CA2310216A1 - Alkylaromatic process using a solid alkylation catalyst - Google Patents

Abstract

An integrated alkylaromatic process using a solid alkylation catalyst is disclosed for alkylating aromatics with olefins and for regenerating the solid alkylation catalyst. A
relatively low-purity aromatic-containing stream is used in producing alkylaromatics, and a relatively high-purity aromatic-containing stream is used in regenerating the solid alkylation catalyst. In another embodiment, this process is further integrated with a paraffin dehydrogenation zone and an aromatic by-products removal zone. This invention produces the benzene-containing streams that are necessary for alkylating and for regenerating in a more economical manner than prior art processes.

Description

"Alkylaromatic Process Using A Solid Alkylation Catalyst "
FIELD OF THE INVENTION
The invention relates to the alkylation of aromatic compounds with olefins using solid catalyst.
s BACKGROUND OF THE INVENTION
About thirty years ago it became apparent that household laundry detergents made of branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS), which were found to biodegrade io more rapidly than the branched variety.
LABS are manufactured from linear alkylbenzenes (LAB).
The petrochemical industry produces LAB by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of HF. This is the industry's standard process. Over the last decade, is environmental concerns over HF have increased, leading to a search for substitute processes employing catalysts other than HF that are equivalent or superior to the standard process. Solid alkylation catalysts, for example, are the subject of vigorous, ongoing research.
Solid alkylation catalyst processes tend to operate at a higher molar ratio 20 of benzene per olefin than processes that employ HF. Detergent alkylation processes that use HF tend to operate at a benzene/olefin molar ratio of 12:1 to 6:1. Solid alkylation catalyst processes tend to run at higher benzene/olefin ratios, typically 30:1 to 20:1. One reason for this is that solid alkylation catalysts tend to be less selective toward producing monoalkylbenzene, and therefore the 2s benzene/olefin molar ratio must be increased to meet increasingly stringent selectivity requirements. Selectivity is defined as the weight ratio of monoalkylbenzene product to all products.

Solid catalysts deactivate with use. An alkylation process employing a solid alkylation catalyst typically includes means for periodically regenerating it by removing the gum-type polymers that accumulate on the surface of the catalyst and block reaction sites. For a solid alkylation catalyst, therefore, the s catalyst life is measured in terms of time in service at constant conversion between regenerations.
Solid catalyst can be best used in the continuous alkylation of aromatics where effective and inexpensive means of catalyst regeneration are available.
Solid catalysts used for alkylation of aromatic compounds by olefins, io especially those in the 6 to 20 carbon atom range, usually are deactivated by by-products which are preferentially adsorbed by the catalysts. Such by-products include polynuclear hydrocarbons in the 10 to 20 carbon atom range formed in the dehydrogenation of C6 to C2o linear paraffins and also include products of higher molecular weight than the desired monoalkyl is benzenes, e.g., di- and tri-alkyl benzenes, as well as olefin oligomers.
Such catalyst deactivating agents or "poisons" are an adjunct of aromatic alkylation.
These deactivating agents can be readily desorbed from the catalyst by washing the catalyst with the aromatic reactant. Thus, catalyst reactivation, or catalyst regeneration is conveniently effected by flushing the catalyst with aromatic 2o reactants to remove accumulated poisons from the catalyst surface, generally with restoration of 100% of catalyst activity.
Therefore, it is imperative to have means of repeatedly regenerating these catalysts, i.e., to restore their activity, in order to utilize their catalytic effectiveness over long periods of time. It is further desirable to minimize the 2s additional equipment required for regeneration.
Accordingly, an integrated continuous alkylation process with a method of removing catalyst deactivation agents or minimizing catalyst deactivation is sought.

