arc stabilized when the surface tension of the reflux increases down the column, ... INTRODUCTION IN operations of ... of this group is the wetted wall column.

GENIE

)L. 9 195S

The influence of surface phenomena on the performance ofdistillation columns*

F. J. ZuiDEii\vEr, and A. HAUMKXS

Koniiiklijke/ShcH-Liiboratoriuiii, Amsterdam (X.V. l)e Bataafsehe IVtroU-um Maatschappij)

(Received 4 March ll)."»8 ; in ririfieiljornt "£~ March 10.18)

Abstract—A study lias been made «m the inHiifiur of changes in surface tension on the formationof iutcrfacial area in different types of distillation, equipment. It has been found that liquidfilms arc stabilized when the surface tension of the reflux increases down the column, whereaswith decreasing surface tension liquid films break up into narrow rivulets or droplets. Intcrfneial "area in sprays, however, is not much influenced by changes in surface tension.

For :i given liquid mixture, break-up or stabili/.iit ton of films can be demonstrated by reversingthe direction of mass transfer ; the magnitude of the effects depends distinctly on. the magnitudeof the surface tension gradient developing in the reflux.

In apparatus where the mterfachil area is mainly of the film type, mass transfer rates formixtures causing the surface tension, of the reflux to increase may be twice as high (or more)as those in systems in which the surface tension of the rellux decreases. In the case of technicalequipment with intcrfacial area mainly in the form of droplet sprays, this influence of the systemon mass transfer is but small.

Resumed—Les auteurs out ctudic 1'effct dc changements dans la tension superlicielle sur laformation de Tinterface dans differcnts types de colonnes de distillation. Us out trouve^ que lesfilms de liquide sonte stabilises quand l;i tension superficielle du liquide de reflux augmentc,tandis qu'une diminution de la tension supcrficielle cause la, rupture dc ces films en formant depetits ruissenux ou gouttes. Cependant, dans des systcmes de gouttes ('interface n'est pasinfluence'e beaucoup p:ir des changements dans la tension superficielle.

Tour un melange dc liquidcs donne, on peut demontrcr la rupture ou la stabilisation desfilms en changeant hi direction du transfert de masse ; la grandeur des effets depend nettementde la valeur du gradient de la tension superficielle qui se forme dans le liquide en reflux.

Dans les colonnes oil Tinterface a principalemcnt la forme d'un film, les vitesscs du transfertde masse, pour le cos de melanges qui provoqucut ttiie augmentation de la tension superfieielledu liquide de reflux, peuvent ctre deux fois (ou plus) plus clevees que celles qiii se presententlorsque la tension superficielle du liquide de reflux dimmue. Dans le cas d'installations indus-trielles comportant des interfaces principalement en forme dc gouttcs, cette influence du systemesur le transfert de masse n'est que faible.

Zusammenfassung—Eine Untersuchung iibcr den Kinfluss von Anderungen, der Obcrflachen-spannung auf die Grcn/flachenbildung in Dcstillationskolonrtcn verschiedcner Art hat ergeben,dass Klussi^keitsfiluic stabilisicrt wcrdcn, wenn die Obcrflachcnspannung des Riickflusseszunjmmt, wahrcnd bi'i abnehmender Obcrflachcnspammng solchc Kilme zu schmalen Stromenoder Tropfcn zcrrisscn wcrden. Jedoch wird die Girnxfliiche in Tropfensystemen von Ander-ungen der Obcrflachcnspannung nicht stark beeinflusst.

*The contents of this pajK-r were read at an " Interne Arbeitssitxung des Kachausschusses Destination, Rektin;ation und Extraktion der V.D.I. Fnchgruppe Vcrnilirciibtechnik," 21- April, liijfi, 15ingcn-on-Hhiue (Germany) [13]

F. J. ZUIUKRWEG and A. HARMKNS

Fiir em t»e*timmtes Kliissigfceitsgemiseh liisst sirh *l:is XrrrcissenoderStaliilisieren drr FHmenacfmeiseii, indent die I lu-htun™ ties StnfTaii.statischcs umpckclirt wird ; die Effekte wenlendeutlieh von der Crussc dcs sich im Iliw-kRuss entwickclndcn Oberflaelifn.spunmm<:sjjeR*Hesbedingt.

In Apparaturen, wo die Grenzttiiehe hatiptsarhlirh in Form ernes Films nuf t r i t t . kunn beiGemischen, die die OberiHU-henspannuny des KiiekHusses stcijrern der Stoffaustatiseh zweimaiso schnell foder schneller) scin als der bei Syslcmen, wo die Oberflaehenspannun" des Kiu-kflussesabnimmt. Bri technischen Apparaturen mit finer Grcnzfliiche haupEsacIUich in Form eines Sprudel-betts ist dieser Einfltiss dcs Systems auf den Stoffatistausch nur gering.

1. I N T R O D U C T I O N

IN operations of mass transfer between a gasor vapour and a liquid the transfer rates obtaineddepend on the rates of diffusion in the two phasesand the magnitude of the contact area betweenthe phases. These quantities are influenced by thetype of equipment in which the operation iscarried out, the physical properties of the phasesand the operating conditions applied. Frequentlythe effect of these factors is mutually dependentand cannot clearly be separated. As an example,the change in operating conditions may affect themass transfer rates on a bubble cap tray quitedifferently from those in a packed column. Thisis particularly true in connexion with the subjectof the present study, which discusses the influenceof surface forces on vapour-liquid interfacialarea and the resulting mass transfer rates.

In general, little attention has been paid in thepast to the formation of interfacial area in actualvapour-liquid mass transfer operations. Mostof the work done concerns the study of bubblesizes emerging from submerged orifices, the degreeof wetting of packing units in a packed columnor the characteristics of liquid sprays. Theformation of interfacial area under conditions ofmass and/or heat transfer is not covered by theseprevious studies. It is found that the interfacialarea depends primarily on the How conditions ofthe phases and mass forces. Surface forcesinfluence the interfacial area to a minor degree,the general effect being a smaller area withincreasing surface tension.

