Control and improve fluctuating processes

07 September 2021

Martina Walzer discusses how the application of digital capabilities can push the energy and resource transition.

The Carbon2Chem project is working on solutions to use CO2 emissions as raw material for the chemical industry. The project outcome may solve two challenges – to reduce the release of carbon-rich gases to the atmosphere and to substitute fossil raw materials as carbon source. To achieve these goals partners from science and industry are developing solutions for the sustainable management of energy and raw materials. 

The Kyoto Protocol pinpoints carbon dioxide as a greenhouse gas, which heavily influences global warming. This odorless and colorless gas emerges, for example, while burning fossil energy carriers as coal, oil, or natural gas and it is the main contributor to the human-intensified greenhouse effect (anthropogenic carbon dioxide). Various estimates assume a residence time (i.e. the time until natural processes finally remove the CO2 molecule from the atmosphere) of over 100 years. It is, therefore, essential to lower emissions in the coming years. 

Recycling of carbon to support the circular economy is the subject of the Carbon2Chem project. Partners from science and industry are collaborating to develop solutions, funded by the Germany Ministry of Education and Research. The aim is to use renewable energies to convert carbon-containing gases from steelworks into chemical raw materials such as methanol. The process needs to be designed so as to flexibly manage changes in loads. At the end, the exhaust gas from furnaces serves as input material for fuels, plastics or fertilisers. 

The challenges of these new processes lies in the fact that exhaust gas flows vary – in terms of both their volume and chemical composition. What is more, renewable energies – and the hydrogen they are used to produce – are not uniformly available. However, the process automation system must deliver safe, optimal control of these fluctuating processes. 

Reliable basis for AI
"In this project, we collect data from multiple temperature measurements and gas analysers and use AI tools to develop a multivariable control system. This leads to a more energy-efficient process, a longer service life for the catalyst as well as improved selectivity and yields," said Dr. Andreas Menne, head of Low Carbon Technologies department at Fraunhofer UMSICHT. "The data also helps to improve existing process-simulations and kinetic models. Both affect the cost-effectiveness and sustainability of the overall process due to model-based predictive control, predictive maintenance or co-simulation enable far-reaching optimisation."

Instrumentation and control systems from Siemens are effectively supporting the progress of developing the methanol synthesis out of gas streams, which fluctuate in terms of quantity and composition.

Siemens' scalable and flexible Simatic PCS 7 process control system is installed for process control and visualisation in the project. The data collection of temperature and exhaust gas compositions forms the basis for a downstream, multivariable control solution that is being developed by Siemens together with the Fraunhofer Institute UMSICHT, within the framework of Carbon2Chem. Thanks to its scalable system architecture, Simatic PCS 7 enables cost-effective implementation of dedicated automation solutions and economic operation of processes. 

The scalability applies to all parts of the system. The architecture is designed in a way that allows instrumentation and control to perfectly fit requirements. As Dörte Sack, head of Technical Concepts for the Chemical Industry at Siemens in Karlsruhe, explains: "Our technology is very flexible in this respect. The instrumentation and control can be expanded at any time if the plant is scaled up or if there are technological modifications. At the control level alone, multiple compatible automation systems, with appropriate price/performance ratios, are available to the user. The performance can therefore be optimally adapted to the tasks of the process." 

Accurate temperature profiles
The SITRANS TO500 system uses fibre-optic methods to determine the temperature profiles in a reactor. It enables many temperature measurements to be captured and evaluated simultaneously. Therefore several sensors (Fibre Bragg grating (FBG)) are arranged on a very slim fibre-optic measuring lance. 

A FBG is a microstructure typically of a few millimeters in length that is inscribed in the core of a standard single-mode fibre. It acts as a resonance structure for the selective reflection of wave lengths: it is a narrow-band filter. Changes in temperature and pressure cause a change in wavelength. To eliminate the influence of pressure, the FBG must not be subjected to strain when used as a temperature sensor. For this reason, the individual optical fibres, which are referred to as lances, are installed in protective tubes (metal capillaries). Due to the measured value transmission (reflection of light) in the same fibre, no additional cables are necessary. This means the cross-section of the protective tubes for the measurement setup can be substantially reduced. 

The transmitter emits light in a range of 1500 to 1600 nm. Based on the reflections of the individual FBG sensors, the system calculates the temperature profile and provides the values via a Profibus interface for evaluation in the control system. This information forms the basis for implementation and optimisation of the processes control as well as for management of the installed assets.

Reliable information is vital for realising innovative processes to meet the goals of the Paris climate agreement without jeopardising the competitiveness of energy-intensive sectors such as the chemical, steel or cement industries in Europe. Precise data evaluation in dedicated applications and implementation of the results into improved control algorithms enable economical and sustainable process control. The digitalised plant thus enables industrial applications that advance defossilisation and modernisation in line with the EU Commission's Green Deal.

Martina Walzer is manager technical concepts at Siemens Digital Industries

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