US2921015A - Preparation of aromatics from a naphtha feed - Google Patents

Description

R. N. SHIRAS 2,921,015

PREPARATION OF AROMATICS FROM A NAPHTHA FEED Jan. 12, 1960 Filed July 29, 1957 HIS TTORNEY T in 5 m mm m N.

5 v 0Q m1 mm (0 II) I 0 1 INVENTOR mm mm on United States Patent PREPARATION OF AROMATICS FROM A NAPHTHA FEED Russell N. Shires, Oakland, Calif., assignor to Shell Development Company, New York, N.'Y., a corporation of Delaware Application July 29, 1957, Serial No. 674,789

2 Claims. (Cl. 208-79) This invention relates to an improved process: for the production and recovery of aromatic hydrocarbons from mixtures of hydrocarbons and to a method which permits the recovery of a heavy aromatic concentrate, and, ifdesired, a light aromatic concentrate.

Itis well known that aromatic hydrocarbons may be separated. from hydrocarbon mixtures containing them by -extraction with any of several available solvents. The eflicient extraction of the aromatics often is no great problem. as the solvent and conditions may be selected to obtain an extract phase containing substantially all or. only a part of the aromatics in the feed. A chief difliculty in. recovering aromatics by extraction is that most solvents selective for aromatics will also dissolve a sig-v nificant. amount of the non-aromatic material which gives rise to a problem in the fractionation of the extract as aromatics and non-aromatics of the same. boiling range cannot be efiiciently separated by the usual distillation techniques. As a result, the separated aromatic fractions are frequently not suitable for the more exacting needs. With the presently large requirement for concentrates for fuel blending, there is a cleardernand for processes that will permit a more sharp separation of aromatics and non-aromatic hydrocarbons.

It is an object of the present invention to provide an improved methodfor the separation of aromatic hydrocarbons from non-aromatic hydrocarbons and in particular for the preparation of aromatic concentrates. Another. object of the present invention istofurnish a processwhich may be used for the joint production of both aheavy aromatic concentrate and a light. aromatic concentrate.

The improved process of this invention is especially suitablefor the separation of aromatic concentrates from catalytically reformed, e.g. hydroformed, such as platformed naphtha feeds. As pointed out earlier, an acute problem exists. in separating aromatics and saturates of thesameboiling range; however, by the practiceof the process. ofthe instant invention, the two types of compounds may be readily and efficiently segregated.

The advantages of the present invention and its prac.

tice will be better understood from the detailed description of it which will be made with reference to the accompanying drawing wherein the sole figure is a schematicrepresentation of a preferred system for the practice of the improved process.

Thepresent invention provides a process in which the naphtha feed is first subjected to a sizing operation to divide the feed into a predominantly light saturate naphtha fraction and into a heavy naphtha fraction made up principally of heavy saturates. The heavy naphtha fraction is then subjected to aromatization, for example, by hydroforming over a platinum-on-alumina catalyst to increase appreciably its heavy aromatic content and of course simultaneously to reduce its heavy saturate content. The aromatized heavy naphtha is then passed to an ,,inter mediate section of a countercurrent extractionv zone. A selective solvent for aromatic hydrocarbons.

