Method for increasing the capacity of an ammonia plant

Abstract

A method for treatment of process condensate (1) in an ammonia plant, wherein the ammonia plant comprises a front-end section producing a make-up gas and a synthesis section where the make-up gas is reacted to ammonia, and said process condensate (1) is collected from one or more equipment of the ammonia plant and is an aqueous solution comprising ammonia, carbon dioxide and methanol. Said method comprises: stripping said process condensate with low-pressure steam (4), obtaining a vapour phase (5) comprising ammonia, carbon dioxide and methanol stripped from the process condensate; condensing said vapour phase, obtaining a solution (11) enriched of ammonia and methanol; re-introducing a first portion (12) of said solution to said stripping environment; recycling a second portion (13) of said solution to said ammonia plant.

Claims

1. A method for treatment of process condensate in an ammonia plant, wherein: the ammonia plant comprises a front-end section producing a make-up gas from reforming of a hydrocarbon source, and a synthesis section where the make-up gas is reacted to ammonia; said process condensate is collected from one or more equipment of the ammonia plant and is an aqueous solution comprising ammonia, carbon dioxide and methanol, the method comprising: stripping of said process condensate in a stripping environment with low-pressure steam having a pressure of no more than 10 bar, obtaining a vapour phase comprising ammonia, carbon dioxide and methanol stripped from the process condensate; condensing said vapour phase, obtaining a condensate solution enriched of ammonia and methanol; re-introducing a first portion of said condensate solution to said stripping environment wherein said first portion of solution is reintroduced directly into said stripping environment; recycling a second portion of said condensate solution to said ammonia plant, wherein said second portion is no more than 4% by volume of said condensate solution.

2. The method according to claim 1, wherein the pressure of said stripping steam is 5 bar or less.

3. The method according to claim 1, wherein said second portion of solution is recycled to the front-end section of the ammonia plant.

4. The method according to claim 3, wherein said second portion of solution is recycled to a primary reformer of the front-end section of the ammonia plant, where said solution is added to the process stream subjected to reforming.

5. The method according to claim 1, wherein said condensate solution contains 5 to 10 mol % of carbon dioxide, 3 to 8 mol % of methanol and 7 to 13 mol % of ammonia.

6. The method according to claim 1, wherein said second portion is no more than 3% by volume of said condensate solution.

7. The method according to claim 1, wherein said second portion is 2% by volume of said condensate solution.

8. The method according to claim 2, wherein the pressure of said stripping steam is 3 to 5 bar.

9. The method according to claim 1, wherein said second portion is no more than 2% by volume of said condensate solution.

Description

DETAILED DESCRIPTION

(1) FIG. 1 shows a process condensate treatment section of an ammonia plant.

(2) Stream 1 denotes a process condensate which is basically an aqueous solution containing ammonia, methanol and carbon dioxide, in a concentration typically of some thousands of ppm, and possibly traces of other contaminants such as alcohols and other hydrocarbons.

(3) Said process condensate 1 after pre-heating in a heat exchanger 2 is sent to a stripping column 3 where it is contacted with a low-pressure stripping steam 4. Said steam 4 is preferably at a pressure of 3 to 5 bar. The stripping column 3 receives pre-heated process condensate 6 and a recirculated solution 7 which will be described later.

(4) The overhead vapour 5 emerging from the stripping column 3 contains water, ammonia, methanol and carbon dioxide stripped from the condensate 1. This vapour 5 is condensed in a condenser 8, for example discharging the condensation heat to water or steam, and the condensate 9 is sent to a separator 10.

(5) The liquid phase 11 drawn from said separator 10 is an ammonia-rich and methanol-rich solution which is split into a first portion 12 and a second portion 13.

(6) The first portion 12 is sent back to the stripping column 3 via a first pump 14 which delivers the recirculated stream 7; the second portion 13 is recycled to the ammonia plant via a second pump 15.

(7) The first portion 12 is sent directly to the stripping column 3 via said pump 14. The second portion 13 for example contains 2% or about 2% of the overhead vapour 5.

(8) Preferably said second portion 13 of the solution is recycled to a primary reformer where it is injected into the process side of the reformer, i.e. mixed with the reforming gas. To this purpose, the second pump 15 delivers a liquid stream 16 at the pressure of primary reforming, for example in the range 25 to 45 bar. This stream 16 can also be termed distilled stream.

(9) The liquid 17 from bottom of the stripping column 3, which is purified water, is partly recirculated into the column 3 after passing through a reboiler 18. Re-heated liquid 19 is introduced back into the lower part of the stripping column 3. Preferably, as shown, the reboiler 18 is heated by low-pressure stripping steam 4. Said low-pressure stripping steam 4 can also be directly fed to the stripping column 3. The remaining portion 20 is preferably cooled in the process condensate pre-heater 2 and the cooled purified water 21 is discharged or exported.

(10) The gas phase 22 from the separator 10 can be used e.g. as a fuel gas.

(11) In a plant having a capacity of 1200 MTD, wherein the process condensate 1 contains about 1000 ppm of ammonia and 1000 ppm of methanol, the recycle of stream 16 allows producing additional 4-5 MTD of ammonia.

(12) It must be noted that the invention can be used to revamp an existing plant for the synthesis of ammonia. A revamping according to the invention requires the addition of the items of FIG. 1, plus auxiliaries such as piping, valves, etc. which in any case are not expensive. Hence the plant can be revamped at an affordable cost. In some cases, a pre-existing treatment section based on stripping and discharge into atmosphere or combustion of the stripped gas can be modified to set-up a section arrangement similar to the one indicated in FIG. 1, thus eliminating a polluting emission.