Hibbard, R R

Ordin, Paul M

Walsh, Thomas J

English

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NACA 
RESEARCH MEMORANDUM 
DILUTION OF LIQUID OXYGEN WHEN NITROGEN IS 
USED FOR PRESSURIZATION
By Thomas J
.
 Walsh, R. R. Hibbard, and Paul M. Ordin 
Lewis Flight Propulsion Laboratory
Cleveland, Ohio 
NATIONAL ADVISORY COMMITTEE 
FOR AERONAUTICS 
WASH I NGTON 
April 1, 1958
https://ntrs.nasa.gov/search.jsp?R=19930089940 2020-06-17T06:53:09+00:00Z
NACA RN E58AO3a 
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
RESEARCH MEMORMJDUM 
DILUTION OF LIQUID OXYGEN WHEN NITROGEN IS USED FOR PRESSURIZATION

By Thomas J. Walsh, R. R. Hibbard, and Faul M. Ordin 
SUMMARY 
A graphic method for determining the composition and phases exist-
ing within a propellant tank when liquid oxygen is pressurized with ni-
trogen gas is presented, and an example of the calculations involved is 
included. The assumed limiting conditions are that the pressurizing gas 
flows adiabatically from the pressure tank and that the equilibrium state 
exists in the oxygen-nitrogen system. Enthalpy-concentration plots for 
the oxygen-nitrogen mixtures at constant pressures from 100 to 600 
pounds per square inch absolute are presented. Both the equilibrium 
considerations and a small amount of experimental testing show that gas-
eous nitrogen is unsuited for pressurizing liquid-oxygen systems. 
IMRODUCTION 
Gas-pressurization systems provide a simple and commonly used means 
for feeding propellants into rocket combustion chambers. Nitrogen and 
helium are often so used. Ideally, the pressurizing gas should neither 
dissolve in the propellants nor condense in the tanks; helium is pre-
ferred on both counts. 
- Nitrogen is considerably cheaper than helium and is widely used, 
especially with systems operated at ambient temperatures. However, for 
use at the low temperatures required with liquid oxygen, nitrogen is far 
less attractive than helium. At these low temperatures and at the pres-
sures of interest, the nitrogen can dissolve in the oxygen to the extent 
that the propellant is seriously diluted. The dilution may vary from 
negligible amounts to equilibrium concentrations, depending on the 
elapsed time between pressurizing the tank and discharging its contents 
and on the rate at which equilibrium is approached. Therefore, both the 
equilibrium state of the oxygen-nitrogen system and the rate of solution 
are factors in considering the feasibility of using nitrogen as a pres-
surizing gas. 
This report considers the vapor-liquid equilibrium for the oxygen-
nitrogen system with the assumption that the pressurization is an
2	 NACA RM E58A03a 
adiabatic process. The extent to which liquid oxygen must be diluted to 
attain a given equilibrium pressure then can be determined by using 
enthalpy-concentration diagrams, which are presented herein for pressures 
from 100 to 600 pounds per square inch absolute. Also presented is a 
limited amount of experimental data that roughly indicate the amount of 
dilution experienced in one propellant system. The rate at which equilib-
rium is approached may be expected to be influenced both by system design 
and by method of operation. Therefore, the experimental results will not 
be directly applicable to other systems. 
PROCEDURE 
An enthalpy-concentration diagram for oxygen-nitrogen mixtures at 1 
atmosphere is given by Dcdge in reference 1. This diagram is based on 
the enthalpy-temperature data of Millar and Sullivan for pure oxygen and 
nitrogen (ref. 2), where the zero enthalpy base for both components is 
taken at the normal boiling point of nitrogen (139.3° R). Similar dia-
grams were prepared for higher pressures by the following method (enthalpy 
data from ref. 2 were used throughout). 
In evaluating vapor-liquid equilibriums, the experimental data of 
D3dge (ref. 3) at low pressures were combined with the calculated data 
of Koch and Williams (ref. 4) at high pressures by preparing a plot of 
pressure against the mole percent oxygen in the vapor with constant mole 
percent oxygen in the liquid as a parameter. Smooth curves were drawn 
through the two sets of data and are shown as figure 1. 
The methods used for determining the enthalpy of oxygen-nitrogen 
mixtures at various temperatures can be shown by referring to figure 2, 
where curves for the pure components were first drawn. 
The enthalpy-temperature relations for saturated liquids are shown 
as solid lines in this figure. Below the critical temperature for ni-
trogen the enthalpy of mixtures is simply a linear function of composi-
tion, and curves were so drawn. Next, the critical conditions for the 
pure components were connected by a straight line (shown by dot-dash in 
fig. 2), and it was assumed that the enthalpies of mixtures at their 
critical temperature lie on this line and are directly proportional to 
molar composition. This fixed the critical-point condition for any mix-
ture and permitted the saturated-liquid curves to be extended from the 
lower temperature region by fairing through the critical points. 
The enthalpies of mixtures in the saturated-vapor state (dashed 
lines in fig. 2) were estimated by adding the latent heats of vaporiza-
tion to the enthalpies of the saturated liquids. Latent heats of vapori-
zation were estimated by using the relation between latent heat and re-
duced temperature developed by Watson (ref. 5) to determine the latent
NACA RM E58A03a	 3 
heat of the mixture. This relation indicates that the ratio of the 
latent heat at a given temperature to the latent heat at the normal 
boiling point is a unique function of the reduced temperature. The par-
ticular relation for this system was developed by plotting the latent-
heat ratio against reduced temperature for oxygen and for nitrogen on a 
common plot and drawing the best smooth curve through the combined data. 
The single curve was used for all mixtures and was read in reverse to 
determine latent heat as a function of temperature for the various mix-
tures of nitrogen and oxygen. The latent heat thus determined was added 
