PLANT FOR UREA PRODUCTION

Abstract

Plant for the synthesis of urea, comprising: a synthesis section comprising at least one reactor, a compressor for supplying CO.sub.2 to said synthesis section, a gas turbine for the operation of said CO.sub.2 compressor and a heat recovery steam generator; the heat source of said heat recovery steam generator consists of the exhaust gases of said gas turbine, and at least one steam flow produced by said heat recovery steam generator is used as heat source for at least one component of said urea plant.

Claims

1) A plant for the synthesis of urea, comprising: a synthesis section comprising at least one reactor, a compressor feeding CO2 to said synthesis section, characterized in that it comprises a gas turbine for the operation of said CO2 compressor and in that it comprises a heat recovery steam generator, wherein the hot source of said heat recovery steam generator is represented by exhaust gases of said gas turbine, and wherein at least one steam flow produced by said heat recovery steam generator is used as heat source for at least one component of said urea plant.

2) The plant according to claim 1, wherein said gas turbine directly drives said CO.sub.2 compressor.

3) The plant according to claim 1, wherein the synthesis section also comprises at least one stripper and a condenser, and wherein a steam flow produced by said heat recovery steam generator is used as heating fluid for said stripper.

4) The plant according to claim 3, wherein said heat recovery steam generator produces a flow of saturated steam at a pressure of between 10 and 30 bar, preferably about 20 bar, and said saturated steam is supplied to the stripper of the high-pressure synthesis section as heating fluid.

5) The plant according to claim 3, wherein: the plant comprises a degasser which produces a flow of demineralized water, said degasser is supplied with steam obtained from cooling of said condenser of the synthesis loop, and said heat recovery steam generator is supplied with demineralized water produced in said degasser.

6) The plant according to claim 5, wherein said steam which supplies the degasser has a pressure lower than 6 bar relative, and preferably about 3 bar relative.

7) A method for revamping a plant for the synthesis of urea from ammonia and CO.sub.2, wherein: the plant includes a synthesis section, operating at a predefined synthesis pressure, comprising at least one reactor, the plant also comprises a CO.sub.2 compressor for feeding CO.sub.2 to at least one of the components of said synthesis loop; the method being characterized by the additional installation of a gas turbine for the operation of said CO.sub.2 compressor, and the additional installation of a heat recovery steam generator, wherein the heat source of said heat recovery steam generator is represented by the exhaust gases of said gas turbine, and wherein at least one steam flow produced by said heat recovery steam generator is used as heat source for at least one component of said plant.

8) The method according to claim 8, wherein the synthesis section comprises a stripper and a condenser, and the method envisages that a steam flow produced by said heat recovery steam generator is used as heat source for the stripper of the synthesis section.

Description

[0026] These and other advantages will become clear from the detailed description below, with reference to FIG. 1 which schematically illustrates a preferred embodiment of the invention.

DETAILED DESCRIPTION

[0027] FIG. 1 illustrates schematically a urea synthesis loop 1 which comprises a reactor 2, a stripper 3, a condenser 4 and a scrubber 31. The synthesis loop 1 is supplied with ammonia 5 and carbon dioxide 6.

[0028] Said loop 1 is well-known to the person skilled in the art and does not need to be described in detail. The example relates to a CO.sub.2-stripping plant in which the ammonia 5 is supplied to the condenser 4 and the carbon dioxide 6 is supplied to the stripper 3; in other embodiments the ammonia and the CO.sub.2 may be supplied to other points of the plant. The aqueous urea-containing solution 20 leaving the reactor 2 is concentrated in the stripper 3 and the solution 21 leaving the stripper is further treated in a recovery section using a technique known per se.

[0029] The stripper 3 is heated by steam at about 20 bar, entering via line 7 and exiting via line 8. The condenser 4 releases heat, producing a flow of steam 9 at a low pressure, for example 3 bar.

[0030] The components of the loop 1 operate at a predefined high pressure, which substantially is the same for the reactor 2, the stripper 3 and the condenser 4.

[0031] The carbon dioxide 6 is raised to said pressure of the loop 1 by a compressor 10. Said compressor is driven mechanically by a gas turbine unit 11 which essentially comprises an air compressor 12, a combustor 13 and a turbine 14. For example, and preferably, the CO.sub.2 compressor 10 is mounted on the shaft of said turbine 14. The gas turbine unit is fed with a fuel 15, for example natural gas or a suitable synthesis gas.

[0032] The exhaust gases 16 of the gas turbine 14, which are at a high temperature, are conveyed to a heat recovery steam generator (HRSG) 17. Said generator 17 produces the steam 7 intended to supply heat to the stripper. In particular, said steam 7 is obtained by heating feed water 30 previously degassed inside a degasser 19; said degasser 19 is heated by steam 9 produced in the condenser 4 of the loop 1. The line 22 indicates the cooled gases leaving the generator 17.

[0033] Usually the steam production in the condenser 4 exceeds the requirement of the degasser 19, therefore the steam 9 which supplies the degasser 19 may be a fraction of the steam which is actually produced. The remaining part 9′ of the steam may be used by other users of the plant or exported. Moreover, part of the steam 7 may also be destined for other plant users or exported, as indicated by the line 7′.

[0034] As can be understood from the figure, in this way high efficiency and good integration between the steam system and the urea plant are achieved. The fuel 15 in fact provides both the mechanical work of the compressor 10 and the heat for the stripper 3, owing to the efficient use in the turbine-recovery steam generator assembly; the invention also ensures optimum use of the steam 9 with a low energy level.