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Gas-liquid phase equilibrium below 0°C in the system NH3N2H2 and in the system NH3Kr
Michels, A. Dumoulin, E. Van Dijk, J. J. Th. 1959
Physica 25 840-848
GAS-LIQUID PHASE EQUILIBRIUM BELOW 0°C IN THE SYSTEM NH3-N2-H 2 AND IN THE SYSTEM NH3-Kr b y A. M I C H E L S , E. D U M O U L I N ~ a n d J. J. Th. V A N D I J K 165th publication of the Van der Waals Fund Van der Waals-laboratorium, Gerneente Universiteit, Amsterdam, Nederland
Synopsis In a previous publication 1) an instrument has been described for studying the gasliquid phase equilibrium of a two- or multicomponent system. For testing purposes the system NH3-N2-H2 had been used, which, as a first v,pproximation, is assumed to behave like a binary system. The temperature range covered spread from 0-120°C. The present publication describes the extension of this range to lower temperatures (-- 14.90°C and --30.96°C). For operation below room temperature it was necessary t o make two alterations: first, in the temperature control; secondly, in the analyzing apparatus to meet the difficulties connected with the low concentrations encountered of the gas dissolved in the liquid phase. In addition, as an example of a real two-component mixture, the system NHB-Kr has been studied at the temperatures --5.1°C and --20. l°C; the pressure range was limited to 200 atm. and only the composition of the liquid phase was measured. The results for both systems are presented in diagrams, which give the isotherms in the P - x projection.
§ 1. Introduction. The equilibrium vessel, the a r r a n g e m e n t for the loading with gas, a n d the pressure d e t e r m i n a t i o n are the s a m e as h a s been described in detail in a p r e v i o u s p u b l i c a t i o n 1). F o r a b e t t e r u n d e r s t a n d i n g of the new t e m p e r a t u r e regulation of the e q u i l i b r i u m e q u i p m e n t , it is necessary to realize t h a t two actions, which oppose each other, m u s t be present, i.e. a h e a t influx, which tends to raise the t e m p e r a t u r e , a n d a h e a t loss, which t e n d s to lower it. F o r equilibrium the a d j u s t m e n t m u s t be such, t h a t at the desired t e m p e r a t u r e the two opposing factors c o m p e n s a t e each other. This can be a c h i e v e d b y regulating either one or b o t h of these factors. H o w e v e r , in practice the control of the h e a t influx is to be preferred, since the h e a t loss a l r e a d y varies with the u n p r e d i c t a b l e changes in the t e m p e r a t u r e of the s u r r o u n d ings. The h e a t influx is easily controlled b y an electric*al device. Therefore the n o r m a l t e m p e r a t u r e regulation is b a s e d on an a u t o m a t i c a d j u s t m e n t of the h e a t to c o m p e n s a t e the h e a t loss. A b o v e r o o m t e m p e r a t u r e a s a t i s f a c t o r y a r r a n g e m e n t can be m a d e b y the --
840
--
GAS-LIQUID PHASE EQUILIBRIUM IN N H 3 - N g . - H 2 AND N H s - K r
841
use of either a therrnoregula.ted oil bath or a metal block. The heat sink is naturally available in the surroundings, i.e., in the ro6m itself. Below room temperature the use of a metal block is to be preferred, since the high viscosity of the oil necessitates vigorous stirring, which introduces an uncontrollable heat input b y friction. Moreover, it is necessary to supply an artificial heat sink. An instrument is designed to meet the requirements for the measurements below room temperature.
§ 2. Apparatus. A diagrammatic sketch of the apparatus is given in Fig. 1.
.
