i.fié;-j > o abrteen 1 e ot ‘ ™~ Contract No. W-T405-eng-26 CHEMICAL TECHNOLOGY DIVISION ‘ ACTOR TO PROCESSING OF MOLTEN-SALT BREEDER RE L. E. McNeese ORNL-TM-1T30 CESTI PEICES HC. scflfl; MN__-é:f_ I Y BELEASED FOR ANNOUNCEMEKT - "IN NUCLEAR SCIENCE ABSTRspRg MARCH 1967 B racy, Completeness, o usefulness of the information ed, v Ith reapect 4, the accy- Tt, or that the use t_flscloa_ed in this reporg may not infringe If of the Commisasop Includes iuh_v em- e " { OT employee of Such eoniractor, ¢o the S ;;wb amplqygg OF contractor of the Commission, o employee of guch co:m'lcto premat ' MiBBeminatag, o Provides accegs to, Ormation " Frepares, OAK RIDGE NATTONAL LABORATORY .~ 0ak Ridge, Tennessee _operated by | UNION CARBIDE CORPORATION - for the | aay tion Pursuant to g o i88lon, or hig *mployment with gyep contractor mplomt o contracy ' ‘U S. ATOMIC ENERGY COMMISSION $ iif CONTENTS AbstraCtl e & -8 @ » -8 -8 - e« - .8 * * - e -0 * -9 - . . e " e . lo IntrOdUCtion ¢« -5 ® s & 8 -% @» LR L I L I A I AL I 2, Distillation at Low Pressure . . v «.« o « o« o o o s o « 2.1 Equilibrium Distillation . . . . v ¢ ¢ .o v o o & . 2,2 Molecular Distillation .« .. o o « o o o = « « o o » 2.3 Mean Free Path . . . ; s s s e s s s e e s v e e 2,4t Langmuir Vaporization Rate . . « v v o «. v o + « 4 , 2.5. Probable-opérating Mbde for MSBR‘Processing.; o o '3, Relative Volatility . . . ¢ o v ¢ ¢ .0 0 o ¢ e o o o s s 3.1 ‘Relative Volatility for Equilibrium Distillation . ‘3,2 Relative Volatility for Molecular Distillation . . 3.3 .Comparison of Experimental and Calculated Relative Volatilities for Rare Earth Fluorides . . .. . . 4, -Separation Potential of‘Various.Distillation'MEthods . o 4.1 Continuous Distillation . . « & ¢ v ¢ ¢ 0o v v o .0 s 4.2 Semicontinuous Distillation with Continuous Feed 4.3 Semicontinuous Distillation with Rectification . . 4L 4 Batch DLSEL11ation o . v s o v v b oie e e e 4.5 Semicontinuous Distillation Followed by Batch : Distillation ... . .,.,; b e e e ee b nieeen 'Vh'6 Comparison of Methods COnsidered . ¢ eie.e.ae 6. Conclusions and Recommendations ;I.];.......;.....,,.,. """Referenc_es v s e tooi" 7!.-"-“'-"'"'T"‘,""'-" * -Page Vi EF W W N M 10 11 12 b 16 16 16 19 21 28 29 .32 5. Prevention of Buildup of Nonvolatiles at a'Vapprizing Surface by Liquid Phase. Mixing .a.‘;.. ...'.,.,....,f. o le ea .36 L3 - 43 e b Wl t) 0 S wh CONSIDERATIONS OF ‘LOW PRESSURE DISTILLATION AND ITS APPLICATION TO. PROCESSING OF MOLTEN-SALT BREEDER REACTOR FUELS _L.-E.-McNeese | ABSTRACT‘ Distillation at low ‘pressure was examined as a: method for removing rare- earth fluorides from the fuel stream of -a molten-salt breeder reactor,' ‘It was concluded that 'dis- - . tillation allows: adequate rare earth fluoride removal with ‘the simulténeous recovery of more ‘than 99,5% of the fuel salt. Characteristics of equilibrium and molecular = distillation were noted and expressions for the relative ! volatility of rare earth fluorides were derived for these types of distillation. 7 . e - - Expressions for the separation potential of several modes of distillation were derived and reported rare earth fluoride relative volatilities were shown‘to:allow_, & great deal of latitude in still design and operational - mode. It was concluded that a single contact stage such as a well mixed liquid pool provides adequate rare earth. -fluoride removal and that rectification is not required . The buildup of rare earth fluorides at the vapori- zation surface was shown to seriously reduce the R effectiveness of a distillation system, .Liquid circu- lation was shown to be an effective means for preventing "buildup of rare earth fluorides at vaporization surfaces.' "lgjlleRObUCTION | The molten*salt breeder reactor (MSBR) is a reactor concept having ‘the possibilities of economic nuclear power production and simultaneous breeding of fissile'material using the thorium-uranium - 'ri'fuel cycle. The reactor will be fueled with a mixture of molten fluoride salts’ which will c1rcu1ate continuously through the reactor core where fission occurs and through the primary heat exchanger" ';where most of the fission energy is removed The reactor will employ'r a blanket of molten fluorides containing a fertile material. (thorium) in order. to: increase the neutron- economy of the system by the con-. version of thorium to fissile uran1um-233 A close-coupled processing facility,for removal of fission products, corrosion products, and fissile materials from these fused fluoride mixtures will be an integral part of the reactor system.. 1t has been proposed that the rare earth fluorides (REF) and fluorides of Ba, Sr, ‘and Y be removed from the fuél stream by - vacuum distillation. The purpose of this report is to examine various factors pertinent to such an operation and to compare several methods for effecting ‘the . distilIation_ . 2. }DISTILLATION’AT,LOW‘PRESSURE - The vaporization of a liquid is normally carriedlout under . - conditions such that the 1liquid and vapor phases are essentially ' in thermodynamic equilibrium. -This condition may cease to exist 1f the distillation pressure is reduced sufficiently; and phenomena peculiar to low pressure distillation may be observed. -In discussing digtillation at lowfpressure,,it'is convenient to'make-a distinction between two modes of distillation-' equilibrium | distillation and molecular distillation. During equilibrium distillation, a kinetic equilibrium exists at the liquid-vapor interface owing to the presence of a vapor atmosphere above the liquid which has the net effect of immediately returning most of the vaporizing molecules to theoliquid surface. In contrast, molecular ‘distillation is carried out in the absence of such an atmosphere and the vaporizing molecules reach the condensing surface without experiencing collisions ‘with other gas molecules or with the walls of the system. In the following