Ethylene plants are at the beginning of the petrochemical value chain. They produce the base material for a large number of important downstream products. Thermal steam cracking converts liquid and gaseous hydrocarbons such as naphtha, ethane and other mineral-oil based mixtures into ethylene (C2H4), propylene (C3H6) and various olefins and aromatics.
The production process takes place in three stages:
In order to achieve a consistently high throughput with constant quality, operators must maintain several critical process parameters precisely - pressure, temperature, flow rate, level - in the respective sections of their plants.
For these tasks, WIKA offers a wide-ranging product portfolio for the aforementioned measured variables. Individual instruments and systems are adapted to customer-specific requirements. The necessary calibration equipment is also available for the applications. Plant operators can also make use of the manufacturer-independent calibration service. The instruments can be tested in WIKA's own accredited calibration laboratories or directly at the customer's site.
WIKA also supports its customers with other services. Service technicians around the world will make sure that measuring instruments, in case of breakdowns and repairs, are fully functional again in a short space of time. They install complex measuring units and provide support for the commissioning of the instrumentation.
In the cracking or pyrolysis furnace of the steam cracker, the long-chain hydrocarbons are thermally cracked. In most plants, this is done through the addition of hot steam.
Inside the cracker, numerous coils run parallel to each other. After a preheating phase, the CxHx-H2O mixture is sent, in less than a second, from the furnace inlet through the coils, which are fired from the outside with burners. The homogeneous distribution of the media flow is crucial for consistent product quality. This process is supported by specially manufactured Venturi nozzles with a high surface quality and a precisely designed rounded profile to prevent uneven coking.
Another critical variable is the temperature of the cracked hydrocarbon at the exit of the furnace, the Coil Outlet Temperature (COT). It ranges between 750 and 900 °C, depending on the feedstock. The temperature measuring instruments used there must withstand a highly abrasive media flow. Invasive measuring points therefore require Stellit® thermowells/protection tubes, either made of bar stock material or in a coated version. As an alternative, surface temperature sensors with fast response times can be considered.
In addition to the steam-based cracking method, the industry also utilises the catalyst method. This enables a higher yield of propylene, for example. Temperature monitoring in catalytic cracking is mainly based around multipoint thermometers.
The gas at the end of the cracking process is chemically unstable and highly reactive. It must therefore be cooled down by several hundred degrees, immediately after leaving the cracking furnace. This is done in the cracking gas cooler/transfer line exchanger. The heat energy released from this is used in the heat exchange process to produce high-pressure steam for the plant.
Cooling continues in the subsequent process steps of oil quenching and water quenching. In these two processes, pyrolytic oils, pyrolytic benzene and solid particles are removed from the media flow simultaneously.
The oil quench as a primary fractionator is of primary importance. The differential pressure between the different levels of the column is the key indicator for its optimal function. A rising value implies the possible presence of deposits that can reduce the throughput. Precisely balanced differential pressure transmitters, in combination with two flush diaphragm seals, whose diaphragms are made of media-specific material, ensure accurate monitoring of this parameter.
Compressors compress the prepared gas mixture in four to six successive stages. Acid gas scrubbing is an intermediate step to remove unwanted hydrogen sulphide (H2S) and carbon dioxide.
In the course of the compression, the medium is intercooled and the resulting condensate is separated in separators. Their level must be monitored continuously to prevent any potential overflow. The compressors and downstream plant areas are thus protected from excessive moisture content.
Modular bypass level measuring instruments are recommended for this task, as they transmit the level precisely and, via an optional switching function, trigger a warning in good time. At particularly critical points, versions with a double chamber enable redundant measurement. In addition to the bypass indicator, other measuring systems can be integrated, for example, reed chain, magnetostrictive sensors, radar and vibrating fork.
For smooth operation of the compressors, the temperature in their heavily stressed bearings must be monitored. Highly vibration-resistant temperature sensors ensure the necessary accuracy for this.
After compression and further cooling in a refrigeration plant, the processed gas mixture is separated into the end products. This process takes place in several distillation columns, connected in series (demethaniser, deethaniser, depropaniser, debutaniser), and splitters, mainly for C2, C3 and C4 compounds.
