U.S. patent number 3,779,904 [Application Number 05/210,404] was granted by the patent office on 1973-12-18 for process for the separation of aromatic hydrocarbons from a mixed hydrocarbon feedstock.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Alexander Jean-Marie Kosseim, Daniel John Kubek, George Solomon Somekh.
United States Patent |
3,779,904 |
Kubek , et al. |
December 18, 1973 |
PROCESS FOR THE SEPARATION OF AROMATIC HYDROCARBONS FROM A MIXED
HYDROCARBON FEEDSTOCK
Abstract
The process involves a combination of continuous solvent
extraction-steam distillation for the recovery of aromatic
hydrocarbons from a mixed feedstock. The feedstock is contacted
with a solvent-water mixture at temperatures in the range of about
75.degree. to 200.degree. C and the extract and raffinate streams
are separated into their components. The purity of the aromatics
recovered from the extract is improved by introducing light
aliphatic hydrocarbons consisting essentially of aliphatic and
cycloaliphatic hydrocarbons of no more than five carbon atoms with
the feedstock at the middle theoretical stage of the extraction
column. The light aliphatics are introduced in amounts in the range
of 5 to 12 percent, based on the weight of the feedstock.
Inventors: |
Kubek; Daniel John (North
Tarrytown, NY), Kosseim; Alexander Jean-Marie (Yorktown
Heights, NY), Somekh; George Solomon (New Rochelle, NY) |
Assignee: |
Union Carbide Corporation (New
York, NY)
|
Family
ID: |
22782772 |
Appl.
No.: |
05/210,404 |
Filed: |
December 21, 1971 |
Current U.S.
Class: |
208/333; 208/321;
208/323; 585/834; 585/837; 585/840; 585/857; 585/863; 585/864;
585/865; 585/866 |
Current CPC
Class: |
C10G
21/12 (20130101); C10G 21/00 (20130101) |
Current International
Class: |
C10G
21/00 (20060101); C10G 21/12 (20060101); C10g
021/16 (); C07c 007/10 () |
Field of
Search: |
;260/674SE
;208/321,323,333 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gantz; Delbert E.
Assistant Examiner: Spresser; C. E.
Claims
What is claimed is:
1. In a continuous solvent extraction distillation process for the
recovery of aromatic hydrocarbons having boiling points in the
range of about 80.degree.C. to about 175.degree.C. from a feedstock
containing aliphatic hydrocarbons and at least about 40 percent by
weight, based on the weight of the feedstock, of said aromatic
hydrocarbons wherein the feedstock contains no more than about 4
percent by weight, based on the weight of the feedstock, of light
aliphatics consisting essentially of aliphatic and cycloaliphatic
hydrocarbons, each having no more than 5 carbon atoms, a boiling
point no higher than about 50.degree.C. and being condensable at a
pressure of no more than about 3 atmospheres and at a temperature
of no less than about 50.degree.C.; a single extraction column
having three to 25 theoretical stages is used for the solvent
extraction to provide an extract; and a single distillation column
is used to distill the extract to provide a mixture of the aromatic
hydrocarbons comprising the following steps:
a. introducing the feedstock into the extraction column at the
middle theoretical stage thereof;
b. contacting the feedstock in the extraction column with a mixture
of water and a solvent, said solvent being a water-miscible organic
liquid having a boiling point of at least about 200.degree.C. and
having a decomposition temperature of at least about 225.degree.C.,
and with reflux hydrocarbons introduced into the extraction column
below the bottom theoretical stage thereof to provide the extract
comprising aromatic hydrocarbons, reflux aliphatic hydrocarbons,
solvent, and water and a raffinate comprising essentially aliphatic
hydrocarbons;
c. introducing the extract into the distillation column to separate
the aromatic hydrocarbons and the reflux hydrocarbons from the
extract;
d. recycling the reflux hydrocarbons from step (c) to the
extraction column as provided in step (b); and
e. recovering the aromatic hydrocarbons of step (c);
the improvement comprising introducing a sufficient amount of the
light aliphatics defined above into the extraction column at the
middle theoretical stage thereof to provide a total percent by
weight of said light aliphatics, based on the weight of the
feedstock, in the range of about 5 percent to about 12 percent.
2. The process of claim 1 comprising the following additional
steps:
f. introducing the raffinate formed in step (b) into a raffinate
distillation zone to separate the light aliphatics defined in claim
1 therefrom;
g. recycling the defined light aliphatics from step (f) to the
middle theoretical stage of the extraction column; and
h. recovering the balance of the raffinate from step (f).
3. The process of claim 2 wherein the feedstock and the defined
light aliphatics enter the extraction column at about the same
point.
4. The process of claim 3 wherein the temperature in the extraction
column is in the range of about 100.degree.C. to about
200.degree.C., the pressure in the extraction column is in the
range of about 75 psig to about 200 psig, the temperature in the
distillation column is in the range of about 135.degree.C. to about
200.degree.C., and the pressure in the distillation column is in
the range of about 10 psig to about 35 psig.
5. The process of claim 4 wherein the feedstock contains at least
about 80 percent by weight, based on the weight of the feedstock,
of aromatic hydrocarbons.
