Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 22 (1) 75−83 (2016) CI&CEQ

75

MEHDI ASADOLLAHZADEH1,2

SHAHROKH SHAHHOSSEINI1

MEISAM TORAB-MOSTAEDI2

AHAD GHAEMI1 1Department of Chemical

Engineering, Iran University of Science and Technology (IUST),

Tehran, Iran

2Nuclear Fuel Cycle Research School, Nuclear Science and

Technology Research Institute, Tehran, Iran

SCIENTIFIC PAPER

UDC 54:66.063

DOI 10.2298/CICEQ150426022A

THE EFFECTS OF OPERATING PARAMETERS ON STAGE EFFICIENCY IN AN OLDSHUE-RUSHTON COLUMN

Article Highlights • Stage efficiency of the investigated column is high in comparison with other extractors • Stage efficiency is strongly dependent on the agitation rate and interfacial tension • Stage efficiency is better when the mass transfer direction is from continuous to

dispersed phase • Empirical correlation is derived for prediction of stage efficiency Abstract

In this research, stage efficiency was measured in a 113 mm Oldshue-Rushton column for two systems including toluene-acetone-water and n-butyl acetate--acetone-water. The experiments were performed in two directions of mass transfer. The effects of different parameters such as rotor speed, dispersed and continuous phase velocities and direction of mass transfer on the stage efficiency were investigated. The experimental data show that the stage effi-ciency is strongly dependent on the agitation rate and interfacial tension, but only slightly dependent on phase velocities. It was observed that the stage efficiency is better when the mass transfer direction of acetone is from the con-tinuous to the dispersed phase in comparison to opposite direction due to the presence of oscillations created by surface tension gradient. The investigated column is one of the extraction columns with high stage efficiency. An empi-rical correlation is proposed to describe the stage efficiency in terms of Rey-nolds and Froude numbers. The predictions of the equation had good agree-ment with the experimental data.

Keywords: Oldshue-Rushton column, stage efficiency, axial mixing, throughput.

Solvent extraction is one of the key unit oper-ations in the processes including the petrochemical, pharmaceutical, hydrometallurgical, and environmen-tal industries. Among various types of solvent ext-raction units, the extraction column is emerging as one of the best choices because of a high throughput and stage efficiency [1].

The droplet size and the degree of turbulence are dependent on the mechanical agitation in the extraction column. Mixing can intensify the stage effi-ciency due to the large interfacial area with small dis-persed drops [2,3]. As drop size decreases with agit-

Correspondence: S. Shahhosseini, Department of Chemical Engineering, Iran University of Science and Technology (IUST), P.O. Box 16765-163, Tehran, Iran. E-mail: shahrokh@iust.ac.ir Paper received: 26 April, 2015 Paper revised: 24 June, 2015 Paper accepted: 2 July, 2015

ation speed, the relative velocity between the dis-persed phase and continuous phase decreases like-wise, which lowers the throughput. In addition, the agitation can increase the axial mixing and reduce the extraction efficiency by decreasing solute concen-tration gradients and as a consequence the mass transfer rate. Neglecting the effect of axial mixing when designing an extraction column can lead to overestimation of mass transfer efficiency of about 30% or more [4]. Thus, the mechanical agitation can be used to control the droplet size, dispersed phase holdup, stage efficiency and, consequently, the per-formance of the extraction columns [5].

In several studies, a number of authors have reported different methods to decrease axial mixing by coalescing small drops in the section between stages. Internal column geometry reduces axial mix-ing, increases droplet coalescence and breakage

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rates resulting in increased mass transfer rates, and affects the mean residence time of the dispersed phase, which allows the handling of large loads with small differences of interfacial tension and density, improving the hydrodynamic performance of the col-umn and, subsequently, the extraction efficiency [6,7].

The experimental set up with the mixing part and the packing part alternately to promote drop coalescence in the packing part has been reported by Scheibel [8]. For the same objective, a three-dimen-sional lattice as a partition of the mixing stages was investigated by Steiner et al. [9].

