Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to a method or the
production of urea starting from pure carbon dioxide and
ammonia.
In a Eew of the methods known heretofore, ca.rbon
dioxide was reacted with the yaseous ammonia as produced by
the splitting of the ammon~um carbamate, the gaseous ammonia
having previously been exploited as thestrippin~ agent of
the urea solution.
The gaseous mixture thus obtained, which was
composed by the dissociated gases, carbon dioxide and ammniar
was cooled by circulation of water so as to form carbamate .
to be sent to the reactor used for the synthesis in order
to carry out the dehydration to urea. From the reactor, the
mixture to be used for the syn~hesis was sent to the stripper
for s-tripping carbon dioxide and ammonia (as formed by the
thermal splitting of the carbamate~, such stripping being
carried into effect in a dissociation apparatus, through the
bottom of which gaseous ammonia was introduced, whereas the
mixture for the synthesis was introduced at the top~ Two
20 streams emerged from the dissociation apparatus, viz. one
stream composed by the solution of urea which still contained
a certain quantity of carbamate, and the other stream composed
by the dissociated gases and ammonia, which, as outlined
above, were fed to the condenser.
The most conspicuous drawback of this kin.d of
procedure was the high concentration of ammonium carbamate
which was experienced in the as produce solution, the result :
being a decreased urea yield. This shortcoming was due to
the requirement of operating with constant NH3 to CO2 ratios,
30 both for the condenser and the reactor.
It has surprisingly been found, according to the
present invention, that urea can be produced from ammonia and ?
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carbon dioxide with improved ~ields by usin~ instead of thé
condensation coil, a film absorber, the operation of which
will be described
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hereinafter and by sending, i~ addition to gaseous ammonia
at the bottom o~ the stripper, liquid ammonia, partly to the
absorbed and partly to the synthesis reactor, so as to obtain
in bo-th such apparatus difEerent molar ratios oE NH3 to CO2
and, more detailedly, a ratio of from 2.5 to 7 in the reactor,
and of from 2 to 4 in the absorber, and by reacting adiabatically
C2 and NH3 at the bottom of said absorber.
Assuming that P is the total pressure and taking a
certain H2O to CO2 ratio as the basis, and setting a ratio of
NH3 to CO2, which must be comparatively high in the reactor and
the stripper and comparatively low in the absorber, it is now
possible to begin to describe the improved method according to
the present in~ention.
~ he single FIGU~E of the accompanying drawing is a
plant layout intencled to illustrate the method according to this
invention.
All the carbon dioxide is fed to the bottom o an
absorber, 1, together with a portion of liquid ammonia via the
line 4~ whereas at the top of the absorber, via the line 5, the
recycle absorbing solution is fed, which is composed by an
aqueous ammoniacal solution of ammonium carbonate.
The overall NE[3 to CO2 ratio is regulated by governing
the rate of flow of the liquid ammonia at the absorber bottom.
In the lower section of the absorber 1, the carbon
dioxide reacts with the ammonia adiabatically, to produce
ammonium carbamate. The built-up heat rises the temperature
oE the mixture to higher or less high values consisten-tly with
the total pressure P, the contents of water and the NH3 to CO2
ratio.
The gases which are evolved from the adiabatic bath
and which are in chemicaI equilibrium therewith at that temper-
ature, are absorbed partly by the recycle solution which falls
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filmwise from the top section. The part of the gas which has
not been absorbed, exits the absorber l and, via the line 6,
is combined with the stream 7 which carries gaseous ammonia to
the stripper.
The absorption heat is communicated to water which
flows through the absorber jacket and, inasmuch as the compo-
sitions of the gas and the absorbing liquor in equilibrium
through the entire absorber have such an N~13 to CO2 ratio as
to have the evolution of heat taking place at a high temperature,
it is possible to produce from -the water coolant, steam having
a thermal level which is high enough as to be used for subsequent
requirements of the installation.
The carbamate is then passed, via the line 8, to the
reactor 2, in which the dehydration to urea takes place. The
NH3 to CO2 ratio and the reactor temperature axe adjusted by
feeding liquid ammonia, a small fraction indeed, through the
bottom via the line 9 and gaseous ammonia, predominant1y, with
gases emerging from the stripper 3 via the line lO.
