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    Tech Talk

#15 Thermal Conductivity of Aqueous Mixtures

The thermal conductivity is an important parameter in engineering calculation. It enters into models and correlations for heat exchange in pipe and process equipment.

The thermal conductivity (λ) is defined as

where q is the heat flow per unit area and dT/dx is the temperature gradient in the direction of the heat flow.

For oil and gases, Calsep recommends using a model based on the corresponding state principle. The basis for the model was developed by Peter Christensen (now Senior Principal Consultant at Calsep) during his Ph.D. study in the late nineteen seventies (Christensen and Fredenslund, 1980).

For water, PVTsim Nova uses a model developed by Meyer (1967).

In cold areas, to avoid hydrate formation, it is necessary to add a hydrate inhibitor to well streams carrying water.

The most commonly used hydrate inhibitors are mono ethylene glycol (MEG) and methanol (MeOH). Alcohols and glycols have much lower thermal conductivities than water. A correct representation of the thermal conductivity of a hydrate inhibited water phase therefore requires a correlation that takes the content of inhibitor into account.

The thermal conductivities of alcohols and glycols vary slightly with temperature and PVTsim Nova uses a linear correlation in temperature for the thermal conductivity (λL0) at atmospheric pressure

A pressure correction proposed by Missenard (1970) is used for methanol and ethanol.  For glycols, the pressure dependence is modest and ignored in PVTsim Nova.

The thermal conductivity of a mixture (mix) of water (H2O) and a hydrate inhibitor (inh) is in PVTsim Nova calculated from

 

where w stands for weight fraction.

Figure 1 shows experimental and simulated thermal conductivities of pure MEG (left hand plot) and pure MeOH (right hand plot).

Figure 2 shows experimental and simulated thermal conductivities of MEG + H2O  (left hand plot) and MeOH+H2O (right hand plot). These two plots clearly illustrate the importance of taking the inhibitor concentration into consideration when calculating the thermal conductivity of an aqueous phase.

A verification report for thermal conductivities of aqueous mixtures covering experimental data and PVTsim Nova simulation results is available to PVTsim Nova clients with a support and maintenance arrangement for PVTsim.

References

Christensen, P.L. and Fredenslund, Aa., “A corresponding states model for the thermal conductivity of gases and liquids”, Chem. Eng. Sci. 35, 1980, pp. 281-283.

Meyer, C.A., McClintock, R.B., Silverstri, G.J. and Spencer, R.C., Jr., ”Thermodynamic and Transport Properties of Steam, 1967 ASME Steam Tables”, Second Ed., ASME, 1967.

Missenard, F. A., Thermal Conductivity of Organic Liquids of a Series or a Group of Liquids, Rev. Gen. Thermodyn. 101, 1970, pp 649-660.