This page contains news about my research such as new publications, new project grants, happenings, collaborations, or other topics of significance which I think could be of interest to others.


New paper on Ice formation and growth in alcohol mixtures

Our paper entitled Ice formation and growth in supercooled water–alcohol mixtures: Theory and experiments with dual fiber sensors has just been published in Fluid Phase Equilibria.

Increased knowledge on fluid-solid phase transitions is needed, both when they are undesired and can impair processes operation, and when strict control is required. In the paper, we present experimental results and theoretical predictions for the solid-formation and melting temperatures of ice in four binary water–alcohol mixtures containing methanol, ethanol, 1-propanol and 1-butanol. A dual fiber sensor set-up with a fiber Bragg grating sensor and a thin-core interferometer is used to detect the solid-formation. The predictions of melting temperatures with the cubic plus association equation of state combined with an ice model are in good agreement with experiments, but deviations are observed at higher alcohol concentrations. The measured degree of supercooling displays a highly non-linear dependence on the alcohol concentration. A heterogeneous nucleation model is developed to predict the solid-formation temperatures of the binary alcohol–water mixtures. The predictions from this model are in reasonable agreement with the measurements, but follow a qualitatively different trend that results in systematic deviations. In particular, the predicted degree of supercooling is found to be an essentially colligative property that increases smoothly with alcohol concentration. Experimental results are also presented for the growth rate of ice crystals in water–ethanol mixtures. For pure water, the measured crystal growth rate is 10.2 cm/s at 16 K supercooling. This is in excellent agreement with previous results from the literature. The crystal growth rate observed in ethanol–water mixtures however, can be orders of magnitude lower, where a mixture with 2% mole fraction ethanol has a growth rate of 2 mm/s. Further work is required to explain the large reduction in crystal growth rate with increasing alcohol concentration and to reproduce the behavior of the solid-formation temperatures with heterogeneous nucleation theory.

The paper is the last chapter in the PhD theses of both Dr. Ailo Aasen and Dr. M. S. Wahl, who both successfully defended their degrees recently. The video below is from the experimental set-up used in the paper and shows the formation of a water crystal forming in an ethanol-water mixture (similar mixture to wine). After the crystal forms by heterogeneous nucleation at the bottom of the container, it rises due to buoyancy, attaches to the sensor and continues to grow. The video is real-time, so the growth process is quite fast. We also find that the growth rate depends strongly on the specifics of the mixture.


Diego Kingston successfully defends his PhD

Diego Kingston successfully defends his PhD on energy efficient design and operation of process equipment such as chemical reactors and distillation columns. During his PhD, Diego had two research stays at PoreLab; a first period between March and May 2019, and a second period between September and November 2019. Under the supervision of myself and Prof. Signe Kjelstrup, he did important work on Minimum entropy production in a distillation column for air separation described by a continuous non-equilibrium model, and how coupling between heat and mass transfer as well as interfacial resistances influence the performance and design of processes, which constitutes a second paper that is still under revision. Diego was awarded his PhD with the highest distinction from the University of Buenos Aires, Argentina.

We are very grateful that Diego chose to visit us and very proud of him for successfully defending his PhD with the highest distinction. The title of his PhD is: “Application of Non-Equilibrium Thermodynamics to the development of models of interest to Chemical and Material’s Science Engineering“


New paper on the thermodynamics of thin films

The paper entitled Thermodynamic stability of volatile droplets and thin films governed by the disjoining pressure in open and closed containers has just been accepted for publication in Langmuir.

