Note: Descriptions are shown in the official language in which they were submitted.
9506
~5S~76
This inv~ntion r¢l~te~ to an improvement ln
~ proces~ for the ~par~lon o~ sromatie hydrocarbon~
from ~ mlxed hytrocarbon ~e~d~tock and~ mor~ particularly~
to the recovery of high puri~y ~romatic hydrocarbon~ in
hi8h yields while making efficient use of process compon-
ent~.
Descrl~ n of the Prior Ar~E
With thQ advent of the benzene-toluene~CB
aromatic~ fraction ~kn~wn and herein~ter re~erred ~o
; as BTX) as the prlncipal r8w m~terial in the manufacture
of petroch~mic~ls, o~tripp~ng ethylono in this rogard,
and the increased domand or aromatics as a compon~nt in
gasol~ne to increa~e its octan~ r~ting and ~hu~ r~duce
or ellminate the need for lead, which has been unter ire
~s a pollutant, aromatics separation process~ availed of
ln the past have come under close ~crutiny with an e~e
towsrd improving proce~s eco~omic~.
Improved proces~ economics can be tra~lated
~0 lnto, among other thlngs, the use of less flpparatus, the
lowering o heat~ng r~quirements, and the more e~fective
use of proce~s co~ponents a~ aid~ in the ~epar~tion proces~.
Variou proees~es h~ve been used for aromat~c~
separ~tion~, e.g., (1) a process u~ing an extraction
column which ~ends a glycol 801ven~/w~t0r solutio~, BTX
~nd r~1u~ to a two ~t~p di3tillation column. Th~ re~ulting
- 2
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lV5C ~ g506
BTX 18 then redistill~d to remov~ wat~r ~nd entrai~ed
~lycol; (2) a similar process using ~wo distillation
columns, BTX and w~er belng dlstlll~d Ln th~ aecond
column; (3) anoth~r simil~r proc~3~ UlBi~g two distlllation
column~, in the ~ccond column of which ]3TX and glycol are
distilledO
Generally, these proce~ses use two ~eparate
wa er circuits. One circuit i8 the ~tripping w~ter
circuit for removing arom~tics from the glycol in the
~tripper and thè oth~r i~ ~ water wash circuit. Both
water stre~ms are revaporized in these processe~. The
mak~-up o~ ~he w~ter wa~h circuit is such ~hflt the water
~rst washe~ raffinst~ and then ls distilled. Unfort-
unately, distillation does not remove ~11 dissolved ~nd
entrained ~liph~t~cs from the w~ter and yet lt is then
used to wash glycol from the aromatics product ~esulting
in decre~ed product purity. In the stripping water
circuit, 8tripping water from the reflux d~canter ~l~o
contains some aliph~tics. Finally, the use o~ two or
more di~tillation columns i8 the rule rath~r than the
exceptlon ln this type of 8y8tem.
In one ~provement over the foregoing, revaporiz-
ation i~ avo~ded in the water wash circuit; however~ a
water rectifier i8 nece~sary ~n the stripping water
; circuit and in another improveme~t, the water recti~i2r
is avoided, but v~rious ~ntreflted w~ter 3treams are
~ combined to r~cover the glycol. Although an ~ttempt i~
;',
- 3 -
~o~s~7~ 9506
m~de ln bo~h improvements to displaee aliphatlcR with
aromatics in the proc~s~ wa~er, i~ i~ apparent tha~
aliphAtic~ are necessarily prese~t in the fin~l product
thu3 reduclng purity.
In sum, a~l of the proces~es men,tioned heretoore,
while viable ccmmerci~lly, have not succeeded in optimizing
process economics together with purity.
Great strides hav~ bsen made in such optimi~ation
~nd in obtaining hlgh purity benzene; a~, or example, :
in Uhited Sta~e~ pat~nt 3,714,033; however, processes
which improve process economicsJ obtain high purit~ benz~ne,
and, further, achieve hlgher toluene and xylene puritie~
are still being sought ater.
Summa~y of the Invention
An ob3ect of this lnvention, therefore, 1~ to
provide an improvQment in a process for the ~eparation
of aroma~ic hydrocarbon~ from a mixed hydroc~rbon feedstock
ln which a ~olvent-w~ter composition 1~ utilized wh~reby
aromatic~ are recovered in high purity u~ing a mlnimum o
spparatus and heat and making more effective u~e o~
. , .
process componen~s.
O~her objects and atvantagas will beca~e apparent
,
herein~fter.
