MXPA01005894A - Dehydrocyclization process with downstream dimethylbutane removal. - Google Patents

Abstract

In the present invention, dimethylbutanes are removed from the raffinate component of the feed to a dehydrocyclization process. Thus, according to a preferred embodiment, a process is provided for producing aromatics by the following steps: (a) contacting fresh paraffins rich feed hydrocarbons, containing 0.1 to 20.0 wt.% dimethylbutanes with a highly selective dehydrocyclization catalyst in a reaction zone, under dehydrocyclization reaction conditions, to convert paraffins to aromatics and obtain an aromatics rich effluent; (b) separating aromatics from the effluent to obtain an aromatics lean raffinate; (c) removing dimethylbutanes from the raffinate to obtain a raffinate of reduced dimethylbutane content; and (d) recycling the raffinate of reduced dimethylbutane content to the reaction zone. Preferably, the dehydrocyclization catalyst used is a nonacidic, monofunctional catalyst. Platinum on L zeolite is a particularly preferred highly selective dehydrocyclization catalyst for use in the process.

Description

DEHYDROCYCLING PROCESS WITH DISPOSAL OF DIMETHYLBUTANE CURRENT DOWN.

FIELD OF THE INVENTION The present invention is directed to a dehydrocyclization using a highly selective dehydrocyclization catalyst, and particularly to remove dimbutanol from recycle to a highly selective dehydrocyclization catalytic zone. BACKGROUND OF THE INVENTION In the process of the present invention aromatics are formed from a hydrocarbon feed by dehydrocyclization. Other reactions may take place in the reaction zone, and the reaction step may be referred to more generally as reformation. Thus, in the reaction step of the process of the present invention, in addition to the dehydrocyclization or aromatization reaction, other reactions may occur, such as dehydrogenation, isomerization, hydroisomerization, cyclization and thermal hydrodegeneration. The main reaction is to _ reí. u? oß 'the dehydrocyclization to form paraffin aromatics. U.S. Patent No. 4,104,320, which was issued on August 1, 1978, discloses that it is possible to dehydrocyclize paraffins to produce aromatics with high selectivity using a non-acid monofunctional catalyst. Preferably, the catalyst comprises a L-type Zeolite. The L-type zeolite preferred in the patent process 10 North American No, 4,104,320 is one that has interchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium. The catalyst used in U.S. Patent No. 4,104,320 also contains at least one noble metal 15 of group VIII (or Tin or Germanium). In particular, the catalysts having platinum in an L zeolite, wherein the potassium in the zeolite L has been exchanged to replace a portion of the potassium with Rubidium or Cesium, are claimed in the patent. 20? 320 achieve or perform an exceptionally high selectivity for the reformation of n-hexane to benzene. As described in U.S. Patent No. 4,104,320, the zeolite L used is --- A --._---! - «---.- > , ---.-.-_ synthesizes typically in the form of potassium. A portion, usually not more than 80%, of the potassium cations can be exchanged so that other cations, preferably rubidium or cesium, replace the exchangeable potassium. After the North American Patent No. 4,104,320 in 1978, an important advanced stage is described in US Patents Nos. 4,434,311; 4,435,283; 4,447,316; and 4,517,306, all 10 whose patents were granted in 1984 and 1985. These patents describe the dehydrocyclization catalysts comprising a large pore zeolite exchanged with an alkaline earth metal (barium, strontium or calcium, preferably Barium) and wherein the catalyst contains one or more metals of group VIII, more preferably platinum. An essential element in the catalyst of these patents is the alkaline earth metal. Especially when the alkaline earth metal is 20 barium, and the large pore zeolite is zeolite L, it was found that the catalyst of these patents provide higher selectivities than the alkaline exchange zeolite L catalysts ---, -, __ -.- > --- ». corresponding described in US Patent No. 4,104,320. These platinum catalysts in zeolite L referred to in the two previous paragraphs, whether in the form of potassium, or another form of alkali metal, or in the exchanged form of alkaline earth metal, are substantially "non-acidic". These non-acid catalysts have been referred to as "monofunctional" catalysts. Such catalysts 10 non-acids, monofunctionals are highly selective to form aromatics via dehydrocyclization of paraffins. Having a highly selective catalyst, the marketing seems direct. However, this does not 15 is the case. It was found that high selectivity L zeolite catalysts containing the group VIII metal are unexpectedly susceptible to sulfur poisoning at ultra low levels of sulfur in the feed. The North American Patent No. No. 4,456,527 descr this discovery. Specifically, it was found that the concentration of sulfur in the hydrocarbon feed should be reduced to ultra low levels, preferably ^ g ^ g ^ less than 50 parts per billion, to achieve acceptable stability, that is, long stay in runs, for the catalyst when used in the dehydrocyclization process. With the progress of US Pat. No. 4,456,527, more attention is given to the arrangement of the process under which the total dehydrocyclization process is carried out to produce aromatics. U.S. Patent Nos. 4,648,961 and 4,650,565 disclose processes using a highly selective dehydrocyclization catalyst wherein a paraffin-rich feed is contacted with the catalyst to form aromatics, then the aromatics are separated from the effluent from the reaction zone by of a solvent extraction step, or via a molecular sieve separation, and a stream rich in refined paraffins from the solvent extraction step or the molecular sieve separation step, is recycled to the reaction zone of the dehydrocyclization . US Patent No. 4,594,145 and Reissued Patent 33,323 disclose a process using a highly selective dehydrocyclization catalyst wherein a paraffin-rich feed is contacted with the catalyst to form aromatics, then the aromatics are separated from the effluent from the zone. of reaction, and the remaining paraffins are recirculated to the reaction zone. U.S. Patent Nos. 4,568,656 and 4,595,668 descrthe use of a highly selective dehydrocyclization catalyst wherein the metal component of Group VIII of the catalyst is highly dispersed in a support of zeolite L. These two patents state (see column 16, line 