control of a continuous ion exchange system …

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ARH-SA-82t

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CONTROL OF A CONTINUOUS

ION EXCHANGE SYSTEM

R,L,HOBART

OCTOBER 20,1970

This Document was Prepared UnderAEC Contract AT (45 -1)- 2130

THIS DOCUMENT CONFIRMED ASUNCLASSIFIED

DIVISION OF CLASSIFICATION

BY (1·1116,4-)'».1.DATE 9/7-Y /71

Atlantic Richfield Hanford CompanyRichland, Washington

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1-This report was prepared as an account of work

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legal liability or responsibility for the accuracy, com-

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CONTROL OF A CONTINUOUS ION EXCHANGE SYSTEM'

R. L. HoWart

Separations Process EngineeringOperations Support Engineering Department

Chemical Processing Division

October 20, 1970

Work performed.under Contract AT(45-1)-2130 between the·

Atomic Energy Commission and Atlantic Richfield Hanford Company.

For presentation at the"Pacific Northwest Regional.AIChE Meeting, Richland, Washington.

October 23, 1970

ATLANTIC RICHFIELD HANFORD COMPANYRICHLAND, WASHINGTON

UNCLASSIFIED

DISTRIJUTION W THIS I CUMEMT IS UNLIMIT

UNCLASSIFIED ' ARH-SA-82

CONTROL OF A CONTINUOUS ION EXCHANGE SYSTEM

.

INTRODUCTION

A continuous anion exchange system has been.employed in the Hanford

Purex Plant for twelve years,to provide final purification of the

plutonium product solution. At·the time of installation,.the plant

plutonium product.contained considerable impurities, The limited

· performance of the solvent extraction systems and the·.corr6sion occur-

ring in the ..final product concentration. equipment were largely respon- i

sible for this contamination problem.

Anion exchange is,a particularly effective method for purifying and

concentrating plutonium. Nearly.all metals form cations in nitric

acid and are not attracted to.the resin. Plutonium., however, forms a=

plutonium-nitrate complex anion, Pu(N03)6 , in strong nitric acid.and

is very strongly bound by.resin (Figure 1). At low acid.concentrations

the equilibrium is reversed'and plutonium returns to the aqueous phase.

- Unfortunately, zirconium-niobium,* (Zr-Nb-95) the major fission product

contaminants of note.in .the Purex plutonium nitrate stream, are: also

attracted to the anion resin, but· are not as tightly bound as plutoniurri

- and can be "scrubbed off" the resin. without

displacing plutonium.

The chemistry and'kinetics of.the plutonium anion exchange·process

have..been.previously described in detail by Ryan. and,Wheelwrightl,2

* These radioisotopes have very similar gamma ray emission spectrums·andare.routinely measured together.

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and the early equipment.performance by Oberg and Swift.3 The subject

of this paper is the control system which is necessary to operate. the,

continuous anion exchange equipment.

PROCESS .DESCRIPTION

The·Purex Plant continuous anion exchange unit consists of:a figure-

eight loop which provides two columns- of resin (Figure 2). One. column

is,used for absorbing plutgnium from. a -(MHN03 solution and.scrubbing

the·loaded resin with 7M HN03 solution; the other for eluting product

from the resin with 16- HN03 solution. The loop contents are periodi-

- cally given, a "push" which transfers a segment of lgaded resin to. the

elution column.and a. segment of depleted resin to the absorption column., \

This push.is counter-current to the flow of. the aqueous streams in both

celumns. During resin movement, the acid composition of each column is

altered: 7M HN03 ·enters the bottom of the elution column, while lM HN03

enters the bottom ef the absorption column. A brief period of time is

then required for each column to be returned·to its ·normal operating

condition.. During this transient period, effluent from the elution

(X€) column contains a high nitric acid concentration, but no. product,

and must be diverted to waste. Subsequently, the elution column efflu-

ent decreases in acidity while the plutonium concentration increases,

and, the effluent is diverted to the product concentrator. Similar

transient conditions,occur in the lower part of .the .absorption (XA)

column. The.initial effluent stream contains dilute' nitric acid and,

therefore, has an appreciable product concentration. | This liquid phase

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is displaced early in·the time cycle aI16wing the restoration of. the

normal 7 molar acidity conditions.

RESIN · "PUSH" SYSTEM AND CONTROLS

The first consideration in. controlling the continuous anion exchange·

process is the design and instrumentation of the resin "push" system. .

