Rectification of Benzene-Toluene EFFECT OF OPERATING VARIABLES ON PLATE EFFICIENCY
Meters, controls, and reflux proportioner
Condenser, feed tanks, and column top
Reboiler, cooler, tank, and piping
Reboiler, tanks, and bottoms cooler
Figure 1.
Views of Twelve-Plate Column 752
John Griswold and Paul
B.
Stewart'
THE UNIVERSITY OF TEXAS, AUSTIN, TEXAS
P l a t e efficiencies are presented for rectification of benzene-toluene a t total reflux and at partial reflux for a 6-inch diameter twelve-plate column having two b u b b l e caps per plate. Incomplete mixing o f the l i q u i d flowing across the plate i s shown t o have a major influence o n plate efficiency and its trend with l i q u i d composition. The over-all separation or column efficiency i s l o w ered when the feed i s introduced t o o high i n the column. H i g h e r plate efficiencies observed at the column top, feed zone, and sometimes bottom are askribed t o effects of c o l d reflux, of c o l d feed, and of the reboiler, respectively. Values of Murphree plate effi-
ciencier (vapor) are 2 t o 10% higher than column efficiencies (theoretical plates d i v i d e d by actual plates) for most of the runs. Local efficiencies calculated from the Murphree plate efficiencies by means of the Lewis relation (case 111) are reasonably constant. L o c a l efficiencies show n o significant trend with l i q u i d composition, with reflux ratio, or with vapor velocity over a rather wide range of operating conditions. The average local or p o i n t efficiency i s 50 t o 55% for this column on benzene-toluene, which agrees well with values predicted by the recent correlation of Geddes.
P
is of major significance, as in coniiriewial rolunins. The number of plates is sufficient t o shoir tho c$Terts of cold rcflus, of cold iced, and of t,hc reboilrr on individual plate ctfficiencies. The results and conclusions of t,hr present, irork explain to somc -tent th(5 discrrpancics h c t n w n those of other invc.st,igations. ncc it appc~:tri.that all important phenomena of rectification in buhhlc-cap d u n i n s arc cspI:tinatde on logical bases, it is liIcc,ly that all of the major m,rial)lcs arc knon.n or undrrstood as such. Thew wmains the accuniulation of data on multiplate columns of sufficient accuracy and detail t o cnablc accurate pretiirtion of platc efficiencies.
L.ITE efficiencies on coninic.rcia1 and on 1ahr:it ory eolumns have been reporred in a nunilicr of itrticlcs, hut many more data are needed. Conimrrcial colunins are idcal for the study of plate efficiencies but cannot be spared from product.ion in ordcr to obtain plate cfficiencies over n-idr rangcs of and during abnormal operating conditions. Hlcnccx rommcr.cinl in this data on an individual column arc never comprc~hcnciac~ respect. D a t a and conclusions to datcs from cspc~rinicntson sniall or laboratory scale columns v a r ~ -so among diffcrent invcstigators that, the quantitative pffc,cts as n-ell its ninny of the qualitative effects of operating variahlrs arc still controvrrsial. This article presents fractionation data on hciir;cnc.-toluc~i~, at partial as well as at total reflux in a multiplatc colunin of wmiworks capacity. The design is such that the, e o n ~ ~ n t r : i t i ogran dient or degree of nonmising of the liquid flowing it(w-:: n plate
FRACTIONATING EQUiPMENT AND MATERIALS
The column (Figure 1') with all acccssoriw and 1inc.s c~stcndcd from the first floor through a 1i:itchway d m o s t t o thv wiling of tlic ac~iondfloor. Figurc 2 is a di:rpram vhowing thc awvhsorics
Present address, E . I. du Pont de Xemours 6; Company, I n c . , IVilmingt o n , Del. 1
Figure 2
753
754
INDUSTRIAL AND ENGINEERING CHEMISTRY
of 50-gallon capacity, and it fccd tank oi 171gallon capacity. Transfers from the run tank$ to the feed tank are made hy a 5O-gallon-p and toluene, respect'ively. A Sational Bureau of Standards compilation of 5pecific and latent heats of benzene is available (4). Thew tlata converted to English units are plottcd in Figure 5 . K O such compilation was found for toluene at. the time of the work. Experimental data on spccific and latent heats for toluene are meager and often inconsistent among different investigators. -4 thermodynamically consistent set of values was worked out according to the folloning proccdure: T h e molar latent heat at varIouP trniperaturcxs n-as calculated by Othmer's "refrrencr substance" method ( I Y ) , using the latent heat of benzene and the vapor pressures of henzcnc and of toluene. .\ linear tcnipcraturc>-lieat capacity relation is assumed for liquid toluene, and a ctraight lint. \ v a ~drawn through the data of Kc~lley(11) :tr~tlof Smith and . I n d r e w (It?), whose data are interconsistent and arch ron~idcredto he the best available. Molar heat capaciticLs of thct vapor were t h c n ralculatcsd by t h ? relation,
Brass Bobs Pda; 30° I n cluded h 9 / e
c- 2+Figure
4.
