Articolele autorului Dan Vladimir Nichita
Link la profilul stiintific al lui Dan Vladimir Nichita

A comparison of conventional and reduction approaches for phase equilibrium calculations

A major part of the computational effort in chemical process simulation and compositional reservoir simulation is spent by phase equilibrium calculations: phase split and phase stability analysis. The calculation algorithms must be efficient and highly robust. The reduction methods offer an attractive alternative to conventional methods. Several recent papers have questioned on the efficiency of the reduction methods as compared to conventional methods,

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Rapid and robust resolution of Underwood equations using convex transformations

In this work, a new method is proposed for solving Underwood's equations. Newton methods cannot be used without interval control, and may require many iterations or experience severe convergence problems if the roots are near poles and the initial guess is poor. It is shown that removing only one adjacent asymptote leads to convex functions, while removing both asymptotes leads to quasi convex functions which are close to linearity on wide intervals.

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A quasi-analytical solution of Frost–Kalkwarf vapor pressure equation
Multiphase equilibrium calculations using the semi-continuous thermodynamics of hydrocarbon mixtures

Natural hydrocarbon mixtures are composed of very many components. Usually, several components are lumped into pseudo-components to reduce the dimensionality of the equilibrium flash calculations. An interesting alternative to lumping is the use of semi-continuous thermodynamics, which enables a better characterization of mixture compositions. Traditionally, in the semi-continuous approach, the feed composition is approximated by a standard distribution

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Robust and efficient Trust-Region based stability analysis and multiphase flash calculations

Multiphase flash calculations and phase stability analysis are central in compositional reservoir and chemical process simulators. For instance, in some simulations, a huge amount of phase equilibrium calculations is required (the most important part of the computational effort). Moreover, a single failure may cause significant error propagations leading to false solutions. Thus, it is imperative that calculation algorithms are efficient and highly

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New basis functions for the representation of vapor pressure data

In this work, Wagner's formulation and methodology for the representation of vapor pressure data have been extended by using new basis functions, obtained by analytical integration of Clausius–Clapeyron equation assuming a Watson like formulation for the heat of vaporization. The proposed rational base functions are more flexible and lead to higher accuracy in data fitting, comparing favorably to Wagner's equations. Even for a few components, a

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Phase stability analysis using a reduction method

Phase stability testing is an important sub-problem in phase equilibrium calculations, which require the most important computational effort in reservoir compositional simulations and some process simulators. In this work, a new reduction method for phase stability analysis is proposed. Based on the multi-linear expression of the logarithm of fugacity coefficients as functions of some component properties (elements of the reduction matrix) and of

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SRK and PR alpha functions for ice

In order to avoid possible phase inconsistencies in water saturation point calculations, this paper proposes to treat ice as a regular component. For SRK and PR cubic equations of state, the goal of this paper is to provide the parameters of an alpha function able to very accurately reproduce the vapor pressure over ice between 173 K and 273.16 K.

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A rapid and robust method for solving the Rachford–Rice equation using convex transformations

The Rachford–Rice equation must be solved in an inner loop of flash calculations. It consists in a sum of hyperbolas, and the desired root is located between two asymptotes which define the negative flash window. First, a simple procedure to remove the adjacent poles by changing the variable is proposed. Then, a class of convex transformations is identified, and several elementary functions that respect the general convexity condition are recognized.

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A new reduction method for phase equilibrium calculations

Phase equilibrium calculations require the most important computational effort in many process simulators and in reservoir compositional simulations. In this work, a new reduction method for phase equilibrium calculations is proposed. A new set of independent variables (and the related set of error equations) is introduced, based on the observation that, under certain conditions, the equilibrium ratios can be related only to some component properties

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