Analysis of binary cryogenic mixtures containing nitrogen and Freon in cryocoolers

Cryogenics 36 (1996) 243-241 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 001 l-2275/96/$15.00 ELSEVIER

Analysis of binary cryogenic mixtures containing nitrogen and Freon in cryocoolers Mingyao

Xu, Yaling

He and Zhongqi

School of Energy and Power Engineering, Shaanxi 710049, China Received

15 June

Chen Xi’an Jiaotong

University,

Xi’an,

1995

Several new non-inflammable binary cryogenic mixtures of nitrogen and Freon are suggested in this paper. The working mechanism of using these mixtures in a JouleThomson cryocooler is analysed according to thermodynamic theory. The PengRobinson equation of state is used to calculate the mixed free enthalpy of these mixtures and a method to calculate the mutual solubility is also suggested. Finally, the vapour-liquid equilibria, liquid-liquid equilibria and vapour-liquid-liquid equilibria of these mixtures are analysed. It is shown that the suggested mixtures can be used as alternatives to mixtures of nitrogen and alkanes or alkenes. Keywords:

binary cryogenic

mixtures;

nitrogen;

Freon; phase equilibria;

Nomenclature

Subscripts

AG K x

i j 1 2

Mixed free enthalpy Interaction parameter Mole fraction in liquid phase

cryocoolers

Component identity Component identity Nitrogen-poor liquid phase Nitrogen-rich liquid phase

L

I

It is an attractive proposition to use a mixture as the working fluid in a miniature Joule-Thomsom (J-T) cryocooler because of its higher refrigerating capacity, lower inlet pressure and higher efficiency of the cycle. At present, mixtures of nitrogen and several alkane or alkene components are commonly used’-‘. Such mixtures are, however, flammable. In 1985, Little’ found that these mixtures could become non-flammable if a certain amount of CF,Br was added to them. But CF,Br has relatively high values of ozone depletion potential (ODP) and global warming potential (GWP), so its use will be largely restricted in the near future. In this paper, several new non-flammable binary cryogenic mixtures of nitrogen and Freon are suggested.

Calculation

method

To analyse the working mechanism of using the suggested mixtures in a J-T cryocooler, the Peng-Robinson (P-R) equation is used to calculate the mixed free enthalpy and the phase equilibria, as reported in reference 4. But compared with the calculations for the binary cryogenic mixtures of nitrogen and alkanes or alkenes reported in that

paper, the calculations in this paper are more difficult. The reason for this is that the components in a binary mixture of nitrogen and alkanes or alkenes are the main components of natural gas, and their characteristics of phase equilibria are readily available and their phase equilibrium data are much more widely reported than those of binary cryogenic mixtures of nitrogen and Freon. In addition, the mutual solubilities of nitrogen and alkanes or alkenes are usually known, while they are almost unknown for binary mixtures of nitrogen and Freon. Therefore, further analysis of phase equilibria of binary mixtures of nitrogen and Freon should be of significance.

Calculated

results and discussion

The vapour-liquid equilibria of various mixtures for different values of the binary interaction parameter K, are shown in Figure 1. From Figure I it can be seen that the bubble point curves often have a horizontal section over a large composition range near liquid nitrogen temperature except for the Nz-R14 system. This behaviour, which is similar to that of the binary cryogenic mixtures reported in reference 4, is the reason why the cooling temperature can be kept constant and close to liquid nitrogen temperature.

Cryogenics

1996 Volume

36, Number

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243

Binary a

mixtures

250

of nitrogen

I

I

and Freon: M.Y. Xu et al. I

_

A

y ‘d

200

ir\

\

‘,

I--_ ---_ ,\“ --_______-==_3-_== \ ‘-______ _----_

\’ ’

5@

’

0.0

1

I

1

0.2

0.4

0.6

Nitrogen

b

----= ===

mole

I 0.8

1 .o

fraction

250

11

4 3 \:

is always less than AG over the range between point A and B. Thus, the liquid phases in these mixtures are unstable and these three kinds of binary mixtures are partially soluble over the range between A and B, like the partially soluble mixtures containing nitrogen and alkanes or alkenes. To analyse the effect of KY on the mutual solubility of nitrogen and Freon, the effects of K, on the relationship between the mixed free enthalpy and the mole fraction of nitrogen for the suggested mixtures are calculated. Figure 3 shows the effect of Kti on this relationship for the mixture of N2 and R23. It can be seen that different values of KU result in different curves of the mixed free enthalpy and the mole fraction of nitrogen, but the partial mutual solubility of N2 and R23 is not changed in the suitably adjusted range of K,. Similar behaviour patterns are found in the other mixtures, including Nz-Rl 16, N2-R32 and N2-R22. The phase equilibria diagrams for the mixtures of nitrogen and Freon, including N,-R116, N2-R23, N,-R32 and N,-R22 at 0.1 MPa are shown in Figure 4. It can be seen that vapour-liquid-liquid equilibrium triple points appear in all of the four mixtures. But for the nitrogen-rich liquid, the triple points cannot be seen with the scale used in Figures 4c and d. The triple points of binary mixtures containing nitrogen and Freon are shown in Table 1. If nitrogen-Freon mixtures are employed as working media in a J-T cryocooler and cooled sufficiently in the heat exchanger, then expanded through the throttling valve to become liquidliquid-vapour mixtures, the heat initially added to the evaporator will not raise the evaporating temperature. This behaviour is similar to that of the partial mutually soluble mixtures of nitrogen and alkanes or alkenes reported in reference 4.

