Recovery of maleic anhydride

Abstract

A method and system for extracting maleic anhydride from a gas mixture, wherein the gas mixture is contacted with an extraction solvent capable of absorbing maleic anhydride at a temperature of about-60 DEG C to 250 DEG C and a pressure of up to about 50 ps i gauge.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 63 / 405,166, filed on September 9, 2022. The application is incorporated herein by reference.

[0003] STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT ACTIVITIES

[0004] not applicable. Technical Field

[0005] The present invention generally relates to the recovery of maleic anhydride from a gas mixture comprising maleic anhydride, and in particular to the recovery of maleic anhydride by using an extraction solvent comprising a phosphate ester. Background Art

[0006] Maleic anhydride (maleic anhydride) can be used alone or with other acids to prepare alkyd resins and polyester resins. It is also a widely used chemical synthesis intermediate. The global demand for maleic anhydride has been growing steadily and is expected to continue to grow in the next few years.

[0007] Maleic anhydride can be prepared industrially by contacting a feed gas containing molecular oxygen and a suitable hydrocarbon (including but not limited to n-butane and butene) with a vanadium-phosphorus-oxygen fixed bed or fluidized catalyst in a reactor. The reaction product gas discharged from the reactor contains maleic anhydride and oxidation by-products such as carbon monoxide, carbon dioxide, water vapor, acrylic acid and acetic acid, as well as inert gases present in air (when air is used as a molecular oxygen source).

[0008] There are various methods in the prior art for recovering maleic anhydride from the reaction product gases. In at least one example, maleic anhydride is recovered by directly condensing the reaction product gases or washing the gases with water and dehydrating the resulting aqueous mixture by azeotropic distillation with xylene. However, due to the improved product yield, the preferred recovery method comprises selective absorption of maleic anhydride in a suitable solvent and subsequent stripping of the maleic anhydride from the rich solvent to obtain a crude maleic anhydride product.

[0009] Currently, the solvent commonly used for selective absorption of maleic anhydride is dibutyl phthalate (CAS#84-74-2) (DBP). However, due to toxicological reasons, DBP is under various scrutiny by local governments, which may hinder its use in future separation and recovery methods. Therefore, there is a need to continuously develop improved solvents that have minimal toxicity and can replace DBP to selectively absorb maleic anhydride from the reaction product gas. Summary of the invention

[0010] The present invention describes a method for recovering maleic anhydride from a gas mixture containing maleic anhydride. The method comprises the step (a) of contacting the gas mixture with an extraction solvent containing a phosphate ester, wherein the contacting comprises transferring at least a portion of the maleic anhydride from the gas mixture to the extraction solvent. The method further comprises the step (b) of stripping the solvent effluent stream obtained in step (a) to produce a stream containing a crude maleic anhydride product and a regenerable extraction solvent substantially free of maleic anhydride but containing the extraction solvent. The method may further comprise the step (c) of recycling at least a portion of the regenerable extraction solvent containing the extraction solvent to step (a).

[0011] The present invention also provides a system for recovering maleic anhydride from a gas mixture containing maleic anhydride. The system comprises an absorption zone operable to contact the gas mixture with an extraction solvent containing a phosphate ester so as to transfer maleic anhydride from the gas mixture to the extraction solvent, and a stripping zone operable to separate maleic anhydride from the extraction solvent, wherein the absorption zone comprises a solvent storage tank containing the extraction solvent. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying figure is a process flow diagram summarizing methods and systems for recovering maleic anhydride according to some embodiments described herein. DETAILED DESCRIPTION

