Apparatus and method for treatment of process vapours coming from a vacuum concentration section of a urea plant

Abstract

An apparatus for treatment of process vapours coming from a vacuum concentration section of a urea plant, comprising a vacuum system having a plurality of successive condensation stages, connected in series by respective line portions and crossed in series by process vapours to be treated; the apparatus has at least one primary steam condensate inlet for feeding steam condensate to the vacuum system and positioned, with reference to a circulation direction of the process vapours in the vacuum system, upstream of at least one selected condensation stage, or in at least one selected condensation stage.

Claims

1. An apparatus comprising: a vacuum system having a plurality of successive condensation stages to treat, in series, process vapours, the plurality of successive condensation stages connected, in series, by a plurality of successive line portions; at least one primary steam condensate inlet that feeds steam condensate to the vacuum system, wherein the at least one primary steam condensate inlet defines a main injection point of steam condensate in the vacuum system where at least 50%, by weight, of all steam condensate is injected to the vacuum system and the at least one primary steam condensate inlet is arranged at at least one of: upstream, relative to a circulation direction of the process vapours through the vacuum system, of a selected one of the condensation stages, and in the selected one of the condensation stages; and a washing unit connected to a last one of the condensation stages by a last line portion of the plurality of successive line portions, the washing unit having: an inlet connected to a supply line fed with steam condensate, a condensate outlet connected to a condensate line from which steam condensate is used for washing is collectable, and a gas outlet connected to a discharge line from which incondensable purge gases are collectable.

2. The apparatus of claim 1, wherein: each condensation stage has an inlet connected to a respective one of the plurality of successive line portions that feeds the process vapours to treat; and the at least one primary steam condensate inlet is arranged on at least one of the line portions and downstream of a pressure boost device arranged along the line portion, the pressure boost device to increases a pressure of a flow of process vapours circulating in the line portion.

3. The apparatus of claim 1, wherein the at least one primary steam condensate inlet is positioned inside a condenser of the selected one of the condensation stages.

4. The apparatus of claim 1, wherein: the vacuum system comprises, in the circulation direction of the process vapours, a first condensation stage and at least one further condensation stage including at least one second condensation stage up to the last one of the condensation stages, and the at least one primary steam condensate inlet is positioned at one of: upstream of the at least one further condensation stage following the first condensation stage, and inside the at least one further condensation stage.

5. The apparatus of claim 4, wherein the selected one of the condensation stages is the second condensation stage in the circulation direction of the process vapours through the vacuum system.

6. The apparatus of claim 5, the at least one primary steam condensate inlet is positioned upstream of the second condensation stage, on the line portion connecting the first condensation stage to the second condensation stage and downstream of a pressure boost device arranged at one of: along the line portion connecting the first condensation stage to the second condensation stage and inside a condenser of the second condensation stage.

7. The apparatus of claim 1, further comprising a plurality of primary steam condensate inlets, wherein each primary steam condensate inlet is positioned at at least one of: upstream of a respective one of the condensation stages of the vacuum system and in a respective one of the condensation stages of the vacuum system.

8. The apparatus of claim 1, wherein: the condensation stages operate at increasing pressure, and each line portion connecting two successive condensation stages is coupled with a pressure boost device to increase a pressure of a flow of process vapours circulating in the line portion from one of the two successive condensation stages to the next of the two successive condensation stages.

9. The apparatus of claim 8, wherein the pressure boost devices comprise steam ejectors fed with steam acting as a motive fluid.

10. The apparatus of claim 1, wherein the condensation stages comprise respective condensers including respective heat exchangers in which a flow of process vapours to treat transfers heat to a cooling fluid circulating in a cooling circuit connecting, in parallel and with respect to the cooling fluid, at least two of the condensers.

11. The apparatus of claim 10, wherein the cooling circuit connects, in parallel, the condensers of a plurality of condensation stages following a first condensation stage which the flow of process vapours to treat first pass through.

12. The apparatus of claim 1, wherein the vacuum system comprises a pressure boost device positioned along the last line portion exiting from the last one of the condensation stages to increase a pressure of a flow of vent gas discharged from the vacuum system.

