A method for controlling a process including a steam system coupled to a reactor system

Abstract

A method for controlling a process including a steam system coupled to a reactor system, the steam system comprising a steam vessel that supplies a stream of liquid water under pressure to the reactor system to cool the reactor system, thereby generating a steam stream, and receives a steam stream from the reactor system, the method comprising: (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 device; (iii) calculating, using a control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) activating an alarm using the control system if 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 of the first total liquid level measurement and the second total liquid level measurement. A method is described.

Description

Technical Field

[0001] The present invention relates to a method for controlling a process including a steam system used to provide cooling to a reactor system, and more particularly to a method for a reactor system used to produce hydrocarbons by the Fischer-Tropsch process.

Background Art

[0002] The Fischer-Tropsch process involves a series of catalytic chemical reactions in a reactor system that produce various hydrocarbons having the formula (C n H 2n+2 ) from a feed gas containing hydrogen and carbon monoxide. The process can be operated in one or more Fischer-Tropsch reactors using an iron or cobalt-based catalyst at a pressure in the range of 0.1 to 10 MPa and a temperature in the range of 170 to 350 °C. The process can be operated to generate waxy hydrocarbons, which can be further processed into fuel in downstream processing.

[0003] The Fischer-Tropsch reaction is exothermic, and various configurations have been developed to prevent overheating and damage to the Fischer-Tropsch reactor and catalyst. In some configurations, a reactor system including a fixed bed of Fischer-Tropsch catalyst is cooled in heat exchange with boiling water under pressure. In some configurations, water can flow through coolant tubes within a bed of particulate catalyst. In other configurations, catalyst tubes containing particulate catalyst can be immersed in water. In any case, heat is transferred to the water, which boils and generates steam. Thus, the steam system is typically coupled to the reactor system to provide water and receive steam generated by heat exchange within the reactor system.

[0004] During an operating period in which the steam system is operating at a lower pressure than the reactor system, there is a risk that hydrocarbon products of the Fischer-Tropsch reaction may leak into the steam system. The steam system typically comprises a steam vessel, often described as a steam drum, that supplies pressurized water to the reactor system and receives steam from the reactor system. Thus, the steam vessel contains both steam and liquid water.

[0005] The Applicants recognized that hydrocarbons can accumulate in the steam vessel due to their lower density (than water). Conventional steam drum level monitoring devices typically operate by measuring the displacement or pressure difference between two heights within the drum, so as hydrocarbons accumulate in the steam vessel over time, the effectiveness of these devices can be gradually impaired. If a difference occurs between the measured liquid level and the water level, there is a risk of insufficient cooling of the reactor, leading to potential runaways and damage to the catalyst or reactor.

[0006] The Applicants have found that by using a combination of distinct technologies, the operation and control of the process can be improved.

Summary of the Invention

[0007] Accordingly, the present invention provides a method for controlling a process including a steam system coupled to a reactor system, the steam system comprising a steam vessel that supplies a stream of liquid water under pressure to the reactor system to cool the reactor system, thereby generating a steam stream and receiving a steam stream from the reactor system, the method comprising: (i) obtaining a first total liquid level measurement within the steam vessel using an inferential level device; (ii) obtaining a second total liquid level measurement within the steam vessel using a direct level measurement device; (iii) calculating, using a control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) activating an alarm using the control system if 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 of the first total liquid level measurement and the second total liquid level measurement.

[0008] The present invention provides a method for controlling a process including a steam system coupled to a reactor system, the steam system comprising a steam vessel that supplies a stream of liquid water under pressure to the reactor system to cool the reactor system, thereby generating a steam stream and receiving a steam stream from the reactor system, the method comprising: (i) obtaining a first total liquid level measurement within the steam vessel using an inferential level device; (ii) obtaining a second total liquid level measurement within the steam vessel using a direct level measurement device; (iii) calculating, using a control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) activating an alarm using the control system if the difference between the first total liquid level measurement and the second total liquid level measurement is greater than or equal to 5% of the lower of the first total liquid level measurement and the second total liquid level measurement.

[0009] The present invention can be applied when the reactor system is operated at a higher pressure than the steam system, but it is also possible to operate the present invention when the pressures of the systems are the same or when the pressure of the steam system is higher than the pressure of the reactor system.

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

[0011] The reactor system can be a Fischer-Tropsch reactor system containing a Fischer-Tropsch catalyst, but the present invention can be applied to any exothermic reactor system coupled to a steam system that provides cooling as desired.

