Pursuit for enhanced functionality is common for materials and process research, both looking continuously for new solutions that save the usage of materials while also being energy efficient and environmentally benign. The success of a new application is affected not only by its internal physical and chemical interactions but also by the surrounding environment. The expert group led by research professor Pertti Koukkari has developed new thermodynamic calculation methods, which can be diversely used for predicting and steering both manufacturing of functional materials and development of efficient chemical and biochemical processes.
In 2007, Professor Koukkari and Senior Scientist Risto Pajarre were given the Best Paper Award of the CAPLHAD Journal (1) for their publication on the constrained Gibbs energy minimization method (CFE). The paper presented, for the first time, a method by which thermodynamic free energy calculation can be generalized for systems subdued to physical or dynamic work factors. With the help of the method presented in the publication, it is possible to handle both time-dependent kinetics and various physical constraints in the classical thermodynamic calculation routines, and thus significantly expand the scope of their deployment. Since then the awarded method has been used in several practical applications that have been presented both in VTT’s research reports and on international science forums. The new approach has also increased the interest in the use of VTT’s ChemSheet calculation software around the world. Prof. Koukkari has now compiled the background on the CFE method into a new publication in the VTT Technology publication series, and two dissertations related to this demanding subject are about to be finalised.
Inclusion of dynamic changes in equilibrium calculations
Mastering of complex chemical and physical interactions is a lasting problem in process and materials science. Physico-chemical relations are utilized for the purposes of developing structural and operational material properties and controlling the durability of materials under various conditions. In a similar manner, the process technology aims at achieving optimal functionality with a focus on both saving raw materials and energy and reducing the environmental impacts of the overall process.
The chemical and physical interactions occurring in reactive flows and material structures can be mathematically described using the thermodynamic free energy. The minimization of the free energy combines the basic laws of the thermodynamic theory and quantifies all changes generated by the physical and chemical driving forces in various materials. The spontaneously occurring chemical change is guided by the tendency of the reaction system to reach equilibrium (Gibbs free energy minimum). As thermodynamics also gives a mathematical form to the respective energetic changes, the approach provides its masters an access to an efficient and systematic methodology for managing both material properties and behaviour of processes. Over the past few decades, the rapid development of computers has contributed to transforming the abstract thermodynamic theories to practical solutions. Fortuitously, solving thermodynamic problems does not usually require large computational capacity, as the numerical performance of personal workstations is quite sufficient in most cases.
Conventional thermodynamic calculation methods focus on phase changes and on products of chemical reactions as function of temperature and pressure. Therefore the methods in wide international use are specifically suited for calculating chemical equilibrium compositions and for drawing diagrams describing multicomponent phase equilibria. Both approaches are extensively used in chemical technology and material sciences.
However, in practical processes equilibrium is seldom reached, or the composition of the material being processed is affected also by other than purely chemical factors. Accordingly, a new approach was needed for problems in which both the time dynamics of the chemical change and various physical preconditions must be taken into account. The scientific significance of the CFE method is expressly linked to its ability to define the thermodynamic potentials of the constituents of the system in a versatile manner, and, in accordance with the second law, also during the change, not just in a state of equilibrium. It therefore also facilitates calculation of cases in which the change can be effected in the reverse (non-spontaneous) direction by application of external energy input, for example, when charging batteries using electric current.
The computational programs developed by VTT have been commercialised in collaboration with the German SME GTT Technologies GmbH, which markets and distributes thermodynamic calculation software and databases worldwide. VTT’s ChemSheet software has been a part of this product family since 1999. ChemSheet and the simulation program for industrial rotary kiln systems (KilnSimu) are being used in more than 20 countries by both research and industry. In Japan the programs are represented by the Tokyo-based SME RCCM. Furthermore, the Finnish SME Process Flow Ltd OY, specialising in process simulation services, has also been involved in the international collaboration (see figure 2).
Process technology: energy savings and new solutions
Customers for VTT’s advanced thermochemical modelling represent a wide range of industrial fields including energy and power production, pulp and paper manufacturing, chemical, metallurgical and steel industries. In many cases complex reactions occur in reactors with challenging conditions where direct measurements are but impossible. The new method allows for the computational simulation of such processes with high level of accuracy. As the thermodynamic method also takes into account the chemical and energy changes concurrently significant improvements in raw material and energy efficiencies of the process can be achieved simply by adjusting the operational conditions on the basis of the computer simulation. For example, in zinc recycling taking place in a rotary kiln reactor at high temperatures, it has thus been possible to reduce the carbon footprint of the kiln by as much as 40 per cent. The new process, which applied ChemSheet modelling in its developing stage, was recently approved as European BAT (Best Available Technology).