sections, consideration will be given to characteristics of these modes of distillation, to. values ‘i of the mean free path under conditions of Anterest for MSBR , N ’ processing, and to calculated values of maximum vaporization rates to be expected ry cl 2.1 Equilibrium Distillation Equilibrium distillation can be further divided into ebullient distillation and evaporative distillation. Ebullient distillation N occurs when bubbles of vapor form within the bulk of the liquid which remains at a temperature such that -the vapor preSSure is equal to the total external pressure acting on the liquid (in the absence of other gases Boiling‘prOmotes mixing in the liquid .and the surface from which vaporization occurs is not depleted in the ‘more- volatile species, “Evaporative distillation occurs when the distillation is carried out at a temperature below.the bdiling‘point of the liquid. Under ‘these conditions there ie no formation of bubbles at points below the ‘1iquid surface and no visiblemovenent‘of the ‘1iquid surface, Transfer of the more volatile species to the. liquidisurface occurs by a combination of molécular’diffusionfiand convective mixinglso:' that depletion of this species in“the viéinity of the- surface is possible, _However, the rate of distillation is relatively low owing to the kinetic equilibrium which exists ‘at the liquid-vapor -interface and the liquid surface may have ‘a composition near that of the bulk ‘1iquid. ‘o.2 'Molecularr‘Distillation | Mblecular distillation is quite similar to evaporative distillation -in that vaporization occurs only from a quiescent liquid surface end in that ‘the vaporizing species 1s transferred .to the surface by - molecular. diffusion and convective mixing. However, few of the vaporizing molecules are returned to ‘the liquid surface by "collisions in the vapor space above the liquid and vaporizdtion - : 1proceeds at the greatest rate’ possible at the operating temperature. | f In order to achieve this condition, the distance between the vaporizing surface andstheieondensing-surface should_theoretically l,beqleSS'than_the "meanflfree"path“of-thehdistilling moleculeS. This condition - is seldom realized in practice, however ‘the distance should he”no‘gréater than a few mean free paths, greater buildup of rare earth fluorides at the 1iquid surface than | do those- of evaporative distillation at the ‘same temperature where vaporization is impeded by the vapor - atmosphere above the liquid | ‘which serves to Teturn most of the vaporized molecules to the liquid lfi, surface, “In a distillation‘system, the gases in the region between the - vaporizing liquid and the condenser normally consist of a mixture 2.3 Mean Free Path These conditions favor a of ‘the distilling molecules and molecules of noncondensable gases.-“ . The calculation of the mean free path in this region is complicated by the fact that the vaporizing molecules, which have a net velocity component directed away from the liquid surface, pass into 8as vhose molecules are in random motion. ‘1 molecule in type 2 molecules may be obtained from a relation given bijoeb2 as ' 01,0z = collision diameters of type 1 and.type:2*molecules', M,2 - The :mean free path of a type oo+ 022, €SP v (2422 “41*6?5 mean free path of a type 1 molecule moving among type 2 molecules ) ‘n = number of type 2 molecules per unit volume - By making appropriate substitutions into this relatiofi, one can obtain the following relation for the mean free path-of a type 1 molecule in type 2 molecules at a pressure P, both gases.being.at;thec,;' temperature T. | o C1,Cz = average velocity of -type 1 and type 2 molecules. @ [ n} &1 where ; R = gas constant, (mm Hg) (em®)/(°K)(gmole) 03,02 = collision;diameters of type 1 and 2 molecules CMy,Mp = molecular weights of type 1 and 2 m°le°files"~ Values for the mean free path of LiF in Ar and in LiF at 1000 C :;are given in Fig. 1. It should be noted that the mean free path of 'LiF at a pressure of 1 nm Hg-1s approximately 0. O4 cm and that at a pressure of 0.0l mm Hg, the mean free path of LiF is approximately 4 cm., These distances are probably quite small in comparison with ' ~ the distance between the condenser and the surface from which vaporization will occur- in an MSBR distillation system. ‘Hence, the rate of distillation in an MSBR system will be set by the pressure :-drop ‘between the liquid surface-and the condensing surface. . The ~values for the mean free path are sufficiently large that slip-flow may be of importance in presgure drop considerations, 2.4k Langmuir Vaporization Rate ‘The maximum rate of evaporation of a pure substance was shown by Lsngmiur5 to be =¥r__ | = W = 0.0583 EP - (3) where | W = evaporation rate, gms /cmZ 1 sec M= molecular weight =~ ‘T= absolute temperature, éKV - '7P-e vapor pressure, mm Hg. '_'A derivation of this relation will be given in order to show the ”region of its applicability., Consider a plane liquid gsurface at - a temperature below ite’ boiling point. At equilibrium, the rate *of vaporization from the surface will equal the rate of condensation - on the surface. Langmiur postulated that the rate of vaporization ;_in a high vacuum is the same as - the rate of vaporization in the presence of a saturated vapor . and that the rate of condensation in PRESSURE (wn Hg) B | o ORNL DWG 67-19 10 ¢ T |fii-1i|], T T T 11T — T T T T T 1T - 0.1 b— — e - « N 2 r- - = i - oorl 0 vl v vl 4 0.01 0.1 o ! L | " MEAN FREE PATH {em) Fig. 1. Mean Free Path of LiF in Ar and in LiF at 1000°C. g o i e . T 4] o 1)) so that " a high vacuum is determined by the pressure of the vapor. At equilibrium the rates of vaporization and condensation are equal and the rate of vaporization can be calculated from the rate of -_condenSatlon. ‘The vapor contained in a—onit;cube in contact with the liquid “surface is in equilibrium when the number of molecules ‘moving toward the surface equals the number moving away from the surface. ‘For n molecules of mass 'm in the volume v, the quantity of vapor approaching the liquid surface will be i omn 1 o o . : where p is the vapor density._ The average component of velocity of molecules moving toward the surface is é U, where U.is the arithmetic mean velocity of the molecules. The mass of vapor - striking a unit area ‘of the liquid surface per unit time is then T I If the vapor is an ideal gas, PM - - and ' PV=RT=3-mnC.. | B _ | (7) Solving for (Ca) /2 yields _“__h : - | . 