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(11) | EP 0 093 475 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Conversion of certain hydrocarbons using silicate catalyst |
| (57) By-product effluent streams from pyrolytic hydrocarbon cracking processes containing
monoolefins and diolefins, are treated to hydrogenate the olefins and to aromatize
the aliphatics, with a catalyst essentially comprising a silicalite molecular sieve. |
[0001] This application is related to two patent applications filed concurrently herewith (attorney's docket nos. Eur 1236 and Eur 1237).
' BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] This invention relates to the preparation of streams containing recoverable benzene, toluene, and xylenes ("BTX") from initial by-product effluent streams that contain other components, notably monoolefins and diolefins. In one aspect, the invention concerns the removal by conversion of these other components which ordinarily prevent recovery by distillation or solvent extraction of benzene-toluene-xylenes aromatics from the streams. In another aspect, it concerns a low severity process for treating the by-product streams with a specified catalyst, and under defined reaction conditions, both to produce benzene-toluene-xylenes from the initial stream and to reduce or eliminate those components that otherwise would interfere with the economic recovery of these aromatics from the streams.
2. Description of Prior Art
[0003] The preparation of light olefins and diolefins, mainly ethylene, propylene, and butadiene, by the thermal pyrolysis, or cracking, of petroleum fractions is well known and widely practiced. (See for example, Kirk & Othmer's "Encyclopedia of Chemical Technology", Second Edition, Vol. 8, pp. 503-514.) In these pyrolitic cracking processes, hydrocarbons ranging from ethane, through LPG (liquefied petroleum gas, chiefly propane with a few percent butanes), naphtha, heavy gas oil, to even crude petroleum oil, are subjected to high temperature conditions, at low pressure and for a short time, to produce a maximum of the desired product. These thermal processes vary widely, and the yields from any one process depend not only on process equipment and conditions, but on such extraneous factors as the presence or absence of diluents and other reactants, e.g., oxygen, hydrogen, steam, etc.
[0004] Even the best of the pyrolitic processes is less than ideally selective. As a consequence, the total reactor effluent will contain not only the desired olefin or diolefin, but a variety of other components, ranging from methane gas to high boiling polycyclic hydrocarbons. These by-products are conventionally separated, usually by distillation and/or absorption, so as to concentrate the main desired products for ultimate recovery, and to produce one or more by-product effluent streams.
[0005] The by-product effluents contain a mixture of hydrocarbon types, including paraffins, monoolefins, diolefins,. aromatics, cyclics, and various substituted and polynuclear aromatics. Unless the by-product effluent stream or streams contains a particularly valuable or desirable component, making removal economical, the by-product effluent streams are of only limited utility. The lighter gases are useful only as . fuel, while the heavier, normally liquid, components usually termed "dripolene," if not hydrogenated and then subjected to BTX extraction, are customarily either burned locally as fuel or else hydrogenated to saturate the unstable diolefins, and then blended with other gasoline fractions as motor fuel.
[0006] It has long been recognized that some of these by-product effluent streams, particularly the dripolene frac- - tions, contain potentially valuable benzene, toluene, and xylenes (including ethylbenzene). Unfortunately, they also contain diolefins and monoolefins, which effectively interfere with most existing solvent extraction processes, such as the Udex and Sulfolane processes, for the extraction of aromatics from paraffins. Some of these olefins have boiling points similar to those of the BTX aromatics, and hence cannot be removed by fractional distillation. Selective hydrogenation to saturate the olefins and diolefins is practiced, and widely so, but the process tends to be expensive. Moreover, the diolefins in dripolene tend to be thermally unstable, forming catalyst-deactivating and exchanger-fouling carbonaceous deposits.
[0007] A variety of catalysts has been proposed for treating one or more of the by-product effluents from pyrolitic cracking processes so as to render the streams more valuable or more amenable to subsequent processing. (A tabulation of representative references identifying many of these processes, and many catalysts having conceivably useful activity for these processes, is appended.)
