Because of increased environmental pressure, there is currently a movement away from more traditional refrigerants such as HCFC's toward refrigerants with lower global warming potential such as carbon dioxide (CO₂).

However, the use of CO₂ as a refrigerant requires a refrigeration cycle with greater extremes of temperature and pressure. It is essential that equipment using CO₂ as refrigerant release as little as possible gas (ideally zero) to the atmosphere. The integrity of the system depends on the material used in its packings and seals. CO₂ is known to cause damage to many elastomeric materials used in seals such as O-rings. Furthermore, the rapid decompression that occurs in a refrigeration cycle is also known to be detrimental to the integrity of a number of polymers.

In this work we have measured the solubility, diffusivity and permeability of gaseous CO₂ in various polymer materials employed as packing and sealing material in CO₂ refrigeration plants. These include the hard polymers PVDF, PEEK and PTFE which are used as packing material as well as the rubbery polymers Neoprene, Hydrogenated Nitrile Butadiene Rubber (HNBR), VITON and EPDM which are used in seals such as O-rings.

The experiments were performed on two high pressure apparatus - a high-pressure microbalance (solubility and diffusivity) and a high-pressure permeation cell (permeability and diffusivity). Previous results using these apparatus have been published and include polymers used in the offshore oil and gas industry^1-5. The combination of the two apparatus means that reliable measurements can be made for diffusion coefficients. This is not possible using each apparatus alone.

The results for solubility are modelled using an equation of state for polymers (simplified PC-SAFT) developed in our laboratories⁶. The modelling is extended to operating regimes for CO₂ refrigeration plants (e.g. CO₂ liquid) for which experimental results are not available. The application of equation of state methods requires parameters for the pure components, which are not readily available for polymers, particularly in the case of the complex polymers studied here. However we have applied a previously published method⁷ in order to obtain the required parameters.

References

1. N. von Solms and J. Kristensen, “Refrigeration Plants Using Carbon Dioxide as Refrigerant: Measuring and Modelling the Solubility and Diffusion of Carbon Dioxide in Polymers Used as Sealing Materials,” submitted to Int. J. Refrig. (2008).

2. N. von Solms, N. Zecchin, A. Rubin, S.I. Andersen and E. H. Stenby, “Direct Measurement of Gas Solubility and Diffusivity in Poly(vinylidene fluoride) with a High-Pressure Microbalance,” Eur. Polym. J. 41, 341 (2005).

3. N. von Solms, A. Rubin, S.I. Andersen and E. H. Stenby,, “Direct Measurement of High Temperature/High Pressure Solubility of Methane and Carbon Dioxide in Polyamide (PA-11) using a High-Pressure Microbalance,” Int. J. Thermophys. 26, 115 (2005).

4. N. von Solms, J.K. Nielsen, O. Hassager, A. Rubin, A.Y. Dandekar, S.I. Andersen and E. H. Stenby, “Direct Measurement of Gas Solubilities in Polymers with a High-Pressure Microbalance” J. Appl. Polym. Sci. 91, 1476 (2004).

5. N. von Solms, M. L. Michelsen and G. M. Kontogeorgis, “Prediction and Correlation of High-Pressure Gas Solubility in Polymers with Simplified PC-SAFT,” Ind. Eng. Chem. Res. 44, 3330 (2005).

6. N. von Solms, M.L. Michelsen and G.M. Kontogeorgis, “Computational and Physical Performance of a Modified PC-SAFT Equation of State for Highly Asymmetric and Associating Mixtures,” Ind. Eng. Chem. Res. 42, 1098 (2003).

7. I.A. Kouskoumvekaki, N. von Solms, T. Lindvig, M.L. Michelsen and G.M. Kontogeorgis, “A Novel Method for Estimating Pure-Component Parameters for Polymers, Application to the PC-SAFT Equation of State,” Ind. Eng. Chem. Res. 43, 2830 (2004).