve the regeneration of ion exchange resin or USP 2,730,486 which discloses the use of a solution of alkali metal hydroxide and oxygen gas to regenerate ion exchange material exhausted with mercaptans. However, these methods present problems with regeneration of the adsorption media.

All these adsorbents can work at ambient temperature and have a substantial capacity for removal of sulfur compounds at relatively high concentrations. While all these products have been useful for gas and liquid stream purification of sulfur- contaminated compounds, they need special arrangements to get full regeneration. It is a main aspect of the present invention to provide improved adsorbents and processes to remove sulfur containing compounds which do not have the disadvantages of the regeneration mentioned above.

Accordingly, it is an aspect of the invention to provide an adsorbent and a method for purification of sulfur-contaminated feed streams with improved regeneration capabilities.

SUMMARY

The present invention is a method for removing mercaptans from a gas feedstream comprising mercaptans comprising, consisting essentially of, consisting of the steps of: (a) passing the gas feedstream through an anion exchange resin, preferably a strongly basic ion exchange resin, to provide a mercaptan-lean gas feedstream and a mercaptan-loaded anion exchange resin; (b) further treating, recovering, transporting, liquefying, or flaring the mercaptan-lean natural gas stream,

(c) regenerating the loaded anion exchange resin by treatment with an aqueous mixture comprising, consisting essentially of, consisting of (i) a base and (ii) a liquid oxidant wherein the adsorbed mercaptans are desorbed from the loaded anion exchange resin forming a regenerated anion exchange resin and an aqueous phase comprising desorbed mercaptans, and (d) discharging the aqueous phase comprising desorbed mercaptans wherein the regenerated anion exchange resin may be reused as an adsorbent for the separation of additional mercaptans from the same gas feedstream, i.e., in step (a), or different gas feedstream, wherein the base is a hydroxide, sodium carbonate, potassium carbonate, potassium phosphate, an alkyl amine, or an alkanolamine, or the mixture thereof.

One embodiment is the method disclosed herein above wherein the anion exchange resin comprises a crosslinked copolymer matrix derived from reaction of at least one mono vinyl monomer and a polyvinyl aromatic crosslinking monomer. For example, the monovinyl monomer comprises styrene and the polyvinyl aromatic crosslinking monomer comprises divinylbenzene.

One another embodiment of the method described herein above, wherein the anion exchange resin is a gel-type resin.

In one embodiment of the method disclosed herein above, the hydroxide is ammonium hydroxide, an ammonium alkyl hydroxide, or a metal hydroxide. For example, sodium hydroxide, potassium hydroxide, cesium hydroxide, calcium hydroxide, and the like.

In one embodiment of the method disclosed herein above, the oxidant is a liquid oxidant. For example, hydrogen peroxide; a metal peroxide, such as sodium peroxide, potassium peroxide, calcium peroxide, or cesium peroxide; ammonium hypochlorite; an alkyl ammonium hypochlorite; a metal hypochlorite, such as sodium hypochlorite, potassium hypochlorite, cesium hypochlorite, or calcium hypochlorite; ammonium chlorate; an alkyl ammonium chlorate; a metal chlorate, such as sodium chlorate, potassium chlorate, cesium chlorate, or calcium chlorate; ammonium perchlorate; an alkyl ammonium perchlorate; or a metal perchlorate, such as sodium perchlorate, potassium perchlorate, cesium perchlorate, or calcium perchlorate.

DETAILED DESCRIPTION

Raw natural gas comes from three types of wells: oil wells, gas wells, and condensate wells. Natural gas that comes from oil wells is typically termed“associated gas”. This gas can exist separate from oil in the formation (free gas), or dissolved in the crude oil (dissolved gas). Natural gas from gas and condensate wells, in which there is little or no crude oil, is termed“non-associated gas”. Gas wells typically produce raw natural gas by itself, while condensate wells produce free natural gas along with a semi-liquid hydrocarbon condensate. Whatever the source of the natural gas, once separated from crude oil (if present) it commonly exists as methane in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes and to a lesser extent heavier hydrocarbons.