2"
4
Cutaway V i e w of Reflux Proportioner
Figure
5
INDUSTRIAL AND ENGINEERING CHEMISTRY
756
T.\BLEIT. SIXNARYO F
( ' . u , ( : c - I , . ~ T I I J s ~ FOR
Vol. 39, No. 6
P.~RTI.u.R E ~ L I -I{( X s': ___
l l o l e s ~ l l i n u t eX 104
OverRun So.
15 16 17 18 19 20 21 23 25 26 27 28 29 32 33 34 35 36
head product 95 97 98 95 95 94 94 98 96 97 98 99 99 92 92 84 91 83
0
7
8 6 0 2 6 8 2
7
8 4
4 3 7 2 9
5
Mole % Benzene Feed Botplate toins liquid Feed 17.5 15.0 13.5 16.5 15.0 13.5 4.0 33.5 24.5 29.0 30.3 31.0 25.4 2.5 2.3 2.4 1.8
6.5
(lvicp)liq.(t2- t l )
+
78.7 89.4 93.7 81.0 77.0
75.7 53.5 69.5 66.8 65.5 6 7 ,5 6 2 .n 66.8 45.0 .65.0 26.7
5i.l 63.9 69.2 66 4 61.9 63.5 67.4 57.1 57.4 53.7 5 2 ,2 51.1 57.8
57 1
51.8 91.9 83.5
=
Vapur \-elocity Av. Feed a t T o p o f i ' t o plate Plate Coluniri. c o ~ i c o m p n . S o . Ft.;Ser. denser
67 2 73.7 76.2 70.6 66.3 63.8
51.: 68.4
71.6 78.9 76.5 77.4 48.1 49.2 51.8 44.8 59.4
in
10 10 10
in
10 6 6 8 6
n
455 0 922
1 505
0.498 0,432 n 433 0,504 n , 435 0,496 0.496 n , 480 0.395 0.435 0.516 0,516 0,501 0.788 0.713
142 26i 454 141 124 124 143 125 141 141 137 114 124 145 145 140 220 198
~ v ! c p ) ~-~ t~l ) ,+( t( M~X ) ~
1. coil-
N
to col-
umn 9% "'8 404 93 74 74 88 75 84 84 84 65 65 74 74 64 199 168
1)
where ,MCp = average heat capacity betn-een t i and t z Mh = latent heat a t subscript temperature
The resulting values rang(' from 4 to 105%lower than othcr. c:tlculated from spectrometric data ( 2 9 ) . The enthalpy data for toluene are plotted in Figure 6. Enthalpy of the saturated hinary vapor is substantially linear with molar composition and was so assumed. Figure 7 is a chart of enthalpies of saturated liquid agd aaturatrd vapor uscd io], colunin calculations. OPERATING PROCEDURES AND METHOD OF CALCULATION
AA iiumber of trial runs were made under total reflux for t i l ( * purpose of checking instrument calibrations anti determining heat 1 0 s ~ . Complete heat, balances \\-ere made around tlic entire column, and skin tcniperatures were taken in a number of places. Total heat loss was reasonably constant a t 140 B.t.u. per minute.
