Title of Invention

A PROCESS FOR THE PREPARATION OF UREA

Abstract

ABSTRACT Process for the preparation of urea in which the gas stream released during the synthesis of melamine consisting predominantly of ammonia and carbon dioxide, is returned to the urea process without any further treatment and is used there for the synthesis of urea, characterized in that the gas stream coming from the high-pressure melamine process consisting predominantly of ammonia and carbon dioxide, is supplied to a high pressure section of a urea stripping plant.

Full Text

The invention relates to a process for the preparation of urea in which the gas stream released during the synthesis of melamine, consisting predominantly of ammonia and carbon dioxide, is returned to the urea process without any further treatment and is used there for the synthesis of urea. Such a process is described in GB-A-1309275. This patent describes the preparation of urea in which the waste gas, consisting predominantly of ammonia and carbon dioxide, obtained in the preparation of melamine in a high-pressure melamine process, is used for the synthesis of urea. In this process the gas stream from the gas liquid separator of the melamine plant, consisting predominantly of ammonia and carbon dioxide, is transferred to a low-pressure section of a conventional high-pressure urea plant. In this low-pressure section, in an extra reactor, a urea solution is prepared from the ammonia and carbon dioxide originating from the melamine plant. This urea solution is subsequently compressed and transferred to the high-pressure section of the same urea plant. Urea can be prepared by introducing ammonia and carbon dioxide into a synthesis zone at a suitable pressure (for example 12.5-35 MPa) and at a suitable temperature {for example 160-250 °C), ammonium carbamate being formed first according to the following reaction: 2 NHg + COj -» HjN-CO-ONH, From the ammonium carbamate formed, urea is subsequently formed through dehydration according to the following equilibrium reaction: HjN-CO-ONHj - HjN-CO-NHj + HjO The degree to which the latter conversion takes place depends among other things on the temperature and the ammonia excess applied. As the reaction product a solution is obtained which predominantly consists of urea, water, ammonium carbamate and unbound ammonia. The ammonium carbamate and the ammonia need to be removed from the solution and are in most cases returned to the synthesis zone. The synthesis zone may consist of separate zones for the formation of ammonium carbamate and urea. However, these zones may also be combined in one piece of equipment. A conventional high-pressuie uiea plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual excess ammonia take place at a pressure which is essentially lower than the pressure in the synthesis reactor itself. In a conventional high-pressure urea plant the synthesis reactor is usually operated at a temperature of 180-210*C and a pressure of 18-30 MPa. In a conventional high-pressure urea plant the unconverted reactants are returned, after expansion, dissociation and condensation to the urea synthesis at a pressure of between 1.5 and 10 MPa. In addition, in a conventional high-pressure urea plant ammonia and carbon dioxide are directly fed to the urea reactor. The molar NH3/CO2 ratio (= N/C ratio) in the urea synthesis lies between 3 and 5 in a conventional high-pressure urea process. The disadvantage of the process described in GB-A-1309275 is that an extra reactor is needed because the gas stream supplied from the melamine plant, which consists predominantly of ammonia and carbon dioxide, has a pressure which is too low, even in the case of a high-pressure meiamine process, to be used directly in a conventional high-pressure urea plant. Furthermore, in the process according to GE-a-1309275 an extra pump is needed to transfer the urea produced at a low pressure to the high-pressure section. The aim of the invention is to find a process which does not have