DESCRIPTION

Field of the invention The present invention refers to a method for revamping of a self-stripping urea plant. The invention also relates to a novel self-stripping process for synthesis of urea and related plant.

Prior Art

A disclosure of the self-stripping or thermal-stripping process and related plant can be found e.g. in GB 1 542 371 . The process is also referred to by the name of its developer Snamprogetti. A description is also found in Kirk-Othmer, Encyclopedia of Chemical Technology, 3 ^rd Ed, Vol 23, p. 548-562.

A self-stripping urea plant comprises: a high-pressure loop, usually operating at about 140 - 160 bar; a medium-pressure (MP) section and a low-pressure (LP) recovery section where urea solution delivered by the high-pressure loop is treated to decompose carbamate and recycle ammonia. As an indication, the MP section may operate around 10 - 20 bar and the LP section may operate at a few (e.g. 3-4) bar pressure. Common values are for example 150 - 17 - 3.5 bar for high, medium and low-pressure section, respectively. The high-pressure loop typically comprises a synthesis reactor, a steam-heated stripper, and a horizontal kettle condenser. All these items operate substantially at the same pressure.

Ammonia and CO2 are reacted in said synthesis reactor obtaining a urea aqueous solution, comprising ammonia and unconverted ammonium carbamate; said solution is heated in the high-pressure stripper to decompose the carbamate and recover ammonia; a vapour phase containing ammonia and CO ₂ produced in the stripper is condensed in the high-pressure condenser, and recycled to said reactor. The stripping process takes place with the help of heat furnished e.g. by hot steam, and without addition of an external stripping medium such as carbon dioxide (thermal stripping), or using part of the ammonia available in the solution, as a stripping agent (ammonia stripping).

In this description, the term self-stripping urea plant is used to mean a urea plant comprising a high-pressure thermal or ammonia stripping section.

There is a strong interest in the revamping of existing self-stripping urea plants. When an existing plant is revamped, one should try to maintain, whenever possible, the available equipments, for economical reasons. The applicant has noted that a major bottleneck is the capacity of the medium- and low-pressure sections. For example, when the high-pressure loop is boosted, a greater flow rate of urea solution is discharged to the downstream MP/LP sections, which means a considerably larger quantity of carbamate to decompose and ammonia to recover. The available components of said MP and LP sections, including decomposers, condensers, etc... are usually close to their maximum capacity and are not able to comply with this increased duty. Hence, the bottleneck is actually found in the MP and LP sections. Revamping also the MP and/or the LP section, together with the high-pressure section, would involve the addition or replacement of several pressure vessels and then would involve a considerable cost that may render the overall intervention less attractive.

Summary of the invention

The invention is aimed to solve the above drawback and to provide an efficient solution for boosting a self-stripping urea plant. The idea underlying the invention is to add a further pressure level to the processing of the urea solution delivered by the HP section or by the reactor. A further step of carbamate decomposition and carbamate recovery is added upstream the MP section, i.e. at a pressure greater than the original medium pressure of the plant. According to different embodiments of the invention, said further step of carbamate decomposition may receive a portion of the urea solution delivered by the reactor and/or a portion or the full amount of the concentrated urea solution obtained from high-pressure stripping.

Hence it is proposed a new method for modifying a self-stripping urea plant, said plant comprising a high pressure urea synthesis section which includes at least a reactor, a thermal or ammonia stripper and a condenser, a medium pressure treatment section and a low pressure recovery section, the method being characterized by the following steps:

- an intermediate section is installed and connected between the high- pressure synthesis section and the medium-pressure section;

- a stream of urea solution, which is produced in the high pressure section, is directed to said intermediate section; - a flow of concentrated solution from said intermediate section is directed to said medium pressure section;

- said intermediate section being suitable to decompose the carbamate and recover carbamate contained in said urea solution at an intermediate pressure which is lower than working pressure of said synthesis section and greater than the working pressure of said medium pressure section, obtaining said flow of concentrated solution at said intermediate pressure.

The added intermediate section comprises preferably at least a decomposition section and a recovery section, realized e.g. with a decomposer and a condenser, respectively. The steps of redirecting the urea solution to the intermediate section, and directing the concentrated solution to the downstream MP section, are carried out by providing the necessary piping, valves, pumps and auxiliary equipments, according to a per se known technique.

