Title of Invention

A PROCESS FOR UREA SYNTHESIS FROM AMMONIA AND CARBON DIOXIDE WITH AMMONIUM CARBAMATE

Abstract (57) Abstract: The present invention relates to a process for urea synthesis by starting frorn ammonia and carbon dioxide, comprising a. reaction 3r.one under high pressure and temperature conditions, a section wherein a portion of unreacted ammonia and carbon dioxide are stripped and recycled to the reactor, which section operates under substantially the same reactor pressure, and a subsequent section, opei-ating under medium and/or low pressure conditions, for- urea purification and simultaneous recovery of" residual carbon dioxide and a portion of residual amonia contained in the effluent strearm from the stripping, section, as an aqueous solution of amnnonium carbamate, in which said aqueous amonium carbanate solution is either totally or partially fed to said stripping section. Said process makes it possible a conversion of carbon dioxide into urea to be obtained in the reactor which is higher than 70%. PRICE: THIRTY RUPEES
Full Text


The present invention relates to a process for high yield synthesis of urea.
In particular, the present invention relates to an improved-yield process for urea synthesis, comprising the reaction of ammonia and carbon dioxide undei high temperature and pressure conditions, the following separation of urea from the mixture containing the unreacted products, and the recycle of the latter to the reactor.
All industrial processes for urea preparation are based on the direct synthesis according to the rtiaction:

This reaction takes place according to well distinct reaction steps:

In the first step, an exothermic equilibrium reaction occurs, having a high reaction rate at room temperature, which, however, requires high pressures it order to reach a favourable equilibrium under the high temperature conditions required by (lb) step.
During the second step, and endotherniic reaction takes place which reaches a meaningful rate only at high temperatures (>150°C), with an equilibriim state which at 185°C leads to a CO2 conversion of only ap))roximately 53% in a reaction mixture containing a stoichiometric ratio of the

reactants. This unsatisfactory conveirsion can be advantageously increased by increasing the ratio of NHj:C02, but is further reduced in the presence of v»ater. The latter has furthermore an unfavourable effect also on'the overall process kinetics.
Both above reaction steps are not nonrally carried out in separate zones of the reactor, but they occur simultaneously inside the reaction mixture, v/hich, therefore, comprises urea, water, ammonia, carlxjn dioxide and ammonium carbamate, with relative concentrations in the different reactor areas, which depend on the several thermodynamic and kinetic factors which contribute to the procjesf..
Processes for obtaining urea by direct synthesis by starting from ammonia and carbon dioxide liavo been widely reported and disclosed in technical literature specifically dealing with this area. A vast review of the most common processes for producing urea can be found, e.g., in "Encyclopedia of Chemical Technology* Kirk-0:hm«jr, Ed., Wiley Interscience, third edition The industrial processes for urea production normally perform the synthesis inside a reactor to which NHj, COj and the aqueous solutions of ammonium carbonate and/or carbamate from the unconverted reactants recycle strtiams are fed, at temperatures comprised within the range of from 170 to 200*C, under pressures of at least 130 atm, with a molar ratio of

NHjtCOj comprised within the range of from 2.!) to 4.5^ as based on total feed streams. The molar ratio of HgOiCOj fed to the reactor is generally comprised within the range of from 0.5 to 0.6. Under such conditions, the product discharged from the reactor shows conversion rates comprised v*^ithin the range of from 50 to 65%, based on fed COg. Besides v/ater formed and the excess of fed NHj, the effluent stream from the reactor additionally contains large amounts of COj, prevliilingly as unconverted ammonium carbamate.
The separation of urea from these products is carried out in a plurality of sections operating at high tenperature and under decreasing pressures, inside which both the decomposition of ammonium carbamate into NHj and COj (products made available for being recycled to the reactor), and the evaporation of reaction water take place, with high-purity urea being finally obtained which is sent to the subsequent prilling step. The carbamate separation and recycle section requires investment costs for the fa From this section, all COj and a p>ortiori of HH^j, owing to their simultaneous presence, are made available for being recycled as ammonium salts (carbonate and/or hydrogencarbonate and/or carbamate, according to temperature, imposing the use of water as solvent medium to circulate them, in crder to prevent salts from - precipitating and consequently blocking the

