IL30096A - Process for hydrogenating hydrocarbons - Google Patents

Description

Process for hydrogenating hydrocarbons UNIVERSAL OIL PRODUCTS COMPANY la This invention rentes to the hydrogenation of hydrocarbons, and particularly relates to the stabilization of pyrolysis gasoline by selectively removing diolefins and olefins from the product gasoline obtained in light olefin manufacture.

It is known in the art that one of the commercially attractive routes to the production of valuable normally gaseous olefinic hydrocarbons such as ethylene, or propylene, is the thermal cracking or pyrolysis of hydrocarbons such as the light, paraffinic hydrocarbons and/or naphtha fractions obtained from petroleum. Usually, the pyrolysis reaction is effected in the absence of a catalyst at high temperatures, and often in the presence of a diluent such as superheated steam, utilizing a tubular reactor or a plurality of cracking furnace coils. Depending upon the charge stock and specific pyrolysis operating conditions employed, the effluent from the cracking zone may comprise light olefinic hydrocarbons such as ethylene, propylene, and butylene, or mixtures thereof, all of which may constitute the principal product or products. In addition to these light olefinic gases there is also produced a significant quantity of pyrolysis gasoline which contains undesirable amounts of diolefinic hydrocarbons and/or sulfur compounds. The pyrolysis gasoline frequently is rich in aromatic hydrocarbons. It has been found that the aromatic poction of the pyrolysis gasoline is usually heavily contaminated with olefinic hydrocarbons also, which renders recovery of the aromatics in high purity extremely difficult.

Prior art schemes for producing light olefin gases, such as ethylene, may charge ethane, propane or a straight-run naphtha fraction containing about 5% aromatic hydrocarbons, to a pyrolysis unit. The pyrolysis effluent is separated into desired fractions. One fraction usually comprises pyrolysis gasoline having a boiling range of about Ccj to 204°C . which represents, for example, approximately 1% to 40% by weight of the original naphtha feed depending upon the charge stock and severity of cracking. Since the pyrolysis gasoline is heavily contaminated, as previously mentioned, it is hydrotreated for saturation of olefins and/or diolefins and for removal of sulfur compounds. Not infrequently, the prior art schemes also charge the hydrotreated pyrolysis gasoline fraction to an aromatic extraction unit for recovery of the aromatic hydrocarbons such as benzene, toluene, and xylene therefrom. Typical extraction procedures utilizing a solvent such as sulfonane or the glycols are well known to those skilled in the art for aromatic extraction purposes.

However, as is known by those skilled in the art, the diene content of such pyrolysis gasoline, as measured by its well known Diene Value, is usually within the range from 20 to 70 for such pyrolysis gasolines fractions. The diolefin compounds pose particular difficulty in the operation of the hydrotreating facilities since these compounds cause extensive equipment fouling and catalyst bed fouling. So far as is known, the prior art hydrotreating process will experience this fouling from polymer formation to some extent. Usually, prior art schemes will attempt to improve the efficiency of the hydrotreating unit by either promoting the polymerization reaction prior to the hydrotreating step thereby preventing the polymer from reaching downstream equipment, and/or utilizing operating techniques and schemes which tend to minimize polymer formation. None of the prior art approaches are completely successful in overcoming the fouling difficulty resulting from the diolefinic compounds present in the pyrolysis gasoline .

More important, however, the prior art schemes do not provide high selectivity in the hydrotreating unit. For example, the hydro-genation reaction may not stop with the conversion of diolefins to olefins but will frequently saturate the amount of olefins completely or and even hydrogenate substantial p jZ>tions of aromatic hydrocarbons. ie Such nonselectivity, of course, results in a decreased yeiid of desirable products in the pyrolysis gasoline. Even though aromatic most hydrocarbons may not be hydrogenated, me>«*e frequently, olefinic hydrocarbons are completely saturated, thereby significantly decreasing the octane blending value of that portion of the pyrolysis gasoline which is normally utilized in motor fuel.