[0008] It is an object of the present invention to provide a process for preparing a stream from which benzene-toluene-xylenes may be recovered, by catalytically treating by-product effluent streams from pyrolytic hydrocarbon cracking processes. A further object is to provide a process for treating such by-product effluent stream in a simplified, low severity, operation so as both to produce benzene-toluene-xylenes (BTX), and, simultaneously, to decrease the content of interfering components. Still another object is to remove those monoolefins and diolefins which have heretofore interfered with the solvent extraction of BTX from dripolene and the like.
SUMMARY OF THE INVENTION
[0009] Briefly, in accordance with the invention, a stream from which benzene, toluene, and xylenes may be recovered readily is prepared by contacting a pyrolitic hydrocarbon cracking by-product effluent stream, containing substantial amounts of interfering monoolefins and diolefins, with a silicalite molecular sieve catalyst under low severity hydrocarbon processing conditions. As a result of this treatment, not only are the olefins hydrogenated to non-interfering aliphatics, but a substantial fraction of the aliphatics is dehydrocyclized to benzene-toluene-xylenes.
[0010] On of the remarkable aspects of the invention is that the same low severity conditions of temperature, pressure, and space velocity, which are suitable for hydrogenation with a silicalite catalyst, are also suitable for the dehydrogenation reaction involved in aromatization. Thus, a simple processing scheme, with only a single reactor stage, is often adequate both to reduce to a minimal content, or eliminate the olefinic constituents that would interfere with the economic recovery of aromatics, and to produce benzene-toluene-xylenes from the feed stream.
[0011] A further important advantage of the invention resides in its ability to process any of a variety of the by-product effluent streams from pyrolitic cracking processes. As set out more fully below, these by-product effluent streams customarily include a C4 fraction composed predominantly of butanes, butenes, and butadiene; a C5 fraction composed mainly of pentanes, pentenes, pentadienes and cyclic C5 compounds; a C6-C8 "dripolene" fraction containing BTX aromatics together with interfering olefins (i.e., having a similar boiling range): and a C9-plus fraction, including some BTX along with higher alkylated benzenes and polynuclear aromatics and aliphatics. Each of these streams, plus others that may be present in a particular plant may be processes according to the invention.
[0012] The silicalite catalyst of the present invention, to be identified more fully below, is described in Grose et al. U.S. 4,061,724, and its structure is described in an article by Flanigen et al., "Silicalite, a new hydrophobic crystalline silica molecular sieve," Nature 271 512 (9 February 1978). It is a crystalline silica polymorph having identifiable X-ray diffraction characteristics and other properties that have been described in the two references above. An interpretation of the descriptive data is included in an article by Olson et al., "Chemical and physical properties of the ZSM-5 substitutional series," J. Catal., 61 (1980). It has been reported that silicalite may exist in two crystallographically distinct forms, termed, "silicalite-l" and "silicalite-2," according to Bibby, "NH4-tetra-alky1 ammonium system in the synthesis of zeolites," Nature, 285, 3-31 (1 May 1980).
DESCRIPTION OF PREFERRED EMBODIMENTS
1. Pyrolitic Cracking Processes
[0014] Pyrolitic cracking processes for the preparation of light olefins and diolefins such as ethylene, propylene, and/or butadiene, have been described in the literature, and accordingly no detailed exposition is called for here.
[0015] In essence, the thermal pyrolysis, or cracking, of petroleum fractions may utilize as feed stocks hydrocarbons such as ethane, LPG (liquefiled petroleum gas, chiefly propane with a few percent butanes), naphtha, heavy gas oil, or crude petroleum oil. These are subjected to controlled high temperature, low pressure, short time, pyrolitic cracking to produce the desired product or products. Thereafter the reactor effluent is subjected to a combination of condensation, fractional distillation, absorption, and perhaps other unit operations, to segregate various effluent streams enriched in one or more desirable components. The precise arrangement of product recovery streams forms no part of the present invention, and indeed it is probable that no two pyrolitic cracking plants utilize the same recovery scheme.
