CA1234159A - Aromatization of propylene - Google Patents

Abstract

ABSTRACT
An improved aromatization process is disclosed for converting a low molecular weight hydrocarbon stream. The process achieves an improvement in the selectivity to benzene, toluene and xylenes in the liquid product.
The feed, a mixture of C2-C4 olefins, is diluted with C1-C4 paraffins at a feed to diluent mole ratio of 1:4 to 4:1 and passed over a ZSM-5-type crystalline aluminosilicate zeolite catalyst.

Description

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FIELD OF THE INVENTION

This invention relates to an improvement in the preparation of aromatic compounds. In particular, this process relates to improving selectivity for benzene, toluene and xylenes (BTX) in the liquid product obtained using olefinic C2-C4 feedstocks passed over a ZSM-5 type catalyst.

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RIOR ART

The conversion of hydrocarbons by contacting with solid siliceous catalysts inrluding those re~erred to as crystalline aluminosilicate zeolites has long been known in the fi~ld. The contacting of hydrocarbons with the aforementioned catalysts has been carried out for a wide variety of reactions including cracking, isomerization and hydrocracking. U.S. Patents disclosing and claiming these processes are U.S. Patenk Nos. 3,140,249; 3,140,251;
3,140,253 and 3,140,322.

The contacting of hydrocarbon feeds with said catalysts to obtain aromatics is also known in the field. These procedures are described in U.S. Patent Nos. 3,760,024 and 4,304,657. These patents disclose contacting of C2-C4 olefins, paraffins and mixtures thereof with ZSM-S type zeolites for conversion to aromatics.

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The ~nstant application is concerned with the improvement of benzene, toluene and xylenes selectivity in the liquid product upon dilution of an olefin feed with Cl-C4 paraffins, when the dilution results in particular mixed feeds, and the aromatization i9 carried out under particular conditions, as set forth hereinafter.

Cattanach United States Patent 3,760,024 and Givens et al. United States Patent 3,827,968 disclose production of aromatic hydrocarbons from feed mixtures of C2 to C5 paraffinic and olefinic hydrocarbons with H-ZSM-5 catalyst. Although mixed gases are disclosed as feeds, experimental results are given only for pure propylene as feed and there is no disclosure of particular mixed feeds and no disclosure that mixtures give superior resul~s to pure propylene. There is no disclosure of feedstocks containing methane.

Cattanach United States Patent 3,756,g42 discloses conversion of normally liquid C5 plus hydrocarbons to aromatlc hydrocarbons over ZSM-5 type catalysts, discloses ~hat at 550C paraffins in the feed are converted to aromatics, whereas at 390C the aromatics come primarily from converting olefins to aromatics and discloses that lower temperatures can be used with feeds containing paraffin and olefin hydrocarbons than with a substantially paraffinic feed material. There is no disclosure of normally gaseous feed materials, nor that at a given temperature, conversion to aromatics i9 greater with paraffins and olefins in the feed than with olefins alone.

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Givens et al United States Patent 3,960,978 discloses in Example 2 thereof aromatization of a mixture of methane, ethane, ethylene, propane and propylene at 900-950F with Zn-HZSM-5 catalyst; the weight ratio of olefin to paraffin is 2.3 to 1 and the mole ratio of olefin to paraffin is 1.8 to 1; the weight ratio of olefin to methane is about 14.7 to 1, and the mole ratio of olefin to methane is about 5.9 to 1.
In Example 3 of this patent, a mixture of ethane, ethylene, propane and propylene is aromatized at a temperature of 100F with Cr-HZSM-5 catalyst; the weight ratio of olefin to paraffin is 3.0 to 1 and the mole ratio of olefin to paraffin is 2.8 to 1. There is no disclosure in this patent of a mole ratio of olefin to paraffin less than 1.6 to 1, nor of a mole ratio of olefin to methane less than 5.9 to 1, nor any disclosure of any effect of the saturated hydrocarbons in the feed on BTX production.

