Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a combined process for
the productlon of urea and ammonia.
More detailedly, the present in~ention relates to
an inteyrated urea-ammonia production process which is im-
provedunder the respect of the absorption of carbon dioxide
in the zone of formation of the ammonium carbamate.
It is known from the Italian Patent Specification
N 907 469 that the absorption of the CO2 con-tained in the
raw gases for the synthesis of ammonia is carried ou-t with
an ammoniacal aqueous solution in a film-exchanger, the
aqueous ammoniacal solution being fed both at the bot-tom
and a-t the top of such film-type heat exchanger.
Regrettably, during the absorption step, the
evaporation of about the 20~ of ammonia takes place, so
that such ammonia must partially be condensed and fed back
to the CO2-absorption apparatus. The remaining fraction
of ammonia which is still contained in the gases emerging
from the CO2-absorption zone, which is the zone in which
ammonium carbamate is formed, is sent, on completion of the
partial condensation mentioned, just now, to the ammonia-
absorber, wherein such ammonia is absorbed, along with
the ammonia which comes from the ammonia-synthesis apparatus,
with water.
Processsing cycle complications and considerable
expenditures lnvolved when working by absorbing CO2 according
to the conventional art resumed above have been done away
with by the method according to the present invention.
The object of the present invention is to provide
a process for the production of urea, combined with the
production of ammonia, according to which the absorption
of the CO2 contained in the raw gases intended for the
synthesis of ammonia is carried out by introducing said
-- 1 --
' ~
.
C02-containing gases at the bottom, or in the ~icinlty Of
the bottom, of an absorber, the latter being split into
two secti.ons, i.e. the lower section of the film-type, and ..
the upper section of the plate-type.
. . . .
~8'185
The other operations of the combined process are the
known ones, and essentially consisting in producing ammonia in an
ammonia synthesis apparatus, absorbing the ammonia, as it exits
from the synthesis reactor, wi~h water so as to obtain a concen-
trated aqueous solution of ammonia, using the concentrated solu-
tion for absorbing the CO2 contained in the raw gases for the syn-
thesis of ammonia, absorbing the CO2 according to the teachings
of the present invention and forming ammonium carbamate, partial-
ly converting the ammonium carbamateinto urea in a urea-synthesis
reactor, thermally decomposing the unconverted carbamate into urea
and stripping the decomposition products with a gas selected from
the group consisting of ammonia, C02 and inerts, ammonia being
preferred, discharging from the stripping zone a solution of urea
which still contains carbamate, recycling the carbamate decomposi-
tion products to the urea-synthesis reactor, distilling under a
pressure of from 3 to 30 atmospheres, in one or more stages, the
solution of urea to obtain liquid ammonia and one or more ammonia-
cal solutions of ammonium carbonate on one side, and an aqueous
solution of urea free of ammonium-carbamate on the other side.
As aforesaid carbon dioxide is absorbed in two discrete
sections of the same apparatus. More particularly, the CO2-con-
taining gas is introduced at the bottom, or near the bottom, of
an absorber which is split into two sections, the lower section
being a film-absorber and`the upper section being a plate-absorber,
the absorbing solution in the upper section being the concent~ated
aqueous solution of ammonia aforesaid, the absorbing solution
in the lower section being one or more of the aqueous ammoniacal
solutions of ammonium carbonate as obtained by distillation of the
solution of urea under a pressure of from 3 to 30 atm, with ad-
3~ dition of liquid ammonia.
Thus, the main fraction of the CO2 is absorbed in a
section which is equipped with tubes, wherein the absorbing solu-
tion, which is an ammoniacal solution of ammonium carbonate as
-- 2
L85
obtained by distillation of the solution of urea under a low pres-
sure after that the major fraction of the a~nonium carbamate has
been withdrawnunder a pressure substantially equal to the synthe-
sizing pressure, smoothly runs along the tube walls in the form
of a thin film, the absorption heat being removed by the agency of
a coolant fluid which flows outside the tube walls. The remai-
ning portion of C02 is removed in the plate section, which is
essentially adiabatic, the absorbing fluid being a concentrated
ammoni`acal aqueous solutlon.
: 10 The pressure under which the urea solution ls distilled,
in order to obtain the ammoniacal solution of ammonium
carbonate ranges from 3 to 30 atm.
In the plate absorbing section, liquid ammonia is
also used with advan-tage, concurxently with the concentrated
ammoniacal aqueous solution. The preferred weight ratio
of liquid ammonia to concentrated ammoniacal aqueous
solution ranges from 1 to 5~
The absorbing liquor is fed to the film-type
absorption section by means of a distributor which is
located in a zone which is intermediate between the two
sections.
Likewise, the absorbing liquor (concentrated
ammoniacal aqueous solution possibly supplemented by
liquid ammonia) is fed to an area placed at the top, or
near the top, of the plate absorption section. The solution
emerging from the plate section is directly sent to the
urea-synthesis reactor: as an alternative, it can flow
throught the film-absorption section by means of an appro-
priate distributor and can be distributed in film-form onto
the surfaces of the tubes of the film-absorption section
together with the ammoniacal solution of ammonium carbonate
which is directly fed to the film-absorption section.