Figure 6
D 50 30 50
48 50 50 50 50
57 57
53 49 59 71 il i6 21 30
densed by heat l.cI111lo-s demed above by R feed 22 11 50 11 76 11 15 11 17 11 17 11 13 25 15 26 16 18 16 26 16 34 12 34 11 34 12 25 12 18 12 32 35 42 "7 42
I. ) I ~~
At tou
plate 0.695 0 876 n. 906 n ,692 0.645 0.645 0,615 0.643 0 662 0.637 0.653 0 611 0.562 0 547 0 547 n ,500 0.918
0,865
in
enriching Sertion
0.645 0.673 0,697 0 663 0.688 0.657
~~
~
~
.i\ ,
Interniedinte
.lv,
Above feed plate 0 i14 0 880 0 908 0 712 0 671 0.671 0 678 0 698 0 69i 0 688 0 716 0 693 0.651 0 610 0.606 0.587 n 929 0 888
topoi
.
bot:,>tlj
of
enrichilia e i i ~ i c h i r i c
section
section
0 705
...
n
RR
0.Yi
0 io 0.66 0.66 0.63 0.66 0.68 0.65 0 67 0 635 0 585 0.56 0 575 0.525 n 925 n 875
0 66 0 6b 0 6i5 0 io 0 6i5 n 610 0 60
0 5bi
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1947
757
TABLEI1 (Continued) Correction t o Efficiency Theoreticnl I'1:itei
.
5 70-1 6-1 6 il-1 5 69-1 6 36-1 6 25-1 10.6-1 7 69-1 ti :3-1 7 , 03-1 7 69-1 9.84-1 lo 4-1
15 8-1 10-1 Iiidet. Iiidet Indet
\vl'r(b
Over-all Efficiency.
5
39 2 41 6 47.6 39.1 44.; 43. I 80.0 55.7 44.3 50.2 55 i (3.i ii 0 192, i i5.0 Indet. Indet. Indet.
For
-
65 2 2 8 7 11.2
r01-
5 6
,-I
1 3 2 7
-1s
, Y
3 I 6 6 13 9 11 5 12 4 -16 0 -15.s - 13 2
.. . . .
o l ) c ~ t i t ~ c:ind l acljustcd. T1.r. quiin-
vapor x-elocity 0 0 0 0 0 -1.3 0 0 0 0 0 0 -1.5 0 0 0 0 0
For r.onlp".
1.3 5.2 6.7 3 4 0.8 -0 7 -8.3
1.8 2.1 4.0 8.4
;.:
-10.4 -9.5 -8.0 . . , .
Total 1.3 5.2 6.7 3.4 0.8 -2.0 -8.3 1.8 2.1 4.0 8.4 6.9 6.0 -10.4 -9.3 -8.0
....
Efficiency Corrected to 1- = 1 Ft 1 Sec. a n d iav. = 6 5 5 Benzene 40.5 46.8 54.3 41.5 45,s 41.7 71.7 60.5 46.4 54 1 64.1 80.6 83.0 122.3 65,5 Indet.
Location of Feed Plate Theoretical, Actual, fraction fraction Ratio, from from actual./ riilnmn top roliiinn t o p theoretical 0.565 0.231 0.409 0.550 0.231 0.410 0.535 0.231 0.432 0.459 0.231 0.50:j 0.647 0,231 0.35; 0.460 0.231 0.502 0.313 0.538 Liz2 0.797 0,538 0.677 0.385 0.657 0.586 0.756 0.538 0.713 0.663 0 692 1.042 0.692 0.83i 0.82i ' 0.635 0.692 1.080 0.538 0 318 -1.605 0.33i 1.14' 0.385
...
, . .
,..
....
,..
...
