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Abstract
Simultaneous chemical and phase equilibrium (CPE) calculations constitute a major class of challenging equilibrium problems, with applications in diverse scientiﬁc disciplines and engineering ﬁelds, such as the chemical industry, oil and gas production, and geochemistry. Robustness and eﬃciency of computational procedures are essential for demanding simulations of industrial processes, such as reactive distillation, heterogeneous organic synthesis, and fuel synthesis from renewable feedstocks. Most association equations of state, such as the popular SAFT family models, are essentially special cases of physical models incorporating chemical (association) equilibrium. Solution and further improvement of these association models can beneﬁt from the advance in CPE calculations. Over 70 years of research on CPE computation have resulted in a long list of algorithms with many variants but there seems to be no clear consensus on the most adequate methods. The deterministic algorithms can be roughly divided into stoichiometric and nonstoichiometric methods. The stoichiometric methods are more intuitive but less eﬃcient for systems with many reactions. They are usually implemented with ineﬃcient nested loops, whereas quadratic formulation can involve quite a cumbersome implementation for multiple phases. The nonstoichiometric methods are less common but suitable to systems with many reactions. However, most applications of nonstoichiometric methods are for ideal singlephase mixtures tos lightly nonideal twophase systems and the reported algorithms are mostly nonquadratic for nonideal systems. The primary aim of this work is to develop a general and systematic nonstoichiometric approach which can determine the equilibrium state of multicomponent multiphase systems with multiple reactions at speciﬁed temperature and pressure. Two methods based on Gibbs energy minimization under material balance constraints are derived and presented in their extended form for nonideal multiphase reaction systems. Both can be classiﬁed under the same category of using the Lagrange multipliers (and the phase molar amounts) as variables. For distinction, they are called the Lagrange multipliers method and the modiﬁed RAND method, respectively. In the Lagrange multipliers method, successive substitution is employed to solve a modiﬁed set of equations originating from the Lagrangian conditions at the minimum. Convergence is quadratic for ideal systems (ideal gas/ideal solution) and linear for nonideal systems. In the modiﬁed RAND method, one of the Lagrangian conditions is linearized around the current estimate of mole numbers. Composition derivatives of fugacity or activity coeﬃcients are utilized to achieve quadratic convergence. The methods can be combined to form a robust and eﬃcient approach: the Lagrange multipliers method is used for the ﬁrst iterations of successive substitution and the modiﬁed RAND method for the secondorder convergence. The resulting algorithm is called the combined algorithm in this thesis. For comparison, a successive substitution based algorithm using only the ﬁrstorder Lagrange multipliers method is also investigated in this study. Both algorithms incorporate a reliable initialization procedure, where initial estimates are provided by the minimization of a convex function, and stability analysis to introduce additional phases when needed. The combined algorithm, as the recommended approach for CPE problems, has several advantages including a smaller system of equations (fewer variables), less sensitivity to initial estimates, the same treatment for all components and all phases, and the ability to monitor the decrease in Gibbs energy in the modiﬁed RAND steps to guide convergence. The algorithms were applied to vaporliquid (VLE), liquidliquid (LLE) and vaporliquidliquid (VLLE) equilibrium of ideal as well as nonideal systems that are commonly tested in the literature, including acid/alcohol esteriﬁcations, alkene/alcohol ethereﬁcations, hydration, hydrogenation and isomer separation. Additionally, predictions were made for the more complex transesteriﬁcation of two individual triglycerides with methanol, which entails ﬁve chemical reactions and can result in one, two or even threephase equilibrium. Finally, CPE calculations were attempted for electrolyte systems. The electroneutrality equation is satisﬁed by the material balance constraints, therefore there is no need to change the working equations of the algorithms. The equilibrium solution was obtained for aqueous mixtures of electrolytes in contact with a vapor and a solid phase. Consideration of the solid phase did not aﬀect the convergence of the initialization procedure or the CPE calculations. This makes the algorithms potentially applicable to more complicated geological systems with an electrolyte aqueous phase and multiple solids. From the simple onereaction ideal systems to the highly nonideal electrolyte mixtures with speciation reactions and solids, both algorithms could converge without problems to the equilibrium solution. The CPU time and the reasonable number of iterations, allowed us to conclude that the methods presented are eﬃcient and robust for the equilibrium determination of reaction systems. The thesis also involves a small study on the dimethyl ether (DME) phase equilibrium modeling. DME is a slightly polar compound able to dissolve in both water/brine and hydrocarbon phases. It has been considered as a novel solvent in enhanced oil recovery, and more speciﬁcally in DME enhanced waterﬂood (DEW) process. DME is dissolved in water/brine and injected into the reservoir. It partitions preferably into the oil phase to improve the mobility of the oil by swelling it and reducing its viscosity. DME itself is ﬁrstcontact miscible with the oil. Accurate phase equilibrium modeling is necessary in DEW simulations. Parameters for CPA and PR/SRK EoS with HuronVidal mixing rules are regressed from experimental data of DME binary systems with water, hydrocarbons and inert gases. With satisfactory phase equilibrium modeling, predictions are made focusing on the Kvalue of DME between oil and aqueous phases in DME/water/oil mixtures (oil modeled as a mixture of methane, nbutane and ndecane). Diﬀerent oil compositions appear to slightly aﬀect the partitioning of DME, which could possibly simplify simulations of the DEW process. Finally, sensitivity of the Kvalue is investigated with respect to temperature, pressure and salinity of the aqueous phase. Kvalues increase with temperature and salinity but slightly decrease with pressure. Dependence on temperature is larger, while high salinity in the aqueous phase favors markedly the DME partitioning into the oil phase.
Original language  English 

Publisher  Technical University of Denmark 

Number of pages  189 
Publication status  Published  2018 
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 1 Finished

Computation of Simultaneous Phase and Chemical Equilibrium
Tsanas, C., Yan, W., Stenby, E. H., Kontogeorgis, G., Jessen, K. & Solbraa, E.
Technical University of Denmark
15/12/2014 → 22/05/2018
Project: PhD