According to the pres~nt invention) high p~rity
aromatic hydrocarbons are effect~vely recover~d u~ing
min~mal 8ppar~tu8 and heat by a oontinuou~ ~01~2n~
.- 4
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~ ~ 5 ~8 7 6 9506
extr~ction - ~te~m dl~Z~illation proc~s~ ~or theZ recovery
of sromatic hydrocflrbons ha~ing boiling polnts in the
range of about 80C. to about 175C. from ~ fe~datock
containing ~liphatlc hydrocarbo~ ant said aromatic hydro-
carbons ccmpri~ing the following ~tep~:
~a) introducing the ~eed~tock into ~n e~x~raction
- zone at about the midpoint Zthereof;
(b) introduc1ng a mixturc o~ water and 301ve,nt
into the extraction zone ~t about the top of ~aid extraction
zone, ssld ~olvent being a water-misc~,le organic liquid
having a bolling point o~ at least about 200C. and having
a decomposition teZ~perature of at least about 225C,;
(c) inZ~roducing re1ux hydro¢~rbons into th~
extraction zone at about the bo~ttom thereof;
: (d) contact~ng ~e f~Zedatock in the eZxtraction
one with the mixture of w~ter and a solvent, the water
phase of stsp (J), and ~he reflux hydro~arbons ~o provide
an extract compris~ng aromatic hytrocarbons, reflux ali-
phatic hydrocarbons, solvent, and water and a ra~inate
co,~prising e~s2ntially aliphatic hydrocarbons;
~) contacting the ex~ract with steam in a
di~,Ztillation zone ~o provide an overhead distillate
compri~ng a reflux hydrocarbon~ phas~ and a water phase,
B a ~ide cut di~tillate co~ rising ~n aromatic~ hydrocarbo~s
pha3e and a w~ter ph~e~ and bo~t~oms comprising a mixture
o~ solvenZ~ ~nd w~r;
_ 5
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. . . .
: . . , . . ~ .
9506
~Sj~37~
: ~ .
(f~ contacting the ra~finate with the water
phase of step (g~ to provide a raffinate aliphatic :
hydrocarbons phase and a water phase;
~ (g) contacting the raffinate aliphatic hydro-
- carbons of step (f) with the water phase of (f) to
the side cut distillate to provide a raffinate aliphatic
. hydrocarbons phase and a water phase;
(h) contacting ~he water phase of the overhead `~
distillate with an aromatic hydrocarbons stream containing
at least 95 percent aromatic hydrocarbons, the amount of
said stream being in the range of akout 0.1 percent to -:
about 5 percent by weight of ~he total aromatic hyclrocarbons : ~
, in the eedstock to form an aromatic hydrocarbons phase .
and a water phase;
(i) contactlng the aromatic hydrocarbons phase
. ;
`.^f of the side-cut distillate with the water phase of (h) ~ ~
j
to form an aromatic hydrocarbons phase and a water phase; `~
(j) recycling the water phase of step (f) to
.' the extraction zo~e.at about the midpoint thereof;
~: 20 (k) recycling the water phase of step (i) to the
. ~ ,.. .
~. distillation zone where said water phase is essentially ~ :
,::
converted to steam; ~ .
. .. . ..
(1) recycling the reflux hydrocarbons phase of
the overhead distillate and the bottoms of step (e) to
J the extraction zone to provide, respectively, reflux :~
3 hydrocarbons for step (c) and mixture of water and solvent
~ .. ,
for step (b); and
(m) recovering the aromatic hydrocarbons phase of step
6 - :
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~05 ~7 ~ 9506
~i) and the raffinate ~liphatic hydrocarbon~ phase o s~p (g)~
~I e~ 9~l~5¦9~___ __ __ _.win~
The ~ole flgure is a schemat~c flow diagram
of fln illustrative embodiment of the pr~sent inv~ntion~ :
1~5lJ~95L~5 ~____ __ __ d~ E~
A~ noted aboveS there i8 ~n indu~trlal need ~or
BTX, whlch i~ available in hi~h propo~ti,on9 e.g.~ greater
~han 30 percent by weight, ln a wide varlety o~ hydro-
carbon feed8tocks ~uch as re~onmed ga~oline8; coke oven
light oila; cracked ga301ine~; and dripol2ne~ which,
ater hydrogenztion, can contain as much a~ 70 to 98
percent BTX. These feedstocks also contain both ~liphatic
and cycloaliphatic hydrocarbon~ (herein reerred to collect-
ively a8 aliphatic h~droearbon~). Since the individual
hydrocsrbon compounts which make up the3e feedstocks are
~: well known, they w~ll not be discuss~d extensively; how-
. ever, ~t can be pointed out that the ma~or components o
:. the ~eed~tocks u8ed herein are hydrocarbon8 with boiling
points ranging from 25C. to 175C~ including straight-
chain and branched-chain pa~affln~ and naphthenes, such
as n-heptane, i~ooctane, and methyl cyclohexane9 a~d
ar3matics such 89 BTX.