38, of the '666 patent) "Since the catalyst is monofunctional and does not promote isomerization without cyclization, feed compounds such as dimethylbutanes are not effective". Another patent oriented to another process using a high selectivity dehydrocyclization catalyst is European Patent 335,540. This patent descr a process wherein the hydrocarbon feed is (a) separated within a first fraction comprising C5 minus hydrocarbons and dimethylbutanes, and a second fraction comprising C5 plus hydrocarbons; (b) separating the second fraction within (i) a light fraction comprising not more than 10% by volume of dimethylbutanes, the light fraction being selected from a C5, a C7, a C8, a C6- C7, a C7-C8, C6-Ce fraction and a fraction consisting essentially of C and C8 hydrocarbons; and (ii) a heavy fraction; and (c) dehydrocyclize the light fraction under conditions 10 dehydrocyclization in the presence of a monofunctional catalyst. In this way, according to the process of EP 335,540, dimethylbutanes are removed from the fresh hydrocarbon feed before the dehydrocyclization of the 15 fresh feed to form aromatics. BRIEF DESCRIPTION OF THE INVENTION According to the present invention, there is provided a process for dehydrocyclization, wherein the process comprises: (a) contacting feed hydrocarbons rich in fresh paraffins containing from 0.1 to 20% by weight of dimethylbutanes with a highly dehydrocyclization catalyst selective α-Mti-MH-il-U-il in a reaction zone under dehydrocyclization reaction conditions to convert the paraffins to aromatics and obtain an aromatic-rich effluent; 5 (b) separating the aromatics from the effluent to obtain a refining rich in paraffins; (c) removing the dimethylbutanes from the raffinate to obtain a refined content of reduced dimethylbutanes; and 10 (d) recirculating the refining of the reduced dimethobutane content to the reaction zone. Preferably, the highly selective dehydrocyclization catalyst is a non-acid catalyst. The catalyst preferably includes a Group VIII metal, more preferably platinum, and preferably includes a one-dimensional crystalline aluminosilicate. The term "one dimensional" means that the pores or channels in the crystals travel substantially only in one direction along the length or on the C axis of the crystals. The preferred aluminosilicates are zeolite L, omega zeolite, ZSM-10 and mordenite.

The most preferred one is aluminosilicate, zeolite L, zeolite L is one-dimensional. In the process of the present invention, an important aspect centrally is the separation of a larger portion of dimethylbutanes downstream of the dehydrocyclization reaction zone. In this way, dimethylbutanes are not separated mainly from the fresh feed to the process. Instead of this, in the present invention, the dimethylbutanes are mainly separated from the effluent of the dehydrocyclization reaction. The removal of the downstream dimethylbutano can be carried out after the aromatics are separated from the effluent of the reaction zone, or before the separation of the aromatics; but, in any case, downstream of the dehydrocyclization reaction zone. The separation of the aromatics from the effluent of the reaction stage is preferably done by solvent extraction, distillation or molecular sieve extraction. More preferably, the separation of the aromatics is made by the extraction of the solvent. This step of separating the aromatics results in a product stream rich in aromatics, and a refining stream rich in paraffins. In the process of the present invention, the dimethylbutanes are preferably separated from the refining stream. After the refining is recirculated to the reaction stage. Among other factors, the present invention is based on my conception of elimination of the dimethylbutanes mainly downstream of the dehydrocyclization reaction step instead of removing the dimethylbutanes from the fresh feed to the reaction zone. Furthermore, the present invention is based on my discovery that this unusual point of elimination of dimethylbutanes results in unexpected advantages in terms of performance and operating cost for the high selectivity dehydrocyclization process. According to one embodiment of the present invention, as described above, removal of the downstream dimethylbutyroid is carried out after the aromatics are removed from the effluent of the dehydrocyclization step. According to another embodiment of the present invention, the dimethylbutanes are removed downstream from the dehydrocyclization reaction zone but before the treatment of the effluent from the reaction zone separates the aromatics from the paraffins. Thus, according to this embodiment, a process for the dehydrocyclization of hydrocarbons is provided which comprises: (a) contacting the feed hydrocarbon rich in fresh paraffins containing from 0.1 to 20.0% by weight of dimethylbutanes with a highly selective dehydrocyclization catalyst in a reaction zone under dehydrocyclization reaction conditions to convert the paraffins to aromatics and obtain an aromatic-rich effluent; (b) removing the dimethylbutanes from the aromatic-rich effluent to obtain a mixture of paraffin-aromatics of reduced dimethyl-butylated content; (c) separating the aromatics from the paraffin-aromatic mixture to obtain a poor refining in aromatics; and (d) recirculating the raffinate to the reaction zone. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a simplified schematic process flow diagram illustrating a preferred embodiment of the present invention, wherein the dimethylbutanes are removed downstream from the dehydrocyclization reaction zone. DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1 simplified in more detail, the fresh feed is introduced to the dehydrocyclization reaction zone via lines 1 and 2. Suitable feed materials are the non-aromatic organic compounds, preferably the Feeding material with the range of the boiling point of naphtha, such as paraffins C6-C? 0. more preferably C6-C8. Since dehydrocyclization is the key reaction in the reaction zone 3, the feed material preferably is C6 or better non-aromatic organic compounds. Examples of the preferred C6-C8 paraffinic feedstock components include n-hexane, n-heptane and n-octane. 