MXPA99002019A - Process for the preparation of urea - Google Patents

Abstract

In the present process for the preparation of urea, an off-gas stream released during the synthesis of melamine in a high-pressure melamine process which consists predominantly of ammonia and carbon dioxide, is introduced into at least one high-pressure section of a urea stripping plant and is used in the synthesis of urea. The off-gas stream can be used directly without any further treatment.

Description

PROCEDURE FOR THE PREPARATION OF UREA BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a process for the preparation of. urea in which the gas stream released from the melamine process, which consists predominantly of ammonium and carbon dioxide, is recovered and, without any other treatment, is used in the urea synthesis of the urea process. Most particularly, the recovered gas stream is fed to a high pressure section of the urea plant. 2. DESCRIPTION OF THE RELATED TECHNIQUE The urea can be prepared by the introduction of ammonium and carbon dioxide to a synthesis zone at a suitable pressure, for example, 12.5-35 MPa, and at a suitable temperature, for example 160-250 ° C. The ammonium carbamate is formed first according to the following reaction: 2 NH3 + CO2? H2N-CO-ONH4 Subsequently, urea is formed by dehydrating the ammonium carbamate according to the following equilibrium reaction: H2N-CO-ONH4 < r ~ H2N-CO-NH2 + H2O The degree to which the conversion is subsequently carried out depends on the temperature of the ammonium excess applied, among other factors. The solution obtained as the predominant reaction product consists of urea, water, ammonium carbamate and unbound ammonium. The carbamate * 5 ammonium and ammonia need to be removed from the solution. Once removed, they are typically returned to the synthesis zone. The synthesis zone may include separate zones for the formation of ammonium carbamate and urea. However, said zones can also be combined in a piece of equipment. 10 Urea can be prepared in a conventional urea plant.

A plant of. Conventional high pressure urea is one in which the decomposition of the ammonium carbamate that has not been converted to a urea and the ejection of the normal excess ammonia are carried out at a pressure between 1.5 and 10 MPa, which is essentially lower than the pressure in the reactor of urea synthesis. The synthesis reactor is conventionally operated at a temperature of about 180 ° C to about 210 ° C and at a pressure of about MPa.a_aptoximaclar. 30. Mpa .. Ammonia-and. dioxide-carbon are supplied directly to the urea reactor. The molar ratio of s NH3 / CO2. (molar ratio N / C) in the synthesis of urea is generally between about 3 and about 5 in the conventional high pressure urea processes. The reagents discovered are recycled to the urea synthesis reactor, after expansion, dissociation and condensation. A variant of a conventional process for the preparation of urea is described in GB-A-1309275. At In the process described, the evolved gas, also commonly known as gas loss, obtained in the preparation of melamine in a high pressure melamine process is used for the synthesis of urea.

The gas released from melamine consists predominantly of ammonia and carbon dioxide. The release gas stream from the gas / liquid separator of the melamine plant is transferred by a scrubber to only a low pressure section, ie, a first low pressure urea synthesis section. In said low pressure section, a urea solution is prepared in an additional reactor using the ammonia and carbon dioxide originating in the melamine plant. Said urea solution is comorbid and subsequently transferred to a high pressure section in the same urea plant. The process of GB-A-1309275 has many drawbacks. An additional reactor is required because the pressure of the released gas stream supplied from the melamine plant is very low, even though it originates from a high pressure melamine process. Likewise, one or more additional pumps are required to transport the urea produced in the first low pressure synthesis section to the high pressure urea synthesis section (s). Despite these and other efforts to effectively integrate urea and melamine production facilities, there is still a need for an industrially intensive and less capital intensive process for the recovery and use of detached gases and waste composed of ammonia and carbon dioxide from a high pressure melamine plant directly in a high pressure urea plant.

