Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ ~ ~ 7
001 -1~
002 BACKGROUND,AND SUMMARY OF THE INVENTION
003 This invention relates to a combination process for
004 upgrading a naphtha fraction. In one aspect, the invention
005 relates to a process combining reforming and aromatization over
006 a ZSM-5-type zeolite, to produce a product useful as a high-
007 octane gasoline blending stock or a source from which benzene,
008 toluene and xylene can be recovered. In another aspect, the
009 invention relates to a process combining reforming, isomeri-
010 zation and hydrocracking in the presence of a ZSM-5-type
011 zeolite to produce a product useful as a high-octane gasoline
012 blending stock and as a source for recovering benzene, toluene
013 and xylene.
014 In view of the current concern over air pollution and
015 environmental control, processes which will increase the octane
016 number of gasoline while minimizing or eliminating the need for
017 additives are being sought.
018 One traditional way of increasing the octane of a
019 naphtha fraction has been to subject it to catalytic reforming,
020 usually over a platinum-containing or bimetallic catalyst. In
021 the reformer, naphthenes and paraffins are converted to aro-
022 matics, both reactions which substantially increase the octane
023 number of the hydrocarbons involved. Naphthenes are reformed
024 to aromatics with high selectivity. ~owever, the selectivity
025 with which paraffins are converted to aromatics decreases with
026 the number of carbon atoms per paraffin molecule. Only a minor
027 fraction of C6 paraffins is converted to benzene. Other reac-
028 tions which occur in the reformer are isomerization and crack-
029 ing of paraffins. The cracking to C3- hydrocarbons represents
030 an irreversible yield loss, and the isomerization of paraffins
031 mainly to singly branched paraffins is a reversible reaction in
032 which a relatively high concentration of low-octane n-paraffins
033 remain in thermal equiIibrium with the branched isomers. Thus,
U34 inclusion of C6 paraffinic hydrocarbons in a reformer feed is a
035 less efficient use of the catalyst and reactor facilities than
036 the inclusion of C6 naphthenes.
037 Another way of improving the octane number of hydro-
038 carbon fractions is by contacting them with a ZSM-5 type of
~7~Q~
001 -2-
002 aluminosilicate zeolite catalyst to produce new aromatic rings
003 from aliphatic compounds. For example, U.S. Patent 3,761,389
004 teaches aromatization of a hydrocarbon fraction boiling within
005 the range of C2 to 400F with a ZSM-5 type of synthetic alumino-
006 silicate zeolite catalyst, and U.S. Patent 3,756,942 teaches
007 aromatization of a feed consisting essentially of C5+ paraf-
008 fins, olefins and/or naphthenes over a ZSM-5-type catalyst to
009 produce a predominantly aromatic liquid and a light hydrocarbon
010 gas. If the aromatization is performed at high temperature
011 (e.g., about 538C) and low pressure (e.g., about 1 atmosphere)
012 without added H2~ the light gas includes C2-C4 olefins as well
013 as Cl-C4 paraffins.
014 The art discloses several combinations of reacting a
015 hydrocarbon stream over a reforming catalyst and over a ZSM-5-
016 type catalyst. For example: in U.S. Patent 3,729,409 there is
017 described a process for upgrading a reformate by contacting the
018 reformate and hydrogen with a ZSM-5-type zeolite, to selec-
019 tively crack the normal paraffins and to form and alkylate aro-
020 matic compounds. In U.S. Patent 3,849,290 there is described a
021 process for reforming a naphtha and then removing normal and
022 singly branched hydrocarbons by selective cracking to leave an
023 aromatics-enriched product. In these two processes, some
024 cracking of the alkyl side chain on the aromatic ring occurs,
025 resulting in production of unwanted light ends. In U.S. Patent
026 3l770,614 there is disclosed a process in which a reformate is
027 fractionated and the light reformate fraction (C6 to 116 or
028 127C) passed over a ZSM-5-type zeolite to alkylate mono-
029 aromatics. In U.S. Patent 3,950,241 there is disclosed a
030 process for upgrading naphtha by separating it into low- and
û31 high-boiling fractions, reforming the low-boiling fraction, and
032 combining the high-boiling naphtha with the reformate for
033 contact with a ZSM-5-type catalyst to crack the paraffins.