these disadvantages. Applicant has found that the above-mentioned disadvantages can be eliminated by supplying the gas stream coming from the high-pressure meiamine process, consisting predominantly of ammonia and carbon dioxide, to a high-pressure section of a urea stripping plant. A urea stripping plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual excess ammonia for the most part take place at a pressure which is substantially equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper, whether or not with addition of a stripping medium, in a stripping process, carbon dioxide and/or ammonia can be used as stripping gas before these components are fed to the reactor. This stripping takes place in a stripper situated downstream of the reactor, with the solution emerging from the urea reactor, which besides urea, ammonium carbamate and water also contains ammonia and carbon dioxide, being stripped with addition of heat. Thermal stripping can also be applied here. Thermal stripping means that ammonium carbamate is decomposed and the ammonia and carbon dioxide present are removed from the urea solution solely through the addition of heat. The ammonia and carbon dioxide containing streams coming from the stripper are returned to the reactor via a carbamate condenser. The reactor, the stripper and the carbamate condenser are the most important elements of the high-pressure section of the urea stripping plant. In a urea (0 Hi CI 0) ■o rt- 1-3 n m U rt rt- rt o rt 01 rt O Q) ■d (0 rt- 3 (a o o 01 rt 1-' tu a M O a- o rt- * M fi) m a Q> (D a M a 0) (D M 0) (0 (D a ID rt ID n rt- 3 o 3 p- rt ID rt M- (0 a 0) (0 (D rt n 0> tr rt H- P rt 1 (P (0 K a Hi rt O Q> p- p" rt p- ■a tr 01 p. rt- iQ I-" 01 a- rt iQ o> 3 l-J pi rt a P* n (D CD ID ID p- a 3 tJ M 01 ■d ■a rt- a -o D) w 3 ID O -o 0) 3 (0 -a rt rt 3- rt- 3 0) M O a a O p- p- TJ • rt- ■d Q X rt 01 (D ID rt rt 3 01 a pj a p* tJl Q p- M O (D M I-"- 0> !-■■ n H* (D rt- rt- o a H- o cr 0) Q rt- ID a 1 a 3 (0 0) e rt- o 3 a o M Q> 1 n ID M 01 0) 5 a c (D M- ^ 3 a 01 rt 0) o a •d rt- tJ 3 3 a a- - Hi a •d -J pi 3 o M M- 01 X I-"- 01 a. rt rt M O a H rt (D H* M 01 p- CD 01 a H 1^ TJ • rt- -d Oi l_J o a a rt- H- rt w 3 o rt O 3 O 01 £u ID H- rt a 3 rt rt- rt 0) 0 3* l_i i;i p" o iQ (D 3 P i-t lO M rt M- 0) a rt 01 a 1 •d Id CD H- P O p o o ID 01 pi 0> r+ 0) 3 n> Q. M o P a- tJ 3 rt- a Q rt n •d 01 O o rt rt p 01 CD a s 3 ET a M- a rt- rt- (0 a rh rt- ID -a 3- (D 0> rt (D rt- o a Hi a O 3 (D 0) o rt rt 01 «Q rt ^5 rt- rt- O a a o tr H- a- o rt H- (D o O a 3 M* X rt rt a rt o 0 P> 0) p> CD cr 01 0) 0) CD H- iQ rT (D a (D o 3 O O Dl rt o m •a rt p- - H- 0 3 t-J •d 0) o - m p- * 3 rt- o ri- a- 3 M- 3 a rt- 3 ffi (0 tJ -d rt "d rt 3 (D a (J rt H rt 3 01 •d a- Bt H- rti Q C rt a* O a rt Q) H- 3 rt rt H- 0) ■o H- H- 3 (D ^ i-t o i-i M n a- I-" rt £u rt- H- M- D. a cr * o 3 o a O rt- o- 3 P> 1 Q) rt a a- rt ff 3" H cr a o (D (D iQ iQ a 3" (0 •0 p. iQ 0 a iQ 3- H a V O 01 M- O 01 O rt p- p- rt ID ID 0) CO (U Q) 3 •a H- a P H- ■ •d 0) (0 a ID a rt rt rt p- o o o rt- rt > 3 h-" 3 n 1 a t-' U (D rt- Hi M* o ID iQ 0) M H- rt 01 O rt 0 -d X a- P) « a P a B h-t ri- cr M ® •a iQ * rt- KJ &- o (D o 11 (-■■ M • O O (D H- Hi 1 rt rt • Q ro o. W rt- H* M* n M- (0 M Hi a* T) H 0) rt a o rt t, n tJ P> rt rt M a Q rt- iQ a rt rt O 01 O a 3 3 rt- 0 0) o 1 •a a t-" rt rt- m 0> P t-" 3 ID a rt O m 0 Kl p) rti p* O m ^ O 0 a C 3 p) •a ID I-' 3- P- H- • to IB M D) H- O rt a •d p- p* 3 • rt 01 a (+ M a 1-1 M rt D. rt ► M 01 m UI P- P- Q> CD ^ rt- a i-h (D rt p O a- o H P Hi rt (D (D cr 3" a 0) rt rt O 0\ rt (D M 3 hi a" o d « I-" •a 3 M o a H o a rt ■d CD It) a 01 3 Hi 01 3* 01 o CD a M M- o o (0 (D a fa !-■■ Dl !