It is widely recognized that the transport property has a significantly impact on fluid heat transfer, consequently the design of the heat exchangers. The impact is well highlighted when the heat exchanger is designed for the service of bitumen. Bitumen is viscous and typically blended with diluent for transportation and processing. The viscosity estimation of diluted bitumen (Dilbit) is highly related to the mixing rule being used. There are dozens of mixing rules available for estimating viscosity of Dilbit. However, predicted viscosities of Dilbit blends of these mixing rules are significantly different (the relative prediction error can be as high as > ±80%), leaving potential risk of viscosity calculation error or uncertainty. This will affect the design of the associated heat exchangers, as well as the engineering cost. Pre-evaluating the mixing rule performance can avoid the high uncertainty in viscosity prediction of Dilbit, and reduce the uncertainty to ±30%. Further study by HTRI indicates that even with the same uncertainty in viscosity estimate, the impact of the uncertainty on exchanger sizing (cost) for viscous fluid is different compared to that imposed by the less viscous fluid.
A case study of Dilbit cooler unit design is conducted based on an original 3 parallel x 4 series exchanger unit design to investigate the impact of the viscosity uncertainty on exchanger sizing. The glycol water is used as the cooling medium flowing through the tube side. Hot Dilbit is put on the shell side. The viscosity of Dilbit at inlet is changed while other properties are kept constant to test impact of viscosity. The study demonstrates that with the ±30% uncertanity in Dilbit viscosity calculation, its impact on exchanger sizing is about ±5% when the Dilbit viscosity is less than 100 cP. However, a design margin of 10% is required for the exchanger sizing if the Dilbit viscosity is higher than 100 cP to cover the ±30% uncertainty in Dilbit viscosity prediction. As a result, impact of Dilbit viscosity uncertainty on engineering cost will be twice as much for the viscous Dilbit (>100 cP) as compared to the impact of uncertainty for less viscous Dilbit.