6. The process of claim 4 wherein
i. the ratio of solvent to feedstock in the extraction column is in
the range of about 3 to about 12 parts by weight of solvent to one
part by weight of feedstock;
ii. the amount of water in the extraction column is about 1 percent
to about 8 percent by weight based on the weight of the solvent in
said column;
iii. the ratio of reflux to feedstock in the extraction column is
in the range of about 0.5 to about 1.5 parts by weight of reflux to
one part by weight of feedstock; and
iv. the ratio of water to aromatic hydrocarbons in the distillation
column is in the range of about 0.1 to about 0.5 part by weight of
water to one part by weight of aromatic hydrocarbons in said
column.
7. The process of claim 6 wherein the solvent is a polyalkylene
glycol.
8. The process of claim 7 wherein the solvent is tetraethylene
glycol.
9. The process of claim 8 wherein the feedstock contains at least
about 80 percent by weight, based on the weight of the feedstock,
of aromatic hydrocarbons.
10. The process of claim 8 wherein the extraction column has five
to 12 theoretical stages.
11. In a continuous solvent extraction-distillation process for the
recovery of aromatic hydrocarbons having boiling points in the
range of about 80.degree.C. to about 175.degree.C. from a feedstock
containing aliphatic hydrocarbons and at least about 40 percent by
weight, based on the weight of the feedstock, of said aromatic
hydrocarbons wherein the feedstock contains no more than about 4
percent by weight, based on the weight of the feedstock, of light
aliphatics consisting essentially of aliphatic and cycloaliphatic
hydrocarbons, each having no more than five carbon atoms, a boiling
point no higher than about 50.degree.C. and being condensable at a
pressure of no more than about 3 atmospheres and at a temperature
of no less than about 50.degree.C.; a single extraction column
having three to 25 theoretical stages is used for the solvent
extraction to provide an extract; and a single distillation column
is used to distill the extract to provide a mixture of the aromatic
hydrocarbons comprising the following steps:
a. introducing the feedstock into the extraction column at the
middle theoretical stage thereof;
b. contacting the feedstock in the extraction column with a mixture
of water and a solvent, said solvent being a water-miscible organic
liquid having a boiling point of at least about 200.degree.C. and
having a decomposition temperature of at least about 225.degree.C.,
and with reflux hydrocarbons introduced into the extraction column
below the bottom theoretical stage thereof to provide the extract
comprising aromatic hydrocarbons, reflux aliphatic hydrocarbons,
solvent, and water and a raffinate comprising essentially aliphatic
hydrocarbons;
c. contacting the extract with steam in the distillation column to
provide an overhead distillate comprising a reflux hydrocarbons
phase and a water phase, a side cut distillate comprising an
aromatic hydrocarbons phase and a water phase, and bottoms
comprising a mixture of solvent and water;
d. dividing the water phase of the overhead distillate into first
and second streams;
e. contacting the raffinate with the first stream to provide an
aliphatic hydrocarbons phase and a water phase;
f. contacting the second stream with an aromatic hydrocarbons
stream containing at least 95 percent aromatic hydrocarbons, the
amount of said stream being in the range of about 0.1 percent to
about 5 percent by weight of the total aromatic hydrocarbons in the
feedstock, to form an aromatic hydrocarbons phase and a water
phase;
g. contacting the aromatic hydrocarbons phase of the side-cut
distillate with the water phase of (f) to form an aromatic
hydrocarbons phase and a water phase;
h. contacting the water phase of step (e) with an aromatic
hydrocarbons stream containing at least 95 percent aromatic
hydrocarbons, the amount of said stream being in the range of about
0.1 percent to about 5 percent by weight of the total aromatic
hydrocarbons in the feedstock, to form an aromatic hydrocarbons
phase and a water phase;
i. recycling the water phases of steps (g) and (h) to the
distillation zone where said water phases are essentially converted
to steam;
j. recycling the reflux hydrocarbons phase of the overhead
distillate and the bottoms of step (c) to the extraction zone to
provide reflux hydrocarbons and mixture of water and solvent,
respectively, for step (b); and
k. recovering the aromatic hydrocarbons phase of step (g) and the
aliphatic hydrocarbons phase of step (e);
the improvement comprising introducing a sufficient amount of the
light aliphatics defined above into the extraction column at the
middle theoretical stage thereof to provide a total percent by
weight of said light aliphatics, based on the weight of the
feedstock, in the range of about 5 percent to about 12 percent.
