WO2005102992A1 - Process for the preparation of urea from carbon dioxide and ammonia in a urea plant - Google Patents

Process for the preparation of urea from carbon dioxide and ammonia in a urea plant Download PDF

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Publication number
WO2005102992A1
WO2005102992A1 PCT/NL2005/000248 NL2005000248W WO2005102992A1 WO 2005102992 A1 WO2005102992 A1 WO 2005102992A1 NL 2005000248 W NL2005000248 W NL 2005000248W WO 2005102992 A1 WO2005102992 A1 WO 2005102992A1
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Prior art keywords
pressure
urea
section
ammonium carbamate
pressure section
Prior art date
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PCT/NL2005/000248
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French (fr)
Inventor
Leonardus Henricus Gertrudis Maria Heijnen
Luc Louis Maria Dieltjens
Original Assignee
Dsm Ip Assets B.V.
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Publication date
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Publication of WO2005102992A1 publication Critical patent/WO2005102992A1/en

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    • C—CHEMISTRY; METALLURGY
    • C07—ORGANIC CHEMISTRY
    • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C273/00—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups
    • C07C273/02—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds
    • C07C273/04—Preparation of urea or its derivatives, i.e. compounds containing any of the groups, the nitrogen atoms not being part of nitro or nitroso groups of urea, its salts, complexes or addition compounds from carbon dioxide and ammonia