ESS;~IIII%--al/
S\\\\'i
~
N
D
K
:...'s,,,a
u A ]3 D E
Fig. 1. Apparatus F Sensing element Metal block H Alcohol and C Insulating layers I4 Cooling tubes Equilibrium vessel N Alcohol bath Line heaters
The equilibrium vessel D is surrounded b y a cylindrical metal block A, which has electrical line heaters E on the outer surface. Two heaters are Physica 25
842
A. MICHELS, E. DUMOULIN AND J. J. TH. VAN D I J K
used, one to deliver the bulk of the energy, the other to supply the additional heat needed for the temperature control. The latter is governed b y the sensing element inserted in the metal block at F. A cylindrical bath N, filled with alcohol H, surrounds the metal block to act as a heat sink. In this bath are installed two spiralled copper tubes K, which form part of a refrigeration circuit described in § 3. Through an adjustable nozzle hquid freon is continuously injected. It vaporizes in the tubes, where the heat of evaporation is supplied b y the alcohol. B y adjusting the vapour pressure of the freon the temperature of the alcohol is roughly regulated. The reservoir is surrounded b y an insulating cover to reduce the influx of heat from outside.. For proper operation it is essential that the heat losses of the metal block shall be small and, as far as possible, homogeneous over the outer surface, to ensure a satisfactory heat distribution b y conduction in the block itself. To reduce the heat losses the layers B and C of insulating material are inserted between the metal block and the alcohol reservoir; they act as a thermal resistance. According to a general treatment of temperature control given b y L e v e 1t 2) the maximum temperature variation, A Treg., which occurs in a system with sensitive temperature regulation, is given b y
IAZ,,a.I
__ca =
O,2-Z-S--
L (Tin - - T1),
where: L: heat transport coefficient from metal block to surroundings. 2: heat transport coefficient from sensing element to metal block. cB: heat capacity of the metal block. cs: heat capacity of the sensing element. T1 : the temperature reached when only the large heater is on. (this temperature has to be below the desired one) T2: the temperature reached with both heaters on. (this temperature has to be above the desired one). This general equation implies, that for A Treu. to be small, CB/cs and 2 must be large, and L small. The large mass ratio of metal block to sensing element ensures a correspondingly large ratio of the heat capacities. B y close fitting of the regulator in the metal block the condition of maxim u m value of 2 is fulfilled. A small heat transport coefficient L can be obtained b y a right choice of the insulating layers B and C. The thermal resistance of this layers and the heat capacity of the equilibrium vessel then perform a smoothing action analogous to R -- C damping in an electrical circuit. With the dimensions taken it was possible to reduce the temperature fluctuation to well below 0.01°C.
GAS-LIQUID
PHASE
EQUILIBRIUM
IN
NH3-N2-H~. AND NH3-Kr
843
Aluminium was chosen for the metal block for its low specific weight, although in principle copper has the advantage of a 'still higher heat conductivity. The aluminium block showed to be satisfactory. The accuracy of the results of the present measurements is limited b y the chemical analysis of the samples taken. As heaters pyrotenax cables are chosen for their low heat capacity, which results in a rapid decay of the heat flow to the metal block when the heating is switched off. To ensure a good thermal contact between them and the aluminium block, they are imbedded in grooves in the outer surface.
§ 3. Description o/ the [reon re/rigeration circuit. The cooling system is based on the same principles as the commercial refrigeration units. The details have been chosen to satisfy the requirements of the experiment. A sketch of the circuit is given in Fig. 2. liquid tr¢on conduction (6otto) =
H~
.
.
.
condensor
.
.
.
l
Z
/"
nozzle
I
l' -'7
~Stofa
16atrn mac
e tagnk 5.1 L..
O0
for evac allan
I
J
[ I
LI
-
I I $ I
N /
II
® by pass
I
IF 1
L_ . . . . _1 cryostat
oil separator back fcedin9 conduction " _ for oil ~ J to sump ~
F I
. ~r
,~
low pressure I conduction .