034 Reacting heavy naphthas over ZSM-5-type catalysts in
035 the absence of H2 and at high temperatures leads to rapid cata-
036 lyst deactivation, while processing naphthenes over them leads
037 primarily to cracking, which reduces liquid yields.
J.~;Z71`~9L
001 -3-
002 ~e have found it advantageous to separate a naphtha
003 into a light fraction and a heavy fraction, reform the heavy
004 fraction, and pass at least a portion of the reformate over
005 ZSM-5-type catalyst to produce a C5+ product fraction enriched
006 in aromatic hydrocarbons.
007 In particular, we have provided a process for
008 upgrading a l.apntha-boiling-range hydrocarbon to useful
009 products which comprises:
010 ~1) separating said naphtha into a light naphtha fraction
011 boiling below methylcyclopentane and containing C6 aliphatics
012 and lower-boiling hydrocarbons, and a heavy naphtha fraction
013 containing methylcyclopentane and higher-boiling hydrocarbons;
014 (2) reforming said heavy naphtha fraction under reforming
015 conditions to produce a reformate stream enriched in aromatics
016 compared with said heavy naphtha fraction;
017 (3) passing at least a portion of said reforma~e in
018 contact with a ZSM-5-type catalyst at an elevated temperature
019 and recovering from the ZSM-5 contacting operation a
020 hydrocarbon effluent enriched in aromatic hydrocarbons; and
021 (4) separating said effluent into a C4- stream and a C5+
022 product stream.
023 DESCRIPTION OF THE FIGURES
024 Figures 1 and 2 are schematic block-type flow diagram
025 of preferred embodiments of the present invention in which each
026 block represents one particular step or zone oE the process.
027 Conventional items such as pumps, compressors, miscellaneous
028 valving, etc., are believed to be well within the skill in the
02~ art and have been omitted from the drawing. Likewise, with
030 respect to the piping throughout the process system, only the
031 major streams required to illustrate the relationships between
032 the various stages are presented. Accordingly, various recycle
033 lines and vent gas streams, etc., have also been omitted.
034 DETAILED DESCRIPTION
035 Referring to the flow diagram depicted in Figure 1, a
036 C5 to C8 straight-run naphtha obtained from an Alaskan North
037 Slope crude oil is charged through line 20 into fractionator
'Z7~
001 -4-
002 21. The naphtha has an approximate composition of 55 volume
003 percent paraffins, 35 volume percent naphthenes and 10 volume
004 percent aromatics. In the fractionator, the naphtha is sepa-
005 rated into a light fraction comprising C5 and C6 hydrocarbons
006 boiling under about 66C tl50F) and a heavy fraction including
007 methylcyclopentane boiling above about 66C ~150F). The light
008 naphtha frac~ion, about 20 volume percent of the feed, is
009 removed from fractionator 21 via line 25. The heavy naphtha
010 fraction, about 80 volume percent of the feed, is charged via
011 line 26 to reformer 27 in which it is reformed under conven-
012 tional mild reforming conditions with a platinum-rhenium-
013 chloride reforming catalyst (see, for example, U~S. Patent
014 3,415,737, inoorpor~t~ heroin by rcfer~ncc~ to increase the
015 aromatics content and octane number of the naphtha. The refor-
016 mate, in a yield of approximately 90 LV% and substantially
017 depleted of naphthenes, is passed to separation zone 30 via
018 line 28. In separation zone 30, which may comprise one or more
019 stages, hydrogen is recovered for recycle to the reformer (not
020 shown), light gases are removed and one or more reformate frac-
021 tions are recovered.
022 In one embodiment of the present invention, for maxi-
023 mizing BTX production, the naphtha feed is preferably a C5 to
024 C8 straight-run naphtha, and all of the C3+ reformate is passed
OZ5 via line 31, combined with the light naphtha fraction in line
026 25 and passed into ZSM reaction zone 38. In the ZSM reaction
027 zone, the normal and lightly branched paraffins undergo aromati-
028 zation, and aromatics, particularly xylenes, are isomerized.