The BTX fr~ction c~n include benzene, tolue~e,
the~Cg aromatics including ortho-xylens, meta~xylene,
pars-xylffne, ~nd ethyl benz~ne, and Cg aromaticfi which9
i~ pre~ent at all~ a~pear ln the gmall~3~ proportion in
relation to the other co~ponent~.
The 801ve~t~ used in ~ub~ect proce~ are, a8
. 7
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9506
7 ~
described above, wa~er-miscible organi~ liquids (at process
temperatures) having a boillng point of at least about
200C. and having a decomposition temperature of at
least about 225C. The term "water-miscible" includes
those solvents which are complete miscible over a wide
range of temperatures and th~ e 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 car~on, hydrogen and oxygen with some exceptions.
Examples of solvents which may be used in the process ~-
of this invention are dipropylene glycol, tripropylene
glycol, diethylene glycol, N-methyl-2-pyrrolidonc,
triethylene glycol, tetraethylene glycol, acetamide~
diethylenetriamine, triethylenetetramine, diethanolamine~
para-cresol and meta-cresol, and mixtures thereof. The
preferred group of solvents is the polyalkylene glycols
and the preferred solvent is tetraethylene glycol.
, : ,
The apparatus used in the process both for the main
extraction and the distillation is conventional, e.g., an
,i :
extraction column of the multistage reciprocating type
containing a plurality o~ perforated plates centrally
mounted on a vertical shaft driven by a motor in an
oscillatory manner can be used as well as columns containing
,1
pumps w~ h 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. Counter-
current flows are utilized in both extraction and
` distillation columns.
` .:
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. ~' ~ ''.
'. ' , . , . ~. , ' ' ' ' ' ' . .. , " ' ,'~ ' ' ' '
'' ', , " :,',
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.. . .. . ..
9506
.
~55~'7~;
Heat exchangers, decanters, reservoir and solvent
regenerator are also conventional as well as various
extractors other than the main extrac~or~ These oth~r
extractors are preferably single stage miixer settlers, but
can be any of the well known types.
The solvent is used as an aqueous solution thereof
and in the case, e.g. of tetraethylene g;Lycol, it usually
contains about 4 percent to about 6 percent by weight of
wa~er based on the combined weight of the solvent and water
; 10 and preferably contains water in an amoune of about 4.5
percent to about 5 percent by weight. Generally, however,
the aqueous solutions contain about 1 to about 8 percent
.; .
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(3S~ 7 ~ 9~06
water and preerably about 2 to ~bout 5 percen~.
Thi~ aqueous ~olution 18 r~f~rred to h~re~ft~r ~n some
lng~nce8 ~8 a solvent~water mixture. In thl~ psrticular
prQcess, the foregoing amounts of water ~re thoHe used initi-
ally and those that will ~ppear in ths ~t:Lll bottoms and
the upper portion of the extractor, i.e., ~bove the mid~
point or feed-polnt. It ~g well known that these amount~
of water c~n be ad~usted by ad~usting the ~emperature ~nd
pressure at the bottom of the ~till ~shown in the drawing
as stripper 23). The amounts of water in the lGwer portion
of the extractor, i.e" below the midpoint, will b~
discussed hereina~ter.
GeneraLly, to ~ccomplish th~ extr~ction, the ratio
of solvent (exclusive of water) to ~eedstock in the
extractor i3 in the range of about 4 to about 8 parts by
weight of ~olvent to one part by weight of feedstock. Thi~
broad range can be expanded upon whera nonpreferred ~olvents
are u~ed. A broad r~nge of abou~ 3 to about 12 parts by
weigh~ of solvqnt t~ one part by welght o ~eedstock ~nd
a p~e~erred r~nga of about 5 part~ to about 7 par~s of
solvent per part o~ ~eedstock csn be used successfully
~or the ~olvent of preference ~d other like 801vent8.
~ In ~in~l analysis, h~wev~r, ~hQ ratio i8 ~elected by the
i technician based on experience with the particular feed
stock and depends i~ part upon whether high recovery or
~` high purity is bein8 emphas~zad, although the ~nst~nt
process will lmprov~ puri~y in any case.
The re~lux to the extraction zone i~ generally
- 10 -
~ ~S 5~7 ~ 95~6
made up of ~bout 20 percent to about 5Q percent by weigh~
aliph~tics havlng from 5 to 7carbon atoms and about 50
parcent to about 80 percent by w~ight ar~m~tlc~9 both b~sod
on the tot~l weight of the re1ux. The rstio of ~eflux to
feedstock 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 welght of feedsto¢k and preferably
about 0.5 to about 1.0 p~rt9 by welght o~ reflux to one
par~ by weight of feedstock, but, agaln, is selected by
the technician ~ust as ~he ra~lo of solvent to ~edstock.