13. A method for treating process vapours coming from a vacuum concentration section of a urea plant, the method comprising: condensing the process vapours in a plurality of successive condensation stages, connected in series, of a vacuum system to create vacuum conditions in the vacuum concentration section positioned upstream of the condensation stages, wherein the vacuum system comprises the condensation stages and a washing unit connected to a last one of the condensation stages; supplying steam condensate at at least one main injection point where at least 50%, by weight, of all steam condensate is fed to the vacuum system, the at least one main injection point being positioned at at least one of: upstream, relative to a circulation direction of the process vapours through the vacuum system, of at least one selected condensation stage and inside the at least one selected condensation stage; and washing, in the washing unit, incondensable purge gas exiting from the last one of the condensation stages with steam condensate to reduce an amount of NH3.

14. The method of claim 13, further comprising increasing the pressure of a flow of process vapours upstream of the at least one selected condensation stage by a pressure boost device arranged upstream of the at least one selected condensation stage, wherein the main injection point is positioned downstream of the pressure boost device.

15. The method of claim 13, wherein the main injection point is positioned inside a condenser defining the at least one selected condensation stage.

16. The method of claim 13, wherein: the process vapours run through the condensation stages in a predetermined circulation direction, passing through a first condensation stage and then to at least one further condensation stage including at least a second of the condensation stages up to the last one of the condensation stages, and the main injection point is positioned at one of: a position upstream of one of the further condensation stages following the first condensation stage, and a position inside one of the further condensation stages.

17. The method of claim 16, wherein the main injection point is positioned at one of: a position upstream the second condensation stage and a position inside the second condensation stage.

18. The method of claim 13, further comprising suppling steam condensate at a plurality of injection points, wherein each injection point is positioned, with respect to the circulation direction of the process vapours in the vacuum system, upstream of a respective one of the condensation stages and inside a respective one of the condensation stages.

19. The method of claim 13, further comprising increasing a pressure of a flow of process vapours between each condensation stage and the next, if any, condensation stage.

20. The method of claim 19, wherein the pressure rise is obtained by steam ejectors fed with steam as a motive fluid.

21. The method of claim 13, wherein in said condensation stages, heat is transferred from the process vapours by heat exchange with a cooling fluid which is fed in parallel to at least one of the condensation stages.

22. The method of claim 21, wherein the cooling fluid is fed in parallel to a plurality of condensation stages following a first condensation stage that the process vapours first pass through.

23. The method of claim 13, further comprising increasing a pressure of a flow of vent gas exiting from the last one of the condensation stages by a steam ejector.

24. A system comprising: a vacuum concentration section, and an apparatus to treat process vapours coming from the vacuum concentration section, the apparatus comprising: a vacuum system having a plurality of successive condensation stages to treat, in series, the process vapours, the plurality of successive condensation stages connected in series by a plurality of successive line portions; at least one primary steam condensate inlet that feeds steam condensate to the vacuum system, wherein the at least one primary steam condensate inlet defines a main injection point of steam condensate in the vacuum system where at least 50%, by weight, of all steam condensate is injected to the vacuum system and the at least one primary steam condensate inlet is arranged at at least one of: upstream, relative to a circulation direction of the process vapours through the vacuum system, of a selected one of the condensation stages, and in the selected one of the condensation stages; and a washing unit connected to a last one of the condensation stages by a last line portion of the plurality of successive line portions, the washing unit having: an inlet connected to a supply line fed with steam condensate, a condensate outlet connected to a condensate line from which steam condensate is used for washing is collectable, and a gas outlet connected to a discharge line from which incondensable purge gases are collectable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further characteristics and advantages of the present disclosure will become clearer from the description of the following non-limitative embodiments, referring to figures in the accompanying drawings, in which:

(2) FIG. 1 is a schematic view of a urea production plant (urea plant) comprising a vacuum concentration section and an apparatus for the treatment of process vapours coming from the vacuum concentration section, in accordance with the disclosure;

(3) FIG. 2 is a schematic view in greater detail of the vacuum concentration section of the urea plant in FIG. 1 and integrating the apparatus in accordance with the disclosure;

(4) FIG. 3 is a schematic view of a first embodiment of the apparatus in accordance with the disclosure; and

(5) FIGS. 4 and 5 show further alternative embodiments of the apparatus of the disclosure.

DETAILED DESCRIPTION

(6) FIG. 1 shows, in a relatively extremely schematic manner, a urea plant 1 (i.e., a plant for the production of urea).

