U.S. patent application number 09/735452 was filed with the patent office on 2001-05-24 for apparatus and process for improved aromatic extraction from gasoline.
Invention is credited to Merchav, Zvi, winter, George.
Application Number | 20010001451 09/735452 |
Document ID | / |
Family ID | 32043774 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001451 |
Kind Code |
A1 |
winter, George ; et
al. |
May 24, 2001 |
Apparatus and process for improved aromatic extraction from
gasoline
Abstract
The improved process and apparatus of the present invention for
extracting high purity aromatics from gasoline using a glycol
solvent based extraction process decrease liquid-vapor flashing,
reduce reflux flow rate, and use heat of enthalpy produces at one
point as a source of energy used at another point, decreasing
energy consumption while significantly increasing purity and amount
of product obtained.
Inventors: |
winter, George; (Prospect,
IL) ; Merchav, Zvi; (Haifa, IL) |
Correspondence
Address: |
Kajane McManus
P.O.Box 344
Wonder Lake
IL
60097
US
|
Family ID: |
32043774 |
Appl. No.: |
09/735452 |
Filed: |
December 12, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09735452 |
Dec 12, 2000 |
|
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09298428 |
Apr 23, 1999 |
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Current U.S.
Class: |
208/311 ;
208/308; 208/313; 208/314 |
Current CPC
Class: |
C10G 21/00 20130101;
C10G 21/02 20130101 |
Class at
Publication: |
208/311 ;
208/313; 208/314; 208/308 |
International
Class: |
C10G 017/04 |
Claims
We claim:
1. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with non aromatic
hydrocarbon, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising non
aromatic hydrocarbons and a first rich solvent stream comprising
solvent, aromatic hydrocarbons and non aromatic hydrocarbons; b)
passing the first rich solvent stream to a flash drum wherein the
stream encounters a reduction in pressure causing the stream to
separate into two streams comprising a vapor hydrocarbon stream
produced by flashing and a second rich solvent stream; c) passing
the second rich solvent stream to an isolated top tray of a
stripper column wherein the stream encounters a further reduction
in pressure causing the stream to separate again into a vapor
hydrocarbon stream produced by flashing and a third rich solvent
stream; d) passing the third rich solvent stream to a multitiered
tray portion of the stripper column, contacting the stream with an
upwardly flowing vapor hydrocarbon-steam stream under conditions
suitable for stripping and recovering a first flashless vapor
stream; e) combining the vapor streams from the flash drum,
isolated top tray and multitiered tray portion into a condensed
overhead mixed hydrocarbon stream and passing said mixed
hydrocarbon vapor stream to said extr8Xcolumn as at least a portion
of said recycle stream; f) separating water from the overhead vapor
stream and combining the separated water with water from a
raffinate wash system and further water streams created by the
process and sending the combined water stream to a vaporizer where
the steam of the upwardly flowing hydrocarbon-steam stream of the
stripper column is generated; g) recovering an aromatic product
stream together with a portion of the upwardly flowing steam from
said upwardly flowing vapor stream and withdrawing a lean solvent
stream from the bottom of the stripping column; and h) returning
the lean solvent stream to the extractor column for recirculation;
the improvement comprising: creating a barrier across the
multitiered tray portion of the stripper column at a position below
that at which the aromatic product stream is recovered to produce a
top tray portion and a bottom tray portion; diverting the lean
solvent stream exiting the bottom portion of the stripper column
through a heat exchanger; diverting the third hydrocarbon stream
from the top tray portion through the heat exchanger to which heat
is supplied from the diverted lean solvent stream from the stripper
column; and creating a fourth heated hydrocarbon stream which is
then fed to the bottom tray portion of the stripper column for
continuation through steps d) through h) above, the heating of the
fourth hydrocarbon stream generated significantly improving the
degree of extraction of the aromatic product.
2. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into two streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
causing the stream to separate again into a vapor hydrocarbon
stream produced by flashing a third rich solvent stream; d) passing
the third rich solvent stream to a multitiered tray portion of the
stripper column, contacting the stream with an upwardly flowing
vapor hydrocarbon-steam stream under conditions suitable for
stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated top tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon to said
extractor column as at least a portion of said recycle stream; f)
separating water from the overhead vapor stream and combining the
separated water with water from a raffinate wash system and further
water streams created by the process and sending the combined water
stream to a vaporizer where the steam of the upwardly flowing
hydrocarbon-steam stream of the stripper column is generated; g)
recovering an aromatic product stream together with a portion of
the upwardly flowing steam from said upwardly flowing vapor stream
and withdrawing a lean solvent stream from the bottom of the
stripping column, and h) returning the lean solvent stream to the
extractor column for recirculation; the improvement comprising:
cooling the lean solvent stream to remove sufficient energy from
the stream so that the flash drum and the top tray of the stripper
column produce no vapor.
3. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into tow streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
casing the stream to separate again into a vapor hydrocarbon stream
produced by flashing and a third rich solvent stream; d) passing
the third rich solvent stream to a multitiered tray portion of the
stripper column, contacting the stream with an upwardly flowing
vapor hydrocarbon-steam stream under conditions suitable for
stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated top tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process and sending
the combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon-steam stream of the stripper column is
generated; g) recovering an aromatic product stream together with a
portion of the upwardly flowing steam from said upwardly flowing
vapor stream and withdrawing a lean solvent stream from the bottom
of the stripping column; and h) returning the lean solvent stream
to the extractor the improvement comprising: reducing the flow of
reflux to the extractor correspondingly with a reduction in vapor
created in the flash drum and the top for recirculation; tray of
the stripper by removal of energy from the lean solvent stream
flowing therethrough.
4. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into two streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
causing the stream to separate again into a vapor hydrocarbon
stream produced by flashing a third rich solvent stream; d) passing
the third rich solvent stream to a multitiered tray portion of the
stripper column, contacting the stream with an upwardly flowing
vapor hydrocarbon-steam stream under conditions suitable for
stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated top tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process and sending
the combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon -steam stream of the stripper column
is generated; g) recovering an aromatic product stream together
with a portion of the upwardly flowing steam from said upwardly
flowing vapor stream and withdrawing a lean solvent stream from the
bottom of the stripping column; and h) returning the lean solvent
stream to the extractor column for recirculation; the improvement
comprising: reducing the flow of solvent to the extractor
correspondingly with a reduction of flow of reflux produced by
removing energy from the stream entering the stripper column.
5. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into two streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
causing the stream to separate again into a vapor hydrocarbon
stream produced by flashing and a third rich solvent stream; d)
passing the third rich solvent stream to a multitiered tray portion
of the stripper column, contacting the stream with an upwardly
flowing vapor hydrocarbon-steam stream under conditions suitable
for stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated op tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process and sending
the combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon-steam stream of the stripper column is
generated; g) recovering an aromatic product stream together with a
portion of the upwardly flowing steam from said upwardly flowing
vapor stream and withdrawing a lean solvent stream from the bottom
of the stripping column; and h) returning the lean solvent stream
to the extractor column for recirculation; the improvement
comprising: reducing the stripping steam in the stripper column
correspondingly with a reduction of flow of the lean solvent
stream.
6. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into two streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
causing the stream to separate again into a vapor hydrocarbon
stream produced by flashing and a third rich solvent stream; d)
passing the third rich solvent stream to a multitiered tray portion
of the stripper column, contacting the stream with an upwardly
flowing vapor hydrocarbon-steam stream under conditions suitable
for stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated top tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process sending the
combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon-steam stream of the stripper column is
generated; g) recovering an aromatic product stream together with a
portion of the upwardly flowing steam from said upwardly flowing
vapor stream and withdrawing a lean solvent stream from the bottom
of the stripping column; and h) returning the lean solvent stream
to the extractor column for recirculation; the improvement
comprising: using a control system that detects flow of vapor from
the flash drum and top tray of the stripper column to control
removal of energy from the stream entering the stripper column.
7. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into tow streams comprising a vapor hydrocarbon stream produced by
flashing and second rich solvent stream; c) passing the second rich
solvent stream to an isolated top tray of a stripper column wherein
the stream encounters a further reduction in pressure causing the
stream to separate again into a vapor hydrocarbon stream produced
by flashing and a third rich solvent stream; d) passing the third
rich solvent stream to a multitiered tray portion of the stripper
column, contacting the stream with an upwardly flowing vapor
hydrocarbon-steam stream under conditions suitable for stripping
and recovering a first flashless vapor stream; e) combining the
vapor streams from the flash drum, isolated top tray and
multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process and sending
the combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon-steam stream of the stripper column is
generated; g) recovering an aromatic product stream together with a
portion of the upwardly flowing steam from said upwardly flowing
vapor stream and withdrawing a lean solvent stream from the bottom
of the stripping column; and h) returning the lean solvent stream
to the extractor column for recirculation; the improvement
comprising: creating a barrier in the stripper column to detour the
down-flowing stream to an external heat exchanger, heating the
detoured stream with heat from the lean solvent stream in the
external heat exchanger, and returning the stream to the stripper
column at a point just below the barrier, the barrier being
positioned between the point of recover of the aromatic product and
the bottom of the column, the heating to the detoured stream
decreasing the amount of upward flowing stripping steam required in
the stripper column.