BRIEF DESCRIPTION AND OBJECTS OF THE INVENTION The present invention offers an attractive solution for many and recognized industry needs by effectively utilizing the ? 5 release gas stream from a high pressure melamine process directly into a high pressure section of a urea separation plant. The release gas stream of the high pressure melamine process consists predominantly of ammonia and carbon dioxide. Predominantly means that more than 90% by weight of the release gas stream consists of ammonia and carbon dioxide, preferably more than 95% by weight. In addition, the release gas stream may contain, for example, small amounts of melamine, urea, isocyanic acid and / or hydrogen. The molar ratio of NH3 / CO2 in the stripping gas stream is about 2 or greater, preferably between 2.2 and about 4. A high pressure section of the urea recovery plant can, for example, be a urea reactor, a separator, a carbamate s condenser, an additional pre-separator placed between the urea reactor and the separator, an instantaneous vaporization vessel additionally installed between the separator and the carbamate condenser, or the conduits between any of said equipment. An object of the present invention relates to the improvement of the efficiency of high pressure urea plants. Said objective can be achieved by using a virtually water-free release gas stream consisting predominantly of ammonia and ammonia dioxide obtained from a high pressure melamine plant in a high pressure section of a urea recovery plant. The above results in an efficiency compared to. the current supply of carbamate containing water from the melamine plant to a urea plant. * 5 Another related object is to avoid a requirement to subject the stripping gas stream from a melamine plant to absorb or concentrate the steps before entering the urea plant. The foregoing is achieved in the present invention because the release gas stream can be found virtually free of water and has a pressure of high enough. Another object is to obtain energy efficiencies increased in the production of urea. The above can be achieved with the present invention because the additional heat released in the condensation of the release gas stream from the high pressure melamine plant can be recirculate and use to produce additional steam (low pressure).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart of urea and synthesis of melamine 20 with recycle of release gas from the high pressure melamine plant to the carbamate condenser of the urea plant according to the present invention; Figure 2 is a flow chart of urea and synthesis of melamine with recycling of release gas from the high melamine plant Pressure to a flash chromatography vessel installed between the separator and the carbamate condenser of the urea plant according to the present invention; Figure 3 is a flow diagram of urea and synthesis of melamine with recycle of release gas stream from the high pressure melamine plant to the separator of the urea plant according to the present invention. Figure 4 is a flow diagram of urea and melamine synthesis with recycling of release gas from the high pressure melamine plant to a pre-separator installed between the urea reactor and the urea plant separator according to the present invention; and Figure 5 is a flow diagram of urea and synthesis of melamine with recycle of release gas from the high pressure melamine plant directly in a high pressure line of the urea plant. Figure 6 shows in more detail the delivery of gas stream to a high pressure pre-separator in a urea plant according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT ILLUSTRATIVE MODALITIES OF THE INVENTION The present invention relates to the preparation of urea in the urea recovery plant having at least one high pressure section in which the release gas stream released during the melamine high pressure synthesis is supplied in at least one high pressure section of the urea separation plant, where the release gas flow consists essentially of ammonia and carbon oxide.

