Filtration

Question 1

What are the potential driving forces for filtration?

Answer 1

• gravity
• vacuum
• pressure
• centrifugal force
  (only last 3 are used on an industrial scale)

Question 2

Filter Press

Answer 2

• 2 plates squeeze together to form a filter cake
• slurry distributed between 2 plates and the plate compression removes liquid
• often operated in parallel so that one can be cleaned while other is used
• uses vibration to remove cake

Question 3

Belt Press

Answer 3

• completely continuous
• belt is a filter material, so liquid from slurry falls through belt (slight vacuum (0.8-0.5 atm) under belt or gravity used to pull liquid from system)
• cannot get very dry filter cake, but can handle very large flowrates.

Question 4

Rotary Drum

Answer 4

• drum rotates with rotating material being the filter
• slurry is fed at the basin (slight vacuum at centre of drum which pulls liquid into it)
• can handle very large flowrates, but expensive

Question 5

Filter theory 2 types of filters:

Answer 5

1. Surface filter (particles collected at surface as size of solids > pore size of filter) (used for solid concentration greater than 1% by volume)
2. depth filter (small particles can penetrate into filter media)
   - want solid concentration less than 1% by volume (otherwise can be very expensive to replace)

Question 6

Necessary properties of a filter media

Answer 6

• must remove solids with certain size (to give clear filtrate)
• pores should not become plugged enough that filtration becomes too slow
• must be strong enough not to tear
• must be chemically resistant (not react with slurry)

Question 7

What are the 2 properties that may be kept constant when filtering

Answer 7

1.constant pressure
- reduce rpm of pump to get lower flowrate

2. constant flowrate
   - increase rpm of pump to get higher pressure

Question 8

Cycle Time

Answer 8

• the total time for filtration to occur (time to cycle, time to filter and dead time - wash time, disassembly/assembly time)
• is cyclical