concerned lines. This procedure implies an increase in water amount contained in the several liquid process streams and inside the reactor, with the consequent negcitive effects on conversion, as mentioned hereinabove. Fnown processes operating according to the above general scheme are, e.g., disclosed in U.S. patent No. 4,092,358; U.S. patent No. 4,208,347; U.S. patent No. 4,801,745; and J.S. patent No. 4,354,040.
In order to better clarify the above, we think it useful to stress that the water amount recycled to thts reactor for said handling is of the order of magnitude of water amount produced during the reaction. Therefore, the traditional reactor is particularly penalized because since from the very beginning of the process, it is interested by the high water concentration coming from the recycle lines. .?urthermore, the maximal water concentration is precisely found in the end reactor zone in which, on the contrary, having an as low as possible water concentration would be much more useful, in order to favour the rightwards shift of equilibrium in (lb) step, precisely in said end zone in which urea c;oncentration is already relatively high.
In order to obviate the above said drawbacks and increase as far as possible COj conversion into urea in traditional facilities, the operators attempted to opetrate under still higher temperature and-pressure conditions, eilthough such an

perating mode may imply a further increase in investment and perations costs. Unfortunately, even so, conversion levels f 60-65% cannot be exceeded.
The present Applicant found now a process which makes it ossible the difficulties and limitations affecting the raditional industrial processes, as mentioned above, to be vercome, with COj conversions into urea of higher than 70% eing reached.
Therefore, the object of the present invention is an mproved process for urea synthesis from amjnonla and carbon ioxide, with ammonium carbamate being formed as an ntermediate species, which comprises the following steps:
a) feeding ammonia and carbon dioxide to at least one reactor and causing them to react with each other, with a molar ratio of NHjtCOj, either as such, or as anunonium carbamate, comprised within the range o:: from 2.1 to 10, preferably of from 2.1 to 6.0, with a first liquid mixture containing urea, ammonium carbamate, water and ammonia being formed;
b) transferring said first liquid mixture to a decomposition-stripping step;
c) heating said first liquid mixture in said decomposition-stripping step, operating substantially under the same pressure as existing in said reactor, to cause a portion of ammonium carbcimate to get decomposed into ammonia and

carbon dioxide, and siinultaneoualy submitting said liquid mixture to stripping, with a firs*; gas mixture containing ammonia and carbon dioxidts^ and a second liquid mixture containing urea, water, aimronia, and the undecomposed portion of ammonium carbamate, being formed;
transferring, possibly through an ejector, said first gas mixture to a condensation step substantially operating under the same reactor pressure cind condensing said mixture, with a third liqxiid mixture being formed which contains ammonium carbamate and ammonia, which third liquid mixture is recycled, possibly through an ejector, to the reactor of the (a) step; recovering urea contained in said liquid mixture in one or more subsequent decomposition/separation step(8) with substantially pure urea, a fourth liquid mixture containing water, ammonia and ammoniuin carbamate and, possibly, a fifth stream substantially containing ammonia, being formed;
wherein said fourth liquid mixture formed in (e) step is totally or partially, preferably from 50 to 100% thereof, combined with said first liqu;.d mixture and is sent to said first decomposition-stripping step, with the residual portion, if any, being sent to the reactor, or, preferably, to said condensation s-ep.

9
According to the process of the present invention, which is usually carried out continuously on a 6u;.tal)le facility, fresh ammonia and fresh carbon dioxide are continuously fed to the facility in order to compensate for (I.e., replenish)-the corresponding amounts of reactants convert.ed into urea and discharged from the end separation and prilling sections.
Fresh ammonia and fresh carbon dioxide can be directly fed to the reactor, but they are preferably at least partially used as the drive fluid for one or more ejeci:or{s), in order to supply the necessary drive for said f:.rst: gas stream discharged from the (c) stripping st.ep, and/or ammonium carbamate coming from (d) condensation step to circulate. Ammonia is particularly preferred for use for that purpose.
According to an alternative route, or also s;i.multaneously to the use of the ejector(s), fresh ammonia or fresh carbon dioxide can be used either totally or pai-tially, as the stripping fluid in the stripper and/or can be directly sent to the condenser.
The synthesis reactor normally operates at temperatures comprised within the range of from 160 to 215°C, preferably comprised within the range of from 170 to ;:05°C, and under pressures comprised within the range of from 90 to 250 abs.atm, preferably of from 120 to 180 abs.atm, with molar ratios of ammonia:carbon dioxide preferably comprised within the range of- from 2.1 .to 6.0, more preferably' of from 2.5 to