170 r = e T | (02)/ ifi‘ | - - ® - vwhere M = pv. The mean veloc1ty U is related to the root mean 7square velocity, (—E)l/e :?:i~;7l”-" ,-"hfl-;;‘u-'hf-?;Bl ..-”n e .' U=J%fi@ S : ,. L e (10) Thus or | W = 0.0583 P—T P The assumptions implicit in the use of this relation for cal- : .. -culating the rate of vaporization from a liquid surface include P the following: ' (1) The liquid surface is plane. (2) The liquid surface is of infinite extent, i.e. collisions -of molecules with the vessel walls in the vapor space must - exert a negligible influence on the rate of vaporization. a e (3) The vapor behaves as an ideal gas.ki, -F (4) Every part of the liquid surface is within a fraction of fl- o the mean free path from every other part or from a condensing surface,-i.e., thereffect of collisions between evaporating molecules on the ratehof vaporization is negligible. | S - « (5) The number of molecules-leaving the liquid surface is not affected by the number striking the surface. | (6) Vapor molecules striking_therliquid surface are absorbed and revaporized in a direction given by a cosine relation which is independent of the direction of aoproach atithe moment of absorption. When applied to the vaporization of LiF-BeF2 mixtures, the poorest .of these assumptions is likely that of considering the vapor “i__m to behave as an ideal gas since it is known that gaseous_LiF ‘tends to associate. The vaporization rate given by Eq. 11 represents the maximum rate at which vaporization will occur and hence sets an upper limit on the vaporization rate. Values forvthe-Langmulr vaporization rate of LiF are given in Fig. 2. The vaporisation‘rates- observed in practice may be considerably lower than the Langmuir rate since the fourth assumption is rarely met, + VAPORIZATION RATE (g/cm. sec) . 0.0001 0.04 - 0.0 ORNL DWG 67-20R1 800 | TEMPERAIURE (-c) Sl S e | Fig2 La.ngmuir Vaporization Rate for LiF. 10 2.5 Probable QOperating Mode for MSBR Processing The mode of distillation currently envisioned for proceSSing MSBR fuel salt is that of single stage equilibrium distillation at 950°~1050°C and at a pressure of -approximately 1 mm Hg. The - composition of liquid in equilibrium with vapor having the composition of MSBR fuel salt (64 mole 4 LiF - 36 mole ¢ BeFy) is approximately 88 mole % LiF - 12 mole % BeF. The vapor pressure of liquid of this composition is ~ 1.5 mm Hg at~lQOO°C_.5 -Hence, evaporative distillation, with surface vaporization onli, will occur if the distillation is carried out at a pressure greater than 1.5 mm Hg. However, if the distillation is carried out at a pressure lower than ‘1.5 mm Hg, boiling could occur below the liquid surface. At a pressure of 0.5 mm Hg, boiling could occur to a depth of about 0.7 cm. The actual depth to which boiling would occur is-dependent on the vertical variation of liquid temperature and composition (and hence vapor pressure) and on the extent of superheating of the liquid; the value of 0.7 cm assumes a constant temperature and concentration and no superheat throughout the bulk of the liquid. Boiling in the vicinity of the liquid surface would promote convective mixing which would result in a lower rare earth fluoride concentration at the liquid surface than would be observed without such mixing. The lower surface concentrations would in turn decrease the relative rate of volatilization of REF with respect to LiF. .Molecular distillation offers two advantages over either type of equilibrium distillation in.that (1) the distillation proceeds at the maximum rate, and (2) a greater separation of rare earth fluorides from the MSBR fuel salt is possible as will be discussed ~ in the section on relative volatilities. 1Its chief disadvantages are the low pressure required to achieve this type of ‘distillation and the increased likelihood of an undesirable buildup of rare eafth fluoride at the liquid surface. The MSBR distillatlon system will probably be operated at the vapor pressure of the liquid at the vaporization surface or possibly oy L) 11 .at a pressure:0,5-1.0 mm Hg lower than the vapor pressure. A decrease in pressure could yield an increase :in distillation rate-and/or -a deerease-in operating-temperature. Entraimment -at the lower pressures should'be»consideree.'\It'is improbable thatjthe'advantages to be gained'by'molecnlar'distillation justify the;efforthnecessary to attain this mode of operation. " %, " RELATIVE VOLATILITY - The relative volatility;is;a convenientrform4for,presenting ‘data relating the compositionnof‘liQuid and vapor phases at equilibrium and,is-defined as Yy, AR Qe = . (12) AB xAle | vhere | o ‘ - | . :aAs =:relativepvolatility,of componentiA,referreo to oomponent'B Yprg =.mole fraction of component A, B in vapor X, 2%y = mole fraction of component A, B in liquid. If the conoentration of component A is low. compared to that of ‘the major component (B), the relative volatility can be expressed in a useful approxrmate form | % "T, @) where o I C, = moles A/unit volume of .condensed vapor 1Cg-= moles:A/unit volnmé‘of'lionid" . and where the condensed vapor and liquid are at the temperature at which | '~vaporization is . carried out, In a binary system, the error ‘introduced - _in relative volatility by -this approximation depends on. the concentration ~of component A, the relative volatility, and the. relative molar volumes -of components A and B. The error can be evaluated as follows. 