[0016] For example, the reactor effluent liquid may be subjected to fractional distillation to separate one or more fractions rich in benzene (B.P. 80.103°C.), toluene (B.P. 110.623°.), and/or the xylenes, namely ethylbenzene (B.P. l36.l87°C), p-xylene (B.P. 138.348°C.), m-xylene (B.P. 139. 102°C.), and o-xylene (B.P. 144,414°C.). This fraction, or fractions is desirably solvent extracted, as for example by the Udex or Sulfolane process, to recover the BTX aromatic/ aromatics.
[0017] In the absence of prior treatment, such as by the process of the present invention, solvent extraction is ineffective to extract the aromatics from the remaining aliphatics, inasmuch as solvents selective for aromatics will also extract many olefins and diolefins. However, the diolefins and the aromatics cannot be separated by fractional distillation; for example, benzene, with a boiling point of 80.103°C., is not easily distilled from the 2,4 hexadienes, which boil at about 80.0°C. Similarly, the various dimethylpentenes boil within a range of 72.2°C. to 85.0°C.
[0018] Be that as it may, and howsoever produced or constituted, there inevitably will be one or more by-product effluent streams which contain diverse mixtures of hydrocarbon (and perhaps non-hydrocarbon) components, varying both with respect to boiling point and chemical classification. It is this diversity that either complicates or prevents the recovery of useable components.
[0019] By way of example, in an illustrative pyrolitic cracking plant, the total reactor effluent may be segregated into a predominantly gaseous fraction including recoverable ethylene and propylene; a crude C4 fraction, a distillation cut which includes hydrocarbons with primarily four carbon atoms each; a crude C5 fraction, another distillation cut which primarily contains hydrocarbon molecules with five carbon atoms each, and generally containing a large quantity of unsaturated and cyclic compounds, including olefins and lesser amounts of C4's and ligher, C6's and heavier; a C6-C8 fraction, sometimes referred to as pyrolysis gasoline or dripolene; and a Cg plus fraction, a heavier distillation cut which primarily includes hydrocarbons with at least nine carbon atoms, along with lesser amounts of C5-C8 hydrocarbons. The C9 fraction generally is produced as the distillation bottoms from the processing of dripolene to remove pyrolysis gasoline, and contains components as widely varying as styrene, ethyl- toluenes, and trimethylbenzenesto heavier compounds including ethylnaphthalene, diphenyl, and dimethylnaphthalene.
[0020] An illustrative C4 fraction, giving both the range and a typical composition, is set out in Table I below:
[0021] It will be appreciated, as noted earlier, that these compositions may vary quite widely, depending upon the initial feed to the pyrolitic cracking unit, the type of pyrolitic cracking unit, conditions in the pyrolitic unit, and the type and conditions of the product recovery section. The by-product effluent streams may likewise be blended with each other where this is desired, or may include recycle components from elsewhere in the product recovery section.
2. Catalyst
[0022] The catalyst used in the present process is commonly termed silicalite, a newly discovered crystalline silica polymorph described in Grose et al. U.S. 4,061,724 and in the other references cited previously. It consists of silica, but unlike many of the more common forms of silica, has an open porous structure, with a pore diameter of about 6 Angstrom units, and a pore volume of about 0.18 cc/gram as determined by adsorption. It has a density (as-synthesized) of 1.99 + 0.05 g/cc. Its refractive index is 1.48 + 0.01 as synthesized, or 1.39 + 0.01 after calcining at 600°C. for one hour.
[0023] The X-ray powder diffraction pattern of silicalite (600°C. calcination in air for one hour) has as its six strongest lines (i.e., interplanar spacings) those set forth in the table below, where "S" is strong, and "VS" is very strong:
[0024] The chemical analysis of silicalite (after 600°C. calcination and washing with 1 N HC1) shows that it consists of Si02, with only insignificant impurities. Alumina, a common impurity, is present to the extent of less than 1 aluminum atom per unit cell of 92 Si atoms. Silicalite exhibits essentially no detectible ion exchange properties, in contrast to the conventional zeolitic molecular sieves.