12. In a continuous solvent extraction-distillation process for the
recovery of aromatic hydrocarbons having boiling points in the
range of about 80.degree.C. to about 175.degree.C. from a
feed-stock containing aliphatic hydrocarbons and at least about 40
percent by weight, based on the weight of the feedstock, of said
aromatic hydrocarbons wherein the feedstock contains no more than
about 4 percent by weight, based on the weight of the feedstock, of
light aliphatics consisting essentially of aliphatic and
cycloaliphatic hydrocarbons, each having no more than five carbon
atoms, a boiling point no higher than about 50.degree.C. and being
condensable at a pressure of no more than about 3 atmospheres and
at a temperature of no less than about 50.degree.C.; a single
extraction column having three to 25 theoretical stages is used for
the solvent extraction to provide an extract; and a single
distillation column is used to distill the extract to provide a
mixture of the aromatic hydrocarbons comprising the following
steps:
a. introducing the feedstock into the extraction column at the
middle theoretical stage thereof;
b. contacting the feedstock in the extraction column with a mixture
of water and a solvent, said solvent being a water-miscible organic
liquid having a boiling point of at least about 200.degree.C. and
having a decomposition temperature of at least about 225.degree.C.,
and with reflux hydrocarbons introduced into the extraction column
below the bottom theoretical stage thereof to provide the extract
comprising aromatic hydrocarbons, reflux aliphatic hydrocarbons,
solvent, and water and a raffinate comprising essentially aliphatic
hydrocarbons;
c. contacting the extract with steam in the distillation column to
provide an overhead distillate comprising a reflux hydrocarbons
phase and a water phase, a side cut distillate comprising an
aromatic hydrocarbons phase and a water phase, and bottoms
comprising a mixture of solvent and water;
d. contacting the raffinate with the water phase of the overhead
distillate to provide an aliphatic hydrocarbons phase and a water
phase;
e. contacting the water phase of step (d) with an aromatic
hydrocarbons stream containing at least 95 percent aromatic
hydrocarbons, the amount of said stream being in the range of about
0.1 percent to about 5 percent by weight of the total aromatic
hydrocarbons in the feedstock, to form an aromatic hydrocarbons
phase and a water phase;
f. contacting the aromatic hydrocarbons phase of the side-cut
distillate with the water phase of (e) to form an aromatic
hydrocarbons phase and a water phase;
g. recycling the water phase of step (f) to the distillate zone
where said water phase is essentially converted to steam;
h. recycling the reflux hydrocarbons phase of the overhead
distillate and the bottoms of step (c) to the extraction zone to
provide reflux hydrocarbons and mixture of water and solvent,
respectively, for step (b); and
i. recovering the aromatic hydrocarbons phase of step (f) and the
aliphatic hydrocarbons phase of step (d);
the improvement comprising introducing a sufficient amount of the
light aliphatics defined above into the extraction column at the
middle theoretical stage thereof to provide a total percent by
weight of said light aliphatics, based on the weight of the
feedstock, in the range of about 5 percent to about 12 percent.
13. The process of claim 11 wherein the solvent is a polyalkylene
glycol.
14. The process of claim 13 wherein the solvent is tetraethylene
glycol.
15. The process of claim 12 wherein the solvent is a polyalkylene
glycol.
16. The process of claim 15 wherein the solvent is tetraethylene
glycol.
Description
FIELD OF THE INVENTION
This invention relates to an improvement in a process for the
separation of aromatic hydrocarbons from a fixed hydrocarbon
feedstock and, more particularly, to the recovery of high purity
aromatic hydrocarbons in high yields while making efficient use of
process components.
DESCRIPTION OF THE PRIOR ART
With the advent of the benzene-toluene-C.sub.8 aromatics fraction
(known and hereinafter referred to as BTX) as the principal raw
material in the manufacture of petrochemicals, outstripping
ethylene in this regard, and the increased demand for aromatics as
a component in gasoline to increase its octane rating and thus
reduce or eliminate the need for lead, which has been under fire as
a pollutant, aromatics separation processes availed of in the past
have come under close scrutiny with an eye toward improving process
economics.
Improved economics can be translated into, among other things, the
lowering of heating requirements and the more effective use of
process components as aids in the separation process.
Various processes have been used for aromatics separations in
systems of the single extractor-single distillation column (or
stripper) type, which have one particular step, among others, in
common, i.e., the reflux hydrocarbons are derived from the
distillation of the extract and are recycled into the lower portion
of the extraction column below the bottom plate or below the lowest
theoretical stage. To improve the purity of the aromatic products,
lower boiling aliphatics, not precisely the light aliphatics
defined below, have been added to this reflux. These lower boiling
aliphatics were obtained from fractionation of the feedstock,
raffinate, and/or reflux and always introduced with or as a part of
the reflux at the bottom of the extractor.
While purities have been improved and heating requirements lowered
using this technique, optimization has not been achieved.
SUMMARY OF THE INVENTION
An object of this invention, therefore, is to improve the reflux in
aqueous solvent single extractor-single distillation column systems
whereby heating requirements are reduced and the purity of the
aromatics is raised to previously unattained levels.
Other objects and advantages will become apparent hereinafter.
According to the present invention, high purity aromatic
hydrocarbons are effectively recovered using minimal heat in a
continuous solvent extraction-distillation process for the recovery
of aromatic hydrocarbons having boiling points in the range of
about 80.degree.C. to about 175.degree.C. from a feedstock
containing aliphatic hydrocarbons and at least about 40 percent by
weight, based on the weight of the feedstock, of aromatic
hydrocarbons wherein the feedstock contains no more than about 4
percent by weight, based on the weight of the feedstock, of light
aliphatics consisting essentially of aliphatic and cycloaliphatic
hydrocarbons, each having no more than 5 carbon atoms, a boiling
point no higher than about 50.degree.C., and being condensable at a
pressure of nc more than about 3 atmospheres and at a temperature
of no less than about 50.degree.C.; a single extraction column
having about 3 to about 25 theoretical stages is used for the
solvent extraction to provide an extract; and a single distillation
column is used to distill the extract to provide a mixture of the
aromatic hydrocarbons comprising the following steps:
a. introducing the feedstock into the extraction column at the
middle theoretical stage thereof;
b. contacting the feedstock in the extraction column with a mixture
of water and a solvent, said solvent being a water-miscible organic
liquid having a boiling point of at least about 200.degree.C. and
having a decomposition temperature of at least about 225.degree.C.,
and with reflux hydrocarbons introduced into the extraction column
below the bottom theoretical stage thereof to provide an extract
comprising aromatic hydrocarbons, reflux aliphatic hydrocarbons,
solvent, and water and a raffinate comprising essentially aliphatic
hydrocarbons;
c. introducing the extract into the distillation column to separate
the aromatic hydrocarbons and the reflux hydrocarbons from the
extract;
d. recycling the reflux hydrocarbons from step (c) to the
extraction column as provided in step (b); and
e. recovering the aromatic hydrocarbons of step (d);
the improvement comprising introducing a sufficient amount of the
light aliphatics defined above into the extraction column at the
middle theoretical stage thereof to provide a total percent by
weight of said light aliphatics, based on the weight of the
feedstock, in the range of about 5 percent to about 12 percent.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE is a schematic flow diagram of an illustrative
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As noted, there is an industrial need for BTX, which is available
in high proportion,e.g., at least about 40 percent by weight, in a
wide variety of hydrocarbon feedstocks such as reformed gasolines;
coke oven light oils; cracked gasolines; and dripolenes, which,
after hydrogenation, can contain as much as 70 to 98 percent BTX.
