Estimating Thermal Conductivity of Hydrocarbons

The thermal conductivity of hydrocarbons is an essential parameter that needs to be known when designing heat transfer equipment. Presented here is a simple-to-use correlation that was developed for predicting thermal conductivities of liquid paraffin hydrocarbons, petroleum fractions and atmospheric natural hydrocarbon gases as a function of temperature and molecular weight or relative density. Results show that the proposed correlation has a very good agreement with reported data.

IntroductionThe thermal conductivity is an important property of liquids providing a measure of a materials’ ability to conduct heat. It is normally defined in terms of the quantity of heat transmitted due to a unit temperature gradient, under steady conditions, in a direction normal to a surface of unit area. Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole.
From a process engineer’s view point, a convenient and easy-to-use approach for predicting physical properties is the use of commercial software and the appropriate equations of state. However, such an approach does not work equally well for all properties. Accurate and reliable values can be determined for some properties, such as gas-phase densities, volumes and Z-factors, whereas less accurate — but still reliable — results are predicted for liquid volumes and densities using traditional methods. However, experience has shown that that equations of state are not suitable for predicting thermal conductivities, viscosities, and surface tensions.

For many simple organic liquids, the thermal conductivities are much higher than those of low-pressure gases at the same temperature [1]. Pressure has little effect on thermal conductivities of liquids; however, the thermal conductivity will usually decrease as the temperature increases.
The reason for this behavior can be understood on the molecular level. In the gas phase, molecules are relatively free to move about and transfer momentum and energy by collisions. In the liquid phase, however, this hypothesis is not even approximately true. Because of the close proximity of molecules in the liquid phase, the intermolecular attractive forces become important, so the molecules are not free to wander around. This leads to the low values of liquid diffusion coefficients, and often a liquid is modeled as a lattice with each molecule caged by its nearest neighbors. Energy and momentum are primarily exchanged by oscillations of molecules in the shared force fields surrounding each molecule. To date, theory has not been successful in formulating useful and accurate expressions to calculate liquid thermal conductivities. Therefore, approximations must be employed for engineering applications [1].
In many instances, the reported data are not believed to be particularly reliable and the estimation errors are in the same range as the experimental uncertainty. And yet, thermal conductivity data are very important in designing heat exchangers.
Heat-transfer coefficients in these components are usually computed using correlations that require thermal conductivity data. Due to the importance of two-phase, heat-transfer processes in many applications, thermal conductivity of the saturated liquid and vapor are of greatest importance. It is also difficult, however, to measure the thermal conductivity at saturation, and thus, single-phase measurements will be extrapolated to saturation conditions. The higher thermal conductivities and larger temperature gradients cause a greater heat flux in a one-dimensional system with correspondingly larger responses to changes in gas thermal conductivity. The physical mechanism of thermal-energy conduction in liquids is qualitatively the same as in gases; however, the situation is highly more complex because the molecules are more closely spaced and molecular force fields exert a strong influence on the energy exchange in the collision process.

Article Source : http://che.com/