Therefore, it would be desirable to provide a process for with selectively hydrogenating pyrolysis gasoline wfctefc- minimized polymer "which formation, minimized product degradation, and/operates in a facile and economical manner.

Accordingly, it is an object of this invention to provide a method for hydrogenating hydrocarbons to stabilize pyrolysis gasoline.

It is a specific object of this invention to provide a method for removing diolefins from pyrolysis gasoline without destroying the olefins, while simultaneously removing diolefins, olefins, and sulfur compounds from the aromatic portion of the pyrolysis gasoline in a facile and economical manner.

Accordingly, the present invention provides a process for hydrocarbon feed stock containing olefins and diolefins and boiling with 'a range of from about C5 hydrocarbons to about 204°C. , with hydrogen and a recycled hydrocarbon stream, hydrogenating diolefins contained in said admixture in a first reaction zone in the presence of a catalyst containing palladium at a temperature within a range of from about 93°C. to about 260°C. , passing the resulting effluent to a first separation zone, separating from said effluent a first gaseous fraction comprising hydrogen, and a first liquid fraction, returning a portion of said first liquid fraction to the reaction zone as said recycled hydrocarbon stream, passing the remainder of said first liquid fraction to a second separation zone, separating from said first liquid fraction a gasoline hydrocarbon product stream and an aromatic hydrocarbon concentrate, admixing a portion of said first gaseous stream with said hydrocarbon concentrate and passing the re-suiting admixture to a second reaction zone, therein hydrogenating olefins and sulfur compounds contained in said resulting admixture in the presence of a hydrodesulfurization catalyst, and recovering from the resulting reactor effluent an aromatic hydrocarbon product stream and a second gaseous fraction comprising hydrogen and hydrogen sulfide.

Another feature of the present invention provides the process hereinabove set forth wherein the first separation zone is made up of two separators. The first reactor effluent is separated in the first of these separators to provide a vapor and relatively heavy liquid phase. This vapor phase is then cooled, which causes conden— sation of relatively light hydrocarbons. The resulting cooled stream is then separated in the second separator of said first separation zone to provide another vapor phase, and a relatively light liquid phase. This second vapor phase corresponds to the first gaseous fraction hereinabove described. A portion of the relatively light liquid phase is recycled to admix with the feedstock, as the first liquid fraction hereinabove described. The remainder of the relatively light liquid phase is then admixed with the relatively heavy liquid phase from the first separator. The resulting admixture then comprises the remainder of the first liquid fraction hereinabove described, which fraction is fed forward to the second separation zone.

The selectivity provided by the present invention results from the discovery that the unique two-stage system for hydrogenation accomplishes the desired results of removing sulfur compounds simultaneously from various fractions of pyrolysis gasoline so that maximum recovery of desired products may be obtained from the pyrolysis of th elatane, propane and/or naphthas to produce, for example, ethylene. ie This invention ach#yves these results in an economical and facile manner. For example, the use of the palladium catalyst and a relativel low temperature in the first reaction zone achieves selectively the conversion of diolefins to olefins without either substantial desul-furization or substantial saturation of the olefins. The relatively low temperature is that which is below desulfurization temperatures for the same system.

Satisfactory operating conditions for the first reaction zone include the hereinbefore mentioned temperature of from about 93°C. to about 260°C. , a pressure from 6.8 to 81.5 atmospheres, gauge, a liquid hourly space velocity from 1 to 10, based on combined charge, and a molar excess of hydrogen typically within the range from The operation performed in the second reaction zone of the present invention is primarily one of desulfurization and saturation of olefins boiling within the Cg to Cg boiling range, utilizing any of the well known desulfurization catalysts. It has now been found that the conventional nickel-containing desulfurization catalyst was particularly satisfactory for use in the present inventive process to remove sulfur from the Cg to Cg aromatic concentrate fraction while simultaneously saturating any olefin compounds therein. By proper selection of operating conditions it has been found that no substantial saturation of the aromatic hydrocarbons is achieved. Particularly satisfactory operating conditions for the second reaction zone of the present invention include a relatively high temperature in the range o o from 288 C. to 399 C. , a pressure from 46.2 to 54.4 atmospheres, gauge, a liquid hourly space velocity from 1 to 10, and a molar excess of hydrogen such as from 0.089 to 3.46 SCML of charge. A particularly preferred catalyst for desulfurization and olefin saturation in the second reaction zone is, for example, nickel molybdate supported on alumina.