It should be observed that in the film-absorption
zone, the CO2 is absorbed to such an extent as to have
residual gas values of 2% - 3% on a volume basis.
During this stage, a certain evaporation of
ammonia takes place, so that the gas deprived of the carbon
dioxide has a content of ammonia equal to about 10% - 12%
by volume. This gas subsequently enters the second top
section, the plate section, wherein, by the scrubbing
mentioned above, it becomes possible completely to absorb
both the carbon dioxide and the ammonia which are present.
- 4
It should be observed that by operating according
to the method of thls invention, it becomes possible not
only to reduce the exchange surfaces which are necessary,
but also the work under conditions of great safety on accoun-t
of the considerable excess of ammonia and the relative
thermal volume thereof.
It is possible, according to the present invention,
as it is obvious, to absorb the CO2 in two serially arranged
absorption sections, that is, sections which are not super-
posed to one another: in the first section the absorption
takes place withthe film method, whereas the second section
has a set of plates for absorbing the residual CO2 which
had not been absorbed in the film-absorption section.
In this case, the raw gas for the synthesis of
ammonia, which contains CO2 is fed at the bottom, or near
the bottom, of the film-absorption section in which the
absorbing liquor (ammoniacal solution of ammonium carbonate)
is fed at the top, or near the top, of said section, whereas
the gas discharged from the top of the film-absorption
section is fed at the bottom, or near the bottom, of the
plate-absorption section: the absorbing liquor consisting
of the concentrated ammoniacal aqueous solution, possibly
supplemented by liquid ammonia, is conversely fed at the
top of the plate section. The gas, deprived of its CO2
is discharged from the top of the plate-absorption section,
whereas the solution as obtained at the bottom of the plate
section can, if desired and with advantage, be exploited as
an additional absorbing liquor in the film-absorption
section.
An example will now be given, which is intended
for better illustrating the invention without limiting
it in any wise:
-- 5 --
3S
PR~CTIC~L EXAMPLE
-
To produce 1,000 metric tons an hour of urea,
there is fed at the bottom of the absorber 1, shown in the
accompanying diagram and which works under a pressure of
195 kg/sq.cm, a raw gas 2 having the following percentage
composition, by volume:
Ar 215 nor.cu.meters an hour 0.24% by volume
H255,425 do 61.52% do
N218,161 do 19.99% do
CH4 251 do 0.28% do
CO 390 do 0.43% do
C215,804 do 17.54% do
The working temperature is 175C.
At 3 the following ammoniacal solution, having a
temperature of 50C, is introduced:
NH3 25,370 kg an hour 80% by weight
H2O 6,343 do 20% do
total 31,713 do
At the top portion of the film absorber 1, the
recycle carbonate, proceeding from 4, is fed at a temperature
of 103C.
NH3 5,558 kg an hour 26.75% by weight
C2 7,076 do34.24% do
H2O 8,059 do39,01% do
total 20,663 do100.00% do
The concentrated solution of carbamate exits the
bottom and shall be sent to the urea reactor 5, at the
temperature of 140C.
-- 6 --
NH3 24,834 kg an hour 35.43% by weight
C2 31,343 do 44.71% do
H2O 13,920 do 19,86% do
total 70,097 do 100.00% do
From the film section the gas, partially s-tripped of
its CO2emerges and is sent at a temperature of 125C to
the plate section 6:
Ar215 nor.cu.meters an 0.25% by volume
hour
H255,425 do63.44% do
N218,161 do20.78% do
CH4251 do 0.29% do
CO390 do 0.45% do
C23,451 do 3.95% do
NH39,467 do10.84% do
total 87,360 do100.00% do
The absorption hea-t is withdrawn from the jacket
side of the absorber, to produce low-pressure steam.
At the top of the plate section, there are fed:
Ammoniacal solution (temperature 50C), from 3
NH3 12,684 kg an hour 80% by weigh-t
H2O 3,172 do 20% do
Recycled anhydrous ammonia, at a temperature of
38C, from 7 : 35,874 kg an hour
From the bottom of the plate section, the following
solution emerges via 8 at the temperature of 117C:
NH3 45,939 kg an hour 82.20% by weight
C2 6,778 do 12.13% do
H2O 3,172 do 5.67% do
total 55,889 do100.00% do
8~
Such solution is sent to the urea-synthesis reactor.
From the top of the plate section the CO2-stripped
gas emerges via 9 at a temperature of 43C:
Ar 215 nor.cu.m an hour 0.25% l~y volume
H2 55,425 do68.44% do
N2 18,161 do20.79% do
CH4 251 do 0.29% do
CO 390 do0.45% do
NH3 12,918 do 14.78% do
total87,360 do100.00% do
This gas is sent to methanization.
.. , .. . . . . _ _ , .
