COMPARISON OF DISK & DOUGHNUT AND SIEVE PLATE INTERNALS IN PULSED COLUMNS AT EXTREME PHASE RATIOS. Baruch Grinbaum 1, Geoff Stevens 2

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1 COMPARISON OF DISK & DOUGHNUT AND SIEVE PLATE INTERNALS IN PULSED COLUMNS AT EXTREME PHASE RATIOS Baruch Grinbaum 1, Geoff Stevens 2 1 Bateman Solvent Extraction, 5 Hamada St., Yokneam 20692, Israel; 2 Chemical and Biomolecular Engineering, University of Melbourne, Victoria, Australia 3010 Corresponding author address: baruchgr@bateman.co.il ABSTRACT The purpose of this work was to compare the dispersion created in pulsed columns with Sieve Plate (SP) and Disc and Doughnut (DD) internals and their combination, at high phase ratios. The tested system was DEHPA in Kerosene and an aqueous solution of ZnSO 4. At phase ratio 1:1 both types of internals created regular drops even at low energy input. At extreme phase ratios in the SP regular drops were obtained while in DD lumps were created. An attempt to use sieve plates as a distributor in a DD column, in order to improve the dispersion, did not help. The sieve plates, with the right wetting properties, were installed near the entrance of the dispersed phase, before the DD internals. They created the required small drops, but after a few compartments of DD the effect faded and the characteristic lumps were obtained. INTRODUCTION: Both Disk & Doughnut (DD) and Sieve Plate (SP) internals are well known and utilized in pulsed extraction columns. Previous studies [1, 2] showed that there is little difference in the performance of both types of internals, as far as the HTU and flux are concerned. These comparisons was done on standard systems: physical extraction of acetone from water to toluene [1], and chemical extraction of H 2 SO 4 by Alamine 336 [1] and Zn ++ by DEHPA [2]. All three are relatively "easy" processes, with quick kinetics and moderate phase ratio (O:A between 1:4 and 4:1). It is difficult to deduce from these investigations what happens in the more difficult cases, when the dispersed: continuous (d:c) phase ratio exceeds 10:1 or the dispersed phase clearly does not create well-defined drops. Unfortunately, many processes in hydrometallurgy follow this description. It happens either when extracting small concentration of the metal from feed, e.g. extraction of uranium, or in stripping and scrubbing batteries, where small amount of aqueous solution has to scrub/stripping huge volume of solvent. For SP internals, data about scrubbing of Uranium, in TBP based solvent, at O:W phase ratio of 17:1 and stripping at O:W = 25:1 were published by Ritcey [3]. The highest reported phase ratio for DD was O:A=1:7, in extraction of uranium [4], with the major phase dispersed. In DD columns a few processes at phase ratio of 20 were carried out, e.g. stripping of Ni in O:A of 20 and extraction of Uranium at A:O

2 of 20, but they have not yet been published (proprietary data). In all of them the major phase was the dispersed one, in order to provide reasonable mass transfer. The purpose of this work was to compare qualitatively the drop size, while operating a pulsed column with both SP and DD internals and their combination at high phase ratio. Drop Size The importance of drop size is common knowledge and is analyzed in many reference books [5, 6, 7]. The drop size, combined with the holdup, determines the mass transfer area. As the rate of mass transfer is proportional to the mass transfer area, small drop size usually improves the mass transfer. For different column types (SP vs. DD) there may be a huge difference in the drop size, that may lead to significant difference in the mass transfer characteristics. The Process EXPERIMENTAL The experiments were done in the standard system of extraction and stripping of Zn ++, using DEHPA in Kerosene. The column was operated both in the extraction and the stripping steps, at ambient temperature. The solvent contained 10% DEHPA in Kerosene, the aqueous feed 60 g/l Zn ++ and the stripping solution 175 g/l H 2 SO 4. At these compositions mass transfer above 50% was expected both in extraction and stripping. As the main goal of the experiments was to compare visually the drop size and shape, no mass transfer tests were carried out. In order to view clearly the dispersion, the aqueous phase was colored by traces of Tartrazine (acid yellow 23), and the organic by Waxolin blue. For each type of internals both aqueous and organic continuous dispersion were tested. In the extraction the dispersion was organic continuous and the aqueous phase was the major one. In the stripping the dispersion was aqueous continuous and the organic was the major phase. All the tests (regardless of who the continuous phase was) were carried out at phase ratio of d:c=10:1 and above. Pilot Equipment The pulsed column had 72.5 mm internal diameter, and its active section was 1 m high. The column had two small decanters, above and below the active section. Both inlets flowed through a 0.5" pipe, without any distributor. Four sets of internals were used: 1. Stainless steel (SS) sieve plates (SP), 1.5 mm thick, with 50 mm spacing between them. The internal diameter of the holes was 2.64 mm and the free flow area was 23%. 2. Plastic (PVC) sieve plates, with thickness, spacing, open area and diameter of holes similar to set Stainless steel (SS) disk & doughnut (DD) plates, 1 mm thick, 12 mm spacing between each disk and doughnut (26 mm compartment height i.e. disk to disk). The free flow area was 23%. 4. Plastic (Teflon) DD plates, with spacing and dimensions similar to set 3. The internals were thicker (3 mm each) and the free flow area was 26%.