The coalescence-dispersion pulsed-sieve-plate extraction column (CDPSEC) is a modified pulsed-sieve-plate extraction column (PSEC). It was reported that the CDPSEC with 50 mm in the plate spacing was of 120% overall mass transfer efficiency over the standard PSEC [10]. However, when the plate spacing of the CDPSEC was reduced to 25 mm, it was reported that the mass transfer efficiency of the CDPSEC was only about 50% that of the standard PSEC, although the interface renewal frequency was doubled [11].

Horvath and Hartland achieved the high stage efficiency with a mixer-settler extraction column in which the inter-stage mixing was extremely small, similar to the throughput of the column [12]. Schwei-tzer reported a rectangular mixer-settler tower with horizontal arrangement of the mixer and settler in each stage. The arrangement between stages can reduce the axial mixing and result in the enhance-ment of separation efficiency [13]. The comparison of performance of various columns is shown in Table 1.

The Oldshue-Rushton column manufactured by the mixing equipment company and commonly known as the Mixco column was developed in 1940 thanks to the best endeavors of Rushton and Oldshue. The unit consists of an outer shell in which horizontal stage separators are constructed to form the desired num-ber of processing stages, each equivalent to a sepa-

rate mixing operation [14]. Experimental work in Old-shue-Rushton columns is limited and the studies about stage efficiencies in the column have rarely been referred to in the literature.

The objective of the present work is to inves-tigate the influence of operating parameters such as rotor speed and velocity of dispersed and continuous phase on the stage efficiency for mass transfer dir-ections as well as the two systems. An empirical correlation for prediction of stage efficiency is recom-mended in terms of physical properties of liquid sys-tems and operating conditions.

EXPERIMENTAL

A pilot plant Oldshue-Rushton extraction column is used in these experiments. The column built in a cylindrical glass section was equipped with impellers with accurate speed control and the internal parts were constructed from stainless steel; a schematic diagram of the Oldshue-Rushton column used in this study is presented in Figure 1. The specifications of this column and range of operating variables are listed in Table 2.

In normal operation, two types of immiscible liquids with different densities flow counter-currently through the apparatus. One of them is in large quan-tity (continuous phase), while the other, being in minute quantity (few percent), is dispersed as drops. Two flow meters are employed to supply and monitor the fixed flow rates of continuous and dispersed phases. The inlet and outlet of the column are con-nected to four tanks, each of 85 L capacity. The inter-face is maintained at the required level by using an optical sensor as previously described.

Two chemical systems for instance toluene-acetone-water (high interfacial tension), and n-butyl acetate-acetone-water (medium interfacial tension) are examined on the extraction column for both mass transfer directions. The European Federation of

Table 1. Comparison of performance of various columns [12]

Column Diameter

m Stage height

m Stage efficiency

% NTS/m

1/m Throughput m3 m–2 h–1

Mass transfer direction

Enhanced coalescing column 0.072 0.060 45 3-7.5 10-60 d→c

Kühni column 0.060 0.350 100/130 2.9-3.7 4-10 d→c

Packed column 0.070 - - 1.8-2.5 15-30 d→c

Pulsed packed column 0.070 - - 3.8-5.8 18-20 d→c

Pulsed sieve plate column 0.050 0.100 60 3.5-6.0 20-30 d→c

Karr reciprocating column 0.050 0.025 15 3.5-6.0 30-40 d→c

Rotating disc column 0.070 - - 2.8-3.5 15-35 d→c

Mixer settler extraction column 0.152 0.150 97 6.5 2-6 c→d

Mixer settler extraction column 0.152 0.150 170 11.3 2-4 d→c

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Chemical Engineering (EFCE) has adopted these systems as recommended systems.

Table 2. Technical description of the Oldshue-Rushton column

Parameter Value Unit

Column height (H) 700 mm

Column internal diameter (Dc) 113 mm

Diameter of the rotor 50 mm

Settler diameter 169 mm

No. number of stages 9 -

Height of the stages 67 mm

Fractional free cross section area 25 %

Continuous phase flow rate 18-36 l/h

Dispersed phase flow rate 18-36 l/h

Rotor speed 100-240 rpm

All experiments are carried out far from flooding conditions. Conditions became steady, as evidenced by a constant interface level, after three or four col-umn volume of operation depending on the phase flow rates and rotor speed. At the end of each expe-riment, the average hold-up of the column was mea-sured by using the shutdown procedure (interface position changes).

