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n OUTLINE

n EXECUTIVE SUMMARY

Centrifugal Extractors in the Back End of Fuel Cycleand other Applications

Centrifugal Extractors (CE) are efficient liquid – liquid contacting devices wherein valuable solutes can be extractedselectively by solvents based on their distribution coefficients. There are varieties of solvent extraction equipmentoperating in the chemical, petrochemical, pharmaceutical and metallurgical industries based on un-agitated columnslike spray, packed and perforated plate columns, mechanically agitated columns like, pulsed columns, reciprocatingcolumns, rotary devices like, rotating disk column (RDC) and mixer settlers. Centrifugal extractors score over others dueto intense energy input and phase separation by centrifugal force of 100-500g leading to very compact size. For fastkinetics systems, the residence time is few seconds (5-10) as opposed to 3-5 minutes in liquid pulsed column and 5-10minutes in mixer-settler.

Centrifugal extractor is a liquid-liquid mass transfer device wherein solutetransfer can occur between two immiscible liquid phases. Centrifugal extractorsare more attractive in fuel reprocessing step of Fast Breeder Reactor (FBR) fuels

with the burn-up values of 10 times more compared to CANDU fuel. Due tohigher activity of the aqueous solutions the solvent Tri-Butyl Phosphate (TBP)undergo degradation leading to loss of U and Pu. Since the stage residence timein centrifugal extractors are typically 5-10 seconds as against 3-5 minutes incolumn type of extractors and 5-10 minutes in mixer settler, these units arefavored. Fig.1 shows the typical working of the annular centrifugal extractor. The annular centrifugal extractor devised by Agonne National Laboratory (ANL)simplifies the design for remote maintenance compared to Oak Ridge NationalLaboratory (ORNL) design. Both heavy and light phases are fed into the annulargap between the inner rotating bowl and outer stationary bowl leading to one of the phase being dispersed in the other continuous phase. Thus annular gapserves as a mixer and the mixed phase (dispersion) flow down the annular gapand enters below the rotary bowl and the stationary baffles at the bottom of thestationary bowl and enters the rotatingbowl.

Within the rotating bowl the dispersion experiences the centrifugal force many times that of gravity and the heavy phase separates near to the inner wall of thebowl and the light phase separates towards the centre of the bowl. The lightphase over flows over the light phase weir ring and thrown out in to the lightphase collection chamber exiting the unit through a pipe. The heavy phase flowsthrough the under flow passage at the I.D. of the bowl and subsequently flowsover heavy phase weir andcollects in the heavy phase collection chamber exitingthrough a pipe. At IGCAR, two types of units (fixed inter-phase and variableinter-phase units) were employed in CORAL facility for the processing of mixedcarbide fuel irradiated to 100 GWd/t and the experience of these units wereobtained. Fig.2 is the schematic of the fixed weir type wherein the interfaceposition is adjusted to be in the centre of the separating zone for the given liquiddensities. Interface position will be dictated by the pressure balance betweenheavy and light phase thickness under the respective weirs. Fig.3 shows twoheavy phase weirs and one light phase weir for adjusting the interface position.In general the light phase weir dimension will be fixed based on the rotor sizeselected for a desired throughput. Fixed weir configuration will be adopted if the

densities of each phase are known whereas variable interface design can workfor any densities of liquid pair without physically changing the heavy phase weirby applying thecalculated airpressure on theprimaryheavy weir.

 The interface position r can be calculated from pressure balance of the light

phase and heavy phase under light phase weir and the heavy phase under theheavy phase weir taking account of the head between the two weir plates and

-15

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Fig.1 : Separation of Phases in Centrifugal

Extractor

where, r is the interface position, r , and r , are the actual liquid radii at heavy 

phase weir and the light phase weir and and are the densities of heavy and

light phases. The actual position of the liquid under the weirs depends on theflow rate of therespectivephases andangularvelocity of the rotating bowl.

