(204n) Conceptual Design and Optimization of Petlyuk System Based On a Rigorous Method | AIChE

(204n) Conceptual Design and Optimization of Petlyuk System Based On a Rigorous Method

Authors 

Fernández-Martínez, E. H. - Presenter, Universidad Autónoma de Tlaxcala
Castro-Agüero, A., Universidad Autónoma de Tlaxcala
Ramírez-Corona, N., Universidad de las Americas Puebla
Ponce-Ortega, J. M., Universidad Michoacana de San Nicolás de Hidalgo



Despite its high energy consumption and its low thermodynamics efficiency, distillation is so far the most commonly used separation method for purifying fluid mixtures; these features significantly increase the operating cost for the process industry. As consequence, this has motivated the research to promote significant energy savings, as well as the small reboiler and condenser; in this context, distillation schemes using thermally coupled distillation arrangements have represented attractive options in the processes area. For fully thermally coupled distillation systems (FTCDS) the thermodynamic efficiency is better respect to standard columns (Triantafyllou and Smith, 1992) and an average of 30% of energy savings have been achieved in comparison with conventional sequences (Dünnebier and Pantelides, 1999).

Models based on shortcut and simplified methods have been recommended for designing and optimizing FTCDS (see Ramírez, et al (2010); Muralikrishna et al (2002)); these design methods have provided an initialization for a rigorous simulation (Annakous and Mizsey, 1996), but they have considered some simplified assumptions such as constant relative volatilities and constant internal molar flows across the column. When the relative volatilities and internal flows ratio vary widely throughout the column, methods more rigorous  can be applied for design.

To overcome the drawbacks associated to the use of simplifying methods for designing FTCDS systems, in this work stochastic programming tools to rigorously solve a simple design procedure based on Ponchon-Savarit method is applied to Petlyuk system. The proposed mathematical procedure solves simultaneously the material and energy balance equations and also vapor-liquid equilibrium relationships, through vector summations, that generate a vector space inside the Enthalpy-Composition diagram, this method allows a direct estimation of design variables and avoids iterative calculations using a commercial simulator.

Petlyuk configuration has two columns connected by saturated interlinking streams (see Figure 1a). For designing, this system can be modeled as an equivalent scheme of three separate columns; the first is called prefractionator and has a total condenser and total reboiler. The sections II and III represent the upper and lower parts of the main column as shown in Figure 1b, and these are coupled as one column through the Nikolaides and Malone criterion (1987); where the maximum value of the minimum reflux is taken and the dominant section is specified, therefore the vapor that flows at bottom of section II and the top of section III must be equal. Stages are determinate with the methodology proposed by Doherty and Malone (2001) and for feed location; the minimum distance criterion is applied (Gutiérrez and Jiménez, 2007).

             (a)

                           (b)

Figure 1. Schematic representation of (a) Petlyuk configuration, (b) Equivalent system.

Highly nonlinear and nonconvex equations are generated in the distillation models using a tray by tray scheme, because of these conditions, the optimization of such systems using mathematical programming strategies can be trapped prematurely in sub-optimal solutions. In this work stochastic searches such as genetic algorithms are suited for designing Petlyuk column together with the proposed rigorous design method, the reflux ratio, flows and compositions of interlinking streams are used as search variables in the proposed optimization approach and the objective function is accounted for minimizing the total annual cost for the system, which includes the capital and operational costs. Results are compared with shortcuts methods and rigorous simulations and the solution is found feasible.

References

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Doherty, M. F. y Malone, M. F. “Conceptual Design of Distillation Systems”, Editorial Mc Graw Hill, primer edición, (2001).

Dünnebier, G. and Pantelides, C.C. “Optimal Design of Thermally Coupled Distillation Columns”, Ind. Eng.Chem. Res., 38, p. 162-176, (1999).

Gutiérrez, C. and Jiménez, A., “Method for the Design of Azeotropic Distillation Columns”, Ind. Eng. Chem. Res., 46, p. 6635-6644, (2007).

Muralikrishna, K., Madhavan, K. P. and Shah, S. S., “Development of Dividing Wall Distillation Column Design Space for a Specified Separation”, Trans IChemE., 80, p. 155-166, (2002).

Nikolaides, I. P. and Malone, M. F., “Approximate Design of Multiple-Feed/Side Stream Distillation Systems”, Ind. Eng. Chem. Res., 26, p. 1839-1845, (1987).

Triantafyllou, C. and Smith, R. “The Design and Operation of Fully Thermally Coupled Distillation Columns”, Trans. Inst. Chem. Eng., 70p.118-132, (1992).

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