In all experiments, dilute solutions were inves-tigated with approximately 3.5 wt.% acetone in the organic phase. The acetone content of the aqueous and organic stream was measured by UV-Vis spectroscopy. The physical properties of the liquid–liquid systems used in these experiments are listed in

Table 3 [15]. In the present work, the values of phys-ical properties have been assumed to correspond to the arithmetic-mean concentrations of the continuous and dispersed phases at the inlet and outlet of the column.

Table 3. Physical properties of liquid systems at 20 °C [15]

Physical property

Toluene/acetone/water n-Butyl acetate/ /acetone/water

ρc / kg m–3 994.4-995.7 994.3-995.8

ρd / kg m–3 864.4-865.2 879.6-881.4

μc / mPa s 1.059-1.075 1.075-1.088

μd / mPa s 0.574-0.584 0.723-0.738

σ / mN m–1 27.5-30.1 12.4-13.2

Dc / m2 s–1 1.09-1.14×10-9 1.01-1.06×10-9

Dd / m2 s–1 2.7-2.8×10-9 2.16-2.18×10-9

The drops were photographed by a very high- -resolution Nikon D5000 camera. Next, droplet dimen-sions were compared with the thickness of stators as a reference. It is found that the curved surface of the glass extraction column and significant differences between air and the glass refractive indices leads to a parallax deformation of the objects photographed in the extraction column. In order to omit this pheno-menon, a container filled with water was attached to the extraction column and the photographic approach was used to calculate the size of stator thickness served as the reference for drop size measurements. Consequently, digital image analysis software was

Figure 1. Schematic flow diagram of Oldshue-Rushton column.

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applied in order to investigate the taken high quality photograph. A minimum of 1000 drops was analyzed for each experimental condition in order to guarantee the statistical significance of the determined size distributions.

In the case of non-spherical droplets, the major and minor axes, d1 and d2, were measured and the equivalent diameter, de, was calculated from Eq. (1):

( )1/32e 1 2d d d= (1)

The Sauter mean diameter was then calculated according to the following equation:

3

132

2

1

N

i iiN

i ii

n dd

n d

=

=

=

(2)

where ni is the number of droplets of the mean diameter di within a narrow size range i.

RESULTS AND DISCUSSION

The performance of an extraction column with well-defined stages can be expressed in terms of

stage efficiency. The stage efficiency, Eoy, based on the concentration of organic phase is illustrated as follows:

( )1

oy *1

( )n n

n n

y yEy y

−

−

−=−

(3)

where *ny = mxn is the organic phase concentration in

equilibrium with the aqueous phase of nth stage, the value of m is 0.68 and 0.91 for toluene-water and n-butyl acetate-water, respectively. Figure 2 illus-trates the typical concentration profile for the organic and continues phase for two systems and two dir-ections of mass transfer. The experimental results obtained in these experiments are given in Tables A.1 and A.2 in Appendix and the pictures of drop sizes for two systems is shown in Figure 3.

Effect of agitation speed

Figure 4a shows the effect of changing the agi-tation speed on the stage efficiency for both systems from dispersed to continuous phase mass transfer. It was observed that the stage efficiency in both sys-tems is heavily dependent on the agitation speed. At low speeds, the stage efficiency is low due to inade-quate mixing, resulting in low holdup and large drops

Figure 2. Typical concentration profile along the column (N=140 rpm, Vd= Vc=0.66 mm/s).

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(low interfacial area) that is also observed in Figure 3. The stage efficiency increases with an increase in the agitation speed and reaches a maximum of 58% for toluene-acetone-water system at a speed of 220 rpm and a maximum of 59% for n-butyl acetate-acetone- -water system at a speed of 180 rpm.

Figure 4. Effect of rotor speed on the stage efficiency: a) surface tension and b) direction of mass transfer (Vc= Vd= 0.66 mm/s).

Having reached its maximum, the stage effi-ciency falls to further increasing at agitation speed. A decrease in the stage efficiency could contribute to the a significant decrease in mass transfer rates due to small droplets behaving as rigid spheres, in which case molecular diffusion would govern mass transfer in the system.