which is relatively immiscible with non-aromatic hydro-. carbons is admitted to one end of the zone in extractive contact with the aromatized heavy naphtha. vent should have a relatively selective solubility for the heavy aromatics at the elevated temperature of the ex-. traction, preferably in excess of 300 F., and a low solubility at the temperature of operation of the desorbing zone described infra, generally in the neighborhood of F. Since the solvent and extracted material are not separated by distillation, there is greater latitude in the selection of the. solvent because no spread in volatility is essential. This may prove to be an advantage where the naphtha being processed extends into the kerosene range. Among the preferred solvents are the glycols, includingthe polyglycols, and particularly diethylene glycol. There is formed in the extraction zone an aromatic-enriched extract phase and a raffinate phase containing predominantly heavy saturates. The process employs a countersolvent in this first extraction zone, Which is a portion of the light naphtha from the earlier mentioned naphtha feed fractionation, to bring about a higher rejection of the heavy saturates from the extract phase. The extract phase upon its removal from the extraction zone is cooled to effect the formation of a solvent phase and a hydro-. carbon phase and the cooled material is then passed to the top of a desorbing (second solvent extraction) zone. Another portion of the light naphtha (from the naphtha feed fraction) is admitted to the bottom of this desorbing zone to obtain a desorbed solvent which is substantially free of heavy aromatics and a raffinate containing principally heavy aromatics and light naphtha. The rafiinate is fractionated into a heavy aromatic concentrate and a light naphtha fraction. The light naphtha fraction may be used as a gasoline blending stock or as a source of specialties but it is preferred to process it further to provide a source of light aromatics, as by catalytic reforming or to subject it to an isomerization process to produce high octane, branched isomers.

Various ones of the common selective solvents for aromatics, e.g. phenol, nitrobenzene, the sulfolanes, aceto nitrile, furfural, or one of the several glycols may be used. A preferred solvent is diethylene glycol which may be used alone or diluted with, say 2 to 10% by weight of water. Representative of others of the glycol solvents which may be utilized with somewhat less ad-.

vantageous results are ethylene glycol, triethylene glycol,

suitable sulfolanes are sulfolane itself and many of their derivatives such as hydrocarbon-substituted sulfolanes, including alkyl sulfolanes, e.g. 3-methylsulfolane, pref erably containing not more than 14 carbon atoms; hydroxy sulfolanes such as S-sulfolanol, 3-methyl-4-sulfolanol, etc. sulfolanyl others such as methyl-3-sulfolanyl ether; and sulfolanyl esters such as 3-sulfolanyl acetate.

It is preferred that the naphtha feed (before catalytically reforming) be debutanized and have a boiling range within about 60 F. to 400 F. The feed is preferably divided into a heavy naphtha fraction containing prim-- cipally the C and heavier saturates and the C and heavier aromatics and into a light naphtha fraction made up principally of the C and lighter saturates and C and C aromatics. The C saturate hydrocarbons will preferably be split between the two fractions and generally the normal C saturates will be found in the heavy naphtha fraction and the branched C saturates in the light naphtha fraction.

The aromatization of the heavy naphtha feed may be had by catalytically reforming, for example, by either hydroforming or platforming in accordance with standard operating conditions.

matic, say 60% aromatic and 40% saturates. It is e'ssen- The 501-.

It is preferred that the reformed material should have a composition which is chiefly aro ran'e as the aromatics of the reforrnate be removed therefrom. This removal is facilitated in the extraction zone by the use of a portion of the light naphtha (from the naphtha feed fractionation) as countersolvent. In such usage the light saturates of the countersolvent tend to displace the heavy saturates dissolved in the extract phase. This supplanting makes for better separation of the saturates and the aromatics in the subsequent fractionation procedure. The separation afforded by simple solvent extracting in the absence of the light saturate countersolvent is not adequate for a sharp subsequent fractionation of saturates and aromatics. The rafiinate from the extraction zone, which contains chiefly heavy saturates, may be recycled in part or wholly to the aromatization step. The extract is stripped from the solvent by a desorbing procedure promoted by cooling of the extract phase. Preferably, the extract phase out of the extraction zone is passed in indirect heat exchange with desorbed solvent from the desorbing zone. This procedure assists in the needed cooling of the extract phase and simultaneously raises the temperature of the recycled solvent which is introduced to the extraction zone at an elevated temperature. The extract phase in the desorbing zone is counter extracted with an additional portion of the light naphtha to assist in the stripping of the solvent. It may be desirable in some instances to strip at least a portion of the solvent (from the desorber) with a quantity of heavy naphtha which may then be recycled to form a portion of the naphtha feed. This additional stripping of the desorbed solvent will lower still further its light saturate content.

