METHOD FOR THE CONTROL OF PRESSURE IN A LOOP FOR THE PREPARATION OF AMMONIA OR METHANOL

Abstract

Method for the control of pressure in a loop for the preparation of ammonia or methanol by means of an anti-surge control valve of a compressor and/or a compressor flow regulation valve for the recirculation of loop recirculation gas at variating flow supply of fresh synthesis gas.

Claims

1. Method for the control of pressure in a loop for the preparation of ammonia or methanol comprising the steps of (a) providing a fresh ammonia or methanol synthesis gas; (b) providing a loop recirculation gas; (c) providing a loop recirculation compressor with an anti-surge valve and/or a compressor flow regulation valve; (d) providing an ammonia or methanol synthesis loop; (e) adding the fresh ammonia or methanol synthesis gas into the loop recirculation gas; (f) pressurizing the loop recirculation gas from step (e) in the loop recirculation compressor; and (g) monitoring pressure in the ammonia or methanol synthesis loop, wherein flow of the loop recirculation gas through the anti-surge valve and/or the recirculation compressor flow regulation valve is controlled to obtain a substantially constant pressure in the ammonia or methanol synthesis loop.

2. The method of claim 1, wherein compressor flow regulation valve is arranged in parallel with the antisurge valve.

3. The method of claim 1, wherein flow of the fresh ammonia or methanol synthesis gas is controlled by an antisurge valve of a compressor for the fresh synthesis gas.

4. The method of claim 1, comprising the further step of controlling temperature in a high-pressure loop separator arranged in the loop for the preparation of ammonia or methanol.

5. The method of claim 1, wherein hydrogen in the fresh ammonia or methanol synthesis gas is provided by means of electrolysis of water.

6. The method of claim 5, wherein the electrolysis of water is performed in a solid oxide electrolysis cell.

7. The method of claims 1, wherein the fresh methanol synthesis gas is provided by co-electrolysis of water and carbon dioxide.

8. The method of claim 1, wherein nitrogen in the fresh ammonia synthesis gas is provided by means of air separation.

9. The method of claim 1, wherein the fresh ammonia synthesis gas is prepared of water and air in a solid oxide electrolysis cell.

10. The method of claim 1, wherein flow of the loop recirculation gas is additionally controlled by a loop pressure controller downstream or upstream the recirculation compressor.

11. The method of claim 1, wherein the module of the fresh ammonia synthesis gas is controlled by a ratio controller of hydrogen and nitrogen flow in ammonia synthesis gas by controlling the nitrogen flow rate relative to the hydrogen flow rate.

12. The method of claim 11, wherein the ratio controller is compensated by a real-time analyzer.

Description

[0013] Pursuant to the above finding, the present invention provides a method for the control of pressure in a loop for the preparation of ammonia or methanol comprising the steps of [0014] (a) providing a fresh ammonia or methanol synthesis gas; [0015] (b) providing a loop recirculation gas; [0016] (c) providing a loop recirculation compressor with an antisurge valve and/or a compressor flow regulation valve; [0017] (d) providing an ammonia or methanol synthesis loop; [0018] (e) adding the fresh ammonia or methanol synthesis gas into the loop recirculation gas; [0019] (f) pressurizing the loop recirculation gas from step (e) in the loop recirculation compressor; and [0020] (g) monitoring pressure in the ammonia or methanol synthesis loop, [0021] wherein flow of the loop recirculation gas through the anti-surge valve and/or the recirculation compressor flow regulation valve is controlled to obtain a substantially constant pressure in the ammonia or methanol synthesis loop.

[0022] In case of slow load variation (days or weeks), the method according to the invention can be supplemented by control of the temperature in a high pressure ammonia or methanol loop separator. Thereby reactivity of the loop recirculation loop gas flowing to the ammonia converter can be reduced when the concentration of ammonia in the feed gas is increased. Higher temperature leads to less reactivity and higher loop pressure.

[0023] Thus, in an embodiment of the invention the method comprises the further step of controlling temperature in a loop separator arranged in the loop for the preparation of ammonia or methanol.

[0024] The loop separator separates liquid ammonia or methanol product from the unconverted gas effluent from the synthesis converter at equilibrium between gas and liquid at the given pressure and temperature. At constant pressure and higher temperature gives higher content of product in unconverted gas to be recycled back to the synthesis converter. This will lower the potential conversion per pass since the synthesis reaction is limited by equilibrium resulting in reduced capacity of the synthesis loop at constant pressure.

[0025] One of the advantages of the invention is that energy for operating various equipment for the preparation of ammonia synthesis gas can be renewable energy generated by windmills, solar cells, hydraulic energy or other renewables.

[0026] Preferably, the equipment comprises one or more electrolysis units, such as solid oxide electrolysis cells.

[0027] Thus, in an embodiment of the invention, hydrogen contained in the fresh ammonia or methanol synthesis gas is provided by means of electrolysis of water.