[0013] The present invention generally provides a method for extracting maleic anhydride from a gas mixture, wherein by contacting the gas mixture with an extraction solvent comprising a phosphoric acid ester, a gas stream substantially free of maleic anhydride and a solvent effluent stream comprising absorbed maleic anhydride and an extraction solvent are produced. The absorbed maleic anhydride can be recovered from the solvent effluent stream in a subsequent separation step to produce a crude maleic anhydride product. In certain embodiments, the extraction solvent is regenerable, because maleic anhydride can be released from the solvent effluent stream, and the extraction solvent can be circulated to contact more gas mixtures. It has been surprisingly found that, since the extraction solvent comprising a phosphoric acid ester can effectively absorb maleic anhydride from the gas mixture, it can be used to replace the liquid extraction solvent comprising dibutyl phthalate of the current prior art, thereby alleviating toxicity problems without losing performance. In addition, in the method of the prior art, the gas mixture leaving the reactor after the partial oxidation of C4-hydrocarbons must usually be cooled in one or more heat exchangers before recovery. However, due to the various impurities contained in the gas mixture, this step is known to cause scaling of the heat exchanger, thereby requiring an additional cleaning step and increasing operating costs. The extraction solvent of the present invention is capable of recovering maleic anhydride from the gas mixture at a relatively high temperature, thereby eliminating this cooling step and increasing the efficiency and cost-effectiveness of the process. Finally, due to the relatively high volatility of the extraction solvent of the present invention, the maleic anhydride absorbed therein can be separated from the extraction solvent using relatively little energy.

[0014] Before explaining various aspects of the present invention in detail, it should be understood that the application of the present invention is not limited to the configuration of structural details and components or steps or the methods described below. The present invention can have other embodiments or be implemented or executed in different ways. In addition, it should be understood that the words and terms used herein are only for descriptive purposes and should not be regarded as limiting.

[0015] The term "comprising" and its derivative statements appearing in this article are not used to exclude the existence of any additional components, steps or processes, whether they are disclosed here or not. In order to avoid any doubt, all compositions claimed by using the term "comprising" herein can include any additional additives, auxiliary agents or compounds, unless otherwise stated. On the contrary, if the term "substantially consisting of..." appears in this article, any other components, steps or processes of any subsequent citation are excluded from the scope, except those that are not important for operability, and if the term "consisting of..." is applied, any components, steps or processes that are not specifically described or listed are excluded. Unless otherwise stated, the terms "or" and "and / or" refer to each listed element and any combination thereof. For example, "A or B" and "A and / or B" refer to A, B, or A and B.

[0016] The indefinite articles used herein refer to one or more (i.e., at least one) of the grammatical subjects of the article. For example, "solvent" refers to one solvent or more than one solvent. The phrases "in one embodiment," "according to one embodiment," and similar terms generally refer to the features, structures, or characteristics following the term being included in at least one embodiment of the present invention, and may be included in multiple embodiments of the present invention. Importantly, such terms do not necessarily refer to the same embodiment. If the specification states that a component or feature "may," "can," "can," or "might" be included or have a certain characteristic, the component or feature does not necessarily have to be included or have the characteristic.

[0017] The terms "preferred" and "preferably" refer to embodiments that may provide certain benefits in certain circumstances. However, other embodiments may also be preferred under the same or other circumstances. In addition, the description of one or more preferred embodiments does not mean that other embodiments are not applicable, nor is it intended that other embodiments are excluded from the scope of the present invention.

[0018] As used herein, the term "about" may allow a certain degree of variation in a numerical value or range, for example, it may be within 10%, 5% or 1% of the stated value or the limit of the stated range.

[0019] The use of ordinal terms (i.e., "first," "second," "third," "fourth," etc.) is intended solely to distinguish between two or more options and, unless otherwise stated, does not imply any order or sequence or importance of one option relative to another or any order of addition.

[0020] Values ​​expressed in range format should be understood in a flexible manner to include not only the values ​​explicitly listed as the limits of the range, but also all individual values ​​or subranges contained within the range, as if each value and subrange were explicitly listed. For example, a range such as 1-6 should be understood to have specifically disclosed subranges such as 1-3, 2-4, 3-6, etc., as well as individual values ​​within the range such as 1, 2, 3, 4, 5, and 6. This applies no matter how broad the range is.