The rafiinate from the extraction zone is preferably water-washed to recover the solvent dissolved therein and the heavy aromatic and light naphtha raflinate' from the desorbing zone is likewise water-washed to reduce solvent loss. The latter water-washed raflinate is passed to a conventional fractionating tower where it is readily separated into an overhead light naphtha fraction and into a heavy aromatic concentrate. The heavy aromatic concentrate will generally constitute approximately an 85% recovery of the aromatics from the reforming (aromatization). The sharp separation of saturates and aromatics is made possible because of the earlier processing steps devoted to the removal of heavy saturates from the aromatic stream. i

In the following description of a system for performing the improved process of the invention, the solvent used is diethylene glycol which is the preferred material; it is to be understood, however, that others of the common high boiling aromatic solvents may be utilized, in which event the conditions of operation will be suitably adjusted.

Referring to the drawing, a debutanized naphtha feed (having preferably about a 60-390 F. boiling range) of say 1,000 barrels per day is introduced through a line to a central section of a fractionating tower or naphtha splitter 11. The bottom of the tower is preferably maintained at a temperature suitable to give a bottom product having a boiling range of approximately 250.-390 F.

while the top of the tower is operated to provide a dis-.

tillate with the boiling range of approximately 60250 F. The naphtha feed is split into a heavy naphtha fraction of say 440 barrels per day and a light naphtha fraction of 560 barrels per day. The proportions of the light and heavy fractions may be varied somewhat but generally a split providing 40 to 60% heavy naphtha gives best results.

. The heavy naphtha fraction is passed by a line 12 to a catalytic reforming unit 14 where it is subjected to a conventional reforming operation to increase significantly its aromaticcontent to say 60%. The reformate is removed from the reformer through a line 15 which opens into a central portion of a reformate extractor 17. This extractor is operated at an elevated temperature and when diethylene glycol is employed as a solvent, a suitable temperature is about 350375 F. The solvent enters the extraction zone at its top through a conduit 18, from whence it passes in counterflow to the reformate, dissolving a principal portion of the heavy aromatics produced in the reformer. The heavy saturates of the reformate pass upwardly through the extractor and leave in the rafiinate phase through a conduit 20. A portion of the light naphtha fraction from the splitter 11, say about 124 barrelsper day is introduced by a line 13 to the base of the extractor as a counter solvent, i.e. a solvent selective for paraffin hydrocarbons and at least partially immiscible with the polar solvent. The light naphtha substantially supplants the heavy saturates that are contained in the extract phase.

Theraffinate from the extractor will contain princi pally heavy saturates, a small amount of heavy aromatics and some light naphtha. In a typical operation based on 1,000 barrels per day of naphtha feed (and which employs a moderate recycling of of the rafiinate to the platformer) the rafiinate may be expected to comprise approximately 159 barrels per day of heavy saturates, 32 barrels per day of heavy aromatics and 44 barrels per day of light naphtha. The raffinate is waterwashed in a tower 22 to remove therefrom the small amount of solvent that it contains and leaves that tower via line 92 which branches into lines 93 and 94. At times it may be desirable to recycle a substantial quantity of the heavy saturate raflinate via the line 93 back to the reformer. In other instances it may be desirable to transfer all or a portion of the rafilnate to a product tank 95. The wash water leaves the washing tower 22 through a line 24 and is introduced to the bottom portion of a water solvent fractionating tower 26 via a line 27. Water is removed overhead by a line 29 and the recovered solvent is returned via line 30 to the solvent system. The extract phase exits from the bottom of the extractor in a line 32 which first opens into a heat exchanger 33 and from the latter unit, the still warm extract phase passes to a water cooler 34, from whence it is moved through line 36 to a top portion of a desorber tower 37. A typical composition of the extract phase (again based on 1,000 barrels of naphtha feed daily) will be about 120 barrels of light naphtha, l1 barrels of heavy saturates, 216 barrels of heavy aromatics and approxivfor aromatics is quite temperature dependent and with this appreciable drop in temperature, the extract phase forms asolvent phase and a hydrocarbon phase. The introduced cooled material to the desorber tower forms an overlying hydrocarbon layer which is removed from the tower through a line 39. The desorbed solvent leaves the tower at its base through a line 41 which carries the cool solvent through the aforementioned heat exchanger 33 where its temperature is raised to the neighborhood of 350 F. and from whence it passes through a second heat exchanger 43, steam heated, where its temperature is further elevated to the operating temperature of say 375 F. of the extractor tower. A second portion of the light naphtha fraction from the naphtha splitter 11 is transferred through a line 45 to a bottom section of the desorber tower 37. This material which is introduced in an appreciable quantity, say 240 barrels per day (again based on 1,000 barrels per day naphtha feed) passes countercurrently to the downwardly descending solvent within the desorber. The light naphtha material acts as a desorbent and assists in the stripping of the dissolved aromatic hydrocarbons from the solvent.

