Centrifugation

How are solids discharged in the split-bowl operating mode for a disc bowl centrifuge?

By pulling apart the top and bottom of the bowl intermittently.

How is gravitational force increased during centrifugation?

By rotating the mixture about a vertical axis at high speeds.

What are basket centrifuges?

Centrifuges with filters on the outer boundaries of the chamber. These utilize centrifugal force to drive the filtration process. The sediment builds up on top of the filter as a filter cake.

What are decanting centrifuges?

Centrifuges with internal archimedes screws that propel high (solids) and low density (liquids) streams in opposite directions during the centrifugation process. These effectively push out material after it sediments.

What is the function of a centripetal pump discharge?

It discharges liquid from a centrifuge chamber as the clarified rotating liquid enters the upper discharge chamber of the bowl during the centrifugation run.

How do gravity settling setups work to sediment particles?

Mixtures are passed over a long basin with a low flow rate and allowed to settle over the distance of the basin based on gravity.

What are decanting centrifuges used for?

Mixtures with high solid content.

What are the three different operating modes in a disc bowl centrifuge?

Solids retention Nozzle discharge Split-bowl discharge

What does a "-" value imply for a sedimentation velocity?

The particle/liquid will rise.

In continuous centrifugation separation processes, what must the influx of liquid flow be equivalent to?

The rate of solid sedimentation

In what horizontal direction does sedimentation occur relative to the centre of rotation in a centrifuge?

Away from the centre

What are four types of process scale centrifuges?

Disc bowl centrifuges Tubular centrifuges Decanting centrifuges Basket centrifuges (centrifugal filter)

For a spherical particle, what are the three forces acting on it when not being centrifuged?

Fw (resisting force) -> up Fa (lifting force) -> up Fg (weight) -> down

How are liquids of varying densities separated (via design) in a tubular centrifuge?

Given that liquids of lower density tend to float towards the centre of a centrifuge, the lower density liquids enter a tube in the centre whereas the higher density liquids are thrown into tubes on the outside of the centrifuge chamber.

At process scales, what general centrifugation speed would you use for most operations?

Relatively low speeds

What does the Stokes equation calculate?

Sedimentation velocity

What are tubular centrifuges good for?

Separating liquid mixtures with low volume flow rates and very good separation or for mixtures with high flow rates and moderate separation.

What does centrifugation essentially speed up?

The settling of particles/immiscible liquid by gravity.

What is the difference between a tubular centrifuge designed to discharge one separated liquid and one designed to separate two?

The tubular centrifuges are designed to contain separate discharge streams for each discharged liquid (thus, the quantity varies).

What is the function of variable shot control implementations in a disc bowl centrifuge?

They allow for control of the amount of sediment discharged during a centrifugation so as to increase product yield (from less losses) and shorten the harvest time.

How do automatic solids ejection mechanisms work?

They detect the buildup of sediment and open a small gap to let out the solids.

Why does it only take a short time for particles to sediment in a disk-bowl centrifuge?

They only have to travel a small distance.

What is the function of the nozzle discharge operating mode for a disc bowl centrifuge?

To continuously discharge the solid slurry from the running centrifuge. This is achieved by opening the nozzle slightly.

Why are baffles fitted into centrifuge tubes/chambers?

To direct lighter material to the centre while the heavier material is thrown to the outside.

What is the function of the split-bowl operating mode for a disc bowl centrifuge?

To intermittently discharge solid slurry from the running centrifuge. The upper and lower halves of the mixture are allowed to separate slightly prior to discharging the slurry.

What is the main purpose of centrifugation?

To remove cells/cell debris or to separate immiscible phases

What is the function of the solids retention operating mode for a disc bowl centrifuge?

To retain solids during the centrifuge run. Solids build up inside the rotor against the outside wall and are removed by taking apart the bowl.

When would you need to employ filter aids when using a decanting centrifuge?

When separating out microorganisms (given that this process provides too little of a separation effect to separate them out).