(404c) Development and Application of a CFD Model for Gas-Droplet Multiphase Flow with Evaporation in Optimizing Spray Injection and Atomization for Control of Reactor Selectivity

This presentation describes the development and application of a comprehensive computational fluid dynamics (CFD) model pertaining to a spray technology.  The spray nozzle was proposed for recycle of a liquid stream into the feed pipe of a gas-phase reactor.  The reactor produces two chemical products sold into two different markets.  The recycle injection impacts reactor selectivity, thus improves the process capability in responding to transient market conditions. Process reliability requires a complete evaporation of liquid droplets within the feeding pipe prior to entering the reactor with only a fraction of a second residence time available.  In addition, contact between the droplets and the feed pipe wall needs to be minimized in order to prevent a liquid film wall layer leading to potential fouling due to thermal polymerization.

The developed CFD model integrated comprehensive physics - including turbulent hydrodynamics, multiphase flow, convective heat transfer, phase change (droplet evaporation), and latent heat effect - into an Euler-Lagrange framework with 2-way coupling for gas/droplet interactions.  The motion of droplets is affected by the continuous gas phase convection, and as well the droplets and their evaporations affect the flow of the gas phase. The droplet size distribution used in the model was based on experimental measurements. The dispersion of droplets due to turbulence was predicted using the stochastic tracking (Random Walk) model that includes the effect of instantaneous turbulent velocity fluctuations on the droplet trajectories during integration through the use of stochastic methods. However, achieving a fully converged solution for such a complicated model was time-consuming.  Therefore, different simplification strategies were adopted in different phases to accelerate the design process by reducing the turnaround time for CFD simulations.  The developed spray technology was implemented in the plant and has shown the desired improvement in product flexibility.