Raw natural gas and sometimes treated natural gas often contain a significant amount of impurities, such as water or acid gases, for example carbon dioxide (CO₂), hydrogen sulfide (H₂S), hydrogen cyanide (HCN), and low molecular weight sulfur compounds, such as, but not limited to sulfur dioxide (SO₂), disulfides, such as carbon disulfide (CS₂), sulfides, such as carbonyl sulfide (COS) and dimethyl sulfide, thiophenes, thiophanes or mercaptans as impurities. Mercaptan compounds include but not limited to a Ci mercaptan, such as methanethiol, or C₂ to Ce mercaptans, for example ethanethiol, l-propanethiol, 2- propanethiol, l-butanethiol, 2-butanethiol, cyclohexanethiol, or greater than Ce mercaptans, such as benzenethiol, a-toluenethiol, or disulfides which include but are not limited to dimethyl sulfide, diethylsulfide, methylethylsulfide, or combinations thereof. The term“gas feedstream” as used in the method of the present invention includes any liquid or gas source comprising methane in mixtures with other hydrocarbons, including, but not limited to, raw natural gas or raw natural gas that has been treated one or more times to remove water and/or other impurities.

The method is the use of an adsorbent to remove low molecular weight compounds, specifically sulfur dioxide (S0₂), carbon disulfide (CS₂), carbonyl sulfide (COS), and/or mercaptans from a gas feedstream. Suitable adsorbents are anionic ion exchange resins, such as strongly basic anion exchange resins. For example, the adsorbent resin may include quaternary ammonium groups, and the resins may show as strongly basic materials similar to inorganic bases such as sodium hydroxide or potassium hydroxide.

In one embodiment, the adsorbent media, is an anion exchange resin that is used to remove mercaptans from a gas feedstream (e.g., a natural gas feedstream) comprising mercaptans and optionally one or more other impurities. The mechanism by which the anion exchange resin extracts the mercaptans from the natural gas stream may be a combination of adsorption and absorption, the dominating mechanism at least is believed to be adsorption. Accordingly, the terms "adsorption" and "adsorbent" are used throughout this specification, although this is done primarily for convenience.

The invention is not considered to be limited to any particular mechanism.

When an anion exchange resin has adsorbed any amount of mercaptans it is referred to as“loaded”. Loaded includes a range of adsorbance from a low level of mercaptans up to and including saturation with mercaptans.

There are a variety of anion exchange resins suitable for use in accordance with the present invention. For example, the anion exchange resin is a strongly basic ion exchange resin. In general, a strongly basic ion exchange resin is one which on titration with hydrochloric acid in water free from electrolytes has a pH above approximately 7.0 when the amount of hydrochloric acid added is one-half of that required to reach the inflection point (equivalence point). A weakly basic ion exchange resin under the same conditions has a pH below approximately 7.0 when one-half of the acid required to reach equivalence point has been added.

In one embodiment, the anion exchange resin is an amine anion exchange resins. The anion exchange resins may comprise primary, secondary, tertiary and/or quaternary amine functional groups. Those anion exchange resins comprising predominantly tertiary amine functional groups, e.g., dimethylaminomethyl functional groups, may be among the more effective anion exchange resins. In another embodiment, the anion exchange resins comprise cross-linked monoethylenically unsaturated monomerpolyvinylidene monomer copolymer matrices having desirable surface area and high pore diameter properties affording greater access to a larger number of functional groups. Cross-linked styrene polyvinyl copolymers are a notable example. Other monoethylenically unsaturated monomers, for example, alpha methylstyrene, mono- and polychlorostyrenes, vinyltoluene, vinylanisole, vinylnaphthalene, and the like, have been disclosed as being copolymerizable with other polyvinylidiene monomers, for example, trivinylbenzene, di vinylnaphthalene, divinylethene, trivinylpropene, and the like, to form desirable cross-linked copolymer matrices.

Exemplary anion exchange resins include amine anion exchange resins comprising a porous styrene divinylbenzene cross-linked copolymer matrix, including such amine anion exchange resins having primary, secondary, tertiary, and/or quaternary amine functional groups. AMBERLYST™ A-21, described as a weakly anion exchange resin comprising a porous cross-linked styrene divinylbenzene copolymer matrix having tertiary amine functional groups, is a preferred anion exchange resin. Anion exchange resins manufactured under the trade name

AMBERLYST™ A-29 and DUOLITE™ A-7 are exemplary of a commercial anion exchange resin which can be employed. The former is described as an intermediate strength anion exchange resin and the latter is described as a weakly anion exchange resin comprising secondary and tertiary amine functional groups.

The anion exchange resin is generally available as a spherical particulate of greater than 0 to 50 mesh (e.g., 0.41 mm to 0. 1 mm diameter).

Generally, anion exchange resins have a surface area of from approximately 30 to 40 square meters per gram, an average pore diameter of from approximately 900 to 1,300 angstroms, and a porosity of from approximately 44% to 56%.