SUMMARY OF THE INVENTION
In one embodiment, this invention is an integrated process for producing alkyl aromatics from paraffins and aromatics, for regenerating deactivated solid alkylation catalyst, and optionally for preventing catalyst-deactivating by-products s from contacting the solid alkylation catalyst. In this invention, the effluent of a solid catalyst alkylation reactor producing alkyl aromatics (e.g., detergent-grade alkyl aromatics) is separated to produce a relatively low-purity aromatic-containing (e.g. benzene-containing) stream which is suitable for recycling to an on-stream solid catalyst reactor and a relatively high-purity aromatic-containing io stream (e.g., benzene-containing) stream which is suitable for passing to an off-stream alkylation reactor containing deactivated catalyst which is undergoing regeneration. A rectifier provides an economical way of producing the relatively low purity aromatic containing stream and maintaining a relatively high molar ratio of aromatic (e.g., benzene) per olefin in an on-stream solid catalyst bed, is thereby helping to retard deactivation and extend the life of the solid alkylation catalyst. The bottom stream of the rectifier may pass to an aromatic fractionation column, to produce the relatively high-purity aromatic-containing stream. Although some capital and operating costs are incurred in producing this relatively high-purity stream, the aromatic column does not needlessly 2o recycle only the relatively high-purity stream to the on-stream alkylation reactor.
Thus, savings accrue to the extent that the relatively low-purity stream such as that obtained from a rectifier, instead of the relatively high-purity fractionation column overhead stream, is recycled to the on-stream alkylation reactor.
In another aspect, this invention can be further integrated with a sorptive removal 2s unit for removing aromatic by-products formed during paraffin dehydrogenation, because the high-purity stream of the is also suitable for regenerating an off-stream sorptive bed in the sorptive removal unit.
One arrangement of the invention can use a single column to provide the low purity and high purity aromatic-containing streams along with a bottom 3o stream comprising feed aromatics and alkylaromatics. In such an arrangement the single column will ordinarily take the high purity stream comprising the aromatics which in most cases will contain benzene from the single column as a net overhead stream. The relatively low purity aromatic or benzene-containing stream will ordinarily be taken as a side cut stream at an intermediate tray level s of the single fractionation column. The side cut will ordinarily be taken from an intermediate elevation of the rectification section within the fractionation column.
The bottom stream from the single column will contain the alkylaromatic products from the alkylation process along with any heavy alkylate by-product generated in the alkylation zone in any paraffin recycle components.
io When applied to a detergent alkylation process, the present invention can use a rectifier to decrease the cost of recycling benzene to alkylation reactors that are producing detergent alkylate. The higher the benzene/olefin molar ratio in the on-stream detergent alkylation reactor, the greater is the benefit of this is invention. This benefit arises not only because rectification is a more economical method of separating the alkylation reactor effluent than the fractionation columns employed in the prior art processes, but also because rectification produces a recycle stream that is sufficiently, but not overly, pure benzene-containing stream for recycling to the on-stream detergent alkylation 2o reactor. Thus, by using less of the relatively high-purity stream when the relatively low-purity overhead stream suffices, this invention decreases the costs of recycling benzene to the alkylation reactor.
In one specific form, the benzene rectifier zone bottom stream passes to a fractionation column, commonly known as the benzene column, which removes 2s most of the remaining benzene that was in the alkylation reactor effluent and produces a benzene column overhead stream having a higher purity than that of the overhead stream produced by the benzene rectifier. Of course, it is within the scope of this invention that some of the benzene column overhead stream may be recycled to the on-stream detergent alkylation reactor, but the benefit of 3o this invention is greatest when the flow of relatively high-purity benzene from the benzene column overhead to the on-stream alkylation reactor is minimized.
One of the significant ways in which this invention can reduce the costs associated with recycling benzene to an on-stream alkylation reactor is by significantly decreasing the size of the benzene column. By removing some of s the benzene from the on-stream alkylation reactor effluent prior to passing the remainder of the reactor effluent to the benzene column, the diameter, height, and reboiler duty of the benzene column are reduced, because the benzene throughput through the benzene column has been decreased. Although new solid catalyst alkylation units can benefit from this advantage, this advantage has io far-reaching implications for solid catalyst alkylation processes that are built by converting existing HF detergent alkylation processes to solid alkylation catalysts. This is because enough benzene can be removed from the alkylation reactor effluent using a benzene rectifier that the remaining benzene in the benzene rectifier bottom stream is not greater than the benzene content in the is HF stripper bottom stream in an HF alkylation process. Therefore, with the use of a benzene rectifier between the alkylation reactor effluent and the benzene column, the entire existing fractionation train of an existing HF alkylation process can be re-used when the catalyst is switched from HF to a solid alkylation catalyst, resulting in large savings in investment capital for converting to a solid 2o catalyst alkylation unit. Additional savings are possible because the existing HF
stripper of the HF alkylation process can be readily modified and then used as the benzene rectifier in the solid alkylation process and therefore much of the cost of a new benzene rectifier is avoided.
Accordingly, in one embodiment, this invention is a process for producing 2s alkylaromatics. Olefins and feed aromatics are reacted to form alkylaromatics in an on-stream alkylation zone in the presence of solid alkylation catalyst at alkylation conditions. The alkylation conditions are sufficient to at least partially deactivate at least a portion of the solid alkylation catalyst in the on-stream alkylation zone. An on-stream effluent stream comprising alkylaromatics and 3o feed aromatics is withdrawn from the on-stream alkylation zone. At least a -s-portion of the on-stream effluent stream is separated into a relatively impure stream comprising feed aromatics and depleted in alkylaromatics and relatively impure stream comprising feed aromatics in a higher purity than that of the pure stream and depleted in alkylaromatics and a bottom stream comprising feed s aromatics and enriched in alkylaromatics. At least a portion of the relatively pure stream is recycled to the on-stream alkylation zone. Alkylaromatics are recovered from the bottom stream. At least a portion of the relatively impure stream comprising feed aromatics passes to an off-stream alkylation zone containing at least partially deactivated solid alkylation catalyst. The relatively io impure stream contacts partially deactivated solid alkylation catalyst in the off-stream alkylation zone to partially regenerate the solid alkylation catalyst and to produce at least partially regenerated solid alkylation catalyst in the off-stream alkylation zone. An off-stream effluent stream comprising feed aromatics is withdrawn from the off-stream alkylation zone. Periodically the functions of the is on-stream and off-stream alkylation zones are shifted by operating the off-stream alkylation zone to function as the on-stream alkylation zone and operating the on-stream alkylation zone to function as the off-stream alkylation zone.
INFORMATION DISCLOSURE
2o US-A-5,648,579 (Kulprathinpanja et al.) teaches that solid catalyst used for the alkylation of aromatic compounds by olefins usually are deactivated by by-products which are preferentially adsorbed by the solid catalysts, and that the deactivating agents can be readily desorbed from the solid alkylation catalyst by washing the catalyst with the aromatic reactant.
2s US-A-5,276,231 (Kocal et al.) teaches an alkylaromatic process with removal of aromatic by-products which are normally formed in paraffin dehydrogenation by sorbing the aromatic by-products on a sorbent and contacting the sorbent with liquid benzene to regenerate the sorbent.