The reflux a}iphfltic~ pa~ into the sxtract rather than
belng t~ksn overhead with the raf~inate and are recycled
~o the extr~ctor from the re~lux decanter ~s wilL be aeen
hereinafter.
The temperature in the extraction zone i3 maintained
in the range o~ about 100C. to about 200-C. and i~ prefer-
ably in ~he range of about 130C. to about 180C., e~peci~
~lly for the solvent of preference.
The pressure ln the oxtraction zone i8 maintained ~n
th~ range of ~bout 75 p8ig to about 200 psig. As i~ well
known in the art, however, one selected pressure i5 not
-~ maintained throughout the extractlon zone, b~, ra~her,
a high pressure wi~hin the stated range is pre~ent at the
bo~tom of the zone and a low pressure again within the
stated range i9 present at the top of the zone with an
i~t~rmediate pre~3ure in the middle of the zon~ The
pressures in the zone depend on the ~esign of the
... . .
,
1~)5~7~ 95û6
equlpment ~nd the temp~ratur~, both o which are ~d~u~ted
ts ~intain the preasure wlthin the ~tated range.
The temperature at the top of ~he di~t~llation
zone, which, in term~l of ~he spparatu~ used; may ~e
referred to ~ a di~t~llation eolumn or stripp~r, i~ at
the bolling point of the mixture of aromatic~ pr2s~n~ in
the zone wh~le the te~pera~cure at the bottom of th~ ~3tripper
i8 g~nerally in the range of: about 135C. to about 200C.
The pre~sure at the top of the ~tripper, an upper
fla~h zone in this case,, i8 in the range of about 20 p8ig
to about 35 psig. In a lower flash zone ~U8t b~ath the
upper flash ~one and connected thereto, the pre~sure i~ in
the r~ng~ o~ about 10 p~ig to about 20 p9ig ~nd i8 about
10 or 15 p8ig l~wer than the pressure ln the upper 1ash
zone. The pr~s~ure in ~he re~t of the distillation zone
i~ m~intained in the range of about 5 p~ig ~o about 30
p8ig with som~ variation throughout the zone.
The 3t~am brought into the bottom of the distillation
zone ~nter3 at a tempera~ure o~ abou~ lOO~C. to ~bout 150C.
and i9 under a pres~ure of about 10 p~ig to about 25 p91g.
The total water pre~ent ~n the di3tillation column i8
es~entiAlly in vapor form and i3 generally in the r~nge
of about Q.l parts to . bout 0.5 part~ by weight of water-
to one par~ by weight of aromatics in the zone ~nd prefer- -
ably in the range of about 0.1 parts to about 0~3 p~rt~
by weight of ~a~er to one part by weight of arom~tic~.
The water u~ed for the ~e~m m~y be called 8trip~ping water~
- 12 -
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9506
~5~37t;
A small ~mount of water i~ preaent in liquid form in the
di3tlllation zone dissolved in the solvent.
Referrirlg to the drawing:
The feedstock i~ introduced through line 1 into
heat exchanger 2 where it i8 preheated to a temperattlre
in the range of about 50C. to about lOO'~C. It then
continues through line 1 to enter extractor 3 at about
the middle tray thereo~. An aqueou~ solvent solution
: havtng 8 te~perature in the r~nge o~ abou~ 135C. to ~bout
200C. enters ~t the top tray o~ extractor 3 through line
4 and percolates down the column removing aromatics rom
the ~edstock.
The rafinate, essentially free o aromatics,
leaves the top of the column through hea~ exchanger 2 ~ ;
wher~ it i8 used to preheat the ~eed~tock and iB cooled ln -~
turn to a temperature in the range of 75C. to about 125C.
The ra~finate comprises about 95 percent to about 98
percent by weight aliphatic~, abo~t 1 p~rcent to about 3
percQnt by weight dissolved and entrained ~olvent, and
abo~t 0 percent to about 3 p~rcent by weigh~ aromatics.