[0021] In the methods described herein, all operations may be performed in any order without departing from the principles of the invention, except where a temporal order or order of operations is clearly described. In addition, specific operations may be performed simultaneously unless there is clear limiting language indicating that they are to be performed separately. For example, a desired X operation and a desired Y operation may be performed simultaneously in a single operation, and the resulting method falls within the literal scope of the desired method.

[0022] The term "optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0023] The term "aryl" refers to an unsaturated aromatic hydrocarbon ring system group. Aromatic ring series include phenyl, naphthyl, azulenyl, anthracenyl, etc.

[0024] The term "regenerable" means that the media can be treated to restore its functions and properties to close to its original state.

[0025] The term "absorption" or variations thereof generally refers to a reversible process by which some components of a gas mixture are surrounded by solvent molecules and interact with them, thereby dissolving and forming a solution.

[0026] The term "gas stripping" includes general processes for removing, separating or (forced) excluding gaseous compounds from a liquid stream.

[0027] The term "substantially free of" refers to a composition in which the presence of a particular component or part has no substantial effect on the entire composition. In some embodiments, "substantially free of" may refer to a composition in which the presence of a particular component or part in the composition is less than about 5 wt%, or less than about 4 wt%, or less than about 3 wt%, or less than about 2 wt%, or less than about 1 wt%, or less than about 0.5 wt%, or less than about 0.1 wt%, or less than about 0.05 wt%, or even less than about 0.01 wt%, or the particular component or part is not present in the corresponding composition.

[0028] According to one embodiment, the present invention provides a method for recovering maleic anhydride from a gas mixture comprising maleic anhydride, the method comprising step (a): contacting the gas mixture with an extraction solvent comprising a phosphate ester, wherein the contacting comprises transferring at least a portion of the maleic anhydride from the gas mixture to the extraction solvent. In some embodiments, the contacting step (a) may be carried out in an absorber, thereby providing a waste gas stream substantially free of maleic anhydride and a solvent effluent stream comprising the extraction solvent and maleic anhydride.

[0029] A representative gas mixture includes a reaction product gas obtained by oxidizing a gaseous hydrocarbon (such as n-butane or butene) in a reactor via a fixed bed or fluidized bed catalyst (such as a vanadium-phosphorus-oxygen catalyst). The reaction product gas may contain about 0.5-2.5 mol% maleic anhydride, about 4-20 mol% water vapor, as well as nitrogen, oxygen, unreacted hydrocarbons and oxidation byproducts, including those described above (such as carbon monoxide, carbon dioxide, acrylic acid, maleic acid and acetic acid). Maleic anhydride and part of the oxidation byproducts (as well as other various impurities, such as carboxylic acids and organic and inorganic phosphates) are absorbed by the extraction solvent, while the remaining oxidation byproducts and inert gases are contained in the waste gas stream.

[0030] In one embodiment, the phosphate ester is a compound having the following general formula:

[0031]

[0032] Where R 3 is a straight chain or branched alkyl group, an aryl group, or an -aryl-(C1-C 10 )alkyl.

[0033] In one embodiment, the phosphate ester is a monoalkyl diphenyl phosphate, wherein R 3 is a straight or branched chain alkyl group having up to 22 carbon atoms, more specifically 6 to 14 carbon atoms. 3 It is a branched chain alkyl group containing 6 to 10 carbon atoms.

[0034] Examples of monoalkyl diphenyl phosphates that may be used include, but are not limited to, 2-ethylhexyl diphenyl phosphate, nonyl diphenyl phosphate, iso-octyl diphenyl phosphate, 2-methylpentyl diphenyl phosphate, 2-n-propylheptyl diphenyl phosphate, 2-butyloctyl diphenyl phosphate, 2-ethylbutyl diphenyl phosphate, isodecyl diphenyl phosphate, and mixtures thereof.

[0035] In another embodiment, the phosphate ester is R 3 is -aryl-(C1-C 10 ) alkyl compounds, such as R 3 is -aryl-(C1-C4)alkyl. In one embodiment, R 3 It is tolyl, cresol, xylyl, tert-butylphenyl or isopropylphenyl.

