EFFECT OF MIXING ENERGY, MIXING TIME AND SETTLING TIME ON DISPERSION EFFECTIVENESS IN TWO BENCH-SCALE TESTING SYSTEMS

摘要

Dispersion experiments were conducted in baffled-flask and paddle-jar mixing systems at five energy dissipation rates ranging from 4.8 × 10~(-4) to 1.6 × 10~(-1) J/kg-s. The objective of these experiments was to investigate the effects of mixing energy, mixing time, and settling time on dispersion effectiveness and size distribution of the chemically dispersed oil droplets. Two separate combinations of evaporatively weathered Mars crude oil premixed with dispersants differing in hydrophile-lipophile balance (HLB) (12 and 10) but having the same chemical composition (Tween 80 and Span 80 in dodecane) were used. Dispersion effectiveness increased with energy dissipation rate to a maximum and then leveled for all cases studied. In the baffled flask, dispersion effectiveness reached a maximum of 82 ± 5% irrespective of oil-dispersant combination. In the paddle jar, the maximum value of dispersion effectiveness was oil-dispersant specific, being at 87 ± 9% and 30 ± 11% for dispersant HLB 12 and 10, respectively. Mixing time did not seem to have a significant effect on dispersion effectiveness in comparison to the effects of energy dissipation rates and oil-dispersant combinations. The normalized volume distributions of the dispersed oil droplets were tri-modal in both systems, suggesting that multiple mechanisms of droplet formation occurred. The largest droplet mode disappeared from the size distribution in dispersions produced in the baffled flask when the mixing energy was ≥1.6 × 10~(-2) Jlkg-s. A similar behavior was also observed in the paddle jar for the oil-dispersant combination of HLB 12, but not for HLB 10. Inclusion of a settling period of 20 minutes before collecting sample decreased the dispersion effectiveness in paddle jar but no significant changes were observed in the baffled flask system. The differences observed were due to the differences in the size distributions of the dispersed oil droplets generated in these two systems.