According to some embodiments, a stream of urea solution, which is delivered by the high pressure section and originally directed to the medium pressure section, is at least partly redirected to said intermediate section. In preferred embodiments, the full flow of urea solution delivered by the high pressure section is redirected to the newly installed intermediate section.

According to other embodiments, a stream of urea solution, which is delivered by said reactor and originally directed to said high-pressure thermal or ammonia stripper, is partly redirected to said intermediate section.

The above embodiments may be combined, i.e. the invention encompasses embodiments where a urea solution as delivered by the reactor and a more concentrated urea solution delivered by the high-pressure stripper are both directed to said intermediate section. Eventually said urea solution and concentrated (stripped) urea solution can be mixed before entering the intermediate section. In further embodiments of the invention, the intermediate section is paralleled to the high-pressure stripping section. For example, the urea solution delivered by the reactor is split into two fractions; a first fraction is directed to a high-pressure stripper, and a second fraction is directed to said intermediate section.

In some embodiments, the capacity of the high-pressure synthesis section can be increased. This increase of capacity may be achieved by modifying any of the components of the synthesis section such as reactor, stripper and condenser, or by adding further equipments to the synthesis section. The nominal pressure of said intermediate section is preferably in the range 30 to 100 bar, more preferably 70 to 80 bar. In a preferred embodiment, the equipments of the intermediate section include at least a decomposer, a condenser and a rectification column. The advantage of the invention is that the MP and LP section are de- bottlenecked by redirecting at least a part of the urea solution coming from the high-pressure stripper, or coming from the reactor, to the added intermediate section. If the synthesis section is boosted, the capacity of said synthesis section can be consistently increased while, at the same time, the duty of the downstream MP and LP sections is maintained substantially unaltered, since the extra duty is accomplished by the newly-installed intermediate section. In other words, the invention allows to boost the HP section in a significant manner and to debottleneck the downstream MP and LP section by the addition of the intermediate section. Installing an intermediate section in parallel with the high- pressure stripper, in accordance with some embodiments of the invention, has the advantage that the processing of the urea solution effluent from the reactor is fractioned between the HP stripper and the new intermediate section. This advantage is useful for example in cases where the capacity of the reactor is boosted, or where the HP stripper is the bottleneck of the plant, or where another or a further reactor is installed.

The modified plant operates, in practice, with a modified self-stripping process with a further pressure level, compared to the conventional process. Said process is a further aspect of the invention. Another aspect of the invention is a urea plant natively designed to operate according to the new process, as set out in the attached claims.

More in detail, an aspect of the invention is a self-stripping process for synthesis of urea, including high-pressure reaction, high-pressure thermal or ammonia stripping, and medium pressure and low pressure recovery, the process being characterized in that: a stream of urea solution, which comprises urea solution delivered by the reactor and/or concentrated urea solution delivered by said thermal or ammonia stripping, is subject to a process step of decomposition at an intermediate pressure, said intermediate pressure being lower than said high pressure and greater than said medium pressure, thus obtaining a concentrated solution at intermediate pressure, and in that said concentrated solution at intermediate pressure is directed to further treatment at said medium pressure.

A related urea plant comprises: a high-pressure section including at least a urea reactor, a thermal or ammonia stripper and a condenser; a medium-pressure urea recovery section and a low-pressure urea recovery section; an intermediate section which is installed and connected between said high- pressure section and said medium-pressure section, said intermediate section comprising at least a decomposition section and a recovery section operating at an intermediate pressure which is lower than working pressure of said synthesis section and greater than the working pressure of said medium pressure section, said intermediate section being connected to the high-pressure and medium pressure sections so that said decomposition section receives a stream of urea solution which comprises urea solution delivered by said reactor and/or concentrated urea solution delivered by said thermal or ammonia stripper, and a concentrated solution delivered by said intermediate decomposition section is directed to said medium pressure section.

In all the embodiments of the invention the high-pressure section may include one or more reactor(s), stripper(s) or condenser(s).

Further characteristics and advantages of the invention shall become clearer from the following description of some example embodiments, with reference to the attached drawings.

Brief description of the drawings

Fig. 1 is a scheme of a conventional self-stripping plant.

Fig. 2 is a scheme of the plant of Fig. 1 modified according to a first embodiment of the invention.

Fig. 3 is a scheme of a second embodiment of the invention. Fig. 4 is a scheme of a third embodiment of the invention.