TABLE 111. CIXIPARISOSO F OVER-ALLASD ~ I L - R P H R EFFICIESCIE" EE
t i t > - nioothc%;tplots. .It partial reflux, scattering of the plate efficiencies becomes more pronounced and is worst at, the loner reflux ratios. Inordinately high plate efficiencies usually occur a t the top and feed zones and, in some cases, at' the hott,om of the column also. Introduction of cold reflux and cold feed requires the resprctivc plates to function as partial condensers as well as
0 40 0 41 0 44 0 47 0 54 0 64 0 82 1 11
=
0 31 F t . Sec ; LIT7 = 1 0 0.40 0.41 0.44 0 47 0.54 0.64 0.82 1 12
Run 21, Partial Reflux: Feed on Plate 6 : ij .iv.( L : r),,= 0.64; -41.. ( L 12 11 10 9 8 7 6
5
4 3 2
1
87.0 80 i 75 0 69.4 64.1 58.8 53.5 43.5 33.6 24.3 16 0 9.3
0 0 0 0 0 0 0 0 1 1 1
46 50 54 60 65 72 79 94 13 36 64 1 90
87.5 65.8 60 4 57 8 65 9
65 66
54 51 56 55 54
2 G
79 8 Av., plates 1-7
67.4; T. = 0.50 Tt r.)m = 1.17
=
0.72 0.78 0 85 0.94 1.02 1.13 0.68 0.80 0.97 1.16 1.40 1.62
75 58
. i .8 . -
55
See.;
94 3 56.5 55 2 53 7 48.5
55.5
51.5 118 0 92.7 74 2 90.2 95 5 AT.., plates 1-11
73 47 46 44 40 44 44 87 ti7 34 59 58 54
R u n 35, Enriching Column: 1. = 0.79 Ft., Sec.; .\v. L ' T . = 0.925 12 11 10 9 8 7 6 5 4
89.5 86., 82 2 75 6 65.0 50 8 35.5 22.5 13 9
3Iurphree efficiency. b Local efficiency. a
0 44 0 46 0.50 0 55 0 60 0 82 1.09 1 41
1 ,2
0 48 0 50 0 54 0 60 0.65 0.89 1.1s 1.53 1.86
34 9 3.5 1 40.8 53 3 63 2 72 9 7s 1 86 7 127.0 .\v.,plates4-11
33 :33 :3 7
47 53 57 56 57 68
jl
758
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1
Vol. 39, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune 1947
tO
0 Figure 8
759
40
80
60
100
MOLEPER CENT E r n r ~ ~ ONr PLATE Figure 9.
Effect of Concentration on Column Performance
C O L U M N EFFICIENCY A N D FEED P L A T E L O C A T I O N
l'liiwetical plates required for the ovc~r-allwp:trationb t v ( ' t ' ~ ' calculated by IIcCabe-Thiele d i a g r a m (Figure 11 I . The. 011rrating line> corrected for changes in reflux ratio ducb t o ht.:tt lobs were constructed as f o l l o w : From t h t ' opcrating data a n d known heat loss. D T' and L;T' at the cwrnpo~ition011 any plait. could be determined. The enrirhing liiic \va> dran.n in, starting at the overhead composition and adjusthg t h e slope according to the inerc.asr in L,'T7due t,o heat loss. SII ,significant diffcrc~iic~c~ in any hut the tn-o t~~.~.lve-plate-enrichirig runs resulted f n i n i drawing in the enriching line straight at :tn average L,:V- 0 1 t,correcting the slop(. plate by plate. The fecd entered a s c ~ ~ l t l liquid. and t h e plate. sarnplcb was con-iticsrahly ric.hrxr i n III~IIf c b c d
I
0.4
1
Y4POR Y € L o C / t r 47-
Figure
1
I
0.8
TOP
I
I
12 O F COLUnl*
I
-
/6
~n 20
Fr PfRffc,
10. Effect of Vapor Velocity on Efficiency
( e i i r i c h g plates) /(total plates) _.__ (theoretical enriching plates) ,/(total theoretical plxtes)
-~
Thc tlicwretical ctiric>liing plates anti the total t IIIYJ~I~;~I~:~~ plat('.:iw obtained from the 1lcCabc-Thiele diagram. 'Yh pliit of I l i v j)iLrtial t ~ f l u rruns as over-all efficiency against F1,F i h givvn i n I.'igurc 12. It is evident that introduction of the fcotl too high i n T hi1 colunin results in a poorer separation. Theoretically, :t tu:ixitiiuiii on Figure 12 ,should be obtaincd at FLF = 1.0. .'ii~tu:illy. t i i ~ teiiougli runs are available n-irh thc fwd loiv in the ccilulnn to (+tablish a maximum. Figure 12 i. offered as tentstivc, fat, :ip1)rosimating the effect of feed location on over-all separation. et the equilibrium Curve only i n t h c Thcx operating lines int t \~elve-platc-en~iching opcration. In no other case could thc. 01)crating lines be maintained closc to intersection n.ith the cquilit)rium curve. Reduction in rt~fluxratio merely reduced over-all aixparation.
