DD204250A5 - PROCESS FOR PREPARING UREA - Google Patents

Abstract

The invention relates to a process for the production of urea from ammonia and carbon dioxide at elevated temperatures and pressures. The aim of the invention is to reduce the required amount of high-pressure steam. According to the invention, the technological sequence of the process is directed such that the heat of reaction from the reaction zone is available at a high temperature level and thus can be used more effectively such that the reaction zone exchanging heat with the stripping zone is maintained at a pressure in the range from about 125 to 250 bar and that the stripping zone is maintained at a pressure which is below the pressure prevailing in the reaction zone. The pressure difference can be 20 to 120 bar, preferably 40 to 60 bar. The conversion of ammonium carbamate to urea is continued in the reaction zone until the quantity of urea formed equals at least 50%, preferably 70%, of that amount of urea which would have been achieved under the reaction conditions in the reaction zone in the presence of the equilibrium state.

Description

4 2 8 8 0 7 Berlin, January 16, 1983

_ ^ _ AP C 07 C / 242 880/7 61 318 18

Process for preparing τοη urea

Field of application of the invention

The present invention relates to a process for the production of urea from ammonia and carbon dioxide at elevated temperatures and pressures, whereby the Uetto ™ heat produced by the urea synthesis reaction can be used efficiently and economically to an increased extent.

Characteristic of the known technical solutions

One of the methods widely used in practice today for the production of urea is described in "European Chemical News Urea Supplement ^15" of 17 January 1969 on pages 17 to 20. In the process disclosed there, the urea synthesis solution is in a high pressure and high temperature held reaction zone and then subjected to a stripping treatment under synthesis pressure by heating this solution and brought in countercurrent with a carbon dioxide stripping gas, thus decomposing a major part of the ammonium carbamate contained therein · The thus formed and from ammonia and carbon dioxide together with stripping gas and a small amount of water vapor gas mixture is removed from the remaining urea »product stream and introduced into a condensation zone in which it is cooled to form an aqueous ammonium carbamate solution * This

17.FEB1383 * 0G974i

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aqueous Karbamatlösung as well as the remaining, non-condensed gas mixture is fed to the reaction zone for conversion into urea again. The condensation of this recycled to the reaction zone gas mixture provides the heat energy, which is required for the conversion of ammonium carbamate in urea, so that no heat energy from the outside the reaction zone must be supplied ·

The heat energy needed for the stripping treatment is provided by the condensation of high pressure steam on the outside of tubes in a vertical heat exchanger in which stripping occurs.

According to this publication, about 1000 kg of steam are required at a pressure of about 25 bar per tonne of urea. Practically, the consumption of high-pressure steam (25 bar) has been reduced to about 850 kg per tonne of urea, with the heat energy used for stripping treatment partly due to condensation However, this condensation takes place at a relatively low temperature level, with the result that only 3 to 5 bar of steam is produced, for which there are only relatively few uses both inside and outside the process For reasons and in particular in view of the continuously rising energy costs, it is highly desirable to limit the consumption of high-pressure steam to the greatest possible extent, whereby it depends on local conditions.

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Conditions and requirements - may equally be desirable _s the production of additional low-pressure steam to spare or at least to reduce a large extent

Several proposals have already been made to use the heat released from ammonia and carbon dioxide in the formation of carbamate in order to cover at least part of the heat demand required for the stripping treatment. "For various reasons, however, these proposals have found no practical application. Thus, for example, British Patent 1,147,734 discloses a process in which the ammonium carbamate and urea synthesis is carried out in two successive reaction zones. Fresh ammonia and at least a portion of the fresh carbon dioxide are fed into the first reaction zone and those in this reaction zone heat released by the formation of ammonium carbamate is fed via built-in heat exchangers at a pressure of the urea synthesis solution formed in the second reaction zone which corresponds to that pressure or falls below the pressure prevailing in the first reaction zone; this results in the decomposition of the unreacted ammonium carbamate 8. The decomposition products thus formed are stripped off from the remaining urea solution by means of an inert gas.

In carrying out this known method, the fresh ammonia and the fresh carbon dioxide must be at a high temperature, for example, at the synthesis temperature maintained in the first reaction zone.