This situation, however, may change consider-ably if the formation of ititerfacial area takesplace under mass and/or heat transfer, particu-larly if owing to these phenomena .surface tensiongradients develop along rhc vapour-liquid inter-face. It has loiiff been known that surface tension

gradients cause rapid movements in the surfartand may lead to a spreading or a contraction ofthe surface [7], Such contraction phenomenahave indeed been observed under certain con-ditions in the absorption of ethyl alcohol vapour[8], and ammonia [1] into water films, It isreadily understandable that the contraction ofthe liquid surface into small rivulets will have aprofound effect on the overall mass transferrates. Presumably many hitherto unexplainedanomalies in mass transfer rates reported inliterature [3, 10] have to be ascribed to thistype of surface effects.

It is noteworthy that the relation betweenmass transfer and interfaeial area has recentlyreceived attention in the ease of liquic uidcontacting [n]. The stability of dispersed dropletshas been shown to depend on changes of surfacetension caused by the transfer of a solute throughthe interface.

Complex surface phenomena as quoted aboveare in general very difficult to analyse theoreti-cally. The accumulation of a general insight intothese problems is a first objective that is bestobtainable by carrying out adequate experiments.It has been found that in the case of vapour-liquid contacting (distillation) the latter can berealized relatively easily. The first purpose ofthis paper is to describe such experiments, andto give a discussion and possible explanationof the results.

2 . S u u f A C K T K X S I O X C H A N G E S I ND I S T I L L A T I O N

The surface tension or intcrfaeial tension of Hu-n-flux is subject to changes during the downward(low of the liquid in the fractionating column.This is caused by the changes in composition andthe increasing temperature. It is possible that

90

•* S 1 lie imiUClUl- til > m T < t i i jrtu mmivn

mperature and composition effects reinforce or•utralizc each other in this respect. Since inipour-liquid systems temperature ;md composi->m are closely interrelated, the possible variations

surface tension can easily be calculated fromie equilibrium compositions and temperaturesvd the surface tension of the pure components: the same temperatures.It is then found that for systems composed of

impounds of homologous series, the possiblexanges in surface tension are about 2-3 dyn/cm.or mixtures of compounds of a less related natureinch larger variations in surface tensions arc,owever, possible. Mixtures of hydrocarbons andleohols, for instance, may show surface tensionmges of about 5 dyn/cm at their boiling points.'ure hydrocarbon systems can also show largeurfaee tension gradients, if they are composedf paraffins and aromatics. For instance, the-irface tensions in the system benzenc-;t-heptancuige between 21 and 12 dyn/cm, those of thelixture H-lieptanc-tolucue between 12 andS-5 dyn/cm. From these data it is seen that innormal distillation the reflux running down in a

rac iting column decreases in surface tensioni the urst case, whereas with the second mixturen increase occurs. It will be clear however, thathese changes in surface tension only develop." the operation of- the column is such that aradient in concentration and temperature occurslong the column. In the case of operation closeo minimum reflux the changes in composition,nd therefore also the gradient in surface tensionn the reflux stream are smalls even if there is a-"ide gap between the surface tensions of the pureomponents of the system. This applies in alore general sense to all mixtures where relativeolatility is small. In that ease, even at totalcfiux, the concentration gradient in the reflux:»ay be negligible. An example of such a system* the widely used test mixture ?i-heptane-lethylq/efohexane. Though the surface tensionsf the components (12 and 15 dyn/cm) differonsiderably, the low y. value (1-07) does notHow of the building up of noticeable surfaceension gradients in the reflux under normalonditions of distillation.From the above description it is seen that three

«Tf..rm--m.-.' ..f i f k t i f f . i f HIM t ((/iiiim-i

typos of systems can be distinguished with respectto the changes in surface tension developing in thercllnx flow. For convenience these types will bedenoted as negative, positive and neutral re-spectively. In negative systems, the refluxdecreases in surface tension ; in positive systemsit increases. In both cases the relative volatilitiesin the system have to be large enough to allowthe development of appreciable gradients. Theterm neutral is given to those systems in whicheither the components do not show a differencein surface tension or in which the relative vola-tility is very low and the gradients in surfacetension are consequently always small.

The mixtures employed in the present studyare listed in Table I.

Table 1. SKITC.IJ of binary systems used

.,,„„,.jt-heptane-methylff/cMiexaiieH-heptane-toluenebcnzcne-H-heptanebenzene-toluenebenzene-cycfobexane(azeotrope)ethanol-2.2.4-trimethylpentane(azeotrope)

Hailing point

98-4101-8

WS-4110-7SO-208-480-2

110-7SO-280-877-578-399-171

Surface tensionat boiling point

(dtjn/cm)

121512

18-5

21 ~v ,^ --12 ^

21 Y'18-5--'21

17-5201811-S16

Most of the experiments were done withthe negative benzene—n-heptane, the positiven-heptane~toluene and «-heptane-methylci/clo-hexane mixtures. Because of the low a value,the latter mixture will, however, be taken as arepresentative of a neutral system.

Two systems are of particular interest—viz.the mixtures ethanol-2.2.4-trimethylpentane andbenzene-c#o/ohcxane. These mixtures both showan azeotrope at about cquimolar compositions.As a result the changes in surface tension aredifferent in sign, according to whether the mixture

01

££?. • ••- r^;^;?"v>;jO^<*-' ?^^^ ;'•-*;•; H'v'V V':"

— : • -

M

distilled has a low or a high content of the low-boiling constituent.

3. E Q U I P M E N T U S E D

The formation of interfacial area differsaccording to the type of equipment used.Roughly, two groups can be distinguished. Inthe first, interfacial area is obtained by spreadingthe liquid in thin films over area already presentin the equipment. In this case therefore, one canspeak of supported interfacial area. A prototypeof this group is the wetted wall column. Packedcolumns and modified wetted wall columns suchas Yigreux columns also belong to this group.

In the second type of equipment, the inter-facial area is obtained by mixing the liquid andvapour phases. As a result, the formation ofvapour bubbles and droplets occurs ; the inter-facial area in this case is not directly supportedby the equipment. Examples are all types oftray columns which may operate either withpredominantly liquid film intcrfaeial area (foamand bubble systems) or droplet interfaeial area.Pure droplet interfaeial area is obtained in spray

Table 2. Survey of equipment used

(1) Concentric tube columnLength 200 mm, intcriuil diameter (i mm, width ofannular space 0-GO mm. Only the outer wall wetted,inner wall dry.

