Reducing Diluent Cost in Dilbit Preparation
Why are diluents needed?
Bitumen produced from the oil sands is rich in long-chain hydrocarbon molecules. The long hydrocarbon chains are the main reason for the high viscosity of the bitumen. In order to transport bitumen through pipelines, its viscosity must be lowered to meet pipeline specifications. One treatment that is commonly used to lower the viscosity is to blend bitumen with a less viscous diluent producing what is known as Dilbit. However, the addition of diluent adds significant cost affecting the profit margins of the bitumen product.
How to reduce costs?
To reduce the costs associated with the diluent, part of the diluent can be substituted by a second, cheaper, diluent such as butane. Blending butane into the Dilbit means that less diluent will be needed to achieve the desired viscosity. One might ask, if butane can be used to lower viscosity, then why not use only butane instead of the diluent? The answer lies in a second pipeline constraint, vapor pressure. The more butane that is blended into the bitumen the lower the vapor pressure gets. However, for Dilbit to be transported in the pipeline it must meet a minimum vapor pressure specification as well as the viscosity. Therefore, mathematically the process of blending butane and diluent can be translated into an optimization problem.
The Optimization Problem
A process diagram depicting the butane and diluent blending process is shown in the Figure below. The process poses an optimization problem to maximize profit as the diluent and butane have different associated costs. The objective function to be minimized can be formulated as follows:
Minimize (diluent cost * diluent flow rate + butane cost * butane flow rate)
Note that the objective function is subject to the viscosity and vapor pressure constraints as previously stated. To add complexity to the problem, the two constraints vary throughout the year. For example, a higher vapor pressure might be allowed in the wintertime versus in the summertime. This means that the optimum flowrate values vary throughout the year.
Figure 1: Process diagram Dilbit preparation
With the problem formulated, the only thing left is to create a model so a solution can be reached. The model can be created in a process simulation software set to minimize the objective function. Special care must be taken with the viscosity and vapor pressure properties as these are the critical constraints for the problem at hand.
The viscosity of bitumen-diluent blends is determined via a complex relationship among properties of both bitumen and diluent and their ratios in the blend. The lack of blending models with theoretical foundations for the prediction of viscosity of mixtures has led to the development of various empirical mixing rules. As a result of large differences between the viscosities of bitumen and diluent, the predictive ability of even the most suitable blending rules may not accurately match the measured lab data. As such, there is a need to identify which empirical blending rule is most suitable to the specific bitumen-diluent mixture as well as adjust the parameters of the blending rules to match the data more closely and improve predictability.
To accurately predict the vapor pressure of the Dilbit mixture, the bitumen and diluent must be appropriately characterized. Once characterized an appropriate fluid package must be chosen to model the interactions. Since bitumen is a highly complex mixture of different hydrocarbon molecules it might be worthwhile to conduct lab experiments to get experimental data. With the experimental data, the interaction parameters in the fluid package can be tuned to better represent the oil. However, in the absence of experimental data, the existing measured data with analogues bitumen and solvent can be used.
How can we help?
Process Ecology has performed blending studies on different bitumen-diluent mixtures with great feedback from our clients. Experimental data for various bitumen-simple diluent and bitumen-mixed diluents blends were obtained from various published papers and university thesis and compiled by Process Ecology into a database. From our experience, significant cost savings can be achieved through the blending of butane into Dilbit. With the help of our Bitumen/Diluent database, knowledge of various high-performing viscosity models, and expertise in process modelling we can help identify potential strategies in your Dilbit project. Don’t hesitate to reach out to us today!