Definitions

  • the invention relates to a process for the preparation of urea from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure, an aqueous ammonium carbamate solution being formed in the recovery section(s) at a lower pressure, which solution is returned to the section having a higher pressure.
  • a process for the preparation of urea is common knowledge and is described in for example Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350.
  • a drawback of that process is that in many cases much water is returned along with the ammonium carbamate, a valuable raw material in the preparation of urea.
  • the presence of water has an adverse effect on the position of the equilibrium in the reaction of ammonium carbamate to urea at elevated temperature.
  • the conversion to urea decreases with increasing amounts of water, resulting in decreasing urea production in a urea plant.
  • the invention is characterized in that the crystallization point of the ammonium carbamate solution is determined in the recovery section(s). It is thus achieved that the water content of the said ammonium carbamate solution can be accurately controlled as a result of which less water is returned to the high-pressure section of the urea plant and urea can be produced more efficiently and with a higher degree of conversion.
  • Urea is prepared from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure.
  • Urea can be prepared introducing into a reactor excess ammonia together with carbon dioxide at high pressure (for example 12-40 MPa) and increased temperature (for example 160-250°C) which first results in the formation of ammonium carbamate according to the reaction:
  • a conventional urea plant is understood to mean a urea plant in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the customary excess ammonia take place at an essentially lower pressure than the pressure in the synthesis reactor itself.
  • a conventional high- pressure urea plant following expansion, dissociation and condensation at a pressure of between 1.5 and 10 MPa, the reactants that are not converted into urea are returned as a carbamate stream to the urea synthesis section.
  • ammonia and carbon dioxide are directly supplied to the urea reactor.
  • Such conventional urea plants are also designed as so-called Conventional Recycle Processes wherein all non-converted ammonia and carbon dioxide are returned to the urea reactor.
  • Such recirculation is effected in two stages.
  • the first recirculation stage the urea synthesis solution coming from the reactor is heated in a heater whereby ammonium carbamate decomposes into gaseous ammonia and carbon dioxide and in addition the excess ammonia evaporates here.
  • This gas mixture is subsequently converted in a rectifying column into pure ammonia and an aqueous ammonium carbamate stream.
  • Both streams are returned to ⁇ the urea reactor.
  • the urea solution from the first recirculation stage is reheated and then separated.
  • the gas stream thus obtained is condensed and then supplied to the first-stage rectifying column.
  • urea is liberated from the urea solution coming from the second recirculation stage by evaporating water.
  • the two recirculation stages and the evaporation together form the key element of the urea recovery.
  • a urea stripping plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that is not converted into urea and the expulsion of the customary excess ammonia largely take place at a pressure that is essentially virtually equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper, with or without addition of a stripping medium.
  • carbon dioxide and/or ammonia can be used as a stripping gas before the said components are added to the reactor.
  • Such stripping is effected in a stripper downstream of the reactor whereby the solution coming from the reactor, which solution contains besides urea, ammonium carbamate and water also ammonia and carbon dioxide, is stripped with the stripping gas with addition of heat. It is also possible to apply thermal stripping here, which means that ammonium carbamate is decomposed exclusively by addition of heat and the ammonia and carbon dioxide that are present are removed from the urea solution.
  • the gas stream released from the stripper, which contains ammonia and carbon dioxide, is returned to the reactor via a high-pressure carbamate condenser. The gas mixture that has not reacted in the urea synthesis is removed from the synthesis section.
  • this gas mixture also contains inert gases such as nitrogen, oxygen and possibly hydrogen.
  • inert gases originate from the feedstocks and from the air added to the carbon dioxide feed to the synthesis to protect the materials against corrosion.
  • This gas stream is discharged from the synthesis section downstream of for example the reactor or the high-pressure carbamate condensation, depending on the process route. It is preferred, however, for the condensable components (ammonia and carbon dioxide) to be absorbed in a high-pressure scrubber at synthesis pressure before the inert gases are discharged. In such a high-pressure scrubber the condensable components, ammonia and carbon dioxide, are absorbed from the synthesis off-gas in the carbamate stream from a recovery section operating at lower pressure.
  • This scrubbing process in the high-pressure scrubber can be promoted by application of a heat exchanger that extracts heat from the process.
  • the carbamate stream from the high- pressure scrubber which stream contains ammonia and carbon dioxide absorbed from the synthesis off-gas, returns via the high-pressure carbamate condenser to the reactor.
  • the reactor, high-pressure scrubber, stripper and high-pressure carbamate condenser are the main elements of the high-pressure section of a urea plant.
  • the crystallization point of the ammonium carbamate solution formed in it is determined.
  • the recovery section may comprise a low-pressure section or a medium-pressure and a low-pressure section.
  • the pressure in a high-pressure section is for example 12-40 MPa, in a medium-pressure section for example 1-3 MPa and in a low-pressure section for example 0,1-0,5 MPa.
  • the crystallization point of an ammonium carbamate solution returning to the high-pressure section is determined.
  • the ammonium carbamate solution is preferably returned to the high-pressure scrubber.
  • the ammonium carbamate solution may also be returned from the low-pressure scrubber to the medium-pressure section.
  • the ammonium carbamate solution is preferably returned to the medium-pressure condenser.
  • crystallization point of an ammonium carbamate solution that leaves the low-pressure carbamate condenser It is preferred to determine the crystallization point of an ammonium carbamate solution that leaves the low-pressure carbamate condenser.
  • the crystallization point is determined of an ammonium carbamate solution that is returned to the high-pressure section. Such ammonium carbamate solution is preferably returned to the high-pressure scrubber. It is preferred to determine the crystallization point of an ammonium carbamate solution that leaves the medium-pressure carbamate condenser.
  • the crystallization point of an ammonium carbamate solution can be determined in various ways. For example by optical measuring techniques.
  • the purpose of determining the crystallization point is to prevent the ammonium carbamate in the solution from crystallizing, which causes ammonium carbamate solution to be less readily transportable and may cause piping to clog up.
  • the ammonium carbamate solution must contain an amount of water such that crystallization does not occur, and the water content of the solution must also be kept to a minimum because of the aforementioned adverse effect on the degree of conversion.
  • the water content of the ammonium carbamate solution can be increased by for example adding water or an aqueous solution.
  • the water content can be decreased by for example concentrating the ammonium carbamate solution through evaporation.
  • the crystallization point is preferably determined by measurement with ultrasonic sound.
  • ultrasonic sound Such a technique is described in for example the German patent application DE 19741667 A1. It describes measuring probes with which the crystallization point of a substance dissolved in a liquid is determined with the aid of ultrasonic sound.
  • a measuring probe comprising an ultrasonic transmitter and receiver as well as a temperature sensor. The latter is used because the crystallization point of a substance in solution is heavily dependent on temperature.
  • the measuring probe and temperature sensor are preferably installed on a bypass line through which the ammonium carbamate solution passes.
  • the invention also relates to a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure wherein the recovery section(s) of the urea plant comprise(s) an apparatus for determination of the crystallization point of an ammonium carbamate solution.
  • the low-pressure section or the medium-pressure section and the low-pressure section comprise an apparatus for determination of the crystallization point of an ammonium carbamate solution by means of ultrasonic sound.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

Abstract

The invention relates to a process for the preparation of urea from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure, an aqueous ammonium carbamate solution being formed in the recovery section(s) at a lower pressure, which solution is returned to the section having a higher pressure, wherein the crystallization point of the ammonium carbamate solution is determined in the recovery section(s) of the urea plant.