The effect of rotor speed on the values of the stage efficiencies in the water-acetone-n-butyl acetate test system (medium interfacial tension) is greater than that of the water-acetone-toluene test system (high interfacial tension). The size of the droplets in higher interfacial tension test systems is larger than the droplet size in the lower interfacial tension test systems (Figure 3), which results in a decrease in their residence time in the column. Finally, the slip velocities increase and, consequently, the value of the dispersed phase holdup and stage efficiency will decrease; consequently, the column will operate in a more-stable manner.

Effect of mass transfer direction

The effect of the mass transfer direction on the stage efficiency is shown in Figure 4b. It is found from this figure that the mass transfer direction has a considerable effect on the stage efficiency. The stage efficiency in the continuous to dispersed phase trans-fer is lower than that in the opposite direction. This is due to the interfacial tension gradients that leads to the smaller drop sizes in continuous to dispersed phase transfer and larger drop sizes in the opposite direction. Therefore, the higher values of the stage efficiency in the case of the dispersed to continuous phase transfer are resulted from the increased mass transfer rates in drops of bigger sizes due to the pre-sence of oscillations created by coalescence between the droplets enhanced by the Marangoni effect [16].

Figure 3. Variation of drop sizes with rotor speed and interfacial tension for toluene-acetone-water: a) 140, b) 160, c) 180 rpm, and for

n-butyl acetate-acetone-water: d) 140, e) 160, f) 180 rpm.

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Effect of dispersed phase velocity

As shown in Figure 5a, the stage efficiencies increase with an increase in dispersed phase velocity for mass transfer direction from the dispersed to the continuous phase. This observation could be attri-buted to an increase in mean drop sizes because of an increase in drop formation and higher coalescence frequency. The increment of the number of dispersed droplets leads to an increase in the dispersed phase holdup. It is observed that the effect of the holdup on the interfacial area is larger than that of mean drop size, i.e., the interfacial area increases with an inc-rease in the dispersed phase velocity; albeit an inc-rease in the dispersed phase velocity leads to the reduction of mass transfer coefficient, a decrease is more predominant when the increase in the interfacial area is considered. Therefore, the stage efficiency decreases along the column.

Figure 5. Effect of dispersed phase velocity on the stage

efficiency: a) surface tension and b) direction of mass transfer (Vc= 0.66 mm/s).

Effect of continuous phase velocity

The effect of the continuous phase velocity on the stage efficiency is shown in Figure 6a. This effect

leads to an increment in the holdup due to the red-uction of the relative velocity between the drops and continuous phase, but it is not appreciable on the drop sizes. Therefore, the interfacial area increases with the positive effect of the holdup. An increase in drag forces arising from the relative velocity between the continuous and dispersed phases leads to the circulation in a drop and consequently, overall mass transfer coefficient increases with an increase in Vc. The stage efficiency increases with both increase in overall mass transfer coefficient and interfacial area. As mentioned earlier, it is observed from Figures 5b and 6b that the stage efficiency in the dispersed to continuous phase transfer is higher than that in the opposite direction.

Figure 6. Effect of continuous phase velocity on the stage

efficiency: a) surface tension and b) direction of mass transfer (Vd= 0.66 mm/s).

Comparison of other type of extractors with present column

A comparison of the separation performance of the Oldshue-Rushton column with some other type of extraction extractors is described in Figure 7. The pat-tern is, as proposed by Pratt and Stevens, the number

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of theoretical stages per unit of length against total volumetric throughput of both phases [5]. This plot is of value in facilitating the comparison of the relative areas of application of various extractor types, despite being based on the data for a single system, viz. toluene-acetone-water in a phase ratio of 1.5. The present Oldshue-Rushton column reached values of between 5.14 and 6.55 NTS/m at low total through-puts. Therefore, it can be concluded that the present column has high stage efficiency while its throughput is low.

Figure 7. Comparison of extractor performance; toluene-

–acetone-water system, Vd/ Vc = 1.5.

