Effect of system properties on the performance of Liquid—Liquid extraction columns—II

EFFECT OF SYSTEM PROPERTIES ON THE PERFORMANCE OF LIQUID-LIQUID EXTRACTION COLUMNS-II OLDSHUERUSHTON I KOMASAWAt Postgraduate

School of Chemrcal

Engmeermg,

(Recewed

COLUMN

and J INGHAM

The Umversrty England

22 February

of Bradford,

1977, accepted

Bradford,

West Yorkshtre,

BD7 1DP.

14 June 1977)

Abstrac-Mean drop sue. fractronrd hold-up of drspersed phase and axral mrxmg charactenstrcs have been determmed m a 72 mm dtameter mechamcally agttated extractron column of Oldshue-Rushton type, usmg the two hqmd-hquid mass transfer systems, toluene-acetone-water and MlBK-acetic acid-water As for normal condrtrons of packed column operatton descrrbed m Part I, solute presence and the drrectron of mass transfer has a stgntficant effect on mean drop srze, fracttonal hold-up and to a lesser extent, axraf mwng m the drspersed phase Probably the most dramattc effect however IS the manner m whtch solute transfer affects dispersed phase behavrour Highly coalescmg conditions with transfer from the Qspersed to the contrnuous phase can make the column practically unoperabie As for the packed column, axial mrxmg m the contmuous phase ts una8ected except m so far as solute presence and due&on of mass transfer affect the hold-up of dtspersed phase

1 INTRODUCTION

3llEsuLT8

wechamcally agitated extraction columns represent an extreme case of flow regune, owmg to the prevdmg Hugh levels of turbulence Under such con&tions It may be expected that the effects of any special mterfacial mass transfer process would be dominated by the unposed turbulent flow regnne, smce the effects of droplet-droplet mteractlon should be much more pronounced 2EKpEI(IMENTAL

3 1 Drop sue Mean droplet sizes for the system toluene-acetonewater are shown m Fig 1 as a function of the rotor speed N The presence of solute as an eqmhbrated solution causes a reduction m the mean drop size compared to the values obtamed with the solute free system Mean drop sues for solute transfer from the contmuous to the hspersed phase are also reduced by the reduced tendency for coalescence For all the above cases, the mean drop size decreases as expected with mcreasmg rotor speed For solute transfer from the dispersed to the

The column conslsted of a 72 mm preclslon QVF tube contauung 16 mrxed stages, each 32 mm m he&t The column compartments were agitated by centrally located, standard 6 bladed turbme impellers, 25 mm in dmmeter Bafflmg of the compartments was effected by 4 verucal wall baffles, 6 mm In depth and 1 mm width spaced equally around the column circumference The stator plate openmg dmmeter was 36 mm Special end sectlons, fabncated from a Teflon (PTFE) block, glass wool and brass honeycomb secuon were used to muunuse end effects The experlmental system was generally the same, as described m Part I Drop size evaluations were based on the 8th compartment numbenng from the base and sue reference was gven by the rotor dunenstons. In order to reduce end effects, the interface was held at the base of the top honeycomb section for determmatlons of contmuous phase axial mlxmg and at the column top for the dispersed phase Analysis of the tracer response curves, as described m Part I, gave values of M = [F, L/2 El X,] and hence the Peclet Number Pe = [2 A&H/L]

FU

?Present address Department of Chemtcal Engmeermg, Faculty of Engmeenng Science, Umverstty of Osaka, Toyon&a, Osaka, Japan

Toluene-acetone-w&er 0, wlthout solute, V, equmbrated wnh solute (Cf8 wt%). l . solute transferred &sp +cont phase (C,, 7wt%), A, solute transferred cant +dasp phase (cf. 8 5 wt%), I$ = 8 2, F, = 9 34 cm3/cmzmm

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Mean drop srxe. toluene-acetone-system Effect of rotor speed, solute presence and drrectron of solute transfer