(2) Vigreux columnsVarying length, diameter 2J mm, distance betweensimilar indentations 40 nun.

Packed column(3) Length 250 mm, diameter -M» nun. I'arking : porce-

lain llaschig rings, 6 mm diameter.

(•!•) Packed columnLength 100 mm, diameter 25 mm. Pat-kin^: metalFenske helices, 3 nun, or JDixou gauze rings, a nun.

(5) Spray columnIS rotating discs, 40 nun diameter, spaced :;o mmapart on axis. Column diameter 100 mm.

(G) Oldershaw sieve plate column [2]Diameter 23 mm, plate spacing: modified to ion mm,2 or 5 plates.

(7) Perforated plate atlttinttDiameter 4.>0 mm. If; per rt-nt open iirra plate,10 mm hole .si/e. IMnte spacing -MHi mm.

:o and A. UAK.MKNS

columns in which the liquid is dispersed bymechanical atomizing device.

It is found that, with respect to the effect <changes in surface tension, supported interfaci;area behaves quite differently from unsupporttinterfacial area. Therefore, representative typeof equipment of both groups defined above werused in the present study. Particulars about thequipment are listed in Table 2.

4. SURFACE TENSION EFFECTS WITHS U P P O R T E D I N T E R F A C I A L AREA

(1) Wetted wall type columns

A most striking example of the influence osurface phenomena on the performance of dbtillation equipment is obtained when opcratiti:a wetted wall column. IIETP data measured fothe small concentric tube column at total rellu:are given in Fig. 1. In the case of the u-hcptane-methyle#f/ohcxane system, the liquid phas

200

6-2O O4O 05OVapour velocity, m/r«c

FIG. 1. Separating pmver of euncentm* tube columnwith different test mixtures.

^ «-hcptane-niethylf//rfohexane , . . - . '.m~]>iK.'ii/.ene-H-heptane -<•?.- -. -

shovvctl complete wet t ing of the column wall amthe separating p*»wor observed agreed very we!with the theoretically prrdictcd values [1^]

The influence <>r surface phenomena on the performance of distillation columns

However, with the negative bcn/.cno-H-hcplanesystem very bad separation was obtained, theHETP being about ten times the theoreticalvalue. It was observed that in this ease the columnwall was poorly wetted ; the liquid flowed downin narrow rivulets. Improvement eould beobtained by fixing a wire coil of low pitch (about•2 mm) along the column wall. Because ofcapillary forces, the liquid was now retained andspread out on the wall and nearly the sameHKTP was observed for the benzenes-heptanemixture as for H-heptane-mothylcf/r/ohexane(Fig. 1).

A similar difference in separating powercaused by a different degree in wetting is observedwith the Vigrcux columns. Total reflux datawere obtained for a variety of systems under awide range of load conditions. In the range ofvapour velocities of about 0-4 to 0-G in/see theseparating power was approximately constant.The average HETP values observed in thisrange are plotted in Fig. 2 against averagemixture composition. As shown, the HKTPt for the negative mixture are nearly twice

£ 4OO

3OOoTui 200I

100

ChoT

•mailing

Wetting" -, ^ o

|

V 1

O 2O 4O 6O 8O 10OMole /o volatile component in mixture

FIG. 2. Separating power of "25 mm Vigreux column withvarious test mixtures. Vapour velocity 0-4 0-tJ in/sec.

O w-heptaiie-loluene c" - ^->>A 7i-hcptane-mctliyJcf/c£olifXJine

those observed for the H-hcptanc-metliylr#cfo-licxane and the »-hcptanc-tolucne systems. Thewetting of the column wall observed visually wasin accordance with these results : iu the case ofbfiizene-»-heptanc partial wetting and the for-mation of rivulets, as illustrated iu Fig. 8,

occurred, whereas the other two mixtures showedalmost complete welting. The similar behaviourof the latter two explains the almost identicalI1I"IT values for these mixtures.

The observation that particularly the benxcne-»-hcptanc mixture gave poor wetting only seemsexplicable by surface effects. Since there is nomajor difference in the " static " surface tensionsof the different systems employed, it was believedthat the sign uf the changes in the reflux surfacetension might perhaps be responsible. In orderto test this supposition, some more experimentswere done with the Vigrcux column. First,total reflux distillations were carried out withthe mixture bt-n/cnc q/r/ohcxane. At highbenzene concentrations, tins constituent is theless volatile because ot the azcotrope in thesystem. Therefore, reflux surface tension in-creases downwards in the column (lt positive "mixture). Complete wetting was observed in thiscase. AYilh a low ben/cnc content of the mixture,(N/e/e/hexanc is the less volatile component andthe surface tension of the reilux decreases("negative" system). As with benzene-Jt-hcptanechannelling of the reflux was now observed.The separating power measured for the twomixture compositions was in accordance with thewotting characteristics found (see Fig. 2).

In a second series of experiments it was attempt-ed to measure mass transfer rates with thenormally " positive " systems Ji-heptanc-tolueneand fi-heptane-methyln/dohexane under " nega-tive " conditions. This cannot be realized innormal distillation ; however, by conntcrcurrentabsorption of vapour rich in volatile componentinto a liquid rich in the heavy component thedirection of mass transfer may be reversed andthus also the sign of the surface tension changesin the downllowing liquid. Moreover, by changingthe length of the Vigrcux column or by adjustingthe compositions of entering liquid and vapour,the surface tension gradient in the liquid caneasily be varied and its influence determined.

Data on the absorption experiments withVigreux columns of various lengths arc collectedin Table 3. Besides top and bottom flow ratesand composition, the average driving force,i.e. the average difference, between the operating

^£i&SSSSKSfc

*':3 f'1

T-i a jrii-aK;- 1

ffis

3§Sm

F. J. ZUIDERWKG and A. HARMON'S

line and equilibrium line is given (in terms ofliquid composition). The latter is representativefor the average changes in concentration (andhence for changes in surface tension) in the refluxover one theoretical tray. At a high drivingforce, mass transfer is rapid and a large gradientin the reflux surface tension results. At lowdriving forces, the reverse is true.

