A glass ebullioscope was built and at atmospheric pressure, liquid-vapor equilibria relative to the Br2-HBr-H2O system, in the concentration range of interest for evaluation of the Mark 13 cycle was studied. Measurements were performed for the brome-azeotrope (HBr-H2O) pseudo-binary system and for the ternary system at temperatures lower than 125 C and in the bromine concentration range up to 13% wt

Benizri, R.

Courvoisier, P.

Lessart, P.

English

NASA Technical Reports Server

NASA TECHNICAL YEMON\NDUM NASA TM-77787 
N85- 1 5  836 IaPsA-Tfi-77787) STIUDP CF T E E  LIQUID VAECB 
a 2 t l X L I B R X W  XB1 'LtiE BSC8INE-HYDLOBBOIIC 
ACID-YATBR SYSTEM (National  Aeronautics and 
Space B d a i n i s t r a t i o n )  17 F HC A02/aE A01 Ooclas 
CSCL 07D 63/25 13473 
STUDY OF THE LIQUID-VATTOR EQUILIBRIUM IN 
THE BD!311NE-H'ITDROBROMIC ACID-WATER SYSTEM 
R. Benizri, P. Lessart, P. Courvoisier 
Translation of "Etude de l'equilibre de 
phases liquides-qaz dans le systeme brome- 
acide bromhydrique-eau" . In: Conmimicatinn of 
the European Communities I t .  Report EUR 6783, 
1980, Hydrogen Energy Vector, pp 101-116. 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
XASHINGTON D.C. 20546 DECElYRER 1984 
https://ntrs.nasa.gov/search.jsp?R=19850007527 2020-03-20T19:45:37+00:00Z
WAUO- v m e  - . _ -  
1 L 0h.rrmC A W O B ~ ~  k. a a m . y r * a  LC.. 
TJHE .LIQUID+-VAPOR- 
EQUILIBRIUM IN THE BROMINE-HYDROBROMIC 
ACID-VATER SYSTEM 
t. *&add 
R. B e n i z r i ,  P. Lessart and P. Comisier 
W S C E C F  - CEN/sACLAY 
9. p.rL.r*ao &amti m a t h a  w.n .  a d  A-BO 
8. Codo-ro OIa*lca.c(n Roew No. 
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WASv -4004 
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a t  
VULUWtoB, D.C. a546 w&--m4-wo. 
- .  . .. - - . . - - - - 1s *-w.k. 
Translation of "Etude de l'equilibre de phases liquides- 
gaz dans le system brome-acide bromhydrique-eau". 
Communication of the European Communities. Report 
EUR 6783, 1980, Hydrogen Energy Vector, pp 101-116. 
I& rrhmr,' 
A glass ebullioscope was built and a method developed to study, at 
atmospheric pressure, liquid-vapour equilibria relative to the Br - 
HBr-i 0 system, in the concentration range of interest for evaluakon 
of ti8 MARI= 13 cycle. 
azeotrope (HBr-H 0) pseudo-binary system and for the ternary system 
a t  tempercltzcs ?wcr t-su 1-3 cI z.2 ii t;.c Lroiiiiie coiiccirtratioit 
range up to 13% w t .  
'4easurements were preformed for h\e brome- 
-eo n 
ii 
STUDY OF LIQUID-GAS PHASE EQUILIBRIUM IN THE 
BROMINE-HYDROBROMIC ACID-WATER SYSTEM 
1. INTRODUCTION 
/102* 
The "Mark 13" chemical decomposition cycle of water that was analyzed 
at the Centre d'Ispra (and set up in a laboratory loop operating con- 
tinuously and producing 100 1.h-l of hydrogen) Cl] includes 3 steps: 
(1) Br2 + SO2 + 2 H20 -> H,S04 + 2HBr 
L 
(2)  H2S04 + H20 + SO2 + 402 
(3) 2HBr + Br2 + H2 (electrolysis) 
To calculate the amount of energy needed to separate the bromine pro- 
duced by electrolysis from the hydrobromic acid in the presence of 
water through distillation, and to determine the size of the corres- 
ponding columns, it is necessary to know the concentrations at the 
liquid-vapor phase equilibrium of the H20-HBr-Jr2 ternary system for 
certain ranges of interest for the process. As requested by the 
Ispra engineers, this equilibrium was studied at Saclay as part of a 
contract with the Commission of the European Communities (project 
H/A22). 
A summary of the bibliographic study concerning the H20-HBr-Br2 
ternary system may be found in the reports from the preceding semi- 
nar on hydrogen organized by the CEC [2]. It is reminded that the 
liquid-vapor equilibrium diagram of binary system H20-:IBr is known 
at atmospheric pressure [3J. The miscibility range of the pseudo- 
ternary bromine azeotrope (H20-HBr 47%) was defined at 2OoC [4]; 
this is shown in Figure 1. The properties of the solutions formed 
by the 3 compounds were also studied in 5 , 
In order to determine the liquid-gas phase equilibriums of ternary 
system Br2-HBr-H20 we built a glass ebullioscope and designed a 
procedure c21.  Measurements were taken at atmospheric pressure in 
the concentration range of interest for the Mark 13 process. 