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TECHNOLOGY-5

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Fig. 3 Weir arrangement for Interface Adjustment

Reprocessing Group, IGCAR initiated the development of Centrifugal Extractors and the first unit with 50 mm bowl wasfabricated in 1977 and an annular 8 stage CE bank successfully processed the irradiated thorium fuel for the recovery of U-233 in twocampaignswith a gap of 12 years. Prior to 1992, IGCAR hasdemonstrated theworkingof up to 100 mm CE bowls. The first CE used in large reprocessing application was at Savannah River plant. ANL has introduced annular mixingconcept and simplified the design and made amenable for remote maintenance. CINC a US company, based on the annular

ANL technology, scaled up the units and commercialized for chemical, pharmaceutical and hydrometallurgy industry andeven applied to cleaning of oil spills in ocean. Apart from CINC, a French company, Robatel is also the major commercialsupplier. Recently Heavy Water Board (HWB) is interested in building large size centrifugal extractors for recovery of uraniumfrom phosphoric acidand large throughput unit work was initiated in collaboration withIGCAR and BARC.

  The dimensionless dispersion number N proposed by ANL workers can be used for the design of the centrifugalextractors andN is defined as givenbelow;

where, Q is the volumetric flow rate of the combined phas

(r - r ) and a is the

average acceleration in the separation zone defined by theabove equation. For a given solvent extraction system Nvalues can be experimentally obtained by conductingexperiments in the small centrifugal extractor or even frombatch gravity settling data and the same N values can beused for scale up.

is the mixing intensity W/kg, can be calculated as givenbelow. ANL workers arbitrarily classified 435 W/kg as high,114 W/kg as medium and 27 W/kg as low and they havegiven a correlation for power consumption in the annulargap, wherein 'm' is the mass of the fluid in the annular gap,'H ' is the liquid height in the annulus, 'R ' is the outsideradius of the rotating bowl, ' ' is annular gap width, 'N' isthe rpm, ' ' is the viscosity of the continuous phase, ' ' is thefluid dispersion density and' ' is theangularvelocity.

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Further inquiries:Dr. S.B. Koganti,Reprocessing Research and Development Division

Reprocessing Group, IGCAR, e-mail:koganti@igcar.gov.in

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1. B.D. Kadam, J. B. Joshi, S.B. Koganti and R. N. Patil, , (In Press)2. Sandesh S.Deshmukh,Vedantam Sripriya, Jyeshtharaj B.Joshi and SudhirB.Koganti, ., (2007)8343.3. Sreepriya Vedantam, Jyeshtharaj B. Joshi and Sudhir B. Koganti, (2006) 279.4. Sreepriya V edantam, J yeshtharaj B. J oshia nd Sudhir B. K oganti, , (2006) 6360.

ChemicalEngineeringResearch&Design Ind. Eng.Chem.Res 

TheCanadian J.Chem.Engg.,Ind. Eng.Chem.Res.

4684

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n ADDITIONAL INFORMATION

interface controlled banks of 16 stage each (1C and 2A).Interface controlled banks have a novel passive rotary waterseal incorporated in each stage. Though there is nomaintenance requirement of the seal, often the water has tobe filled in each rotating water seal. Based on the cumulativeoperating experience over the last three years it is decidedthat fixed weir type of centrifugal extractors are simple tooperate and are preferred for Demonstration Fuel

Reprocessing Plant (DFRP).

n DESIGN OF CENTRIFUGAL EXTRACTOR

n PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK

CORAL is a unique facility with all SX cycles operated with centrifugal extractors for processing irradiated mixed carbidefuel with burn up of 100 GWd/t. Valuable experience is obtained in terms of the extraction efficiency and remotemaintainability. Large sized centrifugal extractors (250 mm, 500 mm) are being developed for other than the reprocessingapplications. IGCAR collaborates with academic institutes in realizing the goals.

n ACHIEVEMENT

TECHNOLOGY-5

  The amount of the liquid build-up H in the annular gapdepends on the annular gap, the gap clearance between thebottom of the rotary bowl and the radial baffles, baffle heightand the diameter of opening at the bottom of the rotary bowl.HC can be either experimentally evaluated or through CFDanalysis.

Compact Reprocessing of Advanced Fuel in Lead Cells(CORAL) facility for irradiated U, Pu carbide fuel has three CEbanks of16 stage each fixed weirtype (H , H , 1A) and two

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