[0028] In further an embodiment, nitrogen contained in the fresh ammonia synthesis gas is provided by means of air separation.

[0029] In still an embodiment of the invention, the fresh methanol synthesis gas is provided by co-electrolysis of water and carbon dioxide.

[0030] In further an embodiment of the invention, the fresh ammonia synthesis gas is prepared in a solid oxide electrolysis cell of water and air.

[0031] FIG. 1 shows a typical configuration of the make-up gas compressor, recirculator and synthesis loop.

[0032] If the antisurge valve is open, then less flow will pass on to the reactor. During start-up where the synthesis reactor is heated up by circulating gas in the loop and having the start-up heater ignited then the antisurge will initially be fully open in order to protect the recirculator from surge and to reduce the flow rate to the reactor for easy control of the heating up phase.

[0033] The same valve (antisurge valve) is used simultaneously as compressor protection and flow control valve to the reactor. This is feasible as the two functions are never contradictory and in any case the machine protection will overrule all other set point to the valve. This concept is well proven for start-up of the synthesis.

[0034] Using renewable energy for production of synthesis gas will provide fluctuations throughout a day in feed gas flow rate resulting in many and possibly also abrupt synthesis pressure fluctuations. This can be smoothed out or even eliminated by the method according to the invention.

[0035] In normal operation, the recirculator antisurge valve can be used for control of the loop pressure. At full capacity the valve will remain closed and if less make up gas is available then the recirculation gas flow will be reduced correspondingly by controlled opening of the valve.

[0036] This will limit the conversion of synthesis gas in the loop to exactly the amount of make-up gas available resulting in keeping the same amount of gas in the loop and thus constant loop pressure.

[0037] There might be an understanding of the loop pressure is also controlled by the make-up compressor speed, but this is not the case as the make-up gas compressor will deliver the required pressure for a given conversion in the loop.

[0038] Since the method of the invention controls the conversion in the loop to maintain a constant loop pressure then the make-up gas compressor will follow the loop requirement. The only way the make-up gas compressor can do that and still be within its operating window (flow versus discharge pressure) is by opening its own antisurge valve(s) to compensate for the lower make-up gas flow available (see FIGS. 1 and 2).

[0039] There could be cases where it is not allowed to use the antisurge valve for loop pressure control valve. Then the alternative would be to install a control valve in parallel without jeopardizing the compressor surge protection as the antisurge valve opening is still governed by the compressor requirement measured as resulting flow from two control valves to the suction of the recirculator (see FIG. 2).

[0040] Since the conversion equilibrium temperature remains constant, a control which ensure the ratio between make-up gas and converter feed gas remains constant will nearly eliminate pressure and temperature fluctuations in the converter and ammonia loop.

[0041] Because the anti-surge valve has a security function, the flow from the compressor discharge side to suction side may additionally or completely be regulated by means of compressor flow regulation valve during feed gas flow variations.

[0042] The examples of FIGS. 1 and 2, will have a limitation on the turn down of the gas flow since the minimum flow to the converter will depend on the pressure drop ratio between the converter and the anti-surge valve.

[0043] FIG. 3 shows a configuration where the gas flow to the converter can be controlled down to a zero flow by means of a loop pressure controller and optionally a small bypass valve. When reducing or closing the loop pressure controller, the synthesis gas in the synthesis reactor is retained in the reactor and maintains the reactor pressure. This will allow the loop pressure to be controlled down to very low load and still keep the loop pressure up and the converter in hot conditions. This is important in the case where suddenly the renewable energy and thus synthesis gas production comes back from low load to high load, then the conversion of synthesis gas into ammonia or methanol can take place essentially instantaneous.

[0044] FIG. 4 shows a similar process layout as shown in FIG. 3, where one or more valves are foreseen to control converter inlet flow, recirculator anti-surge flow, and make-up gas compressor anti-surge flow. The module of the make-up gas is controlled by a ratio controller of hydrogen and nitrogen flow in ammonia synthesis gas by controlling the nitrogen flow rate relative to the hydrogen flow rate. With many fluctuations perhaps daily in energy supply, and thus directly impacting the hydrogen flow rate and also the nitrogen flow rate, the measurement of hydrogen and nitrogen flow might get a bit off set at each fluctuation. A small change in the make-up gas module will be amplified in the module of the loop recirculation gas and for this reason it is desirable to improve the module controller by having a near real-time analyzer on the make-up gas. Typically, a common gas chromatography analyzer is used for multiple sampling point leading to long tubing from each sampling point to the analyzer, which results in long cycle time for each analysis. Long cycle time of 10-20 min. is not suitable for adjustment of the module controller. A real-time analyzer can provide a cycle time of 10-20 sec. and the module controller can act in time before a wrong module gets amplified in the loop resulting in loss of capacity and/or pressure increase when high capacity is required.

[0045] In the figures, A defines an analysis point, F a flow measurement point, and P a pressure measurement point.