to the enthalpy curves for the liquid state to produce a smooth loop 
for each 10-percent increment of oxygen in the mixture, as shown in 
figure 2. 
Figure 2 was used to determine the enthalpy for the enthalpy-
concentration plots at various pressures. The assumption of no heat of 
mixing is implied by this method. Dalton t s and Raoult t s laws were used 
to calculate the equilibrium composition for the liquid existing at a 
given temperature and at a predetermined total pressure. The enthalpy 
was read, and a point was marked on the saturated-liquid (bubble-point) 
line. The equilibrium vapor composition was read from figure 1, and the 
vapor enthalpy was determined; this fixed a point on the saturated-
vapor (dewpoint) line. Several such points determined both lines. The 
resulting enthalpy-concentration charts are presented for several pres-
sures ranging from 100 to 600 pounds per square inch absolute in figure 
3.
APPLICATION 
The use of figure 3 to estimate equilibrium compositions is shown 
by the following example. The enthalpies of gaseous nitrogen and liquid 
oxygen are required, and figure 4 gives the enthalpy for nitrogen over 
the region of interest; that for liquid oxygen can be taken from figure 
2.
Assume that liquid oxygen at 1 atmosphere pressure and at its normal 
boiling point (162° R) is contained in a tank with negligible ullage, 
and must be pressurized to 200 pounds per square inch absolute with 
gaseous nitrogen at 1000 pounds per square inch absolute (68 atm) and 
520° R. The molar enthalpy of the liquid oxygen is 280 Btu (fig. 2) and 
for the gaseous nitrogen is estimated to be 4885 Btu (fig. 4). These 
points are located on figure 3(c) (for 200 lb/sq in. abs, with the nitro-
gen point obtained by extrapolation). A straight line is then drawn be-
tween the points; the intercept of this line with the saturated-liquid 
curve (point x on fig. 3(c)) gives the composition of the liquid phase 
at adiabatic equilibrium. In this case, sufficient nitrogen would have 
to be added to give a liquid phase which is 83 mole percent oxygen and 
17 percent nitrogen. The temperature would then be 218 0 R, and the vapor 
phase would be 42 mole percent oxygen and 58 percent nitrogen.
4	 NACA RM E58AO3a 
EXPERIMENTAL RESULTS 
The preceding example shows that the use of nitrogen to pressurize 
liquid-oxygen tanks can result in a substantial dilution of the propel-
lant. At higher pressures the dilution would have been more severe. 
However, in the practical case, nitrogen might be used without 
significant dilution, because the nitrogen would condense only at the 
surface of the oxygen and would be transferred into the body of the liq-
uid by diffusion or mixing. If these diffusion or mixing processes were 
sufficiently slow, much of the oxygen could be drawn off as substantially 
undiluted liquid. 
To test this possibility, an insulated tank having a volume of about 
cubic feet was fitted with a dip tube to the bottom to take off liquid 
phase and with a pressure line to the top (fig. 5). This tank was 60 to 
80 percent filled with liquid oxygen and was pressurized from a nitrogen 
supply at 1500 pounds per square inch absolute and 520° H. After holding 
for varying times, the liquid oxygen was released at rates of 0.1 to 0.2 
pound per second, and samples were taken from the product line at times 
corresponding to varying percentages of liquid withdrawn. These samples 
were analyzed for oxygen content. 
While no consistent data were obtained, the results from two such 
runs are shown in figure 6, where the percent of oxygen found in the liq-
uid product is plotted against the percent of liquid taken from the tank. 
Data from rrns made at 450 and 900 pounds per square inch absolute and 
with different holding times are shown. The calculated composition for 
the equilibrium liquid is 72 mole percent oxygen and 28 percent nitrogen 
at 450 pounds per square inch absolute. At 900 pounds per square inch 
absolute the adiabatic model predicts that there would be no liquid phase 
at equilibrium, since 900 pounds per square inch absolute is well above 
the critical pressure and cannot be generated from a simple vapor-
pressure effect. It may be assumed, however, that equilibrium conditions 
were not obtained for any of the tests. In both cases the first portions 
of the liquid taken from the tank were undiluted oxygen; this shows that 
nitrogen had not diffused to the bottom of the tank in the holding times 
used. However, only a part of the tank contents could be withdrawn as 
substantially undiluted oxygen, and this amount differed between these 
two runs in an unexplainable manner. These data are contrary to expecta-
tions, since more undiluted oxygen was available for the higher pressure - 
longer holding time run. However, variations in the degree of mixing, 
heat input, temperature stratification, and solubility may account for 
these initial results. Nevertheless, in both cases the liquid withdrawn 
during the later portions of these runs was seriously diluted. Rocket 
performance would be appreciably reduced with such a propellant. Pres-
surization with helium gas under the same conditions and at a pressure 
of 900 pounds per square inch indicated no contamination of the oxygen.
NACA RM E58AO3a	 5 
CONCLUSIONS 
With an adiabatic process assumed, enthalpy-concentration diagrams 
are developed herein that can be used to estimate the equilibrium com-
position of the liquid phase after liquid oxygen is pressurized with 
gaseous nitrogen. At equilibrium the liquid contains substantial quan-
tities of nitrogen. Limited experimental work also indicates a serious 
dilution of the oxidant when nitrogen is used as the pressurizing gas. 
However, a barrier at the liquid-gas interface would permit the use of 
nitrogen. A float might be used, or possibly small amounts of helium 
added to the oxidant tank prior to pressurization would serve as an 
adequate barrier to diffusion. 
Lewis Flight Propulsion Laboratory 
National Advisory Conimittee for Aeronautics

