Control of an ammonia synthesis loop at partial load
A process for synthesis of ammonia wherein an ammonia synthesis loop includes an ammonia converter where a makeup gas is reacted to form ammonia, and the loop is controlled at a partial load by reducing the synthesis pressure and maintaining the reduced pressure within a desired range by controlling a bypass line of make-up gas of the converter.
1 . A process for synthesis of ammonia, the process comprising:
producing an ammonia make-up synthesis gas in a front-end;
raising the pressure of said make-up gas in a first compressor;
feeding high-pressure make-up synthesis gas delivered by said first compressor to an ammonia synthesis loop;
wherein said ammonia synthesis loop includes at least:
a converter where ammonia is synthesized catalytically;
a circulator, which is a compressor configured to maintain circulation in the loop and to deliver a feed gas, which includes the make-up synthesis gas, to said converter;
a converter feed line from the circulator to the converter;
a condensation section arranged downstream from the ammonia synthesis loop to receive an ammonia-containing gaseous product; and
a separation section wherein a condensate produced in said condensation section is separated into an ammonia liquid product and a gaseous recycle stream;
a recycle line from the separation section to a suction of the circulator;
wherein the ammonia synthesis loop has a full load condition corresponding to the processing of a nominal flow rate of make-up gas transferred from the front end to the ammonia synthesis loop; and
controlling the loop at a partial load condition, wherein the flow rate of make-up gas transferred from the front end to the loop is smaller than said nominal flow rate, by the following steps:
the pressure at which ammonia is synthesized is lowered to a reduced ammonia synthesis pressure, which is less than a nominal synthesis pressure at said full load condition of the ammonia synthesis loop; and
the synthesis pressure is maintained within a target range which includes said reduced synthesis pressure, by controlling a flow rate of feed gas bypassing the converter.
2 . The process of claim 1 , wherein said reduced synthesis pressure is in the range of 50% to 80% of the nominal synthesis pressure.
3 . The process of claim 1 , wherein said target range is centred at the reduced synthesis pressure, and wherein the target range is +/−15% of the reduced synthesis pressure.
4 . The process of claim 3 wherein said target range is +/−10% of said reduced synthesis pressure.
5 . The process of claim 3 wherein said target range is +/−5% of said reduced synthesis pressure.
6 . The process of claim 1 , further comprising detecting the temperature of the feed gas of at least one catalytic bed of the converter and determining a bypass flow of the converter in accordance with the detected temperature, wherein the converter includes a plurality of catalytic beds arranged in series and traversed sequentially by the feed gas, and the process includes detecting the temperature of the first catalytic bed of the sequence.
7 . The process of claim 1 , further comprising detecting the difference of temperature across the converter, which is the difference between the temperature of the feed gas entering the converter and the temperature of the ammonia-containing product withdrawn from the converter.
8 . The process of claim 1 , further comprising, when a drop or surge of the flow rate of makeup gas transferred from the front end to the synthesis loop is detected, increasing the amount of feed gas bypassing the converter in the event of a drop of flow or decreasing said amount in the event of a surge of flow.
9 . The process of claim 1 , wherein said condition of partial load includes loads until the make-up gas transferred from the frontend to the synthesis loop is 15% of the nominal flow rate.
10 . The process of claim 1 , wherein the production of makeup gas in the front end includes the production of hydrogen from a renewable energy source.