(305d) Investigation of Operating Conditions On the Composition of Wax Deposit | AIChE

(305d) Investigation of Operating Conditions On the Composition of Wax Deposit

Authors 

Zheng, S. - Presenter, The University of Michigan - Ann Arbor
Huang, Z., Multiphase Solutions Kenny, Incorporated
Fogler, H. S., University of Michigan


Investigation of the Impact of Composition of Wax Deposit on Pigging Frequency Design

Sheng Zheng, Fan Zhang, Zhenyu Huang and H. Scott Fogler

Yield stress of a wax deposit is a critical parameter for the design of pigging operations because the breaking force generated by the pig needs to overcome the yield stress of the deposit in order to break the deposit.  The yield stress of a wax deposit depends on multiple factors, one of the most important being its n-paraffin carbon number distribution.  Generally, the increase of n-paraffin content in the deposit leads to a higher deposit yield stress (i.e., a “harder” deposit).[1],[2]  However, for wax deposits with identical n-paraffin content, the yield stress can vary significantly with the carbon number distribution (CND) of the wax contained in the deposit.[3]-[4][5][6]  In a previous study of Senra et al based on model mixtures (C32 and C36 in C10), it was reported that 2% C32+ 2% C36 in C10 produces a much weaker deposit than 2% C28+2% C36 in C10 because of the co-crystalization of C32 and C366.  More recently, Zhang et al.5 reported that the variation in the n-paraffin distribution among a wide range of carbon numbers (C17-C55) can significantly change the yield stress of the deposit.  Therefore, identifying the CND in wax deposits provides important insights to the yield stress of the deposits and thus is extremely important for pipeline pigging design.

Accurately predicting the CND of the deposit involves accurate understanding of fluid dynamics,  ftransport phenomena and paraffin thermodynamics  in pipelines. Various wax models including the Michigan Wax Predictor (MWP), Olga-Wax, PVTSim-DepoWax and GUTS/TUWax have been developed and used to model wax deposition in sub-sea oil pipelines.[7]-[8][9][10]  However, only PVTSim-DepoWax is capable of predicting the CND of the deposit.[11] Unfortunately, even for PVTSim-DepoWax, the prediction of the CND in a deposit is based on the assumption that the concentrations of dissolved wax in bulk fluid and at wall/interface are in thermodynamic equilibrium.11  Using this assumption can lead to significant under-prediction of wax deposition.7  Consequently, the prediction by PVTSim-DepoWax of the CND in the deposit might not be reliable.

Different from PVTSim-DepoWax, the MWP does not assume thermodynamic equilibrium. The MWP calculates both the axial and radial concentration profiles by numerically solving the transport equations.7  In this project, we propose to develop a compositional deposition model based on the current MWP by simulating the molecular diffusion of multiple n-paraffin components.  In addition to deposit thickness and wax content, the enhanced MWP will predict CND of the deposit at different locations and times.  The CND predicted by the MWP is expected to provide insights to the yield stress of the deposit and improve pipeline pigging design.




[1] Venkatesan, R. The Deposition and Rheology of Organic Gels., Ph.D. Thesis, University of Michigan, 2004

[2] Venkatesan, R.; Nagarajan, N. R.; Paso, K.; Yi, Y.-B.; Sastry, A. M.; Fogler, H. S. “The strength of paraffin gels formed under static and flow conditions” Chem. Eng. Sci. 2005, 60, 3587−3598

[3] Guo, X.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K.; Adamson, D. H.; Fetters, L. J. “Crystallization of mixed paraffin from model waxy oils and the influence of micro-crystalline poly(ethylenebutene) random copolymers” Energy Fuels 2004, 18, 930−937.

[4] Imai, T.; Nakamura, K.; Shibata, M. “Relationship between the hardness of an oil-wax gel and the surface structure of the wax crystals”  Colloids Surf., A 2001, 194, 233−237.

[5] Bai, C.; Zhang, J. “Effect of Carbon Number Distribution of Wax on the Yield Stress of Waxy Oil Gels” Ind. Chem. Eng. Res. 2013, 52, 2732-2739

[6] Senra, M.; Scholand, T.; Maxey, C.; Fogler, H. S. “Role of polydispersity and cocrystallization on the gelation of long-chained nalkanes in solution” Energy and Fuels 2009, 23, 5947−5957.

[7] Huang, Z., Lee, H.S., Senra, M., Fogler, H.S., “A Fundamental Model of Wax Deposition in Subsea Oil Pipelines” AIChE J.  2011, 57, 2955-2964

[8] PVTSim 19 Method Documentation P150

[9] Edmonds, B., Moorwood, T., Szczepanski, R., Zhang, X., “Simulating Wax Deposition in Pipelines for Flow Assurance”, Energy and Fuels 2008, 22, 729-741

[10]Apte, M.S., Matzain, A., Zhang, H-Q., Volk, M., Brill, J.P., Creek, J.L., “Investigation of Paraffin Deposition During Multiphase Flow in Pipelines and Wellbores – Part 2: Modeling”, J.  Energy Resour.  Technol.  2001, 123, 150-157

[11] Lindeloff, N. and Krejbjerg, K., “A Compositional Model Simulating Wax Deposition in Pipeline Systems”, Energy and Fuels, 2002, 16, 887-891


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