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This preheating of the reactants consumes a considerable amount of high-pressure steam. However, only a small part of the excess heat formed in the urea synthesis can be used as steam A greater proportion of this excess heat is recovered at a relatively low temperature level in the desorption column operating at the pressure of the second reaction zone. In this column, the urea product stream stripped ammonia and carbon dioxide are separated from the inert stripping gas. The steam thus produced has a relatively low pressure of only about 2 bar, but there are few uses for this within the process n ·

In another process, as described in US Pat. No. 3,957,868, the urea synthesis reaction takes place in the shell side of a vertically disposed tubular heat exchanger in which the tips of the tubes open into the shell from the heat exchanger the urea synthesis solution formed in the shell-side reaction zone flows into and down the inside of the tubes, decomposing in the tubes the ammonium carbamate by means of that heat of reaction transmitted through the tube walls; the ammonia and carbon dioxide liberated in this way is stripped off the solution.

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In order to maximize the decomposition of ammonium carbamate, it is necessary to have the heat of reaction at the highest possible temperature level and. To ensure optimum heat transfer to the solution to be stripped. The measures proposed to achieve this aim include conducting the urea synthesis and stripping at a temperature of 210 to 245 ° C and a pressure of 250 to 600 atmospheres while maintaining a relatively high molar HH ^ / COg ratio in the liquid phase of the reaction zone and the return of a portion of the gas mixture discharged from the stripping zone together with a mixture of gas mixture in the bottom part of the reaction zone, said extra amount of gas mixture is kept available for circulation, that only a portion of the gaseous ammonia and carbon dioxide fed to the reaction zone is condensed to ammonium carbamate.

In this way, the mass of urea produced will be low relative to the amounts of ammonia and carbon dioxide supplied to the reaction zone »

Due to the high temperatures used in this process, the construction materials used in this process must meet high corrosion resistance requirements. "In addition, the high pressure used and the special design of the combined reactor wiper make the equipment relatively expensive to use require

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derlieh · If all these factors are taken into consideration, this known process has little or no economic advantage in its practical application compared to the widely used process described in the European Chemical News Urea Supplement.

Aim of the invention;

The object of the present invention is to provide a process for the production of urea, in which the amount of high-pressure steam required for the decomposition of unconverted ammonium carbamate is substantially reduced, while at the same time avoiding the disadvantages of the above-mentioned, already known process solutions.

Explanation of the essence of, invention

The invention has for its object to direct the technological process of the process so that the heat of reaction from the reaction zone is available at a high temperature level and thus can be used more effectively and efficiently ·

According to the invention, in a reaction zone of carbon dioxide and excess ammonia, a solution containing ammonium carbamate and unconverted ammonia is produced at elevated temperature and pressure, the ammonium carbamate being partially converted to urea and at least one

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The amount of heat required in the stripping zone is provided by heat exchange with that reaction zone in which the carbamate is formed. The stripped urea synthesis solution is further processed such that urea either According to the invention, the stripping zone is kept under a pressure which is below the pressure of the reaction zone to which the stripping zone is connected by heat exchange. The pressure in this latter reaction zone is maintained at 125 to 250 bar »

The condensation of ammonia and carbon dioxide in the reaction zone is preferably carried out in the presence of relatively large amounts of urea and water; this causes a considerable increase in the average temperature in the reaction zone. The reaction heat from the reaction zone is exchanged with a stripping zone in which this heat is used to decompose ammonium carbamate in the urea product stream at a pressure below the pressure prevailing in the reaction zone. In such operation, the heat of reaction from the reaction zone is at one high temperature level available and can thus be used more effectively and efficiently than was the case in the previous procedural solutions in the decomposition and stripping of Ammoniumkarba »from the urea» product stream ·

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As the pressure in the reaction zone increases, condensation of carbon dioxide and ammonia to ammonium carbamate takes place at higher temperatures, thus more heat will be available for decomposition of carbamate in the stripping zone. Preferably, the pressure differential between the heat exchanging reaction zone and stripping zones should be at least about 20 bar amount; this has a sufficiently high difference in the average temperature and thus a sufficient driving force between the two zones in the interest of effective heat exchange result "a" pressure difference of more than 120 bar is theoretically possible, but this complicates the design of the reactor stripper plant »Am ideally, the reaction zone should be maintained at a pressure which is 40 to 60 bar above the pressure in the stripping zone. "This provides a suitable driving force for heat exchange without the necessity of installing more complicated devices; moreover, there is the advantage over using higher pressure differentials that the gas mixture liberated in the Mederdruck stripping zone does not have to be compressed excessively high in order to be fed back to the reaction zone. "If the pressure difference is in the range of about 40 to 60 bar, the gas mixture can be cooled without the need for intermediate cooling k while the heat in the reaction zone will still be available at a sufficiently high temperature level

The amount of heat transferred from the reaction zone to the stripping zone can be increased by adding a certain amount of carbamate to urea and in the reaction zone

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Water is converted * Due to the presence of this urea and water, in which the ammonia and carbon dioxide are absorbed and in which the ammonium carbamate formed dissolves, the temperature in the reaction zone rises by an additional amount * depending on what is maintained in the reaction zone Pressure, this temperature rise can make up about 10 to 20 ⁰ C.

