Position-dependent rate simulation

When running a CFD simulation, a decision must be made as to whether to use a laminar-flow or a turbulent-flow model. For many flow situations, the transition from laminar to turbulent flow with increasing flow rate is quite sharp, for example, at Re — 2100 for flow in an empty tube. For flow in a fixed bed, the situation is more complicated, with the laminar to turbulent transition taking place over a range of Re, which is dependent on the type of packing and on the position within the bed. 

Figures 16.38 and 16.39 demonstrate that the form of the particle size distributions is once again almost constant during the process time, and consequently the pneumatic recycled dust is not used for seed production. Dust is deposited on the particles because the nozzle position is close to the dust recycle tube (uniform wetted dust), and this leads to an enlarged particle growth. The measured time-dependent gas outlet temperature and the measured time-dependent conversion corresponds with simulations (Fig 16.40). The bed mass growth is linear at constant liquid injection rates (Fig. 16.41). The change in particle size distribution value and of the Sauter diameter is, again, declining.

Another illustration of the power of molecular dynamics simulation can be drawn from the sphere of enzyme catalysis. Many enzyme-catalyzed reactions proceed at a rate that depends on the diffusion-limited association of the substrate with the active site. Sharp et al. [28] have carried out Brownian dynamics simulations of the association of superoxide anions with superoxide dismutase (SOD). The active center in SOD is a positively charged copper atom. The distribution of charge over the enzyme is not uniform, and so an electric field is produced. Using their model, Sharp et al. [28] have shown that the electric field enhances the association of the substrate with the enzyme by a factor of 30 or more. Their calculations also predict correctly the response of the association rate to changes in ionic strength and amino... 

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