BRIEF DESCRIPTION OF THE DRAWING
The drawing is a process flow diagram of an embodiment of this invention.
DETAILED DESCRIPTION OF THE INVENTION
s This invention is an integrated process for producing alkyl aromatics by alkylating aromatics with olefins using a solid alkylation catalyst and for regenerating deactivated solid alkylation catalyst. The feedstocks which are used in the practice of the invention normally result from the dehydrogenation of paraffins. The entire dehydrogenation reaction mixture often is used.
io The polyolefins formed during dehydrogenation are minimized in the feedstocks used in the practice of this invention. Consequently the feedstocks are a mixture largely of unreacted paraffins, branched monoolefins, and unbranched or linear monoolefins. These paraffins and monoolefins typically are in the C6-C22 range, although those in the Cg-C16 range are preferred in the practice of this invention, is and those in the Cip-C~4 range are even more preferred. The monoolefins in the feedstock are reacted with benzene or alkylated derivatives of benzene that are charged to the subject process. Suitable alkylated derivatives of benzene (alkylaromatics) include, but are not limited to, toluene, xylenes, and higher methylated benzenes; ethylbenzene, diethylbenzene, and triethylbenzenes;
2o isopropylbenzene (cumene), n-propylbenzene, and higher propylbenzenes;
butylbenzenes; and pentylbenzenes. Thus, the alkylated derivative of benzene may have one or more alkyl groups, and each alkyl group may have from 1 to 5 or even more carbon atoms.
The most widely practiced alkylaromatic process to which the present 2s invention is applicable is the production of linear alkylbenzenes (LAB).
An LAB process usually charges normal paraffins to the dehydrogenation reactor. Branched olefins formed in dehydrogenation are usually not removed, Branched monoolefins in the feedstock are usually present in small concentrations. As for the monoolefins in the feedstock, unsaturation may appear anywhere on the monoolefin chain, since there is no requirement as to the position of the double bond. The monoolefins in the feedstock are reacted with benzene, since the product of alkylating a monoolefin with an alkylated derivative of benzene may not be as suitable a detergent precursor as alkylated s benzene. Although the stoichiometry of the alkylation reaction requires only molar proportion of benzene per mole of total linear monoolefins, the use of a 1:1 mole proportion results in excessive olefin polymerization and polyalkylation creating large amounts of the dialkylbenzenes, trialkylbenzenes, possibly higher polyalkylated benzenes, olefin dimers, trimers, etc., and unreacted benzene.
To io carry out alkylation with the conversion, selectivity, and linearity required, a total benzene: monoolefin molar ratio of from 5:1 up to as high as 30:1 is recommended, with ratios of between about 8:1 and about 20:1 preferred.
The benzene and linear monoolefins are reacted in the presence of a solid alkylation catalyst under alkylation conditions. These alkylation conditions is include a temperature in the range between about 80°C (176°F) and about 140°C (284°F), most usually at a temperature not exceeding 135°C (275°F).
Since the alkylation is conducted as a liquid phase process, pressures must be sufficient to maintain the reactants in the liquid state. The requisite pressure necessarily depends upon the feedstock and temperature, but normally is in the 2o range of 1480-7000 kPa absolute (200-1000 psi(g)), and most usually 2170-3550 kPa(g) (300-500 psi(g)).
Solid alkylation catalysts typically have an acid function and are, therefore, better known as solid acid catalysts. Such solid acid catalysts include, materials such as amorphous silica-alumina, crystalline aluminosilicate materials 2s such as zeolites and molecular sieves, naturally occurring and man-made clays including pillared clays, sulfated oxides such as sulfonated zirconia, traditional Friedel-Crafts catalyst such as aluminum chloride and zinc chloride, and solid Lewis acids generally. Solid alkylation catalysts are illustrated which disclose an extruded catalyst comprising clay and at least one multi-valent metal; U.S.
Patent 3o No. 5,034,564 issued to J.A. Kocal which discloses a catalyst comprising a pillared _8_ clay and a binder; U.S. Patent Nos. 5,196,574 and 5,344,997, both issued to J.
A.
Kocal, which disclose a fluorided silica-alumina catalyst; U.S. Patent No.