Fig. 5 is a scheme of another embodiment of the invention.

Fig. 6 is a scheme of a further variant of the invention.

Detailed description of preferred embodiments Referring to the conventional scheme of Fig. 1 , the high-pressure (HP) section of the plant comprises a reactor 1 , a thermal stripper or ammonia stripper 2, and a condenser 3. The medium pressure (MP) section of the plant comprises a decomposition section 4 and an ammonia recovery section, which is indicated as block 5. The low pressure (LP) section comprises a decomposition section 6 and a low pressure recovery section 7. Block 8 designates a vacuum concentration section.

A carbon dioxide feed is sent to the synthesis reactor 1 via line 10. An ejector 1 1 feeds the reactor 1 with fresh ammonia 12 and ammonia 13 recovered from the MP and LP sections. A urea solution 14 comprising urea, unconverted carbamate and free ammonia is discharged from the reactor 1 . In some embodiments the fresh ammonia 12 may be fed to the recovery section 5, so that line 13 contains fresh and recovered ammonia.

The urea solution 14 is sent to the stripper 2, where said solution 14 is decomposed with the help of heat. A concentrated urea solution 15, produced in the stripper 2 and containing residual carbamate and ammonia, is fed to MP decomposition section 4; a further concentrated urea solution 16 is fed to the LP decomposition section 6. The solution 17 delivered by the LP section is further treated in the vacuum section 8, obtaining the urea product U.

A vapour phase 18 separated in the MP decomposition section 4, and containing mainly CO2, ammonia and water, is treated in the ammonia recovery section 5, which delivers a liquid ammonia stream 13 and a solution 19 containing residual carbamate. To this purpose, said ammonia recovery section 5 may comprise for example a rectifying column where excess ammonia in the stream 18 is separated, and an ammonia condenser where the liquid ammonia stream 13 is produced. Bottom liquid from said rectifying column forms the carbamate solution 19.

Said carbamate solution 19 is mixed with overhead vapors 20 released by the HP stripper 2; the resulting mixed stream 21 is condensed in the high-pressure condenser 3, obtaining a condensate stream which is returned to the reactor 1 via the ejector 1 1 . A vapour phase 22 separated in the LP decomposing section 6 is condensed in the LP recovery section 7, obtaining ammonia solution 23 which is pumped to the MP ammonia recovery section 5, or eventually mixed with the vapour stream 18. Further ammonia is recycled via line 24 from the vacuum section 8 to the LP section of the plant. The non-condensable gases in the output stream 25 of the condenser 3 are removed in a separator (not shown) which is located upstream the ejector 1 1 .

The stripper 2 is typically a tube-bundle, steam-heated exchanger. The urea solution 14 from reactor 1 forms a liquid film inside the tubes of the tube bundle, whereas hot steam flowing outside the tubes supplies the heat necessary to decompose the carbamate contained in the solution. Ammonia and carbon dioxide are recovered in gaseous phase 20 at the top of the stripper. The condenser 3 is usually a horizontal shell-and-tube kettle unit, where condensation is effected on the tube side, and the condensation heat is used to produce steam. The above details are known to a skilled person, and no further described.

Fig. 2 discloses a scheme of the plant modified according to the invention. The urea solution from the stripper 2 is now indicated as stream 15A, which may be larger than original stream 15 in embodiments where the capacity of the high pressure synthesis section is increased.

The concentrated solution from the HP stripper 2, namely the urea solution 15A, which is originally directed to the MP decomposing section 4, is now redirected to a newly-installed intermediate section 30, comprising a decomposing section 31 and a recovery section 32. In Fig. 2 the full output of urea solution, coming from the boosted high pressure section 1 , is redirected to said intermediate section 30, although other embodiments of the invention provide that a portion of the urea solution is redirected to the new section 30, and a remaining portion passes from stripper 2 to the MP section (Fig. 5).

The working pressure of said intermediate section 30 is between the pressure of the high-pressure section and that of MP section, for example 30 to 100 bar and more preferably 70 to 80 bar. For example the pressure of reactor 1 , stripper 2 and condenser 3 is greater than 100 bar, the medium pressure is in the range 10 to 20 bar, the low pressure is less than 5 bar. Equipments of a section operating at a certain pressure, such as reactor 1 , stripper 2 and condenser 3, operate substantially all at the same pressure, with small differences that may be due to pressure losses in the connections, valves, etc...

