Method for the production of urea and purification of water

In a urea producing method by synthesis, the improvement which consists in feeding liquid ammonia in excess to the reactor so that a urea solution which contains ammonium carbamate is produced, the carbamate is decomposed in a high-pressure decomposer and the stripping agent is an inert, oxygen-containing gaseous stream, the carbamate decomposition products are sent to a high pressure condenser and ammonium carbamate is formed, whereafter the carbamate is separated from the inerts and the solution of urea coming from the high-pressure decomposer is fed to a medium-pressure decomposer which is fed through its bottom with inerts. The heat for operating the medium-pressure decomposer is a hot condensate from the high-pressure-decomposer-heating steam, and the products of decomposition of the carbamate coming from the medium-pressure decomposer are sent to a medium-pressure condenser together with a solution of ammonium carbonate coming from the low-pressure sections of the installation. Virtually pure, nonpolluting waters can be discharged from the installation.

This invention relates to a method for the production of urea and 
purification of water. 
More particularly, the present invention relates to a method for the 
production of urea in which the power consumption is minimized and the 
water dumped from the installation does not present any pollution 
problems. 
There are known in the art a number of methods for the synthesis of urea 
and among these a few which provide for an isobaric loop followed by a 
low-pressure section in which the urea solution is stripped by 
distillation of its ammonia and carbon dioxide contents which is in the 
form of ammonium carbamate and free ammonia. 
Such conventional methods use, for the distillation of the urea solution 
coming from the high-pressure loop, costly live steam coming from the 
outside. 
This fact, as it is apparent, is a considerable aggravating factor of the 
running costs of the installation. 
In addition, the waters obtained in the methods according to the known art 
and which are sent to the sewers generally contain high urea values and 
high ammonia values, so that they are a considerable source of pollution. 
All these shortcomings, and others, of the conventional urea plants are 
overcome by the method according to this invention. 
The subject-matter of the present invention is a method for the production 
of urea and recovery of the water, which comprises the following steps: 
a high-pressure urea-synthesizing reactor is fed with liquid ammonia in 
excess with respect to the stoichiometric quantity and with carbon 
dioxide, a urea solution being thus produced which contains ammonium 
carbamate. 
ammonium carbamate is decomposed to CO.sub.2 and NH.sub.3 in a decomposer, 
substantially under the same pressure as in the synthesis (high-pressure 
decomposer) using as the stripper a stream of oxygen-containing inerts: 
the distiller is preferably of the film type, 
the products of the decomposition of the carbamate are fed to a condenser 
substantially under the same pressure of the synthesis run in which the 
formation of the ammonium carbamate takes place, 
in a separator the carbamate is separated from the inerts (which contain 
oxygen) which are introduced in the high-pressure decomposer as a 
stripping agent, 
the urea solution emerging from the high-pressure decomposer is then sent 
to a medium-pressure decomposer (generally a pressure of about 18 
atmospheres is preferred) through the bottom of which are introduced, in 
counterflow relationship with respect to the solution, the inert gases 
which contain oxygen and which have been separated in the carbamate 
stripper aforementioned, 
the heat which is required for the operation of the medium-pressure 
decomposer is supplied by the hot condensate as obtained from the steam 
used for heating the high-pressure decomposer, 
the decomposition products of carbamate coming from the medium-pressure 
decomposer are sent to a medium-pressure condenser (the pressure therein 
is substantially the same as in the medium-pressure decomposer) together 
with a carbamate solution coming from the low-pressure portion of the 
installation. 
It is important to observe that according to an aspect of the present 
invention, the medium-pressure condenser operates when it is empty of any 
liquid. 
The condensate emerging from the medium-pressure condenser is sent to a 
rectification column from which ammonia is separated as a head product and 
a solution of ammonium carbonate is the tail product, said solution being 
recycled to the high-pressure carbamate condenser. The head NH.sub.2 is 
recycled to the synthesis. The urea solution emerging from the bottom of 
the medium-pressure decomposer is sent to a low-pressure decomposer by 
operating in such a way as to obtain an aqueous solution of urea which 
still contains a consistent residue of ammonia (2 to 3% by weight) and of 
CO.sub.2 (from 1% to 1.5%, by weight) and, on the head, there are vapors 
of NH.sub.3 and CO.sub.2 and H.sub.2 O which are properly condensed in a 
low-pressure condenser which is also empty of any liquid and the 
condensate is recycled to the medium-pressure condenser aforementioned. 