4.5.
The reactor is normally equipped with a plurality of trays, of a type selected from the different types known in the art, so as to realize the optimal conditicjns of plug flow, possibly also in the presence of two-phase systems. The reactor can also comprise a plurality of reaction zones, suitably interconnected with each other, possibly with different feed streams.
Heat developed and, more generally, the temp^erature level of the reactor in the (a) step is controlled by acting on the temperature level of carbon dioxide and/or ammonia stream(8) fed to the reactor and/or based on the sub The reactor must display a value of liquid hold-up which will enable the residence time of said liquid inside the reactor to be comprised within the range of from a few minutes up to some ten minutes, in order to allow ammonium carbamate formed by the reaction of ammonia with carbon dioxide in the (d) condensation step and/or inside the same reactor, to undergo dehydration into urea.
In the process according to the present invention, in the (a) reaction step, also a second gas stream can be separated, as an effluent stream from reactor top, which is rich in inert species which, must be..removed. Such a gats stream is preferably

submitted to condensation in order to recover axmnonia and carbon dioxide contained in it, which are then directly recycled to the reactor. According to a second embodiment, such a gas stream is sent to the decomposition-stripping step, and the inert species are then subsequently separated from recycled carbamate stream to reactor.
In the process according to the prosenr invention, in which the reactor is caused to operate with cin excess of ammonia over the stoichiometric ratio to carboa dio'xide as necessary to produce ammonium carbamate, and, :he:i, urea (2:1, by mol), the reactor leaving stream end, generally, most liquid streams which are formed in the px-ocess, usually ■ contain ammonia in excess. In the instant disclosure, reference is made to the composition of such liquid (or also two-phase) streams and mixtures, conventional.'.y assuming that all carbon dioxide is present as ammonium carbamate, and any residual ammonia excess is present as free timmonia, or, more simply, ammonia.
Furthermore, in order to simplify the present disclosure, the term "liquid" is indifferently used with jreference to streams or mixtures of the process according to the present invention, which are constituted either by one single liquid phase, or by a mixed liquid-vapour phase. On tt.e contrary, the tenn "gas" is used for those streams or mixtures in which the liquid phase_.ia-subst.ai)tially absent.

The (c) decomposition-stripping step is normally carried out in a stripper usually heated by indirect, high pressure Bteain. The temperature of the stripper is normally comprised within the range of from 160 to 220'C, and the pressure inside it is equal to, or slightly lower than, the pressure inside the reactor, so as to make possible the decomposition products (first gas stream) to be recycled using, an driving means, only, and possibly, ejectors.
Under the above said conditions, ammoniuii corbamate tends to rapidly decompose forming ammonia and carbon dioxide, and urea already formed inside the reactor remains substantially unchanged. The stripping can be carried oat by using fresh ammonia or fresh carbon dioxide as th According to a preferred embodiment of the present invention, the decomposition-stripping step is carried out by using, as the drive gas, the same anmionia excefss present in the reactor leaving stream. Further detjils on such a
preferred technology can be found, e.g. . in lis*-, patent
lit 9^^
>7-fr3-^T-fi^fc -to -SNAMPB.QGETTI, the contents o:: which are

incorporated hereto by reference. Thitj last technology is referred to as "self-stripping".
According to the present invention, th«i decomposition-stripping step can also be carried out inside two equipment pieces (strippers) in cascade, possibly of different types from each other and operating under different conditions from each other, as disclosed, e.g., in G.B, patent 1,581,505, the contents of which are incorporated hereto by reference.
According to the present invention, from the (c) decomposition-stripping step, a first gas ni.cture of ammonia and carbon dioxide having a very low water content, is obtained. The water content is normally comprised within the range of from 0.0 to 10%, preferably oi fron. 0.0 to 5.0% by weight, based on total gas mixture weight. Such a small water content is that content which is normally obtained from the high-pressure stripping steps carried out according to the processes cited above.
In general, the (c) decompo6itic»n-s tripping step is carried out inside tube-bundle apparatuses with f.alling liquid film. The effluent mixture from the reactor, together with the fourth liquid mixture coming from the ttepus dovnstream from the stripper, is preferably fed to the head of the apparatus and forms a falling film along the walls of t;he tube bundle. However, also other well-known types of apparatuses can be used in the^ present, process, which are sv iteble for the

intended purpose.
The (d) condensation step is normally cairiesd out inside suitable condensers, e.g., of tube-bundle or surface types, in which the condensation heat is used in order to heat other fluid streams. The condensation heat is preferably used to generate steam, but it can also be used to supiply heat to one of the subsequent medium or low pressure ammonium carbamate decomposition steps.
According to the present invention, the condensation step can be carried out under the usual conditions (temperature, pressure and composition) used in the proo«sseu known from the prior art, provided that these are such as to prevent solid ammonium carbamate from forming in the condenser and/or in the outlet lines from it.
The separation of urea from ammonia aid ammonium carbamate still present in the second liquid effluent stream from the decomposition-stripping step is carried out, according to the (e) step of the prestmt process, in subsequent decomposition and separation secti.ont!, operating under medium (from 15 to 25 abs.atm) and/or low 'from 3 to 8 abs.atm) pressure conditions. For the purposes of the present invention, such (e) separation step can be carried out by means of any of the methods described in the upecific sector literature, which make it possible a liquid re:;ycle stream to be obtained which contains an aqueous solut:ion of ammonium