j
I
3otto
max
L_
magnctic['~-~
pressostot '"1 coolin 9 system
J
compressor
motor
used Frton IZ CF2CL 2 vapour pressure O099atrn a t . - - 3 0 a c
Fig. 2. Freon refrigeration circuit. A storage tank contains liquid freon (CF2C12), the vapour pressure of which is about 6 atm. at room temperature. From the tank the liquid freon, forced b y its vapour pressure, flows to an adjustable nozzle, through which it is continuously injected into the cooling tubes, as described earlier. A compressor, driven b y an l½ H.P. motor, sucks the evaporated freon and recompresses it. The gas then passes through an oil separator and a water-
844
A. MICHELS, E. DUMOULIN AND J. J. TH. VAN D I J K
cooled heat exchanger, where it liquefies. The liquid freon .flows through an silica-gel filter, where residual traces of oil and other impurities are absorbed and then returns to the storage tank. The cooling unit has been provided with two pressostats. The first is connected to the high-pressure side of the compressor and acts purely as a safety device. If accidently the pressure tends to surpass the safety limit, this pressostat stops the motor. The second pressostat is coupled to the suction side of the compressor to maintain the vapour pressure in the cooling tubes within the range set b y the operaton As soon as the pressure reaches the upper limit the motor starts; it stops again when the pressure has dropped about 0.4 atm. The corresponding temperature fluctuation of the alcohol bath in the experimental region is at most 0.5°C. For a smooth operation of the compressor it was necessary to add a buffertank to the low pressure side 6f the compressor. The oil separator has a back feed connection to the oil sump of the compressor. As soon as the oil level in the separator rises above a given limit a valve opens and the oil drains back to the sump. Before use the freon m a y be dried. This is performed b y leading the liquid freon through an electrically heated tube, where i t v a p o r i z e s and then b y passing the vapour through a tube filled with BaO.
§ 4. Analysis o] the gas and liquid phase. The equilibrium composition of gas and liquid phase is determined from samples taken. As described previously the method of sampling guarantees an unchanged pressure and composition in the equilibrium cell. The technique of analysis had to be modified so as to allow the determination of the low concentrations of gas dissolved in the liquid with sufficient precision. The flask containing the sample is placed in a thermoregulated bath and connected to the glass system sketched in Fig. 3. The volume between the stopcocks 1, 2 and 3 and the mercury level in the differential manometer C is kept small as compared to the volume of the flask. With 3 closed the glass system is evacuated, after which 3 is opened. Then l i s closed and b y raising or lowering the reservoir G the mercury meniscus in the right hand leg of C is adjusted at as high a level as possible to reduce the dead volume. The pressure Pl is then determined from the mercury levels in E and C. A measured amount (I 0 cc) sulfuric acid is added to the gas mixture in the flask, where it absorbs the ammonia present. After a readjusting of the mercury level in the differentialmanometer C in the initial position, the pressure p~ of the residual gas is measured. F r o m the pressure ratio (Pl--P2)/Pl the percentage of ammonia is calculated. Corrections have to be applied for the volume of the sulfuric acid and for the solubility of the gas in the acid.
GAS-LIQUID PHASE EQUILIBRIUM IN NH3-N2-H2 AND NHs-Kr
845
For security sake before taking the sample, the quantity of any air left in the flask was measured with a Mac Leod manometer.' At low temperatures the solubility of gas in the liquid phase is low. This results in a low residual pressure during the analysis of the liquid phase, so that the determination of the pressure p2 with the manometer system is not sufficiently accurate. The residual pressure P2 is then measured with the help of a Mac Leod connected at B.
to
VQcuum pump
H2SO4 4
i-L
3
to Moc Leod
c
water both
Fig. 3. Apparatus for the analysis of the gas and liquid phase.
The accuracy of the method of analysis described is estimated to be one part in thousand.
§ 5. Results o/the system NHa-N2-H2. The compositions of liquid and gas phase at the different temperatures and pressures are represented in Fig. 4a and Fig. 4b and in Table I. The columns give respectively: the total pressure during each experiment; the concentrations of ammonia in gas and liquid phases; the partial vapour pressure Pl of ammonia; and the ratio of this
846
A. MICHELS, E. DUMOULIN AND I" J. TH. VAN D I J K
I000
system NH3--Nz-.Hz liquid phase
aim
P
"~14"9oOc
~--30"96°C
1
T l
3%
z
J
OO
o
Fig. 4a. The pressure-composition diagram of the gas phase.