In the simplest and preferred embodiments of the present invention the release gas stream is supplied to a carbamate condenser in a high pressure section of the urea separation plant or to the line between the separator and the carbamate condenser. The pressure of said stripping gas stream supplied from the high pressure melamine plant is generally above about 12.5 MPa. In general, the pressure is below about 80 MPa, preferably below about MPa and most preferably about 20 MPa. In particular, the pressure of the stripping gas stream coming from the high pressure melamine plant is from about 0 to about 10 MPa, particularly 0-3 MPa and very specifically about 0-2 MPa greater than the pressure at the urea reactor. The temperature of said stripping gas stream is generally above 160 ° C, and preferably above 175 ° C. The temperature of said stripping gas stream is generally below 285 ° C, preferably below 275 ° C and most preferably below 235 ° C. As contemplated herein, a urea separation plant generally refers to a urea plant in which the decomposition V of the ammonium carbamate that has not been converted to urea and the carbon dioxide ejection and the normal excess of ammonia are conducted at a pressure which is substantially equal to the pressure in the synthesis reactor. Said decomposition / expulsion takes place in a separator, with or without the addition of a separating means. In a separation process, carbon dioxide, ammonia or both can be used as separation gas before said components are introduced into the synthesis reactor. Said separation is carried out in a separator that can be installed downstream of the reactor. The solution that arises from the urea reactor contains urea, * 5 ammonium carbamate, water and also ammonia and carbon dioxide. The solution can be purified by applying additional heat. The solution is also ? can be purified by using thermal purification techniques in which the sodium carbamate decomposes and the ammonia and ammonium carbamate present are removed from the urea solution only by the addition of heat. Ammonia and carbon dioxide containing streams from the separator are returned to the reactor by a carbamate condenser. The reactor, the separator and the carbamate condenser are among the most important components of the high pressure section of urea synthesis. In a urea separation plant, the synthesis reactor is preferably operated at a temperature at a temperature of about 160 to about 220 ° C and at a pressure of about 12.5 to about MPa. The ratio of N / C in the synthesis in a separation plant is typically between about 2.5 and about 4.

The present invention can be applied in this manner to widely used methods for the preparation of urea by the urea separation processes, such as those described in European chemical News, Urea Supplement, January 17, 1969, pages 17-20 , the complete description of which is incorporated herein by reference. In this procedure, The urea synthesis solution is formed in the synthesis zone at a high temperature and pressure, and, while the heat is added, it undergoes a separation treatment in the synthesis pressure when it is brought into countercurrent contact with the dioxide of gaseous carbon. In said purification operation, most of the ammonium carbamate present in the solution is decomposed into ammonia and carbon dioxide. Said decomposition products are then expelled from the solution in gaseous form and discharged with a small amount of water vapor and the carbon dioxide used for the separation. Said separation treatment can be carried out using carbon dioxide (gas) as separation medium as described for example in the patent of US Pat. No. 3,356,723, the complete description of which is incorporated by reference. The separation can also be carried out using the thermal separation technique, or it can be controlled using gaseous ammonia as the separation gas. In addition, the separation can be carried out using a combination of the aforementioned separation techniques. The gas mixture obtained from the separation treatment is for more than 95% condensed and absorbed in a carbamate condenser. The ammonium carbamate formed in this way is transferred to the synthesis zone for the formation of urea. The gas mixture that is not condensed and absorbed can comprise, for example, inert gases. The synthesis of urea can be carried out in one or two reactors. For example, pure ammonia and carbon dioxide can be used in a first reactor. A mixture of pure ammonia and carbon dioxide plus recycled ammonia and carbon dioxide can be used in a second reactor. Preferably, the synthesis is carried out in a reactor. Similarly, the separation of the urea synthesis solution with the aid of a gaseous separation medium can be carried out in more than one separator.