Proposed correlation for stage efficiencies

There is no correlation for prediction of stage efficiency in the Oldshue-Rushton column. The expe-rimental data on the stage efficiency are correlated in terms of dimensionless numbers Re and Fr for both mass transfer directions as well as the two systems by using the least square method, as follows:

0.203 0.169oy 1.399E Re Fr− −= (4)

where:

2R

gFrd N

= (5)

c 32 s

c

d VRe ρμ

= (6)

The experimental data are compared with the calculated results from the above equation in Figure 8. The stage efficiency calculated according to this correlation reproduces the experimental data with an average error of 4.64%. Thus, the proposed correl-

ation can predict the stage efficiency of the column accurately.

Figure 8. Comparison between experimental data and the

proposed correlation.

CONCLUSION

Stage efficiency was measured in a 113 mm Oldshue-Rushton column for two systems. It is shown in this work that the performance of the column depends largely on the rotor speed. The stage effi-ciency increased with agitation speed and reached a maximum, but after having reached its maximum, it fell to further increase in agitation speed. The com-parison between the stage efficiencies for the two drops under the the same conditions of the two sys-tems shows that the drop in n-butyl acetate-acetone-water system with a lower value of interfacial tension has a higher value of Eoy. It was observed that the stage efficiency is higher when the mass transfer direction is from the continuous to the dispersed phase. The comparison of Oldshue-Rushton column with some other types of extractors revealed that the stage efficiency is high in this column.

Nomenclature

d32 Sauter mean drop diameter (m) D molecular diffusivity (m2/s) Dc column diameter (m) dR rotor diameter (m) Eoy stage efficiency g acceleration due to gravity (m/s2) N rotor speed (1/s) NTS number of stage efficiency Re Reynolds number Fr Froude number m distribution ratio

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t time (s) V superficial velocity (m/s) Vs slip velocity (m/s) yn mass fraction of acetone in dispersed phase x mass fraction of acetone in continuous phase

Greek letters ρ density (kg/m3) σ interfacial tension (N/m) μ viscosity (Pa s) φ dispersed phase holdup

Subscripts c continuous phase d dispersed phase o overall value

Superscripts * equilibrium value

REFERENCES

[1] J. Rydberg, C. Musikas, G.R. Choppin, Solvent extraction principles and practice, CRC Press, New York, 2004, p. 15

[2] J.C. Godfrey, M.J. Slater, Liquid-Liquid Extraction Equip-ment, Wiley, New York, 1995, p. 40

[3] G.M. Ritcey, A.W. Ashbrook, Solvent extraction: prin-ciples and applications to process metallurgy, Vol. 1, Elsevier, New York, 1984, p.100

[4] M.S.A. Nabli, P. Guiraud, C. Gourdon, Chem. Eng. Res. Des. 76 (1998) 951-960

[5] H.R.C. Pratt, G.W. Stevens, in Science and Practice in Liquid–Liquid Extraction, Oxford University Press, Oxford, 1992, pp. 491-502

[6] W. Batey, J.D. Thornton, Ind. Eng. Chem. Res. 76 (1989) 1096-1101

[7] M. Jaradat, M. Attarakih, H. J. Bart, Ind. Eng. Chem. Res. 50 (2011) 14121-14135

[8] E.G. Scheibel, Chem. Eng. Prog. 44 (1948) 681-690

[9] L. Steiner, E.V. Fisher, S. Hartland, AIChE Symp. Ser. 80 (1984) 130-138

[10] H.B. Li, G.S. Luo, W.Y. Fei, J.D. Wang, Chem. Eng. J. 78 (2000) 225-229

[11] X.J. Tang, G.S. Luo, H.B. Li, J.D. Wang, Pet. Technol. 32 (2003) 1046-1050

[12] M. Horvoth, S. Hartland, Ind. Eng. Chem. Process Des. Dev. 24 (1985) 1220-1225

[13] P.A. Schweitzer, Hanson Mixer-Settler Handbook of Separation Techniques for Chemical Engineering, 3rd ed., McGraw-Hill, New York, 1997, p. 230

[14] J.H. Rushton, S. Nagata, T.B. Rooney, AIChE J. 10 (1964) 298-302

[15] T. Míšek, R. Berger, J. Schroter, EFCE Publ. Ser. 46 (1985)

[16] M. Wegener, J. Grünig, J. Stüber, A.R. Paschedag, M. Kraume, Chem. Eng. Sci. 62 (2007) 2967-2978.