800~L-~ °'mill
P
system NH3- N2-Hz
I
9asphas,
oOc
NH3
0
lo
20
30%
Fig. 4b. The pressure-composition diagram of the liquid phase.
°••
GAS-LIQUID PHASE EQUILIBRIUM IN NHs-N2-H2 AND NHs-Kr _5.IC
NH3_Kr
C°
utm 200
i
150
-~
~
I00
% Kr•
5O
~_~. 3
4
847
Z
I
O
Fig. 5. The pressure-composition diagram of the Hquid phase. TABLE
I
N Ha-N2-Ha-equilibrium t: - - 3 0 . 9 6 ° C P atm.
Po: 1.126 a t m .
% N H a in g a s
9.25 24.17 49.33 74.27 99.32 300.00 398.78 598.82 799.43
% N H 3 in l i q u i d
PI: p a r t i a l v a p o u r p r e s s u r e of a m m o n i a
Pt]Po
991946 99.895 99.842 99.760 99.705 99.240 98.94 98.61 98.33
1.0101 1.1577 1.1936 1.374 o 1.489 o 2.280 o 2.8313 3.7726 4.7166
0.897 1.028 1.060 1.220 1.323 2.025 2.514 3.350 4.189
10.92 4.79 2.42 1.85 1.80 0.76 0.71 0.63 0.58
t: - - 1 4 . 9 0 ° C P atm. 9.22 24.30 49.37 74.34 99.36 200.60 300.06 498.68 800.95
po: 2 . 3 3 4 a t m .
% N H 3 in g a s
99.961 99.878 99.790 99.664 99.537 99.180 98.85 98.35 97.66
2.190 2.342 2.493 2.669 2.981 3,852 4.741 6.483 9.291
23.75 9.64 5.05 3.59 3.00 1.92 1.58 1.30 1.16 TABLE
NH3-Kr
% N H 3 in l i q u i d
Pt: partial vapour pressure of a m m o n i a
IIA
t : - - 2 0 . 1 °C
TABLE NHs-Kr
Pt/Po 0.938 1.004 1.068 1.143 1.277 1.650 2.031 2.777
3.980 lIB $: - - 5 . 1 ° C
P atm.
% N H 3 in l i q u i d
P atm.
% N H 3 in li~luid
25.082 50.039 100.095 200.447
99.34 98.80 98.18 97.61
25.114 50.077 100.095 175.085
99.38 98.53 97.41 96.70
848
GAS-LIQUID PHASE EQUILIBRIUM IN
NHs-N2-H2
AND
NHs-Kr
partial pressure to the vapour pressure P0 of pure ammonia at the same temperature. From the last column it is obvious that Raoult's law breaks down and that at high pressures the vapour pressure of ammonia has increased several times by the influence of the inert gas.
§ 6. Results o~ the system NH3-Kr. Measurements were made of the compositions of the liquid phase only, at two temperatures --5.1 °C and --20.1 °C and up to pressures of 175 atm. and 200 atm. respectively. The concentration of ammonia in the liquid phase is given in Fig. 5 and Tables IIA and liB at the different pressures and temperatures. The measurements were limited to 200 atm. only and not extended in the gaseous region for lack of a sufficient quantity of krypton, § 7. Discussion. Comparison of the two systems shows that under equal circumstances the solubility of krypton in liquid, ammonia is much greater than that of the N~-H~ mixture. It was observed that the time necessary to attain equilibrium was much longer for NH3-Kr than for the case of NHs-N2-H2. Apparently the diffusion velocity of krypton- lags far behind that of hydrogen and nitrogen. Several reasons may b e suggested as, for instance, the high molecular weight, the large atomic radius and the higher polarisability. Received 9-6-59
REFERENCES 1) Michels, A., S k e l t o n , G. and D u m o u l i n , E., Physica 16 (1950) 831. 2) L e v e l t , A., Thesis, van Gorcum, Assen (1958).