The carbamate condenser can be, for example, the so-called submerged condenser, such as that described in NL-A-8400839, the complete description of which is incorporated herein by reference. In this case, the gas mixture to be condensed is introduced into the space * 5 side of the shell of a heat exchanger of .coraza and tube, which also introduces a dilution carbamate solution. The heat of dissolution and condensation released is discharged with the aid of a heat absorbing fluid medium flowing through the tubes. For example, a suitable fluid medium is water in which case it can be converted into a steam gives low pressure for later use in the process or plant. He • Submerged condenser can be installed horizontally or vertically. However, it is particularly convenient to carry out the condensation in a horizontally placed submerged condenser because the residence time of the liquid in the condenser is generally longer compared with a vertically placed submerged condenser. The above results in the formation of ucea, which rises to the boiling point, so that the difference in temperature between the carbamate solution increases • Containing yrea and the cooling medium. As a result, a better heat transfer is achieved. A capacitor called a capacitor type picina ea.a illustrative submerged condenser, and one is described, for example, in Nitrogen No. 222, July-August 1996, pp. 29-31, whose full description is incorporated as a reference. If desired, the condensation zone and the synthesis zone can be combined in an apparatus as described for example in NL-A-25 1000416, the complete description of which is incorporated by reference. In the latter case, the formation of the ammonium carbamate and the urea of carbon dioxide and ammonia can be carried out at a pressure of about 12.5 to about 35 MPa in a urea reactor. The urea reactor may have a horizontally placed condensation zone and a heat exchanger. An illustrative urea reactor is the so-called picina type reactor * 5 is described, for example, in Nitrogen No. 222, July-August 1996, pp. 29-31. The ammonia and carbon dioxide are supplied to the urea reactor and are condensed and absorbed to a large extent in the urea synthesis solution. A substantial part of the heat released by the exothermic condensation is recovered using a heat exchanger by means of which steam is produced. The residence time of the solution of the urea synthesis in the urea reactor is selected so that at least 85% of the theoretically feasible amount of urea is obtained. In general, the urea synthesis solution is then processed in a solution of urea or solid urea. After the separation operation, the purified urea synthesis solution is expanded in several steps at a low pressure and concentrated by evaporation and the urea melt obtained in this way can be partially or totally transferred to a melamine plant "related" for the synthesis of melamine. Said operation of urea and melamine can be _ > characterized as an integrated operation. 20 Urea, urea plants and procedures, in general, are described in Messen and others; Urea, Ullmann's Encyclopedia of Industrial Chemistry, Volume A27, pages 333-365 (1996), including references cited therein, the complete descriptions of which are incorporated herein by reference. 25 Urea is the preferred raw material for the preparation of melamine. The urea is preferably used in the form of a melt. Ammonia and carbon dioxide are byproducts formed during the preparation of melamine, which proceeds according to the following equation reaction: 6 CO (NH2) 2? C3N6H6 + 6 NH3 + 3 CO2 The melamine preparation can be carried out at a pressure above 12.4 MPa and generally below 80 MPa, preferably below 40 MPa and most preferably below 20 MPa, without the catalyst being present. The temperature of the reaction may vary, and in general may be between about 300 ° C and about 500 ° C, but preferably between about 350 ° C and about 425 ° C. A plant for the preparation of melamine suitable for the practice of the present invention may include, for example, a urea scavenger, a reactor, whether or not combined with a gas-liquid separator or with a separate gas liquid separator, optionally a post-reactor or aging tank disposed downstream thereof and a product cooler / product manufacturing section. Melamine, melamine synthesis and melamine plants are generally described in Crews and others; Melamine and Guanamines, Ullman's Encyclopedia of Industrial Chemistry, Volume A16, pages 176 to 185 (1990), including references, the descriptions of which are incorporated herein by reference. In one embodiment of the present invention, the melamine is prepared from the urea in a composite plant, for example, from a urea scavenger K, a reactor L for the preparation of melamine, a gas / liquid separator M and a product cooler P. Optionally a post-reactor or aging tank is installed between M and P. In this mode, the synthesis of residual urea obtained from the high-pressure section of a urea separation plant is discharged through the line 5 and it expands through the expansion valve D, resulting in the decomposition of the residual ammonium carbamate and the formation of a gas-liquid mixture. Said mixture is then introduced into the heater E in which another decomposition of the carbamate takes place. From heater E, the mixture is introduced through line 6 to the gas-liquid separator F. The separated gas phase in separator F, consisting mainly of ammonia and carbon dioxide, is recycled to the carbamate C condenser. In an additional urea plant there may be more than one E-collector and the gas-liquid F separator. The urea product stream is discharged through the bottom of the separator F through the line 8 and further expanded through a second expansion valve G before entering the evaporator (not shown) and subsequently to the urea tank H merged The fused urea can be removed through. line 19 o, for the. Synthesis of melamine ^ -is pumped through line 9 by pump I through a heater J to the urea debugger K. The urea melt is introduced to the urea K scrubber at a pressure above 12.5 MPa generally below about 80 MPa, preferably below about 40 MPa and most preferably below 20 MPa, and at a temperature of above the melting point of urea. Although not shown in detail, the urea K-scrubber may be provided with a cooling jacket to ensure additional cooling. The urea scrubber K can also be provided with internal cooling media. In the urea K scrubber, the liquid urea is brought into contact with the reaction gases of the gas-liquid separator M disposed downstream of the reactor. The reaction gases consist of carbon dioxide and ammonia, and in addition, they generally contain an amount of melamine vapor. The fused urea purifies the evolved gas and the melamine is returned to reactor L. The release gases obtained from the scrubber consist predominantly of ammonia and carbon dioxide. The release gases are removed from the superatior part of the K urea scrubber and returned to the high pressure section of the urea plant, in which the urea is prepared by the separation process, as a raw material in the production of urea. urea. The pressure of the stripping gas stream in general is virtually equal to the pressure in the melamine reactor L and is generally above about 12.5 MPa. The pressure is generally 0-10 MPa higher, preferably 0-3 MPa higher, and most preferably 0-2 MPa higher than in the urea reactor. The temperature of said gas stream is preferably between about 175 ° C and 235 ° C. The preheated urea is removed from the urea scavenger K and introduced, together with the melamine scavenger, for example, by a high pressure pump (not shown in detail), to the reactor L, which has a pressure above 12.5. MPa and generally below 80 MPa, preferably below 40 MPa and most preferably below 20 MPa. The molten urea material can also be transferred to the melamine reactor through line 10 with the help of gravity by placing the urea K-scrubber above the reactor L, as shown in the figures.