Description

PROCESS FOR THE PREPARATION OF UREA FROM CARBON DIOXIDE AND AMMONIA IN A UREA PLANT
The invention relates to a process for the preparation of urea from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure, an aqueous ammonium carbamate solution being formed in the recovery section(s) at a lower pressure, which solution is returned to the section having a higher pressure. Such a process for the preparation of urea is common knowledge and is described in for example Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. A drawback of that process is that in many cases much water is returned along with the ammonium carbamate, a valuable raw material in the preparation of urea. The presence of water has an adverse effect on the position of the equilibrium in the reaction of ammonium carbamate to urea at elevated temperature. The conversion to urea decreases with increasing amounts of water, resulting in decreasing urea production in a urea plant. It is the object of the invention to eliminate the aforementioned drawback. The invention is characterized in that the crystallization point of the ammonium carbamate solution is determined in the recovery section(s). It is thus achieved that the water content of the said ammonium carbamate solution can be accurately controlled as a result of which less water is returned to the high-pressure section of the urea plant and urea can be produced more efficiently and with a higher degree of conversion. Urea is prepared from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure. Urea can be prepared introducing into a reactor excess ammonia together with carbon dioxide at high pressure (for example 12-40 MPa) and increased temperature (for example 160-250°C) which first results in the formation of ammonium carbamate according to the reaction:
2NH3 + CO2 → H2N-CO-ONH4 Dehydration of the ammonium carbamate formed then results in the formation of urea according to the equilibrium reaction:
H2N-CO-ONH4<— > H2N-CO-NH2 + H2O
The theoretically attainable conversion of ammonia and carbon dioxide into urea is determined by the thermodynamic position of the equilibrium and depends on for example the NH3/CO2 ratio (N/C ratio), the H2O/CO2 ratio and temperature. From the above reaction equations it can be derived that the application of an excess of water in the synthesis zone has a negative influence on the theoretically attainable conversion. Various processes are employed in practice for preparing urea. Initially, urea used to be prepared in so-called conventional high-pressure urea plants but since the late nineteen sixties urea is mostly prepared by processes that are practiced in so-called urea stripping plants. A conventional urea plant is understood to mean a urea plant in which the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the customary excess ammonia take place at an essentially lower pressure than the pressure in the synthesis reactor itself. In a conventional high- pressure urea plant, following expansion, dissociation and condensation at a pressure of between 1.5 and 10 MPa, the reactants that are not converted into urea are returned as a carbamate stream to the urea synthesis section. Furthermore, in a conventional high-pressure urea plant ammonia and carbon dioxide are directly supplied to the urea reactor. Such conventional urea plants are also designed as so-called Conventional Recycle Processes wherein all non-converted ammonia and carbon dioxide are returned to the urea reactor. Such recirculation is effected in two stages. A first recirculation stage at medium pressure (1.8-2.5 MPa) and a second recirculation stage (0,2-0,5 MPa). In the first recirculation stage the urea synthesis solution coming from the reactor is heated in a heater whereby ammonium carbamate decomposes into gaseous ammonia and carbon dioxide and in addition the excess ammonia evaporates here. This gas mixture is subsequently converted in a rectifying column into pure ammonia and an aqueous ammonium carbamate stream. Both streams are returned to ■ the urea reactor. In the second recirculation stage the urea solution from the first recirculation stage is reheated and then separated. The gas stream thus obtained is condensed and then supplied to the first-stage rectifying column. Next, in the evaporation at reduced pressure, urea is liberated from the urea solution coming from the second recirculation stage by evaporating water. The two recirculation stages and the evaporation together form the key element of the urea recovery. A urea stripping plant is understood to be a urea plant in which the decomposition of the ammonium carbamate that is not converted into urea and the expulsion of the customary excess ammonia largely take place at a pressure that is essentially virtually equal to the pressure in the synthesis reactor. This decomposition/expulsion takes place in a stripper, with or without addition of a stripping medium. In a stripping process carbon dioxide and/or ammonia can be used as a stripping gas before the said components are added to the reactor. Such stripping is effected in a stripper downstream of the reactor whereby the solution coming from the reactor, which solution contains besides urea, ammonium carbamate and water also ammonia and carbon dioxide, is stripped with the stripping gas with addition of heat. It is also possible to apply thermal stripping here, which means that ammonium carbamate is decomposed exclusively by addition of heat and the ammonia and carbon dioxide that are present are removed from the urea