As expected, under alt conditions wherereHux surface tension decreased in the absorptionexperiments, channelling of the reflux occurred.This became more severe the higher the drivingforce applied. Consequently, the HETP alsoincreased with the driving force. This is clearlyillustrated in Fig. -t, in which the HETP data areplotted. It is seen that for both systems thedata fall on one single line. Furthermore, theHETP value obtained in normal distillation withthe " channelling " benzene—n-heptane mixturecorrelates well with this line if plotted at theaverage driving force applied in the distillationexperiments.

Fig. 4 also shows the data obtained under fullywetting conditions with various mixtures. Thevalue for the n-heptaiie-mcthylrz/ciohexanc sys-tem at high driving force was obtained by the

absorption of methylq/r/yhcxane into n-heptm(see Table 3). It is seen that under coiulitiotof positive changes in surface tension there is rinfluence of the driving force oh the HETIpresumably because the wetting is almocomplete. Further, it may be concluded fro-Fig. 4 that at low driving forces, the HETP tnegative systems seems to approach that of tlpositive mixtures, i.e. wetting becomesand channelling disappears.

ouu

a.U 2OO

X" w

Chonneirn

_-^£"

•tfcr-g

3 ---"*''""* A

U--

0-10 0-2O O-30 0-40 O-50

Driving force, mote fraction

FIG. 4. Effect of driving force on separating power oVigreiix column, tested with various test mixture*

Vapour velocity 0-3-0-4 rn/see.£3 benzt-ne-H -heptane

A A w-heptnne-methylt'i/cfc/hexiineO • w-ht-ptane-toluene

Table 3. Absorption experiments in Vtgreux columns. Average vapour velocities 0-3 m/sec.Concentrations in mole per cent n-keptane

jVy.v/fw*

n-heptiine-toluene,,f jJ}

„n-heptane-

methylq/nff/hexune„SJ

J(

,,tl

»

o wm/iten^itt

(in \

1-781-4S

1-00

0-880-65

1-781-4S1-OG

o-ss065

048

0-48

Feed rates(mnlc/hr)

top

IG-817-S17-G

1G-G

18-1

21-920-9

16-5

21-1

15»

16-31D-7

buttvni.

lti-0

15*51G-G1G-717-4

15-51G-G

15-1

1G-915-7

1G-3

18-1

Product rates(turtle /hr)

,

tup

10-010-1

11-912-3

14-7

11-S12-111-G

14-G

13-014-1

16-2

bottom

22-823-2

22-3

20-5

20-8

•J5-G

25-4

20-0

23-418-fi

IS 5lit G

Compositions, (mole per cent)Feeds

top

00

000

o0

oo0

0

100

botfnm

100

100

100

KJO

100

100

100100

100

100

1000

Products

top

14

14-5

30

34-5

44

3-08-5

23-5

23

43

55

71

bottom

~M~~

GO-5

58GO

52-5

5i*Gl€357 -5544G

as

Averagedriving Tttforetictiifore? j plntt.v

trnole %)

1G-517

25-52JS

37

145

192G-5

27-5

44

5533

i

4-9

4-3

2-;i2-1

1-3

5-1

3S

2-32-1

1-3

ll-S2-0

HETP(mm)

•MO;uo4GO

420500

.150

3!»0

4GO

420500

cuo240

•5»«•£z$is«•\\$m8%

' .'i/'*A& -^-v;.r-?;Ate.f*.-'.V*itfrS*$»•'•*•»*

The influence of surface phenomena tin the performance of di>t illation columns

There is one remarkable feature in the eorrcla-iuii of the HETP data in Fig. 4 which should beicntioncd. The difference in surface tension•ctwcen n-hcptane and methylr^cfohcxanc is lesshan half the difference in surface tension be-ween w-heptane and toluene or benzene. This:ieans that since for all mixtures the range ofiqukl concentration under channelling conditions-; about tlie same, the surface tension gradient inhe reflux during the u-heptane-aromntics tests>/as about double the gradient in the «-hcplane-ncthyltt/cfohexanc experiments. In spite of thisact the intensity of channelling in the formation>f interfacial area seems to be the same.

The observations of channelling liquid phasescported in literature are also to be interpretedn the basis of decreasing surface tensions.IOLSTAD and PAH SLY [8] found that the contrac-ion of water films in grid type tower packingsccurred when ethanol vapour was absorbed. Inhis case surface tensions of the liquid film decrease>ecausc of the increasing ethanol concentration.

BOND and DOXALD [1] observed the break-upf water films when hydrogen chloride or ammonia••as absorbed in a rippling film in a wetted wallolumn. Owing to the heat of absorption, a.ecrease in surface tension also develops in theseases. It is interesting that the break-up did notccur when rippling of the liquid film wastrevented.

2) Packed columns

In packed columns the interfacial area is sup->orted by randomly distributed packing elements.u view of the results obtained with the wetted/ail columns, the effect of the direction of surfaceension changes on the HKTP was also deter-mined for this type of columns. Two types oflacking were investigated in relatively smallolumns : 6 mm porcelain Raschig rings (in a0 mm diameter column) and two fine metalBackings (in a 25 mm diameter column). In thease of the Raschig rings, efficiency varied butittle with the boil-up rate at vapour velocities>f O-2 to 0-3 m/sec. HETP data measured underhcse conditions are plotted in Fig. 5. It is seenhat just as with Yigreux columns, the HKTPor the rt-hcptanc-methylfj/t'/whcxane and ii-licp-

tanc-toluenc mixtures are about the same,whereas for the negative benzene—n-hcptanemixture about the double HETP values arerecorded.

JOU

oT

X

i•'• i

!•

a0 <*

•— -

O 20 4O 6O SO 1OO

Mole °/o volatile component in mixture

Kic. 5. Separating power of 6 mm Raschig rings, withvarious test mixtures. Vapour velocity 0-2-0-3 m/sec.

£3 benzciie-H -heptaneA ?^-heptane-toIueneO M-heptane-methylci/efohexane

A similar difference between HETP valuesobserved with positive and negative systems wasfound for the fine metal packings such as Fenskehelices and Dixon gauze rings. Table 4 shows thedata obtained :

Table 4. HETP data for fine metal packings*Average liquid composition 65 mole per cent volatile

component

Packing

Fenske helices

Dixon rings

Si/stcai '

n -heptane-toluene

benzene-H-hcptane

M -heptane-toluene

benzene-Ti-heptane

Vapour velocity(m/sec)

0-OG

0-06

0-12

0-15

HETP(nun)

15

30

20

35

Though in the cases of the packed columns theactual flow of liquid over the packing elementswas dillicult to observe, it may be safely assumedthat the differences in separating power with thedifferent systems must again be ascribed entirelyto a different decree in wetting.