Distributed thin films of water and their coexistence with droplets are investigated using a capillary description based on a thermodynamic fundamental relation for the film Helmholtz energy, derived from disjoining pressure isotherms and an accurate equation of state. Gas-film and film-solid interfacial tensions are derived, and the latter has a dependence on the film thickness. The resulting energy functionals from the capillary description are discretized and stationary states are identified. The thermodynamic stability of configurations with thin films in systems that are closed (canonical ensemble) or connected to a particle reservoir (grand canonical ensemble) is evaluated by comparing the eigenvalues of the corresponding Hessian matrices. The conventional stability criterion from the literature states that thin flat films are stable when the derivative of the disjoining pressure with respect to the film thickness is negative. This criterion is found to apply only in open systems. A closer inspection of the eigenvectors of the negative eigenvalues shows that condensation/evaporation destabilizes the film in an open system. In closed systems, thin films can be stable even though the disjoining pressure derivative is positive, and their stability is governed by mechanical instabilities of a similar kind to those responsible for spinodal dewetting in non-volatile systems. The films are stabilized when their thickness and disjoining pressure derivative are such that the minimum unstable wavelength is larger than the container diameter. Droplets in coexistence with thin films are found to be unstable for all considered examples in open systems. In closed systems, they are found to be stable under certain conditions. The unstable droplets in both open and closed systems are saddle points in their respective energy landscapes. In the closed system they represent the activation barrier for the transition between a stable film and a stable droplet. In the open system, the unstable droplets represent the activation barrier for the transition from a film into a bulk liquid phase. Thin films are found to be the equilibrium configuration up to a certain value for the total density in a closed system. Beyond this value, there is a morphological phase transition to stable droplet configurations.

Most of the work behind this paper was done by Magnus Aa. Gjennestad, where the thermodynamic stability and the transition between stable (green) and unstable (red) has been shown in the figure below.


New paper on the use of fiber optic sensors to study phase transitions

The paper entitled Using Fiber-Optic Sensors to Give Insight into Liquid-Solid Phase Transitions in Pure Fluids and Mixtures, has just been accepted for publication in the journal entitled "Experimental Thermal and Fluid Science".

Fiber optic sensors offer a new and unique way to detect and analyze phase transitions, due to their small thermal mass and inert material. This paper presents and demonstrates a dual-sensor system to detect and analyze phase transitions in pure water and aqueous ethanol mixtures. With a multi-mode interferometer based on a thin-core fiber, and a fiber Bragg grating sensor, it is possible to differentiate between refractive index, temperature and strain in the sample system. The three parameters supply important information during a phase transition, but also in the characterization of the liquid and solid phases. Binary mixtures at non-eutectic concentrations are expected to separate into a solid phase consisting of only one constituent, and the sensors are demonstrated to be able to estimate the concentration in the remaining liquid phase. For pure water and low ethanol concentrations, the progression of the phase transition was found to be limited by heat transfer, whereas for higher concentrations the process becomes mass transfer limited. In pure water, strain due to thermal expansion of the ice hinders temperature measurements in the solid phase. The reflection-based geometry enables insertion probes that measure the properties inside the samples, with little or no disturbance of the system. By interpreting the sensor response in a known system, the sensing capabilities in unknown substances can be evaluated. The sensor system is able to capture the dynamics of the phase transition, which can be difficult to predict theoretically due to the multitude of contributing effects. Analysis of the combined signal from the two sensors enables the determination of the ethanol mixture melting points in agreement with the literature, within the uncertainty of the system (0.25 K).

The key person behind this work is Dr. Markus S. Wahl, who just defended his PhD on this topic. The work was also carried out in collaboration with Prof. Dag Roar Hjelme from NTNU. The figure below shows some of the results from the measurements and illustrates how these can be used to gain information about the phase transition.


Markus S. Wahl defends his PhD thesis

Markus Solberg Wahl successfully defends his PhD thesis entitled "Detection and analysis of liquid-solid phase transitions with fiber-optic sensors", 19th of May, 2020.

The thesis addresses the very interesting topic of using fiber optic sensors to detect phase transitions, and is a collection of three papers. In the first paper, we use experiments and modelling to study how the interference spectrum and reproducibility depends on how the fiber is manufactured. In particular, the cleave-angle is discussed in detail. It is concluded that the spliced region, which is more difficult to measure, plays a significant role in the produced spectrum. The thesis proceeds to demonstrate how multi-mode fiber interferometers can be used in conjunction with a fiber-Bragg grating (FBG) to analyze phase transitions in binary mixtures of ethanol and water. The temperature and strain sensitivity of the FBG is used to decouple these parameters from the multi-mode interferometer (MMI) response through a unique procedure. The remaining RI sensitivity of the MMI is used to measure the increased ethanol concentration caused by pure ice forming in the mixture. The measured melting points show excellent agreement with tabulated values. The sensor system developed is then eventually used to study ice formation in supercooled water-alcohol mixtures. The results are compared to theoretical predictions from heterogeneous nucleation theory. The effect on the nucleation barrier from solute type and concentration is studied, as well as the reduction in this barrier as a function of container material and pre-experiment rinsing procedures. Because of the dependency of the nucleation rate on the self-diffusivity of water, ice growth rates are measured in different ethanol concentrations to estimate the diffusivity at the liquid-solid interface.