solution. The gas stream released from the stripper, which contains ammonia and carbon dioxide, is returned to the reactor via a high-pressure carbamate condenser. The gas mixture that has not reacted in the urea synthesis is removed from the synthesis section. Besides condensable ammonia and carbon dioxide, this gas mixture also contains inert gases such as nitrogen, oxygen and possibly hydrogen. Such inert gases originate from the feedstocks and from the air added to the carbon dioxide feed to the synthesis to protect the materials against corrosion. This gas stream is discharged from the synthesis section downstream of for example the reactor or the high-pressure carbamate condensation, depending on the process route. It is preferred, however, for the condensable components (ammonia and carbon dioxide) to be absorbed in a high-pressure scrubber at synthesis pressure before the inert gases are discharged. In such a high-pressure scrubber the condensable components, ammonia and carbon dioxide, are absorbed from the synthesis off-gas in the carbamate stream from a recovery section operating at lower pressure. This scrubbing process in the high-pressure scrubber can be promoted by application of a heat exchanger that extracts heat from the process. The carbamate stream from the high- pressure scrubber, which stream contains ammonia and carbon dioxide absorbed from the synthesis off-gas, returns via the high-pressure carbamate condenser to the reactor. The reactor, high-pressure scrubber, stripper and high-pressure carbamate condenser are the main elements of the high-pressure section of a urea plant. In the recovery section of a urea plant, the crystallization point of the ammonium carbamate solution formed in it is determined. Depending on the urea process, the recovery section may comprise a low-pressure section or a medium-pressure and a low-pressure section. The pressure in a high-pressure section is for example 12-40 MPa, in a medium-pressure section for example 1-3 MPa and in a low-pressure section for example 0,1-0,5 MPa. In the low-pressure section the crystallization point of an ammonium carbamate solution returning to the high-pressure section is determined. The ammonium carbamate solution is preferably returned to the high-pressure scrubber. The ammonium carbamate solution may also be returned from the low-pressure scrubber to the medium-pressure section. The ammonium carbamate solution is preferably returned to the medium-pressure condenser.
It is preferred to determine the crystallization point of an ammonium carbamate solution that leaves the low-pressure carbamate condenser. In the medium-pressure section the crystallization point is determined of an ammonium carbamate solution that is returned to the high-pressure section. Such ammonium carbamate solution is preferably returned to the high-pressure scrubber. It is preferred to determine the crystallization point of an ammonium carbamate solution that leaves the medium-pressure carbamate condenser. The crystallization point of an ammonium carbamate solution can be determined in various ways. For example by optical measuring techniques. The purpose of determining the crystallization point is to prevent the ammonium carbamate in the solution from crystallizing, which causes ammonium carbamate solution to be less readily transportable and may cause piping to clog up. The ammonium carbamate solution must contain an amount of water such that crystallization does not occur, and the water content of the solution must also be kept to a minimum because of the aforementioned adverse effect on the degree of conversion. Following the determination of the crystallization point it is possible to adjust the water content of the ammonium carbamate solution by increasing or decreasing the water content. The water content can be increased by for example adding water or an aqueous solution. The water content can be decreased by for example concentrating the ammonium carbamate solution through evaporation. The crystallization point is preferably determined by measurement with ultrasonic sound. Such a technique is described in for example the German patent application DE 19741667 A1. It describes measuring probes with which the crystallization point of a substance dissolved in a liquid is determined with the aid of ultrasonic sound. For this measurement are employed a measuring probe comprising an ultrasonic transmitter and receiver as well as a temperature sensor. The latter is used because the crystallization point of a substance in solution is heavily dependent on temperature. The measuring probe and temperature sensor are preferably installed on a bypass line through which the ammonium carbamate solution passes. The invention also relates to a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure wherein the recovery section(s) of the urea plant comprise(s) an apparatus for determination of the crystallization point of an ammonium carbamate solution. In a urea plant comprising a high-pressure section and a low- pressure section or a high-pressure section, a medium-pressure section and a low- pressure section, the low-pressure section or the medium-pressure section and the low-pressure section comprise an apparatus for determination of the crystallization point of an ammonium carbamate solution by means of ultrasonic sound. In such plant it is preferred to determine the crystallization point of an ammonium carbamate solution that is leaving a carbamate condenser in the low-pressure section and/or the medium pressure section.