, The rafin~te then pa~ses through cooler 6 where it is
; f~rther cooled to about 25C~ to about 50~C. and proceeds
along line 5 to mixer-settler i where it is contacted with
a portion of ~he wa~er phase comlng from`mixer~settler 8
via llnes 11, 12, 20~ and 5 combined with a recirculated
portio~ of its uwn water phase, w~ich pa88e9 through lines
10, 20, and 5. The first ~tage raffinate wa~h takes
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9506
5~
plsce in mixer-settler 8 and the ~econd stage raf~lnate
wa~h takes place ln mixer-settler 7~ ïn both mixer-set~lers
; 7 and 8 a raffinate aliphatic hydrocarbon~ phase and a wa~er
phase are formed, e~ch water phase belng contaminated with
aliphatics. It should be poin~ed out thst ~h~ mixer-
~ettler i3 represented by a box to denote a ~one. In fac'c,
a mixer-se~tler i~ two pieces o~ apparatus, the ~xer in
whi~h the mlxing i~ accomplished and the settler in whlch
the settling i~ accomplished, Single stage mixer-settlers
are preferred here, but other conv~n~ional types of extractor
can be used.
Tt also ~hould be noted that the "phase" i8 named
after its main component, which i8 pr~sent in the phase in
an amount of at least S0 percent by weight and, in mo~t
ca~es, in an amount of at lea~t 90% ~y we~ght.
The aliphatic hydrocarbons phase leaving mixer-~ettler
7 via line 5 can 8till be referred to as the raffinate ~nd
: now contains about 96 percent to about 99 percent by weight
aliphatics, about 0 percent to about 1 pcrce~t by weight
dissolved and entrained solvent~ and about 0 percent to
about 3 percent by weight aromaticæ. The water phase,
;. on the other hand, cont~lns about 48 percent to ~bout 84
.,
percent by weight water, about 15 percent to about 50
percent by welght sol~ent, and about 1 percent to about
;`! 2 percent by weight aliphatics.
The raf~inate continues overhead through line 5
into mixer-se~tler 8.
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9506
~ ~ 5~7 ~
The balance, generally a major proportion or more
than 50% by weight, of ~he combined water phases passing
through line 20 is preferably sent through line 20a to
join line 1 and enter extractor 3 at about its midpoint.
The raffinate wash which takes pLace in mixer-settler
8 is called the first stage raffinate wash simply because
the water used in this wash comes directly from the
system whereas the water used in ~he raffinate wash in
mixer-settler 7 comes from mixer-settler 8. See above.
The washes are counter-current. It should be noted that
a portion of the water phase from mixer-settler 8 is
optionally recirculated to mixer-settler 8 via lines 11,
9a, 9b, and 5. Aside from the rec-Lrculated water phase,
the water used in mixer-settler 8 is pre~erably obtained
from aromatics decanter 36 via lines 37, 38, 32, 9b, and
5, but can be alternatively obtained from reflux decanter
29 via lines 31, 32, 9b, and 5. The aromatics decanter
36 source is used, e.g., where the feed is a high aromatics
one and the pentanes present in the feed reduce the water
flow from reflux decanter 29. The technician has to make
this choice depending on the availability of water from
the mentioned sources. In some cases both sources can
be used.
Another option open to the technician and also
dependent on the composition of the feed is whether a two
stage raffinate wash or a single stage raffinate wash is to
be used. In most cases the two stage raffinate wash described
;,
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9506
lC~5~iY7~;
here i~ to be preferred. Th~ ~ingle ~tag~ raffinate w~sh
iB aecompli~h~d, howevar~ by eliminatlng mixer~settLer 7,
line 10, flnd l~ne 9b Recirculation to mixer~set~ler 8 would
then follow the path along lines 11, 12, 20, and 5. The
single stage wa~h i8 generally effected in cases wh~re
there i~ ~ small raf~inate, e.g., in dripolenes.
The componen~ in mixer-settler 8 ~ep~ra~e into
fln ~liphatlc hydrocarbons pha~e (still called the raff:anate)
which i~ e~sentially free of 801vent 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 rafinate water phase as bottom~ which contain~ about
95 percent to about 98 percent by weight water, about 0.1
percent to about 3.0 p~rcent by weight solvent, and about
1 percent to about 2 percent by weight aliphatic~. The
.
~eparstion in mixer-e~ttler 7 i8 about ~he ~ame, but the
ra~in~te ~till ha9 ~ome solvent and water 80 the percent-
age of aliphatics ha~ to be revised downward.
As pointed out, p~rt of the raf~inate w~ter pha~e
can optionally be recirculated through the m~xer-settler
rom which it came. In the cas0 o~ mixer-settler 8, the
r2circulation woult take pla¢e along lines 11, 9a, 9b, and
5. In the csse of mixer~aettler 7~ the reclrculation would
take pl~ce al~ng lin2s 10, 20, ~nd 5. Thi~ recirculation is
convent~bnal with a mixsr-settler arrangement, but may not
be ad~antageous with other typ2s of extractors.
