Process for the Preparation of Ethyl Benzene

1. Field of the Invention

The present invention relates to a process for the alkylation of organic aromatic compounds. More particularly the invention relates to a process for the concurrent alkylation and distillation of reaction components (reactants and products) in a catalyst bed wherein the catalyst also serves as the distillation structure.

2. Related Art

Recently a new method of carrying out catalytic reactions has been developed, wherein the components of the reaction system are concurrently separable by distillation, using the catalyst structures as the distillation structures. Such systems are described variously in U.S. Pat. Nos. 4,215,011; 4,232,177; 4,242,530; 4,250,052; 4,302,356; and 4,307,254 commonly assigned herewith.

Briefly, a structure described there is a cloth belt with a plurality of pockets spaced along the belt, which is then wound in a helix about a spacing material such as stainless steel knitted mesh. These units are then disposed in the distillation column reactor.

In addition, commonly assigned U.S. Pat. No. 4,443,559 discloses a variety of catalyst structures for this use and is incorporated herein.

Ethyl benzene and cumene are currently produced by the reaction of benzene and the respective olefin, i.e., ethylene and propylene by acid catalysis. In some known processes the catalyst is highly corrosive and has a relatively short life, e.g., AlCl.sub.3, H.sub.3 PO.sub.4 on clay, BF.sub.3 on alumina, and others require periodic regeneration, e.g., molecular sieves. The exothermicity of the reaction and the tendency to produce polysubstituted benzene require low benzene conversions per pass with large volume recycle in conventional processes. Advantages of the present invention are that the catalyst are not highly corrosive and do not require periodic regeneration, the heat of reaction is used efficiently, only low volume of recycle is required and the feed ratios can approach unity.

Briefly, the present invention is a process for the preparation of ethyl benzene by contacting the benzene with ethylene in a distillation column reactor containing a fixed bed acidic catalytic distillation structure comprising molecular sieve in a distillation reaction zone thereby catalytically reacting said benzene and said ethylene to produce an alkylated benzene product and concurrently in said fixed bed fractionating the resultant alkylated benzene product from the unreacted materials and separating ethyl benzene from said alkylated benzene product (preferably by fractional distillation). The principal alkylated benzene product is ethyl benzene. In addition there are other alkylated products including di and tri isopropyl benzene, n-propyl benzene, ethyl benzene, toluene, di- and triethyl benzene and di-n-propyl benzene, which are believed to be disproportion and isomerization products of ethyl benzene. In a preferred process the residual alkylated products remaining after ethyl benzene separation are passed to a transalkylation reactor operated under conditions to transalkylate polyalkylated benzene, e.g., diethyl benzene and triethyl to ethyl benzene, which is separated from the other materials in transalkylation product stream and may be combined with the ethyl benzene from first separation. Alternatively the two ethyl benzene containing streams may be combined and the ethyl benzene separated therefrom with the residual stream or a portion thereof being the feed to the transalkylation reactor. The principal residual alkylated product is diethyl benzene.

The transalkylation reactor may be operated as a straight pass reactor. The transalkylation is preferably carried out in liquid phase using a mole sieve catalyst. The transalkylated product contains a substantial concentration of ethyl benzene which may be recovered by fractional distillation, for example and combined with the ethyl benzene recovered from the alkylation product or the two streams may be combined for recovery of ethyl benzene. The catalytic distillation structure provides both the catalytic sites and the distillation sites. The alkylated benzene product is withdrawn from the distillation column reactor at a point below the fixed bed and unreacted organic aromatic compound may be taken off as an overhead. Preferred acidic catalysts are molecular sieves (mole sieves) .

More specifically the mole sieve catalyst packing is of such a nature as to allow vapor flow through the bed, yet provide a sufficient surface area for catalytic contact as described in the previously noted U.S. Pat. No. 4,443,559, and U.S. Pat. Nos. 4,215,011 and 4,302,356 which are incorporated herein in their entirety. The catalyst packing is preferably arranged in the upper portion of the distillation column reactor, more preferably occupying about one-third to one half of the column and extending substantially to the upper end thereof.