[0025] Silicalite is readily prepared by the procedure of the Grose et al. patent. This involves the hydrothermal crystallization of.a reaction mixture comprising water, a source of silica, and an alkylonium compound at a pH of 10 to 14 to form a hydrous crystalline precursor, and subsequently calcining that precursor to decompose aklylonium moieties. Thereafter, the calcined precursor is desirably washed to remove any metals or trace contaminants.
[0026] When used in the present process, silicalits may be employed either alone or in intimate admixture with independently active catalytic components, as for example the noble metals such as platinum, or other catalytically active metals such as molybdemun, vanadium, zinc, etc. The techniques of introducing catalytically active metals to a molecular sieve zeolite are disclosed in the literature, and with the exception of cation exchange, preexisting metal incorporation techniques are suitable. See, for example, Rabo et al. U.S. 3,236,761 and U.S. 3,236,762.
[0027] The physical form of the silicalite catalyst depends on the type of catalytic reactor being employed. Silicalite by itself is a fine-grain granule or powder, and is desirably compacted into a more readily usable form (e.g., larger agglomerates), usually with a silica or alumina binder for fluidized bed reaction, or pills, prills, spheres, extrudates, or other shapes of controlled size to accord adequate catalyst-reactant contact. As indicated, the catalyst may be employed either as a fluidized catalyst, or in a fixed or moving bed, and in one or more reaction stages.
3. Conversion Parameters
[0028] An unusual, if not unique, feature of the present invention is that the reaction conditions are low severity as compared with many preexisting processes. Indeed, the conversion parameters, while broad, may be selected to provide a high degree of versatility, depending upon the feed composition and on the desired product quality.
[0029] With respect to temperature, a temperature within the range of about 300-700°C., more preferably within the range of about 350-600°C., is adequate for many, if not all, conversions. Higher temperatures give more rapid and more complete reaction, but tend to produce undesirable by-products, chiefly coke, and may otherwise disturb the optimum balance of product composition with on-stream ease of operation.
[0030] The pressure, almost uniquely, is desirably quite low. Atmospheric pressure operation has been used successfully in the laboratory, but under specific conditions may be as high as 100 atmospheres or more. A desirable range is from atmospheric pressure to about 7 atmospheres. High pressures facilitate hydrogenation; lower pressures facilitate dehydrocyclization. The optimum pressure will therefore depend on process economics, considering whether it is more desirable to hydrogenate olefins than to produce a high yield of BTX aromatics.
[0031] Process stream flow rate, as expressed in units of weight hourly space velocity (WHSV), or weight of hydrocarbon feed per unit weight of catalyst, is suitably within the range of about 0.1 to about 20, more desirably about 0.5-5.0. High WHSV's permit more economic plant construction, while lower WHSV's permit more complete reaction at given temperature- pressure conditions.
[0032] If desired, a gaseous or gasifiable diluent may be introduced along with the hydrocarbon feed to the silicalite catalyst. This diluent may be inert, typified by steam, nitrogen or a low boiling paraffin, or may be reactive with the feed under catalysis conditions (e.g., hydrogen). Hydrogen is particularly desirable as it minimizes coke formation and deposition on the catalyst, with resulting premature deactivation, and also facilitates hydrogenation. As demonstrated below, however, the technique of the present invention need not utilize hydrogen.
[0033] If either an inert or a reactive gas is employed, diluent/hydrocarbon molar (gas volume) ratios, optimally, of from 0.1 to about 10 may be employed.
[0034] It is usually necessary that the catalyst be regenerated, either periodically or continuously, to remove the carbonaceous coke-like deposits from the catalyst. In a fluidized bed operation, a portion of the catalyst is continuously withdrawn from the reactor and then subjected to regeneration by combustion with air or other oxygen containing gas, after which it is continuously recycled to the reactor. In a moving bed operation, the removal of catalyst followed by regeneration may be effected either continuously or periodically. In a fixed bed operation, it is generally desirable that two or more reactors be used in parallel, so that when one is processing the hydrocarbon feed, the other is out of service and being regenerated. Regeneration conditions of approximately 450-650°C., preferably 500-600°C. may be employed.
