Purge gas conditioning of NH3 plants
Embodiments of the disclosure pertain to the conditioning of the purge gas stream in an NH 3 synthesis plant comprising a water electrolysis unit to produce a H 2 stream, ammonia synthesis loop, and a treatment section for treating purge gas at 10-70 bar(a) using scrubbing and membrane separation.
1 . A process for conditioning a purge gas stream in an ammonia synthesis plant comprising a water electrolysis unit to produce a H 2 stream and an ammonia synthesis loop for reacting said H 2 with an N 2 feed, the process comprising:
a) obtaining a purge stream from said ammonia synthesis loop, wherein said purge stream is gaseous and comprises N 2 , H 2 , NH 3 and Ar;
b) subjecting said purge stream to a treatment at a pressure of 10-70 bar (a) comprising scrubbing in a scrubber and membrane separation in a membrane separation unit to remove at least a part of the NH 3 and at least part of the H 2 from the purge gas stream, to provide a treated purge stream, a gaseous stream comprising H 2 and a liquid stream, wherein the membrane separation uses a hydrogen-selective membrane having a permeate side and a retentate side, and comprises supplying a sweep gas comprising N 2 at the permeate side of the membrane; and
c) recycling the gaseous stream in part or entirely to the ammonia synthesis loop, wherein step a) comprises reducing the pressure of the purge stream from the pressure of the ammonia synthesis loop to a pressure in the range of 10 to 70 bar(a) with a pressure reducing element that comprises an ejector that also receives a gas stream that is obtained from a step of flashing a high pressure NH 3 liquid stream in the ammonia synthesis plant.
2 . The process according to claim 1 , wherein said sweep gas stream originates from an outlet for a N 2 gas stream of a nitrogen generation unit, which unit also provides said N 2 feed.
3 . The process according to claim 2 , wherein the nitrogen generation unit is an air separation unit.
4 . The process according to claim 1 , wherein the sweep gas has a pressure of 5-20 bar, and comprises at least 98 vol. % N 2 .
5 . The process according to claim 1 , wherein the scrubbing and the membrane separation are carried out at 20-50 bar(a).
6 . The process according to claim 1 , wherein the treated purge stream is recycled in part to the ammonia synthesis loop.
7 . The process according to claim 1 , wherein the treated purge stream is in part or entirely vented or supplied to Ar recovery.
8 . The process according to claim 7 , wherein the venting is without combustion.
9 . The process according to claim 1 , wherein the scrubbing is carried out upstream of the membrane separation, wherein the scrubbing provides the liquid stream which comprises a solution of ammonia or ammonium salt, and a scrubbed purge stream, wherein the membrane separation comprises contacting the scrubbed purge stream in the membrane separation unit with the hydrogen-selective membrane having a permeate side and a retentate side, thereby obtaining the gaseous stream at the permeate side and the treated purge stream at the retentate side.
10 . The process according to claim 1 , wherein the membrane is a polymeric membrane.
11 . The process according to claim 10 , wherein the membrane is a hollow fiber membrane.
12 . The process according to claim 1 , wherein the scrubbing comprises acid scrubbing.
13 . An ammonia production process, comprising:
producing an H 2 stream by water electrolysis in the water electrolysis unit and reacting said H 2 stream with N 2 feed to form NH 3 in said an ammonia synthesis loop;
obtaining and conditioning a purge stream from the ammonia synthesis loop with the process of claim 1 .
14 . The process according to claim 1 , wherein said ammonia synthesis loop is operated at a pressure of above 100 bar(a).
15 . The process according to claim 1 , wherein said ammonia synthesis loop is operated at a pressure in the range 150-300 bar(a).
16 . An ammonia synthesis plant comprising a water electrolysis unit to produce a H 2 stream; an ammonia synthesis loop for reacting said H 2 with an N 2 feed and having an outlet for a gaseous purge stream; and a treatment section comprising a scrubber and a membrane separation unit for subjecting said purge stream to a treatment at a pressure of 10-70 bar(a) comprising scrubbing in the scrubber and membrane separation in the membrane separation unit to remove at least a part of the NH 3 and at least part of the H 2 from the purge gas stream, to provide a treated purge stream, a gaseous stream comprising H 2 and a liquid stream, wherein the membrane separation unit comprises a hydrogen-selective membrane, a permeate side compartment and a retentate side compartment, and a supply for a sweep gas to the permeate side compartment; and
a flow line for recycling the gaseous stream in part or entirely to the ammonia synthesis loop, wherein the ammonia synthesis plant comprises a flash vessel for flashing a high pressure NH 3 liquid stream from the ammonia synthesis loop, which flash vessel has a gas outlet and a liquid outlet, wherein said gas outlet is connected to an ejector provided in the flow line for the purge stream from the ammonia synthesis loop to said treatment section.
17 . The plant according to claim 16 , wherein the scrubber is arranged upstream of the membrane separation unit, wherein the scrubber has an outlet for the liquid stream which comprises a solution of ammonia or ammonium salt, and an outlet for a scrubbed purge stream, wherein the membrane separation unit has an inlet for the scrubbed purge stream in the retentate side compartment and an outlet for the gaseous stream from the permeate side compartment and an outlet for the treated purge stream from the retentate side compartment.
18 . The plant according to claim 16 , wherein said plant comprises a nitrogen generation unit providing said N 2 feed, wherein said nitrogen generation unit also has an outlet for a N 2 gas stream that is connected to said supply for said sweep gas.
19 . The plant according to claim 18 , wherein the nitrogen generation unit is an air separation unit.
20 . The plant according to claim 16 , wherein the outlet for the treated purge stream is connected to a recycle flow line to the ammonia synthesis loop.
21 . The plant according to claim 16 , wherein the outlet for the treated purge stream is connected to a vent and/or to a unit for Ar recovery.
22 . A method of modifying an ammonia synthesis plant comprising a water electrolysis unit to produce a H 2 stream; an ammonia synthesis loop for reacting said H 2 with N 2 feed and having an outlet for a gaseous purge stream; wherein the method comprises: providing the plant with
a section comprising a scrubber and a membrane separation unit for subjecting said purge stream to a treatment at a pressure of 10-70 bar(a) comprising scrubbing in the scrubber and membrane separation in the membrane separation unit to remove at least a part of the NH 3 and at least part of the H 2 from the purge gas stream, to provide a treated purge stream, a gaseous stream comprising H 2 and a liquid stream, wherein the membrane separation unit comprises a hydrogen-selective membrane, a permeate side compartment and a retentate side compartment, and a supply for a sweep gas to the permeate side compartment; and
a flow line for recycling the gaseous stream in part or entirely to the ammonia synthesis loop, wherein the ammonia synthesis plant comprises a flash vessel for flashing a high pressure NH 3 liquid stream from the ammonia synthesis loop, which flash vessel has a gas outlet and a liquid outlet, wherein said gas outlet is connected to an ejector provided in the flow line for the purge stream from the synthesis loop to said treatment section.