The urea solution as it emerges from the low-pressure decomposer is 
concentrated by evaporation of its water in a vacuum concentration system; 
the as-obtained urea (melted urea) is sent to prilling or to granulation 
whenever required, whereas the vapors (water, ammonia and carbon dioxide) 
are condensed and fed to the hydrolysis stage which is operated at a 
temperature of from 170.degree. C. to 250.degree. C., preferably at about 
190.degree. C., for a time of from 30 to 80 minutes, preferably from 40 to 
60 minutes, in the presence of air. 
The hydrolyzed solution is then sent to a rectification column to separate 
water as a tail product and, as a head product, upon condensation, an 
ammoniated solution, having a feeble concentration, of ammonium carbonate 
which is partly fed back as a reflux stream to the head of the 
rectification column and partly to the low-pressure condenser of the urea 
synthesis. The water is substantially pure. 
It is worth noting that, quite apart from that which has been said in 
connection with the heat recovery as in the method of the present 
invention, that the oxygen-containing inerts are fed as stripping agents 
to the high-pressure decomposer, are then fed to the high-pressure 
condenser, are subsequently stripped of their carbamate and used anew as 
stripping agents in the medium-pressure decomposers, thus passivating all 
the high- and medium-pressure implementations. The inerts are then passed, 
in fact, to the medium-pressure condenser and the subsequent rectification 
column and are eventually separated from NH.sub.3 which has been condensed 
and obtained as a head product. 
This fact is an original aspect of the method according to this invention. 
It is interesting to observe, moreover, that both the medium-pressure 
condenser and the low-pressure condenser are operated without any liquid 
in their interior and that both, apart the vapors coming from the 
condenser upstream thereof, are fed with solution of ammonium carbonate. 
The low-pressure condenser, above all, is fed with the solution of 
ammonium carbonate as obtained by condensation of the vapors as obtained 
in the hydrolyser and by condensation of the vapors coming from the column 
which rectifies the aqueous solution coming from the hydrolyser, whereas 
the medium-pressure condenser is fed with the condensate as obtained in 
the low-pressure condenser. 
A critical condition is that the maximum weight ratio of the vapors to the 
solution(s) of carbonate fed to the two medium-and low-pressure condenser 
is not higher than 2.5 and that the minimum temperature of the coolant is 
30.degree. C. 
It is interesting to observe that also the low-pressure condenser does not 
boost too much the decomposition reaction and causes to be left in the 
urea solution still a comparatively high amount of ammonia and carbon 
dioxide combined together in the carbamate form, and that the hydrolysis 
of the water which contains urea, ammonia and carbon dioxide is carried 
out in the presence of air.

The urea-synthesizing reactor, 1, operated at 150 kgs/sq. cm and 
190.degree. C., is fed with CO.sub.2, 2, by the compressor, 3, and with 
NH.sub.3, 4, by the pump 5. Ammonia, 4, prior to entering the reactor, 1, 
draws from the ejector, 6, the solution of recycled carbamate, 7. The urea 
solution, 8, from the reactor, 1, feeds the high-pressure decomposer or 
stripper, 9, which uses steam at 26 abs. atmospheres and 225.degree. C. 
The ammonia, carbon dioxide and water vapors, 10, emerging from the head of 
the stripper, 9, feed the carbamate condenser, 11, along with the solution 
of carbonate coming from the sections located downstream of the 
high-pressure loop now described, wherein they are exothermically 
condensed. 
The heat evolved in the condenser 11 is removed by producing steam at 4.5 
abs. atmosphere and at 147.degree. C. Such steam is used up in the 
sections located downstream of the loop, as will be explained hereinafter. 
The solution of condensed carbamate, 13, feeds the separator, 14, wherein 
the incondensable, oxygen-containing gases fed by 58 are separated and, 
from 14 the solution is recycled through 7 to the reactor 1 by means of 
the ejector 6. 
The urea solution, 15, discharged from the bottom of the stripper 9 and 
sent to the subsequent handling in the downstream sections, has the 
following properties: 
temperature--210.degree. C. 
pressure--150 kgs/sq.cm 
NH.sub.3 --22% by weight 
CO.sub.2 --5% by weight 
urea--48% by weight 
H.sub.2 O--25% by weight 
The solution, 15, feeds the medium-pressure decomposer, 16, which uses as a 
heating fluid the condensed steam coming from the stripper, 9, at 26 
kgs/sq.cm and 225.degree. C. 