8. An improved process for the recovery of aromatic hydrocarbons
from a feed comprising a mixture thereof with nonaromatic
hydrocarbons, the process comprising at least the steps of: a)
contacting the feed with a lean solvent stream and a recycle stream
in an extractor column operated at extraction conditions effective
to separate the feed into a raffinate stream comprising nonaromatic
hydrocarbons and a first rich solvent stream comprising solvent,
aromatic hydrocarbons and nonaromatic hydrocarbons; b) passing the
first rich solvent stream to a flash drum wherein the stream
encounters a reduction in pressure causing the stream to separate
into tow streams comprising a vapor hydrocarbon stream produced by
flashing and a second rich solvent stream; c) passing the second
rich solvent stream to an isolated top tray of a stripper column
wherein the stream encounters a further reduction in pressure
causing the stream to separate again into a vapor hydrocarbon
stream produced by flashing and a third rich solvent stream; d)
passing the third rich solvent stream to a multitiered tray portion
of the stripper column, contacting the stream with and upwardly
flowing vapor hydrocarbon-steam stream under conditions suitable
for stripping and recovering a first flashless vapor stream; e)
combining the vapor streams from the flash drum, isolated top tray
and multitiered tray portion into a condensed overhead mixed
hydrocarbon vapor stream and passing said mixed hydrocarbon vapor
stream to said extractor column as at least a portion of said
recycle stream; f) separating water from the overhead vapor stream
and combining the separated water with water from a raffinate wash
system and further water streams created by the process and sending
the combined water stream to a vaporizer where the steam of the
upwardly flowing hydrocarbon-steam stream of the stripper column is
generated; g) recovering an aromatic product stream together with a
portion of the upwardly flowing steam from said upwardly flowing
vapor stream and withdrawing a lean solvent stream from the bottom
of the stripping and h) returning the lean solvent stream to the
extractor column for recirculation; the improvement comprising:
removing the nonaromatic hydrocarbons from the water used in
producing the stripping steam and removing further energy from the
lean solvent stream.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation In Part of U.S.
application Ser. No. 09/298,428 filed Apr. 23, 1999, of the same
title, now abandoned.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an improved apparatus and
process for extracting high-purity aromatics from gasoline which
increases system capacity while reducing energy consumption.
[0004] 2. Prior Art
[0005] Several commercially proven processes and apparatus are
available for extracting high-purity aromatics from gasoline, coke
oven light oil, and pyrolysis naphtha. Most include a liquid-liquid
extractor followed by an extractive distillation column for
extracting a high-purity aromatic stream, apparatus for removing
solvent from the product streams, and solvent conditioning
facilities.
[0006] The extraction of benzene and heavier aromatic homologs has
been practiced commercially for about a century. Prior to the
preparation of high-octane gasoline from crude oil, aromatics were
extracted from liquids produced during the coking and gasification
of coal. With the advent of platinum reforming ("Platforming") in
the late 1940's, a large source of less expensive aromatics became
available in oil refineries.
[0007] At about the same time Dow and others were developing
commercial plants to produce ethylene glycol for the automotive
antifreeze market. One of the heavy byproducts of this process was
di-ethylene glycol. Dow found that this material could be used to
extract aromatics from gasoline, and developed a process to
accomplish this,
[0008] Dow made an arrangement for UOP to market the process once
it was proved, naming it UDEX in honor of the new partnership
promoting the process. This process dominated the extraction field
through the 1950's until the Shell Sulfolane process supplanted it
in the 1960's.
[0009] The early UDEX units used di-ethylene glycol ("DEG") and
di-glycol amine ("DGA") for solvents. The consumption of energy was
typically in the range of 1200 to 1500 BTU/pound of extract. In the
early 1960's, tri-ethylene glycol replaced most of the DEG/DGA,
reducing energy consumption to 1000 to 1200 BTU/pound of extract,
and increasing unit capacity by 20 to 30%. In the 1970's,
tetra-ethylene glycol replaced most of the tri-ethylene. With this
change, the energy consumption dropped to the range of 800 to 1000
BTU/pound of extract and the capacity increased another 10 to 20%.
A solvent additive called "Carom" was used in the 1960's to
decrease the energy consumption and increase capacity, each
changing in the range of 5 to 10%.
[0010] The introduction of the Shell Sulfolane process in the
1960's ended the design and construction of most UDEX apparatus.
The Shell Sulfolane apparatus usually consumes less than 700
BTU/pound of extract. While this is a strong advantage, the process
has two important disadvantages.
[0011] First, the Sulfolane process requires four large columns
rather than the two required for the UDEX process. This increases
capital cost.
[0012] Second, the solvent can become corrosive. Reboiler
replacement and exotic metallurgy are not uncommon due to this
corrosive nature. Entire columns have had to be replaced at
times.
[0013] Thus, because of the low capital cost and non-corrosive
nature of the glycol units, a UDEX apparatus having a low
consumption of energy would have substantial application in the
aromatics field.
SUMMARY OF THE INVENTION
[0014] Accordingly, it is a primary object of the invention to
produce an improved process and apparatus for glycol based
extraction of aromatics from gasoline and the like commonly
referred to as the UDEX process.
