A Model For The Viscosity Of Bitumen/Bitumen Fractions - Diluent Blends
Authors: Mehrotra A.K. 
Published: March 2010

A procedure is presented for predicting the mixture viscosity of bitumen or bitumen fractions ("cuts") blended with liquid diluents, such ar toluene. The following simple (additive) viscosity mixing rule is evaluated for binary and multicomponent mixtures of bitumen fractions and toluene (as the diluent):

Equation (available in full paper)

where u, denotes the geometric mean of mass and mole fractions. The viscosity predictions from the above mixing rule are shown to be well within an order-of-magnitude of literature data for 22 reconstituted binary mixtures. The predictions improve significantly by introducing a binary viscous interaction parameter Bi, as:

Equation (available in full paper)

Introduction

Bitumens and heavy-oils of Alberta are extremely viscous at reservoir conditions, hence suitable steps to decrease their viscosity are essential in all enhanced oil recovery (EOR) and transport processes. Calculation methods for the viscosity of bitumen-diluent mixtures are needed for reservoir simulation, pipeline design, and many upstream and downstream processes. Because bitumens are mixtures of complex hydrocarbons, the available viscosity calculation methods are empirical and/or semi-empirical (Johnson et al. 1987; Mehrotra and Svrcek 1987; Puttagunta et al. 1988; Svrcek and Mehrotra 1988). The viscosity of bitumens and bitumen fractions decreases dramatically with an increase in temperature (AOSTRA 1984; Mehrotra et al. 1989). Another well-known alternative for decreasing the bitumen viscosity is dilution with liquid diluents.

The solubility of bitumens in liquid diluents is dependent on the nature of both the diluent and the bitumen. Bitumens can be diluted with C5 paraffinic liquid hydrocarbons but only up to a critical dilution ratio at which the asphaltene fraction begins to separate from the liquid mixture (Mitchell and Speight 1974; James and Mehrotra 1988). However, solvents such as benzene, toluene and xylcnes are known to be miscible with bitumens in all proportions. With liquid diluents, a reduction in bitumen Viscosity can be achieved at any pressure provided il is high enough to retain the diluent in the liquid (mixture) phase. One drawback with the use of liquid diluents may be their higher cost; therefore, effective diluent recovery and recycle methods need to be implemented for improving the process economics.

Of particular relevance to the present investigation on the ViScosity of bitumen-diluent mixtures are three recent studies by Eastick and Mehrotra (1990), Mehrotra (1990) and Mehrotra et al. (1989). The viscosity-temperature-composition data in these studies are used here to develop a generalized method for the viscosity of diluted bitumen as a function of temperature and mixture composition. Viscosity mixing rules are proposed and validated for the diluted bitumen blends comprising as many as six constituents, The effect of temperature on the viscosity of all liquid-mixture constituents is incorporated through a one-parameter generalized equation. For bitumen fractions, the single parameter is shown to be related directly to the molar mass and density. The calculation method is straight forward and provides excellent viscosity, predictions over a range of temperature and diluent/bitumen ratio.

Viscosity-temperature Equation

The viscosity, data for five fractions of Cold Lake bitumen are plotted in Figure 1, and these differ by several orders-of-magnitude (Mehrotra et al. 1989).