Physica 25 840-848
GAS-LIQUID PHASE EQUILIBRIUM BELOW 0°C IN THE SYSTEM NH3-N2-H 2 AND IN THE SYSTEM NH3-Kr b y A. M I C H E L S , E. D U M O U L I N ~ a n d J. J. Th. V A N D I J K 165th publication of the Van der Waals Fund Van der Waals-laboratorium, Gerneente Universiteit, Amsterdam, Nederland
Synopsis In a previous publication 1) an instrument has been described for studying the gasliquid phase equilibrium of a two- or multicomponent system. For testing purposes the system NH3-N2-H2 had been used, which, as a first v,pproximation, is assumed to behave like a binary system. The temperature range covered spread from 0-120°C. The present publication describes the extension of this range to lower temperatures (-- 14.90°C and --30.96°C). For operation below room temperature it was necessary t o make two alterations: first, in the temperature control; secondly, in the analyzing apparatus to meet the difficulties connected with the low concentrations encountered of the gas dissolved in the liquid phase. In addition, as an example of a real two-component mixture, the system NHB-Kr has been studied at the temperatures --5.1°C and --20. l°C; the pressure range was limited to 200 atm. and only the composition of the liquid phase was measured. The results for both systems are presented in diagrams, which give the isotherms in the P - x projection.
§ 1. Introduction. The equilibrium vessel, the a r r a n g e m e n t for the loading with gas, a n d the pressure d e t e r m i n a t i o n are the s a m e as h a s been described in detail in a p r e v i o u s p u b l i c a t i o n 1). F o r a b e t t e r u n d e r s t a n d i n g of the new t e m p e r a t u r e regulation of the e q u i l i b r i u m e q u i p m e n t , it is necessary to realize t h a t two actions, which oppose each other, m u s t be present, i.e. a h e a t influx, which tends to raise the t e m p e r a t u r e , a n d a h e a t loss, which t e n d s to lower it. F o r equilibrium the a d j u s t m e n t m u s t be such, t h a t at the desired t e m p e r a t u r e the two opposing factors c o m p e n s a t e each other. This can be a c h i e v e d b y regulating either one or b o t h of these factors. H o w e v e r , in practice the control of the h e a t influx is to be preferred, since the h e a t loss a l r e a d y varies with the u n p r e d i c t a b l e changes in the t e m p e r a t u r e of the s u r r o u n d ings. The h e a t influx is easily controlled b y an electric*al device. Therefore the n o r m a l t e m p e r a t u r e regulation is b a s e d on an a u t o m a t i c a d j u s t m e n t of the h e a t to c o m p e n s a t e the h e a t loss. A b o v e r o o m t e m p e r a t u r e a s a t i s f a c t o r y a r r a n g e m e n t can be m a d e b y the --
840
--
GAS-LIQUID PHASE EQUILIBRIUM IN N H 3 - N g . - H 2 AND N H s - K r
841
use of either a therrnoregula.ted oil bath or a metal block. The heat sink is naturally available in the surroundings, i.e., in the ro6m itself. Below room temperature the use of a metal block is to be preferred, since the high viscosity of the oil necessitates vigorous stirring, which introduces an uncontrollable heat input b y friction. Moreover, it is necessary to supply an artificial heat sink. An instrument is designed to meet the requirements for the measurements below room temperature.
§ 2. Apparatus. A diagrammatic sketch of the apparatus is given in Fig. 1.
.