[0015] Such object, as well as others, is accomplished by the
process and apparatus of the present invention which decrease
liquid-vapor flashing, reduce reflux flow rate, and use heat of
enthalpy produced at one point as a source of energy used at
another point, decreasing energy consumption while significantly
increasing purity and amount of product obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic diagram of the prior art apparatus,
commonly referred to as UDEX, used in the glycol based process of
extracting aromatics from gasoline.
[0017] FIG. 2 is a schematic diagram showing the apparatus of FIG.
1 shown in phantom incorporating structure not shown in phantom
proposed for addition for carrying out the process of the present
invention in an improved manner, increasing yield while at the same
time decreasing energy consumption.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] Traditional apparatus for carrying out the UDEX process is
shown in FIG. 1. Inasmuch as the apparatus of the present invention
adds onto existing apparatus, for the sake of brevity, the
description of FIG. 2 will incorporate a description of the
apparatus of FIG. 1.
[0019] FIG. 2 shows the UDEX apparatus of FIG. 1 in phantom and
incorporates added structure to provide the improved apparatus of
the present invention commonly referred to by reference numeral 10.
The description will further define process flow as it relates to
the apparatus 10.
[0020] A feed stream 11, normally at ambient temperature, is first
heated with raffinate in a suitable heat exchanger 12 and further
heated in a suitable heater 13 before being sent to an extractor
column 14.
[0021] Raffinate stream 15 exits at the top of the extractor column
14, is cooled in the heat exchanger 12 and is cooled again in a
suitable cooler 16. The cooled raffinate stream 15 is mixed with a
recycled water stream 17 to extract solvent from the raffinated
stream 15, and the combined stream 15 is sent to a separator 18
where the water and solvent separate from the raffinate into what
is defined as a heavy phase 85.
[0022] The raffinate, defined as a light hydrocarbon phase 45, from
separator 18 is mixed with a second water stream 19 and sent to a
separator 20. This second mixing of water and raffinate further
reduces the solvent concentration in the raffinate which becomes
product stream 21.
[0023] Returning to the extractor column 14, a rich solvent stream
22 is removed at the bottom of the column 14, heated in a lean/rich
heat exchanger 23, and is typically sent to a flash drum A at the
top of a stripper column 24. While physically part of stripper
column 24, the flash drum A is an isolated unit and operates at a
higher pressure than tray portions BC of the stripper column 24 but
at a lower pressure than extractor column 14. As stream 22
encounters the lower pressure in the flash drum A some of the
hydrocarbons dissolved in the rich solvent stream 22 flash and exit
stripper column 24 as a vapor stream 25. Because of low volatility
of the solvent at the temperature and pressure in the flash drum A,
vapor stream 25 contains virtually no solvent.
[0024] The solvent, carrying dissolved hydrocarbons therein, exits
flash drum A as stream 26, and enters at an uppermost or top tray
B1 position within the multitiered upper tray portion B of the
stripper column 24. Because pressure in the stripper column tray
portions BC is lower than in the flash drum A, another portion of
the hydrocarbons dissolved in stream 26 flashes and exits the
stripper column 24 as a vapor stream 28. The liquid portion of
stream 26 which has not flashed flows down and across tiered trays
B2 isolated from the top tray B1 in tray portion B, contacting an
upwardly flowing vapor stream 29 to be defined hereinafter. As a
result of the vapor-liquid contact between streams 26 and 29 in
trays B2, the vapor stream 29 strips most of the non-aromatic
hydrocarbons out of stream 26 and carries them out of the stripper
column 24.
[0025] The vapor streams 28 and 29 flow into vapor stream 25 and
the combined stream 125 flows to condenser 32. Once condensed, the
stream 125 flows to a receiver 33.
[0026] Condensed hydrocarbons in the receiver 33 are recycled to
the bottom of extractor column 14 as stream 34. The purpose of
stream 34 is to control the purity of the extract 40. As stream 34
flow rate is increased, a similar increase must be seen in streams
25, 28 and/or 29. Whatever portion of the flow increase occurs in
streams 25 or 28 provides no improvement in purity. In fact,
increasing flow of either stream 25 or 28 often reduces purity, as
shown recently in both simulation models and empirical tests. Only
the portion of the flow increase produced with stream 29 improves
the purity of extract 40.
[0027] The means by which streams 25, 28 and 29 are generated helps
explain why they produce such different results. Streams 25 and 28
are produced by the enthalpy of the rich solvent stream 26. Stream
29 is produced by stripping of non-aromatic hydrocarbons from
stream 26 in trays B2 of portion B of the stripper column 24. As
the flow rate of stream 29 increases, more of the non-aromatic
hydrocarbons are removed.
[0028] Inside stripper column 24, the down-flowing rich solvent
stream 26 contacts the up-flowing vapor stream 29. Such
counter-current contact removes the non-aromatic hydrocarbons from
the rich solvent stream 26. Because of a high degree of
non-ideality introduced by the presence of solvent in stream 26,
even non-aromatic hydrocarbons with up to 9 carbon atoms become
volatile and can be removed from the rich solvent stream 26. Thus,
the tiered trays B2 of portion B of the stripper column 24 function
as an extractive distillation column.