I

480

KOMASAWA

contmuous phase, the mean drop size 1s however conslderably Increased and unhke the previous sltuatlons reaches a value which is apparently Independent of rotor speed This IS also shown by the results for the MIBK system, shown m Fig 2 Transfer from the dispersed to the contmuous phase promotes droplet coalescence thus mcreasmg mean drop size Droplet coalescence IS enhanced at high rotor speeds by the increasing probablllty of droplet collrslon Thus at high rotor speed, the enhanced rate of coalescence overcomes the increased tendency for droplet breakdown and the drop size 1s apparently stablllsed Mean drop sizes for the MIBK system follow the same trends as for the toluene system and similar explanations apply Mean drop sizes however are generally smaller for the lower mterfacml tension system The empmcal correlation of Vermeulen et al {l] predicts values of mean drop size 111the range 0054cm0 031 cm for the toluene system and drops in the range 0 026-O 015 cm for the MIBK system respectively over a 250-500rpm range of impeller speeds These values are about 0 25-O 14 the magrutude of the drop sues observed m the present study Recent work by Kubol et al [2] has shown that droplet residence times of the order of 2 mm are required to obtam 90% of the equlllbrmm drop size, compared to the mean residence times of 0 45-l 1 mm employed in this study Thus it appears that the drops m the present study may not have reached equfibrlum size 3 2 Droplet behaurour The dispersed phase was supplied via the dlstnbutmg nozzle into the first stage of the column, m the form of drops of 0 1-O 5 cm dlam There was no slgmficant change of drop size along the column, for the case of the solute free system In the presence of solute transfer,

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E, G

01 007 005

Fig 2

Mean drop size, MIBK-acetlc acid system Effect of rotor speed, solute presence and due&on of solute transfer

h41BK acetIc a&-water 0, without solute, 0, solute transferred disp *cant phase (cdf 4%), A, solute transferred cant + dmp phase, (C,,, 5%) Fd = 5 85 cm”/cm’ mm, F, = 9 34 cm3/cm2 mm

and J

INGHAM

however, a change of drop size was observed, especially for the case of solute transfer from the dispersed phase to continuous phase Under condltlons of low dispersed phase flow and h@ flow rate of continuous phase, the solute transfer tends to approach completion as the drops nse through the higher parts of the column In this case, the drop size in the lower SIX stages of the column was about 0 3 cm and m the upper SIX stages approximately 0 1 cm Under condltlons of high flow rate of dispersed phase and low flow rate of contmuous phase, the solute concentration in the continuous phase tends to approach equihbrmm with the dispersed phase Under these con&tlons, the drop size varied from about 0 3 cm in the upper stages to 0 1-O 3 cm m the bottom three stages of the column Such special condltlons were discounted m the analysis I+ghly variable operating characterlstlcs were obtamed for both systems for the solute free system and with solute transfer m the two opposing dlrectlons of mass transfer These are illustrated for the case of the tolueneacetone-water m Fig 3 For the solute free system, a wide datibutlon of drop sizes, ranging from l-3 mm was observed at a rotor speed of 350 rpm Increasmg the rotor speed to 500 rpm gave a reduced size dlstrlbutlon and increased hold-up of dmpersed phase At 700rpm phase mverslon condltlons occurred With acetone transfer from the contmuous aqueous phase to the dispersed toluene phase, the coalescence tendency at 350rpm was reduced, giving a wide dlstrlbutlon of smaller drop sizes m the range 0 5-2 mm Increased rotor speed gave condltlons of high hold-up with drop sizes iess than about 0 5 mm At 7OOrpm, condltlons m the column were turbid and emulslficatlon occurred, owing to the very fine distribution of drop sizes Acetone transfer from the dispersed toluene phase to the aqueous contmuous phase corresponded to a greatly enhanced rate of droplet coalescence At 350 rpm, fairly large droplets were formed which travelled quickly through the column Increased coalescence and frequent droplet redlsperslon behavlour was observed at the mcreased rotor speed of 5OOrpm, and at 700 rpm, the remarkable effect of the formatlon of single toluene drops, almost filling each compartment was observed For the toluene solute free system, a mrmmum rotor speed of about 250 rpm was required to obtain adequate phase dlsperslon This was reduced to about 200 rpm with acetone transfer from the contmuous to the dlspersed phase and for the eqtuhbrated solute system With acetone transfer from dispersed to contmuous phase, however, a muumum stlrrmg speed of about 500 rpm was required for proper operation At low rotor speeds, a tendency for the drops to collect under the honzontal stator baffle plates was observed, correspondmg to behavlour previously observed by Blbaud and Treybal [ 31 Drop stzes were much smaller for the low interfacial tension system MIBK-acetlc acid-water and a lower mmlmum rotor speed of 150rpm was required for adequate phase dlsperslon Sumlar behavlour to that of