-• (D p: cr ID p rt- •d 3- H- rt H* rt- rt- ID ID 3 P- 1 P) [Q 1 a O cr rt O •d (D O. (D o ■d Q> rt 3" •a o 0) H- w a m a ID O ID O rt- p* p- Q) rt tr w rt M rt- rt- rt rt O M (D 3 a H- a ft 0) rt o •d M CD rt 0 tJ 3 rt- CD p- lU rt o O ■. tr ■a a- tr H- Q a> rt m a H- lO a H- ID rt- a K ID rt H- a p- a tJ a O a >Q * a- rt CU rt- o a 0) Di o O m cr (D 01 a H- 3 0» 0) (D 01 o ID rt ■d p" rt o Pt 3 1 o H> a € o H- CO rt- H rt- ID O rt- a a CD 01 a- o p" - p> p- ■o 01 CQ Q- (D Q a "rt 0 rt n P P rt rt- ■o T3 h" 3 01 M- p- n ID rt > (D to rt 0> CO cr - rt p- p) CD 3 3 -d M- a u (D H* -a rt M- D. tJ ID 0 (D a B -a rt "D cr 0 tJ Q o 1 rt- H* (-"■ H- 01 ID Hi rt rt- rt p- o a- rt CO 3" (D (-•- H- rt- •o a- CD o IS 0 O 3 (D 01 3" rt- 3 01 rt CD rt 0) H 1 P- p CD 0) rti •a a- CO M- (D ID M 3 H. rt n rt ID iQ P- rt "^ 3" CD p- o •d ' w (D O t-i ■o rt 01 rt « -d n O (D CD -d 0) o n ID rt ID p- 73 rt 1-1 l-h a P H* a rt rt rt 3- H- Q> QJ rt 0) fi) rt 3 -d a 01 X ID 0 u rt 0) rt- Q p n to M- O 3" a- (D H- ■d Ui a o h-" O 0 iQ o Hi rt 01 01 Hi CD cr M- a rt- ■o (D o o ID o 01 ■D a n M rt 0) rt rt M cr 3 01 p- (Q p" O a- 0> Ml M (D ID Hi (0 ► 01 -d p a CO ' C rt- (D rt- (D H- 01 iQ c rti rt O M- Dl rt ID P) n 3 (-> rt C M O O a a (Q 0) rt (D ■• P 3 01 n a 3- P) CD CD Q) rt 0> o - {Q (D a a ID a rt CD H DI • m a a Q to an adiabatically operated flash vessel. It has been found that this process results in a saving in terms of high-pressure steam and in improved stripping operation. It has also been found that additional steam production was obtained in the carbamate condenser. The latter is an advantage in particular if the melamine plant and the urea plant are highly tied-in. Plants that are highly tied-in mean that a relatively large amount of the urea produced is used by the melamine plant. The advantage of using the gas stream from the high-pressure melamine plant is that a virtually water-free gas stream consisting of ammonia and carbon dioxide is obtained for the urea stripping plant which, by virtue of its virtually water-free character, results in improved efficiency of the urea plant compared with a urea plant to which a water-containing carbamate stream is supplied from the melamine plant. Moreover, according to this process no absorption and/or concentration step of the gas stream coming from the melamine plant is necessary because the gas stream is already virtually water-free and has a sufficiently high pressure. Further, the extra heat released in condensing the gas stream from the high-pressure melamine plant can be used for extra production of (low pressure) steam. The pressure of the gas stream coming from the high-pressure melamine plant, which consists predominantly of ammonia and carbon dioxide, lies above 12.5 MPa. The pressure of the gas stream coming from the high-pressure melamine plant lies generally below 80 MPa, preferably below 40 MPa and more preferably below 20 MPa. In particular, the pressure of the gas stream coming from the high-pressure melamine plant is 0-10 MPa, more specifically 0-3 MPa and even more specifically 0-2 MPa higher than the pressure in the urea reactor. The temperature of this gas stream lies between 160 and 275°C, preferably between 175 and 235°C. Urea synthesis A widely used method for the preparation of urea via a stripping process is described in European Chemicsl News, Urea Supplement, of 17 January 1969, pages 17-20. In this process the urea synthesis solution formed in the synthesis zone at a high temperature and pressure is subjected to a stripping treatment at the synthesis pressure by being contacted countedcurrently with gaseous carbon dioxide wile heat is added. In this operation, the greater part of the ammonium carbamate present in the solution is decomposed into ammonia and carbon dioxide. These decomposition products are then expelled from the solution in gaseous form and discharged together with a small amount of water vapour and the carbon dioxide used for stripping. Such a stripping treatment can be carried out not only with carbon dioxide, as described in this publication, but also thermally or with the aid of gaseous ammonia as stripping gas or with a mixture of the above-mentioned methods. The gas mixture obtained in the stripping treatment is for the greater part condensed and adsorbed in a carbamate condenser, upon