Peptide Coupling Reagents, More than a Letter Soup Ayman El-Faham* and Fernando Albericio* Institute for Research in Biomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028-Barcelona, Spain Alexandria University, Faculty of Science, Department of Chemistry, P.O. Box 426, Ibrahimia, 21321 Alexandria, Egypt CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, Barcelona Science Park, Baldiri Reixac 10, 08028-Barcelona, Spain. Department of Organic Chemistry, University of Barcelona, Martí i Franqu es 1-11, 08028-Barcelona, Spain

Peptide coupling reagents, more than a letter soup

Multiphase equilibria calculation by direct minimization of Gibbs free energy with a global optimization method

http://rua.ua.es/dspace/bitstream/10045/66521/5/2017_Marcilla_etal_FluidPhaseEquilibria_accepted.pdf

Should we trust all the published LLE correlation parameters in phase equilibria? Necessity of their assessment prior to publication

Evaluation of genetic algorithms and simulated annealing for phase equilibrium and stability problems

The reliable prediction of phase stability is a challenging computational problem in chemical process simulation, optimization, and design. The phase stability problem can be formulated either as a minimization problem or as an equivalent nonlinear equation-solving problem. Conventional solution methods are initialization dependent and may fail by converging to trivial or nonphysical solutions or to a point that is a local but not a global minimum. Thus, there has been considerable recent interest in developing more reliable techniques for stability analysis. Recently, the authors have demonstrated, using cubic equation of state models, a technique that can solve the phase stability problem with complete reliability. The technique, which is based on interval analysis, is initialization independent and, if properly implemented, provides a mathematical guarantee that the correct solution to the phase stability problem has been found. However, there is much room for improvement in the computational efficiency of the technique. In this paper, the authors consider two means of enhancing the efficiency of the method, both based on sharpening the range of interval function evaluations. Results indicate that, by using the enhanced method, computation times can be reduced by nearly an order of magnitude in some cases.

/pdf/enhanced-interval-analysis-for-phase-stability-cubic-wcm8r2zklf.pdf

Enhanced Interval Analysis for Phase Stability: Cubic Equation of State Models

A global algorithm for implementing the Gibbs tangent plane stability test in multicomponent, multiphase liquid mixtures is described. The algorithm is self-starting and significantly improves the reliability and robustness of multiphase equilibrium calculations. The main improvement is a result of a new approach for locating the stationary points of the tangent plane distance function. This relies on the geometric properties of the tangent plane distance function but is independent of the specific fluid model. For ternary mixtures, it is shown that stationary points in the tangent plane distance function satisfy a global topological constraint Nmax + Nmin − Nsad = 1 where Nmax is the number of maxima, Nmin is the number of minima, and Nsad is the number of saddles. This provides a global consistency check on the numerical results that is independent of the specific model or parameters used to describe the mixture. The method has been implemented and tested on a variety of problems with three and four com...

Global Stability Analysis and Calculation of Liquid−Liquid Equilibrium in Multicomponent Mixtures†

Multistage equilibrium separation processes involving binary homogeneous mixtures are analyzed. For several sets of specifications, it is shown that these process models admit a unique steady-state solution without the need for a constant molar overflow assumption or other energy balance or phase equilibria simplifications. The steady-state model equations are analyzed directly. Consequently, the analysis is constructive and gives insight into the behavior and development of algorithms for solving multistage separation process model equations in both the binary and multicomponent cases. To illustrate this, a new generalized Broyden acceleration technique is developed and used in the context of a sum-rates algorithm to solve several example problems. Numerical results show that the proposed acceleration method is comparable to Newton acceleration and clearly outperforms traditional Broyden acceleration. Other properties predicted by the analysis are also illustrated. 21 refs., 4 figs.

Analysis and algorithms for multistage separation processes

Multicomponent, multistage separation processes with fixed temperature and pressure profiles are analyzed by using classical fixed-point theory. In this paper it is shown that these processes exhibit unique vapor-liquid solutions, provided the fluid phases are homogeneous and satisfy the Gibbs-Duban equation and provided there are no retrograde effects. The role of homogeneity in determining solution uniqueness is clearly identified, and many new related analysis results are presented. In particular, it is shown that, under the conditions stated, the number of solutions is solely determined by the homogeneity of the fluid phases, indicating that mixtures that are potentially heterogeneous have the possibility of exhibiting turning points, bifurcations, and solution multiplicity. Auxiliary analysis results, such as the fact that the eigenvalues associated with any individual stage in the separator must lie in the closed interval (0,1), are also presented, and specific examples are used, whenever possible, for illustrative purposes.

Lakshmi N. Sridhar

Papers

Global Stability Analysis and Calculation of Liquid−Liquid Equilibrium in Multicomponent Mixtures†

Analysis and algorithms for multistage separation processes

Analysis of multicomponent, multistage separation processes: fixed temperature and fixed pressure profiles