Method for controlling a process comprising a steam system coupled to a reactor system

Abstract

A method for controlling a process including a steam system coupled to a reactor system is described, where the steam system includes a steam vessel that supplies a pressurized liquid water stream to the reactor system to cool the reactor system to generate a steam stream, and receives the steam stream from the reactor system, the method comprises the steps of: (i) obtaining a first total liquid level measurement in the vapour vessel using an inferred liquid level device; (ii) obtaining a second total liquid level measurement in the steam vessel using the direct liquid level measurement device; (iii) calculating, using the control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) initiating an alarm using the control system when the difference between the first total liquid level measurement and the second total liquid level measurement is greater than or equal to 1% of the lower one of the first total liquid level measurement and the second total liquid level measurement.

Description

[0001] The present invention relates to a method for controlling a process comprising a steam system for cooling a reactor system, in particular a reactor system for producing hydrocarbons according to the Fischer-Tropsch process.

[0002] The Fischer-Tropsch process involves producing a carbon monoxide having the formula (C n H 2n+2 ) in a reactor system. The process may be operated in one or more Fischer-Tropsch reactors using an iron-based catalyst or a cobalt-based catalyst at a pressure in the range of 0.1 MPa to 10 MPa and at a temperature in the range of 170°C to 350°C. The process may be operated to produce waxy hydrocarbons that may be further processed into fuels in downstream processing.

[0003] The Fischer-Tropsch reaction is an exothermic reaction, and various arrangements have been developed to prevent overheating of the Fischer-Tropsch reactor and catalyst and to prevent damage to them. In some arrangements, the reactor system including the fixed bed of the Fischer-Tropsch catalyst is cooled while exchanging heat with pressurized boiling water. In some arrangements, water may flow through coolant tubes within the bed of particulate catalyst. In other arrangements, catalyst tubes containing particulate catalysts may be soaked in water. In any case, heat is transferred to the water, which boils to generate steam. Therefore, a steam system is typically coupled to the reactor system to provide water and receive steam generated by the exchange of heat within the reactor system.

[0004] There is a risk that hydrocarbon products of the Fischer-Tropsch reaction may leak into the steam system during operation of the steam system at a lower pressure than the reactor system. The steam system typically includes a steam vessel (often described as a steam drum) that supplies pressurized water to the reactor system and receives steam from the reactor system. The steam vessel therefore contains both steam and liquid water.

[0005] The applicant has recognized that hydrocarbons can accumulate in steam vessels due to their lower density (lower than water). Long-term accumulation of hydrocarbons in steam vessels can undermine the effectiveness of conventional drum level monitoring instruments, as these instruments typically work by measuring the displacement or pressure difference between two heights within the drum. If a difference occurs between the measured liquid level and the water level, there is a risk of insufficient cooling of the reactor, resulting in potential loss of control and catalyst or reactor damage.

[0006] The applicant has found that the use of a combination of different techniques can improve the operation and control of the process.

[0007] Therefore, the present invention provides a method for controlling a process including a steam system coupled to a reactor system, wherein the steam system includes a steam container that supplies a pressurized liquid water flow to the reactor system to cool the reactor system to generate a steam flow, and receives a steam flow from the reactor system, the method comprising the following steps: (i) using an inferred level device to obtain a first total liquid level measurement result in the steam container; (ii) using a direct level measurement device to obtain a second total liquid level measurement result in the steam container; (iii) using a control system to calculate the difference between the first total liquid level measurement result and the second total liquid level measurement result; and (iv) when the difference between the first total liquid level measurement result and the second total liquid level measurement result is ≥ 1% of the lower of the first total liquid level measurement result and the second total liquid level measurement result, using the control system to activate an alarm.

[0008] The present invention includes a method for controlling a process including a steam system coupled to a reactor system, wherein the steam system includes a steam vessel that supplies a pressurized liquid water flow to the reactor system to cool the reactor system to generate a steam flow, and receives a steam flow from the reactor system, the method comprising the following steps: (i) obtaining a first total liquid level measurement result in the steam vessel using an inferred level device; (ii) obtaining a second total liquid level measurement result in the steam vessel using a direct level measurement device; (iii) calculating a difference between the first total liquid level measurement result and the second total liquid level measurement result using a control system; and (iv) initiating an alarm using the control system when the difference between the first total liquid level measurement result and the second total liquid level measurement result is ≥ 5% of the lower of the first total liquid level measurement result and the second total liquid level measurement result.