4. Example
[0035] A specific example of the present invention is presented below. From the data, it is apparent that olefins and diolefins are converted by hydrogenation; that acyclic and cyclic nonaromatic components are dehydrocyclized to aromatic compounds; that Cg plus alkybenzenes, indan, indene, and methylstyrenes are converted in substantial part to the more desirable C6-Cg aromatics; and that, apparently, higher alkylnaphthalenes are converted by hydrogenolysis to recoverable naphthalene and methyl naphthalenes. Additionally, it is likely that the organic sulfur and nitrogen contents are lowered.
[0036] In the example herein, a C9 plus by-product hydrocarbon effluent from the thermal pyrolysis unit was reacted over 37g of 1/16 inch extrudates of silicalite with a 20% alumina binder. The reaction vessel was a 3/4 inch OD stainless steel tubular reactor.
[0037] The reaction pressure was ambient; the reaction temperature was approximately 503°C.; and the space velocity of the feed varied from 0.57-0.69 g/feed/g catalyst/hr.
[0038] Samples of the liquid products, trapped in an ice water condenser, were taken after one hour and after six hours on stream. Five gas product samples were taken periodically.
[0040] The gas samples were analyzed on a Hewlett Packard 5830A gas chromatograph equipped with a thermal conductivity detector. A forty foot stainless steel column with an OD of 1/8 inch packed with 20% tributylphosphate on 35/80 mesh Chromasorb P (acid washed), was used. The thermal conductivity detector temperature was set at 250°C., and the column temperature was ambient (approximately 20-22°C.). Gas samples were injected into the column off-line, through an eight port gas switching valve, via a gas syringe. The sample gas volume was approximately 0.3cc; the carrier gas rate was 30cc/min of helium.
[0041] Liquid samples for both product and feed were analyzed on a Hewett Packard 5730A gas chromatograph, using a 5705A thermal conductivity detector. A ten foot stainless steel column with an OD of 1/8 inch, packed with 15% Carbowax 20M on 40/60 mesh Chromasorb P acid wash, was used. The detector temperature was set at 250°C. The column was maintained in an oven, with a temperature programmed form 55°C. to 190°C. at 4°C./min; the injector temperature was 250°C.' A sample size of approximately 2 ml. was used, and the helium carrier gas rate was 30 cc/min.
1. A low severity process for the preparation of a benzene-toluene-xylenes containing
stream having a minimal - monoolefin and diolefin content, from a feed stream comprising
a by-product effluent of a process for the pyrolitic cracking of hydrocarbons to produce
light olefins or diolefins, said by-product effluent stream containing olefins and
diolefins, said process comprising contacting said by-product effluent stream, under
low severity conditions including a temperature within the range of about 300-700°C.,
a pressure within the range of about 0 to 100 atmospheres, and a weight hourly space
velocity within the range of about 0.1 to about 20, with a catalyst essentially comprising
silicalite, said silicalite being a crystalline silica polymorph having as the six
strongest d-values of its X-ray powder diffraction pattern those set forth in Table
IV.
2. Process of claim 1 wherein said conditions include a temperature within the range
of about 350-600°C., a pressure within the range of about 1-7 atmospheres, and a weight
hourly space velocity within the range of about 0.5-5.0.
3. Process of claim 1 wherein said by-product effluent comprises a C4 stream.
4. Process of claim 1 wherein said by-product effluent comprises a C5 stream.
5. Process of claim 1 wherein said by-product effluent comprises a whole or fractionated
dripolene stream.
6. Process of claim 1 wherein said feed stream is admixed with a diluent.
7. Process of claim 6 wherein said diluent is steam.
8. Process of claim 6 wherein said diluent'is hydrogen.
9. Process of claim 6 wherein said diluent is a low boiling paraffin.
10. Process of claim 6 wherein said diluent is a mixture of a low boiling paraffin,
hydrogen and/or steam.