净化塔的外文资料

Comparison of the Performance of a Pulsed
Disc and Doughnut Column with a Pulsed
Sieve Plate Liquid Extraction Column
Ali B.Jahya,H.R.Clive Pratt,and Geoffrey W.Stevens
Department of Chemical and Biomolecular Engineering,
The University of Melbourne,Australia
Abstract:The hydrodynamic and mass-transfer performance of a 75mm diameter pulsed disc and doughnut column (PDD)and a pulsed sieve-plate column (PSP)are presented and compared for a toluene–acetone–water system under similar operating conditions.It was found that the pulsed disc and doughnut column flooded earlier than the pulsed sieve-plate column,indicating that the total throughput per unit cross-sectional area through the pulsed disc and doughnut column was less.At similar operating conditions (i.e.,flowrates,pulse frequency,and amplitude),the mass-transfer performance of the pulsed disc and doughnut column was higher,and its holdup higher.The mass-transfer performance of the PSP column,when compared at similar holdup to the PDD,was found to be higher,and so it is concluded that it is a more efficient column for this system.
Keywords:Hydrodynamics,pulse disk and donut column,pulse sieve plate column,mass transfer
INTRODUCTION
The selection of a suitable extractor involves consideration of a range of conflicting requirements.Morello and Poffenberger presented a useful survey of extractor types,[1]and this was extended by Pratt in the form of an extractor selection chart giving numerical ratings for each range.[2]This was updated more recently by Pratt and Stevens.[3]There is a large amount Received 26March 2004,Accepted 23October 2004
Address correspondence to Geoffrey W.Stevens,Department of Chemical and Biomolecular Engineering,The University of Melbourne,Australia.
Solvent Extraction and Ion Exchange ,23:307–317,2005
Copyright #Taylor &Francis,Inc.
ISSN 0736-6299print /1532-2262online
DOI:
10.1081/SEI-200045257
307
of literature available on the pulsed sieve-plate column,which has been widely used in many industrial applications.However,recently the pulsed disc and doughnut column has been installed in a number of metallurgical applications around the world,for example at Western Mining Corporation Olympic Dam operation in South Australia,ten columns have been operating for the extraction of uranium.It has also been used in the nuclear industry and in the chemical industry,yet very little information is available on its performance,particularly relative to other columns.This paper attempts to present a comparison between these two columns to enable effective selection of the appropriate column for a given application.The system that was chosen for the comparison is the toluene–acetone–water system,which is a standard test system for liquid extraction as recommended by the European Federation for Chemical Engineering.[4]Physical and transport properties for this system are given in the European Federation of Chemical Engineering Publication.[4]
EXPERIMENTAL SECTION
Description of Apparatus
Pulsed Disc and Doughnut (PDD)Column
The main column section comprised a 1.0m long precision-bore glass tube of 72.5mm internal diameter,enclosing a stack of alternating stationary discs and doughnuts.This was surmounted by a 250mm length of 76mm nominal i.d.QVF T-piece.Below the plate section was a 75–100mm expanded glass section enclosing a stainless-steel solvent distributor that was supported on a piston-type pulsing unit,which imparted a sinusoidal motion to the fluids in the column.The amplitude used in all experiments was defined as the maximum displacement of the interface in the column.Figure 1shows the schematic arrangement of the experimental rig.
In total,30pairs of stainless-steel (SS)discs and doughnuts were arranged alternately and spaced 12.5mm apart in the column,resulting in a 25mm compartment height.They were held in place by means of three 3.2mm o.d.SS tie rods with spacer sleeves located on a 42.6mm triangular pitch.The discs were of 63.5mm diameter and the doughnut apertures were 35mm,giving open free areas for each of 23.3%.
The continuous phase was introduced through a 10mm i.d.SS tube located approximately 50mm above the top plate.The dispersed phase distributor consisted of a 40mm length of 75mm o.d.SS tube,closed by a nozzle plate at the top containing 44Â2.64mm i.d.SS nozzles.