[0009] The present invention is applicable in scenarios where the reactor system operates at a higher pressure than the steam system, but the present invention may also operate in scenarios where the pressures of the systems are the same or even where the pressure of the steam system is greater than the pressure of the reactor system.

[0010] The temperature of the liquid water coolant provided by the steam system may be in the range of 150° C. to 250° C. The pressure of the liquid coolant and steam in the steam vessel may be in the range of 0.4 to 4.0 MPa(a).

[0011] The reactor system may be a Fischer-Tropsch reactor system comprising a Fischer-Tropsch catalyst, but the invention is applicable to any exothermic reactor system coupled to a steam system to provide cooling, if desired.

[0012] The steam vessel contains both steam and liquid water. The method uses two different types of measuring devices. This overcomes the drawback of using two devices of the same type.

[0013] The method includes obtaining a first total liquid level measurement in a steam container using an inferred level device. The inferred level device uses the difference in density or pressure in the container to measure the total liquid level of the liquid in the steam container by inference. Such devices include positive displacement or differential pressure devices. Such devices are used in the chemical industry and are commercially available. The liquid level of the liquid is the total liquid level, because the liquid may include both liquid water and another liquid, which may be immiscible with water. In a preferred embodiment, the total liquid level includes the sum of the liquid water level and the immiscible liquid level such as the liquid hydrocarbon level. When the immiscible liquid is a mixture of hydrocarbons or hydrocarbons, such as a Fischer-Tropsch liquid, its density is usually lower than water, and it therefore forms a layer on the top of the liquid water in the steam container. Therefore, the detected liquid level will be the level of the hydrocarbons on the top of the liquid water in the steam container. The presence of a liquid hydrocarbon layer may cause an error in the inferred level device measurement result, because the inferred liquid level is usually based on the density of water. Therefore, the presence of a less dense liquid will cause the inferred level device to read a lower level than the true level because the weight of the liquid it is measuring will be less.

[0014] The method also includes obtaining a second total liquid level measurement in the steam vessel simultaneously or sequentially using a direct level measurement device. The direct level measurement device may employ a direct scanning method to measure the total level of liquid in the steam vessel. Such devices include ultrasonic or guided wave radar (GWR) devices. Such devices are used in the chemical industry and are commercially available. Alternatively, a float level device may be connected to the steam vessel and used to directly measure the liquid water level using a magnet connected to the float to indicate the water level.

[0015] Where the reactor system comprises a Fischer-Tropsch reactor, the use of both inferred and direct level measurement devices has a particular benefit in that if hydrocarbons begin to accumulate in the liquid being measured, the level readings will tend to diverge.

[0016] In some arrangements, guided wave radar (GWR) instrumentation may be provided on the steam vessel in addition to existing displacement or differential pressure instrumentation.

[0017] The method includes the step of using a control system to compare a first total liquid level measurement and a second total liquid level measurement to calculate a difference between the first total liquid level measurement and the second total liquid level measurement. The comparison includes subtracting a smaller or lower liquid level measurement from a larger or higher liquid measurement. Thus, the difference is a positive number. The comparison may utilize statistical techniques known in the art, such as an n-point moving average of the two values ​​being compared over time, where n is a number between 2 and 100.

[0018] The method steps (i), (ii) and (iii) can be operated continuously or regularly and frequently every few seconds or minutes or hours. The calculations are performed using a control system. The control system can be any suitable control system for controlling chemical reactors and processes. The control system can be a distributed control system (DCS). Distributed control systems are used to control many chemical processes and are commercially available.

[0019] In the method of the present invention, including using a control system to calculate the difference between the first total liquid level measurement and the second total liquid level measurement, and when the difference between the first total liquid level measurement and the second total liquid level measurement is ≥ 1% or 5% of the lower of the first total liquid level measurement and the second total liquid level measurement, using the control system to activate an alarm. If the difference is below this value, the control system does not activate the alarm, but simply repeats the previous steps to monitor the steam container. This can be done continuously or periodically.