[0036] In another embodiment, the extraction solvent may further include an organic solvent. The organic solvent may include, but is not limited to, maleates, phthalates, carbonates, benzoates, ketones, aromatic hydrocarbons, anhydrides, halogenated hydrocarbons, halogenated oxygen hydrocarbons, acetate ethers, naphthalene, ethers, and non-phosphate esters. More specifically, the organic solvent may include dimethyl maleate, diethyl maleate, propylene carbonate, propyl benzoate, isobutyl benzoate, isophorone, e-caprolactone, isobutyl heptyl ketone, di-n-amyl ketone, diisoamyl ketone, hexylbenzene, mixed aromatic hydrocarbons, n-valeric anhydride, C9 alkyl acetate, C 10 Alkyl acetate, 1,4-butanediol diacetate, dipropyl malonate, dimethyl phthalate and C5-C 10 Esters of neo-acids and mono-polyols. In one embodiment, the extraction solvent is substantially free of these organic solvents.

[0037] In some embodiments, the contacting step (a) is carried out at a temperature greater than about -60°C, or greater than about -30°C, or greater than about 10°C, or greater than about 50°C, or greater than about 100°C, or greater than about 120°C, or greater than about 140°C, or greater than about 160°C, or greater than about 180°C, or greater than about 200°C, or greater than about 220°C. In other embodiments, the contacting step (a) can be carried out at a temperature less than about 250°C, or less than about 240°C, or less than about 230°C, or less than about 220°C, or less than about 210°C, or less than about 200°C, or less than about 190°C, or less than about 180°C, or less than about 170°C. In other implementations, the contacting step (a) can be carried out at a temperature in the range of about -60°C to about 250°C.

[0038] In some embodiments, the contacting step (a) can be carried out at a pressure greater than atmospheric pressure, for example, a pressure of up to about 10 psig, or up to about 20 psig, or up to about 30 psig, or up to about 40 psig, or up to about 50 psig.

[0039] In another embodiment, the method further comprises a step (b): stripping the solvent effluent stream obtained in step (a) to produce a first stream comprising a crude maleic anhydride product and a regenerable extraction solvent substantially free of maleic anhydride but comprising an extraction solvent and impurities. In some embodiments, step (b) may be implemented in a distillation column. Thus, in one embodiment, the extraction solvent is capable of being distilled at a temperature of greater than 180°C to about 250°C, thereby fully separating maleic anhydride from the solvent effluent stream. In at least one example, the extraction solvent may comprise a phosphate having a normal boiling point of at least about 180°C. In other embodiments, the extraction solvent may comprise a phosphate having a normal boiling point of about 185-250°C, for example about 190-245°C.

[0040] In other embodiments, gas stripping is performed at subatmospheric pressure. Specifically, the distillation column is operated at subatmospheric pressure so that the absolute pressure in the column is about 25-90 mm Hg near the bottom of the column, and about 10-50 mm Hg near the top of the column. The temperature in the column is about 180-250° C. near the bottom of the column, and about 90-105° C. near the top of the column.

[0041] According to another embodiment, the method further comprises the step (c): recycling at least a portion of the regenerable extraction solvent comprising the extraction solvent and the impurities to step (a). In some embodiments, the regenerable extraction solvent is treated prior to recycling to remove at least a portion of the impurities. Such treatment may include, but is not limited to, vacuum and steam distillation, stripping, absorption (e.g., using a solid absorbent), and filtration. In one embodiment, the regenerable extraction solvent is treated by contacting the regenerable extraction solvent with an aqueous liquid extractant in a stirred extraction zone to produce a mixture comprising an aqueous extract phase and an organic raffinate phase, the aqueous extract phase being substantially immiscible with the regenerable extraction solvent.

[0042] As a result of the contact, the water-soluble impurities in the regenerable extraction solvent are thereby transferred to the aqueous liquid extractant, and a two-phase mixture comprising an aqueous extract phase and an organic raffinate phase is produced. The organic raffinate phase contains the regenerable extraction solvent with a reduced concentration of impurities. After the aqueous extract phase and the organic raffinate phase are separated, the organic raffinate phase can be recycled to step (a) to further contact more gas mixture for absorption of maleic anhydride.