1*5

• *sS' "<sXri'-<fi$,!%&$¥&•&iv$tetttefef3

-> -i'vwav^^?i-C^f•,-.•: . tr• ?:';?h5>;

•'"pi 'fl.fll

.'•'*$£$(:;>v4.$"^^|i

^

>V-''rt<

i!ii'•.'--V^v

• t i-~ .-*ii;-ra

->

V)

C

o3"

magi

3<crCfr

C

r*.

if.'3

r;

P

C*3

£.

rt"

O

X

O

£.s

S9I3o

p"1

C

3*c

"JcV)

rt~*

p(T[y,O•

r?

C"

seem

<ji<—•rt-

"

sfO

c/:

3C

P>CT3P-iPT

in

^3 O 35 r; S O

O £MM £ £

3 rf

O =c- -

X •—• 3

" ~J O £

O 2

P r:rt

o ~X

oc _

'yi ^ y,

-; 7 P

~ cr3 *<

3 ft>c 2

*" H " »^ ta-I

a

^P = ""

O

X

C » > C

Numberof theoretical trays /spray

o O O O o

S 3

JL ro Co Ji LO O O O C

— i

/

1I

A-

S P ooji- «• c!•* . . ci-(T ^ rt-^ 2 2P o 5"3 „

-.' -

s. 7 ,3 rt-S £,""' Co 2*i 3m n>w ja3d *•*

§ &3 c-o

K-|

P Lo

£. 0

— . X. s T "-a ^ s ?5 3 c S

1 = ' 0 £ r t .

£.0 2. f•— o >—• •-

P H^ *^ 3 O O

o"

CT1 — .

Ej p"

«-»• J^ _

~ * c3 "

i-. ^>

N i*

x y

C •*"

1' £.3 33. £X £j

0 c"P :'-

~ i T"

3" rf ^

™ ^* 5-"" -«J "T

3 _; 7?T 75 P|.

3/ o

? 1 "H.

^" ~ ?*Q ?• fj"X

C il, w

"" *" 3"C HT. v;

J*5 " ^ 3^ (•*•0-. r> r-

C i_ p"

i— v"P "• rt-0* r-t- C

3^ ° (?

'J3" 2 •"V)

y; tr rt-"n '< 2••£ "

* 3 y;

o--B 2O Cp- 3' p

iJ C #

=: ^ |P 3.•" 3

^' r* n-i

1—1 ,-».

£ ^-

or^- C"

• f7 5-

rt- 75

71 2

rt-

2- %"^ ^~'2 ?< ~"

rr 2.»*d c

o 1;rt ^

2 "?i3 ?*

£, P"2 ?

PM w

^ 53 tnrt- '•*•;

v.

»' J

§ g

S p

2- 3?

's FS c

— — —" S C rt C

X A- i

2 2 t3*f? ra •-•

T

X

(7

*1 -« !Jiti ti C

vp vp vP0s* 6s- «

5 2N NC nf? 5^ i

^ ?ET o3re

S»"Hin>

ss is?^|

5"*

\

3

tgemole

>$.o^>_a

_o Awwuge iray CTTiCiency./o -*• y. Q >-< ^

* l o S ^ g g R i=HoEr-»' PI-sr ^'

< 'Jn

•5 "3 ^ O5" 2 -8 16e -i o* S S<•* -.? o o o

^ o 1*If 3

?" s

s- go

i ii

4

^; «f_(_I_i_i f11 1'i i

';•

-r,-/

— /—rj

** tl « '/I r-t- «-S* o o — '

2 "&P 2 *~ p

1 1 1 aii_. 1 m «.] C-

rr: t^-j * £J° --i —

" 0 £ _ «'

• 1 i 1' $?" rt- ?D" O^••5 — * CT" r?"p >-•

5* I? ^ a ^f? S n 3- "

£ '

"TJ

*~~* f- rt-g-jo o o g- d. o

3 5

-: p

r M

o ">-. M

^ ^3-

^.^•mws^frr--"v• s-'wsaSa eg&~5s!MK&t?3M*3t5r'aa*-&3eStib,

S^^ »?^^» w^^^^A i-sa -liel*^a-»? S J». JC±i_-w-«M.-wsKs^5C?rK ,>isi

ssKr^Sff^ r^KBagJS^^s^^a^

54.2 ; r.'*^,-.-»-?,^^?c%-<r;Hi ii*3-&£3

Fin. H. Tray art ion mi OMersliaw sieve plates witliben/.cne-H-hcptanu (left) and »-hcptanc-toluciMr system

respectiwly. Vapour velocity «1»i»iit »•« MI'sec.

iWTO'PW^JW^^

;•;,-.: .v*y:S-'"•' .;'-'-• . /P"

' I'A fr'i'.'bl

-.ir^'^v'-.V:;. •',•«!

!-^-^- '• j'' l*.^-'^

•£$?&$;CS^- |

Table 5. Efficiency of Olderskay: sieve plates

Syxtrm

...h<MiliHu?-(fiiptliy1<v/r/<>lioxiiiH"

tenxene- toluene

•eiizene-M-licrptane

(-heptane-toluene

•tIianol-2,2.4-triinethylpentjme

.•hYT«!*r mofr %rultitilt' runtp.

50

50

85

40

.15

40

75

87

92

7

I'apanr velocity(in/sec)

0-10

°"f

0-32o-aoO-4So-o.sO-21O-320-400-400.520-300-.1S0-470-550-GS0-140-230-300-330-50 •0-G20-130-2SQ-440-G70-130-210-2»

0-150-300-480-130-30O-470-350-290-30

0-480-200-25

Trtitj efficiency

' /of

5751.