[0020] If the difference between the measurements is ≥ 1% or 5% of the lower of the first total liquid level measurement and the second total liquid level measurement, an alarm is activated so that an operator of the process can investigate the source of the difference. This may include one or more steps performed in response to the alarm: monitoring the flow rate and temperature of liquid water from the steam vessel; monitoring the temperature of the reaction vessel in the reactor system; and; monitoring the chemical composition of the liquid in the steam vessel, such as by collecting and analyzing the pH of a liquid sample.

[0021] A display system with a visual alarm or an audible alarm can be connected to the control system. The connection of the control system to any alarm system can be wireless or by a direct hardwired connection.

[0022] The operator can use the reactor system to shut down the process. When the calculated difference is ≥1% or 5% or more, particularly 5% or more, it may become necessary to shut down the reactor system. Therefore, in some embodiments, the method includes another step, i.e. shutting down the reactor system to prevent overheating of the reactor and the catalyst. The method of shutting down the reactor system is known. The method of shutting down the Fischer-Tropsch reactor system is described in, for example, GB2223237A, US10329492 and WO2022 / 11784A1, the contents of which are incorporated herein by reference. In such methods, the steam vessel can be depressurized to cool the liquid water coolant, and the supply of fresh synthesis gas can be stopped. Alternatively or in addition to this, if necessary, an inert gas such as nitrogen can be injected into the reactor system.

[0023] The method is practically applicable to a process comprising a Fischer-Tropsch reactor containing a cooled Fischer-Tropsch catalyst fed together with a reaction gas mixture and operating in a loop.

[0024] The reaction gas mixture supplied to the Fischer-Tropsch reactor typically comprises synthesis gas plus recycle gas recovered from the Fischer-Tropsch reactor product stream. The synthesis gas used in the Fischer-Tropsch process comprises hydrogen and carbon monoxide. The recycle gas typically comprises unreacted synthesis gas, carbon dioxide and potentially light hydrocarbons.

[0025] The Fischer-Tropsch process involves a series of chemical reactions that produce various hydrocarbons, ideally having the formula (C n H 2n+2 ). A more useful reaction produces alkanes from the reaction gas mixture as follows:

[0026] (2n+1)H2+nCO→C n H 2n+2 +n H2O,

[0027] Where n is generally from 5 to 100 or more, preferred products have n in the range of 10 to 20.

[0028] The Fischer-Tropsch reactor is usually operated in a synthesis loop, i.e., a reaction gas mixture is supplied to the Fischer-Tropsch reactor, in which the reaction gas mixture reacts on a Fischer-Tropsch catalyst to form a product mixture comprising liquid hydrocarbons and gaseous hydrocarbons, steam and unreacted gas. The product gas mixture is cooled after leaving the Fischer-Tropsch reactor to condense steam and facilitate the recovery of liquid hydrocarbons. A portion of the unreacted gas, optionally after separation of light hydrocarbons, is returned to the Fischer-Tropsch reactor as a circulating gas, thereby forming a synthesis loop. The circulating gas is combined with the synthesis gas to form a reaction gas mixture outside the Fischer-Tropsch reactor, thereby allowing more effective temperature control of the feed to the Fischer-Tropsch reactor. Operating the Fischer-Tropsch reactor in a loop improves the conversion efficiency of the process. In order to prevent the accumulation of inert gases, a purge can be performed from the loop as a Fischer-Tropsch tail gas, which can be further processed.

[0029] The Fischer-Tropsch reactor can be operated at a pressure in the range of 10 bar absolute to 100 bar absolute (0.1 MPa to 10 MPa) and at a temperature in the range of 170° C. to 350° C. Operation over a cobalt catalyst can be carried out at a pressure of 20 bar absolute to 50 bar absolute and at a temperature of 200° C. to 320° C. The gas hourly space velocity (GHSV) for continuous operation can be 1000 hr. -1 Up to 25000hr -1 within the range.

[0030] The Fischer-Tropsch reactor comprises a Fischer-Tropsch catalyst which is indirectly cooled by pressurized water. The Fischer-Tropsch catalyst can be provided in the form of a bed through which tubes or plates carrying a coolant are placed, or the catalyst can be provided in a plurality of reaction tubes which are immersed in a coolant flowing around the outside thereof. The latter reactor technology is preferred.