[0043] According to another embodiment, a system for preparing and recovering maleic anhydride is provided. The system generally comprises a reaction zone for preparing a gas mixture containing maleic anhydride, an absorption zone for contacting the gas mixture with an extraction solvent of the present invention so as to transfer maleic anhydride from the gas mixture to the extraction solvent, and a stripping zone for separating maleic anhydride from the extraction solvent, and an optional regeneration zone.

[0044] The reaction zone can be operated to catalytically oxidize hydrocarbons and produce a gas mixture containing maleic anhydride, which is then transferred to an absorption zone. The reaction zone typically comprises a reactor containing a vanadium-phosphorus-oxygen catalyst, the reactor having an inlet for introducing a raw gas containing molecular oxygen and hydrocarbons into the reactor and an outlet for discharging the gas mixture from the reactor. The reaction zone can also include one or more heat exchangers operable to cool the discharged gas mixture. In other embodiments, the reaction zone does not substantially include such a heat exchanger.

[0045] The system also includes an absorption zone operable to contact the gas mixture with the extraction solvent of the present invention, wherein maleic anhydride is transferred from the gas mixture to the extraction solvent and produces an off-gas substantially free of maleic anhydride and a solvent effluent stream comprising the extraction solvent, maleic anhydride and impurities. The absorption zone may also be operated to convey the solvent effluent stream to a stripping zone.

[0046] The absorption zone includes an absorber having a lower inlet for receiving a gas mixture, an upper inlet for receiving an extraction solvent, an exhaust conduit for exhausting exhaust gases, and an outlet for discharging a solvent effluent stream. The absorption zone also includes a solvent storage tank containing an extraction solvent of the present invention. The solvent storage tank is operable to deliver the extraction solvent to the absorber. The storage tank includes one or more inlets for receiving additional extraction solvent and regenerable extraction solvent, and an outlet for withdrawing the extraction solvent.

[0047] The system also includes a stripping zone operable to separate maleic anhydride from the solvent effluent stream, thereby producing a crude maleic anhydride product and a regenerable extraction solvent comprising extraction solvent and impurities. The stripping zone is also operable to transport the regenerable extraction solvent back to the absorption zone.

[0048] The stripping zone includes a rectification column having an inlet for receiving a solvent effluent stream, an upper outlet for discharging a crude maleic anhydride product, and a lower outlet for discharging a regenerable extraction solvent.

[0049] The system optionally includes a regeneration zone operable to treat at least a portion of the regenerable extraction solvent to reduce the concentration of impurities (i.e., water-soluble impurities) therein. The regeneration zone can also be operated to transport the regenerable extraction solvent having a reduced concentration of water-soluble impurities to the absorption zone.

[0050] The regeneration zone includes an extractor having an inlet for receiving a regenerable extraction solvent and an aqueous liquid extractant, an agitator for contacting and mixing the regenerable extraction solvent with the aqueous liquid extractant to produce an aqueous extract phase containing water-soluble impurities and an organic raffinate phase containing the regenerable extraction solvent, and an outlet for discharging the aqueous extract phase and the organic raffinate phase, wherein the organic raffinate phase is recycled to the absorption zone.

[0051] In another embodiment, a system for recovering maleic anhydride is provided (i.e., eliminating the reaction zone from the above system), which generally comprises: an absorption zone for contacting a gas mixture comprising maleic anhydride with an extraction solvent to transfer maleic anhydride from the gas mixture to the extraction solvent, a stripping zone for separating the maleic anhydride from the extraction solvent, and optionally a regeneration zone for reducing the concentration of impurities in the extraction solvent as described above.