It is noted from the description hereinabove of the features to the present invention that a portion of the liquid effluent from the first reaction zone is recycled in admixture with the feed to the first reaction zone to produce a combined charge to the reactor. The purpose of this admixing is to reduce the Diene Value of the total feed to the reaction zone to a relatively low value. Preferably, the Diene Value of the combined charge to the first reaction zone is less than 30 and, typically, will be less than 20, for example from 10 to 15. It has been found that the pyrolysis gasoline usually contains, for example, 5% to 35% by weight conjugated diolefin hydrocarbons, generally concentrated in the C5 fraction. These diolefins would contribute significantly to polymer formation in the reactor. However, utilizing the process of the present invention, the operating conditions previously mentioned, and the satisfactory palladium catalyst, the diolefins are selectively converted to olefins at a temperature from 93°C. to 260°C, preferably from about 182°C. to 260°C.

It is also to be noted that the fresh hydrogen is added to the system only through the first reaction zone. Thus, the sole source of hydrogen for the second reaction zone is obtained from the first zone. That is not to say, however, that a recycle stream may not be provided around the second reactor.. However, the amount of fresh hydrogen necessary to replace the hydrogen consumed in the second re- comes only from the first reaction zone. By operating in this manner an important benefit is obtained. The hydrogen to the first reaction zone is substantially free of hydrogen__sulfide. Therefore, there is virtually no chance of forming mercaptans from hydrogen sulfide passing over the palladium catalyst of the first reaction zone. Similarly, the desulfurization catalyst in the second reaction zone is notably sulfur resistant. Therefore, the recycle hydrogen stream to the second reactor contains hydrogen sulfide with substantially no adverse effects being noticed on the desired reactions.

By way of emphasis it is to be further noted that the present invention is based on the discovery that the palladium-containing catalyst is particularly useful in effect]& i&€t the desired reactions in the first reaction zone. Contrary to teachings found in the prior art, a platinum-containing catalyst was not satisfactory in the practice of the present invention. It has now also been discovered that palladium deposited on. lithiated alumina support achieved remarkable results in reducing gum formation caused by polymerization of the dienes on the acid sites of the catalyst.

The prefered palladium-containing catalyst employed in the present invention is prepared utilizing spherical alumina particles formed in accordance with the well-known oil-drop method as described in the art. As a specific example, these prefered catalyst contain either 0.75% or 0.375% by weight of palladium incorporated by way of an impregnation technique using the proper quantities of dinitro-dianisole palladium. Following evaporation to visual dryness and o - - ■ drying in air for about an hour at 38 C.^the palladium impregnated alumina is calcined at about 593°C. for about two hours. The lithium component is then incorporated using the necessary quantities of lithium nitrate to produce catalysts of 0.33% and 0.5% lithium in an impregnation procedure and the composite again is dried and calcined. A distinctly preferred diene catalyst includes 0.4% by weight palladium and 0.5% by weight lithium on a 6.4 mm. spherical alumina T base. B-?ea«iiy->—fefee**r he particularly preferred catalyst for the first reaction zone of the present invention comprises iifekiated- and from about 0.05 to about 5.0 by weight of lithium, alumina containing from about 0.05% to about 5.0% by weight of palladium The practice of the present invention, as previously noted, is particularly applicable to an aromatic hydrocarbon feedstock obtained from the pyrolysis of hydrocarbons such as naphthas for the production of light olefinic gases such as ethylene. As used herein, the term "aromatic hydrocarbon feedstock" is intended to include those feedstocks containing sufficient quanitites of aromatic hydrocarbons as a separate product stream substantially free of olefinic ?iVdrocarbons and sulfur compounds.