APPENDIX

Table A.1. Experimental data obtained in the experiments for toluene-acetone-water system

Qd / l h–1 Qc / l h

–1 rpm d to c transfer c to d transfer

φ d32 / mm φ d32 / mm

24 24 140 0.0687 2.48 0.072 2.44

24 24 160 0.0755 2.22 0.0792 2.09

24 24 180 0.089 1.9 0.0945 1.805

24 24 200 0.111 1.48 0.116 1.41

24 24 220 0.115 1.35 0.125 1.282

24 24 240 0.128 1.12 0.134 1.02

24 18 160 0.0703 2.23 0.0751 2.1

24 30 160 0.0768 2.19 0.08391 2.06

24 36 160 0.0805 2.17 0.0876 2.01

24 18 200 0.108 1.5 0.111 1.45

24 30 200 0.116 1.47 0.1205 1.37

24 36 200 0.119 1.46 0.1264 1.34

18 24 160 0.0671 2.16 0.0716 2.04

30 24 160 0.0818 2.28 0.0879 2.14

36 24 160 0.0893 2.39 0.0966 2.17

18 24 200 0.1045 1.42 0.1073 1.38

30 24 200 0.1212 1.53 0.1248 1.45

36 24 200 0.1331 1.63 0.1373 1.52

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Table A.2. Experimental data obtained in the experiments for n-butyl acetate-acetone-water system

Qd / l h–1 Qc / l h

–1 rpm φ d32 / mm

24 24 100 0.0748 2.02

24 24 120 0.0893 1.7092

24 24 140 0.0978 1.4191

24 24 160 0.119 1.25

24 24 180 0.129 1.08

24 24 200 0.14 0.95

24 18 120 0.0848 1.7186

24 30 120 0.0943 1.6882

24 36 120 0.098 1.66

24 18 160 0.1145 1.263

24 30 160 0.1231 1.242

24 36 160 0.1262 1.22

18 24 120 0.0828 1.6512

30 24 120 0.0998 1.7412

36 24 120 0.1086 1.8092

18 24 160 0.1125 1.224

30 24 160 0.1256 1.309

36 24 160 0.1337 1.367

MEHDI ASADOLLAHZADEH1,2

SHAHROKH SHAHHOSSEINI1

MEISAM TORAB-MOSTAEDI2

AHAD GHAEMI1 1Department of Chemical

Engineering, Iran University of Science and Technology (IUST),

Tehran, Iran

2Nuclear Fuel Cycle Research School, Nuclear Science and

Technology Research Institute, Tehran, Iran

NAUČNI RAD

EFEKTI RADNIH PARAMETARA NA EFIKASNOST STUPNJA OLDŠUE-RUŠTONOVE KOLONE

U ovom istraživanju, efikasnost stupnja je ispitivana u Oldšue-Ruštonovoj koloni, prečnika 113 mm, za dva sistema: toluen-aceton-voda i n-butil acetat-aceton- voda. Eksperimenti su uključili oba pravca prenosa mase. Ispitivan je uticaj različitih parametara, kao što su: brzina mešanja, brzine strujanja dispergovane i kontinualne faze i pravac prenosa mase, na efikasnost stupnja. Eksperimentalni podaci pokazuju da efikasnost stupnja jako zavisi od brzine mešanja i međufaznog napona, a malo od brzine strujanja faza. Primećeno je da je efikasnost stupnja bolja kada je smer prenosa mase acetona od kontinualne prema dis-pergovanoj fazi u odnosu na suprotan smer zbog prisustva oscilacija stvorenih gradijentom površinskog napona. Ispitivana kolona je jedna od ekstrakcionih kolona sa visokom efikas-nošću stupnja. Predložena je empirijska korelacija koja povezuje efikasnost stupnja sa Rejnoldsovim i Frudovim brojem. Predviđanja jednačine se dobro slažu sa eksperimen-talnim podacima.

Ključne reči: Oldšue-Ruštonova kolona, efikasnost stupnja, aksijalna mešanje, kapacitet.