While many of the conventional process models are focussing on the main reaction components, ChemSheet with its systematic thermodynamic description covers all system elements and compounds. Consequently, harmful emissions and potential by-products are also included in the detailed simulation analysis. Such approach has been applied, for example, at the UBE Industries cement factories in Japan for the elimination of undesired chlorides and sulphates from the cement manufacturing process. Additional applications in Finland include removal of non-process compounds from the chemical recovery circuit of pulp-making plants. It is characteristic to ChemSheet versatility that in Japan the focus is on gas circulation of cement kilns operating at above 1,400-degree temperatures and in Finland on control of foreign substances in forest industry’s water circulation systems.
Another typical industrial application is the introduction of a totally new process chemistry concept with the help of successful simulation; one example of this was the extensive transfer to neutral paper manufacturing processes implemented in the industry in the early 2000s. In this case, the key innovation was to include the built-in electric charge of cellulose fibre – so-called ion exchange potential of the fibres – in the paper making chemistry models by using the CFE technique. By using the thermodynamic ion exchange potential in the calculations, it was possible to control the acidity of machine stock and to define the renewed dosages of chemicals required for the neutral operating conditions.
The challenge today is the in-line production of precipitated calcium carbonate (In-Line PCC, to be used as filler and pigment) by direct crystallization process in the paper machine pipeline. To reach this goal, VTT researchers collaborate with the Savonlinna based SME Wetend Technologies Oy, which markets and develops the in-line technology. The pilot tests conducted by Wetend in 2014 have produced momentous new results to achieve such a breakthrough technology. The In-Line PCC technology, with highly competitive production costs, provides major benefits to its users by forming strong fibre carbonate composite structures, by replacing valuable fibres in paper and board with filler materials and by cleaning the plant’s water circulation.
Future challenges in BAT technologies and process control
The on-going global digitalisation of process industry provides new opportunities and brings new challenges for the thermodynamic methodology. Many of the traditional technologies within process industries have been designed at a time when advanced computer techniques, such as ChemSheet, were not available. Modelling has been found to be a tool of paramount importance when developing new BAT-level techniques and striving for maximum efficiency with minimal environmental impacts.
The multi-component models of unit processes can be further coupled with the control system for the entire process. If the process-specific time constants allow, even thermodynamic models can be used as components in the automation software. An example is the ChemSheet simulation of the Xstrata Nickel company’s multi-phase nickel smelter in Canada, which is used as support for the on-line control of the multi-stage process. Similar solutions are currently being studied and developed in Finland in the ongoing SHOK (Strategic Centre for Science, Technology and Innovation) programme projects. The other up-to-date field for process applications includes various undertakings to improve recovery and recycling of process chemicals and to develop new industrial Cleantech solutions. For example in Austria, ChemSheet has been used when developing technology for the recycling of phosphate nutrients from recycled sewage sludges.
New openings in materials science
Even though VTT researchers have mainly focused on the renewal of process technologies, the usage of CFE methods has also produced several new openings in material sciences. In materials technology, the challenge is in most cases the mastering of the structure-property-performance chain. Advanced thermodynamics often helps to reach practical solutions by its predictions of work functions in various material combinations. VTT researchers look for international collaboration to explore new areas of materials research, yet also publications on ChemSheet provide information of novel CFE technology developments. The new technology offers, for example, a means to examine the functionality of surface layers formed in alloys and mixtures in their manufacturing phase and affecting both their processing conditions and product end uses. The Norwegian SINTEF research institute has used ChemSheet surface phase analysis both for the development of the smelting process of siliciferous metal alloys (ferro-silicon) and again in research and development of the surface properties of polycrystalline silicon used in solar cells. In Japan, Osaka University and its collaborators have applied surface energy research to the manufacturing technologies of new magnetic materials of high susceptibility. The objective has been to take advantage of the surface energy of nanosize powders so that the magnetisation of the alloy product is optimised with reduced input of the expensive highly magnetized component. In the development of bending displays, the problem is how to find materials with high enough electrical conductivity, but also with sufficient surface cohesion at the grain boundaries. With the help of calculation models linking the interactions of surface energy and material composition it is possible to study suitable material combinations where both of these properties are optimised.