Performance

FIN 3 Dispersed

the toluene

system

was

obtamed,

under

the

of hquld-hquld

phase hehavlour,

dtienng

The lower coalescmg tendency for this system, however, falled to produce the large smgle drop behaviour observed for the toluene system It IS apparent that the effect of solute presence and due&on of solute transfer on the dispersed phase dlstnbuhon represents a dominant effect m the OMshueRushton column operation, which may be attibuted directly to the dtiermg system physical propertles and consequent droplet coalescence behavlour systems

of mass

transfer

3 3 Hold-up Typical results for the toluene-acetone-water system are shown m Fig 4 Hold-up increases with increasing solute concentration both for the eqmhbrated system and for mass transfer m the dlrectlon contmuous to dispersed phase owmg to suppressed coalescence tendencies For transfer from dispersed to the contmuous phase, the coalescence tendency 1s increased glvmg larger drops

extraction

columns

481

toluene-acetone-system

N *‘, representmg the mcreased droplet break-up due to the increased turbulence at higher rotor speeds and the correspondmgly reduced drop rise velocltles Overall results are correlated in Fig 7 All the hold-up values increased markedly with increasing flow of dlspersed phase The highest values of hold-up were obtamed with the equilibrated solution, representing hmltmg conditions very close to floodmg Mass transfer from the contmuous to dispersed phase gave hold-up values between those for the equilibrated and solute free systems In these cases, solute presence causes a 05

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Tohem-acetone-WOI-

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with higher rise velocities and hence reduced hold-up values The toluene-acetone-water system ISvery highly coalescing and, at a rotor speed of 350rpm it was lmpossible to obtam a reahstlc droplet size dlstrlbutlon with mass transfer from dispersed to contmuous phase Increasing flow of the contmuous phase would be expected to mcrease dispersed phase hold-up owmg to the reduction in the relative slip velocity between the drops and continuous phase This is shown m Fig 5 for the case of solute transfer from contmuous to dspersed phase The effect of mcreased coalescence tendency for solute transfer from dispersed phase to contmuous phase IS, however, sufficient to overcome the effects of reduced slip velocity, such that the hold-up tends to decrease with mcreasmg flow of contmuous phase Figure 6 shows the effect of rotor speed on hold-up for the toluene solute free system Hold-up increases with mcreasmg rotor speed according to the relation Xd 0:

Fig 4 Fractzonal hold-up of hpersed system Effect of solute concentration

A

V 0

phase, toluene-acetone and due&on of transfer

Duectlon

Fd

F’

N

cant +dlsp equdlbrated dlsp +cont

824

933

350 350 500

482

I

KOMASAWA and J

INGAAM

003

002 i

FIN 7 Correlauon

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3

5 5,

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5 Fractmnal hold-up of dispersed phase, toluene-acetonewater system Effect of the flowrate of contmuous phase

Ihrection

Feed cone wt%

A contmuous A + dispersed 0, dispersed + @ continuous 0

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c,, 10 60 2 82 c,, 1140 860 463

9

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ff. rm Fw 6 Fractional hold-up of dmpersed phase, toluene-water (solute free) system Effect of rotor speed