The decomposition section 31 comprises at least a decomposer and operates substantially in the same way as the MP decomposition section 4, producing the concentrated solution 15B and a vapour phase 33 containing CO2, ammonia and water, which is condensed in the recovery section 32. The liquid carbamate solution 19 from the MP ammonia recovery section 5 is deviated and pumped to said intermediate recovery section 32, to provide condensation of said vapour phase 33. The condensate 34 is a carbamate solution containing ammonia; this solution 34 is then directed to the condenser 3 (as shown) or, if appropriate, to the reactor 1 . Directing some liquid carbamate solution to the reactor instead of condenser may be appropriate in order to avoid excessive liquid amount in the ejector 1 1 . ln some embodiments the flow of said carbamate solution 19 is split into two parts. A first part is directed to the intermediate recovery section 32 and the remaining part is directed to the condenser or reactor, preferably being mixed with the carbamate solution 34. Fig. 6 is an example of one of such embodiments, where a portion 19A of the carbamate solution 19 delivered by the MP recovery section 5 is deviated to the new recovery section 32. The condensate 34 is mixed with the remaining fraction 19B of said carbamate solution 19, and the mixture is then directed to the condenser or to the reactor.

Oxygen which is introduced for passivation is preferably passed also through the new section 30. For example a gaseous current containing oxygen (not shown) is usually directed from condenser 3 to the MP decomposition section 4 and finally vented from the ammonia recovery section 5; in the modified plant this current will be directed from the condenser 3 to decomposition section 31 , then from decomposition section 31 to recovery section 32, and again to MP sections 4 and 5.

The carbamate solution 34 is available at the pressure of the intermediate section 30 (e.g. 70 bar) and must be elevated at the higher pressure of the condenser 3 or reactor 1 . To this purpose, an option of the invention is to use high-pressure pumps that are already available on site and were originally used to pump the stream 19 from MP recovery section 5 to the condenser 3. Said pumps could require slight adaptation due to higher suction pressure but their use may help reducing costs. New pumps will be installed to raise the pressure of the solution from medium to intermediate level (e.g. from 20 bar to 70 bar): these pumps are however less expensive than those adapted to reach the higher pressure of the synthesis loop.

Fig. 3 illustrates an embodiment where a portion of the urea solution delivered by the reactor is sent directly to intermediate-pressure decomposition, bypassing the high-pressure stripping. Urea solution effluent 14 from the reactor 1 is divided into two portions. A first portion 14A bypasses the HP stripper 2 and is directed to the intermediate section 30, e.g. to decomposition section 31 . Said first portion 14A of the urea solution can be merged with the stripped solution 15A before entering the section 31 , or can be sent separately to section 31 , these two variants being equally applicable. A remaining portion 14B of the reactor effluent 14 is sent to the HP stripper 2. The concentrated solution 15B delivered by said decomposition section 31 is then directed to the MP section 4.

Fig. 4 illustrates an embodiment where the high-pressure stripping section and the intermediate-pressure section operate substantially in parallel. A first part 14A of the urea solution 14 is directed to the intermediate-pressure section 30; a second part 14B of said urea solution 14 is directed to the HP stripper 2. The high-pressure concentrated solution 15 and the intermediate-pressure concentrated solution 15B delivered by said section 30 are merged to form a stream 15C directed to the MP section 4.

Fig. 5 illustrates a further embodiment where the urea solution 15A directed to the intermediate section 30 is a portion of the concentrated urea solution coming from the HP stripper 2; a remaining portion 15 of said urea solution passes from stripper 2 to the MP section 4. As an example, said remaining portion 15 is merged with the concentrated urea solution 15B coming from the section 31 . The embodiment of Fig. 5 is substantially a variant of the embodiment of Fig. 2; the same variant is also applicable to other embodiments of the invention such as those of Figs. 3 or Fig. 4.

Expansion valves and pumps/compressors are not shown. As apparent to a skilled reader, any stream passing from an item at a higher pressure to an item operating at a lower pressure will be suitably expanded. Vice versa, streams passing to a higher pressure will need compression / pumping. It should be noted that schemes of Figs. 1 to 6 are simplified block schemes and auxiliary items, as well as compressors, pumps and valves, are not shown.