These feedstocks also contain both aliphatic and cycloaliphatic
hydrocarbons (herein sometimes referred to collectively as
aliphatic hydrocarbons). Since the individual hydrocarbon compounds
which make up these feedstocks are well known, they will not be
discussed extensively; however, it can be pointed out that the
major components of the feedstocks used herein are hydrocarbons
with boiling points ranging from 25.degree.C. to 175.degree.C.
including straight-chain and branched-chain paraffins and
naphthenes, such as n-heptane, isooctane, and methyl cyclohexane,
and aromatics such as BTX.
The BTX fraction can include benzene, toluene, the C.sub.8
aromatics including ortho-xylene, meta-xylene, para-xylene, and
ethyl benzene, and C.sub.9 aromatics, which, if present at all,
usually appear in the smallest proportion in relation to the other
components.
It is also important to point out that conventional feedstocks used
in extraction-distillation systems of the type discussed herein for
the recovery of BTX contain about 1 to 3 percent and, in some
cases, up to about 4 percent by weight based on the weight of the
feedstock, of light aliphatics which are defined herein as
aliphatic and cycloaliphatic hydrocarbons having no more than 5
carbon atoms in each compound, said compound having a boiling point
no higher than about 50.degree.C. and being condensable at a
pressure of no more than 3 atmospheres and a temperature of no less
than 50.degree.C. (hereinafter referred to as the "defined light
aliphatics").
These defined light aliphatics provide the crux of subject
improvement since it has been found that when at a particular
percentage level and when introduced at a particular point in the
extraction column, they enhance the reflux to a point where there
is a noticeable and advantageous drop in heating requirement
together with a higher level of purity.
The use of this improvement is found to be advantageous in any
process falling within the process definition set out heretofore
under the summary of the invention; however, it is found to be
particularly advantageous when applied to the preferred embodiment
described hereinafter.
The process described here, exclusive of subject improvement, is
the subject of our copending application Ser. No. 180,996, filed on
Sept. 16, 1971, now U.S. Pat. No. 3,714,033. This application bears
the same title as the instant application and is incorporated by
reference herein.
A typical breakdown of the defined light aliphatic fraction, in
percent by weight based on the total weight of the fraction, is
about 0 percent to about 95 percent of the full range of C.sub.5
aliphatics, about 0 percent to about 5 percent of C.sub.4
aliphatics, and a small proportion, about 0 percent to about 1
percent, of C.sub.1 to C.sub.3 aliphatics.
The solvents used in the subject process are, as described above,
water-miscible organic liquids, (at process temperatures) having a
boiling point of at least about 200.degree.C. and having a
decomposition temperature of at least about 225.degree.C. The term
"water-miscible" includes those solvents which are completely
miscible over a wide range of temperatures and those solvents which
have a high partial miscibility at room temperature since the
latter are usually completely miscible at process temperatures. The
solvents are also polar and are generally comprised of carbon,
hydrogen and oxygen with some exceptions. Examples of solvents
which may be used in the process of this invention are 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.
The preferred group of solvents is the polyalkylene glycols and the
preferred solvent is tetraethylene glycol.
Additional solvents, which may be used alone or together, or with
the aforementioned solvents are amides such as formamide,
acetamide, dimethylformamide, diethylformamide, and
dimethylacetamide; amines such as diethylenetriamine and
triethylenetetramine; alkanolamines such as monoethanolamine,
diethanolamine, and triethanolamine; nitriles such as
beta,beta.sup.1 -oxydipropionitrile and beta,beta.sup.1
-thiodipropionitrile; phenol and the cresols; the methyl
sulfolanes; sulfoxides such as dimethyl sulfoxide and diethyl
sulfoxides; lactones such as gamma-propiolactone and
gamma-butyrolactone.
The apparatus used in the process both for the main extraction and
distillation is conventional, e.g., an extraction column of the
multistage reciprocating type containing a plurality of perforated
plates centrally mounted on a vertical shaft driven by a motor in
an oscillatory manner can be used as well as columns containing
pumps with settling zones, sieve trays with upcomers, or even a
hollow tube while the distillation can be conducted in a packed or
bubble plate fractionating column. Countercurrent flows are
generally utilized in both extraction and distillation columns.