Movemeht is accomplished hydraulically by using two reservoirs: one

for exerting pressure, the other for receiving displaced liquid and

resin (Figure 3). When.resin movement is desired, the reservoirs are

isolated, the large ball.valves in:the system are,opened to permit

resin movement, and air pressure is, applied to the .liquid surface in

the push tank. Liquid is·thus pushed through the equipment and up into

the resin receiver tank. This liquid fluidizes the resin beads start-

ing at the top of each column.. The pressure drop across·the.resin beds

is substantially reduced when fluidization becomes complete, allowing\

rapid-plug flow of the resin to occur. The air pressure is then re-

leased, continuity between the·two reservoirs is restored, resin in the

receiver tank· settles into the · absorption column, leg,,and the large

ball valves are closed to.isolate the two columns.

Efficient performance of the ion exchange system requires that.the

resin movement.be uniform. The sudden resin:movement.produced.by this

method necessitates precise .control of the push. A capacitive liquid

level measurement device is used in the push tank to obtain this con-

trol (Figure 4). The capacitance probe system consists of a.Teflon-

coated steel rod, a constant current source, a voltage sensitive

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relay, and a pulse frequency. recorder. The system operates.as fol-

lows:

The nitric acid solution,.st.eel rod and dielectric material form a

capacitor whose. capacitance varies directly· with the .height of.

liquid on the probe.. The constant .current source charges the capa-

citor until a predetermined voltage is reached. The capacitor

is then discharged,.generating a pulse.with a fre3uency inversely

proportional to the height of liquid on the probe.

Resin:movement.ic of·major impurlance in a continuous,ion exchange

system. The operating·efficiency of·the XA absorption column scrub

section is directly· related· to the amount of reverse fluid flow occur-

ring during the. resin movement interval. Considerable reverse flow

can nearly·eliminate the effectiveness of the scrub section. The aver-

age.upflow can, under certain conditions, approach the·.time averaged.

downflow. Important considerations relating to resin movement are as ,

follows:.

1. The XA absorption column resin loading must be limited because the

absorption reaction reduces the size of the resin beads. A resin

column with a small.average bead size is difficult to.move.because

of fluid flow restriction during the resin push.

2. The stream flow entering the columns should be continuous.if possi-

ble. Oscillating flow will tend to pack the resin columns.causing

movement difficulties when the resin push occurs. Diaphragm pumps.

are used in the·Purex Plant, so that it was necessary to install

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damping chambers to eliminate.flow oscillations.

(See Figure 5.)

3. Restricting passages in the·.ion exchange equipment .should be,·avoided.

If these areas.contain resin phase, a high fluid. pressure. drop will

occur during the resin movement. Large bore ball valves were

installed in the·Purex Plant equipment to correct this problem.

4. The resin bead size distribution is very important. If the· average

bead size is smaller· than.30 mesh size or if the fraction of beadd in the

30-50 mesh size is .appreciable., movement .problems,will.be encountered.

A special apparatus was developed in the. Purex Plant for classifying

the resin beads and resin movement behavior was. significantly

improved.

5, Air pockets within the resin.loop prevent. a.uniform fluid,movement

during the resin'push interval. Venting valves.weie,installed.on the

Purex Plant. equipment to eliminate .this problem.

ABSORPTION-SCRUB COLUMN

Operation of the absorption scrub column departs little from that·of a

stationary absorption system. The column itself consists of a 6-foot absorp-

tion section below a 4-foot.scrubbing section (Figure 6). Both the feed

and the scrub·streams contain P- HNO.3 · Periodically, the resin is moved

counter-current to·the process streams in order.to expose, fresh resin to.

the feed stream and to move loaded.and scrubbed resin:toward the elution

column. The streams flow as shown on·the diagram. during the initial 6

minutes.of the programmed cycle. Resin movement occurs.during the 7th

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minute and requires that the XA:column feed and waste valves be closed.

Process control of the· XA absorption-scrub column:is associated· princi-

pally with·limiting the plutonium content of the.resin phase;

ELUTION COLUMN.

In contrast to the simplicity of the· absorption-scrub column, operational

control of the 7-foot elution column is intricate.(Figure 7). Difficulty

in controlling the column stems from the. 7M HN03 present . after resin

movement. This acid would-limit the attainable plutonium concentration if·

it were included,in the product.stream; consequently, the acid is routed

to other portions of the plant for receyery of the acid · and the .small

amount of product. The column elution time is fixed.at two minutes.by

elution kinetics and,the stationary resin time is,fixed at six minutes by

absorption-scrub column operation; thus,.it··is necessary t.o displace the

7M.HN03 within about four minutes. Elution effluent · flows to the product

concentrator during the following 2·minutes. The actual amount of acid

to be.displaced is dependent,upon.the variable amount of fluid, slippage,

during resin movement.. Hence, the eluant flow rate must ·be adjusted to

control the displacement time.' Flow adjustments are.performed manually

and will not compensate.for the variable amount:of fluid slippage during

the·.resin movement. Therefore, a simple timer..is not- an effective ..method

for controlling effluent.diversion. Since resin elution occurs only in

dilute nitric media, a modified commercial conductance system is used.

