CONTROL OF AN AMMONIA SYNTHESIS LOOP AT PARTIAL LOAD

Abstract

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.

Claims

1-16. (canceled)

17. 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 the synthesis section 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 the 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 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 converter; 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.

18. The process of claim 17, wherein said reduced synthesis pressure is in the range 50% to 80% of the nominal synthesis pressure.

19. The process of claim 17 wherein said target range is centred at the reduced synthesis pressure, wherein the target range is +/−15% of the reduced synthesis pressure,

20. The process of claim 19 wherein said target range is +/−10% of said pressure.

21. The process of claim 19 wherein said target range is +/−5% of said pressure.

22. The process of claim 17, further comprising detecting the temperature of the inlet gas of at least one catalytic bed of the converter and determining a bypass flow of the converter in accordance with the detected temperature(s), wherein the converter includes a plurality of catalytic beds arranged in series and traversed sequentially by the gas flow, and the process includes detecting the temperature of the first catalytic bed of the sequence.

23. The process of claim 17, further comprising detecting the difference of temperature across the converter, which is the difference between the temperature of the gas feed entering the converter and the temperature of the ammonia-containing product withdrawn from the converter;

24. The process of claim 17, further comprising detecting a drop or surge of the flow rate of flow rate of makeup gas transferred from the front end to the synthesis loop, and increasing the amount of gas in the bypass stream in the event of a drop of flow or decreasing said amount in the event of a surge of flow.

25. The process of claim 17, wherein said condition of partial load include loads until the make-up gas transferred from the frontend to the synthesis loop is 15% of the nominal flow rate.

26. The process of claim 17, wherein the production of makeup gas in the front end includes the production of hydrogen from a renewable energy source.

27. A method for controlling an ammonia synthesis loop running at a partial load, wherein said ammonia synthesis loop includes: 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 the synthesis section to receive an ammonia-containing gaseous product; 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 the 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 synthesis loop, and said partial load corresponds to a condition wherein an amount less than said nominal flow rate is transferred from the front end to the loop, the method comprising: a) reducing the pressure at which ammonia is synthesized to a reduced ammonia synthesis pressure, which is less than a nominal synthesis pressure at full load of the converter, being 50% to 80% of the nominal synthesis pressure; b) controlling the synthesis pressure, in accordance with the load of the converter, so that the synthesis pressure remains within a target range which includes said reduced synthesis pressure; c) said step b) includes bypassing the converter with a portion of the converter feed gas.

28. The method of claim 27 wherein the step c) includes: separating a gas stream from said converter feed line, at a point upstream of the converter, to form a bypass stream and reintroducing said bypass stream at the suction side of the circulator or into the ammonia synthesis loop at a point downstream of said separation section.

29. The method of claim 27 wherein the target range of step b) is centred at the reduced synthesis pressure, said target range being +/−15% of the reduced synthesis pressure,

30. The method of claim 29 wherein said target range is +/−10% of said pressure

31. The method of claim 29 wherein said target range is +/−5% of said pressure.

32. The method of claim 27 wherein the step b) includes detecting the temperature of the inlet gas of at least one catalytic bed of the converter and determining a bypass flow of the converter in accordance with the detected temperature(s), wherein the converter includes a plurality of catalytic bed arranged in series and traversed sequentially by the gas flow and the method includes detecting the temperature of the first catalytic bed of the sequence.

33. The method of claim 27 wherein the step b) includes: detecting the difference of temperature across the converter, which is the difference between the temperature of the gas feed entering the converter and the temperature of the ammonia-containing product withdrawn from the converter; determining a bypass flow of the converter in accordance with the detected difference of temperature.

34. The method of claim 27, further comprising detecting a drop or surge of the flow rate of flow rate of makeup gas (2) transferred from the front end (1) to the synthesis loop, and increasing the amount of gas in the bypass stream in the event of a drop of flow or decreasing said amount in the event of a surge of flow.

35. A synthesis loop for the synthesis of ammonia from ammonia make-up synthesis gas, the synthesis loop comprising: 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 the synthesis section to receive an ammonia-containing gaseous product; a separation section wherein a condensate produced in said condensation section is separated into an ammonia liquid product and a gaseous recycle stream; and a recycle line from the separation section to the suction of the circulator; wherein the loop further includes: a bypass line arranged to take a gas stream from said converter feed line, at a point upstream of the converter and downstream of the circulator, and to reintroduce said bypass stream at the suction side of the circulator or into the ammonia synthesis loop at a point downstream of said separation section; a control system of the converter configured to control the loop at a partial load with the method according to claim 27.

36. The synthesis loop of claim 35, further comprising a flow control valve installed on said bypass line, the control system being configured to control the opening of said valve and therefore an amount of gas bypassing the converter via the bypass line.

Description

DESCRIPTION OF THE FIGURE(S)

[0076] The invention is now further elucidated with reference to FIG. 1 which illustrates a scheme of an ammonia synthesis loop according to an embodiment of the invention.

[0077] In FIG. 1, the block 1 denotes a front-end which produces a make-up ammonia synthesis gas (syngas) 2. The make-up gas 2 is fed to a main compressor 3 which delivers a compressed gas 4 to a synthesis loop 5.

[0078] The loop 5 comprises basically a circulator 6, a converter 7, a condenser 8 and a separator 9. The condenser 8 forms a condensation section and the separator 9 forms a separation section.