In order to be able to further increase and utilize the heat energy released in the reaction zone, it is desirable to ensure intensive mixing of the reaction zone contents; Then the stripping can be carried out in a known manner within the tubes of a vertically arranged heat exchanger, wherein the shell side of the exchanger serves as a reaction zone, the contents of the reaction zone is to be mixed so extensively that the temperature difference between the top and the bottom of the reaction zone to a maximum of 5 ⁰ C bleibte limited in order to maximize heat transfer, this temperature difference should preferably be kept to a maximum of about 2 ⁰ C.

According to the invention, the effluent from the reaction zone is introduced into a post-reaction zone in which an additional portion of ammonium carbamate is converted to urea to form a urea product stream comprising urea and unconverted ammonium carbamate

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wherein said urea product stream is then introduced into said stripping zone, and finally in said post-reaction zone, the conversion of ammonium carbamate into urea is in an amount which is at least 90 % of that amount of urea which is below that in said Reaction zone given reaction conditions would have been achieved in the presence of the equilibrium state *

The urea product stream from the post-reaction zone is treated prior to its introduction into the stripping zone in a heater-decomposer in which at least a portion of the ammonium carbamate is decomposed by heating, a gas mixture of ammonia and carbon dioxide and a residual urea product stream having a reduced carbamate content and wherein said gas mixture is introduced into said reaction zone and said residual urea product stream in said stripping zone.

In a further embodiment of the process of the invention, only a portion of the urea product stream from the post-reaction zone is introduced into the stripping zone, and a remaining portion of the urea product stream from the post-reaction zone is passed into a second decomposition zone in which ammonium carbonate decomposed to form an ammonia-containing and carbon dioxide-containing gas mixture, and this gas mixture is separated from the remaining urea product stream.

The decomposition zone is kept under the same pressure as the stripping zone «

The invention is based on the Pig. 1 to 4 illustrates

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Pig. Figure 1 illustrates an embodiment of the invention in which the urea is prepared in two steps, each step being carried out under the same pressure and wherein a portion of the gas mixture recovered in the stripping zone is fed to the reaction zone, which is in heat exchange with the stripping zone;

Pig 2: illustrates a similar embodiment for the production of urea in two stages, wherein the second reaction step is carried out at a lower pressure than the first reaction step;

Pig. Figure 3 illustrates an embodiment of the invention in which the stripping treatment is performed in two successive steps;

Pig. 4: illustrates an embodiment of the invention in which the stripping treatment took place in two stripping zones arranged parallel to one another

-2 ^ 111983 * 083042

O 7

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12 - ^: ,. , -:

is the one with the book "countersigned plant in the reactor" stripper, shown as a vertical tube heat exchanger

^ons: zöne in the shell side and iii 'is the tubes »Also ^ ^ B called the letter a Karbamateihen laughing reactor, D ^fl a Heiger ^ decomposers, E a ^ ^x liaöchkölGnne, and ä.en with the letters F, The equipment used is the same

Verdi-chter, an ammonia © Amraoniakpumpe _e L is a Karbamatpumpe, M is a gas compressor and S ^- is a

M-Pigö '^ i represented embodiment of the invention

rnstoff '-' SyntheBelösung which urea, ^a Wäiieff ^{nieht; i-converted} carbamate and free ammonia containing About ^°: line 31 into the reactor Abstreifsone "e # i ^{Ä Ä: i} #IngefUhrt in which they are heated and m ^6! with a stream of gaseous carbon dioxide

becomes via line 1 a »

brought to a pressure of beibar and line 2 of Ab ° - initiated streifzöSe · The carbon dioxide is air or ⁵ irg% ftdel: h other saüerstoffhaltiges inert gas mixture train ^ esetziti ^ö To DIE Materials _$ the MILP at high temperatures deni% mmoniumkarbamathaltigen solutions in contact kommehj ^£ keep in Einern passive state "The destruction of

in the stripping zone required

The heat energy is dissipated by means of heat exchange

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the reaction zone delivered; the gas mixture of ammonia and carbon dioxide formed in this way is expelled together with the introduced fresh carbon dioxide from the urea synthesis solution and discharged via line 4 from the stripping zone of the reactor stripper A. The stripped urea product stream is removed via line 3 to the stripping zone discharged and subsequently treated in order to eliminate the remaining small amount of ammonium carbamate in any of the known ways, and then further processed for the formation of concentrated urea solution or of solid urea.

