CA2214305A1 - Process for the separation of aromatic hydrocarbons from a mixed feed stock - Google Patents
Process for the separation of aromatic hydrocarbons from a mixed feed stock Download PDFInfo
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- CA2214305A1 CA2214305A1 CA 2214305 CA2214305A CA2214305A1 CA 2214305 A1 CA2214305 A1 CA 2214305A1 CA 2214305 CA2214305 CA 2214305 CA 2214305 A CA2214305 A CA 2214305A CA 2214305 A1 CA2214305 A1 CA 2214305A1
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- miscible solvent
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G21/00—Refining of hydrocarbon oils, in the absence of hydrogen, by extraction with selective solvents
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C7/00—Purification; Separation; Use of additives
- C07C7/04—Purification; Separation; Use of additives by distillation
- C07C7/05—Purification; Separation; Use of additives by distillation with the aid of auxiliary compounds
- C07C7/08—Purification; Separation; Use of additives by distillation with the aid of auxiliary compounds by extractive distillation
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- C—CHEMISTRY; METALLURGY
- C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
- C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- C10G7/00—Distillation of hydrocarbon oils
- C10G7/08—Azeotropic or extractive distillation
Abstract
The capacity of a continuous combined solvent extraction, distillation process for the separation of aromatics from a mixed feed stream may be increased by one or more of:
(1) recycling a portion, typically from 20 to 45% of a recycle stream of water and a water miscible solvent to the feed stream prior to the extractor;
(2) adding 3 to 10 weight % of a saturated non-cyclic C3-5 stream to the feed stock prior to the extractor; and (3) providing a side reboiler to the stripper column to add 10 -50%
of the total heat requirement at the extractor draw location.
(1) recycling a portion, typically from 20 to 45% of a recycle stream of water and a water miscible solvent to the feed stream prior to the extractor;
(2) adding 3 to 10 weight % of a saturated non-cyclic C3-5 stream to the feed stock prior to the extractor; and (3) providing a side reboiler to the stripper column to add 10 -50%
of the total heat requirement at the extractor draw location.
Description
CA 0221430~ 1997-08-29 Fl ocess For The Separation Of Aromatic Hydrocarbons From A Mixed Feed Stock FIELD OF THE INVENTION
This invention relates to a continuous solvent extraction-steam-distillation process for the recovery of aromatic hydrocarbons from a feed stream or feed stock containing such aromatic hydrocarbons and non-aromatic hydrocarbons. The invention relates to the recovery of mixtures of benzene, toluene, xylenes (BTX) and other aromatics typically C6 12 at purity levels required for petrochemical uses. More particularly, the subject invention relates to one or more of the following steps: recycling a portion of the stream of water and water miscible solvent directly to the feed stream prior to the extractor;
adding saturated non-cyclic (acyclic) C5's to the feed prior to the extractor and adding a side reboiler to the extractor draw location of the stripper.
BACKGROUND OF THE INVENTION
Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feed stocks including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feed stock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, tt/jm/spec/9137can.doc 2 CA 0221430~ 1997-08-29 and an extract phase comprising solvent having aromatic components dissolved therein.
An important consideration in the operation of an aromatic extraction process beyond the recovery and the purity of the products is increasing the capacity in a cost efficient manner.
The solvent extraction and stripping process to recover BTX
o from a stream of one or more aliphatic compounds is quite well developed. There is some art on recycling part of the raffinate (aliphatic) stream back to the feed stock and there is some art on feeding part of the aromatic side stream (from the stripper) back to the feed stream. Applicant has been unable to locate any art on directly feeding part of the water and water miscible recycle stream directly to the feed stream prior to delivery to the middle of the extractor.
U.S. patent 3,779,904 issued Dec. 18, 1973 (now expired) assigned to Union Carbide Corporation teaches that a portion of the light raffinate stream is fed to the feed stream so that its content is from 5 to 12 weight % (whereas the original feed stream had no more than 1 to 4 weight % of light aliphatics). The patent does not teach feeding a portion of the recycled mixture of water and water miscible solvent (lean solvent) to the feed stream prior to the extractor column.
The patent also teaches a reboiler (28) at the bottom of the stripper column and a reboiler 63 at the bottom of a column to separate the light aliphatics and the heavier aliphatics. However, the patent does not suggest the use of a reboiler about the middle of the stripper tt/jm/spec/9137can.doc 3 CA 0221430~ 1997-08-29 column. The reference teaches away form the major aspects of the present invention.
U.S. patent 3,789,077 issued Jan. 29, 1974 (now expired) assigned to Union Carbide Corporation teaches recycling part of the aromatic stream through line 144 to the feed prior to entering the extractor column (see col. 8, lines 55-60). The patent does not teach o recycling the "lean" solvent to the feed. Rather the lean solvent from the bottom of the extractor column is recycled directly to the top of the primary extractor. The patent does not teach or suggest that the "lean solvent" could be added to the feed stream prior to the extractor column.
U.S. 3,966,589 issued June 29, 1976 (now expired) assigned to Union Carbide Corporation teaches recycling a part of the aromatic stream back to the feed stream (via line 16, see col. 9 lines 53-58).
There are reboilers in the process (28) but they deliver product below the bottom plate of the stripper (col. 8, lines 45-50). The patent fails to teach or disclose feeding part of the recycle stream of lean solvent to the feed stock prior to the extractor. Rather the recycle stream of water and water miscible solvent is recycled to the top of the extractor.
U.s. patent 3,714,033 issued Jan. 30, 1973 (now expired) assigned to Union Carbide Corporation is also of interest. The recycle of the "neat or lean" solvent is to the top of the extractor column (col. 5, lines 63-65 and col. 7 lines 50-60). However, the patent teaches recycling an aromatic slip stream to the feed (through line 16, col. 6, lines 64-70). Again the patent teaches against the subject matter of IU;" ,/~.ec/~1 37can.doc 4 CA 0221430~ 1997-08-29 the present patent application as it teaches feeding the neat solvent to the top of the extractor without prior contact with the feed stock.
SUMMARY OF THE INVENTION
The present invention seeks to provide higher capacity for the removal of aromatic hydrocarbons from aliphatic hydrocarbons on existing or new solvent extraction and separation processes.
Preferably the process is carried out using a single extraction column and a single separator or stripper (steam stripping column).
Accordingly, in one embodiment, the present invention provides in a continuous solvent extraction process for the recovery of aromatic hydrocarbons having a boiling point in the range from 80~C to 175~C
from a feed stream comprising said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising contacting said feed stream with a mixture of water and a water miscible organic liquid having a boiling point of at least 200~C; in an extractor column having from 3 to 25 theoretical plates at a temperature from 100~C to 200~C
and at a pressure from 75 psig to 200 psig and removing at least 95%
of said aliphatics from feed stream and passing the resulting azeotropic mixture of water, water miscible solvent and aromatic hydrocarbons to stripper column operated at temperatures from 90~C
to 200~C at a pressure from 5 psig to 35 psig to separate at least 95%
of said water and water miscible solvent from said aromatic hydrocarbons and generating a recycle stream of water and water miscible solvent, the improvement comprising feeding from 20 to 45 tt/jrn/spec/9137can.doc 5 CA 0221430~ 1997-08-29 weight % of said recycle stream of water and water miscible solvent into said feed stream prior to said extractor and feeding the recycle water stream and the balance of the recycle stream of water and water miscible solvent to the head of the extractor column.
