Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~73636
This invention relates to an isotopic exchange
procass of the dual-temperature type.
In isotopic exchange processes of this type, iso-
topic exchange is carried out between a gas and a liquid
circulating in countercurrent flow within two successive
towers which operate at different temperatures and are
commo~ly designated as the hot tower and cold tower. Transfer
of the gas or of the liquid from the cold tower to the hot
tower and conversely therefore entails the need for heating `
and cooling of the gas or the liquid before these latter are
admitted respectively into each tower, Moreover, in order to
achieve optimum exchange conditions, the gas which enters the
hot tower must be saturated with the vapor of the exchangç
liquid ; conversely, as the gas enters the cold to~er, it is
necessary to condense the vapor which it contains in order to
introduce a relatively dry gas into said tower.
In order to limit the energy which is necessary for
the heating and cooling process just mentioned, a number of
different solutions have been proposed up to the present time
for recovering the heat transported by the hot fluids as these
latter circulate within the installation and for transferring
the heat to the cold fluids which are intended to undergo a
temperature rise. Consideration has similarly been given to
the use of intermediate exchange-liquid loops in order to
saturate with vapor the cold gas which enters the hot tower.
French patent No 1,409,860 granted to the present
Applicant illustrates one of these solutions which is adapted
to an installation for the production of heavy water by dual-
temperature i~otopic exchange between a gas and a liquid.
This solution which is based on the use of auxiliary exchange-
liquid loops makes it possible to effect a txansfer of heat
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from the hot gas to the cold gas which enters the hot tower
: while at the same time ensuring vapor saturation of said
cold gas by the exchange liqui.~ which circulates within these
independent loops. The heat-recovery system comprises an
auxiliary liquid loop for cooling the hot exit gas from the
hot tower within an indirect heat exchanger and transferring
the recovered heat to the cold gas before this latter enters
the hot tower within a direct~contact heat exchanger. The
system further comprises a second liquid loop for re~oving
the heat from the gas within a direct-contact heat exchanger
before said gas enters the cold tower and for tran's~erring
the heat to a third liquid loop by means of an indirect
liquid-liquid exchanger ; s~id third loop also feeds the
dlrect-contact exchanger which is placed upstream of the hot-
' tower inlet.
. ,,This system of heat re,covery is, subject, however,to a few disadvantages arising more esp~cially from the
plurality of independent liquid loops ~hich entail the need
for costly intermediate heat exchangers between these latter.
Moreover, the system cannot be~employed in the case of an
isotopic exchange process between.a gas and a liquid which
contains a catalyst in solution since the use of two
independent exchange-liquid loops for saturating the gas with
vapor in a direct-contact heat exchanger results in an
; - accumulation of.catalyst within the auxiliary loops. In-fact,
the catalyst is continuously introduced into said loops by
means of an addition of exchange liquid which is necessary in ~,~
ordex to compensate for the losses of said liquid resulting
from entrainment by the gas ; this addition is advantageously
effected by means of the exit exchange liquid from the hot
: tower in order to prevent any harmful effect on isotopic
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equilibria.
The present lnvention is directed to a process and
a device for dual-temperature isotopic exchange which over-
come the different drawbacks mentioned above and make it
possible in particular to ensure better recovery of the heat
transported by the gas and the~liquid which take part in the
exchange reaction while effecting a heat transfer from ~he hot
- gas to the cold liquid and employing the totaL flow of
exchange liquid from the hot tower for heating and saturating
with vapor the cold gas which enters said tower.
In accordance with ~he invention t the dual
temperature isotopic exchange process is carried out in an
installation comprising a hot tower and a cold tower in which
a gas and a liquid are circulated in countercurren~ flow and
essentially consists in passing the cold exit gas from the
cold tower into a first then into a second direct-contact heat
exchanger which are fed in countercurrent f`low wlth hot liquid
having a flow rate which varies from one heat exchanger to
the other with a view to heating and vapor-saturating said
cold gas upstream of the hot tower by means of said hot liquid,
the liquid stream which passes into the second heat exchanger
being constituted partly by a first liquid stream derived from
the hot tower and part,ly by a second liquid stream derived
from an auxiliary circuit in which said second liquid stream
ls heated by indlrect exchange with the hot exit gas from the
hot tower, the liquid stream which passes into the first heat
exchanger being conqtituted solely by the liquid stream
delivered from the hot tower via the second heat exchanger.
