Note: Descriptions are shown in the official language in which they were submitted.
6~i9
INTEGRATED AMMONIA-UREA PROCESS
This invention relates to an integrated ammonia-
urea process.
A number of integrated processes are known for
the synthesis of urea in combination with the synthesis of
ammonia.
,
One of these is, more particularly, the one dis-
closed in the Canadian Patent application N 292.378 filed
on December 5, 1977.
10 - The method described in the patent application
aforementioned comprises the steps of feeding to a urea-
- synthesis reactor a stream of anhydrous ammonia and/or aquéous
solution of ammonia and a stream contalning ammonium carbamate,
reaction carbon dioxide with the ammonia in said urea-syn-
thesis reactor, discharging from the urea-synthesis reactor
. . .
a solution of urea which contains unconverted ammonium
carbamate, thermally decomposing about the 50% of the
carbamate contaîned in the urea solution and separating the
decompositlon products, recycling the decomposition products
~- 20 to the urea-synthesis reactor, adiabatically decomposing,
~, . . .
i.e. without admlnistering heat from an external source, the
ammonium carbamate still contained in the urea solution in
an adiabatic stripper in which there is introduced, as the
~ stripping agent, the gas stream obtained by steam-
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~106~9
reforming or partial oxidation of liquid or gaseous
hydrocarbons~ essentially composed by C02, N2 and H2 to
be used generally for the synthesis of ammonia, remov-
ing from the adiabatic stripper both C02 and the NH3
deriving from the decomposition of the residual ammo-
nium carbamate and the stripping agent, feeding the gas
mixture aforesaid to a C02-absorber in which said C02
is absorbed by an ammoniacal solution thus obtaining a
stream containing ammonium carbamate to be fed to the
urea-synthesis reactor~ discharging from the adiabatic
stripper the solution of urea substantially devoid of
carbamate, which is sent to the subsequent treatments
of low-pressure decomposition and concentration under
vacuum.
A shortcoming of the method outlined above is that
the contents of C02 of the solution going from the
bottom of the adiabatic stripper to the low-pressure
decomposition stage, is rather high (10% to 15% by wt.)
the result being an oversizing of the decomposition
stage and an increase of the consumptions in the same
..~
apparatus.
On the other hand~ the run of the adiabatic str~P-
per according to the patent application aforementioned
was bound to the use of C02 among the stripping agents~
inasmuch as said C02 permitted~ by its partial reaction
with the ammonia contained in the urea solution~ to
supply at least partially the heat which was required
-; for the decomposition of the ammonium carbamate as
contained in the urea solution. Regrettably~ as out-
lined above~ the C02 contents remains high so that the
low-pressure decomposition stage must be significantly
oversized.
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3.
It has been surprisingly found that it is possible
to dispense with C02 as the stripping agent~ and con-
sequently to have-an end product substantially depri-
ved of C02 without thereby giving up the adiabatic
stripping.
~his achievement has been made possible by select-
ing appropriate ratios between H20 and C02 and between
NH3 and C02 in the urea synthesis reactor.
An object of the present inven~ion is to provide
an integrated ammonia-urea process comprising the follow-
ing stages :
a) to send the gaseous stream as obtained from steam-
reforming or partial oxidation of liquid or gaseous
hydrocarbons which make up the raw gas stream for
the synthesis of ammonia and which essentially
comprise H2, N2 and C02~ to a C02-absorption system
using a concentrated aqueous solution of ammonia
; and which is more particularly composed by two
: serially arranged discrete absorption stages, in
the first of which the absorbing liquor is a con-
centrated aqueous solution of ammonia (concentra-
~ tion above 70% by wt of ammonia~ 80% being pre-
.; ferred), whereas in the second stage the absorpt-
`~ ion liquor is an aqueous ammoniated solution of
ammonium carbonate as obtained from the low-pres-
sure decomposition stage~ or, in the case in which
the latter is not provided~ from the stage of de-
composition under vacuum of the urea solution;
`: b) to discharge from the C02-absorption section a
~: 30 gas stream which is essentially composed by N2 and
H2 (with possible traces of NH3 and C02) together
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659
4.
with a liquid stream which essentially comprises
an aqueous solution of ammonium carbamate;
c) to feed with the aqueous solution of ammonium
carbamate a urea-synthesis reactor wherein the
S ammonium carbamate is partially converted into
urea;
d) to discharge from the urea-synthesis reactor an
aqueous solution of urea which contains the un-
converted ammonium carbamate and the excess of ammonia
over the stoichiometric amount and possibly a
gaseous stream from the reactor top, said stream
essentially comprising inerts with a certain
3 2;
e) to feed the aqueous solution of urea from stage d)
~ 15 hereinabove to a decomposer in which the ammonium
; carbamate is decomposed into ammonia and carbon
: dioxide~ the latter being both drawn from said
decomposer together with the water which evaporated
:~ off~ to be recycled in the vapor phase to the urea-
synthesis reactor;
. .
f~ to discharge from the decomposer an aqueous solution
~ of urea which contains about the 50% of the carba-
,-~ mate originally contained in the urea solution exit-
ing the synthesis reactor and to feed said aqueous
solution to an adiabatic stripper wherein it is
. . .
