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AFWAL-TR-88-2081

JET FUEL DEOXYGENATION

S. Darrah

Geo-Centers, Inc.

7 Wells Avenue

Newton Centre, MA 02159 CD

October 1988

Ln

0

N

Interim Report for Period March 1987 - July 1988

Approved for Public Release; Distribution Unlimited

DTIC

ELECTE

E

AERO PROPULSION LABORATORY

AIR FORCE WRIGHT AERONAUTICAL LABORATORIES

AIR FORCE SYSTEMS COMMAND

WRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433-6563
NOTICE

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This technical report has been reviewed and is approved for publication.

CHARES RTEL R. CHARLES L. DELANEY, Chie

Project Engineer Fuels Branch

Fuels Branch Fuels and Lubrication Division

FOR THE COMMANDER

BENITO P. BOTTERI, Assistant Chief

Fuels and Lubrication Division

Aero Propulsion & Power Laboratory

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Geo-Centers, Inc. Air Force Wright Aeronautical Labs, AFSC

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Wright-Patterson AFB OH 45433-6563 ELEMENT NO. NO. NO. ACCESSION NO.

___________________________ 61101F 30805 46

11. TITLE (Include Security Classification) ' 3 0

Jet Fuel Deoxygenation

-12. P RSONAL AUTHOR(S) "

S Darrah

13o. TYPE OF I REPORT ~ rMFROM 13b. TIME Mar COVERED 87/ 14. DATE OF REPORT (Year, Month, Day) 15. PAGE COUNT

Interim Oo "ju 81988 October 26

16. SUPPLEMENTARY NOTATION

17. COSATI CODES 18. JECT TERMS (Continue on reverse If necessary and identify by block number)

FIELD J21~~~ GROUP SUB-GROUP t4J ~~ Fuels) iM:w ' -

W[ J21 , 05 Deoxygenation, "- -

ABSTRACT (Continue on reverse if necessary and identify by block number)

This program was initiated to identify and characterize methods of deoxygenating quantities

of jet fuel to improve fuel thermal stability. Three methods, chemical getters, molecular

sieves and nitrogen sparging, were evaluated in our laboratory. In the case of nitrogen

sparging, additional results were obtained by comparing laboratory experimental results

with the output from ULLAGE, a computer based mathematical model. Each method was shown

to reduce the oxygen content of jet fuel. Economic and system considerations favor

nitrogen sparging for large quantities of fuel.

2

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FOREWORD

This report documents work conducted under contract F33615-84-C2412, Task No. 13, with Geo-Centers Inc. This contract is

documented under project 3048, task 05, work unit 46. Funding for

Task 13 was provided by Independent Laboratory In House Research

funds (ILIR), work unit ILIRP705, program element 61101F. Dr

Shirley Darrah, Geo-Centers Inc., was the principal investigator,

and Mr Charles R. Martel, AFWAL/POSF, was the program manager.

Accession For

Ni tX 01Ac

IDTIC TAB

Unannounced

Justifivatio

By

Distribution/

Availability Codes

vo1.and/or

'Dist Special

ift
ABSTRACT

This program was initiated to identify and characterize

methods of deoxygenating quantities of jet fuel to improve fuel

thermal stability. Three methods, chemical getters, molecular

sieves and nitrogen spurging, were evaluated in our laboratory.

In the case of nitrogen sparging additional results were

obtained by comparing laboratory experimental results with the

output from ULLAGE, a computer based mathematical model. Each

method was shown to reduce the oxygen content of jet fuel.