029 The ZSM reaction zone is operated at reaction conditions
030 including a temperature of 538C (1000F), no added hydrogen,
031 and an LHSV of 2 V/V/Hr. The effluent from ZSM reaction zone
032 38 is passed via line 39 to separation zone 40 wherein a C5+
033 product stream in an amount of about 70 weight percent of the
034 combined feed in line 25 and rich in aromatics is separated and
035 sent via line 48 for recovery of benzene, toluene and xylene
036 values therefrom. The C4- component of the effluent is prefer-
037 ably separated into an H2/Cl/C2 fraction which is removed via
~Z71~
001 -5-
002 line 42 and a C3/C4 fraction, in an amount of about 18 weight
003 percent of the combined feed, which may be removed from the
004 process via line 47 but which is preferably recycled via line
005 46 to ZSM reaction zone 38. The C3/C4 fraction may contain
006 propylene and butenes in addition to propane and butane.
007 In another embodiment of the present invention, the
008 naphtha feed is preferably a C5 to C8 straight-run naphtha, and
009 the reformer effluent is separated in separation zone 30 into a
010 C3 to 135C - (275F-) fraction which is sent via line 31 to the
011 ZSM reaction zone operated at conditions to form and isomerize
012 aromatics, and a 135C+ t275F+) fraction rich in xylenes which
013 may be passed via line 32 to a conventional xylene isomeri-
014 zation zone, such as that described in U.S. Patent 3,948,758
015 ir;aorpor~ted horein by rcfcrcnc~. When the feed is a fu]l-
016 boiling-range naphtha, a Cg+ fraction may be removed from sepa-
017 ration zone 30 via line 33 and used as a high-octane (research
018 octane number of about 116 Clear) gasoline blending stock.
019 Advantages of this embodiment include a longer ZSM catalyst
020 life because higher-end-point hydrocarbons which tend to coke
021 the ZSM catalyst are not passed over it and a more efficient
022 use of the ZSM catalyst and reactor because feeding the C8+ aro-
023 matics to the ZSM reaction zone would not contribute to higher
024 aromatics yields and would reduce catalyst life.
025 In yet another embodiment of the present invention,
026 particularly useful for the production of high-octane gasoline,
027 a full-boiling-range straight-run naphtha is the feedstock, the
028 reformer effluent is separated in separation zone 30 into a
029 light reformate fraction tc3 to 104C or 220F), usually about
030 70 volume percent of the reformate, which is passed via line 31
031 to the ZSM reaction zone, and a heavy reformate fraction (lOC+
032 or 220F+), usually having a research octane number of about
033 106 (Clear), which is sent to a gasoline pool (not shown) via
034 line 32. The light reformate fraction in line 31 is combined
035 with the light naphtha fraction in line 25 and preferably also
036 with a recycle C3/C4 stream, and passed in contact with a ZSM-5-
037 type zeolite catalyst at reaction conditions previously men-
038 tioned. The effluent from zone 38 is passed via line 39 to
1~271~9L
001 -6-
002 separation zone 40, from which a C5+ fraction having a research
003 octane number of about 116 (Clear) is removed via line 48 and
004 combined with the heavy reformate fraction in line 32 to form a
005 high-octane gasoline blendin~ stock having a research octane
006 number of at least 90 (Clear), and preferably 95 and still more
007 preferably at least 100 or more.
008 An advantage of this embodiment is that in splitting
009 the reformate at 104C (220F), C7 paraffins having a low
010 octane number are included in the feedstock to the ZSM reaction
011 zone in which they will undergo aromatization, but must of the
012 toluene fraction is excluded since feeding it to the ZSM reac-
013 tion zone would not increase aromatics yield~. Another
014 advantage of this embodiment is that the reformer is used for
015 what it does efficiently and in high yield: dehydrocyclization
016 of naphthenes, while C6- paraffins are aromaticized over the
017 ZSM catalyt to form high-octane aromatic compounds rather than
018 cracked to C4- in the reformer, with the attendant yield loss.