`~ exploited as a stripping agent the gaseous stream
of b) above;
;~ g) to discharge through the bottom of the adiabatic
stripper the solution of urea substantially devoid
of ammonium carbamate, and to discharge from the
. ~ top of the stripper the stripping agent (N2+H2)
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with the products of decomposition of the carba-
mate (NH3+C02) and the evaporated water;
h) to introduce the gaseous mixture drawn from the
top of the adiabatic stripper to a condenser
wherein the ammonia and the carbon dioxide are
condensed by cooling as a result of an indirect
heat exchange with a cold fluid in the presence
of an ammoniated solution of ammonium carbonate
: coming from the low-pressure decomposition stage
whereas the stream with H2 and N2 is discharged
at the top and fed, upon methanization, to the
ammonia synthesis together with the inert gases
N2 and H2 exiting the C0~-absorption system;
i) to send the condensate from h) to the C02-absorber;
15 1) to send the urea solution of stage g) to the stage
of concentration under vacuum~ either directly or
through a previous low-pressure decomposition stage
(4 to 5 atmospheres), there being obtained both
. from the head of the low-pressure decomposition
stage and from that of the concentration a gas mix-
ture composed bv ammonia, C02 and water~ which,when
condensed~ is the ammoniated solution of ammonium
: carbonate to be used for the steps a) and h),
.: melted urea being obtained through the bottom of
the concentration stage under vacuum.
: The method according to the present ~nvention will
now be illustrated in a preferred embodiment with the aid
~ of the accompanying drawing.
; The raw gas which essentially comprises C02, N2
~ 30 and H2~ is sent~ after having been compressed~ via the
:. piping 1~ to the C02-absorber 17~ wherein the absorbing
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:1111~659
liquid is mainly composed by an aqueous solution of am-
monia which comes via the piping 6 from the ammonia
synthesis reactor: the ammonium carbamate coming from
the absorber 17 is sent via the piping 7 to the urea-.
synthesis reactor 18.
The decarbonated gas exits the absorber 17 and,
via the piping 2, goes to the NH3-stripping condenser
19~ in which NH3 is absorbed by an aqueous solution of
ammonium carbonate coming, through the pipe 16, from
the low-pressure liquor-recovering section by the
agency of the pump 20.
The ammoniated solution formed at 19 is sent via
the piping 13 to the absorber 17.
. The purified gas is sent via the piping 3 to the
adiabatic stripping column 21 ~rhich is fed via the
piping 10, with the urea ~olution coming from the car-
bamate decomposer 22. The gas stream exits the column
21 and~ via the piping 4~ is fed to the carbamate con-
denser 23 where it is combined with an aqueous solu-
tion of ammonium carbonate 14, as fed via the pump 20
and the piping 15 from the low-pressure liquor-recovery
. section. The carbamate produced at 23 is sent via the
piping 12 and the pump 24 to the C02-absorber 17.
The purified gases exiting the condenser 23 are
. 25 sent via the piping 5 to methanization and, therefrom~
to the ammonia synthesis in which an ammoniated solu-
: tion is produced to be sent to the absorber 17 via the
piping 6 as outlined above.
J .
. The carbamate solution~ via the piping 7~ is sent
.~ 30 to the urea reactor 18 in which the reaction of dehy-
. dration of the carbamate into urea takes place. The
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thusly obtained solution of urea reaches, via the pipe
8~ the carbamate decomposer 22 wherein said carbamate
is decomposed into C02 and NH3 and these latter are
recycled via the piping 9 to the reactor 18.
The urea solution, along with the carbamate which
has not been decomposed~ is sent via the piping 10 to
the adiabatic stripping column 21 which has been de-
scribed above.
The solution of urea exiting the bottom of the
column 21 is sent via to the section of evaporation
under vacuum (not shown)~ wherein it is treated in
the conventional manner.
It is quite surprising that, when operating with
the process according to the present invention, it
becomes possible to obtain at the exit of the adiaba-
tic stripper a solutlon of urea which is so highly
~ concentrated as to be sent directly to the final treat-
;~ ment u~der vacuum. By so doing~ the considerable ad-
vantage is achieved that the expensive operations of
~; 20 decomposition of the undecomposed carbamate under me-
`~ dium (18 atm) and low (4.5 atm) pressure are dispensed
with and so is the recondensation of the produced vapors.
.:
This is the contrary of the teachings of the prior
` ~ art, according to which the solution to be sent to the
,~ 25 evaporation under vacuum is obtained at the expense of
`` a considerable power useup.
~ All that which has been described above requires
`~ in the urea reactor a selection of the ratio of H20 to
C2 within the range from 0.9 and 1.3~ the preferred
; 3 value being 1.1, and of the ratios of NH3 to C02 com
l prised between 4.5 and 6.5~ the preferred value being
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A modification which has not been shown in the
drawings consists in using, instead of the pump 24 for
recycling the carbonate towards the absorber 17, an
ejector, the working fluid for which is the 80% ammo-
niated solution coming from the ammonia synthesis react-
or via the piping 6.
As regards the pressures, the method according to
the present invention works under a pressure comprised
between 100 kg/cm2 and 300 kg/cm , a pressure which is
virtually equal to that at which the ammonia synthesis
reactor operates, and in such a case a pump may be re-
quired for sending the ammoniated solution from the am-
monia synthesis reactor to the cycle described above~ or
under a pressure which is from 10 kg/cm to 100 kg/cm
below the pressure at which the ammonia synthesis reactor
works~ and in this case the pump can be dispensed with.
A practical example will now be given in order
that the invention may be better illustrated but without
li~iting it in any way.
EXAMPLE
:~ Reference will be had to the single FIGURE of
` the drawing.
TABLES 1 and 2 report the working conditions, the
concentrations and the rates of flow.
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