The ethylene feed to the reaction is preferably made below the catalyst bed thereby allowing mixing of the reactants before contact with the catalyst bed. In another embodiment the olefin feed to the reaction is preferably made into the catalyst bed, such as between the bottom of the fixed bed, and the upper one-fourth section thereof preferably in the middle one-half of the bed. For example, in the case of alkylation of benzene (B.P. 80.degree. C.) with ethylene, the olefin feed may be located below the bed.

The benzene feed may be added at any point in the reactor, however, preferably it is added to the fixed bed or to the reflux as makeup. In the operation of the transalkylator a sufficient quantity of benzene is required to allow the reaction to proceed. The benzene may be added to the feed to the transalkylator along with the residual from the ethyl benzene separation.

Also, in order to achieve high selectivity toward monosubstitution (which is a preferred aspect of the present invention), there is a large excess of the organic aromatic compound to the olefin in the reactor in the range of 2 to 100 moles of benzene per mole of olefin, that is, the net molar feed ratio of aromatic organic compound: olefin may be close to 1:1, although the system is operated so as to maintain a substantial molar excess of organic aromatic compound to olefin in the reaction zone. In the event the make up benzene for the transalkylation is to be derived from the overhead/reflux stream of the alkylation reactor the feed molar ratio of the benzene: ethylene may exceed 1:1, e.g. 1.1-2:1.

The alkylated product is the highest boiling material and is separated in the lower portion of the column usually as bottoms. The benzene compound is the second highest boiling component (excluding inerts) as noted above, however, by operating with a large excess of benzene and a sufficient height of catalyst packing in the reactor, the major portion of the olefin is reacted, thereby reducing the separation and recovery problems. The success of catalytic distillation lies in an understanding of the principles associated with distillation. First, because the reaction is occurring concurrently with distillation, the initial reaction product is removed from the reaction zone as quickly as it is formed. The removal the alkylation product minimizes polysubstitution and decomposition of the alkylation product. Second, because the organic aromatic compound is boiling, the temperature of the reaction is controlled by the boiling point of that component at the system pressure. The heat of the reaction simply creates more boil up, but no increase in temperature. Third, the reaction has an increased driving force because the reaction products have been removed and cannot contribute to a reverse reaction (Le Chatelier's Principle).

As a result, a great deal of control over the rate of reaction and distribution of products can be achieved by regulating the system pressure. Also, adjusting the through-put (residence time=liquid hourly space velocity.sup.-1) gives further control of product distribution and degree of olefin conversion.

The temperature in the reactor is determined by the boiling point of the liquid mixture present at any given pressure. The temperature in the lower portions of the column will reflect the constitution of the material in that part of the column, which will be higher than the overhead; that is, at constant pressure a change in the temperature of the system indicates a change in the composition in the column. To change the temperature the pressure is changed. Temperature control in the reaction zone is thus controlled by the pressure; by increasing the pressure, the temperature in the system is increased, and vice versa. It can also be appreciated that in catalytic distillation as in any distillation there is both a liquid phase (internal reflux) and a vapor phase. Thus, the reactants are partially in liquid phase which allows for a more dense concentration of molecules for reaction, whereas, the concurrent fractionation separates product and unreacted materials, providing the benefits of a liquid phase system (and a vapor phase system) while avoiding the detriment of having all of the components of the reaction system continually in contact with the catalyst which would limit the conversion to the equilibrium of the reaction system components.

FIG. 1 is a schematic representation of a preferred embodiment of one species of the present invention for producing ethyl benzene.

FIG. 2 is schematic of the same process as FIG. 1 with an alternative ethyl benzene recovery.

The ethylene feed is preferably as high a purity as possible and generally contains 5 to 99.9% ethylene with the balance being alkanes such as ethane, propane, butane and the like which are inerts in this process. The presence of other olefins in materials may be substantially detrimental to the production of high purity ethyl benzene if that is required. Preferably the ethylene feed to the alkylation reactor will contain less than 1.5% other olefinic material.

