A METHOD OF PRODUCING AND SEPARATING DURENE

Abstract: Durene is recovered from a mixture containing durene and hydrocarbons boiling in the gasoline range by fractionat­ ing to mixture to produce lower and higher boiling fractions and then cooling the higher boiling fraction to a point where crystallization occurs. The crystallized durene is separated from the mother liquor and subsequently is washed with a wash fluid. The wash fluid, which can be methanol, is returned to the process stream wherein it is converted to gasoline and durene. The separated mother liquor is added to the gasoline fraction. The original mixture of durene and gasoline is obtained by the catalytic conversion of synthesis gas or alcohols and ethers.

Description: A MET tslETH3D OF PRODUCING AND SEPARATING DURENE This invention relates to the production and separation of durene, 1,2,4,5-tetramethylbenzene. Of the tetraalkyl benzene isomers the most important currently is durene, 1,2,4,5-tetramethylbenzene. Durene is an important intermediate in the preparation of pyromellitic anhydride used in the manufacture of polyimide resins. These resins are used in the aerospace industry, as electrical wire coatings, and in laminate applications. Until recently durene has been produced by crystallizing it from hydrocarbon fractions such as naphtha reformate fractions boiling in the range 3500F to 4250F (1770C to 2180C). U.S. Patent Nos. 2,914,582; 2,914,586; and 3,150,197 are directed to aspects of this type of recovery. Processes utilizing crystalline zeolites for converting synthesis gas as well as alcohols (primarily methanol) to gasoline have now been established in the art. In the conversion of synthesis gas to gasoline, synthesis gas is contacted in a first stage with a catalyst to form oxygenated compounds such as methanol and dimethyl ether. These products are then converted into hydrocarbons boiling in the gasoline range by passing them through a second catalytic bed containing a selected crystalline zeolite. U.S. Patent Nos. 3,894,102 and 4,076,761 exemplify such a process. In two-stage processes for converting lower alcohols and ethers, such as methanol, ethanol, propanol, dimethyl ether, diethyl ether or mixtures thereof to gasoline these materials are contacted with a catalyst to produce a mixture having predominantly aliphatic or organic intediate products. In a second stage this product with or without further modification is contacted with a catalyst such as crystalline zeolite and converted to a final product having a preponderance of nyd--oOrtor,s boiling within the range of gasoline and distillate. r.r process is exemplified by U.S. Patent Nos. 3,928,483 and 3,794tl,-- Xsolirle produced by these processes ordinarily contains durene. Excessive amounts of durene in gasoline can, however, be troublesome, since durene is a solid at normal temperatures and pressures with a melting point of 17SoF (79or), and hence it may crystallize out of the gasoline in unheated transfer lines or in storage tanks. Accordingly, it is highly desirable to remove the durene from the gasoline product or at least reduce the durene concentration. In its broadest aspect, the present invention resides in a method for separating durene from a gasoline-durene mixture comprising: (a) fractionating said mixture to produce a lower boiling fraction and a higher boiling fraction; (b) cooling said higher boiling fraction to crystallize durene therefrom; and (c) separating the durene crystals formed from the mother liquor. More specifically, the present method comprises separating a mixture of durene and hydrocarbons boiling in the range of gasoline in which the mixture is first fractionated into at least two fractions, one fraction, a heavy or bottoms fraction, being particularly concentrated in durene. This fraction ordinarily will be the higher fraction having a heavier boiling point. The remaining fraction or fractions will ordinarily constitute gasoline products. The heavy or bottoms fraction is cooled to a temperature at which durene begins to crystallize, whereafter the crystallized durene is separated from the mother liquor. The separated crystalline durene may then be washed with a solvent to remove residual mother liquor and the solvent returned to the process stream where it is converted to additional gasoline or other hydrocarbons. The durene-containing feedstock may be produced from synthesis gas by contacting the synthesis gas with a catalyst thereby converting the synthesis gas to a mixture of ethers, alcohols and hydrocarbons, contacting the mixture with a second catalyst thereby converting the mixture to a stream rich in durene and hydrocarbons boiling within the range of gasoline and then further distilling the product stream to provide a bottoms product concentrated in durene and an overhead product comprising gasoline. The durene is then removed from the product as described above. Alternatively, the feedstock may be produced by catalytic conversion of alcohols and/or ethers to a mixture rich in durene and gasoline. This mixture is then separated into gasoline and crystallized