DSMC simulation of vapor flow in molecular distillation

• Based on the VHS molecular model, a new rigorous model is established.
• Evaporating temperature has a more significant effect on flow properties of vapors.
• Number density and velocity always show a reverse variation tendency.
• The distribution of collision frequency and mean free path are opposite.
• The results are more consistent with actual situations that occur in collisions.

Molecular distillation process is a type of vapors transport process at high vacuum in the distillation space between evaporator and condenser. To get a better understanding of vapor flow characteristics in molecular distillation, vapor phase behavior is simulated using the direct simulation Monte Carlo (DSMC) method. The variable hard sphere (VHS) model is adopted to calculate molecular collisions. Molecular rotation and translation have both been taken into account while dealing with molecular energy distribution after collisions. Flow characteristics obtained for velocity distribution and number density are used to calculate the collision frequency and mean free path in the distillation space. The effects of evaporating temperature, condensing temperature and distillation gap on the vapors flow characteristics are discussed. It is found that for gas molecules by increasing the evaporating temperature, the number density and velocity, collision frequency in distillation space increase, while the mean free path decreases. As the condensing temperature increases, the number density and collision frequency in the distillation space increases, while velocity and mean free path decreases. It is also observed that much narrower distillation gap results in steeper variation tendency of flow characteristics.