; The NH3 to CO2 ratio in the reactor 2 is kept at a
higher rating than in the absorber l, the conversion yields o~
C2 to urea being thereby increased.
The N~13 to CO2 ratios, the temperature and the yields
are so selected as to have the absorption condensation heat of
the gas from the stripper 3 enabling the thermal balance of the
reactor 2 at the preselected total pressure to be kept constant.
By so doing, it becomes possible to increase the ~ -
yields in the reactor while abating the consumption of steam for
the str1pper and raising the thermal level of the steam produced
by the absorber~
Inasmuch as the pressure in the reactor-stripper
~ sys-tem is virtually the same, it is regulated by discharging
; from the reactor top the inert gases coming from the stripper
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bottom and rom the absorber for carbon dioxide. Such a
pressure can be either heavier or lighter than the pressure
obtaining in the absorber, according to whether the ~ransfer
of the caxbamate takes place either by barometric pull or by
the agency of pumps.
Inasmuch as the conversion yields in the reactor are
outstandingly high and on.taking into account the physicochemical
properties o the solution which exits the reactor.
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through the line ll, carbon dioxide.and water separated in
the stripper are a small amount as compared with the ~uantity
of ammonia. In addition to this particular reason, the heat
to be administered to the system is not a great deal, since
the solution of urea is already close to its critical con-
ditions, whereby ammonia can easily be distilled. ~ :
The critical temperature is decreased as the NH3to CO2 ratio is increased and as the water contents is
decreased. This circumstance is such that exchange surfaces
in the stripper are required, which are lesser than the sur~
faces required for the conventional strippers and, in addition,
lower heat-exchanger temperatures are necessary, which enable
steam at a lower thermal level to be used, the running costs ~ ;
being thereby reduced.
An example will now be reported in order better
to elucidate the invention without, however, limiting it ~ .
: anyhow.
E X A M P L E
To produce 72,550 kilograms an hour oE urea, the
absorber 1 is fed with :
53,167 kilograms an hour of CO2 at the temperature
of 150~C
32~853 kilograms an hour of NH3 at the temperature
of 50C
along with the recycle carbonate which is sent to the top of
the absorber l at a temperature of 76C and with the following
composition : ~
NH3 27,323 kilograms an hour 47.62% by weight ~ ;
C2 lO/127 17,65% " ~ ;
; 30 H2O 19,926 " " 34.73~ "
The absorber:works under the pressure of the
entire urea-synthesis loop which tpressure drops allowed for)
is 180 kiloyrams/s~. centimeters. CO2 and NH3 react in the
absorber and the reaction heat is remo~ed for producing
steam.
From the bottom of the absorbf~r a carbamate
solution emerges at the temperature of 180C and has the
following composition :
NH3 50,794 kilograms an hour 42.23% by weight
C2 50~794 kilograms an hour 42.23%
H20 18,699 " ~ 15.54% "
Said solution is sent to the reactor 2.
The uncondensed vapors exit the top of the
- absorber 1 at the temperature of 180C and have the following
composition :
NH3 9,382 kilograms an hour 40,60% by weight
C2 12,500 " " 54.09% 1l .
H20 1,227 ~ - 5.31%
Said vapors are fed to the bottom of the
stripper 3.
The reactor 2 is fed, in addition to the solution
of carbamate, with 47,840 kilograms an hour of ammonia at
100C and with the vapors exiting the stripped 3, which
have a temperature of 190C and the following composition :
NH3 50,218 kilograms an hour 59.10% by weight
C2 26,259 " " 30.90% "
H2O '3,497 ~ .. 10.00% 17
The solution exiting the reactor is sent -to the
stripper 3 at a temperature of 185C and has the following
composition :
Urea 72,500 kilograms an hour 28.64% by weight
NH3107,768 " " 42.58% "
C223,886 " " 9.44% "
H2O48~946 " " 19.34%
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Through the stripper bott~m exist a stream at
210C which is sent to a conventional concentration section,
and has the ~ollowing composition : .
~rea 72,500 kilograms an hour 37~91% by weight
NH3 66,932
~: ~ C2 10,127 " " 5.30~ .
H2O 41,676 " " 21.79%
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