The pyrolysis reaction for the conversion of hydrocarbons into normally gaseous olefinic hydrocarbons is generally obtained at operating conditions including a temperature from 538°C. to 927°C. , preferably, 732°C. to 843°C. , a pressure from 0 to 1.36 atmospheres, gauge, preferably 0.34 to 0.68 atmospheres, gauge; and a residence time in the pyrolysis reaction zone of from 0.5 to 25 seconds, preferably from 3 to 10 seconds. In order for the pyrolysis reaction to proceed without undue plugging of the. reaction, an inert diluent such as steam, light gases, and the like, is used. The prior art distinctly prefers to use superheated steam as the diluent which is added to the pyrolysis reaction zone in an amount from 0.2 to 1.0 kilograms of steam per kilogram of hydrocarbon, preferably from 0.3 to 0.7 kilograms per kilogram, and typically, about 0.5 kilograms per kilogram.

The invention may be more fully understood with reference to the appended drawing which is a schematic representation of apparatus which may be utilized in practicing one embodiment of the invention.

Referring now to the drawing, a typical naphtha stream is introduced into the system via line 10 and pyrolyzed to desirable light olefinic gases in ethylene production facilities 11. The desirable minus hydrocarbons including the particularly desired ethylene stream is separated from the system via line 12. A typical pyrolysis gasoline comprising material separated from the effluent of a steam pyrolysis reaction zone is passed via line 13, pump 14, heater 15, into prefractionation or separation facilities 16.

Typically, the separation zone 16 comprises a distillation column maintained under conditions to separate gum materials and relatively non-volatile materials (such as those boiling above about 204°C. ) may be removed via line 17. The desirable feedstock fraction comprising, say, Cr to 204°C. hydrocarbons is withdrawn from separation zone 16 via line 19.

The feedstock is heated to substantially reaction temperature in heater 20, admixed with hydrogen from line 21, and further admixed with a hereinafter specified recycle stream from line 22. This admixture comprising hydrogen and a combined hydrocarbon charge is passed via line 23 into reactor 24.

• It has been found that optimum reaction conditions may be obtained by minimizing the degree to which the feedstock is heated and maximizing the heat input through the recycle liquid stream and the hydrogen stream. These conditions are consistent with effective vaporization and the preferable limiting of temperature of any single stream not to exceed 288°C. The temperature of the fresh feed in line 19 is preferably limited to a temperature of no higher than about 216°C. It should be further noted that although the invention has been described as admixing the materials in lines 19, 21, and 22 prior to introduction into the reactor, the term "admixing said feedstock" is intended to include first admixing the feedstock and hydrogen, or first admixing the hydrogen and recycle liquid which is ' then admixed with the feedstock or any other combination of introducing these three streams into the first reaction zone.

The combined charge material plus a molar excess of hydrogen is passed through reactor 24 over the preferred palladium catalyst under conditions sufficient to substantially convert diolefin compounds to olefin compounds without substantial saturation of olefin compounds. The total effluent from reactor 24 is cooled in exchanger 26 in an amount sufficient to produce in separator vessel 27 a significant 30096/2 quantity of relatively heavy liquid.. Preferably, the amount of relatively heavy liquid, more fully discussed hereinafter, which is separated in separator 27 comprises from 30% to 70% by weight, -typically about 50% by weight of the hydrocarbons in the effluent stream. · Operating conditions suitable for the achievement of the proper liquid phase in separator 27 include a temperature of from 121°C. to 232°C. , typically, about 166°C.

Operating under these conditions a relatively light fraction is withdrawn from separator 27 via line 29, passed through condense · 30 into secondary separator 31. The relatively light liquid which is condensed from the light fraction is withdrawn from separator 31 via line 22 and passed in part to first reactor 24, as .previously mentioned, utilizing pump 59 and heater 60. The separated gaseous phase or first gnseous fraction comprising hydrogen, is withdrawn from 'separator.31 by aI line 21 and recycled to reactor 24 as previously mentioned, utilizing pump 59' and- heater 6Θ· Sufficient make-up hydrogen is added to the system via line 56.