which the ammonium carbamate formed is transferred to the synthesis zone for urea formation. The synthesis can be carried out in one or two reactors. For example in two reactors in which in the first reactor pure ammonia and carbondioxide are used and in the second reactor a mixture of pure ammonia and carbondioxide and recycled ammonia and carbondioxide or recycled ammonia and carbondioxide alone. Preferably the synthesis is carried out in one reactor. Likewise, the stripping of the urea synthesis solution with the aid of a stripping medium can be carried out in more than one stripper. The carbamate condenser can for example be designed as a so-called submerged condenser as described in NL-A-8400839. In this case the gas mixture to be condensed is led into the shell-side space of a shell-and-tube heat exchanger, into which space in addition a dilute carbamate solution is led, and the heat of dissolution and condensation released is discharged with the aid of a medium flowing through the tubes, for example water, which is converted into low-pressure steam in the process. The submerged condenser may be positioned horizontally or vertically. However, it is particularly advantageous to carry out the condensation in a horizontally positioned submerged condenser {a so-called pool condenser; see for example Nitrogen No 222, July-August 1996, pp. 29-31), as the residence time of the liquid in the condenser is generally longer compared with a vertically positioned submerged condenser. This results in the formation of urea, which raises the boiling point, so that the difference in temperature between the urea-containing carbamate solution and the cooling medium increases, as a result of which a better heat transfer is achieved. It is also possible to combine the condensation zone and the synthesis zone in one apparatus as described in for example NL-A-1000416. In this case the formation of ammonium carbamate and urea from carbon dioxide and ammonia is carried out at a pressure of 12,5-35 HPa in a urea reactor. This urea reactor comprises a horizontal positioned condensation zone and heat exchanger (a so-called pool reactor; see for example Nitrogen No 222, July-August 1996, pp. 29-31), with ammonia and carbon dioxide being fed to the reactor and largely being adsorbed in the urea synthesis solution. With the aid of the heat exchanger a substantial part of the heat formed by condensation is carried off. The residence time of the urea synthesis solution in the reactor is chosen so that at least 85% of the theoretically feasible amount of urea is prepared, upon which the urea synthesis solution is processed into a urea solution or solid urea. After the stripping operation the stripped urea synthesis solution is expanded to a low pressure and concentrated through evaporation and the urea melt thus obtained is entirely or partly transferred to the melamine plant. Melamine synthesis In the preparation of melamine urea is preferably used as raw material, preferably in the form of a melt. Ammonia and carbon dioxide are by-products formed during the preparation of melamine, which proceeds according to the following reaction equation: 6 CO(NH2)2 -» CaNgHfi + 6 NH3 + 3 CO2 The preparation can for the purpose of this process be carried out at a pressure above 12.5 MPa and generally below 80 MPa, preferably below 40 MPa and more preferably below 20 MPa, without a catalyst being present. The temperature of the reaction varies between 300 and SOCC and is preferably between 350 and 425°C. A plant for the preparation of melamine suitable for the present invention may for example consist of a urea scrubber, a reactor, whether or not combined with a gas-liquid separator or with a separate gas-liguid separator, optionally an after-reactor or ageing tank arranged downstream thereof and a product cooler/product working-up section. In one embodiment of the process, melamine is prepared from urea in a plant consisting of for example a urea scrubber, a reactor for melamine preparation, optionally