In a further embodiment the present invention provides a continuous solvent extraction-distillation process for the recovery of aromatic hydrocarbons having boiling points in the range of about 80~C
to about 175~C from a feed stream comprising at least about 40 weight % of said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising:
(a) contacting said feed stream with from 20 to 45 weight % of a recycle stream of water and a water miscible solvent having a boiling point of at least 200~C and a decomposition temperature~0 of at least 225~C;
(aa) preferably adding to said feed stock 3-10 weight % of an aliphatic non-cyclic C3 5 stream (preferably obtained from an existing or new C5 column) prior to the extractor;
(b) introducing the feed stock and said 20 to 45 weight % of a recycle stream of water and a water miscible solvent (and preferably containing said 3-10 weight % of non-cyclic C5's) into the middle of a single extractor column having from 3 to 25 theoretical plates operated at a temperature from 100~C to 200~C and at a pressure from 75 psig to 200 psig;
tt/jm/spec/9137can.doc 6 CA 022l430~ l997-08-29 (c) contacting the feed stream and said 20 to 45 weight % of a recycle stream of water and water miscible solvent together with reflux hydrocarbons introduced into the extractor column below the bottom theoretical plate in the extractor zone with the remaining 80 to 55 weight % of said recycle stream of water and a water miscible solvent introduced at the top of said extractor in a counter current flow to provide an overhead raffinate stream comprising at least 95% of aliphatic hydrocarbons and a bottom stream comprising aromatic hydrocarbons, aliphatic hydrocarbons, water and the water miscible hydrocarbon solvent;
(d) feeding the bottom stream from the extractor to the top of a steam stripper operated at a temperature from 90~C to 200~C
and at a pressure from 5 psig to 35 psig to provide an overhead distillate comprising a reflux hydrocarbon stream; a side stream of a predominantly aromatic phase and a bottom stream of water and a water miscible solvent;
(e) recycling said reflux hydrocarbon stream to below the bottom plate of said extractor;
(f) dividing said recycle stream of water and water miscible solvent into two portions comprising from 20 to 45 weight % of said stream and from 80 to 55 weight % of said stream;
(g) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 20 to 45 weight % of said recycle stream to said feed stream in step (a); and l~",/~ec/9137can.doc 7 CA 0221430~ 1997-08-29 (h) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 80 to 55 weight % of said recycle stream to the top of the extractor in step (c).
Most preferably a side reboiler is added to the stripper at the extract vapor draw location to add 10 to 50% of the stripper heat requirement.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic drawing of a process according to the present invention.
BEST MODE
Hydrocarbon feed stocks suitable for utilization in the method of the present invention include many different aromatic-aliphatic mixtures having a sufficient concentration of aromatic hydrocarbons to justify the recovery of the aromatic hydrocarbons as a separate product stream. The present invention is particularly applicable to hydrocarbon feed mixtures containing at least 15% by weight aromatic hydrocarbons. Typical aromatic feed stock charged to an extraction step will contain from about 25% to about 80% by weight aromatic hydrocarbons with aromatic hydrocarbon concentrations as high as 3 0 98% being suitable in some instances. A suitable carbon range for the hydrocarbon feed stock is from about 5 carbon atoms per molecule to about 20 carbon atoms per molecule, preferably from 5 to 12 carbon atoms per molecule.
One suitable source of hydrocarbon feed stock is the output from a conventional catalytic unit for reforming a naphtha feed stock.
tt~m/spec/9137can.doc 8 CA 0221430~ 1997-08-29 Another suitable source of feed stock is the liquid by-product from a pyrolysis gasoline unit which has been hydrotreated to saturate olefins and diolefins, thereby producing an aromatic hydrocarbon concentrate suitable for the solvent extraction technique described herein.
Typically the hydrocarbons in the feed stock will have boiling points from about 25~C to 1 75~C. The aliphatic portion may include o cyclic and straight and branched chained paraffins and naphthenes such as pentane, cyclopentane, hexane, cyclohexane, n-heptane, isooctane and methyl cyclohexane. The aromatic fraction (BTX) includes benzene, toluene, and C8 aromatics such as ortho-xylene, meta-xylene and para-xylene, and ethyl benzene and the Cg aromatics.
Solvent compositions which may be utilized in the practice of the present invention are those selected from solvents having a high selectivity for aromatic hydrocarbons. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group, such as a hydroxyl, amino, cyano, carboxyl or nitro radical. In order to be effective, the organic compounds of the solvent composition having the polar radical must have a boiling point substantially greater than the boiling point of water since water is typically included in the solvent composition for enhancing its selectivity. In general, the solvent must also have a boiling point greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture.
Typically the solvent should have a boiling point of at least 200~C and a decomposition temperature greater than 225~C.
Ilfi, I ,/s~,ec/S1 37can.doc 9 CA 0221430~ 1997-08-29 Organic compounds suitable for use as part of the solvent composition are preferably selected from the group of those organic-containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The mono- and poly-alkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms, such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl and butyl ethers of the glycol hydroxyl groups and the acetic acid esters and mixtures of the above, constitute a satisfactory class of organic solvents useful in admixture with water as the solvent composition for use in the present invention. Most preferably the water miscible solvent will be selected from the group consisting of dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monomethyl ether, sulfolane, N-methyl pyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycol diethyl ether, propylene glycol monoethyl ether, pentaethylene glycol, hexaethylene glycol and mixtures thereof. A
particularly preferred water miscible solvent is tetraethylene glycol.
The water miscible solvent may be used alone or in conjunction with one or more co-solvents. Suitable co-solvents include amides such as formamide, acetamide, dimethylformamide, diethylformamide, and diethylacetamide; amines such as ethylenetiramine and tt/jm/spec/9137can.doc 1 0 CA 0221430~ 1997-08-29 triethyenetetramine; alkanoiamines such as nonoethanolamine, diethanolamine and triethanolamine; nitriles such as ~"~1_ oxydipropionitrile and ~,~1-thiodipropionitrile; phenol and the cresols;
the methyl sulfolanes; sulfoxides such as dimethyl sulfoxide and diethyl sulfoxide and lactones such as ~-propiolactone and ~-butyrolactone.
The aromatic selectivity of the solvent may be improved by the addition of water to the solvent. The solvents utilized in the practice of this invention could contain small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbons without substantially reducing the solubility of the solvent of the solvent phase for aromatic hydrocarbons. Accordingly, the solvent composition of the present invention preferably contains from about 0.1% to about 20% by weight water and, preferably, about 3 to~0 about 15% by weight, most preferably from 5 to 12 weight % (based on the weight of the water miscible solvent) depending upon the particular solvent utilized and the process conditions at which the extraction zone and the extractor-stripper are operated.
Generally, to achieve the extraction the ratio of solvent (exclusive of water) to feed stock in the extractor may be in the range of about 3 to about 12 parts by weight per part by weight of feed stream. For preferred solvents the range may be from 4 to 8, most preferably from 5 to 7 parts by weight of solvent (exclusive of water) per part by weight of feed stream or feed stock. The foregoing ratios are the ratio of total weight of water miscible solvent (exclusive of water) in the extractor (i.e. the weight of solvent fed to the feed stream ~ ",/~-ec/9137can.doc 1 1 CA 0221430~ 1997-08-29 prior to entering the extractor and the amount of solvent fed to the top of the extractor column).
In accordance with one embodiment of the present invention the recycle stream of water and water miscible solvent taken from the bottom of the stripper is split into two streams. Generally from 20 to 45 weight % of the recycle stream of water and water miscible solvent, o preferably from 30 to 40 weight % of the recycle stream of water and water miscible solvent, most preferably from 30 to 35 weight % of the recycle stream of water and water miscible solvent is fed directly to the feed stream prior to the extractor. The balance of the recycle stream of water and water miscible solvent (i.e. 80 to 55, preferably 70 to 60, most preferably 70 to 65 weight % of the recycle stream) is fed to the top of the extractor column.
The resulting feed stream comprising a mixture of aromatic and aliphatic hydrocarbons, a water miscible solvent and water is fed to the middle of an extractor column. The extractor column is a multistage counter current column typically comprising from about 3 to 25, preferably from about 5 to 12 perforated plates mounted in the column.
The plates may be mounted on a vertical shaft which may be driven by a motor in an oscillatory manner. The extractor column may also contain pumps with settling zones, sieve trays with upcomers or possibly even hollow tubes.
The extractor column may be operated at temperatures from 100~C to 200~C, preferably from about 125~C to 175~C for the preferred water miscible solvent. The pressure in the extractor column tVjm/spec/9137can.doc 12 CA 0221430~ 1997-08-29 may be from about 75 to 200 psig (pounds per square inch gage) (i.e. above atmospheric pressure ).