[0029] Although not normally needed for most applications,
non-aromatic hydrocarbons with 10 or more carbon atoms can also be
made as volatile as benzene by increasing concentration of solvent
in the down-flowing stream 26. The condensed hydrocarbon stream 34
recycled to the extractor column 14 thus contains almost all of the
non-aromatic hydrocarbons stripped from rich solvent stream 26,
plus a substantial amount of benzene and heavier aromatics.
[0030] Moving now to a bottom portion C of the stripper column 24,
an upward-flowing steam vapor stream 30 is generated by a
combination of a stripping stream 35 and a vapor stream 49
generated by a reboiler 36. In the bottom portion C of stripper
column 24, upward-flowing vapor stream 30 strips dissolved aromatic
hydrocarbons from the down-flowing rich liquid solvent stream 26
since virtually all of the non-aromatic hydrocarbons were removed
in the upper portion B of the stripper column 24. A first portion
of vapor stream 30 is removed as side-cut stream 37, is sent to a
side-cut condenser 38 and flows to a side-cut receiver 39. The
aromatic hydrocarbons are removed from the side-cut receiver 39 as
the high purity aromatic extract 40. Another portion of stream 30
rises through portion B, becoming vapor stream 29 by stripping the
non-aromatic hydrocarbons from fluid stream 26.
[0031] Water condensed from the stream 30 by condenser 38 and
received in the side-cut receiver 39 is split into three streams. A
first stream 41 is recycled to mix with the condensed side-cut
stream 37 downstream of side-cut condenser 38 to ensure the removal
of solvent strained within stream 37. A second stream 42 is sent to
an accumulator 43. The third stream 19 is fed into separator 20 for
washing the stream 45.
[0032] Since a substantial portion of vapor stream 30, which splits
into side-cut stream 37 and vapor stream 29, is steam, a portion of
vapor stream 29 rising through portion B of the stripper column 24
must also be steam. Therefore, some of the condensed material in
receiver 33 will also be water which flows to the accumulator 43 as
water stream 44.
[0033] A stream 46 from accumulator 43 flows to a tube and shell
vaporizer 47 where a substantial portion of stream 46 is converted
to vapor or steam, which generates stripping stream 35. It should
be noted that several variations of this basic design of UDEX
apparatus exist with some incorporating water columns and some
having the vaporizer 47 on the rich solvent line 22 or 54.
[0034] Portion 48 of the down-flowing stream 26 which reaches a
bottom area of tray portion C of stripper column 24 is looped
through reboiler 36 and returned as vapor stream 49. A lean solvent
stream 50, the net bottoms product from stripper column 24 flows to
a tube side of the vaporizer 47. Boiling of water on a shell side
of the vaporizer 47 reduces the temperature of the lean solvent
stream 50. The lean solvent stream 50 then flows to lean/rich heat
exchanger 23 (in a design in which a lean/rich exchanger is
included) along line 51, and returns to the top of extractor column
14 as stream 52.
[0035] In an apparatus 10 where no exchanger 23 is provided, it
will be understood that the stream 52 will be identical to stream
51, and will obviously be hotter than a stream 52 exiting an
exchanger 23.
[0036] The down-flowing solvent stream 52 in the extractor column
14 contacts the upward-flowing hydrocarbon streams 11 and 34. This
counter-current flow extracts virtually all of the aromatics and an
equilibrium amount of non-aromatics from the upward-flowing stream
34, generating rich solvent stream 22.
[0037] Using gathered data, a steady state simulation model of the
typical UDEX apparatus described above was prepared. Inasmuch as
the model was set up to calculate compositions of internal as well
as external streams, with multiple runs it became possible to
understand what occurs in the flash drum A and at the isolated top
tray B1 of portion B of the stripper column 24.
[0038] Based on the pressure in the flash drum A and in the top
tray B1 of portion B in the stripper column 24, it is assumed that
the vapor-liquid flashes taking place therein reduce vapor loading
or stream velocity in the trays B2 of portion B of the stripper
column 24. In this way, the diameter of the stripper column 24 is
most probably reduced.
[0039] With the steady state model, it was possible to run a series
of calculations in which the amount of vapor flashed in flash drum
A to form stream 25 was reduced to zero. With every decrease in the
amount of vapor flashed, purity of the extract 40 improved. Since
the flow of hydrocarbon stream 34 was held constant, each decrease
in the flow of vapor stream 25 from the flash drum A required an
equal increase in flow of vapor streams 28 and 29 from stripper
column 24. Thus, it appears that not all reflux materials, vapor
streams 25, 28 and 29, effect extract 40 purity equally.
[0040] The next set of calculations methodically reduced the flow
rate of hydrocarbon stream 34 until the purity of the extract 40
was returned to a starting point purity. This showed that the same
extract 40 purity could be generated at significantly different
flow rates of hydrocarbon stream 34. Thus, the unexpected result
was that the purity of the extract 40 increased when the flow of
hydrocarbon stream 34 was reduced in a specific manner.