The anion exchange resin may be a polystryrenic anion resin in hydroxide form such as DOWEX™ , MARATHON™ MSA, MONOSPHERE™ 550A, and

AMBERLYST™ A26 available from The Dow Chemical Company and the NRW series from Purolite. In one embodiment, the anionic exchange resin is a strong base anion exchange resin, e.g., provided in bead form having a median diameter from 10 to 2,000 microns and/or from 100 to 1,000 microns. The beads may have a Gaussian particle size distribution or may have a relatively uniform particle size distribution, i.e.“monodisperse” that is, at least 90 volume percent of the beads have a particle diameter from approximately 0.8 to 1.2 and/or 0.85 to 1.15 times the volume average particle diameter. Monodisperse beads are preferred.

The subject anion exchange resins are preferably gel-type. The terms “microporous,”“gellular,”“gel” and“gel-type” are synonyms that describe copolymer resins having pore sizes less than approximately 20 Angstroms (A). In distinction, macroporous resins have both mesopores of from 20 A to 500 A and macropores of greater than about 500 A. Gel-type copolymer beads, as well as their preparation are described in USP 4,256,840 and USP 5,244,926. One exemplary method is known in the art as a“seeded” polymerization, sometimes also referred to as batch or multi-batch (as generally described in EP 62088A1 and EP 179133A1); and continuous or semi-continuous staged polymerizations (as generally described in USP 4,419,245; USP 4,564,644; and USP 5,244,926). A seeded polymerization process typically adds monomers in two or more increments. Each increment is followed by complete or substantial polymerization of the monomers therein before adding a subsequent increment. A seeded polymerization is advantageously conducted as a suspension polymerization wherein monomers or mixtures of monomers and seed particles are dispersed and polymerized within a continuous suspending medium. In such a process, staged polymerization is readily

accomplished by forming an initial suspension of monomers, wholly or partially polymerizing the monomers to form seed particles, and subsequently adding remaining monomers in one or more increments. Each increment may be added at once or continuously. Due to the insolubility of the monomers in the suspending medium and their solubility within the seed particles, the monomers are imbibed by the seed particles and polymerized therein. Multi-staged polymerization techniques can vary in the amount and type of monomers employed for each stage as well as the polymerizing conditions employed. The seed particles employed may be prepared by known suspension polymerization techniques. In general the seed particles may be prepared by forming a suspension of a first monomer mixture in an agitated, continuous suspending medium as described by F. Helfferich in Ion Exchange, (McGraw-Hill 1962) at pp. 35-36. The first monomer mixture comprises: 1) a first monovinylidene monomer, 2) a first crosslinking monomer, and 3) an effective amount of a first free-radical initiator. The suspending medium may contain one or more suspending agents commonly employed in the art. Polymerization is initiated by heating the suspension to a temperature of generally from approximately 50-90°C. The suspension is maintained at such temperature or optionally increased temperatures of approximately 90-150° C until reaching a desired degree of conversion of monomer to copolymer. Other suitable polymerization methods are described in USP 4,444,961; USP 4,623,706; USP 4,666,673; and USP 5,244,926 - each of which is incorporated herein in its entirety.

The monovinylidene aromatic monomers employed herein may be those of which reference is made to Polymer Processes, edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Inc., New York, Chapter III, “Polymerization in Suspension” at pp. 69-109. Table II (pp. 78-81) of Schildknecht lists diverse types of monomers which are suitable in practicing the present invention. Of the monomers listed, styrene and substituted styrene are preferred. The term “substituted styrene” includes substituents of either/or both the vinylidene group and phenyl group of styrene and include: vinyl naphthalene, alpha alkyl substituted styrene (e.g., alpha methyl styrene) alkylene-substituted styrenes (particularly monoalkyl- substituted styrenes such as vinyltoluene and ethylvinylbenzene) and halo-substituted styrenes, such as bromo or chlorostyrene and vinylbenzyl chloride. Additional monomers may be included along with the monovinylidene aromatic monomers, including monovinylidene non-styrenics such as: esters of a,b-ethylenically unsaturated carboxylic acids, particularly acrylic or methacrylic acid, methyl methacrylate, isobomyl- methacrylate, ethylacrylate, and butadiene, ethylene, propylene, acrylonitrile, and vinyl chloride; and mixtures of one or more of said monomers.

Preferred monovinylidene monomers include styrene and substituted styrene such as ethylvinylbenzene. The term“monovinylidene monomer” is intended to include homogeneous monomer mixtures and mixtures of different types of monomers, e.g. styrene and isobornylmethacrylate. The seed polymer component may comprise a styrenic content greater than 50 molar percent, greater than 75, and/or greater than 95 molar percent (based upon the total molar content). The term“styrenic content” refers to the quantity of monovinylidene monomer units of styrene and/or substituted styrene utilized to form the copolymer. “Substituted styrene” includes substituents of either/or both the vinylidene group and phenyl group of styrene as described above. In exemplary embodiments, the first monomer mixture used to form the first polymer component (e.g. seed) comprises at least 75 molar percent, at least 85 molar percent, and/or at least 95 molar percent of styrene.

