Urea production process and plant

Abstract

Process for the preparation of urea granules comprising the steps of obtaining an aqueous urea solution from one or more synthesis and recovery steps wherein ammonia and carbon dioxide are reacted together, subjecting the aqueous urea solution to an evaporation step wherein water is removed to obtain a urea melt (1), processing and treating said urea melt in a granulation step (7) and optionally in a cooling section (10) to obtain solid urea granules (14); the process further comprises a scrubbing step (3) of granulation offgas and an atmospheric evaporation step (32) to recover a urea solution (2) and a water-saturated air stream (18): the water-saturated air stream is fed back to the scrubbing section (3) without condensation, and the recovered urea solution is conveyed to the granulation step (7).

Claims

1. A process for the preparation of urea granules comprising the steps of: a) reacting ammonia and carbon dioxide at urea synthesis pressure to obtain a urea-containing effluent and processing said effluent in at least one recovery step at a lower pressure obtaining an aqueous urea solution; b) subjecting the aqueous urea solution to an evaporation step wherein water is removed from said aqueous urea solution so as to obtain a urea melt and an aqueous vapour phase; c) subjecting said urea melt to granulation in presence of fluidising air obtaining solid urea granules and a granulation offgas containing air and urea dust; d) conveying the granulation offgas to a scrubbing step performed with an aqueous scrubbing agent to remove urea dust from the offgas and to produce an aqueous urea solution containing urea removed from the offgas and a purified gaseous stream; e) contacting a first portion of the aqueous urea solution of step (d) with water to yield said aqueous scrubbing agent, and recirculating said aqueous scrubbing agent to the scrubbing step; f) feeding a second portion of said aqueous urea solution of step (d) to an evaporation step wherein said evaporation step is conducted in presence of an air stream to yield a recovered urea solution and a water-saturated air stream and wherein said evaporation step is conducted at atmospheric pressure or below atmospheric pressure; g) recirculating the recovered urea solution obtained at step (f) to the granulation step so that said recovered urea solution is subject to granulation together with the urea melt; h) recirculating the water-saturated air stream to step (d) without passing through a condensation step, so that said water-saturated air stream is subjected to a scrubbing step together with the granulation offgas of step (c).

2. The process according to claim 1, further comprising the step of: j) subjecting the urea granules to a cooling process in presence of cooling air to yield solid urea granules and a cooling off-gas stream; k) conveying the cooling off-gas stream to the scrubber step together with the granulation offgas and with the water-saturated air stream.

3. The process according to claim 1, wherein the evaporation step according to feature f) is performed at a pressure in the range of 0 bar rel to minus 0.2 bar rel.

4. The process according to claim 1, wherein the fluidising air and the air stream are separated from a main air stream.

5. The process according to claim 1, wherein the aqueous urea solution exiting the scrubbing step has a concentration in the range of 43 to 47% wt.

6. The process according to claim 1, wherein the urea melt of step (b) contains 95 to 99.7% wt urea.

7. The process according to claim 1, wherein the recovered urea solution has a urea concentration in the range of 95 to 98% wt.

8. The process according to claim 1, wherein the air stream is subjected to a heating stage before being fed to the evaporation step.

9. The process according to claim 8, wherein the temperature of the air stream after the heating stage is in the range of 120 to 170? C.

10. The process according to claim 1, wherein the purified gaseous stream is vented to the atmosphere.

11. The process according to claim 3, wherein the evaporation step according to feature f) is performed at a pressure in the range of 0 bar rel to minus 0.1 bar rel.

12. The process according to claim 6, wherein the urea melt of step (b) contains 95 to 98.8% wt.

13. The process according to claim 7, wherein the recovered urea solution has a urea concentration of 96% wt.

14. The process according to claim 9, wherein the temperature of the air stream after the heating stage is in the range of 130 to 140? C.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic representation of a urea granules production process according to an embodiment of the invention.

DETAILED DESCRIPTION

(2) A urea melt 1 is obtained in a urea plant comprising at least a synthesis section, a recovery section, a section for vacuum evaporation (concentration) and a water treatment section. These sections are of standard design and are well known to the skilled person in the art and for those reasons, are not represented herein. The urea plant producing the melt 1 may be a urea stripping plant.

(3) The urea melt 1 is sent together with a recovered urea solution 2 to a granulator 7 (fluidised bed) wherein the urea melt is solidified in the form of granules 9. The heat of solidification is removed via a fluidising air 5 that leaves the granulator 7 as granulation offgas 8 retaining some urea dust.

(4) Heat is removed in the cooling section 10 from the solidified urea particles 9 in order to cool them to a suitable temperature for safe and conform storage and transport of this final product. Solid urea granules 14, ready for transportation or storage, are extracted from the cooling section 10.

(5) The cooling air 12 supplied by an air blower 11 comes into direct contact with the solidified urea particles in the cooling section 10. Therefore, also the cooling air withdrawn from the cooling section 10 (cooling offgas stream 13) is contaminated with some urea dust.

(6) The granulation offgas 8 and the cooling off-gas stream 13 are sent to a scrubber 3 for removal of the urea dust contained therein. More specifically the granulation offgas 8 and the cooling offgas 13 are mixed with a water-saturated air stream 18 to yield a gas mixture 30. Said water-saturated air stream 18 is extracted from an atmospheric evaporator 32 which will be described below.

(7) The gas mixture 30 is treated in the scrubber 3 with an aqueous scrubbing agent 50 to yield an aqueous urea solution 22 (containing the urea removed from the gas) and a purified gaseous stream 24.

(8) A first portion 23 of the aqueous urea solution 22 is recycled into the scrubber 3 together with a make-up water 25. The recycled solution and the make-up water 25 form the above-mentioned aqueous scrubbing agent 50. The purified gaseous stream 24 may be vented to the atmosphere.

(9) A second portion 31 of the aqueous urea solution 22 is fed to atmospheric or sub-atmospheric evaporator 32 in presence of an air stream 6 to yield a recovered urea solution 2 and a water-saturated air stream 18. The air stream 6 lowers the partial pressure of water in the evaporator 32 and facilitates the evaporation process.

(10) The heat required for the evaporation of water is provided through steam 19.

(11) The evaporator 32 is a shell-and-tube apparatus wherein the evaporation is performed inside the tubes in a falling-film condition. The tubes are externally heated by the steam 19; the solution 31 evaporates while descending in the tubes in counter-current direction with an ascending flow of the air 6.

(12) The recovered urea solution 2 is at a concentration such that it can be directly recirculated back to the granulator via a pump 21 whilst the water-saturated air stream 18 is recirculated back and mixed with the granulation offgas 8 and with the cooling off-gas stream 13.

(13) The air stream 6 is supplied to the evaporator 32 preferably after a pre-heating stage in the heat exchanger 26.

(14) In FIG. 1 it can be appreciated that the line carrying the air stream 6 and the line carrying the fluidising air 5 are branched off from a main collector carrying a main air stream 60 so to limit the number of air blower 10 to just one. Pre-heating of the main air stream is carried out via the heat exchanger 4.