5 The naphtha feedstock can be directly derived from crude oil by distillation, or it can be indirectly derived from an oil feedstock by separating a stream in the range of the point of 10 boiling of the naphtha from the hydrodisintegration effluent, carbonization, or catalytic disintegration. Suitable naphtha feedstocks can boil between about 60 ° C and 220 ° C. Preferably, the feeding material Naphtha is predominantly (more than one half by weight) paraffins, and most preferably C6-C8 paraffins. Ce paraffins, especially n-hexane and also methylpentanes, are particularly preferred feedstocks. As discussed in more detail below, these feed materials almost invariably contain dimethylbutanes, since the boiling point of dimethylbutanes is close to that of IHM-a-i-tf-E? -i-k-t-uii- hexanes. Table 1 below lists the boiling points of hydrocarbons in the range of paraffin C5-C7. TABLE 1 Boiling points of selected C5-C-7 hydrocarbons. 2, 2-dimethylbutane 121.5 50 2, 3-dimet ilbut ano 136 58 2 -methylpentane 140.5 60 CßHi-j 3-methylpentane 145.9 63 n-hexane 156 69 As can be seen in Table 1, the boiling of dimethylbutanes is close to that of n-hexane, and also close to that of methylpentanes, both 5 which are preferred as feedstock for the dehydrocyclization reaction zone. The fresh feed of net weight for the process of the present invention will contain dimethylbutanes, typically from 0.1 to 20% by weight of dimethylbutanes, preferably from 0.3 to 18%, more preferably from 0.5 to 18%, and more preferably from 1.0 to 15. %, based on the weight of the fresh or net hydrocarbon feed for the process. Since dimethylbutanes are not desired in the dehydrocyclization reaction zone 3, the prior art shows the elimination of the dimethylbutanes from the fresh feed. The separation of the dimethylbutanes from the fresh feed can be done by distillation, as shown by the previously mentioned EP Patent 335,540. -.-- t --- amifc > .

However, in the process of the present invention dimethylbutanes are not primarily removed from the fresh feed to the reaction zone 3. In the present invention, the dimethylbutanes are mainly removed downstream in the process, after the aromatics have been formed in the process. the dehydrocyclization reaction zone. The downstream removal of the dimethylbutanes is further described below. The term "fresh feed" is used here to indicate the hydrocarbon feed net to the process, which has not passed through the dehydrocyclization zone. In the present invention, the reaction zone 3 uses a dehydrocyclization catalyst which has a high selectivity to form aromatics. The term "selectivity" as used in the present invention is defined as a percentage of moles of acyclic hydrocarbons converted to aromatics compared to the aromatic converted moles and disintegrated products. In this way, the percent selectivity can be defined by the following formula: 100 x moles of acyclic hydrocarbons That is, selectivity: converted to aromatic Moles of acyclic hydrocarbons converted to aromatics and disintegrated products.

The isomerization of paraffins and interreformation paraffins and the alkylcyclopentanes having the same number of carbon atoms per molecule are not considered in the determination of selectivity. The selectivity for converting acyclic to aromatic hydrocarbons is a measure of the effectiveness of the dehydrocyclization reaction step in the reformation of acyclic hydrocarbons to valuable products and desired: aromatics and hydrogen, as opposed to less desirable by-products, such as products for hydrodisintegration in the dehydrocyclization reaction zone. Highly selective catalysts produce more hydrogen than less selective catalysts - 1! because hydrogen is produced when acyclic hydrocarbons are converted to aromatics, since hydrogen is consumed when the acyclic hydrocarbons are converted into disintegrated products. Increasing the selectivity of the process increases the amount of hydrogen produced (more aromatization) and decreases the amount of hydrogen consumed (less disintegration). The preferred catalysts for use in the The reaction zone 3 are highly selective dehydrocyclization catalysts, particularly the catalysts comprising a Group VIII metal dispersed in a one-dimensional crystalline aluminosilicate. The catalyst of Highly selective dehydrocyclization is preferably a non-acidic catalyst, and preferably it is a monofunctional catalyst. Preferably, the catalyst used in the reaction zone 3 contains one or more metals of the Group VIII, for example, nickel, iridium, rhodium, palladium, rhenium or platinum. The most preferred Group VIII metals for use in the dehydrocyclization catalyst are iridium, palladium and platinum, ______ --._. - .----, --- more especially platinum. The amount of Group VIII metal in the catalyst is preferably between 0.1 to 5% by weight based on the total catalyst, more preferably 0.3 to 2% by weight. Metal or Group VIII metals can be introduced to the catalyst such as the zeolite component of the catalyst, by synthesis, impregnation, or ion exchange carried out in an aqueous solution of an appropriate salt. When it is desired to introduce two Group VIII metals into the zeolite component of the catalyst, the operation can be carried out simultaneously or sequentially. By way of example, the platinum can be introduced by impregnation of the zeolite with an aqueous solution of tetramine (II) nitrate, chloroplatinic acid, chloroplastic acid, diamine-platinum or tet raminoplat ino (II) chlorite. In an ion exchange process, platinum can be introduced by using a complex of cationic platinum such as tet raminoplat ino (II) nitrate. Preferably, an inorganic oxide is used as a carrier to bind the zeolite containing the Group VIII metal and provides firmness and integrity for the catalyst as a particulate. The carrier can be an inorganic oxide produced synthetically or naturally or a combination of inorganic oxides. Typical inorganic oxide supports that can be used include clays, alumina and silica wherein the acid sites are preferably exchanged for cations which do not impart strong acidity (such as sodium, potassium, rubidium, cesium, calcium, strontium or barium) ). The preferred one-dimensional aluminosilicates for use in the dehydrocyclization catalyst are zeolite L, omega zeolite, ZSM-10, cancrinite and mordenite. The zeolite L is especially preferred for the catalyst used in the reaction zone 3. The L-type zeolites are the synthetic zeolites. A theoretical formula is Mg / n [A102) 9 (Si02) 27] where M is a cation having the valence n. The actual formula can vary without changing the crystalline structure; for example, the molar ratio of silicon 65 to aluminum (Si / Al) can vary from 1.0 to 3.5.