[0079] A gas feed is provided to the converter 7 via a converter feed line 10. A hot ammonia-containing gaseous product at line 11 is withdrawn from the converter 7 and is condensed in the condenser 8; the condensate in line 12 is separated in the separator 9 into a liquid ammonia product exported via line 13 and a gaseous phase in line 14 containing some unreacted hydrogen and nitrogen and residual vapors of ammonia, which is recycled to the suction of the circulator 6.

[0080] The feed line 10 from the circulator 6 to the converter 7 is connected to a bypass line 15 which bypasses the converter 7, the condenser 8 and the separator 9, thus connecting the delivery side of the circulator 7 back to its suction. The bypass line 15 optionally comprises a by-pass cooler 16.

[0081] The lines 10, 11 and 14 may include heat exchangers (not shown).

[0082] A valve 17 is provided on the bypass line 15 to control the flow rate through said line 15. In the example, the valve 17 has a controller 18 connected to a control unit 19.

[0083] The control unit 19 is connected to a flow gauge 20 which is arranged to detect the incoming flow rate of makeup gas from the front-end 1. For example the flow gauge 20 senses the flow rate of makeup gas 2 at the suction of the main compressor 3.

[0084] The control unit 19 is also connected to a loop pressure sensor 21 which detects the pressure at the converter inlet, for example on the line 10.

[0085] Based on the input signals from the flow gauge 20 and the loop pressure sensor 21, the control unit 19 calculates the appropriate opening of the valve 17 and, consequently, the amount of gas flowing in the bypass line 15.

[0086] An anti-surge line 22 of the main compressor 3 is also illustrated. Said line 22 includes a gas cooler 23. With the anti-surge line 22, some gas taken from the line 4 can be sent back to the suction of the main compressor 3.

[0087] In operation, the circulator 6 receives at its suction inlet 24 the compressed make-up gas 4 delivered by the main compressor 3, mixed with the gaseous phase via line 14 from top of the loop separator 9 and possibly mixed with bypass gas in the line 15.

[0088] The flow rate at the delivery side 25 of the circulator 6 can be partially deviated to the bypass line 15 according to the position of the valve 17; the remaining portion is fed to the converter 7 via the delivery line 10.

[0089] The converter 7 has a nominal ammonia synthesis pressure (also termed loop pressure) at 100% capacity, for example about 140 bar. At partial loads, the control unit 19 operates the valve 17 to vary the amount of make-up gas actually admitted to the converter 7, to keep the pressure in the loop and the converter, for example the pressure detected by the sensor 21, within a target range.

[0090] In another embodiment, the circulation in the loop and the bypass flow rate in the line 15 can be controlled on the basis of the converter delta-T, e.g. by taking a converter inlet temperature T.sub.10 at converter input line 10 and a converter output temperature T.sub.11 at line 11. In this embodiment, the control unit 19 may be configured to keep the converter delta-T (T.sub.11−T.sub.10) within a target range. Particularly, the system may be configured to avoid overheating of the converter and to avoid that the temperature falls below a minimum, which may cause the converter to lose the self-sustaining condition.

[0091] Additionally, the control unit 19 may be configured to react to a rapid change in the flow rate measured by the gauge 20. For example, the control unit 19 may command a pre-opening of the valve 17 in case of a sudden drop of the flow rate of makeup gas 2. In this step the unit 19 may operate with a feed-forward control technique. Then, the unit 19 switches to normal control to keep the loop pressure stable. Similarly, the control unit 19 may react to a surge of the flow by closing the valve.

Example 1

[0092] The following example 1 relates to a small-scale ammonia production plant with a capacity of 3 metric tons per day (MTD) of ammonia. The symbol m.sup.3/h.sub.EFF denotes the cubic meters per hour at the conditions of temperature and pressure of the synthesis loop. The symbol Nm.sup.3/h denotes cubic meters per hour at normal conditions of atmospheric pressure and 0° C. The table indicates the inlet temperature of the first catalytic bed which triggers the opening of the bypass valve. The pressure is given in bar gauge (bar g).

TABLE-US-00001 EXAMPLE 1 100% load 60% load 30% load Pressure (bar g) 226 175 175 MUG flow (Nm.sup.3/h) 338 203 101 Circulation m.sup.3/h.sub.EFF 12.04 11.80 5.9 Converter ΔT (Tout-Tin) 122.4 95.4 95.4 First bed inlet temperature 375 375 375 triggering the bypass (° C.) Ammonia @ converter inlet 11.3 13.21 13.21 (mol %) Ammonia @ converter outlet 20.99 20.52 20.52 (mol %) Delta Ammonia (OUT-IN)— 9.69 7.31 7.31 mol %

Example 2

[0093] The following Example 2 relates to a large ammonia production plant rated at 1000 MTD of ammonia. The parameters are same as in the Example 1.

TABLE-US-00002 EXAMPLE 2 100% load 50% load 25% load Pressure (bar g) 138.5 96.6 96.6 MUG flow (Nm.sup.3/h) 109737 55098 27549 Circulation m.sup.3/h.sub.EFF 4713 2941 1470 Converter ΔT (Tout-Tin) 265.7 220 220 Inlet temperature of first 360/400/380 360/400/380 360/400/380 bed/second bed/third bed triggering the bypass (° C.) Ammonia @ converter inlet 2.58 3.51 3.51 (mol %) Ammonia @ converter outlet 20.64 20.50 20.50 (mol %) Delta Ammonia (OUT-IN)— 18.06 16.99 16.99 mol %