Revisiting the birth of NaCl crystals using molecular dynamics simulation.

Molecular dynamics simulations were used to investigate the initial stage of phase separation mechanisms for an oversaturated electrolytic solution. We developed a low computational cost methodology to determine the simulation frames where the first ionic clusters are formed. By discretizing the simulation box, we obtain a density profile in the moments preceding and succeeding the nuclei's formation. The growth of the clusters identified with our methodology was analyzed until the end of the simulation. Calculation of the Steinhardt parameter showed symmetry of the solid, giving indications that the classical nucleation theory explains the mechanism of the solid formation. The methodology developed was useful for identifying phase separation mechanisms in the nucleation process. At lower concentrations, there was no formation of stable clusters. At intermediate concentrations, the analyses indicate a transition of phases in one stage, from a oversaturate electrolytic solution to a crystalline solid. At high concentration, a transition of phases in two stages, initially, is the formation of a dense liquid, and only after that, crystalline solid formed inside the dense liquid. The change in phase separation mechanism due to increasing oversaturation underscores the importance of precise determination of the driving force for phase separation and concentration limits for each mechanism.

Simultaneous absorption of UV-vis and circular dichroism to measure enantiomeric concentrations of praziquantel under nonlinear conditions.

Beer-Lambert-Bouguer law is for a limiting case and, therefore, it is not useful to describe the relationship between absorption signal and enantiomer concentration in a stream when there are nonlinear phenomena present. In this work, the Chiral Detector (CD-2095 JASCO) equipment was used to measure simultaneously the UV-Vis and circular dichroism (CD) signals of a stream with different compositions of praziquantel enantiomers. The tested models were calibrated (parameter estimation) and validated using the Leave-One-Out Cross Validation (LOOCV) method. Both UV-vis and CD signals were absorbed differently in mixtures in comparison to pure solutions, indicating a nonlinear relationship between the absorbed signal and the enantiomer concentration in a mixture stream. Empirical mathematical relationships were tested for each signal (UV-vis and CD) and the pair of equations was evaluated using the Mean Square Error (MSE) metric for each enantiomer concentration (MSEL and MSED) and the pair of equations with the smallest MSEt (=MSEL + MSED) metric was chosen. Confidence interval analysis helped to find even simpler equations in comparison to the chosen ones. Higher nonlinearity was observed for a mixture with low L-PZQ concentration. The developed methodology allowed the choice of an empiric model to give good predictions in a wide range of concentration, what is of utmost importance for monitoring and automatic control purposes, for instance.

Nonlinear model predictive control applied to the separation of praziquantel in simulated moving bed chromatography.

An adaptive nonlinear model predictive control of a simulated moving bed unit for the enantioseparation of praziquantel is presented. A first principle model was applied at the proposed purity control scheme. The main concern about this kind of model in a control framework is in regard to the computational effort to solve it; however, a fast enough solution was achieved. In order to evaluate the controller's performance, several cases were simulated, including external pumps and switching valve malfunctions. The problem of plant-model mismatch was also investigated, and for that reason a parameter estimation step was introduced in the control strategy. In every studied scenario, the controller was able to maintain the purity levels at their set points, which were set to 99% and 98.6% for extract and raffinate, respectively. Additionally, fast responses and smooth actuation were achieved.