The cell, located at the bottom 6f the-.column, allows a continuous. pass-

age. of liquid past.high purity electrodes, which are: excited by a radio

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frequency current of 1000 cycles per·second in,order to prevent polariza-

tion and the resulting electrolysis of,the liquid. A bridge circuit,con-

stantly analyzes the solution's conductivity, which is·proportional.to

acidity. The control·system is. set to route effluent.to the product con-

centrator only when the acidity falls below 5M.

PROGRAMMER

Coordinating all of these operations is the function.of·an intricate master

programmer (Figure 8). The programmer. is made up of switches and timers

which, with input from the conductance:and capacitance probes, controltthe

sequence of operation. To regulate the-process, the programmer provides

electriq signals.to solenoid valves.supplying air to pneumatic cylinders

operating ball and plug valves in the processing equipment. At the start

of the resin push, the programmer:

1. Recycles feed.to the feed tank;

2. Cuts off effluent flow from the columns;

3. Isolates the push and receiving reservoirs;

4. Opens the large ball valves.which allow resin movement;.and,

5. Applies air pressure. to the push tank.

When the capacitive liquid level controller in the push tank indicates

that a sufficient amount of liquid.has moved through the loop, the pro-

grammer:

1. Vents the·push tank;

2. Closes the .large ball valves which .allowed resin .movement ;

3. Restores.continuity immediately between.the push and receiving reser-

voirs; UNCLASSIFIED


4. Opens the waste valves of the columns, and:

5. Routes feed.to the absorption column.

When the conductance.recorder-controller indicates that effluent,from the

elution column is less than 31 HN03, the.programmer re-routes the elution

effluent to the product receiver.

The·typical,time.cycle for the unit (from the start,of one push to the

start of the .next) is about seven minutes. This includes approximately

30 to 60 seconds·.for resin.pushing, 4 minutes.for diversion of·high acid

elution waste, and 2 minutes for c6llection of ptoduct elution. Absorption.r

scrubbing operation.occurs simultaneously during 6 of the 7 minutes.

ALPHA MONITORS

Monitors,were.installed te continuously determine the plutonium content in

the·waste streams from.the two columns. Early detection of·losses.mini-

mizes rework. and assists in·. diagnosing processing problems. These monitors

detect.alpha particles emitted by plutonium as it,decays to uranium. The

waste.solution is in.direct contact with a special cerium. activated glass

whi ch emits a. flash of light as alpha particles are absorbed. The light

flashes are amplified by a ph9tomultiplier tube and. amplifier and.then

converted to pulses.. Thus the pulse ·rate is, proportional. to the plutonium

content of the solution.

OPERATING HISTORY,

Although the continuous ion exchange unit is complex, it has performed

well. The system described in this paper has evolved over a period.of 12

'

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UNCLASSIFIED ARH-SA-82.Page 9

years. The problems which have necessitated design changes have:been.

mechanical in nature; the chemical separation behavior has been excellent.

The capacitive liquid. level controller and flow-through conductivity cell

are two. of·the changes,made:to improve -control of·the unit. ·In,addition,

the-alpha monitors installed on,the waste.streams have been an invaluable

operating aid.

Recent reports of spontaneous combustion reactions in highly-nitrated

anion resin,systems have provided the basis for reassessing·the applica-

tion of anion exchange·to.plutonium purification. The · decision has been4

made to install a remotely-operated solvent.extraction cycle.in place of

the ion exchange system because of the hazards specifically related to

plutonium anion exchange.

CONCLUSION

In summary, the continuous anion-exchange. unit-and associated controls

have ·proven to be effedtive in the·.final purification of plutonium.

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REFERENCES

1Ryan, J. L., E. J. Wheelwright, Industrial Engineering Chemistry , 11,Page 60 (1959).

2Ryan, J. L., E. J, Wheelwright, USAEO Ddclassified Report, HW-55893,January.2, 1959.

3oberg, G. C. and W. H. Swift, "Continuous Anion Exchange Processingof Plutonium," USAEC Report, HW-SA-2290 (Wnclassified), October 3,1961.

4Miles, F..W., "Ion Exchange - Re§in System Failure in.Processing·Acti-vities," Nuclear Safety, Vol. 9 - No. 5, September-October, 1968.

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1

figure 1 EABSORPTION - ELUTION R/4

CHEMISTRY *XA COLUMNABSORPTION CONDITIONS7 MOLAR NITRIC ACID MEDIA60° C.

