Urea process with high-temperature stripping

Abstract

A shell-and-tube stripper for carbamate decomposition and ammonia recovery from a urea solution comprising a bundle of heated tubes, said tubes being fed with said urea solution and carbon dioxide as stripping medium, the urea solutions forming a liquid falling film on the internal surface of the tubes and the carbon dioxide forming a counter-current gaseous flow; said tubes comprise an external layer made of super austenitic or super duplex stainless steel and an internal layer made of zirconium, said internal layer reaching temperatures higher than 220 C.

Claims

1. A process of stripping of an aqueous urea solution comprising unconverted carbamate ammonium and ammonia, wherein said solution is contacted with gaseous carbon dioxide acting as a stripping medium, and the stripping takes place at a synthesis pressure of said solution, wherein the process of stripping of the aqueous urea solution is performed in a falling-film shell-and-tube stripper, wherein a liquid film of said solution formed in the tubes of said stripper has a peak temperature of at least 220 C.

2. The process according to claim 1, said peak temperature being in the range 220 to 250 C.

3. The process according to claim 1, wherein the tubes of said stripper include an outer layer of a stainless steel and a heat and corrosion resistant inner layer.

4. The process according to claim 3, said outer layer being of a super austenitic stainless steel or a super duplex stainless steel.

5. The process according to claim 3, said heat and corrosion resistant inner layer being made of zirconium.

6. The process according to claim 1, wherein said tubes are heated by a condensing steam at a pressure of at least 30 bar.

7. The process according to claim 1, further comprising providing a stripped solution comprising urea in a concentration of at least 70% by weight.

8. The process according to claim 1, further comprising providing a stripped solution containing an amount of unconverted ammonia corresponding to a concentration not greater than 2% by weight and/or an amount of unconverted ammonium carbamate corresponding to a concentration not greater than 4% by weight.

9. The process according to claim 1, said liquid film having: a first temperature at the tube inlet, said first temperature being in the range 175 to 195 C.; a peak temperature in an intermediate region of the tube in the range 220 to 250 C.; a second temperature at the tube outlet, said second temperature being 5 to 10 C. greater than said first temperature.

10. A process for the synthesis of urea from ammonia and carbon dioxide, including the formation of an aqueous urea solution at a synthesis pressure, said solution comprising unconverted ammonium carbamate and ammonia, and further including a stripping of said solution with gaseous carbon dioxide as a stripping medium and at said synthesis pressure, said stripping being performed in a falling-film shell-and-tube stripper and producing a stream of a stripped solution, wherein during the stripping process a liquid film of said solution formed in the tubes of said stripper has a peak temperature of at least 220 C.

11. The process according to claim 10, wherein said stripped solution has a urea concentration of at least 70% by weight and/or contains an amount of unconverted ammonia corresponding to a concentration of not greater than 2% by weight, and/or an amount of unconverted ammonium carbamate corresponding to a concentration not greater than 4% by weight.

12. The process according to claim 11, wherein at least part of said stripped solution is sent directly to a vacuum concentration section.

13. The process according to claim 8, wherein the stripped solution contains an amount of unconverted ammonia corresponding to a concentration not greater than 1% by weight and/or an amount of unconverted ammonium carbamate corresponding to a concentration not greater than 3% by weight.

14. The process according to claim 9, wherein said first temperature being in the range 180 to 185 C., and/or said peak temperature being 240 C., and/or said second temperature being in the range 190 to 195 C.

15. The process according to claim 11, wherein said stripped solution contains an amount of unconverted ammonia corresponding to a concentration not greater than 1% by weight and/or an amount of unconverted ammonium carbamate corresponding to a concentration not greater than 3% by weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a block scheme of a urea plant according to an embodiment of the invention.

(2) FIG. 2 is a first embodiment of the high-pressure synthesis loop of the plant of

(3) FIG. 1.

(4) FIG. 3 is another embodiment of the high-pressure synthesis loop of the plant of FIG. 1.

(5) FIG. 4 is a schematic illustration of the stripper of the synthesis loop of FIG. 2 or 3.