5,302,732 issued to K.Z. Steigleder et al which describes an ultra-low sodium silica-alumina catalyst; and U.S. Patent No. 5,491,271 issued to Marinangeli et al. which s discloses the use of either delaminated or pillared tetrahedrally charged clays.
The effluent of the alkylation reaction zone, preferrably passes to a rectifier. A rectifier differs distinctly from a stripper. The differences between a rectifier and a stripper are readily apparent by considering distillation processes in general. Distillation processes rely on the well-known tendency that when io liquid and vapor phases contact, the more volatile components tend to concentrate more in the vapor phase than in the liquid phase. In multi-stage operation, a liquid descends a vertical distillation column and passes through a number of stages in which it is contacted countercurrently by ascending vapor.
The point at which feed is introduced to the distillation column divides the column is into two sections. The stripping section is below the feed point, and the rectifying section is above the feed point. In the stripping section, the more volatile component is stripped from the descending liquid. In the rectifying section, the concentration of the less volatile component in the vapor is reduced. In practice, the stages in which the streams of liquid and vapor contact each other may be 2o trays or packing material. Therefore, in a rectifier the feed is at the bottom of a number of stages, in comparison to a stripper where the feed to a stripper is at the top of a number of stages. Furthermore, a rectifier reduces the concentration of the less volatile component in the vapor, whereas a stripper strips the more volatile component from the descending liquid.
2s A rectifier has generally from about 10 to about 20 separation stages and usually uses sieve trays with a tray efficiency of about 60%. Thus, the rectifier generally has from about 15 to about 25 trays, and typically 20 trays. Fewer than 15 trays could be used, and some or all of the trays could be replaced with a vapor-liquid contacting media, such as regular-shaped Berl saddles or Raschig 3o rings in a random arrangement or such as structured elements in an ordered arrangement. The benzene rectifier usually employs a reboiler, either external or internal to the benzene rectifier, a feed preheater, or both. The benzene rectifier typically also employs a total condenser, which condenses a vapor or mixture of vapors, condensing generally more than 95 wt-%, and more commonly, more s than 99.5 wt-% of the vapors. A portion of the condensed overhead stream is typically refluxed to the upper portion of the benzene rectifier. The remaining portion of the condensed overhead stream recycles to an on-stream alkylation reactor. As used herein in the context of a portion of a stream, the term "portion"
means an aliquot portion or a nonaliquot portion, unless otherwise stated.
io An aliquot portion of a stream is a portion of the stream that has essentially the same composition as the stream.
The operating conditions of the benzene rectifier typically include a pressure of from about 50 to about 70 psi(g) (345 to 483 kPa(g)), although higher pressures up to the design limit of the vessel may be employed. The is overhead and bottom temperatures of the benzene rectifier are normally about 300°F (149°C) such that the benzene rectifier operates with relatively little difference between the overhead and bottom temperatures. The benzene rectifier generally produces a bottom stream that contains a sufficient amount of benzene such that the boiling point of the bottom stream is relatively close to that zo of the overhead stream. Generally about 50 percent to about 70 percent of the benzene entering the rectifier exits with the net overhead stream.
The purity of the relatively impure benzene-containing stream such as that recovered from the net overhead of the benzene rectifier, is relatively low in comparison with the relatively high purity stream which may be recovered from 2s the net overhead of a benzene column. The relatively impure benzene-containing net stream recovered from the overhead of the benzene rectifier generally has a benzene concentration of from about 80 to about 98 mol-%. For the benzene rectifier overhead stream, the concentration of paraffins is generally from 2 to 20 mol-% and preferably from 2 to 5 mol-%, and the concentration of 3o alkylated benzenes (alkylaromatics) is generally less than 100 wppm. The - io -presence of paraffins in contact with the catalyst at normal alkylation temperatures is believed to not have a significant detrimental effect on the solid alkylation catalysts, other than occupying volume in the reactor that could be producing alkylated benzenes. Accordingly, despite its paraffin content, the a s relatively impure liquid stream such as that of the benzene rectifier is a suitable source of benzene for recycling a portion or an aliquot portion thereof to the alkylation reactor.
The paraffins present in the net overhead stream of the benzene rectifier generally have from 5 to 22 carbon atoms. A first source of paraffins in the io benzene rectifier overhead stream is the paraffins that accompany the monoolefin-containing feedstock. Such paraffins typically have the same number of carbon atoms as that of the monoolefins in the feedstock. Paraffins also enter the benzene rectifier overhead stream with the benzene-containing charge stream. The paraffins in this benzene charge stream have boiling points is that are generally close to that of benzene.
A net bottom stream of the benzene rectifier, having a molar ratio of benzene per alkylaromatic of about 7:1, may pass to a benzene column.
The benzene column can remove the remainder of the benzene using typically from 45 to 55 sieve trays, usually about 50 sieve trays. The benzene rectifier zo bottoms stream enters at or around sieve tray 30, as numbered from the top of the benzene column. Makeup benzene, which need not be previously dried, may also be fed to the benzene column. The benzene column usually employs a reboiler as well as a total condenser for the overhead stream, which refluxes liquid to the top of the benzene column. The operating conditions of the 2s benzene column include a pressure of about ) 170 kPa absolute (10 psi(g)), an overhead temperature of about 93°C (200°F), and a bottom temperature of about 232°C (450°F). The benzene column produces a net overhead stream which has a benzene concentration of usually more than 95 mol-%, preferably more than 99.9 mol-%, and more preferably more than 99.99 mol-%. The 3o benzene column net overhead stream may also contain a small concentration of -ii-paraffins of generally less than 5 mol-%, preferably less than 0.1 mol-%, more preferably less than 100 wppm, and even more preferably less than 10 wppm.
In the benzene column net overhead stream, alkylated benzenes (alkylaromatics), if any, are generally present at lower concentrations than that of s paraffins. Thus, in accordance with this invention the purity of the benzene stream recovered from the overhead of the benzene column is generally greater than that of the benzene stream recovered from the overhead of the benzene rectifier.
The net overhead stream of the benzene column may contain paraffins io having from 5 to 22 carbon atoms. The particular paraffins present in the benzene column net overhead stream depend primarily on the paraffins in the monoolefinic feedstock, the benzene-containing charge, and the purge stream, if any, of an aromatic by-products removal zone, if used.
In accord with this invention, a portion such as an aliquot portion of the net is overhead liquid stream of the benzene column is passed to an off-stream alkylation reactor containing solid alkylation catalyst that is undergoing reactivation or regeneration. It is believed that the purity of the benzene that is used for regeneration of the solid alkylation catalyst is an important variable, in combination with the regeneration temperature, for insuring that the regenerated 2o catalyst is returned to an acceptable level of activity for alkylating reactions.
Without being bound to any particular theory, it is believed that the presence of paraffins in contact with the alkylation catalyst at the relatively high temperatures employed during regeneration has a detrimental effect on the catalyst.
Therefore, it is believed that a relatively pure stream, such as the net overhead 2s liquid stream of the benzene column, with its lower paraffin concentration relative to a relatively impure stream such as the net overhead liquid stream of the benzene rectifier, is a suitable stream for regenerating deactivated solid alkylation catalyst.