, V In some iii'stancesin'order"to loweithe lightnaphtha loss tojtlie" heavy" saturate raffinate 'of -the extractor 17, itwill 'lieeonie desirable to'pa's's at" leastf'a portion of the desorbe'd solyent'from the tower 37 through a line 47 to the 16p of awash tower 49- where the solvent is passed in" counter-current flow to "a"heavy-saturate stream introduced'to thebase of that'towervia a line 50. The heavy naphtha employed" heremaybe a portion of the heavy naphtha from the splitter 11 or it may be a portion of the heavy saturate recycled from the raflinate wash tower 22 in which event it is transferred to line 50 via line 51 which is a continuation of the heavy recycle line 93. This countercurrent washing of the solvent with the heavy naphtha will lower still further the hydrocarbon content of the solvent. The stripped solvent exits from the base of the hydrocarbon wash tower 49 through a line 52 which opens into the earlier mentioned line 41 from the desorber at a point immediately preceding the heat exchanger 33. The hydrocarbon wash materialis removed from the tower 49 by a line 54 to the base of a water wash tower 56. The purpose of this latter tower is to recover any solvent that may have been carried over with the hydrocarbon stream from the preceding tower 49. The wash water for this purpose is admitted to the top of the tower 56 by a line 58. The solvent-containing wash water is transferred via a line 59 to the earlier mentioned water-solvent fractionating tower 26 (or to a separate fractionating tower). The waterwashed heavy aromatic material from tower 56 is removed by a line 97 and added to the naphtha feed in line at a point preceding the splitter 11. The heavy naphtha material is recycled in this fashion since it contains a portion of light naphtha and should consequently be resplit before undergoing further processing.

The raflinate contained in the line 39 is transferred to the base of a waterwash tower 61 where it is passed in 'countercurrent flow to downwardly descending water, extracting any solvent that may be present and is removed through a line 63 from the top of the tower. On a daily basis of this example the rafiinate is made up of approximately 208 barrels of heavy aromatic and 300 barrels of light naphtha. These two principal components of the raffinate have significantly different boiling ranges which difference permits an effective and sharp fractionation of the ralfinate in a distillation column 66 which is designed and operated under conditions to achieve recovery of about 98% of heavy aromatics in the feed to this column 66 and approximately a 93% rejection of the light naphthas of the column feed. For example, the column may have say 60 bubble trays and be operated with from 2 to 2.5 moles of reflux per mole of column feed. The heavy aromatic concentrate which is removed to storage 68 as a liquid from the base of the tower through a line 67 will generally represent an overall recovery of approximately 85% of the heavy aromatics originally contained in the reformate.