12 Let O denote‘the aotual relative "oolatility as defined by - - - Eq, .12, and- a* denote the relative vdlatility in the approximate form defined by Eq. 15. From Eq. 135, ' 'Y, X +(1 "}i')v a* = ‘A A X, Y, V, + (1 -¢¥ )v The relati.on between Y and X 1is given by Eq..12 and its use with A the expression for a* yields the fractional error in o as o .o X (1 a)(v /v ) o o S rrfrac error = o 1 "X [1 a(v /V)] . The fractional error in afilis given in Eig.‘j as a function of'_X.A for various values of & for the case where the molar volumes of A and B are equal. It should be noted that the error in @ introduced by Eq. 13 is less than 184 for X < 0.15 mole’ fraction if @ < 2,46, For rare earth fluorides in LiF, the error in.d' will be approximately three times the values shown for & < 1072 since the molar volume of rare earth fluorides 1is ~approximately ‘three times that of LiF. The definition of relative volatility given by Eq..12 has . - ‘been used throughout ‘this report except in Section 5 where the definition given by Eq. 13 was used - | The appropriate forms of the relative voIetility will be derived in the following sections for both equilibrium and molecular distillation, and a comparison of experimental and calCuleted'values of relative volatility for several rare earth fluorides in LiF will be made. 3.1 Relative Volatility for-‘Equllibriinn,‘ Distillation - - In equilibrium distillation, the relative volatility relates -the composition of liquid and vapor which are -in thermodynamic equilibrium. .For the ith component of a system which obeys Raoult' Law, one can write = S o C T , - A &x¥ I 1Y | CWt-0 a 5 ORNL DWG 67-21 - ST T T T T 0-6— 04 b o »N I B | a <10 -0.4 | - 001 . o | | X A. finofe froc!lon of cmpomnl A In liguid) 0.5 Fig | 3. - Error Introduced in Relative Vola'bility by Use of Approximate Form of Relative Volatility. S 1k Ty =By vhere | o = total pressure P, = vapor pressure of component i Vg = mole fraction of i in vapor *1'=;9°Ie fraction 'of i in liquid. Substitution of this relation into the definition for'reletive volatility of component A referred to component B yields % =T ey T F, _ *A B Raoult ¢ Law implies the absence of chemical interaction between the components under consideration. Interaction may be taken into account if information is available on the activity of the components ‘since one can write for the ith component Ty = Bxg o (@9) where _71 = activity coefficient of conponent-i,in liquid of the composition under consideration. ‘The relative volatility may then be written as P (o AB 7B 7B 3.2 Relative Volatility for Molecular Distillation With molecular distillation, the liquid and vapor phases are’ not "in thermodynamic equilibrium,,instead, the composition of the vaporiting~material i{s related to that of the liquid by a dynamic equilibrium which is dependent on the relative rates for naporization of the various components of the liquid. An expression for the rate of vaporization of a pure liquid was derived in Section 2.4, PA"A/PB"B P - | ,(111) | /S o (e) :\ r () £ 15 Division of the- rate equation (Eq. 11) by M, the molecular . weight ‘yields the molar rate of vaporization as P g L == F . - . (n Thus, the molar rate of vaporization for any substance, atlaigiven' ‘temperature, is governed by the ratio of P/{M. "In a binary system, if Raoult's Law is assumed, Py =% By - (18) where _ : X, = mole fraction of component i in the liquid Pi,='vapor pressure of pure component i pi = partial pressure-of-componentii at liquid:surface. . - The mole ratio of components vaporizing from the liquid surface is then ' ‘ B o ATA r MB A (19) - T ;] R T x13 | | Since the quantity /mB is related to the ratio of the mole fraction of components A and_B in the vapor as (20) it ‘éfilfifi oF ifi»g | where L o ~yi~; mole fraction of component i in vapor _i one obtains af the relative volatility as' o = A/x:fié‘l_; (&) - This should be compared with the relation for relative volatility for equilibrium distillation which was P /P | | 16 3.3 Comparison of Experimental and Calculated Relative . Volatilities for Rare Earth Fluorides ‘Relative volatilities for several rare earth fluorides in LiF have been measured at 1000°C bj‘Hightoweré and the relative volatility of LaFs in 87.5 -11.9 - 0.6 mole % LiF-BeFp-LaFg has been measured at '1000° and 1028°C by Cantor.7 These - experimental . data and calculated relative volatilities for rare earth fluorides - 8 »9,10 for which vapor pressure data '’ are available are given in Tebie‘l. Calculated values were obtained using Eq. 14 and Eq. 21. 4. SEPARATION POTENTIAL OF VARIOUS DISTILLATIONIMETHODSr : Several modes of operetion~are=availab1erfor.the.distillation of MSBR fuel selt; the_choice_betweenttheee-will_ipvolve coneideration . of their separation potential as well as factors 'such as degree of complexity, economics, etc, In the following sections, a compariseh “ will be made of the separation potential of distillation systems employing continuous, batch, and semicontinuous methods. A semi- continuous system employing rectification will also be considered. 4.1 Continuous Distillation Consider a continuous distillation syetem,of the type shown in Fig. 4. 8Salt containing Cf moles REF/mole LiF is fed to the | system at a rate of F moles LiF/unit time. A vapor stream containing OC moles REF/mole LiF is withdrawn at the rate of v moles ‘LiF/unit time and salt containing C moles REF/mole LiF is discarded at the rate of F-v moles LiF/unit time, A material balance on REF yields the relation | ' FC wa + F-V)C. £ - The fraction of the REF removed by the distillation system is then given as QO Table 1. Experimental and Calculated Relative Volatilities of- Several Rare Earth Fluorides Referred to LiF at 1000°C Vapor Pressure o mm Hg .