Method for revamping a self-stripping urea plant

Abstract

A method for revamping a self-stripping urea plant comprising the installation of a new CO2-stripping synthesis section (6), wherein at least part of the aqueous urea solution (10) leaving said new section (6) is directed to the existing low pressure recovery section (4) of the self-stripping plant, by-passing the existing self-stripping high-pressure section (2) and medium pressure treatment section (3).

Claims

1. A method for revamping a self-stripping urea plant, said plant comprising: a high pressure urea synthesis section including at least a reactor, a thermal stripper or an ammonia stripper and a condenser; a medium pressure treatment section; a low pressure recovery section, the method comprising the following steps: installation of a CO.sub.2-stripping synthesis section, where ammonia and carbon dioxide are converted to an aqueous urea solution according to CO.sub.2-stripping process, wherein said new CO.sub.2-stripping section comprises at least: a synthesis reactor, a stripper, and a condenser, which are part of a high-pressure loop, and wherein at least a portion of the aqueous urea solution leaving said new CO.sub.2-stripping section is directed to said existing low pressure recovery section, by-passing the existing self-stripping high-pressure section and medium pressure treatment section.

2. The method according to claim 1, wherein said new CO.sub.2-stripping section further comprises a scrubber.

3. The method according to claim 1, said high-pressure loop operating substantially at the same pressure as the existing self-stripping synthesis section.

4. The method according to claim 1, wherein a first portion of the aqueous urea solution leaving said new CO.sub.2-stripping section is directed to said low pressure recovery section, while a second remaining portion of said solution is directed to said medium pressure treatment section.

5. The method according to claim 4, said first portion of urea solution being greater than said second portion.

6. The method according to claim 1, wherein a portion of liquid ammonia and/or a portion of carbamate solution, which are recovered from the existing medium-pressure section, is fed to the new CO.sub.2-stripping section.

7. The method according to claim 6, wherein said portion of liquid ammonia and/or portion of carbamate is fed to said CO.sub.2-stripping section via existing pumps of the self-stripping plants.

8. The method according to claim 1, wherein: the existing self-stripping urea plant comprises more than one urea production lines in parallel, the aqueous urea solution leaving said new CO.sub.2-stripping section and by-passing the medium-pressure sections is split into a plurality of streams, and each of said streams is directed to a respective low-pressure recovery section of the existing urea production lines.

9. The method according to claim 8, wherein some of the urea solution from the new CO.sub.2 stripping section is also directed to one or more of the medium-pressure sections.

10. The method according to claim 8, wherein respective portions of liquid ammonia and/or carbamate solution, recovered from the medium-pressure sections of the urea production lines, are directed to the new CO.sub.2 stripping section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme of a self-stripping urea plant revamped in accordance with an embodiment of the invention.

(2) FIG. 2 is a scheme of a self-stripping urea plant with two lines in parallel, which is also revamped according to another embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(3) Referring to FIG. 1, the block 1 denotes a carbon-dioxide compression section and blocks 2 to 5 denote a conventional self-stripping urea plant. More in detail, block 2 denotes a self-stripping HP section, block 3 is a medium-pressure section, block 4 is a low-pressure recovery section and block 5 is an optional finishing section.

(4) These sections are known in the art and need not be described in a full detail. Basically, the high-pressure section 2 comprises a reactor, a thermal stripper or ammonia stripper, and a condenser; the medium pressure section 3 normally comprises a decomposition section and an ammonia recovery section; the low pressure section 4 comprises another decomposition section and an ammonia recovery section, operating at a lower pressure.

(5) The finishing section 5 may include a vacuum concentration section and equipments suitable for the conversion of concentrated urea into a solid product U, such as a granulator or a prilling tower.

(6) Usually the section 2 is run at a pressure of around 120-140 bar, the section 3 at around 20 bar and the section 3 at around 2-4 bar, but these values may vary according to the particular plant which is revamped.

(7) FIG. 1 shows also the inputs of fresh carbon dioxide CO2 and the input of fresh ammonia NH3. The fresh ammonia may enter the medium-pressure section 3 (as shown) and thereafter reach the reactor of section 2 together with ammonia recovered from the MP section 3 and LP section 4.