[0031] Any Fischer-Tropsch catalyst can be used, but iron and cobalt Fischer-Tropsch catalysts are preferred. Cobalt-based Fischer-Tropsch catalysts are superior to iron-based catalysts due to their lower carbon dioxide selectivity. In a particularly preferred arrangement, the Fischer-Tropsch catalyst is used in combination with a catalyst carrier suitable for a tubular Fischer-Tropsch reactor, wherein the catalyst carrier containing the catalyst is arranged in one or more tubes cooled by pressurized circulating water. The so-called "catalyst carrier" refers to a catalyst container, for example in the form of a cup or can, which is configured to allow gas and / or liquid to flow into and out of the carrier and flow through a bed of catalyst or catalyst precursor arranged in the carrier. Any suitable catalyst carrier can be used. In one arrangement, the catalyst carrier is a catalyst carrier described in WO2011 / 048361, the contents of which are incorporated herein by reference. In an alternative arrangement, the catalyst carrier may include a monolithic catalyst disclosed in WO2012 / 136971, the contents of which are also incorporated herein by reference. In another alternative arrangement, the catalyst support may be a catalyst support disclosed in WO2016 / 050520, the contents of which are also incorporated herein by reference. In a preferred embodiment, the reactor system comprises a tubular Fischer-Tropsch reactor in which a catalyst support comprising a Fischer-Tropsch catalyst is disposed within one or more tubes cooled by a cooling medium.

[0032] The present invention is further described with reference to the accompanying drawings, in which:

[0033] Figure 1 is a depiction of one embodiment of a system to which the methods of the present invention may be applied.

[0034] It will be appreciated by those skilled in the art that the drawings are illustrative and that other items of equipment may be required in a commercial plant, such as feedstock barrels, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, liquid level controllers, collection tanks, storage tanks, etc. The provision of such ancillary equipment does not form part of the present invention and is consistent with conventional chemical engineering practice.

[0035] exist Figure 1In the embodiment of the present invention, a steam system 10 is coupled to a Fischer-Tropsch reactor system 12. A DCS control system 14 controls the steam system and the reactor system using valves 16, 18, and 20. The steam system 10 includes a steam container 22, which is supplied with a boiler feed water flow via a pipeline 24, which in turn supplies a pressurized liquid water flow to a Fischer-Tropsch reactor 28 via a pipeline 26, in which a pressurized liquid water flow is used to cool a plurality of tubes 30 containing a Fischer-Tropsch catalyst. A supply gas 32 including a fresh synthesis gas stream 34 and a recycle gas stream 36 is supplied to the reactor 28, in which it reacts on a catalyst in the tube 30 to generate a hydrocarbon liquid product, which is recovered from the reactor as a product stream 38 together with unreacted gas and by-product water for further processing. The fresh synthesis gas stream 34 and the recycle gas stream 36 are compressed by a compressor (not shown). The pressure of the supply gas mixture 32 is greater than the pressure of the pressurized water supplied via the pipeline 26. Recycle gas stream 36 is recovered from product stream 38 using one or more gas liquid separators (not shown).

[0036] The formation of hydrocarbon liquids generates heat that converts a portion of the liquid water provided by line 26 into steam within reactor 28. A mixture of steam and liquid water is recovered from reactor 28 and supplied to steam vessel 22 via line 40. Steam system 10 also includes an inferred level device 42 that obtains a first total liquid level measurement in steam vessel 22 and a direct level measurement device 44 that obtains a second total liquid level measurement in steam vessel 22. The levels detected by devices 42, 44 are transmitted (as shown by dashed lines 48 and 50) to controller 14, which compares the first total liquid level measurement and the second total liquid level measurement and calculates the difference between them. Control system 14 is connected (as shown by dashed line 52) to display system 46 having a visual alarm and an audible alarm. When the difference between the first total liquid level measurement and the second total liquid level measurement is ≥ 5% of the lower of the first total liquid level measurement and the second total liquid level measurement, an alarm is activated in display system 46 using control system 14.

