Cyclones

Question 1

Methods to separate solids from gases

Answer 1

gravity settling (dp > 100 microns) - essentially settling tanks

filtration

wet scrubbing (spraying liquid to catch soilds and pull to group)

electrostatic percipitation (charge solids, pass through charged plate to remove solids from gas)

cyclones (least effective at separating, but cheap and efficient)

Question 2

inlet of cyclone is:

Answer 2

rectangualr shaped

(want narrow to increase friction forces and initializes swirling motion better)

Question 3

Cyclone outlet gas line must

Answer 3

protrude into cyclone, past the inlet feed (to prevent solids from blowing up into the outlet)

Question 4

2 types of cyclones:

Answer 4

reverse flow (gas flows done and then spirals up)

uniflow (cone to initialize spinning flow, and catcher at edge higher up to cause solids to fall back down cyclone)

Question 5

Multistage cyclones

Answer 5

series: allows to pull smaller particles out of sequential cyclones (can change inlet flowrate - speed up flowrate to remove smaller particles)

parallel: cyclones can only be so big, so for large flow, separate flow into different cyclones

analogous to pumps (series = can get higher pressure, parallel move more liquid)

Question 6

Issue with high velocity inlet in cyclones

Answer 6

• as velocity increases, pressure drop increases (more costly)
• high velocity can cause particle attrition (breakage of particles) (hence why would put cyclones in series to first remove large particles instead of breaking them apart)

Question 7

Advantages of cyclones

Answer 7

• no mechanical parts (no moving parts)
• relatively inexpensive
• relatively low pressure drops
• low maintenance costs (cohesive powders may build up in cyclone, or could get plugage of solids)

Question 8

Disadvantages of cyclones

Answer 8

• particle attrition
• wall erosion (high velocity particles are hitting wall, causing erosion)

Question 9

Cyclone solid loading

Answer 9

cyclone efficiency increases with solid loading (kg solids/m^3 of gas or kg of gas)

Question 10

Cyclone length

Answer 10

• efficiency improves with increased cyclone length until height > 6.15 cyclone diameter, then efficiency decreases (gas has too much turbulence and blows particles out with it)

Question 11

How do cyclones have a pressure drop?

Answer 11

• looses pressure with friction forces
• changing direction (direction changes is a change in velocity (vector) so causes a change in pressure)

Question 12

cyclone dp,th

Answer 12

all particles with dp>dp,th should be collected at 100% efficiency

Question 13

critical particle size, xcrit)

Answer 13

where about 50% of particles are collected in actuality, but theory says 100% should be collected

*Uses this one in calculations, not dp,th