The balance of the combined raffinate water phases
~; - 16 -
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9506
~ 7 ~
whlch was ~aken alon~ llnes 20, 20a, and 1 to enter
extractor 3 disso1~e~ ~n the rich ~o1vent passlng down
the extractor past the midpoint and pass~!s into the l~wer
ha1f of the ~xtractor co1umn. This addi~:isna1 water in the
l~wer half o the extractor causes a1iphatic3 to come out
of the extract ~olution by disp1acement t:hus purifying
~he aromatics having a particu1ar effect on toluene and
xylane puritie~. The lower water content at ~he top of
the extractor increa~es so1vency in the top of the extractor
for a1iphatics while at th~ bottom so1vency i8 reducecl
and se1ectivity for aromatics increased. It wa~ notecl
above that ~he water in the upper half o the e~trActor
iB used in the amount of about 4 to about 6 percent by
weight based on the combined weight of the soLve~t and
water and preferably about 4.S to about 5 percent. The
water in the lower ha1f o~ ~he extractor is about 0.5 ~o
abou~ 1.5 percent above that in the upper ha1 of the
e~tractor and i9 preferably about 0.75 to abou~ 1.25
percent above. The percentages here are again based on
the total weight of the mlxture of so1vent and water in the
e~traction zone.
If there is insufficient water in the raffinate
water pha~e, or anywhere in the system, outside water can,
of course, be used; however, side stream water via lines
37, 38, 32, 9b and 5 can be and is preferab1y u~ed to
make up any 1ack in the rafinate water phase. The use
of thi~ side stream water does reduce th~ ~mount of
- 17 -
9506
~LOS~
~ripping water ~vailable, but this can be ~imply taken
care of by rai~ing the bottoms temperature of s~ripper 23.
The recycle of the raffinate water pha~e ~o the midpoint
of the extractor al~o permits the reduc~lon of the sol~ent
` to feedstock ratio referred to above ~hu~ providing a
sa~ing in solvent.
Another option is the addition of water to the bottom
of extractor 3 via line~ 20b and 18. This water can be
all or part of the balance of the combined ra~inate water
phases which, as noted above, was preferably ~en~ along
line 20a. While not increasing the water content of the
solvent solution below the feed point, this option does
: result in the oiling out o~ an addltlonal hydrocarbon
layer at the bottom of extractor 3. This hydrocarbon layer
passes up the lowcr half of extractor 3 and increase~ the
purification in that zone.
It has been noted above that the aqueous solvent
percolates down extractor 3 carrying with it the aromatics
and Joining the r~ffinate water phase. In the lower half
o~ extractor 3, the solvent solution of aromatics comes
into countercurrent contact with a reflux liquid, which
enter~ extractor 3 below the bottom tray along li~e 18.
Th~ reflux percolates up the lower half of extractor 3
progressively dis~olving lh and purifying the ~olvent
solution of aromatics. The ~olution which is ~ormed, i.e.,
the extract, compri~es about 5 percen~ to about 10 percent
by weight feed~tock aromatics, about 3 percent to abou
: .,
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~ ~ S 5~7 ~
6 percent by weight water, about 75 percent ~o about
85 percent by weight 901vent, about 4 percent ~o about 8
percent by weight re~lux aromatics, and abou~ 3 percen~ ~o
about 6 percent by weight reflux allphat:ic~, all based on
- the total weight of the extract.
The extract le~ves the bottom o~ ~xtractor 3
through line 19 and p~83e8 through heat exchanger 22
where it i8 cooled to a temperature in the range of about
100C to about 140C. The extract proceeds along line
19 and enters stripper 23, the distillation zone, a~
upper ~lash chamber 24, which, as noted heretofore, i8
at a lower pressure than the ~xtractor. Part of the extract
flashes on entering the ~lash chamber and is taken over-
head through line 18 in vapor form. Another part o~ the
extract pa~ses as a llquid into lower flash chamber 21,
:i which is operated a~ an even lower pressure and further
flashing occurs. It should be noted that fla~hing is
minimized in the present process. The flashed vapors
Join the fractionated vapors and pasB through line 30 to
20 ~oin the vapors passing through line 18. The balance o~
the e$trsct ~at least about 80 percent by weight)
percolates down the column into the fractionation zone
where it c~mes into countercurrent contact with the
stripping vapors, i.e., steam, and more vapors are gener~
ated. A part of the vapor rises to the top of the column
and mixes ~th ~he flashed vapor~ ~n flash chamber 21 a~
noted. The overhead distillate comprises about 40 to
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~0 5 ~f~ :
about 75 percent by wslght aromatirs, ~bout 20 to about
40 percent allphatic~, abou~ 2 percen~ t:o about lO percent
by weight water, and about 0 percent ~o ~bout 5 percent
by we~ght solv~nt, all based on the total weight of the
overhead distillate.