I have had the privilege to be the co-supervisor of Markus, and Prof. Dag Roar Wahl from NTNU has been his main-supervisor. The first opponent of the defense was Professor Yuliya Semenova, TU Dublin in Ireland. The second opponent was Senior Research Scientist Sigurd Weidemann Løvseth, my colleague from SINTEF Energy Research. The administrator of the committee was Adjunct Professor Peter James Thomas from the Department of Electronic Systems, NTNU. Due to the ongoing virus crisis, it was impossible for the opponents to come to Norway. Moreover, since most of us are staying at home, the PhD defense was successfully executed interactively. I feel very grateful for the chance to work with Markus these years. We have had both scientifically interesting and fun discussions. Markus has created a new, very interesting branch in the research portfolio of the group that includes unique experimental investigations of solid-fluid phase transitions.


Ailo Aasen defends his PhD thesis

Ailo Aasen successfully defends his PhD thesis entitled "Bulk and interfacial thermodynamics of mixtures: From aqueous systems to ultracryogenic fluids", Friday 27th of March.

The thesis deals with a variety of topics such as predicting the thermodynamic properties of fluids that exhibit strong quantum effects, such as hydrogen, helium, neon, deuterium and their mixtures. The thesis also discusses nucleation in fluid mixtures (see the following Gemini article for a popular scientific summary) and solid formation in mixtures.

I have had the privilege to be the main supervisor of Ailo, and Dr. Hammer from SINTEF Energy Research has been his co-supervisor. The main opponent at his thesis defence was Prof. Carlos Vega from the Department of Physical Chemistry at the University Complutense of Madrid in Spain, and the second opponent was Dr. Thijs van Westen from Stuttgart University in Germany. The administrator of the committee was Prof. Maria Fernandino from NTNU. Due to the ongoing virus crisis, it was impossible for the opponents to come to Norway. Moreover, since most of us is staying at home, the PhD defense was successfully executed interactively. The committee was very satisfied with the work, and stated that it belonged to the top 5% best theses in the world. I feel very grateful for all the fun that we have had these years, and it's a bit sad that the chapter of Ailos PhD has been completed. Fortunately, Ailo joins me as a colleague in SINTEF Energy Research. A couple of pictures can be found below, with Ailo and Maria, and of the opponents.


New article on thermodynamic perturbation theory

Our paper entitled Choice of reference, influence of non-additivity, and present challenges in thermodynamic perturbation theory for mixtures has just been published in the Journal of Chemical Physics.

In the article, we discuss how thermodynamic perturbation theory should be extended to fluid mixtures, and how different formulations and references will result in different accuracies. All formulations examined in the paper display challenges for Lennard-Jones mixtures with significantly different particle sizes (sigma) or significantly different well depths of their potentials (epsilon). We found that an inaccurate representation of higher order perturbation terms was the reason for this, giving a large overprediction of the pressures in the critical region. This overprediction presently causes problems in the prediction of the thermodynamic properties of fluids such as helium-neon mixtures. We are currently working to improve the theory and overcome these challenges.

The work in the paper has been done in collaboration with Morten Hammer, Ailo Aasen and Åsmund Ervik from SINTEF Energy Research.


The first accurate representation of quantum fluid mixtures

Our paper entitled Equation of state and force fields for Feynman–Hibbs-corrected Mie fluids. II. Application to mixtures of helium, neon, hydrogen, and deuterium becomes Editors pick in J. Chem. Phys.