Heat exchangers, decanters, reservoir, solvent regenerator, and
raffinate still are also conventional as well as various extractors
other than the main extractor. These other extractors are
preferably single stage mixer-settlers, but can be any of the well
known types.
The number of theoretical stages in the extraction column can be
about three to about 25 stages and is preferably about five to
about 12 stages. The "middle theoretical stage" of the column is
defined to include the theoretical stages running from 0.25 times
the theoretical stages in the column plus one to 0.75 times the
theoretical stages in the same column, e.g., in a column having
four theoretical stages, the "middle theoretical stage" would
include, as defined herein, the second and third theoretical stages
and in a column having 24 theoretical stages, the "middle
theoretical stage" would include the seventh to the 18th
theoretical stages, inclusive. It will be recognized by those
skilled in the art that the number of theoretical stages is to be
considered as based on functioning theoretical stages and that the
bottom theoretical stage is located just above the reflux inlet no
matter what the actual physical layout of the column is. The feed
and the defined light aliphatics can be introduced anywhere in the
middle theoretical stage; however, it is preferred that both be
introduced at the same point in the middle theoretical stage to
obtain the full benefits of the process.
The solvent is used as an aqueous solution thereof containing water
in an amount of about 1 percent to about 8 percent by weight based
on the weight of the solvent and preferably containing water in an
amount of about 2 percent to about 5 percent by weight. This
aqueous solution is referred to hereafter in some instances as a
solvent-water mixture.
Generally, to accomplish the extraction, the ratio of solvent
(exclusive of water) to feedstock in the extractor is in the range
of about 4 to about 8 parts by weight of solvent to one part by
weight of feedstock. This broad range can be expanded upon where
nonpreferred solvents are used. A broad range of about 3 to about
12 parts by weight of solvent to one part by weight of feedstock
and a preferred range of about 5 parts to about 7 parts of solvent
per part of feedstock can be used successfully for the solvent of
preference and other like solvents. In final analysis, however, the
ratio is selected by the technician based on experience with the
particular feedstock and depends in part upon whether high recovery
or high purity is being emphasized, although the instant process
will improve purity in any case.
The reflux to the extraction zone is generally made up of about 20
percent to about 50 percent by weight aliphatics having from five
to seven carbon atoms and about 50 percent to about 80 percent by
weight aromatics, both based on the total weight of the reflux. The
reflux, percentages, and ratios discussed in this paragraph are not
considered to include and do not apply to the defined light
aliphatics, although the latter might be referred to as an
auxiliary reflux. There is some overlap in composition, however, in
that some of the components which make up the reflux are the same
as some of the defined light aliphatics. These similar components
in the reflux should not be included when determining whether the
percentage of defined light aliphatics meets the stated
requirements. In any event, the sources, other than the feedstock,
are different for both in that the major source of the reflux is
the distillate from the main distillation column while the source
of the defined light aliphatics is from raffinate distillation or
from outside the system. The ratio of reflux to feed-stock in the
extraction zone is, generally, maintained in the range of about 0.5
to about 1.5 parts by weight of reflux to one part by weight of
feedstock and preferably about 0.6 to about 1.25 parts by weight of
reflux to one part by weight of feedstock, but, again, is selected
by the technician just as the ratio of solvent to feedstock. The
reflux aliphatics pass into the extract rather than being taken
overhead with the raffinate and are recycled to the extractor from
the reflux decanter as will be seen hereinafter.
The temperature in the extraction zone is maintained in the range
of about 100.degree.C. to about 200.degree.C. and is preferably in
the range of about 125.degree.C. to about 150.degree.C., especially
for the solvent of preference.
The pressure in the extraction zone is maintained in the range of
about 75 psig. to about 200 psig. As is well known in the art,
however, one selected pressure is not maintained throughout the
extraction zone, but, rather, a high pressure within the stated
range is present at the bottom of the zone and a low pressure again
within the stated range is present at the top of the zone with an
intermediate pressure in the middle of the zone. The pressures in
the zone depend on the design of the equipment and the temperature,
both of which are adjusted to maintain the pressure within the
stated range.
The temperature at the top of the distillation zone, which, in
terms of the apparatus used, may be referred to as a distillation
column or stripper, is at the boiling point of the mixture of
aromatics present in the zone while the temperature at the bottom
of the stripper is generally in the range of about 135.degree.C. to
about 200.degree.C.
The pressure at the top of the stripper, an upper flash zone in
this case, is in the range of about 20 psig to about 35 psig. In a
lower flash zone just beneath the upper flash zone and connected
thereto, the pressure is in the range of about 10 psig to about 20
psig and is about 10 or 15 psig lower than the pressure in the
upper flash zone. The pressure in the rest of the distillation zone
is maintained in the range of about 15 psig to about 25 psig with
some variation throughout the zone.
The steam brought into the bottom of the distillation zone enters
at a temperature of about 100.degree.C. to about 150.degree.C. and
is under a pressure of about 15 psig to about 25 psig. The total
water present in the distillation column is essentially in vapor
form and is generally in the range of about 0.1 parts to about 0.5
parts by weight of water to one part by weight of aromatics in the
zone and preferably in the range of about 0.1 parts to about 0.3
parts by weight of water to one part by weight of aromatics. The
water used for the steam may be called stripping water. A small
amount of water is present in liquid form in the distillation zone
dissolved in the solvent.