The light naphtha feed leaves the tower through a line 69 which combines with a line 70 from the top of the naphtha feed splitter 11 to form a line 71. The line 70 transfers the excess light naphtha which has not been utilized either as counter solvent in the extractor 17 or as desorbant in the desorbing tower 37. The quantity of 7 flow through the excess light naphtha line 70 will vary from time to time and there may even be instances where no material is transferred therethrough depending on how the naphtha feed is split and the proportions of light naphtha used as counter solvent and desorbant. The light naphtha carried by the combined line 71 may be transferred to a storage tank 73 from which it may be removed as needed for use, say as a gasoline blending stock or as a specialty source. In an alternative the material may be further processed in a separate light aromatic synthesis plant which may or may not involve 6 naphtha isbmerization andmay be'designed' for the" pro= duction of nitration grade'benzeneand/or toluene.

In' the'drawing" the light naphtha of line 71 is passedto an aromatizatio'n unit 75 where the material is sub-' jected to aconventional processing. to furnish a catalytically reformed material containing a large quantity of light aromatics. The'aromatized material flows through a line 76 to a solventextractor 77 where it is contacted with a solventgpreferably the same solvent employed for extracting. the heavy. naphtha. This solvent enters the extractor adjacent it s top through a line 78. The various solvents and conditions discussed earlier in connection with the other extraction may be employed here and again the solvent should be thermally stable as the extraction is preferably carried on atan elevated temperature. The extract phase leaves from the base of the tower via conduit 79 and is passed through the heat exchanger 81 against the lean solvent carried by the line 78. The extract phase is introduced to a central section of a solvent stripping tower 83. Heat is supplied to the stripping section at the base of the tower through a reboiler 84. A light aromatic stream is removed at an intermediate point of the tower through a line 85 and passed to storage 95. A gaseous stream containing a higher concentration of light saturates is removed from the top of the stripping tower through a line 87 and returned to the base of the solvent extractor 77 where it serves as a back wash, passing in countercurrent flow to the downwardly descending extract phase. Reflux for the separations made in tower 83 is provided by the condenser 86. The temperature of operation of the tower 83 and amount of reflux will depend chiefly on the particular solvent employed and the composition of the extract. A light saturate rafiinate is transferred from the top of the extraction tower 77 in a conduit 88 to a water washing tower 89 where it is passed in countercurrent flow to water introduced through a line 90. Solvent containing wash water is transferred from the Wash tower by a line 91 to the previously described Water solvent distillation column 26. The light saturate stream out of the wash tower 89 may be recycled in part as a portion of the feed to the reformer 75 through a line 93 and the line 71 or it may be moved in part or in whole via a line 94 to storage or further processing.

1 claim as my invention:

1'. A process for the production of aromatics from a naphtha feed which comprises dividing the naphtha feed into a light naphtha fraction containing predominantly light saturates and into a heavy naphtha fraction containing predominantly heavy saturates; subjecting the heavy naphtha to aromatization, thereby increasing appreciably its heavy aromatic content and reducing its heavy saturate content; introducing the aromatized heavy naphtha into a counter flow extractive zone at an intermediate point thereof; admitting a polar solvent selective for aromatics and relatively immiscible with non-aromatic hydrocarbons into one end of the zone in extractive contact with the aromatized heavy naphtha to form an aromatic-enriched extract phase and a rafiinate phase containing predominantly heavy saturates and introducing to the other end of the extraction zone a counter solvent comprising a portion of said light naphtha, thus effecting a higher rejection of the heavy saturates from the extract phase; withdrawing the extract phase from said other end of the zone, cooling said extract phase to effect the formation of a solvent phase and a hydrocarbon phase, and passing the cooled material to a desorbing zone; sweeping the desorbing zone with another portion of said light naphtha to obtain a solvent substantially free of the heavy aromatic and a raffinate containing heavier aromatics and light naphtha; fractionating the latter rafl'lnate into a heavy aromatic concentrate and a light naphtha fraction; subjecting the light naphtha fraction to an aromatization operation, thereby providing a reformed light naphtha rich in light aromatics; extracting the light aremade-enriched reformed material :with a high boiling preferential solvent for aromatics to obtain a light saturate-enriched rafiinate and an aromatic-enriched extract phase; and passing the latter extract phase to a fractionation zone and there recovering alight aromatic concentrate.