Claims

1. Process for the preparation of urea from carbon dioxide and ammonia in a urea plant comprising a high-pressure section and at least one recovery section at a lower pressure, an aqueous ammonium carbamate solution being formed in the recovery section(s) at a lower pressure, which solution is returned to the section having a higher pressure, characterized in that the crystallization point of the ammonium carbamate solution is determined in the recovery section(s) of the urea plant.
2. Process according to Claim 1 , wherein the urea plant comprises a high- pressure section and a low-pressure section and wherein in the low-pressure section an aqueous ammonium carbamate solution is formed that is returned to the high-pressure section, characterized in that the crystallization point of the ammonium carbamate solution in the low-pressure section of the urea plant is determined.
3. Process according to Claim 1 , wherein the urea plant comprises a high- pressure section, a medium-pressure section and a low-pressure section and wherein in the medium-pressure section and the low-pressure section an aqueous ammonium carbamate solution is formed that is returned to the section having a higher pressure, characterized in that the crystallization point of the ammonium carbamate solution is determined in the medium-pressure section and in the low-pressure section of the urea plant.
4. Process according to either of Claims 1-2, characterized in that the ammonium carbamate solution is returned to the high-pressure scrubber.
5. Process according to any one of Claims 1-4, characterized in that the crystallization point is determined of the ammonium carbamate solution leaving a carbamate condenser.
6. Process according to any one of Claims 1-5, characterized in that the crystallization point of the ammonium carbamate solution is determined by measurement with ultrasonic sound.
7. Process according to any one of Claims 1-6, characterized in that, following the determination of the crystallization point, the amount of water in the ammonium carbamate solution is adjusted.
8. Urea plant comprising a high-pressure section and at least one recovery - section at a lower pressure, characterized in that the recovery section(s) of the urea plant comprise(s) an apparatus for determination of the crystallization point of an ammonium carbamate solution.
9. Urea plant comprising a high-pressure section and a low-pressure section, characterized in that the low-pressure section comprises an apparatus for determining the crystallization point of an ammonium carbamate solution by means of ultrasonic sound.
10. Urea plant comprising a high-pressure section, a medium-pressure section and a low-pressure section, characterized in that the medium-pressure section and the low-pressure section comprise an apparatus for determining the crystallization point of an ammonium carbamate solution by means of ultrasonic sound.
11. Urea plant according to any one of Claims 8-10, characterized in that the crystallization point is determined of an ammonium carbamate solution that is leaving a carbamate condenser.
PCT/NL2005/000248 2004-04-22 2005-04-04 Process for the preparation of urea from carbon dioxide and ammonia in a urea plant WO2005102992A1 (en)

Applications Claiming Priority (2)

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NL1026003 2004-04-22
NL1026003A NL1026003C2 (en) 2004-04-22 2004-04-22 Process for the preparation of urea from carbon dioxide and ammonia in a urea plant.

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3075725A1 (en) * 2015-03-30 2016-10-05 Casale SA Sonication in a urea or melamine synthesis process

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3940440A (en) * 1970-11-10 1976-02-24 Ivo Mavrovic Method of controlling urea system
EP0114442A1 (en) * 1982-12-24 1984-08-01 Stamicarbon B.V. Process for determining and controlling the composition of aqueous solutions of NH3 and CO2
DE19741667A1 (en) * 1997-09-17 1999-10-07 Sensotech Ges Fuer Sensorentwi Process for deriving crystallization process control and on-line monitoring parameters
WO2001030748A1 (en) * 1999-10-26 2001-05-03 Dsm N.V. Process for the preparation of urea

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3940440A (en) * 1970-11-10 1976-02-24 Ivo Mavrovic Method of controlling urea system
EP0114442A1 (en) * 1982-12-24 1984-08-01 Stamicarbon B.V. Process for determining and controlling the composition of aqueous solutions of NH3 and CO2
DE19741667A1 (en) * 1997-09-17 1999-10-07 Sensotech Ges Fuer Sensorentwi Process for deriving crystallization process control and on-line monitoring parameters
WO2001030748A1 (en) * 1999-10-26 2001-05-03 Dsm N.V. Process for the preparation of urea

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3075725A1 (en) * 2015-03-30 2016-10-05 Casale SA Sonication in a urea or melamine synthesis process
WO2016156015A1 (en) 2015-03-30 2016-10-06 Casale Sa Sonication in a urea or melamine synthesis process
CN107428679A (en) * 2015-03-30 2017-12-01 卡萨勒有限公司 Sonication is used for urea or melamine synthetic method
US10507451B2 (en) 2015-03-30 2019-12-17 Casale Sa Sonication in a urea or melamine synthesis process
RU2718901C2 (en) * 2015-03-30 2020-04-15 Касале Са Ultrasonic treatment during synthesis of urea or melamine
CN107428679B (en) * 2015-03-30 2020-09-01 卡萨勒有限公司 Sonication process for urea or melamine synthesis

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