Thus in a preferred embodiment, the net overhead liquid stream of a benzene column is passed to a bed of solid alkylation catalyst which is undergoing regeneration. The effluent of the reactor that is undergoing regeneration contains benzene, paraffins, alkylated benzenes, and heavy s components desorbed from the catalyst. Although this effluent stream from the off-stream alkylation reactor could be passed to a benzene rectifier in the same manner as the alkylation reactor effluent during normal operation, it is preferred that this regeneration effluent stream passes to the benzene column. Thus, the benzene column may be fed not only with benzene from the bottom of the io benzene rectifier, and makeup benzene, but also benzene from the alkylation reactor that is undergoing regeneration.
In one commonly employed arrangement, the bottom stream of the benzene column passes to a paraffin column which produces an overhead liquid stream containing unreacted paraffins, which normally is recycled as a recycle is stream to the dehydrogenation zone, and a bottoms stream containing the product alkylate and any higher molecular weight side product hydrocarbons formed in the selective alkylation zone. This bottoms stream is passed into a rerun column which produces an overhead alkylate product stream containing the detergent alkylate and a bottoms stream containing polymerized olefins and 2o polyalkylated benzenes (heavy alkylate).
In another embodiment, this invention is an integrated process for producing alkyl aromatics by dehydrogenating linear paraffins to linear olefins and then alkylating benzene with the linear olefins in the presence of a solid alkylation catalyst, for regenerating deactivated solid alkylation catalyst, and in 2s addition for preventing catalyst-deactivating by-products from contacting the solid alkylation catalyst.
The dehydrogenation section will preferably be configured substantially in the manner shown in the drawing of US-A-5,276,231. A feed stream containing paraffins combines with recycled hydrogen and recycled unreacted paraffins from the alkylation section. This forms a reactant stream which is heated and passed through a bed of a suitable catalyst maintained at the proper dehydrogenation conditions of temperature, pressure, etc. Dehydrogenation catalysts are well known in the dehydrogenation art, as exemplified by US-A-s 3,274,287; US-A-3,315,007; US-A-3,315,008; US-A-3,745,112; and US-A-4,430,517, and need not be described here in great detail. The effluent of this catalyst bed or reactor effluent stream is usually cooled, partially condensed, and separated to provide an effluent that is passed to the alkylation section.
A common variant of this embodiment includes the selective io hydrogenation of diolefins that are normally present in the dehydrogenated product stream. It is well known that diolefins are formed during the catalytic dehydrogenation of paraffins. Selective diolefin hydrogenation converts diolefins to monoolefins, which are the desired product of the dehydrogenation section, and produces a selective diolefin hydrogenation product stream. Selective is diolefin hydrogenation is taught in US-A-4,520,214 and US-A-5,012,021.
An aromatics removal zone eliminates or significantly reduces the aromatic by-products in the feedstock to the selective alkylation zone in the present embodiment for the production of alkylated aromatic compounds.
Removal of the aromatic by-products reduces the deactivation rate of solid 2o alkylation catalyst and, thereby, produces a significantly higher yield of linear alkylated aromatic compounds.
It is well known that aromatic by-products are formed during the catalytic dehydrogenation of paraffins. These aromatic by-products are believed to include alkylated benzenes, naphthalenes, other polynuclear aromatics, zs alkylated polynuclear hydrocarbons in the C~p-C15 range, indanes, and tetralins, and may be viewed as aromatized normal paraffins. Typically, from about 0.2 to about 0.7 weight percent, and generally no more than 1 weight percent, of the feed paraffinic compounds to a dehydrogenation zone form aromatic by-products. It is believed that these by-products are formed at least to a small extent at suitable dehydrogenation conditions in the presence of most, if not all, commercially available dehydrogenation catalysts. In processes without removal of aromatic by-products, the concentration of aromatic by-products in the dehydrogenation effluent stream can typically accumulate to 4-10 weight s percent, which leads to rapid deactivation of solid alkylation catalyst.
This embodiment of the invention selectively removes at least a portion of the aromatic by-products in the dehydrogenated product stream using at least one aromatics removal zone. The aromatics removal zone is preferably located between the dehydrogenation zone and the selective alkylation zone because io the aromatic by-products are preferably selectively removed prior to entering the selective alkylation zone. Suitable aromatics removal zones for this embodiment of the invention include sorptive separation zones Where the aromatics removal zone is a sorptive separation zone, our invention can be practiced in fixed bed or moving sorbent bed systems, but the fixed bed system is preferred. The flow of is the stream containing the aromatic by-products through the sorptive separation zones is preferably performed in a parallel manner so that when one of the sorbent beds or chambers is spent by the accumulation of the aromatic by-products thereon, the spent zone may be by-passed while continuing uninterrupted operation through the parallel zone.
2o Suitable sorbents may be selected from materials which exhibit the primary requirement of selectivity for the aromatic by-products and which are otherwise convenient to use. Suitable sorbents include, for example, molecular sieves, silica, activated carbon activated charcoal, activated alumina, silica-alumina, clay, cellulose acetate, synthetic magnesium silicate, 2s macroporous magnesium silicate, and/or macroporous polystyrene gel. It should be understood that the above-mentioned sorbents are not necessarily equivalent in their effectiveness. The choice of sorbent will depend on several considerations including the capacity of the sorbent to retain aromatic by-products, the selectivity of the sorbent to retain the aromatic by-products 3o which are more detrimental to solid alkylation catalysts, and the cost of the -is-sorbent. The preferred sorbent is a molecular sieve, and the preferred molecular sieve is 13 X zeolite (sodium zeolite X).
Those skilled in the art are able to select the appropriate conditions for operation of the sorbent without undue experimentation. For example, a fixed s bed sorptive separation zone containing 13 X zeolite may be maintained at a temperature generally from about 20°C to 300°C (68°F to about 572°F) and preferably from about 100°C to 200°C (212°F to about 392°F), a pressure effective to maintain the stream containing the aromatic by-products in a liquid phase at the chosen temperature. and a liquid hourly space velocity from about io 1 hr' to about 10 hr' and preferably from about 1 hr' to about 3 hr'. The flow of the stream containing the aromatic by-products through the sorptive separation zone may be conducted in an upflow, downflow or radial-flow manner.
Although both liquid and vapor phase operations can be used in many sorptive separation processes, liquid phase operation is preferred for the sorptive is separation zone because of the lower temperature requirements and because of the higher sorption yields of the aromatic by-products that can be obtained with liquid phase operation. Therefore, the temperature and pressure of the sorptive separation zone during sorption of the aromatic by-products are preferably selected to maintain in a liquid phase the stream from which the aromatic 2o by-products are selectively removed. However, the operating conditions of a sorptive separation zone can be optimized by those skilled in the art to operate over wide ranges which are expected to include the conditions in the reaction zones of our invention and its variants. Therefore, this embodiment of our invention includes a sorptive separation zone contained in a common reaction zs vessel with the dehydrogenation zone, the selective diolefin hydrogenation zone, the selective alkylation zone or the selective monoolefin hydrogenation zone.
Following an appropriate processing period the sorbent, is regenerated by removing the sorbed aromatic by-products from the sorbent. There are numerous methods of regenerating the sorbent using and any suitable regeneration method may be used, including altering the temperature and pressure of the sorbent and treating the sorbent with a relatively pure stream such as that obtained from the benzene column overhead stream to displace or desorb the sorbed aromatic by-products. The flow direction of the benzene s column overhead stream through the sorptive separation zone may be upflow or radial flow, but the preferred direction is downflow. The phase of the benzene column overhead stream mixture through the sorptive separation zone may be liquid phase and/or vapor phase.
An effluent stream is withdrawn from the aromatics removal zone which io contains benzene, a purge hydrocarbon such as pentanes where the zone was purged with a purge hydrocarbon prior to being contacted with the benzene-containing stream, and aromatic by-products produced during dehydrogenation.