~ Relative Volatility ',Rare Earth'E1fiotide__ . *Smfia: e ,cngf . ,LaFa o ' g:1.6-x 10f§fl-'”“' 1.3 x 104 . .2.8x105 8 ._6 X ‘_210-"4 1.09 x 103 | Calculated ” - ~ Equilibrium = Molecular . *gExperimental31-Distillation - Distillation 6x10% 31x10% 1.1x10* 5x10% - - - 3x103 = 24k x104 8.7x 105 3x10% 52x10° 1.9x10° b *Measured in 87.5-11.9-0.6 mole % LiF-BeF,-LaFs at 1028°C. ‘bMéasuréd;ifi'87,54;1!9-0.6'm01e ¢ LiF-BeFy-LaF4 at 1000°C. LT ORNL DWG 67-22 Vapofi.ze:d Salt v, a C : L Discard 'Sc_a_,ltzr, | . F-v, C Feed Salt I | F, Cf Fig. 4. Continuous Still Having a Uniform Concehti'ation of Rare Barth Fluoride in Liquid. : . . :fOr‘V CQnstant 19 fraction of_REF removed = (E_- V)C.= 1 _ (22) FCg 1+ If the fraction of the LiF fed to the system which is vaporized .is denoted as f, where | v t= F ‘then fraetion'of REF removed = ———;—EE-. (23) LT - The fraction of REF. removed was .calculated using Eq. 23 for various values of £ andtx -Ls shown in Fig. 5. 4,2 Semicontinuous Distillation with Continuous Feed Consider a single-stage distillation system which contains V -moles LiF at any time and a quantity of BeF, such that vapor in o equilibrium with the liquid has the composition of the MSBR fuel ~ salt, Assume that MSBR fuel salt containing X moles REF /mole LiF is fed to the system at a rate ‘of F moles LiF/unit time where -1t mixes with the ‘liquid in the- system, Let the initial REF concentration in the'liquidube'x moleS'REF/mole LiF and let the concentration at any time t be X moles REF/mole LiF. From a mass balance on REF, _ : _' s - ‘E(VX) = FXO "FOfX IR = (24) ¢Wh1thhasxthe solution_ Xe=g -@-a)exp(-Fae/V)] - (25) The total quantity of REF fed to the system at time ‘t is -(Ft + V)X and ‘the quantity of REF remaining in the liquid at ‘that L0 g o oo FRACTION OF RARE EARTH FLUORIDE NOT VAPORIZED e o 04— 20 | ORNL DWG &7-2 , T —— @ =0.0001 \ 30 _00_057 02 I | | 0.945 0.97 0.98 0.99 1.0 ' FRACTION OF LiF VAPORIZED Fig. 5. Rare Earth Fluoride Removal in a Continuous Still as.a Function of LiF Recovery and Rare Earth Fluoride Relative Volatility. .- el | tifiehierXf Thus, the fraction of the ‘REF not vaporized at. time t: is o 'l'e,' VX e 1 - (1 - a) exp (-EJCIV) fReF = (Ft + Nk, T (%1:_ . 1) ,, (26) where fREF = fraction of REF not vaporized at time t, - ,The fraction of the LiF vaporized at time t. is given by the relation __Ft iR (2m) where f = fraction of LiF vaporized at time t. rSubstitution of Eq. 27 into Eq. 26 yields the desired relation: 'kafj= L= [l -r(l - @) exp (-af/l - f)] o (28) Values for the fraction of REF not volatilized as a function of the. fraction of the LiF volatilized are - shown in,Fig 6 for various values - of REF relative volatility ' ‘H;5 ‘Semicontinuous Distillationrwith Rectification Consider a distillation system as shown in Fig. T, which consists of a reboiler and one ‘theoretical plate to which reflux is» returned, The feed stream to the reboiler consists of F moles LiF/unit time plus X moles REF/mole LiF and Z moles BeFa/mole ';LiF. The following assumptions will be made-' 3;(1) At any time the reboiler contains V moles LiF, where V. is L s constant. e ' S l'*;f(z) The initial REF concentration in the reboiler 1is X . .moles REF/mole LiF.-_' ) ' (3) The concentrations of BeFfi%‘moles BeFz/mole LiF) throughout _the system are- ‘the steady state values, i, e., the values o which WOUId“be ofitained at steady state. with a feed ‘Stream : containing Z moles BeFalmole LiF and no REF 22 ORNL DWG 67-24 1.0 08— 0.4 [— FRACTION OF RARE EARTH FLUORIDE NOT VAPORIZED 0.2} 0.965 057 0.98 0.9 1.0 ' FRACTION OF i.iF VAPORIZED - Fig. 6. Rare Earth Fluoride Removal in & Semicontinuous Still as e Function of LiF Recovery and Rare Earth Fluoride Relative Volatility. 23 ~ ORNL DWG" 67-25 ~ Condenser (R + fl F 'aREF'x‘-. i —P»F %er*y: 7, | rRF Crieriodin | ®reF*y Zo v X One Theoretical Plate Z S of Rectificaflon Y . ererXs CBeF, Zs 2 - A X,xv Z1 . Pl - Xo | rRebOIIer S ”'_zo_ ' Fig. 7. Semicontinuous Distillation System with Rectification. 2k (4) The vaporization rates for LiF and BeFy are unaffeéted_by fhe presence of REF and hencéiare the-steady state vaporization rates. This assumption is;made-ifi_view-of the-lowféoncentration of REF in thé vapor from the ' reboiler and hence in the liquid on the plate and in the distillate. - o (5) The holdup in all parts of ‘the system excluding the | ~ 1liquid in the reboiler is negligible. ' (6) The relative volatilities of BeFp and REF referred to | 'LiF are constant. For Ealéulatidh of tfie»sféadf ététe‘Bng concentration and the eteady state vaporization rates fo;;BéFa and LiF, it is assdfié& - that the heat input rate to the-reb§iler-is equal to the heat withdrawal rate in the condensef-(fiegligiblé;heat of mixing) which 18 = (R + 1)F (lLiF'T'ZoxBeFé) heat input rate of reboiler qH O n = reflux ratio " feed rate of LiF to reboiler, moles/unit time if = molar heat of vaporiiation for' LiF F xBeF = molar heat of vaporization for Bng 2 Z o = concentration of BeFs in feed and distillate, moles BeFp mole/LiF. , The rates of vaporization of LiF and BeF, from. the reboiler are = then moles LiF vaporized/unit time = $———7 Q 7 - (29) - - Muar * %Ber, Zs MBers g, 2 ' moles BeFp vaporized/unit time = +_a? 7 (30) ALiF Bng s 7\BEFE vhere aBéF = relative volatility of BeF, referred to LiF e st e AR B0 SRR & gL gt 1t where 25 ‘Zs.= concentration of - Bng_in liquid in reboiler, moles. . BeFs/mole LiF, A material_balanceeon_LiF around plate7leyie1ds 9 ‘ _ + RF = (R_-l-‘l)F. +X MaF * %Ber, Zs MBer, from which ‘ . Q _ X = - - F Mar * %er, z_s‘ NpeF, X = moles Li.F returned to reboiler/unit time from plate 1. A material balance on BeF2 around the reboiler gives ' ' QZ1 7\Li. BeF 2 Zg ?\Bng . z - FZy +FZ) = e . 7\L:i.F BeF2 s ,7\BeF2 'Substitution of the definition of Q from Eq. 29 with the reletti.on~ - Zo‘ '_ZJ_ =__a[A o | BeFo into Eq. 33 yields R + aBeF 2 - azo_ — R+l 7\L:LF * % 7‘13:-;@'2 “Bera [RBeF - ] e ek BeF2 ?