54

54

52

53

51

40

4S

4050

50

434G4ft5553

55

55535283S3857897

100100880001

7380

7505

G3G364

30-407O

Foam Height(itim)

10

20

10

20

10

20

10

20

40

fiO

40

20

15

5-1O50-100

>y carrying out absorption experiments. The " negatively *" i?» this region of concentration.tzeotropic mixture cthanol-2.2.4-trimcthylpcn-ane was used. At cthanol concentration** below

33 mole per cent, this component is the mostvolatile. Since the surface tension of -J.2.4—

At ethnnol concentrations above 53 mole per cent,the octane is the most volatile component andthe mixture shows " positive " properties. Thisconclusion is confirmed in a spectacular manner

rimcthylpentanc is lowest, f h c system behaves by the disti l lation tests. Fi«r. 0 shows

ife;- - - .^^^fS^r'^^K^Sr^S^-^

F. J. ZvuirctwEG and ;\

^^sf^-4ig3?&S3

a»

-V!3sM

of the liquid-vapour behaviour for low and highethanol concentrations at about the same vapourvelocities. The tray efficiencies (30—H) per centand about 70 per cent respectively) arc in accor-dance with the formation of mtcrfuciul areaobserved.

Absorption experiments with the Oldcrshawsieve plates were carried out with //-heptane-niethylczfc/ohexane and ^-heptane-toluene mix-tures under conditions of increasing surfacetension changes in the reflux flow. Table G givestbe collected data. Tray efficiencies and visuallyobserved foam heights of tbe absorption and totalreflux distillation experiments arc plotted againstliquid driving force in Figs. 10 and 11, which arehence similar to the 1IETP graph given in Fig. 4for the Vigreux column.

A distinct similarity is noted between thedependence of tray efficiency and foam heightson the average driving forces applied. The degreeof foaming and increase in tray efficiency withthe two mixtures, however, is clearly different andin line with the surface tension difference's inboth systems. This is in contrast to the Vigreuxcolumn, in which—with decreasing surface ten-sions and using a channelling mixture—no suchinfluences of the system could be detected.

Figs. 10 and 11 show that at low driving force,the foaming effect seems to disappear and the50-55 per cent tray efficiency is approached.In normal distillation practice these low drivingforces are obtained at low or high concentrationsor under " pinched *' conditions. In these cir-cumstances, a loss in fray elliciency is therefore

u5 eo

"D

w 60

?40

$20(_V .

O^j- 1 *

/f^^ i&.'• a

Spray

Foam «

-a ,

3 & O-1ODriving force, mole fraction

O20

l''io. 10. Inlluence of driving force on tray efficiencyOldersliavv sieve plates for various test mixtures. Vajto

velocity about 0-a m/see.

benzenes-heptanebenzene-toluene»-heptane-toIuene»- heptane -met hylci/cfohexane

Open points : normal distillationClosed points : absorption experiments

DV

o-ioDriving force, mrfe fraction

O-2O

•Kic. 11. Influence of driving force on foam height onOldershaw sieve plates with increasing surface tensions

of reflux.

O • w-heptaiie-tolueneA A w-neptune-methyhv/f/ohexaiieOpen points ; normal distillationClosed points : absorption experiments

Table G. Absorption experiments icilft Oldrrshctw sieve tray column. Column zcith 5 plates, averagevapour velocity 0-3 m/sec. Concentrutiutts in mole per cent n-heptune

System

n-heptane-methvlq/ftohexane

w-heptune-tolucne

Feed rates(tnole/hr)

top

l.'I-S

14-y].»!.

I«-H

bottnnt

I7-S

1S-21H-11«-1

Product rates(intile/hr)

tup j bottom

14-1 1 17-.1I

lit. ; i7o10- 1 | 17-5!;>•» f :-'!•!

ComfKtsitinmt. (mole per cent)JPeeeLt

fo/i

WO

7S-35S

KM)

bottom

0

*-!»'_'«

0

Pfoditclx

ton

78-5

07-05I-.1

11 1-0

buttani

U-j

2«-0;!!•()il-0

Averagedrivingforce

(mole %)

L'{)

U7

!iO

Theoreticalplates

4-2

4-t:j-74-:s

Plateefficiency

\

84

SS71Sfi

FoamItt-ififit(turn)

;j."i

40;'.<)50

o be expected when a positive mixture is distilled,i'or total reflux distillation this is shown explicitly-, Fig. 12.

The concentration effect on plate cOU-ieney asiemonstratcd in Fig. I'-i has also been reported:a the literature ; in all eases in which noticeable

O 2O 4O 6O SO 1OOt.% Mote fa volatile component in mixture

FIG. 12. Effect of concentration on tray efficiency ofOltlershaw sieve plates in distillation with n-heptane-

toluene. Averaj^e vapour velocity 0-3 in/sec.

effects were recorded the binary system used canhe identified as positive. Fig. 13 shows data ofTUIJSSKN [10, 11] for the distillation of mcthyl-ryefohexane—toluene and n-heptane-methylf#Ho-hexane in a small perforated plate column (:J8 mm

_o TOO

SO

f 60'O

fc *>

1 20i-

-^i

f~—i — — —Jiylcyclctw ane— tolufrr

h*ptane-rt rthylcyelc*

k-Aexane — i-

0 2O 4O 6O 3O TOO

Mole /o volatile component in mixture

FIG, 33, Concentration effect on tray efficiency ofperforated plates observed by THIJSSKN. Average vapour

velocity about 0-2 ni/see.

diameter). The behaviour of the first system wasto be expected because of its high driving forcesand large difference in surface tension(3-5 dyn/cm). It is of interest to note that alsothe 7t-hcptane-methylci/d0hexane mixture, inspite of its low relative volatility, shows theconcentration effects under extreme compositions.This mixture should therefore be considered asslightly positive instead of neutral.

Tray efficiency maxima at mid-concentrationsas found by TJIUSSEM and by the present authorsfor hydrocarbon systems have also been observedfor ethanol-water mixtures [4, 5], ttecuusc of thelow surface tension and high volatility of ethanolthis system should also be identified as positive.

Further, some interesting cases are reportedfor the distillation of nitrogen-oxygen mixtures.Nitrogen is the most volatile component and hasa low surface tension at the boiling point (about9 dyn/cm), whereas the heavy component oxygenhas a high surface tension of about 13-5 dyn/cm.Accordingly, GKASSMANX and FUANK [3] foundthat when transferring oxygen from vapourbubbles into oxygen-nitrogen liquid mixtures,higher transfer rates were observed than for theopposite case in which nitrogen vapour wastransferred to the liquid. From the surfacetensions of the pure compounds it is clear thatin the first case positive surface tension changesdevelop in the liquid, which changes stabilizethe bubble formation and hence favour masstransfer rates.

