Using Pyrolysis with Catalytic Oxidation Generated in ...

Plutonium Stabilization - Research & Development

The Stabilization of Polymeric MaterialsGenerated in Nuclear Materials Applications

Using Pyrolysis with Catalytic OxidationDaniel J. Kathios

Los Alamos National LaboratoryNuclear Materials Technology Group, NMT-2

Mail Stop E511, Los Alamos, New Mexico 87545

American Institute of Chemical Engineers1998 Annual Meeting

General Topics in Environmental Reaction Engineering Session

Miami, FloridaNovember 18, 1998

LA-UR-98-4697


Assessment of Problem

• Polycubes are mixturesof plutonium anduranium oxides that arecast in a polystyrenematrix.

• The polycubes werefabricated during theCold War for thepurpose of conductingcriticality studies.


Assessment of Problem

• Approximately 1600 polycubes are in storage at Hanford.

> The polycubes are in a variety of sizes, the largest ofwhich is 2 inches x 2 inches x 2 inches.

> Some of the polycubes are coated with aluminum paint,PVC tape, or Shurtape.

> The polycubes are now packaged in vented food packcans with 5 to 8 cubes per can.

> The presence of Pu240 and Am in the polycubes causesthem to represent a significant exposure hazard.

• The polycubes are not suitable for long-term storage.


Goal of Stabilization Effort

• The goal of this stabilization effort is to design, optimize,and build a pyrolysis process to stabilize the remaininginventory of polycubes at Hanford.

• Requirements for the pyrolysis process:

> It must effectively destroy the polymer matrix andremove it from the oxides of plutonium and uranium.

> It must be suitable for glovebox operations.

> It must allow for minimal handling of the polycubes.

> It must be complete with off-gas treatment to oxidizethe hydrocarbons resulting from the decomposition ofthe polymer matrix.


The Pyrolysis Reactor

Firebrick

2500 WattFurnaceReactor

Insert

Polycube

FurnaceThermocouple

Pyrolysis ReactorHeatShield

Off-Gas Ar Feed

• A pyrolysis reactorwas designed andbuilt at Los Alamosspecifically for thisapplication.

• The reactor is built tobe “user-friendly” forglovebox operations.


The Oxidation Processes

• Three oxidationprocesses wereevaluated toprocess thereactor off-gas.

Pyrolysis Unit(2500 Watt Furnace)

Furnace Controller

PressureGauge

HouseArgon

Reactor Off-Gas

HouseAir

House Air

Oxidation Process

1. Thermal Oxidation2. Catalytic Conversion3. Silent Discharge Plasma

PressureGauge

CO2, H2O, N2, etc.


The Performance of the Pyrolysis Reactor

• Composition of a polycube:

> Polystyrene: 134.7 grams> Al paint: 1.4 grams> PVC tape: 4.2 grams> Shurtape: 6.9 grams

> Total mass: 147.2 grams

Polycube Before Pyrolysis



• Composition of the dry solids:

> The dry solids are presumedto be a form of carbon.


• Only 4.7% of the original massof the polymer matrix remains inthe reactor.

• The dry solids are to be calcinedto remove any residual carbon,and then packaged in a 3013storage container.

Dry Solids Left in ReactorAfter Pyrolysis



Liquid Phase Fraction ofthe Reactor Off-Gas

• Composition of the liquid phase:

> Styrene: 52.4 grams> Toluene: 11.6 grams> Ethylbenzene: 10.2 grams> Other cmpds.: 53.1 grams


• Most (86.5%) of the decompositionproducts of the polymer matrix areliquid phase compounds.



• Composition of the Vapor Phase:

> The vapor phase contained volatilized versions ofthe compounds found in the liquid phase (i.e.,styrene, toluene, etc.).

> The vapor phase also had appreciable amounts ofvinyl chloride, chloroethane, and 1,2-dichloroethaneresulting from the decomposition of the PVC tape.


• Approximately 8.8% of the decomposition products ofthe polymer matrix are vapor phase compounds.


