Electro-reduction pulsed column for the purex-process: Operational and theoretical results

Chemical EngineeringScience,Vol. 41. No. 4, pp. 981-988. 1986. Printedin Great Britain.

@

ELECTRO-REDUCTION PULSED OPERATIONAL AND

G.Petrich.

U.Galla,

COLUMN FOR THEORETICAL

0009-2X9/86 53.00 + 0.00 1986.Pergamon Press Ltd.

THE PUREX-PROCESS: RESULTS

H.Goldacker,

H.Scbmieder

Institut fir HeiSe Chemie Kernforschungszentrum Karlsruhe D-7500 Karlsruhe, F.R.G. Postfach 3640,

ABSTRACT Electrochemical in-situ reduction of plutonium and uranium for the separation of U/Pu and purification of Pu shows all the advantages of the external feed U(IV)-process but avoids its In continuation of our previous work with electro reduction mixer settlers (EMMA) drawbacks. of technical scale was designed and installed in a an electro reduction pulsed column (ELKE) Total throughput amounts pulsed column test facility (PUTE) and is in operation since 1982. to 270 kg Pu up to now. ELKE operated reliably both for U/Pu separation in the 1st Purex extraction cycle and for Pu purification in the 2nd extraction cycle. The wide range of operational parameters indicates a technical process of high inherent stability. Theoretical predictions for a process without the need for hydrazine under certain flowsheet conditions were confirmed. leading to the computer model VISCO for the Theoretical work was conducted in parallel, simulation of countercurrent extraction in mixer settlers and in pulsed columns. The paper includes the basis of the model and the design principles of the electroreduction column and some operational results. A sensitivity analysis of this extremely complex process involving countercurrent multicomponent extraction with coupled chemical and electrochemical redox reactions is presented. KEYWORDS Purex process; solvent extraction; plutonium; computer model; computer

separation simulation.

process;

pulsed

column;

electrolysis;

uranium;

INTRODUCTION The Purex process is based on the highly selective extraction of uranium(VI) and solution of tributyl phosphate in kerosene. All plutonium(lV) with usually a 20 or 30 vol.% extraction and reextraction operations are performed continuously in counter-current U(V1) and Pu(IV) and a small fraction of the fission products are coextracted in extractors. a first (highly radioactive) extraction cycle. After partially scrubbing the fission products plutonium is reextracted by adjusting its from the loaded solvent in further extractors, valency: in contrast to Pu(IV) the trivalent species of Plutonium is characterized by an extremely low extractability from the aqueous into the organic phase. Therefore, the basis of U and Pu separation is found in the order of organic to aqueous distribution coefficients D: Du(VI)

= DPu(IV)

= DHN02

'

DU(IV)

' DHN03

"

DPu(III)

= Dfission

products

to Pu(III), By reducing Pu(IV) separation is performed in the first extraction cycle of most Purex flowsheets. Purification of Pu then occurs in the second and third Pu extraction cycles by extracting Pu(IV) after reoxidation into the organic phase with a following backstrip under reductive conditions. Chemical reductants are almost exclusively applied in industrial plants.

URANIUM-PLUTONIUM Four

U/Pu

separation

SEPARATION

processes

were

PROCESSES

developed

to

technical

o Ferrosulfamate process, e.g. Walser (1970); q Uranium(IV) process, e.g. Jenkins (1959), Schlea(1963); o Hydroxylamine nitrate (HAN) process, e.g. Orth (1971), q Electra reduction process, e.g. Baumgartner (1978);

maturity:

Patigny

(1974);

Ferrosulfamate-process This process is well-proven the plants. Unfortunately,

with regard to separation oxidation product of the

981

efficiency and reductant cannot

reliability be recycled,

in different however,

G.

982 and may cause corrosion (20 to 40 fold). Waste concentrations. Hydroxylamine

problems. costs are

PETRICH

I-2

et al.