[0041] The next set of simulation model runs involved reducing the
amount of vapor flashed at the top tray B1 of portion B of stripper
column 24. As with the vapor stream 25 produced in the flash drum
A, the vapor stream 28 produced at the top tray B1 of portion B of
the stripper column 24 had little effect on the purity of the
extract 40. When the flow rate of the hydrocarbon stream 34 was
held constant while the vapor stream 28 produced at the top tray B1
of portion B was reduced, the purity of the extract flow of 40
increased. Likewise, when the purity of hydrocarbon stream 34
decreased to the starting purity, less flow of stream 34 was
required. Again, the surprising result of greater purity of extract
40 with less stream 34 flow was observed.
[0042] The vapor streams 25 and 28 produced at the flash drum A and
at the top tray B1 of portion B of the stripper column 24,
respectively, share one common factor: they do not contact a
counter-current flow of liquid solvent stream 26 as do the
remaining trays B2 of portion B of the stripper column 24. Thus, it
appears that sequential flashes in the absence of contact with a
counter-current flow of liquid solvent do not produce as much
purification of extract 40 as does contacting the vapor stream 29
with the counter-current liquid solvent stream 26.
[0043] With existing technology, the normal range for the
volumetric ratio of reflux to extract ("R/E") is 1.1 to 2.5,
depending on the composition of the feed and the requirements for
purity of the extract.
[0044] With the improvements proposed herein in place, this ratio
drops to a range of 0.5 to 1.0. The reason for this reduction is
that the new technology eliminates the portion of the reflux that
does not enhance purity. The reflux generated in the flash drum A
and on the top tray B1 of the stripper column 24 does not improve
purity. Only the portion generated in the counter-current trayed
section of the stripper column 24 selectively removes
impurities.
[0045] To eliminate reflux in the flash drum A and the top tray B1
in the stripper column 24, energy must be removed from the rich
solvent stream upstream of the flash drum A. This is accomplished
with the new solvent cooler.
[0046] Most units now run at about 1.1 R/E. This should drop to
approximately 0.6 with the proposed improvements, so most units
will see a reduction of about 0.5.
[0047] By comparison, the vapor stream 29 flow from trays B2 of
portion B of the stripper column 24 for the above models showed
that, for constant extract 40 purity, the flow rate of the vapor
stream 29 remained constant. A few more runs showed that this flow
correlated well with extract 40 purity. Thus, the flow rate of
stream 29 was found to determine the purity of the extract 40.
[0048] In addition to purity issues, the simulation model provided
another surprise. As the flow of hydrocarbon stream 34 was reduced,
the model showed that purity of the raffinate 21 increased. With
less aromatics in the raffinate 21, the recovery of aromatics
increased and the flow of extract 40 increased slightly. Therefore,
a series of model runs was made to return the raffinate 21 purity
to the starting point purity. Since recovery of aromatics is
affected by lean solvent stream 52 flow, this flow was reduced.
[0049] With the current technology, the normal range for the
volumetric ratio of solvent to feed ("S/F") is 3.0 to 5.0,
depending on the composition of the feed.
[0050] With the proposed improvements, this will drop to a range of
2.0 to 2.8. The range reflects the difficulty of processing the
particular feed.
[0051] As an example, for a feed comprising 50% aromatics and a
solvent to feed ratio of 4.0, the solvent to extract ratio will be
8.0. This can be used to show how the improvement in the technology
holds constant the concentration of hydrocarbons in the rich
solvent stream 22 leaving the bottom of the extractor 14. The
hydrocarbons consist of the extract and the reflux. Using E for
extract flow, R for reflux flow and S for solvent flow, the
equation for calculating the concentration of hydrocarbons in the
rich solvent is:
Hydrocarbons in solvent= (E=R)/(E+R+S)
[0052] Dividing each term by E provides an equation that is easier
to use:
= (E/E+R/E)/(E/E+R/E+S/E)
[0053] Starting with a solvent to feed ratio of 4.0 and 50%
aromatics in the feed, the solvent to extract ratio will be 8.0.
Likewise, with a reflux to feed ratio of 0.8, the reflux to extract
ratio would be 1.6. Using these values, the concentration of
hydrocarbons in the solvent is about:
(1.0+1.6)/(1.0+1.6+8.0)=24.5%
[0054] With the proposed improvements, the S/E ratio will drop to
5.3 and the R/E will drop to 0.7:
(1.0+0.7)/(1.0/+0.7+5.3)=24.3%
[0055] Thus, even with the modifications to the flow scheme, the
heat balance and the key ratios, the concentration of the
hydrocarbons in the solvent stream 22 at the bottom of the
extractor 14 are about the same with both the new and old
technologies. As a result, the selectivities will be about the same
even though the energy consumption will be substantially
reduced.