The extractor is operated in a counter current flow manner. A
portion of the recycle mixture of water and water miscible solvent is fed to the top of the extractor. The feed stock mixture comprising aromatic and aliphatic hydrocarbons, water and a water miscible solvent is fed o to the middle theoretical stage or plate of the extractor. The middle theoretical stage of the extractor is defined to include those stages within the range from (one quarter of the number of stages or plates in the column) plus one to three quarters of the stages or plates in the same column. For example, in a column having 12 stages or plates the middle of the column would range from (1/4 X 12) + 1 = 4 to 3/4 X 12 = 9. In the extractor the bottom theoretical stage or plate is located just above the inlet for the reflux from the stripper column.
In the extractor column the water and water miscible solvent flows down from the top of the column; the feed stream enters the column in the middle (as explained above) and the aliphatic portion of the feed stream with some aromatics travels up the column counter current to the water and water miscible solvent flowing down from the top. The aromatics, water miscible solvent and water in the feed stream generally flow down the column. The reflux stream entering the column below the bottom theoretical stage or plate of the column flows or percolates up the column contacting the water miscible solvent in which the aromatic portion of the reflux is entrained (dissolves). This stream leaves the bottom of the extractor column as the extract. The tt/jm/spec/9137can.doc 1 3 CA 0221430~ 1997-08-29 stream may comprise, based on the weight of the extract, from about 5 to 10 weight % of the feed stream aromatics, about 3 to 10 weight %
water, about 66 to 85 weight % of water miscible solvent, about 4 to 8 weight % of reflux aromatics and about 3 to 6 weight % of reflux aliphatics. The aliphatic stream leaving the top of the extractor (raffinate) has a significantly reduced aromatic hydrocarbon content and typically comprises from 87 to 99 weight % of aliphatic hydrocarbons, from about 1 to 3 weight % of dissolved and entrained solvent and from about 0 to 10 weight % of aromatics.
The aliphatic raffinate stream may be treated in a number of manners as disclosed, for example, in United States Patent 3,966,589 and United States patent 3,714,033. The raffinate leaving the extractor column may pass through a heat exchanger with the raw feed stock to heat the feed stock (prior to mixing with the water and water miscible solvent mixture). The raffinate may be cooled to a temperature from about 75~C to 125~C at this point. The raffinate may then further be washed with water separated from the raffinate in a settling tank (and passed through a reflux decanter) and cooled and passed to a settling tank where the water separates from the raffinate feed. The water phase comprising 75 to 90 weight % water,10 to 25 weight % of water miscible solvent and about 0.1 to 1 weight % of aliphatics, separates from the resulting raffinate comprising from 90 to 100 weight % of aliphatics and from 0 to 10 weight % of aromatics.
The raffinate may then be further processed.
tVjm/spec/9137can.doc 14 CA 0221430~ 1997-08-29 In a particularly preferred embodiment of the invention from about 3 to 10 weight %, most preferably from about 4 to 8 weight %, based on the weight of the raw feed stream (i.e. before mixing with the water and water miscible solvent) of a non-cyclic (straight chained) aliphatic C3 5, most preferably a C5 hydrocarbon stream may be fed to the raw feed stream.
lo The extract from the extractor column is fed to a stripper or distillation column. The stripper column is divided into two zones. The upper zone or extractive distillation section removes the non-aromatic hydrocarbons from the extract. The lower zone or stripping section removes the aromatic hydrocarbons from the lean solvent. The stripper is operated at pressures from about 5 to 20 psig and at temperatures from about 90~C up to about 200~C, preferably not greater than about 1 75~C.
In the top of the stripper a portion of the extract from the extractor flashes. The liquid phase comprising some aliphatic hydrocarbons, water and solvent, and aromatic hydrocarbons descends farther down the stripper. As the liquid descends the aliphatic hydrocarbons are vaporized from the liquid and rise and are mixed in the overhead section of the stripper with the flashed aliphatics. This overhead portion is removed from the stripper as an overhead distillate. The overhead distillate may comprise from about 40 to about 75 weight % of aromatics, about 20 to 40 weight % of aliphatics, about 2 to 10 weight % of water and about 0 to 5 weight %
of solvent ( the sum of the components adding to 100 weight %). The tVjm/spec/9137can.doc 1 5 CA 0221430~ 1997-08-29 overhead stream is returned to the extractor column as a reflux stream introduced below the bottom plate of the extractor.
As the liquid phase descends down the stripper column it is freed of aliphatics. At about half way down the column typically all of the aliphatics have been removed. At about half way down the stripper a side stream in the form of a vapor is removed. The side stream o comprises about 65 to 90 weight % of aromatics, about 10 to 30 weight % of water and about 1 to 10 weight % of solvent based on the total weight of the side stream. The side stream is further treated to separate the aromatic hydrocarbons from the water, typically by first passing through a condenser and then a decanter. The aromatic phase comprising about 99.8 to 99.9 weight % of the aromatics and 0.1 to 0.2 weight % of solvent is separated from the side stream by this process.
The water and solvent descend through the stripper and are returned to the front end of the process. As noted above, the recycle lean solvent stream is split into two parts, one of which (about 20 to 45 weight %) is fed directly into the feed stream and one of which (the balance) is fed to the top of the extractor column.
The water and solvent not forming part of the recycle "lean solvent" are to the best extent possible recovered from the streams in which they are entrained and returned to the "lean solvent" stream or the extractor.
The lean solvent stream leaving the stripper may pass through a reboiler to recover as much heat as possible for the stripper.
tt/jm/spec/9137can.doc 1 6 CA 0221430~ 1997-08-29 In a preferred embodiment of the present invention, low pressure steam, typically about 50 pound pressure steam from the refinery, most preferably from a C5 splitter installed to improve the quality of a C5 product and to provide saturated non-cyclic C5's to be added to the extractor feed stock; is fed to a reboiler at the middle of the stripper column (e.g. at the extract draw location) to add heat to the o column and reduce the vapor and liquid loads in the lower section of the stripper column. The low pressure steam may be the exhaust steam from a turbine used to drive a heat pump on a C5 splitter. Most preferably, the reboiler adds from 10 to 50 % of the stripper heat requirement.
The present invention will now be described in conjunction with figure 1.
In figure 1, the feed stream of mixed aliphatics and aromatic hydrocarbons is fed through line 1 to extractor 2. In the preferred embodiment, a stream of non-cyclic C5 hydrocarbons from a C5 splitter (already existing or added to improve C5 product quality and increase extractor unit capacity) is fed to the feed stream by line 3. A portion of the recycled "lean" solvent is fed through line 4 into feed line 1 before the feed stream enters the extractor column 2. In the extractor column the feed steam is separated into a largely aliphatic stream or raffinate which is taken off as the overhead stream through line 5. The extract which is comprised of water, water miscible solvent, aromatics and entrained aliphatics, leaves the extractor through bottom line 6 which feeds into the upper end of the separation column 7. In the upper tVjm/spec/9137can.doc 1 7 CA 0221430~ 1997-08-29 portion of the separation column the aliphatics and low boiling aromatics together with water and some solvent are flashed off and taken as a reflux through over head line 8 and returned to the extractor column below the lowest plate in the extractor column. The aromatics, water miscible solvent, water and entrained aliphatics descend through the column with descending stream being freed of aliphatics about half o way down the column. The aliphatics rise and are taken out of the separator column as part of the reflux. About half way down the column a side stream of aromatics, water and water miscible solvent is taken off through line 9. The aromatic stream is further treated downstream to remove water and, if possible, water miscible solvent.
The remaining water and water miscible solvent descend through the separator and generally pass through a reboiler 10 before leaving the splitter through bottom line 11 which is split at line 4 so part of the recycle "lean solvent" goes to the feed line 1 and the remainder goes to the top of the extractor.
In a particularly preferred embodiment, steam from a C5 splitter in the refinery is used by line 13 to power a reboiler 12 at about mid way down the stripper (i.e. just above the plate for line 9) to remove the side stream).
