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CaBr 2 HYDROLYSIS FOR HBr PRODUCTION USING A DIRECT SPARGING CONTACTOR
Reaction constant
Determination of the reaction constant from the experimentally measured rate of HBr formation is
achieved by applying the reaction given by Eq. (1) to a single steam bubble:
d
[ HBr]
Vb
k f a H2O
dt c
[ P ][ CaBr] n
= (6)
The reaction is assumed to be first order in steam partial pressure and n th order (to be determined
from the kinetic data) with CaBr 2 concentration. Similarly, the rate equation for consumption of H 2 O is
given by:
d
[ H O]
k
= (7)
[ PH
O ][ CaBr ] n
2
f
Vb
− a
2
2
dt c 2
which can be simplified to the following form for integration over the contact time, t c :
where:
dP
dt
H2 O
−k′
PH
2O
c
= (8)
k ( a V
)
′ (9)
f
b
k =
2
2ρm
[ CaBr ] n
It should be noted that the grouping is an effective reaction rate as commonly used in gas-liquid
reactors.
Integration of Eq. (8) yields the
k f a rate of steam conversion for a given contact time t c .
The steam conversion, x c , is given by:
−k t
( t = ) e
c
′
P H O = PH
2O
c 0
2
(10)
x c = 1 – P H2O /P H2O (t c = 0) (11)
The rate of steam consumption is equal to the steam flow rate times the steam conversion, and
the rate of HBr formation is twice the rate of steam consumption. The formation of HBr at a given
reaction time t R depends upon the melt composition. A second-order reaction of CaBr 2 was found to
match the experimentally measured reaction rates far better than a first-order reaction. The reaction
constant is then derived from the rate of HBr formation, which is experimentally measured. The
observed kinetic constant was 2.17 10 –12 kmol s –1 m –2 MPa –1 (1.30 10 –4 g-mol min –1 cm –2 bar –1 ) for the
hydrolysis reaction, which is 24 times greater than the constant reported for solid CaBr 2 reaction. This
higher rate promises to significantly reduce the size and design complexity of the hydrolysis reactor.
Conclusions and recommendations
This modelling and experimental investigation of steam sparging into molten CaBr 2 supports the
following conclusions:
• Early modelling of the system provided critical guidance to developing the apparatus and
interpreting results.
• The operation of the hydrolysis reaction was quite stable, with undetectable carry-over of
molten salt, which proves that it should be possible to design a continuous CaBr 2 hydrolysis
reactor.
• The use of alumina packing to better distribute the steam bubbles was also investigated, and
the kinetic results show that there was enhanced contact of steam and CaBr 2 . Therefore,
various design options for steam distribution should be investigated further.
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