* Numbers i n  margin  i n d i c a t e  f o r e i g n  p a g i n a t i o n .  
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2. EBULLIOSCOPE AND MEASUREMENT THEORY 
The ebullioscope has t dual function: first, it enables the liquid / l i ~ 4  
and vapor phases to be put into equilibrium, with measurement of the 
temperature at fixed.values of pressure; second,. it enables the 
separation and taking of samples of the phases in equilibrium. 
The samples are then analyzed. The CATHALA ebullioscope L63 used 
is schematized in Figure 3. A preheated liquid and a superheated 
vapor flow into a mixer at a constant rate. 
of two perforated plates and a COTTRELL nozzle equipped with 
VIGREUX points. The closely mixed liquid-vapor mixture wets a ther- 
mometric sheath before it completely separates into two phases which 
are cooled and condensed and then sent to the sampling outlets. 
The mixer is composed 
The instrument underwent a preliminary calibration to determine: 
- The flow rates not to be exceeded in order to prevent 
droplets of liquid from being drawn along with the vapor; 
- The amount of time the fluids remain in the space between 
the thermocouple sheaths and the bleed valves. 
The various equilibrium points are studied by varying the supply 
flows, the temperatures, and the concentrations of the entering li- 
quids. 
Proper operation of the ebullioscope greatly depends upon the 
supply conditions. 
The primary problems encountered were due to the highly active nature of 
the fluids studied. Only viton and teflon should be used for the 
flexible connections of the i n s t r u m e n t -  Beyond 100°C teflon does 
not withstand prolonged contact with the liquid phases. 
3 
Thermocouple sheath 
Vapor p k e  outlet 
Perforated plates 
- - - - - - _ -  - -  - - - -_ 
I 
Density ;iBr % mass I NHBr N B r Z  
eq. H B r / l  Zq. B r / l  
a 
~ 20,7 8,04 3,53 1,653 39,30 
21,6 8,44 1,91 1,584 43,12 
21,6 7,22 7,71 1,865 31,56 
22,4 4,81 3,66 1,487 26,24 
20,6 5 , 7 0  5,33 1,634 28,30 
20,6 5,50 7,95 1,776 25,14 
20,8 6,04 12,93 2,104 23,40 
20,6 7,95 4,02 1,682 38,33 
Fig. 3 - CATHALA ebullioscope 
TABLE I 
Densities of HBr - B r 2  - H20 solution for 
varying vaiues of normality N and temperature T 
B r 2  % mass 
17,06 
49,47 
9,62 
33.28 
19,72 
19r14 
35,89 
26,14 
4 
3. DENSITIES OF SOLUTIONS 
Proportioning of the liquid and vapor phases in equilibrium leads to 
determination of the number of moles of hydrobromic acid and bromine 
per liter of solution. 
tages W by the densities of the corresponding solutions according 
to the following formulas: 
/lo6 
These molarities are linked to mass percen- 
8.092 MHBr - 
'HBr d 
15.9808 MBr, 
L -  
'Br2 d 
where M represents the molar concentrations of HBr and Br2 in the 
solution of density d. 
We had first used density values evaluated acr-ording to documentation 
based on the empirical law: d = A(YHBr + NBr 
are constants and NHBr and NBr are normalit?es of the solution; den- 
2 
sity is expressed linearly depending on the total number of bromine 
equivalents contained in a liter of solution, with the bromine equi- 
valents originating from the HBr or Br2 having the same'beight''. 
' +  B, where A and B 
For cross-checking purposes, and in ordQr to more closely evaluate 
the mass percentages, we performed some experimental determinations 
of the density of the HBr - Br2 - H20 solutions. In order to do this 
we worked at ambient temperature using pycnometers whose volumes were 
determined at the temperature considered in relation to water at 4OC. 
The results, according to the temperature and normality of the solu-  
tions, are given in Table I. Within the limits of experimental accur- 
acy, our results agree with those of the existing documentation. 
In addition, we calculated, through the method of the least squares, the 
equations that are likely to correlata with greater accuracy than the 
linear lay +he densities given at 2 O o C  by the.documentation and the nor- 
malities of the bromine and hydrobromic acid solutions. 
5 
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..I d - N N N 0 W 0 W, W, CI (0 w 0, 
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  