(7) The general configuration of the urea plant 1 can be of one of several types, as can the urea production process implemented in the plant.

(8) Here, reference is made, purely by way of example, to a urea production plant/process according to the known “Snamprogetti” technology. It is understood that the disclosure is also applicable to other urea production plants/processes in which a vacuum concentration section or, in any case, at least one vacuum system is used.

(9) In the non-limitative configuration shown, the urea plant 1 comprises, albeit not necessarily: a urea synthesis reactor 6 where a reaction of urea synthesis from ammonia and carbon dioxide takes place; recovery sections 7, 8 and 9, in particular a high-pressure recovery section 7, a medium-pressure recovery section 8 and a low-pressure recovery section 9, where a urea solution produced in the reactor 6 becomes progressively concentrated with the removal of unreacted ammonia and carbon dioxide and water, and the recovered components are recirculated; a vacuum concentration section 10 provided with a vacuum system 11; a process condensate treatment section 12; and a finishing/solidification section 13, comprising, for example, a granulator or prilling tower.

(10) The reactor 6 is fed with NH3 and CO2 through respective feed lines 14 and 15, connected to respective inlets 14a and 15a from which NH3 and CO2 enter the urea plant 1. A urea circuit 16 gradually carries the urea solution produced in the reactor 6 away to the recovery sections 7, 8 and 9 and the vacuum concentration section 10, where the urea is progressively concentrated and separated from the unreacted reagents, before being sent to the finishing/solidification section 13. A recovery circuit 17 recirculates the unreacted reagents recovered by the recovery sections 7, 8 and 9 and by the process condensate treatment section 12 back to the reactor 6.

(11) Not all of the components of the various sections and the circuits that connect them are indicated and described herein, but only those useful for understanding the present disclosure.

(12) Referring also to FIG. 2, the urea plant 1 comprises an apparatus 20 for the treatment of process vapours coming from the vacuum concentration section 10 and integrating the vacuum system 11.

(13) The vacuum concentration section 10 is connected by a urea inlet line 21 and a urea outlet line 22 forming part of the urea circuit 16 to the low-pressure recovery section 9 and the finishing/solidification section 13, respectively.

(14) The vacuum concentration section 10 comprises at least one concentration stage 23, connected to the vacuum system 11 of the apparatus 20 and operating in a vacuum to process (concentrate) the urea solution arriving from the low-pressure recovery section 9.

(15) Again, depending on the type of finishing required for the product (granulated or prilled urea), the vacuum concentration section 10 may comprise several concentration stages 23 in series. For example, in the embodiment provided purely by way of example in FIG. 2, the vacuum concentration section 10 comprises two concentration stages 23.

(16) Each concentration stage 23 can be configured in various ways. In the example shown, each concentration stage 23 comprises a concentrator 24 associated with a separator 25. For example, the concentrator 24 is a heat exchanger fed with steam (generated, in particular, by the high-pressure recovery section 7), and the separator 25 is a liquid-vapour phase separator connected to the concentrator 24.

(17) The separator 25 has a top outlet 26 for the outflow of process vapours, and a bottom outlet 27 for the outflow of a concentrated urea solution.

(18) The top outlet 26 is connected to the apparatus 20 through a vapour line 28; the bottom outlet 27 is connected to the finishing/solidification section 13 through the urea outlet line 22, or to the next concentration stage 23 (if several concentration stages 23 are provided) through a connecting line 29.

(19) The vacuum system 11 ensures the vacuum conditions required in the respective concentration stage 23 (i.e., the operating pressure of the concentration stage 23 to which it is connected, through the condensation of the process vapours coming from the concentration stage 23).

(20) If several concentration stages 23 are present, they are opportunely associated with respective vacuum systems 11. The apparatus 20 that treats the process vapours of the vacuum concentration section 10 therefore comprises one or more vacuum systems 11.

(21) FIG. 3 shows a single vacuum system 11 forming part of the apparatus 20 of the disclosure.

(22) The vacuum system 11 comprises a plurality of successive condensation stages 30 at increasing pressures, connected in series by respective line portions 31 and crossed in series by the process vapours to treat.