3 RESULTS AND DISCUSSION Uniform Internals For each type of internals both aqueous and organic continuous dispersion were tested, at phase ratio of d:c=10:1 and above. For organic continuous the aqueous was the major phase and vice versa for the aqueous continuous. For some configurations 1:1 phase ratio was also tested. The amplitude was always 10 mm in the column, while the frequency was varied. The pulsation is expressed as pulse intensity (PI = amplitude x frequency), [cm/s]. Tables 1 and 2 summarize the results. Table 1: Hydraulic Test for Stripping with Uniform Internals - Aqueous Continuous Type of internals R Flux PI Holdup Flow Regime (O:A) (m 3 /m 2 /h) (cm/s) (%) SP ss MS Drops Appearance SP ss Dispersion Fine Drops SP PVC Dispersion Drops SP PVC Dispersion Clusters of drops SP PVC MS Hazy bulks DD ss Dispersion Drops DD ss Dispersion Lumps DD ss MS Lumps /Big Drops DD ss MS Drops DD Teflon MS Lumps DD Teflon MS Lumps DD Teflon MS Lumps DD Teflon MS Lumps DD Teflon Dispersion Drops Remark: MS means mixer-settler flow regime, i.e. separated layer may be observed in the compartment. Visual observations: a. At phase ratio 1:1 drops were always present using SP, but with DD drops occurred only with metal internals. b. At O:W=10 and low energy the organic dispersed to drops in the SP, but remained in lumps in the DD. With high energy drops were obtained along the entire column with both types of internals

4 c. The drops (if present) appear to be at their final size already after the first plate in both types of columns. As the column was below one meter, it is difficult to predict what would happen in a longer column. d. The drops are smaller approaching the sieve plate and larger after passing it. The size of the drops was always larger than the diameter of the holes in the plate. The results show that SP internals from the appropriate material increased the chance of obtaining small drops at high phase ratio, without the risk of flooding or phase inversion. With aqueous continuity, metal SP resulted in fine drops, while for organic continuous, the plastic SP produced a haze, that seemed to have good mass transfer characteristics. The disk & doughnut internals did not produce reasonable drops at high phase ratio in MS or even dispersion regime. As disk & doughnut internals have their obvious merits, an attempt was made to test a combined column. Table 2: Hydraulic Test for Extraction with Uniform Internals - Organic Continuous Type of internals R Flux PI Holdup Flow Regime (A:O) m 3 /m 2 /h (cm/min) (%) Appearance SP ss MS Lumps /Big Drops SP ss Emulsion Drops SP PVC Dispersion Drops SP PVC Dispersion Fine drops DD ss Dispersion Lumps DD Teflon Dispersion Drops DD Teflon Dispersion Lumps Combined Column All possible options, i.e. both metal and plastic SP at bottom of column with plastic DD internals and plastic SP at top of column with metal DD internals, were tested. They were installed (sequentially of course) in the same column, to make the comparison as exact as possible. SP at bottom of a Teflon DD column The results with plastic (PVC) SP are summarized in Table 3, and with metal (SS) SP in Table 4.

5 Table 3: Two Plastic SP at Top of Plastic DD Internals- Organic Continuous Dispersion Flux ( m3 Phase PI Flow Holdup Flood Remarks /m 2 /h) ratio (cm/s) Regime (%) (d:c) Dispersion N Drops in SP, big drops+lumps in DD Dispersion N Drops in SP & DD MS 20 N Drops in SP, lumps in DD Dispersion 30 N Drops in SP, lumps in DD Dispersion 35 N Drops in all parts Dispersion Y Flooding in SP Flux Table 4: Metal (SS) SP at Top of Plastic DD - Organic Continuous Dispersion (m 3 /m 2 /h) Phase ratio (d:c) PI (cm/s) Flow Regime Holdup (%) Flood Remarks Dispersion N Drops in SP, big drops in DD Dispersion N Drops in SP & DD MS 20 N Drops in SP, lumps in DD Dispersion 30 N Drops in SP, lumps in DD Dispersion 35 N Drops in all parts Dispersion Y Flood in SP Plastic (PVC) SP at top of metal (SS) DD Internals The experiment was carried out at aqueous continuous dispersion with two SS sieve plates installed 50 mm below the last disk. The phenomenon was very similar to the previous test: at O:A=10 at low energy the organic dispersed as well defined small drops in the SP, but within one compartment height they had coalesced and got the bulky shape that they had without the SP. With high pulsation intensities, drops were obtained along the entire column. At phase ratio 1:1 drops were obtained along the entire column even at low energy. Flooding When flooding happened, it always developed in the SP section, both in aqueous continuous and organic continuous dispersions. The flooding was usually from the