durene. CONVERSION OF SYNTHESIS GAS TO A MIXTURE OF DURENE AND GASOLINE Techniques for deriving synthesis gas (a mixture of carbon monoxide and hydrogen) from coal, natural gas, naphthas, crude oil, shale oil and residua are well known. If desired the composition of the gas may be adjusted to provide a predetermined volumetric ratio of hydrogen to carbon monoxide plus carbon dioxide. The synthesis gas is then contacted with a carbon monoxide reduction catalyst in a first reaction zone to produce a reduction product containing typically of the order of 20 weight percent of ethers, alcohols, and other hydrocarbon products. The reduction product is catalytically converted by contact with a catalyst which preferably is a crystalline zeolite of the ZSM-5 type to form a major fraction of aromatics-rich high octane gasoline and a lesser fraction of useful products that include durene and a hydrogen-rich gaseous mixture that may be recycled to the fossil fuel gasifier. Representative of the ZSM-5 type zeolites are ZSM-5, ZSM-ll, ZSM-12, ZSM-23, ZSM-35 and ZSM-38. ZSM-5 is disclosed in U.S. Patent No. 3,702,886 and U.S. Patent RE 29,948; ZSM-ll is disclosed in U.S. Patent No. 3,709,979; ZSM-12 is disclosed in U.S. Patent No. 3,832,449; ZSM-23 is disclosed in U.S. Patent No. 4,076,842; ZSM-35 is disclosed in U.S. Patent Wo. 4,106,245 and ZSM-38 is disclosed in U.S. Patent No. 4,406,839. PRODUCTION OF DURENE/GASOLINE MIXTURE FROM LOWER ALCDHDLS AND/OR ETHERS Th; ec * fizz . of gasoline and durene processed may also be derived from, 4C alcohols and/or ethers by a multi-stage process, in the first stage r lich the alcohol reactant is contacted with a con densation catalyst to produce a predominantly aliphatic organic intermediate product. In a second stage the intermediate product with or without further modification is contacted with a crystalline zeolite preferably of the ZSM-5 type to convert the intermediate product to a final product which is a hydrocarbon mixture having a preponderance of normally fluid hydrocarbons in the gasoline boiling range of up to 415OF (2130C). Suitable lower alcohols that may be charged to the first stage of the process include methanol, ethanol, normal propanol and isopropanol and/or their corresponding ethers. The feed may consist of a relatively pure single alcohol or ether or a mixture thereof. The preferred charge is to this first stage of the process is ethanol, dimethyl ether and/or methanol Particularly preferred charges include methanol and mixtures of methanol and dimethylether. In the first stage of this conversion process the alcohol reactant is contacted with a condensation catalyst to produce water and a predominantly aliphatic organic intermediate product. The condensation catalyst may be any catalyst which results in the intermolecular dehydration of the alcohol reactant to form an aliphatic product of higher carbon to oxygen ratio than the feed. Solid inorganic and organic acidic catalysts such as phosphoric acid supported on kieselguhr, high surface area silica-alumina, acidic alumina, acid treated clays, bauxite and polystyrene sulfonic acids of the ion exchange type can be used. The predominantly aliphatic intermediate, for example, dimethylether produced in the first stage of this process, without further conversion is contacted with a crystalline zeolite which preferably is selected from the group consisting of ZSM-5, ZSM-ll, ZSM-12, ZSM-21 and TEA Mordenite. Of these ZSM-5 is particularly preferred. Operating conditions for this process are disclosed in U.S. Patent No. 3,928,483. As disclosed in this patent, it may also be desirable to follow the initial alcohol condensation stage by a further conversion stage. This intermediate conversion can be carried out in the effective presence of either the same catalyst as used for the first stage, or deactivated last stage catalyst. The reaction product from the last stage of the reaction contains aliphatic and aromatic gasoline boiling-range hydrocarbons mixed with durene. CRYSTALLIZATION AND WASHING OF DURENE The liquid mixture of durene and gasoline, preferably containing at least 10, or more preferably at least 30 percent by weight of durene, is separated by distillation into at least two fractions to provide a lower boiling fraction or fractions that will constitute primarily the gasoline product and will be processed as may be further desired to convert it to a final gasoline product. The higher boiling point fraction preferably has a boiling point of at least 3000F (1490C) and preferably above 3300F (1660C). It represents a fraction sufficiently concentrated in durene so that as it is further cooled or chilled durene begins to crystallize. As a result the durene crystallizes out at a relatively high temperature of about 1000F (380C). For efficient operation for durene recovery a temperature of 30 to 100OF (-1 to 380C) can be used but a temperature of 65 to 800F (18 to 27or) is preferred. The crystalline durene is then separated from the mother liquor by centrifugation, filtration or other suitable means. The separation step is preferably conducted