Economic and system considerations favor nitrogen sparging for

large quantities of fuel.

iv
TABLE OF CONTENTS

PAGE

TABLE OF CONTENTS . .. .. . .... .. . .. .. ........................... v

1 .0 INTRODUCTION. . ........ . ... . .. ........................ 1I

2 .0 CHEMICAL GETTrER.......................... ........ ........ 2

3 .0 MOL.ECUJLAR SIEVES ..... .. ....... . o ....... . ............... o 5

4.0 NITROGEN SPARGING. ... . ... .. .. . .. ............ .. .......... 10

5.0 CONCLUSIONS ....... .. .. .. .. .. . .......................... 21
LIST OF FIGURES

FIGURES PAGE

1 Percent oxygen removed from JP-5 with type 3A molecular sieves ................................................ 7

2 Deoxygenation of JP-5 with 99.999% N2 sparging gas at

different flow rates ................................. 12

3 Deoxygenation of JP-5 with 99.5% N2 sparging gas at

different flow rates .................................. 13

4 Deoxygenation of JP-5 with 0.1 liter/min of 99.999% N2

sparging gas at different temperatures ................ 14

5 Deoxygenation of 1 liter of JP-5 at 220 C with 99.999% N2

at various flow rates and 95% efficiency .............. 16

6 Deoxygenation of 10,000 gallons of JP-5 at 600 C and

various fuel flow rates. Scrub gas is 99.95% N2 at 0.4

lb/min and 95% efficiency ............................. 18

7 Deoxygneation of 10,000 gallons of JP-5 at 60°F and

100 gal/min fuel flow. Scrub gas if 99.5% N2 at various

flow rates and 95% efficiency .......................... 19

8 Deoxygenation of 10,000 gallons of JP-4 at 60°F and

various fuel flow rates. Scrub gas is 99.95% N2 at

0.5 lb/min and 95% efficiency .......................... 20

vi
1.0 INTRODUCTION

The thermal stability of jet fuels has been a problem of

increasing importance to both commercial and military users for

many years. With the development of higher speed aircraft, which

have higher temperature engines, the thermal stability of fuel is

becoming increasingly critical. Thermal instability refers to the

propensity of the fuel to form carbonaceous deposits on hot metal

surfaces. This effect is a function of fuel components (especially

trace contaminants), the composition of the metal surface and

temperature.

In aircraft, fuel thermal instability can cause problems in

three major areas. The first two, deposits on fuel injection

nozzles and metering valves, directly and perhaps catastrophically

affect engine performance. The third, deposits on walls of heat

exchangers, reduces the efficiency of heat transfer to fuel which

is used as a coolant. The latter effect is becoming more important

as higher Mach number aircraft are developed, which increasingly

rely on fuel as a coolant of airframe and engine components.

A major contributor to the thermal decomposition of fuels is

dissolved molecular oxygen. When heated, oxygen readily forms free

radicals which initiate and propagate autoxidation reactions of

hydrocarbons. When the concentration of dissolved molecular oxygen

is reduced to levels of lppm, this mechanism of decomposition

becomes unimportant, and the pyrolysis of the fuel itself is the

dominant mechanism. Pyrolysis, however, occurs at much higher

temperatures than autoxidation. Thus, by decreasing the oxygen

concentration to lppm, the fuel can be subjected to higher

temperatures before thermal decomposition occurs.
GEO-CENTERS, INC. has conducted a program to identify and

evaluate methods of deoxygenating jet fuels. These methods were

considered for potential application to either a ground-based, fuel

preconditioning and servicing system or to an on-board, aircraft

fuel system.

In evaluating methods of deoxygenation, several technical

factors must be considered. Kinetics and thermodynamics must

strongly favor oxygen removal to affect a low oxygen concentration

in a short time. It is important that no new contaminants be

introduced into the fuel and that no beneficial components be

removed. In addition, safety considerations involving the fire

hazard of exothermic reactions, toxicity of materials including the

input stream and waste products, and the disposal of waste products

must eventually be addressed. Economic factors, including the

costs of materials, equipment and manpower, will eventually

determine the viability of building large scale deoxygenation

facilities. While we are mindful of safety and economic implications of each method examined, the thrust of this effort has been

directed to technical feasibility issues.