The positive characteristics of the nitrogen-oxygen system can also be derived from thedistillation experiments with perforated platesearned out by Lixmz [6]. In the middle concen-tration range foaming was observed, whereas atextreme mixture compositions only a low spraybed developed. The same was found for the systemnitrogen-argon, but foaming was absent with themixture argon-oxygen. These results are inaccordance with the surface tensions and relativevolatilities in the respective s}*stems (Nitrogen-argon : boiling point difference 10°C, surface ten-sions 9 and 12-5 dyn/cm respectively; argon-oxy-gen : boiling point difference about 1°C, surfacetensions 12-5 and 13-5 dyn/cm respectively).

The effects reported above with perforatedplate columns in general pertain to plates withvery small open area and small perforations,conditions which are favourable for foam forma-tion. In technical columns, however, a higherfree area and large perforations are used. There-fore, and also because of higher vapour velocities,a relatively larger amount of spray is formed andfoaming diminishes. The results obtained with thesprny column give rise to the expectation that

99

F. .T. ZrinF.invEG ami A.

SSI

n 100(

£ 60'y£ 60

20

O4 O-8

Vapour velocity.1-2m/sec

1-6

Fro. 14. Tray efficiency of a perforated tray. 1C, per centopen area, 10 mm holes.

* n-heptane-toluene, about 50%^ ?i-hept:nie-niethylry/f/ohexane, about 50%V benzene-toluene, about 50%

there would therefore be less difference betweenpositive and negative mixtures with respect totray efficiency. This is indeed the case and can beseen from Fig. 14 in which tray efficienciesobserved for a perforated plate with 16 per centopen area, 10 mm hole size, (diameter 0-45 m) arcplotted for three different systems.

G. D I S C U S S I O N *The experiments reported in the preceding-

paragraphs have shown that, depending on thetype of equipment used, decreasing or increasingsurface tensions of the reflux may influenceinterfacial area and • separating power quitemarkedly. A survey of the main trends observedis given in Table 7. For convenience, the proper-ties pertaining to systems in which no surfacetension changes occur (neutral systems) havebeen denoted in this table as ** u n i t v "*.

In order to give an explanation of these pi,nouicna, it should be recalled that liquid surfacof high interfacial tension contract when contact-with a surface of lower surface tension. Tl.is known as the 4t Maragnoni effect ". MARA*

[7], however, did not consider the influemof vapour -liquid interaction in his studies.

A very well-known phenomenon in whicliquid film contraction occurs as ,1 result of ma-transfcr effects is the so-called " wine-gin^effect ". Wine creeps up the wall of the gla-bccause of its wetting tendency. When a filrhas been formed, alcohol preferentially evaporatesleaving a residue of higher water content an<therefore higher surface tension. When fresfwine creeps up the wall, and comes into contacwith the aqueous film, the latter contracts intismall rivulets.

A similar explanation can be given for tinchannelling phenomenon taking place whetdistilling a negative mixture in a wetted waltype of column. Liquid films descending in sucl.a column are never of uniform thickness, andhence, locally, thin places will become morerapidly saturated with heavy component than theremainder of the film. Since the heavy componenthas the lowest surface tension, areas of lowersurface tension develop locally; the remainingparts with higher surface tensions thereforecontract and the liquid film breaks up intorivulets.*

*K«i*entially the same explanation lias been given byU<>\*[» and DON.VI.IJ [t ] for the i r fibservatiotw of the brt-;ik-down of water ( U r n s in the c-:i-;e of ammonia absorption.

Surrey of surface phrnvmena in distillation equipment

Supported interfacial areaSurface tension

Unsupported iaterfitciitl area

changes

no changes(neutral)

increasing(positive)

Wetting

complete

complete

tit-creasing ' partial(negative)

Separating \ nfptwer* driving force

I none

^ I

< I

none

severe

Tt//tf ofintfrfacial art'it

spray

foam

spray

Separatingpoicer*

1

> 1

~ 1

Effect ofdriving force

none

.severe

none

*Srparating power denotn! un i ty fur " no

100

The influentr nf surface hH mi OH tin1 prrroriiiimn-

Once the rivulets arc formed, the edges, becausef their smaller volume, will ahvays be more(turated with the heavy component. Surface•nsion is thus lower at the edges and this stabilizeslie contraction of the liquid (see sketch in Fig.5).It is easily understood that the differences of

.nicentration in the liquid surfaces will become.iqhcr and contraction will become more severef the mass transfer per unit area is higher.Hie latter is produced either by an increased

of liquid film into rivulets

Stabilisation of rivulet

Fio. l~t. Schematic illustration of break-up of a liquid:ilm in a " negative *' system. Shaded areas denote

liquid of lower surface tension.

driving force or because of higher mass transfercoefficients (i.e. smaller HETP values). For theYigreux column, the driving force effect hasalready been illustrated in Fig. 4. The influenceof the HETP on the severity of the channellingphenomenon is recognized when comparing theresults from the small concentric tube column(Fig. 1) and the Vigrcux column (Fig. 2). Fig. 16

o-s

0-6

0-4

0-2

Cor>cerrtr.c tube * wire coil

Frne wir« packing

_6mm raschig rings, _

Vigr-eux,

'ccoceitnc tutje

FIG. 10.HETP

O 100 20O

HETP, wetting system

Ht-Litiou In-tween loss in separating power andwhen distilling benzene- » -heptane in various

types of equipment.

gives a graphical representation of the relationbetween the ratio of IIETlrs with channellingand complete wetting and the UETP of thecolumns with complete wetting. Furthermore,the same HETP ratios observed for the packedcolumns arc plotted in this graph.

It is thus seen that at the same HETP, thepacked columns suffer a smaller loss in cllicieneythan the wetted wall type of columns. This iseasily explained by the fact that in the packedcolumns liquid is retained by capillary forces ininterstices between the packing elements. Thecontraction of the liquid surface in this casewill be less effective and the loss in intcrfacialarea will therefore be smaller. Reduction of thechannelling effect in this sense has also beenfound when providing the concentric tube columnwith a thin wire spiral.