Examples of suitable crosslinking monomers (i.e., polyvinylidene compounds) include polyvinylidene aromatics such as divinylbenzene, divinyltoluene,

divinylxylene, divinylnaphthalene, trivinylbenzene, divinyldiphenylsulfone, as well as diverse alkylene diacrylates and alkylene dimethacrylates. Exemplary crosslinking monomers are divinylbenzene, trivinylbenzene, and ethylene glycol dimethacrylate. The terms“crosslinking agent,”“crosslinker” and“crosslinking monomer” are used herein as synonyms and are intended to include both a single species of crosslinking agent along with combinations of different types of crosslinking agents. The proportion of crosslinking monomer in the copolymer seed particles is preferably sufficient to render the particles insoluble in subsequent polymerization steps (and also on conversion to an ion-exchange resin), yet still allow for adequate imbibition of an optional phase-separating diluent and monomers of the second monomer mixture. In some embodiments, no crosslinking monomer will be used. Generally, a suitable amount of crosslinking monomer in the seed particles is minor, i.e., desirably from approximately 0.01 to 12 molar percent based on total moles of monomers in the first monomer mixture used to prepare the seed particles. In a exemplary embodiment, the first polymer component (e.g. seed) is derived from polymerization of a first monomer mixture comprising at least 85 molar percent of styrene (or substituted styrene such as ethyl vinylbenzene) and from 0.01 to 10 molar percent of divinylbenzene.

Polymerization of the first monomer mixture may be conducted to a point short of substantially complete conversion of the monomers to copolymer or alternatively, to substantially complete conversion. If incomplete conversion is desired, the resulting partially polymerized seed particles advantageously contain a free-radical source therein capable of initiating further polymerization in subsequent polymerization stages. The term“free-radical source” refers to the presence of free-radicals, a residual amount of free-radical initiator or both, which is capable of inducing further polymerization of ethylenically unsaturated monomers. In such an embodiment of the invention, there is from approximately 20 to 95 weight percent and/or 50 to 90 weight percent of the first monomer mixture, based on weight of the monomers therein, converted to copolymer. Based on the presence of the free radical source, the use of a free-radical initiator in a subsequent polymerization stage would be optional. For embodiments where conversion of the first monomer mixture is substantially complete, it may be necessary to use a free -radical initiator in subsequent polymerization stages.

The free-radical initiator may be any one or a combination of conventional initiators for generating free radicals in the polymerization of ethylenically unsaturated monomers. Representative initiators are UV radiation and chemical initiators, such as azo-compounds including azobisisobutyronitrile; and peroxygen compounds such as benzoyl peroxide, t-butylperoctoate, t-butylperbenzoate and isopropylpercarbonate. Other suitable initiators are mentioned in USP 4,192,921, USP 4,246,386 and USP 4,283,499 - each of which is incorporated in its entirety. The free-radical initiators are employed in amounts sufficient to induce polymerization of the monomers in a particular monomer mixture. The amount will vary as those skilled in the art can appreciate and will depend generally on the type of initiators employed, as well as the type and proportion of monomers being polymerized. Generally, an amount of from approximately 0.02 to 2 weight percent is adequate, based on total weight of the monomer mixture.

The first monomer mixture used to prepare the seed particles is advantageously suspended within an agitated suspending medium comprising a liquid that is substantially immiscible with the monomers, (e.g. preferably water). Generally, the suspending medium is employed in an amount from approximately 30 to 70 and/or from approximately 35 to 50 weight percent based on total weight of the monomer mixture and suspending medium. Various suspending agents are conventionally employed to assist with maintaining a relatively uniform suspension of monomer droplets within the suspending medium. Illustrative suspending agents are gelatin, polyvinyl alcohol, magnesium hydroxide, hydroxyethylcellulose, methylhydroxyethyl cellulose methylcellulose and carboxymethyl methylcellulose. Other suitable suspending agents are disclosed in USP 4,419,245. The amount of suspending agent used can vary widely depending on the monomers and suspending agents employed. Latex inhibitors such as sodium dichromate may be used to minimize latex formation.