The challenges of nuclear technology include a range of chemical problems, where multicomponent thermodynamics is a must. Such is the prevention of potential radioactive emissions of caesium and iodine compounds which may form during serious accidents. With the help of ChemSheet, VTT has linked the Gibbs’ian thermochemistry as part of the US-made MELCOR nuclear power plant simulation software, and this combined technology has been awarded qualification as a tool abiding the requirements of nuclear safety related work set by the Nuclear Regulatory Commission (NRC) in the U.S.A. In addition, quite recently research scientist Henri Loukusa has written a new Gibbs energy minimization program in order to calculate the amounts of different chemical compounds formed in nuclear fuel. Tens of fission products are formed through fission in nuclear fuel during irradiation. Their chemistry affects the material properties of fuel rods, which are important in safety analyses of nuclear reactors. The modelling of the chemistry of this complex system is almost impossible with other than thermodynamic methods.
Strength and corrosion-resistance for steel products
Steel industry has been a key user of the conventional phase diagrams mentioned at the beginning. Steel gains its properties, besides from the alloy components used, also from the phase changes occurring in the solidification processes during casting. Such changes may further take place in annealing and rolling. They seldom lead to equilibrium compositions, but are targeted to a product with specific physical, mechanical and chemical properties that often are reached with a particular (non-equilibrium) phase structure.
For example, in manufacturing ultra-high-strength steels, so-called paraequilibria can be applied, where non-metallic substances (such as carbon) are apt to reach equilibrium while metallic components (such as iron, chromium, and nickel) occur at constant ratios in the various phases. The manufacturing of these special alloys is proven with practical experience. The constrained Gibbs free energy technique then provides new possibilities for developments when the theoretical paraequilibrium phase descriptions can be leveraged through computer simulation. While the paraequilibria calculations follow the same principles as those used for kinetically constrained chemical processes, in steelmaking it is also possible to use other methods offered by CFE technology. For example, thermodynamic models on mechanical processing (rolling) or, e.g. on magnetically controlled austenite/ferrite changes may help in the development of new product properties.
Another excellent example of a material application utilizing ChemSheet technology is the SteaMax expert system developed within Valmet Power. SteaMax is used for material durability control and corrosion prevention in new biofuel boilers supplied by Valmet for power production. In this technology it is essential to maintain high efficiency with reduced emissions notwithstanding the varying composition and quality of the biomass and waste fuels. They characteristically contain high amounts of alkali metals and chlorine, often in combination with low amounts of sulfur.
The presence of these elements increases the risk for operational problems such as corrosion, fouling and bed sintering. SteaMax expert system based on the ChemSheet program helps to avoid service malfunctions and to secure operating life of critical plant components. In plant design, it applies both for determining the maximum steam temperature and for the selection of heat exchanger materials and superheater locations. During operation, SteaMax can be used to find the optimum fuel mixture for the boiler, e.g. to find the maximum allowed fraction of a high chlorine waste fraction. Thus, the innovative ChemSheet based system also has opened a lead to a new service business based on customer needs while operating the plant with different fuel mixtures and when solving maintenance problems.
Publications and theses
The recent issue of the VTT Technology series published by professor Koukkari presents the new method by simple basic examples. The dissertation by M.Sc. (Eng.) Risto Pajarre at Aalto University describes, besides the thermodynamics of the CFE technique, also a number of new application fields, such as boundary surface phenomena of key importance in material sciences and the interaction of external force fields and chemical changes.
The thesis of Lic.Sc.(Tech.) Petteri Kangas on the use of CFE technology in the development of new simulation models for gasification and pyrolysis technologies is also on its final stretches at Åbo Akademi university. “A thermodynamic approach provides a common basis for easy comparison of raw material and energy efficiencies between different thermal processing technologies,” says Lic. Kangas.