carbamate and, possibly, also a stream which is essentially constituted by ammonia. Suitable separation-purification sections for the purposes of the present invesntion are, e.g., those which are schematically represented in Figures 1-5 of "Encyclopedia of Chemical Technology" ibid.
Urea so separated from ammonium carbama1:e and ammonia is generally obtained as an aqueous solution which is then submitted to an end step of vacuum dehydration (down to 0.1 abs.atm) with on the one side water, and on the other side, substantially pure urea being obtained which is then sent to normal prilling processes, and so forth.
According to the present invention, the (e) urea separation-purification step also includes th'^ erd dehydration step and the section of purification of waste effluent waters from synthesis facility. According to a preferred embodiment of the present invention, the several, either liquid or two-phase streams containing ammonium carbamate, coming from the several sub-sections of the (e) step (medium- and low-pressure carbamate decomposition, re-condensation of carbamate, urea dehydration, waste streams purification) are gathjered , one single recycle stream which constitutes said fourth liquid mixture which is then either totally or parcially sent to the first decomposition-stripping step. According to certain modes of embodiment of urea separation and purification, however encompassed—by- the scope of the present invention, recycle

9
aimnonia and carbon dioxide can be prese.it as anunonium carbonate, hydrogencarbonate or carbamate, or a mixture thereof, according to mixture temperature eind pressure conditions.
The process according to the present in\'ention makes it possible the water amount fed to the reactor to be meaningfully decreased, down to such a level as to have, at reactor inlet, a molar ratio of H^iCO^ which is always lower than 0.3 (in which COj inside the reactor ie conventionally considered as being in carbamate form, as specified hereinabove). In such a way, a considerable ceneral decrease is obtained also in water amount present in the reaction zone. This makes it possible urea synthesis to be carried out with particularly high conversion values, prefe:rabLy comprised within the range of from 70 to 75% per each cycle, with no need for resorting to particularly complex and burdensome technical solutions in order to accomplish :hat result. In fact, the many advantages of the presert process are surprisingly obtained by means of the adoption of the simple measure of feeding to the stripper a considerable portion of carbamate containing streams coming from the sections downstream from the same stripper, and carrying out the stripping under such conditions as to have a low uater content in the separated gas stream.
The present process furthermore displays the advantage

of being suitable for being easily and surprisingly carried out by supplying a small number of simple modifications to an already existing traditional facility, provided it is equipped with a high-pressure stripping step. In particular, it will '^ be enough to modify the facility in such a way as to send to said stripping step, either totally, or partially, the recycled carbamate containing stream coming from the steps downstream from the same stripper.
Therefore, a further object of the present invention is a method for improving the yield of an existing process for urea production, which operates with a high-pressure synthesis section comprising at least one (self)stripping step, and subsequent urea purification and concentration steps, from which an aqueous ammonium carbamate solution is obtained, characterized in that said aqueous solution is either totally or partially, preferably from 50 to 100% thereof, fed to said (self)stripping step.


Accordingly the present invention provides A process for urea synthesis from ammonia and carbon dioxide, with ammonium carbamate being formed as an intermediate species, which comprises the following steps: (a) feeding ammonia and carbon dioxide to at least one reactor and causing them to react with each other, with a molar ratio of NHsiCOi, either as such, or as ammonium carbamate, comprised within the range of from 2.1 to 10, with a first liquid mixture containing urea, ammonium carbamate, water and ammonia being formed at a temperature from 170 to 250°C and under pressures from 120 to 180 abs. dtm. (b) transferring said first liquid mixture to a decomposition-stripping step; (c) heating said first liquid mixture in said decomposition stripping step at a temperature of 160 to 220° C operating substantially under the same pressure as existing in said reactor, to cause a portion of ammonium carbamate to get decomposed into ammonia and carbon dioxide, and simultaneously stripping said liquid mixture with a first gas mixture containing ammonia and carbon dioxide, and a second liquid mixture containing urea, water, amonia, and the undecomposed portion of ammonia carbamate, being formed; (d) transferring, through an ejector, said first gas mixture to a condensation step substantially operating under the same reactor pressure and condensing said mixture, with a third liquid mixture being formed which contains ammonium carbamate and ammonia, which third liquid mixture is recycled, through an ejector, to the reactor of the step(a); (e) recovering urea contained in said second liquid mixture in one or more subsequent decomposition/separation step (s) with substantially pure urea, a fourth liquid mixture containing water, ammonia and ammonium carbamate and.