Garwood et al United States Patent 4,1501602 discloses conversion f C2 to C4 olefins to high octane gasoline components including BTX at temperatures up to 800F with ZSM-5 type catlayst, water being used as diluent to control coking or aging of the catalyst and Cl to C4 saturated hydrocarbons being disclosed as optional additional diluents. There is no disclosure of temperatures above 800F (427C), nor of any effect of the saturated hydrocarbon diluent on BTX production.
Miller U.S. Pate~t 4,304,657 discloses arcmatization of hydrocarbon feedstock boiling in the range from C2 to 70C, containing paraffins and/or olefins and/or naphthenes, diluted with carbon monoxide, carbon dioxide or nitrogen, in order to decrease the hydrogen to methane mole ratio of the product and decrease C10+ aromatics production. Experimental results are given only for naphtha feedstocks and there is no disclosure of a particular normally gaseous feedstoc3c other than propane. Ihere is no disclosure that selectivity for BTX production is higher from gaseous feeds when mixtures of paraffins and olefins are used. Ihere is no disclosure of feedstocks containing methane.

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SUMMARY OF THE INVENTION

The current invention provides for a process for converting an olefinic feed to a product stream in which the liquid is comprlsed mainly of benzene, toluene and xylenes, by contacting said feedstock with an aromatization catalyst. This contact occurs at a temperature in the range of 400 to 600C and a pressure of one atmosphere. The improvement comprises diluting said feedstock with a diluent comprising Cl-C4 paraffins, at a mole ratio of feed to diluent ranging from 1:4 to 4:1.

The impro~ement results in an increase in the selectivity to benzene, toluene and xylenes in the liquid product.

DETAILE~ DESCRIPTION OF THE PROPOSED EMBODIMENT

The present invention involves the contacting of a hydrocarbon stream, consisting of C2-C4 olefins or mixtures thereof diluted with C1-G4 paraffins, or mixtures thereof, with an aromatization catalyst at a temperature in the range of 400 to 600C to produce benæene, toluene and xylenes.

The process of the invention is, as shown in the examples, capable of produclng an aggregate weight percent yield of benzene, toluene and xylenes, based on total liqu~d product, when a mixture of feedstock and diluent i8 used, which is greater than that obtained in the absence of diluent under otherwise similar conditions of catalyst, temperature, pressure and total flow rate ~feedstock and diluent if any) per unit . ~

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amount of catalyst. The process of the invention results in grea~er selectiviey ~or conversion of feedstock to benzene, ~oluene or xylene, and preferably in greater selectivity for the aggregate of benzene9 toluene and xylen~s.

The feedstock for this invention may be either a single oleinic species diluted with a single paraffinic component or it may be a mixture of olefins and diluents. Such mixtures may be those which are availa~le at refineries, which are often used for internal heating within the plant.
In the latter case, after the process according to the invention, 80-90 wt% of the original gas stream may be returned for heating purposes and at a higher H/C ratio.

The term 'aromatization catalyst' refers to ZSM-5 catalysts which are capable of producing aromatics from a C2-C4 ole~inic feed. The ZSM-5 type of zeolite is generally considered to include ZSM-5, ZSM-8, ZSM-11, ZSM-12 and other similarly behaving catalysts. U.S. Patent No. 3,702,886 describes methods o~ preparation of the ZSM-5 zeolites.

It is known in the art that zeolite~, particularly synthetic zeolites, may have their compositions modified or altered by impregnation of certain metals thereon or by exchanging variou~ anions and/or cations into the crystal structure of the zeolite. The ZSM-5 family of catalyst has been found to be especially active for aromatization if it has been exchanged with ~he following cations: ammonium, chromium or nickel. In particular the ammonium-exchanged ZSM-5 which results in the hydrogen form upon calcination i8 preferred. Typically~ the exchange i8 achieved . ~

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by contacting the sald zeolite with a salt of the desired replacing cation. Although a wide variety of salts may be used partioular preference is given to chlorides, nitrates and sulfates. It is also known in the art to disperse the zeolite catalyst in a porous mixture such as clay, alumina, silica and mixtures thereof. Techniques for incorporating the zeolites into a matrix are set forth in U.S. Patent No. 3,140,253.

In accordance with the present invention a feed of C2-C4 olefin or mixture thereof is diluted with C1-C4 paraffins, or mixtures thereof and contacted with an aromatizat~on catalyst. It ls preferred to use methane as the diluent and H-ZSM-5 as the aromatization catalyst. The contact is made at a temperature in the range of about 400~C to about 600C and preferably 435C to 550DC., more pre~erably 460C to 510C. ~igher temperatures favor selecti~ity for BTX, but if the temperature is too high, the increase in selectivity will not ~ustify the higher energy consumption.

