Development of An Augmented Free-Water Three-Phase Rachford-Rice Algorithm for CO2/Hydrocarbons/Water Mixtures

• Author / Creator

  Pang,Wanying

• In this work, we develop a robust and efficient algorithm to perform three-phase 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 so-called augmented free-water flash, considering that it is a modified version of the conventional free-water 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 hydrocarbon-rich liquid phase); in the inner loop, we solve the Rachford-Rice (RR) equation that has been simplified based on the augmented free-water assumption. Moreover, this new augmented free-water three-phase flash algorithm is incorporated into a flash package which can handle single-phase, two-phase, and three-phase equilibria calculations. The flash package first tests the stability of the feed. If the feed is found to be stable, a single-phase equilibrium can be concluded. Otherwise, the augmented free-water three-phase flash algorithm is initiated. If the phase fractions obtained from this augmented free-water three-phase algorithm do not belong to [0, 1] or if an open feasible region occurs during the iterations, two-phase flash will be conducted. The flash package that couples the augmented free-water flash requires less computational time and a fewer number of iterations than the conventional full three-phase flash package. Furthermore, the augmented free-water flash method has been extended to the methane-containing 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 K-values for the non-water components, but use the equation suggested by Lapene et al. (2010) to initialize the K-values 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 high-pressure/high-temperature conditions, justifying the use of our augmented algorithm (instead of the free-water algorithm) to perform flash computations for the methane-containing hydrocarbons/water mixtures. The example calculations for water/hydrocarbon mixtures using the augmented free-water algorithm prove its robustness and effectiveness over a wide range of pressure and temperature. The results obtained by the augmented free-water method are more accurate than the traditional free-water 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 free-water 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

  • Three-phase equilibrium
  • Augmented free-water method
  • Negative flash

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3JW87213

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial 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.