In the paper, we extend the statistical associating fluid theory of quantum corrected Mie potentials (SAFT-VRQ Mie), previously developed for pure fluids, to fluid mixtures. In this model, particles interact via Mie potentials with Feynman–Hibbs quantum corrections of first order (Mie-FH1) or second order (Mie-FH2). This is done using a third-order Barker–Henderson expansion of the Helmholtz energy from a non-additive hard-sphere reference system. We survey existing experimental measurements and ab initio calculations of thermodynamic properties of mixtures of neon, helium, deuterium, and hydrogen and use them to optimize the Mie-FH1 and Mie-FH2 force fields for binary interactions. Simulations employing the optimized force fields are shown to follow the experimental results closely over the entire phase envelopes. SAFT-VRQ Mie reproduces results from simulations employing these force fields, with the exception of near-critical states for mixtures containing helium. This breakdown is explained in terms of the extremely low dispersive energy of helium and the challenges inherent in current implementations of the Barker–Henderson expansion for mixtures. The interaction parameters of two cubic equations of state (Soave–Redlich–Kwong and Peng–Robinson) are also fitted to experiments and used as performance benchmarks. There are large gaps in the ranges and properties that have been experimentally measured for these systems, making the force fields presented especially useful.

The paper is one of the last papers in the PhD thesis of Ailo Aasen , but several people have made significant contributions to the work, such as Morten Hammer at SINTEF Energy Research and Erich A. Müller from Imperial College in London. The figure below shows a comparison to experimental results for the Helium-Neon mixture (crosses) and the first accurate theoretical description of this mixture (bullet points). The perturbation theory for mixtures (solid lines) still needs some more development before it can cope with the challenging aspects of quantum fluid mixtures.


In a paper in Chem. Eng. Sci., we study minimum entropy production in distillation

Our paper entitled Minimum entropy production in a distillation column for air separation described by a continuous non-equilibrium model has been published in Chemical Engineering Science.

In this work, we apply the rate-based model of Taylor and Krishna to describe the separation of air in a low-pressure, packed distillation column. By use of numerical optimization, we identify the temperature profile for heat exchange with the column and its surroundings that minimizes the total entropy production. Optimal operation of the column reduces the entropy production by nearly 50%, and the total heating- and cooling duties by 30% and 50%, respectively. We find that the local entropy production is more uniform for the optimal solution than in the adiabatic column, a property that may be helpful for new designs. Using the equilibrium stage model, the state of minimum entropy production has higher cooling/heating duties than in the rate-based model case. This shows that more sophisticated models can be beneficial for the development of reliable strategies to improve the energy efficiency of distillation columns.

The work was performed mainly by Diego Kingston, a visiting PhD student from the University of Buenos Aires in Argentina and in collaboration with Signe Kjelstrup from NTNU.


In a letter in Phys. Rev. Lett., we address the challenges of binary nucleation

Our letter entitled Curvature corrections remove the inconsistencies of binary classical nucleation theory has been accepted for publication in Phys. Rev. Lett.

In the letter we have studied the condensation process in vapors of supersatured alcohol—water mixtures. The nanoscopic droplets forming in such vapors have highly curved surfaces, making their surface tension very different from that of a macroscopic, planar surface. We developed a method to estimate curvature effects on surface tension, and found that it has a dramatic impact on model predictions for the rate of formation of such droplets. Current models, that do not account for curvature effects, yield rate predictions that deviate many orders of magnitude from experiments, and are moreover marred by physical inconsistencies. A key inconsistency is that classical nucleation theory (CNT) can give a negative number of particles of some of the components in the critical cluster. We show in the letter that incorporating curvature effects removes these inconsistencies and restores near-quantitative agreement with experiments. The figure below shows the characteristic "hump" in the activity plot from CNT for the highly surface active water-propanol mixture, which due to the first nucleation theorem means that one of the components has a negative number of particles in the critical cluster. The figure further shows that curvature corrected nucleation theory removes this peak and restores good agreement with experiments. We also show that this is true for the water-methanol and water-ethanol mixtures.


New article on the heat exchangers of a large-scale hydrogen liquefaction process

Our article entitled Comparing exergy losses and evaluating the potential of catalyst-filled plate-fin and spiral-wound heat exchangers in a large-scale Claude hydrogen liquefaction process has been published in the International Journal of Hydrogen Energy.