[0052] A schematic diagram of the method and system 100 of the present invention is given in the accompanying drawings. A raw gas mixture 101 containing molecular oxygen and hydrocarbons is introduced into a reactor 103 containing an oxidation catalyst to partially oxidize the hydrocarbons to form a gas mixture containing maleic anhydride. The raw gas mixture 101 can be produced by mixing a gas containing molecular oxygen (preferably air) with a gaseous hydrocarbon feedstock containing hydrocarbons having no less than four straight-chain carbon atoms (e.g., n-butane, 1-butene, and 2-butene). The hydrocarbons contained in the raw gas mixture 101 are converted into maleic anhydride by contacting the raw gas mixture with a vanadium-phosphorus-oxygen catalyst at elevated temperatures in the reactor 103. The gas mixture 102 flows out of the reactor 103 and can then be cooled in a heat exchanger 105. As discussed above, the extraction solvent of the present invention can allow the subsequent absorption step to be carried out at a higher temperature than that currently possible with prior art solvents, so the gas mixture 102 may not need to be cooled before the absorption step. Therefore, in some embodiments, the heat exchanger 105 can be removed from the system 100. Since such heat exchangers typically require significant operating expenses to maintain normal operation, eliminating heat exchanger 105 can save significant operating expenses and associated capital.

[0053] The gas mixture 102 flows into the absorber 104, which includes a facility for contacting the gas mixture 102 with the extraction solvent 106. The absorber 104 may include packing (e.g., saddles, rings, etc.) for promoting gas / liquid contact and mass transfer of maleic anhydride from the gas phase to the liquid phase. Alternatively, the absorber 104 may include a plate tower in which gas / liquid contact is implemented on the plate. The gas mixture 102 may be introduced near the bottom of the absorber 104, and the extraction solvent 106 from the solvent storage tank 122 may be introduced near the top of the absorber 104. The gas mixture 102 and the extraction solvent 106 flow countercurrently through the absorber 104. Maleic anhydride and partial oxidation byproducts are absorbed by the extraction solvent 106, while the oxidation byproducts and inert gases remaining in the gas mixture 102 are discharged from the exhaust port at the top of the absorber 104 as a waste gas 108 substantially free of maleic anhydride.

[0054] In some embodiments, absorber 104 can be operated at a temperature of about 25-260°C. In one embodiment, the amount of oxygen in gas mixture 102 and the physical properties of extraction solvent 106 are used to determine the maximum operating temperature of absorber 104. In another example, the maximum operating temperature of absorber 104 is set to be equal to the flash point of extraction solvent 106, including a reasonable safety margin of at least 10°C. In addition, the minimum operating temperature of absorber 104 is set to the melting point of extraction solvent 106. In at least some embodiments, absorber 104 operates at a pressure higher than atmospheric pressure. For example, the pressure of absorber 104 is at most about 50 psi gauge. When absorber 104 is operated under pressure, the extraction solvent 106 described herein can absorb maleic anhydride at a temperature of about -60°C to about 250°C.

[0055] In other embodiments, the ratio of extractive solvent 106 to gas mixture 102 introduced into absorber 104 also varies over a wide range, but for economic reasons, it may be preferred to set the ratio so that the solvent effluent stream 110 leaving absorber 104 contains about 5-45 wt %, more preferably about 15-20 wt %, of maleic anhydride based on its total weight. For example, for a gas mixture containing about 0.7 mol % maleic anhydride and about 7 mol % water, the ratio of extractive solvent 106 to gas mixture 102 is about 0.07-0.3 kg of extractive solvent per cubic meter of gas mixture.

[0056] The solvent effluent stream 110 can be removed near the bottom of the absorber 104 and sent to a distillation column 112, wherein the solvent effluent stream 110 is stripped of maleic anhydride in a stripping step to recover a crude maleic anhydride product 114 and a regenerable extraction solvent 116 containing an extraction solvent and impurities. The distillation column 112 includes a shell of suitable size having a plurality of vapor spaces and a packing area filled with suitable packing. The solvent effluent stream 110 can be introduced into the lower half of the distillation column 112.