[0056] As the flow of lean solvent stream 52 was reduced, several
things happened. First, as expected, the raffinate 21 was found to
include more aromatics. Second, the lower flow rates of solvent
stream 52 and hydrocarbon stream 34 decreased tray loadings in the
extractor column 14 and stripper column 24. Third, with less
solvent stream 52 flow, the flow of stripping stream 35 from the
vaporizer 47 to the bottom of portion C of the stripper column 24
could be decreased.
[0057] From the standpoint of the flow of energy, the reboiler 36
of stripper column 24 provides the energy, for the process. Most of
this energy is removed in the condensers 32 and 38 associated with
the stripper column 24. Since the feed 11 and products 21 and 40
enter and leave the process at the same temperature, the reboiler
36 duty must balance the duties of the two condensers 32 and 38 and
of the raffinate cooler 16. Thus, any change that reduces the duty
of condenser 32 and/or condenser 38 must produce an equal change at
the reboiler 36.
[0058] Reducing the flow of material to the flash drum A and the
top tray B1 of portion B of the stripper column 24 reduces flow to
the condenser 32. In practice, the energy to vaporize the
hydrocarbons in the flash drum A and at the top tray B1 of portion
B of the stripper column 24 actually comes from reboiler 36 and
travels to the flash drum A in the enthalpy of the solvent stream
50, 51, 52, and 22. Thus, as the flow of hydrocarbon stream 34
decreases, the duties of the reboiler 36 and condenser decrease
bythe same amount.
[0059] As the flow of lean solvent stream 52 decreases, the flow of
stripping steam in stream 35 can be decreased. With less steam in
stream 35 flowing up the stripper column 24, less will flow to both
condensers 32 and 38. Thus, lower flow of solvent stream 52 also
decreases the consumption of energy.
[0060] With the current technology, the ratio of stripping steam to
solvent flow is in the range of 2 to 5% with the average about 4%.
As described above, the solvent flow drops by about one-third.
Since the basic job of stripping the hydrocarbons out of the
solvent does not change with the new technology, the ratio of
stripping steam to lean solvent will not change. Because the flow
rate of solvent decreases by one-third, the flow of stripping steam
also decreases by one-third.
[0061] As seen in the model, upon elimination of flashing of the
rich solvent stream 22, the consumption of energy at the stripper
reboiler 36 decreased from over 900 to about 600 BTU/pound of
extract 40. In addition, upon decreasing flow rates of streams 35
and 26 in the stripper column 24, the tray loading in portions B
and C of stripper column 24 dropped by nearly one half of previous
valves.
[0062] The relative expense for the capacity of the UDEX apparatus
10 is strongly related to apparatus 10 capacity, relative primarily
to extractor column 14 size, stripper column 24 size, and the need
for the two stripper condensers 32 and 38 and the stripper reboiler
36.
[0063] Thus, modifying the UDEX process flow scheme as empirically
determined and set forth above the load on all of the structures
except for a top section of the extractor column 14.
[0064] Therefore, with the modified apparatus 10 described
hereinbelow, used in the empirical testing, the feed stream 11 flow
rate could be doubled after incorporating the above modifications
into the improved process.
[0065] Turning now to a study of the modified apparatus 10 proposed
herein for carrying out the improved process, it was first
appreciated that hydrocarbon flashing in flash drum A needed to be
eliminated.
[0066] To eliminate hydrocarbon flashing in flash drum A, it was
determined that energy, in the form of heat, or enthalpy, must be
removed from the solvent stream 22. The enthalpy in the solvent
stream 22 was found to be useable in other areas of the apparatus
10.
[0067] The excess energy is in the solvent stream. This excess is
removed in a cooler 70 on the solvent stream. While the cooler 70
can be in either the lean solvent or the rich solvent streams, the
preferred embodiment is to insert the new cooler in the lean
solvent line 52.
[0068] First, direct flow of liquid stream 26 from portion B2 to
portion C of stripper column 24 was eliminated by placement of a
suitable barrier 71 therebetween. As is known in the art, the
stripper column 24 includes doors, (not shown) commonly referred to
as manways, through which a worker can enter the column 24 with
appropriate parts to create the barrier 71 by assemblage of the
parts inside the column 24. Those skilled in the art will have full
knowledge of use of such manways and erection of the barrier 7hin
the column 24. It will be understood that a further barrier 79 is
also commonly used to stop flow of vapor stream 29 to top tray B1
from the multitiered trays B2 located therebeneath in portion B2 of
the stripper column 24. Then, a shell and tube heat exchange
reboiler 60 could draw bottom stream 80 from portion B2, at a point
just above the side-cut in the stripper column 24, therethrough, to
heat same prior to routing the stream 80 into an upper area of
portion C, the stream 80 being heated by transfer of heat thereto
from lean solvent stream 50. Such increased temperature requires
less flow of stripping steam in stream 35, further reducing the
consumption of energy since the temperature of the lean solvent
stream 50 leaving the bottom of the stripper column 24 is high
enough to provide an adequate temperature difference for such heat
exchange.