(6) FIG. 5 represents a cross-section of a tube of the stripper of FIG. 4.

(7) FIG. 6 illustrates a thermal profile of the liquid film flowing inside the tube of FIG. 5.

(8) FIG. 7 is a scheme of a urea plant according to another embodiment of the invention.

(9) FIG. 8 is a scheme of the high-pressure synthesis loop of the plant of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(10) FIG. 1 illustrates a block scheme of a urea plant according to a first embodiment of the invention.

(11) The urea plant comprises basically: a high-pressure synthesis section (SYN) 1, a recovery section (REC) 2, a vacuum concentration section (CONC) 3, a finishing section (FIN) 4, a condensation and recycle section (COND) 5, a vacuum condensation unit (VAC) 6 and a waste water treatment section (WTS) 7.

(12) Ammonia feed 10 and carbon dioxide feed 11 are reacted in the synthesis section 1 to produce an aqueous solution 12 of urea; said solution 12 is further treated in the recovery section 2 which comprises at least one decomposition unit where unconverted carbamate contained in the solution 12 is decomposed to ammonia and carbon dioxide.

(13) The output of said section 2 is an aqueous solution 13 which is sent to the vacuum concentration unit 3 where water is removed until a desired concentration is reached, suitable for finishing in the subsequent section 4.

(14) The stream 14 delivered by said unit 3 for example may be a solution of 95% or more urea, which is suitable for granulation, or a urea melt having a purity of 99.7% or more which is suitable for prilling. Urea 15 in a solid form is produced in the section 4.

(15) Water removed from the solution 13 is sent via line 16 to the condensation unit 6 and waste water treatment section 7. The stream 16 is still contaminated with some ammonia and carbon dioxide, which are recycled to the synthesis section 1 via line 17, through the condensation and recycle section 5 and carbamate recycle line 18.

(16) As illustrated in FIG. 2, the synthesis section 1 includes a reactor 100, a stripper 101, a condenser 102 and a scrubber 103.

(17) The effluent 104 of the reactor 100 is sent to the stripper 101 which produces the solution 12; overhead gas 105 of the stripper, which is predominantly ammonia and carbon dioxide, is condensed in the condenser 102 and recycled to the reactor 100.

(18) The overhead gas 106 of the reactor 100 is scrubbed in the scrubber 103, which is also supplied with the carbamate recycle 18, to separate inert gas 19; the carbamate solution from the scrubber is sent to the reactor, together with the ammonia feed 10, via the condenser 102.

(19) The inert gas 19 from the scrubber 103 can be purged to the atmosphere or better can be further treated in a washing column prior to the discharge,

(20) FIG. 3 illustrates a scheme of the synthesis section 1 according to the Split-Flow-Loop technology and the Full-Condenser technology.

(21) A portion 10a of the ammonia feed is sent to the reactor 100 and a portion 10b is sent to the condenser 102.

(22) The effluent 104 of the reactor 100 is sent to the stripper 101 together with carbon dioxide 11, producing the solution 12 and the gas 105 predominantly containing ammonia and carbon dioxide.

(23) A first portion 105a of the overhead gas of the stripper 101 is sent to the condenser 102, while a second portion 105b is sent to the reactor 100. Preferably, said first portion 105a is about two thirds of the total gas 105 coming from the stripper and the second portion 105b is the remaining one thirds.

(24) Said first portion 105a is at least partially condensed to carbamate 117, and feeds the reactor 100 together with ammonia 10a. The remaining non-condensed vapours 118, which contain some inert gases, are sent the scrubber 103 together with the inert gases they contain. Said non-condensed vapours 118 are a small amount with respect to the condensate 117.

(25) The overhead gas 106 of the reactor 100 is scrubbed in the scrubber 103 to separate inert gases 19 contained therein and provide a carbamate solution 119. Said carbamate solution 119 is sent to the condenser 102 together with the ammonia feed 10b and said inert gases 19 are purged to the atmosphere or further treated in a washing column prior to the discharge.