CATION ANION RESIN PHASE

Pu+4+6NOT- Pu(NO,)6=-R:Pu(NO,)i

XC COLUMNELUTION CONDITIONS1-5 MOLAR NITRIC ACID MEDIA30" C.- 90

2 RESIN PHASE ANION CATION 97S R:Pu(NO,)6.- Pu(NO,)6- Pu+4+6N0T EF3 8

figure 2INDICATES RESIN FLOW--.4, PUREX PLANT 4

CONTINUOUS 5ANION EXCHANGE

(C-4 SYSTEM

XA SCRUB -

XA COLUMN 1,=-- t« -- XC ELUTANT

XA FEED-

--=,E XC COLUMN

„t iZ J'XC

PRODUCTXAF RECYCLE

WFEED PRODUCT PRODUCT

5 --0STORAGE CONC STORAGE

9 puM,4 #0M VV V

XA WASTE XC WASTE 'cl D>

4 970 P W

M pm0 1u g

r

figure 3 WCENTRAL Capacitanc• RESIN

MOVEMENT RecorderPROGRAMMER SYSTEMController

Transducer1 1 VIENT 8RESIN RECEIVER TANK 0 (V)4 -' AIR SUPPLY

on I (C 1 1 lit C.)r rl·r--·-·-L -'a'-#'- -XA SCRUB --*4--* - - E PRV

_ Wt... RESIN MOVEMENTp-ACCUMMULATOR

off .„ ) C- (PUSH TANK)

$ 4- XC ELUTANTXA FEED --*4-D

XA XCABSORPTION lmm.- 4* ELUTION

COLUMN COLUMN

o'« . off

- *4--+ XC PRODUCT

off

# I 1 8 itXA WASTE XC WASTE mi3 Ywz-00 1 . 2

figure 4 ECAPACITIVE LIQUID LEVEL CONTROL *v

A gSEMICONDUCTOR

SUPPLY TRANSDUCER

MODULE CONTROL

ROOM CENTRAL

1PROGRAMMER

1 1

CONSTANT CURRENTSOURCE COUPLING CAPACITANCE RECORDER

TRANSFORMER--------- READOUT --- -(frequency) CONTROLLER

0- f..../-*./.4

CAPACITANCE 9,=....ELECTRODE '----

_ RESIN MOVEMENTACCUMMULATOR

(PUSH TANK )M

E1= EQUIPMENT WENCLOSURE

M Al P>0 1

23 *-4

r--

f

C

mi

figure 5 MDIAPHRAGM PUMP CHARACTERISTIC

/3

AVERAGE FLOW Vmax. = 3.1 /

SMOOTHED FLOW Vavg·withSUPPRESSOR INSTALLED

P W -----\-- --- ---/0.. *

TIME 0'I.

I. .----'

1*09 X

M .69 8Gr

im

figure 6XA ABSORPTION COLUMN

9

-

LOADED RESIN

3< 9XA SCRUB

7 MOLAR NITRIC ACID

- I

XA FEED 3/77 MOLAR NITRIC ACID

,<XA WASTE j , C

i < C & 4 FRESH RESIN u 'I>9 5097

9 499 00ro

L. 09figure 7 ELUTION COLUMN CYCLE XC Effluent to

XC ELUTION COLUMN XC Effluent to Waste Concentrator

OPERATION R•sin , Mtj

Move

:':,N 60' "ME- mi.....234 5 6 7

C >Cl

*-*|*- XC ELUTANTCENTRAL

PROGRAMMER

7 MOLAR NITRIC ACID.,E711 MOLAR NITRIC ACID..r=]

1

CONDUCTANCERECORDER

CONTROLLER1

XC PRODUClto i

CONCENTRATORLOADED POINT (C)

RESIN

12(- CONDUCTANCE -

1

CELL

1*rn XC WASTE 4 V 9 9 34 t.ipol SCINTILATION

-4 WASTE MONITOR p > -g

e

figure 8OVERALL CONTROL SYSTEM H

39

MASTER

- PROGRAMMER 1

1 1 1

I l

CAPACITIVE CONDUCTANCESOLENOIDRECORDER RECORDER

VALVES CONTROLLER CONTROLLER

CAPACITIVELEVEL XC COLUMN

XA COLUMN READOUT

r--

6 1 I PUSH *rll .1 14*- i g TANK LI - fl ' 1FEED

:

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STORAGE 4--7 1 E-4 1-k iE -t ' IIll, i

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1, 1 1.- 1% PUMP .

CONDUCTANCE

CELL

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control of a continuous ion exchange system …

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