In accordance with a first advantageous feature of
~O the invention, the liquid entering the hot tower and the hot
exit gas from said tower are passed countercurrentwise
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through at least one indirect-contact heat exchanger in order
to effect the heating of said liquid by said hot gas.
In an advantageous mode of practical application of
the invention, the hot exit gas from the hot tower, the
liquid entering the hot tower and the llquid of the auxiliary ~
circuit are passed through the same indirect-contact heat
èxchanger in order to effect the heating of said liquids by
said hot gas. Steps can also be taken to ensure that only the
hot exit gas from the hot tower and the liquid of the
auxiliary circuit pass through said indirect-contact hea~
excha~ger.
In accordance with another advantageous feature of
the invention, the exit gas~from the cold tower is compressed
between the two direct-contact heat exchangers.
In a preferred application of the invention, the
process is employed for the production of deuterium-enriched
hydrogen by isotopic exchange between hydrogen gas and a
liquid amine in which a catalyst is dissolved, the amine
being preferably selected from the group comprising mono-
~-- 20 methylamine or a mixture of monomethylamine and trimethylamine,
an~ the catalyst being preferentially potassium methyl-amide.
The dual-temperature isotopic exchange device for
~ the practical application of the process in accordance with
; the invention essentially comprlses in combination :
, , .
- a hot exchange tower,
- a cold exchange tower,
- a circuit composed of a liquid loop designated hereinafter
; as the main liquid cLrcuit comprising a first pipe for
c~nnecting the base of the cold tower to the top of the hot
tower via at least one indirect-contact heat exchanger, a
second pipe for connecting the base of the hot tqwer to the
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3~i,36
top of the cold tower via two successlve direct-contact
heat exchangers and if necessary an exit pipe upstream of
the cold tower and an inlet pipe downstream o said exit
pipe and within the cold tower so as to permit wlthdrawal
and introduction of make-up liquid in order to feed the
loop with the desired isotope by ~eans of a transfer
carried out exte-rnally of said loop,
an auxiliary liquid circuit ~o~nected to the pipe which
joins the base of the hot tower to the top of the cold
tower upstream and downstream of the direct-contact heat
exchanger which is located nearest the hot tower and also
passes through an indirect-contact heat exçhanger,
a circuit composed of a~ gas loop comprising a first pipe
for connecting the top of the hot tower to the~base of the
cold tower via said indirect-contact heat exchanger or
exchangers, a second pipe for connecting the top of the
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cold tower to the base of the hot tower via said direct- -
~: contact heat exchangers and if necessary an exit pipe
. ~ located be~ween the hot and cold towers and:an inlet pipe ~ ~:
;~ ~24 between the two towers and downstream of said exit pipe
. . so as to permit withdrawal and introduction o make-up ~as
and to effect withdrawals of the deslred isotope,
- means for circu.lating the gas ~rom the top of the hot
tower to the base of the cold tower and from the top of
: the cold tower to the base of the hot tower,
- and means for circulating the liquid from the ~ase o the
, hot tower to the top of the cold tower and from the base
of the cold tower to the top of the hot tower.
In accordance with an advantageous feature of the
device according to the invention, the first plpe of the ,:;.
liquld circuit and the second pipe of the gas circuit are
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10~3636 - -
provided wlth a plurality of common indirect-contact heat
exchangers. : .
In accordance with another advantag~ous feature of
the device according to the invention, the indirect heat
exchanger which is associated with the auxiliary liquid
circuit and the gas circuit and the indirect heat exchanger
located nearest the hot tower which i5 associated with the
main liquid circuit and the gas circuit are a common ~hree-
1uid heat exchanger. -
~
In accordance with a further advantageous feature
of the device according to.the invention, the means for clr-
culating the gas from the top of the hot tower to the base of
the col~ tower and from the~ top of the cold tower to the base
- of the hot tower are constituted by a compressor placed in
the gas circuit at a point located between the two direct-
contact heat exchangers.