ESS;~IIII%--al/
S\\\\'i
~
N
D
K
:...'s,,,a
u A ]3 D E
Fig. 1. Apparatus F Sensing element Metal block H Alcohol and C Insulating layers I4 Cooling tubes Equilibrium vessel N Alcohol bath Line heaters
The equilibrium vessel D is surrounded b y a cylindrical metal block A, which has electrical line heaters E on the outer surface. Two heaters are Physica 25
842
A. MICHELS, E. DUMOULIN AND J. J. TH. VAN D I J K
used, one to deliver the bulk of the energy, the other to supply the additional heat needed for the temperature control. The latter is governed b y the sensing element inserted in the metal block at F. A cylindrical bath N, filled with alcohol H, surrounds the metal block to act as a heat sink. In this bath are installed two spiralled copper tubes K, which form part of a refrigeration circuit described in § 3. Through an adjustable nozzle hquid freon is continuously injected. It vaporizes in the tubes, where the heat of evaporation is supplied b y the alcohol. B y adjusting the vapour pressure of the freon the temperature of the alcohol is roughly regulated. The reservoir is surrounded b y an insulating cover to reduce the influx of heat from outside.. For proper operation it is essential that the heat losses of the metal block shall be small and, as far as possible, homogeneous over the outer surface, to ensure a satisfactory heat distribution b y conduction in the block itself. To reduce the heat losses the layers B and C of insulating material are inserted between the metal block and the alcohol reservoir; they act as a thermal resistance. According to a general treatment of temperature control given b y L e v e 1t 2) the maximum temperature variation, A Treg., which occurs in a system with sensitive temperature regulation, is given b y
IAZ,,a.I
__ca =
O,2-Z-S--
L (Tin - - T1),
where: L: heat transport coefficient from metal block to surroundings. 2: heat transport coefficient from sensing element to metal block. cB: heat capacity of the metal block. cs: heat capacity of the sensing element. T1 : the temperature reached when only the large heater is on. (this temperature has to be below the desired one) T2: the temperature reached with both heaters on. (this temperature has to be above the desired one). This general equation implies, that for A Treu. to be small, CB/cs and 2 must be large, and L small. The large mass ratio of metal block to sensing element ensures a correspondingly large ratio of the heat capacities. B y close fitting of the regulator in the metal block the condition of maxim u m value of 2 is fulfilled. A small heat transport coefficient L can be obtained b y a right choice of the insulating layers B and C. The thermal resistance of this layers and the heat capacity of the equilibrium vessel then perform a smoothing action analogous to R -- C damping in an electrical circuit. With the dimensions taken it was possible to reduce the temperature fluctuation to well below 0.01°C.
GAS-LIQUID
PHASE
EQUILIBRIUM
IN
NH3-N2-H~. AND NH3-Kr
843
Aluminium was chosen for the metal block for its low specific weight, although in principle copper has the advantage of a 'still higher heat conductivity. The aluminium block showed to be satisfactory. The accuracy of the results of the present measurements is limited b y the chemical analysis of the samples taken. As heaters pyrotenax cables are chosen for their low heat capacity, which results in a rapid decay of the heat flow to the metal block when the heating is switched off. To ensure a good thermal contact between them and the aluminium block, they are imbedded in grooves in the outer surface.
§ 3. Description o/ the [reon re/rigeration circuit. The cooling system is based on the same principles as the commercial refrigeration units. The details have been chosen to satisfy the requirements of the experiment. A sketch of the circuit is given in Fig. 2. liquid tr¢on conduction (6otto) =
H~
.
.
.
condensor
.
.
.
l
Z
/"
nozzle
I
l' -'7
~Stofa
16atrn mac
e tagnk 5.1 L..
O0
for evac allan
I
J
[ I
LI
-
I I $ I
N /
II
® by pass
I
IF 1
L_ . . . . _1 cryostat
oil separator back fcedin9 conduction " _ for oil ~ J to sump ~
F I
. ~r
,~
low pressure I conduction .