[0037] The control system 14 is connected (as shown by dashed lines 54, 56 and 58) to valves 16, 18, 20, and valves 16, 18 and 20 can be adjusted to cause a controlled shutdown of the reactor system 12 based on the temperature of the catalyst in the tube 30, for example, according to the instructions of the control system or the operator. For example, the valve 16 can be opened to depressurize the steam container 22, thereby reducing the temperature of the liquid water and quenching the Fischer-Tropsch reaction. Alternatively or in addition, the supply of fresh synthesis gas can be stopped by closing the valve 20. The loop compressor continues to supply the recycle gas flow 32 to the reactor. Optionally, a pressure vessel (not shown) containing high-pressure nitrogen at a pressure greater than the supply gas 32 can be connected to the recycle gas pipeline 36 or the supply pipeline 32 for injecting nitrogen into the tube 30 filled with the catalyst in an emergency. The valve 18 controlling the supply of boiler feed water to the steam container 22 can also be closed to cut off the supply water.

Claims

1. A method for controlling a process comprising a steam system coupled to a reactor system, wherein: The steam system includes a steam vessel that supplies a pressurized liquid water flow to the reactor system to cool the reactor system, thereby generating a steam flow, and receives the steam flow from the reactor system, the method comprising the steps of: (i) obtaining a first total liquid level measurement in the steam vessel using an inferred level device; (ii) obtaining a second total liquid level measurement in the steam vessel using a direct level measurement device; (iii) calculating a difference between the first total liquid level measurement and the second total liquid level measurement using a control system; and (iv) activating an alarm using the control system when the difference between the first total liquid level measurement and the second total liquid level measurement is ≥ 1% of the lower of the first total liquid level measurement and the second total liquid level measurement.

2. The method according to claim 1, wherein: The reactor system operates at a higher pressure than the steam system.

3. The method according to claim 1 or claim 2, wherein: The inferred liquid level device is a positive displacement device or a pressure differential device.

4. The method according to any one of claims 1 to 3, wherein: The direct liquid level measurement device is a guided wave radar device or a float device.

5. The method according to any one of claims 1 to 4, wherein: The process steps (i), (ii) and (iii) are performed continuously.

6. The method according to any one of claims 1 to 4, wherein: Steps (i), (ii) and (iii) of the method are performed in such a way that measurements are taken every few seconds, minutes or hours.

7. The method according to claim 1, wherein: The difference between the first total liquid level measurement and the second total liquid level measurement is calculated using a time averaging method or a statistical method.

8. The method according to any one of claims 1 to 7, wherein: The control system is a distributed control system.

9. The method according to any one of claims 1 to 8, wherein: A display system with a visual alarm or an audible alarm is connected to the control system.

10. The method according to any one of claims 1 to 9, wherein: In response to the alarm, the method further includes one or more of the following steps: monitoring the flow and temperature of the liquid water from the steam container; monitoring the temperature of the reaction vessel in the reactor system; and; monitoring the chemical composition of the liquid in the steam container.

11. The method according to any one of claims 1 to 10, wherein: In response to the alarm, the method comprises the further step of shutting down the reactor system.

12. The method according to any one of claims 1 to 11, wherein: The reactor system is a Fischer-Tropsch reactor system comprising a Fischer-Tropsch catalyst indirectly cooled by pressurized water.

13. The method according to claim 12, wherein: The Fischer-Tropsch catalyst is provided in the form of a bed of water-carrying tubes or plates, or the Fischer-Tropsch catalyst is provided in a plurality of water-cooled reactor tubes.

14. The method according to claim 12 or claim 13, wherein: The Fischer-Tropsch catalyst is used in combination with a catalyst support in a tubular Fischer-Tropsch reactor, wherein the catalyst support containing the Fischer-Tropsch catalyst is arranged in one or more tubes cooled by pressurized circulating water.

15. The method for controlling a process comprising a steam system coupled to a reactor system according to claim 1, wherein: The steam system includes a steam vessel that supplies a pressurized liquid water flow to the reactor system to cool the reactor system, thereby generating a steam flow, and receives the steam flow from the reactor system, the method comprising the steps of: (i) obtaining a first total liquid level measurement in the steam vessel using an inferred level device; (ii) obtaining a second total liquid level measurement in the steam vessel using a direct level measurement device; (iii) calculating a difference between the first total liquid level measurement and the second total liquid level measurement using a control system; and (iv) activating an alarm using the control system when the difference between the first total liquid level measurement and the second total liquid level measurement is ≥ 5% of the lower of the first total liquid level measurement and the second total liquid level measurement.

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