The foregoing describes the subject matter of the present invention. However, other embodiments of the subject matter of the present invention would be apparent to those skilled in the art.
The following non-limiting examples are intended to illustrate the present invention without limiting it.
tt/jm/spec/9137can.doc 1 8 CA 0221430~ 1997-08-29 A computer simulation (PRO ll simulator using unifac liquid-liquid vapor-liquid equilibrium data) of the operation of the extractor and stripper sections of a commercial refinery in the Sarnia area was developed to closely model the plant operation prior to the invention (that is without feeding a portion of the "lean" solvent to the feed stream; the addition of a non-cyclic C5 stream to the feed stream and without the reboiler at the midpoint of the stripper).
From the computer model the rate of feed was about 120,000 Ib./hour of mixed aromatic and aliphatic stream. The raffinate stream was 21,276 Ib. per hour with about 5.7 weight % of aromatics entrained in the raffinate. The extractor tray loadings were calculated to be those set forth in table 1.
Table 1: Calculated Extractor Tray Loads Section HeavyPhase Light Phase (Lb. Mol/Hr.) (Lb. Mol/Hr.) Top 11,377 1886 Bottom 12,796 1960 The calculated tray loads in the stripper are set forth in table 2.
Table 2: Calculated Stripper Tray Loads Section Liquid Vapor (Lb. Mol/Hr.) (Lb. Mol/Hr.) Top 1,713 1803 Bottom - 1,980 4233 The extract rate was 98,788 Ib./hour with about 1000 ppm of non aromatic component. The reflux rate was 159,370 Ib./hour and rate for recycle solvent was about 900,0001b./hour.
tt~m/spec/9137can.doc 1 9 CA 0221430~ 1997-08-29 The model was re-run using the same size extractor and stripper except that about 25% of the lean solvent stream was fed directly into the feed stream. Additionally, a feed of 10,000 Ib./hour of C5 non-cyclic aliphatic hydrocarbons were fed to the feed stream and 50 pound steam from a C5 splitter turbine was used to power a reboiler at the middle of the stripper column (i.e. the extract off-take) to add 30 million Btu/hour to the stripper. At equilibrium it was found that the rate of total feed could be increased to 170,000 Ib./hour (in view of the added 10,000 Ib./hr of C5 this was an increase in feed rate of 40,000 Ib./hour over a base rate of 120,000 Ib./hour, or about a 30%
increase in capacity). The rate of raffinate production was 37,792 Ib./hour with about 2.11 weight % of aromatics (i.e. the rate and purity of the raffinate stream increased).
The calculated loading rates of the trays in the extractor columns is set forth in table 3.
Table 3: Calculated Extractor Tray Loads Section Heavy Phase Light Phase Lb. Mol/Hr. Lb. Mol/Hr.
Top 7417 1509 Bottom 12,688 1920 The extractor tray loads have gone down even though the rate of raffinate production has gone up.
The calculated tray loading in the stripper are set forth in table 4.
tt/jm/spec/9137can.doc 20 CA 0221430~ 1997-08-29 Table 4: Calculated St,i,-"er Tray Loads Section Liquid Vapor Lb. Mol/Hr. Lb. Mol/Hr.
Top 11,933 1880 Bottom 10,947 3890 The loadings at the top of the stripper have gone up slightly but the loadings at the bottom of the stripper have gone down slightly.
The rate of production of extract was 132,483 Ib./hour with 1000 ppm of non-aromatics. The rate of reflux was 150,195 Ib./hour which had gone down slightly. The total rate of recycle of lean solvent remained the same at 900,000 Ib./hour.
The above example demonstrates that the subject matter of the present invention is capable of increasing the capacity of an existing extractor stripper without increasing the size of the stripper or extractor.
There would be some minor engineering costs associated with the expansion of feed lines.
tt~m/spec/9 1 37can .doc 21
This invention relates to a continuous solvent extraction-steam-distillation process for the recovery of aromatic hydrocarbons from a feed stream or feed stock containing such aromatic hydrocarbons and non-aromatic hydrocarbons. The invention relates to the recovery of mixtures of benzene, toluene, xylenes (BTX) and other aromatics typically C6 12 at purity levels required for petrochemical uses. More particularly, the subject invention relates to one or more of the following steps: recycling a portion of the stream of water and water miscible solvent directly to the feed stream prior to the extractor;
adding saturated non-cyclic (acyclic) C5's to the feed prior to the extractor and adding a side reboiler to the extractor draw location of the stripper.
BACKGROUND OF THE INVENTION
Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feed stocks including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilize an aromatic selective solvent. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feed stock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, tt/jm/spec/9137can.doc 2 CA 0221430~ 1997-08-29 and an extract phase comprising solvent having aromatic components dissolved therein.
An important consideration in the operation of an aromatic extraction process beyond the recovery and the purity of the products is increasing the capacity in a cost efficient manner.
The solvent extraction and stripping process to recover BTX
o from a stream of one or more aliphatic compounds is quite well developed. There is some art on recycling part of the raffinate (aliphatic) stream back to the feed stock and there is some art on feeding part of the aromatic side stream (from the stripper) back to the feed stream. Applicant has been unable to locate any art on directly feeding part of the water and water miscible recycle stream directly to the feed stream prior to delivery to the middle of the extractor.
U.S. patent 3,779,904 issued Dec. 18, 1973 (now expired) assigned to Union Carbide Corporation teaches that a portion of the light raffinate stream is fed to the feed stream so that its content is from 5 to 12 weight % (whereas the original feed stream had no more than 1 to 4 weight % of light aliphatics). The patent does not teach feeding a portion of the recycled mixture of water and water miscible solvent (lean solvent) to the feed stream prior to the extractor column.
The patent also teaches a reboiler (28) at the bottom of the stripper column and a reboiler 63 at the bottom of a column to separate the light aliphatics and the heavier aliphatics. However, the patent does not suggest the use of a reboiler about the middle of the stripper tt/jm/spec/9137can.doc 3 CA 0221430~ 1997-08-29 column. The reference teaches away form the major aspects of the present invention.
U.S. patent 3,789,077 issued Jan. 29, 1974 (now expired) assigned to Union Carbide Corporation teaches recycling part of the aromatic stream through line 144 to the feed prior to entering the extractor column (see col. 8, lines 55-60). The patent does not teach o recycling the "lean" solvent to the feed. Rather the lean solvent from the bottom of the extractor column is recycled directly to the top of the primary extractor. The patent does not teach or suggest that the "lean solvent" could be added to the feed stream prior to the extractor column.
U.S. 3,966,589 issued June 29, 1976 (now expired) assigned to Union Carbide Corporation teaches recycling a part of the aromatic stream back to the feed stream (via line 16, see col. 9 lines 53-58).
There are reboilers in the process (28) but they deliver product below the bottom plate of the stripper (col. 8, lines 45-50). The patent fails to teach or disclose feeding part of the recycle stream of lean solvent to the feed stock prior to the extractor. Rather the recycle stream of water and water miscible solvent is recycled to the top of the extractor.
U.s. patent 3,714,033 issued Jan. 30, 1973 (now expired) assigned to Union Carbide Corporation is also of interest. The recycle of the "neat or lean" solvent is to the top of the extractor column (col. 5, lines 63-65 and col. 7 lines 50-60). However, the patent teaches recycling an aromatic slip stream to the feed (through line 16, col. 6, lines 64-70). Again the patent teaches against the subject matter of IU;" ,/~.ec/~1 37can.doc 4 CA 0221430~ 1997-08-29 the present patent application as it teaches feeding the neat solvent to the top of the extractor without prior contact with the feed stock.
SUMMARY OF THE INVENTION
The present invention seeks to provide higher capacity for the removal of aromatic hydrocarbons from aliphatic hydrocarbons on existing or new solvent extraction and separation processes.
Preferably the process is carried out using a single extraction column and a single separator or stripper (steam stripping column).