Referring to the drawing:
The feedstock and defined light aliphatics are introduced through
line 1 into heat exchanger 2 where they are preheated to a
temperature in the range of about 50.degree.C. to about
150.degree.C. The mixture then continues through line 1 to enter
extractor 3 at the middle tray thereof, which is the equivalent of
the middle theoretical stage. An aqueous solvent solution having a
temperature in the range of about 125.degree.C. to about
175.degree.C. enters at the top tray of extractor 3 through line 4
and percolates down the column removing aromatics from the
feedstock.
The raffinate, essentially free of aromatics, leaves the top of the
column and passes through heat exchanger 2 where it is used to
preheat the feedstock and is cooled in turn to a temperature in the
range of 75.degree.C. to about 125.degree.C. The raffinate
comprises about 95 percent to about 98 percent by weight
aliphatics, about 1 percent to about 3 percent by weight dissolved
and entrained solvent, and about 0 percent to about 3 percent by
weight aromatics. The raffinate then passes through cooler 6 where
it is further cooled to about 25.degree.C. to about 50.degree.C.
and proceeds along line 5 to decanter 7 where it separates into two
phases, an aliphatic hydrocarbons phase and a solvent phase, the
solvent being contaminated with aliphatics.
It should be noted that the "phase" is named after its main
component, which is present in the phase in an amount of at least
50 percent by weight and, in most cases, in an amount of at least
90 percent by weight.
The aliphatic hydrocarbons phase, which can still be referred to as
the raffinate, now contains about 96 percent to about 99 percent by
weight aliphatics, about 0 percent to about 1 percent by weight
dissolved and entrained solvent, and about 0 percent to about 3
percent by weight aromatics. The solvent phase, on the other hand,
contains about 90 percent to about 96 percent by weight solvent,
about 2 percent to about 5 percent by weight water, and about 2
percent to about 4 percent by weight aliphatics.
The raffinate continues overhead through line 5 into raffinate
extractor 8, which can be a single stage mixer-settler or other
conventional type of extractor.
The solvent phase passes through line 10 to join line 12 referred
to hereinafter or it can be optionally recycled to the top of
extractor 3 along line 20 (connection to extractor not shown).
The raffinate is washed with a portion of the water phase from
reflux decanter 29 and separated in raffinate extractor 8 into an
aliphatic hydrocarbons phase (still called the raffinate) which is
essentially free of solvent and water and contains about 97 percent
to about 100 percent by weight aliphatics and about 0 percent to
about 3 percent by weight aromatics, and a raffinate water phase as
bottoms which contains about 75 percent to about 90 percent by
weight water, about 10 percent to about 25 percent by weight
solvent, and about 0.1 percent to about 1 percent by weight
aliphatics.
The raffinate proceeds along line 55 to raffinate still 56 where
the defined light aliphatics are removed as a vapor overhead and
pass through condenser 58. A portion of the condensate is returned
to the top tray of raffinate still 56 as a reflux which aids in
purifying the lights by knocking down the high boilers. The heavier
aliphatics are removed as bottoms and pass through line 62 where a
portion is diverted through reboiler 63 and returns to raffinate
still 56 below the bottom tray as a vapor to provide most of the
heating requirement. The balance of the heavy aliphatics proceeds
along line 62 to storage (not shown).
The balance of the condensate proceeds along line 60 where it joins
line 1 thus supplying a high proportion of the defined light
aliphatics for the process. This raffinate distillation is the
preferred and most satisfactory mode for supplying the bulk of the
defined light aliphatics for the process. About 50 percent to about
100 percent by weight of the defined light aliphatics can be
supplied in this manner although initially the necessary defined
light aliphatics must be provided from an outside source (not
shown) to line 1 and make-up defined light aliphatics may be added
from time to time to maintain the required level. The raffinate
still is similar to stripper 23 except that it can be half the
height and half the diameter thus reducing its cost and heating
requirements. The temperature in the bottom of the raffinate still
is maintained in the range of about 45 .degree.C. to about 200
.degree.C., the pressure is maintained in the range of about 5 psig
to about 40 psig, and the reflux ratio is about 0.5:1 to about
3.0:1.
Part of the raffinate water phase can optionally be recirculated
through extractor 8 via line 11, line 9 and line 5 as shown. This
recirculation is conventional with a mixer-settler arrangement, but
may not be advantageous with other types of extractors. As noted,
this water phase still contains, along with the water and solvent,
a small amount of aliphatics. All of the balance of the water phase
is, therefore, directed from line 11 along line 12 to extractor 13,
which can again be a single-stage mixer-settler.
Feeding into line 12 via line 50 is an aromatics slipstream, which
at its source (see line 14) is an essentially pure stream of
aromatics, i.e., having a purity of at least 95 percent by weight,
or in other words, at least 95 percent by weight of the slipstream
is aromatic hydrocarbons. The purity of the slipstream is
preferably about 98 percent and for optimum performance, i.e., to
obtain the highest purity product, about 99 percent or even higher.