The discharged via line 4 from the stripping gas mixture is brought by means of gas compressor M to an elevated pressure of, for example, 190 bar and then divided into two subsets A portion is introduced via line 10 in the lower part of the shell side reaction zone of the reactor stripper A, the other subset is supplied via line 12 to the carbamate condenser B, which is maintained under the same pressure as prevails in the Reaktionszpne. The ammonia required for the synthesis reaction is fed to the reaction zone primarily directly via line 3, pump K, ammonia heater G and line 11. However, part of the required ammonia is also passed via line 39 to the carbamate condenser B. In the carbamate condenser B, the supplied via line 12 from the stripping zone gas mixture in the presence of a line 16 supplied by the wash column E ammonium carbamate solution and in the presence of via line 39 angelie-

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Completely condensed ammonia ammonia, wherein the heat of condensation is dissipated and used to generate steam, for example 3 to 6 bar pressure * Is sufficient hydrostatic pressure to transport the Karbamatlösung of the wash column E to K carbamate condenser B due to a small difference in level between the two Parts of the system is not enough, then the ejector N, which is driven by ammonia brought in via line 39, can be used to support this transport.

The ammonium carbamate solution formed in the carbamate »condenser B is introduced via line 17 into the bottom part of the reaction zone of the reactor stripper A. In this reaction zone, a large part of the ammonia and carbon dioxide supplied via the lines 10 and 11 is converted into iimaonium carbamate , said ammonium carbamate together with that ammonium carbamate contained in the Karbamatlb'sung from line 17, is partly converted to urea and water _e in the conversion of ammonia and carbon dioxide to ammonium carbamate is a heat releasing exothermic reaction, whereas the However, the net reaction results in a heat surplus which, by means of heat exchange, is transferred from the reaction zone to the stripping zone of the reactor stripper A. Since the formation of ammonium carbamate and urea in accordance with the present invention results in the conversion of carbonate to urea and water implementation

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In the reaction zone at a pressure greater than the pressure maintained in the stripper zone, the excess heat will be at a relatively high temperature level. Accordingly, a substantial portion of the stripping zone present in the urea product line and line 31 may be present be fed decomposed Aminoniumkarbamats without this additional heat must be brought via an external heat source *

In addition, a further increase in temperature in the Reaktionsaone is achieved by the fact that the conversion of ammonium carbamate to urea and water is continued until more than half of the obtainable under the given reaction conditions equilibrium amount has been formed · This also increases the amount of transferred to the stripping zone Excess Heat "This effect increases as the reaction in the synthesis zone moves toward the equilibrium condition. For example, in a typical embodiment of the Pig 1 process, at a pressure of about 240 bar, a molar ratio of ammonia to carbon dioxide of $ 40: 1 and a molar ratio of water to carbon dioxide of about 0.46: 1 is about 50 %. the. Urea-equilibrium amount formed, so the average! Temperature in the reaction zone is about 195 ⁰ C will be "At 60, 70, 80 and 90% of the formed urea-equilibrium amount, the temperature corresponding to 196, 199 '201 or · 203 ⁰ C be ·

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The temperature increases due to higher conversion rates of ammonium carbamate to urea and water are significant with respect to the difference in temperature across the heat exchanger flanges in the reactor stripper. In general, the conditions are such that the amount of urea formed exceeds 70 % of that under equilibrium conditions However, since the reaction rate decreases greatly as it approaches equilibrium, urea formation over 90 % of the amount achievable under equilibrium conditions requires considerably longer residence times and thus unattractive large volume in the reaction zone

At a given pressure in the reaction zone, "given amount of excess ammonia, given molar ratio of HpO / CO" ₉ and given residence time, the temperature achievable in the reaction zone represents a determinable fixed value. The quantity of gas supplied via line 10 is at gas mixture then equally firm; inasmuch as this amount of gas mixture affects the residence time, whereby the amount of heat transferred can not be further increased by increasing the amount of gas mixture introduced. The unused heat is recovered from the process by recovering from the stripping zone condensed gas mixture in the carbamate condenser B and in the scrubbing column Ξ and in that steam is formed from the thus released heat or process streams can be heated *

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It is highly desirable to maintain a more intense make-up in the reaction zone so as to achieve the highest possible uniform heat distribution and maximize heat transfer to the desorption zone. "Some mixing is already achieved by mixing the gas mixture and the ammonium carbamate solution from flow down through the reaction zone "However, this mixing effect can be increased by incorporating baffles, baffles or similar components into the reaction zone, resulting in a further improvement in heat transfer."