' ' The relatively heavy liquid fraction recovered in separator 27 is withdrawn via line 28, admixed with the remainder of the relatively light liquid in line 22 from line 32, and the admixture passed via line 33 into fractionating column 34. These streams, of course, can be introduced separately into column 34 at different column locations, if desired.

Fractionating column 34 is 'maintained under suitable conditions to separate, as an' overhead product, the C5 portion of the effluent which is subsequently passed via line 35 to, for example, stabilization' and further handling in accordance with well known practices in the " art. The bottoms from fractionating column 34 comprises the C6+ material and is withdrawn via line 36 and passed into second fractionating column 37. Suitable distillation conditions are maintained in column .37 to separate as a bottoms product, a relatively heavy, gasoline fraction which is suitable for blending into motor fuel. It should also be noted that the material in line 38 comprises generally Cg+ hydrocarbons and the material in line 35 generally comprises C5 olefin-containing hydrocarbons. Both of these streams comprise hydrocarbons suitable for gasoline blending stock. The overhead 37 from distillation column ffl comprises an aromatic hydrocarbon concentrate and is withdrawn via line 39 through exchanger 40 into heater 43 via line 41 after admixture with hydrogen from line 42.

The heated aromatic concentrate-hydrogen mixture is passed via line 44 into second reactor 45 containing the preferred nickel-molybdate desulfurization catalyst. Proper operating conditions are maintained in reactor 45, as previously mentioned, to effectuEtjbei saturation of the olefins contained in the aromatic concentrate stream as well as the substantial conversion of any sulfur compounds present therein to hydrogen sulfide.

The total effluent from reactor 45 is withdrawn via line 46, passed through cooler 47 into separator 48, wherein a hydrogen fraction containing hydrogen sulfide gas is withdrawn via line 50 and recycled to reactor 45 utilizing compressor 52 in line 42. It should also be noted at this point that sufficient make-up hydrogen is provided to the second reaction zone system via lines 21 and 51. Thus, no fresh make-up hydrogen need be added to the reactor 45 system thereby enabling the confinement of the H2S gases to the second reaction zone which contains a traditionally sulfur resistant catalyst. Otherwise, should hydrogen sulfide be passed through reactor 24, there would be a strong tendency for the palladium catalyst to convert or effectuate reaction between the hydrocarbons and hydrogen sulfide to produce undesirable mercaptans.

Referring again to separator 48, the condensed aromatic product stream is withdrawn from sj fearator 48 via line 49 and passed into fractionating column 53. Suitable operating conditions are maintained in fractionating column 53 to produce a light ends fraction containing hydrogen and H2S which is removed via line 54 and an aromatic hydrocarbon products stream which is removed via line 55 and which may be sent, for example, to solvent extraction for recovery of benzene, toluene, and/or xylene therefrom utilizing techniques well known to those skilled in the art.

As a matter of preference, the amount of relatively light liquid material in line 22 which is recycled to reactor 24 via line 23 is sufficient to produce a combined charge to reactor 24 which has a Diene Value less than 30 and particularly has a Diene Value less than 20 and which typically is about 15.

In a preferred feature of the present invention, the hydrogen introduced into the first reaction zone is substantially free of hydrogen sulfide and the hydrogen present in the second reaction zone contains hydrogen sulfide.