Accordingly, in one embodiment, the present invention provides in a continuous solvent extraction process for the recovery of aromatic hydrocarbons having a boiling point in the range from 80~C to 175~C
from a feed stream comprising said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising contacting said feed stream with a mixture of water and a water miscible organic liquid having a boiling point of at least 200~C; in an extractor column having from 3 to 25 theoretical plates at a temperature from 100~C to 200~C
and at a pressure from 75 psig to 200 psig and removing at least 95%
of said aliphatics from feed stream and passing the resulting azeotropic mixture of water, water miscible solvent and aromatic hydrocarbons to stripper column operated at temperatures from 90~C
to 200~C at a pressure from 5 psig to 35 psig to separate at least 95%
of said water and water miscible solvent from said aromatic hydrocarbons and generating a recycle stream of water and water miscible solvent, the improvement comprising feeding from 20 to 45 tt/jrn/spec/9137can.doc 5 CA 0221430~ 1997-08-29 weight % of said recycle stream of water and water miscible solvent into said feed stream prior to said extractor and feeding the recycle water stream and the balance of the recycle stream of water and water miscible solvent to the head of the extractor column.
In a further embodiment the present invention provides a continuous solvent extraction-distillation process for the recovery of aromatic hydrocarbons having boiling points in the range of about 80~C
to about 175~C from a feed stream comprising at least about 40 weight % of said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising:
(a) contacting said feed stream with from 20 to 45 weight % of a recycle stream of water and a water miscible solvent having a boiling point of at least 200~C and a decomposition temperature~0 of at least 225~C;
(aa) preferably adding to said feed stock 3-10 weight % of an aliphatic non-cyclic C3 5 stream (preferably obtained from an existing or new C5 column) prior to the extractor;
(b) introducing the feed stock and said 20 to 45 weight % of a recycle stream of water and a water miscible solvent (and preferably containing said 3-10 weight % of non-cyclic C5's) into the middle of a single extractor column having from 3 to 25 theoretical plates operated at a temperature from 100~C to 200~C and at a pressure from 75 psig to 200 psig;
tt/jm/spec/9137can.doc 6 CA 022l430~ l997-08-29 (c) contacting the feed stream and said 20 to 45 weight % of a recycle stream of water and water miscible solvent together with reflux hydrocarbons introduced into the extractor column below the bottom theoretical plate in the extractor zone with the remaining 80 to 55 weight % of said recycle stream of water and a water miscible solvent introduced at the top of said extractor in a counter current flow to provide an overhead raffinate stream comprising at least 95% of aliphatic hydrocarbons and a bottom stream comprising aromatic hydrocarbons, aliphatic hydrocarbons, water and the water miscible hydrocarbon solvent;
(d) feeding the bottom stream from the extractor to the top of a steam stripper operated at a temperature from 90~C to 200~C
and at a pressure from 5 psig to 35 psig to provide an overhead distillate comprising a reflux hydrocarbon stream; a side stream of a predominantly aromatic phase and a bottom stream of water and a water miscible solvent;
(e) recycling said reflux hydrocarbon stream to below the bottom plate of said extractor;
(f) dividing said recycle stream of water and water miscible solvent into two portions comprising from 20 to 45 weight % of said stream and from 80 to 55 weight % of said stream;
(g) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 20 to 45 weight % of said recycle stream to said feed stream in step (a); and l~",/~ec/9137can.doc 7 CA 0221430~ 1997-08-29 (h) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 80 to 55 weight % of said recycle stream to the top of the extractor in step (c).
Most preferably a side reboiler is added to the stripper at the extract vapor draw location to add 10 to 50% of the stripper heat requirement.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic drawing of a process according to the present invention.
BEST MODE
Hydrocarbon feed stocks suitable for utilization in the method of the present invention include many different aromatic-aliphatic mixtures having a sufficient concentration of aromatic hydrocarbons to justify the recovery of the aromatic hydrocarbons as a separate product stream. The present invention is particularly applicable to hydrocarbon feed mixtures containing at least 15% by weight aromatic hydrocarbons. Typical aromatic feed stock charged to an extraction step will contain from about 25% to about 80% by weight aromatic hydrocarbons with aromatic hydrocarbon concentrations as high as 3 0 98% being suitable in some instances. A suitable carbon range for the hydrocarbon feed stock is from about 5 carbon atoms per molecule to about 20 carbon atoms per molecule, preferably from 5 to 12 carbon atoms per molecule.
One suitable source of hydrocarbon feed stock is the output from a conventional catalytic unit for reforming a naphtha feed stock.
tt~m/spec/9137can.doc 8 CA 0221430~ 1997-08-29 Another suitable source of feed stock is the liquid by-product from a pyrolysis gasoline unit which has been hydrotreated to saturate olefins and diolefins, thereby producing an aromatic hydrocarbon concentrate suitable for the solvent extraction technique described herein.
Typically the hydrocarbons in the feed stock will have boiling points from about 25~C to 1 75~C. The aliphatic portion may include o cyclic and straight and branched chained paraffins and naphthenes such as pentane, cyclopentane, hexane, cyclohexane, n-heptane, isooctane and methyl cyclohexane. The aromatic fraction (BTX) includes benzene, toluene, and C8 aromatics such as ortho-xylene, meta-xylene and para-xylene, and ethyl benzene and the Cg aromatics.
Solvent compositions which may be utilized in the practice of the present invention are those selected from solvents having a high selectivity for aromatic hydrocarbons. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group, such as a hydroxyl, amino, cyano, carboxyl or nitro radical. In order to be effective, the organic compounds of the solvent composition having the polar radical must have a boiling point substantially greater than the boiling point of water since water is typically included in the solvent composition for enhancing its selectivity. In general, the solvent must also have a boiling point greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture.
Typically the solvent should have a boiling point of at least 200~C and a decomposition temperature greater than 225~C.
Ilfi, I ,/s~,ec/S1 37can.doc 9 CA 0221430~ 1997-08-29 Organic compounds suitable for use as part of the solvent composition are preferably selected from the group of those organic-containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The mono- and poly-alkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms, such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl and butyl ethers of the glycol hydroxyl groups and the acetic acid esters and mixtures of the above, constitute a satisfactory class of organic solvents useful in admixture with water as the solvent composition for use in the present invention. Most preferably the water miscible solvent will be selected from the group consisting of dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monomethyl ether, sulfolane, N-methyl pyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycol diethyl ether, propylene glycol monoethyl ether, pentaethylene glycol, hexaethylene glycol and mixtures thereof. A
particularly preferred water miscible solvent is tetraethylene glycol.
The water miscible solvent may be used alone or in conjunction with one or more co-solvents. Suitable co-solvents include amides such as formamide, acetamide, dimethylformamide, diethylformamide, and diethylacetamide; amines such as ethylenetiramine and tt/jm/spec/9137can.doc 1 0 CA 0221430~ 1997-08-29 triethyenetetramine; alkanoiamines such as nonoethanolamine, diethanolamine and triethanolamine; nitriles such as ~"~1_ oxydipropionitrile and ~,~1-thiodipropionitrile; phenol and the cresols;
the methyl sulfolanes; sulfoxides such as dimethyl sulfoxide and diethyl sulfoxide and lactones such as ~-propiolactone and ~-butyrolactone.
The aromatic selectivity of the solvent may be improved by the addition of water to the solvent. The solvents utilized in the practice of this invention could contain small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbons without substantially reducing the solubility of the solvent of the solvent phase for aromatic hydrocarbons. Accordingly, the solvent composition of the present invention preferably contains from about 0.1% to about 20% by weight water and, preferably, about 3 to~0 about 15% by weight, most preferably from 5 to 12 weight % (based on the weight of the water miscible solvent) depending upon the particular solvent utilized and the process conditions at which the extraction zone and the extractor-stripper are operated.
Generally, to achieve the extraction the ratio of solvent (exclusive of water) to feed stock in the extractor may be in the range of about 3 to about 12 parts by weight per part by weight of feed stream. For preferred solvents the range may be from 4 to 8, most preferably from 5 to 7 parts by weight of solvent (exclusive of water) per part by weight of feed stream or feed stock. The foregoing ratios are the ratio of total weight of water miscible solvent (exclusive of water) in the extractor (i.e. the weight of solvent fed to the feed stream ~ ",/~-ec/9137can.doc 1 1 CA 0221430~ 1997-08-29 prior to entering the extractor and the amount of solvent fed to the top of the extractor column).