The characteristic features of the invention will
be more specifically brought out by the following description
which is given by way of example ~and not in any limiting
.~ 20 sense, reference being made to the accompanying drawings,
wherein : ~ -
: - Figs. la and lb are two diagxammatic illustra-
tions of a dual~temperature isotopic exchange installation
which is adapted to the application of the method according
,
to the invention ;
Fig. 2 is a graphical representation of the curve.s
of enthalpy of the gas and of the operatlon of the heat
exchangers 10 and 11. :
~he installation shown in.Fig. la comprises a
. 30 cold~exchange tower 1, a hot exchange tower 2., .a circuit
shown in full lines for the circulation of the exchange
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liquid between the two towers in the direction indicated
by the arrows and a circuit shown in dashe~ li`~æ~ ~r-~h~
circulation of the gas between the two towers in the directlan
indicated by the arrows.
There is shown in double lines an auxiliary
exchange-liquid circuit which is connected to the main -~
exchange-liquid circuit at the level of the hot tower 2.
The main exchange-liquid circuit aforesaid
comprises a first pipe 3 which connects the base of the cold
tower l to the top of the hot tower 2 and a second pipe 4 for
connecting the base of the hot tower 2 to the top of the cold ~ ~:
tower l, and two pipes 21 and 22 which permit exchanges of
j,
: liquid having two different' isotopic contents with the
exterior of the loop constituted by the pipes 3 and 4.
The first pipe 3 is equipped with a plurality of
indirect-contact heat exchangers, three of which are shown at
5, 6 and 7, and with a heater 8. The 6econd pipe 4 is fitted :~
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with a heater 9, with two direct-contact heat exchangers 10
and ll, a cooler 12 and a clrculating pump 13.
: 20 : In the gas circuit, a first pipe 14 which connects
the top of the hot tower 2 to the bottom of the cold tower 1
traverses the indirect-contact heat exchangers 7, 6 and 5 as
well as coolers 15, 16 and 17 located downstream of each
indirect-contact heat exchanger such as the exchanger 7 ; a
second pipe 18 connects the top of the cold tower 1 to the
bottom of the hot tower 2 and traverses the direct-contact
heat exchangers ll and lO, and two plpes 23 and 24 permit
exchanges of gas having two different isotopic contents with
the exterior of the loop constituted by the p1pes 14 and l&.
The pipe~l8 aforesaid is provided in addLtion with
a compressor l9 located between the two direct-contact heat
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exchangers 11 and 10.
The auxiliary liquid circuit shown in double lines
comprises a pipe 20 branched off the pipe 4 of the main
liquid circuit upstream of- the heater 9 and downstream of the
direct-contact heat exchanger 10~ Said pipe 20 also passes
through the indirect-contact heat exchanger 7.
In an alternative form of construction of the
installation for separation of isotopes as illustrated in
Fig. lb, heati~g of the liquid of the auxiliary loop 20 is
~10 effected by means of the indirect-contact heat exchanger 7b
~hrou~h which flows the hot e~it gas from the hot tower where-
as a second heat exchanger 7a which is also of the indirect-
contact type ensures heat exchange between the entering
liquid and the exit gas from the hot tower. The r~spective
order of these two heat exchangers in the circuit is
unimportant and is not a characterlstic featurs of ths
invention.
~ he operation of this installation will now be
describsd by taking as a non-limitative example the
particular case of an installation for the production o~
.
deuterium-enriched hydrogen by dual-temperature lsotopic
exchange between hydrogen gas and a liquid amine such as
monomethylamine containing ln ~olution ~otassium methyl-amide
which serves as a catalyst ~or the exchange reaction.
.
The monomethylamins is introduced into the cold
tower 1 which operates at a temperature of -50C and is
enriched in deuterium as it flows through the tower in contact
; w1th hydrogen gas which is circulsted countercurrentwi5e.
Said monomethylamine leaves said tower 1 via the pipe 3, then
passes successively through the indirect-contact heat
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~ exchangers 5, 6 and 7, then into the heater~8. Within ssid
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73636
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indirect-contact heat exchangers, the monomethylamine under-
goes a Progressive heat build-up and recovers the heat
released by the hydrogen gas which flows countercurrentwise
within said heat exchangers at a higher temperature since it
comes from the hot tower. The temperature of the monomethyl-
amine thus rises from -50C to - 34C within the heat ex- - :
changer 5, from - 34C to ~ 10C within the heat exchanger 6
and its temperature is ~ 25C at the outlet of the heat ex- ~ .
changer 7. By passing it through the heater 8t the monomethyl- :
amine is brought to the temperature of the hot tower, namely
:~ 30C. ,~. ~
: In the hot tower 2, the monomethylamine is depleted
in deuterium,(said deuterium being transferred into the
hydrogen gas as this latter circulates in counterc~urrent flow),
then passes out of said tower via the pipe ~ into which the
: pipe 20 of the auxiliary circuit also opens. The flow rate of
~`, monomethylamine which circulates within said pipe 4 is then
increased by the addition of the monomethylamine contained .
in said auxiliary circuit.
