Energy & Fuels, Vol.18, No.4, 1156-1168, 2004
Lowering the viscosity of Doba - Chad heavy crude oil for pipeline transportation - The hydrovisbreaking approach
This study was instigated in view of the recent commercial exploitation of the Doba oil field in landlocked Chad, which is a region from which crude oil is extracted and expected to be routed to the Atlantic shore through pipeline transportation. Thus, nonisothermal kinetic hydrovisbreaking tests of Doba crude oil were conducted in a mechanically stirred baffled autoclave reactor under various conditions to alter the rheological properties of the treated crude. The crude hydrovisbreaking kinetics was modeled based on the four-parameter reaction severity concept (omega, gamma, E, beta(o)), as a function of the conversion in polyaromatics and polar maltenic subfractions. The hydrovisbreaking of Doba crude was observed to be a pseudo-first-order reaction (i.e., in excess of H-2), with respect to the polyaromatics and polar maltenic conversion. The following assortment of kinetic parameters was identified under noncatalytic hydrovisbreaking conditions: heterogeneity coefficient, gamma = 1; characteristic temperature, omega = 29.95 K; and activation energy, E = 140.9 kJ/mol. Four viscosity mitigation scenarios involving catalytic (FeS, MoS2) and noncatalytic hydrovisbreaking of Doba heavy crude oil were investigated. It was found that the minor proportions of the fractions that distilled before 250 degreesC and the small asphaltene yields marginally affected the crude viscosity. It was therefore determined that it is possible to meet the viscosity specification for pipeline transportation via (noncatalytic) hydrovisbreaking, which requires neither predistillation (topping) nor post-deasphalting units. The treated crudes and the syncrudes (mixtures of untreated and treated crudes) were observed to exhibit nonelastic viscous Newtonian behavior over the temperature range typical of crude transportation via pipeline. Treated crudes at 440 degreesC for 25 min and syncrudes that were the result of mixing 50 wt % of untreated crudes with crudes treated at 460 degreesC for 15 min yielded kinematic viscosities within the pumping specifications (i.e., less than or equal to 25cSt @ 50 degreesC). The use of catalysts led to even less-viscous maltenes subfractions; however, post-deasphalting was required, because the catalyst-coke mixture, as well as asphaltenes, inflated the viscosity above the norm. An iron sulfide catalyst outperformed a molybdenum sulfide catalyst, in terms of the deasphalted crude viscosity. Aging tests over two-month periods indicated that the higher the treatment severity, the more stable the viscosity of the Doba treated crudes, which is potentially compatible with the residence times of syncrudes within the 1050-km-long transportation pipeline between Chad and Cameroon.