Although there is a number of cations that may be present in the zeolite L, in one embodiment, it is preferred to synthesize the potassium form of the zeolite, ie, the way in which the interchangeable cations present are substantially all potassium ions. The compliance reagents used are readily available and are generally soluble in water. The interchangeable cations present in the zeolite can conveniently be replaced by other interchangeable cations, as will be shown below, thereby producing an isomorphic form of zeolite L. In a method of manufacturing zeolite L, the potassium form of zeolite L is prepared by adequately heating a mixture of aqueous metal aluminosilicate whose composition, expressed in terms of the molar proportions of the oxides, fall within the range: K20 / (K20 + Na20) From about 0.33 to about 1 (K20 + Na20) / Si02 From about 0.35 to about 0.5 Si02 / Al2 03 From about 10 to about 28 H20 / (K20 + Na20) From about 15 to about 41 20 - ¿• • EMF-d-b The desired product is therefore crystallized relatively free of zeolite from the dissimilar crystal structure. The potassium form of zeolite L can also be prepared in another method in conjunction with other zeolite compounds which employ a reaction mixture whose composition, expressed in terms of molar ratios of oxides, falls within the range: K20 / (K20 + Na20) From about 0.26 to about 1 (K20 + Na20) / Si02 From about 0.34 to about 0.5 Si02 / Al2 03 From about 15 to about 28 H20 / (K20 + Na20) From about 15 to about 51 It should be noted that the presence of sodium in the reaction mixture is not critical to the present invention. When the zeolite is prepared from reaction mixtures containing sodium, the sodium ions are also generally included within the product as part of the interchangeable cations together with the potassium ions. The product obtained from the above ranges has a composition, expressed in terms of moles of oxides, corresponding to the formula: 0.9-1.3 [(1-x) K20? XNa20]: A1203: 5.2-6.9 Si02: H20 where " x "can be any value from 0 to about 0.75 and" and "can be any value from 0 to about 9. In the manufacture of zeolite L, the representative reagents are activated alumina, gamma alumina, alumina trihydrate, and sodium aluminate. as a source of alumina. Silica can be obtained from sodium or potassium silicate, silica gels, silicic acid, sols (liquid colloids) of aqueous colloidal silica, and reactive amorphous solid silicas. The preparation of typical silica sols which are suitable for use in the process of the present invention are described in US Pat. American No. 2,574,902 and Pat. North American No. 2,597,872. Amorphous solid reactive group silicas, typical, preferably having a final size of less than 1 micron, are such materials as smoked silica, precipitated silica sols and chemically precipitated. Sodium and potassium hydroxide can supply the metal cation and help control pH. In the manufacture of zeolite L, the usual method comprises dissolving potassium aluminate or sodium and alkali viz., Sodium or potassium hydroxide, in water. This solution is mixed with a solution of sodium silicate water, or preferably with water-silicate mixture derived from at least part of an aqueous colloidal silica sol. The resulting reaction mixture is placed in a vessel made, for example, of metal or glass. The container must be closed to prevent water loss. The reaction mixture is then stirred to ensure homogeneity. The zeolite can be satisfactorily prepared at temperatures of about 90 ° C to 200 ° C, the pressure being atmospheric or at least corresponding to the vapor pressure of the water in equilibrium with the reagent mixture at the highest temperature. Any suitable heating apparatus may be used, for example, an oven, a sand bath, an oil bath or a jacketed autoclave. The heating is continued until the desired crystalline zeolite product is formed. The zeolite crystals are then filtered and washed to separate these from the reactive mother liquor. The zeolite crystals must be washed, preferably with distilled water, until the effluent washed with water, in equilibrium with the product, has a pH between approximately 9 and 12. As the zeolite crystals are washed, the interchangeable cation of the zeolite it can be partially removed and it is believed that it is replaced by hydrogen cations. If the wash is discontinued when the pH of the effluent washed with water is between about 10 and 11, the molar ratio (K20 + Na20)) / Al203 of the crystalline product will be about 1.0. Then, the zeolite crystals can be dried, conveniently in a vented oven. Zeolite L has been characterized in "Zeolite Molecular Sieves" by Donald. Breck, John Wiley & Sons, 1974, as having a structure that includes cells of the type of cancrinite of 18 units of tetrahedra joined by 6 double rings in columns and crosslinked by oxygen bridges ......or*. simple to form rings of 12 flat members. These 12-member rings produce wide channels parallel to the C axis with non-stacked imperfections. Unlike the eroinite and cancrinite, the cancrinite cells are symmetrically placed transverse to the units of 6 double rings. There are four types of cation locations: A in the 6 double rings, B in the cells of the cancrinite type, C between the cells of the cancrinite type, and D in the channel wall. The cations in place D appear to be the only exchangeable cations at room temperature. During dehydration the cations at site D are probably removed from the walls of the channel to a fifth site, the site or site E, which is located between sites A. The hydrocarbon absorption pores are approximately 7 to 8 Angstroms in diameter. A more complete description of these zeolites is given, for example, in Pat. No. 3,216,789 which, more particularly, provides a conventional description of these zeolites. U.S. Patent No. 3,216,789 is therefore incorporated herein by reference to show a type L zeolite useful in the present invention. Zeolite L differs from other large pore zeolites in a variety of ways, in addition to the X-ray diffraction pattern. One of the most pronounced differences is in the zeolite L channel system. Zeolite L has a system of