: 20 The total flow passes through the heater.9 so as to
bring this latter to a temperature of 40C, then passes into
; the direct-contact heat exchanger 10.
Said heat exchanger 10 is Eed countercurrentwise
with hydrogen gas at a temperature of - 9C which is intro-
duced through the pipe 18 from the cold towçr 1 via the heat -:
exchanger 11. Part of the monomethylamine is vaporized
within said heat exchanger 10 and thus saturates the hydrogen
ga~ with which it is in contact. ~ ~ :
. : Furthermore, the heat.released by the monomethyl-
amine is recovered.by the hydrogen gas which is thus brought :
to the temperature of the hot tower, namely 30C. ~ ~
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A~ the exit of the heat exchanger 10, the mono-
methylamine stream is at a temperature of 10C and dlvided
into two parts. One part is withdrawn'through the pipe 20 of
the auxiliary circuit ; the other part corresponding to the
stream which circulates within the towers 1 and'2 is directed
towards the second dlrect-contact heat exchanger 11, this -
latter being fed countercurrentwise with hydrogen gas at a
temperature of - 50C whlch comes di,rectly from the cold
. tower 1.
10 Similarly, there takes place within the second
heat exchan~er 11 a vapor saturation of the hydrogen gas and
heat transfer from the monome-thylamine into the hydrogen,
thus permitting a temperatu~e rise of the hydro~en gas from
- 50C to - 9C. ~he monomethylamine leaves said second heat
exchanger at a temperature of - 45C ; by passing through the '
., cooler 12, its temperature is reduced to - 50C before being
.
returned to the cold tower 1. How~ver, part~of the mono-
methylamine stream is withdrawn upstream of the cold tower 1
through the pipe 21 in order to re-charge the li~uid phase
with deuterium, for example Ly ~ontacting with a deutçrium-
feed gas phase which can be constituted by pure hydrogen or
hydrogen mixed with an inert gas such as nitrogen in the : ~:
case of ammonia synthesis gas. Once it has been re-charged,
the monomethylamine is returned to the tower 1 and introduced
''therein at a level whlch depends on the characteristics of
~'. the ].ow diagram which is adopted but does not have any
bearing on the object of the present invention.
. .
Prlor to return into the pipe 4, the monomethyl-
amine stream of the auxillary circuit 20 ls heated to a
temperature of 25C by passing'through the indirect-contact
' heat exchanger 7, by means of the hydrogen gas l~aving the
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hot tower 2. Said heat exchanger 7 which is also traversed
by the pipe 3 oE the main monomethylamine circuit i5 there-
fore a triple heat exchanger constituted for example by a
plurality of elements mounted in parallel and each formed by
a certain number of double tubes, the monomethylamine being
circulated with.in the lnner tubes of these latter and derived
in some cases from the main circuit and in other cases from
the auxiliary circuit~
So far as the hydrogen gas~is concerned, the flow
diagram is as follows : at the exit of the hot tower 2, the
hydrogen which is at a ~emperature of + 30C passes into the
triple indirect-contact heat exchanger 7 in which its
. temperature drops to 16C,,'then into ~he .cooler 15 which it
leaves at a temperature of 15C. The increased ~low rate of
monomethylamine which passes through said triple heat ex-
changer makes it possible in this~exchange process to obtain
condensation o~ the vapor contained in ~he hydrogen, this
condensation being already substantial. This condensed
monomethylamine vapor is separated from the.hydrogen within a
separator (not shown) and returned into the main monomethyl-
amine circuit. The hydrogen gas then passes successively into
the heat exchanger 6 whlch it leaves at a temperature o~
- 3C, then into the cooler 16 which brln~s it to - 30C, then
into another separator tnot shown), then into the indirect-
~ contact heat exchanger 5 whi.ch it leaves at a temperature of- 41.5C, then into the cooler 17 so as to attain the
temperature of the cold tower, namely - 50C~ and lnto a
flnal separatox which is not lllustrated. As it passes
~etween the hot tower and the cold tower, preferably at the
.30 exit of the cooler 17, a fraction of t~e hydrogen o maximum
isatopic content at this polnt is withdrawn thraugh the pipe :
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73636 - -
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23 in order to be passed into a further enrichment stage if
necessary. An equivalent flow is introduced into the loop
through the inlet pipe 24 located between the hot and cold
towers downstream of the pipe 23 an~ preferably after the
cooler 17. - -
At the exit of the cold tower 1, the hydrogen isdirected towards the direct-contact heat exchanger 11 in
which it is heated to - 9C, then passed into the compressor
19, then into the direct-contact heat exchanger 10 in which
it is:heated to + 30C.