The solution of urea, water, ammonia and unconverted carbamate discharged from the reaction zone of the reactor stripper A, together with a certain amount of gas mixture consisting of ammonia, carbon dioxide, water and inert gas, is passed via line 19 into the Hach reactor C In Üiach Reactor C, the conversion of ammonium carbamate to urea is continued until at least 90 % of the amount of urea has formed, which is below that prevailing in the Hach reactor Achieving equilibrium conditions would be _e The required for this transformation. The heat energy is provided by feeding to the Ua Ch reactor via line 40 an uncondensed part of the gaseous mixture fed to the carbamate condenser B via line 12. This gas mixture is condensed in the post-reactor C, whereby both the condensate and the condensate are condensed.

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On the other hand, this uncondensed gas mixture from the carbamate condenser B can be fed to the reaction zone of the reactor stripper Δ together with the carbamate solution formed in the carbamate condenser B. In the latter Pail, however, the heat required in the post-reactor C must be obtained in different ways, for example, by passing a larger amount of gas mixture from the reaction zone of the reactor stripper A sum ÜJach-C reactor and condensed in this «

The urea synthesis solution formed in the post-reactor G is depressurized in the pressure reducing valve 41 to the pressure prevailing in the stripping zone, for example from 190 to 140 bar, and the gas-liquid mixture thus formed is conveyed via line 20 to the gas-liquid separator S The gas mixture consisting of ammonia, carbon dioxide and water vapor is separated from the remaining urea product st rom. This gas mixture is added via line 42 to that gas mixture which has been discharged via line 4 from the stripping zone of the reactor stripper A residual urea product stream from the gas-liquid separator S is introduced via line 31 into the stripping zone of the reactor stripper A.

From the Hach reactor C, a gas mixture of inert gases, ammonia, carbon dioxide and water vapor is discharged and fed via line 21 of the scrubbing column E, which

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is maintained under the same pressure as prevailed in the reaction zone of the reactor ~ stripper A, the carbamate condenser B and the Efach ^ reactor G. In the scrubbing column E, ammonia and carbon dioxide are washed by washing with water or a dilute Karbamatlösung sued via line 23 The recovered Ammoniumkarbamat solution is fed to the carbamate = »condenser B via line 16 and, if necessary, by means of the ejector N 3, while the remaining exhaust gas mixture via line 24 from the scrubbing column E is discharged · In this embodiment no high-pressure steam is used for the decomposition of unconverted ammonium carbamate. "

In the process embodiment according to Pig 2, only the reaction zone of the reactor stripper A is maintained at a high pressure of, for example, 190 bar, whereas the stripping zone, the carbamate condenser B, the post-reactor C and the scrubbing column E are all at the same lower Thus, only that portion of the gas mixture discharged from the stripping zone has to be compressed to the higher pressure prevailing in the reaction zone, which is fed to the reaction zone via line 10. Accordingly, a smaller compressor can be used compared to the method embodiment shown in Figure 1. The pressure of the urea product solution formed in the reaction zone is reduced to the pressure prevailing in the Mederdruck part of the process resulting from stripper A , the carbamate condensate

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Sop B _{8 is composed of} the ¥ ach «reactor C and the scrubbing column E? the pump L is used ₉ to compress the carbamate solution formed in the carbamate condenser B for introduction into the reaction zone to the prevailing pressure. In this process embodiment according to the invention, too, the unconverted ammonium carbamate is decomposed without the use of high-pressure steam.