(8) The invention provides the installation of a CO2-stripping synthesis section 6, where ammonia and carbon dioxide are converted to an aqueous urea solution according to the known CO2-stripping process.

(9) Said new section 6 receives a carbon dioxide input 7 from the existing carbon-dioxide compression section 1, although in some embodiments, if necessary, a new compression section may be installed or the existing one may be revamped.

(10) Ammonia input for said section 6 is given by an ammonia flow 8 and a flow of carbamate 9 coming from the existing MP section 3, which means that at least part of the ammonia and carbamate obtained in said section 3 (originally directed to the section 2) will be redirected to the new CO2-stripping section 6.

(11) In some cases, fresh ammonia may also be fed to the new CO2-stripping section 6. Anyway, the way of feeding fresh ammonia is not essential for the invention.

(12) Said new section 6 operates at a high pressure which is preferably the same pressure (or substantially the same pressure) of the section 2. As mentioned before, said pressure is preferably in the range 120 to 150 bar and more preferably is around 140 bar.

(13) The aqueous urea solution 10 leaving said CO2-stripping section 6, as shown, is directed to the low pressure recovery section 4, by-passing the existing self-stripping high-pressure section 2 and the medium pressure treatment section 3. In some embodiments of the invention, a portion of the urea solution leaving the CO2-stripping section 6 may be also directed to the medium-pressure section 3, as shown by the dotted flow line 17. This embodiment may be applied, for example, when the available MP section is still able to cope with a certain increase of capacity. Since, however, the margin of the MP section, if any, will be limited, the portion of said urea solution directed to the LP section 4 is normally greater than the portion sent to the MP section 3.

(14) It should be noted that the urea solution 10 has a low content of ammonia, compared to the effluent of the original HP section 2 which, in most cases, contains a large ammonia excess. Hence, an advantageous effect of the invention is that the N/C ratio in the LP section 4 is lowered and, as explained above, less water will be found in the reactor of section 2, to the advantage of the conversion yield.

(15) FIG. 2 shows an example of a preferred application of the invention to a urea plant with several production lines in parallel. Two lines A, B are shown, but the invention is equally applicable with any number of urea production lines in parallel.

(16) Each line includes a high-pressure section 2A, 2B, a medium-pressure section 3A, 3B, a low pressure section 4A, 4B and optionally a finishing section 5A, 5B. The compression section 1 may be separate or in common. In each line A, B, the flows exchanged between said sections are similar to FIG. 1 and, hence, they are not described in detail.

(17) The urea effluent 10 from the new section 6, and bypassing the MP sections, is split into a first part 10A directed to the low pressure section 4A of the first line, and a second part 10B directed to the low pressure section 4B of the second line. Similarly, the ammonia input to the section 6 is provided by respective streams 8A, 8B of ammonia and 9A, 9B of carbamate, coming from the medium-pressure sections of the urea lines A and B. In some cases, as mentioned above, fresh ammonia may also be fed to the new CO2-stripping section 6.

(18) In a similar way, the invention can be applied to plants with any number of urea production lines. Also in the embodiment of FIG. 2, some of the urea effluent from the new section 6 may be directed to one or more of the medium-pressure section(s), whenever it is possible and deemed appropriate, as denoted by line 17.

(19) Turning back to FIG. 1, the lines 11 and 12 denote the aqueous solution of urea from the HP section 2 to the MP section 3 and from the latter to the LP section 4. The flow line 13 indicates the concentrated urea (usually around 70%) leaving the LP section 4, which can be exported as such or further processed in the finishing section 5.

(20) The lines 14 and 14′ denotes generally the recycle from the MP section to the HP synthesis section, which may comprise: a first current of liquid ammonia stream which is mixed with a fresh ammonia input and fed to the high-pressure reactor of the section 2; a second current comprising the bottom solution of a rectifying column, which is mixed with overhead vapors of the high-pressure stripper and sent to the high-pressure condenser of the section 2. Said two currents, denoted by the lines 14 and 14′, may be mixed in some embodiments.

(21) The line 15 denotes a recycle from LP to MP section, which may consist predominantly of ammonia-rich carbonate solution condensed in a low pressure condenser of said LP section 4. The line 16 denotes a further current of carbonate solution which is recycled from the finishing section 5, for example in a vacuum concentration section.