2. A process in accordance with claim 1 wherein at least a portion of the light saturate rafiinate is recycled to said light naphtha aromatization operation.

References Cited in the file of this patent UNITED STATES PATENTS 2,176,396 Fenske et al., Oct. 17, 1939 8 Haensel et al. Nov. 17, 1953 Haensel Jan. 4, 1955 Shelton et al Nov. 22, 1955 Georgian Dec; 20, 1955 Evans Nov.6 1956 Grote L. Nov. 13, 1956 Kassel et al. June 10, 1958 Haensel Sept. 23,1958

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF AROMATICS FROM A NAPHTHA FEED WHICH COMPRISES DIVIDING THE NAPHTHA FEED INTO A LIGHT NAPHTHA FRACTION CONTAINING PREDOMINANTLY LIGHT SATURATES AND INTO A HEAVY NAPHTHA FRACTION CONTAINING PREDOMINANTLY HEAVY SATURATES, SUBJECTING THE HEAVY NAPHTHA TO AROMATIZATION, THEREBY INCREASING APPRECIABLY ITS HEAVY AROMATIC CONTENTENT AND REDUCING ITS HEAVY SATURATE CONTENT, INTRODUCING THE AROMATIZED HEAVY NAPHTHA INTO A COUNTER FLOW EXTRACTIVE ZONE AT AN INTERMEDIATE POINT THEREOF, ADMITTING A POLAR SOLVENT SELECTIVE FOR AROMATICS AND RELATIVELY IMMISCIBLE WITH NON-AROMATIC HYDROCARBONS INTO ONE END OF THE ZONE IN EXTRACTIVE CONTACT WITH THE AROMATIZED HEAVY NAPHTHA TO FORM AN AROMATIC-ENRICHED EXTRACT PHASE AND A RAFFINATE PHASE CONTAINING PREDOMINANTLY HEAVY SATURATES AND INTRODUCING TO THE OTHER END OF THE EXTRACTION ZONE A COUNTER SOLVENT COMPRISING A PORTION OF SAID LIGHT NAPHTHA, THUS EFFECTING A HIGHER REJECTION OF THE HEAVY SATURATES FROM THE EXTRACT PHASE, WITHDRAWING THE EXTRACT PHASE FROM SAID OTHER END OF THE ZONE, COOLING SAID EXTRACT PHASE TO EFFECT THE FORMATION OF A SOLVENT PHASE AND A HYDROCARBON PHASE, AND PASSING THE COOLED MATERIAL TO A DESORBING ZONE, SWEEPING THE DESORBING ZONE WITH ANOTHER PORTION OF SAID LIGHT NAPHTHA TO OBTAIN A SOLVENT SUBSTANTIALLY FREE OF THE HEAVY AROMATIC AND A RAFFINATE CONTAINING HEAVIER AROMATICS AND LIGHT NAPHTHA, FRACTIONATING THE LATTER RAFFINATE INTO A HEAVY AROMATIC CONCENTRATE AND A LIGHT NAPHTHA FRACTION, SUBJECTING THE LIGHT NAPHTHA FRACTION TO AN AROMATIZATION OPERATION, THEREBY PROVIDING A REFORMED LIGHT NAPHTHA RICH IN LIGHT AROMATICS, EXTRACTING THE LIGHT AROMATIC-ENRICHED REFORMED MATERIAL WITH A HIGH BOILING PREFERENTIAL SOLVENT FOR AROMATICS TO OBTAIN A LIGHT SATURATE-ENRICHED RAFFINATE AND AN AROMATIC-ENRICHED EXTRACT PHASE, AND PASSING THE LATTER EXTRACT PHASE TO A FRACTIONATION ZONE AND THERE RECOVERING A LIGHT AROMATIC CONCENTRATE.

Images