Azeotropic Purification of Styrene LLOYD BERG', J. 31. HARRISOX, . ~ N DC. W. M0NTC;OMERY Gulf Research & Development Cornpan?,. Pittshtrrph. 1 ' ~ .
I h c rcverw c'iide, where thc of entrainers include phase separation, solvent extraction, arid pressure rectification. \ comparison betireen the amount of the s t j rene reco\ered and the coluiiin capacit3 and heat requirements indicates that thew factor. iire renitelit. The ION boiling entrainers, s u c h a s water or acetic acid. had the c*apac*it!and heat requireeffect of iticreaCing the (.oI~iniii nients, 3s heren. those h i l i n g higher, s ~ i c has 1-hictanol or nicth?l ( ellosolte, gate ii poorer >ield of st>retie. The optiiiiuni entrainers .ippeared to be intermediate Iioiliny conipouricls. *uch as i-obu tatiol o r 1-nitropropane. h e o tropic. tnethotls cilharicc the +epsration of st>renc from ethrl1,ciirerrc \+it11respec t to the quaiitit3 of st>rerle reco>erecl i n .I gi\eti coluniti, or with reapect to the sa\iiig of rolumii (:ip.ic i t ? and heat rccpirements for a gi\eii H? :I conibination of ordinar? rectification and pitrity azeotropic (list illatioii, probably ail? lijdrocarhon i t i i puritT init> lw ic.tno\ed from st?reiie.
I ,
k:Xl'k:I have bccn repnrtcd from 30 to 805, (fd). The fact that during dehydrogenation only :i portion of the etliylbenzeiie under treatment is converted does riot represent n serious loss, provided the styrene can be readily a i d completely separated from the ethylbenzene and the latter recovered for recycling. Furthermore, in many of its uses styrene must be had in a relatively pure state, and large quantities of ethylbenzene admixed cannot bc tolerated. Ethylbeiizciit. is separated from styrelic conirnercially by continuous rectification. This is not easily accomplished, however, because thc difference in boiling pointi: is not large and the deviation from Raoult's law is reported t o be negative (1.3). Thus, a column of a large number of plates i.5 required to obtain rcasonably pure ethylbenzene and styrene, I t !vas the purpose of this investigntiuii to determine wlietlier the difficulty of this separation might be reduced by application of azeotropic distillatioii methods. Previous investigators reported the azeotropic separation of styrene from ethylbenzene using, as cntrainers, tlic loner fatty acids ( 1 , 2 ) and steam to form :i heterogeneous azeotrope ( I O ) . The use of ethylene glycol monomethyl ether (methyl Cellosolve) to separate styrene from cod tar naphtha arid xylol fractions 1j-a~ also suggested (.3, 5 ) . TTPES OF ENTRAISERS
Separatiuiis by :izeotropic distillatioii are accomplished by the formation and proper utilization of constant boiling mixtures. The entrainers capable of accomplishing the separation of styrene from other close boiling hydrocarbons fall into t v o general groups : (a)The entrainer forms RII azeotrope with the nonstyrene component of the mixture but not with the styrene. To be useful, the vapor-liquid equilibrium relations between the azeotropc formed and styrene must be such that separation by rectification is enhanced. Such an entrainer will be referred t o as sclective. 1
Prewnt addrew. M o n t a n a State College, Bozeman, 3Iont.
:md styrents v ; r ~ohtaiiid (run1 >I.I.I.IIRIAI.~.Etlir-lbeiizc~ni~ Dow C'iicii.nic:d C'on1p:iiiy tind v-erc sufficicntl>~purr i'11r uso \ \ . i t h u t iurtiicxr proccs,.ing. .\I1 nxitcrid.: inr-c~stig:.ntctl :IS pcssihlc tbjf*i.tcdt o :I 1)relinii1iU')~1:ibor:ttory rectifitxt io11 t h t i'ruc.tioii lmiling C;. fronl thcb noriiid boiling p o i i i t . Thc' builiiig puin t GO r i m . (IC mercury :iritl the rel'ructivc. i i i d i ( ~ . li.;tcvl in Td)l(tI nrc til,,:ic,tual valiic~. for tlic inipnunds u w i l c3zpcrinic.nt :illy. .\S.\LYTICIL MEIHOD. .\ii:tly~t\? t)i :\zvotropii.cmqmitioiiu \vc'rc ni:~tl(:11). t,c,ir:ic.tivt. i i i c l e s using :L \':iIcwtiii~: refrni-tometcr l -tlicoretienl plates t lic
I< ESCLI'S