[0069] Second, since water stream 85 from the raffinate wash
contains a significant amount of soluble non-aromatics, a stream 77
tapped off of stream 85 can be sent to a small hydrocarbon
stripping column 74 which can be used to keep these non-aromatics
out of the bottom portion C of the stripper column 24.
Contamination of the extract 40 can be eliminated by sending these
non-aromatics via line 78 to condenser 32. Further, the flow of
stream 34 can be reduced by reducing energy input from reboiler 36.
Lean solvent stream 50, even after coursing through reboiler 60,
would still have a temperature to supply energy to a reboiler 75
for the small column 74. A bottoms stream 76 from the column 74
containing water and solvent, in a preferred embodiment, would flow
into the vaporizer 47.
[0070] Third, after coursing through reboiler 75 on the hydrocarbon
stripping column 74, the lean solvent stream 50 would still be hot
enough to provide energy to the vaporizer 47. Process calculations
also indicated that the enthalpy of the solvent stream 50 would
still be high enough to vaporize some of the hydrocarbons in the
solvent stream 52. Therefore, a cooler 70, probably in the form of
a cooling water exchanger 70, would be required to cool a
substantial portion of the stream 52 prior to its entry into the
extractor column 14, completing the improved apparatus 10.
[0071] Since sub-cooling rich solvent stream 22 upstream of flash
drum A would increase reboiler 60 duty, it is proposed to control
the flow of lean solvent stream 22 though the cooling water
exchanger 70 by creating a secondary route 53 bypassing the cooler
70. Flow through the secondary route 53 is controlled by a valve 55
which is operated under control of a flow meter 72 electronically
coupled thereto and provided for sensing an instantaneous rate of
flow of the stream 25 from the flash drum A, with elimination of
substantially all flow of the stream 25 being desired.
[0072] Substantially complete elimination of flow of stream 25 can
be produced by manipulation of the valve 55 in a predetermined
manner relative to a desired temperature for the solvent stream 52,
by producing a degree of valve 55 closure sufficient to
substantially eliminate flow of stream 25 by increasing the volume
of lean solvent stream 52 flowing through the cooler 70 without
sub-cooling the stream 52. Conversely, should it be found that
sub-cooling of the stream 52 is taking place, it may be desirable
to allow creation of a negligible rate of flow of stream 25 to a
predefined upper limit by producing a degree of valve 55 opening to
decrease the volume of stream 52 flowing through the cooler 70,
thereby warming the stream 52 prior to its flowing into the
extractor column 14.
[0073] With the current technology, there is no issue on removing
energy from the solvent entering the flash drum A.
[0074] With the proposed improved flow scheme, the amount of energy
in the solvent can be optimized. If the temperature is too high (as
it is with the current technology), then some of the hydrocarbons
in the rich solvent stream 22 vaporize in the flash drum A. This
leads to more reflux flow, more solvent flow, more stripping steam
flow, lower capacity and higher consumption of energy. If, on the
other hand, the temperature of the rich solvent stream 22 is too
low, then the stream 22 must be reheated to its bubble point with
vapors from the reboiler consuming more energy. At the optimum, the
rich solvent stream 22 entering the flash drum will be at its
bubble point and the use of energy will be minimized.
[0075] To adjust the removal of energy from the solvent so that the
stream 22 entering the flash drum A is at its point, it is possible
to monitor the flow of vapor from the flash drum A. To accomplish
this, the pressure of the drum must be set at the pressure of the
top of the stripper column 24. At this pressure, any vapor
generated in the flash drum A represents the degree to which the
solvent stream 22 contains too much energy. Therefore, the
measurement of the flow of vapor from the flash drum A provides a
signal or index that can be used to remove the excess heat from the
solvent steam 22. With a cooler 70 on the solvent and control
valves in place as proposed, the flow of vapor from the flash drum
A can be used to adjust removal of excess energy from the
solvent.
[0076] The last set of process calculations performed with the
model investigated the water concentration in the lean solvent
stream 50. After modifying the process flow scheme, the best
combination of purity and recovery was obtained at about a 5 to 6%
concentration of water. Higher concentrations lost recovery faster
than gaining purity while lower concentrations produced the
opposite effect.
[0077] As a side note, solvent additives such as ether glycol,
while having significant effects at high water concentrations, were
found to have negligible significance at the optimum water
concentrations.
[0078] Almost all present day UDEX apparatus are believed to have
the same process flow scheme, making the proposed modifications
substantially universally applicable. Likewise, similar
modifications appear applicable for use in other glycol solvent
systems.
[0079] As described above, the method of the present invention
provides a number of advantages, some of which have been described
above and others of which are inherent in the invention. Also,
modifications may be proposed to the method without departing from
the teachings herein. Accordingly, the of the invention is only to
be limited as necessitated by the accompanying claims.
* * * * *