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. A system for controlling pressure in a loop for the synthesis of ammonia or methanol, comprising: a make-up gas compressor configured to pressurize a make-up synthesis gas, a synthesis loop configured to receive a pressurized make-up synthesis gas from the make-up gas compressor, the synthesis loop comprising a synthesis reactor, a synthesis loop cooler, a loop recirculation compressor, a synthesis loop heater, and a pressure monitor; the synthesis reactor arranged to receive a heated loop recirculation gas from the synthesis loop heater and configured to provide an at least partially synthesized gas to the synthesis loop cooler; the synthesis loop cooler configured to cool the at least partially synthesized gas and separate into a product stream and a loop recirculation gas; the loop recirculation compressor arranged to receive the loop recirculation gas and configured to pressurize and provide a pressurized loop recirculation gas to the synthesis loop heater; wherein the loop recirculation compressor comprises an anti-surge valve and/or a compressor flow regulation valve in communication with the pressure monitor, the anti-surge valve and/or a compressor flow regulation valve being configured to control the flow of the loop recirculation gas through the anti-surge valve and/or a compressor flow regulation valve to obtain a substantially constant pressure in the ammonia or methanol synthesis loop.

2. The system according to claim 1, wherein the compressor flow regulation valve is arranged in parallel with the anti-surge valve.

3. The system according to claim 1, further comprising a second anti-surge valve configured to control the flow of ammonia or methanol make-up synthesis gas provided by the make-up gas compressor to the synthesis loop.

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

5. The system according to claim 1, further comprising a loop pressure controller downstream or upstream of the recirculation compressor for additionally controlling the loop recirculation gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a configuration of the make-up gas compressor, recirculator and synthesis loop.

(2) FIG. 2 shows a configuration using an additional pressure control valve instead of using recirculatory anti-surge valve loop control.

(3) FIG. 3 shows a configuration where the gas flow to the converter is controlled down to a zero flow by a loop pressure controller and optionally a small bypass valve.

(4) FIG. 4 shows process layout similar to FIG. 3 where one or more valves control converter inlet flow, recirculatory anti-surge flow, and make-up gas compressor flow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) FIG. 1 shows a typical configuration of the make-up gas compressor, recirculator and synthesis loop.

(6) 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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.

(11) 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.

(12) 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).

(13) 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).

(14) 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.

(15) 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.

(16) 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.

(17) 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.

(18) 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.

(19) In the figures, A defines an analysis point, F a flow measurement point, and P a pressure measurement point.