Preferably, the reaction is conducted at atmospheric pressure; other pressures can also be employed, but higher pressures may adversely affect BTX yield and selectivity and are therefore not preferred. The feed to diluent mole ratio is preferably in ~he range from abou~ 1:4 to about

4:1. When dlluents other than methane are used, the ratio should not be greater than 2.4:1. Preferably the feed to diluent mole ratio is in the range from about 1:2.5 to about 2.4:1. ~ligher ratios tend to reduce yield and selectivity for BTX. Lower ratios, although they give good yield and s~lectivity for BTX, increase the sl~e of equipment and the .. ~
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amount of diluent required to be handled in processing of a given amount of olefin hydrocarbon feedstock.

Preferably, the space velocity (~HSV) is at least three gra~s of total feed per gram of catalyst per hour. Below that rate, coking of the catalyst may increase. Higher space velocities can be employed; a person skilled in the art can select suitable space velocities in the light of the present specification.

Although the Cl to C4 hydrocarbons which are added to the feedstock in the practice of the invention are refer~ed ~o as diluents, it is understood that they may also enter into the reactions involved in the process of the invention. The use of the term, diluent, does not indicate that the material referred to is necessarily inert.

In the following examples 1,3,4 and 5 a stainless steel (316) reactor and preheater were used. The tublng, from the cylinders to preheater inlee and from condenser outlet was Teflon~ while all other tubing was stainless steel. The preheater was a ~" OD s~ainless steel tube packed to 4 inch depth with pyrex beads. A fixed-bed upflow reactor was used with an axial thermowell. The catalyst in the reactor was supported by plugs of quartz eape. Tubing between preheater and reactor, and reactor and condenser was heated with an electrical heating tape. Pressure gauges were mounted at the inlet to the preheater and the inlet to the :

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reactor. The flow was monitored, controlled and mixed with a Matheson Model No. 7352 flow controller equipped with No. 610A tubes.

In example 2 the system was similar except that the preheater was a ~ OD
stainless steel tube filled with pyrex beads with the gas flowing up, the preheater outlet temperature was about 250 to 300C. The stainless steel reactor was as in the other examples except that gas flow was down through the reactor in example 2.

In the fol]owing examples, fresh catalyst samples were used in Runs 4,5,14,17,22,28,32 and 35. In Run 7, a mixture of fresh and regenerated catalyst was used. In the other runs, catalyst samples which had been regenerated from one to eight times were used. The regeneration procedure, performed after the completion of a run, and without cooling from reaction temperature, is as follows:

Nitrogen is passed over the catalyst to purge the system. Then air is passed o~er the catalyst at 30-40 mllmin. for 3-4 hours after which the system is again purged with nitrogen. Hydrogen i~ then passed over the catalyst at 10-15 ml/min. for 15-18 hours after which the catalyst is ready for another run.

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~'æ3~59 Example 1 Th2 following runs illustrate the effects of dilution of the feed on the selectivity to benzene, toluene and xylenes in the liquid product.
Selectivity is calculated as the weight percent of the aggregate of benzene, toluene and xylene in the liquid product of the reaction. The olefinic feed was propylene and the diluent was methane. The propylene/methane mixture was contacted with a HZSM-5 catalyst at atmospheric pressure. The dilution of the olefinic feed is seen to improve the selectivity to benzene, toluene and xylenes. Dilution changes the thermal profile in the reactor and, as a consequence, affects the catalyst activi~y; these factors contribute to a favoring of benzene, toluene and xylenes in the liquid product. The yields of benzene, toluene and xylenes ~based on propylene in the feed) are also found to increase. This is believed to be due to the combined effects of diluting the stream with methane and of the contribution of methane as a reactant;
however, the invention is not to be limited by any theory as to the reaction mechanism.

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~'~3~59 Table 1 Propylene Feed Contacted With H-ZSM-5 at AtmosF~ric Pressure With and Without Methane _ luent Runs 1-9 10 11 12 13 Diluent nonel CH4 CH4 CH4 CH4 Dilution (mol ratio of feed to diluent) 3:1 0.95:1 0.98:1 1:3 Reactor Temp (C) 527 530 523 496 518 WHSV (g olefin HC-(g cat) hr 1) 9.7 7.28 4.74 4.73 2.37 Catalyst leading (g) 0.5 0.5 0.5 0.5 0.5 Total Flow rate (ml/min.) 47.0 47.1 46.7 51.0 46.7 Yield BTX Yield (wtX of C3H6 in) 26.0 36.7 37.8 24.2 36.8 BTX selectivity (wt~ of liquid)65.080.9 87.3 74.9 94.0 1. These pure propylene results are averages of 9 runs. The estimated standard deviations of these averages are: temperature +16C, yields ~6.5 wt% and selectivity 110 wt%. The BTX selectivity range of extreme values was 57 to 78 wt%. The data for the individual runs are as follows:

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Bed BTX BTX Liquid Total Temp. Selectivity Yield Yield State of Flow Run No. C wt % wt % wt ~Catalyst mL/min 1 543 74.2 26.2 35.2 R 47 2 541 77.6 32.0 41.3 R 47 3 538 68.3 23.6 34.7 R 47 4 541 59.3 23.3 39.3 F 47 520 77.8 32.8 42.2 F 48 6 499 59.7 21.8 36.4 R 47 7 504 47.2 15.0 32.0F(0.25g) 47 and R(0.25g) 8 532 57.4 30.0 51.4 R 47 9 526 68.2 24.9 36.7 R 47 F - Fresh Ca~alyst R = Regenerated Catalyst Comparison of Runs 10,11 and 13 with Runs 1-9 shows that dilution of propylene with methane increases the BTX yield and selectivity at equivalen~ total flow rates per unit amount of ca~alyst and at generally equivalent temperatures. The selectivity increases as the ratio of feed to diluent decreases. In Run 12, the yield and selectivity were lower than in Run 11, probably because of the lower temperature in Run 12.

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Example 2 The following runs further illustrate the effects of dilution of the feed on the selectivity to benzene, toluene and xylenes in the liquid product.

The catalyst used ln these runs was prepared as followso 2.29 g SiO2 was dissolved in 17.7 mL of 25~ solution of tetrapropyl-ammonium hydroxide. 0.32 g of NaA12O was dissolved in 2 mL of water.
These two solutions were combined and mixed well and then poured into a pyrex tube. An additional 2 mL of water was used to wash residue out of the beaker into the pyrex tube~ The tube was sealed and left for 7 days at 170C. The solid was filtered, washed and dried at 110C overnight and then calcined at 600C overnight. The catalyst was then determined to be ~SM-5 type by X-ray diffraction. The catalyst was ion exchanged in a solution of 2M NH4NO3 (250 mL/g of catalyst) for 24 hours. The solid was filtered, washed and dryed overnight at 110C and then calcined at 600C overnight.

The procedure was otherwise as disclosed in Example 1. Table 2 gives the results obtained:

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Table 2 Propylene Feed Contacted with H-ZSM-5 at Atmospheric Pressure, With and Without Methane Diluent Runs 14 15 16 17 18 19 Diluentnone none CH~ CH4 CH4 CH4 Dilution (mol ratio feed to diluent) - - 1.00:1 0.96:1 1.00:1 0.90:1 Reactor Temp (C)546 534 541 523 482 572 WHSV (g) o~efin ~C
(g. cat hr. ) 8.87 8.99 4.28 4.42 4,50 4.46 Catalyst loading (g) 0.5 0.5 0.5 0.5 0.5 0.5 Total flow rate (ml/min) 43.0 43.6 41.5 43.6 43.5 45.9 BTX yield (wt.~
of C3H6 in) 17.1 19.3 29.5 24.9 24.3 34.8 BTX selectivity (wt.~ of liquld) 51.8 61.3 74.6 80.2 80.2 91.2 Comparison of Runs 16, 17, 18 and 19 with Runs 14 and 15 again shows superior BTX yield and selectivity at comparable temperature~ using methane as diluent. Since the catalyst in Example 2 was from a different batch from the catalyst used in Example 1, the results in Examples 1 and 2 are not directly comparable. For example, the lower yield and selectivity in Run 17 as compared with Run 11 may be primarily the result of the different catalyst used.

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In another run with the same caealyst as in Runs 16 to 19, with dilution of 0.92:1, reactor te~perature of 433C, WHSV of 4.42, catalyst loading of 0.5 and total flow rate of 44.1, BTX yield of 19.2 and BTX selectivity of 60.9 were obtained, indicating the poorer results obt~ined with lower reactor temperature.

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Example 3 The following runs illustrate the efEects of other dlluents on benzene, toluene and xylenes seleceivity with propylene as the olefinic feed. The feeds were contacted ~7ith the ~-ZSM-5 catalyst of Example 1, at atmospheric pressure. The diluents, N2 and C3~8 (the latter in mole ratios of feed to diluent of 0.91:1 and 0.31:1) improved the selectivity to benzene~ toluene and xylenes compared to the undiluted stream. The results for Runs 1-9 are included for comparison purposes.