In the article, we determine detailed heat exchanger designs by matching intermediate temperatures in a large-scale Claude refrigeration process for liquefaction of hydrogen with a capacity of 125 tons/day. A comparison is made of catalyst filled plate-fin and spiral-wound heat exchangers by use of a flexible and robust modeling framework for multi-stream heat exchangers that incorporates conversion of ortho-to para-hydrogen in the hydrogen feed stream, accurate thermophysical models and a distributed resolution of all streams and wall temperatures. Maps of the local exergy destruction in the heat exchangers are presented, which enable the identification of several avenues to improve their performances. The heat exchanger duties vary between 1 and 31 MW and their second law energy efficiencies vary between 72.3% and 96.6%. Due to geometrical constraints imposed by the heat exchanger manufacturers, it is necessary to employ between one to four parallel plate-fin heat exchanger modules, while it is possible to use single modules in series for the spiral-wound heat exchangers. Due to the lower surface density and heat transfer coefficients in the spiral-wound heat exchangers, their weights are 2–14 times higher than those of the plate-fin heat exchangers. In the first heat exchanger, hydrogen feed gas is cooled from ambient temperature to about 120 K by use of a single mixed refrigerant cycle. Here, most of the exergy destruction occurs when the high-pressure mixed refrigerant enters the single-phase regime. A dual mixed refrigerant or a cascade process holds the potential to remove a large part of this exergy destruction and improve the efficiency. In many of the heat exchangers, uneven local exergy destruction reveals a potential for further optimization of geometrical parameters, in combination with process parameters and constraints. The framework presented makes it possible to compare different sources of exergy destruction on equal terms and enables a qualified specification on the maximum allowed pressure drops in the streams. The mole fraction of para-hydrogen is significantly closer to the equilibrium composition through the entire process for the spiral-wound heat exchangers due to the longer residence time. This reduces the exergy destruction from the conversion of ortho-hydrogen and results in a higher outlet mole fraction of para-hydrogen from the process. We show that because of the higher surface densities of the plate-fin heat exchangers, they are the preferred technology for hydrogen liquefaction, unless a higher conversion to heat exchange ratio is desired. A figure showing the evolution of the para-hydrogen mole fraction through all the heat exchangers can be found below.

The work was done in collaboration with G. Skaugen and D. Berstad, both from SINTEF Energy Research.


Three articles on the most downloaded list in Fluid Phase Equilibria

Three of our recent articles are now on the list of the most downloaded papers in the excellent journal Fluid Phase Equilibria:

  1. Thermodynamic stability of droplets, bubbles and thick films in open and closed pores
  2. Thermodynamic models to accurately describe the PVTxy-behavior of water/carbon dioxide mixtures
  3. The spinodal of single- and multi-component fluids and its role in the development of modern equations of state

We are very happy that our articles are well received by the competent readers of Fluid phase equilibria. This is very motivating and encourages us to continue our research on thermodynamics.


New project granted on moisture migration through porous media

The competence building project “PredictCUI: Prediction of water liquid and vapour migration for mitigating corrosion under insulation” received funding by the Norwegian Research Council. Here, I will in collaboration with Prof. Alex Hansen from NTNU/Porelab, researchers from SINTEF Energy Research and the industry deal with one of the major challenges at the Norwegian Continental Shelf, Corrosion under insulation (CUI). The piping in process plants is frequently insulated. CUI occurs under externally clad or jacketed insulation due to the penetration of water. It constitutes a major challenge for the oil and gas industry. If not mitigated, CUI leads to structural failure with serious consequences, ranging from leakages to explosions. One out of five major oil and gas incidents within the EU since 1984 and 50% of hydrocarbon leaks on the Norwegian Continental Shelf have been caused by CUI. Hence, to mitigate CUI is very important. The figure below shows how water can be adsorbed by a porous insulation material (taken by Åsmund Ervik from SINTEF Energy Research). The typical insulation material resembles the negative image of a porous rock: the porosity is very high (above 90%), and the pore space resides around interwoven fibers. In the project, one of the goals is to develop a predictive model for transport of water vapor and liquid through the highly porous insulation material in order to understand where the moisture migrates and where corrosion could potentially occur. One PhD will be educated on this topic in the group.