[0057] In some embodiments, the distillation column 112 can be operated at less than atmospheric pressure, so that the absolute pressure within the distillation column 112 can vary between about 25-90 mm Hg near the bottom of the column 112 and about 10-50 mm Hg near the top of the column 112. The temperature within the distillation column 112 can vary between about 180-250° C. near the bottom of the column 112 and about 90-105° C. near the top of the column 112.

[0058] Crude maleic anhydride product 114 may be withdrawn in gaseous or liquid form near the top of distillation column 112. In embodiments where crude maleic anhydride product 114 is withdrawn in gaseous form, it may subsequently be condensed in a condenser (not shown) to obtain a liquid crude maleic anhydride product.

[0059] The regenerable extraction solvent 116 may be recovered from the bottom of the distillation column 112 and circulated through the system 100. In at least one embodiment, the regenerable extraction solvent 116 recovered from the bottom of the distillation column 112 may be circulated to the solvent storage tank 122. In some embodiments, at least a portion of the impurities present in the regenerable extraction solvent 116 may be removed by a treatment step in a regenerator prior to recycling.

[0060] Thus, in one embodiment, at least a portion of the regenerable extraction solvent 116 is fed to an extractor 118, wherein the regenerable extraction solvent stream is contacted with an aqueous liquid extractant under agitation. The aqueous liquid extractant is substantially immiscible with the regenerable extraction solvent, and in some embodiments, the aqueous liquid extractant comprises deionized water. Upon contacting (and agitating) the regenerable extraction solvent with the aqueous extractant, water-soluble impurities contained in the regenerable extraction solvent are transferred to the aqueous liquid extractant, and a two-phase mixture comprising an aqueous extract phase and an organic raffinate phase is produced, wherein the impurity concentration of the regenerable extraction solvent in the organic raffinate phase is reduced.

[0061] Extractor 118 may include a suitably sized pressure vessel having an inlet for a regenerable extraction solvent, an inlet for an aqueous liquid extractant, and an outlet for withdrawing an organic raffinate phase from the extractor. Extractor 118 may include paddles for agitating the aqueous and organic phase mixture within the extraction zone to promote intimate turbulent contact between the phases and mass transfer of water-soluble impurities from the regenerable extraction solvent to the aqueous liquid extractant.

[0062] The extraction may be carried out under moderate process conditions (including but not limited to a temperature of about 40-80° C. and a pressure of about 20-100 psig). The volume ratio of the aqueous liquid extractant to the regenerable extraction solvent entering the extractor 118 may be about 1:1 to about 1:10, or about 1:4 to about 1:6.

[0063] The separated aqueous extract phase, containing water-soluble impurities transferred from the regenerable extraction solvent, may be discharged from the system 100 as waste, while the separated organic raffinate phase may be discharged from the extractor 118 and combined with any regenerable extraction solvent 116 that bypassed the extractor 118, or transferred directly (not shown) to the solvent storage tank 122. In another embodiment (not shown), the regenerable extraction solvent (treated or untreated) may be directly re-entered into the absorber 104.

[0064] In one embodiment, some of the extraction solvent may decompose during the absorption and stripping steps. In this case, fresh extraction solvent can be added to the solvent storage tank 122 through the solvent replenishment pipeline 124. In order to prevent the loss of maleic anhydride product and extraction solvent in large quantities when circulating through the system 100, phosphate esters with lower water solubility are generally preferred. Therefore, the solubility of the improved extraction solvent in water is preferably less than about 100 mg / L. In at least one example, the solubility of the improved extraction solvent in water is less than about 50 mg / L. In at least one additional example, the solubility of the improved extraction solvent in water is less than about 25 mg / L or less than about 10 mg / L.

[0065] The low-boiling by-products and a portion of the impurities contained in the solvent effluent stream transferred to the distillation column 112 are discharged from the top of the distillation column 112 as a gas stream 120. In at least some cases, a small amount of maleic anhydride may also be present in the gas stream 120. In this case, the maleic anhydride can be recovered from the gas stream 120 by extraction in a scrubber (not shown), and the scrubbed stream containing maleic anhydride can be returned to the system 100, or combined with the crude maleic anhydride product 114 flowing out of the system 100.