The liquid–liquid interface was maintained approximately 60mm above the top compartment.This was achieved by regulating the outflow of the A.B.Jahya et al.308
continuous phase with a needle valve.In all experiments,the aqueous continu-ous phase leaving the column was drained into the appropriate storage vessel.
Pulsed Sieve Plate(PSP)Column
On completion of the experiments with the pulsed disc and doughnut(PDD) column,the existing disc and doughnut internal compartments were replaced by sieve plates for comparison purposes.These consisted of17SS sieve plates of3.2mm perforation diameter and21.3%free area,spaced 50mm apart by means of three3.2mm o.d.tie rods with6.4mm SS spacer sleeves,located on a56.5mm triangular pitch.The effective height of the column was830mm.All other equipment used for the pulsed sieve plate column was exactly the same as those in the PDD column.
Liquid–Liquid System
A toluene–acetone–water system with acetone transferring from the water, which is the continuous phase,to the toluene,which is the dispersed phase, was used in this study.This system is recommended as a standard test
system by the European Federation for Chemical Engineering.[4]Technical
grade toluene and acetone,and tap water were used in all experiments.The concentration of acetone was determined by the hydroxylamine titration technique.[4]This system was chosen because of the large amount of data available in the literature on the pulsed sieve-plate column and other columns,thus enabling a more complete comparison.
Procedure
Dispersed-phase Holdup
Before carrying out the experiments,both phases were mutually saturated,and the pulse amplitude and frequency were adjusted to the desired values.The continuous-phase and the dispersed-phase flow rates were then set to the required flow rate,and the system was stabilized to allow steady state to be reached.Then,the inlet and outlet flows were stopped simultaneously.The dispersion was then allowed to coalesce at the interface.The holdup was then measured either by determining the change of interfacial height or displacing the solvent layer into a measuring cylinder.
Axial Dispersion Coefficient and the Height-of-transfer
Unit of the Continuous Phase
When starting a run,the solvent and water phases were first mutually saturated,after which solute was added to the continuous phase to give a concentration of about 3wt%acetone.Samples of each phase were taken at their inlets to the column and used for the determination of the initial solute concentration using the hydroxylamine titration technique.[4]
The amplitude and frequency of pulsation were next adjusted to the desired values and,after filling the column with the continuous phase,the dispersed phase was introduced.The interface position was then maintained at the desired height,and the system was allowed to reach steady state,which usually necessitated 3–4changes of the column volume.
The continuous phase axial dispersion coefficient quantifies the extent of mixing of the continuous phase in the column and is used in the calculation of the height of the column.The unsteady tracer pulse-injection technique [5]was then used to determine the axial dispersion coefficient.For this purpose,tartrazine yellow tracer was injected as a pulse at the continuous-phase inlet.
The transient responses of the tracer were measured online by sampling the continuous phase at rates of 1.0–1.5%of the total continuous flow rate with two peristaltic pumps.The samples were taken from two plates approxi-mately 300mm apart in the centre of the column and passed through two flow cells located in two UV /Vis spectrophotometers,which were set to measure adsorption of a wavelength of 425nm.The outputs of the spectrophotometers were recorded at intervals of 15–30seconds until the transient responses A.B.Jahya et al.310
Pulsed Disc/Doughnut vs.Pulsed Sieve Plate Liquid Extraction Column311 approached the original value.The responses,which are directly proportional to the tracer concentrations,were then plotted against time,giving the residence time distribution(RTD)of the tracer in the column;this was then analyzed using Mathematica software for the determination of the axial dispersion coefficient of the continuous phase,E c,using the technique described by Dongaonkar.[5]
At the end of each experiment,samples of the organic and aqueous phases were taken at their respective outlets.The solute concentrations were then determined by titration.Together with the axial dispersion coefficients, these results were used for calculating the height of the transfer unit of the continuous phase as described by Pratt and Stevens.[3]Axial dispersion in the dispersed phase was assumed to be negligible in this case and so was ignored in the analysis.