The distillation of " positive " mixtures in theVigrcux column and the Rasehig rings columndid not reveal any significant differences withrespect to the distillation of a. " neutral " mixturein the same equipment. Considering for thiscase again the non-uniformities in the liquidfilms, it is now seen that thin and weak spotsbecome preferentially saturated with the com-ponent with the highest surface tension. Theweak spots are thus reinforced, -and as a. resultthe film tends to become of more uniform thicknessand break-up will be prevented (see the sketchin Fig. 17). Wetting of the supporting solidsurface with a film of the positive mixture willtherefore be approximately the same as with

KIG. 17. Schematic illustration of the stabilization of aliquid film in a "positive" system. Shaded areas denote

liquid of lower surface tension.

mixtures in which surface tension gradients areabsent.

The latter conclusion, however, only holdsgood for supported interfacial area. In a systemwhere the vapour is dispersed into a liquid, thelifetime of the bubbles formed may increaseconsiderably because of the reinforcement of

101

F. .1. XriiiKttvvKr; ami A. HAKMKNS

ara

weak spots. These weak spots occur betweenadjacent bubbles (see Fig. IS); the preferentialsaturation of these spots with heavy component

Fir.. 18. Schematic illustration of the stabilization ofvapour bubbles in a ** positive " system. Shaded areas

denote liquid of lower surface tension,

causes the surface tension to rise locally. There-fore, liquid is drawn in between the bubbles andcoalescence of the adjacent bubbles is prevented.If the changes in surface tension are large enough,even a foam may develop in this manner.

Since the degree of bubble stabilization willdepend on the change in surface tension of theliquid on the trays, the relation of foam heightand tray efficiency to driving force as shown inFigs. 10 and 11 seems quite logical.

Finally, the behaviour of neutral systems andnegative mixtures in a perforated plate columnwill be briefly considered. For hydrocarbonmixtures in which no appreciable changes insurface tension occur, the stabilising factortending to produce prolonged bubble life isabsent. Accordingly, bubbling is poor wi th suchsystems and spray formation occurs preferentially.From what has been said about the ins tab i l i tyof liquid films when dist i l l ing a negative mixture,it will be clear that in this case the bubble stabi-lizing factor is also absent. Approximately, thesame interfacial area can therefore be expectedfor the two types of systems ; accordingly aboutequal tray efficiencies are observed for the" neutral *' and negative systems.

7. CoNCJ.r .s ioxs

The experimental facts observed and the quali-tative interpretation provided give rise to somedefinite conclusions regarding the formation of

interfacial area in vapour liquid contact Iequipment.

The main conclusion is tint changes in surfctension occurring in the liquid phase can havispectacular effect on the interfacial area. Breaup of liquid films results from decreasing surfatensions and is of particular importance iwetted wall columns and packed columiStabilization of liquid films occurs when t!interfacial tension increases ; even foaming m;develop in this case. The latter effect is of paticular interest for the operation of plate columnSince mass transfer rates are directly proportion.to interfacial area, the surface tension effecare reflected in the JIETP values of the varioitypes of equipment.

The above phenomena become especially notictable when the differences in surface tensiobetween the components of the mixtures at thboiling point are higher than about 2 dyn/ciand at driving forces above 5 mole per cent.

A further conclusion is that equipment witihigh mass transfer efficiencies is relatively morsensitive to the surface effects than less efficienapparatus. From a practical point of view thimeans that the effects are of particular importancefor laboratory apparatus, whilst technical equipmcnt will be much less sensitive to the surfaci

• phenomena described.Finally, it is of interest to note that tin

influence of the changes in surface tension onmass transfer in general is quite severe andappreciably more pronounced than the effect olvariations in the static properties of the liquidphase, such as density, viscosity and diffusivity.TIIIJ>SKN- [10, 11], also MOI.STAU [8] ct al.t havein fact argued that variations in these propertiescould not account for their observations. In thisconnexion it may be pointed out that whenstudies are made in which the static propertiesof the phases are the main subject, (e.g. whenthe contribution of the vapour and liquid phasesto the resistance against mass transfer is assessed)care must be taken to select experimental con-ditions such that the surface effects do notaffect the observations. From the results of thepresent study it mav be concluded that suchconditions cannot easily be realized.

lOi!

jttv.vv.--T •>•? *.«r*W.s yK*,tu.»u..»u. «« * KM j.«Tform:iiuT' of distillation <-<tluinns

T? K1' K K K X f K S

[1] BoM> J. and DONALD M. 15. C'Awii. £»£»£. AYf. 1057 6 237.

[2] COLLINS F. C. and LANTZ V. Jmlusttr. JKngHg. Chem. (.-ttnil.) 1956 IS <>7:i.

|:S] GIIASPMANN P. and FIEANK A. Interne Arbt'itfixitznng (Ms f\«7m«s.sT/tH-sscs Deslilffifwn, Kcktijifffttirtn witl Extractiontier V.DJ. Fttchgnippe rerftihrenstfchntk. Gottinyt-n 1JI57.

[4] KEYES D. B. and BVMAN L. LT»ic. Jit. JSngng. Erp. Sta. Butt. Scr. »28 7041.

[5] KiRscisitAVM E. Cltcm.-lng..Tcch. 1951 23 213.

[6] LINIIK G. Chem.-l»g. Tech. 1955 27 GG1.

(7] MABACNOXI C. Ann. Pays. Lpz. 1871 143 337.

[S] MOUSTAD M. C. and PAHSLY L. F. Client. Engng. Progr. l!»5t> 46 20.

[9] SicwAan K. Z. IVr. jD/wft. /?/5. 1956 98 453.

10] THIJSSKX H. A. C. Effect of Liquid Conipttsition »n I'lttie Efficiency in the Kectiftcution of Ithittrif J//.r/nrcs. Vcrsl.JLandbouwk. Onderzoek, The Hague Xo. 61.12. 1955.

11] WUK W. R. VAN and TIIIJSSEN H. A. (\ Engng. Sci. l»5i 3 153.

12] WILLINGICAM C. H., SEDIJIK V. A., ROSSINI F. D. and WKSTIIAVKK J. /m/rwfr. Engng. Client. 1947 39 706.

13] ZOIDEUWEG F. J. Cltein.-Ing. Tech. 1950 23 587.