It is called a slipstream or sidestream because the amount of
aromatics fed into the water phase passing through line 12 is very
small. The amount of slipstream aromatic hydrocarbons used in the
process is in the range of about 0.1 percent to about 5 percent by
weight of the aromatic hydrocarbons in the feedstock and is
preferably in the range of about 0.5 percent to about 2.0 percent
by weight of such aromatic hydrocarbons. The slipstream washes the
water in extractor 13 to remove the small amount of aliphatics,
which is so detrimental to the efficiency of the process. This
aromatics slipstream can be recycled along line 15 through
extractor 13 to further wash the water phase where a mixer settler
extractor is used and it is then, preferably, sent along line 16 to
line 1 where it is reintroduced into the feedstock and passes into
the system once more. The water, which is essentially devoid of
aliphatics, but contains solvent, then passes as bottoms from
extractor 13 through line 17 and into water reservoir 51 via line
37.
Returning to extractor 3, it has been noted above that the aqueous
solvent percolates down the column carrying with it the aromatics.
In the middle of the column, the aqueous solvent comes into contact
with both the feedstock and the defined light aliphatics. Although
the mechanism involved is not clear it is believed that the defined
light aliphatics initially build up in the extract until a
saturation point is reached and then they pass into the raffinate.
The effect of the defined light aliphatics once the saturation
point is reached is to reduce the amount of aliphatics which pass
into the extract or, in other words, to improve the selectivity of
the solvent solution and enhance the activity of the reflux. In the
lower half of extractor 3, the solvent solution of aromatics comes
into countercurrent contact with a reflux liquid, which enters
extractor 3 below the bottom tray (or theoretical stage) along line
18. The reflux percolates up the lower half of extractor 3
progressively dissolving in and purifying the solvent solution of
aromatics. The solution which is formed, i.e., the extract,
comprises about 10 percent to about 20 percent by weight feedstock
aromatics, about 2 percent to about 5 percent by weight water,
about 65 percent to about 75 percent by weight solvent, about 4
percent to about 8 percent by weight reflux aromatics, and about 3
percent to about 6 percent by weight reflux aliphatics, all based
on the total weight of the extract.
The extract leaves the bottom of extractor 3 through line 19 and
passes through heat exchanger 22 where it is cooled to a
temperature in the range of about 100.degree.C. to about
125.degree.C. The extract proceeds along line 19 and enters
stripper 23, the distillation zone, at upper flash chamber 24,
which, as noted heretofore, is at a lower pressure than the
extractor. Part of the extract flashes on entering the flash
chamber and is taken overhead through line 18 in vapor form.
Another part of the extract passes as a liquid into lower flash
chamber 21, which is operated at an even lower pressure and further
flashing occurs. These flashed vapors join the fractionated vapors
and pass through line 30 to join the vapors passing through line
18. The balance of the extract (at least about 80 percent by
weight) percolates down the column into the fractionation zone
where it comes into countercurrent contact with the stripping
vapors, i.e., steam, and more vapors are generated. A part of the
vapor rises to the top of the column and mixes with the flashed
vapors in flash chamber 21 as noted. The overhead distillate
comprises about 40 to about 75 percent by weight aromatics, about
20 to about 40 percent aliphatics, about 2 percent to about 10
percent by weight water, and about 0 percent to about 5 percent by
weight solvent, all based on the total weight of the overhead
distillate.
After the aqueous solvent descends about halfway down the column,
it becomes essentially free of aliphatics. At this point, a vapor
side-stream distillate is removed through line 26. The side-stream
distillate is comprised of about 65 to about 90 percent by weight
aromatics, about 10 to about 30 percent by weight water, and about
1 percent to about 10 percent by weight of solvent, based on the
total weight of the side-stream distillate.
The bulk of the solvent and water solution, an amount equal to over
98 percent by weight of the solvent and water entering stripper 23
through line 19, leaves the bottom of stripper 23 through line 4. A
portion of this solution is diverted into reboiler 28 and returns
as a vapor to a point below the bottom tray of stripper 23 to
provide most of the stripper's heating requirements. The balance of
the water and solvent extraction is recycled to the top tray of
extractor 3 through line 4. Recycled stripping water containing
some dissolved solvent enters stripper 23 through line 27 from
water reservoir 51 after essentially all of it is converted in heat
exchanger 22 to steam.
Returning to the overhead distillate mentioned heretofore, such
overhead distillate is a combination of flashed vapors and
fractionated vapors having the aforementioned composition. This
overhead distillate is also known as a reflux distillate. The vapor
is first condensed and cooled to between about 38.degree.C. and
94.degree.C. in reflux condenser 25. The condensate then passes
into reflux decanter 29 where a reflux hydrocarbons phase is
decanted from a water phase. The reflux hydrocarbons phase
comprises about 20 to 50 percent by weight aliphatics having from
five to seven carbon atoms, and about 50 to about 80 percent by
weight aromatics and is recycled as reflux through line 18 to
extractor 3 as previously described.
The water phase contains about 95 to about 99 percent by weight
water, about 0 to about 5 percent by weight solvent, and about 0.1
to about 0.5 percent by weight aliphatics. It passes through line
31 and is split in two streams, lines 32 and 33, a raffinate wash
stream and an aromatics wash stream, respectively. These washes can
take place as shown by splitting the stream or the entire stream
can be used to wash the raffinate first and then the aromatics
providing that the water is treated with an aromatics slipstream
before the aromatics wash.