2.0 CHFMTCAL GETTERS

Chemical methods were the first approach evaluated during this

program. Among the chemical methods, "getters", metals or alloys

with chemically active surfaces, appeared attractive be cause they

react with oxygen to form insoluble oxides. They have been widely

used in the semiconductor and nuclear fuels industries. Many of

these are transition metals which were deemed unacceptable for this

application because of the potential for catalyzing reactions in

fuel. Barium metal, however, is a commonly used getter and looked

2
attractive. Barium reacts readily with oxygen and water, but the

reaction is not as exothermic as the reaction of other metals such

as sodium. The relevant reactions of barium are:

2Ba + 02 - 2BaO

Ba + H2 0 - BaO + H2

BaO + H2 0 - Ba(OH)2

Laboratory experiments were performed to determine the

effectiveness of this method. The initial experiments were

conducted with JP-5 in a threeneck round bottom flask. Two of the

necks contained stopcocks attached to ground glass joints. One

stopcock was used to admit nitrogen gas while the other was used

to vent the flask. The third neck held the probe for a Yellow

Springs Instrument International, Inc. model 58 dissolved oxygen

meter. One liter of JP-5 fuel was used. The barium metal was

purchased in 6 mm diameter sticks (granulated barium metal is not

currently available from laboratory suppliers). These sticks were

cut into pieces approximately 6 mm in length. The barium was

stored in oil under a nitrogen atmosphere in an attempt to prevent

its surface from oxidizing. The JP-5 fuel was saturated with air

prior to the experiment by bubbling air through it.

In the first experiment, fuel and barium were stirred together

in the flask under a nitrogen atmosphere. After 30 minutes, the

oxygen concentration had decreased to 50% of the initial concentration. It was observed, however, that this result could be obtained

simply by stirring the fuel under a nitrogen atmosphere without the

barium metal. The presence of white solid did indicate that some

barium had reacted. Subsequent experiments were performed with air

initially present and no nitrogen purging of the flask.

3
In order to increase the exposed surface area of barium, the

threenecked flask was replaced with a blender. The lid was sealed

to the container to prevent air leakage after the deoxygenation

process began. The system was simplified by using cyclohexane

rather than JP-5. The cyclohexane was mixed with the pieces of

barium in the blender until the barium was broken into very small

particles. After 30 minutes, the oxygen concentration was 4% of

the initial concentration. The absolute concentration of oxygen

in the cyclohexane could not be determined by our measuring

instrumentation which gives a relative indication, however,

assuming the air saturated concentration to be 50 ppm then 4% would

be 2 ppm. This value may well reflect the air-tightness of the

blender rather than the limit of the deoxygenation capability of

the barium. A white solid was observed in the liquid. At the

conclusion of the experiment, solid barium metal remained in the

container. The white solid was filtered from the liquid with

Whatman No. 2V filter paper. This paper retains solids larger than

8 microns in size. The filtered solution was observed to be clear

and did not scatter light, thus indicating that all solid

by-products had been removed by the filter. It was also observed

that a coating of white solid remained on the walls of the

container.

Calculations of the quantity of barium metal required to

stoichiometrically react with 10,000 gallons of fuel with 50 ppm

oxygen yield a value of 35 lbs of barium. Additionally, twenty-two

lbs of barium react with 70 ppm of water in 10,000 gallons of fuel.

The actual quantity of barium metal required to react with both the

oxygen and water is less than the total of these two values because

barium oxide itself reacts with water. Barium sells commercially

for approximately $20/lb which is in the range of economically

feasibility. However, barium is costly enough that recycling

technique should be explored for large throughput operations.

4
3.0 MOLECULAR SIEVES

The use of molecular sieves as deoxygenating agents was also

investigated in this program. They are commonly used in industrial

processes to remove water and contaminant gases. Molecular sieves

have advantages over chemical means of deoxygenation in that they

are nontoxic, nonflammable, and can be regenerated by physical

means, thus avoiding disposal problems.

Zeolite molecular sieves have pores of a uniform size which

can adsorb small molecules such as oxygen, while excluding the

larger molecules present in jet fuel. These materials have a high

internal surface area available for adsorption due to the channels

or pores which uniformly penetrate the entire volume of the solid.

The available surface area is very much larger than the external

surface of the adsorbert particles, which contributes only a small

percentage of the total area. The particular sieves to be used are

chosen by matching the size of the molecules to be occluded with

the diameter of the channels.