one-dimensional channel parallel to the C axis, while most other zeolites have either three-dimensional or two-dimensional channel systems. All zeolites A, X and Y have three-dimensional channel systems. The mordenite (Large pore) has a system of main channels parallel to the C axis, and another very restricted channel system parallel to the axis B. The Omega zeolite (Mazzite) has a system of one-dimensional channels. Another pronounced difference is in the structure of the various zeolites. Zeolite L has cells of the cancrinite type joined by six double rings in columns and crosslinked by the oxygen bridges to form 12 flat rings. Zeolite A has a cubic arrangement of beta-octahedral cells, -M > * ^ * truncated joined by units of four double rings. The X and Y zeolites both have truncated octahedral beta cells bound tetrahedrically through six double rings in an arrangement similar to the carbon atoms in a diamond. The modernite has complex chains of five rings degraded by chains of four rings. The omega has a fourteenhedro of the gmelinite type joined by oxygen bridges in columns parallel to the axis C. The ZSM-10 is built of alternating columns of cells of the type of cancrinite and 6 double rings. In the ZSM-10, there are two systems of pores of 12 one-dimensional rings which run parallel to the axis C. A 12-ring channel is apparently identical to a wave channel in the structure of the zeolite L, the other is apparently identical to the channel of 12 rings in the structure of offretita. In this way, it is believed that the ZSM-10 has two systems of 12-ring channels, parallel, different. ZSM-10 is described in "ZSM-10; Synthesis and Tetrahedral Framework Structure", Zeolites, 16, 4, 236-244 (April 1996). The synthesis of ZSM-10 is also described in U.S. Patent No. 3,692,470, which is incorporated herein by reference. Several factors have an effect on the diffraction pattern of the X-rays of a zeolite. Such factors include temperature, pressure, crystal size, impurities, and the type of cations present. For example, as the crystal size of the zeolite type L becomes smaller, the X-ray diffraction pattern becomes wider and less precise. In this manner, the term "zeolite L" includes any of the zeolites made from cancrinite cells having an X-ray diffraction pattern substantially similar to the X-ray diffraction pattern shown in US Patent No. 3., 216,789. L-type zeolites are conventionally more broadly synthesized in the potassium form, ie, in the theoretical formula provided previously, most of the M cations are potassium. The M cations are interchangeable, so as to provide the type of zeolite L, for example, a type of zeolite L in a potassium form, can be used to obtain the type of zeolites L containing other cations, by subjecting the type of zeolite L for an ion exchange treatment in an aqueous solution of appropriate salts. However, it is difficult to exchange all the original cations, for example, potassium, since some of the interchangeable cations in the zeolite are in places which are difficult to reach for the reactants. As mentioned previously, preferably the catalyst used in the reaction zone 3 is non-acidic. The non-acidity can be achieved by the use of alkali metals or alkaline earth metals in the zeolite component, for example, as described in US Pat. Nos. 4, 104, 320 and 4, 435, 311. The term " "non-acid" is used in contrast to double acid-function catalysts such as palladium in alumina haloidea, or rhenium platinum in alumina haloidea, or platinum in an ammonium exchange zeolite (and then calcined). For examples of these dual function acid reforming / hydrocyclization catalysts, see North American Patent No. 3,006,841: US Patent No. 3,415,737; and U.S. Patent No. 3,783,123 (such as example 16, which belong to an exchange zeolite support). Although halides, particularly chloride, have been used in the past to achieve a measure of the acidity of the dehydrocyclization catalysts or reformation of the double function of platinum alumina, it should be noted that the halides can be involved in the preparation of the catalysts. of highly selective non-acid dehydrocyclicization, particularly when the support is an aluminosilicate such as zeolite L. Recent catalysts which involve the use of halides in the preparation of zeolite L catalysts of non-acidic Group VIII metals are exemplified by U.S. Patent No. 4,661,865; U.S. Patent No. 5,073,652; U.S. Patent No. 5,196,631; and U.S. Patent No. 5,294,579. All of these recently cited patents exemplify the monofunctional, non-acidic dehydrocyclicization catalysts which have the selectivity to convert paraffins to aromatics. Referring again to the drawing, in the reaction zone 3, the feed is contacted with a highly selective dehydrocyclization catalyst under the reaction conditions of dehydrocyclization to convert the paraffins to aromatics. The term "highly selective dehydrocyclization catalyst" is used here 10 to refer to the catalysts which, in the reformation of n-hexane to aromatics under the conditions of the dehydrocyclization reaction as described above results in a selectivity for aromatics of at least 40% by weight, Preferably of at least 50%, more preferably at least 70%, and more preferably at least 80% selectivity for the reformation of n-hexane to aromatics, in bases stepwise (excluding the recirculated reformation). Preferably the highly selective dehydrocyclization catalyst is used in the present invention under reaction conditions, such as those described above, -ü ^ aMa-éß-i-K-w -? - d-É-h. effective to perform stepwise reformation of paraffins to aromatics and other hydrocarbons of at least 50% by weight, more preferably of at least 60%, and more preferably of at least 70%. The performance of the desired aromatics products, 0 a step-by-step basis, is the selectivity of the times of reform by steps. Suitable reaction conditions include a temperature between 400 ° C and 600 ° C so that the highly selective dehydrocyclization reaction occurs with acceptable speed and selectivity. Preferably, the dehydrocyclization is carried out in the presence of a hydrogen at a pressure adjusted to favor the reaction thermodynamically and limits undesirable hydrodesmtegration reactions by kinetic means. The pressure used preferably varies from 1 atmosphere at 5.245 x 10"3 kg / cm2, more preferably 50 atmospheres at 2.185 x 10" 3 kg / cm2. The molar ratio of hydrogen to the hydrocarbons is preferably from 1: 1 to 10: 1, more preferably from 2: 1 to 6: 1.

