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Development of An Augmented FreeWater ThreePhase RachfordRice Algorithm for CO2/Hydrocarbons/Water Mixtures

 Author / Creator
 Pang,Wanying

In this work, we develop a robust and efficient algorithm to perform threephase flash calculations for CO2/water/hydrocarbons mixtures on the basis of the assumption that only CO2 and water are considered in the aqueous phase. We name this new algorithm as the socalled augmented freewater flash, considering that it is a modified version of the conventional freewater flash which assumes the presence of pure water in the aqueous phase. The new algorithm is comprised of two loops: in the outer loop, we first develop a pragmatic method for initializing the equilibrium ratios of CO2 and water in the aqueous phase with respect to the reference phase (i.e., the hydrocarbonrich liquid phase); in the inner loop, we solve the RachfordRice (RR) equation that has been simplified based on the augmented freewater assumption. Moreover, this new augmented freewater threephase flash algorithm is incorporated into a flash package which can handle singlephase, twophase, and threephase equilibria calculations. The flash package first tests the stability of the feed. If the feed is found to be stable, a singlephase equilibrium can be concluded. Otherwise, the augmented freewater threephase flash algorithm is initiated. If the phase fractions obtained from this augmented freewater threephase algorithm do not belong to [0, 1] or if an open feasible region occurs during the iterations, twophase flash will be conducted. The flash package that couples the augmented freewater flash requires less computational time and a fewer number of iterations than the conventional full threephase flash package. Furthermore, the augmented freewater flash method has been extended to the methanecontaining hydrocarbons/water mixtures where the solubility of methane in the aqueous phase might not be negligible under certain conditions. Similarly, in the new algorithm, we only consider the presence of water and methane in the aqueous phase. The general framework of the flash algorithm is the same as the one that is previously developed for the CO2/hydrocarbons/water mixtures. But, we use the Wilson equation to initialize the Kvalues for the nonwater components, but use the equation suggested by Lapene et al. (2010) to initialize the Kvalues for water. Two case studies have been used to test the performance of the new algorithm. The testing results show that the amount of methane dissolved in water is less than that of CO2 under the same conditions. But the solubility of methane in the aqueous phase can be also quite high at highpressure/hightemperature conditions, justifying the use of our augmented algorithm (instead of the freewater algorithm) to perform flash computations for the methanecontaining hydrocarbons/water mixtures. The example calculations for water/hydrocarbon mixtures using the augmented freewater algorithm prove its robustness and effectiveness over a wide range of pressure and temperature. The results obtained by the augmented freewater method are more accurate than the traditional freewater method since the solubility of methane is considered in the augmented one. The computational time and number of iterations are significantly decreased with the use of the new flash package featuring the augmented algorithm. This is because of the following reasons: 1) A fewer number of parameters are involved in the calculations due to the use of the augmented freewater concept; 2) the number of iterations are reduced due to a more accurate initialization of equilibrium ratios compared with the conventional method; and 3) A fewer number of stability tests are required in the new flash package compared with the conventional method.

 Subjects / Keywords

 Graduation date
 201711

 Type of Item
 Thesis

 Degree
 Master of Science

 License
 This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for noncommercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.