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

[0013] This method includes obtaining a first total liquid level measurement value in a vapor vessel using an inferred level device. The inferred level device measures by inferring the total level of liquid in the vapor vessel using the difference in density or pressure within the vessel. Such devices include volumetric devices or differential pressure devices. Such devices are used in the chemical industry and are commercially available. Since the liquid can include both liquid water and another liquid that may be immiscible with water, the level of the liquid is the total liquid level. In a preferred embodiment, the total liquid level includes the total amount of the liquid water level and the immiscible liquid level, such as a liquid hydrocarbon level. When the immiscible liquid is a hydrocarbon or a mixture of hydrocarbons such as a Fischer-Tropsch liquid, this is typically less dense than water and thus forms a layer on top of the liquid water in the vapor vessel. Accordingly, the detected level will be the level of the hydrocarbon above the liquid water in the vapor vessel. Since the inferred level is typically based on the density of water, the presence of the liquid hydrocarbon layer can introduce an error in the measurement by the inferred level device. Thus, the presence of the lower density liquid causes the inferred level device to read a lower level than the actual level because the weight of the liquid being measured by the inferred level device is lighter.

[0014] This method also includes obtaining a second total liquid level measurement value in the vapor vessel simultaneously or sequentially using a direct level measurement device. The direct level measurement device can measure the total level of liquid in the vapor vessel using a direct scanning method. 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 can be connected to the vapor vessel and used to directly measure the liquid water level using a magnet connected to a float that indicates the level.

[0015] When the reactor system includes a Fischer-Tropsch reactor, using both the inferred level device and the direct level measurement device provides certain benefits, such as a tendency for the level indication value to deviate when hydrocarbons begin to accumulate in the liquid during measurement.

[0016] In some configurations, in addition to existing displacement or differential pressure devices, a guided wave radar (GWR) device may be provided to the vapor vessel.

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

[0018] Method steps (i), (ii), and (iii) can be operated while measuring continuously, or at regular frequencies such as every few seconds, or every few minutes, or every few hours. The calculation is performed using a control system. The control system can be any suitable control system used to control a chemical reactor and process. The control system can be a distributed control system (DCS). Distributed control systems are used to control a number of chemical processes and are commercially available.

[0019] In the method of the present invention, using a control system to calculate the difference between the first total liquid level measurement value and the second total liquid level measurement value, when the difference between the first total liquid level measurement value and the second total liquid level measurement value is 1% or more or 5% or more of the lower of the first total liquid level measurement value and the second total liquid level measurement value, it includes using the control system to activate an alarm. When the difference is lower than this value, the control system simply repeats the previous step to monitor the vapor vessel without activating an alarm. This can be done continuously or periodically.

[0020] If the difference between the measured values is 1% or more, or 5% or more, of the lower of the first total liquid level measurement value and the second total liquid level measurement value, an alarm is activated so that the operator of the process can investigate the cause of the difference. This may include one or more of the steps of monitoring the flow and temperature of liquid water from the vapor vessel, monitoring the temperature of the reaction vessel within the reactor system, and monitoring the chemical composition of the liquid within the vapor vessel, for example, by taking a liquid sample and performing a pH analysis thereof.

[0021] A display system having a visual or audible alarm may be connected to the control system. The connection to any alarm system of the control system may be wireless or by direct hardwiring connection.

[0022] The operator may use the reactor system to shut down the process. When the calculated difference is 1% or more, or 5% or more, particularly 5% or more, it may be necessary to shut down the reactor system. Thus, in some embodiments, the method includes a further step of shutting down the reactor system to prevent overheating of the reactor and catalyst. Methods for shutting down reactor systems are known. Methods for shutting down Fischer-Tropsch reactor systems are described, for example, in UK Patent Application Publication No. 2223237(A), US Patent No. 10329492, and International Publication No. 2022 / 11784(A1), the contents of which are incorporated herein by reference. In such methods, the vapor vessel may be depressurized to cool the liquid water coolant and the supply of fresh synthesis gas may be stopped. Alternatively or additionally, an inert gas such as nitrogen gas may be injected into the reactor system if desired.

[0023] The method may be usefully applied to a process including a Fischer-Tropsch reactor containing a cooled Fischer-Tropsch catalyst to which a reaction gas mixture is supplied and operated within a loop.

[0024] The reaction gas mixture fed to the Fischer-Tropsch reactor typically includes, in addition to synthesis gas, recycled gas recovered from the Fischer-Tropsch reactor product stream. Synthesis gas for the Fischer-Tropsch process contains hydrogen and carbon monoxide. The recycled gas will typically contain unreacted synthesis gas, carbon dioxide, and possibly light hydrocarbons.

[0025] The Fischer-Tropsch process ideally involves a series of chemical reactions that produce various hydrocarbons having the formula (C n H 2n+2 ). A more useful reaction produces alkanes from the reaction gas mixture as follows: (2n + 1)H2 + nCO → C n H 2n+2 + nH2O where n is typically 5 to 100 or more, and the preferred products have n in the range of 10 to 20.