0 @

824 1149

933 933

of fracuonal hold-up acetone-water system

results,

toluene-

reduced tendency for coalescence and the formatlon of smaller drops For mass transfer from the dispersed phase to the contmuous phase, the hold-up values are reduced considerably due to increased coalescmg tendency Results for the MIBK-acetlc acid-water system are very similar to those obtamed for the toluene system, as shown m Figs 8 and 9 For the MIBK system, the functlonal dependence of hdd-up on rotor speed, shown m Pig 9, may be represented as X, a IV*’ Overall results are correlated m Fa 10 In this case, there appears to be no slgnlficant dtierence m the hold-up varmtions for the eqmhbrated system and mass transfer from contmuous to dispersed phase Increased coalescmg tendencies for solute transfer from the dispersed to contmuous phase are agam evidenced by a large reductlon m hold-up 3 4 Axral muring As for the packed column results descmbed m Part I, excellent agreement between the measured response cmves and the theoretical curves predicted by the onedlmenslonal dlsperslon model were agam obtamed for all flow con&tions 3 4 1 Axral murrng wzthe contrnuous phase Ax14 mixmg results for the single aqueous contmuous phase and the aqueous contmuous phase with toluene and MIBK dispersed are shown m Fig 11 Although showing normal scatter, the results, for the single phase, the equlltbrated phase systems, and both drrectlons of solute transfer for the system toluene-acetone-water, all fall on a common relationstip which IS in good agreement wth results obtamed previously by Mlyauchl et al [4] and Blbaud and Treybal[3] The term X,, fractional hold-up of contmuous phase, 111the generahsed correlation may be taken to mdlcate two major effects of the dispersed phase as postulated by Ingham[q VLZ (a), a flow straghtenmg effect, and (b), the hmdrance of the free passage of back-mured contmuous phase This common correlation holds for rotor speeds greater than about 2OOrpm where

483

Performance of liqmd-hqmd extraction columns 05

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Fa 8 Fractional hold-up of dispersed phase, MIBK-acehc acidwater system Effect of solute concentration and duectlon of transfer

Duectlon A

Fd

F,

-

2

3

5

7 5

Fig

10 Correlation

IO ,

20

cm3/cm2

30

min

of fractional hold-up results, acid-water system

MIBK-acetlc

N

cant --,&sp eqmhbrated chsp +cont

835 933 400 0 Xii 0 axml rmxLng 1s controlled by the act1011 of the impellers and normal operatmg con&tions apply Thus azual mtxlng zn the contmuous phase appears to be tiected by system proV

peties only to the extent to which these &ect hold-up lIus result IS 111agreement with the findmgs of other mvestigators [6], and the results previously reported for the packed column 111Part I 3 4 2 Axral muerng m the dupersed phase The effect of the dispersed phase flow rate on axial mixing 111the dispersed phase for the two solute free systems are 0

20

40

(RN/F,)(s/D)*,

sp

01

loo

FQ 11 Contmuous phase axml mlxm8 A, smgle phase, 0, toluene-water, 0, acetone transferred from

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Ispersed

-

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FIN 9 Fractional hold-up of &spersed phase, MIBK-water (solute free) system Effect of rotor speed 4 0, @

F, 585 1081

933 933

to contmuous phase, 0, acetone transferred from contmuous to dmpersed phase, 0, MIBK-water

shown m Fig 12 The reciprocal Peclet number decreases with mcreasmg flow rate for each case according to a functional dependence of the form Pd = Fz3 Smce hold-up varred with chspersed phase flow accordmg to x, a Fdo ‘-’ ‘, the axed mlxmg m the dispersed phase cannot be considered to be a simple function of hold-up as obtamed for the contmuous phase Thus was also found by Blbaud and Treybal[3] The effect of solute mass transfer for the system toluene-acetone-water IS shown m FGg 13 For solute transfer from the continuous to dispersed phase, and the eqmhbrated phase system, correspondmg to reduced coalescence tendency, the recaprocal Pecfet number IS mcreased correspondmg to increased axial mlxmg For solute transfer from dlspened to contmuous phase, lower values of reciprocal Peclet number are obtamed, correspondmg to reduced

484

I KOMASAWA and J

INGHAM

3

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3

5

Fig

12 Dispersed

IO

7

6,

20

I

II

III

-acetone-water

30

cm’km2 mh

phase ax~al mlxmg (solute free

F, cm3/cm2mtn

I

I Toluene

systems)

Nrpm

9 33 9 33

350 300

Fig

tuluene-water MIBK-water

14 Correlatatlon of dispersed phase axial mlxmg, acetone-water system

toluene-

0, without solute, V, eqmhbrated, to dlsp

03-

02

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I 5 CM.