RESULTS AND DISCUSSION
On comparing these columns,every attempt was made to keep all of the operating and geometric variables the same for both columns.The one variable where this is not possible is the compartment height in the PDD column.This is determined by the column diameter and the free area.Thus, for a75mm diameter column with23%free area,this corresponds to a com-partment height of2.5cm.This is obtained from the area available forflow in the horizontal direction along the plate being equal to the area of the hole in the doughnut plate and equal to the area of the space around the disc.As these columns are scaled up,the fraction of free area is kept constant,and so the compartment height is varied with the diameter of the column.
This imposes a limit on the smallest-scale column that can be used for scale up,because,as the column is made smaller,the gap between the disc and the wall decreases and ultimately can hinder droplet movement.For a75mm diameter column with a23%free area,the gap is4.65mm,and with an average drop size of3mm this is the minimum size column that could be used.
For the PSP column however,the height of the compartment is constant and independent of diameter,and so there is no such limit on the minimum size of column that can be used.In practice,the minimum size is determined from wall effects on two phaseflow in the compartments,and many experi-menters have used columns down to50mm diameter,although there is still some discussion on the reliability of simple scale rules from columns of this size.The most common compartment height reported in the literature is 5cm,which is what has been used for these tests.[6]Decreasing the compart-ment height without changing the pulse amptitude would increase axial dispersion and significantly degrade the performance of the column.[7] It also gives an extra degree of freedom in PSP that PSD does not have to optimize its performance.
As can be seen from Table 1,the degree of continuous phase axial dis-persion is much higher in the PDD column probably because of the small plate spacing relative to amplitude of pulsation.This causes mixing zones that are longer than the plate spacing and lead to increased axial mixing.
It was found that flooding in PDD columns occurs at much lower through-puts than those obtained by Thornton,[8]for the PSP column with the same column diameter (75mm in the present case).This effect is thought to be due to a relatively high pressure drop as the fluid moves in alternate vertical and horizontal direction within the PDD column,whereas,in the PSP column,the fluids move only in the vertical direction.As shown in Figs.2and 3,the dispersed-phase holdup of PDD column was found to be higher Table 1.Axial dispersion coefficients for toluene–acetone–water system in PDD and PSP columns
A (cm)
f (Hz)V c (m /s)V d (m /s)E c (PDD)(m 2/s)E c (PSP)a (m 2/s)0.64 3.0 3.75Â1023 6.35Â1024
1.23Â1023 1.23Â10247.21Â1024
1.26Â1023 1.24Â10248.50Â1024
1.42Â1023 1.25Â10249.23Â1024
1.41Â1023 1.26Â1024
2.0
3.75Â1023 6.35Â1024
1.18Â1023 1.19Â10249.23Â1024
1.29Â1023 1.21Â10241.05Â1023
1.54Â1023 1.21Â10241.39Â1023
1.61Â1023 1.23Â10243.0 1.83Â1023 6.35Â1024
8.80Â10248.53Â10259.23Â1024
9.60Â10248.86Â10251.07Â1023
1.02Â10238.99Â10251.22Â1023
1.04Â10239.12Â1025
2.0 1.83Â10239.23Â1024
8.70Â10248.14Â10251.28Â1023
9.20Â10248.40Â10251.47Â1023
1.14Â10238.51Â10251.66Â1023
1.24Â10239.61Â10251.0
2.0
3.75Â1023 6.35Â1024
1.40Â1023 1.32Â10247.21Â1024
1.58Â1023 1.34Â10249.23Â1024
1.56Â1023 1.35Â10241.12Â1023
1.64Â1023 1.37Â1024
2.0 1.83Â1023 6.35Â1024
9.72Â10249.66Â10259.23Â1024
1.09Â1023 1.01Â10251.13Â1023
1.10Â1023 1.03Â10251.33Â1023
1.34Â1023 1.05Â10253.0 1.83Â1023
6.35Â1024
1.12Â1023 1.07Â10247.43Â1024
1.02Â1023 1.09Â10249.23Â1024 1.27Â1023 1.12Â1024a Predicted from correlation presented by Prvcic.[7]
A.B.Jahya et al.
312
than those obtained for PSP column for similar conditions,which corresponds with the observation of earlier flooding in the PDD column.