In the so-called“batch-seeded” process, seed particles comprising from approximately 10 to 50 weight percent of the copolymer may be suspended within a continuous suspending medium. A second monomer mixture containing a free radical initiator may then be added to the suspended seed particles, imbibed thereby, and then polymerized. Although less preferred, the seed particles can be imbibed with the second monomer mixture prior to being suspended in the continuous suspending medium. The second monomer mixture may be added in one amount or in stages. The second monomer mixture may be imbibed by the seed particles under conditions such that substantially no polymerization occurs until the mixture is substantially fully imbibed by the seed particles. The time required to substantially imbibe the monomers will vary depending on the copolymer seed composition and the monomers imbibed therein. However, the extent of imbibition can generally be determined by microscopic examination of the seed particles, or suspending media, seed particles and monomer droplets. The second monomer mixture desirably may contain from approximately 0.5 to 25 molar percent, from 2 to 17 molar percent, and/or from 2.5 to 8.5 molar percent of crosslinking monomer based on total weight of monomers in the second monomer mixture with the balance comprising a monovinylidene monomer. Whereas the selection of crosslinking monomer and monovinylidene monomer may be the same as those described above with reference to the preparation of the first monomer mixture, (i.e. seed preparation). As with the seed preparation, the monovinylidene monomer may include styrene and/or a substituted styrene. In an exemplary embodiment, the second polymer component (i.e. second monomer mixture, or“imbibed” polymer component) has a styrenic content greater than 50 molar percent and/or at least 75 molar percent (based upon the total molar content of the second monomer mixture). In an exemplary embodiment, the second polymer component is derived from

polymerization of a second monomer mixture comprising at least 75 molar percent of styrene (and/or substituted styrene such as ethylvinylbenzene) and from approximately 1 to 20 molar percent divinylbenzene. In an in-situ batch-seeded process, seed particles comprising from

approximately 10 to 80 weight percent of the copolymer product are initially formed by suspension polymerization of the first monomer mixture. The seed particles can have a free-radical source therein as previously described, which is capable of initiating further polymerization. Optionally, a polymerization initiator can be added with the second monomer mixture where the seed particles do not contain an adequate free radical source or where additional initiator is desired. In this embodiment, seed preparation and subsequent polymerization stages are conducted in-situ within a single reactor. A second monomer mixture is then added to the suspended seed particles, imbibed thereby, and polymerized. The second monomer mixture may be added under polymerizing conditions, but alternatively may be added to the suspending medium under conditions such that substantially no polymerization occurs until the mixture is substantially fully imbibed by the seed particles. The composition of the second monomer mixture preferably corresponds to the description previously given for the batch- seeded embodiment.

The copolymer product may be chloromethylated and subsequently am mated. The specific means and conditions for chloromethylating the copolymers are not particularly limited and many applicable techniques are documented in the literature, as illustrated by: G. Jones,“Chloromethylation of Polystyrene,” Industrial and

Engineering Chemistry, Vol. 44, No. 1, pgs. 2686-2692, (Nov 1952), along with US 2008/0289949 and USP 6,756,462 - both of which are incorporated herein in their entirety. Chloromethylation is typically conducted by combining the polymer with a chloromethylation reagent in the presence of a catalyst at a temperature of from approximately 15 to l00°C, e.g., 35 to 70°C for approximately 1 to 8 hours. An exemplary chloromethylation reagent is chloromethyl methyl ether (CMME); however, other reagents may be used including CMME-forming reactants such as the combination of formaldehyde, methanol and hydrogen chloride or chlorosulfonic acid (as described in US 2004/0256597), or hydrogen chloride with methylated formalin.

The chloromethylating reagent is typically combined with the polymer in an amount of from approximately 0.5 to 20 and/or 1.5 to 8 mole of CMME per mole of polymer. While less preferred, other chloromethylation reagents may be used including but not limited to: bis-chloromethyl ether (BCME), BCME-forming reactants such as formaldehyde and hydrogen chloride, and long chain alkyl chloromethyl ethers as described in USP 4,568,700.

Catalyst useful for conducting chloromethylation reactions are well known and are often referred to in the art as“Lewis acid” or“Friedel-Crafts” catalyst. Non- limiting examples include: zinc chloride, zinc oxide, ferric chloride, ferric oxide, tin chloride, tin oxide, titanium chloride, zirconium chloride, aluminum chloride and sulfuric acid along with combinations thereof. Halogens other than chloride may also be used in the preceding examples. An exemplary catalyst is ferric chloride. The catalyst is typically used in an amount corresponding to approximately 0.01 to 0.2 and/or 0.02 to 0.1 mole catalyst per mole of polymer repeating unit. Catalyst may be used in combination with optional catalyst adjuncts such as calcium chloride and activating agents such as silicon tetrachloride. More than one catalyst may be used to achieve the desired chloromethylation reaction profile.