The molten urea is subjected to heat and the pressure conditions in the melamine reactor L to convert the urea into melamine, carbon dioxide and ammonia. In general, the temperature is on a scale of about 300 ° C to 500 ° C, and preferably about 350 ° C to 425 ° C. The pressure is above 12.5 MPa but generally below about 80 MPa, preferably below about 40 MPa and most preferably below about 20 MPa. The ammonia can be metered, for example, by supplying reactor L through line 17. The ammonia supplied to the melamine reactor, for example, functions as a purification agent to prevent clogging of the lower part of the reactor or to prevent the formation of melamine condensation products such as melam, melem and melon, due to the shape and location of its introduction, to supply the mixture to reactor L. The amount of ammonia supplied to the reactor is from about 0 to about 10 moles per mole of urea, preferably 0-5 moles of ammonia, although in particular about 0.1 to about 2 moles of ammonia can be used per mole of urea. The separation section, for example, can be a section in the upper part of the melamine reactor, or, for example, a separator M installed downstream of the reactor as shown in the figures. Carbon dioxide and ammonia are separated from the liquid melamine in the form of a gas mixture. Said gas mixture is introduced into the urea scavenger K to remove the entrained melamine vapor and to preheat the molten urea material. The liquid melamine is removed from the melamine reactor L and, for example, can be transferred via line 11 to the gas-liquid separator M and then to a product enfirater P.

In the gas-liquid separator M the liquid malamina can once again be contacted with about 0.01 to about 10 moles of ammonia per mole of melamine and preferably about 0.1 to about 2 moles of ammonia per mole of melamine, introduced. for example through line 18. The residence time in the gas-liquid separator M is generally between 1 minute and 10 hours, but preferably between 1 minute and 3 hours. The pressure in the gas-liquid separator M is, in genes, virtually the same as in the reactor where the urea is converted to melamine or may be lower. The temperature may be higher or lower than the temperature of the reactor, and preferably it will be between 200-500 ° C, and in particular between 330-440 ° C. The liquid melamine present in the gas-liquid separator M is discharged from the gas-liquid separator M and is transferred through line 14, through the expansion valve N to the product cooler P. The liquid melamine in the product cooler P is cooled upon contact with a cooling medium as described, for example, in U.S. Patent No. 5,514,796, the entire disclosure of which is incorporated herein by reference. The cooling medium is preferably ammonia, such as the liquid ammonia which is introduced, for example, through the line 15. Alternatively, the pressure and temperature can be selected in such a way that the evaporation of the ammonia dissolved in the melamine fused it is used to cool the melamine as described in WO-A-97/20826, the entire disclosure of which is incorporated herein by reference. The melamine is converted to a powder in the process and discharged from the cooling unit through line 16 into the lower part of the product cooler P.