possibly, a fifth stream substantially containing ammonia, being formed; characterized in that said fourth liquid mixture formed in step (e) is totally or partially, preferably from 50 to 100% thereof, combined with said first liquid mixture and sent to said first decomposition stripping step, with the residual portion, if any, being sent to the reactor, or preferably, to said condensation step.
The invention will now be described more in detail with reference to embodiments given by way of example and shown in the accompanying drawings, in which;
Figure 1 schematically represents the implementation of the reaction and decomposition-stripping steps (synthesis loop) of a process for urea synthesis, which constitutes a preferred embodiment of the present invention;
Figure 2 schematically represents an an alogous synthesis


loop, based on a typical traditional procesti.
In above said Figure 1, the dotted lines represent alternative possibilities, not mutually exclusive, available for implementing the process according to the present invention. In said figures, the functional unit, such as pumps, valves snd still other pieces of equipment which are not necessary for the purpose of fully uiderstanding the schematically shown processes, are not illuntratsd. In no case the process according to the present invention shall be understood as being limited to the einbodimeats reported and disclosed in the accompanying figures, which are supplied for merely exemplifying purposes.
In the Flow Scheme reported in Figure 1, the reactor Rl can be seen which, through the overflovi T
connected either with the reactor {line l-i) or with the condenser (line lb) or with the stripper bottom (line Ic), or, also, with a plurality of such equipment pieces. The line 3 from the top of reactor Rl is connected with the condenser CI (line 3a) and/or with the stripper SI (line 3b) or the condenser C2 (line 3c).
The Flow Scheme reported in Figure 2 substantially reproduces the same elements, with the name meaning, as contained in the Flow Scheme of Figure 1, bat with reference to a traditional process for urea synthesia. The meaningful differences relatively to Figure 1 are contitituted by the absence of line 5a, the line 5 being directlj connected with the line 7-7b leading to the condenser C2, the absence of ejector El in the outlet line from stripper SI top (and the corresponding absence of the drive stream 10b), fresh COg being totally fed to the reactor, through lir^e 1.
Referring to Figure 1, some possibilities of embodiment of the process of the present invention are disclosed now, with anyway such a disclosure being non-liniite.tive of the overall purview of the same invention.
Fresh ammonia, compressed and fed through line 2, is combined with recovered ammonia (line 9), coming from section P, and the resulting stream is partially sent to reactor Rl through line 10 and ejector E2, and partially -^o carbamate condenser C2L through, .ejector El (line 10b). \ccording to an

ilternative route, as needed, ammonia can hi: e:.ther totally Dr partially fed to stripper SI through liae 10a; in this ::a8e, line 10b (and, consequently, ejector BIl) can be absent. This is the case when stripping is carried out with ammonia, fiowever, preferably, from 30 to 90% of ammot.ia from lines 2 and 9 is fed to the ejector El, via 10b, with the residual portion thereof being sent to the ejector E2, via line 10.
Under usual operating conditions of the process according to the present invention, said streams 10, 10a and 10b prevailingly contain ammonia in liquid .state .
Fresh COg (line 1) can be analogously sent through lines la and/or lb, according to the enthalpy requirements of reactor Rl, but can also be sent, through line Ic, to stripper SI, in which case it will be also used as stripping medium.
Preferably, fresh carbon dioxide, after b«ing compressed, Ls mostly directly sent to the reacto;: (more than 50% thereof), and partially sent to condenser C2.
The total reactor feed is constituted by streams la and 11, the latter with a very limited water content, partly deriving from the possible urea formation already in the-ammonium carbamate condenser C2. According to the present invention, in that way the reactor preferably operates with an overall feed in which the molar ratio of water to COg is lower than 0.2.
The liquid stream discharged from reactoi- R] through the

overflow T and line 4, containing urea, wauer, anmionia and anunonivun carbamate/ is sent to line 5a containing at least 50% of aqueous recovered stream coming, through ] inei 5, from urea separation and purification section P, and is sent (line 4a) to the stripper SI. Preferably, the stream 5a contains from 70 to 100% of the recovered stream 5. The possibly residual portion of that recovery stream is sent to the condenser C2, either directly through line 5c, or indirectly through 5b.
According to a preferred embodiment of the present invention, the feed 4a to stripper SI is subdivided into partial feed streams fed to said stripper at different levels thereof.
The gas phase present in reactor ovesrhead is sent, through lines 3 and 3a, to the condenser CI in which the inert species present in fresh NHj and COj feed streams are separated; according to an alternative route, a portion of said gas phase can be sent, by means of the line 3b, to stripper SI, to act as the stripping mediun, or it can be directly sent to the condenser C2 (line 3c).
The gas stream 7 discharged from stripper head, containing NHj and COj and having a lovr water content, preferably lower than 10% by weight, and, still more preferably, lower than 5% by weight, is recycled to the condenser C2 (lines 7a and 7b) through the ejector El, using NHj as the -drive fluid. Here, it is condensed, under a