(23) In particular, the vacuum system 11 comprises (in the order the process vapours pass through the vacuum system 11) a first condensation stage 30a and one or more further condensation stages 30b, including at least a second condensation stage 30c, up to a last condensation stage 30n.

(24) For example, each condensation stage 30 is defined by a condenser 32, in particular constituted by a heat exchanger in which the flow of process vapours to treat transfers heat to a cooling fluid, for example cooling water.

(25) In the example shown in FIG. 3, the cooling fluid (e.g., water) circulates in the condensation stages 30 in series, running through a cooling circuit 33 that connects the condensers 32 in series (in particular, the condensers 32 of the further condensation stages 30b, with exclusion of the first condensation stage 30a). The cooling fluid is first fed to the last condensation stage 30n and then in series to the further condensation stages 30b, with exclusion of the first condensation stage 30a.

(26) Each condenser 32 (heat exchanger), defining a condensation stage 30, has an inlet 35 and an outlet 36 connected to respective line portions 31 for the inflow and outflow of process vapours, and a condensate outlet 37 connected to a condensate line 38, from which the process condensates that have condensed in the condenser 32 are recovered.

(27) Each condensation stage 30 thus has an inlet 35 connected to a respective line portion 31 for feeding a flow of process vapours to be treated into the condensation stage 30, and an outlet 36 connected to a further respective line portion 31 for the outflow of process vapours treated in the condensation stage 30.

(28) The condensers 32 (i.e., the respective condensation stages 30) are connected in series by the line portions 31 and operate at increasing pressure.

(29) Each line portion 31 is provided with a pressure boost device 39, for example a steam ejector fed with steam (acting as the motive fluid), such as coming from the high-pressure recovery section 7. In the example shown in FIG. 3, the line portions 31 that connect together the further condensation stages 30b following the first condensation stage 30a are provided with respective pressure boost devices 39; it is understood that the line portion 31 that feeds the first condensation stage 30a could also be provided with a pressure boost device 39.

(30) The pressure boost devices 39 are configured to increase the pressure of the flow of process vapours circulating in the line portions 31 from one condensation stage 30 to the next.

(31) The outlet 36 of the last condensation stage 30n is connected to a washing unit 40 by a further line portion 31.

(32) The washing unit 40 is, for example, a scrubber fed with steam condensate through an inlet 41 connected to a supply line 42.

(33) The washing unit 40 has a condensate outlet 43 connected to a condensate line 44, from which the steam condensate used for washing is collected, and a gas outlet 45 connected to a discharge line 46, from which the remaining (incondensable) purge gases are collected, which are then sent to a specially provided discharge system (not shown).

(34) The process condensates coming from each condensation stage 30 and from the washing unit 40 pass through the respective lines 38 and 44 and are transferred, possibly after being collected in a tank 47 (FIG. 2) optionally fitted with a vent, through a further condensate line 48 to the process condensate treatment section 12.

(35) The vacuum system 11 of the apparatus 20 has at least one primary steam condensate inlet 50, connected by a steam condensate feed line 51 to a unit (not specifically shown; for example, located in the process condensate treatment section 12) of the urea plant 1 where steam condensate is produced (i.e., where the steam used in the urea plant 1 is condensed).

(36) In particular, the primary inlet 50 defines the main injection point of the steam condensate in the vacuum system 11, intended as the point where the main or larger part (i.e., greater or equal to any other parts and/or greater or equal to 50% by weight), of all the steam condensate fed as a whole to the vacuum system 11 is injected.

(37) In accordance with the disclosure, the primary inlet 50 is positioned upstream of at least one of the condensation stages 30 of the vacuum system 11, or in at least one of the condensation stages 30.

(38) In the embodiment shown in FIG. 3, the selected condensation stage 30, fitted with the primary steam condensate inlet 50, is the second condensation stage 30c (always in the circulation direction of the process vapours in the vacuum system 11). In other words, the primary inlet 50 is positioned upstream of the second condensation stage 30c (always with reference to the circulation direction of the process vapours in the vacuum system 11), (i.e., between the first condensation stage 30a and the next (second) condensation stage 30c).

(39) The primary inlet 50 can be positioned, for example, on the line portion 31 that connects the first condensation stage 30a to the second condensation stage 30c, downstream of the pressure boost device 39 (steam ejector) located along said line portion 31 (as shown by way of example in FIG. 3), or directly inside the condenser 32 of the second condensation stage 30c (being defined, for example, by one or more nozzles positioned on the shell side of the heat exchanger defining said condenser 32).