Figure 11.
McCabe-Thiele Diagrams
June 1947
76 1
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y I
able t h a t local efficiency is substantially indcpendent of reflus ratio.
Q
VARIOUS METHODS OF CORRELATION
At least tn-o distinct factors require platcl efficiency to vary with liquid concentration-the effect of the liquid diffusional film and the effect of iioniiiising of rhe liquid flowing across a plate (vapor-liquid cro.w f l o ~ ) . I .is secn from their derivations and noted I75 the Murphrec vapor and liquid eft~lx~n.hcre, ficiencies are rigorous only for local conditions and not over a concentration rangc of both liquid and vapor. -1number of diffusional efficiency expressions may be derived, depending upon Figure 12. Effect of Feed Plate Location on Efficiency which film is assumed t o be controlling and upon other simplifying assuniptioni.. T h e 3Iurphrce vapor film cfficiency (1.5 i i n mod(~rnnomrnclaturt~ This tabulation predicts the greatest trend of plate efficiency ivitli 1s. concentration for the t~~-elve-plate-enriching oprration and t h e least trend for partial reflux operation n-ith center frcd. This a g r r w with the present esperiniental data, ronfirming t h r prcdominance of the liquid-vapor cross f l o ~mechanism in this column. -4 l I u r p h i w ~ - t y p crfficirricy relztion for liquid film controlling I n a column of similar plate design, S o r d :I61 reported a feiv fo1lon.s from the additional ininor assumption that rquilibrium plate efficiency runs on benzene-toluene a t total reflus. His vapor romposition hc~tn.wny,,- and y,,i.G linear in .r (8,1 2 ) : numqrical values and their trend with concentration both a g r w with t h e present d a t a . Drickamer and Bradford ( 5 ) p r r w n t r d a correlation of plate cfficicxncics of commercial columns (a\ ging 7 feet in diamctcr) \There q = efficiency, %/>00 in tcsrms of liquid viscosity only. .4lthough these columns are J;., . f L = rapor arid liquid diffusional film coefficients, respectively obviously not comparablcl t o tho csperi -4, 8 = effective interfacial area and time of contact berelation predicts a plate efficiency of 4 tween liquid and vapor, respectively (80" C.! and 5 4 5 a t the toluene cnd (11 UL = slope of equilibrium curve betn-eeri yn and y n the trend of visccsity with composition is t o ciecrease the efii. riency \Tit11 increasing hixnzcnc concentration. -4rrordinp t o Equations 2 anti 3, the csfficicncy calculated from Gcddcs ( 6 ) presented. a remarkahlc correlation of point c.fF.pl:itcl ,saniplt+ is thc zaiiic in tithcr -that iq, i; not directllrt,l:ttcd to composition whcre tli(' vapor film is controlling, but varicss w i t h i t ! i ~ i i t ltht.why n i t h .c n-hore t h e liquid film in controlling. a. numcriral valuc for f~j.-lBiof 0.02 ( $ 3 . ~for a constmt, P(~lt~c~ting vxl)oi' film r,fficiency of 0.601, thc. liquid film c~fficii~nc,y c~alculaicd f l o i i i t:quation 3 and Figurc 13 murt risc froni 0.33 at Oc;- henzc,nc% t o 0.91 at 1 0 0 5 Iwnzc~iic~.Tho offvct of flon-ing a ~ i v r sa plat(. rc~quiri~s the cfic~ic~iiry to dwr , It i.c cac7rtairi that thc liquid filni pro t o (liffusion: but in t h r . prcw'nt c~zpc~rinit~nt.; thrb tix'nd of c ~ f i cic>iirywith c.oric,c,ntr:itic,n indiratcas t h a t tht. conrcntration rffcct oi t 1 1 ~ 8 liquid film is s u h r d i n : ~ t c to ~ and completc~1~maskcti 11)ot 1it.r c,ffFc,ctF, nota\ily noninizing of plate liquid. I'\-(.lU .. >:i .\ e of t,hc ahnomially high efficiencics at the fred and r~iidsof the column, local c~ficic~ncics arc reasonably constant for tliv three t>-pcsof operation. It follows that local efficicncics are 11(wly indcxpendent of concentration, of reflus ratio, and of liquid and vapor loads over wide ranges in this investigation. T h avcrage local cfficicncy for all conditions is 50 to 55cc. ()uaIitativc values of mT,':L as a n intics of t h e divergence betn-cvn local and plate efficicwics and of the trend of plate efficicmy n-itli composition for t h e various types of oprration and acc.tions of a column are as follows:
-
Stripping Section i s Small) Ill >I 1- I, at partial reflus 1 ! t i l 7 I, at partjalrcflus a t total refluxand platcvnriching colpartial reflus umn I
Enriching Section (5 Large) 1 a t total reflux Same as a t partial reflux
az
0.4
MOL€fR4CT/ON
aa
0.6 BEN2 €NE
Figure 1 3
/ N !t
/4WO
LO
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
162
Rr
I
0
Vol. 39, No. 6
locity. on local as well as on plate cfficiencies, and or1 platerj p a r transfc.r unit. DiscrPpancies between results on short columns and t h v present data arc partly explainable by consideration of the column end effects noted earlicr. Since this is apparently the first article st,udying plate cfficicncies on a laboratory multiplate iduinri using center f e d . marly more part,ial reflux studiw on columns of ten or more p1atr.s 3hould he made. There sceni.* t o I I P no reason ~ v h ygeneral results and dependable predictions of pllatc cfficii.ncic~scannot he established. FUTURE P L A T E EFFICIENCY STUDIES
1
0
I
0
I
I
I
I
I
I
I
I
I
20 40 60 80 /00 MOLE&@CENT BEUZENEou PLATE Figure 14. Local EFficiencies of Typical Runs
I I I ic*guicl t o sinall diameter columns, considerat ioiis t ~ i rt tit. olitaining of :tccurate, reproducible data, free from u i i i q u ~or ~ linrwogiiizc.cl influences of the equipment and c~xpcrinit~nt i ~ r arch . listid in tlu. i t i t c ~ i ~ of i ~ future ~t platr efficicnc 1. Sninll columns are hubject to major and rapid fluc.tii:tticii~t!i Bon- rate. and hence of point concentrations. 2. Heat 1 0 s is apt t o be inordinate and, unless accounted or conipenwted for, may invalidate flon- rate, reflux ratio, and eficiriiq- calculations. 3. Nixing of the liquid lJ11a plate is inadequate to eliniiiiute concentration gradients evrn in rather small columns. This gives rise t o erratic sample concentrations, errors in plate liquid coinerratic Murphree efficiencies, and may lead tu to a trend of plate efficiency n-ith liquid ( ~ ~ r n -
4. Cold reflux and c d d feed require the plates in their respective zones t o a r t a:. partial condensers rather than as adis results in abnormally high plate abatic diffusional devic efficiencies in the>esectioi 5 . IAW boiling oon returned t o the still (a3 i- often donc iii expeiimmtal studies) tends t o flash through the hot still liquid without thorough mixing. The still vapor may then be much richer than that corresponding t o equilibrium n-ith the bottoms product stream. 6. Most of these efferts hecorne relatively greater tlie fewer the plates in the column, and also for lionideal 3ystetns tvliose ide spread b e t r e e n their boiling points. em-for example, acetone-water---plateand temperature gradients are large a t lo^ concentrations and small at high concentrations. Sonideal and azeotropic Sy'trms art= not t h r best for the stud>- of I j l a t r efficirncirs.
1.0 0
1.0 0 0.5 '1
0.2 0.2
1.0
1 0 1. o
1. 0 1 0 1. 0 1.0 0.5
('aicnlatad a t SOa C'. f o r .i'u = 1 0 niid
11,5:j
.?
I!
0.50 0 4; 48
15 II
n
:x'
-, _
0 52
11(Io( ' ior . r a = 0 .