pressure which is either equal to, or slightl/ lower than, the pressure existing inside the reactor, and at an as high as possible temperature, preferably higher than 150°C, in order to obtain a liquid stream (the third liquid mixture) prevailingly containing ammonium carbamate and ammonia and minor water amounts and, possibly, urea amounts. The latter is formed during the condensation step, wi';h the operating conditions being already favourable to pari:ia:.ly shift the preceding reported chemical equilibrium (lb) to the right. The so obtained liquid stream si fed to the reactor through line 8 and ejector E2.
The stream 6 discharged from the bottom of stripper SI, containing all produced urea, is sent to the subsequent purification and concentration steps, which are schematically combined in P section, in the Flow Scheme o:! Figure 1. From here the recovered NHj and carbamate streams, already cited above, are derived, and pure urea and watei' are discharged through lines 15 and, respectively, 14.
In order to better illustrate the purpose and advantages of the present invention, a practical exajnple thereof is supplied in the following, which by no v/ay ssha.Ll constitute a limitation to the scope of the claims.
In the following examples, the compositions of the several streams are reported by reforrincf to the basic components_urfia, ammonia, carbon dioxide and, water.

independently on carbon dioxide, in those liquid streams which contain ammonia, substantially being in ammonium carbamate form.
Example 1
A process for urea synthesis is operated by feeding to the stripper the recovery stream coming from urea separation/purification section (synthesis loop according to the present invention). Reference is made to the Flow Scheme reported in Figure 1.
From lines la, lb and 2 respectively, fresch streams of 45,461 Kg/h and 8,573 kg/h ofCOg and 41,753 kg/h of NHj, co.italning a total amount of 636 kg/h of inerts, are fed to reactor R\.
The reactor operates under 157 abs.atn. and ISB^C, the condenser C2 operates under 152 abs.atm and .it approximately 155*'C.
From the purification/concentration section P downstream of the stripper SI, an aqueous stream 5 rich in carbamate is recovered, which is constituted, in particular, by: HgO = 17,181 kg/h COj = 9,415 Kg/h NH3 = 19,401 kg/h Total
stream 45,997 kg/h
which is totally sent again back to stripper ;31 through lines 5a and 4a, after being combined with the effluent stream 4

from the reactor.
From the same section P a stream of neat NH3 is simultaneously recovered through line 9 and ;.s then combined with fresh airanonia coming from line 2.
From the total NH3 stream thus obtained, 20,793 kg/h are sent, through line 10b and ejector El, to condenser C2, and 41,753 kg/h are sent to the reactor through line 10 and-ejector E2.
The total stream fed to the reactor Rl through line 11 is as follows:
COj = 72,973 Kg/h NHj = 96,241 kg/h
Urea = 960 kg/h (formed in C2 and equivalent to
704 and 455 kg/h of COj and NH3,
respectively)
HjO = 288 Kg/h
Inerts = 636 Kg/h Total stream 171,098 kg/h
The liquid stream 4 discharged from realtor overflow T
contains all produced urea, and, in jjarticular, is
characterized by:
Urea = 73,682 Kg/h
HjO = 22,105 kg/h
CO2 =. 1,9,643 ...Kg/h

NHj = 55,032 kg/h
The reactor overhead gas stream 3 is totally sent to condenser CI, allowed to operate at 75°C; from this, a stream is discharged which contains the amount of 636 kg/h of inerts originally contained in the feed to the reactor, besides negligible amounts of NH3 and CO2, not taken into consideration in the above balance.
The stream 4 is sent to the stripper SI, through line 4a, after being combined with stream 5a; the stripper operates under 148 abs.atm, with a bottom temperature of 2;05°C, with no feed of stripping gas (self-stripper).
From the head of the stripper SI a gas ' stream is discharged which is substantially free from water and is characterized by the following composition: CO2 = 19,643 Kg/h NH3 = 34,239 kg/h
From stripper bottom a liquid stre.am 6 is discharged, which is constituted by: Urea = 73,682 Kg/h H2O = 39,286 kg/h COj = 9,415 Kg/h NHj = 40,194 kg/h Total
stream 162,532 Kg/h which is sent to the following steps of urea purification and

concentration. These are substantially constituted, in this particular case, by the typical medium- and low-pressure separation sections, and by the concentration section which characterize the traditional SNAMPRCX3ETTI urea process the general scheme of which is reported, e.g., on page 561 of "Encyclopedia of Chemical Technology", ibid.
The process for urea synthesis exemplified above is characterized by a conversion of COj into ireci, i.e. by a molar ratio of (produced urea): (total COj fed) of 0.73. The liquid stream discharged from the stripper and sent to the subsequent urea separation/purificatioa section is characterized by a molar ratio of urea COj == 5.74.
Example 2 (Comparison example)
A process for urea synthesis operates with t.he recovered stream from urea separation/purification secti.on (traditional synthesis loop) being fed to the high-pressura condenser. Reference is made to the Flow Scheme reportec: in Figure 2.
The reactor Rl, the condenser C2 and the stripper SI (self-stripper) operate under temperature and pressure conditions which are exactly the same as ol; the preceding Example 1.
The reactor Rl is fed with:
(a) a fresh COg stream through line 1;
(b) a liquid stream, through line 8, constituted by an aqueaus- solution- containing carbamate and ammonia.