Solvents and/or swelling agents may also be used in the chloromethylation reaction. Examples of suitable solvents including but are not limited to one or more of: an aliphatic hydrocarbon halides such as ethylene dichloride, dichloropropane, dichloromethane, chloroform, diethyl ether, dipropyl ether, dibutyl ether and diisoamyl ether. When CMME is used as the chloromethylation agent, such solvents and/or swelling agents are often not necessary.

The chloromethylated vinyl aromatic polymer is reacted with an amine to form an ion exchange resin including functional amine groups. The amination is preferably conducted by combining a tertiary amine and a vinyl aromatic polymer comprising benzyl chloride groups within an alcohol-based solvent to form a reaction mixture. An exemplary tertiary amine is represented by the Formula 1.

Formula 1: k ί"^'"k₃

R₂ wherein Ri, FT and R₃ are each independently selected from: alkyl and alkoxy groups each having from 1 to 6 carbon atoms and/or 1 to 2 carbon atoms. Each alkyl or alkoxy group (Ri, FT and R₃) may independently be: straight (e.g. ethyl, propyl, butyl, pentyl, etc.) or branched (e.g. isopropyl, isobutyl, etc.), and may be unsubstituted or substituted (e.g. substituted with such groups as a hydroxyl). In one series of embodiments, the three alkyl groups (Ri, R₂ and R₃) are independently selected from unsubstituted alkyl groups which may be straight or branched. In other embodiments,“mixed species” of the subject tertiary amines may be used. Representative amines include:

trimethylamine, dimethylaminoethanol, triethylamine, tripropylamine and

tributylamine. An exemplary amine is trimethyl amine. A reaction product of the subject amination, i.e. a vinyl aromatic polymer including a quaternary ammonium functionality including a nitrogen atom bonded to a benzyl carbon of the polymer and three alkyl or alkoxy groups. By way of illustration, Formula 2 provides a structural formula of a repeating unit of vinyl aromatic polymer including quaternary ammonium functionality.

Formula 2:

wherein the benzyl carbon is located at the meta, ortho or para position (typically including a combination of species but with predominantly para substitution) of the aromatic ring; and wherein Ri, R₂ and R₃ are the same as previously described with respect to Formula 1. Representative amination reactions are described in: USP 5,134,169, USP 6,756,462, USP 6,924,317, USP 8,273,799 and US 2004/0256597. Representative strong base anion exchange resins include DOWEX™ 1x4 and

DOWEX MARATHON™ 11, both commercially available from The Dow Chemical Company.

Suitable procedures for using an anion exchange resin include the following. When starting with a fresh anion exchange material which is received from the supplier in the form of a salt of a strong acid, for example a chloride, the exchange material is treated with an aqueous solution of an alkali metal hydroxide, for example sodium hydroxide. It has been found that treating one volume of exchange resin in the chloride form with 40 volumes of 4% aqueous sodium hydroxide converts the exchange resin to the hydroxyl form.

In one embodiment, the method for removing mercaptans from a gas feedstream comprising mercaptans comprising the stages of: (a) passing the gas feedstream through an anion exchange resin to provide a mercaptan-lean gas feedstream and a mercaptan-loaded anion exchange resin; (b) further treating, recovering, transporting, liquefying, or flaring the mercaptan- lean natural gas stream, (c) regenerating the loaded anion exchange resin, for example, by treatment with an aqueous mixture comprising, consisting essentially of, consisting of (i) a base, preferably a hydroxide, more preferably a metal hydroxide, or another base such as sodium carbonate, potassium carbonate, potassium phosphate, an alkyl amine, or an alkanolamine, or the mixture thereof, and (ii) an oxidant wherein the adsorbed mercaptans are desorbed from the loaded anion exchange resin forming a regenerated anion exchange resin and an aqueous phase comprising desorbed mercaptans, and (d) discharging the aqueous phase comprising desorbed mercaptans wherein the regenerated anion exchange resin can be reused as an adsorbent for the separation of additional mercaptans from the same, i.e., in step (a) or different gas feedstream.

In one embodiment, the anion exchange resin is in a packed column.

In another embodiment of the method, the anion exchange resin may be in a bed within a column or there may be a multiple adsorbent beds within a column.

In one embodiment, the adsorbent bed(s) may be regenerated in-place.

Generally, the adsorption step and/or the regeneration step of the method may operate as a batch process.