Furthermore huge excess amounts of therefore intolerably high for fuel

reductant are with large Pu

necessary

nitrate-process

Principal advantages of this process are the possibility of complete decomposition of the reductant to inert gases and the large reduction rate. This rate is drastically reduced, with the square of the Pu(III) concentration and the fourth power of the nitric acid however, concentration which are reaction products. Therefore, for a technical process elevated temperature and a large reductant excess are needed. Furthermore Pu complexed by dibutylphosphate can only be incompletely stripped with this reductant. in contrast to U(IV) as reductant. Maximum Pu concentration factors are 2.5-3. Not well understood is the role of hydrazine which is usually additionally fed close to the organic feed point. Market availability of concentrated HAN solutions is limited. Uranium(IV)-process The extractable reductant U(IV) should be ideally suited for a high separation efficiency. In practice, however, separation deteriorates at least in mixer-settlers due to the autocatalytic oxidation reaction of Pu(II1) with nitrous acid and the combined oxidation of U(IV) in the organic phase. For plant operation a large excess of U(IV) is normally applied (4 to 6 fold stoichiometry) and at least two U(IV) feed-points are necessary. This reduces also the danger of a local depletion of the stabilizer and reductant which was occasionally observed as a spontaneous malfunction in several plants. Along with the U(IV> production cell (fed with about 30-60X of recycled product uranium for Fast Breeder Reactor fuel) this means significant additional equipment. Electra-reduction-process This process shows all the advantages of the U(IV)-process but avoids its drawbacks. Neither product-U recycle nor additional U(IV) feeds are necessary. Local depletion of stabilizer and reductant has never been observed in the experiments (the reductant is homogeneously produced within the extractor from process inherent U). Under certain flowsheet conditions the stabilizer hydrazine may be avoided. The electrodes (Ti, Pt) show a very high corrosion stability. Advantages and Further details 1985).

drawbacks of the of a quantitative

different process

The

development of the electra-reduction After the period of initial seventies. technical scale equipment was developed: q

an electro-reduction plant WAK for the

mixer settler second plutonium mixer extraction

processes comparison

are schematically were previously

process was experimentation

(PB-EMMA)

in our institute laboratory scale

(lB-EMMA) was constructed of WAK since 1985.

q

another electro-reduction installed in the first

q

an electro-reduction pulsed sieve plate column (ELKE) of pilot plant scale with a throughput of 100 kg LWR fuel per day was installed in the institute's pulsed column facility (PUTE) and operates as either a partitioning column (lB-ELKE) or a Pu reextraction column (>B-ELKE) since 1982. HAN

Fe(ll) separation process Pu

efficiency stability

concentration

rcd”c+=“t PU,,“, reductant stabilizer &=“(I”, Pu-HDBP

++

++

++

++

-

++

+

++

-

-

++

++

++

?

E

disposal

tte.z feed

-

+

-

--

---

strip

operating

temperature

corrosion

problems

reductant

+

waste

--

equipment

costs

-

--

costs

+

+-c -

favorable

+

-

conditions

+

-++

--

+

unfavorable

and

Electra

U(IV)

+

-recycle

tested

++

factor

feed feed

and

++

+

+

++

++

_

+

++

++

conditions

I Fig.

1.

Comparison

of

different

U/Pu

separation

processes.

1983;

in the mixer settlers

was installed in the German reprocessing cycle and operates successfully since

extraction

settler cycle

started with

shown in Fig. 1. reported (Petrich

1979.

is

test

983

Purex process

I-2 While results (e.g. Schmieder 1982c)) of the 1982a, 1982b, the present work will extend THE The design described follows:

1983)) and theoretical basis (e.g. (1974, 1980, electro mixer settlers were described in several basis and results to electro pulsed columns.

ELECTRO-REDUCTION

COLUMN

Petrich Papers

(1981a, before,

ELKE

principles of the electro-reduction Schmieder (1981) and before, e.g.

pulsed Goldacker

sieve plate column (1983) and can be

ELKE (Fig. 2) summarized as

were

no diaphragms are used in ELKE to separate catholyte and anolyte, since reoxidation is sufficiently reduced by an appropriate anode design; casing and sieve trays are titanium made and serve as cathode; the platinized tantalum central rod serves as anode; insulation between cathode and anode is achieved by the use of highly sintered ceramic material; the electrode gas generated by the process contains hydrogen and is separated from the liquid phases by means of metal sheets in the top decanter and diluted with air.