recovered from urea separation/purification section P,
and from stripper SI through the condenser C2, which is
driven by means of an ejector E2 using NH, as the drive
fluid.
The total feed to the reactor is*
2O2 = 73,677 Kg/h
m^ = 96,785 kg/h
ijO = 17,181 Kg/h
rnerts = 636 Kg/h
Total
stream 188,279 kg/h In particular:
(a) the fresh COj stream is constituted by (b) the fresh NH3 stream is constituted by 35,148 kg/h; the stream of NH3 recovered as such from P, is constituted by 23,573 kg/h;
(c) the liquid stream recovered from P is cinstituted by: H2O = 17,181 kg/h
CO2 = 28,216 kg/h
NH3 = 38,064 kg/h
Thus, to the system consisting of the c:ondenser C2 and reactor Rl, total amounts of NH3 and COg are fed, which are equivalent to those as of the preceding example, but, in this case, with the ratio of H20:C02 being increased up to 0.57 owing to the_xecycle..of the aqueous stream 5 tD the condenser.

Sumnarizing, the reactor Rl operates, in tnat case, under operating conditions equal to those as of the preceding Example 1, except for the larger amount of water deriving from the stream from condenser C2.
The stream discharged from said reactor through the overflow is constituted by:
Urea = 62,291 Kg/h
HjO = 35,868 kg/h
CO2 = 27,797 Kg/h
NH3 = 61,487 kg/h
Inert species = 636 kg/h
Total effluent = 188,279 Kg/h
This stream is sent to the stripper SI through the line 4.
The gas stream 7 discharged from the head of the stripper is directly sent to the condenser C2. The liquid stream 6 discharged from stripper bottom and containing all produced urea, is sent to the subsequent purification/concentration section P, which also allows excess NHj and carbamate, still present in the same stream, to be recycled through lines 5 and 9.
The traditional synthesis loop taken into cionsideration in this comparison example is characterized by a conversion of COj into urea, i.e., by a molar ratio of (produced urea-): (total- fed COj) of 0.62. The liquid stream discharged

from the stripper and sent to the subsequent separation/purification steps is characterized, by a molar ratio of urea COj = 4.84.
From a comparison between the conversion values which characterize the process according to the prosent invention, as disclosed in Example 1, and a typical traditional process for urea synthesis, a very meaningful increaise in conversion rate clearly appears (with all the other factors possibly playing a role being the same), and consequently of the production capacity of the facility, which is increased by approximately 20%.
It is furthermore evidenced that the decrease in water level circulating through the condensor-reactor loop of the present invention also allows the reactor size to be appreciably reduced, thanks to the more favourable kinetics of the same reaction.
Obviously, a large number of changes ard modifications can be supplied to the process as disclosed above, which shall anyway be regarded as being fully encompassed by the purview of the present invention.



WE CLAIM:
1. A process for urea synthesis from ammonia and carbon dioxide, with ammonium carbamate being formed as an intermediate species, which comprises the following steps: (a) feeding ammonia and carbon dioxide to at least one reactor and causing them to react with each other, with a molar ratio of NH3:C02, either as such, or as ammonium carbamate, comprised within the range of from 2.1 to 10, with a first liquid mixture containing urea, ammonium carbamate, water and ammonia being formed at a temperature from 170 to 250°C and under pressures from 120 to 180 abs. atm. (b) transferring said first liquid mixture to a decomposition-stripping step; (c) heating said first liquid mixture in said decomposition stripping step at a temperature of 160 to 220° C operating substantially under the same pressure as existing in said reactor, to cause a portion of ammonium carbamate to get decomposed into ammonia and carbon dioxide, and simultaneously stripping said liquid mixture, with a first gas mixture containing ammonia and carbon dioxide, and a second liquid mixture containing urea, water, amonia, and the undecomposed portion of ammonia carbamate, being formed; (d) transferring, through an ejector, said first gas mixture to a condensation step substantially operating under the same reactor pressure and condensing said mixture, with a third liquid mixture being formed which contains ammonium carbamate and ammonia, which third liquid mixture is recycled, through an ejector, to the reactor of the step(a); (e) recovering urea contained in said second liquid mixture in one or more subsequent decomposition/separation step (s) with substantially pure urea, a fourth liquid mixture containing water, ammonia and