[0026] The Fischer-Tropsch reactor is typically operated within a synthesis loop. That is, the reaction gas mixture is fed to the Fischer-Tropsch reactor where it reacts over a Fischer-Tropsch catalyst to form a product mixture containing liquid and gaseous hydrocarbons, steam, and unreacted gas. The product gas mixture is cooled after exiting the Fischer-Tropsch reactor to condense steam and facilitate recovery of the liquid hydrocarbons. A portion of the unreacted gas is optionally returned to the Fischer-Tropsch reactor as recycled gas after separation of the light hydrocarbons, thereby forming the synthesis loop. The recycled gas is combined with the synthesis gas to form the reaction gas mixture outside the Fischer-Tropsch reactor, which allows for more efficient temperature control of the feed to the Fischer-Tropsch reactor. Operating the Fischer-Tropsch reactor within the loop improves the conversion efficiency of the process. A purge may be taken from the loop as Fischer-Tropsch tail gas for further processing to prevent accumulation of inert gases.

[0027] The Fischer-Tropsch reactor can be operated at pressures in the range of 10 to 100 bar absolute (0.1 to 10 MPa) and temperatures in the range of 170 to 350 °C. Operation on a cobalt catalyst can be at 20 to 50 bar absolute and 200 to 320 °C. The gas-hourly-space velocity (GHSV) for continuous operation can be in the range of 1000 to 25000 hr -1 -1.

[0028] The Fischer-Tropsch reactor contains a Fischer-Tropsch catalyst that is indirectly cooled by water under pressure. The Fischer-Tropsch catalyst can be provided as a bed in which tubes or plates carrying a coolant are installed, or the catalyst can be provided in a plurality of reaction tubes immersed in a coolant flowing around the outside of the plurality of reaction tubes. The latter reactor technology is preferred.

[0029] Any Fischer-Tropsch catalyst can be used, although iron and cobalt Fischer-Tropsch catalysts are preferred. Cobalt-based Fischer-Tropsch catalysts are preferred over 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 support suitable for use in a tubular Fischer-Tropsch reactor, and the catalyst support containing the catalyst is disposed within one or more tubes that are cooled by circulating water under pressure. A "catalyst support" means a catalyst container, such as in the form of a cup or can, configured to allow gases and / or liquids to enter and exit the support and flow through a bed of catalyst or catalyst precursor disposed within the support. Any suitable catalyst support may be used. In one configuration, the catalyst support is as described in International Publication No. WO 2011 / 048361, the contents of which are incorporated herein by reference. In an alternative configuration, the catalyst support may include a catalyst monolith as disclosed in International Publication No. WO 2012 / 136971. The contents of this document are also incorporated herein by reference. In yet another alternative configuration, the catalyst support may be as disclosed in International Publication No. WO 2016 / 050520. The contents of this document are also incorporated herein by reference. In a preferred embodiment, the reactor system includes a tubular Fischer-Tropsch reactor in which a catalyst support containing a Fischer-Tropsch catalyst is disposed within one or more tubes that are cooled by a cooling medium.

Brief Description of the Drawings

[0030] The present invention will be further described with reference to the drawings.

Figure 1

[0031] One skilled in the art will understand that the drawings are schematic and that in a commercial plant, additional items of equipment such as raw material drums, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, etc. may be required. The provision of such ancillary equipment does not form part of the present invention and follows conventional chemical engineering practice.

DETAILED DESCRIPTION OF THE INVENTION

[0032] In FIG. 1, 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 by means of valves 16, 18, 20. The steam system 10 includes a steam vessel 22 to which a stream of boiler feed water is supplied via line 24, and this steam vessel supplies a stream of liquid water under pressure via line 26 to a Fischer-Tropsch reactor 28 where the liquid water is used to cool tubes 30 containing a plurality of Fischer-Tropsch catalysts. A feed gas 32 including a fresh synthesis gas stream 34 and a recycle gas stream 36 is supplied to the reactor 28 where the feed gas reacts over the catalyst in the tubes 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 feed gas mixture 32 is higher than the pressure of the water under pressure supplied via line 26. The recycle gas stream 36 is recovered from the product stream 38 using one or more gas-liquid separators (not shown).

[0033] The formation of the hydrocarbon liquid generates heat, and this heat 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 via line 40 to steam vessel 22. The steam system 10 further comprises an inferential level device 42 that obtains a first total liquid level measurement within steam vessel 22 and a direct level measurement device 44 that obtains a second total liquid level measurement within steam vessel 22. The levels detected by devices 42, 44 are communicated to controller 14 (shown by dashed lines 48 and 50), and this controller compares the first total liquid level measurement with the second total liquid level measurement and calculates the difference therebetween. The control system 14 is connected to a display system 46 having visual and audible alarms (as shown by dashed line 52). If the difference between the first total liquid level measurement and the second total liquid level measurement is greater than or equal to 5% of the lower of the first total liquid level measurement and the second total liquid level measurement, an alarm is activated in the display system 46 using the control system 14.