The hourly space velocity of the hydrocarbon liquid through the catalyst bed in the reaction zone 3 is preferably between 0.3 to 5, more preferably between 0.5 and 2.0. As the dehydrocyclization reaction is endothermic, the combined fresh feed and recirculated feed, and the recirculated hydrogen, are heated in one or more furnaces before introduction to the catalytic reaction zone. The furnace is not shown as a separate aspect in the simplified block flow diagram. Also, the reaction zone shown in the simplified diagram is not divided into a series of dehydrocyclization or reformation reactor vessels, although preferably the dehydrocyclization reaction is carried out in a series of reactor vessels. In addition, the simplified diagram does not show the stages of heat exchange. The effluent from the reaction zone 3 is passed to the separation zone 5, which may comprise a series of separation steps. The hydrogen is recycled from the separation zone 5 via line 6 it returns to the reaction zone 3. The materials rich in aromatic products are removed via line 7 of the separation zone 5 and passed to the separation zone 8 In the separation zone 8, the aromatics are separated and removed as a product via line 10. The non-aromatics are removed via line 9. The separation of the aromatics from the non-aromatics in zone 8 can be done by extraction using a solvent, distillation or by the use of molecular sieves. In any of these separation means, for convenience we will use the term "refined" to refer to the paraffin-rich stream separated from the aromatic product. The term "paraffin-rich" is used for any more than 50% by weight of paraffins. Preferably, the raffinate contains more than 80% by weight of paraffins. The use of molecular sieves for the separation of aromatics product of the refined paraffin-rich can be done by passing the aromatics and paraffins through a bed of molecular sieves. The molecular sieves absorb the normal paraffins and some of the isoparaffins present, but not the aromatics. To cause such separation, the molecular sieve must have an effective pore diameter of 4.5 to 5.5 Angstroms. Examples of such molecular sieves are silicalite, zeolite L, zeolite A, zeolite X, zeolite Y, offertite and zeolite ZSM-5, with cations properly used to adjust the size of the orifices of the zeolite to accommodate the desired separation. Alternatively, the separation zone 8 can be carried out by distillation to separate a "refined" stream which is rich in paraffins from the aromatics product. More preferably, the paraffins are separated from the aromatics in zone 8 by the extraction of the solvent, that is, the aromatics are absorbed into a solvent and thus are extracted from the paraffin-aromatic stream effluent from the zone of reaction. The extracted aromatics can be separated from the solvent by distillation. Solvents that can be used in the solvent extraction method include phenol, sulfolane and N-formyl morpholine. The solvent extraction means preferred for the present process, particularly including extractive distillation, as further described in US Patent No. 5,401,365, the disclosure of which is incorporated herein by reference. After the step of removing the aromatics, the dimethylbutanes remaining in the paraffin-rich stream may be in the range of 5 to 50% by weight, preferably 10 to 40% by weight, and more preferably 15 to 35% by weight. weight, and more preferably 20 to 30% by weight, based on the total weight of the refining stream (paraffin-rich stream). According to this preferred embodiment of the present invention, prior to the recirculation of the paraffinic refiners to the reaction zone 3, the dimethylbutanes are separated from the raffinate. Preferably, the dimethylbutanes are removed by distillation. We find that the total dehydrocyclization process is unexpectedly efficient when the dimethylbutanes are removed mainly downstream against the elimination - ~ * ~ - "" of most dimethylbutanes upstream of the dehydrocyclization reaction zone. The dimethylbutanes are removed from the separation zone 11 via line 12. After the removal of the dimethylbutanes from the paraffin-rich stream described above, the remaining dimethylbutanes are preferably from about 0.01 to 15% by weight, more preferably from 0.1 to 10%, and more 10 preferably from 1 to 10% by weight of the paraffin-rich stream. Preferably, the removal of the dimethylbutanes downstream of this embodiment is carried out to eliminate 70 to 99.8% of the 15 dimethylbutanes of the paraffin-rich stream, more preferably from 75 to 99%, and more preferably from 80 to 95% of the dimethylbutanes, based on the total weight of the paraffin-rich stream. The drawing shows the elimination of 20 downstream dimethylbutanes after the aromatics are separated from the recirculated paraffins. According to another preferred embodiment of the present invention, dimethylbutanes are removed The downstream of the dehydrocyclization reaction zone but before for the treatment of the effluent from the reaction zone to separate the aromatics from the paraffins, in this way, according to this modality, a process is provided. for the dehydrocyclisation of hydrocarbons which comprises: (a) contacting feed hydrocarbons rich in fresh paraffins containing from 0.1 to 20% by weight of dimethylbutanes with a highly selective dehydrocyclization catalyst, in a reaction zone under the conditions of dehydrocyclization reaction to convert the paraffins to aromatics and obtain an aromatic-rich effluent, (b) remove the dimethylbutanes from the aromatic-rich effluent to obtain a mixture of aromatic paraffins with reduced dimethobutane content, (c) separate the aromatics from the mixture of paraffin-aromatics to obtain a refined scarce aromatics, and (d) recirculate the refined to the area of reaction.