an after-reactor or ageing tank and a product cooler. In this embodiment urea melt from a urea plant is fed to a urea scrubber at a pressure above 12.5 MPa and generally below 80 MPa, preferably below 40 HPa and more preferably below 20 MPa, and at a temperature above the melting point of urea. This urea scrubber may be provided with a jacket to ensure extra cooling in the scrubber. The urea scrubber may also be provided with internal cooling means. In the urea scrubber the liquid urea comes into contact with reaction gases from the melamine reactor or from a separate separator arranged downstream of the reactor. The reaction gases consist predominantly of carbon dioxide and ammonia and in addition contain an amount of melamine vapour. The molten urea scrubs the off-gas and carries along this melamine back to the reactor. The off-gases consisting predominantly of ammonia and carbon dioxide are removed from the top of the urea scrubber and returned to the high-pressure section of a urea plant, in which urea is prepared via the stripping process, to be used there as a raw material for urea production. The pressure of this gas stream is virtually equal to the pressure in the melamine reactor and lies above 12.5 MPa. Preferably, the pressure is 0-10 MPa, more preferably 0-3 MPa, and still more in particular 0-2 MPa higher than in the urea reactor. The temperature of this gas stream is preferably between 175 and 235°C. The preheated urea is withdrawn from the urea scrubber and is fed, together with the scrubbed-out melamine, via for example a high pressure pump, to the reactor, which has a pressure above 12.5 MPa and generally below 80 MPa, preferably below 40 MPa and more preferably below 20 MPa. The urea melt may also be transferred to the melamine reactor with the aid of gravity by placing the urea scrubber above the reactor. In the reactor the molten urea is heated to a temperature of 300 to 500"'C, preferably about 350 to 425°Cr at a pressure of above 12.5 MPa and generally below 80 MPa, preferably below 40 MPa and more preferably below 20 HPa, under which conditions the urea is converted into melamine, carbon dioxide and ammonia. An amount of ammonia can be dosed to the reactor. The ammonia supplied may for example serve as a purifying agent to prevent clogging of the reactor bottom or to avoid the formation of melamine condensation products such as melam, melem and melon or to promote mixing in the reactor. The amount of ammonia supplied to the reactor is 0-10 mol per mol urea, preferably 0-5 mol ammonia is used and in particular 0-2 mol ammonia per mol urea. The carbon dioxide and ammonia formed during the reaction and the extra ammonia supplied collect in the separation section, for example in the top of the reactor, although a separate separator placed downstream of the reactor is also possible, and are separated from the liquid melamine in gaseous form. The gas mixture formed is led to the urea scrubber for removing melamine vapour and for preheating the urea melt. The liquid melamine is withdrawn from the reactor and in this embodiment for example transferred to an after-reactor, although direct transfer to the product cooler is also possible. When an after-reactor or an ageing tank is used the liquid melamine is once again contacted with 0.01-10 mol ammonia per mole of melamine and preferably 0.1-2 mol ammonia per mole of melamine. The contact time in the after-reactor or in the ageing vessel is between 1 minutes and 10 hours preferably between 1 minute and 3 hours. The temperature and the pressure in the after-reactor or in the ageing vessel are virtually the same as in the reactor where urea is converted into melamine or it may be lower. The liquid melamine present in the after-reactor or ageing vessel is discharged from the after-reactor or ageing vessel and transferred to a product cooler. In the product cooler the liquid melamine is cooled by being contacted with a cooling medium as described in for example US 4565867 or 