An Efficient Method to Calculate Three-Phase Free-Water Flash for Water−Hydrocarbon Systems (original) (raw)
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Three-phase free-water flash calculations using a new Modified Rachford–Rice equation
Fluid Phase Equilibria, 2010
A novel Modified Rachford-Rice equation is developed for three-phase equilibrium calculations in hydrocarbon-water systems, based on the free-water assumption, i.e., the water-rich liquid phase consists of pure water only. In the inner loop of the flash algorithm, the three-phase problem (consisting in a system of two non-linear equations) is replaced by a pseudo-two-phase problem (consisting in a non-linear equation). Unlike previous formulations, the new Modified Rachford-Rice function is guaranteed to monotonically decrease between two adjacent asymptotes. The negative flash concept is used, and a search window is proposed for the vapor fraction. The new free-water flash method proved to be robust, and excellent agreement between full three-phase flash and pseudo-two-phase free-water flash was obtained for various test problems. The proposed method is very useful in compositional reservoir simulation of certain oil recovery methods and in process simulation.
Multiphase equilibria for water/hydrocarbon mixtures are frequently encountered in hydrocarbon reservoirs. The presence of water in these mixtures can lead to a higher number of equilibrating phases, increasing the complexity of the multiphase split calculations. It is a common approach to treating water as the bulk free phase and performing only two-phase split calculations on the hydrocarbon-rich liquid phase and vapor phase. The free-water flash algorithm uses a different approach; it considers the effect of water presence on the overall phase equilibrium of water/hydrocarbon mixtures, albeit also assuming the aqueous phase to be pure water. The free-water algorithm might be less accurate in some cases where the solubility of methane in the aqueous phase cannot be neglected. In this study, a modified version of the free-water flash method previously developed by the authors, i.e., the so-called augmented free-water flash, is extended to perform three-phase vapor-oleic-aqueous (VOA) flash calculations for water/hydrocarbons mixtures on the basis of the assumption that only the existence of water and methane is considered in the aqueous phase. The flash package incorporating this augmented free-water method can handle the single-phase, two-phase, or three-phase equilibrium calculations. Example calculations made on two water/hydrocarbons mixtures demonstrate that the phase compositions and phase mole fractions calculated by augmented free-water method provide better predictions compared with the traditional free-water method since the solubility of methane is considered in the aqueous phase. Our new algorithm is also shown to be computationally more efficient than the conventional full three-phase flash algorithm. Therefore, the augmented free-water approach strikes a good balance between computational efficiency and prediction accuracy.
2017
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 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 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 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 iii 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 methanecontaining 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. iv ACKNOWLEDGMENTS My sincere gratitude would go first and foremost to my supervisor, Dr. Huazhou Andy Li who teaches me how to be a good researcher. Without his instruction and guidance, the present work would not have been accomplished. I'm also grateful to Dr. Zhehui Jin and Dr. Nobuo Maeda for being my examination committee members and providing constructive suggestions. My thanks would also go to my beloved parents: Mom (Fengyu Gao) and Dad (Xiangyang Pang), and my boyfriend (Jialin Shi) for their care and confidence in me. I would also wish to thank all my friends in Edmonton; their friendship is a treasure to me. In addition, I am thankful for the friendship and technical support provided by the past and present
Three-Phase Flash in Compositional Simulation Using a Reduced Method
SPE Journal, 2010
Summary CO2 flooding at low temperatures often results in three or more hydrocarbon phases. Multiphase compositional simulation must simulate such gasfloods accurately. Drawbacks of modeling three hydrocarbon phases are the increased computational time and convergence problems associated with flash calculations. Use of a reduced method is a potential solution to these problems. We first demonstrate the importance of using three-phase flash calculations in compositional simulation by investigating difficulties with two-phase equilibrium approximations proposed in the literature. We then extend an algorithm for reduced two-phase flash calculations to three-phase calculations and show the efficiency and robustness of our algorithm. The reduced three-phase flash algorithm is implemented in a multiphase compositional simulator to demonstrate the speed-up and increased robustness of simulations in various case studies. Results show that use of a two-phase equilibrium approximation in rese...
The Canadian Journal of Chemical Engineering, 1994
The paper presents a novel approach for the solution of the isothermal multiphase flash problem with particular application to systems exhibiting liquid-liquid-vapor equilibria. The approach includes a rigorous method for thermodynamic stability analysis as a first step and an efficient phase identification procedure. The stability analysis, exercised only once, uses a modification of the Gibbs tangent plane criterion. The identification procedure implements the results of the stability test in a sequence of liquid-liquid and liquid-vapor calculations only till the phase configuration with a minimum Gibbs energy is determined.
A method for flash calculations in reactive mixtures
2008
En este trabajo se reporta un método alternativo para realizar cálculos flash en sistemas reactivos, el cual ha sido desarrollado empleando la teoría de transformación de variables de Ung y Doherty (1995). En este método se utiliza un enfoque de resolución simultánea de ecuaciones para realizar los cálculos de equilibrio bifásico en este tipo de sistemas. El comportamiento numérico del método propuesto es evaluado empleando varios casos de estudio y los resultados obtenidos indican que éste es eficiente y generalmente puede converger a las composiciones de equilibrio. Palabras clave: Equilibrio Químico. Equilibrio de fases. Cálculos flash. Variables transformadas.
Three-phase equilibria using equations of state
AIChE Journal, 1974
It is demonstrated that a single equation of state may be used to describe all three phases in liquid-liquid-vapor equilibrium situations. Wilson's version of the Redlich-Kwong equation is shown to predict accurately the water solubility in normal paraffins with interaction parameters k12 = 0.50. A simple procedure for three-phase flash computations is presented. Results obtained using this procedure for the system methane, n-butane, water exhibit many of the characteristics of the experimental data of McKetta and Katz (1948). In particular, the water content of the vapor phase and the liquid hydrocarbon phase are accurately predicted.
ESTIMATION OF WATER-ALCOHOL MIXTURE FLASH POINT
Mixtures of water with alcohols are important in numerous engineering applications. Knowledge of mixture flash points is needed for their safe handling, storage and transportation. A procedure for the estimation of flash points of water-alcohol mixtures by using the equations of Margules, van Laar and Wilson is presented in this paper.
Estimation of the flash points of saturated and unsaturated hydrocarbons
2012
This work introduces a novel, reliable and simple correlation for predicting the flash point of various types of saturated and unsaturated hydrocarbons containing cyclic and acyclic paraffins, olefins, alkynes and aromatic hydrocarbons. It is shown that the new method is based on the number of carbon and hydrogen atoms and some specific molecular moieties, which can easily be used for any types of hydrocarbons. The largest available experimental data consisting of 441 diverse hydrocarbons are used here in order to derive and test the general correlation. For a data set containing 423 of these 441 hydrocarbons, the root mean square (rms) and average absolute deviations are 7.7 and 5.7 K, respectively. The estimated flash points for 18 further hydrocarbons containing complex molecular structures have been compared with one of the best available new group additivity methods, which give much lower values of the rms and average absolute deviations.