at or only slightly above the crystallization temperature to prevent loss of durene to the solubilizing mother liquor. The mother liquor occluded on the surface of the durene can be removed by washing the separated crystals with a solvent, preferably methanol. The washing step is preferably effected at a temperature of 650F to 800F (18 to 270C). Methanol is preferred as a wash solvent because it can be recycled to and, processed as part of, the feed stream. Methanol thus requires no separate recovery system as might other solvents. Other wash solvents which can be used include lower aliphatic hydrocarbons such as paraffins, olefins, alcohols, ketones, ethers, esters and cyclic hydrocarbons. Examples include propane, bane, pentane, propene and cyclopentane. Examples of solvents which, like methanol, can be recycled to the ether alcohol feed converter include ethanol, propane and dimethylether. The washed crystalline durene product can have a purity as high as 96% or higher meeting the requirements for commodity grade durene. The quantity of wash liquid used to remove the mother liquor adsorbed or occluded on the durene crystals depends to a large extent upon the degree of purity desired and the particular solvent utilized. Generally the quantity of washed liquid varies between 5 and 200 weight percent of the durene. A preferred range is between 10 and 50 weight percent. The durene crystals can be washed with solvent in batch wise or continuous fashion with recycle of the mother liquor when advantageous. After the washing step the crystallized durene is dried and subjected to further processing as desired. For example, to decrease the volume of the durene product and consequently packaging costs, the durene can be melted before packaging. As noted previously the source of the mixture of durene and gasoline boiling range hydrocarbons can be derived from a number of processes. Primary among these processes will be the conversion of synthesis gas to gasoline boiling range hydrocarbons and the conversion of methanol to gasoline. Either one of these processes will result in the feedstock to be processed as described previously. The accompanying drawing is a flow sheet of a process according to one example of the invention. Referring to the drawing, coal, natural gas, naphthas, crude oil, shale oil, or residua, or a combination thereof, is conveyed via line 4 to a synthesis gas plant 5, where it is converted to synthesis gas. If hydrogen sulfide is produced in this plant, it may be separated and sent via line 6 to a treatment plant (not shown) for sulfur recovery. Synthesis gas, previously treated in a catalytic carbon monoxide shift converter and then reduced in carbon dioxide content by selective sorption, is conveyed via line 7 to a first reaction zone 8, where it is at least partially catalytically converted to produce a carbon monoxide reduction product that contains at least 20% by weight of alcohols, ethers and other hydrocarbon products. Part or all of the unconverted synthesis gas may be separated from such reduction product and recycled via line 10, but it is preferred to convey the total mixture via line 9 to a second reaction zone 11 where catalytic conversion to hydrocarbons and steam occurs. The reaction products from the second reaction zone 11 are conveyed via line 12 to a cooler, 13, and the cooled products are then conveyed via line 14 to a separator 15 (the cooler 13, line 14 and separator 15 may be one integral unit). Water is removed from separator 15 via line 16, gases via line 17, and liquid hydrocarbon products via line 18. The liquid hydrocarbon products are conveyed via line 18 to a distillation tower 19. In distillation tower 19 the liquid hydrocarbon products are fractionated into an overhead product of propane and butanes (LPG) which is removed via line 20, an intermediate gasoline cut which is removed via line 22 and a heavier gasoline cut, which is rich in durene and is removed via line 21. This heavier-durene cut has a boiling point above 3300F to 380OF (1650C to 1930C). The actual temperature at which this cut is made will depend in part on how much durene it is desired to recover. It is this cut which is processed further to separate the durene therefrom. The heavier gasoline cut is then diverted by way of line 21 through a heat exchanger (not shown) and thence into a crystallization system (cooler 24 and crystallizer 25) wherein the gasoline-durene mixture is cooled to a temperature sufficient to induce the durene present to crystallize, thereby forming a mixture of durene crystals and mother liquor. With the process described the stream is surprisingly rich in durene and low in durene isomers which are difficult to separate. The durene crystals are then separated from the mother liquor, which is returned by way of line 23 to line 22 carrying the intermediate gasoline cut. The separated durene crystals are then washed with methanol in a washer 27, removed from the washer and then dried. The effluent methanol wash liquid is passed back into the methanol conversion system by way of line 26. The intermediate