In accordance with one embodiment of the present invention the recycle stream of water and water miscible solvent taken from the bottom of the stripper is split into two streams. Generally from 20 to 45 weight % of the recycle stream of water and water miscible solvent, o preferably from 30 to 40 weight % of the recycle stream of water and water miscible solvent, most preferably from 30 to 35 weight % of the recycle stream of water and water miscible solvent is fed directly to the feed stream prior to the extractor. The balance of the recycle stream of water and water miscible solvent (i.e. 80 to 55, preferably 70 to 60, most preferably 70 to 65 weight % of the recycle stream) is fed to the top of the extractor column.
The resulting feed stream comprising a mixture of aromatic and aliphatic hydrocarbons, a water miscible solvent and water is fed to the middle of an extractor column. The extractor column is a multistage counter current column typically comprising from about 3 to 25, preferably from about 5 to 12 perforated plates mounted in the column.
The plates may be mounted on a vertical shaft which may be driven by a motor in an oscillatory manner. The extractor column may also contain pumps with settling zones, sieve trays with upcomers or possibly even hollow tubes.
The extractor column may be operated at temperatures from 100~C to 200~C, preferably from about 125~C to 175~C for the preferred water miscible solvent. The pressure in the extractor column tVjm/spec/9137can.doc 12 CA 0221430~ 1997-08-29 may be from about 75 to 200 psig (pounds per square inch gage) (i.e. above atmospheric pressure ).
The extractor is operated in a counter current flow manner. A
portion of the recycle mixture of water and water miscible solvent is fed to the top of the extractor. The feed stock mixture comprising aromatic and aliphatic hydrocarbons, water and a water miscible solvent is fed o to the middle theoretical stage or plate of the extractor. The middle theoretical stage of the extractor is defined to include those stages within the range from (one quarter of the number of stages or plates in the column) plus one to three quarters of the stages or plates in the same column. For example, in a column having 12 stages or plates the middle of the column would range from (1/4 X 12) + 1 = 4 to 3/4 X 12 = 9. In the extractor the bottom theoretical stage or plate is located just above the inlet for the reflux from the stripper column.
In the extractor column the water and water miscible solvent flows down from the top of the column; the feed stream enters the column in the middle (as explained above) and the aliphatic portion of the feed stream with some aromatics travels up the column counter current to the water and water miscible solvent flowing down from the top. The aromatics, water miscible solvent and water in the feed stream generally flow down the column. The reflux stream entering the column below the bottom theoretical stage or plate of the column flows or percolates up the column contacting the water miscible solvent in which the aromatic portion of the reflux is entrained (dissolves). This stream leaves the bottom of the extractor column as the extract. The tt/jm/spec/9137can.doc 1 3 CA 0221430~ 1997-08-29 stream may comprise, based on the weight of the extract, from about 5 to 10 weight % of the feed stream aromatics, about 3 to 10 weight %
water, about 66 to 85 weight % of water miscible solvent, about 4 to 8 weight % of reflux aromatics and about 3 to 6 weight % of reflux aliphatics. The aliphatic stream leaving the top of the extractor (raffinate) has a significantly reduced aromatic hydrocarbon content and typically comprises from 87 to 99 weight % of aliphatic hydrocarbons, from about 1 to 3 weight % of dissolved and entrained solvent and from about 0 to 10 weight % of aromatics.
The aliphatic raffinate stream may be treated in a number of manners as disclosed, for example, in United States Patent 3,966,589 and United States patent 3,714,033. The raffinate leaving the extractor column may pass through a heat exchanger with the raw feed stock to heat the feed stock (prior to mixing with the water and water miscible solvent mixture). The raffinate may be cooled to a temperature from about 75~C to 125~C at this point. The raffinate may then further be washed with water separated from the raffinate in a settling tank (and passed through a reflux decanter) and cooled and passed to a settling tank where the water separates from the raffinate feed. The water phase comprising 75 to 90 weight % water,10 to 25 weight % of water miscible solvent and about 0.1 to 1 weight % of aliphatics, separates from the resulting raffinate comprising from 90 to 100 weight % of aliphatics and from 0 to 10 weight % of aromatics.
The raffinate may then be further processed.
tVjm/spec/9137can.doc 14 CA 0221430~ 1997-08-29 In a particularly preferred embodiment of the invention from about 3 to 10 weight %, most preferably from about 4 to 8 weight %, based on the weight of the raw feed stream (i.e. before mixing with the water and water miscible solvent) of a non-cyclic (straight chained) aliphatic C3 5, most preferably a C5 hydrocarbon stream may be fed to the raw feed stream.
lo The extract from the extractor column is fed to a stripper or distillation column. The stripper column is divided into two zones. The upper zone or extractive distillation section removes the non-aromatic hydrocarbons from the extract. The lower zone or stripping section removes the aromatic hydrocarbons from the lean solvent. The stripper is operated at pressures from about 5 to 20 psig and at temperatures from about 90~C up to about 200~C, preferably not greater than about 1 75~C.
In the top of the stripper a portion of the extract from the extractor flashes. The liquid phase comprising some aliphatic hydrocarbons, water and solvent, and aromatic hydrocarbons descends farther down the stripper. As the liquid descends the aliphatic hydrocarbons are vaporized from the liquid and rise and are mixed in the overhead section of the stripper with the flashed aliphatics. This overhead portion is removed from the stripper as an overhead distillate. The overhead distillate may comprise from about 40 to about 75 weight % of aromatics, about 20 to 40 weight % of aliphatics, about 2 to 10 weight % of water and about 0 to 5 weight %
of solvent ( the sum of the components adding to 100 weight %). The tVjm/spec/9137can.doc 1 5 CA 0221430~ 1997-08-29 overhead stream is returned to the extractor column as a reflux stream introduced below the bottom plate of the extractor.
As the liquid phase descends down the stripper column it is freed of aliphatics. At about half way down the column typically all of the aliphatics have been removed. At about half way down the stripper a side stream in the form of a vapor is removed. The side stream o comprises about 65 to 90 weight % of aromatics, about 10 to 30 weight % of water and about 1 to 10 weight % of solvent based on the total weight of the side stream. The side stream is further treated to separate the aromatic hydrocarbons from the water, typically by first passing through a condenser and then a decanter. The aromatic phase comprising about 99.8 to 99.9 weight % of the aromatics and 0.1 to 0.2 weight % of solvent is separated from the side stream by this process.
The water and solvent descend through the stripper and are returned to the front end of the process. As noted above, the recycle lean solvent stream is split into two parts, one of which (about 20 to 45 weight %) is fed directly into the feed stream and one of which (the balance) is fed to the top of the extractor column.
The water and solvent not forming part of the recycle "lean solvent" are to the best extent possible recovered from the streams in which they are entrained and returned to the "lean solvent" stream or the extractor.
The lean solvent stream leaving the stripper may pass through a reboiler to recover as much heat as possible for the stripper.
tt/jm/spec/9137can.doc 1 6 CA 0221430~ 1997-08-29 In a preferred embodiment of the present invention, low pressure steam, typically about 50 pound pressure steam from the refinery, most preferably from a C5 splitter installed to improve the quality of a C5 product and to provide saturated non-cyclic C5's to be added to the extractor feed stock; is fed to a reboiler at the middle of the stripper column (e.g. at the extract draw location) to add heat to the o column and reduce the vapor and liquid loads in the lower section of the stripper column. The low pressure steam may be the exhaust steam from a turbine used to drive a heat pump on a C5 splitter. Most preferably, the reboiler adds from 10 to 50 % of the stripper heat requirement.
The present invention will now be described in conjunction with figure 1.