Initially, Type 3A sieves in 1/16 inch pellets were chosen.

These were chosen because they occlude only molecules which are 3

angstroms in size or smaller. Thus they should occlude oxygen in

the channels but exclude the larger fuel molecules. The

5
experiments were performed in a one liter round bottom flask. The

sieves were placed in the flask and activated by heating them while

the flask was being evacuated. JP-5 jet fuel was then allowed into

the flask. The small volume in the flask which was not occupied

by the 1 liter of fuel was allowed to fill with air so that the

system was under atmospheric pressure. The probe for the oxygen

meter which had previously been calibrated on air-saturated fuel

was inserted into the flask. The mixture was stirred until

equilibrium was attained (the meter readings were constant).

The results of these experiments are shown in Figure 1.

Deoxygenation does occur and is a function of the quantity of

sieves used. The efficiency of these particular sieves, however,

is not sufficient for fuel deoxygenation because even at a ratio

of 150g sieves/liter fuel a reduction of oxygen concentration by

only 27% was obtained.

Because oxygen molecules are near the limit of the Type 3A

channels in size, experiments were also performed with Type 4A

sieves in the form of 8-12 mesh beads. Although the larger size

of the channels allow slightly larger molecules such as C02 to be

occluded, it was hoped that they would be more efficient

deoxygenating agents. As in the previous experiments, a 1 liter,

three necked flask was used. The beads were activated by heating

them under vacuum. After cooling, fuel was admitted to the flask

and the mixture was stirred until equilibrium was obtained. The

concentration of oxygen in the fuel relative to the concentration

of oxygen in air saturated fuel was measured. The concentrations

of oxygen in the gas space above the fuel and in the sieves were

calculated.

6
100

75

0

Q)

50

X

0

41

25 a)

04

00

0 50 100 150 200

Grams molecular sieves/liter fuel

Figure 1. Percent oxygen removed from JP-5

with Type 3A molecular sieves

7
Since the flask is under vacuum when the fuel is admitted to

it, gases boil out of the air saturated fuel as the flask is being

filled. Therefore, the flask can not be completely filled with

fuel. In order to evaluate the effect of this phenomenon,

experiments were first performed simply with an evacuated flask

and no sieves present. The total volume of the flask was actually

1.215 liter. With no sieves present, the fuel occupied a volume

of 1.15 liter. Assuming a concentration of 40 ppm in air saturated

fuel, the fuel contained 46.1 mg of oxygen. When the flask was

filled with fuel, 10 ml of air at atmospheric pressure which was

in the tubing was also admitted into the Figure 1 flask. It

contained 2.8 mg of oxygen. Thus, the total oxygen content of the

flask was 48.9 mg. After equilibrium, the oxygen concentration in

the flask was 31.0 ppm or a total content of 35.5 mg. From Henry's

law and the ideal gas law, the gas phase contained 13.9 mg of

oxygen. The total oxygen in the flask was 49.4 mg which agrees

well with the calculation of the quantity of oxygen initially

admitted to the flask.

For the experiments with the sieves, the initial and final

concentrations of oxygen in the fuel were measured with the oxygen

meter. The oxygen contents of the gaseous and liquid phases were

calculated by the same procedure used when no sieves were present.

The oxygen content of the sieves was the difference between the

initial and final contents of the gaseous and liquid phases. The

results of the measurements and calculations are given in Table 1.

Although the sieves do remove oxygen from the fuel, their efficiency is not good and in practice massive quantities would be

required.

8
TABLE 1. Results of Jet Fuel Deoxygenation Experiments

Using Type 4A Molecular Sieves

Wt. of sieves (g) 0 50 100 150

Vol. of sieves (ml) 0 30 60 90

Vol. of fuel (ml) 1150 1110 1075 1035

Vol. of gas (ml) 65 75 80 90

Initial 02 in fuel (mg) 46.1 44.5 42.9 41.3

Initial total 02 (mg) 48.9 47.4 45.8 44.2

Final 02 in fuel (mg) 35.5 30.2 24.0 17.8

Final 02 in gas (mg) 13.9 14.1 12.3 10.7

Final 02 in sieves 0 3.1 9.5 15.7

9
4.0 NITROGEN SPARGING

The deoxygenation of fuel by nitrogen sparging has also been

evaluated during this program. Sparging is the method which has

been used in laboratory studies on thermal stability of

deoxygenated fuel. It has the advantage of not introducing

possible contaminants or reactive materials to the fuel and also

poses no disposal problem.