Claims (15)

1. Process for hydrogenating hydrocarbons which comprises admixing an aromatic hydrocarbon feed stock containing olefins and di-olefins and boiling within a range of from about C,_ hydrocarbons to o about 204 C. , with hydrogen and a recycled hydrocarbon stream, hydro-genating diolefins contained in said admixture in a first reaction zone in the presence of a catalyst containing palladium, at a temper- o o ature within a range of from about 93 C. to about 260 C. , passing the resulting effluent to a first separation zone, separating from said effluent a first gaseous fraction comprising hydrogen, and a first liquid fraction, returning a portion of said first liquid fraction to the reaction zone as said recycled hydrocarbon stream, passing the remainder of said first liquid fraction to a second separation zone, separating from said first liquid fraction a gasoline hydrocarbon product stream and an aromatic hydrocarbon concentrate, admixing a por-tion of said first gaseous stream with said hydrocarbon concentrate and passing the resulting admixture to a second reaction zone, therein hydrogenating olefins and sulfur compounds contained in said resulting admixture in the presence of a hydrodesulfurization catalyst, and recovering from the resulting reactor effluent an aromatic hydrocarbon product stream and a, second gaseous fraction comprising hydrogen and hydrogen sulfide.

2. The process of Claim 1, further characterized in that the remainder of the first gaseous fraction is recycled to the first reaction zone,

3. The process of either of Claims 1 or 2, further char recycled to the second reaction zone.

4. The process of any of Claims 1 to 3, further characterized in that the first separation zone comprises two separators, the first reactor' effluent is separated in the first separator of said first separator zone to provide a first vapor phase and a relatively heavy liquid phase, the first vapor phase is cooled and passed to the second separator of said first separation zone, the resulting cooled stream is separated therein to provide the first gaseous fraction and a relatively light liquid phase, passing a portion of said relatively, light liquid phase to the first reaction zone as the liquid fraction recycle stream, admixing the remainder said relatively light liquid phase with said relatively heavy liquid phase, and passing the resulting admixture to the second separation zone as the remainder of the liquid fraction.

5. The process of any of Claims 1 to 4, further characterized in that the first liquid fraction is separated in said second separation zone to provide the gasoline hydrocarbon product stream, the aromatic concentrate,—and a light hydrocarbon fraction comprising pentanes .

6. The process of any of Claims 4 to 5, further characterized in that the relatively heavy liquid phase comprises from about 30 to about 70 weight percent of the effluent from the first reaction zone.

7. The process of any of Claims 4 to 6, further characterized in that the amount of relatively light liquid phase recycled to e the first reaction zone is controlled to maintain the Dien Value of the combined hydrocarbon charge to the first reaction zone less than 30.

8· The process of any of Claims 1 to 7, further characterized in that the catalyst in the firat reaction zone comprises palladium on lithiated alumina,

9· The process of Claim 8, further characterized in that the catalyst in the first reaction zone comprises from about 0.05 to about 5·0# by weight of palladium*

10· The process of either of Claims 8 or 9» further characterized in that the catalyst in the first reaction zone comprises from 0,05 to about «0# by weight .of lithium.

11. The process of any of Claims 1 to 10, further characters ized in that the hydrodesulfurizatiOn catalyst in the second reaction zone comprises nickel molybdate on alumina.

12. The process of any of Claims 1 to 11, further characterized in that the first reaction zone is maintained at a pressure within a range of from 6.8 to 81.5 atmospheres gauge, a liquid hourly space velocity within a range of from 1 to 10, and a hydrogen to hydrooarbon ratio within a range of from 0,089 to 3* 6 standard cubic meters of hydrogen per liter of combined charge.

13· The process of any of Claims 1 to 12, further characterized in that the second reaction zone is maintained at a temperature within a range of from 288°C to 399°C, a pressure within a range of from 46.2 to 54.4 atmospheres, gauge, a liquid hourly space velocity within a range of from 1 to 10, and a hydrogen to hydrooarbon ratio with a ran e of from 0,089 to 3· 56 standard cubic meters of hydrogen per liter of combined charge·

14· The process of any of Claims 1 to 13, further characterized in that the aromatic hydrocarbon feed stock is derived from the pyrolysls of a naphtha charge stock, and said feedstock is fraction*-ated to remove gums and non-volatile materials prior to charging the feedstock to the first reaction zone.

15. A process for hydrogenating hydrocarbons substantially