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 article gives insight on energy efficient membrane separation
Our article entitled Energy efficient design of membranes processes by use of entropy production minimization has just been accepted for publication in Computers & Chemical Engineering. In the paper, we discuss how to develop more energy efficient membrane separation processes by use of entropy production minimization. For our example, which is a unit for separation of CO2 from syngas, minimization of the entropy production is equivalent to minimization of the work requirement, e.g. due to re-compression.
In the paper, we use optimal control theory to minimize the total entropy production of a membrane unit for separation of CO2 from natural gas, by control of the partial and total pressures on the permeate side. We find that with the control of the permeate partial pressures, the total entropy production can be significantly reduced (circa 38%), while the reduction is lower with the control of the total permeate pressure only (circa 6.4%). The continuous optimal results can serve as ideal limits for the practical design. We find that a threestep permeate pressure that approximates the optimum reduces the entropy production by 5.3%. This corresponds to a reduction of the compressor power of 3.8%, when the permeate gas is re-compressed to be further processed. This concept is illustrated in the figure below. We expect the results and findings in our work to be of general validity, also for other membrane separation processes.
The work was carried out in collaboration with Elisa Magnanelli and Signe Kjelstrup, both from the Department of Chemistry at NTNU, and Eivind Johannessen from EquiNor (previously Statoil)
New article on the surface tension of multicomponent droplets and bubbles is Editors choice
Our article entitled Tolman lengths and rigidity constants of multicomponent fluids: Fundamental theory and numerical examples has been accepted for publication in the Journal of Chemical Physics. The paper was very positively received by the reviewers and was chosen to be Editors pick in the journal. The work has previously drawn attention at international conferences, where my PhD student Ailo Aasen won the prize for best poster when he presented the initial results last year.
The curvature dependence of the surface tension can be described by the Tolman length (first order correction) and the rigidity constants (second order corrections) through the Helfrich expansion. In the paper, we explain the general theory for this dependence for multicomponent fluids and calculate the Tolman length and rigidity constants for a hexane-heptane mixture by use of density gradient theory. We show that the Tolman length of multicomponent fluids is independent of the choice of dividing surface and present simple formulae that capture the change in the rigidity constants for different choices of dividing surface.
For multicomponent fluids, the Tolman length, rigidity constants and the accuracy of the Helfrich expansion depend on the choice of path in composition and pressure space along which droplets and bubbles are considered. This is illustrated in the figure below, where the chemical potential of the droplet state can be reached by both State A and State B on the saturation curve. Since the surface tension of the droplet is independent of the reference state and the surface tension of the planar surface is different for States A and B (since the surface tension depends on the composition), the Tolman length and the rigidity constants are path dependent.
For the hexane-heptane mixture, we found that the most accurate choice of path was the direction of constant liquid-phase composition. For this path, the Tolman length and rigidity constants were close to linear in the mole-fraction of the liquid-phase and the Helfrich expansion represented the surface tension of hexane–heptane droplets and bubbles within 0.1% down to radii of 3 nanometers.
The framework presented in the paper can be applied to a wide range of fluid mixtures and can be used to accurately represent the surface tension of nanoscopic bubbles and droplets, or surfaces with more exotic curvatures. A future challenge where the framework will be applied is in overcoming some of the challenges of multicomponent nucleation theory. The work was carried out in collaboration with my PhD-student Ailo Aasen and Ass. Prof. Edgar M. Blokhuis from the University of Leiden.
Received Norwegian Research Councils Award for Young Outstanding Researchers
1st of March I recieved the prize by the Norwegian Research Councils Award for Young Outstanding Researchers within Natural Sciences on my research on surfaces. The research would not have been possible without my excellent colleagues in Norway and in Spain and good collaboration. The award was mentioned in both national and international media such as Adressa , Trondheim by , Youtube , SINTEFs webpages , SINTEF blog by my boss Dr. Mølnvik , Nordic life sciences , The Norwegian American and also other webpages. I am very honored and super proud to receive this prize. The award money of 500 000 NOK will be used on further research on the topic.