Mercaptan adsorption/desorption onto/from the anionic ion exchange resin is a reversible process. A loaded or partially loaded anionic exchange resin is regenerated by passing an aqueous solution comprising, consisting essentially of, or consisting of (i) a base, e.g., a hydroxide, such as ammonium hydroxide, an alkyl ammonium hydroxide, a metal hydroxide or other base such as sodium carbonate, potassium carbonate, potassium phosphate, an alkyl amine, or an alkanolamine, or the mixture thereof and (ii) an oxidant, preferably a liquid oxidant (e.g., a liquid at ambient conditions of temperature and pressure). In one embodiment, the metal hydroxide is sodium hydroxide, potassium hydroxide, cesium hydroxide, calcium hydroxide, and the like.

The hydroxide is present in amount equal to or greater than 1 weight percent, equal to or greater than 2 weight percent, equal to or greater than 4 weight percent, and/or equal to or greater than 5 weight percent, weight percents based on the total weight of the aqueous hydroxide/oxidant solution. The hydroxide is present in amount equal to or less than 50 weight percent, equal to or less than 40 weight percent, equal to or less than 30 weight percent, equal to or less than 20 weight percent, and/or equal to or less than 10 weight percent, weight percents based on the total weight of the aqueous hydroxide/oxidant solution.

In one embodiment, the oxidant is hydrogen peroxide; a metal peroxide, such as sodium peroxide, potassium peroxide, calcium peroxide, cesium peroxide, and the like; ammonium hypochlorite; alkyl ammonium hypochlorite; a metal hypochlorite, such as sodium hypochlorite, potassium hypochlorite, cesium hypochlorite, calcium

hypochlorite, and the like; ammonium chlorate; an alkyl ammonium chlorate; a metal chlorate, such as sodium chlorate, potassium chlorate, cesium chlorate, calcium chlorate, and the like; ammonium perchlorate; an alkyl ammonium perchlorate; a metal perchlorate, such as sodium perchlorate, potassium perchlorate, cesium perchlorate, calcium perchlorate, and the like (or a mixture thereof).

The oxidant is present in amount equal to or greater than 0.01 weight percent, equal to or greater than 0.1 weight percent, equal to or greater than 0.5 weight percent, and/or equal to or greater than 1 weight percent, weight percents based on the total weight of the aqueous hydroxide/oxidant solution. The oxidant is present in amount equal to or less than 30 weight percent, equal to or less than 20 weight percent, equal to or less than 10 weight percent, equal to or less than 5 weight percent, and/or equal to or less than 2 weight percent, weight percents based on the total weight of the aqueous hydroxide/oxidant solution.

A weight ratio of the base to the oxidant in the aqueous solution may be from 20:1 to 1:20 (e.g., from 15: 1 to 1:15, from 10:1 to 1:10, from 5:1 to 1:5, from 5:1 to 1:1, etc.). In exemplary embodiments, the amount of the base may be more than the amount of the oxidant in the aqueous solution. The aqueous solution may consistent essentially of the base, oxidant, and water. For example, the aqueous solution may consistent essentially of (i) a base that sodium hydroxide, potassium hydroxide, cesium hydroxide, calcium hydroxide, or a mixture thereof, (ii) an oxidant that is hydrogen peroxide, and (iii) water.

The balance of the aqueous solution of the hydroxide/oxidant is water.

Example 1.

The anion exchange resin used is DOWEX™ MARATHON™ MSA chloride form anion exchange resin packed in a 3/8 inch by 2.5 inch stainless steel column is exposed to a gas stream containing 1,000 ppm methyl mercaptan in nitrogen with a flow rate of 500 scc/min at atmospheric pressure and 25°C. The mercaptan breakthrough is monitored using an Agilent 490 Micro GC equipped with an Agilent Porabond Q column. Breakthrough of methyl mercaptan is immediately observed.

Example 2.

For a first regeneration stage, according to a comparative example, the anion exchange resin from Example 1 is treated with 25 mL aqueous solution of 4% sodium hydroxide as the base and the oxidant is excluded, and then rinsed with water until the pH of the effluent is neutral. It is then exposed to a gas stream containing 1,000 ppm methyl mercaptan in nitrogen with a flow rate of 500 scc/min at atmospheric pressure and 25°C. The mercaptan breakthrough is monitored using an Agilent 490 Micro GC equipped with an Agilent Porabond Q column. Breakthrough of methyl mercaptan is observed after 50 minutes.

Example 3.

For a second regeneration stage, the loaded anion exchange resin from Example 2 is regenerated with 25 mL aqueous solution of 4% sodium hydroxide as the base and 1% hydrogen peroxide as the oxidant, and then rinsed with water until the pH of the effluent is neutral. It is then exposed to a gas stream containing 1,000 ppm methyl mercaptan in nitrogen with a flow rate of 500 scc/min at atmospheric pressure and 25°C. Breakthrough of methyl mercaptan is observed after 60 minutes, whereas a higher breakthrough time shows a higher mercaptan absorption capacity. The experiment is stopped when complete breakthrough is observed. Accordingly, it is shown that by use of the combination of the base and oxidant in the regeneration stage the mercaptan absorption capacity can be improved by 20% relative to use of the base alone.