As noted heretofore, the side-stream distillate is withdrawn in
vapor form from stripper 23 through line 26 and condensed in
aromatics condenser 34 and further cooled to a temperature in the
range of about 25.degree.C. to about 50.degree.C. in cooler 35,
which can be a heat exchanger or other type of cooling device. The
condensate then passes into aromatics decanter 36 where an aromatic
hydrocarbons phase containing about 99.8 to about 99.9 percent by
weight aromatics, and about 0.1 to about 0.2 percent by weight
solvent and a water phase containing about 90 percent to about 98
percent by weight water, about 2 percent to about 10 percent by
weight solvent, and about 0.1 percent to about 0.5 percent by
weight aromatics are formed. The water phase passes through line 37
to water reservoir 51. Optionally, all of part of the water phase
can be directed through valved line 38 to join line 32 for use as
raffinate wash.
The aromatic hydrocarbons phase proceeds from decanter 36 through
line 26 along which an aromatics slipstream is taken through line
14 to wash water coming from reflux decanter 29 along line 33. As
noted, this slipstream can be in the range of about 0.10 percent to
about 5.0 percent of the total aromatics in the feedstock and is
preferably in the range of about 0.50 percent to about 2.0 percent
of the total aromatics in the feedstock. These percentages are by
weight.
In practice, the weight of the total aromatics is determined by
analysis of a sample portion of the feedstock. Aromatics added,
e.g., as slipstream, during the process cycle are included in the
determination.
The slipstream can, alternatively, be obtained from another source
such as the overhead product of a benzene fractionating column,
which is not shown in the drawing, or from a source completely
removed from the system. As long as the slipstream has the
previously noted high aromatics content, it will be satisfactory in
this process.
The combined streams of lines 33 and 14 proceed into wash extractor
39, which can be a single stage mixer-settler or other form of
extractor. Where a mixer-settler is used, it is advantageous to use
an aromatics recycle which passes along line 42 and joins lines 33
and 14 returning to wash extractor 39. The slipstream, now
containing a small amount of aliphatics, passes overhead from wash
extractor 39 into line 42 and along line 50 to join lines 12 and 15
and proceeds into wash extractor 13 as discussed previously.
Reflux water, now essentially free of aliphatics, is withdrawn from
wash extractor 39 and proceeds along line 43, which joins line 26,
and passes into aromatics extractor 44, which can be a single stage
mixer-settler or other type of extractor. This reflux water, along
with water recycled from the settling zone in the case of a
mixer-settler via line 45, which joins line 43, and process makeup
water from line 46 (source not shown) contacts the aromatic product
proceeding along line 26 into aromatics extractor 44 and recovers
essentially all of the small amount of solvent remaining in the
aromatics. This water with solvent then proceeds along line 47 to
join line 17, which joins line 37 and enters water reservoir 51.
High purity aromatic product is withdrawn from the process through
line 26.
Removal of certain impurities, which may include some aliphatics of
a type which can build up in the system and affect it in a
deleterious manner, is accomplished by taking a small purge of the
water circuit. To accomplish this purge, water is withdrawn from
any of the decanters and discarded periodically or continuously.
One such purge can be accomplished through line 48. It is found
that only a small proportion of the solvent is lost by such a
purge; however, this solvent can be recovered if desired. The water
purge stream can be in the range of about 0.25 percent to about 2.0
percent by weight of the total water in the system and is
preferably in the range of about 0.5 percent to about 1.0 percent
by weight of the water in the system.
The total water in the system can be determined easily because the
amount of water introduced can be controlled. Allowances must be
made for water losses through leakage, entrainment and upsets,
however.
Solvent can be recovered from this purge by directing the water
through line 49 to join line 53 and enter solvent regenerator 52
where the solvent is separated from low boiling and high boiling
impurities by steam distillation under vacuum. The solvent is
recovered and recycled along line 54 to extractor 3 (connection not
shown) and the water and impurities discarded.
It will be noted that in the preferred embodiment of subject
process the slipstream taken through line 14 is first used to wash
the water phase from reflux decanter 29 (i.e., one stream) and then
the water phases from raffinate decanter 7 (optional) and raffinate
extractor 8. This procedure can be varied so that a different
slipstream from a different source is used for each wash or, as
previously mentioned, a single slipstream is used to wash one water
phase where stream 31 is not split, but is first used to wash
raffinate.
In the preferred embodiment, it was stated heretofore that the
slipstream picks up some aliphatics in extractor 39 before
proceeding to extractor 13. It should be pointed out that the
purity of this slipstream containing the small amount of aliphatics
is only reduced by about one percent and that it still has a purity
of at least about 95 percent by weight and preferably about 98
percent so that the definition of the slipstream with respect to
purity is fulfilled.
The description of the invention is in terms of a continuous
process which has already been initiated. In order to initiate the
process, it is necessary to supply to the system from an outside
source, sufficient of the defined light aliphatics to first
saturate the extract or the reflux loop and then a sufficient
amount to bring the defined light aliphatics up to the prescribed
level. This is also done for the solvent and water. Once the
process is initiated, these components are recycled with make-up
being added when necessary.
Subject process is found to be particularly advantageous for
feedstock containing at least about 80 percent by weight aromatics
and even more so for those containing about 90 percent or more
aromatics. Optimum performance is achieved when the benzene content
is also high, i.e., at least about 25 percent by weight of the
feedstock. The process is found to be beneficial for feedstocks
having an aromatics content of at least about 40 percent by weight,
however.
* * * * *