However, operating the reaction with far less than a stoichiometric amount of olefin in the reaction zone, as described, will normally keep the olefin level in the bottoms low or entirely eliminated. There may be some olefin going overhead even with the large molar excess of the organic aromatic compound present in the reaction zone. In those instances the overhead may be condensed to remove a major portion of the organic aromatic compound and the olefin and inerts removed for further separation or use.

Similarly inerts such as the alkane of the particular olefin(s) which are often found in olefin streams will be a possible contaminant.

The mole ratio of organic aromatic compound to olefin in the reactor may be in the range of 2 to 100:1, preferably 2 to 50:1 and more desirably about 2 to 10:1. The greater the excess of organic aromatic compound the more the selectivity to the monosubstituted product is improved. Alkylation is forced to completion, since the simultaneous and concurrent fractionation and removal of the alkylation product from the distillation column reactor does not allow the products to contribute to the reverse reaction (Le Chatelier's Principle). However, very large molar excesses of organic aromatic compounds require a very high reflux ratio, and a low unit productivity.

The length of the catalyst bed, particularly that portion wherein the reactants are in contact and the major portion of the reaction occurs, depends on the reactants, location of the olefin feed and the acceptable unreacted olefin in the streams leaving the tower.

The present alkylation reaction can be carried out at sub-through super atmospheric pressure, e.g., 0.20 to 50 atmospheres. The temperature will vary depending on the reactants and product. Furthermore, the temperature along the column will be as in any distillation column, the highest temperature will be in the bottom and the temperature along the column will be the boiling point of the composition at that point in the column under the particular conditions of pressure. Moreover, the exothermic heat of reaction does not change the temperature in the column, but merely causes more boil up. However, the temperatures within the column with the above considerations in mind will generally be in the range of 50.degree. C., e.g. 50.degree. C. to 300.degree. C. and more preferably in the range of about 80.degree. C. to 250.degree. C. at pressures of 0.5 to 30 atmospheres.

Molecular sieves are porous crystalline, three-dimensional alumina-silicates of the zeolite mineral group. The crystal skeleton is composed of silicon and aluminum atoms each surrounded by four oxygen atoms to form a small pyramid or tetrahedron (tetrahedral coordination). The term molecular sieve can be applied to both naturally occurring zeolites and synthetic zeolites. Naturally occurring zeolites have irregular pore size and are not generally considered as equivalent to synthetic zeolites. In the present invention, however, naturally occurring zeolites are acceptable so long as they are substantially pure. The balance of the present discussion shall be directed to the synthetic zeolites with the understanding that natural zeolites are considered equivalent thereto as indicated above, i.e., in so far as the natural zeolites are the functional equivalents to the synthetic zeolites.

Usually synthetic zeolites are prepared in the sodium form, that is, with a sodium cation in close proximity to each aluminum tetrahedron and balancing its charge. To date seven principal types of molecular sieves have been reported, A, X, Y, L, erionite, omega and mordenite. The A type have relative small pore size. By the term pore size is meant the effective pore size (diameter) rather than the free pore size (diameter). Types X and Y have larger pore size (approximately 10 A.) and differ as to the range of ratio of Al.sub.2 O.sub.3 to SiO.sub.2 as:

Type X - - - Al.sub.2 O.sub.3 /2.0-3.0 SiO.sub.2

Type Y - - - Al.sub.2 O.sub.3 /3.0-6.0 SiO.sub.2

Type L and other types listed above have still higher ratios of SiO.sub.2 to Al.sub.2 O.sub.3

The mole sieve catalysts employed in the present invention are the acid form mole sieves or exhibit acidic characteristics. The acid form of the mole sieves is commercially available, but also may be prepared by treating the mole sieves with acid to exchange Na for hydrogen. Another method to produce the acid form is to treat the mole sieve with decomposable cations (generally ammonium ions) to replace Na with the decomposable ions and thereafter to heat the mole sieve to decompose the cation, leaving the acid form. Generally the Na form mole sieve is treated with soluble ammonium salts to remove the Na and thereafter the mole sieve is heated to a temperature of about 350.degree. C. to remove of the ammonia. The removal of Na.sup.+ ions with NH.sup.+.sub.4 is more easily carried out than with multivalent ions as described below and these catalysts are generally more active, but less stable to heat than the multivalent cation exchange forms. Mole sieves, which have had their alkali metal reduced to low levels by partial treatment with NH.sup.+.sub.4 and partial multivalent metal cation exchange, possess increased activity and increased stability.

