Packed Columns

Question 1

What is a packed column’s primary function

Answer 1

To contact two phases with each other to perform mass and heat transfer operations.

Question 2

What are the two major differences between packed columns and packed beds?

Answer 2

• Packed columns have much larger packing elements that generate a higher Re and turbulence.
• Packing columns packing will be hollow/complex shapes to maximise contact area and minimise pressure drop.

Question 3

How does a gas liquid contacting column work generally?

Answer 3

Liquid is distributed over the top of the packing and travels down through it, forming a film over the packings surface. Gas is then fed at the base (counter to fluid flow) contacting the higher surface area fluid film. The liquid and gaseous products are than extracted from the base and top respectively.

Question 4

How does distillation in a packing column work?

Answer 4

Hot vapour enters the cooler packing and the less volatile components condense on the high surface area packing. This allows efficient separation of the components with a column of minimum height/cost.

Question 5

What two important factors need to be taken into account when designing a packing bed support?

Answer 5

• Withstand the weight of the packing.
• Have at least 75% open area to allow a low pressure drops for gas passage whilst allowing fluid to flow back and be collected.

Question 6

What are the four categories of packing?

Answer 6

• Broken solids
• Shaped packings
• Grids
• Structured packings

Question 7

What are the 3 important factors to consider when selecting packing materials?

Answer 7

• Material
• Size
• Complexity

Question 8

Why is the material packing is made out of important?

Answer 8

The material must be compatible with gases and fluids. If crystallisation is possible then porous packing may be damaged. Polymers can be very corrosion resistant and inert but can be difficult to wet (especially on start up).

Question 9

Why is the packing size important?

Answer 9

Smaller packings of the same geometry will have better transfer characteristics allowing shorter columns at the expense of a bigger pressure drop.

Question 10

Why is the complexity of the packing important?

Answer 10

More complex packings are more expensive but could have better transfer characteristics allowing shorter columns.

Question 11

What is the loading point in a packed column?

Answer 11

The point where the gas starts to hold up the fluid in the packing giving a steeper increase in the pressure drop.

Question 12

What is the holding point?

Answer 12

The fluid cannot move down the column due to the gas velocity being to high. The change in pressure is also very high and so the column will essentially stop working.

Question 13

What is G’?

Answer 13

Gas mass flow rate per column section area.

Question 14

What is L’?

Answer 14

The liquid mass flow rate per column section area.

Question 15

What is F?

Answer 15

The packing factor of a material. Can be found in tables and manuals.

Question 16

What is the volumetric hold up?

Answer 16

The amount of liquid present in the column packing at any one time.

Question 17

How do you work out the hold up for rings and saddle packing?

Answer 17

Hw = 0.143 (L’ / d)^0.6

Hw = Volumetric hold up
L' = The liquid mass flow rate per column section area
d = column diameter