Process simulators dynamic simulations

While the classical approach to simulation of slow activated events, as described above, has received extensive attention in the literature and the methods are in general well established, the methods for quantum-dynamical simulation of reactive processes in complex systems in the condensed phase are still under development. We briefly consider electron and proton quantum dynamics. 

Bala, P., Lesyng, B., McCammon, J.A. Application of quantum-classical and quantum-stochastic molecular dynamics simulations for proton transfer processes. Chem. Phys. 180 (1994) 271-285. 

Permeation process of small molecules across lipid membranes studied by molecular dynamics simulations. J. Phys. Chem. 100 (1996) 16729-16738. 

The principal idea behind the CSP approach is to use input from Classical Molecular Dynamics simulations, carried out for the process of interest as a first preliminary step, in order to simplify a quantum mechanical calculation, implemented in a subsequent, second step. This takes advantage of the fact that classical dynamics offers a reasonable description of many properties of molecular systems, in particular of average quantities. More specifically, the method uses classical MD simulations in order to determine effective... 

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

D. Fincham and B. J. Ralston, Molecular dynamics simulation using the Cray-1 vector processing computer , Comp. Phys. Comm., Vol 23, no 2, 127-34, 1981. 

One drawback to a molecular dynamics simulation is that the trajectory length calculated in a reasonable time is several orders of magnitude shorter than any chemical process and most physical processes, which occur in nanoseconds or longer. This allows yon to study properties that change w ithin shorter time periods (such as energy finctnations and atomic positions), but not long-term processes like protein folding. 

To examine the soUd as it approaches equUibrium (atom energies of 0.025 eV) requires molecular dynamic simulations. Molecular dynamic (MD) simulations foUow the spatial and temporal evolution of atoms in a cascade as the atoms regain thermal equiUbrium in about 10 ps. By use of MD, one can foUow the physical and chemical effects that induence the final cascade state. Molecular dynamics have been used to study a variety of cascade phenomena. These include defect evolution, recombination dynamics, Hquid-like core effects, and final defect states. MD programs have also been used to model sputtering processes. 

The computer effort required to get a solution to a simulation problem is important because, ia the cases of optimization of desiga and dynamic simulation for control, many simulator mns must be made. At times the models of process units are simplified and often linearized to speed up the convergence. 

T. Sinhora and S. Kasada, "A Real Time Dynamic Simulator for the Residue ECC Process," paper presented at Eastern Simulation Conference, Odando, Fla., Apr. 18-21,1988. 

Process Safety Considerations. Unit optimization studies combined with dynamic simulations of the process may identify operating conditions that are unsafe regarding fire safety, equipment damage potential, and operating sensitivity. Several instances of fires and deflagrations in ethylene oxide production units have been reported in the past (160). These incidents have occurred in both the reaction cycle and ethylene oxide refining areas. Therefore, ethylene oxide units should always be designed to prevent the formation of explosive gas mixtures. 

Dynamical simulations monitor time-dependent processes in molecular systems in order to smdy their structural, dynamic, and thennodynamic properties by numerically solving an equation of motion, which is the formulation of the rules that govern the motion executed by the molecule. That is, molecular dynamics (MD) provides information about the time dependence and magnitude of fluctuations in both positions and velocities, whereas the Monte Carlo approach provides mainly positional information and gives only little information on time dependence. 

For 25 years, molecular dynamics simulations of proteins have provided detailed insights into the role of dynamics in biological activity and function [1-3]. The earliest simulations of proteins probed fast vibrational dynamics on a picosecond time scale. Fifteen years later, it proved possible to simulate protein dynamics on a nanosecond time scale. At present it is possible to simulate the dynamics of a solvated protein on the microsecond time scale [4]. These gains have been made through a combination of improved computer processing (Moore s law) and clever computational algorithms [5]. 

The interaction between ligands and their receptors is clearly a dynamic process. Once the static model of ligand-receptor interaction has been obtained, the stability of ligand-receptor complexes should be evaluated by means of molecular dynamics simulations [18]. 

During process design, the greatest opportunity to benefit from dynamic simulation is after adequate design information is available to develop the model,... 

Ernest, J.B. and C.A. Depew, 1995, Use of Dynamic Simulation to Model HPU Reactor Depressuring, Hydrocarbon Processing, January 1995, p. 72. 

S. Hess. Constraints in molecular dynamics, nonequihbrium processes in fluids via computer simulations. In K. H. Hoffmann, M. Schreiber, eds. Computational Physics. Heidelberg Springer-Verlag, 1996, pp. 268-293. 

M. R. Philpott, J. N. Glosli. Molecular dynamics simulation of interfacial electrochemical processes electric double layer screening. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 2, pp. 13-30. 

Whenever the polymer crystal assumes a loosely packed hexagonal structure at high pressure, the ECC structure is found to be realized. Hikosaka [165] then proposed the sliding diffusion of a polymer chain as dominant transport process. Molecular dynamics simulations will be helpful for the understanding of this shding diffusion. Folding phenomena of chains are also studied intensively by Monte Carlo methods and generalizations [166,167]. 

DFT molecular dynamics simulations were used to investigate the kinetics of the chemical reactions that occur during the induction phase of acid-catalyzed polymerization of 205 [97JA7218]. These calculations support the experimental finding that the induction phase is characterized by the protolysis of 205 followed by a rapid decomposition into two formaldehyde molecules plus a methylenic carbocation (Scheme 135). For the second phase of the polymerization process, a reaction of the protonated 1,3,5-trioxane 208 with formaldehyde yielding 1,3,5,7-tetroxane 209 is discussed (Scheme 136). 

SMB technology is now a mature technology adopted by pharmaceutical industry. The existence of an organized body of knowledge [39, 40] was helpful in optimizing SMB systems and making them acceptable by the industry. The future will require dynamic simulation for systems with small number of columns, e.g., configurations of the type 1-2-2-1 as encountered in some cases and also in view of process control to improve process performance. 

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