. . . . . . . . . .  9 . S .  
~ W ~ Q ~ W ~ Q O ~ W Q N V ~  
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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  
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N v N r- w o m o a3 o (u o r- UI 
F O O ~ - ~ . O N - Q O N - N  . . . . . . . . . . . . . .  
- 0 N 0 0 0 0 0 0 0 0 0 0 0  
V O ~ V ~ N - ~  a o 4 - o  
m m 0 - n m O n m m ~ a m ~  
p . w ~ r - m m w r - m n m r - v m  
. . . . . . . . . . . . . .  
+ 
r m - m n n n n o - o o n w  N 9 
v 0 w 0 0 - 0 0 0 0 0.0- 0 0 
0 0 0 0 0 0 0 0 0 0 0 0 0 0  . .. . . . . . . . . .  
o o o r - m ~ m - m - + w w v  
m o m - n a . m p . v w m m m w  
m w N m m a N m r - - - t N W  
0 0 ~ 0 0 0 3 0 0 0 0  3 0 0  
. . . . . . . . . . . . . .  
It appears that the best correlation is of the following type: 
d A.NHBr + B*NBr + C*NHBr’NBr2 + D  
2 
In this formula the bromine equivalents no longer have the same 
influence, depending on w’lether they come from hydrobromic acid or 
bromine; nevertheless their respective ‘height# are .xry similar, 
with constants A and B being very close. For information purposes 
the values of the constants are as follows: 
A = 5.5859 log2 
C = -9.7082 10” 
B = 5.8nb9 1CmB2 
D = 1.00C2 
4. EXPERIMENTAL RESULTS 
The results of the ebullioscope measurements for the pseudo-binary 
mixture of Br2-azemtrope 
and represented in Figure 2. The experimental results of the phase 
equilibrium study for terLiary system Br2-HBr-H20 are shown in Table I11 
for small concentrations of bromine and in Table IV for larger con- 
certrations. 
(HBr-H20) are grouped together in Table IT 
The experimental points of Table IV were located in a ry diagram: 
in Figures 4 and 5.the points bearing t he  same numbers (defining the 
experiment - refer to Table IV) correspond to the phases in equi- 
librium; the points reFresented by circles give the compcsitlon of the 
vapor phase, whereas those represented by a cross gi7.C . ‘ -* t  of the 
liquid phase. As for Table 111, the proximity of th6  r imen t a 1 
points makes any representation on a ternary diagram A ,,~sible. 
The compositions of the liquid and vapor phases in equilibrium are 
shown for different temperature values T (“C) (first column). The 
results given in the other Columr,~ are described as follows: 
10 
Q s a 
11 
- "Moles H B r / l "  r e p r e s e n t s  t h e  mola r i ty  of H B r  i n  t h e  l i q u i d  o r  vapor 
phase a t  ambient temperature .  This mola r i ty  i s  ob ta ined  by metric 
ac id-base  yH propor t ion ing .  
1113 
- "Moles B r 2 / 1 "  r e p r e s e n t s  t h e  mola r i+*p  of B r 2  i n  t h e  l i q u i d  o r  
vapor phase a t  ambient temperature.  This m o l a r i t y  is ob ta ined  by 
po ten t iome t r i c  oxydat ion- reduct ion  p ropor t ion ing  of i o d i n e  by sodium 
h y 2 o s u l f i t e  accord ing  t o  t h e  fo l lowing  r e a c t i o n :  
-> s 02- + 2 I- 2 s20;- + I2 4 6  
The bromine, which i s  a s t r o n g e r  o x y d i z e r t h a n i o d i n e ,  oxydizes  t h e  
t h i o s u l f a t e  i n t o  s u l f a t e  under v a r i a b l e  c o n d i t i o n s .  2ilS i s  why 
t h e  p ropor t ion  of bromine is c o n t r o l e d  by adding  excess  K I  b e f o r e  
t i t r a t i n g  t h e  r e l e a s e d  i o d i n e  wi th  t h i o s u l f a t e  
Br2 + 2 1- -> I + 2Br- 2 
I n  a very a c i d  environment, s i n c e  t h e  i o d i n e  can  be ox id ized  by t h e  
oxygen i n  t h e  a i r ,  p ropor t ion ing  i s  performed i n  an  argcil atmosphere 
and i n  a s o l u t i o n  a d j u s t e d  t o  a pH number of 5 (acetate s o l u t i o n ) .  
Dens i ty"  r e p r e s e n t s  t h e  mass pe r  u n i t  volume i n  g/cm' of t h e  l i q u i d  8 )  
o r  vapor phases a t  ambient temperature .  
"HBr mass Z" r e p r e s e n t s  t h e  mass of H B r  con ta ined  i n  100 grams of 
s o l u t i o n ,  e i t h e r  i n  t h e  l i q u i d  o r  vapor phase,  a t  ambient tempera- 
t u r e .  
This  va lue  i s  c a l c u l a t e d  us ing  t h e  molar mass of H B r ,  t h e  mola r i ty  
determined expe r imen ta l ly ,  and t h e  d e n s i t y  5 . a l u a t e d  accord ing  t o  
t h e  documentation. 