After ~he aqueou~ solvent descends ~bout halfway
down the column, it becomes essentially ~ree of al~pha~ies.
At thi3 point, a vapor side-~tream distillate is removed
through line 26. The side-~tream distillate i~ comprised
of about 65 to about 90 percent by w~ight arom~tics, about
10 to ~bout 30 percent by welght water, and about 1 percent
to about 10 percent by wei~ht o solvent, based o~ the
total weight of the side-stream distillate.
: T~a bulk of the ~olvent and water solution~ an
- ~mount equal to over 99 percent by weight of the solvent
-, and water entering stripper 23 through line 19, leaves
the bo~tom of s~ripper 23 through line 4. A portion of
this solution is diYerted into reboiLer 28 and returns
as a vapor to a point below the bottom tray of stripper
20 23 to provlde most of the Btripperls heating requirements.
The bal~ce of the w~ter and solvent solutionL i~ recycled
. .
to the top tray of extractor 3 through line 4. Recycled
strlpping water contalning some di~solved 801vent enters
~tripper 23 through line 2~ from water re~ervolr 51
a~er es~entially all of it ig converted ln hea~ exchanger
22 to steam. Returning to the overhead dlstilla~e mentioned
heretofore, such overhead distillate i8 a combination of
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~ ~ 5 ~ ~ 7 ~flashed vapors and fr~ctionated vapor~ h~ving the afore-
men~ioned composition. This overhe~d di~tillate 1~ ~190
known as a reflux dis~illate. The ~por i8 first condensed
and cooled to be~ween about 38C. and 94C. ln refl~c
condenser 25. The condensa~e then paese~ into reflux
decanter 29 where a reflux hydrocarbons phase i9 deCQnted
from a water pha~e. The reflux hydrocarbons ph~e comprises
about 20 to 50 percent by weight aliphatics hnving from 5
to 7 c~rbon atoms, and about 50 to abvut 80 percent by
weight aromatic~ and is recycl~d ~s reflux through l~ne
18 to extractor 3 ~19 previou~ly de~cribed.
The water phase con~alns about 95 to abou~ 99
p~rcent by weight water, about 0 to ~bout 5 psrcent by
weight solvent~ and about 0.1 to a~out 0.5 percent by
weight aliphatics. It passes through line 31 and may be
~plit into two streams, lines 32 and 33, a raffinate wa~h
stre~m and an aromatics wash stream, respectively, depending,
as noted above, on the amount of water available. The
preerred mode of operation, how0ver, i9 to use the refl~x
distillate water phase or the aromatics wash ~nd the side
cut distillate water phase for ths raffinate wash.
As noted heretofore9 the side- tream distillate is
withdrawn in vapor fonm from stripper 23 through line 26
and conden~ed in arom~tics condenser 34 and further co~led
to R ~emperature in the range of about 25DC. to about 50C.
in cooler 35, which can be a heat exchanger or other type
of cooling device. The condensate ~hen pa~se~ into aromatics
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.
,, ', .. ' ' . ~.. . ' .
9506
~ ~ 5 ~
decanter 36 where ~n aromatic hydroc~rbons ph~s~ con~ain-
ing about 99.8 to abou~ 99.9 perc~nt by w~ght aromatics,
and about 0.} to about 0.2 percent by weigh~ ~olven~ ant
a water ph~e containlng abou~ 90 p~rcent to about 98
percent by welght water, about 2 pQrcent to about 10 psrc~nt
by welght solvent, ~nd about 0.1 percen~ to about 0.5 percen~
by weight sromatics are ~ormst. The w~e~r pha0e may pa88
through line 37 to water re~er~oir 51. Preer~bly, however,
all or part o~ the water phaæ~ i5 dir~cted throu~h ~lvet
line 38 to Join line 32 for u~e as raffinate wash.
The aromatic hydrocarbon~ phase proceeds rom
decanter 36 through line 26 a}ong which an aromatics slip-
gtream i9 taken through line 14 to wash w~t~r coming from
reflux decanter 29 along line 33. The aroma~ic~ slip~tream
at it~ source 1~ ~n easentially pure ~tre~m o~ ~romatic~, i.
e., having a purity of at lea3t 95 percent by w~ight, or in
other words, at least 95 perc~nt by weight o the 011p-
~tream i8 sromatic hydrocarbon~. The purity d ~he slip-
stream i9 preferably about 98 percent and or optim~m p~r-
ormance, l.e., to obt~in the highest purity product, about
99 percent. I'c i~ called a ~lipstream or ~ldestream becau~e
the amount of aromatic~ fet into th~ water pha~ p~ing
throttgh line 33 i9 very ~mall. Th~ arnount of sllp~tresm
aroma~ic hydroc~rbo~s used ln the proc~ss i~ in the ra~ge
of about 0.1 percent to about 5 p~reent by wetght oi~ t:he
Broma~CiC hydrscarbons in the ~eed~tol:k and i~ pref~rllbly
in ~he r~nge o~ ~botl~ 0.5 p~rc~n'c to about 2.0 perc:~nt by
- 22 ~
.