\LiF o xBng - R+1 A material balance around plate 1 for REF yields " Fa “REF’ l)(7‘Luf.1? + 0 Fg Zg Npep,) * Q (31) (32) (33) - (34) ”'-‘(35)_ (36) (R + 1)Fa Fxl ¥ QXl CEG e 7\LiF Bng s ?\Bng ' ' qa e | REF _. = .+ RFQ 1 R 7\L;I.F BeFa z 7‘BeF2 _REFX from vhich R K= REF L 26 . A material balance around the reboiler on REF yields dX_ Q.. X "REF "8 - . , V=—>=FK_ - +Xx (37 de ° Mur * %ers Zs Mger, which can be written as & o 8 _F t; _ SR it -y (Ko - FXQ) S B® where (Apyp + zol‘L ) O‘REF 2 * Frer Oeg~t -~ 1 R + 1 7\L1F+7\BeF2[z +R+l aBeFaz] The solution to Eq._58, with the,condition X = X when t = 0 is B = 8 X =-- [1 - (1 - B) exp (-Fat/v)] - (39) from which the fraction of the total REF not vaporized is given by o b = S5E 1L - (1 - 8) exp (-BEAL - £)] (ko) where _ fREF = fraction of total REF fed to éystem whlch is contained in liquid in reboiler = fraction of total LiF ‘added to system.which is vaporized Values for the fraction of REF not vaporized as a function of ‘the fraction of the LiF vaporized are shown in Fig. 8 for various values of the rare earth fluoride relative volatility. Data used in the calculation were as follows ZO = 0.45 moles BeFalmole LiF R=1 | rlaBEFg =5 kLiF = 53.8 kcal/mole Ngep, = 50-1 keal/mole 27 oS L ORNL DWG 7-26 R - S a001 | 0.96 p— 0.88}— S & x g . O = & £ & (.80 {— L Q Z 6 g g 0.72 b S 0.965 ,--0.97.—,;‘:;_:;;_5 Sl 098 08 1.0 FRACTION OF LiF VAPORIZED o Fig. 8. Rare Earbh Fluoride Removal in & Semicontinuous S5till ' Having One Theoretical Stage wit.h Rectification a.nd a Reflux Ratio of Unity. . . 28 L. Batch Distillation Consider 2 liquid of uniform composition consisting of components 1, 2, and 3 which have ‘molecular weights Ml, Ma, and Mg, respectively. Let the weights of the components in the liquid at any time be Wl, Wg, -and Ws, respectively. For the vaporization of a differential quantity of the number of moles of each component in the vapor thus is d (—1§ (—2), and d (W )e' Since the mole fraction 1 in the vapor fs given by - a(w, ) ) o) o) one can write Yi ¥a In the liquid WafMa _Xa i _Za'd = g = ! . WM X M Xy W Thus My WM Viza Wy Wz ya Mg y3 X W3 Then d(log W;) ¥,/ o d(log W3) x/xg ~ i3 the liquid produced- of component ) (v2) (43) (bb) _where ¢, is the relative volatility'of component i‘fiith'respect to- i3 component 3. The quantitytxi3_is-defined as afif (moles i/mole 3), vapor i3 = _ _ (mole frac i/mole frac 3), vapork,hs) (moles i/mole 55:'liguid (mole frac i/mole frac 3) liquid' E3l | range of LiF recoveries since for o 29 Ifa13 is constant during the distillation, Eq. 44 can be integrated, Thus if Qi and Q3 are the initial weights of»components.i and 3 in the liquid, | W, Wy [ d( log Wi) = aij_ ! d( log W5) W fw\ %3 "\ where the quantity W /Qi represents the fraction of component i which remains in the liquid -1f the components 1, 2, and 3 are now designated to be a rare earth fluoride (REF), BeF2 and LiF ; respectively, | R - ‘a MREE i (WLiF REF Qir/ - o« VBer, _ (WLiF‘ ‘BeFz Re™ The fraction of the rare earth fluoride not vaporiaed as a function of the fraction of the LiF vaporiZed for various values of aREf was calculated from Eq. 48 and is shown in Fig. 9. The; - vaporization of BeF2 can be regarded as- complete for ‘the: probable Be F ~LiF = 5.0, VaPDrization' of 90% of the LiF results in vaporization of 99. 9% of the Bng._ h 5 Semicontinuous Distillation Followed by Batch Distillation' (46) (k) (48) (49) Consider a still containing V‘ moles LlF If a feed stream o consisting of F moles LiF/unit time which contains X moles REF/mole LiF and which may also contain BeFp is fed to the still 'with ‘the condition. that the initial REF concentration in the ~still is X o’ the concentration of REF in the liquid at time t is given by 1.0 0.96 0.88 FRACTION OF RARE EARTH FLUORIDE NOT VAPORIZED 0.72 - 30 'J - ORNL DWG 67-27 a=0.001 - @=0005 “a=QOI a=0.02 - 0.965 0.97 - 0.9 _ 0.99 | S 1.0 FRACTION OF Lif VAPORIZED | . o , Fig. 9. Rere Earth Fluoride Removal in a Batch Still as a Function of LiF Recovery and Rare Earth Fluoride Volatility. = ¥ 31 e X=52[1-(1-a)exp (-E/Y)] (50 where REF-COncentratiqn in still liquid, moles REF/mole LiF n REF concentration in feed, moles REF/mole ‘LiF relative volatility of REF referred to LiF feed rate, moles LiF/unit time o QMM " = time V' = moles LiF contained in liquid in still, 1f at time t the feed is stopped and batch distillation is carried out on the liquid in the still to give a.final liquid.nolume con- taining V moles LiF, the fraction of the REF which was present in the still at the begrnning of batch distillation which remains in the still is then (V/V') . The moles of REF at the beginning of batch operation is VX 0 that the moles of REF in the still at the end of batch operation is V'X(V/V ) The total moles of REF fed to the system isu(Ft,+,Vq)xo-_‘Henceethe fraction of the totalrn REF which remains in the still is which can be written in the form_ m [l .'.L - Ot) exp (-rfl)]g - (52) | fREF = fraction of REF which remains in still . g - vV . . = final LiF content ofiEtiifi,—noleem 'V = initial LiF. content of still ~moles - n = Fe/V', number: of st111 volumes fed prior to batch operation ' Ei; feed rate to st111 during semicontinuous operation, .' ' moles/unit time ;_¢;};~- s | 'tt = time , ia“ ) T , @ = relative volatility of REF referred to LiF. 32 The fraction of the LiF which is vaporized by both methods of operation is given by .ij_F— 1 n +1 , L - (53) where fLiF fraction of LiF vaporized by both methods of distillation. The fraction of REF not volatilized as e function of the fraction of the LiF volatilized for ¢ = 0.1 (final volume of 10% of still volume) is shown in Fig. 10 for various values of the REF relative volatility, " | 4.6 Comparison of Methods Considered A comparison of the various distillation methods must take account of numerous factors such as separation potential operability, economics, etc. Consideration of all of these factors is beyond the scope of this report, however, the two