5514796. The cooling medium is preferably ammonia, in particular liquid ammonia. Alternatively the pressure and temperature may be such, that the evaporation of ammonia dissolved in the melted melamine is used to cool the melamine as described in WO-A-97/20826. The melamine Is converted into a powder in the process and is discharged from the cooling unit via the bottom of the product cooler. In yet another embodiment of the process an evaporation step is included between the reactor or optionally the after-reactor and the product cooler. In this evaporation step liquid melamine is converted into gaseous melamine, with the by-products, such as melam, remaining behind in the evaporator. The advantage of this is that the quantity of by-products in the melamine is reduced. In this way melamine of very high purity is obtained. It is also possible to supply extra ammonia during evaporation. According to this process gaseous melamine is then cooled afterwards in the product cooler with ammonia. An embodiment with a pre-strlpper will be elucidated in more detail with reference to Fig. 1. In Fig. 1, A represents a urea reactor in which urea is prepared from ammonia and carbon dioxide. The urea synthesis solution consisting of urea, ammonium carbamate, water and ammonia is supplied to pre-stripper B via line 1. A gas stream consisting predominantly of ammonia and carbon dioxide comes from the high-pressure melamine plant via line 2 and partly strips the urea synthesis solution in B. Via line 3 the urea synthesis solution is transferred to the stripper C where the urea synthesis solution is stripped with the stripping medium supplied via line 5. In this operation the urea synthesis solution is separated into a gas stream and a urea solution. This urea solution is discharged via line 6 for further processing. The gas stream coming from the stripper via line 7, consisting predominantly of ammonia and carbon dioxide, is combined with the gas sfream coming from the pre-stripper via line 4, which also consists predominantly of ammonia and carbon dioxide, and jointly fed to the carbamate condenser D. The liquid ammonium carabamate solution coming from the carbamate condeser is transferred to the urea reactor A via line 8. Accordingly the present invention provides a process for the preparation of urea, starting from carbon dioxide and ammonia, in which process a gas stream, released from a high-pressure process for making melamine operating at a pressure between 12.5 MPa and 80 MPa, is fed to said process for the preparation of urea, whereby said gas stream is used for the synthesis of urea, characterized in that the process for the preparation of urea is a stripping process, done m a urea sfripping plant having at least one high-pressure section, and in that said gas sfream is fed to at least one high-pressure section of said urea stripping plant, whereby said gas stream has a temperature between 160'C and 285 C and consists essentially of ammonia and carbon dioxide. The invention will be explained in detail with reference to the following examples. Example 1 A gas consisting predominantly of ammonia and carbon dioxide with an N/C ration of 2.7 at a temperature of 200 and a pressure of 15 MPa emerges from a high-pressure melamine syntiiesis with a capacity of 5 tonnes of melamine per hour from the top of the melamine scrubber. This sfream is fed directly to the carbamate condenser of a 1200-tonne/day urea sfripping plant in which the pressure is 14 MPa, as a result of which 7.6 tonnes less high-pressure steam (of 2.7 MPa) per hour need to be imported and 1.3 tonnes less low-pressure steam (of 0.4 MPa) are exported from the urea plant compared with the working-up of the carbamate from the tie-in stage of a 0.7 MPa low-pressure melamine plant. In the tie-in stage the carbamate stream coming from the melamine plant is concentrated to make it suitable for use in the urea stripping plant. In addition, 20 tomies of high-presssure steam (of 2.7 MPa) per hour are saved in the tie-in