.i numbc.r u i compourids were found wliich actctl as suit:iblc cntraiiiers t u effect the azeotropic separation of ethylbeneenc from styrene. Three of these acted 3,s selcctivc entrainers over n presjure range of 760 to .50 mm.of mercury. Thl. three sdcctive entraincis \\-ere 1-nitropropalie, irohut,yl propionate, arid ethylene bromidv. Of these 1-nitropropane was thc m o d szkisfactory. Ethylene bromide decomposed considerably on prolonged boiliiig md, in witlition. cntraitictl only :L small treight proportion of ethylbenzene. Isobutyl propiouute gavc :I tlrprewion of lese th:m 1" C., arid the, equilibrium relntion is :uch t1i:it complete sepalation is difficult in an ordinary column. The rczult of a batch rectificatioii usiitg 1-iiitroprupiirie :is :ui eiitraincr is show1 in Figure 1. Suficiciit 1-nitropropane 1 ~ : ~ s added to a 50-50 weight 5%mixture of cthylbermne and styrviic to form an azeotrope with all the ethylbenzene preseut. The ethylboiizciie-1-nitropropalie azeotrope distilled over, and thc refractive index remained substantidly constant at a point intermediate between that of etliylhenaeiic and 1-nitropropano. Khen t,he azeotrope was esll:iu>teci, the v:ipor temperature rose rapidly to the boiling point of $tyrene, and substantially prirc styrene, as indicated by the refractive index, mas removed as overhead. If there is a large excess of 1-nitropropane prescwt,
1149
INDUSTRIAL AND ENGINEERING CHEMISTRY
1150
Vol. 38, No. 11
I6000
1.8000 STYRENE
I5800
1.5800 ISOBUTANOL
I5600
44
60
1.5600
I 5400 x
I5200
E I5000
9-
t-
I4800
2 U
I4600
a
I4400 I4200 0
10
20
30
WElWT
40 PERCENT
SO
OF
60
70
CHARGE
BO
90
100
DISTILLED VAPOR
F i g u r e 1. Azeotropic Distillation Curve Using I-Nitropropane
I
I
l
l
I
I
l
l
/
I
1
0
1.6000
64
20 30 40 WEIGHT PERCENT
Figure 2.
1.5800
60
10
TEMPERATURE
'
)
1
50 60 70 80 OF CHARGE DISTILLED
{lJ8m
SO
100
Azeotropic Distillation Curve Using Isobutanol
1.5600
t
1.4000
40
.
1.3000
1.3600
0
10
20 30 40 50 60 70 00 WEIGHT PERCENT OF CHARGE D I S T I L L E D
F i g u r e 3.
80
100
Azeotropic Distillation Curve Using Acetic Acid
no loss in quantity or quality of the styrene is encountered because the excess l-nitropropane can be recovered after the ethylbenzene-1-nitropropane azeotrope is exhausted. An excess of 1-nitropropane in a continuous system requires that the stripping section of the column be capable of separating it from styrene. The azeotropic separation of ethylbenzene from styrene can also be effected with a number of compounds which act as nonselective entrainers. These compounds include the loner aliphatic monobasic acids, a number of alcohols of more than two carbon atoms, methyl Cellosolve, and water. As the boiling point of the entrainer approaches or exceeds that of ethylbenzene, the boiling temperature of the azeotrope becomes only slightly lower than that of ethylbenzene itself, and most of the advantages of azeotroping are lost. Therefore, the compounds boiling below
ethylbenzene are usually more suitable as entrainers than those boiling at or above ethylbenzene. The effect of pressure often is such that an entrainer which is poor a t one pressure is satisfactory at another. The alcohols normally boiling from 95' to 136O C. can be used as nonselective entrainers. The compositions as well as the boiling points of the azeotropes are markedly dependent upon the pressure of the system. For example, at atmospheric pressure, sec-butanol forms no azeotrope with either ethylbenzene or styrene, but a t 60 mm. of mercury pressure, aec-butanol forms an azeotrope with each. At atmospheric pressure isobutanol is a selective entrainer, forming an azeotrope with ethylbenzene but not n i t h styrene, whereaa at 60 mm. isobutanol forms azeotropes n i t h both ethylbenzene and styrene. Tert-butanol forms an azeotrope with ethylbenzene only below approximately 60 mm. The change in azeotropic composition with pressure may be exemplified as follows: At 60 mm. of mercury the isobutanol' ethylbenzene azeotrope contains approximately 39 weight % ethylbenzene, whereas a t atmospheric pressure the azeotrope composition is only 15Q/, hydrocarbon. Thus, the total pressure on the system may determine whether any azeotrope exists, whether or not the entrainer is selective, and the compositions of the azeotropes when formed. An example of the use of an alcohol (isobutanol) as an entrainer is shown in Figure 2. Only a small excess of isobutanol was used, so that a minimum of styrene was removed as an azeotrope. The lower aliphatic monobasic acids, particularly formic and acetic, are suitable nonselective entrainers for this separation. A batch rectification using acetic acid is shown in Figure 3. An example of the use of water as a nonselective entrainer is shown in Figure 4. RECOVERY OF ENTRAINERS
To make a process economical, it is essential that the entrainer be recovered from the nonstyrene component (or components) with which it is azeotroped. Three general methods were considered to effect that separation-namely, solvent extraction, pressure rectification, and phase separation.
November, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
1151
14400
14200
I3800
-,
VAPOR
TEMPERATURE
I3600
30 _ _ _ _ _REFRACTIVE _ _ _ - _ _ _I NOEX -_----341-
LOWER
I3400
-
PHASE
28
I3200 0
10
20
30
WEIGHT
Figure 4.
40
50
PERCENT OF
70 80 CHARGE DISTILLED
60
90
100
Azeotropic Distillation Curve Using Water
If a solvent can be found which is capable of effecting a complete separation by estraction alone and which is readily separated from the extract, solvent estraction is an attractive method of recovering the original entrainer. I n many cases, however, the selective solvent is capable of concentrating only one constituent with respect to the other, or the recovery of the solvent is complicated by additional azeotrope formation. Pressure rectification offers a simple means of recovering the entrainer from its azeotrope, provided the vapor pressure-temperaturc relation is such that the Composition of the azeotrope varies considerably with pressure (9, 13). If the data yield nonparallel lines when they are plotted on a Cos-type chart (Figure 5 ) , change in azeotropic composition with pressure will be considerable; but when these lines are almost parallel, usually little change in azeotropic composition occurs. Thus from Figure 5 one might expect a decreasing amount of alcohol in the isobu-
tanol-ethylbcnzene azeotrope as the total pressurc is incrcased, and but little change in composition of the l-nitropropanc-ethylbenzene azeotrope. This is confirmed by esperiment. When i t is possible, phase separation is the simplest means of entrainer recovery. Of the entrainers investigated, only Tyater could be readily and completely separated by decantation of thc condensed distillate. The recovery of alcohols or methyl Cellosolve from cthylhenzene can be readily accomplished by pressure rectification. T h e change in the composition of the isobutanol-ethylbenzene azeotrope with pressure is shown in the following table: Abs.
Pressure. Lb./Sq. In. 1.16 14.3 20.2 24 9 38.2 70
TABLE I. PROPERTIES OF COMPOUNDS AND AZEOTROPES AT 60 MM. Hg ABSOLUTE PRESSURE ~ ~ i P2int, C.
Compound Styrene 68 Ethylbenzene 60.5 n-Butyl carbinol 70.5 Isoamyl alcohol 70.5 sec-Butyl carbinol 70 Propionic acid 69 Isobutyl propionate 66.5 2-Pentanol 62 Formic acid 61.5" Methyl isopropyl carbinol 61 1-Butanol GO 1-Nitropropane 58.5 Ethylene bromide 58.2 Methyl Cellosolve 57.5 Diethyl carbinol 53 Acetic acid 51.3 Isobutanol 50 tert-Amyl alcohol 50 2-Butanol 45.5 1-Propanol 43.2 Water 41.5 tert-Butanol 28.3 a A t 200 mm. Hg/pressure. b Two phase.
Azeotrope with: Refractive l i ~ ~ Ethylbenzene Styrene Index, B.P., Compn., $.p., Conipn.. n y wt. % wt. % 1.5468 . 1.4953 . .. ... ... 1,4120 57.5 80 ... 1.4076 58.5 74 64.8 57 1.4098 56 67 60 48 1.3872 58.5 90 ... 1.3973 GO 87 N O azeotrbpk 1.4083 54 50 60 31 GOo 316 *., 1.3703
c.
. .
...
51 53 56.4 57 51 61 48 48 45 44 41 33.5 28
38 63 39 13 57 50 25 39 17 16 32 676 5
c.
... ...
... ...
...
1.4097 1.3993 1.4015 1.5386 1.4021 1.4091 1.3724 1.3965 1.4057 1.4069 1.3858 1.3330 1.3878
...
...
.... ..
... ...
49
. .4. ...
57 41 No azeotrope S o azeotrope
.,.
... 45
...
...
... 25
...
No azeotrope
Approx. Coinpn.,
wt. yo
Ethylbenzene
4
0 (estd.)
Figure 6 is a flow sheet for the azeotropic separation of ethylbenzene from styrene using isobutanol as an entrainer. Thc weight balance indicated is obtained when the azcotropc column is operated a t 1.2 pounds per square inch absolute pressure and the ethylbenzene recovery column at about 70 pounds per square inch pressure. The separation of the lower fatty acids from ethylbenzene could be d m e by water washing. Subsequent recovery of the acid from aqueous solutiori is well known (8, 1.8). I n the case of the 1nitropropane-ethylbenzene azeotrope, neit'her pressure rectification nor solvent estraction proved effective. Other methods, such as chemical trcatment, could be used. COMPARISON OF RESULTS
A contrast between the azeotropic systenis and
ordinary rectification is shown by comparing the azeotropic batch rectification curves (Figures 1-4) with the nonazeotropic curves (Figure 7).