[0066] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be envisaged without departing from the basic scope thereof, the scope of which is determined by the appended claims.

Claims

1. A method for recovering maleic anhydride from a gas mixture comprising maleic anhydride, the method comprising the step (a): contacting the gas mixture with an extraction solvent comprising a phosphate ester, wherein the contacting comprises transferring at least a portion of the maleic anhydride from the gas mixture to the extraction solvent.

2. The method of claim 1, wherein the phosphate ester is a compound of the following general formula: Where R 3 is a straight-chain or branched alkyl, aryl or -aryl-(C1-C 10 )alkyl.

3. The method of claim 2, wherein the phosphate ester is a monoalkyl diphenyl phosphate, wherein R 3 It is a straight-chain or branched alkyl group containing up to 22 carbon atoms.

4. The method of claim 3, wherein the monoalkyl phosphate comprises 2-ethylhexyl diphenyl phosphate, nonyl diphenyl phosphate, iso-octyl diphenyl phosphate, 2-methylpentyl diphenyl phosphate, 2-n-propylheptyl diphenyl phosphate, 2-butyloctyl diphenyl phosphate, 2-ethylbutyl diphenyl phosphate, isodecyl diphenyl phosphate, or mixtures thereof.

5. The method of claim 2, wherein R 3 It is -aryl-(C1-C4)alkyl.

6. The method of claim 5, wherein R 3 It is tolyl, cresol, xylyl, tert-butylphenyl or isopropylphenyl.

7. The method of claim 1, wherein the extraction solvent further comprises an organic solvent.

8. The method of claim 1, wherein the extraction solvent is substantially free of organic solvents.

9. The process of claim 1, wherein the contacting step (a) is carried out at a temperature of about -60°C to about 250°C.

10. The process of claim 1, further comprising the step (b) of stripping the solvent effluent stream obtained in step (a) to produce a first stream comprising a crude maleic anhydride product and a regenerable extraction solvent substantially free of maleic anhydride but comprising the extraction solvent and impurities.

11. The process of claim 1, wherein step (b) is performed in a distillation column, the distillation column being operated at a temperature of about 180-250°C.

12. The method of claim 11, wherein step (b) is performed at subatmospheric pressure.

13. The method of claim 1, further comprising the step (c) of recycling at least a portion of the regenerable extraction solvent comprising the extraction solvent and the impurities to step (a).

14. The method of claim 13, wherein the regenerable extraction solvent comprising the extraction solvent and the impurities is treated prior to recycling to reduce the concentration of the impurities in the regenerable extraction solvent.

15. The process of claim 14, wherein the regenerable extraction solvent is treated by contacting the regenerable extraction solvent with an aqueous liquid extractant to transfer at least a portion of the impurities from the regenerable extraction solvent to the aqueous liquid extractant.

16. The method of claim 15, wherein the contacting produces a two-phase mixture comprising an aqueous extract phase and an organic raffinate phase, wherein the aqueous extract phase comprises at least a portion of the impurities and the organic raffinate phase comprises the regenerable extraction solvent having a reduced concentration of impurities.

17. A system for producing and recovering maleic anhydride, the system comprising a reaction zone operable to produce a gas mixture comprising maleic anhydride, an absorption zone operable to contact the gas mixture with an extraction solvent comprising a phosphate ester to transfer the maleic anhydride from the gas mixture to the extraction solvent, and a stripping zone operable to separate the maleic anhydride from the extraction solvent, wherein the absorption zone comprises a solvent storage tank comprising the extraction solvent.

18. A system for recovering maleic anhydride, the system comprising an absorption zone operable to contact a gas mixture comprising maleic anhydride with an extraction solvent comprising a phosphate ester so as to transfer the maleic anhydride from the gas mixture to the extraction solvent, and a stripping zone operable to separate the maleic anhydride from the extraction solvent, wherein the absorption zone comprises a solvent storage tank comprising the extraction solvent.

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