From this hydrogenLgas flow diagram, it is
apparent that one of the main advantages offered by the
process in accordance with, the invention lies in the possi-
bility of ensuring heating and vapor satu'ration o~ said
hydrogen at the inlet to the hot,tower by direct contacting
with the exit monomethylamine from said tower.
', : ' : -
The fact of carrying out this contacting operation
in two successive stages with different liquid flow rates
makes it possible in addition to ensure better heat recovery
between the gas and the Iiquid at the time of vapor satura-
.:.
tion of said gas as can be seen by referring to Fig. 2 which
is a diagram representing the enthalpy curve of the gas and
the operation of the heat exchangers 10 and 11.
In this figure, the curve Q represents the
enthalpy curve of hydrogen gas saturated with monomethylamine
as a functlon of temperatures.
On thls curve, the point A having - 15C as abscissa ,,
.
represents the state of the gas at the exit of the cold tower
1 and the point B having + 30C as abscissa represents the
stat,e of the gas at the inlat to the'hot tower 2. '`'
The heat which is necessary to induce transition of
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the gas from state A to state B i5 thereore represented by
the quantity QB ~ QA ' QB and QA being the ordinates oE the
points B and A.
Part of this hea~ is trans~erred to the gas by the
liquid which circulates withln the direct-contact heat
exchangers 10 and 11. ;
The curves representing the operation of the heat
exchangers 10 and 11 have been plotted in the same diagram.
Said curves are approximately straight lines having
a slope~which is substantially equal to the ratio of the
liquid and gas flow rates within each heat exchanger.
- Thus-the segment CD illustrates the operation of the -
heat exchanger 10 `and the ~egment DE illustrates the operation
of the heat exchanger 11 in which the ~low rate is lower. The
dashed-line segment C'D indicates the operation of the heat
exchanger 10 in the event that the flow rate is the same as
- in th~ heat exchanger 11. ~ -
It is apparent that the substantial curvature af
the enthalpy curve prevents a high liquid flow rate within the
tower.
The addition of necessary heat for saturating the
gas is represented by the segment C'H' ; in fact, the exit
liquid from the heat exchanger 7 cannot be at a temperature
below 30 - ~t (lf 30 ls the gas inlet temperature in said
heat exchanger 7) ; in the case of the figure, ~t = 5.
If the liquid flow rate within the tower is
increased, lt is apparent that the segment C'H' becomes
shorter and consequently that the addition of necessa~y heat
CH is smaller. It can be understood, however, that in order
to flatten the enthalpy curve to th~ maximum extent, it will
be an advantage on the other hand to have a lower flow rate
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of liquid within the heat exchanger 11
A further important advantage obtained by contacting
the cold gas and the exit liquid from the hot tower in two
successive stages lies in the possibility of effecting re-
compression of the gas, not at the inlet to the hot tower but
between these two successive stages, therefore at a lower
intermediate temperature, thus entailing the need for lower
'
recompression power.
.- Compression upstream of the heat exchanger 11 does
not represent any advantage, however, since this would make it
necessary to provide a compr~ssor which operates at Iow
temperature and would result in unfavorable heating of the
cold liquidO , . .
The method according to the invention a~so makes it
possible to achieve a substantial savlng of power in the
cooling of the gas which passes from the hot tower to the : :
cold tower, especially when this latter operates at low
temperature. In fact, by making use of a plurality of
successive indirect-contact heat exchangers and intermediate
coolers, this avoids the nead for additional cooling at the
level of the lowest temperat:ure , from this it follows that,
- in some cases, coollng can be carried out directly with
water within the first cooler 15.
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