This effluent stream is typically passed to a desorbent fractionation column, which produces a heavy bottom stream comprising the aromatic by-products. If is a benzene column overhead stream containing any feed paraffins passed to the aromatics removal zone, these feed paraffins would be present in the effluent stream, and would ultimately appear in the heavy bottom stream of the desorbent column. This is because the aromatics by-products and the paraffins generally have the same carbon number, and hence both the aromatic by-2o products and the paraffins co-boil at approximately the same temperature.
Because the paraffins, which can potentially be converted into desirable alkylated aromatics, and the aromatic by-products, which cannot be readily converted into the desired alkylaromatics, are recovered in the same stream, the rejection of the aromatic by-products from the desorbent column results also in 2s the rejection of the paraffins. The higher the concentration of paraffins in the benzene column overhead stream to the aromatics removal zone, the greater is the loss of these paraffins from the desorbent column with the aromatic by-products. Accordingly, it is preferred that the regeneration stream for the aromatics removal zone be a relatively pure stream such as that from the net 30 overhead liquid of the benzene column rather than a relatively impure stream such as the net overhead liquid of the benzene rectifier because of its higher purity. For this reason the relatively pure stream such as the benzene column overhead stream, contains preferably less than 0.1 mol-% paraffins, more preferably less than 100 wppm paraffins, and even more preferably less than s 10 wppm paraffins.
The desorbent column also produces a net overhead stream which contains the lighter components, namely benzene and a purge compound such as pentane. This net overhead stream passes to a fractionation column which separates the purge compound from the benzene. In the case where the purge io compound is pentane, this separation zone is a depentanizing fractionation column, which produces a net overhead stream comprising pentanes and a net bottom stream comprising benzene. The net overhead stream is recovered for use in purging the aromatics removal zone, and the net bottom stream is recycled to the solid catalyst alkylation zone. In this manner, some of the is benzene requirements for the solid catalyst alkylation zone is supplied by the fractionation column downstream of the desorbent column associated with the aromatics removal zone, which in the aforementioned case is a depentanizer.
A complete operation of the process can be more fully understood from a process flow for a preferred embodiment. Referring now to the drawing, a line 20 212 charges a paraffin feed stream comprising an admixture of C1o-C,5 normal paraffins. (which will usually include recycle paraffins from line 174) to a dehydrogenation section 210, that contacts the paraffins with a dehydrogenation catalyst in the presence of hydrogen at conditions that convert a significant amount of the paraffins to the corresponding olefins. The product of the 2s dehydrogenation section comprises monoolefins, unreacted paraffins, and aromatic by-products and passes via line 214 to an aromatic by-product removal zone having a bed 230 on-stream for removing aromatic by-products and bed 220 off-stream for regeneration of the sorbent. Valve 222 is open and valve is closed. The dehydrogenation section product passes through lines 218 and 30 224 and open valve 222 to enter on-stream bed 230, which removes aromatic -i8_ by-products. The effluent of bed 230 flows through lines 232, 234, and 258 to depentanizer column 280, valve 240 being open and valve 250 being closed.
Most of the olefinic and paraffinic hydrocarbons entering depentanizer 280 through line 258 are heavier than pentane, and therefore exit via the bottom of s depentanizer 280 through line 278. Preferably, the concentration of C~-minus paraffins in the bottom stream in line 278 is small. The hydrocarbons in line combine with the benzene-containing net overhead liquid stream from benzene rectifier 150, which flows through line 164.
The combined stream of olefins, paraffins, and benzene flows through io lines 116, 122, and 286 via open valve 126 to alkylation reactor 110 to alkylate benzene with olefins. The on-stream reactor effluent passes through open valve 13 and line 136. Open valve 134 and line 138 combine with the on-stream effluent of off-stream reactor 120 with the on-stream effluent. The combined stream flows through line 142, is heated in heat exchanger 130, flows through is line 146, is further heated in heat exchanger 140, flows through line 152, and enters benzene rectifier 150. Heat exchanger 130 supplies heat from the benzene rectifier overhead vapor stream in line 144. The benzene-containing overhead vapor stream in line 148 of the benzene rectifier 150 is further condensed in condenser 160, flows through line 154, and enters overhead 2o receiver 170. A small net stream of light, uncondensed hydrocarbons is withdrawn from receiver 170 via line 156. Line 170 supplies liquid to the benzene rectifier 150 as reflux via line 166 and recycle to the on-stream alkylation reactor 110 via line 164.
The benzene rectifier bottom stream flows through line 158 to benzene 2s fractionation column 180. In this depicted process arrangement, the benzene fractionation column 180 is a separate vessel from the benzene rectifier 150.
A
benzene-containing makeup stream enters benzene column 180 through line 114. The bottom stream of the benzene column 180 flows through line 172 to conventional product recovery facilities 190. The streams withdrawn from 3o recovery facilities 190 include a paraffin recycle stream 174, a heavy alkylate stream 178, and an alkylaromatic product stream 176. After condensing the net liquid overhead stream from benzene column 180 flows via line 168 to off-stream bed 220 and to solid alkylation catalyst undegoing regeneration in off-stream reactor 120.
s Accordingly, one portion of the stream in line 168 flows through lines 268 and 228 to bed 220, valve 272 being open and valve 266 being closed. Thus, with bed 220 being off-stream there is no flow through line 216, and with bed being on-stream there is no flow through line 264. The effluent flows through lines 236, 244, and 256 to desorbent column 270, valve 242 being open and io valve 250 being closed. Because bed 220 is off-stream with valve 250 closed, there is no flow through line 248, and because bed 230 is on-stream with valve 238 closed there is no flow through line 246. Desorbent column 270 produces a bottom stream 276 comprising aromatic by-products which is withdrawn from the process and can be used for fuel. The desorbent column also produces an is overhead stream 274 comprising benzene and pentanes which flows to depentanizer 280. Depentanizer column 280 produces an overhead stream 260 comprising pentane, which is routed to storage facilities (not shown) for maintaining a pentane inventory available for purging sorptive bed 230 when that bed is taken off-stream.
2o The other portion of the benzene-containing stream in line 168 flows through lines 252, 254, and 288, and into off-stream reactor 120. Valve 262 is open and valve 284 is closed. With reactor 120 being off-stream and reactor 110 being on-stream, there is no flow through line 282 nor is there flow through line 124. The benzene entering reactor 120 washes heavy by-products from the 2s alkylation catalyst that cause the catalyst to deactivate. Thus, the effluent from off-stream reactor 120 contains benzene as well as these by-products, which can include polynuclear hydrocarbons, polyalkylated aromatics, and olefin oligomers.
The by-products in effluent of off-stream reactor 120 tend to concentrate in the bottom streams of benzene rectifier 150 and benzene column 180, and are 3o ultimately recovered by product recovery facilities 190 in the heavy alkylate stream 178. As an alternative to flowing to benzene rectifier 150, the effluent of off-stream reactor 120 could instead bypass the benzene rectifier 150 and flow directly to benzene column 180.
The arrangement of the lines and valves upstream and downstream of the s reactors 110 and 120 permit the functions of the on-stream reactor 110 and the off-stream reactor 120 to be periodically shifted. This shifting of functions is performed when the catalyst in the on-stream reactor 110 becomes sufficiently deactivated as to render continued on-stream operation impractical or uneconomical, or when the catalyst in the off-stream reactor 120 becomes to sufficiently reactivated as to render it to be practicably or economically operated on-stream, or both. This shifting of functions can be accomplished by opening the valves 128 and 284 which are closed and closing the valves 126 and 262 which are open. In an analogous manner, the arrangement of the lines and valves upstream and downstream of the beds 220 and 230 also permit the Is functions of the on-stream bed 230 and the off-stream bed 220 to be periodically shifted. on-stream bed 230 functions as the off-stream bed 220 and the off-stream bed 220 functions as the on-stream bed 230.