topics of separation potential and operational simplicity will be discussed For currently envisioned processing rates, the required removal. efficiency for the rare earth fluorides is approximately 90% for the more important neutron poisons (Pm, Nd, Sm) and less,for other members of this group. It should be recalled that the real requirement is the rate at which neutron poisons are removed from the reactor system, hence the "required removal efficiency"” can be -lowered at the expense of an increased throughput in the processing plant. With this in mind,!a rare earth‘fluoride;removal'efficiency of 90% will be used as a basis for discussion of separation potential for the various distillation methods. As shown in Table 1, recent measurements indicate the relative volatilities (referred to LiF) of some of the fore important rare | earth fluorides to be 6 x 10 % for NdFs, 5 x 10 % for SmFs, and 3 x 10 # for LaFs at 1000°C in LiF, The relative volatility of CeFg was found to be 3 x 10-3,.however; the required removal effiCienoy.' o for CeFs is only 84. 33 ORNL DWG 67-28 | . a=0001 o w N g . O 0.96 | — Z | 5 & g a*0.005 T - . Ed . A w - « 0.92 p— - . O z 0 0 < , | E L S . . - - a=0.01 088 1 . ,.J - 0965 057 098 :'*r_0.99»:-_:~ 1.0 FRACTION OF LtF VAPOR!ZED | Fig. lO. Var:!.ation of Rare Earth Fluoride Removal in & Semi- - continuous Still Using a Final Batch Volume Reduction to 10% of the Still Volume. C , The fraction of REF not vaporizéd for the various distillatibn ~ methods considered is given in Table 2. The values were calculated for a REF relative volatility'of.fi.x'10f4-and.a'LiF'recovery of 99.5%. As would be expected, the éontinuous system yields_the -‘pporest-REF'removal.(91%) although this is an adequate removal efficiency. The removal efficiency fof a semicontinuous system is. somewhat higher (95.2%). The combination of semicontinuous dis- " tillation followed by batch operation to yield a 10% heel results- in a REF removal of 99.5% which is comparable to that obtained with 'batch'bpération (99.744). The effectiveness of rectification is ~ pointed. out by the REF removal (99. 99674) for a semicontinuous f ; system containing one theoretical plate in addition to the reboiler.i Table 2. Fraction of Rare Earth Fluoride not Vaporized for Several; Distillation Methods for a REF Relative Volatility of 5 x 10 ¢ . and a LiF Recovery of 99.5%. Distillation Method Fraction REF not Vaporized Continuous 0.91 -Semicontinuous 0.952 Semicontinuous + batch (10%) heel 0.995 Batch 0.9974 Semicontinuous with Rectification 0.999967 The continuous distillation system is considered to be the - simplest operationally. This system will operate with a near constant heat load (due to fission product'decay) and liquid level vhich will allow prediction and control of‘concentfafion-and‘ f ' temperature gradients in the liquid phése and will simplify‘instfu- menting the system. Waste salt could be removed from~the?systémrat .frequent intervals rather than continuously, The semicontinuous system is considered to be slightly more complex operationally than the continuous system; this complexity 35 arises primarily from the,transient“nature.ofhthisimode of operation and the associated variation of.the‘fiSSion'product°heat load with 'time. Reduction of the liquid volume in the system near the end of a cycle by batch operation will not complicate instrumenting the ”: system if the heel volume is not less ‘than approximately 10% of the initial still volume. - | | fi Operation of a batch system W1ll be significantly more compli- _ cated than either of the above systems for a number of reasons. The cycle time for this system should be relatively short in order to maintain an acceptably low salt 1nventory, hence the system must be charged and discharged frequently. Control of temperature and 7 concentration gradients in the liquid will be complicated by the continual variation of both liquid level and heat generation per unit volume, Near the end of a cycle, the liquid volume in the ”i' system will be approximately 0., 5% of the initial volume° accurate -measurement of liquid level at’ this point may be difficult ' Conceivably, a continuous distillation system employing ~rectification would be as simple operationally as a- ‘continuous ‘system without rectification._ However, a system using rectification_ probably requires a greater extension of present technology than any of the systems considered Problems 'such as vapor- liquid contact are aggravated by the necessity for low pressure, high temperature operation. The very high rare ‘earth removal efficiency achievable with rectification is not required for MSBR processing.ii:} Besed on the factors considered the distillation methods can be listed in order of decreasing desirability as follows-'- 51,,1continuous . . R i - 7"'2.”_semicontinuous With batch reduction to yield a 10% heel - 3., semicontinuous S e ' 4. batch ) L - .5.' continuous with rectification’h . | i i 1 | A 36 5. PREVENTION OF BUILDUP OF 'NONVOLATILES AT A VAPORIZING SUREACE S 7 . : BY LIQUID PHASE MIXING , o During.vaporization of a multicomponent mixture, materials less volatile than the bulk of the mixture tend to remain in the‘.v S | liquid phase and are removed from the liquid surface by the processesjfiqq -of convection and molecular diffusion. As noted in Section 11, Low pressure distillation will result in little if any convective mixing in ‘the liquid and ‘an appreciable variation in the concentration of materials of low volatility is possible if these materials are " '_ ~ removed by diffusion only. The extent of surface buildup and the - effectiveness of liquid phase mixing will be examined for a continuous | still although the phenomena 1is common to all types of stills.