ammonium carbamate and, possibly, a fifth stream substantially containing ammonia, being formed; characterized in that said fourth liquid mixture formed in step (e) is totally or partially, preferably from 50 to 100% thereof, combined with said first liquid mixture and sent to said first decomposition stripping step, with the residual portion, if any, being sent to the reactor, or preferably, to said condensation step.
2. The process according to claim 1, in which said ratio of
NH3:C02 inside the reactor is in the range of from 2.5 to 4.5.
3. The process according to claims 1 or 2, in which said reactor is
provided with trays and operates under plug flow state conditions.
4. The process according to any one of the preceding claims, in
which said reactor comprises a plurality of reaction zones, suitably
interconnected with each other, preferably having different feed streams.
5. The process according to any one of the preceding claims, in
which the drive fluid of the possible ejector of step (d) for the recycle of said
third liquid mixture, is constituted by at least a portion of feed ammonia.
6. The process according to any one of the preceding claims from
1 to 4, in which ammonia is at least partially directly fed to said
decomposition-stripping step.


7. The process according to any one of the preceding claims from
1 to 4, in which at least a portion of feed ammonia is used as the drive fluid
in a ejector used to circulate said first gas mixture.
8. The process according to any one of the preceding claims from
1 to 4 in which said decomposition-stripping step operates under self-
stripping conditions.
9. The process according to any one of the preceding claims from
1 to 4, in which fresh carbon dioxide is at least partially directly fed to said
decomposition-stripping step.
10. The process according to any one of the preceding claims from
1 to 4, in which carbon dioxide is at least partially fed to the condensation
step (d).
11. The process according to any one of the preceding claims, in
which the decomposition-stripping step 'c' is carried out inside a stipper and
heated by means of indirect high-pressure steam.
12. The process according to any one of the preceding claims in
which decomposition-stripping step (c) is carried out in two pieces of
equipment in cascade, preferablyof different types from each other and
operating under different conditions from each other.


I
13. The process according to any of the preceding claims in which said first gas mixture contains a water level comprised within the range of from 0 to 5.0% by weight, based on total mixture weight.
14. The process according to any one of the preceding claims, in which the average molar ratio of H2O:CO2 in the feed to said reactor of the step (a) is lower than 0.3, preferably lower than 0.2.
15. The process according to any one of the preceding claims, in which carbon dioxide conversion is comprised within the range of from 70 to 75% per each cycle.
16. The process according to any one of the preceding claims, in which at least a portion of carbon dioxide conversion into urea takes place during the course of condensation step (d).
17. The process according to any one of the preceding claims, in which from 70 to 100% of said fourth liquid mixture formed in the step (e) is combined with said first liquid mixture and the resulting combined mixture is sent to said first decomposition-stripping step (c ).
18. The process according to any one of the preceding claims, in which the feed to said decomposition-stripping step is subdivided and the resulting partial streams are fed at different stripper levels.


19. A method for improving the yield of an existing process for
urea production, which operates with a high-pressure synthesis section
comprising a (self)stripping step and subsequent urea purification and
concentration steps, from which an aqueous solution of ammonia carbamate
is obtained, characterized in that said aqueous solution is either totally or
partially, preferably from 50 to 100% thereof, fed to said (self) stripping
step.
20. The method according to claim 13, in which from 70 to 100%
of said aqueous ammonium carbamate solution is fed to said (self )stripping
step.
21. A process for urea synthesis from ammonia and carbon dioxide
substantially as herein describes with reference to the accompanying
drawings.


Documents:

0157-mas-1996 abstract.pdf

0157-mas-1996 claims.pdf

0157-mas-1996 correspondence others.pdf

0157-mas-1996 correspondence po.pdf

0157-mas-1996 description (complete).pdf

0157-mas-1996 drawings.pdf

0157-mas-1996 form-1.pdf

0157-mas-1996 form-26.pdf

0157-mas-1996 form-4.pdf

0157-mas-1996 form-6.pdf

0157-mas-1996 others.pdf

0157-mas-1996 petition.pdf


Patent Number 194091
Indian Patent Application Number 157/MAS/1996
PG Journal Number 02/2006
Publication Date 13-Jan-2006
Grant Date 28-Oct-2005
Date of Filing 31-Jan-1996
Name of Patentee M/S. SNAMPROGETTI S P A
Applicant Address VIALE DE GASPERI 16 20097 S. DONATO MILANESE
Inventors:
# Inventor's Name Inventor's Address
1 CARLO RESCALLI VIA LIBERTA 20-S. DONATO MILANESE
PCT International Classification Number C07C273/04
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 MI 95/A 000281 1995-02-16 Italy