Removal of TBP From Aqueous Phase

Removal of TBP From Aqueous PhaseABSTRACTPUREX process involves the use of 30% TBP in Dodecane to extract the fissile materials. However, referable to mutual solubility some amount of TBP gets transferred into the aqueous phase. This transferred TBP leads to many environmental problems. Removal of this TBP from aqueous phase is of prime concern which can be through by contacting it with an organic diluent. UNIFAC and Uniquac methods develop been apply to describe Liquid Liquid equilibrium (LLE) in TBP-Diluent-HNO3 system. Uniquac and UNIFAC Group interaction parameters have been founded to fit the experimental data. non-homogeneous alloy nitrates are also present in the organic phase. These metallic element nitrates affect the solubility of TBP in aqueous phase. Metal nitrates like Sodium and atomic number 20 nitrate have also been incorporated in model to find out interaction parameters in the front end of metal nitrates like Sodium and Calcium. The obtained parameters w illing be useful in predicting LLE for the above system and will back up in safe disposal of nuclear waste.INTRODUCTIONReprocessing of the used nuclear give the axe has always been carried out to reduce the volume of high level radioactive waste and also for their safe disposal. The plutonium uranium extraction (PUREX) process is widely used for reprocessing. This process uses 30% Tri-n-butyl phosphate (TBP) in an inert paraffinic diluent for the separation of uranium and plutonium from the aqueous phase containing nitric acid. Mutual solubility of aqueous and the organic phase leads to the transfer of certain finite amount of TBP in aqueous phase. This transferred TBP decomposes very slowly in the presence of water and nitric acid by hydrolysis to lower organo-phosphate acids at normal operating temperatures leading to many environmental problems. Removal of such dissolved TBP is of direct interest in reprocessing processes as this would enable manifold evaporation of aqueous s tream without harming the environment. Various metal elements are also present in the highly radioactive molten waste solution in nitrate form. The salting out of TBP takes place in the presence of these inorganic nitrates in the aqueous phase. Studies in the presence of several(a) metal nitrates will aid in efficient removal of TBP from aqueous phase.In order to predict the extent to which TBP could be removed from aqueous phase, a model must be developed to predict the phase behaviour. Such models can be used for designing remediation projects. Estimation of activity coefficients of the mixtures is chief(prenominal) for predicting the phase behaviour In order to predict the extent of mass transfer, chemical compositions of the two-phase system at equilibrium needs be predicted first.Investigators have used various models to predict the LLE. Cheng et al. have work out the thermodynamic equilibrium constant for the system HNO3-TBP-n-C7H16.The activity coefficient of nitric acid was calculated using Pitzers compare and those of the components in organic phase was derived from experimental data. Ding et al. have calculated the activity coefficient for 20 binary and 7 ternary systems composed of nC6H14, nC7H16, nC8H18, C6H6, cy-C6H6, CCl4, CHCl3, (C4H9)3PO4 and UO2(NO3)2.2((C4H9)3PO4) using head-space throttle valve chromatography. The results are compared with Scatchard-Hilderbald, NRTL and UNIQUAC models. Li et al. has calculated the Vapor-Liquid and Vapor-Liquid-Liquid equilibria of 19 tributyl phosphate systems. Aqueous phase activity coefficients are calculated using Pitzers equation. UNIFAC method is used for correlating and predicting the data in organic phase. All the above authors have done work on the systems in the absence of metal nitrates. Interaction parameters for systems in the presence of metal nitrates have not been reported yet.The objective of the present work to find the UNIFAC and Uniquac class interaction parameters regressed for the experimental data for three systems. These systems comprise of TBP-diluent-HNO3, TBP-diluent-HNO3-NaNO3, TBP-diluent-HNO3-Ca(NO3)2. These parameters will aid in