For complex chemical plants to survive in an extremely competitive market, failures can be the difference between profit and loss, or in the worst case an environmental disaster. Obtaining reliable statements on the behaviour of a plant requires comprehensive and accurate information. To achieve this level of understanding, the use of computer-aided engineering (CAE) is continually increasing.
The term 'large-scale plant' has changed its meaning over the years. Traditionally methanol production would have a daily production rate of 2000 tons per day, today the production rate is typically 5000 tons per day and some up to a capacity of 10000. The challenge is not only the rapidly increasing production rate for engineering departments, but the knowledge of how to get there. More and more often multi-stranded plants are substituted by single-stranded production lines. In such cases, faults can impact on the whole plant, therefore high reliability of processes is absolutely necessary.
One of the enterprises that has a central position in the market is the Frankfurt, Germany- based Lurgi GmbH. The company which employs 1450 people worldwide, 750 of them in Germany, is engaged in the construction of plants for the production of petrochemical intermediates and final products as well as synthetic fuels and oleo chemistry. One of the main fields of activity for plants is gas production and treatment. Lurgi itself does not have a production line, but orders are given to contractors.
Simulation is essential
Process technology requires highly accurate planning and outstanding engineering. The increasing complexities, even for experienced engineers, often mean experience alone is not enough for an efficient and stable solution.
To simulate and analyse models which are unable to be calculated manually, computer based simulation programs are often being used. Computer aided engineering (CAE) programs allow reliability planning to be enhanced significantly. Failures can be avoided or corrected before they cost additional time and money.
The list of computational fluid dynamics (CFD) applications used is comprehensive, including FEM analysis (Ansys) and process simulation (Aspen) as well as 3D CFD programs (Ansys, Fluent). For the system simulation of the mile long and branched piping systems, Lurgi GmbH has successfully used Flowmaster for more than 10 years.
Fast model generation
Unlike 3D CFD programs which require CAD-geometry to model systems, Flowmaster only requires schematic models which can be generated using standard components and drag and drop functionality.
The behaviour of these components (pipes, valves, pumps,etc) is described by physical equations and/or empirical data which are stored in thematically oriented libraries.
Alternatively, own calculations and tests can be used to understand the behaviour of components using characteristic curves or arrays obtained from the originator.
For clarity and to minimise the potential number of error sources, the models are kept as simple as possible. For example, the typical behaviour of a whole sub-system can be described by its pressure loss; nevertheless, the analysed models often obtain up to 5000 components.
"In principle we try to generate our models as simply as possible," explains Jürgen Bohle, Chief Engineer at Lurgi GmbH. "but to calculate for example pressure losses and distribution, besides the literal system you need the character and position of in- and outlet - that's just physics."
Today, Flowmaster is an essential component of the engineering process at Lurgi, especially as it is the only software code which allows the calculation of pressure losses in the widely branched piping systems. The field of application ranges from research, development and plant design through to failure analysis resulting in preventive activities for future projects.
Jürgen Bohle said: "Our main focus is on the dimensioning and optimisation of new components or devices which can be used as so called 'prop equipment' in future projects to tackle distribution problems. Our main objective is the best possible homogenous gas distribution in piping systems.
"A typical example of this kind of problem is the injection system of an MTP plant (methanol to propylen) where liquid DME (Dimethylether) is blown into a hot gas flow. This process runs in a reactor where a large number of jets on different levels have to be fed over a widely branched piping system. For a highly homogenous spray and evaporation flow we optimised the feeding and atomisation using Flowmaster," continues Jürgen Bohle.
More than 800 pipes can diverge from the manifolds in such plants. Without reliable system simulation, the engineers would not be able to ensure a homogeneous inflow to the catalytic converters and as a result a stable and efficient process.
Flowmaster is even used for safety relevant research. One of many examples is the analysis and optimisation of the opening process of safety valves for a fire suppression system to ensure that enough water is available for all hydrants.
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Ulrich Feldhaus is with Flowmaster GmbH, Idstein, Germany. www.flowmaster.com