In figure 1, the feed stream of mixed aliphatics and aromatic hydrocarbons is fed through line 1 to extractor 2. In the preferred embodiment, a stream of non-cyclic C5 hydrocarbons from a C5 splitter (already existing or added to improve C5 product quality and increase extractor unit capacity) is fed to the feed stream by line 3. A portion of the recycled "lean" solvent is fed through line 4 into feed line 1 before the feed stream enters the extractor column 2. In the extractor column the feed steam is separated into a largely aliphatic stream or raffinate which is taken off as the overhead stream through line 5. The extract which is comprised of water, water miscible solvent, aromatics and entrained aliphatics, leaves the extractor through bottom line 6 which feeds into the upper end of the separation column 7. In the upper tVjm/spec/9137can.doc 1 7 CA 0221430~ 1997-08-29 portion of the separation column the aliphatics and low boiling aromatics together with water and some solvent are flashed off and taken as a reflux through over head line 8 and returned to the extractor column below the lowest plate in the extractor column. The aromatics, water miscible solvent, water and entrained aliphatics descend through the column with descending stream being freed of aliphatics about half o way down the column. The aliphatics rise and are taken out of the separator column as part of the reflux. About half way down the column a side stream of aromatics, water and water miscible solvent is taken off through line 9. The aromatic stream is further treated downstream to remove water and, if possible, water miscible solvent.
The remaining water and water miscible solvent descend through the separator and generally pass through a reboiler 10 before leaving the splitter through bottom line 11 which is split at line 4 so part of the recycle "lean solvent" goes to the feed line 1 and the remainder goes to the top of the extractor.
In a particularly preferred embodiment, steam from a C5 splitter in the refinery is used by line 13 to power a reboiler 12 at about mid way down the stripper (i.e. just above the plate for line 9) to remove the side stream).
The foregoing describes the subject matter of the present invention. However, other embodiments of the subject matter of the present invention would be apparent to those skilled in the art.
The following non-limiting examples are intended to illustrate the present invention without limiting it.
tt/jm/spec/9137can.doc 1 8 CA 0221430~ 1997-08-29 A computer simulation (PRO ll simulator using unifac liquid-liquid vapor-liquid equilibrium data) of the operation of the extractor and stripper sections of a commercial refinery in the Sarnia area was developed to closely model the plant operation prior to the invention (that is without feeding a portion of the "lean" solvent to the feed stream; the addition of a non-cyclic C5 stream to the feed stream and without the reboiler at the midpoint of the stripper).
From the computer model the rate of feed was about 120,000 Ib./hour of mixed aromatic and aliphatic stream. The raffinate stream was 21,276 Ib. per hour with about 5.7 weight % of aromatics entrained in the raffinate. The extractor tray loadings were calculated to be those set forth in table 1.
Table 1: Calculated Extractor Tray Loads Section HeavyPhase Light Phase (Lb. Mol/Hr.) (Lb. Mol/Hr.) Top 11,377 1886 Bottom 12,796 1960 The calculated tray loads in the stripper are set forth in table 2.
Table 2: Calculated Stripper Tray Loads Section Liquid Vapor (Lb. Mol/Hr.) (Lb. Mol/Hr.) Top 1,713 1803 Bottom - 1,980 4233 The extract rate was 98,788 Ib./hour with about 1000 ppm of non aromatic component. The reflux rate was 159,370 Ib./hour and rate for recycle solvent was about 900,0001b./hour.
tt~m/spec/9137can.doc 1 9 CA 0221430~ 1997-08-29 The model was re-run using the same size extractor and stripper except that about 25% of the lean solvent stream was fed directly into the feed stream. Additionally, a feed of 10,000 Ib./hour of C5 non-cyclic aliphatic hydrocarbons were fed to the feed stream and 50 pound steam from a C5 splitter turbine was used to power a reboiler at the middle of the stripper column (i.e. the extract off-take) to add 30 million Btu/hour to the stripper. At equilibrium it was found that the rate of total feed could be increased to 170,000 Ib./hour (in view of the added 10,000 Ib./hr of C5 this was an increase in feed rate of 40,000 Ib./hour over a base rate of 120,000 Ib./hour, or about a 30%
increase in capacity). The rate of raffinate production was 37,792 Ib./hour with about 2.11 weight % of aromatics (i.e. the rate and purity of the raffinate stream increased).
The calculated loading rates of the trays in the extractor columns is set forth in table 3.
Table 3: Calculated Extractor Tray Loads Section Heavy Phase Light Phase Lb. Mol/Hr. Lb. Mol/Hr.
Top 7417 1509 Bottom 12,688 1920 The extractor tray loads have gone down even though the rate of raffinate production has gone up.
The calculated tray loading in the stripper are set forth in table 4.
tt/jm/spec/9137can.doc 20 CA 0221430~ 1997-08-29 Table 4: Calculated St,i,-"er Tray Loads Section Liquid Vapor Lb. Mol/Hr. Lb. Mol/Hr.
Top 11,933 1880 Bottom 10,947 3890 The loadings at the top of the stripper have gone up slightly but the loadings at the bottom of the stripper have gone down slightly.
The rate of production of extract was 132,483 Ib./hour with 1000 ppm of non-aromatics. The rate of reflux was 150,195 Ib./hour which had gone down slightly. The total rate of recycle of lean solvent remained the same at 900,000 Ib./hour.
The above example demonstrates that the subject matter of the present invention is capable of increasing the capacity of an existing extractor stripper without increasing the size of the stripper or extractor.
There would be some minor engineering costs associated with the expansion of feed lines.
tt~m/spec/9 1 37can .doc 21
Claims (29)
1. In a continuous solvent extraction process for the recovery of aromatic hydrocarbons having a boiling point in the range from 80°C to 175°C from a feed stream comprising said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising contacting said feed stream with a mixture of water and a water miscible organic liquid having a boiling point of at least 200°C; in an extractor column having from 3 to 25 theoretical plates at a temperature from 100°C to 200°C
and at a pressure from 75 psig to 200 psig and removing at least 95%
of said aliphatics from feed stream and passing the resulting azeotropic mixture of water, water miscible solvent and aromatic hydrocarbons to stripper column operated at temperatures from 90°C
to 200°C at a pressure from 5 psig to 35 psig to separate at least 95%
of said water and water miscible solvent from said aromatic hydrocarbons and generating a recycle stream of water and water miscible solvent the improvement comprising feeding from 20 to 45 weight % of said recycle stream of water and water miscible solvent into said feed stream prior to said extractor and feeding the recycle water stream and the balance of the recycle stream of water and water miscible solvent to the head of the extractor column.
and at a pressure from 75 psig to 200 psig and removing at least 95%
of said aliphatics from feed stream and passing the resulting azeotropic mixture of water, water miscible solvent and aromatic hydrocarbons to stripper column operated at temperatures from 90°C
to 200°C at a pressure from 5 psig to 35 psig to separate at least 95%
of said water and water miscible solvent from said aromatic hydrocarbons and generating a recycle stream of water and water miscible solvent the improvement comprising feeding from 20 to 45 weight % of said recycle stream of water and water miscible solvent into said feed stream prior to said extractor and feeding the recycle water stream and the balance of the recycle stream of water and water miscible solvent to the head of the extractor column.
2. A process according to claim 1 wherein the ratio of water miscible solvent to feed stock is from 3 to 12 parts by weight per part of feed stream and the ratio of water to water miscible solvent is from 0.1 to 20 weight % based on the weight of the water miscible solvent.
3. A process according to claim 2 further comprising operating said extractor column and said stripper column to provide a reflux condition to provide a reflux stream to the base of said extractor column in a ratio to provide from 0.5 to 1.5 parts by weight of reflux from said stripper to said extractor column per part by weight of feed stream.
4. A process according to claim 3, wherein said aliphatic hydrocarbons comprise a mixture of aliphatic hydrocarbons having boiling points ranging from 25°C to 175°C.
5. A process according to claim 4 wherein said aromatic hydrocarbons comprise at least 95% of benzene, toluene and xylene.
6. The process according to claim 5 wherein said water miscible solvent is selected from the group consisting of dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, sulfolane, N-methyl pyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycol diethyl ether, propylene glycol monoethyl ether, pentaethylene glycol, hexaethylene glycol and mixtures thereof.