In the decomposer, 16, the steam condensates are cooled to 160.degree. C. 
and the heat which has thus been yielded is supplied to the urea solution 
coming from 9 which is thus stripped of the major fraction of the ammonia 
and carbon dioxide contained therein. 
The solution, 17, discharged from the bottom of 16 has the following 
properties: 
temperature--155.degree. C. 
pressure--18 kgs/sq.cm 
NH.sub.3 --6.5% by weight 
CO.sub.2 --2.0% by weight 
Urea--64.0% by weight 
H.sub.2 O--27.5% by weight 
To the bottom of 16 are fed, in counterflow relationship with respect to 
the urea solution 15, the inert gases 18 coming from 14, with the 
advantage of unfolding a stripping effect and thus a low residual contents 
of NH.sub.3 and CO.sub.2 in the solution, even operating at the 
comparatively low temperatures (155.degree. C.) to which one is compelled 
to work when using as a heating means the condensate instead of the live 
steam and with the further advantage of passivating the decomposer 16 
inasmuch as oxygen is present in the inert gas stream. 
The pressure in 16 and thus also the temperature at which the urea solution 
must be heated in order that a maximum distillation of ammonia and carbon 
dioxide may be obtained is bound to the system of recovery and recycling 
of the vapors produced at 16. 
It is advisable to work at the lowest possible pressure, but the bottom 
value of the pressure is determined by the temperature of the cooling 
means employed in the head condenser 19 of the distillation column 20. The 
vapors coming from the head of 16 and composed by ammonia, carbon dioxide 
and H.sub.2 O feed the condenser 21, the latter working at 18 kgs/sq.cm 
and 70.degree. C. 
In addition to the condenser 21 is fed the diluted solution, 22, of 
carbonate coming from the low-pressure recycling section. The uncondensed 
coming from 21 and composed by inert gases, ammonia, and carbon dioxide 
and residual water along with the condensate feed through 23 the 
rectification column 20 in which, by a head reflux of pure ammonia, the 
complete absorption of CO.sub.2 and H.sub.2 O is obtained, pure ammonia 
being concurrently obtained as the head product, at 24. 
From the bottom of the column 20 the carbonate solution, 12, is dumped, 
which is fed back to the carbamate condenser, 11, of the high-pressure 
loop by means of the pump 25. 
The carbonate solution has the following properties: 
Temperature--65.degree. C. 
pressure--18 kgs/sq.cm 
NH.sub.3 --45.5% by weight 
CO.sub.2 --18.5% by weight 
H.sub.2 O--36.0% by weight 
From the head to the column 20, one discharge, through the main 24: 
pure gaseous ammonia (a few parts per million of CO.sub.2 and H.sub.2 O as 
residues) and 
inert gases, 
at a temperature of 43.degree. C. and a pressure of 17.5 kgs/sq.cm. 
The gas discharged from 20 feeds the condenser 19 wherein the major 
fraction of ammonia is condensed and is collected in the storage tank 26 
together with the fresh ammonia 4 to be fed to the installation. The 
inerts emerge from 26. From the tank 26 the liquid ammonia is, for the 
major aliquot, sent to the reactor 1 through 27 and the pumps 28 and 5 and 
is partly sent to the head of the column 20 through the pump 28. As 
outlined above, it is imperative that the decomposer 16 is operated at the 
lowest possible pressure and it has been found, according to the 
invention, that it is necessary that the condenser 21 works without any 
liquid in its interior. 
It was surmised, according to that which was customary, that it were 
impossible to achieve such a result, that is to work with empty carbamate 
condensers, and the condenser of the vapors of ammonia, carbon dioxide and 
water usually work full of the processing liquor (solution of carbamate 
and carbonate) to make homogeneous the solution and to prevent 
crystallization phenomena and consequential cloggings and plant stoppage. 
The admixtures of vapors of ammonia, carbon dioxide and water by being 
fractionally condensed usually originate, in the liquidless processing 
condensers (empty), areas with a high concentration of carbon dioxide and 
thus crystallization problems. 
This is prevented in the method according to the present invention with the 
following conditions: 
a maximum weight ratio of the vapors to the diluted carbonate solution not 
higher than 2.5; 
a minimum temperature of the coolant medium of 30.degree. C. 
The stream 17 coming from the decomposer 16 is fed to a decomposer 29 which 
works under a pressure of about 4.5 atmospheres at a temperature of about 
138.degree. C. (bottom temperature). 
