Browsing by Author "Shirazi Manesh, Amir Ahmad"

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    Computational Improvements for PT Flash Calculations: A New Look at the Initiation and Underemphasized Aspects of Calculation Method Modifications
    (2024-09-20) Shirazi Manesh, Amir Ahmad; Clarke, Matthew; Maini, Brij; Ponnurangam, Sathish; Yarranton, Harvey; Mehta, Sudarshan; Zendehboudi, Sohrab
    Phase equilibrium calculations at constant pressure and constant temperature, PT flash calculations, are at the heart of thermodynamic modeling and simulations. Reliability and efficiency of simulations are heavily dependent on these calculations. Accordingly, there has been many attempts to improve these calculations using the available mathematical tools and the knowledge amassed in this regard is considerable. However, in relation to the characteristics of the problem, it seems that there are still capacities in the mathematical methods and these capacities should be used to attain better results. This study is an attempt to employ the connection between the special structure of the flash problem and the author’s understanding of mathematical methods to improve efficiency of the calculations while preserving reliability of the results. It was found a consideration should be made in design of the unidirectional search algorithms or in application of the line search methods to the problem and effect of making this consideration on flash results was discussed. It was shown that line search methods with a specific search direction can be applied to improve the efficiency of stability test calculations in hard cases. The efficiency of the line search methods with different types of factorization was evaluated and similarity of their abilities was found. With respect to initial guesses required to conduct stability analysis in PT flash calculations, as a global optimization problem, it was shown that considerably smaller sets of guesses, compared to the sets suggested in the literature, can provide the same level of results reliability. Application of these sets to the calculations can lead to a significant computational burden reduction. Two approaches which are customarily used in the stability analysis were evaluated and it was shown that searching for the lowest negative minimum of modified tangent plane distance function can cause considerably extra computational cost for the optimization-based calculations. Using the results, an approach to initialization of the calculations with a previously recommended stability test and prioritization of the guesses in the reduced size sets was suggested. A case of failure of successive substitution method in phase split calculations was reported and a combined line search-trust region method to remedy was suggested. Patterns of stability test and phase split iterations convergence behaviour with respect to the location of successive substitution method basin of attraction in stability test calculations were reported. A modification for line search methods to improve the efficiency of stability test calculations in difficult to converge specifications was introduced.
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    Thermodynamic Modeling of Asphaltene Precipitation Using Cubic Plus Association Equation of State
    (2015-09-28) Shirazi Manesh, Amir Ahmad; Abedi, Jalal
    The ability to predict the thermodynamic conditions which result in asphaltene precipitation and its severity has many applications in industry. Asphaltene precipitation can be interpreted as a result of the competition between physical and association interactions between asphaltene molecules and asphaltene molecules with other mixture components. Hence, the CPA EoS is an excellent choice for modelling this phenomenon. In this thesis, a new version of the CPA EoS was developed. Efficient and reliable phase equilibrium calculation programs including isothermal flash and phase envelope constructor programs were prepared. A comparison of the results using the developed programs and commercial software revealed abilities of the developed programs. The asphaltene precipitation modelling results were compared to those of Li and Firoozabadi who used a slightly different version of the CPA EoS. The case studies and conditions studied in this thesis presented some unnoticed behaviours and a general improvement over their results.