- , Consider a continuous still of the type shown in Fig., . Fuel carrier salt. (LiF-Bng) containing fission product fluorides is fed to the bottom of the system continuously. Most of the LiF-BeFp fed to the system is vaporized and a salt stream containing most _77 of the nonvolatile materials is withdrawn continuously. The S positive x direction will be taken as vertically upward and the liquid withdrawal point and the liquid surface will be located at _ x = 0 and x = £, respectively. Assume that above the liquid ‘withdrawal point, molten LiF containing rare earth fluorides (REF) flows upward at a constant velocity V. At the surface, -a fraction | v/V of the LiF vaporizes and the remaining LiF is returned to the bottom of the still, o Above the withdrawal point, the concentration of REF satisfies | .the relation ' ' | dec dc DaxZ "~ Viax oo and the boundary conditions are: | at x = £ -D = + VC_ = V¢ ; (Vv - v)C (55) x = 8 s 8 : vk Recirculating. Salt V-V, c . \.Figo- llo _ ORNL DWG 67-29 | ____» Produ X T x=t I | l ct Salt Wnthdrowal Point > Dlscord Salt F-v, C Continuous Still Having External Circulation and & Nonuniform. Liguid Phase _Rare Earth Fluoride Concentration Gradient. 38 and at x = 0 -p £ Ix‘=‘ UG = (V- WG FC - (Fov)e, (56) where o , o D= diffusivity of REF in molten salt of still pot eoncentretion,' ( cmzlsec A o - - C.= concentration of REF in molten salt at position x, moles REF/cm® salt | | X = position in molten salt‘measured from liquid withdrawal point, cm ' ' V = velocity of molten salt with respect to liquid surface, cm/sec ' ' ' -cs;= concentration of REF at X = 3 moles REF/cm salt ‘em® LiF (1liquid) | cm® vaporizing surface-.sec O = relative volatility of REF referred to LiF 'co = concentration of REF at x = O, moles REF/cm® salt Cs Equation Sk has the solution FC {1 -3 (1 - @) [1 - exp(-v(¢ - X)/Di v.= LiF vaporization rate, . = concentration of REF in feed salt, moles REF/cma salt . v 1-3(1- @) [L - exp (-v2/D)] C(x) = (57T W + (F - v) {1 -¥(1-a)1 - exp(-Vfl/D)]} The fraction of the REF removed by the still is (F - v)C fraction REF removed —_— : 'FCf = = — o (8) F -v + ‘ The fractional removal of REF for a continuous system hav1ng a perfectly'mixed liquid phase was derived earlier and is given by Eq. 23. The ratio of the fractional removal of REF in a system 29 ‘having a nonuniform_concentration to that in a still having a uniform concentration will be denoted as ¢ and can be obtained by - dividing Eq. 58 by Eq. 23. Thus :;li+ ¥ < a i i ~ (59) 8! i - 1 - %’-.-(1 = a)[1 - exp(-V4/D)] 1+ Values of ¢ calculated for"a still in which 99.5% of the LiF fed to the still is vaporizedj(v/F - v = 199) and in which the | relative'volatility.of'REFis.S.xth-* are given in Fig. 12. The following two effects should be noted: -’ 1. The value of ¢ is essentially unity for V£/D < 0.1 for any value of v/Vf(fraction of LiF vaporized per circu- ‘lation through'still) Within this region, a near uniform REF concentration is maintained by diffusion of REF within the liquid and mixing by liquid circu— lation is not required. 2. The value of ¢'is?strong1y dependent on v/V for - VE/D > l.-7Within thisrregion, a near uniform REF concentration can beJmaintainedronlygif liquid circu- ‘lation is-provided" For V£/D = 100, @ has a value -of 0.0055 with no 1iquid circulation and a value of 0. 99 1f 90% of the LiF is returned to the bottom of the still h,fiAn actual still would probably operate in the region Vfl/D > 1lso '.that the importance of liquid circulation can not be over emphasized Liquid phase mixing by circulation is believed to be an essential B feature of an effective distillation system. In the case . considered - *.circulation was .provided by an external loop for mathematical con-t '*'Vvenience. In an actual still internal circulation could be prov1ded by a toroidal liquid flow path which for a liquid having ‘a strong volume heat source (provided in the present case by fission product decay), would result if more cooling were provided to the still in an outer annular region than in the center. . 'ORNL DWG 67-30 ‘.0"- 7111[ \\_l I I3 . o v | - ~ NG =01 : Nyt cos i h v 0.9 DLI S o 0.0' — ——— - \V 'nr'- ' - - v . 0.001 L1t 1 ratal 1 P 1 1’1'5' L 4 ;'1'i‘t KX v w0 D 100 D Fig. 12. Ratio of Fraction of Rare Earth Fluoride Removed in Still Eeving Nonuniform Concentration to That Still Having Uniform Concentration. | | ' 41 6. CONCLUSIONS ‘AND .:RECOMENDATIONS " The following conclusions. have been drawn from the information considered in- this report- 1. Distillation"at'1ow.pressure.allows.the‘adequate"renoval - of rare earth fluorides from MSBR.fuel salt and the ~simultaneous recovery of~more than 99.5¢ of.theafuel salt. Recently reported relative volatilities of several rare - earth fluorides allow a great deal of latitude in still design and‘operetional mode. A single contact stage such as a liquid pool is adequate and rectification is not necessary. The effectiveness\offa distillation system can be'seriously decreased by the buildup of rare earth fluorides at-the surface of the vaporizing liquid. Liquid circulation can provide adequate liquid phase - mixing and is an essential feature of an effective dis- tillation gystem. The following recommendations ‘are made- jlo Further consideration should be given to the use -of single- stage,.continuous distillation for removal of rare.earth - fluorides from=the fuel~stream of an MSBR. - dThe ‘study of liquid phase temperature and concentration 'profiles ‘should be extended to stills having configurations of interest for‘MSBR processing. ‘Methods :should be gdevised for the calculation of velocity, temperature and concentration in the liquidiphase of a three dimensional i, -still'havingVa_distributed-heat source, The effect of .'_rvariations;in heatigeneration rate ‘should be considered. 3. Removal of fission product decay heat from a distillation 42 system should be étudied. Heat removal_systems which ‘maintain the temperature within acceptable limits in the event of failure of the primary cooling-systém-should;be?v,