When a post-reactor or an aging tank is used, the liquid sheet is contacted once again with about 0.01 to about 10 moles of ammonia per mole of melamine and preferably about 0.1 to about 2 moles of ammonia per - 5 mol of melamine. The residence time in the post-reactor or in the storage vessel is generally between 1 minute and 3 hours. The temperatures and the pressure in the post-reactor or in the aging vessel are, in general, within the same scale as described for the gas / liquid separator. It is preferred to use a relatively low temperature. An evaporation step can be provided by installing an evaporator between the gas-liquid separator and the product cooler. The melamine is converted into the melamine gas evaporation step through the byproducts, such as melam, which remains behind the evaporator. Therefore, the amount of impurities in the product derived in melamine. Accordingly, melamine having a very high purity is obtained. In addition, if desired, the ammonia can be supplied during the evaporation step. The gaseous melamine is then cooled in the product cooler with the selected cooling medium, such as ammonia or the like. The release gas from the melamine plant can be introduced into a high pressure section of the urea separation plant. The section receiving the gas mixture may, for example, be in a location which is located in a high pressure section from the separator and including the same urea reactor. In this way, the gas flow of The release of the high-pressure melamine process can be introduced, for example, into a urea reactor, into a prevapolator, into a? 8 carbamate condenser, to a prevapolator additionally placed between the vaporizer and the carbamate condenser, or the pipes between them. Figure 1 illustrates a first embodiment in which the stripping gas stream coming from the high melamine process The pressure is introduced via line 13 to the carbamate C condenser of the high pressure urea membrane. In said embodiment, as in the other embodiments of the present invention, the line 13 may include one or more control values (not shown in particular). According to said method, the step of absorption and / or concentration of the stream of the release gas coming from the melamine plant is not necessary because the gas stream is virtually free of water and has a pressure of high enough Example 1 below provides a more detailed explanation of said embodiment of the invention. As will be appreciated, an advantage of said modality and the modalities of figures 3 and 5, described below, when compared with the embodiments of FIGS. 2 and 4/6, is that the advantages of introducing the release gas stream from the melamine plant directly to a high-pressure section of the urea plant can be carried out without the potentially high cost of the additional installation of the containers or other components in the floor. Figure 2 illustrates another embodiment in which the release gas stream coming from a high-pressure melamine process is introduced through line 213 to an instantaneous vaporization vessel Q additionally installed between separator B and the condenser of carbamate C. The above is an advantage if the pressure in the melamine process is substantially higher than the pressure in the urea process.