3 illustrates an embodiment of the process according to the invention in which the urea synthesis solution containing ammonium carbamate formed in the Hach reactor C is fed via line 43 to the heater decomposer D, in which part of the ammonium carbamate is decomposed and removed from the remaining urea product solution. The pressure in the heater-decomposer D is preferably maintained at about the same level as prevails in the reaction zone and in the post-reactor C, but it may also be below the pressure prevailing in the reaction zone. In this heater-decomposer D it can Both the direct current and the countercurrent heater can be direct, ie the direction of flow of the liquid stream and of the released gas flow can be synchronous or counter-rotating. Via line 44, the heater decomposer D receives a small amount of air small amount of another oxygen-containing gas mixture supplied to the corrosion of those part of the plant e, passivating and reducing the high-temperature Aimaoniumkarbamat «solution,

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In the heater Zersetser D, the urea synthesis solution is heated to a temperature of for example 180 to 220 ⁰ C using steam, wherein a cable of the existing Ammoniumkarbaiciats decomposed and consisting of ammonia, carbon dioxide, water vapor and inert gas mixture is designed * This gas mixture is discharged via line 10 and introduced into the reaction zone of reactor stripper A with the aid of ejector H. * This ejector H is driven by fresh ammonia, which is delivered via pump K through heater G and line 11. Degreaser D leaving and still containing a certain amount of ammonium carbamate urea Proauktstrom is relaxed in Druckredusierventil 41, and the thus formed gas »liquid mixture is introduced via line 20 into the separator S, in which developed by relaxation gas mixture from the urea product stream deposited and via lines 42 and 12 the carbamate condenser B is supplied «. The urea product stream from the separator S is then fed into the stripping zone of the reactor stripper A to decompose a further amount of ammonium carbamate. Since a portion of the ammonium carbamate in the heater decomposer D has already been decomposed, this decomposition in the stripping zone can now with a Thus, a portion of the amount of carbon dioxide required for the process can optionally be introduced directly into the reaction zone via line 45, rather than all of the carbon dioxide passing through the system Stripping zone is introduced via line 2 in the process.

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The gas leaving the Naeh reactor 0 gas mixture is fed via expansion valve 22 for the purpose of recovery of ammonia and carbon dioxide to the scrubbing column B. In the present embodiment, the scrubbing column E is driven at the same pressure as in the stripping zone of the reactor stripper A and in Karbamat »Condenser B is maintained«

It is also possible in accordance with the one in Pig. 4 to be treated in the stripping zone of the reactor stripper A and to allow the treatment of the remaining part to be carried out in the heater-decomposer D, preferably in the part of the urea synthesis solution formed in the post-reactor C. Heater-decomposer D maintaining the same pressure as in the stripping zone, but it is also possible to choose a lower pressure in the heater-decomposer D; In this Palle the released gas mixture can be supplied by means of an ejector to the carbamate condenser B, wherein the ejector can be driven, for example by a subset of the fresh Ammoniakeintrages In this embodiment, as in the embodiment of FIG. 3, only a part of the After-reactor C leaving ammonium carbamate decomposed and eliminated in the stripping zone of the reactor stripper A; thus, it may be that a smaller Wänneaustauschfläehe and a smaller amount of carbon dioxide stripping gas in the stripping zone is sufficient.

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The illustrated in the KLG _e 1 and 2 embodiments of the invention will be described in more detail with reference to the following embodiments.

Urea is produced in accordance with the process configuration shown in FIG. 1. Per ton of urea produced, 400 kg of ammonia are fed via the ammonia pump K, brought to a temperature of 110 ^° C. in the heater G and via line 11 into the reaction zone of the reactor stripper A introduced; 733 kg of carbon dioxide and 29 kg of inert gas (namely air) are also introduced into the reaction zone of the reactor stripper A via line 2 and compressor P. The pressures maintained in the reaction zone or the stripping zone of the reactor stripper A are 186 bar and 137, respectively bar.

Furthermore, the reaction zone of the reactor stripper A via line 10 with a gas mixture of 513 kg of ammonia, 721 kg of carbon dioxide, 43 kg of water and 21 kg of inert gas at a temperature of 227 ⁰ C and via line 17 with a Karbamatlösung of 175 ⁰ C. supplied, which consists of 624 kg of ammonia, 540 kg of carbon dioxide and 173 kg of water.