-

pruduuorpMd0

mod0 imulclw cclhdc.

Fig.

2.

Principles

of

the

electro-reduction

chvc

plcloc

pulsed

column

(ELKE).

ELKE started operation as an electrolytic uranium reduction column in 1981. In 1982 ELKE was first operated as a 2B-column for plutonium reextraction in the plutonium purification cycle. The main parameters under investigation were the Pu to U feed ratio and the organic to aqueous flow ratio. Plutonium decontamination factors (DF) ranged from 2.000 to 900,000 depending on flowsheet conditions. Even at the highest Pu/U ratios of about 16 and at the highest Pu product concentrations of about 24 g/l, excellent separation was obtained. As an example Fig. 3 shows the measured and calculated concentration profiles of one of the test runs. The series of 2B-ELKE experiments was continued in 1984. Keeping the flow ratio in the range 2.5 to 3 and the Pu to U feed concentration in the range 1.5 to 4, the product concentration was increased up to a value of 40 g Pu/l. Total throughput varied between 20 and 50% flooding capacity and was limited by the PUTE C-column capacity for uranium reextraction. Typical Pu decontamination factors of several 10' were observed. In 1983 ELKE was operated under 1B conditions, i.e. U/Pu separation in the first extraction cycle of the Purex process. Pu product concentrations ranged from 2.4 to 8 g/l with organic Pu raffinate concentrations between 0.5 and 5.5 mg/l. The experiments proved the theoretical prediction that process specifications can be met by the lB-ELKE even in the absence of the stabilizing agent hydrazine if the nitric acid concentration is kept below 0.5 M. The high Pu raffinate losses were observed at the maximum organic to aqueous flow ratio of 8.2 where ELKE had to operate at only 30"ia of the flooding capacity; this resulted in a mean residence time of the aqueous phase of 8 hours. The experiment proved the dominance of axial mixing effects at exceedingly low superficial velocities of the continuous phase. BASICS

OF

MODELLING

THE

Great care was taken to separate the This ensures maximum independance of shows schematically the interactions

ELECTRO

REDUCTION

COLUMN

individual modules of the model as much as possible. the submodels from particular process steps. Fig. 4 considered by the model. In contrast to mixer-settlers

G. PETRICH

984 we such

have as

column,

to

consider

pulsed

the

a continuous

columns.

lighter

The

organic

heavier

phase

L-2

et al.

space variable z for COntinUOUS counter-current eXtraCtOrS aqueous phase (concentrations x3 enters at the top of the (concentrations y) enters at the bottom and flows upward.

The Purex process requires a division of most of its pulsed columns into individual sections by either adding additional center feeds to a column (multifunction column, such as an extraction section plus a scrubbing section) or by splitting one column into several separate columns coupled to one common counter-current flow system. Discontinuities in geometry (e.g. diameter or electrodes) result in a further division for the model. Each section is modelled individually while numerical integration of all sections has to be performed simultaneously. Each solute i contributes to each section one pair of parabolic equations to the system:

Q,axi/at Qyayi/at

The

= QXEXa2xi/a22 = QyEyaayi/a2Z

+

amxxi)/az

- aWyYi)/'b

- oQ,B~(D.x.

11

+ oQyBi(Dixi

- Y.) 1

+ RR xi

- yi)

+ ZR

. Yi

submodels

for the distribution coefficients Di and the mass transfer coefficients 6. are 1 below. The coefficients Q are the holdup fractions of the column cross section the two phases, o is the interfacial area per unit volume, F are the volumetric flow the phases. E are the dispersion coefficients of the dispersion model which is used phases, approximating the axial mixing in the column. The additive terms IR. 1 describe the additional chemical reaction kinetics for solute i encountered in modelling the Purex partitioning and purification columns (B-columns) and are detailed below.

described taken by rates of for both

Pulse amplitude and pulse frequency enter the model equations only indirectly via the dispersion coefficients E and the holdup fractions of the two phases, and the drop size. Britsch (1983) measured dispersion coefficients and holdup fractions under Purex conditions as a function of flow rates and pulse parameters. His results cover both regular pulsed columns, and electro pulsed columns with axial mixing increased by the electrode gas. No reliable method to predict drop sizes in pulsed columns of the Purex process is known to us. A maximum of the drop diameter distributions of the order of l-2 mm was usually observed in our laboratory.