The decomposer 29 aforesaid operates in such a way as to maintain a 
comparatively high residual contents of ammonia and carbon dioxide in the 
solution of urea, with the following advantages: 
(1) a low temperature of the urea solution which makes possible an 
economically acceptable use of steam at 4.5 atmospheres (147.degree. C.), 
(2) the presence of ammonia in the solution of urea sent to the final 
treatment (concentration in a vacuo and crystallization), which minimizes 
the urea decomposition phenomena. 
The urea solution at the outlet of the decomposer 29 is composed by: 
NH.sub.3 --2% by weight 
CO.sub.2 --1% by weight 
Urea--71% by weight 
H.sub.2 O--26% by weight 
In the conventional procedure in which, in this stage, a very accurate 
purification of the urea solution is carried out (NH.sub.3 less than 1%) 
it is necessary to operate, the pressure being the same, at a temperature 
higher than 150.degree. C., thus rendering economically unacceptable the 
use of low-pressure steam coming from the carbamate condenser. The vapors 
of ammonia, carbon dioxide and water coming from the head of the 
decomposer 29 (stream 30) are totally condensed in the condenser 31. The 
carbonate solution thus obtained is sent via 32 to the tank 33 and 
recycled via the pipings 34 and 36 and the pump 35 to the condenser 21. 
Also in this latter section of the installation, it is vital that the 
working pressure of the decomposer 29 be the lowest possible since it is 
determined by the temperature of the coolant medium in the condenser 31 
and the minimum temperature which can be attained in the condenser 31 
without having to cope with crystallization phenomena. 
In this case, too, the condenser 31 is empty and the diluted ammoniated 
solution 37 emerging from the sewage water treatment section is fed to the 
condenser 31 and encourages the total condensation of the vapors. The urea 
solution, 38, coming from 29 is fed to the final vacuum concentration 
treatment (in the specific example shown in the drawing) in order to 
obtain waterless urea. 
The water vapors, polluted with ammonia, carbon dioxide and urea, are 
introduced, via the lines 39 and 40, into the vacuum section 41, 42 
wherein they are condensed and are the sewage water of the installation, 
which is collected in the vessel 43 and properly treated as will be 
described hereinafter. 
The discharge water coming from 43 has the following composition: 
NH.sub.3 --4 to 5% by weight 
CO.sub.2 --1.5 to 2.5% by weight 
urea--0.5 to 2.0% by weight 
water--balance to 100%. 
Such a water, through the pump 44 is sent, after having been heated in the 
heat-exchangers 45 and 46, to the hydrolyser 47 which operates at least 
180.degree. C. and 18 atmospheres. 
In the hydrolyser 47 there is air blown from 48, continually removed from 
47 and used to the purpose of reducing the partial pressure of ammonia and 
carbon dioxide so as to encourage hydrolysis. The aqueous solution remains 
in the hydrolyser 47 for 40-60 mins. at 169.degree. C. and the virtually 
total hydrolysis of the urea which is present is obtained (the residual 
contents is less than 200 parts per million). 
The aqueous solution in the hydrolyser is heated with steam in the 
conventional manner. 
The hydrolyzed solution emerges from the hydrolyser 47 via 49 and preheats 
in the exchanger 46 the solution to be fed to the hydrolyser up to about 
173.degree. C. The solution is then fed through 50 to the rectification 
column 51 where the ammonia and carbon dioxide contained therein are 
removed. The column 51 has a bottom reboiler 52 which is heated by steam. 
The head vapors 53 formed by a mixture of ammonia, carbon dioxide and 
water are sent to a condenser 54 where they are also reached by the vapors 
55 coming from the hydrolyser 47. The water which has been treated and 
emerges from the bottom of 51 has the following residual impurity 
contents: 
NH.sub.3 --from 25 to 50 parts per million 
Urea--from 100 to 200 parts per million 
It preheates the water to be sent to the hydrolyser 47 in the exchanger 45. 
The ammoniated solution obtained in 54 is collected in the tank 56 and 
therefrom, via the pump 57 is partly refluxed to the head of the column 51 
and partly to the condenser 31. Air is vented from 56 via the pipe 58. 
In the example shown in the drawing, the water coming from 51 after the 
recovery of heat as aforesaid is sent to the apparatus for the recovery of 
the urea dust 59 and the aqueous solution of urea thus obtained, 60, is 
sent to the recovery of urea in the vacuum concentration stage. 
The melted urea 61 is sent from the concentrators to the prilling tower 62. 
If the dusts are not recovered, the water coming from 51 is dumped.