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Characterization of Materials: when molecular simulation enhances experimentation

June 2016

The application of molecular simulation methods to material property calculations is a fast-developing field, both from a scientific and industrial application perspective.

IFPEN has used Monte Carlo simulation (see Box 1), in addition to experimental measurements, to determine the barrier properties of polymers under very high gas pressure. This work was carried out as part of a research partnership with Paris Sud and Lyon 1 universities as well as the French National Center for Scientific Research (CNRS) and Technip, to resolve the problem of acid gas permeation through the impermeable sheathes of flexible oil and gas pipelines (Box 2).

Molecular simulation helps provide predictions for conditions in which it is difficult or impossible to perform experiments. Based on rigorous physics, this type of simulation generates “pseudo-experimental” points which can then be used like test results.

This is shown in Figure 1 for the solubility of CO2 in polyethylene at 60°C, for pressures of up to 1,000 bar. Experimental data for up to 200 bar helped, within this range, to validate the orders of magnitude obtained through molecular simulation [1].

This was then used in a 100% predictive process to investigate the polymer's behavior under very high CO2 pressure [2].

The work showed the different sorption modes for CO2 solubility into the polymer, leading to the creation of a simple model to account for this phenomenon (eq. 13 of [2]). Using the molecular simulation data as a benchmark, this model definitively provides a precise description of the studied behavior, for a very large pressure range.

            Figure 1. Change of mass concentration of CO2 in Polyethylene at 60°C.
          Filled symbols: Monte Carlo simulations. Open symbols: experimental data.


Monte Carlo simulation

Monte Carlo simulation is an atomistic molecular simulation method which models a system measuring several nanometers (see Figure 2). Using precise calculation of the interactions between molecules through accurate force fields and accounting for all the positions that these molecules could have through random Monte Carlo moves, this approach helps calculate several properties for a given system, be it in gaseous, liquid or solid state (crystalline or amorphous). The method is increasingly used to meet needs in a variety of industrial sectors, and has boomed with the increasing performance of computers. The speed and calculating power of modern computers are increasing rapidly in comparison to their price while experimental studies are increasingly expensive.

             Figure 2. Simulation box containing molecular chains of fluoropolymer
                                              (polyvinylidene fluoride)


Flexible pipelines

Flexible pipelines, used for offshore oil and gas production, are made by combining polymer and metal layers (Figure 3).

The polymer sheathes are made of thermoplastics which guarantee the leakproofness of the pipeline for both the transported liquid and the sea water, and which reduce heat dissipation.

As regards metallic layers, they ensure that the structure is extremely mechanically resistant and flexible during the installation and use phases. The main issues with these materials involve controlling corrosion and guaranteeing durability. These properties are sensitive to the acid gases contained in the transported liquids which may leak through the internal impermeable sheath.

Good knowledge of the barrier properties of the polymer sheathes is needed to provide an answer to these issues. The relevant physical quantity associated to such barrier properties is the permeability, a property which comes from the solubilization of fluids in the polymer and from their diffusion through the material. For many years now, IFPEN has had a variety of experimental means for measuring these properties. Molecular simulation now offers an alternative and complementary approach in addition to experimental work.

                            Figure 3. Example of flexible pipeline structure


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  1. P. Memari, V. Lachet, M.-H. Klopffer, B. Flaconnèche, B. Rousseau, "Gas mixture solubilities in polyethylene below its melting temperature: experimental and molecular simulation studies", Journal of Membrane Science, 390-391, 194 (2012).
    >> doi:10.1016/j.memsci.2011.11.035
  2. F. Sarrasin, P. Memari, M.H. Klopffer, V. Lachet, C. Taravel Condat, B. Rousseau, E. Espuche, "Influence of high pressures on CH4, CO2 and H2S solubility in polyethylene: Experimental and molecular simulation approaches for pure gas and gas mixtures. Modelling of the sorption isotherms", Journal of Membrane Science, 490, 380 (2015).
    >> doi:10.1016/j.memsci.2015.04.040

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