Reactor-condenser for the synthesis of urea

Abstract

A combined reactor and condenser for the synthesis of urea from ammonia and carbon dioxide, including a condenser section coupled to a reaction section, comprising inputs directed to said condenser section for a gaseous stream comprising ammonia and carbon dioxide and for a solution containing ammonium carbamate and liquid ammonia, and wherein the effluent of the condenser section is sent to the reaction section; the reaction section comprises a plurality of compartments and a plurality of mixers, at least one inside each of said compartments.

Claims

1. A combined reactor and condenser for the synthesis of urea from ammonia and carbon dioxide, including a condenser section coupled to a reaction section, comprising at least one input directed to said condenser section for a gaseous stream comprising ammonia and carbon dioxide, and for a solution containing liquid ammonia and/or ammonium carbamate, and wherein the effluent of the condenser section is sent to the reaction section, characterized in that: the reaction section comprises a plurality of compartments; the reaction section comprises a plurality of mixers, including at least one mixer inside each of said compartments; wherein said mixers are rotating impellers; and wherein one of said impellers is suitable to impart an axial thrust to said effluent of the condenser section, acting as a drive impeller of the flow through the reaction section.

2. The reactor according to claim 1, said impellers being mounted on a common single shaft passing through the reaction section.

3. The reactor according to claim 1, wherein said drive impeller is also mounted on said common shaft.

4. The reactor according to claim 1, comprising an ejector acting as a phase mixer of said gaseous stream comprising ammonia and carbon dioxide and of said solution containing ammonium carbamate, before entering the condensation section.

5. The reactor according to claim 4, said ejector being designed to boost the pressure of said gaseous stream comprising ammonia and carbon dioxide.

6. The reactor according to claim 1, the reaction section comprising a plurality of segmental baffles or trays to delimit said compartments.

7. The reactor according to claim 1, the reaction section comprising at least four compartments.

8. The reactor according to claim 1, the condenser section being of shell-and-tube type.

9. A reactor according to claim 8, the condenser section comprising a bundle of U-tubes.

10. The reactor according claim 1, wherein the condenser section and the reaction section are arranged according to one of the following: the condenser section and the reaction section are both vertical, the reaction section being arranged above the condenser section; the condenser section is horizontal and the reaction section is vertical; condenser section and the reaction section are both horizontal.

11. A plant for the synthesis of urea from ammonia and carbon dioxide, comprising a synthesis section which includes a stripper and a combined reactor and condenser, said combined reactor and condenser including a condenser section coupled to a reaction section and comprising at least one input directed to said condenser section for a gaseous stream coming from said stripper and comprising ammonia and carbon dioxide, and for a solution containing liquid ammonia and/or ammonium carbamate, and wherein the effluent of the condenser section is sent to the reaction section, wherein: said reaction section of the combined reactor and condenser comprises a plurality of compartments; said reaction section comprises a plurality of mixers, wherein said mixers are rotating impellers, including at least one mixer inside each of said compartments; one of said impellers is suitable to impart an axial thrust to said effluent of the condenser section, acting as a drive impeller of the flow through the reaction section and providing a suitable head for fluid circulation within the synthesis section, and said combined reactor and condenser is installed at ground level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a combined reactor-condenser for the synthesis of urea according to a first embodiment of the invention.

(2) FIG. 2 shows a second embodiment of the invention.

(3) FIG. 3 shows a third embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) FIG. 1 is a schematic illustration of a combined reactor-condenser 1 which comprises essentially a reaction section 2, a condensation section 3, an ejector 4 and a tank 5. Both sections 2 and 3 are vertically arranged and the reaction section 2 is above the condensation section 3.

(5) The reaction section 2 includes a plurality of compartments 2A, 2B, 20 and 2D delimited by baffles or trays 6. Each of said compartments has a respective impeller 7A, 7B, 70 and 7D. The impellers are mounted on a common shaft 8 and are powered by a motor 9 installed on top of reactor 1.

(6) The lower impeller 7A, near the condensation section, is designed to give an axial thrust to the fluid, thus providing the motive force for the upward flow through the vertical reaction section 2.

(7) The condensation section 3 comprises a bundle of U-tubes 10. Only one tube is shown in the figure for clarity.

(8) The ejector 4 receives a first input flow 11 of gaseous carbon dioxide and ammonia (for example from a stripper) and a second input flow 12 of ammonia and recycled carbamate (which comes for example from a recovery section). Typically the gas flow 11 has a pressure of 130 to 160 bar and a temperature of 100 to 200 C.; the carbamate flow 12 has a pressure of 150 to 250 bar and a temperature of 30 to 100 C.

(9) The ejector 4 intimately contacts the gas flow 11 and liquid flow 12 to achieve an output flow which is a two-phase mixture where the gas is completely dispersed into the liquid (dispersed flow). Said two-phase mixture is directed into the tubes of the tube bundle 10 by means of a feeding partition 13. The ejector 4 also boosts the pressure of the gas flow 11, using the energy of the liquid flow 12, to facilitate the passage through the bundle 10.

(10) Said two-phase mixture is partially condensed in the tube bundle 10; heat of condensation is used to produce steam 15 which is collected by the steam drum 5. Level of water 14 in the steam drum 15 is controlled by feeding steam condensate 16 when appropriate.

(11) Most of ammonia and carbon dioxide contained in the mixture is converted to carbamate in the tube bundle 10; the conversion is controlled acting on a pressure controller installed in the steam drum 5.

(12) The outlet flow 17 from the tube bundle 10 still contains some ammonia and carbon dioxide in a gas phase. This is due to the presence of passivation air injected along with the stream 11 and also to the need of feeding some gaseous carbon dioxide to the reactor since the heat released by condensation of said carbon dioxide will compensate for the endothermic reaction of dehydration of carbamate.

(13) Said output flow 17 enters the reaction section 2 and traverses the compartments (reaction stages) 2A to 2D. The lower impeller 2A is designed to impart an axial thrust to the flow 17, allowing the upward flow. Each compartment operates as a continuous stirred tank reactor thanks to the respective impeller.

(14) The output of reaction section 2 is a stream 18 which is essentially a solution of urea, water, carbamate and some unconverted ammonia, and is sent to a high-pressure stripper (not shown) which produces the gaseous stream 11.

(15) A separator can be provided to receive the stream 18 and separate gaseous and liquid components. The liquid components are normally sent to the high pressure stripper while the gaseous components can be directed to a scrubber (not shown) to wash the residual ammonia with water rich solution such as the recycle carbamate 12.

(16) FIG. 2 shows a second embodiment where the condensation section 3 is horizontally arranged. In this embodiment the ejector 4 is located inside the vessel of the condensation section, in the feeding partition 13.

(17) FIG. 3 shows a third embodiment having a horizontal layout of both the reaction section 2 and condensation section 3. In this embodiment the drive impeller 7A can be omitted as the flow through the reactor does not have to overcome the height of the reaction section.

(18) The combined reactor of the invention can be inserted in a urea synthesis loop, for example by connection with a stripper. Hence the invention is useful to new urea plants and also to the revamping of existing plants.