1152
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 38, No. 11
I -rilcSe(lurvesyilo\vtil(! r(t>ll~t,> 70 CHARGE- WEIGHT PERCENT of batch rectifications of 50-50 I E' STYRENE 50 \\.right mixturrs of styreiii, a n d ethylbenzene run in tlii, .-:imc column at rcflus ratios 01 5 tn 1, 10 to 1, nnd 20 tl) 1: they nlso shniv the, c,ffcct 01 varying rcfliis r:itiii* IIII tlii. REFLUX R A T I O I ionazr~otropicsyst tml. 20-1 0 - -10-1 0 - . . euming the mol:ir I : i t o i t t 5-1 e ---_ lieat of vaporizntion of thl. variow compound? to I)r :ilmnt equal, a comparison of column rapacity and hcnt rcqiiircmentc I5400 for these systems i u t y be 01)1 t :tined by determining thc, 605 40E I5200 niolei of vapor conrlcnsed per mole of 50-50 styrt~ne-etliylK benzene mixtiiri. cliargccl. Y Lr I 5 : STYRENE __ .--c -L-- . I 4 8 0 0 I Table I1 slio~vvu tlii, columii A; I S O B U T A N O L I I 1 0 IO 20 30 40 50 60 70 80 90 100 capacity requiremcut ns perWEIGHT PERCENT OF CHARGE D I S T I L L E D centage of capacity rctluired Figure 6. Flow Sheet for Azeotropic Separation of ior the nonnzcot ropita scparaFigure 7. Coniparison of ~ t , r e i i e - E t h y l b e i i a e i i ~ , Styrene from Ethylbention at 20 to 1 rc+lux ratio. Distillation f ' i i r x r s ai Se\c=raI Refllis Ratio. sene Usine Ieobutanol The per cent st,yrene rrrovercci in a nuritv. of 9jri; or Iiir1ic.r Lhe nit,ropropant: c u i tiiris reinovo ixrtaiii I~ydrucarboiiaboiling ('l'nbk 11, N as determined froni experimental data on h t c l i higher t h m styrene-l'oi iJ.i:implt-, nonnrnn1:itic.s of tlrr t'nmenc i~iiis. The assumption was made that a11 the styrene c.li.irgctl h i l i n g rnngc. could Iir rrcnvered ns overhead by nienns of a proper c ~ l i r i w i
1 ,
-
I .
(:O\(:LUSIOhb
rizttotropic distil1:ttiuii iiicrc,nses tlte quailtit)
T A R TTT. U Lnirain
Nonr None Nonc 1-Butand Methyl Celiomvt 1-Nitropropan" Isobutanoi Water Acetic a , i(.
(:(J~IPARISOS OF
Iteflux Ratio 20.1 10:1 .:i 1
5:1 5:1 5:1 5:l
5:l 5:1
$1
DISTILLATIOS sYSTEMb
St,yrene Recovery in 95So of lligh~P r urity 72 62 30 50
Cvlumn G u ~ a c i i > Rpquireil, % 100
4 i
gG I ,
i0
io
zz.:,
'8.3 5G 60
81..-> 93 1 l:i 180
Tllc ~ ~ ~ J C : " i l l l l ~ l l td:rt:i :li indicate t h t . i i i g ~ t i i ~ i ~ :t ihIi J, . s f ' C I I tritinera mntaining :L high 1iydror:irhon cwntcnt i i t tli(*ir vtliylbenzene azcotropc. s11cIi :la 1-but:inol or inctligl ( ' ( ~ l l i i ~ o i v iy~k, k l n r m a k r amount v i high purity styr?iif,. The Mviug i l l i . o h n i i c:ipacity and lieat rcquiremcnts i. offscat 11y the rctliiction i i i yieltl I I acceptable ~ stywire obtaincd. 011the other hand. ih pounds which entraiii only :t s~iiallmole iraction ill' (.tliyI such as water or ncet,ic acid, usually yield high piii,ity >tyrtlne i i i good qiimtity but increa,se column rnp:xity : r ~ i d i i ( , : i t ri>quircments. The must sntisfact,ory eiitr:tintxrs :LW Iiro\i:tbly t intermediate compound&,such as isohutanol :ant1 l-nitrol)rni,:lrit,, \r-liicl> com pro mi SP fwtv-een t 1iesc t 1 x - n fnr t o r i . 1112
~ P Y L ~ C A Y IOF ~ Soriim .\iixr~i~b:.
Tlieae elitrainera are uot8limitcd t o t,he sepalatioii ( 1 1 ,-tyreiit, from ethylbenzene but, may be used t,o s(qmrat-, r. \ t l : i n l i c Cit,y, X. J .