019 Referring to the flow diagram depicted in Figure 2,
020 an Arabian straight-run naphtha fraction boiling in the range
021 of C5-193C (380F) is charged through line 121 into fractiona-
022 tor 122. The naphtha has an approximate composition of 66
023 volume percent paraffins, 2] volume percent naphthenes and 13
024 volume percent aromatics. In the fractionator, the naphtha is
025 separated at about 66C (150F) into a light naphtha fraction
026 comprising C5 and C6 hydrocarbons boiling under about 66C
027 (150F) and a heavy naphtha fraction boiling above about 66C
028 (1505F) which includes most of the methylcyclopentane. The
029 light naphtha fraction, about ]0 volume percent of the feed, is
030 remo~ed from fractionator 122 via line 124. The heavy naphtha
031 fraction, about 90 volume percent of the feed, is charged via
032 line 126 into reformer 127 in which it is reformed under conven-
033 tional reforming conditions with a platinum-rhenium-chloride
034 reforming catalyst (see, for examplel U.S. Patent 3,415,737,
035 inco~por~Qd ~cre~ eefcrcncc). The reformer acts to
036 increase the aromatics content and octane number of the
037 naphtha. The reformate, having a research octane number of
71~
001 ~7~
002 about 98, in a yield of approximately 83 liquid volume percent
003 and substantially depleted of naphthenes, is passed to sepa-
004 ration zone 129 via line 128. In separation zone 129, which
005 may comprise one or more stages, hydrogen is recovered for
006 recycle to the reformer (not shown), C4- light gases are
007 removed and the C5+ reformate is split into two fractions. The
008 first fraction is a C5-77C (170F) light reformate which is
009 removed from the separator via line 134. The light reformate
010 has a research octane number of about 71 and is about 10 liquid
011 volume percent of the total reformate.
012 The light reformate in line 134 is combined with the
013 light straight-run naphtha fraction having a research octane
014 number of about 65 in line 124 and passed as a composite stream
015 via line 124 into isomerization zone 150, where it is contacted
016 in the presence of hydrogen (supply line not shown) with a
017 hydrocarbon isomerization catalyst. The isomerization condi-
018 tions in zone 140 include a temperature of approximately 150C
019 (302F), a pressure of 15 atmospheres, a liquid hourly space
020 velocity of 1.5 V/V/Hr and a hydrogen to hydrocarbon mol ratio
021 of 3 . The resulting isomerizate is withdrawn in line 15] and
022 passed to separator 152 wherein it is separated into a C4- frac-
023 tion and a liquid hydrocarbon fraction in about a 95 liquid
024 volume percent yield and having a research octane number of
025 about 79. The C4- fraction is withdrawn from separator 152 via
026 line 153 and the hydrogen component therein may be separated
027 and recycled to isomerization zone 150, if desired. The C5+
028 fraction from separator 152 is withdrawn via line 156 and
029 passed to a gasoline pool as a high-octane gasoline blending
030 stock.
031 Referring again to separator 129, the portion of the
032 reformate boiling above 77C (170F) is passed via line 133 to
033 ZSM conversion zone 134. About 90 liquid volume percent of the
034 reformate leaves separator 129 via line 133 and this heavy
035 reformate fraction has a research octane number of approxi-
036 mately 100. In the ZSM reaction zone, the heavy reformate is
037 contacted with an H-ZSM-5 catalyst comprising 50% zeolite and
l~.Z7~
001 -8-
002 50% Catapal matrix. ZSM reaction conditions include a tempera-
003 ture of 343C (650F), a pressure of 28 atmospher~s (400 psig),
004 a hydrogen to hydrocarbon mol ratio of 5, and a liquid hourly
005 space velocity of 2 V/~/Hr. The ZSM reactor is operated at
006 reaction conditions to crack out paraffins, leaving mainly aro-
007 matics in the effluent, and to isomerize the xylenes as well as
008 generally upgrade the octane of the effluent. The effluent
009 from the ZSM reactor is withdrawn via line 35 and is charged to
010 separator 136. A C4- fraction is withdrawn from the top of the
011 separator via line 137 and a C5+ fraction is removed from sepa-
012 rator 136 via line 138. The C5+ fraction has a research octane
013 number of approximately 113, contains approximately 45 volume
014 percent BTX and represents about 85 volume percent of the feed
015 to the ZSM reaction zone. The C5+ fraction in line 138 may be
016 sent via line 140 to form a part of the high-octane gasoline
017 pool, or some or all of it may be sent via line 139 to a BTX
018 separation step, and if desired the BTX-depleted stream may be
019 returned to the gasoline pool.