Claims (10)

1. A process for producing alkylaromatics comprising:
a) reacting olefins and feed aromatics to form alkylaromatics in an on-stream alkylation zone in the presence of solid alkylation catalyst at alkylation conditions, the alkylation conditions being sufficient to at least partially deactivating at least a portion of the solid alkylation catalyst in the on-stream alkylation zone, and withdrawing from the on-stream alkylation zone an on-stream effluent stream comprising alkylaromatics and feed aromatics;
b) separating at least a portion of the on-stream effluent stream into a relatively low purity stream comprising feed aromatics and depleted in alkylaromatics; a relatively higher purity stream comprising feed aromatics in a higher purity than the relatively low purity stream and depleted in alkylaromatics, and a bottom stream comprising feed aromatics and enriched in alkylaromatics;
c) recycling at least a portion of the relatively low purity stream to the on-stream alkylation zone;
d) recovering alkylaromatics from the bottom stream;
e) passing at least a portion of the relatively high purity stream to an off-stream alkylation zone containing at least partially deactivated solid alkylation catalyst, contacting the at least partially deactivated solid alkylation catalyst in the off-stream alkylation zone with at least a portion of the relatively high purity stream to at least partially regenerate the solid alkylation catalyst and to produce at least partially regenerated solid alkylation catalyst in the off-stream alkylation zone, and withdrawing an off-stream effluent stream comprising feed aromatics from the off-stream alkylation zone;
f) returning at least a portion of the off-stream effluent stream for separation in Step (b); and g) periodically shifting the functions of the on-stream and off-stream alkylation zones by operating the off-stream alkylation zone to function as the on-stream alkylation zone in Steps (a) and (c) and operating the on-stream alkylation zone to function as the off-stream alkylation zone in Step (e).