0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 20 40 60 80 100 120

Rate of Off-Gas Generation- Pyrolysis of a Single Polycube -

Polystyrene cubePolystyrene pellets (crushed cube simulant)Cube with Al paintCube with PVC tape and ShurtapeCube with Al paint, PVC tape, and Shurtape

Off

-Gas

Gen

erat

ion

Rat

e (g

ram

s/m

inut

e)

Time (minutes)

Polystyrene cube 4.6 g/min

Polystyrene pellets 4.8 g/min

Cube with Al paint 4.6 g/min

Cube with PVC tape 5.2 g/minand Shurtape

Cube with Al paint, 5.2 g/minPVC tape and Shurtape


Pyrolysis Process with Catalytic Conversion- Configured with Condense and Treat Option -

Pump CollectionReservoir

Air Preheater

HeatExchanger

Gas Sensors

CatalyticConverter

RVChamber

PyrolysisUnit

AirFeed

ArgonFeed

Surge Tank


The Catalytic Converter

Catalyst • The catalyst evaluated is thePRO*HHC VOC catalyst fromPrototech Company.

> The catalyst is designed tobe resistant to acids formedwhen oxidizing halogenatedhydrocarbons.

> The catalyst is in the formof 1/4 inch pellets. Thisfacilitates handling inglovebox applications.


The Catalytic Converter

Catalyst Bed(1/4 inch pellets)

Support Bed(1/8 inch Al2O3 pellets)

Dispersion Bed(1/8 inch Al2O3 pellets)

HeatingMantle

• Three factors are used toevaluate the performance ofthe catalytic converter.

> Oxidation efficiency> Selectivity> Catalyst longevity


0

500

1000

1500

2000

2500

3000

3500

4000

0.0 2.0 4.0 6.0 8.0 10.0

Composition of the Effluent StreamLeaving the Catalytic Converter

Con

cent

rati

on i

n E

fflu

ent

Str

eam

(pp

m)

Time (hours)

CO2

CO

TotalHydrocarbons


The Performance of the Catalytic Converter

• Oxidation Efficiency:

> The oxidation efficiency of the catalytic converter isgreater than 99.0%.

> The oxidation efficiency surpasses the 99.0% designspecification required by Hanford.

• Selectivity:

> The [CO2]/[CO] selectivity is greater than 800.

> The catalytic converter not only oxidizes thehydrocarbons, it oxidizes them completely to CO2.


99.150%

99.200%

99.250%

99.300%

0 5 10 15 20 25 30

Oxidation Efficiency of the Catalytic Converter

Number of Cubes Processed

OxidationEfficiency =

[THC] feed

{ [CO2] + [CO] }

formed100 x

Oxidation efficiency of the catalyticconverter at processing the liquid andvapor phase fractions of the reactoroff-gas.

Oxi

datio

n E

ffic

ienc

y


0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30

Selectivity of the Oxidation Process

Sel

ecti

vity

Number of Cubes Processed

Selectivity = [CO

2]

formed

[CO] formed

Selectivity of the oxidation process atprocessing the liquid and vapor phasefractions of the reactor off-gas.


The Overall Performance of the Process

• Catalytic Converter:

> The performance of the catalyst decreased only slightlyduring the duration of the pilot-scale test. The oxidationefficiency decreased approximately 0.05% per 30 polycubesprocessed.

• Other Units:

> There was no noticeable deterioration in the performance ofthe pyrolysis unit, the air preheater, the pump, etc.


0% 20% 40% 60% 80% 100%

Reduction in Mass of CharacteristicGlovebox Materials Subject to Pyrolysis

Mass Percent of Material Remaining after Pyrolysis

Phenolic screw caps (reduced 47.0%)

Cellulose rags soaked in 15.7 M HNO3 and dried (reduced 82.7%)

Cellulose rags (reduced 79.7%)

PVC bag-out bag (reduced 83.0%)

Yellow vinyl tape (reduced 71.7%)

Elastic cord from bag-out bag (reduced 85.7%)

Tygon tubing (reduced 91.3%)

Dowex 11 anion exchange resin, chloride form (reduced 89.6%)

Polypropylene rags (reduced 99.0%)

Polypropylene rags soaked in 15.7 M HNO3 and dried (reduced 99.1%)

Polyethylene plastic bags (reduced 98.7%)

Polyethylene plastic bottles (reduced 98.3%)

Teflon gasket material (reduced 95.3%)

Teflon pipe fittings (reduced 96.3%)

Nylon wire ties (reduced 96.0%)

Polystyrene foam (reduced 100.0%)

Category 1Materials

Category 2Materials

Category 3Materials

Category 4Materials

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