j
I
3otto
max
L_
magnctic['~-~
pressostot '"1 coolin 9 system
J
compressor
motor
used Frton IZ CF2CL 2 vapour pressure O099atrn a t . - - 3 0 a c
Fig. 2. Freon refrigeration circuit. A storage tank contains liquid freon (CF2C12), the vapour pressure of which is about 6 atm. at room temperature. From the tank the liquid freon, forced b y its vapour pressure, flows to an adjustable nozzle, through which it is continuously injected into the cooling tubes, as described earlier. A compressor, driven b y an l½ H.P. motor, sucks the evaporated freon and recompresses it. The gas then passes through an oil separator and a water-
844
A. MICHELS, E. DUMOULIN AND J. J. TH. VAN D I J K
cooled heat exchanger, where it liquefies. The liquid freon .flows through an silica-gel filter, where residual traces of oil and other impurities are absorbed and then returns to the storage tank. The cooling unit has been provided with two pressostats. The first is connected to the high-pressure side of the compressor and acts purely as a safety device. If accidently the pressure tends to surpass the safety limit, this pressostat stops the motor. The second pressostat is coupled to the suction side of the compressor to maintain the vapour pressure in the cooling tubes within the range set b y the operaton As soon as the pressure reaches the upper limit the motor starts; it stops again when the pressure has dropped about 0.4 atm. The corresponding temperature fluctuation of the alcohol bath in the experimental region is at most 0.5°C. For a smooth operation of the compressor it was necessary to add a buffertank to the low pressure side 6f the compressor. The oil separator has a back feed connection to the oil sump of the compressor. As soon as the oil level in the separator rises above a given limit a valve opens and the oil drains back to the sump. Before use the freon m a y be dried. This is performed b y leading the liquid freon through an electrically heated tube, where i t v a p o r i z e s and then b y passing the vapour through a tube filled with BaO.
§ 4. Analysis o] the gas and liquid phase. The equilibrium composition of gas and liquid phase is determined from samples taken. As described previously the method of sampling guarantees an unchanged pressure and composition in the equilibrium cell. The technique of analysis had to be modified so as to allow the determination of the low concentrations of gas dissolved in the liquid with sufficient precision. The flask containing the sample is placed in a thermoregulated bath and connected to the glass system sketched in Fig. 3. The volume between the stopcocks 1, 2 and 3 and the mercury level in the differential manometer C is kept small as compared to the volume of the flask. With 3 closed the glass system is evacuated, after which 3 is opened. Then l i s closed and b y raising or lowering the reservoir G the mercury meniscus in the right hand leg of C is adjusted at as high a level as possible to reduce the dead volume. The pressure Pl is then determined from the mercury levels in E and C. A measured amount (I 0 cc) sulfuric acid is added to the gas mixture in the flask, where it absorbs the ammonia present. After a readjusting of the mercury level in the differentialmanometer C in the initial position, the pressure p~ of the residual gas is measured. F r o m the pressure ratio (Pl--P2)/Pl the percentage of ammonia is calculated. Corrections have to be applied for the volume of the sulfuric acid and for the solubility of the gas in the acid.
GAS-LIQUID PHASE EQUILIBRIUM IN NH3-N2-H2 AND NHs-Kr
845
For security sake before taking the sample, the quantity of any air left in the flask was measured with a Mac Leod manometer.' At low temperatures the solubility of gas in the liquid phase is low. This results in a low residual pressure during the analysis of the liquid phase, so that the determination of the pressure p2 with the manometer system is not sufficiently accurate. The residual pressure P2 is then measured with the help of a Mac Leod connected at B.
to
VQcuum pump
H2SO4 4
i-L
3
to Moc Leod
c
water both
Fig. 3. Apparatus for the analysis of the gas and liquid phase.
The accuracy of the method of analysis described is estimated to be one part in thousand.
§ 5. Results o/the system NHa-N2-H2. The compositions of liquid and gas phase at the different temperatures and pressures are represented in Fig. 4a and Fig. 4b and in Table I. The columns give respectively: the total pressure during each experiment; the concentrations of ammonia in gas and liquid phases; the partial vapour pressure Pl of ammonia; and the ratio of this
846
A. MICHELS, E. DUMOULIN AND I" J. TH. VAN D I J K
I000
system NH3--Nz-.Hz liquid phase
aim
P
"~14"9oOc
~--30"96°C
1
T l
3%
z
J
OO
o
Fig. 4a. The pressure-composition diagram of the gas phase.
