Liquid Fuel Part I – Kinematic Viscosity of Binary Mixture

Specifying equipment consuming liquid fuel often requires process data of fuel used which typically includes the viscosity, density, heat capacity, heating value and etc. which can ideally be obtained from the stream data.  When the process data at different conditions is desired otherwise, the stream data at those conditions can be obtained by utilising the properties of pseudo-components provided by the Licensor through the use of simulation model. This process requires validation of existing stream data with the thermophysical properties obtained through simulation. This step becomes crucial if the liquid fuel is the resulting mixing of two different petroleum products.

Setting up a simulation model and validate the model with existing stream data solely for the purpose of estimating viscosity of a blend is unnecessary because the process is time-consuming and a shortcut method utilising minimal inputs is available.

While one property package may give rise to a series of thermophysical properties which match with existing stream data, the viscosity is always underestimated (particularly for heavy oils such as the atmospheric residue and FCC slurry oil) despite the efforts made to search for the most suitable property package which gives the closest estimate of viscosities. This is because the pseudo-component properties used covers only molecular weight, normal boiling point and liquid density at 15°C which contain no information of the viscosity. It is the fuel specification or the lab-measured sample which contains that information – the kinematic viscosity established at 2 temperature points.

Fortunately, there is an existing API method can be used to estimate the viscosity of blended petroleum product within reasonable accuracy using only the following information as inputs:

  • Kinematic viscosity established at 2 temperature points; and the
  • Mixing ratio.

This calculation below uses the API method provides the estimated parameters of log-linear-relationship between the kinematic viscosity and temperature.

While the measured sample may not always be available, two pairs of dynamic viscosity (in cP) and density from the existing stream data may be used in place of the kinematic viscosity (in cSt) established at 2 temperature points.

Note: This calculation is also available as an Excel spreadsheet with the flexibilities of using different units and estimating viscosity of each light/heavy oil at different temperatures of interest.