Several simplifying sssumptions are made for the model due to our present lack of relevant experimental data: D no concentration gradients over the column radius for both phases; o droplet concentrations are uniform over the drop radius; o mean diameters are used instead of drop size distributions; o dispersion coefficients, holdup fractions, and drop diameters are kept constant for each column section. The use of the dispersion model to represent random velocity fluctuations superimposed on plug flow is not believed to be adequate for the drop phase (Levenspiel, 1983). Numerical experiments for different Purex flowsheets showed, however, that axial mixa of the drop phase has almost no influence on concentration profiles in this system, due to the large linear velocity of the drops. q decanters for phase separation at the top or bottom of a column are not yet taken into account (dispersed phase holdup variation unknown). 28-ELKE

20.09.02

12:30

I

Fig. 3. Example of measured and theoretical concentration profiles in the ELKE electro pulsed column under conditions of the 2nd Pu
985

Purex process

I-2

The numerical solution of the partial differential equations given above is complicated by the fact that we are dealing with a multi-point boundary value problem of the Dankwerts type. Under certain circumstances the equations may be of considerable stiffness and are difficult From a number of trials it appears that solving the equations is fastest by to solve. This may lead to numerical instabilities and employing a central differences approximation. our computer code VISCO then switches to a one-sided differences approximation. In this case the number of pivotal points has to be increased in order to account for the loss in accuracy. In spite of the model well with experiment flowsheet conditions. Distribution

simplifications and the model

the numerical validated in

was

results numerous

do generally agree cases for a great

sufficiently variety of

Model

The basis of all our modalling efforts was to collect measured distribution data from the literature and to fill in the sparsely populated concentration regions with our own By setting up two basically different numerical models and measurements (Petrich, 1981b). grouping the data in the 5-dimensional space spanned by the independent variables U, Pu, TBP and temperature it was possible to discriminate against the unavoidable far-out HN03. results

and

to

quantify

the

experimental

scatter

of

the

by

data.

Simulation of the reductive uranium-plutonium separation and of the second and third plutonium extraction cycles requires an extension of the above model to include nitrous acid and hydrazine nitrate. Along with additional macro-quantities of U(W), Pu(III), extensions for Np(IV) and Np(V1) the submodel for the distribution equilibria is now generally used in our routine calculations. Mass

Transfer

Kinetics

From the work of Baumggrtner Pu(IV) and HN03 B for U(VI), time

change

of

the

drop

and Finsterwalder were derived. B

dc drop/dt Here

o

is

the

ratio

U(W)

PulWl

of

c

concentration

drop

reduction

surface

at

=

(1970) the overall mass transfer coefficients defined by the first order relation for the

drop:

o-8.1 to

is

drop

cdrop(t

=

-)

- cdrop(t)

1

volume.

cathode

rsductian at cathode

PulIN) oxidation at anode Pu(IV) reduction by UUV) Pu(IV)

reduction

by

hydrazinc

Pu (1111 PulWi) oxidation by nitrite lautocatalytk)

HNO, HNO,

nltrlte

destruction

by hydrazine mass

hydrazine

oxidation at

anode

\ t

transfer

=f(concentrationa temperature.

axial nixing = f tpu1se .flows,gtonttry, electrode gas1

of fluiddynamic Fig. 4. Schematics phases of an electro pulsed column

and chemical of the Purex

interactions process.

in

the

bulk

and

between

the

G.

986

The

original data of BaumgXrtner (1970) covered New measurements (Petrich, 1981a) for dodecane. correlation (Petrich, 1980) B = with

without

any

calculated

further from

tuning

the

For the separation additional species The

successful

Chemical

Reduction of d[Pu(IV)]/dt

0 Reduction

q

of

0

=

of

=

k=O.O38/min,

EA=93

reduction =

of

of

Reaction

of

Reaction

of

and

KU

constants

are k

readily

to

justify

this

assumption.

columns, redox

the

1959) to

Barney

be

a

factor

(1976):