In addition to mole sieves which are acidic according to the Bronsted Theory those mole sieves which exhibit acidic characteristics under the Lewis Theory, for example, calcium exchanged mole sieves are suitable for the present reaction. By exchanging the univalent cations (e.g. Na.sup.+) with multivalent cation, strong ionic activity is imparted. The ratio of SiO.sub.2 : Al.sub.2 O.sub.3, valence and radius of the cation and the extent of exchange all affect the catalyst activity. In general activity increases with (1) increased SiO.sub.2 Al.sub.2 O.sub.3 ratio, (2) decreased cation radius and an increase in cation valence. The effect of replacing univalent ions (e.g. Na.sup.+) with bivalent (e.g. Ca.sup.++) is much greater than replacing the bivalent ions with cations of greater valence.

The various types of mole sieves having reduced alkali metal content are characterized as the acid form molecular sieve and are all contemplated as useful in the present invention.

It would appear that the pore size within the crystal lattice may affect the selectivity. According to one theory of molecular sieve catalytic activity, zeolite catalysis occurs primarily inside the uniform crystal cavities, consequently zeolitic catalyst activity depends on the number of aluminum atoms in the crystal and thus on the chemical composition of the crystal. Moreover, these catalytic sites are fixed within the rigid structure of the crystal, so that access to the site can be altered by altering the structure of the crystal.

The acid form mole sieves are generally produced and available as particles in the range of <10 micron (powders) to 0.2 inch in diameter (beads).

In this form the mole sieves form too compact a bed and will not function adequately in a distillation, since there is a very large pressure drop through the bed and the free flow of internal reflux and rising vapor is impeded. Mole sieves in the shape of conventional distillation structures, such as rings, saddles, and the like may be used in the present invention. The particulate mole sieves may be employed by enclosing them in a porous container such as cloth, screen wire or polymeric mesh. The material used to make the container must be inert to the reactants and conditions in the reaction system. The cloth may be any material which meets this requirement such as cotton, fiber glass, polyester, nylon and the like. The screen wire may be aluminum, steel, stainless steel and the like. The polymer mesh may be nylon, teflon or the like. The mesh or threads per inch of the material used to make the container is such that the catalyst is retained therein and will not pass through the openings in the material. Particles of about 0.15 mm size or powders may be used and particles up to about 1/4 inch diameter may be employed in the containers.

The container employed to hold the catalyst particles may have any configuration, such as the pockets disclosed in the commonly assigned patents above or the container may be a single cylinder, sphere, doughnut, cube, tube or the like.

Each container containing a solid catalytic material comprises a catalyst component. Each catalyst component is intimately associated with a spacing component which is comprised of at least 70 volume % open space up to about 95 volume % open space. This component may be rigid or resilient or a combination thereof. The combination of catalyst component and spacing component form the catalytic distillation structure. The total volume of open space for the catalytic distillation structure should be at least 10 volume % and preferably at least 20 volume % up to about 65 volume %. Thus desirably the spacing component or material should comprise about 30 volume % of the catalytic distillation structure, preferably about 30 volume % to 70 volume %. Resilient materials are preferred. One suitable such material is open mesh knitted stainless wire, known generally as demister wire or an expanded aluminum. Other resilient components may be similar open mesh knitted polymeric filaments of nylon, teflon and the like. Other materials such as highly open structures foamed material, e.g., reticulated polyurethane foam (rigid or resilient) may be formed in place or applied around the catalyst component.

In the case of larger catalyst components such as from about 1/4 inch to 1/2 pellets, spheres, pills and the like each such larger component may be individually intimately associated with or surrounded by the spacing component as described above.