2. The process of Claim 1 wherein a single column provides the relatively high purity stream as an overhead stream, the relatively low purity stream as a sidecut stream, and the bottom stream and wherein the effluents from the offstream and on stream alkylation zones are returned to the single column.

3. The process of Claim 1 wherein at least a portion of the on-stream effluent stream passes to a rectifier, the relatively low purity stream is an overhead from the rectifier, the rectifier provides a rectifier bottoms stream at least a portion of the rectifier bottoms stream passes to a fractionation zone and the fractionation zone produces the relatively higher purity stream as a fractionation overhead stream and the bottom stream as a fractionation bottoms stream.

4. The process of Claim 3 further characterized in that the at least a portion of the rectifer overhead stream comprises an aliquot portion of the rectifier overhead stream and in that the at least a portion of the fractionation overhead stream comprises aliquot portion of the fractionation overhead stream.

5. The process of Claims 1, 2 and 3 or 4 further characterized in that the relatively low purity stream has a concentration of feed aromatics of from about 80 to about 98 mol-% and the relatively high purity system has a concentration of feed aromatics of greater than about 95 mol-%.

6. The process of Claims 1, 2, 3 or 4 further characterized in that the olefins comprise olefinic hydrocarbons having from 6 to 22 carbon atoms and the feed aromatics comprise benzene and alkylated derivatives of benzene.

7. The process of Claims 1, 2, 3 or 4 further characterized in that the alkylation conditions comprise a molar ratio of olefins per feed aromatic of from about 5:1 to about 30:1.

8. The process of Claims 4 further characterized in that the rectifier and the fractionation zone are contained in separate vessels.

9. The process of Claim 1, 2 or 3 further characterized in that the relatively low purity stream has a concentration of alkylaromatics of less than 100 wppm.

10. The process of Claim 1, 2, 3 or 4 further comprising the steps of dehydrogenating a feed stream containing C6-C22 paraffins in a dehydrogenation zone and recovering therefrom a dehydrogenated product stream containing paraffins, monoolefins, C9-minus hydrocarbons, diolefins, and aromatic by-products;
passing at least a portion of the dehydrogenated product stream to the on stream alkylation zone to supply the olefins;
selectively removing at least a portion of the aromatic by-products from at least a portion of the dehydrogenated product stream, to a level of no more than 2 wt.% in at least one on-stream aromatic by-products removal zone containing a sorbent at sorptive conditions effective to selectively sorb aromatic by-products and reduce the concentration of aromatic by-products;
and passing a portion of the relatively pure stream; sorbent in at least one off-stream aromatic by-products removal zone containing sorbent, the sorbent containing sorbed aromatic by-products, to at least partially desorb aromatic by-products from the sorbent in the off-stream aromatic by-products removal zone; recovering from the at least one off-stream aromatic by-products removal zone a reject stream containing aromatic by-products and a desorbent stream containing aromatic compound; and, periodically shifting the functions of the at least one on-stream aromatic by-products removal zone and the at least one off-stream aromatic by-products removal zone.