Process and plant for the synthesis of urea and melamine

Patent metadata

Field	Value
Publication number	US-9957239-B2
Application number	US-201515126212-A
Country	US
Kind code	B2
Filing date	Apr 16, 2015
Priority date	Apr 28, 2014
Publication date	May 1, 2018
Grant date	May 1, 2018

Abstract

An integrated process for the synthesis of urea and melamine, wherein: urea is synthesized with a stripping process in a high-pressure synthesis loop comprising a reactor, a stripper and a carbamate condenser, and the urea solution leaving said stripper is sent to a recovery section to produce a concentrated urea product and a recovered carbamate solution; at least part of said urea product is converted to melamine, and the off-gas from the synthesis of melamine are recycled to the urea synthesis by mixing with the gas phase from the stripper and with said recovered carbamate solution, thus forming a mixed flow which is then condensed in said carbamate condenser, and the condensate is eventually directed to the reactor.

First claim

The invention claimed is:

1. A process for the combined synthesis of urea and melamine, wherein:
urea is synthesized from ammonia and carbon dioxide with a stripping process, said stripping process including at least the steps of reacting ammonia and carbon dioxide in a reaction section, to form an aqueous solution comprising urea, ammonium carbamate and unconverted ammonia, and treating said solution in a stripping section, obtaining a urea solution and a gas phase containing ammonia and carbon dioxide, also including a step of condensation in a condensation section,
at least a portion of synthesized urea is used to produce melamine in a tied-in melamine plant, obtaining also a flow of melamine off gas which contain ammonia and carbon dioxide, and said flow of melamine off gas is recycled back to said process for the synthesis of urea, either in a gaseous state or in a liquid state after condensation,
wherein a first portion of said gaseous phase obtained from the stripping process is fed directly to said reaction section in a gaseous state, and
wherein said urea solution leaving the stripping section is further processed in at least one recovery section obtaining a liquid carbamate solution and a more concentrated solution, and a first portion of said concentrated solution is used to produce melamine, and
wherein the melamine off gas are recycled to the process for synthesis of urea by:
mixing said flow of melamine off gas with a second portion of said gaseous phase from the stripping treatment, and with a second portion of said liquid carbamate solution coming from said recovery section, thus obtaining a gaseous-liquid mixed flow,
condensing said mixed flow in said condensation section, obtaining a condensate, and
feeding said condensate to said reaction section.

2. The process according to claim 1 , wherein a portion of said gaseous phase from the stripping section is fed directly to said reaction section, and a remaining portion of said gaseous phase is sent to said condensation section.

3. The process according to claim 1 , wherein said condensate is further separated into a gaseous phase comprising vapors and non-condensable inert gases and a liquid phase, said gas phase is directed to a process of scrubbing and further condensation, and said liquid phase is fed to said reaction section.

4. The process according to claim 1 , said flow of melamine off gas having a pressure of 80 bar or greater, and being substantially free of water.

5. The process according to claim 1 , said flow of melamine off gas being released at a medium pressure of no more than 30 bar, and said off gas being recycled to the urea synthesis in a liquid state after condensation.

6. The process according to claim 1 , said stripping process for the synthesis of urea being a self-stripping or ammonia stripping process.

7. The process according to claim 1 , said stripping process for the synthesis of urea being a CO2-stripping process.

8. The process according to claim 1 , wherein the condensation step in said carbamate condensation section is a substantially total condensation.

9. The process according to claim 1 , wherein at least 50% of the synthesized urea is used to produce melamine.

10. The process according to claim 9 , wherein all the synthesized urea is used to produce melamine.

11. The process according to claim 1 , wherein a portion of synthesized urea is used to produce melamine, and a remaining part of the synthesized urea is exported as such or sent to a finishing section.

12. The plant for the synthesis of urea and melamine according to the process of claim 1 , the plant comprising a urea synthesis section and a melamine synthesis section, wherein:
said urea synthesis section includes a synthesis loop which in turn comprises at least a reactor, a stripper, and a condenser,
the urea section comprises a flow line to feed at least a portion of said gaseous phase leaving said stripper directly to said reactor, and
the urea synthesis section comprises at least one recovery section where the urea solution leaving the stripper is further processed, a liquid carbamate solution and a more concentrated urea solution being obtained in said at least one recovery section,
said melamine section receives at least a portion of the urea produced in the urea synthesis section, and produces melamine and a flow of melamine off gas containing ammonia and carbon dioxide,
the plant comprises flow lines for recycling said flow of melamine off gas back to the urea section,
the flow lines arranged to mix said flow of melamine off gas with a first portion of a gaseous phase emerging from said stripper, and with a portion of said liquid carbamate solution coming from said at least one recovery section, thus obtaining a gaseous-liquid mixed flow which is condensed in said condenser of the urea section.

13. A modification of a urea plant operating according to a stripping process, said urea plant including at least a reactor, a stripper and a condenser, the urea plant further comprising at least one recovery section where a urea solution leaving the stripper is further processed, a liquid carbamate solution and a more concentrated urea solution being obtained in said at least one recovery section, and the modification including at least the steps of:
adding a tied-in melamine plant which converts into melamine a portion of the urea synthesized by said urea plant;
recycling the off gas containing ammonia and carbon dioxide withdrawn from of said melamine plant to the urea plant, and
directing a first portion of the gaseous phase separated in withdrawn from separated in the stripper of the urea plant, containing ammonia and carbon dioxide, to the reactor of the urea plant, the modification further including:
mixing said off gas withdrawn from the melamine plant with a second portion of said gaseous phase from the stripper, and with a portion of said liquid carbamate solution obtained from said at least one recovery section, thus forming a gaseous-liquid mixed flow which is condensed in said condenser of the urea plant.

14. The process according to claim 1 , said flow of melamine off gas being released at a medium pressure of around 20 bar, and said off gas being recycled to the urea synthesis in a liquid state after condensation.

Assignees

• Casale Sa

Inventors

• Bertini Paolo
• Di Carlo Gabriele

Classifications

• Y02P20/125

  Cross-Sectional Technologies · mapped topic

• B01J4/02

  for feeding measured {, i.e. prescribed} quantities of reagents · CPC title

• B01J3/04

  Pressure vessels, e.g. autoclaves · CPC title

• B01J10/00 Primary

  Chemical processes in general for reacting liquid with gaseous media other than in the presence of solid particles, or apparatus specially adapted therefor (B01J19/08 takes precedence; separation, e.g. distillation, also combined with chemical reactions B01D, {e.g. B01D3/009}) · CPC title

• C07D251/60 Primary

  from urea or from carbon dioxide and ammonia · CPC title