predicting the equilibrium and calculating the number of stages for designing the equipment to remove dissolved TBP.LIQUID LIQUID EXTRACTION EQUILIBRIAThe organic phase consists of (1) diluent (NPH), (2) TBP, (3) TBP.HNO3The dissolution of TBP in NPH and HNO3 can be represented by eq 1and eq 2.xTBPorg (TBP)x org(1)TBPorg + H+aq + NO3-aq HNO3.TBPorg (2)where the subscripts aq and org denote the species in the aqueous and organic phase.The thermodynamic equilibrium constant for response (2) can be calculated asa HNO3.TBP(org) a 3 (3)K ==a H+(aq) a NO3-(aq) a TBP(org)a2HNO3 a 2x3 3K = (4)m2 2 x2 2where a is the thermodynamic activity, x is the jetty fraction of the component in the organic phase and is the check activity coefficient. m is the entertain molality concentration of electrolyte in the aqueous phase and is th e mean ionic activity coefficient of corresponding electrolytes. Similar procedure as described by Chen et al has been used to predict the LLE with one extraction reaction.Mean ionic activity coefficient of electrolytesPitzer equation is used to calculate the mean ionic coefficient of HNO3 in all the cases and of Ca(NO3)2, NaNO3 in the presence of metal nitrates.ln = A+ m (5)where b=1.2, =2, A = 0.391. I is the ionic strength of solution. The Pitzer parameters for HNO3, NaNO3 and Ca(NO3)2 are listed in Table 1.Activity Coefficients of components in organic phase can be estimated using UNIFAC and Uniquac equation.UNIFAC EQUATIONIn a multi-component mixture, the UNIFAC equation for the activity coefficient of component i is given by Equation 3.1.ln i = ln iC +ln iR (6)The integrative part of the UNIFAC model considers the shape and the size of the molecules in the mixture.ln iC = 1 Ji + ln Ji 5qi( ln (i /i )+ 1 (i /i ) ) (7)where, Ji = ( i /xi)The molecule volume fraction i , and the molecule wax sphere fraction i , are given byrespectively,i = xi*ri/j xj*rj and i = xi*qi/j xj*qj (8)In Equations (3.3), relative molecular(a) volume rs, and relative molecule surface area q, are given byri = k k(i)* Rk and qi = k k(i)* Qk (9)The quantity vk is the number of subgroups of part k in a molecule of species i. ri is the relative molecular volume and qi is the relative molecular surface area. Group parameters Rk and Qk are reported by Fredenslund et al.The eternal rest part of the activity coefficient is given by Equation 10.ln iR = k k(i ) ln k ln k(i) (10)where,k denotes each group in the mixturek(i ) is the number of groups of type k in molecule ik is the group residual activity coefficientk(i) is the residual activity coefficient of group k in a credit rating solution containing only molecules of type i.The group residual activity coefficient is found by Equation 11.ln k = Qk 1- ln(m m mk) m (m km /n n nm ) (11)where,Qk is a group surface area pa rameterm is the area fraction of group mmn is the group energy of interaction parametermn = exp(-amn + bmn/T) (12)Where,amn and bmn is the group-interaction parameter.T is the temperatureGroup assignment as proposed by Chen et al. has been followed. C7H16, TBP and TBP.HNO3 has been broken down into groups CH3, CH2, (CH2O)3PO, HNO3.(CH2O)3PO. Group volume and surface parameters for above groups have been reported by Cheng et al.The UNIQUAC model thus consists of two terms a combinatorial or entropic term, a residual or enthalpic term.The combinatorial and the residual terms are identical to the terms used in the traditional UNIQUAC equation. The combinatorial, entropic term isln iC = ln(i/xi) +1 (i/xi) (z/2)*qi ln(i/i) + 1- (i/i) (13) z = 10 is the co-ordination number. xi is the mole fraction, i is the volume fraction, i and is the surface area fraction of component i.i = xi*ri/j xj*rj and i = xi*qi/j xj*qj (14)ri and qi are volume and surface area parameters for component i. The residual, enthalpic term isln iR = qi 1 ln( k kki) k kik/(l llk) (15)The parameter ki is given byki = exp(-uki-uii/T) (16)uki and uii are interaction energy parameters.Uniquac r and q parameters for C7H16, TBP and TBP.HNO3 have been reported by Li et al.RESULTS AND DISCUSSIONEQUILIBRIUM PREDICTION.The following equations can be used to predict equilibriumx1 + x2 + x3 = 1 (17)x20 = (x2 + x3)/( x1 + x2 + x3) (18)x3 3K = (19)m2 2 x2 22 and 3 values have been calculated using UNIFAC and Uniquac equation. Knowing equilibrium constant K, m calculated mole fractions can be found out., The group interaction parameters are regressed by Least Square Technique to minimize the error between experimental and calculated mole fraction values. The obtained regressed group interaction values in the absence of metal nitrates, in the presence of NaNO3 and Ca(NO3)2 using Uniquac and UNIFAC have been listed in Table. The standard implicit deviation of components in the organic phase is listed i n table. The experimental and calculated values of mole fractions have been reported graphically in fig ratiocinationThe experimental mole fraction data were correlated using UNIFAC and Uniquac model. The Uniquac and UNIFAC group interaction parameters are capable of predicting mole fraction for TBP-Diluent-HNO3 in the absence and presence of metal nitrates. Thus these can be effectively used to predict the equilibrium for the removal of dissolved TBP in Nuclear engineering. UNIFAC gives a better expectation as compared to Uniquac in all the casesLIST OF TABLESPitzer parameters for calculation of mean ionic activity coefficientUniquac Group interaction parameters in the absence of metal nitratesUnifac Group interaction parameters in the absence of metal nitratesUniquac Group interaction parameters in the presence of Sodium metal nitrateUnifac Group interaction parameters in the presence of atomic number 11 nitrateUniquac Group interaction parameters in the presence of atomic numb er 20 metal nitrateUnifac Group interaction parameters in the presence of calcium nitrate sample absolute deviations between predicted and experimental mole fraction of extracted complexesTable 1. Pitzer parameters for calculation of mean ionic activity coefficientComponentsooHNO30.11190.32060.001NaNO30.00680.1783-0.0007Ca(NO3)0.21081.409-0.02014Table 2. Uniquac Group interaction parameters in the absence of metal nitratesa (m,n)NPHTBPHNO3.TBPNPH02.057410.679624TBP-3.045490-1.1645HNO3.TBP-1.033281.1439790b (m,n)NPHTBPHNO3.TBPNPH01.0078420.999125TBP0.98353201.095824HNO3.TBP0.9931731.0196490Table 3. Unifac Group interaction parameters in the absence of metal nitratesa (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2002.56180892-48.1996CH3002.56180892-48.1996(CH2O)3PO-5.15816-5.158160-6.13375HNO3(CH2O)3PO-2.57271-2.5727134.7816850b (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2000.9973091.126187CH3000.9973091.126187(CH2O)3PO1.0168191.01681901.131422HNO3(CH2O)3PO1.0153451.0153450.8903010Table 4. Uniquac Grou p interaction parameters in the presence of Sodium metal nitratea (m,n)NPHTBPHNO3.TBPNPH0-1.826236.863001TBP0.8782100.552002HNO3.TBP1.3806762.2726630b (m,n)NPHTBPHNO3.TBPNPH00.9907711.019671TBP0.99959200.998537HNO3.TBP1.0012811.0042690Table 5. Unifac Group interaction parameters in the presence of sodium nitratea (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2000.742770-0.6378CH3000.742770-0.6378(CH2O)3PO1.0964261.0964260-0.373895HNO3(CH2O)3PO0.7481110.748111-0.209662060b (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2001.0008351.005434CH3001.0008331.005434(CH2O)3PO0.999680.99968801.004477HNO3(CH2O)3PO1.0008171.0008181.0039270Table 6. Uniquac Group interaction parameters in the presence of Calcium metal nitratea (m,n)NPHTBPHNO3.TBPNPH00.3249-0.4026TBP0.952210-1.40706HNO3.TBP1.1685451.397970b (m,n)NPHTBPHNO3.TBPNPH00.9978780.99529TBP0.99983600.99200HNO3.TBP1.000581.0001380Table 7. Unifac Group interaction parameters in the presence of calcium nitratea (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2005.94292.14979CH3 005.94292.14979(CH2O)3PO2.69322.69320-2.59369HNO3(CH2O)3PO3.88893.88893.87400b (m,n)CH2CH3(CH2O)3POHNO3(CH2O)3POCH2000.98390.99622CH3000.98390.99622(CH2O)3PO0.994470.9944701.011186HNO3(CH2O)3PO0.9906120.9906120.9906330Table 8. Standard absolute deviations between predicted and experimental mole fraction of extracted complexes.System x (Uniquac) x (Unifac)HNO3/TBP/Diluent0.0160.0051HNO3/TBP/Diluent/NaNO30.04350.0429HNO3/TBP/Diluent/Ca(NO3)20.0150.0051