Ammonia has, for many years, been established as an important basic material in the field of fertiliser production (urea and ammonium salts). Using the Haber-Bosch process, hydrogen and nitrogen are converted into ammonia (NH3) under pressures of 250 bar and temperatures of 550 °C. The individual plant concept is chosen depending on the geographically available raw material and the desired production capacity.
The production process takes place in four stages:
In order to reduce the energy required and achieve a consistently high throughput with constant quality, the operators are dependent on maintaining several critical process parameters – pressure, temperature, flow, level – in the respective plant sections.
Rather than coal, naphtha and oil, today, natural gas is mainly used as the feedstock for ammonia production. Unwanted natural gas components such as sulphur are bound in the form of hydrogen sulphide (H2S) through targeted hydrogenation and then removed in the desulphurisation reactor.
In order to monitor the activity of the catalyst in the hydrogenation reactor constantly over the entire bed, specially adapted multipoint thermometers are used to monitor multiple points. Correct functioning must be permanently guaranteed, since any sulphur components that slip through would poison the downstream process steps. The correct gas flow is detected by using compact orifice plates and Venturi tubes with installed differential pressure transmitters.
Depending on the feedstock and the selected plant design, one, two or three types of reformers are installed in series for steam methane reforming:
This combination is the basis for the optimal recovery of hydrogen and syngas. In the catalyst bed-based cylindrical pre-reformer, the first pre-reforming stage takes place under the supply of hot steam at around 500 °C and 30 bar.
In order to monitor the activity and ageing of the catalyst constantly over the entire bed, specially adapted multipoint thermometers are used to record multiple points. The ratio between gas and steam must be monitored precisely to prevent coking and damage to the catalyst. Compact orifice plates and Venturi tubes with mounted differential pressure transmitters perform this task reliably and durably.
In the primary reformer (known internationally as a steam methane reformer – SMR), reformer tubes are mounted vertically in several rows, which are continuously fired with burners from the outside. The conversion of the gas-steam mixture into hydrogen, carbon monoxide and carbon dioxide takes place in the catalyst-filled interior of the tube. The walls of the reformer tubes, which are usually specially alloyed, are permanently under particular stress due to the high temperatures. A crack during regular operation due to overtemperature is not uncommon. This can make it necessary to shut down production or could even damage the entire plant. The alternative operation of the process – as is often practised – with a reduced flame leads to a reduced throughput and the plant operator must accept permanent losses in efficiency.
Through the targeted analysis of the individual set-up and perfect positioning of tube-surface temperature sensors (XTRACTO-PAD®) matched to the tube materials, we offer the perfect solution. The exact tube surface temperature, which is independent of the flame strike, is recorded 24/7 due to the special shielded design. The SMR can now actually be controlled for longevity with the highest throughput. Furthermore, in addition to the tube surfaces, the temperatures of the flue gases produced by firing are also monitored in the chamber. Sensor solutions suitable for this purpose, with thermowells made of materials which are resistant over the long term, are developed specifically for your application.
After initially converting about one third of the gas passed through the primary reformer, the next stage “ignites” in the secondary reformer at temperatures of over 1,000 °C and pressures of over 30 bar to achieve a conversion of close to one hundred percent. The autothermal reformer (ATR) has a refractory lining in the interior. The steam-gas mixture and, via a large burner, preheated process air (oxygen and nitrogen components) are fed in from above. First, partial combustion – partial oxidation – takes place. Then the mixture flows through the catalytic bed, located in the middle part of the reformer, and the remaining methane components are finally converted.
In order to monitor this demanding process properly and continuously, the correct temperature is recorded in many catalyst beds by means of specially designed WIKA multipoint temperature sensors. Due to their robust design and the protection of the sensor elements against hydrogen poisoning, these sensors offer redundant measurement which is stable over the long term.
The carbon monoxide produced during the reforming process would negatively influence the subsequent ammonia synthesis by poisoning the catalyst used there. With the addition of steam, carbon monoxide is converted into carbon dioxide in the high-temperature shift (HTS) reactor and the low-temperature shift (LTS) reactor. The CO2 can be separated from the media flow even better in the subsequent process steps in the scrubber and methanator.
The uniform activity of the catalysts in the HTS and LTS reactors is crucial for the reaction. Unconverted carbon monoxide must not enter the next process steps. The state of activity and ageing can be determined, precisely, by using multipoint thermometers with multiple points distributed in the bed. This enables a measurement data-based prediction of the end of life cycle of the catalyst.
In the methanation reactor, unwanted residual carbon monoxide and carbon dioxide are removed from the hydrogen-nitrogen media stream. With the addition of hydrogen, the methanation of CO and CO2 to methane and water takes place using a catalytic reaction.
The uniform activity of the catalysts is crucial for the reaction. Unconverted carbon monoxide must not enter the next process steps. The state of activity and ageing can be determined, precisely, by using multipoint thermometers with multiple points distributed in the bed. This enables a measurement data-based prediction of the end of life cycle of the catalyst.
In the cryogenic purification unit, impurities, such as argon, are removed in an isolated cold box at temperatures below -170 °C and the desired ratio of hydrogen to nitrogen for ammonia synthesis is adjusted.
For such low temperatures, a range of WIKA instrumentation valves in monoflange and needle valve design are used (block, block-and-bleed and double block-and-bleed). This enables a safe and permanently sealed connection of the pressure transmitter and differential pressure transmitter solutions.
Special thermowell-temperature sensor combinations, that suit the low temperatures and the special installation conditions in the insulated double walls, are often used as a complete solution with a remote SIL temperature transmitter.
In the ammonia synthesis reactor, the synthesis of hydrogen and nitrogen into ammonia (NH3), under pressures of 150 ... 250 bar and temperatures of 400 ... 520 °C, takes place. The upright or horizontal reactors contain several catalyst-filled beds in which the gas mixture is converted in several passes.
To ensure that the required pressure for the desired synthesis is available continuously, measurements are taken at the reactor inlet using a temperature-compensated transmitter/diaphragm seal system (exact, even under high temperatures).
During the multistage reaction, a lot of heat is generated, which is removed using intercooling stages. The critical temperature distribution along the reactor beds is monitored by several fast-response multipoint thermometers with rapid heat transfer.
After the ammonia synthesis, the gas is cooled down significantly and liquefied in several stages using an interconnected system of heat exchangers and ammonia chillers. Diverse-redundant sensors are often used for critical level monitoring. Differential pressure transmitters, with fully welded, flush diaphragm seals connected via capillaries, output the hydrostatic level column.
Level indicators with reed chain or magnetostrictive transducers with two-chamber systems allow gas bubbles created by boiling ammonia gas to simply pass by the float, and thus provide an operationally proven continuous analogue measurement with a stable signal.
Precise calibration instruments are the starting point for resolving your test requirements. However, they only form one part of a high-performance calibration system. From our extensive product range, we can design a complete and individual solution for you which contains all the relevant components – with adaptability for test items, pressure and vacuum supply, components for pressure control and fine adjustment, through to voltage supply and multimeters for the calibration of electrical test items. Our particular strength lies in the project planning, development and the building of complete, individual, user-specific systems – from simple manual workstations through to fully automatic test systems in production lines.
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