(26) FIG. 4 illustrates a preferred embodiment of the stripper 101 (high-temperature stripper). The stripper 101 is a steam-heated tube-bundle equipment including a bundle of tubes 110 externally heated by steam entering at steam inlet 111 and leaving at steam or condensate outlet 112. The reactor effluent 104 is fed to the inlet of tubes 110 (tube side) via inlet 113 and suitable distribution means. Carbon dioxide 11 is also fed to the tubes via carbon dioxide inlet 114 to act as a stripping agent. Inside the tubes, the aqueous solution 104 forms a liquid film which is contacted with the rising gaseous stream of carbon dioxide 11, providing the stripped solution 12 and the overhead gas 105.

(27) FIG. 5 shows a preferred embodiment of tubes 110. Each tube 110 is preferably a bimetallic tube including an outer layer 120 made of a suitable stainless steel and an inner layer or inner coating 121 made of zirconium. Preferably each tube 110 has an inner diameter of around 25 mm and a thickness of around 3 mm.

(28) FIG. 6 shows a temperature profile of the liquid film F flowing inside a tube 110. The temperature is pictured as a function of a coordinate x along the axis of the tube 110. The origin of the coordinate x is assumed at the liquid inlet, i.e. upper end of the tube 110.

(29) The liquid film F has an inlet tube temperature Ti at coordinate x=0 which is for example 185 C. and an outlet tube temperature To at coordinate x=Lt where Lt is the length of the tube. The outlet temperature To is slightly greater than said inlet temperature, for example 190 C. The temperature T is ascending in a first portion of tube 110, namely from coordinate x=0 to x* and then descending from x* to L. The peak temperature T* measured at coordinate x* is above 220 C., for example 240 C. in the shown example. The temperature drop in the second portion of the tube (x* to Lt) is due to the cooling effect of the fresh carbon dioxide 11. The evaporation of ammonia and some ammonium carbamate due to mass transfer removes some enthalpy from the liquid phase and, as a consequence, the temperature decreases.

(30) It can be noted that the temperature of the film is above 200 C. along a substantial portion of the tube 110. As a consequence, most of the unconverted carbamate contained in the effluent 104 is dissociated and the output solution 12 has a reduced amount of residual carbamate if compared to the prior art.

(31) Looking at FIG. 1, it shall be noted that the invention reduces drastically the flow rate of condensate 18 compared to a prior art process using a conventional stripper. The condensate flow 18 contains water which is detrimental to the synthesis of urea; then the invention allows improve the conversion yield in the synthesis section 1.

(32) FIG. 7 is a further embodiment of the invention which is a variant of FIG. 1 where the effluent 12 is directly sent to the vacuum concentration section 3, that is without the need of recovery section 2. This results in a drastic simplification of the plant and reduction of the related cost. The embodiment of FIG. 7 is made possible by the inventive stripper which is able to deliver a solution 12 containing only 1-2% of residual carbamate. Such a solution can be admitted directly to the vacuum concentration section 3.

(33) For example the stream 12 may have a pressure of 140 bar, a temperature around 190 C. and the following composition (% weight): Urea 70%; water 25%; ammonia not greater than 1-2%; ammonium carbamate not greater than 3-4%.

(34) FIG. 8 illustrates a high pressure synthesis loop of the plant of FIG. 7 where the reactor 100 and the condenser 102 are integrated in a single equipment 101. This solution further contributes to reduce the plant cost and to simplify the scheme of the plant.

(35) The invention can be applied also to revamping of existing urea plants. A method for revamping a urea plant may include replacing an existing stripper with a stripper according to the invention. In some embodiments, recovery section can be bypassed thanks to the lower content of unconverted carbamate delivered by the new stripper.

(36) For example, a urea plant for the synthesis of urea comprising a high-pressure synthesis loop with a conventional stripper, a recovery section and a vacuum concentration unit, can be revamped by: replacing the stripper with a stripper according to the invention and providing a line to feed the effluent of the newly-installed stripper directly to the vacuum concentration unit.