020 The advantage to this flow scheme of the present
021 invention is that it allows the isomerization zone, the reform-
022 ing zone and the ZSM reaction zone each to be operated with the
023 feedstocks that are converted most effectively therein. The
024 CS-66C portion of the straight-run naphtha is more advan-
025 tageously isomerized than reformed, because the lower tempera-
026 tures in the isomerization zone favor production of more highly
027 branched paraffins with a resulting higher octane. In the
028 reformer, C5 and C6 paraffins would suffer a yield loss due to
029 cracking as well as having a relatively high equilibrium amount
030 of low-octane normal and slightly branched paraffins. Thus,
031 the process of the present invention provides for sending the
032 portion of the naphtha containing C6+ naphthenes (such as
033 methylcyclopentane) and higher-boiling hydrocarbons to the
034 reforming zone where the naphthenes are efficiently converted
035 to aromatics, while the light naphtha containing C5 and most of
036 the C6 paraffins is sent to the isomerization zone.
037 The advantage of splitting the reformate into a light
~1271~
001 -9-
002 fraction (boiling.from C5 to 77C) is that this fraction may be
003 isomerized to further increase the octane number with little
004 decrease in yield. The heavy reformate containing benzene and
005 higher-boiling hydrocarbons is advantageously contacted with
006 the ZSM catalyst at cracking conditions to remove any C7~ paraf-
007 fins and leave a product stream concentrated in aromatics such
008 as benzene, toluene and xylene.
009 Process Feeds
010 Feedstocks suitable for use in the process of the
011 present invention include full-boiling-range na~htha hydro-
012 carbon materials boiling in the range of C5 hydrocarbons up to
013 about 175C (347F) or 200C (392F) which contain low-octane
014 paraffinic C5 and C6 components and preferably are C5 to C8
015 straight-run naphthas. The C6 naphthenes in these naphthas are
016 excellent reformer feedstock components, for they are
017 efficiently converted to aromatics. The paraffins are undesir-
018 able as components of a gasoline pool because of their low
019 research octane numbers, usually under about 70 (Clear, ASTM
020 Method). The C7+ paraffins are suitable reformer feed compo-
021 nents, but the C6- paraffins are marginal because of the yield
022 loss when they are cracked and the e~uilibrium amount of rela-
023 tively low-octane normal and singly branched paraffins which
024 remain after isomerization.
025 Reforming Sta~e
026 The reformer of the present invention is a conven-
027 tional one in which the feedstock is contacted with a platinum-
028 containing reforming catalyst, preferably a bimetallic catalyst
029 such as platinum-rhenium-chloride on alumina, under reaction
030 conditions such as a temperature from 427 to 552C (800-
031 1025F), preferably from 454-538C (850-1000F), a pressure
032 from atmospheric to 50 atmospheres or higher, preferably from
033 6.8 to 40 atmospheres, a liquid hourly space velocity from Q.l
034 to 10, preferably from 0.5 to 5, and a hydrogen to hydrocarbon
035 mol ratio from 0.5 to 20, and preferably from 1 to 10. During
036 reforming a multitude of reactions takes place, including
~ 271Q~
dehydrogenation, isomerization, dehydrocyclization, hydrocracking, and com-
binations thereof to yield a product having an increased content of aro-
matics and branched-chain hydrocarbons. The reformer is especially efficient
when used to convert naphthenes to aromatics.
ZSM Reaction Zone
At least a portion of the reformate is passed over a ZSM-5-type
catalyst in a ZSM reaction zone operated at conditions including an elevated
temperature. Broadly; the temperature will be from 343 to 649C (650 to
1200F) and the pressure will be from 0.5 to 55 atmospheres and a liquid
hourly space velocity from 0.1 to 20 and preferably 1 to 5.
In the preferred embodiment in which the ZSM reaction zone is op-
erated at conditions to crack paraffins and isomerize C8 aromatics in the
feed, the preferred reaction conditions include a temperature from 371-482C
(700-900F), a hydrogen pressure from 1.7 to 55 atmospheres, and a hydrogen
to hydrocarbon mol ratio from 1 to 10 and preferably 3 to 8.