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Table 3 Propylene/Propane and Propylene/Nitrogen Feeds Contacted with H-ZSM-5 at Atmospheric Pressure Runs 1-9 20 21 22 23 Diluent noneC3~8 C3H8 C3H8 2 Dilution (mol ratio of feed to diluent) - 0.91:1 3.03:1 0.31:1 0.90.1 Reactor Temp (C) 527 526 520 500 532 W~ISV (g olefinic ac- (g cat) l-hr 1) 9.7 4.547.26 2.32 4.75 Catalyst loading (g) 0.5 0.5 0.5 0.5 0.5 Total Flow rate (ml/min.) 47.0 46.2 46.847.4 48.5 Yield BTX Yield ~wt% of C3H6 in) 26.0 29.7 15.327.6 28.0 BTX selectivity (wt70 of liquid) 65.0 75.457.7 83.2 73.2 Comparing Runs 20, 21 and 22 with Runs 1-9 indicates that superior yield and selectivity for BTX are obtained with propane diluent as compared with no diluent when the feed to d~luent mole ratio is about 1:1 (0.91:1) and about 1:3 tO.31:1), but not when the ratio is about 3:1 (3.03:1).

Comparing Run 20 and 23 lndicates that propane and ni~rogen, as diluent, give similar BTX yield and selectivity, with propane apparently somewhat better. Comparing Run 23 with Run 11 in Example 1 indicates that methane as diluent gives substantially increased B'FX yield and selectivity as compared with nitrogen as diluentO

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Example 4 The following runs further illustrate the effects of paraffin hydrocarbon diluents on benzene, toluene and xylenes se~ectivity with olefinic feeds comprising ethylene and propylene. The feeds were contacted wlth H-ZSM-5 catalyst at atmospheric pressure. The feed composition was as follows:

Component Weight X
Oleflns Ethylene 12.5 + 0.3 Propylene 24.4 + 0.5 Isobutene 3.46 + 0.07 1.3 - butadiene 0.51 ~ 0.01 Trans-2-butene 1.49 + 0.03 Cis-2-butene 1.32 + 0.01 43.6 Paraffins Methane 0.085 ~ 0.002 Ethane 29.4 ~ 0.6 Propane 20.0 + 0.4 Isobutane L.65 + 0.03 n-butane 5.22 ~ 0.1 56.355 , :' ` ~L2~5~

The catalyst used was the same as in Example 1. The following results were obtained. The results for Runs 1-9 wherein no paraffin hydrocarbons were present, are shown for comparison.

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Table 4 C1 to C4 Olefin/Paraffin (P) Mixtures Contacted with H ZSM-5 at Atmospheric Pressure Runs 1-9 24 25 26 27 Diluent none P P P P

Dilution ~mol ratio of C3H6 to diluent) - 0.74 0.74 0.74 0.74 Reactor Temp (C) 527 504 550 543 462 WHSV (g) o~efin ~C
(g. cat- hr.- ~ 9.7 3.59 3.5 3.5 3.5 Catalyst loading (g) 0.5 0.5 0.5 0.5 0.5 Total flow rate (ml/min) 47.0 46.2 4409 44.9 44.9 BTX yield (wt.%
of olefin in) 26.0 28.3 35.B 32.5 24.2 BTX selectivity (wt.% of liquid)65.068.3 82.6 72.9 69.8 Comparing Run~ 24, 25 and 26 wi~h Runs 1-9 indicates the superior yield and selectivity obtained with the paraffin hydrocarbon dlluents. Run 27 gave lower yield and/or selectivi~y9 apparently because of lower reac~ion temperature.

In additional runs with the catalyst used in Example 2, the following results were obtained, the results for Runs 14 and 15 being shown for comparison.

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Table 5 C~ to C4 Olefin/Paraffin (P) Mix~ures Contacted with H-ZSM-5 at Atmospheric Pressur Runs 14 15 28 29 Diluent none none P P
Dilution (mol ratio of feed to diluent) - - 0.74 0.74 Reactor Temp (C) 546 534 511 514 WHSV (g olefinic HC-(g cat) hr 1) 8.87 8.99 3.70 7.06 Catalyst loading (g) 0.5 0.5 0.5 0.5 Total Flow rate (ml/min.) 43.0 43.6 47.6 90.6 BTX Yield (wt% of C3H6 in) 17.1 19.3 18.3 17.7 BTX Selectivity (wt~ of liquid 51.8 61.3 64.4 55.7 .

