L7/8- Multiphase systems Flashcards
Continuous phase
Mass transfer more rapid - turbulent diffusion
Disperse phase
Proceeds by droplet breakup and recoalescence
When is pH only controlled?
When aqueous phase continuous
What is insensitive to agitation?
Phase boundaries are insensitive to agitation
How does increasing agitator speed affect continuity?
Increasing agitator speed destabilises heavy phase continuity/ stabilises light phase continuity
What are the axis on a phase diagram related to phase continuity?
Volume fraction light phase v agitator speed/power
Controlling phase dispersion
Add phase to be dispersed to agitated vessel, until volume fraction is required
What phase will be continuous of an agitator is stopped and re-started?
Phase agitator sits in will become continuous (this is usually the heavy phase)
What type of liquid is best to have as disperse phase?
High viscosity liquid as disperse phase;
Effective mixture viscosity reduced & heat transfer coefficients improved
How to go from heavy continuous to light continuous phase?
Add light phase / increase volume fraction
(vice versa)
Effect of baffles & what can happen without?
Good dispersion, otherwise surface vortices form and overtopping danger
Agitated vessel; mean specific power consumption, epsilon m
epsilon m = P/(V*p)
Critical rotation speed, NJD:
If agitator started in heavy phase, there is a critical rotation speed NJD where the light layer above is dispersed in droplets (light in heavy)
(vice versa)
How would a decreased vessel size and heavy-in-light system affect power required?
Both would require more work/rotational power
What impeller best used for Solid suspensions?
Angled-blade turbine or marine propeller:
Directs flow downward to scour bottom and creates an upward velocity