800~L-~ °'mill
P
system NH3- N2-Hz
I
9asphas,
oOc
NH3
0
lo
20
30%
Fig. 4b. The pressure-composition diagram of the liquid phase.
°••
GAS-LIQUID PHASE EQUILIBRIUM IN NHs-N2-H2 AND NHs-Kr _5.IC
NH3_Kr
C°
utm 200
i
150
-~
~
I00
% Kr•
5O
~_~. 3
4
847
Z
I
O
Fig. 5. The pressure-composition diagram of the Hquid phase. TABLE
I
N Ha-N2-Ha-equilibrium t: - - 3 0 . 9 6 ° C P atm.
Po: 1.126 a t m .
% N H a in g a s
9.25 24.17 49.33 74.27 99.32 300.00 398.78 598.82 799.43
% N H 3 in l i q u i d
PI: p a r t i a l v a p o u r p r e s s u r e of a m m o n i a
Pt]Po
991946 99.895 99.842 99.760 99.705 99.240 98.94 98.61 98.33
1.0101 1.1577 1.1936 1.374 o 1.489 o 2.280 o 2.8313 3.7726 4.7166
0.897 1.028 1.060 1.220 1.323 2.025 2.514 3.350 4.189
10.92 4.79 2.42 1.85 1.80 0.76 0.71 0.63 0.58
t: - - 1 4 . 9 0 ° C P atm. 9.22 24.30 49.37 74.34 99.36 200.60 300.06 498.68 800.95
po: 2 . 3 3 4 a t m .
% N H 3 in g a s
99.961 99.878 99.790 99.664 99.537 99.180 98.85 98.35 97.66
2.190 2.342 2.493 2.669 2.981 3,852 4.741 6.483 9.291
23.75 9.64 5.05 3.59 3.00 1.92 1.58 1.30 1.16 TABLE
NH3-Kr
% N H 3 in l i q u i d
Pt: partial vapour pressure of a m m o n i a
IIA
t : - - 2 0 . 1 °C
TABLE NHs-Kr
Pt/Po 0.938 1.004 1.068 1.143 1.277 1.650 2.031 2.777
3.980 lIB $: - - 5 . 1 ° C
P atm.
% N H 3 in l i q u i d
P atm.
% N H 3 in li~luid
25.082 50.039 100.095 200.447
99.34 98.80 98.18 97.61
25.114 50.077 100.095 175.085
99.38 98.53 97.41 96.70
848
GAS-LIQUID PHASE EQUILIBRIUM IN
NHs-N2-H2
AND
NHs-Kr
partial pressure to the vapour pressure P0 of pure ammonia at the same temperature. From the last column it is obvious that Raoult's law breaks down and that at high pressures the vapour pressure of ammonia has increased several times by the influence of the inert gas.
§ 6. Results o~ the system NH3-Kr. Measurements were made of the compositions of the liquid phase only, at two temperatures --5.1 °C and --20.1 °C and up to pressures of 175 atm. and 200 atm. respectively. The concentration of ammonia in the liquid phase is given in Fig. 5 and Tables IIA and liB at the different pressures and temperatures. The measurements were limited to 200 atm. only and not extended in the gaseous region for lack of a sufficient quantity of krypton, § 7. Discussion. Comparison of the two systems shows that under equal circumstances the solubility of krypton in liquid, ammonia is much greater than that of the N~-H~ mixture. It was observed that the time necessary to attain equilibrium was much longer for NH3-Kr than for the case of NHs-N2-H2. Apparently the diffusion velocity of krypton- lags far behind that of hydrogen and nitrogen. Several reasons may b e suggested as, for instance, the high molecular weight, the large atomic radius and the higher polarisability. Received 9-6-59
REFERENCES 1) Michels, A., S k e l t o n , G. and D u m o u l i n , E., Physica 16 (1950) 831. 2) L e v e l t , A., Thesis, van Gorcum, Assen (1958).