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phase

0

A, solute transferredfrom cant transferred from drsp to cant phase

solute

The correlation obtamed for the MIBK-acetic acldwater system 1s shown m Fig 15 The results for the solute free system show a slmllar tendency to that shown for the toluene-water system The results for the other cases, 1 e equilibrated system and solute transfer in both duectlons, however, show considerable scatter and no observable trends can be dlstmgmshed Axial mlxmg values are about 30-40?6 less than those obtained for the h&er interfacial tension toluene system

wt %

CONCLUSIONS

4

13 I)lspersed phase axial mlxmg toluene-acetone-water system Effect of solute concentration and duection of transfer Fd, 8 24, FE, 9 33cm3/cm2 mm

mlxmg under enhanced coalescing conditions Results for the MIBK-acetic acid-water system followed a common relationship, irrespective of the dlrectlon of transfer Any special effects due to an enhancement in the rate of coalescence for this system with solute transfer from dispersed to the continuous phase are thus neghable, compared to those of the toluene system Overall results for the toluene-acetone-water system are correlated m Fig 14 Axial mmg m the dispersed phase 1s reduced progressively as the system changes from conditions of retarded to enhanced coalescence m accordance with the change from the equilibrated system, solute transfer from contmuous to dispersed phase, the solute free system, and solute transfer from dlspersed to contmuous phase With the exception of the eqmhbrated phase system, axial mixing decreases w&h mcreasmg rotor speed Difficulty was found m obtammg accurate results for the equilibrated system owmg to the tendency to unstable operation at high hold-up values Results for solute transfer from dispersed to continuous phase were hmlted to high rotor speeds (N = 500 rpm), owing to the dficulties of operation m this highly coalescing system axial

Drop size determmatlons for the Oldshue-Rushton column with both systems showed the same tendencies as those observed for the MIBK system 111the packed column, described in Part I Thus solute transfer has an appreciable effect on drop size under both quiescent and aatated flow conditions In determmmg the V~.IYOUSmodes of operation, as 3

I

I

I

,,,I,

MIBK- acetic acid - water

05c

Fig

15 Correlation

-I

of dispersed phase axlal acetic acid-water system

mixing,

MIBK-

0, wlthout solute, V, equilibrated with solute, A, solute transferred from cant to dtsp phase, l , solute transferred from disp to cant phase

Performance of hquld-hquld extractlon columns shown m Fig 3, solute presence and dIrectIon of mass transfer have their most predommant effect on column operation, which may completely outweigh other conslderatlons such as drop size, hold-up and axial mlxmg Hold-up values m the Oldshue-Rushton column mcreased markedly as the system changed from solute transfer from dispersed to contmuous phase, solute free, solute transfer from contmuous to dispersed phase to the eqmhbrated system, m accordance with progressively reducmg coalescmg tendency Solute presence and direction of mass transfer has no sign&ant effect on the axial mlxmg m the contmuous phase, other than mamfested by a simple hold-up correction term, and affected only the dispersed phase axial mlxmg m the lugher mterfaclal tension system, tolueneacetone-water to an appreciable degree This effect was again consistent with either enhanced or suppressed coalescence tendencies m the dispersed phase, brought about by the appropriate system changes NOTATION

d

YS

c Et F, H

Sauter mean drop diameter, cm solute concentration, wt% eddy dlsperslon coefficient for phase I, cm’/mm superficial flow velocity for phase I, cm3/cm2 mm height of compartment, cm

L M

485

contactor length, cm dunenslonless coefficient

for axial mlxmg

=- F&l-.

N Pe,

2EXI rotor speed, mm-’ dunenslonless Peclet

number

for uhase I

=- F,H EX R S X,

rotor diameter, cm stator openmg diameter, cm fractional hold-up for phase I

Subscripts C

d f

contmuous phase dispersed phase feed value REFERENCES

[l] Vermeulen T , Wllhams G M and LangloIs G E , Chern Engng Progr 1955 5185F [2] Kubol R , Komasawa I and Otake T , Paper presented 8th Autumnal Research Meetmg of the Sot Chem Engng Japan 1974 [3] Blbaud R E and Treybal R E , A ICh EJ 1962 12 472 [4] Mlyauchl T , Mltsuatake H , Harase I , A I Ch E J 1966 12 508 [5] Ingham J , Trans Instn Chem Engrs 1977250 372 [6] Ingham J , Recent Aduances m Solvent Extractron (Edlted by Hanson C ) Pergamon Press, New York 1971