Cleveland, Ohio, January 9, 1958 
REFERENCES 
1. Dodge, Barnett F.: Chemical Engineering Thermodynamics. McGraw-Hill 
Book Co., Inc., 1944, p. 402. 
2. Millar, Russell W., and Sullivan, John D.: Thermodynamic Properties 
of Oxygen and Nitrogen. Tech. Paper No. 424, Bur. Mines, 1928. 
3. Dodge, B. F., and Dunbar, A. K.: Investigation of the Co-Existing 
Liquid and Vapor Phases of Solutions of Oxygen and Nitrogen. Jour. 
Am. Chem. Soc., vol. 49, 1927, pp. 591-610. 
4. Koch, H. A., Jr., and Williams, V. C.: Graphical Residual, Its Ap-
plication to Phase Relations of Oxygen and Nitrogen. Chem. Eng. 
Prog., vol. 43, no. 11, 1947, pp. 623-632. 
5. Watson, K. M.: Prediction of Critical Temperatures and Heats of 
Vaporization. md. and Eng. Chem., vol. 23, 1931, pp. 360-364.

NACA RM E5BAO3a
Temperature, 
Figure 2. - Variation of enthalpy with temperature for saturated solutions of oxygen and 
nitrogen.
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3 
2 
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NACA RN E58AO3a 
0	 20	 40	 60	 80	 100
Oxygen, percent 
(a) Pressure, 100 pounds per square inch absolute. 
Figure 3. - Variation of enthalpy with concentration for oxygen-
nitrogen mixtures at constant pressures.
NACA RN E58AO3a 
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Oxygen, percent 
(b) Pressure, 150 pounds per square inch absolute. 
Figure 3. - Continued. Variation of enthalpy with concentration 
for oxygen-nitrogen mixtures at constant pressures. 
9 
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NACA EM E58A03a 
0	 20	 40	 60	 80	 100
Oxygen, percent 
(c) Pressure, 200 pounds per square inch absolute. 
Figure 3. - Continued. Variation of enthalpy with concentra-

tion for oxygen-nitrogen mixtures at constant pressures.
a) 
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NACA RN E58AO3a
	
11 
0	 20	 40	 60	 80	 100
Oxygen, percent 
(d) Pressure, 300 pounds per square inch absolute. 
Figure 3. - Continued. Variation of enthalpy with concentra-

tion for oxygen-nitrogen mixtures at constant pressures.
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NACA RN E5SAO3a 
0	 20	 40	 60	 80	 100
Oxygen, percent 
(e) Pressure, 400 pounds per square inch absolute. 
Figure 3. - Continued. Variation of enthalpy with concentra-

tion for oxygen-nitrogen mixtures at constant pressures. 
NACA RN E58AO3a 
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Oxygen, percent 
(r) Pressure, 500 pounds per square inch absolute. 
Figure 3. - Continued. Variation of enthalpy with concentra-

tion for oxygen-nitrogen mixtures at constant pressures. 

NACA RN E58AO3a
	
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Dilution of liquid oxygen when nitrogen is used for pressurization

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