The volume of the reaction zone of the reactor stripper A and, accordingly, the residence time in this reaction zone are designed so that at the prevailing pressure

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and the associated temperature of about 193 ⁰ O, a gas-liquid mixture of 1084 kg of ammonia, 674 kg of carbon dioxide, 800 kg of urea, 456 kg of water and 21 kg of inert gas is formed "this via line 19 from the synthesis zone applied gas-K? Mixture is reacted together with a gas mixture discharged from the carbamate condenser B through line 40 in the second reaction zone C, which is maintained at the same pressure as the reaction zone (186 bar), to form a urea synthesis solution , which has a temperature of 196 C and 84? kg of ammonia, 436 kg of carbon dioxide, 1000 kg of urea and 510 kg of water. The quantity of urea formed in the reaction zone constitutes about 76% of that set of _s which would have been formed when the conversion of ammonium carbamate to urea would have been continued to the equilibrium state ·

After the expansion of this urea synthesis solution in the expansion valve 41 to 137 bar, the resulting gas mixture of 44 kg of ammonia, 21 kg of carbon dioxide and 9 kg of water vapor in the gas-liquid separator S is separated, wherein a urea product solution goes off, which is a temperature of 188 ⁰ C and forming from 798 kg Ammoniakj 415 kg carbon dioxide, 1000 kg of urea and 501 kg of water is · This urea product stream is then introduced into the stripping zone of reactor-stripper A, in which it is heated and stripped countercurrently with fresh carbon dioxide resulting in a residual urea product consisting of 1000 kg urea, 450 kg water, 124 kg ammonia and 166 kg carbon dioxide «

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The gases released in the stripping treatment together with the carbon dioxide stripping gas used form a gas mixture of 674 kg of ammonia, 987 kg of carbon dioxide, 51 kg of water vapor and 29 kg of inert gas, this gas mixture together with the deposited in the gas-liquid separator S gas mixture in the compressor M is compressed to a pressure of 186 bar. A portion of this combined and compressed gas mixture is fed to the reaction zone of the reactor stripper A, the remainder consisting of 205 kg of ammonia, 287 kg of carbon dioxide, 17 kg of steam and 8 kg of inert gas together with 186 kg of fresh ammonia and the ammonium carbamate solution formed in the wax column E condensed partially in the carbamate condenser B, wherein said Ammoniumkarbamat »solution consists of 227 kg of ammonia, 271 kg of carbon dioxide and 157 kg of water« The liberated in this condensation Heat energy is recovered and used to produce 355 kg of saturated steam In this embodiment of the invention, the consumption of high pressure steam (20 to 30 bar) for decomposing unconverted carbamate is zero.

Embodiment JI

Urea is produced in accordance with the process configuration shown in Pig. 2. "Per ton of urea produced are fed to the reaction zone of the reactor stripper A 400 kg of ammonia at 110 ^° C. via pump K, ammonia heater G and line 11; fed in at the same time is a gas mixture of 221 ^° C.,

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consisting of 569 kg of ammonia, 781 kg of carbon dioxide, 45 kg of water and 22 kg of inert gas, via line 10 and - via line 17 - a carbamate solution of 611 kg of ammonia, 516 kg of carbon dioxide and 172 kg of water at a temperature of 1 & 5 ⁰ Ce In the reaction zone at a temperature of about 193 ⁰ C, a gas-liquid mixture is formed, which consists of 1127 kg of ammonia, 710 kg of carbon dioxide, 800 kg of urea, 457 kg of water and 22 kg of inert gas · The quantity of in the Urea formed by the reaction zone accounts for about 67 % of the amount of urea which would have been obtained by continuing the conversion of ammonium carbamate to urea to equilibrium. This gas-liquid mixture is depressurized to a pressure of about 137 bar and is co-ducted with it Brought 40 and brought 18 kg of ammonia, 15 kg of carbon dioxide, 1 kg of water and 7 kg of inert gas-containing gas mixture in the post-reactor 0 for the reaction to b further 200 kg of urea This finally results in 2862 kg of urea synthesis solution containing 1000 kg of urea. This urea synthesis solution with a temperature of 182 ^° C. and a pressure of 137 bar, which, in addition to the 1000 kg urea, still contains 883 kg of ammonia, 469 kg of carbon dioxide and 510 kg of water, is fed directly to the stripping zone of the reactor stripper A, in which it is stripped at a pressure of 137 bar with 733 kg of gaseous carbon dioxide _? which results in a residual urea product stream of 1000 kg urea, 450 kg water, 124 kg ammonia and 166 kg carbon dioxide «

Of the discharged from the stripping zone of the reactor stripper A gas mixture 1417 kg in the compressor M on

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compressed a pressure of 186 bar and then fed into the reaction zone, whereas the remaining portion _? consisting of 190 kg of ammonia, 260 kg of coal & ioxLd, 15 kg of water and 7 kg of inert gas with the aid of 167 kg of ammonia and derived from the wash column E Karbamatlösung from 272 kg of ammonia, 271 kg of carbon dioxide and 157 kg of water in the carbamate condenser B partially condensed The resulting ammonium carbamate solution is - as mentioned above - via pump L and line. 17 of the reaction zone of the reactor stripper A. Fed, and the remaining non-condensed gas mixture is - as mentioned above * * fed via line 40 to the Liach reactor »