In this preferred embodiment, the dimethylbutanes in the aromatic-paraffin mixture prior to removal of the dimethylbutanes downstream may be in the range of 0.1% to 20% by weight, preferably 0.5% to 15%, more preferably 1 % to 12%, and more preferably 2% to 10%, based on the weight of the mixture. The dimethylbutanes remaining in the aromatic-paraffin mixture after the removal of the dimethobutane may be in the range of 0.01% to 8%, preferably 0.05% to 6%, more preferably 0.1% to 4%, and more preferably 0.2% to 1%, based on the weight of the mixture. Preferably, the removal of the downstream dimbutane is carried out by distillation. The distillation step can be carried out in one or more distillation columns, using conventional distillation techniques conducted in accordance with the removal requirements of the dimethylbutanes of the present invention. In the embodiment wherein the elimination of the downstream ilbutane dimet is carried out prior to the separation of the aromatics, the preferred distillation separation step removes 60-99.5% of the dimethylbutanes from the aromatic-rich effluent, more preferably from the 70 -99%, more preferably 80-90% of the dimethylbutanes in the aromatic-rich effluent. It can be noted that the elimination referred to in this paragraph is based on the feed to the elimination stage downstream of the dimethylbutanes, not in the fresh feed. The percentage of elimination based on fresh food is the basis of the next two paragraphs. In the present invention, for any of the embodiments, the dimethylbutanes are removed mainly downstream of the highly selective dehydrocyclization step. The term "mainly" is used for any less than 50% based on total dimethylbutanes in the fresh feed, exclusive that the portion of the dimethylbutanes that are effectively eliminated from the system due to the degeneration of other dimethyl butyral reforming reactions in the dihydrocyclization reaction stage. The usual amounts of the elimination of dimethylbutanes by reformation in the reaction zone of dehydrocyclization is 10% to 50%, more typically from 15% to 45% based on the content of dimethylbutanes in the fresh feed. Preferably, at least 50% of the dimethylbutanes that are removed by distillation or other physical separation (as opposed to removal by the reactive conversion of other materials in the reaction zone) are removed by the downstream removal, more preferably by less 60% of the dimethylbutanes are removed from the feed to the dehydrocyclization step by the downstream removal. This relatively high percentage of elimination is in contrast so that the portion of dimethylbutanes which can be removed upstream, for example, from the fresh feed when the fresh feed, alone or with recirculated paraffins, is pre-fractioned to remove C5- and other hydrocarbons . The downstream removal refers to downstream of the reaction zone. The percent removal of dimethylbutanes, unless otherwise indicated, is based on dimethylbutanes in the fresh feed. In order to carry out the elimination of dimethylbutanes downstream in a relatively high percentage of the reaction zone, as required by the present invention, the elimination percent of the dimethylbutanes from the feed to the dimbutanol downstream removal process It can be high. Thus, as previously indicated, preferably the percent elimination of dimethylbutanes based on the paraffins in the feed to the downstream dimethylbutene elimination step is at least 70%, and more preferably at least 80%. Referring again to the drawing, prior to the dehydrocyclization step, the fresh feed, or the combined fresh feed and the recirculated paraffins, such as the raffinate shown in line 13, can be distilled to remove the C5_ hydrocarbons. The C5_ are not suitable for the dehydrification reaction stage. The predivision or pre-drive stage, which is the preferred stage for eliminating the C5-, is not shown in the drawing. The predivision of the fresh feed or the combined fresh feed and the recirculated raffinate stream can remove the 5 dimethylbutanes together with the removal of C5- and other hydrocarbons. However, according to the present invention, as indicated previously, the amount of dimethylbutanes eliminated by such upstream predivision or distillation is lower 10 that 50% of the total dimethylbutanes, based on the fresh feed, preferably less than 40% of the dimethylbutanes removed by distillation or by other physical separation. EXAMPLES 15 The first configuration of this example is one where the refining of an aromatics extraction unit followed by a highly selective dehydrocyclization reforming reaction step is combined with the fresh feed, and 20 then the distillate to remove the C5_ and a smaller portion of the dimethylbutanes. After the fresh distilled feed and the refined ^^^^ feed the dehydrocyclization stage. This is referred to as the base case. In a second case, according to the invention, the refining of the extraction unit of the aromatics is distilled to remove a greater portion of the dimethylbutanes, and then the distillate refining, in conjunction with the fresh feed, are combined and they go to the dehydrocyclization stage. 10 In both cases, the fresh feed is composed of the following components: C5_, dimethylbutanes (DMBs), 2-met ilpent anos, 3-metilpentanos, met ilciclopent ano, n-hexane, cyclohexane. In this example, the dimethylbutane portion of the feed contains 18% of 2,2-dimethylbutane and 82% of 2,3-dimethylbutyrate. The relative feed compositions for the two cases as shown below, or a percent basis of the volume of the liquid. (Percent by weight of this 20 type of feeds should be close to the percent in volume of the liquid). ^^^^^ '- ^ m. ^^.