950&
~ ~5 ~ ~7 ~
weigh~ of such aromatic hydrocarbon~. Th¢ ~lipstream
wa~hes the water in extractor 39 to remove the small
amoun~ of aliphatics, which i9 80 detrimental~to ~h~
efficiency of the proceas. Thi~ aromat~c~ slipstream ~8
then, preerably, 3ent along line 16 to line 1 w~ere it~i~
rein~roduced into the feedstock and pas~es lnto the Rystem
once more, or, alternatively, it i9 removed fram the
8ystem.
I~ practice, the weight of the tot~l aroma~lcs is
determlned by analysi~ of ~ sampl~ portion of the feed-
; stoclc. Aromatlcs added, e.g., as slipstream, during the
process oycLe are included in the det~rmination.
The slip~tream can, alternati~ely, be obt~ined from
anoth~r source such as the overhe~d product of a benzene
: ractionating column, which ~8 not shown in the drawin~,
or rom a source completely removed from the system. A~
long as the slipstream has the previously noted high
~rom~tics content, it will be Ratisactory in this process.
The comblnet streams of lines 33 and 14 proceed into
wash extractor 39, which can be a slngle stage mixer-~ettLer
or other form of extractor. Where a mi~er-settler i~ used,
it is adv~ntageous to u8e an aromatics recycle which pa~ses
along line 42 and ~oins lines 33 and 14 returni~g to wa~h
extractor 39. The slip8tream9 now containing a ~mall
amount of aliphatlcs9 pa~e~ from w~sh extractor 39 into
line 16 88 discussed prevLously.
Reflux water, now essentially free of ~liphatic~,
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. .
. ~
,~ 9506
10 5 ~
is withdrawn from wssh ex~rac~or 3~ and proc~eds along
line 43, which ~oln~ line 26, and pa~es ~nto arom~tic~
extractor 44, which c~n be a single stage mixer-settler
or other type of extractor. This re1ux water, a}ong with
wster recyclet ~rom the settlLng zone in lthe case of a
mixer-~ettler via line 45, which ~olns line 43, ~nd proces~
m~keup w~ter from line 46 (80urce not shown) contacts the
- aromatic product proceeding ~long line 26 into aromatics
extractor 44 and recover3 essentlally all of the ~mall
amount of solvent remaining in the arom~tlcs. Th~s water
with solvent then proceeds along line 47 to Join line 17,
which ~oins line 37 and enters water re~ervoir Sl. This
wflter c~n, op~ion~ly, b~ ~ent Prom line 17 to line 38
(connectlng line and ~alve not ~hown) for use in the raf~in-
-ate wash. High purity arom~tic product i~ withdrawn from
the process through line 26.
There i3 a provision for the removal o certain
impurities~ which may include some aliphatics, of a type
which can build up in the system and affect it in a deleter-
ious manner. Thia i9 accomplished by taking a small purge
o the water circuit. To ~ccompLish this purge1 water i~
withdrawn from any of the decanters ~nd discarded periodi-
cally or continuousl~. One ~uch purga can be accomplished
through line 48. It i3 ~ound that only a small proportivn
o~ the solvent is lost by such a purge; h~wever, this
solvent can be recovered if desired. The water purge
stream can be in the range of about 0.25 percent to about
:
- 24 -
:
: . . .
~ . . .. , :
9506
, . . .
~ SS~
2.0 perc~nt by weight of the ~otal water ln the system
and iR preferably in the r~nge of about 0.5 percent to about
1.0 percen~ by weight of he wster ~n the 3y8tem.
The total water in the ~ystem can be determined
easily because th¢ ~mount of water introduc2d can be
controlled. All~wance~ mu~t be made for water 108se9
~;~ through leakage, entrainment and upsets, however.
Solvent can be recovered from thi~ purge by direct-
ing the water through line 49 to ~oin llne 53 and enter
~olvent regenerator 52 where the solvent i8 separated from
low boiling and high boiling impurities by ste~m distillation
under vacuum. The solvent i8 recovered and recycled along
line 54 to extractor 3 (connection not shown~ ~nd the
water and impurities discarded.
- 25 -
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