Sparging is a physical process in which the composition of the

atmosphere which is in contact with the fuel is changed. As the

composition of the atmosphere changes, then the composition of the

fuel must also change in order to maintain equilibrium. The

relationship between the concentrations of oxygen in the two phases

is give by Henry's Law:

pj = xjkj

where pj is the partial pressure of the solute (oxygen) in the

vapor above the solution, xj is the mole fraction of the solute in

the fuel and k. is a constant. Calculations using Henry's law show

that fuel which contains 40 ppm oxygen when air saturated should

contain 1 ppm oxygen when in equilibrium with an atmosphere which

has no more than 0.5% oxygen.

Experiments were performed to study the effect of fuel

temperature and sparging gas concentration and flow rate on

deoxygenation rate and final oxygen concentration. A three-neck

round bottom flask containing 1 liter of JP-5 fuel was used. The

fuel was first saturated with air and then deoxygenated by bubbling

the sparging gas through the fuel with a glass tube with a fritted

10
cylinder. The fuel was stirred during deoxygenation in order to

maximize the efficiency of the deoxygenation and also to ensure

proper response of the oxygen metpr. The two different gases used

for sparging were a special mixture containing 99.5% N2 and 0.5%

02 and "ultrapure" nitrogen, containing 99.999% N2.

With the gas containing 99.5% N2 , a final concentration of

2.5% of the air saturated oxygen concentration was obtained.

Assuming the air saturated JP-5 contains 40 ppm of 02 then 2.5% of

this value is 1 ppm. Figures 2 and 3 show graphically deoxygenation experiments with "ultrapure" and 99.5% N2 . Although, a 1 ppm

level can be attained with either gas, the time of deoxygenation

and volume of sparging gas required were significantly less for

"ultrapure" N2 rather than the 99.5% mixture. Thus, the 99.5%

mixture is adequate to maintain fuel at the 1 ppm dissolved oxygen

concentration, but higher purity nitrogen is more efficient for the

initial deoxygenation process. In actual practice, different

methods may be advantageous for purifying the nitrogen gas used at

different steps of the fuel handling and storage process.

In Figure 4, the effect of temperature is illustrated. The

deoxygenation process is as expected from kinetics arguments slower

at lower temperatures. Therefore, the range of temperatures to

which the fuel could be subjected in the deoxygenation process must

be considered in setting the requirements for the sparging system.

Following these laboratory experiments, the application of

nitrogen sparging to deoxygenation of full-scale systems was

studied by performing calculations with the computer based

mathematical model, ULLAGE. This model is described in the report

11
100 @ 0. 1 Vmnin

0 0.05 Vmin

* 0.02 L'min

00

cc:

z

0

z

0 20 0

cm

0

0 4

0002 04 06

TIE(-NTS

Fiue2 exgnaino P5wth9.9%Sagn

o 20
100 * 0.i1 Vmin

a 0.05 Vmin

w 80-

U- 0

t- 60 - z

w

CC,

Paz

0

tR 40-

z

w

z

0

0

c 20

0

0 10 20 30 40 50 60

IME (MINUTES)

Figure 3. Deoxygenation of JP-5 with 99.5% N 2Sparging

Gas at Different Flow Rates.

13
100 -

R 5*C

w 80- I-

,<

I--

C)

LL

0

60-

0

cc

9w,

z

0

F- 40 - cc

IT"

z

w

z

0

0

0 c 20

0

0 10 20 30 40 50 60

IME (MINUTES)

Figure 4. Deoxygenation of JP-5 with 0.1 Itmin of 99.999%

N2 Sparging Gas at Different Temperatures.