Xoltunov

(1974):

k(Pt-catalyzed)=k+0.22*Rc

=

by

HNO2

(Heilgeist,

A

=59 by

stirred)=O.23

cm/min

stirred)=0.25

cm/min

(1980): cm/min

Schmieder k(35'C,

(aqueous

(1980): Pt-anode.

phase),

Dukes

(19601,

with

HN02

(organic

hydrazine,

=

phase),

Biddle

hydroxylamine

Biddle

(1965). =

modified: -20d[HN02]/dt

(1968):

=

nitrate,

Barney

(1971):

-k-[HN02]'[NH30H+J~[HN031

kJ/mol

and for

-2*d[HN02]/dt

-60000'[HN02]'[N2H5+]*[HN03]2

=

EA=48

hydrolysis

modified:

kJ/mol

-exp{7.28-([HN03]+0.8)}~[Pu(III)]~[HN02] with

1983)

(1980): Ti-cathode,

-k'[Pu(III)]'[HN02]'([HN03]-0.4)o[N03-]

d[NH30H+]/dt

given

Schmieder k=0.008

of Pu(XII), -kmRa*[Pu(III)],

E

HN02

are

seems

transfer kinetics and pulsed are selected from the following the reaction stoichiometries.

nitrate,

k(35'C,

k=600/(mol'~min), Kp

is

drop

above.

transfer model is also used for the no separate transfer measurements exist.

extractors

(Newton,

Schmieder

Pu(II1)

=

kJ/mol

hydrazine,

= d[N2H5+]/dt

d[HN02]/dt

this which

accontjac

slope

well known. It is assumed reaction (Biddle 1965).

Pu(IV),

HN02

d[HN02]/dt

The

kJ/mol

kJ/mol. of

=

d[Pu(III)]/dt

D

by

Pu(III)

d[Pu(III)]/dt

q

these

-k*Rc*[Pu(IV)],

k=144/(mol'*min), o Oxidation

of

8.

discussed

[NH30H+]2/~[Pu(III)]+*[HN03]Y~([N03-]+Kp)~)

Cathodic reduction of U(VI), d[U(VI)]/dt = -k=nc~[U(VI)],

Oxidation

and

-k'[Pu(IV)]'[N2H5+]/([HN03J+Kp)

D Anodic reoxidation d[Pu(III)]/dt = q

a

model

extractors and HN02 for

hydroxylamine

EA=130

Pu(IV)

d[Pu(IV)]/dt q

by

-k*[Pu(IV)]='

d[Pu(IV>]/dt

Cathodic

EA=104 is not aqueous

Pu(IV)

mo15/min,

Reduction

parameters

distribution

submodels for phase equilibria, for the B-column calculations balances are calculated from

reaction than the

d[Pu(IV)]/dt kc1.74

the

simulation

mol/min,

organic slower

]' X=0.63+-0.16

Pu(IV) by U
k(aqu)=15200 The 100

cont'acdrop

5 to 20 weight% TBP in (30 ~01%) fit the general

Kinetics

In addition to the reaction equations reactions. Material q

of

and purification U(IV), Pu(III)

Reaction

the range from 36 weight", TBP

cz=0.0038+-O.O009cm/sec,

equilibrium

numerical

ac

a-(

I-Z

et AI.

PETRICH

dissociation

3O'C.

Rc

and

constants Ra

are

the

for

Pu

specific

and

U

cathode

respectively. and

anode

The areas

reaction of

the

for electro-reduction of U and Pu were measured elactro-process in l/cm. The rate constants in individual laboratory experiments (Schmieder, 1974; 1983). These constants are functions of the degree of agitation of the fluid and of the applied current. The constants used in the model are estimated from the average current density applied to the extractor which is assumed to be constant over the full electrode area.