– An interesting area of application is the chemical and thermodynamic simulation of combustion engines, where optimisation of the use of, for example, new biofuels will constitute a future challenge. The traditional combustion technology models are either chemically simplified thermodynamic energy models or mechanistic chemistry models, consisting of several hundreds, or even tens of thousands reaction equations. Using the latter, it is difficult to manage the thermodynamic variables that are essential in engine technology. In this application, the international benchmark used for VTT’s CFE technology is the RCCE (rate controlled chemical equilibrium) modelling technology developed at MIT in the 1990s-2000s, particularly focussed in combining the reaction kinetics and Gibbs’ian thermodynamics of combustion processes. Constrained Gibbs free energy techniques could leverage the benefits of various techniques and still produce sufficient information on both state properties and combustion chemistry for practical applications, says Kangas.
Dr (Tech.) Pertti Koukkari acts as research professor of sustainable chemical processes and systems at VTT’s Process Chemistry business area. Professor Koukkari is specialised in application of multi-phase chemical thermodynamics on the development of cost-effective process solutions and functional materials. The free energy technologies and ChemSheet and KilnSimu calculation programs, developed at VTT under his leadership, are globally recognised and used by industry, universities, and research institutes in more than 20 countries.
1 Calphad Journal, published since the 1970s, is a special publication, focusing on computational thermodynamics of material structures. It is held in high esteem within the material sciences sector in particular. Over the past few decades, the computation methods of phase diagrams and chemical equilibria developed by the Calphad community have been adopted all over the world, and they are currently applied by both universities and the industry. Computer simulations based on Calphad methods have played a crucial role in the strong development within the material sciences over the past few decades.
Figure 2. The thermochemistry can be combined with 3-dimensional fluid dynamic (CFD) modeling when doing detailed simulations of complex reactors. The figure shows temperature distributions received with a combined CFD and thermodynamic model for a counter-current rotary kiln. The colour code below gives the radial temperature distribution in the gas phase and the curves above the iterative solution of the axial average temperature of the solid ’bed phase’. In the 2-way coupling the thermodynamic and aerodynamic model apply sequential boundary conditions from each other and give the best simulation result. (http://www.kilnsimu-fks.com/ 2014)
Figure 3. On the left, a phase diagram describing eutectic melting of tin-bismuth (Sn-Sb) alloy, one for the bulk material and the other for a 10-nanometre scale particle (the asterisk shows the experimental measuring point). The surface energy of nano-scale particles lowers the melting point of the alloy. (Lee, Penttilä & al., JOM-J Min. Met. Mat. S. 2005)
On the right, the topology of boiling points and dew points in reactive mixture of ethanol and acetic acid shown as ‘iso-affinity surfaces’ in relation to the extent of reaction ( ξ ). The reactive phase diagrams could be helpful when developing efficient recovery processes for new biochemicals. (Koukkari& Pajarre, Pure App. Chem. 2011)
Figure 4. Rotary kilns are widely used in different applications of the process industry.
Figure 5. The phase diagrams for carbon containing Fe-Cr (iron-chromium) alloy. The phase structure of the paraequilibrium (below) is much more simple than global equilibrium (above). Mastering of paraequilibria supports the development of high-strength steels. (Pelton, Koukkari & al. J. Chem. Thermodynamics 2014)
Figure 6. New solution models for multicomponent solution equilibria are needed for various cleantech and mining applications. On the left solubilities of sulphate salts for hot (100 C) aqueous solutions. (Pajarre 2014).
On the right : Validation of the chemical safety model for nuclear power plants. The pH control of the water pools in the nuclear plant safety system is designed to prevent the release of radioactive compounds into the atmosphere. (Penttilä et al. 2009).
Figure 7. Examples of modelling of the demanding unit processes of forest industry and thermal conversion of biomass.
Left: enrichment of sulfur and alkali metals in the flue gases of the recovery boiler. In the CFE super-equilibrium model sulfur and alkali compounds are determined with enrichment constraints while the rest of the flue gas is in local equilibrium. (Kangas et al., J-FOR 2013).
Middle: The main components formed in biomass gasification. The CFE model includes kinetic constraints for the formation of methane, tars and char. The rest of the system reaches thermodynamic equilibrium. (Kangas et al., Fuel 2014).
Right: Carbon conversion of biomass during pyrolysis. The CFE-model includes constraint for the pyrolysis reaction of solid biomass. (Kangas et al., Energy & Fuels 2014).