Some pictures from the award-ceremony can be found below. The first picture shows my Department leader at SINTEF, Dr. Mølnvik, the Norwegian Minister of Research and Innovation Iselin Nybø, me and my team leader Dr. Halvor Lund. The second picture shows me and the prime minister at the Norwegian University of Science and Technology, Gunnar Bovim, and the third picture shows my mentor, close collaborator and PhD-supervisor Prof. Dick Bedeaux.
Visiting the group of Prof. Müller at Imperial College London
Since January, I have been visiting the group of Prof. Erich Müller together with my PhD-student Ailo Aasen and my SINTEF colleague Morten Hammer. We will stay here until May, and have been very well taken care of so far. We are all enjoying the "London-experience" meanwhile developing an equation of state capable of handling quantum fluids such as helium, neon and hydrogen for use in development of more energy efficient hydrogen liquefaction processes.
The first picture below shows me and Prof. Müller during a presentation at the Chemical Engineering Department at Imperial College in London. The second picture shows Ailo trying to read the book on quantum mechanics by R. Feynman as we are aiming to consistently incorporate quantum corrections into the theory. Since Ailo had forgotten his computer charger, the mobile screen had to suffice.
New article in International Journal of Hydrogen Energy
Our paper entitled: Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes has been published in the International Journal of Hydrogen Energy.
A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency.
In the work, we present a detailed mathematical model of a plate-fin heat exchanger that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Plate fin heat exchangers have fins that increase the heat transfer area. Catalyst is placed in the layers where there is conversion from ortho-hydrogen to para-hydrogen as illustrated in the figure below.
Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. The correlation (solid lines) is compared to some of the available experimental data in the figure below.
In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.
The work describes new routes to follow in order to improve the efficiency and reduce the power requirement to liquefy hydrogen and was carried out in in collaboration with D. Berstad, Ailo Aasen, Petter Nekså and Geir Skaugen from SINTEF Energy Research and the Norwegian university of science and technology.
New article in Phys. Rev. E on nonlocal entropic contributions to interfacial properties
Our paper entitled: Temperature anisotropy at equilibrium reveals nonlocal entropic contributions to interfacial properties has been accepted for publication in Physical Review E.
In the work, we show that the configurational part of the temperature has different contributions from the parallel and perpendicular directions at the vapor-liquid interface, even at equilibrium. This has been illustrated in the figure below. Let us assume that north/south are the directions perpendicular, and east/west are the directions parallel to the vapor-liquid interface. Particles located about 1 nanometer towards the vapor-side of the equimolar surface would feel contributions to the configurational temperature from the north/south direction which, if they would have been in a single-phase fluid, would correspond to a hot temperature. These directions would perhaps feel like the Sahara desert. From the east/west directions on the other hand, the contributions would be equivalent to those of a cold temperature in a single-phase fluid. These directions would perhaps feel like the arctic winter. The hot and cold contributions compensate each other, such that the particle at the interface experiences the equilibrium temperature overall.
The article starts by explaining why anisotropy in the contributions to the configurational temperature is expected across the vapor-liquid interface from a theroretical point of view. We next show that the anisotropy can also be found in molecular dynamics simulations and obtain a qualitative agreement between theory and simulations. The theory shows that the temperature anisotropy originates in nonlocal entropic contributions, which are missing from the classical description of interfacial phenomena.
The nonlocal entropic contributions discussed in this work are likely to play a role in the description of both equilibrium and nonequilibrium properties of interfaces. At equilibrium, they influence the temperature- and curvature-dependence of the surface tension. Across the vapor-liquid interface of the Lennard Jones fluid, we find that the maximum in the temperature anisotropy coincides precisely with the maximum in the thermal resistivity relative to the equimolar surface, where the integral of the thermal resistivity gives the Kapitza resistance. This links the temperature anisotropy at equilibrium to the Kapitza resistance of the vapor-liquid interface at nonequilibrium.
I believe the work to be of importance to future research on interfacial phenomena, in particular for the description of nonequilibrium interfacial processes. The work was performed in in collaboration with Thuat T. Trinh and Anders Lervik from the Department of Chemistry at the Norwegian University of Science and technology.