gasoline cut leaving the distillation tower 19 by way of line 22 is subjected to further processing, such as hydrotreating, to yield a desired gasoline product. In some cases, however, it may be preferred to omit line 22 and pass the entire gasoline cut by way of the line 21 through the durene crystallization system 24, 25, 27. Having described the invention, the following example is given to illustrate the invention without limiting the scope thereof. Parts and percentages are by weight unless expressly stated otherwise and the reference numerals designate parts of the flow sheet previously discussed. EXAMPLE A feed stream of ethers, alcohols and other hydrocarbons is introduced into converter 11 and converted to an aromatic stream containing approximately 8S of durene. The stream is passed through cooler 13 and into separator 15 where the liquid aromatic product is recovered and passed into the distillation tower 19. In distillation tower 19 the aromatic product obtained is fractionated into an overhead stream, a light gasoline stream and a heavier gasoline stream. The distillation column operates at a temperature of between 340 and 350OF (171 and 1770C) and a pressure of 0 psig (101 kPa). The gasoline product emerging from the distillation column 19 contains between 40 and 60% of durene. It is desired in this case to remove approximately 50% of the durene from the gasoline product to meet the durene concentration limitation of the gasoline pool. All the gasoline product is removed through line 21 and passed through the crystallization system. The product stream is passed first through a crystallizer 25 operating at a temperature of 700F (210C) where the crystallized durene is recovered and washed at 27 with methanol at a temperature of about 700F (210C). The crystallized durene is then removed and dried yielding a product of 97% purity.

Claims: CLAIMS: 1. A method for separating durene from a gasoline-durene mixture comprising: (a) fractionating said mixture to produce a lower boiling fraction and a higher boiling fraction; (b) cooling said higher boiling fraction to crystallize durene therefrom; and (c) separating the durene crystals formed from the mother liquor. 2. A method for producing durene from synthesis gas comprising: (a) catalytically converting said synthesis gas in a reaction zone to a product stream comprising durene and liquid hydrocarbons boiling in the gasoline boiling range; (b) fractionating the product stream of (a) to provide at least one lower boiling point fraction and a higher boiling point fraction containing a sufficient concentration of durene so as to crystallize when cooled; (c) cooling said higher boiling point fraction to a temperature at which durene begins to crystallize from said fraction; (d) crystallizing durene from said high boiling point fraction; (e) separating crystallized durene from said fraction leaving behind a mother liquor; and (f) returning said mother liquor to the lower boiling point fraction of (b). 3. A method for producing durene from a feed mixture comprising C1- % alcohols or ethers or mixtures thereof comprising: (a) catalytically converting said feed mixture in a reaction zone to a product stream comprising durene and liquid hydrocarbons boiling in the gasoline boiling range; (b) fractionating the stream of (a) to provide at least one lower boiling point fraction and a higher boiling point fraction containing a sufficient concentration of durene so as to crystallize when cooled; (c) cooling said higher boiling point fraction to a temperature at which durene begins to crystallize from said fraction; (d) crystallizing dufene'from said high boiling point fraction; (e) separating crystallized durene from said fraction leaving behind a mother liquor; and (f) returning said mother liquor to the lower boiling point fraction of (b). 4. The method of any preceding claim wherein separated crystallized durene is washed with a solvent and said solvent is thereafter converted catalytically to a mixture comprising durene and liquid hydrocarbons and said mixture constitutes a part of the product stream subjected to fractionation. 5. The method of claim 2 or claim 3 wherein the separated crystallized durene is washed with a solvent and the solvent is thereafter added to the feed mixture flowing to said reaction zone. 6. The method of claim 4 or claim 5 wherein the solvent is methanol. 7. The method of claim 2 or claim 3 wherein the catalytic conversion (a) is conducted in the presence of a zeolite catalyst chosen from ZSM-5, ZSM-ll, ZSM-12, ZSM-23, ZSM-35 and ZSM-38. 8. The method of any preceding claim wherein crystallization of the durene is conducted at a temperature of 65 to 800F (18 to 270C). 9. The method of any preceding claim wherein the higher boiling fraction boils at a temperature above 330OF (166 C). 10. The method of any preceding claim wherein the higher boiling fraction contains at least 10% by weight of durene.

Inventors: FOWLES, PATRICK ERNEST, YAN, TSOUNG-YUAN
Applicants: MOBIL OIL CORPORATION
Application year: 1983
Main classification: C10G31
IPC codes: C07C7, C10G3, C10G31