stage, but an extra 5.5 tonnes of high-pressure steam (of w.7 MPa) are needed in the evaporation section of the urea plant. The overall saving achieved Is 5.5 tonnes of high-pressure steam (of 2.7 MPa) per tonne of melamine, while the export of low-pressure steam (of 0.4 MPa) is reduced by 1.4 tonnes per tomie of melamine. Example 2 A gas consisting predominantly of ammonia and carbon dioxide with an N/C ratio of 2.7 at a temperature of 200° and a pressure of 15 MPa emerges from a high-pressure melamine synthesis with a capacity of 5 tonnes of melamine per hour from the top of the melamine scrubber. This stream is directly fed to an adiabatically operated pre-stripper present between the urea reactor and the COj stripper, as a result of which 2.2 tonnes less high-pressure steam (of 2.7 MPa) are needed in the stripper and 2.4 tonnes less low-pressure steam (of 0.4 MPa) are generated in the carbamate condenser compared with the situation in Example 1. WE CLAIM 1. A process for preparation of urea, starting from carbon dioxide and ammonia in which process a gas stream, released from a high-pressure process for making melamine operating at a pressure between 12.5 MPa and 80 MPa, is fed to said process for the preparation of urea, whereby said gas stream is used for the synthesis of urea, characterized in that the process for the preparation of urea is a stripping process, done in a urea stripping plant having at least one high-pressure section, and in that said gas stream is fed to at least one high-pressure section of said urea stripping plant, whereby said gas stream has a temperature between 160'C and 285'C and consists essentially of ammonia and carbon dioxide. 2. The process as claimed in Clang 1, wherein the gas stream coming from a high-pressure melamine process is fed to a urea reactor, to a stripper, to a carbamate condenser, to a pre-stripper additionally placed between the reactor and the stripper, to a flash vessel additionally installed between the stripper and the carbamate condenser or to pipelines between these. 3. The process as claimed in Claim 2, wherein the gas stream coming Trom a high-pressure melamine process is fed to an adiabatically operated pre-stripper additionally placed between a urea reactor and a stripper. 4. The process as claimed in Claim 2, wherein the gas stream coming from a high-pressure melamine process is fed to an adiabatically operated flash vessel installed between the stripper and the carbamate condenser. 5. The process as claimed in Claims 1 to 4, wherein the synthesis reactor in the urea plant is operated at a temperature of 160 to 220*'C. 6. The process as claimed in Claims 1 to 5, wherein the synthesis reactor in the urea plant is operated at a pressure of 12.5-17.5 MPa. 7. The process as claimed in Claims 1 to 6, wherein the gas stream released from the melamine process has a temperature of between 175 and 235°C. 8. The process as claimed in Claims 1 to 7, wherein the gas stream released from the melamine process has a pressure above 12.5 MPa. 9. The process as claimed in Claim 8, wherein the gas stream released from the melamine process has a pressure which is 0 to 10 MPa higher than the pressure in the urea reactor. 10. The process as claimed in Claim 8, wherein the gas stream released from die melamine process has a pressure which is 0 to 2 MPa higher than the pressure in the urea reactor. 11. A process for the preparation of urea, substantially as herein described and exemplified.

Documents:

1900-mas-1997 abstract duplicate.pdf

1900-mas-1997 assignment.pdf

1900-mas-1997 claims duplicate.pdf

1900-mas-1997 description (complete) duplicate.pdf

1900-mas-1997 form-19.pdf

1900-mas-1997 petition.pdf

1990-mas-1997 abstract.pdf

1990-mas-1997 claims.pdf

1990-mas-1997 correspondnece-others.pdf

1990-mas-1997 correspondnece-po.pdf

1990-mas-1997 description (complete).pdf

1990-mas-1997 drawings.pdf

1990-mas-1997 form-2.pdf

1990-mas-1997 form-26.pdf

1990-mas-1997 form-4.pdf

1990-mas-1997 form-6.pdf