Comparison of Runs 28 and 29 with Runs 14 and 15 indicates that compara~le BTX yield and selectivity can be obtained at lower temperatures, with diluent as compared to without diluent.

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Example 5 The following runs further illustrate the effects of paraffin hydrocarbon diluents on benzene, toluene and xylenes selectivity with olefinic feeds comprising ethylene and propylene. The feeds were contacted with HZSM-5 catalyst at atmospheric pressure.

The catalyst preparation was as follows: 1.37 grams of SiO2 was pnrtially dissolved in 11.5 ml. of 25% tetrapropylammonium hydroxide solution by heating to a temperature of about 100C. There was then added a mixture of 0.19 grams NaAlO2 dissolved in 1.07 ml. of water.
The mixture was placed in a pyrex tube and sealed. The tube was heated to l50C for 9 days. The resultant solid product was cooled to room temperature, filtered3 washed with 2 liters of water and dried at 110C
overnight. A portion of this catalyst was analyzed by X-ray diffraction and determined to be ZSM-5 type. The rest of the catalyst was calcined at 600C overnight. The catalyst was then ion exchanged in 250 ml. of 2M NH4NO3 solution per gram of catalyst for 24 hours. The exchanged catalyst was filtered, washed with distilled water, dried o~ernight at 110C and calcined at 600C overnight.

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The feed composition for Runs 32 and 33 was as follows:

Component Wei~ht %

Ethylene 3 Ethane 40 Propylene 17 Propane 40 The followin~ results were obtained. The results of Runs 30 and 31 wherein no paraffinic hydrocarbons were present, are shown for comparison.

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Table 5 Cl to C4 olefin/paraffin (P) mixtures contacted with H-ZSM-5 a~ atmos~heric pressure Runs 30 31 32 33 Diluent none none P P
Dllution (mol ratio of C3H6 to diluent) - - 0.18 0.18 Reactor Temp (C) 552 565 552 565 WHSV (g ol~finic HC'(g cat) hr 1)9.71 9.71 2.28 1.69 Catalyst loading (g) 0.5 0O5 0.5 0.5 Total Flow rate (ml/min.) 47 47 64 47.3 BTX Yield (wtX of olefin in) 30.4 27.6 30.0 35.0 BTX selectivity (wt% of liquid 72.0 67.4 97.0 97.5 The results indicate an improvement in selectivity for BTX by use of the paraffin diluents in Runs 32 and 33 as compared with Runs 30 and 31 respectively, and also an increase in BTX yield by use of the paraffin diluents in Run 33, as compared with Run 31.

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Claims (11)

What is claimed is:

1. A process for converting a feedstock consisting essentially of C2 to C4 olefin hydrocarbons or mixtures thereof to aromatic hydrocarbons which comprises: contacting a mixture consisting essentially of said feedstock and C1 to C4 paraffin hydrocarbons or mixtures thereof with an aromatization catalyst at a temperature in the range from about 435°C. to about 600°C., the mole ratio of feedstock to diluent being in the range from about 1:5 to about 1.6:1, thereby to obtain greater selectivity for conversion of feedstock to benzene, toluene and xylene than in the absence of said diluent.

2. Process according to claim 1 wherein said temperature is in the range from about 460°C. to about 510°C.

3. Process according to claim 1 wherein the weight hourly space velocity is at least three grams of hydrocarbon feed per gram of catalyst per hour.

4. Process according to claim 1 wherein said feedstock is propylene.

5. Process according to claim 1 wherein said feedstock comprises ethylene and propylene and said diluent comprises ethane and propane.

6. Process according to claim 1 wherein said aromatization catalyst is H-ZSM-5.

7. A process for converting a feedstock consisting essentially of C2 to C4 olefin hydrocarbons or mixtures thereof to aromatic hydrocarbons which comprises: contacting a mixture consisting essentially of said feedstock and methane with an aromatization catalyst at a temperature in the range from about 435°C. to about 600°C., the mole ratio of feedstock to methane being in the range from about 1:4 to about 4:1, thereby to obtain greater selectivity for conversion of feedstock to benzene, toluene and xylene than in the absence of said diluent.

8. Process according to claim 7 wherein said temperature is in the range from about 460°C. to about 510°C.

9. Process according to claim 7 wherein the weight hourly space velocity is at least three grams of hydrocarbon feed per gram of catalyst per hour.

10. Process according to claim 7 wherein said feedstock is propylene.

11. Process according to claim 7 wherein said aromatization catalyst is H-ZSM-5.