Figures 4and 5show that values of the height of an overall transfer unit based on the continuous phase,H oc,for the PDD column are lower than
those Figure 2.Holdup comparison between PDD and PSP column for the toluene–acetone–water
system.
Figure 3.Holdup comparison between PDD and PSP columns for the toluene–acetone–water system.
Pulsed Disc /Doughnut vs.Pulsed Sieve Plate Liquid Extraction Column 313
for the PSP in most cases.This may be expected,due to the large dispersed-phase holdup of the PDD column,which in turn provides a greater mass-transfer area than that of the PSP column under similar operating conditions and for the same system.
A further comparison of the PDD and PSP columns at the same dispersed phase holdup is presented in Figs.6and 7.These show that the values of H oc for the PDD column are higher than those for the PSP column.This result differs from the comparison presented above (Figs.4and 5),where the dispersed-phase holdup was significantly different in both columns,and so the area for mass transfer was also different.Thus,for the same
holdup,Figure parison of mass-transfer performances of the PDD and PSP
columns.Figure parison of mass-transfer performances of the PDD and PSP columns.A.B.Jahya et al.314
the performance of the PSP column is shown to be better than that obtained for the PDD column,probably due to the higher shear force on the dispersed-phase droplets when passing through the perforations and the lower continu-ous phase axial dispersion.This indicates that if there is a choice between the two columns for a given operation,the pulsed sieve-plate column would be smaller in diameter and height for a given duty.This conclusion is based
on Figure parison of mass-transfer performances of the PDD and PSP
columns.Figure parison of mass-transfer performances of the PDD and PSP columns.
Pulsed Disc /Doughnut vs.Pulsed Sieve Plate Liquid Extraction Column 315
the comparison of columns at one diameter only because of the different methods of scaling up these columns.It is recommended that a further study be undertaken on different column diameters and a range of systems before a general conclusion can be made about the performance of these columns.CONCLUSIONS
The dispersed-phase holdup of a PDD column is higher that obtained from PSP column under the same pulsation and flow conditions.Thus,the PDD column floods earlier than the PSP column.
The height-of-transfer unit obtained in the PDD column is lower than that obtained from PSP column under the same operating condition.However,if the height-of-transfer units are compared when the columns are operated at the same dispersed phase holdup,the PSP has the lower height-of-transfer unit and thus higher efficiency.
For the same duty,the PSP would have a smaller diameter and height based on these data.
NOMENCLATURE
A
pulse amplitude (cm)f
pulse frequency (Hz)h c
compartment height (m)H oc
height of an overall transfer unit based on the continuous phase (m)V j
superficial velocity of phase j x d dispersed-phase holdup
Subscripts
c
continuous phase d dispersed phase
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2.Pratt,H.R.C.Liquid liquid extraction in theory and practice part 2a.Ind.Chem.1954,30,475,597.
3.Pratt,H.R.C.;Stevens,G.W.Generalised design equations for backmixed liquid extraction columns with non-linear equilibria.Instn.Engrs.Chem.Res.1991,30,733. A.B.Jahya et al.316
Pulsed Disc/Doughnut vs.Pulsed Sieve Plate Liquid Extraction Column317 4.Misek,T.;Berger,R.;Schnoter,J.European Federation of Chem.Eng.standard test
systems of liquid extraction.EFCE1985,46.
5.Dongaonkar,R.;Pratt,H.R.C.;Stevens,G.W.Generalised solution to the transient
backflow model for stagewise liquid extraction columns.Instn.Engrs.Chem.Res.
1993,32,1167.
6.Thornton,J.D.Science and Practice of Liquid–Liquid Extraction;Oxford,1992;
Vol.1.
7.Prvcic,L.M.;Pratt,H.R.C.;Stevens,G.W.Axial dispersion in pulsed perforated
plate extraction column.A.I.Ch.E.J.1989,35,1845.
8.Thornton,J.D.Liquid liquid extraction part XIII:the effect of pulse waveform and
plate geometry on the performance and throughput of a pulsed column.Trans.Instn.
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