B r 2  % mass" l i k e w i s e  r e p r e s e n t s  t h e  weight t i t r e  of t h e  s o l u t i o n  I t  
i n  B r 2 .  
The c o n c e n t r a t i o m o f  H B r  and B r 2  given f o r  t h e  vapor  phase r e p r e s e n t  
12 
an average concentration of two liquid phases generally obtained 
after condensation of the vapor. 
for the liquid phase does not exceed 13.3 X bromine 
/ i14 
The maximum concentration obtained 
5 .  DISCUSSION 
In the course of this woLk about sixty different liquid-vapor 
phase equilibrim points for the HBr-Br -H 0 system were determined 
for the corresponding temperatures. 
of the ternary diagram was explored. 
2 2  
However, only a limited area 
If one wished to extend the stddy of this diagram, two major diffi- 
culties would be encountered: the existence of a vast zone of partial 
mixing in the liquid phase and the extremely corrosive nature of the 
products . 
Since the gaseous phase in equilibrium with the liquid phase is very 
rich in bromine, in order to extend the realm of study it would be 
necessary to modify the Cathala ebullioscope to enable injection of 
a liquid phase having a similar composition to that desired, as well 
as two gaseous phases in suitable proportions - one rich in Br2 and 
therefore very poor in HBr and H20, the other very poor in Br2 and 
with an HBr concentration such that, when the two gaseous phases are 
mixed, a gas of similar composition to that of the gaseous phase in 
equilibrium with the added liquid phase is obtained. In this case 
the Cathala ebullioscope would be equipped with a preheated-liquid 
input and two quartz atomizers with different heating capacities. 
The input lines should be welded to avoid having couplings in the 
hot zone. The other products should be supplied without pulsations 
by a nonvermisular proportioning pump; flexible lines are quickly de- 
teriorated by concentrated bromine solutions (in some of the experiments 
the hoses were damaged in less than 6 hours). 
13 
60 CONCLUSION 
As part of the studies concerning thecyclical Mark 13 process of I 1 1 5  
hydrogen production, the isobaric diagram of liquid-vapor equili- 
brium for the Br -HBt-H20 system was analyzed at atmospheric pressure 
and at temperatures greater than 300 K. 
studied, which is o f  interest for the process, is limited by a 
large range of partial mixing of the system at ambient temperature 
and by the aggressive natureof the mixtures. Another ebullioscope 
would have to be designed t o  complete analysis of the system. 
2 
The concentration range 
After careful examination cf the results, observations concerning 
the separation of bromine, hydrobromic acid, and water after elec- 
trolysis, may be made (step 3 of the cycle - refer to 51). 
the gaseous phase in equilibrium is very rich in bromine when com- 
pared to the liquid phase, separation should be easy and energy 
consgmption moderate provided it is acceptable that the hyrobromic 
acid recycled into electrolysis still contain a little bromine at a 
concentration of a few thousandths. However, elimination of this 
small amount of bromine appears to be difficult, especially as the 
composition of the mixture becomes similar to that ofthe azeotrope 
HBr-H20. 
As 
REFERENCES 
(1) D . V A N  V E L Z E N  e t  a l .  
P r o c e e d i n g s  of  t h e  2nd W o r l d  H y d r o g e n  E n e r g y  C o n f e r e n c e .  
Z u r i c h .  p . 6 4 9 ,  v o l .  2 ,  1978. 
( 2 )  P .  L E S S A R T ,  R .  B E N I Z R I  
S E M I N A R  O N  H Y D R O G E N  A S  A N  E N E R G Y  V E C T O R .  B r u x e l l e s  1978. 
p .  71 - R a p p o r t  E U R  6085 D E / E N / F R / I T  
( 3 )  E .  CARRIERE and CERVEAU 
Comptes Rendus Academie des Sciences, Vol. 177, pp 46-48,> 1923 
( 4 )  M . A .  M E N K O V S K I I ,  N . A .  P E T F O U ,  K . I .  L I T V I N  
J. o f  I n o r g a n i c  C h e m i s t r y .  U S S R .  vol. I, 7, 1956, 
pp. 1658-64. 
14 
(5 )  W .  GLASS 
Hydrogen-bromine f u e l  c e l l  
AFCRL-65-34, 1 9 6 4 .  
(6) J. CATHALA 
Ebullioscopy, Annales du Genie Chimique, 11, PRIVAT, 1963. 
1 5  


Study of the liquid vapor equilibrium in the bromine-hydrobromic acid-water system

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