In the preferred embodiment in which the light naphtha fraction is
combined with at least a portion of the reformate and optionally a recycle
or externally supplied C3/C4 stream, the reaction zone is preferably main-
tained under reaction conditions promoting aromatization of paraffinic com-
poundsJ such as a temperature from 454 to 566C (850 to 1050F) and more
preferably from 500 to 540C (932 to 1004F), a pressure from 0.5 to 35
atmospheres or higher, more preferably from 1 to 10 atmospheres and still
more preferably 1 to 5 atmospheres, and preferably in the absence of added
hydrogen.
The ZSM-5-type zeolite itself is known in the art per se, and is
exemplified by ZSM-5, ZSM-8, ZSM-ll and ZSM-35 and other similar materials.
ZSM-5-type zeolites, described in United States Patents 3,702,886, 3,729,409
and 3,770,614, describe the ZSM-5 preparation, composition and use as well
as related information. The H-ZSM-5 or Zn-H-ZSM-5 forms of the ZSM-5-type
zeolites are
- 10 -
,. . .
2t7~
001
002 preferred for use herein and may be obtained by conventional
003 base and/or ion-exchange methods well known to the art. It is
004 especially beneficial with respect to the catalyst life and
005 coke formation for the ZSM-5 zeolite to have a silica-to-
006 alumina mol ratio from 40 to 160, and preferably from 60 to
007 120.
008 The catalyst of the ZSM reaction zone may be in any
009 convenient form, that is, as required for conventional fixed,
010 fluid or slurry usage. Preferably, the ZSM-5-type is a fixed-
011 bed type with the zeolite being composited with an inorganic
012 binder or matrix such as alumina, silica, silica-alumina mix-
013 tures, naturally occurring and conventionally processed clays,
014 e.g., kaolin and the like, as well as silica-magnesia, silica-
015 zirconia, etc., and mixtures of any of them. ~he composite is
016 preferably prepared by mixing the binder or matrix in the form
017 of a gel or a cogel with the zeolite, followed by shaping or ex-
018 truding to the desired form and size customary for the intended
019 use. The relative proportions of zeolite and binder may vary
020 widely, from 5% to 95% by weight, with preferably 35% to 80%
021 and more preferably about 65% of the composition being zeolite.
022 The preferred binder is alumina.
023 Isomerization Zone
024 The isomerization zone is used to isomerize C5 and C6
025 paraffins in the light naphtha and light reformate fractions.
026 Any suitable light paraffinic hydrocarbon isomerization cata-
027 lyst and method may be used and numerous suitable methods have
028 been described in the prior art and descriptions of representa-
029 tive methods are given in an article entitled "Advances in
030 Isomerization" by P. A. Lawrence et al, Proceedings of the
031 Seventh World Petroleum Congress, Volume IV, pp. 135-145,
032 Elsevier Publishing Company (1967). Other examples of isomer-
033 ization processes for paraffins appear in the following U.S.
034 Patents: 2,834,823, 3,190,939, 3,527,835, 3,577,479, 3,578,725,
035 and 3,789,082.
036 In a preferred embodiment, the isomerization is
037 carried out using as the catalyst a chlorided composite of
l~Z7~
001 -12-
002 platinum dispersed upon porous alumina (e.g., see U.S. Patent
003 3,789,082, Example 1). In another preferred embodiment, the
004 catalyst employed is a composite of palladium and ultra-stable
005 Y crystalline aluminosilicate molecular sieve in the H form
006 (see, for example, U.S. Patent 3,293,192). This catalyst is
007 prepared by any suitable method. For example an aqueous
008 solution of the palladium salt is admixed with acid-peptized
009 alumina hydrogel and, thereafter, the palladium is
010 gravimetrically precipitated in a finely divided form by ad-
011 mixing a minor amount of 1,2,3-benzotriazole in hydrogel (see
012 for example U.S. Patent 3,978,001). Next, the H-Y sieve is
013 ad-mixed with the hydrogel and the resulting composite is
014 shaped, dried and calcined for use. Sufficient amounts of the
015 components are used to provide on a dry basis for each 100
016 parts by weight a composite containing alumina, ultra-stable
017 Y-sieve and palladium in an amount of 35, 65 and 0.3 parts,
018 respectively. The relative amounts of these components may be
019 varied widely as in the conventional practice, and yet the
020 catalyst will be effective for isomerizing the feed herein.
021 The isomerization of the C5-C6 paraffinic hydrocarbon feed
022 under hydrocarbon isomerizing conditions per se is not
023 considered as inventive.