The heat released in the carbamate condenser B is recovered and used to produce 327 kg of saturated steam at a pressure of 3 * 5 bar. Also in this embodiment of the invention no high pressure steam (20 to 30 bar) is needed for the decomposition of unreacted ammonium carbamate *

Claims (13)

61 318 18 - 28 - Claim for invention 1 A process for the production of ammonia and carbon dioxide Hametoff at an elevated temperature and pressure using a reaction zone and a stripping zone, wherein in said reaction zone carbon dioxide and a portion of said ammonia to ammonium carbamate and a portion of said ammonium carbamate converted to urea to form a product effluent from the reaction zone of product urea, unreacted ammonium carbamate and excess ammonia, said transformations taking place to form net heat, and in said stripping zone a urea product stream of unconverted ammonium carbamate for decomposition at least a portion of said ammonium carbamate is heated by heat exchange with said reaction zone and stripped to remove the gaseous ammonia and carbon dioxide thus formed from said urea product stream, characterized in that said reaction zone which exchanges heat with the stripping zone is maintained at a pressure in the range from about 125 to 250 bar and in that said stripping zone is maintained at a pressure which is below the pressure prevailing in said reaction zone Method according to item 1, characterized in that said stripping zone is kept under a pressure which is about 20 to 120 bar below the pressure maintained in the reaction zone. 24288 0 7 - ²⁹ - Method according to item 2, characterized in that said desorption zone is maintained under a pressure which is about 40 to 60 bar lower than the pressure maintained in said reaction zone. Process according to any one of items 1 to 3, characterized in that the conversion of ammonium carbamate to urea in said reaction zone is continued until the quantity of urea formed equals at least 50 % of the quantity of urea which under the reaction conditions given in said reaction zone would have been achieved in the presence of the equilibrium state · Process according to item 1, characterized in that the conversion of ammonium carbamate into urea in said reaction zone is continued until the quantity of urea formed corresponds to at least 70 % of the quantity of urea which is below the reaction conditions in said reaction zone in the presence of the equilibrium state would have been achieved « Process according to any one of items 1 to 5, characterized in that the contents of said reaction zone are thoroughly mixed. The method according to item 6, characterized in that said reaction zone and said stripping zone are within a vertical tubular heat exchanger, said stripping zone being within the tubes of said heat exchanger and 61 318 18 24 28 8 0 7 -so- wherein said reaction zone is within the housing of said heat exchanger, and finally the temperature difference between the upper and lower part of said reaction zone is limited to at most 5 ° C * 8 · Process according to item 7, characterized in that the temperature difference between the upper and lower part of the said reaction zone is limited to a maximum of 2 ^° C. « Process according to any one of items 1 to 8, characterized in that the medium effluent from the reaction zone is introduced into a reaction zone in which an additional portion of ammonium carbamate is converted to urea to form a urea product stream; which comprises urea and unconverted ammonium carbamate, said urea product stream then being introduced into said stripping zone, and finally converting said ammonium carbamate into urea in said NaCh reaction zone in an amount corresponding to at least 90 % of that amount of urea would have been achieved under the reaction conditions given in the Uach reaction zone in the presence of the equilibrium state. " 10. The method of item 9 _9, characterized in that the urea product stream from said post-reaction zone is treated prior to its introduction into said stripping zone in a heater-decomposer, in which at least a portion of the ammonium carbamate is decomposed by heating to a Gas mixture of ammonia and coal 61 318 18 7 -31- dioxide and a residual urea product stream having a reduced carbamate content, and wherein said gas mixture is introduced into said reaction zone and said residual urea product stream into said stripping zone. The process of any one of items 1 to 9, characterized in that the urea product stream from said stripping zone is introduced into a second stripping zone in which an additional amount of ammonium carbamate is decomposed and removed from the urea product stream. Process according to item 9, characterized in that only part of the urea product stream from said Uach reaction zone is introduced into said stripping zone and that a remaining part of the urea product stream is introduced from said Kach reaction zone into a second stripping zone is decomposed in which ammonium carbamate to form a gas mixture containing ammonia and carbon dioxide and this gas mixture is separated from the remaining urea product stream. Method according to item 11 or 12, characterized in that said second stripping zone is kept under the same pressure as said stripping zone » For this 4 pages drawings