6 cork type, that is very rare in the DD but quite common in SP. Apparently at the tested conditions the DD internals may work at higher flux than the SP. Discussion As one may see from the results, at high phase ratio (either organic or aqueous continuous) the dispersed phase tends to agglomerate and creates lumps, especially when internals with the wrong wetting properties are used. It could have been expected, as at d:c high phase ratio, the holdup of the dispersed phase increases (as noticed by Gayler & Pratt [8] 50 years ago) and it occupies larger part of the column, causing, in some cases, coalescence of the drops to big lumps. Sieve plates create better dispersion, i.e. more regular and smaller drops than disk & doughnuts at high phase ratio, in both dispersions. While at phase ratio about 1:1 the dispersion is similar, at phase ratio of 10 and above it the difference is huge. This result suggests that at high phase ratio there may be an advantage to SP over DD internals. Still the drops that go though the holes in the plates are larger than the holes themselves. An attempt was made to create a combined column, i.e. to use sieve plates with the right wetting properties, as a distributor for a DD internals. The results were poor: during the passage through the SP the phase was dispersed to regular small drops, as any good distributor would do, but within few compartments of DD their influence had faded. After flow through less than 50 cm in the column the distribution was exactly as it was without any SP, and the characteristic lumps were obtained. CONCLUSIONS While at standard phase ratios there is no obvious difference neither between the dispersion created by both types of internals nor the available flux and HTU, at high d:c ratio the DD pulsed column tends to create huge lumps instead of drops, potentially harming the mass transfer (although it was not proven experimentally). The phenomenon is more pronounced when using internals that are not wetted by the continuous phase. The SP internals create reasonable drops even at high phase ratio. Addition of a sieve plate with the right wetting properties, on top of the DD internals, to serve as a distributor, does not help. The SP creates the required small drops, but within few compartments of DD their influence fades and the characteristic lumps are regenerated. Unfortunately, it means that the idea of a combined SP + DD column does not help to get better dispersion in the column. Accordingly there was no point to make any mass transfer tests to further investigate this point. REFERENCES 1. Jahya A., Pratt, HRC, and Stevens, G.W., Comparison of the Performance of a Pulsed Disc and Doughnut Column with a Pulsed Sieve Plate Liquid Extraction Column, Solv. Ext. & Ion Exch., 23, 3, May 2005, pp Colman E., Ilan I., Grinbaum B., and Stevens G., Comparison of Disk & Doughnut vs. Sieve Tray internals, ISEC 2005 proceedings, Beijing 3. G. M. Ritcey, Solvent Extraction, vol. 2, p , Gordon M. Ritcey, Canada, 2006.

7 4. Movsowitz, R.L., Kleinberger, R., Buchalter, E.M., and Grinbaum, B., Comparison of the Performance of Full Scale Pulsed Columns vs. Mixer-Settlers for Uranium SX Solvent Extraction for the 21st Century, Cox, M., Hidalgo, and M. Valiente, Eds., 2001, SCI, London, UK. (2, pp ). 5. Lo, T. C., Baird, M. H. I., and Hanson, C., Eds., Handbook of Solvent Extraction; Wiley: New York, Bart H.J., Reactive Extraction. Berlin: Springer, Bart H.J. and Stevens G.W., Reactive solvent extraction in Ion Exchange and Solvent Extraction, 17: 37-83, Marcus Y and Sengupta A.K. (Eds), Marcel Dekker (2004). 8. Gayler R., and Pratt, H.R.C., Holdup and pressure Drop in Packed Columns, Trans. Instn. Chem. Engr. 35, 273 (1957)

Hydrodynamic and Mass Transfer Performance of a Hybrid Pulsed Column for Solvent Extraction

Hydrodynamic and Mass Transfer Performance of a Hybrid Pulsed Column for Solvent Extraction Proceedings of the 2 nd World Congress on Momentum, Heat and Mass Transfer (MHMT 17) Barcelona, Spain April 6 8, 2017 Paper No. ICMTOD 101 ISSN: 2371-5316 DOI: 10.11159/icmtod17.101 Hydrodynamic and Mass