Figure 3 illustrates another embodiment in which the stripping gas stream coming from the high pressure melamine process is introduced through line 313 to the main separator B in a high pressure urea plant. The advantage is that the release gas stream is used as a separation gas with additional heat separation. The flow through the urea and melamine plants in said exemplary embodiment is described in another way above with reference to Figure 1. In another embodiment of the process, the release gas stream from the melamine process is introduced into the separator additionally installed between the urea reactor A and the main separator B as shown in figures 4 and 6. In this embodiment, the urea synthesis solution is separates in the pre-separator R with the help of the stripping gas stream supplied from the high pressure melamine process through line 413. Again, the stripping gas stream consists predominantly of ammonia and carbon dioxide . The above results in a saving of extra high pressure steam and in an improved separation effect. In addition, it was discovered that the additional production of apor is obtained in the carbamate C condenser. Pre-separator R is preferably an adiabatically operable pre-separator. The latter is an advantage, particularly if the melamine plant and the urea plant are highly related. Plants that are highly related means that the relatively large amount of urea produced which is used by the melamine plant is, for example, more than 50% than the urea produced in the urea separation plant which is used for the production of melamine, very particularly more than 80%. It will be appreciated that the "related" melamine and urea plants of the present invention are not restricted to said embodiment. The delivery of the release gas stream to the high pressure pre-separator R in the urea plant is described in greater detail in Figure 6. As in Figure 4, A represents a urea reactor in which the Urea is prepared from ammonia and carbon dioxide. A urea synthesis solution consisting of urea, ammonium carbamate, water and ammonia is supplied to pre-separator R through line 2. A release gas stream consisting predominantly of ammonia and carbon dioxide, comes from the high pressure melamine plant through line 413 and partially recover the urea synthesis solution in B. The urea synthesis solution is transferred to the main B separator via line 2 'where the synthesis solution of urea is separated with the sepsiracjon medium supplied via line 20. In this operation, the solution of Isi urea synthesis is separated into a stream of urea and a solution of urea. Said urea solution is discharged through line 5 for further processing. The gas stream coming from the separator through line 3, consisting predominantly of ammonia and carbon dioxide, is combined with the gas stream coming from the pre-separator R through line 3'_ which also consists of predominantly ammonia and carbon dioxide, and together they are introduced to the carbamate C condenser. The liquid sodium carbamate solution coming from the carbamate condenser is transferred to the urea reactor A through line 4. Figure 5 illustrates another embodiment in which the release gas stream coming from the pressure melamine process High is introduced through line 513 directly to the high pressure line in a high pressure urea plant as illustrated; line 3 between B and C is preferably a high pressure line in this respect. The flow through the balance of the urea and melamine plants in said illustrative embodiment is the same as described above with reference to Figure 1. The production of melamine and urea, including the introduction of gas consisting predominantly of carbon dioxide and ammonia from a high pressure melamine plant in a high pressure section of a urea plant, are disclosed in Dutch Patent Applications 1003923 and 1004475, issued respectively on August 30, 1996 and November 8, 1996 ^ .. whose descriptions-aa are incorporated herein by reference in their entirety.

EXAMPLES 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 ratio of 2.7 at a temperature of 200 ° C and a pressure of 15 MPa arises from a synthesis of high pressure melamine with a capacity of 5 melamine tons per hour from the top of the melamine separator. Said stream is introduced directly to the carbamate condenser of a urea separation plant of 1200 tons / day in which the pressure is 14 MPa, as a result of which it is necessary to import 7.6 tons less of high pressure steam (from 2.7 MPa) per hour and less than 1.3 tons of low pressure steam (0.4 MPa) are exported from the urea plant compared to the manufacture of carbamate from a conventional MPa related stage of the low pressure melamine plant. In the related conventional stage, the carbamate stream coming from the meiamine plant is concentrated to make possible its use in the urea separation plant. In addition, 20 tons of high pressure steam (2.7 MPa) per hour is saved in the related stage of the present invention, but 5.5 extra tons of high pressure steam (2.7 MPa) are needed in the evaporation section of the Urea plant. The overall savings achieved are 5.5 tons of high pressure steam (2.7 MPa) per metric ton, while the export of low pressure steam (0.4 MPa) is reduced by 1.4 tons per metric ton.

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 synthesis of high pressure melamine with a capacity of 5 metric tons of melamine. time from the top of the melamine separator. Said stream is introduced directly to a high pressure section of a urea separation plant. In said example, the steam is introduced into an adiabatically operated pre-separator, installed between! the urea reactor and the CQ2 separator. As a result, 2.2 tons less high pressure steam (2.7 MPa) are needed in the CO2 separator and 2.4 tons less low pressure steam (0.4 MPa) are generated in the carbamate condenser compared to example 1 .