It is not essential that the spacing component, entirely cover the catalyst component. It is only necessary that the spacing component intimately associated with the catalyst component will act to space the various catalyst components away from one another as described above. Thus, the spacing component provides in effect a matrix of substantially open space in which the catalyst components are randomly but substantially evenly distributed.

A preferred catalytic distillation structure for use herein comprises placing the mole sieve particles into a plurality of pockets in a cloth belt, which is supported in the distillation column reactor by open mesh knitted stainless steel wire by twisting the two together in a helical form. This allows the requisite flows and prevents loss of catalysts. The cloth may be any material which is inert in the reaction. Cotton or linen are useful, but fiber glass cloth or "Teflon" cloth are preferred.

In the following examples the catalyst packing consisted of bags in the form of a fiber glass cloth belt approximately six inches wide with narrow pockets approximately 3/4 inch wide sewn across the belt. The pockets are spaced about 1/4 inch apart. These pockets are filled with the catalyst particles to form approximately cylindrical containers, and the open ends are then sewn closed to confine the particles. This belt is then twisted into a helical form to fit inside the column. Twisted in with the belt is also a strip of an open mesh knitted stainless steel wire, which serves to separate the mole sieve filled cloth pockets and provide a passage for vapor flow.

The wire mesh provides the support for the catalyst (belt) and provides some degree of vapor passage through the catalyst particles, which otherwise form a very compact bed which has a high pressure drop. Thus, the down flowing liquid is in intimate contact with the rising vapors in the column.

In commercial-scale operations, it is contemplated, catalyst packing would be made up of alternating layers of mole sieve filled cloth belts similar to the ones described above, and a spacing material which could be of any convenient, suitable substance, such as a corrugated wire screen or wire cloth or a knitted wire mesh. The layers would be arranged vertically or horizontally. For simplicity of fabrication and for better distribution of vapor flow passages, a vertical orientation is preferred. The height of a section of this packing should be of any convenient dimension, from a few inches to several feet. For ease of assembly and installation, the packing would be made into sections of the desired shape and size, each section fastened together with circumferential bands of tie wires depending on its size and shape. A complete assembly in a column would consist of several sections, arranged in layers, with possibly the orientation of the catalyst-filled belts turned at right angles in successive layers to improve liquid and vapor flow distribution.

FIG. 1 illustrates one embodiment of the present invention, for the production of ethyl benzene by alkylating benzene with ethylene.

Referring to the drawing, distillation column/reactor 10 is divided into three sections. In the middle section the catalyst packing (catalytic distillation structures) 12 is positioned as described. Linde molecular sieve LZ-Y82 1/16" (Union Carbide Corp.) is deposited in the pockets of fiber glass belts and formed in to a helix with stainless steel mesh as described.

The reactor 10 is a four inch diameter pilot column 70 feet tall with 35 feet of the catalyst packing in the middle portion. The lower portion of the column is a conventional distillation column configuration (equivalent 25 trays). Benzene is conveniently added as makeup via 14 into reflux accumulator. The benzene can also be added through a separate line (36). The ethylene is fed to the column via 8 at the lower point of the catalyst packing 12. The reaction is exothermic and initiated by contacting the two reactants in the catalyst packing. Ethyl benzene and diethyl benzene are the principal reaction products. Both of these products as well as other polyalkyates are higher boiling than benzene and ethylene and are recovered via 18 as a bottoms product. The feed of ethylene may be such that there is a molar excess of benzene in the reactor, such that the overhead 20 is primarily benzene, the ethylene having been almost totally reacted. Alternatively the benzene reflux may be increased to increase the ratio of benzene to ethylene in the reactor, while the feed ratios may be about 1 to 1. In addition to benzene, some ethylene and other lights go off overhead. The overhead is passed to condenser 22 which is operated to condense substantially all of the benzene which passes via 24 to accumulator 16 and hence, by reflux via 26 to column 10. The benzene used in the reaction and lost with the lights (which exit condenser 22 via 28) may be made up by fresh benzene feed 14.

In a different embodiment line 14 is used as a draw to supply benzene from the accumulator to the transalkylator and in this embodiment the benzene enters reactor 10 via line 36. The feed of benzene is also increased to a molar ratio in excess of 1:1 to ethylene.