987

Purex process

I-2

PARAMETER

SENSITIVITIES

The course of the experiments proved that Total ELKE throughput was 270 kg Pu up to now. to achieve the desired separation operating conditions have to be carefully chosen in order 5 were obtained by the computer code The parameter sensitivities indicated in Fig. results. VISCO. Detailed results and a step by step comparison with the corresponding experimental findings would exceed the frame of this paper. The following comments and Fig. 5 are therefore an attempt to summarize interpretation of how the Pu purification process works. Up to a certain should also be applicable to the U/Pu separation process.

in a concise degree the

form our comments

In the absence of pulsation throughput is very low and the column tends to flood. Throughput increases when pulse amplitude or pulse frequency are increased. However, axial mixing intensifies at the same time leading to smaller concentration profile gradients and therefore to decreased separation at the same time. Theoretical optimization of pulse conditions is not yet sufficiently reliable due to the present uncertainties in dispersion coefficients and interfacial area predictions. In practice ELRE is usually operated at 1 Hz and 1.5 cm/s pulse amplitude. At the standard low dispersed phase holdup values only the continuous phase axial mixing contributes significantly. The effect of axial mixing is most pronounced at low flow rates since mixing increases with residence time and concentration profiles are flattened out. Decreased dispersed phase holdup at constant flow rates lowers contact time between the Mass transfer is reduced and separation may be reduced. phases. Increased drop size at constant dispersed phase holdup decreases the interfacial area and thus reduces mass transfer for all species. Since the stabilizing agent hydrazine does not the autacatalvtic formation of nitrites by reoxidation of extract into the organic phase, Pu(II1) in the organic phase is favored by a decreased interfacial area. The ratio of Pu(IV) to Pu(III) increases, overall Pu extractability is enhanced and separation Is reduced. Increasing nitric acid concentration lowers U and Pu reextraction from the organic phase. Therefore less U(V1) is available for electrolytic reduction which takes place in the Pu(IV) reduction rate and aqueous phase only: lower U(IV) concentrations reduce the total thus separation. By an increase of the U(V1) feed concentration this effect may be compensated. Increased Pu feed concentration at constant flow rates obviously requires a larger supply in reductants for compensation. But it also increases the salting-out effects by the higher Pu(II1) concentration (Schmieder, 1981b), therefore reducing reextraction and separation. Hydrazine may be necessary to prevent reoxidation becoming dominant. But hydrazine also acts as a salting-out agent and therefore reduces separation, depending on nitric acid concentration. A rise in temperature increases reaction rates but may have complicated effects on extractability and reoxidation rate. fluiddynamics. At least for well designed flowsheets it seems to enhance separation.

lowers impact axial mixing Pulse

improves of

ITlaSS

transfer

Improves reaction rates

lowers reoxid&ion

+ -

Drop

-

/

-{+I

I

+

+

PU

Fig. 5. Primary increase of the separation. For separation.

overall effect on efficiency

+(--I

o-fbw

U

improves reductant Pullvt ratio

-

Holdup

A-flow

lowers salting out effects

+

-

_

+

•t

I I

+

effects of ELKE operating conditions on efficiency. + indicates left column operating parameter improves separation, - indicates increasing the pulse energy increases axial mixing and example,

I

that an lower thus lowers

G.

988

PETRICH

et al.

The model links the influence of an external electric field only to the reduction and oxidation rates at the electrodes. No experimental evidence was yet found that would require to consider the effect of a field to the hydrodynamic behavior of the column or to changes in conductivity arising from changes in nitric acid concentration. All phenomena described above were either predicted by the model calculations or could be verified at least qualitatively by the program after the corresponding experimental observations were made.

Results of the electro-reduction pulsed column ELKE can be highlighted as follows: q ELKE does operate reliably even under unfavourable conditions both for U/Pu separation in the first extraction cycle of the PUREX process and for Pu purification in the second Pu extraction cycle. 0 2nd extraction cycle operation (U/Pu=O.l-1) resulted in decontamination factors of 10' to 106. Pu product concentrations as high as 40 g/l were obtained. Theoretical calculations predict the possibility of even higher concentrations. 0 For 1st extraction cycle separation conditions (U/Pu=lOO) flow ratios organic to aqueous operated successfully. Pu decontamination factors of 200 to 2000 were of up to 6 were achieved. The low DFs resulted from tests where the column was operated far below its nominal capacity at residence times of the aqueous continuous phase of up to 8 h. Under certain U/Pu flowsheet conditions ELKE may be operated without hydrazine if nitric acid concentrations are kept low enough. 0 The theoretical model and its computer implementation VISCO proved to be a powerful tool for design and evaluation of experiments and flowsheets.

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