[0034] The control system 14 is connected to valves 16, 18, 20 (as shown by dashed lines 54, 56, and 58) and the valves 16, 18, and 20 can be adjusted to effect a controlled shutdown of the reactor system 12 based on instructions from the control system or an operator, for example based on the temperature of the catalyst within tube 30. For example, valve 16 can be opened to depressurize the steam vessel 22, thereby reducing the temperature of the liquid water and suppressing the Fischer-Tropsch reaction. Alternatively or additionally, the supply of fresh synthesis gas can be stopped by closing valve 20. The recycle compressor continues to supply a recycle gas stream 32 to the reactor. Optionally, a pressure vessel (not shown) containing high-pressure nitrogen at a pressure higher than that of supply gas 32 can be connected to recycle gas line 36 or supply line 32 to inject nitrogen gas into the catalyst fill tube 30 in an emergency. Thereafter, valve 18, which controls the supply of boiler feed water to the steam vessel 22, can also be closed to shut off the supply water.

Claims

**Claim 1** A method for controlling a process comprising a steam system coupled to a reactor system, wherein the steam system supplies a stream of liquid water under pressure to the reactor system to cool the reactor system, thereby generating a steam stream, and comprises a steam vessel that receives the steam stream from the reactor system, the method comprising: (i) obtaining a first total liquid level measurement within the steam vessel using an inferential level device; (ii) obtaining a second total liquid level measurement within the steam vessel using a direct level measurement device; (iii) calculating, using a control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) activating an alarm using the control system if 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 of the first total liquid level measurement and the second total liquid level measurement. A method for controlling a process comprising a steam system coupled to a reactor system. **Claim 2** The method of claim 1, wherein the reactor system is operated at a higher pressure than the steam system. **Claim 3** The method of claim 1 or 2, wherein the inferential level device is a volumetric device or a differential pressure device. **Claim 4** The method of any one of claims 1 to 3, wherein the direct level measurement device is a guided wave radar device or a float device. **Claim 5** The method of any one of claims 1 to 4, wherein method steps (i), (ii), and (iii) are operated continuously. **Claim 6** The method of any one of claims 1 to 4, wherein method steps (i), (ii), and (iii) are performed by measurements taken every few seconds, every few minutes, or every few hours. **Claim 7** The method of claim 1, wherein the difference between the first total liquid level measurement and the second total liquid level measurement is calculated using a time average or a statistical method. **Claim 8** The method of any one of claims 1 to 7, wherein the control system is a distributed control system. **Claim 9** The method of any one of claims 1 to 8, wherein a display system having a visual or audible alarm is connected to the control system. **Claim 10** The method according to any one of claims 1 to 9, further comprising, in response to the alarm, one or more of the steps of monitoring the flow and temperature of the liquid water from the vapor vessel, monitoring the temperature of the reaction vessel in the reactor system, and monitoring the chemical composition of the liquid in the vapor vessel.

11. The method according to any one of claims 1 to 10, wherein, in response to the alarm, the method further comprises the 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 water under pressure.

13. The method according to claim 12, wherein the Fischer-Tropsch catalyst is provided as a floor with pipes or plates for carrying water, or the Fischer-Tropsch catalyst is provided in a plurality of reaction tubes cooled by water.

14. The method according to claim 12 or 13, wherein the Fischer-Tropsch catalyst is used in combination with a catalyst carrier in a tubular Fischer-Tropsch reactor, and the catalyst carrier containing the Fischer-Tropsch catalyst is disposed in one or more tubes cooled by circulating water under pressure.

15. The method according to claim 1 for controlling a process comprising a steam system coupled to a reactor system, wherein the steam system comprises a steam vessel that supplies a stream of liquid water under pressure to the reactor system to cool the reactor system, thereby generating a steam stream, and receives the steam stream from the reactor system, the method comprising: (i) obtaining a first total liquid level measurement within the steam vessel using an inferential level device; (ii) obtaining a second total liquid level measurement within the steam vessel using a direct level measurement device; (iii) calculating, using a control system, a difference between the first total liquid level measurement and the second total liquid level measurement; and (iv) activating an alarm using the control system if the difference between the first total liquid level measurement and the second total liquid level measurement is greater than or equal to 5% of the lower of the first total liquid level measurement and the second total liquid level measurement. A method for controlling a process comprising a steam system coupled to a reactor system.

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