Claims (3)

1. NOVELTY OF THE INVENTION CLAIMS • 1. A process for the preparation of urea in a urea separation plant having at least one high pressure section wherein said process comprises the step of supplying a gas stream released from a high pressure process to making melamine to at least one high pressure section of said urea separation plant, 10 characterized in that said gas stream consists essentially of ammonia and carbon dioxide.
2. 2. A method according to claim 1, further characterized in that said high-pressure section to which said gas stream is supplied, comprises at least one of a reactor of Urea, a separator, a carbamate condenser, a pre-separator installed between the reactor and the separator, a flash chromatography vessel installed between the separator and the carbamate condenser, or a pipe between any of them.
3. 3. A method according to claim 2, Further characterized in that said high pressure section comprises a high pressure urea reactor and a high pressure separator, and said gas stream is supplied to an adiabatically operated pre-separator, installed between a urea reactor and said separator. . A - A method according to claim 2, Further characterized in that said high pressure section of the urea separation plant includes a carbamate condenser, a separator and a line therebetween, wherein said gas stream is supplied to said carbamate condenser or said line therebetween. . 5. A process according to one of claims 1-4, further characterized in that said urea separation plant includes a urea synthesis reactor and said urea synthesis reactor is operated at a temperature of 160 ° C to 220 ° C. ° C. 6. A process according to one of claims 1-4, further characterized in that said urea separation plant includes a urea synthesis reactor operated at a pressure of 12.5-17.5 MPa. 7. A process according to one of claims 1-4, further characterized in that said gas stream released from the melamine process has a temperature between about 175 ° C to about 235 ° C. 8. A method according to one of the claims. 1-4, further characterized in that the gas stream released from the melamine process has a pressure greater than 12.5 MPa. 9. A method according to claim 8, further characterized in that said gas stream released from the melamine process has a pressure that is from 0 to 10 MPa greater than the pressure in said high pressure section. 10. A method according to claim 8 further characterized in that said gas stream released from the melamine process has a pressure that is 0 to 2 MPa greater than the pressure in said high pressure section. 11. - A urea separation plant for the preparation of urea, the plant having at least one high pressure section comprising at least one of a urea reactor, a separator, a carbamate condenser and fluid flow lines interconnecting thereto, and a melamine release gas line for supplying a release gas stream released during the high pressure process, wherein said gas stream consists essentially of ammonia and carbon dioxide. 12. A plant according to claim 11, further characterized in that at least one high pressure section to which said gas stream is supplied comprises at least one pre-separator installed between the reactor and the separator, a container of instantaneous vaporization installed between the separator and the carbamate condenser and pipes between them. 13. A plant according to claim 12, further characterized in that said high pressure section comprises an adiabatically operated pre-separator, installed between a urea reactor and a separator. 14. An apparatus for urea and melamine synthesis containing: a urea plant that includes a high pressure section having a urea reactor, a separator, a carbamate condenser, and fluid flow lines interconnecting them; a high pressure melamine plant including a urea separator for receiving the urea melt, a melamine reactor and a product cooler for generating melamine powder, and fluid flow lines interconnecting them; and a release gas supply line for delivering the release gas from at least the melamine reactor or urea separator directly to said high pressure section at a pressure equal to or greater than the pressure prevailing in said high pressure section. . 15. An apparatus according to claim 14, further characterized in that said high pressure section to which said release gas stream is further supplied comprises at least one pre-separator installed between the reactor and the separator, a container of instantaneous vaporization between the separator and the carbamate condenser, and pipes between any of them. 16. An apparatus according to claim 15, further characterized in that said release gas supply line is operatively coupled to an adiabatically operated pre-separator, installed between the urea reactor and the separator. 2T RESET OF THE INVENTION In the present process for the preparation of urea, a stripping gas stream during the synthesis of melamine in a high pressure melamine process consisting predominantly of ammonia and carbon dioxide is introduced into at least one high pressure section of a urea separation plant and used in the synthesis of urea; The gas stream can be used directly without any other treatment. JJ / cgt * P99 / 256F