---G--q

‘~ \ \. \

Conclusions .-_

-__=---

---===~~~_=~~_-_

‘-_____----_\ -_

.

_

-

-

-_

z

=s

_--T--

0.2

0.4

Nitrogen

_-

S.-

mole

/

1

0.6

0.8

1 .o

fraction

Figure 1 Vapour-liquid phase equilibria at 0.1 MPa. (a) K,= 0: 1, N,-R14; 2, N,-R23; 3, N,-R116; 4, N,-R32; 5, N,R22. (b) 1, N,-R14, K, = 0.02; 2, N,-R23, Kjj = 0.04; 3, N,-RI 16, K, = 0.09; 4, N,-R32, Kij= 0.05; 5, N,-R22, K;, = 0.06

The relationship between mixed free enthalpy and the mole fraction of nitrogen in the binary systems N,-R14, Np-Rl 16, N,-R32 and N,-R22 at 70 K and 0.1 MPa are shown in Figure 2, in which AG’ denotes the total mixed free enthalpy when one liquid phase is separated into two liquid phases. From Figure 2a it can be found that AG’ is always greater than AG over the whole composition range for the binary mixture of N2 and R14, as for the mixture of nitrogen and methane shown in reference 4. This implies that N2 is completely mutually soluble with R14, and the liquid phase in this mixture cannot be separated into two liquid phases. But for the mixtures N2-Rl 16, N,-R32 and N,-R22 shown in Figures 2b, c and d, respectively, AC’

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1996 Volume

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Several new non-flammable binary cryogenic mixtures of nitrogen and Freon are suggested. The P-R equation of state is used to calculate their phase equilibria and mutual solubilities, and the working mechanism of using these mixtures in a J-T cryocooler is analysed. It is shown that the suggested mixtures can be used as alternatives to mixtures of nitrogen and alkanes or alkenes.

Acknowledgement The financial support provided by the Doctorate’s Foundation of The Education Commission of China is greatly appreciated.

References Alfeev, V.N. et al. UK Patent 1 336 892 (1973) Brodyansky, V.M. et al. The use of mixtures as the working gas in throttle J-T cryogenic refrigerator, in: Proc 13th Int Congress of Refrigeration Vol 1, US National Committee for IIR-NAS-NRC, USA (1973) 43 Little, W.A. Recent developments in J-T cooling gases, coolers and in: Proc 5th Cryocoolers Conf Naval Postgraduate compressors, School, USA (1988) Xu, M.Y. et al. Analysis of using binary cryogenic mixtures containing nitrogen and alkanes or alkenes in cryocoolers Cryogenics (in press)

Binary

a

Xl

x11

5 -50

7 V

and Freon: M.Y. Xu et al.

T

0

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of nitrogen

b

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A

E

mixtures

AG \

J

g-100 u L 4 5

AG'

-150

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aJ aJ i -200 73 CL, .- x zii

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I

I

I

I

0.4

0.2

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Nitrogen

mole

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1

0

i

0.6

0.2

0.4

0.6

fractiiY-7

d

I

I

= -l”8.1, ’

I

1

I

Nitrogen’

1 .o

Nitrogen

4n70ieo’FractiY$

Mixed free enthalpy Figure2 (c) N,-R32; (d) N,-R22

versus mole fraction of nitrogen for nitrogen mixtures

Table 1 nitrogen

equilibria

Liquid-liquid-vapour mixtures at 0.1 MPa

mole

1

I

I

I

1 .o

0.8

0.6

I

0.2

fraction

at 70 K and 0.1 MPa. (a) N,-R14;

(b) N,-R116;

triple points of binary

Component 2 mole fraction V

Component 2

Temperature (K)

;-A,

;:,

R116 R23 R32 R22

77.266 77.251 77.212 77.212

80.61 94.52 99.51 98.50

0.685 6.353 x lo-’ 0.488 9.655 x 1O-9 0.02 1.95 x IO-” 0.02 2.473 x 10~”

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1996 Volume

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Binary

mixtures

of nitrogen

and Freon: M.Y. Xu et al.

100

a A

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m,

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72 -

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Nitrogen

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300

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Nitrogen

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0.8

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100 a F

o-

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+ ;

+ -%

.X >

50

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-1001 0.0

I

I

I

I

0.2

0.4

0.6

0.8

Nitrogen

mole

1 .o

0 -5c -108

I

I

0.2

0.4

Nitrogen

fraction

Figure 3 Mixed free enthalpy versus mole fraction of nitrogen for binary mixture containing Kij = 0; (b) K, = 0.02; (c) Kij = 0.04; (d) K;; = 0.06

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II 0.6

mole N, and R23

I

I

0.8

1 .o

I

fraction at 70 K and

0.1

MPa.

(a)

Binary

mixtures

of nitrogen

and Freon: M. Y. Xu et al.

b 180

0.5

)

Nitrogen c

60 0 .O

1.0

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Nitrogen

240

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_---

Nitrogen Phase equilibria

rnye

diagrams

_-

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fraction

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Y 80

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Figure 4

mole

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f

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fraction

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__st__-----\

-_

__---

40 0.0

I 0.5

Nitrogen

for mixtures of nitrogen and Freon at 0.1 MPa. (a) N,-R116;

Cryogenics

mole (b) N,-R23;

1996 Volume

i 1 .( I

fraction (c) N,-R32;

(d) N,-R22

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247