The bottoms in reactor 10 contain a mixture of ethyl benzene and diethyl benzene which pass via 18 to splitter 30, which is a conventional distillation column operated to fractionate ethyl benzene and diethyl benzene. The ethyl benzene is recovered as overhead 32 and the diethyl benzene and other polyalkylates recovered as a bottoms product. In this preferred embodiment the diethyl benzene is sent via 34 to the transalkylator 40 containing the same mole sieve catalyst as column 10. This is a single or multiple fixed bed 38 through which the polyalkylated benzenes and benzene 48 pass at 140.degree. to 210.degree. C. under sufficient pressure to maintain the liquid phases at LHSV of 1 to 5.

However, in this preferred embodiment it is desired to maximize ethyl benzene production. There is an equilibrium between benzene and diethyl benzene in the catalyst in the transalkylator as:

Benzene+Diethyl Benzene.fwdarw.Ethyl benzene

There is substantially no ethylene in the transalkylator and a large volume of benzene along with the polyalkylated reaction products such as diethyl benzene, hence, the reversible reaction favors the production of ethyl benzene, which is being continuously removed from the catalytic zone as the stream passes through.

In the embodiment of FIG. 1 this product stream 42 passes to splitter where the ethyl benzene is recovered overhead and heavies, including unconverted benzene polyalkylates are removed through 46.

In the embodiment of FIG. 2 this last step is different since the alkylation product is recovered via line 42 and returned as additional feed to splitter 30. The only caution in such a closed system as this is that a draw must be provided to remove built up heavies or other potentially detrimental materials as required.

Such conventional items as valves, reboilers, slip streams, etc. are not shown, but would be obvious expedients to those setting up such equipment.

The reactor was a 1 inch, six foot stainless tube, composed of 2 foot sections bolted together. The bottom and top two feet contained conventional distillation packing, the middle two feet contained mole sieve in pockets (four pockets twisted with demister wire as described above. Benzene was fed under nitrogen pressure through a rotameter to the tower about 6" above the top of the catalyst bed. The olefin, either ethylene or propylene was fed from a tank to a point below the catalyst bed using a micrometering valve. The rate of feed of liquid olefin was adjusted to maintain the tower pressure with slow constant bleed of gas overhead. The rate of olefin addition was slightly larger than the rate of reaction. The benzene feed rate and bottoms withdraw rate are related. The benzene rotameter was set at a given value and the bottom withdrawal rate was adjusted to maintain a constant bottoms level.

The catalyst was dried initially by taking off some benzene and water overhead and an occasional small amount of liquid material was taken off overhead during runs to maintain the dry catalyst and to remove any low boiling by-products. Bottoms samples were analyzed by, gas liquid phase chromatography using a 50 meter SE-30 capillary column and FID.

The conditions and results of several runs are set forth in TABLE I.

Using an apparatus substantially as described in regard to FIG. 2, the production of ethyl benzene is carried over a 4 month period of time. The catalyst in the distillation structure is Union Carbide LZY-82 mole sieve. Thirty feet of the helical catalytic structures were packed in the lower portion of a 35 ft. 4" column. The benzene feed is 99.9% benzene (nitration grade) with the balance being toluene and C.sub.6 and C.sub.7 aliphatics, and the ethylene is 90-95% (polymer grade) with 5-10% of ethane. The ethylene is fed below the catalyst bed and the benzene was feed into line 48. Benzene make up is not added to the reflux (or benzene is removed from the overhead accumulator and used as feed to the transalkylator). The feed rates and other conditions of the alkylation are set out in TABLE II.

The alkylation product from the bottom of the tower is fractionated in a three inch by twenty foot packed tower and the ethyl benzene as reported in TABLE II is collected. The bottoms from the fractionation are passed with make up benzene through a 4 inch, ten foot long reactor packed with the particular catalyst used in the catalytic structures. Results of the transalkylation are also reported in TABLE II.

The net result of the combined alkylation and transalkylation is an average selectivity to ethyl benzene of 99.7 mole % (considering 100% conversion based of ethylene).