Example 4.

For a third regeneration stage, the loaded anion exchange resin from Example 3 is regenerated with 25 mL aqueous solution of 4% sodium hydroxide as the base and the oxidant is excluded, and then rinsed with water until the pH of the effluent is neutral. The effluent has the characteristic odor of mercaptan. The ion exchange material is then exposed to a gas stream containing 1,000 ppm methyl mercaptan in nitrogen with a flow rate of 500 scc/min at atmospheric pressure and 25°C.

Breakthrough of methyl mercaptan is observed after 50 minutes. The experiment is stopped when complete breakthrough is observed. Accordingly, it shown that the anion exchange resin is still regenerable after the second regeneration stage where the combination of the base and oxidant are used.

Example 5.

For a fourth regeneration stage, the loaded anion exchange resin from Example 4 is then regenerated with 25 mL aqueous solution of 4% sodium hydroxide and 1% hydrogen peroxide, and then rinsed with water until the pH of the effluent is neutral. It is then exposed to a gas stream containing 1,000 ppm methyl mercaptan in nitrogen with a flow rate of 500 scc/min at atmospheric pressure and 25°C. Breakthrough of methyl mercaptan is observed after 60 minutes. The experiment is stopped when complete breakthrough is observed. Accordingly, it is shown that the use of the combination of the base and oxidant in the regeneration stage still allows for additional regeneration stages, while still realizing a mercaptan absorption capacity that is improved by 20% relative to use of the base alone.

Claims

What is claimed is:

1. A method for removing mercaptans from a gas feedstream comprising mercaptans, the method comprising the stages of:

(a) passing the gas feedstream through an anion exchange resin to provide a mercaptan-lean gas feedstream and a mercaptan-loaded anion exchange resin;

(b) further treating, recovering, transporting, liquefying, or flaring the mercaptan-lean natural gas stream;

(c) regenerating the loaded anion exchange resin for reuse in step (a) by treatment with an aqueous mixture includes (i) a base and (ii) an oxidant, wherein the adsorbed mercaptans are desorbed from the loaded anion exchange resin forming a regenerated anion exchange resin and an aqueous phase comprising desorbed mercaptans; and

(d) discharging the aqueous phase comprising desorbed mercaptans.

2. The method as claimed in claim 1, wherein the anion exchange resin includes a crosslinked copolymer matrix derived from reaction of at least one monovinyl monomer and a polyvinyl aromatic crosslinking monomer.

3. The method as claimed in claim 2, wherein the mono vinyl monomer includes styrene and the polyvinyl aromatic crosslinking monomer includes divinylbenzene.

4. The method as claimed in any one of claims 1 to 3, wherein the anion exchange resin is a gel-type resin.

5. The method as claimed in any one of claims 1 and 4, wherein the base is ammonium hydroxide, an ammonium alkyl hydroxide, a metal hydroxide, sodium carbonate, potassium carbonate, potassium phosphate, an alkyl amine, an alkanolamine, or a mixture thereof.

6. The method as claimed in claim 5, wherein the metal hydroxide is sodium hydroxide, potassium hydroxide, cesium hydroxide, calcium hydroxide, or a mixture thereof.

7. The method as claimed in any one of claims 1 to 6, wherein the oxidant is a liquid oxidant.

8. The method as claimed in any one of claims 1 to 7, wherein the oxidant is hydrogen peroxide, a metal peroxide, ammonium hypochlorite, an alkyl ammonium hypochlorite, a metal hypochlorite, ammonium chlorate, an alkyl ammonium chlorate, a metal chlorate, ammonium perchlorate, an alkyl ammonium perchlorate, a metal perchlorate, or a mixture thereof.

9. The method as claimed in claim 8, wherein the oxidant is sodium peroxide, potassium peroxide, calcium peroxide, cesium peroxide, sodium hypochlorite, potassium hypochlorite, cesium hypochlorite, calcium hypochlorite, sodium chlorate, potassium chlorate, cesium chlorate, calcium chlorate, sodium perchlorate, potassium perchlorate, cesium perchlorate, calcium perchlorate, or a mixture thereof.

10. The method as claimed in any one of claims 1 to 4, wherein the base is sodium hydroxide, potassium hydroxide, cesium hydroxide, calcium hydroxide, or a mixture thereof and the oxidant is hydrogen peroxide.