(40) In other embodiments, schematically shown with broken lines in FIG. 3, instead of being positioned upstream of or inside the second condensation stage 30c, the primary inlet 50 is positioned upstream of or inside the first condensation stage 30a, or any one of the further condensation stages 30b.

(41) In general, the primary inlet 50 can be positioned upstream of or inside any one of the condensation stages 30.

(42) In other embodiments yet, the vacuum system 11 of the apparatus 20 has a plurality of primary steam condensate inlets 50, connected, for example, by respective steam condensate feed lines 51, to the steam condensate production unit and positioned upstream of respective condensation stages 30 of the vacuum system 11 and/or in respective condensation stages 30 (any two or more of the condensation stages 30, or even all the condensation stages 30).

(43) Each primary inlet 50 can be positioned upstream of the respective condensation stage 30, on the line portion 31 that feeds steam condensate to the same condensation stage 30 and downstream of the pressure boost device 39 arranged along the same line portion 31, or inside the condenser 32 of the respective condensation stage 30.

(44) The flow of steam condensate fed to each condensation stage 30 through the respective primary inlet 50 can be different depending on the position of the condensation stage 30 (in other words, the primary inlets 50 and the respective lines 51 are configured to supply the same or different flows of steam condensate to the respective condensation stages 30).

(45) The main or larger part (i.e., greater or equal to any other parts and/or greater or equal to 50% by weight), of all the steam condensate fed as a whole to the vacuum system 11, is fed to the vacuum system 11 through the single primary inlet 50 or the plurality of primary inlets 50 as a whole.

(46) In use, in implementation of the method in accordance with the disclosure, the apparatus 20 operates in the following manner.

(47) The vacuum concentration section 10 receives an aqueous urea solution, also containing NH3 and CO2, through the urea inlet line 21 (FIG. 2).

(48) The urea solution becomes concentrated in the vacuum concentration section 10, producing a flow of process vapours that is sent through the steam line 28 (or respective steam lines 28) to the apparatus 20 and precisely to the vacuum system 11 (or to respective vacuum systems 11).

(49) In the vacuum system 11, the process vapours are treated in the condensation stages 30 in series: in each condensation stage 30, a process condensate is separated, which is collected through the condensate outlet 37 and sent to the process condensate treatment section 12 to recover the reagents, and a flow of uncondensed process vapours is passed to the next condensation stage 30.

(50) A flow of (incondensable) purge gas exits from condensation stage 30n, which is washed in the washing unit 40 with steam condensate, supplied from inlet 41, to reduce NH3.

(51) The process condensates collected from the various condensation stages 30, as well as those circulating in the vacuum system 11 and finally collected by the washing unit 40, are sent to the process condensate treatment section 12 to recover the reagents, which are recycled to the reactor 6.

(52) In the embodiment in FIG. 4, where details similar or identical to those already described are indicated with the same reference numerals, the cooling fluid (e.g., water) circulates in the condensation stages 30 in parallel (instead of in series as shown in FIG. 3), running through the cooling circuit 33 that connects the condensers 32 in parallel with respect to the cooling fluid (in particular, the condensers 32 of the further condensation stages 30b, with exclusion of the first condensation stage 30a).

(53) In the embodiment in FIG. 5, where details similar or identical to those already described are indicated with the same reference numerals, the vacuum system 11 is provided with a further final pressure boost device 52, in particular a steam ejector, located along the line portion 31 leaving the last condensation stage 30n, to increase the pressure of the purge gas leaving the vacuum system 11 to a suitable level to send said purge gas to a specific treatment unit (not shown).

(54) It should be appreciated that in the embodiments in FIGS. 4 and 5, the apparatus 20 may comprise a single primary inlet 50, positioned upstream of or inside any one of the condensation stages 30, or a plurality of primary inlets 50, positioned upstream of or inside any respective condensation stages 30.

(55) Finally, it is understood that further modifications and variants can be made regarding the apparatus and method described and illustrated herein without departing from the scope of the appended claims. Accordingly, various changes and modifications to the presently disclosed embodiments will be apparent to those skilled in the art.