14
AFWAL-TR-87-2060, "Volume I - Airplane Fuel Tank Ullage Computer

Model". In this model, aircraft fuel tanks are simulated and

conditions such as temperature and flow rates can be varied. The

software was modified to calculate the concentration of oxygen

dissolved in the fuel in ppm (mg oxygen/liter fuel) and to output

this value as a function of time.

Initially, the model was used to simulate the conditions which

had been used in previous laboratory experiments. These conditions

were 1 liter of JP-5 at 720F in a volume of 1.2 liter with a

surface area of 0.102 ft. 2 Calculations were performed for 99.999%

N2 at flow rates of .01 liter/min, 0.05 liter/min and 0.02

liter/min with an efficiency of 0.96. Comparing Figure 2 with

Figure 5 shows there is good agreement between the experimental

and calculated results.

Calculations were also performed to show the effect of

temperature on deoxygenation. These res,.lts did not agree well

with experiments. The calculated rates of deoxygenation showed an

insignificant change with change in temperature unlike the

laboratory experiments. The reason for this discrepancy is not

understood at this time. However, the laboratory results shows a

consistent temperature effect which leads us to believe that the

computer model should be examined with respect to its handling of

temperature parameters.

The deoxygenation of fuel in storage tanks was next simulated.

For the calculations, 10,000 gallons of fuel in a 1500 ft3 tank

with 113 ft2 surface area was used. The calculations were

performed for a variety of conditions including an empty tank being

filled with fuel at various flow rates and a previously filled

tank. The fuel and upper skin temperatures were varied as were the

scrub flow rate, scrub gas concentration and scrub efficiency.

15
061 0-006-MM1 02022-IL

60-

*A A=0.10OVmin

B =0.05 Vmin

50 C = 0.02 Vmin

* 0.1 Vmin

40* 0.05 Vmin

40 0 *0.02 Vmin

0

0

0

CC

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0 20 1 A04 06

Tie(ints

Fi ure5 exgnto f1 fJ t20wt 999 2a

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C1
The time and quantity of nitrogen required to deoxygenate a

tank of fuel is highly dependent on the fuel flow rate. Figure 6

shows oxygen concentration versus time for several flow rates plus

for a previously filled tank. As can be seen, deoxygenating a

previously filled tank is the least efficient process. At the

given conditions of 600F, 0.4 lb/min scrub rate, 99.95% N2 and 95%

efficiency, the 250 gal/min flow rate is most efficient because

the tank is filled in forty minutes and a 1 ppm 02 level is reached

after 62 minutes. At 100 gal/min the 1 ppm 02 concentration level

is reached in less time, but the tank is not filled until 100

minutes have elapsed. Therefore, more nitrogen is also required

to maintain scrub conditions through the longer time interval.

These results show that it is important to optimize the fuel

flow and scrub flow rates in order to produce a tank of fuel

deoxygenated to the 1 ppm oxygen level most efficiently. Although

a fast fuel flow rate will fill the tank more quickly, a slower

rate may actually produce deoxygenated fuel more quickly depending

on the scrub flow rate. On the other hand, a fast scrub rate may

result in excess nitrogen being used if the fuel flow rate is low.

Figure 7 illustrates this by showing the deoxygenation of 10,000

gallons of JP-5 with fuel flow rate of 100 gals/min. Scrub flow

rates of 0.40 and 0.18 lbs/min were used. Although both produce

the desired result in 100 minutes, the time required to fill the

tank, the faster scrub flow required an excess of 32 lbs of

nitrogen over the slower scrub rate.

Other variables of the system were also tested in these

calculations. Figure 8 shows the deoxygenation of JP-4 under the

same conditions as the JP-5 shown in Figure 6. Because of the

higher initial 02 concentration, the deoxygenation proceeds more

17
60-

A=full tank

B =1000 gal/min

C =500 gal/min

50 D0 250 gal/min

E =100 gal/min

40 -

10 - A

00

0

0 10 20 .30 40 50 60 70 60 90 100

TIME f'.IINUTES)

Figure 6. Deoxygenation of 10.000 gals of JP-5 at 60*F and various fuel flow

rates. Scrub gas is 99.95% N2 at 0.4 lb/min and 95% efficiency.