7. The process according to claim 6, further comprising feeding to said feed stream prior to said extractor column from 1 to 10 weight %
based on the weight of the feed stream of additional C3-5 aliphatic hydrocarbon feed.
based on the weight of the feed stream of additional C3-5 aliphatic hydrocarbon feed.
8. The process according to claim 6, further comprising a low pressure steam reboiler about the middle of said stripper.
9. The process according to claim 7, wherein said additional C3-5 aliphatic hydrocarbon feed is added to said feed stream in an amount from 4 to 8 weight % based on the weight of said feed stream.
10. The process according to claim 9, further comprising a low pressure steam reboiler about the middle of said stripper.
11. The process according to claim 6, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
12. The process according to claim 7, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
13. The process according to claim 8, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
14. The process according to claim 9, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
15. The process according to claim 10, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
16. A continuous solvent extraction-distillation process for the recovery of aromatic hydrocarbons having boiling points in the range of about 80°C to about 175°C from a feed stream comprising at least about 40 weight % of said aromatic hydrocarbons in admixture with aliphatic hydrocarbons comprising:
(a) contacting said feed stream with from 20 to 45 weight %
of a recycle stream of water and a water miscible solvent having a boiling point of at least 200°C and a decomposition temperature of at least 225°C;
(b) introducing the feed stock and said 20 to 45 weight % of a recycle stream of water and a water miscible solvent into the middle of a single extractor column having from 3 to 25 theoretical plates operated at a temperature from 100°C to 200°C and at a pressure from 75 psig to 200 psig;
(c) contacting the feed stream and said 20 to 45 weight % of a recycle stream of water and water miscible solvent together with reflux hydrocarbons introduced into the extractor column below the bottom theoretical plate in the extractor zone with the remaining 80 to 55 weight % of said recycle stream of water and a water miscible solvent introduced at the top of said extractor in a counter current flow to provide an overhead raffinate stream comprising at least 95% of aliphatic hydrocarbons and a bottom stream comprising aromatic hydrocarbons, aliphatic hydrocarbons, water and the water miscible hydrocarbon solvent;
(d) feeding the bottom stream from the extractor to the top of a steam stripper operated at a temperature from 90°C to 200°C and at a pressure from 5 psig to 35 psig to provide an overhead distillate comprising a reflux hydrocarbon stream; a side stream of a predominantly aromatic phase and a bottom stream of water and a water miscible solvent;
(e) recycling said reflux hydrocarbon stream to below the bottom plate of said extractor;
(f) dividing said recycle stream of water and water miscible solvent into two portions comprising from 20 to 45 weight % of said stream and from 80 to 55 weight % of said stream;
(g) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 20 to 45 weight % of said recycle steam to said feed stream in step (a); and (h) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 80 to 55 weight % of said recycle stream to the top of the extractor in step (c).
(a) contacting said feed stream with from 20 to 45 weight %
of a recycle stream of water and a water miscible solvent having a boiling point of at least 200°C and a decomposition temperature of at least 225°C;
(b) introducing the feed stock and said 20 to 45 weight % of a recycle stream of water and a water miscible solvent into the middle of a single extractor column having from 3 to 25 theoretical plates operated at a temperature from 100°C to 200°C and at a pressure from 75 psig to 200 psig;
(c) contacting the feed stream and said 20 to 45 weight % of a recycle stream of water and water miscible solvent together with reflux hydrocarbons introduced into the extractor column below the bottom theoretical plate in the extractor zone with the remaining 80 to 55 weight % of said recycle stream of water and a water miscible solvent introduced at the top of said extractor in a counter current flow to provide an overhead raffinate stream comprising at least 95% of aliphatic hydrocarbons and a bottom stream comprising aromatic hydrocarbons, aliphatic hydrocarbons, water and the water miscible hydrocarbon solvent;
(d) feeding the bottom stream from the extractor to the top of a steam stripper operated at a temperature from 90°C to 200°C and at a pressure from 5 psig to 35 psig to provide an overhead distillate comprising a reflux hydrocarbon stream; a side stream of a predominantly aromatic phase and a bottom stream of water and a water miscible solvent;
(e) recycling said reflux hydrocarbon stream to below the bottom plate of said extractor;
(f) dividing said recycle stream of water and water miscible solvent into two portions comprising from 20 to 45 weight % of said stream and from 80 to 55 weight % of said stream;
(g) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 20 to 45 weight % of said recycle steam to said feed stream in step (a); and (h) feeding the portion of said recycle stream comprising water and a water miscible solvent comprising 80 to 55 weight % of said recycle stream to the top of the extractor in step (c).
17. A process according to claim 16 wherein the ratio of water miscible solvent to feed stock is from 3 to 12 parts by weight per part of feed stream and the ratio of water to water miscible solvent is from 0.1 to 20 weight % based on the weight of the water miscible solvent.
18. A process according to claim 17 further comprising operating said extractor column and said stripper column to provide a reflux condition to provide a reflux stream to the base of said extractor column in a ratio to provide from 0.5 to 1.5 parts by weight of reflux from said stripper to said extractor column per part by weight of feed stream.
19. A process according to claim 18, wherein said aliphatic hydrocarbons comprise a mixture of aliphatic hydrocarbons having boiling points ranging from 25°C to 175°C.
20. A process according to claim 19 wherein said aromatic hydrocarbons comprise at least 95% of benzene, toluene and xylene.
21. The process according to claim 20 wherein said water miscible solvent is selected from the group consisting of dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, ethylene glycol, diethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, sulfolane, N-methyl pyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycol diethyl ether, propylene glycol monoethyl ether, pentaethylene glycol, hexaethylene glycol and mixtures thereof.
22. The process according to claim 21, further comprising feeding to said feed stream prior to said extractor column from 3 to 10 weight %
based on the weight of the feed stream of additional C3-5 aliphatic hydrocarbon feed.
based on the weight of the feed stream of additional C3-5 aliphatic hydrocarbon feed.
23. The process according to claim 22, further comprising a low pressure steam reboiler about the middle of said stripper.
24. The process according to claim 23, wherein said additional C3-5 aliphatic hydrocarbon feed is added to said feed stream in an amount from 4 to 8 weight % based on the weight of said feed stream.
25. The process according to claim 24, further comprising a low pressure steam reboiler about the middle of said stripper.
26. The process according to claim 22, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
27. The process according to claim 23, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
28. The process according to claim 24, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
29. The process according to claim 25, wherein from 30 to 40 weight % of said recycle stream of water miscible solvent is fed to said feed stream prior to said extractor column.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2214305 CA2214305A1 (en) | 1997-08-29 | 1997-08-29 | Process for the separation of aromatic hydrocarbons from a mixed feed stock |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2214305 CA2214305A1 (en) | 1997-08-29 | 1997-08-29 | Process for the separation of aromatic hydrocarbons from a mixed feed stock |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2214305A1 true CA2214305A1 (en) | 1999-02-28 |
Family
ID=4161371
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2214305 Abandoned CA2214305A1 (en) | 1997-08-29 | 1997-08-29 | Process for the separation of aromatic hydrocarbons from a mixed feed stock |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2214305A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2020212315A1 (en) * | 2019-04-18 | 2020-10-22 | Shell Internationale Research Maatschappij B.V. | Recovery of aliphatic hydrocarbons |
| US11920094B2 (en) | 2016-12-08 | 2024-03-05 | Shell Usa, Inc. | Method of pretreating and converting hydrocarbons |
-
1997
- 1997-08-29 CA CA 2214305 patent/CA2214305A1/en not_active Abandoned
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US11920094B2 (en) | 2016-12-08 | 2024-03-05 | Shell Usa, Inc. | Method of pretreating and converting hydrocarbons |
| WO2020212315A1 (en) * | 2019-04-18 | 2020-10-22 | Shell Internationale Research Maatschappij B.V. | Recovery of aliphatic hydrocarbons |
| US11891574B2 (en) | 2019-04-18 | 2024-02-06 | Shell Usa, Inc. | Recovery of aliphatic hydrocarbons |
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