Component of Base case Invention food c5- 0.1 0.1 Dimethylbutanes 21.5 7.3 2-methylpentane 76.4 67.8 3-methylpentane 61.0 62.0 Methylcyclopentane 40.2 43.7 n-hexane 100.0 100.0 Cyclohexane 9.8 10.3 Step reforming in the highly selective dehydrocyclization step in this example is shown below, as one percent by weight of each feed component. The catalyst used in the non-acidic PtL zeolite. Dimethylbutanes 29.8 2-methylpentane 80.8 3-methylpentane 84.9 Methylcyclopentane 90.5 n-hexane 95.0 Cyclohexane 98.6 The yield of benzene for the two processes are shown below in pounds of benzene per pound of feed to the reaction zone: Base case Invention 0.65 0.68 The production of benzene in the case of the invention is 3% higher in the feed base compared to the base case. This is a substantial economic advantage for the preferred case, because the cost of the feedstock is a greater portion of the total cost of the production of the aromatic products. The costs of the distillation are also unexpected and advantageous for the case of the invention against the base case. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (1)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process for the dehydrocyclization of hydrocarbons which is characterized in that it comprises: (a) contacting feed hydrocarbons rich in fresh paraffins which they contain 10 from 0.1 to 20% by weight of dimethylbutanes with a highly selective dehydrocyclization catalyst, in a reaction zone under dehydrocyclization reaction conditions to convert the paraffins to aromatics and obtain an effluent rich in 15 aromatics; (b) separating the aromatics from the effluent to obtain a poor refining in aromatics; (c) removing the dimethylbutanes from the raffinate to obtain a refining of dimethylbutane content 20 reduced; and (d) recirculating the refined content of reduced dimethylbutanes to the reaction zone. _-i ---- t ----- i -----? 2. A process according to claim 1, characterized in that the highly selective dehydrocyclization catalyst is a non-acid catalyst. 3. A process according to claim 2, characterized in that the catalyst comprises a Group VIII metal in a one-dimensional crystalline aluminosilicate. 4. A process according to claim 3, characterized in that the aluminosilicate is zeolite L, omega zeolite or mordenite. 5. A process according to claim 4, characterized in that the aluminosilicate is zeolite L and the metal of Group VIII is platinum. 6. A process according to claim 1, characterized in that the aromatics are separated from the raffinate by solvent extraction, distillation, or extraction by molecular screening. 7. A process according to claim 6, characterized in that the aromatics are separated from the raffinate by the extraction of the solvent. 8. A process for the dehydrocyclization of hydrocarbons, characterized in that it comprises: (a) contacting feed hydrocarbons rich in fresh paraffins containing from 0.1 to 20% by weight of dimethylbutanes with a highly selective dehydrocyclization catalyst, in an area of reaction under conditions of dehydrocyclization reaction to convert the paraffins to aromatics and obtain an effluent rich in aromatics; (b) removing the dimethylbutanes from the aromatic-rich effluent to obtain a mixture of aromatic paraffins with reduced dimethylbutane content; (c) separating the aromatics from the paraffin-aromatic mixture to obtain a poor refining in aromatics; and (d) recirculating the raffinate to the reaction zone. 9. A process according to claim 8, characterized in that the catalyst ^^ M --- M - M - ^ - Aa-M highly selective dehydrocyclization is a non-acid catalyst. 10. A process according to claim 9, characterized in that the catalyst 5 comprises a Group VIII metal in zeolite L. 10 fifteen twenty --Á-Mib-fai DIMETHYLBUTANE CURRENT DOWN. SUMMARY OF THE INVENTION In the present invention, dimethylbutanes are removed from the refined component of the feed to a dehydrocyclization process. Thus according to a preferred embodiment, a process for producing aromatics is provided by the following steps: (a) contacting feed hydrocarbons rich in fresh paraffins containing from 0.1 to 20% by weight of dimethylbutanes with a catalyst of highly selective dehydrocyclization, in a reaction zone under dehydrocyclization reaction conditions to convert the paraffins to aromatics and obtain an aromatic-rich effluent; (b) separating the aromatics from the effluent to obtain a poor refining in aromatics; (c) removing the dimethylbutanes from the raffinate to obtain a refined content of reduced dimethylbutanes; and (d) recirculating the refined content of reduced dimethylbutanes to the reaction zone. Preferably, the cyclization dehydration catalyst used is a monofunctional, non-acid catalyst. Platinum or zeolite L are highly selective dehydrocyclization catalysts particularly preferred for use in the process.