18
60

A = 0.18 lb/min

B = 0.40 lb/min

40

* 30

20-

z

(,)

1 20

A

0 ' - ----- -- - .. .. I....----

0 10 20 30 40 50 60 70 s0 90 100

TIME (MINUTES)

Figure 7. Deoxygenation of 10,000 gals of JP-5 at 60°F and 100 gal/min fuel flow.

Scrub gas is 99.95% N 2 at various flow rates and 95% efficiency.

19
80-

A = ull tank

B = 1000 gat/min

70 C = 500 gal/min

D0=250 gal/min

E =100 gal/min

A

c~50

z

< 40-

Lii

z

0 S30-

00

10-

0 10 20 30 40 50 60 70 so 90 100

TIME (MINUTES)

Figure 8. Deoxygenation of 10,000 gals of JP-4 at 60OF and various fuel flow

rates. Scrub gas is 99.95% N2 at 0.4 lb/min and 95% efficiency.

20
slowly. Simulations in which the temperature of the fuel or upper

skin were varied showed a very slight, less than 1 percent, change

in deoxygenation rate. As discussed above this conflicts with

laboratory results. Also, simulations were performed with the vent

valve set at an overpressure. In these cases, the deoxygenation

was delayed until sufficient scrub gas was added to equal the

9 overpressure and then the deoxygenation proceeded normally for the

given conditions.

5.0 CONCLUSIONS

Several methods of deoxygenating jet fuel have been considered

and evaluated. These methods were based on both the chemical and

physical properties of oxygen. The chemical methods suffer from

several disadvantages. The best reducing agents are difficult to

handle because they must be protected from air and often react so

exothermically as to be a fire hazard. Also, good reducing agents

are not easily regenerated after exposure to oxygen. The large

volumes of fuel which must be deoxygenated then result in large

quantities of used agents which must either be subjected to costly

regeneration processes or disposed of in a safe manner.

Molecular sieves do not suffer from these disadvantages

because they are inert, nontoxic, nonflammable materials. Also,

because they function by the physical process of adsorption rather

than by chemical reaction, they are more easily regenerated. The

adsorption of gases is an equilibrium process and by placing the

sieves under vacuum, the gases can be removed. The disadvantage

of sieves is that massive quantities of sieves will be required in

order to deoxygenate jet fuel to the low oxygen concentrations

required to improve thermal stability.

21
Sparging fuel with nitrogen gas is also a physical process of

removing oxygen involving an equilibrium between the fuel and

the atmosphere surrounding it. As the oxygen concentration of the

atmosphere is decreased, the oxygen concentration in the fuel also

decreases in order to maintain the equilibrium. In this study, it

was found that deoxygenation is best achieved by using high purity

nitrogen and deoxygenating as fuel flows into a storage container.

The fuel flow rate should be adjusted according to the scrub gas

f low rate so that a 1 ppm oxygen level is attained in the time that

it takes to fill the tank. It is very important that the proper

relationship be maintained between the two flow rates in order to

most efficiently obtain a full tank of deoxygenated fuel. It may

even be more efficient to use a slower fuel flow rate in order to

achieve this goal. The scrub gas should also be dispersed into

small bubbles in order to present the maximum surface area to the

fuel, resulting in maximum efficiency.

Although high purity nitrogen is best for the deoxygenation

process, gas which is 99.5% nitrogen is sufficient to maintain the

deoxygenated fuel. This number is significant because it is

achievable by processing air with hollow fiber membrane filters.

Thus, these filters could be used to supply the atmosphere in

storage tanks as well as to provide demand gas on the aircraft.

This eliminates the need to provide prepurified nitrogen on the

aircraft. Such systems are available commercially from Dow

Chemical under the trademark GENERON or Permea Inc., a Monsanto

Company under the trade name Prism Alpha Nitrogen Systems.

*U.S.Government Printing Office: 1989 - 648-056/04168 22