Due to the different temperatures in the distillation columns, the components with a lower boiling point rise to the top and are skimmed off there, while the substances with a higher boiling point sink to the bottom and are fed from there to the next column. Gradually, the following media are obtained in this way - ethylene, propylene, butene and other hydrocarbon fractions. Ethylene is the largest fraction in terms of volume.
For a constant and clean separation of the challenging media, the process pressure and process temperature must be in perfect balance at all times. This requires both their high availability and the corresponding accuracy of the installed measuring instruments.
Measurement quality is not the only critical factor in instrumentation for monitoring pressure and temperature, but it is also true for level and flow - the instruments must be resistant to the process conditions in order to fulfil their task as required and, at the same time, not jeopardise the product quality. All measuring instruments are therefore made of application-specific materials, and weld seams are also subjected to non-destructive tests.
The acetylene and methyl acetylene/propadiene (MA/PD) produced during the distillation/cracking process are converted into ethylene and propylene, respectively, via hydrogenation with hydrogen. (C2H2 + H2 => C2H4; C3H4 + H2 => C3H6). In this way, the total yield of these two high-quality end products increases.
The selective hydrogenation process takes place in fixed-bed reactors filled with the relevant catalysts. The control of the process depends critically on the flow of acetylene and MA/PD being subject to a homogeneous temperature. Multiple and optimally distributed temperature measuring points in the catalyst bed enable immediate detection of hot and cold spots. The monitoring function thus creates the prerequisite for the reaction to proceed uniformly. At the same time, it allows conclusions to be drawn about the ageing of the catalyst.
Two versions of multipoint thermometers can be used for the measurement. The version without a thermowell/protection tube enables the measuring points to be set directly in the catalyst bed. This solution offers the fastest response time. An alternative to this are multipoint thermometers in slide design with a thermowell/protection tube. Here, the sensors for the measuring points are positioned individually in the thermowell/protection tube by means of a guide rod and pressed against its wall to achieve a comparatively short response time. The advantage: If necessary, the temperature sensors can be removed individually from the thermowell/protection tube during operation - i.e. with the process remaining closed - and their measuring performance can be checked.
In addition to the temperature, the pressure in the reactor must also be monitored. With the measurement, it is important to prevent rising temperatures in the hydrogen from causing drift. In this case, diaphragm seals with gold-coated diaphragms ensure permanently reliable pressure transmission to the respective measuring instrument.
All products are stored temporarily in a gaseous or liquid state in large tanks until further use. Ethylene, for example, must be cooled down to -103 °C in order that a large quantity can be stored in a relatively small space. The filling process requires monitoring of the temperature conditions in the tank. To achieve this, multipoint thermometers are inserted into the tank from above.
In view of the large difference between the media temperature and the ambient temperature, additional sensors are located on the steel shell of the tank. During initial filling, they serve to warn, in good time, that the tank shell is cooling down too quickly, thus preventing potential cracking.
Temperature sensors in the concrete base of the tank and in the bunds under the external piping fulfil a comparable purpose - if they detect a sudden cooling at the measuring point, this is an indication of a leak in the tank vessel.
Every ethylene plant has at least one flare system. This is in operation around the clock to flare off, if necessary, any unwanted or excess gases from all parts of the production in a controlled manner.
The flare system is thus also a fundamental part of plant safety. Its measuring instrumentation includes thermocouples. They monitor the pilot burners and also provide information as soon as a sufficient amount of gas produces a flame.
Consultation, design, implementation – all from one source
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, application-specific systems – from simple manual work stations through to fully automated test systems in production lines.
Calibration technology and calibration services
Are you looking for suitable calibration equipment for your applications? Get an overview of our wide range of calibrators. Use our calibration service for pressure, temperature, force, flow and electrical measurands. We calibrate your references and test equipment independently of manufacturers in our accredited calibration laboratories, or directly on your premises.
Additional services
WIKA will support you with additional services from our trained experts. Service technicians will make sure that your measuring instruments, in case of breakdowns and repairs, are fully functional again in a short space of time. From pressure controllers and diaphragm seal systems to calibration baths - all from one source. We install your measuring units and provide support for the commissioning of the instrumentation. Through our local experts, we can be reached worldwide, are quickly available and tuned to individual circumstances. Try it for yourself.